Surface Structure Controls Self-Metalation: In-Situ IR Studies of Anchored Porphyrins on Atomically-Defined Cobalt Oxide Surfaces

The functionality of porphyrin/oxide interfaces depends on both the structure of the organic layer and the metalation of the porphyrin units. While the structure of the porphyrin film is known to be controlled by the oxide surface, little is known on the structure dependence of the self-metalation reaction. In this work, we investigated both the anchoring and the self-metalation reaction of 5-(4-carboxyphenyl)-10,15,20-triphenylporphyrin (MCTPP) on three atomically-defined oxide thin films, namely Co₃O₄(111), CoO(111), and CoO(100), under ultrahigh vacuum conditions. We used infrared reflection absorption spectroscopy to follow the growth and self-metalation reaction in-situ as a function of coverage and temperature. At 300 K, MCTPP binds to Co₃O₄(111) in the form of a chelating surface bidentate carboxylate, whereas bridging carboxylates are found on CoO(111) and CoO(100). MCTPP multilayers desorb between 450 and 460 K on all oxides. However, monolayer species reside on the Co₃O₄(111) surface up to temperatures of 590 ± 5 K, similar as observed for CoO(100) (575 ± 5 K). On CoO(111), the anchored carboxylates are more strongly bound and remain up to 690 ± 5 K. Finally, we observed that the self-metalation reaction is strongly dependent on the surface structure and temperature. At 300 K, the degree of metalation is low (<10%) on all surfaces. At 450 K, however, we observe self-metalation of 60 ± 10% of the porphyrins on CoO(111) and CoO(100) in the monolayer. In sharp contrast, no increase in the metalation rate is observed for the deposition of MCTPP on Co₃O₄(111) at 450 K. Our results show that the adsorption motif, the molecular orientation, and the metalation reaction are strongly dependent on the oxide surface structure and, in particular, on the arrangement and distance of the Co cations in the surface region.