Visible Light Driven Bromide Oxidation and Ligand Substitution Photochemistry of a Ru Diimine Complex.

The complex [Ru(deeb)(bpz)] (RuBPZ, deeb = 4,4'-diethylester-2,2'-bipyridine, bpz = 2,2'-bipyrazine) forms a single ion pair with bromide, [RuBPZ, Br], with K = 8400 ± 200 M in acetone. The RuBPZ displayed photoluminescence (PL) at room temperature with a lifetime of 1.75 μs. The addition of bromide to a RuBPZ acetone solution led to significant PL quenching and Stern-Volmer plots showed upward curvature. Time-resolved PL measurements identified two excited state quenching pathways, static and dynamic, which were operative toward [RuBPZ, Br] and free RuBPZ, respectively. The single ion-pair [RuBPZ, Br]* had a lifetime of 45 ± 5 ns, consistent with an electron transfer rate constant, k = (2.2 ± 0.3) × 10 s. In contrast, RuBPZ* was dynamically quenched by bromide with a quenching rate constant, k = (8.1 ± 0.1) × 10 M s. Nanosecond transient absorption revealed that both the static and dynamic pathways yielded RuBPZ and Br products that underwent recombination to regenerate the ground state with a second-order rate constant, k = (2.3 ± 0.5) × 10 M s. Kinetic analysis revealed that RuBPZ was a primary photoproduct, while Br was secondary product formed by the reaction of a Br with Br, k = (1.1 ± 0.2) × 10 M s. Marcus theory afforded an estimate of the formal reduction potential for E(Br) in acetone, 1.42 V vs NHE. A H NMR analysis indicated that the ion-paired bromide was preferentially situated close to the Ru center. Prolonged steady state photolysis of RuBPZ and bromide yielded two ligand-substituted photoproducts, cis- and trans-Ru(deeb)(bpz)Br. A photochemical intermediate, proposed to be [Ru(deeb)(bpz)(κ-bpz)(Br)], was found to absorb a second photon to yield cis- and trans-Ru(deeb)(bpz)Br photoproducts.