TMPD–CA revisited: Ionicity, stack dimerization, and phase transition of a key mixed stack charge transfer crystal.

Raman and single crystal polarized reflectance spectra (from 400 to 10 000 cm-1) of the tetramethylparaphenylenediamine–chloranil (TMPD–CA) charge transfer complex are reported. A careful interpretation of the data leads one to estimate for TMPD–CA a degree of ionicity of 0.64 at room temperature, indicating a quasi-ionic ground state. Moreover, we show that the mixed stack formed by TMPD and CA is dimerized at all temperatures. The latter finding is substantiated by an a priori calculation of the infrared conductivity spectrum polarized along the stack axis. The calculation is based on a model taking into account the coupling of the molecular vibrations with the charge transfer excitations through the modulation of both on-site energies and charge transfer integrals; moreover the intrinsic intensity of the infrared active modes is introduced semiempirically. The spectral simulation finds a further application in interpreting the subtle spectral changes observed in correspondence with the TMPD–CA 250 K phase transition: it is suggested that the phase transition is a disorder–order, orientational one. An extensive comparison with the results previously obtained by other experimental techniques allows one to solve early contradictions and definitely clear up the nature of the ground state of this key charge transfer crystal. [ABSTRACT FROM AUTHOR]