Cholesterol-appended cyanostyryl thiophene positional isomers with multistimuli responsive emission switching and liquid crystalline properties

Abstract

The present work describes the synthesis of two cholesterol appended cyanostyryl thiophene positional isomers CS-1 and CS-2 and their stimuli responsive solid state emission and liquid crystalline properties. Both compounds showed enhanced emission in the solid state due to their aggregation-induced emission (AIE). Interestingly, CS-1 and CS-2 showed emission switching under the influence of thermal and/or mechanical stimuli. The difference in emission properties of pristine CS-1 and its melt processed sample CS-1M is attributed to the formation of two different crystalline phases. The powder X-ray diffractogram simulated from the single crystal X-ray diffraction data of CS-1 and CS-1M showed similar features indicating that the molecules in the melt cooled phase adopt a packing similar to the one observed in the single crystal. In addition to heat-induced changes in fluorescence properties, compound CS-2 showed mechanofluorochromism and methanol vapour-induced reversible emission switching. The observed emission switching in CS-2 under thermal and shear stress was due to crystalline to amorphous transition. The emission recovery from the ground sample of CS-2 under methanol vapours was attributed to the reversible transition from amorphous to a crystalline phase similar to pristine CS-2. A detailed analysis of various photophysical parameters of the compounds under different stimuli is reported. The liquid crystalline properties of the compounds were studied using polarizing optical microscopy (POM), differential scanning calorimetry (DSC), and X-ray diffraction (XRD) measurements. Both CS-1 and CS-2 showed the formation of an enantiotropic cholesteric (N*) liquid crystalline phase. Interestingly, the N* phase in CS-1 and CS-2 was stabilized down to room temperature in a glassy state.