Adsorption-induced symmetry reduction of metal-phthalocyanines studied by vibrational spectroscopy

TL;DR

This study uses vibrational spectroscopy to analyze how metal-phthalocyanines on silver surfaces undergo symmetry reduction due to adsorption, revealing charge transfer effects and changes in molecular symmetry.

Contribution

It demonstrates a vibrational spectroscopy approach to detect symmetry reduction and charge transfer in metal-phthalocyanines adsorbed on surfaces, providing insights into electronic structure modifications.

Findings

Molecules exhibit long-range order in monolayers.

Vibrational spectra indicate a flat adsorption geometry.

Symmetry reduction from D4h to C2v due to charge transfer.

Abstract

We investigate the vibrational properties of Pt- and Pd-phthalocyanine (PtPc and PdPc) molecules on Ag(111) with high resolution electron energy loss spectroscopy (HREELS). In the monolayer regime, both molecules exhibit long range order. The vibrational spectra prove a flat adsorption geometry. The red shift of vibrational modes and the presence of asymmetric vibrational peaks suggest a moderate interaction of the molecules with the substrate, accompanied by a static charge transfer from the metal to the molecules. The appearance of a particular vibrational mode, which (i) belongs to the $\mathrm{B_{1g}}$ representation of the original fourfold $\mathrm{D_{4h}}$ molecular symmetry group and which (ii) exhibits interfacial dynamical charge transfer (IDCT), proves that a preferential charge transfer from the Ag surface into one of the originally doubly degenerate lowest unoccupied molecular orbitals (LUMOs) of $\mathrm{E_g}$ symmetry takes place, i.e. the electronic degeneracy is lifted and the molecule-surface complex acquires the twofold symmetry group $\mathrm{C_{2v}}$. The vibration-based analysis of orbital degeneracies, as carried out here for PtPc/Ag(111) and PdPc/Ag(111), is not restricted to these cases. It is particularly useful whenever the presence of multiple molecular in-plane orientations at the interface makes the analysis of orbital degeneracies with angle-resolved photoemission spectroscopy difficult.