Langmuir 1997, 13, 7099-7105
Characterization, Adsorption, and Photochemistry of Self-Assembled Monolayers of 10-Thiodecyl 2-Anthryl Ether on Gold Marye Anne Fox* and Marilyn D. Wooten Department of Chemistry and Biochemistry, The University of Texas at Austin, Austin, Texas 78712 Received August 8, 1997. In Final Form: October 17, 1997X
The photodimerization and its photochemical reversal of the pendent anthryl groups in 10-thiodecyl 2-anthryl ether (1) as a self-assembled monolayer on polycrystalline gold are evaluated as a photochromic surface fluorescence switch. Evidence for SAM-photoresponsiveness was gathered from surface fluorescence and surface reflectance infrared spectroscopic measurements in which changes in the monolayer were monitored before and after irradiation at 350 and 254 nm. Contact angle measurements confirmed the presence of a hydrophobic surface in the self-assembled monolayer Au-1. Electrochemical measurements of the resulting monolayer, before and after irradiation at 350 and 254 nm, indicate that the stable monolayers insulate the gold electrode from electrical contact with redox couples present in a contacting solution. These effects are interpreted by comparison with the solution phase photochemistry of the acetyl precursor of 1.
Introduction Self-assembled monolayers (SAMs) on metal surfaces have received much attention because of the importance of establishing how surface properties can be altered by light and how the behavior of a SAM is affected by its structural rigidity.1,2 SAMs are well-ordered, oriented, stable, and easy to produce and can incorporate a variety of functional groups within the packed array or at the atmosphere-monolayer interface. The density and orientation of the SAM lead to a well-defined interface of known composition, structure, and thickness. These features, together with the SAM’s stability, demonstrate the potential applicability of SAMs in catalysis, corrosion inhibition, lubrication, adhesion, optical imaging, and other applications.1-4 Although SAMs of a variety of substrate-adsorbate combinations have been described, alkanethiolates on gold represent the most thoroughly investigated class of SAMs.1-5 The photoreactivity of anthracene has been known for over a century, dating back to Fritzsche’s isolation of the photodimer in 1869 upon irradiation of the monomer in benzene.8 Anthracene is a planar, highly aromatic molecule containing three cyclic, fused, benzene-like rings. When illuminated with light at 350 nm, anthracene dimerizes at the 9,10 positions, and this photocycloaddition can be reversed by irradiation at 254 nm.10 The structure of the dimer (9,10-dihydrodianthracene) had been postulated as early as 1892 by Linebarger, on the basis of the already-known reactivity of the 9,10 (meso) positions of X Abstract published in Advance ACS Abstracts, December 1, 1997.
(1) Whitesides, G. M.; Folkers, P. J.; Zerkowski, J. A.; Laibinis, P. E.; Seto, C. T. J. Am. Chem. Soc. 1992, 114, 6156. (2) Strong, L.; Whitesides, G. M. Langmuir 1988, 4, 546. (3) Porter, M. D.; Bright, T. B.; Allara, D. L; Chidsey, C. E. D. J. Am. Chem. Soc. 1987, 109, 3559. (4) Ulman, A. Chem. Rev. 1996, 96, 1533. (5) Ulman, A. An Introduction to Ultrathin Organic Films; Academic Press: New York, 1991. (6) Searle, R.; Applequist, D. E. J. Am. Chem. Soc. 1964, 86, 1389. (7) Schottelius, M. J.; Chen, P. J. J. Am. Chem. Soc. 1996, 118, 4896. (8) Fritzsche, J. J. Prakt. Chem. 1867, 101, 333. (9) Linebarger, C. E. Am. Chem. J. 1892, 14, 597. (10) Turro, N. J. Modern Molecular Photochemistry; Benjamin/ Cummings: San Francisco, CA, 1978; Chapter 5.
S0743-7463(97)00894-9 CCC: $14.00
the central ring of anthracene.9 The dimer also can be thermally dissociated at elevated temperatures (335-344 °C) in solution or in the solid state.6 We show here that the well-known solution phase photochemistry of the anthryl group, when examined instead at a monolayer-atmosphere interface, is useful as a probe for the effects of monolayer order and of electronic coupling to the metal surface on excited state reactivity (Figure 1). The photodimerization and dimer photocleavage of an acetyl derivative of a new thioalkyl anthryl ether, 10-(acetylthio)decyl 2-anthryl ether (4), are parallel to those of anthracene itself,6-8 and this reactivity is comparable to changes that occur in a SAM of 1 on gold.
