Photochemical reduction of water-soluble fullerene C60(C4H10N+) in titanium dioxide suspensions (EPR-study):: a comparison with radiolytically-induced reduction

N-methylfulleropynolidinium (C-60(C4H10N+)/I-) was found to be efficiently reduced in photoexcited TiO2 suspensions in a toluene/ acetonitrile/2-propanol (10:10:1 v/v) solvent mixture. The formation of the pi-radical anion, (C-60(.-)) (C4H10N+), 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(C4H10N+)/I- towards reduction is, however, very complex. Although the N-methylfulleropyrrolidinium is sufficiently water-soluble, no reduction could be observed in irradiated aqueous TiO2 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(.-))(C4H10N+) was observed in low yields. Using gamma-cyclodextrin (gamma-CD) encapsulated fullerene, C-60(C4H10N+)/gamma-CD, instead of the surfactant capped fullerene, the fullerene's pi-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(C4H10N+)}-clusters by (CH3)(2)C-.(OH) radicals relative to the analogous reduction of C-60(C4H10N+)/gamma-CD. This emerged from competitive experiments with applying 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) as a spin trap in aqueous TiO2 suspensions, containing 2-propanol. (C) 1998 Elsevier Science S.A. All rights reserved.