Syntheses and Chiroptical Properties of Optically Active Helical Poly(phenylacetylene)s Bearing [60]Fullerene Pendants

Abstract

Two novel C60-bound chiral (3) and achiral (2) phenylacetylenes were synthesized by the treatment of C60 with (R)-N-[1-(4-ethynylphenyl)ethyl]glycine and N-(4-ethynylbenzyl)glycine by the Prato reaction in the presence of paraformaldehyde and dipentadecyl ketone, respectively. The homopolymerization of the achiral 2 bearing long alkyl chains and copolymerization with optically active phenylacetylenes including 3 using a rhodium catalyst in chloroform homogeneously occurred, giving the C60-containing helical poly(phenylacetylene)s soluble in chloroform, while the homopolymers of the analogous achiral C60-bound phenylacetylenes with no alkyl chain (1) and 3 were totally insoluble in common organic solvents. The obtained copolymers exhibited an induced circular dichroism (ICD) both in the main-chain region and in the fullerene chromophoric region, indicating that the optically active copolymers possess a preferred-handed helical conformation and that the pendant fullerene groups are arranged in helical arrays with a predominant screw-sense along the polymer backbones. The formation of a hierarchical supramolecular assembly of the homopolymers of 1 and 3 bearing fullerene pendants on each repeating unit was also studied by electron microscopy.