Photochemical Patterning of a Self-Assembled Monolayer of 7

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Photochemical Patterning of a Self-Assembled Monolayer of 7-Diazomethylcarbonyl-2,4,9-trithiaadmantane on Gold Films via Wolff Rearrangement Jun Hu,* Yubiao Liu, Chalermchai Khemtong, Jouliana M. El Khoury, Timothy J. McAfoos,† and Ian S. Taschner† Department of Chemistry, The University of Akron, Akron, Ohio 44325-3601 Received February 11, 2004 Photolithographic attachment of functional organic molecules via ester or amide linkages to self-assembled monolayers (SAMs) on gold thin films was achieved by employing a novel photoreactive surface anchor, 7-diazomethylcarbonyl-2,4,9-trithiaadmantane. The photoreactive SAM was prepared by the spontaneous physical adsorption of the photoreactive surface anchor onto gold surfaces. The R-diazo ketone moiety of the SAM was found to display the classical Wolff rearrangement reactivity to produce a ketene intermediate on the exposed area. Organic molecules such as alcohols and amines can thus be attached to the gold surfaces selectively by the facile in situ formation of ester or amide linkages. The structure and reactivity of the photoreactive surface anchor were characterized by real-time FT-IR, fluorescence, and polarization modulation infrared reflectance absorption spectroscopy (PM-IRRAS). The Wolff rearrangement reactivity of the SAM suggested that a “surface-isolated” carbonylcarbene may be generated when the SAM was exposed to 255-nm irradiation.

Introduction We report a new type of photoreactive self-assembled monolayer (SAM) for chemical modification and patterning of SAMs on gold films. The self-assembled monolayer is formed by spontaneous physical adsorption of 7-(diazomethylcarbonyl)-2,4,9-trithiaadmantane (1) on a gold surface. The 2,4,9-trithiaadmantane moiety serves as the tridentate surface anchor for reversible surface binding, while the diazomethylcarbonyl group displays the classical Wolff rearrangement reactivity upon UV irradiation. The photochemical reaction produces a ketene intermediate (2) and subsequent ground-state chemical reactions of the ketene moiety allow clean and facile attachment of organic molecules to the photoexposed SAM surfaces (Scheme 1). Self-assembled monolayers of organic molecules on metal surfaces have been studied extensively because of their importance in nanoscience and nanofabrication. A wealth of information is available concerning the fundamental understanding of molecular recognition, selfassembling processes, and structure-property relationships in these molecular-based materials and interfaces.1 Applications of the self-assembling techniques and the resulting functional organic thin films include chemical and biological sensors and sensing arrays.2 In particular, SAMs based on S-Au chemistry have been studied for controlling the surface properties and forming wellorganized molecular assemblies at gold surfaces.3 Similarly, SAMs can also be prepared by physical adsorption of thioethers on gold surfaces. Although thiolates bind * To whom correspondence should be addressed. † Undergraduate students. (1) (a) Laibinis, P. E.; Whitesides, G. M. J. Am. Chem. Soc. 1992, 114, 9022. (b) Lopez, G. P.; Biebuyck, H. A.; Whitesides, G. M. Langmuir 1993, 9, 1513. (2) (a) Brockman, J. M.; Nelson, B. P.; Corn, R. M. Annu. Rev. Phys. Chem. 2000, 51, 41. (b) Wegner, G. J.; Lee, H. J.; Marriott, G.; Corn, R. M. Anal. Chem. 2003, 75, 4740. (c) Knoll, W. Annu. Rev. Phys. Chem. 1998, 49, 569. (d) Homola, J. Anal. Bioanal. Chem. 2003, 377, 528. (3) (a) Singhvi, R.; Kumar, A.; Lopez, G. P.; Stephanopoulos, G. N.; Wang, D. I.; Whitesides, G. M.; Ingber, D. E. Science 1994, 264, 696. (b) Kumar, A.; Whitesides, G. M. Science 1994, 263, 60.

