Azulene-Fused Linear Polycyclic Aromatic Hydrocarbons with Small Bandgap, High Stability, and Reversible Stimuli Responsiveness

Synthesis and Properties of Twisted and Helical Azulene Oligomers and Azulene-Based Polycyclic Hydrocarbons

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.

Diazulenorubicene as a Non-benzenoid Isomer of peri-Tetracene with Two Sets of 5/7/5 Membered Rings Showing Good Semiconducting Properties

Non-benzenoid polycyclic aromatic hydrocarbons (PAHs) have received a lot of attention because of their unique optical, electronic, and magnetic properties, but their synthesis remains challenging. Herein, we report a non-benzenoid isomer of peri-tetracene, diazulenorubicene (DAR), with two sets of 5/7/5 membered rings synthesized by a (3+2) annulation reaction. Compared with the precursor containing only 5/7 membered rings, the newly formed five membered rings switch the aromaticity of the original heptagon/pentagon from antiaromatic/aromatic to non-aromatic/antiaromatic respectively, modify the intermolecular packing modes, and lower the LUMO levels. Notably, compound 2 b (DAR-TMS) shows p-type semiconducting properties with a hole mobility up to 1.27 cm2  V-1  s-1 . Moreover, further extension to larger non-benzenoid PAHs with 19 rings was achieved through on-surface chemistry from the DAR derivative with one alkynyl group.

Merging of Azulene and Perylene Diimide for Optical pH Sensors

Polycyclic aromatic hydrocarbons (PAHs) have emerged as promising materials for organic electronics, including organic photovoltaics (OPVs), organic field-effect transistors (OFETs), and organic light-emitting diodes (OLEDs). Particularly, non-hexagonal ring-fused PAHs are highly desirable due to their unique optoelectronic properties. Herein, a new redox-active azulene-perylene diimide triad 1 and its ring-fused counterpart, diazulenocoronene diimide 2, were synthesized and fully characterized by a combination of NMR, cyclic voltammetry, and UV-visible absorption spectroscopy. Direct comparison of their electronic properties leads us to the conclusion that a significant change in the localization of HOMO and LUMO occurs upon the fusion of azulene and perylene diimide in 2, leading to the lack of intramolecular charge-transfer character for transitions in the visible spectral region. Density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations were performed to gain further insight into various electronic transitions. Moreover, we found that the adaptive response to acids and bases manifests itself in a reversible two-color change that can be attributed to changes in the chemical structures. Our findings pave the way for manipulating the relative HOMO and LUMO energy levels of organic chromophores by fusing non-alternant azulenes to an otherwise flat PAH, which could possibly lead to applications in organic electronics and optical sensors.

Extension of Non-alternant Nanographenes Containing Nitrogen-Doped Stone-Thrower-Wales Defects

Non-alternant topologies have attracted considerable attention due to their unique physiochemical characteristics in recent years. Here, three novel topological nanographenes molecular models of nitrogen-doped Stone-Thrower-Wales (S-T-W) defects were achieved through intramolecular direct arylation. Their chemical structures were unambiguously elucidated by single-crystal analysis. Among them, threefold intramolecular direct arylation compound (C42 H21 N) is the largest nanographene bearing a N-doped non-alternant topology to date, in which the non-benzenoid rings account for 83 % of the total molecular skeleton. The absorption maxima of this compound was located in the near-infrared region with a long tail up to 900 nm, which was much longer than those reported for similarly sized N-doped nanographene with six-membered rings (C40 H15 N). In addition, the electronic energy gaps of these series compounds clearly decreased with the introduction of non-alternant topologies (from 2.27 eV to 1.50 eV). It is noteworthy that C42 H21 N possesses such a low energy gap (Eg opt =1.40 eV; Eg cv =1.50 eV), yet is highly stable under ambient conditions. Our work reported herein demonstrates that the non-alternant topology could significantly influence the electronic configurations of nanocarbons, where the introduction of a non-alternanting topology may be an effective way to narrow the energy gap without extending the molecular π-conjugation.

