6616
Langmuir 1996, 12, 6616-6626
Synthesis and Surface-Confined Electrochemistry of Dimethoxynaphthalene Fused through Rigid Norbornylogous Spacers to Thiolate, Dithiolate, and Disulfide Groups Timothy T. Wooster, Paul R. Gamm, and William E. Geiger* Department of Chemistry, University of Vermont, Burlington, Vermont 05405
Anna M. Oliver, Andrew J. Black, Donald C. Craig, and Michael N. Paddon-Row* School of Chemistry, University of New South Wales, Sydney, 2052 Australia Received September 16, 1996X A series of compounds (1-8) has been prepared in which a 1,4-dimethoxynaphthalene (DMN) group and either two thiol groups or a disulfide moiety are attached to the respective termini of rigid “norbornylogous” hydrocarbon bridges, 3, 5, 7, 9, and 13 bonds in length. Each of the molecules forms an adsorbate monolayer when its CH2Cl2 solutions are exposed to etched gold. Chemically reversible one-electron oxidation of the DMN group is observed in CH2Cl2 or CH3CN at potentials of 0.54-0.67 V vs ferrocene. Repeated potential cycling demonstates less monolayer degradation (a) in CH2Cl2 compared to CH3CN and (b) at subambient temperatures. Fast cyclic voltammetry experiments establish that the electron-transfer rates between the naphthalene group and the electrode surface are very fast (>103 s-1).
Introduction The modification of Au electrode surfaces is being widely exploited by means of covalent attachment of molecules, especially long chain alkane thiols1,2 which self-assemble on gold surfaces.3-13 Consistent with the goals14 of research on chemically modified electrodes, variations of the electroactive group in the attached molecule may be employed in order to achieve chemical specificity. Relatively few types of electroactive headgroups have, however, been attached through thio tethers to gold,5,6,15-18 the majority of studies using the ferrocene/ferrocenium X Abstract published in Advance ACS Abstracts, December 15, 1996.
(1) Ulman, A. An Introduction to Ultra Thin Organic Films from Langmuir-Blodgett to Self-Assembly; Academic Press: New York, 1991. (2) Bard, A. J. Integrated Chemical Systems; John Wiley and Sons: New York, 1994; Chapter 4. (3) Nuzzo, R. G.; Allara, D. J. Am. Chem. Soc. 1983, 105, 4481. (4) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J. Am. Chem. Soc. 1987, 109, 3559. (5) Willner, I.; Katz, E.; Riklin, A.; Kasha, R. J. Am. Chem. Soc. 1992, 114, 10965. (6) Sabatini, E.; Cohen-Boulakia, J.; Bruening, M.; Rubenstein, I. Langmuir 1993, 9, 2947. (7) Bain, C. D.; Troughton, E. B.; Too, Y.-T.; Evall, J.; Whitesides, G. M.; Nuzzo, R. G. J. Am. Chem. Soc. 1989, 111, 321. (8) Strong, L.; Whitesides, G. M. Langmuir 1988, 4, 546. (9) Chidsey, C. E. D.; Liu, G.-Y.; Rowntree, P.; Scoles, G. J. Chem. Phys. 1989, 91, 4421. (10) Finklea, H. O.; Robinson, L. R.; Blackburn, A.; Richter, B.; Allara, D.; Bright, T. Langmuir 1986, 2, 239. (11) Uosaki, K.; Sato, Y.; Kita, H. Langmuir 1991, 7, 1510. (12) Caldwell, W. B.; Chen, K.; Mirkin, C. A.; Babinec, S. J. Langmuir 1993, 9, 1945. (13) Chidsey, C. E. D. Science 1991, 251, 919. (14) Murray, R. W. In Electroanalytical Chemistry; Bard, A. J., Ed.; Marcel Dekker: New York, 1984; Vol. 13, p 191. (15) Curtin, L. S.; Peck, S. R.; Tender, L. M.; Murray, R. W.; Rowe, G. K.; Creager, S. E. Anal. Chem. 1993, 65, 386 and references therein. (16) Yip, C. M.; Ward, M. D. Langmuir 1994, 10, 549. (17) (a) Rojas, M. T.; Kro¨niger, R.; Stoddart, J. F.; Kaifer, A. E. J. Am. Chem. Soc. 1995, 117, 336. (b) Bharathi, S.; Yegnaraman, V.; Rao, G. P. Langmuir 1993, 9, 1614. (c) Zhang, L.; Lu, T.; Gokel, G. W.; Kaifer, A. E. Langmuir 1993, 9, 786. (18) Black, A. J.; Wooster, T. T.; Geiger, W. E.; Paddon-Row, M. N. J. Am. Chem. Soc. 1993, 115, 7924.
S0743-7463(96)00902-X CCC: $12.00
couple13,15,19-21 or the [Ru(NH3)5L]2+/3+ couple22-24 (L ) derivatized pyridine or thiol). In expanding the types of electrochemically active functionalities in these systems, it is desirable to include headgroups which give rise to reversible redox processes in nonaqueous solvents. The present work reports the synthesis and surface electrochemical properties of a new class of molecules with a 1,4-dimethoxynaphthalene (DMN) headgroup which undergoes reversible one-electron oxidation in nonaqueous media. Naphthalene and its derivatives and congeners are attractive headgroups not only from the viewpoint of their electron-transfer22,26 and optical27 properties but also because these groups are known to participate in other important phenomena such as donor-acceptor interactions and intercalation processes.28 (19) Hickman, J. J.; Ofer, D.; Zou, C.; Wrighton, M. S.; Laibinis, P. E.; Whitesides, G. M. J. Am. Chem. Soc. 1991, 113, 1128. (20) Creager, S. E.; Hockett, L. A.; Rowe, G. K. Langmuir 1992, 8, 854. (21) Chidsey, C. E. D.; Bertozzi, C. R.; Putvinski, T. M.; Mujsce, A. M. J. Am. Chem. Soc. 1990, 112, 4301. (22) Obeng, Y. S.; Laing, M. E.; Friedli, A. C.; Yang, H. C.; Wang, D.; Thulstrup, E. W.; Bard, A. J.; Michl, J. J. Am. Chem. Soc. 1992, 114, 9943. (23) Finklea, H. O.; Hanshew, D. D. J. Am. Chem. Soc. 1992, 114, 3174. (24) Ravenscroft, M. S.; Finklea, H. O. J. Phys. Chem. 1994, 98, 3843. (25) Zweig, A.; Maurer, A. H.; Roberts, B. G. J. Org. Chem. 1967, 32, 1322. (26) (a) Paddon-Row, M. N.; Cotsaris, E.; Patney, H. K. Tetrahedron 1986, 42, 1779. (b) Patney, H. K.; Paddon-Row, M. N. Synthesis 1986, 326. (c) Paddon-Row, M. N.; Patney, H. K. Synthesis 1986, 328. (d) Craig, D. C.; Lawson, J. M.; Oliver, A. M.; Paddon-Row, M. N. J. Chem. Soc., Perkin Trans. 1 1990, 3305. (e) Antolovich, M.; Oliver, A. M.; Paddon-Row, M. N. J. Chem. Soc., Perkin Trans. 2 1989, 783. (f) Golka, A.; Keyte, P. J.; Paddon-Row, M. N. Tetrahedron 1992, 48, 7663. (g) Craig, D. C.; Oliver, A. M.; Paddon-Row, M. N. J. Chem. Soc., Perkin Trans. 1 1993, 197. (h) Atkinson, E. J.; Oliver, A. M.; Paddon-Row, M. N. Tetrahedron Lett. 1993, 34, 6147. (i) Lawson, J. M.; Paddon-Row, M. N. J. Chem. Soc., Chem. Commun. 1993, 1641. (j) Oevering, H.; Paddon-Row, M. N.; Heppener, H.; Oliver, A. M.; Cotsaris, E.; Verhoeven, J. W.; Hush, N. S. J. Am. Chem. Soc. 1987, 109, 3258. (27) Mathauer, K.; Frank, C. W. Langmuir 1993, 9, 3002. (28) (a) Foster, R. Organic Charge Transfer Complexes; Academic Press: New York, 1969. (b) Wilson, W. D.; Johes, R. L. In Intercalation Chemistry; Whittingham, M. S., Jacobsen, A. J., Eds.; Academic Press: New York, 1982; Chapter 14.
