A new type of neutral-ionic interface in mixed-stack organic charge-tranfer crystals: Temperature induced ionicity change in ClMePD-DMeDCNQI

Abstract

Raman and polarized infrared spectra of the mixed stack charge transfer crystal 2-chloro-5methyl-p-phenylendiamine- -2,5-dimethyl-dicyanoquinonediimine (ClMePD-DMeDCNQI) are reported as a function of temperature. A detailed spectral interpretation allows us to gain new insight into the temperature induced neutral-ionic transition in this compound. In particular, the crossing of the neutral-ionic borderline appears to be quite different from that of the few known temperature induced neutral-ionic phase transitions. First of all, the ionicity change is continuous. Furthermore, the onset of stack dimerization precedes, rather than accompanies, the neutral-ionic crossing. The (second order) phase transition is then driven by the dimerization, but the extent of dimerization is in turn affected by the ionicity change.