Tunable design strategy for fluorescence probes based on 4-substituted BODIPY chromophore: improvement of highly sensitive fluorescence probe for nitric oxide

Ex Tenebris Lux: Illuminating Reactive Oxygen and Nitrogen Species with Small Molecule Probes

Reactive oxygen and nitrogen species are small reactive molecules derived from elements in the air─oxygen and nitrogen. They are produced in biological systems to mediate fundamental aspects of cellular signaling but must be very tightly balanced to prevent indiscriminate damage to biological molecules. Small molecule probes can transmute the specific nature of each reactive oxygen and nitrogen species into an observable luminescent signal (or even an acoustic wave) to offer sensitive and selective imaging in living cells and whole animals. This review focuses specifically on small molecule probes for superoxide, hydrogen peroxide, hypochlorite, nitric oxide, and peroxynitrite that provide a luminescent or photoacoustic signal. Important background information on general photophysical phenomena, common probe designs, mechanisms, and imaging modalities will be provided, and then, probes for each analyte will be thoroughly evaluated. A discussion of the successes of the field will be presented, followed by recommendations for improvement and a future outlook of emerging trends. Our objectives are to provide an informative, useful, and thorough field guide to small molecule probes for reactive oxygen and nitrogen species as well as important context to compare the ecosystem of chemistries and molecular scaffolds that has manifested within the field.

Benzo-Crown-Ether Functionalized O-BODIPY Probes for Cations - A Selective Fluorescent Probe for Ba2

Herein, we report the synthesis and sensing characteristics of 4,4'-methoxy-substituted BODIPY fluorescent probes (O-BODIPYs) 3, 4 and 5 equipped with differently sized benzo-crown ethers (cf. Scheme 1, 3 (benzo-15-crown-5), 4 (benzo-18-crown-6) and 5 (benzo-21-crown7)). O-BODIPYs 3, 4 and 5 exhibited in comparison to their known F-BODIPY analogues 3a, 4a and 5a (cf. Scheme 1) an improved solubility in aqueous medium and higher fluorescence quantum yields. Fluorometric study in aqueous solutions of 3, 4 and 5 in the presence of different cations show cation induced fluorescence enhancements (FE). Compared to the benzo-crown ether substituted F-BODIPY analogues 3a, 4a and 5a, we found for the free O-BODIPYs 3, 4 and 5 higher fluorescence quantum yields (φf) but lower cation induced FEs. We show that in aqueous medium the fluorescence quenching process (OFF switching), a photoinduced electron transfer, in O-BODIPYs 3, 4 and 5 is less effective and consequently sensitive and selective ON switching of the fluorescence by cations, too. Albeit these observations the novel benzo-21-crown-7 equipped fluorescent probe 5 exhibits a good fluorometric Ba2+ selectivity and Ba2+ sensitivity in conjunction to their aqueous solubility.

Novel salicylic acid derivatives connecting to five-membered cycle through an acyl hydrazone bond as multi-stimuli responsive fluorescent smart materials with photoswitching properties

In order to exploit novel multi-stimuli responsive fluorescent materials, a series of novel fluorescent molecules of salicylic acid derivatives were designed and synthesized via introducing pyrazole or cyclopentane to the salicylic acid scaffold through a special Schiff base-acylhydrazone, and the molecular structures of representative compounds A2 and A4 were verified via single crystal X-ray diffraction. All title molecules could exhibit obvious solvatofluorochromism from cyan to indigo in several solutions with different polarity. The fluorescence titration data exhibited compound A2 and complex A2-Cu2+ with prime detection limits to Cu2+ (0.24 μM) and S2- ions (2.83 μM). The sensitive recognition of A2 to trifluoroacetic (TFA) and A2-TFA to triethylamine (TEA) were also confirmed via fluorescent titration experiments in various solutions, respectively. What's more, the 1H NMR and UV/Vis absorption spectra further explained the mechanism between molecules and ions or molecules and TFA/TEA. Besides, the photoswitching properties of the compounds A2 and A3 could be demonstrated via the irradiation of special wavelength light or heating accompanied with changes in quantum yields. In addition, these phenomena of multiple responses were explained via Density Functional Theory (DFT) based on the Gaussian calculations. Thus, this work provided a preliminary basis for the research of multi-stimuli responsive fluorescent molecules with photoswitching properties.

