"Switching Shapes: Reversible Three Species Photoisomerization of Substituted 1,2-Dihydro-1,2-azaborinines" in Journal of the American Chemical Society
Derivatives of 1,2-dihydro-1,2-azaborinines generally undergo selective photochemical electrocyclic ring-closure reactions to the corresponding Dewar isomers (2-aza-3-borabicyclo[2.2.0]hex-5-ene). Depending on the substitution pattern, these photoreactions can also yield benzvalene (3-aza-4-boratricyclo[3.1.0.02.6]hexane) analogues. Here, we report the synthesis of 1,2,3,5-tetrasubstituted dihydroazaborinines by transition-metal-catalyzed late-stage functionalization and the investigation of their photophysical and photochemical properties using transient absorption spectroscopy. The introduction of aryl groups at the 3- and 5-positions induces a pronounced bathochromic shift of the absorption maximum. Under broad-spectrum irradiation (280–400 nm), quantitative conversion to the benzvalene isomer can be achieved. The initial photoisomerization proceeds via excitation to the short-lived singlet excited state (S1) yielding the Dewar isomer, whereas the subsequent conversion of this intermediate occurs through a long-lived excited state. Notably, the second isomerization step is accompanied by an interchange of the carbons C3 and C4. Once formed, the benzvalene isomers exhibit exceptional thermal stability. Cycloreversion to the Dewar isomer and even to the dihydroazaborinine structure can be triggered photochemically through targeted excitation and during both processes the substituents return to the C3 and C5 positions. The thermal cycloreversion of the benzvalene isomer can yield either the educt BN-benzene isomer (1,2,3,5-substitued) or its 1,2,4,5-substituted isomer. Computational studies revealed a stepwise mechanism for the thermal back reaction reforming the educt, while a concerted, energetically less-favorable pathway leads to the 1,2,4,5-substituted analogue.
"Dispersion-Controlled Excited-State Dynamics in Azobenzene Photoisomerization" in Journal of the American Chemical Society
Weak interactions, like London dispersion forces, are cumulative in nature and have been thought to be essential for only the structure and stability of large molecular systems. Only recently has their relevance for chemical reactivity been recognized. Until today, their role in photoreactions and subsequent ultrafast excited-state processes has remained elusive. Here, we show the impact of London dispersion on the excited-state behavior and the outcome of the photoreaction of the iconic photoswitch azobenzene as a representative example. Increased dispersion interactions between substituents decisively prolong the excited-state lifetimes by preventing direct passage through the conical intersection. This significantly alters the dynamics of the Z to E photoisomerization. We expect our findings to lead to increased research interest in such “dispersion-controlled excited-state dynamics” relevant for the steering of ultrafast processes.
