Comparative Photophysics of C61H2 Isomers

Photophysical properties have been measured and compared for the two known isomers ([6,6]-closed and [5,6]-open) of C 61 H 2, the parent C 60 methano adduct. Optical absorption spectra from the ground and triplet states are consistent with C 60 π-electron systems that are significantly less perturbed by [5,6] than [6,6] derivatization. However, intrinsic nonradiative decay of triplet state [5,6]-C 61 H 2 is the most rapid yet found in any C 60 adduct, occurring with a characteristic time of 2 μs, a factor of 70 faster than in pristine C 60 . By contrast, the triplet decay time of [6,6]-C 61 H 2 is 65 μs under the same conditions. The S 1 and T 1 energies of both isomers have been deduced by various methods. It is found that the S 1 −T 1 energy gap of [5,6]-C 61 H 2 is larger than that of C 60, whereas the opposite relation holds for [6,6]-C 61 H 2 . Quantum yields of triplet state formation and fluorescence were found to be 0.06 and <2 × 10 -5 for the [5,6] isomer and 0.93 and 7.8 × 10 -4 for the [6,6] isomer, respectively. The anomalously low quantum yields for [5,6]-C 61 H 2 indicate efficient nonradiative decay from its S 1 state with a rate constant estimated at ca. 1.5 × 10 10 s -1 . It is concluded that isomeric forms of simple C 60 monoadducts can show major differences in excited-state relaxation.