Crystal structures and emission properties of the BF₂ complex 1-phenyl-3-(3,5-dimethoxyphenyl)-propane-1,3-dione: multiple chromisms, aggregation- or crystallization-induced emission, and the self-assembly effect.

It is known that electron donating groups have quite a different effect on the π-delocalization of a conjugate system when bonded at ortho and para as compared to meta positions in the phenyl ring. In the present work, the BF2 complex of 1-phenyl-3-(3,5-dimethoxyphenyl)-propane-1,3-dione (1), a molecule with two methoxy groups in one of the phenyl rings at meta positions, was prepared. Compound 1 exists as two polymorphs having different mutual orientations of the two methoxy groups: in polymorph A away from each other (termed anti), while in polymorph B one methoxy group is oriented toward the other (syn-anti). In both crystals, the molecules which are antiparallel (the subPh rings as well as dioxaborine are on opposite sides) form stacks through face-to-face π-π interactions, while in polymorph A the crystal packing is further stabilized by intermolecular C(phenyl)-H···F and C(methoxy)-H···F hydrogen bonds. Solid A possesses numerous chromic effects, including mechano-, thermo-, and chronochromism, though the latter to a lesser extent, as well as the effect of rearrangement of the amorphous phase into a more stable crystalline phase A, associated with crystallization-induced emission enhancement (CIEE). The solid-state emission can be repeatedly switched regarding its color and efficiency with excellent reversibility by external stimuli. On the other hand, crystalline solid B undergoes thermal interconversion of syn-anti to the anti conformer. Compound 1 shows a solvatochromic effect (SE), is aggregation-induced emission (AIE) active, and through the sublimation process displays self-assembling crystalline platelike microstructures or microfibers that reveal an obvious optical waveguide effect.