Tuning the Work Function of Gold with Self-Assembled Monolayers

Jan 9, 1999 - Changes in the work function of gold, ΔΦAu, induced by the formation of chemisorbed self-assembled monolayers derived from the series ...
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Langmuir 1999, 15, 1121-1127

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Tuning the Work Function of Gold with Self-Assembled Monolayers Derived from X-[C6H4-CtC-]nC6H4-SH (n ) 0, 1, 2; X ) H, F, CH3, CF3, and OCH3) Robert W. Zehner, Bradley F. Parsons, Richard P. Hsung,† and Lawrence R. Sita*,‡ Searle Chemical Laboratory, Department of Chemistry, The University of Chicago, 5735 South Ellis Avenue, Chicago, Illinois 60637 Received August 26, 1998. In Final Form: November 10, 1998 Changes in the work function of gold, ∆ΦAu, induced by the formation of chemisorbed self-assembled monolayers derived from the series of titled arenethiol adsorbates X - (C6H4 - CtC-)nC6H4-SH (1, n ) 0; 2, n ) 1; 3, n ) 2) have been measured by the Kelvin probe method. An analysis of these data indicates that variation in work function for a given chain length can be effectively modeled with two dipole sheets, as has been shown for other classes of self-assembled monolayers. Importantly, it is shown that monolayers of class 1 produce larger relative changes in work function for a given substituent X than those of either 2 or 3, presumably as a result of the larger polarizability of the longer chains present in the latter. Accordingly, these commercially available adsorbates are attractive for use in device applications requiring work function modification.

Introduction Although considerable progress has been made in the development of organic thin film-based electronic devices,1-4 for organic electronics to become commercially competitive with traditional silicon technology, remaining problems associated with device stability, efficiency, and lifetime must be overcome. One strategy currently being pursued to achieve this is to utilize chemisorbed selfassembled monolayers (SAMs) formed on the surface of an inorganic (metal) component in contact with the organic thin film as a means by which to fine-tune interfacial properties.5-9 Features of SAMs that are attractive for this purpose include a high degree of chemical robustness, superior passivating properties, ease of formation, and above all, the ability to manipulate their structural and electronic characteristics through design and synthesis.10 Recently, there has been considerable interest in the structure, properties, and utility of SAMs derived from oligo(phenylethynyl)phenylthiols of the general structure X-[C6H4-CtC-]nC6H4-SH (I) (Chart 1) as components in electronic devices since studies using both microscopic † Present address: Department of Chemistry, University of Minnesota. ‡ Present address: Department of Chemistry & Biochemistry, University of Maryland, College Park.

(1) Jackson, N.; Lin, Y. Y.; Gundlach, D. J.; Klauk, H. IEEE J. Sel. Top. Quantum Electron. 1998, 4, 100-104. (2) Dodabalapur, A. Solid State Commun. 1997, 102, 259-267. (3) Arai, M.; Nakaya, K.; Onitsuka, O.; Inoue, T.; Codama, M.; Tanaka, M.; Tanabe, H. Synth. Met. 1997, 91, 21-25. (4) Sheats, J. R. Science 1997, 277, 191-192. (5) Lo, R. K.; Ritchie, J. E.; Zhou, J. P.; Zhao, J. N.; McDevitt, J. T.; Xu, F.; Mirkin, C. A. J. Am. Chem. Soc. 1996, 118, 11295-11296. (6) Evans, S. D.; Ulman, A. Chem. Phys. Lett. 1990, 170, 462-466. (7) Bruening, M.; Moons, E.; Yaron-Marcovich, D.; Cahen, D.; Libman, J.; Shanzer, A. J. Am. Chem. Soc. 1994, 116, 2972-2977. (8) Bruening, M.; Moons, E.; Cahen, D.; Shanzer, A. J. Phys. Chem. 1995, 99, 8368-8373. (9) Campbell, I. H.; Rubin, S.; Zawodinski, T. A.; Kress, J. D.; Martin, R. L.; Smith, D. L.; Barashkov, N. N.; Ferraris, J. P. Phys. Rev. B 1996, 54, 14321-14324. (10) Ulman, A. An introduction to ultrathin organic films: from Langmuir-Blodgett to self-assembly, 1st ed.; Academic Press: San Diego, CA, 1991.

Chart 1

and macroscopic techniques have shown that this class of monolayer subunit greatly facilitates electron transfer across the SAM interfacial barrier as compared to nalkanethiols of similar lengths.11,21 In this regard, Campbell et al.19 recently demonstrated that SAMs prepared from compounds 3a and 3b could be used to respectively decrease and increase the work function of copper, ΦCu, (11) Dhirani, A.-A.; Zehner, R. W.; Hsung, R. P.; Guyot-Sionnest, P.; Sita, L. R. J. Am. Chem. Soc. 1996, 118, 3319-3320. (12) Dhirani, A.-A.; Lin, P.-H.; Guyot-Sionnest, P.; Zehner, R. W.; Sita, L. R. J. Chem. Phys. 1996, 106, 5249-5253. (13) Zehner, R. W.; Sita, L. R. Langmuir 1997, 13, 2973-2979. (14) Cygan, M. T.; Dunbar, T. D.; Arnold, J. J.; Bumm, L. A.; Shedlock, N. F.; Burgin, T. P.; Jones, L., II; Allara, D. L.; Tour, J. M.; Weiss, P. S. J. Am. Chem. Soc. 1998, 120, 2721-2732. (15) Bumm, L. A.; Arnold, J. J.; Cygan, M. T.; Dunbar, T. D.; Burgin, T. P.; Jones, L., II; Allara, D. L.; Tour, J. M.; Weiss, P. S. Science 1996, 271, 1705-1707. (16) Tour, J. M.; Jones, L.; Pearson, D. L.; Lamba, J. J. S.; Burgin, T. P.; Whitesides, G. M.; Allara, D. L.; Parikh, A. N.; Atre, S. V. J. Am. Chem. Soc. 1995, 117, 9529-9534. (17) Samori, P.; Francke, V.; Mangel, T.; Mullen, K.; Rabe, J. P. Opt. Mater. 1998, 9, 390-393. (18) Francke, V.; Mangel, T.; Mullen, K. Macromolecules 1998, 31, 2447-2453. (19) Campbell, I. H.; Kress, J. D.; Martin, R. L.; Smith, D. L. Appl. Phys. Lett. 1997, 71, 3528-3530. (20) Sachs, S. B.; Dudek, S. B.; Hsung, R. P.; Sita, L. R.; Smalley, J. F.; Newton, M. D.; Feldberg, S. W.; Chidsey, C. E. D. J. Am. Chem. Soc. 1997, 119, 10563-10564. (21) For recent studies of the electronic properties of SAMs derived from paraphenylene oligomers, see: (a) Reed, M. A.; Zhou, C.; Muller, C. J.; Burgin, T. P.; Tour, J. M. Science 1997, 278, 252-254. (b) Tian, W.; Datta, S.; Hong, S.; Reifenberger, R.; Henderson, J. I.; Kubiak, C. P. J. Chem. Phys. 1998, 109, 2874-2882.

