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Creating Bicolor Patterns via Selective Photobleaching with A Single Dye Species Liguo Gao,† Nan Lu,*,† Juanyuan Hao,† Wei Hu,† Gang Shi,† Yue Wang,† and Lifeng Chi*,†,‡ State Key Laboratory of Supramolecular Structure and Materials, Jilin UniVersity, Changchun, 130012, People’s Republic of China, and Physikalisches Institut and Center for Nanotechnology (CeNTech), Westfa¨lische Wilhelms-UniVersita¨t Mu¨nster, D-48149 Mu¨nster, Germany ReceiVed December 17, 2008. ReVised Manuscript ReceiVed January 21, 2009 Bicolor fluorescent pattern in thin polymer film is fabricated via a photobleaching process. Dye molecules exhibit monomer emission when they are dispersed inside the polymer and aggregate emission when they are on the surface of the polymer. Thus, a mixed emission of monomer and aggregate can be obtained by evaporating a single dye species on the polymer film. Bicolor pattern in thin polymer film is readily formed by selective photobleaching. This process is particularly attractive for the fabrication of bicolor patterns on flat substrates using a single dye species, which is of potential applications in photonic/electronic devices.
Introduction Fluorescence (FL) imaging in thin polymer films is of growing interest due to its potential applications in photonic/electronic devices such as optical data storage1,2 and displays.3-6 Functional FL images were first fabricated by selective immobilization of organic dyes in polymer films.7-9 Since the “precursor approach” for preparing fluorescent patterns in polymer films was presented by Kim et al., much effort has been devoted to this approach, which allows for the development of functional micrometersized patterning without employing wet developing processes.2-4,10-19 Prominent examples include protonation induced by a photoacid generator2,3,10-13 and chemical amplification catalyzed by a photoacid generator.14-18 Recently, photobase generators4 and photogenerated free radicals19 were also applied * Corresponding author. E-mail:
[email protected];
[email protected]. † Jilin University. ‡ Westfa¨lische Wilhelms-Universita¨t Mu¨nster.
(1) Tomasulo, M.; Raymo, F. M. J. Mater. Chem. 2005, 15, 4354. (2) Belfield, K. D.; Schafer, K. J. Chem. Mater. 2002, 14, 3656. (3) Pistolis, G.; Boyatzis, S.; Chatzichristidi, M.; Argitis, P. Chem. Mater. 2002, 14, 790. (4) Chae, K. H.; Kim, Y. H. AdV. Funct. Mater. 2007, 17, 3470. (5) Kocher, C.; Montali, A.; Smith, P.; Weder, C. AdV. Funct. Mater. 2001, 11, 31. (6) Vasilopoulou, M.; Georgiadou, D.; Pistolis, G.; Argitis, P. AdV. Funct. Mater. 2007, 17, 3477. (7) Vekselman, A. M.; Zhang, C.; Darling, G. D. Chem. Mater. 1997, 9, 1942. (8) Zhang, C.; Vekselman, A. M.; Darling, G. D. Chem. Mater. 1995, 7, 850. (9) Schilling, M.; Katz, H. E.; Houlihan, F. M.; Kometani, J. M.; Stein, S. M.; Nalamasu, O. Macromolecules 1995, 28, 110. (10) Kim, S.; Park, S. Y. AdV. Mater. 2003, 15, 1341. (11) Kim, J.-M.; Chang, T.-E.; Kang, J.-H.; Park, K. H.; Han, D.-K.; Ahn, K.-D. Angew. Chem., Int. Ed. 2000, 39, 1780. (12) Tian, H.; Gan, J.; Chen, K. C.; He, J.; Song, Q. L.; Hou, X. Y. J. Mater. Chem. 2002, 12, 1262. (13) Lee, C. W.; Yuan, Z.; Ahn, K. D.; Lee, S. H. Chem. Mater. 2002, 14, 4572. (14) Kim, J.-M.; Chang, T.-E.; Kang, J.-H.; Han, D.-K.; Ahn, K.-D. AdV. Mater. 1999, 11, 1499. (15) Kim, J.-M.; Min, S. J.; Lee, S. W.; Bok, J. H.; Kim, J. M. Chem. Commun. 2005, 3427. (16) Yang, N. C.; Choi, H. W.; Lee, J. K.; Hwang, J. I.; Suh, D. H. Chem. Lett. 2002, 824. (17) Frenette, M.; Coenjarts, C.; Scaiano, J. C. Macromol. Rapid Commun. 2004, 25, 1628. (18) Kim, J.-M.; Kang, J.-H.; Han, D.-K.; Lee, C.-W.; Ahn, K.-D. Chem. Mater. 1998, 10, 2332. (19) Coenjarts, C.; Garcia, O.; Llauger, L.; Palfreyman, J.; Vinette, A. L.; Scaiano, J. C. J. Am. Chem. Soc. 2003, 125, 620.
