Intercomponent processes

The absorption spectrum of a supramolecular system can differ substantially from the sum of the spectra of the molecular components. Aside from those small shifts that can be dealt with in terms of perturbation of the spectra of the single components upon bridging, some totally new bands can be present in the spectrum of the supermolecule. These bands correspond to optical electron transfer transitions, commonly denominated charge-tranter and intervalence transfer transitions, in the organic [26] and inorganic [23-25] literature, respectively (process 7 in Fig. 4, eq 1). The factors that determine the spectroscopic characteristics of such bands 

The energy of an optical electron transfer transition, E p, is correlated according to Hush theory [24,25] to the energy gradient between the minima of the A.B and curves, AE, and to the reorganizational energy, X (eq 2). Detailed expressions for the calculation of 1 in terms of 

The optical electron transfer band is expected to be Gaussian-shaped, with a halfwidth, AVj 2 is directly related to the reorganiza-tional energy by eq 3 [24,25]. 

The intensity of the optical electron transfer band can be correlated according to Hush [24,25] to the magnitude of the electronic coupling matrix element The relationship is given by eq 4, where niax 

Very similar arguments can be used to analyze the possibility of the inverse radiative process, namely charge transfer emission (process 8 in Fig. 4, eq 5). The main difference is, of course, that the emission must take 

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In covalently linked systems, the most important intercomponent processes are PET and EnT (Figures 1 and 2). The theoretical aspects of such processes are dealt with in detail in Volume I, Part 1, Chapter 1. For the sake of convenience, this section summarizes some the basic concepts that will be used in the following discussion. 

A comprehensive view of the intercomponent processes taking place in this family of [3]catenates is illustrated schematically in Figure 17. 

The possibility of observing photoinduced intercomponent processes in hetero-dinuclear knotted complexes was made possible with the knots bearing a phenylene unit, as interchromophoric spacer [52b] (Figure 21). In fact, in this case the synthetic yield of the parent dicopper complex is high enough to allow further decomplexation recomplexation steps, as required. 

Recent work on polynuclear metal complexes [3], i.e., supramolecular systems containing covalently linked transition metal complexes as molecular components, is described in this article. Such systems have been designed in order to study intercomponent processes relevant to photoinduced charge separation and antenna functions. The possibility to use supramolecular anterma systems in the spectral sensitization of wide-bandgap semiconductors is also discussed in some detail. 

As we have seen above (Section IV.C), in the polynuclear complexes dealt with in this review it is possible to identify components which can undergo photoexcitation independently from one another. The excited component can then give rise to intercomponent energy transfer processes, in competition with intracomponent decay. For most of the components which constitute the examined systems, the lifetime of the lowest excited state is long enough to allow the occurrence of energy transfer to nearby components when suitable energetic and electronic conditions are satisfied. This is not usually the case for upper excited states, which usually decay very rapidly (picosecond time scale) to the lowest excited state within each component. 

In dumbbell-shaped component 326+, all the redox processes of the incorporated units are present at almost the same potentials as in the separated units (Fig. 13.30) this finding shows that there are no substantial intercomponent electronic interactions. On going from the dumbbell component to rotaxane 316+, some processes are affected while others are not (Fig. 13.30). 

Positional changes of atoms in a molecule or supermolecule correspond on the molecular scale to mechanical processes at the macroscopic level. One may therefore imagine the engineering of molecular machines that would be thermally, photochem-ically or electrochemically activated [1.7,1.9,8.3,8.109,8.278]. Mechanical switching processes consist of the reversible conversion of a bistable (or multistable) entity between two (or more) structurally or conformationally different states. Hindered internal rotation, configurational changes (for instance, cis-trans isomerization in azobenzene derivatives), intercomponent reorientations in supramolecular species (see Section 4.5) embody mechanical aspects of molecular behaviour. 

It should be noted that the 1,3-dimethoxybenzene and 2-naphthyl chromophoric units contained in the branches of the dendrimer are not involved in metal coordination. In some way, they belong to a second coordination sphere. If is considered a large metal complex, the absorption and emission bands of the 1,3-dimethoxybenzene and 2-naphthyl chromophoric units can formally be classified as LC. However, can be more properly viewed as a supramolecular (multicomponent) species (12). In such species, each chromophoric imit displays its own absorption spectrum since there is no appreciable interactions among them in the ground state, but in the excited state even weak interactions can cause intercomponent energy or electron-transfer processes. This kind of reasoning can also be applied to all the other systems discussed in this chapter. 

In interlocked compounds such as rotaxanes and catenanes, electron-donor and -acceptor units not only cause the presence of CT interactions, but are also responsible for the occurrence of intercomponent and intermolecular oxidation and reduction processes. Such processes weaken or even destroy the CT interactions that stabilize the structure of the compound, with a consequent change in its coconformation. External inputs, like electrons or photons, can be used to cause the redox processes and the structural rearrangements that follow. Suitably designed rotaxanes and catenanes can therefore exhibit machine-like movements that correspond to a binary logic behavior. 

In conclusion, for the RuM.8"+ [2]catenates family, photoinduced intercomponent energy and/or electron transfer processes are evideneed. The direction of such processes can be tuned upon a suitable choice of the metal ion complexing the [M(phen)2] +-type eoordination center, according to the schematic representation of Figure 19. 

It should be recalled, however, that even an interaction of a few cm (which cannot be noticed in spectroscopic experiments) may be sufficient to cause intercomponent energy transfer or electron transfer processes. As already mentioned, the nature and length of the bridging ligand can contribute strongly to the rate of the photoinduced processes. Many compounds have been labeled wire molecules, but in most cases the wire-type behavior could not be observed. However, one should first define what is a molecular wire and what are the expectations for such a system. 

The associated bottom-up process thus relies on the identification of building blocks connected by weak (i.e., noncovalent) interactions and possessing specific physicochemical properties, which can be further tuned by intercomponent communications operating at the (supra)molec-ular level (Lehn, 1995). 

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