Reversible mechanofluorochromic properties of phenothiazine-based D-A-D′ aza-N,O-chelated boron difluoride complexes

Abstract

Two new phenothiazine-based D-A-D' aza- N,O-Chelated boron difluoride complexes (E)-5-(4-(dimethylamino)phenyl)-3-((10-ethyl-10H-phenothiazin-3-yl)methylene)-2,2-difluoro-2,3-dihydro-1,3,4,2-oxadiazaborol-3-ium-2-uide (DPDFB) and (E)-3-((10-ethyl-10H-phenothiazin-3-yl)methylene)-2,2-difluoro-5-phenyl-2,3-dihydro-1,3,4,2-oxadiazaborol-3-ium-2-uide (PDDFB) were designed and synthesized. The two BF2 chelating donor–acceptor type aza-N,O-chelated five-membered-ring organoboron complexes displayed large Stokes shifts and remarkable solvatofluorochromism in solutions, as well as good thermal stability in the solid state. Strikingly, the attachment of N,N-dimethyl groups to the terminal benzene ring unit induced significant reversible mechanofluorochromic (MFC) properties (with emission wavelength changes from 617 nm to 660 nm). And the MFC mechanism was ascribed to the phase transformation between crystalline and amorphous state, which was revealed by powder X-ray diffraction (PXRD), differential scanning calorimetry (DSC) and field emission scanning electron microscopy (FESEM) analyses. The introduction of the N,N-dimethyl group demonstrated in this work would be of great help in understanding and designing new MFC materials.