Adsorption Characteristics of Tripodal Thiol-Functionalized Porphyrins

Furthermore, the surface binding characteristics of the SAMs are not sensitive to ... tripodal units having each leg of the tripod bearing a sulfur an...
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J. Phys. Chem. B 2005, 109, 23963-23971

23963

Adsorption Characteristics of Tripodal Thiol-Functionalized Porphyrins on Gold Lingyun Wei,† Hugo Tiznado,† Guangming Liu,† Kisari Padmaja,‡ Jonathan S. Lindsey,*,‡ Francisco Zaera,*,† and David F. Bocian*,† Department of Chemistry, UniVersity of California, RiVerside, California 92521-0403, and Department of Chemistry, North Carolina State UniVersity, Raleigh, North Carolina 27695-8204 ReceiVed: July 6, 2005; In Final Form: October 13, 2005

X-ray photoelectron and Fourier transform infrared spectroscopy studies are reported for self-assembled monolayers (SAMs) of two tripodal thiol-functionalized metalloporphyrins (Zn and Cu) and three benchmark tripods on gold substrates. The tripodal unit common to all five molecules is 1-(phenyl)-1,1,1-tris(4mercaptomethylphenyl)methane (Tpd). Both porphyrins contain S-acetyl-protected thiols and are linked to the 4-position of the phenyl ring of Tpd via a phenylethyne group. The benchmark molecules include (1) two tripods containing a bromine atom at the 4-position of the apical phenyl ring, one a free thiol and the other its S-acetyl-protected analogue, and (2) a S-acetyl-protected tripod containing a phenylethyne unit at the 4-position of the apical phenyl group. Together, the spectroscopic studies reveal that none of the five tripodal molecules bond to the gold surface via all three sulfur atoms. Instead, the average number of bound thiols ranges from 1.5 to 2, with the porphyrinic molecules generally falling at the middle to upper end of the range and the smallest benchmark tripods falling at the lower end. Similar surface binding is found for the S-acetyl-protected and free benchmark tripods, indicating that the presence of the protecting group does not influence binding. Furthermore, the surface binding characteristics of the SAMs are not sensitive to deposition conditions such as solvent type, deposition time, or temperature of the solution.

I. Introduction

CHART 1

Owing to their potential applications in nanopatterning, chemical sensors, and molecular electronics, self-assembled monolayers (SAMs) of thiol-derivatized molecules on gold substrates have been widely studied.1-3 Typically, these molecules are functionalized with a single thiol for attachment to the gold substrate,4 because the small size of a single sulfuratom anchor may afford superior packing in the SAM. On the other hand, the integrity of the surface attachment with a singleatom anchor is pinned solely on that Au-S bond. One strategy for achieving enhanced robustness of surface attachment and better control of molecular orientation is to use multiple anchoring atoms. Toward this end, a variety of molecules have been designed with multiple thiol tethers, including tripodal units having each leg of the tripod bearing a sulfur anchoring atom. A comprehensive list of references to the tripodal literature is provided in the Supporting Information. Our group has been engaged in a program aimed at constructing devices that use the redox states of porphyrinic molecules as the active medium for information storage.5-17 In this design, a molecular monolayer is attached to an electroactive surface such as gold or silicon. In our past studies of porphyrinic molecules on gold, a number of thiol anchors have been used for surface attachment. In a recent study, we examined SAMs of molecules functionalized with a tetraarylmethane-based tripodal thiol anchoring group (Chart 1),9 similar to that employed by others to prepare molecular SAMs for a variety of proposed applications, including organic light-emitting diodes, * Authors to whom correspondence should be addressed (e-mail [email protected], [email protected], or [email protected]). † University of California. ‡ North Carolina State University.

sensors, and molecular electronic devices.18-20 An unexpected observation from our studies was that the surface coverage of the molecules with the tripodal thiol anchoring group was relatively low (∼3 × 10-11 mol‚cm-2),9 ∼3-fold lower than that achieved with analogous molecules containing a monopodal thiol anchor (∼8 × 10-11 mol‚cm-2).13,14 We speculated that the low coverage obtained for our molecular tripods might be due to the relatively large molecular footprint of the tetraarylmethane base unit. This result prompted us to investigate the surface-binding characteristics of the molecular tripods in more detail. Herein, we examine the surface-bonding and structural characteristics of SAMs of two tripodal thiol-functionalized metalloporphyrins (Zn and Cu) and three benchmark tripods (Chart 2). The tripodal unit common to all five molecules is

