Magnetic field effects on the photoinduced electron-transfer reactions in a ruthenium porphyrin-C₆₀ ligand complex

Magnetic field effects (MFEs) on a photoinduced electron-transfer reaction in a semi-rigid linked system (RuP-C₆₀) were examined in toluene and o-dichlorobenzene. The transient absorption spectra indicated that the photogenerated biradical produced by an intramolecular electron-transfer reaction was observed only in o-dichlorobenzene. The decay rate constants (k_d) for the biradical decreased steeply at lower magnetic fields (≤0.06 T) and rapidly recovered at 0.08 T. The k_d value was observed to decrease gradually above 0.2 T. The MFEs strongly indicated the formation of the singlet-born biradical for RuP-C₆₀. The dip in the MFE at 0.06 T was ascribed to the S-T level crossing mechanism. The MFEs above 0.2 T can be explained by a spin-spin relaxation mechanism due to the anisotropic Zeeman interaction being related to the exchange interaction.