Structural Modulation of Electrically Conducting TCNQ Salts Using Na⁺(crown ether) Supramolecular Cations

The single crystals of Na+([12]crown-4)2(TCNQ)2 (1), Na+([15]crown-5)(TCNQ)2 (2), Na+([18]crown-6)(TCNQ)2.5 (3), and Na+([24]crown-8)(TCNQ)2 (4) (TCNQ = 7,7,8,8-tetracyano-p-quinodimethane) were prepared and defined by single-crystal X-ray structural analyses. Of these, salts 1 and 2 showed structural phase transitions at 320 and 185 K, respectively, upon heating. According to differential scanning calorimetry, temperature (T)-dependent crystal structures, and dielectric measurements, the motional freedom and orientation change of the spherical sandwich-type Na+([12]crown-4)2 supramolecular cation activated the phase transition. Furthermore, a symmetry change in salt 2 from the low-T Pc to the high-T P21/c was observed and a symmetry change in the Na+···NC-coordination environment is the origin of the structural phase transition at 185.4 K. Conversely, no distinct structural phase transitions were observed for salts 3 and 4. In all of the crystals, the πdimer of the partially reduced TCNQ formed a segregated Ï€-stacking structure that interacted further by interdimer interactions to form electrically conducting TCNQ layers. Nonuniform and one-dimensional (1D) Ï€-stacking columns of TCNQ were observed in salts 1, 3, and 4, while two-dimensional (2D) spanning-overlap interdimer interactions were confirmed in salt 2. The electrical resistivity and activation energy of 2D salt 2 were 10-10000 and 2 times lower, respectively, than those of the 1D electronic systems of salts 1, 3, and 4.