Synthesis of bispyrenyl sugar-aza-crown ethers as new fluorescent molecular sensors for Cu(II)

Two N-pyrenylacetamide-substituted sugar-aza-crown ethers have been synthesized as new fluorescent chemosensors. The designed ligands 1 and 2 exhibit fluorescence characteristics of a pyrene monomer and a dynamic excimer emission when compared to N-pyrenylacetamide as a model compound. Both ligands displayed a Cu2+-sensitive fluorescence quenching with a 1:1 stoichiometry and high stability constants (log K = 6.7 for 1 and 7.8 for 2). The quenching effect was rationalized on the basis of photoinduced electron transfer from the excited pyrene to the complexed Cu2+ cation, while the changes in excimer-to-monomer ratio were explained by a conformational analysis through DFT calculations. The predicted structure suggests that the Cu2+ cation is coordinated with the two carbonyl groups and the sugar-aza-crown ethers which rigidified the complex structure and placed the two pyrene moieties far apart.     

Guest-responsive excimer fluorescence of β-cyclodextrin bearing a pendant group with two pyrene moieties

β-Cyclodextrin derivative 1 , bearing a group with two pyrene moieties, exhibits predominant excimer emission in a 20% DMSO aqueous solution. In spite of the fact that pyrene is too large to be included in the β-cyclodextrin cavity and consequently both pyrene moieties are located outside the cavity, 1 varies the excimer emission intensity through the formation of inclusion complexes with guest species, thus acting as a chemosensor for molecular recognition.     

Ratiometric determination of Hg2+ ions based on simple molecular motifs of pyrene and dioxaoctanediamide

A new podand-type dioxaoctanediamide-based chemosensor having two pyrene moieties was prepared, and its fluoroionophoric behaviors toward transition-metal ions were investigated. Pyrene-appended dioxaoctanediamide 1 showed a selective fluorescence quenching toward Hg2+ ions over other transition-metal ions in an aqueous methanol solution. Unique responses in pyrene monomer and excimer emissions allowed selective ratiometric determination of Hg2+ ions in aqueous environments, and the detection limit was found to be 1.6 x 10(-6) M. [structure: see text]     

Triazole-modified calix[4]crown as a novel fluorescent on-off switchable chemosensor

A novel fluorescent on-off switchable chemosensor 1 with two different types of cationic binding sites is synthesized, which is composed of a triazole-modified calix[4]crown in the 1,3-alternate conformation. Among 15 metal ions examined, the fluorescence of 1 was strongly quenched by Hg2+, Cu2+, Cr3+, and Pb2+; however, the revival of emission from the strongly quenched 1.Pb2+ complex was achievable by the addition of K+, Ba2+, or Zn2+ ions. Thus, metal ion exchange can trigger an on-off switchable fluorescent chemosensor.     

A fluoride-selective PCT chemosensor based on formation of a static pyrene excimer

[reaction: see text] Calixarene-based fluorescent chemosensor 1 with two fluorogenic pyrene units conjugated to amide groups as guest recognition sites is synthesized. Complexation of F(-) by 1 causes a red shift of its absorption band to 400 nm (Deltalambda = 54 nm) and a blue shift of the excimer emission to 470 nm (Deltalambda = 12 nm) together with enhanced fluorescence intensity. The blue-shifted excimer emission is attributed to a pyrene dimer formed in the ground state, a so-called static excimer.     

Cu2+-induced blue shift of the pyrene excimer emission: a new signal transduction mode of pyrene probes

A pentiptycene-bispyrenyl system (1) has been synthesized and investigated as a fluorescent chemosensor for metal ions. A novel blue shift along with an intensity enhancement of the pyrene excimer emission is observed for 1 in the presence of Cu(2+). Such a new signal transduction mode of pyrene probes results from the formation of a static pyrene excimer that has very different characteristics from its dynamic counterpart.     

A calixarene based fluorescent Sr2+ and Ca2+ probe

A fluorescent probe, PyCalix, which has two pyrene moieties at the lower rim of a calix[4]arene fixed in the cone conformation was synthesized and its complexation behavior with alkali and alkaline earth cations was studied by fluorescence spectrometry. The compound showed intramolecular excimer emission at approximately 480 nm in the fluorescence spectra. Upon complexation with alkaline earth metal cations, a decrease of excimer emission was observed. The decrease of excimer emission was accompanied by an increase of monomer emission of pyrenes at 397 nm. The order of complexation constants of PyCalix with metal ions was Sr(+ approximately Ca2+ > Ba2+ > Mg2+ > K+ > Na+ > Cs+ for all reagents. PyCalix doped polyvinyl chloride (PVC) membrane was fabricated and our results showed that this membrane can be used for selective detection of Sr2+.     

