Effects of Branched Ligands on the Structure and Stability of

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Effects of Branched Ligands on the Structure and Stability of Monolayers on Gold Nanoparticles Ralph Paulini, Benjamin L. Frankamp, and Vincent M. Rotello* Department of Chemistry, University of Massachusetts, Amherst, Massachusetts 01003 Received August 16, 2001. In Final Form: November 5, 2001 The ability to control the structure of monolayers on radial and faceted surfaces is an important goal for the successful application of nanoparticles in devices and materials. To investigate the effect of steric bulk in the monolayer periphery on monolayer protected gold cluster (MPC) properties, we have synthesized a series of gold nanoparticles having amide functionality within the monolayer chains and altering the peripheral end groups of the monolayer. Investigation of these MPCs using IR spectroscopy and cyanidemediated decomposition of the gold core reveals a strong correlation between the strength of the intramonolayer hydrogen bonding and the maximal decomposition rate. Additionally, comparison with corresponding ester-functionalized derivatives indicates that the hydrogen bonding amide unit in the monolayer emphasizes the effects of the peripheral groups on monolayer surface packing and MPC properties.

Introduction Monolayer protected gold clusters (MPCs) are versatile systems used for a broad spectrum of applications ranging from material sciences to biology.1 The controlled synthetic access to monodisperse colloidal particles of different core sizes via the Brust-Schiffrin protocol,2 the ease of functionalization of these clusters using the Murray placedisplacement reaction,3 and the applicability of a variety of analytical methods to their characterization4 make them highly useful components. MPCs have been previously used as catalysts,5 chemical and biological sensors,6 nanoreactors,7 and polymer additives8 and as building blocks for the self-assembly of nanoparticle aggregates.9 Future areas of application include such diverse applications as drug delivery, gene transcription,10 nanoscale electronics,11 and electro-optics.12 (1) For reviews, see inter alia: (a) Templeton, A. C.; Wuelfing, W. P.; Murray, R. W. Acc. Chem. Res. 2000, 33, 27-36. (b) Ulman, A. Chem. Rev. 1996, 96, 1533-1554. (2) Brust, M.; Walker, M.; Bethell, D.; Schiffrin, D. J.; Whyman, R. J. Chem. Soc., Chem. Commun. 1994, 801-802. (3) (a) Hostetler, M. J.; Green, S. J.; Stokes, J. J.; Murray, R. W. J. Am. Chem. Soc. 1996, 118, 4212-4213. (b) Hostetler, M. J.; Templeton, A. C.; Murray, R. W. Langmuir 1999, 15, 3782-3789. (4) Hostetler, M. J.; Murray, R. W. Curr. Opin. Colloid Interface Sci. 1997, 2, 42-50. (5) Li, H.; Luk, Y.-Y.; Mrkish, M. Langmuir 1999, 15, 4957-4959. (6) (a) Bruchez, M., Jr.; Moronne, M.; Gin, P.; Weiss, S.; Alivisatos, A. P. Science 1998, 281, 2013-2016. (b) Chan, W. C. W.; Nie, S. Science 1998, 281, 2016-2018. (c) Van Erp, R.; Gribneau, T. C. J.; Van Sommeren, A. P. G.; Bloemers, H. P. J. J. Immunoassay 1991, 12 (3), 425-443. (d) Sampath, S.; Lev, O. Adv. Mater. 1997, 9, 410. (7) (a) Kaifer, A. E. Acc. Chem. Res. 1999, 32, 62-71. (b) Liu, J.; Xu, R. L.; Kaifer, A. E. Langmuir 1998, 14, 7337-7339. (8) Watson, K. J.; Zhu, J.; Nguyen, S. T.; Mirkin, C. A. J. Am. Chem. Soc. 1999, 121, 402-403. (9) (a) Mucic, R. C.; Storhoff, J. J.; Mirkin, C. A.; Letsinger, R. L. J. Am. Chem. Soc. 1998, 120, 12674-12675. (b) Niemeyer, C. M.; Bu¨rger, W.; Peplies, J. Angew. Chem., Int. Ed. Engl. 1998, 37, 2265-2268. (c) Brousseau, L. C.; Novak, J. P.; Marinakos, S. M.; Feldheim, D. L. Adv. Mater. 1999, 11, 447-449. (d) Mirkin, C. A.; Letsinger, R. L.; Mucic, R. C.; Storrhoff, J. J. Nature 1996, 382, 607-609. (e) Alivisatos, A. P.; Johnsson, K. P.; Pneg, X.; Wilson, T. E.; Lowteh, C. J.; Bruchez, M. P., Jr.; Schultz, P. G. Nature 1996, 382, 609-622. (f) Boal, A. K.; Ilhan, F.; DeRouchey, J. E.; Thurn-Albrecht, T.; Russel, T. P.; Rotello, V. M. Nature 2000, 404, 746-748. (10) McIntosh, C. M.; Esposito, E. A.; Boal, A. K.; Simard, J. M.; Martin, C. T.; Rotello, V. M. J. Am. Chem. Soc. 2001, 122, 7626-7629. (11) (a) Schon, G.; Sion, U. Colloid Polym. Sci. 1995, 273, 101-117. (b) Schon, G.; Simon, U. Colloid Polym. Sci. 1995, 273, 202-218.

