Redox Control of Photoinduced Electron Transfer in Axial Terpyridoxy Porphyrin Complexes

The photophysical properties of axial-bonding types (terpyridoxy)aluminum(III) porphyrin (Al(PTP)), bis(terpyridoxy)tin(IV) porphyrin (Sn(PTP)2), and bis(terpyridoxy)phosphorus(V) porphyrin ([P(PTP)2]+) are reported. Compared with their hydroxy analogues, the fluorescence quantum yields and singlet-state lifetimes were found to be lower for Sn(PTP)2 and [P(PTP)2]+, whereas no difference was observed for Al(PTP). At low temperature, all of the compounds show spin-polarized transient electron paramagnetic resonance (TREPR) spectra that are assigned to the lowest excited triplet state of the porphyrin populated by intersystem crossing. In contrast, at room temperature, a triplet radical-pair spectrum that decays to the porphyrin triplet state with a lifetime of 175 ns is observed for [P(PTP)2]+, whereas no spin-polarized TREPR spectrum is found for Sn(PTP)2 and only the porphyrin triplet populated by intersystem crossing is seen for Al(PTP). These results clarify the role of the internal molecular structure and the reduction potential for electron transfer from the terpyridine ligand to the excited porphyrin. It is argued that the efficiency of this process is dependent on the oxidation state of the metal/metalloid present in the porphyrin and the reorganization energy of the solvent.