| Hydrogen has long been considered as an ideal substitute for fossil fuels because of its high energy density. Conversion of solar energy into molecular hydrogen by splitting water has attracted more and more attention in recent years. Development of cheap, efficient and environmentally benign homogeneous systems for photocatalytic hydrogen production is an important and challenging task. In this dissertation, homogeneous water reduction systems were constructed based on organic dyes and two types of proton-reduction catalysts,[2Fe2S] model complexes and cobaloximes, and the processes of electron transfers were studied.[2Fe2S] model complexes, inspired by natural hydrogenases, can electro-and photochemically catalyze the reduction of protons to hydrogen. Most of the reported homogeneous hydrogen production systems employed noble-metal complexes as photosensitizers. These iron-based systems work in organic solvents or mixed aqueous organic solutions at rigorous pH values and with short lifetimes for photochemical hydrogen production due to poor water solubility of [2Fe2S] complexes and their instabilities under long-term irradiation.A [2Fe2S] model complex [(μ-pdt)Fe2(CO)5L](1, pdt=SCH2CH2CH2S, L=P(CH2OH)3) containing a hydrophilic P(CH2OH)3ligand and cheap organic dyes Eosin Yellow (EY), Rose Bengal (RB) and Eosin Bluish (EB) were used to constitute homogeneous molecular catalyst systems, which can photochemically catalyze water reduction to hydrogen in an environmentally benign solvent (EtOH/F2O=1:1) with a TON of226over15-h irradiation of visible light. The comparative experiments indicate that the deactivation of the systems is mainly due to decomposition of organic dyes and the catalyst is only partially decomposed. The intermediates generated in situ by electron transfers were detected by transient absorbance, spectroelectrochemical measurements, time-resolved UV-vis and in-situ EPR spectroscopy. According to the results obtained, a new pathway for electron transfer was proposed for these systems.Cyclodextrins were introduced into the homogenous photocatalytic systems with organic dyes as photosensitizers and a [2Fe2S] model complex [μ-SCH2N(C6H4SO3-)CH2S][Fe(CO)3]2(3) as catalyst. Formation of a host-guest inclusion of complex3with cyclodextrin mimicks the protein environment surrounding the diiron hydrogenase active site. The influence of cyclodextrin on the stabilities of photosensitizers and catalyst were studied by time-resolved UV-vis and in-situ IR spectroscopy. The improvements of fluorescence lifetimes and quantum yields of photosensitizers with addition of cyclodextrins are responsible for the promotion of hydrogen production. It is noteworthy that these systems photochemically catalyze H2production in aqueous solution with lifetimes up to24hours and the maximum TON reaches190, which is the highest TON reported for noble-and toxic-metal-free homogeneous systems producing H2in aqueous solution.Cobaloximes are considered as functional hydrogenase models, which have similar structures of the cobalt complex in Vitamin B12and exhibit activity in electro-and photocatalytic hydrogen production. The homogeneous hydrogen production systems were constructed by taking organic dyes (EY, RB) as photosensitizers and cobaloxime [CoⅢ(dmgH)2PyCl](4, dmgH=dimethylglyoxime, Py=pyridine) as catalyst. The lifetimes of the systems were prolonged from3-5h to8h by introduction of cyclodextrins and formation of host-guest complexes. A TON of117was obtained over8-h irradiation in aqueous solution at pH7. The experimental results indicate that the deactivation of the systems is mainly due to the dissociation of ligands from the metal center of the cobalt catalyst although the axil pyridine ligand of the cobalt catalyst has been included into the cavity of cyclodextrin. The possible pathways for electron transfers and the influence of cyclodextrins on the F2-evolving reaction were studied and discussed on the basis of fluorescence lifetimes and transient absorption spectra. |
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