Formation of Porphyrin-and Sapphyrin-Containing Monolayers on

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Formation of Porphyrin- and Sapphyrin-Containing Monolayers on Electrochemically Prepared Gold Substrates: A FT Raman Spectroscopic Study Kamil Za´ruba,† Pavel Mateˇjka,† Radko Volf,† Karel Volka,† Vladimı´r Kra´l,*,† and Jonathan L. Sessler‡ Department of Analytical Chemistry, Institute of Chemical Technology, Technicka´ 5, CZ-166 28 Prague 6, Czech Republic, and Department of Chemistry and Biochemistry and Institute for Cellular and Molecular Biology, University of Texas at Austin, Austin, Texas 78712-1167 Received March 21, 2002. In Final Form: June 21, 2002 The formation of porphyrin- and sapphyrin-containing self-assembled monolayers (SAMs) on electrochemically prepared gold surfaces by a multistep approach is described. After electrochemical preparation, the gold substrate was characterized by surface-enhanced Raman scattering (SERS) spectroscopy and cyclic voltammetry. SERS characterization was also used to study the basic architecture and properties of the porphyrin- and sapphyrin-modified SAMs and their interactions with model compounds, that is, fluoranthene, 1,10-phenanthroline, and adenine. The results obtained lead to the conclusion that in a general sense, macrocyclic oligopyrrole functionalized SAMs could prove useful in the analysis and detection of polyaromatic and heterocyclic compounds.

1. Introduction In recent years, a wide range of increasingly elegant synthetic receptors have been used to study molecular recognition events in solution. However, in nature, a range of recognition and assembly processes take place on surfaces and at interfaces. Developing models for these kinds of interactions is thus important. Among the approaches that can be envisioned, the possibility of preparing monolayers of molecules (receptors, hosts) which are able to interact with substrates (guests, analytes) of different kinds appears particularly attractive. It offers, among other things, the possibility of constructing special analytical devices, for example, sensors and ion-selective electrodes (ISEs). The fact that well-ordered monolayer films can generally be formed by the adsorption of (longchain) alkanethiols from solution onto the surface of gold particles leads to the further prediction that self-assembled monolayers (SAMs)1 can be generated that will prove useful in the study of molecular recognition phenomena. SAMs are also attractive because of their simple characterization and the wide possibilities they offer for further chemical modification.2 In this paper, we report the synthesis of several novel porphyrin- and sapphyrin-functionalized SAMs. These systems, several of which are without precedent in the literature, were studied by surface-enhanced Raman scattering (SERS) spectroscopy. SERS spectroscopy has been proven to be an effective technique for structure determination of layers fixed on some metal surfaces (e.g., Au, Ag, Cu).3-5 The interaction of the excitation laser beam with the surface of noble metals, for example, gold, is * To whom correspondence should be addressed. E-mail: kralv@ vscht.cz. † Institute of Chemical Technology. ‡ University of Texas at Austin. (1) Ulman, A. Chem. Rev. 1996, 96, 1533. (2) Imahori, H.; Norieda, H.; Nishimura, Y.; Yamazaki, I.; Higuchi, K.; Kato, N.; Motohiro, T.; Yamada, H.; Tamaki, K.; Arimura, M.; Sakata, Y. J. Phys. Chem. B 2000, 104, 1253. (3) Carron, K. T.; Hurley, G. J. Phys. Chem. 1991, 95, 9979. (4) Lewis, M.; Tarlov, M. J. Am. Chem. Soc. 1995, 117, 9574.

accompanied by excitation of surface plasmons. Surface plasmon waves cause enormous (usually ca. 106-fold) enhancements in the Raman scattering efficiency for molecules near the surface.6,7 Such excitation methods are thus attractive for studying functionalized SAMs made of porphyrin and sapphyrin, work that is the focus of this report. Prior to the present work, no self-assembled monolayers containing sapphyrins had been reported, although a variety of porphyrin-based SAMs were known. A number of the latter were prepared via the self-assembly of their thiol or disulfide derivatives.8-10 Others, particularly those targeted for use in spectroscopic studies, were generated by fixing the porphyrin skeleton to the surface via axial binding interactions involving the metalloporphyrin center and amine,11 imidazole,12,13 and pyridine14-16 functionalized thiol-based SAMs. In somewhat related work, the covalent binding of carboxyl derivatives of oligopyrrolic macrocycles to amino-functionalized surfaces (e.g., silica gel) has been used to prepare porphyrin and expanded porphyrin modified sorbents.17-19 (5) Delamarche, E.; Michel, B.; Biebuyck, H. A.; Gerber, C. Adv. Mater. 1996, 8, 719. (6) Moskovits, M. Rev. Mod. Phys. 1985, 57, 783. (7) van Duyne, R. P. In Chemical and Biochemical Applications of Lasers; Moore, C. B., Ed.; Academic Press: New York, 1979; Chapter 5. (8) Redman, J. E.; Sanders, K. M. Org. Lett. 2000, 2, 4141. (9) Ishida, A.; Majima, T. Chem. Commun. 1999, 1299. (10) Gryko, D. T.; Clausen, C.; Lindsey, J. S. J. Org. Chem. 1999, 64, 8635. (11) Zhang, Z.; Hu, R.; Liu, Z. Langmuir 2000, 16, 1158. (12) Ashkenasy, G.; Kalyuzhny, G.; Libman, J.; Rubinstein, I.; Shanzer, A. Angew. Chem., Int. Ed. 1999, 38, 1257. (13) Matthew, S.; Eberspacher, T. A.; Griffin, J. H.; Chidsey, C. E. D.; Collman, J. P. J. Am. Chem. Soc. 1998, 120, 4478. (14) Zhang, Z.; Hou, S.; Zhu, Z.; Liu, Z. Langmuir 2000, 16, 537. (15) Kalyuzhny, G.; Vaskevich, A.; Matlis, S.; Rubinstein, I. Rev. Anal. Chem. 1999, 18, 237. (16) Kanayama, N.; Kanbara, T.; Kitano, H. J. Phys. Chem. B 2000, 104, 271. (17) Chen, S.; Meyerhoff, M. E. Anal. Chem. 1998, 70, 2523. (18) Iverson, B. L.; Thomas, R. E.; Kra´l, V.; Sessler, J. L. J. Am. Chem. Soc. 1994, 116, 2663. (19) Kibbey, C. E.; Meyerhoff, M. E. Anal. Chem. 1993, 65, 2189.

