The accurate modeling of absorption and fluorescence spectra of molecules in condensed phase systems is highly challenging, as both interactions between the chromophore and the environment and the coupling of the optical excitation to nuclear degrees of freedom have to be accounted for. Often, optical spectra are simulated through the Franck-Condon principle, where the ground- and excited state potential energy surfaces are approximated as harmonic around their respective minima and the environment is represented through a polarizable continuum model. However, this approach can lead to large errors in modelled spectra in semi-flexible systems or systems with strong environmental interactions. An alternative approach is the cumulant method, where the optical spectra are computed from correlation functions of energy gap fluctuations generated from molecular dynamics (MD) simulations, directly sampling the (generally anharmonic) potential energy surfaces of the ground and excited state. In this talk, we showcase the advantages of the cumulant approach using the example of Crystal Violet, a semi-flexible dye whose absorption lineshape is highly dependent on solvent environment. The absorption spectrum of Crystal Violet shows two prominent peaks, whose distance is dependent on the solvent screening of a counter ion that breaks the degeneracy of the S1 and the S2 excited state. While such strong environmental effects cannot be reproduced in a simple Franck-Condon scheme, the Cumulant approach based on MD correctly predicts the change in the S1-S2 peak splitting between Toluene and Methanol solvent environments, as well as reproducing the overall lineshape. Furthermore, we show that the cumulant methodology can be extended approximately account for Herzberg-Teller effects, by calculating correlation functions of transition dipole moment fluctuations sampled through MD. We demonstrate that the resulting methodology can predict the splitting of the Q-bands in the low-energy absorption spectrum of porphyrin-based molecules in solution, a prominent example of the influence of Herzberg-Teller effects on optical spectra.

Beyond the harmonic approximation: Modeling linear optical spectra using molecular dynamics simulations

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ACS Fall 2022

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.