Photoresponsive Monolayers Containing In-Chain Azobenzene

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Langmuir 1996, 12, 5838-5844

Photoresponsive Monolayers Containing In-Chain Azobenzene L. M. Siewierski, W. J. Brittain,* S. Petrash, and M. D. Foster Institute of Polymer Science, The University of Akron, Akron, Ohio 44325 Received May 23, 1996. In Final Form: September 11, 1996X Azobenzene monolayers on silicon have been prepared by two synthetic methods: (1) chemisorption of triethoxysilanes, and (2) acylation of amine-functionalized self-assembled monolayers with acid chloride derivatives. A series of films were prepared with different methylene spacer lengths and different terminal end groups (pentyl vs hydrogen). The resulting films were characterized using water contact angles, X-ray photoelectron spectroscopy, attenuated total reflectance infrared spectroscopy, and X-ray reflectivity (XR). Despite evidence for successful attachment of azobenzene to the surfaces, film thicknesses as determined by XR suggest uniform, but incomplete, monolayers. Irradiation of the films with 354 nm light effected a decrease in the water contact angle. The largest photoinduced changes in contact angles (9°) were observed for films prepared by acylation and with terminal pentyl groups; for one of these films, XR monitoring of film thicknesses showed a 1 Å increase in film thickness during 354 nm irradiation.

Introduction The trans-cis isomerization of azobenzene is a wellknown photochemical phenomenon.1 Photoirradiation of polymers containing azobenzene either in the backbone or as a pendant group can cause reversible changes in physical properties such as surface wettability, viscosity, and solubility.2-4 These types of specialty polymers containing photochromic groups which induce reversible property changes are termed photoresponsive polymers.5,6 One potential application of photoresponsive polymers is for biomaterials surfaces that have light-controllable surface wettability. Ishihara and co-workers7 synthesized p-phenylazoacrylanilide (PAAn) polymers and copolymers with 2-hydroxyethyl methacrylate (PAAn-HEMA). They observed that photoirradiation of the polymer caused a reduction in water contact angles; subsequent exposure to visible light resulted in complete recovery of the surface wettability. They attributed this observation to a structural change in the azobenzene moiety in the polymer chain. Ishihara and co-workers8,9 also demonstrated that photoresponsive polymers have photocontrolled affinity for protein adsorption. Under ambient lighting conditions (corresponding to the trans configuration of azobenzene), the PAAn and PAAn-HEMA polymers were more hydrophobic and proteins adsorbed to the polymer. Upon photoirradiation, a decrease in protein adsorption was observed, which was attributed to the increased polarity of the polymer surface (trans to cis isomerization of the azobenzene). Our goal in this research is to create well-defined coatings containing azobenzene. We are especially in* To whom correspondence should be addressed: phone, (330) 972-5147; fax, (330) 972-5290; e-mail, [email protected]. X Abstract published in Advance ACS Abstracts, November 15, 1996. (1) Rau, H. In Photochemistry and Photophysics; Rabek, J., Ed.; CRC Press Inc.: Boca Raton, FL, 1990; Vol. II, Chapter 4. (2) Ishihara, K.; Okazaki, A.; Negjishi, N.; Shinohara, I.; Okano, T.; Kataoka, K.; Sakurai, Y. J. Appl. Polym. Sci. 1982, 27, 239. (3) Irie, M.; Hirano, Y.; Hashimoto, S.; Hayashi, K. Macromolecules 1981, 14, 262. (4) Irie, M.; Tanaka, H. Macromolecules 1983, 16, 210. (5) Irie, M. Adv. Polym. Sci. 1990, 94, 27. (6) Irie, M. Pure Appl. Chem. 1990, 62, 1495. (7) Ishihara, K.; Okazaki, A.; Negishi, N.; Shinohara, I.; Okano, T.; Kataoka, K.; Sakurai, Y. J. Appl. Polym. Sci. 1982, 27, 239. (8) Negishi, N.; Ishihar, K.; Shinohara, I.; Okano, T.; Kataoka, K.; Sakurai, Y. Akaike, T. Chem. Lett. 1981, 681. (9) Ishihara, K.; Negishi, N.; Shinohara, I. J. Appl. Polym. Sci. 1982, 27, 1897.

