Chemosensors based on N-heterocyclic dyes: advances in sensing highly toxic ions such as CN- and Hg2

Microwave-assisted synthesis and functionalization of 2-arylimidazo[1,2-a]pyrimidin-5(8H)-ones

Despite the limited applications and scarcity of commercial examples of imidazo[1,2-a]pyrimidines, their exceptional properties hold great potential, representing a significant challenge in discovering more critical applications. Herein, we present a microwave-assisted approach for preparing 2-arylimidazo[1,2-a]pyrimidin-5(8H)-ones and their alkylation and bromination products using easily accessible and inexpensive reagents, thus offering a promising avenue for further search. Notably, the photophysical properties of an N-alkyl derivative were investigated, and the results highlight the high potential of these compounds as modular fluorophores. All the products were obtained with high yields using highly efficient protocols, and the regioselectivity of the reactions was determined on the basis of NMR measurements and X-ray diffraction analysis.

The complexometric behavior of selected aroyl-S,N-ketene acetals shows that they are more than AIEgens

Using the established synthetic methods, aroyl-S,N-ketene acetals and subsequent bi- and multichromophores can be readily synthesized. Aside from pronounced AIE (aggregation induced emission) properties, these selected examples possess distinct complexometric behavior for various metals purely based on the underlying structural motifs. This affects the fluorescence properties of the materials which can be readily exploited for metal ion detection and for the formation of different metal-aroyl-S,N-ketene acetal complexes that were confirmed by Job plot analysis. In particular, gold(I), iron(III), and ruthenium (III) ions reveal complexation enhanced or quenched emission. For most dyes, weakly coodinating complexes were observed, only in case of a phenanthroline aroyl-S,N-ketene acetal multichromophore, measurements indicate the formation of a strongly coordinating complex. For this multichromophore, the complexation results in a loss of fluorescence intensity whereas for dimethylamino-aroyl-S,N-ketene acetals and bipyridine bichromophores, the observed quantum yield is nearly tripled upon complexation. Even if no stable complexes are formed, changes in absorption and emission properties allow for a simple ion detection.

Small-molecule fluorogenic probes based on indole scaffold

Indoles are the most versatile organic N-heterocyclic compounds widely present in bioactive natural products and used in different fields such as coordination chemistry, pharmacy, dyes, and medicine, as well as in the biology and polymer industries. More recently, the indole scaffold has been widely used in analytical chemistry for the design and development of small-molecule fluorescent chemosensors in the fields of molecular recognition and molecular imaging. The indole-based chemosensor derivatives contain heteroatoms like N-, O-, and S-, through which they interact with analytes (cations, anions, and neutral species), producing measurable analytical signals that can be used for the fluorimetric and colorimetric detection of different analytes in biological, agricultural and environmental samples. This review focuses on indole-based small-molecule fluorimetric and colorimetric chemosensors for detecting cations, anions, and neutral species in a comprehensive manner. Furthermore, the recognition mechanisms are discussed in detail, which could help researchers design and develop more powerful and efficient fluorescent chemosensors in the near future.

Tuning sensing efficacy of anthraimidazoledione-based charge transfer dyes: nitro group positioning impact

Anthraimidazoledione-based optical sensors have been designed by varying the position of the nitro functional group. All three positional isomers showed highly colored, photostable optical signals owing to intramolecular charge transfer interactions. Despite having the same anion-binding site (imidazole unit), the selectivity and sensitivity of the compounds depend on the positioning of the nitro group. The selectivity was fairly good for the meta isomer, followed by the ortho and para isomers, respectively. In contrast, the sensitivity towards anions followed a completely opposite trend, with the para isomer being the most sensitive one towards anions. Interestingly, the color changing response along the turn-on fluorescence signal was observed only with CN- ions in a semi-aqueous environment. Though the introduction of water as a co-solvent could improve the selectivity, the sensitivity was found to be slightly less than that observed in pure organic medium. Mechanistic studies indicated hydrogen bonding interactions between the imidazole -NH proton and cyanide, which further facilitated the extent of intramolecular charge transfer.

