Antiaromatic Sapphyrin Isomer: Transformation into Contracted Porphyrinoids with Variable Aromaticity

Sapphyrin is a pentapyrrolic expanded porphyrin with a 22π aromatic character. Herein, we report the synthesis of a 20π antiaromatic sapphyrin isomer 1 by oxidative cyclization of a pentapyrrane precursor P₅ with a terminal β-linked pyrrole. The resulting isomer 1 , containing a mis-linked bipyrrole unit in the skeleton, exhibits a reactivity for further oxidation due to the distinct antiaromatic electronic structure, affording a fused macrocycle 2 , possessing a spiro-carbon-containing [5.6.5.6]-tetracyclic structure. Subsequent treatment with an acid afforded a weakly aromatic pyrrolone-appended N-confused corrole 3 , and thermal fusion gave a [5.6.5.7]-tetracyclic-ring-embedded 14π aromatic triphyrin(2.1.1) analog 4 . The cyclization at the mis-linked pyrrole moiety of P₅ played a crucial role in synthesizing the antiaromatic porphyrinoid susceptible to facile transformation to novel porphyrinoids with variable aromaticity.     

A Switchable Near-Infrared-Absorbing Dye Based on Redox-Bistable Benzitetraazaporphyrin

Activatable near-infrared (NIR) dyes responsive to external stimuli are used in medical and other applications. Here, we describe the design and synthesis of bench-stable 18π- and 20π-electron benzitetraazaporphyrins (BzTAPs) possessing redox-switchable NIR properties. X-Ray, NMR, and UV/Visible-NIR analyses revealed that 20π-electron BzTAP 1 exhibits NIR absorption and antiaromaticity with a paratropic ring-current, while 18π-electron BzTAP 2 shows weakly aromatic character with NIR inertness. Notably, the NIR-silent BzTAP 2 was readily converted to the NIR-active BzTAP 1 in the presence of mild reducing agents such as amine. The intense NIR absorption band of BzTAP 1 is in sharp contrast to the very weak absorption bands of previously reported antiaromatic porphyrinoids. Molecular orbital analysis revealed that symmetry-lowering perturbation of the 20π-electron porphyrinoid skeleton enables the HOMO–LUMO transition of 1 to be electric-dipole-allowed. BzTAPs are expected to be useful for constructing activatable NIR probes working in reductive environments.     

Quantification of the (anti)aromaticity of fulvenes subject to ring size

Tria-, penta-, hepta- and nonafulvenes (1-4) have been studied theoretically at the MP2 ab initio level of theory. For the global minimum structures, the occupation of the bonding π_CC orbital of the exocyclic CC double bond, obtained by NBO analysis, quantitatively proves π-electron delocalization which can reveal partial 2-, 6- and 10-π-electron aromaticity, and 4-, 8- and 12-π-electron antiaromaticity of the ring moieties. Beside the corresponding occupation number, this conjugation was quantified by the length of the exocyclic CC double bond whilst the (anti)aromaticity of the ring moieties of 1-4 was visualized and quantified by through space NMR shielding surfaces (TSNMRS).     

Predicting an Antiaromatic Benzene Ring in the Ground State Caused by Hyperconjugation

Benzene, the prototype of aromatics, has six equivalent C?C bonds (1.397 Å), which are intermediate between a C?C double bond and a C?C single bond. For over 80 years, chemists have spent much effort on freezing a localized structure to obtain a distorted bond‐length alternating benzene ring in the ground state, leading to various localized trisannelated benzene rings. However, most of the central benzene rings are still aromatic or nonaromatic. Here we report an antiaromatic benzene ring caused by hyperconjugation. Specifically, symmetric annulation of 5,5‐difluorocyclopentadiene results in an antiaromatic benzene ring, which is supported by various aromaticity indices, including nucleus‐independent chemical shift, anisotropy of the induced current density, π‐separated electron‐localization function and heat of hydrogenation. Our findings highlight a strong power of hyperconjugation, a “weak” interaction in organic chemistry, paving the way for designing and realizing more novel (anti)aromatics.     

A Novel Phenylene Topology: Total Syntheses of Zigzag [4]- and [5]Phenylene

In agreement with theory , the title compounds 1 and 2, which were prepared by CpCo-catalyzed cycloisomerizations of appropriate oligoalkynylbenzenes, display physical properties that are in contrast with those of the corresponding linear isomers, but that are very similar to those of the angular topomers.     

