Reactions in Supramolecular Systems

Lucia Zakharova, Alla Mirgorodskaya, Elena Zhiltsova, Ludmila Kudryavtseva, 

Arbuzov Institute of Organic and Physical Chemistry of the Russian Academy of Science, ul. Akad. Arbuzov 8, 420088 Kazan (Russia) 

At first, the catalytic effect of the single micellar solutions based on surfactants of different structure was studied. The data covering this period are reviewed in refs. The regularities when generalized made it possible to proceed with the more complicated systems, in particular, the binary surfactant solutions, the polymer-surfactant and polymer-cyclophane (calixarene, pyrimidinophane, and etc.) systems. For these systems, the terms supramolecular systems (ensembles, compositions, complexes) are further used. 

Molecular Encapsulation Organic Reactions in Constrained Systems Edited by Udo H. Brinker and Jean-Luc Mieusset 2010 John Wiley Sons, Ltd 

2 The Single Micellar Systems Factors of Concentration and Micellar Microenvironment 

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Optimizing the structure and interactions of a supramolecular system still requires sophisticated design, synthesis, and experimental feedback even in the ground state, and should be much more difficult for supramolecular photochemical reactions. In this context, the combined use of external factors, such as temperature, solvent and pressure, provides a versatile and convenient tool for manipulating photochemical reactions in supramolecular system. The role of these external factors is closely correlated with the properties, in particular rigidity, of supramolecular host employed, and the outcomes are often significantly different from those observed in conventional photoreactions, which would be another reason for performing photochemical reaction in supramolecular system. 

L. Ya. Zakharova, A. B. Mirgorodskaya, E. P. Zhil tsova, L. A. Kudryavtseva, A. I. Konovalov, Catalysis of nucleophilic substitution reactions in supramolecular systems, Russ. Chem. Bull., 2004, 53, 1385-1401. 

In supramolecular systems, electronic interactions between metal-polypyridine and other redox-active or units are too small to perturb ground-state electrochemical and spectroscopic properties but are sufficient to enable very fast intramolecular electron-transfer reactions upon excitation. 

Photoinduced electron transfer reactions in supramolecular model systems based on metalloporphyrins 97YGK557. 

While most studies on supramolecular photochemistry have focused on the design and synthesis of suitable hosts that provide desirable interaction or enviromnent for aimed photosubstrates, recent studies by Inoue et al. suggest that the external factors, such as temperature, solvent and pressure, may also play a crucial role in controlling the reactivity and selectivity of photoreactions in supramolecular systems. Changing environmental variants, which is much easier to practice than the optimization of host structure involving tedious syntheses, can provide not only a convenient, yet useful, tool for controlhng photochemical outcomes, but also important information regarding the mechanism of supramolecular photochemical reaction. 

The constant C has units of concentration and denotes the concentration at which intramolecular and intermolecular reactions have the same rate. In the 1970s, the term effective molarity (EM) was widely adopted to describe this parameter and characterize the rate enhancements observed for intramolecular reactions. The EM defined by Eq. (1) is a kinetic EM, because it is based on the ratio of two reaction rates, but it is possible to define a thermodynamic EM in the same way, if equibbrium constants are substituted for rate constants (Eq. (2)). In this review, we will use thermodynamic EM to quantify the chelate cooperativity associated with multivalent noncovalent interactions in supramolecular systems. 

Promising approach to charge transport phenomena in supramolecular systems is physicochemical and kinetical consideration of these reaction in terms of local properties of the media and reactants local permittivity, microviscosity, local concentrations, pH, pX, Ag, etc.). Relationships... 

Since in supramolecular systems electron-and energy transfer processes are no longer limited by diffusion, they take place by first order kinetics and in suitably designed supramolecular systems they can involve even very short lived excited states. The reactions described by (2.13-2.15) correspond to the key step (first order rate constant kg. Fig. 2.1) occurring in the analogous bimolecular reactions (2.1-2.3) taking place in the encounters formed by diffusion. The parameters affecting the rates of such unimolecular reactions will be discussed in Sects. 2.3 and 2.5. 

The main supramolecular self-assembled species involved in analytical chemistry are micelles (direct and reversed), microemulsions (oil/water and water/oil), liposomes, and vesicles, Langmuir-Blodgett films composed of diphilic surfactant molecules or ions. They can form in aqueous, nonaqueous liquid media and on the surface. The other species involved in supramolecular analytical chemistry are molecules-receptors such as calixarenes, cyclodextrins, cyclophanes, cyclopeptides, crown ethers etc. Furthermore, new supramolecular host-guest systems arise due to analytical reaction or process. 

The transition state of concerted Diels-Alder reactions has stringent regio- and stereochemical requirements and can assume settled configurations if the reaction is carried out in a molecular cavity. Cyclodextrins, porphyrin derivatives and cyclophanes are the supramolecular systems that have been most investigated. 

After more than 20 years, Walde et al. (1994) returned in a way to coacervate experiments, although using other methods. Walde (from the Luisi group) repeated nucleotide polymerisation of ADP to give polyadenylic acid, catalysed by polynucleotide phosphorylase (PNPase). But instead of Oparin s coacervates, the Zurich group used micelles and self-forming vesicles. They were able to demonstrate that enzyme-catalysed reactions can take place in these molecular structures, which can thus serve as protocell models. Two different supramolecular systems were used ... 

This monograph described the techniques that can be used for the study of fast processes in supramolecular dynamics. Some of the techniques have been widely used while others have only been applied recently. Many of the techniques rely on the presence of a chromophore in the guest or host that can be used to monitor changes in absorption or fluorescence as the reaction proceeds. This feature is a reflection of the ease in building detection systems for absorption and fluorescence measurements, and in the case of fluorescence it is also related to the high sensitivity of this method. For systems that do not contain a chromophore the dynamics can be... 

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