Blue emissive amidinate-based tetra-coordinated boron compounds

A series of novel amidinate ligated four-coordinated boron compounds, [(Ar)-C(tBuN)2BF2] (1BF2-6BF2), were synthesised and structurally characterised (Ar = 1-phenyl, 2-naphthyl, 2-anthryl, 9-anthryl, 9-phenanthryl and 1-pyrene). The increased π-conjugation of Ar-substitution on the amidinate ligand results in dark blue-emission in compounds 3BF2-6BF2. All these compounds are emissive in the solution state. The 2-anthryl substituted compound 3BF2 was found to exhibit a maximum quantum yield of 48% in dichloromethane. Theoretical studies were carried out which validate the hypothesis about the increased π-conjugation.

Extending the Chemistry of Reaction between BODIPY and Cyanide Ions: An Application in Selective Sensing of Fluoride and Cyanide Ions

A novel colorimetric BODIPY-based probe for selective detection of fluoride and cyanide has been developed. The color of the solution significantly changes upon addition of fluoride and cyanide ions with detection limits of 2.2 × 10^-7 and 1.8 × 10^-7 M calculated by UV-vis absorption method for F^- and CN^- respectively. An unprecedented phenomenon about the interaction of cyanide ions with the probe was discovered which has not been reported yet. The green color of the paper strip in the presence of cyanide ions changes with time. This observation indicates that unlike fluoride, the cyanide ion interaction with the probe is beyond mere deprotonation of the phenolic group rather envisaged as nucleophilic addition reaction. The phenomenon was also observed in the solution phase and subsequently the reaction order and rate constant of the reaction were determined from absorption versus time graph which were found to be first order and 0.3465 s^-1 respectively. The emission spectra also showed different behavior of interaction with time for the two ions. The rate of the reaction was found to be independent of the solvent polarity. The plausible mechanism of the reaction between cyanide and fluoride ions with the probe was proposed based on ¹H NMR titration experiments and mass spectrometry.

Triarylborane-triphenylamine based luminophore for the mitochondria targeted live cell imaging and colorimetric detection of aqueous fluoride

Bioimaging of subcellular organelles such as mitochondria is crucial for detecting physiological abnormalities induced by fluctuations in the levels of various analytes. Herein, we report the design and synthesis of two novel water-soluble cationic Lewis acid triarylborane-triarylamine conjugates 1 and 2. The optical characteristics of 1 and 2 and their precursor compounds BTPA-NMe2 and BTPA-2NMe2 were evaluated, which show similar absorption and fluorescence spectra, with 1 and 2 exhibiting higher quantum yields of 0.73 and 0.64, respectively, than those of the precursors BTPA-NMe2 and BTPA-2NMe2, indicating the partial disruption of the ICT process and the activation of alternative emission bands in 1 and 2. The live cell imaging ability of compound 2 was examined in HeLa cells using a confocal microscope. Moreover, mitochondrial internalisation using compound 2 was effective and it was found to have high photostability under UV light conditions. Furthermore, compound 2 demonstrated an evident colorimetric response with a colour change to dark yellow in aqueous environments, indicating that it could be used for anion sensing. The spectral changes were observed in UV-visible and fluorescence titration experiments, which were strongly supported by DFT calculations. In short, compound 2 synthesized by us can be exclusively utilized for the selective localization of mitochondria with less cytotoxicity and shows excellent colorimetric response to aqueous inorganic fluoride at levels as low as 0.1 ppm with high selectivity.

The sensing mechanism of a flavone-based ESIPT fluorescent chemodosimeter for selective recognition towards fluoride: a theoretical

