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Reversible Wettability of Photoresponsive Pyrimidine-Coated Surfaces Scott Abbott, John Ralston,* Geoffrey Reynolds,† and Robert Hayes‡ Ian Wark Research Institute, University of South Australia, The Levels Campus, Mawson Lakes, South Australia 5095, Australia Received May 7, 1999. In Final Form: July 29, 1999
Thin coatings of photoresponsive, pyrimidine-terminated molecules, attached to gold or quartz substrates in contact with water, undergo dimerization and wettability changes when irradiated with UV light at 280 and 240 nm. Self-assembled monolayers of long chain thymine-terminated thiols give the largest, reversible photoinduced contact angle changes. The latter are caused by a decrease in surface charge as the thymine monomer dimerizes upon irradiation, a process which is accompanied by an increase in the acidity constant of the dimer. Uracil self-assembled monolayers photodimerize but do not photocleave; there is an irreversible change in contact angle. Spin-cast films of thymines give smaller contact angle changes, the maximum values corresponding to films which are composed of a mixture of crystalline and amorphous states.
Introduction The Young equation, describing the force (or energy) balance that exists at the three-phase line of contact between solid, liquid, and vapor has been known for many years. It incorporates the contact angle; however, the influence of the surface charge of the solid on the latter, caused by a change in the pH or ionic strength of the aqueous phase, is not widely recognized, as we and several others have noted.1-3 Yet it provides a key to altering surface wettability in a controlled fashion. The formation of thin, strongly bound organic films on solid surfaces that respond to an external influence that changes their surface charge thus offers exciting possibilities for changing wettability without altering the bulk properties of the bathing liquid phase. The external influence may be light, for example. An organic film that incorporates photochromic molecules can switch between two states when stimulated by light irradiation.4,5 Hence there has been a great deal of activity in searching for molecules which may have application as switching devices in areas such as photoresists, nonlinear optics, computer data storage, and molecular recognition and self-assembly. A change in wettability may result when there is a change in the dipole moment of the two forms of the photochromic species. This has been exploited in the case of molecules containing azobenzenes and spyropyrans, as well as some polymers, with rather small contact angle changes of 9° or so.6-9 Very recently, * Corresponding author. E-mail:
[email protected]. † School of Chemical Technology, University of South Australia, The Levels Campus, Mawson Lakes SA 5095. ‡ Philips Research Eindhoven, Building WAp 3 16, Prof. Holstlaan 4 (WA04), 5656 AA Eindhoven, The Netherlands. (1) Fokkink, L. G. J.; Ralston, J. Colloids Surf. 1989, 36, 69. (2) Chatelier, R.; Drummond, C. J.; Chan, D. Y.; Vasic, Z.; Gengenbach, T.; Griesser, H. J. Langmuir 1995, 11, 4122. (3) Billett, D. F.; Hough, D. B.; Ottewill, R. H. J. Electroanal. Chem. 1976, 74, 107. (4) Durr, H.; Bouas-Laurent, H. Photochromism, Molecules and Systems; Elsevier: Amsterdam, 1990. (5) Inaki, Y. Polym. News 1992, 17, 367. (6) Siewierski, L. M.; Brittain, W. J.; Petrash, S.; Foster, M. D. Langmuir 1996, 12, 5838. (7) Moller, G.; Harke, M.; Motschmann, H. Langmuir 1998, 14, 4955.