to obtain FL imaging in polymer films with high contrast and resolution. Besides the use of photogenerators, a very simple approach, the spatially resolved change of the chemical structure of the active species by irradiation through an appropriate mask (i.e., photobleaching), has been widely applied. The key concept of the photobleaching is to induce fluorescence change (either intensity or wavelength) of luminescent molecules in the UVirradiated areas. Typically, the result is a finely resolved FL image in which certain areas of the film are highly fluorescent and others are virtually nonemissive.20-23 Bicolor fluorescent imaging is also obtained by combination of two dyes with different photosensitivities.24 In this communication, we report an alternative method for fabricating bicolor pattern with a single dye species based on selective photobleaching in which a flat polymer film with blendlight-emitting was initially prepared by evaporating an amount of a single dye species onto the spin-coated polymer and performing an incubation process.25 Then a masked photobleaching process was conducted to selectively quench the aggregate emission, leaving only monomer emission on the exposed area of the polymer film, which results in the generation of the bicolor fluorescent pattern. The formation of bicolor patterns in thin polymer films will be of potential interest for the incorporation of photonic/electronic polymers in optical data storage devices or displays.
Results and Discussions The generation of bicolor fluorescent patterns is schematically shown in Figure 1. The organic molecule, 3-(9-anthrye) pyrazole (ANP) (see Figure 2), is selected for fabricating the bicolor patterns due to its luminescent properties upon crystallization.26 In this approach, ANP molecules were first evaporated onto the (20) Ohshita, J.; Uemura, T.; Kim, D. H.; Kunai, A. Macromolecules 2005, 38, 730–735. (21) Kwak, G.; Fujiki, M.; Sakaguchi, T.; Masuda, T. Macromolecules 2006, 39, 319. (22) Aoki, A.; Miyashita, T. Polymer 2001, 42, 7307. (23) Lee, J. K.; Kim, H.-J.; Kim, T. H.; Lee, C.-H.; Park, W. H.; Kim, J.; Lee, T. S. Macromolecules 2005, 38, 9427. (24) Kim, J. M.; Lee, Y. B.; Chae, S. K.; Ahn, D. J. AdV. Funct. Mater. 2006, 16, 2103. (25) Gao, L. G.; Lu, N.; Hao, J. Y.; Hu, W.; Wang, W. C.; Wu, Y.; Wang, Y.; Chi, L. F. Langmuir 2008, 24, 12745.
10.1021/la804145p CCC: $40.75 2009 American Chemical Society Published on Web 02/10/2009
Bicolor Patterns Via SelectiVe Photobleaching
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Figure 1. Schematic illustration of the fabrication of bicolor patterns on PMMA film via selective photobleaching. (1) Vacuum evaporating dye molecules on the flat PMMA film; (2) some dye molecules diffusing into the PMMA film and exhibiting monomer emission (blue) after incubation; and (3) creating bicolor pattern on the PMMA film via selective photobleaching.
Figure 2. Molecular structure of ANP (3-(9-anthryl) pyrazole).
