Azulene–Pyridine-Fused Heteroaromatics

Heptacyclic aromatic hydrocarbon isomers with two azulene units fused

Azulene, known for its unique electronic properties and structural asymmetry, serves as a promising building block for the design of novel non-benzenoid polycyclic aromatic hydrocarbons (PAHs). Herein, we present the synthesis, characterization, and physical properties of three diazulene-fused heptacyclic aromatic hydrocarbons, 8,17-dioctyldiazuleno[2,1-a:2',1'-h]anthracene (trans configuration), 16,18-dioctyldiazuleno[2,1-a:1',2'-j]anthracene (cis configuration) and 3,18-dioctyldiazuleno[2,1-a:1',2'-i]phenanthrene (zigzag configuration). Three compounds are configurational isomers with different fusing patterns of aromatic rings. All three isomers exhibit pronounced aromaticity, as revealed by nuclear magnetic resonance spectroscopy and theoretical calculations. They exhibit characteristics of both azulene and benzenoid PAHs and are much more stable than their all-benzene analogues. The optical and electrochemical properties of these three isomers were investigated through UV-vis absorption spectra and cyclic voltammetry, revealing distinct behaviors influenced by their molecular configurations. Furthermore, the isomer in trans configuration exhibits promising semiconducting properties with a hole mobility of up to 0.22 cm2 V-1 s-1, indicating its potential in organic electronics applications.

Axially Chiral Nonbenzenoid Nanographene with Second Harmonic Generation Property

Chiral nanographenes (NGs) have garnered significant interest as optoelectronic materials in recent years. While helically chiral NGs have been extensively studied, axially chiral NGs have only witnessed limited examples, with no prior reports of axially chiral nonbenzenoid NGs. Herein we report an axially chiral nonbenzenoid nanographene featuring six pentagons and four heptagons. This compound, denoted as 2, was efficiently synthesized via an efficient Pd-catalyzed aryl silane homocoupling reaction. The presence of two bulky 3,5-di-tert-butylphenyl groups around the axis connecting the two nonbenzenoid PAH (AHR) segments endows 2 with atropisomeric chirality and high racemization energy barrier, effectively preventing racemization of both R- and S-enantiomers at room temperature. Optically pure R-2 and S-2 were obtained by chiral HPLC separation, and they exhibit circular dichroism (CD) activity at wavelengths up to 660 nm, one of the longest wavelengths with CD responses reported for the chiral NGs. Interestingly, racemic 2 forms a homoconfiguration π-dimer in the crystal lattice, belonging to the I222 chiral space group. Consequently, this unique structure renders crystals of 2 with a second harmonic generation (SHG) response, distinguishing it from all the reported axially chiral benzenoid NGs. Moreover, R-2 and S-2 also exhibit SHG-CD properties.

Molecular-Scale Design of Azulene-Based Triplet Photosensitizers: Insights from Time-Dependent Optimally Tuned Range-Separated Hybrid

Metal-free triplet photosensitizers are ubiquitous in photocatalysis, photodynamic therapy, photovoltaics, and so forth. Their photosensitization efficiency strongly depends on the ability of the low-lying excited spin-triplet to be populated through intersystem crossing. Small singlet-triplet gaps and considerable spin-orbit coupling between the excited spin-singlet and spin-triplet facilitate efficient intersystem crossing. Azulene (Az), a classic example of Anti-Kasha's blue emitter with considerable fluorescence quantum yield, holds great promise because of its chemical stability, rich electronic properties, and high structural rigidity. Here, we provide computationally modeled Az-derived photosensitizers, namely, Az-CHO and Az-CHS, implementing polarization consistent time-dependent optimally tuned range-separated hybrid. Calculations reveal energetic reordering of low-lying ππ* and nπ* singlet states between Az-CHO and Az-CHS and, thereby, rendering the latter to a nonfluorescent one. Importantly, a small singlet-triplet gap and large spin-orbit coupling for Az-CHX with X = O and S produce remarkably high intersystem crossing rates. Furthermore, strong nonadiabatic coupling between the S1(nπ*) and S2(ππ*) in Az-CHS due to substantially smaller energy gap causes enhanced S1 population via fast internal conversion. These research findings provide new insights into the development of functional Az and or related heavy-atom-free small organic molecule-based triplet photosensitizers.