10.1021/la981114f CCC: $18.00 © 1999 American Chemical Society Published on Web 01/09/1999

1122 Langmuir, Vol. 15, No. 4, 1999

Figure 1. Schematic diagram of an organic light-emitting diode in which the work function of the metal electrode for hole injection, ΦM, is (a) poorly matched with the valence band of the organic polymer creating the Schottky barrier, φh; (b) decreased by a dipole layer, thereby increasing φh; and (c) increased by a dipole layer of opposite polarity to (b), thereby decreasing φh.

in a predictable manner and that these changes in ΦCu are correlated with the degradation or improvement, respectively, of hole injection into the electroluminescent polymer poly[2-methoxy-5-(2′-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) used in an organic light-emitting diode (LED). As schematically depicted in Figure 1, it is proposed that the dipole moment of the SAMs derived from 3a and 3b serve to augment or attenuate, respectively, the Schottky barrier associated with hole injection, φh, of the Cu/MEH-PPV interface. In addition to providing a lower effective tunneling barrier for electron transfer than SAMs derived from n-alkanethiols, these authors also state that one reason for choosing arenethiols of class 3 for this work is that they are known to form dense and well-ordered monolayers.11,13 It is also of merit to note that defect sites, which can contribute to changes in work function in an unpredictable manner, can be “patched” in SAMs derived from 3 through a process involving subsequent adsorption of hexadecanethiol (HDT).13 On the other hand, though, it is equally important to point out that, to date, there is no evidence to suggest that an ordered SAM is required for applications involving changes in work function or that arenethiols of class 3 will necessarily outperform those of classes 1 or 2 of structure I. To better understand the structural requirements of both the molecular subunit, and of the SAMs derived from I, in applications that can benefit from the fine-tuning of work function, we undertook a thorough investigation of the changes in work function of gold, ∆ΦAu, that are induced by SAMs derived from a wide variety of derivatives of 1-3. Herein, we present the results of this investigation in which we apply a popular model for work function modification to quantify systematic changes that are observed in the properties of these systems. Importantly, this study serves to show that monolayers of class 1 actually produce larger relative changes in work function for a given substituent X than those of either 2 or 3, presumably as a result of the larger polarizability of the longer chains present in the latter. While it remains to be seen whether other factors, such as the lack of microscopic order, density of defect sites, and higher tunneling barrier heights, affect their utility in thin-film devices, this result suggests that SAMs derived from commercially available derivatives of 1 are, in fact, perhaps better targets for further investigation in applications requiring work function modification than those derived from 2 and 3, which are obtained through multistep syntheses. Experimental Section General. Hexadecanethiol (HDT) and compounds 1a-e were obtained from commerical sources and used without further purification. Compounds 2a and 4 were prepared according to

