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Langmuir 1997, 13, 5504-5506
Notes Fatigue-Resistance Property of Diarylethene LB Films in Repeating Photochromic Reaction
Scheme 1
Shigeaki Abe,† Kingo Uchida,‡,§ Iwao Yamazaki,*,†,| and Masahiro Irie‡ Department of Molecular Chemistry, Graduate School of Engineering, Hokkaido University, Sapporo 060, Japan, and Department of Chemistry and Biochemistry, Graduate School of Engineering, Kyushu University, Hakozaki, Hukuoka 812, Japan Received May 28, 1997. In Final Form: July 30, 1997
Introduction Diarylethenes were found to exhibit a superior property in the photochromic reaction by Irie et al.1-3 A distinct fatigue resistibility to repeating forward and backward photoreactions was shown for diarylethene in fluid solution, making a striking contrast to other photochromic molecules like spiropyran, azobenzene, and fulgides. Moreover, it was shown that a diarylethene derivative with a long alkyl chain is incorporated suitably into an LB monolayer film mixed with tripalmitin and that the photochromic reaction occurs in LB films with relatively high quantum yields, Φ (open formfclosed form) ) 0.27 and Φ (open formrclosed form) ) 0.10, which are similar to those in solution.4 A problem still remains concerning the fatigue-resistance property in LB film, i.e., whether or not diarylethene shows a good stability in LB film similarly to that in fluid solution. We here reports the stability of diarylethene 1 in LB film through a comparative study with spiropyran/merocyanine 2a/2b and 3a/3b (Scheme 1). Diarylethene is interchanged between a ring-open form 1a and a closed form 1b by light irradiation of 300 and 550 nm, respectively. In usual solution experiments, the reaction properties are examined simply by monitoring the absorption intensity change of the reactant or the product with high accuracy. In contrast, an LB monolayer film of diarylethene shows only an extremely weak absorption of 0.0002 at most in absorbance. The weakness in absorption prevents a detailed analysis of the reaction property in the LB film from being made. However, by applying the excitation energy transfer in an LB double layer film consisting of diarylethene monolayer (acceptor) and dye monolayer (donor), a highly sensitive measurement of the reaction property is possible through detecting the donor fluorescence. This method of the energy transfer was reported previously by Polymeropoulos and Mo¨bius5 †
Hokkaido University. Kyushu University. § Present address: Department of Materials Chemistry, Ryukoku University, Seta, Otsu 520-21, Japan. | Telephone: 81-11-706-6606. FAX: 81-11-709-2037. E-mail:
[email protected]. ‡
(1) Irie, M.; Sakemura, K.; Okinaka, M.; Uchida, K. J. Org. Chem. 1995, 60, 8305. (2) Uchida, K.; Irie, M. Chem. Lett. 1995, 969. (3) Irie, M.; Uchida, K.; Eriguchi, T.; Tsuzaki, H. Chem. Lett. 1995, 899. (4) Abe, S.; Sugai, A.; Yamazaki, I.; Irie, M. Chem. Lett. 1995, 69. (5) Polymeropoulos, E. E.; Mo¨bius, D. Ber. Bunsen-Ges. Phys. Chem. 1979, 1215.
