Photocatalytic Hydrogen Evolution from Plastoquinol Analogues as a Potential Functional Model of Photosystem I

The recent development of a functional model of photosystem II (PSII) has paved a new way to connect the PSII model with a functional model of photosystem I (PSI). However, PSI functional models have yet to be reported. We report herein the first potential functional model of PSI, in which plastoquinol (PQH2) analogues were oxidized to plastoquinone (PQ) analogues, accompanied by hydrogen (H2) evolution. Photoirradiation of a deaerated acetonitrile (MeCN) solution containing hydroquinone derivatives (X-QH2) as a hydrogen source, 9-mesityl-10-methylacridinium ion (Acr+-Mes) as a photoredox catalyst, and a cobalt(III) complex, CoIII(dmgH)2pyCl (dmgH = dimethylglyoximate monoanion; py = pyridine) as a redox catalyst resulted in the evolution of H2 and formation of the corresponding p-benzoquinone derivatives (X-Q) quantitatively. The maximum quantum yield for photocatalytic H2 evolution from tetrachlorohydroquinone (Cl4QH2) with Acr+-Mes and CoIII(dmgH)2pyCl and H2O in deaerated MeCN was determined to be 10%. Photocatalytic H2 evolution is started by electron transfer (ET) from Cl4QH2 to the triplet ET state of Acr+-Mes to produce Cl4QH2•+ and Acr•-Mes with a rate constant of 7.2 × 107 M-1 s-1, followed by ET from Acr•-Mes to CoIII(dmgH)2pyCl to produce CoII(dmgH)2pyCl-, accompanied by the regeneration of Acr+-Mes. On the other hand, Cl4QH2•+ is deprotonated to produce Cl4QH•, which transfers either a hydrogen-atom transfer or a proton-coupled electron transfer to CoII(dmgH)2pyCl- to produce a cobalt(III) hydride complex, CoIII(H)(dmgH)2pyCl-, which reacts with H+ to evolve H2, accompanied by the regeneration of CoIII(dmgH)2pyCl. The formation of CoII(dmgH)2pyCl- was detected by electron paramagnetic resonance measurements.