Langmuir 2000, 16, 2981-2984
Selective Cleavage of the Carbon-Halide Bond in Substituted Benzaldimine Monolayers by Synchrotron Soft X-ray: Anomalously Large Cleavage Rate of the Carbon-Bromide Bond
2981 Scheme 1
Joong Ho Moon, Young-Hye La, Jeo Young Shim, Bong Jin Hong, Ki Jeong Kim,† Tai-Hee Kang,† Bongsoo Kim,† Heon Kang,‡ and Joon Won Park* Department of Chemistry, Polymer Research Institute, Pohang University of Science and Technology, San 31 Hyoja-dong, Pohang, 790-784, Korea Received August 16, 1999. In Final Form: November 19, 1999
Experimental Section Introduction Bond cleavage or damage to organic materials, induced by X-ray irradiation, has been observed.1-7 The phenomena are commonly observed while taking X-ray photoelectron spectra. Recent advances in preparing well-characterized molecular layers8 have made quantitative studies of the cleavage possible. Whitesides et al. examined the damage to the monolayer and demonstrated that induced photoelectrons from the surfaces, rather than incident X-ray, are mainly responsible for the damage.2 Also, Sagiv et al. applied Fourier transform infrared (FT-IR) spectroscopy to obtain a sensitive view of the damage.3 Provided that the damage were chemoselective, i.e., a particular bond were cleaved more efficiently than the others, and the transformation in accompany with the damage led to a new functional group selectively, then the transformation upon irradiation could be used to tailor the surface property with a spatial resolution.8,9 We previously reported that C-N bond cleavage in the nitrobenzaldimine monolayer is indeed selective.10 Herein, we report that carbon-halide bond cleavage of the halogen-substituted benzaldimines is selective, and among the homologous series, the carbon-bromide bond exhibits an anomalously large cleavage rate. * To whom correspondence should be addressed. Phone: +82562-279-2119. Fax: +82-562-279-3399. E-mail: jwpark@ vision.postech.ac.kr. † Pohang Accelerator Laboratory, Pohang University of Science and Technology, Pohang 790-784, Korea. ‡ Department of Chemistry and Center for Ion-Surface Reaction, Pohang University of Science and Technology. (1) (a) Briggs, D.; Seah, M. P. Practical Surface Analysis, 2nd ed.; John Wiley & Sons: Chichester, 1990. (b) Beamson, G.; Seah, M. P. High-Resolution XPS of Organic Polymers; John Wiley & Sons: Chichester, 1992. (2) (a) Graham, R. L.; Bain, C. D.; Biebuyck, H. A.; Laibinis, P. E.; Whitesides, G. M. J. Phys. Chem. 1993, 97, 9456-9464. (b) Laibinis, P. E.; Graham, R. L.; Biebuyck, H. A.; Whitesides, G. M. Science 1991, 254, 981-983. (3) Frydman, E.; Cohen, H.; Maoz, R.; Sagiv, J. Langmuir 1997, 13, 5089-5106. (4) Rieke, P. C.; Baer, D. R.; Fryxell, G. E.; Engelhard, M. H.; Porter, M. S. J. Vac. Sci. Technol., A 1993, 11, 2292-2297. (5) Duvault-Herrera, Y.; Jaffrezic-Renault, N.; Morel, D.; Serpinet, J.; Duvault, J.; Hollinger, G. Colloids Surf. 1991, 53, 169-182. (6) Horr, T. J.; Arora, P. S. Colloids Surf., A 1997, 126, 113-121. (7) Wasserman, S. R.; Tao, Y.-T.; Whitesides, G. M. Langmuir 1989, 5, 1074-1087. (8) Ulman A. An Introduction to Ultrathin Organic Films from Langmuir-Blodgett to Self-Assembly; Academic: New York, 1991. (9) Xia, Y.; Whitesides, G. M. Angew. Chem., Int. Ed. Engl. 1998, 37, 551-575. (10) Moon, J. H.; Kim, K. J.; Kang, T. H.; Kim, B.; Kang, H.; Park, J. W. Langmuir 1998, 14, 5673-5675.
