Redox Potentials and Electronic States of Iron Porphyrin IX Adsorbed on Single Crystal Gold Electrode Surfaces

Abstract

Metalloporphyrins are active sites in metalloproteins and synthetic catalysts. They have also been studied extensively by electrochemistry as well as being prominent targets in electrochemical scanning tunneling microscopy (STM). Previous studies of FePPIX adsorbed on graphite and alkylthiol modified Au electrodes showed a pair of reversible Fe(III/II)PPIX peaks at about -0.41 V (vs NHE) at high solution pH. We recently used iron protoporphyrin IX (FePPIX) as an intercalating probe for long-range electrochemical electron transfer through a G-quadruplex oligonucleotide (DNAzyme); this study disclosed two, rather than a single pair of voltammetric peaks with a new and dominating peak, shifted 200 mV positive relative to the ≈-0.4 V peak. Prompted by this unexpected observation, we report here a study of the voltammetry of FePPIX itself on single-crystal Au(111), (100), and (110) and polycrystalline Au electrode surfaces. In all cases the dominating pair of new Fe(III/II)PPIX redox peaks, shifted positively by more than 200 mV compared to those of previous studies appeared. This observation is supported by density functional theory (DFT) which shows that strong dispersion forces in the FePPIX/Au electronic interaction drive the midpoint potential toward positive values. The FePPIX spin states depend on interaction with the Au(111) interface, converting all the Fe(II)/(III)PPIX species into low-spin states. These results support electrochemical evidence for the nature of the electronic coupling between FePPIX and Au-surfaces, and the electronic states of adsorbate molecules, with a bearing also on recent reports of magnetic FePPIX/Au(111) interactions in ultrahigh vacuum (UHV).

Oxygen Reduction Reaction at Single‐Site Catalysts: A Combined Electrochemical Scanning Tunnelling Microscopy and DFT Investigation on Iron Octaethylporphyrin Chloride on HOPG**

“…In the opposite direction (not reported) a flat background attests the absence of any faradaic processes for the exception of oxygen evolution reaction (OER). Conversely, CV for FeOEP@HOPG, recorded at 50 mV s −1 , shows a well defined reduction peak with E p =0.315 V, which is partially reversible as confirmed by the presence of a small anodic counterpart picked at E p =0.40 V. Based on the observed CVs and previous reports, the reversible pair of peaks with E p,c =0.315 V and E p,a =0.400 V vs. RHE can be assigned to the first one‐electron transfer step resulting in a reduction/oxidation of the central iron atom from Fe III OEP to Fe II OEP and vice versa [21–24] …”

Electrochemical Atomic Force Microscopy and First-Principles Calculations of Ferriprotoporphyrin Adsorption and Polymerization

“…Because the layers in graphite, whose experimental interlayer distance measures ∼3.4 Å, interact with each other via weak dispersion forces, including a second layer is unlikely to affect the spin state of the adsorbate. For example, recent DFT calculations that used a finite C 28 H 14 aromatic molecule to model the surface, successfully studied the interaction between FePPIX (iron protoporphyrin IX) and graphite, including the change in the spin state . The vdW-DF-optPBE-optimized lattice constant for Pt and the experimental lattice constant for graphite were used for building the models for the Pt(111) and graphite surfaces.…”

“…For example, recent DFT calculations that used a finite C 28 H 14 aromatic molecule to model the surface, successfully studied the interaction between FePPIX (iron protoporphyrin IX) and graphite, including the change in the spin state. 17 The vdW-DF-optPBE-optimized lattice constant for Pt and the experimental lattice constant for graphite were used for building the models for the Pt(111) and graphite surfaces. The surface coverages were approximately 0.258 and 0.236 molecules/nm 2 for the models of the Pt(111) and graphite surfaces, respectively.…”

“…Metalloporphyrins have been studied for a variety of applications including catalysis, chemical sensors, medicine, biotechnology, and water remediation to name a few. − Additionally, there is specific interest in understanding the interaction between porphyrins and surfaces for a variety of new technologies. − Interest in this class of compounds is generated by their stability, structural flexibility, and the ability to adjust the porphyrin properties for a particular application by altering either the central metal atom coordinated to the ring or the peripheral sustitutents. ,, The metal atom is particularly important in electrocatalytic processes because of its redox potential.…”

Chemistry of cysteine assembly on Au(100): electrochemistry, in situ STM and molecular modeling

“…[71][72][73][74] Cys monolayer adsorption on Au(100) does not accord with physical adsorption, such as for metalloporphyrin monolayers, for which both the adsorbed porphyrin monolayer and reconstructed substrate underneath are visible. 70,75 New stripe-like features along both [110] and [1Ī0] directions following exactly the atomic rows of the Au(100) substrate underneath and parallel to the terrace steps appear on Cys adsorption, Figure 5A-C. The length of the stripes ranges from a few nm to several hundred nm, and the distance between the stripes from 2-3 to 6-10 nm, depending on the adsorption conditions such as soaking time, Cys concentration, and potential.…”