Self-assembly of 8-; 5- and 2-hydroxylquinolines on Au(111) single crystal in perchloric acid

Abstract

Self-assembled monolayers of 8-; 5-; and 2-hydroxyquinolines (8-; 5- & 2-hqs) are obtained for the first time at the electrified Au(111)|100mM HClO4 interface via electrochemical scanning tunneling microscopy (EC-STM). We observed that 8- and 2-hqs form stripe patterns, both on the negatively charged reconstructed Au(111)-(p×√3) as well as on unreconstructed Au(111)-(1×1) surfaces. The adlayers are composed of dimer rows stabilized by both cyclic O-H···N hydrogen bonds and van der Waal interaction between the neighboring molecules. The O-H···N bond is made between the 8 and 2 position -OH acting as hydrogen bond donor and N in the pyridine ring as the hydrogen bond acceptor. When the 8 and 2 position of hydroxyl group in the pyridine ring is replaced to the 5 position in the benzene ring, we observed a monolayer with grid-like structure (trimeric pattern) that are qualitatively different from those of 8- and 2-hqs. Dimer formation is completely broken in the case of 5-hq, in which C-H···O and C-H···N hydrogen bonds play a key role in company with coadsorption of solvent molecules or electrolyte ions to support the stability of the adlayer. However, the H-bond networks of these adlayers are destabilized at the positive charge densities of Au(111). While 2-hq molecules form disordered chemisorbed adlayer, 8- and 5-hqs are subjected to electrochemical oxidation.