© 1996 American Chemical Society
Synthesis of DMN with Rigid Norbornylogous Spacers
Langmuir, Vol. 12, No. 26, 1996 6617 Chart 1
A strategy of this work, similar to that recently employed by Obeng et al.,22 was to introduce rigidity into the “alkyl” backbone of the connecting chain. This could eliminate (1) the folding of the backbone at defect sites and (2) the need for chain-to-chain interactions (i.e., self-assembly) to define distances between the electrode surface and the naphthalene electron-transfer site. The presently described molecules use rigid “norbornylogous” bridges comprising linearly fused norbornane and cyclobutane rings29 to separate the tethering group from the electroactive group. As adsorbates, these molecules may prove useful in investigating the distance dependence of the rates of electron-transfer reactions. This subject has received relatively little attention regarding ordered electrode surfaces23 compared to the large body of general literature on the subject.31 Besides introduction of the dimethoxynaphthalene group as the redox site and the norbornylogous bridges as spacer groups, a third aspect of the molecular architecture was provision of two surface attachment points by means of proximal thiol groups or a disulfide group. It was hoped that this would enhance the adsorptive stability of the molecule. A recent study of thiol desorption processes has suggested that multiple attachment sites might indeed significantly enhance the stabilities of S-tethered films.32 This paper reports the preparation of a series of compounds (1-8) in which the DMN group and either a disulfide moiety or one or two thiol groups are attached (see Charts 1 and 2 for idealized structures) to the respective termini of norbornylogous bridges 3, 5, 7, 9, and 13 bonds in length. Full experimental details are in the Supporting Information. These compounds adsorb strongly onto gold and, in the cases studied, platinum electrodes. The surface-bound species give rise to reversible redox processes in CH2Cl2 and CH3CN media. A preliminary account of results on one member of this series (2) has appeared.18 (29) Paddon-Row, M. N. Acc. Chem. Res. 1994, 27, 18. (30) (a) Li, T. T.-T.; Weaver, M. J. J. Am. Chem. Soc. 1984, 106, 6107. (b) Forster, R. J.; Faulkner, L. R. J. Am. Chem. Soc. 1994, 116, 5453. (c) Carter, M. T.; Rowe, G. K.; Richardson, J. N.; Tender, L. M.; Terrill, R. H.; Murray, R. W. J. Am. Chem. Soc. 1995, 117, 2896. (d) Feldberg, S. W. J. Electroanal. Chem. 1986, 198, 1. (31) For leading references, see (a) Newton, M. D. Chem. Rev. 1991, 91, 767. (b) Jordan, K. D.; Paddon-Row, M. N. Chem. Rev. 1992, 92, 395. (c) Wuttke, D. S.; Bjerrum, M. J.; Winkler, J. R.; Gray, H. B. Science 1992, 256, 1007. (32) Schlenoff, J. B.; Li, M.; Ly, H. J. Am. Chem. Soc. 1995, 117, 12528.
Chart 2
Experimental Section Crystallography. Crystal Data for 4. C27H28O2S2, M 448.6, monoclinic, space group P21/c; a 17.376(1), b 6.4772(4), c 23.612(1) Å; β 125.354(2)°; V 2167.5(2) Å3, Dc 1.37 g cm-3, Z 4, µCu 23.49 cm-1; crystal size 0.08 × 0.20 × 0.34 mm; 2θmax 140°, min and max transmission factors 0.52 and 0.84. The number of reflections was 3065, considered observed out of 4086 unique data, with Rmerge 0.020 for 75 pairs of equivalent hk0 reflections. Final residuals R and Rw were 0.041 and 0.060 for the observed data. Crystal Data for 27. C27H28O4, M 416.5, triclinic, space group P1 h ; a 8.466(1), b 11.298(2), c 11.563(2) Å; R 98.823(9)°, β 98.257(9)°, γ 104.043(8)°; V 1041.1(3) Å3, Dc 1.33 g cm-3, Z 2, µCu 6.65 cm-1; crystal size 0.04 × 0.20 × 0.38 mm; 2θmax 130°, min and max transmission factors 0.82 and 0.97. The number of reflections was 2168, considered observed out of 3546 unique data. Final residuals R and Rw were 0.046 and 0.059 for the observed data. Structure Determination. Reflection data were measured with an Enraf-Nonius CAD-4 diffractometer in θ/2θ scan mode using nickel-filtered copper radiation (λ 1.5418 Å). Data were corrected for absorption using the analytical method of de Meulenaer and Tompa.33 Reflections with I > 3σ(I) were considered observed. The structures were determined by direct phasing and Fourier methods. Hydrogen atoms were included in calculated positions and were assigned thermal parameters equal to those of the atom to which they were bonded. Positional and anisotropic thermal parameters for the non-hydrogen atoms were refined using full-matrix least-squares. Reflection weights used were 1/σ2(Fo), with σ(Fo) being derived from σ(Io) ) [σ2(Io) + (0.04Io)2]1/2. The weighted residual is defined as Rw ) (Σw∆2/ΣwFo2)1/2. Atomic scattering factors and anomalous dispersion parameters were from International Tables for X-ray Crystallography.34 Structure solutions were by MULTAN80,35 and refinement used BLOCKLS, (33) De Meulenaer, J.; Tompa, H. Acta Crystallogr. 1965, 19, 1014. (34) Ibers, J. A., Hamilton, W. C., Eds. International Tables for X-ray Crystallography; Kynoch Press: Birmingham, 1974; Vol. 4. (35) Main, P. MULTAN80; University of York: England, 1980.
6618 Langmuir, Vol. 12, No. 26, 1996
Wooster et al.
Figure 1. ORTEP diagram of molecule 4.
