Photochemistry and Photophysics of Coordination Compounds
Photochemistry and Photophysics of Coordination Compounds
Photochemistry and Photophysics of Coordination Compounds
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152 S. Campagna et al.<br />
coherent hopping mechanism, <strong>and</strong> cannot be considered a through-bond,<br />
superexchange-assisted energy transfer. Interestingly, the use <strong>of</strong> the same<br />
spacers has already been mentioned for the couple Ru/Os (see above), where<br />
a normal through-bond superexchange mechanism was proposed to be operative:<br />
the reason for the difference between Ru/Os <strong>and</strong> Ir/Ru series is the<br />
energy level <strong>of</strong> the donor excited state. In Ir(III) complexes, such a state is<br />
much higher in energy <strong>and</strong> can interact significantly with oligophenyl-based<br />
excited states.<br />
An interesting series <strong>of</strong> papers dealing with photoinduced electron transfer<br />
in a series <strong>of</strong> dinuclear Ru(II)–Co(III) species allowed the accumulation<br />
<strong>of</strong> further information on the role <strong>of</strong> the bridging lig<strong>and</strong> [207, 235, 236]. Typical<br />
studied complexes were [(terpy)Ru(terpy-terpy)Co(terpy)] 5+ , [(terpy)Ru<br />
(terpy-ph-terpy)Co(terpy)] 5+ , <strong>and</strong> [(bpy)2Ru(tpphz)Co(bpy)2] 5+ (terpy-terpy<br />
=6,6 ′ -bis(2-pyridyl)-2,2 ′ :4 ′ ,4 ′′ :2 ′′ ,2 ′′′ -quarterpyridine, that is, the bridge with<br />
n =0in5; terpy-ph-terpy is the bridge with n =1in5; tpphzisthebridgein<br />
3). In these studies, the quantum yields <strong>of</strong> the thermally equilibrated product<br />
<strong>of</strong> the photoinduced electron transfer from the Ru-based MLCT state to<br />
the Co(III) subunit were carefully measured [207]. Interestingly, ∗ Ru(II)-to-<br />
Co(III) electron transfer takes place in less than 10 ps in all three abovementioned<br />
species; however, the quantum yields <strong>of</strong> the thermally equilibrated<br />
electron transfer product were quite different in the series, with the tpphzbridged<br />
dinuclear species exhibiting a quantum yield <strong>of</strong> about 0.8 <strong>and</strong> the<br />
other two compounds featuring significantly smaller values (0.53 <strong>and</strong> 0.41 for<br />
the terpy-terpy <strong>and</strong> the terpy-ph-terpy species, respectively) in butyronitrile<br />
at 298 K [207]. The authors proposed that the relatively low yield <strong>of</strong> photoinduced<br />
electron transfer in the two terpy-based complexes is due to formation<br />
<strong>of</strong> the d–d (MC) excited state <strong>of</strong> the [Co(terpy)2] 3+ moiety during solvent