Acta Crystallographica Section B

Structural Science

Volume 59, Part 4 (August 2003)

research papers

Acta Cryst. (2003). B59, 505-511 [ doi:10.1107/S0108768103011893 ]

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. Salmerón-Valverde, S. Bernès and J. G. Robles-Martínez

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 [alpha] crystallizes in space group P2₁/n with Z' = 1, while form [beta] 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 DA interlayer separation of 3.452 Å for the disordered [beta] polymorph and interlayer separations of 3.378 and 3.483 Å in the case of the ordered [alpha] form. These variations are reflected in the degree of charge transfer, [bold 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) ( = 0.35 and 0.31, respectively) is consistent with the structural features observed for each crystalline form.

Keywords: polymorphism; charge-transfer complex; organic conductor; TTF.


	Structure factor file (CIF format) (225.1 kbytes) Contains datablock cdc2tnf

	Structure factor file (CIF format) (114.2 kbytes) Contains datablock cdc2tnfb

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