OrganicMagnetic Diradicals (Radical–Coupler–Radical):Standardization of Couplers for Strong Ferromagnetism.

The intramolecular magnetic couplingconstant (J) values of sets of diradicals linkedto bis-DTDA, OVER, and NN radicals(DTDA, OVER, and NN groups) through an aromatic coupler were studiedby unrestricted density functional theory calculations (UB3LYP/6-311++G(d,p)).Among 15 aromatic couplers, 9 compounds with an odd number of carbonatoms along its spin coupling path were found to interact ferromagneticallyupon coupling with bisradicals while the other 6 couplers with aneven number of carbon atoms along its spin coupling path give riseto antiferromagnetic coupling. The overall trends in the strengthof magnetic interactions of aromatic couplers were preserved for DTDA,OVER, and NN groups so that the trend can be utilized as an indexfor the magnetic strength of a given coupler. It was found that thedifferences in the nucleus-independent chemical shift (NICS), bondorder of connecting bonds, and Mulliken atomic spin density at connectedatoms between triplet and BS states are closely related to the intramolecularmagnetic behavior. 2,4- and 2,5-phosphole couplers exhibit the strongestintramolecular ferromagnetic and antiferromagnetic interactions among15 aromatic couplers when linked to diverse bisradicals. [ABSTRACT FROM AUTHOR]