Ratiometric fluorescence chemosensors for copper(II) and mercury(II) based on FRET systems

A system based on FRET mechanism, comprising a coumarin donor and a rhodamine acceptor, was developed for the selective and quantitative detection of metal ions. Fluorescent chemosensors RCs, linked by 1,2-diethylamine, exhibit significant fluorescence enhancement and excellent selectivity toward Cu²⁺. Fluorescent probes CRB and CR6G, linked by hydrazide, function as ratiometric receptors for Cu²⁺ chromogentically and fluorogentically in organic-aqueous media. Furthermore, the characteristic rhodamine-based fluorescence response of CRB (excitation at 550 nm) exhibits high selectivity for Hg(II). The construction of this kind of universal FRET system opens a broader prospect for future design of ratiometric fluorescent probes.     

A simple click generated probe for highly selective sequential recognition of Cu(II) and pyrophosphate

A click generated quinoline derivative (1) has been synthesized and used as a fluorescent probe for sequential recognition of Cu²⁺ and pyrophosphate (PPi) in DMSO/H₂O (1:1, v/v, HEPES 20 mM, pH = 7.4) solution. Probe 1 displays high selectivity to Cu²⁺ ions, and the in-situ prepared probe 1-Cu²⁺ exhibits high selectivity toward pyrophosphate (PPi) with emission recovery of probe 1. Therefore, 1-Cu²⁺ complex can be applied as a fluorescence turn-on probe for PPi with high selectivity and sensitivity.     

Synthesis and photophysical properties of novel push-pull-type solvatochromic 7-deaza-2′-deoxypurine nucleosides

We have synthesized two novel push-pull-type fluorescent 7-deazapurine nucleosides, ^CNZA and ^CNZG, and investigated their photophysical properties. In particular, ^CNZA was found to exhibit a remarkable solvatofluorochromicity (Δλ_fl.max = 60 nm). We incorporated ^CNZA into oligonucleotides and found that ^CNZA can form a stable base pair with both thymine and cytosine. Such environmentally sensitive fluorescent nucleosides have a potential as a fluorescence sensor for structural studies of nucleic acids.     

Anthracene-labeled pyridinium-based symmetrical chiral chemosensor for enantioselective recognition of l-tartrate

A new anthracene-based chiral chemosensor 1 has been designed and synthesized. l-Valine has been used as the chiral source in the design. The chemosensor 1 has been established as an efficient enantioselective sensor for l-tartrate. While in the presence of l-tartrate the fluorescent sensor 1 in DMSO exhibits considerable increase in emission, the isomeric tartrate brings relatively small change. The enantiomeric fluorescence difference ratio (ef) has been determined to be 29.38.     

Neutral penta-coordinated diorganotin(IV) complexes derived from ortho-aminophenol Schiff bases: Synthesis, characterization and molecular structures

The one pot reactions carried among ortho-aminophenol, R₂SnO (R = Me or Ph) and acetyl acetone, 2-hydroxyacetophenone and 2-hydroxy-3-methylacetophenone led to six new diorganotin(IV) compounds Me₂SnL1 (1), Ph₂SnL1 (2), Me₂SnL2 (3) Ph₂SnL2 (4), Me₂SnL3 (5) and Ph₂SnL3 (6) (H2L1 = 2-(3-hydroxy-1-methyl-but-2-enylideneamino)-phenol, H2L2 and H2L3 = 2-[1-(2-hydroxyaryl)alkylideneamino]-phenol) in good yields. Combination of IR, ¹H, ¹³C and ¹¹⁹Sn NMR and X-ray diffraction techniques along with elemental analyses evidenced the formation of penta-coordinated monomeric species. The crystal structures of ligand H2L1 and complexes 1, 3 and 4 were determined by single crystal X-ray diffraction study. In the solid state, the ligand H2L1 exists as keto-enamine tautomeric form. There are N-H…O intra-molecular hydrogen bonds between amine and carbonyl groups. Diorganotin(IV) complexes 1, 3 and 4 are monomers with TBP (trigonal bipyramidal) geometry surrounding the tin atom. The O, N, O- tridentate ligand places its two oxygen donating atoms in the axial positions, and the nitrogen atom occupies one equatorial position. The two R groups attached to tin occupy the other two equatorial positions. The solution structures were predicted by ¹¹⁹Sn NMR spectroscopy.     

