On the nature of charge-transfer excitations for molecules in aqueous solution: a polarizable QM/MM study

We illustrate the effect of solvation on the nature of electronic excitations of organic molecules which possess excited states of charge-transfer character. The analysis is carried out using both a continuum model and a polarizable QM/MM method that treats the solvent atomistically and embeds each atom in the solvent with a fluctuating charge which responds to the solute quantum-mechanical electrostatic potential in a self-consistent manner. We also show how solvation dynamics can influence the nature of the excited state of molecular systems. The application of the model to aqueous solutions of doxorubicin and a substituted polythiophene derivative shows that the solvent significantly affects the nature the excited states, which results in an enhanced or reduced charge-transfer character as measured using two of the most popular indices for evaluating the distance traveled by the electrons upon excitation.