Synthesis and Properties of Carbo- and Heterocyclic Benz[a]azulenes

A new and convenient synthesis of aryl-substituted naphtho[2,1-a]azulenes by the combination of Suzuki-Miyaura, Sonogashira, and cycloisomerization reactions is reported. The methodology was applied to the synthesis of hitherto unknown azuleno[1,2-h]quinolines, cyclohepta[1,2]indeno[4,5-b]thiophenes, and cyclohepta[1,2]indeno[4,5-c]thiophenes. The impact of different fused-heterocyclic rings on the photophysical and electrochemical properties of these azulene derivatives was studied by experimental and theoretical methods and hence provides a rationale for the preparation of novel azulene derivatives with improved properties for application as organic materials.

Synthesis of a Dicyclohepta[a,g]heptalene-Containing Polycyclic Conjugated Hydrocarbon and the Impact of Non-Alternant Topologies

Incorporating non-hexagonal rings into polycyclic conjugated hydrocarbons (PCHs) can significantly affect their electronic and optoelectronic properties and chemical reactivities. Here, we report the first bottom-up synthesis of a dicyclohepta[a,g]heptalene-embedded PCH (1) with four continuous heptagons, which are arranged in a "Z" shape. Compared with its structural isomer bischrysene 1 R with only hexagonal rings, compound 1 presents a distinct antiaromatic character, especially the inner heptalene core, which possesses clear antiaromatic nature. In addition, PCH 1 exhibits a narrower highest occupied molecular orbital (HOMO)-lowest unoccupied molecular orbital (LUMO) energy gap than its benzenoid contrast 1 R, as verified by experimental measurements and theoretical calculations. Our work reported herein not only provides a new way to synthesize novel PCHs with non-alternant topologies but also offers the possibility to tune their electronic and optical properties.

Synthesis of Defective Nanographenes Containing Joined Pentagons and Heptagons

Defective nanographenes containing joined pentagons and heptagons exhibit striking physicochemical properties from both experimental and theoretical perspectives compared with their pure hexagonal counterparts. Thus, the synthesis and characterization of these unique polyarenes with well-defined defective topologies have attracted increasing attention. Despite extensive research on nonalternant molecules since the last century, most studies focused on the corresponding mutagenic and carcinogenic activities. Recently, researchers have realized that the defective domain induces geometric bending and causes electronic perturbation, thus leading to significant alteration of the photophysical properties. This review discusses the synthesis and characterization of small nonalternant polycyclic hydrocarbons in the early stage and recent developments in embedding pentagon-heptagon (5-7) pairs into large carbon skeletons through in-solution chemistry.

Azulene-Embedded [n]Helicenes (n=5, 6 and 7)

Azulene is a non-benzenoid aromatic building block with unique chemical structure and physicochemical properties. By using the "bottom-up" synthetic strategy, we synthesized three azulene-embedded [n]helicenes ([n]AzHs, n=5, 6 and 7), in which one terminal azulene subunit was fused with n-2 benzene rings. P- and M-enantiomers were observed in the packing diagrams of [5]-, and [6]AzHs. However, P- and M-[7]AzHs could be isolated by recrystallization of the racemic mixture. These [n]AzHs were endowed with new properties through the azulene moiety such as low-lying first electric state (S₁ ), small optical energy gap and anti-Kasha emission. [6]-, and [7]AzHs exhibit strong chiroptical responses with high absorption dissymmetry factor (g_abs ) maxima of about 0.02, which is among the highest |g_abs | values of helicenes in the visible range. These azulene-embedded [n]helicenes contribute to the non-benzenoid helicene library and allow the structure-property relationships to be better understood.

