Solid state emission and mechanochromic luminescence of boron 2-(2′-pyridyl)imidazole complexes

• Boron-difluoride dyes bearing different amino moieties were synthesized.
• The amino groups play an important role in their photophysical properties.
• Mechanochromic luminescence was found for two boron-difluoride dyes.
• Mechanochromism is ascribed to the transition of crystalline to amorphous state.

A series of boron 2-(2′-pyridyl)imidazole complexes were found to exhibit solid state emission and mechanochromic properties, meanwhile the terminal substituent play an important role in their properties. When the boron-difluoride dye bears dimethylamino moieties which can freely rotate, it emits poorly in most organic solvents and in the amorphous powder, but visible emission was observed in the crystalline state. Comparatively, the dyes comprising cyclohexylamino and diphenylamino substitutes show higher fluorescent efficiencies both in solutions and in the solid powder state, due to the larger steric hindrance. However, mechanochromism was observed only for the dyes bearing dimethylamino and diphenylamino moieties. According to XRD analysis, the transition of crystalline to amorphous state by grinding is ascribed to be the driving force for the mechanochromic character, and the flexibility of cyclohexylamino groups makes the dye inclined to be in amorphous state and non-responsive to mechanical force.

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