Bithiophene crown ether

Scheme 1 Bithiophene crown ether in planar conformation with 7i orbital overlapping in the absence of bound metal. After the chelation of the metal ion, the bithiophene is forced out plane and the 7t orbitals no longer have maximal overlapping [12]...

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. 

Crown ether-functionalized calixarene 2.168 (Chart 1.36) in which two bithiophene units were attached at the termini of a calixarene moiety was prepared by a similar procedure to that described above for 2.167... 

The selective detection of metal ions has also been achieved by the covalent attachment of molecular recognition moieties to the ICP backbone. The usual approach has been to synthesize a monomer or dimer containing the appropriate recognition group and this is subsequently polymerized (21). Swager (22) have prepared polythiophenes containing crown ethers and calixarenes covalently boimd to the bithiophene repeat imits, which exhibit controllable selectivity toward Li+, Na+, and K+ ions. 

Copolymers resulting from the chemical polymerization of 3-hexylthiophene with 3-thiopheneelhylacetate (XVII) [51], bithiophene with a crown ether linked bithi-... 

With the development of 3-(w-bromoalkyl)thiophenes [100], several new types of crown ether functionalized PTs have been synthesized. While electropolymerization of monomeric derivatives (32) failed, probably for steric reasons, bithiophene (33) and terthiophene (34) were easily electropolymerized to the corresponding crown ether derivatized PTs [101],... 

Another strategy proposed by Sannicolo et al. consists of the use of cyclopentabithiophene precursors with either a 16-crown-5-ether ring coplanar to the bithiophene moiety (74) or a 15-crown-5-ether perpendicular to it (75) [180]. This approach considerably reduces the steric interactions between the complexing site and the conjugated backbone. Consequently, the precursors were readily electropolymerized into extensively conjugated polymers as confirmed by optical data. The analysis of electrochemical properties in the presence of alkali cations showed that while poly(75) is completely insensitive to change of the cationic species, the redox potential of poly(74) undergoes a 350 mV positive shift in the presence of Na" " [ 180]. 

Zotti et al. reported two cyclopentadithiophene precursors comprising a 16-crown-5-ether ring which is coplanar relative to the bithiophene unit (2.176) and a 15-crown-5-ether 2.177 perpendicular to the bithiophene unit (Chart 1.37) [272]. Electropolymerization of the monomers resulted in extensively conjugated... 

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