10.1021/la025766m CCC: $22.00 © 2002 American Chemical Society Published on Web 08/02/2002

Porphyrin- and Sapphyrin-Containing Monolayers

Figure 1. Scheme of modification of the gold substrate.

In the present work, we detail the use of a two-step synthetic strategy (Figure 1), involving the adsorption of 2-mercaptoethanol on a gold surface, followed by reaction of the free hydroxyl groups with mono- and dicarboxy derivatives of tetraphenylporphyrin and sapphyrin monoand dicarboxylic acids to prepare several new oligopyrrolecontaining SAMs (Figure 2). The mono- and dicarboxy derivatives of both types of oligopyrrole macrocyclic compounds were selected to investigate the effects of the number of binding sites on the properties of SAMs containing either porphyrin or sapphyrin macrocycles. The resulting monolayers were characterized using both spectroscopic and electrochemical methods, with the potential molecular recognition properties of the SAMs containing both porphyrin and sapphyrin derivatives being tested in preliminary fashion by studying their interactions with some model compounds. As model compounds, fluoranthene and o-phenanthroline (Figure 3) were chosen in the case of the SAM systems containing the porphyrin derivatives. Fluoranthene (a representative polycyclic aromatic hydrocarbon important in environmental analysis) was expected to interact with the porphyrin-containing SAMs as the result of noncovalent interactions only (i.e., via π-π stacking), while 1,10-phenanthroline, being a heterocylic aromatic compound, was expected to interact with these same SAMs via both π-π stacking and metal ion coordination. In the case of the sapphyrin SAMs, adenine was used as the model compound; it is prototypical of the biochemically important class of nucleic acid bases. 2. Experimental Section Chemicals. Potassium dicyanoaurate(I) (K[Au(CN)2]) (Safina, Czech Republic), alumina powder (Al2O3) (Staart Enterprises Ltd, U.K.), 2-mercaptoethanol (min 98%) (Aldrich), 4-(dimethylamino)pyridine (Aldrich), N,N′-diisopropylcarbodiimide (DIC), 1-hydroxybenzotriazole (HOBT) (Fluka), 1,10-phenanthroline (analytical grade) (Penta, Czech Republic), fluoranthene (min 98%) (Aldrich), Mohr’s salt (NH4)2Fe(SO4)2‚6H2O (analytical grade) (Penta), and KBr (Spectranal (rr)) (Riedel-deHae¨n) were used without further purification. Absolute ethanol, dimethylformamide (DMF), and methylene chloride were dried over molecular sieves (3 Å, Aldrich) prior to use. 5-(4′-Carboxyphenyl)-10,15,20-tris(4′-methylphenyl)porphyrin (1), 5,15-bis(4′-carboxyphenyl)-10,20-bis(4′-methylphenyl)porphyrin (2), 3,8,17,22-tetraethyl-12-(carboxyethyl)-2,7,13,18,23-pentamethylsaphyrin (3), and 3,12,13,22-tetraethyl-8,17bis(carboxyethyl)-2,7,18,23-tetramethylsaphyrin (4) were used for the preparation of functionalized SAMs. The structures of these precursors are given in Figure 2. They were synthesized as described previously.20-22 Methods. Preparation of the Polished Gold Substrate. Gold targets were prepared by electrochemical deposition of the (20) Kra´l, V.; Sessler, J. L. Tetrahedron 1995, 51, 539. (21) Little, R. G.; Loach, A. J.; Ibers, J. A. J. Heterocycl. Chem. 1967, 32, 476. (22) Anton, J. A.; Loach, P. A. J. Heterocycl. Chem. 1975, 12, 573.

Langmuir, Vol. 18, No. 18, 2002 6897 requisite gold onto platinum plates (thickness, 0.4 mm; diameter, 7 mm) as described below. Platinum plates were polished with emery paper (SIA, Switzerland), aluminum oxide, and calcium carbonate. After a thorough rinse with aqueous hydrochloric acid (3%) and distilled water, the resulting individual platinum plate was connected as the cathode in an electrolytic cell. The electrolytic solution, containing per liter of water 50 g of citric acid, 30 g of sodium citrate, and 12 g of K[Au(CN)2], was adjusted with saturated NaOH to pH ) 5.2. A massive gold electrode was used as the anode. Electrolysis was carried out over the course of 12 min (i ) 3 mA). The resulting gilded platinum plates were polished with Al2O3 and CaCO3 and rinsed with aqueous hydrochloric acid (3%) and distilled water prior to use. Preparation of the Rough Gold Substrate. The polished gold substrates (prepared as described above) were cleaned with hot, fresh piranha solution (concentrated H2SO4/H2O2, 3:1 v/v). After such treatment and a thorough rinse with water, a rough layer of gold was deposited by electrolysis using conditions somewhat modified from those described above (i ) 5 mA, t ) 4 min). The light-brown layer of gold deposited in this way was rinsed thoroughly with water. In preliminary experiments, a high level of adsorbed cyanide was observed by Raman spectroscopy. Subjecting the substrates to oxidizing peroxodisulfate treatment eliminated this adsorbed impurity. Specifically, the substrates in question were bathed in an aqueous potassium peroxodisulfate solution (4 g per 200 mL of water) for 4 h at 50 °C. The rough gold substrate was then allowed to undergo leaching in water for 5 h. Preparation of SAMs Bearing 2-Mercaptoethanol. This class of monolayers were prepared by immersing the gold substrates into a dilute solution (ca. 1 mmol L-1) of 2-mercaptoethanol in absolute ethanol. After an incubation time of at least 24 h, the resulting SAMs were removed from the solution and rinsed carefully with pure ethanol. Measurements were performed as soon as possible after this preparative procedure was complete. Preparation of SAMs of Oligopyrrolic Derivatives. The relevant porphyrin or sapphyrin acids (3 µmol) and HOBT (0.2 mg, 1 µmol) (for abbreviations, see Chemicals) were dissolved in the appropriate dry solvent (3 mL). DMF was used as a solvent for acids 1 and 2, and dichloromethane was used as a solvent for acids 3 and 4. The resulting solutions were cooled to 0 °C, and DIC (0.03 mmol) was added. The reaction mixture was then stirred for 15 min with 4-(dimethylamino)pyridine (0.3 mg, 2 µmol) being added dropwise during this time. Following this latter addition, the SAM of 2-mercaptoethanol was immersed into the reaction mixture. After a 5 day incubation period, the samples were removed from the solution and rinsed carefully with pure solvent (DMF or dichloromethane, as appropriate). Measurements were then performed as soon as possible (within 24 h). Preparation of Systems with Polycyclic Aromatic Compounds. Aqueous solutions of 1,10-phenanthroline (8 µg L-1) and of fluoranthene (9 ng L-1) were prepared. SAMs bearing the fixed porphyrin derivatives 1 and 2 were then immersed into the individual aqueous solutions for 24 h. Following such incubation, the samples were rinsed thoroughly with redistilled water. The Fourier transform (FT) Raman spectra were then recorded immediately to detect the presence of polycyclic aromatic compounds. After these latter measurements, we tested the possibility of removing polycyclic compounds via a change in solvent polarity. Toward this end, some of the samples were immersed in petroleum ether for at least 10 min. They were then dipped into a dichloromethane/methanol mixture (1:1 v/v) for 2 min. Finally, the samples were washed briefly with petroleum ether, dichloromethane, and methanol. The Raman spectra were then immediately remeasured. Considering the complexation ability of 1,10-phenanthroline, the samples treated with 1,10-phenanthroline were immersed into an aqueous solution of Mohr’s salt (concentration of Fe2+ ∼ 0.01 mol L-1) for 10 min. This set of samples was then rinsed with water and methanol and again with water. They were then held at ambient laboratory temperature to allow any residual solvents to evaporate. Dry samples were again subject to Raman spectral analysis.

