Photoinduced Mixed Valency in Zinc Porphyrin Dimer of Triruthenium Cluster Dyads

The preparation, electrochemistry, and spectroscopic characterization of three new species, (ZnTPPpy)­Ru₃O­(OAc)₆(CO)-pz-Ru₃O­(OAc)₆(CO)­L, where ZnTPPpy = zinc­(II) 5-(4-pyridyl)-10,15,20-triphenylporphyin, L = pyridyl ligand, and pz = pyrazine, are reported. These porphyrin-coordinated Ru₃O–BL–Ru₃O (BL = bridging ligand) dyads are capable of undergoing intramolecular electron transfer from the photoexcited Zn porphyrin to Ru₃O donor–bridge–acceptor dimer systems. Seven reversible redox processes are observed in the cyclic voltammograms of the newly synthesized dyads, showing no significant electrochemical interaction between the redox active porphyrin and the pyrazine-bridged ruthenium dimer of Ru₃O trimers. From the electrochemical behavior of the dyads, large comproportionation constants (<i>K</i>_c = 6.0 × 10⁷ for L = dmap) were calculated from the reduction potentials of the Ru^IIIRu^IIIRu^II clusters, indicating a stable mixed-valence state. Electronic absorption spectra of the singly reduced mixed-valence species show two intervalence charge transfer (IVCT) bands assigned within the Brunschwig–Creutz–Sutin semiclassical three-state model as metal-to-bridge and metal-to-metal in character. The progression from most to least delocalized mixed-valence dimer ions, as determined by the divergence of the IVCT bands and in agreement with electrochemical data, follows the order of L = 4-dimethylaminopyridine (dmap) > pyridine (py) > 4-cyanopyridine (cpy). These systems show dynamic coalescence of the infrared spectra in the ν­(CO) region of the singly reduced state. This sets the time scale of electron exchange at <10 ps. The electron transfer from the S₁ excited state of the coordinated porphyrin to the dimer is predicted to be thermodynamically favorable, with Δ<i>G</i>_FET⁰ ranging from −0.54 eV for L = dmap to −0.62 eV for L = cpy. Observation of IVCT band growth under continual photolysis (λ_exc = 568 nm) confirms a phototriggered intramolecular electron transfer process resulting in a strongly coupled singly reduced mixed-valence species