Structural Factors Controlling
the Spin–Spin
Exchange Coupling: EPR Spectroscopic Studies of Highly Asymmetric
Trityl–Nitroxide Biradicals
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Abstract
Highly asymmetric exchange-coupled biradicals, e.g.,
the trityl–nitroxides
(TNs), possess particular magnetic properties that have opened new
possibilities for their application in biophysical, physicochemical,
and biological studies. In the present work, we investigated the effect
of the linker length on the spin–spin coupling interaction
(<i>J</i>) in TN biradicals using the newly synthesized
biradicals CT02-GT, CT02-AT, CT02-VT, and CT02-PPT as well as the
previously reported biradicals TNN14 and TN1. The results show that
the magnitude of <i>J</i> can be easily tuned from ∼4
G (conformer 1 in CT02-PPT) to >1200 G (in TNN14) by varying the
linker
separating the two radical moieties and changing the temperature.
Computer simulations of EPR spectra were carried out to estimate <i>J</i> values of the TN biradicals directly. In addition to the
spin–spin coupling interaction of TN biradicals, their <i>g</i>, hyperfine-splitting, and zero-field-splitting interactions
were explored at low temperature (220 K). Our present study clearly
shows that varying the spin–spin interaction as a function
of linker distance and temperature provides an effective strategy
for the development of new TN biradicals that can find wide applications
in relevant fields