pubs.acs.org/Langmuir © 2009 American Chemical Society
Design of Pyrimidine-Based Photoresponsive Surfaces and Light-Regulated Wettability Anuttam Patra, John Ralston,* Rossen Sedev, and Jingfang Zhou Ian Wark Research Institute, University of South Australia, Mawson Lakes, SA 5095, Australia Received April 13, 2009. Revised Manuscript Received June 16, 2009 Photoresponsive surfaces were prepared by attaching pyrimidine-terminated molecules to flat gold substrates (as thiol self-assembled monolayers) or by dip-coating quartz surfaces. Both types of films underwent photodimerization (two pyrimidine rings react with one another and form a cyclobutane type dimer through the C5dC6 double bond) when irradiated with light of 280 nm wavelength. The reverse reaction was carried out by irradiating the dimerized surface with light of 240 nm wavelength. The photoinduced chemical changes are accompanied by a change in the physical properties of the surface (e.g., wettability and acidity), and are highly dependent on the structure of the photoactive molecules. The surface dimerization reaction follows a pseudo-first order reaction. The rate constant is determined by the structure of the pyrimidine headgroup. In self-assembled monolayers, uracil derivatives dimerize faster than thymine derivatives due to a reduced steric repulsion near the reaction center. In dip-coated films, however, uracil derivatives appear to be less ordered and, correspondingly, the efficiency of the reaction is lower. The reaction rate is also very sensitive to the ordering within the layer, which can be manipulated through the structure of the tail. In SAMs, faster dimerization occurs with molecules containing flexible chains. In dip-coated films, the presence of a polar group at the chain terminus favors dimerization.
Introduction Photochromic compounds can undergo reversible structural changes by irradiation with light.1 For example, azobenzenes2 undergo cis-trans isomerization, while other species can phototautomerize, photodissociate or photodimerize.1 These photoinduced changes lead to alteration in the molecule’s physical properties. There has been a great deal of activity in searching for organic molecules which may have an application in switching devices, e.g., photoresists,3 computer data storage,4 molecular recognition,5 self-assembly,5 colloid stability,6 and wettability.7 The photochromic behavior of pyrimidines in solution is wellknown.1,8,9 When irradiated with UV light of 280 nm wavelength, two pyrimidine molecules readily react with one another to form a cyclobutane type dimer through the C5dC6 double bond (Figure 1a). This cyclization reaction can be totally reversed by exposing the dimer species to UV light of 240 nm wavelength, depending on the molecular structure studied. This type of photochromic behavior has also been studied in detail in thin solid organic films,3,10 where, using X-ray crystallography, the essential conditions for the dimerization reaction to occur were identified. The rings should approach one another in a planar *Corresponding author. (1) Horspool, W.; Lenci, F. CRC Handbook of Organic Photochemistry and Photobiology; 2nd ed.; CRC Press: Boca Raton, FL, 2004. (2) Akiyama, H.; Tamada, K.; Nagasawa, J.; Abe, K.; Tamaki, T. J. Phys. Chem. B 2003, 107, 130–135. (3) Inaki, Y. Polym. News 1992, 17, 367–371. (4) Lohse, B.; Hvilsted, S.; Berg, R. H.; Ramanujam, P. S. Chem. Mater. 2006, 18, 4808–4816. (5) Ding, D.; Zhang, Z.; Shi, B.; Luo, X.; Liang, Y. Colloids Surf., A: Physicochem. Eng. Aspects 1996, 112, 25–30. (6) Zhou, J.; Beattie, D. A.; Ralston, J.; Sedev, R. Langmuir 2007, 23, 12096– 12103. (7) Ralston, J. Aust. J. Chem. 2005, 58, 644–654. (8) Shugar, D.; Fox, J. J. Biochem. Biophys. Acta 1952, 9, 199–218. (9) Wang, S. Y. Photochemistry and photobiology of nucleic acids; Academic Press: San Diego, CA, 1976; Vols. 1 & 2. (10) Inaki, Y. CRC Handbook of Organic Photochemistry and Photobiology; 2nd ed.; CRC Press: Boca Raton, FL, 2004; Vol. 104/1-104/34.