The sensing mechanism of 3-hydroxyflavone-based (3-HF) fluorescent chemodosimeter 3-triisopropylsilylflavone (3-TPSF) for detecting fluoride (F⁻) has been theoretically investigated. The calculated Laplacian bond order confirms that the Si–O bond of 3-TPSF is the reaction site of F⁻. The free energy barrier of 18.33 kcal mol⁻¹ indicates that F-triggered desilylation reaction can occur and then form the anionic state of 3-HF (3-HF⁻) with a fluorescence peak at 545 nm. 3-HF⁻ captures H⁺ of the mixed aqueous medium to be transformed into 3-HF with an intramolecular hydrogen bond (O₁–H⋯O₂). The energy barrier of 1.86 kcal mol⁻¹ in the S₁ state obtained from the constructed potential energy curves confirms that the excited state intramolecular proton transfer (ESIPT) in 3-HF occurs to form a tautomer structure, which produces a long-wavelength emission of 549 nm. The fluorescence emitted from both 3-HF⁻ and 3-HF agrees with the experimental value of 530 nm appearing after adding F⁻. Charge transfer analyses indicate that the extent of intramolecular charge transfer in 3-HF⁻ is more intense than that of 3-TPSF, which induces a large Stokes shift of 180 nm. Therefore, the sensing mechanism is attributed to the combination of a large charge transfer feature and ESIPT that are caused by desilylation reaction. The significant fluorescence change makes 3-TPSF a chemodosimeter for detecting F⁻.

The sensing mechanism of 3-hydroxyflavone-based (3-HF) fluorescent chemodosimeter 3-triisopropylsilylflavone (3-TPSF) for detecting fluoride (F⁻) has been theoretically investigated.

A strongly Lewis-acidic and fluorescent borenium cation supported by a tridentate formazanate ligand

Lewis acids are highly sought after for their applications in sensing, small-molecule activation, and catalysis. When combined with π-conjugated molecular frameworks, Lewis acids with unique optoelectronic properties can be realized. Here, we use a tridentate formazanate ligand to create a planar, redox-active, fluorescent, and strongly Lewis-acidic borenium cation. We also demonstrate that this compound can act as a colourimetric probe for reactivity.

Cationic Boron Formazanate Dyes*

Incorporation of cationic boron atoms into molecular frameworks is an established strategy for creating chemical species with unusual bonding and reactivity but is rarely thought of as a way of enhancing molecular optoelectronic properties. Using boron formazanate dyes as examples, we demonstrate that the wavelengths, intensities, and type of the first electronic transitions in BN heterocycles can be modulated by varying the charge, coordination number, and supporting ligands at the cationic boron atom. UV-vis absorption spectroscopy measurements and density-functional (DFT) calculations show that these modulations are caused by changes in the geometry and extent of π-conjugation of the boron formazanate ring. These findings suggest a new strategy for designing optoelectronic materials based on π-conjugated heterocycles containing boron and other main-group elements.

Rapid synthesis of multifunctional β-cyclodextrin nanospheres as alkali-responsive nanocarriers and selective antibiotic adsorbents

β-Cyclodextrin nanospheres are synthesized through a new strategy and demonstrate excellent performance as alkali-responsive nanocarriers and selective antibiotic adsorbents.

Fluorine-18-Labeled Fluorescent Dyes for Dual-Mode Molecular Imaging

Recent progress realized in the development of optical imaging (OPI) probes and devices has made this technique more and more affordable for imaging studies and fluorescence-guided surgery procedures. However, this imaging modality still suffers from a low depth of penetration, thus limiting its use to shallow tissues or endoscopy-based procedures. In contrast, positron emission tomography (PET) presents a high depth of penetration and the resulting signal is less attenuated, allowing for imaging in-depth tissues. Thus, association of these imaging techniques has the potential to push back the limits of each single modality. Recently, several research groups have been involved in the development of radiolabeled fluorophores with the aim of affording dual-mode PET/OPI probes used in preclinical imaging studies of diverse pathological conditions such as cancer, Alzheimer's disease, or cardiovascular diseases. Among all the available PET-active radionuclides, 18F stands out as the most widely used for clinical imaging thanks to its advantageous characteristics (t1/2 = 109.77 min; 97% β+ emitter). This review focuses on the recent efforts in the synthesis and radiofluorination of fluorescent scaffolds such as 4,4-difluoro-4-bora-diazaindacenes (BODIPYs), cyanines, and xanthene derivatives and their use in preclinical imaging studies using both PET and OPI technologies.

