Search

5,18‐Dimesitylorangarin and its BF2 complex were synthesized by double SNAr reaction of 3,5‐dibromo‐BODIPY with 2‐pyrrolydipyrrin as the first examples of meso‐aryl‐substituted orangarin. These orangarins, delineated as [20]pentaphyrin(1.0.1.0.0), are strongly antiaromatic but rather stable. The free base orangarin was coupled by oxidation with MnO2 to give a 11,11′‐linked dimer, a cyclooctatetraene(COT)‐centered trimer, and a spiro‐trimer. Fused COT‐centered 3H‐orangarin dimer was oxidized to the corresponding 2H‐orangarin dimer, which was further coupled to give a triply COT‐centered 2H‐orangarin tetramer. 3H‐Orangarin oligomers are all antiaromatic as evinced by extremely low‐field‐shifted 1H NMR signals of the inner NH and ill‐defined absorption spectra with broad tails. In contrast, COT‐centered 2H‐orangarin dimer and tetramer show moderately low‐field‐shifted NH signals and intense NIR absorbance over 900 nm, suggesting effective π‐conjugation through the COT bridge and almost non‐antiaromatic character. These orangarin oligomers exhibit many reversible redox potentials owing to the intramolecular electronic interactions. Regardless of the different aromatic characters, all the orangarin monomers and oligomers exhibit very rapid excited‐state decays. [ABSTRACT FROM AUTHOR]

Aromaticity and antiaromaticity are foundational principes in organic chemistry, regularly invoked to explain stability, structure, and magnetic and electronic properties. There are ongoing challenges in assigning molecules as aromatic or antiaromatic using optical spectroscopy. Here we report spectroelectrochemical and computational analyses of porphyrin (18π neutral, aromatic) and norcorrole (16π neutral, antiaromatic), and their oxidized (16π porphyrin dication) and reduced (norcorrole 18π dianion) forms. Our results show that while the visible spectra are characteristic of (anti)aromaticity consistent with Hückel's rules, the IR spectra are much less informative, owing to the relative rigidity of norcorrole. The results have implications for the assignment of (anti)aromaticity in both ground‐state and time‐resolved spectra. [ABSTRACT FROM AUTHOR]

Organic diradicaloids have lately emerged as potential spintronic materials. We report the unprecedented synthesis of a near-IR absorbing indeno[2,1-a]fluorene derivative that displays remarkably low LUMO (−4.15 eV) and a small HOMO–LUMO gap (0.85 eV). NMR/EPR studies indicated its open-shell diradical property, which was supported by DFT calculations while suggesting a 30% diradical character and a small singlet (S)–triplet (T) gap (−2.52 kcal mol−1). A large bond length alternation of the as-indacene core for its single-crystals indicated a quinoidal contribution with greater antiaromaticity, which is in line with the small diradical character despite showing a small S–T gap. [ABSTRACT FROM AUTHOR]

Investigations of the nature and degree of antiaromaticity of cycloheptatrienyl anion derivatives using both experimental and computational tools are presented. The ground state of cycloheptatrienyl anion in the gas phase is triplet, planar and Baird‐aromatic. In DMSO, it assumes a singlet distorted allylic form with a paratropic ring current. The other derivatives in both phases assume either allylic or diallylic conformations depending on the substituent pattern. A combination of experimental and computational methods was used to determine the pKa values of 16 derivatives in DMSO, which ranged from 36 to −10.7. We revealed that the stronger stabilization of the anionic system, which correlates with acidity, does not necessarily imply a lower degree of antiaromaticity in terms of magnetic properties. Conversely, the substitution pattern first affects the geometry of the ring through the bulkiness of the substituents and their better conjugation with a more distorted system. Consequently, the distortion reduces the cyclic conjugation in the π‐system and thereby decreases the paratropic current in a magnetic field, which manifests itself as a decrease in the NICS. The triplet‐state geometries and magnetic properties are nearly independent on the substitution pattern, which is typical for simple aromatic systems. [ABSTRACT FROM AUTHOR]

Antiaromaticity is a fundamental concept in chemistry, but the study of molecular wires incorporating antiaromatic units is limited. Despite initial predictions, very few studies show that antiaromaticity has a beneficial effect on electron transport. Dibenzo[a,e]pentalene (DBP) is a stable structure that displays appreciable antiaromaticity within the five‐membered rings of the pentalene core. We have investigated derivatives of DBP furnished with pyridyl (Py) and F4‐pyridyl (PyF4) anchor groups, and compared the conductance with purely aromatic phenyl and anthracene analogues. We find that the low‐bias conductance of DBP‐Py is approximately 60 % larger than that of the anthracene analogue Anth‐Py and 250 % larger compared to the phenyl derivative Ph‐Py. This is due to a better alignment of the LUMO with the gold Fermi level, which we confirm by conductance‐voltage spectroscopy where the conductance of DBP‐Py shows the greatest voltage‐dependence. The F4‐pyridyl compounds, which have lower LUMO energies compared to the pyridyl analogues, did not, however, form detectable molecular junctions. The strongly electron‐withdrawing fluorine atoms reduce the donor capability of the nitrogen lone‐pair to the point where stable N−Au bonds no longer form. [ABSTRACT FROM AUTHOR]

A recent publication by Wu and co‐workers demonstrated that the workhorse functional B3LYP tends to overemphasize the delocalization in antiaromatic molecules, leading to poor agreement between the calculated proton NMR values and the experimentally obtained numbers. Rather, they showed that the M11 functional affords the best agreement between theory and experiment. We have computationally re‐examined our previously published NICS‐XY scan data using M11‐determined geometries and find that, aside from the placement of s‐indacene, the antiaromaticity trends for both sets of data are essentially identical. [ABSTRACT FROM AUTHOR]