One key characteristic of MPCs that makes their design for specific applications equally challenging and interesting is the radial nature of the monolayer arising from the faceted surface of the metal core.13 Steric bulk and electronic interactions between the peripheral groups at the monolayer surface can have considerable effects on surface packing and therefore MPC properties. Exploring and understanding steric and electronic differences of peripheral substituents and functionalities is therefore a key requirement for the control of monolayers on radial, faceted surfaces and represents an important step in the rational and successful design of complex nanoscale systems. To study these effects, we have prepared a series of MPCs with the generic structure 1, in which the monolayer was systematically varied by altering the R group at the monolayer periphery.

The amide functionality within the system serves two purposes: it provides convenient synthetic access to a variety of different derivatives by variation of the alkyl residue R and also constitutes a probe in the monolayer which can be used to monitor the effect of changes on the packing inside the monolayer due to alteration of the external substituent. Amides are known to form intermolecular hydrogen bonds in the solid state and in solution, the strength of which can be determined from IR data (i.e., comparing the N-H stretching frequencies, which shift to lower wavenumbers as the strength of the hydrogen bonding increases). This tendency to self-assemble is (12) (a) Murray, C. B.; Kagan, C. R.; Bawendi, M. G. Annu. Rev. Mater. Sci. 2000, 30, 545-610. (b) Sun, S.; Murray, C. B.; Weller, D.; Folks, L.; Moser, A. Science 2000, 287, 1989-1992. (13) (a) Whetten, R. L.; Khoury, J. T.; Alvarez, M. M.; Murphy, S.; Vezmar, I.; Wang, Z. L.; Stephens, P. W.; Cleveland, C.; Luedtke, W. D.; Landmann, U. Adv. Mater. 1996, 8, 428. (b) Luedtke, W. D.; Landemann, U. J. Phys. Chem. 1996, 100, 13323-13329.

10.1021/la0155395 CCC: $22.00 © 2002 American Chemical Society Published on Web 02/08/2002

Structure and Stability of Monolayers on Gold Scheme 1. Synthesis of Amide-Functionalized Thiols 2a-i

Langmuir, Vol. 18, No. 6, 2002 2369 Scheme 2. Synthesis of MPCs 1a-g

Scheme 3. Synthesis of MPCs 7a, 7e, and 7g

increased in colloidal systems in which the close proximity of the individual organic molecules favors the assembly of an extended hydrogen bonding network.14 In recent studies,15 we have demonstrated that in systems where R is a nonbranched hydrocarbon, intermolecular hydrogen bonding of amides in the MPC monolayer is a function of the distance from the surface of the metal core, reaching a maximum when the amide functionality is placed at the C4 position (i.e., four carbons from the thiol group). The decrease in intramonolayer hydrogen bonding as amide functionality is moved further from the core indicates a decrease in monolayer ordering arising from the radial structure of the monolayer shell. One way that the degree of ordering at the monolayer periphery could be controlled is through the use of branched ligands. The “wedge” shape of such ligands would be expected to more effectively fill the space available at the outside of the monolayer surface. To test this hypothesis, we have fabricated a family of MPCs featuring a branched functionality. We report here the use of IR spectroscopy to characterize the self-assembly of these monolayers and the correlation of this monolayer structure to the stability of the MPCs under degradative conditions. MPC Synthesis Amide-functionalized thiols 2a-i (Scheme 1) were synthesized by either one of two synthetic routes: via an amide coupling using 4-bromobutyric acid to give the corresponding ω-bromo amide which was subsequently displaced with potassium thioacetate to provide the thioacetates 3a and 3b or via an amide coupling of vinylacetic acid and the desired amine derivative RNH2 under Schotten-Baumann conditions followed by the antiMarkovnikov addition of thioacetic acid to the terminal double bond of the resulting amides yielding thioacetates 3c-i. Removal of the acyl-protecting group with NaOMe gave access to the desired thiols 2a-i. Once synthesized, thiols 2a-i were then used as capping reagents in a Brust-Schiffrin reaction yielding gold MPCs (14) (a) Clegg, R. S.; Reed, S. M.; Hutchison, J. E. J Am. Chem. Soc. 1998, 120, 2486-2487. (b) Clegg, R. S.; Reed, S. M.; Smith, R. K.; Barron, B. L.; Rear, J. A.; Hutchison, J. E. Langmuir 1999, 15, 8876-8883. (15) Boal, A. K.; Rotello, V. M. Langmuir 2000, 16, 9527-9532.

1a-g (Scheme 2) which were purified using precipitation from ethanol (MPCs 1a,b) or acetonitrile (MPCs 1c-g). As observed previously, derivatives with short branched or nonbranched R residues (MPCs 1h and 1i) could not be characterized because of their tendency to form intermonolayer16 hydrogen bonds which leads to aggregation and insolubility of the MPCs. Analogous esterfunctionalized MPCs 7a,e,g were synthesized in analogous fashion (Scheme 3). IR Characterization of Hydrogen Bonding Hydrogen bonding was investigated by solid-state infrared (IR) spectroscopy using films produced by dropcasting from CH2Cl2 solution. Stretches of the relevant bands are summarized in Table 1 and Figure 1. Comparison of the location of the MPC N-H stretches among MPCs 1a-g shows that there is strong correlation between intramonolayer hydrogen bonding ability and steric demand of the peripheral groups. However, the IR evidence also reveals that the nature of peripheral intramonolayer interactions is complex and in some cases sensitive to small structural changes. In general, three main effects can be observed when changing the MPC periphery: (1) Different R groups can enhance the strength of the intramonolayer hydrogen bonding through better monolayer surface packing. This is demonstrated via an (16) The term intermonolayer refers to interactions between two or more colloidal particles, and intramonolayer refers to interactions between monolayer molecules on the same colloid.

