Self-Assembly of Tetraphenylporphyrin Monolayers on Gold

Jun 1, 2000 - Determination of surface coverage for tetraphenylporphyrin monolayers using ultraviolet visible absorption and x-ray photoelectron spect...
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Langmuir 2000, 16, 5644-5653

Self-Assembly of Tetraphenylporphyrin Monolayers on Gold Substrates Maximiliane S. Boeckl,† Ariana L. Bramblett,‡ Kip D. Hauch,‡ Tomikazu Sasaki,*,† Buddy D. Ratner,*,‡,§ and J. W. Rogers, Jr.‡,| Department of Chemistry, University of Washington, Box 351700, Seattle, Washington 98195-1700, Department of Chemical Engineering, University of Washington, Box 351750, Seattle, Washington 98195-1750, Department of Bioengineering, University of Washington, Box 351720, Seattle, Washington 98195-1720, and Pacific Northwest National Laboratory, 902 Battelle Boulevard, Box 999, Richland, Washington 99352 Received November 18, 1999. In Final Form: February 17, 2000 Two tetraphenylporphyrins, TPP-P-disulfide and TPP-P-thiol, were synthesized and used to form selfassembled monolayers (SAMs) on gold surfaces. The monolayers were characterized using X-ray photoelectron spectroscopy (XPS), ultraviolet/visible absorption spectroscopy (UV/vis), and scanning tunneling microscopy (STM). XPS binding energy shifts revealed that the porphyrins were chemisorbed to the surface through a sulfur-gold bond. A red shift without a significant blue-shifted component of the Soret band in the absorption spectra demonstrated that the porphyrin molecules are aligned on the gold surface in a side-by-side orientation. Round STM features, approximately 2 nm in diameter, correspond closely to the diameter of tetraphenylporphyrin (1.8 nm). Taken together, these data indicated the formation of monolayers of uniformly spaced TPP-P-disulfide and TPP-P-thiol molecules, with the porphyrin ring oriented parallel to the gold surface. Furthermore, porphyrin monolayers were stable for at least a week at ambient conditions. These monolayers have the potential to anchor biorecognition molecules in an ideal spacing for protein and cell attachment, making them appropriate models for the development of new biorecognition surfaces.

1. Introduction The formation of terminally functionalized n-alkylthiol self-assembled monolayers (SAMs) on gold and other metals is readily achieved and represents a powerful method for precise chemical modification of surfaces.1 In recent years, this has become a common technique for many research applications, including microelectronics, intermolecular interactions, nanopatterning, and biomaterials.2 One of the recent developments in biomaterials is to utilize mixed self-assembled monolayers for the development of biorecognition surfaces.3-5 Here, the purpose of the first alkanethiol is to act as a nonreactive spacer molecule, which separates the second alkanethiol, functionalized with a biologically active group, to maximize protein interaction. One obstacle in producing these mixed monolayers is the differential adsorption rate between the two end-functionalized alkanethiols. Consequently, the composition of the resulting monolayer on the surface cannot be predicted easily from the given solution concentrations.4,6 Another problem is the tendency for like * To whom correspondence should be addressed. † Department of Chemistry, University of Washington. ‡ Department of Chemical Engineering, University of Washington. § Department of Bioengineering, University of Washington. | Pacific Northwest National Laboratory. (1) (a) Ulman, A. An Introduction to Ultrathin Organic Films: From Langmuir-Blodgett to Self-Assembly; Academic Press: Boston, MA, 1991. (b) Dubois, L. H.; Nuzzo, R. G. Annu. Rev. Phys. Chem. 1992, 43, 437-463. (2) Ulman, A. Chem. Rev. 1996, 96, 1533-1554. (3) Nelson, K. E.; Jung, L. S.; Gamble, L.; Boeckl, M. S.; Naeemi, E.; Campbell, C. T.; Golledge, S. L.; Sasaki, T.; Castner, D. G.; Stayton, P. S. To be submitted for publication. (4) (a) Ha¨ussling, L.; Michel, B.; Ringsdorf, H.; Rohrer, H. Angew. Chem., Int. Ed. Engl. 1991, 30, 569-572. (b) Ha¨ussling, L.; Ringsdorf, H.; Schmitt, F.-J.; Knoll, W. Langmuir 1991, 7, 1837-1840.

