The Baird‐aromaticity of BN/CC cyclooctatetraene isosteres (azaboracyclooctatetraenes) in their lowest triplet states (T1) has been explored through computations of various aromaticity indices that describe the different aspects of aromaticity (magnetic, electronic, energetic and geometric). While cyclooctatetraene (COT) is aromatic in its T1 state following Baird's 4n rule, we now reveal that the degrees of Baird‐aromaticity of its BN/CC isosteres vary with aromaticity aspect considered. The thermodynamically most stable octagonal B4N4H8 isomer, having an alternating B and N pattern (borazocine, B4N4COT‐A), is only weakly aromatic or nonaromatic in T1 according to energetic and electronic indices, while magnetic descriptors suggest it to have about two thirds the Baird‐aromaticity of T1 state COT (3COT). The extent of Baird‐aromaticity of intermediate BN/CC isosteres also varies markedly with aromaticity aspect considered. The strong aromaticity of 3B4N4COT‐A according to magnetic descriptors can be linked to the symmetries of the orbitals involved in the virtual transitions from occupied to unoccupied orbitals, which describe the response of a molecule in an external magnetic field. However, the magnetic aspect of T1 state Baird‐aromaticity (response aromaticity) is not related to the electronic and energetic aspects (intrinsic aromaticity), findings that underline earlier observations on differences between the various aspects of the aromaticity phenomenon (or phenomena).

The hemiterpene isoprene is a volatile C5 hydrocarbon with industrial applications. It is generated today from fossil resources, but can also be made in biological processes. We have utilized engineered photosynthetic cyanobacteria for direct, light-driven production of bio-isoprene from carbon dioxide, and show that isoprene in a subsequent photochemical step, using either near-UV or simulated or natural solar light, can be dimerized into limonene, paradiprene, and isomeric C10H16 hydrocarbons (monoterpenes) in high yields under photosensitized conditions (above 90% after 44 hours with near-UV and 61% with simulated solar light). The optimal sensitizer in our experiments is di(naphth-1-yl)methanone which we use with a loading of 0.1 mol%. It can also easily be recycled for subsequent photodimerization cycles. The isoprene dimers generated are a mixture of [2+2], [4+2] and [4+4] cycloadducts, and after hydrogenation this mixture is nearly ideal as a drop-in jet fuel. Importantly the photodimerization can be carried out at ambient conditions. However, the high content of hydrogenated [2+2] dimers in our isoprene dimer mix lowers the flash point below the threshold (38 °C), yet, these dimers can be converted thermally into [4+2] and [4+4] dimers. When hydrogenated these monoterpenoids fully satisfy the criteria for drop-in jet fuels with regard to energy density, flashpoint, kinematic viscosity, density, and freezing point. Life-cycle assessment results show a potential to produce the fuel in an environmentally sustainable way.

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The hemiterpene isoprene is a volatile C5 hydrocarbon with industrial applications. It is generated today from fossil resources, but can also be made in biological processes. We have utilized engineered...

Besides the most common form of aromaticity involving a π‐ring, hexaiodobenzene and hexakis(phenylselenyl)benzene dications also present σ‐aromaticity in the outer ring formed by the main group substituents. These two compounds are considered σ‐ and π‐double aromatic, and their characterization is of special interest to the fields of organic and structural chemistry. In this work, we decided to explore the double aromaticity in substituted tropylium cations for three reasons: (i) the seven neutral halogen substituents of the tropylium cations will, without oxidation, lead to 14 σ‐electrons (a 4n + 2 Hückel number); (ii) tropylium cations are highly stable and can be easily generated experimentally; and (iii) whereas in substituted benzenes the distances between substituents in the optimized structures or X‐ray crystals are too large to allow strong σ‐aromaticity, these distances are expected to be shorter in substituted tropylium cations. Yet, instead of the expected σ‐aromaticity, we found that the most stable geometries are highly puckered, meaning that delocalization in both π‐ and σ‐systems is lost. Our results, which include also the tropylium anion and trication in the singlet and triplet state, show that there is a need to open a lone pair hole by oxidation to generate σ‐aromaticity. Among the systems studied, only triplet C7Br7+3 with an internal Hückel aromatic tropylium ring and an external incipient Baird aromatic Br7 ring shows double π‐ and σ‐aromaticity. This result, however, is functional‐dependent and reveals that 3C7Br73+ is at the borderline for onset of double aromaticity.