Molecular Vibration Analysis of lonicity and Phase Transition in TMPD-TCNQ (1:1) Charge Transfer Salt

Abstract

The infrared spectra of the (1:1) charge transfer salt of N, N, N′, N ‘-tetramethyl-p-phenylendiamine (TMPD) with 7, 7, 8, 8-tetracyanoquinodimethane (TCNQ) and with fully deuterated TCNQ are reported. The assignment of the vibrational data is obtained on the basis of the comparison with that of neutral and ionized parent molecules; additional relevant information is extracted from the low temperature (∼15 K) infrared and Raman spectra. The vibrational analysis leads one to estimate for TMPD-TCNQ a degree of ionicity of 0.9₂, which corresponds to an almost fully ionic ground state. The interpretation of the vibrational data has been carried out in the light of an appropriate model for the electron-molecular vibration interaction in regular or dimerized mixed stack charge transfer complexes. The study of the temperature dependence of the infrared spectra of TMPD-TCNQ confirms that the already known phase transition at about 220 K is likely associated with a transition from a regular to a dimerized mixed stack.