Magnetic field effects and time-resolved EPR studies on the photogenerated biradicals in phenothiazine–C 60 linked systems: Clarification of the mechanism of novel magnetic field effects by dependences of methylene chain length and temperature

Abstract

Magnetic field effects (MFEs) on photogenerated biradical in phenothiazine (Ph)–C 60 linked compound with four methylene group (Ph(4)C 60 ) have been investigated. Transient absorption spectra showed photoinduced intramolecular electron-transfer reactions from the Ph to triplet excited state of C 60 ( 3 C 60 ∗ ) . 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)C 60 was clearly different from those in other linked compounds ( Ph( n)C 60 ( 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 ∣2 J∣. In time-resolved EPR measurements, the spectra in Ph( n)C 60 ( 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.