FT-IR Micro-Spectroscopic Studies of Several Charge-Transfer Organic Electrical Conductors JOHN R. FERRARO,* HAU H. WANG, JOHN RYAN, and JACK M. WILLIAMS Loyola University, Department of Chemistry, Chicago, Illinois 60626 (J.R.F.); Argonne National Laboratory, Chemistry and Materials Science Divisions, Argonne, Illinois 60439 (H.H.W., J.M.W.); and IBM Corp., Oak Brook, Illinois 60521 (J.R.) A series of infrared reflectance measurements using FT-IR Micro meth- ods were conducted on bis(ethylenedithio)tetrathiafulvalene (BEDT-TTF or "ET") and bis(propylenedithio)tetrathiafulvalene (BPDT or "PT") salts of the type (ET)2X, where X = I~ , AuCI2-, AuBr2 , AuI2-, I2Br , IBr~ , ICI2 , ReO4 , CIOj, and CH3-©-SO3-; and (PT)~X, where X = I3, IBr~-, I2Br, and ICI2 . The technique is useful for characterizing, at room temperature, differences between B-, ~/'- and a'-type structures vs. a-structures. The B-, B'-, and a'-type structures show a vibrational feature at ~1280 cm ' which has been assigned as the -C-C-H bending vibration and is absent in the spectrum of the a-type salts. This vibration shifts toward higher frequency in going from the PT to the ET salts, and the results indicate stronger hydrogen bonding and harder lattices in the ET salts as opposed to the PT salts. This frequency shift also appears to be a function of the electronegativity of the halogen in the anion moiety of the charge-transfer salt. Index Headings: FT-IR Micro-spectroscopic studies; Charge-transfer organic electrical conductors; Vibronic effects; Reflectance spectra. INTRODUCTION Vibrational spectroscopy has proven to be very useful in the study of 1-dimensional (l-D) organic charge-trans- fer (CT) compounds. 1-12These studies have been helpful in determining the degree of charge transfer in these systems and have also provided information on the nature of the interaction between the conduction elec- trons and molecular vibrations (vibronic effects). These studies have recently been extended to the 2-D charge transfer compounds, such as the bis(ethylenedithio)- tetrathiafulvalene (BEDT-TTF or "ET"). 1~-25 In tetrathiafulvalene (TTF) and related molecules, it has been observed that, when reflectance measurements are made, vibrational structure appears on the broad absorption observed for these materials. These features have been ascribed to vibronic interactions between the C=C and C-S symmetrical modes (intramolecular tran- sitions) and the conduction electrons, u and occur in the range of ~ 1600 to 500 cm -1. In the unoxidized ET mol- ecule, these features occur at frequencies <1600 cm -1. The vibrations at 1554 and 1494 cm i have been assigned as the central C=C stretching vibrations, and the vibra- tion found at 876 cm -~ has been assigned as the external C-S stretching motion. Other C-S modes have been assigned at 507 cm 1 (internal stretch) and 495 cm -1 (ex- ternal stretch). For the salt a-(ET)2Is, which has a metal- insulator transition at 135 K, two bands that are super- imposed on the broad feature (half-bandwidth of ~500 Received 1 May 1987. * Author to whom correspondence should be sent. Mailing address: 568 Saylor Ave., Elmhurst, IL 60126. cm ~), and that occur at 1275 and 1160 cm -1, have been assigned by Meneghetti et al. 14 to the coupling of con- duction electrons to the C=C stretching modes. These modes are of the A~.type and, normally, are only Raman- active. However, when coupling occurs, a lowering of symmetry follows in the ET molecule stacks and the vibronic spectrum becomes infrared-active. Similar fea- tures have been observed in the reflectance spectrum of the ambient-pressure organic superconductor (To ~ 1.5 K) ~-(ET)2I~. I3,t~ 19,2t For (ET)2C104(C2H3CI~)0.~, which is metallic to 1.4 K, a broad continuum from 340 to 25,000 cm 1, without vibrational structure in the room-temper- ature reflectance spectrum, was observed. 6,2' For (ET)2PF6, the room-temperature reflectance spectrum has been reported, 2°,2t and vibrational structure was ob- served on broad bands at ~2500 and ~1200 cm 1. A sharp peak was also observed at ~300 cm 1. This paper reports on the infrared reflectance results obtained with several (ET)2X salts, where X = I3-, AuC12-, AuBr2-, IBr2-, I2Br-, IC12-, Re04, Br04 , and CH~-(I}-SO~ , involving /3, fl', a, and a' structures, and several bis(propylenedithio)tetrathiafulvalene (BPDT- TTF or "PT") salts (e.g., (PT)2X, where X = I3 , IBr2-, I2Br-, and IC12-). Room-temperature spectra were ob- tained by micro-Fourier transform interferometry, by use of reflectance techniques, and comparisons between the various salts are discussed. EXPERIMENTAL The materials studied in this paper have been syn- thesized according to a procedure previously reported. 26 As only small amounts of single crystalline material were available for each substance, we relied on FT-IR micro- spectroscopy, a relatively new technique for our mea- surements. 27 An IBM FT-IR microscope was interfaced with an IBM IR/44 interferometer, and the binocular microscope was mounted externally. The sample was placed on a sodium chloride window, and the data were collected by reflectance methods, since the materials are opaque. A mercury-cadmium-telluride detector and an aperture of 80 t~m were used. Data were taken at 4-cm -1 resolution with 1000 scans. All data were measured at room temperature and were unpolarized, inasmuch as low-temperature capabilities and polarizability mea- surements were unavailable in our system. RESULTS Numerous differences in the reflectance spectra are noted for the range of charge transfer salts studied (vide infra). For the (ET)2X salts, four different structures are Volume 41, Number 8, 1987 ooo~-7o~8/smtos-t~7752.oo/o APPLIED SPECTROSCOPY 1377 © 1987 Society for Applied Spectroscopy