Bipyridine-containing π-conjugated polymers bearing optically active amide groups: mechanistic aspects of formation of chiral higher-order structures

Abstract

π-Conjugated polymers with optically active substituents possibly form chiral higher-order structures. In the present study, novel bipyridine- and biphenyl-containing poly(phenyleneethynylene)s [poly(1–2) and poly(1–3)] were synthesized by the Sonogashira–Hagihara coupling polymerization of N-(3,5-diethynylbenzoyl)-L-alanine dodecyl ester (1) with 4,4′-dibromo-2,2′-bipyridine (2) and 3,3′-dibromo-1,1′-biphenyl (3). Poly(1–2) exhibited strong CD signals assignable to the main chain chromophore at 300–370 nm in CHCl₂CHCl₂, while poly(1–3) exhibited no CD signal. CD and UV–vis absorption spectroscopic analyses, DLS measurements, conformational analysis by quantum mechanical calculations and molecular dynamics simulations suggested that each poly(1–2) molecule adopts a left-handed folded helical conformation in CHCl₂CHCl₂ and that the molecules are partly aggregated. Poly(1–2) coordinated to platinum to shift the UV–vis absorption peak toward a longer wavelength region and for the appearance of a CD signal at a longer wavelength, accompanied by weakening of photoluminescence.