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Surface-Confined Monomers on Electrode Surfaces. 4. Electrochemical and Spectroscopic Characterization of Undec-10-ene-1-thiol Self-Assembled Monolayers on Au John S. Peanasky† and Robin L. McCarley* Choppin Laboratories of Chemistry, Louisiana State University, Baton Rouge, Louisiana 70803-1804 Received July 23, 1997. In Final Form: October 31, 1997X The synthesis, structure, and reactivity of undec-10-ene-1-thiol monolayers assembled on planar and nanocrystalline (curved) Au is presented. Cyclic voltammetry and infrared spectroscopy are used to probe the structural changes in the monolayers (on planar Au) upon irradiation with γ-rays. Oligomerization of the monolayers during the γ-ray exposures is indicated by the observed decrease in the intensities of infrared bands associated with the olefin functionality. From infrared spectra obtained during γ-ray exposures of the undec-10-ene-1-thiol monolayers on planar Au, it is proposed that the oligomerization reaction is controlled by the distance the tethered olefin groups can move. That is to say the reaction is stress limited. Dropcast films of undec-10-ene-1-thiol/Au nanoclusters (1.3 and 3.4 nm diameter Au crystals) do not exhibit decreases in the olefin infrared bands after large γ-ray exposures. This decrease in reactivity for the olefin monolayers supported on the Au nanocrystals is suggested to be the result of interdigitation of the alkane chains from neighboring alkanethiolate Au clusters that exist in the dropcast films.

Introduction monolayers,1

Since their discovery, self-assembled SAMs, particularly alkanethiol SAMs on Au(111) surfaces,2 have shown great promise in a variety of areas. Much has been envisioned of these highly ordered and easily prepared monolayers.3 While potential uses have been expounded upon, more recent interests focus on the use of SAMs in studying and modulating chemical reactions in molecular volumes4 and sensing chemically different regions on the nanometer scale.5 The former topic has many elements to it, but can be summarized by stating that well-established solution-phase chemical reactions may have an extra degree of freedom added or removed upon moving the reactant(s) to a flat (or curved) surface. Kurth et al. have shown that a classical solutionphase SN2 reaction fails to occur when one of the reactants is immobilized on a silica surface.4a From a materials point of view, Frostman et al. have demonstrated the preferential growth of molecular crystals with a given * To whom correspondence should be addressed: phone, (504) 388-3239; facsimile, (504) 388-3458; e-mail, [email protected]. † Current address: Corning Incorporated, SP-DV-01-4, Corning, NY 14831. X Abstract published in Advance ACS Abstracts, December 15, 1997. (1) Bigelow, W. C.; Pickett, D. L.; Zisman, W. A. J. Colloid Sci. 1946, 1, 513. (2) Nuzzo, R. G.; Allara, D. L. J. Am. Chem. Soc. 1983, 105, 4481. (3) (a) Ulman, A. In An Introduction to Ultrathin Films From Langmuir-Blodgett to Self-Assembly; Academic: San Diego, CA, 1991. (b) Ulman, A. Chem. Rev. (Washington, D.C.) 1996, 96, 1533. (c) Dubois, L. H.; Nuzzo, R. G. Annu. Rev. Phys. Chem. 1992, 43, 437. (d) Whitesides, G. M.; Mathias, J. P.; Seto, C. T. Science 1991, 254, 1312. (e) Zhong, C.-J.; Porter, M. D. Anal. Chem. 1995, 67, 709A. (4) (a) Kurth, G. G.; Bein, T. Langmuir 1993, 9, 2965. (b) Frostman, L.; Bader, M. M.; Ward, M. D. Langmuir 1994, 10, 576. (c) Batchelder, D. N.; Evans, S. D.; Freeman, T. L.; Haüssling, L.; Ringsdorf, H.; Wolf, H. J. Am. Chem. Soc. 1994, 116, 1050. (d) Kim, T.; Crooks, R. M.; Tsen, M.; Sun, L. J. Am. Chem. Soc. 1995, 117, 3963. (5) (a) Frisbie, C. D.; Rozsnyai, L. F.; Noy, A.; Wrighton, M. S.; Lieber, C. M.; Science 1994, 265, 2071. (b) Green, J. B.; McDermott, M. T.; Porter, M. D.; Siperko, L. M. J. Phys. Chem. 1995, 99, 10960. (c) Browning-Kelley, M. E.; Hari, V.; Liu, G.-y. Langmuir 1997, 13, 343.

crystallographic orientation on alkanethiol SAMs.4b In addition, Batchelder4c and Kim4d have accomplished topochemical polymerizations in monomer-functionalized alkanethiol monolayers. From a high resolution (spatial) chemical sensing point of view, frictional force microscopy with chemically modified tips is particularly attractive. Chemical force microscopy,5 CFM, makes use of probe tips modified by SAMs of variable but controlled chemistry to map the chemical composition of surfaces6 on the ∼10 nm scale. In addition, modified probe tips have been used to measure binding constants of biologicals.5c Recently, the two areas of controlled chemistry in molecular volumes and chemically modified scanning probe microscopy tips have been unified to allow for the preparation of nanoscale structures.7 Thus, it is apparent that the ability to vary the chemical and physical properties of materials by manipulation of the material’s size, shape, and/or crystalline order on the molecular scale, as well as the ability to measure (sense) these changes in molecular environment, will become increasingly important for a variety of current and future applications.8 One technical difficulty associated with the use of alkanethiol SAMs is the lability of the Au-S surface linkage. Removal of adsorbed thiol molecules on Au may occur by changes in environmental conditions, such as exposure to organic solvents9 or heating10 of the substrate. Scanning probe microscopy studies of alkanethiol-modified surfaces and CFM experiments (thiol-modified tips) of (6) Joyce, S. A.; Thomas, R. C.; Houston, J. E.; Michalske, T. A.; Crooks, R. M. Phys. Rev. Lett. 1992, 68, 2790. (7) Muëller, W. T.; Klein, D. L.; Lee, T.; Clarke, J.; McEuen, P. L.; Schultz, P. G. Science 1995, 268, 272. (8) (a) Ozin, G. A. Adv. Mater. 1992, 4, 612. (b) Bard, A. J. In Integrated Chemical Systems: A Chemical Approach to Nanotechnology; Wiley: New York, 1994. (c) Hostetler, M. J.; Murray, R. W. Curr. Opin. Colloid Interface Sci. 1997, 2, 42. (9) (a) Everett, W. R.; Fritsch-Faules, I. Anal. Chim. Acta 1995, 307, 253. (b) Everett, W. R.; Fritsch, I.; Scott, J. R.; Yao, J.; Wilkins, C. L. Submitted for publication. (10) Nishida, N.; Hara, M.; Knoll, W. Jpn. J. Appl. Phys. 1996, 35, 5866.

