Electrochemistry and photoelectrochemistry of chlorophyll on a metallic electrode: Part I. properties of chlorophyll adsorption films on metals

Abstract

According to absorption and fluorescence spectra, chlorophyll solubilized in aqueous solution exists in these systems in monomeric form. It adsorbs on an amalgamated gold electrode firmly and irreversibly. Two redox reactions of adsorbed chlorophyll can be observed at −0.5 and −1.0 V (vs. SHE). The first reaction is accompanied by protonation and is ascribed to the reduction of the carbonyl group of the cyclopentanone ring. The second reaction does not involve proton consumption and is due to the addition of one electron to the porphyrin ring. The irreversible “adsorption isotherm” reveals a phenomenon similar to surface condensation. The limiting coverage corresponds to a monolayer of chlorophyll molecules adsorbed in a flat orientation. The absorption spectra of adsorbed chlorophyll at its low coverages are fairly similar to its spectrum in monomeric solution. At high coverages, a long wave shift of the red maximum typical of strong intermolecular interaction of pigment molecules is observed. Spectra in s-polarized light confirm the flat orientation of the adsorbed molecules. The extinction coefficient of adsorbed chlorophyll exceeds the same in solution by two orders of magnitude. Unusually firm adsorption taking place without any destruction of the organic molecule, formation of the chlorophyll radical anion in aqueous solution without proton attachment, an abnormally high extinction coefficient — all these peculiarities can be explained by a very strong interaction of the π-electrons of the porphyrin ring with the metal.