Electronic Substitution Effect on the Ground and Excited State Properties of Indole Chromophore: A Computational Study**

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 substitution effect on the ground and excited electronic states of indole using computational quantum chemistry. 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 evaluated the performance of time‐dependent density functional theory against available experimental and ab initio results from the literature. The electronic effects on the excited states of indole and indole derivatives e. g. tryptophan, serotonin and melatonin are reported. A bathochromic shift has been observed in the absorption spectrum for the La state. The absorption wavelength increases in the order of indole<tryptophan <serotonin <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 absorption spectra calculated including the vibronic coupling are in good agreement with experiments. These results can be used to estimate the error in photophysical observables of indole derivatives calculated considering indole as a prototypical system.

High-resolution electronic and vibrational spectroscopy of small-to-medium sized molecules with ab initio potential energy surface

“…In this particular case, while the B3LYP functional reproduces the peak maxima and band-shape of the experimental absorption spectrum quite accurately, LC-BLYP fails to reproduce the peak position and band shape of the experimental absorption spectra. Further discussions on this can be found in [3].…”

“…In this method, the exponential time evolution operator is divided into several parts to reduce the numerical error. The wave packet at time t + ∆t is calculated as: ψ(R, r, θ, t + ∆t) = e −i V(R,r,θ)∆t/2ℏ e −i T(θ)∆t/2ℏ e −i T(R,r)∆t/ℏ e −i T(θ)∆t/2ℏ × e −i V(R,r,θ)∆t/2ℏ ψ(R, r, θ, t) + O(∆t) 3 , (60) where O(∆t) 3 is the error of third order, V(R, r, θ) is the potential energy of the triatomic system, T(θ) and T(R, r) are the angular and radial KE operators, respectively.…”

“…Spectra of conjugated organic chromophores, charge transfer metal complexes, and large biomolecules lie in the UV-visible region. Using the UV-visible spectra of organic chromophores, the influence of the adjacent groups and redox states on the main chromophore framework can be traced [3,4]. Electronic spectroscopy also plays a vital role in charge transfer complexes.…”

“…This molecular ion has been studied via photoelectron spectroscopy as well as different computational approaches [151][152][153][154][155][156][157]. The lowest electronic state with 3 A ′′ symmetry is the ground electronic state of N + 3 ion. In a spectroscopic measurement, it was suggested that the ground state of the N + 3 ion is most stable for a linear centrosymmetric structure [154].…”

Unveiling the Excited State Dynamics of Indole in Solution

“…Even though indole photophysics has been deeply investigated by several authors, both from an experimental − and theoretical point of view, − some unclear aspects still remain. For example, the exact mechanism of the excited electronic state relaxation (energy transfer) in polar solvents, such as water is still a matter of debate.…”