Tuning the adsorption orientation of heteroaromatic molecules on passivated metal surfaces by porphyrin layer intercalation

This study investigates the fundamental mechanisms governing the controlled orientation and stacking of heteroaromatic molecules at organic-inorganic interfaces under ultra-high vacuum conditions. We focus on the adsorption behavior of N,N’-di(4-pyridyl)−1,8:4,5-naphthalenetetracarboxydiimide (DPNDI) molecules on a Fe(001)-p(1×1)O surface, both with and without an intermediate layer of ZnII-tetraphenylporphyrin (ZnTPP). By strategically introducing the ZnTPP layer, we demonstrate the ability to tune the molecular arrangement of DPNDI, switching between a lying down orientation on the bare Fe(001)-p(1×1)O surface and a standing-up orientation when coordinated to zinc(II) ions. Using a combination of near edge x-ray absorption fine structure (NEXAFS) spectroscopy, low-energy electron diffraction (LEED), and scanning tunneling microscopy (STM), we explore the competing van-der-Waals and axial coordination interactions, elucidating how these forces dictate the molecular adsorption configuration. By integrating surface-sensitive experiments with density functional theory calculations, we provide a quantitative description of the adsorption energetics and elucidate the role of molecular coverage in determining adsorption geometry and molecular orientation. Our experimental results are reinforced by first principle calculations, which shed light on the energetic landscape underlying these arrangements. The theoretical insights reveal how molecule-substrate and intermolecular interactions promote or inhibit the formation of vertically stacked heterostructures. The comprehensive understanding regarding intermediate molecular layers as agent to control molecular orientation may pave the way to the precise design of functional heterostructures in applications such as organic electronics and molecular sensing