Concept of Excitation-Driven Boron Complexes and Their Applications for Functional Luminescent Materials

It is commonly known that flexible structures and movable substituents are unfavorable for realizing intense luminescence with organic dyes. On the other hand, we show in this short review that excitation-driven boron complexes are promising platforms for obtaining stimuli-responsive luminescent materials with high sensitivity. Initially, we explained transformation of the valence of boron in heterofluorene from four to three-coordinate state through excitation-driven bond cleavage at the boron dative bond. The mechanism and stimuli-responsive luminescence with high sensitivity are illustrated. The next topic regards rational design of aggregation-induced emission (AIE)-active molecules, which are able to provide enhanced emission by aggregation. By employing theoretical calculations, it was possible to find a series of novel AIE-active skeletons from scratch. Stimuli-responsive luminescent chromism was also observed from the resulting molecules. The final topic is concerned with aryl-o-carborane dyads. It was recently found that rotation of the o-carborane unit proceeds triggered by photo-excitation. By regulating rotation behaviors, not only solid-state luminescence but also dual emission were obtained. The concept and several examples to offer applications of this new class of boron complexes are reviewed.