Reverse phenomena of magnetic field effects and time-resolved EPR spectra in the photogenerated biradical from intramolecular electron-transfer in a phenothiazine-C_(60) linked compound with a semi-rigid spacer

摘要

Photoinduced electron-transfer reactions and magnetic field effects (MFEs) on the decay rates of the photogenerated biradical in a phenothiazine (Ph)-C_(60) linked compound with a biphenyl group (Ph(BP)C_(60)) were examined in benzonitrile and benzene. Fluorescence and transient absorption spectra indicate that the intramolecular electron-transfer for Ph(BP)C_(60) from the Ph to the singlet or triplet excited state of C_(60) 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)C_(60) were strongly enhanced with increasing temperature and similar to those in Ph(n)C_(60) (n = 6-12). The MFEs in Ph(BP)C _(60) can be governed by spin-lattice relaxation and/or spin-spin relaxation mechanisms as observed in Ph(n)C_(60) (n = 6-12). Time-resolved EPR spectra of Ph(BP)C_(60) showed absorption, emission, absorption and emission patterns, and are quite different from those in Ph(n)C_(60) (n = 4-12). The result indicates that the magnitude and distribution of the exchange interaction |2J| in Ph(BP)C_(60) are smaller than those in Ph(n)C_(60) (n = 4-12) and charge recombination occurs in the inverted region because the sign of the J is positive.