Triphenylene-Based Discotic Liquid Crystals as Self-Assembled

Using thiol-functionalized triphenylene derivatives to form monolayers, two simple types of molecular packing can be envisaged (Figure 1). Unsymmetric...
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3790

Langmuir 1999, 15, 3790-3797

Triphenylene-Based Discotic Liquid Crystals as Self-Assembled Monolayers Neville Boden, Richard J. Bushby,* and Philip S. Martin Centre for Self-Organising Molecular Systems, University of Leeds, Leeds LS2 9JT, U.K.

Stephen D. Evans, Robert W. Owens, and D. Alastair Smith Department of Physics and Astronomy, University of Leeds, Leeds LS2 9JT, U.K. Received November 30, 1998. In Final Form: March 18, 1999 A series of ω-thiol-substituted 2,3,6,7,10,11-hexaalkoxytriphenylenes was synthesised for the purpose of forming self-assembled monolayers (SAMs) on gold in which the self-organizing, discogenic headgroups have a known orientation. In principle, these headgroups can be oriented either with their short axes perpendicular (disc “face-on”) on their short axes parallel (disc “edge-on”) to the surface, depending on the number and position of the tethering points. To obtain a “face-on” orientation of the headgroups a trithiol (three points of attachment) was synthesized and to obtain an “edge-on” orientation both monothiol and ortho-dithiol derivatives (one or two points of attachment) were made. Within the SAMs it was expected that the headgroups of the “face-on” discogen would self-organize into a hexagonal array and those of the “edge-on” discogens into columns that would run parallel to the surface. The kinetics of formation and the structures of the monolayers were characterized by contact angle measurements, ellipsometry, and scanning tunneling microscopy (STM). In the case of the SAMs obtained from the “edge-on” materials, these confirmed that the desired orientation of the headgroups had been achieved. STM images of a SAM obtained from one of the “edge-on” materials also clearly showed the desired organization of the headgroups into columnar aggregates running parallel to the surface. Clearer STM images were obtained after the SAM had been immersed into a solution of TNF (trinitrofluorenone), a compound known to interdigitate between the triphenylene nuclei. In the case of the SAMs obtained from the “face-on” material, scanning tunneling microscopy gave no indication of the expected hexagonal ordering and ellipsometry suggested that these systems form multilayer rather than monolayer structures.

Introduction Self-assembled monolayers (SAMs) are widely viewed as the successor to Langmuir-Blodgett films, providing a surface with fewer defects, possessing actual covalent binding to the substrate, and possessing a variable functionality at the terminus.1 SAMs have found wide application in areas such as photoresists2 and molecular switching devices.3,4 Recently, our detailed work on the alignment of a calamitic liquid crystal (8-cyanobiphenyl 8CB) using alkanethiol SAMs showed that control over the orientation of the bulk sample could be effected by the modification of the terminal headgroup.5,6,7 The focus for this present study is in creating surfaces that are able to control the orientation of bulk samples of applied discotic liquid crystals, a topic that has only just begun to receive attention.8,9,10 By tailoring the end functional group of a SAM to mimic the shape and nature of the discotic adlayer, * To whom the correspondence should be addressed. E-mail: [email protected]. (1) Ulman, A. An Introduction to Ultrathin Organic Films; Academic Press: London, 1991. (2) Huang, J.; Dahlgren, D. A.; Hemminger, J. C. Langmuir 1994, 10, 626-628. (3) Aoki, K.; Seki, T.; Suzuki, Y.; Tamaski, T.; Hosoki, A.; Ichimura, K. Langmuir 1992, 8, 1007-1013. (4) Cammillone, N.; Chidsey, C. E. D.; Liu, G.-Y.; Scoles, G. J. Chem. Phys. 1993, 98, 3503-3511. (5) Evans, S. D.; Allinson, H.; Boden, N.; Henderson, J. R. Faraday Discuss. 1996, 104, 37-48. (6) Evans, S. D.; Allinson, H.; Boden, N.; Flynn, T. M.; Henderson, J. R. J. Phys. Chem. B 1997, 101, 2143-2148. (7) Alkhairala, H.; Boden, N.; Evans, S. D.; Henderson, J. R. Phys. Rev. E 1999, 59, 3033-3039. (8) Allinson, H.; Boden, N.; Bushby, R. J.; Evans, S. D.; Martin, P. S. Mol. Cryst, Liq. Cryst. 1997, 303, 273-278.

