Photoinduced electron-transfer and electron-mediating processes of fullerenes and phenothiazine oligomers in a polar solvent

Photoinduced electron transfer between fullerenes (C₆₀ and C₇₀) and three phenothiazine oligomers ((PTZ)_ns; n = 1, 2, and 3) was studied by using a transient absorption method in the visible and near-IR regions. Electron transfer took place from (PTZ)_ns to the triplet states of fullerenes (³C₆₀* and ³C₇₀*), giving the radical anions of fullerenes (C₆₀^•-) and C₇₀^•) and the radical cations of (PTZ)_ns, which exhibited strong absorptions in visible and near-IR region for n = 2 and 3. From these absorptions, positions of the radical cation in (PTZ)₂^•+ and (PTZ) ₃^•+generated by electron donation were also determined. Rate constants and quantum efficiencies of the electron transfer were found to be quite high, because of the strong electron-donating abilities of (PTZ)_ns; the order of the rates of electron transfer to ³C₆₀^* and ³C₇₀ ^* is (PTZ)₁ > (PTZ)₂ = (PTZ) ₃, which corresponds to the decreasing order of their oxidation potentials. Upon addition of hexylviologen dication (HV²⁺), an electron-mediating process occurred from C₆₀^•to HV²⁺, yielding the viologen radical cation (HV^•+). In the presence of a sacrificial donor, HV^•+ persisted for a long time.