Structure and degree of charge transfer of two polymorphs of a 1:1 molecular complex: [2,2'-bis-1,3-dithiole][9-dicyanomethylene-4,5,7-trinitrofluorene-2-carboxylic acid methyl ester] (TTF-MeDC2TNF).

A new organic charge-transfer complex, C(18)H(7)N(5)O(8).C(6)H(4)S(4), has been prepared using TTF (tetrathiafulvalene) as a donor (D) and MeDC2TNF (9-dicyanomethylene-4,5,7-trinitro-fluorene-2-carboxylic acid methyl ester) as an acceptor (A). Two monoclinic polymorphs of the 1:1 salt were crystallized and characterized by X-ray diffraction. Form alpha crystallizes in space group P2(1)/n with Z' = 1, while form beta crystallizes in space group C2/c with Z' = 1/2 and the MeDC2TNF moiety disordered across a twofold axis. Both phases have mixed-stack crystal packings, which are characteristic of semiconducting one-dimensional organic complexes. However, the observed crystal packings are significantly different, with a single D...A interlayer separation of 3.452 A for the disordered beta polymorph and interlayer separations of 3.378 and 3.483 A in the case of the ordered alpha form. These variations are reflected in the degree of charge transfer, delta, which was estimated on the basis of the b(2u) stretching-mode frequency shift observed in the IR spectra for the cyano groups of the MeDC2TNF radical anion. The fact that the charge transfer is more efficient for alpha-(TTF-MeDC2TNF) than for beta-(TTF-MeDC2TNF) (delta = 0.35 and 0.31, respectively) is consistent with the structural features observed for each crystalline form.