Molecular Distortions and Chemical Bonding of a Largeπ-Conjugated Molecule on a Metal Surface

Abstract

Normal incidence x-ray standing wave experiments and density functional theory reveal that 3,4,9,10-perylene-tetracarboxylic-dianhydride chemisorbs on Ag(111) in a nonplanar but vertically distorted configuration. The carboxylic O atoms are 0.18 +/- 0.03 angstroms closer to the surface than the perylene core. The distortion is related to weak, local bonds between carboxylic O atoms and the Ag surface which are coupled--through charge transfer into the former lowest unoccupied molecular orbital--to the primary, extended chemisorption bond via the perylene skeleton.

Theoretical study of PTCDA adsorbed on the coinage metal surfaces, Ag(111), Au(111) and Cu(111)

“…Hence, the metal is approximated with a finite number of metallic layers and a vacuum gap is inserted between the periodic images to decouple them. We used three metallic layers to perform the structural optimization of the systems in-line with previous investigations [10], [13]- [15], [17]- [19]. In all cases, the size of the unit cell along the surface normal was chosen to be 26 Å, which guarantees a sufficiently large vacuum gap (between 16 and 25 Å) for all studied systems.…”

“…Studies employing the generalized gradient approximation (GGA) [10], [13]- [15] reported adsorption distances much larger than the ones observed in experiment together with unrealistically small adsorption energies. In [16], the authors found better agreement with experiment, however, in connection with a strong arching of the molecule.…”

“…The former can be directly connected with adsorptioninduced work-function modifications while the latter reveals changes in orbital alignment and occupation [41]. In this section, we will investigate these effects for the monolayer being placed at the adsorption distances determined from x-ray standing wave (XSW) experiments [10]- [12]. This is achieved by performing a structural optimization of monolayer and slab while applying several constraints.…”

“…In particular, recent photoemission-based experiments carried out for PTCDA adsorbed onto Ag(111), Au(111) and Cu(111) have shown characteristic trends for workfunction change, alignment of molecular levels and vertical binding distances. More specifically, upon monolayer deposition (i) the work function of the metallic surfaces is modified in such a way that initial differences are greatly reduced [9], (ii) the lowest unoccupied molecular orbital (LUMO) of PTCDA becomes occupied for the Ag and Cu surfaces, while it remains empty in the Au case [9] and (iii) the adsorption distance is smallest for Cu, intermediate for Ag and largest for Au [10]- [12]. A coherent explanation for these phenomena is challenging, and support from the theoretical side is required.…”

molgw 1: Many-body perturbation theory software for atoms, molecules, and clusters

“…In the past, the application of the GW approximation to finite systems was rather infrequent [10,11,12,13]. But there has been much recent increase in interest [14,15,16,17,18,19,20,21,22,23] in the application of the GW approximation to problems involving atoms, molecules, and clusters, in part driven by the quest to develop efficient techniques to address mixed systems such as molecular junctions [24,25,26,27,28].…”