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

Abstract

Raman and polarized infrared spectra of the mixed stack charge transfer crystals 2-chloro-5-methyl-p-phenylenediamine–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, 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.