Molecular redesign unlocks 99.89% light-driven control of azobiphenyl liquid crystals

Abstract

Two series consisting of 14 unsymmetric azocholesterol dimers linking chiral cholesterol to photoswitchable azobenzene via spacers of varying length (n = 3–9) with terminal cyano (ACN-n, electron-withdrawing) or ester (ACEt-n, electron-donating) groups were synthesized. Differential scanning calorimetry (DSC), polarizing optical microscopy (POM), and X-ray diffraction (XRD) reveal enantiotropic chiral nematic (N*) phases for most derivatives in the cyano series and diverse polymorphism (N*, SmC*, SmA*) in the ester series, with spacer parity and terminal polarity dictating phase sequence and thermal stability. All compounds exhibit reversible photoisomerization in solution, with the ester derivative ACEt-5 showing nearly quantitative E → Z conversion (99.89%). A prototype optical storage device based on the structurally related cyano dimer ACN-8 (5 wt% in E7) demonstrates efficient switching under 365 nm UV irradiation, with a photo-stationary state reached in about 150 s and thermal back-relaxation occurring over approximately 160 min, highlighting the strong device relevant photo-responsiveness of this molecular design in a guest–host system. Density functional theory (DFT) elucidates structure–property relationships via HOMO–LUMO gaps, electrostatic potentials, and polarizability trends, revealing molecular design principles for tunable photoresponsive liquid crystalline materials.