| Abstract
| - Photophysical properties have been measured and compared for the two known isomers ([6,6]-closed and[5,6]-open) of C61H2, the parent C60 methano adduct. Optical absorption spectra from the ground and tripletstates are consistent with C60 π-electron systems that are significantly less perturbed by [5,6] than [6,6]derivatization. However, intrinsic nonradiative decay of triplet state [5,6]-C61H2 is the most rapid yet foundin any C60 adduct, occurring with a characteristic time of 2 μs, a factor of 70 faster than in pristine C60. Bycontrast, the triplet decay time of [6,6]-C61H2 is 65 μs under the same conditions. The S1 and T1 energies ofboth isomers have been deduced by various methods. It is found that the S1−T1 energy gap of [5,6]-C61H2 islarger than that of C60, whereas the opposite relation holds for [6,6]-C61H2. Quantum yields of triplet stateformation 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]-C61H2 indicate efficientnonradiative decay from its S1 state with a rate constant estimated at ca. 1.5 × 1010 s-1. It is concluded thatisomeric forms of simple C60 monoadducts can show major differences in excited-state relaxation.
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