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.

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.

Synthesis of sterically congested double helicene by alkyne cycloisomerization.

Alkyne cycloisomerization of 2,7,10,15-tetra(ortho-alkynylphenyl)benzo[g,p]chrysene containing bulky 4-alkoxy-2,6-dimethylphenyl groups at the alkyne terminals selectively proceeded at the sterically crowded bay-region. The obtained double helicene adopts a distorted structure with a high racemization barrier due to the intramolecular steric repulsion.

Norcorroles as antiaromatic π-electronic systems that form dimension-controlled assemblies.

Norcorrole derivatives with 3,4,5-trialkoxyphenyl moieties at the meso positions were synthesized to form various stacking assemblies in single crystals and thermotropic liquid crystals (LCs) depending on aliphatic chain lengths. Triple-decker stacking structures were formed via the interactions between the antiaromatic systems formed for the butoxy and dodecyloxy derivatives in the single-crystal and LC states, respectively. In particular, the LC state exhibited discotic columnar structures comprising triple deckers to exhibit high electric conductivity, as supported by molecular dynamics simulations.

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.

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.

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.

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.

Syntheses of Thiophene-Thiophene-Linked Corrorin Dimers with Tunable Near-Infrared Absorption and Distinctive Reactivity.

Thiahexaphyrinone 1 and thia-dipyrrin-appended corrorin 2 have been synthesized. Surprisingly, further oxidation of compound 2 with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) in dichloromethane afforded dimer 3 with two molecules of compound 2 linked at the α-carbon atoms of the thienyl units. Treatment of compound 3 with DDQ in MeOH and SnCl2 in tetrahydrofuran/H2O afforded the dimethoxy-attached dimer 4 and hydrogenated dihydroxy-attached dimer 5, respectively. These results provide the first examples for synthesizing thiophene-linked porphyrinoid dimers with tunable near-infrared absorption and chirality.

Excimer Formation Driven by Excited-State Structural Relaxation in a Covalent Aminonaphthalimide Dimer.

Strongly coupled excimer formation from interchromophoric charge transfer driven by the ultrafast excited-state structural dynamics of a 5,5'-linked 4-amino-1,8-naphthalimide covalent homodimer was investigated by ultrafast transient spectroscopy and chemical calculations. Theoretical calculations indicate that the structural relaxation associated with the dihedral motion leads to significantly enhanced interchromophoric charge transfer (CT) coupling, which favors the formation of an excimer-like symmetry-broken CT state. The formation and relaxation dynamics of the excimer state in the dimer are identified via ultrafast transient absorption and fluorescence spectroscopy. The structural relaxation following the photoexcitation occurs in tens of picoseconds and stabilizes the dimer to the strongly coupled excimer state. The highly polar solvents further stabilize the excimer state and enhance the CT character, which enable efficient electron and excitation energy transport in covalent molecular aggregates.

Effect of Internal Substituents on the Properties of Dibenzo[g,p]chrysene.

We investigated the chemical and physical properties of internally functionalized dibenzo[g,p]chrysene (DBC) derivatives. These molecules exhibit chiral double-helicene-like structures that are configurationally stable at ambient temperatures. The internal substituents control the conformational change in the excited state, thereby modulating the emission intensity. Notably, the DBC derivative with a methylenedioxy unit undergoes aromatization through elimination of the internal substituent upon photoexcitation, resulting in the formation of DBC.

Intrinsic Photostability in Dithiolonaphthalimide Achieved by Disulfide Bond-Induced Excited-State Quenching.

Disulfide bridges common in proteins show excellent photostability achieved by ultrafast internal conversion and maintain the stability of the tertiary structure. When disulfide bonds exist in aromatic compounds, the rigid chemical structure may affect the cleavage and reforming dynamics of disulfide bonds. In this work, a model compound with a disulfide five-membered-ring structure, 4,5-dithiolo-N-(2,6-dimethylphenyl)-1,8-naphthalimide (DTDPNI), is selected to elaborate the effect of disulfide modification on the excited-state deactivation mechanism. Quantum chemical calculations show that the S-S stretching leads to a dramatic decrease in the energy gap between the S1 and S0 states, similar to the situation in 1,2-dithiane. Due to the efficient nonradiative process, the excited-state lifetime of DTDPNI resolved by ultrafast spectroscopy is determined to be ∼20 ps. It is found that the excellent photostability is achieved by ultrafast excited-state quenching induced by the S-S stretching, rather than the cleavage of the disulfide bond; even the disulfide bridge is in a very rigid aromatic molecular system.

Diazazethrene bisimide: a strongly electron-accepting π-system synthesized via the incorporation of both imide substituents and imine-type nitrogen atoms into zethrene.

The development of highly electron-accepting π-systems is a fundamentally challenging issue despite their potential applications as high-performance n-type organic semiconductors, organic rechargeable batteries, and stable redox-active organocatalysts. Herein, we demonstrate that the incorporation of both imide substituents and imine-type nitrogen atoms into zethrene affords the strongly electron-accepting π-system diazazethrene bisimide (DAZBI). DAZBI has a low-lying LUMO (-4.3 eV vs. vacuum) and is readily reduced by the weak reductant l-ascorbic acid to afford the corresponding dihydro species. The injection of two electrons into DAZBI provides the corresponding dianion. These reduced species display remarkable stability, even under ambient conditions, and an intense red fluorescence. A DAZBI dimer, which was also synthesized, effectively accommodated four electrons upon electron injection.

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.

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.

Dinaphthooxepine Bisimide Undergoes Oxygen Extrusion Reaction upon Electron Injection at Room Temperature.

We report the synthesis and properties of a dinaphthooxepine bisimide (DNOBI), a nonplanar perylene bisimide (PBI) analogue with an inserted oxygen atom. A DNOBI underwent an oxygen-extrusion reaction smoothly upon electron injection at room temperature, affording PBI in good yield. Studies on the reaction mechanism suggest that the injection of two electrons triggers the isomerization of DNOBI to dinaphthooxanorcaradiene bisimide, which is a key step in inducing the oxygen-extrusion reaction.

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.

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.

Stepwise N-Methylation of Ruthenium and Cobalt 5,15-Diazaporphyrins: Post-Functionalization of Porphyrinoid Catalysts.

Post-functionalization of porphyrinoid catalysts provides a powerful tool for fine-tuning their electronic structure. We have succeeded in the stepwise methylation of the peripheral nitrogen atoms in ruthenium and cobalt 5,15-diazaporphyrins. The axial coordination of an anion to the metal center accelerates the second methylation through charge neutralization. N-Methylation of the diazaporphyrin complexes effectively controls their electron deficiency, Lewis acidity, and catalytic activity.