UC San Diego

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 < 0.1 ps), preventing complete time-resolved exploration of the mixed -valence state. Although the charge recombination itself could not be observed, the yield of unrelaxed ground states supports the conclusion that delocalization takes place at least partially on a sub-100 fs time scale. Ultrafast 2D-IR spectroscopy results conclude thermal electron transfer rates must be occurring slower than 15 ps and the dynamic carbonyl coalescence observed in strongly coupled mixed-valence dimers may be the result of bridge-torsional angle motions, which modulate electronic coupling across the molecule