Metallaantiaromaticity of 10-Platinacorrole Complexes.

The aromaticity of cyclic π-conjugated organometallic compounds is known as metallaaromaticity. π-Conjugated metallacycles, such as metallabenzenes and metallapentalenes, have been investigated in order to understand the involvement of the d electrons from the metal center in the π-conjugated systems of the organic ligands. Here, we report the synthesis of Pd(II) 10-platinacorrole complexes with cyclooctadiene (COD) and norbornadiene (NBD) ligands. While the Pd(II) 10-platinacorrole COD complex adopts a distorted structure without showing appreciable antiaromaticity, the corresponding NBD complex exhibits a distinct antiaromatic character due to its highly planar conformation. Detailed density functional theory (DFT) calculations revealed that two d orbitals are involved in macrocyclic π-conjugation. We furthermore demonstrated that Craig-Möbius antiaromaticity is not present in the studied system. The synthesis of 10-platinacorroles enables a systematic comparison of the antiaromaticity and aromaticity of closely related porphyrin analogues, providing a better understanding of π-conjugation that involves d orbitals.

End-to-End Bent Perylene Bisimide Cyclophanes by Double Sulfur Extrusion.

Bending inherently planar π-cores consisting of only six-membered rings has traditionally been challenging because a powerful transformation is required to compensate for the significant strain energy associated with bending. Herein, we demonstrate that sulfur extrusion can achieve substantial molecular bending of a perylene structure to form a substructure of a Vögtle belt, a proposed yet hitherto elusive carbon nanotube fragment. Bent perylene bisimide (PBI) derivatives were synthesized through a double-sulfur-extrusion reaction from the corresponding sulfur-containing V-shaped precursors with an internal alkyl tether. The effect of bending the inherently planar PBI core, which is a recent topic of interest for the design of advanced organic electronic and optoelectronic materials, was investigated systematically. Increasing the curvature leads to a red shift in the absorption and emission spectra, while the fluorescence quantum yields remain high. This stands in contrast with the nonemissive features of previously reported nonplanar PBI derivatives based on conjugative tethers. Detailed photophysical measurements indicated that the increasing curvature with shorter alkyl tethers (i) slightly facilitates intersystem crossing and (ii) significantly suppresses the internal conversion in the excited state of the present bent PBI derivatives. The latter characteristics originate from the restricted dynamic motion associated with the charge-transfer (CT) character between the core chromophores and the N-aryl units.

Close Stacking of Antiaromatic Ni(II) Norcorrole Originating from a Four-Electron Multicentered Bonding Interaction.

A π-conjugated molecule with one electronic spin often forms a π-stacked dimer through molecular orbital interactions between two unpaired electrons. The bonding is recognized as a multicentered two-electron interaction between the two π-conjugated molecules. Here, we disclose a multicentered bonding interaction between two antiaromatic molecules involving four electrons. We have synthesized an antiaromatic porphyrin analogue, Ni(II) bis(pentafluorophenyl)norcorrole. Its dimer adopts a face-to-face stacked structure with an extremely short stacking distance of 2.97 Å. The close stacking originates from a multicenter four-electron bonding interaction between the two molecules. The bonding electrons were experimentally observed via synchrotron X-ray diffraction analysis and corroborated by theoretical calculations. The intermolecular interaction of the molecular orbitals imparts the stacked dimer with aromatic character that is distinctly different from that of its monomer.

Homochiral and Heterochiral Self-Sorting Assemblies of Antiaromatic Ni(II) Norcorrole Dimers.

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.

Changing aromatic properties through stacking: the face-to-face dimer of Ni(II) bis(pentafluorophenyl)norcorrole.

Nuclear magnetic resonance (NMR) shielding constants have been calculated for Ni(II) bis(pentafluorophenyl)norcorrole and its face-to-face stacked dimer at the Hartree-Fock (HF), second-order Møller-Plesset perturbation theory (MP2), complete-active-space self-consistent-field (CASSCF) levels as well as at density functional theory (DFT) levels using several functionals. The calculated 1H NMR shielding constants agree rather well with the experimental ones. The shielding constants of N and Ni calculated at DFT, HF, and MP2 levels differ from those obtained in the CASSCF calculations due to near-degeneracy effects at the Ni atom. The calculated magnetically induced current densities show that the monomer is antiaromatic, sustaining a strong global paratropic ring current, and the dimer is aromatic, sustaining a strong diatropic ring current. Qualitatively the same current density is obtained at the employed levels of theory. The most accurate ring-current strengths are probably obtained at the MP2 level. The aromatic dimer has a short intermolecular distance of less than 3 Å. The intermolecular interaction changes the nature of the frontier orbitals leading to a formal double bond between the norcorrole macrocycles.