Understanding the Diverse Reactivity of Pentaphenylborole toward Epoxides

Density functional theory calculations have been performed to study the diverse reactivity of pentaphenylborole toward different epoxides. We systematically investigated the effect of substituents on epoxides for the preference/competition of three experimentally observed pathways, that is, intramolecular proton transfer, direct ring expansion via insertion of one epoxide molecule, and ring expansion via insertion of two epoxide molecules. Our calculations also predicted a high competitivity between the proton transfer and direct ring expansion pathways for the epoxide containing both alkyl and aryl substituents.

BODIPYS and aza-BODIPY derivatives as promising fluorophores for in vivo molecular imaging and theranostic applications

Since their discovery in 1968, the BODIPYs dyes (4,4-difluoro-4-bora-3a, 4a diaza-s-indacene) have found an exponentially increasing number of applications in a large variety of scientific fields. In particular, studies reporting bioapplications of BODIPYs have increased dramatically. However, most of the time, only in vitro investigations have been reported. The in vivo potential of BODIPYs and aza-BODIPYs is more recent, but considering the number of in vivo studies with BODIPY and aza-BODIPY which have been reported in the last five years, we can now affirm that this family of fluorophores can be considered important as cyanine dyes for future in vivo and even clinical applications. This review aims to present representative examples of recent in vivo applications of BODIPYs or aza-BODIPYs, and to highlight the potential of these dyes for optical molecular imaging.

β-carboline-based turn-on fluorescence chemosensor for quantitative detection of fluoride at PPB level

A novel β-carboline-based chemosensor, having an acidic NH proton that leads to fluoride-induced deprotonation involving a vivid color change from colorless to yellow is described. The absorption spectrum of the chemosensor in acetonitrile has a peak at 375 nm, which changes to 428 nm with the gradual addition of only fluoride in the solution with a clear isosbestic points at 357 nm and 392 nm. More interestingly, the chemosensor gives a turn-on type of fluorescence at 554 nm in the presence of fluoride. Further it was found that the sensor is highly selective towards fluoride over other anions including chloride, bromide, iodide, nitrate, borate, perchlorate and can quantitatively detect fluoride at ppb level with a limit of detection of 0.02 mg/ L or 20 ppb. The chemosensor was successfully demonstrated to assess the fluoride concentration in the tap water.

Radiolabeled BODIPYs: An overview

Fluorescence and SPECT/PET imaging are powerful tools currently in use by the scientific community and receiving a great attention for the development of dual-modality imaging agents. BODIPYs are among the most promising candidates to be used for such functions due their excellent absorbance and fluorescence properties as well as their ease of radiolabeling without compromising their biological properties. In this manuscript we present an overview of BODIPY radiolabeling methods and their relevance to the development of multimodality agents.

A water-soluble BODIPY-based fluorescent probe for rapid and selective detection of hypochlorous acid in living cells

We designed and synthesized 4,4-di-(4'-methylmercaptophenoxy)-8-(N-methylpyridinium-2-yl)-1,3,5,7-tetramethyl-4-bora-3a,4a-diaza-s-indacene (probe 1) as a water-soluble BODIPY derivative for rapid and selective detection of hypochlorous acid. The pyridinium-2-yl linked at the meso position of BODIPY core was used to maintain highly fluorescent nature and to increase water solubility. Methylmercaptophenoxy was selected as responsive site installed on the boron atom (to replace the fluorine atom) and induced the photoinduced electron-transfer (PeT) effect to quench the fluorescence of BODIPY. The probe exhibited 83.9 μg mL^-1 solubility in PBS (10 mM, pH 7.4), and possessed very low fluorescence (Φ_f = 0.0013). Upon addition of HClO, the probe could display a distinct response in 1 min and generate marked fluorescence enhancement by 100-fold due to the oxidation of thioether into sulfoxide to terminate PeT process. A limit of detection of 53 nM was calculated for HClO in the linear response range from 0 μM to 10 μM, and the probe was successfully applied to image HClO in living cells.

