Electron spin polarization in an excited triplet-radical pair system: Generation and decay of the state

A computational model to simulate electron spin polarization in the three-spin-1/2 system composed of the molecular excited triplet state of (tetraphenylporphinato)zinc(II) (ZnTPP) and the doublet ground state of the 3-(N-nitronyl-notroxide) pyridine (3-NOPy) stable radical is proposed. The model is based on numerical solutions of the stochastic Liouville equation for the diffusively rotating system where the magnetic dipolar, isotropic Heisenberg exchange, and anisotropic Zeeman electron spin interactions are taken into account in a full measure, whereas the intersystem crossing processes between the singlet and triplet states of ZnTPP are considered in terms of kinetic equations for the relevant spin density matrices. Additional longitudinal and transversal paramagnetic relaxation caused by relative rotation motions of the ZnTPP and 3-NOPy moieties is taken into consideration in the form of the generalized relaxation operator.