Benzo[a]fluoranthene (4a) is one of non‐alternate polycyclic aromatic hydrocarbons. A reaction of 4a in CF3SO3H yielded a dark yellow solution. Direct NMR observation indicated the exclusive formation of carbocation 4aH+ by protonation at the C(8) position. The most deshielded 1H and 13C signals were observed at 8.99 ppm for H(12) and 8.29 ppm for H(1), 182.6 ppm for C(12b), 155.6 ppm for C(8a), and 149.4 ppm for C(7a). The signals for H(4) and H(5) were significantly more shielded than those of 4a. A reaction of 3‐tert‐butylbenzo[a]fluoranthene (4b) in CF3SO3H afforded carbocation 4bH+ by the protonation at the C(8) position. 4bH+ was gradually converted to 4aH+. The changes in 13C NMR chemical shifts (Δδ13C) suggested that positive charge was delocalized into mainly seven carbons in 4aH+ and 4bH+. The observed cations were found to be the most stable cations among the possible protonation cations by the DFT method. The NICS(1)zz values for the five‐membered rings were calculated to be 35.6 for 4aH+ and 34.4 for 4bH+ by GIAO‐B3LYP/6‐311+G(2d,p). The experimental NMR and the NICS(1)zz data indicated that the five‐membered rings in 4aH+ and 4bH+ exhibited anti‐aromaticity. [ABSTRACT FROM AUTHOR]

Recently, π–π stacked antiaromatic π‐systems have received considerable attention because they can exhibit stacked‐ring aromaticity due to substantial intermolecular orbital interactions. Here, we report three antiaromatic norcorrole dimers that self‐assemble to form supramolecular architectures through chiral self‐sorting. A 2,2'‐linked norcorrole dimer with 3,5‐di‐tert‐butylphenyl groups forms a π‐stacked dimer both in solid and solution states via homochiral self‐sorting. Its association constant in solution is (3.6±1.7)×105 M−1 at 20 °C. In the solid state, 3,3'‐linked norcorrole dimers with 3,5‐di‐tert‐butylphenyl and phenyl groups afford macrocyclic and helical supramolecular assemblies via heterochiral and homochiral self‐sorting, respectively. Notably, the subtle modification in the substituent resulted in a complete change in the structure of the aggregates and the chiral self‐sorting mode. The present findings demonstrate that structural manipulation in antiaromatic monomer units leads to the formation of various supramolecular assemblies on the basis of the attractive interactions between antiaromatic π‐systems. [ABSTRACT FROM AUTHOR]

We have calculated the magnetically induced current density susceptibility for gold-containing organometallic molecular rings using the gauge-including magnetically induced currents (GIMIC) method. The aromatic nature has been determined by calculating the strength of the magnetically induced ring current susceptibility, which is often called ring current. To our knowledge, we show here for the first time that gold-containing organometallic rings may be aromatic or antiaromatic sustaining ring currents in the presence of an external magnetic field. The calculated aromatic character of the rings agrees with the aromatic nature one expects when using Hückel's aromaticity rules. The studied auracycles and diauracycles with 4n electrons in the conjugated orbitals generally sustain a weak paratropic ring current, whereas those having 4n + 2 electrons in the conjugated orbitals sustain a diatropic ring current that is almost as strong as that of benzene. The number of electrons are obtained by assuming that each C, N and Au atom of the ring contribute one electron, and a H atom connected to a N atom in the ring increases the number of electrons by one. An electron-attracting ligand at Au removes one electron from the ring. Formation of a short Au–Au bonding diauracycles reduces the number of electrons in the ring by two. [ABSTRACT FROM AUTHOR]

Stabilization of hexaphyrin(1.0.1.0.1.0) (named "rosarin") in its 25π radical state is achieved using a hetero‐bimetal‐coordination strategy. The antiaromatic BF2 complex B‐1 was first synthesized, and then rhodium ion was inserted into B‐1 to produce the BF2/Rh(CO)2 mixed complex Rh‐B‐1 as a highly air‐stable radical. The structures of B‐1 and Rh‐B‐1 were determined by single‐crystal X‐ray diffractions, and the antiaromatic or radical character was identified by various spectroscopy evidence and theoretical calculations. Rh‐B‐1 exhibits excellent redox properties, enabling amphoteric aromatic‐antiaromatic conversion to their 24/26π states. Compared to the 24/26π conjugation systems on the same skeleton, Rh‐B‐1 has the narrowest electrochemical and optical band gaps, with the longest absorption band at 1010 nm. The ring‐current analysis reveals intense paratropic currents for B‐1 and co‐existing diatropic‐paratropic currents for Rh‐B‐1. This hetero‐bimetal‐coordination system provides a novel platform for organic radical stabilization on porphyrinoids, showing the prospect of modulating ligand oxidation states through rational coordination design. [ABSTRACT FROM AUTHOR]

Although second near‐infrared (NIR‐II, 1000–1500 nm) light has attracted considerable attention, especially for life sciences applications, the development of organic dyes with NIR‐II absorption remains a formidable challenge. Herein we report the design, synthesis, and electronic properties of 20π‐electron antiaromatic benziphthalocyanines (BPcs) that exhibit intense absorption bands in the NIR region. The strong, low‐energy absorption of the antiaromatic BPcs is attributed to electric‐dipole‐allowed HOMO‐LUMO transitions with narrow band gaps, enabled by the reduced structural symmetry of BPc compared with regular porphyrins and phthalocyanines. The combination of peripheral substituents and a central metal decreases the HOMO‐LUMO energy gaps, leading to the extension of the absorption bands into the NIR‐II region (reaching 1100 nm) under reductive conditions. [ABSTRACT FROM AUTHOR]

