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Novel Anthracene Materials for Applications in Lithography and Reversible Photoswitching by Light and Air Werner Fudickar and Torsten Linker* Department of Chemistry, University of Potsdam, Karl-Liebknecht-Strasse 24-25, 14476 Potsdam/Golm, Germany Received September 14, 2009. Revised Manuscript Received December 15, 2009 Herein we demonstrate how the photoreaction between anthracenes and singlet oxygen (1O2) is employed for applications either as photoswitch or as photoresist. Thin films of the diaryl-alkyl anthracene 1 and the analogous oligomeric species 2 were irradiated under photomasks to generate pattern structures composed of 1/1-O2 and 2/2-O2. Kelvin probe force microscopy (KPFM) provided a powerful and nondestructive method to image the pattern information. The following studies based on AFM, KPFM and contact angle measurements unfold that the two species 1 and 2 underwent different progressions after the imaging step. Degrading is observed for the monomeric compound 1 and the pattern eventually becomes recognizable in topography. In the oxidized state (1-O2) the monomeric species remains physically stable. In consequence, the unreacted portion is removable and the remaining oxygenated form 1-O2 is sufficiently stable to protect an underlying substrate (e.g., silver) from etching. Thus, the system 1/1-O2 operates as photoresist. On the other hand, both states of the oligomer 2 remain stable. The film is stable up to temperatures >120 C required to erase the pattern within acceptable time by cycloreversion. Anthracene 2 therefore acts as erasable and rewritable photochromic switch. The different behavior between 1 and 2 is explained by phase transitions which cause crystallization and finally ablation. Such transitions affect only the monomeric system 1/1-O2 and not the oligomeric system 2/2-O2. In conclusion, we designed two very similar materials based on diarylanthracenes, which can act either as a photoresist or as a rewritable photochromic switch.
Introduction Among the manifold of photoresponsive materials bistable systems deserve particular interest due to their significant applications in optical data storage.1,2 The interconversion between the two states is triggered from photo and/or thermal stimuli. A central issue in the design of such photochromics is the achievement of a nondestructive read-out process and the enhancement of fatigue-resistance during the cycles of writing and erasing.3-8 This is realized by use of nonfluorescent read-out methods which do not require optical excitation at the wavelength of the switching signal, e.g., by following changes of the refractive index9 and magnetic properties10 as well as by utilization of energy-transfer processes4,5 and IR read-out.7 Most extensively studied in this field are the families of diarylethenes and diazobenzenes which exhibit high stability and fatigue resistance.2 On the other hand, compounds undergoing irreversible photoreactions suit well as *Telephone: þ49 331 977 5212. Fax: þ49 331 977 5056. E-mail: linker@ uni-potsdam.de. (1) For a comprehensive overview, see: D€urr, H.; Bouas-Laurent, H., Eds., Photochromism, Molecules and Systems; Elsevier: Amsterdam, 2003. (2) Irie, M. Chem. Rev. 2000, 100, 1685–1716. (3) Li, F.; Zhuang, J.; Jiang, G.; Tang, H.; Xia, A.; Jiang, L.; Song, Y.; Li, Y.; Zhu, D. Chem. Mater. 2008, 20, 1194–1196. (4) Tyson, D. S.; Bignozzi, C. A.; Castellano, F. N. J. Am. Chem. Soc. 2002, 124, 4562–4563. (5) Lim, S.-J.; Seo, J.; Park, S. Y. J. Am. Chem. Soc. 2006, 128, 14542–14547. (6) Tamaoki, N.; Wada, M. J. Am. Chem. Soc. 2006, 128, 6284–6285. (7) Uchida, K.; Saito, M.; Murakami, A.; Kobayashi, T.; Nakamura, S.; Irie, M. Chem.;Eur. J. 2005, 11, 534–542. (8) Fernandez-Acebes, A.; Lehn, J.-M. Chem.;Eur. J. 1999, 5, 3285–3292. (9) Biteau, J.; Chaput, F.; Lahlil, K.; Boilot, J.-P.; Tsivgoulis, M.; Lehn, J.-M.; Darracq, B.; Marois, C.; Levy, Y. Chem. Mater. 1998, 10, 1945–1950. (10) Matsuda, K.; Irie, M. J. Am. Chem. Soc. 2001, 123, 9896–9897. (11) Reichmanis, E.; Thompson, L. F. Chem. Rev. 1989, 89, 1273–1289. (12) Willson, C. G. Introduction to Microlithography, 2nd ed.; ACS Professional Reference Book; American Chemical Society: Washington, DC, 1994; Chapter 3. (13) Walraff, G. M.; Hinsberg, W. D. Chem. Rev. 1999, 99, 1801–1821.
