On-Surface Synthesis of Nonbenzenoid PAHs Using Intermolecular π-Radical C-C Coupling.

On-surface synthesis has emerged as a new research field, ideal for designing low-dimensional carbon-based nanomaterials. One of the central problems with this synthetic approach is the understanding of reaction mechanisms, which is a key point for advancing the design of novel, highly selective reactions. The concept of π-radical-mediated reactions has been rarely considered in the context of on-surface synthesis so far. Here, we demonstrate that a π-radical-mediated reaction can provide an efficient mechanism of regioselective carbon-carbon coupling. Namely, π-radical coupling enables the dimerization of two π-expanded acenaphthene units, which facilitates the formation of complex nonbenzenoid PAHs. Our work contributes to the understanding of reaction mechanisms at the fundamental level, thus bridging the gap between in-solution radical chemistry and on-surface synthesis. We demonstrate a highly selective reaction in which the crucial C-C coupling step proceeds without direct catalytic involvement of the gold surface. This mechanistic insight suggests that π-radical coupling is a promising strategy that could be potentially expanded to inert surfaces, providing suitable π-radical activation.