BODIPY-Based Molecules for Biomedical Applications

BODIPY (4,4-difluoro-4-bora-3a,4a-diaza-s-indacene) derivatives have attracted attention as probes in applications like imaging and sensing due to their unique properties like (1) strong absorption and emission in the visible and near-infrared regions of the electromagnetic spectrum, (2) strong fluorescence and (3) supreme photostability. They have also been employed in areas like photodynamic therapy. Over the last decade, BODIPY-based molecules have even emerged as candidates for cancer treatments. Cancer remains a significant health issue world-wide, necessitating a continuing search for novel therapeutic options. BODIPY is a flexible fluorophore with distinct photophysical characteristics and is a fascinating drug development platform. This review provides a comprehensive overview of the most recent breakthroughs in BODIPY-based small molecules for cancer or disease detection and therapy, including their functional potential.

A BODIPY-picolinium-cation conjugate as a blue-light-responsive caged group

Caged compounds protected with photolabile protecting groups (PPGs) are useful for controlling various biological events with high spatiotemporal resolution. Most of the commonly used PPGs are controlled by ultraviolet light irradiation, but it is desirable to have PPGs controlled by visible light irradiation in order to minimize tissue damage. Here, we describe a boron-dipyrromethene (BODIPY)-picolinium conjugate (BPc group) that functions as a blue-light-controllable PPG. ESR experiments indicate that the photolysis mechanism is based on intramolecular photoinduced electron transfer. We illustrate the applicability of the BPc group to biologically active compounds by employing it firstly to photocontrol release of histamine, and secondly to photocontrol release of a soluble guanylyl cyclase (sGC) activator, GSK2181236A, which induces photovasodilation. The BPc group is expected to be a useful PPG for controlling various biological events with blue light irradiation.

Substitution at boron in BODIPYs

Recent synthetic achievements have led to 4,4-disubstituted-4-bora-3a,4a-diaza-s-indacenes (BODIPYs) with varying substituents at the meso, pyrrolic and/or boron sites, with each influencing photophysical properties and utility. This Feature article gives an overview of chemistry at the boron atom in BODIPYs, highlighting our contributions that evolved from synthetic curiosities and now offer this dipyrrolic skeleton potential across a wider range of applications. We first summarise preparative routes to BODIPYs through complexation of boron with the dipyrrinato ligand. The role of boron in protecting dipyrrins is then discussed, followed by strategies by which to achieve facile substitution at the boron atom.

Asymmetrically bridged aroyl-S,N-ketene acetal-based multichromophores with aggregation-induced tunable emission

Asymmetrically bridged aroyl-S,N-ketene acetals and aroyl-S,N-ketene acetal multichromophores can be readily synthesized in consecutive three-, four-, or five-component syntheses in good to excellent yields by several successive Suzuki-couplings of aroyl-S,N-ketene acetals and bis(boronic)acid esters. Different aroyl-S,N-ketene acetals as well as linker molecules yield a library of 23 multichromophores with substitution and linker pattern-tunable emission properties. This allows control of different communication pathways between the chromophores and of aggregation-induced emission (AIE) and energy transfer (ET) properties, providing elaborate aggregation-based fluorescence switches.

A library of 23 asymmetrically linked aroyl-S,N-ketene acetal solid-state emissive multichromophores accessed by one-pot multicomponent reactions exhibits AIE- and AIEE-active behavior as well as dual emission and potential energy transfer.

Regioselective Stepwise Bromination of [14]Triphyrins(2.1.1) and Their Effects on Structural, Spectral, and Redox Properties

Regioselective stepwise bromination of meso-tetraaryl [14]triphyrins(2.1.1) was explored to investigate the effect of bromine substitution at the β-pyrrole carbons of triphyrin(2.1.1) on the structural, spectral, photophysical, and redox properties. A series of β-monobromo to β-hexabromo triphyrins(2.1.1) 2-7 were synthesized by treating triphyrin(2.1.1) 1 with appropriate equivalents of N-bromosuccinimide at ambient temperature in decent yields. The regiochemistry of bromines in β-brominated triphyrins(2.1.1) 3-5 and 7 was confirmed by X-ray crystallography, and the analysis revealed the effect of bromination of triphyrin(2.1.1) on the structural framework was significant in the case of hexabromotriphyrin(2.1.1) 7 compared to other macrocycles. Absorption spectroscopy showed that stepwise substitution of bromines at β-pyrrole carbons of triphyrin(2.1.1) resulting in bathochromic shifts of absorption bands relative to triphyrin(2.1.1) 1 and hexabromotriphyrin(2.1.1) 7 exhibited absorption bands at longer wavelengths. The redox studies revealed that compounds 2-7 were easier to reduce than triphyrin(2.1.1) 1, and the first reduction potential wave shifted anodically with an increase in the number of bromine substituents at β-pyrrole carbons of triphyrin(2.1.1) 1 from one to six. These structural, spectral, and electrochemical properties were also predicted by density functional theory calculations, and the analysis was consistent with the experimental observations.

