Crystal Structure and Madelung Energy for the Charge-Transfer Complex Tetrathiafulvalene-2,5-Difluoro-7,7,8,8-Tetracyano-p-Quinodimethane, TTF-2,5-TCNQF2. Observations on a Dimerized Segregated Stack Motif.

Abstract

The twinned crystal structure of the charge-transfer complex TTF-2,5-TCNQF2 has been determined by single-crystal X-ray diffraction techniques. From intensity measurement on one twin component, a full structural model has been developed. The model encompasses segregated stacks of TTF donors and 2,5-TCNQF2 acceptors, with each chain exhibiting significant dimerization. The alternate interplanar spacings in the TTF stack are 3.34A and 3.60A, while those in the 2,5-TCNQF2 stack are 3.23A and 3.55A. Nearly eclipsed molecular overlap patterns are found within each of the dimerized pairs. The Madelung energy of TTF-2,5-TCNQF2 has been calculated at − 2.65 eV per donor/acceptor pair for full charge transfer, utilizing partial atomic charges obtained from CNDO/INDO molecular orbital calculations. Full charge transfer is considered appropriate for this complex based primarily on the observed component molecular geometries. The dimerized structural motif observed at room temperature for the TTF-2,5-TCNQF2 system may reflect a phase transition at a higher temperature, owing to either an electronic-or a spin-Peierls instability. Diffraction data to temperatures on the order of 390 K do not indicate a solid-state phase transformation before crystal decomposition.