Crystal structure of Eu-doped M3MgSi2O8 (M: Ba, Sr, Ca) compounds and their emission properties

Emission properties of Eu²⁺-doped M₃MgSi₂O₈ (M: Ba, Sr, Ca) are discussed in terms of the crystal structure. When Ba²⁺ ions account for over one third of M²⁺ ions, M₃MgSi₂O₈ crystallizes in glaserite-type trigonal structure, while Ba-free compounds crystallize in merwinite-type monoclinic structure. Under UV excitation, the Eu²⁺-doped glaserite-type compounds exhibit an intense blue emission assigned to 5d-4f electron transition at about 435 nm, regardless of the molar ratio of Ba²⁺, Sr²⁺ and Ca²⁺ ions. By contrast, the Eu²⁺-doped merwinite-type compounds show an emission color sensitive to the ratio. A detailed analysis of the emission spectra reveals that the emission chromaticity for the Eu²⁺-doped M₃MgSi₂O₈ is composed of two emission peaks reflecting two different sites accommodating M²⁺ ion.     

Complex formation between azacrown derivatives of dibenzylidenecyclopentanone and alkali-earth metal ions

The complex formation of 2,5-bis[4-(1,7,10,13-tetraoxa-4-azacyclopentadec-4-yl)benzylidene]cyclopentanone and several model compounds, prospective metal-sensitive fluorescent probes, with Mg²⁺, Ca²⁺, and Ba²⁺ ions in acetonitrile was studied. The azacrown derivatives of dibenzylidenecyclopentanone have two complex formation centers, azacrown cycle and carbonyl group. The sequence of binding to these sites is different for different ions. The efficient ejection of the Ca²⁺ and Ba²⁺ ions from their complexes with azacoronands was observed in the excited state, whereas in the case of the Mg²⁺ ion, this process occurred only partially.     

Zn selective luminescent ‘off-on’ probes derived from diaryl oxadiazole and aza-15-crown-5

New photo-induced electron transfer (PET) probes OMOX and OBOX, carrying an additional binding site in the form of ‘oxadiazole nitrogen’ have been designed to evaluate binding interactions with biologically significant Li⁺, Na⁺, K⁺, Ca²⁺, Mg²⁺, and Zn²⁺ including environmentally toxic Ba²⁺ and Cd²⁺ using optical spectral techniques. While Li⁺, Na⁺, and K⁺ did not appreciably perturb either the absorption or emission spectra, Ba²⁺, Ca²⁺, Mg²⁺, Zn²⁺, and Cd²⁺ induced slight red shifts (2-8 nm) in the UV-visible spectra as well as pronounced chelation induced enhanced fluorescence (CHEF). Both OMOX and OBOX exhibited the highest CHEF in contact with the zinc ion, whereas Ba²⁺, Ca²⁺, Mg²⁺, and Cd²⁺ induced relatively less emission enhancements. OBOX, which is a poorer emitter (Φ_f=0.0062) than OMOX (Φ_f=0.015), showed highly promising 160-fold emission enhancement in the presence of Zn²⁺. Potential, therefore is available in OBOX to function as a selective luminescent ‘off-on’ sensor for Zn²⁺ in the presence of coordinatively competing Ba²⁺, Ca²⁺, Mg²⁺, and Cd²⁺ ions.     

New PCT probes featuring N-phenylmonoaza-18-crown-6 and aryl/pyridyl oxadiazoles: optical spectral studies of solvent effects and selected metal ions

