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The Journal of Physical Chemistry, Vol. 83, No. 13, 1979
M. A. Fox and K.-Din
A New Carbanionic Photogalvanic Cell Marye Anne Fox* and Kablr-ud-Din Department of Chemistry, University of Texas at Austin, Austin, Texas 78712 (Received January 12, 1979) Publication costs assisted by the U S . Department of Energy
Cyclooctatetraenyl dianion 1 has been shown to function as a electron source when excited with visible light at the surface of a TiOzsemiconductor electrode in a liquid NH3 photoelectrochemicalcell. Since the conduction band of TiOz in ammonia is located below the oxidation potential of 1 (disodium or -potassium salts of cyclooctatetraene)at - 4 . 5 5 eV vs. Ag, the efficiency of the photoinduced electron transfer may rely on high-lying surface states, kinetic retardation of electron exchanges at the ammonia-semiconductor interface, or on other relaxation phenomena.
Introduction The utility of organic compounds as photosensitizers in photoelectrochemical cells has been limited by two factors: the lack of absorption of visible light by most common organic substrates and, as a consequence, the difficulty of achieving modestly activated electron transfer, i.e., their relatively high oxidation potentials. These two problems can be easily overcome, however, if neutral compounds are converted to anions. Compared to their neutral precursors, for example, hydrocarbon anions exhibit both dramatically red-shifted absorption spectra and significantly more negative oxidation potentia1s.l As a class, they would appear to be ideal candidates as sensitizers for photoelectrochemical cells if a third criterion, the inhibition of electron recapture by the species formed by the photoinduced electron transfer, can be met. One method for controlling this back-electron transfer is to provide a chemical reaction path for the photooxidation product. Our interests in examining photoinduced electron transfer reactions in organic anions led us to consider the cyclooctatetraenyl dianion 1 as a reasonable prospect since its alkali metal salts absorb into the visible,24 it has been shown from fluorescence studies to undergo efficient photoejection upon photolysis with ultraviolet light," its electrochemical behavior has been characterized in a variety of solvent^,^ and the disproportionation equilibrium of its oxidation product, the cyclooctatetraenyl radical anion, 2, can be shifted (by greater than ten orders of magnitude) by changing the identity of the solvent and associated cation.6 In addition, 2 is chemically stable (except with respect to disproportionation) in the absence of oxygen, electrophiles, and weak acids (e.g., moisture or protic solvents). Our hope, then, was to construct a liquid phase photoelectrochemical cell in which 1 would be irradiated at the surface of an electrode to produce 2 under conditions in which a rapid disproportionation would occur, driving the indicated equilibrium to the right (Scheme I). This equilibrium generates the neutral hydrocarbon 3 and the aromatic starting material. The ejected electrons would be available through an external circuit to effect a dark cathodic reduction of 3 to 1 which would complete a catalytic cycle in which absorbed light is converted to electrical current. Experimental Section Our thermostated (-40 to -60 "C) photoelectrochemical cell is analogous t o those which have been previously described.' A thin layer (3-5 mm) of a 20:l mixture of 1:3 (ca. 0.02 M)8 in dried, degassed aprotic solvent was illuminated near the surface of the anode with wavelengths 0022-3654/79/2083-1800$01 .OO/O
Scheme I
M;Na,K
t2e-
> 400 nm to ensure absorption
only by 1, Le., to exclude direct electrode excitation. Furthermore, under the irradiation conditions the photolysis layer was opaque at X < 420 nm. Photocurrents were monitored on a Leeds and Northrup galvanometer. In a typical experiment, the degassed 3 (Aldrich, distilled) was condensed into a mixture of ca. 25 mL of rigorously p ~ r i f i e ddried, ,~ and degassed aprotic solvent and dried inert electrolyte contained within a three-armed electrochemical cell equipped with a 2 X 2 cm Pyrex optical flat. Approximately 3 mm intervened between the flat and the 2 X 2 cm electrode used as photoanode. Perpendicular to the direction of the incident light in the rear of the cell were two platinum electrodes which were available for preparative electrolysis and/or cyclic voltammetric monitoring of solution concentrations of electroactive species or as counterelectrodes for the photoelectrochemical experiment. A Ag reference electrode was separated from the main body of the cell by a glass frit. Tetrabutyl- or tetramethylammonium perchlorate (Southwestern Analytical, vacuum oven dried) was used as the electrolyte in DMF and CH3CN. KI or NaC104 was used as electrolyte in NHB. A GE 275-W sunlamp was used as the irradiation source, with or without a Corning 3-73 filter (X > 400 nm) to ensure absorption only by the anion, i.e., no direct excitation of the electrode. 3 was partially converted in NH3 to 1 either by preparative electrolysis (PAR Model 173 potentiostat and PAR 179 digital coulometer) or by the addition of alkali metal (Na or K). The cell was maintained at f0.5 "C in the range of -40 to -60 "C in a 2-propanol bath. Photocurrents were determined as a difference between the negligible dark current and current observed during irradiation.
