Light-Driven Evolution Of Hydrogen From Aqueous Solution Using BODIPY Photosensitizers And The Search For Molecular Catalysts

Hydrogen has long been considered as an ideal alternative to traditional fossil fules because of its high efficiency and environmentally friendly. And photocatalytic hydrogen evolution from water splitting is regarded as one of the Holy Grails of 21 st century, being a key transformation toward the conversion of solar energy into hydrogen. Photocatalytic systems for H2 generate evolution typically consist of a photosensitizer, a sacrificial electron donor and a proton reduction catalyst. Traditionally, there is a plethora of systems employing noble mentals in the chromophore and/or catalyst. While the limition and high-cost of noble mentals restrict the development and application of photocatalytic hydrogen production directly. As part of efforts to explore and design new photocatalytic hydrogen evolution systems which are cheap, efficient and stable. it is necessary to employ non-noble metals, we herein describe new homogeneous molecular catalytic system for the photocatalytic production of H2 from aqueous protons using organic dyes and cobalt molecular catalyst.Firstly, in this study a series of BODIPY dyes Bn(n=1~6) have been designed and synthesized. After characterized by NMR, absorption, fluorescence and X-ray single-crystal diffraction, they were used as photosensitizers for generation of hydrogen via the reduction of water using cobaloximes Cn(n=1~4) as the catalysts. Besides, we explored the internal and external factors which affect the efficiency of hydrogen production system in detail. Under optimized conditions, the TONs of hydrogen evolution for iodinated BODIPY are differ by the subsitituent group, among which the 8-bit ortho-substituted pyridyl B6 show the highest activity of hydrogen evolution, with a turnover number of approximately 164. A reductive quenching pathway, namely, the intermolecular electron transfer from Bn?-(n=4~6) to the cobalt centers, in the photochemically driven step is possible for the hydrogen production, as evaluated from the electrochemical and photophysical data as well as theoretical calculations.On the basis of previous system, novel aza-BODIPY derivatives substituted with iodine were synthesized, which were applied to the last optimized conditions as photosensitizers. Regretfully, both of them show no ability to hydrogen evolution from water splitting in the same conditions. Subsequent theoretical calculations show that due to the oxidation-reduction potential of Aza-B2 level does not match with the catalysts, therefore Aza-B2 is not able to act as a photosensitizer for hydrogen production.Lastly, a cobalt phthalocyanine([Coâ…¡Pc(-2)]) is found to have impressive catalytic activity for the photocatalytic proton reduction to H2 in a noble-metal-free system composed of organic dye fluorescein(FL) as the PS and triethylamine(TEA) as the sacrificial electron donor under highly basic solutions(pH = 12.0). And the system maintains its activity for ca.12 hours with a turnover number of approximately 2650. Still we explored the reason for the deactivation of the system. It is speculated that the catalytic mechanism may use organo hydrides not cobalt hydrides for the evolution of H2 through a bimolecular pathway, as evaluated from the electrochemical and photophysical data as well as theoretical calculations.