© Copyright 2001 American Chemical Society
NOVEMBER 27, 2001 VOLUME 17, NUMBER 24
Letters In Situ Monitoring of Photo-Cross-Linking Reaction of Anthracene Chromophores in Polymer Langmuir-Blodgett Films by an Integrated Optical Waveguide Technique Masaya Mitsuishi, Tomohiro Tanuma, Jun Matsui, Jinfeng Chen, and Tokuji Miyashita* Institute of Multidisciplinary Research for Advanced Materials (IMRAM), Tohoku University, Katahira 2-1-1, Aoba-ku, Sendai 980-8577, Japan Received June 4, 2001. In Final Form: August 7, 2001 This paper focuses on waveguide technique versatility for direct observation of photochemical reactions in polymer ultrathin films. Monolayers from the copolymer of 9-anthrylmethyl methacrylate (AMMA) with neopentyl methacrylamide (nPMA) were deposited on a tapered quartz waveguide by the LangmuirBlodgett (LB) technique, and spectroscopic properties of the copolymer (p(nPMA-AMMA)) monolayer were precisely monitored by an integrated optical waveguide technique. Magnitude of absorbance increased by a factor of 200, compared with transmission UV-vis spectroscopy. Consequently, a photo-cross-linking reaction of p(nPMA-AMMA) LB films was characterized easily and very accurately as a function of irradiation time.
Introduction Recently, much effort has been devoted to construct functional nanoassemblies. As one powerful tool for this purpose, the Langmuir-Blodgett (LB) technique provides deposition of ultrathin layer assemblies at the molecular scale. For example, the thickness of a poly(alkyl acrylamide) LB monolayer varied in the range of 1-2 nm.1,2 This means that various functional groups can be confined in the nanostructure with high orientation and desired arrangement. It is important to understand spectroscopic properties of nanoassemblies such as the LB film because such information gives insights into organic nanophotonic device nanofabrication. However, spectroscopic study of ultrathin films by conventional transmission UV-vis absorption spectroscopy presents difficulty in resolution due to shortage of optical path length. * To whom correspondence may be addressed. (1) Miyashita, T. Prog. Polym. Sci. 1993, 18, 294. (2) Taniguchi, T.; Yokoyama, Y.; Miyashita, T. Macromolecules 1997, 30, 3646.
Since Swalen and his colleagues3 reported monolayer spectroscopic properties in 1978, much attention has been paid to spectroscopy based on waveguide optics; a slab optical waveguide (SOWG) technique has been intensively used by some researchers.4-6 In general, optical technique allows some observation advantages, including timeresolved measurement and in situ monitoring. This paper presents waveguide technique applications for in situ observation of an anthracene group photochemical reaction in polymer LB films. Experimental Section The copolymer of 9-anthrylmethyl methacrylate (AMMA) with neopentyl methacrylamide (nPMA), p(nPMA-AMMA), was syn(3) Swalen, J. D.; Tacke, M.; Santo, R.; Rieckhoff, K. E.; Fischer, J. Helv. Chim. Acta 1978, 61, 960. (4) Stephens, D. A.; Bohn, P. W. Anal. Chem. 1989, 61, 386. (5) Matsuda, N.; Takatsu, A.; Kato, K. Chem. Lett. 1996, 105. (6) Plowman, T. E.; Saavedra, S. S.; Reichert, W. M. Biomaterials 1998, 19, 341.
10.1021/la010822p CCC: $20.00 © 2001 American Chemical Society Published on Web 11/01/2001
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Figure 1. Chemical structure of p(nPMA-AMMA).
Figure 3. Absorption spectra of the p(nPMA-AMMA) monolayer. The absorption spectrum was obtained by waveguide spectroscopy (solid line), and the spectrum was measured with a conventional transmission spectrometer (dashed line).
Figure 2. Experimental setup used for absorption spectra measurements. thesized by free radical polymerization (Figure 1). Mole content of the AMMA group was determined with UV-vis absorption spectroscopy. A quartz slide (0.2 mm thickness, 65 mm × 20 mm) tapered at both sides by 60° was used as a substrate. The p(nPMA-AMMA) monolayer was prepared using a Langmuir trough (HBM, Kyowa Interface Science, Co., Ltd.). The monolayer was deposited on the waveguide surface by the vertical dipping method keeping surface pressure and temperature at 20 mN/m and 15 °C, respectively. This allows recording of absorption spectra in the wide range of wavelengths from 250 to 800 nm. Absorption spectra of p(nPMA-AMMA) LB films were measured by surface and interface spectrometer (SIS-50, System Instruments Co., Ltd.). Figure 2 shows the experimental setup schematically. White light from a 150-W Xe lamp is utilized as a signal. The light impinges on the tapered quartz substrate through a quartz fiber and a focusing lens. Optical waves propagate in the quartz slide, undergoing total internal reflection at the slide interface. The output signal was detected by a CCD spectrograph and spectra were stored in a personal computer. For photochemical reaction observation, a deep UV light (MDX500MA, USHIO) was utilized as an irradiation source. Irradiated light power density was equal to 1.0 mW/cm2 with neutral density filters and a UV cutoff filter (>350 nm). Propagation direction of near-UV light (>350 nm) was set perpendicular to the waveguide plane as shown in Figure 2. Since power was fairly low, effects of irradiated light on absorption spectra were negligible. Absorption spectra were simultaneously recorded as a function of irradiation time. Measurements were made in an argon atmosphere to prevent photooxidation of AMMA.
