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-tert-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.