Photochemistry and Photophysics of Coordination Compounds

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240 M.T. Indelli et al. Fig. 12 Block diagram representation of a photochemical molecular device for photoinduced electron collection and two-electron redox catalysis nents capable of storing electrons and using them in multielectron redox processes. A possible PEC scheme is shown in Fig. 12. In this scheme, P are electron transfer photosensitizers, C an electron store component, and BL rigid bridging ligands. D is a sacrificial electron donor, and A2 is a two-electron reduced product (e.g., H2 starting from 2H + ). The key molecular component C must have the ability to store two electrons following photoinduced electron transfer from P, and to deliver them to the substrate A + in a low-activation two-electron process that leads to the desired product A2. Very few homogeneous systems for photoinduced electron collection (PEC) have been reported by now [115–122]. Trimetallic complexes incorporating polyazine bridging ligands have been designed and studied by Brewer’s group for potential applications as PEC devices [123–126]. Most of these complexes have general formula [(bpy)2Ru(BL)]2MCl2 n+ with BL = 2,3-bis(2-pyridyl)pyrazine (dpp), 2,3-bis(2-pyridyl)quinoxaline (dpq), or 2,3bis(2-pyridyl)benzoquinoxaline (dpb) and M = Ir(III) or Rh(III) [123, 125]. The first functioning PEC system, [(bpy)2Ru(dpb)]2IrCl2 5+ ,wasreported in 1994, employing π systems of polyazine bridging ligands to collect electrons [123]. Very recently, an analogous supramolecular trimetallic species has been reported where the central Ir-based moiety is replaced by a Rh(III) complex [127]. This new system, [(bpy)2Ru(dpp)]2RhCl2 5+ (25), was obtained coupling two Ru chromophoric units which play the role of P, through
Photochemistry and Photophysics of Coordination Compounds: Rhodium 241 polyazine bridges (BL), to a central Rh core (C). In the presence of dimethylaniline (DMA) as sacrificial electron donor (D), 25 undergoes two-electron photoreduction at the rhodium center, producing the stable Rh(I) form [(bpy)2Ru(dpp)]2Rh 5+ via loss of two chlorides. This result is demonstrated by the observation that the spectroscopic changes associated with the photochemical reduction are identical with those seen in the electrochemical reduction experiments. On the basis of quenching experiments in the presence of different concentrations of DMA, the authors suggest that the monoreduced Rh(II) species can be formed by two alternative pathways following excitation of the peripheral Ru(II)-base chromophores: i) photoinduced electron transfer from the excited Ru(II)-based units followed by regeneration of the Ru(II)-based chromophores by oxidation of the sacrificial donor or ii) bimolecular quenching of the excited Ru(II)-based units by the sacrificial donor followed by reduction of the central Rh(III). No clear indication is given as to the mechanism for the formation of the stable two-electron-reduced Rh(I) product from the Rh(II) intermediate. According to the authors, the ability of [(bpy)2Ru(dpp)]2RhCl2 5+ to undergo photoreduction at the rhodium center by multiple electrons and the fact that the photoreduced product [(bpy)2Ru(dpp)]2Rh 5+ is coordinatively unsaturated and thus available to interact with substrates are promising features in view of potential applications in light energy harvesting to produce fuels [127]. 4 Rhodium Complexes as DNA Intercalators 4.1 Specific Binding to DNA and Photocleavage The development and the study of transition metal complexes able to bind selectively to DNA sites, emulating the behavior of the DNA-binding proteins, ranks among the most fascinating and challenging issues in the field of current chemical and biological research [128–131]. This topic has been extensively investigated by Barton and coworkers, who devoted an impressive number of studies to the use of Rh(III) complexes with ligands containing nitrogen donors as DNA binding agents [129, 130]. Most of these studies center around complexes of the ligand 9,10-phenanthrenequinonediimine (phi) (26) [130, 132–138]. The phi Rh(III) complexes are indeed excellent DNA intercalators given the flat aromatic heterocyclic moiety of the phi ligand that deeply inserts and stacks in between the DNA base pairs (binding affinity constants range from 10 6 –10 9 M –1 ) [132, 139, 140]. The photophysical properties of phi Rh(III) complexes [50] have been discussed in Sect. 2.1. When bound to DNA, upon photoactivation with UV light, they are able to promote DNA strand cleav-
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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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Page 279 and 280: Subject Index Alkyne bridges 148 An
Page 281: Subject Index 273 Rh(III) cyclometa
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