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TECHNICAL NOTES
Spectroelectrochemicai Thin-Layer Cell for Nonaqueous Solvent Systems Josef Salbeck Institut fur Organische Chemie, Uniuersitiit Regensburg, Uniuersitatsstrasse 31, D - W 8400 Regensburg, Germany
INTRODUCTION Since its inception in the mid-l960s, the technique of absorption spectroelectrochemistry has been widely utilized to study the redox chemistry of numerous systems and has proven to be a valuable tool in electroanalytical chemistry, as is illustrated by several review articles.’“ With a thinlayer configuration, the rapid and exhaustive electrolysis intrinsic to thin-layer electrochemistry allows one to detect spectra of species with a half-life of more than some seconds without interference from the bulk solution and offers the convenience of controlling the O/R value by the potential which is applied to the electrochemical cell. Complete electrolysis of electroactive materials in the thin layer can be achieved in seconds with diffusion as the sole mode of mass transport, and spectral data can be gathered on a static solution composition. The thin-layer configuration is also useful for studying homogeneous chemical reactions of electrogenerated reactive oxidation statea. The cell thickness typically ranges between 10 and 300 wm, smaller than the semiinfinite electrochemical diffusion layer thickness. Because the demands for the electrochemical and spectroscopic responses and the ease of fabrication and handling of the cell are somewhat opposed, each cell design is a compromise. For this reason, various thin-layer spectroelectrochemical cell designs have been deacribed.&l0 In this paper a spectroelectrochemicalcell is reported which is especially designed for use with organic solvents; no epoxy cement or other sealing compounds are required and the solution comes in contact with only quartz or glass, Teflon, and the electrodes. This is an advantage over many previous cell designs, which were incompatible for use with all solvents because some of the cell components dissolved in some solvents. Moreover, the new cell allows for work under anaerobic conditions by applying a slight pressure of nitrogen to the cell during the spectroelectrochemical experiment. The cell is easy to fabricate and the same construction can be used either with IT0 electrodes (glassescoated with indium oxide/ tin oxide) or with minigrid electrodes. In addition, experiments over the full spectral range (UV/visible/near-IR) are possible. To illustrate the advantages of the described cell, spectroelectrochemical investigations of highly charged species like the radical trication of N,”,”-trimethyl-4,4’,4’’tripyridocyanine (TMTPC) and the radical trianion of (1) Kuwana, T.; Winograd, N. Electroanal. Chem. 1974, 7, 1. (2) Heineman, W. R.; Hawkridge, F. M.; Blount, H. N. Electroanal. Chem. 1984,13, 1. (3) Robineon, J. R. Electrochemistry 1984, 9, 101. (4) Pragst, F. Z. Chem. 1981,22, 241. (5) Kuwana, T.; Heineman, W. R. Acc. Chem. Res. 1976,9,241. (6) Kissinger, P. T.; Reilley, C. N. Anal. Chem. 1970,42,12. (7) Rhodes, R. K.; Kadish, K. M. Anal. Chem. 1981,53,1539. (8) Scherson, D. A.; Sarangapani, S.;Urbach, F. L. Anal. Chem. 1985, 57, 1501. (9) Lin, X. Q.; Kadish, K. M. Anal. Chem. 1986,58,1493. (10) Gui, Y.; Soper, S. A.; Kuwana, T. Anal. Chem. 1988, 60, 1645. 0003-2700/93/0365-2169$04.00/0
13,13,14,14-tetracyano-5,12-naphthacenoquinodimethane (TCNNQ) are shown.
