Modeling solvation effects on absorption and fluorescence spectra of indole in aqueous solution

Abstract

Modeling the optical spectra of molecules in solution presents a challenge, so it is important to understand which of the solvation effects (i.e., electrostatics, mutual polarization, and hydrogen bonding interactions between solute and solvent molecules) are crucial in reproducing the various features of the absorption and fluorescence spectra and to identify a sufficient theoretical model that accurately captures these effects with minimal computational cost. In this study, we use various implicit and explicit solvation models, such as molecular dynamics coupled with non-polarizable and polarizable force fields, as well as Car–Parrinello molecular dynamics, to model the absorption and fluorescence spectra of indole in aqueous solution. The excited states are computed using the equation of motion coupled cluster with single and double excitations combined with the effective fragment potential to represent water molecules, which we found to be a computationally efficient approach for modeling large solute–solvent clusters at a high level of quantum theory. We find that modeling mutual polarization, compared to other solvation effects, is a dominating factor for accurately reproducing the position of the peaks and spectral line shape of the absorption spectrum of indole in solution. We present an in-depth analysis of the influence that different solvation models have on the electronic excited states responsible for the features of the absorption spectra. Modeling fluorescence is more challenging since it is hard to reproduce even the correct emitting state, and force field parameters need to be re-evaluated.

The quantum chemical solvation of indole: accounting for strong solute–solvent interactions using implicit/explicit models

“…While significantly more expensive, we can exactly treat for both mutual polarisation of the solute-solvent mixture, in addition to direct charge interactions. [11][12][13] Common quantum chemical practice dictates that the computation of excitation energies is governed by one of two methods: a state specific method or a linear response method. The former solves the nonlinear Schro ¨dinger equation for only the requested state, and calculates transition energies as differences of the total solute-solvent free energy.…”

Modeling the Electronic Absorption Spectra of the Indocarbocyanine Cy3

“…For that reason, the FC approach cannot be used to model the spectrum of bound-to-continuum transitions. Ensemble methods are also likely more appropriate in systems where electronic states rather than vibrational levels are responsible for the spectral shapes [97].…”

Effective Fragment Potentials for Microsolvated Excited and Anionic States

“…The Journal of Physical Chemistry A which were published in previous work. 37 In one MD simulation all bonds of the indole solute and water molecules were constrained (C) and in the other they were unconstrained (UC), free to vibrate. In those MD simulations a nonpolarizable force field was used.…”

“…Ten configurations of indole·4H 2 O were extracted from two molecular dynamics (MD) simulations, which were published in previous work . In one MD simulation all bonds of the indole solute and water molecules were constrained (C) and in the other they were unconstrained (UC), free to vibrate.…”