Unusual rod-shaped aggregation of C 70 in acetonitrile–benzonitrile solvent mixtures Sukhendu Nath, Haridas Pal 1 , Avinash V. Sapre * Radiation Chemistry and Chemical Dynamics Division, Bhabha Atomic Research Centre, Modular Labs, Trombay, Mumbai 400 085, India Received 5 November 2002; in final form 23 December 2002 Abstract Unusual rod-shaped aggregation has been observed in acetonitrile–benzonitrile solvent mixtures. C 70 remains as monomers, small aggregates and large rod-shaped aggregates in acetonitrile compositions of <60%, between 60– 75%and >75%,respectively.Thetransitionofthesmallaggregatesintolargerod-shapedaggregatesisuniqueforC 70 , as its analogue, C 60 , is known to form only small spherical aggregates under similar conditions. The rod-shaped C 70 aggregatescouldbeeasilyseparatedfromthesolutionandcanbedispersedindifferentpolarsolventsincludingwater. Ó 2003 Elsevier Science B.V. All rights reserved. 1. Introduction Aggregation of fullerenes, C 60 andC 70 in solu- tion is the subject of intense research in the past few years [1–7]. Since the first report by Sun and Bunker [1] on the aggregation of C 70 in toluene (TL)–acetonitrile (AN) solvent mixtures, the ag- gregation behaviour of both C 60 and C 70 has ex- tensively been investigated by several research groups. Our investigations on the aggregation of C 60 and C 70 in different solvents and solvent mixtures has revealed that the solvent polarity playsanimportantroleintheaggregationprocess [8,9]. Thus a critical solvent polarity, which is the characteristic of each of fullerenes, C 60 andC 70 ,is essential for the aggregation to take place. Ful- lereneaggregationinsolutionhasbeenunderstood to be due to physical interactions among the monomers, and an equilibrium exists between monomers and aggregates in the solution. Very few studies on the aggregation of fullerenes have been reported in neat organic solvents. Slow for- mation (few days) of C 60 aggregates in benzene (BZ) solution has been reported by Ying et al. [10,11] and Rudalevige et al. [12]. According to Ying et al. [10,11], however, the C 60 aggregates in BZ are very unstable and break even on mechan- ical shaking. We have studied the aggregation processes of both C 60 and C 70 in different neat solvents [8,9,13]. Unlike the observation of Ying et al. [10,11], it is found that C 60 forms stable aggregates in benzonitrile (BZN) [13] and benzyl alcohol (BZA) [8]. It is seen that in neat solvents, Chemical Physics Letters 369 (2003) 394–401 www.elsevier.com/locate/cplett * Corresponding author. Fax: +91-22-2550-5151. E-mail addresses: snath@magnum.barc.ernet.in (S. Nath), cdscd@magnum.barc.ernet.in (A.V. Sapre). 1 Also corresponding author. 0009-2614/03/$ - see front matter Ó 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0009-2614(02)02041-9