Energy transfer quenching is the mechanism that natural photosystems use to harvest light over large areas using an 'antenna system' and subsequently funnel this energy to a specific site. In green plants, the antenna is composed of thousands of chlorophyll molecules which, when photoexcited to the triplet state (3Chl), rapidly pass their energy to neighboring molecules eventually resulting in the electronic excitation of the special pair of chlorophyll molecules in the photochemical reaction center of PSI or PSII.113 The whole process occurs on the order of a few tens of picoseconds, thus by using an antenna system, a relatively dilute photon flux that may typically excite a single reaction center chromophore on the order of a few excitations per second is dramatically enhanced.113
Similarly, efficient energy transfer between ruthenium(II) and osmium(II) poly-pyridyl complexes has been the basis for numerous artificial antenna complexes which act to harvest visible light energy and funnel it to of specific metal site.114,115 Energy-transfer from [Ru(bpy)3]2+* to [Os(bpy)3]2+ is found to occur at a rate close to the diffusion limit (kq = 1x109 M-1s-1) for the bimolecular reaction116 and covalently linking these complexes is a common strategy for the construction of antenna complexes. Energy transfer rates in these multi-metallic assemblies are reasonably fast even over large internuclear distances provided that the bridging ligand can provide some electronic coupling. The role of the bridging ligand and the distance dependence of energy transfer in donor-acceptor dyads has been extensively investigated with bridges constructed of non-conjugated hydrocarbons,117-120 polyalkyny-lenes,121 polyenenes,122 oligothienylenes,123 and oligophenylenes,124-129 and others.8,130-132 For example, energy transfer rate constants were measured in acetonitrile for a series dimers in which a Ru(bpy)22+ unit is bridged to a Os(bpy)22+ unit by rigid conjugated ligands of the type bpy-(ph)n-bpy (where n = 3, 5 and 7).133 In this case, the rate constants for energy transfer from the Ru 3MLCT to Os 3MLCT were practically temperature independent and decreased with increasing length of the oligophenylene spacer on the order of ken = 6.7x108 s-1 for n = 3; ken = 1.0x107 s-1 for n = 5; ken = 1.3x106 s-1 for n = 7 at 293 K. Typically, the energy-transfer process in these conjugated systems occurs via a superexchange (Dexter) mechanism where the structure of the bridge plays an integral role in the ET process. Importantly, such systems show that vectoral energy transfer is possible at reasonable rates over large distances or as more commonly referred to 'molecular wires' are possible. From the above example, the Ru-Os distance is 4.2 nm in the [(bpy)2Ru(bpy-(ph)7-bpy)Os(bpy)2]4+ dimer and the rate constant is still 1.3x106 s-1.
In addition to simple dyads, dendritic assemblies of Ru(II) and Os(II) polypyridyl complexes have been extensively investigated as the branching pattern of a dendri-mer is ideally suited for orienting numerous chromophores to funnel energy to a single site.114,134-137 For example, the Ru3Os tetramer 4 shows quantitative energy transfer from the three peripheral Ru sites to the central Os(II) center as was expected as the Os(II) site has the lowest lying excited state.138 When more complex dendrimers such as the decametallic 5 (shown schematically having the same bridging motif as 4) are examined, the energy migration pathways were less straightforward with energy transfer from the intermediate Ru sites (Rui) going to both the central Os(II) site and the peripheral Ru sites (Rup).139 The bidirectionality is due to the relative energy ordering of the local 3MLCT states which is Rui>Rup>Os, thus energy transfer from Rui is downhill whether it goes towards the Os or the Rup. While this has limited the utility of these complexes as antenna which funnel the energy to a single site, recent developments suggest this can be overcome. Campag-na, Scandola and co-workers showed that complex 6 which contains seven Ru chro-mophores (4 Rup, 2 Rui, 1 Ru(dpp)2Cl2 apical) undergoes energy transfer exclusively to the apical site even though the Rui is again the highest energy 3MLCT state.140 The result is understood to involve an unusual two-step electron transfer process, importantly suggesting that indeed such complexes can funnel captured solar energy to a single site!
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