Tunable Cis-cisoid Helical Conformation of Poly(3,5-disubstibuted phenylacetylene)s Stabilized by n→π* Interaction

A series of novel cis poly(phenylacetylene)s (PPAs) substituted at meta -position(s) by two alkoxycarbonyl pendants, i.e. , sP-Me-C8/rP-Me-C8 , P-Me-C12 , sP-Et-C4 , sP-2C4 and sP-Oct-C4 , were synthesized under the catalysis of [Rh(nbd)Cl] 2 (nbd = norbornadiene). The dependence of elongation, screw sense, and stimuli response of helical polyene backbone on the structure of pendant, solvent, and temperature was systematically investigated in both solution and solid states. Because of n → π * interaction between vicinal carbonyl groups, sP-Me-C8/rP-Me-C8 could adopt contracted cis-cisoid helix in THF, toluene, CH 2 Cl 2 , and CHCl 3 . Such an intramolecular interaction was sensitive to the hydrogen bond donating ability of solvent and temperature, but insensitive to the dielectric constant and polarity of solvent. In poly(3-methoxycarbonyl-5-alkoxycarbonylphenylacetylene), the longer the chiral alkyl chain was, the easier the stable cis-cisoid helix could be achieved. However, when the methoxycarbonyl was changed to ethoxycarbonyl, sec -butyloxycarbonyl, and octyloxycarbonyl pendant groups, only cis-transoid helix was obtained at room temperature due to the increased steric hindrance. Moreover, lowering temperature was found to facilitate the stabilization of n → π * interactions, and reversible temperature-dependent stereomutations were achieved in sP-Me-C8 and sP-Et-C4 depending on the solvent where they were dissolved. These results suggested that the long alkyl chain, small pendant size, and lower temperature favored the stabilization of intramolecular n → π * interactions and the formation of contracted, cis-cisoid helices for poly(3,5-diester substituted phenylacetylene)s.