Effect of conjugated system extension on structural features and electron-density distribution in charge-transfer difluoroborates

A comparative structural study of two related donor-acceptor pyridine-based BF₂ complexes, namely, 3-(dimethylamino)-1,1-difluoro-1H-pyrido[1,2-c][1,3,5,2]oxadiazaborinin-9-ium-1-uide, C₈H₁₀BF₂N₃O (1), and 3-{(1E,3E)-4-[4-(dimethylamino)phenyl]buta-1,3-dien-1-yl}-1,1-difluoro-1H-pyrido[1,2-c][1,3,5,2]oxadiazaborinin-9-ium-1-uide, C₁₈H₁₈BF₂N₃O (2), containing a dimethylamino group and either the shortest (in 1) or the longest (in 2) charge-transfer path known until now in this family of compounds, is presented. Single-crystal X-ray diffraction analysis supported by computational investigations shed more light on these systems, indicating, among other aspects, the predominance of C-H...F contacts in 1, the formation of antiparallel dimers held together by π-π interactions in both compounds, and the involvement of fused BF₂-bearing rings in the charge-transfer process.

Synthesis and properties of covalently linked di-p-benzihomoporphyrin-BODIPY conjugates

Two mono meso-functionalized [20]di-[Formula: see text]-benzihomoporphyrins containing [Formula: see text]-formylphenyl and [Formula: see text]-iodophenyl groups at meso-position respectively were synthesized by condensing one equivalent of appropriate tetrapyrrane with one equivalent of [Formula: see text]-formyl benzaldehyde/[Formula: see text]-iodo benzaldehyde in CH2Cl2 under mild acid catalyzed conditions. The meso-formylphenyl and meso-iodophenyl functionalized di-[Formula: see text]-benzihomoporphyrins were used to synthesize two covalently linked di-[Formula: see text]-benzihomoporphyrins-BODIPY conjugates. The meso-formylphenyl-functionalized di-[Formula: see text]-benzihomoporphyrin was converted to corresponding meso-dipyrrolyl substituted di-[Formula: see text]-benzihomoporphyrin by treating with excess pyrrole under acid catalyzed conditions. In the next step, the meso-dipyrrolyl di-[Formula: see text]-benzihomoporphyrin was subjected to oxidation followed by BF2 complexation to afford the directly linked di-[Formula: see text]-benzihomoporphyrin-BODIPY conjugate. The meso-iodophenyl functionalized di-[Formula: see text]-benzihomoporphyrin was coupled with ethynyl-functionalized BODIPY under mild Pd(0) coupling condition to synthesize diphenylethyne-bridged di-[Formula: see text]-benzihomoporphyrin-BODIPY conjugate. The two conjugates were characterized by HR-MS, NMR, absorption, electrochemical, fluorescence and DFT studies. The spectral and electrochemical studies indicated that the two constituents, di-[Formula: see text]-benzihomoporphyrin and BODIPY units in the conjugates interact weakly and retain their individual characteristic features. DFT studies indicated a possibility of charge transfer between di-[Formula: see text]-benzihomoporphyrin and BODIPY units in conjugates.

Synthesis and photolysis of new BODIPY derivatives with chelated boron centre

New borondipyromethene (BODIPY) derivatives chelated at the boron centre with different catecholate and salicylate ligands were synthesized via substituting the fluoride atoms with the aid of aluminum chloride that activates the boron-fluoride bond for substitution. The photophysical properties of the novel BODIPYs were investigated by normalized UV-vis absorption as well as the fluorescence emission spectra. Moreover, the fluorescence quantum yields of the chelated BODIPYs were also calculated and the ultraviolet irradiation of the salicylate derivatives was studied.

Anchoring BODIPY photosensitizers enable pan-microbial photoinactivation

Photodynamic antimicrobial chemotherapy (PACT) is an effective strategy to inactivate pathogenic and resistant microbes. However, pan-microbial photoinactivation has hardly achieved. In this manuscript, we built anti-microbial PSs based on 2,6-diiodo-1,3,5,7-tetramethyl BODIPY (2I-BDP) using anchoring strategy through modifications on boron atom with bis-cationic moieties. With appropriate bis-cationic anchoring, we could achieve effective PACT for pan-microbial photoinactivation via straight forward modifications. Our studies suggested that integration of an efficient photosensitizer, good amphiphilicity, as well as tight interaction with microbial membrane could be essential for effective PACT.

Facile deprotection of F-BODIPYs using methylboronic acid

4,4-Difluoro-4-bora-3a,4a-diaza-s-indacenes (F-BODIPYs) are deprotected through removal of the -BF₂ moiety upon treatment with methylboronic acid. The tolerance of various substitution patterns about the dipyrrinato core is demonstrated via the deprotection of thirteen F-BODIPYs and an F-aza-BODIPY. Work-up with aq. HBr affords the desired dipyrin HBr salt in quantitative yield without need for purification.