less strongly to the noble metal surfaces, SAMs formed by thioethers are less susceptible to oxidative damages such as electrochemical cycling or photooxidation.4 We are developing surface plasmon resonance imaging (SPRI) based biosensing arrays that require high throughput addressable attachment of biomolecules on gold thin films.2 Patterning SAMs on gold surfaces by microcontact printing and other techniques involving wet chemistry have been developed previously.5 Alternatively, photolithography is a reliable and high throughput method commonly used in the microelectronics and DNA chip fabrications.6 Photoisomerization and photorearrangement reactions have been observed in SAMs on gold surfaces, which indicated that the photolithographic method is promising for fabricating microarrays necessary for SPRI applications.7 Chemical modifications of alkylthiolate SAMs were found to be difficult on well-packed alkylthiolate SAMs on gold surfaces, most likely due to the steric hindrances of the end groups. A previous study by Jocys and Workentin of the photochemistry of a SAM formed by 2-diazo-13-mercaptotridecan-2-one showed complete photochemical conversion of the R-diazocarbonylfunctionalized SAM. Only the sterically least demanding methanol was reported to form the corresponding esters in the study so far.8 Although the van der Waals packing in the SAMs can be partially disrupted by the incorporation of sterically irregular surface linkers, the steric hindrance problem sometimes can be alleviated by the formation of mixed SAMs with short chain alkylthiolates as the diluents, kinetically. Fox and Whitesell developed an (4) Kittredge, K. W.; Minton, M. A.; Fox, M. A.; Whitesell, J. K. Helv. Chim. Acta 2002, 85, 788. (5) (a) Harada, Y.; Girolami, G. S.; Nuzzo, R. G. Langmuir 2003, 19, 5104. (b) Yan, L.; Zhao, X.-M.; Whitesides, G. M. J. Am. Chem. Soc. 1998, 120, 6179. (c) Abbott, N. L.; Gorman, C. B.; Whitesides, G. M. Langmuir 1995, 11, 16. (6) Heller, M. J. Annu. Rev. Biomed. Eng. 2002, 4, 129. (7) (a) Hu, J.; Zhang, J.; Liu, F.; Kittredge, K.; Whitesell, J. K.; Fox, M. A. J. Am. Chem. Soc. 2001, 123, 1464. (b) Li, W.; Lynch, V.; Thompson, H.; Fox, M. A. J. Am. Chem. Soc. 1997, 119, 7211. (c) Wolf, M. O.; Fox, M. A. J. Am. Chem. Soc. 1995, 117, 1845. (8) Jocys, G. J.; Workentin, M. S. Chem. Commun. 1999, 9, 839.

10.1021/la049629w CCC: $27.50 © 2004 American Chemical Society Published on Web 04/23/2004

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Scheme 1. A New Photolithographic Surface Linker Based on Photoinduced Wolff Rearrangement

elegant approach by using tripodal surface anchors to reduce the end group density of the resulting SAMs to about one-third of the full coverage.4 Solvation and reactions on the SAMs were strikingly accelerated by the reduction of the steric hindrance of the end groups of the SAMs. To combine the advantages of tripodal surface linkers and photolithographic chip manufacturing techniques, we designed and synthesized 7-diazomethylcarbonyl-2,4,9-trithiaadmantane as the photoreactive surface anchor. Known as the Wolff rearrangement, R-diazo ketones undergo photoinduced rearrangement via singlet carbonylcarbene intermediates to form ketenes, which react rapidly with alcohols and amines to produce ester and amide linkages in solution.9 The reaction is remarkably clean, with light as the reagent and N2 gas as the only byproduct. We report herein the photochemistry of the 7-diazomethylcarbonyl-2,4,9-trithiaadmantane in a SAM on a gold surface. The photochemical reaction was investigated by real-time FT-IR and fluorescence spectroscopy. We show that the R-diazo ketone displays solution phase reactivity in a SAM on gold surfaces and allows for facile photolithographic attachment of noctadecylamine and 1-hydroxylmethylpyrene to form chemically modified/patterned SAMs. The observed chemical reactivity of the R-diazomethylcarbonyl-functionalized SAM strongly suggests that carbonylcarbene or a related carbenoid transient species is likely formed on the isolated SAM surface upon photolysis. Experimental Section General Procedures. All air and/or moisture sensitive reactions were conducted under argon. Common reagents were purchased from Aldrich and used as received. Diethyl ether used in SAM preparation was freshly distilled from sodium benzophenone ketyl under argon. Pure water (Milli-Q) had a resistance of higher than 4 MΩ. Infrared spectra were recorded on a Nicolet Nexus 870 Fourier transform spectrometer. The instrument was equipped with a Thunderdom ATR accessory for bulk sample analysis and external accessories for recording polarization modulation reflectance absorption spectra (PMFTIRRAS) of SAMs. 1H (300 MHz) and 13C (77 MHz) NMR spectra were recorded in CDCl3 on a Varian Gemini-300 NMR spectrometer. X-ray data were measured at 100 K (Bruker KRYOFLEX) on a Bruker SMART APEX CCD-based X-ray diffractometer system equipped with a Mo-target X-ray tube (λ ) 0.710 73 Å) operated at 2000 W. Fluorescence spectra were recorded with a Perkin-Elmer LS 55 luminescence spectrometer using a 343-nm excitation wavelength and slit widths of 2 nm for both excitation and emission. Data were collected using the Perkin-Elmer FL WinLab molecular spectroscopy software (9) The formation of the ketene may be concerted with the dissociation of the nitrogen molecule: Kirmse, W. Euro. J. Org. Chem. 2002, 14, 2193.

