The Heterocyclic Diradical Benzo-1,2:4,5-bis(1,3,2-dithiazolyl). Electronic, Molecular and Solid State Structure

The preparation and purification of the heterocyclic diradical benzo-1,2:4,5-bis(1,3,2-dithiazolyl) (BBDTA) is described. Crystals of BBDTA, grown by fractional sublimation at 120−180 °C/10 -3 Torr, are monoclinic, space group P 2 1 / c, with a = 4.144(2), b = 9.0344(13), and c = 10.7424(16) Å, β = 91.11(3)°, and Z = 2. The crystal structure consists of discrete, unassociated molecules of BBDTA. The molecules form slipped stacks along the x direction, with the mean molecular plane making an angle of 32.4° to the x axis. The interplanar separation between consecutive molecules along the stacking direction is 3.49 Å. The ESR spectrum of BBDTA is solvent dependent, displaying signals attributable to partially associated materials ( not exchange coupled) and to free diradical (exchange coupled). Ab initio molecular orbital calculations suggest a small ( ca . 0.5 kcal/mol) separation between the triplet and singlet diradical states. Cyclic voltammetry on BBDTA and the related benzo-1,3,2-dithiazolyl BDTA, coupled with gas phase photoelectron studies on BDTA, establish that both compounds are strong electron donors. Magnetic susceptibility measurements show that BBDTA is essentially diamagnetic up to room temperature; variable temperature single-crystal conductivity measurements provide a band gap of 0.22 eV. The transport properties are discussed in the light of extended Hückel band structure calculations, which suggest a highly three-dimensional electronic structure for bulk BBDTA in the solid state.