Catalytic Action of a Cu(111) Surface on Tetraazaperopyrene Polymerization

The polymerization of tetraazaperopyrene molecules on a Cu(111) substrate, as observed in recent scanning tunneling microscopy experiments, has been investigated in detail by first-principles calculations. Tautomerization is the first step required for the formation of molecular dimers and polymers. The substrate has been found to catalyze this tautomerization. SECTION Surfaces, Interfaces, Catalysis T he ongoing miniaturization of electronic devices drives the search for alternatives to the present approaches of lithographic manufacturing. In this context, the spontaneous ordering and assembly of atoms and molecules on atomically well-defined surfaces, the so-called bottom-up approach, appears to be a very promising way to fabricate functional systems with nanometer dimensions. Organicmolecules are used as building blocks to construct supramolecular structures for potential applications. During the self-assembling process, the intermolecular interactions ranging from indirect, substrate-mediated interplay, direct Coulomb forces, weak dispersion interactions, and metal complexation to hydrogen or covalent bonds, set the initial stage for a large number of possibilities to form oneand twodimensional molecular networks of varying robustness. For applications that require a high thermal and chemical stability, the stabilization through covalent bonds is essential. While it can be expected that the formation of covalent bonds between surface adsorbed molecules is significantly different compared to the corresponding reaction in solution or in bulk films, only little is known in detail about the impact of the surface on the reaction between the adsorbates. In this article, we present first-principles calculations of the tautomerization reaction of 1,3,8,10-tetraazaperopyrene (TAPP) on a Cu(111) surface. This special system has previously been investigated experimentally byMatena et al. The authors found that the copper substrate is crucial for the formation of the various aggregates they observed, i.e., differently ordered close-packed as well as porous networks on one hand, and covalently bonded linear chains on the other hand. From a surface science point of view, especially the latter are of great interest, since their formation requires a multiple step chemical reaction that does not occur in gas phase. The Cu(111) surface exhibits only a weak interaction with the adsorbed molecules, but obviously is sufficient to change the reaction kinetics. Recent density functional theory (DFT) calculations focused on the energetic stability of the molecular network or the chain, respectively. However, the detailed mechanism for the tautomerization reaction has not been investigated yet. With the work presented here, we want to close this gap. Our DFTcalculations were performed using the Vienna Ab Initio SimulationPackage (VASP) and thePW91 functional of the generalized gradient approximation (GGA) tomodel the electron exchange and correlation. In order to account approximately for the influence of the dispersion interaction, a semiempirical scheme based on the London dispersion formula was used. The electron-ion interaction was described by the projector-augmented wave (PAW) method, which allows for an accurate treatment of the first-row elements as well as the Cu 3d electrons with a relatively moderate energy cutoff of 340 eV. The adstructures were modeled in periodically repeated supercells, containing two atomic Cu layers arranged in a 14 0 3 6 !