N,N’‐dicyanoquinonediimines as a molecular constituent of organic conductors: Vibrational behavior and electron–molecular vibration coupling

The results of a vibrational analysis of neutral N,N’‐dicyanoquinonediimine (DCNQI) and of 2,5‐X,Y‐DCNQI (X■Y■Cl; X■Cl, Y■CH3; X■Y■CH3 or CD3) are presented. A sound assignment of the in‐plane fundamental vibrational modes is discussed also in terms of a normal coordinate analysis (NCA) based on a modified valence force field. For light and deuterated 2,5‐dimethyl‐dicyanoquinonediimine (DMDCNQI) the vibrational analysis is extended to the corresponding radical anion. The comparison of the vibrational assignment of neutral and fully ionized species, together with the results of the NCA, leads to the identification of a strong Duschinsky effect, as well as of the frequency shifts relative to the ionization process. The buν47 fundamental mode (quinoid C■N stretch) is recognized as diagnostic of the degree of charge transfer (CT) for DMDCNQI component units of CT systems. The relative values of the linear electron–molecular vibration (e–mv) coupling constants for the neutral and ionized DMDCNQI molecular stru...