Fluorescence properties of polyamines bearing two terminal quinoline fragments in water

Fluorescence properties of polyamines bearing two terminal quinoline fragments with different polyamine chain length, such as ethylenediamine (L0), diethylenetriamine (L1), and triethylenetetramine (L2), have been studied in water. These ligands show Zn²⁺-induced fluorescence enhancement, while showing almost no enhancement with other cations. However, stability constants for Zn²⁺ coordination and fluorescence response against Zn²⁺ depend strongly on the polyamine chain length. The chain length also affects the fluorescence wavelength. The Zn²⁺-L1 and Zn²⁺-L2 complexes show emission at 410 nm, while Zn²⁺-L0 complexes show a blue-shifted emission at 375 nm due to the partial charge transfer from the excited state quinoline to the Zn²⁺ center.     

Synthesis of novel push-pull-type solvatochromic 2′-deoxyguanosine derivatives with longer wavelength emission

We synthesized novel push-pull-type fluorescent guanosine derivatives, ^CNG and ^AcG containing 1,6- and 2,7-disubstituted pyrene chromophores. 1,6-Disubstituted pyrene derivatives, 1,6-^CNG (3b) and 1,6-^AcG (3c), exhibited highly solvatochromic fluorescence emission at longer wavelength (∼540 nm). The environmentally sensitive fluorescent deoxyguanosines such as 3b and 3c can be used as powerful tools for structural studies of nucleic acids and molecular diagnostics.     

Synthesis and photophysical properties of 8-arylbutadienyl 2′-deoxyguanosines

We have developed novel push-pull-type 8-arylbutadienyl 2′-deoxyguanosine derivatives, ^ABG (1a) and ^CBG (1b). These nucleosides exhibit strong solvent polarity dependent fluorescence emission at long wavelength (ca. 490-550 nm). These environmentally sensitive fluorescent deoxyguanosines are powerful tools for structural studies of nucleic acids and also in molecular diagnostics.     

A logic gate-based fluorogenic probe for Hg detection and its applications in cellular imaging

A new colorimetric and fluorogenic probe (RN₃) based on rhodamine-B has been successfully designed and synthesized. It displays a selective response to Hg²⁺ in the aqueous buffer solution over the other competing metals. Upon addition of Hg²⁺, the solution of RN₃ exhibits a ‘naked eye’ observable color change from colorless to red and an intensive fluorescence with about 105-fold enhancement. The changes in the color and fluorescence are ascribed to the ring-opening of spirolactam in rhodamine fluorophore, which is induced by a binding of the constructed receptor to Hg²⁺ with the association and dissociation constants of 0.22 × 10⁵ M⁻¹ and 25.2 μM, respectively. The Job's plot experiment determines a 1:1 binding stoichiometry between RN₃ and Hg²⁺. The resultant “turn-on” fluorescence in buffer solution, allows the application of a method to determine Hg²⁺ levels in the range of 4.0–15.0 μM, with the limit of detection (LOD) calculated at 60.7 nM (3σ/slope). In addition, the fluorescence ‘turn-off’ and color ‘fading-out’ happen to the mixture of RN₃-Hg²⁺ by further addition of I⁻ or S²⁻. The reversible switching cycles of fluorescence intensity upon alternate additions of Hg²⁺ and S²⁻ demonstrate that RN₃ can perform as an INHIBIT logic gate. Furthermore, the potential of RN₃ as a fluorescent probe has been demonstrated for cellular imaging.     

