Accurate Singlet–Triplet Excited States Energy Gap Can Be Mastered by Time-Dependent Density Functional Theory Calculations Based on a Dielectric-Screened Range-Separated Hybrid Functional
Roshan Khatri, Barry D. Dunietz
J. Phys. Chem. C..
,
XXX
(2024) XXX
The energy gap between the lowest singlet and lowest triplet excited states of molecular emitters is a key property affecting their thermally activated delayed fluorescence (TADF) functionality. In cases in which these states are marked by internal charge transfer, the molecular environment of the emitter significantly stabilizes these states. To address this challenging relationship, a recently developed density functional theory (DFT) framework that alleviates the fundamental orbital gap in the gas phase can be employed, offering a highly effective and predictive description of these states. These modern functionals, range-separated hybrid functionals, are based on a generalized Kohn–Sham exchange functional expression where electronic interactions are separated into long and short ranges. For representing the significant condensed phase effects, especially in the case of charge transfer states, dielectric screening can be affected by functional expression. Dielectric-screened range-separated functionals that are invoked within a polarizable continuum model were shown to properly address the orbital gap challenge in the condensed phase and consequently benchmark well in calculating triplet and charge transfer excited states. We address the excited singlet–triplet state energy gap using such a screened range-separated functional and find that the calculated gaps tend to fall within 0.1 eV of the measured energies for a widely addressed benchmark set. Here, we emphasize the success of the dielectric-screened DFT framework in calculating these important energy gaps, reproducing well the benchmark values and therefore alleviating concerns about the applicability of existing functionals. We posit that dielectric-screened calculations will bear increasing impact on the design of organic materials aiming to enhance optoelectronic applications and in particular of realized TADF efficiencies.
.......................................................................................................................................
Anisotropic Dielectric Screened Range-Separated Hybrid Density Functional Theory Calculations of Charge Transfer States across an Anthracene–TCNQ Donor–Acceptor Interface
Chandrima Chakravarty, Maximilian A. C. Saller, Huseyin Aksu, Barry D. Dunietz
J. Chem. Theory Comput.
,
20
(2024) 10751
A density functional theory framework is developed to study electronic excited states affected by an anisotropic dielectric environment. In particular, an anisotropic dielectric screened range-separated hybrid (SRSH[r]) functional is defined and combined with an anisotropic polarizable continuum model (PCM) implemented through a generalized Poisson equation solver. We develop the SRSH-PCM(r) approach and use it to quantify the effect of anisotropy on an excited charge transfer (CT) state energy. In particular, the dielectric interface effect on the CT state within a donor–acceptor molecular complex of antrancene and tetracyanoquinodimethane is studied. The donor–acceptor complex and the dielectric interface are used to represent the interface between thin films consisting of these materials. We report the effect of such a dielectric interface on the energy of a CT and follow its dependence on the donor–acceptor distance. We also benchmark the anisotropy-affected energy by comparing to homogeneous dielectric calculated energies. Due to the planar interface, the anisotropic energies are expected to to match with those obtained based on isotropic calculations of the larger dielectric constant at large enough distances. The approach is applicable, in general, to more complicated dielectric constant distributions as expected to be found in actual interfaces of such thin films or in other systems, for example, for CT processes within photosystems.
.......................................................................................................................................
Antioxidative Triplet Excitation Energy Transfer in Bacterial Reaction Center Using a Screened Range Separated Hybrid Functional
Khadiza Begam, Huseyin Aksu, Barry D Dunietz
J. Phys. Chem. B.
,
128
(2024) 4315-4324
Excess energy absorbed by photosystems (PSs) can result in photoinduced oxidative damage. Transfer of such energy within the core pigments of the reaction center in the form of triplet excitation is important in regulating and preserving the functionality of PSs. In the bacterial reaction center (BRC), the special pair (P) is understood to act as the electron donor in a photoinduced charge transfer process, triggering the charge separation process through the photoactive branch A pigments that experience a higher polarizing environment. At this work, triplet excitation energy transfer (TEET) in BRC is studied using a computational perspective to gain insights into the roles of the dielectric environment and interpigment orientations. We find in agreement with experimental observations that TEET proceeds through branch B. The TEET process toward branch B pigment is found to be significantly faster than the hypothetical process proceeding through branch A pigments with ps and ms time scales, respectively. Our calculations find that conformational differences play a major role in this branch asymmetry in TEET, where the dielectric environment asymmetry plays only a secondary role in directing the TEET to proceed through branch B. We also address TEET processes asserting the role of carotenoid as the final triplet energy acceptor and in a mutant form, where the branch pigments adjacent to P are replaced by bacteriopheophytins. The necessary electronic excitation energies and electronic state couplings are calculated by the recently developed polarization-consistent framework combining a screened range-separated hybrid functional and a polarizable continuum mode. The polarization-consistent potential energy surfaces are used to parametrize the quantum mechanical approach, implementing Fermi’s golden rule expression of the TEET rate calculations.
.......................................................................................................................................
Visible Light-Activatable Platinum (IV) Prodrugs Harnessing CD36 for Ovarian Cancer Therapy
A. M. D. S. Jayawardhana, S. Bhandari, A. Kaspi-Kaneti, M. Kshetri, Z. Qiu, M. M. Cheline, H. Shen, \underline{B. D. Dunietz}, Y. Zheng
Dalton Trans.
53
(2023) 10942-50
We hereby engineered photoactivatable Pt(IV) metallodrugs that harness CD36 to target ovarian cancer cells. Pt(IV) compounds mimic the structure of fatty acids and take advantage of CD36 as a “Trojan horse” to gain entry into the cells. We confirmed that CD36-dependent entry occurs using graphite furnace atomic absorption spectroscopy with ovarian cancer cells expressing different levels of CD36 and a CD36 inhibitor, SSO. Once the Pt(IV) metallodrugs enter the cancer cells, they can be activated to form Pt(II) with characteristics of cisplatin under visible light (490 nm) irradiation, promoting photoinduced electron transfer from the attached fluorophore to the metal center. This light-induced activation can increase the cytotoxicity of the Pt(IV) metallodrugs by up to 20 times toward ovarian cancer cells, inducing DNA damage and enabling efficient elimination of drug-resistant cancer cells.
.......................................................................................................................................
A Computational Study of the Electronic Energy and Charge Transfer Rates and Pathways in the Tetraphenyldibenzoperiflanthene/Fullerene Interfacial Dyad
A Schubert, S Bhandari, E Geva, Barry D. Dunietz
J. Phys. Chem. Lett.
,
14
(2023) 9569-9583
The electronic transition rates and pathways underlying interfacial charge separation in tetraphenyldibenzoperiflanthene:fullerene (DBP:C70) blends are investigated computationally. The analysis is based on a polarization-consistent framework employing screened range-separated hybrid functional in a polarizable continuum model to parametrize Fermi’s golden rule rate theory. The model considers the possible transitions within the 25 lowest excited states of a DBP:C70 dyad that are accessible by photoexcitation. The different identified pathways contributing to charge carrier generation include electron and hole transfer and backtransfer, exciton transfer, and internal relaxation steps. The larger density of states of C70 appears to explain the previously observed larger efficiency for charge separation through hole transfer mechanism. We also analyze the validity of the high-temperature and short-time semiclassical approximations of the FGR theory, where both overestimated and underestimated Marcus theory based constants can be affected.
.......................................................................................................................................
Polarization Consistent Dielectric Screening in Polarizable Continuum Model Calculations of Solvation Energies
Roshan Khatri and Barry D. Dunietz
J. Chem. Phys.
,
159
(2023) 071103
A polarization consistent framework, where dielectric screening is affected consistently in polarizable continuum model (PCM) calculations, is employed for the study of solvation energies. The computational framework combines a screened range-separated-hybrid functional (SRSH) with PCM calculations, SRSH-PCM, where dielectric screening is imposed in both PCM self-consistent reaction field (SCRF) iterations and the electronic structure Hamiltonian. We begin by demonstrating the impact of modifying the Hamiltonian to include such dielectric screening in SCRF iterations by considering the solutions of electrostatically embedded Hartree–Fock (HF) exact exchange equations. Long-range screened HF-PCM calculations are shown to capture properly the linear dependence of gap energy of frontier orbitals on the inverse of the dielectric constant, whereas unscreened HF-PCM orbital energies are fallaciously semi-constant with respect to the dielectric constant and, therefore, inconsistent with the ionization energy gaps. Similar trends affect density functional theory (DFT) calculations that aim to achieve predictive quality. Importantly, the dielectric screened calculations are shown to significantly affect DFT- and HF PCM-based solvation energies, where screened solvation energies are smaller compared to the unscreened values. Importantly, SRSH-PCM, therefore, appears to reduce the tendency of DFT-PCM to overestimate solvation energies, where we find the effect to increase with the dielectric constant and the polarity of the molecular solute, trends that enhance the quality of DFT-PCM calculations of solvation energy. Understanding the relationship of dielectric screening in the Hamiltonian and DFT-PCM calculations can ultimately benefit on-going efforts for the design of predictive and parameter free descriptions of solvation energies.
.......................................................................................................................................
Luminescent 1H-1,3-benzazaphospholes
Sloane Evariste, Alexandra M Harrison, Sunandan Sarkar, Arnold L Rheingold, Barry
D Dunietz, Joachim W Heinicke, Emalyn Delgado Rosario, Sungwoon Yoon, Thomas
S Teets, John D Protasiewicz
RSC Adv.
,
13
(2023) 594
2-R-1H-1,3-Benzazaphospholes (R-BAPs) are an interesting class of σ2P heterocycles containing P[double bond, length as m-dash]C bonds. While closely related 2-R-1,3-benzoxaphospholes (R-BOPs) have been shown to be highly photoluminescent materials depending on specific R substituents, photoluminescence of R-BAPs has been previously limited to an example having a fused carbazole ring system. Here we detail the synthesis and structural characterization of a new R-BAP (3c, R = 2,2′-dithiophene), and compare its photoluminescence against two previously reported R-BAPs (3a, R, R′ = Me and 3b, R = 2-thiophene). The significant fluorescence displayed by the thiophene derivatives 3b (φ = 0.53) and 3c (φ = 0.12) stands in contrast to the weakly emissive methyl substituted analogue 3a (φ = 0.08). Comparative computational investigations of 3a–c offer insights into the interplay between structure–function relationships affecting excited state relaxation processes.
.......................................................................................................................................
Engineering Giant Excitonic Coupling in Bioinspired, Covalently Bridged BODIPY Dyads
Sara Ansteatt, Brian Uthe, Bikash Mandal, Rachel Gelfand, Barry D.Dunietz, Matthew
Pelton, and Marcin Ptaszek
Phys. Chem. Chem. Phys.
,
25
(2023) 8013
Strong excitonic coupling in photosynthetic systems is believed to enable efficient light absorption and quantitative charge separation, motivating the development of artificial multi-chromophore arrays with equally strong or even stronger excitonic coupling. However, large excitonic coupling strengths have typically been accompanied by fast non-radiative recombination, limiting the potential of the arrays for solar energy conversion as well as other applications such as fluorescent labeling. Here, we report giant excitonic coupling leading to broad optical absorption in bioinspired BODIPY dyads that have high photostability, excited-state lifetimes at the nanosecond scale, and fluorescence quantum yields of nearly 50%. Through the synthesis, spectroscopic characterization, and computational modeling of a series of dyads with different linking moieties, we show that the strongest coupling is obtained with diethynylmaleimide linkers, for which the coupling occurs through space between BODIPY units with small separations and slipped co-facial orientations. Other linkers allow for broad tuning of both the relative through-bond and through-space coupling contributions and the overall strength of interpigment coupling, with a tradeoff observed in general between the strength of the two coupling mechanisms. These findings open the door to the synthesis of molecular systems that function effectively as light-harvesting antennas and as electron donors or acceptors for solar energy conversion.
.......................................................................................................................................
Effects of Solvent Dielectric on Thermally Activated Delayed Fluorescence: A Predictive Computational Polarization Consistent Approach
Bikash Mandal and Barry D. Dunietz
J. Phys. Chem. A.
,
127
(2023) 216
We study computationally thermally activated delayed fluorescence (TADF) in donor–acceptor compounds. The relevant electronic excited states that are strongly affected by the dielectric environment are treated by a polarization consistent framework. The high fidelity potential energy surfaces are used following a quantum-mechanical Fermi’s golden rule (FGR) picture to calculate rates of intersystem crossing (ISC) and reverse intersystem crossing (RISC). To demonstrate the potency of the approach, we consider isomers of benzonitrile functionalized tert-butyl-substituted dimethylacridine (DMAC-BN), which were recently found to perform well as TADF emitters. The calculated excited state energies that appear to reproduce well measured spectral trends with respect to the dielectric constant are used to parametrize ISC/RISC FGR rates. The calculated rates reproduce well measured rates, whereas semiclassical based rates are grossly underestimated. In particular, we find in agreement with the recent experimental study [Phys. Rev. Appl.2019, 12, 044021] that the ortho and meta isomers are significantly more effective as TADF emitters. The computational framework provides valuable insight at the molecular level into RISC rates and therefore can contribute to the design of materials of increased TADF efficiency.
.......................................................................................................................................
Solvent Dependent Nuclear Magnetic Resonance Molecular Parameters Based on a Polarization Consistent Screened Range Separated Hybrid Density Functional Theory Framework
Khadiza Begam, Lilian Cohen, Gil Goobes, Barry D Dunietz
J. Chem. Theory Comput.
,
18
(2022) 5259
Nuclear magnetic resonance (NMR) properties of solvated molecules are significantly affected by the solvent. We, therefore, employ a polarization consistent framework that efficiently addresses the solvent polarizing environment effects. Toward this goal a dielectric screened range separated hybrid (SRSH) functional is invoked with a polarizable continuum model (PCM) to properly represent the orbital gap in the condensed phase. We build on the success of range separated hybrid (RSH) functionals to address the erroneous tendency of traditional density functional theory (DFT) to collapse the orbital gap. Recently, the impact of RSH that properly opens up the orbital gap in gas-phase calculations on NMR properties has been assessed. Here, we report the use of SRSH-PCM that produces properly solute orbital gaps in calculating isotropic nuclear magnetic shielding and chemical shift parameters of molecular systems in the condensed phase. We show that in contrast to simpler DFT-PCM approaches, SRSH-PCM successfully follows expected dielectric constant trends.
.......................................................................................................................................
Role of Dielectric Screening in Calculating Excited States of Solvated Azobenzene: A Benchmark Study Comparing Quantum Embedding and Polarizable Continuum Model for Representing the Solvent
C. Chakravarty, H. Aksu, J. A. Martinez B., P. Ramos, M. Pavanello, and B. D. Dunietz
J. Phys. Chem. Lett.
,
13
(2022) 4849
The low energy excited states of the conformational isomers of solvated azobenzene are calculated with several DFT methods accounting for the solute–solvent interaction implicitly with the polarizable continuum model or explicitly with subsystem DFT. For the latter, embedding potentials are calculated for 21 sampled snapshots of the solvent molecules. First, we find that accounting for the solvent implicitly or explicitly has little effect on the predicted cis–trans S1 excitation energy gap. Second, we find that azobenzene’s S1 cis and trans energies are accurate as long as a screened range-separated hybrid exchange-correlation functional is employed. Finally, we also tested a simplified workflow whereby a single, averaged, embedding potential is used. Unfortunately, we find larger deviations against the experiment for the simplified workflow. This highlights a basic flaw in the approach, where the time scale of solvent averaging is much longer than that of the solute’s electronic polarization.
