R. Wojciechowski

A New Organic Conductor and a Novel Structural Phase Transition in the BEDT-TTF Trihalide Family**

decrease of transmission (absorption increase) appears. This decrease cannot be explained by the photodegradation effect and can be accounted for by the photoinduced reorientation of azo groups. The increase of the transmission from D (a minimum point in curve a) to E indicates that azo chromophores with their long axis oriented in the direction perpendicular to the pump polarization also become photodegraded. In curve b, a decrease of transmission after the initial fast jump is observed (see the inset of Fig. 4), which is attributed predominantly to the photoinduced orientation effects. After this decrease, photodegradation in the direction perpendicular to the pump polarization dominates and therefore the transmission increases. Compared to curve b, the fast decrease of the transmission from C to D in curve a is due to the fact that the concentration of the azo dye trans isomers with their orientation parallel to the p? pump polarization is higher due to the prior pk pump excitation. We have reported on the experimental investigation of PIB in thin films fabricated by electrostatic sequential adsorption of CRP films. The formation of this large birefringence is attributed to the polarization-selective photodegradation of the azo dyes within the matrix of the polyelectrolyte. The investigations on photoinduced absorption change, dichroism, Raman and infrared spectra support the conclusion. Photoinduced alignment of azobenzene chromophores results in only a small contribution to the birefringence. The molecular structural details of the polarization-dependent PDAC matrix controlled photodegradation of CR are not completely clear at the present time. Further investigation of the photochemical reaction is necessary in order to thoroughly understand the mechanism involved in the polarization-selective photodegradation process. Nevertheless, we have successfully recorded holographic gratings based on this unusual PIB effect. A diffraction efficiency of 9 % in the first order was obtained with just a 1.3 mm thick film. The recorded birefringent gratings were stable and did not exhibit any long-term decay. We also observed that heating does not remarkably change the diffraction of the recorded grating on the film up to ~290.0 C. Above this temperature, discoloration and loss of materials from the film surface is observed. With azo dye‐containing polymers where photoinduced pure orientation effects play a major role similar or even better refractive index modulations in special cases and stability while maintaining reversibility have been achieved. Given the stable (including thermal stability) PIB exhibited by the electrostatic self-assembly CRP films, the assembled dye‐molecular composite films are especially suited to irreversible applications, such as long-term optical data storage, light-weight diffractive optical elements, and low-cost phase retarders at red or near infrared wavelength.

New conducting molecular metal/polycarbonate bilayered composites: (ET)2IBr2/PC-, (BET)2IBr2/PC- and (BET)2I3/PC-films*

Abstract Preparation of thin layers of oriented microcrystals of molecular metals using chemical oxidation by either I 2 . IBr or ICl vaporous of the polycarbonate films containing 2 % of ET or BET is presented. Compositions and crystal structures of the conducting layers were analyzed by X-ray, ESR and Raman spectroscopies. The polycrystalline β-(BET) 2 I 3 layer reveals a metal-like R(T) dependence (d.c.) down to low temperature.

Transformation of (BEDT-TTF)2I3 networks in polymer films into superconducting βt phase as studied by resonant Raman spectroscopy

Abstract Reticulate doped polymer composites containing polyiodide of bis(ethylenedithio)tetrathiafulvalene (BEDT-TTF or ET) in polycarbonate (PC) films were investigated by resonant Raman spectroscopy (RRS). We found that RRS spectra of non-annealed composites are similar to the spectra of single crystals of the α-ET2I3 phase but show also an unexpected additional band at 108 cm−1 due to vibration of the I3Poststeggerd− molecule. The RRS spectra of annealed composites are nearly identical to the spectrum of single crystals of the βt-ET2I3 phase. The reproducibility of the RRS spectra taken at different spots on the surface of the composites is very good. Raman microscopy indicated that the surface of the composites is homogeneous.

Raman study of charge disproportionation in α-(BEDT-TTF)2I3

Raman studies of α-(BEDT-TTF) 2 I 3 [BEDT-TTF is bis-ethylenedithiolo(tetrathiafulvalene)] have been performed to investigate the charge distribution between BEDT-TTFs below and above the metal-insulator phase transition at T M-1 =135 K. Following the crystal-symmetry lowering at T M-1 , clear ±0.3e deviation from average 0.5e charge has been found in the nonmagnetic insulating phase. In the paramagnetic semi-metal-like phase, the split of one of the Raman active mode suggests charge nonequivalence between donors.

Mixtures of Trihalide Anions in Isostructural BEDT-TTF Salts as Seen by Raman Spectroscopy

Abstract Low frequency Raman spectra have been recorded at room temperature for β-type structure of the (BEDT-TTF)2X single crystals (where BEDT-TTF is bis(ethylenedithio) tetrathiafulvalene and X stands for mono or mixed trihalide anions). This technique has permitted us to detect a presence of the (Br-I-Cl)−, (I-I-Cl)− and (Cl-I-Cl)− anions in addition to a mixture of the (I-I-I)−, (I-I-Br)− and (Br-I-Br)− anions in some recently grown single crystals. The observed low-frequency Raman bands were compared with the ab initio calculated vibrational frequencies of trihalide anions and a very good agreement between experimental and calculated data was found.

