Chelation-enhanced quenching

Figure 3.8. Titration of the fluorescence of anthrylazamacrocydes 5c and 5e (10 and 100 fiM, respectively) by added metal ions. The three experiments shown are ( ) titration of 5c in pH 12 buffer with Zn(Cl04)2. (o) titration of 5c in pH 12 buffer withZnfClO and 1 M NaC104 (a) titration of 5c in pH 6 buffer with HgfClOah. The chelation-enhanced quenching (CHEQ) of 5e by Hg(ll) ion was accomplished at pH 6 because, while the azamacrocycle is sufficiently deprotonated at this pH to bind...

Yoon J, Czarnik AW. Fluorescent chemosensors of carbohydrates. A means of chemically communicating the binding of polyols in water based on chelation-enhanced quenching. Journal of the American Chemical Society 1992, 114, 5874—5875. 

Yoon, J. and Czarnik, A. W. (1992) Fluorescent chemosensors of carbohydrates. A means of chemically communicating the binding of polyols in water based on chelation-enhanced quenching, J. Am.Chem. Soc., 114, 5874-5875 Mohler, L. K. and Czarnik, A. W. (1993) a-Amino acid chelative complexation by an arylboronic acid. Ibid., 115, 2998-2999. 

Scheme IV. A working hypothesis for the observed chelation-enhanced quenching (CHEQ) effect observed in the equilibrium shown in Scheme HI. (Reproduce with permission from reference 16. Copyright 1993 Pergamon...

Class 2 fluorophores that can reversibly bind an analyte. If the analyte is a proton, the term fluorescent pH indicator is often used. If the analyte is an ion, the term fluorescent chelating agent is appropriate. Fluorescence can be either quenched upon binding (CEQ type Chelation Enhancement of Quenching), or enhanced (CEF type Chelation Enhancement of Fluorescence). In the latter case, the compound is said to be fluorogenic [e.g. 8-hydroxyquinoline (oxine)]. 

Thr), leucine (Leu), phenylglycine (Phg), and phenylalanine (Phe), in form of tetrabutylammonium salts were used in this study. Addition of amino acid anions to solution of 21 or 22 caused chelation enhanced fluorescence quenching (CHEQ) for both anion receptors. The CHEQ effect for host 21 can be ascribed to photoinduced charge transfer (PCX), whereas for 22 quenching can be attributed to photoinduced electron transfer processes (PET). The DNB protected amino acids displayed larger... 

The principle of the action of fluorescent enolimine/ketoenamine chemosensors is usually based on two fundamentally different effects - CHEF (Chelation-Enhanced Fluorescence) and CHEQ (Chelation-Enhanced Fluorescence Quenching). The former is connected with hindering the enolimine C=N bond rotation by complexation with cations. As a result, the fluorescence intensity of compounds 23 (Figure 10.15) is selectively and significantly enhanced by adding of, for example, Cd " " cations [92, 93]. This enhancement was attributed to the formation of a 1 1 complex in which the rotation around the acyclic C=N bond is frozen. 

CEF Chelation or Complexation Enhancement of Fluorescence CEQ Chelation or Complexation Enhancement of Quenching Fig. 10.1. Main classes of fluorescent molecular sensors of ions or molecules. 

Enhancement of fluorescence due to the complexation of metal ions with fluoroionophores has been used as a well-precedented technique to analyze for the presence of metal ions [189-191], A number of studies have reported chelating fluorophores whose emission spectra change upon the addition of metal ions [192-198]. One remarkable result of this emission intensity enhancement is shown in Scheme 23, where the chelation of zinc chloride to 9,10-bis(((2-(dimethylamino)ethyl)methylamino)methyl)anthracene drastically enhances the observed fluorescence by a factor greater than 1000-fold [199], In the absence of Zn2+, the singlet excited state of anthracene moiety is strongly quenched by intramolecular photoinduced electron transfer from the amine to the anthracene moiety. The complex formation of Zn2+ with the amine moiety may result in the largely positive shift of the one-electron oxidation potential. Thus, intramolecular photoinduced electron transfer is strongly suppressed by the complexation of the amine moiety with Zn2+,... 

X 109 and 8. 9 x 109 s 1 for Ndm and Ybm, respectively. Both the above-described in-termolecular mechanism, as well as an intramolecular pathway in the ternary complex with aad which forms in solution, are responsible for the observation of NIR luminescence in these systems. Addition of water to the toluene solutions quenches the NIR luminescence, while it enhances the visible CL emission of the corresponding solution of Eum and Tbm (Voloshin et al., 2000c). Neodymium and ytterbium tris(benzoyltrifluoroacetonates) display the same CL as tta complexes, although for Ybm its intensity is about 2.5 times lower than for the tta chelate. On the other hand, almost no CL is detected for acetylacetonate complexes (Voloshin et al., 2000a). Thermal or photochemical decomposition of aad also triggers CL from [Pr(dpm)3] and Pr(fod)3], both in the visible (from the 3Pi, 3Po, and 1D2 levels) and in the NIR at 850 nm ( Do -> 3F2 transition) and 1100 nm ( D2 3F4 transition). The excited... 

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