Azulene-boronate esters: colorimetric indicators for fluoride in drinking water

Highly Selective and Scalable Molecular Fluoride Sensor for Naked-Eye Detection

Fluoride is widely present in nature, and human exposure to it is generally regarded as inevitable. High levels of fluoride intake induce acute and chronic illnesses. To reduce potential harm to the general public, it is essential to create selective fluoride detectors capable of providing a colorimetric response for naked-eye detection without the need for sophisticated equipment. Here, we report a one-pot synthesis of four different diaminomaleonitrile-derived Schiff base sensors. The terephthalaldehyde adduct provided a strong color change visible to the naked eye at a F- concentration level as low as 2 ppm. From the evaluation against other anions, such as CN-, I-, Br-, Cl-, NO3-, PO43-, OAc-, and HSO4-, the molecular sensor displayed a visible color change exclusively upon exposure to fluoride, underscoring exceptional selectivity. As a key intermediate for understanding the mechanism, HF2- was confirmed by 19F nuclear magnetic resonance. Theoretical calculations suggested a deprotonation-triggered bathochromic shift brought about by the unique electronic structure of the sensor. Furthermore, the simple synthetic protocol from economically accessible materials allowed for the preparation of the compound on a large scale, rendering it a highly practical visual fluoride sensor.

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.

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.

Higher-Order Cycloaddition Reactions for the Construction of Polycyclic Aromatic and Polycyclic Heteroaromatic Compounds

Cycloadditions are an important class of reactions in materials science for the construction of polycyclic aromatic and polycyclic heteroaromatic compounds. Recently, cycloadditions have been expanded beyond the "classical" group of cycloadditions involving six π-electrons, and it is now possible to control cycloadditions for an extended number of π-electrons by applying organocatalysis. This novel field of cycloadditions-termed higher-order cycloadditions-allows new synthetic methodologies to construct polycyclic carbo- and heteroaromatic compounds in two or three dimensions. This concept presents higher-order cycloadditions as a method for accessing two- and three-dimensional azulenes and cyclazines, as well as three dimensional indenes, as polycyclic aromatic and polycyclic heteroaromatic compounds.

Near-Infrared Turn-On Fluorescent Probe for Aqueous Fluoride Ion Detection and Cell Imaging

Fluoride ions are one of the most essential anions in the human body and have been implicated in various pathological and physiological processes. The detection of fluoride ions in aqueous solution, as well as the imaging of fluoride ions in living cells, remains a challenge. We herein report a BODIPY-based fluorescent probe employing a pinacol borate group as the recognition moiety for the detection of fluoride ions in aqueous solutions. This probe shows high selectivity and sensitivity to fluoride ions with a significant near-infrared fluorescence turn-on response. In addition, this probe was successfully employed in fluorescence bioimaging of fluoride ions in the human cervical cancer cell and mouse mammary cancer cell, demonstrating its good cell permeability and stability under physiological conditions.

Highly sensitive sensing and quantitative detection of sulfate ion with a SERS chip-based on boric acid's Lewis effect

Based on the Lewis acid's coordination principle, a surface-enhanced Raman scattering (SERS) chip strategy had been developed for the ultrasensitive quantitation of SO₄^2-. Through the immobilization of silver nanoparticles (Ag NPs) and the construction of the boric acid-based sensing unit, the chip system displayed outstanding merits on the direct sensing of SO₄^2-, e.g., simple operation, ultra-high sensitivity, reproducibility, excellent selectivity and specificity. Moreover, an accurate evaluation was obtained by ratiometric calculations on characteristic peaks (1382 and 1070 cm^-1) for quantitative detection of SO₄^2-. The detection limit was down to 10 nM. Tap water, beer, and mineral water samples were tested, and high recoveries were achieved (97.12-110.12%). Besides, such SERS chip also displayed strong applicability for the evaluation of SO₃^2-. Therefore, this SERS chip provided a promising idea for the quantification of trace amounts of SO₄^2- and SO₃^2- in the fields of food safety and environmental monitoring.

Fluorescent probes and functional materials for biomedical applications

Due to their simplicity in preparation, sensitivity and selectivity, fluorescent probes have become the analytical tool of choice in a wide range of research and industrial fields, facilitating the rapid detection of chemical substances of interest as well as the study of important physiological and pathological processes at the cellular level. In addition, many long-wavelength fluorescent probes developed have also proven applicable for in vivo biomedical applications including fluorescence-guided disease diagnosis and theranostics (e.g., fluorogenic prodrugs). Impressive progresses have been made in the development of sensing agents and materials for the detection of ions, organic small molecules, and biomacromolecules including enzymes, DNAs/RNAs, lipids, and carbohydrates that play crucial roles in biological and disease-relevant events. Here, we highlight examples of fluorescent probes and functional materials for biological applications selected from the special issues “Fluorescent Probes” and “Molecular Sensors and Logic Gates” recently published in this journal, offering insights into the future development of powerful fluorescence-based chemical tools for basic biological studies and clinical translation.