Excited-State (Anti)Aromaticity Explains Why Azulene Disobeys Kasha's Rule

Fluorescence exclusively occurs from the lowest excited state of a given multiplicity according to Kasha's rule. However, this rule is not obeyed by a handful of anti-Kasha fluorophores whose underlying mechanism is still understood merely on a phenomenological basis. This lack of understanding prevents the rational design and property-tuning of anti-Kasha fluorophores. Here, we propose a model explaining the photophysical properties of an archetypal anti-Kasha fluorophore, azulene, based on its ground- and excited-state (anti)aromaticity. We derived our model from a detailed analysis of the electronic structure of the ground singlet, first excited triplet, and quintet states and of the first and second excited singlet states using the perturbational molecular orbital theory and quantum-chemical aromaticity indices. Our model reveals that the anti-Kasha properties of azulene and its derivatives result from (i) the contrasting (anti)aromaticity of its first and second singlet excited states (S1 and S2, respectively) and (ii) an easily accessible antiaromaticity relief pathway of the S1 state. This explanation of the fundamental cause of anti-Kasha behavior may pave the way for new classes of anti-Kasha fluorophores and materials with long-lived, high-energy excited states.

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.

Conceptual advances in nucleophilic organophosphine-promoted transformations

Catalysis by trivalent nucleophilic organophosphines has emerged as an essential tool in organic synthesis. Several new organic transformations promoted by phosphines substantiate and complement the existing synthetic chemistry tools. Mere design of the substrate and reagent combinations has introduced new modes of reactivity patterns, which are otherwise difficult to achieve. These design considerations have led to the rapid build-up of complex molecular entities and laid a solid foundation to synthesise bioactive natural products and pharmaceuticals. This article presents an overview of some of the conceptual advances, including our contributions to nucleophilic organophosphine chemistry. The scope, limitations, mechanistic insights, and applications of these metal-free transformations are discussed elaborately.

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.

Electronic Structure Modulation of Nanographenes for Second Order Nonlinear Optical Molecular Materials

Nanographenes (NGs) have drawn extensive attention as promising candidates for next-generation optoelectronic and nonlinear optical (NLO) materials, owing to its unique optoelectronic properties and high thermal stability. However, the weak polarity or even non-polarity of NGs (resulting in weak even order NLO properties) and the high chemical reactivity of zigzag edged NGs hinder their further applications in nonlinear optics, thus stabilization (lowering the chemical reactivity) and polarizing the charge distribution in NGs are necessary for such applications of NGs. The fusion of heptagon and pentagon endows the azulene with the character of donor-acceptor, and the B=N unit is isoelectronic to C=C unit. The introduction of polar azulene and BN are idea to polarize and stabilize the electronic structure of NGs for NLO applications. In the present review, a survey on the functionalization and applications of NGs in nonlinear optics is conducted. The engineering of the electronic structure of NGs by topological defects, doping and edge modulation is summarized. Finally, a summary of challenges and perspectives for carbon-based NLO nanomaterials is presented.

Reshaping aromatic frameworks: expansion of aromatic system drives metallabenzenoids to metallapentalenes

Reshaping an aromatic framework to generate other skeletons is a challenging issue due to the stabilization energy of aromaticity. Such reconfigurations of aromatics commonly generate non-aromatic products and hardly ever reshape to a different aromatic framework. Herein, we present the transformation of metallaindenols to metallapentalenes and metallaindenes in divergent pathways, converting one aromatic framework to another with an extension of the conjugation framework. The mechanistic study of this transformation shows that phosphorus ligands play different roles in the divergent processes. Further theoretical studies indicate that the expansion of the aromatic system is the driving force promoting this skeletal rearrangement. Our findings offer a new concept and strategy to reshape and construct aromatic compounds.