.......................................................................................................................................
Correlating Interfacial Charge Transfer Rates with Interfacial Molecular Structure in the Tetraphenyldibenzoperiflanthene/C70 Organic Photovoltaic System
J. Tinnin, S. Bhandari, P. Zhang, E. Geva, B. D. Dunietz, X. Sun, M. S. Cheung
J. Phys. Chem. Lett.
,
13
(2022) 763
Organic photovoltaics (OPV) is an emerging solar cell technology that offers vast advantages such as low-cost manufacturing, transparency, and solution processability. However, because the performance of OPV devices is still disappointing compared to their inorganic counterparts, better understanding of how controlling the molecular-level morphology can impact performance is needed. To this end, one has to overcome significant challenges that stem from the complexity and heterogeneity of the underlying electronic structure and molecular morphology. In this Letter, we address this challenge in the context of the DBP/C70 OPV system by employing a modular workflow that combines recent advances in electronic structure, molecular dynamics, and rate theory. We show how the wide range of interfacial pairs can be classified into four types of interfacial donor–acceptor geometries and find that the least populated interfacial geometry gives rise to the fastest charge transfer (CT) rates.
.......................................................................................................................................
Enhancing fluorescence and lowering the optical gap through C=P doping of a pi-conjugated molecular backbone: A computational-based design approach
S. Sarkar, P. Durairaj, J. D. Protasiewicz, B. D. Dunietz
J. Photochem. Photobio.
,
8
(2021) 100089
Design principles of organic fluorescent materials are investigated at the molecular level using a first-principles-based computational approach. The design approach incorporates hetero atoms into a conjugated cyclic skeleton by introducing Cdouble bondP bonds. In particular we report the design of a molecular system of an extended conjugation system that maintains its planarity across the full system, achieving low lying electronic excited states of large oscillator strengths. We also present calculations that confirm the competing process of internal system crossing to be too slow for affecting the actual relaxation following photexcitation.
.......................................................................................................................................
Electronic Spectra of C-60 Films Using Screened Range Separated Hybrid Functionals
Chandrima Chakravarty,
Huseyin Aksu,
Buddhadev Maiti,
Barry D. Dunietz,
J. Phys. Chem. A.
,
125
(2021) 7625
We study computationally the electronic spectra of C-60 thin films using the recently developed density functional theory (DFT) framework combining a screened range separated hybrid (SRSH) functional with a polarizable continuum model (PCM). The SRSH-PCM approach achieves excellent correspondence between the frontier orbital's energy levels and the ionization potential and electron affinity of the molecular system at the condensed phase and consequently leads to high quality electronic excitation energies when used in time-dependent DFT calculations. Our calculated excited states reproduce the experimentally main reported spectral peaks at the 3.6-4.6 eV energy range and when addressing excitonic effects also reproduce the red-shifted spectral feature. Notably, we analyze the low-lying peak at 2.7 eV and associate it to an excitonic state.
.......................................................................................................................................
Software for the frontiers of quantum chemistry: An overview of developments in the Q-Chem 5 package
Epifanovsky, E et al.
J. Chem. Phys.
,
155
(2021) 084801
This article summarizes technical advances contained in the fifth major release of the Q-Chem quantum chemistry program package, covering developments since 2015. A comprehensive library of exchange-correlation functionals, along with a suite of correlated many-body methods, continues to be a hallmark of the Q-Chem software. The many-body methods include novel variants of both coupled-cluster and configuration-interaction approaches along with methods based on the algebraic diagrammatic construction and variational reduced density-matrix methods. Methods highlighted in Q-Chem 5 include a suite of tools for modeling core-level spectroscopy, methods for describing metastable resonances, methods for computing vibronic spectra, the nuclear-electronic orbital method, and several different energy decomposition analysis techniques. High-performance capabilities including multithreaded parallelism and support for calculations on graphics processing units are described. Q-Chem boasts a community of well over 100 active academic developers, and the continuing evolution of the software is supported by an "open teamware" model and an increasingly modular design.
.......................................................................................................................................
Intersystem Crossing in Tetrapyrrolic Macrocycles. A First-Principles Analysis
Srijana Bhandari,
Sunandan Sarkar,
Alexander Schubert,
Atsushi Yamada,
Jameson Payne,
Marcin Ptaszek,
Eitan Geva,
Barry D. Dunietz,
J. Phys. Chem. C.
,
125
(2021) 13493-13500
We analyze reported trends of the photophysical properties in a series of tetrapyrrolic macrocycles with varied saturation level. We compare rates of intersystem crossing (ISC) and fluorescence upon photoabsorption in porphine (P), chlorin (CH), and bacteriochlorin (BC). CH and BC result from single hydrogenation and double hydrogenation of P, respectively. A first-principles time-dependent density functional theory based on a novel framework is used to implement a quantum-mechanical Fermi's golden rule (FGR) rate theory. We employ the recently developed screened range-separated hybrid (SRSH) functionals and a polarizable continuum model (PCM) achieving a polar- ization-consistent description of the embedded molecular electronic structure. We find, in agreement with the measurements, an increase of the ISC rate upon hydrogenation originating in an increase of the spin-orbit coupling (SOC). This trend is traced back to the overlap of attachment and detachment densities of the relevant singlet and triplet states. Simultaneously, we find an increase in the fluorescence rate competing with the ISC, which, overall, results in a lower ISC yield with increasing degree of hydrogenation despite the increased SOC. Crucially, both the quantum mechanical perspective in the FGR theory and the polarization consistent formulation achieved by the screened RSH used in the DFT calculations are required for achieving predictive quality in the calculated rates.
.......................................................................................................................................
CTRAMER: An open-source software package for correlating interfacial charge transfer rate constants with donor/acceptor geometries in organic photovoltaic materials
Jacob Tinnin, Huseyin Aksu, Zhengqing Tong, Pengzhi Zhang, Eita Geva, Barry D. Dunietz, Xiang Sun, Margaret S. Cheung.
J. Chem. Phys.
,
154
(2021) 214108
In this paper, we present CTRAMER (Charge-Transfer RAtes from Molecular dynamics, Electronic structure, and Rate theory)-an open-source software package for calculating interfacial charge-transfer (CT) rate constants in organic photovoltaic (OPV) materials based on ab initio calculations and molecular dynamics simulations. The software is based on identifying representative donor/acceptor geometries within interfacial structures obtained from molecular dynamics simulation of donor/acceptor blends and calculating the corresponding Fermi's golden rule CT rate constants within the framework of the linearized-semiclassical approximation. While the methods used are well established, the integration of these state-of-the-art tools originating from different disciplines to study photoinduced CT processes with explicit treatment of the environment, in our opinion, makes this package unique and innovative. The software also provides tools for investigating other observables of interest. After outlining the features and implementation details, the usage and performance of the software are demonstrated with results from an example OPV system.
.......................................................................................................................................
Simulating energy transfer dynamics in the Fenna–Matthews–Olson complex via the modified generalized quantum master equation
Ellen Mulvihill, Kristina M. Lenn, Xing Gao, Alexander Schubert, Barry D. Dunietz, and Eitan Geva
J. Chem. Phys.
,
154
(2021) 204109
The generalized quantum master equation (GQME) provides a general and formally exact framework for simulating the reduced dynamics of open quantum systems. The recently introduced modified approach to the GQME (M-GQME) corresponds to a specific implementation of the GQME that is geared toward simulating the dynamics of the electronic reduced density matrix in systems governed by an excitonic Hamiltonian. Such a Hamiltonian, which is often used for describing energy and charge transfer dynamics in complex molecular systems, is given in terms of diabatic electronic states that are coupled to each other and correspond to different nuclear Hamiltonians. Within the M-GQME approach, the effect of the nuclear degrees of freedom on the time evolution of the electronic density matrix is fully captured by a memory kernel superoperator, which can be obtained from short-lived (compared to the time scale of energy/charge transfer) projection-free inputs. In this paper, we test the ability of the M-GQME to predict the energy transfer dynamics within a seven-state benchmark model of the Fenna–Matthews–Olson (FMO) complex, with the short-lived projection-free inputs obtained via the Ehrenfest method. The M-GQME with Ehrenfest-based inputs is shown to yield accurate results across a wide parameter range. It is also found to dramatically outperform the direct application of the Ehrenfest method and to provide better-behaved convergence with respect to memory time in comparison to an alternative implementation of the GQME approach previously applied to the same FMO model.
.......................................................................................................................................
Three-state harmonic models for photoinduced charge transfer
Dominikus Brian, Zengkui Liu, Barry D. Dunietz, Eitan Geva, and Xiang Sun
J. Chem. Phys.
,
154
(2021) 174105
A widely used strategy for simulating the charge transfer between donor and acceptor electronic states in an all-atom anharmonic condensed-phase system is based on invoking linear response theory to describe the system in terms of an effective spin-boson model Hamiltonian. Extending this strategy to photoinduced charge transfer processes requires also taking into consideration the ground electronic state in addition to the excited donor and acceptor electronic states. In this paper, we revisit the problem of describing such nonequilibrium processes in terms of an effective three-state harmonic model. We do so within the framework of nonequilibrium Fermi’s golden rule (NE-FGR) in the context of photoinduced charge transfer in the carotenoid–porphyrin–C60 (CPC60) molecular triad dissolved in explicit tetrahydrofuran (THF). To this end, we consider different ways for obtaining a three-state harmonic model from the equilibrium autocorrelation functions of the donor–acceptor, donor–ground, and acceptor–ground energy gaps, as obtained from all-atom molecular dynamics simulations of the CPC60/THF system. The quantum-mechanically exact time-dependent NE-FGR rate coefficients for two different charge transfer processes in two different triad conformations are then calculated using the effective three-state model Hamiltonians as well as a hierarchy of more approximate expressions that lead to the instantaneous Marcus theory limit. Our results show that the photoinduced charge transfer in CPC60/THF can be described accurately by the effective harmonic three-state models and that nuclear quantum effects are small in this system.
.......................................................................................................................................
Heat Flow Enhancement in a Nanoscale Plasmonic Junction Induced by Kondo Resonances and Electron-Phonon Coupling
Ali Goker, Huseyin Aksu, Barry D Dunietz
Physica E: Low-dimensional Systems and Nanostructures
,
127
(2021) 114536
Recently, we showed that plasmon-exciton coupling can increase entropy current through a bridge coupled to plasmonic metal nanoparticles. Here we show that electron-phonon coupling can also be used to control the entropy current in similar systems. Entropy current tends to decrease due to electron-phonon coupling and to exhibit a monotonous decrease upon temperature ramping. However, an anomaly affecting the current where it is enhanced by electron-phonon coupling is indicated at around 42 times the system’s Kondo temperature. We therefore report means to control heat flow by tuning the Kondo resonance through the electron-phonon coupling. We analyze the conditions that bring about these trends due to electron-phonon coupling by employing non-equilibrium Green’s function formulation addressing the entropy current and the derived heat flow.
.......................................................................................................................................
Cyanide Bridged Platinum‐Iron Complexes as Cisplatin Prodrug Systems: Design and Computational Study
Ariela W. Kaspi-Kaneti, Srijana Bhandari, Alexander Schubert, Songping Huang, Barry Dunietz
ChemPhysChem
,
22
(2021) 106-111
The potential role of cyanide-bridged platinum-iron complexes as an anti-cancer Pt(IV) prodrug is studied. We present design principles of a dual-function prodrug that can upon reduction dissociate and release concurrently six cisplatin units and a ferricyanide anion per prodrug unit. The prodrug molecule is a unique complex of hepta metal centers consisting of a ferricyanide core with six Pt(IV) centers each bonded to the Fe (III) core through a cyano ligand. The functionality of the prodrug is addressed through density functional theory (DFT) calculations.
.......................................................................................................................................
Achieving Predictive Description of Negative Differential Resistance in Molecular Junctions Using a Range‐Separated Hybrid Functional
Srijana Bhandari, Atsushi Yamada, Austin Hoskins, Jameson Payne, Huseyin Aksu, Barry D. Dunietz
Adv. Theory Simul.
,
(2021) 2000016
Range‐separated hybrid (RSH) functionals have been recently used to overcome the tendency of traditional density functional theory (DFT) calculations to overestimate the conductance of molecular junctions. Non‐equilibrium conditions are addressed following non‐equilibrium Green's function (NEGF) formulation with RSH functionals to study negative differential resistance (NDR) in molecular junctions of oligo phenylene ethylene derivatives linking gold electrodes. It is shown that the RSH‐NEGF calculations indicate NDR onset bias that agrees well with measured trends, associate NDR to orbital localization at the drain contact, and analyze the role of junction asymmetry in NDR. The RSH‐NEGF results are also compared with alternative DFT‐NEGF combinations to highlight the importance of basing the computational study on a functional that achieves physically significant frontier orbitals. Finally, the effects of thermally accessible molecular fluctuations to enhance the NDR conductance drop are also discussed.
.......................................................................................................................................
How Well Does a Solvated Octa-acid Capsule Shield the Embedded Chromophore? A Computational Analysis Based on an Anisotropic Dielectric Continuum Model
H. Aksu, S. K. Paul, J. M. Herbert, and B. D. Dunietz
J. Phys. Chem. B.
,
124
(2020) 6998
The optical properties of chromophores embedded in a water-solvated dimer of octa-acid that forms a molecular-shaped capsule are investigated. In particular, we address the anisotropic dielectric environment that appears to blue-shift excitation energies compared to the free aqueous chromophores. Recently we reported that using an effective scalar dielectric constant ε ≈ 3 appears to reproduce the measured spectra of the embedded coumarins, suggesting that the capsule provides a significant, albeit not perfect, screening of the aqueous dielectric environment. Here, we report absorption energies using a theoretical treatment that includes continuum solvation affected by an anisotropic dielectric function reflecting the high-dielectric environment outside of the capsule and the low-dielectric region within. We report time-dependent density functional theory calculations using a range-separated functional with the Poisson boundary conditions that model the anisotropic dielectric environment. Our calculations find that the anisotropic environment due to the water-solvated hydrophobic capsule is equivalent to a homogeneous effective dielectric constant of ≈3. The calculated values also appear to reproduce measured absorption of the embedded coumarin, where we study the effect of the hydrophobic capsule on the excited state.
.......................................................................................................................................
