Unique mechanofluorochromism and reversible multicolor switching of triphenylamine-functionalized sulfonyl hydrazone derivatives

Two new simple triphenylamine-based sulfonyl hydrazone derivatives DBBS ((E)-N'-(4-(diphenylamino)benzylidene)benzenesulfonohydrazide) and DBMS ((E)-N'-(4-(diphenylamino)benzylidene)-4-methylbenzenesulfonohydrazide) were designed and synthesized via one-step aldimine condensation reaction to investigate the influence of terminal methyl group on their photophysical, crystal stacking and mechanofluorochromic (MFC) properties. Interestingly, reversible multi-color fluorescence tuning and switching mechanofluorochromism was found for DBBS due to the nature of polymorphs, and DBMS exhibited reversible bicolor switching mechanofluorochromism. Additionally, the reversible MFC behavior was ascribed to the phase transition between the crystalline and amorphous states, which have been confirmed by powder X-ray diffraction (PXRD), field emission scanning electron microscopy (FESEM) and differential scanning calorimetry (DSC) results. The obtained results showed that terminal methyl group did not only regulate molecular conformations and/or intermolecular interactions in crystals, but also affect the force-stimuli responsiveness. The present study provided insight into the development of multicolor switching materials by rational modification of classic luminogens.