Reverse phenomena of magnetic field effects and time-resolved EPR spectra in the photogenerated biradical from intramolecular electron-transfer in a phenothiazine-C60 linked compound with a semi-rigid spacer
Photoinduced electron-transfer reactions and magnetic field effects (MFEs) on the decay rates of the photogenerated biradical in a phenothiazine (Ph)-C60 linked compound with a biphenyl group (Ph(BP)C60) were examined in benzonitrile and benzene. Fluorescence and transient absorption spectra indicate that the intramolecular electron-transfer for Ph(BP)C60 from the Ph to the singlet or triplet excited state of C60 was suppressed by the biphenyl group. The decay rates of the photogenerated biradical decreased in the 0-0.2 T magnetic field range and increased in the 0.2-1 T magnetic field range. The reverse phenomena of the MFEs in Ph(BP)C60 were strongly enhanced with increasing temperature and similar to those in Ph(n)C60 (n = 6-12). The MFEs in Ph(BP)C60 can be governed by spin-lattice relaxation and/or spin-spin relaxation mechanisms as observed in Ph(n)C60 (n = 6-12). Time-resolved EPR spectra of Ph(BP)C60 showed absorption, emission, absorption and emission patterns, and are quite different from those in Ph(n)C60 (n = 4-12). The result indicates that the magnitude and distribution of the exchange interaction |2J| in Ph(BP)C60 are smaller than those in Ph(n)C60 (n = 4-12) and charge recombination occurs in the inverted region because the sign of the J is positive.