A PCT-based, pyrene-armed calix[4]crown fluoroionophore

A photoinduced charge transfer (PCT)-based 1,3-alternate calix[4]crown fluoroionophore containing two cation recognition sites, a crown ether ring and two facing pyreneamide groups, is synthesized. Upon addition of K+, Pb2+, or Cu2+, wavelength changes are observed in both the fluorescence and absorption spectra, but with different binding modes. With K+, fluorescence emissions of the ligand scarcely change, while addition of Pb2+ or Cu2+ produces a remarkable change in both the excimer and monomer emissions. The observed data indicate that the metal cation is encapsulated in the crown-5 ring for K+ and by the two facing amide groups in the latter case, which is verified by a metal ion exchange experiment. The wavelength shifts in both fluorescence and absorption spectra upon addition of Cu2+ show that, in contrast to Pb2+, Cu2+ interacts with the nitrogen atoms of the amide groups, resulting in a PCT mechanism.     

Synthesis of double-armed lariat ethers with pyrene moieties at each end of two sidearms and their fluorescence properties in the presence of alkali metal and alkaline earth metal cations

Two types of double-armed lariat ether derivatives having pyrene moieties at each end of two sidearms, (3x + 1)-crown-x derivatives 1 (x = 5), 2 (x = 6), and 3 (x = 4) (type A) and 3y-crown-y derivatives, 6 (y = 5) and 7 (y = 6) (type B), were synthesized, and their complexation behavior toward alkali metal and alkaline earth metal cations was examined by fluorescence spectroscopy. Pyrene excimer emission decreased accompanied by an increase in monomer emission upon metal ion complexation. This finding is ascribed to the change of the spatial distance of two pyrene rings by movement suppression of both the crown ring and one of the two sidearms based on complexation with the metal cation. The selectivity for alkaline earth metal cations was highly dependent on the fitness of the host cavity and the guest size. Although most of the fluorophores did not respond to alkali metal cations, only trans-7a containing an 18-crown-6 ring showed K(+) selectivity.     

Pyrene-linked triazole-modified homooxacalix[3]arene: a unique C3 symmetry ratiometric fluorescent chemosensor for Pb2+

A new type of fluorescent chemosensor based on homooxacalix[3]arene was synthesized. The fluorescent sensor was highly selective for Pb(2+) in comparison with other metal ions tested by enhancement of the monomer emission of pyrene. The C(3) symmetric structure of homooxacalix[3]arene has potential application in the development of a new ratiometric fluorescent chemosensor for heavy metal ions.     

Ratiometry of monomer/excimer emissions of dipyrenyl thiacalix[4]arene for Cu detection: a potential Cu and K switched INHIBIT logic gate with NOT and YES logic functions

A new thiacalix[4]arene derivative 2 of 1,3-alternate conformation possessing two pyrene groups has been synthesized and examined for its cation recognition abilities towards different cations such as lithium, sodium, potassium, nickel, zinc, cadmium, silver, mercury, lead and copper by fluorescence spectroscopy. In CH₃CN/CH₂Cl₂ (1:1), the presence of Cu(II) induces the formation of a 1:2 ligand/metal complex, which exhibits increasing monomer emission at 376 nm at the expense of the fluorescent excimer emission of 2 centered at 476 nm. In the presence of K⁺, the intensity of the excimer emission increases along with the formation of a new blue shifted band at 435 nm which corresponds to a static dimer. The compound behaves as a fluorescent molecular switch upon chemical input of Cu²⁺ and K⁺.     

Highly selective fluorescent sensing of Cu ion by an arylisoxazole modified calix[4]arene

A novel fluorescent chemosensor 1 with two anthraceneisoxazolymethyl groups at the lower rim of calix[4]arene has been synthesized, which revealed a dual emission (monomer and excimer) when excited at 375 nm. This chemosensor displayed a selective fluorescence quenching only with Cu²⁺ ion over all other metal ions examined. When Cu²⁺ ion was bound to 1, the fluorescence intensities of both monomer and excimer were quenched. Furthermore, the association constant for the 1:1 complex of 1·Cu²⁺ was determined to be (1.58 ± 0.03) × 10⁴ M⁻¹.     

Intramolecular excimer formation and delayed fluorescence in sterically constrained pyrene dimers

The synthesis is described for a series of five molecular dyads comprising pyrene-based terminals covalently linked through a 1,3-disubstituted phenylene spacer. The extent of through-space communication between the pyrene units is modulated by steric interactions imposed by bulky moieties attached at the 6,8-positions of each pyrene unit. For the control compound, only hydrogen atoms occupy the 6,8 positions (DP1), whereas the remaining compounds incorporate ethynylene groups terminated with either triisopropylsilyl (DP2), 1-tert-butylbenzene (DP3), 2,6-di-tert-butylbenzene (DP4) or 1-tert-butyl-3,5-dimethylbenzene (DP5) units. Each compound shows a mixture of monomer and excimer fluorescence in fluid solution at room temperature, but only monomer emission in a glassy matrix at 77 K. The ratio of monomer to excimer fluorescence depends markedly on the molecular structure; DP1 is heavily biased in favour of the excimer and DP4 is enriched with monomer fluorescence. Photophysical properties, including laser induced and delayed fluorescence data, are reported for each compound. Delayed fluorescence occurs by both intramolecular and bimolecular steps, but these events take place on different timescales. The possibility is raised for using intramolecular triplet-triplet annihilation as a means of molecular imaging.     