In a series of earlier studies, the effect of benzo‐annelation was found to be a useful tool for tuning the aromaticity in polycyclic conjugated compounds to desired level. In this work we studied the (anti)aromaticity of benzo‐annelated derivatives of three conjugated hydrocarbons (anthracene, fluoranthene and biphenylene) in their lowest lying singlet (S0) and triplet (T1) states by means of the energy effect (ef), harmonic oscillator model of aromaticity (HOMA), multicentre delocalization indices (MCI), magnetically induced current densities (MICDs) and nucleus independent chemical shifts (NICS). We showed that benzo‐annelation is a topology‐based effect which can be used to modify the T1 state excitation energies (E(T1)). A quantitative model was established being able to accurately predict the E(T1) based only on the numbers of angularly, linearly and geminally annelated benzene rings. In addition, it was demonstrated that the E(T1) can be directly related to the (anti)aromatic character of the central ring in the studied molecules in their S0 state. [ABSTRACT FROM AUTHOR]

Carbazole‐incorporated smaragdyrin BF2‐complex 3 was synthesized by SNAr reaction of 3,5‐dibromo‐8‐mesityl‐BODIPY 1 with 3,6‐di(tert‐butyl)‐1,8‐di(pyrrol‐2‐yl)carbazole 2 as a nucleophile. Demetalation of 3 with ZrCl4 gave the corresponding smaragdyrin free base 4 in a good yield. Oxidations of 3 and 4 with MnO2 gave smaragdyrins 5 and 6, respectively, both followed by aromaticity switching, since the oxidized products showed a moderate paratropic ring current owing to their 20π‐electronic circuits. Further, treatment of 4 with [RhCl(CO)2]2 in the presence of NaOAc gave RhI complex 7, and oxidation of 3 with RuCl3 in the presence of triethylamine led to the formation of a spiro dimer product, 8. [ABSTRACT FROM AUTHOR]

Analysis of the variations of the off‐nucleus isotropic magnetic shielding, σiso(r), around thiophene, thienothiophenes, dithienothiophenes and sulflowers in their electronic ground (S0) and lowest triplet (T1) states reveals that some of the features of aromaticity and bonding in these molecules do not fit in with predictions based on the popular Hückel's and Baird's rules. Despite having 4n π electrons, the S0 states of the sulflowers are shown to be aromatic, due to the local aromaticities of the individual thiophene rings. To reduce its T1 antiaromaticity, the geometry of thiophene changes considerably between S0 and T1: In addition to losing planarity, the carbon‐carbon two 'double' and one 'single' bonds in S0 turn into two 'single' and one 'double' bonds in T1. Well‐defined Baird‐style aromaticity reversals are observed between the S0 and T1 states of only three of the twelve thiophene‐based compounds investigated in this work, in contrast, the sulflower with six thiophene rings which is weakly aromatic in S0 becomes more aromatic in T1. The results suggest that the change in aromaticity between the S0 and T1 states in longer chains of fused rings is likely to affect mostly the central ring (or the pair of central rings); rings sufficiently far away from the central ring(s) can retain aromatic character. [ABSTRACT FROM AUTHOR]

Copyright of Nachrichten aus der Chemie is the property of Wiley-Blackwell and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Copyright of Nachrichten aus der Chemie is the property of Wiley-Blackwell and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Long-wavelength, near-infrared small-molecule dyes are attractive in biophotonics. Conventionally, they rely on expanded aromatic structures for redshift, which comes at the cost of application performance such as photostability, cell permeability, and functionality. Here, we report a ground-state antiaromatic strategy and showcase the concise synthesis of 14 cationic aminofluorene dyes with mini structures (molecular weights: 299–504 Da) and distinct spectra covering 700–1600 nm. Aminofluorene dyes are cell-permeable and achieve rapid renal clearance via a simple 44 Da carboxylation. This accelerates optical diagnostics of renal injury by 50 min compared to existing macromolecular approaches. We develop a compact molecular sensing platform for in vivo intracellular sensing, and demonstrate the versatile applications of these dyes in multispectral fluorescence and optoacoustic imaging. We find that aromaticity reversal upon electronic excitation, as indicated by magnetic descriptors, not only reduces the energy bandgap but also induces strong vibronic coupling, resulting in ultrafast excited-state dynamics and unparalleled photostability. These results support the argument for ground-state antiaromaticity as a useful design rule of dye development, enabling performances essential for modern biophotonics. Near-infrared small molecule dyes are attractive in biophotonics, but generally rely on expanded aromatic structures to achieve this redshift. Here the authors report the use of a ground state antiaromatic strategy for 14 aminofluorene dyes, and their use in imaging kidney injury. [ABSTRACT FROM AUTHOR]

Comprehensive Summary: Binary polyantimony clusters, namely [Cu2Sb14]4– and [Ag2Sb14]4–, containing coinage metals, were successfully synthesized and characterized, in which two homoatomic Sb73– subunits are bridged by two coinage metals in η4:η1 and η1:η1 coordination modes, respectively. In [Cu2Sb14]4–, two bridging Cu ions and two Sb atoms form a planar rhombic unit, which was revealed to have antiaromatic properties by theoretical calculations. Further electron structure and bonding analysis confirmed the presence of delocalized bonds in the rhombic unit and the metallophilic interaction between two formal M+ ions. [ABSTRACT FROM AUTHOR]

