Schiff base derivatives based on diaminomaleonitrile: Colorimetric and fluorescent recognition of Cu(II), cell imaging application, polymorph-dependent fluorescence and aggregation-enhanced emission

• Two Schiff base derivatives based on diaminomaleonitrile were synthesized.
• Two compounds behave as selective UV and off–on fluorescent sensors toward Cu2+ ions.
• The mixtures of Cu2+ with 1 or 2 shows great potential application in bioimaging.
• Two different polymorphs of compound 1 were obtained.
• Compound 2 was aggregation-enhanced emission (AEE)-active.

Two Schiff base derivatives based on diaminomaleonitrile were synthesized and characterized. Both of the two compounds displayed a highly selective UV and off-on type fluorescent response to Cu2+ in acetonitrile solution. The mixtures of Cu2+ with 1 or 2 showed great potential application in bioimaging. Two different polymorphs of compound 1 were obtained from ethanol and ethyl acetate, respectively, which exhibit obvious differences in the wavelength and intensity of the fluorescence emission. This interesting phenomenon may originate from the tightness of packing and π–π interactions in the stacking structure. Compound 2 was aggregation-enhanced emission (AEE)-active in tetrahydrofuran (THF)/water mixtures. Moreover, compound 2 in a solid amorphous state was collected though special methods, which showed completely different fluorescence compared with that of the crystal due to the irregular molecular aggregation pattern.