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bare metals indicate that the removal,11 displacement,12 and even transfer of alkanethiols can occur.13 In either case, such Au-S lability results in removal of the adsorbate molecules or their rearrangement on the surface, making prolonged use of the modified surface unfeasible. To improve adherence of alkanethiols on the coinage metals (Au, Cu, Ag), one may either increase the interaction of the sulfur headgroup of the SAM with the substrate or increase the lateral interaction between the alkane chains comprising the SAM. The former may be realized by careful manipulation of the substrate surface14 or perhaps a different chemistry.4c,d,15-17 The latter, the subject of this paper, may be realized by tethering the chains together via polymerization.4c,d,15,16 Thus, the interaction of the molecules with the surface is proportional to the degree of polymerization. Our particular interests concern polymerization schemes where a monomolecular film is adsorbed on a surface, and then reaction between alkane chains occurssnot the study of a solution-deposited, thiol-functionalized polymer.18 While there are a few examples focusing on the polymerization of functional groups in self-assembled alkanethiol monolayers,4c,d,15 we are interested in understanding what factors affect the reactivity and the resulting degree of polymerization of the monolayer. Such factors to be studied include the degree of chain packing, position of polymerizable moiety within the alkane chain, and radius of curvature of the substrate surface. There has been much interest in γ-ray, X-ray, and electron beam-induced polymerizations of LangmuirBlodgett (L-B) films for lithographic purposes, as well as fundamental studies of solid-state polymerizations.3a,19 Studies involving polymerizations initiated by γ-rays in crystalline environments were first performed in 1956 with the solid state polymerization of vinyl stearate and more recently with L-B films supported on dielectrics and salts.19a-c Electron-beam-induced polymerizations have been studied on a variety of L-B multilayers on dielectric surfaces.19d,f In all of the systems described above, it is thought that the polymerizations are restricted by the distance that the alkane chains can move about within the organized layer. That is to say, the stress that is developed in the crystal during the coupling of the chains puts a limit on the length of the polymer.19f A drawing of such a polymer structure would resemble a corn shock. (11) (a) Kim, Y.-T.; Bard, A. J. Langmuir 1992, 8, 1096. (b) Ross, C. B.; Crooks, R. M. Langmuir 1993, 9, 632. (12) (a) Liu, G.-y; Salmeron, M. B. Langmuir 1994, 10, 367. (b) Xu, S.; Liu, G.-y. Langmuir 1997, 13, 127. (13) Porter, M. D. Private communication, 1996. (14) Jennings, G. K.; Laibinis, P. E. J. Am. Chem. Soc. 1997, 119, 5208. (15) (a) Willicut, R. J.; McCarley, R. L. J. Am. Chem. Soc. 1994, 116, 10823. (b) Kim, T.; Crooks, R. M. Tetrahedron Lett. 1994, 35, 9501. (c) Chan, K. C.; Kim, T.; Schoer, J. K.; Crooks, R. M. J. Am. Chem. Soc. 1995, 117, 5875. (d) Lin, S.; McCarley, R. L. Unpublished results, 1997. (e) Ford, J. F.; Vickers, T. J.; Mann, C. K.; Schlenoff, J. B. Langmuir 1996, 12, 1944. (f) Hayes, W. A.; Shannon, C. Langmuir 1996, 12, 3688. (16) Huang, D. Y.; Tao, Y. T. Bull. Inst. Chem., Acad. Sin. 1986, 33, 73. (17) Clegg, R. S.; Hutchinson, J. E. Langmuir 1996, 12, 5239. (18) (a) Sun, F.; Castner, D. G.; Mao, G.; Wang, W.; McKeown, P.; Grainger, D. W. J. Am. Chem. Soc. 1996, 118, 1856. (b) Schlenoff, J. B.; Dharia, J. R.; Xu, H.; Wen, L.; Li, M. Macromolecules 1995, 28, 4290. (19) (a) Restaino, A. J.; Mesrobian, R. B.; Morawetz, H.; Ballantine, D. S.; Dienes, G. J.; Metz, D. J. J. Am. Chem. Soc. 1956, 78, 2939. (b) Hatada, M.; Nishii, M.; Hirota, K. J. Colloid Interface Sci. 1973, 45, 502. (c) Cemel, A.; Fort, T., Jr.; Lando, J. B. J. Polym. Sci. A-1 1972, 10, 2061. (d) Barraud, A.; Rosilio, C.; Ruaudel, A. J. Colloid Interface Sci. 1977, 62, 509. (e) Barraud, A. Thin Solid Films 1983, 99, 317. (f) Barraud, A. Mol. Cryst. Liq. Cryst. 1983, 96, 353. (g) Ogawa, K.; Tamura, H.; Hatada, M.; Ishihara, T. Langmuir 1988, 4, 1229. (h) Ogawa, K.; Tamura, H.; Hatada, M.; Ishihara, T. Colloid Polym. Sci. 1988, 266, 525.

Peanasky and McCarley

Thus, any changes in the mobility of the monomer chains or the distance between the monomer functional groups will have a substantial effect on the degree of polymerization/oligomerization. Interestingly, we are not aware of any previous work involving the study of such polymerization reactions in single L-B monolayers. In this paper we present spectroscopic and electrochemical data describing the structure of 11-carbon, ω-substituted alkanethiol SAMs on planar Au. Small, polarizable ω termini (olefin, hydroxyl) give rise to monolayers that are similar to those derived from nalkanethiols.20 Exposure of undec-10-ene-1-thiol monolayers on planar Au to γ-rays is investigated with reflection-absorption infrared spectroscopy. Spectra of the olefin-terminated monolayers as a function of γ-ray exposure indicate oligomerization in the monolayers. Voltammetric and spectroscopic data lead us to propose that the oligomerization process is stress limited. Finally, to model polymerization reactions in SAMs on scanning probe microscopy (SPM) tips, monolayers of undec-10ene-1-thiol on 1.3 and 3.4 nm nanocrystals were exposed to γ-rays. These studies have led to a preliminary understanding of how changes in monolayer environment affect oligomerization reactions under conditions where the radius of curvature of the substrate approaches the thickness of the monolayer. Experimental Section Reagents. All solvents were of spectrophotometric grade or better and were used without further purification. Undec-10ene 1-bromide [7766-50-9] (Lancaster, >99% pure by gas chromatography-mass spectrometry (GC-MS, electron ionization)), 11-bromoundecanoic acid [2834-05-1] (Aldrich, 99% by GC-MS), 11-bromoundecanol (Janssen, 99% by GC-MS), thiourea [6256-6] (Aldrich, 99%), semiconductor grade potassium hydroxide and sodium hydroxide (Aldrich), sodium borohydride (Aldrich), HAuCl4‚xH2O (Strem Chemicals), hexammineruthenium(III) chloride [14282-91-8] (Strem, 99%), and potassium ferricyanide [13746-66-2] (Aldrich, 99+%) were used without further purification. Distilled water was passed through a Barnstead reverse osmosis filter followed by a Nanopure water system to yield water with a resistivity of 18 MΩ cm. Other reagents were used as received. Synthesis of Thiols. All thiols were obtained from the corresponding bromide using the well-known thiourea route.21 Typical syntheses were carried out as follows. To 100 mL of Ar-purged (40 min) ethanol or methanol in a three-neck flask equipped with a reflux condenser and bubbler was added 0.025 mol of thiourea. After the mixture was stirred to dissolve the thiourea and occasional purges with Ar, roughly 0.008-0.010 mol of the bromide was added and the solution heated to reflux for 8-16 h. At this point, thin-layer chromatography (TLC) indicated complete conversion of the bromide to the corresponding thiouronium salt. The reaction was allowed to cool to room temperature under an Ar purge, and then 10 mL of an aqueous 10% solution (w/w) of NaOH was added. The solution was heated to reflux for approximately 4 h, at which time TLC indicated all of the thiouronium salt had been hydrolyzed. Upon cooling to room temperature, the hydrolyzed solution was titrated with dilute HCl to pH 7 (pH 5 for the carboxylic acid). After extraction with 200 mL of diethyl ether, the organic phase was rinsed three times with 18 MΩ cm water, dried over anhydrous magnesium sulfate, filtered, and then evaporated under nominal vacuum to yield crude products. After the neat undec-10-ene-1-thiol was passed through a minicolumn of silica gel, an almost colorless liquid that readily froze at -20 °C was obtained in 90% yield. The material was tested by GC-MS and found to be >97% pure. Negative ion (20) (a) A previous report20b regarding the voltammetric blocking behavior of ω-substituted alkanethiol SAMs indicated similar results. (b) Chidsey, C. E. D.; Loiacono, D. N. Langmuir 1990, 6, 682. (21) Chemistry of the Thiol Group; Patai, S., Ed.; Wiley: New York, 1974.