Core-Modified Pentaphyrins(2.1.1.1.1) and Bis(difluoroborane) Complex: Synthesis, Structure, and Spectral and Redox Properties

A series of hetero analogues of pentaphyrins(2.1.1.1.1) such as oxapentaphyrins(2.1.1.1.1) and thiapentaphyrins(2.1.1.1.1) were synthesized by 3 + 2 condensation of dipyrroethenedicarbinol with 16-oxatripyrrane/16-thiatripyrrane under mild acid-catalyzed reaction conditions. The stable macrocycles are freely soluble in organic solvents, and their identities were confirmed by a corresponding molecular-ion peak in high-resolution mass spectrometry, 1D and 2D NMR spectroscopy, and X-ray structure obtained for one of the oxapentaphyrin(2.1.1.1.1) macrocycles. The crystal structure and NMR studies indicated that the heterocyclic ring, such as furan in oxapentaphyrins(2.1.1.1.1) and thiophene in thiapentaphyrins(2.1.1.1.1), was inverted. In absorption spectra, the macrocycles showed one sharp band at ∼516 nm and one broad band at ∼744 nm. The spectral and X-ray studies supported the nonaromatic nature of these macrocycles. This is in contrast to the recently reported aza analogue of pentaphyrins(2.1.1.1.1), which showed antiaromatic behavior. Upon protonation, the core-modified pentaphyrins(2.1.1.1.1) macrocycles exhibited bathochromically shifted absorption bands with a distinct change in the color of the solution. The ¹H NMR, nucleus-independent chemical shift, and anisotropy-induced current density studies indicated the presence of Mobius aromaticity in the protonated macrocycles. The core-modified pentaphyrins(2.1.1.1.1) can act as good coordinating ligands, as shown here by synthesizing a bis(difluoroborane) complex of one of the thiapentaphyrins(2.1.1.1.1).

BODIPYs as Chemically Stable Fluorescent Tags for Synthetic Glycosylation Strategies towards Fluorescently Labeled Saccharides

A series of fluorescent boron-dipyrromethene (BODIPY, 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene) dyes have been designed to participate, as aglycons, in synthetic oligosaccharide protocols. As such, they served a dual purpose: first, by being incorporated at the beginning of the process (at the reducing-end of the growing saccharide moiety), they can function as fluorescent glycosyl tags, facilitating the detection and purification of the desired glycosidic intermediates, and secondly, the presence of these chromophores on the ensuing compounds grants access to fluorescently labeled saccharides. In this context, a sought-after feature of the fluorescent dyes has been their chemical robustness. Accordingly, some BODIPY derivatives described in this work can withstand the reaction conditions commonly employed in the chemical synthesis of saccharides; namely, glycosylation and protecting-group manipulations. Regarding their photophysical properties, the BODIPY-labeled saccharides obtained in this work display remarkable fluorescence efficiency in water, reaching quantum yield values up to 82 %, as well as notable lasing efficiencies and photostabilities.

Dibenzofuran/Dibenzothiophene-Embedded Dithia-bis(calix)-sapphyrins

A series of first examples of dibenzofuran (DBF)/dibenzothiophene (DBT)-embedded dithia-bis(calix)-sapphyrins were synthesized by condensing 1 equiv of dibenzofuran/dibenzothiophene-based tripyrrane with 1 equiv of [2,2'-bithiophene]-5,5'-diylbis(aryl)methanol under mild acid-catalyzed conditions in CH₂Cl₂ followed by oxidation with DDQ (2,3-dichloro-5,6-dicyano-1,4-benzoquinone) and alumina column chromatographic purification afforded new dithia-bis(calix)-sapphyrins with two meso-sp³ carbons in 5-7% yields. The DBF/DBT-embedded dithia-bis(calix)-sapphyrins were characterized by HRMS (high-resolution mass spectrometry), ¹H and ¹³C NMR, ¹H-¹H COSY, ¹H-¹H NOESY, ¹H-¹³C HSQC, and ¹H-¹³C HMBC spectroscopy, absorption spectroscopy, cyclic voltammetry, and density functional theory (DFT) studies. The macrocycles showed one broad absorption band at ∼553 nm with a shoulder peak at the higher energy side along with a sharp intense band at ∼415 nm. However, the protonated dithia-bis(calix)-sapphyrins showed large bathochromic shifts in the absorption bands, indicating that the electronic properties of dithia-bis(calix)-sapphyrins were altered significantly upon protonation of dithia-bis(calix)-sapphyrins. The electrochemical study indicated that dithia-bis(calix)-sapphyrins are relatively easier to reduce but difficult to oxidize. The DFT studies revealed that macrocycles adopt a highly folded half-chair conformation due to the disruption of conjugation of the macrocycle because of the presence of two sp³ meso-carbons. The DFT studies also support the significant bathochromic shifts observed for protonated dithia-bis(calix)-sapphyrins macrocycles.