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terested in monolayer films that undergo reversible changes in surface wettability and adhesion upon exposure to controlled wavelengths of light. Self-assembled monolayers, SAMs, formed by spontaneous chemisorption of a surface active reagent are the most attractive type of monolayer method in terms of controlling molecular structure at the surface.10 Thus far, common methods for producing SAMs include alkyltrichlorosilanes and alkyltriethoxysilanes on oxide surfaces,11-14 alkanethiols or disulfides on gold,15-17 alcohols and amines on platinum,18 and carboxylic acids on aluminum oxide.19,20 Our work is focused on the modification of silicate surfaces, and thus we are particularly interested in organosilane deposition. Whitesides and Prime21 studied protein adsorption to SAMs in which the surface chemistry was tailored by coadsorbing mixtures of methyl- and hydroxy-terminated alkanethiols on gold. They found that protein adsorption decreased as the surface became more hydrophilic. Therefore, the affinity of a surface for proteins was shown to be directly tied to surface wettability. There are several reports in the literature on the preparation of organic thin films containing photoresponsive groups. Ichimura and co-workers prepared photoresponsive films by the chemisorption of azobenzenetriethoxysilanes to quartz substrates22,23 or by in situ reactions of functionalized azobenzenes with precoated substrates.24-26 They constructed cells by sandwiching (10) Ulman, A. An Introduction to Ultrathin Organic Films: from Langmuir-Blodgett to Self-Assembly; Academic: San Diego, CA, 1991. (11) Wasserman, S. R.; Tao, Y.-T.; Whitesides, G. M. Langmuir 1989, 5, 1074. (12) Tidswell, I. M.; Rabedeau, T. T.; Pershan, P. S.; Kosowsky, S. D.; Folkers, J. P.; Whitesides, G. M. J. Chem. Phys. 1991, 95, 2854. (13) Balachander, N.; Sukenik, C. N. Langmuir 1990, 6, 1621. (14) Margel, S.; Sivan, O.; Dolitzky, Y. Langmuir 1991, 7, 2317. (15) Bain, C. D. Ph.D. Thesis, Harvard University, 1989. (16) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J. Am. Chem. Soc. 1987, 109, 3559. (17) Whitesides, G. M.; Laibinis, P. E. Langmuir 1990, 6, 87. (18) Troughten, E. B.; Bain, C. D.; Whitesides, G. M.; Nuzzo, R. G.; Allara, D. L.; Porter, M. D. Langmuir 1988, 4, 365. (19) Allara, D. L.; Nuzzo, R. G. Langmuir 1985, 1, 45. (20) Allara, D. L.; Nuzzo, R. G. Langmuir 1985, 1, 52. (21) Prime, K. L.; Whitesides, G. M. Science 1991, 252, 1164. (22) Ichimura, K.; Suzuki, Y.; Seki, T.;Hosoki, A.; Aoki, K. Langmuir 1988,4, 1214. (23) Aoki, K.; Seki, T.; Suzuki, Y.; Tamaki, T.; Hosoki, A.; Ichimura, K. Langmuir 1992, 8, 1007. (24) Kawanishi, Y.; Tamaki, T.; Sakuragi, M.; Seki, T.; Suzuki, Y.; Ichimura, K. Langmuir 1992, 8, 2601.

© 1996 American Chemical Society

Photoresponsive Monolayers

liquid crystals between two quartz plates modified with azobenzene units and found that the alignment of liquid crystals could be regulated by the isomerization of the azobenzene moiety. Knoll and co-workers27 have studied the absorption spectra of azobenzene monolayers on quartz slides using linearly polarized light. The azobenzene films exhibited a persistent dichroism which depended on irradiation time; the dichroism could be written/erased for several cycles. Mirkin and co-workers28 reported the structural characterization and electrochemical properties of SAMs prepared from azobenzene-containing alkanethiols on gold substrates. From atomic force microscopy and X-ray diffraction measurements, they found that the SAMs formed a highly ordered structure and that the azobenzene group plays a large role in dictating the overall monolayer structure. Wolf and Fox29 reported the incorporation of stilbene moieties into thiols and the preparation of monolayers from these materials on gold substrates. The surface wettability could be altered depending upon the wavelength of light and photopatterning of the stilbeneSAMs was demonstrated. In this report, we describe the preparation of thin films which contain azobenzene. We have applied multiple surface analytical techniques to the characterization of these thin films. The photoresponsive nature of these films has been probed by contact angle changes. Results and Discussion We have prepared monolayer assemblies containing azobenzene on silicate substrata using two methods: direct deposition of alkyltriethoxysilanes and covalent attachment of azobenzene derivatives to functionalized SAMs. Self-assembly was chosen for surface preparation in the hope of making well-ordered and reproducible surfaces. Our initial attempts at preparing photoresponsive monolayers involved the direct chemisorption of an alkyltrichlorosilane containing azobenzene on silicon.30 Analytical data for these azobenzene films were not consistent with a well-defined structure. For example, experimental carbon/nitrogen (C/N) ratios (X-ray photoelectron spectroscopy, XPS) were approximately three times greater than the calculated values. In addition, film thicknesses measured by X-ray reflectometry (XR) were approximately twice the calculated value, which suggested multilayer formation. One of the issues in the synthesis of trichlorosilane-functionalized azobenzene was the difficulty in purifying the final product. The difficulty in obtaining high-purity trichlorosilane and lack of reproducibility among different monolayer samples prompted an investigation into alternative synthetic methods for azobenzene SAMs. The first method is the direct deposition of alkyltriethoxysilanes containing azobenzene (Scheme 1). These alkoxysilanes were prepared by a reaction between azobenzenecarboxylic acid derivatives and (3-aminopropyl)triethoxysilane in the presence of 1,3-dicyclohexylcarbodiimide (DCC). We prepared a series of triethoxysilanes in which the terminal group (hydrogen vs n-pentyl) and spacer length between the azobenzene and (25) Aoki, K.; Seki, T.; Sakuagi, M.; Ichimura, K. Makromol. Chem. 1992, 193, 2163. (26) Aoki, K.; Ichimura, K.; Tamaki, T.; Seki, T.; Kawanishi, Y. Kobunshi Ronbunshu 1990, 47, 1990. (27) Sekkat, Z.; Wood, J.; Geerts, Y.; Knoll, W. Langmuir 1995, 11, 2856. (28) Caldwell, W. B.; Campbell, D. J.; Chen, K.; Herr, B. R.; Mirkin, C. A.; Malik, A.; Burbin, M. K.; Dutta, P.; Huang, K. G. J. Am. Chem. Soc. 1995, 117, 6071. (29) Wolf, M. O.; Fox, M. A. J. Am. Chem. Soc. 1995, 117, 1845. (30) Siewierski, L. M.; Brittain, W. J. Polym. Prepr., Am. Chem. Soc., Div. Polym. Chem. 1994, 35, 490.