Figure 2. ORTEP diagram of molecule 27. a local version of ORFLS.36 ORTEP-II37 running on a Macintosh IIcx was used for the structural diagrams, and a DEC AlphaAXP workstation was used for calculations. The structures and atom-numbering schemes are shown in Figures 1 and 2. Material deposited with this journal comprises all atom and thermal parameters, interatomic distances, angles and torsional angles, and observed and calculated structure factors. Electrochemistry. [NBu4][PF6] was prepared by metathesis of [NBu4]I and [NH4][PF6] in acetone/water, thrice recrystallized from 95% EtOH, and vacuum-dried at 373 K. Reagent-grade dichloromethane was refluxed for at least 2 days over CaH2 before being stored under vacuum. It was vacuum-transferred into dry receiving bulbs as needed. HPLC-grade acetonitrile was predried with type 4A molecular sieves and then stored over and vacuum-distilled from CaH2. CH3(CH2)17SH (Aldrich) was used as received. Except for a glassy carbon disk electrode (Bioanalytical Systems), the electrodes for this study were constructed at the University of Vermont. Gold wire of at least 99.99% purity (Johnson-Matthey) was sealed either in an insulating epoxy (Shell EPON 825, m-phenylenediamine curing agent) for 1 mm diameter wire and cut flat to give a 1 mm disk in a 6 mm insulating plane or in soft glass for 125 µm diameter wire. The latter was flattened on a grinding wheel leaving a nominally 125 µm Au disk. Both electrode surfaces were further prepared by successive polishings with 600-10 µM alumina/water slurries before being subjected to electrochemical pretreatments. Immediately prior to use these electrodes were hand-polished successively with 10, 5, 1, and 0.3 µm alumina, being rinsed with purified water between polishings. Finally the electrodes were etched in dilute aqua regia (4:3:1 H2O:HCl:HNO3) for 3 min before final rinses with water and absolute EtOH. Now ready for measurements, the electrodes were immediately transferred to the antechamber of a drybox and kept under N2. Pt microdisks of 500 and 125 µm diameter were pretreated with alumina polishings followed by rinsings with nanaopure water. The areas used in surface coverage calculations were the electrochemical areas (A) measured in calibration experiments using 0.89 mM ferrocene in CH3CN/0.1 M [NEt4][ClO4] at 273 K, for which a diffusion coefficient of 1.7 × 10-5 cm2/s was assumed.38 With electrodes of diameter > 0.5 mm, chronoamperometry was employed (step times 3-15 s) and the Cottrell equation used to calculate A.39 For microdisks with diameter < (36) Busing, W. R.; Martin, K. O.; Levy, H. A. ORFLS; Oak Ridge National Laboratory: Oak Ridge, TN, 1962. (37) Johnson, C. K. ORTEP-II; Oak Ridge National Laboratory: Oak Ridge, TN, 1976. (38) Hershberger, J. W.; Klingler, R. J.; Kochi, J. K. J. Am. Chem. Soc. 1983, 105, 61. (39) Bard, A. J.; Faulkner, L. R. Electrochemical Methods; John Wiley: New York, 1980; p 143.
Figure 3. Coverage vs soak time plot of 2 on Au. At least 10 continuous CV cycles were performed at ν ) 0.2 V/s in CH2Cl2 at 243 K prior to scan used in calculation. 0.5 mm, steady-state voltammetry was used, the ν-independent plateau current being used to obtain A (ν 1-5 mV/s).40 For the set of larger electrodes, the electrochemically calculated areas were within 10% of the geometrically calculated areas based on the nominal diameter of the wire. Larger differences were noted, however, for the set of small diameter disks: Pt disk from 125 µm diameter wire, 1.00 × 10-4 cm2 (nominal 1.23 × 10-4 cm2); two Au disks from 125 µm diameter wire, 1.85 × 10-4 and 2.65 × 10-4 cm2 (nominal 1.23 × 10-4 cm2). A PARC Model 173 potentiostat with a Model 176 currentto-voltage convertor (rise time 1 µs) was used in conjunction with a Model 175 function generator to perform voltammetry experiments. CV scans were recorded on several devices, including a Yokogawa X/Y recorder (Model 302313) and a Nicolet Model 4094C digital oscilloscope. Conductivity measurements were performed with a Beckman Model C-18 conductivity bridge. The conductivity cell (Model 3402, Yellow Springs Inst. Co.) was calibrated with 0.01 M KCl. Integration of voltammograms was accomplished with a Summagraphic board and area calculational software locally written. Protocol for Monolayer Studies. Solutions of the thiols, dithiols, and disulfides were handled inside a drybox under N2. In the case of the disulfides, the measurements were conducted under low-light levels since they appeared to deteriorate slowly upon exposure to sunlight and air. The precision ((0.1 °C) of the solution temperature was set by immersion of the electrochemical cell in a heptane bath controlled by an FTS Systems temperature controller. Monolayer formation was accomplished by immersing a bare electrode in a freshly prepared soak solution (1.0 mM in adsorbent in CH2Cl2). After a given period of time the electrode was removed from the soak solution, washed with pure CH2Cl2, and immersed in an electrolyte solution of either CH2Cl2 or CH3CN containing 0.1 M or greater [NBu4][PF6] for electrochemical analysis. Soak periods varied from minutes to hours, as discussed in the Results section. When experiments were carried out under less rigorous conditions (e.g., under N2 flow on the bench top), significant interferences from anodic background features were observed. The presence of trace H2O and/or other nucleophiles was thought to be responsible for these features which disappeared if the electrochemical test solution was stirred over 1 g Super-I neutral alumina (ICN Biomedicals). Surface Coverage Measurements. Surface coverages, Γ, were calculated by integration of the voltammetric currents (vs time) in CV scans. The separate values obtained in each scan from the anodic and cathodic responses are referred to as the anodic and cathodic areas, respectively, Γa and Γc. The most positive excursion of the anodic wave (ca. +1 V) was terminated on the foot of a second, irreversible wave which (if included) led to apparent destruction of the monolayer. (40) (a) Amatore, C. A.; Saveant, J. M.; Tessier, D. J. Electroanal. Chem. 1983, 147, 39. (b) Aoki, K.; Akimoto, K.; Tokuda, K.; Matsuda, H.; Osteryoung, J. G. J. Electroanal. Chem. 1984, 171, 219.
Synthesis of DMN with Rigid Norbornylogous Spacers Scheme 1
Electron-Transfer Rate Measurements. The standard electron-transfer rates, ks, were measured by monitoring the manner in which peak potentials (Ep) shifted with scan rate (ν), using the correlation either of Ep with log ν or of ∆Ep with ν to obtain ks, as discussed in the Results section.41 Both methods require Ep values which are not significantly influenced by ohmic losses (iRu) arising from the uncompensated electrolyte resistance (Ru). This is a major limitation when measuring large ks values in resistive solvents.42 We employed small working electrodes (ca. 125-500 µm diameter disks) in order to minimize faradaic currents. Furthermore, we corrected each voltammogram for iRu loss by the method of Bowyer and Evans.43 The correction procedure recognizes that the charging current influences the effective scan rate, a consequence that must be accounted for if accuracy is desired in cases where the charging current is large. Values of Ru used in the correction procedures were calculated from the equation for disk electrodes placed at least 5 times the disk diameter from the reference electrode: Ru ) F/2d, where F is the solution resistivity and d is the disk diameter.44 Resistivities used in this equation were measured at 298 K as 0.3 M [NBu4][PF6] in CH3CN, 47 Ω‚cm (lit.45 49.7); 0.1 M [NBu4][PF6] in CH3CN, 102 Ω‚cm (lit.45 109); 0.5 M [NBu4][PF6] in CH2Cl2, 168 Ω‚cm. The rise time of the electrochemical cell was less than 100 µs.