In situ Raman spectroscopy of thermal phase transformation of ET2I3 polycrystalline network in polymer films

Abstract Raman microscopy was used for studying the thermal transformation of α- to β t -phase of polycrystalline ET 2 I 3 thin layers (ET=bis(ethylenodithio)tetrathiafulvalene) in polycarbonate film prepared by the two step reticulate doping technique (RDT). The in situ resonant Raman spectroscopy provides the unique possibility to observe the stretching frequency change of the I 3 − anion in ET 2 I 3 salt during annealing. This potentially enables to find the optimum conditions for phase transformation. For the polycarbonate (PC)+α-ET 2 I 3 system, 1-h annealing at 107±1°C was found to be optimal, as confirmed by the temperature dependence of the resistivity.

Thin layers of ET2I3 obtained by in situ crystallization — the role of polymer matrix

Unconventional method of preparation of thin layers of organic crystals by in situ crystallization consists in using a polymer matrix as a reservoir of molecular compounds and also as a medium of crystallization. We discuss an influence of properties of the polymer matrix on the crystallization and properties of the α-ET 2 I 3 polycrystalline layers, and on their transformation by annealing into the layers of superconducting β t -ET 2 I 3 phase. Polycarbonate, polystyrene and poly(methyl methacrylate) were used as the matrices. The properties and structure of the polycrystalline layers were investigated by means of direct current and microwave conductivity measurements in a broad temperature range, by Raman spectroscopy, AFM and SEM. It was found that in the systems with polymer matrices of lower T g , the α->β t transformation occurs easier and at lower temperatures, yielding metallically conducting layers which show in some cases an onset of superconducting transition at liquid helium temperatures.

Raman Spectroscopy of Polymer Composites with Thin Layers of Bedt-Ttf Trihalide Salts

Conducting polycrystalline layers in polymer composites have been prepared by a two step reticulate doping technique i.e. by exposing solid solution of bis-(ethylenedithiotetrathia) fulvalene (BEDT-TTF) in the polymer matrix to vapours of solvent containing I 2 or IBr. Characterisation of trihalide anion mixtures in the obtained layers of BEDT-TTF salts was completed by low frequency Raman spectroscopy. The surface morphology and the chemical composition of the layers were investigated by means of scanning electron microscopy with energy dispersive X-ray microanalysis and atomic force microscopy.

Low frequency Raman spectroscopy of β″-(ET)2Br0.5ICl1.5 single crystals

Abstract The Raman spectra of β″-(ET) 2 Br 0.5 ICl 1.5 single crystals (where ET=bis(ethylenodithio) tetrathiafulvalene) were investigated at 25 and at 80 K for the (001) crystallographic plane. The comparison with the spectra for β″-(ET) 2 ICl 2 and β-(ET) 2 IBr 2 indicates the presence of a mixture of (Br–I–Br) − , (Br–I–Cl) − and (Cl–I–Cl) − anions in the β″-(ET) 2 Br 0.5 ICl 1.5 single crystals. The angular dependence of the Raman spectra obtained at 80 K by rotating the single crystal by π /8 for fixed polarisation of the incident laser light confirms the assignment of the observed bands to three different trihalide anions. It was found for β″-(ET) 2 Br 0.5 ICl 1.5 and β″-ET 2 ICl 2 , that internal vibrations of ET molecule are the strongest and very well resolved for polarisation parallel to the [100] .

Micro-Raman spectroscopy of single crystals of ET salts with mixed trihalide anions**

We present a Raman study of isostructural organic conductors β-ET2X (ET is (bis(ethylenedithio)tetrathiafulvalene and X is I3, I2Br, IBr2 or nominally (I3)0.3(IBr2)0.7) grown by chemical oxidation. The low frequency region is studied down to 20 K for the 514 and 676 nm laser lines. In the case of β-ET2I3 single crystals we assign the 98 cm−1 band to the out-of-phase stretch of the symmetrical linear I3− anions. The band at 139 cm−1 in the Raman spectrum of β-ET2I2Br single crystals we assign to the in-phase stretch of the I2Br− anions, while the band at 120 cm−1 to the in-phase stretch of the I3− impurities. Domination of the band at 120 cm−1 we explain by high concentration of the I3− anions impurities for which Raman scattering occurs in resonant condition. The Raman spectra of β-ET2(I3)0.3(IBr2)0.7 single crystals, whose nominal stoichiometry was derived from X-ray diffraction, show that in fact the IBr2− and also the I2Br− anions dominate in the structure, while the I3− anions concentration is very small.