package. The sample is placed at a 60° incident angle for the excitation light in the front phase mode so that the luminescence of the gold substrate was minimized.10 Synthesis of 7-Hydroxycarbonyl-2,4,9-Trithiaadmantane (TPCOOH). To a solution of 7-methoxycarbonyl-2,4,9trithiaadmantane (73 mg, 0.30 mmol) in a mixed solvent of THF and H2O (3 mL, v/v ) 2/1) in a 25-mL round-bottom flask was added LiOH‚H2O (0.13 g, 3.0 mmol) at ambient conditions. The reaction mixture was stirred at room temperature for 1 h and refluxed for additional 30 min. The reaction mixture was acidified and cooled to 5 °C using an ice bath. The reaction mixture was filtered and a yellow solid product was collected. The solid was dried under vacuum overnight to yield 66 mg of product (92%). The acid was used in the following reaction without further purification. 1H NMR δ: 4.36 (t, 3 H, CH), 2.96. (d, 6 H, CH2), 13C NMR δ: 67.9, 30.6, 29.5, 29.5, 29.4, 29.3, 29.2, 29.1, 29.0, 28.8, 26.1. FTIR (cm-1): 2933, 2920, 1695, 1445, 1421, 1300, 1277, 1184, 1103, 1042, 1000, 930, 830. Synthesis of 7-Diazomethylcarbonyl-2,4,9-trithiaadmantane (TPCOCHN2, 1). Diazomethane was prepared using a Diazald glassware set from Aldrich.11 The freshly distilled diazomethane etheric solution (ca. 1 mmol, 8 mL) was slowly added to a solution of 7-chlorocarbonyl-2,4,9-trithiaadmantane (43 mg, 0.17 mmol) made in situ from 2,4,9-trithiaadmantane7-carboxylic acid with thionyl chloride in CH2Cl2. Slow evaporation of diethyl ether yielded a yellow crystal (41 mg, 95% yield from TPCOOH). 1H NMR δ: 5.48 (s, 1H, CHN2), 4.36 (t, 3H, SCHS), 2.82 (d, 6H, CH2). 13C NMR δ: 196.80, 44.47, 41.30, 40.58, 39.97, 39.38. FTIR (cm-1): 3078, 2848, 2917, 2849, 2105, 1726, 1620, 1429, 1370, 1348, 1268, 1214, 1161, 1049, 1008, 858, 836, 705. X-ray crystal graph data: C9H12O2S3, M ) 248.37, orthorhombic, a ) 10.9660(10) Å, b ) 7.0388(6) Å, c ) 27.433(3) Å, U ) 3741.0(9) Å3, T ) 173 K, space group Pbca, Z ) 8, µ(Mo KR) ) 0.669 mm-1, 16 895 reflections were measured, 2549 being unique (Rint ) 0.0433) and used in all calculations. The final wR(F2) was 0.1074 (all data). Synthesis of 7-(n-Octadecylamino)carbonyl-2,4,9-trithiaadmantane (4). To freshly prepared 7-chlorocarbonyl-2,4,9trithiaadmantane (11 mg, 0.042 mmol) in a 25 mL round-bottom flask was added a solution of n-octadecylamine (11 mg, 0.042 mmol) in THF (5 mL). The resulting reaction mixture was stirred at 40 °C for 30 min, until TLC monitoring showed the completion of the reaction. The reaction mixture was neutralized with saturated aqueous bicarbonate and extracted with diethyl ether. The combined organic layers were dried over anhydrous Na2SO4. Evaporation of the solvent gave a yellow solid residue, which was further purified by TLC to yield the sample for the SAM preparation (17 mg, 91% yield from TPCOOH). 1H NMR δ: 4.36 (t, 3H, SCHS), 3.28 (m, 2H, NHCH2), 2.88 (d, 6H, CH2), 1.52 (m, 2H, NHCH2CH2), 1.24 (bs, 30H, (CH2)15), 0.89 (t, 3H, CH3) 4.36 (t, 3 H), 3.28 (m, 2H), 2.88 (d, 6 H), 1.52 (m, 2H), 1.24 (s, 30H), 0.89 (t, 3H). 13C NMR δ: 175.5, 42.00, 40.33, 38.42, 34.02, 32.11, (10) (a) Bruening, M. L.; Zhou, Y.; Aguilar, G.; Agee, R.; Bergbreiter, D. E.; Crooks, R. M. Langmuir 1997, 13, 770. (b) Fox, M. A.; Li, W.; Wooten, M.; McKerrow, A.; Whitesell, J. K. Thin Solid Films 1998, 327, 477. (11) Black, T. H. Aldrichimica Acta 1983, 16, 3.

