Appl. Magn. Reson. 23, 369-375 (2003) Applied Magnetic Resonance 9 Springer-Verlag 2003 Printed in Austria Time-Resolved EPR Studies on Magnetic Interactions between Excited Triplet (Tetraphenylporphinato) Zinc and Doublet Nitroxide Radical K. Ishii, T. Ishizaki, and N. Kobayashi Dcpartment of Chemistry, Graduatc School of Science, Tohoku University, S•ndai, Japan Received August 29, 2002; revised October 25, 2002 Abstract. By time-resolved electron paramagnetic resonance (TREPR), four (tetraphenylporphinato) zinc (ZnTPP) complexes coordinated by ah 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 spec- trum of ZnTPP coordinated by NR10 is almost the same as the one of ZnTPP coordinated by pyri- dine, 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 Q~ state constituted by the ZnTPP and the nitroxide radical. In the case of the ZnTPP-NR8 complex, both T~ and Q~ TREPR signals are seen, which may originate from two conformations or degenerate T~ states of ZnTPP. This EPR study is useful for understanding the photophysical and photochemical properties of chromophores. 1 lntroduction Interactions between porphyrinic chromophores in the lowest excited triplet state (Ti) and other paramagnetic species result in some important phenom- ena in photophysical and photochemical processes, e.g., excited-state quench- ing [1], formation of excited singlet oxygen [2, 3], and photocontrol of mag- netic properties [4, 5]. While a key feature of these phenomena is the forma- tion of excited multiplet states constituted by the T a chromophore and other paramagnetic species, it had been very difficult to observe the excited mul- tiplet states directly because of the short lifetime and weak luminescence. Recently, time-resolved electron paramagnetic resonance (TREPR) has been shown to be a useful method for investigating this kind of short-lived excited multiplet states, such as the lowest excited doublet (Dl) and quartet (Q~) states constituted by the T a chromophore and the doublet nitroxide radical [6-21].