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developable photoresists for lithographic applications.11-14 The idea to design new systems that offer both applications seems to be very attractive. Additionally, the materials should be conveniently accessible and a writing process under mild and environmentally friendly conditions would be advantageous. The chemistry of singlet oxygen (1O2) satisfies these requirements. The excited species of oxygen represents a reactive oxidant which can be generated from ambient air by sensitized irradiation with visible light.15-17 We applied this reagent for stereoselective synthesis18-21 and more recently as trigger for molecular switches22-24 and the patterning of surfaces.25-27 For the last two applications anthracenes are very suitable substrates, since they react with 1O2 to endoperoxides reversibly with no decomposition. Furthermore, singlet oxygen is generated from air by direct irradiation of the anthracene moiety without necessity of a sensitizer. The cleavage of the endoperoxides to the parent anthracenes is either triggered by heat or light stimuli and (14) Bratton, D.; Yang, D.; Dai, J.; Ober, C. K. Polym. Adv. Technol. 2006, 17, 94–103. (15) Greer, A. Acc. Chem. Res. 2006, 39, 797–804. (16) Schweitzer, C.; Schmidt, R. Chem. Rev. 2003, 103, 1685–1758. (17) Aubry, J.-M.; Pierlot, C.; Rigaudy, J.; Schmidt, R. Acc. Chem. Res. 2003, 36, 668–675. (18) Linker, T.; Fr€ohlich, L. Angew. Chem., Int. Ed. Engl. 1994, 33, 1971–1972. (19) Linker, T.; Fr€ohlich, L. J. Am. Chem. Soc. 1995, 117, 2694–2697. (20) Nardello, V.; Aubry, J.-M.; Linker, T. Photochem. Photobiol. 1999, 70, 524–530. (21) Fudickar, W.; Vorndran, K.; Linker, T. Tetrahedron 2006, 62, 10639– 10646. (22) Zehm, D.; Fudickar, W.; Linker, T. Angew. Chem., Int. Ed. 2007, 46, 7689– 7692. (23) Zehm, D.; Fudickar, W.; Hans, M.; Schilde, U.; Kelling, A.; Linker, T. Chem.;Eur. J. 2008, 14, 11429–11441. (24) Fudickar, W.; Linker, T. Chem. Commun. 2008, 1771–1773. (25) Fudickar, W.; Fery, A.; Linker, T. J. Am. Chem. Soc. 2005, 127, 9386–9388. (26) Fudickar, W.; Linker, T. Chem.;Eur. J. 2006, 12, 9276–9283. (27) Fudickar, W.; Linker, T. Langmuir 2009, 25, 9797–9803.
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proceeds with almost quantitative yield.17,28,29 Compared to common bistable photoswitches, the oxidation is bimolecular, as oxygen is incorporated into the product. The substantial structural transformation may suit for following irreversible developing steps, common for photoresists. We were interested in the investigation of the photooxygenation on a microscopic scale to disclose the factors controlling the physical reversibility of the process, which eventually determines the possibility of further utilization as photoswitch or photoresist. Since the anthracene-endoperoxide switching occurs by a change of delocalization of electrons in π-states, Kelvin probe force microscopy (KPFM) could be used to map the local distribution of the electronic surface potential.30 This method is simple and has the advantage of being a nondestructive read-out. Fluorescence detections of anthracenes, for example, cause rapid bleaching of the chromophore. KPFM has successfully been employed to disclose pattern structures on monolayers which exhibited no structure in topography.31-34 However, the technique is also applicable to nonordered layers when functional groups undergo detectable transformations.35,36 Studies on thin films of π-conjugated systems unfold also a particular contribution of the electronic delocalization to the contrast of a KPFM image.37 Herein we describe the application of two novel photoresponsive 9,10-diarylanthracene films which are easily available in only few steps, as either lithographic materials or reversible photoswitches. The local changes of material properties are investigated by KPFM in combination with measurements of topography and wettability on a microscopic level. The processes are simply triggered by light and air, and a selective and nondestructive read-out method by means of KPFM has been realized. It will be demonstrated how the molecular design controls over the physical stability, which eventually determines the suitability for use as a thermally stable photoswitch or as a photoresist for lithographic applications.