Patterning of a SAM of TPCOCHN2 on Au

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Scheme 2. Synthesis of 7-(Diazomethylcarbonyl)-2,4,9-trithiaadmantane and Its Crystal Structure

29.96, 27.14, 22.95, 14.39. FTIR (cm-1): 3309, 2918, 2850, 1634, 1552, 1521, 1467, 1306, 707. Formation of TPCOCHN2 SAMs. Gold surfaces were produced by sequential thermal deposition of Ti (2000 Å) and 99.99% pure gold (1800 Å) on glass slides. The gold films consist exclusively of polycrystalline Au[111] surface with the surface roughness factor determined to be 1.1. All the SAMs in this study were prepared in 25-mL glass weighing bottles at room temperature. Prior to each experiment, the weighing bottle and the gold thin film substrate were cleaned by soaking in a piranha solution (7:3 concentrated H2SO4/30% H2O2) at about 90 °C for 30 min. The bottle and the substrate were then rinsed with absolute ethanol and blow-dried with N2. The freshly prepared gold slide was immediately characterized using PM-FTIRRAS as the background and then immersed into the monomer solution for 2 days at room temperature for the self-assembling. The resulting SAM sample was thoroughly rinsed with diethyl ether and water and then blow-dried with nitrogen before the subsequent characterization and use. FT-IR Characterizations of the TPCOCHN2 SAMs. The quality and composition of the SAMs were verified by PMFTIRRAS technique using a Nicolet Nexus 870 Fourier transform infrared spectrometer. The experiments were performed with a liquid nitrogen cooled MCT detector and a Hinds Instruments PEM-90 photoelastic modulator operating at 100 kHz.12 The incoming infrared radiation was reflected from the sample at an angle of incidence of 80°. The spectra were usually collected in 4000 scans at a spectral resolution of 4 cm-1. Real-time FT-IR spectroscopy was performed with a Thunderdom ATR accessory with a Ge single crystal. Wolff Rearrangement Reaction and Photopatterning of TPCOCHN2 SAM. The photochemical reactivity and photodirected surface chemical patterning of the TPCOCHN2 SAMs were studied in solutions or under solvent-free reaction conditions. All UV exposures were carried out with a SuperBright 2000SW light source equipped with a superior Hoya Optics U-325C SW filter for 5-10 min. The irradiation intensity for the 254.7 nm peak is about 110 µW/cm2, as indicated by the manufacturer specifications. All the reactions were carried out under argon or N2 protection unless otherwise specified. The surface image was created by using a paper mask clipped on the SAM slides before the irradiations. The reacted SAMs were rinsed thoroughly with diethyl ether and characterized by PM-IRRAS, surface fluorescence, and fluorescence imaging.