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electrospray mass spectrometry (Finnigan, MAT-900) did not indicate the presence of any oligomeric species.22 The 11mercaptoundecanol was obtained in quantitative yield and was a white solid >99.5% pure by GC-MS. No changes in purity could be determined upon recrystallization from hexane. The melting point of the crystals was found to be 35-38 °C. A 90% yield of the 11-mercaptoundecanoic acid was achieved; this white solid was >99.5% pure as determined by GC-MS. Further recrystallizations from hexane did not affect the purity or melting point of 48-51 °C. Surface Derivatization. Electrodes and IR substrates were prepared from 99.999% Au (Refining Systems). Emerald muscovite mica, ASTM V-2, grade 3, was obtained from Lawrence and Co. (New Bedford, MA) and cleaved just prior to evaporations. Glass substrates (1 in. × 3 in. microscope slides) were placed in a specially prepared microslide holder made from 1/8 in. glass rods. The entire assembly was allowed to soak for about 30 min in freshly prepared piranha solution (70/30 (v/v) mixture of H2SO4/30% H2O2). Caution: The cleaning solution is highly oxidizing and should be handled with extreme care. The solution should be disposed of upon completion of substrate cleaning to avoid explosions. The assembly was rinsed for 10 min in 18 MΩ cm water and then dried with nitrogen gas. These substrates were immediately placed in an Edwards Auto 306 evaporator system where metal evaporation took place. Base pressures were never more than 8 × 10-7 Torr. Glass substrates had a 3 nm Cr adhesion layer. Au films for use as electrodes were prepared by evaporation of 100 nm of Au through a mask that allowed for fabrication of a disk (0.172 cm2) with an attached thin line of Au that could be used as a contact. All Au disk electrodes were prepared on mica and subsequently annealed at 325 °C for 3-3.5 h so as to create a flat, Au(111) surface.23,24 Scanning tunneling microscopy (STM) measurements confirmed the presence of atomically flat 300 nm wide terraces. Au substrates for IR studies were prepared in a similar fashion except no mask was used. Upon removal from the evaporator, the Au substrates were rinsed with copious amounts of ethanol and then immediately immersed “wet” in 1 mM solutions of a given thiol in argon-degassed absolute ethanol. Au disk electrodes on mica that had been annealed were cleaned by using an UV-ozone apparatus (DHK Industries, Santa Clara, CA) for 5 min, rinsed with copious amounts of ethanol, and then placed in the thiol/ ethanol solutions. Derivatization solutions were stored under nitrogen or argon in glovebags or Plexiglas box “purge chambers”. Such solutions could be stored this way for >4 weeks with no apparent presence of dialkyl disulfides. Thiol-Capped Au Nanocrystals. The method of Brust et al..25 was used to synthesize small undec-10-ene-1-thiol/Au clusters, with the modification that all reactions were carried out at 0 °C. Larger diameter undec-10-ene-1-thiol/Au clusters were synthesized at room temperature by adding a 1.1 molar equivalent of undec-10-ene-1-thiol dissolved in toluene to a toluene/Oct4NBr solution of Au crystals prepared as previously reported.25b The large thiol-capped clusters were precipitated by addition of absolute ethanol followed by storage at 0 °C under argon for 1 day. The clusters were isolated by centrifugation, rinsed with absolute ethanol, and then isolated again by centrifugation. This procedure was repeated a total of eight times to ensure that all of the free thiol and tetraoctylammonium bromide were removed. The isolated nanoclusters were then dried under a stream of nitrogen. Voltammetric Measurements. All voltammetry was performed using an EG&G Princeton Applied Research Model 273A potentiostat/galvanostat. Cyclic staircase voltammetry was carried out using locally written routines in Headstart (Version 1.7) computer software (Princeton Applied Research). All measurements were obtained in glovebags (Instruments for

Research and Industry, Cheltenham, PA) fitted with a Plexiglas feedthrough panel for the electrical and gas connections. During measurements, the bags were inflated to positive pressure at all times. Voltammetry was performed with a platinum wire counterelectrode and a saturated sodium calomel electrode (SSCE) reference. Infrared Spectroscopy. The neat liquid and surface infrared spectroscopy measurements were carried out with a Nicolet 740 Fourier transform infrared spectrometer using a liquid-nitrogencooled, wide-band MCT detector. Liquid measurements were obtained with a 15 µm path length NaCl cell. External reflection measurements were taken at 80° incidence using the Versatile Reflection Attachment with Retro-Mirror Accessory (Harrick, Ossining, NY). The retro mirror was an Al-coated optical flat. All measurements were performed using a glovebag that surrounded the sample chamber and was connected to an auxiliary nitrogen purge line. Purge correction software provided by Nicolet was usually not necessary when using this setup. Spectra from samples prepared on glass substrates with a Cr adhesion layer were indistinguishable from those prepared on annealed mica substrates. All reflection-absorption infrared (RAIR) spectra are the result of 2048 scans with a C18D37SH/Au background. Happ-Genzel apodization was employed throughout this study. Transmission IR spectra of the Au nanoclusters (dropcast film on NaCl plate) were obtained using a PerkinElmer 1760 infrared spectrometer equipped with a triglycine sulfate (TGS) detector. All spectra reported here were obtained at 2 cm-1 resolution. It was found that band frequencies have reproducibilities of (1 cm-1. Infrared bands were integrated from a baseline (defined by the band of interest) using Origin 4.0 (Microcal). Transmission Electron Microscopy. Samples for transmission electron microscopy (TEM) were prepared by immersing carbon-coated (15-18 nm) Collodion (2% in amyl acetate) films on copper grids (300 mesh) in an approximately 1 mg mL-1 cluster solution in hexane, followed by drying in air for 1-30 min. Images were obtained with a JEOL 100CX electron microscope operating at 80 keV. Average Au core size was obtained by measuring features (>300 particles) of enlarged micrographs (250000×) from different regions on the TEM grid. The small clusters were found to have an average diameter of 1.3 ( 0.7 nm, while the larger ones are 3.4 ( 1.2 nm in diameter. The observed difference in size for the larger clusters (3.4 ( 1.2 nm) versus that observed by Brust (8 ( 2 nm)25b for similar preparation conditions is most likely a result of the fast rate of NaBH4 addition used here. No rates for reductant addition were reported by Brust et al.25b γ-ray Exposure of Monolayers. All sample preparations took place in a nitrogen- or argon-filled glovebag. Samples to be exposed to γ-rays were first placed in Ziploc bags and then glass canister jars outfitted with an O-ring seal (J. C. Penney). After a 5 min purge, the Ziploc bag was closed and placed in the canister jar, and then the jar was sealed and removed from the glovebag. The canister jar containing the samples in the Ziploc bags was then placed inside a stainless steel bomb measuring 6 in. in diameter by 18 in. in height (outside dimensions). The bomb was fitted with an argon gas line inlet/outlet system. Samples were purged with argon for 1-2 h prior to their being placed in the 60Co source. Using a pulley system, the bomb was lowered to a special holder at the bottom of a 6 m well filled with water. The spatial variance of exposure doses across the entire bomb was obtained using a Fricke dosimetry.26 The source yielded exposures of 1000-1300 rad/min. After the desired exposure time had been reached, the bomb was raised and the argon atmosphere maintained for at least 90 min prior to sample removal from the bomb. Samples were kept under nitrogen or argon until they were analyzed.

(22) McCarley, T. D.; McCarley, R. L. Anal. Chem. 1997, 69, 130. (23) Chidsey, C. E. D.; Loiacono, D. N.; Sleator, T.; Nakahara, S. Surf. Sci. 1988, 200, 45. (24) (a) Widrig, C. A.; Chung, C.; Porter, M. D. J. Electroanal. Chem. 1991, 310, 335. (b) Walczak, M. A.; Alves, C. A.; Lamp, B. D.; Porter, M. D. J. Electroanal. Chem. 1995, 396, 103. (25) (a) Brust, M.; Walker, M.; Bethell, D.; Schiffrin, D. J.; Whyman, R. J. Chem. Soc. Chem. Commun. 1994, 801. (b) Brust, M.; Bethell, D.; Schiffrin, D. J.; Kiely, C. J. Adv. Mater. 1995, 7, 795.

Infrared Spectroscopy of Undec-10-ene-1-thiol Monolayers. We begin our discussions of the olefin monolayer structure by comparing the infrared spectra of monolayers on planar and nanocluster Au to the neat

Results and Discussion

(26) Fricke dosimetry was used as described at the Nuclear Science Center, Louisiana State University; Max Scott - Director.

