A liquid crystalline homopolymer that has photoisomerizable methoxyazobenzene groups in the side chain has been synthesized and characterized. Thin films of the nematic glassy phase of this polymer have been processed in order to study the absorption spectra and the vibrational and electronic circular dichroism responses by irradiation with 488 nm circularly polarized light (CPL). Selective reflection of visible light demonstrates that the irradiation of this glassy nematic azopolymer induces a helix as a consequence of the chiral arrangement of the azobenzene units. Moreover, a wedge cell with an aligning layer for planar orientation was filled with the polymer with the aim of investigating the change in the macroscopic optical properties and optical textures of the azopolymer on irradiation with CPL. The transfer of chirality from CPL to azopolymer through chiral conformations is proposed as a model for explaining the supramolecular chirality.

No abstract

No abstract

No abstract

The 2‐(2′‐furyl)‐1H‐imidazole (1) has been prepared and characterized using infrared, Raman, and multidimensional nuclear magnetic resonance spectroscopies. Two conformations of this species obtained by rotation of 180°, approximately, around the CC inter‐ring bond are detected. Likewise, the crystal and molecular structure of 1 has been analyzed by X‐ray diffraction methods and it evidenced that both conformations are present in the lattice with equal occupancy and linked in alternate way to the NH‐‐‐N bonded polymeric chains along the crystal [101] direction. Theoretical calculations have been carried out by employing the density functional theory (DFT)/B3LYP method, in order to optimize the geometry in gas phase of both conformers and to support the assignments of the vibrational bands of 1 to their normal modes. For a complete assignment of the compound, DFT calculations were combined with Pulay's SQMFF methodology in order to fit the theoretical frequency values to the experimental one. The nuclear magnetic resonance spectrum observed for 1 is successfully compared with the calculated chemicals shifts at B3LYP/6‐311++G** level of theory for the two conformers. Furthermore, natural bond orbital (NBO) analysis and topological (AIM) calculations for an oligomer, containing three alternated units of the NH‐‐‐N bonded chain found in the crystal, reveals the characteristics of the hydrogen bonds present in the polymeric structure. Copyright © 2008 John Wiley & Sons, Ltd.

In this study 2-(2(-furyl)-4,5-1H-dihydroimidazole (1) was prepared and then characterized by infrared, Raman, and multidimensional nuclear magnetic resonance (NMR) spectroscopies. The crystal and molecular structures of 1 were determined by X-ray diffraction methods. The density functional theory (DFT) and second-order Møller-Plesset theory (MP2) with Pople's basis set show that there are two conformers for the title molecule that have been theoretically determined in the gas phase, and that only one of them, conformer I, is present in the solid phase. NMR spectra observed for 1 were successfully compared with the calculated chemical shifts at the B3LYP/6-311RRG ** level theorized for this conformer. The harmonic vibrational frequencies for the optimized geometry of the latter conformer were calculated at the B3LYP/6-311RRG ** level in the approximation of the isolated molecule. For a complete assignment of the IR and Raman spectra in the solid phase of 1, DFT calculations were combined with Pulaý s scaled quantum mechanics force field (SQMFF) methodology to fit the theoretical frequency values to the experimental ones.

No abstract

We report on a combined experimental and computational study of the chiral recognition of the amino acid serine in protonated form (L/D-SerH(+)), by the crown ether (all-S)-(18-crown-6)-2,3,11,12-tetracarboxylic acid (S-18c6H4). Infrared and vibrational circular dichroism spectroscopies (IR-VCD) are employed to characterize the chiroptical response of the complexes formed by S-18c6H4 with the L-SerH(+) and D-SerH(+) enantiomers in dried thin films obtained from aqueous solutions. The study focuses on vibrational modes directly related to the intermolecular hydrogen bonds between the crown ether derivative and serine, responsible for crown-serine binding, namely, the C═O and C-O stretching modes, and on the C-O-H bending mode, which yield intense IR and VCD signals in the range of wavenumbers 900-2000 cm(-1). The experimental spectra are analyzed in combination with a computational structural survey and optimization at different levels of density functional theory. The conformational landscape of the complexes is found to be primarily governed by a bowl-like structure of the crown ether host and a tripodal coordination of the protonated R-NH3(+) group of serine with the oxygen atoms of the central ether ring. Additionally, one or two of the carboxylic side groups of the crown ether interact with the -COH and -COOH groups of serine. Chiral selectivity is probed by recording the IR and VCD spectra of dried thin films obtained from aqueous solutions with equimolar concentrations of the two serine enantiomers and the macrocycle. The results demonstrate a marked chiral recognition of L-SerH(+) relative to D-SerH(+) by the S-18c6H4 substrate, which arises from the favorable host-guest coordination through H-bonds at optimum distances and collinear orientations, also involving a limited distortion of the crown ether backbone.