2′-Hydroxybenzylideneanilines and 2-Hydroxyazobenzenes. The Role of Intramolecular O-H•••N Resonance Assisted Hydrogen Bonds

A short review on the effect of the OH groups in the ortho position on the liquid crystalline properties of benzylideneanilines and azobenzenes is presented. Numerous examples show that there are no general rules governing the behaviour of these compounds. However one finds most frequently a broadening of the existence range of mesophases both due to a decrease of freezing temperature and an increase of the isotropisation temperature. In contrast, the presence of strongly polar groups such as -NO2 and -CN leads to a reverse behaviour that can be explained by assuming a strong it-electron conjugation of these groups with the conjugate-chelate ring formed by intramolecular O-H center dot center dot center dot N hydrogen bond. These rings are analyzed by using data related to the structure of molecules and infra-red spectra in the region of the stretching nu(OH) and bending delta(OH) and gamma(OH) modes.