Aggregation-induced emission and distinct mechanochromic luminescence based on symmetrical D-A-D type and unsymmetrical D-A type carbazole functionalized dicyanovinyl derivatives

The π-conjugated luminophores CZ-DPMN and DCZ-DPMN, which are composed of electron-rich carbazole group and electron-deficient dicyanovinyl unit, are designed and synthesized. Spectral studies and theoretical calculations affirm that both compounds show unique ICT (intramolecular charge transfer) process and donor-acceptor (D-A) type molecular structure with highly distorted conformation. CZ-DPMN and DCZ-DPMN also exhibit AIE (aggregation-induced emission) characteristics at high water fraction in THF/water mixtures. Importantly, CZ-DPMN and DCZ-DPMN display remarkable fluorescent response to mechanical force. The as-synthesized solids of CZ-DPMN and DCZ-DPMN emit bright green and yellow fluorescence, and the emitting-colors change into yellow and orange-yellow after grinding, respectively. The spectral shifts are 62 nm and 22 nm upon the treatment of grinding. This MCL (mechanochromic luminescence) phenomenon is reversible during repeating grinding-fuming treatment. PXRD diffraction patterns indicate that the change of molecular internal structure from crystalline state to amorphous state after the treatment of grinding is the direct cause of MCL phenomenon.