inorganic films deposited onto glass substrates have captured attention due to their optical and self-cleaning properties.10 Although the photochromic behavior of molecules from the pyrimidine and related families has been known for some time,4,5 its possible influence on surface charge and hence contact angle has not been recognized. The monomer and dimer forms possess different pKa values that, if the pyrimidines are grafted or spin-coated onto suitable surfaces and then irradiated with light of an appropriate wavelength, could potentially result in very marked changes in surface wettability.11 In this investigation we demonstrate that molecules from the pyrimidine family can, when grafted to an alkyl chain that, in turn, is terminated by a thiol and then chemisorbed to the surface of gold, undergo reversible photodimerization, accompanied by a large change in contact angle/wettability. Similar behavior occurs for spincoated surfaces in the absence of a thiol group, although the correct orientation is considerably easier to achieve in a self-assembled monolayer on a gold surface.12 We have specifically focused on thymine as a representative of the pyrimidine family in this particular study with a very limited investigation of uracil. Other photochromic species that can ionize in aqueous and nonaqueous media are the subject of our future investigations. Materials and Methods Substrate Preparation. Glass microscope cover slips (51 mm × 22 mm) were cleaned by rinsing in ultrahigh quality (UHQ) water (surface tension of 72.8 mNm-1 at 20 °C, κ < 0.5 µS) and then dried in a stream of nitrogen. A final plasma-cleaning step was performed by exposing each cover slip to an argon plasma (Harrick Scientific PDC-32G plasma cleaner, 60W) for 5 min. Quartz slides (45 mm × 12.5 mm) were cleaned in 4 M KOH for 10 min at room temperature, rinsed in UHQ water, dried in nitrogen, and plasma cleaned. (8) Terrettaz, S.; Tachibana, H.; Matsumoto, M. Langmuir 1998, 14, 7511. (9) Irie, M.; Iga, R. Macromol. Chem., Rapid Commun. 1987, 8, 569. (10) Hasimoto, K.; et al. Nature 1997, 388, 431. (11) Shugar, D.; Fox, J. J. Biochim. Biophys. Acta 1952, 9, 199. (12) Ulman, A. (Ed.) Organic Thin Films, Directions for the Nineties; Academic Press: New York, 1995.
10.1021/la990558o CCC: $18.00 © 1999 American Chemical Society Published on Web 10/21/1999
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Figure 1. Synthesis of N-1-mercapto-10-thymyldecane. Gold substrates were prepared by resistive evaporation of gold (Aldrich, 99.99%), from a molybdenum holder, onto chromiumprecoated glass cover slips at room temperature. The cover slips were held in a sample holder to which metallic strips were attached. The sample was then uniformly rotated during the coating process with the aid of an external magnet, permitting both sides of the cover slip to be coated. The chromium (Aldrich, 99.996%) adhesion layer was evaporated, prior to deposition of the gold layer, from a tungsten holder. The deposition rate and sample thicknesses were monitored with a quartz crystal oscillator. Film thicknesses ranged from 2.5 to 67 nm for the chromium and gold layers, respectively. The deposition rates were typically 0.3 nm s-1. The evaporation chamber was kept at 1 × 10-6 Torr during evaporation, with the aid of a diffusion pump, while the substrate temperature was at or slightly above room temperature. The roughness of the gold slides was determined by AFM imaging, with a Nanoscope III (Digital Instruments) scanning probe microscope. Three completely different areas, each of 1 µm2, were scanned. The root-mean square (rms) roughness over this area was 0.680 ( 0.036 nm. This did not detectably change after the formation of the self-assembled monolayer, nor after irradiation. For comparison, gold deposited onto a mica surface using the hot stage method was also examined. In this case the rms value was 1.000 ( 0.542 nm over the same scan area, reducing to 0.294 + 0.161 nm if major defects were excluded. The gold cover slip slides were of a satisfactory degree of smoothness for this investigation. Thiol Synthesis. N-1-Mercapto-10-thymyldecane (C1) was synthesized according to the scheme13,19 shown in Figure 1. N-1-Mercapto-10-uracyldecane (C2) was also synthesized by a similar route. The structures were verified by FTNMR spectroscopy. N1-Octylthymine (C3), N1-tridecylthymine (C4), and N-2-mercaptoethyl-2-thymylpropionamide (C5) were obtained from Dr Y. Inaki from the Department of Materials and Life Science, Graduate School of Engineering at the University of Osaka. They were used as received for the preparation of spincoated surfaces. (13) Takemoto, K.; Inaki, Y. Functional Monomers and Polymers; Marcel Dekker: New York, 1987, Chapter 4. (14) Shackleford, D., B. App. Sci. (Hons.) Thesis, University of South Australia, 1996. (15) Frens, G. Nature Phys. Sci. 1973, 241, 20. (16) Hunter, R. J. Foundations of Colloid Science; OUP: 1987; Vol. 1. (17) Hayes, R. A.; Ralston, J. J. Colloid Interface Sci. 1993, 159, 429. (18) Fornasiero, D.; Li, F.; Ralston, J.; Smart, R. St. C. J. Colloid Interface Sci. 1994, 164, 333. (19) Troughton, E. B.; Bain, C. D.; Whitesides, G. M.; Nuzzo, R. G.; Allara, D. L.; Porter, M. D. Langmuir, 1988, 4, 365.
