Three different fluorescent responses to transition metal ions using receptors based on 1,2-bis- and 1,2,4,5-tetrakis-(8-hydroxyquinolinoxymethyl)benzene

The dipod 1,2-bis(8-hydroxyquinolinoxymethyl)benzene (3) and tetrapod 1,2,4,5-tetrakis(8-hydroxyquinolinoxymethyl)benzene (5) have been synthesized through nucleophilic substitution of respective 1,2-bis(bromomethyl)benzene (2) and 1,2,4,5-tetra(bromomethyl)benzene (4) with 8-hydroxyquinoline (1). For comparison, 1,3,5-tris(8-hydroxyquinolinoxymethyl)benzene derivatives (7a and 7b) have been obtained. The complexation behavior of these podands towards Ag⁺, Co²⁺, Ni²⁺, Cu²⁺, Zn²⁺, and Cd²⁺ metal ions has been investigated in acetonitrile by fluorescence spectroscopy. The sterically crowded 1,2,4,5-tetrapod 5 displays unique fluorescence ‘ON-OFF-ON’ switching through fluorescence quenching (λ_max 395 nm, switch OFF) with <1.0 equiv of Ag⁺ and fluorescence enhancement (λ_max 495 nm, switch ON) with >3 equiv Ag⁺ and can be used for estimation of two different concentrations of Ag⁺ at two different wavelengths. The addition of Cu²⁺, Ni²⁺, and Co²⁺ metal ions to tetrapod 5 causes fluorescence quenching, i.e., ‘ON-OFF’ phenomena at λ_max 395 nm for <10 μM (1 equiv) of these ions but addition of Zn²⁺ and Cd²⁺ to tetrapod 5 results in fluorescence enhancement with a gradual shift of λ_em from 395 to 432 and 418 nm, respectively. Similarly, dipod 3 behaves as an ‘ON-OFF-ON’ switch with Ag⁺, an ‘ON-OFF’ switch with Cu²⁺, and an ‘OFF-ON’ switch with Zn²⁺. The placement of quinolinoxymethyl groups at the 1,3,5-positions of benzene ring in tripod 7a-b leads to simultaneous fluorescence quenching at λ_max 380 nm and enhancement at λ_max 490 nm with both Ag⁺ and Cu²⁺. This behavior is in parallel with 8-methoxyquinoline 8. The rationalization of these results in terms of metal ion coordination and protonation of podands shows that 1,2 placement of quinoline units in tetrapod 5 and dipod 3 causes three different fluorescent responses, i.e., ‘ON-OFF-ON’, ‘ON-OFF’, and ‘OFF-ON’ due to metal ion coordination of different transition metal ions and 1, 3, and 5 placement of three quinolines in tripod 7, the protonation of quinolines is preferred over metal ion coordination. In general, the greater number of quinoline units coordinated per metal ion in 5 compared with the other podands points to organization of the four quinoline moieties around metal ions in the case of 5.     

A turn-on Schiff base fluorescence sensor for zinc ion

A simple Schiff base type fluorescent receptor 1 was prepared and evaluated for its fluorescence response to heavy metal ions. Receptor 1 exhibits an ‘off-on-type’ mode with high selectivity in the presence of Zn²⁺ ion. The selectivity of 1 for Zn²⁺ is the consequence of combined effects of chelation-enhanced fluorescence (CHEF), CN isomerization, and inhibition of photoinduced electron transfer (PET).     

Highly selective fluorescent chemosensors for cadmium in water

The design, synthesis and photophysical evaluation of two new chemosensors 1 and 2 is described for the selective detection of Cd(II) in water at pH 7.4. Both are based on the use of aromatic iminodiacetate receptors that connected to an anthracene fluorophore by covalent methyl spacers. These are highly water-soluble sensors where the fluorescence is ‘switched off’ between pH 3-11, due to photoinduced electron transfer (PET) quenching of the anthracene excited state by the receptor. Upon protonation of the receptor, the emission was however, ‘switched on’. From these changes pK_as of 1.8 and 2.5 were determined for 1 and 2 respectively. Both showed good selectivity for Cd(II) over competitive ions such as group II and Zn(II), Cu(II), Co(II). For 1, having a single receptor, only a weak monomer anthracene emission was observed for the free sensor at pH 7.4 (HEPES buffer, 135 mM NaCl). Upon Zn(II) titration, a broad red shifted emission occurred, centred at 468 nm. In the presence of Cd(II), a similar red shifted emission was also observed, however, this time centred at 506 nm. In contrast to these results, the fluorescence of 2 in the presence of Zn(II) gave rise to typical monomeric anthracene emission, due to suppression of PET, that is, the anthracene emission was ‘switched on’. Nevertheless, in the presence of Cd(II) a broad emission centred at 500 nm was observed, similar to that seen for 1. These ion induced long wavelength emission bands were assigned to the formation of charge-transfer complexes (exciplexes) between the anthracene moieties and the ion-receptor complexes. Importantly, for both 1 and 2, a selective detection of Cd(II) was possible, even in the presence of Zn(II).     

