posted on 2018-03-06, 00:00authored byLing Zhang, Kasper P. Kepp, Jens Ulstrup, Jingdong Zhang
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).