Modeling the Effect of H-Bonding of Excited Quadrupolar Molecules with a Solvent on Charge Transfer Symmetry Breaking

A model of the H-bonding effect on charge transfer symmetry breaking in excited quadrupolar dyes is proposed. A dye is assumed to have two symmetrically arranged H-bond acceptors. The effect of H-bonding is described in terms of two quantities: (i) the free energy of H-bond formation by an excited symmetric dye with a protic solvent and (ii) a parameter that determines the susceptibility of the H-bond strength to the charge of the H-bond acceptor. The model assumes that an increase in dye dissymmetry results in a charge shift from one acceptor to the other, making one acceptor more negative and the other less negative. As a result, the strength of the H-bond on one branch increases, while on the other branch, it decreases. An analytical solution of the mathematical model is obtained. Regardless of the strength of the H-bond, the effect of the H-bonding on the symmetry breaking degree is shown to be small, as long as the free energy of formation of the weaker H-bond is negative. A strong effect is expected only if this free energy becomes positive, that is, when the H-bond is formed on only one arm. An unexpected result of weakening of the dissymmetry degree caused by the strengthening of the H-bond is predicted and discussed. An approach for quantitative determination of the susceptibility of the H-bond strength to the charge of the H-bond acceptor is proposed.