Aryl/pyridyl oxadiazole chromophores 6, 8 and 10, carrying N-phenyl aza-18-crown-6 have been synthesized as new photo-induced charge transfer (PCT) probes. While, the absorption spectra of the hosts experienced a slight negative solvatochromism, however the emission bands were dramatically red shifted (Stokes shifts up to 178 nm) in solvents of increasing polarity. Among the metal ions tested, Li⁺, Na⁺, K⁺ and Mg²⁺ did not appreciably perturbed the optical properties of the hosts. On the other hand, Ba²⁺ and to a lesser extent Ca²⁺ induced marked blue shifts in both the absorption and emission spectra of the hosts. The magnitude of cation induced spectral blue shifts corresponded with the increasing acceptor strength of the attached aryl/pyridyl groups in the host molecules. The blue shifts and the stability constants were found to follow the order Ba²⁺ > Ca²⁺ ? Mg²⁺ > Na⁺ > Li⁺ > K⁺. Competitive experiments performed with a matrix of ions also revealed superior binding affinity of Ba²⁺ with all the hosts examined. Noteworthily, the deep yellow solution (λ_max, 386 nm) of the host 10 was completely bleached (λ_max, 320 nm), in the company of Ba²⁺ thereby allowing the naked eye detection of this ion.     

Metal Ion Interaction of Two New Tritopic N2O13 Macrobicycles with B15C5 Moieties in Acetonitrile Solution

The synthesis of two new tritopic crown ligands (L1 and L2) bearing two benzo-15-crown-5 lateral moieties linked through a dibenzo-trioxa chain together with their interaction with metal ions, in acetonitrile and acetonitrile–water (50%, v/v) solutions is reported. The influence of K⁺, Na⁺, Li⁺, Ca²⁺, Ba²⁺, Cu²⁺, Zn²⁺, Cd²⁺, Pb²⁺ and Al³⁺, on the spectroscopic properties of these diaza-polyoxa ligands was investigated by absorption spectrophotometry and in some cases by fluorescence emission spectroscopy. Coordination with alkaline (Na⁺, K⁺ and Li⁺) and alkaline earth (Ca²⁺and Ba²⁺) metal ions is assumed to be weak with both macrobicyclic ligands, while the interaction with both imine and amine derivatives causes a minor effect in the absorption spectra. Coordination with Cu²⁺, Zn²⁺ and Pb²⁺ in acetonitrile solution causes a major change in the absorption spectra of the chromophores. In the case of Cu²⁺, addition of the metal to L1 or L2 leads to a blue–violet complex in solution with an absorbance maximum centred at 590 nm. Interaction of the Schiff-base L1 with Pb²⁺ leads to a short wavelength shift in the absorption bands, comparable with the ZnL1 complex. Presence of transition metal ions such as Co²⁺, Ni²⁺and Cd²⁺ do not remarkably affect the absorption spectra of L1 and L2 in solution. Trivalent aluminium has a modest effect in the absorption bands of both N₂O₁₃ donor set bismacrocyclic ligands. The fluorescence study of L2 in the presence of Na⁺, K⁺, Ca²⁺, Ba²⁺, Co²⁺, Cu²⁺, Ni²⁺, Pb²⁺ and Al³⁺shows that Cu²⁺, Pb²⁺ and Al³⁺ complexes form non-fluorescent complexes.     

Two electroactive ferrocenyl chalcones as original optical chemosensors for Ca and Ba cations in CH3CN

The optical study of ferrocenyl ligands 1-2 is presented, and reveals several interesting points. Contrary to their monosubstituted counterparts, these ligands exhibit fluorescence properties in acetonitrile. They can detect calcium, and also barium, by four different techniques: NMR, electrochemistry, UV-Vis absorption spectroscopy and fluorimetry in the same solvent. Each ligand detects both salts in the same manner by UV-Vis absorption and by fluorimetry. The response depends on the nature of the N terminal groups of the ligand. In each case, the ligand-calcium interaction is complex and involves 3-5 species in equilibrium in solution. Their association constants have been determined by fitting the UV-Vis data. Remarkably, for 2 and the calcium salts, nearly the same set of association constants can be used to fit not only the UV-Vis data obtained with calcium triflate (in a restricted range of concentration) or with calcium perchlorate, but also the NMR data obtained with calcium triflate. Interestingly, these results strengthen the fact that, in this family of compounds, the azacrown derivatives are less sensitive to high calcium triflate concentrations than their simple N-alkyl homologues. It is noteworthy that the complex non-monotonous fluorescence behaviour of compounds 1 and 2 upon Ca²⁺ or Ba²⁺ addition is quite original for ferrocenyl chalcones. These ligands constitute scarce examples of multi-signalling fluorescent ferrocenyl chemosensors for Ca²⁺ and Ba²⁺ cations in CH₃CN.     

