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"Electronic Structure Based Studies"

Our research program studies complex molecular systems using computational tools. Group members collaborate closely with each other and with researchers from other groups that include both experimental and computational experts. We have been able to incorporate in our activity students at the high school, undergraduate and graduate levels, whose contributions are acknowledged and reported in several scientific publications.

In one major research thrust we study photo induced processes in pigments related to natural photosystems. Photosynthesis is the natural process of harvesting solar energy. The objective of the research activity is to advance predictive models for understanding the mechanisms through which photosynthesis proceeds, and therefore extracting important lessons that can benefit various technologies.1-3 Towards the goal of achieving predictive modeling of photosynthesis we go beyond the ubiquitous semiclassical Marcus theory for obtaining rates of transfer reactions,4,5 and use high quality first-principles calculations6-8 of the relevant potential energy surfaces for generating the required inputs.9,10

In another main research thrust we investigate molecular junctions under voltage-biased conditions to understand structure-function relationships affecting current at the molecular scale.12-15 Our studies elaborate on the conditions leading to non equilibrium effects as of charge rectification and negative differential resistance. We are developing state of the art tools based on range-separated hybrid functional that have been shown to benchmark well against available experimental measurements of conductance and more recently in the study of negative differential resistance (NDR).

For example, in Figure 1 we confirm the success of our approach based on modern DFT, noted in the figure by using the BNL functional. We compare the calculated I-V relationships based on three functionals; LDA, B3LYP and BNL, a range separated hybrid (RSH) functional.11 The RSH based framework finds the NDR onset bias in agreement with the reported measurement at about 1.8 V.16 The onset of the NDR based on LDA and B3LYP is at 0.8 and 1.3V, respectively, and which are significantly lower than the measured onset of 1.85V. The success of the calculation is clearly linked to using DFT that achieves correct orbital gaps.12 As also reflected in the figure the overall drop in conductance is overestimated by LDA and B3LYP compared to the reported sample I-V trend, while the BNL drop appears significantly underestimated.

Fig. 1: I-V relationships based on LDA (red line with circles), B3LYP (green line with diamonds) and BNL (blue line with stars) functionals and upon ring rotation, compared to measured trend (red line with diamonds).11 BNL reproduces well the NDR trend, while with LDA and B3LYP NDR occurs at a significantly lower bias. Current significantly drops upon ring rotation as shown by the BNL calculation.

1. Lee, M. H., Dunietz, B. D., Geva, E.: Donor-to-Donor vs Donor-to-Acceptor Interfacial Charge Transfer States in the Phthalocyanine–Fullerene Organic Photovoltaic System, J. Phys. Chem. Lett., 2014, 5, 3810–3816
2. Manna, A. K., Balamurugan, D., Cheung, M. S., Dunietz, B. D.: Unraveling the Mechanism of Photoinduced Charge Transfer in Carotenoid - Porphyrin - C60 Molecular Triad, J. Phys. Chem. Lett., 2015, 6, 1231–1237
3. Song, Y., Schubert, A., Maret, E., Burdick, R. K., Dunietz, B. D., Geva, E., Ogilvie, J. P.: Vibronic Structure of Photosynthetic Pigments Probed by Polarized Two-dimensional Electronic Spectroscopy and ab initio Calculations, Chem. Sci., 2019, 10, 8143–8153
4. Mulvihill, E., Gao, X., Liu, Y., Schubert, A., Dunietz, B. D., Geva, E.: Combining the Mapping Hamiltonian Linearized Semiclassical Approach with the Generalized Quantum Master Equation to Simulate Electronically Nonadiabatic Molecular Dynamics, J. Chem. Phys., 2019, 151, 074103–16
5. Tong, Z., Gao, X., Cheung, M. S., Dunietz, B. D., Geva, E., Sun, X.: Charge Transfer Rate Constants for the Carotenoid-Porphyrin-C60 Molecular Triad Dissolved in Tetrahydrofuran: The Spin-Boson Model vs the Linearized Semiclassical Approximation, J. Chem. Phys., 2020, 153, 044105
6. Bhandari, S., Cheung, M., Geva, E., Kronik, L., Dunietz, B. D.: Fundamental Gaps of Condensed-Phase Organic Semiconductors From Single-Molecule Polarization-Consistent Optimally Tuned Screened Range-Separated Hybrid Functionals, J. Chem. Theory Comput., 2018, 14, 6287–6294
7. Bhandari, S., Dunietz, B. D.: Quantitative Accuracy in Calculating Charge Transfer State Energies in Solvated Molecular Dimers Using Screened Range Separated Hybrid Functional Within a Polarized Continuum Model, J. Chem. Theory Comput., 2019, 15, 4305
8. Begam, K., Bhandari, S., Maiti, B., Dunietz, B. D.: Screened Range-Separated Hybrid Functional with Polarizable Continuum Model Overcomes Challenges in Describing Triplet Excitations in the Condensed Phase Using TDDFT, J. Chem. Theory Comput., 2020, 16, 3287
9. Sun, X., Zhang, P., Lai, Y., Williams, K. L., Cheung, M. S., Dunietz, B. D., Geva, E.: Computational Study of Charge-Transfer Dynamics in the Carotenoid–Porphyrin–C60 Molecular Triad Solvated in Explicit Tetrahydrofuran and Its Spectroscopic Signature, J. Phys. Chem. C, 2018, 122, 11288–11299
10. Song, Y., Schubert, A., Liu, X., Bhandari, S., Forrest, S. R., Dunietz, B. D., Geva, E., Ogilvie, J. P.: Efficient Charge Generation via Hole Transfer in Dilute Organic Donor–Fullerene Blends, J. Phys. Chem. Lett., 2020, 11, 2203–2210
11. Bhandari, S., Yamada, A., Hoskins, A., Payne, J., Aksu, H., Dunietz, B. D.: Achieving Predictive Description of Negative Differential Resistance in Molecular Junctions Using a Range-Separated Hybrid Functional, Adv. Theory Simul., 2021, 4, 2000016
12. Yamada, A., Feng, Q., Hoskins, A., Fenk, K. D., Dunietz, B. D.: Achieving Predictive Description of Molecular Conductance by Using a Range-Separated Hybrid Functional, Nano. Lett., 2016, 16, 6092
13. Feng, Q., Yamada, A., Baer, R., Dunietz, B. D.: Deleterious Effects of Exact Exchange Functionals on Predictions of Molecular Conductance, J. Chem. Theory Comput., 2016, 12, 3431–3435
14. Yamada, A., Feng, Q., Q. Zhou, Hoskins, A., Lewis, K. M., Dunietz, B. D.: Conductance of Junctions with Acetyl-Functionalized Thiols: A First-Principles-Based Analysis, J. Phys. Chem. C, 2017, 121, 10298
15. Zhou, Q., Yamada, A., Feng, Q. G., Hoskins, H., Dunietz, B. D., Lewis, K. M.: Modification of Molecular Conductance by In-Situ Deprotection of Thiol-Based Porphyrin, ACS Appl. Mater. Interfaces, 2017, 9, 15901
16. Xiao, X., Nagahara, L. A., Rawlett, A. M., Tao, N.: Electrochemical gate-controlled conductance of single oligo (phenylene ethynylene) s, J. Am. Chem. Soc., 2005, 127, 9235–9240