Photochemistry and Photophysics of Coordination Compounds
Photochemistry and Photophysics of Coordination Compounds
Photochemistry and Photophysics of Coordination Compounds
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<strong>Photochemistry</strong> <strong>and</strong> <strong>Photophysics</strong> <strong>of</strong> <strong>Coordination</strong> <strong>Compounds</strong>: Ruthenium 153<br />
relaxation within the electron transfer excited state. In fact, fast tunneling<br />
transition <strong>of</strong> the nonrelaxed electron transfer product to the lowest d–d excited<br />
states <strong>of</strong> the [Co(terpy)2] 3+ moiety can take place via hole transfer from<br />
[Ru(terpy)2] 3+ to the [Co(terpy)2] 2+ , generating a dπ 6 dσ ∗ configuration.<br />
Strong through-lig<strong>and</strong> electronic coupling <strong>of</strong> dπ(Ru)–dπ(Co), as estimated<br />
from the strong intensity <strong>of</strong> the intervalence b<strong>and</strong> <strong>of</strong> [(terpy)Ru(terpyterpy)Ru(terpy)]<br />
5+ , can effectively mediate the fast hole transfer process.<br />
For the tpphz-bridged system, through-lig<strong>and</strong> electronic coupling between<br />
dπ(Ru III ) <strong>and</strong> dπ(Co II ) orbitals is much smaller, as suggested by the absence<br />
<strong>of</strong> any sizeable intervalence b<strong>and</strong> in [(bpy)2Ru(tpphz)Ru(bpy)2] 5+ [207]. It<br />
turns out that the weak tpphz superexchange interaction between dπ(Ru III )<br />
<strong>and</strong> dπ(Co II ) orbitals may be unable to open the channel <strong>of</strong> hole transfer<br />
during the relaxation <strong>of</strong> the electron transfer product, leading to a higher<br />
quantum yield <strong>of</strong> the charge-separated, thermally equilibrated product. However,<br />
the charge recombination rate constant was fast in all cases: in butyronitrile<br />
at room temperature it was 2.1 × 10 7 s –1 for the tpphz species <strong>and</strong><br />
biphasic <strong>and</strong> faster for the other two compounds (81 × 10 9 <strong>and</strong> 5 × 10 9 s –1<br />
for the terpy-ph-terpy containing species <strong>and</strong> 52 × 10 10 <strong>and</strong> 3 × 10 10 s –1 for<br />
the terpy-terpy species). Even in the charge recombination (back electron<br />
transfer) process, the different coupling <strong>of</strong>fered by the bridging lig<strong>and</strong>s could<br />
explain the results.<br />
5.2<br />
Photoactive Multinuclear Ruthenium Species<br />
Exhibiting Particular Topologies<br />
5.2.1<br />
Racks <strong>and</strong> Grids<br />
Rack-type metal complexes are linearly arranged species [237], but differ<br />
from the species discussed in the former section since they are made <strong>of</strong><br />
several repeating, roughly identical, metal-based subunits orthogonally appended<br />
to a roughly linear <strong>and</strong> rigid polytopic molecular str<strong>and</strong>. The metal<br />
centers are never aligned along the main axis <strong>of</strong> the bridging lig<strong>and</strong>.<br />
The first rack-type Ru(II) polypyridine complex investigated from a photochemical<br />
viewpoint is 28 [238]. In this species, the anthryl group has only<br />
the function <strong>of</strong> absorbing additional light energy; in fact, its triplet state is<br />
higher in energy than the MLCT triplet state(s) <strong>of</strong> the Ru(II) subunits (here,<br />
the lowest-lying MLCT states involve the bridging lig<strong>and</strong>, which is very easily<br />
reduced). For 28, near-IR emission occurs (λem = 845 nm) with a relatively<br />
long lifetime (60 ns). Such an emission is totally quenched in the somewhat<br />
related tetranuclear Ru – Fe grid 29 [239], where energy transfer from the Rubased<br />
MLCT state to the Fe-based MC levels is likely to occur. Kinetic data for<br />
the quenching processes were not reported.