Axial Chirality can lead to Chiral Smectic and Cholesteric Liquid Crystals

Influence of helical twisting power on the photoswitching behavior of chiral azobenzene compounds: applications to high-performance switching devices

Five photochromic chiral azobenzene compounds and one nonphotochromic chiral compound were synthesized and characterized by IR, 1H NMR spectroscopy, and elemental analysis. Cholesteric liquid crystalline phases were induced by mixing of the nonphotochromic chiral compound and one of the photochromic chiral azobenzene compounds in a host nematic liquid crystal (E44). The helical pitch of the induced cholesteric phase was determined by Cano's wedge method and the helical twisting power (HTP) of each sample was thus determined. The helical twisting powers of azobenzene compounds were decreased upon UV irradiation, due to trans-->cis photoisomerization of azobenzene molecules. Among the azobenzene compounds synthesized in our study, Azo-5, with isomannide (radical) as chiral photochromic dopant, showed the highest HTP and contrast ratio (Tmax/Tmin). Photoswitching between compensated nematic phase and cholesteric phase was achieved through reversible trans<-->cis photoisomerization of the chiral azobenzene molecules through irradiation with UV and visible light, respectively. Transmission rates (contrast ratios) increased with decreasing helical pitch length in the induced cholesteric phase. The influence of helical twisting power on the photoswitching behavior of chiral azobenzene compounds is discussed in detail.

Flexibility vs Rigidity of Singly and Doubly Tethered Biphenyls: Structure, Dynamic Stereochemistry, and Resolution of Tribenzo[a,c,f]cyclooctane, Tetrabenzo[a,de,h,kl]bicyclo[6.6.0]tetradecane, and Their Alkyl Derivatives

The barrier for enantiomerization of tribenzo[a,c,f]cyclooctane (1a) as acquired from dynamic (1)H NMR experiments was found to be DeltaG() = 19.5-20.1 kcal mol(-)(1), and DeltaG() = 17.2-17.6 kcal mol(-)(1) for its alkylated derivatives (1b-e) at the coalescence temperatures. Tetrabenzo[a,de,h,kl]bicyclo[6.6.0]tetradecane (2a) and its tetraethyl derivative (2b) were separated to their enantiomers by chiral HPLC. The kinetic values for the enantiomerization were obtained by following the racemization rate of the pure enantiomer. Barriers of DeltaG()(596) = 49.6 kcal mol(-)(1) and DeltaG()(561) = 46.3 kcal mol(-)(1) for 2a and 2b, respectively, were obtained. Strong solvent and temperature effects on the (1)H NMR chemical shifts were observed. X-ray crystallography revealed torsional conformations for 1a, 1b, and 2a with dihedral angles of 57.8 degrees, 56.1 degrees /58.2 degrees (two conformations), and 57.7 degrees, respectively. Semiempirical calculations (AM1) reveal higher relative strain energies and smaller biphenyl dihedral angles for the bulky alkyl substituents, which correspond to their lower DeltaG() values.