Site isolation effect

The absorption and fluorescence spectra of a neat film made of RdB-den-drimer are shown in Fig. 2. The absorption spectrum in visible-wavelength region was similar to that obtained from a solution of RdB with a concentration less than 0.1 mmol/1. Interpretation of the fluorescence in terms of the Frank-Condon mechanism indicated that the core RdB chromophore behaved with a site-isolation effect and had little interaction with the neighboring chro-... 

Taming Carbon-Carbon Bond Cleavage on Metallic Surfaces, the "Site Isolation" Effect... 

In core- (and focal point-) functionalized dendrimers, the catalyst may benefit from the site isolation created by the environment of the dendritic structure. Site-isolation effects in dendrimers can also be beneficial for other functionalities (a review of this topic has appeared in Reference (10)). When reactions are deactivated by excess ligand and when a bimetallic deactivation mechanism is operative, core-functionalized dendrimers can minimize the deactivation. 

When the catalyst is located in the core of a dendrimer, its stability can also be increased by site-isolation effects. Core-functionalized dendritic catalysts supported on a carbosilane backbone were reported by Oosterom et al. 19). A novel route was developed to synthesize dendritic wedges with arylbromide as the focal point. These wedges were divergently coupled to a ferrocenyl diphosphine core to form dppf-like ligands (5). Other core-functionalized phosphine dendritic ligands have also been prepared by the same strategy 20). 

Polyolefins possessing properties distinctly different from those produced in homogeneous solution, characterised by an unusually high molecular weight, have recently been obtained by using catalysts with c/ .sc/-metallocene precursors supported directly on an unpretreated silica gel carrier [204,206], This may be due to the active site isolation effect, i.e. to the strict suppression of participation of other sites in the polymerisation [30],... 

The hb-PAEs of hb-P13 and hb-P15 contain NLO-active azo-functionalities, which are soluble, film-forming, and morphologically stable (Tg > 180 °C). Their poled films exhibited high SHG coefficients ( 33 up to 177pm/V), thanks to the chromophore-separation and site-isolation effects of the hyperbranched structures of the polymers in the three-dimensional space (Table 5) [28]. The optical nonlinearities of the poled films of the polymers are thermally stable with no drop in d33 observable when heated to 152 °C (Fig. 8), due to the facile cross-linking of the multiple acetylenic triple bonds in the hb-PAEs at moderate temperatures (e.g., 88 °C). 

Site isolation effects catalytically active centres such as metal complexes can be dispersed over the zeolite matrix. In this way, reaction between adjacent metal centres can be impeded for steric reasons (7). In other well-documented cases, intramolecular reactions are preferred over intermolecular reactions due to the spatial isolation of molecules adsorbed in low concentrations (8)... 

A spectacular, site-isolation effect in heterogeneous asymmetric catalysis was first reported by Pugin et al. The asymmetric hydrogenation of imine 1 is important for the commercial production of fS -metolachlor, a herbicide presently produced at >10000 tons per year. In this reaction, whereas homogeneous Ir-BPPM (2) catalyst prepared with [Ir(COD)Cl]2 was deactivated after 26% conversion (turnover frequency (TOP) min = 0), the covalently immobilized Ir catalysts, Si-PPM (3)-Ir, were much more active and productive (TOP min = up to 5.1 Scheme 2.1)... 

It is known that Ir-complexes in the presence of hydrogen can form catalytically inactive hydrogen-bridged dimers. The finding that the activities of immobilized catalysts increase with decreasing catalyst loading (Scheme 2.1) indicates that the dimer formation can be prevented by a site-isolation effect. 

The homocoupling of aryl halides, like the previously mentioned reaction, is caused by oxygen contaminant in the reaction medium. It may occur in spite of the site isolation effect caused by the polymer. Vigorous exclusion of oxygen in the coupling medium suppressed this side reaction efficiently. 

A general problem of alkene cross-metathesis is the formation of self-condensation products from the starting alkenes. On the solid phase, dimerization of polymer-bound alkene should be miniinized by the use of excess alkene in solution combined with the effective dilution of the resin-bound alkene by site isolation effects (see Suzuki coupling). Homocoupled products of the solution phase alkene are simply washed way during the resin washes. 

Another approach is to produce dendron-substituted polymers (or dendronized polymers) by combining the site-isolation effect of dendrimers with the good processability of linear polymers. This strategy provides a greater flexibility in designing suitable molecular structures for realizing high-performance EO materials. 

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