Results Syntheses. Syntheses of the various mercaptomethyl and disulfide compounds were readily achieved using standard bridge-building techniques.46 The synthetic procedures that were used are outlined in Schemes 1-5. The depicted stereochemistry of 1-3, 6, and 7 is based on solid precedent;26 these compounds were synthesized as racemic mixtures and used as such. Complete synthetic details are provided as Supporting Information. The depicted exo stereochemistry of the dithiane ring in 4 and the mercaptomethyl group in 5 are predicted on the basis of the expected stereochemical outcome of the respective Diels-Alder reactions used in their syntheses (Scheme 3). The exo configuration of these molecules was confirmed from the single-crystal X-ray structures of 4 and 27, ORTEP projections of which are displayed in Figures 1 and 2, respectively. The dithiane ring in 4 adopts a boat conformation possessing approximate local Cs symmetry; the dihedral angles C26-S1-S2-C27 and C26-C2-C3-C27 are 10.3° and 3.4°, respectively. The S-S and C-S bond lengths are 2.070 and 1.805 Å, respectively. General Adsorption Characteristics. Each of the sulfur-containing compounds adsorbs rapidly onto polycrystalline gold or platinum electrodes20,47 from CH2Cl2 soak solutions. In less than 1 min there is a well-formed CV wave when these electrodes are removed from the (41) Laviron, E. J. Electroanal. Chem. 1979, 101, 19. (42) Milner, D. F.; Weaver, M. J. Anal. Chim. Acta 1987, 198, 24. (43) Bowyer, W. J.; Engelman, E. E.; Evans, D. H. J. Electroanal. Chem. 1989, 262, 67. (44) Newman, J. J. Electrochem. Soc. 1966, 113, 501. (45) (a) House, H. O.; Feng, E.; Peet, N. P. J. Org. Chem. 1971, 36, 2371. (b) Kadish, K. M.; Ding, J. Q.; Malinski, T. Anal. Chem. 1984, 56, 1741. (46) Oliver, A. M.; Paddon-Row, M. N. J. Chem. Soc., Perkin Trans. 1 1990, 1145. (47) Creager, S.; Rowe, G. Langmuir 1991, 7, 2307.
Langmuir, Vol. 12, No. 26, 1996 6619
soak solution, rinsed with CH2Cl2, and immersed in an electrolyte blank consisting of either CH2Cl2 or CH3CN containing 0.1 M [NBu4][PF6]. When the electrode is put back into the soak solution, coverage continues to increase over 1-2 h, depending on the compound, until an apparent plateau is reached (Figure 3). Coverage statistics were affected somewhat by the measurement tactics, owing, we believe, to minor adsorbate stripping that accompanies the voltammetric scans. Both anodic and cathodic peak currents were proportional to scan rate over a wide range of ν, as befits a surface-bound species.48 Full diagnostics are available.49 Each adsorbed compound exhibited an anodic and coupled cathodic wave close to those reported for freely diffusing dimethoxynaphthalene (E° ) 0.70 V),50 showing that the electroactivity of the films arises from oxidation of the naphthyl headgroup. No reduction of the film was seen to -2.3 V, the negative potential limit investigated. The oxidation of the naphthyl headgroup is chemically reversible under dry solvent conditions. The susceptibility of oxidized aromatic hydrocarbons to trace water has long been known51 and is apparently still a problem when the hydrocarbon cation radical is generated in the surfacebound molecule. A second oxidation wave at Epa (ca. 0.95 V) was irreversible. Scanning through the second oxidation rendered the film electroinactive, so scans into the second oxidation wave were generally avoided. Better voltammetric behavior was observed in CH2Cl2 than in CH3CN, especially regarding the chemical reversibility of the couple and the lessening of the electrochemical response with each succeeding scan. The quality of the CV responses improved at lower temperatures, so most CV scans were obtained at 273 or 243 K. The potentials measured for the species involved in this investigation are collected in Table 1. Because compound 2 was studied in greatest detail, results on this system are presented first. Adsorption Characteristics of 2 on Au. Figure 4 shows typical CV traces (in CH2Cl2 at 243 K) of the dithiol 2 adsorbed on gold. The voltammograms deviate from those of an ideal film48,52 in that (1) ∆Ep values reach a lower limit of ca. 54 mV (theory: 0 mV) and (2) the anodic and cathodic components are both broader than expected. Moreover, the anodic branch was broader than its cathodic counterpart, with the differences being somewhat dependent on surface coverage. Even the narrowest waves (fwhm ca. 120 mV) exceeded the ideal peak width52 (74 mV at T ) 243 K). Amont the causes of peak broadening47,53 is the possibility that there is a distribution of adsorbate sites in which the DMN headgroups differ slightly in their E° values.54 The chemical reversibility of the couple was greater in CH2Cl2 than in CH3CN. Chemical reversibility in CH3CN was, however, sufficient to allow electron-transfer rates to be measured in this solvent. Solvent purity plays an important role in the CV characteristics of the adsorbed films. It is very important (48) Reference 39, pp 521 ff. (49) Gamm, P. M.S. Thesis, University of Vermont, Burlington, VT, 1994. (50) In ref 25 the E1/2 of 1,4-dimethoxynaphthalene is reported as +1.10 V vs aqueous SCE in CH3CN/0.1 M [NPr4][ClO4]. Subtraction of 0.40 V (ferrocene vs SCE in CH3CN) yields the value quoted. (51) Perichon, J. In Encyclopedia of Electrochemistry of the Elements; Bard, A. J., Ed.; Marcel Dekker: New York, 1978; Vol. XI, pp 136 ff. The radical cation of 1,4-dimethoxynaphthalene is reported to have a half-life of 0.2 s in DMF at room temperature (ref 25). (52) Laviron, E. J. Electroanal. Chem. 1974, 52, 355, 395. (53) (a) Rowe, G. K.; Creager, S. E. J. Phys. Chem. 1994, 98, 5500. (b) Smith, C. P.; White, H. S. Anal. Chem. 1992, 64, 2398. (54) Rowe, G. K.; Carter, M. T.; Richardson, J. N.; Murray, R. W. Langmuir 1995, 11, 1797.
6620 Langmuir, Vol. 12, No. 26, 1996
Wooster et al. Scheme 2
Scheme 3
Scheme 4
that all solvents be dry and nucleophile-free. Handling of all solutions, including the soak solution, was accomplished within an inert atmosphere box to assure reproducible film characteristics. When poor response characteristics were obtained owing apparently to solution wetness (Figure 5), a clean response could be restored after addition of alumina to the solution, a procedure known to scavenge trace nucleophiles from nonaqueous solvents.56 In spite of the sensitivity of this couple to water, dry monolayers exposed to laboratory air for periods of tens of minutes showed no significant degradation when reimmersed in electrolyte solutions. (55) (a) Hammerich, O.; Parker, V. D. Electrochim. Acta 1973, 18, 537. (b) Jensen, B. S.; Parker, V. D. Chem. Commun. 1974, 367. (56) A referee has suggested that the discrepancy between the anodic and cathodic peak areas could also be caused by mediated oxidation of some impurity in the electrolyte.
