Effects of Packing and Orientation on the Hydrolysis of Ester

at 0.3 nm/s onto freshly cleaved sheets of mica (B&M Trading) or onto glass ... area) of 1.1 ( 0.1; the gold-coated glass substrates were not annealed...
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Langmuir 2001, 17, 6569-6576

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Effects of Packing and Orientation on the Hydrolysis of Ester Monolayers on Gold Bikas Vaidya, Jinhua Chen, Marc D. Porter,*,† and Robert J. Angelici*,‡ Ames LaboratorysUSDOE, Microanalytical Instrumentation Center and Department of Chemistry, Iowa State University, Ames, Iowa 50011 Received April 23, 2001. In Final Form: July 3, 2001 Two novel nitrophenyl disulfide esters, di(4-nitrophenyl)-4,4′-dithiobisbenzoate (pNBD) and di(4nitrophenyl)-3,3′-dithiobisbenzoate (mNBD), were prepared, adsorbed spontaneously as thiolate monolayers on gold surfaces, and used as models for examining factors affecting rates of interfacial reactions. These monolayers were characterized by infrared reflection spectroscopy (IRS), X-ray photoelectron spectroscopy, single sweep voltammetry, and contact angle goniometry. The adsorbed form of mNBD (mNBT) undergoes a pseudo-first-order base-catalyzed hydrolysis at a rate that is 7 × 104 slower than that for mNBD in solution. The monolayer of the adsorbed para isomer of the ester is even less reactive; that is, it is not detectably hydrolyzed after 3 days of immersion in 0.5 M KOH. The lower packing density of the meta isomer ester accounts for its faster rate of hydrolysis as compared with that of the para isomer ester. The progression of the IRS data suggests that the orientation of the nitrophenyl ring in the mNBT monolayer undergoes a significant change during the early stages of hydrolysis but that this change in orientation does not detectably affect the rate of hydrolysis. Reaction of the mNBT monolayer with n-butylamine, which forms the corresponding amide via pseudo-first-order kinetics, does not cause a change in the orientation of the nitrophenyl ring. Thus, the change in orientation during hydrolysis reflects an increase in the free volume of the adlayer that arises from loss of the nitrophenylate product to solution.

Introduction Base-catalyzed ester hydrolysis is one of the most widely studied homogeneous chemical reactions.1-3 However, only a few studies on the base-catalyzed hydrolysis of ester monolayers have been reported.4-10 Various techniques such as infrared reflection spectroscopy (IRS), cyclic voltammetry, surface plasmon resonance, atomic force microscopy, and contact angle investigations have been used to determine the rate of base-catalyzed hydrolysis of spontaneously adsorbed ester-thiol and -disulfide monolayers on gold.8-10 The use of these systems as models for studying interfacial reactions stems from their level of structural definition, stability, and ease of preparation.11-14 In a study of monolayers of 11-mercaptoundecyl isonicotinate and those mixed with decanethiol on gold, Ryswyk † Corresponding author phone: 515-294-6433; fax: 515-294-3254; e-mail: [email protected]. ‡ Corresponding author phone: 515-294-2603; fax: 515-294-0105; e-mail: [email protected].

(1) Zhan, C.; Landry, D. W.; Ornstein, R. L. J. Am. Chem. Soc. 2000, 122, 2621. (2) Caplow, M.; Jencks, W. P. Biochemistry 1962, 1, 883. (3) Kirsch, J. F.; Clewell, W.; Simon, A. J. Org. Chem. 1968, 33, 127. (4) Alexander, A. E.; Schulan, J. H. Proc. R. Soc. London, Ser. A 1937, 161, 115. (5) Alexander, A. E.; Rideal, E. K. Proc. R. Soc. London, Ser. A 1937, 163, 70. (6) Valenty, S. J. J. Am. Chem. Soc. 1979, 101, 1. (7) Ahmad, J.; Astin, K. B. Langmuir 1990, 6, 1797. (8) Ryswyk, H. V.; Turtle, E. D.; Watson-Clark, R.; Tanzer, T. A.; Herman, T. K.; Chong, P. Y.; Walle, P. J.; Taurog, A. L.; Wagner, C. E. Langmuir 1996, 12, 6143. (9) Wang, J.; Kenseth, J. R.; Jones, V. W.; Green, J. D.; McDermott, M. T.; Porter, M. D. J. Am. Chem. Soc. 1997, 119, 12796. (10) Scho¨nherr, H.; Chechik, V.; Stirling, C. J. M.; Vancso, G. J. J. Am. Chem. Soc. 2000, 122, 3679. (11) Whitesides, G. M.; Laibinis, P. E. Langmuir 1990, 6, 87. (12) Ulman, A. An Introduction to Ultrathin Organic Films From Langmuir-Blodgett to Self-Assembly; Academic Press: San Diego, 1991. (13) Popenoe, D. D.; Deinhammer, R. S.; Porter, M. D. Langmuir 1992, 8, 2521. (14) Simmons, N. J.; Chin, K. O. A.; Harnisch, J. A.; Vaidya, B.; Trahanovsky, W. S.; Porter, M. D.; Angelici, R. J. J. Electroanal. Chem. 2000, 482, 178.