Zehner et al. published procedures.22-24 Compounds 2b-e were prepared by the palladium-catalyzed coupling of 4 with known corresponding p-substituted phenylacetylenes followed by reductive removal of the thioacetate group according to procedures described below in detail for the syntheses of compounds 3a-e.25 All solvents employed were of HPLC grade, and tetrahydrofuran (THF) and diethyl ether (Et2O) were redistilled under nitrogen from sodium benzophenone. Anhydrous diisopropylethylamine (Hunig’s base) was obtained in a septum-sealed bottle and used as received. Unless otherwise noted, all synthetic procedures were carried out under an inert atmosphere using standard Schlenk-line techniques. 1H NMR spectra were recorded in chloroform-d or dichloromethane-d2 and are referenced relative to residual protonated solvent peaks (7.26 ppm for CDCl3, 5.32 ppm for CD2Cl2). SAMs derived from HDT and 1-3 were prepared on evaporated gold films supported on polished crystalline silicon wafers as previously described.13 Importantly, all bare gold substrates were first cleaned in fresh acidic peroxide (75% H2SO4, 25% H2O2 solution; CAUTION: reacts explosively with organics!) for 20 min, rinsed with copious amounts of deionized water, and then electrochemically cycled between -0.1 and -0.8 V vs SCE in 1 M NaCl for 5 min to remove the resulting oxide layer. In agreement with the results of McLendon and co-workers,26 we find that this protocol produces monolayers of the best quality. SAMs derived from HDT and 1 were formed from a 1 mM solution of this adsorbate in absolute ethanol. Due to their extremely low solubility in this solvent, however, SAMs derived from 2 and 3 were formed from 1 mM solutions of these adsorbates in either a 9:1 chloroform/methanol solvent mixture or pure dichloromethane. Gold film substrates were derivatized in solution for a period of 12-24 h, removed from solution, rinsed with copious quantities of dichloromethane and 2-propanol, and finally rinsed and dried on a spin caster. Monolayer thicknesses were determined by using an automated optical ellipsometer (Gaertner L110) interfaced to a personal computer. Optical constants for each gold film were measured before and after immersion into the solution containing the adsorbate. Any samples displaying a visible surface film after derivatization, or deviating by more than 1 standard deviation from the average thickness for that adsorbate, were discarded. Contact potential differences (CPDs) of the bare and SAMderivatized gold films were determined with a commercial Kelvin probe (McAllister) interfaced to a personal computer. The probe enclosure containing the sample was purged with a flow of highpurity nitrogen during all measurements, and measurements were recorded once the CPD value had stabilized. Data presented are the average of at least three different samples. Electronic structure calculations for the thiol adsorbates were performed with the Gaussian 94 package27 on Silicon Graphics workstations. Following the methodology of Campbell et al.,9,19 the minimum-energy geometry of the thiol adsorbate was determined by the PM3 semiempirical method. The thiol hydrogen atom was then removed, and the dipole for the thiol radical was calculated at the unrestricted Hartree-Fock level of theory employing a 6-31G* basis set. The inclusion of polarization functions on sulfur and carbon was deemed necessary to better model the polar nature of the arenethiol adsorbates. The resulting dipoles were projected onto the metal surface normal, assuming a tilt of 0° for the arenethiols and 30° for HDT.10 (22) Jones, L., II; Schumm, J. S.; Tour, J. M. J. Org. Chem. 1997, 62, 1388-1410. (23) Pearson, D. L.; Tour, J. M. J. Org. Chem. 1997, 62, 1376-1387. (24) (a) Hsung, R. P.; Chidsey, C. E. D.; Sita, L. R. Organometallics 1995, 14, 4808-4815. (b) Hsung, R. P.; Babcock, J. R.; Chidsey, C. E. D.; Sita, L. R. Tetrahedron Lett. 1995, 36, 4525-4528. (25) Additional information provided in the Supporting Information. (26) Guo, L. H.; Facci, J. S.; McLendon, G.; Mosher, R. Langmuir 1994, 10, 4588-4593. (27) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Gill, P. M. W.; Johnson, B. G.; Robb, M. A.; Cheeseman, J. R.; Keith, T.; Petersson, G. A.; Montgomery, J. A.; Raghavachari, K.; Al-Laham, M. A.; Zakrzewski, V. G.; Ortiz, J. V.; Foresman, J. B.; Cioslowski, J.; Stefanov, B. B.; Nanayakkara, A.; Challacombe, M.; Peng, C. Y.; Ayala, P. Y.; Chen, W.; Wong, M. W.; Andres, J. L.; Replogle, E. S.; Gomperts, R.; Martin, R. L.; Fox, D. J.; Binkley, J. S.; Defrees, D. J.; Baker, J.; Stewart, J. P.; Head-Gordon, M.; Gonzalez, C.; Pople, J. A. Gaussian 94; E.2 ed.; Gaussian, Inc.: Pittsburgh, PA, 1995.

Tuning the Work Function of Gold with SAMs Preparation of 1-[(Triisopropylsilyl)ethynyl]-4-[(trimethylsilyl)ethynyl]benzene (6). In a 100 mL Schlenk flask, 3.0 g (10.6 mmol) of 1,4-diiodobenzene were combined with 2.7 mL (11.7 mmol) of (triisopropylsilyl)acetylene, 301 mg (4 mol %) of Pd(PPh3)2Cl2, and 43 mg (4 mol %) of CuI. Diisopropylamine (50 mL) was then added via syringe and the reaction was stirred at room temperature for 24 h, at which time, 3.0 mL (2 eq) of (trimethylsilyl)acetylene was added by syringe. After an additional 24 h, the volatiles were removed in vacuo, and the crude product was purified by column chromatography on silica gel with hexane as the eluant to provide 3.50 g (92% yield) of 5 as a light yellow oil. Data for 5: 1H NMR δ 0.25 (s, 9 H), 1.12 (s, 21 H), and 7.39 (s, 4 H). A portion of this product (559 mg, 1.58 mmol) was dissolved in 7 mL of a 1:1 THF/methanol solvent mixture to which 3 mL of aqueous 1 M NaOH was added. The reaction mixture was then stirred for 3 h, quenched with 25 mL of water, and extracted with 3 × 15 mL portions of hexane. The combined organic fractions were dried over anhydrous Na2SO4 and the solvents were removed in vacuo to yield 445.4 mg (100% yield, 92% combined yield for the two steps) of 6 as a white crystalline solid. Analytically pure crystals of 6 were obtained by recrystallization from ethanol. Data for 6: mp 52-53 °C; 1H NMR δ 1.13 (s, 21 H), 3.16 (s, 1 H), and 7.43 (s, 4 H); 13C NMR δ 11.5, 18.9, 79.0, 83.5, 93.1, 106.6, 122.2, 124.2, and 132.1; Anal. Calcd for C19H26Si: C, 80.78; H, 9.28. Found: C, 80.83; H, 9.49. General Procedure for Palladium-Catalyzed Coupling of Para-Substituted Aryl Iodides with Phenylacetylenes. A 4-substituted iodobenzene and a phenylacetylene were combined with Pd(PPh3)2Cl2 (4 mol %) and CuI (4 mol %) in a 1:1 THF/Hunig’s base solvent mixture in a Schlenk flask. The reaction mixture was deoxygenated by performing three freezepump-thaw cycles, after which the flask was backfilled with dinitrogen and sealed. The mixture was then stirred for 2-3 days at 55 °C until thin-layer chromatography (TLC) showed the reaction to be complete. The volatiles were then removed in vacuo and the remaining solids were taken up in a 1:1 CH2Cl2/hexane solvent mixture and filtered through a short (3 cm) silica gel column assembled in a fritted funnel. After removal of the solvent, further purification of the crude material was achieved by column chromatography on silica gel. Preparation of 8a. Compound 6 (100 mg, 350 µmol) was coupled with iodobenzene (80 mg, 385 µmol) in 20 mL of solvent under the general procedure given above to provide 76 mg (60% yield) of 7a as a colorless oil. The entire quantity of this product was dissolved in 3 mL of THF, to which was then added 0.21 mL (1 equiv) of a 1M solution of tetrabutylammonium fluoride (TBAF) in THF. After the mixture was stirred for 12 h, the solvent was removed in vacuo and the resulting crude material was purified by column chromatography on silica gel to yield 33 mg of 8a as a pale yellow powder (46% combined yield). Data for 8a: mp 69-71 °C; Rf ) 0.23 (hexane); 1H NMR δ 3.15 (s, 1 H), 7.30 (m, 3 H), 7.42 (br s, 4 H), and 7.47 (m, 2 H); 13C NMR δ 78.9, 83.3, 88.8, 91.4, 121.8, 122.9, 123.7, 128.4, 128.5, 131.4, 131.6, and 132.0; IR (neat) 3287s, 2924w, 1442m, 836s, and 756s; MS (EI) m/e (% relative intensity) 202 (100) M+, 192 (2), 175 (3), 150 (3), and 101 (6). Preparation of 8b. Compound 6 (1 g, 3.5 mmol) was coupled with 1-fluoro-4-iodobenzene (825 mg, 3.6 mmol) in 60 mL of solvent according to the general procedure to give 1.26 g of 7b (95% yield) as a brown oil. This entire product was dissolved in 250 mL of THF, and a 1 M TBAF/THF solution was added dropwise until the reaction was complete as indicated by TLC. The solvent was then removed in vacuo and the crude product was purified by chromatography on silica gel with a 9:1 hexane/ CH2Cl2 solvent mixture as eluant to provide 519 mg of 8b as a white powder (67% combined yield). Data for 8b: 1H NMR δ 3.25 (s, 1 H), 7.08 (t, 2 H, J ) 8.8 Hz), 7.48 (s, 4 H), and 7.54 (dd, 2 H, J ) 8.8 Hz, J ) 5.5 Hz); 13C NMR δ 79.3, 83.4, 88.7, 90.5, 116.0, 116.2, 119.5, 122.3, 124.0, 131.8, 132.5, 133.9, 134.0, 162.1, and 164.1; Anal. Calcd for C16H9F: C, 87.26; H, 4.12. Found: C, 87.17; H, 3.90. Preparation of 8c. Compound 6 (500 mg, 1.77 mmol) and 4-iodoanisole (455.6 mg, 1.95 mmol) were combined in 40 mL of solvent and coupled by the general procedure to provide 527 mg of 7c as a yellow crystalline material (98% yield). The entire quantity of this product was dissolved in 100 mL of CH2Cl2, to

