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Quantum Chemical Studies of Charge-Separated States and X-Ray Absorption Spectroscopy with Time Dependent Kohn-Sham Theory

Alam, Bushra
http://rave.ohiolink.edu/etdc/view?acc_num=osu162947841641183

Abstract Details

2021, Doctor of Philosophy, Ohio State University, Chemistry.
In the following studies, we explore several different categories of excited states encountered in chemistry in the solid, liquid and gaseous phase. These investigations are either aimed at understanding the disputed behaviour of certain excited states, or the development of methodology that can provide a more complete description of them. First , we have looked at the extent of charge separation in the low lying excited states in pentacene which is an organic photovoltaic material that can generate multiple charge carriers from a single excitation. Theoretical studies have reached differing conclusions regarding the role of charge separation in this material and to to investigate this, we have used a fragment based model to examine size-dependent trends, in models ranging from a dimer to hexamer. We have done this to understand the change in the energetics from the gas phase to the bulk. We have also conducted calculations using theoretical tools which have been optimized to give an accurate description of the underlying electronic structure of these systems. Our results suggest that the low lying excited states of this material are indeed charge separated, and this is more pronounced as the system size increases. Hence , studies based on small models or methodology unsuited for these systems, can miss out on the bulk interactions present in a crystal environment, and reach erroneous conclusions about charge separation in these states. This resolves a long standing debate between high level quantum chemistry studies on small clusters and periodic calculations which simulate the bulk crystal. Another theoretical methodology used to study charge separated states and multiexciton states (characterized by excitations in multiple centres) in systems like pentacene, is provided by the restricted active space spin-flip (RAS-SF) family of methods. However, in it's current framework, these can only be conducted in the vacuum phase. However, charge separated states are known to interact strongly with a solvent,which can differentially stabilize the different species involved in such reactions, and hence impact the reaction mechanism. We have thus adapted this method to include the effects of non-equilibrium solvation, which is an accurate method of including solvent polarization to excited states , when the change in the charge density of a system is substantial (such as in charge separated states). We have benchmarked this against systems known for their charge separated excited states, and demonstrated the asymptotic trends that the excitation energies are expected to follow with increasing distance of charge separation. Another category of excited states that we have studied are excitations originating from core electrons. The calculation of these via more conventional techniques is prohibitive due to the high cost and memory requirements, and we have demonstrated the utility of using the time dependent Kohn Sham (TDKS) approach developed in our group for the calculation of these. This TDKS implementation , which includes the use of complex absorbing potentials (CAPs) and Pade accelerated transforms offer significant advantages over an implementation that does not use these, as it significantly reduces the total time required to obtain a converged spectra. We further demonstrate this cost-effective TDKS approach by application to a copper dithiolene complex, where binding of a ligand is detectable via shifts in the sulfur K-edge.
John Herbert (Advisor)
113 p.
Chemistry; Physical Chemistry

Recommended Citations

Citations

  • Alam, B. (2021). Quantum Chemical Studies of Charge-Separated States and X-Ray Absorption Spectroscopy with Time Dependent Kohn-Sham Theory [Doctoral dissertation, Ohio State University]. OhioLINK Electronic Theses and Dissertations Center. http://rave.ohiolink.edu/etdc/view?acc_num=osu162947841641183

    APA Style (7th edition)

  • Alam, Bushra. Quantum Chemical Studies of Charge-Separated States and X-Ray Absorption Spectroscopy with Time Dependent Kohn-Sham Theory. 2021. Ohio State University, Doctoral dissertation. OhioLINK Electronic Theses and Dissertations Center, http://rave.ohiolink.edu/etdc/view?acc_num=osu162947841641183.

    MLA Style (8th edition)

  • Alam, Bushra. "Quantum Chemical Studies of Charge-Separated States and X-Ray Absorption Spectroscopy with Time Dependent Kohn-Sham Theory." Doctoral dissertation, Ohio State University, 2021. http://rave.ohiolink.edu/etdc/view?acc_num=osu162947841641183

    Chicago Manual of Style (17th edition)

osu162947841641183
151
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This open access ETD is published by The Ohio State University and OhioLINK.