Synthesis, Reactivity, and Properties of Benz[a]azulenes via the [8 + 2] Cycloaddition of 2H-Cyclohepta[b]furan-2-ones with an Enamine

Starting with the reaction of 2H-cyclohepta[b]furan-2-ones with an enamine, which was prepared from 4-tert-butylcyclohexanone and pyrrolidine, benz[a]azulenes having both formyl and tert-butyl groups were obtained in the three-step sequence. Subsequently, both the formyl and tert-butyl groups were eliminated by heating the benz[a]azulene derivatives in 100% H₃PO₄ to give benz[a]azulenes without these substituents in high yields. In terms of product yield, this method is the best one ever reported for the synthesis of the parent benz[a]azulene so far. The conversion of the benz[a]azulene derivatives with a formyl group into cyclohept[a]acenaphthylen-3-one derivatives was also investigated via Knoevenagel condensation with dimethyl malonate, followed by Brønsted acid-mediated intramolecular cyclization. The structural features including the bond alternation in the benz[a]azulene derivatives were revealed by NMR studies, NICS calculations, and a single-crystal X-ray structural analysis. The optical and electrochemical properties of a series of benz[a]azulene derivatives were evaluated by UV/Vis, fluorescence spectroscopy, and voltammetry experiments. As a result, we found that some benz[a]azulene derivatives showed remarkable luminescence in acidic media. In addition, the benz[a]azulene derivatives with the electron-withdrawing group and cyclohept[a]acenaphthylen-3-one derivative displayed good reversibility in the spectral changes under the electrochemical redox conditions.

Design, Synthesis, and Biological Evaluation of New Azulene-Containing Chalcones

Azulene-containing chalcones have been synthesized via Claisen-Schmidt condensation reaction. Their chemical structure has been established by spectroscopic methods where the ¹H-NMR spectra suggested that the title chalcones were geometrically pure and configured trans (J = 15 Hz). The influence of functional groups from azulene-containing chalcones on the biological activity of the 2-propen-1-one unit was investigated for the first time. This study presents optical and fluorescent investigations, QSAR studies, and biological activity of 10 novel compounds. These chalcones were evaluated for their antimicrobial activity against Gram-positive and Gram-negative bacteria. The results revealed that most of the synthesized compounds showed inhibition against Gram-negative microorganisms, independent of the substitution of azulene scaffold. Instead, all azulene-containing chalcones exhibited good antifungal activity against Candida parapsilosis, with MIC values ranging between 0.156 and 0.312 mg/mL. The most active compound was chalcone containing azulene moieties on both sides of the 2-propene-1-one bond, exhibiting good activity against both bacteria-type strains and good antifungal activity. This antifungal activity combined with low toxicity makes azulene-containing chalcones a new class of bioorganic compounds.

Construction of Heptagon-Containing Molecular Nanocarbons

Molecular nanocarbons containing heptagonal rings have attracted increasing interest due to their dynamic behavior, electronic properties, aromaticity, and solid-state packing. Heptagon incorporation can not only induce negative curvature within nanocarbon scaffolds, but also confer significantly altered properties through interaction with adjacent non-hexagonal rings. Despite the disclosure of several beautiful examples in recent years, synthetic strategies toward heptagon-embedded molecular nanocarbons remain relatively limited due to the intrinsic challenges of heptagon formation and incorporation into polyarene frameworks. In this Review, recent advances in solution-mediated and surface-assisted synthesis of heptagon-containing molecular nanocarbons, as well as the intriguing properties of these frameworks, will be discussed.

Synthesis of Azulene Derivatives from 2H-Cyclohepta[b]furan-2-ones as Starting Materials: Their Reactivity and Properties

A variety of synthetic methods have been developed for azulene derivatives due to their potential applications in pharmaceuticals and organic materials. Particularly, 2H-cyclohepta[b]furan-2-one and its derivatives have been frequently used as promising precursors for the synthesis of azulenes. In this review, we describe the development of the synthesis of azulenes by the reaction of 2H-cyclohepta[b]furan-2-ones with olefins, active methylenes, enamines, and silyl enol ethers as well as their reactivity and properties.