it was hoped that any form of self-assembly within the monolayer would impart a similar alignment to the bulk sample. Using thiol-functionalized triphenylene derivatives to form monolayers, two simple types of molecular packing can be envisaged (Figure 1). Unsymmetrical one-point or two-point substitution of the core should result in an “edgeon” tethering of the discogen (Figure 1b), producing a columnar arrangement11 with the principal symmetry axis in the plane of the monolayer. A symmetric substitution of three thiol groups around the triphenylene core (three-point tethering) should produce a “face-on” orientation of the discogen relative to the substrate and perhaps a hexagonal array (Figure 1c). Experimental Section Materials. The synthesis of 2-(2-(2-thioethoxy)ethoxy)-3,6,7,10,11-pentakis(hexyloxy)triphenylene (1) has been reported previously and is shown in outline in Scheme 1.8 The synthesis of 2-((5-thiopentyl)oxy)-3,6,7,10,11-pentakis(hexyloxy)triphenylene (2) and 2,3-bis((5-thiopentyl)oxy)-6,7,10,11-tetrakis(hexyloxy)triphenylene (3) are also summarized in Scheme 1,12,13 and (9) Schonherr, H.; Kremer, F. J. B.; Kumar, S.; Rego, J. A.; Wolf, H.; Ringsdorf, H.; Jaschke, M.; Butt, H.-J.; Bamberg, E. J. Am. Chem. Soc. 1996, 118, 13051-13057. (10) Gidalevitz, D.; Mindyuk, O. Y.; Heiney, P. A.; Ocko, B. M.; Henderson, P.; Ringsdorf, H.; Boden, N.; Bushby, R. J.; Martin, P. S.; Strazalka, J.; McCauley, J. P.; Smith, A. B. J. Phys. Chem. B 1997, 101, 10870-10875. (11) Chandresekhar, S.; Sadashiva, B. K.; Suresh, K. A. Pramana 1977, 9, 471-480. (12) Boden, N.; Bushby, R. J.; Cammidge, A. N. J. Chem. Soc., Chem. Commun. 1994, 465-466. (13) Boden, N.; Bushby, R. J.; Cammidge, A. N.; Headdock, G. Synthesis 1995, 31-32.

10.1021/la9816555 CCC: $18.00 © 1999 American Chemical Society Published on Web 05/25/1999

Discotic Self-Assembled Monolayers

Langmuir, Vol. 15, No. 11, 1999 3791 Scheme 1. General Synthesis of the “Edge-on” Triphenylene Derivatives 1-3

Figure 1. (a) A schematic representation of a discotic-based SAM molecule and ideal ordering cases of (b) “edge-on” and (c) “face-on” orientation on a suitable substrate. that of 2,6,10-tris((5-thiopentyl)oxy)-3,7,11-tris(hexyloxy)triphenylene (4) is shown in Scheme 2.14 Details of the characterization and the syntheses of these compounds and the intermediates that are shown in these schemes are given below. 1-((5-Bromopentyl)oxy)-2-(hexyloxy)benzene (7). 1,5Dibromopentane (16 g; 75 mmol), 2-(hexyloxy)phenol (6 g; 30 mmol), and anhydrous potassium carbonate (4.5 g) were stirred in refluxing ethanol (50 mL) for 72 h. The mixture was poured onto water (50 mL) and extracted with dichloromethane (2 × 50 mL), and the solvent was evaporated. The crude product was purified by column chromatography [silica; light petroleumdichloromethane (4:1)] to give 1-((5-bromopentyl)oxy)-2-(hexyloxy)triphenylene (7) (8.02 g; 78%) as a clear, colorless oil. 1H NMR (CDCl3): δ 6.87 (s, 4H, ArH), 3.97 (t, 4H, J ) 7 Hz, ArOCH2R), 3.45 (t, 2H, J ) 7 Hz, CH2Br), 1.85 (m, 4H, J ) 7 Hz, OCH2CH2), 1.3-1.7 (m, 10H, CH2), 0.97 (t, 3H, J ) 7 Hz, CH3). Anal. Calc for C17H27O2Br: C, 59.48; H, 7.93. Found: C, 59.75; H, 7.95. 2-((5-Bromopentyl)oxy)-3,6,7,10,11-pentakis(hexyloxy)triphenylene (10).15 Anhydrous iron(III) chloride (5.76 g) was carefully added to a vigorously stirred solution of 3,3′,4,4′-tetrakis(hexyloxy)biphenyl (5) (4.92 g; 8.88 mmol) and 1-((5-bromopentyl)oxy)-2-(hexyloxy)benzene (7) (3.35 g; 9.76 mmol) in dichloromethane (50 mL). The solution was stirred for 1 h, before being carefully poured onto methanol (100 mL), and the resultant precipitate was filtered off. The crude product was purified by column chromatography [silica; light petroleum-dichloromethane (1:1)] to give 2-(5-bromopentyloxy)-3,6,7,10,11-pentakis(hexy(14) Boden, N.; Borner, R. C.; Bushby, R. J.; Cammidge, A. N.; Jesudason, M. V. Liq. Cryst. 1993, 15, 851-858.

