Drastic Mechanochromic Luminescence Through Fluorescence–Phosphorescence Switching in a Heavy‐Atom‐Free Organoboron Compound

Abstract Fluorescence and phosphorescence are emissions from molecules in the lowest excited singlet state (S ₁ ) and the lowest triplet state (T ₁ ), respectively. Generally, as the S ₁ and T ₁ energy levels are different, the fluorescence and phosphorescence wavelengths are also different. By switching between these emissions through mechanical stimuli, a drastic change in emission color can be achieved. However, organic molecules typically have low intersystem crossing efficiency, making it difficult for them to exhibit phosphorescence. Therefore, organic dyes that exhibit remarkable mechanochromic luminescence (MCL) via the switching between thermally activated delayed fluorescence (TADF) and room‐temperature phosphorescence (RTP) are extremely limited. In this study, heavy‐atom‐free molecules are synthesized. One of them, which has a bulky amino group to a benzodithienoborepin core, exhibited green TADF in solution and the amorphous state, and orange RTP in the crystalline state. This switching between TADF and RTP is enabled by mechanical grinding. Furthermore, DFT calculations and single‐crystal X‐ray structural analysis revealed that the twisting of the bulky amino group in the crystalline state induces the switching between TADF and RTP.