The surface coverage of 2 is approximately that expected for a monolayer of the compound. When measured from the charge passed on the initial anodic sweep, coverages (3-5 × 10-10 mol/cm2) were found to be within the range of values measured for monolayers of the ferrocenyl-capped thiol Fc(CH2)16SH at the same electrodes and are consistent with literature data for monolayers of ferrocenecapped thiols.48 The coverage measured from the anodic branch (Γa) decreased significantly on second and subsequent scans, however. Continuous cycling of the potential gave a steady-state response in which Γa and Γc (the apparent coverage measured from the cathodic return wave) were generally ca. 2 × 10-10 mol/cm2. The monolayer voltammetric characteristics in Table 2 are all derived from steady-state voltammograms. We now consider how and why the ratio of Γa/Γc changes with the scan history of the film. Values of 2-3 are measured for the first CV scan after lengthy soak periods (several hours), whereas values of 1.2-1.3 are normal in steady-state voltammograms. A value of 1.0 is expected for an ideal monolayer undergoing reversible electron transfer. Measured values of Γa may be biased high because of the onset of the second oxidation wave and the baseline extrapolation involved in the measurement. This possible error is unlikely, however, to change significantly
Synthesis of DMN with Rigid Norbornylogous Spacers
Langmuir, Vol. 12, No. 26, 1996 6621 Scheme 5
Table 1. Formal Potentials (vs Ferrocene) of One-Electron Oxidations, Supporting Electrolyte ) [NBu4][PF6]a compd DMNd nor-DMNd 1 5 2 4 3 6 7 8
stateb
temp (K)
solvent
E°′ (V)c
soln soln soln ads ads soln ads ads soln ads ads soln ads ads soln ads ads soln ads ads soln ads ads
ambient ambient ambient 243 243 ambient 243 243 ambient 243 243 ambient 243 243 ambient 243 243 ambient 243 243 ambient 243 ambient
CH3CN CH3CN CH2Cl2 CH2Cl2 CH2Cl2
0.70 (ref 25) 0.70 (ref 26j) 0.67 0.67 0.54 0.52 0.57 0.54 0.54 0.57 0.53 0.52 0.57 0.54 0.52 0.56 0.64 0.63 0.67 0.52 0.51 0.58 0.55
CH3CN CH2Cl2 CH3CN CH2Cl2 CH3CN CH2Cl2 CH3CN CH2Cl2 CH3CN CH2Cl2 CH3CN CH2Cl2
a Concentration ) 0.3 M in CH CN, 0.5 M in CH Cl . b Adsorbed 3 2 2 on Au or in solution at glassy carbon. c Calculated from (Epa + d Epc)/2. DMN ) 1,4-dimethoxynaphthalene; nor-DMN ) 1,2,3,4tetrahydro-1,4-methano-9,10-dimethoxyanthracene (structure 26).
with different coverage levels. Rather, two possibilities arising from variations in the adsorption mode of 2 are more likely to be responsible: either oxidatively induced adsorption is occurring on the anodic scan or oxidation leads to loss of more weakly bound adsorbates from the electrode surface.56 Addressing the first possibility, it is feasible that molecules of 2 are adsorbed through attachment of just one sulfur to the surface, leaving a free SH group near the electrode surface. Oxidation of the SH group might release a proton, giving rise to a doubly S-bound molecule which then would be oxidized at the DMN group like other doubly bound molecules. This oxidative adsorption would in-
Figure 4. CV scans (ν ) 0.2 V/s) in CH2Cl2/0.5 M [Bu4N][PF6] at 243 K of monolayer of 2 on Au: (top) after ca. 60 min soak in 1 mM solution of 2 in CH2Cl2 and (bottom) after exposing the same coated electrode to air for 30 min.
crease the value of Γa/Γc over that expected for simple oxidation and rereduction of the DMN headgroup. The similarity, however, of the results for 2 with those for the monothiol system 5 (see below), which cannot undergo this process, appears to eliminate oxidative adsorption as an important mechanism for the dithiol systems. A more likely explanation for increases in Γa/Γc with long soak times and in initial CV scans is that dithiol molecules may bind to the gold surface with different energies, some molecules being bound less strongly than others. If oxidation of the weakly bound molecules facilitates their surface release, the decreased cathodic response may be rationalized. Other observations that are consistent with this “strong site, weak site” model will be pointed out below. Blockage Experiments Using 2-Coated Electrodes. Tests were performed to see if gold electrodes treated with the dithiol impeded electron-transfer processes that are normally facile57 at a bare gold electrode. When immersed in a 0.5 M H2SO4 solution, the electrode treated with 2
6622 Langmuir, Vol. 12, No. 26, 1996
Wooster et al.
Figure 5. CV scans (ν ) 0.2 V/s) in CH2Cl2 at 243 K of monolayer of 2 on Au: (top) in “wet” CH2Cl2 and (bottom) after stirring the same solution over activated alumina.
Figure 6. Multiple-cycle CV scans (0.2 V/s) of monolayers of 2 on Au: (a) CH2Cl2 at 295 K, (b) CH2Cl2 at 243 K, (c) CH3CN at 295 K, and (d) CH3CN at 243 K. Table 2. Steady-Statea Coverage Parameters for Adsorbates on Au, T ) 243 K, Measured in CH2Cl2/0.5 M [NBu4][PF6] with ν ) 0.2 V/s; Γa and Γc Calculated by Integration of CV Responses of Anodic and Cathodic Waves, Respectively parent ∆Ep compd (mV)b 1 5 2 4 3 6 7 8 c
89 (11) 54 (3) 40 (7) 38 (7) 33 (7) 64 (3) 34 (4) 40
fwhmc (mV) 160 (10) 177 (8) 135 (12) 117 (7) 121 (1) 132 (1) 133 (7) 120
Γa Γc (×1010 mol/cm2) (×1010 mol/cm2) Γa/Γc 0.34 4.0 2.6 1.7 0.63 1.7 2.3 2.1
0.24 3.7 2.5 1.3 0.58 1.4 2.3 1.1
1.4 1.1 1.0 1.3 1.1 1.2 1.0 1.9
a Attained after about 10 cyclic scans. b RSD in parentheses. Full width at half-maximum of the cathodic response.
failed to retard the formation of AuO in CV scans; electrodes treated with CH3(CH2)17SH were much more efficient in the same electron-transfer blockage. The monolayer films of 2 on Au do not, therefore, deny access of small molecules to the electrode surface. This may be due to pinhole imperfections in the monolayer. The electrochemical response of freely diffusing ferrocene was also tested at coated and untreated electrodes. No apparent differences were observed in peak shapes, postions, or ∆Ep values in CV scans at ν ) 0.1 V/s. Adsorbed 2 therefore appears to be an effective electron(57) (a) Hamelin, A.; Lipkowski, J. J. Electroanal. Chem. 1984, 171, 317. (b) Engelsmann, K.; Lorenz, W. J.; Schmidt, E. J. Electroanal. Chem. 1980, 114, 1.