A novel fluorescent off–on probe for the sensitive and selective detection of fluoride ions

A highly sensitive and selective fluorescent probe for fluoride ions has been developed by incorporating the dimethylphosphinothionyl group as a recognition moiety into the fluorophore of coumarin. The detection mechanism is based on the fluoride ion-triggered cleavage of the dimethylphosphinothionyl group, followed by the release of coumarin, which leads to a large fluorescence enhancement at 455 nm (λ_ex = 385 nm). Under the optimized conditions, the fluorescence enhancement of the probe is directly proportional to the concentration of fluoride ions in the range of 0–30 μM with a detection limit of 0.29 μM, which is much lower than the maximum content of fluoride ions guided by WHO. Notably, satisfying results have been obtained by utilizing the probe to determine fluoride ions in real-water samples and commercially available toothpaste samples. The proposed probe is rather simple and may be useful in the detection of fluoride ions in more real samples.

A sensitive and selective fluorescent off–on probe is developed for fluoride ion detection, and its applicability has been demonstrated by determining fluoride ions in real-water samples and toothpaste samples.

Influence of the catalyst structure in the cycloaddition of isocyanates to oxiranes promoted by tetraarylstibonium cations

In the context of our work on electron deficient group 15 cations as Lewis acid catalysts, we have synthesized the triflate salts of a series of tetraarylstibonium cations of general formula [ArSbPh3]+ with Ar = Mes (4+), o-(dimethylamino)phenyl (5+), and o-((dimethylamino)methyl)phenyl (6+). These new cationic antimony derivatives, along with the known [Ph4Sb]+ (1+), 1-naphthyltriphenylstibonium (2+), and [(Ant)SbPh3]+ (3+), have been evaluated as catalysts for the cycloaddition of oxiranes and isocyanates under mild conditions. While all stibonium cations favor the 3,4-oxazolidinone products, the reactivities of 5+ and 6+ are hindered by the ancillary amino donor which quenches the Lewis acidity of the antimony center. A comparison of the other stibonium cations shows that 4+ is the most selective catalyst.

Tetrafluorobenzo-Fused BODIPY: A Platform for Regioselective Synthesis of BODIPY Dye Derivatives

A novel route for the synthesis of unsymmetrical benzo-fused BODIPYs is reported using 4,5,6,7-tetrafluoroisoindole as a precursor. The reactivity of the 3,5-dibromo tetrafluorobenzo-fused BODIPY was investigated under nucleophilic substitution and Pd(0)-catalyzed cross-coupling reaction conditions. In addition to the 3,5-bromines, one α-fluoro group on the benzo-fused ring can also be functionalized, and an unusual homocoupling with formation of a bisBODIPY was observed. This new class of fluorinated BODIPYs could find various applications in medicine and materials.

Ultrasensitive reversible chromophore reaction of BODIPY functions as high ratio double turn on probe

Chromophore reactions with changes to conjugation degree, especially those between the conjugated and unconjugated state, will bring a large spectral variation. To realize such a process, a meso-naked BODIPY (MNBOD) with two electron-withdrawing groups around the core is designed and synthesized. The resulting system is extremely sensitive to bases. The red, highly fluorescent solution readily becomes colorless and non-fluorescent after base addition; however, the color and fluorescence can be totally and instantly restored by addition of acid or formaldehyde. Analyses show that two identical MNBODs are connected by a C–C single bond (sp³) at the meso-position through a radical reaction that results in an unconjugated, colorless dimer complexed with bases. When the bases are consumed, the dimer immediately dissociates into the red, highly fluorescent, conjugated MNBOD monomer. With 260 nm spectral change and over 120,000 turn-on ratio, this chromophore-reaction can be utilized as a sensitive reaction-based dual-signal probe.

BODIPY dyes, though widely explored, have not been pursued as chromophore reaction based chemical probes. Here, the authors synthesize a meso-naked BODIPY core flanked with two electron-withdrawing groups, which undergoes a reversible change in conjugated structure in the presence of base and functions as a dual signal and ultrahigh turn-on ratio chemical probe.

Synthesis and photophysical properties of a porphyrin–BODIPY dyad and a porphyrin–o-carborane–BODIPY triad

The photophysical and electrochemical properties of a newly synthesized porphyrin–BODIPY dyad and a porphyrin–o-carborane–BODIPY triad have been investigated.

