2660
Langmuir 2005, 21, 2660-2661
Reactions of CdC-Terminated Self-Assembled Monolayers with Gas-Phase Ozone Larry R. Fiegland, Marilyn McCorn Saint Fleur,† and John R. Morris* Department of Chemistry, Virginia Tech, Blacksburg, Virginia 24061 Received January 6, 2005 Reactions of gas-phase ozone with alkene-terminated alkanethiol self-assembled monolayers on Au are explored using reflection-absorption infrared spectroscopy (RAIRS). The experiments are performed by using a directional doser to control the ozone flux onto the surface and in situ high vacuum techniques to eliminate reactions with atmospheric contaminants. We find that reactions between ozone and the CdC terminal group proceed through the formation of a carboxylic acid moiety that subsequently converts to an interchain carboxylic acid anhydride.
The reactions of ozone with unsaturated organic surfaces play an important role in atmospheric chemistry. Many man-made materials contain CdC functional groups, which are potential reaction sites for the atmospheric pollutant ozone. Surfactants coating the human lungs contain unsaturated phospholipids that are also highly reactive toward ozone. Although reactions of ozone with CdC moieties in gas and condensed phases have been extensively explored,1 less is known about the reactions and mechanisms at the gas-surface interface. Uosaki,2 Thomas,3 and Dubowski4 have shown that when unsaturated organic monolayers on silicon react with ozone, carboxylic acid-containing products are formed. Manhimmey and Yates used FTIR to observe the direct formation of carboxylic acid moieties from the reaction of amorphous carbon with ozone.5 In the same infrared spectrum, they also observed the formation of a broad shoulder at 1860 cm-1 that was attributed to the formation of a strained carbonyl species, possibly an anhydride.5 In addition, ozonolysis has been shown to oxidize the AusS bonds in saturated self-assembled monolayers (SAMs) of alkanethiols on Au.6-8 The study described below explores the reaction mechanisms of ozone with unsaturated organic surfaces by using CdC-terminated alkanethiol SAMs (CdC-T SAMs) to afford a highly characterized, well-ordered organic surface with the double bond isolated at the gas-surface interface. We find direct evidence for the formation of interchain carboxylic anhydride groups from the reaction of CdC-terminated alkenethiols on Au with gas-phase ozone. The experiments follow the reactions of undec-10-ene1-thiol SAMs on Au with gas-phase ozone using reflectionabsorption infrared spectroscopy (RAIRS). The alkenethiols were synthesized according to published procedures,9 and the monolayer surfaces were prepared as previously reported.10 A Bruker IFS 66v/S spectrometer, equipped with a liquid N2 cooled mercury cadmium telluride (MCT) * Corresponding author. E-mail:
[email protected]. † Current address: Tufts University, Medford, MA 02155. (1) Bailey, P. S. Ozonation in Organic Chemistry; Academic Press: New York, 1978; Vol. I. (2) Uosaki, K.; Quayum, M. E.; Nihonyanagi, S.; Kondo, T. Langmuir 2004, 20, 1207-1212. (3) Thomas, E. R.; Frost, G. J.; Rudich, Y. J. Geophys. Res. 2001, 106 (D3), 3045-3056. (4) Dubowski, Y.; Vieceli, J.; Tobias, D. J.; Gomez, A.; Lin, A.; Nizkorodov, S. A.; McIntire, T. M.; Finlayson-Pitts, B. J. J. Phys. Chem. A 2004, 108, 10473-10485. (5) Mawhinney, D. B.; Yates, J. T., Jr. Carbon 2001, 39, 1167-1173. (6) Zhang, Y.; Terrill, R. H.; Tanzer, T. A.; Bohn, P. W. J. Am. Chem. Soc. 1998, 120, 2654-2655. (7) Poirier, G. E.; Herne, T. M.; Miller, C. C.; Tarlov, M. J. J. Am. Chem. Soc. 1999, 121, 9703-9711. (8) Norrod, K. L.; Rowlen, K. L. J. Am. Chem. Soc. 1998, 120, 26562657.
Figure 1. RAIR spectra of a CdC-T SAM on Au: (A) before exposure to ozone; (B) after exposure to ∼1000 L of ozone.
detector, was used to record the RAIR spectra. Each spectrum was collected using p-polarized light at an incident angle of 86° and represents the average of 100 scans. A resolution of 2 cm-1 was used with clean gold substrates as background references, unless otherwise noted. The experiments were performed in high vacuum (5 × 10-7 Torr) to minimize the effects of background water and other contaminants. Figure 1A shows a RAIR spectrum of an unexposed CdC-T SAM. The absorbance at 3085 cm-1 is attributed to the dCH2 asymmetric stretching mode of the CdC-T monolayer.9 The peaks at 2920 and 2851 cm-1 are due to the asymmetric and symmetric stretching modes of the methylene units within the hydrocarbon chains, respectively.9 Three additional peaks (1644, 994, and 911 cm-1) observed in the spectrum are due to the CdC stretch, CdC out-of-plane deformation, and the CsH out-of-plane deformation of the dCH2 moiety, respectively.9 After recording the spectrum of the CdC-T SAM, the surface was exposed to a highly pure source of ozone through a specially designed directional doser attached to an ozone storage system.11 Mass spectra of the ozone source indicate only minor impurities of O2, CO2, and H2O. Figure 1B shows the RAIR spectrum of the CdC-T SAM after exposure to ∼1000 Langmuirs (L) of ozone. After (9) Peanasky, J. S.; McCarley, R. L. Langmuir 1998, 14, 113-123. (10) Ferguson, M. K.; Low, E. R.; Morris, J. R. Langmuir 2004, 20, 3319-3323. (11) Zhukov, V.; Popova, I.; Yates, J. T., Jr. J. Vac. Sci. Technol., A 2000, 18, 992-994.
