Probing the electron-accepting reactivity of isomeric bis(pyrrolidinium) fullerene salts in aqueous solutions

Series of water-soluble isomer bis(pyrrolidinium) salts, with C-60(C4H10N+)(2) as cationic moiety (2a-2d), were probed in radical- and light-induced reduction studies and compared to bis(carboxylates) C-60[C(COO-)(2)](2) and to gamma-CD-encapsulated C-60 Pulse radiolytic reduction of 2a-2d with hydrated electrons and (CH3)(2). COH radicals leads to the formation of the fullerene pi-radical anion, exhibiting fingerprint absorption characteristics in the near-IR region. Because of the electron-withdrawing nature of the pyrrolidinium groups the electron-acceptor properties of the investigated bis(pyrrolidinium) salts are markedly improved relative to the bis(carboxylates) (C-60[C(COO-)(2)](2)) and also relative to C-60. For example, the rate constants for the fullerene reduction of 2a-2d with hydrated electrons ((0.88-2.2) x 10(10) M-1 s(-1)) and (CH3)(2). COH radicals ((4.7-7.1) x 10(8) M-1 s(-1)) are clearly faster than those noted for C-60[C(COO-)(2)](2) (e(aq)(-): (0.19-0.34) x 10(10) M-1 s(-1); (CH3)(2). COH: (0.9-2.2) x 10(8) M-1 s(-1)), and C-60 (e(aq)(-): 1.8 x 10(10) M-1 s(-1); (CH3)(2). COH: 2.7 x 10(8) M-1 s-'). Photolysis of 2a-2d gives rise to singlet excited state absorptions that closely resemble earlier observations for the pyrrolidine precursor, e.g., C-60(C3H7N)(2) (1a-1d) in deoxygenated toluene solutions. The triplet lifetimes of 2a-2d, as measured by nanosecond-resolved photolysis, are typically around 33 mu s, similar to the triplet lifetimes of truly monomeric fullerene solutions. Quenching experiments with diazabicyclooctane (DABCO), involving the tripler excited states of 2a-2d, yielded rate constants which vary between 2.5 x 10(6) and 4.7 x 10(6) M-1 s(-1) and reveal a slow-down relative to C-60 (7.4 x 10(7) M-1 s(-1)).