Structure and liquid crystalline properties of 2-hydroxyazobenzenes. X-ray diffraction, infra-red and DFT theoretical studies

The family of 20-hydroxy-40-alkyloxyazobenzenes containing in position 4 either CH3 or Cl group with varying number of carbon atoms in alkyl group was analysed from the point of view of liquid crystalline properties and intramolecular OH· · ·N hydrogen bonding. The phase transition diagrams show that insertion of the OH group in 20 position leads to a marked extention of mesophases as compared with parent azobenzenes and appearance in addition to nematic of the smectic SmA phase in the case of 4-chloro-derivatives. The role of chelate conjugating OH· · ·N hydrogen bonds in modification of mesophases is discussed based on DFT theoretical and X-ray as well as infrared studies. The importance of the molecule pairing is emphasized. Most important feature of mesophases and crystalline state of compounds under study is appearance of a continuous absorption down to 500 cm21 which is absent in solutions and Ar matrices. q 2004 Elsevier B.V. All rights reserved