Natural and engineered charge-transfer and diradical state mixing and retinal chromophore dynamics

Recent experimental and computational results show that a second singlet electronic state may be transiently populated during the photoisomerization of retinal chromophores. Such a state is dominated by a doubly excited electronic configuration, and it is qualitatively different from the spectroscopically allowed state featuring a dominant singly excited configuration and charge-transfer character. In this contribution, we review and discuss the consequences of the mixing of these two states on the photoinduced dynamics of different models of the retinal chromophore. In doing so, we consider the results of simulations carried out using quantum-classical trajectories driven by Born-Oppenheimer forces, as well as using methods where both electron and nuclear motions are simultaneously propagated with the quantum Ehrenfest approach. We conclude that electronic state mixing is a viable strategy for modulating the retinal chromophore dynamics and reactivity in both natural and artificial systems.