Self-Assembly Mechanism and Analysis of Smectic Mesophase Induction in Binary Systems of 4-(4-Alkoxyphenylazo)benzonitriles and Dodecyloxybenzoic Acid

Using a combination of experimental methods (polarizing optical microscopy, IR spectroscopy), the two-component systems consisting of 4-dodecyloxybenzoic acid (BA) and 4-(4-alkoxyphenylazo)benzonitriles (BN-n, n = 8, 10, 12) have been studied. The mesophase existence ranges of the systems depending on the length of alkyl substituents of the component BN-n were established. The possibility of formation as well as stability of heterodimeric complexes with a hydrogen bond –COOH···NC – were analyzed by DFT/B97D/6-311++G** method. In all obtained equimolar BN-n:BA (1:1) systems SmA phase was induced, which manifests itself enantiotropically in wide temperature ranges. With the help of computer simulation, it was shown that, unlike homodimers, the expected BN···BA H-complex has an angular structure, which cannot ensure the formation of the SmA phase. To elucidate the mechanism of mesophase formation in BN-n:BA systems, we calculated the energies of intermolecular interaction in homo- and heterodimers, and analyzed the experimental IR spectra of equimolar BN-n:BA systems. The spectra represent a superposition of the spectra of associates BN···BN and cyclic dimers of acid BA···BA. Therefore, we suggest that the BN···BA type H-complexes cannot form in equimolar BN-n:BA mixtures. Instead, a structure consisting of alternating layers of BA···BA dimers and BN···BN associates is formed. The layers of the acid dimer play the main role in the formation of SmA phase. It is noted that –COOH group, in contrast to such proton-donor groups as –NH2, –NH–, –OH, does not form hydrogen bonds with the proton-acceptor –CN group.