Design and Luminescence Chromism of Fused Boron Complexes Having Constant Emission Efficiencies in Solution and in the Amorphous and Crystalline States

Mechanochromic luminescent molecules often suffer from critical loss of luminescence intensity during phase transitions in the solid state. In this study, we present mechanochromic luminescent molecules that can provide clear color changes with constant emission intensity during phase transitions. It was reported that the class of aggregation‐induced emission (AIE)‐active boron complexes can present intense emission by freezing intramolecular motions. According to this mechanism, we designed a complex with a fused ligand to preserve the enhanced emission in the aggregated state in the AIE during phase transitions between the solution, amorphous, and crystalline states. From optical measurements, it was shown that the synthesized boron complexes present bright emission in various phases. In particular, as we expected, the luminescence color was clearly altered during phase transition, whereas the emission intensity changed only slightly. Finally, we also found that diverse luminescence colors were obtained from each polymorph and for various substituents as induced by the substituent effect. The results in the series of mechanistic studies show that electronic interactions and π–π interactions can modulate the electronic conjugation system of the complexes without causing loss of emission efficiency. Novel boron complexes were designed and synthesized for realizing constant emission during phase transitions. The synthesized complexes show bright emission in various phases. Additionally, the luminescence color changes drastically during phase transition. We also found that diverse luminescence colors were obtained for each polymorph and for different substituents.