Langmuir, Vol. 12, No. 24, 1996 5839 Scheme 1

the triethoxysilane functionality (2, 5, or 10 methylene carbons) were varied. Purification of the triethoxysilanes was not attempted; on the basis of 1H NMR, we estimated purity at 90% with unreacted starting material present. Because the starting material is not a surface-active reagent, we did not regard its presence as a problem for monolayer formation. One goal was to optimize the magnitude of photoinduced film changes by varying the location of the azobenzene chromophore in the film. Atomistic simulations of SAMs containing azobenzene have indicated film thickness changes induced by transto-cis isomerization of the azobenzene are influenced by the relative location of the azobenzene within the chain.31 Ichimura and co-workers22,23 prepared a series of monolayers from triethoxysilanes containing azobenzene using a similar method. In more recent work, Ichimura and co-workers33 prepared triethoxysilanes by the reaction of an azobenzene acid chloride with (3-aminopropyl)triethoxysilane; they reported better yields with this reaction compared to the reaction of the azobenzenecarboxylic acids. However, we obtained good yields of the desired azobenzenetriethoxysilanes by the DCC coupling method. The focus of Ichimura’s work was device-oriented, and they did not fully characterize their films. We characterized our films using contact angles, attenuated total reflectance (ATR)-IR spectroscopy, X-ray photoelectron spectroscopy (XPS) and X-ray reflectivity (XR). Advancing and receding water contact angles were measured for the entire series of monolayers. The contact angle hysteresis for these monolayers was in the range of 16-24°. Because we used sessile contact angles to study the photoisomerization, we are reporting only sessile values here. Table 1 contains the sessile contact angles for the series of azobenzene films prepared by triethoxysilane deposition. The sessile contact angles for these azobenzene monolayers are similar in magnitude to the values reported by Wolf and Fox29 for a cis-stilbene film (θ ) 60°) and by Mathauer and Frank32 for monolayers prepared from naphthyl-terminated trichlorosilanes (θ ) 85°). Also, the contact angles were comparable with SAMs (31) Xing, L.; Mattice, W. L. Langmuir 1996, 12, 3024. (32) Mathauer, K.; Frank, C. W. Langmuir 1993, 9, 3002. (33) Ichimura, K.; Akiyama, H.; Kudo, K.; Ishizuki, N.; Yamamura, S. Liq. Cryst. 1996, 20, 423.

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Table 1. Water Contact Angles (deg)a for Films Prepared by the Chemisorption of Azobenzene-Containing Triethoxysilanes sampleb

θ, as prepared

θ, (hν)c

θ, (visible)d

∆θe

5Az10TES 5Az5TES 5Az2TES HAz10TES HAz5TES HAz2TES

76 ( 2 84 ( 1 77 ( 1 72 ( 2 72 ( 1 75 ( 2

73 ( 1 79 ( 1 73 ( 1 69 ( 1 69 ( 1 73 ( 1

75 ( 2 84 ( 2 76 ( 2 72 ( 2 73 ( 1 75 ( 3

3 5 4 3 3 2

a Sessile contact angles. b See Scheme 1 for sample acronyms. Water contact angle after 10 min exposure to 354 nm irradiation. d Water contact angle after 24 h under ambient laboratory conditions. e Change in contact angle. c

Figure 2. X-ray reflectivity for 5Az10TES film prepared by triethoxysilane chemisorption to a silicon wafer.

Figure 1. ATR-IR spectrum of 5Az10TES film prepared by triethoxysilane chemisorption to an ATR silicon crystal. Table 2. XPS Carbon-Nitrogen Ratio (C/N) for Films Prepared by the Chemisorption of Azobenzene-Containing Triethoxysilanes samplea

C/N observedb

C/N calculated

5Az10TES 5Az5TES 5Az2TES HAz10TES HAz5TES HAz2TES

7.0 ( 0.5 9(1 7.3 ( 0.5 7.2 ( 0.5 7.2 ( 0.5 6.6 ( 0.5

10.3 8.7 7.7 8.7 7.0 6.0

a See Scheme 1 for acronym definitions. b Experimental error based on 0.2 atom % accuracy for the nitrogen and carbon peaks.

prepared from azobenzene-containing trichlorosilanes30 and with those reported by Ichimura22,23 and others for similar azobenzene monolayers. For a hexyl-terminated azobenzene monolayer prepared by the chemisorption of a triethoxysilane derivative (similar to 5Az5TES) on quartz, Ichimura and co-workers34 reported a sessile contact angle of 80°. Ichimura and co-workers34 reported a sessile contact angle of 64° for a film prepared from HAz5TES. We obtained slightly higher contact angles for the analogous systems. The ATR-IR spectrum of the monolayer prepared by the chemisorption of 5Az10TES on a ATR silicon crystal is shown in Figure 1. Band assignments were made as follows: ν(NH) ) 3250, ν(CdC) ) 3074, νa(CH2) ) 2928, νs(CH2) ) 2854, ν(CdO) ) 1617 cm-1. Similar spectra were obtained for the other triethoxysilanes. The XPS spectra contained a peak at 399 eV, which corresponds to the 1s nitrogen ionization. The experimental and calculated C/N ratios (Table 2) are in a reasonable agreement for the entire series except for the film prepared from 5Az10TES for which the experimentally observed C/N ratio is greater than that predicted. While contact angles, XPS, and ATR-IR provide evidence for films containing covalently attached azobenzene, they (34) Aoki, K.; Kawanishi, Y.; Seki, T.; Sakuragi, M.; Tamaki, T.; Ichimura, K. Liq. Cryst. 1995, 19, 119.

