Photophysical properties of mono- and multiply-functionalized fullerene derivatives

Time-resolved and steady-state techniques have been performed to investigate the photophysical properties of C-60[C(COOEt)(2)] (Ia), e-C-60[COOEt)(2)](2) (IIa) (e = equatorial), trans-3-C-60[C(COOEt)(2)](2) (IIb), trans-2-C-60[C(COOEt)(2)](2) (IIc), and e,e,e-C-60[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 C-60 (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 C-3(60) to 650 nm for e,e,e-(C-3(60))[C(COOEt)(2)](3) (III) Energy transfer from radiolytically excited biphenyl ((BP)-B-3) 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 C-60, again sensitively reflect the perturbation of the fullerene's pi-system as a function of the degree and site of functionalization. Cyclic voltammetry and reductive quenching of excited triplet fullerenes demonstrate that functionalization of C-60 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 C-60 and e,e,e-(C-3(60))[C(COOEt)(2)](3) (III), respectively).