Figure 7. CV scans of monolayers on Au: (top) 9-bond system 7 at ν ) 0.2 V/s in CH2Cl2 at 243 K and (bottom) 13-bond system 8 at ν ) 50 V/s in CH3CN.
transfer mediator to redox-active molecules in solution, although the unaffected voltammetry of ferrocene may also arise from permeability of ferrocene to the electrode surface through pinholes. In contrast, electrodes in CH2Cl2 treated with long chain alkane thiols showed significant blockage of ferrocene voltammetry (unpublished work at University of Vermont) similar to that reported for aqueous solutions of (hydroxymethyl)ferrocene measured at electrodes containing thiol monolayers.20 Adsorption of 2 onto Pt. Compound 2 adsorbed readily onto polycrystalline Pt electrodes subjected to the same process described above for Au electrodes. The voltammetric behaviors of the resulting monolayers (peak shapes, peak separations, response to scan rate) were essentially indistinguishable between the two metals. As implied by this statement, the measured electron-transfer rates were virtually independent of electrode material (see below). There is precedent for adsorption of thiols onto Pt, although the mechanism of adsorption is not yet clear.58 Response of 2 in Acetonitrile. The voltammetry of 2 on Au in CH3CN suffered by comparison to that in CH2Cl2 in three ways: (i) increased interferences from spurious current responses, (ii) lower apparent chemical reversibility, and (iii) greater tendency for monolayer stripping after voltammetry scans. Figure 6 indicates typical differences in response of 2 on Au in the two solvents. In CH3CN (i) a “prewave” peak is observed on the first anodic scan, (ii) the relative cathodic response is lower than seen with CH2Cl2, and (iii) loss of voltammetric activity is greater when successive scans are performed in CH3CN. Observations (i) and (ii) are explicable in terms of the greater difficulty of ridding CH3CN of traces of nucleophiles, and (iii) is consistent with the higher polarity of CH3CN compared to CH2Cl2. Alkanethiols have been shown to detatch more readily in nonaqueous solvents of higher polarity.32 Monolayer instability, especially at room temperature, has been noted in other studies of thiotethered systems in CH3CN.15,16,19,58 In spite of the above difficulties, electron-transfer rate measurements were pursued in CH3CN because of the lower resistance inherent to its electrolyte solutions, and these results are quoted below in comparison to the results in CH2Cl2. (58) (a) Hickman, J. J.; Laibinis, P. E.; Auerbach, D.; Zou, C.; Gardner, T. J.; Whitesides, G. M.; Wrighton, M. S. Langmuir 1992, 8, 357. (b) Tsutsumi, H.; Furumoto, S.; Morita, M.; Matsuda, Y. J. Electroanal. Chem. 1992, 139, 1522. (c) Purcell, S. T.; Garcia, N.; Binh, V. T.; Jones, L., II; Tour, J. M. J. Am. Chem. Soc. 1994, 116, 11985. (d) Hines, M. A.; Todd, J. A.; Guyot-Sionnest, P. Langmuir 1995, 11, 493.
Synthesis of DMN with Rigid Norbornylogous Spacers
Langmuir, Vol. 12, No. 26, 1996 6623
Figure 8. CV scans (ν ) 0.2 V/s) of monolayer of monothiol system 5 on Au in CH2Cl2 at 295 K.
Adsorption Characteristics of 7 and 8. Compounds 7 and 8 gave monolayers on Au having voltammetric characteristics similar to those of 2. Figure 7 shows CVs in pure CH2Cl2 of an Au electrode with monolayers of 7 (at low ν) or 8 (at higher ν). After ca. 10 scans, these monolayers had close to ideal voltammetric characteristics, with Γc/Γa ≈ 1.03 and ∆Ep < 30 mV. These electrodes still suffered initial loss of electroactivity when scanned in CH3CN. Monothiol 5. As expected, the monothiol system 5 also adsorbs onto Au, giving an anodic peak at a potential appropriate to the oxidation of the naphthyl headgroup. Although its general responses to changes in solvents, temperatures, and scan rates mimic those of 2, the points of difference are instructive about the adsorbate/electrode interaction sites. Recall that for 2 the anodic wave was somewhat broader than the cathodic wave and that Γa > Γc, the difference between the two diminishing after each of the first few scans. These tendencies are exaggerated with 5, with large decreases in the anodic responses between the first and second scans (Figure 8) and a broad anodic wave (typically 170-180 mV) being seen even in steady-state voltammograms. Another important difference is that the overall coverages calculated for 5 are 1.5-2 times higher than those of 2. These data are consistent with there being some sites on the polycrystalline gold surface that are available (perhaps exclusively so) for bonding to one sulfur atom and that molecules bonded to these sites desorb more easily in nonaqueous media. This model suggests that the dithiol 2 may bond through one or two sulfurs, weaker adsorption being found for the former. Adsorption of Disulfides 3 and 4. The trans-disulfide 3 and the cis-disulfide 4 were studied. Compared to the dithiol 2 of the same structure, the disulfides showed somewhat slower adsorption onto Au, comparable loss of electrochemical activity with scans, and lower steadystate coverages. Taking the cathodically calculated coverages from Table 2, Γc (×1010) ) 0.6 for 3, 1.3 for 4, 2.5 for 2, and 2.3 for 7 (the two dithiols 2 and 7 have trans-SH groups). According to the criteria of peak shapes (fwhm ) 115-120 mV) and peak separations (25-40 mV at ν ) 0.2 V/s), the voltammetric behavior of the disulfides was closer to ideal than those of the dithiol. This fact is likely to be connected to the lower coverages seen for the disulfides and their inability to occupy electrode sites with a single sulfur. Adsorption onto Pt was also effective, with a nicely shaped CV response (Figure 9). Adsorption of 1 and 6. These compounds are discussed together because the cyclobutane ring is adjacent to the adsorbing tail rather than to the electroactive head, the latter being the case in the other compounds studied. An apparent consequence of this structural difference is that the oxidation of 1 and 6 is found at 0.64-0.67 V (Table
Figure 9. CV scans of disulfide system 4 on d ) 125 µm Pt disk in CH3CN at 243 K: (top) ν ) 5 V/s and (bottom) scan rates as shown, currents normalized for scan rate. The peak current was 10 µA at ν ) 500 V/s.