Experimental Section Materials. The monomeric compound 1 was synthesized according to our previously published procedure.26 The protocol of the synthesis of oligomer 2 can be found in the Supporting Information.38 GPC analyses of the oligomer provided weight and number averages of the molecular weight of Mw = 16000 and Mn = 5680, respectively. Differential scanning calorimetry (DSC) and thermogravimetric analysis of 1 and 2 were measured providing melting transitions of 126 and 175 C for 1 and 2, respectively. In addition to that, another transition peak for 1 was detected at 4 C at the second heating cycle.38 (28) Turro, N. J.; Chow, M.-F.; Rigaudy, J. J. Am. Chem. Soc. 1981, 103, 7218– 7224. (29) Schmidt, R.; Schaffner, K.; Trost, W.; Brauer, H.-D. J. Phys. Chem. 1984, 88, 956–958. (30) Palermo, V.; Palma, M.; Samori, P. Adv. Mater. 2006, 18, 145–164. (31) Stiller, B.; Karageorgiev, P.; Perez-Enciso, E.; Velez, M.; Vieira, S.; Reiche, J.; Knochenhauer, G.; Prescher, D.; Brehmer, L. Surf. Interface Anal. 2000, 30, 549–551. (32) Hayashi, K.; Saito, N.; Sugimura, H.; Takai, O.; Nakagiri, N. Langmuir 2002, 18, 7469–7472. (33) L€u, J.; Delamarche, E.; Eng, L.; Bennewitz, R.; Meyer, E.; G€untherodt, H.-J. Langmuir 1999, 15, 8184–8188. (34) Bush, B. G.; DelRio, F. W.; Opatkiewicz, J.; Maboudian, R.; Carraro, C. J. Phys. Chem. A 2007, 111, 12339–12343. (35) Millaruelo, M.; Eng, M. L.; Mertig, M.; Pilch, B.; Oertel, U.; Opitz, J.; Sieczkowska, B.; Simon, F.; Voit, B. Langmuir 2006, 22, 9446–9452. (36) Schlapak, R.; Armitage, D.; Saucedo-Zeni, N.; Latini, G.; Gruber, H. J.; Mesquida, P.; Samotskaya, Y.; Hohage, M.; Cacialli, F.; Howorka, S. Langmuir 2007, 23, 8916–8924. (37) Palermo, V.; Palma, M.; Tomovi, Z.; Watson, M. D.; Friedlein, R.; M€ullen, K.; Samori, P. ChemPhysChem 2005, 6, 2371–2375. (38) See Supporting Information.
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Substrate Preparation. As substrates for the spin coating films, P/boron doped silicon wafers were used with a sheet resistance of 25 mΩcm. Cleaning was performed by dipping the substrates into a mixture of ammonia/H2O2/water (1:1:2) for 5 min at 60 C. For hydrophobization, the substrates were first coated with octadecyltrichlorosilane by dipping the cleaned wafers into a solution of octadecyltrichlorosilane (Aldrich, 1%) in CHCl3. Then, the hydrophobized sample was covered with the anthracene films. Spin coating was carried out from solutions of the two compounds (1%) in CHCl3 at a spinning rate of 300 rounds per second. Photopatterning. The films were covered with copper grids used for electron microscopy. The standard bar size of a grid is 20 μm. A 15 W low-pressure mercury lamp (Desaga) with an emission maximum at 365 nm was used for irradiation. The lamp was held 10 cm above the sample. A light intensity of 80 lx was measured at this distance with a lux meter. Irradiation was carried out under air. At a wavelength of 365 nm, the anthracene chromophore is electronically excited and sensitizes directly oxygen into its excited singlet state. Lithographic Application.
Preparation of the Silver Layer.
First, a glass slide was cleaned as described above for silicon wafers. Then, an aqueous solution of AgNO3 (50 mg in 10 mL water) was prepared and treated with ammonia until a transparent solution was obtained after which glucose (50 mg) was added. The silver film formed immediately on a glass slide upon dipping into the solution. The glass slide was heated to 100 C before dipping. Coating and Imaging. A film of anthracene 1 was spin coated on top of the silver layer and irradiation through a transparency carrying an image or through a TEM copper grid was carried out under the conditions described above. Development and Etching. The sample was dipped into hexane and kept there for 5 min. During this time, the image becomes visible at the irradiated region. Subsequently, the sample was dried and transferred into a vessel containing 10% nitric acid. The etching procedure was finished after ∼2 min and the sample was then gently rinsed with water. Finally, the sample was washed with CHCl3 to remove the remaining portion of the film. Fluorescence Imaging. The pattern was imaged by a Zeiss Aixostar microscope using a high-pressure mercury lamp for excitation at 365 nm. Fluorescence from the sample was imaged through a 400 nm long pass (cutoff) filter. KPFM and AFM Measurements. The KPFM technique investigates the contact potential difference (CPD) between the sample and the probe tip, as they were electrically connected. By setting an oscillating voltage Vac on the tip, the tip senses the electrostatic force of the CPD which causes the tip to oscillate. The amplitude of this oscillation is directly proportional to the CPD between tip and sample. An additional feedback loop applies an additional dc voltage on the tip to compensate the potential difference causing to minimize the amplitude. This voltage finally provides the signal for the image. The measurements were carried out with an Innova microscope (Veeco) operating in noncontact mode on air. Antimony doped silicon tips with a Pt/Ir coating, a resonant frequency of ∼100 kHz and a force constant of ∼5 N m were used. The first scan of each line measured and recorded the topography; then, the second scan measured the surface potential whereby the tip was lifted by 100 nm with respect to the first scan. The applied oscillating voltage was 8 V. For the in situ heating the piezo-scanner was capped with a thermal application controller (Digital Instruments). Contact Angle Measurements. Dynamic water contact angles were measured on a goniometer (OEG) using the sessile drop method. The average measurement error is (2.
