Host-guest systems, continued

We mentioned in the previous section that the bulk NLO properties of a poled-polymer host-guest system might be expected to be proportional to the chromophore fi/3 multiplied by the number density of chromophores. However, for high loadings of molecules with large pi, the d33 and r33 coefficients do not continue to increase linearly with chromophore loading, but exhibit maxima these are attributed to competition between the interaction of the chromophores and the poling... 

Different methods can be used to determine the guest-to-host ratio, or stoichiometry, of a host-guest system. Among them, the continuous variations, also called Job s method [50], and the mole ratio method [51], are certainly the easiest and most common techniques. 

The lattice gas approach is valid within certain limits for typical metallic hydrides, binaries as well as ternaries. Deviation from this idealized picture indicates that metallic hydrides are not pure host-guest systems, but real chemical compounds. An important difference between the model of hydrogen as a lattice gas, liquid, or solid and real metal hydrides lies in the nature of the phase transitions. Whereas the crystallization of a material is a first-order transition according to Landau s theory, an order-disorder transition in a hydride can be of first or second order. The structural relationships between ordered and disordered phases of metal hydrides have been proven in many cases by crystallographic group-subgroup relationships, which suggests the possibility of second-order (continuous) phase transitions. However, in many cases hints for a transition of first order were found due to a surface contamination of the sample that kinetically hinders the transition to proceed. 

In this procedure which is a continuation of the study of amphiphilic macromolecules containing PLA [48-50], the tosylated p CD was used as initiator for ring opening polymerization of lactide (9) and of 2-ethyl-2-oxazoline (10), leading to copolymer B. The TEM (transmission electron microscope) results have shown a tube-like self-assembly of formed copolymers B. Due to this type of self-assembly, the obtained copolymers are promising for use as host-guest systems and in the guest delivery. 

The host guest chemistry of crown ethers continues to be exploited for the development of new ionophores for the selective recovery of Hg(II). A novel crown ring system containing a redox switchable trithiadiazapentalene/trithiotriuret unit (17/18) allows control of Hg(II) in solvent extraction experiments between chloroform and water. The thiocar-bonyl sulfur donor sites outside the macrocycUc cavity of (18) are responsible for strong complexation and extractabiUty into chloroform. 

The development of new and improved NMR techniques is continual and fast-paced, with the range of systems that are suitable for study and the information that can be elucidated from them increasing aU the time. Recent years have seen a shift away from one-dimensional NMR techniques towards two-dimensional techniques, such as MQMAS, which are often capable of providing higher resolution. In particular, the recent development of the SATRAS technique opens up new possibilities. In addition to the applications outlined above, Antonijevic and coworkers [28] have reported on the use of SATRAS to study host-guest interactions and guest dynamics in framework solids. Nuclei that could be studied by this method include 0, Na, Al and Ga. 

These results continue to support the hypothesis that all members of oligomer series 23 exist in solution in conformationally well-ordered states with chiral cylindrical cavities capable of binding chiral rodlike guest molecules such as 39. Co-modularity of host—guest oligomeric pairs such as the system described here raises a number of engaging possibili-... 

The host Ni(4-mepy)4(NCS)2 If forms isomorphous 1 1 inclusion compounds with many monocyclic aromatic guests. In continuing attempts to determine the basis for the selectivity shown by towards pairs of such guests, distribution data between solid and liquid phases were reported for seven ternary systems at room temperature. These consist of -xylene, and each of the following in turn -bromotoluene, -chlorotoluene, -fluorotoluene, -dichlorobenzene, 4-meth-ylpyridine and benzene, as well as the system I — -chloro-toluene— -dichlorobenzene, The results, as well as those already published, were reviewed. 

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