Electronic substitution effect on the ground and excited state properties of indole chromophore: A computational study

11 July 2022, Version 1
This content is a preprint and has not undergone peer review at the time of posting.

Abstract

Indole, being the main chromophore of amino acid tryptophan and several other biologically relevant molecules like serotonin, melatonin, has prompted considerable theoretical and experimental interest. The current work focuses on the investigation of photophysical and photochemical properties of indole and indole derivatives e.g. tryptophan, serotonin and melatonin using theoretical and computational methodologies. Having three close-lying excited electronic states, the vibronic coupling effect becomes extremely important yet challenging for the photophysics and photochemistry of indole. Here, we have used density functional theory (DFT) extensively and evaluated the performance of DFT in compared to available experimental and ab initio results from literature. The benchmarking of the method is followed by investigation of the chemical and geometrical effects of ring substitution in indole. A bathochromic shift has been observed in the HOMO-LUMO gap as well as vertical excitation energy from indole to melatonin. While the contribution of the in-plane small adjacent groups increases the electron density of the indole ring, the out-of-plane long substituent groups have minor effect. The comparison of singlet-triplet gaps suggests highest probability of inter-system crossing for tryptophan which is in line with previous experiment. The absorption spectra calculated including the vibronic coupling are in good agreement with experiment. These results can be used to estimate the error in photophysical observables of indole derivatives calculated considering indole as prototypical system. This study also demonstrates the merits and demerits of using DFT functionals to compute the photophysical properties of indole derivatives.

Keywords

Indole derivatives
chromophore
photophysics
electronic substitution
vibronic coupling
excited states

Supplementary materials

Title
Description
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Title
Electronic substitution effect on the ground and excited state properties of indole chromophore: A Computational Study
Description
Partial charge distribution- effect of substitution, vertical and adiabatic excitation energies.
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