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)

Abstract

A new organic charge-transfer complex, C18H7N5O8·C6H4S4, 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 α crystallizes in space group P21/n with Z′ = 1, while form β crystallizes in space group C2/c with Z′ = 1\over 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 Å for the disordered β polymorph and interlayer separations of 3.378 and 3.483 Å in the case of the ordered α form. These variations are reflected in the degree of charge transfer, δ, 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 α-(TTF-MeDC2TNF) than for β-(TTF-MeDC2TNF) (δ = 0.35 and 0.31, respectively) is consistent with the structural features observed for each crystalline form.