Light-Induced Hydrogen Generating System Based On Metalloporphyrin And Hydrogenase Active Site Model

With the development of society, conversion of solar energy into molecular hydrogen, which is known as a clean fuel, has attracted considerable attention in recent decades. Iron-Iron hydrogenase is a good choice as a proton reduction catalyst in the photochemical H₂-evolving systems for its high efficiency in catalyzing the proton into hydrogen; metalloporphyrin has a wide range of absorption and a high reduction potential in the excited states, making it a good photosensitizer. However, in view of previous photochemical H₂-evolving models in which the photosensitizer and the proton reduction catalyst are covalently connected, the back electron transfer is a serious problem. Therefore in this paper a self-assembled system in which the photosensitizer and the proton reduction is linked noncovalently is employed as a photochemical H₂-evolving model.In this paper, zinc tetraphenylporphyrin is employed as the photosensitizer in the photochemical H₂-evolving systems, and three dithiolate diiron complexes are prepared to mimic the active center of the iron-iron hydrogenase, the complex 8 , which is pyridyl-functionalized({(μ-SCH₂)₂N(CH₂CH₂OOCPy)}-Fe₂(CO)₆), the complex 9, which is pyrazyl-functionnalized({(μ-SCH₂)₂N(CH₂CH₂OOCPrz)}-Fe₂(CO)₆)), as the proton reduction catalyst and the complex 7 ({(μ-SCH₂)₂N(CH₂CH₂OH)}-Fe₂(CO)₆) as a reference. Based on the analysis of static (UV-Vis and fluorescence) and dynamic absorption spectra obtained from the coexistence systems of ZnTPP with complex 7 or 8 and the similar systems consisted of ZnTPP-Dimer with 7 or 9, we found that light-induced electron transfer occurred from ZnTPP to complex 5, but not from ZnTPP to complex 7, not for the ZnTPP-Dimer with complex 7 or 9, either.Of most importance, Visible light-induced hydrogen generation was observed from the self-assembled system of ZnTPP and complex 8 in the presence of CF₃COOH and 2-mercaptobenzoic acid. The reference experiments showed that the hydrogen was not produced if complex 7 is substituted for complex 8. And ZnTPP, complex 8, CF₃COOH and 2-mercaptobenzoic acid are all indispensable in this system for H₂-evolving. Meanwhile, the mechanism for light-driven reduction of protons using the self-assembled system is proposed based on the experimental results and relevar literatures.