The fluorescent 9'-anthracenyl-functionalized dithiadiazolyl radical (3) exhibits four structurally determined crystalline phases, all of which are monomeric in the solid state. Polymorph 3 alpha (monoclinic P2(1)/c, Z' = 2) is isolated when the radical is condensed onto a cold substrate (enthalpically favored polymorph), whereas 3 beta (orthorhombic P2(1)2(1)2(1), Z' = 3) is collected on a warm substrate (entropically favored polymorph). The alpha and beta polymorphs exhibit chemically distinct structures with 3 alpha exhibiting face-to-face pi-pi interactions between anthracenyl groups, while 3 beta exhibits edge-to-face pi-pi interactions. 3 alpha undergoes an irreversible conversion to 3 beta on warming to 120 degrees C (393 K). The beta-phase undergoes a series of reversible solid-state transformations on cooling; below 300 K a phase transition occurs to form 3 gamma (monoclinic P2(1)/c, Z' = 1), and on further cooling below 165 K, a further transition is observed to 3 delta (monoclinic P2(1)/n, Z' = 2). Both 3 beta -> 3 gamma and 3 gamma -> 3 delta transitions are reversible (single -crystal X-ray diffraction), and the 3 gamma -> 3 delta process exhibits thermal hysteresis with a clear feature observed by heat capacity measurements. Heating 3 beta above 160 degrees C generates a fifth polymorph (3 epsilon) which is distinct from 3 alpha-3 delta based on powder X-ray diffraction data. The magnetic behavior of both 3 alpha and the 3 beta/3 gamma/3 delta system reflect an S = 1/2 paramagnet with weak antiferromagnetic coupling. The reversible 3 delta -> 3 gamma phase transition exhibits thermal hysteresis of 20 K. Below 50 K, the value of chi T-m for 3 delta 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 (lambda(em) similar to 440 nm) upon excitation at 255 nm with quantum yields of 4% (3) and 30% ([3] [GaCl4]) affording a switching ratio [Phi(F)(3(+))/Phi(F)(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.