Polymorphism in a pi-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 lot grows as purple/black blocks or rods and crystallizes in the noncentric space group P2(1), while the thermodynamically favored 1 beta phase forms green/black shards and crystallizes in the centric space group P2(1)/n. The structures of both 1 alpha and 1 beta comprise layers of head-to-head pi*-pi* dimers in the ab plane. The two structures differ in the orientation of the pi-dimers along the stacks. In 1 alpha dimers are aligned head-to-head, while in 1 beta the orientation of the dimers alternates in a head-wover-tail fashion, so as to produce an approximate doubling of the crystallographic c-axis in relation to 1 alpha. Variable temperature magnetic susceptibility measurements reveal that both phases are essentially diamagnetic at low temperatures. Above 180 K (for 1 beta) and above 250 K (for 1 alpha) the free Curie spin count for both polymorphs begins to rise, indicating a weakening of the.pi*-pi* dimers in the solid state.