Effects of metal cation coordination on fluorescence properties of a diethylenetriamine bearing two end pyrene fragments

Fluorescence properties of a diethylenetriamine bearing two end pyrene fragments (L) have been studied in water, where effects of adding metal cations (Zn2+, Cd2+, Cu2+, Hg2+, Ag+) on the emission properties of L have been studied. Without metal cations, L shows dual-mode fluorescence consisting of monomer and excimer emissions. The monomer emission intensity (I(M)) is strong at acidic pH but decreases with a pH increase because of an electron transfer (ET) from the unprotonated nitrogen atoms to the excited pyrene fragment. The excimer emission is due to the static excimer formed via a direct photoexcitation of the intramolecular ground-state dimer (GSD) of the end pyrene fragments. The excimer emission intensity (I(E)) is weak at acidic pH but increases with a pH increase because of the GSD stability increase associated with the deprotonation of the polyamine chain. Addition of metal cations leads to I(M) decrease, where chelation-driven I(M) enhancement does not occur even with diamagnetic Zn2+ and Cd2+ at any pH. This is because a pyrene-metal cation pi-complex, formed via a donation of pi-electron of the pyrene fragment to the adjacent metal center, suppresses the monomer photoexcitation. I(E) also decreases upon addition of metal cations because the pyrene-metal cation pi-complex weakens pi-stacking interaction of the end pyrene fragments, leading to GSD stability decrease. The emission properties of L-Zn2+ complexes were studied by means of time-resolved fluorescence decay measurements, and the effects of adding a less-polar organic solvent were also studied to clarify the detailed emission properties.     

New BODIPY derivatives as OFF-ON fluorescent chemosensor and fluorescent chemodosimeter for Cu2+: cooperative selectivity enhancement toward Cu2+

New 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (BODIPY) derivatives (1 and 2) were synthesized as an "off-on" fluorescent chemosensor and fluorescent chemodosimeter for Cu2+ and Pb2+. Compound 1 displayed selective and large chelation enhanced fluorescence effects with Pb2+ and Cu2+ among the metal ions examined. On the other hand, compound 2, a fluorescent chemodosimeter, effectively recognized Cu2+ via a selective hydrolysis of the acetyl group.     

Indium(III)-induced fluorescent excimer formation and extinction in calix[4]arene-fluoroionophores

New fluorogenic or/and chromogenic calix[4]arenes 1-3 with two facing amide groups linked to fluorescent pyrene units are synthesized. Orientations of the pyrene units are remote from each other in 1 and face-to-face pi-stacked in 2, which produces different photophysical properties. In the excited state, the two pyrene units of 2 form a strong intramolecular excimer displaying an emission at 472 nm with a relatively weak monomer emission at 395 nm. In contrast, 1 exhibits only a monomer emission at 398 nm because intramolecular hydrogen bonding between the phenolic OH oxygens and the amide hydrogens prevents pi-stacking of the two pyrene groups. Fluorescence changes upon addition of various metal ions show that 1 has a remarkably high selectivity for In(3+) over the other metal ions tested. Compound 1 forms 2:1 (metal:ligand), as well as 1:1 complexes, with In(3+), with fluorescence varying uniquely with the complex stoichiometry. Compound 3, which possess two pyrene units and two chromogenic azo groups, shows almost the same binding behavior toward metal ions as does 1, together with additional bathochromic shifts of the absorption maximum. Compared with 1, compound 3 emits a considerably weaker fluorescence, which is attributed to electron transfer from the pyrene units to the nitro groups of the phenylazo moieties.     

Switching and fluorescence signal amplification with metal ions in allosteric systems based on 1, 3-alternate thiacalix[4]crown

New thiacalix[4]crown derivatives 4 and 5 of 1,3-alternate conformation possessing anthracene and quinoline moieties respectively were synthesised and examined for their cation recognition abilities toward different cations by fluorescence spectroscopy. Selective optical recognition with fluorescence amplification was observed for Fe(3+)/K(+) and Hg(2+)/K(+) in chemosensors 4 and 5 respectively. Further it was observed that the formation of 4·Fe(3+) complex triggers the decomplexation of K(+) ion. Similar allosteric behaviour between Hg(2+) and K(+) was observed in chemosensor 5.     

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.     

Unique blue shift due to the formation of static pyrene excimer: highly selective fluorescent chemosensor for Cu

New binaphthyl derivatives bearing pyrene groups have been synthesized and studied as fluorescent chemosensors for Cu²⁺ ions. A unique blue shift along with fluorescent enhancement in pyrene excimer emission was observed, which were induced by the formation of a static pyrene excimer.     

A water-soluble "switching on" fluorescent chemosensor of selectivity to Cd2+

Compound 1, a new fluorescent chemosensor signaling via significantly enhanced fluorescence when bound with cation analytes, was synthesized and characterized. This fluorescent chemosensor exhibits its selectivity to Cd2+ among a series of cations in HEPES buffer solution. Its in vitro sensitivity to Cd2+ was demonstrated in the HK-2 cell line with use of confocal microscopy. The mechanistic selectivity and sensitivity of compound 1 to Cd2+ was discussed on the basis of fluorescence, 1H NMR, and mass spectroscopic results.