Synthesis and Helix Pitch Control of π-Conjugated Helical Polymers with Accordion-like Oscillation

Poly(alkyl propiolate)s (PAP), e.g., Poly(n-heptyl propiolate) (PnHepP) and Poly((s)-2-octyl propiolate) (Ps2OctP) having an achiral or a chiral ester side chain were stereoregularly prepared with a Rh complex catalyst, [Rh(nbd)Cl](2) (nbd = norbornadiene), at 40 degrees C in methanol, respectively. The temperature dependence H-1 and C-12 NMR spectra of PAPs and their spectral simulations indicated that the restricted rotation around the ester O-C bond takes place through the so-called 3-site jump conformation. The spectra also exhibited that the helical conjugated main-chain of Ps2OctP was largely oscillating between the sequences of contracted helix and stretched helix in the time scale of the NMRs, although the degree of the oscilation of PnHepP was fairly small compared to that of Ps2OctP. The UV-vis and IR spectra of them supported that PAPs have at least two helical conformations, i.e., contracted and stretched helices. Therefore, these spectra supported the oscillation of the helix along the main-chain axis. The proposed model calculated by MMFF94 also indicated the accordion-like helix oscillation of contracted and stretched helices (HELIOS) of PAPs which was synchronized with restricted 3-site jump rotation around the ester O-C bond in the side-chain.