Cavities with aromatic guests

These hosts were used to study inclusion complexation with aromatic guests in aqueous solution.23 The differences in binding between the phenethylamines and the benzylamines are shown in Table 1. Host 13 and 14 bound aromatic guests better than host 15 and 16. It is possible that the ammonium groups of the benzyl amine hosts are directed into the cavity this reduces its hydrophobicity and results in a low binding constant. There was also a strong... 

The calixarenes are phenol-formaldehyde cyclic oligomers which possess hydro-phobic cavities capable of forming inclusion complexes with aromatic guest molecules in the solid state [1,2]. They also have the ability to function as ion and molecular carriers as well as enzyme mimics [1, 3,4]. 

Diederich and coworkers [10] synthesized so-called dendrophanes (Figure 13.6) containing a paracyclophane core embedded in dendritic poly(ether-amide) shells. X-ray crystal-structure analysis indicated that these dendrimers had an open cavity binding site in the center, suitable for the binding of aromatic guests. NMR and fluorescence titration experiments revealed a site specific binding between these dendrimers and 6-(p-toluidino)naphthalene-2-sulfonate (TNS) with a 1 1 association. Also, the fluorescence spectral shift of TNS, which is... 

Of particular interest in the application of cyclodextrins is the enhancement of luminescence from molecules when they are present in a cyclodextrin cavity. Polynuclear aromatic hydrocarbons show virtually no phosphorescence in solution. If, however, these compounds in solution are encapsulated with 1,2-dibromoethane (enhances intersystem crossing by increasing spin-orbit coupling external heavy atom effect) in the cavities of P-cyclodextrin and nitrogen gas passed, intense phosphorescence emission occurs at room temperature. Cyclodextrins form complexes with guest molecules, which fit into the cavity so that the microenvironment around the guest molecule is different from that in... 

In order to increase the n-n interaction with the aromatic guests, two molecular clips with naphthalene walls (compounds 8 and 9) were synthesized. These molecules had quite unexpected properties. Surprisingly, the clip with 1,4-dimethoxynaphthalene cavity walls (8) did not bind guest molecules [15]. The reason for this behavior became clear when the X-ray structure was solved [16] (Fig. 6). All four methoxy groups of 8 were found to point into the cavity, completely blocking the access of a potential guest to the carbonyl groups. A similar structure may also be present in solution. 

Compounds 11 and 12 also form complexes with aromatic diammonium salts (Table 5). The dG values of binding vary from 44-54 kJ/mol. In the case of p-xylylenediammonium picrate, the aromatic protons of the guest give two signals, because two of them are in the shielding zone of the cavity walls, and the other two are in the deshielding zone. 

Trithiocyanuric acid (TCA) and 4,4 -bipyridyl (BP) form hydrogen-bonded co-crystals with aromatic compounds such as benzene, toluene, /7-xylene and anthracene. The TCA-BP co-crystal is composed of cavities formed by the N-H - N hydrogen bonds between the two molecules, and the three-dimensional structure contains channels of approximately 10 A where aromatic molecules are accommodated. The molar ratios of TCA, BP and the aromatic compound in the co-crystals are 2 1 1 or 2 1 0.5. Benzene, toluene and /7-xylene are removed from the channels around 190, 183 and 170 °C respectively, and these aromatic guests can be reintroduced into the empty channels of the apo-hosts. The apo-hosts with empty channels have reasonable thermal stability and exhibit shape selectivity in that the empty channels accommodate /7-xylene but not m- or o-xylene or mesitylene. 

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