| The stimuli-responsive organic light-emitting material is intelligent light-emitting material in which the fluorescence color or intensity significantly changes under external stimuli,such as light,force,heat,solvent,and acid-base,and therefore has a great potential in optical recording,data storage,anti-counterfeit,trademarks,et al.In this paper,phenanthroimidazole is used as the main builder,and several compounds were obtained by introducing cyanostyrene,the classical aggregation-induced emission and mechanofluorochromism motif with twisted-structure,at the N1 and C2 positions of imidazole via Debus–Radziszewski reaction and Knoevenagel condensation reaction.And we focused on their aggregation-induced emission and mechanically stimuli responsive properties.Specific work results are summarized as below:(1)Triphenylamine-substituted cyanostyrene was introduced at the N1 position of the imidazole and four phenanthroimidazole derivatives M1-M4 were obtained by adjusting different substituents at the C2 position.The photo-physical properties of the solution state,aggregation state and solid state were studied by means of UV-visible absorption spectrum,fluorescence emission spectrum.Experimental investigation indicates that they all show aggregation-induced emission and solvatochromism properties.Especially,rare mechanical induced enhancement emission was observed in the solid state,in which the fluorescence quantum yield of compound M1 increases 78-fold after mechanical grinding with “off-on” mode.Previously reported phenanthroimidazole derivatives only exhibit luminescent color change under mechanical stimulation.It is the first report for phenanthroimidazole system exhibiting mechanical luminescence enhancement effect.X-ray powder diffraction(PXRD)spectroscopy indicated that the luminescence enhancement was caused by variation of the molecular packing pattern with transformation from the ordered crystalline structure into disordered amorphous state upon grinding.(2)For further exploration of the mechanism of the above-mentioned obvious mechanical induced emission enhancement properties of M1,we synthesized four contrast molecules M5-M8 to examine the role of phenanthroimidazole and triphenylamine-substituted cyanostyrene.Photo-physical properties investigation of the solution state,aggregation state and the solid state indicated that only when both phenanthroimidazole and triphenylamine substituted cyanostyrene are included in one molecule can show mechanically enhanced luminescence properties.X-ray single crystal structure analysis,SEM measurement and theoretical calculation revealed that mechanically enhanced luminescence is due to the fact that the strength of the triphenylamine-substituted cyanostyrene moiety influences the stacking pattern of the phenanthroimidazole moiety and thus changes the ST energy difference of molecular aggregates,while the existence of the heavy atom Br is beneficial to enhance the coupling of the orbital transition when the energy difference is low,and facilitates the intersystem crossing(ISC)of the electron from the S excited state to the T excited state.Therefore,the initial state of the M1 exhibits weak fluorescence;while polishing disturbs this special stacking mode and blocks ISC process and thus enhances emission strength.(3)By connecting two phenanthrazole imidazoles at N1 and C2 positions of imidazole via dicyanodistyrylbenzene(DCS),we obtained two compounds and investigate the effect of substitution position on the luminescence properties.UV-vis absorption spectra and fluorescence emission spectra showed that only the N1-substituted compound is AIE active.Solid state fluorescent measurement presented that both of them had mechanically enhanced luminescence while the C2-substituted compound had obvious mechanochromic properties.Thus,the substitution position has a great influence on the luminescence behavior of the aggregation state and solid state. |
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