do not provide information regarding the surface coverage of azobenzene. Comparison of the experimental film thickness determined via XR with the calculated film thickness provides information about the ordering of the monolayer. It is worth noting that most of the previous work on monolayers containing in-chain chromophores did not examine monolayer ordering in detail. XR data for the film prepared from 5Az10TES is shown in Figure 2 along with the scattering length density (b/V) profile for a model curve which closely matches the experimental data. The reflectivity technique is very sensitive to SAM uniformity, roughness of interfaces, and the overall monolayer thickness, though it probes typically a sample area on the order of 1 cm2 and therefore provides a global, average picture. Qualitative and semiquantitative statements about the film structure are available from simple analysis of the data, especially for a specimen belonging to a class of samples with which the researcher has prior experience. Detailed information is obtained by nonlinear regression of the data. It is not possible, in general, to invert the data to give the scattering length density profile of the sample. Rather, as in almost all scattering techniques, a candidate model for the structure is assumed and parameters describing this model are varied to obtain a best fit to the experimental data. The models considered are constrained by knowledge of the sample composition and deposition parameters. In the case at hand, there is no reason to expect a very complex variation in scattering length density within the layer. The scattering length density throughout the film has to lie within a range characteristic of a predominantly hydrocarbon layer. With these constraints, the overall film thickness and overall film roughness can only vary within small limits for physically reasonable models. We were unable to find a model with an uniform density through the entire film thickness which could fit the data. Therefore a somewhat less dense region (8 Å thickness) has been envisioned next to the surface in the model shown. This is plausible due to the difference in packing and mass density of the azobenzene moiety in the chain and the terminal alkyl section. It is also possible that there is some lateral variation in the film thickness and that this is reflected by the failure of a perfectly uniform film model to fit the data. A complete characterization of the internal structure of the film is beyond the scope of the present study. However, had the sample been grossly nonuniform in thickness, for example as happens in ill-defined multilayer deposition, this would have been readily apparent from the XR data. For the model shown, the overall thickness is 23 ( 2 Å and the air/film roughness is 2-3 Å (root-mean-squares),

Photoresponsive Monolayers

and these are the findings of central interest. This thickness is considerably less than a theoretical thickness of 35 Å, which one would calculate for this molecule in a monolayer assuming a tilt angle of 20°.35 This discrepancy between calculated and experimental thicknesses probably reflects incomplete monolayer formation. Assuming film thickness scales with coverage, a rough estimate of film coverage would be 60%. Photoisomerization was effected via irradiation with a high-intensity UV lamp (354 nm). Photoinduced changes in the films were studied using contact angles. Sessile water contact angles (θ) were measured after 10 min of exposure (Table 1). While a small decrease in contact angle was observed upon irradiation, the change was close to experimental error for all samples. After UV exposure, the films were allowed to stand under ambient laboratory conditions for 24 h. The contact angles returned to their original values (Table 1). Films prepared from 5Az5TES exhibited the highest initial water contact angle and exhibited the largest change upon photoisomerization (∆θ ) 5°). The photoinduced decrease in the contact angle may be a reflection of increased disorder in the film and/ or the increase in the dipole moment of the azobenzene moiety upon trans-to-cis isomerization.1 Ichimura and co-workers34 reported a 6° decrease in the water contact angle for the 354 nm irradiation of a film prepared from the hexyl-terminated analog of 5Az5TES; this was the largest photoinduced change in contact angle reported by Ichimura for triethoxysilane films. The small magnitude of the photoinduced change in wettability prompted an investigation into an alternative method of preparation for the photoresponsive films. We decided to explore the in situ reaction of azobenzene derivatives with functionalized and well-ordered SAMs. This method benefits from the use of high-purity azobenzene derivatives (recrystallization was possible in most cases and purity exceeded 95% for most azobenzene derivatives) in contrast to the trichloro- and triethoxysilanes. The in situ reaction investigated was an acylation of an amine-functionalized SAM with an azobenzene acid chloride derivative (Scheme 2). Ichimura and coworkers24-26 studied the coupling of acid chlorides to silicate substrates derivatized with (3-aminopropyl)triethoxysilane. However this method suffers from complications associated with the (3-aminopropyl)triethoxysilane deposition; depending on conditions, widely different film structures can be obtained.36 For example, there is a competition for chemisorption between the amine and triethoxysilane; also, multilayer formation is common.36 We decided to improve on this method by starting well-defined amine-functionalized SAMs. The amine-functionalized SAMs were prepared by the sequential reaction of a bromine-terminated SAM with sodium azide and then lithium aluminum hydride according to literature procedures.37 The progress of these reactions was followed using ellipsometry, ATR-IR, XPS, and contact angles.38 The results were consistent with our previous work.39,40 The film thickness of the bromineand amine-functionalized SAMs was measured using XR. The bromine-functionalized SAM has an experimental (35) Wasserman, S. R.; Tao, Y.-T.; Whitesides, G. M. Langmuir 1989, 5, 1074. (36) Vandenberg, E. T.; Bertilsson, L.; Leidberg, B.; Uvdal, K.; Erlandsson, R.; Ewling, H.; Lundstrom, I. J. Colloid Interface Sci. 1991, 47, 103. (37) Balachander, N.; Sukenik, C. N. Langmuir 1990, 6, 1621. (38) Siewierski, L. M. Ph.D. Thesis, The University of Akron, 1995. (39) Vogler, E. A.; Graper, J. C.; Harper, G. R.; Sugg, H. W.; Lander, L. M.; Brittain, W. J. J. Biomed. Mater. Res. 1995, 29, 1005. (40) Liebmann-Vinson, A.; Lander, L. M.; Foster, M. D.; Brittain, W. J.; Vogler, E. A.; Majkrzak, C. F.; Satija, S. Langmuir 1996, 12, 2256.