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fashion and the distance between the two rings should be within the range 3.6-4.1 A˚. The reverse reaction may then be driven by 240 nm irradiation. This totally reversible behavior takes place over many cycles with no detectable breakdown products or side reactions. If the molecules in the film are not properly ordered, only partial reversibility is achieved. We have demonstrated11-13 that similar reactions involving self-assembled monolayers of pyrimidines can also occur on surfaces, reversibly altering the physical properties of that surface. Furthermore we have also examined the properties of these molecules attached to gold nanoparticles.14 Their mode of attachment and colloid stability in the presence of electrolyte and UV light at 280 and 240 nm was examined.6,15,16Importantly, for both the flat gold and nanoparticle surfaces decorated with these ultrathin films, no breakdown products or side-reactions were detected, in accord with the investigations of Inaki et al.3,10 Recall that in order to produce photoresponsive materials that display both high sensitivity and reversibility, it is necessary to control the orientation of the thymine molecules. In our earlier work, we showed how altering the structure of the pyrimidine headgroup can influence surface wettability and dimerization.11-13 The focus of this present investigation is to examine the influence of dimerization upon wettability and surface ionization constants. Thus a series of pyrimidine thiol derivatives (Figure 1a) were synthesized and deposited as SAMs on smooth gold surfaces. In a complementary study, three alkylpyrimidine derivatives (Figure 1b) were deposited as dip-coated, thin solid films on quartz surfaces.10,11 (11) Abbott, S.; Ralston, J.; Reynolds, G.; Hayes, R. Langmuir 1999, 15, 8923– 8928. (12) Lake, N.; Ralston, J.; Reynolds, G. Langmuir 2005, 21, 11922–11931. (13) Richards, N.; Ralston, J.; Reynolds, G. Contact Angle, Wettability Adhes. 2003, 3, 361–372. (14) Zhou, J.; Beattie, D. A.; Sedev, R.; Ralston, J. Langmuir 2007, 23, 9170– 9177. (15) Zhou, J.; Sedev, R.; Beattie, D.; Ralston, J. Langmuir 2008, 24, 4506–4511. (16) Zhou, J.; Ralston, J.; Sedev, R.; Beattie, D. A. J. Colloid Interface Sci. 2009, 331, 251–262.
Published on Web 08/24/2009
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Figure 1. (a) Schematic of the pyrimidine photochemical reaction. Structures of pyrimidine alkylthiols synthesized. (b) Structures of alkylpyrimidine synthesized.
The molecules synthesized were characterized by FTIR and 1H and 13C NMR spectroscopy, as well as by elemental analysis. Advancing and receding water contact angle measurements were used to assess surface wettability and to provide evidence on the influence of molecular structure. Surface acidity constants were determined by contact angle titrations, while the rate of dimerization was monitored by UV spectroscopy. These primary techniques were supplemented by AFM imaging, RA-FTIR, XPS and ToF-SIMS as diagnostic tools.