An important goal of this study is to determine whether photochemical manipulation of an organic chromophore as in 1, when present at the terminus of a SAM on a gold surface, can be measured and controlled. Previously, we have demonstrated that nonablative photochemical methods permit alteration of self-assembled monolayers on gold.11-13 In parallel to these photochemical geometric isomerizations and photocycloadditions,13 we now show that the surface absorption and emission spectra of 1 selfassembled onto polycrystalline gold can be switched by ultraviolet irradiation. Although contacting metals frequently quench the excited states of directly absorbed chromophores,14 the observation of substantial photoreactivity of monolayer assemblies containing chromophores placed at some distance from the metal surface establishes that at least some photochemical pathways can compete with this rapid relaxation mode.11-13 Here, we show that indeed a monolayer of 1 on gold can be photochemically altered and that the spectral changes accompanying this partially reversible photodimerization (11) Fox, M. A.; Wolf, M. O. J. Am. Chem. Soc. 1995, 117, 1845. (12) Wolf, M. O.; Fox, M. A. Langmuir 1996, 12, 955. (13) Li, W.; Lynch, V.; Thompson, H. W.; Fox, M. A. J. Am. Chem. Soc., in press. (14) Mo¨bius, D.; Vaubel, G.; Baessler, H. Chem. Phys. Lett. 1971, 10, 334.
© 1997 American Chemical Society
7100 Langmuir, Vol. 13, No. 26, 1997
Figure 1. Photochemical switching in Au-1, a SAM of 1.
can be predictably monitored. The reversible photomodification of a SAM of 1, by irradiation at λ ) 350 nm and then at λ ) 254 nm, was characterized by sessile drop contact angle measurements, fluorescence spectroscopy, and grazing angle surface (reflectance-absorption) FTIR spectra. These measurements indicated that the local structure of a monolayer of 1 on gold can be manipulated photochemically without ablatively destroying the monolayer. Experimental Section General Procedures. Gold surfaces were prepared by argon plasma sputter-deposition of 80 Å of chromium (Cr) as an adhesion promoter, followed by 2000 Å of Au, onto a single silicon(111) wafer (Silicon Sense). Surfaces were placed in deoxygenated ethanol as soon as possible after the nitrogen-filled vacuum chamber was opened, exposing the gold surfaces to the atmosphere for a few minutes at most. The gold substrates were then used immediately for adsorption studies or were stored under degassed ethanol. Gold surfaces not used within a day were cleansed by rinsing with a pirannha solution (3:1 H2O2 (30%)/ concentrated H2SO4), deionized water, and degassed ethanol and were dried with a stream of N2. Once the surfaces were immersed in a beaker of degassed ethanol containing the alkanethiol, the beaker was sealed, wrapped in foil, and left to stand for at least 12 h and sometimes up to 4 days. The monolayered gold surfaces were rinsed sequentially with degassed ethanol, deionized water, and degassed ethanol, before being dried with a stream of N2 prior to characterization. Advancing contact angle measurements with a free-standing droplet of deionized water15 were obtained on a Rame-Hart NRL 100 goniometer at room temperature. All measurements were replicated for three different sections of a given plate and were repeated twice. Cyclic voltammetry was performed on a Bioanalytical Systems (BAS-100) electrochemical analyzer. The bare and covered gold surfaces were used as working electrodes, having an exposed area of 0.5 cm2, with a Pt flag as the counter electrode in a standard three-electrode cell. All potentials were referenced to a saturated calomel reference electrode (SCE). Electrolyte solutions of 0.1 M KCl in ASTM grade water (Millipore), purged with argon, were prepared immediately prior to use. Voltammograms of 1.2 mM Fe(CN)64- in 0.1 M KCl solutions were obtained at 25 °C. The scan rate was at 100 mV/s. Absorption spectra were obtained on a Nicolet 550 FTIR spectrometer fitted with a nitrogen purge and a Spectra-Tech FT-80 grazing angle reflectance accessory. The polarized light was incident to the surface at 80°, with a spectral resolution of 4 cm-1. To minimize the signal-to-noise ratio, 1056 scans were obtained. The reported spectra represent the averages thus acquired. (15) Evans, S. D.; Urankar, E.; Ferris, N.; Ulman, A. J. Am. Chem. Soc. 1991, 113, 4121.
