Fox, Nobs, Voynick
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Attachment of Arenes to Sn02-Semict mductor Electrodes
three-electron F--SF4+ bond. At the same time, an electron rearrangement occurs in the SF4+part of the molecule so that there are two three-center four-electron bonds, giving SF5 the structure of a tetragonal pyramid:30 F
F
F
i
SF6 is built up from SF5 and F by the formation of a strong three-center four-electron bond when F approaches SF5 from the opposite side of the F--SF4+ bond. In SF6 the maximum number of ligands is reached and by symmetry all S-F bonds are equivalent; Le., SF6 has three orthogonal three-center four-electron bonds. Thus this transformation of lone-pair electrons on the central sulfur atom to at first a weak two-center three-electron bond and then to a strong three-center four-electron bond permits sulfur to have a hypervalent character without introducing the concepts of valence excitation and d-orbital hybridization. Moreover, this simple picture also accounts for the observed alternation of the stepwise bond dissociation energies in going from SF6 to SF2. It may be expected that similar behavior will be found in the successive bond dissociation energies of other hypervalent compounds. Acknowledgments. We thank D. L. Hildenbrand, R. H. Holm, and W. R. Wadt for useful discussions. This work was supported by the Army Research Office under DAAG 2977-(3-015 1. References and Notes (1) Hildenbrand, D. L. Adv. High Temp. Chem. 1967, 7, 193. (2) (a) Lewis, G. N. J. Am. Chem. SOC. 1916, 38, 762. (b) Langmuir, I. Ibid. 1919, 41,868. 1543. (3) (a) Pauling, L. "The Nature of the Chemical Bond", 3rd ed.; Cornell University Press: Ithaca, N.Y.. 1960. (b) J. Am. Chem. SOC. 1931, 53, 1367. (4) Musher, J. I. Angew. Chem., ht. Ed. Engl. 1969, 8, 54. (5) (a) Chernick, C. L.; Claasen, H. H.; Fields, P. R.; Hyman, H. H.; Malm, J. G.; Manning, W. M.; Matheson, M. S.; Quaterman, L. A,; Schreiner, F.; Selig, H. H.; Sheft, I.; Siegel, S.;Sloth, E. N.; Stein, L.; Studier, M. H.; Weeks, J. L.; Zirin, M. H. Science 1962, 138, 136. (b)Claasen. H. H.; Selig, H.; Malm, J. G. J. Am. Chem. SOC.1962, 84, 3593. (c) Weeks, J. L.; Chernick, C. L.;
4029
Matheson, M. S. /bid. 1962, 84, 4612. (6) Kiang, T.; Estler, R. C.; Zare, R. N. J. Chem. Phys. 1979, 70, 5925. (7) Zare, R. N. Ber. Bunsenges. Phys. Chem. 1974, 78, 153. (8) Oldenborg, R. C.; Dickson, C. R.; Zare, R. N. J. Mol. Spectrosc. 1975, 58, 283. (9) Estler, R. C.; Zare, R. N. Chem. Phys. 1976, 28, 253. (10) Dagdigian, P. J.; Cruse, H. W.; Zare, R. N. J. Chem. Phys. 1974, 60, 2330. (11) Menzinger, M. Can. J. Chem. 1974, 52, 1688. (12) Steimle, T. C.; Domaille, P. J.; Harris, D. 0. J. Mol. Spectrosc. 1977, 68, 134. (13) Gaily, T. D.; Rosner, S.D.; Holt, R. A. Rev. Sci. Instrum. 