10.1021/jp0537005 CCC: $30.25 © 2005 American Chemical Society Published on Web 11/24/2005

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CHART 2

1-(phenyl)-1,1,1-tris(4-mercaptomethylphenyl)methane (Tpd). The two metalloporphyrins (ZnP-Tpd and CuP-Tpd) bear mesityl groups at the three nonlinking meso positions, are linked to the 4-position of the phenyl ring of Tpd via a phenylethyne group attached to the fourth meso position, and contain S-acetylprotected thiols. The benchmark molecules include (1) two tripods each containing a bromine atom at the 4-position of the apical phenyl ring, one a free thiol (Tpd-1′) and the other its S-acetyl-protected analogue (Tpd-1), and (2) a S-acetylprotected tripod containing a phenylethyne unit at the 4-position of the apical phenyl ring (Tpd-2). The surface-bonding and structural characteristics of the five tripods were examined using X-ray photoelectron (XPS) and Fourier transform infrared (FTIR) spectroscopy. The salient observation that emerges from these studies is that none of the five molecular tripods bind to the gold surface via all three sulfur atoms. II. Experimental Section A. Synthesis. 1. General Procedures. Dry toluene and dry triethylamine (TEA) were prepared by distillation over CaH2. The CHCl3 was of reagent grade and contained 0.9% v/v ethanol. All other chemicals were of reagent grade and were used as received. All of the palladium-catalyzed couplings were carried out under argon using standard Schlenk techniques. Adsorption column chromatography was performed using flash

silica (particle size ) 32-63 µm) for sample purification. Absorption spectra were collected in toluene for sample characterization. 1H NMR (300 MHz) and 13C NMR (75 MHz) spectra were obtained in CDCl3 unless otherwise noted. Additional characterization was carried out by laser desorption mass spectrometry (LD-MS) in the absence of a matrix and/or by fast atom bombardment mass spectrometry (FAB-MS). 2. a. ZnP-Tpd was prepared as described in the literature.9 2. b. 5,10,15-Trimesityl-20-[4-(3-methyl-3-hydroxybut-1-yn1-yl)phenyl]porphinatocopper(II) (CuP-3). A sample of 5,10,15trimesityl-20-[4-(3-methyl-3-hydroxybut-1-yn-1-yl)phenyl]porphyrin (P-3)21 was treated with copper acetate monohydrate to give the copper chelate CuP-3 (Scheme 1). Following a general procedure,22 a solution of P-3 (206 mg, 0.250 mmol) in CH2Cl2/MeOH (1:1) (5 mL) was treated with Cu(OAc)2‚H2O (1.25 g, 6.25 mmol) at room temperature. The progress of the reaction was monitored by TLC. The reaction was complete in 1.5 h. Chromatographic purification (silica, CH2Cl2) afforded a reddish purple solid (217 mg, 98%): LD-MS obsd 883.4; FAB-MS obsd 883.3461, calcd 883.3537 (C58H52CuN4O); λabs 418, 541 nm. 2. c. 5-(4-Ethynylphenyl)-10,15,20-trimesitylporphinatocopper(II) (CuP-4). Reaction of CuP-3 with sodium hydroxide in refluxing toluene provided CuP-4 in quantitative yield (Scheme 1). Following a general procedure,15 a solution of porphyrin

