Self-Metalation of Porphyrins by Cobalt Oxide: Photoemission Spectroscopic Investigation

Abstract

Understanding the electronic structure between porphyrins and ultrathin metal-oxide film interfaces is vital for the rational design of functional molecular architectures. Here, we have studied the interfacial electronic properties of porphyrin molecular thin films on well-ordered thin layers of CoO(111) and metallic Co(100) surfaces by means of X-ray and ultraviolet photoemission spectroscopies. In this work, we observed the direct self-metalation of free-base porphyrin molecules at the interface to CoO(111) at room temperature by monitoring the N 1s core-level evolution. On metallic Co(100) films, no metalation occurs, but it can be induced by exposure to oxygen. The oxygen-induced metalation has, to the best of our knowledge, not been reported until now and might play an important role in processes where porphyrins are exposed to metals in a reactive environment.

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

“…If the metalating ion has to be released from the surface, we expect that the abundance of the metal ions, the energy barriers, and the overall driving force should depend on the surface structure. However, there are very few studies yet in which the influence of the surface structure on self-metalation was probed experimentally on well-defined oxide surfaces. − , On nanoparticulate systems, however, there is clear experimental evidence that the defect density of the surface (steps, corners, edges) plays a critical role in the metalation reaction. ,,,, …”

“…However, there are very few studies yet in which the influence of the surface structure on self-metalation was probed experimentally on well-defined oxide surfaces. [23][24][25][26][27]45 On nanoparticulate systems, however, there is clear experimental evidence that the defect density of the surface (steps, corners, edges) plays a critical role in the metalation reaction. 23,32,33,46,47 In this work, we present the results of the first study, in which the metalation reaction of anchored porphyrins is compared on three atomically-defined oxide surfaces under identical conditions.…”

Self‐Metalation of Porphyrins at the Solid–Gas Interface

“…Thei ntensity ratio of the resonances at 1327 and 1369 cm À1 can be exploited as am etalation fingerprint when the 2H-TPP monolayer is deposited on the oxygen pre-covered Pd(100) termination and subsequently annealed to 410 K, thus inducing metalation and water production at variance with the 2H-TPP/Pd(100) case. [8] In analogy to the cobalt substrates in vacuo, [10,15,18] we exploit here the same metalation marker to extend the investigation beyond the UHV limits,t hanks to the unique selection rules of the sum-frequency generation process. [19,20] In the top part of Figure 1weplot the IR-Vis SFG spectrum of the as-prepared system (bottom), at room temperature,but in presence of 1mbar O 2 .Already at afirst glance an overall, general diminution of the amplitudes is evident.…”

“… [14] The potential role of an oxygen atmosphere has been addressed in the porphyrin self‐metalation mechanisms observed on Co and Ag terminations in UHV. [ 15 , 16 ] However, the concomitant oxidation of the surface makes it hard to distinguish between the role of a direct (Eley–Rideal like) mechanism induced by gaseous O 2 and a surface‐mediated process. At variance with the mentioned systems, on palladium we find that no surface oxidation takes place under the considered conditions, since the porphyrin monolayer prevents the evolution of a sizeable oxygen surface coverage, and specific insights about the role of gaseous O2 can be thus achieved.…”

Self‐Metalation of Porphyrins at the Solid–Gas Interface

“…Whereas recent studies have definitely proven the role in UHV of adsorbed atomic oxygen in promoting the metalation reaction via the formation of a sitting‐atop complex and through the production of molecular hydrogen or water, [8, 10–13] the extension of this level of insight up to NAP conditions is unprecedented yet [14] . The potential role of an oxygen atmosphere has been addressed in the porphyrin self‐metalation mechanisms observed on Co and Ag terminations in UHV [15, 16] . However, the concomitant oxidation of the surface makes it hard to distinguish between the role of a direct (Eley–Rideal like) mechanism induced by gaseous O 2 and a surface‐mediated process.…”