1), ca. 0.1 V positive of the potentials of 2-5 and 7. The former are consistent with the literature value for 1,4dimethoxynaphthalene itself25 and 1,2,3,4-tetrahydo-1,4methano-9,10-dimethoxyanthracene26j (both ca. 0.7 V). Fusing a cyclobutane group to the 2,3-positions of the DMN group significantly lowers the oxidation potential of the molecule. The adsorption characteristics of 1 were quite poor. Its coverages on gold were generally less than one-fifth of those seen for all other compounds, and ∆Ep and fwhm values were the largest of the series (ca. 90 mV at ν ) 0.2 V/s and 160 mV, respectively). The chemical reversibility was low, and the films were readily stripped. Compound 6 gave films on Au with behavior largely analogous to the fused DMN-cyclobutane systems 2 and 7 (Table 2) but showing a somewhat greater tendency than these compounds toward stripping upon oxidation. The chemical reversibility of 6 was sufficient, however, to allow electrontransfer rate measurements on the adsorbed species (see below). Competitive Adsorption Experiments. In a number of cases we investigated the electrochemical character of Au electrodes subjected to sequential soak regimens of different adsorbents. The results were informative about the relative affinities and rates of adsorption of thiol groups vs disulfides toward Au. Observations were restricted to 2-4, compounds having the same chain length but differing in the chemical functionality of the adsorption linkage. When either the cis- or trans-disulfide 4 or 3, respectively, was used in sequence with the dithiol 2, virtually identical results were obtained, as exemplified by Figure 10. The electrode was soaked in the disulfide solution for 75 min, over which time the coverage was checked five times in single scans (region I of Figure 10). Then the soak solution was changed to one of dithiol 2, and the
6624 Langmuir, Vol. 12, No. 26, 1996
Wooster et al. Table 3. Standard Electron-Transfer Rates for Adsorbates
Figure 10. Coverage vs soak time plots for sequence of soaks in solutions of 1 mM 4 in CH2Cl2 (region I) followed by immersion of same electrode in 1 mM 2 (region II) with subsequent reimmersion in 1 mM 4.
parent
electrode
solvent
temp (K)
log ksa
log ks(corr)b
methodc
5 2 2 2 2 2 4 3 6 7 8 8 8 + C18d
Au Au Pt Au Au Au Au Au Au Au Au Au Au
CH3CN CH3CN CH3CN CH2Cl2 CH2Cl2 CH2Cl2 CH3CN CH3CN CH3CN CH3CN CH2Cl2 CH3CN CH3CN
243 243 243 243 295 295 243 243 243 243 295 243 243
3.84 3.86 3.85 2.84 3.35 2.78 3.70 3.85 3.76 3.72 3.30 3.60 3.30
4.07 4.08 4.07 3.28 3.60 3.30 4.11 4.18 3.95 3.93 3.60 3.74 3.48
QR QR QR QR IR QR QR QR QR QR QR QR QR
a Electron-transfer rate (s-1) from data uncorrected for iR loss. u Electron-transfer rate (s-1) from data corrected for iRu loss. c ks calculation based on either: QR, ∆Ep values below 200 mV, or IR, Ep shifts when ∆Ep > 200 mV. d In the presence of 1 mM CH3(CH2)17SH. b
Figure 11. CV scans of monolayer of 2 on Au (d ) 150 µm) in CH2Cl2 at 295 K at various scan rates. Currents normalized for scan rate with current marker appropriate for 100 V/s scan.
coverage measurements were repeated (region II). Finally in region III the electrode was returned to the disulfide soak solution. Trends in the anodic and cathodic coverages were noted. The latter continued a slow but steady increase over the entire sequence of the experiment, whereas the former has three regions of essentially plateau values, that of region II (the dithiol region) being dramatically higher than those of regions I and III. Two variations on this experiment are worth noting: (a) sequential soaks of the cis- and trans-disulfides do not show the anodic increase in region II and (b) the sequence disulfide/dithiol/pure CH2Cl2 produced the same result as found in the disulfide/ dithiol/disulfide sequence of Figure 10. These results are discussed below in terms of monolayer sites which differ in adsorption energies.50 Electron-Transfer Rate Measurements. Each redox-active monolayer except 1 was investigated by cyclic voltammetry for the purpose of determining its standard rate constant, ks, defining the electron-transfer rate from the DMN headgroup to the electrode surface. Rate constants were obtained using Laviron’s calculations,52 which assume Butler-Volmer kinetics and a Langmuir adsorption isotherm. This method relies on the relationship between ks values and shifts in peak potentials with ν. At very high sweep rates the Ep shifts are certainly affected by ohmic errors in the solvents employed in this study. We attempted to compensate for ohmic effects by the correction procedure detailed in the Experimental
Figure 12. Peak potential vs scan rate for monolayer of 2 on Au (d ) 150 µm) in CH3CN at 243 K: (top) corrected for iRu effect and (bottom) uncorrected data. Lines give behavior calculated from ref 41 on basis of rate constants in Table 3.
Section. The iRu-corrected ks values are very large (≈104/ s), however, and near the practical limit of accurate measurement by cyclic voltammetry. With the possible exception of the 13-bond system 8, we view the ks values in Table 3 as lower limits to the true ks values. A set of CV scans is shown in Figure 11 (for 2 in CH2Cl2), and typical plots of Ep vs log ν are seen in Figure 12. Comparison of the raw data set (Figure 12 bottom) with the iRu-corrected set (top) indicates the utility of the ohmic correction through the fits of Epa and Epc vs log ν with theory52 (curved line) at high scan rates. At very large ν a linear region of Ep vs log ν is expected as the redox couple enters the totally irreversible charge-transfer regime. Extrapolation of the linear anodic and cathodic branches to their intersection point is one way to obtain ks.52 The values so obtained were acceptably close to those
Synthesis of DMN with Rigid Norbornylogous Spacers
computed from the quasi-reversible region (see following). Because of the extraordinarily high scan rates needed to attain the irreversible region, however, we have more confidence in the quasi-reversible treatment and report values from the latter in this paper. When ν is between ca. 10 and 500-1000 V/s for these systems, the difference in peak potentials (∆Ep) is less than 200 mV, and it is expected to vary with 1/m in a prescribed fashion [where m ) (RT/F)(ks/ν) for a oneelectron process]. Using a working curve for R ) 0.5,52 we obtained the values for ks listed in Table 3. Inspection of even the uncorrected ks values indicates that the electrontransfer rates are very high in these surface-bound molecules, approaching 104/s in CH3CN and 103/s in CH2Cl2. The ohmic-corrected data puts most of the values [designated as ks (corr)] above 104/s and closes the gap between rates in CH3CN and those in the more resistive medium of CH2Cl2. A significant limitation in the treatment of these systems by the Laviron method is that the redox-active monolayers do not display ideal peak splitting at very low ν, where ∆Ep ) 0 is expected. Rather, ∆Ep is virtually constant (ca. 30-50 mV) with ν at low ν. Calculation of ks from peak splittings was therefore performed on scans in which ∆Ep was between ca. 70 and 200 mV (see, for example, Figure 9). Nonideal ∆Ep values are frequently observed for surfacebound species.47,53,54 When these values are independent of ν, there is most likely a distribution of E°′ values of the redox-active site within the monolayer. The consequences of such a potential distribution on measurement of chargetransfer kinetics have recently been addressed for selfassembled monolayers.54 Within the context of a treatment assuming a distribution of E°′ values, the rate constants we report may be designated as ks(avg), an average for substrates in different surface environments. Finally, the ks values measured for the compound with the longest spacer, 8, deserve special attention. Excellent agreement between theory and experiment were obtained for ks ) 5.5 × 103 s-1 in CH3CN with ohmic losses too small to appreciably affect the measured rate constant. Furthermore, the iRu-corrected ks value in CH2Cl2 is virtually the same as the value in CH3CN. These observations lend confidence to the view that the ks values reported in Table 3 for 8 are its “true” values rather than lower limits. We repeated the high-speed CV experiments for several systems in the presence of alkanethiols which might be expected to close pinhole imperfections in the monolayer. Virtually identical peak splittings were obtained. The single exception was with 8. When a monolayer of 8 on Au was soaked for 1 h in a 1 mM solution of C18-alkanethiol and returned to a solution containing the same concentration of alkanethiol, somewhat increased peak separations were noted and the apparent ks value decreased to ca. 3 × 103 s-1. Discussion The present results show that adsorbates containing the DMN group undergo a reversible one-electron transfer reaction in dry CH2Cl2 or CH3CN, expanding options for attachment of electroactive moieties to electrode surfaces. Access to a diversity of electroactive headgroups promises to be important as the study of monolayers is extended more to nonaqueous solvents.15,16,19,23,31c,58-60 Compounds 1-8 are almost certainly attached to the electrode as thiolates, rather than through the DMN group. (59) Groat, K. A.; Creager, S. E. Langmuir 1993, 9, 3668. (60) Willicut, R. J.; McCarley, R. L. Langmuir 1995, 11, 296.