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Figure 2. Structures of porphyrin and sapphyrin carboxylic acids used in this study: 5-(4′-carboxyphenyl)-10,15,20-tris(4′methylphenyl)porphyrin (1), 5,15-bis(4′-carboxyphenyl)-10,20-bis(4′-methylphenyl)porphyrin (2), 3,8,17,22-tetraethyl-12-(carboxyethyl)-2,7,13,18,23-pentamethylsapphyrin (3), 3,12,13,22-tetraethyl-8,17-bis(carboxyethyl)-2,7,18,23-tetramethylsapphyrin (4).

Figure 3. Structures of aromatic compounds used to study the ability of the porphyrin and sapphyrin SAMs to interact with model substrates: fluoranthene (a), 1,10-phenanthroline (b), and adenine (c). Preparation of Systems with Adenine. Adenine, min 99% (Sigma), was dissolved in water (c(adenine) ) 0.05 mol L-1, pH ) 7.7); afterward, a part of the solution was acidified with HCl (c(HCl) ) 0.1 mol L-1) to achieve pH ) 2.5 (the pH values were measured by a digital pH meter Cole-Parmer with a glass pH microelectrode (9802BN Orion)). SAMs bearing the fixed sapphyrin derivatives 3 and 4 were immersed into the individual aqueous solutions for 24 h. Afterward, the samples were rinsed thoroughly and repeatedly with redistilled water. Apparatus. Raman Spectroscopy. Raman spectra were collected using a Fourier transform near-infrared (FT-NIR) spectrometer Equinox 55/S (Bruker, Germany) equipped with a FT Raman module FRA 106/S (Bruker). A defocused laser beam (30 mW) of a Nd:YAG laser (1064 nm, Coherent) was used to excite the Raman effect. Pure samples of 1, 2, 3, and 4 were made up in glass vials and placed on a motorized X-Y-Z sample stage. The scattered light was collected using a backscattering geometry. Interferograms were obtained using a quartz beam splitter and a Ge detector (liquid N2 cooled). Typically, 128 separate interferograms were accumulated and then processed by Fourier transformation using Blackman-Harris 4-term apodization and a zerofilling factor of 8 in order to obtain individual FT Raman spectra with 2 cm-1 resolution. Samples of SAMs were measured using a Bruker Ramanscope, which was connected to a FRA 106/S module by optical fibers. The laser power, less than 50 mW at the microscope objective, was found to be acceptable in terms of obtaining reliable and reproducible spectra. At least three sites on each side of a given gold target were measured, with each of these sites being subject to at least two separate measurements. The final spectrum of each SAM was then calculated as the arithmetical mean of all spectra measured using a given target. Cyclic Voltammetry. Voltammetric data were obtained using a potentiostat PAR model 263 (EG&G Princeton Applied Research). A standard one-compartment, three-electrode cell equipped with inlet and outlet ports to allow N2 flow was used for all experiments. The individual targets bearing the SAMs (prepared as described above) were used as the working electrode.

A platinum wire and a Ag/AgCl (3 mol L-1 KCl) electrode were used as the counter and reference electrodes, respectively. Solutions were purged for ca. 10 min with nitrogen to remove oxygen from the solution. An overpressure of nitrogen was maintained in the cell during the measurements. Infrared Spectroscopy. Infrared spectra were measured using a FTIR spectrometer Nicolet 210 (128 scans at a resolution of 4 cm-1). Samples were prepared in the form of KBr pellets (0.5 mg of the particular compound in question, 230 mg of KBr). Scanning Electron Microscopy. Micrographs were measured using a Hitachi S-4700 (Japan) scanning electron microscope, with an accelerating voltage of 5 kV.

3. Results and Discussion Preparation of Gold Substrates. Several electrochemical preparation procedures involving a gold layer on a Pt electrode were tested so as to obtain systems without interfering inorganic ions and with reproducible properties. Two types of the gold surfaces on Pt plates were prepared by electrolysis of potassium dicyanoaurate from citrate electrolyte: (1) polished gold substrates and (2) rough gold substrates. The first one was prepared by electrolytic deposition using a current density of ca. 3.8 mA cm-2. The gold layer that formed under these conditions was then polished with Al2O3 and CaCO3. In the case of this substrate, the SAM derived from 2-mercaptoethanol displayed characteristic bands, assigned to methylene group stretching vibrations (ca. 2924 and ca. 2875 cm-1), in the FT Raman spectra obtained (Figure 4a). Unfortunately, the spectral features in the 1800-300 cm-1 range proved very weak and could not be reliably interpreted. A strong band at ca. 263 cm-1 was observed. Considering the shape and intensity of this band, it was tentatively attributed to a ν(Au-S) vibration in analogy to the band of ν(Au-Cl).23 The other possibility involved assigning this band to a δ(C-C-S) mode, based on the interpretation of SERS spectra of thiol monolayers on gold by Sandhyarani and Pradeep.24 On the other hand, no band of analogous position, intensity, and shape was observed in the spectrum of pure 2-mercaptoethanol. Thus in any case, this band was considered to reflect binding of 2-mercaptoethanol molecules to the Au surface. A band in the range 255-295 cm-1 is also observed in other systems, but in such cases the precise position, intensity, (23) Kania, S.; Holze, R. Surf. Sci. 1998, 408, 252. (24) Sandhyarani, N.; Pradeep, T. Vacuum 1998, 49, 279.