1976, 47, 143. (14) (a)Havey. M. D.; Balling, L. C.; Wright, J. J. Phys. Rev. A 1976, 73, 1269. (b) Giusfredi, G.; Minguui, P.: Strumia. F.; Tonelli, M. 2.Phys. A 1975, 274, Balling, L. C.; Wright, J. J. Phys. Len. A 1975, 53, 279. (c) Furcinitti, P. S.; 75. (15) See: Karny, 2 . ; Zare, R. N. J. Chem. Phys. 1978, 68, 3360. These bond energies agree within the stated uncertainties with the values reported in Hildenbrand, D. L. /bid. 1968, 48, 3647. (16) Kiang, T.; Zare. R. N., in preparation. (17) Harland, P. W.; Thynne, J. C. J. J. Phys. Chem. 1971, 75, 3517. (18) Bott, J. F. J. Chem. Phys. 1971, 54, 181. (19) Since ref 6 was completed, B. S.Rabinovitch (private communication) has pointed out to us that the calculation of &O(FS-S-F) from the shock tube dissociation data (Bott, J. F.; Jacobs, T. A. J. Chem. Phys. 1969, 50,3850. Lyman, J. L. /bid. 1977, 67, 1868. Benson, S.W. Chem. Rev. 1978, 78, 23) may be an overestimate since no variation of the collision efficiency with the degree of falloff was considered. See: Rabinovitch, 8. S.;Keil, D. G.; Burkhalter, J. F.;Skinner, G. B. Proceedings of the Tenth International Shock Tube Symposium, Kyoto, Japan, 1975. (20) (a) Stull, D. R..Ed. "JANAF Thermochemical Tables"; Dow Chemical Co.: Midland, Mich.. 1965. (b) Chase et al. J. Phys. Chem. Ref. Data, 1976, 7, 897-917. (21) Hildenbrand, D. L. J. Phys. Chem. 1973, 77, 897. (22) (a) Skinner, H. A. Trans. Faraday SOC. 1949, 45, 20. (b) /bid. 1955, 57, 1036. (23) (a) Willett, R . D. Theor. Chim. Acta 1964, 2, 393. (b) Santry, D. P.; Segal, G. A. J. Chem. Phys. 1967, 47, 158. (c) Santry, D. P. J. Am. Chem. SOC. 1968, 90,3309. (d) Brown, R. D.; Peel, J. B. Aust. J. Chem. 1968, 21, 2589, 2605, 2617. (e) Gavin, R. M. Jr. J. Chem. Educ. 1969, 46, 413. (f) Companion, A. L. Theor. Chim. Acta 1972, 25, 268. (g) Kouteclj, V. 6.; Musher, J. I. /bid. 1974, 33, 227. (h) Chen, M. M. L.; Hoffmann, R. J. Am. Chem. Soc. 1976, 98, 1647. (i) Rbsch, N.; Smith, V. H.; Whangbo. M. H. Inorg. Chem. 1976, 75, 1768. (i)Cowley, A. H.; Lattman, M.; Walker, M. L. J. Am. Chem. SOC. 1979, 101,4074. (24) (a) Radom, L.; Schaefer, H. F. 111. Aust. J. Chem. 1975, 28, 2069. (b) Hay, P. J. J. Am. Chem. SOC. 1977, 99, 1003, 1013. (25) Pimentel, G. C. J. Chem. Phys. 1951, 19. 446. (26) (a) Hach. R. J.; Rundle, R. E. J. Am. Chem. SOC. 1951, 73, 4321. (b) Rundle, R. E. Rec. Chem. Prog. 1962, 23, 195. (c) J. Am. Chem. SOC. 1963, 85, 112. (d) Surv. Prog. Chem. 1963, 1, 81. (27) (a) Cornwell, C. D.; Yamasaki, R. S. J. Chem. Phys. 1957, 27, 1060. (b) Yamasaki, R. S.;Cornwell. C. D. Ibid. 1959, 30, 1265. (c) Wiebenga. E. H.; Havinga, E. E.; Boswijk, K. H. Adv. horg. Chem. Radiochem. 1961, 3, 133. (d) Bersohn, R. J. Chem. Phys. 1962, 36, 3445. (e) Jortner, J.; Rice, S. A.; Wilson, E. G. Ibid. 1963, 38, 2302. (1) Bersohn, R. /bid. 1963, 38, 2913. (9) Pitzer. K. S.Science 1963, 739, 414. (h) Malm, J. G.; Selig, H.; Jortner, J.; Rice, S. A. Chem. Rev. 1965, 65, 199. (i) Coulson. C. A. J. Chem. Phys. 1966, 44,468. (28) Tolles, W. M.; Gwinn. W. D. J. Chem. Phys. 1962, 36, 1119. (29) Muetterties, E. L.; Schunn, R. A. 0.Rev., Chem. SOC.1966, 20, 245. (30) Gregory, A. R . Chem. Phys. Lett. 1974, 28, 552.