A Triply Linked Porphyrin-Norcorrole Hybrid with Singlet Diradical Character.

Norcorrole Ni(II) complexes have recently received considerable attention because they are readily accessible antiaromatic molecules. Their high stability under ambient conditions and ease of synthesis have enabled the exploration of the intrinsic properties of antiaromatic molecules. Here, we report the synthesis and properties of meso-meso singly linked porphyrin-norcorrole hybrids and a triply linked porphyrin-norcorrole hybrid. The singly linked and triply linked porphyrin-norcorrole hybrids were fully characterized, including an X-ray structural analysis. Due to their orthogonal conformation, the singly linked hybrids maintain the individual electronic properties of their porphyrin and norcorrole subunits, while the triply linked hybrid shows a significantly smaller electrochemical HOMO-LUMO gap (0.45 eV) than that of Ni(II) dimesitylnorcorrole (1.08 eV). Furthermore, the triply linked hybrid exhibits singlet diradical characteristics, as confirmed by VT NMR, ESR, and SQUID experiments.

Realization of Stacked-Ring Aromaticity in a Water-Soluble Micellar Capsule.

Stacked-ring aromaticity arising from the close stacking of antiaromatic π-systems has recently received considerable attention. Here, we realize stacked-ring aromaticity via a rational supramolecular approach. A nanocapsule composed of bent polyaromatic amphiphiles was employed to encapsulate several antiaromatic norcorrole Ni(II) complexes (NCs) in water. The resulting micellar capsules display high stability toward heating and concentration change. The encapsulation resulted in the appearance of a broad absorption band in the near-infrared region, which is characteristic of norcorroles with close face-to-face stacking. Importantly, a meso-isopropyl NC, which does not exhibit π-stacking even in a concentrated solution or the crystalline phase, adopted π-stacking with stacked-ring aromaticity in the supramolecular micellar capsule.

Near-Infrared-Responsive Hydrocarbons Designed by π-Extension of Indeno[1,2,3,4-pgra]perylene at the 1,2,12-Positions.

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.

Easily Switchable 18π-, 19π-, and 20π-Conjugation of Diazaporphyrin Double-Pincer Bispalladium Complexes.

NiII 3,7,13,17-tetrapyridyl-5,15-diazaporphyrin serves as a double tridentate ligand to PdII ions to provide a pincer-type bispalladium complex. Electrochemical analysis revealed that the bispalladium complex shows excellent ability to accept electrons and reversible redox properties due to the coordination of the two cationic PdII centers to the meso-nitrogen atoms. We isolated and characterized one- and two-electron reduction species of the bispalladium complex. The 20π antiaromatic nature of the two-electron reduction species was confirmed by 1 H NMR spectroscopy, UV/Vis-near-IR (NIR) absorption spectra, and density functional theory (DFT) calculations. X-ray diffraction revealed highly twisted structures for the bispalladium complexes regardless of the oxidation state.

Indeno[1,2,3,4-pqra]Perylene: A Medium-Sized Aromatic Hydrocarbon Exhibiting Full-Range Visible-Light Absorption.

We report the synthesis and properties of indeno[1,2,3,4-pqra]perylene, which was prepared by the fusion of one anthracene unit with one naphthalene unit via three carbon-carbon bonds. The synthetic route through two-fold C-H arylation enabled not only the synthesis of unsubstituted indenoperylene, but also rapid access to its arylated derivatives on the gram scale. Indenoperylene is a medium-sized aromatic hydrocarbon with the composition C24 H12 that is isomeric to coronene. Nevertheless, its absorption covers the entire visible region owing to its small HOMO-LUMO gap. Furthermore, indenoperylene exhibits high stability despite the absence of peripheral substituents. We propose that the unique electronic structure of indenoperylene originates from the coexistence of an electron-withdrawing subunit (benzoaceanthrylene) and an electron-donating subunit (perylene). The electronic properties of indenoperylene were modulated via post-functionalization through regioselective bromination. The current research demonstrates that indenoperylene is a promising candidate as a main skeleton for near-infrared-responsive and redox-active materials.

D-Mannose-appended 5,15-diazaporphyrin for photodynamic therapy.

5,15-Diazaporphyrin appended with D-mannose moieties was prepared through Suzuki-Miyaura cross-coupling reaction and SN2 alkylation. The resultant diazaporphyrin was hydrophilic enough to exhibit sufficient solubility in aqueous media. Because of the photosensitizing ability of diazaporphyrins, the in vitro activity of the D-mannose-appended diazaporphyrin in photodynamic therapy (PDT) was investigated. The specific internalization of the functionalized diazaporphyrin into human breast adenocarcinoma (MDA-MB-231) cells through mannose receptors was confirmed by confocal microscopy imaging. We also demonstrated the strong PDT activity of the functionalized diazaporphyrin at a nanomolar level with short light irradiation time.