Langmuir, Vol. 15, No. 4, 1999 1123 which 1 equiv of a 1 M TBAF/THF solution was added via syringe. The reaction mixture was then stirred for 15 min, the solvent was removed in vacuo, and the resulting crude product was purified by chromatography on silica gel using a 1:1 CH2Cl2/ hexane solvent mixture as eluant to provide 273 mg (66% combined yield) of 8c as light yellow, flaky crystals. Data for 8c:1H NMR δ 3.25 (s, 1 H), 3.82 (s, 3 H), 6.89 (d, 2 H, J ) 8.9 Hz), 7.47 (d, 2 H, J ) 8.9 Hz), and 7.48 (s, 4 H); 13C NMR δ 55.3, 78.7, 83.1, 87.4, 91.4, 114.1, 114.8, 121.5, 124.1, 131.2, 132.0, 133.1, and 160.0; Anal. Calcd for C17H12O: C, 87.91; H, 5.21. Found: C, 87.81; H, 5.08. Preparation of 8d. Compound 6 (758 mg, 2.68 mmol) and 702 mg (3.22 mmol) of 4-iodotoluene were combined in 30 mL of solvent and coupled under the general conditions given above for 48 h. The resulting product was first passed through a column of silica gel with a 9:1 hexane/CH2Cl2 solvent mixture as the eluant to provide a crude material that was taken up in 150 mL CH2Cl2. To this, 2.7 mL (1 equiv) of a 1 M TBAF/THF solution was added and the reaction mixture was stirred for 20 min. The solvent was then removed in vacuo and the crude product was purified by column chromatography on silica gel with a 4:1 hexane/CH2Cl2 solvent mixture as the eluant to yield 297 mg of 8d (57% combined yield) as a pale yellow crystalline powder. Data for 8d: 1H NMR δ 2.37 (s, 3 H), 3.17 (s, 1 H), 7.16 (d, 2 H, J ) 8.0 Hz), 7.42 (d, 2 H, J ) 8.0 Hz), and 7.46 (s, 4 H). Anal. Calcd for C17H12: C, 94.41; H, 5.59. Found: C, 93.81, H, 5.33. Preparation of 8e. Compound 6 (662 mg, 2.34 mmol) and 765 mg (2.81 mmol) of 1-iodo-4-(trifluoromethyl)benzene were dissolved in 30 mL of solvent and coupled under the general conditions given above for 48 h. The resulting product was first passed through a column of silica gel with a 9:1 hexane/CH2Cl2 solvent mixture as the eluant to provide a crude material that was taken up in 150 mL of CH2Cl2. To this was added 2.3 mL (1 equiv) of a 1 M TBAF/THF solution, and the reaction mixture was stirred for 20 min. The solvent was then removed in vacuo and the crude product was purified by column chromatography on silica gel with a 4:1 hexane/CH2Cl2 solvent mixture as the eluant to yield 465 mg of 8e as a white, powdery solid (73% combined yield). Data for 8e: 1H NMR δ 3.19 (s, 1 H), 7.49 (s, 4 H), and 7.62 (br s, 4 H). Anal. Calcd for C17H9F3: C, 75.55; H, 3.36. Found: C, 75.36; H, 3.52. Preparation of 9a. Compound 8a (202 mg, 1.00 mmol) was coupled with 278 mg (1.01 mmol) of 4 according to the general procedure to provide 330 mg (94% yield) of the thioacetate 9a as pale yellow needles after purification by column chromatography on silica gel with a 1:3 CH2Cl2 / hexane solvent mixture as the eluant. Data for 9a: mp 188-190 °C; Rf ) 0.39 (1:1 CH2Cl2/ hexane); 1H NMR δ 2.42 (s, 3 H), 7.30 (m, 3 H), 7.35 (d, 2 H, J ) 8.1 Hz), 7.46 (br s, 4 H), 7.49 (d, 2 H, J ) 8.1 Hz), and 7.51 (d, 2 H, J ) 8.4 Hz); 13C NMR δ 30.3, 89.0, 90.4, 90.7, 91.4, 122.7, 123.0, 123.4, 124.3, 128.2, 128.4, 128.5, 131.5, 131.6, 131.7, 132.2, 134.2, and 193.4; IR (neat) 2956w, 2933w, 2920m, 2848w, 1700s, 1111m, 1105m, 839s, 828s, and 756m; MS (EI) m/e (% relative intensity) 352 (49) M+, 310 (100), 276 (8), 263 (8), 223 (4), 149 (20), and 110 (25); HRMS m/e for C24H16OS, calcd 352.0922, found 352.0933. Anal. Calcd for C24H16OS: C, 81.79; H, 4.58; S, 9.10. Found: C, 80.95; H, 4.57; S, 9.24. Preparation of 9b. Compound 8b (66.9 mg, 0.29 mmol) was coupled with 80 mg (0.29 mmol) of 4 according to the general procedure to provide 100 mg (91% yield) of the thioacetate 9b as a pale yellow powder after purification by column chromatography on silica gel with a 1:3 CH2Cl2/hexane solvent mixture as the eluant. Analytically pure material was obtained by recrystallization from hot ethyl acetate. Data for 9b: 1H NMR δ 2.44 (s, 3 H), 7.10 (t, 2 H, J ) 8.5 Hz), 7.39 (d, 2 H, J ) 8.0 Hz), 7.48 (s, 4 H), 7.51 (m, 2 H), 7.57 (d, 2 H, J ) 8.0 Hz); 13C NMR δ 30.5, 88.9, 90.6, 90.7, 90.9, 116.0, 116.2, 123.1, 123.6, 124.5, 129.1, 131.9, 132.0, 132.4, 133.9, 134.0, 134.7, and 193.6; MS (EI) m/e (% relative intensity) 370.1 (47) M+ and 328.1 (100); HRMS m/e for C24H15OFS, calcd 370.0828, found 370.0830. Preparation of 9c. Compound 8c (186.0 mg, 0.85 mmol) was coupled with 258 mg (0.94 mmol) of 4 according to the general procedure to provide 250 mg (80% yield) of the thioacetate 9c as a pale yellow powder after purification by column chromatography on silica gel with a 1:3 CH2Cl2/hexane solvent mixture as the eluant. Analytically pure material was obtained by recrystal-

