UHPLC and NMR for monitoring of E↔Z isomerization in chiral liquid crystalline azobenzene derivatives

This is the authors' abstract. We don't add key points for this paper.

BACKGROUND: Photosensitive liquid crystals are of considerable technological interest since the light-induced structural changes enable reversible control of material properties. Methods for assessing their E↔Z isomerization are needed during design of new compounds to elucidate the relationship between molecular structure and materials' properties. In this study, ultra(high)performance liquid chromatography (UHPLC) and nuclear magnetic resonance (NMR) were employed to monitor the thermal relaxation of homologous series of twelve chiral liquid crystalline compounds containing a photoisomerizable azobenzene group. Azobenzene is a key photoactive component in photosensitive liquid crystalline materials. RESULTS: Both UHPLC and NMR enable quantitation of E and Z isomers in the investigated materials, however, they are based on fundamentally different measurement principles. NMR offers universal and immediate analysis, while UHPLC allows faster individual runs and higher sample throughput. Differences in thermal relaxation rates depending on the length of alkyl chain in the molecular structure as well as on the measurement conditions were observed. SIGNIFICANCE: This work compares the strengths and limitations of these two approaches for investigating photoresponsive liquid crystalline materials and demonstrates their respective applicability. The observed discrepancies were mainly attributed to manual, non-automated steps in the analytical workflow. Both methods provided reliable results and serve the purpose of investigation of photo responsiveness and subsequent thermal relaxation in novel materials and can thus guide the design of new mesomorphic materials.