S0743-7463(97)00555-6 CCC: $14.00
and recently by Yamazaki et al.6 In the present study, the fatigue resistance property has been examined for diarylethene LB films with the energy transfer method and compared with two types of spiropyran LB films. Experimental Section Diarylethene derivative 1 was synthesized by a method reported previously.7,8 Spiropyran derivatives 2 and 3 were purchased from Nippon Kanko Shikiso Co. and were used without further purification. Oxacarbocyanine 4 (Nippon Kanko Shikiso Co.) was purified by repeated recrystallization from ethanol. The LB films were prepared with a Langmuir trough (San-Yesu Instrument Co., FSD-20). The subphase containing CdCl2 (3 × 10-4 M) was adjusted to be pH 6.7 by adding NaHCO3 buffer solution. The trough temperature was held constant at 15 °C during deposition of monolayer into a substrate. After evaporation of solvent, the monolayer was compressed and deposited on a quartz plate under a constant surface pressure. The photochromic layer containing 1, 2, or 3 was prepared as a mixed layer with tripalmitin (molecular ratio 1:4). The donor layer of oxacarbocyanine 4 was prepared also as a mixed layer (molecular ratio 1:19) with a mixture of arachidic acid and methyl arachidate in a ratio of 5:1. First, three layers of cadmium arachidate were precoated to make the glass surface uniform and hydrophobic at 25 mN/m. Second, the donor layer was deposited at 23 mN/m, and then the photochromic layer was deposited at 15 mN/m for 1 and at 20 mN/m for 2 and 3. Finally two layers of cadmium arachidate were covered. The structure of the LB multilayers is illustrated schematically in Figure 1. The photochromic switching and the fluorescence measurement were performed by means of a gated spectrophotometer,6 equipped with two light sources, a controlling light (20 mW/cm2) for inducing the photochromic reaction and an excitation light (0.5 mW/cm2) for generating a fluorescence of donor 4. The sample was set in a vacuum vessel with four quartz windows. The fluorescence was collected at right angles to the excitation light and monitored with a monochromator and a photomultiplier (Hamamatsu R-446). The sample was irradiated with the controlling light: UV light (300 nm for 1 and 366 nm for spiropyran 2 and 3) for coloration and vis light (577 nm) for (6) Yamazaki, I.; Okazaki, S.; Minami, T.; Ohta, N. Appl. Opt. 1994, 33, 7561. (7) Nakayama, Y.; Hayashi, K.; Irie, M. J. Org. Chem. 1990, 55, 2592. (8) Nakayama, Y.; Hayashi, K.; Irie, M. Bull. Chem. Soc. Jpn. 1991, 64, 789.
© 1997 American Chemical Society
Notes
Langmuir, Vol. 13, No. 20, 1997 5505
Figure 1. Schematic illustration of structure of LB multilayers.
Figure 2. Absorption spectra of diarylethenes 1a (dashed line) and 1b (solid line) (a) and spiropyrans 3a (dashed line) and 3b (solid line) (b) in LB monolayer films. Absorption (solid line) and fluorescence spectra (dotted line) of oxacarbocyanine 4 (c) in LB monolayer film. decoloration. This controlling light UV or vis light irradiates the sample for 5 s and induces the photochromic reaction. Then the donor fluorescence was observed at 510 nm with excitation at 460 nm for 1 s. The unit process of irradiation and measurement was repeated 30 times, and then the controlling light was switched from UV to vis and vice versa.
Results and Discussion Parts a and b of Figure 2 show absorption spectra of diarylethene and spiropyran in LB films. For the two isomers of diarylethene, i.e., the ring-open form 1a and the closed form 1b, the open form exhibits an absorption in a short wavelength region less than 350 nm, and the closed form exhibits an absorption band centered at 550 nm. For spiropyran 3a and merocyanine 3b, spiropyran exhibits an absorption in a short wavelength region less than 400 nm, and merocyanine shows an absorption band around 550-600 nm. The spectra of 2a and 2b are almost identical with those of 3a and 3b except for slight red shifts of 2-5 nm. Oxacarbocyanine 4, a donor molecule
Figure 3. Fluorescence intensity changes of oxacarbocyanine 4 due to the energy transfer to the acceptor of which the concentration is changed by a photochromic reaction: (a) oxacarbocyanine-diarylethene 1 and (b and c) oxacarbocyanine-spiropyran 2 and 3, respectively, in LB multilayers.
