ELSEVIER Synthetic Metals 70 (1995) 987-988 Photoluminescence study of BEDT-TrF and its ion radical salt M.E.Kozlov°, Y.Tanaka, M.Tokumoto and T.Tani Electrotechnical Laboratory, 1-1-4 Umezono, Tsukuba, Ibaraki 305, Japan Abstract Photoluminescence spectra of the organic donor compound, bis(ethylenedithio)tetrathiafulvalene (BEDT-TI'F), has been observed in crystals and rigid solution at liquid He temperature. The spectra have similar shape with a maximum at 1.96 eV and an intense side-band near 1.79 eV. A solid state effect reveals itself in low-temperature excitation spectra of the crystalline sample as a drastic increase of the excitation efficiency in the low energy region (2.1-3.1 eV) comparing to the high energy one (3.1-4.0 eV). The emission of BEDT- TrF is quenched by a modification of degree of oxidation of molecules forming the crystal and is very weak in the case of monovalent (BEDT-TI'F)Br salt. The recorded spectra are interpreted assuming a dimer-like arrangement of molecules in the investigated compounds. Much of the early spectroscopic work on complexes of BEDT- TTF was devoted to investigation of optical spectra of conducting crystals as well as the initial insulating compound [1]. To date however no systematic study of luminescence properties has been done. In the report we present emission and excitation spectra of BEDT-TFF measured in crystals, thin pressed pellet and rigid solution at low temperature as well as data on (BEDT-'I"rF)Br monovalent salt. For the luminescence measurements we used a mosaic formed from crystals of the neutral compound, thin (about 0.2 mm in section) pellet pressed from its powder, dilute solution (0.5x10 5 mol/l) of BEDT-TI'F in 2-methyhetrahydrofuran and pressed pellet of (BEDT-TI'F)Br salt. The study of absorption spectra was carried out on ground powder of BEDT-TI'F as KBr pellet, solution (about 0.2x10-3 mol/1) of the neutral substance in dichloromethane as well as solution of the salt having all similar absorbance in the region investigated. The emission and excitation spectra were recorded using a Hitachi M 850 Fluorescence Spectrometer (resolution of about 0.01 eV) equipped with a Oxford CF1204 cryostat at liquid He temperature. The spectra were corrected for the system response. Visible and near i.r. absorption spectra were recorded with a Hitachi U-3400 spectrophotometer in 4.5-1.1 eV region at room temperature. The luminescence signal from crystals of the neutral BEDT- TI'F is very strong at liquid He temperature, exhibits a maximum near 1.96 eV and a distinctive peculiarity near 1.79 eV. The specmam of the compound in the frozen solution has maximum at 1.97 eV and the sharp side-band near 1.80 eV as well as essentially lower intensity. This indicates that molecules of BEDT-TTF are responsible for the emission. The side-band near 1.79 eV has apparently a vibronic nature. Its position in the spectrum corresponds to the energy of the most intense Raman band of the neutral BEDT-'ITF equal to 0.185 eV (1494 cm4, v 3 totally symmetric vibration). According to [1] the value of coupling constant is maximal for this mode. In the case of the monovalent salt, the luminescence spectrum has a similar shape, but its intensity is more than 2 orders lower than corresponding for crystals of BEDT-TTF. The changes in excitation spectra of BEDT-q"rF (Fig.2) are drastic; the efficiency of the luminescence excitation in low (2.1- 3.1 eV) and high (3.1-4.0 eV) energy regions differs strongly for crystals and the dispersion of molecules in the rigid solution. The latter spectrum reveals also bands at 2.70, 3.55 and 3.76 eV. In absorption spectra of BEDT-TI'F solution (Fig.3) two intense bands are clearly observed at 3.86 and 3.58 eV and more weak one near 2.6 eV. A large energy difference between the peaks in the absorption and emission spectra equal to 0.64 eV for the latter transition indicates an essential change in the molecular configuration. The band at 2.6 eV markedly increases its relative intensity in the spectrum of the BEDT-TTF powder (Fig.3) as well as a pronounced shoulder in vicinity of 3.0 eV appears. Position of the two high energy bands changes very little in the spectrum of the radical cation salt (3.86 and 3.56 eV, correspondingly) but as, considered above, the third band exhibits fine structure with a very weak shoulder near 2.91 eV and two A .~ ~ x50 ~ ....... . \ I I ~ ........... ,,, 1.6 1.8 2.0 2.2 Energy (eV) Fig.1. Emission spectra of BEDT-TI'F in 2-methyltetrahydrofuran solution (dashed line), BEDT-TI'F crystals (solid line) and (BEDT-TTF)Br pellet (dotted line) at 5 K. E~=3.3 eV. *On leave from Institute of Semiconductors, UAS, Kiev, Ukraine. 0379-6779/95/$09.50 © 1995 Elsevier Science S.A. All rights reserved SSDI 0379-6779(94)02731-D