AIE-active D-A-D(D′) dicyanodiaryl ethylene derivatives: solvatochromism, aggregation-induced emission and high-contrast mechanofluorochromism

Abstract

Two AIE-active dicyanodiaryl ethylene derivatives (E)-2-(1,3-bis(4-(diphenylamino)phenyl)allylidene)malononitrile (DPAM) and (E)-2-(3-(4-(diphenylamino)phenyl)-1-(4-(1,2,2-triphenylvinyl)phenyl)allylidene)malononitrile (DPTPAM) based on triphenylamine or tetraphenylethylene have been synthesized and characterized. Photophysical property studies and theoretical calculation results indicated that they have non-planar highly distorted conformations, D-A-D(D′) type molecular structures, and well-defined intramolecular charge transfer (ICT) properties. Furthermore, both molecules, DPAM and DPTPAM, demonstrated interesting aggregation-induced emission (AIE) characteristics in a mixed system comprising THF and water. Additionally, it was observed that both DPAM and DPTPAM were sensitive to external forces with considerable alteration of luminescence colors and intensities. Upon grinding, the maximum emission of DPAM and DPTPAM exhibited a red shift of 49 nm and 70 nm, respectively. Reversible mechanofluorochromism originated from the crystalline-amorphous phase transition in the solid states, which was supported by powder X-ray diffraction experiments (PXRD), differential scanning calorimetry (DSC) and field emission scanning electron microscopy (FESEM) results. The crystal structure analysis further explained that four specific hydrogen bond interactions could be easily disrupted toward to mechanical forces. The planarization of the molecular conformation and subsequent planar intramolecular charge transfer (PICT) could be responsible for the red-shift of fluorescence spectra upon grinding.