Tuning Dipole Orientation of 2,6-Azulene Units in Conjugated Copolymers by C–H Activation Strategy toward High-Performance Organic Semiconductor

Azulene has aroused widespread interest for constructing optoelectronic materials. However, controlling the dipole orientation of 2,6-azulene units in the conjugated polymer backbone is a significant challenge so far. Herein, by C-H activation strategy, three 2,6-azulene-TPD-based conjugated copolymers with different dipole arrangements were synthesized, where TPD = thieno[3,4-c]pyrrole-4,6-dione. The dipole arrangements of 2,6-azulene units were random for P(AzTPD-1), head-to-head/tail-to-tail for P(AzTPD-2), and head-to-tail for P(AzTPD-3). These polymers exhibited unipolar n-type semiconductor characteristics in organic field effect transistors. Moreover, regioregular polymer P(AzTPD-3) displayed the best device performance with an electron mobility of up to 0.33 cm² V^-1 s^-1, which makes P(AzTPD-3) a high-performance n-type polymeric semiconductor. These results demonstrate that incorporation of 2,6-azulene units into the polymeric backbone together with the regulation of the dipole orientation of 2,6-azulene units is an effective strategy for obtaining high-performance organic optoelectronic materials.

Tailoring liquid crystalline self-assembly and de Vries behavior of azulenes via lateral and core substitution

The azulene moiety is a highly attractive building block in optoelectronic applications due to its unique properties. For high-performing devices, the molecular orientation is crucial and can be controlled through liquid-crystalline self-assembly. Recent work showed that liquid crystalline derivatives bearing the 2-phenyl-azulene-1-nitrile core formed broad de Vries-type SmA and SmC phases. For exact understanding of the structure-property relationship, a series of 2-(hetero)aryl-azulenes has been synthesized varying the chain linkage, the lateral substituent, and the aromatic ring. Small changes of the molecular structure determined whether the orthogonal SmA phase or the tilted SmC phase is predominant. Implementation of alkyne chains instead of alkoxy chains resulted in the reduction of phase transition temperatures and formation of mesophases at room temperature. Furthermore, de Vries-like behavior was investigated and reduction values between R = 0.35 and 0.74 were measured which supported the hypothesis that in this system de Vries-like behavior is caused by steric repulsion of the lateral substituent. The control of the phase geometry by the molecular structure might be used for improved molecular orientation in optoelectronic materials.

Construction of a 2,2'-biazulene framework via Brønsted acid-promoted annulation of 2,3-di(1-azulenyl)benzofurans

Dibenzofurans featuring a 2,2'-biazulene framework were prepared in good yields by Brønsted acid-promoted annulation of 2,3-di(1-azulenyl)benzofurans in 100% H₃PO₄. NMR, UV-Vis, and fluorescence spectroscopies were used to investigate the structural and optical properties of the products prepared. Remarkably, the annulated products exhibited fluorescence, with the longest wavelength of azulene derivatives reported to date, which extended into the near-infrared region under acidic conditions.

A Heptacene Analogue Entailing a Quinoidal Benzodi[7]annulene (7/6/7 Ring) Core with a Tunable Configuration and Multiple Redox Properties

Non-benzenoid acenes containing heptagons have received increasing attention. We herein report a heptacene analogue containing a quinoidal benzodi[7]annulene core. Derivatives of this new non-benzenoid acene were obtained through an efficient synthetic strategy involving an Aldol condensation and a Diels-Alder reaction as key steps. The configuration of this heptacene analogue can be modulated from a wavy to a curved one by just varying the substituents from a (triisopropylsilyl)ethynyl group to a 2,4,6-triisopropylphenyl (Trip) group. When mesityl (Mes) groups are linked to the heptagons, the resulting non-benzenoid acene displays polymorphism with a tunable configuration from a curved to a wavy one upon varying the crystallization conditions. In addition, this new non-benzenoid acene can be oxidized or reduced by NOSbF₆ or KC₈ to the respective radical cation or radical anion. Compared with the neutral acene, the radical anion shows a wavy configuration and the central hexagon becomes aromatic.

Zigzag-Edged Polycyclic Aromatic Hydrocarbons from Benzo[m]tetraphene Precursors

A series of zigzag-edged polycyclic aromatic hydrocarbons (PAHs) (Z1-Z3) were synthesized from 2,12-dibromo-7,14-diphenyl-benzo[m]tetraphene (9) as a versatile building block. Their structures were unambiguously confirmed by laser desorption/ionization time-of-flight mass spectrometry, ¹ H NMR, Raman, and Fourier-transformed infrared (FTIR) spectroscopies as well as scanning tunneling microscopy. The fingerprint vibrational modes were elucidated with theoretical support. The edge- and size-dependent optical properties were characterized by UV-Vis absorption and fluorescence spectroscopy and DFT calculations. Moreover, ultrafast transient absorption spectroscopy revealed distinct modulation of the photophysical properties upon π-extension from Z1 to Z2, the latter having a gulf edge.