polymethylmethacrylate (PMMA) film spin-coated on silicon wafer and incubated for a certain time to make ANP diffuse into PMMA film, and then the bicolor pattern was created on the PMMA film via a masked photobleaching. It is well-known that the aggregation of organic dye molecules can induce spectra shift, which could cover a wide spectral region.27-30 For ANP molecules, they can diffuse into polymers when they are deposited on both elastic and plastic polymers by vacuum evaporation.31 Their monomer and aggregate characteristics are maintained inside and outside the polymer, respectively.25 The spin-coated PMMA film with the thickness of 100 nm was chosen as a matrix, and its thickness can be controlled by adjusting the spin speed. Then 8 nm ANP were evaporated on the flat PMMA substrate under a base pressure of 5 × 10-4 Pa. The thickness of ANP was monitored by a quartz crystal microbalance (QCM) installed beside the sample holder. The photoluminescence (PL) spectra shown in Figure 3 indicate that only aggregate emission is detectable right after evaporation. After 5 weeks of incubation, both monomer and aggregate emissions can be observed on 100 nm PMMA substrate because some ANP molecules were “dissolved” into PMMA film and others still aggregated on the surface, leading to the mixture emission. The aggregates outside the polymer can be easily quenched using UV exposure, which allows for the fabrication of bicolor patterns by selective photobleaching. Photobleaching experiments were carried out under ambient conditions, mounted on a supporting frame. A beam of light from a mercury-arc lamp, with a certain extent of intensity in the UV regime, was focused on the sample. To investigate the photobleaching behavior of ANP aggregates on PMMA film (26) Zhang, H. Y.; Zhang, Z. L.; Ye, K. Q.; Zhang, J. Y.; Wang, Y. AdV. Mater. 2006, 18, 2369. (27) Liu, Y.; Nishiura, M.; Wang, Y.; Hou, Z. J. Am. Chem. Soc. 2006, 128, 5592. (28) Zhao, Y. S.; Fu, H. B.; Hu, F. Q.; Peng, A. D.; Yao, J. N. AdV. Mater. 2007, 19, 3554. (29) Brunner, K.; van Haare, J. A. E. H.; Langeveld-Voss, B. M. W.; Schoo, H. F. M.; Hofstraat, J. W.; van Dijken, A. J. Phys. Chem. B 2002, 106, 6834. (30) Zhang, G. Q.; Kooi, S. E.; Demas, J. N.; Fraser, C. L. AdV. Mater. 2008, 20, 2099. (31) Hu, W.; Lu, N.; Zhang, H. Y.; Wang, Y.; Kehagias, N.; Reboud, V.; Sotomayor Torres, C. M.; Hao, J. Y.; Li, W.; Fuchs, H.; Chi, L. F. AdV. Mater. 2007, 19, 2119.
Figure 3. PL spectra of 8 nm ANP on 100 nm PMMA film collected right after evaporation (solid line) and after 5 weeks of incubation (dashed line). (λex ) 370 nm).
upon the UV exposure time, the PL spectra of the evaporated ANP film (8 nm) were collected at different exposure times. As shown in Figure 4a, the PL intensity of aggregates decreases with extending the exposure time, and less than 11% is maintained after 30 min exposure. It can be observed that the PL intensity exponentially decreases (Figure 4a, inset), consistent with the first-order decomposition kinetics, which suggests that the PL intensity can be controlled by changing the energy of light. The blue-green and blue emissions can be observed on the film before and after 30 min of UV exposure, respectively (see Figure 4b,c). It should be noted that the effect of photobleaching in the present study is irreversible. To fabricate the fluorescent patterns based on the different emissions of a single molecule species, the following experiments were carried out. After the 8 nm ANP film was evaporated on PMMA film and incubated for 5 weeks, the film was exposured to UV light (40 mW/cm2) through a shadow mask with the periodicity of 13 µm. As revealed in Figure 5a, the quenching of some aggregates led to a decrease in the intensity of aggregate emission, which made the monomer emission more obvious. Figure 5b presents a blue and blue-green bicolor fluorescent pattern generated with selective photobleaching. By comparing Figure 5b with Figure 4b,c, it can be concluded that the blue and blue-green emissions are from the exposed and unexposed areas, respectively. Temperature plays an important role on the incubation time. Increasing the incubation temperatue can promote the diffusion of dye molecules into polymer film, which can result in a shorter fabrication time of bicolor fluorescent patterns. Figure 6a (solid line) shows that the monomer emission can be detected after incubation for 60 h at 60 °C, which is much shorter than that incubated at room temperature (5 weeks). For a certain polymer, the intensity ratio of blue to green-blue emissions on the bicolor pattern can be tuned by adjusting the
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Figure 4. (a) PL spectra of 8 nm ANP on PMMA film after 5 weeks of incubation with different UV exposure time (from top to bottom, after exposure for 0, 1, 3, 5, 7, 10, 20, and 30 min). The inset shows the development of the PL intensity measured at 490 nm. (λex ) 370 nm). (b) and (c) are the fluorescence images of the samples before exposure and after 30 min of UV exposure, respectively.
Figure 5. (a) PL spectra of the sample presented in Figure 4b after selective photobleaching (λex ) 370 nm). (b) The corresponding fluorescence microscopy image.