1124 Langmuir, Vol. 15, No. 4, 1999

Zehner et al. Scheme 1

Scheme 2

lization from hot ethyl acetate. Data for 9c: 1H NMR δ 2.42 (s, 3 H), 3.82 (s, 3 H), 6.88 (d, 2 H, J ) 8.8 Hz), 7.38 (d, 2 H, J ) 8.3 Hz), 7.46 (d, 2 H, J ) 8.8 Hz), 7.47 (s, 4 H), 7.54 (d, 2 H, J ) 8.3 Hz); 13C NMR δ 30.3, 55.3, 87.8, 90.2, 90.8, 91.5, 114.1, 115.1, 122.3, 123.8, 124.3, 128.2, 131.4, 131.6, 132.1, 133.1, 134.2, 159.8, and 193.3; MS (EI) m/e (% relative intensity) 382 (73) M+, 340 (100), and 325 (21); HRMS m/e for C25H18O2S, calcd 382.1027, found 382.1025. Preparation of 9d. Compound 8d (264.4 mg, 1.22 mmol) was coupled with 357 mg (1.28 mmol) of 4 according to the general procedure to provide 211 mg (47% yield) of the thioacetate 9c as a pale yellow powder after purification by recrystallization from hot ethyl acetate. Data for 9d: 1H NMR δ 2.38 (s, 3 H), 2.44 (s, 3 H), 7.16 (d, 2 H, J ) 8.1 Hz), 7.40 (d, 2 H, J ) 8.3 Hz), 7.43 (d, 2 H, J ) 8.1 Hz), 7.50 (s, 4 H), 7.55 (d, 2 H, J ) 8.3 Hz); MS (EI) m/e (% relative intensity) 366 (17) M+, 326 (22), 325 (88), 324 (37); 306 (17), 258 (11), 245 (10), 243 (13), 217 (21), 163 (30), and 91 (100); HRMS m/e for C25H18OS, calcd 366.1065, found 366.1087. Preparation of 9e. Compound 8e (430 mg, 1.59 mmol) was coupled with 465 mg (1.67 mmol) of 4 according to the general procedure to provide 320 mg (48% yield) of the thioacetate 9e as a pale yellow powder after purification by recrystallization from hot ethyl acetate. Data for 9e: 1H NMR δ 2.45 (s, 3 H), 7.41 (d, 2 H, J ) 8.5 Hz), 7.53 (s, 4 H), 7.57 (d, 2 H, J ) 8.5 Hz), 7.63 (br s, 4 H); MS (EI) m/e (% relative intensity) 420 (34) M+, 379 (25), 378 (100), and 377 (18); HRMS m/e for C25H15F3OS, calcd 420.0800, found 420.0796. Preparation of Arenethiols 3a-e from the Thioacetates 9a-e. As a general procedure, 0.25 mmol of the thioacetate 9 was dissolved in 150 mL of Et2O, forming a pale yellow solution. To this was added 38 mg (4 equiv) of LiAlH4, and the reaction mixture was stirred for 35 min, at which time an additional 19 mg of LiAlH4 was added. After the reaction mixture was stirred for an additional 15 min, 40 mg of zinc dust and 3 mL of glacial acetic acid were sequentially added, and the mixture opened to air and stirred for 10 min. This mixture was then filtered through a 1 cm pad of silica gel in a fritted glass funnel, which was washed with an additional 100 mL of Et2O. The filtrates were combined and the solvent was removed in vacuo to yield the pure arenethiol 3 as a pale yellow powder in 70-90% yield.

