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

Abstract

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 structure, evaluated by a spectroscopic version of the complete neglect of differential overlap (CNDO/S) calculation associated with the eigenvectors coming from the NCA, are also reported. The infrared vibronic features of the Ba(DMDCNQI)2 salt are sorted out by exploiting their intensity evolution with the temperature and are interpreted by applying Rice’s model for a dimerized linear chain together with the calculated e–mv coupling constants. Furthermore, the visible absorption spectrum of the DMDCNQI radical anion is analyzed and its vibronic structure is attributed to a coupling with the three vibrational modes agν′5, 1585; ν8, 1290; and ν′18, 395 cm−1. The dependence of the vibrational frequencies on ionicity and the vibronic features of the optical spectra are discussed with a view to their application in the investigation of the physical properties and the temperature and/or pressure-dependent phase transitions of conductors based on the DCNQI structure, particularly in the case of the open questions raised by the physical properties of the highly conducting Cu (DMDCNQI)2 salt.