4-Chloro-2′-hydroxy-4′-alkoxyazobenzenes. Intramolecular OH··N hydrogen bonding and liquid crystalline properties

A series of 4-chloro-2′-hydroxy-4′-alkoxyazobenzenes with n=1-12 was synthesized. Liquid crystalline properties were studied by using the DSC and thermo-optical (TOA) techniques and observation of textures in the polarizing microscope. For the members with n=5-8 the nematic and smectic A phases were evidenced. For n=4 only the nematic phase is visible. The presence of an OH group in 2′-position leads to a lowering of crystallization temperature and marked extension of the smectic A phase. The temperature dependence of IR spectra of neat compounds in thin layers was studied and analysed based on density functional theory (DFT) calculations. The results obtained show that in a thin layer (KRS cell windows) the molecules tend to orient perpendicularly to the surfaces of windows. This leads to spectacular changes in IR band intensities going from isotropic through nematic and smectic A to crystalline phases. These changes are opposite when comparing modes in which predominate in-plane or out-of-plane vibrations.