loxy)triphenylene (10) (3.9 g; 47%) as a white solid. [K (70.8) D (75.8) I]. MS (m/z) 894: (Μ+, 100%). 1H NMR (CDCl3): δ 7.84 (s, 6H, ArH), 3.97 (t, 12H, J ) 7 Hz, ArOCH2R), 3.45 (t, 2H, J ) 7 Hz, CH2Br), 1.85 (m, 12H, J ) 7 Hz, OCH2CH2), 1.3-1.7 (m, 34H, CH2), 0.97 (t, 15H, J ) 7 Hz, CH3). 2-((5-(Acetylthio)pentyl)oxy)-3,6,7,10,11-pentakis(hexyloxy)triphenylene (13). Thioacetic acid (310 mg; 4.1 mmol) was added to a mixture of ethanol (50 mL) and sodium ethoxide (270 mg; 4.0 mmol), and the mixture was stirred for 1 h. 2-((5Bromopentyl)oxy)-3,6,7,10,11-pentakis(hexyloxy)triphenylene (10) (3 g; 3.35 mmol) was added and the solution refluxed for 24 h. The cooled mixture was poured carefully onto dilute aqueous hydrochloric acid (50 mL; 2 M) and extracted with dichloromethane (2 × 50 mL), the solvent was evaporated, and the product was purified by column chromatography [silica; dichloromethane-light petroleum (2:3)] to give 2-((5-(acetylthio)pentyl)oxy)-3,6,7,10,11-pentaks(hexyloxy)triphenylene (13) (800 mg, 27%) as a white solid. [K (56.9) D (58.6) I]. MS (m/z): 888 (M+, 100%). 1H NMR (CDCl3): δ 7.84 (s, 6H, ArH), 3.97 (t, 12H, J ) 7 Hz, ArOCH2R), 2.94 (t, 2H, J ) 7 Hz, CH2SCOCH3), 2.33 (s, 3H, SCOCH3), 1.85 (m, 12H, J ) 7 Hz, OCH2CH2), 1.3-1.7 (m, 34H, CH2), 0.97 (t, 15H, J ) 7 Hz, CH3). 2-((5-Thiopentyl)oxy)-3,6,7,10,11-pentakis(hexyloxy)triphenylene (2). Sodium hydroxide (45 mg) was added to a vigorously stirred solution of 2-((5-(acetylthio)pentyl)oxy)-3,6,7,10,11-pentakis(hexyloxy)triphenylene (13) (720 mg, 0.81 mol) in THF (50 mL) and water (10 mL) under a nitrogen atmosphere. The mixture was stirred for 24 h, poured onto dilute aqueous ammonium chloride (50 mL; 1 M), and extracted with chloroform (2 × 50 mL). The solvent was evaporated under vacuum at room temperature, and the product was purified by column chromatography [silica; dichloromethane-light petroleum (1:1)] and reprecipitated from chloroform (5 mL)/methanol (20 mL) to give 2-((5-thiopentyl)oxy)-3,6,7,10,11-pentakis(hexyloxy)triphenylene (2) (650 mg, 95%) as a white solid. MS (m/z): 846 (M+, 100%). 1H NMR (CDCl3): δ 7.84 (s, 6H, ArH), 3.97 (t, 12H, J ) 7 Hz, ArOCH2R), 2.78 (t, 2H, J ) 7 Hz, CH2SH), 1.85 (m, 12H,

3792 Langmuir, Vol. 15, No. 11, 1999 Scheme 2. Synthesis of the Mixed Isomer “Face-on” Triphenylene Derivative 4

J ) 7 Hz, OCH2CH2), 1.3-1.7 (m, 34H, CH2), 0.97 (t, 15H, J ) 7 Hz, CH3). Anal. Calc for C55H84O7S: C, 75.2; H, 9.69; S, 3.78. Found: C, 74.9; H, 9.9; S, 3.7. 1,2-Bis((5-bromopentyl)oxy)benzene (8).16 1,5-Dibromopentane (20 g; 87 mmol), catechol (1.6 g; 14.5 mmol), and anhydrous potassium carbonate (4.0 g) were stirred in refluxing ethanol (50 mL) for 4 days. The mixture was decanted onto water (50 mL) and extracted with dichloromethane (2 × 50 mL), and the solvent was evaporated. The crude product was purified by column chromatography [silica; light petroleum-dichloromethane (2:1)] to give 1,2-bis((5-bromopentyl)oxy)triphenylene (8) (4.08 g; 69%) as a clear, colorless oil. 1H NMR (CDCl3): δ 6.87 (s, 4H, ArH), 3.97 (t, 4H, J ) 7 Hz, ArOCH2R), 3.45 (t, 4H, J ) 7 Hz, CH2Br), 1.85 (m, 8H, OCH2CH2/CH2CH2Br), 1.62 (m, 4H, CH2). 2,3-Bis((5-bromopentyl)oxy)-6,7,10,11-tetrakis(hexyloxy)triphenylene (11). Anhydrous iron(III) chloride (6.65 g) was carefully added to a vigorously stirred solution of 3,3′,4,4′,tetrakis(hexyloxy)biphenyl (5) (5.4 g; 9.7 mmol) and 1,2-bis((5bromopentyl)oxy)benzene (8) (4.76 g; 11.7 mmol) in dichloromethane (50 mL). The solution was stirred for 1 h, before being carefully poured onto methanol (100 mL), and the resultant precipitate was filtered off. The crude product was purified by column chromatography [silica; light petroleum-dichloromethane (1:1)] to give 2,3-bis((5-bromopentyl)oxy)-6,7,10,11-tetrakis(hexyloxy)triphenylene (11) (3.8 g; 41%) as a white solid (mp 71.0 °C). 1H NMR (CDCl3): δ 7.84 (s, 6H, ArH), 4.23 (t, 12H, J ) 7 Hz, ArOCH2R), 3.45 (t, 4H, J ) 7 Hz, CH2Br), 1.85 (m, 12H, OCH2CH2), 1.3-1.7 (m, 32H, CH2), 0.97 (t, 12H, J ) 7 Hz, CH3). Anal. Calc for C52H78O6Br2: C, 65.13; H, 8.20. Found: C, 64.85; H, 7.95. 2,3-Bis((5-(acetylthio)pentyl)oxy)-6,7,10,11-tetrakis(hexyloxy)triphenylene (14). Thioacetic acid (349 mg; 4.59 mmol) was added to a mixture of dry ethanol (50 mL) and sodium ethoxide (273 mg; 4.02 mmol), and the mixture was stirred for 1 h. 2,3-Bis((5-bromopentyl)oxy)-6,7,10,11-tetrakis(hexyloxy)triphenylene (11) (2.0 g; 2.09 mmol) was added, and the solution was refluxed for 24 h. The cooled mixture was poured carefully onto dilute aqueous hydrochloric acid (50 mL; 2 M) and extracted with dichloromethane (2 × 50 mL), the solvent was evaporated, and the product was purified by column chromatography [silica;

