Melt-Extruded Films of a Commercial Polymer with Intense Chiral Optical Response of Achiral Guests

Abstract

The degree of circular polarization (g-value) of infrared and UV–vis bands of melt-extruded syndiotactic polystyrene (s-PS) films of different thickness, as induced by cocrystallization with a nonracemic guest ((R)-(−) or (S)-(+) carvone), and of their nonracemic and achiral guests has been studied. The induced circular dichroism (CD) phenomena, both for polymer host and for chromophore guests, are maximized for films with thickness of few micrometers. Surprisingly, for all the examined thicknesses and spectral ranges, the chiral response of the considered achiral guests (azulene and 4-nitroaniline) is definitely higher than for the chirality-inducing nonracemic guest (carvone). This clearly indicates that the origin of the chiral optical response is extrinsic to the site of photon absorption. In agreement with this extrinsic nature, a comparison of g-value for host and guest peaks of melt-extruded and spin-coated s-PS films indicates that the observed chiral response is an extensive property. The reported results also confirm that the intense chiral responses of s-PS films are associated with a nonracemic helical morphology of crystallites, as induced by the nonracemic guest.