Helical Bilayer Nonbenzenoid Nanographene Bearing a [10]Helicene with Two Embedded Heptagons

The precision synthesis of helical bilayer nanographenes (NGs) with new topology is of substantial interest because of their exotic physicochemical properties. However, helical bilayer NGs bearing non-hexagonal rings remain synthetically challenging. Here we present the efficient synthesis of the first helical bilayer nonbenzenoid nanographene (HBNG1) from a tailor-made azulene-embedded precursor, which contains a novel [10]helicene backbone with two embedded heptagons. Single-crystal X-ray analysis reveals its highly twisted bilayer geometry with a record small interlayer distance of 3.2 Å among the reported helical bilayer NGs. Notably, the close interlayer distance between the two layers offers intramolecular through-space conjugation as revealed by in situ spectroelectrochemistry studies together with DFT simulations. Furthermore, the chiroptical properties of the P/M enantiomers of HBNG1 are also evaluated by circular dichroism and circularly polarized luminescence.

Liquid crystalline self-assembly of azulene-thiophene hybrids and their applications as OFET materials

Orientational control within thin films is crucial for the preparation of organic field effect transistors (OFETs). The highly ordered liquid crystalline smectic E phase (SmE) is known as a powerful template for solution processed thin films. Here, we describe the synthesis and characterization of three novel azulene-thiophene hybrid materials. Liquid crystalline characterization showed the presence of wide SmE phases. Thin films were prepared by spin-coating at mesophase temperature. Due to the self-aligning properties of the SmE phase uniformly flat films with good molecular alignment were manufactured. Top contact bottom gate OFETs showed mobilities up to (3.3 ± 0.5) × 10^-3 cm² V^-1 s^-1.

Small-molecule ambipolar transistors

Ambipolar transistor properties have been observed in various small-molecule materials. Since a small energy gap is necessary, many types of molecular designs including extended π-skeletons as well as the incorporation of donor and acceptor units have been attempted. In addition to the energy levels, an inert passivation layer is important to observe ambipolar transistor properties. Ambipolar transport has been observed in extraordinary π-electron systems such as antiaromatic compounds, biradicals, radicals, metal complexes, and hydrogen-bonded materials. Several donor/acceptor cocrystals show ambipolar transport as well.

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.

Bis-periazulene (Cyclohepta[def]fluorene) as a Nonalternant Isomer of Pyrene: Synthesis and Characterization of Its Triaryl Derivatives

Bis-periazulene (cyclohepta[def]fluorene), which is an unknown pyrene isomer, was synthesized as kinetically protected forms. Its triaryl derivatives 1c-e exhibited the superimposed electronic structures of peripheral, polarized, and open-shell π-conjugated systems. In contrast to previous theoretical predictions, bis-periazulene derivatives were in the singlet ground state. Changing an aryl group controlled the energy gap between the lowest singlet-triplet states.

Properties Assessment by Quantum Mechanical Calculations for Azulenes Substituted with Thiophen– or Furan–Vinyl–Pyridine

In this paper, azulenes substituted with thiophen– or furan–vinyl–pyridine are reported as heavy metal ligands in systems based on chemically modified electrodes. We undertook a computational study of their structures using density functional theory (DFT). Based on these computations, we obtained properties and key molecular descriptors related to chemical reactivity and electrochemical behavior. We investigated the correlation between some quantum parameters associated with the chemical reactivity and the complexing properties of the modified electrodes based on these ligands. The best correlations for the parameters were retained. We showed that the linear correlation between DFT-computed HOMO/LUMO energies and experimental redox potentials is very good.

Perylene Five-membered Ring Diimide for Organic Semiconductors and π-Expanded Conjugated Molecules

A perylene five-membered ring diimide PDI39 was developed as a new electron-deficient building block for n-type semiconductors. The π-expanded conjugated molecules entailing azulenes were synthesized from PDI39. These conjuagted molecules...

