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.