Investigation of Luminescence and Absorption Spectra of Bedt-Ttf and its Ion Radical Salt
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M.E.KOZLOV*, Y.TANAKA , M.TOKUMOTO** AND T.TANI** "Instituteof Semiconductors, Ukraine Academy of Sciences, Kiev, Ukraine "*Electrotechnical Laboratory, 1-1-4 Umezono, Tsukuba, Ibaraki 305, Japan
ABSTRACT Luminescence with maximum near 1.96 eV of organic donor compound bis(ethylenedithio)tetrathiafulvalene (BEDT-TTF) was observed in crystals and frozen at low temperature solution. It was found that solid state effects sharply reveal themselves in excitation spectra of the substance and have almost no influence on the shape of the emission. Intensity of the emission is strong at liquid He temperature but rapidly decreases with heating of the sample as well as after modification of charged state of molecules forming the crystal and is very weak in case of monovalent (BEDT-TTF)Br salt. Shape of the luminescence spectra correlates with the absorption of the investigated compounds. The data obtained can be interpreted assuming dimer-like arrangement of molecules in the crystal structure. INTRODUCTION Organic donor compound bis(ethylenedithio)tetrathiafulvalene (BEDT-TTF) became well-known in the past decade after discovery of superconductivity in several of its chargetransfer salts with rather high for organic substances critical temperatures (up to 12.8 K), see [1]. A lot of works were devoted to the investigation of optical properties of crystals of conducting as well as initial insulating compound [1], analysis of molecular vibrations and calculation of electron-molecular vibration coupling constants [2]. Only their luminescence properties up to now have never been the subject of the systematic research. As continuation of our examination of materials of this class, we report now luminescence spectra of BEDTTTF measured in crystals, pressed pellet and as a dispersion in the rigid solvent at low temperature and discuss their nature using comparison with data obtained for (BEDT-TFF)Br monovalent salt as well as with absorption spectra of both compounds. EXPERIMENTAL BEDT-TTF crystals and (BEDT-TTF)Br powder were obtained by method described earlier [2]. The ca 1:1 stoichiometry of the latter salt was verified by elemental analysis. As a sample for the luminescence measurements we used mosaic formed from crystals of the neutral compound (typical size of 0.5x0.5x3 mm) and pressed pellet of its monovalent salt. Besides, the spectra of a thin (about 0.2 mm in section) pellet pressed from the powder of neutral substance and its dilute solution (0.5x10.5 mol/1) in 2-methyltetrahydrofuran were also obtained. The emission and excitation spectra were recorded on standard Hitachi M 850 Fluorescence 343 Mat. Res. Soc. Symp. Proc. Vol. 328. @1994 Materials Research Society
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Spectrometer (resolution of about 0.01 eV) in 4.5-1.5 eV region. The cryostat system allowed us to perform measurements at several fixe
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