Charge transfer chemistry of benzo[2,1-c:3,4-c′]bis(1,2,3-dithiazole) (BT).: Preparation and structural characterization of [BT][ClO4] and [BT]3[X]2 (X = ClO4- and FSO3-)

The heterocyclic pi-donor benzo[2,1-c:3,4-c']bis(1,2,3-dithiazole) (BT) can be prepared in 60% yield by the condensation of 1,4-diaminobenzene-2,3-dithiol with sulfur monochloride. BT is also formed, in low yield, along with its 5-chloro and 5,6-dichloro derivatives, in the reaction of p-phenylenediamine with sulfur monochloride. Cyclic voltammetry on BT reveals two reversible oxidation waves at 0.61 and 1.10 V (in CH3CN, ref SCE). The ESR signal (g = 2.0175) of the radical cation BT+ [in SO2(1)] exhibits a 1:2:3:2:1 hyperfine structure with a(N) = 0.161 mT. BT forms radical ion salts by electro-oxidation in the presence of inorganic counterions. In addition to a 1:1 salt [BT][ClO4], the structure of which consists of a simple ionic arrangement of BT+ radical cations and ClO4- anions, BT also forms 3:2 salts, i.e., [BT](3)[X](2), with X = ClO4- and FSO3-. The crystal structures of these 3:2 salts consist of triple-decker units of radical cation dimers, i.e., [BT2](2+), and neutral BT molecules. These units are packed into ribbonlike arrays interspersed (to either side) by counterions. The ClO4- and FSO3- salts differ in the way these ribbons are layered on top of one another. Both can be considered as forming slipped pi-stacks, but the degree of offset between consecutive layers is considerably larger in the ClO4- salt. The room-temperature conductivity of [BT][ClO4] is ca. 10(-5) S cm(-1), while that of the two 3:2 salts is ca. 10(-2) S cm(-1).