Photoresponsive behavior of hydrophilic/hydrophobic-based novel azobenzene mesogens: synthesis, characterization and their application in optical storage devices

Three series of alkoxy chain-bearing azobenzene-derived quaternary ammonium iodides with an alkoxy chain at one end, namely N,N-diethanol-6-(4-((4′-alkyloxyphenyl)diazenyl)phenoxy)hexan-1-ammonium iodides, N-ethyl-N-ethanol-6-(4-((4′-alkyloxyphenyl)diazenyl)phenoxy)hexan-1-ammonium iodides and N,N-diethyl-6-(4-((4′-alkyloxyphenyl)diazenyl)phenoxy)hexan-1-ammonium iodides were synthesized and characterized. Their mesomorphic and photoswitching properties were examined via polarising optical microscopy (POM), differential scanning calorimetry (DSC) and UV-vis spectrophotometry. The liquid crystalline tilted schlieren texture of smectic C, non-tilted natural focal conic texture of smectic A and smectic B phases were observed in the N,N-diethanol- and N-ethyl-N-ethanol-bearing ammonium group substituted at the terminal via the alkoxy chain of the azo moiety. In these azo moieties, the equilibrium time for trans–cis isomerization was about 1 min and cis–trans isomerization occurred at around 590 min, which had the highest alkoxy chain and no hydroxyl group on their head group. The absence of a hydroxyl group on the terminal head group resulted in slow thermal back relaxation, whereas the hydroxyl group-bearing head group showed fast thermal back relaxation. These results suggest that the influence of the substituent on the cationic ammonium head group and alkoxy chain length on the photoisomerization of the azo compounds is vital for optical storage devices. Furthermore, the device fabricated using these materials demonstrated that they are excellent candidates for optical image storage applications.