In Situ Infrared Study of 4,4‘-Bipyridine Adsorption on Thin Gold Films

The phase formation of 4,4‘-bipyridine (4,4‘-BP) and its coadsorption with interfacial water on quasi-Au(111) film electrodes (20 nm) from 0.05 M KClO 4 has been studied employing in-situ surface enhanced infrared reflection adsorption spectroscopy (SEIRAS). Organic molecules form, dependent on the electrode potential and in the absence of Faradaic reactions, three monolayers of distincly different orientation. The high coverage adlayer I is composed of perpendicularly oriented 4,4‘-BP molecules coordinated with one nitrogen atom to the underlying positively charged electrodes ( C 2 v symmetry). Changing the electrode potential toward negative values causes two first-order phase transitions giving rise to two low-coverage organic adlayers. These transitions are accompanied by an in-plane tilting of the N-coordinated molecule. The conclusions on the interfacial orientation of 4,4‘-BP are supported by a comparative analysis of in situ SEIRAS, surface enhanced Raman spectroscopy, and sum frequency generation spectra. SEIRAS spectra also indicate that 4,4‘-BP is coadsorbed with water molecules and modifies the interfacial hydrogen-bonded network of the later. The nature of these interactions and their consequences for the creation of functionalized adlayers on metal electrodes are compared with related N-heterocyclic molecules.