Boden et al. dichloromethane-light petroleum (1:1)] to give 2,3-bis((5(acetylthio)pentyl)oxy)-6,7,10,11-tetrakis(hexyloxy)triphenylene (14) (1.2 g, 60%) as a white solid. [K (58.6) D (66.0) I]. 1H NMR (CDCl3): δ 7.84 (s, 6H, ArH), 4.23 (t, 12H, J ) 7 Hz, ArOCH2R), 2.94 (t, 4H, J ) 7 Hz, CH2SCOCH3), 2.33 (s, 6H, SCOCH3), 1.85 (m, 12H, OCH2CH2), 1.3-1.7 (m, 32H, CH2), 0.97 (t, 12H, J ) 7 Hz, CH3). Anal. Calc for C56H84O8S2: C, 70.85; H, 8.92. Found: C, 70.55; H, 9.1. 2,3-Bis((5-thiopentyl)oxy)-6,7,10,11-tetrakis(hexyloxy)triphenylene (3). Sodium hydroxide (80 mg) was added to a vigorously stirred solution of 2,3-bis((5-(acetylthio)pentyl)oxy)6,7,10,11-tetrakis(hexyloxy)triphenylene (14) (830 mg, 0.88 mmol) in THF (50 mL) and water (5 mL) under a nitrogen atmosphere. The mixture was stirred for 24 h, poured onto dilute aqueous ammonium chloride (50 mL; 1 M), and extracted with chloroform (2 × 50 mL). The solvent was evaporated under vacuum at room temperature, and the product was purified by column chromatography [silica; light petroleum-dichloromethane (1:1)] and reprecipitated from chloroform (5 mL)/methanol (20 mL) to give 2-((5-thiopentyl)oxy)-3,6,7,10,11-pentakis(hexyloxy)triphenylene (4) (400 mg, 52%) as a white solid. MS (m/z): 862 (M+, 100%). 1H NMR (CDCl3): δ 7.84 (s, 6H, ArH), 4.23 (t, 12H, J ) 7 Hz, ArOCH2R), 2.78 (t, 4H, J ) 7 Hz, CH2SH), 1.85 (m, 12H, OCH2CH2), 1.3-1.7 (m, 32H, CH2), 0.97 (t, 12H, J ) 7 Hz, CH3). Anal. Calc for C52H80O6S2: C, 72.2; H, 9.26; S, 7.41. Found: C, 71.9; H, 9.35; S, 7.7. 2,6,10-Trimethoxy-3,7,11-tris(hexyloxy)triphenylene (17).17 Anhydrous ferric chloride (70 g) was added to a vigorously stirred solution of 1-methoxy-2-(hexyloxy)benzene (16) (30 g; 0.14 mol) and sulfuric acid (1 drop) in dichloromethane (100 mL). After 1 h, the mixture was carefully poured onto methanol and left at 0 °C for 2 h before the resultant precipitate was filtered off. Column chromatography [silica; dichloromethane] gave 2,6,10-trimethoxy-3,7,11-tris(hexyloxy)triphenylene (17) and 2,6,11-trimethoxy-3,7,10-tris(hexyloxy)triphenylene (15.3 g; 51.5%) as a pale yellow solid isomeric mixture. 1H NMR (CDCl3): δ 7.66 (s, 6H, ArH), 4.15 (t, 6H, J ) 7 Hz, ArOCH2), 4.61 (s, 9H, ArOCH3), 1.85 (t, 6H, J ) 7 Hz, OCH2CH2), 1.3-1.7 (m, 18H, CH2), 0.97 (t, 9H, J ) 7 Hz, CH3). 2,6,10-Trihydroxy-3,7,11-tris(hexyloxy)triphenylene (18).18 2,6,10-Trimethoxy-3,7,11-tris(hexyloxy)triphenylene (17) (4 g; 6.5 mmol) was added to a solution of lithium diphenylphosphide (4.5 g) in THF (50 mL) under nitrogen, and the mixture was stirred at room temperature for 12 h. The resulting suspension was poured onto dilute aqueous hydrochloric acid (50 mL; 2 M) and extracted with dichloromethane (2 × 50 mL). The solvents were evaporated, and the residue was purified by column chromatography [silica; dichloromethane] and then precipitated from methanol (50 mL) to give 2,6,10-trihydroxy3,7,11-tris(hexyloxy)triphenylene (18) as a white solid (2.9 g; 75%), which was immediately used for the next step. 2,6,10-Tris((5-bromopentyl)oxy)-3,7,11-tris(hexyloxy) triphenylene (19). 2,6,10-Trihydroxy-3,7,11-tris(hexyloxy)triphenylene (18) (800 mg; 1.39 mmol) was stirred with 1,5dibromopentane (10 mL) and anhydrous potassium carbonate (10 g) in refluxing ethanol (50 mL) for 72 h, upon which time the solution was carefully decanted into water (50 mL) and extracted with dichloromethane (2 × 50 mL), the organic layer was washed with dilute aqueous hydrochloric acid (2 M; 50 mL), and then the solvent was evaporated. Column chromatography (silica; dichloromethane) gave 2,6,10-tris((5-bromopentyl)oxy)-3,7,11-tris(hexyloxy)triphenylene (19) (325 mg; 23%) as an off-white solid. MS (m/z): 1024 (M+, 100%). 1H NMR (CDCl3): δ: 7.84 (s, 6H, ArH), 4.23 (t, 12H, J ) 7 Hz, ArOCH2R), 3.45 (t, 6H, J ) 7 Hz, CH2Br), 1.95 (m, 12H, OCH2CH2), 1.3-1.7 (m, 30H, CH2), 0.97 (t, 9H, J ) 7 Hz, CH3). Anal. Calc for C51H75O6Br3: C, 59.82; H, 7.38. Found: C, 59.75; H, 7.59. 2,6,10-Tris((5-(acetylthio)pentyl)oxy)-3,7,11-tris(hexyloxy)triphenylene (20). Sodium chips (66 mg) were stirred vigorously in dry ethanol (25 mL) under argon until the reaction ceased. Thioacetic acid (207 mg) was then added, and the solution was stirred for 1 h. 2,6,10-Tris((5-bromopentyl)oxy)-3,7,11-tris(hexyloxy)triphenylene (19) (214 mg; 0.21 mmol) was added, and the solution was refluxed for 24 h. On cooling, the mixture was carefully poured onto dilute aqueous hydrochloric acid (25 mL; 2 M) and the mixture was extracted with dichloromethane (2 ×