Results and Discussion Photoinduced current (-0.1-0.5 pA/cm2) was observed when 1, 2M+ (M = Na, K) was irradiated at single crystal or polycrystalline n-Ti02 in ammonia. Negligible photocurrents were observable at Pt or polycrystalline SnOz in NH3, CH3CN, or dimethylformamide (DMF). Currents were absent upon irradiation of a mixture 1 and 2 at n-TiOz in CH,CN or DMF. In the latter case, degradation of 1 to a complex mixture of products rapidly occurred upon irradiation, although in the dark the solution was fairly stable and the redox reactions were nearly completely reversible. In the former case, acetonitrile (or the elec0 1979 American Chemical Society
The Journal of Pbysical Chemistry, Vol. 83, No. 13, 1979
A New Carbanionic Photogalvanic Cell Scheme
I1
n L LUMO " !I
Ec
HOMO
- "E Semiconductor
Excited substrate
trolyte) apparently protonated the highly basic excited state of 1, leading to electroactive trienes, in a process analogous to that described earlier.1° In the n-Ti02/NH3 experiment, no decrease in 1 concentration nor appearance of electroactive products could be observed after 5 days of irradiation. The direct dependence of the observed current on illumination intensity was established by employing neutral density filters. Although the currents observed are significantly lower than those produced by direct irradiation of the semiconductor, their observation suggest that the photoresponse of more sensitive semiconductors can be extended to longer wavelengths and, perhaps more importantly, that such cells can be effectively used in mechanistic studies of photoinduced electron transfer reactions of carbanions. The detailed mechanism by which such a cell functions is intriguing. In agreement with our observations, dyesensitized photogalvanic cells should theoretically function most effectively at semiconductor electrodes.'l If the redox potential of an absorptive substrate lies between the valence and conduction bands and if photolysis generates an excited state in which an occupied molecular orbital lies above the conduction band, photoinduced electron transfer from the substrate to the electrode should occur, producing an anodic photocurrent, even with less energetic excitation wavelengths than required for photoejection from the substrate to vacuum (Scheme II),llJz An inhibition of back-electron transfer, achieved by the separation of the energy levels of the semiconductor conduction band and the ground state HOMO of the substrate and by band bending in the semiconductor, would similarly be anticipated. The band positions (Ec and Ev) of n-TiOz semiconductor electrodes have been characterized in water,13but are known to vary with pH14 and solvents7 In particular, Ec in ammonia was unknown although it had been demonstrated that the Ec in CH3CN was sensitive to the presence of amines.15 Our observation that, in NH3, the reduction of 3 was reversible at Pt but irreversible at n-TiOz, as a polycrystalline or a single crystal, was consistent with an Ec (n-Ti02)above the oxidation potential of 1 (E, 0.8 eV vs. Ag). Our determination of flat band potential by either a Schottky-Mott plot (capacitance-2 vs. potential)16or by the potential of onset of photoinduced current,17 however, placed the conduction band for n-Ti02
-
1801
in NH3 at ca. -0.55 f 0.10 eV. A similar determination placed the conduction band for SnOz (NESA) in NH3 at ca. -0.45 V. We assume, then, that the photocurrent observed may depend, among other factors, on am anomalous high lying surface state of n-Ti02 in ammonia, on significant kinetic retardation of electron transfers at the Ti02-ammonia-substrate, or on relaxation phenomena at the semiconductor ~ u r f a c e . ~ Our efforts toward understanding the photoreactions of carbanions at electrode surfaces are continuing.
