Acylhydrazones liquid crystals: effect of structure over thermal behaviour and molecular switching

Abstract

ABSTRACT Compared to other photochromophores that show E-Z isomerisation, acylhydrazones are still relatively little explored in the field of liquid crystalline materials. To improve the knowledge about this group and their viability as the mesogenic core, herein a series of 10 molecules were synthesised and had their thermal and photoisomerisation properties investigated and correlated to structural changes. Mesomorphism was investigated by POM, DSC, XRD and SAXS techniques (using synchrotron radiation), while photoisomerisation was accompanied by UV-Vis absorption spectroscopy and 1H NMR measurements. It is demonstrated that the higher the alkoxy chain number, the lower the transition temperatures and the greater the propensity for a columnar arrangement, so that the hexacatenar compounds maintained a Colh mesophase at room temperature. On the other hand, a SmC phase was stabilised for low alkoxy chain number and the mesomorphism was extinguished for the non-symmetric structures. All compounds successfully presented E-Z photoisomerisation in solution, being faster for the molecules with the mesogenic core expanded by the addition of a benzoate unit. Overall, this work demonstrates the viability and effectiveness of the use of acylhydrazones in photoisomerizable liquid crystals, showing the advantages compared to other groups and opening an important path for applications based on this photochromophor.