Orientation and periodicity in the c(4x8) and p(2x1) structures of 3-thiophene carboxylic acid on Cu(110)

The chemisorption of 3-thiophene carboxylic acid on Cu(110) between 300 and 350 K has hen 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 x 8) and p(2 x 1) structures. HREELS shows that at low coverage, the molecule lays flat with its rr orbitals interacting with the surface. Increasing the coverage induces the molecules to reorient perpendicular to the surface and form a c(4 x 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 x 8) periodicity and a coverage of 0.25 ML. With increasing coverage, the c(4 x 8) structure changes to a p(2 x 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 x 1) structure results in rotation of the thiophene ring by an estimated 30 degrees 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.