Evaluation of molecular orbital symmetry via oxygen-induced charge transfer quenching at a metal-organic interface

Abstract

Thin molecular films under model conditions are often exploited as benchmarks and case studies to investigate the electronic and structural changes occurring on the surface of metallic electrodes. Here we show that the modification of a metallic surface induced by oxygen adsorption allows the preservation of the geometry of a molecular adlayer, giving access to the determination of molecular orbital symmetries by means of near-edge x-ray absorption fine structure spectroscopy, NEXAFS. As a prototypical example, we exploited Nickel Tetraphenyl Porphyrin molecules deposited on a bare and on an oxygen pre-covered Cu(100) surface. We find that adsorbed atomic oxygen quenches the charge transfer at the metal-organic interface but, in contrast to a thin film sample, maintains the ordered adsorption geometry of the organic molecules. In this way, it is possible to disentangle π* and σ* symmetry orbitals, hence estimating the relative oscillator strength of core level transitions directly from the experimental data, as well as to evaluate and localize the degree of charge transfer in a coupled system. In particular, we neatly single out the σ* contribution associated with the N 1s transition to the mixed N 2px,y-Ni 3dx 2 -y 2 orbital, which falls close to the leading π*-symmetry LUMO resonance.

Ferrous to Ferric Transition in Fe‐Phthalocyanine Driven by NO2 Exposure

“…Figure 1 a shows the momentum integrated photoelectron spectrum of the FePc/Cu(100) interface measured at 30 eV using p ‐polarized synchrotron radiation. While the valence band spectrum of the bare copper substrate shows a rather featureless plateau associated with sp‐bands, [17] two prominent features are present in the FePc/Cu(100) spectrum, peaked at BEs 1.6 eV and 0.6 eV. To identify their origin, the momentum maps at corresponding BE were measured and the results are presented in the bottom row of Figure 1 b.…”

“…The introduction of a buffer layer at the organic–metal interface can be exploited to tune the molecular‐surface interaction, [17, 18] even for restoring the gas phase triplet spin state of the iron ion together with its net magnetic moment. The latter has been achieved on the oxygen‐reconstructed copper surface, where the covalent nature of the Cu−O interaction yields a strong localization of the surface electrons inhibiting the charge transfer from the metal to the organic overlayer [17, 18] …”

Reversible redox reactions in metal-supported porphyrin: the role of spin and oxidation state

“…27,28 At saturation, Cu(100) displays a characteristic (O2 Â 2O2)R451-O reconstruction, where a significant quenching of the charge transfer at the organicmetal interface is expected. [29][30][31] Similarly to the free molecule, CoTPP deposited on O-Cu(100), has a LS state with a single unpaired electron occupying the 3d z 2-based MO (Fig. S1, right panel, ESI †).…”

Stabilization and activation of molecular oxygen at biomimetic tetrapyrroles on surfaces: from UHV to near-ambient pressure

“…This occurs for the NiTPP/Cu(100) system. 45,63 The strong metal-substrate interaction induces a sizeable charge transfer to the molecule, yielding population of the LUMOs up to the 3+ level and the formation of a Ni(I) ion, at variance to the Ni(II) oxidation state typical of the gas phase porphyrin. On the contrary, in the NiTPP/O/Cu(100) layer, oxygen decouples the molecule from the substrate and the Ni(II) oxidation state is preserved.…”