Fox and Wooten Surface fluorescence spectra were obtained on a SPEX Fluorolog 2 fluorometer at an angle of incidence of 11° in the front face mode. This instrument is equipped with a 450 W Xe lamp, a Hammatsu R508 photomultiplier, and double-grating monochromators on both the excitation and emission sides. Spectral resolution was 1 nm. Melting points were determined using a Mel-Temp apparatus and are uncorrected. 1H and 13C NMR spectra were recorded in CDCl3 on a Varian 300 or 500 MHz spectrometer. Chemical shifts were reported in ppm relative to the CHCl3 peak at 7.24 ppm. Absorption spectra were obtained on a Hewlett-Packard 8451A diode array spectrophotometer. Emission spectra were obtained using a SLM Aminco SPF-500C spectrofluorometer. FTIR spectra were acquired on a Nicolet 550 FTIR spectrometer. High-resolution mass spectral analysis, FAB and CI, was performed at the University of Texas on a Finnigan TSQ70 or a Fisons TS270 spectrometer. All solvents used for photochemistry were degassed with N2 for 1 h prior to use. A Rayonet Photochemical Reactor equipped with RPR lamps blazed at 3500 or 2537 Å was used for the photolyses. During irradiation through quartz or Pyrex ampules, a stream of nitrogen was passed through the solution to remove dissolved air. The quartz water jacket covering the ampule was kept at 10 °C during the photodimerization and photodissociation. Irradiation of the monolayers proceeded in a fashion similar to that of the solutions. The monolayered plate was irradiated while inside a Pyrex or quartz tube, which was sealed and purged with N2 for 30 min prior to and during the irradiation. Synthesis. Thiol 1 was prepared by the route shown in Scheme 1. 2-Anthracenol (2),16 prepared from 2-chloroanthracene,17,18 was alkylated with excess 1,10-dibromodecane, producing 10-bromodecyl 2-anthryl ether (3) in 40% yield. Conversion to the corresponding thioacetate, 10-(acetylthio)decyl 2-anthryl ether (4), followed by hydrolysis with potassium hydroxide in 2-propanol, gave 1. Oxidation to the corresponding disulfide or sulfonic acid was avoided by conducting the hydrolysis under Ar.19 All reactions were carried out under an atmosphere of dry oxygen-free Ar using standard Schlenck line techniques. Glassware was dried in an oven (200 °C) and/or with a heat gun prior to use. Benzene and THF were distilled from sodium/benzophenone under nitrogen. All solvents used for photolysis were spectral grade and degassed with N2. The following solvents and reagents were used as received from Aldrich: anthracene, ethanol, 2-propanol, tert-butyl alcohol, dichloromethane (CH2Cl2), ethyl acetate, hexanes, 2-chloroanthracene, 1,10-dibromodecane, borane-THF complex, hydrogen peroxide (30%), potassium hydroxide, and potassium acetate. 2-Anthracenol (2).16 Lithium (20% excess) was rinsed three times with degassed hexanes. 2-Chloroanthracene18 (1.0 g, 4.7 mmol), a borane-THF complex (7 mL), and THF (10 mL) were added, and the mixture was stirred for 2.5 h. Unreacted lithium was removed, and a mixture of 23 mL of H2O2 and 15 mL (3.0 M) of NaOH was added at 0 °C. The solution was stirred for 30 min. Water was added, and the impure product was extracted with CH2Cl2 and purified by flash column chromatography on silica (hexanes/ethyl acetate (4:1)), yielding 2 (657 mg, 72%) as a yellow solid. Mp: 252-254 °C. 1H NMR δ: 8.34 (s, 1 H), 8.20 (s, 1 H), 7.93 (m, 3 H), 7.41 (m, 2 H), 7.13 (dd, 2H, J ) 9.0, 2.6 Hz), 5.17 (s, broad, 1 H). 13C NMR δ: 152.8, 132.6, 132.3, 130.4, 130.3, 128.3, 128.2, 127.6, 126.5, 125.6, 124.5, 123.8, 119.3, 107.6. FTIR (cm-1): OH stretch, 3505 (free hydroxyl)5 and 3356; aromatic CH stretch, 3059; aryl CC ring stretch, 1633, 1483, and 1452; COC stretch, 1205 and 1174; aromatic CH bend, 887. HRMS (m/z) calc for C14H11O (M + H)+, 195.0810; found, 195.0809. 10-Bromodecyl 2-Anthryl Ether (3). A solution of 2 (0.60 mg, 3.1 mmol), 1,10-dibromodecane (3.7 g, 12 mmol), potassium (16) Lide, D. R., Milne, G. W. A., Eds. Handbook of Data on Organic Compounds, 3rd ed.; CRC Press: Ann Arbor, MI, 1994; Vol. 1, p 295. (17) Pickles, G. M.; Thorpe, F. G.; Podesta, J. C. J. Organomet. Chem. 1977, 128, 305. (18) Pouchert, C. J., Behnke, J., Eds. The Aldrich Library of 13C and 1H FT NMR Spectra, 1st ed.; Aldrich Chemical: 1993; Vol. 2, p 174. (19) Pale-Grosdemange, C.; Simon, E. S.; Prime, K. L.; Whitesides, G. M. J. Am. Chem. Soc. 1991, 113, 12.