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Table 1. IR Stretching Frequencies of Thiols and MPCs thiol MPC

alkyl group R

1a 1b 1c 1d 1e 1f 1g 1h 1i

diphenylmethyl 2,2-diphenyl-ethyl isobutyl 2-methyl-butyl propyl neopentyl tert-butyl ethyl isopropyl

a

nanoparticle

N-H N-H stretcha amide Ia stretcha amide Ia 3245 3297 3300 3299 3299 3313 3314 3292 3291

1636 1645 1644 1641 1642 1645 1646 1642 1644

3285 3286 3293 3299 3301 3315 3315 aggb aggb

1643 1643 1645 1646 1648 1651 1651 aggb aggb

In cm-1. b Aggregated nanoparticles.

Figure 2. Normalized absorbance decay at 500 nm caused by NaCN-induced decomposition of MPCs 1a-g.

Figure 1. NH stretching frequency of drop-cast colloids 1a-g and corresponding thiols.

increased H-bonding ability shown by the isobutyl derivative 1c with a stretching frequency of 3293 cm-1 compared to the propyl derivative 1e, where the frequency was 3301 cm-1. Elongation of the R group by one extra methyl group resulted in little change in H-bonding strength of the 2-methyl-butyl derivative 1d, denoted by a frequency of 3299 cm-1. This derivative should, by added bulk, lead to better hydrogen bonding, but that is not the case. One explanation for this discrepancy is that in contrast to the isobutyl group, branching occurs midchain for 1d. This branching could negate the added bulk and leave a poorly packed monolayer which would be indicated by the smaller than expected frequency shift. (2) Bulkier R groups can decrease the ability to form H-bonds through increased steric demand. tert-Butyl groups at the periphery of the monolayer of MPCs 1f and 1g weaken H-bonding quite significantly most likely due to the angular requirements of hydrogen bonding. These two tert-butyl derivatives possessing the weakest intramonolayer hydrogen bonds have an identical N-H stretch (3315 cm-1) indicating that the location of the branching point does not have a large influence on the steric effect of the substituent, provided it is at the end of the chain. (3) Aromatic R groups were observed to enhance H-bonding through favorable π-stacking interactions. The diphenylmethyl and the 2,2-diphenylethyl derivatives 1a and 1b exhibit IR evidence for the strongest H-bonding in this series (3285 and 3286 cm-1, respectively), probably due to π-π stacking interactions of the phenyl units which align to provide a maximal monolayer packing effect. The interaction of these chains is further supported by 1H NMR data, where an upfield shift for the aromatic protons of MPCs 1a and 1b compared to the corresponding thiols 2a and 2b from 7.30 and 7.29 ppm to 7.07 and 7.09 ppm, respectively, was observed. Further insight into the monolayer order can be obtained from the CH2 symmetric and asymmetric stretches. There is a 6 cm-1 shift in the symmetric d+ CH2 region from the

1a at 2854 cm-1 to 1f at 2860 cm-1 with one anomaly being 1g at 2853 cm-1. Since 2850 cm-1 corresponds to a fully ordered all-trans CH2 region, we can conclude that all the MPCs reported are disordered, but a complementary trend is observed (aside from 1g) corresponding with the N-H ordering data. The asymmetric d- stretching showed little change from 1a to 1f alternating from 2923 to 2924 cm-1 and rising to 2926 cm-1 for 1g. Investigation of the MPC Stability by NaCN-Induced Decomposition Research on SAMs adsorbed onto gold surfaces and colloids has shown that cyanide-induced decomposition of MPCs can be used to establish the stability of nanocluster systems.17 The rate of decomposition of MPC systems can be monitored by UV-vis spectroscopy and correlates to the protective barrier that the thiolate monolayer provides to the gold surface. NaCN-induced decomposition of the MPCs 1a-h following a similar procedure to that described by Murray et al.17a indicates that there is a strong correlation between the shift of the IR N-H stretching frequency and the maximal rate of decomposition of the MPCs. With our MPC systems, we have observed that the absorbance decay shows a short characteristic induction period, followed by a rapid increase which levels off to a final absorbance probably due to light scattering by the inorganic reaction product (Figure 2).18 To correlate decomposition rate and monolayer structure, the maximal decomposition rate corresponding to the fastest drop in absorbance between two points of the absorbance decay curve was used. We observe that MPC monolayers exhibiting strong H-bonding show the slowest decomposition rates and protect the gold core most efficiently whereas decomposition rates increase up to 9-fold for the MPC with weakest H-bonding (Figure 3). The strong correlation between the IR and the kinetics of NaCN decomposition raises the question of cause and effect. It is reasonable to assume that stronger hydrogen (17) (a) Templeton, A. C.; Hostetler, M. J.; Kraft, C. T.; Murray, R. W. J. Am. Chem. Soc. 1998, 120, 1906-1911. (b) Kumar, A.; Whitesides, G. M. Appl. Phys. Lett. 1993, 63, 2002-2004. (c) Weisbecker, C. S.; Merritt, M. V.; Whitesides, G. M. Langmuir 1996, 12, 3763-3772. (d) Gorman, C. B.; Biebuyck, H. A.; Whitesides, G. M. Chem. Mater. 1995, 7, 252-254. (18) A first-order fit of the absorbance decay failed to give interpretable reaction rates.

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Figure 5. Comparison of maximal decomposition rates of amides and corresponding ester MPCs.

Figure 3. Correlation between maximal decomposition rate vmax and IR N-H stretching frequency. Esters 7a,g,h were placed at the same frequency position as their corresponding amides for illustrative purposes.

