Chelation-enhanced fluorescence effect

Another fluorogenic probe 5 (Fig. 28.2), tren-N-tricalix[4]arene, bearing three pyrene-amide units at the periphery of the tren-calix[4]arene unit, has strong complexation ability with Al " [31] addition of small amounts of Al " ions results in a ratiometric response, with a decrease in the excimer emission at 480 nm and an increase in the monomer emission at 394 nm, attributed to reduced jr-jc interactions due to the complexation of Al " by the tren part. Further addition of Al " leads to an overall increase in the monomer and excimer fluorescence emissions attributed to the CHEF (chelation-enhanced fluorescence) effect observed upon complexation. Mono- and difunctionalized derivatives were not suitable for fluorogenic detection of Al " ions. 

Enantiopure dioctyl-substituted acridino-18-crown-6 ether-type Hgands such as 85, shown in Scheme 30, are potentiometric sensor molecules when incorporated into plasticized PVC membrane electrodes (2009T(A)2795). Another article introduced two acridines 86 as fluorescent chemosensors for and Cd in aqueous solution. These acridines displayed selective chelation-enhanced fluorescence effects with or Cd among the... 

The methylene-blue sensitized photo-oxidation of mesodiphenyl-helianthrene has been proposed as a convenient new chemical actinometer for the extended wavelength range 610-670 nm. Chelation enhanced fluorescence can be used to detect non-metal ions such as carboxylate, sulphate, and phosphate groups. Anthrylpolyamines can be employed as effective probes for this purpose. 

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. 

Kleinerman and co-workers (158) reported an enhancement of fluorescent yield of chelated lanthanide ions by Lewis bases. They observed that in liquid, plastic, and glassy solutions containing terbium europium, and samarium chelates, the use of Lewis bases accomplishes the same effect as substituting deuterium for hydrogen. Not all bases, however, are equally effective. The molecular size of the base does not appear to be particularly important, since strong enhancement effects can be obtained with both bulky and small molecules. The nature of the atom of the base having the unshared electron pair is not a determining factor in the enhancement phenomenon. 

When the fluorescing atoms or molecules are placed inside such a microcavity, the fluorescence gets coupled to the MDR as an electromagnetic field. This results in alternatively enhancement or inhibition of the fluorescence depending on whether or not the fluorescence emission spectrally coincides with a cavity resonance. The effect of MDR on the radiative rate of chelated Europium ions [2] as well as the shortening of fluorescence lifetime of Rhodamine 6G due to the effect of MDR have been reported in microdroplets [3]. 

Titration with chelators of a metalloenzyme preparation from which extraneous metals and chelators have been removed produces a characteristic enhancement of the intrinsic protein fluorescence (excitation at 280 nm, emission at 350 nm) (13). This fluorescence enhancement by nonfluorescent chelators is instantaneous, reversible by excess added divalent metal ions, and can occur without loss of activity. Different chelators give different characteristic amounts of fluorescence enhancement at saturation, demonstrating the specific effect of the chelator on the fluorescence of the apparent metalloenzyme-chelator complex. In contrast, if the effect of chelators were simply to complex with Mg2 after its dissociation from the metalloenzyme, the resulting apoenzyme should have identical fluorescence properties regardless of which chelator was utilized. 

Freeman, Crosby, Lawson (13), Kropp, and Windsor 17, 18) had reported that hydrated europium chloride crystals and aqueous solutions of europium salts, respectively, showed considerable enhancement of ion fluorescence and lifetimes upon substitution of H2O by D2O, the latter workers have reported (37) that wet hydrocarbon solutions of the thenoyltrifluoroacetone chelate of europium showed no enhancement of quantum yield and lifetime upon replacing the H2O present by D2O. They suggested that whereas deuteration enhanced the fluorescent properties of the free ions by inhibiting nonradiative deactivation of the Do level, the quantum efficiency of Do luminescence obtained upon direct excitation of this level (JO, 11) is already so high in the fluorinated diketone chelates e.g. thenoyltrifluoroacetone) that deuteration of the environment should have little further enhancing effect. 

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