Aggregation of fullerene, C-60, in benzonitrile

C-60 solutions in benzonitrile have been found to show concentration depend ent optical absorption behaviour. At lower concentrations (<100 mu M) the U V-vis absorption characteristics of C-60 in benzonitrile are exactly simila r to those in benzene and decalin. At higher concentrations (>100 mu M), ho wever, the C-60 solutions in benzonitrile show very broad absorption tail, extending beyond 900 nm. At higher concentrations the solutions are also vi sually opaque. From picosecond laser flash photolysis experiments it is see n that the triplet quantum yield of C-60 in benzonitrile at higher concentr ations (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 inde pendent of the C-60 concentration. Dynamic light scattering experiments ind icate the presence of particles of mean size of about 250 nm in C-60 soluti ons in benzonitrile with concentration > 100 mu M, while in < 100 mu M solu tions no such particles have been observed. Such particles are also not obs erved for C60 solutions in benzene and decalin, even up to similar to 500 m u M. Scanning electron microscopy also shows particles of size similar to 2 50 nm. It is inferred that C-60 forms aggregates in benzonitrile at concent rations >100 mu M and that the aggregated and the monomeric form of C-60 ar e in equilibrium.