Tripodal Thiol-Functionalized Porphyrins

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SCHEME 1

SCHEME 2

CuP-3 (193 mg, 218 µmol) in toluene (15 mL) was treated with powdered NaOH (348 mg, 8.70 mmol) under reflux conditions. The reaction was monitored by TLC. The reaction was stopped after 5 h. The reaction mixture was then cooled to room temperature, washed with water, and extracted with CH2Cl2. The organic layers were combined and concentrated. The resulting residue was dissolved in CH2Cl2 and filtered through a short pad of silica, affording a dark pink solid. The reaction was quantitative: LD-MS obsd 825.3; FAB-MS obsd 825.3027, calcd 825.3018 (C55H46CuN4); λabs 418, 542 nm. 2. d. 1-(4-Phenylethynylphenyl)-1,1,1-tris[4-(S-acetylthiomethyl)phenyl]methane (Tpd-2). The key reaction for the synthesis of the target molecules is the palladium-mediated Sonogashira coupling of a bromoaryl and an ethynylaryl molecule (Scheme 2), which was carried out using conditions developed for porphyrin substrates.23,24 A Schlenk flask containing Tpd-120 (132 mg, 199 µmol), Pd2(dba)3 (18 mg, 20 µmol), and P(o-tol)3 (48 mg, 160 µmol) was alternatively evacuated and purged with argon three times. Toluene/TEA (5:1, 5.3 mL) and phenylacetylene (1.0 mL of 0.136 mM solution in toluene) were added, and the mixture was stirred at 65 °C. The progress of the reaction was monitored by TLC. After 2 h, additional portions of phenylacetylene (50.0 µL, 455 µmol), Pd2(dba)3 (18

mg, 20. µmol), and P(o-tol)3 (48 mg, 160 µmol) were added. The stirring was continued at 65 °C. Three additional portions of Pd2(dba)3 (18 mg, 20. µmol) and P(o-tol)3 (48 mg, 160 µmol) were added at 5.25, 8.75, and 11.5 h. After 15 h, additional portions of Pd2(dba)3 (36 mg, 39 µmol), and P(o-tol)3 (96 mg, 320 µmol) were added, followed by the addition of phenylacetylene (100 µL, 910 µmol). The stirring was continued at 65 °C. The reaction was stopped after a total of 36 h. The solution was concentrated. The resulting residue was chromatographed [silica, hexanes/ethyl acetate (85:15)], affording a colorless liquid (29.6 mg, 22%): 1H NMR (400 MHz) δ 2.302.42 (m, 9H), 4.08 (s, 6H), 7.00-7.24 (m, 14H), 7.28-7.44 (m, 5H), 7.46-7.56 (m, 2H); 13C NMR δ 30.57, 33.11, 64.39, 89.32, 89.68, 121.05, 127.74, 128.07, 128.12, 128.26, 128.44, 128.53, 131.02, 131.11, 131.33, 131.43, 131.80, 132.16, 135.23, 135.46, 145.44, 146.97, 195.39; FAB-MS obsd 684.1848, calcd 684.1827 (C42H36O3S3). 2. e. 5-[4-[2-[4-[1,1,1-Tris[4-(S-acetylthiomethyl)phenyl]methyl]phenyl]ethynyl]phenyl]-10,15,20-trimesitylporphinatocopper(II) (CuP-Tpd). A similar reaction of Tpd-120 with CuP-4 provided CuP-Tpd (Scheme 3). A Schlenk flask containing Tpd-1 (121 mg, 182 µmol), CuP-4 (100 mg, 121 µmol), Pd2(dba)3 (16 mg, 18 µmol), and P(o-tol)3 (44 mg, 150 µmol) was alternatively evacuated and purged with argon three times. Toluene/TEA (5:1, 8.4 mL) was added, and the mixture was stirred at 65 °C. The progress of the reaction was monitored by TLC. The reaction was complete in 5.5 h. The solvent was evaporated and the residue chromatographed (silica, hexanes/ CH2Cl2 1:1), affording a reddish purple solid (58 mg, 34%): LD-MS obsd 1407.3; FAB-MS obsd 1407.4319, calcd 1407.4376 (C89H76CuN4O3S3); λabs 420, 540 nm. 2. f. 1-(4-Bromophenyl)-1,1,1-tris(4-mercaptomethylphenyl)methane (Tpd-1′). Reductive cleavage of the thioacetyl functionality of Tpd-120 with LiAlH4 provided the free thiol tripod Tpd-1′ in 14% yield (Scheme 4). Following a general procedure,25 lithium aluminum hydride (1 M solution in THF, 4.6 mL) was added dropwise to a solution of Tpd-1 (514 mg, 0.770