molecules to aggregate into islands on the surface. These islands decrease the ability for proteins to specifically bind to the monolayer.7 The development of a uniform monolayer, with controlled, optimal spacing of biorecognition groups, would address these problems. This can be achieved using a template molecule, which can be tailored to complement different protein sizes, while chemically binding an appropriate biorecognition group to the substrate surface; tetraphenylporphyrin is such a template molecule. The porphyrin’s aromatic ring structure gives rigidity to the molecule and can be easily modified by varying the substituents in the meso position to control the template’s size and properties. A thiol or disulfide linker attached to a metal center incorporated into the porphyrin ring anchors the molecule to the gold surface through a sulfurgold bond. The linker’s length can be adjusted to vary the distance between the porphyrin ring and the surface. A second linker, 180° opposite from the surface bound linker, can be modified with a biorecognition group. Furthermore, the sulfur containing tetraphenylporphyrin derivatives are expected to form self-assembled (5) (a) Kasmi, A. E.; Wallace, J. M.; Bowden, E. F.; Binet, S. M.; Linderman, R. J. J. Am. Chem. Soc. 1998, 120, 225-226. (b) Spinke, J.; Liley, M.; Guder, H.-J.; Angermaier, L.; Knoll, W. Langmuir 1993, 9, 1821-1825. (c) Patel, N.; Davies, M. C.; Hartshorne, M.; Heaton, R. J.; Roberts, C. J.; Tendler, S. J. B.; Williams, P. M. Langmuir 1997, 13, 6485-8490. (d) Duschl, C.; Se´vin-Landais, A.-F.; Vogel, H. Biophys. J. 1996, 90, 1985-1995. (e) Prime, K. L.; Whitesides, G. M. Science 1991, 252, 1164-1167. (f) Prime, K. L.; Whitesides, G. M. J. Am. Chem. Soc. 1993, 115, 10714-10721. (g) Sigal, G. B.; Ramdad, C.; Barberis, A.; Strominger, J.; Whitesides, G. M. Anal. Chem. 1996, 68, 490-497. (h) Mrksich, M.; Sigal, G. B.; Whitesides, G. M. Langmuir 1995, 11, 43834385. (6) Kang, J. F.; Liao, S.; Jordan, R.; Ulman, A. J. Am. Chem. Soc. 1998, 120, 9662-9667. (7) Stranick, S. J.; Parikh, A. N.; Tao, Y.-T.; Allara, D. L.; Weiss, P. S. J. Phys. Chem. 1994, 98, 7636-7646.

10.1021/la991513q CCC: $19.00 © 2000 American Chemical Society Published on Web 06/01/2000

Self-Assembly of Tetraphenylporphyrin Monolayers

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Figure 1. Chem3D Plus ball and stick representation of TPP-P-disulfide and TPP-P-thiol molecules (side view). Their chemical structures are also shown along with those of bound TPP-P-disulfide, bound mercaptoethanol, and bound TPP-P-thiol. The dimensions are taken from the Chem3D Plus model.

monolayers on gold substrates. A variety of porphyrins and metalloporphyrins have been assembled on gold to study their electrochemical properties.8-10 Such porphyrin systems bound to gold electrodes have been used to investigate the reduction of oxygen to hydrogen peroxide.11-13 In the above systems, the sulfur containing moieties are attached to the periphery of the porphyrin molecules. The two monolayers presented here are unique in that the sulfur group is attached to the porphyrin through its metal center. In this work, two new tetraphenylporphyrin compounds, TPP-P-disulfide and TPP-P-thiol (Figure 1), were synthesized to investigate the formation of self-assembled porphyrin monolayers as the first step toward designing (8) (a) Imahori, H.; Norieda, H.; Ozawa, S.; Ushida, K.; Yamada, H.; Azuma, T.; Tamaki, K.; Sakata, Y. Langmuir 1998, 14, 5335-5338. (b) Akiyama, T.; Imahori, H.; Sakata, Y. Chem. Lett. 1994, 1447-1450. (9) Shimazu, K.; Takechi, M.; Fujii, H.; Suzuki, M.; Saiki, H.; Yoshimura, T.; Uosaki, K. Thin Solid Films 1996, 273, 250-253. (10) (a) Akiyama, T.; Imahori, H.; Ajawakom, A.; Sakata, Y. Chem. Lett. 1996, 907-908. (b) Uosaki, K.; Kondo, T.; Zhang, X.-Q.; Yanagida, M. J. Am. Chem. Soc. 1997, 119, 8367-8368. (c) Kondo, T.; Ito, T.; Nomura, S.-i.; Uosaki, K. Thin Solid Films 1996, 284-285, 652-655. (d) Ishida, A.; Sakata, Y.; Majima, T. Chem. Commun. 1998, 57. (e) Ishida, A.; Sakata, Y.; Majima, T. Chem. Lett. 1998, 267. (11) Zak, J.; Yuan, H.; Ho, M.; Woo, L. K.; Porter, M. D. Langmuir 1993, 9, 2772-2774. (12) (a) Hutchison, J. E.; Postlethwaite, T. A.; Chen, C.-h.; Hathcock, K. W.; Ingram, R. S.; Ou, W.; Linton, R. W.; Murray, R. W. Langmuir 1997, 13, 2143-2148. (b) Hutchison, J. E.; Postlethwaite, T. A.; Murray, R. W. Langmuir 1993, 9, 3277-3283. (13) Postlethwaite, T. A.; Hutchison, J. E.; Hathcock, K. W.; Murray, R. W. Langmuir 1995, 11, 4109-4116.

