Long-lived light-induced electron spin polarization in porphyrin triplet states and the dynamic Jahn-Teller effect

The time dependence of the light-induced spin polarization of a series of porphyrins is reported. The porphyrins contain central elements from three distinct groups in the Periodic Table with different oxidation states, types, and numbers of axial ligands, as well as different peripheral substituents on the porphyrin ring. Shortly after photoexcitation, in all cases, the primary multiplet polarization of the porphyrin triplet state evolves into long-lived net polarization whose lifetime is determined by the lifetime of the triplet state. The zero-field splitting parameters and sign of the multiplet polarization vary significantly among the porphyrins, but the transient EPR spectra taken at later times are remarkably similar. The development of long-lived net polarization is proposed to involve the dynamic Jahn-Teller effect, in which hopping between the two lowest triplet states occurs.