SAMs of 10-Thiodecyl 2-Anthryl Ether on Gold
Langmuir, Vol. 13, No. 26, 1997 7101
Scheme 1. Preparation of thiol 1
hydroxide (0.19 mg, 3.4 mmol), tert-butyl alcohol (25 mL), and water (3 mL) was heated to reflux for 20 h. Water was added, and impure product was extracted with CH2Cl2. The CH2Cl2 was removed in vacuum, yielding a dark amber oil. Excess dibromodecane was separated from product by column chromatography on silica with hexanes as the eluent. The product was eluted from the column with hexanes/ethyl acetate (4:1), yielding 3 (511 mg, 40%) as a light yellow solid. Mp: 99-100 °C. 1H NMR δ: 8.31 (s, 1 H), 8.23 (s, 1 H), 7.93 (t, 2 H, J ) 10.0 Hz), 7.87 (d, 1 H, J ) 8.9 Hz), 7.40 (m, 2 H), 7.15 (m, 2 H), 4.1 (t, 2 H, J ) 6.4 Hz), 3.39 (t, 2 H, J ) 6.7 Hz), 1.85 (m, 4 H), 1.31-1.51 (m, 12 H). 13C NMR δ: 156.6, 132.8, 132.1, 130.3, 129.7, 128.3, 128.2, 127.5, 126.1, 125.4, 124.3, 124.0, 120.8, 104.2, 67.9, 34.0, 32.8, 29.4, 29.4, 29.4, 29.2, 28.3, 28.1, 26.1. FTIR (cm-1): aromatic CH stretch, 3055; CH stretch, 2934, 2918, and 2851; aryl CC ring stretch, 1633 and 1460; CO stretch, 1211, 1182, and 1012; aromatic CH bend, 887. HRMS (m/z): calc for C24H30OBr (M + H)+, 413.1480; found, 413.1464. 10-(Acetylthioacetyl)decyl 2-Anthryl Ether (4). An Arpurged solution of 3 (500 mg, 1.2 mmol) and potassium thioacetate (140 mg, 1.2 mmol) in ethanol (10 mL) was placed in the flask and heated to reflux for 24 h. The flask was cooled to room temperature, and the solid was washed with ethyl acetate, yielding 4 as a light yellow solid (440 mg, 90%). Mp: 106-108 °C. 1H NMR δ: 8.31 (s, 1 H), 8.23 (s, 1 H), 7.90 (m, 3 H), 7.40 (m, 2 H), 7.15 (m, 2 H), 4.09 (t, 2 H, J ) 6.6 Hz), 2.85 (t, 2 H, J ) 7.4), 2.30 (s, 3 H), 1.86 (m, 4 H), 1.29-1.58 (m, 12 H). 13C NMR δ: 196.0, 156.7, 132.8, 132.2, 130.3, 129.7, 128.3, 128.2, 127.5, 126.1, 125.4, 124.3, 124.0, 120.9, 104.2, 67.9, 30.6, 29.5, 29.5, 29.4, 29.3, 29.2, 29.1, 29.0, 28.8, 26.1. FTIR (cm-1): 3051, 2916, 2847, 1697, 1631, 1579, 1481, 1412, 1393, 1268, 1211, 1180, 1139, 1011, 888, and 845. HRMS (m/z) calc for C24H33OS (M + H)+, 409.2201; found, 409.2204. 10-Thiodecyl 2-Anthryl Ether (1). A solution of thioacetate 4 (400 mg, 1.1 mmol, 1 equiv), potassium hydroxide (68 mg, 1.2 mmol, 1.1 equiv), and 2-propanol (20 mL) was heated to reflux for 2.5 h. Water was added and impure product extracted with CH2Cl2. Rotary vacuum evaporation of the solvent yielded 1 (298 mg, 83%) as a beige solid after recrystallization from THF/ hexanes. Mp: 145-148 °C. 1H NMR δ: 8.30 (s, 1 H), 8.22 (s, 1 H), 7.89 (m, 3 H), 7.39 (m, 2 H), 7.14 (m, 2 H), 4.09 (t, 2 H, J ) 6.5 Hz), 2.50 (q, 2 H, J ) 7.4 Hz), 1.85 (m, 2 H), 1.17-1.58 (m, 15 H). 13C NMR δ: 156.6, 132.8, 132.2, 130.3, 129.7, 128.3, 128.2, 127.5, 126.1, 125.4, 124.3, 124.0, 120.9, 104.3, 67.9, 34.0, 29.5, 29.4, 29.3, 29.2, 29.0, 28.4, 26.1, 24.6. FTIR (cm-1): 3053, 2920,
Scheme 2. Photodimerization of 4
2852, 1633, 1580, 1483, 1460, 1414, 1394, 1271, 1211, 1180, 1142, 1008, 887, 845. λmax (nm) (): 334 (2134), 350 (5564), 371 (4876), 394 (4275). HRMS (m/z): calc for C24H31OS (M + H)+, 367.2096; found, 367.2103. 10-(Acetylthio)decyl 9,10-[2-((10-(Acetylthio)decyl)oxy)anthracene-9,10-diyl]-9,10-dihydro-2-anthryl Ether (5). The irradiation of the acetylthio precursor 10-(acetylthio)decyl 2-anthryl ether (4) in dichloromethane at 25 °C for 60 h at 350 nm led to photodimerization (Scheme 2). (Thiol 1 was not used for these studies because it is susceptible to slow air oxidation to the corresponding disulfide, sulfonic acid, or anthraquinone,12 as determined by 1H NMR and HRMS (C) analysis.) The progress of the photodimerization was monitored by TLC (hexanes/ethyl acetate (5:1)). Upon completion of the irradiation, solvent was removed in vacuo and the residue separated by chromatography (silica, hexanes/ethyl acetate (5:1)), yielding a yellowish white solid as a mixture of dimers 5. TLC indicated three overlapping bands, determined by CI MS to be isomers, but the individual isomers were not isolable. The presence of isomers was also indicated by broadened NMR peaks. 1H NMR δ 6.87 (m, 6 H), 6.77 (m, 6 H), 6.48 (d, 2 H, J )2.4 Hz), 6.28 (dd, 2 H, J ) 9.2 Hz, 2.4 Hz), 4.42 (d, 4 H, J ) 5.2 Hz), 3.75 (m, 4 H), 2.85 (t, 4 H, J ) 7.2), 2.30 (s, 6 H), 1.2-1.9 (m, 24 H).