controlled biorecognition surfaces. Several methods were used to characterize these novel SAMs. X-ray photoelectron spectroscopy (XPS) showed the elemental composition of the monolayer, the formation of the sulfur-gold bond, and the overlayer thickness. Ultraviolet/visible absorption spectroscopy (UV/vis) revealed the orientation of the porphyrin molecules relative to one another, as well as the surface concentration. Finally, scanning tunneling microscopy (STM) illustrated the spatial arrangement of the porphyrin molecules on the gold surface. 2. Experimental Section 2.1. Synthesis of TPP-P-disulfide and TPP-P-thiol. The phosphorus center was introduced by refluxing 5,10,15,20tetraphenylporphine with phosphorus oxychloride to yield (5,10,15,20-tetraphenylporphinato)dichlorophosphorus(V) chloride (1).14-16 The axial chlorine ligands on phosphorus were replaced by alkoxy groups by heating 1 at approximately 70 °C in neat dihydroxyethyl disulfide to give TPP-P-disulfide (2).14,17,18 TPP-P-thiol (3) was synthesized from 1 in a manner similar to (14) Boeckl, M. S.; Baas, T.; Fujita, A.; Hwang, K.-H.; Bramblett, A. L.; Ratner, B. D.; J. W. Rogers, J.; Sasaki, T. Biopolymers 1998, 47, 185-193. (15) Barbour, T.; Belcher, W. J.; Brothers, P. J.; Rickard, C. E. F.; Ware, D. C. Inorg. Chem. 1992, 31, 746-754. (16) Marrese, C. A.; Carrano, C. J. Inorg. Chem. 1983, 22, 18581862. (17) Segawa, H.; Kunimoto, K.; Susumu, K.; Taniguchi, M.; Shimidzu, T. J. Am. Chem. Soc. 1994, 116, 11193-11194. (18) Segawa, H.; Kunimoto, K.; Nakamoto, A.; Shimidzu, T. J. Chem. Soc., Perkin Trans. 1 1992, 939-940.

