Investigating the molecule-substrate interaction of prototypic tetrapyrrole compounds: Adsorption and self-metalation of porphine on Cu(111)

Abstract

We report on the adsorption and self-metalation of a prototypic tetrapyrrole compound, the free-base porphine (2H-P), on the Cu(111) surface. Our multitechnique study combines scanning tunneling microscopy (STM) results with near-edge X-ray absorption fine-structure (NEXAFS) and X-ray photoelectron spectroscopy (XPS) data whose interpretation is supported by density functional theory calculations. In the first layer in contact with the copper substrate the molecules adsorb coplanar with the surface as shown by angle-resolved NEXAFS measurements. The quenching of the first resonance in the magic angle spectra of both carbon and nitrogen regions indicates a substantial electron transfer from the substrate to the LUMO of the molecule. The stepwise annealing of a bilayer of 2H-P molecules sequentially transforms the XP and NEXAFS signatures of the nitrogen regions into those indicative of the coordinated nitrogen species of the metalated copper porphine (Cu-P), i.e., we observe a temperature-induced self-metalation of the system. Pre- and post-metalation species are clearly discriminable by STM, corroborating the spectroscopic results. Similar to the free-base porphine, the Cu-P adsorbs flat in the first layer without distortion of the macrocycle. Additionally, the electron transfer from the copper surface to the molecule is preserved upon metalation. This behavior contrasts the self-metalation of tetraphenylporphyrin (2H-TPP) on Cu(111), where both the molecular conformation and the interaction with the substrate are strongly affected by the metalation process.

Surface chemistry of porphyrins and phthalocyanines

“…On Ag(1 1 1), 2HP showed a similar adsorption behaviour as was discussed above for free-base phthalocyanine (2HPc), i.e., 2HP adsorbed with the molecular plane parallel to the surface [432][433][434] and the self-assembly was driven by a delicate balance between maximization of the adsorption energy, lateral repulsion, and retention of the preferred adsorption site (cf. Section 4.1.…”

“…Although not further discussed in refs. [432,433], this is a very remarkable result, because both the high evaporation temperature of 2HP (493 K) and the multilayer growth on Ag indicate that multilayers are thermodynamically stable at room temperature. The self-limiting growth can thus only result from kinetic hindrance, i.e., from an extremely low sticking coefficient, which is very unusual for the physisorption of large organic molecules.…”

“…The multilayer structure of 2HP on Ag(1 1 1) and Cu(1 1 1) was shown to be sensitive to both the nature of the substrate and the preparation conditions [432][433][434]. On both surfaces, deposition at 150 K resulted in the formation of a multilayer in which the molecules were tilted by 301 relative to the surface, as was shown by NEXAFS [432].…”

“…At room temperature, the two substrates showed different growth behaviour: On Ag (1 1 1), multilayer growth was easily possible and the molecules were oriented perpendicular to the surface [432]. On Cu (1 1 1), however, adsorption was self-limited to approximately two monolayers [432,433]. Although not further discussed in refs.…”

Reconstruction of steps on the Cu(111) surface induced by sulfur

“…[5][6][7] (Molecules with some other functional groups, such as cyano groups, can also self-metallate on these surfaces. 8,9 ) Sulfur on Cu(111) is one such system. On Cu(111), Feibelman first proposed a Cu 3 S 3 complex as an agent of mass transport on the basis of density functional theory (DFT).…”