Reversible Cis-Cisoid to Cis-Transoid Helical Structure Transition in Poly(3,5-disubstituted phenylacetylene)s

A series of novel 3,5-disubstituted phenylacetylenes, rM-1, sM-1 to sM-5, bearing an achiral methoxycarbonyl pendant group and various chiral N-alkylamide pendant groups, were synthesized. They were converted to the corresponding polymers, rP-1, sP-1 to sP-5, with high cis-structure contents under the catalysis of [Rh(nbd)Cl](2), aiming to understand how the environmental variation and the structure of pendant group influence the chiroptical properties of polymers. sP-1/rP-1 were soluble in CHCl3 and THF at the molecular level and exhibited much larger optical rotations with opposite signs to those of sM-1/rM-1 and displayed the intense Cotton effects centered at 360 rim in the circular dichroism (CD) spectra, ascribed to the one-handed, contracted cis-cisoid helical polyene backbone. The reversible conformation transition between the contracted cis-cisoid helix and the frustrated, extended cis-transoid helix was achieved by alternately adding trifluoroacetic acid (TFA) and triethylamine into the hydrogen bond donating solvent (i.e., CHCl3), as evidenced by UV-vis absorption and CD spectroscopy, dynamic and static laser light scattering, DSC, and WAXD results. However, the addition of TFA into the sP-1 solution in the hydrogen bond accepting solvent (i.e., THF) caused no discernible halochromism. The competing interaction of THF with TFA was considered to account for the observed difference in acid induced chromism. The small modification in the chiral alkylamide pendant group was found to remarkably affect the solubility and helical conformation of the polymer. sP-2 was insoluble in all the solvents tested, sP-3 and sP-4 dissolved in polar DMF, while sP-5 dissolved in both polar and apolar solvents. Depending on the nature of solvents and additives, sP-3 and sP-4 took either contracted or frustrated helical conformation, whereas sP-5 took only a stretched helical conformation due to the highly branched alkyl group.National Natural Science Foundation of China [21274003]; Research Fund for Doctoral Program of Higher Education of MOE [20110001110084]SCI(E)ARTICLExhwan@pku.edu.cn228407-84174