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2, using mercaptoethanol, but only heated to about 60 °C to avoid decomposition. The isolated products were shown to be TPP-Pdisulfide and TPP-P-thiol using nuclear magnetic resonance (1H NMR and 31P NMR), UV/vis, infrared spectroscopy (FTIR), and electrospray mass spectrometry (ES-MS). 2.1.1. Methods and Materials. All reagents, solvents, and chemicals were purchased from Aldrich, Fisher Scientific, and J. T. Baker and were used as received. NMR spectra were obtained with a Bruker dpx200, Bruker ac200, or Bruker af300 instrument. Infrared spectroscopy was performed on a Perkin-Elmer 1600 series FTIR. ES-MS were taken using a Kratos Profile HV4 electrospray mass spectrometer, where a 50% methanol/water solvent was used with gramicidin as the internal standard. UV/ vis absorption spectra were obtained on a Hewlett-Packard HP8452A diode array spectrophotometer. 2.1.2. (5,10,15,20-Tetraphenylporphinato)dichlorophosphorus(V) Chloride (1).14-16 Phosphorus oxychloride (6.0 mL, 6.44 × 10-2 mol) was slowly added over a period of 5 min to 5,10,15,20tetraphenyl-21H,23H-porphine (1.021 g, 1.66 × 10-3 mol), dissolved in 45 mL of anhydrous pyridine. The reaction refluxed for 40 h, whereupon the mixture was cooled to room temperature and the solvent evaporated off under vacuum. The residue was purified by flash column chromatography using a short, wide alumina column (neutral, Grade I). The starting material formed a burgundy band, which was eluted using methylene chloride. The product, a dark green band, was eluted using 5-15% methanol in methylene chloride. Typically, this procedure was repeated using a second column to further purify the product. The solvent containing the collected product was evaporated, and the residue was triturated with benzene for 30 min. After filtration and thorough drying, 1 was obtained as dark blue crystals (0.874 g, 70.2%). 1H NMR (CDCl3, 200 MHz): δ (ppm) 7.78 (m, 12H, Hm,p), 7.98 (m, 8H, Ho), 9.13 (d, 8H, Hpyrrole). 31P NMR (CDCl3, 200 MHz): δ (ppm) -179.5. FTIR (thin film, cm-1): 3072.0 (νC-H), 2742.9 (νC-H), 1637.0, 1607.6, 1542.9, 1484.2, 1237.4, 1043.5, 802.6, 755.6, 680.0, 570.0 (νP-Cl). ES-MS: 713 [M+ - Cl]. UV/vis (CHCl3): λmax (nm) (log ) 438 (5.5), 567 (4.2), 611 (4.0). 2.1.3. TPP-P-disulfide (2).14 Compound 1 (0.1547 g, 2.06 × 10-4 mol) and dihydroxyethyl disulfide (2.60 mL, 2.12 × 10-2 mol) were reacted at 65-75 °C. After 4 h the reaction was cooled to room temperature, and the reaction mixture was purified using a short, wide silica gel column (5 × 10 cm). The reaction mixture was loaded on the column using methylene chloride with 0.5% acetone and was washed with 300 mL of chloroform to remove dihydroxyethyl disulfide. The remaining materials were eluted by slowly increasing the amount of methanol in methylene chloride from 1 to 15%. The starting material (green band), 1, eluted quickly, and the desired product (burgundy band) started to elute at 7%/93% methanol-methylene chloride. After evaporation of the solvent, the residue was triturated with benzene (3×) and ethyl acetate (3×). The porphyrin film was then dissolved in a minimal amount of methylene chloride, and this mixture was slowly added to an excess of hexane. The precipitated TPPP-disulfide was collected by filtration and washed with cold hexane (0.1467 g, 72.3%). 1H NMR (acetone-d6, 200 MHz): δ (ppm) -2.01 (dt, 4H, HR), -0.10 (t, 4H, Hβ), 1.54 (t, 4 H, Hγ), 2.91 (t, 4H, Hδ), 7.85 (m, 12H, Hm,p), 8.19 (m, 8H, Ho), 9.25 (d, 8 H, Hpyrrole). 31P NMR (CDCl3, 200 MHz): δ (ppm) -180.3. FTIR (KBr, cm-1): 3448.1 (νO-H), 3271.8 (νC-H), 3054.7(νC-H), 2942.7 (νC-H), 2872.2 (νC-H), 1643.6, 1488.1, 1441.5, 1380.3, 1354.9, 1237.4, 1074.8(νC-O), 1037.2 (νC-O), 1014.1 (νP-O), 802.1, 755.1, 703.4. MS: 949 [M + H - Cl]+. UV/vis (CHCl3): λmax (nm) (log ) 432 (5.4), 560 (4.0), 602 (3.5). 2.1.4. TPP-P-thiol (3). The synthesis of TPP-P-thiol is similar to TPP-P-disulfide. Compound 1 (0.200 g, 2.67 × 10-4 mol) and 2-mercaptoethanol (1.90 mL, 2.70 × 10-2 mol) were reacted at 50-65 °C under nitrogen. After 7 h, residual mercaptoethanol was removed under reduced pressure, and the crude product was purified by flash column chromatography. The porphyrin containing mixture was loaded with methylene chloride onto a silica gel column. The starting material (green band) was removed with 2% methanol, and the product (burgundy band) was eluted with 10% methanol in methylene chloride. After removal of the solvent, the residue was dissolved in a minimal amount of methylene chloride and was slowly added to a large excess of