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

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

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.

A spore-based portable kit for on-site detection of fluoride ions

The excess residues of fluoride ions cause serious human health problems, making their detection highly valuable. In this work, a whole-cell-based biosensor was presented for the detection of fluoride ions, which can inhibit the color reaction of 3,3',5,5',-tetramethylbenzidine (TMB) catalyzed by the CotA-laccase of spore surface. This reaction for the detection of fluoride ions could be read out through UV-vis spectrophotometer, smartphone, or standard colorimetric card within 10 min. Under optimum conditions, a linear range of 1-600 μmol L^-1 with a detection limit of 0.12 μmol L^-1 (3σ/k) was achieved for fluoride ions detection by using UV-vis spectrophotometer. The biosensor coupling with smartphone had a good linear response to fluoride ions concentration in the range of 5-600 μmol L^-1 with LOD of 0.90 μmol L^-1 (3σ/k). The standard colorimetric card can be directly used for recognizing the fluoride ions level via naked-eyes. A portable kit based on a colorimetric card and smartphone was developed and has been successfully applied for fluoride ions monitoring in surface waters and groundwater. This developed method has several advantages such as rapid, outstanding selectivity and anti-interference, low-cost, ease of operation and storage, and eco-friendliness, meeting the demands of point-of-care testing of fluoride ions and disease prevention.

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.

All Silica Micro-Fluidic Flow Injection Sensor System for Colorimetric Chemical Sensing

This paper presents a miniature, all-silica, flow-injection sensor. The sensor consists of an optical fiber-coupled microcell for spectral absorption measurements and a microfluidic reagent injection system. The proposed sensor operates in back reflection mode and, with its compact dimensions, (no more than 200 µm in diameter) enables operation in small spaces and at very low flow rates of analyte and reagent, thus allowing for on-line or in-line colorimetric chemical sensing.

Smartphone as a simple device for visual and on-site detection of fluoride in groundwater

Developing a portable device for visual and on-site detection of fluoride in groundwater is highly anticipated. In this paper, 2-(tert-butyl-diphenylsilanyloxy)-5-nitro-1H-benzoimidazole (1) has been rationally designed via a silanization reaction for self-calibration detection of fluoride, and the detection limit was calculated as 0.11 μM. The contact of 1 with fluoride would induce the cleavage of Si-O bond and trigger the emergence of excited state intramolecular proton transfer (ESIPT) process, and then the enol-like emission at 437 nm decreased accompanying with the increase of keto-like tautomerism emission at 550 nm. More importantly, considering the demand of field detection for fluoride in groundwater and combining the function of smartphone to obtain the chroma of photos. The chroma value of the fluorescence color changes from blue to yellow could be conveniently determined through a color recognizer application installed in smartphone. The device can accurately reflect the concentration of fluoride by analyzing the chroma value. The test in actual water samples confirmed that the simple device based on smartphone could be used efficiently for visual, on-site and accurate detection of fluoride in groundwater.

High-contrast colourimetric probes for fluoride and trace water based on tautomerization of naphthalimide and application in fingerprint imaging

Three probes for fluoride ion and trace water based on naphthalimide were designed and synthesized. A new sensing mechanism based on naphthalimide tautomerization induced by fluoride ion and water was explored in the aprotic organic solvent. In the fluoride ion sensing process, the probes exhibited a remarkable absorption peak centred at 560 nm in the visible range of 400-700 nm. When trace water presented, the newly formed absorption peak centred at 560 nm gradually disappeared. The sensitive colour variation of the probe also was used in fingerprint imaging. Accordingly, the significant changes in chemical shift of dept135 and ¹HNMR spectrum confirmed the structural transformation of the probes with high contrast. Furthermore, this work also presented an optimization strategy for the sensitivity of the probe based on regulatory tautomerization.

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.

Ferrocene-appended boronated ester: effect of cyanovinylene group on the nonlinear optical properties and colorimetric detection of fluoride ion

A centrosymmetric ferrocenyl boronated ester shows second-order NLO effect, selective and sensitive detection of fluoride ion.

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.

