The kinetics of the E-Z-E isomerisation and liquid-crystalline properties of selected azobenzene derivatives investigated by the prism of the ester group inversion

Abstract

Two new groups of azobenzene ester derivatives were synthesised: alkyl 4-[4-(nonyloxy)phenyl]diazenyl]benzoates and 4-[4-(nonyloxy)phenyl]diazenyl]phenyl alkanoates. All 35 presented homologues are mesogenic. Moreover, some of the above-mentioned compounds exhibit rich liquid-crystalline polymorphism likewise tetramorphism. During this investigation by the use of polarising optical microscopy, differential scanning calorimetry and X-Ray studies, six types of mesophases were detected: nematic, smectics (A, C, I, F) and G. Furthermore, due to the presence of the photosensitive azo moiety, the E-Z isomerisation reaction is possible. This process, which is initiated by the UV irradiation, causes significant changes in the UV-Vis absorption spectra of investigated compounds. However, the photoisomerisation is a reversible process and in the dark the thermal relaxation of Z isomer takes place. Based on the achieved data, the kinetic constants of the isomerisation and relaxation processes were calculated. It shows that conversion of the ester bond makes some changes in the optical properties. The shift of about 7 nm of the absorbance maximum was observed. Surprisingly, the inversion of the ester group has significant influence on the liquid-crystalline polymorphism replacing one mesophase (for benzoates) into four (for alkanoates).

Chiral separation of novel diazenes on a polysaccharide-based stationary phase in the reversed-phase mode

“…When photons of a suitable wavelength are utilized the azo‐compound transition from its more thermodynamically stable E configuration to its labile Z form occurs. This phenomenon is called light‐induced reversible isomerization, and its detailed mechanism, kinetics, and thermodynamics are still under investigation . The mentioned isomerization capability of these LCMs is useful and applicable in modern display technology as well as other fields such as separation and biomaterial sciences .…”

A new approach to the chiral separation of novel diazenes

“…The absorption of UV light corresponds to the π–π* electron transition within the azo group; this absorption is accompanied by the transformation of the rod‐like E shape of the molecule to the bent Z configuration. Moreover, this phenomenon is reversible, and the relaxation back from the Z to the E configuration can be induced by visible light in the range of 400–550 nm as well as by heating .…”

Thermal and Photophysical Studies of Binary Mixtures of Liquid Crystal with Different Geometrical Mesogens

“…In both types of binary mixtures, the intermolecular interactions between the two mesogenic cores lead to a significant variation in the optical activity [21][22][23][24]. In previous work [25], it was observed that the mesophase stability is enhanced by an increment of the polarity and/or polarizability of the mesogenic core of the molecule. In general, the linking groups in the mesophase formation, type, thermal stability, and temperature ranges of the LCs compounds [26][27][28][29][30][31][32].…”