Supramolecular Helical Fluid Columns from Self‐Assembly of Homomeric Dipeptides

Abstract

Herein, we demonstrate that with the widespread theme of residue patterning and stereochemical restraints of self-complimenting proteinogenic amino acids, a new and rich class of homomeric dipeptides exhibiting two-dimensional fluid aggregates with hierarchical ordering can be obtained. In particular, a simple way of achieving a class of functional dipeptides, wherein the first and the second residues chosen are L-/D-alanines and L-/D-leucines, has been accomplished. The supramolecules synthesized can be regarded as intermediates between polycatenars and taper-shaped amphiphiles because they possess two lipophilic segments interlinked by a peptide unit (spacer). Two pairs of enantiomers and their respective diastereomers derived from these amino acids are evidenced to self-organize into a helical columnar phase through hydrogen bonding by means of FTIR, UV/Vis, and chiroptical circular dichroism (CD) spectral analyses as well as by optical, calorimetric, electrical switching, and X-ray studies. The CD and X-ray studies have revealed that the form chirality (handedness) and the magnitude of out-of-plane fluctuations of the lattice planes of the fluid supramolecular columnar structures are solely directed by the stereochemistry encoded in the spacer. Of special significance, the less frequently found oblique helical columnar phase formed by a pair of enantiomers derived from L-/D-alanines, unlike those derived from other amino acids, exhibit ferroelectric behavior; the measured spontaneous polarization is as high as 440 nC cm(-2). Besides, all these supramolecules form stable organogels in ethanol and the CD and SEM studies on a representative gel suggest the presence of helical structures.

Interplay between polarity and chirality in the electric-field-responsive columnar phase of a dipeptide derivative

“…The SHG signal was only observed under an electric field, suggesting that this material is apolar in the absence of an electric field. This result is inconsistent with observations made previously by Yelamaggad et al, 8 who observed a single reversal current peak, which is characteristic of a ferroelectric liquid crystal. The SHG intensity increases continuously and saturates at nearly the same voltage, 25 V PP mm À1 , as the EO response, meaning that the polar order is closely related to the rotation of the optical axis.…”

“…The electric field dependence of the SHG signal and the reversal current measurements show that the Col ob phase is macroscopically apolar in the absence of an electric field. Yelamaggad et al 8 suggested the formation of a helical structure along the column axis. From the in-plane anisotropy of the IR absorption and the SHG intensity, however, it is obvious that the molecules do not form a helix but stack with their long axes and molecular planes nearly parallel to one another in each column.…”

“…Recently, Yelamaggad et al 8 synthesized unique discotic columnar (Col) liquid crystals composed of dipeptide derivatives. Some of the derivatives exhibited a single polarization reversal current peak during application of a triangular wave voltage.…”

“…We also discovered an asymmetric incidence-angle dependence of the SHG signals and the IR absorption, which is inconsistent with the proposed model of a helical molecular arrangement along the column axis. 8 We propose a new model based on the experimental observations described in this work.…”

Hydrogen Bonding for Supramolecular Liquid Crystals

“…5.20) [100]. Recently, Tshierske and coworkers have combined 2,3,4-and 3,4,5-trisubstituted aromatics in benzoylated benzylidene hydrazines enhancing the formation of thermally stable, wide range room temperature columnar liquid crystals (Fig.…”