Topology Effects in Molecular Organic Electronic Materials: Pyrene and Azupyrene*

Pyrene derivatives play a prominent role in organic electronic devices, including field effect transistors, light emitting diodes, and solar cells. The flexibility in the desired properties has previously been achieved by variation of substituents at the periphery of the pyrene backbone. In contrast, the influence of the topology of the central π‐electron system on the relevant properties such as the band gap or the fluorescence behavior has not yet been addressed. In this work, pyrene is compared with its structural isomer azupyrene, which has a π‐electron system with non‐alternant topology. Using photoelectron spectroscopy, near edge X‐ray absorption fine structure spectroscopy, and other methods, it is shown that the electronic band gap of azupyrene is by 0.72 eV smaller than that of pyrene. The difference of the optical band gaps is even larger with 1.09 eV, as determined by ultraviolet–visible absorption spectroscopy. The non‐alternant nature of azupyrene is also associated with a more localized charge distribution. Further insight is provided by density functional theory (DFT) calculations of the molecular properties and ab initio coupled cluster calculations of the optical transitions. The concept of aromaticity is used to interpret the major topology‐related differences.

Azulene as an ingredient for visible-light- and stimuli-responsive photoswitches

The azulene molecule features a unique combination of optical, luminescence, and stimuli-responsive properties. This makes the azulene motif a promising functional group to be introduced in photoswitches. Recent challenges in the field of photochromic compounds require the development of new approaches to molecules that are switched by visible light (400-760 nm), are proton responsive and have advanced luminescent properties. Merging azulene with photoswitches opens prospects for fulfilling these requirements. Herein, we highlight recent results on the application of this hydrocarbon motif in various photochromic systems, such as stilbenes, diarylethenes, and azobenzenes.

Azulene-Substituted Donor-Acceptor Polymethines and 1,6'-Bi-, 1,6';3,6''-Ter-, and Quinqueazulenes via Zincke Salts: Synthesis, and Structural, Optical, and Electrochemical Properties

Azulene-substituted donor-acceptor polymethines, bi-, ter-, and quinqueazulenes composed of the 1,6'-biazulene unit have been successfully prepared from corresponding Zincke salts. The synthesis of polymethines through the reaction of Zincke salts with several amines, followed by a Knoevenagel reaction with malononitrile, was accomplished in moderate to high yields (40-92 %). Meanwhile, the reaction of Zincke salts with secondary amines and the subsequent sequential condensation-cyclization with cyclopentadienide ions, so-called Ziegler-Hafner method, produced the corresponding 1,6'-biazulenes, 1,6';3,6''-terazulenes, and quinqueazulene, respectively. The structural, optical, and electrochemical properties of the azulene-substituted donor-acceptor polymethines, bi-, ter-, and quinqueazulenes were revealed by single-crystal X-ray structure analysis, UV/vis spectroscopy, voltammetry analysis, spectroelectrochemistry, and theoretical calculations. These results suggested that the substituents on the azulene ring and their substitution positions directly affect their reactivities, optical and electrochemical properties.

