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 P2 1 /c, with a ) 4.144(2), b ) 9.0344(13), and c ) 10.7424(16) Å, ) 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.

The preparation of the bis(1,2,3,5-dithiadiazolyl) radicals [S2N2C]−X−[CN2S2], where X is a heterocyclic bridging group (X = piperazine, thiophene, bithiophene) is described. Crystal structures of the piperazine- and bithiophene-bridged diradicals have been determined by single-crystal X-ray diffraction. The two compounds are isostructural, belonging to the monoclinic space group P21/n; for the piperazine derivative a = 5.9913(6) Å, b = 19.1958(16) Å, c = 9.1244(6) Å, β = 100.314(7)°, and Z = 4. For the bithiophene derivative a = 5.9907(17) Å, b = 24.272(3) Å, c = 9.1740(14) Å, β = 102.961(18)°, and Z = 4. The diradicals associate as cofacial dimers which adopt a herringbone packing pattern analogous to that observed previously for the 1,4-phenylene-bridged derivative. In all three structures the herringbone arrays are characterized by a series of close interdimer S---S contacts along the spine. Consistently, the results of extended Hückel band structure calculations indicate highly developed two-dimensional electronic structures.

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