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Figure 5. Voltammogram showing the decomposition of a 2-mercaptoethanol layer (starting potential, -0.4 V; final potential, -1.4 V; scan, 100 mV s-1; surface area, 0.77 cm2).

Figure 4. FT Raman spectra of a layer of 2-mercaptoethanol on a polished (a) and a rough (b) gold substrate compared with the spectrum of pure 2-mercaptoethanol (c).

and shape of this band differs dramatically, probably due to the effect of the different systems and their organization. The next step in the study involved preparing and characterizing SAMs containing oligopyrrolic macrocycles. There are generally two strategies established for the preparation of functional SAMs: (1) the one-step approach, that is, the spontaneous adsorption of a derivative that contains a thiol group(s)25,26 and (2) the two- or multistep approach, that is, the spontaneous adsorption of a bifunctional thiol molecule in a first step, followed by a derivatization reaction. In this later step, the molecule that will establish the functionality of the SAM is attached onto the thiol monolayer.27,28 The second strategy was adopted in this study. The SAMs containing oligopyrrolic macrocycles were prepared by esterifying the hydroxyl groups of the SAMs bearing 2-mercaptoethanol functionality using the carboxyl groups of macrocyclic acids 1, 2, 3, and 4 as the acid source. Prior to coupling, the macrocyclic acids were activated by reaction with DIC (i.e., to form active esters). Unfortunately, the signal-tonoise ratio for the resulting putative SAMs proved very low in all FT Raman spectral measurements. This lessthan-pleasing observation could reflect several factors: (25) Troughton, E. B.; Bain, C. D.; Whitesides, G. M.; Allara, D. L.; Porter, M. D. Langmuir 1988, 4, 365. (26) Bain, C. D.; Troughton, E. B.; Tao, Y.-T.; Evall, J.; Whitesides, G. M.; Nuzzo, R. G. J. Am. Chem. Soc. 1989, 111, 321. (27) Tour, J. M.; Jones, L., II; Pearson, D. L.; Lamba, J. J. S.; Burgin, T. P.; Whitesides, G. M.; Allara, D. L.; Parikh, A. N.; Atre, S. V. J. Am. Chem. Soc. 1995, 117, 9529. (28) Arnold, S.; Feng, Z. Q.; Kakiuchi, T.; Knoll, W.; Niki, K. J. Electroanal. Chem. 1997, 438, 91.

(1) the prepared SAM of 2-mercaptoethanol is not dense enough, (2) the SERS activity of the polished gold substrate is too weak, or (3) yield of the reaction of macrocyclic acids with the SAM of 2-mercaptoethanol is very low. Some efforts were made to distinguish between these limiting possibilities. Figure 5 represents the voltammogram of decomposition of the SAM of 2-mercaptoethanol that has been carried out in ethanolic potassium hydroxide solution (0.1 mol L-1). There are two observable maxima on the voltammetric curve (ca. -0.9 and ca -1.1 V, vs Ag/AgCl), that correspond to the formation of two energetically distinct surface phases. This result, similar to ones published previously,29,30 is consistent with a reasonably dense surface coverage. Separately, as discussed below, it was found that the coupling yield between 2-mercaptoethanol SAMs and various polypyrrolic macrocycles was satisfactory for a different type of gold substrate. On this basis, we conclude that the polished gold substrate does not exhibit a sufficiently high level of SERS activity, especially, as in the present instance, when NIR excitation of Raman scattering is used. In light of the above findings, rough gold layers were prepared as a second type of Au substrate. Because of the higher current density applied in this case as compared to that used for the polished substrate (ca. 6.5 vs 3.8 mA cm-1), the gold layers formed with different surface morphology compared to the polished ones (Figure 6a,b). The gold particles of the size of hundreds of nanometers observed on the rough surface (Figure 6c) are proposed to be well suited for allowing a high level of surface enhancement of Raman scattering via an electromagnetic mechanism.31 It is important to appreciate that the gold layers formed under the conditions of higher current were not later mechanically modified. Still, the amount of cyanide adsorbed on the surface had to be scaled down because a very strong band at ca. 2128 cm-1 assigned to ν(CtN) was observed in the FT Raman spectra, both of the substrate itself and of the layers formed on this substrate. An oxidizing peroxosulfate treatment was therefore used to decrease the amount of cyanide. The band of ν(CtN) vibration was still observable, but its intensity was about 102-fold lower than before the oxidizing treatment. The (29) Hatchett, D. W.; Stevenson, K. J.; Lacy, W. B.; Harris, J. M.; White, H. S. J. Am. Chem. Soc. 1997, 119, 6596. (30) Weisshaar, D. E.; Lamp, B. D.; Porter, M. D. J. Am. Chem. Soc. 1992, 114, 5860. (31) Chang, R. K.; Furtak, T. E. Surface Enhanced Raman Scattering; Plenum Press: New York, 1982.

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Za´ ruba et al. Table 1. Comparison of Band Positions (cm-1) in the FT Raman Spectra of 1, 1a, 2, and 2a with Those for TPPa and TCPPb and Tentative Assignments of Vibrational Modes 1

1a

2

2a

TPP

TCPP

1710 1607 1546 1525 1360 1330 1287 1240 1187 1076 1015

1706 1605

1715 1608

1720 1608 1551

1597 1550

1609 1552

1536 1352 1323 1279 1245

1524 1358 1329 1288 1238 1187 1076 1014

889 840 784 697 670 653 562 416 334 247 168

1061 1013 1000 967 842 705

429 341 244

842 786 696 668 652 565 417 335 247

1348 1331 1236 1182 1076 1002

1357 1327 1291 1234 1076

888 844

1238 1185 1084

889 822

714 643

674 638

413 334 167

415 327 244

assignmentc,d ν(CdO) φ(phenyl) ν(Cb-Cb)e ν(Cb-Cb) ν(Ca-N) δ(N-H) ν(Ca-Cb) ν(Cm-Ph) φ(phenyl) δ(Cb-H), ν(porph) ν(porph) π(phenyl) ν(porph) π(phenyl) π(phenyl) δ(N-H) φ(phenyl) γ(porph) π(phenyl) δ(porph) δ(porph) δ(porph)

a

References 32 and 33. b References 34 and 35. c Assignments were made according to refs 32-36. d φ(phenyl), phenyl in-plane; π(phenyl), phenyl out-of-plane; ν(porph), porphyrin in-plane; γ(porph), porphyrin out-of-plane; δ(porph), nonspecified porphyrin vibration. e Description of porphyrin core atoms: Ca, carbon in the structure of pyrrole in positions 2 and 5; Cb, carbon in the structure of pyrrole in positions 3 and 4; Cm, carbon of the methine bridge of the porphyrin core.

