Covalently connected yet electronically decoupled fluorenyl diradical

Covalently linked diradicals provide an important platform for investigating interactions between radical centers; however, in most systems these interactions are dominated by exchange coupling through π-conjugation. Here we report the synthesis and characterization of a fluorenyl-based diradical composed of two covalently connected fluorenyl radical (FLY) units. ESR spectroscopy, electronic absorption measurements, and DFT calculations reveal that the two radical centers are electronically almost independent, exhibiting a very large diradical character (y0 = 97%) and nearly degenerate singlet and triplet states. The weak interaction between the unpaired electrons originates from the bonding topology at the 4,4'-positions of the FLY units, which disfavors spin pairing due to the loss of aromatic sextets. Despite the negligible π-conjugation between the two radical centers, frozen-solution ESR spectra clearly show magnetic dipolar interaction arising from their spatial proximity. Electrochemical measurements further indicate through-space Coulomb repulsion between the FLY units. These results demonstrate a good example of a covalently connected yet electronically decoupled diradical, providing insight into through-space interactions in π-radical systems.