Figure 6. (a) PL spectra of the different amounts of ANP on 100 nm PMMA film collected after incubating for 60 h at 60 °C (λex ) 370 nm): 4 nm (dotted line), 6 nm (dashed line), and 8 nm (solid line). (b) The correlation of the emission intensity ratio of ANP monomer to aggregate and incubation time; the amount of ANP is 4 nm (9), 6 nm (2), and 8 nm (b); the thickness of PMMA film is 100 nm; the incubation temperature is 60 °C.
amount of ANP molecules or the thickness of the polymer. In this work, ANP molecules were evaporated onto the 100 nm PMMA films with three different thicknesses: 4 nm, 6 nm, and 8 nm. The monomer emission can be observed on all the samples after incubating for 60 h at 60 °C. As shown in Figure 6a, more ANP molecules remain on the surface of 8 nm samples than on 4 and 6 nm samples, which induces stronger aggregate emission on the sample of 8 nm. Figure 6b demonstrates that the intensity ratio of monomer to aggregate emissions increased at the beginning and kept constant after 60 h on these three samples. This indicates that the amounts of ANP on the surfaces of the samples are different after a complete diffusing process, which can induce different intensity ratios of monomer to aggregate emissions.
Conclusion A method for fabricating bicolor patterns via selective photobleaching using a single dye species is presented. With
ANP molecules, the pattern of blue and blue-green emissions was created on the flat PMMA film with shadow masked photobleaching. The intensity ratio of the monomer to aggregate emissions can be tuned by adjusting the amounts of ANP molecules or the thickness of PMMA film. This bicolor patterning method may have potential applications in full color displays and multiple optical data storage media.
Experimental Section Materials. 3-(9-Anthrye)pyrazole (ANP) was prepared as previously described.32,33 Ethanol, chloroform, and acetone were purchased form commercial sources at the highest available purity and used without further purification. PMMA (molecular weight Mw ) 50 kDa) was purchased from Microresist Technology GmbH, Germany. Si wafers were purchased from a commercial source. (32) Trofimenko, S.; Calabrese, J. C.; Thompson, J. S. Inorg. Chem. 1987, 26, 1007. (33) Trofimenko, S.; Calabrese, J. C.; Kochi, J. K.; Wolowiec, S.; Hulsbergen, F. B.; Reedijk, J. Inorg. Chem. 1992, 31, 3943.
Bicolor Patterns Via SelectiVe Photobleaching Fabrication of Bicolor Patterns. Si slices (approximately 2 × 2 cm2) were pretreated with oxygen plasma (PVA TePla O-Plasma System 100) at 300 W and 660 mTorr for 3 min and thoroughly cleaned with acetone and ethanol in an ultrasonic bath for 3 min, subsequently. Then, the substrates were rinsed with deionized water and dried under nitrogen gas flow before use. A PMMA film was spin-coated on the Si substrate; its thickness is controlled by adjusting spin speed. Then ANP was evaporated on a homemade evaporation system, and the amount of evaporated ANP was measured with a QCM. The base pressure was 5 × 10-4 Pa during the deposition, and the deposition rate was adjusted by changing the current for heating. The photobleaching process was performed in a cleanroom; all the other processes were carried out in a standard chemistry laboratory. The samples were exposed to UV light amplified by an object lens from a High Pressure Mercury Vapor Short Arc lamp (OSRAM, HBO103W/2). The irradiation power was 40 mW/cm2, which was controlled by using several neutral-density (ND) filters
Langmuir, Vol. 25, No. 6, 2009 3897 and was monitored by a UV-light meter (Lutron, YK-34UV). For the purpose of bicolor patterning, a lined structure with the period of 13 µm was used as a shadow mask. Typically, we observed that a 30 min exposure resulted in the best pattern. Characterization. Fluorescence images were taken on a fluorescence microscope (Olympus Reflected Fluorescence System BX51, Olympus, Japan). PL spectra were obtained on a fluorimeter (Shimadzu RF-5301PC, Shimadzu, Japan).
Acknowledgment. We gratefully acknowledge Prof. Yihua Zhang for PL spectra measurements. Financial support was given by the National Natural Science Foundation of China (20773052, 20373019, and 50520130316), the Program for New Century Excellent Talents in University, Program “111”, and the National Basic Research Program (2007CB808003 and 2009CB939701). LA804145P
