Structures of tetrathiabenzoquinone derivatives and the order-disorder phase transition

The tetrathiabenzoquinone derivatives 2,3,7,8-tetrahydro-benzo[1,2-b:4,5-b']bis[1,4]dithiin-5,10-dione (1), C(10)H(8)O(2)S(4), and 3,4,9,10-tetrahydro-2H,8H-benzo[1,2-b:4,5-b']bis[1,4]dithiepin-6,12-dione (2), C(12)H(12)O(2)S(4), have attracted attention as new organic pigments. Compound (1) has also been known to exhibit a large third-order non-linear susceptibility. In order to study the electronic structure in the solid state, the crystal structure has been investigated. Compound (1) was found to possess two crystal phases. Phase I is a room-temperature phase in which the molecular conformation is disordered. The other phase II is an ordered phase found at 223 K. Compound (2) is found to be isomorphous with (1) (phase II). Both (1) and (2) form characteristic one-dimensional molecular columns along the stacking c axis and two-dimensional or quasi two-dimensional molecular sheets parallel to the ab plane. Several close van der Waals contacts were observed between the neighboring molecules. Compound (1) was also found to undergo a reversible order-disorder phase transition at ca 265-269 K by X-ray diffraction as well as differential scanning calorimetry.