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Determination of Acidity Constant (pKa). Pyrimidine molecules exhibit different pKa values, depending on their exact molecular structure.11 Thymine is reported to have a pKa of 9.9. This was verified by acid-base titration, monitoring the changes in the UV absorbance spectra as ionization occurred upon addition of base. Upon alkylation, the pKa of thymine increased to 10.3.14 In dimerized form it increased to over 13.14 Formation of Self-Assembled Monolayers. Self-assembled thiol monolayers were formed spontaneously by immersing the gold substrates into a freshly prepared 1 × 10-3 M solution of the thiol in distilled ethanol. The substrate was removed after 24 h, washed with distilled ethanol and UHQ water, and dried in a stream of nitrogen gas. Contact angle measurements and irradiation were then performed. Formation of Spin-Cast Films. Alkylthymine spin-cast films were formed on quartz slides. The alkylthymine was dissolved in chloroform (2.5%, w/v) and rapidly spread over the slide (mounted on a turntable) to a thickness of approximately 1 µm, to provide as uniform a coverage as possible. The slide was rotated at 2000 rpm for 60 s, ensuring chloroform evaporation and that a transparent and uniform film was obtained. The casting process is critical, for if it is improperly performed, yielding films of nonuniform thickness, then opaque films exhibiting poor dimerization yields are obtained. The films were annealed by placing the spin-coated slide on a flat hot-plate, held at a temperature just below the melting point of the alkylthymine (100-110 °C), for specific periods of time. They were then allowed to cool to room temperature. Preparation of Gold Colloids. To facilitate the characterization of pyrimidine self-assembled monolayers on gold surfaces, particularly through ATR-FTIR spectroscopy, gold colloids were prepared, according to the method of Frens.15 The particles were reasonably monodisperse, falling within a particle size range between 94 and 158 nm in diameter. Irradiation. Irradiation of the self-assembled monolayers and spin-cast surfaces was performed using a Nihon-Bunko Model CRM-FA Spectroirradiator equipped with a 2 kW Xe arc lamp providing monochromatic light of high flux (13.6 mJ cm-2 s-1). Photodimerization was performed at a wavelength of 280 nm while photocleavage occurred at 240 nm.5 Irradiation was performed at ambient temperature; no sample heating was observed even upon extended irradiation times. A full irradiation cycle corresponded to irradiation at 280 nm for the required time and then irradiation at 240 nm. Irradiation was generally performed for 10-20 min for a complete cycle, depending on the nature of the surface. Contact Angle Determinations. Contact angles were determined using the sessile drop technique.16 The treated plates were placed in a flat-walled glass container in a clean room at a constant temperature of 25 °C. Drops of 20 µL volume were formed with the use of a micropipet and placed directly onto the sample. The drops were observed with a camera using 5× magnification; the latter was enhanced to 13× with the use of a focusing lens. Both the camera and the sample were mounted on stages designed to minimize vibration. The focal length was 1.1 m. The signal from the camera was fed to a monitor from which very good images were taken using a Fuji Film Digital Camera Fine Pix (1.5 megapixels). Calibration was performed via droplet aspect ratio. Advancing contact angles were determined immediately after placing the droplet on the sample. The droplet volume was decreased to about 90% of its original volume in order to obtain receding contact angles. All measurements were taken in at least triplicate for each sample, on different areas of the sample, as well as for different samples.