Results and Discussion

the “end-capping” reagent 4, followed by removal of the thioacetate group with lithium aluminum hydride according to Scheme 1. For the synthesis of longer chain oligomers, such as 3a-e, however, the ready availability of these compounds for study, and specifically, for the non-synthetic-oriented specialist, a concise, high-yielding synthetic route is of paramount importance. Accordingly, to achieve this, we developed the synthetic approach to compounds 3a-e that is depicted in Schemes 2 and 3. To begin, Scheme 2 provides the procedures used to obtain the key building blocks 4 and 6. This synthesis of compound 4 has already been previously reported,22,23 and the preparation of 6 follows very closely that reported by Hoger and Enkelmann28,29 for a related derivative. Importantly, both synthetic preparations leading to compounds 4 and 6 can be scaled up considerably to rapidly provide multigram quantities of these reagents. As shown in Scheme 3, a variety of commercially available p-substituted iodo- or bromobenzenes can be coupled with 6 in standard palladium-catalyzed fashion to provide the derivatives 7a-e that are immediately subjected to silyl group cleavage with TBAF in THF to provide compounds 8a-e in modest to excellent yields for the two-step procedure. At this stage, compounds 8a-e can be purified to a high degree by column chromatography on silica gel. By now employing the end-capping reagent 4, final chain extension of 8a-e can be effected once more by the palladium-catalyzed coupling protocol to provide the thioacetates 9a-e, again in modest to high yields. As all of these compounds are crystalline materials, it is easy to obtain them in a high state of purity through recrystallizations from hot ethyl acetate, and this state of purity is important since the free thiols 3a-e themselves are all quite prone to an oxidation that leads to the corresponding insoluble disulfides. Accordingly, once 3a-e are generated from 9a-e by use of first LiAlH4 to remove the thioacetate group, and then Zn/acetic acid reduction of any initially formed disulfide, they are in a pure enough state after

Synthesis of Arenethiols 3a-e. A few different strategies for the synthesis of thiol-functionalized phenylethynyl oligomers are now available.22-24 By use of these known procedures, the shorter chain adsorbates, 2a-e, are readily available from the simple palladium-catalyzed coupling of known p-substituted phenylacetylenes with

(28) For alternative syntheses of this compound, see: (a) Misumi, S. Bull. Chem. Soc. Jpn. 1961, 34, 1827-1832. (b) Haenninen, E.; Takalo, H.; Kankare, J. Acta Chem. Scand. Ser. B 1988, 42, 614-619. (c) Lavastre, O.; Cabioch, S.; Dixneuf, P. H.; Vohlidal, J. Tetrahedron 1997, 53, 7595-7604. (29) Hoger, S.; Enkelmann, V. Angew. Chem., Int. Ed. Engl. 1995, 34, 2713-2716.

Tuning the Work Function of Gold with SAMs

Langmuir, Vol. 15, No. 4, 1999 1125 Scheme 3

Reagents and conditions: (a) Pd(PPh3)2Cl2 (4 mol %), CuI (4 mol %), THF/Hunig’s base (1:1), 50 °C, 48 h. (b) THF, TBAF (1 M in THF, 1 equiv), 30 min. (c) Pd(PPh3)2Cl2 (4 mol %), CuI (4 mol %), 3 (1.1 equiv) THF/Hunig’s base (1:1), 50 °C, 48 h. (d) (i) THF, LAH (6 equiv), 25 °C, 30 min; (ii) HOAc, Zn, 25 °C, 15 min. Table 1. Measured and Calculated Properties of SAMs Derived from 1-3 thickness (Å) adsorbate

calca

meas

calc µ⊥ (D)

CPD (mV)

∆Φ (mV)

1a 1b 1c 1d 1e 2a 2b 2c 2d 2e 3a 3b 3c 3d 3e HDT bare Au

6 6 8.5 7.5 7.5 13 13 15.5 14.5 14.5 20.5 20.5 23 22 22 19

9 5 6 6 4 12 13 19 16 17 22 26 28 26 27 20

2.29 0.64 3.02 2.78 -0.97 2.95 0.99 3.36 3.31 -0.92 3.06 1.06 3.52 3.43 -1.37 1.98

111 -670 343 340 -817 -145 -620 95 -64 -1290 88 -481 139 93 -797 347 53

-58 723 -290 -287 870 198 673 -42 117 1343 -35 534 -86 -40 850 -294 0

a Estimated from the end-to-end length (excluding the thiol hydrogen) of the adsorbate as measured on a CPK space-filling model minimized at the PM3 level of theory incorporated in the Biosym 4.0 (MSI) suite of computational programs.

removal of the inorganic solids to be used immediately for monolayer formation. Although an alternative method of “in situ” thioacetate group removal leading to a derivatization of gold surfaces has been reported,14-16 we have not explored this procedure ourselves and therefore cannot comment on its relative merits for the present study. Finally, it can be mentioned here that the overall procedure of Scheme 3 has also been used to prepare the additional derivatives of 3 where X ) NMe2 and NO2; however, since these arenethiol adsorbates do not appear to form welldefined SAMs, they were not included in the present work. SAM Characterization. Table 1 summarizes the calculated and measured macroscopic properties of the entire range of SAMs derived from 1a-e, 2a-e, and 3ae. The corresponding values for a SAM derived from HDT are included as a reference. As can be seen, in each case, the measured SAM thickness, as determined by optical ellipsometry, is very close to that calculated, thus indicating that these adsorbates form well-defined close-packed monolayers. The values shown in column 5 are the contact potential differences (CPDs) measured by the Kelvin probe technique.30 Briefly, the Kelvin probe comprises a parallel (30) Christman, K. Introduction to Surface Physical Chemistry; Baumga¨rtel, H., Franck, E. U., Gru¨bein, W., Eds.; Springer-Verlag: New York, 1991.

