Chiral Optical Films Based on Achiral Chromophore Guests

Abstract

Films constituted by an achiral host polymer and an achiral guest chromophore, which exhibit intense chiral optical responses for both host and guest peaks in the infrared as well as in the UV-visible regions, have been obtained. Such films can have any thickness from the nanoscale up to the macroscopic scale. This unexpected chiral optical behavior is easily achieved by crystallization of a robust commercial stereoregular host polymer (syndiotactic polystyrene, s-PS) from the amorphous phase as induced by a nonracemic guest, followed by exchange of the nonracemic guest with the achiral guest, leading to the formation of polymer-host/chromophore-guest cocrystalline phases. This finding opens the possibility to achieve s-PS-based films with chiral optical response at selected wavelengths. It has been also found that the chiral optical cocrystalline phase of s-PS with azulene exhibits a monoclinic δ-clathrate form with equal proportions of left- and right-handed helices. This confirms that the observed chiral optical behavior arises by the nonracemic native morphology of the crystalline regions, which has been induced by the temporary nonracemic guest.

Nanoporous polymeric materials: A new class of materials with enhanced properties

“…The reason for this difference resides in the presence of a dense layer that covered the films developed by Yokoyama et al [54]. Finally, Rizzo et al [234] and Guerra and coworkers [184] found that s-PS films (thickness % 20 lm; pore size % 1 nm; porosity: 7%) exhibited chiral optical responses both in the infrared and in the UV-visible regions, opening the opportunity to achieve s-PS-based films with tunable chiral optical properties. Thus, nanoporous polymers seem to present enhanced optical behavior compared to the solid, which is mainly governed by the porosity of the system and not by the pore size.…”

Chiral Optical Response of Achiral and Enantiomeric Guests in Syndiotactic Polystyrene Films

“…Recently, s‐PS based films with intense chiral optical responses at visible wavelengths have been obtained by exchanging the non‐racemic guest with achiral chromophores …”

“…In this paper, we show the chiral optical responses of sPS films and of their achiral guests like for instance azulene and 4‐nitro‐aniline (NA) included in δ‐clathrate monoclinic and triclinic crystalline forms, respectively. Moreover the chiral optical responses of the non‐racemic enantiomeric guests when they have been replaced in the crystalline cavities of s‐PS co‐crystalline phases will be also presented.…”

Intense Chiral Optical Phenomena in Racemic Polymers by Cocrystallization With Chiral Guest Molecules: A Brief Overview

“…According to a supramolecular mechanism, a nonracemic guest induces the formation of nonracemic helical crystallites, whose unit cell is instead racemic, including both right‐ and left‐handed polymer helices. This behavior is shown by syndiotactic polystyrene (s‐PS) . The chiral optical response of s‐PS films after substitution of the optically active guest with achiral chromophores or with other nonracemic guest molecules has been also presented .…”

“…This behavior is shown by syndiotactic polystyrene (s-PS). 9,[13][14][15][16][17][18][19][20][21] The chiral optical response of s-PS films after substitution of the optically active guest with achiral chromophores 9 or with other nonracemic guest molecules has been also presented. 20,21 In the final section of the review, recent additional evidence on the supramolecular nature of the chiral optical response for s-PS cocrystalline films are collected.…”