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The relationship between the overall electronic structure of π‐conjugated molecules and the arrangement of their constituent elements is of fundamental importance. Establishing rational design guidelines for conjugated hydrocarbons with narrow HOMO‐LUMO gaps is useful to develop near‐infrared (NIR) responsive dyes and redox‐active materials. This study describes the synthesis and properties of three conjugated hydrocarbons, i. e. an indenonaphthoperylene, an indenoterrylene, and a diindenoterrylene. These molecules exhibit NIR absorption despite the absence of significant antiaromaticity and diradical character. Notably, the indenonaphthoperylene exhibits red‐to‐NIR emission in the 620–850 nm region. The indenoterrylene and the diindenoterrylene exhibit NIR absorption tailing to 870 and 940 nm, respectively. Moreover, the effect of the π‐extension of indenoperylene is disclosed in order to propose guidelines for achieving a narrow HOMO‐LUMO gap with negligible antiaromaticity and diradical character. [ABSTRACT FROM AUTHOR]

The switching of cyclic π‐conjugation pathways using external stimuli is an attractive research topic in the field of organic chemistry. Here, we synthesized C4h‐symmetric octaaza[8]circulenes with four peripherally arranged amidine moieties that exhibit enhanced antiaromaticity upon protonation. Titration experiments with methanesulfonic acid revealed the formation of the tetraprotonated forms of the octaaza[8]circulenes in solution. Single‐crystal X‐ray diffraction analyses and theoretical calculations indicated that the contribution of the 8π antiaromatic character of the octaaza[8]circulenes is enhanced by the delocalization of charge through the protonation of the pyridine rings. [ABSTRACT FROM AUTHOR]

We investigated the effect of excited‐state antiaromaticity on the ligand‐to‐metal charge transfer (LMCT) process of metallohexaphyrins. According to Baird rule, the aromatic metallohexaphyrins in the S0 state become antiaromatic in the S1 state. Because of the metal–ligand redox‐like nature of LMCT process, the LMCT process can release the antiaromatic nature and its energetic destabilization. The metallohexaphyrins show a correlation between the S0‐state aromaticity and time constants for the LMCT process, which well demonstrates that the excited‐state antiaromaticity at the Franck–Condon state acts as a driving force for the LMCT process. These findings elucidate the effect of excited‐state antiaromaticity on the excited‐state behaviors and further provide an insight into the photochemistry of organic molecules. [ABSTRACT FROM AUTHOR]

3,7,23,27‐Tetrabromo[36]octaphyrin 2 was synthesized as a novel octaphyrin bearing two meso‐free positions. Surprisingly, its ZnII and NiII complexation reactions produced a directly fused porphyrin(2.1.1.1) dimer 6, and a meso‐α fused N‐confused porphyrin (NCP) dimer 7, as the first example of NCP tape, respectively, via transannular C−C bond formation. While 6 exhibits a diatropic ring‐current effect owing to the global 36π Möbius aromaticity, 7 shows a paratropic ring‐current effect due to the global Hückel 36π antiaromaticity. In addition, the oxidation of 7 with PbO2 allowed for formation of its two‐electron oxidized species 9 that exhibited a diatropic ring‐current effect due to the global Hückel 34π aromaticity. This work has demonstrated that meso‐free large expanded porphyrins can be a promising platform to produce novel fused porphyrinoids. [ABSTRACT FROM AUTHOR]

s‐Indacene is a classical non‐alternant hydrocarbon that contains 12 π‐electrons in a cyclic π‐conjugation system. Herein, we report its nitrogen‐doped analogue, 1,5‐diaza‐s‐indacene. 1,5‐Diaza‐s‐indacenes were readily prepared from commercially available 2,5‐dichlorobenzene‐1,4‐diamine through a two‐step transformation consisting of a palladium‐catalyzed Larock cyclization with diaryl acetylenes followed by hydrogen abstraction. The thus obtained 1,5‐diaza‐s‐indacenes exhibited distinct antiaromaticity, as manifested in clear bond‐length alternation, a forbidden HOMO–LUMO transition, and a paratropic ring current. As compared to the parent s‐indacene, the 1,5‐diaza‐s‐indacenes showed higher electron‐accepting ability owing to the presence of imine‐type nitrogen atoms. The 1,5‐diaza‐s‐indacene core is effectively conjugated with the peripheral aryl groups, which enables fine‐tuning of the absorption spectra and redox properties. The two possible localized forms of 1,5‐diaza‐s‐indacene were compared in terms of their energetic aspects. [ABSTRACT FROM AUTHOR]

