Study of the photoinduced supramolecular chirality in columnar liquid crystals by infrared and VCD spectroscopies

IR and VCD spectroscopies are employed to clarify the molecular origins of supramolecular chirality in azobenzene-containing columnar liquid crystals. The different columnar mesomorphic assemblies, Col r and Col h, of an achiral and a chiral propeller-like hydrogen-bonded complex, respectively, are used for this study. The mesomorphic behavior of the achiral complex is studied here for the first time, and the structural parameters of its Col r mesophase are determined by X-ray diffraction. Both complexes bear azobenzene units and this makes it possible to implement photoresponsive columnar architectures, the chirality of which can be induced and modulated by irradiation with 488 nm circularly polarized light (CPL). Thin films of the respective rectangular and hexagonal columnar phases of these complexes have been processed in order to study the IR absorption spectra and the vibrational circular dichroism responses upon irradiation with CPL. These studies allow confirming that the outer part of the columns, consisting on azobenzene groups, is mainly involved in the photoinduced supramolecular chirality, rather than the inner part of the columns where a melamine core is located. This supports a structural model based on the helical disposition of the azobenzene groups along the stacked propeller-like complexes. © 2012 American Chemical Society.J.R.A.M. expresses thanks to the Ministry of Education and Science of Spain for the Post-Doc Grant SB2006-0119 and to the Andalusia Government for the Post-Doc Grant P08-FQM-04096. F.V. thanks the MEC for a grant. The authors thank the University of Jaén and the Andalusia Government for financial support, the MICINN Project MAT2009-14636-C03-01, FEDER founding (EU), and the Aragon Government.Peer Reviewe