7102 Langmuir, Vol. 13, No. 26, 1997 FTIR (cm-1): 3079, 3048, 3023, 2931, 2860, 1693, 1672 (s), 1596, 1499, 1479, 1433, 1332, 1312, 1244, 1153, 1122, 1046, 1009, 852. Solution Phase Photolysis of 5. A 10-3 M degassed solution of 5 in CH2Cl2 was irradiated at 254 nm for 24 h using the Rayonet photochemical reactor. Because of the small scale of the reaction, product formation was monitored by fluorescence. Steady state fluorescence spectra of 1 and 4 in benzene show strong emission at 422 nm with fine structure typically observed in substituted anthracenes.6 The observable fluorescence from the mixture of dimers 5 was negligible at the same concentration but reappeared upon irradiation, indicating the photoinduced cleavage of the anthracene dimer, as would be expected upon the reversal of the dimerization.6 HRMS and fluorescence spectroscopy indicated 4 to be present as the major photoproduct. The intensity from the solution mixture indicated that at completion 4 produced by the photocleavage had an intensity that was 45% of that expected after 24 h, and further irradiation showed no increase in intensity. Photolysis of the SAMs. Functionalized monolayers on gold surfaces were placed in sealed Pyrex or quartz tubes (depending on the irradiation wavelength) and purged with N2 for 30 min prior to and during their photolysis. The surfaces were continuously monitored by surface FTIR and surface fluorescence spectroscopy to observe changes induced by the photodimerization and photocleavage (Figure 1).
Results and Discussion Photoresponsiveness of a SAM of 1. Upon immersing a freshly deposited gold surface into a dilute solution of 1, an electrochemically blocking self-assembled monolayer Au-1 was produced (Figure 1). Spectral measurements were used to determine if long wavelength ultraviolet irradiation produces surface-bound dimer, Au-1350. This photodimerization can be reversed by short wavelength ultraviolet irradiation (at 254 nm), so that the resulting monolayer, Au-1254, is chemically similar to Au1, but with some photodecomposition caused by the partial irreversibility of the dimer photocleavage. This photoreactivity is completely parallel to that observed with the acetylthio precursor 10-(acetylthio)decyl 2-anthryl ether (4) in dichloromethane at 350 nm, which also leads to photodimerization (Scheme 2). (Unfortunately, separation of the regioisomeric products 5 proved impossible in the solution phase irradiation.) Short wavelength (254 nm) irradiation of the dimer mixture caused substantial reversion to 4, but for each cycle as much as 50% of the original material was lost. Contact Angle Measurements. Since wetting behavior is very sensitive to surface composition,20 the hydrophobicity of Au-1 before and after irradiation was determined by sessile drop contact angle measurements of a free-standing droplet of water. Whitesides and Nuzzo have shown that the wetting properties of a monolayer are determined by the terminal groups.21,22 As a result, the alkyl chains that result in a close-packed monolayer do not directly affect the wetting behavior. Changes in surface chemistry were found to accompany the photoconversion of a monolayer of 1 on gold (Au-1) to the dimer (Au-1350) and its reversion to monomer (Au-1254) (Figure 1). These contact angle measurements showed that Au-1 was more hydrophobic than was a bare gold surface. A contact angle of 53° was observed for bare gold, in good correspondence with previously reported angles in the (20) Laibinis, P. E.; Whitesides, G. M.; Allara, D. L.; Tao Y.-T.; Parikh, A. N.; Nuzzo, R. G. J. Am. Chem. Soc. 1991, 113, 7152. (21) Waldeck, D. H.; Alivisatos, P. A.; Harris, C. B. Surf. Sci. 1985, 158, 103. (22) Pale-Grosdemange, C.; Simon, E. S.; Prime, K. L.; Whitesides, G. M. J. Am. Chem. Soc. 1991, 113, 12.