Langmuir, Vol. 12, No. 24, 1996 5841 Scheme 2

Table 3. Water Contact Angles (deg)a for Films Prepared by the Coupling Reaction of Azobenzene Acid Chlorides with Amine-Functionalized SAMs sampleb

θ, as prepared

θ, (hν)c

θ, (visible)d

∆θe

5Az10C11 5Az5C11 5Az2C11 HAz10C11 HAz5C11 HAz2C11

85 ( 1 92 ( 1 63 ( 1 75 ( 1 75 ( 1 62 ( 2

76 ( 2 84 ( 1 58 ( 3 70 ( 2 69 ( 1 58 ( 1

85 ( 1 89 ( 2 62 ( 1 76 ( 1 75 ( 1 61 ( 4

9 9 5 5 5 4

a Sessile contact angles. b See Scheme 2 for sample acronyms. Water contact angle after 10 min of exposure to 354 nm irradiation. d Water contact angle after 24 h under ambient laboratory conditions. e Change in contact angle. c

thickness of 18 ( 1 Å compared to a calculated thickness of 19 Å (tilt angle of ≈20°). The film thickness of the amine-functionalized SAM was 15 ( 1 Å, which is slightly lower than the calculated thickness of 18 Å. On the basis of the surface analytical results, we conclude that the amine-functionalized SAMs are well-ordered monolayers with a high degree of functionality. Azobenzene carboxylic acid chloride derivatives were synthesized by a Williamson ether synthesis of azobenzene phenols with ω-halocarboxylic acids and a subsequent reaction with thionyl chloride. The derivatives obtained were in high purity as confirmed by NMR and IR spectroscopy. A series of azobenzene films were prepared (Scheme 2). Coupling reactions of the acid chloride with the amine-functionalized SAMs were done in an inert atmosphere by placing freshly prepared SAMs in a benzene solution of the azobenzene acid chloride and pyridine. Coupling reactions were conducted for 10 h at 60-70 °C. The coupling reactions were monitored by contact angles, XPS, ATR-IR, and XR. Sessile contact angles are given in Table 3 for the coupled azobenzene films. The pentyl-terminated systems with the 5- and 10-carbon spacers (5Az5/C11 and 5Az10/C11) had higher contact angles than any of the corresponding films from the triethoxysilanes. This synthetic method of using a preformed SAM as the template for azobenzene derivatization must lead to a structurally different system

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Figure 3. ATR-IR spectrum of 5Az10/C11 film prepared by acylation of amine-functionalized monolayer. Table 4. XPS Carbon-Nitrogen Ratio (C/N) for Films Prepared by the Coupling Reaction of Azobenzene Acid Chlorides with Amine-Functionalized SAMs samplea

C/N observedb

C/N calculated

5Az10C11 5Az5C11 5Az2C11 HAz10C11 HAz5C11 HAz2C11

17 ( 2 13 ( 1 11 ( 2 14 ( 2 10 ( 1 11 ( 1

13.0 11.3 10.3 11.3 9.7 8.7

a See Scheme 2 for acronym definitions. b Experimental error based on 0.2 atom % accuracy for the nitrogen and carbon peaks.

compared to the triethoxysilane, as evidenced by the higher contact angles. The other coupled azobenzene films had similar contact angles as the triethoxysilane films. ATRIR spectra for the coupled azobenzene films were consistent with the anticipated structures; like the triethoxysilane systems, the presence of absorptions for the amide linkage was taken as proof of structure. Figure 3 displays the ATR-IR spectrum for the 5Az10/C11 film; bands at 3292 (N-H) and 1646 (CdO) cm-1 are assigned to the amide function. Table 4 contains XPS data for the coupled azobenzene films. The calculated C/N ratio assumes quantitative coupling of terminal amine groups. There is reasonable agreement between experimental and calculated C/N ratios for all of the films except 5Az10/C11. We cannot account for the higher C/N ratio for the 5Az10/C11 film. The film thickness of the 5Az10/C11 film was determined by XR. The measured film thickness increased from 15 ( 1 Å (details on the XR of the amine-functionalized monolayer can be found in a previous publication40) to 28 ( 2 Å after acylation. For a fully extended chain, the calculated film thickness is 43 Å assuming a ≈20° tilt angle. Assuming the film thickness scales linearly with percent coverage (which is equivalent to the yield of the coupling reaction), we estimate 50% surface coverage for this film. Results for the photoinduced changes in water contact angles are given in Table 3. Similar to the results for the triethoxysilanes, exposure to 354 nm light caused a decrease in the water contact angle. The 5Az10/C11 and 5Az5/C11 films displayed the largest photoinduced changes in wettability. The other samples in Table 3 displayed modest changes in contact angle with magnitudes close to experimental error (similar to the results in Table 1 for the triethoxysilanes). Exposure of the azobenzene mono-