et al.8 showed that the rate of ester hydrolysis is controlled by (i) steric factors (e.g., packing density) that dictate ion and solvent access to reactive sites within the monolayer and (ii) the amount of charge accumulated at the monolayer/solution interface (e.g., conversion of an immobilized ester to its carboxylate ion analogue). Their results also indicated that the rate of hydrolysis for a mixed monolayer (1:3 11-mercaptoundecyl isonicotinate:decanethiol) is ∼200 times faster than that of the “undiluted” 11-mercaptoundecyl isonicotinate monolayer.8 Scho¨nherr et al.10 recently demonstrated in a study of a set of monolayers of an ester-terminated thiol (11-mercaptoundecyl acetate), a symmetric ester-terminated disulfide (bis(11-undecyl acetate)disulfide), and an asymmetric disulfide with esterand alcohol-terminated groups (11-undecyl acetate-11undecanol disulfide) on gold that monolayers prepared from ester-terminated thiol and the corresponding symmetric disulfide are hydrolyzed much slower than the monolayer prepared from the asymmetric disulfide.10 Our earlier IRS investigation of a dithiobis(succinimido undecanoate) monolayer on gold found that the rate of hydrolysis of the surface-confined ester was ∼1000 times slower than the non-mercapto solution analogue.9 Our study also found that the hydrolysis of the adlayer is accelerated by movement of the tip-sample microcontact when imaging with atomic force microscopy.9 Together, these studies indicate that the base-catalyzed hydrolysis of an ester monolayer that has few defects and is more densely packed is slower than that of an ester monolayer that is more loosely packed or has many defects. This paper presents the preparation, characterization, and kinetic investigations of the base-catalyzed hydrolysis of two new ester-terminated monolayers. The first is formed from di(4-nitrophenyl)-4,4′-dithiobisbenzoate (pNBD); the second is formed from di(4-nitrophenyl)-3,3′dithiobisbenzoate (mNBD). Both pNBD and mNBD chemisorb through cleavage of S-S bonds to yield the corresponding gold-bound thiolates pNBT and mNBT. Scheme

10.1021/la010590u CCC: $20.00 © 2001 American Chemical Society Published on Web 09/20/2001

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1 depicts their idealized interfacial structures. These interfaces therefore provide a means to explore how the orientation and/or packing density of the ester groups affect the rate of hydrolysis. The following describes the findings of this investigation, which includes evidence for a change in the orientation of the terminal group as the hydrolysis proceeds and an assessment by using aminolysis of whether the reorientation has an impact on the observed hydrolysis rate. Experimental Section Reagents. Bis(p-carboxyphenyl)disulfide and bis(m-carboxyphenyl)disulfide were obtained from Toronto Research Chemicals. HPLC grade acetonitrile and p-nitrophenol were purchased from Fisher Scientific. All other chemicals were acquired from Aldrich and were used as received. Aqueous solutions were prepared using Milli-Q (Millipore) deionized water. Di(4-nitrophenyl)-4,4′-dithiobisbenzoate (pNBD). Bis(pcarboxyphenyl)disulfide (615 mg, 2.00 mmol) was dissolved in 3.0 mL of thionyl chloride and refluxed for 3 h. Excess thionyl chloride was removed under reduced pressure, and diethyl ether (5.0 mL) was then added. Next, the resulting solution was mixed with a solution of p-nitrophenol (560 mg, 4.00 mmol) and triethylamine (3 mL) in dried ether (20 mL). A solid formed immediately. After the solution was refluxed for 2 h, chloroform was added to dissolve the solid. The solution was washed with water and saturated sodium chloride consecutively. After being dried with sodium sulfate and concentrated under vacuum, the desired product was obtained by column chromatography on silica gel using chloroform as the eluent. pNBD (765 mg, 70%) was obtained. 1H NMR (300 MHz, CDCl3): δ 8.32 (d, 4H, J ) 9.3 Hz), 8.13 (d, 4H, J ) 8.4 Hz), 7.64 (d, 4H, J ) 8.4 Hz), 7.40 (d, 4H, J ) 9.3 Hz). IR (KBr): 1734, 1615, 1591, 1518, 1489, 1344, 1261, 1208, 1175, 1061, 1010, 888, 746 cm-1. EIMS: 547.9 (M+), 410.1 (M+ - OC6H4NO2), 135.9 (SC6H4CO), 107.6 (SC6H4), 244 (C6H4SSC6H4CO). mp: 215-217 °C. Anal. Calcd for C26H16N2O8S2: C, 56.93; H, 2.94; N, 5.11. Found: C, 56.61; H, 2.96; N, 4.93. Di(4-nitrophenyl)-3,3′-dithiobisbenzoate (mNBD). Bis(m-carboxyphenyl)disulfide (613 mg, 2.00 mmol) was dissolved in 3.0 mL of thionyl chloride and refluxed for 2 h. Excess thionyl chloride was removed under reduced pressure, and diethyl ether (5.0 mL) was added. This solution was then added to a solution of p-nitrophenol (560 mg, 4.00 mmol) and triethylamine (3 mL) in ether (20 mL). A solid formed immediately. After the solution was refluxed for 2 h, chloroform was added to the diethyl ether solution to dissolve the solid. This solution was washed with