Acknowledgment. Financial support by the U S . Department of Energy is gratefully acknowledged. The electrochemical apparatus was purchased, in part, with funds from the University of Texas Research Institute. We are grateful for helpful discussions with Professor A. J. Bard and for the gift of several single crystalline TiOz electrodes from Dr. R. Wilson of General Electric Research. References and Notes (1) (2) (3) (4) (5)
M. A. Fox, Chem. Rev., in press. P. G. Farrell and S. F. Mason, Z. Naturforsch., 166, 848 (1961). P. I. Kimmel and H. L. Strauss, J . Phys. Cbem., 72, 2813 (1988). V. Dvorak and J. Michl, J . Am. Chem. SOC., 98, 1080 (1976). For example, W. H. Smith and A. J. Bard, J. Electroanal. Chem., 76, 19 (1977); R. D. Allendoerfer, J. Am. Chem. Soc., 97,218 (1975); A. J. Fry, C. S. Hutchins, and L. L. Chung, ibid., 97, 591 (1975); 8. S.Jensen, A. Ronlan, and V. D. Parker, Acta Cbem. Scand., Sect. 5, 29, 394 (1975); H. Lehmkuhl, S. Kintopf, and E. Janssen, J. Organomefal. Chem., 56,41 (1973); D. R. Thelen and L. B. Anderson, J. Am. Chem. Soc., 94, 2521 (1972); E. J. Huebert and D. E. Smith, J . Electroanal. Cbem. InterfacialNectrochern.,31, 333 (1971); R. D. Allendoerfer and P. H. Rieger, J . Am. Chem. Soc., 87, 2336 (1965); T. J. Katz, W. H. Reinmuth, and D. E. Smith, bid., 84, 802 (1962). (6) F. J. Smentowski and G. R. Stevenson, J. Phys. Cbem., 73, 340 (1969), and references cited therein. (7) S.N. Frank and A. J. Bard, J. Am. Chem. SOC.,97, 7427 (1975). (8) The photocurrent observed depends on the ratio of 1 to 3. 3 is required as a cathodic reductant but if its concentration was too high a decrease in photocurrent was obs8rved. Presumably, ground state 3 can compete with the semiconductor as an electron acceptor, providing another nonproduct deactivation pathway for excited I. The absence of even trace quantities of oxygen and water is critical for the production of photocurrent. (9) Ammonia was purified by a double distillation from sodium metal chunks (after storage overnight over Na). CH,CN and DMF were purified by standard procedures: S. N. Frank, A. J. Bard and A. Ledwith, J . Electrochem. SOC., 122, 898 (1975); H. Kojima and A. J. Bard, J . Electroanal. Cbem., 63, 117 (1975). (IO) J. I. Brauman, J. Schwartz and E. E. van Tamelen, J. Am. Chem. Soc., 90, 5328 (1968). (1 1) H. Gerischer in "Physical Chemistry: An Advanced Treatise", Vol. 9A, H. Eyring, D. Henderson, and W. Jost, Ed., Academlc Press, New York, 1970; H. Gerischer and F. Willig, Forfschr. Chem. Forscb., 61, 31 (1976). (12) H. Tributsch and M. Calvin, Photochem. Photobiol., 14, 95 (1971). (13) R. N. Noufi, P. A. Kohl, S. N. Frank, and A. J. Bard, J. Electrochem. Soc., 125, 246 (1978). (14) T. Watanabe, A. Fujishima, and K. Honda, Chem. Left., 897 (1974). (15) K. Nakatani and H. Tsubomura, Bull. Cbem. Soc. Jpn., 50, 783 (1977). (16) V. A. Myamlin and Y. V. Pleskov, "Electrochemistry of Semiconductors", Plenum Press, New York, 1967. (17) P. A. Kohl and A. J. Bard, J . Am. Chem. Soc., 99, 7531 (1977).
