PHOTOPHYSICS OF SELECTED C_(84) ISOMERS

摘要

Photophysical properties of C_(84) isomers have been measured following preparation by recycling HPLC methods. A sample of purified C_(84) was separated into three fractions. The first contained a mixture of the three most abundant isomers, with symmetry labels D_2(Ⅳ), D_(2d)(Ⅱ), and C_s(a). UV-vis absorption spectroscopy confirmed that the second and third peaks contained predominately the C_2 and C_s(b) isomers, respectively. A kinetics-based method was used to find the effective molar absorptivities of these fractions. Sensitive time-resolved absorption measurements following optical excitation revealed a surprisingly wide range of triplet state lifetimes among these isomers. Using samples dissolved in a solid polymer film to suppress bimolecular triplet decay processes, kinetic components were observed with lifetimes ranging from 5 to 710 μs. The species with the longest triplet lifetime shows a T_n ← T_1 absorption peak at 740 nm. Quenching studies indicate that one C_(84) isomer has a triplet state energy well below that of oxygen's ~1Δ_g state, while two others have triplet energies close to, but still belowm the ~1Δ_g energy. These findings provide the first evidence for variations of photophysical properties among isomeric forms of a fullerene.