Bis(triisopropylsilylethynyl)pentacene/Au (111) Interface: Coupling, Molecular Orientation and Thermal Stability

The assembly and the orientation of functionalized pentacene at the interface with inorganics strongly influence both the electric contact and the charge transport in organic electronic devices. In bis(triisopropylsilylethnyl)pentacene (TIPS-Pc) the TIPS functional groups not only improve the solubility in common organic solvents, but also features good transport properties arising from the "brick-wall" face-to-face arrangement of the molecular planes, that increases the intermolecular interactions with respect to the edge-to-face "herringbone" stacking of pentacene. In order to understand and control the assembling of TIPS-Pc on the Au(111) surface, we have combined electronic spectroscopies and theoretical modeling to investigate the properties TIPS-Pc/Au(111) interface as a function of the molecular coverage, to compare the molecular state in the gas- and in the adsorbed-phase and to determine the thermal stability of TIPS-Pc in contact with gold. Our results show that in the free molecule only the acene atoms directly bonded to the ligands are affected by the functionalization. Adsorption on Au(111) leads to a weak coupling which causes only modest binding energy shifts in the TIPS-Pc and substrate core level spectra. In the first monolayer the acene plane form an angle of 33º ± 2º with the Au(111) surface at variance with the vertical geometry reported for thicker solution-processed or evaporated films, whereas the presence of configurational disorder was observed in the multilayer. The thermal annealing of the TIPS-Pc/Au(111) interface reveals the ligand desorption at ~ 470 K, which leaves the backbone of the decomposed molecule flat-lying on the metal surface as in the case of the unmodified pentacene. The weak interaction with the metal substrate causes the molecular dissociation to occur 60 K above the thermal decomposition taking place in thick drop-cast films.