Vaidya et al. water and saturated sodium chloride consecutively, dried with sodium sulfate, concentrated, and then purified by column chromatography on silica gel using 3:1 chloroform/hexane as the eluent. The solid was recrystallized from chloroform and hexane to give mNBD (821 mg, 75%). 1H NMR (300 MHz, CDCl3): δ 8.32 (m, 6H), 8.08 (m, 2H), 7.81 (m, 2H), 7.51 (t, 2H, J ) 7.5 Hz), 7.39 (d, 4H, J ) 9.3 Hz). IR (KBr): 1745, 1615, 1592, 1523, 1490, 1347, 1249, 1207, 1163, 1078, 1057, 865, 739 cm-1. EIMS: 548 (M+), 410 (M+ - OC6H4NO2), 136 (SC6H4CO), 108 (SC6H4), 244 (C6H4SSC6H4CO). mp: 148-150 °C. Anal. Calcd for C26H16N2O8S2: C, 56.93; H, 2.94; N, 5.11. Found: C, 56.72; H, 3.08; N, 4.94. Substrate Preparation. Substrates were prepared using an Edwards E306A coater for the deposition of 300 nm of gold (99.9%) at 0.3 nm/s onto freshly cleaved sheets of mica (B&M Trading) or onto glass microscope slides primed with a 15-nm chromium adhesive layer. The mica sheets (25 mm × 75 mm) were cleaved just prior to loading into the vacuum coater. Glass substrates were cleaned by sonication in a dilute Micro (Cole-Parmer) solution, rinsed with deionized water and methanol, dried under a stream of ultrahigh purity argon (Air Products), and loaded into the vacuum coater. During deposition, the pressure in the coater was ∼8 × 10-6 Torr. After being cooled, the gold-coated mica substrates were removed and annealed in a muffle furnace at 300 °C for 5 h, which produces gold films with a predominant (111) surface crystallinity and a roughness factor (i.e., the ratio of the real surface area with respect to the geometric surface area) of 1.1 ( 0.1; the gold-coated glass substrates were not annealed and have a roughness factor of 1.3 ( 0.1.15-19 The goldcoated mica substrates were used only in the electrochemical assessments of adlayer surface concentration (see below). Ester-containing monolayers were formed on the gold substrates by immersion for 1 h in 0.5 mM solutions of pNBD or mNBD in acetonitrile. The substrates were then rinsed with acetonitrile and dried in a stream of high purity argon or nitrogen. Instrumentation. Infrared spectra (2-cm-1 resolution) were obtained with a Nicolet MAGNA 750 Fourier transform infrared spectrometer using a liquid nitrogen cooled MCT detector. Reflectance spectra were collected with p-polarized light incident at 80° with respect to the surface normal. These spectra are presented as -log(R/Ro), where R is the sample reflectance and Ro is the reference reflectance (i.e., an octadecanethiolate-d37 monolayer on gold). Each monolayer spectrum represents the average of 512 sample and reference scans. Electrochemical experiments were conducted at room temperature using a CV-27 potentiostat (Bioanalytical Systems), X-Y recorder (Houston Instruments), and a standard threeelectrode cell (Pt auxiliary electrode and a Ag/AgCl/saturated NaCl electrode). The geometric area of the working electrode (0.60 cm2) was defined by the opening in an inert elastomer gasket. The aqueous supporting electrolyte (pH 12) contained 0.5 M NaClO4, with the pH adjusted with NaOH. Electrolyte solutions were deoxygenated immediately before use with high purity argon. The X-ray photoelectron spectroscopy (XPS) data were collected with a Physical Electronics Industries 5500 surface analysis system equipped with a hemispherical analyzer, torroidal monochromator, and multichannel detector. Monochromatic aluminum KR radiation (1486.6 eV) at 250 W was used for excitation with photoelectrons collected at 45° with respect to the surface normal. Further details of the infrared and XPS analyses have been presented previously.15,20 Contact angles were measured with a Rame-Hart model 10000 goniometer equipped with a CCD camera and an environment control chamber. NMR spectra were obtained at 300 MHz in (15) Walczak, M. M.; Alves, C. A.; Lamp, B. D.; Porter, M. D. J. Electroanal. Chem. 1995, 396, 103. (16) Hallmark, V. M.; Chiang, S.; Rabolt, J. F.; Swalen, J. D.; Wilson, R. J. Phys. Rev. Lett. 1987, 59, 2879. (17) Chidsey, C. E. D.; Loiacono, D. N.; Sleator, T.; Nakahara, S. Surf. Sci. 1988, 200, 45. (18) Widrig, C. A.; Chung, C.; Porter, M. D. J. Electroanal. Chem. 1991, 310, 335. (19) Widrig, C. A.; Alves, C. A.; Porter, M. D. J. Am. Chem. Soc. 1991, 113, 2805. (20) Walczak, M. M.; Chung, C.; Stole, S. M.; Widrig, C. A.; Porter, M. D. J. Am. Chem. Soc. 1991, 113, 2370.

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Figure 1. Low-energy infrared spectra of (a) pNBD dispersed in KBr and (b) pNBT monolayer adsorbed on gold. CDCl3 on a Varian VXR-300 NMR spectrometer. Electron impact mass spectra were obtained on a Finnigan TSQ700 mass spectrometer.

