Electron-Transfer Reactions of Electronically Excited Zinc Tetraphenylporphyrin with Multinuclear Ruthenium Complexes.

Transient absorption decay rate constants (kobs) for reactions of electronically excited zinc tetraphenylporphyrin ((3)ZnTPP*) with triruthenium oxo-centered acetate-bridged clusters [Ru3(μ3-O)(μ-CH3CO2)6(CO)(L)]2(μ-pz), where pz = pyrazine and L = 4-cyanopyridine (cpy) (1), pyridine (py) (2), or 4-dimethylaminopyridine (dmap) (3), were obtained from nanosecond flash-quench spectroscopic data (quenching constants, kq, for (3)ZnTPP*/1-3 are 3.0 × 10(9), 1.5 × 10 (9), and 1.1 × 10(9) M(-1) s(-1), respectively). Values of kq for reactions of (3)ZnTPP* with 1-3 and Ru3(μ3-O)(μ-CH3CO2)6(CO)(L)2 [L = cpy (4), py (5), dmap (6)] monomeric analogues suggest that photoinduced electron transfer is the main pathway of excited-state decay; this mechanistic proposal is consistent with results from a photolysis control experiment, where growth of characteristic near-IR absorption bands attributable to reduced (mixed-valence) Ru3O-cluster products were observed.