Time-resolved EPR studies on magnetic interactions between excited triplet (tetraphenylporphinato) zinc and doublet nitroxide radical

By time-resolved electron paramagnetic resonance (TREPR), four (tetraphenylporphinato) zinc (ZnTPP) complexes coordinated by an axial ligand containing a nitroxide radical (NRX; X = 4, 5, 8, and 10, denotes the bond number from zinc to nitroxide nitrogen) have been studied in terms of magnetic interactions between the photoexcited triplet state of ZnTPP and NRX. The TREPR spectrum of ZnTPP coordinated by NR10 is almost the same as the one of ZnTPP coordinated by pyridine, indicating that the electron exchange interaction, J, between ZnTPP and the doublet nitroxide is negligibly small. On the other hand, TREPR spectra of the NR4 and NR5 complexes are assigned to the Q₁ state constituted by the ZnTPP and the nitroxide radical. In the case of the ZnTPP-NR8 complex, both T₁ and Q₁ TREPR signals are seen, which may originate from two conformations or degenerate T₁ states of ZnTPP. This EPR study is useful for understanding the photophysical and photochemical properties of chromophores.