Results and Discussion The next sections first describe the results of characterizing the as-formed, partially, and fully base-hydrolyzed monolayers by using IRS, XPS, electrochemical, and wettability measurements. The findings from kinetic studies of the base-catalyzed hydrolysis and aminolysis follow. The aminolysis experiments were conducted to test one of the structural interpretations developed from the hydrolysis data. IRS Characterizations. Spectra from 2000 to 800 cm-1 for pNBD and mNBD when dispersed in KBr and adsorbed on gold (pNBT and mNBT) are presented in Figures 1 and 2, respectively. The band assignments21-24 and peak positions, including those at higher energy, are summarized in Table 1. For pNBT (Figure 1b), the ν(CdO) band at 1736 and the two ν(CsO) bands at 1272 and 1219 cm-1 are diagnostic of the ester group, and the ν(CdC) bands at 1617, 1588, 1557, 1494, and 1396 cm-1 are assigned to the two different aromatic rings. The bands for νas(NO2) at 1539 cm-1, νs(NO2) at 1354 cm-1, and ν(CsN) at 865 cm-1 are indicative of the presence of the nitro group. Inspection of the spectra for mNBT (Figure 2) yields the same set of conclusions. These data confirm the immobilization of the two different adlayers. An examination of the spectra in more detail provides insight into the orientation, and therefore packing density, of the two adlayers. These insights develop through considerations of the so-called infrared surface selection (21) Nakanishi, K.; Solomon, P. H. Infrared Absorption Spectroscopy, 2nd ed.; Holden-Day: Oakland, 1977. (22) Silverstein, R. M.; Bassler, G. C.; Morrill, T. C. Spectrometric Identification of Organic Compounds, 5th ed.; Wiley: New York, 1991. (23) Colthup, N. B.; Daly, L. H.; Wiberley, S. E. Introduction to Infrared and Raman Spectroscopy, 3rd ed.; Wiley: New York, 1990. (24) Varsanyi, G. Assignments for Vibrational Spectra of Seven Hundred Benzene Derivatives; Wiley: New York, 1974; Vols. 1 and 2.

Figure 2. Low-energy infrared spectra of (a) mNBD dispersed in KBr, (b) mNBT, (c) partially hydrolyzed mNBT (3 h in 0.5 M KOH), and (d) almost fully hydrolyzed mNBT monolayer (19 h in 0.5 M KOH) adsorbed on gold. Table 1. Infrared Spectroscopic Peak Positions and Band Assignments for pNBD and mNBD Dispersed in KBr and pNBT and mNBT Adsorbed at Au/Glassa peak position (cm-1) band assignt

pNBD in KBr

pNBT monolayer

mNBD in KBr

mNBT monolayer

ν(CdO) ν(CdC)b

1734d 1615, 1590, 1490 1569, 1400 1518 1344 1261, 1208 886, 746 864

1736 1617, 1588, 1494 1557, 1396 1539 1354 1272, 1219

1746 1615, 1592, 1490 1572, 1395 1523d 1347 1249, 1207 893, 739 865

1746 1618, 1595, 1493 1565, 1413 1541 1353 1256, 1217

ν(CdC)c νas(NO2) νs(NO2) ν(CsO) δ(CH)op ν(CsN)

865

866

a

Key: ν, stretch; δ, bend; op, out-of-plane; as, asymmetric; s, symmetric. b Nitrophenyl ring. c Benzoate ring. d Splitting of the band observed in some samples.

rule,25,26 which is a surface effect on metals that results in the strongly preferential excitation of vibrations with their transition dipoles aligned at the surface normal. Thus, a comparison of the amplitudes of νs(NO2) and νas(NO2), which have transition dipoles orthogonal to each other, serves as a basis to qualitatively describe the average orientation of the nitro group. For the pNBT monolayer (Figure 1b), the amplitude of νs(NO2) is about 4 times that of νas(NO2), whereas the same set of bands has comparable magnitudes for pNBD in KBr (Figure 1a). This difference indicates that the nitro group in the pNBT monolayer is oriented along (i.e., close to) the surface (25) Greenler, R. G. J. Chem. Phys. 1966, 44, 310. (26) Porter, M. D. Anal. Chem. 1988, 60, 1143A.

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Figure 3. XPS spectra in the C(1s), O(1s), N(1s), and S(2p) regions for monolayers of (a) pNBT, (b) mNBT, (c) partially hydrolyzed mNBT (3 h in 0.5 M NaOH), and (d) almost fully hydrolyzed mNBT monolayer adsorbed on gold.

normal. This analysis can also be applied to the aromatic rings and again relies on contrasting the amplitudes of bands for the pNBT monolayer with respect to pNBD in KBr. That is, in comparison to the spectrum in Figure 1a, the relatively weak ν(CdO), the stronger ν(CsO) at 1272 cm-1 relative to ν(CsO) at 1219 cm-1, and the absence of the two δ(CH) in Figure 1b argue that both aromatic rings are oriented along the surface normal. The same type of description can be developed for the mNBT monolayer from the spectra in Figure 2. That is, the relatively weak ν(CdO), the strong ν(CsO), and the absence of the two δ(CH) for mNBT (Figure 2b) indicate that the aromatic rings in the mNBT monolayer are oriented also along the surface normal. However, the smaller difference in the relative absorbances of νas(NO2) and νs(NO2) for the pNBT monolayer with respect to the same two bands for pNBD shows that the nitro group is tilted further from the surface normal in the mNBT monolayer than in the pNBT monolayer. This difference in orientation is also indicative of a qualitative difference in the packing densities of the two different monolayers, with the pNBT monolayer being more densely packed than the mNBT monolayer. Idealized structures for both monolayers are presented in Scheme 1. The difference in packing density can also be inferred from the position of the CsO vibrational modes.10,27 The ester ν(CsO) at 1272 cm-1 in the pNBT monolayer is more than 10 cm-1 higher in energy than that for pNBD in KBr, which is at 1261 cm-1. A shift to higher frequency of only 7 cm-1 is observed for the ester ν(CsO) in the mNBT (27) Sondag, A. H. M.; Tol, A. J. W.; Touwslager, F. J. Langmuir 1992, 8, 1127.

