Exploration the inherent mechanism of polymorphism and mechanochromism based on isomerism and AIE theory

Polymorphism plays an important role in the fields of drugs, pigments and multicolor conversion materials, however, obtaining polymorphism mainly depend on accident and fortune until now. To explore the intrinsic mechanism of polymorphism and mechanochromism, TPEB2T and TPEB3T were designed and synthesized based on isomerism effect and aggregation-induced emission (AIE) theory. Photo-physical properties of the two complexes indicate they exhibit solvent-dependent fluorescence spectra, similar AIE property and markedly different mechanochromism and polymorphism. By diffusion n-hexane into dichloromethane solution of luminogens, the crystals of TPEB2T (2O) and TPEB3T (3Y and 3O) were obtained. The emission maxima of crystals 2O, 3O and 3Y display 23, 40 and 66 nm red-shift respectively after grinding, accompanied by significantly enhanced fluorescence quantum efficiency for 3Y from 0.09 to 0.39 and over 50% decline for 2O and 3O. Compared with 2O and 3O, the high contrast mechanochromism and weak emission intensity for crystal 3Y were attributed to different intermolecular head to head overlap rather than heavy atom effect of sulfur based on single crystal analysis and theoretical calculation. By extracting molecular pairs from crystals and constructing hypothetical molecular pairs based on molecular isomerization between sulphur and boron atoms, the minimum energy and dipole moment calculation results reveal that the formation polymorphism mainly depend on dipole moment of molecular pairs and culture solvent.