Symmetry-breaking charge separation in a nitrogen-bridged naphthalene monoimide dimer.

The photophysical properties of 4-aminonaphthalene-1,8-imide-based derivatives, bis-ANI, consisting of two naphthalimide (NI) units linked by a butylamine bridge and its monomer ANI have been intensively investigated by steady-state and transient spectroscopy combined with quantum chemical calculations. The excited state relaxation dynamics of the two molecules are studied in three solvents of varying polarity - from hexane to tetrahydrofuran to acetone. A strong reduction in the fluorescence quantum yields and larger red shifts of the emission spectra are observed when going from the monomer ANI to dimer bis-ANI with increasing solvent polarity. It is found that the presence of the central amino linker in bis-ANI facilitates the formation of an asymmetric CS state between the ANI and NI moieties in bis-ANI, where NI˙- is the dominant radical anion unit after CS, evidenced by the femtosecond transient absorption measurements and spectroelectrochemistry in polar solvents. Femtosecond transient absorption spectra and quantum chemical calculations reveal the conformational change after the formation of the symmetry-breaking charge separation (SBCS) state upon photoexcitation, while a near-orthogonal structure in the excited state of bis-ANI retards charge recombination. In addition, it is evidenced that the rate of SBCS can be tuned by changing the different polar solvents.

A Heme-Acquisition Protein Reconstructed with a Cobalt 5-Oxaporphyrinium Cation and Its Growth-Inhibition Activity Toward Multidrug-Resistant Pseudomonas aeruginosa.

Using artificial hemes for the reconstruction of natural heme proteins represents a fascinating approach to enhance the bioactivity of the latter. We report the synthesis of various metal 5-oxaporphyrinium cations as cofactors, and a cobalt 5-oxaporphyrinium cation was successfully incorporated into the heme-acquisition protein (HasA) secreted by Pseudomonas aeruginosa. We hypothesize that the oxaporphyrinium cation strongly binds to the HasA-specific outer membrane receptor (HasR) due to its cationic charge, which prevents the subsequent acquisition of heme. In fact, the reconstructed HasA inhibited the growth of Pseudomonas aeruginosa and even of multidrug-resistant P. aeruginosa.

A Supramolecular Polymer Constituted of Antiaromatic NiII Norcorroles.

For the creation of next-generation organic electronic materials, the integration of π-systems has recently become a central theme. Such functional materials can be assembled by supramolecular polymerization when aromatic π-systems are used as monomers, and the properties of the resulting supramolecular polymer strongly depend on the electronic structure of the monomers. Here, we demonstrate the construction of a supramolecular polymer consisting of an antiaromatic π-system as the monomer. An amide-functionalized NiII norcorrole derivative formed a one-dimensional supramolecular polymer through π-π stacking and hydrogen-bonding interactions, ensuring the persistency of the conducting pathway against thermal perturbation, which results in higher charge mobility along the tightly bound linear aggregates than that of the aromatic analogue composed of ZnII porphyrins.

Determinant Factors of Three-Dimensional Aromaticity in Antiaromatic Cyclophanes.

Three-dimensional aromaticity arising from the close stacking of two antiaromatic π-conjugated macrocycles has recently received considerable attention. Here, a cyclophane consisting of two antiaromatic Ni(II) norcorrole units tethered with two flexible alkyl chains was synthesized. The norcorrole cyclophane showed crystal polymorphism providing three different solid-state structures. Surprisingly, one of them adopted an aligned face-to-face stacking arrangement with negligible displacement along the slipping axis. Although the exchange repulsion between two π-clouds should be maximized in this orientation, the π-π distance is remarkably close (3.258 Å). Three-dimensional aromaticity in this conformation has been supported experimentally and theoretically as evidenced by small bond length alternations as well as the presence of a diatropic ring current. An analogous cyclophane with two aromatic Ni(II) porphyrin units was prepared for comparison. The porphyrin cyclophane exhibited a slipped-stacking conformation with a larger displacement (2.9 Å) and a larger interplanar distance (3.402 Å) without noticeable change of the aromaticity of each porphyrin unit. In solution, the norcorrole cyclophane forms a twist stacking arrangement with effective interplanar orbital overlap and exists in an equilibrium between stacked and nonstacked structures. Thermodynamic parameters of the stacking process were estimated, revealing an inherently large attractive interaction operating between two norcorrole units, which has been further supported by energy decomposition analysis.

