Magnetic field effects and time-resolved EPR studies on photogenerated biradical from intramolecular electron transfer reactions in zinc-tetraphenylporphyrin-C₆₀ linked compounds: contribution of relaxation mechanism due to spin–spin relaxation

Abstract

Magnetic field effects (MFEs) and time-resolved EPR on photogenerated biradical from intramolecular electron-transfer reactions in zinc-tetraphenylporphyrin (ZnP)–C₆₀ linked compounds (ZnP(4)C₆₀ and ZnP(8)C₆₀) with flexible four and eight methylene groups have been investigated in benzonitrile. At low temperature (288 K), the decay rate constant of the biradical decreased steeply in low magnetic fields (<0.1 T), and then increased gradually in middle magnetic fields (0.1 ≤ H ≤ 0.6 T) and finally became almost constant in high magnetic fields (0.6 ≤ H ≤ 1 T) in both ZnP(4)C₆₀ and ZnP(8)C₆₀. At high temperature (323 K), in contrast, the decay rate constant decreased slightly in low magnetic fields (<0.1 T), and then increased gradually in higher magnetic fields (0.1 ≤ H ≤ 1 T). Interestingly, the decay rate constants in higher magnetic fields (0.4 ≤ H ≤ 1 T) were larger than that in zero magnetic field. The reverse phenomena of the MFEs around 0.1 T and temperature dependence on the MFEs are explained by the contribution of spin–spin relaxation due to anisotropic Zeeman interaction to the relaxation mechanism and most likely ascribed to the properties of C₆₀ anion radical due to the spherical π-system. The mechanism is also supported by the time-resolved EPR spectra.

Acknowledgements

The authors are grateful to Mr H. Horiuchi for the preparation of quartz cells for transient absorption spectral experiments. The authors also thank The Centre for Advanced Instrumental Analysis, Kyushu University, for ¹H-NMR measurements. The present study was financially supported by grants from the Inamori Foundation and the society for the support of Kyushu University, and by Grant-in-Aids for Scientific Research: Priority Area “Innovative Utilization of Strong Magnetic Fields” (Area 767, No. 15085203) and “Molecular Nano Dynamics” (Area 432, No. 17034051), Scientific Research (C) (No. 17550131), and 21st Century COE Programme “Function Innovation of Molecular Informatics” from MEXT of the Japan.