N
TMTPC
TCNNQ
EXPERIMENTAL SECTION Cell Design. A schematic illustration of the cell is shown in Figure 1. The central part consists of optically transparent electrodes in a sandwich configuration of the general type described by DeAngelis and Heineman.” In contrast to many previous designs,this thin-layer assemblyis not glued with epoxy cement but clamped together with two Teflon U-profiles. Consequently, after completion of the experiment the cell can be easily disassembled and the components cleaned by conventionalmeans. In the Pyrexversionof the cell, two IT0 electrodes12 in a twin-electrode arrangement were used, and in the quartz version a gold minigrid electrode13(5 mm X 15mm) between two quartz slides was used. The cell thickness in both cases was defined by two glass strips with a thickness of 100 or 150pm (cut from microscope slide cover glass) on the left and right side between the two slides. Electricalcontact to the working electrode was achieved by a platinum strip of the same 100 or 150 pm thicknessbetween the slidesalong the upper edge, sincethe Teflon U-profiles on the left and right side of this thin-layer arrangement act as springs and cause a good contact between the platinum strip and the IT0 electrode. The upper part of the Teflon U-profilesare cylindricalso as to fit in two holes in the underside of the Teflon head. These Teflon profiies additionallywere slitted for passing through the platinum strip. The Teflon head of the cell has the form of a standard groundjoint (NS29)and is further equipped with the electricalcontacts of the three electrodesmade by tight-fitting brass grub screws and three screwed HPLC fittings. Two of these fittings are used with HPLC Teflon tubes for nitrogen flusing of the assembled cell. The third fitting is equipped with a septum and used for filling the cell vessel with the electrolyte by means of a Hamilton syringe. The counter electrode is a circular disk placed on the bottom of the cell, which was made from stainless V4A steel,electroplated with nickel and gold. An AgC1-coated silver wire serves as ‘pseudo” reference electrode. The reference electrode ends at the lower edge of the thin-layer assembly. The electrodes are fixed with 1-mmscrews on the brass contacts of the Teflon head, so the Teflon head bears the completeequipment of the cell. The thin-layer assembly and the electrodes are then put into the protecting vessel, which also is equipped with a standard ground (11) DeAngelis, T. P.; Heineman, W. R. J. Chem. Educ. 1979,53,529. (12) Baltracon Z 20, Fa. Balzers, Fiirstentum Liechtenstein. (13) Gold electroformed mesh, 500 wiredin., transmission 60%, Interconics, Buckbee-Mears Operation, St. Paul, MN.
0 1993 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 65, NO. 15, AUGUST 1, 1993
Figure 1. Schematic illustration of the spectroelectrochemical cell: (a) fittings and HPLC Teflon tubes for nitrogen flushing; (b) glass or quartz protectingvessel; (c) third fitting; (d) septum; (e) Teflon U-profiles in front view and top view; (f) I T 0 electrode or quartz slide; (9) assembled thin-layer part, front view; (h) thin-layer part, top view; (i) platinum strip for electrical connect to the OTE; 0) I T 0 electrode; (k) glass strips; (I) thin-layer part with minigrid electrode; (m) Teflon head, top view; (n) Teflon head, side view (not all holes were drawn); ( 0 ) Teflon head, bottom view; (p) brass contacts; (4) counter electrode, side view and top view; (r) reference electrode; (s) screws; (t) assembled cell (not all parts were drawn).
joint for a tight fit of the Teflon head. This protecting vessel consists of a cylindrical Pyrex glass or quartz cuvette, with a diameter of 3.0 cm and a height of 5.3 cm, and can be made from a standard 29/32 female Pyrex or quartz joint by a glassblower. Pyrex glass is convenient with the use of IT0 electrodeson glass and allows studies in the spectral range from 320 to 2500 nm. If the vessel is made from quartz, and minigrid electrodesbetween two quartz slides are used, the spectral range from 200 to 3200 nm can be monitored. If the assembled cell is filled with ca. 5 mL of the electrolyte solution to a height of 5 mm, so that the lower edge of the thinlayer part comes in contact with the solution,the thin-layer cell is filled by capillary action. The cell is mounted in the compartment of the spectrophotometer, and the spectrophotometer beam is placed in the center of the thin-layer assembly without interference by the bulk solution, Instrumentation. Electrochemical measurements were carried out with an Amel 553 potentiostat/galvanostat equipped with positive feedback for iR compensation in conjunction with an Amel 568function generator. On-lineaquisition of the current/ voltage outputs was done by means of a multichannel analogue/ digital converter ANA86/3 (Computer Centre, University of Regensburg) on a 80386SX PC. Programming the function generator was done by means of the RS232 interface, and the analysisand evaluation of the voltammogramswere accomplished with graphicoriented computerprogramswritten in TurboPascal. The working electrode was polished with diamond paste (0.25 pm) to a mirror finish. Potentials are reported relative to the Ag/ AgCl quasi-reference electrode, if not otherwise indicated. After each experiment, calibration with ferrocene was performed.14 Thin-layer voltammetry was done with a modified CV celP at a layer thickness of 20 f 5 pm with a platinum disk electrode (5.5 X le2 cm2). Spectroelectrochemicalmeasurements with the introduced cellwere performed without iR compensation. The electronic spectra were obtained by a Perkin-Elmer Lambda 9 spectrophotometer (spectral range from 190 to 3200 nm). Computerized on-line aquisition was applied. Reagents. Tetrabutylammonium hexafluorophosphate (TBAHFP)was prepared by metathesis of tetrabutylammonium bromide and ammonium hexafluorophosphate in acetone/water (14) Gritzner, G.; Kutta, J. Pure Appl. Chem. 1984,56, 461. (15) Modification of a CV cell according to: Carlier, R.; Simonet, J. Bull. SOC.Chim. Fr. 1988,831.
according to a procedure of Fry and Britton,lg The product was recrystallizedfour times from ethanol/water and dried at 100 "C on a high-vacuum line (