Results and Discussion The two compounds, the monomeric anthracene 1 and the oligomeric anthracene 2 used in this studies exhibit both a 9,10-diarylanthracene unit as the reactive center and a long chain Langmuir 2010, 26(6), 4421–4428
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Figure 1. Reversible reactions of the anthracenes 1 and 2 to give the endoperoxides 1-O2 and 2-O2.
to improve solubility (Figure 1). It appeared reasonable to control the parameters affecting physical stability by the change from a monomeric compound to an oligomer, where the backbone may countervail physical transitions. Except this feature, the two compounds carry very similar skeletons which facilitate a comparison. Both systems are investigated in terms of their thermal stability and physical transitions are disclosed. Photo Conversion of the Monomer 1 and the Oligomer 2. For all experiments, we prepared 60-100 nm thick spin coating films and carried out the conversion of the anthracenes by direct excitation of the anthracene chromophore at 365 nm, which sensitizes in turn oxygen into its excited singlet state. The course of the photooxidation was monitored by measuring the UVvis spectra of the films on transparent glass slides as shown in Figure 2. The characteristic maxima of the anthracene chromophore between 340 and 400 nm disappeared during irradiation. For both compounds, a conversion of more than 80% is reached after 240 s. A notable slowdown of the rate at higher conversion was observed which can be explained by the depletion of oxygen sensitization: As the concentration of the chromophore drops less 1 O2 is produced. Thus, irradiation was stopped after 10 min at ∼90% conversion. An additional blank experiment with a film of 1 carried out in the absence of oxygen by irradiation in a glovebox showed that the spectrum remained unchanged. Therefore, a possible dimerization by a [4 þ 4]-cycloaddition as known from non- or monoaryl-substituted anthracenes has not occurred.39 NMR analysis of the product obtained from irradiation in solution under air confirmed also the formation of the endoperoxide.25 In order to prove that the anthracenes converted reversibly into endoperoxides the samples were heated at 100 C for 60 min. In both cases the anthracene chromophore reappeared which accounts for a reversible process (Figure 2). After the regeneration of the anthracenes a second cycle was carried out (see Supporting Information Figure S2). In the case of the oligomer 2, the kinetic course at the second run of irradiation is (39) Kaupp, G.; Teufe, E. Chem. Ber. 1980, 113, 3669–3674.
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the same as at the first run. In contrast, the kinetics of the second run of the monomer 1 has slowed down significantly. The slower conversion can have two origins. First, the generation of singlet oxygen is reduced or second, the diffusion of singlet oxygen is different due to a change of the physical properties of the film. These observations show that the two materials behave differently after oxygenation. The oligomer allows multiple cycles of irradiation and thermolysis whereas the monomer undergoes a physical transition. For the further investigation of physical changes, it is reasonable to generate pattern structures where both species can be compared in parallel. Photo Patterning of Thin Films of the Anthracene Monomer 1. For the generation of pattern structures on silicon supports irradiation was carried out through a photo mask in the same way as described above. We prepared a thin film with a thickness of ∼60-100 nm.40 The AFM and KPFM images of the patterned film are shown in Figure 3. The film exhibits substantial roughness with an rms value of 9.5 nm over the whole area. Apparent is that the pattern structure is not represented in the topography (Figure 3a) and also not optically visible (Figure 3c). However, the pattern motif of a fluorescence micrograph (shown in the inset of Figure 3a) clearly confirms the imaging process. The bright area in the image corresponds to the fluorescence of the anthracene chromophore whereas the decrease of the fluorescence in dark area resulted from a disruption of the conjugated system by formation of the endoperoxide after irradiation. In contrast to the topography, the scan of the surface potential (Figure 3b) clearly constitutes the pattern structure. The potential difference between the darker regions and the light yellow regions is -13.5 ( 2 mV. The higher grains in the topography appear also in the potential image (white spots) which arises from interference between topography and KPFM measurement. The dark spots (40) According to our previous work (ref 27), a singlet oxygen molecule penetrates into the film also at regions which are covered by the mask with a depth of ∼15 nm. In consequence, the outer section of the film is completely oxidized and therefore not contributing to the surface potential contrast between oxidized and nonoxidized regions.