Screened Range-Separated Hybrid Functional with Polarizable Continuum Model Overcomes Challenges in Describing Triplet Excitations in the Condensed Phase Using TDDFT'
K. Begam, S. Bhandari, B. Maiti, B. D. Dunietz
J. Chem. Theo. Comput.
,
16
(2020) 3287
Long range-corrected (LRC) or range-separated hybrid (RSH) functionals where the long-range (LR) limit of electronic interactions is set to the exact exchange have been shown to correct the tendency of traditional density functional theory (DFT) to underestimate the frontier orbital gap. Consequently, the use of such functionals in calculating electronic excited states using linear response based time-dependent DFT (TDDFT) has been successful in correcting the tendency for underestimating the energies of charge transfer states by DFT-based calculations. More recently formulations of functionals that attenuate the LR limit to address condensed-phase effects to polarize the electronic density have been reported. In particular screened RSH (SRSH) combined with polarizable continuum model (PCM) was benchmarked successfully in reproducing the fundamental gap and charge transfer state energies of molecular systems in the condensed phase. Here we use SRSH-PCM to address triplet excited states, and show its success in obtaining correspondence of the low-lying triplet states to the singlet–triplet gap in a similar way that the fundamental orbital gap corresponds to electron removal and addition energies. Importantly, the accuracy of the SRSH-PCM in calculating triplet excitations stands on the polarization consistent framework in addressing the scalar dielectric constant and without affecting the optimal tuning by triplet energies. The prospect of even further improving the SRSH-PCM accuracy in calculating triplet states can be achieved by optimal tuning on the basis of the spin multiplicity gap.
.......................................................................................................................................
Discovery and characterization of an acridine radical photoreductant
Ian A. MacKenzie, Leifeng Wang, Nicholas P. R. Onuska, Olivia F. Williams, Khadiza Begam, Andrew M. Moran, Barry D. Dunietz & David A. Nicewicz
,
580
(2020) 76-80
Photoinduced electron transfer (PET) is a phenomenon whereby the absorption of light by a chemical species provides an energetic driving force for an electron-transfer reaction. This mechanism is relevant in many areas of chemistry, including the study of natural and artificial photosynthesis, photovoltaics and photosensitive materials. In recent years, research in the area of photoredox catalysis has enabled the use of PET for the catalytic generation of both neutral and charged organic free-radical species. These technologies have enabled previously inaccessible chemical transformations and have been widely used in both academic and industrial settings. Such reactions are often catalysed by visible-light-absorbing organic molecules or transition-metal complexes of ruthenium, iridium, chromium or copper. Although various closed-shell organic molecules have been shown to behave as competent electron-transfer catalysts in photoredox reactions, there are only limited reports of PET reactions involving neutral organic radicals as excited-state donors or acceptors. This is unsurprising because the lifetimes of doublet excited states of neutral organic radicals are typically several orders of magnitude shorter than the singlet lifetimes of known transition-metal photoredox catalysts. Here we document the discovery, characterization and reactivity of a neutral acridine radical with a maximum excited-state oxidation potential of −3.36 volts versus a saturated calomel electrode, which is similarly reducing to elemental lithium, making this radical one of the most potent chemical reductants reported12. Spectroscopic, computational and chemical studies indicate that the formation of a twisted intramolecular charge-transfer species enables the population of higher-energy doublet excited states, leading to the observed potent photoreducing behaviour. We demonstrate that this catalytically generated PET catalyst facilitates several chemical reactions that typically require alkali metal reductants and can be used in other organic transformations that require dissolving metal reductants.
.......................................................................................................................................
Photoinduced Charge Transfer Dynamics in the Carotenoid–Porphyrin–C60 Triad via the Linearized Semiclassical Nonequilibrium Fermi’s Golden Rule
Zhubin Hu, Zhengqing Tong, Margaret S. Cheung, Barry D. Dunietz, Eitan Geva, and Xiang Sun
J. Phys. Chem. B.
,
124
(2020) 9579-91
The nonequilibrium Fermi’s golden rule (NE-FGR) describes the time-dependent rate coefficient for electronic transitions when the nuclear degrees of freedom start out in a nonequilibrium state. In this paper, the linearized semiclassical (LSC) approximation of the NE-FGR is used to calculate the photoinduced charge transfer (CT) rates in the carotenoid–porphyrin–C60 molecular triad dissolved in explicit tetrahydrofuran. The initial nonequilibrium state corresponds to impulsive photoexcitation from the equilibrated ground state to the ππ* state, and the porphyrin-to-C60 and carotenoid-to-C60 CT rates are calculated. Our results show that accounting for the nonequilibrium nature of the initial state significantly enhances the transition rate of the porphyrin-to-C60 CT process. We also derive the instantaneous Marcus theory (IMT) from LSC NE-FGR, which casts the CT rate coefficients in terms of a Marcus-like expression, with explicitly time-dependent reorganization energy and reaction free energy. IMT is found to reproduce the CT rates in the system under consideration remarkably well.
.......................................................................................................................................
On the Interplay between Electronic Structure and Polarizable Force Fields When Calculating Solution-Phase Charge-Transfer Rates
Jaebeom Han, Pengzhi Zhang, Huseyin Aksu, Buddhadev Maiti, Xiang Sun*, Eitan Geva*, Barry D. Dunietz*, and Margaret S. Cheung*
J. Chem. Theory Comput.
,
16
(2020) 6481-6490
We present a comprehensive analysis of the interplay between the choice of an electronic structure method and the effect of using polarizable force fields vs. nonpolarizable force fields when calculating solution-phase charge-transfer (CT) rates. The analysis is based on an integrative approach that combines inputs from electronic structure calculations and molecular dynamics simulations and is performed in the context of the carotenoid–porphyrin–C60 molecular triad dissolved in an explicit tetrahydrofuran (THF) liquid solvent. Marcus theory rate constants are calculated for the multiple CT processes that occur in this system based on either polarizable or nonpolarizable force fields, parameterized using density functional theory (DFT) with either the B3LYP or the Baer–Neuhauser–Livshits (BNL) density functionals. We find that the effect of switching from nonpolarizable to polarizable force fields on the CT rates is strongly dependent on the choice of the density functional. More specifically, the rate constants obtained using polarizable and nonpolarizable force fields differ significantly when B3LYP is used, while much smaller changes are observed when BNL is used. It is shown that this behavior can be traced back to the tendency of B3LYP to overstabilize CT states, thereby pushing the underlying electronic transitions to the deep inverted region, where even small changes in the force fields can lead to significant changes in the CT rate constants. Our results demonstrate the importance of combining polarizable force fields with an electronic structure method that can accurately capture the energies of excited CT states when calculating charge-transfer rates
.......................................................................................................................................
Photoinduced Charge Transfer in Zn and Au-Ligated Symmetric and Asymmetric Bacteriochlorin Dyads: A Computational Study
H. Aksu, B. Maiti, M. Ptaszek, B. D. Dunietz,
J. Chem. Phys.
,
153
(2020) 134111
The excited-state properties and photoinduced charge-transfer (CT) kinetics in a series of symmetrical and asymmetrical Zn- and Au-ligated meso–meso-connected bacteriochlorin (BChl) complexes are studied computationally. BChl derivatives, which are excellent near-IR absorbing chromophores, are found to play a central role in bacterial photosynthetic reaction centers but are rarely used in artificial solar energy harvesting systems. The optical properties of chemically linked BChl complexes can be tuned by varying the linking group and involving different ligated metal ions. We investigate charge transfer in BChl dyads that are either directly linked or through a phenylene ring (1,4-phenylene) and which are ligating Zn or Au ions. The directly linked dyads with a nearly perpendicular arrangement of the BChl units bear markedly different properties than phenylene linked dyads. In addition, we find that the dielectric dependence of the intramolecular CT rate is very strong in neutral Zn-ligated dyads, whereas cationic Au-ligated dyads show negligible dielectric dependence of the CT rate. Rate constants of the photo induced CT process are calculated at the semiclassical Marcus level and are compared to fully quantum mechanical Fermi’s golden rule based values. The rates are calculated using a screened range separated hybrid functional that offers a consistent framework for addressing environment polarization. We study solvated systems in two solvents of a low and a high scalar dielectric constant.
.......................................................................................................................................
Molecular-Level Exploration of the Structure-Function Relations Underlying Interfacial Charge Transfer in the Subphthalocyanine/C60 Organic Photovoltaic System
J. Tinnin, S. Bhandari, P. Zhang, H. Aksu, B. Maiti, E. Geva, B. D. Dunietz, X. Sun, M. S. Cheung,
Phys. Rev. Applied
,
13
(2020) 054075
The arrangement of organic molecules at the donor-acceptor interface in an organic photovoltaic (OPV) cell can have a strong effect on the generation of charge carriers and thereby cell performance. In this paper, we report the molecular-level exploration of the ensemble of interfacial donor-acceptor pair geometries and the charge-transfer (CT) rates to which they give rise. Our approach combines molecular-dynamics simulations, electronic structure calculations, machine learning, and rate theory. This approach is applied to the boron subphthalocyanine chloride (donor) and C60 (acceptor) OPV system. We find that the interface is dominated by a previously unreported donor-acceptor pair edge geometry, which contributes significantly to device performance in a manner that depends on the initial conditions. Quantitative relations between the morphology and CT rates are established, which can be used to advance the design of more efficient OPV devices.
.......................................................................................................................................
Charge Transfer Rate Constants for the Carotenoid-Porphyrin-{C60} Molecular Triad Dissolved in Tetrahydrofuran: The Spin-Boson Model vs the Linearized Semiclassical Approximation
Z. Tong, X. Gao, M. S. Cheung, B. D. Dunietz, E. Geva, X. Sun
J. Chem. Phys.
,
153
(2020) 044105
Charge transfer rate constants were calculated for the carotenoid-porphyrin-C60 (CPC60) molecular triad dissolved in explicit tetrahydrofuran. The calculation was based on mapping the all-atom anharmonic Hamiltonian of this system onto the spin-boson Hamiltonian. The mapping was based on discretizing the spectral density from the time correlation function of the donor–acceptor potential energy gap, as obtained from all-atom molecular dynamics simulations. Different spin-boson Hamiltonians were constructed for each of the possible transitions between the three excited electronic states in two different triad conformations. The rate constants of three possible transitions were calculated via the quantum-mechanically exact Fermi’s golden rule (FGR), as well as a progression of more approximate expressions that lead to the classical Marcus expression. The advantage of the spin-boson approach is that once the mapping is established, the quantum-mechanically exact FGR and the hierarchy of approximations are known in closed form. The classical Marcus charge transfer rate constants obtained with the spin-boson Hamiltonians were found to reproduce those obtained from all-atom simulations with the linearized semiclassical approximation, thereby confirming the equivalence of the two approaches for this system. Within the spin-boson Hamiltonian, we also found that the quantum-mechanically exact FGR rate constants were significantly enhanced compared to the classical Marcus theory rate constants for two out of three transitions in one of the two conformations under consideration. The results confirm that mapping to the spin-boson model can yield accurate predictions for charge transfer rate constants in a system as complex as CPC60 dissolved in tetrahydrofuran.
.......................................................................................................................................
Efficient Charge Generation via Hole Transfer in Dilute Organic Donor–Fullerene Blends
Y. Song, A. Schubert, X. Liu, S. Bhandari, S. R. Forrest, B. D. Dunietz, E. Geva, J. P. Ogilvie,
J. Phys. Chem. Lett.
,
11
(2020) 2203-2210
Efficient organic photovoltaics (OPVs) require broadband charge photogeneration with near-unity quantum yield. This can only be achieved by exploiting all pathways that generate charge. Electron transfer from organic donors to acceptors has been well-studied and is considered the primary path to charge photogeneration in OPVs. In contrast, much less is known about the hole transfer pathway. Here we study charge photogeneration in an archetypal system comprising tetraphenyldibenzoperiflanthene:C70 blends using our recently developed multispectral two-dimensional electronic spectroscopy (M-2DES), supported by time-dependent density functional theory and fully quantum-mechanical Fermi’s golden rule rate calculations. Our approach identifies in real time two rapid charge transfer pathways that are confirmed through computational analysis. Surprisingly, we find that both electron and hole transfer occur with comparable rates and efficiencies, facilitated by donor–acceptor electronic interactions. Our results highlight the importance of the hole transfer pathway for optimizing the efficiency of OPV devices employing small-molecule heterojunctions.
.......................................................................................................................................
On the Role of the Special Pair in Photosystems as a Charge Transfer Rectifier
H. Aksu, A. Schubert, S. Bhandari, A. Yamada, E. Geva, B. D. Dunietz,
J. Phys. Chem. B.
,
124
(2020) 1987-1994
The special pair, a bacteriochlorophyll a (BChl) dimer found at the core of bacterial reaction centers, is known to play a key role in the functionality of photosystems as a precursor to the photosynthesis process. In this paper, we analyze the inherent affinity of the special pair to rectify the intrapair photo-induced charge transfer (CT). In particular, we show that the molecular environment affects the nuclear geometry, resulting in symmetry breaking between the two possible intrapair CT processes. To this end, we study the relationships of the intrapair CT and the molecular geometry with respect to the effective dielectric constant provided by the molecular environment. We identify the special pair structural feature that breaks the symmetry between the two molecules, leading to CT rectification. Excited state energies, oscillator strengths, and electronic coupling values are obtained via time-dependent density functional theory, employing a recently developed framework based on a screened range-separated hybrid functional within a polarizable continuum model (SRSH-PCM). We analyze the rectification capability of the special pair by calculating the CT rates using a first-principles-based Fermi’s golden rule approach.
.......................................................................................................................................
Enhancing Charge Mobilities in Self-Assembled N⋯I Halogen Bonded Organic Semiconductors: A Design Approach Based on Experimental and Computational Perspectives
B. Maiti, K. Wang, S. D. Bunge, R. J. Twieg, B. D. Dunietz
Org. Electron.
,
79
(2020) 105637
Charge transport in pyridine-ethynyliodophenyl supramolecules that involve intermolecular halogen bonding is studied by a combined experimental and computational approach. Selective fluorination of the molecules de-termines their crystallization pattern and is found to potentially increase the charge mobilities in the crystal. We report the synthesis of the molecules, full chemical characterization and resolved crystal structures. Computa-tional analysis of the charge transport is provided to understand at the molecular level the structure-function relationships determining the charge mobilities. Combination of selective fluorination, halogen bonding motif and increased π system is highlighted as bearing the potential to achieve both enhanced hole and electron mobilities.
.......................................................................................................................................
Explaining Spectral Asymmetries and Excitonic Characters of the Core Pigment Pairs in the Bacterial Reaction Center Using a Screened Range-Separated Hybrid Functional
Husein Aksu, Alexander Schubert, Eitan Geva, and Barry D. Dunietz
J. Phys. Chem. B.
,
10
(2019) 8143
Spectral peaks of the special pair (P) and adjacent pigments in the bacterial reaction center (BRC) are investigated computationally. We employ a novel framework based on a polarization-consistent treatment of the dielectric environment, combining the polarizable continuum model (PCM) with time-dependent screened range-separated hybrid (SRSH) density functional theory. Our calculations quantitatively reproduce recently measured spectral peak splits between P excitonic states and spectral asymmetries within the pairs of excited states of the adjacent bacteriochlorophyll a (BChl) and bacteriopheophytin a (BPhe) pigments. For the special pair, a splitting energy between the absorptive state and a blue-shifted semidark state of 0.07 eV is found in close agreement with the measured value. The spectral asymmetries within the pseudosymmetric pairs of BChl and BPhe pigments are interpreted to result from locally different effective dielectric environments in the A and the B branch, where the latter are exposed to a lesser polarizing environment. We base our analysis on X-ray-resolved structures and where the effect of neighboring pigments on the electronic structure is addressed through an effective dielectric environment. We show that the spectral trends are only reproduced using a polarization-consistent framework based on a screened range-separated hybrid functional, whereas B3LYP-PCM energies fail to provide the correct trends.