Fox and Wooten Table 1. Contact Angle Measurements for Au-1 before and after Irradiation for t hoursa substrate bare gold Au-1 Au-1350 Au-1254
contact angle, θ with H2O (( 2°)
0 0 8 0.75
53 81 86 81
a Irradiation with RPR-phosphor-coated Hg vapor lamps blazed at 350 nm and with uncoated lamps blazed at 254 nm.
30-70° range.23,24 Table 1 summarizes an average of the data obtained at three different sites on the substrate on a total of five separate SAMs. The contact angle obtained for Au-1 (81°) compares well with values reported by Sabatani et al. in the range 80-85° (( 2°) for SAMs of thiophenol, p-biphenyl mercaptan, and p-terphenyl mercaptan.13 Au-1350, obtained by irradiation of Au-1 at 350 nm, had a contact angle θ of 86°, indicating that this photolysis produces a slightly more hydrophobic monolayer. The increased contact angle observed in Au-1350 can be rationalized by the assumption that the sp3-hybridization at the meso positions of the dimer should inhibit delocalization in the terminal aryl group. Conjugated terminal groups are more hydrophilic and, thus, have smaller water contact angles than nonconjugated groups. For example, the water contact angles for monolayers on gold of (C6H5)SH, HS(CH2)17CHdCH2, and long chain alkanethiols are, respectively, 80°,25 107°,24 and 110-115°.20,26 The increase in surface hydrophobicity of Au-1350 over Au-1 is thus consistent with the proposed photodimerization. When Au-1350 was photolyzed at 254 nm, the surface contact angle of the resulting surface, Au-1254, reverted to the initial value observed for Au-1. The observation that Au-1254 has the same contact angle reading as Au-1 indicates at least partial recovery of the initial state of the monolayer. Electrochemical Measurements. Cyclic voltammetry was employed to evaluate the blocking behavior of Au-1 toward the oxidation of Fe(CN)64- present in a contacting solution of aqueous 0.1 M KCl. When a gold surface is covered with a densely-packed nonconducting monolayer, electron transfer between the surface and any electroactive species in an electrolytic solution is suppressed.27 Accordingly, any direct interaction with the solution phase redox couple indicates exposed sites on the gold surface. Figure 2 illustrates that Au-1, Au-1350, and Au-1254 all inhibit the oxidation of Fe(CN)64- present in a contacting electrolyte solution. This blocking of the redox couple suggests dense packing in the monolayer that is not affected by irradiation. Grazing Surface Angle FTIR Measurements. Band assignments for grazing angle FTIR spectra of 1 in KBr and for Au-1 before and after irradiation (Table 2) were based on published work for similar structures.28-30 The similarity of the FTIR spectra of Au-1 and of 1 in a KBr (23) Schrader, M. E.; Loeb, G. I. Modern Approaches to Wettability: Theory and Applications; Plenum Press: New York, 1991. (24) Bain, C. D.; Troughton, E. B.; Tao, Y.-T.; Evall, J.; Whitesides, G. M.; Nuzzo, R. G. J. Am. Chem. Soc. 1989, 111, 321. (25) Sabatani, E.; Cohen-Boulakia, J.; Bruening, M.; Rubenstein, I. Langmuir 1993, 9, 2974. (26) Bertilsson, L.; Lindberg, B. Langmuir 1993, 9, 141. (27) Chidsey, C.; Loiacono, D. Langmuir 1990, 6, 682. (28) Allara, D.; Nuzzo, R. G. Langmuir 1985, 1, 52. (29) Silverstein, R. M.; Bassler, G. C.; Morril, T. C. Spectrometric Identification of Organic Compounds; Wiley: New York, 1991; Chapter 3. (30) Chang, S.-C.; Chao, I.; Tao, Y.-T. J. Am. Chem. Soc. 1994, 116, 6792.
SAMs of 10-Thiodecyl 2-Anthryl Ether on Gold
Langmuir, Vol. 13, No. 26, 1997 7103
Figure 2. Cyclic voltammograms of a 1 mM solution of Fe(CN)64- in degassed aqueous 0.1 M KCl at 25 °C on (a) bare gold, (b) Au-1, (c) Au-1350, and (d) Au-1254. Scan rate ) 100 mV/s.
