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

Abstract

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 (PQH(2)) analogues were oxidized to plastoquinone (PQ) analogues, accompanied by hydrogen (H-2) evolution. Photoirradiation of a deaerated acetonitrile (MeCN) solution containing hydroquinone derivatives (X-QH(2)) as a hydrogen source, 9-mesityl-10-methylacridinium ion (Ace-Mes) as a photoredox catalyst, and a cobalt(III) complex, Co-III(dmgH)(2) pyCl (dmgH = dimethylglyoximate monoanion; py = pyridine) as a redox catalyst resulted in the evolution of H-2 and formation of the corresponding p-benzoquinone derivatives (X-Q) quantitatively. The maximum quantum yield for photocatalytic H-2 evolution from tetrachlorohydroquinone (Cl(4)QH(2)) with Acr(+)-Mes and Co-III(dmgH)(2)pyCl and H2O in deaerated MeCN was determined to be 10%. Photocatalytic H-2 evolution is started by electron transfer (ET) from Cl(4)QH(2) to the triplet ET state of Acr(+)-Mes to produce Cl(4)QH(2)(center dot+) and Acr(center dot)-Mes with a rate constant of 7.2 X 10(7) M(-1)s(-1,) followed by ET from Acr(center dot)-Mes to Co-III (dmgH)(2)pyCl to produce [Co-III(dmgH)(2)pyCl](-), accompanied by the regeneration of Acr(+)-Mes. On the other hand, Cl(4)QH(2)(center dot+) is deprotonated to produce Cl(4)QH(center dot), which transfers either a hydrogen-atom transfer or a proton-coupled electron transfer to [Co-II(dmgH)(2)pyCl](-) to produce a cobalt(III) hydride complex, [Co-III(H)(dmgH)(2)pyCl](-), which reacts with H+ to evolve H-2, accompanied by the regeneration of Co-III (dmgH)(2)pyCl. The formation of [Co-II(dmgH)(2)pyCl](-) was detected by electron paramagnetic resonance measurements.