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Table 1. Infrared Band Assignments for Undec-10-ene-1-thiol band frequency (cm-1) H2CdCH(CH2)9SH/Au planar

H2CdCH(CH2)9SH/Au cluster (1.3 nm)

H2CdCH(CH2)9SH neat liquid

mode

3085 3007 2983 2922 2851 1828 1644 1467 N/Oa 1439 1418 b 993 914

3078 2996 2976 2919 2850 ∼1812 1638 1462c N/Oa 1439 1414 ∼1160-1380 989 906

3077 2997 2977 2926 2855 1820 1641 1465 1460 1439 1415 ∼1160-1380 988 910

νa(dCH2) ν(H2CdC-H) νs(dCH2) νa(CH2) νs(CH2) ω(dCH2) overtone ν(CdC) δ(CH2) δG(CH2) δD(CH2) δS(CH2) + δ(dCH2) (CH2)Wag/Twist CdC oop def ω(dCH2)

a N/O denotes not observed. b Signal is too small. In the case of planar Au, although there appear to be bands in this region, the S/N ratio is too small to make band assignments. c Due to the value for this particular mode in the nanoclusters, it is entirely possible that the band for the δ(CH2) mode for the all-trans chain overlaps with the δG(CH2) band or none of the chains are in the trans conformation. From the other spectroscopic bands noted for the nanocluster sample, we feel that the latter case is certainly not plausible.

Figure 1. Infrared spectra of undec-10-ene-1-thiol: A, on planar Au (RAIR spectrum); B, on 1.3 nm Au nanocrystals (dropcast film on NaCl); C, as the neat liquid. The features in A at roughly 1250 and 1000-1120 cm-1 are due to a polarizer artifact and Si-O absorptions, respectively.

transmission spectrum (isotropic). Mode assignments27,28,29b,31 are presented in Table 1. From a stability point of view, monolayers on both the Au nanoclusters and planar gold samples display no changes in the band positions or band intensities upon exposure to the laboratory ambient for times up to 1 week. Modes Associated with the Methylene Backbone. Shown in Figure 1 are the IR spectra of undec-10-ene-1-thiol on planar and 1.3 nm diameter Au (dropcast film on NaCl plate), and as the neat liquid. In all three spectra, bands due to νa(CH2), νs(CH2), and δ(CH2) modes are readily (27) Allara, D. L.; Nuzzo, R. G. Langmuir 1985, 1, 52. (28) (a) Snyder, R. G.; Schachtschneider, J. H. Spectrochim. Acta 1963, 19, 85. (b) Schachtschneider, J. H.; Snyder, R. G. Spectrochim. Acta 1963, 19, 117. (c) Snyder, R. G. J. Chem. Phys. 1967, 47, 1316. (d) Maroncelli, M.; Qi, S. P.; Strauss, H. L.; Snyder, R. G. J. Am. Chem. Soc. 1982, 104, 6237. (29) (a) Terrill, R. H.; Postlewaite, T. A.; Chen, C.-H.; Poon, C.-D.; Terzis, A.; Chen, A.; Hutchinson, J. E.; Clark, M. R.; Wignall, G.; Londono, J. D.; Superfine, R.; Falvo, M.; Johnson, C. S., Jr.; Samulski, E. T.; Murray, R. W. J. Am. Chem. Soc. 1995, 117, 12537. (b) Hostetler, M. J.; Stokes, J. J.; Murray, R. W. Langmuir 1996, 12, 3604. (c) Hostetler, M. J.; Green, S. J.; Stokes, J. J.; Murray, R. W. J. Am. Chem. Soc. 1996, 118, 4212. (d) Hostetler, M. J.; Wingate, J. E.; Zhong, C.-Z.; Harris, J. E.; Vachet, R. W.; Clark, M. R.; Londono, J. D.; Green, S. J.; Stokes, J. J.; Wignall, G. D.; Glish, G. L.; Porter, M. D.; Evans, N. E.; Murray, R. W. Submitted for publication in Langmuir.

apparent. As expected, the neat undec-10-ene-1-thiol spectrum exhibits νa(CH2) and νs(CH2) band frequencies (2926 and 2855 cm-1, respectively) indicative of a very disordered chain environment.28 Upon adsorption onto planar or nanocrystalline (curved) Au, these bands are red shifted, reflecting a more ordered, crystalline molecular environment. This effect is greater for the olefinthiol on the curved Au (7 cm-1 shift versus 4 cm-1)sthis is most likely due to interdigitation of the alkane chains from neighboring Au nanoclusters, vide infra.25,29,30a Bands associated with methylene C-H scissoring28,31 are observed in all three of the spectra in Figure 1, 15001400 cm-1. The band at ∼1465 cm-1 (δ) can be assigned to the scissoring of an all-trans methylene chain. An indication of gauche defects in the alkane chains of the various samples can be obtained by close examination of the spectra for the presence of bands near 1460 and 1440 cm-1. The former band has previously been assigned to a scissoring motion of a methylene unit next to a gauche bond, δG. As expected, the isotropic spectrum of undec10-ene-1-thiol liquid exhibits a shoulder near 1460 cm-1. Spectra of the olefin-thiol on the two Au surfaces do not indicate the presence of the band associated with the δG mode, a result that is in agreement with previous studies of n-alkanethiol/Au clusters.29b Although some controversy exists concerning bands near 1440 cm-1 in the spectra of n-alkanes, Murray and co-workers have concluded that this band can be assigned to the scissoring motions of a methylene group adjacent to a chain endgauche defect.29b We observe a band near 1439 cm-1 in each of the spectra in Figure 1 and assign this as the mode due to methylene scissoring next to a chain endgauche defect. In the case of the neat liquid and the curved Au samples, the chain end-gauche defect is confirmed by the presence of a band near 1345 cm-1. Previous studies have shown that the 1345 cm-1 band is associated with a methylene wagging mode for chain end-gauche defects.28 For the planar Au spectrum, the signal in the 1400-1300 cm-1 range is extremely weak, and thus confirmation of (30) (a) Badia, A.; Singh, S.; Demers, L.; Cuccia, L.; Brown, G. R.; Lennox, R. B. Chem. Eur. J. 1996, 2, 359. (b) Whetten, R. L.; Khoury, J. T.; Alvarez, M. M.; Murthy, S.; Vezmar, I.; Wang, Z. L.; Stephens, P. W.; Cleveland, C. L.; Luedtke, W. D.; Landman, U. Adv. Mater. 1996, 8, 428. (c) Heath, J. R.; Knobler, C. M.; Leff, D. V. J. Phys. Chem. B 1997, 101, 189. (31) (a) Socrates, In Infrared Characteristic Group Frequencies: Tables and Charts; Wiley: New York, 1994. (b) Bellamy, L. J. In The Infrared Spectra of Complex Molecules; Wiley: New York, 1975.


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the presence of chain end-gauche defects (indicated by the 1439 cm-1 band, δD) cannot be obtained. Finally, a band within 2 cm-1 of 1416 cm-1 is found in all three spectra and could be assigned to methylene scissoring next to the S-Au bond (δS).29b We suggest that this band results from overlap of the methylene scissoring mode of the terminal dCH2 group, δ(dCH2),31b with the δS mode. Modes Associated with the ω-olefin Group. The terminal olefin group gives rise to six readily observable bands in the IR region of 3500-800 cm-1.27,31 All six of these bands are present in the IR spectra of the three sample types in Figure 1. The bands at roughly 3075 and 2975 cm-1 can be assigned to the dCH2 antisymmetric (νa(dCH2)) and symmetric (νs(dCH2)) modes, while that near 3000 cm-1 is due to the β C-H stretch (ν(H2CdC-H)). All three of these vinyl C-H stretches are blue shifted with respect to the neat spectrum in the case of planar Au. In addition, the νa(dCH2) mode exhibits a much narrower line width. These phenomena reflect the change in environment of the vinyl group upon assembly. In particular, this change comes about as a result of decreased hydrogen bonding of the terminal vinyl group. This result is expectedssimilar trends have been observed previously for various olefins and acetylenes upon moving from the liquid to the gas phase.27,32 Allara and Nuzzo have also noted almost identical changes for olefin-terminated carboxylic acids upon adsorption onto alumina surfaces.27 The band maxima positions for νa(dCH2), νs(dCH2), and ν(H2CdCH) in the spectra of the olefin-thiol on the Au nanocluster surface are virtually indistinguishable from the isotropic sample, indicating that the molecular environment of the terminal vinyl group is similar, if not identical, to that of the neat liquid. This result can be rationalized as follows. Interdigitation of the alkane chains of neighboring clusters would result in a very aliphatic environment for the olefin groups, very much like that expected for the neat liquid. TEM images of 1.3 nm diameter undec-10-ene-1-thiol/Au clusters and various n-alkanethiol-capped Au clusters25a,29a,29d,30a indicate a substantial degree of chain interdigitation for films made from very dilute cluster solutions. This interdigitation has been proposed to explain the apparent crystallinity of relatively short n-alkane chains (as judged by the frequencies of the νa(CH2) and νs(CH2) bands) in alkanethiol/Au nanoclusters. As discussed above, we observe a significant difference in the band positions of the νa(CH2) and νs(CH2) modes when comparing the IR spectra of planar and curved Au surfaces coated with the undec-10-ene-1-thiol. Thus, we conclude that the increases in the vinyl C-H mode frequencies and the apparent chain crystallinity are due to interdigitation of the alkane chains. Further evidence for this conclusion is presented below in the section discussing the reduced activity of the olefin group upon exposure to ionizing radiation. All three of the spectra in Figure 1 exhibit bands associated with the CdC stretch (ν(CdC)) at ∼1640 cm-1, CdC out-of-plane deformation near 990 cm-1, and the C-H out-of-plane deformation for the dCH2 group at roughly 910 cm-1, ω(dCH2). The band maxima positions for these modes on the various Au substrates are almost identical ((4 cm-1) to those of the neat liquid spectrum. Although we have not yet carried out quantitative orientation analysis on the undec-10-ene-1-thiol monolayers on planar Au,33-34 we can make some qualitative

