Aggregation of fullerene, C60, in benzonitrile

C-60 solutions in benzonitrile have been found to show concentration dependent optical absorption behaviour. At lower concentrations (<100 mu M) the UV-vis absorption characteristics of C-60 in benzonitrile are exactly similar to those in benzene and decalin. At higher concentrations (>100 mu M), however, the C-60 solutions in benzonitrile show very broad absorption tail, extending beyond 900 nm. At higher concentrations the solutions are also visually opaque. From picosecond laser flash photolysis experiments it is seen that the triplet quantum yield of C-60 in benzonitrile at higher concentrations (similar to 400 mu M) is much less than unity and increases with the dilution while in decalin and benzene it is always close to unity and independent of the C-60 concentration. Dynamic light scattering experiments indicate the presence of particles of mean size of about 250 nm in C-60 solutions in benzonitrile with concentration > 100 mu M, while in < 100 mu M solutions no such particles have been observed. Such particles are also not observed for C60 solutions in benzene and decalin, even up to similar to 500 mu M. Scanning electron microscopy also shows particles of size similar to 250 nm. It is inferred that C-60 forms aggregates in benzonitrile at concentrations >100 mu M and that the aggregated and the monomeric form of C-60 are in equilibrium.