Exploring the Surface Chemistry of Cobalt Porphyrin Complexed with Iodine Using Single Molecule Microscopy and Theory

Understanding the interactions between molecules on surfaces is crucial for advancing technologies in sensing, catalysis, and energy harvesting. In this study, we explore the complex surface chemistry resulting from the interaction of Co(II)octaethylporphyrin (CoOEP) and iodine, I2, both in solution and at the phenyloctane/highly oriented pyrolytic graphite (HOPG) interface. In pursuit of this goal, we report results from electrochemistry, NMR and UV–vis spectroscopy, X-ray crystallography, scanning tunneling microscopy (STM), and density functional theory (DFT). Both spectroscopic methods of analysis confirmed that at and above the stoichiometric ratio of one CoOEP to one I2 the reaction product was metal-centered CoIII(OEP)I. X-ray crystallography verified that a single iodine is bonded to each cobalt ion in the triclinic P1̅ system. The surface chemistry of CoOEP and I2 is complicated and remarkably dependent on the iodine concentration. STM images of CoOEP and I2 in phenyloctane on HOPG at low halogen concentrations (1:<2 Co/I ratios) presented random individual Co(OEP)I molecules weakly adsorbed onto a hexagonal (HEX) CoOEP monolayer. Images of 1:≥2 Co/I ratio solutions showed phase-segregated HEX CoOEP and pseudorectangular (REC) Co(OEP)I incorporating one solvent molecule per Co(OEP)I. The REC structure formed in long parallel rows with the number of rows increasing with increasing solution I2. In this case, the presence of CoOEP on the surface was attributed to the spontaneous reduction of Co(OEP)I by the graphite substrate. DFT calculations indicate that the REC Co(OEP)I:PhO form is energetically more stable than the HEX form of Co(OEP)I on HOPG. Experimental STM images and DFT calculated adsorption energies support our interpretation of the observed structures.