Conformation Crown ether moieties

The slightly less negative value found for the entropy term can be explained by the preorganization of the ligand in the 1,3-alternate conformation, where only a small part of the crown ether moiety is rather flexible. This flexibility is lost with the large cesium cation, which fits very well into the cavity created by the polyether ring and the aromatic nuclei.34... 

Kimura et al. have reported a dramatic conformational change of a Crystal Violet derivative with three crown ether moieties, 96, on its cesium-ion... 

All 1,2-oligosilyl dipotassium compounds studied were found to acquire an approximate trans conformation of the potassium-crown ether moieties, but only 3b was found to adopt an exact trans conformation [9], Although 3b can acquire both a meso- and a rac-type configuration, only the meso form was found to exist in the crystal. 

Potassium and rubidium perchlorates have been observed to decrease the photocyclisation quantum yield of l,2-bis(2,4-dimethylthien-3-yl)perfluorocy-clopentene having two benzo-15-crown-5 ethers (101), and this has been ascribed to increases in the ratio of the photoinactive parallel conformational analogue of (101) by intramolecular interaction of the two crown ether moieties with a metal ion. ... 

For solid phases of CEPcHj and CEPcCu orthorombic structures were found by X-ray determinations at small angles. In these forms two-dimensional rectangular arrays of the substituted phthalocyanines lead to corrugated planes whereby the Pc macrocycles form an angle with the crown ether moieties. The substituents are arranged in an eclipsed conformation and the crown ether macrocycles form channels. Metastable mesophases of the crown ether phthalocyanines are constituted of two-dimensional square lattices superposed in a staggered conformation. The behavior of these mesophases bas been compared to that of liquid crystalline phthalocyanines [179]. 

Fluorescent chemosensor 25 [53] (Fig. 28.6) with calix[4]arene in the 1,3-altemate conformation, a conjugated crown ether unit, and an anthracenyl amine showed strong fluorescence under acidic conditions but very weak emission in neutral and basic media. The strong fluorescence is due to the inhibition of PET to the anthracene moiety from the amine units. The addition of K ions leads to the formation of complexes with the crown ether moiety, which induce deprotonation of ammonium ion, causing resumption of the PET process. This metal ion-induced deprotonation is interesting for the design of improved receptors for specific cations. 

The E,Z-isomerization of the styiyl dyes 2c containing aza-15-crown-5 ether moiety is characterized by great hypsochromic shifts, equal to 170 nm, of the spectrum of [(Z)-2c]Ca2+ with respect to the spectrum of [(E)-2c] Ca2+ (see Figure 1). Apparently, it can be explained by the fact that in anion- capped complex the molecule of dye acquires a twisted conformation with marked disruption of the conjugation in the chromophoric system. When ( )-2a is converted into the Z-form, the stability of complex increases approximately 2.5 fold. On going from the cationic dye 2a to the betaine 2c, stability of the complexes formed by Z-isomer increases by more than three orders of magnitude [16]. 

Calixarenes were developed later than crown ethers and cyclodextrins but have stillbeen extensively researched. Macrocycles of calix[n]arenes are constructed by linking a number of phenol residues via methylene moieties (Fig. 2.16). Like crown ethers, the name calixarene reflects the structures of these molecules, since a calix is a chalice. Calixarenes with various cavity sizes have been designed, each of which has conformation isomers, and their phenolic hydroxyl groups are often modified. These structural characteristics allow us to create calixarene derivatives with various structural modifications. 

In addition to crown-ethers, calix[4]resorcinarenes 19 have also been used as head groups to obtain azobenzene amphipbiles with sufficient photo-isomerization in LBK films. For this purpose, azobenzene moieties have been tethered to the lower rim of the crown conformer of the calixarene. O-octacarboxymethoxylated calix[4]resorcinarenes 19 (X = CH2-COOH) display efficient trans to cis photoisomerizability in densely packed mono-layers on a water surface, in LBK films, and in surface-adsorbed monolayers, whereas the noncarboxymetylated 19 (X = H) derivative gives films that are too densely packed. However, the aggregation is already suppressed efficiently compared to the azobenzene derivative without calixaren. ... 

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