Triplet State Dissociation of C120, the Dimer of C60

Photophysical and photochemical properties of C 120, the [2+2]-dimer of C 60, have been investigated. In toluene solution, the 327 nm ground-state absorption peak has a molar absorptivity of 113 000 dm 3 mol -1 cm -1, somewhat more than twice that of the analogous 336 nm peak of C 60 . The S 1 and T 1 origin energies in C 120 are found to be lower than those in C 60 by approximately 1070 and 625 cm -1, respectively. The T 1 state's intrinsic exponential lifetime at 296 K is 42 ± 2 μs, or a factor of 3.4 times shorter than for C 60 . The T n ← T 1 absorption spectrum of C 120 shows bands at 710 and 1060 nm. The former is similar to those seen in [6,6]-adducts of C 60, but the latter may reflect electronic coupling between the two halves of the dimer molecule. In degassed room-temperature toluene solution, C 120 photodissociates to C 60 with a quantum yield of 2.3 × 10 -3 . This dissociation occurs from the T 1 state and is strongly suppressed by dissolved oxygen. The rate constant for T 1 dissociation increases from ca. 55 s -1 at 297 K to 800 s -1 at 333 K, implying an Arrhenius activation energy near 64 kJ mol -1 (5360 cm -1 ) and a prefactor in the range of vibrational frequencies.