On-surface Chemistry Explored by STM and XPSOn surface chemical reactions are of profound importance for the modification of surfaces by adding specific covalent/ionic molecular assemblies. Fast, highly directive reactions with small functional units serving as reactants, so called “click” reactions, are well established in nature’s biochemistry and are particularly useful also to design specific ad-surface molecular layers in subsequent reactions. Current understanding, however, of chemistry in the solvent-free on-surface environment is just at its start: experience gathered and reaction mechanisms derived for “in-solution” chemistry are not necessarily transferrable to solution-free processes occurring at surfaces.
In order to control on-surface reactions, it is possible to modify either the reactants or the substrate itself. The former can be changed by, for example, replacing substituents, while the latter can be modified by several methods including chemical modifications as well as varying the number of active sites by changing roughness. The approach we chose in our study  involves modifying the reactive surface with an atomically thin layer of adsorbates and investigating its influence on the on-surface metalation reaction of a 2HTPP porphyrin.
Currently we are investigating the influence of other adsorbate-induced (e.g. N and Cl) surface superstructures on the availability of reactive metal atoms by studying the porphyrin metalation. We are able not only to promote a reaction, but also inhibit it completely.
In our work , a charge-transfer (CT) reaction between 7,7,8,8-tetracyano-p-quinodimethane (TCNQ) and NaCl, both sublimed onto a Au(111) surface in a sub-monolayer regime, has been studied. STM has been used to probe self-assembly of TCNQ/Au(111) and NaCl+TCNQ/Au(111) (Fig. 1a). In the former case, self assembly is mainly ascribed to H-bonding of the cyano groups . In the latter, arrangement of TCNQ is significantly influenced by the addition of NaCl.
XPS data (Fig. 2) indicate the reduction of TCNQ0 to TCNQ- due to an on-surface CT reaction with NaCl. Further photoelectron spectroscopy studies prove that the CT occurs between TCNQ and NaCl without involving the Au(111) substrate. As a result, Cl2 gas is released. Thanks to this reaction, extended 2D molecular structure could be obtained.
Another on-surface chemical reaction  was observed for a zinc porphyrin (ZnTPP, see Fig. 1b) on Au(111) substrate. When exposed to TCNQ molecule at room temperature, a covalent bond between the two molecules is formed and a new molecule (Fig. 1b) arises. The XPS (Fig. 3) data unequivocally prove the covalent bond formation and creation of the ZnTPP-TCNQ complex.
References Porphyrin metalation providing an example of a redox reaction facilitated by a surface reconstruction
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