Saturation of magnetic exchange coupling with increasing cumulene coupler length in diarylmethyl diradicals

We have computationally designed nine diarylmethyl diradicals to understand the increasing trend of magnetic exchange coupling constant with the cumulene length. We have computed the magnetic exchange coupling constant with four different DFT functionals namely B3LYP, MN12SX, M06L and BP86 to check the consistency of the results. The molecular orbital analysis and their energy gap have also been studied. The bond length alternation increases with an increase in the length of the coupler. This suggests a clear shift in the molecule's electronic distribution with the increase in the cumulene length due to the interaction of the radical centers and the cumulene. All the functionals give consistent ferromagnetic interactions for the designed diradicals. After a certain length of the cumulene coupler the magnetic exchange coupling constant of the diradicals reached a saturation point. The MO analysis shows that the saturation of magnetic exchange comes from the reduced helical nature of LUMO.