Premium
Supramolecular Chirality and Reversible Chiroptical Switching in New Chiral Liquid‐Crystal Azopolymers
Author(s) -
Barberá Joaquín,
Giorgini Loris,
Paris Fabio,
Salatelli Elisabetta,
Tejedor Rosa M.,
Angiolini Luigi
Publication year - 2008
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.200801246
Subject(s) - chirality (physics) , circular dichroism , supramolecular chirality , crystallography , materials science , polymer , supramolecular chemistry , dispersity , polymerization , monomer , liquid crystal , side chain , cotton effect , chemistry , crystal structure , polymer chemistry , chiral symmetry , optoelectronics , physics , quantum mechanics , nambu–jona lasinio model , composite material , quark
Abstract The synthesis of chiral liquid‐crystalline polymers of well‐controlled structure (linear and three‐armed star‐shaped) with distinct average chain lengths and low polydispersity was achieved by atom transfer radical polymerisation (ATRP) of a new optically active monomer ( S )‐4‐[6‐(2‐methacryloyloxypropanoyloxy)hexyloxy)]‐4′‐ethoxyazobenzene [( S )‐ ML6A ], containing the L ‐lactic residue of one absolute configuration in the side‐chain. All the obtained polymeric samples, characterised by differential scanning calorimetry (DSC), X‐ray diffraction (XRD) and polarised optical microscopy (POM), exhibit a smectic A 1/2 (fully interdigitated) liquid‐crystalline phase and high cleaning points, with transition temperatures dependent on the average polymerisation degree and the macromolecular structure. The chirality originated at the molecular level by the asymmetric functionality of the L ‐lactic acid residue provides the polymers, in the smectic phase, of highly homogeneous conformations with a prevailing chirality related to the presence of H‐aggregates having conformational dissymmetry of one prevailing screw‐sense. By irradiating with circularly polarised light (CPL), it is possible to photomodulate the chiroptical properties of these intrinsically chiral polymeric thin films. Upon irradiation with left‐handed CPL ( l ‐CPL), the circular dichroism (CD) spectra of the films show enhancement of ellipticity and a net inversion of sign. The effect is reversible and the mirror image of the CD spectrum can be restored by pumping with right‐handed CPL radiation ( r ‐CPL). The results show the ability of l ‐CPL to invert the supramolecular chirality of the materials and demonstrate the essential role of azoaromatic aggregates.