Polymorphous meso-Dicyanovinyl-Substituted Tetraphenylethylene Derivative Showing Distinguishing Two-Color Emission and Ultrahigh-Contrast Mechano-Responsive Luminescence Behavior

The development of versatile luminescent materials, especially those exhibiting high-contrast mechano-responsive luminescence (MRL) characteristics, is urgently desirable but a daunting challenge. Herein, two novel MRL materials named TPEP-m-2DCV and TPE-m-2DCV were designed and synthesized, in which the strategy of incorporating two dicyanovinyl (DCV) units into meso-positions of one benzene ring of the TPEP and TPE skeleton was realized for the first time. Both TPEP-m-2DCV and TPE-m-2DCV exhibited solvatochromism, aggregation-induced emission (AIE), and reversible MRL properties. More interestingly, two different polymorphs of TPEP-m-2DCV (TPEP-m-2DCV(B) and TPEP-m-2DCV(O)) with distinguishing emission (468 and 580 nm) could be separated by adjusting the solvent polarity in the process of growing single crystals. Furthermore, TPEP-m-2DCV(B) with blue fluorescence showed ultrahigh red-shifted emission up to 141 nm after grinding, while TPEP-m-2DCV(O) with orange emission did not exhibit MRL property. Single-crystal structural analysis of two polymorphs and corresponding theoretical calculations were used to explain their significant differences in photophysical properties. TPEP-m-2DCV(O) possessed a higher coplanarity and a more tightly packed dimer. According to the interaction region indicator (IRI) and density functional theory (DFT) analysis, the TPEP-m-2DCV(O) dimer showed stronger noncovalent interactions (NCI) and a significantly reduced energy gap. TPEP-m-2DCV(B) underwent a morphology transformation from crystalline to amorphous state upon grinding, with a tremendous red shift in emission. However, TPEP-m-2DCV(O) was MRL inactive because it could remain in the original crystalline state with the aid of fused hydrogen-bonded rings in the crystal packing when suffering an external force. It is believed that this work reveals the relationship between intermolecular interactions/molecular conformations and MRL performance and provides useful insights for designing high-contrast MRL materials.