Direct Probing of Photoinduced Electron Transfer in a Self-Assembled Biomimetic [2Fe2S]-hydrogenase Complex Using Ultrafast Vibrational Spectroscopy

Inorg Chem. 2014 May 19;53(10):5373-83. doi: 10.1021/ic500777d. Epub 2014 Apr 26.

Abstract

A pyridyl-functionalized diiron dithiolate complex, [μ-(4-pyCH2-NMI-S2)Fe2(CO)6] (3, py = pyridine (ligand), NMI = naphthalene monoimide) was synthesized and fully characterized. In the presence of zinc tetraphenylporphyrin (ZnTPP), a self-assembled 3·ZnTPP complex was readily formed in CH2Cl2 by the coordination of the pyridyl nitrogen to the porphyrin zinc center. Ultrafast photoinduced electron transfer from excited ZnTPP to complex 3 in the supramolecular assembly was observed in real time by monitoring the ν(C≡O) and ν(C═O)NMI spectral changes with femtosecond time-resolved infrared (TRIR) spectroscopy. We have confirmed that photoinduced charge separation produced the monoreduced species by comparing the time-resolved IR spectra with the conventional IR spectra of 3(•-) generated by reversible electrochemical reduction. The lifetimes for the charge separation and charge recombination processes were found to be τCS = 40 ± 3 ps and τCR = 205 ± 14 ps, respectively. The charge recombination is much slower than that in an analogous covalent complex, demonstrating the potential of a supramolecular approach to extend the lifetime of the charge-separated state in photocatalytic complexes. The observed vibrational frequency shifts provide a very sensitive probe of the delocalization of the electron-spin density over the different parts of the Fe2S2 complex. The TR and spectro-electrochemical IR spectra, electron paramagnetic resonance spectra, and density functional theory calculations all show that the spin density in 3(•-) is delocalized over the diiron core and the NMI bridge. This delocalization explains why the complex exhibits low catalytic dihydrogen production even though it features a very efficient photoinduced electron transfer. The ultrafast porphyrin-to-NMI-S2-Fe2(CO)6 photoinduced electron transfer is the first reported example of a supramolecular Fe2S2-hydrogenase model studied by femtosecond TRIR spectroscopy. Our results show that TRIR spectroscopy is a powerful tool to investigate photoinduced electron transfer in potential dihydrogen-producing catalytic complexes, and that way to optimize their performance by rational approaches.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Electron Transport
  • Iron-Sulfur Proteins / chemistry
  • Iron-Sulfur Proteins / metabolism*
  • Models, Molecular
  • Molecular Conformation
  • Photochemical Processes
  • Quantum Theory
  • Spectrometry, Fluorescence
  • Time Factors

Substances

  • Iron-Sulfur Proteins