Far-red/near-infrared emitting, two-photon absorbing, and bio-stable amino-Si-pyronin dyes

Organic fluorophores emitting in the far-red/near-infrared (NIR) wavelength region are in great demand for minimal autofluorescence and reduced light scattering in deep tissue or whole body imaging. Currently, only a few classes of far-red/NIR fluorophores are available including widely used cyanine dyes, which are susceptible to photobleaching and form nonfluorescent aggregates. Even rare are those far-red/NIR emitting dyes that have two-photon imaging capability. Here we report a new class of far-red/NIR-emitting dyes that are photo-stable, very bright, biocompatible, and also two-photon absorbing. The introduction of an electron-withdrawing group such as N-acyl or N-alkoxycarbonyl groups on the C-10-amino substituent of the new julolidine-derived amino-Si-pyronin dyes (ASiPj), which emit in the far-red region, causes large bathochromic shifts, leading to NIR-emitting amino-Si-pyronin dyes (NIR-ASiPj) having high cellular stability. Furthermore, the ASiPj-NIR-ASiPj couple offers a novel ratiometric bioimaging platform with a large spectral gap, as demonstrated here with a boronate-containing NIR-ASiPj derivative that is converted to the corresponding ASiPj dye upon reaction with hydrogen peroxide.

Effects of Core Modification on Electronic Properties of para-Benziporphyrins

In contrast to the nonaromatic meta-benziporphyrins, the para-benziporphyrins possess aromatic character depending on the type of five-membered ring present in the macrocyclic core. The effects of changing the para-benziporphyrinic core from C₂N₃ to C₂NSN, C₂NSeN, and C₂NTeN by replacing the pyrrole with other five-membered heterocycles such as thiophene, selenophene, and tellurophene on aromatic properties of p-benziporphyrins are described here using spectral, electrochemical, X-ray, and density functional theory (DFT) studies. The missing core-modified p-benziporphyrins with C₂NSeN and C₂NTeN cores were synthesized by condensing 1 equiv of benzitripyrrane and 1,3-benzene-bis((4-phenyl)methanol with an appropriate diol such as 2,5-bis[(p-tolyl)hyroxymethyl]selenophene and 2,5-bis(hydroxymethyl)tellurophene under mild acid-catalyzed conditions at room temperature and characterized in detail by high-resolution mass spectrometry (HR-MS), one- & two-dimensional NMR, and X-ray crystallography of the one of the macrocycles, Selena p-benziporphyrin. The X-ray structure of Selena p-benziporphyrin revealed that the macrocycle was almost planar apart from the p-phenylene ring, which was deviated by 49.71° from the mean plane of the macrocycle defined by four meso carbons, unlike Selena m-benziporphyrin, which is relatively more distorted. NMR studies revealed that, as the core changes from C₂N₃ to C₂NSN, C₂NSeN, and C₂NTeN, the diatropic ring current decreases, indicating that the aromatic character also decreases in the same order. X-ray structure and DFT studies also revealed that the distortion in the macrocycle increases as the pyrrole ring of p-benziporphyrin was replaced with other heterocycles such as furan, thiophene, selenophene, and tellurophene and that the tellura p-benziporphyrin was the most distorted macrocycle among core-modified p-benziporphyrins. Absorption and electrochemical properties were in agreement with these observations. Our repeated attempts on metalation of these p-benziporphyrins resulted in the successful synthesis of a Pd(II) complex of tellura p-benziporphyrin. The Pd(II) complex was characterized by HR-MS and NMR techniques, and the structure was optimized by DFT. The studies indicated that the Pd(II) ion was bonded to one of the pyrrolic nitrogens, tellurophene, tellurium, and two chloride ions in distorted square-planar geometry.

gem-Dibromovinyl boron dipyrrins: synthesis, spectral properties and crystal structures

A family of new asymmetric and symmetric 1,3,7,9-tetramethyl-4,4-bora difluoro-diaza-s-indacene (BODIPY) derivatives, bearing gem-dibromovinyl substituents, was synthesized by the Corey-Fuchs olefination method. One or two gem-dibromovinyl moieties were attached at either the p-position of 5-phenyl, or the β-position of the pyrrole ring, directly or, through phenyl spacers. The assigned structures were supported by MS, NMR (¹H, ¹³C, ¹⁹F), X-ray diffraction analysis and for some compounds 2D HSQC and ¹¹B NMR as well as optical spectroscopy. Their absorption and fluorescence properties and solvatochromism in different solvents were investigated. The highest absorption and emission maxima were obtained for compounds having two gem-dibromovinyl groups attached directly or through the phenyl spacer. The best correlation (R-coefficient) between the solvent and spectral properties of the BODIPYs were obtained using the refractive index of the solvent. Although these compounds are structurally quite similar, their solid states show remarkable differences in the crystal system, clearly revealing two distinct patterns of gem-dibromovinyl orientation and torsion angles of the 5-phenyl ring and the indacene plane. Hirshfeld surface analysis data were used to visualize various intermolecular interactions.

