Influence of the Central Metal Ion on the Desorption Kinetics of a Porphyrin from the Solution/HOPG Interface

Abstract

The changes in desorption kinetics that result from incorporating a metal ion into a porphyrin ring are studied by scanning tunneling microscopy (STM). Desorption studies of cobalt(II) octaethylporphyrin (CoOEP) and free base octaethylporphyrin (H2OEP) at the 1-phenyloctane/HOPG interface were performed in the 20–110 °C temperature range. These studies of mixtures of CoOEP and H2OEP have shown that the resulting monolayer compositions are stable for more than one year at 20 °C, and are controlled by kinetics to above 100 °C. Quantitative temperature and time dependent surface coverage studies were performed on both CoOEP and H2OEP at 90, 100, and 110 °C. The desorption activation energies for both porphyrins were found to be (1.25 ± 0.05) × 102 kJ/mol. The rate of desorption and the rate of adsorption for CoOEP are similar to the corresponding rates for H2OEP, indicating that replacing the central protons with a cobalt ion has only a minor influence on adsorption. Thus, the adsorption strength is dominate...