Covalent Attachment of Arenes to SnOz-Semiconductor Electrodes Marye Anne Fox,* Frederic J. Nobs, and Tamara A. Voynick Contributionfrom the Department of Chemistry, University of Texas at Austin, Austin, Texas 7871 2. Received August 14, 1979
Abstract: The synthesis and chemical and physical properties of some arene-derivatized tin oxide semiconductor electrodes are described. Several techniques for attachment are employed (esterification, silanation, silanation/amidation, and the use of cyanuric chloride as a linking agent) and the merits of each are evaluated. The relationship between the observed properties of the attached arenes and th'e potential utility of the derivatized electrodes as anodes in photogalvanic cells is discussed.
Introduction Covalent modification of semiconductor electrodes used in photoelectrochemical cells has been studied recently as a 0002-7863/80/ 1502-4029$01.OO/O
method for improving electrode stability,' for opening new electrocatalytic pathways,'V2 and for attaching absorptive sensitizing dye^.^-^ Our own interest in electron injection from
0 I980 American Chemical Society
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excited carbanions' prompted us to compare several methods for attaching arenes to semiconductors. If reliable methods for varying the separation between an attached molecule and a flat electrode surface become available, valuable information on the geometry required for efficient dye sensitization would become accessible. We describe in this paper our efforts at attaching several model aromatic compounds of varying reduction potential to a tin oxide semiconductor electrode.
Results Arene Derivatives. The photochemistry and electrochemistry of appropriate arene derivatives were first examined. Friedel-Crafts acylation of anthracene with the half-ester acid chloride of adipic acid (1) afforded a mixture of methyl ester 2a (60%) and its 9-anthryl isomer 3a (5%).
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Covalent Modification of SnO2 Electrodes. The appropriate arene derivatives were attached to antimony-doped tin oxide coated NESA glasses (PPG Industries). The modification of these pretreated, polycrystalline surfaces to give electrodes of type 9-19 was attempted (Ar = aryl).
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The phosphorescence spectrum of 2a shows an intense band at 42.6 kcal/mol and a highly structured band at 69.5 kcall mol. These were assigned as normal phosphorescence* and delayed fluorescence,9 respectively, by analogy to reported anthryl derivatives. The fluorescence spectrum of 2a (Ae,,, = 454 nm, t1/2 = 220 ps) confirmed the latter assignment. The triplet lifetime of 2a (2-MeTHF, 77 K) was 23.5 ms. Upon photolysis at 254 or ca. 300 nm, 2a gave a mixture of at least three dimers10 by a low quantum yield pathway. Irradiation with longer wavelength light (>350 nm) gave no detectable reaction and attempts to sensitize a triplet reaction in 2a with b e n d , eosin, or pyrene led to no discernible reaction. By cyclic voltammetry (reduction of 2a at Pt in anhydrous CH3CN), a reversible one-electron wave was observed at 1.95 eV, while a second nearly irreversible wave was found at -2.55 eV (vs. SCE). Although the radical anion appeared to be stable on a cyclic voltammetric scale (5 s), preparative electrochemical reduction of 2a (1 faraday) in acetonitrile led to a complex mixture of products. Among these were anthracene and a compound tentatively identified as 4, which was obtained independently by the Clemmensen reduction of 2a. 4 presumably arises via initial reduction of ketone 2a to an alcohol, analogues of which have been reduced electrochemically." l-Naphthyltrichlorosilane12 ( 5 ) was prepared by the attack of 1aaphthylmagnesium bromide on tetrachlorosilane. Cyclic voltammetry of 5 at Pt in anhydrous DMF revealed two reversible reduction waves at -0.48 and -2.55 eV (vs. SCE). The solvolysis product of 5, 6,13 gave a single, reversible cathodic wave at -2.52 eV (vs. SCE). 5 is much more photosensitive than 6. After photolysis of a T H F ( low2 M) solution of 6 with low-pressure mercury lamps (254 nm) for 30 min, some traces of polymer and naphthalene could be detected but 6 could be recovered largely (
