Catenation of Two Singlet Diradicals: Synthesis of a Stable Tetraradical (Tetraradicaloid)

Counting the calories: The coupling of two PBPB diradicals through an antiferromagnetic linker affords a singlet tetraradical, whereas when they are coupled through a ferromagnetic linker, a bis(bicyclic) system is obtained (see picture). The energy difference between the planar form and the bicyclic isomer of substituted PBPB derivatives is only a matter of a few kcal mol−1, thus a few kcal mol−1 can make a tremendous difference.