Azulene-Based π-Functional Materials: Design, Synthesis, and Applications

ConspectusAzulene, an isomer of naphthalene, is a molecule of historical interest for its unusual photophysical properties, including a beautiful blue color derived from the narrow HOMO-LUMO energy gap and anti-Kasha fluorescence from S₂ to S₀. More recently, it has attracted increasing attention for its novel electronic structure, including an electron-rich five-membered ring and an electron-deficient seven-membered ring with a dipole moment of 1.08 D resulting from resonance delocalization, its different reactivities at odd and even positions, and its stimuli-responsive behavior. As a key building block, azulene has been used in various fields because of its unique physicochemical properties. Recent studies have demonstrated the great potential of azulene for constructing advanced organic materials. However, exploring azulene-based materials has long been hindered by challenges in molecular design and synthesis. Most of the reported azulene-based materials have the azulene unit incorporated through the five-membered ring or seven-membered ring. Creating azulene-based novel building blocks for optoelectronics and using 2,6-connected azulene units to construct conjugated polymers that can adequately utilize the "donor-acceptor" structure of azulene remained underexplored before our contributions. Besides, for most azulene-fused polycyclic aromatic hydrocarbons (PAHs) and heteroaromatics, the azulene substructures were created during later synthesis stages, and the use of azulene derivatives as starting materials to design and synthesize PAHs and heteroaromatics intelligently is still limited.In this Account, we summarize our efforts on the design, synthesis, and applications of azulene-based π-functional materials. Our studies start with the creation of novel π-conjugated structures based on azulene. The design strategy, synthesis, and optoelectronic performance of the first class of azulene-based aromatic diimides, 2,2'-biazulene-1,1',3,3'-tetracarboxylic diimide (BAzDI) and its π-extended and π-bridged derivatives, are presented. Notably, antiparallel stacking between adjacent azulene units derived from azulene's dipole was observed in single crystals of BAzDI and its derivatives. Besides, we developed an azulene-fused isoindigo analogue, azulenoisoindigo, which combines the merits of both isoindigo and azulene, including reversible redox behavior and reversible proton responsiveness. Then we discuss our contributions to the design and synthesis of 2,6-azulene-based conjugated polymers. By incorporation of 2,6-connected azulene units into the polymeric backbone, two conjugated polymers with high organic field-effect transistor (OFET) performance were developed. Two 2,6-azulene-based polymers with proton responsiveness and high electrical conductivity upon protonation were also provided. We also discuss our recent studies on azulene-based heteroaromatics. Two azulene-fused BN-heteroaromatics were designed and synthesized, and they exhibited a selective response to fluoride ion and unexpected deboronization upon the addition of trifluoroacetic acid. An unexpected synthesis of azulene-pyridine-fused heteroaromatics (Az-Py) by reductive cyclization of 1-nitroazulenes and the OFET performance of Az-Py-1 are included. Afterward, we discuss several examples of azulene-capped organic conjugated molecules. The molecules capped with the five-membered ring of azulene favor hole transport, whereas the ones capped with the seven-membered ring favor electron transport.

C4-aldehyde of guaiazulene: synthesis and derivatisation

Guaiazulene is an alkyl-substituted azulene available from natural sources and is a much lower cost starting material for the synthesis of azulene derivatives than azulene itself. Here we report an approach for the selective functionalisation of guaiazulene which takes advantage of the acidity of the protons on the guaiazulene C4 methyl group. The aldehyde produced by this approach constitutes a building block for the construction of azulenes substituted on the seven-membered ring. Derivatives of this aldehyde synthesised by alkenylation, reduction and condensation are reported, and the halochromic properties of a subset of these derivatives have been studied.

NIR-Absorbing π-Extended Azulene: Non-Alternant Isomer of Terrylene Bisimide

The first planar π-extended azulene that retains aromaticity of odd-membered rings was synthesized by [3+3] peri-annulation of two naphthalene imides at both long-edge sides of azulene. Using bromination and subsequent nucleophilic substitution by methoxide and morpholine, selective functionalization of the π-extended azulene was achieved. Whilst these new azulenes can be regarded as isomers of terrylene bisimide they exhibit entirely different properties, which include very narrow optical and electrochemical gaps. DFT, TD-DFT, as well as nucleus-independent chemical shift calculations were applied to explain the structural and functional properties of these new π scaffolds. Furthermore, X-ray crystallography confirmed the planarity of the reported π-scaffolds and aromaticity of their azulene moiety.