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Figure 2. UV-vis spectra showing the photoconversion of the films of anthracenes 1 and 2 (dark solid lines) and the thermal regeneration (dashed red curve).
Figure 3. Photopatterning of a thin film of the monomer 1: (a) Topographic image with a Z range of 284 nm; (b) surface potential image with a Z range of 39 mV and (c) optical micrograph (b). The inset in (a) shows the fluorescence of the pattern.
having a diameter of ∼20 μm fit well with the holes of the photomask and can therefore be assigned to the irradiated portion of the film. The photooxygenation thus causes a drop of the surface potential. For a better understanding of the relation between molecular properties and the value of the surface potential the following theoretical model can be proposed. The surface potential for a single defined layer is defined by the Helmholtz equation for a parallel plate capacitor (eq 1) V ¼-
φsample -φtip μ^ þ e Aεlayer ε0
ð1Þ
where φsample and φtip are the work functions of the substrate and the probe tip, respectively, e is the charge of an electron, μ^ is the perpendicular component of the dipole moment, A the surface area and εlayer and ε0 are the permittivities of the layer and the free space, respectively.41 The potential difference between nonoxidized and oxidized regions, ΔV, is described by eq 2. ΔV ¼ VANT - VEndo ¼
μ^ANT μ - ^Endo AεANT ε0 AεEndo ε0
ð2Þ
The unknown term of eq 1 is eliminated by measurement of the contrast of the surface potential between two spots. Equation 2 shows that only the difference of the surface potential (contrast) provides valuable information whereas absolute values require the knowledge of the work functions. By neglecting a change of the permittivity and assuming a value of 3.0 for εANT and εEndo the (41) Taylor, D. M.; Bayes, G. F. Phys. Rev. E49 1994, 1439–1446.
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contrast becomes a function of the perpendicular vector of the dipole moments. We have estimated dipole moments by semi empirical calculations and derived the vector components being parallel to the molecular axis. Those dipole vectors directing toward the aliphatic chain provided 4.6 and 2.5 D, for the nonoxidized and oxidized form of 1, respectively.38 The smaller dipole moment of the oxidized form is consistent with the reduced electron density in the anthracene moiety. If we consider one molecule covering an approximated surface area of A = 4 nm2 the measured value of 13.5 mV would be reached at a tilt angle of ∼20 assuming that the orientation of the oxidized molecule retains after the reaction (see Figure S3 in the Supporting Information). Higher tilt angles as expected for molecules directly linked to the surface in monolayers would give higher values of the potential shift.32 The application of the Helmholtz equation to the present thicker nonordered spin coating films becomes more intricate as dipoles are not oriented in the same direction and the thickness and permittivity of the film are not uniform anymore. Moreover, strong interactions of dipoles in vertical direction deviate from the model system. However, a preference in orientation of dipoles can be expected as interactions at the interface between film and support haven an inductive effect. Obviously, thin films of monomeric anthracene 1 exhibit interesting material properties and the oxidized compounds show distinctive differences in their surface potentials. Furthermore, this behavior allows a sensitive, selective, and nondestructive read-out method by means of the KPFM technique. Studies of the Thermal Stability of the Patterned Film of 1. To accelerate the process of physical degrading the sample was in situ heated to 60 C. After the sample was cooled to room Langmuir 2010, 26(6), 4421–4428
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Figure 4. Changes of a patterned film of 1. (a) after heating at 60 C for 30 min; (left) SPM topography (Z range 250 nm, rms over the whole area is 9.5 nm), (middle) surface potential (Z range 150 mV), (right) optical image, and (b) after heating for 180 min; (left) SPM topography (Z range 200 nm, rms over the whole area is 15.2 nm), (middle) surface potential (Z range 180 mV), (right) optical image. The two lines in the topography of part b depict the direction of the height profile in Figure 5.