Imino-enamine tautomerism and dynamic prototropy in 1-imino-3-amino-1H-indens

The tautomeric structures and dynamic prototropic behavior of the products 1 and 2 obtained in the condensation reaction of 1,3-indandione and 2-pyridyl-1,3-indandione with p-toluidine, respectively, were investigated by ¹H NMR spectroscopy and X-ray analysis. In the solid state, compound 1 is in an imino-enamine tautomeric form, whereas in solution it coexists with an imino-imino tautomeric form. Dynamic 1,5-prototropic interconversion of the imino-enamine form was revealed to be very fast at room temperature by temperature-dependent ¹H NMR spectra. For 2, the imino-enamine form is the only species present in solution. The hydrogen of the enamine NH is hydrogen-bonded intramolecularly with the nitrogen in the pyridine ring. When the temperature is raised, the NH proton enters into dynamic 1,5-migration, which is accompanied by internal rotation around the pivot bond, which changes the hydrogen-bonding sites. For the condensation product 3 of 2-(2-quinolyl)-1,3-indandione with p-butylaniline, dynamic behavior similar to that found in 2 was observed also in ¹³C NMR spectra.     

NMR investigation and theoretical studies on the tautomerism of β,β′-tricarbonyl compounds

The behaviour on keto–enol tautomeric equilibration of ethyl 2-benzoyl-5-(2-furyl)-3-hydroxy-penta-2,4-dienoate (1) and ethyl 2-acetyl-3-hydroxy-5-phenyl-penta-2,4-dienoate (2) was investigated by ¹H and ¹³C NMR spectroscopy in different solvents and BP86/TZVP density functional theory computations. The spectral assignment to enol and keto tautomers was performed with one- and two-dimensional techniques. The percentage of the keto form in the tautomeric equilibrium depends on solvents and rises by increasing solvent polarity. The enol–enol tautomerism is also discussed on the basis of the coupling constants ²J_C,OH, ³J_C,OH and ⁴J_H,OH, respectively.     

A highly selective and sensitive fluorescein-based chemodosimeter for Hg2+ ions in aqueous media

A new fluorescein-based chemodosimeter (II) for Hg²⁺ ion was designed and synthesized, and it displayed excellent selective and sensitive toward Hg²⁺ ion over other commonly metal ions in aqueous media. II was a colorless, non-fluorescent compound. Upon addition of Hg²⁺ to the solution of II, the thiosemicarbazide moiety of II would undergo an irreversible desulfurization reaction to form its corresponding oxadiazole (IV), a colorful and fluorescent product. During this process, the spirocyclic ring of II was opened, causing instantaneous development of visible color and strong fluorescence emission in the range of 500-600 nm. Based on the above mechanism, a fluorogenic Hg²⁺-selective chemodosimeter was developed. The fluorescence increase is linearly with Hg²⁺ concentration up to 1.0 μmol L⁻¹ with a detection limit of 8.5 × 10⁻¹⁰ mol L⁻¹ (3σ). Compared with the rhodamine-type chemodosimeter, II is more stable in aqueous media and exhibits higher sensitivity toward Hg²⁺. The findings suggest that II will serve as a practical chemodosimeter for rapid detection of Hg²⁺ concentrations in realistic media.     

A pyrene-benzthiazolium conjugate portraying aggregation induced emission,a ratiometric detection and live cell visualization of HSO3

The present study deals with the photophysical property of a pyrene-benzthiazolium conjugate R1, as a strong intramolecular charge transfer (ICT) probe exhibiting long wavelength emission in the red region. Unlike traditional planar polyaromatic hydrocarbons whose aggregation generally quenches the light emission, the pyrene based R1 was found to display aggregation-induced emission (AIE) property along with simultaneous increase in its quantum yield upon increasing the water content of the medium. The R1 exhibits high specificity towards HSO₃⁻/SO₃²⁻ by interrupting its own ICT producing there upon a large ratiometric blue shift of ∼220 nm in its emission spectrum. The lowest detection limit for the above measurement was found to be 8.90 × 10⁻⁸ M. The fluorescent detection of HSO₃⁻ was also demonstrated excellently by test paper strip and silica coated TLC plate incorporating R1. The live cell imaging of HSO₃^─ through R1 in HeLa cells was studied using fluorescence microscopic studies. The particle size and morphological features of R1 and R1-HSO₃⁻ aggregates in aqueous solution were characterized by DLS along with SEM analysis.     