Figure 4. X-ray reflectivity before and during exposure to 354 nm light for 5Az10/C11 film prepared by acylation of aminefunctionalized monolayer.

layers to a second dose of UV light resulted in a similar decreases in water contact angles; thus, the photoresponsive effect was reproducible. The 5Az10/C11 and 5Az5/C11 azobenzene monolayers displayed a photoresponsive effect nearly twice that seen by Ichimura and co-workers.22,23,34 Even though a 5° change in water contact angle is a modest photoinduced change in the wettability, Ichimura and coworkers used this type of monolayer system to effectively control large-scale liquid crystal alignment in sandwich displays.24-26 The effect of the photoinduced trans-cis isomerization on the film thickness was probed by XR for the 5Az10/C11 film. Upon solution photoisomerization of azobenzene, the distance between para carbon atoms decreases from 9 to 5.5 Å.41 Modeling of azobenzene monolayers has predicted ≈3 Å changes in the position of terminal groups upon trans-to-cis isomerization.31 We expected that the film thickness of our azobenzene films would decrease upon irradiation. However, we observed an increase of ∼1 Å in the film thickness upon exposure to 354 nm light as shown in Figure 4. This increase was evidenced by a shifting of the two minima to the left. A larger spacing between some of the XR data points was used to minimize exposure of the film to the high-energy X-rays before the UV irradiation. Since the UV lamp had to be left on while measuring the curve marked “during”, it was also desirable to reduce the length of that measurement. Nonetheless, the positions of the first and second minima were still sufficiently well resolved to discern whether the film thickness grew or decreased with irradiation. This observed film thickness change was reproducible and irreversible (in contrast to the reversible changes in contact angles). At this time we do not have an explanation for this effect. One may ask whether the shift in the positions of the minima in Figure 4 can be ascribed solely to thickness changes, and this was checked. While a sufficiently large increase in the density of the film with a decrease in thickness would lead to a slight shift of the first minimum to the left, such a change would also affect the height of first minimum. The height of the first minimum does not change, though, and the behavior of the second minimum is also not matched by this scenario. The observed behavior can only be explained by increasing the thickness and correspondingly decreasing the film density so as to conserve the total mass of the film. (41) Delang, J. J.; Robertson, J. M.; Woodward, I. Proc. R. Soc. London, Sect. A 1939, 171, 398.

Photoresponsive Monolayers

Interestingly, the 5Az10/C11 film was one of two films which displayed the largest photoinduced wettability change; also, this was the one film for which the largest deviation was observed in the experimental vs calculated XPS C/N ratios. On the basis of the XR data, the azobenzene coverage of this film is 50%. Perhaps, the unexpected photoinduced increase in film thickness is related to this loose packing of the azobenzene units. Currently, we are studying mixed monolayers (composed of azobenzene chains terminated with methyl groups and polymethylene chains terminated with carboxylic acid groups) in which the fraction of azobenzene-containing chains is systematically varied to study the influence of neighboring chain effects on photoisomerization. In addition, we are attempted to prepare systems with a higher coverage of azobenzene chains for XR studies of photoinduced film thickness changes. Summary We have prepared azobenzene monolayers by two synthetic methods. Detailed characterization of these films by contact angles, XPS, ATR-IR, ellipsometry, and XR have revealed imperfect structures which presumably result from the inefficiency of some synthetic steps. For example, XR consistently reveals film thicknesses that are lower than those anticipated for monolayer structures containing fully trans-extended polymethylene chains and trans-azobenzene. ATR-IR and XPS data are consistent with the covalent attachment of azobenzene groups to silicate surfaces. Exposure of these azobenzene monolayers to 354 nm light decreased the water contact angles. Although the effect was small (2-9° changes in contact angles), the effect was reproducible. Two of the azobenzene monolayer films produced the largest photoinduced changes in water contact angles. These corresponded to systems prepared by coupling to an amine-functionalized SAM. An unexplained experimental observation is the increase in film thickness (as measured by XR) upon photoirradiation. We are currently studying this effect in more detail. Experimental Section Silicon wafers were obtained from Semiconductor Processing, Inc., and were cut to size using a diamond-tipped glass cutter. Silicon ATR crystals (10 × 5 × 1 mm, 25 × 5 × 2 mm, and 25 × 5 × 1 mm) were obtained from Harrick Scientific. Quartz slides (45 × 12 × 1 mm) were obtained from Precision Glass, Inc. (3-Aminopropyl)triethoxysilane (Hu¨ls America) was purified by vacuum distillation (bp 120-125 °C at 30 mmHg). 4-Phenylazophenol (Aldrich, 98%) was recrystallized from either 95% ethanol or benzene. 3-Bromopropionic acid (Aldrich, 97%) was recrystallized from carbon tetrachloride. 6-Bromohexanoic acid (Aldrich, 98%) was recrystallized from petroleum ether. Undecylenic bromide (Pfaltz & Bauer) was purified by Kugelrohr distillation (bp ) 130-135 °C at 10 mmHg). Hexadecane (Aldrich, 99%) was percolated twice through neutral Brockmann I aluminum oxide (Aldrich) and stored over 4 Å molecular sieves (Aldrich). Tetrahydrofuran (Aldrich, anhydrous) was distilled from Na0/benzophenone prior to use. Hexanes (Fisher, reagent grade) were distilled from calcium hydride (Aldrich) prior to use. All other reagents were purchased from either Aldrich or Fisher and used as received. All glassware was cleaned in a KOH/isopropyl alcohol bath for 24 h and then rinsed with copious amounts of hot water and distilled water followed by oven drying overnight. Substrate Preparation. Silicon substrates were cleaned by either plasma or “piranha” etching. Silicon wafers for X-ray reflectometry and small silicon wafers were cleaned by “piranha” etching as described by Wasserman et al.42 Plasma cleaning (42) Wasserman, S. R.; Whitesides, G. M.; Tidswell, I. M.; Ocko, B. M.; Pershan, P. S.; Axe, J. D. J. Am. Chem. Soc. 1989, 111, 5852.