Figure 4. Grazing angle FTIR spectra of (a) Au-1, (b) Au-1350, and (c) Au-1254. Arrows point to significant spectral differences. Figure 3. FTIR spectra of (a) Au-1 (solid line) and (b) 1 in KBr (dashed line). Table 2. Infrared Assignments (cm-1) for 1 in KBr and Au-1 before and after Irradiation 1 in KBr
3052 2920 2851 1633
3050 2915 2846 1633
3051 2919 2849 1634 1592
3051 2916 2847 1634
arom CH str aliph CH asym str (CH2) aliph CH sym str (CH2) aryl CC ring str
1580 1460 1394 1304 1271 1211 1183
1583 1460 1392 1302 1271 1211 1184
1458 1392 1303 1267 1210 1182 1120 1040 887
1583 1460 1392 1304 1270 1210 1184
CH bend (scissors, CH2) CH bend (in plane, CH2) CH2 twist and wag C-O-C asym str CH2 twist and wag C-O-C sym str arom CH bend
pellet (Figure 3), confirmed that the structural integrity of the molecule is maintained upon monolayer formation. The conformation and orientation of the intervening methylene chains directly affect the packing density of the SAM, as can be judged by the methylene CH stretches in the 2800-2990 cm-1 region.26,30-32 As shown in Figure 3, a shift to higher energies for both asymmetrical (νa) and symmetrical (νs) CH2 stretches in both spectra implies a crystalline-like state for the monolayer.22,23,28 Hence, a well-packed array with fully extended alkyl chains and with the methylene units aligned in a predominantly trans conformation is indicated for Au-1.21,30 (31) Hill, I. R.; Levin, I. W. J. Chem. Phys. 1979, 70, 842. (32) Fan, J.; Trenary, M. Langmuir 1994, 10, 3649.
The relative intensities of these peaks are also indicative of the orientation of the monolayer.21,27,32 On the basis of the selection rules for surface infrared spectroscopy, a mode is infrared-active if its dipole moment projects at a non-zero angle from the surface normal30,32 as a result of vibrations of bonds parallel to the surface. However, since gold is reflective, an opposing effect on these modes causes the measured absorption to decrease.32 Figure 4 shows that the relative intensities of these stretching modes are significantly decreased in the Au-1 spectrum compared with that observed for 1 in KBr. Thus, the axis of the extended polymethylene array deviates slightly from the surface normal, as has been reported previously for other polymethylene chains of ten carbons or more.3,21,26 The aromatic CH stretch (3000-3100 cm-1) shifts to a slightly lower frequency (3052 to 3050 cm-1) when 1 is confined to a monolayer, suggesting that the terminal anthracene groups are not significantly more ordered in the monolayer than in the bulk. The small difference in intensity of this band in Figure 3a and b indicates the plane of the anthracene moieties is not perpendicular to the surface. This is in contrast to previous studies of monolayers of biphenyl and naphthalene derivatives that were judged to be held perpendicular to the surface. In these substrates, the aromatic component is not at the termini of the methylene ether chain but within it, and close to the sulfur head groups. In the 800-1800 cm-1 range, there is no significant difference between the bulk and adsorbate spectra except for the presence of the aromatic CH out-of-plane bending band at 888 cm-1 in Au-1. This mode was not present in previously investigated monolayers of aryl alkanethiolates where the aryl component was parallel to the surface normal.30 Irradiation of SAM Au-1. Infrared spectroscopy could also be used to follow the conversion of Au-1 to Au-1350
7104 Langmuir, Vol. 13, No. 26, 1997
Figure 5. Surface fluorescence of (a) Au-1, (b) Au-1350, and (c) Au-1254, λex > 350 nm. Inset: Solution phase emission spectra of 1 and dimer 6 in 10-4 M solutions in degassed benzene at room temperature.
