Planar-to-Bent Conformational Changes in Luminescent Tetra-BF₂ Complexes: Toward Smart Stimuli-Responsive Materials

Functional dyes that combine flexibility and rigidity exhibit tunable optical properties in response to structural modifications such as bending, planarization, and twisting, making them promising candidates for smart materials responsive to external stimuli. In this study, we explored stimuli-responsive dyes based on conformational changes between bent and planar structures using highly emissive tetra-BF2 complexes. We synthesized four types of tetra-BF2 complexes, each with different side chains. In solution, all complexes exhibited yellow–green emission with near-unity quantum yield, without substantial changes in their absorption or emission characteristics. However, in the solid state, the emission colors varied from orange to red, and the corresponding quantum yields are up to 49%, depending on the side-chain structure. The single-crystal X-ray diffraction analysis provided structural insights, revealing that the orange- and red-emitting complexes exhibited bent and planar conformations, respectively. Notably, mechanical stimulation of the planar-structured crystals using a spatula induced a remarkable emission color change from red to orange, accompanied by crystal collapse. In certain cases, this stimulation also led to enhanced emission intensity and reversible mechanochromism by grinding and solvent treatments. These findings provide a design strategy for developing stimuli-responsive materials and highlight their remarkable potential for future smart material applications.