Discotic Self-Assembled Monolayers

Figure 2. Triphenylene-based alkanethiol molecules of varying substitution symmetry designed for “edge-on” (1-3) and “faceon” (4) orientation with respect to a gold substrate. 25 mL). The organic solvent was then evaporated, and the product was separated by column chromatography (silica; dichloromethane) and then reprecipitated from methanol (50 mL) to give 2,6,10-tris((5-(acetylthio)pentyl)oxy)-3,7,11-tris(hexyloxy)triphenylene (20) (165 mg; 78%) as a white solid. MS (m/z): 1008, (M+, 100%). 1H NMR (CDCl3): δ 7.84 (s, 6H, ArH), 4.23 (t, 12H, J ) 7 Hz, ArOCH2R), 2.94 (t, 6H, J ) 7 Hz, CH2SCOCH3), 2.33 (s, 9H, SCOCH3), 1.95 (m, 12H, OCH2CH2), 1.3-1.7 (m, 30H, CH2), 0.97 (t, 9H, J ) 7 Hz, CH3). 2,6,10-Tris((5-thiopentyl)oxy)-3,7,11-tris(hexyloxy)triphenylene (4). 2,6,10-Tris((5-(acetylthio)pentyl)oxy)-3,7,11-tris(hexyloxy)triphenylene (20) (152 mg; 0.15 mmol) was stirred in a solution of sodium hydroxide (1 pellet) and water (5 mL) in THF (25 mL) at room temperature under an inert atmosphere for 24 h. After this, the resultant mixture was carefully poured onto dilute aqueous ammonium chloride (25 mL; 1 M), extracted with chloroform (2 × 25 mL) and the solvent evaporated at room temperature. The product was reprecipitated from methanol (25 mL) to give 2,6,10-tris((5-thiopentyl)oxy)-3,7,11-trishexyloxy)triphenylene (4) (100 mg; 75%) as a white solid. 1H NMR (CDCl3): δ 7.84 (s, 6H, ArH), 4.23 (t, 12H, J ) 7 Hz, ArOCH2R), 2.78 (t, 6H, J ) 7 Hz, CH2SH), 1.95 (m, 12H, OCH2CH2), 1.3-1.7 (m, 30H, CH2), 0.97 (t, 9H, J ) 7 Hz, CH3). MS (m/z): 886 (M+, 100%). Anal. Calc for C51H78O6S3: C, 69.4; H, 8.84; S, 10.88. Found: C, 69.55; H, 8.75; S, 10.75. Materials Characterization. NMR spectra were recorded on a General Electric QE300, a Bruker AC200, or a Bruker AM400 instrument. Mass spectra were obtained on a VG Autospec instrument. Column chromatography on silica refers to the use of Merck silica gel 9385 Type 60. The phase behavior was investigated using an Olympus BH-2 optical polarizing microscope with a Mettler FP82 HT hot stage and Perkin-Elmer 7 thermal analysis system (cooling and heating rate, 10 °C/min). These results are summarized in Figure 2. Formation of the SAMs. Preliminary investigations into the orientation of compound 1 shown in Figures 3 and 4 have already been reported.8 However, more detailed results are included here and are compared with the new SAM systems (2-4). Substrates for the SAMs consisted of a 10 nm chromium sublayer and a 100 nm gold layer formed by evaporation. Generally, evaporated metal films have a surface roughness of tens of nanometers and, although these are often used for the preparation of SAMs, they are not ideal. The evaporated gold substrates were therefore subjected to rapid flame annealing (700 °C) and subsequent quenching in methanol. This produced large “flat” gold terraces extending over distances of hundreds of nanometers. For the STM imaging of the SAMs, molecular beam epitaxy (MBE) was used to produce large flat gold terraces.

