Azulene-Based BN-Heteroaromatics

Recent Progress in the Chemistry of Ring-Fused Azulenes: Synthesis, Reactivity and Properties

Azulene, a non-alternative aromatic hydrocarbon, has attracted significant attention due to its unique electronic properties, and potential applications in organic electronics and optoelectronics. This review highlights recent advances in the synthesis, reactivity, and functional properties of ring-fused azulene derivatives. The discussion encompasses classical synthetic routes, including the Ziegler-Hafner and Nozoe methods, as well as novel approaches such as transition metal-catalyzed cyclizations. Key advancements in the construction of benzo[a]azulenes, naphthoazulenes, and other polycyclic azulene frameworks are detailed, emphasizing their regioselective functionalization and enhanced stability. Moreover, the incorporation of azulene moieties into polycyclic aromatic hydrocarbons (PAHs) and heterocyclic systems is explored, highlighting their potential applications in organic light-emitting diodes (OLEDs), field-effect transistors (OFETs), and photovoltaic devices. Special attention is given to azulene-fused helicenes and nanographenes, which demonstrate promising chiroptical properties and extended π-conjugation. This review aims to provide a comprehensive overview of the synthetic strategies and emerging applications of azulene-based compounds, contributing to the development of advanced materials for future electronic and photonic technologies.

Azulene-Fused seco-Hexabenzocoronene-Based Nanographenes

Due to the unique electronic structure, azulene-containing polycyclic aromatic hydrocarbons attract significant attention toward tuning the chemical and electronic properties of benzenoid carbocyclic systems. Here, we report the synthesis and properties of two types of azulene fused, seco-hexabenzocoronene-based nanographenes. The X-ray crystallographic analysis indicated their nonplanar structures with highly torsional periphery in the solid structures. The typical narrow highest occupied molecular orbital-lowest unoccupied molecular orbital (HOMO-LUMO) energy gaps, long-wavelength absorption, and anti-Kasha emissions were investigated by experimental and computational studies. Interestingly, these nanographenes exhibited a pronounced photothermal effect.

Insight into the Excited States in Monomers and π-Stacked Dimers of Azulene-Fused Acenes: ADC(2) and TD-DFT Studies

Charge transfer (CT) states in polycyclic aromatic hydrocarbons play crucial roles in determining their electronic properties and their potential applications in organic electronics. In this work, we investigate the nature of the excited states in monomers and π-stacked dimers of azulene-fused naphthalene and anthracene systems, focusing on the interplay between structure and excited-state properties. Four different isomers for azulene-fused naphthalene (NapAz-A, NapAz-B, NapAz-C, and NapAz-D) and anthracene (AntAz-A, AntAz-B, AntAz-C, and AntAz-D) are considered. The excited-state studies are performed at the SCS-ADC(2) level and at the TD-DFT level using CAM-B3LYP, SCS-ωB2GP-PLYP, and SCS-RSX-QIDH functionals. For the monomers, the SCS-ADC(2) results reveal that states with CT characters are different in naphthalene- and anthracene-based systems. In π-stacked dimers, a few of the excited states are of the charge resonance (CR) type in NapAz-A, NapAz-B, and NapAz-C and the intermolecular CT type in NapAz-D. Similarly, AntAz-A, AntAz-B, and AntAz-D have some CR type excited states, whereas the AntAz-C isomer has intramolecular CT type excited states. Overall, among the three DFT functionals considered, CAM-B3LYP has been found to reproduce well the SCS-ADC(2) excited results in both monomers and π-stacked dimers.

Buckybowl-Based Fullerene Receptors

Buckybowls, bowl-shaped polyaromatic hydrocarbons, have received intensive interest owing to their multifaceted potentials in supramolecular chemistry and materials science. Buckybowls possess unique chemical and physical properties associated with their concave and convex faces. In view of the shape complementarity, which is one of the key factors for host-guest assembly, buckybowls are ideal receptors for fullerenes. In fact, the host-guest assembly between buckybowls and fullerenes is one of the most active topics in buckybowls chemistry, and the resulting supramolecular materials show promising applications in optoelectronics, biomaterials, and so forth. In this tutorial review, we present an overview for the progress on fullerene receptors based on buckybowls over the last decade.

