Polymorphism in a π-Stacked 1,3,2-Dithiazolyl Radical: Pyridyl-1,3,2-Dithiazolyl

Pyridyl-1,3,2-dithiazolyl PyDTA (1) has been synthesized and its electronic structure has been probed by cyclic voltammetry, electron paramagnetic resonance spectroscopy, and density functional theory calculations. The results demonstrate that the spin distribution and redox properties of the radical are intermediate between those observed for the isoelectronic benzo- and pyrazino-1,3,2-dithiazolyl radicals. Crystals of 1 can be grown by vacuum sublimation as two polymorphs. The kinetically favored form 1α grows as purple/black blocks or rods and crystallizes in the noncentric space group P21, while the thermodynamically favored 1β phase forms green/black shards and crystallizes in the centric space group P21/n. The structures of both 1α and 1β comprise layers of head-to-head π*−π* dimers in the ab plane. The two structures differ in the orientation of the π-dimers along the stacks. In 1α dimers are aligned head-to-head, while in 1β the orientation of the dimers alternates in a head-over-tail fashion, so as to produce an approximate doubling of the crystallographic c-axis in relation to 1α. Variable temperature magnetic susceptibility measurements reveal that both phases are essentially diamagnetic at low temperatures. Above 180 K (for 1β) and above 250 K (for 1α) the free Curie spin count for both polymorphs begins to rise, indicating a weakening of the π*−π* dimers in the solid state.