Speaker
Description
The controlled fabrication of ordered molecular architectures with tailored functional properties at solid surfaces represents a key challenge for nanotechnology and device miniaturization. Although coordination polymers are efficiently obtained via conventional solution-based chemistry, their transfer onto substrates with the chemical, spatial, and geometrical homogeneity required for device integration remains challenging. Here, we adopt an in situ, vacuum-based, layer-by-layer self-assembly approach in which the anchoring sites are provided by ordered monolayers of metal(II)-tetraphenylporphyrins (M-TPP, M = Cu, Zn, Co) grown on the rutile TiO₂(110) surface. The metal center embedded in the porphyrinic macrocycle offers a well-defined coordination environment, enabling selective axial binding of a second-layer ligand. The affinity of the different metal ions toward axial coordination is investigated by subsequent deposition of symmetric dipyridyl-naphthalenediimide (DPNDI). Linear dichroism in NEXAFS spectroscopy reveals that DPNDI adopts a standing-up configuration on Zn- and Co-TPP as a result of nitrogen–metal axial coordination, whereas it lies flat on the substrate in the case of Cu-TPP. Calculations for a model pyridine ligand indicate stronger binding to Zn and Co centers, assisted by a surface trans effect, while the weaker Cu–pyridine interaction is overcome by the strong DPNDI–TiO₂ interaction. The homeotropic alignment of the ditopic DPNDI ligand on Zn- and Co-TPP exposes coordination sites suitable for the growth of laterally coherent three-dimensional hetero-organic architectures.