The silane coupling agent (3-aminopropyl)diethoxymethylsilane was purchased from Gelest, Inc. All aldehydes, except 4-iodobenzaldehyde, were purchased from Aldrich Chemical Co. and used as received. 4-Iodobenzaldehyde was purchased from Landcaster Synthesis Ltd. The polished prime Si(100) wafers (dopant, phosphorus; resistivity, 1.5-2.1 Ω‚cm) were purchased from MEMC Electronic Materials, Inc. UV-vis spectra were recorded on a Hewlett-Packard diode-array 8453 spectrophotometer. Deionized water (18 Ω/cm) was obtained by passing distilled water through a Barnstead E-pure 3-Module system. A synchrotron radiation source (Pohang Accelerator Laboratory, Korea)11 was used for the photocleavage and X-ray photoelectron spectroscopy (XPS) analysis of the benzaldimine monolayers. The photon energy was selected to give the most-enhanced photoemission intensity and/or the best-resolved spectra. Electron binding energies were calibrated against the C(1s) emission at Eb ) 284.6 eV or the Si(2p) emission at Eb ) 99.3 eV.1
Results and Discussion The benzaldimine monolayers were prepared by treating the aminosilylated silicon wafer with halide-substituted aldehydes (Scheme 1).10,12 The aldehydes included 4-fluorobenzaldehyde (1), 4-chlorobenzaldehyde (2), 4-bromobenzaldehyde (3), 3,5-dibromosalicylaldehyde (4), and 4-iodobenzaldehyde (5). The absolute density of the imines on the surface was determined by the same method applied to 4-nitrobenzaldehyde,12 and the pertinent extinction coefficients were used for the calculation. For example, the 4-fluorobenzaldimine-formed substrate (1 × 3 cm) was dipped into water of the known volume for the hydrolysis (10 mL), and the concentration of the generated aldehyde was measured by UV-vis spectrophotometer. The absolute density of the imine was obtained from the amount of the aldehyde and the known surface area of the substrate. Invariably absolute density of ca. 3 amines/ nm2 was measured for each sample. The survey spectra of the benzaldimine monolayers, obtained at 820 eV, showed the bands arising from the core level of corresponding halides (such as F(1s), Cl(2p), Br(3d), and I(3d)) and other components (such as C(1s), N(1s), O(1s), Si(2s), and Si(2p)). The positions of these peaks coincide well with the literature values.1 As an example, a survey spectrum of the 4-bromobenzaldimine monolayer is shown in Figure 1. A new Br(3d) band at 71 eV, which appeared after the derivatization of the ami(11) Rah, S.-Y.; Kang, T.-H.; Chung, Y.; Kim, B.; Lee, K.-B. Rev. Sci. Instrum. 1995, 66, 1751-1753. (12) (a) Moon, J. H.; Kim, J. H.; Kim, K.-J.; Kang, T. H.; Kim, B.; Kim, C.-H.; Hahn, J. H.; Park, J. W. Langmuir 1997, 13, 4305-4310. (b) Moon, J. H.; Shin, J. W.; Kim, S. Y.; Park, J. W. Langmuir 1996, 12, 4621-4624.
10.1021/la991061h CCC: $19.00 © 2000 American Chemical Society Published on Web 01/20/2000
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Notes Table 1. Normalized Rate Constants and the Cross Section for the Cleavage of the Carbon-Halide Bonds substituted benzaldimine monolayer 4-F 4-Cl 4-Br 3,5-dibromo-2-hydroxy 4-I 4-NO2c
PEa
(eV)
820 350 250 250 780 500
knormalizedb (×103) (s-1)
cross section (σr) (×1015) (cm2/photon)
0.09(2) 0.18(2) 0.86(9) 0.12(1) 0.20(8) 0.53
0.39(9) 0.83(8) 3.9(4) 0.56(4) 0.9(3) 2.4
a Photon energy used for the irradiation. b The rate constants were normalized using a particular photon flux (2.2 × 1011 photons/ (cm2‚s)) observed at 820 eV. c From the previous result.
Figure 1. XPS survey spectrum of the 4-bromobenzaldimine monolayer.