Materials and Methods Surface Preparation and Characterization. The pyrimidines and bromoalkanes used were of analytical grade (98-99%, Lancaster, U.K.). Potassium thioacetate, hexamethyldisilazane (HMDS) and trimethylchlorosilane (TMCS) were also of analytical grade (98-99%, Sigma-Aldrich). Silica gel was purchased from Merck, Germany. Ultrahigh quality (UHQ) water used in the experiments was produced by reverse osmosis with two stages of ion exchange and two stages of activated carbon prior to final filtration through a 0.22 μm Millipore filter (surface tension 72.2 mJ/m2 at 22 °C; pH 5.8 ( 0.2). Solvents (Crown Scientific, Australia) were of analytical grade and were used as received for chromatography. All solvents used for reaction purpose were freshly dried and distilled prior to use. All glassware used in the synthesis was soaked for 3 h in a concentrated solution of KOH in ethanol, thoroughly rinsed with deionized water and ethanol, and then dried in an oven at 120 °C. The synthetic routes used to obtain the various pyrimidine compounds are described in detail in previous publications,11,14 while the structures are shown in Figure 1. Characterization of the final compounds is given in the Supporting Information.14 Langmuir 2009, 25(19), 11486–11494
Smooth gold surfaces (substrates used for thiol SAM preparation) were prepared17 by physical vapor deposition (PVD) of gold (50 nm thick) on to a clean silicon wafer, with a 5 nm chromium (99.996%) layer between the Si and Au (to improve adhesion between Si and Au). ToF-SIMS surface imaging (PHI TRIFT II (model 2100 CMM)) was used to ensure that the substrate was fully covered with gold-no evidence for exposed Si or Cr was found. Silver surfaces were produced using a similar procedure-the only difference was that a chromium adlayer was not required in this case. After deposition the gold films were annealed18 at 220 °C for 20 h inside a clean furnace. The roughness of the gold films was determined by AFM imaging (NanoScope III SPM (Digital Instruments)). The rms roughness of the gold surface was 0.3 nm over an area of 1 1 μm2 with a maximum peak to valley (ptv) distance of 1 nm. Self-assembled monolayers (SAMs) of different thiol molecules (Figure 2a) on gold were formed by dipping the gold surface into the 1 mM thiol solution for 18 h.12 The SAMs were then annealed in air (inside a clean oven) at 100 °C for 20 min to remove gauche defects present in the monolayer chains.10,12 The roughness of the films did not change detectably during monolayer deposition or during irradiation, as shown by AFM imaging (NanoScope III SPM (Digital Instruments)) in comparison with the smooth gold surface-no islands were detected. The rms and peak to valley remained unchanged from the original smooth gold surface. ToF-SIMS showed an even distribution of, for example, nitrogen over a 100 μm2 area. (17) Kemp, W. Organic spectroscopy, 3rd ed.; Macmillan: New York, 1991. (18) Ulman, A. Self assembled monolayers; Academic Press: San Diego, CA, 1998; Vol. 24.
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280 ( 5 nm with an intensity of 2.2 mW/cm2. Photocleavage was achieved by irradiation of the dimer sample at 240 ( 5 nm with an intensity of 0.7 mW/cm2. Contact Angle Measurement. For all SAMs prepared, the contact angles of water were determined using the sessile drop technique (Dataphysics OCAH200 instrument). Drops of 2.0 μL volume were formed with a microsyringe and placed directly onto the sample. The drops were monitored with a video system (CCD camera HS 3, resolution 752 484 pixels). Both the camera and the sample were mounted on stages designed to minimize vibration. Advancing contact angles were determined immediately after placing the droplet on the sample. Receding contact angles were measured when the water droplet volume was reduced by a syringe pump, ensuring that the three phase contact line retreated. The contact angle was measured by numerically drawing a tangent close to the edge of the droplet. The data obtained were analyzed with Dataphysics SCA20 software (version 3.7.4). All measurements were taken in triplicate at three different surface locations on the sample. The measured values were reproducible to within (1°. The Wilhelmy plate method was further used to check the contact angle changes on T-C10-SH and U-C10-SH. A thin vertical sample was suspended above a liquid surface from the arm of a tared balance (Dataphysics DCAT21 instrument). The weight change, w, recorded on contact with the liquid is given by w = pγ cos θ, where θ is the contact angle, γ is the surface tension, and p is the perimeter of the plate. This equation was used to determine θ. The data obtained were analyzed with Dataphysics SCAT software (version 2.5.1.56). Both the sessile drop and Wilhelmy plate technique yielded very similar contact angles (within (1°).12,19 Since each uracil derivative has a different pKa, contact angles were reported under conditions where there was no surface charge, i.e., at least 1.5 pH units below the pKa. The cosine of the contact angle, at a given pH, can be obtained from:12,19 cos θðpHÞ ¼ Figure 2. (a) Rate of dimerization of different pyrimidine derivatives (SAM). All the films were annealed at 100 °C for 20 min. (b) Rate of photosplitting of different pyrimidine derivatives (SAM).
