Host molecule

Most properties of linear polymers are controlled by two different factors. The chemical constitution of tire monomers detennines tire interaction strengtli between tire chains, tire interactions of tire polymer witli host molecules or witli interfaces. The monomer stmcture also detennines tire possible local confonnations of tire polymer chain. This relationship between the molecular stmcture and any interaction witli surrounding molecules is similar to tliat found for low-molecular-weight compounds. The second important parameter tliat controls polymer properties is tire molecular weight. Contrary to tire situation for low-molecular-weight compounds, it plays a fimdamental role in polymer behaviour. It detennines tire slow-mode dynamics and tire viscosity of polymers in solutions and in tire melt. These properties are of utmost importance in polymer rheology and condition tlieir processability. The mechanical properties, solubility and miscibility of different polymers also depend on tlieir molecular weights. 

Fig. 17. Prototypical host molecules based on more recent clathrate strategies.

Fig. 18. Crystal structures of recent clathrate design (a) coordinatoclathrate between host (39) (Fig. 17) and / -butanol (host—guest hydrogen bonding in the shaded area) (b) perspective view of the hehcal inclusion channel formed by diol host (43) (Fig. 17 all except one host molecule are represented...

More recently, Johnson and Sutherland have used bridged monoazacrowns as host molecules for bis-ammonium salts. The intramolecular bis-crown diammonium complex formed when the size relationships are appropriate is shown below as structure 8. 

Tautomerism involving compounds included in a host molecule (type 6) has been studied, in some cases involving triazoles and pyrazoles [87CL2317 88CL1061 93JCS(CC)1139] [the last reference describes a case of proton transfer for pyrazole included in l,l-di(2,4-dimethylphenyl)but-2-yn-l-ol]. 

Electrochemistry of supramolecular systems with heterocyclic fragments as host molecules (porphirinoids, complexes on the basis of 2,2 -bipyridine and 2,2, 2"-terpyridine, hetero- and heteracyclophanes) 98AG(E)216. 

Positively charged and electroneutral macroheterocycles as host molecules for anions 97CRV1609. 

Complexes of transition metals with macroheterocyclic ligands as host molecules in the construction of molecular containers 98CSR289. 

Facile syntheses of highly symmetric cage-type cryptands consisted of pyridine rings and their properties as host molecules 98YGK604. 

An example of the modular preparation of the cyclophane 3 from the substituted bipyridine 2 and a general tripeptide 1 is shown in Scheme 3-3. The host molecule 3 contains a pre-organized binding pocket. The overall basicity of such molecules also facilitates their intercalation within the lamellas of acidic zirconium phosphate, thus making this chemistry well suited for the desired application. 

In mimicking this type of function, noncyclic artificial carboxylic ionophores having two terminal groups of hydroxyl and carboxylic acid moieties were synthesized and the selective transport of alkali metal cations were examined by Yamazaki et al. 9 10). Noncyclic polyethers take on a pseudo-cyclic structure when coordinating cations and so it is possible to achieve the desired selectivity for specific cations by adjusting the length of the polyether chain 2). However, they were not able to observe any relationship between the selectivity and the structure of the host molecules in an active transport system using ionophores 1-3 10). (Table 1)... 

The result is explained by considering the stacking structure between the quinoline moiety and the benzene ring linked to the carboxylic acid, which gives the cavity size adequate for Li+. (Fig. 3) Several selective host molecules for Li+ such as [13]crown-4 18), [14]crown-4 19), [16]crown-4 20>, or noncyclic polyether amide derivatives 21) also possess trimethylene moiety, and this is an interesting finding from the point of view of molecular design of new host molecules for Li+. 

By considering the stability constant and the lipophilicity of host molecules, Fyles et al. synthesized a series of carboxylic ionophores having a crown ether moiety and energetically developed the active transport of alkali metal cations 27-32). Ionophores 19-21 possess appropriate stability constants for K+ and show effective K+-selective transports (Fig. 5). Although all of the corresponding [15]crown-5 derivatives (22-24) selectively transport Na+, their transport rates are rather slow compared with... 

