Breakdown and Salvation of Statistical Rate Theory in Biomolecular Photoinduced Electron Transfer

Quantifying rates of photoinduced electron transfer (PET) is crucial to understand the remarkable efficiency of biological photosystems involved in photosynthesis and DNA repair. The ability to distinguish between specific PET channels and compare their timescales with competing photophysical events is also essential for the design synthetic light harvesting systems. However, commonly used statistical rate theories often break down owing to strong electronic coupling and the nonequilibrium nature of PET. Experimental tracking of particular photophysical processes with time-resolved techniques also remains a notorious challenge. Here, we employed surface-hopping nonadiabatic excited-state dynamics simulations and statistical rate theories to characterize ultrafast PET in a dimer of stacked adenine nucleobases. We show that the widely used classical Marcus Theory and Fermi's Golden Rule fail to describe ultrafast PET or even reproduce qualitative rate trends. Instead, we propose a chromophore-localized variant of nonadiabatic Rice-Rampsperger-Kassel-Marcus (NA-RRKM) theory, which yields PET timescales that are in excellent agreement with excited-state dynamics simulations. We argue that by accounting for the nonequilibrium nature of PET, the NA-RRKM approach offers strong predictive capacity for the design of efficient photoredox-active chromophores.