Photochemistry and Photophysics of Coordination Compounds

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232 M.T. Indelli et al. Fig. 8 Distance dependence of photoinduced electron transfer rates in the dyads of Chart 1: Ru-ph-Rh, Ru-ph2-Rh, Ru-ph3-Rh (dots), Ru-ph ′ 3-Rh (triangle) This is the behavior predicted for electron transfer in the superexchange regime [5,93,95] if the distance dependence of the reorganizational energy term can be neglected. The β valueobtainedfromtheslopeofthelinein Fig. 8, 0.65 ˚A –1 , should be regarded as an upper limiting value for the attenuation factor of the intercomponent electronic coupling (Eq. 9). This β value is in the range found for other oligophenylene-containing systems (organic dyads [96, 97], metal–molecules–metal junctions [98]). This underlines the goodabilityofthistypeofbridgestomediatedonor–acceptorelectroniccoupling (for comparison, β is typically 0.8–1.2 ˚A –1 for rigid aliphatic bridges). In this regard, it is instructive to compare the electron transfer rate constant observed for Ru-ph-Rh (k = 3.0 × 109 s –1 ) with that mentioned above for dyad 15 containing an aliphatic bis-methylene bridge (k = 1.7 × 108 s –1 ). Despite the longer metal–metal distance (15.5 ˚A for Ru-ph-Rh relative to 13.5 ˚A for 15), the reaction is faster across the phenylene spacer by more than one order of magnitude. An interesting result [6, 92] is the fact that dyad Ru-ph ′ 3-Rh, which is iden- tical to Ru-ph3-Rh except for the presence of two solubilizing hexyl groups on the central phenylene ring, is one order of magnitude slower than its unsubstituted analog (Fig. 8). This is related to the notion that in a superexchange mechanism the rate is sensitive to the electronic coupling between adjacent modules of the spacer [5, 93, 95], and that in polyphenylene bridges this coupling is a sensitive function of the twist angle between adjacent spacers [99]. While the planes of unsubstituted adjacent phenylene units form angles of. 20 ◦ –40 ◦ [100, 101], ring substitution leads to a substantial increase in the twist angle (to ca. 70 ◦ ) [100] and, as a consequence, to a slowing down of the electron transfer process. A number of Ru(II)-Rh(III) dyads have been reported where little or any photoinduced electron transfer quenching of the Ru(II)-based MLCT emis-
Photochemistry and Photophysics of Coordination Compounds: Rhodium 233 sion takes place [75, 76, 102]. In the case of dyad 17, the plausible reason is that, owing to the comparable reduction potentials of the diphenylpyrazine bridging ligand and Ru(III) center, the driving force for intramolecular electron transfer is too small [102]. In the cases of dyads 18 and 19, the presence of cyclometalated ancillary ligands makes the formally Rh(III) center very difficult to reduce and relatively easy to oxidize, thus yielding MLCT states at comparable energies on the two units [75, 76]. Low driving force arguments could also apply to the dyad 20, where relatively slow quenching of the Ru(II) MLCT emission (estimated k, ca. 3.5 × 10 7 s –1 ) was observed and attributed to intramolecular electron transfer [103]. Here, however, a relevant aspect is also the presence a Rh(III)
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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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