Figure 6. Micrographs of gold surfaces obtained by scanning electron microscopy: polished surface (magnification, 15 000×) (a), rough surface (magnification, 15 000×) (b), and rough surface (magnification, 100 000×) (c).

intensity of this band remains practically of the same magnitude and its position does not shift in all further experiments (i.e., SAM formation studies). This observation is consistent with cyanide being strongly fixed on the gold surface and not affected by subsequent formation/ modification of SAMs. Consistent with the SERS-related problems proposed in the case of the smooth substrates, all Raman spectra collected for monolayers formed on the rough gold substrates were found to display a higher signal-to-noise ratio than equivalent spectra measured for SAMs prepared using polished surfaces. Accordingly, all further measurements were made using SAMs prepared on rough gold surfaces. Preparation of SAMs Derived from 2-Mercaptoethanol. The observation of slightly shifted characteristic -CH2- stretching vibration bands for 2-mercaptoethanol (ca. 2934 and 2877 cm-1 and ca. 2933 and 2878 cm-1 for the rough gold substrate SAM and pure 2-mercaptoethanol, respectively) (Figure 4b,c), as well as the presence of some other weak, rather modestly shifted 2-mercaptoethanol bands in the spectrum of the SAM, is consistent with this difunctional molecule being bound to the gold surface. The fact that the characteristic ν(S-H) band at ca. 2566 cm-1 in the spectrum of pure 2-mercaptoethanol

(Figure 4c) was missing in the spectrum of the SAM (Figure 4b) is further consistent with the 2-mercaptoethanol being fixed onto the rough gold substrate via an S-Au bond. In the spectrum of pure 2-mercaptoethanol, a strong band assigned to ν(C-S) was observed at 663 cm-1. In the spectrum of the SAM on the Au substrates, a very weak band tentatively assigned to ν(C-S) is observed at ca. 646 cm-1. To the extent this assignment is correct, the associated shift in band position and the associated changes in relative intensities confirm the proposed surface bonding and exclude the possibility that the 2-mercaptoethanol is simply adsorbed via physisorption.16 Finally, the proposed SAMs were considered to be homogeneous over the full extent of the surface since all bands analyzed displayed the same wavenumber values and intensities when the spectra were collected at four different positions on the gold target. Monolayers of Porphyrins. Once prepared and characterized, the above-mentioned 2-mercaptoethanolbearing SAMs were derivatized by reaction with 1 and 2. Structures proposed for the resulting two-tier SAMs are shown in Figure 7 (1a and 2a). The FT Raman spectra of pure 1 and 2 are very similar (Figure 8), and the characteristic frequencies of these two porphyrins were found to compare well with those characteristic of 5,10,15,20-tetraphenylporphyrin (TPP)32,33 and 5,10,15,20tetrakis(4-carboxyphenyl)porphyrin (TCPP).34,35 These comparisons are summarized in Table 1 wherein an (32) Stein, P.; Ulman, A.; Spiro, T. G. J. Phys. Chem. 1984, 88, 369. (33) Bour, P.; Za´ruba, K.; Urbanova´, M.; Setnicka, V.; Matejka, P.; Fiedler, Z.; Kra´l, V.; Volka, K. Chirality 2000, 12, 191. (34) Cotton, T. M.; Scholtz, S. G.; van Duyne, R. P. J. Am. Chem. Soc. 1982, 104, 6528. (35) Vlcˇkova´, B.; Mateˇjka, P.; Sˇ imonova´, J.; C ˇ erma´kova´, K.; Pancˇosˇka, P.; Baumruk, V. J. Phys. Chem. 1993, 97, 9719.


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Figure 7. Proposed structures of the oligopyrrole macrocycle containing monolayers that are the subject of this study.

assignment of porphyrin core modes and phenyl- and carboxy- substituent modes is also provided. In terms of specifics, a number of bands characteristic of mesotetraphenylporphyrin derivatives can be observed. For instance, both in-plane phenyl vibrations32-36 (φ-modes) (1607, 1187, and 653 cm-1) and out-of-plane vibrations (π-modes) (840 and 416 cm-1) can be distinguished in the spectra of both 1 and 2. Next, bands corresponding to in-plane deformation vibrations of the porphyrin skeleton32-35 (ν-modes) (1360, 1240, 1076, and 1015 cm-1) are also seen. However, only one out-of-plane vibration, assigned to the porphyrin skeleton (γ-modes), was observed (ca. 562 cm-1) in the case of both 1 and 2. In the spectral range of about 1500 cm-1 should be observed bands corresponding to vibrations both of phenyl rings and of the porphyrin skeleton.32-36 An intense and quite broad band with several shoulders and a maximum at ca. 1525 cm-1 is also a feature of the Raman spectra of pure (36) Socrates, G. Infrared Characteristic Group Frequencies; J. Wiley: Chichester, 1980.

1 and 2. On the basis of a literature precedent,32-36 it is attributed to a combination of phenyl ring and porphyrin skeleton modes. Weak bands at 1710 and 1715 cm-1 in the spectra of 1 and 2, respectively, were assigned to carboxy group ν(CdO) vibrations. Analysis of Porphyrin-Containing SAMs. An attachment of a macrocycle to a SAM influences the force constants of the bonds with the exact mode of attachment affecting the resulting Raman spectra. It has been discovered that excitation of surface plasmons affects perpendicular vibration modes more than those associated with motions parallel to a surface.3,37,38 In the case of the present porphyrin-containing SAMs, if the macrocycles end up lying parallel to the surface, it will be primarily the intensities of the out-of-plane mode bands that are enhanced. By contrast, if the porphyrin rings adopt (37) Szafranski, C. A.; Tanner, W.; Laibinis, P. E.; Garrell, R. L. Langmuir 1998, 14, 3580. (38) Szafranski, C. A.; Tanner, W.; Laibinis, P. E.; Garrell, R. L. Langmuir 1998, 14, 3570.

