Fluorescent dye molecules are frequently used for bioimaging, in which they undergo solvation. With such solvent environments, it is known that the environmental effects are extremely relevant to simulate the absorption and emission spectra of these molecular systems. Spectral shifts known as Stokes shifts occur due to specific solute-solvent interactions and due to charge separation in the excited state. By changing the conditions, we explored the differences in the fluorescent signal when a solvent is relaxed in the excited state. This would avail in understanding which theoretical solvation models (linear response, LR vs state specific, SS) more accurately reflect experimental relaxation energies and quantum yields. Furtherly, it would aid in the development of a more effective model to quantify florescent quantum yields based on absorption and emission spectrum simulated in equilibrium and non-equilibrium solvation models. Calculations were done using the GPU accelerated program, TeraChem, on the SeaWulf HPC Cluster. Five DFT exchange–correlation hybrid functionals were explored: B3LYP, B3PW91, cam-B3LYP, PBE0, ωB97X. Five different basis sets: 6-311++g, 6-311++g[2d,2p], 6-311gss, aug-cc-pVDZ, and cc-pVDZ were explored. Absorption and emission energies were calculated using the cc-pVDZ basis set with the LR and SS formalisms, in both equilibrium and non-equilibrium models, with the cam-B3LYP functional. Relaxation energies and fluorescent quantum yields were predicted with the solvent at its relaxed state and its Franck Condon point. Results show that the cam-B3LYP functional with 6-311G** basis set displayed the best fit to the experimental data and therefore was the preferred level of theory to simulate the photochemical properties. Furthermore, the LR formalism surpassed the SS solvation model in simulating absorption and emission energies but were unable to effectively simulate relaxation energies. It was found that relaxation energies did not correlate with fluorescence quantum yield prediction.

Computational study of solvation effects on the spectroscopic properties of dyes

ACS Spring 2022

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