Electron Transfer from the Porphyrin S2 State in a Zinc Porphyrin-Rhenium Bipyridyl Dyad having Carbon Dioxide Reduction Activity

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• 作者：Kuniomi Kiyosawa ; Naoki Shiraishi ; Tetsuya Shimada ; Dai Masui ; Hiroshi Tachibana ; Shinsuke Takagi ; Osamu Ishitani ; Donald A. Tryk ; Haruo Inoue
• 刊名：Journal of Physical Chemistry C
• 出版年：2009
• 出版时间：July 9, 2009
• 年：2009
• 卷：113
• 期：27
• 页码：11667-11673
• 全文大小：216K
• 年卷期：v.113,no.27(July 9, 2009)
• ISSN：1932-7455

文摘

fac-Re(bpy)(CO)₃Cl (bpy = 2,2′-bipyridine) and similar complexes are known to act as photocatalysts for the reduction of CO₂ to CO in the presence of electron donors. However, these complexes do not have significant absorption in the visible region. In a separate publication, we have reported the successful combination of fac-Re(bpy)(CO)₄Cl with a zinc porphyrin that has strong absorption in the visible region to produce the ZnTMP-Re(bpy)(NHAc) dyad(5-{4-[N-4′-(rhenium(I)tricarbonylchloride-4-acetylamino-2,2′-bipyridyl)-carbamoyl]phenyl}-10,15,20-trimesityl-porphyrinatozinc(II)). In that paper we reported the photochemical reduction of CO₂ using excitation of the porphyrin upper excited singlet state, S₂ state (visible region), in the dyad. The very wide visible light region is able to be utilized by the sensitizer. In the present work, as regards the detailed molecular mechanism, we have demonstrated the direct electron transfer from the Zn porphyrin S₂ state to the Re complex in the dyad by ultrafast transient spectroscopy. The fluorescence of the dyad was quenched by 67% compared to that of ZnTMP(COOMe) (5,10,15-trimesityl-20-[4-(methoxycarbonyl)phenyl]porphyrinatozinc) in DMF, measured from the steady-state fluorescence spectrum. The lifetime of the ZnTMP-Re(bpy)(NHAc) S₂ state was revealed to be 0.53 ps by time-resolved fluorescence and 0.56 ps by transient absorption decay, while that of ZnTMP(COOMe) was 1.6 ps. The porphyrin cation radical was actually identified in the time-resolved absorption spectrum. These are concrete evidence of electron transfer from the porphyrin S₂ state to the Re complex. The lifetime of the charge-separated state for ZnTMP-Re(bpy)(NHAc) was also estimated to be 90 ± 10 ps from the time-resolved absorption decay.