monolayer with respect to the mNBD spectrum in KBr. The larger shift in the pNBT monolayer arises from stronger lateral interactions between neighboring carbonyl groups in the pNBT monolayer than in the mNBT monolayer.10,27 This difference indicates that the pNBT monolayer is packed more densely than the mNBT monolayer, a conclusion supported by the findings from the XPS and electrochemical experiments discussed in the next two sections. XPS Characterizations. The two monolayers were also characterized by XPS, and the results are shown in Figure 3. Using the Au(4f7/2) band for reference, a strong band in the C(1s) region at 284.2 eV, a shoulder at ∼286 eV, and a weak band at 288 eV are observed in the spectra of both pNBT (spectrum a) and mNBT (spectrum b). These bands are assigned to the carbon atoms of the aromatic rings not linked to a substituent, the aromatic carbon linked to the nitro group and/or to the oxygen of the ester group, and the ester carbonyl carbon, respectively.28,29 Upon immersion in 0.5 M NaOH, the features for mNBT evolve as generally expected for a hydrolysis. In other words, the shoulder at ∼286 eV gradually decreases, the band at 288 eV exhibits no detectable difference, and the aromatic band shifts to lower binding energy by ∼0.4 eV (spectra c and d). The lack of a detectable change in the band at ∼288 eV is in agreement with the similarity in the binding energies for an ester carbonyl carbon, which (28) Moulder, J. F.; Stickle, W. F.; Sobol, P. E.; Bomben, K. D. Handbook of X-ray Photoelectron Spectroscopy; Chastain, J., Ed.; PerkinElmer: Eden Prairie, MN, 1992. (29) Beamson, G.; Briggs, D. High-resolution XPS of Organic Polymers: The Scienta ESCA300 Database; John Wiley & Sons: New York, 1992.

Effects of Packing and Orientation

is lost as the reaction progresses, and a carbonyl carbon of a carboxylic acid, which is formed as the reaction proceeds.28,29 We are uncertain as to the origin of the 0.4 eV shift. The O(1s) region for both pNBT (spectrum a) and mNBT (spectrum b) monolayers exhibits an asymmetrically shaped envelope at ∼532 eV. The envelope is attributed to the overlap of the bands for the oxygens in both the ester and the nitro groups.28,29 Upon hydrolysis of mNBT, the intensity of the envelope decreases as the aromatic nitro group is lost to solution due to cleavage of the ester moiety. The change in the envelope also begins to reveal the presence of the two carboxyl oxygens,30 which are typically separated by ∼1.6 eV.28,29 The presence of the bands for the carboxylic acid functionality is consistent with the expected form of the reaction product that should remain immobilized on the gold surface. The N(1s) region for both pNBT (spectrum a) and mNBT (spectrum b) has a band at 405.2 eV that is attributed to the nitrogen from the nitro group.31 Upon hydrolysis of mNBT, the N(1s) band decreases and finally disappears (spectrum c and d), further confirming the loss of the nitro group via hydrolysis. The binding energies of the S(2p1/2) and S(2p3/2) couplet are located, respectively, at 161.8 and 162.9 eV. These positions are characteristic of sulfur that is adsorbed as a gold-bound thiolate.15,32-36 The sulfur bands for pNBT monolayer (spectrum a) are slightly stronger than those for the mNBT monolayer. However, the sulfur bands for the two hydrolyzed mNBT monolayers (spectra c and d) are stronger than those for the pNBT monolayer. These differences reflect the differences in the escape depths for the sulfur photoelectrons. Moreover, the areas under each of the bands for the pNBT monolayer are all 10-20% higher than those for the mNBT monolayer, indicating a higher surface concentration for pNBT as compared to mNBT. These results, together with the findings from the IRS characterization, further establish the compositional similarities and subtle architectural differences of the two types of monolayers. Surface Concentration Assessments. This section presents voltammetric studies of pNBT and mNBT adsorbed on gold for the purpose of determining the surface concentration of the pNBT and mNBT monolayers. Figure 4 shows a set of single sweep (scan rate: 50 mV s-1) voltammetric curves for the reductive desorption of pNBT, mNBT, and hydrolyzed (19 h in 0.5 M KOH) mNBT monolayers on gold. Curve a has a large cathodic wave at -740 mV for the pNBT monolayer, whereas curves b and c show cathodic waves at -715 and -535 mV for mNBT and the hydrolyzed mNBT monolayers, respectively. These differences reflect a mixing of contributions, including the strength of the gold-sulfur binding and the effective dielectric strength of the adlayer.15,19,36 The charge passed for reductive desorption, Qrd, found by integrating the area under the cathodic voltammetric (30) The carboxylic acid groups in a hydrolyzed monolayer in a basic solution are expected to exist in the deprotonated carboxylate form when immersed in 0.5 M KOH. However, the carboxylate groups on the surface appear to partially protonate upon rinsing. (31) Allongue, P.; Delamar, M.; Desbat, B.; Fagebaume, O.; Hitmi, R.; Pinson, J.; Saveant, J. J. Am. Chem. Soc. 1997, 119, 201. (32) Weisshaar, D. E.; Walczak, M. M.; Porter, M. D. Langmuir 1993, 9, 323. (33) Laibinis, P. E.; Whitesides, G. M.; Allara, D. L.; Tao, Y.-T.; Parikh, A. N.; Nuzzo, R. G. J. Am. Chem. Soc. 1991, 113, 7152. (34) Zhong, C.-J.; Porter, M. D. J. Am. Chem. Soc. 1994, 116, 11616. (35) Freeman, T. L.; Evans, S. D.; Ulman, A. Langmuir 1995, 11, 4411. (36) Zhong, C.-J.; Zak, J.; Porter, M. D. J. Electroanal. Chem. 1997, 421, 9.

