Mixed-vs-Segregated Stack Polymorphism in the N,N,N',N'-Tetramethylbenzidine-TCNQF₄ Charge Transfer Complex

Organic charge transfer (CT) cocrystals are a wide class of 1-D materials based on stacks of planar π-electron donor (D) and acceptor (A) molecules overlapping their frontier orbitals due to the CT interaction. Most CT crystals have either a segregated or mixed stack motif: in segregated stacks, D and A molecules form separate columns, while in mixed stacks, D and A alternate along the same stack. Although CT crystals often display polymorphism, very few D-A pairs are known to crystallize with both stack motifs. Here, we present a new CT complex presenting both mixed and segregated stack phases, based on the strong donor N,N,N',N'-Tetramethylbenzidine (N-TMB) and the strong acceptor TCNQF₄. The combination of polarized IR and Raman spectroscopy with X-ray diffraction on single crystals found common structural and optical features between the two phases. The low temperature data also suggest that a stack distortion occurs in the segregated stack phase.