Generating antiaromaticity in polycyclic conjugated hydrocarbons by thermally selective skeletal rearrangements at interfaces

Antiaromatic polycyclic conjugated hydrocarbons (PCHs) are attractive research targets because of their interesting structural, electronic and magnetic properties. Unlike aromatic compounds, the synthesis of antiaromatic PCHs is challenging because of their high reactivity and lack of stability, which stems from the small energy gap between their highest occupied and lowest unoccupied molecular orbitals. Here we describe a strategy for the introduction of antiaromatic units in PCHs via thermally selective intra- and intermolecular ring-rearrangement reactions of dibromomethylene-functionalized molecular precursors upon sublimation on a hot Au(111) metal surface, not available in solution chemistry. The synthetic value of these reactions is proven by the integration of pentalene segments into acene-based precursors, which undergo intramolecular ring rearrangement, and the formation of pi-conjugated ladder polymers, linked through cyclobutadiene connections, due to ring-rearrangement and homocoupling reactions of indenofluorene-based precursors. The reaction products are investigated by scanning tunnelling microscopy and non-contact atomic force microscopy, and mechanistic insights are unveiled by computational studies. The synthesis of polycyclic conjugated hydrocarbons with antiaromatic moieties is realized by the thermal rearrangement of dibromomethylene-functionalized molecular precursors on a hot Au(111) metal surface.