Real-space investigations of on-surface intermolecular radical transfer reactions assisted by persistent radicals.

Synthesizing radicals that have both long lifetimes and high chemical reactivity remains a long-term challenge. Here, persistent phenyl radicals are successfully synthesized on Ag(111) by protecting the carbon radical site by designing the precursor molecule with suitable steric hindrance. As carbon-carbon coupling is prohibited, such radicals remain intact for longer than 6 hours at room temperature on Ag(111). Taking advantage of the long lifetimes, the as-synthesized radicals are directly characterized in the real space at the single-chemical-bond scale by means of bond-resolving scanning tunneling microscopy imaging. Accompanied by the excellent stability, the radicals exhibit high chemical reactivities and facilitate the intermolecular radical transfer reactions at extreme low temperature. The preparation of persistent radicals not only favors the characterization of a surface-stabilized radical in the real space but also aids in illuminating the detailed reaction pathways of subsequent radical-assisted reactions directly.