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Figure 8. FT Raman spectra of compounds 1 (curve a) and 2 (curve b).

orientations that are more perpendicular to the surface, then the intensities of the in-plane mode bands will be enhanced. As can be seen by an inspection of Figure 9, the FT Raman spectra of SAMs 1a and 2a differ markedly even though the spectra of the original compounds (1 and 2, Figure 8) are quite similar. This leads us to suggest that these two systems (1a vs 2a) differ structurally. While the positions of the various bands observed for 1, 1a, 2, and 2a are similar (Table 1), the relative intensities differ significantly, especially when the two layered systems 1a and 2a are compared (Figure 9). In the spectrum of 1a, for instance, bands are observed at 1536, 1352, 1245, 1061, and 1013 cm-1 that are assigned to in-plane porphyrin modes. On the other hand, the band at 562 cm-1, assigned to an out-of-plane porphyrin core mode, observed in the spectrum of 1 is not observed in spectrum of 1a. The only in-plane phenyl mode observable in the spectrum of 1a is the band at 1605 cm-1. The ratio of intensities for this band relative to the bands assigned to the in-plane porphyrin core modes is much smaller than in the case of 1. Nevertheless, as compared to the spectrum of 1 the intensities of the bands assigned to the out-of-plane phenyl ring modes (e.g., 842 and 705 cm-1) are enhanced relative to those of the other bands observed in the spectrum of 1a. The bands in the spectrum of 2a at 1551, 1348, 1236, 1182, 1076, and 888 cm-1 have been assigned to in-plane porphyrin core modes. The bands assigned to in-plane phenyl modes (1608, 1182, and 643 cm-1) are clearly identified in the spectrum of 2a. In this case, the ratio of

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Figure 9. FT Raman spectra of porphyrin monolayers 1a (curve a) and 2a (curve b).

intensities of these bands relative to those of the in-plane porphyrin core modes is essentially analogous to the spectrum of the free macrocycle, 2. Further, the bands assigned to phenyl ring out-of-plane modes (844, 714, and 413 cm-1) are quite weak in the spectrum of 2a, just as they are in the spectrum of 2. Considering the above observations, particularly the fact that the out-of-plane porphyrin modes in the case of both 1a and 2a are not selectively enhanced, leads us to conclude that neither macrocycle is coplanar with the gold surface. On the other hand, while the ratios of band intensities for both the in-plane and out-of-plane phenyl ring modes to porphyrin core modes are quite similar in the Raman spectra of 2 and 2a, the corresponding ratios are completely different in the case of the spectra of 1 and 1a. This observation can be rationalized by considering that the angle between the porphyrin macrocycle and the appended meso-phenyl rings changes during the formation of 1a. No such change in internal conformation is invoked in the case of 2a. As a consequence of these differences, stacking interactions between porphyrin molecules are possible in the case of 1a. Moreover, one can further infer that the mutual orientation of the porphyrin rings is actually parallel. Presumably, this reflects the fact that molecules of 1 are tethered via only one ester linkage. By contrast, in system 2a, two ester linkages serve to effect attachment. This affects the orientation of 2 in the SAM (2a), where no features consistent with mutual interactions between the porphyrin rings can be suggested. Monolayers of Sapphyrins. In marked contrast to what is true in the case of porphyrins, the expanded


porphyrin system, sapphyrin, has yet to be the subject of detailed Raman spectral analysis. Initial efforts, therefore, focused on recording the Raman spectra of 3 and 4. Unfortunately, acceptable spectra could not be obtained. Although near-infrared excitation was used and various combinations of laser power and beam focusing were tested, many overlapping Raman bands were observed that were associated with a high level of thermal39 and/or fluorescence40 background. A thermal low-intensity background is observed also in the case of the SAMs, but it does not exceed the intensity of the Raman bands even in the case of the 2-mercaptoethanol monolayers which display quite weak Raman features (Figure 4). This lack of success provided a further important motivation for studying sapphyrin-containing SAMs. Site isolation and the SERS effect could lead to an ability to observe the Raman spectra of sapphyrin derivatives. Given the above, sapphyrin-containing SAMs with the proposed structures 3a and 4a (Figure 7) were prepared by functionalizing the 2-mercaptoethanol-derived SAM with 3 and with 4, respectively. In this case, the Raman spectra could indeed be recorded. They are reproduced in Figure 10. In contrast to what was true for 1a and 2a, only modest spectral differences were observed for these two sapphyrin-based SAMs. For instance, broad bands with two distinct maxima at ca. 2921 and ca. 2856 cm-1 are observed in the spectrum of 3a. These have been assigned to vibrations of -CH2- and -CH3 groups involving both 2-mercaptoethanol and 3. The corresponding bands in the spectrum of 4a were weak, and the two maxima were not well resolved. On the other hand, the spectral features of 3a and 4a are very similar in the 1650-200 cm-1 range (Figure 10). Both spectra (Figure 10a,b) are dominated by two bands at ca. 1604 and 1571 cm-1, two groups of multiple bands in the 1490-1230 and 1130-840 cm-1 separate spectral ranges, and a broad band at ca. 275 cm-1. A number of bands could be assigned to vibrations involving the pyrrole rings, which are recognized as having a number of in-plane deformation bands (1580-1545, about 1490, and 1430-1390 cm-1).36 Likewise, a band attributable to δ(CH2) motion could be distinguished at ca. 1436 cm-1 among the multiple band features present between 1490 and 1230 cm-1. The spectra of 3a and 4a were compared with IR spectra of 3 and 4 (Figure 11). The quite similar IR spectra of 3 and 4 are dominated by the bands of the polar groups. The strong, broad band in the range 1750-1580 cm-1 is attributed to an overlap of ν(CdO) of the carboxylic group (maxima about 1670 cm-1) with ν(CdC) modes of the macrocyclic system (shoulder about 1610 cm-1) observed in the Raman spectra of 3a and 4a about 1605 cm-1. The other strong band observed in the IR spectra at about 1200 cm-1 (Figure 11) is attributed to a ν(C-C-O) vibration; the very weak counterpart in the Raman spectra36 cannot be reliably distinguished in the spectra of 3a and 4a. In other words, the Raman and IR spectra are complementary, providing important insights into the macrocyclic skeleton and the polar substituents, respectively. In summary, the similarity of the Raman spectra collected for 3a and 4a (Figure 10) leads us to conclude that the spatial orientation of 3 and 4 in the SAMs is not necessarily different. The high similarity of the spectra of 3a and 4a contrasts with the dissimilarity observed for the spectra of the porphyrin-containing SAMs (1a and (39) Schrader, B.; Moore, D. S. Pure Appl. Chem. 1997, 69, 1451. (40) Schrader, B.; Schulz, H.; Andreev, G. N.; Klump, H. H.; Sawatzki, J. Talanta 2000, 53, 35.

