We have successfully determined the crystal structure of both the low temperature and solvated phases of 3,3′,5,5′-tetramethylbenzidine-tetracyanoquinodimethane (TMB-TCNQ). The triclinic solvate phase was erroneously believed for years to be a polymorph of TMB-TCNQ, while using X-rays diffraction, infrared spectroscopy, and differential scanning calorimetry, we have determined the presence of crystallization of acetonitrile molecules within the structure and observed the conversion to the room temperature monoclinic phase of TMB-TCNQ upon desolvation promoted both by heating and by grinding. The nonsolvated phase undergoes inversion symmetry breaking below 160 K, giving rise to stack dimerization. A combination of the X-ray crystal structure determination, Hirschfeld surface analysis, and density functional theory calculations suggest a rather complex scenario for the first order phase transition, which implies, besides stack dimerization, molecular inclination and small increase of the degree of charge transfer. The two donor-acceptor pairs within the unit cell are arranged in-phase, so that the low-temperature structure is potentially ferroelectric.
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