Supramolecular systems defined

The dynamics of a supramolecular system are defined by the association and dissociation rate constants of the various components of the system. The time-scale for the dynamic events is influenced by the size (length-scale) and by the complexity of the system. The fastest time for an event to occur in solution is limited by the diffusion of the various components to form encounter complexes. This diffusion limit provides an estimate for the shortest time scale required for kinetic measurements. The diffusion of a small molecule in water over a distance of 1 nm, which is the length-scale for the size of small host systems such as CDs or calixarenes, is 3 ns at room temperature. In general terms, one can define that mobility within host systems can occur on time scales shorter than nanoseconds, while the association/dissociation processes are expected to occur in nanoseconds or on longer time scales. The complexity of a system also influences its dynamics, since various kinetic events can occur over different time scales. An increase in complexity can be related to an increase in the number of building blocks within the system, or complexity can be related to the presence of more than one binding site. 

One may venture to predict that this instructed mixture paradigm will define a major line of development of chemistry in the years to come the spontaneous but controlled build-up of structurally organized and functionally integrated supramolecular systems from a preexisting soup of instructed components following well-defined programmes and interactional algorithms. 

Layered materials are used to produce supramolecular assemblies, in which the properties of the assemblies differ from those of the individual members of the assembly, such that the whole is more than the sum of its components. Such a behavior may be used as a criterion to define a supramolecular system, and nature provides numerous examples. The light-harvesting complex of the photosynthetic apparatus is one such example of outstanding organization. 

The LB technique enables us to obtain ultrathin films with the structures and thicknesses controlled at the molecular level, which promises applications to various fields. One of the challenges is to construct a new type of switching device based on conductive LB films. For this purpose, a supramolecular system is used since the geometrical alignment of the functional units are defined by the molecular design of the supermolecules to be used. 

Balzani et al. (70) employed a strategy focusing on complexes as metals and as ligands (71,72), for developing a number of interesting supramolecular systems. Ditopic polypyridyl ligands, for example, 2,3- and 2,5-dipyridylpyrazine (2,3-dpp and 2,5-dpp), 2,2 -biquinoline (biq), tetrapyrido[3,2-fl 2, 3 -c 3",2"-/z 2", 3 "-/]-phenazine (73,74) (tppz), and 2,3,5,6-tetrakis(2-pyridyl)pyrazine (75-77) (tpypz), in combination with monotopic ligands (e.g., bpy and phen) were utilized in the preparation of a number of homo- and heteropolynuclear complexes of well-defined structures (Fig. 2). 

While non-covalent bonding interactions may define and direct the self-assembly processes that leads to new supramolecular systems, it is important to note that in general such forces still continue to act once the respective systems are formed. As such, they will continue to govern any dynamic processes that occur within the self-assembled structure. 

Catenanes (Latin catena, chain) may be defined as linked supramolecular systems consisting of cyclic subunits, not necessarily identical, held together by mechanical means in an analogous fashion to the links in a chain. Usually the components also show considerable mutual association via one or more non-covalent interactions. 

Bimolecular ET processes are far from ideal from a mechanistic standpoint, as the experimental kinetics is complicated by diffusion effects and the spatial relationship between A and B is completely undefined. If, on the other hand, A and B are pre-assembled in a supramolecular structure, the ET process becomes unim-olecular, the kinetics is expected to be free from diffusional artifacts, and the spatial relationship between A and B may be to some extent defined. Although supramolecular systems based on weak interactions are interesting as well (see Volume III, Part 2, Chapters 4-8), the present review is specifically concerned with systems where the A and B units are covalently linked via a suitable bridging group, L. 

When six coordination sites around octahedral metal ion are occupied by only bidentate ligands, stereoisomers around the metal ion are formed. However, the coordination of symmetric tridentate ligands to a six-coordinate metal ion leads to only one isomer. Furthermore, tridentate bridging ligands connect metals in a linear fashion, resulting in the formation of stereochemically well-defined supramolecular systems. The rigid structure of these systems is suitable for studies of electron or energy transfer events between the donor-acceptor dyads. 

The determination of the rate constants for photoinduced processes in supramolecular systems is possible via time-resolved fluorescence techniques [16] provided that the characteristic times of these processes fall into the experimental time window that is defined by the lifetime of the involved excited states. 

Conversely, the sheer complexity of the supramolecular system may, if it crystallizes at all, lead to a very poorly defined structure. The first problem is then in growing a suitable crystal. If volatile solvents are in any way involved in the crystal lattice the chances are that they will evaporate before or during the data-gathering phase of the crystallographic experiment. Failing this the molecular... 

Remarkably, the H NMR spectrum of the mixture is simply equal to the sum of the H NMR spectra of its components. This spectroscopic earmark indicates that this mixture of molecules undergoes self-sorting. Self-sorting refers to the ability of a molecule or entity to efficiently distinguish between self and non-self even within a complex mixture. Self-sorting is commonly observed in Nature (e.g. the immune system) but is still relatively rare in designed supramolecular systems. The molecular clips described here represent a well-defined model system for studies of self-sorting. 

Supramolecular systems can be defined as a collection of molecular units which are held together by noncovalent bonds. Chemical and physical properties of supramolecular systems, which are different from the sum of the individual properties of each respective unit, are employed to modify chemical reactivity or perform complex functions. 

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