Correlation between Structure and Shape of the Polarized Infrared Absorption Spectra of 4-Chloro-2′-hydroxy-4′-alkyloxyazobenzenes

Abstract

Polarized infrared spectra of liquid crystalline 4-chloro-2'-hydroxy-4'-alkyloxyazobenzenes (CHAB) with C(5), C(6), and C(7) alkyl chains measured at 25 °C were compared, with particular attention being paid to the influence of chain length on formation of ordered boundary layers. The effect of chain elongation is discussed on the basis of calculations of the optimized geometry of dimeric species, which reflects the role of association in ordered phases. The impact of the varying intermolecular interactions on the spectra of C(5), C(6), and C(7) derivatives measured as a function of the polarization angle is analyzed by principal component analysis (PCA). The polarization-angle-dependent results are contrasted with the transition moment directions obtained from the density functional theory (DFT) calculations. The continuum spread down to 500 cm(-1) and a pseudoband at ca. 600 cm(-1) ascribed to the Fermi resonance between ν(OH) and γ(OH) modes is an additional phenomenon whose features also depend on alkyl chain length.