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 K_eq = 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, k_et = (2.2 ± 0.3) × 10⁷ s^-1. In contrast, RuBPZ²⁺* was dynamically quenched by bromide with a quenching rate constant, k_q = (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, k_cr = (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 Ru^II 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.