temperature another scan was taken at the same area (Figure 4). At 60 C the cleavage of the endoperoxide to the nonoxygenated form is still negligible. Any changes in the scanning images would therefore be of physical (nonchemical) origin. The potential differences in the KPFM images change from 13.5 to 32.6 mV after 60 min and finally to 28.2 mV after 180 min. These values correspond to the difference between valleys and hills (dark brown and yellow regions in Figure 4) and do not include the bright peaks which may result from topographic cross-talk. Note that the tip for KPFM image is lifted from the sample by 100 nm for a possible change in topography is not causing an increase in the potential difference after heating. The pattern structure is only slightly degraded and the photooxygenated species are still represented by the dark region. However, a significant change is apparent in the topographic images. As shown by the optical micrographs in Figure 4 the pattern structure becomes gradually more visible during the heating process. It is discernible from the topographies in Figure 4 that the roughness has increased during the period of heating. The new emerging rough portion in the scanned image in Figure 4 constitutes the nonirradiated part of the pattern structure as the peaks are located exactly where the potential in the KPFM image is more positive. The region corresponding to the photooxygenated part with more negative potential (dark brown), on the other hand, exhibits a smoother topography. The different behavior in the change of the roughness becomes clearer by displaying the height profiles across irradiated and nonirradiated regions (Figure 5). During the heating period the roughness has increased from ∼10 to ∼70 nm. The rms values determined exclusively within nonexposed areas increased correspondingly from 9.5 to 13.4 and finally to 23.7 nm, whereas the rms values of exposed regions ranged only from 6.2 to finally 7.2 nm. The effect of roughening of the nonirradiated species and the persistence of the irradiated portion of the film could be reproduced also on samples with larger pattern sectioning (see Figure S5, Supporting Information).38 If the areas of unexposed material became larger (∼1 mm2) the growth of protruding islands became clearly visible. The roughening effect would also explain the decrease in reactivity at the second irradiation cycle (Figure S2, Supporting Information). Langmuir 2010, 26(6), 4421–4428
Degradation of the patterned film of 1 became also visible after a few days when the sample was kept at room temperature. Moreover, deposition and patterning of thin films of 1 on other substrates as glass, gold, silver or copper caused the same effect of roughening. We can therefore conclude that thin films of monomeric anthracene 1 degrade upon moderate heating whereas the corresponding photooxygenated species remains physically stable. The increase of roughness arises from the formation of small crystallites. The propensity to form crystalline domains is likely as the planar anthracene units support intermolecular attractions. Thus, directly after spin coating the film of 1 is still smooth showing incipient formation of grains as recognizable in the AFM-topography (Figure 3a). This process is dramatically advancing at moderate heating (Figure 4 and Figure S5, Supporting Information). The same effect of crystallite formation was even observed on strongly hydrophobized silicon samples which were covered by a SAM of octadecylsilane. As these two extremely different surfaces exhibit both the formation of crystallites, the observed effect can be explained by a mechanism based on physical transitions between crystalline states and an explanation based on different adhesion forces can be ruled out. Indeed, differential scanning calorimetry (DSC) revealed such transitions below the melting point (mp = 126 C) at 5 and 116 C.38 As polymorphism of comparable long chain tethered 9,10-ethynylaryl-substituted anthracenes has already been reported, it is reasonable that compound 1 might behave similar.42 Therefore, the transition between these two states could cause the observed microcrystallization. The lack of such a transition for the oxygenated species could be explained by the loss of aromaticity of the anthracene core. Stacking effects due to π-π interactions between neighboring anthracene moieties contribute to the polymorphism which completely fail upon oxygenation.43 The monomeric system 1/1-O2 obviously acts as memory, storing the (42) Giminez, R.; Pinol, M.; Serrano, J. L. Chem. Mater. 2004, 16, 1377–1383. (43) Stacking effects of anthracene aggregates could be observed by UV spectroscopy. A comparison between the UV spectra of anthracene 1 in solution and as a thin film features a bathochromic shift of only ∼2 nm for the maxima in the film. However, a moderate red-shift of the maxima after heating of the film supports the explanation of a transition due to aggregation effects (see also Supporting Information, Figure S6).
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Figure 5. Height profiles of irradiated (red) and nonirradiated (green) portions of the patterned film of 1. (a) Directly after patterning, (b) after heating at 60 C for 30 min and (c) after heating at 60 C for 180 min.