Thin films were prepared on clean quartz surfaces from the 4 mM solutions of different pyrimidine derivatives in ethanol, using the dip-coating method. The withdrawing velocity was 200 μm per second. ToF-SIMS surface imaging was also used to confirm total surface coverage by the monomers (on gold) after formation of the thin film. The thickness of the film was determined with ellipsometry, and found to be 60 min). We hypothesize that the initial, slower rate of T-Ph-CONHSH dimerization is kinetically determined while the final dimerization state (as indicated by a greater contact angle change) is determined by better packing (thermodynamically favorable) due to the planar, rigid phenyl groups in the chain. The rate of dimerization for T-C10-SH is lower than that for UC10-SH (Figure 2a) due to the extra methyl group in the thymine near the double bond. This increases the steric hindrance when the two rings approach each other and slows down the dimerization reaction. On the other hand, the opposite effect is seen during cleavage-the initial rate of T-C10-SH dimer is higher than for the U-C10-SH dimer. Steric repulsion between methyl groups attached to the cyclobutane ring accelerates the cleavage reaction. U-C10-SH is the best performing compound of the series studied here with respect to reaction rate and overall contact angle change. T-C10-CONHSH is relatively sluggish to dimerize on the surface (Figure 2a). This is possibly due to the kinetic restrictions but also (in contrast to T-Ph-CONHSH) due to a lack of additional chain-chain interactions. It appears that annealing did not improve the ordering (achieving a homogeneous all-trans conformation of the hydrocarbon chains) for this type of monolayer. When SAM chains contain a hydrogen bonding group at the end of the chain, the packing of the monolayer becomes very different18,40-42 from those formed by alkanethiol monolayers. As shown by Sek42 (chains with an amide group) and Jaschke40 (chains with an ester group) the distance between neighboring molecules was 5.5 A˚, with an area per molecule of ∼25 A˚2 (these (39) Ishida, T.; Hara, M.; Kojima, I.; Tsuneda, S.; Nishida, N.; Sasabe, H.; Knoll, W. Langmuir 1998, 14, 2092–2096. (40) Jaschke, M.; Schoenherr, H.; Wolf, H.; Butt, H. J.; Bamberg, E.; Besocke, M. K.; Ringsdorf, H. J. Phys. Chem. B 1996, 100, 2290–2301. (41) Poirier, G. E. Chem. Rev. 1997, 97, 1117–1127. (42) Sek, S.; Palys, B.; Bilewicz, R. J. Phys. Chem. B 2002, 106, 5907–5914.
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values are 15% larger than the corresponding values for alkylthiols SAMs: 4.6 A˚ and 21.6 A˚2). The increased average separation between the reaction centers is reflected in the decreased reaction rates discussed above. In the case of uracil terminated SAMs, the bulky but wellordered tail groups obscure the underlying methylene groups, irrespective of whether or not the number of methylene group are even or odd. Here the contact angle values are governed only by the uracil groups. The observed odd-even effects in the contact angle data are due to the systematic variation of the orientation of the terminal uracil group in the SAMs having odd-numbered versus even-numbered methylene chains. In cases where n is odd, the terminal uracil N1-C bond is normal to the surface, exposing polar oxygen and nitrogen atoms to a much lesser extent-thus reducing the attractive interactions between a water droplet and the SAM relative to the cases where n is even, where most of the polar groups are exposed to the outermost surface. For this reason a systematic variation in wettability as a function of the odd-numbered versus even-numbered chain length of the uracilterminated SAMs was observed, with lower contact angles for even n and higher contact angles for odd n. Previous studies have shown43,44 that such odd-even effects were observed when some organic liquids were used in wettability studies of methyl and phenyl terminated SAMs. The static advancing water contact angle varied only slightly for those types of surfaces with alternate odd and even chain lengths. However, in uracil terminated monolayers, the odd-even effects were clearly observed using water as a wetting liquid. This different wetting behavior is possibly due to the presence of highly polar groups (due to H-bonding), such as CdO and N-H in uracil terminated surfaces, unlike methyl or phenyl terminated surfaces. Chain length variation yielded almost no alteration in the reaction rate or contact angle changes on the surfaces (Figure 3b). This implies that the ordering of the pyrimidine head groups (after annealing of the SAM) is not determined by the ordering of the alkyl chains (chain ordering decreases in the order C10 > C6 > C3). The wettability is mainly governed by the pyrimidine groups and their mutual orientation. However, all T-C3-SH surfaces exhibited larger contact angles, by about 6° (Figure 3b). This may be due to the shorter chain length for C3, or be caused by an odd and even effect. The large value of hysteresis indicated physical and/or chemical