The exploration of annulene's conformation, electronic properties and aromaticity has generated enduring interest over the years, yet it continues to present formidable challenges for annulenes with more than ten carbon atoms. In this study, we present the synthesis of a stable [10]cyclo‐para‐phenylmethine derivative (1), which bears a resemblance to [10]annulene. 1 can be readily oxidized into its respective cations, wherein electrons are effectively delocalized along the backbone, resulting in different conformations and aromaticity. Both 1 and its tetracation (14+ ⋅ 4SbF6−) exhibit a nearly planar conformation with a rectangular shape, akin to the E,Z,E,Z,Z‐[10]annulene. In contrast, the radical cation (1⋅+ ⋅ SbCl6−) possesses a doubly twisted Hückel topology. Furthermore, the dication (12+ ⋅ 2SbCl6−) displays conformational flexibility in solution and crystalizes with the simultaneous presence of Möbius‐twisted (1a2+ ⋅ 2SbCl6−) and Hückel‐planar (1b2+ ⋅ 2SbCl6−) isomers in its unit cell. Detailed experimental measurements and theoretical calculations reveal that: (1) 1 demonstrates localized aromaticity with an alternating benzenoid/quinoid structure; (2) 1a2+ ⋅ 2SbCl6− and 1b2+ ⋅ 2SbCl6− with 48π electrons are weakly Möbius aromatic and Hückel antiaromatic, respectively; (3) 14+ ⋅ 4SbF6− exhibits Hückel aromaticity (46π) and open‐shell diradical character. [ABSTRACT FROM AUTHOR]

Embedding both boron and nitrogen into the backbone of acenes to generate their isoelectronic structures has significantly enriched the acene chemistry to offer appealing properties. However, only small BN‐heteroacenes have been extensively investigated, with BN‐heptacenes as the hitherto longest homologue. Herein, we report the synthesis of three new nonacene BN‐isosteres via incorporating a pair of antiaromatic B2C4 and N2C4 heterocycles, representing a new length record for BN‐heteroacenes. The distance between the B2C4 and N2C4 rings affects the contribution of the charge‐separated resonance forms, leading to tunable antiaromaticity of the two heterocycles. The adjusted local antiaromaticity manifests substantial influence on the molecular orbital arrangement, and consequently, the radiative transition rate of BN‐3 is greatly enhanced compared with BN‐1 and BN‐2, realizing a high fluorescence quantum yield of 92 %. This work provides a novel design concept of large acene BN‐isosteres and reveals the importance of BN/CC isosterism on their luminescent properties. [ABSTRACT FROM AUTHOR]

A new series of biaryls, bi‐linear‐terphenylenes (BLTPs), were prepared using the tert‐butyllithium‐mediated cyclization as the key synthetic step. The three‐dimensional structures of the studied compounds were verified using X‐ray crystallography and DFT calculations. Tetraaryl(ethynyl)‐substituted BLTPs are highly crowded molecules, and the internal rotation around the central C−C bond is restricted due to a high barrier (>50 kcal/mol). These structures contain several aryl/terphenylenyl/aryl sandwiches, where the through‐space π‐π (TSPP) interactions are strongly reflected in the shielding of 1H NMR chemical shifts, reduction of oxidation potentials, increasing aromaticity of the central six‐membered ring and decreasing antiaromaticity of the four‐membered rings in a terphenylenyl moiety based on NICS(0) and iso‐chemical shielding surfaces. Despite the restricted C−C bond associated intramolecular TSPP interactions for BLTPs in the ground state, to our surprise, the electronic coupling between two linear terphenylenes (LTPs) in BLTPs in the excited state is weak, so that the excited‐state behavior is dominated by the corresponding monomeric LTPs. In other words, all BLTPs undergo ultrafast relaxation dynamics via strong exciton‐vibration coupling, acting as a blue‐light absorber with essentially no emission. [ABSTRACT FROM AUTHOR]

This invited Team Profile was created by Prof. Judy I. Wu, Department of Chemistry, University of Houston (USA) and Prof. Michael M. Haley, Department of Chemistry & Biochemistry and the Materials Science Institute, University of Oregon, Eugene (USA). The X‐ray structure of tetra‐t‐butyl‐s‐indacene was reported in 1986 to possess a symmetric, delocalized D2h geometry, which went against conventional wisdom that this antiaromatic should exhibit a bond‐localized C2h structure. Computational studies over the ensuing 35+ years yielded conflicting outcomes as to the preferred geometry. A computational reexamination of this structure by our team found experimental proton chemical shifts matched best with a computed C2h symmetry, confirming a bond‐localized geometry. "Tetra‐tert‐butyl‐s‐indacene is a Bond‐Localized C2h Structure and a Challenge for Computational Chemistry", L. J. Karas, S. Jalife, R. V. Viesser, J. V. Soares, M. M. Haley, J. I. Wu, Angew. Chem. Int. Ed. 2023, 62, e202307379. [ABSTRACT FROM AUTHOR]

This invited Team Profile was created by Prof. Judy I. Wu, Department of Chemistry, University of Houston (USA) and Prof. Michael M. Haley, Department of Chemistry & Biochemistry and the Materials Science Institute, University of Oregon, Eugene (USA). The X‐ray structure of tetra‐t‐butyl‐s‐indacene was reported in 1986 to possess a symmetric, delocalized D2h geometry, which went against conventional wisdom that this antiaromatic should exhibit a bond‐localized C2h structure. Computational studies over the ensuing 35+ years yielded conflicting outcomes as to the preferred geometry. A computational reexamination of this structure by our team found experimental proton chemical shifts matched best with a computed C2h symmetry, confirming a bond‐localized geometry. "Tetra‐tert‐butyl‐s‐indacene is a Bond‐Localized C2h Structure and a Challenge for Computational Chemistry", L. J. Karas, S. Jalife, R. V. Viesser, J. V. Soares, M. M. Haley, J. I. Wu, Angew. Chem. Int. Ed. 2023, 62, e202307379. [ABSTRACT FROM AUTHOR]