Unprecedented iron-assisted room temperature synthesis of AgCN using acetonitrile

A straightforward and convenient approach for producing AgCN at room temperature using acetonitrile as a source has been developed, employing various iron salts. To date, there have been no prior studies documenting the synthesis of AgCN by cleaving the C-CN bond in acetonitrile with the use of iron salts. The resulting highly crystalline material was subjected to characterization through XRD and FT-IR analysis. Additionally, the same process was used for C-CN bond breaking using Ag2S or via the formation of an AgSxOy composite. Consequently, this report is primarily dedicated to exploring the efficacy of different iron salts in breaking the C-CN bond in CH3CN. A theoretical investigation of the proposed experimental scheme has also been performed to confer the feasibility of the reaction.

Update on novel synthetic approaches towards the construction of carbazole nuclei: a review

The carbazole scaffold is a significant entity in organic compounds due to its variety of biological and synthetic applications. Traditionally, carbazole skeletons have been synthesized either via the Grabe-Ullman method, Clemo-Perkin method or Tauber method. With the passage of time, these methods have been modified and explored to accomplish the synthesis of target compounds. These methods include hydroarylations, C-H activations, annulations and cyclization reactions mediated by a variety of catalysts to construct carbazole-based compounds. This brief review article intends to provide recent updates on important methodological developments reported for the synthesis of carbazole nuclei covering 2019-2023.

Aroyl-S,N-Ketene Acetals: Luminous Renaissance of a Class of Heterocyclic Compounds

Aroyl-S,N-ketene acetals represent a peculiar class of heterocyclic merocyanines, compounds bearing pronounced and rather short dipoles with great push-pull characteristics that define their rich properties. They are accessible via a wide array of synthetic concepts and procedures, ranging from addition-elimination and condensation procedures up to rearrangement and metal-mediated reactions. With our work from 2020, aroyl-S,N-ketene acetals have been identified as powerful and promising dyes with pronounced and vastly tunable solid-state emission and aggregation-induced emission properties. One characteristic trademark of this class of dye molecules is the level of control that could be exerted, and which was thoroughly explored. Based on these results, the field was opened to extend the system to bi- and multichromophoric systems by the full toolkit of synthetic organic chemistry thus giving access to even more exciting properties and manifolded substance libraries capitalizing on the AIE properties. This review aims at outlining the reaction-based principles that allow for a swift and facile access to aroyl-S,N-ketene acetals, their methodical and structural evolution and the plethora of fluorescence and aggregation properties.

Efficient fluorescence-enhanced probe for cyanide ions based on a tetraphenylethene pyridine coordinated copper-iodide complex

An efficient fluorescence-enhanced probe was developed for detecting cyanide ions (CN^-) based on a tetraphenylethene coordinated copper-iodide complex (named CIT-Z). The coordination polymers (CPs) prepared were (Z)-1,2-diphenyl-1,2-bis[4-(pyridin-3-ylmethoxy)phenyl]ethene (1Z) and a CuI cluster, where the tetraphenylethylene (TPE) pyridine derivatives acted as organic ligands and the CuI cluster acted as a metal center. The higher-dimensional CIT-Z exhibited a 3-fold-interpenetrating network structure with excellent optical properties and chemical stability. This study also provides insights into the mechanism behind the fluorescence enhancement, which is attributed to the competitive coordination between CN^- and the ligands. The probe showed high selectivity and sensitivity towards CN^-, with a detection limit of 0.1 μM and good recovery in the real water samples.

AIE active luminous dye with a triphenylamine attached benzothiazole core as a portable polymer film for sensitively detecting CN- ions in food samples

Aggregation-induced emission (AIE) active 3-(3-(benzo[d]thiazol-2-yl)-2-hydroxyphenyl)-2-(4'-(diphenylamino)-[1,1'-biphenyl]-4-yl)acrylonitrile (BTPA) has been designed and synthesized herein, with the goal of detecting CN^- ions at a low-level in semi-aqueous medium. The deliberate addition of the electron-deficient alkene BTPA increased its sensitivity and selectivity to CN^- ions, with a better detection limit of 6.4 nM, unveiling the next-generation approach to creating sophisticated CN^- ions selective chemosensors. The ESI-MS and NMR spectra analyses provided strong support for the structures of the chemosensors, while the UV-Vis, photoluminescence, and ¹H-NMR titration experiments provided support for the sensing efficiencies. Subsequently, PVDF/BTPA electrospun nanofibers have been effectively produced as functional films. These nanofiber films exhibit outstanding mechanical strength, photo/thermal stability, and optical responsiveness to CN^- ions, making them a potential choice for on-field emerging contaminant detection.