.......................................................................................................................................
Vibronic Structure of Photosynthetic Pigments Probed by Polarized two-dimensional Electronic Spectroscopy and ab initio Calculations
Yin Song, Alexander Schubert, Elizabeth Maret, Ryan K. Burdick, Barry D. Dunietz, Eitan Geva and Jennifer P. Ogilvie
Chem. Sci.
,
10
(2019) 8143
Bacteriochlorophyll a (Bchl a) and chlorophyll a (Chl a) play important roles as light absorbers in photosynthetic antennae and participate in the initial charge-separation steps in photosynthetic reaction centers. Despite decades of study, questions remain about the interplay of electronic and vibrational states within the Q-band and its effect on the photoexcited dynamics. Here we report results of polarized two-dimensional electronic spectroscopic measurements, performed on penta-coordinated Bchl a and Chl a and their interpretation based on state-of-the-art time-dependent density functional theory calculations and vibrational mode analysis for spectral shapes. We find that the Q-band of Bchl a is comprised of two independent bands, that are assigned following the Gouterman model to Qx and Qy states with orthogonal transition dipole moments. However, we measure the angle to be ∼75°, a finding that is confirmed by ab initio calculations. The internal conversion rate constant from Qx to Qy is found to be 11 ps−1. Unlike Bchl a, the Q-band of Chl a contains three distinct peaks with different polarizations. Ab initio calculations trace these features back to a spectral overlap between two electronic transitions and their vibrational replicas. The smaller energy gap and the mixing of vibronic states result in faster internal conversion rate constants of 38–50 ps−1. We analyze the spectra of penta-coordinated Bchl a and Chl a to highlight the interplay between low-lying vibronic states and their relationship to photoinduced relaxation. Our findings shed new light on the photoexcited dynamics in photosynthetic systems where these chromophores are primary pigments.
.......................................................................................................................................
Quantitative Accuracy in Calculating Charge Transfer State Energies in Solvated Molecular Complexes Using a Screened Range Separated Hybrid Functional within a Polarized Continuum Model.
Srijana Bhandari, and Barry D. Dunietz
J. Chem. Theory Comput.
,
15
(2019) 4305--11
A screened-range separated hybrid (SRSH) functional in combination with a polarized continuum model (PCM) was recently implemented within a consistent dielectric polarization treatment. The SRSH-PCM demonstrated excellent agreement of the calculated fundamental orbital gaps with measured energies in the condensed phase. Here we develop a linear response time-dependent DFT (TDDFT) approach to obtain solvated charge transfer state energies. We show that the calculated excited state energies of solvated electron-donor-acceptor complexes are in excellent agreement with measured benchmark values. Specifically we consider donor-acceptor complexes of functionalized anthracenes with tetracyanoethylene in methylene chloride. Our proposed SRSH-PCM calculated energies earn a mean absolute deviation (MAD) from the benchmark values as low as 0.04 eV with optimal tuning in PCM, whereas values based on simpler RSH-PCM, without proper treatment of dielectric screening, are associated with a 0.27 eV MAD.
.......................................................................................................................................
Combining the Mapping Hamiltonian Linearized Semiclassical Approach with the Generalized Quantum Master Equation to Simulate Electronically Nonadiabatic Molecular Dynamics
Ellen Mulvihill, Alexander Schubert, Xiang Sun, Barry D. Dunietz, and Eitan Geva
J. Chem. Phys.
,
151
(2019) 074103
The generalized quantum master equation (GQME) provides a powerful framework for simulating electronically nonadiabatic molecular dynamics. Within this framework, the effect of the nuclear degrees of freedom on the time evolution of the electronic reduced density matrix is fully captured by a memory kernel superoperator. In this paper, we consider two different procedures for calculating the memory kernel of the GQME from projection-free inputs obtained via the combination of the mapping Hamiltonian (MH) approach and the linearized semiclassical (LSC) approximation. The accuracy and feasibility of the two procedures are demonstrated on the spin-boson model. We find that although simulating the electronic dynamics by direct application of the two LSC-based procedures leads to qualitatively different results that become increasingly less accurate with increasing time, restricting their use to calculating the memory kernel leads to an accurate description of the electronic dynamics. Comparison with a previously proposed procedure for calculating the memory kernel via the Ehrenfest method reveals that MH/LSC methods produce memory kernels that are better behaved at long times and lead to more accurate electronic dynamics.
.......................................................................................................................................
Enhancing charge mobilities in selectively fluorinated oligophenyl organic semiconductors: a design approach based on experimental and computational perspectives
Buddhadev Maiti, Kunlun Wang, Srijana Bhandari, Scott D. Bunge, Robert J. Twieg and Barry D. Dunietz
J. Mater. Chem. C.
,
7
(2019) 3881
Fluorination can be used to tune optoelectronic properties at the molecular level. A series of oligophenyls with various difluorinations of the phenyl rings has been synthesized, crystalized, structurally resolved and computationally analyzed for charge mobility. We find that difluorination of the phenyl rings at para positions leads to oligophenyls that are stacked in symmetrical overlap with significantly enhanced hole mobility as well as the highest electron mobility of the molecules considered. Other difluorinations lead to relatively shifted molecular units in the π-stacked crystal and therefore to lower mobilities. The selectively fluorinated oligophenyls were synthesized using the Suzuki–Miyaura cross coupling reaction. The structures of the products were characterized by X-ray diffraction (XRD), 1H, 13C, 19F NMR spectroscopy and gas chromatography (GC)/mass spectroscopy (MS) measurements. Computational analysis of the materials based on state-of-the-art tools are used to predict their charge transport properties in the crystal phase. In short, we establish a molecular design approach based on fluorination of oligophenyls to achieve enhanced hole mobilities and relatively high electron mobilities.
.......................................................................................................................................
A modified generalized quantum master equation for simulating electronically
nonadiabatic dynamics
Ellen Mulvihill, Alexander Schubert, Xiang Sun, Barry D. Dunietz, and Eitan Geva
J. Chem. Phys.
,
150
(2019) 034101
We present a modified approach for simulating electronically nonadiabatic dynamics based on the Nakajima-Zwanzig generalized quantum master equation (GQME). The modified approach utilizes the fact that the Nakajima-Zwanzig formalism does not require casting the overall Hamiltonian in system-bath form, which is arguably neither natural nor convenient in the case of the Hamiltonian that governs nonadiabatic dynamics. Within the modified approach, the effect of the nuclear degrees of freedom on the time evolution of the electronic reduced density operator is fully captured by a memory kernel super-operator. A methodology for calculating the memory kernel from projection-free inputs is developed. Simulating the electronic dynamics via the modified approach, with a memory kernel obtained using exact or approximate methods, can be more cost effective and/or lead to more accurate results than direct application of those methods. The modified approach is compared to previously proposed GQME-based approaches, and its robustness and accuracy are demonstrated on a benchmark spin-boson model with a memory kernel which is calculated within the Ehrenfest method.
.......................................................................................................................................
Fundamental Gaps of Condensed-Phase Organic Semiconductors from Single-Molecule Calculations using Polarization-Consistent Optimally Tuned Screened Range-Separated Hybrid Functionals
Srijana Bhandari, Margaret S. Cheung, Eitan Geva, Leeor Kronik, and Barry D. Dunietz
J. Chem. Theory Comput.
,
14
(2018) 6287--94
Range-separated hybrid (RSH) functionals have been shown to overcome the tendency of traditional density functional theory to underestimate the fundamental orbital gap. More recently, the screened RSH (SRSH) approach has been developed as a means to extend these functionals to address the effect of the electrostatic environment on the fundamental gap. Here, we report a scheme that combines the SRSH formulation with the polarized continuum model (PCM) within a consistent framework for addressing long-range screened electrostatic interactions, which is further improved by optimal tuning (OT). The quantitative predictive power of the new OT-SRSH-PCM scheme is demonstrated by addressing fundamental gaps in thin films of organic semiconducting materials. This is especially impressive as the approach is based on single molecule calculations. We also discuss the advantages of this approach over alternative schemes combining PCM with RSH. In particular, we show that it avoids the well-documented tendency of standard OT to collapse the range separation parameter when performed within a dielectric continuum.
.......................................................................................................................................
Controlling the Emissive Activity in Heterocyclic Systems Bearing C═P Bonds
Sunandan Sarkar, John D. Protasiewicz, and Barry D. Dunietz
J. Phys. Chem. Lett.
,
9
(2018) 3567--72
The photophysical properties of a series of heteroatom substituted indoles are explored to identify chemical means to control their emissive activity. In particular, we consider impacts of changes in the conjugated backbone, where the C═N bonds of benzoxazoles are replaced by C═P bonds (benzoxaphospholes). The effects of extending the π-conjugation, incorporating various secondary heteroatoms (X–C═P), and enforcing planar rigidity are also examined. Our computational analysis explains the higher fluorescence efficiency observed with extended π-conjugation and highlights the importance of maintaining molecular planarity at both ground- and emissive-state geometries.
.......................................................................................................................................
Computational Study of Charge-Transfer Dynamics in the Carotenoid–Porphyrin–C60 Molecular Triad Solvated in Explicit Tetrahydrofuran and Its Spectroscopic Signature
Xiang Sun, Pengzhi Zhang, Yifan Lai†, Kyle L. Williams, Margaret S. Cheung, Barry D. Dunietz, and Eitan Geva
J. Phys. Chem. C.
,
122
(2018) 11288-99
We investigated the charge-transfer dynamics between distinctive excited states of a carotenoid–porphyrin–C60 molecular triad in tetrahydrofuran solvent. Our approach combines all-atom molecular dynamics simulations with an explicit solvent and electronic-state-specific force fields with a recently proposed hierarchy of approximations based on the linearized semiclassical method. The validity of the second-order cumulant approximation, which leads to a Marcus-like expression for the rate constants, was established by comparing the rate constants calculated with and without resorting to this approximation. We calculated the rate constants between the porphyrin-localized ππ* state, porphyrin-to-C60 charge-transfer state, and carotenoid-to-C60 charge-separated state for the bent and linearly extended conformations. In agreement with our earlier finding, the charge separation was found to occur via a two-step mechanism, where the second step is switched on by the bent-to-linear conformational change. By comparing the rate constants calculated for a flexible and a rigid triad molecule, while allowing the solvent molecules to fluctuate, we showed that the charge-transfer process is driven by the solvent, rather than by the triad’s intramolecular degrees of freedom. We further calculated the triad’s amide I stretch frequency distributions and found them to be highly sensitive to the electronic state, thereby demonstrating the possibility of monitoring charge-transfer dynamics in this system via UV–vis/IR pump–probe spectroscopy.
.......................................................................................................................................
Excitonic Interactions in Bacteriochlorin Homo-Dyads Enable Charge Transfer: A New Approach to the Artificial Photosynthetic Special Pair
Christopher McCleese, Zhanqian Yu, Nopondo N. Esemoto, Charles Kolodziej, Buddhadev Maiti, Srijana Bhandari, Barry D. Dunietz, Clemens Burda, and Marcin Ptaszek
J. Phys. Chem. B.
,
122
(2018) 4131
Excitonically coupled bacteriochlorin (BC) dimers constitute a primary electron donor (special pair) in bacterial photosynthesis and absorbing units in light-harvesting antenna. However, the exact nature of the excited state of these dyads is still not fully understood. Here, we report a detailed spectroscopic and computational investigation of a series of symmetrical bacteriochlorin dimers, where the bacteriochlorins are connected either directly or by a phenylene bridge of variable length. The excited state of these dyads is quenched in high-dielectric solvents, which we attribute to photoinduced charge transfer. The mixing of charge transfer with the excitonic state causes accelerated (within 41 ps) decay of the excited state for the directly linked dyad, which is reduced by orders of magnitude with each additional phenyl ring separating the bacteriochlorins. These results highlight the origins of the excited-state dynamics in symmetric BC dyads and provide a new model for studying the primary processes in photosynthesis and for the development of artificial, biomimetic systems for solar energy conversion.
.......................................................................................................................................
A Comparative Study of Different Methods for Calculating Electronic Transition Rates
Alexei A. Kananenka, Xiang Sun, Alexander Schubert, Barry D. Dunietz, and
Eitan Geva,
J. Chem. Phys.
,
148
(2018) 102304
We present a comprehensive comparison of the following mixed quantum-classical methods for calculating electronic transition rates: (1) nonequilibrium Fermi’s golden rule, (2) mixed quantum-classical Liouville method, (3) mean-field (Ehrenfest) mixed quantum-classical method, and (4) fewest switches surface-hopping method (in diabatic and adiabatic representations). The comparison is performed on the Garg-Onuchic-Ambegaokar benchmark charge-transfer model, over a broad range of temperatures and electronic coupling strengths, with different nonequilibrium initial states, in the normal and inverted regimes. Under weak to moderate electronic coupling, the nonequilibrium Fermi’s golden rule rates are found to be in good agreement with the rates obtained via the mixed quantum-classical Liouville method that coincides with the fully quantum-mechanically exact results for the model system under study. Our results suggest that the nonequilibrium Fermi’s golden rule can serve as an inexpensive yet accurate alternative to Ehrenfest and the fewest switches surface-hopping methods.
.......................................................................................................................................
What Is the Optoelectronic Effect of the Capsule on the Guest Molecule in Aqueous Host/Guest Complexes? A Combined Computational and Spectroscopic Perspective
Srijana Bhandari, Zilong Zheng, Buddhadev Maiti, Chi-Hung Chuang, Mintu Porel, Zhi-Qiang You, Vaidhyanathan Ramamurthy, Clemens Burda, John M. Herbert, and Barry D. Dunietz
J.Phys. Chem. C.
,
121
(2017) 15481–15488
Encapsulation of dye molecules is used as a means to achieve charge separation across different dielectric environments. We analyze the absorption and emission spectra of several coumarin molecules that are encapsulated within an octa-acid dimer forming a molecular capsule. The water-solvated capsule effect on the coumarin’s electronic structure and absorption spectra can be understood as due to an effective dielectric constant where the capsule partially shields electrostatically the dielectric solvent environment. Blue-shifted emission spectra are explained as resulting from a partial intermolecular charge transfer where the capsule is the acceptor, and which reduces the coumarin relaxation in the excited state.
.......................................................................................................................................
Enhancing charge mobilities in organic semiconductors by selective fluorination: a design approach based on a quantum mechanical perspective
Buddhadev Maiti, Alexander Schubert, Sunandan Sarkar, Srijana Bhandari, Kunlun Wang, Zhe Li, Eitan Geva, Robert J. Twieg, and Barry D. Dunietz
Chem. Sci.
,
8
(2017) 6947-6953
Selective fluorination of organic semiconducting molecules is proposed as a means to achieving enhanced hole mobility. Naphthalene is examined here as a root molecular system with fluorination performed at various sites. Our quantum chemical calculations show that selective fluorination can enhance attractive intermolecular interactions while reducing charge trapping. Those observations suggest a design principle whereby fluorination is utilized for achieving high charge mobilities in the crystalline form. The utility of this design principle is demonstrated through an application to perylene, which is an important building block of organic semiconducting materials. We also show that a quantum mechanical perspective of nuclear degrees of freedom is crucial for a reliable description of charge transport.
.......................................................................................................................................