Boeckl et al. hexane. The precipitate was filtered and dried to yield 3 (0.188 g, 84%). 1H NMR (acetone-d6, 300 MHz): δ (ppm) -1.98 (t, 4H, HR), -0.10 (t, 4H, Hβ), 7.85 (m, 12H, Hm,p), 8,18 (m, 8H, Ho), 9.25 (d, 8H, Hpyrrole). 31P NMR (CDCl3, 200 MHz): δ (ppm) -180.6. FTIR (KBr, cm-1): 3354.1, 3060.3 (νC-H), 2931.0 (νC-H), 2554.9 (νS-H), 1490.1, 1443.1, 1378.4, 1354.9, 1237.4, 1072.9 (νC-O), 1020.0 (νP-O), 802.6, 755.6, 702.7, 667.4, 579.3. UV/vis (CHCl3): λmax (nm) (log ) 432 (5.4), 560 (4.0), 602 (3.4). 2.2. Preparation of Gold Substrates. In general, gold was either sputtered or evaporated onto a chemically3 or plasma19 cleaned glass microscope slide or cover slip. The procedure used to prepare the evaporated gold substrates has been published elsewhere.3 For the sputtered gold substrates, a Denton 502A sputtering system with a DC planar magnetron was used with an argon matrix. The cleaned substrates were allowed to outgas at room temperature for approximately 2 h until a vacuum between 3 × 10-6 and 5 × 10-7 Torr was achieved, before sputtering. Initially, approximately 200 Å of gold were deposited at a high rate (∼5-15 Å/s) to ensure good gold adhesion to the glass. The remainder of the gold layer was sputtered at a rate of approximately 2 Å/s.19 Each analytical technique required a different preparation of the gold substrates, and therefore three categories of gold substrates were used. A typical XPS sample consisted of 1000 Å of gold with no adhesion layer (1000 Å Au). Alternatively, 1000 Å of gold were deposited atop a 40-200 Å chromium adhesion layer (1000 Å Au - 200 Å Cr). For UV/vis spectroscopy, an optically transparent gold layer 55 Å thick (55 Å Au) was sputtered onto glass cover slips with no adhesion layer.13 To produce optimal STM images, atomically flat gold substrates were required (1000 Å Au - annealed). STM samples were sputtered as described above, but they were held at 300 °C during the outgas period and while sputtering. After sputtering, the temperature (300 °C) was maintained for 30 min to 14 h, with 2 h giving optimal results.19 Alternatively, 1000 Å Au substrates were annealed by passing them over the blue cone of a Bunsen burner flame for 2-10 s. Gold substrates were either used immediately after preparation or stored in ethanol and purged with nitrogen. 2.3. Preparation of SAMs and Thin Films. Porphyrin selfassembled monolayers were formed by immersing a gold substrate in a porphyrin solution, using the following procedure. First, the gold substrates were rinsed with ethanol, then hexane, and finally the incubation solvent. Sometimes the hexane rinse was omitted, without noticeable effect on the SAMs. Second, the gold slides were submerged in a porphyrin solution for 24 h. Typically, a 1.0 mM porphyrin solution was used; however, concentrations ranging from 0.01 to 1.5 mM were also examined. Both ethanol and methylene chloride were utilized as solvents. Absolute ethanol was acquired from McCormick Distilling Co, and methylene chloride (HPLC grade) was obtained from Fisher Scientific; they were used as received. The vials containing the substrates were protected from light and kept under nitrogen, to prevent photodecomposition and oxidation of the porphyrins. Next, the samples were removed and copiously rinsed with fresh solvent. Finally, they were stored under nitrogen in fresh solvent until analysis. For all experiments, a gold blank was treated in the same manner as the other samples, except that no porphyrin was added to the incubation solution. Thin films of evaporated porphyrin were prepared for comparison to the self-assembled porphyrin monolayers. A drop of porphyrin solution was placed onto a glass microscope slide or cover slip, and the solvent (methylene chloride) was allowed to evaporate from the substrate under atmospheric conditions. 2.4. X-ray Photoelectron Spectroscopic Analysis. Spectra were obtained either on a Surface Science Instruments X-Probe or S-Probe spectrometer. Both instruments utilized an Al KR monochromatic X-ray source (1486.6 eV), a hemispherical analyzer (with 0.1 eV resolution), and a spot size of 1000 × 1700 µm. The takeoff angle (defined as the angle between the detector lens axis and the sample’s surface normal) was typically 55°. An initial survey scan with binding energies ranging from 0 to 1000 eV was taken using a 150 eV pass energy. Detail scans (150 eV (19) Rufael, T. S.; Bramblett, A. L.; J. W. Rogers, J. To be submitted for publication.

