Photosynthesis, the reduction of carbon dioxide into biomass using energy derived from light, is one of the most important biological processes known to mankind. In oxygenic photosynthesis, light-driven water oxidation provides the redox equivalents necessary to reduce carbon dioxide. In natural photosynthesis water oxidation occurs at Photosystem II (PSII) through the Kok cycle following absorption of four photons that result in activation of the Oxygen Evolving Complex (OEC). Carbon dioxide reduction occurs through the light-independent steps of the Calvin-Benson-Bassham cycle.
Water oxidation and carbon dioxide reduction are also key reactions in artificial photosynthesis. Solar driven water splitting into hydrogen and oxygen, 2 H2O → O2 + 2 H2, can provide most needed clean, renewable energy, whereas carbon dioxide reduction can diminish excessive amounts of carbon dioxide in the atmosphere.
Water oxidation is also catalyzed by the ruthenium “blue dimer” cis,cis-[(bpy)2(H2O)RuIIIORuIII(OH2)(bpy)2]4+ and structurally related derivatives. In this work, we present recent breakthroughs in both water oxidation and carbon dioxide reduction. Based on recently gained mechanistic insight on catalytic water oxidation by the ruthenium “blue dimer”, we have devised a strategy for enhancing catalytic water oxidation by addition of the kinetically facile electron transfer mediators, Ru(bpy)2(L-L)2+ (L-L is 2,2'-bipyridine (bpy), 2,2'-bipyrazine (bpz) and 2,2'-bipyrimidine (bpm)) and [Ru(bpm)3]2+. Reduction potentials for their Ru(III/II) couples (Eo' vs NHE) are 1.27, 1.47, 1.40 and 1.69 V, respectively, making the Ru(III) forms of each accessible by Ce(IV) oxidation. These couples undergo facile electron transfer with self-exchange rate constants between the Ru(bpy)2(L)3+ and Ru(bpy)2(L)2+ forms on the order of 108-109 s-1 while the self-exchange rate constant for the Ce(IV/III) couple is orders of magnitude slower.
With the new strategy, rate enhancements for water oxidation by factors of up to ~30 have been obtained. In addition, preliminary electrochemical experiments in 0.1 M HNO3 with a glassy carbon working electrode demonstrate that mediator-assisted electrocatalytic water oxidation is attainable with a turnover number of 19 already achieved.
In a recent communication, we demonstrated the existence of a new type of excited state reactivity, excited state electron proton transfer (ES-EPT), for [Ru(bpy)2(bpz)]2+ (bpy is 2,2'-bipyridine; bpz is 2,2'-bipyrazine). In this work, the transient products generated by EPT reductive quenching of the MLCT excited state of [Ru(bpy)2(bpz)]2+ are successfully used to reduce carbon dioxide in a process that mimics the light-independent reaction of the second stage of plant photosynthesis.