Result and Discussion The photoreactive surface linker was inspired by Fox and Whitesell’s original study of the self-assembled monolayer of trithiaadmantane derivatives.4 7-Hydroxycarbonyl-2,4,9-trithiaadmantane was obtained according to the literature procedure from basic hydrolysis of (12) (a) Jordan, C. E.; Frey, B. L.; Kornguth, S.; Corn, R. M. Langmuir 1994, 10, 3642. (b) Barner, B. J.; Green, M. J.; Saez, E. I.; Corn, R. M. Anal. Chem. 1991, 63, 55.

Figure 1. FTIR for TPCOCHN2: (a) as a thin film on KBr salt plate, (b) as a SAM on a gold thin film (PM-FTIRRAS), and (c) the PM-FTIRRAS of the SAM after the UV irradiation.

7-methoxycarbonyl-2,4,9-trithiaadmantane. The carboxylic acid intermediate was converted to the corresponding acid chloride and then allowed to react with diazomethane to form the R-diazo ketone product (Scheme 2).4 The structure of the photoreactive surface linker was determined by common spectroscopic methods and confirmed by X-ray single-crystal structure analysis (Scheme 2). The solution of the X-ray crystal structure showed an elongated C-N bond, indicating a minor impurity in the crystal. However, the NMR and IR spectra of the corresponding sample are sufficiently clean. Cocrystallization of an impurity such as the acid chloride precursor in the crystal can be ruled out. We believe that the unstable diazo compound decomposes to form the ketene intermediate during the data collection in X-ray crystallography. A photoreactive SAM was prepared by immersing a freshly cleaned gold thin film on a glass slide into a dilute solution of 7-diazomethylcarbonyl-2,4,9-trithiaadmantane in diethyl ether. The resulting SAM of 7-diazomethylcarbonyl-2,4,9-trithiaadmantane was confirmed by PMFTIRRAS. The absorption bands at 2916 and 2849 cm-1 in the solid sample are assigned to the νa and νs modes of the CH2 group, respectively (Figure 1). The peak at 2106 cm-1 is characteristic for the -CHN2 group. The diazomethylcarbonyl CdO stretching peak appears at 1726 cm-1 indicating that the diazomethylcarbonyl group adapts a cis conformation in the SAM as it is in the crystalline form. Before the recrystallization or selfassembling, the diazomethylcarbonyl CdO stretching peak of the same compound appears predominantly at 1620 cm-1, consistent with a trans-diazomethylcarbonyl

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Figure 2. Real-time FTIR of TPCOCHN2 thin film under UV irradiation (254.7 nm, 110 µW/cm2).