Langmuir, Vol. 12, No. 26, 1996 6625
We found no evidence of adsorption from nonaqueous solvents onto Au or Pt for molecules containing the 1,4dimethoxynaphthyl group (1,4-dimethoxynaphthalene itself and 1,2,3,4-tetrahydro-1,4-methano-9,10-dimethoxyanthracene) but lacking the thiol or disulfide group(s). Surface-enhanced Raman spectra are consistent with adsorption of 2 through one or more sulfur atoms in an orientation having the DMN group nearly perpendicular to the Au surface.61 Both these data and the surface coverage results (no systematic dependence of Γ on spacer length) are inconsistent with these molecules laying flat on the electrode surface. Coverage changes upon oxidation suggest that some molecules are bound more strongly to the surface than others; this is not surprising given the variations expected in the strengths of surface-adsorbate binding sites.62 The recent studies of Schlenoff et al.32 demonstrate an analogous phenomena in nonaqueous media. These workers explained that the self-exchange kinetics of labeled octadecanethiol on gold arise from a group of molecules that undergoes fairly rapid exchange with soluble C18thiol and a second group that undergoes comparatively slow exchange. Previous work has established that disulfides adsorb onto Au as thiolates after cleavage of the S-S bond.19,58a,63,64 In the present case, this attachment mode results in an adsorbate with two thiolate-gold bonds for each electroactive moiety. When compared to dithiolates 2 and 6-8 as well as the monothiolate 5, the disulfides 3 and 4 have (a) lower coverages and (b) lower first-scan ratios of Γa/Γc, suggesting that molecules with two attachment points may adhere better to the electrode surface. It is noteworthy that the three longest dithiolate systems with the same attachment endgroup (5-bond 2, 9-bond 7, and 13-bond 8) display virtually interchangeable behavior regarding surface coverages and stabilities of their monolayers, in contrast to alkanethiols, in which longer alkyl chains give more stable monolayers. The rigid dithiolates are not expected to undergo significant “self-assembly” in the monolayer, thereby rationalizing this contrasting behavior. Ambiguous results were obtained from measurements of electron-transfer rates for these adsorbates, which display no apparent systematic dependence on the length of the chain separating the DMN group from the electrode surface. It is possible that the electron-transfer rates for most of these adsorbates exceed those which can be accurately measured by cyclic voltammetry in highly resistive media. Present data cannot rule out the possibility, however, that the charge-transfer rates are enhanced by monolayer defects, for details of the adsorbate structure on the electrode surface are still to be addressed by other methods. The ks value of 8, the longest chain member of the series, may be compared with those estimated for ferrocenes separated by similar numbers of alkyl groups from a gold surface. Based on extrapolations of data reported by Chidsey13 and Carter et al.,31c an estimated range of ks ) 10-200 s-1 is obtained for a redox label separated by 13 -CH2- groups from the sulfur anchor. Our value of ks ) 3-5 × 103 s-1 is higher than this estimate by 1-2 orders of magnitude. Although faster electron exchange is expected through the norbornylogous bridge compared to (61) Prof. R. L. Garrell, unpublished work at University of California at Los Angeles. (62) Sellers, H.; Ulman, A.; Shnidman, Y.; Eilers, J. E. J. Am. Chem. Soc. 1993, 115, 9389. (63) Biebuyck, H. A.; Whitesides, G. M. Langmuir 1993, 9, 1766. (64) Hagenhoff, B.; Benninghoven, A.; Spinke, J.; Liley, M.; Knoll, W. Langmuir 1993, 9, 1622.
6626 Langmuir, Vol. 12, No. 26, 1996
alkyl chains,65 the possible significance of this comparison awaits more detailed investigations of the degree of order of the rigid adsorbate monolayer. Summary The dimethoxynaphthalene-norbornylogous bridgesulfur linkage provides a family of molecules in which an organic radical ion redox couple may be separated by predictable distances from an electrode attachment point. Given the ability of rigid norbornylogous bridges to enhance electron-transfer rates compared to their alkane analogues,30,65 this strategy is promising for linking remote charge-transfer groups to electrode surfaces. Very rapid electron-transfer rates have been measured for this series of adsorbates, although more must be known about the degree of order of the adsorbates before it is concluded that the rates are inherently rapid rather than determined by monolayer defects. The stripping observed during the initial CV scans of fresh monolayers in CH2Cl2 is rationalized if there are at least two types of adsorption sites for these molecules on polycrystalline gold, one providing weaker adhesion than the other (or rest). Disulfides, presumably being restricted to bond to the electrode through two sulfurs, show the least anodically induced stripping; the monothiol 5 shows the most. These results should encourage systematic studies of the influence of the number of adsorption sites (“sticky feet”) on monolayer adhesion stabilities.18,32,66 (65) (a) Penfield, K. W.; Miller, J. R.; Paddon-Row, M. N.; Cotsaris, E.; Oliver, A. M.; Hush, N. S. J. Am. Chem. Soc. 1987, 109, 5061. (b) Warman, J. M.; Hom, M.; Paddon-Row, M. N.; Oliver, A. M.; Kroon, J. Chem. Phys. Lett. 1990, 172, 114.
Wooster et al.
Adsorbates with rigid spacers have a conceptual advantage over those possessing floppy spacers in that selfassembly is not required in order to fix the distance between the electrode and the electron-transfer site. Finally, we emphasize the advantage that the rigid bridge of the norbornylogous-based thiol/disulfide systems may be systematically altered with comparative synthetic ease to change the bridge length and bridge configuration. We are presently synthesizing longer members of the series, including the 17-bond extension of 8. Intensive investigation of the surface structures and properties of these and related rigid molecules appears to be warranted. Acknowledgment. We gratefully acknowledge the financial support of the National Science Foundation (CHE91-16332 and 94-16611) and the Australian Research Council. One of us (A.M.O.) acknowledges the award of an Australian Research Fellowship. We thank Dr. Robin Garrell for obtaining Raman data of 2 and Dr. Stephen Creager for helpful advice and a sample of FcCO2(CH2)12SH. Supporting Information Available: Tables describing geometric parameters for molecules 4 and 27 and data describing the synthesis of all thiols, dithiols, and disulfides (25 pages). Ordering information can be found on any current masthead page. LA9609027 (66) Whitesell, J. K.; Chang, H. K. Science 1993, 261, 73.