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Figure 4. Single sweep voltammograms for (a) pNBT, (b) mNBT, and (c) hydrolyzed mNBT monolayers adsorbed on gold in 0.5 M NaClO4 (pH ∼12).

wave and accounting for the roughness of the electrode surface,15,18,19 can be used to estimate the adlayer surface concentration.37 A straight capacitive baseline is used to approximate the current flow from double layer charging. This analysis yields Qrd values of 175, 142, and 50 µC/cm2 for the desorption waves of pNBT, mNBT, and hydrolyzed mNBT, respectively (the uncertainties in these values are 10%18). However, the values of Qrd, assuming a oneelectron reduction process,18,37 for the pNBT and mNBT adlayers translate into unrealistically high surface concentrations for both adlayers. These results suggest that the observed charge includes not only that for the oneelectron reductive cleavage of the gold-sulfur linkage but also that for the passage of charge close to that expected for an additional two-electron reduction process, most likely associated with the reduction of the nitro group to a nitroso group.38 If we assume that the observed charge is equally distributed between the one-electron desorption process and the two-electron conversion of the nitro group, the Qrd values translate to surface concentrations of (5.5×, 4.5×, and 4.7) × 10-10 mol/cm2 for pNBT, mNBT, and fully hydrolyzed mNBT monolayers, respectively. The difference in surface concentrations of the pNBT and mNBT monolayers determined by the reductive desorption is consistent with our IRS and XPS results. In addition, the near equivalence of the surface concentrations of the as-prepared and fully hydrolyzed mNBT monolayers demonstrates the stability of the linking chemistry over the full course of the hydrolysis. Contact Angle Characterizations. The sessile contact angles for the two monolayers were determined using deionized water as a probe liquid. For as-prepared pNBT and mNBT and for mNBT immersed in 0.5 M NaOH for 2 and 21 h, the contact angles are 60, 57, 52, and 33°, respectively. The uncertainty in these data is (2°. The subtle, but detectably lower contact angle for the as-formed mNBT monolayer as compared to the pNBT monolayer, is attributed to the fact that mNBT is less tightly packed than pNBT and that the probe liquid comes into contact (37) Walczak, M. M.; Popenoe, D. D.; Deinhammer, R. S.; Lamp, B. D.; Chung, C.; Porter, M. D. Langmuir 1991, 7, 2687. (38) Lund, H. Cathodic Reduction of Nitro Compounds; Baizer, M. M., Ed.; Marcel Dekker: New York, 1973; p 315.

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Scheme 2

with some of the ester groups under the nitrophenyl ring. The contact angle further decreases upon loss of the nitrophenol group as the hydrolysis progresses. This decrease reflects the fact that the carboxylic acid/carboxylate group is more polar and hydrophilic than the nitro group. The next section demonstrates how the difference in packing density of the two adlayers, which exhibit a barely detectable difference in wetting properties, affects the rate of base-catalyzed hydrolysis. Base-Catalyzed Hydrolysis. The mNBT monolayer is hydrolyzed in alkaline solution to form a mercaptobenzoic acid monolayer and nitrophenolate in solution, as shown in Scheme 2. Spectra c and d in Figure 2 present a portion of the results for the hydrolysis of the mNBT monolayer in 0.5 M KOH. As is evident, all of the strong bands decrease in magnitude, and a new, fairly broad band at ∼1700 cm-1 appears.30 These changes are all consistent with the conversion of the ester group to a hydrogen-bonded carboxylic acid as the hydrolysis progresses.39 Interestingly, the presence of the side-to-side, hydrogenbonded carboxylic acid suggests that the hydrolysis, once initiated, preferentially continues at the site of initiation, which is probably a pinhole or other structural defect. Furthermore, the changes observed in the relative magnitudes of the nitro and ester bands indicate that the mNBT layer becomes less ordered as the reaction progresses, which is ascribed to an increase in the free volume of the layer due to the loss of nitrophenolate. Before hydrolysis, the magnitude of the νs(NO2) is more than twice that of νas(NO2). This difference decreases as the monolayer hydrolyzes until the relative magnitudes of the two bands approach that of the bulk sample in KBr (spectrum a). The subtle decrease in the energy of the ester ν(CsO) (1256-1253 cm-1) also indicates that the monolayer becomes more disordered with progression of the reaction, noting again that such a shift is consistent with a decrease in the lateral interactions between neighboring carbonyl groups.10,27 Figure 5 plots the logrithimic dependence of the absorbances at 1541 cm-1 (O) and 1353 cm-1 (b), which, respectively, correspond to νs(NO2) and νas(NO2),40 as a function of hydrolysis time (in 0.3 M KOH in 33% acetonitrile/67% water at 25 °C) for the mNBT monolayer. The plot is nonlinear at the early stages of the hydrolysis, evolving into a linear dependence as time increases. The nonlinearity results from changes in the orientation of the nitrophenyl rings, which is discussed later in this section. Only data points in the linear part of the plots were used in the calculation of the rate of hydrolysis of mNBT, yielding a pseudo-first-order rate constant of 6.8 (39) Duevel, R. V.; Corn, R. M. Anal. Chem. 1992, 64, 337. (40) Samples were immersed into the 0.3 M KOH in 33% acetonitrile for a given period of time at room temperature (∼25 °C), withdrawn from the solution, rinsed with water and acetonitrile, blown dry with nitrogen, and the IRS spectrum taken. This process was repeated with the same sample until most of the ester was hydrolyzed.

