Surface-Induced Valence Tautomerism in a Molecular Europium Adsorbate

Abstract

Abstract

Molecular lanthanide compounds are a computationally addressable guidepost whose insight is critical to deconvoluting the structure-property relationships that inform the design of novel materials. Of the many ways to study molecular lanthanide compounds, one underexplored approach is to examine their properties as adsorbates on surfaces. In this work, we present a combined experimental and computational study of the topography and electronic structure of sub-monolayer europium tris(2,2,6,6-tetramethyl-3,5-heptanedionato), Eu(THD)3, molecules adsorbed on Cu(111) and Au(111) using Scanning Tunneling Microscopy (STM), differential conductance (dI/dV) spectroscopy and mapping, and Density Functional Theory (DFT) calculations. STM topographic images of the adsorbed complex revealed adsorbates whose morphology resembles a 5-petal flower. DFT calculations showed the complex undergoes drastic structural rearrangement when adsorbed on the surfaces and corroborated the 5-petal flower on-surface structure. Calculations also demonstrated a metal-centered reduction of the europium complex from Eu(III) to Eu(II) when adsorbed on Cu(111), contrasting with retention of the trivalent state on Au(111). These differences in oxidation state are reflected in experimental dI/dV spectra, which contain features that are prescribed as having 4f character based on calculated partial density of states (PDOS) for the 4f orbitals. Finally, dI/dV mapping showed ligand states that are in good agreement with theory. This work marks the first example of surface-dependent valence tautomerism for an adsorbed lanthanide compound.