arguments concerning the orientation.35 Due to the fact that the intensities of the νa(dCH2) and ν(CdC) bands are so great in the reflection spectra, the H-C-H plane of the olefin must be canted away from the surface normal but not be grossly rotated about the C9-C10 axis. The νa(dCH2) and ν(CdC) modes have transition dipole moments that are orthogonal to each other, Figure 2. Assuming an alkane chain tilt (R) of 30-40° (a fair assumption based on previous IR work on ω-substituted thiols,36 the band intensity of the νs(CH2) mode for the olefin monolayer in comparison to n-undecanethiol monolayers prepared in this and other laboratories, and the electrochemically determined surface coverage of the undec-10-ene-1-thiol monolayer, vide infra) and knowing that the chain twist is ∼49°,35 the olefin group cannot be rotated (γ) more than ∼+60° around the C9-C10 bond. We draw this conclusion based on the large band intensities of the νa(dCH2), ν(CdC), ω(dCH2), and ν(H2CdC-H) modes, in conjunction with the band intensity ratio of νa(dCH2) to ν(CdC). Rotation of the olefin group about the C9-C10 axis (γ) beyond +60° would result in very small band intensities for the ω(dCH2) and ν(CdC) modes and large intensities for the ν(H2CdC-H) and νa(dCH2) modes. If instead the rotation about C9-C10 is counterclockwise, then the band intensity ratio of νa(CH2) to ν(CdC) should be much less than 1 and the ω(dCH2) band would be seriously diminished. Thus it would appear that the olefin is rotated clockwise about the C9-C10 axis between 0 and +60°. We must emphasize that differences in R for the olefin monolayer versus other ω-substituted alkanethiols could lead to a different conclusion about γ. But, such differences in R would result in obvious intensity differences for νa(CH2) and νs(CH2) when comparing infrared spectra of the olefin monolayer to those of n-undecanethiol monolayers.

(32) Boobyer, G. J. Spectrochim. Acta 1967, 23A, 325. (33) (a) Greenler, R. G. J. Chem. Phys. 1966, 44, 310. (b) Allara, D. L.; Nuzzo, R. G. Langmuir 1985, 1, 45. (34) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J. Am. Chem. Soc. 1987, 107, 3559.

(35) (a) Debe, M. K. Appl. Surf. Sci. 1982, 14, 1. (b) Sinniah, K.; Cheng, J.; Terrattaz, S.; Reutt-Robey, J. E.; Miller, C. J. J. Phys. Chem. 1995, 99, 14500. (36) Nuzzo, R. G.; DuBois, L. H.; Allara, D. L. J. Am. Chem. Soc. 1990, 112, 558.

Figure 2. Schematic diagram indicating the direction of the various transition dipole moments for undec-10-ene-1-thiol.

118


Electrochemistry of C11 Thiol Monolayers. Surface Coverage of ω-Terminated C11 Thiols. To determine the surface coverage, Γ (mol cm-2), of the various C11 thiol monolayers, reductive desorption, as described by Porter,24 was used. In this technique, the potential of the working electrode is scanned cathodically in degassed 0.1 M hydroxide/H2O electrolyte. Adsorbed thiol molecules on various metal surfaces give rise to a voltammetric peak that, in the case of Au, is assigned to the Au-SR + 1ef RS- + Au reaction. The resulting cathodic peak is integrated to give the corresponding electrochemical charge for the electrodesorption reaction. Although there were questions in the past regarding the accuracy of this method due to possible contributions from electrochemical double-layer charging, this method is now well established and can indeed be used for comparison of surface coverage values for different thiol monolayers. We chose to compare surface coverage values for the undec-10-ene-1-thiol monolayers to that of n-undecanethiol layers. The n-undecanethiol monolayers were chosen due to the fact that the structure of and the electrochemically determined Γ values for methyl-terminated alkanethiol monolayers are well-known. Surface coverage values of 7.8 ( 0.3 × 10-10 mol cm-2 (seven trials) and 8.2 ( 0.5 × 10-10 mol cm-2 (five trials) were obtained for the n-undecanethiol and undec-10-ene-1-thiol monolayers, respectively, on annealed Au(111)/mica substrates (roughness factor of 1.1). A linear background was used for determining the cathodic peak areas. Previous electrochemical desorption studies of various n-alkanethiol (n > 8) monolayers resulted in surface coverage values of ∼7.8 × 10-10 mol cm-2, a value virtually identical with that predicted from theory.24 The difference in observed surface coverage for the olefin-terminated monolayer versus that of the methyl-terminated monolayer is statistically insignificant. Thus, the undec-10-ene-1-thiol must have an identical headgroup packing density (arrangement) to that of n-undecanethiol on Au(111). This information, taken with the previously discussed νs(CH2) band intensities for the olefin and methyl monolayers, indicates that the structure of the undec-10-ene-1-thiol monolayer (exclusive of the ω-terminus) must be virtually identical with that of the n-undecanethiol. Electrochemical Blocking Characteristics of ω-Terminated C11 Monolayers. To obtain information regarding the blocking capabilities of the undec-10-ene-1-thiol monolayers in the pristine state (before exposure to γ-rays), voltammetry of Ru(NH3)63+ in aqueous KCl electrolyte was obtained. (We chose Ru(NH3)63+ over [Fe(CN)6]3due to the fact that the latter material tends not to be as effective as Ru(NH3)63+ when investigating the integrity of monolayers.) Voltammograms in identically prepared Ru(NH3)63+/KCl solutions at Au(111)/mica electrode surfaces modified with n-undecanethiol and 11-mercaptoundecanol were also obtained. Representative voltammograms are shown in Figure 3. There was no change observed in any of the voltammograms if the electrode was allowed to scan for several minutes. Although rigorous treatment of the voltammograms could possibly yield information on defect size and number,37 conclusions regarding the blocking efficacies of monolayers can be obtained by merely monitoring ∆Ep, (∆Ep ) Ep,c - ∆Ep,a), and the ratio ip,cx/ip,cbare, where ip,cx is the peak cathodic current for a given thiol-modified Au(111)/mica surface and ip,cbare the current at bare Au(111)/mica. These values (37) (a) Amatore, C.; Saveánt, J. M.; Tessier, D. J. J. Electroanal. Chem. 1983, 147, 39. (b) Finklea, H. O.; Snider, D. A.; Fedyk, J.; Sabatani, E.; Gafni, Y.; Rubenstein, I. Langmuir 1993, 9, 3660.


