Radical induced intermolecular linkage and energy level modifications of a porphyrin monolayer
A new method to directly modify the surface structure and energy levels of a porphyrin monolayer was examined in the molecular scale using scanning tunneling microscopy and spectroscopy (STM and STS) and presented in this communication. The exposure to atomic oxygen has induced highly ordered surface cross-linking and changed the occupied and unoccupied orbital levels of a cobalt(II) octaethyl porphyrin (CoOEP) monolayer, and as a result, the HOMO–LUMO gap was reduced by ∼10%. Counterintuitively, the STM/STS data indicated that the reactive central Co atoms did not participate in the gas–surface reactions. Reflection–absorption infrared spectroscopy (RAIRS) measurements further indicated that the STM observed intermolecular linkages are stabilized via hydrogen bonding. This CoOEP + O˙ system also illustrates an example that the six-fold surface packing symmetry predominates the four-fold molecular symmetry in producing a three-fold symmetric surface cross-linking structure.