and then back to Au-1254 (Figure 4 and Table 2). Care was taken to ensure that deaeration was attained with dry N2, as the presence of water impedes the observed conversions. In Au-1350, the aryl CC aromatic stretch modes at 1583 and 1634 cm-1 are attenuated and a new strong peak is observed at 1592 cm-1. The latter is slightly shifted from the 1596 cm-1 peak present in the spectrum of the dimer 5. A new, broad, and intense band appears encompassing shifts at 1123, 1154, 1182, and 1211 cm-1. Upon irradiation of Au-1350 at 254 nm, the spectrum reverts entirely back to that of Au-1, indicating that the product of the photoreversion (Au-1254) has substantial structural similarity to the initial monolayer Au-1. This result affirms the results derived from the electrochemical measurements, namely, that the anthracene dimer partially reverts without significantly destroying monolayer packing. These surface FTIR measurements also clearly indicate largely reversible photochemical manipulation of the monolayer with little, if any, evidence of photodecomposition. This at least partial reversibility argues for the possibility of employing such anthracene-functionalized monolayers as a surface-bound photoinduced switch. Fluorescence Measurements. Monolayer fluorescence was measured by grazing angle surface fluorescence spectroscopy at an incident angle of 11° from the gold surface.13 Upon excitation at 350 nm, strong surface fluorescence was observed for Au-1 at 420 nm, similar to that observed for 1 in dilute solution, (Figure 5). A broad shoulder at 455 nm in Au-1 corresponds to that assigned to anthracene excimer emission.33,34 The intense emission of Au-1 is consistent with a close-packed array in which the fluorophores are held at a distance from the gold surface. (Previously, reports that gold can quench fluorescence of arenes describe aryl groups absorbed directly on the metallic substance.14,35) Therefore, the anthryl chromophores in Au-1 must be organized so that gold cannot quench their emission efficiently. The spectrum of Au-1 also displays more tailing than does a dilute solution of 1, possibly indicating a ground state π-π stacking interaction in addition to excimer formation.36 If so, the anthracene groups must be held in a face-to-face arrangement enforced by the structured environment of the monolayer. Ferguson et al. have described similar spectra for anthracene sandwich dimers in a rigid glass, (33) Hoshino, M.; Imamura, M.; Seki, H.; Yamamoto, S. Chem. Phys. Lett. 1984, 104, 369. (34) Wilkinson, F.; Worrall, D. R.; Williams, S. L. J. Phys. Chem. 1995, 99, 6689. (35) Pineda, A.; Ronis, D. J. J. Chem. Phys. 1985, 83, 5330. (36) Ferguson, J.; Mau, A. W.-H.; Morris, J. M. Aust. J. Chem. 1973, 26, 91.
Fox and Wooten
Figure 6. Excitation spectra of (a) Au-1, (b) Au-1350, and (c) Au-1254; λex ) 420 nm.
explaining the tailing behavior as being due to a “partly dissociated pair” where the anthracenes are aligned in a proper orbital relationship but cannot approach close enough to form excimers. Thus, the observed tailing in the spectrum of Au-1 may be indicative of an unusual anthracene interaction and/or a weak charge transfer emission. The fluorescence excitation spectrum of Au-1 monitored at 420 nm displays bands at 336, 358, and 377 nm, whereas the excitation spectrum of 1 has maxima at 334, 350, 371, and 394 nm (Figure 6). Each band in the monolayer is thus red-shifted from the solution spectra, and the fourth long wavelength band was not observed. These trends parallel those observed in rigid glasses of the anthracene π-stacked dimer. Changes in the fluorecence spectrum of Au-1 were also monitored as the photodimerization and photodissociation took place. Au-1350 displays negligible fluorescence, and its excitation spectrum is similar to the solution phase absorption spectrum of dimers 5 (Figures 5). Au-1254 displays some recovery of fluorescence, but the recovery is much weaker than that in the original Au-1. The emission intensity of Au-1254 after one cycle is approximately 17% of that of Au-1, or about 2/3 that expected from the reversibility of the solution phase photocleavage of 5. Since the surface FTIR spectra indicate excellent recovery of the monomeric species (Figure 4), the emission and changes in Au-1254 can be attributed to restructuring of the monolayer induced by the photolysis cycle, although it is not determined whether the loss of chromophoric activity occurs during photodimerization or photocleavage. Conclusions Spectral characterization indicates that 1 forms a wellordered hydrophobic monolayer on gold. Cyclic voltammetry demonstrated that solution phase electrochemistry was completely blocked in Au-1 and in the monolayers produced in the irradiation cycles. Surface grazing angle FTIR spectra indicate that the terminal anthracene groups are probably held at an angle deviating from parallel to the surface. Surface fluorescence spectra indicate that the terminal anthryl groups are arranged in an unusual environment that promotes π-stacking and/or intermolecular charge transfer interactions. Photodimerization and photocleavage in Au-1 take place in parallel with those same conversions observed in dilute solutions of 4, as indicated by reflection FTIR absorption and fluorescence spectra. The integrity of the monolayer was maintained after irradiation, as shown by cyclic voltammetry and sessile contact angle measurements. These results imply that the terminal group was selectively
SAMs of 10-Thiodecyl 2-Anthryl Ether on Gold
altered without ablative loss of the monolayer. Steady state surface fluorescence measurements (both emission and excitation), however, show substantial loss of intensity after even a single cycle, suggesting an appreciable alteration of the local environment about the chromophore. Although a fully reversible photodimerization of Au-1 would have potential use in photoimaging or as a photoinduced switch, the incomplete recovery of fluorescence makes use of Au-1 as a fully reversible read-write fluorescent device impractical so far.
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Acknowledgment. This research has been supported by the Office of Basic Energy Science of the U.S. Department of Energy. We are grateful to Professor S. E. Webber for the use of his fluorimeter and to Professor J. K. Whitesell for use of his highly sensitive surface FTIR spectrometer.