Synthetic Metals 156 (2006) 75–80 Evaluation of charge transfer degree in the bis(ethylenethio)tetrathiafulvalene salts by Raman spectroscopy Aneta Kowalska a,∗ , Roman Wojciechowski a , Jacek Ulanski a , Marta Mas-Torrent b , Elena Laukhina b , Concepcio Rovira b , Kyuya Yakushi c a Department of Molecular Physics, Technical University of Lodz, 90-924 Lodz, Poland b Institut de Ciencia de Materials de Barcelona, Campus UAB, E-08193 Bellaterra, Spain c Institute for Molecular Science, Okazaki, Aichi 444-8585, Japan Received 25 May 2005; received in revised form 5 October 2005; accepted 25 October 2005 Available online 6 December 2005 Abstract Raman spectroscopy studies for a series of charge-transfer salts based on the bis(ethylenethio)tetrathiafulvalene (BET-TTF) were carried out in order to analyse the charge distribution on the donor molecules in the unit cell of crystals. With the help of the density functional theory calculations for BET-TTF 0 and BET-TTF +1 molecules it was shown that the Raman spectroscopy can be applied to determine the stoichiometry in the BET-TTF salts. For salts exhibiting increase of the resistivity below ca. 100K, the Raman spectra at variable temperature indicate that this behaviour is not related to the charge disproportionation phenomenon. © 2005 Elsevier B.V. All rights reserved. Keywords: Raman spectroscopy; Charge transfer salts; Charge disproportionation 1. Introduction Conducting low dimensional organic crystals are usually formed by planar aromatic molecules that exhibit interfacial staking allowing overlap of the  electron orbitals. This stacking mode leads to one-dimensional systems, which are vulnera- ble at low temperature to Peierls lattice distortions that result in an insulating state [1,2]. Therefore, to avoid this Peierls distortion is important to promote good contacts between the molecules in the adjacent stacks giving rise to a system with conductivity in two-dimensions (2-D). So far the most suc- cessful results have been obtained using halogenated salts of bis(ethylenedithio)tetrathiafulvalene (BEDT-TTF) in which 2- D electronic structures are formed through S···S contacts [1,2]. One of the ways to develop new conducting organic crystals is the synthesis of the new donor molecules, which, similar to ∗ Corresponding author. Tel.: +48 42 6313205; fax: +48 42 6313218. E-mail address: chwiejcz@p.lodz.pl (A. Kowalska). BEDT-TTF, can also promote S···S interactions. The donor bis(ethylenethio)tetrathiafulvalene (BET-TTF) [3] is hence a good candidate material. This molecule does not show the typ- ical conformational isomerism of the ethylene groups found in BEDT-TTF due to the higher rigidity of the five membered rings, but it was found that BET-TTF exists in the two iso- meric forms shown in Fig. 1a and b. There exist a number of the charge transfer BET-TTF-based salts that below 100 K show a metal–insulator transition of unknown nature [3]. Structural phase transitions in molecular crystals can be investigated by X-ray diffraction, which is a standard technique used for deter- mination of the crystal structures. In addition, the analysis of the X-ray data allows one to calculate the electronic structure of molecular crystals. However, there is a need for an experimental technique useful for rapid characterization of newly obtained crystals, since they are often obtained as a mixture of multiple crystalline phases, which have also often different composi- tions [1,2]. Even an approximate stoichiometry evaluation gives some indication if the prepared crystals may potentially have the desired electrical properties. For this purpose, the micro- 0379-6779/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.synthmet.2005.10.012