Diradical B/N-Doped Polycyclic Hydrocarbons

Heterocyclic diradicaloids with atom-precise control over open-shell nature are promising materials for organic electronics and spintronics. Herein, we disclose quinoidal π-extension of a B/N-heterocycle for generating B/N-type organic diradicaloids. Two quinoidal π-extended B/N-doped polycyclic hydrocarbons that feature fusion of the B/N-heterocycle motif with the antiaromatic s-indacene or dicyclopenta[b,g]naphthalene core were synthesized. This quinoidal π-extension and B/N-heterocycle leads to their open-shell electronic nature, which stands in contrast to the multiple-resonance effect of conventional B/N-type emitters. These B/N-type diradicaloids have modulated (anti)aromaticity and enhanced diradical characters comparing with the all-carbon analogues, as well as intriguing properties, such as magnetic activities, narrow energy gaps and highly red-shifted absorptions. This study thus opens the new space for both of B/N-doped polycyclic π-systems and heterocyclic diradicaloids.     

Progress of Indeno-type Organic Diradicaloids

Organic diradicaloids have unusual open-shell nature and properties and are promising materials for organic electronics, spintronics, energy storage and nonlinear optics. In this review, we focus on indeno-type organic diradicaloids and summarize their molecular design and synthesis, as well as topological structures, open-shell characters and diradical properties. The molecular systems are classified into indenofluorenes and diindenoacenes, indeno-based molecules with one-dimensional, two-dimensional and unique topological structures, and heterocyclic indeno-based molecules. By constructing these various topological π-skeletons with tunable conjugation modes and variation of atomic composition, their key open-shell parameters, such as diradical characters and singlet-triplet energy gaps, along with the optical, electronic and magnetic properties, as well as stabilities are efficiently modulated. More attention may be paid to accurate computational analysis, rational design and synthesis, and novel functions of indeno-type diradicaloids, which will promote the development of radical chemistry and materials.     

Red Emissive Double Aza[7]helicenes with Antiaromaticity / Aromaticity Switching via the Redox-Induced Radical Cation and Dication Species

We herein present the synthetic approach to a new antiaromatic double aza[7]helicene C that features NN-embedded polycyclic aromatic hydrocarbons (PAHs). This heteroatom-doped helicene showed a rarely obtained long-wavelength emission and far-red circularly polarized luminescence (CPL) in the solid state. These optical and chiroptical properties could be ascribed to both the NN-PAH core structure and the further extension through angular ring fusions. Such a unique electronic structure also culminated in facile chemical oxidations of neutral C to the positively charged chiral radical ( C ⋅⁺) and dication species ( C ²⁺). Interestingly, DFT computations revealed that the pyridazine central core showed an antiaromaticity-to-aromaticity switching, in contrast to the inversed transition for the helical periphery in cationic states. The reported approaches are anticipated to lead to the development of further redox-active chiral systems for potential applications in chiroptoelectronics, spintronics as well as fluorescent bioimaging.     

A Journey from Benzoborirene to Benzoborole-Supported 1,2-Diimine and Antiaromatic Borolediide

The reaction of benzoborirene with one equivalent of isocyanides leads to benzene-fused boretes bearing one imine function, while the reaction with two equivalents of isocyanide affords 2,3-dihydro-2,3-diiminoboroles with perfect regioselectivity. The isocyanide double insertion products represent a novel type of 1,2-diimine with a benzoborole backbone. The reduction chemistry of the benzoborole-supported 1,2-diimine was investigated. Single- and two-electron reduction allow for the isolation and full characterization of a radical anion and a dianion, respectively. In stark contrast to the ordinary boroles, which should turn aromatic by accepting two electrons, the antiaromatic character of the benzoborole backbone is highlighted upon reduction, thus presenting a rare example of antiaromatic borole dianion. Detailed quantum chemical calculations provide a rationale for the observed regioselectivity and the electronic structure of the reduced borole diimine species.     

Tetra-tert-butyl-s-indacene is a Bond-Localized C2h Structure and a Challenge for Computational Chemistry

Whether tetra-tert-butyl-s-indacene is a symmetric D_2h structure or a bond-alternating C_2h structure remains a standing puzzle. Close agreement between experimental and computed proton chemical shifts based on minima structures optimized at the M06-2X, ωB97X-D, and M11 levels confirm a bond-localized C_2h symmetry, which is consistent with the expected strong antiaromaticity of TtB-s-indacene.