Cesare Pecile

Vibrational spectroscopy of molecular constituents of one‐dimensional organic conductors. Tetrathiofulvalene (TTF), TTF+, and (TTF+)2 dimer

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 ...

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

The infrared and Raman spectra for the room temperature, quasineutral, and the low temperature, quasi‐ionic, 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 use...

Charge-transfer organic crystals: molecular vibrations and spectroscopic effects of electron-molecular vibration coupling of the strong electron acceptor TCNQF4

A fundamental aspect in the study of the charge–transfer (CT) organic crystals with ionic or partially ionic ground states is the investigation of the spectroscopic effect of the electron–molecular vibration coupling. 7,7,8,8‐tetracyano‐2,3,5,6‐tetrafluoroquinodimethane (TCNQF4), an electron acceptor much stronger than TCNQ, is an outstanding component of many interesting CT systems. A thorough vibrational analysis of the title compound and of its monomeric radical anion is reported. The analysis is based on the Raman depolarization ratio measurements and on infrared data of solutions of both neutral and ionic species as well as on polarized infrared spectra of oriented crystals of the neutral molecule. The vibrational assignment, completed by a normal coordinate analysis (NCA), brings to the identification of the ionization frequency shifts and to the choice of the fundamentals (b1uν19 and b2uν33) whose frequencies are diagnostic of the degree of charge transfer for a TCNQF4 moiety partner of a CT system...

Vibrational behavior of molecular constituents of organic superconductors: TMTSF, its radical cation and the sulphur analogs TMTTF and TMTTF+

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 ca...

Structural properties of Langmuir–Blodgett films of charge transfer salts: Pristine and iodine doped conducting films of (N‐docosyl‐pyridinium)TCNQ

We have investigated the molecular constitution and ordering of Langmuir–Blodgett films prepared by deposition of N‐docosylpyridinium–TCNQ charge transfer salt onto CaF2 substrates and by their subsequent doping with iodine vapor to yield reasonably good (∼10−1 Ω−1 cm−1) conducting films. Optical and electron scanning microscopy of the precursor film show that it is made up of a mosaic of highly crystalline platelets lying flat on the substrate though with random orientation. Structural information at the molecular level is gained by measuring and analyzing the ESR spectra and their angular and temperature dependence as well as the UV‐visible and infrared spectra and their linear dichroism. A triplet fine structure is observed in the ESR spectra of the precursor films with zero field splitting parameters ‖D‖=7.48×10−3 cm−1 and ‖E‖=1.2×10−3 cm−1 and singlet–triplet activation energy J=0.20 eV, markedly different from those of the N‐docosylpyridinium–TCNQ powder salt. The UV‐visible and infrared spectra sho...

Vibrational analysis of spectra of quinonoid molecular ions. Part 3.—Vibrational spectra and assignment of 7,7,8,8-tetracyanoquinodimethane radical anion

The Raman and infrared spectra of TCNQ and [2H4]TCNQ radical anions in solution are presented for the first time. The successful assignment of most of the totally symmetric ag modes, obtained directly from the experimental Raman depolarization ratios, corroborates the assignment of the infrared active modes which is based on comparison with the neutral molecules.Radical–radical dimerization in water solution has no relevant influence on the intramolecular fundamentals. The Raman and infrared powder spectra of LiTCNQ and RbTCNQ and of the analogous fully deuterated salts are also reported, and the solid state–solution shifts in the band positions are found not to exceed 15 cm–1.Once the effect of environment on TCNQ– vibrational frequencies had been identified, we were able to evaluate the frequency shifts due to the addition of the extra electron. These shifts of up to about seventy wave numbers are reproduced by a normal coordinate analysis, based on a set of judicious adjustments of the neutral molecule empirical Valence Force Field.The vibrational spectra have been used successfully, as previously with Cs2(TCNQ)3, to demonstrate that in crystalline Cu(TCNQ)2 both neutral and negatively charged TCNQ units are present.

Polarized Infrared Spectra of TCNQ and TCNQ‐d4 Single Crystals

The infrared‐active vibrations of the TCNQ molecular structure have been investigated by using the spectra of the powder and those of the (001) and (010) crystal planes in polarized radiation for both TCNQ and its fully deuterated analog. These measurements give most of the significant indications which can be drawn from infrared spectra for the localization and symmetry class identification of infrared active modes. As a result, a well‐based assignment of infrared‐active fundamentals, as well as some information on crystal field effects and on the optical behavior of this anisotropic crystal in the (010) plane, is given.

A key to understanding ionic mixed stacked organic solids: Tetrathiafulvalene-bromanil (TTF-BA)

Abstract An extensive series of structural, optical, vibrational and magnetic susceptibility measurements are reported for the charge transfer complex tetrathiafulvalene-bromanil (TTF-BA). The complex is shown to be an ionic mixed stack solid at all temperatures, with a phase transition around 50 K to a nonmagnetic state. The origin of this nonmagnetic state in other ionic charge transfer crystals has been the matter of considerable debate. In TTF-BA, it is shown to be associated with a distortion of the stacks.

Accurate electron-molecular vibration coupling constants from powders optical spectra: TCNQ and TTF

Abstract A simple but accurate method is proposed for the experimental determination of electron — molecular vibration (e-mv) coupling constants from the optical spectra of powdered samples of dimerized, segregated stack organic quasi-one-dimensional radical salts. By re-examining various TCNQ-based compounds a transferable set of coupling constants is obtained for this key molecular structure. The application of our method to TTF Br salt leads to TTF e-mv coupling constants which nicely compare with those determined from TTF-Chloranil.

Raman and infrared frequency shifts proceeding from ionization of perhalo‐p‐benzoquinones to radical anions

The vibrational assignment of the fundamental modes of 2,3,5,6‐tetrachloro‐ and 2,3,5,6‐tetrabromo‐p‐benzoquinone (chloranil and bromanil, respectively) radical anions is reported and compared with that of the neutral molecules. For this purpose the single crystal Raman and infrared spectra of bromanil are also given and interpreted. The absence of the vibronic effects present in the infrared spectra of some free radical salts is demonstrated, and the vibrational spectra of the potassium salts of both radical ions are shown to be interpretable in terms of substantially unperturbed molecular structures even in the case of crystalline samples. The vibrational assignments and their normal coordinate analysis allow one to obtain most of the relevant information on the frequency shifts proceeding from the addition of an extra electron to the perhalo‐p‐benzoquinone structure.