An Azulene-Based Crystalline Porous Covalent Organic Framework for Efficient Photothermal Conversion.

The designed synthesis of a crystalline azulene-based covalent organic framework (COF-Azu-TP) is presented and its photothermal property is investigated. Azulene, a distinctive 5-7 fused ring non-benzenoid aromatic compound with a large intramolecular dipole moment and unique photophysical characteristics, is introduced as the key feature in COF-Azu-TP. The incorporation of azulene moiety imparts COF-Azu-TP with broad-spectrum light absorption capability and interlayer dipole interactions, which makes COF-Azu-TP a highly efficient photothermal conversion material. Its polyurethane (PU) composite exhibits a solar-to-vapor conversion efficiency (97.2%) and displays a water evaporation rate (1.43 kg m-2 h-1) under one sun irradiation, even at a very low dosage of COF-Azu-TP (2.2 wt%). Furthermore, COF-Azu-TP is utilized as a filler in a polylactic acid (PLA)/polycaprolactone (PCL) composited shape memory material, enabling rapid shape recovery under laser stimulation. A comparison study with a naphthalene-based COF isomer further emphasizes the crucial role of azulene in enhancing photothermal conversion efficiency. This study demonstrates the significance of incorporating specific building blocks into COFs for the development of functional porous materials with enhanced properties, paving the way for future applications in diverse fields.

Azulene-Containing Bis(squaraine) Dyes: Design, Synthesis and Aggregation Behaviors.

The relationship among chemical structure, physicochemical property and aggregation behavior of organic functional material is an important research topic. Here, we designed and synthesized three bis(squaraine) dyes BSQ1, BSQ2 and BSQ3 through the combination of two kinds of unsymmetrical azulenyl squaraine monomers. Their physicochemical properties were investigated in both molecular and aggregate states. Generally, BSQ1 displayed different assembly behaviors from BSQ2 and BSQ3. Upon fabrication into nanoparticles, BSQ1 tend to form J-aggregates while BSQ2 and BSQ3 tend to form H-aggregates in aqueous medium. When in the form of thin films, three bis(squaraine) dyes all adopted J-aggregation packing modes while only BSQ1 presented the most significant rearrangement of aggregate structures as well as the improvement in the carrier mobilities upon thermal annealing. Our research highlights the discrepancy of aggregation behaviors originating from the molecular structure and surrounding circumstances, providing guidance for the molecular design and functional applications of squaraines.

Efficient Synthesis and Structural Analysis of Chiral 4,4'-Biazulene.

4,4'-Biazulene is a potentially attractive key component of an axially chiral biaryl compound, however, its structure and properties have not been clarified owing to the lack of its efficient synthesis. We report a breakthrough in the reliable synthesis of 4,4'-biazulene, which is achieved by the access to azulen-4-ylboronic acid pinacol ester and 4-iodoazulene as novel key synthetic intermediates for the Suzuki-Miyaura cross-coupling reaction. The X-ray crystallographic analysis of 4,4'-biazulene confirmed its axial chirality. The enantiomers of 4,4'-biazulene were successfully resolved by HPLC on the chiral stationary phase column. The kinetic experiments and DFT calculations indicate that the racemization energy barrier of 4,4'-biazulene is comparable to that of 1,1'-binaphthyl.

Azulene and Its Derivatives as Potential Compounds in the Therapy of Dermatological and Anticancer Diseases: New Perspectives against the Backdrop of Current Research.