Figure 4. Plots showing the 500 nm absorbance decay of amides and corresponding ester derivatives during NaCN-induced MPC decomposition.

bonding within a monolayer could provide a better degree of protection by tethering the bulky surface groups together, resulting in a slower decomposition rate. Conversely, better packing within the cluster monolayer would provide a greater hydrophobic and steric barrier for ionic penetration, while at the same time enhancing intramonolayer hydrogen bonding. To determine the origin of the correlation between IR and kinetic results, structural, ester analogues 7a, 7e, and 7g of MPCs 1a, 1e, and 1g were submitted to cyanide decomposition (Figure 4, Figure 119). Surprisingly, all three ester derivatives showed slower decomposition rates compared to the corresponding amides; the differences between esters and amides increase with decreasing intramonolayer H-bonding of the amide derivatives (Figure 5). A reasonable explanation for the enhanced stability of the ester MPCs 7 is a “vertex” effect20 (Figure 6): as the gold surface of the MPCs is faceted instead of evenly curved,21 intramonolayer hydrogen bonding can occur best (19) For better comparison, data points have been inserted into the graph at the x-values of the corresponding amides. (20) Fudickar, W.; Zimmermann, J.; Ruhlmann, L.; Schneider, J.; Ro¨der, B.; Siggel, U.; Fuhrhop, J.-H. J. Am. Chem. Soc. 1999, 121, 9539-9545. (21) Meister, W.; Guthrie, R. D.; Maxwell, J. L.; Jaeger, D. A.; Cram, D. J. J. Am. Chem. Soc. 1969, 91, 4452-4458.

Figure 6. Vertex effect hypothesis for decomposition rates of amide- and ester-functionalized MPCs: (a) weakly hydrogenbonded systems; (b) strongly hydrogen-bound systems; (c) ester MPCs.

on flat parts of the surface where the thiolate ligands can align in a parallel zigzag conformation. This leads to compact, tightly hydrogen-bound areas on the surface that leave spaces at vertexes. This results in deep channels which make the gold surface easily accessible for CNpenetration. Since all MPC derivatives are tightly hydrogen bound compared to free amide monomers in solution, which typically show NH stretching frequencies of ≈3400-3450 cm-1, it is reasonable to assume that this effect occurs for all of the MPCs 1a-g. However, the intramonolayer hydrogen bonding of clusters 1f and 1g may be strong enough to link coplanar thiol ligands creating compact isles on the gold surface but not sufficient to bridge between facets, partially closing the resulting channels. The ability to bridge between vertexes should correlate with increasing strength of the hydrogen bonds, essentially neutralizing the vertex effect in the case of MPCs 1a and 1b. Interestingly, the ester derivatives show the same stability trend observed for the corresponding amide-

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functionalized MPCs, although the differences between the three systems are smaller. These results indicate that the effects of the R group on the MPC stability discussed above are valid for these derivatives as well and that it is indeed the bulky surface substituents that protect the MPCs from degradation. For MPCs 1a-g, the hydrogen bonding amide unit in the monolayer seems to emphasize the effects of the R groups on the stability of the MPCs. Conclusions The determination and fine-tuning of ligand properties in order to control monolayers on radial and faceted surfaces is an important step for the successful application of MPCs in nanotechnology. Alteration of the periphery of the monolayer can be one approach to the rational design of MPC characteristics such as nanoparticle stability. We have prepared a series of amide-functionalized MPCs where the monolayer periphery was changed systematically. We have then studied intramonolayer hydrogen bonding as well as cluster stability toward cyanide decomposition as a function of the peripheral groups. Hydrogen bonding was weakest when bulky groups were placed at the monolayer periphery and strongest when aromatic units were used. Interestingly, groups with moderate steric demand lead to an increasing hydrogen bonding ability due to space filling and monolayer surface packing effects. This is in line with CN--induced decomposition data, which showed a strong correlation between the strength of the intramonolayer hydrogen bonding and the maximal decomposition rate. Experimental Section General. Amines, alcohols, vinylacetic acid, 4-bromobutyric acid, and all other reagents were purchased from Aldrich Chemical Co. and used as received. CH2Cl2 was distilled from CaH2 under argon, THF was distilled from Na under argon, and all other solvents were reagent grade and used as received. Isopentylamine was prepared via LAH reduction of trimethylacetamide as described elsewhere.22 ω-Bromo esters were prepared following a procedure described in the literature.22 1H NMR spectra were recorded at 200 MHz in CDCl3 (purchased from Cambridge Isotope Labs, Inc.) and referenced internally to TMS at 0.0 ppm. IR spectra were recorded as drop-cast films from CH2Cl2 (neat for liquid thiols) using CaF2 plates and a MIDAC FTIR spectrometer. Thioacetates 3a-b. In a 100 mL round-bottom flask, 4-bromobutyric acid (1 equiv) was dissolved in CH2Cl2 (20 mL). Oxalyl chloride (1.2 equiv) was then added, followed by one drop of DMF. The reaction was stirred until gas evolution ceased and cooled to 0 °C, and the desired amine (2 equiv) and TEA (3 equiv) were added dropwise. The reaction was stirred for 2 h and then washed once with a saturated aqueous NaHCO3 solution and twice with brine. The organic phase was collected and dried over MgSO4. After solvent removal, the crude product was redissolved in DMF and placed in a 100 mL round-bottom flask where potassium thioacetate (3 equiv) was added. The orange suspension was then stirred at 60 °C overnight, cooled, and added to a separatory funnel containing EtOAc and H2O. The aqueous phase was extracted with EtOAc, and the combined organic fractions were extracted twice with H2O, once with a saturated aqueous NaHCO3 solution, and twice with brine before being dried over MgSO4. The crude product, a red solid, was purified by chromatography on silica gel to yield the desired thioacetates as off-white solids. Thioacetate 3a (40% yield). 1H NMR (CDCl3, 200 MHz): δ (ppm) 7.24 (m, 10H), 6.25 (bs, 2H), 2.91 (t, 2H, J ) 7 Hz), 2.33 (t, 2H, J ) 7 Hz), 2.32 (s, 3H), 1.96 (qu, 2H, J ) 7 Hz). Thioacetate 3b (28% yield). 1H NMR (CDCl3, 200 MHz): δ (ppm) 7.27 (m, 10H), 5.59 (bs, 1H), 4.21 (t, 1H, J ) 8 Hz), 3.90 (t, 2H, J ) 7 Hz), 2.75 (t, 2H, J ) 7 Hz), 2.31 (s, 3H), 1.2 (t, 2H, J ) 7 Hz), 1.82 (qu, 2H, J ) 7 Hz). (22) Morin, C.; Michel, V. Tetrahedron 1992, 48, 9277-9282.