1,N-Etheno-2′-deoxytubercidin and pyrrolo-C: synthesis, base pairing, and fluorescence properties of 7-deazapurine nucleosides and oligonucleotides

The synthesis of 1,N⁶-etheno-7-deaza-2′-deoxyadenosine (12b) which was prepared from 7-deaza-2′-deoxyadenosine (5a) with chloroacetaldehyde is described. Also the regioselective glycosylation of the 7-deazapurine-2-one at nitrogen-1 (19) furnishing the pyrrolo-C nucleoside 7a is reported and a side chain derivative with a terminal triple bond (7d) is prepared. The fluorescence properties of these nucleosides and related compounds were determined. The etheno nucleoside 12b is strongly fluorescent showing a Stokes shift of 134 nm and a quantum yield of Φ=0.53. It proved to be stable, both in acidic and in alkaline medium while the parent purine compound 10b is labile under both conditions. Compound 12b was converted into its phosphoramidite 14 and was incorporated into oligonucleotides. Compound 12b destabilizes oligonucleotide duplexes when it is located in the center of the molecule; it stabilizes when it is incorporated in the terminal base pair or acts as an overhanging nucleoside. Temperature-dependent fluorescent measurements yielded sigmoidal melting profiles when compound 12b is stacked to the terminal base pair while a linear decrease of the fluorescence is observed when the molecule is located opposite to the four canonical nucleosides in the center of the duplex.     

Dual responsive chemosensors for anions: the combination of fluorescent PET (Photoinduced Electron Transfer) and colorimetric chemosensors in a single molecule

The design and synthesis of two novel fluorescent PET anion sensors is described, based on the principle of ‘fluorophore-spacer-(anion)receptor’. The sensors 1 and 2 employ simple diaromatic thioureas as anion receptors, and the fluorophore is a naphthalimide moiety that absorbs in the visible part of the spectrum and emits in the green. Upon recognition of anions such as F⁻ and AcO⁻ in DMSO, the fluorescence emission of 1 and 2 was ‘switched off’, with no significant changes in the UV-vis spectra. This recognition shows a 1:1 binding between the receptor and the anions. In the case of F⁻, further additions of the anion, gave rise to large changes in the UV-vis spectra, where the λ_max at 455 nm was shifted to 550 nm. These changes are thought to be due to the deprotonation of the 4-amino moiety of the naphthalimide fluorophore. This was in fact found to be the case, using simple naphthalimide derivatives such as 6. Sensors 1 and 2 can thus display dual sensing action; where at low concentrations, the fluorescence emission is quenched, and at higher concentrations the absorption spectra are modulated.     

A reversible fluorescent–colorimetric Schiff base sensor for Hg ion

A simple (R)-(−)-2-phenylglycinol functionalized Schiff base L1 and its characterization as a fluorescent–colorimetric sensor for Hg²⁺ ion are described. The UV–vis and fluorescence analysis in methanol and aqueous solution show complex formation between L1 and Hg²⁺ ion with a micromolar association constant. Competition experiments performed for the acetate salts of Hg²⁺, Zn²⁺, Co²⁺, Pb²⁺, Cd²⁺, Mn²⁺, Cu²⁺, Ni²⁺, and Ba²⁺ revealed that compound L1 exhibits high selectivity toward Hg²⁺ displaying a color change easily detectable by naked-eye and a turn-off fluorescent effect due to a chelation-enhanced quenching (CHEQ) mechanism. Moreover, addition of EDTA to L1–Hg²⁺ recovers the fluorescence and color offering receptor L1 as a reversible sensor for real-time applications.     