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SCHEME 4

mmol) in dry THF (25 mL) cooled to 0 °C under argon. The reaction mixture was stirred at 0 °C for ∼5 h. Excess LiAlH4 was quenched by dropwise addition of ethyl acetate (∼10 mL), during which a greenish yellow solid separated from the solution. The solvent was removed and the residue thus obtained dissolved in CH2Cl2 and washed with 1 N HCl followed by water. The organic layer was separated, and the aqueous layer was extracted with CH2Cl2. The combined organic layer was

dried (Na2SO4) and concentrated. TLC analysis [silica, hexanes/ CH2Cl2 (1:1)] showed the presence of two components (∼1:1 ratio) with similar retention factors. The resulting residue was chromatographed [silica, hexanes/CH2Cl2 (1:1)] to provide a white solid (58 mg, 14%): 1H NMR δ 1.77 (t, J ) 7.8 Hz, 3H), 3.71 (d, J ) 7.8 Hz, 6H), 7.04-7.24 (m, 14H), 7.337.40 (m, 2H); 13C NMR δ 28.7, 64.1, 120.3, 127.6, 130.9, 131.3, 132.9, 139.1, 145.2, 145.9. A satisfactory mass spectrum was not obtained; however, the 1H NMR spectrum clearly showed the loss of the acetyl groups (absence of the singlet at δ 2.34) from Tpd-1, as well as the appearance of the triplet owing to the SH units (δ 1.77) and the doublet owing to the -CH2moieties (δ 3.71) of the product Tpd-1′. B. Structural Characterization. 1. Chemicals and Materials. The solvents used for the preparation of the SAMs were anhydrous CH2Cl2 (Aldrich, 99.8%), absolute EtOH (Gold Shield), benzonitrile (Aldrich, 99%), THF (Aldrich, 99%), and toluene (Aldrich, 99%); all were used as received. Silicon wafers (Silicon Valley Microelectronics) were purchased as nonoxidized B-doped Si(100) (F ) 13-30 Ω‚cm). Ar and N2 (99.995%) were passed through Drierite (Fisher) and Oxyclear (Supelco) gas purifiers prior to use. 2. Gold Substrate Preparation. The gold films were prepared by e-beam vapor deposition of 20 nm of Cr (99.999%) followed by 200 nm of Au (99.99%) on the surface of precleaned B-doped Si(100) wafers. The chromium and gold films were deposited under vacuum (P < 2.0 × 10-6 Torr) at 0.5 and 1.5 Å‚s-1, respectively. Upon completion of the gold deposition, the wafer was diced into ∼1 cm2 pieces, and each piece was immediately inserted into a VOC-type glass vial fitted with a Teflon-silicon rubber septum. The vials were purged with Ar (99.995%) to maintain the samples under an inert environment. 3. SAM Preparation. The SAMs of the porphyrin and benchmark tripods were prepared from CH2Cl2/absolute EtOH (85:15), THF, and toluene solutions at room temperature. For

Tripodal Thiol-Functionalized Porphyrins SAMs prepared at elevated temperature, benzonitrile was used as the solvent. Saturation coverage SAMs for the XPS and FTIR studies were prepared by depositing successive 50 µL aliquots of a 3 mM solution of the porphyrin or benchmark tripod onto the gold substrate. Previous studies of the porphyrin tripods have shown that saturation coverage is achieved with deposition times of e15 min,9 and FTIR studies revealed that this is also the case for the benchmark tripods. Assembly times of 24 h did not increase the coverage for any of the molecules (as also revealed by FTIR studies). After molecule deposition, the substrate was repeatedly sonicated and rinsed (five times) with CH2Cl2 and dried with a stream of Ar. 4. X-ray Photoelectron Spectroscopy (XPS) Measurements. The XPS data were acquired with a Leybold EA11-MCD spectrometer equipped with a Mg KR X-ray (1253.6 eV) source, a 100 mm hemispherical analyzer, and an 18-channel detector. The main XPS chamber was maintained at a steady base pressure of