S.M. Barlow

Complex organic molecules at metal surfaces: bonding, organisation and chirality

Abstract Surface science techniques have now reached a stage of maturity that has enabled their successful deployment in the study of complex adsorption systems. A particular example of this success has been the understanding that has been gained regarding the behaviour of multi-functional organic molecules at metal surfaces. These organic–metal systems show enormous diversity, starting from their local description which can vary in terms of chemical structure, orientation and bonding. Additionally, in many cases, these complex organic molecules self-organise into beautiful, ordered superstructures held together by networks of intermolecular bonds. Both these aspects enable a single organic molecule–metal system to exhibit a wide-ranging and flexible approach to its environment, leading to a variety of adsorption phases, according to the prevailing temperature and coverage conditions. In this review we have attempted to capture this complexity by constructing adsorption phase diagrams from the available literature for complex carboxylic acids, amino acids, anhydrides and ring systems, all deposited under controlled conditions onto defined metal surfaces. These provide an accessible, pictorial basis of the adsorption phases which are then discussed further in the text of the review. Finally, interest has recently focused on the property of chirality that can be bestowed at an achiral metal surface by the adsorption of these complex organic molecules. The creation of such architectures offers the opportunity for ultimate stereocontrol of reactions and responses at surfaces. We have, therefore, specifically examined the various ways in which chirality can be expressed at a surface and provide a framework for classifying chiral hierarchies that are manifested at surfaces, with particular attention being paid to the progression of chirality from a local to a global level.

Orientation and periodicity in the c(4 × 8) and p(2 × 1) structures of 3-thiophene carboxylic acid on Cu(110)

Abstract The chemisorption of 3-thiophene carboxylic acid on Cu(110) between 300 and 350 K has been investigated by high resolution electron energy loss spectroscopy (HREELS), scanning tunnelling microscopy (STM) and low energy electron diffraction (LEED). Ab initio molecular orbital calculations of the molecular ion aided in vibrational frequency assignments, interpretation of STM images and estimation of intra- and inter-molecular interactions influencing formation of the c(4 × 8) and p(2 × 1) structures. HREELS shows that at low coverage, the molecule lays flat with its π orbitals interacting with the surface. Increasing the coverage induces the molecules to reorient perpendicular to the surface and form a c(4 × 8) intermediate structure. Impact scattering in HREELS demonstrates that the molecules are preferentially aligned with the thiophene ring in the [110] azimuth. STM images suggest that the upright carboxylate species form rows of four adjacent molecules face-to-face along the [001] direction separated by four lattice constants in [110]. Subsequent rows are shifted by two lattice constants along [110], resulting in an overall c(4 × 8) periodicity and a coverage of 0.25 ML. With increasing coverage, the c(4 × 8) structure changes to a p(2 × 1) structure. A model with the carboxylates bound in short bridge sites two lattice constants apart along [110] with a local coverage of 0.5 ML is proposed. Steric repulsion in the p(2 × 1) structure results in rotation of the thiophene ring by an estimated 30° away from the [110] direction, consistent with impact scattering HREELS measurements. Calculated dipole-dipole repulsion between the carboxylate groups is large compared to any dipole-dipole attraction which could result from anti-parallel alignment of the static dipole moments of the thiophene rings.