In situ spectroelectrochemistry demonstrates stability of electrografted diazonium interfaces on conductive oxides & their suitability as anchoring groups for molecular species.
Surface enhanced vibrational spectroscopy shows the correlation between electron transfer kinetics and protonation degree of Fe Hangman complexes on electrodes.
Two
iron porphyrin complexes with either mesityl (FeTMP) or thiophene
(FeT3ThP) peripheral substituents were attached to basal pyrolytic
graphite and Ag electrodes via different immobilization methods. By
combining cyclic voltammetry and in-operando surface-enhanced Raman
spectroscopy along with MD simulations and DFT calculations, their
respective surface attachment, redox chemistry and activity toward
electrocatalytic oxygen reduction was investigated. For both porphyrin
complexes, it could be shown that catalytic activity is restricted
to the first (few) molecular layer(s), although electrodes covered
with thiophene-substituted complexes showed a better capability to
consume the oxygen at a given overpotential even in thicker films.
The spectroscopic data and simulations suggest that both porphyrin
complexes attach to a Ag electrode surface in a way that maximum planarity
and minimum distance between the catalytic iron site and the electrode
is achieved. However, due to the distinctive design of the FeT3ThP
complex, the thiophene rings are capable of occupying a conformation,
via rotation around the bonding axis to the porphyrin, in which all
four sulfur atoms can coordinate to the Ag surface. This effect creates
a dense and planar surface coverage of the porphyrin on the electrode
facilitating a fast (multi) electron transfer via several covalent
Ag–S bonds. In contrast, bulky mesityl groups as peripheral
substituents, which have been initially introduced to prevent aggregation
and improve catalytic behavior in solution, exert
a negative effect on the overall electrocatalytic performance in the immobilized state as a less dense coverage and less
stable interactions with the surface are formed. Our results underline
the importance of rationally designed heterogenized molecular catalysts
to achieve optimal turnover, which not only strictly applies
to the here discussed oxygen reduction reaction but eventually holds
also true for other energy conversion reactions such as carbon dioxide
reduction.
Respiratory complex I (CpI) is a key player in the way organisms obtain energy, being an energy transducer, which couples nicotinamide adenine dinucleotide (NADH)/quinone oxidoreduction with proton translocation by a mechanism that remains elusive so far. In this work, we monitored the function of CpI in a biomimetic, supported lipid membrane system assembled on a 4-aminothiophenol (4-ATP) self-assembled monolayer by surface-enhanced infrared absorption spectroscopy. 4-ATP serves not only as a linker molecule to a nanostructured gold surface but also as pH sensor, as indicated by concomitant density functional theory calculations. In this way, we were able to monitor NADH/quinone oxidoreduction-induced transmembrane proton translocation via the protonation state of 4-ATP, depending on the net orientation of CpI molecules induced by two complementary approaches. An associated change of the amide I/amide II band intensity ratio indicates conformational modifications upon catalysis which may involve movements of transmembrane helices or other secondary structural elements, as suggested in the literature [ Di Luca , Proc. Natl. Acad. Sci. U.S.A. , 2017 , 114 , E6314 - E6321 ].
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