Solvent Hydrogen Bonding Determines Adsorption Geometry of Trimesic Acid on Cu

The adsorption geometry of functional molecules largely determines the interfacial behaviour in various applications like catalysis, photovoltaics and molecular electronics. Understanding adsorbate orientation has remained a challenge due to lack of suitable in situ methods, thus hindering controlled device behaviour. Here, a combined tip-enhanced Raman spectroscopy, density functional theory and molecular dynamics simulation approach elucidates the prevailing controversy about the adsorption geometry of showcase trimesic acid, one of the most widely used functional organic linkers, on Cu. We find that subtle energy differences in adsorbate-substrate-solvent interactions on the scale of 0.25 eV are decisive for the molecular orientation at solid/liquid versus solid/gas interfaces, emphasising the strength of combined theoretical and experimental quantitative in situ nanoscale characterisation for the rational design of functional molecular interfaces.