Toward helical-shaped diradicaloids: cyclobutenyl o -quinodimethane-bridged indeno[1,2- b ]fluorenes

Open-shell diradicaloids have been intensively investigated recently due to their attractive and unique optical, electronic and magnetic properties. However, open-shell diradicaloids with non-planar conjugated frameworks are less studied, especially helical-shaped π systems. In this work, we synthesized tetraindeno-based singlet biradicals in a helical geometry, bridged by a curved cyclobutenyl o-quinodimethane molecule. Large diradical characters and extremely small singlet-triplet energy gaps, as well as unusual half-field transitions (forbidden ΔMs = 2) were revealed for these 3D diradicals.