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.     

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.     

Nucleoside and oligonucleotide pyrene conjugates with 1,2,3-triazolyl or ethynyl linkers: synthesis,duplex stability,and fluorescence changes generated by the DNA-dye connector

Fluorescent nucleosides and oligonucleotides functionalized with pyrene were synthesized using ‘click’ chemistry or the Sonogashira cross-coupling reaction. The dye was connected to position-7 of 7-deaza-2′-deoxyguanosine or to the 2′-deoxyribofuranose moiety. Four different DNA-dye connectors with 1,2,3-triazolyl residues or triple bonds were constructed. Phosphoramidites of the pyrene conjugates (9, 14, 25) were prepared and used in solid-phase synthesis. Short linkers (2, 4) destabilize DNA, while long linkers (1) increased duplex stability. Nucleosides and oligonucleotides with single dye incorporations show linker dependent fluorescence. Linker dependent excimer emission with pyrenes in proximal positions was also observed. A ‘superchromophore’ formed by the 7-deaza-2′-deoxyguanosine ethynylpyrene conjugate shows strong red shifted fluorescence emission at 495 nm.     

A novel chemosensor based on Fe(III)-complexation for selective recognition and rapid detection of fluoride anions in aqueous media

A novel fluorogenic receptor 1 was prepared by the reaction of thiosemicarbazide with 1-naphthaldehyde. Based on in situ-formed 1-Fe(III) complexes having the specific binding affinity for F⁻ anions, this sensory system allows rapid recognition and quantitative detection of fluoride in neutral aqueous media in an ‘off-on’ fashion. The fluoride measurement method not only exhibits a low detection limit but also has strong anti-interference ability to common coexisting ions, as evidenced by competitive experiments.     

FRET-derived ratiometric fluorescence sensor for Cu

New N-(pyrenylmethyl)naphtho-azacrown-5 (1) was synthesized as an ‘On-Off’ fluorescent chemosensor for Cu²⁺. Excited at 240 nm corresponding to the absorption of naphthalene unit (energy donor) of 1, emission at 380 nm from pyrene unit (energy acceptor) is observed, indicating that intramolecular fluorescence resonance energy transfer (FRET-On) occurs in 1. When Cu²⁺ is added to a solution of 1, however, the fluorescence of pyrene is strongly quenched (FRET-Off) whereas that of naphthalene group is revived. Such FRET ‘On-Off’ behavior of 1 is observed only in the case of Cu²⁺ binding, but not for other metal cations. The high selectivity of 1 toward Cu²⁺ can be potentially applied to a new kind of FRET-based chemosensor. The FRET On-Off behavior is supported by computational studies. The calculated molecular orbitals of HOMO and LUMOs suggest the excited-state interactions leading to FRET from naphthalene to pyrene in 1, but no electron density changes in 1·Cu²⁺ complex.     

Design and synthesis of fluorescent 6-aryl[1,2-c]quinazolines serving as selective and sensitive ‘on-off’ chemosensor for Hg in aqueous media

Three fluorescent quinazolines thiophen-2-yl-5,6-dihydrobenzo-[4,5]imidazo[1,2-c]quinazoline (1), pyridin-3-yl-5,6-dihydrobenzo-[4,5]imidazo-[1,2-c]quinazoline (2) and phenyl-5,5′,6,6′-dihydrobenzo-[4,4′,5,5′]imidazo-[1.1′,2-c,2′-c]quinazoline (3) have been synthesized. Structures of 1 and 3 have been authenticated crystallographically. Quinazolines 1-3 exhibit highly selective ‘on-off’ switching for Hg²⁺ ions. The fluorescence intensity displayed a linear relationship with respect to Hg²⁺ concentration (0.1-1.0 μM; R² = 0.99) with detection limit of 2.0 × 10⁻⁷ M.     

