Photophysical properties of functionalized fullerene derivatives; A selective overview

Time-resolved and steady-state techniques are employed to explore the photophysical properties of C60C(COOEt)(2) (1), equatorial-C-60[C(COOEt)(2)](2) (2a), trans(3)-C-60[C(COOEt)(2)](2) (2b), trans 2-C-60[C(COOEt)(2)](2) (2c), and equatorial-C-60[C(COOEt)(2)](3) (3) Picosecond-resolved energy transfer to the fullerene core results in the formation of the excited singlet state with remarkably blue-shifted singlet-singlet (S-1* --> S-n*) transitions in going from C-60 (920 nm) to 3 (868 nm). Rapid formation of the triplet-triplet (T-1* --> T-n*) absorption as a consequence of intersystem crossing to the energetically lower lying excited tripler state suffers a deceleration with increasing number of functionalizing addends. The corresponding triplet-triplet 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 equatorial-(C-3(60))[C(COOEt)(2)](3) (3). Energy transfer from radiolytically excited biphenyl to the fullerene's ground state, corroborates the photolytic data *0 --> 0 Transitions from the lowest level of the excited singlet state (fluorescence) and excited triplet state (phosphorescence) are mirror-images to the reversed 0 --> *0 absorption transitions. Red-shifts of these emission, relative to pristine C-60, again sensitively reflect the degree and site of functionalization. Cyclic voltammetry and reductive quenching of triplet excited fullerenes demonstrate that functionalization of the fullerene's pi-system obstructs the ease of reduction in the ground and excited tripler state. An increasing number of bis(ethoxycarbonyl)methylene groups shifts the redox potential of the ground state from -0.54 to -0.86 V vs SCE and of the excited triplet state from +1.01 V vs SCE to +0.64 V vs SCE for C-60 and 3, respectively.