nosilylated substrate, clearly indicates the presence of the bromine group in the layer. The two Si(2p) peaks originating from silicon oxide (103 eV) and bulk silicon (99.1 eV) confirm that the thus-formed imine layer is thinner than the escape depth of photoelectrons. Analysis of the spectra along the irradiation time showed that the carbon-halide bond was cleaved upon irradiation. As seen in Figure 2a, the intensity of Br(3d) bands diminished as X-ray irradiation progressed. A spectrum obtained immediately after the light-shutter was open is composed of filled squares (9). The second (0), third (b), fourth (O), fifth (2), and sixth (4) data were recorded at time intervals of 10 min under continuous X-ray irradiation at 250 eV. The peaks from the C(1s), N(1s), and O(1s) levels were also examined as a function of the irradiation time (Figure 2b-d). Measurements were made at more than three independent spots on a sample surface to check reproducibility. The shape and intensity of the latter peaks were found to be invariant within the error bound under X-ray irradiation, while the Br(3d) peak was reduced. A simulation using the reference peak intensity data showed that the dissociation of a single carbon atom from the phenyl ring should have been discernible in the spectra, if it occurred concurrently with the cleavage of the carbonbromide bond. Therefore, it is clear that the carbonbromide bond is cleaved selectively by X-ray irradiation, leaving the phenyl ring intact on the surface. The selectivity is reminiscent of the selective C-N bond cleavage of the 4-nitrobenzaldimine monolayer. Examination of the other halide-substituted benzaldimine layers also reveals that selective cleavage is common among the homologous halide-substituted benzaldimine on the surfaces. To examine the effect of the incident photon energy on the carbon-halide bond cleavage, we measured the rate constants of the bromide-substituted benzaldimine monolayer at various energies (150-820 eV). To increase the accuracy of the measurement, we chose the 3,5-dibromosalicylaldimine monolayer, which exhibits a stronger Br(3d) band than the 4-bromobenzaldimine monolayer. The normalized rate constants were 1.83(5) × 10-4 s-1 (at 150 eV), 1.24(8) × 10-4 s-1 (at 250 eV), and 0.83(4) × 10-4 s-1 (at 820 eV). The rate constant decreases slightly with increasing photon energy, and the examined photon energy range is enough to cover the possible photoexcitation and ionization from various Br energy levels, including Br(3s) (256 eV), Br(3p) (182, 189 eV), and Br(3d) (70, 71 eV) levels. This observation proves that the bond cleavage does not occur via direct excitation or ionization from any of these levels. If direct proton-induced cleavage occurred,
the cleavage yield would exhibit a substantial increase at or above the corresponding excitation energy. We further investigated the cleavage process by using a photon of low energy (30 eV). Due to the restriction in the monochromator operation at the beamline, 30 eV is the lowest obtainable photon energy with a suitable photon flux. Figure 3 shows the Br(3d) band region (68-74 eV) of a 4-bromobenzaldimine monolayer before and after the irradiation at 30 eV. The filled squares (9) were obtained right after a light-shutter was open, and the open squares (0) were recorded after irradiation for 30 min. As evident in Figure 3, the Br(3d) band disappeared entirely after the irradiation. When the rate was normalized with the photon flux of 820 eV, it was found that the cleavage rate (knornalized ) 0.7 × 10-3 s-1) is comparable with the rate from irradiation at 250 eV (vide infra). The result again confirms that the cleavage does not involve direct Br(3d) excitation or the Auger processes associated with the Br(3d) excitation. Currently, many of the indirect photoinduced dissociation processes are thought to occur via the electronstimulated desorption (ESD) mechanism.13 Rieley et al. reported that a carbon-chloride bond is cleaved by irradiation on ω-chloroalkanethiol monolayers.13c Recently, Khan et al. observed that physisorbed alkyl bromides on GaAs could be cleaved by photoinduced electrons, resulting in the bromide anion and an alkyl radical.13f In these cases, bond cleavages are proposed to be caused by electron attachment to occupy the σ* orbital. A similar mechanism may be operative in the present case. Table 1 lists the normalized rate constant and the cross section for the bond cleavage of benzaldimine monolayers. The observed rate constant was normalized using a particular photon flux (2.2 × 1011 photons/(cm2‚s)). As in Figure 2a, the diminution of the peak area followed the first-order kinetic law for all the cases, and the rate constant (k) was obtained from the kinetic plot, i.e., plot of ln(area) versus time (eq 1). Typical error in measuring the area (A) was ca. (7%. The error increased slightly with calculation of the rate constant from the fitting. For example, the constant of 0.42 ( 0.04 × 10-3 s-1 was observed for the 4-bromobenzaldimine monolayer from the kinetic plot. Because the photon flux was 1.0 × 1011 photons/(cm2‚s) at the particular condition, the normalization gave the constant of 0.92 ( 0.08 × 10-3 s-1. Three series of measurements finally yielded an averaged (13) (a) Tolk, N. H., Traum, M. M., Tully, J. C., Madey, T. E., Eds. Desorption Induced by Electronic Transitions, DIET I; Springer Series in Chemical Physics 24; Springer-Verlag: Berlin, 1983. (b) Zhou, X.-L.; Coon, S. R.; White, J. M. J. Chem. Phys. 1991, 94, 1613-1625. (c) Rieley, H.; Price, N. J.; Smith, T. L.; Yang, S. J. Chem. Soc., Faraday Trans. 1996, 92 (19), 3629-3634. (d) Kondoh, H.; Nozoye, H. J. Phys. Chem. B 1998, 102, 2367-2372. (e) Olsen, C.; Rowntree, P. A. J. Chem. Phys. 1998, 108, 3750-3764. (f) Khan, K. A.; Camillone, N.; Osgood, R. M. J. Chem. Phys. 1999, 110, 10526-10538.