Figure 5. Logrithimic dependence of absorbances νas(NO2) (O) and νs(NO2) (b) as a function of hydrolysis time with 0.3 M KOH in 33% acetonitrile for the mNBT monolayer.

((0.19) × 10-4 min-1 (n ) 6). The second-order rate constant is 2.3 × 10-3 M-1 min-1 or ∼7 × 104 times lower than that estimated using the Hammett equation for its thiol analogue in solution, 153 M-1 min-1 (using ko ) 48.4 M-1 min-1, F ) 2.006, and σ ) 0.25).3,41 We also note that the hydrolysis rate predicted by using the Hammett equation for p-nitrophenyl 4-mercaptobenzoate (σ ) 0.15) is lower than that for p-nitrophenyl 3-mercaptobenzoate by only 1.58.3,41 The pNBT monolayer, in contrast to the mNBT monolayer, was not hydrolyzed to a detectable extent after immersion for 3 days in 0.3 M KOH (33% acetonitrile) or even in aqueous 0.5 M NaOH for 3 days. The striking difference between the base-catalyzed rates of hydrolysis of the pNBT and mNBT monolayers, despite their similar rates of hydrolysis in solution, is attributed to differences in orientation and packing of the two monolayers.8,9 As indicated by IRS, XPS, and reductive desorption studies, the surface concentration of the pNBT monolayer is ∼15% higher than that of the mNBT monolayer. IRS also indicates that the nitro group, and hence the aromatic ring attached to the nitro group, is aligned more closely to the surface normal than the analogous aromatic ring in mNBT. A more upright orientation would allow the adlayer to be more tightly packed, which would then reduce the rate of OH- attack on the ester group. This interpretation is supported by the small but detectable difference in the wettability of the two as-prepared monolayers. We concluded earlier from IRS data that the nitrophenyl ring in the mNBT monolayer undergoes a significant change in orientation at the early stages of the reaction. As indicated by the relative magnitudes of νas(NO2) at 1541 cm-1 and νs(NO2) at 1353 cm-1, the nitrophenyl ring is aligned close to surface normal prior to hydrolysis. However, the ring becomes more tilted as the hydrolytic loss of nitrophenyl groups increases the interfacial free volume (Scheme 2). Eventually, the nitrophenyl group becomes sufficiently disordered that the relative magnitudes of the two sets of bands become indistinguishable from that of mNBD in KBr. At the beginning of the hydrolysis, Figure 5 shows that the magnitude of νs(NO2) decreases dramatically, whereas that of νas(NO2) undergoes a sharp increase. Moreover, the increase in the magnitude of νas(NO2) occurs despite a decrease in the number of remaining nitrophenyl groups. We attribute these observations to the fact that once there is sufficient free volume for the nitrophenyl rings to (41) Hansch, C.; Leo, A.; Taft, R. W. Chem. Rev. 1991, 91, 165.

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Figure 6. Logrithimic dependence of % N (determined by XPS) on the gold surface as a function of hydrolysis time with 0.3 M KOH in 33% acetonitrile for the mNBT monolayer. The scatter reflects the use of a different sample for each data point. Scheme 3

reorient, the amplitudes of both νas(NO2) and νs(NO2) decrease at the same rate, which follows pseudo-firstorder kinetics. To confirm whether the difference in the plots for νas(NO2) and νs(NO2) is due to a change in the orientation of the nitrophenyl group, the rate of hydrolysis (in 33% acetonitrile/67% water with 0.3 M KOH) was also determined by XPS.42 Figure 6 therefore plots the logrithimic dependence of the N(1s) emission as a function of hydrolysis time (a portion of the collected XPS data is presented in Figure 3). This plot shows a linear dependence from the very beginning of the hydrolysis. The pseudofirst-order rate constant for the hydrolysis (7.8 × 10-4 min-1), which was calculated using the slope of the plot, is similar to that determined by the IRS measurements. This similarity indicates that the opposing dependencies in the amplitudes of νas(NO2) and νs(NO2) in Figure 5 are indeed due to a change in the orientation of the nitrophenyl rings. The experiments described in the next section were designed to test if the observed reorientation arises from a change in interfacial free volume. Aminolysis. When treated with an n-butylamine solution (0.50 M) in acetonitrile, the mNBT monolayer on gold undergoes psuedo-first-order aminolysis when forming the N-(n-Bu)benzamide and p-nitrophenol (Scheme 3).43 A set of IRS data is presented in Figure 7. The (42) Separate samples were exposed to 0.3 M KOH in 33% acetonitrile for different periods of time. Each sample was immersed into the 0.3 M KOH in 33% acetonitrile for a given period of time at room temperature (∼25 °C), withdrawn from the solution, rinsed with water and acetonitrile, blown dry with nitrogen, and then characterized by IRS or XPS. (43) Each of the mNBT monolayer samples was immersed into the 0.50 M n-butylamine in acetonitrile for a given period of time at room temperature (∼25 °C), withdrawn from the solution, rinsed with acetonitrile, blown dry with nitrogen, and then characterized by IRS or XPS. This process was repeated with the same sample until most of the ester was aminolyzed.

