Photoexcited spin triplet states in zinc phthalocyanine studied by transient EPR

Transient EPR experiments were performed between 4 and 80 K on photoexcited spin triplet states of zinc phthalocyanine molecules in solid state powders and in frozen dilute solution of MeTHF. Polarized triplet states of zinc phthalocyanine with very similar zero field splitting (ZFS) parameters were detected in both matrices. However, the spin polarisation and therefore the populations of the three sublevels of the spin triplet state, were different in the two matrices. In particular, the spin polarisation in frozen solution shows that the spin levels are populated via conventional intersystem crossing. In solid state powders the spin triplet states arise only in the presence of molecular oxygen, and the polarisation is not compatible with the intersystem crossing mechanism. The proposed mechanism is a two-stage electron back-transfer mechanism involving the superoxide anion as an intermediate.