Conductance of Junctions with Acetyl-Functionalized Thiols: A First-Principles-Based Analysis
Atsushi Yamada, Qingguo Feng, Qi Zhou, Austin Hoskins, Kim M. Lewis, and Barry D. Dunietz
J. Phys. Chem. C.
,
121
(2017) 10298-10304
Thiol-based contacts are widely used in fabrication of molecular junctions but are associated with several drawbacks due to their chemical reactivity. In particular, their tendency to dimerize by forming sulfur–sulfur bonds is viewed as a barrier for large scale bridge fabrication. Instead, the use of functionalized sulfur end groups in the fabrication of the junctions is promoted. We analyze the effects of thiol functionalization by acetyl on the transport properties of porphyrin based bridges. In scanning tunneling microscopy (STM) experiments, where the conductance is measured as the tip is retracted, we observe molecular conductance steps due to junctions with acetyl protected thiols that are significantly lower than observed for junctions with deprotected thiols (by a factor of ~5). Using a first-principles based computational approach, we explain the lower conductance of junctions with acetyl functionalized thiol and associate it to chemical changes of the sulfur, where essentially a tunneling barrier in the transport pathway is enforced. The acetyl protected thiol lowers the transmission mainly through its direct effect on the electronic structure. We show that the geometrical relaxation upon acetylation where the Au–S bond is elongated plays a smaller role in determining the conductance trends. Interestingly, we find that in a hypothetical deprotected case with an imposed longer Au–S bond distance to that of the protected thiol bond length the transmission is slightly increased.
.......................................................................................................................................
Modification of Molecular Conductance by in Situ Deprotection of Thiol-Based Porphyrin
Qi Zhou, Atsushi Yamada, Qingguo Feng, Austin Hoskins, Barry D. Dunietz, and Kim M. Lewis
ACS Appl. Mater. Interfaces
,
9
(2017) 15901-06
Acetylthio-protected free base porphyrins are used to form scanning tunneling microscope-molecular break junctions. The porphyrin molecules are deprotected in situ, before the self-assembly. Two types of molecular junctions are formed in the junctions: Au-S-Por-SAc-Au and Au-S-Por-S-Au. Lower conductance values and higher conductance values are observed. Computational modeling attributes the lower conductance to the Au-S-Por-SAc-Au junctions and the higher conductance to the Au-S-Por-S-Au junctions. First-principles calculation suggests that the reduced conductance in the protected porphyrin originates from the presence of the acetyl end groups (-COCH3), rather than from the elongation of the sulfur–gold (S-Au) bonds at the tip-molecule interface.
.......................................................................................................................................
Phosphorescence in Bromobenzaldehyde Can Be Enhanced through Intramolecular Heavy Atom Effect
Sunandan Sarkar, Heidi P. Hendrickson, Dongwook Lee, Francis DeVine, Jaehun Jung, Eitan Geva, Jinsang Kim, and Barry D. Dunietz
J. Phys. Chem. C.
,
121
(2017) 3771-3777
A synthetic route to achieve high phosphorescence quantum yield in a purely organic material was achieved by doping a crystal containing heavy bromine atoms with a molecule that contains a triplet producing aromatic carbonyl group. The enhanced phosphorescence originated from intermolecular nonbonding interactions between the bromine and the carbonyl oxygen. In this study we employ a computational approach to design molecules containing both structural motifs, which exhibit enhanced phosphorescence through intramolecular nonbonding interactions between bromine and carbonyl groups.
.......................................................................................................................................
Achieving Predictive Description of Molecular Conductance by using a Range-Separated Hybrid Functional
Atsushi Yamada, Qingguo Feng, Austin Hoskins,Kevin D. Fenk, and Barry D. Dunietz,
Nano Lett.
,
16
(2016) 6092-6098
The conductance of molecular bridges tends to be overestimated by computational studies in comparison to measured
values. While this well-established trend may be related to
difficulties for achieving robust bridges, the employed
computational scheme can also contribute to this tendency. In
par- ticular caveats of the traditional functionals employed
in first- principles based calculations can lead to
discrepancies reflected in exaggerated conductance. Here, we
show that by employing a range-separated hybrid functional
the calculated values are within the same order as the
measured conductance for all four considered cases. On the
other hand, with B3LYP, which is a widely used functional,
the calculated values greatly overestimate the conductance
(by about one to two orders of magnitude). The improved
description of the conductance with a RSH functional builds
on achieving a physically meaningful treatment of the quasi
particles associated with the frontier orbitals.
.......................................................................................................................................
Photoinduced Homolytic Bond Cleavage of the Central Si-C Bond in Porphyrinic Macrocycles Is a Charge Polarization Driven Process
Maiti, Buddhadev; Manna, Arun K; McCleese, Christopher; Doane, Tennyson L ; Chakrapani, Sudha; Burda, Clemens; Dunietz, Barry D.
J. Phys. Chem. A.
,
120
(2016) 7634-7640
Photoinduced cleavage of the bond between the central Si atom in porphyrinic macrocycles and the neighboring carbon atom of an axial alkyl ligand is investigated by both experimental and computational tools. Photolysis and electron paramagnetic res- onance measurements indicate that the Si-C bond cleavage of Si-phthalocyanine occurs through a homolytic process. The ho- molytic process follows a low lying electronic excitation of about 1.8 eV that destabilizes the carbide bond of similar bond dissoci- ation energy. Using electronic structure calculations we provide insight into the nature of the excited state and the resulting pho- tocleavage mechanism. We explain this process by finding that the electronic excited state is of a charge transfer character from the axial ligand towards the macrocycle in the reverse direction of the ground state polarization. We find that the homolytic process yielding the radical intermediate is energetically the most stable mechanistic route. Furthermore, we demonstrate using our com- putational approach that changing the phthalocyanine to smaller ring system enhances the homolytic photocleavage of the Si-C bond by reducing the energetic barrier in the relevant excited states.
.......................................................................................................................................
Deleterious Effects of Exact Exchange Functionals on Predictions of Molecular Conductance
Qingguo Feng, Atsushi Yamada, Roi Baer, and Barry D. Dunietz
J. Chem. Theory Comput.
,
12
(2016) 3431-3435
Kohn–Sham (KS) density functional theory (DFT) describes well the atomistic structure of molecular junctions and their coupling to the semi-infinite metallic electrodes but severely overestimates conductance due to the spuriously large density of charge-carrier states of the KS system. Previous works show that inclusion of appropriate amounts of nonlocal exchange in the functional can fix the problem and provide realistic conductance estimates. Here however we discover that nonlocal exchange can also lead to deleterious effects which artificially overestimate transmittance even beyond the KS-DFT prediction. The effect is a result of exchange coupling between nonoverlapping states of diradical character. We prescribe a practical recipe for eliminating such artifacts.
.......................................................................................................................................
The Effect of Interfacial Geometry on Charge-Transfer States in the Phthalocyanine/Fullerene Organic Photovoltaic System
Myeong H. Lee , Eitan Geva , and Barry D. Dunietz
J. Phys. Chem. A.
,
12
(2016) 2970-2975
The dependence of charge-transfer states on interfacial geometry at
the phthalocyanine/fullerene organic photovoltaic system is
investigated. The effect of deviations from the equilibrium geometry
of the donor-donor-acceptor trimer on the energies of and electronic
coupling between different types of interfacial electronic excited
states is calculated from first-principles. Deviations from the
equilibrium geometry are found to destabilize the donor-to-donor
charge transfer states and to weaken their coupling to the
photoexcited donor-localized states, thereby reducing their ability to
serve as charge traps. At the same time, we find that the energies of
donor-to-acceptor charge transfer states and their coupling to the
donor-localized photoexcited states are either less sensitive to the
interfacial geometry or become more favorable due to modifications
relative to the equilibrium geometry, thereby enhancing their ability
to serve as gateway states for charge separation. Through these
findings, we eludicate how interfacial geometry modifications can play
a key role in achieving charge separation in this widely studied
organic photovoltaic system.
.......................................................................................................................................
Calculating High Energy Charge Transfer States Using Optimally Tuned Range-Separated Hybrid Functionals
Arun K. Manna, Myeong H. Lee , Kayla L. McMahon , and Barry D. Dunietz
J. Chem. Theo. Comput.
,
11
(2015) 1110-1117
Recently developed optimally tuned range-separated hybrid (OT-RHS) functionals within time-dependent density functional theory have been shown to address existing limitations in calculating charge transfer excited state energies. The RSH success in improving the calculation of CT states stems from enforcing the correspondence of the frontier molecular orbitals (FMOs) to physical properties, where the highest occupied MO energy relates to the ionization potential and the lowest unoccupied MO energy relates to the electron affinity. However, in this work, we show that a less accurate description of CT states that involves non-FMOs is afforded by the RSH approach. In order to achieve a high quality description of such higher energy CT states, the parameter tuning procedure, which lies at the foundation of the RSH approach, needs to be generalized to consider the CT process. We demonstrate the need for improved description of such CT states in donor–acceptor systems, where the optimal tuning parameter is accounting for the state itself.
.......................................................................................................................................
Unraveling the Mechanism of Photoinduced Charge Transfer in Carotenoid–Porphyrin–C60 Molecular Triad
Arun K. Manna, B. Balamurugan, Margaret S. Cheung, and Barry D. Dunietz
J. Phys. Chem. Lett.
,
6
(2015) 1231-1237
Photoinduced charge transfer (CT) plays a central role in biologically significant systems and in applications that harvest solar energy. We investigate the relationship of CT kinetics and conformation in a molecular triad. The triad, consisting of carotenoid, porphyrin, and fullerene is structurally flexible and able to acquire significantly varied conformations under ambient conditions. With an integrated approach of quantum calculations and molecular dynamics simulations, we compute the rate of CT at two distinctive conformations. The linearly extended conformation, in which the donor (carotenoid) and the acceptor (fullerene) are separated by nearly 50 Å, enables charge separation through a sequential CT process. A representative bent conformation that is entropically dominant, however, attenuates the CT, although the donor and the acceptor are spatially closer. Our computed rate of CT at the linear conformation is in good agreement with measured values. Our work provides unique fundamental understanding of the photoinduced CT process in the molecular triad.
.......................................................................................................................................
Ultrafast Charge-Transfer Dynamics at the Boron Subphthalocyanine Chloride/C60 Heterojunction: Comparison between Experiment and Theory
Daniel E. Wilcox, Myeong H. Lee, Matthew E. Sykes, Andrew Niedringhaus, Eitan Geva, Barry D. Dunietz, Max Shtein, and Jennifer P. Ogilvie
J. Phys. Chem. Lett.
,
6
(2015) 569-574
Photoinduced charge-transfer (CT) processes play a key role in many systems, particularly those relevant to organic photovoltaics and photosynthesis. Advancing the understanding of CT processes calls for comparing their rates measured via state-of-the-art time-resolved interface-specific spectroscopic techniques with theoretical predictions based on first-principles molecular models. We measure charge-transfer rates across a boron subphthalocyanine chloride (SubPc)/C60 heterojunction, commonly used in organic photovoltaics, via heterodyne-detected time-resolved second-harmonic generation. We compare these results to theoretical predictions based on a Fermi’s golden rule approach, with input parameters obtained using first-principles calculations for two different equilibrium geometries of a molecular donor–acceptor in a dielectric continuum model. The calculated rates (∼2 ps–1) overestimate the measured rates (∼0.1 ps–1), which is consistent with the expectation that the calculated rates represent an upper bound over the experimental ones. The comparison provides valuable understanding of how the structure of the electron donor–acceptor interface affects the CT kinetics in organic photovoltaic systems.
.......................................................................................................................................
Advances in molecular quantum chemistry contained in the Q-Chem 4 program package
Yihan Shao
et. al.
Mol. Phys.
,
113
(2015) 184-215
A summary of the technical advances that are incorporated in the fourth major release of the Q-CHEM quantum chemistry program is provided, covering approximately the last seven years. These include developments in density functional theory methods and algorithms, nuclear magnetic resonance (NMR) property evaluation, coupled cluster and perturbation theories, methods for electronically excited and open-shell species, tools for treating extended environments, algorithms for walking on potential surfaces, analysis tools, energy and electron transfer modelling, parallel computing capabilities, and graphical user interfaces. In addition, a selection of example case studies that illustrate these capabilities is given. These include extensive benchmarks of the comparative accuracy of modern density functionals for bonded and non-bonded interactions, tests of attenuated second order Møller–Plesset (MP2) methods for intermolecular interactions, a variety of parallel performance benchmarks, and tests of the accuracy of implicit solvation models. Some specific chemical examples include calculations on the strongly correlated Cr2 dimer, exploring zeolite-catalysed ethane dehydrogenation, energy decomposition analysis of a charged ter-molecular complex arising from glycerol photoionisation, and natural transition orbitals for a Frenkel exciton state in a nine-unit model of a self-assembling nanotube.
.......................................................................................................................................
Molecular structure, spectroscopy and photo induced kinetics in
tri-nuclear cyanide bridged complex in solution: A first principle
perspective
Zilong Zheng, Arun K. Manna, Heidi Phillips, Morgan Hammer, Chenchen Song, Eitan Geva and Barry D. Dunietz
J. Amer. Chem. Soc. (comm.)
,
136
(2014) 16954–16957
We investigate the molecular structure of the solvated complex, [(NC)6Fe–Pt(NH3)4–Fe(CN)6]4–, and related dinuclear and mononuclear model complexes using first-principles calculations. Mixed nuclear complexes in both solution and crystal phases were widely studied as models for charge transfer (CT) reactions using advanced spectroscopical and electrochemical tools. In contrast to earlier interpretations, we find that the most stable gas phase and solvated geometries are substantially different from the crystal phase geometry, mainly due to variance in the underlying oxidation numbers of the metal centers. Specifically, in the crystal phase a Pt(IV) metal center resulting from Fe ← Pt backward electron transfers is stabilized by an octahedral ligand field, whereas in the solution phase a Pt(II) metal complex that prefers a square planar ligand field forms a CT salt by bridging to the iron complexes through long-range electrostatic interactions. The different geometry is shown to be consistent with spectroscopical data and measured CT rates of the solvated complex. Interestingly, we find that the experimentally indicated photoinduced process in the solvated complex is of backward CT (Fe ← Pt).
.......................................................................................................................................
Donor-to-donor vs. donor-to-acceptor interfacial charge transfer states
in the phthalocyanine-fullerene organic photovoltaic system
Myeong Lee, Eitan Geva, and Barry D. Dunietz
J. Phys. Chem. Lett.
,
5
(2014) 3810-16
Charge transfer (CT) states formed at the donor/acceptor heterointerface are key for photocurrent generation in organic photovoltaics (OPV). Our calculations show that interfacial donor-to-donor CT states in the phthalocyanine–fullerene OPV system may be more stable than donor-to-acceptor CT states and that they may rapidly recombine, thereby constituting a potentially critical and thus far overlooked loss mechanism. Our results provide new insight into processes that may compete with charge separation, and suggest that the efficiency for charge separation may be improved by destabilizing donor-to-donor CT states or decoupling them from other states.
.......................................................................................................................................
