2014 Fiscal Year Annual Research Report
アリールホウ素置換基を有する遷移金属錯体を利用した新規人工光合成系の創製
| Project/Area Number |
13J01659
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| Research Institution | Hokkaido University |
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Principal Investigator |
康 媛媛 北海道大学, 総合化学院, 特別研究員(DC1)
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| Project Period (FY) |
2013-04-01 – 2016-03-31
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| Keywords | aritificial / photosynthesis / rhenium(I) tricarbonyl / charge transfer |
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| Outline of Annual Research Achievements |
Energy shortage from fossil fuels and pollution of the environment are two important issues that must be solved in the near future. In this study, I focus on dπ6-type polypyridine transition metal complexes, especially a tricarbonyl rhenium(I) complex having an aromatic diimine ligand, [Re(CO)3LX] (L = diimine and X = halogen, phosphine or imine) has been studied extensively as they can conduct the photocatalytic reduction of CO2.
Recently, I have reported that the rhenium(I) complex having one (dimesityl)borylethynyl (DBDE) group at the 4-position of 1,10-phenanthroline (ReBphen) or that of 2,2'-bipyridine ligand (ReBbpy) shows low-energy/intense absorption and low-energy/long-lived emission compared with the corresponding reference complex without an arylborane charge transfer unit. Since they also show efficient electron transfer quenching by electron donors, the rhenium(I) arylborane complexes are possible candidates for the photosensitizer/photocatalysts in a future artificial-photosynthesis system.
Therefore, two new and novel tricarbonyl rhenium(I) complexes having two DBDE groups in the diimine ligands (ReB2phen and ReB2bpy) were designed/synthesized, and the excited-state characteristics of the complexes were evaluated in detail.
These spectroscopic, photophysical and photochemical characteristics (intense absorption, long-lived excited state and efficient electron transfer) observed for the ReB-type complexes are highly advantageous toward development of efficient photocatalytic reactions and solar energy conversion systems.
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| Current Status of Research Progress |
Current Status of Research Progress
2: Research has progressed on the whole more than it was originally planned.
Reason
In this year, I designed and synthesized two novel rhenium(I) complexes having two arylborane charge transfer units.
The novel complexes showed larger molar absorption coefficient with the increase of the arylborane charge transfer units due to the larger transition dipole moments for photoexcitation. On the other hand, longer-lived and low-energy emission observed for ReBphens and ReBbpys can be ascribed to the large energy barrier for the thermal deactivation process via nonemitting 4th MLCT excited states.
I also conducted electron-transfer emission quenching experiments of ReB2phen and ReB2bpy by triethanolamine in DMF, and compared with the results on the corresponding reference complexes, the quenching rate constant (kq) value increased with increasing in the number of the arylborane unit. The electron transfer reactions of ReBphens occurred more efficiently while they are thermodynamically disadvantageous, compared with those of Rephen. Therefore, it is clear that the presence of “electron-accepting” triarylborane unit(s) in the complex is quite important for “reductive” electron transfer reactions, indicating that the complex plays an important role in further enhancement of the CO2 photoreduction efficiency.
Since the target of my research is exploitation of novel artificial photosynthesis system by transition metal complexes having arylborane charge transfer units, until now, I have exploited four novel Re(I) complexes which will be used for CO2 reduction next, I believe my research have been conducted smoothly.
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| Strategy for Future Research Activity |
Since the spectroscopic, photophysical and photochemical properties of the ReB-type complexes are highly advantageous for photocatalytic reduction, next, photocatalytic reduction of CO2 by all the four novel complexes will be demonstrated in detail.
The efficiency of producing the OER species of each complex, and the quantum yields, the turnover frequencies (TOF) and numbers (TON) of the reactions by the complexes will be evaluated using the absorption spectral change, gas or liquid chromatography measurements, and so forth.
Finally, I will try to elucidate the reaction mechanism of CO2 photocatalytic reduction by the complexes having arylborane units and the effects of the synergistic MLCT/π(aryl)-p(B) CT excited state of the complexes on the reaction efficiency.
If the excited state of the novel complex can be quenched by CO2 in solution, only one one-electron-reduced species is required for CO2 reduction and large enhancement of efficiency can be expected.
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