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

作者：Li Ping; Amirjalayer Saeed; Hartl Frantisek^*; Lutz Martin; de Bruin Bas; Becker Rene; Woutersen Sander; Reek Joost N H

来源：Inorganic Chemistry, 2014, 53(10): 5373-5383.

DOI：10.1021/ic500777d

A pyridyl-functionalized diiron dithiolate complex, [mu-(4-pyCH(2)-NMI-S-2)Fe-2(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 center dot 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 nu(C O) and nu(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(center dot-) generated by reversible electrochemical reduction. The lifetimes for the charge separation and charge recombination processes were found to be tau(CS) = 40 +/- 3 ps and tau(CR) = 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(center dot-) 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-S-2 Fe-2(CO)(6) photoinduced electron transfer is the first reported example of a suprarnolecular 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.

• 出版日期2014-5-19