Paulini et al. Thioacetates 3c-i. In a 100 mL round-bottom flask, vinylacetic acid (1 equiv) was dissolved in THF (20 mL). Oxalyl chloride (1.2 equiv) was then added, followed by a drop of DMF. The reaction was stirred until gas evolution ceased and was then slowly added into a 250 mL round-bottom flask where a twophase mixture of an aqueous K2CO3-solution (3 equiv) and a solution of the amine (1.2 equiv) in THF was vigorously stirred at 0 °C. The reaction was stirred for 2 h, the layers were separated, and the organic portion was then washed once with a saturated aqueous NaHCO3 solution and twice with brine. The organic phase was collected and dried over MgSO4. After solvent removal, the product was redissolved in toluene and placed in a 100 mL flask where AIBN (0.25 equiv) and thioacetic acid (3 equiv) were added. The orange solution was then refluxed for 3 h and after cooling added to a separatory funnel containing EtOAc and a saturated aqueous NaHCO3 solution. The aqueous phase was extracted with EtOAc, and the combined organic fractions were extracted with saturated aqueous NaHCO3 solution and twice with brine before being dried over MgSO4. The crude product was purified by chromatography on silica gel. Thioacetate 3c (47% yield). 1H NMR (CDCl3, 200 MHz): δ (pm) 5.71 (bs, 1H), 3.09 (t, 2H, J ) 6 Hz), 2.92 (t, 2H, J ) 7 Hz), 2.34 (s, 3H), 2.25 (t, 2H, J ) 7 Hz), 1.93 (qu, 2H, J ) 6 Hz), 1.78 (m, 1H), 0.92 (d, 6H, J ) 7 Hz). Thioacetate 3d (72% yield). 1H NMR (CDCl3, 200 MHz): δ (ppm) 5.68 (bs, 1H), 3.13 (m, 2H), 2.91 (t, 2H, J ) 7 Hz), 2.34 (s, 3H), 2.25 (t, 2H, J ) 7 Hz), 2.21 (qu, 2H, J ) 7 Hz), 1.51 (m, 2H), 1.19 (m, 1H), 0.91 (m, 6H). Thioacetate 3e (54% yield). 1H NMR (CDCl , 200 MHz): δ (ppm) 5.70 (bs, 1H), 3.23 (q, 2H, 3 J ) 7 Hz), 2.91 (t, 2H, J ) 7 Hz), 2.34 (s, 3H), 2.24 (t, 2H, J ) 7 Hz), 1.92 (qu, 2H, J ) 6 Hz), 1.52 (qu, 2H, J ) 7 Hz), 0.93 (t, 3H, J ) 7 Hz). Thioacetate 3f (25% yield). 1H NMR (CDCl3, 200 MHz): δ (ppm) 5.67 (bs, 1H), 3.08 (d, 2 H, J ) 6 Hz), 2.34 (s, 3H), 2.27 (t, 2H, J ) 7 Hz), 1.94 (qu, 2H, J ) 7 H), 0.91 (s, 9H). Thioacetate 3g (33% yield). 1H NMR (CDCl3, 200 MHz): δ (ppm) 5.42 (bs, 1H), 2.90 (t, 2H, J ) 7 Hz), 2.34 (s, 3H), 2.16 (t, 2H, J ) 7 Hz), 1.89 (qu, 2H, J ) 7 Hz), 1.35 (s, 9H). Thioacetate 3h (60% yield). 