Application of Octacalcium Phosphate with an Innovative Household-scale Defluoridator Prototype and Behavioral Determinants of its Adoption in Rural Communities of the East African Rift Valley

Natural fluoride contamination of drinking water is a serious issue that affects several countries of the world. Its negative health impact is well documented in the East African Rift Valley, where water consumption with fluoride ( F - ) concentration greater than 1.5 mg/L can cause fluorosis to people. Within the framework of the European Union (EU) Horizon 2020 FLOWERED project, we first designed an effective defluoridation device based on innovative application of octacalcium phosphate (OCP) and then explored its acceptance within rural communities. The prototype (FLOWERED Defluoridator Device [FDD]) essentially is composed of a 20-L tank and a recirculating pump that guarantees the interaction between water and OCP. The device is powered by a car battery for a fixed pumping working time using a fixed amount of OCP for every defluoridation cycle. The results of tests performed in the rural areas of Tanzania show that a standardized use of the prototype can lower the dissolved F - from an initial concentration of 21 mg/L to below the World Health Organization (WHO) drinkable limit of 1.5 mg/L in 2 h without secondary negative effects on water quality. The approximate cost of this device is around US$220, whereas that of OCP is about $0.03/L of treated water. As with any device, acceptance requires a behavioral change on behalf of rural communities that needed to be investigated. To this end, we piloted a survey to explore how psychological and socioeconomic factors influence the consumption of fluoride-free water. Results show that the adoption of FDD and OCP is more appealing to members of the rural communities who are willing to pay more and have a high consumption of water. Moreover, we suggest that given the low level of knowledge about fluorosis diseases, the government should introduce educational programs to make rural communities aware of the negative health consequences. Integr Environ Assess Manag 2020;16:856-870. © 2020 The Authors. Integrated Environmental Assessment and Management published by Wiley Periodicals LLC on behalf of Society of Environmental Toxicology & Chemistry (SETAC).

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.

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.

1-m-Nitrobenzoyl semicarbazide: reversible colorimetric cascade indicators for fluoride and moisture

A very simple and effective strategy for an instant, in-field reversible colorimetric cascade indicator of fluoride and moisture at very low concentrations has been developed.

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.

Reusable Silicon-Based SERS Chip for Ratiometric Analysis of Fluoride Ion in Aqueous Solutions

An innovative ratiometric surface-enhanced Raman scattering (SERS) sensor using a 4-mercaptoboric acid (4-MPBA)-modified silver nanoparticle-decorated silicon wafer (Si@Ag NPs chip) was proposed for the ultrasensitive determination of F^- ions in aqueous solutions. The principle of sensing strategy is based on fluoride-induced structural symmetry breaking and charge redistribution of phenylboronic acid, leading to a band shift of the C-C stretching mode of 4-MPBA from 1589 to 1576 cm^-1. Accordingly, a ratiometric signal of the area ratio (A₁₅₇₆/A₁₅₈₉) between the fluoride-bond MPBA molecules and unoccupied MPBA molecules can be used for the quantitative response of F^- ions. In comparison with other SERS-based sensing methods, this ratiometric method can avoid a large error resulting from the inhomogeneity of substrates. Under the optimized analytical conditions, the proposed SERS sensor possesses a quick response to F^- ions within 2 min and exhibits high selectivity for F^- ions with the determination limit of 10^-8 M, which is over 3 orders of magnitude lower than the World Health Organization (WHO) guideline value for F^- ions in drinking water. Of particular significance, the present sensor features favorable recyclability, which preserves suitable reproducibility during 6-time cyclic determination of F^- ions. The practical utility of this sensing system for the determination of F^- ions was tested with real water and toothpaste samples, and the results demonstrate that this sensor shows high recoveries (90-110%). Given its simple principle and easy operation, the present silicon-based SERS sensor could serve as a promising sensor for various practical applications.

1,6-Elimination reaction induced detection of fluoride ions in vitro and in vivo based on a NIR fluorescent probe

Near-Infrared "turn on" type fluorescent probes are attractive and promising in the fields of chemical sensing and bioimaging. Here, a new dicyanomethylene-4H-pyran derivative (DCM-Si) NIR fluorescent probe was designed and synthesized for specific lighting up F^- in living cells and bodies. SiO bond was used as F^- trigger, and the release of fluorophore (DCM-NH₂) occurred after substituent reaction and subsequent 1,6-elimination. This NIR probe displayed high sensitivity and selectivity for the sensing of F^-, and the detection limit was calculated to be as low as 157 nM. Moreover, the "off-on" fluorescent signal changes can be realized by adding F^- in living cells and zebrafish embryos.

Colorimetric sensor based on 4-mercaptophenylboronic modified gold nanoparticles for rapid and selective detection of fluoride anion

A highly selective and sensitive colorimetric sensor based on aggregation-induced color change of 4-mercaptophenylboronic modified gold nanoparticles was designed for the determination of fluoride anion. The 4-mercaptophenylboronic modified gold nanoparticles were synthesized by a simple one-pot reaction. The aggregation process occurred when interaction between fluoride anion and 4-mercaptophenylboronic on the surface of gold nanoparticles took place; as a result, fluoroborate anions were formed coupled with changes in the electronic properties of the AuNPs. The change can be measured by UV-Vis absorption spectra. The sensor shows good selectivity and sensitivity for fluoride anion. The linear range is 10.0-30.0 μM for fluoride and the detection limit of fluoride is 3.45 × 10^-7 M according to IUPAC criteria (3σ rule). Furthermore, the sensor has been used for the detection of fluoride anion in tap water, ground water and human serum samples, the recovery can achieve 94.0%-103.3%, 94.7%-101.0% and 89.8-100.9%, respectively. The excellent performance of colorimetric sensor in the detection of the fluoride anion demonstrated the potential application in the detecting fluoride anion present in the complex environmental and biological samples.

