Antiaromaticity by on-surface synthesis

Low-dimensional antiaromatic carbon-based nanostructures have attracted tremendous interest lately due to their distinctive electronic, optical and magnetic properties. These properties make them attractive for a myriad of potential applications in the fields of advanced organic optoelectronics, electronics, spintronics, photovoltaics, and quantum materials. However, their synthesis remains elusive due to their intrinsic electronic instability and high reactivity. In this context, recent advances in on-surface synthesis under ultra-high vacuum conditions have enabled the controlled generation and rationalization at the atomic scale of these compounds. In this review, we first introduce the concept of antiaromaticity and its main progress using solution-based methodologies. Then, we summarize key developments in the formation and characterization of individual antiaromatic molecules, one-dimensional polymers and two-dimensional networks on surfaces under ultra-high vacuum conditions. We highlight how molecular precursors are designed and surface conditions tuned to thermally or electronically direct skeletal rearrangements, enabling the formation of antiaromatic moieties, compounds and polymers, including cyclobutadiene, pentalene or cyclooctatetraene subunits, cyclocarbons, and other 4n π-electron systems. Finally, we discuss the implications of these findings for future applications, offering a perspective on emerging challenges in the surface-assisted chemistry of antiaromatic systems.