Mixed Phenyl and Thiophene Oligomers for Bridging Nitronyl Nitroxides

The
synthesis of four nitronyl nitroxide (NN) biradicals is described
which are conjugatively linked through p-ter-phenyl
(PPP), ter-thiophene (TTT) and alternating phenylene
(P) and thiophene (T) units as PTP and TPT. We first utilized Suzuki
and Stille coupling reactions through protection and deprotection
protocols to synthesize these (NN) biradicals. Single crystals were
efficiently grown for radical precursors of 3, 5, 6, PPP-NNSi, PTP-NNSi, and final
biradicals of TTT-NN, TPT-NN, and PPP-NN, whose structures and molecular packing were examined
by X-ray diffraction studies. As a result, much smaller torsions between
the NN and thiophene units (∼10°) in TTT-NN and TPT-NN than for NN and phenyl units (∼29°)
in PPP-NN were observed due to smaller hindrance for
a five vs a six membered ring. All four biradicals TTT-NN, TPT-NN, PTP-NN, and PPP-NN were investigated by EPR and optical spectroscopy combined with
DFT calculations. The magnetic susceptibility was studied by SQUID
measurements for TTT-NN and TPT-NN. The
intramolecular exchange interactions for TPT-NN and TTT-NN were found in good agreement with the ones calculated
by broken symmetry DFT calculations.