plate capacitor where one of the plates is vibrating and the other is stationary. As the tip vibrates, the capacitance of the system changes, and so current must flow between the two plates. If the time-variant spacing between the plates is of the form d ) d0 + d1 sin (ωt), then the current is given by eq 1. This vibration induced current is

i ) - AV

d1ω cos (ωt) [d0 + d1 sin (ωt)]2

(1)

proportional to the potential across the capacitor, and will thus vanish when the potential goes to zero. This so-called null condition occurs when the applied potential exactly opposes the difference in work function between the two surfaces. Rather than seek this point directly, the automated Kelvin probe used in the current study measures the magnitude of the current at several potentials near the null point and then applies a linear regression to determine its location more accurately.31 The CPD is related to the change in work function by the equation ∆Φ ) -(CPDSAM - CPDref). Since the zero point of the CPD is determined by the work function of the probe tip, an external reference must be used to determine the absolute work functions of these samples. The CPD for a SAM derived from HDT should be a highly reproducible quantity but, to our knowledge, ∆ΦAu has not yet been accurately determined for this system as the reference CPD value for a “clean” bare gold substrate is rather hard to come by. More specifically, from a review of the literature, previous investigators have recorded a ∆ΦAu of -600 mV for the formation of an HDT-derived monolayer,6,32 and if this value is correct, then on the basis of our measured CPD value for a similiar HDT-derived monolayer that is given in Table 1, we would expect a clean, bare gold substrate to have a CPD of approximately -300 mV. In our present study, while bare gold samples that had been exposed to air for an extended period of time did indeed have a CPD near this value, freshly prepared gold surfaces placed into the Kelvin probe within 2 min of being removed from a vacuum evaporator had a CPD of 53 ( 75 mV, which decayed with time toward a more negative value. Figure 2 shows the time evolution of the CPD of one of these freshly evaporated samples where the sharp discontinuities were produced by turning (31) McAllister Technical Services. KP6500 Digital Kelvin Probe User’s Guide, 3rd ed.; McAllister Technical Services: Coeur d’Alene, ID, 1997. (32) Evans, S. D.; Urankar, E.; Ulman, A.; Ferris, N. J. Am. Chem. Soc. 1991, 113, 4121-4131.

1126 Langmuir, Vol. 15, No. 4, 1999

Zehner et al.

Figure 3. Schematic diagram of the two-layer model for a self-assembled monolayer derived from 3a on gold.

Figure 2. Time evolution of the contact potential difference of a clean gold surface, as measured by Kelvin probe. In region a, the flow of purge nitrogen to the probe was turned off, then back on. At b, the probe was opened to air for 10 min, and at c it was again opened for 20 min.

off the nitrogen purge and opening the probe cavity to the laboratory ambient. The slight rebound of the CPD after resealing of the probe is likely the result of the removal of physisorbed adsorbates, such as water, but the data still show an irreversible trend toward lower work function. Thus, for the purposes of the present study, ∆ΦAu values have all been calculated assuming the CPD for clean gold to be 53 mV. Here, it is significant to note that, in contrast to what is observed for a bare gold substrate, the CPD of a SAM-modified gold surface does not vary with time within the margin of error of the measurement, and this observation underscores the ability of SAMs to stabilize the character of the metal surface. Work Function Theory. The change in work function resulting from a dipole layer at a metal surface can be derived from classical electrostatics. If a dipole layer is conceptualized as two parallel charge sheets with density σ, separated by a thin layer of dielectric of thickness l, then the potential drop caused by a dipole layer is simply the integral of the electric field across this distance, as shown in eq 2, where N is the density of molecules on the

Figure 4. Plot of the calculated dipole moments for the thiol radicals of arenethiols of class 3 as a function of the Hammett parameter, σp.

with monolayer adsorption is then given by eq 3, where

[

∆Φ ) -N

]

µAu-S µ⊥,mono + 0κmono 0κAu-S

(3)

surface, µ⊥ is the component of the dipole moment normal to the surface, κ is the dielectric constant of the dipole layer, and 0 is the permittivity of free space. In this case, the dipole is defined so that the change in work function is negative if the negative end of the dipole is nearest the surface. Finally, in work function measurements, the quantity (µ/κ0) is often referred to as the “effective dipole moment,” since experimentally determined dielectric constants of monolayers are often several times greater than those observed in bulk materials of similar composition. This disparity may be the result of dipole interaction between neighboring molecules, or from disorder in the film that allows the dipoles to tilt away from normal. Chemisorbed SAMs are most effectively modeled as two dipole sheets with different dielectric constants, as schematically depicted in Figure 3 for SAMs derived from 3a. In this scheme, one layer comprises the effective dipole moment of the adsorbate, while the other accounts for the effective dipole of the gold-sulfur charge-transfer interaction.6,33,34 Our model for the change in work function

for commensurate, close-packed monolayers with a x3 × x3 R 30° structure, N ) 4.67 × 1018 m-2.10 Equation 3 can be viewed as a linear relationship between dipole moment, µ⊥,mono, and change in work function, ∆Φ, containing three unknown values, namely, µS-Au, κS-Au, and κmono. If we assume that the dielectric of the sulfur-gold bond is invariant, then the values of the remaining two parameters can be independently determined for 1-3. Comparison of Calculations with Experiment. As expected, the calculated dipole moments for the arenethiols 1-3 depend on the electron-donating or -accepting character of the substituent, X. In fact, as shown in Figure 4, which includes calculated dipoles for additional derivatives of 3 where X ) CN (σp, 0.66; µ⊥, -2.35 D) and for X ) NH2 (σp, -0.66; µ⊥, 6.98 D), the dipole moments vary linearly with the Hammett parameter, σp, a measure of the electron-donating ability of a substituent that is widely tabulated.35 A similar relationship has been observed before in work function modification of semiconductor surfaces with p-substituted benzoic acid derivatives.7,8,36 Accordingly, although such correlations are formally inappropriate given the basis for σp, they are clearly useful for providing an empirical method by which to rapidly assess the anticipated impact of a particular substituent on the work function of a substrate without having to resort to more time-consuming computational studies. As shown in Figure 5, a plot of ∆ΦAu versus the calculated dipole moment within a series of arenethiol

(33) Taylor, D. M.; De Oliviera, O. N., Jr.; Morgan, H. J. Colloid Interface Sci. 1990, 139, 508-518. (34) Demchak, R. J.; Fort, T., Jr. J. Colloid Interface Sci. 1974, 46, 191-202.