Complexation of di-benzyloxy ether derivatives of calix[4]arene with alkali earth metal cations

The synthesis and complexing abilities of 26,28-bis-benzyloxy-25,27-dihydroxy-5,11,17,23-tetra-tertbutyl-calix[4]arene towards alkali earth metal ions Mg²⁺, Ca²⁺, Sr²⁺, and Ba²⁺ in a methanol-chloroform mixture have been evaluated at 25°C, using UV-Vis spectrophotometric techniques. The results showed that the ligand is capable to complex all alkali earth cations by 1:1 metal to ligand ratios. The selectivity presented considering the calculated stability constants are in the order Mg²⁺ > Ca²⁺ > Sr²⁺ > Ba²⁺ towards the ligand.     

Tautomeric crown-containing chemosensors for alkali-earth metal cations

Crown-containing arylimines of 5-hydroxy- and 5-hydroxy-6-nitro-2,3-diphenylbenzo[b]furan-4-carbaldehydes were synthesized and their structure, spectral, luminescent, and complexing properties were studied by means of ¹H and ¹³C NMR, IR, UV, and mass-spectrometry. In solution, these compounds exist as equilibrium mixtures of benzenoid and quinoid tautomers. The relative concentration of the quinoid form increases in the order of solvents: toluene, acetonitrile, 2-propanol, chloroform. The presence in the benzo[b]furan moiety of a strong withdrawing 6-NO₂ group favors stabilization of the quinoid tautomer. This finding is in accord with the results of the DFT B3LYP/6-311++g(d,p) calculations. Complexation of 5-hydroxy-6-nitro-2,3-diphenyl-1-benzofuran-4-carbaldehyde crown-containing imines with alkali and alkali-earth metal ions shifts the tautomeric equilibrium to the quinoid forms and is accompanied by blue shifts of the emission spectra. The Schiff bases obtained represent a new type of fluorescent tautomeric chemosensor for Mg²⁺, Ca²⁺ (benzo-15-crown-5 derivative), and Ba²⁺ (benzo-18-crown-6 derivative) displaying diagnostic changes in both absorption and emission spectra.     

Synthesis,spectroscopic and spectrophotometric study of BODIPY appended crown ether sensor for ion detection

A novel fluoroionophore compound was synthesized from a boron dipyrromethene (BODIPY) fluorophore and 4′-formylbenzo-15-crown-5 ionophore groups. Photophysical properties of the BODIPY-crown compound were studied with UV–Vis and fluorescence spectroscopy. The effect of metalic cations (Li⁺, Na⁺, K⁺, Mg²⁺, Ca²⁺, Ba²⁺, Al³⁺, Fe³⁺, Cu²⁺, Co²⁺, Zn²⁺, Ag⁺, Hg²⁺, Pb²⁺) on the absorption and fluorescence spectra of compound 2 was investigated. Blue shifts were detected in UV–Vis spectra upon addition of some metal ions (Al³⁺ > Fe³⁺ > Na⁺). At the same time, the emission intensity of this complex increased due to binding of Na⁺ ion to the benzo crown cavity. Additionally, a decrease in the intensity of the 630 nm emission peak and an increase in the intensity of the 570 nm emission peak was observed in the fluorescence emission spectra following addition of Al³⁺ and Fe³⁺ ions.     