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.

Gold(I)-Catalyzed Intramolecular 7-endo-dig Cyclization of Triene-Yne Systems: New Access towards Azulenothiophenes

We report the direct synthesis of new azulene derivatives through gold-catalyzed cyclization reactions. A five-membered ring as backbone in the applied triene-yne substrates turned out to be crucial to induce the 7-endo-dig cyclization mode necessary to trigger azulene formation. The obtained targets are of high interest due to their potential applications in different fields, like organic materials, medicine or cosmetics. UV/Vis spectra and cyclic voltammetry were measured, based on these the electronic properties were determined. Short two or three step sequences towards the applied starting materials make this approach synthetically highly attractive.

The magic of biaryl linkers: the electronic coupling through them defines the propensity for excited-state symmetry breaking in quadrupolar acceptor-donor-acceptor fluorophores

Charge transfer (CT) is key for molecular photonics, governing the optical properties of chromophores comprising electron-rich and electron-deficient components. In photoexcited dyes with an acceptor-donor-acceptor or donor-acceptor-donor architecture, CT breaks their quadrupolar symmetry and yields dipolar structures manifesting pronounced solvatochromism. Herein, we explore the effects of electronic coupling through biaryl linkers on the excited-state symmetry breaking of such hybrid dyes composed of an electron-rich core, i.e., 1,4-dihydropyrrolo[3,2-b]pyrrole (DHPP), and pyrene substituents that can act as electron acceptors. Experimental and theoretical studies reveal that strengthening the donor-acceptor electronic coupling decreases the CT rates and the propensity for symmetry breaking. We ascribe this unexpected result to effects of electronic coupling on the CT thermodynamics, which in its turn affects the CT kinetics. In cases of intermediate electronic coupling, the pyrene-DHPP conjugates produce fluorescence spectra, spreading over the whole visible range, that in addition to the broad CT emission, show bands from the radiative deactivation of the locally excited states of the donor and the acceptors. Because the radiative deactivation of the low-lying CT states is distinctly slow, fluorescence from upper locally excited states emerge leading to the observed anti-Kasha behaviour. As a result, these dyes exhibit white fluorescence. In addition to demonstrating the multifaceted nature of the effects of electronic coupling on CT dynamics, these chromophores can act as broad-band light sources with practical importance for imaging and photonics.

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.

Synthesis, Structural, and Optical Properties of Azuleno[1,2-c]pyran-1-ones: Bro̷nsted Acid-Mediated Cyclization of 2-Azulenylalkynes

Pyrones and their aromatic ring-fused derivatives have gained significant attention due to their diverse biological activities and potential as foundational frameworks for advanced materials. In this paper, we describe a proficient approach for the preparation of azuleno[1,2-c]pyran-1-ones, which are difficult to produce by using conventional methods. The synthesis was achieved through Bro̷nsted acid-mediated cyclization of 2-azulenylalkynes. The structural and optical properties of azuleno[1,2-c]pyran-1-ones were characterized by single-crystal X-ray analysis, NMR, UV/vis, and fluorescence spectroscopies. Under acidic conditions, these compounds displayed notable spectral alterations and emission, distinct from their spectra in neutral medium. These results suggest that azuleno[1,2-c]pyran-1-ones hold great potential for applications in organic electronic materials and fluorescent pH sensors.

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.

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.

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.

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.

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.

