Liquid crystalline properties and IR spectra of 2′-hydroxy-4′-octyloxyazobenzenes

New azobenzene derivatives, namely 3,5-dichloro, 2,6-dimethyl and 3-chloro-4-methyl derivatives of 2'-hydroxy-4'-octyloxyazobenzene were synthesized and their properties were compared with those of 4-chloro, 4-methyl and 4-nitroderivatives. Special attention was paid to the characteristics of intramolecular O-(HN)-N-. . . hydrogen bond reflected in IR spectra Of CCl4 solutions and neat crystalline samples. The spectra were analyzed based on DFT calculations. In all cases of solutions very strong hydrogen bonds are manifested in broad bands centered at about 2700-2800 cm(-1) of low intensity typical of resonance assisted H-bonds. On the other hand, in cases of neat crystalline samples, a broad, intensified absorption is extended down to similar to 600 cm(-1) forming a continuum resembling a Hadzi's trio. However, neither broad maxima nor minima can be assigned to delta(OH) or gamma(OH) overtones. The studies of the H/D isotope effect on the continuum seem to indicate on the role of resonance couplings between v(OH) vibrators of neighboring molecules as well as hot bands arising from the coupling between high and low frequency modes. These couplings modify the dynamic pattern of the potential for the proton motion leading to a decrease of the barrier for the proton transfer. (C) 2003 Elsevier Science B.V. All rights reserved.