Doublet Chirality Transfer and Reversible Helical Transition in Poly(3,5-disubstituted phenylacetylene)s with Pyrene as a Probe Unit(dagger)

Summary of main observation and conclusion A novel doublet chirality transfer (DCT) model was demonstrated in cis poly(3,5-disubstituted phenylacetylene)s, i.e., S-I, R-I, and S-I-NMe. The chiral message from the stereocenter of alkylamide substituent at 3-position induced the polyene backbone to take cis-transoid helical conformation with a predominant screw sense. And in turn the helical backbone acted as a scaffold to orient the pyrene probes, which was linked to phenyl rings through 5-position, to array in an asymmetric manner. A combinatory analyses of H-1 NMR, Raman, FTIR, UV-vis absorption, CD, and computer simulation suggested that the main-chain stereostructure, solvent nature, and intramolecular hydrogen bonds played important and complex roles on DCT. High cis-structure content and intramolecular hydrogen bonds were beneficial for the realization of DCT. Reversible helix-helix transition was observed in S-I by changing the nature of solvents. In DMF, S-I adopted a relatively contracted helix, where the main chain exhibited strong optical activity, but that of pyrene was weak. In contrast, a relatively stretched helix formed in CHCl3, in which the optical activity of pyrene was much larger, whereas that of the polyene backbone was the weakest. This helix-helix transition was attributed to the intramolecular hydrogen bonds, which was confirmed by solution-state FTIR spectra and computer calculations.