The scientific article focuses on the role of azulene and its derivatives in the therapy of dermatological diseases, presenting the latest laboratory and clinical research as well as prospects for further studies. In a synthetic literature review, various databases such as PubMed, Scopus, Web of Science, and the Database of Polish Scientific Journals were queried to select relevant articles concerning azulene. The conclusions drawn from the thematic analysis of the studies emphasize the multifaceted pharmacological actions of azulene and its derivatives including their anti-inflammatory properties, potential anticancer effects, photoprotective abilities, alleviation of itching, management of atopic dermatitis, and treatment of erectile dysfunction. However, there are certain limitations associated with the application of unmodified azulene on the skin, particularly related to photodecomposition and the generation of reactive oxygen species under UV radiation. These effects, in turn, necessitate further research on the safety of azulene and azulene-derived substances, especially regarding their long-term use and potential application in phototherapy. The authors of this work emphasize the necessity of conducting further preclinical and clinical studies to fully understand the mechanisms of action. Incorporating azulene and its derivatives into the therapy of dermatological disorders may represent an innovative approach, thereby opening new treatment avenues for patients.

Introduction of Electron Donor Groups into the Azulene Structure: The Appearance of Intense Absorption and Emission in the Visible Region.

In this work, through the Suzuki-Miyaura cross-coupling reaction with high yields, new π-conjugated azulene compounds containing diphenylaniline groups at positions 2 and 6 of azulene were synthesized. The obtained diphenylaniline-azulenes have intensely visible-light absorbing and emitting (in the wavelength range from 400 to 600 nm) properties. It has been shown that such unique optical properties, in particular fluorescent emission in the region of blue and green photoluminescence (λem at 495 and 525 nm), which were absent in the original azulene, are the result of the electron donor effect of diphenylaniline groups, which significantly changes the electronic structure of azulene and leads to the allowed HOMO → LUMO electron transition.

Perylene with Split-Azulene Embedding.

Splitting the five and seven-membered rings of azulene and embedding them separately into a conjugated backbone provides azulene-like polycyclic aromatic hydrocarbons (PAHs), which are of great interest in quantum and material chemistry. However, the synthetic accessibility poses a significant challenge. In this study, we present the synthesis of a novel azulene-like PAH, Pery-57, which can be viewed as the integration of a perylene framework into the split azulene. The compact structure of Pery-57 displays several intriguing characteristics, including NIR II absorption at 1200 nm, a substantial dipole moment of 3.5 D, and head-to-tail alternating columnar packing. Furthermore, Pery-57 exhibits remarkable redox properties. The cationic radical Pery-57•+ readily captures a hydrogen atom. Variable-temperature NMR (VT-NMR) and variable-temperature EPR (VT-EPR) studies reveal that the dianion Pery-572- possesses an open-shell singlet ground state and demonstrates significant global anti-aromaticity. The dication Pery-572+ is also predicted to exhibit diradical character. Despite bearing three bulky substituents, Pery-57 displays p-type transport characteristics with a mobility of 0.03 cm2 V-1 s-1, attributed to its unique azulene-like structure. Overall, this work directs interest in azulene-like PAHs, a unique member of nonalternant PAHs showcasing exceptional properties and applications.

Fused Azulenyl Squaraine Derivatives Improve Phototheranostics in the Second Near-Infrared Window by Concentrating Excited State Energy on Non-Radiative Decay Pathways.

The second near-infrared (NIR-II) theranostics offer new opportunities for precise disease phototheranostic due to the enhanced tissue penetration and higher maximum permissible exposure of NIR-II light. However, traditional regimens lacking effective NIR-II absorption and uncontrollable excited-state energy decay pathways often result in insufficient theranostic outcomes. Herein a phototheranostic nano-agent (PS-1 NPs) based on azulenyl squaraine derivatives with a strong NIR-II absorption band centered at 1092 nm is reported, allowing almost all absorbed excitation energy to dissipate through non-radiative decay pathways, leading to high photothermal conversion efficiency (90.98 %) and strong photoacoustic response. Both in vitro and in vivo photoacoustic/photothermal therapy results demonstrate enhanced deep tissue cancer theranostic performance of PS-1 NPs. Even in the 5 mm deep-seated tumor model, PS-1 NPs demonstrated a satisfactory anti-tumor effect in photoacoustic imaging-guided photothermal therapy. Moreover, for the human extracted tooth root canal infection model, the synergistic outcomes of the photothermal effect of PS-1 NPs and 0.5 % NaClO solution resulted in therapeutic efficacy comparable to the clinical gold standard irrigation agent 5.25 % NaClO, opening up possibilities for the expansion of NIR-II theranostic agents in oral medicine.