group. The peak intensity of the diazo group in the SAM is relatively high compared to that in the bulk. Surface selection rules for PM-FTIRRAS for SAMs on gold surfaces dictate that the vibration modes perpendicular to the gold surface should be enhanced.13 Therefore, the diazo group is likely perpendicular to the surface, as it is shown in the crystal structure (Scheme 1). The diazo peak at 2106 cm-1 disappeared upon UV irradiation under N2, indicating that it is photoreactive in the SAM. The IR also showed that the photoproduct of the SAM was consistent with that of a TPCOCHN2 thin film on a Ge crystal surface. A real-time FTIR study of the photolysis of a TPCOCHN2 thin film on a Ge crystal surface was carried out under identical reaction conditions as those on the gold thin film. From the decreasing peak intensity at 2106 cm-1 (-CHN2) and the increasing peak intensity at 1727 cm-1 (CdO of acid, Figure 2), we estimated that over 95% of the TPCOCHN2 underwent photoinduced rearrangement and hydrolysis within 4 min of irradiation (Figure 2). We found that the hydrolysis proceeded rapidly and the ketene intermediate was not observed, even under N2 under solvent-free conditions, due to the trace amount of water on the SAM surfaces and in the purge N2 gas. Because the -OH of the carboxylic acid is most likely hydrogen-bonded to surface water and not oriented on the gold surface, it is not visible in the PM-FTIRRAS as the p-polarization spectrum cancels out the s-polarization spectrum. The SAMs appeared to be stable in the air during the UV irradiation and the subsequent chemical modifications. PM-FTIRRAS of the SAM showed no evidence for the formation of sulfone or other degradation products. Photodirected chemical patterning of the SAMs via amide and ester linkages was studied using n-octadecylamine and 1-hydroxymethylpyrene as the model substrates. For example, attachment of octadecylamine to the SAM surface at the exposed area was achieved by the UV irradiation of the SAM sample in a solution of n-octadecylamine in anhydrous diethyl ether (1.0 mM) under nitrogen. The trapping of the ketene intermediate by the primary amine and alcohol in the photolysis appeared to be instantaneous, and a dark reaction period after the photobleach of the SAM was found to be unnecessary. 7-(n-Octadecylamino)carbonyl-2,4,9-trithiaadmantane (4) can be prepared by treating 7-chlorocarbonyl-2,4,9-trithiaadmantane with n-octadecylamine (Scheme 2). As shown in Figure 3, the SAMs prepared by direct assembly of 4 (13) Chailapakul, O.; Sun, L.; Xu, C.; Crooks, R. M. J. Am. Chem. Soc. 1993, 115, 12459.

Figure 3. (a) FTIR spectrum of bulk 4, (b) FTIR spectrum of a SAM of 4 on a gold surface, and (c) PM-FTIRRAS of the TPCOCHN2 SAM after UV exposure in 1.0 mM octadecylamine solution.

on freshly prepared gold film and the SAM produced by the photoinduced Wolff rearrangement followed by alkylamine trapping on the SAM display a similar surface IR spectrum, indicating the nearly identical surface coverage of the alkyl groups in these two SAMs. In the surface IR spectra of the SAMs, we assigned the peak at 3301 cm-1 to the amide N-H stretching. The peaks at 2927 and 2856 cm-1 arising from the typical νa(CH2) and νs(CH2) modes of alkyl groups in SAMs are slightly shifted from 2917 and 2849 cm-1 in the IR spectrum of the bulk sample. The surface IR studies of the SAMs showed that the trithiaadmantane anchor could be used to form robust SAMs with good molecular coverage of the substrate surfaces. To demonstrate the principles for photolithographic attachment of organic compound on gold surfaces, a 7-diazomethylcarbonyl-2,4,9-trithiaadmantane SAM sample was exposed to UV irradiation with a mask in the n-octadecylamine solution (Scheme 4). The resulting long chain alkyl group covered region in the SAM is hydrophobic. The SAM was then treated by UV irradiation without a mask in water to produce hydrophilic carboxylate end groups in the unmasked region. When the sample was cooled and allowed to condense atmospheric moisture, the differences in hydrophilic and hydrophobic regions of the surface could be observed by naked eyes as water droplets selectively condensed at the hydrophilic area (Figure 4a). By replacing n-octadecylamine with 1-hydroxymethylpyrene in the above reaction, photoluminescent pyrene groups can be chemically attached to the SAM surface in

Patterning of a SAM of TPCOCHN2 on Au

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Scheme 3. Preparation of Standard 7-(n-Octadecylamino)carbonyl-2,4,9-trithiaadmantane SAM

Scheme 4. Photolithographic Chemical Modification of SAMs Using Wolff Rearrangement

Figure 4. (a) Pattern formed by water droplet condensation at the hydrophilic area that is generated by UV exposure in water; (b) fluorescence image of the pyrene-derivatized SAM when illuminated with a UV lamp for thin-layer chromatography at 245 nm.

