Organic
Magnetic Diradicals (Radical–Coupler–Radical):
Standardization of Couplers for Strong Ferromagnetism
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Abstract
The intramolecular magnetic coupling
constant (<i>J</i>) values of sets of diradicals linked
to bis-DTDA, OVER, and NN radicals
(DTDA, OVER, and NN groups) through an aromatic coupler were studied
by unrestricted density functional theory calculations (UB3LYP/6-311++G(d,p)).
Among 15 aromatic couplers, 9 compounds with an odd number of carbon
atoms along its spin coupling path were found to interact ferromagnetically
upon coupling with bisradicals while the other 6 couplers with an
even number of carbon atoms along its spin coupling path give rise
to antiferromagnetic coupling. The overall trends in the strength
of magnetic interactions of aromatic couplers were preserved for DTDA,
OVER, and NN groups so that the trend can be utilized as an index
for the magnetic strength of a given coupler. It was found that the
differences in the nucleus-independent chemical shift (NICS), bond
order of connecting bonds, and Mulliken atomic spin density at connected
atoms between triplet and BS states are closely related to the intramolecular
magnetic behavior. 2,4- and 2,5-phosphole couplers exhibit the strongest
intramolecular ferromagnetic and antiferromagnetic interactions among
15 aromatic couplers when linked to diverse bisradicals