Aggregation-induced emission enhancement and mechanofluorochromism based on dicyanovinyl derivatives decorated carbazole or triphenylamine units: Effects of electronic structures and spatial conformations

Abstract

Based on a rational design strategy by combining an electron-accepting dicyanoethene unit and electron-donating carbazole (CZ) or triphenylamine (TPA) segment, we successfully synthesized two AIE-active luminogens DPMN-CZ and DPMN-TPA using the classical Suzuki-Miyaura coupling and Knoevenagel condensation reactions. It was found that both of them showed specific intramolecular charge transfer (ICT) characteristics and aggregation-induced emission enhancement (AIEE) properties. Spectral studies and theoretical calculations proved that compounds DPMN-CZ and DPMN-TPA had typical donor-acceptor (D-A)π-conjugated structures. More interestingly, DPMN-CZ and DPMN-TPA showed mechanofluorochromic (MFC) behaviors. Upon the treatment of grinding, the initial crystals of DPMN-CZ only showed a low contrast mechanochromism (wavelength changed by 27 nm), while the emitting colors of DPMN-TPA samples changed from orange to red, and a large spectral shift of 57 nm was obtained. The reason was that DPMN-TPA possessed a stronger ICT degree and more twisted molecular conformation than that of DPMN-CZ, which endowed DPMN-TPA with loose molecular packing and weak intermolecular interactions, and its initial crystal was easily destroyed by external force, resulting in a large spectral shift. XRD and DSC studies confirmed that the significant MFC nature of DPMN-CZ and DPMN-TPA was attributed to crystalline-to-amorphous transition.