Effect of solid-state packing on the photophysical properties of two novel carbazole derivatives containing tetraphenylethylene and cyano groups

Two novel carbazole derivatives containing tetraphenylethylene and cyano groups have been synthesized and characterized. The only difference between TPECZ and TPECZBr is at the C-8 and C-1 positions with or without a Br atom. Interestingly, they exhibited similar photophysical properties in solution, while their quantum yields and molecular packing models in solid state were significantly different. It was confirmed by single crystal structure analysis that the presence of a Br atom changed the molecular packing. Meanwhile, the two compounds exhibit mechanochromism (MC) behaviors, that the fluorescence emissions can be changed between yellow and orange. The powder X-ray diffraction (PXRD) and differential scanning calorimetry (DSC) suggested that the MC mechanism of TPECZ and TPECZBr can be correlated with a phase transition between crystalline and amorphous states.