Crystallographic investigation of temperature-induced phase transition of the tetrathiafulvalene-p-bromanil, TTF-BA charge transfer complex

The phase transition of 1:1 mixed-stack organic charge-transfer crystal of tetrathiafulvalene-$p$-bromanil (TTF-BA) has been investigated by high resolution x-ray diffraction. The temperature-induced phase transition to a nonmagnetic state at 53 K was characterized by a dimeric distortion of the donor-acceptor stack similar to that observed in charge transfer complexes displaying a neutral-to-ionic transition. But in this case the dimerization process does not change the charge transfer between donor and acceptor molecules. Molecular packing in mixed stacks along two orthogonal crystallographic axes and the role of inter- and intrastack interactions are discussed.