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Figure 2. Reversible photodimerization with contact angle and ionization change. Some measurements were also performed using the dynamic Wilhelmy plate technique, described elsewhere.17 Attenuated Total Reflectance-Fourier Transform Infrared Spectroscopy. A single beam Nicolet Magna-IR system 750, equipped with a liquid nitrogen cooled mercury-cadmiumtelluride detector was used. ATR (attenuated total reflectance) spectra were recorded with a SPECTRA-TECH Inc. ATR trough cell Model 0055-390T, with a trapezoidal ZnSe crystal and a 45° angle of incidence as the internal reflection element. The ZnSe crystal was coated with a thin layer of the gold particles and then dried under vacuum at room temperature, leaving a uniform layer of particles on the ZnSe surface. Compound C5 was dissolved in tetrachloromethane, CCl4, to form a 1 × 10-4 M solution and allowed to contact the ZnSe crystal in the ATR cell for 1 h, permitting chemisorption to occur. The cell was then flushed with fresh CCl4 to remove any unreacted compound. The cell was then dried in a stream of dry nitrogen. Fifty scans were coadded to obtain each spectrum, with a resolution of four wavenumbers. All spectra were recorded using a blank ATR cell as the background. X-ray Photoelectron Spectroscopy. XPS measurements were performed on modified gold surfaces using a Perkin-Elmer Physical Electronics (PHI) 5100 ESCA System. Details of the calibration are given elsewhere.18 X-rays from the MgKR source were operated at a fixed pass energy of 93.9 eV. Low power settings (200 W, 12.5 kV) together with a takeoff angle of 45° with respect to the sample surface were used.19 Samples were cut into 1 cm2 squares with a diamond etcher and mounted on stainless steel stubs for use in the spectrometer. All signals were referenced to the C1s peak at 284.7 eV. X-ray Diffraction and Optical Microscopy. Powder XRD measurements were made on spin-coated quartz substrates. XRD Spectra for compounds C3 and C4 were collected using a Rigaku RAXIS-CS imaging plate two-dimensional detector using graphite-monochromatized radiation.20 Optical examination of specimens was performed using an Olympus microscope with polarized light capability.
Results and Discussion Concept. Consider the case where a thymine-terminated alkanethiol has been anchored to a gold surface and then subjected to irradiation with light at 280 nm and then at 240 nm. The process is shown in Figure 2. The pKa of the monomer is 10.3 while the photodimer has a pKa which exceeds 13.11,14 If the contacting solution has a pH of 11.1, then there ought to be a change in the wettability of the surface, reflected by the contact angle, when dimerization occurs. The monomer should be ionized at pH 11.1, increasing the surface charge, σ0. The change in wettability and in the contact angle is predicted by1 (20) Tohnai, N.; Inaki, Y.; Miyata, M.; Yasui, N.; Michizuki, E.; Kai, Y. J. Photopolym. Sci. Technol. 1998, 18, 59.