Going beyond the borders: pyrrolo[3,2-b]pyrroles with deep red emission

A two-step route to strongly absorbing and efficiently orange to deep red fluorescent, doubly B/N-doped, ladder-type pyrrolo[3,2-b]pyrroles has been developed. We synthesize and study a series of derivatives of these four-coordinate boron-containing, nominally quadrupolar materials, which mostly exhibit one-photon absorption in the 500-600 nm range with the peak molar extinction coefficients reaching 150 000, and emission in the 520-670 nm range with the fluorescence quantum yields reaching 0.90. Within the family of these ultrastable dyes even small structural changes lead to significant variations of the photophysical properties, in some cases attributed to reversal of energy ordering of alternate-parity excited electronic states. Effective preservation of ground-state inversion symmetry was evidenced by very weak two-photon absorption (2PA) at excitation wavelengths corresponding to the lowest-energy, strongly one-photon allowed purely electronic transition. π-Expanded derivatives and those possessing electron-donating groups showed the most red-shifted absorption- and emission spectra, while displaying remarkably high peak 2PA cross-section (σ _2PA) values reaching ∼2400 GM at around 760 nm, corresponding to a two-photon allowed higher-energy excited state. At the same time, derivatives lacking π-expansion were found to have a relatively weak 2PA peak centered at ca. 800-900 nm with the maximum σ _2PA ∼50-250 GM. Our findings are augmented by theoretical calculations performed using TD-DFT method, which reproduce the main experimental trends, including the 2PA, in a nearly quantitative manner. Electrochemical studies revealed that the HOMO of the new dyes is located at ca. -5.35 eV making them relatively electron rich in spite of the presence of two B^--N⁺ dative bonds. These dyes undergo a fully reversible first oxidation, located on the diphenylpyrrolo[3,2-b]pyrrole core, directly to the di(radical cation) stage.

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.

Azulene-based fluorescent chemosensor for adenosine diphosphate

AzuFluor® 435-DPA-Zn, an azulene fluorophore bearing two zinc(II)-dipicolylamine receptor motifs, exhibits fluorescence enhancement in the presence of adenosine diphosphate. Selectivity for ADP over ATP, AMP and PPi results from appropriate positioning of the receptor motifs, since an isomeric sensor cannot discriminate between ADP and ATP.

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.

Synthesis and Halochromic Properties of 1,2,6-Tri- and 1,2,3,6-Tetra-aryl Azulenes

A series of novel 2,6-functionalized azulene molecules Azu1-3 with varied fluorene substituents at the 1- and 3-positions of azulene as well as at the 5'-position of 2-thiophene group were synthesized. Their electronic absorption and emission spectra at neutral and protonated states were examined. It was found that after functionalization with fluorenyl groups, Azu1-3 exhibited absorption maxima at 445, 451 to 468 nm, respectively. In contrast, their corresponding protonated species showed much redshifted absorption maxima at 560, 582 to 643 nm, respectively, mainly due to the extension of conjugation length and the large dipole moment along the C_2v axis of 2,6-substituted azulene molecules. Azu1-3 are non-fluorescent in their neutral forms, but became emissive in their protonated states. Analysis of absorption and emission spectra shows that substitution of the 1- or 3-position of azulene led to decrease in response to trifluoroacetic acid.

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.

Azulene-A Bright Core for Sensing and Imaging

Azulene is a hydrocarbon isomer of naphthalene known for its unusual colour and fluorescence properties. Through the harnessing of these properties, the literature has been enriched with a series of chemical sensors and dosimeters with distinct colorimetric and fluorescence responses. This review focuses specifically on the latter of these phenomena. The review is subdivided into two sections. Section one discusses turn-on fluorescent sensors employing azulene, for which the literature is dominated by examples of the unusual phenomenon of azulene protonation-dependent fluorescence. Section two focuses on fluorescent azulenes that have been used in the context of biological sensing and imaging. To aid the reader, the azulene skeleton is highlighted in blue in each compound.

Azulene Functionalization by Iron-Mediated Addition to a Cyclohexadiene Scaffold

The functionalization of azulenes via reaction with cationic η5-iron carbonyl diene complexes under mild reaction conditions is demonstrated. A range of azulenes, including derivatives of naturally occurring guaiazulene, were investigated in reactions with three electrophilic iron complexes of varying electronic properties, affording the desired coupling products in 43-98% yield. The products were examined with UV-vis/fluorescence spectroscopy and showed interesting halochromic properties. Decomplexation and further derivatization of the products provide access to several different classes of 1-substituted azulenes, including a conjugated ketone and a fused tetracycle.