Enthalpically and Entropically Favorable Self-Assembly: Synthesis of C4h -Symmetric Tetraazatetrathia[8]circulenes by Regioselective Introduction of Pyridine Rings.

Self-assembly of π-conjugated molecules in solution generally occurs owing to either an enthalpic or an entropic gain; however, designing π-conjugated systems that simultaneously exhibit enthalpically and entropically favorable self-assembly behavior is challenging. Herein, the self-assembly behavior of tetraazatetrathia[8]circulenes is disclosed, which is driven by both enthalpy and entropy. Single-crystal X-ray diffraction analysis demonstrated that molecules of these tetraazatetrathia[8]circulenes form face-to-face stacked dimers with a 1D columnar structure owing to the circularly arranged dipole moments. Importantly, concentration- and temperature-dependent 1 H NMR spectra revealed that the formation of self-assemblies of tetraazatetrathia[8]circulenes in chloroform and methanol is favored by both enthalpic and entropic factors. The unique association behavior is due to the presence of sp2 -hybridized nitrogen atoms, which weakly coordinate to the hydrogen atoms of these solvents and reduce the π-electron density of the circulene cores.

Quadruply BN-Fused Tetrathia[8]circulenes with Flexible Frameworks: Synthesis, Structures and Properties.

Quadruply BN-fused tetrathia[8]circulenes were synthesized through four-fold electrophilic borylation. The single-crystal X-ray diffraction analysis revealed that the BN-fused tetrathia[8]circulene with peripheral phenyl groups exhibits crystal polymorphism, in which the circulene core adopts both planar and saddle conformations in the solid state. The experimental and theoretical studies revealed that the weaker aromaticity of azaborine compared with benzene renders the flexibility of the BN-fused tetrathia[8]circulenes.

Non-Planar Perylene Bisimide Analogues with Inserted Carbonyl and Methylene Subunits.

The synthesis and properties of dinaphthotropone bisimide (DNTrBI) and dinaphthocycloheptatriene bisimide (DNCHepBI) are described. Their molecular design is conceptually based on the insertion of a carbon atom into a perylene bisimide (PBI) core. These molecules adopt non-planar structures due to the presence of a seven-membered ring. The PBI derivative into which a carbonyl group was inserted (DNTrBI) immediately underwent nonradiative decay and/or intersystem crossing in its excited state. The PBI derivative into which a methylene group was inserted (DNCHepBI) was susceptible to deprotonation on account of the two electron-withdrawing naphthalene monoimide units. Subsequent aerobic oxidation resulted in the formation of a C-C bond at the central methylene unit, thus affording a σ-dimer. The formation of this C-C bond is dynamically redox-active, i.e., electron injection into the σ-dimer almost quantitatively regenerated the deprotonated DNCHepBI.

Synthesis and Characterization of 16π Antiaromatic 2,7-Dihydrodiazapyrenes: Antiaromatic Polycyclic Hydrocarbons with Embedded Nitrogen.

We describe the two-electron reduction of N,N'-dimethyl-2,7-diazapyrenium dications (MDAP2+ ), which afforded the corresponding reduced form (MDAP0 ) as a highly electron-rich 16π antiaromatic system. A single-crystal X-ray diffraction analysis of MDAP0 revealed a distorted quinoidal structure with high bond-length alternation. The 1 H NMR spectrum of MDAP0 exhibited a diagnostic proton signal (4.6 ppm) that is distinctly upfield shifted compared to that of aromatic diazapyrene (8.3 ppm). Theoretical calculations supported the existence of a paratropic ring current. These results indicate that MDAP0 exhibits antiaromatic character derived from its peripheral 16π-electron conjugation.

Acridino[2,1,9,8-klmna]acridine Bisimides: An Electron-Deficient π-System for Robust Radical Anions and n-Type Organic Semiconductors.

We report the synthesis and properties of acridino[2,1,9,8-klmna]acridine bisimide (AABI), a nitrogen-doped anthanthrene with two imide functionalities. AABI exhibits excellent electron affinity as evident by its low-lying LUMO level (-4.1 eV vs. vacuum). Single-electron reduction of one AABI derivative afforded the corresponding radical anion, which was stable under ambient conditions. Photoconductivity measurements suggest that the intrinsic electron mobility of an N-phenethyl AABI derivative obeys a band-transport model. Accordingly, an electron mobility of 0.90 cm2 V-1 s-1 was attained with the corresponding single-crystal organic field-effect transistor (OFET) device. The vacuum-deposited OFET device consisting of a polycrystalline sample exhibited high electron mobility of up to 0.27 cm2 V-1 s-1 even in air. This study demonstrates that dual incorporation of both imide substituents and imine-type nitrogen atoms is an effective strategy to create novel electron-deficient π-systems.