Synthesis, characterization, optical and electrochemical properties and antibacterial activity of novel BODIPY glycoconjugated dendrimers

A new class of novel BODIPY glycoconjugated dendrimers with optical, electrochemical and antibacterial properties is reported.

Synthesis and evaluation of a [18F]BODIPY-labeled caspase-inhibitor

BODIPYs (boron dipyrromethenes) are fluorescent dyes which show high stability and quantum yields. They feature the possibility of selective ¹⁸F-fluorination at the boron-core. Attached to a bioactive molecule and labeled with [¹⁸F]fluorine, the resulting compounds are promising tracers for multimodal imaging in vivo and can be used for PET and fluorescence imaging. A BODIPY containing a phenyl and a hydroxy substituent on boron was synthesized and characterized. Fluorinated and hydroxy substituted dyes were coupled to an isatin-based caspase inhibitor via cycloaddition and the resulting compounds were evaluated in vitro in caspase inhibition assays. The metabolic stability and the formed metabolites were investigated by incubation with mouse liver microsomes and LC-MS analysis. Subsequently the fluorophores were labeled with [¹⁸F]fluorine and an in vivo biodistribution study using dynamic PET was performed.

Synthesis and spectroscopic properties of β,β'-dibenzo-3,5,8-triaryl-BODIPYs

A series of β,β'-bicyclo-3,5-diaryl-BODIPYs were synthesized from the corresponding β,β'-bicyclo-3,5-diiodo-BODIPYs (1a,b) via Pd(0)-mediated Suzuki cross-coupling reactions in 82-92% yields. Subsequent aromatization with DDQ afforded the corresponding β,β'-dibenzo-aryl-BODIPYs, which showed red-shifted absorptions and emissions in the near-IR range. The dibenzo-appended BODIPYs showed characteristic 1H-, 13C-, 11B- and 19F-NMR shifts, and nearly planar conformations by X-ray crystallography.

Boron difluorides with formazanate ligands: redox-switchable fluorescent dyes with large stokes shifts

The synthesis of a series of (formazanate)boron difluorides and their 1-electron reduction products is described.

[(18)F]-Group 13 fluoride derivatives as radiotracers for positron emission tomography

The field of (18)F chemistry is rapidly expanding because of the use of this radionuclide in radiotracers for positron emission tomography (PET). Until recently, most [(18)F]-radiotracers were generated by the direct attachment of (18)F to a carbon in the organic backbone of the radiotracer. The past decade has witnessed the emergence of a new strategy based on the formation of an (18)F-group 13 element bond. This approach, which is rooted in the field of fluoride anion complexation/coordination chemistry, has led to the development of a remarkable family of boron, aluminium and gallium [(18)F]-fluoride anion complexing agents which can be conjugated with peptides and small molecules to generate disease specific PET radiotracers. This review is dedicated to the chemistry of these group 13 [(18)F]-fluorides anion complexing agents and their use in PET. Some of the key fluoride-binding motifs covered in this review include the trifluoroborate unit bound to neutral or cationic electron deficient backbones, the BF2 unit of BODIPY dyes, and AlF or GaF3 units coordinated to multidentate Lewis basic ligands. In addition to describing how these moieties can be converted into their [(18)F]-analogs, this review also dicusses their incorporation into bioconjugates for application in PET.

Substituent effects on anion sensing of salicylidene Schiff base derivatives: Tuning sensitivity and selectivity

A series of colorimetric anion sensors using the salicylidene Schiff bases with different substituents, including electron donating group (tert-butyl, in sensor 2), conjugated group (naphthyl, in sensor 3) and electron withdrawing group (chlorine, in sensor 4), respectively, have been developed. The substituents can not only impact chromogenic signal output, but also tune the sensitivity and selectivity of the anion sensing by their specific electron push-pull features. In particular, both 1 (without substituent) and 2 show high selectivity for F(-) over Cl(-), Br(-), I(-), AcO(-) and H2PO4(-), but the sensitivity of 2 is poorer than 1 due to the effect of electron donating groups. Sensor 3 exhibits higher sensitivity for F(-) than 1, but it is disturbed by the weak response to AcO(-) and H2PO4(-). Sensor 4 has the highest sensitivity for F(-), but shows the significant response to AcO(-) and H2PO4(-), which also decreases the selectivity for F(-). Finally, analytical applications of 1 for the detection of F(-) in aqueous medium and toothpaste have been studied.

