Controlled modulation of the helical sense and the elongation of poly(phenylacetylene)s by polar and donor effects

The elasticity (stretched/compressed) and helical sense (clockwise/anticlockwise) of a dynamic helical poly(phenylacetylene) (PPA), bearing (R)-α-methoxytrifluorophenyl acetic acid (MTPA) pendants [poly-(R)-1], can be selectively controlled by the donor and the polar character of the solvent. The basis for this effect lies on the presence in the pendants of two independently tuneable bonds: (O)C–C(–O) and (H–)N–C(O). An increase in polarity shifts the conformational equilibrium of the (O)C–C(–O) bond in favour of the most polar synperiplanar (sp) conformer, that is more sterically demanding, thus forcing the polymer to invert its helix. A donor solvent associates with the (H–)N–C(O) bond favouring the shift to the cis form of the amide and inducing both the elongation of the polymer chain and the inversion of the helical sense (visualized as UV-Vis bathochromic shift and CD inversion respectively). This ability of poly-(R)-1 to respond to donor/polar properties occurs in solution as well as in film state. Poly-(R)-1 presents in CHCl₃ identical helical sense for both the internal (polyene backbone) and for the external (described by the pendants) helices (3/1 clockwise), while in THF their senses are opposite (internal anticlockwise; external 2/1 clockwise), as proven by DSC.