Vibrational spectroscopy of mixed stack organic semiconductors: Neutral and ionic phases of tetrathiafulvalene–chloranil (TTF–CA) charge transfer complex

Abstract

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.

Infrared Investigations of the Neutral-Ionic Phase Transition in TTF-CA and Its Dynamics

“…Furthermore, in the low temperature phase, the TTF and CA molecules dimerize along the a-axis: the initial distance of 3.70 Å decreases to 3.504 Å and 3.685 Å [2], as it is depicted in Figure 1c, respectively. Also, the ionicity ρ increases from 0.3 e to about 0.6 e [2, 4,5,8], which was consistently determined by infrared, Raman and X-ray studies. The strong dimerization and the charge transfer lead to the creation of strong electric dipoles between the TTF and CA molecules.…”

“…In Figure 18, the mid-infrared reflectivity and the optical conductivity of TTF-CA are plotted for temperatures above and below T NI . In this energy range, the intramolecular modes of the TTF and CA molecules can be found, which are the most interesting ones [4,47,49]. Along the a-direction, the symmetric a g as well as the infrared-active b 3u modes of CA and TTF are observed whereas the first one is infrared active only due to the emv-coupling.…”

“…First studies on the crystal structure and the bond lengths of the C=C double bond of TTF and CA as well as the C=O bonds in CA reveal that the molecules are almost neutral at room temperature caused by a small charge transfer between TTF and CA [3] that can be attributed to the large intermolecular distance of 3.70 Å [2]. Indeed, infrared measurements show that the charge transfer is ρ = 0.2 e [4,5] at ambient conditions. Therefore, this state is referred to as the neutral phase.…”

“…Upon cooling, the charge transfer increases slightly from 0.2 e to 0.3 e [2, 4,5,8]. At T NI = 82 K, a phase transition occurs, at which the space group is lowered from P2 1 /n to Pn as identified by structure-resolving techniques.…”

Phenomenology of the Neutral-Ionic Valence Instability in Mixed Stack Charge-Transfer Crystals

“…It is clearly seen that the temperature evolution of ρ is different in the two cases. In TTF-CA, the ionicity gradually increase from about 0.2 at room temperature to about 0.3 at 81 K. Below 81 K, ρ jumps to about 0.5, and gradually increases up to 0.6 at 15 K ( [10], not shown in the Figure). Also in DMeTTF-CA ρ increases gradually by lowering T, reaching ∼0.3 at 64 K. Then there is a small jump and in a restricted temperature interval the C=O stretching band splits (arrows in Figure 1), indicating the presence of two differently charged species.…”