Figure 5. Fluorescence spectra of (a) TPCH2COOCH2Py SAM on a gold surface and (b) a bare gold surface as background with an excitation wavelength of 343 nm.

a similar reaction (Scheme 1). As shown in Figure 4b, a clear fluorescence image of the mask pattern is visible under a low-power UV lamp (Mineralight model UVGL25 for TLC visualization at 245 nm). Fluorescence emission from the surface-anchored pyrene has been used as a local probe for the molecular packing characteristics of SAMs. When the SAM is well packed, the spectrum is dominated by the pyrene excimer emission.14 The fluorescence spectrum of the pyrene-derivatized SAM is shown in Figure 5. We observed relatively low overall emissions from the pyrene moiety, which can be attributed to the fact that the distance between the pyrene group and the metal surface is quite short in this particular case and significant surface quenching of the pyrene excited state occurred. The peaks at 376, 397, and 411 nm are assigned to the characteristic emissions of the substituted pyrene monomer. There is a very weak broad emission at ∼457 nm, indicating that the corresponding excimer emission is insignificant. (14) Wolf, M. O.; Fox, M. A. Langmuir 1996, 12, 955.

The high efficiency of the surface Wolff rearrangement and the facile derivatizations of the SAMs in the subsequent dark reactions indicated that the end groups in the tripod-anchored SAMs are not closely packed. The fluorescence study confirms that the pyrene end groups in the tripod-anchored SAMs are not closely packed and monomeric fluorescence emission can be observed without dilution in a mixed monolayer. These unique structural features can be attributed to the large footprint of the tripod, which allows the formation of SAMs of optimal end group density for our applications. It is worth noting that while the surface-attached chromophores strongly interact with the metal surface electronically, as indicated by the IR and fluorescence studies, the overall reactivity of the R-diazo ketone moiety in the SAM on the gold surface is remarkably similar to what was observed in solution. Known as the Wolff rearrangement, R-diazo ketones eliminate N2 when activated by UV light, heat, microwave irradiation, and transition metal catalysts to form ketenes. The photoinduced Wolff rearrangement is preferable for surface modifications, because light is used as the only “reagent” and N2 is produced as the only byproduct. In addition, the reaction generates ketenes, which are readily trapped by water, alcohols, and amines to produce high yields of carboxylic acids, esters, and amides, respectively. The successful formation of the SAM of TPCOCHN2 on a gold surface provided us with an excellent opportunity to study a classical reactive intermediate in a well-defined physical/chemical environment. However, when the surface photochemistry was monitored with PM-IRRAS, only the secondary trapping products such as carboxylic acid were observed. The trace amount of water on the SAM and in the N2 purge gas rapidly quenches the ketene

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intermediate, rendering the direct observation of the primary photochemistry infeasible. Cryogenic and ultrahigh vacuum are necessary for the future investigations of the detailed reaction mechanisms involving such “surface-isolated” reactive intermediates. In conclusion, we have designed and synthesized a new photoactive surface linker for photolithographic assembling of organic and biological molecules on gold surfaces for SPRI and other bioanalytical arrays. The technique was demonstrated by the surface attachments of an alkylamine and an alkyl alcohol linked fluorescence probe. From a mechanistic point of view, we demonstrated that the photoinduced Wolff-rearrangement is still competitive with the metal-quenching process when the R-diazomethylcarbonyl chromophore is within 1.0 nm from the metal surface. The successful demonstration of the surface linker for photochemical patterning of SAMs by Wolff rearrangement provides strong evidences for the formation

Hu et al.

of the carbonylcarbenoid species in the SAMs. Like “matrix isolation” for spectroscopic characterization of transient species in a frozen gas or solvent matrix, the generation of reactive intermediates in rigid SAMs should pave the way for detailed mechanistic investigations of “surfaceisolated” reactive intermediates on metal surfaces. Acknowledgment. This work was supported by the US National Institute of Health (DK61316-01). J.H. thanks the University of Akron Research Foundation for a startup grant and a faculty research fellowship. Supporting Information Available: Spectra, structures, and crystallographic data of the compounds discussed. This material is available free of charge via the Internet at http://pubs.acs.org. LA049629W