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

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.