Photophysical Properties of Mono- and Multiply-Functionalized Fullerene Derivatives

Abstract

Time-resolved and steady-state techniques have been performed to investigate the photophysical properties of C60[C(COOEt)2] (Ia), e-C60[COOEt)2]2 (IIa) (e = equatorial), trans-3-C60[C(COOEt)2]2 (IIb), trans-2-C60[C(COOEt)2]2 (IIc), and e,e,e-C60[C(COOEt)2]3 (III) (e = equatorial). Picosecond-resolved energy transfer to the fullerene core results in the rapid formation of the excited singlet state with remarkably blue-shifted singlet−singlet (S*1 → S*n) transitions (868 nm (III)) relative to pristine C60 (920 nm). Intersystem crossing to the energetically lower lying excited triplet state exhibits a deceleration with increasing number of functionalizing addends. The corresponding triplet−triplet (T*1 → T*n) absorption energies also show a significant dependence on the degree and site of functionalization, spreading over a range of 100 nm (750 nm for 3C60 to 650 nm for e,e,e-(3C60)[C(COOEt)2]3 (III)). Energy transfer from radiolytically excited biphenyl (3BP) to the fullerene's ground state corroborates the photolytic data. *0 → 0 Emissions from the lowest level of the excited singlet state (fluorescence) are mirror images of the reversed 0 → *0 absorption transitions with minor Stokes shift. Red shifts of fluorescence- and phosphorescence-related emission, relative to pristine C60, again sensitively reflect the perturbation of the fullerene's π-system as a function of the degree and site of functionalization. Cyclic voltammetry and reductive quenching of excited triplet fullerenes demonstrate that functionalization of C60 obstructs the ease of reduction in the ground and excited triplet state. An increasing number of functional groups results in a cathodic shift of the redox potential (ground state −0.54 to −0.86 V; excited singlet state 1.44 to 0.91 V; excited triplet state +1.01 to +0.64 V versus SCE for C60 and e,e,e-(3C60)[C(COOEt)2]3 (III), respectively).