Controlling the Binding of Ligands to Molecular Assemblies of Porphyrin Pedestals on Surfaces
Authors: Papageorgiou Anthoula Chrysa 
Published: September 2023

Metal-organic species and nanoarchitectures at well-defined interfaces bear great potential for key technological applications, spanning over the design of single-site heterogeneous catalysts, novel materials for light harvesting, the fabrication of molecular rotors andnanomachines and the advent of molecular spintronics. The high stability, conformational flexibility and ease of chemical functionalisation of porphyrins make them ideal building blocks of such structures. Here we focus on Ru porphyrins self-assembling on the planar, close-packed silver surface [1]. Our analysis encompasses an integration of microscopy, diffraction, spectroscopy and desorption in ultra-high vacuum combined with theoretical considerations to generate a high degree of precision in the understanding of interfacial processes at the atomic level. By using carbon monoxide (CO) as a probe molecule, crucial differences in the binding to the Ru porphyrin were discovered depending on its conformation: if the central cyclic part of the porphyrin has a saddle-shape, CO can bind; if, instead, the central cyclic part of the molecule is more planar or bowl-shaped, CO does not bind [2]. The surface chemistry of the saddle shape Ru porphyrin was further exploited to control the surface tethering of N-heterocyclic carbenes (NHCs), a current challenge of surface functionalization. The metalloporphyrin acts as a pedestal that orients and by its self-assembly defines the lateral arrangement of NHCs on a planar surface by providing single metal atom sites for discrete binding. We deposited NHCs on a self-assembled metalloporphyrin layer and found that they are suitable pillars for the formation of out-of-plane architectures [3]. The resulting modular arrangement of NHCs allows us to individually address the ligands by controlled manipulation with the tip of a scanning tunnelling microscope, creating patterned structures on the nanometer scale. Whilst performing such positioning, we also found that NHCs act as unprecedented molecular porters. In a dense single layer of metalloporphyrins, NHCs pull complete metalloporphyrins out of the supporting silver surface and transfer them to the second layer, on top of other metalloporphyrins (see graphic). This phenomenon is dynamic and reversible as evidenced by a variety of experimental techniques interrogating both the local and the global molecular assembly. Additional measurements of temperature programmed desorption of the NHCs and determination of the related surfaces’ work function in combination with density functional theory modelling showed the role of the electrostatic potential in this phenomenon [4]. Harnessing these events will serve the engineering of atomically precise NHC-containing complex interfaces.

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Figure 1