Deep insights into polymorphism initiated by exploring multicolor conversion materials

Abstract

The discovery of mechanochromic materials with multicolor conversion has mainly depended on accidents and good fortune until now, but manipulating molecular aggregation modes to achieve polymorphism is highly efficient. To obtain multicolor conversion materials and explore the intrinsic mechanism of polymorphism, TPEB2F and TPEB3F were designed and synthesized based on molecular isomerism and aggregation-induced emission (AIE) theory. Although they have similar solvatochromism and AIE properties, TPEB2F and TPEB3F show a high-contrast polymorphism and mechanochromism in terms of reversibility, fluorescence emission intensity and wavelength shift. Only crystal 2R was obtained for TPEB2F, but four different crystals 3Y, 3R, 3N and 3F exist for TPEB3F in the same solvent environment. Emission maxima of 2R show a 22 nm red shift, and its quantum yield (Φ_F) was significantly increased to 0.37 from 0.09 after hard grinding. 3Y provided a maximum redshift (40 nm) and slightly reduced Φ_F due to different intermolecular stacking and arrangement compared to the others, while 3N exhibited a remarkable blue shift (48 nm) and emission enhancement by gently grinding. X-ray single crystal analysis indicated that mechanochromism was not only related to intermolecular stacking and arrangement but also to the crystal system, space group and density. Theoretical calculations revealed that 2R, 3Y and 3R have an intramolecular charge transfer effect, and furthermore, the key to the formation of polymorphism mainly depends on the dipole moment of stacked molecular pairs and culture solvents rather than the minimum energy of stacked molecular pairs and the dipole moment of a single molecule. In addition, the by-product was purified and characterized in detail for the first time.