Azulenes with aryl substituents bearing pentafluorosulfanyl groups: synthesis, spectroscopic and halochromic properties

Azulenes with SF₅-containing substituents gave significantly different spectroscopic responses to protonation depending on the regioisomer in question.

Organocatalytic [10+4] cycloadditions for the synthesis of functionalised benzo[a]azulenes

A direct and mild strategy for the synthesis of benzo[a]azulenes based on an organocatalytic [10+4] cycloaddition reaction is described.

Synthesis of azulenophthalimides by phosphine-mediated annulation of 1,2-diformylazulenes with maleimides

The reaction of 1,2-diformylazulene with maleimides in the presence of PPh₃ gave the azulenophthalimides. The optical and electrochemical properties of the azulenophthalimides were investigated.

Synthesis of azuleno[2,1-b]thiophenes by cycloaddition of azulenylalkynes with elemental sulfur and their structural, optical and electrochemical properties

The reaction of several azulenylalkynes having an aryl substituent with elemental sulfur afforded the corresponding azuleno[2,1-b]thiophenes in good yields.

Förster Resonance Energy-Transfer-Based Ratiometric Fluorescent Indicator for Quantifying Fluoride Ion in Water and Toothpaste

A Förster resonance energy-transfer (FRET)-based ratiometric fluorescent indicator Cou-FITC-Si toward fluoride ion has been designed and synthesized by combining coumarin unit and fluorescein derivative as energy donor and acceptor, respectively. The fluorescein unit is capped with tert-butyldiphenylchlorosilane. The indicator gives out emission responses based on switch-on of the FRET process that triggered by the desilylation mediated by the fluoride ion. The fluorescence emission spectrum of Cou-FITC-Si presents a significant bathochromic shift of 59 nm after the addition of fluoride ion with up to 180-fold increase of the fluorescence intensity ratio. The limit of detection of the Cou-FITC-Si indicator system toward fluoride ion was estimated to be 3.3 ppb. Furthermore, this indicator has been successfully applied for quantifying the fluoride ion of different concentrations from commercially available toothpaste.

Functionalized Quinoxaline for Chromogenic and Fluorogenic Anion Sensing

This Review article provides a comprehensive analysis of recent examples reported in the field of quinoxaline-based chromogenic and fluorogenic chemosensors for inorganic anions such as fluoride, cyanide, acetate, and phosphate, as well as their utility in biomolecular science. It commences with a discussion of the various structural motifs such as quinoxaline-based oligopyrroles, polymers, sulfonamides, cationic receptors, and miscellaneous receptors bearing mixed recognition sites in the same receptor. Advances are discussed in depth, where the focus of this review is to tackle mainly solution state anion sensing utilizing quinoxaline-based receptors using different spectroscopic techniques with reference to anion selectivity by colorimetric and fluorescence response. The various examples discussed in this Review illustrate how the integration of anion binding elements with the quinoxaline chromophore could result in anion responsive chemosensors. Over the years, it has been observed that structural modification of the quinoxaline moiety with different sets of signaling unit and recognition sites has resulted in a few anion specific chemosensors.

Catalytic Functionalization of C-H Bonds of Azulene by Carbene/Nitrene Incorporation

The selective catalytic functionalization of the C-H bond of azulene upon incorporation of carbene or nitrene units with metal-based catalysts is described. Ethyl diazoacetate or ArI═NTs are employed as carbene or nitrene precursors, respectively. The azulene derivatives are subsequently employed as building blocks toward more complex structures with potential use as biodegradable materials.

Calix[4]arene containing thiourea and coumarin functionality as highly selective fluorescent and colorimetric chemosensor for fluoride ion

A novel calix[4]arene based chemosensor L which contains coumarin and thiourea group has been synthesized and characterized. Interestingly, probe L exhibits both fluorescent and colorimetric response to fluoride anion with high selectivity and sensitivity. The addition of F^- to a solution of probe L resulted in obvious naked-eye color change from colorless to orange under daylight and prominent fluorescence quenching. Further studies showed that the recognition process was less affected by other anions. The binding property of L with F^- was studied by a combination of combination of various spectroscopic techniques, such as absorption spectra, fluorescence titration, Job's plot and ¹H NMR titration. We are anticipating that this architecture with functional group attached to upper rim of calix[4]arene platform may provide a new approach for the development of F^- chemosensor.