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

The complex [Ru(deeb)(bpz)2]2+ (RuBPZ2+, deeb = 4,4'-diethylester-2,2'-bipyridine, bpz = 2,2'-bipyrazine) forms a single ion pair with bromide, [RuBPZ2+, Br-]+, with Keq = 8400 ± 200 M-1 in acetone. The RuBPZ2+ displayed photoluminescence (PL) at room temperature with a lifetime of 1.75 μs. The addition of bromide to a RuBPZ2+ 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 [RuBPZ2+, Br-]+ and free RuBPZ2+, respectively. The single ion-pair [RuBPZ2+, Br-]+* had a lifetime of 45 ± 5 ns, consistent with an electron transfer rate constant, ket = (2.2 ± 0.3) × 107 s-1. In contrast, RuBPZ2+* was dynamically quenched by bromide with a quenching rate constant, kq = (8.1 ± 0.1) × 1010 M-1 s-1. Nanosecond transient absorption revealed that both the static and dynamic pathways yielded RuBPZ+ and Br2•- products that underwent recombination to regenerate the ground state with a second-order rate constant, kcr = (2.3 ± 0.5) × 1010 M-1 s-1. Kinetic analysis revealed that RuBPZ+ was a primary photoproduct, while Br2•- was secondary product formed by the reaction of a Br• with Br-, k = (1.1 ± 0.2) × 1010 M-1 s-1. Marcus theory afforded an estimate of the formal reduction potential for E0(Br•/-) in acetone, 1.42 V vs NHE. A 1H NMR analysis indicated that the ion-paired bromide was preferentially situated close to the RuII center. Prolonged steady state photolysis of RuBPZ2+ and bromide yielded two ligand-substituted photoproducts, cis- and trans-Ru(deeb)(bpz)Br2. A photochemical intermediate, proposed to be [Ru(deeb)(bpz)(κ1-bpz)(Br)]+, was found to absorb a second photon to yield cis- and trans-Ru(deeb)(bpz)Br2 photoproducts.