Ab Initio modelling of photophysical properties of light harvesting molecules
Creators: name: Manian, Anjay
Publisher: name: RMIT University
Year: 2024
Subjects: classificationCode: 510201
subject: Atomic and molecular physics
classificationCode: 510403
subject: Condensed matter modelling and density functional theory

description: In this thesis, I work towards developing a model which can predict all quantum chemical aspects of any given chromophore, to help optimise the synthesis and operation considerations of photon harvesting devices. Using Density Functional Theory based methods to minimise computational complexity while maximising theoretical accuracy, I first examine the formalism to compute relaxation rates within both Franck-Condon and Herzberg-Teller regimes of naphthalene, anthracene, tetracene, pentacene, indole, diketo-pyrrolo-pyrrole, and perylene diimide. An approximation used to estimate second order corrected fluorescence yields positive results for systems in which second order terms are not dominant. Non-radiative rates for indole are all overestimated, due to an under-described solvent model. Second order corrected photoluminescence quantum yields terms are found to be 0.33, 0.19, 0.08, 0.04, 0.21, 0.70, and 0.99 for each chromophore respectively, highlighting the flexibility and power of my model. The efficacy of various solvent models is then investigated, as per the results obtained for indole. Two solvent models are used to predict photophysical properties for both implicit and explicit solutes. It is demonstrated that implicit models overestimate energies and oscillator strengths, with indole-water complexes not displaying energetic level inversion of low lying excited states, suggesting a dielectric medium on its own is insufficient. By contrast, energies are seen to stabilise fairly rapidly for explicit models, with respect to cluster size. Following variation of parameters, an optimum functional and basis set which can adequately match experiment are noted. For indole-water, solvent reorganisation is shown to significantly effect the stabilisation of excited state energies. I then examine molecular aggregation using bulky perylene diimide derivatives, finding that for chromophores prone to aggregation, optically accessible excitations are in close energetic proximity to low-lying triplet charge transfer states, thus facilitating additional intersystem crossing pathways. Evidence was also found of an indirect singlet fission pathway, mediated by intermediary charge transfer states. Finally, excitonic kinetics that may occur within a typical luminescence solar concentrator device were modelled. Limited to singlet exciton dynamics, a rejection-free kinetic monte carlo model was used to show that perylene diimide type substrates display exalted singlet exciton transport properties in comparison to tetracene. Further, substrates with dipole-aligned chromophores are characterised by anisotropic exciton diffusion, with slightly improved transport properties. Following, a hetero-substrate was found to display perylene diimide mediated transport properties due to the asymmetry in relaxation rates with respect to tetracene. While by no means complete yet, the work reported herein presents an important stepping stone in the search for a tool to predict properties of larger, coupled systems. It is hoped that this work aids in the future development of efficient photon harvesting technologies.