Electrochemical and photoelectrochemical behavior of chlorophyll a films adsorbed on mercury

The electrochemical behavior of films of chlorophyll a (Chl) adsorbed on a hanging mercury drop electrode by two different procedures was investigated in the dark by double potential-step chronocoulometry and cyclic voltammetry. Light-on and light-off photocurrents of these films were recorded at different applied potentials. Appreciable photocurrents were observed by adsorbing Chl from solutions of organic solvents containing Chl aggregates. The charge involved in the electroreduction of these films to the Chl radical anion, Chl−, at ∼−1.250 V versus the saturated calomel reference electrode (SCE) points to adsorbed Chl dimers. Electroreduction of Chl to Chl− and its subsequent reoxidation causes a loss of about one half of the adsorbed Chl molecules, with a resulting notable depression of the photocurrents. Stepping the applied potential back to −0.100 V(SCE) after each negative potential jump by using double potential-step chronocoulometry avoids such a loss and shifts Chl electroreduction to ∼−0.900 V(SCE) by causing Chl− protonation. The potentiostatic photocurrents recorded at progressively more negative applied potentials starting from −0.160 V(SCE) attain a stationary value that is first cathodic, but becomes anodic at more negative potentials. An explanation for this behavior is provided.