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

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 degrees 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 degrees C, and are controlled by kinetics to above 100 degrees C. Quantitative temperature and time dependent surface coverage studies were performed on both CoOEP and H2OEP at 90, 100, and 110 degrees C. The desorption activation energies for both porphyrins were found to be (1.25 +/- 0.05) x 10(2) 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 dominated by the interactions between the porphyrin ring and HOPG. Comparison of these results with previously published work for the NiOEP/CoOEP system suggests the presence of weak cooperativity in the desorption process. We also found that setting the sample potential to +/-1.5 V relative to the earth for periods of the order of an hour had no effect on desorption rates at 50 degrees C. On the other hand, a large potential difference between the tip and sample did produce a significant change in desorption rate.