imprinted image and responses with delay upon a change in topography. Application of the Patterned Film in Lithography. Because of a substantial solubility in unpolar solvents and the reduced contact area to the surface the nonoxidized species 1 can be selectively removed from the support with hexane. To exploit this feature for lithographic applications we exposed a patterned film after development to oxidizing acids. The lithographic procedure is shown in Figure 6. First, the photoresist 1 is deposited on a thin layer of silver and exposed to irradiation under a mask. Afterward, the unexposed portion is removed by dipping the sample into hexane. This process requires several minutes where the image gradually becomes visible. After development a 10% nitric acid solution was used to etch off the unprotected silver layer within a few minutes demonstrating a convenient lithographic process with monomeric anthracene 1. The use of smaller photomasks revealed that feature sizes of ∼10 μm can be transferred 4426 DOI: 10.1021/la904299n
into the metallic layer with high reproducibility of the master. The reproducibility of smaller sizes, however, was increasingly hampered by perforations in the remaining portion of the film. The endoperoxide layer protected well the underlying silver layer from the oxidizing acid which indeed makes the system 1/1-O2 suitable as photoresist. Photooxygenation of Thin Films of the Anthracene Oligomer 2. In the next part of our investigations we wanted to apply anthracenes as reversible photoswitches. For this purpose the physical stability of the system had to be remarkable enhanced, since repetitive cycles of writing and erasing would cause degrading of monomer 1. Therefore, we synthesized oligomeric compound 2 from an easily available styrene-substituted anthracene (Supporting Information). Indeed, patterning on thin films of such an oligomer resulted in discernible KPFM and fluorescence images (Figure 7). The averaged potential difference between oxidized and nonoxidized portion is 24 ( 8 mV. The motif Langmuir 2010, 26(6), 4421–4428
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Figure 6. Procedure of pattering, developing and etching of the anthracene photoresist 1 to give a silver-pattern. The photographs show this silver patterns by using a TEM grid and a transparency as masks.
Figure 7. Photopatterning of a thin film of the oligomer 2: (a) Topographic image with a Z range of 268 nm and an rms value of 32.3 nm, (b) surface potential image with a Z range of 180 mV, and (c) optical micrograph of the scanned area. The inset in part a shows the fluorescence of the patterned region.
Figure 8. Thermal treatment of a film of 2 at 130 C for 3 h followed by a second patterning step. (a) Topographic image with a Z range of 178 nm and an rms value of 20.1 nm, (b) surface potential image with a z range of 167 mV, and (c) optical image of the scanned area.
is not represented in the topography and the roughness induces again strong interactions with the KPFM. Apparently both the monomeric species 1 as well as the oligomer 2 undergo changes in the surface potential which allows a mapping of the imprinted pattern motif by KPFM. On the other hand, there are no changes in topography. Thus, either the difference in required space between reacted or unreacted species is too small or, in the case of the more fluid film of the monomer 1, the softness compensates the strain caused by the photoreaction. However, in contrast to the patterned film of the monomeric species heating at 60 C for more than 3 h did not cause any change in topography. Additionally, DSC measurements of the oligomeric anthracene 2 indicated no phase transition below the melting temperature of 175 C and thus, heating to 130 C for cleavage of the endoperoxide 2-O2 appeared reasonable. Indeed, after 60 min of heating the fluorescent pattern has vanished and a second imaging process at the same site where the first mask was located was carried out subsequently. Topography, surface Langmuir 2010, 26(6), 4421–4428
potential and optical and images of the rewriting process are shown in Figure 8. The annealing procedure has dramatically improved the topography and the KPFM contrast (ΔU = 81 ( 5 mV) after the second imaging process. This effect might be attributed to short-range reorientations of the molecules which occurred below the melting temperature but above the temperature required for the cleavage. The film of the oligomeric anthracene 2, however, persists during writing and erasing with no sign of a microscopic degradation. This proves the suitability for the oligomeric species to act as persistent and highly sensible storage medium. Furthermore, the patterned film of the oligomer is stable against the solvents that caused ablation of the monomeric species. The persistence of the oligomeric species in both, the oxidized and the nonoxidized state, could be ascribed to a belting effect of the backbone. Molecular reorientations required for the formation of crystalline domains as observed for the monomeric species 1 are consequently impeded. If π-stacking between two anthracene planes is the driving force for the formation of crystals the intramolecular strain would frustrate DOI: 10.1021/la904299n
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Article
Fudickar and Linker
Table 1. Advancing (θa) and Receding (θr) Contact Angles (deg) and RMS Values (nm) of the Films of the Monomer 1 and the Oligomer 2 and Their Photooxygenated Forms Measured at Room Temperature and after Heating at 80 and 130 C at 80 C
room temperature
at 130 C
anthracene
θa
θr
rms
θa
θr
rms
θa
θr
rms
1 1-O2 2 2-O2
101 100 101 103
81 79 85 82
9.5 9.5 32.3 32.3
89 98 105 102
63 72 82 83
27.7 7.2
98 100 105 107
67 72 75 76
20.1 20.1
