Crown ether based receptors

The lasalocid derivatives 20a,b and 21a,b were less effective enantioselective carriers than the crown-ether-based compounds. Interesting behavior was observed on studying the complexation properties of 21b. Although this receptor showed a remarkable selectivity for L-Trp, negligible enantioselectivity for Phe was observed. 

Scheme 2. Structures of the crown ether-based peptide receptors.

Crown ethers can also be used to develop fluorescent sensors. The De Silva group was the first to use crown-ether-based ionophores and developed the photoinduced electron transfer (PET) sensor. In their system (Figme 3b), a methylene spacer connected an anthracene fiuorophore and an aza-18-crown-6 receptor. The lone pair of the nitrogen atom on the receptor quenches the fluorescence owing to PET. Once potassium ion binds to the aza-18-crown-6, the PET process is turned off and fluorescence is turned on. Electrochemical sensors are also available, where the ion-selective electrode (ISE) made by a modified crown ether is popular (Figure 3c). ... 

The sensor covalently joined a bithiophene unit with a crown ether macrocycle as the monomeric unit for polymerization (Scheme 1). The spatial distribution of oxygen coordination sites around a metal ion causes planarization of the backbone in the bithiophene, eliciting a red-shift upon metal coordination. They expanded upon this bithiophene structure by replacing the crown ether macrocycle with a calixarene-based ion receptor, and worked with both a monomeric model and a polymeric version to compare ion-binding specificity and behavior [13]. The monomer exhibited less specificity for Na+ than the polymer. However, with the gradual addition of Na+, the monomer underwent a steady blue shift in fluorescence emission whereas the polymer appeared to reach a critical concentration where the spectra rapidly transitioned to a shorter wavelength. Scheme 2 illustrates the proposed explanation for blue shift with increasing ion concentration. 

Chirality derived from the readily accessible a-amino acids has been incorporated into the side chains of aza and diaza macrocyclic polyethers. A number of procedures suitable for peptide synthesis have proved (178) to be unsuitable for acylating the relatively unreactive secondary amine groups of aza crown ethers. Eventually, it was discovered that mixed anhydrides of diphenylphos-phinic acid and alkoxycarbonyl-L-alanine derivatives do yield amides, which can be reduced to the corresponding amines, e.g., l-172. By contrast, the corresponding bisamides of diaza-15-crown-S derivatives could not be reduced and so an alternative approach, involving the use of chiral A-chloroacetamido alcohols derived from a-amino acids, has been employed (178) in the synthesis of chiral receptors, such as ll-173 to ll-175, based on this constitution. 

Pyridine-based crown ethers such as 3.107 have also been extensively studied. R,R 3.107 binds the R enantiomer of [a-(l-naphthyl) ethyl] ammonium some 12 kj mol-1 more strongly than the 5 enantiomer. The reason is evident from the stereoviews shown in Figure 3.74. The napthyl substituent on the R enantiomer avoids the bulky phenyl group of the receptor on the same face as the bound cation, while there is a steric clash in the 5enantiomer.49... 

This review surveys our studies devoted to the photoswitchable molecular receptors based on photochromic crown ethers. Photochromic systems described in the review may be classified into three groups according to the reaction types E,Z-isomerization, [2+2]-photocycloaddition reactions and electrocyclic reaction. It has proved the groups to be an especially suitable basis for photochromic systems, and promising for the industrial applications. 

Protons are relatively simple targets for sensor molecules and do not require engineered receptors, however, achievement of selective interactions with other chemical species requires much more elaborate receptors. In the most cases cations are bound via electrostatic or coordinative interactions within the receptors alkali metal cations, which are rather poor central ions and form only very weak coordination bonds, are usually bound within crown ethers, azacrown macrocycles, cryptands, podands, and related types of receptor moieties with oxygen and nitrogen donor atoms [8], Most of the common cation sensors are based on the photoinduced electron transfer (PET) mechanism, so the receptor moiety must have its redox potential (HOMO energy) adjusted to quench luminescence of the fluorophore (Figure 16.3). 

Figure 16.25 Semiconductor nanoparticle-based fluorescent sensors (a) Forster resonant energy transfer (FRET) between two nanoparticles induced by analyte, (b) crown ether receptor for potassium ions, and (c) operation principle of maltose fluorescent sensor. (Adapted from Chen et at. [144] and Medintz et at. [146])...

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