(35) Lowry, T. H.; Richardson, K. S. Mechanism and theory in organic chemistry; 3rd ed.; Harper & Row: New York, 1987. (36) Bastide, S.; Butruille, R.; Cahen, D.; Dutta, A.; Libman, J.; Shanzer, A.; Sun, L.; Vilan, A. J. Phys. Chem. B 1997, 101, 2678-2684.

∆Φ )

)∫0l κσ0 dx ) κσl0 ) Nql κ0

Nµ⊥ κ0

(2)

Tuning the Work Function of Gold with SAMs

Figure 5. Plot of the changes in the work function of Au(111) with the formation of arenethiol-derived monolayers as a function of the calculated dipole moment of the corresponding thiol radicals. (O, - ‚ -) Compounds 1a-e; (0, s) compounds 2a-e; (4, - - -) compounds 3a-e. Table 2. Dielectric Constants and Dipoles Calculated from Kelvin Probe Measurements adsorbate series

κmono

µS-Au (D)

1a-e 2a-e 3a-e

1.8 ( 0.2 1.9 ( 0.2 2.9 ( 0.2

2.5 ( 0.2 3.8 ( 0.2 2.3 ( 0.2

adsorbates belonging to the same class, i.e., 1, 2, or 3, can be fit satisfactorily with a linear regression, thereby confirming the form of the model expressed in eq 3. However, it is also clear from this figure that plots between two different classes are neither collinear nor parallel (see Figure 5). Thus, at least two of the three parameters in the model described above must vary with chain length. Considering first the portion accounting for the molecular dipole, the effective dielectric must vary with chain length to produce a change in the slope. However, in the term derived from the gold-sulfur interaction, we can reasonably assume that the dielectric constant of this region remains constant, and for the purposes of this model, κS-Au will be fixed at 6.4, a value that has been used in other studies.6 We are then left with the conclusion that the sulfur-gold dipole, which can be viewed as a result of charge transfer, must be a function of chain length as well. On the basis of the assumptions made above, substituting the constants into eq 3 provides values for the molecular dielectric constant and the sulfur-gold dipole as a function of the chain lengths described by 1-3, and these derived parameters are presented in Table 2. From an analysis of these data, the dielectric constant of the monolayer is seen to increase with chain length, in contrast with the trend that Evans and Ulman6 report for a series of SAMs derived from n-alkanethiols. As an interpretation of this difference, in a microscopic system such as a SAM, the dielectric constant can be thought of as a measure of the polarizability of the component molecules. The trend in this parameter that we observe is then in keeping with an increase in the delocalization of electron density over a longer conjugated system as chain length increases. It can also be noted that our values are in reasonable agreement with those for extended solids of small, conjugated molecules, cf. 2.5 for naphthalene and 3.0 for phenol.37 On the other hand, no trend is apparent in the goldsulfur dipoles listed in Table 2 (see column 3). Although (37) Weast, R. C. Handbook of Chemistry and Physics, 52nd ed.; Chemical Rubber Company: Cleveland, OH, 1971; Vol. 1.

Langmuir, Vol. 15, No. 4, 1999 1127

the values for 1 and 3 are similar, the value for 2 is half again as large. One would expect the dipole to change monotonically with chain length, if there is indeed a dependence on this parameter. Accordingly, one possible explanation is that SAMs derived from 1, which have been shown to be poorly ordered,11 do not completely displace surface contaminants, such as oxides. As demonstrated in Figure 2, the presence of these species would lower the CPD and thus increase the apparent work function. Such an effect would then result in an artificially low value for the sulfur-gold dipole of class 1. However, in the absence of a concrete explanation for this anomaly, it can be said that these values correlate with the transfer of one-third to one-fifth of an electron from the gold to the sulfur, assuming a sulfur-gold bond length of 2 Å. Considering the parameters of Tables 1 and 2 in a practical sense, if the goal is to achieve the maximum range of work function modification for improved charge injection, then monolayers of 1 would appear to be the best choice. The smaller dielectric constant of these short monolayers results in a proportionately larger effect upon the work function for a given end group. In addition, a wide variety of these p-thiophenol derivatives are commercially available. Thiophenol monolayers, however, are much less ordered, and perhaps of higher defect density, than those of the longer arenethiol 2 and 3,11 which is a difficulty if order is an important factor for achieving efficient charge injection. Furthermore, the differences in the charge-transfer nature of the gold-thiol bond in arenethiols of different lengths may result in differences in monolayer-induced bending of the metal Fermi level, which could affect charge transport across this interface. Measurement of the efficiencies of model LEDs incorporating monolayers derived from 1 should determine whether a change in work function is the only important quantity in mediating carrier injection.19 Conclusion A thorough investigation of a large family of arenethiol monolayers has shown that the variation in work function for a given chain length can be effectively modeled with two dipole sheets, as has been shown for other classes of self-assembled monolayers. Significantly, the dielectric constant for the monolayer has been shown to increase as the chain length increases, in contrast to the behavior reported for n-alkanethiol-derived SAMs. Accordingly, p-thiophenol derivatives, represented by 1, result in the largest proportional changes in work function and should therefore be the best choice for mediating charge injection in organic devices. However, it is questioned whether the increased disorder of these shorter monolayers, or the differing nature of the Au-S bond, will decrease their carrier injection efficiency. Studies along these lines are now in progress. Acknowledgment. This work was supported in part by the MRSEC program under the National Science Foundation (DMR-9400379), for which we are grateful. L.R.S. is a Beckman Young Investigator (1995-1997) and a Camille Dreyfus Teacher-Scholar (1995-2000). Supporting Information Available: 1H NMR spectra for thioacetates of 2 and 3 (9 pages). This material is available free of charge via the Internet at http://pubs.acs.org. LA981114F