Asymmetric N,C-chelate organoboron compounds bearing two different aryl groups at the boron center undergo photoisomerization reactions that involve exclusively the less bulky aryl group, generating various strongly colored "dark isomers". These species thermally isomerize to 4bH-azaborepin molecules by direct hydrogen atom transfer from a borirane cycle to the pyridyl moiety and ring expansion. Mechanistic insight into these highly regioselective transformations was obtained from kinetic data and through computational studies.

Chiral organoboron compounds with a chelate backbone and mesityl/heterocycle substituents (thienyl, furyl, and derivatives thereof) undergo a quantitative phototransformation that yields rare, chiral N,B,X-containing heterocycles, such as base-stabilized 1,2-thiaborinines and 1,2-oxaborinines. Boriranes were observed as intermediates in some of these transformations. The oxaborinines display further reactivity, generating 4a,12b-dihydrobenzo[h][1,2]oxaborinino[4,3-f]quinolines through a sequential conrotatory electrocyclization and a [1,5]-H shift. The N,B,X-containing heterocycles display strong blue-green to orange-red emission in the solid state. Combined DFT//CASP2T calculations suggest that a common biradical intermediate is responsible for the formation of these compounds as well as their interconversion.

Alkyne-functionalized chelate boron compounds with extended π-conjugation on one of the aryl groups attached to boron display thermally reversible and regioselective isomerization on the more delocalized substituent, forming base-stabilized boriranes with an intense color. Linking two of such boron chromophores through a 1,4-phenylene spacer via ethynyl moieties leads to photochemically inert molecules, while connecting them by a nonconjugated silicon bridge yields photochromic systems capable of switching at a single boron center.

Chiral organoboron compounds with a chelate backbone and mesityl/heterocycle substituents (thienyl, furyl, and derivatives thereof) undergo a quantitative phototransformation that yields rare, chiral N,B,X‐containing heterocycles, such as base‐stabilized 1,2‐thiaborinines and 1,2‐oxaborinines. Boriranes were observed as intermediates in some of these transformations. The oxaborinines display further reactivity, generating 4a,12b‐dihydrobenzo[h][1,2]oxaborinino[4,3‐f]quinolines through a sequential conrotatory electrocyclization and a [1,5]‐H shift. The N,B,X‐containing heterocycles display strong blue‐green to orange‐red emission in the solid state. Combined DFT//CASP2T calculations suggest that a common biradical intermediate is responsible for the formation of these compounds as well as their interconversion.

A new family of unsymmetrical, N,C-chelate organoboron compounds B­(ppz)­(Mes)­Ar have been synthesized and found to undergo a rare, regioselective two-stage photoisomerization, involving the Ar group only. The initial transformation is a Zimmerman rearrangement to afford yellow azaboratabisnorcaradiene isomers that are subsequently converted to unprecedented 14aH-diazaborepins via a photochemical “walk” rearrangement. Spectroscopic and computational studies provide insight into the formation and properties of these unique systems.