cos θ (pH) ) cos θ (pHpzc) -
∆Fdl(pH) γlv
(1)
where θ is the contact angle at the solid-liquid-vapor interface, γlv is the liquid-vapor surface tension, pHpzc is the pH where the surface bears zero charge, and ∆Fdl is the free energy of double layer formation. Correspondingly, the free energy of formation of a single double layer is given by21
∫0Ψ σ0 dΨ
∆Fdl ) -
0
(2)
where ψ0 is the electrical potential of the solid-liquid interface. This equation is valid for Nernstian surfaces, i.e for those for which ψ0(pX) (where X is the potential determining ion) obeys the Nernst equation.16 For nonNernstian surfaces, configurational contributions can be included in eq 2.22 The ∆Fdl contribution in eq 2 can be readily calculated from electrical double layer theory. For Nernstian surfaces, this is achieved by calculating σ0(ψ0) from the Poisson-Boltzmann equation and performing the integration in eq 2. In this case, for a flat diffuse double layer,
∆Fdl ) -
{
}
8n0kT zeψ0 cosh κ 2kT
(3)
where n0 is the concentration of the symmetric z:z electrolyte, k is the Boltzmann constant, κ the reciprocal double layer thickness, and e the elementary charge. The link among surface charge, pH, and the influence of ionic strength is now complete. As the monomeric form of thymine ionizes as the pH increases above its pKa, say, by one pH unit, since H+aq is the potential determining ion, the increased surface charge causes the contact angle to decrease and the surface becomes more wettable with respect to the aqueous phase. Upon irradiation with light at 280 nm, dimerization occurs and the surface becomes uncharged again, for the pKa of the dimer now exceeds the pH of the contacting solution; thus, the wettability is decreased and the contact angle increases again. The process is then reversed following irradiation at 240 nm, where cleavage of the dimer occurs and the surface is restored to a charged state. Self-Assembled Monolayers on Gold Surfaces. To optimize photodimerization and hence contact angle changes, good orientation is desirable, along with a high (21) Vervey, E. J. W.; Overbeek, T. Th. G. Theory of the Stability of Lyophobic Colloids; Elsevier: Amsterdam, 1948. (22) Chan, D. Y.; Mitchell, D. J. J. Colloid Interface Sci. 1983, 95, 193.
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Table 1. Wetting Results for a C5 Thymine-Terminated Gold SAM (total time of irradiation, 80 min) advancing contact angles (deg) irradiation state (no. of cycles)
pH 5.8
pH 11.1
0 0.5 1 4 7.5 8
63 ( 2 67 ( 2 62 ( 2 59 ( 2 62 ( 2 61 ( 2
39 ( 2 65 ( 2 40 ( 2 43 ( 2 61 ( 2 47 ( 2
packing density of pyrimidine headgroups. Self-assembled monolayers are a suitable way of obtaining the required molecular ordering.12 Compound C5 was used to form a self-assembled monolayer on the gold-coated cover slips. Incorporation of an amide group in the alkyl chain helps to protect the monolayer against possible thermal decomposition, photooxidation, and exchange through the formation of intermolecular hydrogen bonding.23 Long alkyl chains and the presence of large pendant groups within the structure also enhance the stability of the SAM.24 The surface was characterized using XPS, ATRFTIR, and contact angle measurements. UV absorbance spectroscopy and XRD studies were not feasible due to the opaque character of the gold surface and the insensitivity of XRD to monolayer structure when performed in symmetric reflection geometry. XPS analysis of a fresh C5 SAM before irradiation showed there to be a sulfur 2p peak at a binding energy of 162.1 eV, characteristic of thiols adsorbed onto gold (i.e., R-S-Au). Extended UV irradiation of this SAM showed no evidence of photooxidation. If oxidation had occurred, with the formation of RSO3-, an extra peak with a higher binding energy (167 eV) was expected,18,19 but it was absent. Similar results were obtained for compounds C1 and C2. Thus the thymine SAMs were stable to the intense, cold UV irradiation used in this investigation. For the ATR-FTIR measurements, gold colloids were used rather than flat surfaces in order to maximize the signal intensity by using a large surface area. ATR-FTIR spectra of compound C5 dissolved in tetrachloromethane were compared with C5 SAMs adsorbed on the colloidal gold particle in the wavenumber range between 1000 and 2500 cm-1. In both cases the peaks appeared at similar wavenumbers, albeit those for the SAMs were weaker in intensity and shifted slightly, due to adsorption. The dominant absorption band in the spectrum appeared at 1550 cm-1 and was assigned to the amide II band (N-H bending) of the trans conformer of a secondary amide.25 According to Tam-Chang et al.,23 the presence of an intense amide II band and the absence of any absorption in the 1450 cm-1 region indicate that the amide exists solely in the trans conformation. Contact angle changes at various stages of irradiation are given in Table 1 and Figure 3, using the sessile drop technique, for the C5 SAM. Advancing water contact angles for drops at a near neutral pH of 5.8 are compared with drops at pH 11.1. Measurements were completed within several minutes without any detectable change in pH. A comparison of the drop profiles for the surfaces over a complete cycle is shown in Figure 4. Within experimental (23) Tam-Chang, S. W.; Biebuyck, H. A.; Whitesides, G. M.; Jeon, N.; Nuzzo, R. G. Langmuir, 1995, 11, 4371. (24) Schoenfisch, M. H.; Pemberton, J. E. J. Am. Chem. Soc. 1998, 120, 4502. (25) Williams, D. H.; Fleming, I. Spectroscopic Methods in Organic Chemistry, 3rd ed.; McGraw-Hill Book Co. (UK): Berkshire, England, 1980.
