On-Surface Synthesis of Azobenzene-Linked Porphyrin Derivatives

On-surface synthesis has become an attractive strategy to obtain functionalized carbon nanostructures from small precursor molecules using a bottom-up approach. Although various on-surface reactions have been developed, it is still unclear how the chirality of self-assembled structures prior to reaction affects the coupling process. Here, we investigate homocoupling of nitro-phenyl groups in Pt-porphyrin derivatives on Au(111) surfaces using low-temperature scanning tunneling microscopy. Two different self-assembled structures composed either of linear oligomer of molecules of opposing chirality or of discrete trimers of molecules having the same chirality (i.e., homochiral) were respectively obtained by using differently substituted phenyl and 3,5-di-<i>tert</i>-butylphenyl groups porphyrin cores. A machine-learning-assisted protocol was used for large-scale statistical analysis revealing the distinct difference in reaction yields of azobenzene formation between two different self-assembled structures. Since the azobenzene (dimer) products are composed of two molecules with opposing chirality (i.e., they are heterochiral), the molecules in the homochiral assembly must first be disassembled, which is one of the reasons for the low reaction yield. This study highlights the significant role of porphyrin chirality in the azo coupling reaction process on surface.