Absorption Spectra of Flexible Fluorescent Probes by a Combined Computational Approach: Molecular Dynamics Simulations and Time-Dependent Density Functional Theory.

A detailed understanding and interpretation of absorption spectra of molecular systems, especially in condensed phases, requires computational models that allow their structural and electronic features to be connected to the observed macroscopic spectra. This work is focused on modeling the electronic absorption spectrum of a fluorescent probe, namely, the 9-(4-((bis(2-((2-(ethylthio)ethyl)thio)ethyl)amino)methyl)phenyl)-6-(pyrrolidin-1-yl)-3 H -xanthen-3-one molecule, depicted by a combined classical-quantum chemical approach. Particularly, first classical molecular dynamics (MD) has been used to explore the configurational space, and next, the absorption spectrum has been reconstructed by averaging the results of time-dependent density functional theory (TD-DFT) calculations performed on equispaced molecular conformations extracted from MD to properly sample the configurational space explored at finite temperature. To verify the effect of molecular conformation on the spectral profile, the generated electronic absorption spectra were compared with those obtained considering a single structure corresponding to the optimized one, an approach also referred to as static. This comparison allows one to highlight a sizable though small shift between the maxima of the corresponding reconstructed absorption spectra, highlighting the importance of conformational sampling in the case of this rather flexible molecule. Four different exchange and correlation functionals (PBE, BLYP, PBE0, B3LYP) were considered to compute vertical transition via TD-DFT calculations. From the results obtained in gas and in condensed, here solution, phases, it appears that the magnitude of the shift is actually more affected by the phase in which the system is found than by the functional used. This fact underlines the central importance of conformational mobility, that is flexibility, of this molecule. From a more quantitative point of view, a comparison with available experimental data shows that hybrid functionals, such as PBE0 and B3LYP, enable one to faithfully reproduce the observed absorption maxima.