Langmuir, Vol. 15, No. 11, 1999 3793 This involved evaporation of gold, onto freshly cleaved mica (0.1 Å s-1) at 10-11 Torr, and then subsequent annealing at 450 °C for 2 h. All of the thiols used in the preparation of the SAMs were made up as 1 mM solutions in dry, distilled dichloromethane solvent, and all slides were washed copiously with distilled dichloromethane both before and after exposure to the solution. Characterization of the SAMs. Contact Angle Measurements.19 Contact angle measurements were obtained in a dust free environment by formation of an advancing or receding water droplet from a 10 mL syringe, illuminated by a sodium lamp. Images of the drop were captured (Navitar 6000 II zoom lens attached to a Hamamatsu C3077 CCD), digitized, and analyzed using Avipal imaging software. The triphenylene monolayers were removed from the SAM solution and washed with copious amounts of dichloromethane to remove excess thiol, and then the motional contact angle of an advancing water droplet upon the surface was obtained. The substrate was then re-immersed within the thiol solution and SAM formation resumed. Ellipsometry.20 Ellipsometry was performed using a Beaglehole picometer ellipsometer and Opsys ellipsometric software. Fixed angle measurements were obtained at discrete intervals during monolayer formation, in a manner similar to the contact angle measurements. The imaginary part of the index of refractivity was assumed to be zero (k ) 0) due to the “transparent” nature of the layers, and the real part was assumed to be 1 for air (N ) 1) and 1.5 for the monolayer film (for the discotic monolayers N ) 1.5 was assumed). The real and imaginary part of the refractive index for gold were obtained from ellipsometric measurement of the bare gold surface prior to monolayer formation (as an indication, for gold n ) 0.18, and k ) -3.35), as estimated by a fit to a two-layer model.21,22 An angle close to that of the Brewster angle was adopted for all measurements (∼65°) for the incident light. Estimates of the final film thicknesses were obtained using a three-layer model. The quality of the fits to the data and the sample to sample variation placed a limit of (0.2 nm on the accuracy of the results. The measurements were taken as an average of three readings for each point on the surface of the SAM slide, and measurements were averaged for three different points to give the monolayer thickness. Scanning Tunneling Microscopy (STM).23 Scanning tunneling microscopy (STM) was used to image both the “face-on” (material 4) and one of the “edge-on” (material 2) SAMs. The images were acquired using a Digital Instruments IIIa STM with a low current booster box. Images were obtained with a tunneling current set point of 30-100 pA, with a tip bias of 800-1000 mV, in ambient conditions and using mechanically cut tips on the DI.

Results Organic Synthesis. The family of ω-thioalkoxy functionalized triphenylene derivatives shown in Figure 2 with unsymmetrical monosubstitution (Figure 2, 1, 2) or orthodisubstitution (Figure 2, 3) and symmetric trisubstitution (Figure 2, 4) were synthesized according to the procedures shown in Schemes 1 and 2. The key step in the synthesis of the unsymmetrically substituted triphenylene derivatives 1-3 is the oxidative coupling of biphenyl and phenyl moieties followed by a methanol reductive workup12,13 that gives ω-halogenated triphenylenes. Subsequent steps convert these ω-halo-triphenylenes into initially the acetylprotected thiol, and finally the free thiol by base deprotection. Hence, the coupling reaction of 3,3′,4,4′-tetrakis(hexyloxy)biphenyl (5) with the benzene derivatives 6-8, (15) Boden, N.; Bushby, R. J.; Cammidge, A. N.; El-Mansoury, A.; Martin, P. S.; Lu, Z. J. Mater. Chem., in press. (16) Sircar, I.; Hoefle, M.; Maxwell, R. E. J. Med. Chem. 1983, 26, 1020-1027. (17) Chapuzet, J.-M.; Simonet, J. Tetrahedron 1991, 47, 791-793. (18) Boden, N.; Bushby, R. J.; Lu Z. B. Liq. Cryst. 1998, 25, 47-58. (19) Whitesides G. M.; Ferguson, G. S. Chemtracts: Org. Chem. 1988, 1, 171-187 and references therein. (20) Nuzzo, R. G.; Fusco, F. A.; Allara, D. L. J. Am. Chem. Soc. 1987, 109, 2358-2368. (21) Tompkins, H. G. A Users Guide to Ellipsometry; Academic Press: Boston, 1993. (22) Azzam, R. M. A.; Bashara, N. M. Ellipsometry and Polarised Light; North-Holland: Amsterdam, 1977.

