Disorder in organic charge-transfer single crystals: dipolar disorder in ClMePD-DMeDCNQI

The unusual electronic, vibrational, and structural properties of the title compound are associated with the polar donor D=2-chloro-5-methyl-p-phenylenediamine, which is twofold disordered in single crystals. Its 3 D dipole generates random site energies with standard deviation sigma=0.35 eV that significantly alter the standard description of charge-transfer (CT) salts with nonpolar donors and acceptors. The average structure at 298 and 150 K is centrosymmetric, space group P1, and consistent with increasing degree of CT (or ionicity rho) on cooling. Vibrational spectra indicate that rho increases from approximately 0.3 at 400 K to approximately 0.6 at 80 K, with coincident Raman and infrared (IR) molecular modes in contrast with the centrosymmetric structure. Dipolar disorder is modeled by adding random site energies to Peierls-Hubbard models of CT salts, and sigma=0.35 eV is shown to suppress the Peierls instability for typical bandwidth and lattice stiffness, in agreement with the structural data. Disorder also breaks inversion symmetry and rationalizes coincident Raman and IR modes. The combination of site energies xp and the dipole operator P for systems with periodic boundary conditions leads at molecule p to (partial differentialP/ partial differentialxp)2 for the IR intensity polarized along the DA stack. The ensemble average of (partial differentialP/ partial differentialxp)2 for sigma=0.35 eV as a function of the ground-state ionicity rho accounts for the intensity variations of totally symmetric molecular modes of D and A, either on cooling at ambient pressure or on squeezing at ambient temperature.