Meta-Linked Poly(phenylene ethynylene) Conjugated Polyelectrolyte Featuring a Chiral Side Group:  Helical Folding and Guest Binding

Abstract

A water soluble, meta-linked poly(phenylene ethynylene) featuring chiral and optically active side groups based on l-alanine (mPPE-Ala) has been studied by using absorption, fluorescence, and circular dichroism spectroscopy. Studies of mPPE-Ala in methanol/water solvent mixtures show that the polymer folds into a helical conformation, and the extent of helical folding increases with the volume % water in the solvent. The presence of the helical conformation is signaled by the appearance of a broad, excimer-like visible fluorescence band, combined with a strong bisignate circular dichroism signal in the region of the π,π* absorption of the polymer backbone. The circular dichroism signal exhibits negative chirality, suggesting that the left-handed (M-form) of the helix is in enantiomeric excess. Binding of the metallointercalator [Ru(bpy)2(dppz)]2+ (where bpy = 2,2-bipyridine and dppz = dipyrido[3,2-a:2‘,3‘−c]phenazine) with the helical polymer is accompanied by the appearance of the orange-red photoluminescence from the metal complex. This effect is directly analogous to that observed when [Ru(bpy)2(dppz)]2+ binds to DNA via intercalation, suggesting that the metal complex binds to mPPE-Ala by intercalating between the π-stacked phenylene ethynylene residues. Cationic cyanine dyes also bind to the periphery of the helical polymer in a manner that is interpreted as “groove binding”. A circular dichroism signal is observed that is believed to arise from exciton coupling within the chiral cyanine dye chromophore aggregate that is formed as the dye molecules are oriented by the helical mPPE-Ala “template”