heterogeneity; physical heterogeneity was eliminated as a cause of hysteresis since the surfaces were extremely smooth (AFM), macroscopically homogeneous (ToF-SIMS imaging) and highly ordered (RAIRS). Thus it may be concluded that the large degree of hysteresis originates from molecular level heterogeneity, as the terminal pyrimidine group is itself a combination of polar (N-H and CdO, hydrophilic, static advancing water contact angle 0°) and nonpolar groups (C-H and C-C, hydrophobic, static advancing water contact angle 112°). The area of each of these can be estimated from simple geometric considerations and contact angles on these surfaces can be estimated using the Cassie equation.23 The calculated static advancing water contact angle was 50° which compares well with the experimental data, for all pyrimidine-functionalized surfaces had contact angles in this range (Table 1). The contact angle may be further modified when ionization of the respective terminal molecules45 is considered. Thymine derivatives have higher contact angles than do uracil derivatives, for the respective derivatives1,8 of the former have
higher pKa values than do the latter (Table 1). The hydrophobic methyl group at the C5-position contributes to the higher contact angles values of the thymine derivatives. Uracil derivatives exhibit a larger contact angle change (upon irradiation) compared with thymine derivatives, however the change in pKa on dimerization is greater. We observe that the changes in the surface pKa values can be used to estimate the degree of dimerization. For example, in case of U-C10-SH, calculation predicts a 2.1 unit change in pKa after complete dimerization. Experiment shows that the pKa changes by 2.0 units after U-C10-SH dimerization on surface. For T-C10-SH, the surface pKa change is only 0.7 units, in comparison with the 1.3 unit pKa change as predicted. This indicates that there may be only 50 - 60% conversion of T-C10SH monomer to dimer on the surface after 280 nm light irradiation for 30 min (Table 1). For the thymine derivatives, the extra methyl group at C5 opposes the dimerization reaction, favoring the monomer compared with the uracil derivatives. Of course this then favors photosplitting in the case of the thymine dimer when 240 nm UV light is used. These phenomena were confirmed by surface pKa (Table 1) and contact angle measurements. In a previous study, Lake et al.12 showed that when the pyrimidine derivatives dimerized on surface, in order to bring the pyrimidine groups in proper orientation, the chains bent, exposing the carbon chain in the outermost surface. Pyrimidine groups were then adjacent to the gold surface and, due to electron transfer from oxygen and nitrogen to gold, their oxidation states had changed. This was reflected in the different binding energies of the oxygen and nitrogen, before and after dimerization.12 In the present study, far smoother surfaces are used (rms roughness 0.3 nm compared with 2.0 nm in the previous study) and even purer thiol compounds. A much denser monolayer results with the outcome that and the molecules do not show chain bending during dimerization, as observed previously,12 as there are no free spaces left in the monolayer to do so. Since there is now no interaction between the nitrogen and oxygen with the gold surface, their respective oxidation states remain unchanged before and after dimerization in this present study. Unlike thymine molecules, photoreaction of uracil in solution can undergo not only dimerization, but also hydration.9 However, in solid state, both thymine and uracil only dimerize, producing different isomers, and cleave upon irradiation with different wavelength of light, no other side reaction occurred.10 In this investigation, all the surface spectra, RA-FTIR, XPS and ToF-SIMS, confirmed the molecular structure, the nature of gold - sulfur bond, and the occurrence of photodimerization and cleavage of the uracilthiol-SAMs. As no suspicious or unidentified peaks or species detected in the spectra, other side reactions did not occur at a detectable extent except dimerization and cleavage, in agreement with the previous reports. (b). Dip-coated Films. The reaction kinetic behavior in the dip-coated films is of key interest. The photodimerization process of thymine can be described as one excited thymine monomer reacting with another ground state thymine monomer to form a thymine dimer. Since the concentration of excited thymine monomer can be considered to be constant during photoreaction, thymine dimerization has been shown to follow a pseudofirst order reaction, described by the equation ln(M0/Mt) = ktt, where M0 represents the initial monomer concentration, Mt is the concentration at any time t, and k is the rate constant (s-1).40-48 The curve can be most simply resolved into two
(43) Wong, S.-S.; Takano, H.; Porter, M. D. Anal. Chem. 1998, 70, 5209–5212. (44) Lee, S.; Puck, A.; Graupe, M.; Colorado, R.; Shon, Y.-S.; Lee, T. R.; Perry, S. S. Langmuir 2001, 17, 7364–7370. (45) Fokkink, L. G.; Ralston, J. J. Colloids Surf. 1989, 36, 69–76.
