The fate of molecular excited states: modeling donor–acceptor dyes

Abstract

We investigate the relaxation of a coherently excited molecule in the Redfield approximation. The molecular model, parametrized to describe donor–acceptor dyes that represent a large family of molecules of interest for several applications, accounts for two diabatic electronic states non-adiabatically coupled to a few vibrational coordinates. The proposed approach successfully describes the fast vibrational relaxation, followed by a much slower relaxation towards the ground state, a physically relevant result that is robust vs. the specific model adopted for the system–bath coupling and the specific (reasonable) choice of the bath spectral density. We demonstrate that, when dealing with more than a single vibration, it is important that each vibration is separately coupled to an independent bath so as to avoid the cross-talking of the modes through their coupling to the same bath. Provided that the overall strength of the electron–vibration coupling is maintained constant, the number of molecular vibrations introduced in the model does not affect the system dynamics, supporting the use of effective and easy models for donor–acceptor dyes accounting for a single coupled vibration.