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, Keq 1 > 106 M–1, and
the second ion-pair equilibrium was estimated to be Keq 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, ket = (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,
ΔGw, that were directly correlated
with the measured change in rate constants.