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.

Nanographene molecules can be regarded as building blocks for larger graphene nanostructures. The incorporation of a borazine core (B3N3) into the center of the iconic nanographene hexa-peri-hexabenzo...

1,2-Azaborinines are a promising molecular class for application as molecular solar thermal (MOST) systems offering a solution for solar energy storage. Our computational screening of 4,5-substituted ...

The discovery of nanomaterials with unique electronic band structures, such as flat bands, has drawn significant interest for enabling novel physical phenomena and advanced technological applications....

Substituents of the hydrindacene type allow the isolation of crystalline stable triplet arylnitrenes from photodecomposition of the corresponding aryl azides, while phenyl azide is known to produce si...

The first solution-phase synthesis and reactivity of a nitrilium-type N-hetaryne are described. The 1,2-azaborine-derived 1,6-BN-aryne 2 exhibits [4 + 2], [3 + 2], and [2 + 2] cycloadditions and electrophilic aromatic substitution (EAS) reactivity. The observed regio- and diastereoselectivities of the cycloaddition and EAS products are consistent with a polarized aryne/nitrilium species. The free 1,6-BN-aryne 2 was isolated and characterized under matrix isolation conditions. A Lewis structure description where the 1,6-BN-aryne 2 resonates between two limiting (ketenimine (dominant) vs nitrilium) forms is consistent with DFT calculations. New 1,2-azaborine structures that are functionalized at the C6- and N-positions, including highly strained derivatives that were previously not accessible, can now be accessed using 1,6-BN-aryne 2 as a versatile synthetic building block.

Understanding the electronic structure of polycyclic aromatic compounds is of fundamental importance for their potential applications. The optoelectronic properties of shorter acenes such as tetracene and pentacene have been extensively studied with regard to excitation, emission, and nonlinear effects such as singlet fission. The longer homologues present a unique challenge due to their low stability both in the solid state and in solution. In this work, we synthesized persistent 6,8,15,17-tetrakis((triisopropylsilyl)ethynyl)heptacene and investigated its photophysical properties as well as those of the parent heptacene. Our steady-state electronic absorption and emission experiments combined with transient absorption spectroscopy show that the Franck–Condon electric-dipole-forbidden (“dark”) transition to the 21Ag singlet state is the lowest-energy excited state of heptacene. This contrasts with the optical properties of the well-known shorter acenes. Transient absorption data further suggest singlet fission or intersystem crossing as potential pathways to rapid population of the triplet state facilitated by the dark singlet state.

In contrast to the multichannel photochemistry of benzene, 1,2-dihydro-1,2-azaborinines generally undergo a selective electrocyclic ring-closure reaction, yielding the corresponding Dewar (2-aza-3-borabicyclo[2.2.0]hex-5-ene) isomers. The Dewar isomers react back to the dihydroazaborinines in unimolecular reactions with long half-lives at room temperature, making this isomer pair of relevance as a molecular solar thermal (MOST) energy storage system. A systematic investigation of the influence of C-backbone functionalization on the photoisomerization or the thermal ring opening has not yet been conducted. We report here a study of the late-stage C3 functionalization of 1,2-dihydroazaborinines by cross-coupling and discuss the electronic impact of aryl substitution at C3 on the properties of 1,2-dihydro-1,2-azaborinines itself, as well as on the photoreaction and the thermal back reaction employing a Hammett analysis for the latter. 3-Aryl-substituted dihydroazaborinines react selectively and reversibly in almost quantitative fashion to the Dewar isomer. The Woodward–Hoffmann forbidden thermal ring-opening reaction proceeds by interaction with the adjacent boron center involving a three-center–two-electron interaction and is accelerated by both electron-withdrawing and electron-donating groups. Through the targeted selection of C3 substituents, both the electronic absorption characteristics of the dihydroazaborinine and the half-life of its Dewar isomer can be addressed.

The reversible photoisomerization of 1,2-dihydro-1,2-azaborinines (BN benzenes) to their Dewar isomers (2-aza-3-borabicyclo[2.2.0]hex-5-enes) provides a promising platform for molecular solar thermal ...

Acenes are an important class of polycyclic aromatic hydrocarbons that have gained considerable attention from chemists, physicists, and material scientists, due to their exceptional potential for org...