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Figure 10. FT Raman spectra of sapphyrin monolayers 3a (curve a) and 4a (curve b).

2a). Presumably, this contrast reflects the different mutual positions of the two carboxy groups in the structures of the dicarboxy derivatives in question (2 and 4, Figure 2). There are two quite rigid p-carboxyphenyl substituents in the meso-positions of 2, while the sapphyrin macrocycle 4 is substituted at its β-pyrrole positions by two more flexible 2-carboxyethyl groups. However, on the basis of the present studies, we are unable to tell, in the case of 4a, whether the sapphyrin is actually linked to the underlying 2-mercaptoethanol layer by two carboxyl groups or by only one. Interactions of the Porphyrin-Containing SAMs with Polyaromatic Compounds. To study the interaction of the porphyrin-containing SAMs with model compounds (analytes), we exposed systems 1a and 2a to a solution of a given analyte. Afterward, the systems were washed to remove any excess analyte. The interaction of the analyte with the SAM should lead to the observation of characteristic bands of both the original SAM and the analyte. Here, the bands of the skeletons involved in the interaction could be slightly shifted and/or changed in their relative intensities. In any event, the proposed noncovalent interaction can be proven by the reversibility of the “SAM-analyte complex” composition/decomposition, where the decomposition leads to the initial state of the SAM; that is, the final Raman spectra are the same as the original ones. The Raman spectra of SAMs 1a and 2a recorded after exposure to 1,10-phenanthroline (designated as systems 1b and 2b) are reproduced in Figure 12. Here, the spectra

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Figure 11. FTIR spectra of compounds 3 (curve a) and 4 (curve b).

of 1b and 2b are now quite similar, whereas in the absence of additive those of SAMs 1a and 2a are noticeably different (Figure 9). Many of new spectral features seen in the case of 1b and 2b are attributable to the added 1,10-phenanthroline. On the other hand, these spectral features are themselves changed relative to what is observed for free 1,10-phenanthroline. For instance, the intense bands (ca. 1449, 1419, 1051, 724, and 427 cm-1, 1b) seen in the case of systems 1b and 2b are slightly shifted relative to what is seen in the spectrum of pure 1,10-phenanthroline (ca. 1456, 1419, 1052, 713, and 415 cm-1). Also, a ca. 3053 cm-1 band ascribed to a 1,10phenanthroline ν(C-H) is observed in the spectra of SAMs 1b and 2b, but the maximum is at 3063 cm-1 in the spectrum of pure 1,10-phenanthroline. Other bands in the spectra of 1b and 2b (e.g., 1605, 1552, 1341, 1305, and 1001 cm-1, 2b) are slightly shifted compared to the spectra of 1a and 2a. The shifts in these bands are tentatively ascribed to some as yet not fully understood spatial reorientation of the porphyrin skeleton. In any case, the key point is that this spatial reorientation occurs as the result of the porphyrins present in 1a and 2a interacting with 1,10-phenanthroline in a noncovalent sense. Evidence for the proposed noncovalent interaction in systems 1b and 2b (i.e., in the porphyrin SAMs exposed to 1,10-phenanthroline) came from a recognition that if the binding of 1,10-phenanthroline were supramolecular in nature, the spectral features and the underlying interaction itself should be quite susceptible to changes in solvent polarity. Consequently, we tested the effect that nonpolar solvents had on systems 1b and 2b. These

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Figure 12. FT Raman spectra of porphyrin monolayers recorded after exposure to 1,10-phenathroline (system 1b, curve a; system 2b, curve b) compared with the spectrum of pure 1,10-phenanthroline (curve c).

systems, both made up initially in water, were immersed into petroleum ether and subsequently placed into a mixture of dichloromethane and methanol. The spectra obtained (e.g., Figure 13a) are now more similar to those of the initial SAMs 1a and 2a (Figure 9) than to those of systems 1b and 2b. The strongest 1,10-phenanthroline band, appearing at ca. 1419 cm-1, disappears almost completely following this treatment; other bands attributed to 1,10-phenanthroline (e.g., at 1456 and 713 cm-1, Figure 12c) are considerably weakened and slightly shifted (to e.g., 1455 and 724 cm-1, Figure 13a). We conclude that most of the 1,10-phenanthroline is removed by this procedure. To remove the 1,10-phenanthroline completely from systems 1b and 2b, they were washed with a solution of Mohr’s salt for 10 min. Here, the idea was to use a solution containing an easy-to-complex species, such as Fe2+, to remove the 1,10-phenanthroline via the formation of a coordination complex. In the event, such treatment produces Raman spectra identical to those of the initial SAMs 1a and 2a (Figure 13b). In particular, the bands ascribed to 1,10-phenanthroline in 1b and 2b (ca. 1449, 1419, 1051, 724, and 427 cm-1, 1b) are no longer observed, because the interaction between 1,10-phenanthroline and Fe2+ is stronger than the noncovalent “aromatic” interaction between 1,10-phenanthroline and the porphyrin macrocycle. In a separate series of supramolecular experiments, SAMs 1a and 2a were immersed in a very dilute aqueous


Langmuir, Vol. 18, No. 18, 2002 6905

Figure 13. FT Raman spectra of the porphyrin monolayer after exposure to 1,10-phenanthroline (system 1b) and subsequent washing with nonpolar solvents (curve a) and then a solution of Mohr’s salt (curve b).

Figure 14. FT Raman spectra of porphyrin monolayer 2a recorded after exposure to fluoranthene (system 2c, curve a) and after washing in nonpolar solvents (curve b). Also shown is the spectrum of pure fluoranthene (curve c).