in the energy transfer, shows absorption and fluorescence spectra in a wavelength region much shorter than those of the colored forms of photochromic molecules. The absorption band of the colored forms 1b, 2b, and 3b and the fluorescence band of oxacarbocyanine 4 show substantial spectral overlap, and the Fo¨rster excitation energy transfer can take place from the photoexcited oxacarbocyanine to the ground-state colored forms. In the LB multilayer film, the donor and acceptor molecules are stacked 25 Å apart so that the excitation energy transfer takes place straightforward within 200 ps.6 On the other hand, the S1 energy levels of the decolored forms 1a, 2a and 3a are substantially higher than that of oxacarbocyanine and then the energy transfer cannot proceed. Since the fluorescence emission in oxacarbocyanine is competing with the energy transfer, then the fluorescence intensity of oxacarbocyanine should decrease under UV irradiation and increase under vis irradiation. The experimental results are shown in Figure 3 for the three systems, diarylethene 1-oxacarbocyanine and spiropyrans 2 and 3-oxacarbocyanine. In all the cases, the fluorescence intensity of oxacarbocyanine is changed upon changing irradiation between UV and vis; UV irradiation reduces the fluorescence intensity by about 10% due to the energy transfer to the colored form of diarylethene or spiropyran. The fluctuation in fluorescence intensity as is seen in every case in Figure 3 is due to the fluctuation of the excitation light. Besides this fluctuation, one should note here that the intensity-change profiles are different between diarylethene and spiropyran with respect to amount of the intensity change. In diarylethene, the intensity change is approximately constant during a time period of the present experiment, while in spiropyran it decreases with repeating the photochromic reaction. To examine the intensity change quantitatively, we here take a fraction of the intensity difference in the nth repetition, η(n), defined as η(n) ) {Id(n) - Ic(n)}/{Id(0) - Ic(0)}, where Id(n) and Ic(n) are the fluorescence intensities when the photochromic compound is in decolored and colored forms, respectively. Figure 4 shows plots of η(n) values as a function of repetition number n and the light irradiation time. It is seen that η(n) of diarylethene LB film is constant during light irradiation of 104 s, while those of spiropyran LB films undergo a prompt bleaching with an η(n) value being 0.5 of the initial value at 1500 s. This result indicates that the diarylethene LB film has a superior property with
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Figure 4. Fatigue-resistance property of photochromic compounds in the LB multilayer: (O, b, 0) diarylethene 1 (three measurements); (4) spiropyran 2; (1) and spiropyran 3.
respect to fatigue resistance in comparison with those of spiropyran LB films. The bleaching of spiropyran/merocyanine photoreaction may arise from a structural change of the LB monolayer upon light irradiation or an irreversible side reaction of spiropyran/merocyanine. It can be seen from parts a and b of Figure 3 that any structural change of the LB multilayer is negligible since the intensity of the oxacarbocyanine fluorescence under vis irradiation is almost constant except for the small fluctuation; in other words, a line connecting the upper margin in the modulated curve gives a horizontal line. Then we consider it from a
Notes
viewpoint of the side reaction. According to our previous study,9 the quantum yields of the photochemical reaction of spiropyran in LB films are Φ(spiropyran f merocyanine) ) 0.44 and Φ(spiropyran r merocyanine) ) 0.02. The picosecond time-resolved fluorescence measurement9 revealed that the reaction rate and yield depend strongly on rigidity of the reaction cage surrounding merocyanine chromophore. Note that this reaction is associated with a relatively large conformational change, and the molecular environment must determine a blanching ratio of the side reaction to the primary reaction. Also noteworthy is that the quantum yield of decoloring reaction in spiropyran (0.02) is very low relative to that in diarylethene (0.10). This may suggest that the decoloring reaction is associated with a nonneglible side reaction, and the concentration of merocyanine decreases with repeating the photoreaction as seen in parts a and b of Figures 3. Therefore it can be considered that the prompt bleaching of spiropyran is accounted for as being due to an irreversible side reaction in LB films. In fact, a recent study by Sakuragi et al.10 shows a byproduct with a substantial amount of quantum yield. On the other hand, the photoreaction of diarylethene is associated with a much smaller conformational change, and indeed its byproduct (decomposition) yield is negligibly small (0.25 × 10-5).11 LA970555G (9) Minami, T.; Tamai, N.; Yamazaki, T.; Yamazaki, I. J. Phys. Chem. 1991, 95, 3988. (10) Sakuragi, M.; Aoki, K.; Tamaki, T.; Ichimura, K. Bull. Chem. Soc. Jpn. 1990, 63, 74. (11) Uchida, K.; Nakayama, Y.; Irie, M. Bull. Chem. Soc. Jpn. 1990, 63, 1311.