Alkaline metal ion-enhanced chemiluminescence of bicyclic dioxetanes bearing a hydroxyaryl group with an ‘even’ substitution pattern

Bicyclic dioxetane 5 bearing a 3-hydroxynaphthalen-2-yl group and its analogs 14 and 15 decomposed to give light with efficiencies of only 0.002-0.005% in a tetrabutylammonium fluoride (TBAF)/THF system, which was as expected for dioxetanes with a so-called ‘even’ substitution pattern. However, the chemiluminescence efficiencies (Φ^CL) markedly increased when these dioxetanes were decomposed with alkaline metal t-butoxide in THF. This enhancement of Φ^CL by alkaline metal ion was most likely due to the highly ordered conformation of an aromatic ring by chelate formation of the metal ion with both an oxido anion and oxygen atom of a tetrahydrofuran ring in an intermediary dioxetane like 12. Alkaline metal ion-enhanced chemiluminescence was similarly observed for dioxetane 6 bearing a 2-hydroxyphenyl group.     

A ninhydrin based colorimetric molecular switch for Hg and CH3COO/F

We have prepared a new ninhydrin-based colorimetric molecular switch (receptor 1) which gets ‘ON’ (blue) in the simultaneous presence of Hg²⁺ and CH₃COO⁻/F⁻ while the absence of any one among them leads the system ‘OFF’ (purple). The switching is highly specific in terms of its inputs as no other ion pairs were able to do the same. The corresponding UV-vis spectral changes involve a bathochromic shift in the charge transfer (CT) band of receptor 1 from 535 to 590 nm and get fitted to AND logic function. In the light of involvement of Hg(CH₃COO)₂ as a catalyst in a number of organic reactions, receptor 1 is quite promising.     

A unique ‘ON-OFF-ON’ switch with two perturbations at two different concentrations of Ag

The dipod 1,2-bis(8-quinolinoxymethyl)benzene 3 and tetrapod 1,2,4,5-tetrakis(8-quinolinoxymethyl)benzene 4 show two perturbations in fluorescence with Ag⁺, (i) fluorescence quenching with <1.0 equiv of AgNO₃ at λ_max 395 nm and (ii) fluorescence enhancement at λ_max 500 nm with >3 equiv of AgNO₃. This ‘ON-OFF-ON’ switching of 3 and 4 in comparison with simultaneous fluorescence quenching and enhancement in the case of 8-methoxyquinoline 1 and the tripod 1,3,5-trimethyl-2,4,6-tris(8-quinolinoxymethyl)benzene 2 point to the unique role of molecular architectures arising due to the number and spatial positions of quinoline units in the fluorescence behaviour of an 8-alkoxyquinoline moiety towards Ag⁺.     

A triphenylamine based multi-analyte chemosensor for Hg and Cu ions in MeCN/H2O

A novel triphenylamine based oxidative chemosensor TOC was synthesized. The chromogenic and fluorogenic behaviors of TOC towards Hg²⁺ and Cu²⁺ ions in a binary mixture of MeCN/H₂O (9/1) were dramatically different. TOC displays colorimetric ‘naked eye’ recognition of Hg²⁺ and fluorogenic ‘turn on’ response towards Cu²⁺ via a unique cyclization reaction using two different detection modes. Moreover, TOCAZOL obtained from the oxidative cyclization reaction of TOC with Cu(ClO₄)₂ can be used as a selective fluorescent sensor toward Hg²⁺ ion.     

Dual-mode recognition of transition metal ions by bis-triazoles chained pyrenes

Fluorescent chemosensors 7−10, with variable methylene chain length as spacers between the two triazole methyl ether units, have been synthesized under ‘Click’ condition, where the bistriazoles are used as the metal ion binding sites and the pyrenes as the fluorophores. Compound 10, having the longest methylene chain among 7−10, shows monomer and excimer fluorescence quenching in acetonitrile toward Ni²⁺, Pb²⁺, Cu²⁺, Hg²⁺, and Cr³⁺ ions, however, it shows an enhanced monomer but a decreased excimer emission when complexed with Cd²⁺ and Zn²⁺ ions.     