5,20‐Bis(ethoxycarbonyl)‐[28]hexaphyrin was synthesized by acid catalyzed cross‐condensation of meso‐diaryl‐substituted tripyrrane and ethyl 2‐oxoacetate followed by subsequent oxidation. This hexaphyrin was found to be a stable 28π‐antiaromatic compound with a dumbbell‐like conformation. Upon oxidization with PbO2, this [28]hexaphyrin was converted into an aromatic [26]hexaphyrin with a rectangular shape bearing two ester groups at the edge side. The [28]hexaphyrin can incorporate two NiII or CuII metals by using the ester carbonyl groups and three pyrrolic nitrogen atoms to give bis‐NiII and bis‐CuII complexes with essentially the same dumbbell‐like structure. The antiaromatic properties of the [28]hexaphyrin and its metal complexes have been well characterized. Die of a rose in antiaromatic pain? 5,20‐Bis(ethoxycarbonyl)‐substituted [28]hexaphyrin was synthesized and converted to its bis‐NiII and bis‐CuII complexes as stable antiaromatic species with dumbbell‐like conformations. Oxidation of the [28]hexaphyrin free base gave an aromatic and rectangular [26]hexaphyrin with the ethoxycarbonyl groups at the short side. These compounds have been well characterized by X‐ray analysis and other spectroscopic methods. [ABSTRACT FROM AUTHOR]

Abstract: Silicon complexation of a [38]octaphyrin (1) was accomplished by reaction with an excess amount of MeSiCl3 in the presence of N,N‐diisopropylethylamine, thus giving an aromatic [38]octaphyrin bis(silicon) complex 2. This complex was interconvertible with an antiaromatic [36]octaphyrin congener (3) by oxidation with MnO2 and reduction with NaBH4. Curiously, mild oxidation of 2 with ferrocenium hexafluorophosphate afforded a [37]octaphyrin bis(silicon) complex 4 as an stable radical cation that can be stored under ambient conditions in the solid state. Owing to the two NNNCC‐five‐coordinated Si atoms bearing trigonal bipyramidal geometry, these octaphyrin bis(silicon) complexes take on similar and rigid figure‐of‐eight structures with different consecutive numbers of conjugated π‐electrons (38, 37, and 36), and are all stable. [ABSTRACT FROM AUTHOR]

Understanding of interactions among molecules is essential to elucidate the binding of pharmaceuticals on receptors, the mechanism of protein folding and self-assembling of organic molecules. While interactions between two aromatic molecules have been examined extensively, little is known about the interactions between two antiaromatic molecules. Theoretical investigations have predicted that antiaromatic molecules should be stabilized when they stack with each other by attractive intermolecular interactions. Here, we report the synthesis of a cyclophane, in which two antiaromatic porphyrin moieties adopt a stacked face-to-face geometry with a distance shorter than the sum of the van der Waals radii of the atoms involved. The aromaticity in this cyclophane has been examined experimentally and theoretically. This cyclophane exhibits three-dimensional spatial current channels between the two subunits, which corroborates the existence of attractive interactions between two antiaromatic π-systems. Little is known about interactions between two antiaromatic molecules. Here, the authors synthesised a cyclophane, in which two antiaromatic porphyrin moieties adopt a stacked face-to-face geometry with a distance shorter than the sum of the van der Waals radii of the atoms involved. [ABSTRACT FROM AUTHOR]