Boron complexes often present intense emission originating from high rigidity and planarity. Therefore, these luminescent boron complexes are a promising "element-block", which is defined as a functional unit containing heteroatoms, for constructing advanced optical materials. Recently, it has been reported that several boron complexes having molecular flexibility provided slight emission under diluted conditions where conventional luminescent boron complexes can show intense emission.

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... , we have accomplished to obtain the series of stimuli-responsive luminescent materials based on these flexible boron complexes through controls of their unique behaviors in the excited state. Herein, we review photochemistry and applications of their optical properties of flexible boron element-blocks. We will demonstrate discovery of the flexible boron complexes and mention recent progresses for the development of aggregation-induced emission-active molecules and luminescent chromic materials including optical sensors based on these flexible boron element-blocks.