Results and Discussion Figure 3 shows absorption spectra of the p(nPMAAMMA) LB monolayer, which contains 5 mol % of AMMA. The characteristic band of AMMA groups is clearly observed around 370 nm (solid line). In comparison with absorption spectra of the p(nPMA-AMMA) LB monolayer measured by a transmission UV-vis spectrophotometer (dashed line, Figure 3), the optical waveguide measurement clearly provides us with greatly enhanced sensitivity. Sensitivity increased approximately by a factor of 200. In this case, guided optical waves repeat total internal reflection ca. 130 times.6 This is the predominant contributing factor for increased sensitivity. The photochemi-
Figure 4. Time course of UV absorption spectra of p(nPMAAMMA) LB films with two layers as a function of irradiation time.
cal reaction of AMMA groups in the polymer LB monolayer is investigated to demonstrate this feature. It is known that anthracene groups undergo a photocross-linking reaction by irradiation of near-UV light.7-13 In fact, a fine negative-tone pattern of p(nPMA-AMMA) LB films was found by irradiation (>350 nm).14,15 Consequently, a p(nPMA-AMMA) LB film with two layers was irradiated by near-UV light at >350 nm. Changes in absorption spectra of p(nPMA-AMMA) LB films with two layers are shown in Figure 4. These layers were deposited on the tapered quartz substrate so that the neopentyl side chains face each other between layers. As shown in Figure 4, absorbance in the range around 370 nm is decreased as irradiation time proceeds, while absorbance is increased in the range below 300 nm. (7) Greene, F. D.; Misrock, S. L.; Wolfe, J. R. J. Am. Chem. Soc. 1955, 77, 3852. (8) Birks, J. B. Photophysics of Aromatic Molecules; Wiley: New York, 1970; p 320. (9) De Schryver, F. C.; Anand, L.; Smets, G.; Switten, J. J. Polym. Sci., Part B 1971, 9, 777. (10) Rånby, B.; Rabek, J. F. Photodegradation, Photooxidation and Photostabilization of Polymers; John Wiley & Sons Ltd.: New York, 1975; p 313 and references therein. (11) Guillet, J. Polymer Photophysics and Photochemistry; Cambridge University Press: New York, 1985. (12) Paul, S.; Stein, S.; Knoll, W.; Mu¨llen, K. Acta Polym. 1994, 45, 235. (13) Fox M. A.; Wooten, M. D. Langmuir 1997, 13, 7099. (14) Li, T. Ph.D. Thesis, Tohoku University, 2001. (15) Li, T.; Mitsuishi, M.; Miyashita, T. To be submitted for publication.
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Figure 5. Absorbance change at 367 nm for the p(nPMAAMMA) with monolayer (open circles) and two layers (closed circles) as a function of irradiation time.
If AMMA group photodimerization obeys bimolecular reaction kinetics,16 absorbance should decrease with irradiation time as
1 ) -kt + C A(t) where A(t) is optical density, k and t are a bimolecular rate constant and irradiation time, respectively, and C is a constant. Figure 5 shows plots of the reciprocal of absorbance at 367 nm vs irradiation time. The linear relationship between them clearly shows that the photochemical reaction apparently occurs based on bimolecular reaction kinetics. Interestingly, no significant absorption change was observed in the p(nPMA-AMMA) monolayer within the observed time range, while absorbance of p(nPMA-AMMA) LB films with two layers gradually decreased with irradiation time (Figure 5). The distance of neighboring anthracene groups in the polymer monolayer is calculated to be ca. 2.8 nm, assuming a random distribution.17 On the other hand, monolayer thickness determined from the surface plasmon technique18 equals 1.1 nm. In addition, the anthracene group is located between the neopentyl side chains and oriented almost parallel to the surface from the molecular occupied surface area determined from the surface area-pressure isotherms. Therefore, it seems that the photo-cross-linking reaction occurs more easily between adjacent p(nPMAAMMA) LB monolayers than in the p(nPMA-AMMA) monolayer. In the time range observed, half of the anthracene group remained as shown in Figure 4. It has been reported that the effective photo-cross-linking reac-
Figure 6. Schematic illustration of the photo-cross-linking reaction in p(nPMA-AMMA) LB films with two layers.
tion occurred in cast films due to high mobility of the anthracene group.19 Anthracene group motion, however, is constrained by the neopentyl side chain similar to the pyrene group incorporated in the tert-pentyl acrylamide copolymer LB films.20 These are feasible reasons for slow photochemical reaction in LB films as well as for difficulty of the photodimerization in the monolayer. Photodimerization between the anthracene groups in the p(nPMAAMMA) LB film is schematically illustrated in Figure 6. Detailed analysis is now in progress. In conclusion, high sensitivity of present techniques was demonstrated in the case of observing the photochemical reaction in p(nPMA-AMMA) LB films. Absorption spectra of the p(nPMA-AMMA) monolayer were observed with high sensitivity and accuracy. Sensitivity increased by a factor of 200. Since photofunctional molecules such as anthracene groups have widely various functionality, i.e., electron transfer, energy transfer, and molecular sensing, the combination of photofunctional ultrathin assemblies with highly sensitive spectroscopy will give us a better understanding in the photophysical and photochemical fields and, in particular, the nanophotonic field. Acknowledgment. This work was supported by a Grant-in-Aid for Scientific Research (No. 12450342) by the Ministry of Education, Culture, Sports, Science, and Technology of Japan. Also, J.M. thanks the Japan Society for the Promotion of Science for Young Scientists. LA010822P
(16) Aoki, A.; Nakaya, M.; Miyashita, T. Macromolecules 1998, 31, 7321. (17) Ohmori, S.; Ito, S.; Yamamoto M. Macromolecules 1990, 23, 4047. (18) Knoll, W. Mater. Res. Bull. 1991, 16. 29.
(19) Hargreaves J. S.; Webber, S. E. Macromolecules 1984, 17, 235. (20) Matsui, J.; Mitsuishi, M.; Miyashita, T. Macromolecules 1999, 32, 381.
