Calculating Absorption and Fluorescence Spectra for Chromophores in Solution with Ensemble Franck-Condon Methods

01 July 2024, Version 2
This content is a preprint and has not undergone peer review at the time of posting.

Abstract

Accurately modeling absorption and fluorescence spectra for molecules in solution poses a challenge due to the need to incorporate both vibronic and environmental effects, as well as the necessity of accurate excited state electronic structure calculations. Nuclear ensemble approaches capture explicit environmental effects, Franck-Condon methods capture vibronic effects, and recently introduced ensemble-Franck-Condon approaches combine the advantages of both methods. In this study, we present and analyze simulated absorption and fluorescence spectra generated with combined ensemble-Franck-Condon approaches for three chromophore-solvent systems and compare them to standard ensemble and Franck-Condon spectra, as well as to experiment. Employing configurations obtained from ground and excited state ab initio molecular dynamics, three combined ensemble-Franck-Condon approaches are directly compared to each other to assess the accuracy and relative computational time. We find that the approach employing an average finite-temperature Franck-Condon lineshape generates spectra nearly identical to the direct summation of an ensemble of Franck-Condon spectra at one-fourth of the computational cost. We analyze how the spectral simulation method, as well as the level of electronic structure theory, affects spectral lineshapes and associated Stokes shifts for 7-nitrobenz-2-oxa-1,3-diazol-4-yl (NBD) and Nile Red in dimethyl sulfoxide (DMSO), and 7-methoxy coumarin-4-acetic acid (7MC) in methanol. For the first time, our studies showcase the capability of combined ensemble-Franck-Condon methods for both absorption and fluorescence spectroscopy and provide a powerful tool for simulating linear optical spectra.

Keywords

Absorption
Fluorescence
Spectra
Molecules
Vibronic
Environmental effects
Excited state
Electronic structure calculations
Nuclear ensemble
Franck-Condon methods
Ensemble-Franck-Condon approaches
Chromophore-solvent systems
Computational time
Spectral simulation method
Stokes shifts
emission
spectroscopy
ab inito
molecular dynamics
force fields
GPUs
high-throughput
hydrogen bonding
parameterization
charges
RESP
ChelpG
implicit solvent
linear response
dft
TDDFT
LC-wHPBE
M06-2X
geometry optimization
frequency
TDA
vertical excitation energies
deexcitation energies
dynamics

Supplementary materials

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Title
Supplementary Material: Calculating Absorption and Fluorescence Spectra for Chromophores in Solution with Ensemble Franck-Condon Methods
Description
The Supplementary Material is available free of charge and provided with the article. It contains information on MD and AIMD simulation setup, FC lineshape averaging procedure, experimental spectra transformation to wavenumbers and lineshapes, and unshifted spectral lineshapes of all dyes. For each dye, there are graphs showing VEEs and VDEs across snapshots, pictorial representation of E-FC methods for NR and 7MC, graphs comparing full TDDFT and TDA, as well as DFT functional comparison for vibronic lineshapes.
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Supplementary weblinks

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