Theoretical investigation of solvatochromic effects and emission from TICT states through Becke’s exciton model

Dutta, Adrija (2023) Theoretical investigation of solvatochromic effects and emission from TICT states through Becke’s exciton model. Masters thesis, Indian Institute of Science Education and Research Kolkata.

Text (MS dissertation of Adrija Dutta (18MS183]) Thesis_18MS183.pdf - Submitted Version Restricted to Repository staff only Download (4MB)

Abstract

KS-DFT has evolved as a powerful tool for calculating the ground-state properties of a system.However, there are serious challenges when it is applied to excited states, especially those that cannot be represented by a single Slater determinant. The lowest excited singlet state of various organic chromophores, dyes, and other substances which have significant interest in photochemistry belong to this category. TDDFT has a favorable trade-off between accuracy and computational cost, and it is thus the most popular framework for extracting the excited state properties of a system. Recently, a time-independent KS-DFT scheme known as Becke’s exciton model was proposed, whose success critically depends on the accurate evaluation of the so-called two-electron Kif integral (Coulumb self energy of product of transition orbitals). However, due to the presence of a singularity in the position space, the precise computation of the Kif integral is a challenging task. Therefore, the Kif integral is dealt with by applying the Fourier convolution theorem in the cartesian coordinate grid. In light of the success of this scheme for determining the excitation energies of small molecules as well as large charge transfer organic chromophores, the current work assesses the applicability of this scheme for determining solvatochromic shifts based on environmental polarity. Excitation, as well as emission energies, of these exotic chromophores have been estimated within this TIKS-DFT framework using the CCG as implemented in InDFT. The method is applied to treat emission energies from states of distorted geometry in a specific class of organic chromophores that display twisted intramolecular charge transfer in their excited state as a response to various environmental variables. In most cases, the GGA type B3LYP functional is used, though the range-separated CAM-B3LYP functional has been investigated for performance enhancements in certain systems. Successful agreement with experimental results relies on isolating the correct geometry of the molecule, displaying the vertical transition. A close examination of the results demonstrates the model’s viability, with a 0.5 eV mean deviation from reference experimental data. Hence proving the success of the framework, especially considering the drastic reduction of computational cost it offers. In short, we have demonstrated the viability of a novel KS-DFT procedure for evaluating the transition energies of various organic chromophores within the framework of InDFT as implemented in our laboratory.

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