On-Surface Synthesis of Non-Benzenoid Nanographenes Embedding Azulene and Stone-Wales Topologies.

The incorporation of non-benzenoid motifs in graphene nanostructures significantly impacts their properties, making them attractive for applications in carbon-based electronics. However, understanding how specific non-benzenoid structures influence their properties remains limited, and further investigations are needed to fully comprehend their implications. Here, we report an on-surface synthetic strategy toward fabricating non-benzenoid nanographenes containing different combinations of pentagonal and heptagonal rings. Their structure and electronic properties were investigated via scanning tunneling microscopy and spectroscopy, complemented by computational investigations. After thermal activation of the precursor P on the Au(111) surface, we detected two major nanographene products. Nanographene Aa-a embeds two azulene units formed through oxidative ring-closure of methyl substituents, while Aa-s contains one azulene unit and one Stone-Wales defect, formed by the combination of oxidative ring-closure and skeletal ring-rearrangement reactions. Aa-a exhibits an antiferromagnetic ground state with the highest magnetic exchange coupling reported up to date for a non-benzenoid containing nanographene, coexisting with side-products with closed shell configurations resulted from the combination of ring-closure and ring-rearragement reactions (Ba-a , Ba-s , Bs-a and Bs-s ). Our results provide insights into the single gold atom assisted synthesis of novel NGs containing non-benzenoid motifs and their tailored electronic/magnetic properties.

Synthesis of a Benzotrisazulene via Trioxobenzotrisazulene.

Polycyclic aromatic hydrocarbons (PAHs) containing odd-membered rings (such as pentagons or heptagons) are fascinating synthetic targets. A special case is the introduction of five- and seven-membered rings in form of an azulene unit. Azulene itself is an aromatic compounds known for its deep blue color, which is a result of its internal dipole moment. When azulene is embedded into PAHs it can change the optoelectronic properties of the PAH significantly. Herein, the synthesis and characterization of a PAH containing three azulene units is reported via reduction and elimination of its trioxo derivative.

Structural Expansion of Cyclohepta[def]fluorene towards Azulene-Embedded Non-Benzenoid Nanographenes.

Non-benzenoid non-alternant nanographenes (NGs) have attracted increasing attention on account of their distinct electronic and structural features in comparison to their isomeric benzenoid counterparts. In this work, we present a series of unprecedented azulene-embedded NGs on Au(111) during the attempted synthesis of cyclohepta[def]fluorene-based high-spin non-Kekulé structure. Comprehensive scanning tunneling microscopy (STM) and non-contact atomic force microscopy (nc-AFM) evidence the structures and conformations of these unexpected products. The dynamics of the precursor bearing 9-(2,6-dimethylphenyl)anthracene and dihydro-dibenzo-cyclohepta[def]fluorene units and its reaction products on the surface are analyzed by density functional theory (DFT) and molecular dynamics (MD) simulations. Our study sheds light on the fundamental understanding of precursor design for the fabrication of π-extended non-benzenoid NGs on a metal surface.

Butterfly-Shaped Nanographenes with Excellent Second-Order Nonlinear Optical Properties: The Synergy of B/N and Azulene.

Azulene, a simple polar polycyclic aromatic hydrocarbon with connected electron donor and acceptor (DA), ignites the hope of designing second-order nonlinear optical (NLO) molecular materials from pure nonpolar carbon nanomaterials. In this work, a butterfly-shaped nanographene (π-DA-π) was designed by incorporating azulene between two coronenes. One more electron in a N atom or one electron fewer in a B atom with respect to a C atom can polarize charge distribution in carbon nanomaterials, and further doping of B and N in the designed butterfly-shaped nanographene changes the system from π-DA-π to D-π-A, leading to strong NLO responses. For example, the largest static first hyperpolarizability even reaches 173.89×10-30  esu per heavy atom. The synergetic role of B, N and azulene in the nanographene is scrutinized, and such a doping strategy is found to provide an effective means for the design of carbon-based functional materials. The strong second-order NLO responses of these butterfly-shaped carbon-based nanographenes under external fields, for example, sum frequency generation and difference frequency generation, could inspire future experimental exploration.

