Independence and inverted dependence on temperature of rates of photoinduced electron transfer in double-linked phthalocyanine-fullerene dyads

Photoinduced electron transfer reactions of phthalalocyanine-fullerene dyads, in which donor and acceptor moieties are covalently linked to each other, with one or two malonic linkers, were studied. In the dyads with two linkers, phthalocyanine and fullerene have mutual orientations, face-to-face or face-to-tail, which differ from each other and influence photoinduced electron transfer processes. Quantitative spectroscopic and time-resolved spectroscopic measurements were done in polar and non-polar solvents at room temperature and at several reduced temperatures. The emission spectra of the double-linked dyads were different from that of the reference phthalocyanine showing a shoulder at the red part of the spectrum, emission decays were two-exponential and emission lifetimes depend on monitoring wavelengths. These facts support, for the dyads with two linkers, the formation of an emissive intramolecular exciplex preceding the charge separated state. For these dyads the formation times of the charge separated state, approximately 0.4 ps and 0.8 ps for the face-to-face and face-to-tail isomers, respectively, were independent of temperature and the reaction is considered to be quantum tunneling in nature. The charge recombination times were temperature dependent, but decreased with the decrease of temperature from roughly 1.2 ns at room temperature to 0.7 ns at 190 K.