Chiroptical Circular Dichroism Research Articles

Chiroptical spectrophotometer and analytical method for optically anisotropic samples.

A new dual-polarization-modulated comprehensive chiroptical spectrophotometer (D-CCS) was successfully developed for recording artifact-free chiroptical circular dichroism (CD) and circularly polarized luminescence (CPL) spectra of optically anisotropic samples on the basis of the Stokes-Mueller matrix approach. The D-CCS includes an additional second photoelastic modulator (PEM) set in front of the sample and a second PEM controller being driven independently of the first PEM driver, which are essential for obtaining artifact-free chiroptical signals of optically anisotropic samples containing non-chiral signals, i.e., linear birefringence, linear dichroism, and linearly polarized luminescence. The theory and implementation of the method are described, and examples of its application to CD and CPL samples with macroscopic anisotropies are provided. An important advantage of the developed D-CCS is that it reduces the essential step of sample rotation in the plane, including the gravitational axis, to obtain artifact-free CD signals. Consequently, this dual-polarization method shortens the time required to obtain artifact-free CD signals by half and could be very effective for samples, such as high fluidity samples, gels, liquid crystals, and supramolecules in solution, for which artifact-free chiroptical signals are difficult to obtain using conventional methods.

Supramolecular Helical Fluid Columns from Self‐Assembly of Homomeric Dipeptides

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.