This review surveys the types of host molecules that are applicable to the active transport system. It need scarcely be said that these results, which are based on selective transport in passive transport systems (see the Chaps. 3 and 5), strongly supports this consideration. From this point of view, a systematic investigation into the passive transport system as that by Izatt et al. is noted as one of the best approaches for clarifying the question of membrane transport77). 

Vogtle, F., Muller, W. M., and Watson, W. H. Stereochemistry of the Complexes of Neutral Guests with Neutral Crown Host Molecules, 125, 131-164 (1984). 

Fluorescent small molecules are used as dopants in either electron- or hole-transporting binders. These emitters are selected for their high photoluminescent quantum efficiency and for the color of their emission. Typical examples include perylene and its derivatives 44], quinacridones [45, penlaphenylcyclopenlcne [46], dicyanomethylene pyrans [47, 48], and rubrene [3(3, 49]. The emissive dopant is chosen to have a lower excited state energy than the host, such that if an exciton forms on a host molecule it will spontaneously transfer to the dopant. Relatively small concentrations of dopant are used, typically in the order of 1%, in order to avoid concentration quenching of their luminescence. 

The Zimm model predicts correctly the experimental scaling exponent xx ss M3/2 determined in dilute solutions under 0-conditions. In concentrated solution and melts, the hydrodynamic interaction between the polymer segments of the same chain is screened by the host molecules (Eq. 28) and a flexible polymer coil behaves much like a free-draining chain with a Rouse spectrum in the relaxation times. 

Complexation of uncharged molecules and anions by crown-type host molecules. F. Vogtle, H. Sieger and W, M. Muller, Top. Curr. Chem.,. 1981, 98,107-161 (130). 

A different non-classical approach to the resolution of sulphoxides was reported by Mikolajczyk and Drabowicz269-281. It is based on the fact that sulphinyl compounds very easily form inclusion complexes with /1-cyclodextrin. Since /1-cyclodextrin as the host molecule is chiral, its inclusion complexes with racemic guest substances used in an excess are mixtures of diastereoisomers that should be formed in unequal amounts. In this way a series of alkyl phenyl, alkyl p-tolyl and alkyl benzyl sulphoxides has been resolved. However, the optical purities of the partially resolved sulphoxides do not exceed 22% after... 

Cyclophanes or 7r-spherands have played a central role in the development of supramolecular chemistry forming an important class of organic host molecules for the inclusion of metal ions or organic molecules via n-n interactions. Particular examples are provided by their applications in synthesis [80], in the development of molecular sensors [81], and the development of cavities adequate for molecular reactions with possible applications in catalysis [82]. The classical organic synthesis of cyclophanes can be quite complex [83], so that the preparation of structurally related molecules via coordination or organometallic chemistry might be an interesting alternative. 

A new class of host molecules for the selective complexation of salts [237], alcohols [238], amines [239], and catecholamines [240] has been designed by combining crown ethers of different sizes with a boronic acid or boronate (Figs. 39 and 40). 

As we have implied, the ability of these host molecules to bind guests is often very specific, often linked to the hydrogen-bonding ability of the host," enabling... 

Calixarenes, which are macrocyclic compounds, are one of the best building blocks to design molecular hosts in supramolecular chemistry [158]. Synthesis of calix[4]arenes, which have been adamantylated, has been reported [105, 109]. In calix[4]arenes, adamantane or its ester/carboxylic acid derivatives were introduced as substituents (Fig. 29). The purpose of this synthesis was to learn how to employ the flexible chemistry of adamantane in order to construct different kinds of molecular hosts. The X-ray structure analysis of p-(l-adamantyl)thiacalix[4]arene [109] demonstrated that it contained four CHCI3 molecules, one of which was located inside the host molecule cavity, and the host molecule assumed the cone-like conformational shape (Fig. 30). 

An example for a host molecule with a layerlike structure is graphite. Various types of both organic and inorganic inclusion compounds, as well as stoichiometric and nonstoichiometric compounds, are known. 

Gas hydrates are a special form of clathrates. Here water is the host molecule. The first gas hydrate (with chlorine) was described in 1818 by Sir Humphrey Davy. Naturally-occurring gas hydrates in Siberia are methane hydrates. 

Clathrates and, in particular, gas hydrates can be decomposed very easily by dissolving or melting the crystal lattice of the host molecule. 

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