Structural, Magnetic, and Optical Studies of the Polymorphic 9′-Anthracenyl Dithiadiazolyl Radical

The
fluorescent 9′-anthracenyl-functionalized dithiadiazolyl
radical (3) exhibits four structurally determined crystalline
phases, all of which are monomeric in the solid state. Polymorph 3α (monoclinic P21/c, Z′ = 2) is isolated when the
radical is condensed onto a cold substrate (enthalpically favored
polymorph), whereas 3β (orthorhombic P212121, Z′ = 3) is collected on a warm substrate
(entropically favored polymorph). The α and β polymorphs exhibit chemically distinct structures
with 3α exhibiting face-to-face π–π
interactions between anthracenyl groups, while 3β exhibits edge-to-face π–π interactions. 3α undergoes an irreversible conversion to 3β on warming to 120 °C (393 K). The β-phase undergoes a
series of reversible solid-state transformations on cooling; below
300 K a phase transition occurs to form 3γ (monoclinic P21/c, Z′
= 1), and on further cooling below 165 K, a further transition is
observed to 3δ (monoclinic P21/n, Z′ = 2). Both 3β → 3γ and 3γ → 3δ transitions are reversible (single-crystal
X-ray diffraction), and the 3γ → 3δ process exhibits thermal hysteresis with a clear feature observed
by heat capacity measurements. Heating 3β above
160 °C generates a fifth polymorph (3ε) which
is distinct from 3α–3δ based on powder X-ray diffraction data. The magnetic behavior of
both 3α and the 3β/3γ/3δ system reflect an S = 1/2
paramagnet with weak antiferromagnetic coupling. The reversible 3δ ↔ 3γ phase transition exhibits
thermal hysteresis of 20 K. Below 50 K, the value of χmT for 3δ approaches 0 emu·K·mol–1 consistent with formation of a gapped state with
an S = 0 ground-state configuration. In solution,
both paramagnetic 3 and diamagnetic [3]­[GaCl4] exhibit similar absorption and emission profiles reflecting
similar absorption and emission mechanisms for paramagnetic and diamagnetic
forms. Both emit in the deep-blue region of the visible spectrum (λem ∼ 440 nm) upon excitation at 255 nm with quantum
yields of 4% (3) and 30% ([3]­[GaCl4]) affording a switching ratio [ΦF(3+)/ΦF(3)] of 7.5 in quantum
efficiency with oxidation state. Solid-state films of both 3 and [3]­[GaCl4] exhibit emission bands at
a longer wavelength (490 nm) attributed to excimer emission.