D-A type π-systems containing tetraphenylethene exhibiting aggregation-induced emission and reversible mechanical force-induced luminescent enhancement and chromism

Two novel D-A type tetraphenylethene-based fluorophores, named XA-BZ-TPE and XA-DCE-TPE, were designed and synthesized. Both fluorophores exhibit typical intramolecular charge-transfer features, good solid-state luminescence, and remarkable aggregation-induced emission (AIE) behavior. The AIE factors for XA-BZ-TPE and XA-DCE-TPE were >243 and 12, respectively. Notably, both XA-BZ-TPE and XA-DCE-TPE display high-contrast mechanofluorochromic (MFC) properties. During the grinding of initially prepared solid powders, the emission colors transformed from blue and blue-green to blue-green and yellow, and their maximum emission wavelengths shifted from 449 to 485 nm to 481 and 557 nm, respectively. Importantly, the two fluorophores attained enhancement in luminescence efficiency and chromism when the mechanical force was applied. After grinding, the solid-state luminescence efficiencies were elevated from 0.243 to 0.132 to 0.484 and 0.278, respectively, for XA-BZ-TPE and XA-DCE-TPE. In XA-BZ-TPE and XA-DCE-TPE, according to the analysis using PXRD, fluorescence lifetimes, and spectral data, the MFC behavior can be attributed to the phase transition between the crystalline and amorphous states. The present study contributes to expanding the repertoire of D-A type MFC materials and provides valuable insights for the development of novel MFC materials incorporating tetraphenylethene units, which exhibit excellent responsiveness under solid-state emission stimuli.