Figure 3. Cyclic voltammograms of ω-terminated C11 alkanethiol monolayers on Au(111)/mica: A, olefin; B, methyl; C, hydroxyl; D, bare surface. Scan rate was 20 mV s-1 and nominal electrode area was 0.172 cm2. The redox solution was composed of 1 mM Ru(NH3)63+ in 0.1 M KCl.

are recorded in Table 2 for the different surfaces. Reported values are the average from six separately prepared electrodes. The data follow the trend of more polar termini yielding better blocking of solution-phase redox probes. Similar trends in blocking have been noted previously,20b and in the case of ω-hydroxyalkanethiol monolayers,38 such increased blocking efficacy has permitted information to be obtained regarding tunneling. We leave this section of discussion by making a few comments on precautions necessary for the formation of high-quality undec-10-ene-1-thiol monolayers. As previously mentioned, the olefin monolayers on planar and curved Au do not show any appreciable changes in their IR spectra upon exposure to the lab ambient for periods of up to 1 week (we have not carried out longer exposures). However, if the dosing solution of the undec-10-ene-1thiol in ethanol is left exposed to the lab ambient (O2 and light) for times greater than 1 week, IR spectra of the resulting layers on planar Au are substantially different from those made by exposing Au-coated microscope slides to a freshly prepared undec-10-ene-1-thiol/ethanol solution. Shown in Figure 4 are the IR spectra of an undec10-ene-1-thiol monolayer formed from a fresh dosing solution (Figure 4A) and a film deposited from a dosing solution that is 14 days old (Figure 4B). The band positions and intensities in Figure 4A are identical with that of Figure 1C. Obvious to the observer are the increased band intensities of the νa(CH2) and νs(CH2) modes and decreased band intensities for the modes associated with the vinyl group when comparing Figure 4B to Figure 4A. In addition, the νa(CH2) and νs(CH2) band positions for Figure 4B are found to be at 2928 and 2856 cm-1, respectively. These results point to a highly disordered layer on the Au surface. Voltammetry of Ru(NH3)63+ at such surfaces as those of Figure 4B leads to values of 0.89 and 0.128 V for ip,cx/ip,cbare and ∆Ep, indicating the presence of a poor blocking layer. It is proposed that these observations can be attributed to a film formed by adsorption of oligomeric materials, possibly present in solution as a result of O2/ light-induced oligomerization of the olefin. Negative ion electrospray mass spectra22 of such solutions indicated the presence of dimers. Thus, for all data displayed here, we have used modified Au prepared from freshly made solutions. (38) Becka, A.; Miller, C. J. J. Phys. Chem. 1991, 95, 877.


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Table 2. Voltammetric Parameters as a Function of Monolayer ω-Terminus electrode

bare Au(111)

undec-10-ene-1-thiol

11-mercaptoundecanol

n-undecanethiol

∆Ep, mV ip,cx/ip,cbare

59.7 ( 0.5 1.00

354 ( 22 0.65 ( 0.01

735 ( 56 0.48 ( 0.02

175 ( 16 0.76 ( 0.01

Figure 4. RAIR spectra of undec-10-ene-1-thiol layers on Au prepared from different age dosing solutions: A, freshly prepared solution; B, 14-day-old solution.

Reactions of Undec-10-ene-1-thiol Monolayers on Planar and Curved Au During γ-Irradiation. The reactivity of the olefin-terminated monolayers was tested by exposing them to γ-rays emitted from a 60Co radiation source (sometimes referred to as a radiation swimming pool or “pit”). The O-ring-sealed steel chamber containing the samples was continuously purged with Ar during the γ-ray exposures, unless noted otherwise. Radiation doses were calculated based on data obtained from Fricke dosimetry. During such γ-ray exposures, olefinic species are known to react with one another to form polymers.39 The expected result for the undec-10-ene-1-thiol monolayers is a structure with “stitched together” neighboring chains. Although such reactions could be referred to as polymerization reactions, the number of monomer repeat units in the resulting macromolecule is expected to be small for such immobilized monomers. Thus, it is more proper to label such coupling reactions as oligomerization reactions. Consumption of monomer in the monolayers was obtained by noting the decrease in integrated IR band intensity for the modes associated with the olefin group.19b,d,g,h This reliable method has been used as a routine measure of oligomerization/polymerization in Langmuir-Blodgett films of ω-tricosenoic acid and acrylate esters of fatty carboxylic acids supported on glass and metals. Planar Au. As a control experiment to ensure that any spectral changes noted for the olefin monolayers were due to reactions involving only the olefin functionality, nundecanethiol monolayers were inspected with RAIRS as a function of γ-ray exposure. These methyl-terminated monolayers showed no changes in the band intensities or positions (νa(CH2) at 2920 cm-1 and νs(CH2) at 2852 cm-1) upon γ-irradiation for exposures as large as 2 Mrad. Shown in Figure 5 are RAIR spectra of representative undec-10-ene-1-thiol monolayers at various time points during the exposure. Due to logistics and space constraints (in a continuous run, there was not sufficient space to have more than roughly four sets of Au slides in the steel bomb), the largest exposure used was 2 Mrad for the planar (39) Mitsui, H.; Machi, S.; Hagiwara, M.; Hosoi, F.; Kagiya, T. J. Polym. Sci., A-1 1967, 5, 2731.

Figure 5. RAIR spectra of undec-10-ene-1-thiol monolayers on planar Au obtained at various γ-ray exposures: A, pristine (no exposure); B, 77 krad; C, 0.31 Mrad; D, 2 Mrad.

Figure 6. Plot of percent monomer conversion (integrated band intensity ratio (initial-time point)/initial)) for undec-10-ene1-thiol monolayers on planar Au from Figure 5 as a function of γ-ray exposure: 9, changes in the νa(dCH2) mode; 2, changes in the ν(CdC) mode.

Au samples. As can be seen from Figure 5, the intensities of the νa(dCH2), νs(dCH2), ν(H2CdC-H), and ν(CdC) bands decrease with increasing exposure to the γ-rays. The fraction of the olefin groups converted (ν(CdC) and νa(dCH2)) versus exposure time is shown in Figure 6. During the exposures, no new spectral features appeared, which is expected for reaction of the olefin groups with each other to form oligomers. Other reactions can occur which yield products (oxidized species) that are easily observed in the RAIR spectra, vide infra. Upon close inspection of the spectra in Figure 5, noticeable increases in the band positions for νa(CH2) and νs(CH2) are observed for the longest exposure (∼3 cm-1 shift for each mode upon irradiation for 2 Mrad). This observation is indicative of alkane chain disordering during the irradiation procedure, albeit apparently small. Although this disordering is subtle in terms of a blue shift for the νa(CH2) and νs(CH2) bands, the electrochemical blocking characteristics change dramatically. This is exemplified by Figure 7 which consists of cyclic voltammograms for a pristine undec10-ene-1-thiol monolayer (νa(CH2) at 2922 cm-1 and

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Figure 7. Cyclic voltammograms of Ru(NH3)63+ in 0.1 M KCl at undec-10-ene-1-thiol monolayers on planar Au(111)/mica (A) before and (B) after 2 Mrad exposure to γ-rays.

Figure 9. Schematic of undec-10-ene-1-thiol monolayer tethered to a surface (A) before and (B) after oligomerization to demonstrate possible structural changes incurred. Dashed lines indicate direction of chain axes.

Figure 8. Reductive desorption of undec-10-ene-1-thiol monolayers on Au(111)/mica: A, pristine monolayer; B, monolayer after 77 krad of γ-ray exposure. Electrolyte is 0.1 M KOH.