Notes
Langmuir, Vol. 16, No. 6, 2000 2983
Figure 2. XPS spectra of the 4-bromobenzaldimine monolayer: (a) Br(3d); (b) C(1s); (c) N(1s); (d) O(1s) bands. Each spectrum was obtained after exposure to X-ray for 0 min (9), 10 min (0), 20 min (b), 30 min (O), 40 min (2), and 50 min (4). Inset: A kinetic plot as a function of the irradiation time.
Figure 3. XPS spectra showing the Br(3d) region for the 4-bromobenzaldimine monolayer. The spectrum with filled squares (9) was obtained right after a light-shutter was open. The open squares (0) were recorded after the irradiation at 30 eV for 30 min.
normalized rate constant of 0.86(9) × 10-3 s-1. For reference, the cross section for the cleavage, the term independent of the photon flux, was calculated using eq 2, and it is presented in the table.
ln A ) -kt + ln Ao
(1)
cross section (σr) ) rate constant (k)/photon flux (I) (2) It is evident that these rate constants (knormalized) are comparable to those of the carbon-nitrogen bond cleavage
of the nitrobenzaldimine case. It is interesting to note that the normalized cleavage rate constant for the carbonbromide bond is considerably larger (5-10 times) than that of the other carbon-halide bonds. Though a carboniodide bond is weaker than a carbon-bromide bond in thermal dissociation, the X-ray induced cleavage rate of the carbon-bromide is larger than that of the former bond. It is also noteworthy that the cleavage rate is also sensitive to the kind and position of substituents on the aromatic ring. For a 3,5-dibromo-2-hydroxylbenzaldimine monolayer, the cleavage rate decreased significantly with a hydroxyl group at an ortho position. Also, it is quite reasonable to assume that the cleavage rate is dependent on the position of the substitution, i.e., para-substituted bromide would be different from the ortho- or metasubstituted bromide in terms of cleavage rate. Therefore, the decrease of cleavage rate in 3,5-dibromosalicylaldimine should arise from the combination of the two effects. Although the high lability of halide groups, in particular bromide, during XPS scans has been listed,1b,6,7 the rationale for the abnormally high cleavage efficiency of the carbon-bromide bond is yet to be reported. If the lowest unoccupied molecular orbital (LUMO) of carbon-halide bonds has an antibonding character and lies at an energy accessible to the photoinduced electrons, then the electron injection to the antibonding orbital could lead to the bond cleavage.13 In this context, the anomalously high cleavage rate for the carbon-bromide bond appears to be related to its high propensity for being populated by the photoelectrons. An adequate model explaining this difference requires quantitative calculations of the energy-level shift
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caused by adsorbate-substrate bonding, the band broadening effect, and the energy quenching of the excited adsorbate. Conclusion The cleavage of carbon-halide bond under the irradiation of soft X-rays is selective. The cleavage rate is independent of the incident photon energy, showing that the cleavage occurs via an electron-induced process rather than via the direct photoexcitation. Quantitative analysis of the cleavage rate shows that carbon-bromide bond cleavage is considerably more efficient than cleavage of other carbon-halide bonds.
Notes
Acknowledgment. This work is supported by a Korea Research Foundation Grant via the Polymer Research Institute of POSTECH and the Ministry of Science and Technology (MOST) through PLS.
Supporting Information Available: Survey spectra and spectra showing the Br(3d) region for a 3,5-dibromosalicylbenzaldimine monolayer irradiated at 150, 250, and 820 eV. This material is available free of charge via the Internet at http://pubs.acs.org. LA991061H