Aromaticity, in general, can promote a given reaction by stabilizing a transition state or a product via a mobility of π electrons in a cyclic structure. Similarly, such a promotion could be also achieved by destabilizing an antiaromatic reactant. However, both aromaticity and transition states cannot be directly measured in experiment. Thus, computational chemistry has been becoming a key tool to understand the aromaticity-driven reaction mechanisms. In this review, we will analyze the relationship between aromaticity and reaction mechanism to highlight the importance of density functional theory calculations and present it according to an approach via either aromatizing a transition state/product or destabilizing a reactant by antiaromaticity. Specifically, we will start with a particularly challenging example of dinitrogen activation followed by other small-molecule activation, CF bond activation, rearrangement, as well as metathesis reactions. In addition, antiaromaticity-promoted dihydrogen activation, CO2 capture, and oxygen reduction reactions will be also briefly discussed. Finally, caution must be cast as the magnitude of the aromaticity in the transition states is not particularly high in most cases. Thus, a proof of an adequate electron delocalization rather than a complete ring current is recommended to support the relatively weak aromaticity in these transition states.

Molecular carbons (MCs) are molecular cutouts of carbon materials. Doping with heteroatoms and constructing open‐shell structures are two powerful approaches to achieve unexpected and unique properties of MCs. Herein, we disclose a new strategy to design open‐shell boron‐doped MCs (BMCs), namely by pentagon‐fusion of an organoborane π‐system. We synthesized two diradicaloid BMC molecules that feature C24B and C38B π‐skeletons containing a pentagonal ring. A thorough investigation reveals that such pentagon‐fusion not only leads to their local antiaromaticity, but also incorporates an internal quinoidal substructure and thereby induces open‐shell singlet diradical states. Moreover, their fully fused structures enable efficient π conjugation, which is expanded over the whole frameworks. Consequently, some intriguing physical properties are achieved, such as narrow energy gaps, very broad light absorptions, and superior photothermal capability, along with excellent photostability. Notably, the solid of the C38B molecule exhibits absorption that covers the range of 300–1200 nm and an efficiency of 93.5 % for solar‐driven water evaporation, thus demonstrating the potential of diradicaloid BMCs as high‐performance organic photothermal materials. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The formation of Lewis pairs is an important chemical concept. Recently, the complexation of Lewis acidic tris(pentafluorophenyl)borane with Lewis basic moieties and subsequent reduction has emerged as a fascinating strategy for designing novel reactions and structures. The impact of the complexation and subsequent reduction of antiaromatic systems bearing Lewis base moieties has been investigated. We found how Lewis adduct formation stabilizes an antiaromatic system consisting of 9,10‐dicyanoanthracene and tris(pentafluorophenyl)borane by using synthesis, X‐ray crystallography, spectroscopic analysis, and quantum chemical calculations. [ABSTRACT FROM AUTHOR]

We here disclose that the incorporation of thiophene rings into a seven‐membered 8π azepine in a fused fashion produces a useful antiaromatic core for near‐infrared (NIR) dyes. In contrast to dibenzazepine derivatives with bent structures, dithieno‐fused derivatives with electron‐accepting groups adopt flat conformations in the ground state. The dithieno‐fused derivatives exhibited broad absorption spectra that cover the visible region as well as sharp emission bands in the NIR region, which are considerably red‐shifted relative to those of the dibenzo‐fused congeners. Theoretical study revealed two contradictory effects of the less‐aromatic thiophene‐fused structure, i.e. the enhancement of the antiaromaticity of the adjacent azepine ring and the relief of the antiaromaticity through the contribution of a quinoidal resonance form. The combination of the dithienoazepine core with cationic electron‐accepting groups produced a NIR fluorescent dye with an emission at 878 nm in solution. [ABSTRACT FROM AUTHOR]

The maleimide moiety is present in numerous species of commercial or theoretical interest. The resemblance of maleimide to aromatic pyrrole raise the question of the aromaticity of the former. Are maleimides aromatic or antiaromatic, or neither? Should they be described as N-substituted derivatives of (2,5-dihydro)pyrrole-2,5-dione or as pyrrole-2,5-dioxy biradical? Through an analysis of the energetics of neutral and ionic species alike, the former description is to be preferred with only minimal antiaromaticity associated with its 4 π electrons. [ABSTRACT FROM AUTHOR]

Within these, Modes B A b and B C b , in which the boron atom acts as a Lewis acid, have been widely explored, [6] SP , sp [18][19][20][21][22][23][24][25][26][27][28][29][30][31][32][33][34][35] whereas reports on the phosphorus atom acting as a Lewis base (Mode B B b ) are scarce. [37] In Ph SB 2 sb PBpin, despite the presence of tricoordinate boron and phosphorus centers, there is negligible -dative interaction between the phosphorus lone pair and the p-orbital on the boron. Keywords: pinacol diphenylphosphinoboronate; boroles; borafluorenes; antiaromaticity; Lewis acid EN pinacol diphenylphosphinoboronate boroles borafluorenes antiaromaticity Lewis acid 2193 2196 4 10/31/23 20231127 NES 231127 Graph Phosphinoboranes [1][52][2] (R SB 2 sb P-BR SB 2 sb ) feature trivalent and tricoordinate P and B atoms linked covalently and are valence isoelectronic with aminoboranes (R SB 2 sb N-BR SB 2 sb ; Scheme 1a). Upon coordination, the P(1)-B(2) bonds of the Ph SB 2 sb PBpin elongate slightly [ B 1 b : 1.953(2) A , B 2 b : 1.9432(18) A , cf. free Ph SB 2 sb PBpin: 1.9274(14) A ]. [Extracted from the article]