Figure 7. Low-energy infrared spectra of (a) mNBT monolayer and (b) mNBT monolayer after 16 h in 0.50 M n-butylamine in acetonitrile.

Figure 8. Logrithimic dependence of the absorbance of νs(NO2) as a function of aminolysis time for the reaction of 0.50 M n-butylamine in acetonitrile with the mNBT monolayer. Pseudo-first-order rate constant ) 1.3 × 10-2 min-1. An analysis using νas(NO2) was prohibited by its overlap with the amide II band.

occurrence of this reaction is supported by a decrease in the absorbance of νs(NO2) at 1353 cm-1 as well as by the increases in the absorbances of the amide I and amide II bands of the immobilized product at 1649 and 1539 cm-1, respectively.39,44 Figure 8 plots the logrithimic decay of νs(NO2) as a function of aminolysis time. The linearity of this plot suggests that there are no changes in the orientation of the remaining nitrophenyl groups during the aminolysis reaction, which is in contrast to that observed in the hydrolysis reaction of the mNBT monolayer (Figure 5). This lack of nitrophenyl reorientation is attributed to the formation of an amide with an n-butyl group that is sufficiently bulky to prevent reorientation. Conclusions The synthesis and characterization of two novel di(pnitrophenyl)dithiobisbenzoates have been described. Monolayers of these two esters adsorbed on gold were used as model systems for investigating the base-catalyzed hydrolysis of surface-immobilized esters. IRS studies show (44) Lahiri, J.; Isaacs, L.; Tien, J.; Whitesides, G. M. Anal. Chem. 1999, 71, 777.

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that pNBD forms a pNBT monolayer on gold with the nitrophenyl ring plane oriented close to the surface normal. mNBD, on the other hand, forms a mNBT monolayer on gold with the plane of its nitrophenyl ring tilted further from the surface normal. IRS, XPS, and voltammetric experiments indicate that the surface concentration of pNBT on Au is ∼15% higher than that of mNBT on gold. The mNBT monolayer undergoes pseudo-first-order basecatalyzed hydrolysis at a rate that is ∼7 × 104 slower than the same reaction in solution, whereas the pNBT monolayer is not detectably hydrolyzed even after extensive exposure to 0.5 M NaOH. Thus, the less densely packed mNBT monolayer hydrolyzes much faster than the pNBT monolayer. IRS also shows that the nitrophenyl ring in the mNBT monolayer undergoes a decrease in order in the early stages of hydrolysis and that the disordering does not have a detectable effect on the hydrolysis rate at any time throughout the course of the reaction. The lack of a dependence of the reaction rate on the extent of reaction differs from that reported, for example, in investigations of the base-catalyzed hydrolysis of thiolate monolayers terminated with acetate esters.10 These investigations found that the reaction rate was affected by the extent of the interfacial transformation, with the rate controlled initially by reactions at defect sites. There is, however, a distinct difference in the architecture of our mNBT adlayer in comparison to that of the adlayer terminated with an acetate ester. Our system has a lower packing density. This difference argues that (i) the observed independence of the reaction rate on the extent of reaction is due to an adlayer inherently more permeable to OH-

Vaidya et al.

and (ii) the randomization of the nitrophenyl group does not have an effect on the observed reaction rate. In other words, there is no detectable difference in the accessibility of OH- to the carbonyl carbon of the ester functionalities throughout the complete course of the interfacial transformation. In contrast to the ester hydrolysis, no changes in orientation were observed during the pseudo-first-order reaction of the mNBT monolayer with n-butylamine to form the corresponding benzamide. This result confirms the role of increased free volume in driving the randomization of the nitrophenyl ring. This finding also supports the argument that the permeability of the adlayer to OHis not altered by the randomization of the nitrophenyl terminus, given the fact that the rate for the formation of the N-(n-Bu)benzamide is unchanged throughout the aminolysis process. Finally, we note that the reaction of the mNBT monolayer with amines also offers a new strategy for anchoring amine-containing molecules to gold surfaces. Acknowledgment. We thank James Andregg of Ames Laboratory for expert assistance with the XPS measurements and Patricia Thiel for use of the goniometer. We also acknowledge the insights provided by one of the reviewers in the interpretation of the XPS data. This work was supported by the Office of Science, Basic Energy Sciences, Chemical Science Division. Ames Laboratory is operated for the U.S. Department of Energy by Iowa State University under Contract W-7405-eng-82. LA010590U