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)

摘要

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-trinitrofluorene-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) 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 Angstrom for the disordered beta polymorph and interlayer separations of 3.378 and 3.483 Angstrom 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. [References: 26]