Self-Assembly of Tetraphenylporphyrin Monolayers pass energy, 20 eV window) were signal averaged to enhance the signal-to-noise ratio for the weaker elements: nitrogen, sulfur, and phosphorus. Selected samples were etched using argon or xenon gas from 5 to 15 min, until minimal amounts of carbon were detectable to obtain a spectra of pure gold. Atomic percentages of the monolayers were determined by considering only the relative intensities of the carbon, oxygen, sulfur, nitrogen, and phosphorus signals. To determine the relative change in surface density, the element to gold ratios (C/Au, O/Au, S/Au, and N/Au) were compared. Although both TPP-P-disulfide and TPP-P-thiol contained phosphorus, this signal was weak and not consistently detected and, thus, not considered when comparison between sample sets were made. Spectra taken at higher resolution (50 eV pass energy, 20 eV window) were used to determine the binding energies of the different components and were referenced to the Au(4f)7/2 signal at 83.8 eV.20 A Shirley base line was applied to the gold peaks (Au(4f)7/2 and Au(4f)5/2), and the sulfur peaks were fit with a linear base line. Gaussian curves were used to fit all the peaks. For the sulfur peaks, each doublet (S(2p)3/2 and S(2p)1/2) was fit with its own curve, using a 2:1 peak area ratio and 1.2 eV separation as described by Castner et al.21 Oxidized sulfur was fit with a single peak since it represented a mixture of several different oxidized sulfur species. For angle-resolved XPS analysis, the takeoff angle was varied between 0°, 39°, 55°, 68° and 80°. The relative peak intensities taken from the detailed scans of Au, C, O, S, and N were used to determine composition. Further, the samples were subsequently etched and a series of Au, C, O, S, and N scans performed to determine the thickness of the porphyrin monolayer. XPS was performed on SAMs prepared for UV/vis and STM as well as for XPS, which included all the different gold substrates, to ensure that these variations did not affect the final monolayer composition. There were no significant deviations between these samples, in either the elemental percentages or the appearance of the wide scan spectra. Furthermore, the various porphyrin element (C, O, S, N, and P) to Au ratios were similar to each other. 2.5. Ultraviolet/Visible Absorption Spectroscopy. The solution extinction coefficients for the absorption bands were determined by preparing a series of dilutions in methylene chloride. From the intensity of each concentration’s absorption bands, the solution extinction coefficients were determined using the Beer-Lambert law.22 For absorption spectra of the monolayers, a specially constructed fixture held the cover slips in the path of the light beam. Each 55 Å Au substrate was scanned before self-assembly to obtain a background spectrum for the substrate’s absorption due to gold and glass. After self-assembly, the porphyrin covered substrate was rescanned and the composite spectrum obtained. To determine the porphyrin monolayer’s absorption, the background spectrum was subtracted from the composite spectrum. In a similar manner, the spectra of thin porphyrin films were obtained with the glass cover slip as the background spectrum and the thin film plus glass cover slip as the composite spectrum. The stability of the SAMs was determined by preparing five identical samples (1.0 mM in ethanol), stored at different temperature, light, and fluid conditions (section 3.4.1). Every 24 h the samples were removed, rinsed, scanned, and then returned to their storage condition. Stability was determined by observing changes in the Soret band (at approximately 440 nm) intensity with time. 2.6. Scanning Tunneling Microscopy. A NanoScope II scanning tunneling microscope (Digital Instruments, Santa Barbara, CA), equipped with an STM Head A was used to image porphyrin monolayers under ambient conditions. Tips were prepared by mechanically cutting a platinum/rhodium (87%/13%) wire. Blank gold surfaces were imaged in constant current mode (20) Wagner, C. D.; Riggs, W. M.; Davis, L. E.; Moulder, J. F.; Muilenberg, G. E. Handbook of X-ray Photoelectron Spectroscopy; PerkinElmer Corp.: Eden Prairie, MN, 1979. (21) Castner, D.; Hinds, K.; Grainger, D. W. Langmuir 1996, 12, 5083-5086. (22) Silverstein, R. M.; Bassler, G. C.; Morrill, T. C. Spectrometric Identification of Organic Compounds; 5th ed.; John Wiley & Sons: New York, 1991.

