HYDROGEN BOND IN POTENTIALLY MESOGENIC MOLECULAR COMPLEXES OF 4-(PHENYLAZO)BENZOIC ACID AND 4-(PHENYLAZO)PHENOL WITH PYRIDINE DERIVATIVES

Abstract

Using experimental and theoretical methods, it has been shown that hydrogen-bonded molecular complexes in two-component systems based on 4-pyridyl-4'-n-alkyloxybenzoates (n = 7, 12) with 4-(phenylazo)benzoic acid and 4-(phenylazo)phenol (composition of 1:1) are formed. Quantum chemistry methods (DFT (B3LYP)/cc-pVTZ) were used to determine the conformational properties of 4-(phenylazo)benzoic acid and 4-(phenylazo)phenol molecules. It has been shown that the introduction of 4-carboxy and 4-hydroxy groups into the azobenzene molecule has practically no effect on the energy characteristics of the trans-cis isomerization process. The energy, geometric, and electronic characteristics of intermolecular hydrogen bonds in the H-complexes have been estimated. The calculations showed that the H-complexes of 4-pyridyl-4-n-propyloxybenzoate with 4-(phenylazo)benzoic acid and 4-(phenylazo)phenol differ significantly from each other in geometric structure: the complex with 4-(phenylazo)benzoic acid – rod-shaped, complex with 4-(phenylazo)phenol – angular. A comparison of the energy characteristics of H-complexes, as well as the characteristics of hydrogen bonds, showed that a stronger hydrogen bond is formed in the complex with 4-(phenylazo)benzoic acid than in the complex with 4-(phenylazo)phenol. The results of modeling dimers of molecules of 4-(phenylazo)benzoic acid, 4-(phenylazo)phenol, reproducing intermolecular interactions in crystals, and their hydrogen-bonded complexes allow to conclude that in the process of self-organization of systems with a composition of 1:1, associates of 4-(phenylazo)benzoic acid molecules, associates of 4-(phenylazo)phenol molecules will be broken and H-complexes with 4-pyridyl-4'-alkyloxybenzoates will be formed instead. Changes in the experimental IR spectra recorded for the initial components and for systems with a composition of 1:1 confirm the formation of molecular complexes. The structural units of the studied systems based on 4-pyridyl-4'-alkyloxybenzoates with 4-(phenylazo)benzoic acid and 4-(phenylazo)phenol with a composition of 1:1 can be considered supramolecules, formed by hydrogen bonding. The structure of supermolecules and the presence of their geometric anisotropy indicate their potential mesogenicity. The sample of H-complexes of 4-(phenylazo)phenol and 4-pyridyl-4'-n-dodecyloxybenzoate (composition of 1:1) was studied for the manifestation of mesomorphic properties using the method of polarization thermomicroscopy. The textures registered in a polarizing microscope allow to conclude that the H-complex has a smectic mesophase in the temperature range of 101.4–109.4 °C.