Studies in Supramolecular Systems

Structure 5.14. Bifunctional supermolecule. Reprinted from Structure 5.18 J. Costamagna, G. Ferraudi, J. Canales and J. Vargas, Carbon dioxide activation by aza-macrocyclic complexes. Coordination Chemistry Reviews 148 (1996) 221-248. Copyright 1996, with permission of Elsevier. 

A bifunctional supramolecule (5.14 ) has been synthesized and its redox and CO2 catalytic properties have been investigated. Although this process induced reductive photocleavage of the supramolecule, catalytic studies show twice the amount of CO is produced compared with the multimolecular system composed of Ru(bpy)3, pyridinium salt and Ni(Cyclam) +. This example shows how the Ru(II) unit can be used as an antenna and allows the charge transfer reaction and to obtain the cooperative effects. 

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With the increased use of equations with several fitting parameters, it has been necessary to develop more sophisticated fitting procedures, hi recent years, many such procedures have been described [38,40,41,80-84]. Global analysis [33-37,43,45,48,78,79] and the maximum entropy method (MEM) [38,39] will be described briefly, as these methods are relevant for studies in supramolecular systems and have been developed sufficiently to be applied by most users. 

Biological membranes are highly complicated entities performing various functions. Thus also in this case one has to study simpler supramolecular systems (like those shown in Figure 4.1) to better understand their operation. A... 

Chemical Equilibrium in Supramolecular Systems as Studied by NMR Spectroscopy 113... 

Studies on electron transfer rates through proteins, DNA and in supramolecular systems have shown that generally such rates decrease exponentially with increasing donor-acceptor distance according to the following equations [10] ... 

Wasielewski, M.R. (2002) High time resolution Q-band EPR study of sequential electron transfer in a triad oriented in a liquid crystal, J. Phys. Chem. A 106,1933-1937 Wasielewski, M.R. (1992) Photoinduced electron transfer in supramolecular systems of artificial photosynthesis, Chem. Rev. 92, 435-461. 

The cavities and voids inside dendritic structures are of great importance, particularly in the study of supramolecular systems. The nature of this void, how it is affected by dendrimer size, constitution and solvent are of supreme importance in relation to potential applications [15], It is therefore necessary to discover ways to characterize the microenvironment in the dendrimer. Many investigators have made use of functional probes in order to study dendritic microenvironments [56,57], These probes are either attached covalently to the dendrimer or introduced as guest species and the effects of solvent and dendrimer size on the microenvironment are then studied. 

The theoretical study of supramolecular systems has certainly just begun. Tlte technical means for such studies have been provided within the last decades. In the future, these methods will be improved and refined. Also new approaches will be envisaged, which are tailored for the specific purposes of supramolecular chemistry - some of which have already been discussed in this work. 

Titiloye, J.O., Tschaufeser, P. and Parker, S.C. (1992) In Spectroscopic and Computational Studies of Supramolecular Systems (ed. J.E.D. Davies), Kluwer Academic Publishing, Dordrecht. 

This chapter covered the use of excited states as probes for dynamic studies involving supramolecular stmctures. It was shown that excited singlet and triplet states provide complementary capabilities for these studies. Micelles are the best characterized structures, and most of the methodology currently in use was developed when studying these supramolecular systems. Other self-assemblies, such as vesicles and bile salt aggregates, have been studied to some extent, but the dynamics of guest complexation has not been fully described and will undoubtedly be explored in the future. Cyclodextrins are widely employed as... 

J. Cioslowski, in Spectroscopic and Computational Studies of Supramolecular Systems. J. E. Davies, Ed., Kluwer Academic Publishers, Dordrecht in press, 1992. Ab Initio Electronic Structure Calculations on Endohedral Complexes of the Cgo Cluster. 

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. 

Sandstedt. C.A. Michalski, D. Eckhardt. C.J. Quantitative measurement of guest-host interactions in supramolecular systems A comparative Brillouin scattering study of the Dianin s compound clathrand and two of its isostructural clathrates. J. Chem. Phys. 2000. 112 (7). 7606. 

The induction of chiral structure in supramolecular systems at equilibrium was widely studied, partially because of surging interest in the hierarchical nature of the self-assembly of chiral aggregates. It is also of utmost relevance in the study of chiral information in unnatural systems displaying new properties. 

Phase transitions were reported in other clathrate systems. Although these were not studied systematically, they are important as indications of generality of phase transitions in supramolecular systems. Some are of interest because of the transition mechanism, others are of interest for molecular recognition involving cooperative effects, and still others are for potential use in practical applications. 

The evolution of computational chemistry, facilitated by the development of ever more powerful computers in conjunction with more efficient computational methods, has increasingly contributed to a better understanding of structural and thermodynamical features of chemical systems.In this context, computational studies of supramolecular systems became increasingly popular in order to interpret processes of molecular recognition and supramolecular organization. "... 

It would be completely beyond the scope of this chapter to review all the uses of ferrocene in supramolecular systems. The author will then confine himself to summarize the various ways in which, ferrocene has been used in the design and study of supramolecular systems by his group. 

M.R. Chierotti, R. Gobetto, Solid-state NMR studies of weak interactions in supramolecular systems, Chem. Commun. (2008) 1621—1634. 

Here, we want to discuss diffusion NMR experiments from a pragmatic point of view in order to show what information can be obtained and how reliable it is, focusing attention on supramolecular objects of intermediate dimensions. In particular, after recalling the principles underlying diffusion NMR spectroscopy and the measurement of the translational self-diffusion coefficient (A) (Section 2), we show how accurate hydrodynamic dimensions can be derived from A once the shape and size of the diffusing particles have been correctly taken into account (Section 3). Later on, the application of diffusion NMR to the study of supramolecular systems is described (Section 4) in terms of determination of the average hydro-dynamic dimensions and thermodynamic parameters of the self-assembly processes. 

After a brief introduction focused on the NMR approach to the study of solid-state systems, the chapter is organized into two parts that presents new methods for probing, first, the structure and, second, the dynamics in supramolecular systems. 

Computational methods such as molecular mechanics (MM) and quantum theory calculations have become convenient and reliable techniques in the analysis of CD data. A common approach in chiral supramolecular structural study is to examine if a supramolecular conformation simulated by MM method is consistent with the observed CD spectrum of the sample. However, in order to use this approach, one needs to correlate the stereostructures and CD data by using, for example, exciton chirality method or reference CD spectra of analogs with known structures. Thus, it should be noted that the application of this method has some limitations for evaluation in supramolecular systems. 

The photodimerization of acenaphtylene was also studied in supramolecular hydrogel media upon irradiation at 350 nm for 12 h. The antilsyn photodimer ratio was found to be higher in the gel state compared to micellar systems formed from cationic bile salts, which was... 

Although the development of antenna-sensitizer devices for the spectral sensitization of semiconductors might have some practical impact, it would clearly be unrealistic to consider most of the work described in this article of direct relevance to photocatalysis. It seems likely, however, that the need for increasing efficiency and selectivity will lead towards the development of complex photocatalytic systems of supramolecular nature. Thus, further fundamental studies on ways to control and direct photoinduced energy and electron transfer processes in supramolecular systems seem to be worthwhile. 

The conclusion of this paper [4] clearly shows that lifetimes measurements not only enable the study of the photophysical behavior of molecules, but also allow a glance at structural changes in supramolecular systems. 

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