Photochemical reduction of water-soluble fullerene C 60(C 4H 10N +) in titanium dioxide suspensions (EPR-study): a comparison with radiolytically-induced reduction

Abstract

N-methylfulleropyrrolidinium (C 60(C 4H 10N +)/I −) was found to be efficiently reduced in photoexcited TiO 2 suspensions in a toluene/ acetonitrile/2-propanol (10:10:1 v/v) solvent mixture. The formation of the π-radical anion, (C 60 −) (C 4H 10N +), was monitored by in situ EPR spectroscopy. Despite the fullerene's functionalization, the reduction process proceeds analogously to that reported earlier for pristine C 60. In aqueous solutions, the behaviour of C 60(C 4H 10N +)/I − towards reduction is, however, very complex. Although the N-methylfulleropyrrolidinium is sufficiently water-soluble, no reduction could be observed in irradiated aqueous TiO 2 suspension. This is attributed to the formation of fullerene clusters in aqueous solutions which seems to prevent transfer of an electron. Addition of a surfactant, such as lauryl sulfate, successfully inhibited fullerene clustering and, in turn after prolonged irradiation (C 60 −) (C 4H 10N +) was observed in low yields. Using γ-cyclodextrin (γ-CD) encapsulated fullerene, C 60(C 4H 10N +)/γ-CD, instead of the surfactant capped fullerene, the fullerene's π-anion-radical was formed immediately. These observations underline the dominant role of cluster formation with respect to reduction and other processes, in general, of water-soluble monofunctionalized fullerene derivatives. A considerably lower activity was also noticed for the reduction of C 60(C 4H 10N +)-clusters by (CH 3) 4 .C(OH) radicals relative to the analogous reduction of C 60(C 4H 10N +)/γ-CD. This emerged from competitive experiments with applying 5,5-dimethyl-1-pyrroline- N-oxide (DMPO) as a spin trap in aqueous TiO 2 suspensions, containing 2-propanol.