Naphthalene‐ and phenanthrene‐fused [22]smaragdyrin BF2‐complexes were synthesized by 1) Suzuki‐Miyaura coupling of β‐brominated [22]smaragdyrin BF2 complexes with 2‐formylarylboronates, 2) Witting‐type methoxymethylenation of the formyl group, and 3) methanesulfonic acid‐catalyzed cyclization reaction. Subsequently these BF2 complexes were deboronized and oxidized to the corresponding antiaromatic [20]smaragdyrin free bases. The installed fused structures led to decrease of the aromatic characters of the [22]smaragdyrin BF2 complexes and the antiaromatic characters of the [20]smaragdyrin free bases. [ABSTRACT FROM AUTHOR]

The construction of 1,2‐position‐connected azulene oligomers was achieved. In the crystal packing structure of the terazulene, two molecules of (Ra)‐ and (Sa)‐configurations formed a pair. Variable temperature NMR measurements and theoretical calculations of the quaterazulene suggest that the helical and syn‐type structure with terminal azulene overlap is more stable. Two kinds of fused terazulenes (1,2′′‐closed and 1,8′′‐closed) were also synthesized by intramolecular Pd‐catalyzed C−H/C−Br arylation of the terazulene moieties. X‐ray structure analysis of 1,2′′‐closed terazulene revealed a planar structure, while an analysis of 1,8′′‐closed terazulene performed on a C60 co‐crystal revealed a curved structure forming a 1 : 1 complex covering the co‐crystal. Nucleus‐independent chemical shift (NICS) calculations carried out for the central seven‐membered ring of 1,8′′‐closed terazulene showed a positive value, suggesting anti‐aromatic properties. [ABSTRACT FROM AUTHOR]

With their bent π‐systems, cyclic conjugation and inherent cavities, conjugated nanohoops are attractive for organic electronics applications. For ease of processing and morphological stability, an incorporation into polymers is desirable, but to date was hampered with few exceptions by synthetic difficulties. We herein present a unique strategy for the synthesis of conjugated nanohoop polymers using a dibenzo[a,e]pentalene (DBP) as central connector. We demonstrate this versatility by synthesizing three electronically diverse copolymers with dithienyldiketo(pyrrolopyrrol), fluorene and carbazole comonomers, and report the first donor‐acceptor nanohoop polymer. Optoelectronic investigations reveal the prevalence of cyclic or linear conjugation, depending on the comonomer unit, and ambipolar electrochemical properties through the antiaromatic character of the DBP units. As the first report on using conjugated nanohoops for charge storage as positive electrode materials, we show a significant improvement in battery performance in a nanohoop‐containing polymer compared to an equivalent nanohoop‐free reference polymer. We believe this study will pave the way for the synthesis of a diverse range of nanohoop polymers and further stimulate their exploration for charge storage in batteries. [ABSTRACT FROM AUTHOR]

Near‐infrared (NIR) emitters are of great interest for applications in bioimaging and modern technology. Yet the design of such materials with decent characteristics is challenging due to intrinsic limitations. In a recent article, Murai and Yamaguchi report the synthesis of NIR emitters with appreciable fluorescence quantum yields reaching 0.02 at 878 nm in CH2Cl2 solution. The low band gaps were achieved by a new design strategy exploiting antiaromaticity relief. This concept was realized for compounds consisting of an antiaromatic azepine central ring fused to thiophene moieties. In these systems, thiophene unfolds its dual nature. On the one hand, it contributes to the high antiaromaticity of azepine; on the other hand, it exerts a stabilizing effect on azepine through the formation of a quinoid structure, which reduces its antiaromaticity and shifts the absorption and emission maxima into the NIR region. [ABSTRACT FROM AUTHOR]

Abstract The construction of 1,2‐position‐connected azulene oligomers was achieved. In the crystal packing structure of the terazulene, two molecules of (Ra)‐ and (Sa)‐configurations formed a pair. Variable temperature NMR measurements and theoretical calculations of the quaterazulene suggest that the helical and syn‐type structure with terminal azulene overlap is more stable. Two kinds of fused terazulenes (1,2′′‐closed and 1,8′′‐closed) were also synthesized by intramolecular Pd‐catalyzed C−H/C−Br arylation of the terazulene moieties. X‐ray structure analysis of 1,2′′‐closed terazulene revealed a planar structure, while an analysis of 1,8′′‐closed terazulene performed on a C60 co‐crystal revealed a curved structure forming a 1 : 1 complex covering the co‐crystal. Nucleus‐independent chemical shift (NICS) calculations carried out for the central seven‐membered ring of 1,8′′‐closed terazulene showed a positive value, suggesting anti‐aromatic properties.