Charge-transfer rate constants in Zinc-Porphyrin-Porphyrin-derived dyads: A Fermi golden rule first-principles-based study
Arun K. Manna, and Barry D. Dunietz
J. Chem. Phys. (comm.)
,
141
(2014) 121102
We investigate photoinduced charge transfer (CT) processes within dyads consisting of porphyrin derivatives in which one ring ligates a Zn metal center and where the rings vary by their degree of conjugation. Using a first-principles approach, we show that molecular-scale means can tune CT rates through stabilization affected by the polar environment. Such means of CT tuning are important for achieving high efficiency optoelectronic applications using organic semiconducting materials. Our fully quantum mechanical scheme is necessary for reliably modeling the CT process across different regimes, in contrast to the pervading semi-classical Marcus picture that grossly underestimates transfer in the far-inverted regime.
.......................................................................................................................................
Calculation from First Principles of Golden-Rule Rate Constants for Photo-Induced Subphthalocyanine/Fullerene Interfacial Charge Transfer and Recombination in Organic Photovoltaic Cells
Myeong Lee, Eitan Geva, and Barry D. Dunietz
J. Phys. Chem. C.
,
118
(2014) 9780-9789
The rates of interfacial charge transfer and recombination between the donor and acceptor layers play a key role in determining the performance of organic photovoltaic cells. The time scale and mechanism of these processes are expected to be impacted by the structure of the interface. In this paper we model the kinetics of those processes within the framework of a subphthalocyanine/fullerene donor/acceptor dimer model. Two likely configurations (on-top and hollow) in which the interfacial charge transfer and recombination may occur are studied. The corresponding rate constants are calculated within the fully quantum-mechanical framework of Fermi's golden rule. All the input parameters (excitation energies, electronic coupling coefficients, normal-mode frequencies and coordinates, and Huang–Rhys factors) are obtained from density functional theory calculations with density functionals designed to yield accurate results in the case of noncovalently bound systems and charge transfer states. Multiple pi-pi** and charge-transfer excited states are identified and assigned. The kinetics of photoinduced charge transfer is obtained by solving a master equation using Fermi’s golden rule rate constants for the electronic transitions between the various excited states. Our results suggest that the hollow configuration may be superior to the on-top configuration and that maximizing its prevalence may improve the performance of subphthalocyanine/fullerene-based photovoltaic cells.
.......................................................................................................................................
Orbital Gap Predictions for Rational Design of Organic Photovoltaic Materials
Heidi Phillips, Zilong Zheng, Eitan Geva, and Barry D. Dunietz
Org. Elect.
,
15
(2014) 1509-1520
Ionization potentials (IP) and electron affinities (EA) of organic molecules with applications in photovoltaic devices are calculated using modern density functional theory (DFT). Calculated frontier orbital energies are compared to experimentally determined IPs and EAs at gas phase and thin film environments. Gas phase frontier orbital energies calculated with widely-used DFT functionals accidentally coincide with thin film measurements, reproducing condensed phase results for the wrong reasons. Recently developed range separated hybrid (RSH) functionals, on the other hand, provide gas phase frontier orbital energies that correspond properly to measured IPs and EAs. We also employ a polarizable continuum model to address the effects of the electrostatic environment in the solid state. We find that the environmentally-corrected RSH orbital energies compare well with thin film experimental measurements.
.......................................................................................................................................
Active control of thermal transport in molecular spin valves
Myeong H. Lee and Barry D. Dunietz
Phys. Rev. B.
,
88
(2013) 045421
Active control of heat flow is challenging. We demonstrate that molecular spin valves offer a unique opportunity for achieving this goal. Our first-principles calculations of the transport of electrons and phonons in nickel-benzenedithiol-nickel junctions show that when the magnetization direction of the electrodes is changed from parallel to antiparallel the junctions become thermally insulating. Our findings, therefore, suggest a novel avenue for actively controlling thermal transport via the spin degree of freedom.
.......................................................................................................................................
End-Group Influence on Frontier Molecular Orbital Reorganization and Thermoelectric Properties of Molecular Junctions
Janakiraman Balachandran, Pramod Reddy, Barry D. Dunietz, and Vikram Gavini
J. Phys. Chem. Lett.
,
4
(2013) 3825–3833
The reorganization of frontier molecular orbitals (FMOs) upon formation of molecular junctions plays an important role in the resulting transport and thermoelectric properties. By considering a wide range of Au–molecule–Au junctions—created from phenyl molecules with different lengths and end-groups—we demonstrate that the extent of reorganization of FMOs is based on: (i) stabilization due to the physical presence of electrodes, and (ii) change in the electron–electron (e–e) interactions due to charge transfer. In molecular systems with charge (electron) transfer into the molecule, the opposing effects of stabilization and increased e–e interactions result in a small overall reorganization of FMOs. In contrast, for molecular systems with charge transfer out of the molecule, the complementary effects of stabilization and reduced e–e interactions result in a large overall reorganization of FMOs to lower energies. Further, we present a computationally efficient approach to quantitatively compute the extent of reorganization, which has potential for high-throughput analysis of molecular junctions.
.......................................................................................................................................
Calculation from First Principles of Intramolecular Golden-Rule Rate Constants for Photo-Induced Electron Transfer in Molecular Donor–Acceptor Systems
Myeong H. Lee, Barry D. Dunietz, and Eitan Geva
J. Phys. Chem. C.
,
117
(2013) 23391
The feasibility of calculating photoinduced intramolecular electron transfer rate constants in realistic molecular donor–acceptor systems via Fermi’s golden rule, using inputs obtained from state-of-the-art electronic structure techniques, is demonstrated and tested. To this end, calculations of photoinduced electron transfer rate constants were performed on two benchmark systems: (1) phenylacetylene-bridged carbazole-naphthalimide (meta and para) and (2) C60-(N,N-dimethylaniline). Intramolecular input parameters such as normal-mode frequencies, Huang–Rhys factors, and electronic coupling coefficients were obtained via ground state, time-dependent, and constrained density functional theory. Good agreement between the intramolecular Fermi’s golden rule rate constants and the experimental rate constants is found for both systems without accounting for the solvent reorganization. The relative roles of intramolecular vs intermolecular modes at promoting electron transfer and the validity of several limits of Fermi’s golden rule for describing intramolecular electron transfer are discussed.
.......................................................................................................................................
Solvated charge transfer states of functionalized anthracene and tetracyanoethylene dimers: A computational study based on a range separated hybrid functional and charge constrained self-consistent field with switching Gaussian polarized continuum models.
Shaohui Zheng, Eitan Geva, Barry D Dunietz
J. Chem. Theo. Comp.
,
9
(2013) 1125-1131
We benchmark several protocols for evaluating the energies of excited charge transfer (CT) states of organic molecules dissolved in polar liquids. The protocols combine time-dependent density functional theory using range-separated hybrid functionals, constrained density functional theory, dispersion corrected functional, and dielectric continuum model for representing the solvent. We compare the different protocols against well-established experimental measured charge transfer state energies in solvated dimers of functionalized anthracene and tetracyanoethylene. We find that using the range-separated hybrid functional for the charge-transfer state energies and the combination of constrained density functional theory with recently improved polarizable continuum model (PCM) provide good agreement with the experimental values of the solvated CT states. We also find that using dispersion corrected solvated geometries for the weakly coupled donor-acceptor dimers considered here leads to improved agreement with experimental values.
.......................................................................................................................................
Length Dependence of Frontier Orbital Alignment in Aromatic
Molecular Junctions
Aaron Tan, Janakiraman Balachandran, Barry Dunietz, Sung-Yeon Jang,
Vikram Gavini, Pramod Reddy
Appl. Phys. Lett.
,
101,
(2012) 243107
We report on experiments
and computations performed on a series of aromatic monothiol
molecular junctions (AMMJs) to ascertain both the identity of
the frontier molecular orbitals (FMOs) and their approximate
energetic separation from the chemical potential. Joint
transition voltage spectroscopy and thermoelectric
measurements unambiguously show that the FMOs in all the
studied junctions are the highest occupied molecular orbitals
and that the energetic separation decreases with increasing
molecular length. Our computational studies of energetic
separations and Seebeck coefficients of these AMMJs are in
agreement with the experimentally obtained values and
elucidate the electronic structure origins of the observed
length dependence.
.......................................................................................................................................
On the suppression and significance of ghost transmission in electron transport modeling of single molecule junctions.
Partha Pal, and Barry D. Dunietz
J. Chem. Phys.
,
137
(2012) 194104
The difficulty in achieving experimental control over a metal-molecule-metal junction formation hinders the understanding of the relationship between the contact geometry and electron transmittance. Computational studies on the other hand have the potential to resolve structural effects on the transport in molecular junctions. In a recent computational effort substantial transport was indicated even in the case where all the junction atoms were removed, while their corresponding atomic basis functions were included in the basis set (i.e., ghost atoms). In this report we explain the origin of the artifact termed as "ghost transmission." We provide a systematic analysis of the factors that enhance or suppress the artifact. We find that symmetric electronic densities at the two metal-molecule interfaces can lead to an amplification of the artificial transmission. In addition, interaction between an unpaired electron of the left electrode with one in the right electrode results with a substantial increase in "ghost transmission." Finally we find that a self-consistent single particle Green's function formalism that solves the junction electronic structure self-consistently with respect to the electrodes self-energies, reduces the artifact substantially.
.......................................................................................................................................
Calculating Off-Site Excitations in Symmetric Donor–Acceptor Systems via Time-Dependent Density Functional Theory with Range-Separated Density Functionals
Heidi Phillips, Eitan Geva, and Barry D. Dunietz
J. Chem. Theo. Comp.
,
8
(2012) 2661-2668
Time-dependent density functional theory with range-separated
hybrid functionals is used to calculate off-site excitations,
involving transitions between spatially-separated orbitals, in
weakly-coupled systems. Although such off-site excitations involve
charge transfer, orbital degeneracy in symmetrical systems results in
linear combinations of off-site excitations with equal weights, and
therefore zero net charge transfer character. Like other types of
off-site excitations, such "hidden" off-site excitations are not
accurately captured by conventional density functionals. We show that
the recently introduced Baer-Neuhauser-Livshitz range-separated hybrid
functional accurately characterizes such hidden off-site excitation
energies via applications to the ethene dimer and dye-functionalized
silsesquioxane.
.......................................................................................................................................
End group induced charge transfer in molecular junctions: Effect on thermopower
Janakiraman Balachandran, Pramod Reddy, Barry D. Dunietz and Vikram Gavini
J. Phys. Chem. Lett.
,
3
(2012) 1962–1967
We analyze triphenyl molecules coupled to gold electrodes through five different end groups to understand the effect of end groups on the thermoelectric properties of molecular junctions. Our investigation suggests that end-group-mediated charge transfer between the molecule and electrodes plays an important role in the resulting thermoelectric properties. We find that the direction of charge transfer, which is governed by the electronegativity of the end-group functionalized molecule, is strongly correlated to the degree of reorganization of frontier molecular orbitals (HOMO–LUMO). In particular, isocyanide, nitrile, and amine end-group molecular junctions, with charge (electron) transfer out of the molecule, exhibit a strong overall downward shift in the energies of frontier molecular orbitals, whereas thiol and hydroxyl end-group molecular junctions, with charge transfer into the molecule, exhibit a smaller overall downward shift. Finally, our study shows that the sign of the thermopower of molecular junctions is closely related to the HOMO–LUMO energies and electronegativity of isolated molecules.
.......................................................................................................................................
Ab-initio study of the emissive charge-transfer states of solvated
chromophore-functionalized silsesquioxanes
Shaohui Zheng, Heidi Phillips, Eitan Geva, and Barry D. Dunietz,
J. Amer. Chem. Soc. (comm.)
,
134
(2012) 6944-47
Recent experimental advances in the ability to tune the optical properties of silsesquioxanes by functionalizing them with photoactive ligands have made these compounds attractive candidates for building blocks of photovoltaic materials. We employ state-of-the-art ab initio methodologies to determine the nature of the excited charge-transfer (CT) states that give rise to a large red-shift between absorption and emission in these molecules, in comparison to the corresponding red-shift in the individual ligand. The calculations are based on time-dependent density functional theory and employ the recently developed Baer–Neuhauser–Livshits range-separated hybrid (RSH) functional. Solvent effects are accounted for via a combination of charge-constrained density functional theory and the polarizable continuum model. We find that the experimentally observed red-shift is consistent with identifying the emissive state as a ligand-to-ligand, rather than a ligand-to-silsesquioxane, CT state. We also find that the enhanced red-shift cannot be explained without accounting for solvation effects, and we demonstrate the importance of using a RSH functional to obtain reliable predictions regarding the emissive state.
.......................................................................................................................................
Ab-initio calculation of the electronic absorption of functionalized octahedral silsesquioxanes via time-dependent density functional theory with range separated hybrid functionals
Heidi Phillips, Shaohui Zheng, Alexander Hyla, Richard Laine, Theodore Goodson III, Eitan Geva, and Barry D. Dunietz,
J. Phys. Chem. A.
,
116
(2012) 1137
Recent advances in the ability to functionalize octahedral silsesquioxanes with different photoactive ligands, and thereby tune their optical properties, suggest that these molecules may serve as potential building blocks of light-harvesting, photovoltaic, and photonic devices. In this paper we report extensive ab initio calculations of the excitation energies underlying the absorption spectra of these systems. The calculations are based on density functional theory for the ground electronic state and time-dependent density functional theory for the excited electronic states. The ability of the commonly used B3LYP functional to reproduce the experimentally observed absorption excitation energies is compared to that of recently developed range-separated hybrid functionals. The importance of pairing the range-separated hybrid functionals with basis sets that include diffuse and polarization basis functions is demonstrated in the case of vinyl-functionalized silsesquioxanes. Absorptive excitation energies are then calculated and compared with experiment for octahedral silsesquioxanes functionalized with larger ligands. The tunability of optical properties is demonstrated by considering the effect on the excitation energies of functionalizing the ligands with electron-donating or -withdrawing groups.
.......................................................................................................................................
Effect of Length and Contact Chemistry on the Electronic Structure and Thermoelectric Properties of Molecular Junctions
Aaron Christopher Tan, Janakiraman Balachandran, Seid Hossein Sadat, Vikram Gavini, Barry D. Dunietz, Sung-Yeon Jang, and Pramod Reddy
J. Amer. Chem. Soc.,
133 (2011) 8838
We present a combined experimental and computational study that probes the thermoelectric and electrical transport properties of molecular junctions. Experiments were performed on junctions created by trapping aromatic molecules between gold (Au) electrodes. The end groups (-SH,-NC) of the aromatic molecules were systematically varied to study the effect of contact coupling strength and contact chemistry. When the coupling of the molecule with one of the electrodes was reduced by switching the terminal chemistry from -SH to -H, the electrical conductance of molecular junctions decreased by an order of magnitude, whereas the thermopower varied by only a few percent. This observation, which provides information about the effect of contact coupling on the electronic structure of the junctions, has been predicted computationally in the past and is experimentally demonstrated for the first time. Further, our experiments and computational modeling indicate the prospect of tuning thermoelectric properties at the molecular scale. In particular, the thiol terminated aromatic molecular junctions revealed a positive thermopower that increased linearly with length. This positive thermopower is associated with charge transport primarily through the highest occupied molecular orbital (HOMO) as shown by our computational results. In contrast, a negative thermopower was observed for a corresponding molecular junction terminated by an isocyanide group due to charge transport primarily through the lowest unoccupied molecu- lar orbital (LUMO).
