Chiral alkylated poly(m-phenylene)s: Optical activity and thermal stability of helical structures

Chiral poly[4,6-bis(alkylthio)-1,3-phenylene-alt-2-methyl-1,3-phenylene] was synthesized from 1,3-dibromo-2,6-bis(3-dodecyl-2-methylthio)benzene and 2-methyl-1,3-phenylenebis(pinacol borate) as a precursor of chiral poly(thiaheterohelicene). Circular dichroism (CD) spectra that arise from the poly(1,3-phenylene) backbone inverted according to the chirality of the side chains, which indicated that a helical conformation of the polymer was induced by the interaction between the side chains. The CD intensity of the polymer increased in non-polar solvents such as hexane. The decrease in the molar CD intensity and the broadening of a fluorescence band at higher concentrations suggested that the aggregation of the polymer suppressed the formation of the helical structure. The conformational changes were monitored by the CD and the ¹H NMR spectra at different temperatures. In a good solvent such as dichloromethane, the CD intensity increased, and the ¹H NMR signal of benzene protons shifted to lower fields at low temperatures. In hexane, the CD spectra and the ¹H NMR signals were less dependent on temperatures, as a result of the strong interaction between the chiral alkyl chains in the polymer to freeze the helical conformation.