Benzimidazole-based optical probe for selective detection of multiple-cations via dual-channel analysis

A benzimidazole-based optical probe having pendant carboxyl, amine, and imine groups as ionophore has been prepared for screening various metal ions. The 4-(1H-benzo[d]imidazol-2-yl)-1H-imidazole-5-carboxylic acid (1) has been obtained in good yield and characterized by full battery of complementary physico-chemical techniques including ¹H NMR, UV-Vis, fluorescence spectroscopy, and single crystal X-ray crystallography. Metal ion-binding properties of 1 have been studied using ppm level concentration of representative alkali metal (Na⁺, K⁺), alkaline earth metal (Mg²⁺, Ca²⁺), and transition metal (Zn²⁺, Co²⁺, Fe³⁺, Cd²⁺, Hg²⁺, Pb²⁺, Cu²⁺, Ag⁺) ions and the output signal was monitored via two different channels viz chromogenically and fluorogenically. Selective recognition of Hg²⁺ has been explored with absorption spectra whereas emission spectra of 1 display differential response for multiple cations at parts-per-million (ppm) level concentration that allow selective detection of Ca²⁺, Mg²⁺, and Na⁺ ions. The results have been discussed in light of selectivity, sensitivity, response time, mode of binding/interactions, and sensing properties.     

Biomimetic Method Synthesis of HgS/Polyurethane Composite Film and Its Sensing Properties to Ba2+

Polyurethane-conjugated HgS nanocrystals with tunable sizes prepared by using biomimetic method. The obtained HgS nanoparticles with good dispersibility were characterized by Fourier transform infrared. Scanning electron microscopy are used to envisage the binding of nanoparticles with functional groups. The polyurethane molecules can control nucleation and growth of HgS crystals by binding on the surface of nanocrystals to stabilize nanoparticles. Quantum confinement effect of polyurethane-conjugated HgS nanocrystals was confirmed by UV-Vis spectra. The nanoparticles exhibit a well-defined emission feature at about 291 nm. The fluorescence results reveal that the PU/HgS nanoparticles film is sensitive to Ba²⁺, and a small amount of Ba²⁺ makes the emissions increase rapidly. The emission is hardly affected by other common ions in water. The nanocomposite film is possible to become a special sensor material for Ba²⁺.     

Synthesis and optical characterization of novel enantiopure BODIPY linked azacrown ethers as potential fluorescent chemosensors

Two novel enantiopure BODIPY linked azacrown ether chemosensors were prepared by reacting 3-chloro-5-methoxyBODIPY with new enantiopure monoaza-18-crown-6 ether ligands bearing two methyl and isobutyl groups on their chiral centres, respectively. The latter compounds were synthesized starting from optically active tetraethylene glycols in six steps. The operation of chemosensors is based on the intramolecular charge transfer (ICT) process. They exhibit pronounced off-on type fluorescent responses to some metal ions and chiral primary aralkyl ammonium ions, in particular to Ca²⁺ and Pb²⁺. Even in the case of 1:1 ratio of Ca²⁺ and Pb²⁺ ions to the ionophores, the fluorescence intensities and quantum yield values increased more than 10-fold, as well as both the absorption and emission bands were blue-shifted by about 20-30 nm (hipsochromic effect) in acetonitrile. The formation of relatively stable complexes allowed the determination of log K_s values by the mole-ratio method.     

Novel Ionophores for Barium‐Selective Electrodes: Synthesis and Analytical Characterization

A novel way of synthesis is developed for the Ba²⁺ selective neutral Ionophore 2a : 2,2′‐[1,2‐phenylenebis(oxyethane‐2,1‐diyloxy)]bis(N‐benzyl‐N‐phenylacetamide) and its methyl ( 2b ), buthyl ( 2c ), and hexyl ( 2d ) derivatives. Ba²⁺ selective electrodes based on Ionophores 2a – d are compared with those with commonly used Ionophore 1 : N,N,N′,N′‐tetracyclohexyl‐oxybis(o‐phenyleneoxy) diacetamide. It is shown that Ionophores 2a – d , particularly 2b , are superior for measurements of Ba²⁺ in the presence of Ca²⁺, and in acidic solutions. Segmented sandwich membrane studies suggest formation of complexes IL₂²⁺ for Ba²⁺, Ca²⁺ and Mg²⁺ ions with Ionophore 2b , while H⁺ ions apparently form complexes H₂L²⁺. The values of the complex formation constants are consistent with the selectivity coefficients.     

