A new approach to the chiral separation of novel diazenes

Abstract

Two new types of potential liquid-crystalline azo compounds were synthesized in the form of racemic mixtures and as the individual S enantiomers. Both materials consisting of two substituted aromatic rings in the molecular core and one chiral center at the aliphatic moiety showed mesomorphic behavior. The separation of the R and S enantiomers by chiral high-performance liquid chromatography was unsuccessful when the azo compounds were in their natural E state. However, the irradiation of the compounds in the solution by UV light led to an almost quantitative transition to their Z forms. The chromatographic behavior of the compounds in their Z forms was significantly different, and partial separation of the individual enantiomers on chiral polysaccharide-based stationary phases was obtained. Thus, the proposed procedure represents a novel approach to the enantioseparation of chiral diazenes.

Influence of photoinduced isomerization on the chiral separation of novel liquid crystalline materials with a diazene moiety

“…This report represents our continuous effort to reach sufficient enantiomeric separation for newly synthesized LCMs containing a diazene moiety. Recently, we have demonstrated that individual enantiomers of these simple materials could be partially separated in normal phase separation mode after irradiation by UV light and that the resolution could be significantly improved by applying RP separation mode . The photoinduced isomerization phenomenon for the above mentioned relatively simple structures and its consequence on chromatographic behavior and UV spectra have been discussed.…”

“…As mentioned in Section 1, the irradiation of diazene moiety‐containing materials with photons of the appropriate wavelength initiates the transition of the respective compound from the more stable E isomeric form to the thermodynamically labile Z isomeric form. The time needed for reaching the photostationary state was studied for liquid crystals 1 and 2 utilizing achiral chromatography similar to the procedure described in with an MP composed of a mixture of toluene/methanol, 99.5:0.5 v/v. Table summarizes the relative amounts of the E and Z isomers of liquid crystals 1 and 2 at specific times.…”

“…Recently, we have described the synthesis and chiral chromatographic separation in both normal phase and RP separation modes of two novel diazenes potentially utilizable as liquid crystalline materials (LCMs) in various micro‐ and nanotechnology fields . Their structures were rather simple and featured one chiral center, two aromatic rings with no lateral substitution, and a diazene moiety.…”

“…Furthermore, liquid crystal 1 contains a methacrylate moiety in the achiral part of the molecule. We have shown previously that the presence of the diazene moiety within the molecular structure of LCMs led to a specific response of such compounds to UV irradiation. When photons of a suitable wavelength are applied to the azo compound, a transition from its more thermodynamically stable E configuration to its labile Z isomeric form occurs.…”

“…The thermally directed reverse transition of Z isomer to E isomer takes a relatively long time in many cases. We have demonstrated that for some LCMs, light‐induced reversible isomerization can be beneficially employed to enable chiral separation . In our previous reports, we provided chromatographic data obtained in both normal‐phase and RP modes before and after irradiation by the UV light for relatively simple newly synthesized LCMs .…”