1H NMR (CDCl3, 200 MHz): δ (ppm) 5.71 (bs, 1H), 3.20 (qu, 2H, J ) 6 Hz), 2.91 (t, 2H, J ) 7 Hz), 2.34 (s, 3H), 2.23 (t, 2H, J ) 7 Hz), 1.92 (qu, 2H, J ) 7 Hz), 1.15 (t, 3H, J ) 7 Hz). Thioacetate 3i (47% yield). 1H NMR (CDCl3, 200 MHz): δ (ppm) 5.49 (bs, 1H), 4.10 (m, 1H), 2.90 (t, 2H, J ) 7 Hz), 2.34 (s, 3H), 2.20 (t, 2H, J ) 7 Hz), 1.91 (qu, 2H, J ) 7 Hz), 1.16 (d, 6H, J ) 6 Hz). Thiols 2a-i. A solution of the thioacetate (1 equiv) in MeOH was purged with argon for 30 min. NaOMe (3 equiv of a 30 wt % solution in MeOH) was then added, and argon was bubbled through the reaction for 1 h. The reaction was stirred at 40 °C overnight under argon and quenched by addition of excess saturated aqueous NH4Cl solution. After the MeOH was removed, the resulting slurry was extracted with EtOAc. The organic layer was washed twice with brine and dried over MgSO4. After solvent removal, the crude product was purified by chromatography on silica gel. Thiol 2a (white solid, 71% yield). 1H NMR (CDCl3, 200 MHz): δ (ppm) 7.30 (m, 10 H), 6.25 (d, 1H, J ) 8 Hz), 6.06 (bd, 1H, J ) 7 Hz), 2.58 (q, 2H, J ) 8 Hz), 2.42 (t, 2H, J ) 7 H), 1.98 (qu, 2H, J ) 7 Hz), 1.31 (t, 1H, J ) 8 Hz). IR (CH2Cl2 film): ν 3245, 3051, 2927, 2540, 1636, 1542 cm-1. Anal. Calcd for C15H19NOS: C, 71.54; H, 6.71; N, 4.91. Found: C, 71.31; H, 6.57; N, 4.89. Thiol 2b (white solid, 67% yield). 1H NMR (CDCl3, 200 MHz): δ (ppm) 7.29 (m, 10H), 5.40 (bs, 1H), 4.19 (t, 1H, J ) 8 Hz), 3.90 (t, 2H, J ) 6 Hz), 2.45 (q, 2H, J ) 8 Hz), 2.21 (t, 2H, J ) 7 Hz), 1.85 (qu, 2H, J ) 7 Hz), 1.21 (t, 1H, J ) 8 Hz). IR (CH2Cl2 film): ν 3297, 3060, 3026, 2928, 2563, 1645, 1548 cm-1. Anal. Calcd for C18H21NOS: C, 72.20; H, 7.07; N, 4.68. Found: C, 72.27; H, 7.10; N, 4.67. Thiol 2c (colorless oil, 90% yield). 1H NMR (CDCl3, 200 MHz): δ (ppm) 5.55 (bs, 1H), 3.09 (t, 2H, J ) 6 Hz), 2.60 (q, 2H, J ) 8 Hz), 2.33 (t, 2H, J ) 7 Hz), 1.96 (qu, 2H, J ) 7 Hz), 1.77 (m, 1H), 1.77 (m, 1H), 1.34 (t, 1H, J ) 8 Hz), 0.91 (d, 6H, J ) 7 Hz). IR (neat): ν 3300, 3087, 2959, 2548, 1644, 1555 cm-1. EI-HRMS (m/z): calcd for C8H16NOS (M - 1+), 174.0953; found, 174.0899. Thiol 2d (colorless oil, 75% yield). 1H NMR (CDCl3, 200 MHz): δ (ppm) 5.48 (bs,1H), 3.15 (m, 2H), 2.60 (q, 2H, J ) 8 Hz), 2.33 (t, 2H, J ) 7 Hz), 1.96 (qu, 2H, J ) 7 Hz), 1.50 (m, 2H), 1.34 (t, 1H, J ) 8 Hz), 1.22 (m, 1H), 0.91 (t, 6H, J ) 7 Hz). IR (neat): ν 3299, 3084, 2961, 2545, 1641, 1548 cm-1. EI-HRMS (m/z): calcd for C9H19NOS (M+), 189.1187; found,