Voltammetric study of metal ions binding by biindolizine heterocyclophanes and their acyclic analogues

The binding of ions Li⁺, Na⁺, K⁺, (group I), Mg²⁺, Al³⁺, Ga³⁺ (group II), Ca²⁺, Pb²⁺ (group III) ions, Ba²⁺ and paraquat by heterocyclophanes containing biindolizine and quinoxaline fragments connected by 3,6,9-trioxaundecane and 5,8,11,14,17-pentaoxageneicosane spacers, and also their acyclic analogues, in the acetonitrile-0.1 M Bu₄NBF₄ is studied by cyclic voltammetry. A conclusion is drawn that the ions of the group I are not bound by these compounds; the paraquat is not bound by heterocyclophane with the 5,8,11,14,17-pentaoxageneicosan spacers. For ions of the group II, reversible redox-switchable binding by the macrocycles with the 3,6,9-trioxaundecane and 5,8,11,14,17-pentaoxageneicosan spacers is observed: the initial compounds show the binding; their radical cations and dications do not. The binding of the ions of the group III and Ba²⁺ is determined by the macrocycles’ size. In particular, these ions are bound not only by the heterocyclophane with 3,6,9-trioxaundecane spacers but also by its radical cation or dication. The binding results in the corresponding dication stabilization. The heterocyclophane with the 5,8,11,14,17-pentaoxageneicosan spacers demonstrates the redox-switchable binding of Ca²⁺ and Pb²⁺ ions; no effect of Ba²⁺ ions on the cyclic voltammograms of this heterocyclophane was observed. In the ternary system “heterocyclophane with 3,6,9-trioxaundecane spacers + ions of the group II (Al³⁺, Ga³⁺) + ions of the group III (Ca²⁺, Pb²⁺)” either primary binding of the group III ion Pb2+ or concurrent binding of the ions of the group II and the group III, with the system’s reversible redox-switching from one metal complex to another, was observed.     

Synthesis and spectroscopic properties of new bis-tetrazoles

Syntheses of N,N′-phenyltetrazole podands link with aliphatic chains containing oxygen, nitrogen and sulphur atoms, are described. The complexing properties of these compounds towards metal cations (Fe²⁺, Cu²⁺, Zn²⁺, Co²⁺, Ni²⁺) were investigated by absorption and infrared spectroscopy. The UV–Vis titrations were performed to estimate the stability constant values of the respective complexes with Cu²⁺ ion. Changes in UV–Vis absorption spectra and IR spectra of compound 6 under various concentrations of Cu²⁺ ion in methanol suggest formation of very unstable complex. The structure of ligand 2 has been deduced by X-ray crystallography.     

New ICT probes: synthesis and photophysical studies of N-phenylaza-15-crown-5 aryl/heteroaryl oxadiazoles under acidic condition and in the presence of selected metal ions

Molecular probes 6 and 7, incorporating N-phenylaza-15-crown-5 and aryl/heteroaryl oxadiazole have been designed to function as the new intramolecular charge transfer (ICT) probes. Photophysical properties have been studied under acidic condition as well as in the presence of selected metal ions, Ca²⁺, Ba²⁺, Mg²⁺, Na⁺, K⁺, and Li⁺. The changes in the ICT character of the probes, following the addition of trifluoroacetic acid, were interpreted in terms of site and degree of protonations. Based on the cation affinity, the ICT bands in both UV-vis and emission spectra experienced varying degrees of blue shifts due to removal of the aza-crown ether nitrogen from conjugation. The cation-induced spectral shifts and the stability constants revealed binding strength in the order Ca²⁺>Ba²⁺?Li⁺>Na⁺>K⁺>Mg²⁺. Competitive experiments performed in a matrix of ions also indicated superior interaction of 6 and 7 with Ca²⁺. The excited state decay profiles remained largely unperturbed in the presence of metal ions. The studied probes displayed positive solvatochromism and the Stokes shifts and excited state lifetimes increased with increasing solvent polarity. These findings can be rationalized by invoking highly polar nature of the emittive states. The chemoionophores 6 and 7 constitute potentially interesting Ca²⁺ sensitive probes due to their relatively high binding interaction for Ca²⁺ (log K_s=3.55-3.10) vis-a-vis that of biologically interfering Mg²⁺ (log K_s=1.67-1.30).     

