Recent progresses in the mechanistic studies of aggregation-induced emission-active boron complexes and clusters

We herein review the recent progresses in the development of boron-containing compounds with aggregation-induced emission (AIE) properties. In particular, the contribution of boron to molecular motions in the excited state is mainly focused in this review. Initially, the series of boron complexes with bidentate ligands are surveyed. We will explain their optical properties by classifying into three groups having different types of the dative bonds, such as B–O and/or B–N. As an application, stimuli-responsive polymers and film-type sensors are introduced. Next, we recently found that the tridentate ligands are a promising platform for constructing AIE-active complexes. Influence of skeletal distortions on mechanical motions in the excited state followed by AIE behaviors are explained. Moreover, several examples on the application to near-infrared (NIR)-luminescent materials are described. Finally, we also mention aromatics-connected boron cluster compounds, o-carboranes. Because of preferable structural features of o-carboranes for avoiding aggregation-caused quenching (ACQ) and structural relaxation in the excited state, aryl-modified o-carborane dyads and triads often show not only AIE and crystallization-induced emission (CIE) but also stimuli-responsive luminochromic behaviors. From recent works, these unique luminescent properties and detailed mechanism in o-carborane materials are explained.