.......................................................................................................................................
Efficiency of thermoelectric energy conversion in biphenyl-dithiol junctions: Effect of electron-phonon interactions
Nikolai Sergueev, Seungha Shin, Massoud Kaviany and Barry D. Dunietz,
Phys. Rev. B.,
83 (2011) 195415
The electron-phonon interaction is the dominant mechanism of inelastic scattering in molecular junctions. Here we report on its effect on the thermoelectric properties of single-molecule devices. Using density functional theory and the nonequilibrium Green's function formalism we calculate the thermoelectric figure of merit for a biphenyl-dithiol molecule between two Al electrodes under an applied gate voltage. We find that the effect of electron-phonon coupling on the thermoelectric characteristics strongly varies with the molecular geometry. Two molecular configurations characterized by the torsion angles between the two phenyl rings of 30deg and 90deg exhibit significantly different responses to the inelastic scattering. We also use molecular dynamics calculations to investigate the torsional stability of the biphenyl-dithiol molecule and the phonon thermal transport in the junction.
......................................................................................................................................
Bias effects on the electronic spectrum of a molecular confined and biased bridge
Heidi Philips, Alex Prociuk and Barry D. Dunietz,
J. Chem. Phys.,
134 (2011) 054708
The effect of bias and geometric
breaking on the
electronic spectrum of a model molecular system is studied.
Geometric symmetry breaking can either enhance the dissipative
effect of the bias, where spectral peaks are disabled, or
enable new excitations that are absent under zero bias
conditions. The analysis is performed on a simple model system by
solving for the electronic response to a pulse
perturbation in the dipole approximation. The dynamical response is extracted from the electronic
equations of motion as expressed by the Keldysh formalism. This
expression provides for the accurate treatment of an evolving
bulk-coupled system at the model Hamiltonian level.
......................................................................................................................................
On the conditions for enhanced transport through molecular junctions based on metal centers ligated by pair of pyridazino-derived ligands
Bei Ding, Victoria Washington and Barry D. Dunietz,
Mol. Phys.,
108 (2010) 2591
Transport properties of a Ni bis-&eta ² complex ligated by pairs of
bi-pyridazino derivatives are considered. This complex provides the
opportunity to avoid perpendicular alignment of the ligand $\pi$
planes. We study the effects of &pi bonding and of intramolecular
hydrogen bonding between the ligands as mediated by the metal center
on electron transport.
The complicated effect of the electronic structure
equilibration with the electrodes on the transport is discussed.
The analysis at the electronic structure level
provides guidelines to design a molecular bridge that is based on
metal complexation with effective electronic transport.
......................................................................................................................................
Photo-induced absolute negative current in a molecular electronic system.Alex Prociuk and Barry D. Dunietz,
Phys. Rev. B.,
82 (2010) 125449
The study of current induced by photoradiating a molecular-based device under bias is of fundamental importance to the improvement of photoconductors and photovoltaics. In this technology, electron pumps generate an uphill current that opposes a potential drop and thereby recharges a fuel cell. While the modeled molecular electron pump is completely symmetric, the sign of the photocurrent is solely determined by the existing bias and the nature of photoinduced electronic excitations. The photoradiation induces nonequilibrium population of the electrode-coupled system. The dependence of the photocurrent on electrode coupling, photoradiation field strength, and applied bias are studied at a basic model level.
......................................................................................................................................
Gating dependence of single molecule field effect transistors on contact symmetry
Trilisa Perrine and Barry D. Dunietz,
J. Amer. Chem. Soc.,
132 (2010) 2914-2918
The geometric aspects for the functionality of a molecular-based field
effect transistor (FET) are analyzed. A computational study is
performed on molecular models involving a well defined conjugation
plane coupled to gold-based electrodes through thiol
bonding. Transport gating of the FET is shown to depend on a
symmetry-breaking effect induced by the gating-field. This effect is
also related to the orientation of the field relative to the
gold-thiol bonds, the molecular conjugation plane, and the overall
symmetry of the device. It is found that the presence of a center of
inversion in the bulk-coupled molecular system results in the
cancellation of the transisting response. The presence of a plane of
symmetry which includes the transport vector, in a reduced symmetry
system, results in a gating response only to electric fields oriented
perpendicular to that mirror plane.
......................................................................................................................................
Modeling transient aspects of coherence-driven electron transport
Alex Prociuk, Heidi Phillips and Barry D. Dunietz,
J. Phys.: Conf. Series.,
220 (2010) 012008.
Non-equilibrium Green's function formalism (NEGF) by employing
time-dependent perturbation theory is used to solve the electronic
equations of motion of model systems under potential biasing
conditions. The time propagation is performed in the full frequency
domain of the two time variables representation. We analyze
transient aspects of the conductance under applied direct-current
and alternating current potential perturbation. The coherence
induced response dependence on different aspects of the applied
perturbation are resolved in time and analyzed using TD distribution
of the current operator.
......................................................................................................................................
Beyond 7-Azaindole: Conjugation Effects on Intermolecular Double Hydrogen-Atom Transfer Reactions
Carlos R. Baiz, Sarah J. Ledford, Kevin J. Kubarych and Barry D. Dunietz
J. Phys. Chem. A.
113 (2009) 4862
Conjugation effects on the thermodynamics of ground-state and lowest-singlet excited-state double hydrogen-atom transfer reactions in 7-azaindole and related models are studied with ab initio electronic structure methods. The results indicate that the extended conjugation of the system has a large effect on the relative energies required for hydrogen-atom transfer. The observed energy differences are mainly attributed to stabilization of the tautomer species by enhancing low-energy resonance structures and by allowing for efficient delocalization of excess charge in the reaction center.
......................................................................................................................................
On the Electronic Spectra of a Molecular Bridge Under
Non-Equilibrium Electric Potential Conditions
Alex Prociuk and Barry D. Dunietz,
"Progress in Theoretical Chemistry and Physics" series book chapter in: Atomic and Molecular Systems, Dynamics, Spectroscopy,
Clusters, and Nanostructures, Springer publisher,
20 (2009) 265-277
The linear response of the electronic density of a molecular-based
junction under potential bias conditions to a probing polarizing
perturbation is calculated to model the electronic spectra. It is
shown that steady flux conditions lead to dramatic effects on the
electronic spectra of the confined system. The non-equilibrium
conditions enable electronic transitions that are otherwise forbidden.
The implemented methodology uses the Keldysh contour formalism to
express the electronic equations of motion. The related time
correlation Green Functions are then solved for in the full frequency
representation and at the linear response level.
......................................................................................................................................
Multiadsorption and Coadsorption of Hydrogen on Model Conjugated Systems
Miguel Wong, Benjamin Van-Kuiken, Corneliu Buda and Barry D Dunietz
J. Phys. Chem. C.,
113 (2009) 12571-12579.
Hydrogen interaction with conjugated aromatic molecular systems is
analyzed with high level ab initio electronic structure
methodology. The adsorption of hydrogen molecules on aromatic systems
ranging from a single ring to double ring systems including fused and
bridged molecules is investigated. We propose several
functionalization schemes of the conjugated system to increase
hydrogen intake. These functionalizations consist of using an
asymmetrical conjugation skeleton as inherently present for the
azulene molecule and the introduction of B$(B!](BN heteroatom pairs as
doping sites in the conjugation ring. We have also tested the
possibility of involving more than a single interaction site within
the same molecule by using a molecule involving several rings as the
triptycene and several derived functionalized molecules. These
computational studies provide insight on the interaction of hydrogen
with conjugated molecular species. This insight can be utilized for
designing materials such as metal organic frameworks or other porous
organic polymers with enhanced uptake properties.
......................................................................................................................................
Enhanced Conductance via Induced pi-Stacking Interactions in Cobalt(II) Terpyridine Bridged Complexes
Trilisa M. Perrine, Timothy Berto and Barry D. Dunietz
J. Phys. Chem. b.,
112 (2008) 16070-16075.
Computational model systems are used to explore improving the
transmission through a molecular device based on bridged cobalt(II)
complexes. The bridging ligands and the organic conjugated molecular
ligands are altered to improve the current flow through both an
enhanced $(B&P(B-stacking interaction as well as involving the metal ions
directly in the conduction pathway. With terpyridine as the organic
ligand, both acetate and NH2$(B!](B produce conductive devices, while a
terpyridine complex bridged by Cl$(B!](B is not conductive. The addition of
a fused ring on either end of the conjugated molecule has a complex
effect which is sensitive to the bridged ligand and the particular
geometry of the complex.
......................................................................................................................................
Time-dependent current through electronic channel models using a mixed time-frequency
solution of the equations of motion
Alexander Prociuk and Barry D. Dunietz
Phys. Rev. B,
78
(2008) 165112.
A non-equilibrium Green's-Function (NEGF) model based on time
dependent perturbation theory is developed to propagate electronic
structure and molecular conductance of extended
electrode-molecule-electrode nanostructures. In this model, we use
the two time variable nature of the Kadanoff-Baym equations of motion
to formulate a mixed time-frequency representation for the electronic
density expressed by the appropriate GF (G<). This allows for the
dynamical treatment of open systems. Furthermore, highly informative
time dependent Wigner distributions are used to shed light on the
features of dynamical observables, such as electron current.
Calculations, performed on model systems, resolve the dynamic current
into direct and alternating components. The direct current is due to
electronic open channels near the Fermi level and the alternating
response is due to interference fringes from a superposition of
extended states. We analyze the transient conductance with respect to
the fundamental system's parameters, the effect of bound states and
conductance driven by laser induced coherence affected by detuning due
to an applied DC bias. The amplitude of the alternating transient
current can be adjusted by reshaping the bias pulse or by controlling
the electronic coupling terms. Bound states may yield a persisting
oscillating response depending on their relative electronic densities.
In the analysis we utilize the calculated highly informative
time-dependent current distributions.
......................................................................................................................................
Ab initio study of charge transport of hydrogen functionalized palladium wiresZhen Zhao, and Barry D. Dunietz,
J. Chem. Phys,
129 (2008) 024702.
We present ab initio calculations of transport properties of palladium
wires in the presence of hydrogen. Detailed investigations have been
1conducted with a pure palladium wire and with opening a gap inside the
wire in which the transition between point contact regime and
tunneling regime occurs. The effect of the presence of hydrogen in the
gap is studied for different ranges of the gap size. The hydrogen
mediated transport in the contact and tunneling regimes of the gap are
analyzed and compared. It is predicted that only in large enough
distances the hydrogen presence increases the conductance. The effect
of additional hydrogen molecules on the gap is also studied.
......................................................................................................................................
Synthetic, mechanistic, and computational investigations of nitrile-alkyne cross-metathesis
Andrea M. Geyer, Eric S. Wiedner, J. Brannon Gary, Robyn L. Gdula, Nicola C. Kuhlmann, Marc J. A. Johnson, Barry D. Dunietz, Jeff W. Kampf
J. Amer. Chem. Soc. ,
130 (2008) 8994-8999.
The terminal nitride complexes NW(OC(CF3)2Me)3(DME) (1-DME), [Li(DME)2][NW(OC(CF3)2Me)4] (2), and [NW(OCMe2CF3)3]3 (3) were prepared in good yield by salt elimination from [NWCl3]4. X-ray structures revealed that 1-DME and 2 are monomeric in the solid state. All three complexes catalyze the cross-metathesis of 3-hexyne with assorted nitriles to form propionitrile and the corresponding alkyne. Propylidyne and substituted benzylidyne complexes RCW(OC(CF3)2Me)3 were isolated in good yield upon reaction of 1-DME with 3-hexyne or 1-aryl-1-butyne. The corresponding reactions failed for 3. Instead, EtCW(OC(CF3)Me2)3 (6) was prepared via the reaction of W2(OC(CF3)Me2)6 with 3-hexyne at 95 N0C. Benzylidyne complexes of the form ArCW(OC(CF3)Me2)3 (Ar = aryl) then were prepared by treatment of 6 with the appropriate symmetrical alkyne ArCCAr. Three coupled cycles for the interconversion of 1-DME with the corresponding propylidyne and benzylidyne complexes via [2 + 2] cycloaddition$(B!](Bcycloreversion were examined for reversibility. Stoichiometric reactions revealed that both nitrile-alkyne cross-metathesis (NACM) cycles as well as the alkyne cross-metathesis (ACM) cycle operated reversibly in this system. With catalyst 3, depending on the aryl group used, at least one step in one of the NACM cycles was irreversible. In general, catalyst 1-DME afforded more rapid reaction than did 3 under comparable conditions. However, 3 displayed a slightly improved tolerance of polar functional groups than did 1-DME. For both 1-DME and 3, ACM is more rapid than NACM under typical conditions. Alkyne polymerization (AP) is a competing reaction with both 1-DME and 3. It can be suppressed but not entirely eliminated via manipulation of the catalyst concentration. As AP selectively removes 3-hexyne from the system, tandem NACM-ACM-AP can be used to prepare symmetrically substituted alkynes with good selectivity, including an arylene-ethynylene macrocycle. Alternatively, unsymmetrical alkynes of the form EtCCR (R variable) can be prepared with good selectivity via the reaction of RCN with excess 3-hexyne under conditions that suppress AP. DFT calculations support a [2 + 2] cycloaddition$(B!](Bcycloreversion mechanism analogous to that of alkyne metathesis. The barrier to azametalacyclobutadiene ring formation/breakup is greater than that for the corresponding metalacyclobutadiene. Two distinct high-energy azametalacyclobutadiene intermediates were found. These adopted a distorted square pyramidal geometry with significant bond localization.
......................................................................................................................................
Accessing Metal-Carbide Chemistry. A Computational Analysis of Thermodynamic Considerations
J. Brannon Gary, Corneliu Buda, Marc J. A. Johnson, and Barry D. Dunietz,
Organometallics,
27 (2008) 814.
The electronic structures of terminal metal carbide complexes are
calculated using DFT. This study outlines the factors that give rise
to stable carbide complexes, which can be used to help in the
synthesis of new carbide complexes and to tune their stability as
desired. The calculations reveal the presence of a strong Ru&equiv C triple
(&sigma + 2&pi) bond. The C atom is nearly unhybridized, such that the
C-component of the Ru-C \sigma-bond has 90% 2p character. This leaves a
very stable carbon-based lone pair that is almost entirely 2s in
character, which accounts for the lack of Lewis base character
exhibited experimentally. Calculations predict a Ru-C bond
dissociation energy of 147.4 kcal mol^{-1} in a typical Ru carbide
complex. This large bond strength is not unique to the RuC bond, as
revealed by an extension of the study to identify schemes by which to
chemically tune the metal-carbide bond strength. Methods examined to
achieve this tuning include changing the identity of the central metal
and altering the metal ligation scheme. In general, 16-electron
square-pyramidal M(C)L_4 complexes and 12- or 16-electron tetrahedral
M(C)L_3 complexes of the 4d elements can possess comparably strong
metal-carbide bonds. The calculations also show that the carbide
moiety exerts a very strong trans influence, which explains several
experimental observations. We conclude that the dearth of terminal
carbide complexes is not due to any inherent weakness of M&equiv C
bonds. Many more terminal carbide complexes can be expected in the
future as new routes to their formation are found.