Langmuir, Vol. 12, No. 24, 1996 5843 was performed by exposure to an argon plasma (Harrick Scientific Corp. PDC-32G plasma cleaner, 60 W) for 15-20 min. To remove monolayers from ATR crystals for reuse, prolonged exposure (4560 min) to an argon plasma was necessary. Substrates cleaned by argon plasma etch were used immediately. Synthesis of Azobenzene Carboxylic Acid Derivatives. 3-[4-[(4′-Pentylphenyl)azo]phenoxy]propionoic Acid. The synthesis of 4[4′-(n-pentylphenyl)azo]phenol (5AzOH) has been described previously.30 To a mixture of 5AzOH (5.1 g, 19.0 mmol) and powdered potassium hydroxide (2.4 g, 42.4 mmol) in absolute ethanol (150 mL), 3-bromopropionic acid (3.5 g, 23.1 mmol) was added and the mixture was refluxed for 6 h. After cooling to room temperature, the inorganic salts were removed by filtration. The filtrate was acidified with acetic acid (40 mL), and the mixture was refluxed for 30 min. The solvent was removed in vacuo and chloroform (200 mL) was added. The organic layer was washed three times with water and dried over sodium sulfate. Recrystallization from acetonitrile afforded 1.42 g of orange crystals, 22% yield: mp ) 150-152 °C; 1H NMR (200 MHz, DMSO-d6, TMS) δ (ppm) 0.90 (t, 3H, CH3), 1.30 (m, 4H, CH2), 1.60 (m, 2H, C6H4-CH2-CH2), 2.65 (t, 2H, C6H4-CH2), 2.75 (t, 2H, C6H4O-CH2-CH2-COOH), 4.30 (t, 2H, OCH2), 7.10, 7.35, 7.75, 7.85 (d, 8H, aromatic); 13C NMR (200 MHz, DMSO-d6, TMS) δ (ppm) 13.57, 21.95, 30.46, 30.91, 34.02, 34.98, 64.21, 115.24, 122.58, 124.70, 129.47, 146.08, 146.61, 150.83, 161.29, 172.54; IR (solution, CCl4) 3047(w), 2958, 2929, 2858, 1720, 1601, 1500, 1248 cm-1. 11-[(4-Phenylazo)phenoxy]undecanoic Acid. 11-Bromoundecanoic acid (4.53 g, 17.1 mmol) was added to a mixture of potassium hydroxide (2.01 g, 36.0 mmol) and 4-phenylazophenol (2.74 g, 13.8 mmol) in absolute ethanol. Acidification and recrystallization from acetonitrile afforded 2.92 g of orange crystals in 55% yield: mp ) 116-119 °C; 1H NMR (200 MHz, CDCl3, TMS) δ (ppm) 1.20-1.90 (m, 22H, CH2), 2.30 (t, 2H, CH2COOH), 4.00 (t, 2H, OCH2), 6.95, 7.45, 7.85 (d, 9H, aromatic); 13C NMR (200 MHz, CDCl and DMSO-d ,TMS) δ (ppm) 24.14, 3 6 25.16, 28.33, 28.51, 28.65, 33.45, 67.67, 114.25, 122.00, 124.26, 128.63, 129.97, 146.17, 152.17, 161.39, 175.35; IR (solution, CCl4) 3041(w), 2916, 2848, 1709, 1602, 1498, 1250 cm-1. These are representative preparations, the other azobenzene carboxylic acid derivatives were synthesized in a similar manner (details are available in ref 38). Synthesis of Alkyltriethoxysilanes. N-(3-Triethoxysilylpropyl)-6-[(4-(4′-pentylphenyl)azo)phenoxy]undecanecarboxamide (5Az10TES). In an argon-purged flask, 11[(4-phenylazo)phenoxy]undecanoic acid (0.91 g, 2.02 mmol) was dissolved in anhydrous dichloromethane (15 mL). (3-Aminopropyl)triethoxysilane (0.43 g, 1.95 mmol) was added, and the mixture was stirred for 10 min. 1,3-Dicyclohexylcarbodiimide (0.42 g, 2.03 mmol) in dichloromethane (10 mL) was added, and stirring was continued at room temperature overnight. The urea byproducts were removed by filtration and the solvent was evaporated to afford 1.09 g of orange solid in 86% yield. The product was confirmed by the presence of the secondary amide band at 3200 and 1650 cm-1 in the infrared spectra. The product was used without further purification. 1H NMR analysis of the crude product revealed approximately 90% purity with the starting materials as the primary contaminant. This is a representative preparation. The other azobenzene triethoxysilane derivatives were synthesized in a similar manner (details are available in ref 38). Formation of SAMs by Deposition of Triethoxysilanes. The deposition solution was prepared by dissolving 0.5 g of an azobenzene-containing triethoxysilane in dry toluene (100 mL) which had been bubbled with argon prior to use. Cleaned silicon or quartz substrates were placed in 20 mL scintillation vials, covered with the deposition solution, and heated in a sand bath at 60-80 °C for 3 h. The samples were removed from the warm solution, rinsed in fresh toluene, and allowed to cure under ambient conditions for 24 h. After curing, the wafers were sonicated in toluene and dichloromethane followed by argon drying. Synthesis of Azobenzene Acid Chloride Derivatives. 3-[4-[(4′-Pentylphenyl)azo]phenoxy]propionoyl Chloride. Thionyl chloride (9.0 mL, 123 mmol) was added to 3-[4-[(4′pentylphenyl)azo]phenoxy]propionoic acid (1.2 g, 3.68 mmol), and the mixture was refluxed for 5 h under argon. Upon cooling, the

