Reversible Vapochromic Luminescence Accompanied by Planar-Half-Chair Conformational Change of a Propeller-Shaped Boron β-Diketiminate Complex.

Leakage of volatile organic compounds (VOCs) is one of the most severe industrial accidents because it can cause environmental pollution, global warming, fire, and explosion. Hence the visualization of leakage is an essential technology to detect it at an early stage. Molecular crystals whose fluorescence color can be changed by the exposure to VOCs could potentially serve as the sensing materials for realizing rapid and facile VOC detection. These materials, however, usually require harsh conditions, such as heating or a vacuum, to recover their initial phases for reuse. Therefore, it remains a challenge to obtain completely reversible sensing systems without such energy-consuming recycling processes. Here we report the reversible color change of fluorescence from the crystals of a propeller-shaped boron β-diketiminate complex. The complex was crystallized in distinct crystalline phases having different luminescent colors. Importantly, these phases were interconverted repeatedly upon exposure to the vapors of the appropriate VOCs. The small energy differences between conformers of the complex could lead to this pseudopolymorphic behavior. This finding could be applied for the development of further eco-friendly reversible sensing materials based on four-coordinated boron complexes.