Liquid crystalline properties of and intramolecular hydrogen bonding in 4-methyl-2′-hydroxy-4′-alkoxyazobenzenes

A homologous series of 4-methyl-2′-hydroxy-4′-alkoxyazobenzenes with chain lengths from 1 to 12 carbon atoms was synthesized and the temperatures of melting, freezing and transition from nematic to isotropic phase were determined. The main thermodynamic characteristics for those transitions were estimated. The liquid crystalline properties were compared with those of homologous series without OH groups and show a marked extension of the nematic phase for the OH substituted compounds. The spectroscopic manifestations of the hydrogen-bonded OH ... N chelate ring are discussed, based on a comparison of experimental and Density Functional Theory (DFT) calculated frequencies and IR intensities. The evolution of the IR absorption in the range 1540-1660cm^-1, ascribed to the OH bending vibrations, is presented on going from the dilute CCl₄ solution through the isotropic and nematic phases to the solid state.