Aromatic carboxylic acids are very widely used components of supramolecular self-assemblies, in particular of 3D metal-organic frameworks (MOFs) where they constitute versatile organic linkers. Similarly, they have been investigated on surfaces as building blocks for 2D supramolecular networks where the plane of the aromatic system is orientated parallel to the surface.In contrast, studies on the assembly of such molecules on surfaces in a non-flat adsorption geometry are scarce even though this is of interest for accessing the third dimension as in the layer-by-layer growth of thin film MOFs (SURMOFs) where carboxylic acid moieties provide convenient coordination points for metals.We present our microscopic and spectroscopic studies of upright aromatic carboxylic acid layers on Cu and Ag surfaces. Au(111)/mica substrates modified by under-potential deposited (UPD) layers of Cu and Ag were used as the basis to study the influence of substrate on the formation of self-assembled monolayers (SAMs) of aromatic carboxylic acid molecules. The SAMs were analysed by STM under ambient conditions and by synchrotron based X-ray photoelectron spectroscopy (XPS) and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy. On the Cu surface, STM revealed highly commensurate crystalline structures, with molecules forming very well defined rows aligned along the high symmetry axes of the underlying metal. In contrast, on Ag the molecules were seen to form incommensurate row structures with significant waves and kinks. XPS and NEXAFS reveal pronouncedly different binding and coordination geometries on Cu and Ag modified surfaces. The substrate dependent film structures arise from a change in the balance of molecule-substrate and intermolecular interactions due to the differing coordination strength between the carboxylic acid groups and the Cu and Ag surfaces.

Self-assembly of monolayers of aromatic carboxylic acids on metals at the liquid-solid interface: Comparison of Cu and Ag

247th National Meeting (2014)

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