Tuning the Colors of the Dark Isomers of Photochromic Boron Compounds with Fluoride Ions: Four-State Color Switching

Combining a three-coordinated boron (BMes₂) moiety with a four-coordinated photochromic organoboron unit leads to a series of new diboron compounds that undergo four-state reversible color switching in response to stimuli of light, heat, and fluoride ions. Thus, these hybrid diboron systems allow both convenient color tuning/switching of such photochromic systems, as well as visual fluoride sensing by color or fluorescent emission color change

Photo- and Thermal-Induced Multistructural Transformation of 2‑Phenylazolyl Chelate Boron Compounds

The new N,C-chelate boron compounds B­(2-phenylazolyl)­Mes₂ [Mes = mesityl; azolyl = benzothiazolyl (<b>1a</b>), 4-methylthiazolyl (<b>2a</b>), benzoxazolyl (<b>3a</b>), benzimidazolyl (<b>4a</b>)] undergo an unprecedented multistructural transformation upon light irradiation or heating, sequentially producing isomers <b>b</b>, <b>c</b>, <b>d</b>, and <b>e</b>. The dark isomers <b>b</b> generated by photoisomerization of <b>a</b> undergo a rare thermal intramolecular H-atom transfer (HAT), reducing the azole ring and generating new isomers <b>c</b>, which are further transformed into isomers <b>d</b>. Remarkably, isomers <b>d</b> can be converted to their diastereomers <b>e</b> quantitatively by heating, and <b>e</b> can be converted back to <b>d</b> by irradiation at 300 nm. The structures of isomers <b>1d</b> and <b>1e</b> were established by X-ray diffraction. The unusual HAT reactivity can be attributed to the geometry of the highly energetic isomers <b>b</b> and the relatively low aromaticity of the azole rings. The boryl unit plays a key role in the reversible interconversion of <b>d</b> and <b>e</b>, as shown by mechanistic pathways established through DFT and TD-DFT calculations

Photo- and Thermal-Induced Multistructural Transformation of 2‑Phenylazolyl Chelate Boron Compounds

The new N,C-chelate boron compounds B­(2-phenylazolyl)­Mes₂ [Mes = mesityl; azolyl = benzothiazolyl (<b>1a</b>), 4-methylthiazolyl (<b>2a</b>), benzoxazolyl (<b>3a</b>), benzimidazolyl (<b>4a</b>)] undergo an unprecedented multistructural transformation upon light irradiation or heating, sequentially producing isomers <b>b</b>, <b>c</b>, <b>d</b>, and <b>e</b>. The dark isomers <b>b</b> generated by photoisomerization of <b>a</b> undergo a rare thermal intramolecular H-atom transfer (HAT), reducing the azole ring and generating new isomers <b>c</b>, which are further transformed into isomers <b>d</b>. Remarkably, isomers <b>d</b> can be converted to their diastereomers <b>e</b> quantitatively by heating, and <b>e</b> can be converted back to <b>d</b> by irradiation at 300 nm. The structures of isomers <b>1d</b> and <b>1e</b> were established by X-ray diffraction. The unusual HAT reactivity can be attributed to the geometry of the highly energetic isomers <b>b</b> and the relatively low aromaticity of the azole rings. The boryl unit plays a key role in the reversible interconversion of <b>d</b> and <b>e</b>, as shown by mechanistic pathways established through DFT and TD-DFT calculations