10.1021/la050044r CCC: $30.25 © 2005 American Chemical Society Published on Web 03/04/2005
Letters
Langmuir, Vol. 21, No. 7, 2005 2661 Scheme 1. Schematic Showing a Proposed Mechanism for the Reaction of a CdC-T SAM with Ozone
Figure 2. RAIR difference spectra: (A) CdC-T SAM during exposure to ozone; (B) COOH-T SAM postexposure.
exposure, the peaks associated with the double bond (3085, 1644, 994, and 911 cm-1) disappear, indicative of reactions at the CdC terminal bond. In addition, the modes attributed to the vibration of the methylene units, initially at 2920 and 2851 cm-1, both broaden and blue shift upon exposure. This change in peak position and shape is attributed to monolayer disordering.9 The disordering is likely the result of a combination of effects including reactions at the terminal CdC group and oxidation of the AusS bond. A unique change occurs in the carbonyl region of the postexposure spectrum where new peaks at 1821 and 1750 cm-1 appear. Yan et al. have previously assigned these features to the in-phase and out-of-phase stretching modes, respectively, of an interchain carboxylic anhydride group on an alkanethiol SAM on Au.12 The other new feature in the postexposure spectrum appears at 1040 cm-1, which we assign to a CsO stretch.13 We further investigated the mechanism of this gassurface reaction through in situ RAIRS studies. Figure 2A shows a subset of the RAIR difference spectra of the CdC-T SAM during ozone exposure. For the difference spectra, the unexposed monolayer serves as the background for each spectrum. In this figure, exposure time increases from bottom to top. The bottom spectrum is the unexposed CdC-T SAM, and the top spectrum is the SAM after 1 h of ozone exposure. The exposure between each spectrum increases by ∼100 L. The data show that a peak at 1744 cm-1 begins to emerge after 200 L of exposure and then shifts to 1750 cm-1 as it increases in intensity. At the same time, a new peak emerges at 1821 cm-1 and features associated with the CdC functional group disappear (negative peaks at 1644 and 912 cm-1). The time-resolved spectra suggest that the CdC-T SAM reacts with ozone to initially form a carboxylic acid intermediate that converts to a carboxylic anhydride group upon further exposure. This interpretation is supported by Figure 2B which shows the RAIR difference spectrum of an 11-mercapto-undecanoic acid (COOH-T) SAM exposed to ozone. The COOH-T SAM originally has a peak at ∼1720 cm-1 that is replaced by two peaks at 1823 and (12) Yan, L.; Marzolin, C.; Terfort, A.; Whitesides, G. M. Langmuir 1997, 13, 6704-6712. (13) Socrates, G. Infrared and Raman Characteristic Group Frequencies: Tables and Charts, 3rd ed.; John Wiley and Sons, LTD: New York, 2001.
1760 cm-1 after exposure. The new peaks are consistent with the postexposure CdC-T SAM spectrum and are attributed to the carbonyls of an anhydride. Therefore, it appears that the COOH moiety is a feasible intermediate for the reaction of ozone with a CdC-T SAM to form an anhydride. We also find that the 1821 cm-1 peak, associated with the anhydride, disappears upon exposure to water vapor leaving behind a spectrum very similar to a carboxylic acid-terminated monolayer. Scheme 1 shows a proposed mechanism for the formation of anhydride moieties at the terminal position. We hypothesize that the CdC functional group reacts with ozone to form a primary ozonide1 that decomposes to a diradical intermediate.4 This intermediate rearranges to form a carboxylic acid-terminated monolayer that undergoes a dehydration reaction to form interchain anhydride groups. In addition to changes in the RAIR spectra associated with reactions at the CdC terminal group, the spectra also reveal that the SAM becomes disordered after ozone exposure. Specifically, the methylene CsH stretches blue shift and broaden (see Figure 1). This disordering should enhance the diffusion of ozone through the film to the substrate, allowing further oxidation of the AusS bond. Surprisingly, there is no evidence of further changes in the spectra upon prolonged exposure. This is in contrast to that observed for methyl-terminated SAMs exposed to ozone. Our experiments with methyl-terminated SAMs show that the AusS bonds readily oxidize, resulting in facile and nearly complete removal of the hydrocarbon chains from the substrate. These results are similar to previous studies.6,8 However, for the CdC-T SAM, we observe strong RAIR peaks for the methylene stretches even after prolonged exposure to ozone and rinsing with copious amounts of water. The difference between the reactions of the CdC-T SAMs and the methyl-terminated monolayers is not fully understood, but we speculate that cross-linking of the terminal groups may help stabilize the monolayers toward desorption from the substrate. Future X-ray photoelectron spectroscopy measurements will be used to further probe the mechanistic details of the reactions of ozone at the substrate. Acknowledgment. We thank Robin L. McCarley for helpful advice associated with the synthesis of the C)C-T thiol. We thank Tom Wertalik for his skillful glassblowing. Funding was provided by the National Science Foundation (CAREER Award No. CHE-94269). LA050044R