Novel Biomass-Based Polymeric Dyes: Preparation and Performance Assessment in the Dyeing of Biomass-Derived Aldehyde-Tanned Leather

High-performance chrome-free leather production is currently one of the most concerning needs to warrant the sustainable development of the leather industry due to the serious chrome pollution. Driven by these research challenges, this work explores using biobased polymeric dyes (BPDs) based on dialdehyde starch and reactive small-molecule dye (reactive red 180, RD-180) as novel dyeing agents for leather tanned using a chrome-free, biomass-derived aldehyde tanning agent (BAT). FTIR, ¹H NMR, XPS, and UV-visible spectrometry analyses indicated that a Schiff base structure was generated between the aldehyde group of dialdehyde starch (DST) and the amino group of RD-180, resulting in the successful load of RD-180 on DST to produce BPD. The BPD could first penetrate the BAT-tanned leather efficiently and then be deposited on the leather matrix, thus exhibiting a high uptake ratio. Compared with the crust leathers prepared using a conventional anionic dye (CAD), dyeing, and RD-180 dyeing, the BPD-dyed crust leather not only had better coloring uniformity and fastness but it also showed a higher tensile strength, elongation at break, and fullness. These data suggest that BPD has the potential to be used as a novel sustainable polymeric dye for the high-performance dyeing of organically tanned chrome-free leather, which is paramount to ensuring and promoting the sustainable development of the leather industry.

Straightforward and Efficient Protocol for the Synthesis of Pyrazolo [4,3-b]pyridines and Indazoles

An efficient method for the synthesis of pyrazolo [4,3-b]pyridines has been developed on the basis of readily available 2-chloro-3-nitropyridines via a sequence of SNAr and modified Japp-Klingemann reactions. The method offers a number of advantages including utilization of stable arenediazonium tosylates, operational simplicity as well as combining the azo-coupling, deacylation and pyrazole ring annulation steps in a one-pot manner. An unusual rearrangement (C-N-migration of the acetyl group) was observed and a plausible mechanism was proposed based on the isolated intermediates and NMR experiments. In addition, the developed protocol was successfully applied to the synthesis of 1-arylindazoles combining the Japp-Klingemann reaction and cyclization of the resulting hydrazone as a one-pot procedure.

Photophysical and anion sensing properties of a triphenylamine-dioxaborinine trimeric compound

Herein, we report the synthesis and photophysical characterization of the novel tris(4-(2,2-difluoro-6-methyl-2H-1λ³,3,2λ⁴-dioxaborinin-4-yl)phenyl)amine trimeric probe (A2) via the reaction between triphenylamine (1), acetic anhydride, and BF₃·OEt₂ implying the twelve new bond formation in a one-pot manner. This highly fluorescent compound in solution (φ up to 0.91 at 572 nm) and solid state (φ = 0.24 at 571 nm) showed a better solvatofluorochromism than its analog monomeric A1 due to symmetry-broken charge transfer, which is consistent with high solvent dipolarity (SdP) response in Catalán's multiparametric regression. Notably, A2 had a high sensibility and selectivity for CN^- or F^- in solution (LODCN^-/F^- = 0.18/0.70 μM), and CN^- can be discriminated from F^- by the reaction of A2 with 3.0 equiv. of CN^-. In addition, A2 was impregnated on filter paper to prepare test strips that were applied to naked-eye qualitative sensing of CN^- or F^-. Finally, the octupolar system in A2 allows for better action of two-photon excitation cross-section values when compared with that of the dipolar structure in A1. These findings provide further information for the design of new efficient two-photon absorption dyes.

Current Advances in Diazoles-based Chemosensors for CN- and FDetection

Advances in molecular probes have recently intensified because they are valuable tools in studying species of interest for human health, the environment, and industry. Among these species, cyanide (CN-) and fluoride (F-) stand out as hazardous and toxic ions in trace amounts. Thus, there is a significant interest in probes design for their detection with diverse diazoles (pyrazole and imidazole) used for this purpose. These diazole derivatives are known as functional molecules because of their known synthetic versatility and applicability, as they exhibit essential photophysical properties with helpful recognition centers. This review provides an overview of the recent progress (2017-2021) in diazole-based sensors for CN- and F- detection, using the azolic ring as a signaling or recognition unit. The discussion focuses on the mechanism of the action described for recognizing the anion, the structure of the probes with the best synthetic simplicity, detection limits (LODs), application, and selectivity. In this context, the analysis involves probes for cyanide sensing first, then probes for fluoride sensing, and ultimately, dual probes that allow both species recognition.