5844 Langmuir, Vol. 12, No. 24, 1996 mixture was stirred overnight. The excess thionyl chloride was removed by short path distillation, and the product was placed under vacuum overnight. Complete conversion was confirmed by monitoring the disappearance of the IR band for the carboxylic acid carbonyl at 1710 cm-1 and the appearance of the acid chloride band at 1800-1810 cm-1. Quantitative yields were obtained: 1H NMR (200 MHz, CDCl , TMS): δ (ppm) 0.90 (t, 3H, CH ), 1.30 3 3 (m, 4H, CH2), 1.60 (m, 2H, C6H4-CH2-CH2), 2.65 (t, 2H, C6H4CH2), 3.35 (t, 2H, C6H4-O-CH2-CH2-COCl), 4.30 (t, 2H, OCH2), 7.10, 7.30, 7.80, 7.90 (d, 8H, aromatic); 13C NMR (200 MHz, CDCl3, TMS) δ (ppm) 14.22, 22.74, 31.20, 31.69, 36.12, 46.79, 63.21, 115.38, 123.25, 125.61, 129.69, 147.12, 147.55, 150.77, 161.18, 171.99; IR (solution, CCl4) 3056 (w), 2958, 2931, 2858, 1799, 1601, 1500, 1248 cm-1. This is a representative preparation, the other azobenzene carboxylic acid derivatives were converted to acid chlorides using a similar procedure (details are available in ref 38). Acylation of Amine-Functionalized SAMs. Amine-terminated SAMs35,37 were placed in benzene (10 mL) containing dry pyridine (9.2 mmol). After 5 min, an azobenzene acid chloride derivative (0.68 mmol) in benzene (5 mL) was added. The mixture was warmed at 60-70 °C for 10 h under argon. After cooling to room temperature, the acylated samples were rinsed with benzene and chloroform. Characterization. Contact angle measurements were done with a Rame-Hart NRL-100 contact angle goniometer equipped with an environmental chamber and tilting base mounted on a vibrationless table (Newport Corp). Physiological saline (0.9% sodium chloride, injection, U. >S. P., Abbott Laboratories) was used as the test liquid for all contact angle measurements. Drop volumes were 10 µL. Details of our tensiometric methods have been published previously.43 For photoisomerization studies, sessile contact angle measurements were employed. The environmental chamber on the goniometer was removed and samples were placed directly on a horizontal goniometer stage. Sessile contact angles were measured immediately after placing the droplet on the sample. The samples were exposed to photoirradiation, and after the UV lamp was turned off, sessile water contact angles were quickly remeasured. (43) Lander, L. M.; Siewierski, L. M.; Brittain, W. J.; Vogler, E. A. Langmuir 1993, 9, 2237.

Siewierski et al. ATR-IR spectra were obtained with a Nicolet System 730 spectrometer using a modified 4XF beam condenser (Harrick Scientific). ATR spectra were run at 4 cm-1 resolution, collecting 2000-4000 scans. The sample chamber was purged either with dry air or with nitrogen for at least 60 min. Background spectra were recorded for each ATR crystal and subtracted from the sample spectra. Baselines were adjusted to zero absorbance for measurement of spectral intensities and determination of dichroic ratios. XPS data was collected using a Surface Science SSX-100 spectrometer. The incidence angle of the X-rays was 35° with respect to the sample normal. Survey spectra were taken for each sample (resolution 4 eV, spot size 1000 µm, 2 scans). Highresolution scans of silicon, carbon, oxygen, and nitrogen regions were also obtained. Monolayer film thickness and quality were determined using X-ray reflectometry (XR). XR was performed in air using Cu KR-radiation (λ ) 1.54 Å) generated by a rotating anode (Rigaku). Constant λ resolution with δλ/λ ) 0.022 and goniometer resolution of δθ ) 0.001° were used. X-ray data was corrected for background before analysis. 1H and 13C NMR spectra were recorded using a Varian Gemini 200 MHz spectrometer. Photoisomerization. Photoisomerization of the films was carried out by exposure to a UVP high-intensity longwave UV lamp (11 600 mW/cm2) for 10 min. The lamp was placed 6-8 in. away from the sample (which was already on the adjustable stage of the contact angle goniometer) under ambient laboratory conditions of lighting and temperature. Immediately after irradiation, contact angles were measured by placing three droplets at different sample locations. Contact angles were measured immediately after UV exposure. Thermal equilibration of the samples was effected by allowing samples to stand under ambient laboratory conditions for 24 h.

Acknowledgment. The authors acknowledge the financial support of the U.S. Army Research Office and the Becton Dickinson Research Center (E. A. Vogler and H. Sugg) for XPS data. LA960506O