The power of surfaces: meta-polyaniline from para-aminophenol

On-surface chemistry is a recent research field where the catalytic properties of surfaces are used to synthesize new materials that are not accessible with other techniques. Thus, following this bottom-up strategy, surfaces are used as a platform for enabling new low dimensional molecular networks. In this work, para-aminophenol (p-AP) molecules are used as precursors (building block) for the surface reaction. Surprisingly, we have found that these molecules, when adsorbed on Pt(111) and upon a thermal stimulus, covalently react each other forming oligomers coupled in a metaconfiguration.

We have used a multi-technique approach that includes STM, nc-AFM, STS, XPS, and DFT calculations to study the thermal behavior of the p-AP/Pt(111) system. By STM and nc-AFM, we have observed that starting from individual molecules and by annealing the surface, lead to the formation of oligomer chains. To unveil their chemical nature, we have investigated the thermal behavior of the p-AP/Pt(111) system by means of high-resolution XPS: the changes in the C 1s, N 1s and O 1s XPS core levels spectra of p-AP molecules at different substrate temperatures indicate the initial dehydrogenation of the alcohol group followed of oxygen elimination for higher temperatures, the partial dehydrogenation of the amino group and the formation of C-Pt and azo bonds.

Combining all the experimental results we can conclude that the behavior of p-AP molecules on the Pt surface upon thermal annealing is composed of 3 steps: dehydrogenation of the alcohol groups, partial dehydrogenation of the amine groups, and intermolecular coupling between activated amine groups and carbon atoms at the meta position. This gives rise to an unforeseen result, since we have been able to synthesize meta-polyaniline oligomers starting from a para-functionalized precursor, p-AP. Thus, surprisingly the Pt(111) surface has induced an unexpected selective formal Michael addition between the precursor molecules.