Reactivity of a dihydroboron species: synthesis of a hydroborenium complex and an expedient entry into stable thioxo- and selenoxo-boranes

The reaction of a recently synthesized dihydroboron species complexed with bis(phosphinimino)amide, LBH2 (), (L = [N(Ph2PN(2,4,6-Me3C6H2))2](-)) with 3 equivalents of BH2Cl·SMe2 or one equivalent of BCl3 affords the first stable monohydridoborenium ion, [LBH](+)[HBCl3](-) () that is stable without a weakly coordinating bulky anion. Compound can also be prepared directly by refluxing LH with 3 equivalents of BH2Cl·SMe2. Interestingly, reaction of LBH2 () with elemental sulfur and selenium involves oxidative addition of S and Se into B-H bonds and subsequent release of H2S (or H2Se) from the intermediate LB(SH)2 (or LB(SeH)2) species forming stable compounds with terminal boron-chalcogen double bonds LB[double bond, length as m-dash]S () and LB[double bond, length as m-dash]Se (). The electronic structures of compounds , and were elucidated by high resolution mass spectrometry, multi-nuclear NMR and single crystal X-ray diffraction studies. Ab initio calculations on are in excellent agreement with its experimental structure and clearly support the existence of the boron-sulfur double bond.

Lewis acidity enhancement of triarylborane by appended phosphine oxide groups

A series of mono-, di-, and tri-phosphine oxide substituted triarylboranes, Mes2BAr (1), MesBAr2 (2), and BAr3 (3) (Ar = 4-(Ph2PO)-2,6-Me2-C6H2) were prepared to investigate the effect of a phosphine oxide group (Ph2PO) on Lewis acidity enhancement of triarylboranes. The X-ray crystal structure of 3 revealed peripheral decoration of Ph2PO groups with a C3-axis perpendicular to the trigonal boron center. UV/Vis absorption and PL spectra indicated a significant contribution of π(Mes or phenylene) → pπ(B) charge transfer in the lower-energy electronic transition. The reduction potential measured by cyclic voltammetry showed apparent LUMO stabilization by introduction of phosphine oxide groups, the extent of which gradually increased with the increasing number of phosphine oxide groups. Lewis acidity enhancement was also supported by the gradual increase in fluoride ion affinity in the order 3 > 2 > 1. Theoretical calculations suggest that introduction of a Ph2PO group into a triarylborane significantly enhances the Lewis acidity of the boron center via an inductive electron-withdrawing effect and this effect is additive for multiple phosphine oxide groups.

Efficient electrochemiluminescence of a readily accessible boron difluoride formazanate dye

Systematic studies of the electrochemiluminescence (ECL) of a boron difluoride formazanate dye reveal three distinct, voltage-dependent mechanisms of light emission.

Synthesis of monolateral and bilateral sulfur-heterocycle fused naphthalene diimides (NDIs) from monobromo and dibromo NDIs

Monolateral and bilateral sulfur-heterocycle fused naphthalene diimides (NDIs) are successfully synthesized from monobromo and dibromo NDIs via nucleophilic aromatic substitution reactions and the subsequent oxidative aromatization processes.

Bis-ureidoquinoline as a selective fluoride anion sensor through hydrogen-bond interactions

Bis-ureidoquinoline shows a characteristic UV-vis absorbance and turn-on fluorescence changes in the presence of the fluoride anion. Such selective changes probably originate from the hydrogen-bond interactions, as shown by the (1)H NMR titration and DFT calculations. Bis-ureidoquinoline can be used as a fluoride-selective sensor for the detection of fluoride anions under illumination from a laboratory hand-held UV lamp.