Polycyclic hydrocarbons consisting of two or more directly fused antiaromatic subunits are rare due to their high reactivity. However, it is important to understand how the interactions between the antiaromatic subunits influence the electronic properties of the fused structure. Herein, we present the synthesis of two fused indacene dimer isomers: s‐indaceno[2,1‐a]‐s‐indacene (s‐ID) and as‐indaceno[3,2‐b]‐as‐indacene (as‐ID), containing two fused antiaromatic s‐indacene or as‐indacene units, respectively. Their structures were confirmed by X‐ray crystallographic analysis. 1H NMR/ESR measurements and DFT calculations revealed that both s‐ID and as‐ID have an open‐shell singlet ground state. However, while localized antiaromaticity was observed in s‐ID, as‐ID showed weak global aromaticity. Moreover, as‐ID exhibited a larger diradical character and a smaller singlet‐triplet gap than s‐ID. All the differences can be attributed to their distinct quinoidal substructures. [ABSTRACT FROM AUTHOR]

Whether tetra‐tert‐butyl‐s‐indacene is a symmetric D2h structure or a bond‐alternating C2h 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 C2h symmetry, which is consistent with the expected strong antiaromaticity of TtB‐s‐indacene. [ABSTRACT FROM AUTHOR]

As isomers of the regular porphyrins, N-confused porphyrins have attracted extensive attention of chemists because of their unique chemical structures, chemical reactivities, and physical properties, which result in their promising applications in the fields of catalytic chemistry, biochemistry and material science. Typically, N-confused porphyrins are synthesized via acid catalyzed condensation and following oxidation during which lactams are often formed as the byproducts. Here we report doubly N-confused and ring-contracted [24]hexaphyrin(1.1.0.1.1.0) mono- and bis-Pd-complexes as stable antiaromatic N-confused expanded porphyrins, which are synthesized through Pd-catalyzed Suzuki-Miyaura coupling of 1,14-dibromotripyrrin. These macrocycles show a paratropic ring currents, an ill-defined Soret band, a red-shifted weak absorption tail, and a small HOMO-LUMO gap. NBS bromination of the bis Pd-complex give its mono- and dibromides regioselectively, which are effectively used to synthesize a [24]hexaphyrin dimer and a NiII porphyrin-[24]hexaphyrin-NiII porphyrin triad, respectively. In the chemistry of porphyrinoids, expanded porphyrins and N-confused porphyrins (NCP) are two important newcomers but antiaromatic expanded NCPs have not been isolated before. Here, the authors report the synthesis of doubly N-confused and ring-contracted hexaphyrin PdII complexes. [ABSTRACT FROM AUTHOR]

Antiaromatic molecules have recently received attention because of their intrinsic properties, such as high reactivity and their narrow HOMO–LUMO gaps. Stacking of antiaromatic molecules has been predicted to induce three‐dimensional aromaticity via frontier orbital interactions. Here, we report a covalently linked π‐π stacked rosarin dimer that has been examined experimentally by steady‐state absorption and transient absorption measurements and theoretically by quantum chemical calculations, including time‐dependent density functional theory, anisotropy of induced current density, and nucleus‐independent chemical shift calculations. Relative to the corresponding monomer, the dimer exhibits diminished antiaromaticity upon lowering the temperature to 77 K, a finding ascribed to intramolecular interactions between the macrocyclic rosarin subunits. [ABSTRACT FROM AUTHOR]

We utilized a cycloaromatization reaction driven by relief of excited state antiaromaticity to photouncage aldehydes and ketones. We developed several synthetic routes towards the synthesis of photocaged carbonyls as allylically substituted 3-(2-(arylethynyl)phenyl)prop-2-en-1-ols. A library of photocaged aryl aldehydes and ketones containing donors and acceptors, as well as several photocaged fragrance aldehydes and the steroid 5α-cholestan- 3 -one, were synthesized and demonstrated photouncaging in good to excellent yields. [ABSTRACT FROM AUTHOR]

The structure, energetics, and aromaticity of c.a. 100 constitutional isomers and tautomers of pyrido[m,n]diazepines (m = 1, 2; n = 2, 3, 4, 5; m ≠ n) were studied at the B3LYP/cc-pVTZ level. The pyrido[1,3]diazepines appear the most, while pyrido[2,4]diazepines are the least stable (ca. 26 kcal/mol). In the pyrido[1,n]diazepine group (n = 2–5), the [1,5] isomers are higher in energy by ca. 4.5 kcal/mol and the [1,4] ones by ca. 7 kcal/mol, and the pyrido[1,2]diazepines are the least stable (ca. 20 kcal/mol). All the most stable pyrido[1,n]diazepines have N-atoms near the ring's junction bond but on opposite sites. The most stable [2,n]-forms are also those with the pyridine ring N6-atom near the junction bond. Surprisingly, for the [1,2]-, [1,3]-, and [1,4]-isomer condensation types of pyridine and diazepine rings, the same N9 > N7 > N6 > N8 stability pattern obeys. The stability remains similar in a water medium simulated with the Polarizable Continuum Model of the solvent and is conserved when calculated using the CAM-B3LYP or BHandHlyp functionals. The ring's aromaticity in the pyridine[m,n]diazepines was established based on the integral INICS index resulting from the NICSzz-scan curves' integration. The integral INICS index is physically justified through its relation to the ringcurrent as demonstrated by Berger, R.J.F., et al. Phys. Chem. Chem. Phys. 2022, 24, 624. The six-membered pyrido rings have negative INICSZZ indices and can be aromatic only if they are not protonated at the N-atom. All protonated pyrido and seven-membered rings exhibit meaningful positive INICSZZ values and can be assigned as antiaromatic. However, some non-protonated pyrido rings also have substantial positive INICSZZ indices and are antiaromatic. A weak linear correlation (R2 = 0.72) between the INICSZZ values of the pyridine I(6) and diazepine I(7) rings exists and is a consequence of the communication between the π-electron systems of the two rings. The juxtaposition of the INICS descriptor of the six- and seven-membered rings and diverse electron density parameters at the Ring Critical Points (RCP) revealed good correlations only with the Electrostatic Potentials from the electrons and nuclei (ESPe and ESPn). The relationships with other RCP parameters like electron density and its Laplacian, total energy, and the Hamiltonian form of kinetic energy density were split into two parts: one nearly constant for the six-membered rings and one linearly correlating for the seven-membered rings. Thus, most of the electron density parameters at the RCP of the six-membered rings of pyridodiazepines practically do not change with the diazepine type and the labile proton position. In contrast, those of the seven-membered rings display aromaticity changes in the antiaromatic diazepine with its ring structural modifications. [ABSTRACT FROM AUTHOR]

