Tackling the Cyclacene Challenge

Cyclacenes, i.e., cyclic versions of acenes consisting of linearly fused benzene rings, were first discussed in 1954 by Heilbronner, but - despite considerable synthetic efforts over decades - have remained an elusive class of compounds. Theoretical investigations suggest that cyclacenes are highly reactive due to pronounced polyradical character and significant ring strain. All prior attempts to synthesise cyclacenes failed, mainly because the final synthetic step of establishing a fully conjugated π-system was energetically prohibitive. Despite these challenges, cyclacenes remain intriguing synthetic targets because of their unique chemical, electronic, and structural properties as well as their potential applications in organic (opto-)electronics or spintronics. In this collaborative project, we will develop novel synthetic approaches, which synergistically combine efficient and high-yielding synthetic strategies of belt-type precursors that we will transform to cyclacenes by extrusion of suitable leaving groups under cryogenic matrix isolation conditions, on surfaces, in the solid state, and ultimately even in solution. Our synthetic approach is highly modular and allows to structurally vary the belt-type precursors, and thus the cyclacenes, by placing different substituents at the rims, including peri-annulation or isosteric substitution of CH units by heteroatoms. This research, combined with state-of-the-art computational investigations and characterization even on the single-molecule level, will provide unprecedented insight into the structure-property relationship of the fully conjugated zigzag topology present in cyclacenes and address fundamental questions of chemical reactivity and the interplay of aromaticity, strain and polyradical character. This knowledge will finally allow us to synthesise interlocked structures based on cyclacenes and to apply cyclacenes in controlled reactions on surfaces for creating further elusive materials such as polyacenes.