Calixarenes quenching

Calixarene containing a dioxotetraaza unit, PET-18, is responsive to transition metal ions like Zn2+ and Ni2+. Interaction of Zn2+ with the amino groups induces a fluorescence enhancement according to the PET principle. In contrast, some fluorescence quenching is observed in the case of Ni2+. PET from the fluorophore to the metal ion is a reasonable explanation but energy transfer by electron exchange (Dexter mechanism) cannot be excluded. 

In calixarene-based compound M-8 (Figure 10.28), bearing four anthracene moieties on the lower rim, some changes in fluorescence intensity were observed on binding of alkali metal ions but no excimer emission was detected. Quenching of the fluorescence by Na+ may arise from interaction of four anthracene residues brought in closer proximity to one another enhancement of fluorescence by K+ is difficult to explain. 

The quantitation of undesired enantiomers in drug raw materials is one of the objectives of the pharmaceutical industry. Several calixarene derivatives were investigated as fluorescence sensors for chiral pharmaceutical compounds. The mechanism of these examples is based on different fluorescence quenching of the calixarenes by the two enantiomeric forms of a specific analyte. 

Calixarenes modified with a reporter site are equally competent chemosen-sors for the detection of a variety of analytes according to Scheme 6, especially when the analyte is cationic. The tt-cavity of a tetraanionic resorcin[4]arene readily binds cationic guests, a feature that Inouye et al. have exploited in the development of an acetylcholine chemosensor [358], The fluorescence from a pyrene-modified N-alkylpyridinium is strongly quenched upon its association with the tetraphenolate form of resorcin[4]arene (24). This quenching is consis-... 

The reaction of 13, in which the (3-pyridyl)carbonyloxy groups can be either in syn or in anti position, with spiro[cyclopropene-3,9 -fluorene] creates two new stereogenic centers.35 Thus two diastereomers are possible for each the syn- and the anri-isomer which form a pair of C2-symmetrical enantiomers (R,R/S,S) and a Cs- or Cj-symmetrical meso form (R,S) n The resulting calixarenes 14, bearing dihydroindolizine units, were studied as chromogenic compounds ( calixo-chromes ) in quenching experiments not related to their chirality. 

Enhancement of the excimer band was observed for the interaction of the (R)-enantiomer of phenylalaninol (PA) with the substituted calyx[4]arene 49, which was demonstrated to be due to a change in the calixarene conformation induced by complexation of the guest molecule. The monomeric band was instead quenched by both enantiomers, suggesting that complexation occurred also in the case of the (S)-enantiomer. The enantioselectivity was found to be solvent dependent, being present in the case of chloroform but absent in methanol [65]. 

Grady T, Harris SJ, Smyth MR et al (1996) Determination of the enantiomeric composition of chiral amines based on the quenching of the fluorescence of a chiral calixarene. Anal Chem 68 3775-3782... 

Lynam C, Diamond D (2005) Varying solvent polarity to tune the enantioselective quenching of a calixarene host J Mater Chem 15 307-314... 

Quenching of Pyrene Fluorescence Nitro- and hydroxy-containing aromatic compounds are known to quench the fluorescence of pyrene [53,57-59]. Each of the calixarenes (1), (2), and (3), in this respect, has the ability to quench the fluorescence of pyrene. Accordingly, we found that the above three calixarenes do quench the fluorescence of pyrene within micellar [Cj(,mim][Clj. Plots of versus the concentration of the calixarenes (where and F are the fluorescence intensities in the absence and the presence of the calixarene, respectively) are presented in Figure 9.6. 

Another calixarene with a short distal bridge is the siloxane-bridged calixarene synthesised by Hudrlik et al. [24]. The synthesis of this calixarene was accomplished in four steps, as described in Scheme 10.6. The distal selectivity on the wider rim of the calixarene was obtained through a selective bromine to lithium exchange, followed by quenching with methanol, as described by Larsen and Jprgensen [25]. 

The highest reported binding constant to date for a mono-macrocyclic calixarene or calixarene analogue receptor and Cgo is 3.48 x 10 which was ascribed to the Ag(I) A-heterocyclic carbene-bridged calixarene 48 synthesized by Quin et al. [72]. This binding constant for the 1 1 complexation was determined by fluorescence quenching titration study of in acetonitrile at 298 K (Fig. 33.33). 

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