A ratiometric and exclusively selective Cu fluorescent probe based on internal charge transfer (ICT)

A new ratiometric and exclusively selective fluorescent probe N-butyl-4,5-di[N-(phenyl)-2-(amino)-acetamino]-1,8-naphthalimide (1) was designed and synthesized on the basis of the mechanism of internal charge transfer (ICT). The probe 1 showed exclusively selectivity for Cu^II in the presence of a variety of other metal ions in aqueous ethanol solutions and the binding mode of probe 1 with Cu^II was 1:1 metal-ligand complex. Fluorescent emission spectra of probe 1 in the presence of Cu^II showed a 50 nm blue shift, which is from 521 nm to 471 nm. Furthermore, probe 1 shows the same fluorescent change with the Cu^II in living cells.     

Synthesis and luminescent properties of Zn complex based on 8-hydroxyquinoline group containing 3,5-bis(trifluoromethyl) benzene unit with unique crystal structure

The use of a novel 2-substituted-8-hydroxyquinoline ligand (E)-2-{2-[3,5-bis (trifluoromethyl)phenyl]ethenyl}-8-hydroxyquinoline(4, BFHQ) characterized by EIMS, ¹H NMR spectroscopy, elemental analysis, and FTIR spectroscopy enabled the isolation of a trimeric Zn(II) complex with the formula Zn₃(BFHQ)₆ (5, Scheme 1). X-ray structural analysis shows that 5 exhibits a trinuclear core, which is bridged by four 8-hydroxyquinoline rings. The trinuclear core is surrounded by three pairs of BFHQ ligands with offset π-π stacking, showing propeller-like molecular structure. The aggregation behavior of Zn(AcO)₂ and the ligand 4 in solution was investigated by UV-vis. The luminescence properties of compound 5 were investigated by UV-vis and fluorescence spectra at room temperature. The experimental results show that the complex 5 emits yellow luminescence at 553 nm (λ_{em, max}) in DMSO solution and at 610 nm (λ_em, max) in solid state. The thermogravimetric analysis was carried out to examine the thermal stability.     

Ratiometric fluorescent and colorimetric sensors for Cu based on 4,5-disubstituted-1,8-naphthalimide and sensing cyanide via Cu displacement approach

Two 4,5-disubstituted-1,8-naphthalimide derivatives 1 and 2 were synthesized as ratiometric fluorescent and colorimetric sensors for Cu²⁺, respectively. In 100% aqueous solutions of 1, the presence of Cu²⁺ induces a strong and increasing fluorescent emission centered at 478 nm at the expense of the fluorescent emission of 1 centered at 534 nm. Compound 2 senses Cu²⁺ by means of a colorimetric (primrose yellow to pink) method with a thorough quench in emission attributed to the deprotonation of the secondary amine conjugated to the naphthalimide fluorophore. 1-Cu²⁺ and 2-Cu²⁺ sense cyanide in ratiometric way via colorimetric and fluorescent changes.     

Synthesis, photophysical and photochemical properties of highly soluble phthalocyanines substituted with four 3,5-dimethylpyrazole-1-methoxy groups

The synthesis and characterization of new peripherally tetra-3,5-dimethylpyrazole-1-methoxy substituted metal-free (4), zinc (5), nickel (6), cobalt (7), copper (8) and lead (9) phthalocyanines are described for the first time in this study. The photophysical (fluorescence quantum yields and fluorescence lifetimes) and photochemical (photodegradation and singlet oxygen quantum yields) properties of metal-free (4), zinc (5) and lead (9) phthalocyanines are studied in dimethylsulfoxide (DMSO). Nickel (6), cobalt (7) and copper (8) phthalocyanines (6-8) did not evaluate for this purpose due to transition metal and paramagnetic behavior of central metals in the phthalocyanine cavity. The fluorescence quenching behavior of metal-free (4), zinc (5) and lead (9) phthalocyanines are also investigated. The fluorescence emissions of these phthalocyanines are effectively quenched by 1,4-benzoquinone in DMSO.