AIE Fluorescent Gelators with Thermo-, Mechano-, and Vapochromic Properties

A series of aggregation-induced emission (AIE) fluorescent gelators (TPE-C_n -Chol) were synthesized by attaching tetraphenylethylene (TPE) to cholesterol through an alkyl chain. The properties of the gel, nano-/microaggregate, and condensed phases were studied carefully. TPE-C_n -Chol molecules form AIE fluorescent gels in acetone and in DMF. Their fluorescence can be reversibly switched between the "on" and "off" states by a gel-sol phase transition upon thermal treatment. The AIE properties of aggregated nano-/microstructures in acetone/water mixtures with different water fractions were studied by using fluorescence spectrometry and scanning electron microscopy (SEM). In different acetone/water mixtures, the TPE-C_n -Chol molecules formed different nano-/microaggregates, such as rodlike crystallites and spherical nanoparticles that showed different fluorescence colors. Finally, the condensed phase behavior of TPE-C_n -Chol was studied by using polarizing microscopy (POM), differential scanning calorimetry (DSC), fluorescence spectrometry, fluorescence optical microscopy, and wide-angle X ray scattering (WAXS). The clover-shaped TPE unit introduced into the rodlike cholesterol mesogen inhibits not only the formation of a liquid-crystal phase but also recrystallization upon cooling from the isotropic liquid phase. Very interestingly, TPE-C_n -Chol molecules in the condensed state change their fluorescence color under external stimuli, such as melting, grinding, and solvent fuming. The phase transition is the origin of these thermo-, mechano-, and vapochromic properties. These findings offer a simple and interesting platform for the creation of multistimuli-responsive fluorescent sensors.