M. Masino et al., A new type of neutral-ionic interface in mixed stack organic charge transfer crystals: Temperature induced ionicity change in ClMePD-DMeDCNQI, PHYS CHEM P, 3(10), 2001, pp. 1904-1910
Raman and polarized infrared spectra of the mixed stack charge transfer cry
stals 2-chloro-5-methyl-p-phenylenediamine-2,5-dimethyl-dicyanoquinonediimi
ne (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 cross
ing of the neutral-ionic borderline appears to be quite different from that
of the few known temperature induced neutral-ionic phase transitions. Firs
t, the ionicity change is continuous. Furthermore, the onset of stack dimer
ization 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.