Synthesis of Phlorin Analogues of Dithiacorrphycene and Their Use as Specific Chemodosimetric Sensors for Fe3+ Ions

The first examples of stable phlorin analogues of dithiacorrphycene were synthesized by treating appropriate tetrapyrranes with two equivalents of trifluoroacetic acid in CH₂ Cl₂ followed by oxidation with p-chloranil. The oxidative coupling of tetrapyrrane resulted in the formation of two unexpected phlorin analogues of dithiacorrphycenes that differed from each other at the meso-sp³ -carbon, and the two macrocycles were easily separated and isolated by column chromatography. The novel macrocycles were confirmed by X-ray crystallography and characterized by HRMS, 1D and 2D NMR spectroscopy, absorption spectroscopy, cyclic voltammetry, and time-dependent DFT studies. X-ray structure analysis revealed the highly strained and distorted nature of the macrocycles. The macrocycles showed one ill-defined absorption band at λ≈315 nm and a broad band at λ≈530 nm. Because both macrocycles possess two thiophene and two pyrrole rings with one ionizable inner NH, the cation-sensing properties of the phlorin analogues of dithiacorrphycenes have been investigated. The phlorin analogues of dithiacorrphycenes act as specific chemodosimetric sensors for Fe³⁺ ions by converting phlorin analogues of dithiacorrphycenes into dithiacorrphycene.

Synthesis of Nonaromatic and Aromatic Dithia Benzisapphyrins

A series of [24π] dithia meta-benzisapphyrins and [22π] dithia para-benzisapphyrins were synthesized by 3 + 2 condensation of appropriate benzitripyrrane with bithiophene diol under mild acid catalyzed conditions. The dithia m-benzisapphyrins and dithia p-benzisapphyrins were thoroughly characterized by HR-MS, 1D and 2D NMR, absorption, and electrochemical techniques. Our studies showed that dithia m-benzisapphyrins are nonaromatic, whereas the dithia p-benzisapphyrins are aromatic in nature. Thus, we demonstrated here that the dithiabenzisapphyrins can be made aromatic by replacing the m-phenylene moiety of the benzisapphyrin macrocycle with a p-phenylene unit. Furthermore, the studies also indicated that the aromaticity of the dithia p-benzisapphyrins was relatively more compared to the reported heterosapphyrins. The structural and spectral characteristics including aromaticity of the m-benzisapphyrins and p-benzisapphyrins were also discussed with the help of DFT, NICS, and TD-DFT studies.

Imaging Approaches to Assessments of Toxicological Oxidative Stress Using Genetically-encoded Fluorogenic Sensors

While oxidative stress is a commonly cited toxicological mechanism, conventional methods to study it suffer from a number of shortcomings, including destruction of the sample, introduction of potential artifacts, and a lack of specificity for the reactive species involved. Thus, there is a current need in the field of toxicology for non-destructive, sensitive, and specific methods that can be used to observe and quantify intracellular redox perturbations, more commonly referred to as oxidative stress. Here, we present a method for the use of two genetically-encoded fluorogenic sensors, roGFP2 and HyPer, to be used in live-cell imaging studies to observe xenobiotic-induced oxidative responses. roGFP2 equilibrates with the glutathione redox potential (EGSH), while HyPer directly detects hydrogen peroxide (H2O2). Both sensors can be expressed into various cell types via transfection or transduction, and can be targeted to specific cellular compartments. Most importantly, live-cell microscopy using these sensors offers high spatial and temporal resolution that is not possible using conventional methods. Changes in the fluorescence intensity monitored at 510 nm serves as the readout for both genetically-encoded fluorogenic sensors when sequentially excited by 404 nm and 488 nm light. This property makes both sensors ratiometric, eliminating common microscopy artifacts and correcting for differences in sensor expression between cells. This methodology can be applied across a variety of fluorometric platforms capable of exciting and collecting emissions at the prescribed wavelengths, making it suitable for use with confocal imaging systems, conventional wide-field microscopy, and plate readers. Both genetically-encoded fluorogenic sensors have been used in a variety of cell types and toxicological studies to monitor cellular EGSH and H2O2 generation in real-time. Outlined here is a standardized method that is widely adaptable across cell types and fluorometric platforms for the application of roGFP2 and HyPer in live-cell toxicological assessments of oxidative stress.

