Metalloporphyrins on Oxygen-Passivated Iron: Conformation and Order Beyond the First Layer

TL;DR

This study uses photoemission orbital tomography and DFT calculations to analyze the structure and electronic properties of metalloporphyrins on oxygen-passivated iron surfaces, revealing how substrate interactions induce macrocycle distortions.

Contribution

It provides detailed insights into the conformational changes and electronic structure of metalloporphyrins on passivated iron surfaces, highlighting substrate-induced macrocycle distortions and their effects.

Findings

NiTPP exhibits severe saddle-shape deformation on the surface.

Distortions increase the HOMO-HOMO-1 energy gap.

Multilayer NiTPP layers have flat macrocycles, indicating substrate influence.

Abstract

On-surface metal porphyrins can undergo electronic and conformational changes that play a crucial role in determining the chemical reactivity of the molecular layer. Therefore, understanding those properties is pivotal for the design and implementation of organic-based devices. Here, by means of photoemission orbital tomography supported by density functional theory calculations, we investigate the electronic and geometrical structure of two metallated tetraphenyl porphyrins (MTPPs), namely ZnTPP and NiTPP, adsorbed on the oxygen-passivated Fe(100)-p(1x1)O surface. Both molecules weakly interact with the surface as no charge transfer is observed. In the case of ZnTPP our data correspond to those of moderately distorted molecules, while NiTPP exhibits a severe saddle-shape deformation. From additional experiments on NiTPP multilayer films, we conclude that this distortion is a consequence of the interaction with the substrate, as the NiTPP macrocycle of the second layer turns out to be flat. We further find that distortions in the MTPP macrocycle are accompanied by an increasing energy gap between the highest occupied molecular orbitals (HOMO and HOMO-1). Our results demonstrate that photoemission orbital tomography can simultaneously probe the energy level alignment, the azimuthal orientation, and the adsorption geometry of complex aromatic molecules even in the multilayer regime.