Photochemistry and Photophysics of Coordination Compounds

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184 S. Campagna et al. molecular machines is light. It is therefore almost obvious that several photoactive molecular machines containing Ru(II) polypyridine complexes as the photoactive subunits have been prepared and studied. For an exhaustive discussion, the reader should consult [355–358]. A recent outstanding example [359] is discussed Balzani et al. 2007, in this volume [119]. We mention here the case of photoinduced ring motion in the catenane 65 [360], illustrated in Fig. 17. Visible light excitation of this compound in acetonitrile leads to population of the MLCT triplet state and subsequent formation (via thermal activation) of the MC state which causes the decoordination of the sterically hindered bpy-type ligand. As a result, swinging of the bpy-containing ring occurs, and the catenane structure made of discon- Fig. 17 Structural formula and photochemically and thermally induced motions of a Ru(II) catenane complex
Photochemistry and Photophysics of Coordination Compounds: Ruthenium 185 nected rings is obtained (66). Heating regenerates the starting catenane. The process can be repeated at will, since both the reactions (coordination and decoordination) are quantitative. It can be noted that in this system the photosubstitution process, usually considered as an undesired property for Ru(II) complexes, is instead used to perform the desired function. More recently, light-induced motion on a rotaxane system has also been obtained by using the same strategy [361]. 6 Ruthenium Complexes and Biological Systems The effects of the interaction of photoactive ruthenium complexes with biological structures have been extensively studied. Because of the outstanding excited-state properties of Ru(II) polypyridine complexes, these systems have been employed as probes of biological sites, as well as photocleavage agents and, in recent times, as inhibitors of biological functions [362–369]. Among the species used as luminescent probes, one of the most studied compounds in the last 15 years is 67 [200–204, 362–379]. This complex is very weakly emissive in aqueous solution, but becomes strongly emitting in the presence of DNA, giving rise to the so-called light-switch effect [200, 201]. The reason for such a behavior lies in the electronic properties of this specific chromophore and in the intercalation ability of the dipyrido[3,2-a:2 ′ ,3 ′ - c]phenazine (dppz) ligand. In this species, there are several triplet excited states quite close in energy: (1) a MLCT state directly populated by light excitation, in which the excited electron resides in the LUMO+1 centered on the “bpy-like” portion of dppz ligand; (2) a MLCT state in which the excited electron is located in the LUMO centered on the “phenazine-like” portion of the dppz ligand; and (3) a ligand-centered (dppz-based) excited state. This compound represents another example of interplay between multiple MLCT states, discussed in more detail in Sect. 4.4. The energy gap and order of the three low-lying excited states mentioned above, as well as their dynamics, can be modulated by various parameters, including solvent dielectrics, protic ability of the solvent, and hydrophobic interactions. In a simplified schema-
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280 Topics in Current Chemistry Edi
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Photochemistry and Photophysics of
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Volume Editors Prof. Vincenzo Balza
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Topics in Current Chemistry Also Av
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X Preface nation Compounds. The ven
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Contents Photochemistry and Photoph
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258 Author Index Volumes 251-280 Be
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Subject Index Alkyne bridges 148 An
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Subject Index 273 Rh(III) cyclometa
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