On-Surface Mechanochemistry Synthesis of Non-Alternant Non-Benzenoid Aromatic Compounds Containing 4 to 8-Membered Rings

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Synthesis of polycyclic aromatic hydrocarbons containing various non-benzenoid rings remains a big challenge facing contemporary organic chemistry despite a considerable effort made over the last decades. Herein, we present a novel route, employing on-surface mechanochemistry, to synthesize non-alternant polycyclic aromatic hydrocarbon containing up to four distinct kinds of non-benzenoid rings. We show that the surface-induced mechanical constrains imposed on strained helical reactants play a decisive role leading to the formation of products, energetically unfavorable in solution, with a peculiar ring current stabilizing the aromatic character of the π-conjugated system. Determination of the chemical and electronic structures of the most frequent product reveals its closed-shell character and low band gap. The present study renders a new route for the synthesis of novel non-alternant polycyclic aromatic hydrocarbons or other hydrocarbons not available by traditional approaches of organic chemistry in solution.