On-surface synthesis platform for highly branched oligomers based on sequential C-C coupling and C-H activation of carbenes.

On-surface synthesis is an emerging field for fabricating low-dimensional nanostructures. While carbenes are versatile reactive intermediates in solution-phase organic synthesis, they have rarely been explored in on-surface synthesis. Here, we demonstrate the versatility of carbenes in synthesizing highly branched zero-dimensional oligomers with distinct structures on a metal surface by combining bond-resolved scanning tunneling microscopy imaging, manipulation, X-ray photoelectron spectroscopy, surface infrared spectroscopy, and ab initio theoretical modeling. We synthesize highly symmetric branched oligomers through the C-C coupling of two carbene molecules to form a core of oligomers, followed by C-H activation of the core with up to four additional carbene molecules to create branches. Branched oligomers of lower symmetry are formed through cyclodehydrogenation of the highly symmetric oligomers. Our on-surface synthetic strategy based on C-H activation of carbene building blocks provides a platform for the design and synthesis of highly branched zero-dimensional oligomers with distinct structures.