Magnetic field effects and time-resolved EPR studies on the photogenerated biradicals in phenothiazine-C60 linked systems: Clarification of the mechanism of novel magnetic field effects by dependences of methylene chain length and temperature

Magnetic field effects (MFEs) on photogenerated biradical in phenothiazine (Ph)-C60 linked compound with four methylene group (Ph(4)C60) have been investigated. Transient absorption spectra showed photoinduced intramolecular electron-transfer reactions from the Ph to triplet excited state of C60(3C60∗). With increasing magnetic field, the decay rate constant of the photogenerated biradical decreased steeply at lower magnetic fields (<0.2 T), and then recovered in the 0.2 T < H < 1.0 T region. Temperature dependence on the reverse phenomenon of the MFEs in Ph(4)C60 was clearly different from those in other linked compounds (Ph(n)C60(n = 6, 8, 10, 12)). The present MFEs can be explained by the contribution of not only spin-lattice relaxation mechanism but also spin-spin relaxation mechanism related to ∣2J∣. In time-resolved EPR measurements, the spectra in Ph(n)C60(n = 4-12) are assigned to spin-correlated radical pairs. The methylene chain dependence of the time-resolved EPR spectra also supports the mechanism suggested in MFEs.