Tricoordinate boron-incorporated π-conjugated systems are widely investigated as optoelectronic materials because of their unique p–π* orbital interactions and high Lewis acidity. Among them, thiophene-fused diborinines are characterized by moderate antiaromaticity and extended conjugation. In this work, we have developed two new dithienodiborinines with C2h and C2v symmetries, which exhibited completely different optical properties. The thiophene-fused diborinines synthesized in this study showed excellent fluorescence properties both in solution and in the solid state, with quantum yields of up to 95%. The high antiaromaticity enhanced the Lewis acidity of the boron centers, as proven by the large association constants with fluoride ion estimated from titration experiments. The high Lewis acidity and the superior luminescence property have enabled their application as fluorescent sensor materials for the detection of ammonia vapor. [ABSTRACT FROM AUTHOR]

Fusion of aromatic subunits to stabilize an antiaromatic core allows the isolation and study of otherwise unstable paratropic systems. A complete study of a series of six naphthothiophene‐fused s‐indacene isomers is herein described. Additionally, the structural modifications resulted in increased π–π overlap in the solid state, which was further explored through changing the sterically blocking mesityl group to (triisopropylsilyl)ethynyl in three derivatives. The computed antiaromaticity of the six isomers is compared to the observed physical properties, such as NMR chemical shift, UV‐vis, and CV data. We find that the calculations predict the most antiaromatic isomer and give a general estimation of the relative degree of paratropicity for the remaining isomers, when compared to the experimental results. [ABSTRACT FROM AUTHOR]

Small ring nitrogen heterocycles, azabicyclobutanes and azirines, were investigated by computational methods in order to address the discrepancy between their regioisomers 1‐ and 2‐azabicyclobutane and 1H‐ and 2H‐azirines. Both 1‐azabicyclobutane and 2H‐azirine are well known synthetic starting points to larger nitrogen heterocycles whereas 2‐azabicyclobutane and 1H‐azirine and their derivatives have yet to be reported as isolable compounds. Calculated parameters such as structure, base strength (proton affinities), NICS values and enthalpies of formation from which strain energies are derived are reported. The destabilization of the less stable regioisomers is attributed to homoantiaromaticity in 2‐azabicyclobutane and antiaromaticity in 1H‐azirine. Two stereoisomers exist for 2‐azabicyclobutane with the endo‐ stereoisomer being more stable. This phenomenon is indicative of the hydrogen bond acceptor properties of the neighboring cyclpropane and the π‐bond character of the central bond in 2‐azabicyclobutane. [ABSTRACT FROM AUTHOR]

Molecular emitters simultaneously generating light at different wavelengths have wide applications. With a small molecule, however, it is challenging to realize two independent radiative pathways. We invented the first examples of dual‐emissive single‐benzene fluorophores (SBFs). Two emissive tautomers are generated by synthetic modulation of the hydrogen bond acidity, which opens up pathways for excited‐state proton transfer. White light is produced by a delicate balance between the energy and intensity of the emission from each tautomer. We show that the excited‐state antiaromaticity of the benzene core itself dictates the proton movements driving the tautomer equilibrium. Using this simple benzene platform, a fluorinated SBF was synthesized with a record high solubility in perfluorocarbon solvents. White light‐emitting devices and multicolor imaging of perfluorocarbon nanodroplets in live cells demonstrate the practical utility of these molecules. [ABSTRACT FROM AUTHOR]

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 (C2+). 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. [ABSTRACT FROM AUTHOR]

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]

4H-Pyrazoles are emerging as useful click reagents. Fluorinating the saturated center enables 4H-pyrazoles to react rapidly as Diels–Alder dienes without a catalyst but compromises the stability of these dienes under physiological conditions. To identify more stable 4H-pyrazoles for bioorthogonal chemistry applications, we investigated the Diels–Alder reactivity and biological stability of three 4-oxo-substituted 4H-pyrazoles. We found that these dienes undergo rapid Diels–Alder reactions with endo-bicyclo[6.1.0]non-4-yne (BCN) while being much more stable to biological nucleophiles than their fluorinated counterparts. We attribute the rapid Diels–Alder reactivity of the optimal oxygen-substituted pyrazole to a combination of antiaromaticity, predistortion, and spirocyclization. Their reactivity and stability suggest that 4-oxo-4H-pyrazoles can be useful bioorthogonal reagents. [ABSTRACT FROM AUTHOR]

Gain of aromaticity or relief of antiaromaticity along a reaction path are important factors to consider in mechanism studies. Analysis of such changes along potential energy surfaces has historically focused on reactions in the electronic ground state (S0), but can also be used for excited states. In the lowest ππ* states, the electron counts for aromaticity and antiaromaticity follow Baird's rule where 4n π‐electrons indicate aromaticity and 4n+2 π‐electrons antiaromaticity. Yet, there are also cases where Hückel's rule plays a role in the excited state. The electron count reversals of Baird's rule compared to Hückel's rule explain many altered physicochemical properties upon excitation of (hetero)annulene derivatives. Here we illustrate how the gain of excited‐state aromaticity (ESA) and relief of excited‐state antiaromaticity (ESAA) have an impact on photoreactivity and photostability. Emphasis is placed on recent findings supported by the results of quantum chemical calculations, and photoreactions in a wide variety of areas are covered. [ABSTRACT FROM AUTHOR]