Articles you may be interested inTTF-TCNE a charge transfer π-molecular crystal with partial ionic ground state: Optical properties and electronmolecular vibrations interaction Electronic and structural characterization of a charge transfer crystal with strong electronic correlations through infrared and Raman spectroscopy: TMPD-TCNQF4 J. Chem. Phys. 89, 2704 (1988); 10.1063/1.455022 Electron-molecular vibration (e-mv) coupling in chargetransfer compounds and its consequences on the optical spectra: A theoretical framework J. Chem. Phys. 84, 5655 (1986); 10.1063/1.449926 Structure of an organic chargetransfer salt derived from dibenzotetrathiafulvalene and tetrafluorotetracyanoquinodimethane (DBTTF-TCNQF4). Observation of a hightemperature phase transition
The infrared and Raman spectra for the room temperature, quasineutral, and the low temperature, quasiionic, phases of the mixed stack charge transfer complex tetrathiafulvalene-chloranil (TTF-CA) are reported. The analysis of the analogous data for a newly synthesized room temperature phase point to a dimerized segregated stack structure. All the vibrational data are interpreted and exploited through a clear identification of the differences, for the two types of stacks, in the spectroscopic effects due to the vibronic interaction, i.e., the coupling between electron and molecular vibration (e-mv). It is shown that for distorted mixed stack complexes both Raman and infrared spectra can be substantially influenced by the vibronic interaction, whereas the dimerized segregated stack complexes, as already known, display striking vibronic effects only in infrared. The theoretical model which explains the origin of these effects is briefly summarized and its extension to mixed stack structures successfully used to reproduce the infrared vibronic absorption spectrum of the TTF-CA low temperature distorted phase. By this procedure the experimental values of the e-mv coupling constants of TTF and CA units are extracted. A degree of ionicity p practically equal to one is determined for the segregated stack phase, whereas the fractional charge for the two mixed stack phases results to be: p = 0.2. ± 0.1 at 300 K and p = 0.6, ± 0.1 at 15K. Some aspects of the neutral to ionic phase transition are discussed in terms of the temperature dependence of the vibrational spectra.
An extensive vibrational assignment of TTF and TTF-d4 is achieved, improving the previously reported one through the use of polarized infrared spectra of single crystals of the monoclinic form. Infrared spectra of the monoclinic and triclinic forms are compared and the different crystal field effects discussed. Powder Raman and infrared spectra of (TTF)Br1.0 and (TTF-d4)Br1.0, Raman depolarization ratios and infrared spectra of (TTF)ClO4 and (TTF-d4)ClO4 solutions are reported. The assignment of the ag, b1u, b2u, and b3u fundamental modes of (TTF)+ and (TTF-d4)+ radicals allows the identification of most of the relevant frequency shifts following the ionization of the TTF structure. The possible use of the ionization shifts for the study of the electronic charge distribution in the conducting TTF systems is considered. The parallel investigation of the concentration effects on the visible and infrared absorption spectra of TTF+ in solution let us to identify anomalous infrared absorptions associated with the formation of (TTF+)2 dimer. They are attributable to an intensity borrowing from the charge transfer band due to the electron–molecular vibration (e–mv) interaction and their identification opens the way to an experimental evaluation of the e–mv coupling constants of TTF.
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.
The results of an extensive vibrational analysis of tetramethyltetrathiafulvalene (TMTTF) and tetramethyltetraselenafulvalene (TMTSF) and of their radical cations are presented. The polarized infrared absorption spectra of oriented crystalline samples of neutral TMTTF and TMTSF (4000–80 cm−1) are reported and compared with powder and solution spectra. The polarization data are used as a basis for the symmetry assignment of the infrared active fundamental modes. Powder and solution Raman spectra are presented and discussed considering the values of the depolarization ratios measured for some bands. Raman and infrared spectra of powder and solution samples of the (1:1) bromide and perchlorate salts of TMTTF and TMTSF are reported. Vibronic infrared absorptions originated by the coupling of the unpaired electrons to totally symmetric modes are identified and compared with those previously reported for unsubstituted tetrathiafulvalene (TTF) radical systems. The assignment of the normal modes of the radical cations is based on the measured Raman depolarization ratios and on the correlation with the neutral molecules and with neutral and ionized TTF. The results of a normal coordinate calculation using a modified valence force field for TMTTF, TMTSF, and their radical cations are presented and the calculated shape of the normal modes of the neutral molecules are reported. The frequency changes following ionization observed for the present case of TMTTF and TMTSF and previously reported for TTF are compared and discussed
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