A Novel Strategy to Design and Construct AIE-active Mechanofluorochromic Materials via Regulation of Molecular Structure.

In this work, we first designed and synthesized tetraphenylene-fused aryl-imidazole derivatives TM-1~4 via regulation of molecular structure,which were consisted of 1H-imidazo[4,5-f][1,10]phenanthroline, 1H-phenanthro[9,10-d]imidazole, 4,5-diphenyl-1H-imidazole, 3,3'-(1H-imidazole-4,5-diyl)dipyridine moieties and AIE-active tetraphenylethene units, respectively. The results illustrated that TM-1~4 exhibited obvious AIE characteristics. Meanwhile, TM-2 and TM-3 show excellent solid emission properties (Φ TM-2 =13.73% and Φ TM-3 =36.21%), whereas TM-1 and TM-4 exhibit the opposite properties (Φ TM-1 =1.48% and Φ TM-4 =4.83%). The multiple rotors (pyridine and benzene ring) causes twisted conformations of the molecule that prevents π-π stacking and enhances solid emission(Φ TM-2 <Φ TM-3 , Φ TM-1 <Φ TM-4 ). Significantly, TM-2 and TM-3 also exhibited reversible mechanochromic behavior (Emission red shifts: Δλ TM-2 =43 nm and Δλ TM-3 =41 nm) with color changes between blue and green emissions. The powder X-ray diffraction (PXRD) suggested the disordered state of ground sample could be readily returned to an ordered crystalline. Therefore, the mechanochromisms of TM-2 and TM-3 are ascribable to the phase transformation between crystal and amorphous structure. The single crystal X-ray analysis of TM-2 reveals a twisted conformation for TPE moiety and the absence of π-π intermolecular stacking. These excellent optical properties of TM-2 and TM-3 make them potentially applications in mechanochromic materials and imaging agents.