Enhanced intersystem crossing in three-spin systems: a perturbation theory treatment.

We present a simple model for analyzing the spin dynamics of a three-spin system representing a photoexcited chromophore coupled to a stable radical species. Perturbation theory yields a Fermi's Golden Rule-type rate expression that describes the formation of a local triplet on the chromophore through spin exchange with the radical. The error introduced by perturbation theory is evaluated for a number of parameters. Finally, we explore the effect of different energetic and coupling parameters on the rate of triplet formation and suggest how this model can be used to tune the enhanced intersystem crossing in three-spin systems.