Host-guest systems, reversible

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. 

To pose a question, are the exchange processes between the free and the bound states under examination faster or slower than the timescale of the analytical technique The actual rate constants for the forwards and reverse reactions of a simple host-guest system, ki and k i (24), correspond to the bimolecular complexation (units = M s" ) and the unimolecular decomplexation (units = s ). 

With the demonstration of the existence of a successful host-guest system between BPP34C10 and [PQT][PF6]2, the next question to be addressed was the reversal of the roles of the host and the guest, i.e., the formation of a receptor incorporating bipyridium units, which would complex with a neutral hyrdroquinol derivative as the substrate. To this end, the tetracationic cyclophane [BBIPYBIXYCY][PF6]4 was synthesised Scheme 2) from the known [BBIPYXY][PF6]2 - obtained from 4,4 -bipyridine and... 

The building blocks of supramolecular systems are held together by intramolecular interactions and these systems are reversible. This intrinsic property is not only a consequence of the more labile interactions within supermolecules, compared to covalent bonds in molecules, but reversibility is essential for the function expressed by supramolecular systems. Kinetics can never be inferred from thermodynamic studies. For example, the knowledge of a host-guest equilibrium constant does not... 

The field of supramolecular chemistry is concerned with a large number of systems ranging from simple host-guest complexes to more complicated solution assemblies, as well as two-dimensional (organized monolayers) and three-dimensional assemblies (crystalline solids). Nonco-valent interactions play an important role in the kinetic assembly and thermodynamic stabilization of all these systems and constitute their most distinctive feature. Electron-transfer reactions can obviously be affected by supramolecular structures, but the reverse is also true. It is possible to alter the structure and the thermodynamic stability of supramolecular assemblies using electrochemical (redox) conversions. In other words, electron-transfer reactions can be utilized to exert some degree of control on supramolecular aggregates. Provided in this article is an overview of the interplay between supramolecular structure and electron-transfer reactions. 

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