Azulene as a biphenyl mimetic in orexin/hypocretin receptor agonists.

Azulene is a rare ring structure in drugs, and we investigated whether it could be used as a biphenyl mimetic in known orexin receptor agonist Nag 26, which is binding to both orexin receptors OX1 and OX2 with preference towards OX2. The most potent azulene-based compound was identified as an OX1 orexin receptor agonist (pEC50 = 5.79 ± 0.07, maximum response = 81 ± 8% (s.e.m. of five independent experiments) of the maximum response to orexin-A in Ca2+ elevation assay). However, the azulene ring and the biphenyl scaffold are not identical in their spatial shape and electron distribution, and their derivatives may adopt different binding modes in the binding site.

Synthetic Potential of Regio- and Stereoselective Ring Expansion Reactions of Six-Membered Carbo- and Heterocyclic Ring Systems: A Review.

Ring expansion reactions fascinate synthetic chemists owing to their importance in synthesizing biologically active compounds and their efficacy in medicinal chemistry. The present review summarizes a number of synthetic methodologies, including stereoselective and regioselective pathways adopted by scientists, for framing medium- to large-size carbo- and heterocycles involving lactams, lactone, azepine and azulene derivatives via ring expansion of six-membered carbo- and heterocycles that have been reported from 2007-2022. Numerous rearrangement and cycloaddition reactions involving Tiffeneau-Demjanov rearrangement, Aza-Claisen rearrangement, Schmidt rearrangement, Beckmann rearrangement, etc., have been described in this regard.

Synthesis of Azuleno[2,1-b]quinolones and Quinolines via Brønsted Acid-Catalyzed Cyclization of 2-Arylaminoazulenes.

Quinolone and quinoline derivatives are frequently found as substructures in pharmaceutically active compounds. In this paper, we describe a procedure for the synthesis of azuleno[2,1-b]quinolones and quinolines from 2-arylaminoazulene derivatives, which are readily prepared via the aromatic nucleophilic substitution reaction of a 2-chloroazulene derivative with several arylamines. The synthesis of azuleno[2,1-b]quinolones was established by the Brønsted acid-catalyzed intramolecular cyclization of 2-arylaminoazulene derivatives bearing two ester groups at the five-membered ring. The halogenative aromatization of azuleno[2,1-b]quinolones with POCl3 yielded azuleno[2,1-b]quinolines with a chlorine substituent at the pyridine moiety. The aromatic nucleophilic substitution reaction of azuleno[2,1-b]quinolines bearing chlorine substituent with secondary amines was also investigated to afford the aminoquinoline derivatives. These synthetic methodologies reported in this paper should be valuable in the development of new pharmaceuticals based on the azulene skeleton.

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.

Modular Synthesis of Structurally Diverse Azulene-Embedded Polycyclic Aromatic Hydrocarbons by Knoevenagel-Type Condensation.

The research interest in azulene-embedded polycyclic aromatic hydrocarbons (PAHs) has significantly increased recently, but the lack of efficient synthetic strategies impedes the investigation of their structure-property relationships and further opto-electronic applications. Here we report a modular synthetic strategy towards diverse azulene-embedded PAHs by a tandem Suzuki coupling and base-promoted Knoevenagel-type condensation with good yields and great structural versatility, including non-alternant thiophene-rich PAHs, butterfly- or Z-shaped PAHs bearing two azulene units, and the first example of a two-azulene-embedded double [5]helicene. The structural topology, aromaticity and photophysical properties were investigated by NMR, X-ray crystallography analysis and UV/Vis absorption spectroscopy assisted by DFT calculations. This strategy provides a new platform for rapidly synthesizing unexplored non-alternant PAHs or even graphene nanoribbons with multiple azulene units.

