Real-space resolved surface reactions: deprotonation and metalation of phthalocyanine

Abstract

Upon deposition on a surface, molecules can undergo a plethora of changes, such as reactions with adsorbates and surface atoms and catalytic decomposition. Since different reaction pathways may coexist, spatially averaging techniques can be insufficient for the characterization and distinction of all on-surface products. Here, we present a study of single phthalocyanine molecules on a Cu(111) surface which was performed using high-resolution low-temperature STM. Upon deposition of metal-free H₂Pc, we can identify three distinct molecular species. A thorough investigation reveals that temperature-driven on-surface reactions partially convert H₂Pc into H₀Pc and CuPc. The individual species are differentiated by their topographic appearance and can unambiguously be identified by their STM-induced rotational behavior. While H₂Pc shows a switching between two orientations at low energies, a third orientation can be observed above E > 800 meV, which is induced by tautomerization. Around the Fermi level, the rotational behavior is asymmetric, owing to the excitation of vibrational modes in unoccupied states whereas resonant tunneling occurs in occupied states. A two-step deprotonation of H₂Pc confirms that the second species is H₀Pc. By comparison with CuPc evaporated on Cu(111), we unambiguously reveal that the third species is indeed CuPc, which exhibits an exceptionally low threshold for rotational switching accompanied by an asymmetric behavior around the Fermi level. Varying the post-annealing temperature, we found a sharp threshold for the H₂Pc → CuPc on-surface metalation at around 100 °C. In contrast, the competing process of thermal decomposition from H₂Pc to H₀Pc only increases weakly.