Cation and anion ordering in the layered oxyfluorides Sr3−xAxAlO4F (A=Ba, Ca)

The synthesis and structural characterization of mixed oxyfluorides of the type Sr_3−xA_xAlO₄F is reported, where A is either calcium or barium. In these compounds the fluoride and oxide ions are ordered onto two distinct crystallographic sites. There is also an ordering of the alkaline earth cations over two crystallographic sites upon substitution of Ba²⁺ or Ca²⁺ for Sr²⁺. The solid solubility limits extend to x∼1 for substitution of both barium and calcium, but the larger Ba²⁺ cations show a strong site preference for the ten-coordinate strontium sites, while the smaller Ca²⁺ cations prefer the eight-coordinate strontium sites.     

Spectroscopic study of the complexation of an aza-15-crown-5 containing chromofluoroionophore with Ba2+ and Ca2+ cations

The complexation of 1-[(4-benzothiazolyl)phenyl]-4,7,10,13-tetraoxa-1-aza-cyclopenta-decane with Ba²⁺ and Ca²⁺ cations was investigated spectrophotometrically and spectrofluorometrically. The stability constants of the complexes formed are: for Ba²⁺ logK _st=3.17±0.01 (absorption) and logK _st=2.95±0.03 (fluorescence); for Ca²⁺ logK _st=3.71±0.02 (absorption) and logK _st=3.58±0.05 (fluorescence). Protonation of the ligand leads to fluorescence quenching. AM1 and PPP quantum chemical calculations were used to predict molecular geometry, proton affinities and the spectra of the compounds studied.Dedicated to Prof. Dr. Karl-Heinz Drexhage on the occasion of his 60th birthday     

Determination of calcium binding sites in gas-phase small peptides by tandem mass spectrometry

Low-energy (LE) and high-energy (HE) collisionally activated decompositions (CAD) of calcium/peptide complexes of the form [M-H+Ca]⁺ and [M+Ca]²⁺ reflect the site of calcium binding in various gas-phase peptides that are models of the calcium binding site III of rabbit skeletal troponin C. The Ca²⁺ binding sites involve an aspartic acid, glutamic acid, and asparagine, which are in the metal-binding loops of calcium-binding proteins. Both fast atom bombardment (FAB) and electrospray ionization (ESI) were used to generate the metal/peptide complexes. When submitted to LE CAD, ESI-produced Ca²⁺/peptide complexes undergo fragmentations that are controlled by Ca²⁺ binding and provide information on the Ca²⁺ binding site. The LE CAD spectra are simple, indicating that Ca²⁺ binding involves specific oxygen ligands including acidic side chains and that only a few low-energy fragmentation channels exist. The HE CAD spectra of FAB-produced Ca²⁺/peptide complexes are more complex, owing to the introduction of high internal energy into the precursor ion. Interactions of the other alkaline-earth metal ions Mg²⁺ and Ba²⁺ with these peptides reveal that the ligand preferences of these metal ions are slightly different than those of Ca²⁺.     

N-(4-amino-7-nitrobenzaoxa-1,3-diazole)-substituted aza crown ethers: complexation with alkali, alkaline earth metal ions and ammonium

Three novel aza-crown ether derivatives incorporating 4-amino-7-nitrobenzaoxa-1,3-diazole (NBD) chromophore were synthesized and their structure confirmed by ¹H-NMR, IR and elemental analysis. The influence of the solvent polarity and protonation on the photophysical properties of NBD-15-crown-5 was studied by UV/Vis and fluorescence methods. The influence of the investigated cations on the absorption spectra of the ligands was negligible, however emission was strongly affected. Complexation and binding stability of NBD-aza-15-crown-5 and NBD-aza-18-crown-6 were studied using fluorescence spectroscopy. NBD-aza-18-crown-6 exhibits strong selectivity toward Ca²⁺ and Sr²⁺ ions with formation constants about 10³ times higher than the formation constants with the other ions included in the study.