Jane Susan Henderson
Published: 2015
Total Pages: 10
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Mixed-valence dimers of the type Ru3(O)(OAc)6(CO)L-BL-Ru3(O)(OAc)6(CO)L (BL = bridging ligand and L = a pyridyl ligand) form strongly coupled systems in their [Ru3III,III,II-BL- Ru3III,II,II]- state, observed by intervalence charge transfer (IVCT) bands in the near-IR. Ancillary ligand substitution has been shown to control bimolecular electron transfer rates from electronically excited zinc tetraphenylporphyrin (ZnTPP); quenching constants, kq, for 3ZnTPP* are 3.0 × 109, 1.5 × 109, and 1.1 × 109 M−1 s−1 for BL = pyrazine (pz), L = 4-cyanopyridine (cpy), pyridine (py), or 4-dimethylaminopyridine (dmap), respectively. The preparation, electrochemistry, and spectroscopic characterization of three new species, Ru3(O)(OAc)6(CO)(ZnTPPpy)-pz-Ru3(O)(OAc)6(CO)L, where ZnTPPpy = zinc(II) 5-(4-pyridyl)- 10,15,20-triphenylporphyin and L = dmap, py or cpy, are reported. Observation of IVCT band growth under continual photolysis ([lambda]exc = 568 nm) confirms a phototriggered intramolecular electron transfer from Zn porphyrin to the Ru3O donor-bridge-acceptor dimer, resulting in a strongly coupled mixed-valence species. Femtosecond transient absorption spectroscopy was implemented to follow photoinduced electron transfer reactions in the series of asymmetric porphyrin-coordinated dyads. Excitation of the porphyrin subunit resulted in electron transfer to the Ru3O dimer with a time constant [tau] ≈ 0.6 ps. The intramolecular electron transfer was confirmed by excitation of the Ru3O MLCT, which resulted in the formation of a vibrationally unrelaxed porphyrin ground state. Under both excitation experiments, the back electron transfer was extremely fast ([tau]CR