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respectively, gave the triphenylene derivatives 9-11 in yields of 34%, 47%, and 41%, respectively. Each of these triphenylene derivatives underwent nucleophilic substitution with sodium thioacetate to give the protected thiotriphenylenes 12-14, and finally deprotection at room temperature under an inert atmosphere using basic conditions gave the free thiol derivatives 1-3 in high yield. The key step in the synthesis of the “face-on” triphenylene 4 is the oxidative trimerization of a benzene derivative,14 followed by a reductive workup to give the trimethoxytriphenylene precursor. This direct method of trimerization results in two isomeric products, which can be separated by column chromatography. Triple demethylation, followed by reflux with a large excess of an R,ω-dibrominated alkane and base gives the tribromotriphenylene derivative with limited losses due to oligomerization. From this, preparation of the free thiol is fairly straightforward. As a result, 1-methoxy-2-(hexyloxy)benzene (16) was oxidatively trimerized by 6 equiv of anhydrous ferric chloride, followed by methanol workup to give a regio-isomeric mixture of 2,6,10-trimethoxy-3,7,11-tris(hexyloxy)triphenylene (ABABAB) (17) and 2,6,11-trimethoxy-3,7,10-tris(hexyloxy)triphenylene (ABABBA). Chromatography of the symmetric isomer 17 and demethylation with 3 equiv of lithium diphenylphosphide under an inert anhydrous atmosphere, gave the trihydroxytriphenylene precursor 18, which, in turn, was refluxed with a large excess of 1,5-dibromopentane to give the halogen chain terminated triphenylene precursor 19 in a yield of 23%. Nucleophilic substitution of these end halogens by 3 equiv of sodium thioacetate gave the protected thiol 20, and finally elimination of the protecting group by sodium hydroxide gave 2,6,10-tris((5-thiopentyl)oxy)-3,7,11-tris(hexyloxy)triphenylene (4) in a yield of 75%. Formation of the SAMs. Contact Angle Measurements. The advancing contact angles measured during the formation of the monolayers for the three “edge-on” materials 1-3 are shown in Figure 3a. The advancing contact angles for the mono- and disubstituted triphenylenes were approximately 90°. These are somewhat lower than those typically found for SAMs formed from long chain alkanethiols, implying a lower packing density of alkyl chains at the surface and a certain degree of exposure of the aryl nuclei and ether oxygens. The time required for formation of the SAM was typically around 5 h. It appears that the disubstituted monolayer (compound 3) reached its optimum surface configuration in a shorter time. This is possibly due to the lower degree of freedom allowed due to the double binding of the triphenylene to the surface. In contrast, the monosubstituted material (compound 2) displayed a rapid initial adsorption but a slower rearrangement over the following few hours. The apparent reduction in the advancing water contact angle from the SAMs formed from compound 1 as compared to those formed using alkoxy derivatives 2 and 3 is likely to be a result of the presence of the ethyleneoxy chain. Its influence is 2-fold. First, its hydrophilic nature would be expected to lower the water contact angle, and second, the disorder introduced by having more flexible units might serve to reduce the packing density and hence lead to a greater exposure of both core aromatic and oxygenated regions. For SAMs formed from derivative 4 (Figure 3b) we find that the advancing contact angles are approximately 80°. This is slightly lower than the values found for the derivatives 2 and 3 and similar to that obtained for SAMs of derivative 1. This would be expected if the triphenylene cores are at least partly exposed at the surface.

Boden et al.

Figure 3. Time dependence of the advancing contact angle measurements for (a) the “edge-on” materials (1-3) and (b) the “face-on” material (4) SAMs during monolayer formation. Error bars indicate range of variation between samples.

Figure 4. Film thickness calculated from ellipsometric measurements for (a) “edge-on” materials (1-3) and (b) “face-on” material (4) SAMs shown as a function of formation time.

Ellipsometry. Figure 4a shows the change in thickness of the SAMs for the thiols 1-3 as a function of time. It is seen that there is a rapid initial increase followed by a much slower gradual increase. The final thickness for the “edge-on” materials (typically 2.2 nm) is roughly equivalent to the diameter of the disk, supporting the theory

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Figure 5. (a) 300 nm STM image of annealed gold with the triphenylene-based SAM coating of compound 2. (b) An enhanced image demonstrating the ordering on the surface. (c) Higher resolution image of the SAM. (d) Sketch showing the position of columnar elements in (c). Full range height scale: 5a,b, 5 nm; 5c, 1 nm).

that the monolayers are being formed in which the disks are packing in an “edge-on” orientation. In the case of the “face-on” thiol 4, continuous growth was observed, reaching a thickness of 5 nm, which is well above that of a monolayer (Figure 4b). This is possibly indicative of the growth of a columnar phase from the surface. Alternatively, it seems reasonable to propose that a proportion of the disks are bound “edge-on” through one or two of the three available thiol residues. Eventually, the remaining exposed thiols oxidatively couple with thiols of monomers still free in the solution, leading to a polymeric film held together by covalent disulfide bonds. Indeed, a close inspection of the data might suggest that we are seeing layer-by-layer growth of “edge-on” disks. The formation of the first layer may correspond to the appearance of a plateau at about 2.5 nm after ∼8000 s, followed by the growth of a second layer indicated by a plateau of 5 nm (twice the disk diameter) after 18 000 s, but this is not entirely clear. Within the time scale of our experiments there does not appear to be a limit to the growth.

Scanning Tunneling Microscopy. To determine the molecular orientation within these layers, STM was performed in air. Figures 5 and 6 are constant current images obtained for SAMs of material 2. Figure 5a shows a 300 nm area of the SAM on annealed gold, exhibiting columnar aggregation in the monolayer, while 5b is an enlarged corner of picture 5a. They show an “edge-on” configuration in which the core-core separation between adjacent columns is 2.1-2.4 nm. Figure 5c shows a higher resolution image of a similar sample, it features shapes that are 2.2-2.5 nm wide, with a length of approximately 10 nm. The samples that have been then placed into TNF solution (Figure 6a,b) exhibit the same features but to a better degree of resolution. The columnar ordering is present in the TNF-treated SAM with an average column repeat diameter of 2.1-2.4 nm. The ordering appears less continuous at higher resolution; however, short-range aggregates can still be observed with an apparent length range of about 10 nm.

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Figure 6. Images obtained after TNF treatment of the triphenylene-based SAM coating, displaying improved resolution. Full range height scale: 6a,b, 2.5 nm.

Discussion SAM Formation. Consideration of the wettability of the sufaces produced upon SAM formation by the “edgeon” derivatives 1-3 suggests that relatively high energy surfaces are produced in comparison with those obtained with close-packed alkanethiol monolayers. There are several factors that contribute to the wettability of these surfaces. First the packing for derivatives 1-3 will be dominated by the bulky triphenylene rings, and this will lead to losely packed alkyl (ethyleneoxy) chains. The interfacial energy will then reflect the chemical composition of a surface containing aryl, alkyl, and ether residues. Second, given the steric difficulty in forming close-packed structures (evidenced by their “slow” adsorption isotherms) it is also likely that additional contributions arise from the gold. The small differences between SAMs formed from the derivatives 2 and 3 and those from 1 most likely arise from the incorporation of the hydrophilic ethylenoxy units although we cannot also completely rule out the possibility of a structural contribution.

