Solvation dynamics and electronic structure development of coumarin 120 in methanol: A theoretical modeling study

Abstract

Electronic structure evolution of an organic dye coumarin 120 coupled to polar solvation dynamics is examined by combining ab initio electronic structure calculations and molecular dynamics (MD) simulations. Sets of nonorthogonal Hartree–Fock molecular orbitals optimized in vacuo and in dielectric continuum are utilized for a quantum mechanical description of the solute electronic polarization coupled to the solvent fluctuation. The adiabatic MD simulation for methanol solution is performed to evaluate the equilibrium and nonequilibrium dynamics of the ${\mathrm{(S}}_0{\mathrm{–S}}_1)$ energy gap coordinate and the dipole moments. The absorption and fluorescence spectra are computed via the spectral density functions obtained from the simulation analysis. The results for the quantum polarizable (Q-Pol) model of the solute probe are compared with those for a nonpolarizable fixed-charge (Fix-Z) model. It is shown that the solute electronic polarization notably affects the solvent-induced key quantities such as the reorganization energy, the spectral linewidth, and the Stokes shift (which are mutually related): For example, the computed Stokes shifts are ∼2500 and $\text{∼4000\hspace{0.5em}}{\mathrm{cm}}^{\mathrm{-1}}$ for Fix-Z and Q-Pol, respectively. On the other hand, the solute polarization tends to slightly slow down the methanol solvation, which is not necessarily attributed to reduction of the “solvent force constant” because the effective mass of the coordinate is reduced as well.