......................................................................................................................................
Gating of single molecule transistors: Combining field-effect and chemical control
Trilisa M. Perrine, Ron G. Smith, Christopher Marsh, and Barry D. Dunietz,
Journal of Chemical Physics,
128 (2008) 154706.
Previously we have demonstrated that several structural features
are crucial for the functionality of molecular field effect transistors.
The effect of additional
structural aspects of molecular wires is explored.
These include the type of, the thiol binding location on, and the chemical substitutions of a conjugated
system. Pentacene, porphyrin and the Tour-Reed devices are utilized as model
systems.
The thiol binding location is shown to have a varried effect on the transmission
of a system depending on the molecular orbitals involved.
Substitution by electron withdrawing and donating groups is illustrated
to have a substantial effect on the transmission of single molecule devices.
The substituation effect is either a simple energy shifting effect
or a more complicated
resonance effect, and can be used to effectively tune the electronic behavior of a single molecule field effect transistor.
......................................................................................................................................
Conductance of a cobalt(II) terpyridine complex based molecular transistor: A computational analysis
Trilisa M. Perrine and Barry D. Dunietz,
Journal of Physical Chemistry A: Lester Special Issue,
112 (2008), 2043-2048.
A recent experiment, in which a molecular transistor based on the coordination chemistry of cobalt(II) and organic self-assembly-monolayers is formed by means of self-aligned lithography, is analyzed with a computational approach. The calculations reveal that a complex involving two cobalt(II) ions bridged by acetate ions can effectively span the nano-gap. This bridged complex is shown to be both more flexible and more conductive than the alternative structure involving a
single cobalt(II) ion. The single cobalt(II) ion complex is the more stable structure in a non-confined environment (i.e. in solution), but is found to be less effective at connecting the leads of the fabricated gap and is less
likely to result in a conductive device.
......................................................................................................................................
Carbonyl mediated conductance through metal bound peptides; a computational study
Trilisa M. Perrine and Barry D. Dunietz,
Nanotechnology,
18, (2007), 424003.
Large increases in the conductance of peptides, upon binding to metal ions, have been recently reported experimentally. The mechanism of the conductance switching is examined computationally. It is suggested that oxidation of the metal ion occurs after binding peptide. This is caused by the bias potential placed across the metal-peptide complex. A combination of configurational changes, metal ion involvment and interactions between carbonyl group oxygen atoms and the gold leads are all shown to be necessary for the large improvement in the conductance seen experimentally. Differences in the molecular orbitals of the nickel and copper complexes are noted and serve to explain the variation of the conductance improvement upon binding to either a nickel or copper ion.
......................................................................................................................................
Theoretical Studies of Conjugation Effects on Excited State Intramolecular Hydrogen-atom
Transfer Reactions in Model Systems
Carlos Baiz and Barry D. Dunietz,
Journal of Physical Chemistry A,
111, (2007), 10139-10143.
Intramolecular Hydrogen-atom transfer dependence on electronic
conjugation of curcumin and related molecular models in the ground
state and 1&pi &pi* excited state are computationally studied at
first-principles electronic structure level. The larger, more
conjugated, systems exhibit a lower reaction barrier in the ground
state but a higher barrier in the excited state. This is associated
with a smaller increase in the conjugation upon excitation in the
larger systems. Our studies provide a detailed description and
analysis of these energy trends as well as an insight into the
physical nature of the intramolecular hydrogen-atom transfer
reactions.
......................................................................................................................................
Fragmentation pathways and mechanisms of aromatic compounds in atmospheric pressure studied by GC-DMS and DMS-MS
Shai Kendler, Gordon R. Lambertus, Barry D. Dunietz, Stephen L. Coy, Erkinjon G. Nazarov, Raanan A. Miller, and Richard D. Sacks,
Int. J. of Mass Spec. ,
263, (2007), 137-147.
Differential mobility spectrometry (DMS) is a highly sensitive sensing technology capable of selecting and detecting ions based on the difference between ion mobility at high and low electric field. The combination of a micro-fabricated DMS with gas chromatography (GC) has allowed extensive investigation of the ion chemistry and collisionally induced dissociation (CID) of diaryl molecules on a millisecond timescale at temperatures up to 130 degrees C. DMS-pre-filtered time-of-flight mass spectrometry (DMS-MS) has been used to verify the chemical composition of the ion species resolved by GC-DMS. This work focuses on the fragmentation of diaryl compounds, including diphenyl methane (DPM) and bibenzy] (BB), using information from the DMS and DMS-MS spectra of a series of aromatic compounds. Density functional theory calculations have been used to investigate the geometry and the energy along the reaction coordinate for the loss of benzene from DPM-H+ and BB-H+ for comparison with GC-DMS and DMS-MS experimental results and with previously reported chemical ionization MS. DPM-H+ is observed to undergo field-induced fragmentation in the DMS to produce C7H7+(Bz(+)) and unobserved neutral benzene with a low energy barrier. In contrast, BB-H+ fragments to C8H9+ and benzene with a higher energy barrier. Calculated barriers and experimental results are in qualitative agreement. Depletion of the ionized fragments in favor of ion-neutral clusters was also observed at higher concentrations. It is suggested that CID in DMS can further enhance DMS analytical performance.
......................................................................................................................................
Single-molecule field-effect transistors: A computational study of the effects of contact geometry and gating-field orientation on conductance-switching properties
Trilisa M. Perrine and Barry D. Dunietz,
Phys. Rev. B,
75, (2007), 195319.
The relation of geometric features to the effect of gating electric fields on the conductance through conjugated systems is investigated by electronic transmission calculations employing Green's function based modeling. Switching is only induced if the field is applied in an orientation which results in energy shifting of the molecular orbitals. This is found to depend on the orientation of the field with respect to the plane defined by the molecular conjugation. The switching can be quenched by structural rearrangement of the chemical bonds to the bulk, where the relative position of the electrodes is modified.
......................................................................................................................................
Electron Transport through Heterogeneous Intermolecular Tunnel Junctions
Das, M. and Dunietz, B. D.,
J. Phys. Chem. C.,
111, (2007), 1535--1540.
Quantum charge transport through intermolecular tunnel junctions is studied. Intermolecular tunnel junctions can be defined by the end groups of pairs of self-assembled monolayers of functionalized conjugated alkenes on gold surfaces. Conductivity dependence on the tunnel distance has been compared for various junctions. It is found that for junctions dominated by attractive interactions, for example, systems involving hydrogen bonding, conductivity exhibits less dependence on the tunneling distance than with junctions dominated by dispersive interactions. Junctions with stronger distance dependence conductivty are desired for applications related to chemical sensors. Our study provides insight for designing an efficient chemical sensor that is based on heterogeneous tunnel junction, which may involve conductivity through a combination of the attractive hydrogen-bonding channel and repulsive dispersive interactions.
......................................................................................................................................
Benchmarking the performance of density functional theory based Green's function formalism utilizing different self-energy models in calculating electronic transmission through molecular systems
Prociuk, A. and Dunietz, B.D.,
J. Chem. Phys.,
125, (2006), 204717.
Electronic transmission through a metal-molecule-metal system is calculated by employing a Green's function formalism in the scattering based scheme. Self-energy models representing the bulk and the potential bias are used to describe electron transport through the molecular system. Different self-energies can be defined by varying the partition between device and bulk regions of the metal-molecule-metal model system. In addition, the self-energies are calculated with different representations of the bulk through its Green's function. In this work, the dependence of the calculated transmission on varying the self-energy subspaces is benchmarked. The calculated transmission is monitored with respect to the different choices defining the self-energy model. In this report, we focus on one-dimensional model systems with electronic structures calculated at the density functional level of theory.
......................................................................................................................................
Spin-dependent electronic transport through a porphyrin ring ligating an Fe(II) atom: An ab initio study
Chen, Y. and Prociuk, A. and Perrine, T. and Dunietz, B.D.,
Phys. Rev. B,
74, (2006), 245320.
Conductance calculations employing density functional theory methodology and Landauer formalism predict that a ligated iron atom can be used as a switching device. The iron atom is ligated in our models by a porphyrin molecule. The iron-porphyrin molecular device is shown to lose more than 66% of its conductance by shifting from the low spin coupling state to excited spin states. Further reduction is also correlated with a mechanical distortion of the porphyrin plane. Both the distortions and spin transitions are fast processes that can be invoked by manipulating the iron's ligation scheme through the axial ligands.
......................................................................................................................................
Hydrogen Physisorption on the Organic Linker in Metal Organic Frameworks: Ab Initio Computational Study
Buda, C. and Dunietz, B.D.,
J. Phys. Chem. B.,
110, (2006), 10479--10484.
Research for materials offering efficient hydrogen storage and transport has recently received increased attention. Metal organic frameworks (MOFs) provide one promising group of materials where several recent advances were reported in this direction. In this computational study ab initio methods are employed to study the physisorption of hydrogen on conjugated systems. These systems are used as models for the organic linker within MOFs. Here, we focus on the adsorption sites related to the organic linker with special attention to the edge site, which was only recently reported to exist as the weakest adsorbing site in MOFs. We also investigate chemically modified models of the organic connector that result in enforcing this adsorption site. This may be crucial for improving the uptake properties of these materials to the goal defined by DOE for efficient hydrogen transport materials.
......................................................................................................................................
Metathesis-Enabled Formation of a Terminal Ruthenium Carbide Complex: A Computational Study
Buda, C., Caskey, S.R., Johnson, M.J.A. and Dunietz, B.D.,
Org. Metal.,
25, (2006), 4756-4762.
The energy profile of rare Ru carbide formation starting from an acetoxycarbene complex is studied using DFT methods. Three distinctive reaction pathways that differ in their initiation step are investigated. Two of the proposed reaction mechanisms have relatively similar activation barriers. Therefore, additional calculations have been performed using large size ligands (PCy3), matching exactly the actual experimental system. In addition, the corresponding kinetic isotope effect has been evaluated and compared to the experimental measured value.
......................................................................................................................................
Additional Manuscripts and Papers:
Advances in methods and algorithms in a modern quantum chemistry program package.
Shao, Yihan, Molnar, Laszlo Fusti, Jung, Yousung, Kussmann, Jorg, Ochsenfeld, Christian, Brown, Shawn T., Gilbert, Andrew T.B., Slipchenko, Lyudmila V., Levchenko, Sergey V., O'Neill, Darragh P., Jr, Robert A. DiStasio, Lochan, Rohini C., Wang, Tao, Beran, Gregory J.O., Besley, Nicholas A., Herbert, John M., Lin, Ching Yeh, Voorhis, Troy Van, Chien, Siu Hung, Sodt, Alex, Steele, Ryan P., Rassolov, Vitaly A., Maslen, Paul E., Korambath, Prakashan P., Adamson, Ross D., Austin, Brian, Baker, Jon, Byrd, Edward F. C., Dachsel, Holger, Doerksen, Robert J., Dreuw, Andreas, Dunietz, Barry D., Dutoi, Anthony D., Furlani, Thomas R., Gwaltney, Steven R., Heyden, Andreas, Hirata, So, Hsu, Chao-Ping, Kedziora, Gary, Khalliulin, Rustam Z., Klunzinger, Phil, Lee, Aaron M., Lee, Michael S., Liang, WanZhen, Lotan, Itay, Nair, Nikhil, Peters, Baron, Proynov, Emil I., Pieniazek, Piotr A., Rhee, Young Min, Ritchie, Jim, Rosta, Edina, Sherrill, C. David, Simmonett, Andrew C., Subotnik, Joseph E., III, H. Lee Woodcock, Zhang, Weimin, Bell, Alexis T. and Chakraborty, Arup K.,
Phys. Chem. Chem. Phys.,
8, (2006), 3172-3191.
......................................................................................................................................
Articles Before 2005:
Ugalde, J. M., Dunietz, B., Dreuw, A., Head-Gordon, M. and Boyd, R. J. `The spin
dependence of spatial size of Fe(II) and of the structure of Fe(II)-porphyrins.' J. Phys. Chem. A., 108, (2004), 4653-4657.
Dunietz, B. D., Markovic, N., Ross, P. H. and Head-Gordon, M. `Initiation of Electrooxidation of CO on Pt based electrodes at full coverage conditions simulated by ab-initio electronic structure calculations.' J. Phys. Chem. B., 108, (2004), 9888.
......................................................................................................................................
Saravanan*, C., Dunietz*, B. D., Markovic, N., Somorjai, G. and Head-Gordon,
M. Ross, P. H. `Electro-oxidation of CO on Pt electrodes simulated by electronic structure calculations.' J.Electroanal.Chem., J. Weaver Special memorial issue (v554), (2003), 459.
Dunietz, B. D. and Head-Gordon, M. `Manifestations of symmetry breaking in selfconsistent eld electronic structure calculations.' J. Phys. Chem. A., 107, (2003), 9160.
Head-Gordon, M., Van Voorhis, T., Beran, G. J. O. and Dunietz, B. D. `Local correlation models.' Computational Science - ICCS 2003, Pt IV , 2660, (2003), 96-102.
Dunietz, B. D., Dreuw, A. and Head-Gordon, M. `Initial steps of the photodissociation of the CO ligated heme group.' J. Phys. Chem. B., 107, (2003), 5623-5629.
......................................................................................................................................
Dunietz, B. D., van Voorhis, T. and Head-Gordon, M. `Geometric direct minimization of Hartree Fock calculations involving open shell wavefunctions with spin restricted orbitals.' J. Theo. and Comp. Chem., 1, (2002), 255-261.
Dreuw, A., Dunietz, B. D. and Head-Gordon, M. `Characterization of the relevant
excited states in the photodissociation of the CO-ligated Hemoglobin and Myoglobin.'
J. Am. Chem. Soc., 124, (2002), 12070-12071.
......................................................................................................................................
Dunietz, B. D. and Friesner, R. A. `Application and development of multicongurational localized perturbation theory.' J. Chem. Phys., 115, (2001), 11052.
Friesner, R. A. and Dunietz, B. D. `Large-scale ab-initio quantum chemical calculations on biological systems.' Accounts Chem Res, 34, (2001), 351-358.
Gherman, B. F., Dunietz, B. D., Whittington, D. A., Lippard, S. J. and Friesner, R. A. `Activation of the C-H bond of methane by intermediate Q of methane monooxygenase:
A theoretical study.' J. Am. Chem. Soc., 123, (2001), 3836.
......................................................................................................................................
Dunietz, B. D., Beachy, M. D., Cao, Y. X., Whittington, D. A., Lippard, S. J. and
Friesner, R. A. `Large scale ab-initio quantum chemical calculation of the intermediate
in the soluble methane monooxygenase catalytic cycle.' J. Am. Chem. Soc., 122, (2000), 2828.
......................................................................................................................................
Friesner, R. A., Murphy, R. B., Beachy, M. D., N., Ringnalda M., Pollard, W. T.,
Dunietz, B. D. and Cao, Y. X. `Correlated ab-initio electronic structure calculations
for large molecules.' J. Phys. Chem. A., 103, (1999), 1913.
Dunietz, B. D., Murphy, R. B. and Friesner, R. A. `Calculation of atomization energies by a multicon gurational localized perturbation theory - Application for closed shell cases.' J. Chem. Phys., 110, (1999), 1921.