Structure and Stability of Monolayers on Gold 189.1146. Thiol 2e (colorless oil, 83% yield). 1H NMR (CDCl3, 200 MHz): δ (ppm) 5.49 (bs, 1H), 2.22 (q, 2H, J ) 7 Hz), 2.59 (q, 2H, J ) 8 Hz), 3.32 (t, 2H, J ) 7 Hz), 1.95 (qu, 2H, J ) 7 Hz), 1.52 (m, 2H), 1.33 (t, 1H, J ) 8 Hz), 0.93 (t, 3H, J ) 7 Hz). IR (neat): ν 3299, 3087, 2963, 2549, 1642, 1551 cm-1. EI-HRMS (m/z): calcd for C7H15NOS (M+), 161.0874; found, 161.0847. Thiol 2f (colorless oil, 90% yield). 1H NMR (CDCl3, 200 MHz): δ (ppm) 5.50 (bs, 1H), 3.07 (d, 2H, J ) 6 Hz), 2.60 (q, 2H, J ) 8 Hz), 2.35 (t, 2H, J ) 7 Hz), 1.97 (qu, 2H, J ) 7 Hz), 1.34 (t, 1H, J ) 8 Hz), 0.91 (s, 9 H). IR (neat): ν 3313, 3093, 2960, 2549, 1645, 1556 cm-1. EI-HRMS (m/z): calcd for C9H19NOS (M+), 189.1187; found, 189.1114. Thiol 2g (colorless oil, 95% yield). 1H NMR (CDCl3, 200 MHz): δ (ppm) 5.30 (bs, 1H), 2.56 (q, 2H, J ) 8 Hz), 2.24 (t, 2H, J ) 7 Hz), 1.92 (qu, 2H, J ) 7 Hz), 1.35 (s, 9 H). IR (film): ν 3314, 3197, 3076, 2967, 2551, 1646, 1549 cm-1. EI-HRMS (m/ z): calcd for C8H17NOS (M - 1+), 174.0953; found, 174.0889. Thiol 2h (colorless oil, 88% yield). 1H NMR (CDCl3, 200 MHz): δ (ppm) 5.46 (bs, 1H), 3.29 (qu, 2H, J ) 7 Hz), 2.59 (q, 2H, J ) 8 Hz), 2.31 (t, 2H, J ) 7 Hz), 1.95 (qu, 2H, J ) 7 Hz), 1.34 (t, 1H, J ) 8 Hz), 1.14 (t, 3H, J ) 7 Hz). IR (neat): ν 3287, 3081, 2970, 2545, 1640, 1545 cm-1. Thiol 2i (white solid, 87% yield). 1H NMR (CDCl3, 200 MHz): δ (ppm) 5.31 (bs 1H), 4.08 (m, 1H), 2.59 (q, 2H, J ) 8 Hz), 2.28 (t, 2H, J ) 7 Hz), 1.98 (qu, 2H, J ) 7 Hz), 1.34 (t, 1H, J ) 8 Hz), 1.15 (d, 6H, J ) 7 Hz). IR (CH2Cl2 film): ν 3291, 3079, 2973, 2551, 1644, 1553 cm-1. ω-Bromo Esters 4a,e,g. In a 100 mL round-bottom flask, trifluoroacetic anhydride (2.2 equiv) was added dropwise to a stirred solution of 4-bromobutyric acid (1 equiv) in dry THF cooled to -40 °C. After 30 min stirring at low temperature, a solution of the desired alcohol in THF (15 equiv) was added and the reaction was allowed to stir overnight at room temperature. The reaction mixture was then slowly poured into aqueous NaHCO3 solution, extracted with Et2O, and washed twice with brine before being dried over MgSO4 and evaporated to dryness. Esters 4e and 4g were obtained as pure liquids; ester 4a was purified by column chromatography on silica gel. Ester 4a (30% yield). 1H NMR (CDCl3, 200 MHz): δ (ppm) 7.33 (m, 10H), 6.89 (s, 1H), 3.44 (t, 2H, J ) 6 Hz), 2.63 (t, 2H, J ) 7 Hz), 2.20 (qu, 2H, J ) 6 Hz). Ester 4e (60% yield). 1H NMR (CDCl3, 200 MHz): δ (ppm) 4.05 (t, 2H, J ) 7 Hz), 3.48 (t, 2H, J ) 6 Hz), 2.51 (t, 2H, J ) 7 Hz), 2.18 (qu, 2H, J ) 7 Hz), 1.66 (m, 2H), 0.95 (t, 3H, J ) 7 Hz). Ester 4g (81% yield). 1H NMR (CDCl3, 200 MHz): δ (ppm) 3.46 (t, 2H, J ) 6 Hz), 2.41 (t, 2H, J ) 7 Hz), 2.13 (qu, 2H, J ) 7 Hz), 1.45 (s, 9H). Thioacetates 5a,e,g. In a 100 mL round-bottom flask, the desired ω-bromo ester was dissolved in DMF and potassium thioacetate (3 equiv) was added. The yellow suspension was then stirred at 60 °C overnight, cooled, and added to a separatory funnel containing EtOAc and H2O. The aqueous phase was extracted with EtOAc, and the combined organic fractions were washed twice with H2O, once with a saturated aqueous NaHCO3 solution, and twice with brine before being dried over MgSO4. Column chromatography on silica gel afforded the thioacetates as colorless liquids. Thioacetate 5a (70% yield). 1H NMR (CDCl3, 200 MHz): δ (ppm) 7.33 (m, 10H), 6.88 (s, 1H), 2.90 (t, 2H, J ) 7 Hz), 2.51 (t, 2H, J ) 7 Hz), 2.31 (s, 3H), 1.94 (qu, 2H, J ) 7 Hz). Thioacetate 5e (92% yield). 1H NMR (CDCl3, 200 MHz): δ (ppm) 4.04 (t, 2H, J ) 7 Hz), 2.39 (t, 2H, J ) 7 Hz), 2.33 (s, 3H), 1.91 (qu, 2H, J ) 7 Hz), 1.65 (m, 2H), 0.94 (t, 3H, J ) 7 Hz). Thioacetate 5g (71% yield). 1H NMR (CDCl3, 300 MHz): δ (ppm) 2.91 (t, 2H, J ) 7 Hz), 2.33 (s, 3H), 2.29 (t, 2H, J ) 7 Hz), 1.86 (qu, 2H, J ) 7 Hz), 1.45 (s, 9H). Thiols 6a,e,g. A solution of the thioacetate (1 equiv) in MeOH and a 1 mM solution of NaSMe (1.2 equiv) in MeOH were purged with argon for 60 min. In the case of thiol 5a, both solutions were cooled to 0 °C. Then the NaSMe solution was added to the thioacetate solution, and the reaction was stirred for 1 h. After quenching by addition of excess saturated NH4Cl solution, the resulting slurry was extracted twice with EtOAc. The combined organics were washed with brine and dried over MgSO4. After