Lighting-up protein–ligand interactions with fluorescent PET (photoinduced electron transfer) sensor designs

Extending the versatile fluorescent PET sensing/switching system causes ‘off–on’ signalling when a ligand binds to its appropriate protein.

Thermally-Activated, Delayed Fluorescence in O,B,O- and N,B,O-Strapped Boron Dipyrromethene Derivatives

A small series of boron dipyrromethene (BODIPY) dyes has been synthesized whereby the boron atom is constrained in a five-membered ring formed from either o-dihydroxypyridine or o-aminophenol. In the latter case, the amino group has been converted into the corresponding amide derivative so as to curtail the possibility for light-induced charge transfer from strap to BODIPY. These compounds are weakly emissive in fluid solution but cleavage of the strap, by treatment with a photoacid generator, restores strong fluorescence. Surprisingly, the same compounds remain weakly fluorescent in a rigid glass at 80 K where light-induced charge transfer is most unlikely. In fluid solution, the fluorescence quantum yield increases with increasing temperature due to a thermally activated step but does not correlate with the thermodynamics for intramolecular charge transfer. It is proposed that the strap causes rupture of the potential energy surface for the excited state, creating traps that provide new routes by which the wave packet can return to the ground state. Access to the trap from the excited state is reversible, leading to the delayed emission. Analysis of the temperature dependent emission intensities allows estimation of the kinetic parameters associated with entering and leaving the trap.

Water-soluble Hantzsch ester as switch-on fluorescent probe for efficiently detecting nitric oxide

A water soluble Hantzsch ester derivative of coumarin, DHPS, was synthesized and successfully applied in the fluorescent sensing nitric oxide (NO) in aqueous solution. The fluorescence of probe DHPS is extremely weak, while its fluorescence was greatly switched on upon the addition of NO solution and showed high selectivity and sensitivity to NO. The limitation of the detection was calculated to be 18nM. The NO-induced aromatization of dihydropyridine in DHPS to pyridine derivative (PYS) proved to be the switching mechanism for the fluorescent sensing process, which was confirmed through spectra characterization and computation study. Cytotoxicity assay demonstrated both DHPS and PYS are biocompatible, the DHPS was successfully applied to track the endogenously produced NO in the RAW 264.7 cells.

An Amphiphilic Fluorescent Probe Designed for Extracellular Visualization of Nitric Oxide Released from Living Cells

Nitric oxide (NO) is an intracellular and intercellular messenger involved in numerous physiological and pathophysiological processes. Small-molecule fluorescent probes coupled with fluorescence microscopy provide excellent tools for real-time detection of NO in situ. However, most probes are designed for imaging intracellular NO, which cannot reflect the release behavior of endogenously produced NO. In order to visualize extracellular NO released from living cells, we report herein a particularly designed amphiphilic fluorescent probe, disodium 2,6-disulfonate-1,3-dimethyl-5-hexadecyl-8-(3,4-diaminophenyl)-4,4'-difluoro-4-bora-3a,4a-diaza-s-indacene (DSDMHDAB), in which hydrophilic groups are introduced to keep the fluorophore and recognition domain outside the cell and a hydrophobic C16 alkyl chain acts as the membrane anchor. Based on this design, NO released out of the cells has been visualized on the outer surface of the plasma membrane. Using RAW 264.7 cells and ECV-304 cells as models, the diffusion of NO across the plasma membrane has been directly observed. The amphiphilic design strategy of fluorescent probes holds great promise for developing fluorescent imaging probes to study the release behaviors of other endogenous gasotransmitters.

Live-cell imaging approaches for the investigation of xenobiotic-induced oxidant stress

BACKGROUND

Oxidant stress is arguably a universal feature in toxicology. Research studies on the role of oxidant stress induced by xenobiotic exposures have typically relied on the identification of damaged biomolecules using a variety of conventional biochemical and molecular techniques. However, there is increasing evidence that low-level exposure to a variety of toxicants dysregulates cellular physiology by interfering with redox-dependent processes.

SCOPE OF REVIEW

The study of events involved in redox toxicology requires methodology capable of detecting transient modifications at relatively low signal strength. This article reviews the advantages of live-cell imaging for redox toxicology studies.

MAJOR CONCLUSIONS

Toxicological studies with xenobiotics of supra-physiological reactivity require careful consideration when using fluorogenic sensors in order to avoid potential artifacts and false negatives. Fortunately, experiments conducted for the purpose of validating the use of these sensors in toxicological applications often yield unexpected insights into the mechanisms through which xenobiotic exposure induces oxidant stress.

