Photocatalyst assemblies with two halide ions

Abstract

A series of ruthenium polypyridyl photocatalysts bearing amide functional groups were designed that successfully promoted halide assembly in CH2Cl2 and CH3CN solution. In CH2Cl2, halide assembly was accompanied by a visible color change, and the spectral changes presented clear evidence for two halide binding events, yielding a 1:2 ruthenium:halide assembly. In the more polar solvent CH3CN, 1:2 assembly structures were also observed with chloride, bromide, and iodide, and large equilibrium constants were measured for association of the first and second halide (K11 = 0.04 – 2 × 106 M−1, K12 = 0.01 – 3 × 105 M−1). Varying the functional groups on the ancillary ligands tuned the excited-state reduction potentials (Ru2+⁎/+), resulting in a photocatalyst capable of performing iodide oxidation. Quenching of the photocatalyst excited state resulted in static and dynamic quenching, and a Stern-Volmer analysis yielded two linear regions at low and high iodide concentrations. The dynamic quenching rate constants (kq = 6.8 and 4.0 × 1010 M−1 s−1) and static quenching constants (KS = 2.4 and 0.13 × 104 M−1) at low and high iodide concentrations, respectively, were consistent with dynamic quenching of Ru2+ and [Ru2+,I−]+, and static quenching of [Ru2+,I−]+and [Ru2+,2I−]. Transient absorption spectroscopy revealed that the quenching reaction yielded a reduced ruthenium (Ru+) as the primary photoproduct and diiodide (I2•−) as a secondary photoproduct. The second-order rate constant for I2•− formation was measured to be 2.5 × 1010 M−1 s−1, a value consistent with the diffusion limited reaction. The transient absorption data indicates that oxidized halide photoproducts only result from the diffusional quenching reactions, and not from static quenching with an associated iodide ion. Fast back-electron transfer rates and low cage-escape yields in the ruthenium:iodide assemblies are invoked to explain why the static quenching pathway does not lead to measurable photoproduct yields.