Bromide Photo-oxidation Sensitized to Visible Light in Consecutive Ion Pairs

The titration of bromide into a [Ru(deeb)(bpz)₂]²⁺ (Ru²⁺, deeb = 4,4'-diethylester-2,2'-bipyridine; bpz = 2,2'-bipyrazine) dichloromethane solution led to the formation of two consecutive ion-paired species, [Ru²⁺, Br^-]⁺ and [Ru²⁺, 2Br^-], each with distinct photophysical and electron-transfer properties. Formation of the first ion pair was stoichiometric, K_eq 1 > 10⁶ M^-1, and the second ion-pair equilibrium was estimated to be K_eq 2 = (2.4 ± 0.4) × 10⁵ M^-1. The ¹H NMR spectra recorded in deuterated dichloromethane indicated the presence of contact ion pairs and provided insights into their structures and were complimented by density functional theory calculations. Static quenching of the [Ru(deeb)(bpz)₂]^2+* photoluminescence intensity (PLI) by bromide was observed, and [Ru²⁺, Br^-]^+* was found to be nonluminescent, τ < 10 ns. Further addition of bromide resulted in partial recovery of the PLI, and [Ru²⁺, 2Br^-]* was found to be luminescent with an excited-state lifetime of τ = 65 ± 5 ns. Electron-transfer products were identified as the reduced complex, [Ru(deeb)(bpz)₂]⁺, and dibromide, Br₂^•-. The bromine atom, Br^•, was determined to be the primary excited-state electron-transfer product and was an intermediate in Br₂^•- formation, Br^• + Br^- → Br₂^•-, with a second-order rate constant, k = (5.4 ± 1) × 10⁸ M^-1 s^-1. The unusual enhancement in PLI for [Ru²⁺, 2Br^-]* relative to [Ru²⁺, Br^-]^+* was due to a less favorable Gibbs free energy change for electron transfer that resulted in a smaller rate constant, k_et = (1.5 ± 0.2) × 10⁷ s^-1, in the second ion pair. Natural atomic charge analysis provided estimates of the Coulombic work terms associated with ion pairing, ΔG_w, that were directly correlated with the measured change in rate constants.