solution of fluoranthene (9 ng L-1). This gave rise to systems 1c and 2c, which display slightly different spectra compared to those of 1a and 2a. The small changes in the Raman spectra observable for system 2c are highlighted in Figure 14a. For example, the relative intensity of the 1608 cm-1 band is increased as the result of an overlap of a porphyrin band (1607 cm-1) with a strong band of pure fluoranthene at ca. 1611 cm-1 (Figure 14c). The strong band at 1551 cm-1 with a shoulder at 1570 cm-1 seen in the spectra of system 2a (Figure 9b) is replaced by a broad band with two distinct maxima at ca. 1563 and 1552 cm-1 in the case of system 2c (Figure 14a). The band at 1439 cm-1 seen in the case of 2a is broadened in the spectrum of 2c as the result of an overlap with fluoranthene-derived bands at 1457 and 1423 cm-1 (Figure 14c). The presence of very small amounts of fluoranthene, presumably fixed to SAMs 1c and 2c via noncovalent interactions, can thus be detected readily by this Raman-based method. The proposed noncovalent character of the interaction in systems 1c and 2c was tested in a manner analogous to that used to test systems 1b and 2b, except that only solvents of different polarity were used. After the samples were washed with petroleum ether (Figure 14b, washed 2c), the spectra obtained proved very similar to those recorded for the original porphyrin SAMs, 1a and 2a (Figure 9). For instance, the intensity of the strong band at 1608 cm-1 seen for 1c and 2c is reduced back to a level

comparable to that seen in the case of 1a and 2a. Further, the 1652 cm-1 band has only a weak shoulder at 1570 cm-1, and the bandwidth of the 1439 cm-1 band is decreased after petroleum ether treatment. The reversibility of the spectral changes seen after exposure to fluoranthene and then after petroleum ether washing are consistent with the proposed weak noncovalent interaction between the porphyrins 1a and 2a and fluoranthene. Supporting this conclusion is the observation that the spectrum of 2c (Figure 14) is not the direct additive result of the spectra of 2a and fluoranthene. New specific features, reflective of complex formation, for example, two distinct maxima at ca. 1563 and 1552 cm-1 and a different structure for the multiple bands in the 1350-1100 and 460-380 cm-1 regions, are observed. Interaction of SAMs of Sapphyrins with Adenine. The immersion of SAMs 3a and 4a into aqueous solutions of adenine at two different pH values (2.5 and 7.7) was used to test the ability of sapphyrin SAMs to interact with a model nucleic base, adenine (Ade) (Figure 15). The systems prepared at pH ) 7.7 (3b and 4b) give quite different FT Raman spectra as compared to the spectra of the initial systems 3a and 4a. However, the spectra of 3b (Figure 15a) and 4b (data not shown) are quite similar; characteristic bands of both sapphyrin and Ade can be identified in the spectra of 3b and 4b. For instance, many

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spectra of 3a and 4a, respectively. This observation reflects different protonation states for both Ade and the sapphyrin macrocycle at pH ) 7.7 and pH ) 2.5. The pKa values of Ade and sapphyrin corresponding to proton associations are 4.15 (ref 41) and 4.8 (ref 42), respectively, underscoring the fact that both sapphyrin species change their protonation states between pH ) 7.7 and pH ) 2.5. The diprotonated sapphyrin does not interact with the cationic form of Ade at pH ) 2.5, while the monoprotonated form of sapphyrin interacts with the nonprotonated (i.e., neutral) form of Ade at pH ) 7.7. Ongoing experiments with nucleotides, which will be reported in due course, support the notion that the presence of a phosphate moiety on a nucleic acid base strongly affects the pH dependence of the interaction of such substrates with sapphyrin-based SAMs. 4. Conclusions

Figure 15. FT Raman spectra of sapphyrin monolayer 3a recorded after exposure to adenine at pH ) 7.7 (system 3b, curve a) and at pH ) 2.5 (system 3c, curve b). Also shown is the spectrum of pure adenine (curve c).

bands of 3a (1437, 1354, 1239, 1061, and 987 cm-1) have almost nonshifted counterparts in the spectrum of 3b (1439, 1354, 1236, 1059, and 987 cm-1) and some other bands are only slightly shifted (1305, 1280, and 847 cm-1 and 1308, 1275, and 842 cm-1 for 3a and 3b, respectively). The two most intense bands of Ade (724 and 1334 cm-1) are well distinguished in the spectrum of 3b at somewhat shifted positions (735 and 1343 cm-1). The other characteristic bands of Ade (1484, 1249, 942, and 537 cm-1) could be identified in the spectrum of 3b at ordinary positions as weak bands or shoulders. The 1122 and 1126 cm-1 bands of 3a and Ade, respectively, give rise to a rather broad band with a maximum at 1130 cm-1 and with a well-pronounced shoulder at 1121 cm-1. In summary, Ade is present in systems 3b and 4b. The specific interaction of Ade with the sapphyrin macrocycle at pH ) 7.7 is also inferred for the simple reason that shifts in bands characteristic of both sapphyrin and Ade are observed. On the other hand, no interaction for systems prepared at pH ) 2.5 (3c and 4c) can be identified on the basis of FT Raman spectral analysis. The spectra of 3c (Figure 15b) and 4c (data not shown) are almost the same as the

In summary, a convenient electrochemical-based preparation of a SERS-active gold layer on an underlying platinum substrate has been described. A layer of 2-mercaptoethanol can be readily deposited on the resulting gold surface and further derivatized by treatment with activated porphyrin and sapphyrin carboxylic acids. Such a treatment results in the formation of layers containing macrocyclic species, as confirmed by FT Raman spectroscopy. While the character of porphyrin SAMs depends on the number of binding sites on porphyrin derivatives, an analogous effect was not observed in the case of the sapphyrin SAMs. For the porphyrin SAMs, the ability to interact with two different polycyclic aromatic compounds in a noncovalent manner was demonstrated by a combination of Raman spectral analysis and chemical methods. In the case of sapphyrin SAMs, the ability to interact with a model nucleic base is strongly affected by the pH value of the solution wherein the interaction takes place. The effect of porphyrin metalation on the type of possible interactions in SAMs and the ability of sapphyrin SAMs to interact with nucleotides are the subjects of ongoing study. Nonetheless, the results obtained to date, as reported here, provide support for the notion that macrocyclic oligopyrrole functionalized SAMs could prove useful in the analysis and detection of polyaromatic and heterocyclic compounds. Acknowledgment. Financial support from the Czech Grant Agency (Nos. 203/97/P062 to P.M. and 203/02/0933 to V.K.) and from the Ministry of Education, Youth and Sports of the Czech Republic (Grant MSM 223400008 to K.V.) is gratefully acknowledged. Partial funding for this work came from the National Institutes of Health (GM58907 to J.L.S.). We thank Ms. Z. Cı´lova´, MSc., and Professor V. Hulı´nsky´ for recording the scanning electron micrographs. LA025766M (41) Ts’o, P. O. P. Basic Principles on Nucleic Acid Chemistry; Academic Press: New York, 1974. (42) Kra´l, V.; Furuta, H.; Shreder, K.; Lynch, V.; Sessler, J. L. J. Am. Chem. Soc. 1996, 118, 1595.

Formation of Porphyrin-and Sapphyrin-Containing Monolayers on

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