Figure 3. Reversible wettability over eight cycles at pH 11.1 for a C5 thymine-terminated gold SAM.
Figure 4. Sessile drop profiles for C5-terminated gold SAM over one full irradiation cycle; time of irradiation, 10 min; Flux ) 13.6 mJ cm-2 s-1: (a) no irradiation, (b) irradiation at 280 nm, (c) irradiation at 240 nm. RH drop is at pH 5.8. LH drop is at pH 11.1.
error, the same results were obtained for dynamic Wilhelmy plate determinations. The latter measurements take somewhat longer, however, and sample a far greater area of the SAM surface.17 For the first cycle a change in contact angle of 26° was observed between the monomer and photodimer states. Further irradiation did not change this angle, indicating that the irradiation time had achieved a photostationary state. This contact angle change was completely reversible for the first cycle. This reversibility persisted over eight cycles on the C5 gold SAM with the only difference being a gradual decrease in the magnitude of the contact angle
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Figure 5. Thymine photodimerization is dependent on x (Å) which, in turn, is dependent on annealing time.20,27
change. After eight cycles the magnitude of this change reduced to 14°. The same pattern of behavior and magnitude of changes was observed for the C1 SAM. It mirrors the photodimerization behavior reported for spincast films, as discussed further below. For the C2 case, photodimerization was irreversible, because photocleavage is inhibited for uracil, compared with thymine; in the latter case the presence of the CH3 groups on the cyclobutane ring facilitates photocleavage.26 The contact angle change for the C2 uracil was 16° at pH 10.7. Hysteresis between the advancing and receding contact angles was 30° ( 4° under conditions where the surface was uncharged and 15° ( 4° when the surface was charged. There was no variation with the number of cycles. This hysteresis was evident for both the sessile drop and Wilhelmy plate techniques. There was no evidence of any contact line pinning in the latter case when the surface was scanned. We attribute this hysteresis to the small but detectable roughness of the gold surfaces and the accompanying change in hydrophobicity following ionization. Spin-Cast Films. Spin-cast layers of octylthymine (C3) and tridecylthymine (C4) were prepared on quartz plates. Investigations were performed before and after annealing, for it is known that the latter influences molecular orientation in the film. UV-visible spectroscopy showed that both the C3 and C4 layers had an absorbance maximum at 272 nm, characteristic of thymine. We now consider the two thymines in turn. Octylthymine (C3) Layers. Irradiation at both 280 and 240 nm showed that there was a marked difference between the annealed (110 °C for 10 min) and nonannealed films, in terms of their photoactivity. The nonannealed film exhibited reversible photodimerization while the annealed layer exhibited reduced or no photoactivity, depending on annealing time, a feature noted by Tohnai et al.20,27 There was a gradual decrease in the magnitude of the absorbance change over repeated irradiation cycles. Tohnai et al.20,27 have shown that, prior to annealing, the C5 to C6′ distance in the alkylthymine (Figure 5) is small enough (3.3 Å) to permit photodimerization to occur. Upon annealing, heating causes the molecules to rearrange into their lowest energy configuration, with the result that the C5 to C6′ distance is now too great (4.344 Å) for dimerization to take place. Interestingly, prolonged irradiation of the annealed film caused it to regain partial photactivity. We examined the photoactivity of the nonannealed thymine. In Figure 6 the absorbance change of the (26) Inaki, Y. Private Communication. (27) Tohnai, N.; Inaki, Y.; Miyata, M.; Yasui, N.; Mochizuki, E.; Kai, Y. J. Photopolym. Sci. Technol. Submitted for publication.
