Spectroscopic STM studies of single pentacene molecules on Cu(110)−c(6×2)O

Pentacene adsorbed on the $\mathrm{Cu}(110)\ensuremath{-}\mathrm{c}(6\ifmmode\times\else\texttimes\fi{}2)\mathrm{O}$ surface has been investigated using scanning tunneling microscopy and spectroscopy and density functional theory calculations. The molecules lie almost flat on the surface and are anchored via the end carbon ring to a Cu atom of the topmost surface layer, leading to a break of the mirror symmetry of pentacene parallel to the short molecular axis. As a result the molecular electronic structure of the frontier orbitals exhibits a local and gentle modification around the anchoring site but is essentially unaffected in the remaining part of the molecule. The enhancement of the conductance peak related to the second highest occupied molecular orbital (HOMO-2) at the anchoring site, however, is suggestive of a resonance with a surface state or with the Cu $d$-band.