Ionicity and electron molecular vibration interaction in mixed stack CT systems: M2P–TCNQ and M2P–TCNQF4

Abstract

The powder Raman spectra and the polarized infrared absorption spectra of a polycrystalline oriented film of (1:1) CT complex of 5,10-dihydro-5,10-dimethylphenazine (M2P) with 7,7,8,8-tetracyanoquinodimethane (TCNQ) are reported together with the polarized reflectance infrared spectra of a mosaic of single crystals [(010) crystal face] and the conductivity spectra obtained from a Kramers–Krönig analysis of the reflectance data. The powder Raman and infrared spectra of M2P–TCNQF4 and their temperature dependence (300–15 K) are also presented. The M2P–TCNQ structure is known to be made up of donor–acceptor dimerized mixed stacks. The spectral features related to electron–molecular vibration (e–mv) interaction are successfully interpreted in terms of the dimer model, whose applicability is further validated after its first use in the case of the low temperature phase of tetrathiafulvalene–chloranil complex. The success is testified by the good fit obtained between experimental and calculated vibronic features of the near and midinfrared spectra. The extracted values of the e–mv coupling constants of TCNQ moiety compare well with those obtained from other TCNQ complexes, whereas those of M2P are a first informative evaluation. The value of the degree of ionicity of M2P–TCNQ ( ρ=0.5±0.1) is obtained by completing the assignment of the spectra in terms of fundamental vibrational modes normally active under the molecular symmetry of the component molecules and by using carefully chosen diagnostic infrared frequencies. Results show that M2P–TCNQ is definitely one of the still rare cases of mixed stack CT complexes with intermediate ionicity. By using analogous criteria, the interpretation of room temperature and low temperature (15 K) spectra of M2P–TCNQF4 leads to the conclusion that for this complex ρ∼1.0. Its temperature dependent phase transition (T=120 K) between the room temperature regular mixed stack and the low temperature dimerized one is well characterized. The vibronic features appearing in the low-temperature infrared spectrum agree with predictions by the dimer model for a distorted stack. Consequently, it is likely that the phase transition is attributable to a dimerization of the stack due to a Spin–Peierls instability, in agreement with the previous indication given by magnetic susceptibility measurements.