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Photochemistry and Photophysics of Coordination Compounds

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230 M.T. Indelli et al.<br />

detailed kinetic picture <strong>of</strong> Fig. 6 [84]. The widely different rates <strong>of</strong> the three<br />

ET processes (a, b, d) can be rationalized in terms <strong>of</strong> predominant driving<br />

force effects [84], as shown schematically in Fig. 7.<br />

Fig. 7 Free-energy correlation <strong>of</strong> rate constants for the three electron transfer processes<br />

<strong>of</strong> dyad 15<br />

Since most Ru(II)-Rh(III) polypyridine dyads have very similar energetics,<br />

the qualitative features illustrated above for dyad 15 can be safely generalized<br />

to this whole class <strong>of</strong> compounds. For example, in dyad 16 [85]therateconstants<br />

<strong>of</strong> processes a, b,<strong>and</strong>d are slower by a factor <strong>of</strong> ca. 3 but have the same<br />

relative magnitudes as for dyad 15. The slower rates are likely related to the<br />

longer aliphatic bridge, although for this <strong>and</strong> related [86] dyads, the flexibility<br />

<strong>of</strong> the bridges limits the validity <strong>of</strong> such comparisons.<br />

Within this general type <strong>of</strong> behavior <strong>of</strong> Ru(II)-Rh(III) dyads, a number <strong>of</strong><br />

experimental studies have been specifically aimed at investigating the role <strong>of</strong><br />

the bridge in determining electron transfer rates. As has been the case for<br />

other types <strong>of</strong> bimetallic dyads [87–90], particular attention has been devoted<br />

to Ru(II)-Rh(III) dyads with modular bridges involving p-phenylene<br />

spacer units [6, 91, 92]. The dyads in Chart 1 provide a homogeneous series

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