Reversible Dimerization of [5,6]-C60O

The recently discovered [5,6]-open isomer of C60O has been found to undergo facile dimerization to form a new C 2 symmetry isomer of C120O2, which can be photodissociated with relatively high efficiency to regenerate monomeric [5,6]-C60O. High yield dimerization of [5,6]-C60O proceeds spontaneously in toluene solution near room temperature. On the basis of 13C NMR spectroscopy, ab initio quantum computations, and HPLC retention patterns, the resulting C120O2 product has been deduced to be a nonpolar dimer of C 2 symmetry in which the C60O moieties are linked by two single bonds between sp3-hybridized carbon atoms adjacent to oxygen atoms. Photophysical properties of this dimer have also been measured and compared to those of C120, the [2 + 2]-dimer of C60. The ground-state absorption spectrum of C120O2 in toluene is slightly red-shifted relative to that of C120, with a distinctive peak at 329 nm and an S1−S0 origin band at 704 nm. Its fluorescence spectrum shows two major peaks at 718 and 793 nm. In room-temperature toluene, the measured triplet state intrinsic lifetime of this C120O2 isomer is 34 ± 2 μs, a value somewhat shorter than that of C120 (44 μs). C120O2 undergoes photodissociation from its triplet state to regenerate monomeric [5,6]-C60O with quantum yields of 2.5% at 24 °C and 43% at 70 °C. It can therefore serve as a stable reactant for photolytic production of [5,6]-C60O. As a simple fullerene adduct that reacts under mild conditions, [5,6]-C60O may prove useful in special synthetic applications. Solutions of [5,6]-C60O are also unique because they can provide mixtures of a fullerene monomer and its dimer in a dynamic balance controllable by adjustment of concentration, temperature, and optical irradiation.