νs(CH2) at 2851 cm-1) and an olefin monolayer with a total dose of 2 Mrad (νa(CH2) at 2925 cm-1 and νs(CH2) at 2853 cm-1). Very little electrochemical blocking is observed after 2 Mrad of exposure (∼35% conversion of monomer) as noted by ip,c2Mrad/ip,cbare ) 0.92 ( 0.01 and ∆Ep ) 65.7 ( 2.1 mV. This increase in ion penetration into the monolayer is also observed in the thiol desorption voltammogramssan approximately 100 mV positive shift in Ep,c is found for exposures as small as 77 krad (1 h), Figure 8. No further change in Ep,c (thiol desorption wave) was observed when doses larger than 77 krad were used. This positive shift in Ep,c is expected for a layer that becomes less ordered. Porter has shown that the reduction potential for n-alkanethiol desorption decreases with decreasing chain length and interpreted this result in terms of a decreased efficacy for preventing the electrolyte from approaching the thiol headgroup.24 The shift in Ep,c that is observed upon γ-irradiation cannot be attributed to sulfur desorption (present as a result of radiationinduced cleavage of the C-S bond) due to the fact that the potential for sulfur desorption in 0.1 M hydroxide is -965 mV versus SSCE.40 Another important observation is that there was no change in the amount of charge under the Au-SR + 1e- f RS- + Au voltammetric wave during the γ-ray exposures. This latter result indicates that there is no loss of thiol from the surface due to the incident (40) Zhong, C.-J. Private communication, 1997.

radiation. Thus, the decreases in IR band intensities for the modes associated with the olefin group with increasing radiation dose are due to consumption of the olefin groups, and the blue shifts observed for the νa(CH2) and νs(CH2) bands are attributed to disordering of the alkane chains as a result of olefin consumption.19f,41 The band intensities of the various olefin modes all decrease with increasing γ-ray exposure, pointing to oligomerization of the monolayer. Although the resulting oligomer does not have any new and distinct infraredactive modes (the modes associated with the “new” alkane backbone overlap with those already present), the following data do indeed support oligomer formation in the monolayer. In contrast to the ∼35% conversion of the olefin group at 2 Mrad exposure, we noted a 10-20% increase in the integrated band intensities for the νa(CH2) and νs(CH2) modes. This increase in the methylene intensities can be attributed to reorientation of the alkane chains (chains standing less upright) needed to accommodate the coupled monomer groups. Alkane chain reorientation has been observed in the electron-beam polymerization of ω-tricosenoic acid Langmuir-Blodgett multilayers.19d,41 This reorganization event has been suggested to be associated with the formation of cornshock-like structures during solid-state (both bulk crystals and Langmuir-Blodgett films) polymerizations using ionizing radiation. Such lattice-controlled propagation reactions in L-B multilayers have been shown to be stress limited and thus yield monodisperse polymer.19f We propose a similar corn shock structure for our monolayers that have undergone oligomerization, but with much shorter lengths (fewer repeats), Figure 9. This would not be unexpected when considering the decreased maneu(41) Although not commented on by the authors, Barraud et al..19d displayed spectra in their paper on electron beam polymerization of L-B multilayers of ω-tricosenoic acid that clearly show blue shifts in frequencies (∼3 cm-1) and intensity changes for the νa(CH2) and νs(CH2) modes upon irradiation.


Figure 10. RAIR spectra demonstrating the effect of trace amounts of oxygen present during γ-ray exposure of undec10-ene-1-thiol monolayers on planar Au. The total dose is 18 krad (∼15 min).

verability of the tethered alkane chains (more stress upon coupling). The presence of these “gathered” monomer structures would also give rise to very defective monolayers from a transport point of view. Our voltammetric data support this proposal. Chain scission reactions and unsaturated bond rearrangement reactions (formation of internal olefins) are not consistent with our data. Irradiation of poly(ethylene) causes alkane chain scission and production of a variety of olefin moieties that are not at the chain termini (internal olefins).42 Although radiation-induced scission reactions could possibly occur in organized, crystalline environments, geminate recombination reaction rates should preclude loss of the chain fragment (disproportionation to give a terminal olefin and an alkane).42 In the case of the undec-10-ene-1-thiol and n-undecanethiol monolayers on Au, we have observed no indication of internal olefin production or chain scission reactions. The latter possibility should result in a decrease in the infrared band intensities of the methylene modes as a function of exposure time, similar to the rate of disappearance of the olefin bands. This is not the case. As mentioned above, upon γ-irradiation there is no change in the band intensities or positions for the n-undecanethiol monolayers, and a 10-20% increase in the methylene band intensities is noted for the undec-10-ene-1-thiol monolayers. In addition, isomerization of the terminal olefin to an internal double bond would result in new IR bands in the 960-970 cm-1 region. The ω(dCH2) mode for terminal olefins has a very characteristic absorption near 915 cm-1, while this mode leads to absorptions at ∼965 cm-1 for internal olefins.31 Upon inspection of the spectra of undec-10-ene-1-thiol monolayers after 2 Mrad of γ-ray exposure, the only band present in the 900-970 cm-1 region is the ω(dCH2) mode for the terminal olefin at 914 cm-1. Complete oxidation of the olefin moieties in the undec10-ene-1-thiol monolayers on planar Au can be accomplished by γ-irradiation in the presence of O2 or premature exposure of irradiated monolayers to O2 (no postirradiation argon purge). Shown in Figure 10 are spectra for a pristine undec-10-ene-1-thiol monolayer (A) and the same monolayer after γ-ray exposure (18 krad or ∼15 min) in the presence of trace O2 (B). It is clear that (42) Clough, R. In The Encyclopedia of Polymer Science and Engineering, Vol. 13, Kroschwitz, J. I., Ed.; Wiley: New York, 1985; pp 667-708.

Langmuir, Vol. 14, No. 1, 1998 121

no singular oxygen-containing moiety was formed. On the basis of the breadth of the carbonyl band and the shoulders seen in it, it appears ketones and carboxylic moieties were formed. Virtually all of the monolayer could be removed by irradiation in O2 for longer exposure times (t > ∼2 h). The mechanism for these changes is not immediately apparent, but must certainly involve O2 and/ or O3. Oxygen is known to form ozone in the presence of ionizing radiation42 and thus could readily react with the olefin and produce various oxygen-containing functionalities in the monolayer. In addition, it is well-known that oxygen can quench free-radical polymerization reactions.19a,39 Such oxygen quenching has been shown to lead to peroxides and other oxy species that decompose to form various oxidized organics.39 A very important observation regarding oxygen reactivity is monolayers which have been irradiated for long exposure times (>1 Mrad) under anaerobic conditions, removed from the source, but not purged with Ar for at least 90 min after removal from the 60Co source, exhibited complete oxidation of the olefin functionality. As long as samples were purged for 90-120 min after removal from the γ-ray source, we observed no oxidation. Such “delayed” oxidation reactions point to the existence of long-lived radicals in the monolayer, a conclusion that is consistent with previous reports concerning irradiation of Langmuir-Blodgett films and bulk single crystals of unsaturated fatty acids/fatty acid esters.19a-c The presence of long-lived radical species in the irradiated undec-10-ene-1-thiol monolayers suggests relatively little quenching by the Au substrate. In addition, the observation that samples purged with Ar after irradiation do not exhibit oxidation but instead have only some fraction of the olefin consumed and the fact that long γ-ray exposures lead to moderate monomer conversion (∼35% for 2 Mrad), in conjunction with the observations of complete olefin oxidation (for large exposures) when samples are not postirradiation purged, indicate that the oligomerization reactions are indeed stress limited. From the data and discussion presented here, we conclude that γ-irradiation of undec-10-ene-1-thiol monolayers on planar Au under anaerobic conditions results in consumption of the olefin groups to yield somewhat disordered, oligomeric monolayers. Due to the geometric constraints of the resulting polymer, it is not surprising that extended exposures lead to modest conversion of the olefin groups in the monolayer. This latter observation suggests that the oligomerization reaction of the undec10-ene-1-thiol would be highly sensitive to the physical nature of the underlying substrate. Nanocrystalline or Curved Au. Since the first report by Brust et al.,25 thiol-stabilized Au nanoclusters have become popular due to their possible application in single-electron devices and in the construction of complex molecular architectures.8,29,30,43 In addition to their exciting properties, thiol-stabilized Au nanocrystals offer an avenue of monolayer characterization not readily available for monolayers on planar Au. More importantly, due to the large surface-to-volume ratio and proposed geometry of these systems, the molecular environment of the chain terminus of monolayers on such small Au crystals should be different from that on planar Au and also controllable through variations in the Au core size. This expectation has recently come to fruition in infrared and calorimetry studies of various-sized thiol-stabilized Au clusters. Murray and co-workers have shown that the number of gauche defects in the alkane chains is a function of the (43) (a) Schon, G.; Simon, U. Colloid Polym. Sci. 1995, 273, 101. (b) Schon, G.; Simon, U. Colloid Polym. Sci. 1995, 273, 202.