Cascade Synthesis of Benzotriazulene with Three Embedded Azulene Units and Large Stokes Shifts.

We report here the one-pot synthesis of benzo[1,2-a : 3,4-a' : 5,6-a'']triazulene (BTA), wherein three azulene units are embedded through a tandem reaction comprising two steps, Suzuki coupling and Knoevenagel condensation, between a readily available triborylated truxene precursor and 8-bromo-1-naphthaldehyde. Its nitration leads to a regioselective trinitrated product, namely, BTA-NO2 . Single-crystal X-ray crystallography revealed that the superstructure of BTA consists of a dimer stacked by two enantiomeric helicene conformers, while that of BTA-NO2 consists of an unprecedented π-tetramer stacked from two enantiomeric dimers, that is, four distinct helicene conformers. Both compounds show excellent stability and fluorescence with large Stokes shifts of up to 5100 cm-1 . In addition, BTA-NO2 exhibits a unique solvatochromic effect in different solvents and hydrogen-bonding-induced emission transfer in different ratios of THF/H2 O solutions.

Annulated Azuleno[2,1,8-ija]azulenes: Synthesis and Properties.

Non-alternant non-benzenoid hydrocarbons exhibit very different optical and electronic properties than their well-studied benzenoid analogues. However, preparing such structures with extended conjugation length, remains challenging. Herein, we report the synthesis and properties of azuleno[2,1,8-ija]azulene derivatives using a two-step sequence involving a four-fold aldol condensation between aromatic dialdehydes and readily available tetrahydropentalene-2,5-(1H,3H)-dione. Molecules with band gap values ranging from 1.69 to 2.14 eV and molar extinction coefficients (ϵ) of nearly 3×105  M-1 cm-1 have been prepared. These annulene-like structures exhibit significant diatropic ring currents (aromatic), as supported by 1 H NMR spectroscopy and DFT calculations. Field-effect transistors (OFETs) using azuleno[2,1,8-ija]azulene derivatives as semiconductors exhibit charge mobility values of up to 0.05 cm2  V-1 s-1 .

Redox-Active Azulene-based 2D Conjugated Covalent Organic Framework for Organic Memristors.

As a conjugated and unsymmetric building block composed of an electron-poor seven-membered sp2 carbon ring and an electron-rich five-membered carbon ring, azulene and its derivatives have been recognized as one of the most promising building blocks for novel electronic devices due to its intrinsic redox activity. By using 1,3,5-tris(4-aminophenyl)-benzene and azulene-1,3-dicarbaldehyde as the starting materials, an azulene(Azu)-based 2D conjugated covalent organic framework, COF-Azu, is prepared through liquid-liquid interface polymerization strategy for the first time. The as-fabricated Al/COF-Azu/indium tin oxide (ITO) memristor shows typical non-volatile resistive switching performance due to the electric filed induced intramolecular charge transfer effect. Associated with the unique memristive performance, a simple convolutional neural network is built for image recognition. After 8 epochs of training, image recognition accuracy of 80 % for a neutral network trained on a larger data set is achieved.

π-Extended Pleiadienes by [5+2] Annulation of 1-Boraphenalenes and ortho-Dihaloarenes.

Palladium-catalyzed [5+2] annulation of 1-boraphenalenes with ortho-dihaloarenes afforded negatively curved π-extended pleiadienes. Two benzo[1,2-i:4,5-i']dipleiadienes (BDPs) featuring a seven-six-seven-membered ring arrangement were synthesized and investigated. Their crystal structure revealed a unique packing arrangement and theoretical calculations were employed to shed light onto the dynamic behavior of the BDP moiety and its aromaticity. Further, a naphthalene-fused pleiadiene was stitched together by oxidative cyclodehydrogenation to yield an additional five-membered ring. This formal azulene moiety led to distinct changes in optical and redox properties and increased perturbation of the aromatic system.