(46) Foster, S. J.; Salter, L. F. S. Afr. J. Chem. 1988, 41, 131–135. (47) Johns, H. E. In Advan. Med. Phys., Symp. Ppa. Int. Conf.; 2nd ed., 1971; pp 217-231. (48) Kilfoil, V.; Salter, L. J. Chem. Soc., Chem. Commun. 1988, 23, 732–734.
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dimerization of pyrimidine follows pseudo-first order kinetics (for example, k1 =0.00167 s-1 and k2 =0.00004 s-1 for a T-C12 film on the quartz surface (Figure 8)).
Conclusion
Figure 8. Kinetic data for the reaction of T-C12 on quartz surface (dip-coated film).
independent linear components (Figure 8). We propose that there are two distinct components-both of which are an outcome of the same dimerization reaction (both are first order) running in parallel, with two very different rate constants. The latter reflect the order of the thymine species in the film. We suggest that the fast reaction component consists of the reaction between two thymine molecules lying in the correct orientation (parallel to each other at a reaction distance10 of 3.6-4.1 A˚). Initially, the reaction proceeds rapidly. Within the first few minutes, the ordered thymine molecules are reacted and the remaining kinetic behavior reflects the continuing contribution between relatively disordered molecules. Reaction between these disordered molecules is very slow, as in the two-dimensional solid film the molecular motion (diffusion or tumbling) is very low, and the molecules take an appreciable amount of time to reach the correct orientation for reaction. Consequently, the rate constant for the second disordered reaction is significantly lower compared with the first well-ordered reaction. At this beginning, the combined reaction rate is dominated by the first well-ordered thymine reaction, while at the end, the second, disordered reaction is the major contributor. The second reaction takes hours to complete, compared with the few minutes required for first one. Thus the
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Pyrimidine derivatives with thiol-terminated hydrocarbon chains form photoresponsive SAMs on smooth gold surfaces and can be used to reversibly control the wettability of the surface. The pyrimidine rings dimerized during 280 nm UV irradiation and split when irradiated with 240 nm. Because the reaction is very sensitive to spatial alignment, proper annealing of the SAM was crucial for providing an orientation suitable for dimerization. Uracil derivatives dimerize at a faster rate than thymine derivatives. Flexible chains favor surface dimerization. Dip-coated films of the pyrimidine derivatives on quartz surfaces also showed reversible dimerization. Uracil molecules, however, showed very little efficiency compared with thymine derivatives. In both the cases (SAMs and dip-coated films), for a given pyrimidine molecule, the rate of the reaction (determined by the order of the molecules in the film) is mainly governed by the structure of the chain (in dip-coated films, the presence of a polar group at a chain terminus increases the rate of dimerization). Reaction rates are almost independent of chain length. In dipcoated films, however, no dimerization was detected in the absence of a tail, showing that it plays a key role in achieving a suitable orientation. The reaction kinetics of pyrimidine photodimerization appears to be a combination of two pseudo-first order reactions (one for the highly ordered domains and another for the less-ordered domain), with the respective rates differing by almost 2 orders of magnitude. Acknowledgment. Financial support from the Australian Research Council Special Research Centre Scheme is gratefully acknowledged. Discussions with Dr. David Beattie are warmly appreciated. Supporting Information Available: Text giving detailed characterization and figures showing plots of XPS data. This material is available free of charge via the Internet at http:// pubs.acs.org.
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