Figure 6. Reversible photodimerization of nonannealed octylthymine. Table 2. Spin-Cast Octylthymine Film (nonannealed) (total time of irradiation, 214 min) advancing contact angles (deg) irradiation state (no. of cycles) 0 (monomer) 0.5 (dimer) 1 (monomer) 4 (monomer)
pH 5.8
pH 11.1
62 ( 2 67 ( 2 67 ( 2 61 ( 2
46 ( 2 66 ( 2 47 ( 2 43 ( 2
nonannealed octylthymine film is shown as a function of the irradiation energy. There is a gradual decrease in the magnitude of the absorbance change. It is also apparent that photodimerization and photocleavage occur at different rates. For the case examined here, the annealed film maintained its crystallinity over at least four irradiation cycles, as shown by XRD studies (distinct peaks20,27 were observed, the intensity and position of which did not alter). For the case of the nonannealed film, any crystallinity rapidly decreased and the film became totally amorphous after 1.5 cycles of irradiation. Optical examination of both films under polarized light showed evidence of cracking in the initially clear and uniform texture of the nonannealed film, together with an increase in opacity, after irradiation. For the annealed film there was evidence of crack formation prior to irradiation. After four cycles of irradiation, the annealed film became opaque to light. Quite clearly the texture of these films is altered both by annealing and irradiation. Advancing water contact angles for the octylthymine films are shown in Table 2. At pH 11.1 the nonannealed film gave the largest contact angle change (20°) between the monomer and photodimer states. The process was reversible and was maintained over four cycles. In the annealed case the film was unstable, even before irradiation defects in the film permitted penetration of the aqueous phase and detachment of the film at pH 11.1. At pH 5.8, quite small contact angle changes of 5° or so were observed following photodimerization. Texture changes were detected by optical microscopy, as noted above. Hysteresis effects were not investigated for these octylthymine films. Molecular rearrangement and textural changes, accompanied by an increase in opacity, evidently contribute to the gradual degradation of these octylthymine films. Tridecylthymine Layers. UV absorbance spectra showed that both the annealed (110° for 2 min) and nonannealed films undergo reversible photodimerization
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Figure 7. Reversible photodimerization of annealed tridecylthymine. Table 3. Spin-Cast Tridecylthymine Film (2 min annealing; total time of irradiation, 80 min)
For these spin-cast layers there is clearly a delicate balance in the amount of crystallinity and amorphous character in a film that leads to optimum photoactivity and wettability changes. Indeed Sugiki et al.28 have shown that thymine bases in a spin-cast film can move into a stacked orientation. The latter is preferred for optimum photodimerization when it exists as a mixture of microcrystalline and amorphous states. The nonannealed and 2-min-annealed films of octyland tridecylthymine are indeed a mixture of these two states, as shown by XRD. The largest photoinduced contact angle changes were observed for these spin-cast films. Of the four spin-cast films, the 2-min-annealed tridecylthymine film exhibited the best overall performance. It exhibited the fastest rate of photodimerization and the highest percentage conversion of monomer to dimer, the film was stable and relatively defect-free, and there was a satisfactory reversible change in contact angle upon irradiation. Summary and Conclusions
advancing contact angles (deg) irradiation state (no. of cycles) 0 (monomer) 0.5 (dimer) 1 (monomer) 4 (monomer)
pH 5.8
pH 11.1
79 ( 2 81 ( 2 80 ( 2 76 ( 2
62 ( 2 80 ( 2 60 ( 2 65 ( 2