GENERAL SIGNIFICANCE

Live-cell imaging using a new generation of small molecule and genetically encoded fluorophores with excellent sensitivity and specificity affords unprecedented spatiotemporal resolution that is optimal for redox toxicology studies. This article is part of a Special Issue entitled Air Pollution, edited by Wenjun Ding, Andrew J. Ghio and Weidong Wu.

Fluorescent probes for real-time measurement of nitric oxide in living cells

Nitric oxide (NO) is an important signaling molecule in biology. Both NO excess and insufficiency have been implicated in numerous physiological and pathological conditions. In order to study the diverse biological roles of NO in cells and tissues, many techniques have been developed for assaying NO. Recently, new generations of fluorescent probes have become indispensible tools for the study of NO biology because of their sensitivity, selectivity, spatiotemporal resolution, and experimental feasibility. Rational application of these probes in the study requires the understanding of the molecular mechanism that the probes are involved in. In this review, we will present an arsenal of fluorescent probes used to detect NO in living cells and animal tissues. We will also discuss the molecular mechanisms, actualities and prospects of fluorescent probes in detecting NO in cell biology.

Novel pyrromethene dyes with N-ethyl carbazole at the meso position: a comprehensive photophysical, lasing, photostability and TD-DFT study

Two novel BODIPY (pyrromethene, PM) dyes containing N-ethyl carbazole at the meso position are synthesized and their photophysical properties in different solvents and the photochemical stabilities and laser performances in n-heptane are investigated. The n-heptane solution of the dyes was used as a gain medium in a constructed narrow band dye laser, pumped by a Q-switched (10 Hz) frequency-doubled (532 nm) Nd:YAG laser and the results gave enhanced photo stabilities and similar peak efficiencies of the synthesized dyes as compared to parent dye PM567. When substituted at the meso position with N-alkyl carbazole, photostability is found to be increased in comparison to PM567, and also when substituted at 2- and 6-positions with a benzyl group instead of an ethyl group along with N-ethyl carbazole at the meso position, the photo stability is further increased in n-heptane. A comprehensive study on structural, photophysical and electronic properties of dyes by means of DFT and TD-DFT in the solvents of various polarities has revealed remarkable characteristics of the BODIPY chromophore.

Congeners of Pyrromethene-567 Dye: Perspectives from Synthesis, Photophysics, Photostability, Laser, and TD-DFT Theory

In an attempt to develop photostable and efficient BODIPY (PM) dyes for use in liquid dye lasers, three new congeners of widely used laser dye, PM567, were synthesized and their photophysical properties in various organic solvents, laser performances, and photostabilities in a selected solvent, 1,4-dioxane, have been investigated using a frequency doubled Q-switched (10 Hz) Nd:YAG laser at 532 nm. The results of photostability study in nonpolar 1,4-dioxane revealed the remarkable enhancement in stability of the novel dyes compared to that of PM567 as well as improved laser performances. Cyclic voltammetry study strongly supports the observed enhancement in photostability of the novel dyes compared to that of PM567. The observed properties of the novel dyes in relation to those of PM567 have been rationalized by extensive use of DFT and TD-DFT using the B3LYP/6-31G(d) method of theory.

Synthetic fluorescent probes for studying copper in biological systems

The potent redox activity of copper is required for sustaining life. Mismanagement of its cellular pools, however, can result in oxidative stress and damage connected to aging, neurodegenerative diseases, and metabolic disorders. Therefore, copper homeostasis is tightly regulated by cells and tissues. Whereas copper and other transition metal ions are commonly thought of as static cofactors buried within protein active sites, emerging data points to the presence of additional loosely bound, labile pools that can participate in dynamic signalling pathways. Against this backdrop, we review advances in sensing labile copper pools and understanding their functions using synthetic fluorescent indicators. Following brief introductions to cellular copper homeostasis and considerations in sensor design, we survey available fluorescent copper probes and evaluate their properties in the context of their utility as effective biological screening tools. We emphasize the need for combined chemical and biological evaluation of these reagents, as well as the value of complementing probe data with other techniques for characterizing the different pools of metal ions in biological systems. This holistic approach will maximize the exciting opportunities for these and related chemical technologies in the study and discovery of novel biology of metals.

Modular logic gates: cascading independent logic gates via metal ion signals

Systematic cascading of molecular logic gates is an important issue to be addressed for advancing research in this field. We have demonstrated that photochemically triggered metal ion signals can be utilized towards that goal. Thus, independent logic gates were shown to work together while keeping their identity in more complex logic designs. Communication through the intermediacy of ion signals is clearly inspired from biological processes modulated by such signals, and implemented here with ion responsive molecules.