Magnetic Tuning of All-Organic Binary Alloys between Two Stable Radicals

Mixtures of 2-(4,5,6,7-tetrafluorobenzimidazol-2-yl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazole-3-oxide-1-oxyl (F4BImNN) and 2-(benzimidazol-2-yl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazole-3-oxide-1-oxyl (BImNN) crystallize as solid
solutions (alloys) across a wide range of binary compositions. (F4BImNN)x(BImNN)(1–x) with x < 0.8 gives orthorhombic unit cells,
while x ≥ 0.9 gives monoclinic unit cells.
In all crystalline samples, the dominant intermolecular packing is
controlled by one-dimensional (1D) hydrogen-bonded chains that lead
to quasi-1D ferromagnetic behavior. Magnetic analysis over 0.4–300
K indicates ordering with strong 1D ferromagnetic exchange along the
chains (J/k = 12–22 K). Interchain
exchange is estimated to be 33- to 150-fold weaker, based on antiferromagnetic
ordered phase formation below Néel temperatures in the 0.4–1.2
K range for the various compositions. The ordering temperatures of
the orthorhombic samples increase linearly as (1 – x) increases from 0.25 to 1.00. The variation is attributed
to increased interchain distance corresponding to decreased interchain
exchange, when more F4BImNN is added into the orthorhombic lattice.
The monoclinic samples are not part of the same trend, due to the
different interchain arrangement associated with the phase change.