A Reversible Optical Sensor Film for Mercury Ions Discrimination Based on Isoxazolidine Derivative and Exhibiting pH Sensing

We developed a new optical sensor for tracing Hg(II) ions. The detection affinity examines within a concentration range of 0-4.0 µM Hg(II). The sensor film is based on Methyl 2-hydroxy-3-(((2S,2'R,3a'S,5R)-2-isopropyl-5,5'-dimethyl-4'-oxotetrahydro-2'H-spiro[cy-clohexane-1,6'-im-idazo[1,5-b]isoxazol]-2'-yl)methyl)-5-methylbenzoate (IXZD). The novel synthesized compound could be utilized as an optical turn-on chemosensor for pH. The emission intensity is highly enhanced for the deprotonated form concerning the protonated form. IXZD probe has a characteristic fluorescence peak at 481 nm under excitation of 351 nm with large Stocks shift of approximately 130 nm. In addition, the binding process of IXZD:Hg(II) presents a 1:1 molar ratio which is proved by the large quench of the 481 nm emission peak of IXZD and the growth of a new emission peak at 399 nm (blue shift). The binding configurations with one Hg(II) cation and its electronic characteristics were investigated by applying the Density Functional Theory (DFT) and the time-dependent DFT (TDDFT) calculations. Density functional theory (DFT) and the time-dependent DFT (TDDFT) theoretical results were provided to examine Hg(II)-IXZD structures and their electronic properties in solution. The developed chemical sensor was offered based on the intramolecular charge transfer (ICT) mechanism. The sensor film has a significantly low limit of detection (LOD) for Hg(II) of 0.025 μM in pH 7.4, with a relative standard deviation RSD_r (1%, n = 3). Lastly, the IXZD shows effective binding affinity to mercury ions, and the binding constant K_b was estimated to be 5.80 × 10⁵ M^-1. Hence, this developed optical sensor film has a significant efficiency for tracing mercury ions based on IXZD molecule-doped sensor film.

Recent Advances in Synthesis and Properties of Pyrazoles

Pyrazole-containing compounds represent one of the most influential families of N-heterocycles due to their proven applicability and versatility as synthetic intermediates in preparing relevant chemicals in biological, physical-chemical, material science, and industrial fields. Therefore, synthesizing structurally diverse pyrazole derivatives is highly desirable, and various researchers continue to focus on preparing this functional scaffold and finding new and improved applications; this review highlights some of the most recent and strategic examples regarding the synthesis and properties of different pyrazole derivatives, mainly reported from 2017–present. The discussion involves strategically functionalized rings (i.e., amines, carbaldehydes, halides, etc.) and their use in forming various fused systems, predominantly bicyclic cores with 5:6 fusion taking advantage of our experience in this field and the more recent investigations of our research group.

Sceptrin-Au nano-aggregates (SANA) for overcoming drug-resistant Gram-negative bacteria

One of the recent advances in medical nanotechnology has been the development of nanoformulations to overcome drug-resistant bacterial infections. Herein, we disclose a new nano-antibiotic formulation based on sceptrin-Au nano-aggregates (SANA), which are drug-metal ion multiple complexes. Sceptrin is a natural compound from a marine organism (sponge) and was reported as a potential compound with drug activities. SANA consists of a sceptrin-Au ion and is a self-assembled nano-formation with electrostatic interaction. Interestingly, SANA showed superior antibiotic/antibiofilm activity toward carbapenem-resistant Gram-negative bacteria with low toxicity to red blood cells and endothelial cells. The working mechanism of SANA was identified with analysis of the extracellular reactive oxygen species level and membrane depolarization of bacteria. The feasibility of SANA as a new nano-antibiotic was demonstrated in CRPA-contaminated medical supplies where SANA inhibited the formation of biofilms as well as the growth of CRPA. This work presents a new concept for the development of next-generation nano-antibiotics and a more feasible clinical translational pathway.

