Adsorption and Reactivity of Pyridine Dicarboxylic Acid on Cu(111)

Abstract

A detailed understanding of the reaction of a range of molecules on surfaces will be key to developing targeted strategies for on-surface synthesis. Here, we studied the deprotonation and decarboxylation reactions of 3,5-pyridinedicarboxylic acid (PDC) on Cu(111) using synchrotron radiation photoelectron spectroscopy (SRPES), near-edge X-ray absorption fine structure (NEXAFS) spectroscopy, and density functional theory (DFT). PDC partially deprotonates upon deposition on Cu(111) at room temperature and adsorbs with the plane of its aromatic ring inclined at an average of ∼45° with respect to the surface. By heating to 100 °C, the deprotonation of the molecule increases. When the PDC is partially deprotonated the plane of its aromatic ring adopts a more upright orientation with respect to the surface. Additional heating to 160 °C causes complete deprotonation, upon which the molecule returns to a more planar molecular adsorption geometry. By examining both the N 1s core level and the NEXAFS results, we ascribe these changes in adsorption to a reaction-induced change in the predominant molecule–surface interaction, which is driven by the nitrogen lone pair prior to deprotonation, and by the carboxylate groups after deprotonation of the −COOH groups. These interaction channels and adsorption geometries are supported by our DFT calculations. Heating above 200 °C induces decarboxylation of the molecule; by observing the rate of reaction over a range of fixed temperatures, we extract an activation energy of 1.93 ± 0.17 eV for the decarboxylation reaction. Around this temperature we also begin to observe the ring opening of the molecule, suggesting that if PDC is to be used as a building block for on-surface synthesis of polymers, careful control of temperature is necessary for obtaining decarboxylation and covalent coupling of the molecule without destroying the aromatic core.