Photochemistry and Photophysics of Coordination Compounds

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14 V. Balzani et al. The example discussed above shows that, although the classical definition of supramolecular chemistry as “the chemistry beyond the molecule” [46] is quite useful in general, from a functional viewpoint the distinction between what is molecular and what is supramolecular can be better based on the degree of intercomponent electronic interactions [20, 49–52]. This concept is illustrated, for example, in Fig. 8 [53]. In the case of photon stimulation, a system A∼B, consisting of two units (∼ indicates any type of “bond” that keeps the units together), can be defined as a supramolecular species if light absorption leads to excited states that are substantially localized on either A or B, or causes an electron transfer from A to B (or vice versa). By contrast, when the excited states are substantially delocalized on the entire system, the species can be better considered as a large molecule. Similarly (Fig. 8), oxidation and reduction of a supramolecular species can substantially be described as oxidation and reduction of specific units, whereas oxidation and reduction of a large molecule leads to species where the hole or the electron are delocalized on the entire system. In more general terms, when the interaction energy between units is small compared to the other relevant energy parameters, a system can be considered a supramolecular species, regardless of the nature of the bonds that link the units. Species made of covalently linked (but weakly interacting) components, e.g., 2 and 3 showninFig.7,cantherefore be regarded as belonging to the supramolecular domain when they are stimulated by photons or electrons. It should be noted that the properties of each component of a supramolecular species, i.e., of an assembly of weakly interacting molecular components, can be known from the study of the isolated components or of suitable model molecules. Fig. 8 Schematic representation of the difference between a supramolecular system and alargemoleculebasedontheeffectscausedbyaphotonoranelectroninput.Formore details, see text
Photochemistry and Photophysics of Coordination Compounds 15 4.2 Photoinduced Processes in Supramolecular Systems Clearly, preorganization is a most valuable property from a photochemical viewpoint since a supramolecular system, for example, can be preorganized so as to favor the occurrence of energy- and electron-transfer processes [20]. Consider, for example, an A–L–B supramolecular system, where A is the light-absorbing molecular unit (Eq. 11), B is the other molecular unit to be involved in the light-induced processes, and L is a connecting unit (often called bridge). In such a system, after light excitation of A there is no need to wait for a diffusion-controlled encounter between ∗ A and B, as in molecular photochemistry (vide supra), since the two reaction partners can already be at an interaction distance suitable for electron and energy transfer: A–L–B + hν → ∗ A–L–B photoexcitation (11) ∗ A–L–B → A + –L–B – ∗ A–L–B → A – –L–B + oxidative electron transfer (12) reductive electron transfer (13) ∗ A–L–B → A–L– ∗ B electronic energy transfer . (14) In the absence of chemical complications (e.g., fast decomposition of the oxidized and/or reduced species), photoinduced electron-transfer processes are followed by spontaneous back electron-transfer reactions that regenerate the starting ground-state system (Eqs. 15 and 16), and photoinduced energy transfer is followed by radiative and/or nonradiative deactivation of the excited acceptor (Eq. 17): A + –L–B – → A–L–B back oxidative electron transfer (15) A – –L–B + → A–L–B back reductive electron transfer (16) A–L– ∗ B → A–L–B excited-state decay . (17) In supramolecular systems, electron- and energy-transfer processes take place by first-order kinetics. As a consequence, in suitably designed supramolecular systems these processes can involve even very short lived excited states. In most cases, the interaction between excited and ground-state components in a supramolecular system is weak. When the interaction is strong, new chemical species are formed, which are called excimers (from excited dimers) or exciplexes (from excited complexes), depending on whether the two interacting units have the same or different chemical nature (Fig. 9). It is important to notice that excimer and exciplex formation is a reversible process and that both excimers and exciplexes are sometimes luminescent.
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Photochemistry and Photophysics of Coordination Compounds

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