Layered hosts

Obviously, in a thin layer host material doped device, the EL efficiency is quite low and additional Alq3 emission is observed, however, when a HBL of BCP was used, the EL efficiency dramatically increased and there was no Alq3 emission in the spectra. 

The dimensionality of the host influences various properties. Layered hosts can accommodate large guests as well as small, and Li ions may carry large solvent molecules with them when they enter the layers from the solution in an electrochemical cell. Such cointercalation, discussed in Section 7.5.2, is not a problem in three-dimensional hosts, where the sites are usually too small to accommodate organic molecules. 

Fig. 7.2 Classification of intercalation compounds (a) host of chains weakly bonded together (b) three-dimensional host with one-dimensional lattice of sites for guest ions (c) layered host (two-dimensional host and two-dimensional lattice of sites) (d) three-dimensional host with three-dimensional lattice of sites.

Two-dimensional systems layered host, layers of sites for guests... 

Table 1 Nuclei growth models for solid state reactions. Values possible for intercalation into a layered host are highlighted in bold ...

The values of the exponent n that can be obtained for an intercalation reaction into a layered host are therefore hmited. Possible values are highhghted in bold in Table 1. 

For layered host materials as used in the lithium ion cell, the term intercalation is used for the insertion of guests into the host structure. 

Organic molecules such as aniline, pyrrole and 2,2 -bithiophene have been intercalated and polymerized within the galleries of clay minerals, FeOCl, V2O5 gel and other layered hosts to yield multilayered inorganic/organic polymer nanocomposites... 

Many of the same ionic surfactants used for the assembly of mesostructured molecular sieve catalysts [1-4] and related bulk phases [5] can be intercalated in a variety of layered host structures [6]. We have recently demonstrated that some of these mesostructure - forming surfactants retain their structure directing properties when intercalated in the galleries of smectite clays. In a manner quite analogous to bulk mesostructure formation, the intercalated surfactants direct the assembly of an open framework metal oxide (silica) structure within the constrained gallery regions of the layered host (7). The resulting porous intercalates are referred to as porous clay heterostructures (PCH). 

Here we report the synthesis and catalytic application of a new porous clay heterostructure material derived from synthetic saponite as the layered host. Saponite is a tetrahedrally charged smectite clay wherein the aluminum substitutes for silicon in the tetrahedral sheet of the 2 1 layer lattice structure. In alumina - pillared form saponite is an effective solid acid catalyst [8-10], but its catalytic utility is limited in part by a pore structure in the micropore domain. The PCH form of saponite should be much more accessible for large molecule catalysis. Accordingly, Friedel-Crafts alkylation of bulky 2, 4-di-tert-butylphenol (DBP) (molecular size (A) 9.5x6.1x4.4) with cinnamyl alcohol to produce 6,8-di-tert-butyl-2, 3-dihydro[4H] benzopyran (molecular size (A) 13.5x7.9x 4.9) was used as a probe reaction for SAP-PCH. This large substrate reaction also was selected in part because only mesoporous molecular sieves are known to provide the accessible acid sites for catalysis [11]. Conventional zeolites and pillared clays are poor catalysts for this reaction because the reagents cannot readily access the small micropores. 

It has been pointed out that these pillared intercalates are intrinsically difficult to synthesize in highly crystalline form because the layered hosts are basic, whereas most heteropolyacids are acidic and tend to decompose. Narita et al. (392) tried direct synthesis of a heteropolyanion-pillared layered double hydroxide by a coprecipitation reaction of Zn2+ and A1J+ ions in the presence of a moderately acidic lacunary Keggin anion, a-SiWn039 XRD of the product showed a basal spacing of 14.6 A, which corresponds to a gallery height of 9.9 A. The surface area was found to be 97 m2 g, which is three times that of the layered double hydroxide. 

Group (IV) metal phosphates and phosphonates, transition metal oxides (titanates, silicates, niobates, etc.), layered oxides, and double hydroxides (aluminum, magnesium, iron, etc.) are some of the inorganic compounds used as layered host ma-... 

The layered arrangement makes these materials very interesting from the point of view of host-guest behaviour because ionic or molecular guest species may be inserted between one layer and another causing the layers to expand or swell. Guest intercalation is generally reversible, and it is an important characteristic of layered solids that, rather like zeolites, they can retain their layered host structure... 

The synthesis, characterization and properties of nanomaterials have become very active areas of research in the last few years. In particular, nanostructured materials assembled by means of supramolecular organization offer many exciting possibilities. These include self-assembled monolayers and multilayers with different functionalities, intercalation in preassembled layered hosts and inorganic three-dimensional networks. The reader is referred to the special issue of Chemistry of Materials91 for an overview of present day interests. There are many recent reviews on the varied aspects of nanomaterials. The work of Alivisatos92 on the structural transitions, elec-... 

We have been investigating the use of imogolite as a pillaring agent for smectite clays with layer lattice structures ". The regular intercalation of the tubes within the layered host results in the formation of a tubular silicate-layered silicate (TSLS) complex. These new nanocomposite materials may be viewed as pillared clays in which the pillars themselves are microporous. Significantly, the TSLS structure is thermally stable up to 450 C when montmorillonite is selected as the layered host . 

Layered materials serve as good hosts for accommodating photoresponsive molecular species. Ogawa and Kuroda [150] reviewed the photoprocesses in layered hosts. 

Numerous studies have established the wide scope of inclusion chemistry that is possible with inorganic solids. From layered hosts such as FeOCl and nanoporous membranes to zeolite molecular sieves, very different dimensions are accessible for the encapsulation of conjugated, potentially conducting materials. Zeolites are distinct hosts because they offer well-defined, crystalline pore systems at molecular dimensions, with sizes in mesoporous systems now reaching even beyond the molecular scale. 

Intercalation of organic molecules into layered host lattice produces a variety of organic-inorganic hybrid materials. The solvothermal method provides a reaction system that allows application of high temperatures and therefore is a powerful technique for preparation of intercalation compounds. Exfoliation of layers may occur because of applied high temperatures. For example, exfoliated poly-ethylene/montmoriUonite nanocomposites were reported to be prepared by solvothermal reaction of organophilic montmorillonite with polyethylene in toluene at 170°C for 2... 

Vanadium pentoxide xerogels are very reactive layered host materials which can be intercalated by various means such as cation-exchange, acid-base chemistry, or redox reactions. Vanadium pentoxide xerogel was prepared by polymerization of HVO3 after a few days of reaction at room temperature. The resultant xerogel... 

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