Langmuir, Vol. 16, No. 13, 2000 5647 Scheme 1

with a bias voltage of 72 mV and a set-point current of 2 nA. To image porphyrin monolayers, the bias voltage and set-point current were adjusted between 1200 and 1800 mV and 0.2 nA, respectively. The Digital Instruments data analysis program was used to collect and flatten the images in the x-y dimension. This procedure eliminated the distortion due to the angle between the sample and the tip. NIH Image SXM was used to shadow the image for presentation.23

3. Results and Discussion 3.1. Synthesis and Stability of TPP-P-disulfide and TPP-P-thiol. The synthesis of TPP-P-disulfide and TPPP-thiol (Scheme 1) from 1 was based on previous syntheses by Segawa et al. and Barbour et al., who used various alcohols to replace the axial chlorine ligands in 1.15,17,18 Segawa et al. noted that performing reactions with 1 and alkanediols in a solvent produced oligomers, whereas using pure diol gave only monomers.17 Neat dihydroxyethyl disulfide and mercaptoethanol were used to synthesize TPP-P-disulfide and TPP-P-thiol, respectively, to avoid the formation of oligomers. After performing the syntheses, it was confirmed that TPP-P-disulfide and TPP-P-thiol were produced using NMR, UV/vis, FTIR, and ES-MS. It was notable that the thiol group did not attack phosphorus in TPP-P-thiol, although a thiol is a better nucleophile than a hydroxyl group. Both NMR and FTIR supported this conclusion. In the 1H NMR spectra the methylene protons next to the oxygen (bound to phosphorus) exhibited a characteristic chemical shift of -2.01 and -1.98 ppm for TPP-P-disulfide and TPP-P-thiol, respectively.17 Furthermore, the chemical shift of the phosphorus in the 31P NMR (-180.27 ppm for TPP-P-disulfide and -180.6 ppm for TPP-P-thiol) is in close agreement with literature values for similar porphyrin compounds.17 Finally, very strong P-O stretches at 1014.1 cm-1 for TPP-P-disulfide and 1020.0 cm-1 for TPP-P-thiol in the FTIR spectrum were observed.15 The stability of TPP-P-disulfide and TPP-P-thiol was monitored using thin layer chromatography. Dry TPPP-disulfide and TPP-P-thiol did not exhibit any decomposition products upon exposure to sunlight and oxygen in air for 4 days. However, exposing them to temperatures above 100 °C led to multiple decomposition products within 24 h; none of the decomposition products were characterized. Solutions of TPP-P-disulfide and TPP-P-thiol were stable in most organic solvents. Decomposition, however, occurred rapidly in an acidic solution (2 h in deuterated chloroform). 3.2. Porphyrin Model. Molecular modeling (Chem3D Plus, Cambridge Scientific Computing, Inc.) has been used to estimate the dimensions of the TPP-P-disulfide and TPP-P-thiol molecules, without considering any intermolecular interactions. First, the free energy of the tetraphenylporphyrin was minimized with the plane of the phenyl restrained to an orientation of approximately (23) NIH Image SXM; U.S. National Institutes of Health: Bethesda, MD; version 1.6.1 (http://rsb.info.nih.gov/nih-image/).

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Boeckl et al. Table 1. Elemental Percentagesa

carbon

oxygen

sulfur

nitrogen

phosphorus

TPP-P-Disulfide expected percentages 80.00 6.15 6.15 6.15 1.54 assembled from ethanolb 74 ( 3 15 ( 2 7(2 3.7 ( 0.8 0.3 ( 0.4 assembled from CH2Cl2b,c 73 ( 8 19 ( 6 4(2 2.4 ( 0.8 1.5 ( 1 concentrated (thin film)d 76 ( 2 12 ( 1 6.6 ( 0.8 4.0 ( 0.8 1.3 ( 0.2 TPP-P-Thiol expected percentages 84.21 3.51 3.51 7.02 1.75 assembled from ethanolb,c 75 ( 2 14 ( 2 5.8 ( 0.8 4.1 ( 0.6 1.2 ( 0.7 concentrated (thin film)d 81 10 2.7 5.0 1.4 a Summation of carbon, oxygen, sulfur, nitrogen, and phosphorus total 100%. b Assembled from a 1.0 mM solution. Average values and their standard deviations taken from samples used for STM, UV/vis, and XPS. TPP-P-disulfide/ethanol ) 8 samples, TPP-P-disulfide/ CH2Cl2 ) 7 samples, TPP-P-thiol/ethanol ) 7 samples. c Small amounts (