Nonequilibrium dynamics of photoinduced forward and backward electron transfer reactions.

The biological functions of photoenzymes are often triggered by photoinduced electron transfer (ET) reactions. An ultrafast backward ET (BET) reaction follows the initial photoinduced forward ET (FET), which dissipates the energy of absorbed photons and terminates the biological function in vain. Based upon our previous works, we reasoned that the dynamics of the BET is coupled with that of the FET and other local motions. In this work, the dynamics of the FET and BET is modeled as the master equation of the reduced density operator of a three-state system coupled with a classical harmonic reservoir. The coupling of the FET and BET is reflected in the time-evolution of the charge-transfer state's population, which is generated by a source, the reaction flux for the FET, and annihilated by a sink, the reaction flux for the BET. Surprisingly, numerical simulations show that when the BET is in the Marcus normal region, the BET can be accelerated by nonequilibrium local motions and becomes faster than what is predicted by the Marcus theory. The experimental confirmation of this novel dynamics would provide qualitative evidence for nonequilibrium effects on ultrafast ET dynamics. Additionally, the effects of quantum vibrational modes on the dynamics are discussed. This work can help understand the dynamical interactions between the chain of ultrafast reactions and the complex local environmental motions, revealing the physical nature underlying biological functions.