The Effect of Base and Nucleophile on the Nucleophilic Substitution of Methoxytropone Derivatives: An Easy Access to 4- and 5-Substituted Multifunctional Azulenes

The nucleophilic substitution on 3-substituted 2-methoxytropones to form azulenes is dependent on the nucleophile and base employed. With bulkier nucleophiles (ethyl/methyl cyanoacetate), the reaction proceeds with the abnormal nucleophilic substitution irrespective of the base and with smaller nucleophiles (malononitrile), the reaction follows base-dependent normal and abnormal nucleophilic substitution. Thus, the methodologies are developed to selectively obtain 4- and 5-substituted azulenes based on the nature of bases and nucleophiles employed.

Recent Progress in High Linearly Fused Polycyclic Conjugated Hydrocarbons (PCHs, n > 6) with Well-Defined Structures

Although polycyclic conjugated hydrocarbons (PCHs) and their analogues have gained great progress in the fields of organic photoelectronic materials, the in-depth study on present PCHs is still limited to hexacene or below because longer PCHs are insoluble, unstable, and tediously synthesized. Very recently, various strategies including on-surface synthesis are developed to address these issues and many higher novel PCHs are constructed. Therefore, it is necessary to review these advances. Here, the recent synthetic approach, basic physicochemical properties, single-crystal packing behaviors, and potential applications of the linearly fused PCHs (higher than hexacene), including acenes or π-extended acenes with fused six-membered benzenoid rings and other four-membered, five-membered or even seven-membered and eight-membered fused compounds, are summarized.

A polycyclic aromatic hydrocarbon diradical with pH-responsive magnetic properties

By integrating azulene with a quinoid moiety, a novel non-alternant polycyclic aromatic hydrocarbon molecule BCHF1 exhibiting manifold zwitterionic, quinoidal and diradical behaviors is designed and synthesized. Its zwitterionic feature is evidenced by the changes shown by the ¹H-NMR and absorption spectra when the molecule undergoes reversible protonation and deprotonation reactions at varied pH. The diradical facet, manifesting a small singlet-triplet energy gap (ΔE _S-T), is characterized with a paramagnetic resonance signal detected by the EPR spectroscopy at room temperature. As the diradical properties are not observed in the protonated form, BCHF1+H⁺ , a pH-controlled reversible magnetic switching behavior is illustrated by monitoring the on and off cycles of EPR signals upon successively adding bases and acids to a solution or exposing a thin film of BCHF1+H⁺ to base vapor followed by acid vapor.

A simple scheme for finding magnetic aromatic hydrocarbon molecules

Polycyclic aromatic hydrocarbon (PAH) molecules such as quasi-unidimensional oligo-acene and fused azulene display interesting properties for increasing chain length. However, these molecules can be hard to explore computationally due to the number of atoms involved and the fast-increasing numerical cost when using many-body methods. The identification of magnetic PAH molecules is most relevant for technological applications and hence it would be of particular interest to develop rapid preliminary checks to identify likely candidates for both theoretical and experimental pursuits. In this article, we show that an analysis based on a second-order perturbation treatment of electronic correlations for the Hubbard model qualitatively predicts the outcome of more extensive and accurate methods. Based on these results we propose a simple computational protocol for screening molecules and identifying those worthy of a more sophisticated analysis on the magnetic nature of their ground states. Using this protocol we were able to identify two new magnetic molecules made from the combination of only two naphthalene monomers and two azulene ones (both isomers with formula C₃₄H₂₀). For further confirmation of this result, these molecules were also studied by means of density matrix renormalization group and density functional theory.

Synthesis of phthalimides cross-conjugated with an azulene ring, and their structural, optical and electrochemical properties

The preparation of phthalimides cross-conjugated with an azulene ring was established by a one-pot Diels-Alder reaction of the corresponding 2-aminofuran derivatives with several maleimides, without the isolation of the intermediately formed [4 + 2] cycloadducts. The structure, optical and electrochemical properties of the novel phthalimide derivatives were clarified by single-crystal X-ray analysis, UV/Vis and fluorescence spectra, spectroelectrochemistry and voltammetry experiments, and theoretical calculations. These results indicated that the substituents on the azulene ring greatly affect the optical and electrochemical properties of the molecules.

