Guest isomerization

D. Badjic and N.M. Kostic, Behavior of organic compounds confined in monoliths of sol-gel silica glass. Effects of guest-host hydrogen bonding on uptake, release, and isomerization of the guest compounds. J. Mater. Chem. 11, 408 118 (2001). 

The architectural isomerism described above, in which a framework based on a specific pillar exhibits two different forms depending upon the guest size, suggests that GS hosts with longer pillars would revert to the bilayer framework for the larger guests. Indeed, we demonstrated that the bilayer-to-brick isomerism observed for G2VI with 1,4-dibromobenzene, 1,4-divinylbenzene, 1-nitro-... 

Commercial LASs are complex mixtures of four individual LASs (C10-C13) with 20 possible positional isomers. Isomeric separation can be achieved by solvophobic association with SDS or host-guest interaction with cyclodextrins. Complete resolution of 19 isomers was achieved using 10 mM phosphate buffer (pH 6.8) containing 40 mM SDS and 30% acetonitrile [4]. LAS isomers in technical products were separated using a-cyclodextrin, but complete resolution of all isomers was not achieved [5]. 

Yabe A, Kawabata Y, Niino H, Tanaka M, Ouchi A, Takahashi H, Tamura S, Tagaki W, Nakahara H, Fukuda K. Cis-trans isomerization of azobenzenes included as guests in Langmuir-Blodgett films of amphiphilic beta-cyclodextrin. Chem Lett 1988 l-4. 

Figure 7. Mechanism of catenane formation (amide type) the guest is orthogonally embedded in an intermediate macrocycle, the concave template. Depending on the substitution pattern of the reactants (pathways A and B) isomeric catenanes are obtained. For the sake of clarity the diacid dichloride is drawn here to be the nesting guest even though there is clear indication that the effective interactions take place between the corresponding monoamide and the macrocycle.

In contrast, when the substitution pattern is reversed, i.e. the 2,5-fiirane unit is incorporated in the diamine moiety 25 and reacted with isophthaloyl dichloride (3), not only macrocycle 24, but also the two possible isomeric catenanes 26 and 27 are formed. This suggests that the isophthaloyl unit is a better guest or reacts quicker than the furanoyl moiety. The yields obtained for 26 (20%) and 27 (8%) indicate that the isophthaloyl-guest in the second macrocyclization prefers the west-side a) niche of the host 24. Both isomers, the in/out 26 and the out/out 27 furano catenane could be crystallized and their X-ray crystal structures were... 

To realize the above system, it is required to design a polymer which reversibly changes the molecular properties, such as hydrophilicity, by the external stimulation. Many molecules are known to be reversibly transformed to other isomers by external stimulation, such as photons, electrons or chemicals. Table 1 lists a few examples. Azobenzene shows the property change by photoirradiation. It isomerizes from the trans to the cis form by ultraviolet irradiation, and the dipole moment increases from 0.5 to 3.1 deb ye. The polar cis form returns to the less polar tram form by visible irradiation. Electrochemical oxidation of ferrocene changes the hydrophilicity. When it is oxidized from Fe(II) to Fe(III), the hydrophilicity increases. The Feflll) state returns to the Fe(II) state by either electrochemical or chemical reduction. Host molecules also change the properties in the presence of suitable guest ions. Benzo[18]crown-6, for example, captures potassium ions in the cavity, and increases the hydrophilicity. 

Fig. 6.15 Different forms of inclusion of eo-sin guest molecules in (isomeric) photo-switchable POPAM host compounds. The number and position of the included eosin molecules vary according to the conditions...

In this review are considered combinations of CDs and their derivatives, in which compounds bearing azo unit are chemically bonded or mechanically associated with the host. In this case, the binding properties are modified by light when the guest molecule undergoes trans 5= cis isomerization [6a]. 

Most of the isomeric guests are achiral compounds, and therefore achiral hosts with variable properties are effective enough to selectively form a molecular complex with one of these hosts. 2,2/-Dihydroxy-l,l/-binaphthyl 17, has been found to... 

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