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 degrees C/10(-3) Torr, are monoclinic, space group P2(1)/c, with a = 4.144(2), b = 9.0344(13), a nd c = 10.7424(16) Angstrom, beta = 91.11(3)degrees, and Z = 2. The cr ystal structure consists of discrete, unassociated molecules of BBDTA. The molecules form slipped stacks along the x direction, with the mea n molecular plane making an angle of 32.4 degrees to the x axis. The i nterplanar separation between consecutive molecules along the stacking direction is 3.49 Angstrom. The ESR spectrum of BBDTA is solvent depe ndent, displaying signals attributable to partially associated materia ls (not exchange coupled) and to free diradical (exchange coupled). Ab initio molecular orbital calculations suggest a small (ca. 0.5 kcal/m ol) separation between the triplet and singlet diradical states. Cycli c voltammetry on BBDTA and the related benzo-1,3,2-dithiazolyl BDTA, c oupled with gas phase photoelectron studies on BDTA, establish that bo th compounds are strong electron donors. Magnetic susceptibility measu rements show that BBDTA is essentially diamagnetic up to room temperat ure; variable temperature single-crystal conductivity measurements;pro vide a band gap of 0.22 eV. The transport properties are discussed in the light of extended Huckel band structure calculations, which sugges t a highly three-dimensional electronic structure for bulk BBDTA in th e solid state.