Spin dynamics in strongly coupled spin-correlated radical pairs: Stochastic modulation of the exchange interaction and ST−1 mixing in different magnetic fields

Abstract

We investigate the effect of stochastic modulation of the exchange interaction Jex on singlet (S)-triplet (T) transitions in radical pairs. These transitions limit the lifetime of the photo-generated radical pairs in covalently linked porphyrin-quinone systems that have been developed for biomimetic modeling of photosynthetic electron transfer processes. In order to explain transient electron paramagnetic resonance (EPR) results in different magnetic fields, i.e., with X-band ((0.34 T)/ (9.5 GHz)) and W-band ((3.4 T)/(95 GHz)) time-resolved EPR, we have to assume that Jex is modulated over a range of 20000 G, which is wide enough that S is temporarily almost degenerate with T0 as well as with T−1. This large modulation of Jex is caused by restricted rotational diffusion of the quinone subunit with respect to the porphyrin subunit. However, because of the small interradical distance of about 1.0–1.4 nm, the radical pair is continuously kept in the strong coupling limit and, therefore, we observe only EPR transition between the triplet sublevels. We find an approximation to solve the stochastic Liouville equation valid for rotational diffusion on an intermediate time scale, i.e., the diffusion rate DR is smaller than the singlet electron recombination rate KS ∼ 109 s−1, but larger than the ST transition rates κ0, κ−1 < 106 s−1.