Poly(Zn-salphen)-alt-(p-phenyleneethynylene)s as Dynamic Helical Metallopolymers: Luminescent Properties and Conformational Behavior

The synthesis of soluble, conjugated poly(Zn-salphen)-alt-(p-phenyleneethynylene)s with a coilable structure and bearing acetylated sugar substituents is described, and their photophysical properties and conformational behavior have been investigated. Their CD spectra confirm that these systems are chiral, and signify ordered (helical) coiling of the polymer backbone arising from chirality transfer from the beta-D-glucopyranosyl groups. The consequences of varying side chain substituents as well as the extent of inter- and intrachain [pi-stacking and Zn center dot center dot center dot O(salphen)] interactions have been studied, and comparisons with the corresponding linear-rod and Pt-based congeners have been made. Analysis of the effects of different solvents on these systems by UV-vis absorption, emission, and CD spectroscopy indicates that low-polarity solvents give a tighter, more compressed helical structure with increased CD intensities, while coordinating solvents can perturb the folded conformation to afford a more extended coiled structure with decreased Cotton effects. These observations, together with variable-temperature CD experiments, point to the flexibility and dynamic nature of the helical conformation.