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radius of the Au core.29d This result can be explained by the preponderance of edge and corner adsorption sites on the nanometer-sized Au crystals (cuboctahedron or ideal truncoctahedron) and variations in the ratio of the number of these edge/corner sites to the terrace sites, as well as changes in the radius of curvature of the Au core, as a function of Au core size. Thus, very small Au nanocrystals (diameter less than ∼4 nm) have a fairly large radius of curvature which results in a substantial number of chain end-gauche defects (10-25% of all chains)29b in n-alkanethiol monolayers on such Au crystals. We chose to investigate these model curved Au surfaces in order to gain insight about polymerization reactions in SAMs on Au-coated, high-aspect-ratio AFM tips. To test the effects of substrate curvature on the oligomerization of the olefin monolayers, drop-coated films of the 1.3 nm Au core diameter undec-10-ene-1-thiol/Au clusters on NaCl plates were exposed to γ-rays for various amounts of time. As noted by IR spectroscopy, exposures greater than 2 Mrad did not result in any observable consumption of the olefin group when Ar purging was employed (spectra not shown).44 However, if trace quantities of oxygen were present during the exposures, decreases in the infrared band intensities of the olefin modes and the appearance of bands indicative of carbonyl formation were observed, as noted above with planar Au. Thus, the ω-olefin termini of the alkane chains on the curved Au do not appear to be able to “find” other olefin groups so as to couple and form oligomer, but the olefin is indeed accessible by and reactive with gaseous reagents. There are two possible explanations for these results. One reason for the inability of the olefin groups to encounter each other during γ-ray exposure could be the possible existence of space between olefin groups as a result of the curvature of the underlying Au core. Another possibility is that interdigitation of the alkane chains from neighboring clusters effectively isolates the olefin groups (the olefins are essentially “buried” in the methylene chains), thus precluding reaction with other olefins but not mobile reactants such as oxygen or ozone. From our data it appears that interdigitation of the alkane chains is the most plausible explanation for the lack of olefin group oligomerization during γ-ray exposure of dropcast films of the olefin-thiol/Au clusters. We base this conclusion on several observations. The νa(CH2) and νs(CH2) band positions for dropcast films of the undec10-ene-1-thiol/Au clusters are 2-3 cm-1 lower than those observed on planar Au indicating a higher degree of chain crystallinity. Transmission electron micrographs of undec-10-ene-1-thiol/Au and n-alkanethiol/Au clusters clearly demonstrate that the Au surface-Au surface distance is much less than that expected for Au surfaces separated by two thiol layers. More importantly, the observed νa(dCH2) band position of 3075 cm-1 for the 1.3 nm (Au core) nanoclusters indicates that the environment of the olefin is like that found in the neat liquid alkane in nature.27 Our final piece of evidence suggesting chain interdigitation comes from variation of the Au core size (effectively the radius of curvature). Murray has shown that n-alkanethiol monolayers on Au nanocrystals with diameters greater than ∼4 nm have order (lack of gauche defects as noted by the νa(CH2) and νs(CH2) band positions) virtually identical with those on planar Au. γ Irradiation studies with 3.4 nm diameter Au/undec-10-ene-1-thiol clusters yield results indistinguishable from those found (44) No changes in the integrated band intensities for the olefin modes were noted when using the integrated band intensity of the νs(CH2) mode as an internal reference to account for variations in sample positioning in the IR spectrometer.


Figure 11. Transmission IR spectra of dropcast films of 3.4 nm diameter (Au core) undec-10-ene-1-thiol/Au clusters on NaCl plates (A) before and (B) after 2.5 Mrad exposure to γ-rays.

for the 1.3 nm clusters, Figure 11. Also, the νa(dCH2), νa(CH2), and νs(CH2) band positions for the 3.4 nm Au samples are observed at 3075, 2919, and 2849 cm-1, values virtually identical with those found on 1.3 nm Au. Due to the expectation that the undec-10-ene-1-thiol monolayers on the 3.4 nm Au should behave more like those on planar Au (smaller radius of curvature) and thus oligomerize, another factor affecting the molecular environment of the ω-olefin termini must prevent oligomerization. Although lateral alkane chain folding (gauche chain-end defects)29b would give results similar to chain interdigitation (olefin group isolation), the necessary number of such chain defects would be extraordinary, and their presence would be readily observable in the IR spectra.45 Finally, chain interdigitation between clusters in three dimensions (films were several microns thick) would still give rise to a film “surface” of exposed olefin termini that could and should oligomerize (as long as the curvature of the substrate does not play a role) during γ-ray exposure. In addition, the packing of the Au nanocrystals in three dimensions would not be expected to be “perfect” and should lead to dangling or exposed olefins (not interdigitated) that could oligomerize.46 If a substantial fraction of the olefin termini were exposed, the νa(dCH2) band should be broadened with a shoulder at ∼3085 cm-1sthis is not observed. In either case, the number of oligomerizable olefin sites (approximately 10-8 mol for olefin groups at the dropcast film surface) in comparison to the remaining interdigitated olefins must be very small, such that their initial presence (as noted by a band at ∼3085 cm-1) and subsequent possible consumption46 upon irradiation goes undetected by IR. Thus, we believe that chain interdigitation precludes oligomerization. It is, however, possible that the olefin (45) As mentioned in the body of the text, IR bands at 1439 and 1342 cm-1 (δD and WE modes) indicating the presence of chain end-gauche defects are found for undec-10-ene-1-thiol monolayers on the 1.3 nm Au crystals. These bands are also present for monolayers on the 3.4 nm crystals. A calculation of the percent population of chain end-gauche defects in the monolayers yields values of roughly 5-15% for both the 1.3 and 3.4 nm clusters. The presence of such a small number of defects should not greatly affect the probability of oligomerization, unless the majority of these defects is found in regions where there is no interdigitation of chains from neighboring clusters. (46) This assumes that the chains which are not interdigitated are indeed amenable to oligomerization. That is, the noninterdigitated chains are assumed to contain no chain end-gauche defects. It is very likely that the majority of the chain end-gauche defects45 would be found where interdigitation does not occur. Such a scenario would explain why we do not observe an IR band near 3085 cm-1 (blue-shifted band) for the νa(dCH2) mode.


monolayers on the 3.4 nm diameter Au do not mimic well those on planar Au, which would leave open the possibility that the curvature of the Au affects the coupling reactions. Future work with solution-phase polymerization initiators and dissolved olefin-thiol nanoclusters may lend further insight to the subject of two-dimensional polymerization reactions on curved surfaces. Summary Monolayers of undec-10-ene-1-thiol on planar Au are shown to have structures similar to those of 11-mercaptoundecanol and n-undecanethiol as judged by IR spectroscopy, electrochemical surface coverage determinations, and electrochemical blocking measurements. Oligomerization of undec-10-ene-1-thiol monolayers on planar Au as a function of γ-ray exposure can be followed using infrared spectroscopy. The olefin conversion rate is modest, with roughly 35% conversion occurring at doses of 2 Mrad. Slight disordering of the monolayer occurs during the exposures, as noted by the band positions of the νa(CH2) and νs(CH2) modes. This disordering is more readily detected during voltammetric experimentsslarge

Langmuir, Vol. 14, No. 1, 1998 123

decreases in blocking capabilities are observed. In addition, postirradiation oxidation of the monolayers points to the existence of trapped radical species in the monolayers. All of the data lead us to conclude that the oligomerization reaction is controlled by the distance the tethered monomer groups can move. Transmission IR studies of undec-10-ene-1-thiol/Au nanoclusters irradiated for doses >2 Mrad do not indicate any monomer consumption. This lack of coupling is concluded to be due to cluster-cluster alkane chain interdigitation, which effectively isolates the olefin groups. Future studies focusing on solution polymerizations of dissolved nanoclusters may result in an understanding of the effect of Au substrate curvature on coupling probability. Such studies may also lead to methods that allow for better control of chemical reactions in molecular assemblies. Acknowledgment. We are grateful to the National Science Foundation (CHE-9529770) and the Louisiana Education Quality Support Fund (LEQSF(1993-96)-RDA-09) for financial support of this work. LA970823A

Surface-Confined Monomers on Electrode Surfaces. 4

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