Electroreflectance study of hemin adsorbed on a HOPG electrode: Estimation of molecular orientation and analysis of nonfaradaic electroreflectance signal due to the stark effect

The structural and spectroelectrochemical properties of a layer of hemin (iron(III) protoporphyrin IX chloride) adsorbed on the basal plane surface of a highly oriented pyrolytic graphite (HOPG) electrode were studied using electroreflectance (ER) spectroscopy. The intensity dependence of the ER response to p- or s-polarized incident light on the incident angle was analyzed at the formal potential E^°° of adsorbed hemin. Hemin molecules were found to lie almost flat on the HOPG electrode; i.e., the angle between the porphyrin plane and the surface normal of the electrode is 70-80°. At potentials both far positive and negative from E^°′, adsorbed hemin exhibited a nonfaradaic ER response to p-polarized and s-polarized incident light attributable to the Stark effect, indicating that the adsorbed hemin orients obliquely. The linear dependence in the shift of the ER spectral band due to the Stark effect indicates that the orientation of hemin molecules is insensitive to changes in electrode potential.