Inhibition of Cluster Phenomena in Truly Water Soluble Fullerene Derivatives:  Bimolecular Electron and Energy Transfer Processes

A series of water-soluble fullerene derivatives, namely, e -C 60 [C(COO - ) 2 ] 2 ( 2 ), trans - 3 -C 60 [C(COO - ) 2 ] 2 ( 3 ), trans - 2 -C 60 [C(COO - ) 2 ] 2 ( 4 ), and e,e,e -C 60 [C(COO - ) 2 ] 3 ( 5 ) were probed in light- and radical-induced studies and compared to {( 3 C 60 )C(COO - ) 2 } n clusters ( 1 ). Ground-state absorption spectra of 2 − 5, recorded in a concentration range between 1.0 × 10 -4 M and 5 × 10 -6 M, clearly speak against any appreciable deviation from the Lambert−Beer law. Picosecond-resolved photolysis gives rise to singlet excited state absorptions that closely resemble earlier observations for e -C 60 [C(COOEt) 2 ] 2, trans - 3 -C 60 [C(COOEt) 2 ] 2, trans - 2 -C 60 [C(COOEt) 2 ] 2, and e,e,e -C 60 [C(COOEt) 2 ] 3 in deoxygenated toluene solutions. The triplet lifetimes of 2 − 5, as measured by nanosecond-resolved photolysis, are typically around 40 μs, similar to the triplet lifetimes of truly monomeric fullerene solutions. A strongly enhanced lifetime (by nearly 3 orders of magnitude) relative to {( 3 C 60 )C(COO - ) 2 } n clusters ( 1 ) (τ = 0.4 μs) indicates a truly monomeric appearance of these bis- and tris-functionalized fullerenes in aqueous solutions and confirms that micellar aggregation does, indeed, play only a minor role regarding the reactivity of these derivatives. Quenching experiments with diazabicyclooctane (DABCO), involving the triplet excited states of 2 − 5, yielded rate constants which reveal a strong dependence on the degree of functionalization. They vary between 7.4 × 10 7 M -1 s -1 and 5.1 × 10 5 M -1 s -1 . This clearly indicates a substantial shift in the redox potential of the triplet excited state between pristine C 60 and e,e,e -C 60 [C(COO - ) 2 ] 3 ( 5 ). As a consequence of reductive quenching, the spectral region between 900 and 1100 nm shows the time-resolved growths of the π-radical anion absorptions with maxima at 1040, 995, 880, and 1020 nm, for 2, 3, 4, and 5, respectively. They match the spectra generated by means of hydrated electrons ( k = (0.75−3.4) × 10 9 M -1 s -1 ) and (CH 3 ) 2 • COH radicals ( k = (0.9−2.2) × 10 8 M -1 s -1 ) in pulse radiolysis experiments.