Proton-responsive azulene-based conjugated polymer with nonvolatile memory effects

We report an azulene-based conjugated polymer, PAV, which exhibits proton-gated and electrical-gated changes in its conductivity.

Prompt and Long-Lived Anti-Kasha Emission from Organic Dyes

Anti-Kasha behavior has been the subject of intense debate in the last few years, as demonstrated by the high number of papers appearing in the literature on this topic, dealing with both mechanistic and applicative aspects of this phenomenon. Examples of anomalous emitters reported in the last 10 years are collected in the present review, which is focused on strictly anti-Kasha organic molecules displaying radiative deactivation from S_n and/or T_n, with n greater than 1.

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.

Copper-Catalyzed 3-Positional Amination of 2-Azulenols with O-Benzoylhydroxylamines

A copper-catalyzed ortho-selective amination of 2-azulenols with O-benzoylhydroxylamines (RR'N-OBz) to synthesize ortho-aminoazulenols is reported. A wide range of functional groups on amines are compatible, furnishing the corresponding amino-azulene derivatives in moderate to good yields. The further synthetic elaboration using 3-amino-2-azulenols as starting materials is demonstrated.

One-Pot Synthesis of Triazatriphenylene Using the Povarov Reaction

The Povarov reaction combines aromatic amines, aldehydes, and alkynes in a single step and is regarded as an annulative π-extension reaction of aromatic amines. In this study, the Povarov reaction was investigated as an efficient tool for the synthesis of aza-polycyclic aromatic hydrocarbons via multiple π-extensions. The double Povarov reaction of 1,4-diaminobenzene yielded the 4,7-phenanthroline derivative as the major product, regardless of the steric repulsion in the product. The site selectivity mainly depended on the HOMO distribution of the intermediate rather than the steric factor. Based on these insights, a 1,5,9-triazatriphenylene derivative was synthesized via a triple Povarov reaction. The structures of the synthesized compounds were unambiguously determined by single-crystal X-ray diffraction analysis. The triazatriphenylene derivative formed a smooth and stable thin film upon vacuum vapor deposition and served as a hole-blocking material in organic light-emitting diodes.

Revisiting Acepleiadylene: Two-Step Synthesis and π-Extension toward Nonbenzenoid Nanographene

Acepleiadylene (APD), a nonbenzenoid nonalternant isomer of pyrene, exhibits different electronic properties from pyrene, but has been rarely studied since its first synthesis in 1956, probably due to the difficulties in synthesis and further derivatization. In this work, we revisited this long-known compound and developed a new two-step synthetic route to efficiently access APD on the gram scale. Theoretical and experimental characterizations elucidated the unique properties of APD as compared with its benzenoid isomer pyrene, particularly revealing its dipolar structure with a narrow optical gap. The functionalization of APD was demonstrated for the first time, providing doubly brominated APD as a key precursor for further π-extension. As a proof of concept, a π-extended APD and a cyclotrimer nanographene (C₄₈H₂₄) were constructed, opening up new avenues to nonbenzenoid nanographenes with low HOMO-LUMO gaps.

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.

Synthesis and derivatization of hetera-buckybowls

Buckybowls have concave and convex surfaces with distinct π-electron cloud distribution, and consequently they show unique structural and electronic features as compared to planar aromatic polycycles. Doping the π-framework of buckybowls with heteroatoms is an efficient scheme to tailor inherent properties, because the nature of heteroatoms plays a pivotal role in the structural and electronic characteristics of the resulting hetera-buckybowls. The design, synthesis, and derivatization of hetera-buckybowls open an avenue for obtaining fascinating organic entities not only of fundamental importance but also of promising applications in optoelectronics. In this review, we summarize the advances in hetera-buckybowl chemistry, particularly the synthetic strategies toward these scaffolds.

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.

Transforming electron-rich hetero-buckybowls into electron-deficient polycycles

Oxidation of trichalcogenasumanenes (TCSs) with NO species results in the simultaneous formation of ortho-quinone and diester groups. This reaction enables the transformation of electron-rich TCSs into electron-deficient polycycles.