BF3-Mediated Acetylation of Pyrazolo[1,5-a]pyrimidines and Other π-Excedent (N-Hetero)arenes

An operably simple microwave-assisted BF₃-mediated acetylation reaction of pyrazolo[1,5-a]pyrimidines and a plausible mechanism based on density functional theory (DFT) theoretical calculations for this transformation are reported. Remarkably, and to the best of our knowledge, this is the first example of the direct acetylation for the functional pyrazolo[1,5-a]pyrimidine (PP) core. The synthesis of this essential building block is reported in high yields using mild reaction conditions, inexpensive reagents, and even substrates with electron-deficient or highly hindered groups. In addition, one of the new methyl ketones was successfully used as a substrate for producing novel and valuable bis-electrophilic compounds with yields of up to 90%. Notably, the discovered acetylation method was successfully applied in other π-excedent (N-hetero)aromatic substrates.

A reasonably constructed fluorescent chemosensor based on the dicyanoisophorone skeleton for the discriminative sensing of Fe3+ and Hg2+ as well as imaging in HeLa cells and zebrafish

In this study, a new fluorescent sensor dicyanoisophorone Rhodanine-3-acetic acid (DCI-RDA) (DCI-RDA) has been developed by employing a DCI-based push–pull dye as the fluorophore and RDA as the recognition moiety for the simultaneous sensing of Fe³⁺ and Hg²⁺ with a large Stokes Shift (162 nm), high selectivity and sensitivity, and low LOD (1.468 μM for Fe³⁺ and 0.305 μM for Hg²⁺). In particular, DCI-RDA has a short response time (30 s). The Job's plot method in combination with ¹H NMR titration and theoretical calculations was used to determine the stoichiometry of both DCI-RDA-Fe³⁺/Hg²⁺ complexes to be 1 : 1. Moreover, DCI-RDA is applied as a fluorescent probe for imaging in HeLa cells and zebrafish, indicating that it can be potentially applied for Fe³⁺/Hg²⁺ sensing in the field of biology.

A new fluorescent sensor dicyanoisophorone rhodanine-3-acetic acid has been developed by employing a DCI-based push–pull dye as the fluorophore and RDA as the recognition moiety for the simultaneous sensing of Fe³⁺ and Hg²⁺.

Influence of Steric Effect on the Pseudo-Multicomponent Synthesis of N-Aroylmethyl-4-Arylimidazoles

A pseudo-three-component synthesis of N-aroylmethylimidazoles 3 with three new C-N bonds formed regioselectively under microwave conditions was developed. Products were obtained by reacting two equivalents of aroylmethyl bromide (ArCOCH₂Br, 1) with the appropriate amidine salt (RCN₂H₃.HX, 2) and with K₂CO₃ as a base in acetonitrile. The bicomponent reaction also occurred, giving the expected 4(5)-aryl-1H-imidazoles 4. Notably, the ratio of products 3 and 4 is governed by steric factors of the amidine 2 (i.e., R = H, CH₃, Ph). Therefore, a computational study was carried out to understand the reaction course regarding product ratio (3/4), regioselectivity, and the steric effects of the amidine substituent group.

A novel quinoline derivative as a highly selective and sensitive fluorescent sensor for Fe3+ detection

In this research, a novel selective and sensitive fluorescent sensor for detecting Fe³⁺ was designed and synthesized; it revealed an obvious fluorescence quenching effect upon addition of Fe³⁺, and possessed the quantitative analysis ability on account of the formation of a 1 : 1 metal–ligand complex. Furthermore, the density functional theory calculations were utilized to study the molecular orbitals as well as the spatial structure. Simultaneously, the cell experiments and zebra fish experiments verified the application value of the sensor in the biological field.

A novel selective and sensitive fluorescent sensor for detecting Fe³⁺ was designed and synthesized; it revealed obvious fluorescence quenching effect upon addition of Fe³⁺, and possessed the quantitative analysis ability on account of the formation of a 1 : 1 metal–ligand complex.

Synthesis, “turn-on” fluorescence signals towards Zn2+ and Hg2+ and monoamine oxidase A inhibitory activity using a molecular docking approach of morpholine analogue Schiff base linked organosilanes

A new set of morpholine analogue Schiff base linked organosilanes (5a–5c) was prepared.