A solvent–solute cooperative mechanism for symmetry-breaking charge transfer

Abstract

Symmetry-breaking charge transfer (SBCT) is an important process at the early stages of the photoinduced processes in multichromophore systems such as the photosynthetic apparatus. We investigated the photoinduced SBCT dynamics of 9,9′-bianthracene (BA), a representative molecule showing SBCT, by time-resolved fluorescence (TF) with the highest time-resolution and excited-state quantum mechanics/effective fragment potential molecular dynamics (MD) simulation. TF experiments show that the SBCT kinetics matches quantitatively with the solvation function excluding the initial ultrafast component that is assigned to the inertial motion of the solvent. Therefore, it is established that the SBCT of BA is coupled solely with the rotational diffusion of solvent molecules excluding the inertial motion of solvents. MD simulations show that random rotational fluctuation of solvents mostly in the first solvation shell generates a transient electric field as high as 1.0 × 10⁹ V m⁻¹, which provides an asymmetric environment required for the generation of a CT state in this symmetric dimer. Once the CT state is formed, the dipole moment in the solute causes further rotation of solvent molecules leading to an augmented electric field, which in turn further stabilizes the CT state prohibiting the reverse reaction.