A Flexible, Fused, Azomethine-Boron Complex: Thermochromic Luminescence and Thermosalient Behavior in Structural Transitions between Crystalline Polymorphs.

This manuscript reports the multi-functional boron complex presenting aggregation-induced emission (AIE), crystallization-induced emission enhancement (CIEE), and thermosalient behavior accompanying thermochromic luminescence during crystal-crystal transitions by employing the fused azomethine ligand. In particular, we propose that these properties can be explained by molecular "flexibility" toward external stimuli, including temperature changes and photo-excitation, and two types of crystal polymorphs with different absorption and luminescent properties were obtained. Optical measurements indicated that both polymorphs showed individual AIE and strong CIEE properties. From the investigations, the boron complex showed large structural relaxation and formed the bent structure in the excited state, followed by emission annihilation in the absence of structural restriction. In addition, it was shown that interconversion between these polymorphs could be reversibly induced by heating and cooling. The data from the single-crystal X-ray analyses suggested that alteration of crystal packing and intermolecular interaction should influence the luminescent chromism. Moreover, we also found that the crystals showed unusual mechanical behavior, such as hopping and fragmentation by heating and cooling, respectively, which is called thermosalient behavior. It was suggested that the loosely-fused structure could be responsible for expressing unique optical and mechanical properties.