such association. Therefore, it is also irrelevant whether the anthracene plane is oxidized or fully aromatic. For comparison of the wettability properties between the two types of films we measured advancing and receding water contact angles after irradiation, moderate heating and the erasing process. The two initial forms have high advancing contact angles at ∼100 and receding contact angles at ∼80 (Table 1). Within the experimental error ((2) oxygenation caused no changes on the contact angles though the observed decrease of the surface potentials account for a change in the polarity of the films. The steadiness of the surface wettability, however, is reasonable as the composition of the films has merely changed. The KPFM technique is therefore more responsive to the imaging process as it is not only susceptive for interfacial interactions. Strong changes of wettabilities of films, on the other hand, are rather to be expected when a reaction causes chemical degrading or a transition of the morphology of the material.35 Interestingly, the contact angles of the film of the oligomer 2 and the corresponding oxygenated form remained also constant after heating and erasing. This is indicative of the thermal persistency of the oligomer film. In contrast, the advancing and receding contact angles of the nonoxygenated film of the monomer 1 drop significantly after moderate heating and finally increase after annealing. This observation confirms the change of the morphology of the film of 1 which is also reflected in the change of the rms values. The enhancement of wettability explains also why a patterned film of 1 can be developed. Hexane as the most selective solvent penetrates deeper into the film and causes the ablation of the material which dissolves. The long alkyl chain may have an important effect to provide better solubility in the unpolar solvent. On the other hand, the small wettabilities of the system 2/2-O2 prevent the solvent from removing material. Thus, we found two very similar compounds 1 and 2, which are suitable either for lithographic applications or as stable photochromic materials. Photopatterning leads to structures of 1/1-O2 and 2/2-O2 after which the physical properties of the species undergo changes or persist under gentile thermal treatment. At this point, we might recall that both compounds react reversibly and remain stable on a chemical point of view with only moderate chemical degrading. Even the developable monomeric species allows erasing and rewriting as long as the physical transition has not progressed. However, in contrast to 2, 2-O2, and 1-O2 the morphology of the nonoxidized monomeric species changes with progressing time. If the driving force for this transition rests in the
4428 DOI: 10.1021/la904299n
propensity of the anthracene unit to aggregate it is reasonable that the oxygenation to 1-O2 has a stabilizing effect. The almost identical skeletons and the two long alkyl chains, suggest that the different behavior of 2 stems from the oligomeric nature. Compelling forces induced by the strain of the backbone prevent the anthracenes from aggregation providing an immobile and physically stable film. Additional evidence is gathered from the increased melting point of the oligomer 2 relative to the monomer 1 and the lack of any physical transition upon thermal treatment of 2 up to the melt. In contrast, a transition below the melting point of the monomer 1 indicates the existence of two morphologic forms. Such a transition leads to a change of free volume and the formation of crystalline domains. This effect causes the thermal instability of the nonoxidized portion of the film. In the oxidized state, however, the physical stability is higher and no transitions occur.
Conclusions We described new and easily available materials which exhibit properties as reversible photoswitches and in lithographic applications. Our system is based on the simple oxidation of anthracenes with light and air, providing endoperoxides remarkably influencing the surface potential. This interesting material property allowed the convenient and nondestructive read-out of pattern structures by means of KPFM in micrometer resolution. Topographic investigations at elevated temperatures reveal that thin films of the monomeric compound 1 exhibit degrading as small crystallites are forming whereas its oxidized form 1-O2 remains stable up to the onset of cycloreversion. The effect is explained by a change of the morphology which affects only the unreacted state. This was also confirmed by measurement of the wettabilities of oxidized and nonoxidized states. A film of 1 furnished with a photopattern acts as photoresist to transfer the pattern motif to an underlying silver layer by an etching procedure. In contrast, both the oxidized and nonoxidized states of the system 2/2-O2 are physically stable and the reversion into the original state causes no change in the topography. Thus, erasing and rewriting becomes possible for a film of 2. The system 2/2-O2 operates as photochromic storage media whereas the system 1/1-O2 acts as photoresist for lithographic applications. Since the two functions of the anthracenes can be easily adjusted by switching from monomers to oligomers, we expect various future applications of the new materials. Acknowledgment. We thank Daniel Zehm for DSC measurements and acknowledge generous financial support from the Deutsche Forschungsgemeinschaft (Li 556/9-1). Supporting Information Available: Text giving experimental details for the synthesis of compound 2 including structures and figures showing NMR and UV-vis spectra, structural orientation of the molecule, a DSC diagram of 2, and microscopic and topographic images of an irradiated film of 1. This material is available free of charge via the Internet at http://pubs.acs.org.
Langmuir 2010, 26(6), 4421–4428