Colorimetric detection of Hg2+ with an azulene-containing chemodosimeter via dithioacetal hydrolysis

Azulene is a bicyclic aromatic chromophore that absorbs in the visible region. Its absorption maximum undergoes a hypsochromic shift if a conjugated electron-withdrawing group is introduced at the C1 position. This fact can be exploited in the design of a colorimetric chemodosimeter that functions by the transformation of a dithioacetal to the corresponding aldehyde upon exposure to Hg2+ ions. This chemodosimeter exhibits good chemoselectivity over other metal cations, and responds with an unambiguous colour change clearly visible to the naked eye. Its synthesis is concise and its ease of use makes it appropriate in resource-constrained environments, for example in determing mercury content of drinking water sources in the developing world.

Development of Heterocycle-Substituted and Fused Azulenes in the Last Decade (2010-2020)

Azulene derivatives with heterocyclic moieties in the molecule have been synthesized for applications in materials science by taking advantage of their unique properties. These derivatives have been prepared by various methods, involving electrophilic substitution, condensation, cyclization, and transition metal-catalyzed cross-coupling reactions. Herein, we present the development of the synthetic methods, reactivities, and physical properties for the heterocycle-substituted and heterocycle-fused azulenes reported in the last decade.

Sky-Blue Thermally Activated Delayed Fluorescence with Intramolecular Spatial Charge Transfer Based on a Dibenzothiophene Sulfone Emitter

Intramolecular spatial charge transfer (ISCT) plays a critical role in determining the optical and charge transport properties of thermally activated delayed fluorescence (TADF) materials. Herein, a new donor/acceptor-type TADF compound based on rigid dibenzothiophene sulfone (DBTS) moiety, STF-DBTS, was designed and synthesized. Fluorene unit was used as a rigid linker to position the rigid acceptor and donor subunit in close vicinity with control over their spacing and molecular structure and to achieve high photoluminescence quantum yield (∼53%) and TADF property. For comparison purposes, we constructed the more flexible STF-DPS with a less rotationally constrained diphenylsulphone (DPS) acceptor instead of the rigid DBTS units, and STF-DPS showed no TADF properties and lower PLQY (16.0%). Organic light-emitting diodes (OLEDs) based on STF-DBTS achieve an external quantum efficiency (EQE) of 10.3% at 488 nm, which is a fivefold improvement in EQE with respect to STF-DPS.

A Colorimetric Chemosensor Based on a Nozoe Azulene That Detects Fluoride in Aqueous/Alcoholic Media

Colorimetry is an advantageous method for detecting fluoride in drinking water in a resource-limited context, e. g., in parts of the developing world where excess fluoride intake leads to harmful health effects. Here we report a selective colorimetric chemosensor for fluoride that employs an azulene as the reporter motif and a pinacolborane as the receptor motif. The chemosensor, NAz-6-Bpin, is prepared using the Nozoe azulene synthesis, which allows for its rapid and low-cost synthesis. The chemosensor gives a visually observable response to fluoride both in pure organic solvent and also in water/alcohol binary solvent mixtures.

Three-component reaction of azulene, aryl glyoxal and 1,3-dicarbonyl compound for the synthesis of various azulene derivatives

A three-component reaction of an azulene, an aryl glyoxal and a 1,3-dicarbonyl compound has been elaborated to access a series of azulene derivatives. Some of these azulene-containing adducts were further subjected to post-MCR transformations to assemble azulene–heterocycle conjugates.

Three-component reaction of azulene, aryl glyoxal and 1,3-dicarbonyl compound and subsequent post-transformations provide access to three distinct types of azulene derivatives.

Azulene-Derived Fluorescent Probe for Bioimaging: Detection of Reactive Oxygen and Nitrogen Species by Two-Photon Microscopy

Two-photon fluorescence microscopy has become an indispensable technique for cellular imaging. Whereas most two-photon fluorescent probes rely on well-known fluorophores, here we report a new fluorophore for bioimaging, namely azulene. A chemodosimeter, comprising a boronate ester receptor motif conjugated to an appropriately substituted azulene, is shown to be an effective two-photon fluorescent probe for reactive oxygen species, showing good cell penetration, high selectivity for peroxynitrite, no cytotoxicity, and excellent photostability.