Synthesis of [60]fullerene derivatives bearing five-membered heterocyclic wings and an investigation of their photophysical kinetic properties

The preparation, characterization and photophysical properties of six new stable [6,6]-closed fullerene cycloadducts bearing five-membered heterocycles are described. The modified [60]fullerenes are obtained by a simple and rapid synthesis via a Bingel-type reaction with tetrazole and oxadiazole malonate derivatives. The photophysical kinetics of these new fullerene derivatives in toluene solution under ultraviolet illumination (375 nm, UVA) are studied by electron paramagnetic resonance and free-radical spin-trapping using α-phenyl-N-tert-butyl nitrone as a spin-trap. The results are compared with pure [60]fullerene and [6,6]-phenyl C₆₁ butyric acid methyl ester (C₆₀-PCBM). It is concluded that for all six new compounds as well as pure [60]fullerene and PCBM both superoxide and singlet oxygen are produced in the first stages of UVA illumination following the type I and II mechanisms, respectively. In all cases singlet oxygen is produced as the primary dominant species; however, the type I mechanism always occurs in parallel with type II. In the end, the superoxide is self-dismuted into hydroxyl radicals, thus yielding PBN-OH spin adducts (g = 2.007 and a_hf (¹⁴N) = 1.54 mT). The kinetic reaction constants and their efficiencies in the production of reactive oxygen species at 375 nm and per mW of absorbed power are determined. The experimental results are consistent with an autocatalytic reaction model in which the system evolutes under UVA illumination, with superoxide catalyzing the conversion of singlet oxygen into more superoxide.