Stacking efficiency of diselenadiazolyl π-dimers.: Consequences for electronic structure and transport properties

The preparation and crystal structure of 5-eyanofuran-2-[1,2,3,5-diselenadiazolyl], [RCN2Se2] (R = 5-cyanofuran), is reported. Crystal data for C6H2ON3Se2: monoclinic, space group P2(1), a = 7.1121(7), b= 20,541(2), c =20.923-(2) Angstrom. beta = 99.785(1)degrees, Z = 16. The crystal structure consists of diselenadiazolyl pi -dimer stacks running parallel to the x direction; the asymmetric unit consists of four pi -dimer units. The dimers are aligned into snakelike ribbons along the y direction, with consecutive dimers linked by head-to-tail CN-Se contacts. Each pi -dimer stack is bordered by two out-of-register stacks, but most interstack Se-Se contacts lie outside the van der Waals separation. Along the pi -dimer stacks, the intradimer Se-Se distances range from 3.183(10) to 3.294(1) Angstrom, and the interdimer Se-Se distances range from 3.826(1) to 3.945(1) Angstrom. Like other pi -dimer stacked diselenadiazolyls, [C6H2ON3Se2](2) is diamagnetic over the temperature range 4-380K. Variable temperature single-crystal conductivity measurements reveal a room-temperature conductivity near 10(-5) S cm(-1) and provide a calculated band gap of 0.72 eV. The structural results and transport properties are interpreted in the light of Extended Huckel band structure calculations.