Transient
absorption decay rate constants (k
obs)
for reactions of electronically excited zinc tetraphenylporphyrin
(3ZnTPP*) 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, k
q, for 3ZnTPP*/1–3 are 3.0 × 109, 1.5 × 10 9, and 1.1 × 109 M–1 s–1, respectively). Values
of k
q for reactions of 3ZnTPP*
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.