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.

CATRIN RCPTM Palacký University Olomouc Šlechtitelů 27 77146 Olomouc Czech Republic

Institute of Physics of the Czech Academy of Sciences Cukrovarnická 10 16200 Prague Czech Republic

Organisch Chemisches Institut Universität Heidelberg Im Neuenheimer Feld 270 69120 Heidelberg Germany

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