Langmuir, Vol. 18, No. 6, 2002 2373 solvent removal, the crude product was purified by chromatography on silica gel to yield the desired thiols as colorless liquids. Thiol 6a (40% yield). 1H NMR (CDCl3, 200 MHz): δ (ppm) 7.33 (m, 10H), 6.89 (s, 1H), 2.56 (m, 4H), 1.97 (qu, 2H, J ) 7 Hz), 1.32 (t, 1H, J ) 8 Hz). IR (neat): ν 3029, 2925, 2569, 1736 cm-1. EI-HRMS (m/z): calcd for C17H17O2S (M - 1)+, 285.0949; found, 285.1023. Thiol 6e (58% yield). 1H NMR (CDCl3, 200 MHz): δ (ppm) 4.04 (t, 2H, J ) 7 Hz), 2.59 (q, 2H, J ) 8 Hz), 2.46 (t, 2H, J ) 7 Hz), 1.94 (qu, 2H, J ) 7 Hz), 1.65 (m, 2H), 1.35 (t, 1H, J ) 8 Hz), 0.94 (t, 3H, J ) 7 Hz). IR (neat): ν 2967, 2569, 1732 cm-1. EI-HRMS (m/z): calcd for C7H13O2S (M - 1)+, 161.0636; found, 161.0597. Thiol 6g (62% yield). 1H NMR (CDCl3, 200 MHz): δ (ppm) 2.57 (q, 2H, J ) 8 Hz), 2.36 (t, 2H, J ) 7 Hz), 1.89 (qu, 2H, J ) 7 Hz), 1.45 (s, 9H), 1.35 (t, 1H, J ) 8 Hz). IR (neat): ν 2978, 2571, 1727 cm-1. EI-HRMS (m/z): calcd for C8H15O2S (M - 1)+, 175.0793; found, 175.0769. MPC Synthesis. In a 100 mL round-bottom flask, HAuCl4‚ 3H2O (1 equiv, 0.5 mmol) was dissolved in distilled H2O (15 mL). A solution of tetraoctylammonium bromide (3 equiv) in 30 mL of THF was then added under stirring. To the biphasic mixture containing a colorless aqueous layer and a dark red organic layer, the desired thiol (1.2 equiv) was then added, causing the organic phase to turn orange to yellow. A freshly prepared aqueous NaBH4 solution was then added, leading to gas evolution and a rapid color change of the organic layer to dark brown/black. After 3 h of stirring, the organic phase was collected and the solvent was removed in vacuo yielding a thick brown solid which was precipitated from ethanol or acetonitrile (except colloids 1i and 1j, which precipitated during colloid formation and could not be redissolved) to remove remaining thiol or phase transfer catalyst. MPC 1a. 1H NMR (CDCl3, 200 MHz): δ (ppm) ≈7.06 (bs), ≈3.35 (bm), 1.72 (bs), 1.25 (bm), 0.88 (bm). IR (CH2Cl2 film): ν 3285, 1643, 1530 cm-1. MPC 1b. 1H NMR (CDCl3, 200 MHz): δ (ppm) 1.69 (bs), ≈7.09 (bs), ≈3.34 (bm), 1.69 (bs), 1.27 (bm), 0.89 (bm). IR (CH2Cl2 film): ν 3286, 1644, 1547 cm-1. MPC 1c. 1H NMR (CDCl3, 200 MHz): δ (ppm) 3.29 (bs), 1.67 (bs), 1.25 (bm), 1.88 (bm). IR (CH2Cl2 film): ν 3289, 1644, 1549 cm-1. MPC 1d. 1H NMR (CDCl3, 200 MHz): δ (ppm) 3.34 (bm), 1.92 (bs), 1.67 (bm), 1.25 (bm), 0.88 (bm). IR (CH2Cl2 film): ν 3299, 1646, 1547 cm-1. MPC 1e. 1H NMR (CDCl3, 200 MHz): δ (ppm) ≈3.28 (bm), 1.70 (bs), 1.25 (bm), 0.88 (bm). IR (CH2Cl2 film): ν 3300, 1649, 1552 cm-1. MPC 1f. 1H NMR (CDCl3, 200 MHz): δ (ppm) ≈3.35 (b), ≈1.70 (bm), 1.25 (bm), 0.88 (bm). IR (CH2Cl2 film): ν 3315, 1653, 1551 cm-1. MPC 1g. 1H NMR (CDCl3, 200 MHz): δ (ppm) 3.34 (bm), 1.66 (bs), 1.25 (bm), 0.88 (bm). IR (CH2Cl2 film): ν 3314, 1650, 1543 cm-1. MPC 7a. 1H NMR (CDCl3, 200 MHz): δ (ppm) 7.08 (bm), 3.34 (bm), 2.31 (bm), 1.66 (bs), 1.27 (bm), 0.88 (bm). MPC 7e. 1H NMR (CDCl3, 200 MHz): δ (ppm) 3.32 (bm), 1.65 (bm), 1.28 (bm), 0.88 (bm). MPC 7g. 1H NMR (CDCl3, 200 MHz): δ (ppm) 3.31 (bm), 1.68 (bm), 1.27 (bm), 0.88 (bm). NaCN-Induced MPC Decomposition. To 3 mL of a solution of the desired Au MPC in THF (concentrated ≈ 8.2 µM or 0.1 mg/mL in colloid, final concentration ≈ 7 µM) was added 0.5 mL of an aqueous NaCN solution (20 µM, final concentration ≈ 3 µM) followed by briefly agitating the mixture. The decay in absorbance at 500 nm was monitored beginning 5 s after adding the NaCN solution on a spectrophotometer. The decomposition rates were analyzed by comparing the maximal decomposition rate corresponding to the fastest decay in absorption between two time points of the decay curve.

Acknowledgment. This research was supported by the National Science Foundation (MRSEC instrumentation and CHE-9905492 to V.R.) and the National Institutes of Health (GM62998). V.R. acknowledges support from the Research Corporation and the Camille and Henry Dreyfus Foundation. LA0155395