Adsorption Conformation and Lateral Registry of Cobalt Porphine on Cu(111)

Abstract

The tetrapyrrole macrocycle of porphine is the common core of all porphyrin molecules, an interesting class of π-conjugated molecules with relevance in natural and artificial systems. The functionality of porphines on a solid surface can be tailored by the central metal atom and its interaction with the substrate. In this study, we present a local adsorption geometry determination for cobalt porphine on Cu(111) by means of complementary scanning tunneling microscopy, high-resolution X-ray photoelectron spectroscopy, X-ray standing wave measurements, and density functional theory calculations. Specifically, the Co center was determined to be at an adsorption height of 2.25 ± 0.04 Å occupying a bridge site. The macrocycle adopts a moderate asymmetric saddle-shape conformation, with the two pyrrole groups that are aligned perpendicular to the densely packed direction of the Cu(111) surface tilted away from the surface plane.

Quantitative determination of a model organic/insulator/metal interface structure

“…The XPS binding energy of the Co 1 component (780.3 eV) is significantly greater than that found for Co–P/Cu(111) (778.2 eV),51 Co-TPP/Ag(111) (778.2 eV)61,64 and metallic Co (778.0 eV) 65. The binding energy of Co–P/Cu(111) was ascribed to adsorption-related charge transfer from the copper substrate into the Co ion 51. Therefore, the significant increase in binding energy here and the observed line shape are both indicative of a +2 oxidation state and signal a dramatic decrease in any charge transfer from the substrate.…”

“…Note also that the C 1s and N 1s XP spectra (Fig. S4†) both present a smaller upshift in binding energy of 0.4 eV, compared to Co-P/Cu(111),51 suggesting that the charge transfer into the molecule directly on the metal surface primarily affects the Co center (as discussed in our prior study) 51. The XPS data are summarized in the Table S1 in the ESI †…”

“…1c) on h -BN/Cu(111) as a model system to study the adsorption geometry and the interaction of the molecule with the substrate in a combined experimental approach based on X-ray photoelectron spectroscopy (XPS), XSW, and scanning tunneling microscopy (STM). The geometric structure of both pristine h -BN and Co–P on Cu(111) have been separately determined in a quantitative fashion in prior works 50,51. The h -BN/Cu interface is characterized by coexisting moiré-like superstructures with periodicities ranging from 1 to 15 nm, providing hexagonal arrays of so-called pores separated by wire regions 50,52.…”

“…The h -BN layer was found to display an average adsorption height of 3.38 ± 0.04 Å and exhibits a moderate geometric corrugation of 0.3–0.7 Å 50. Co–P molecules on Cu(111) were determined to adsorb in a single site, with the Co atom bridging surface Cu atoms at a height of 2.25 ± 0.04 Å 51. The molecular backbone of the Co–P molecule on Cu(111) presents an asymmetric saddle-shape conformation as well as a strong interaction between the Co atom and the copper substrate.…”

Validation of the inverted adsorption structure for free-base tetraphenyl porphyrin on Cu(111)

“…In our previous investigation of the adsorption of porphine on Cu(111) 16 and cobalt porphine on Cu(111), 22 the centres of these molecules were found to occupy bridge sites with one of the molecular mirror planes coincident with the surface mirror plane that bisects the bridge site. In the determined PhD structure of porphine on Cu(111) 16 the iminic N atoms occupy two adsorption sites that are mirrored across the [11% 2] direction, which is also a mirror plane of the substrate; by contrast, the mirror plane of the molecule that relates the two aminic N atoms lies in the [% 110] direction, which is not a mirror plane of the substrate.…”