and that the rate is fast, particularly in the annealed case (Figure 7), with the photostationary state reached after just 6 min of irradiation at 280 nm. The percentage of conversion of the monomer to photodimer was greater in the annealed case (94% compared with 68% in the nonannealed film). In Figure 7 it is also clear that the photoreversibility persists over four cycles and shows no sign of any decrease in the magnitude of the maximum absorbance change; neither is there any shift in the baseline. Reversible dimerization is also evident for the nonannealed film, but the magnitude decreases upon repeated irradiation. For the very long chain, tridecylthymine layers, short annealing times improve the photoactivity of the film compared with the nonannealed state. As the annealing time was lengthened, however, we observed that photoactivity decreased. The nonannealed film was amorphous, as observed by XRD examination; it remained so over repeated irradiation cycles. The annealed (2 min) film displayed quite strong crystallinity. Upon irradiation at 280 nm some crystallinity disappeared but was partially regained at 240 nm. The pattern of loss and recovery then remained constant over repeated irradiation cycles. Under the optical microscope the nonannealed film was uniform and clear, turning opaque after irradiation at 280 nm. There was some regeneration when irradiation at 240 nm occurred; however, the surface gradually became patchy, with faint evidence of film cracking, after four cycles. The annealed film was quite dark and opaque in comparison with the nonannealed film. There was little change in its appearance over four irradiation cycles. Advancing water contact angles for the annealed tridecylthymine films are shown in Table 3. Contact angle changes between the monomer and photodimer state at pH 11.1 are 20° after one cycle, decreasing to 11° after four cycles. The changes for the nonannealed film are generally less than 10° and also decrease with the number of irradiation cycles. Hysteresis was not investigated for the tridecylthymine layers.
Thin coatings of photoresponsive pyrimidine-terminated molecules attached to solid substrates undergo dimerization and wettability changes when irradiated with UV light at 280 and 240 nm. Of the surfaces studied, thymine self-assembled monolayers on gold gave the largest reversible photoinduced contact angle change (26°) with respect to water at pH 11.1. This was attributed to optimum molecular ordering compared with the spin-cast films. At pH 11.1, photodimerization induced a change in surface charge (ionized monomer to nonionized dimer), which gave rise to the contact angle changes. Photoinduced reversible wettability was also observed on both octyland tridecylthymine spin-cast films with the largest contact angle change being 20°. Mixed films, displaying both crystallinity and amorphous character, gave optimum results. The tridecylthymine films, annealed at 110 °C for 2 min, gave the largest photoinduced contact angle changes and were the most robust. Uracil self-assembled monolayers also photodimerize and give a change in contact angle of 16° at pH 10.7. However, the difference between the thymine and uracil SAMs lies in the latter’s inability to revert back to monomer when irradiated at 240 nm. The CH3 group in the C5 position of thymine provides the steric driving force behind the photocleavage mechanism. However, the fact that uracil SAMs photodimerize, but do not cleave, opens the door for selective design of photosensitive surfaces, i.e., reversible wettability (e.g. thymine) or irreversible wettability (e.g., uracil) changes. Acknowledgment. The Australian Research Council and the Department of Industry, Science, and Technology are acknowledged for financial support. Dr. Y. Inaki and Mr. N. Tohnai of the Department of Materials and Life Science, University of Osaka, are warmly thanked for the provision of samples, facility access, and very useful discussions. Prof. W. Gong of Wuhan University, People’s Republic of China, is thanked for his advice on the FTIR study. LA990558O (28) Sugiki, T.; Tohnai, N.; Wang, Y.; Wada, T.; Inaki, Y. Bull. Chem. Soc. Jpn. 1996, 69, 1777.
