Tautomerism and Dichroism in Anisotropic Media

Azoquinolin compounds are well known to be in equilibrium with other tautomeric forms. It is described by the intra-molecular proton transfer between the phenol and imine groups in ground and/or excited state. Nature and position of the substituents of the molecule play a significant role on the tautomeric equilibrium, and thus on photo- physical properties.Since the azo group is an electron acceptor. The hydrazone tautomer is favored by electron withdrawing substituents, like NO۲, because the imino group is an electron donor. Further stabilization of the hydrazone tautomer is achieved with intra- or intermolecular hydrogen bonding.It is due to the large differences in the dipole moments and electron donor and acceptor properties of each tautomer. Generally, the equilibrium is influenced strongly by the properties of the solvent. The hydrazone tautomer is promoted by an increase in the polarity of the media. Both tautomers can be detected from the visible absorption spectra. The differences between the absorption spectra of the tautomeric compounds can be used to distinguish them.The anisotropic solvation characteristic of azoquinolins (Fig. ۱) in polar nematic liquid crystals was investigated. The azoquinolin dyes in the polar nematic hosts show an anomalous behavior. From the spectral data (Fig. ۲), the influence of anisotropic environment on the solvation of the compound is complicated. However, the spectral behavior of azoquinolins in nematic solvent should have multiple origins, and several interactions contribute to some extent.