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The “face-on” system 4 possesses an additional thiol residue in place of a methyl group. There are at least two plausible structures for the multilayers formed in this system. In one scenario we have “edge-on” packing of the disks with layers being covalently linked through disulfide bonds. In this case we would expect the degree of order to diminish with increasing thickness, resulting in a higher surface energy configuration of exposed triphenylene cores. The second scenario is that we have the desired “face-on” arrangement in which the first layer is bound to the gold and in which columns then grow outward from this template. In this case we would also expect to see a high surface energy because of the exposed triphenylene cores. The wettability data cannot distinguish between these two. Ordering. Although the ellipsometry results indicate that the monolayers from 1-3 are packing in an “edgeon” orientation, no indication of ordering is obtained. The scanning tunneling microscope images of monolayer 2, however, clearly show a tightly bound monolayer ordered over long range (>300 nm) with only minor observable defects and discontinuities. These columnar aggregates have a regular repeat distance of 2.2 nm, which corresponds reasonably closely to the column-column separation within the bulk Dh phase for the analogous liquid crystal 2,3,6,7,10,11-hexakis(hexyloxy)triphenylene (HAT6). At a closer range, columnar aggregation is not as clearly imaged by STM. TNF is known to form intercalation donor-acceptor complexes with discogens.24 In the case of the SAMs from compound 2, exposure to a solution of TNF appears to give more rigid tighter-packed columnar aggregates and these can certainly be imaged more easily. This is presumably the result of incorporation of TNF within the SAMs. Once again a repeat columnar interval of 2.2 nm is observed for this modified surface. Unlike the straight chain alkanethiol SAMs that close-pack on formation due to their compact rodlike conformation, it is difficult to envisage the discotic thiols both binding and then self-organizing on the surface without encountering steric hindrance from the molecules already bound. However, the results suggest that any steric congestion encountered has no ultimate adverse effect on the final close-packed, reasonably well ordered monolayer observed. Considering the natural state for triphenylene-based mesogens upon any surface, including gold, is as an aligned homeotropic film, the fact that the monolayer orients “edge-on” and within long-range columnar aggregates may be surprising. The difference must reside in the thiol grouping. Presumably, the thiol/gold interaction is stronger than the disk/gold interaction. Conclusions A series of alkanethiol-functionalized triphenylenes was synthesized for the production of self-assembled monolayers possessing orientation in both the “edge-on” and “face-on” conformations. Characterization of the resultant systems revealed that while the triphenylene derivative 4 was intended to form a “face-on” monolayer, some form of the multilayer system was created. For the “edge-on” derivatives 1-3 contact angle results showed a decreased elapse time for formation of the dithiol SAM 3 with respect to that of the monothiol 2 or ethyleneoxythiol 1. They also showed that the characteristics of the pinning chain have a large effect upon the nature of the surface despite the (23) Chen, C. J. Introduction to Scanning Tunnelling Microscopy; Oxford University Press: New York, 1993 (see also references therein). (24) Praefcke, K.; Holbrey, J. D. J. Inclusion Phenom. Mole. Recognit. Chem. 1996, 24, 19-41.

Discotic Self-Assembled Monolayers

fact that this pinning chain is partially shielded by the triphenylene system. Ellipsometry confirmed the edgeon orientation of the monolayers, by giving an average thickness of 2.0-2.5 nm. Scanning tunneling microscopy was used to image these triphenylene SAM systems. The resultant images show clearly the columnar aggregation of the triphenylenes at the surface, revealing long-range self-assembly of the bound monolayer. Enhanced imaging of the triphenylene monolayer at close range was achieved by treatment with a solution of TNF, a compound which is known to interdigitate between the triphenylene nuclei.24 Heiney et al. have recently published an investigation of Langmuir-Blodgett films of hexaalkoxytriphenylenes at the air-water interface,10 where columnar ordering with a repeat diameter of 1.5-1.8 nm for HAT6 was observed by grazing incident X-ray diffractometry. This reduced column-column separation in comparison to the bulk indicates a very tightly clustered monolayer with some adjacent column overlap. These results contrast with our SAM, which is not only bound to the surface, but well ordered, with an interccolumnar distance equivalent to that of the bulk liquid crystal.

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Also, comparison of our imaged monolayers with those published recently by Ringsdorf et al.9 on an analogous system using AFM show similar results, but our columnar aggregates apparently show more alignment in a unique direction. This can be ascribed to the longer tethering chain incorporated within the triphenylene SAM formed by Ringsdorf et al. (C10 (Ringsdorf et al.) to C5 (this paper)). This would allow more motional freedom to the triphenylene headgroups, which would in turn allow more freedom in the alignment of the columnar aggregates. The shorter C5 tether used in our SAM binds the triphenylene closer to the surface and allows less easy variation in the direction of the column. The monolayers we have created should provide a base upon which to align samples of bulk discotic liquid crystals. Investigations into the alignment of related liquid crystals over these SAM systems are now in progress. Acknowledgment. We would like to acknowledge the EPSRC for the funding for this project. LA9816555