Abnormal Nucleophilic Substitution on Methoxytropone Derivatives: Steric Strategy to Synthesize 5-Substituted Azulenes

Azulene is a non-alternant non-benzenoid aromatic system, and in turn, it possesses unusual photophysical properties. Azulene-based conjugated systems have received increasing interest in recent years as optoelectronic materials. Despite the routes available for the preparation of substituted azulene derivatives, there remain few methods that allow regioselective substitution on the seven-membered ring of azulenes due to the subtle reactivity difference among the various positions. This report explores the reactivity of substituted tropolones as the azulene precursors and also provides a new method to create 5-substituted azulenes. The reaction of cyanoacetate enolate with unsubstituted 2-methoxytropone affords azulene through the attack of the nucleophile on the C-2 center (normal pathway). We have observed that 3-substituted 2-methoxytropones undergo steric-guided nucleophilic addition at the C-7 center (abnormal pathway) to afford 5-substituted azulene derivatives. Based on this observation and DFT calculation, a new synthetic strategy is devised for the regioselective synthesis of 5-substituted multifunctional azulenes, which cannot be accessed by any other method.

In the quest for a stable triplet state in small polyaromatic hydrocarbons: an in silico tool for rational design and prediction

Combining the roles of spin frustration and geometry of odd and even numbered rings in polyaromatic hydrocarbons (PAHs), we design small molecules that show exceedingly small singlet-triplet gaps and stable triplet ground states. Furthermore, a computationally efficient protocol with a model spin Hamiltonian is shown to be capable of qualitative agreement with respect to high level multireference calculations and therefore, can be used for fast molecular discovery and screening.

Azulene-Based BN-Heteroaromatics

Azulene, a nonalternant bicyclic aromatic hydrocarbon, has unique chemical and physical properties and is considered to be a promising building block for constructing novel polycyclic aromatic hydrocarbons (PAHs) and heteroaromatics. We present here the first two azulene-based BN-heteroaromatics Az-BN-1 and Az-BN-2. The chemical structures and optical and electrochemical properties of both compounds have been investigated, as well as their sensing behavior in response to fluoride ion. Az-BN-1 and Az-BN-2 show different photophysical properties from other reported BN-embedded PAHs, such as lower band gaps and unusual fluorescence. In addition, Az-BN-1 and Az-BN-2 exhibit unexpected deboronization upon addition of trifluoroacetic acid, which distinguishes them from other reported BN-heteroaromatics and can be ascribed to the unique property of the azulene unit.

Regioselective Functionalization of 9,9-Dimethyl-9-silafluorenes by Borylation, Bromination, and Nitration

Despite the utility of 9-silafluorenes as functional materials and as building blocks, methods for efficient functionalization of their backbone are rare, probably because of the presence of easily cleavable C-Si bonds. Although controlling the regioselectivity of iridium-catalyzed direct borylation of C-H bonds is difficult, we found that bromination and nitration of 2-methoxy-9-silafluorene under mild conditions occurred predominantly at the electron-rich position. The resulting product having methoxy and bromo groups can be utilized as a building block for the synthesis of unsymmetrically substituted 9-silafluorene-containing π-conjugated molecules.

Synthesis and Redox Properties of Pyrrole- and Azulene-Fused Azacoronene

Synthesis of an azacoronene, in which both pyrrole and azulene moieties are circularly fused, was achieved just in three steps. This new azacoronene exhibited multistep reversible oxidations under electrochemical and chemical conditions. Formation of an aromatic 22π-electron conjugation and a tropylium cation (6π-electron conjugation) in the dicationic state was revealed by the single-crystal X-ray crystallographic analysis as well as the nucleus-independent chemical shift (NICS) and anisotropy of the induced current density (ACID) calculations.