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

ABSTRACT

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 3ß (orthorhombic P21 21 21, Z' = 3) is collected on a warm substrate (entropically favored polymorph). The α and ß polymorphs exhibit chemically distinct structures with 3α exhibiting face-to-face π-π interactions between anthracenyl groups, while 3ß exhibits edge-to-face π-π interactions. 3α undergoes an irreversible conversion to 3ß on warming to 120 °C (393 K). The ß-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 3ß â†’ 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 3ß 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 3ß/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 χm T 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.