Crown ethers mesogenic

Until now, i.e., in 25 years of research, only two reviews on liquid crystalline crown ethers have been published [8, 9]. As both reviews cover the field only partially and, e.g., the fascinating polymeric crown ethers as well as taper-shaped liquid crystalline crown ethers are not discussed, we decided to give the first comprehensive review. The present chapter will be structured according to the molecular structure of the mesogens. The discussion of each type of crown ether mesogen will start with small molecules, continue with polymeric compounds, and conclude with possible applications (where applicable). 

The general design of the following crown ether mesogens is a terminal crown ether connected to a calamitic unit carrying flexible side chains and optional functional groups (Fig. 1). 

As indicated above in chiral mesophases, the introduction of a functional group in mesogenic stmctures offers the opportunity to achieve functional LCs. With this aim, mesomorphic crown-ether-isocyanide-gold(I) complexes (26) have been prepared recently [38]. The derivatives with one alkoxy chain show monotropic SmC mesophases at or close to room temperature. In contrast, the complexes with three alkoxy chains behave as monotropic (n = 4) or enantiotropic (n > 4) LCs. The structure of the mesophases could not be fully eluddated because X-ray diffraction studies in the mesophase were unsuccessful and mesophase characterization was made only on the basis of polarized optical microscopy. These complexes are luminescent not only in the solid state and in solution, but also in the mesophase and in the isotropic liquid state at moderate temperatures. The emission spectra of 26a with n=12 were... 

Arias, J., Bardaji, M. and Espinet, P. (2008) Luminescence and Mesogenic Properties in Crown-Ether-Isocyanide or Carbene Gold(I) Complexes Luminescence in Solution, in the Solid, in the Mesophase, and in the Isotropic Liquid State. Inorganic Chemistry, 47, 3559-3567. 

ID and 2D 13C NMR were carried out in a series of novel nematogens by Bayle and co-workers to study the effects on the conformation and order due to the addition of lateral and/or terminal substituents.249 251 For example, lateral flexible substituents are found to adopt a mean conformation more or less parallel to the mesogenic core. As a result, lateral chains are less disordered than terminal chains. Nematogens containing polyoxyethylene ether (POE) chain as a lateral crown ether and terminal chain(s) have been... 

The combination of crown ethers with mesogenic groups is interesting because the resulting hybrid materials possess the properties of both constructive subunits, i.e., liquid crystalline order in the mesophase and the ability to complex specific cations selectively according to the crown ether cavity size. 

Fig. 1 Rod-like mesogen with terminal crown ether...

Tschierske developed a variety of molecules containing a p-terphenyl unit as backbone equipped with two terminal decyloxy chains and laterally attached crown ethers of different sizes [51-53]. The synthesis was straightforward starting from methyl-2,5-dibromobenzoate 30 that was coupled with [4-(decyloxy)phenyl]boronic acid 31 in a subsequent Pd°-catalyzed Suzuki reaction. The 2-hydroxymethyl crown ethers 32 were attached in the last step (Scheme 18) to yield mesogenic 33. 

Scheme 18 Synthesis and mesogenic properties of crown ethers 33 with a lateral p-terphenyl group...

There is only one example that fits into this section and it uses a spectacular way to form the crown ether in the polymer that is worth noting (Scheme 22). In contrast to all other procedures that use crown ethers pre-formed in the monomers, this example by Percec forms the crown ether [13]crown-4 during the polymerization reaction [59]. Cationic cyclocopolymerization of l,2-bis(2-ethenyloxyethoxy)ben-zene 37 with the mesogenic cyano biphenyl unit 38 gave copolymer 39 with a 1 1 ratio of 37 38 displaying a smectic phase. 

The electric conductivity was also measured for complexes of taper-shaped mesogens with oligo(ethylene oxide) central groups. The DC conductivity is in a range of 10 9 to 10 6 S cm-1 and shows a step-like increase at the crystal-columnar phase transition [86]. It was also shown that taper-shaped molecules adjacent to different endo-receptors such as crown ethers or oligo(ethylene oxide) chains were miscible with a poly(methacrylate) matrix and formed isomorphic phases [87]. Applications as columnar reaction media for polymerizations were foreseen. Comprehensive summaries of Percec s taper-shaped molecules can be found in the literature [88, 89]. 

Gitsov presented a series of poly(benzyl ether) monodendrimers capable of cation complexation lacking alkyloxy side chains which were non-mesomorphic [93], Laschat followed a different route to obtain disk-like liquid crystals equipped with crown ether moieties. The crown ether was not attached to the mesogenic... 

The term metallomesogen is often used when a (transition) metal is complexed (by coordinative bonds between the crown heteroatoms and the metal center) by a crown ether. These compounds were discussed in the previous sections as complexed crown ethers (instead of metallomesogens ) together with their uncomplexed precursors. In this section, mesogens with a covalent bond between... 

Pioneering work in the field of luminescence by lanthanidomesogens in the liquid crystal state has been done by Suarez et al. In a seminal paper, they monitored the luminescence intensity and the excited state lifetime of a solvated Eu(N03)3 complex of a 1,7-diaza-l8-crown ether with mesogenic pendant arms (Figure 2.19) as a function of the temperature to detect phase transitions. The integrated intensity of the Do —> transition (fobs) and the Eu( Dq) lifetime (ToBs) were found to... 

Figure 2.19 Europium(III) complex of a l,7-diaza-18-crown ether with mesogenic...

Figure 2.40 Euminescent fluorinated crown-ether-isocyanide gold(I) metallo-mesogens...

A. Crown ether ligand as part of the mesogenic unit... 

---