Raman Optical Activity (ROA) as a New Tool to Elucidate the Helical Structure of Poly(phenylacetylene)s

Abstract

Poly(phenylacetylene)s is a family of helical polymers constituted by conjugated double bonds that are fixed into specific helical structures due to supramolecular and/or steric interactions among pendant groups. Raman spectra of these polymers show a structural fingerprint of the polyene backbone which, in combination with its helical orientation, makes them great candidates to study their helical structure by Raman Optical Activity (ROA). Four different well-known poly(phenylacetylene)s adopting different scaffolds and ten different helical senses have been prepared. For all of these, Raman and ROA spectra were recorded which allows us to establish ROA spectrum/helical sense relationships: a right-handed orientation of the polyene backbone (P helix ) produces a triplet of negative ROA bands, whereas a M helix produces a positive triplet ROA pattern. Raman and ROA spectra of each polymer exhibited the same profile and the sign of the ROA spectrum was opposite to the lowest energy ECD band, thus indicating a resonance effect. Resonance ROA appears then as a useful selective indicator of the helical sense of poly(phenylacetylene)s, especially for those showing an extra Cotton band in the ECD spectrum. In these cases, a wrong helical sense is assigned based on ECD, while ROA alerts of this misassignment.