A highly selective fluorescent chemosensor for silver(I) in water/ethanol mixture

Synthesis, photophysical, and complexation properties of fluorescent chemosensors, L¹ and L², with ‘receptor-spacer-fluorophore’ motif (L¹: NS₂O₂-cyclic receptor, L²: acyclic analogue receptor) are described. Maximum chelation-enhanced fluorescence effect (TURN-ON type) was observed in the presence of Ag⁺ for both fluoroionophores in a 1:1 (v/v) aqueous ethanol solution: remarkably superior selectivity of L¹ (150-fold) with cyclic receptor than that of the L² (50-fold) with acyclic analogue was found. According to the fluorescence and NMR titrations, the excellent selectivity of L¹ is attributed to its topology-based higher affinity in complexation.     

A highly selective and femto-molar sensitive fluorescence ‘turn-on’ chemodosimeter for Hg

A selective fluorescence ‘turn-on’ chemodosimeter N,N′-bis-(4-cyanobenzylidene)-2,4,6-trimethylbenzene-1,3-diamine (3) based on a Schiff base for femto-molar detection of the Hg²⁺ has been described. It presents the highest level of detection limit for Hg²⁺ through Schiff base hydrolysis.     

A selective, sensitive probe for mercury(II) ions based on oxazine-thione

A selective, sensitive probe for Hg(II) ions, 7-(diethylamino)-3-methyl-2H-benzo[b][1,4] oxazine-2-thione (1), is developed. Compound 1 behaves as a ratiometric probe, exhibiting a large blue shift of 100 nm in its absorption spectra upon exposure to Hg(II) ions. The dramatic color change of the solution made ‘naked-eye’ detection of Hg(II) ions possible. Emission spectra of 1 displayed a selective enhancement in intensity in the presence of Hg(II) ions. ESI⁺-MS analysis indicated that Hg²⁺-induced desulfurization caused the large absorption response.     

Fluorescence emission control and switching of oxymethylcrowned spirobenzopyrans by metal ion

Oxymethylcrowned spirobenzopyran 1 and pyrenylspirobenzopyran 2 were synthesized, and fluorescence emission of their corresponding merocyanine form was examined in the presence of metal ions. For 2, fluorescence emission derived from the pyrene moiety was completely quenched by photoinduced electron transfer (PET) of the nitrogen atom when the merocyanine form was not produced, namely, without metal ions. However, when 2 was converted to the merocyanine form by the complexation of its crown ether with a metal ion, fluorescence resonance energy transfer (FRET) from the pyrene to the merocyanine moieties took place to produce fluorescence emission. This result demonstrates that the spirobenzopyran isomerization can function as a fluorescence emission switch. Fluorescence quantum yield measurement for 1 and 2 showed that fluorescence emission depends on the binding metal ion in which the fluorescence quantum yield generally increased with the increase of metal ion radius.     

Synthesis and fluoride-induced chemiluminescent decomposition of bicyclic dioxetanes substituted with a 2-hydroxynaphthyl group

Five bicyclic dioxetanes bearing a 2-hydroxynaphthyl group, 1aA-1eA, were synthesized and their chemiluminescent decomposition was examined by the use of tetrabutylammonium fluoride (TBAF) as a base in DMSO. It was found that these dioxetanes hold completely the ‘odd/even’ relationship between the substitution pattern of hydroxy as a trigger on the naphthalene ring and their chemiluminescent efficiency, and that dioxetane 1aA exhibited chemiluminescence with the highest efficiency among those for the oxynaphthyl-substituted dioxetanes hitherto known. The significant change in chemiluminescent efficiency depending on the substitution pattern was clarified to be attributed to the marked change in singlet-chemiexcitation efficiency for charge-transfer-induced chemiluminescence (CTICL) of 1aA-1eA. In respect of the rate of CTICL-decomposition, ‘odd/even’ relationship was observed for 1aA-1dA.     

Highly selective and sensitive fluorescence turn-on probe for a catalytic amount of Cu(I) ions in water through the click reaction

A simple azide-functionalized coumarin (1) was utilized as a fluorescence turn-on probe for a catalytic amount of Cu(I) ions in HEPES buffer. The probe has shown a selective and sensitive response to the cuprous ions over other various cations through a Cu(I)-mediated click reaction of 1 to an alkyne. When a catalytic amount of copper sulfates was added in the presence of ascorbate, the prominent fluorescence ‘Off-On’ change of 1 was observed so that submicromolar concentration of copper ions was detectable by the naked eye.