  The operation of organic optoelectronic devices relies notably on the bulk
properties of compound molecular species, but even more so on the influence of
interfaces thereof. The identification and characterization of elemental
processes in these critical sections of a device thereby requires well-defined
interfaces with low defect density. In this context titanyl phthalocyanine
(TiOPc) arises as an excellent candidate that reveals the formation of a stable
bilayer structure with a characteristic "up-down" molecular arrangement that
optimizes the dipole-dipole interaction within the bilayer. In our experimental
study, long-range ordered TiOPc bilayers have been grown on Ag(111) surfaces
and analyzed using infrared absorption spectroscopy and scanning tunneling
microscopy. By monitoring the prominent Ti=O stretching mode in IRAS and
identifying local configurations in STM, a microscopic model for the growth of
TiOPc bilayers on Ag(111) is suggested. We demonstrate that defect structures
within these bilayers lead to characteristic vibrational signatures which react
sensitively to the local environment of the molecules. Thermal desorption
spectroscopy reveals a high thermal stability of the TiOPc bilayer up to 500 K,
which is attributed to hydrogen bonds between oxygen of the titanyl unit and
the hydrogen rim of phthalocyanines in the second layer, in addition to
contributions arising from the oppositely oriented axial dipole moments and the
ubiquitous van der Waals interactions.
