Polymorphic Fluorescent Material with Strong Solid State Emission and Multi-Stimuli-Responsive Properties
A bright difluoroboron -diketonate derivative 1 showing four emission colors (green, yellow, orange and red) with high quantum yields (41%-74%) in four polymorphs and one amorphous state is reported. Green-emissive crystals (1-G and 1-G’) exhibit dimeric aggregation structures due to strong molecular π-π interaction but exhibit hypsochromic emission compared to yellow-emissive crystals (1-Y) with monomeric aggregation because of lacking such π-π interaction. These novel emission phenomena are rationalized by theoretical calculations. High fluorescence sensitivity of compound 1 to its molecular packing modes results into excellent responsive behavior to multiple external stimuli thereby showing reversible change of emission colors under mechanical grinding, heating, solvent fuming and hydrostatic pressure.