Sean McConnell, Seth Olsen, and I just finished a paper
A Valence-Bond Nonequilibrium Solvation Model for a Twisting Cyanine Dye
We study a two-state valence-bond electronic Hamiltonian model of non-equilibrium solvation during the excited-state twisting reaction of monomethine cyanines. These dyes are of interest because of the strong environment-dependent enhancement of their fluorescence quantum yield that results from suppression of competing non-radiative decay via twisted internal charge-transfer (TICT) states. For monomethine cyanines, where the ground state is a superposition of structures with different bond and charge localization, there are two twisting pathways with different charge localization in the excited state. The Hamiltonian designed to be as simple as possible consistent with a few well-enumerated assumptions. It is defined by three parameters and is a function of two -bond twisting angle coordinates and a single solvation coordinate. For parameters corresponding to symmetric monomethines, there are two low-energy twisting channels on the excited-state surface that lead to a manifold of twisted intramolecular charge-transfer (TICT) states. For typical monomethines, twisting on the excited state will occur with small or no barrier. We show that changes in the solvation configuration can differentially stabilize TICT states in channels corresponding to different bonds, and that the position of a conical intersection between adiabatic states moves in response to solvent to stabilize either one channel or the other. We show that there is a conical intersection seam that grows along the bottom of the excited-state potential with increasing solvent polarity. For solvents of even moderate polarity, we predict that the intersection seam should completely span the bottom of the excited-state potential in these systems.
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