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Molecular diffusion in a polymer matrix was studied to prevent degradation of photochromic reactions during repeated coloration-decoloration processes. Photochromic diarylethene was dispersed in polydimethylsiloxane (cured polymer), since it promoted exchange of damaged and fresh molecules owing to high diffusivity. The diffusion coefficient was evaluated by measuring a distribution of dye molecules that were colored within a narrow laser beam path. Temporal change of the distribution fitted well to theoretical curves that were drawn according to the 2-D solutions of Fick’s equation. The experimental results indicated a fifteen-fold enhancement of the diffusion coefficient (0.0015 mm2/s) when the polymer was swollen with toluene. Fading of this photochromic polymer was examined by repeating alternative irradiation of violet and green laser beams. Although a non-swollen polymer faded seriously within 1,000 photochromic cycles, a swollen polymer exhibited an excellent photochromic function even after 30,000 cycles.
© 2015 Optical Society of America
1. Introduction
Polydimethylsiloxane (PDMS) oil turns to a transparent solid (elastomer) when the constituent molecules are bridged with one another creating a flexible network. This chemical reaction proceeds by only adding a curing agent to the oil; i.e., neither heating nor light irradiation is needed. Owing to this simple solidification process (molding capability) as well as thermal and chemical stabilities, PDMS are used widely in optical technology including nanoimprinting and LED sealing [1–3]. They also provide a suitable matrix for dispersing organic dyes and nanoparticles [4, 5]. The flexible PDMS matrix is useful to realize a spectral tuning of photonic crystals and laser oscillators [6–11]. Further, this flexible molecular network creates a large free volume in the solid matrix, and accordingly, allows guest molecules to circulate as they do in liquid. PDMS elastomers therefore exhibit a higher molecular diffusivity than other polymers and glasses [12–17]. Recent experiments have revealed that even metal particles circulate in the PDMS matrix [5, 18].
The high molecular diffusivity of PDMS has been used for drug delivery in the fields of medical science [19–21]. This concept of molecular transportation is applicable to lifetime extension of dye lasers, since damaged dye molecules can be replaced by fresh ones owing to their circulation [22]. The diffusion coefficient, however, has to be enhanced further to prevent laser power reduction [4]. In previous studies, we proposed to enhance the diffusivity by swelling PDMS elastomers with toluene [23, 24]. PDMS elastomers can contain a large amount of toluene in its flexible molecular network [1, 7, 19, 25, 26]. Consequently, guest molecules diffuse rapidly through the expanded free volume.
PDMS elastomers provide a suitable matrix particularly for photochromic dyes. Photochromic dyes are promising materials for creating all-optical switches or self-controlled photonic devices, since their absorption coefficient and refractive index are controllable by light irradiation [24, 27–29]. These functions are induced by deformation of the molecular structure (isomerization). Diarylethene, for example, has two stable isomers that are shown in Fig. 1(a). A transparent isomer turns to red by exposure to violet light, and the red isomer is decolored by exposure to green light [23, 28]. Although these photochromic reactions readily take place in dye solutions, the reaction rate and efficiency decrease heavily in solid matrices, since molecules are difficult to deform in a narrow, rigid space [29]. Fading of dye molecules is also a serious problem when creating optical devices [27, 30]. Besides the two isomers shown in Fig. 1(a), diarylethene has a third isomer that exhibits no photochromism. Violet laser irradiation occasionally induces molecular deformation to the third isomer, which leads to the fading phenomenon. Liquid (dye solution) is preferred from the viewpoint of the photochromic efficiency or fade-resistivity, since a solid matrix usually hinders both deformation and diffusion (replacement) of dye molecules. From the viewpoint of device fabrication, however, a solid matrix is advantageous, since solidification not only facilitates material handling but also ensures a stable switching operation in which a molecular state (transparent or colored) has to be preserved during a signal transmission period. Otherwise, if dye molecules are dissolved in liquid, convection causes them to diffuse out of the optical path too rapidly or randomly, and hence, an accidental malfunction takes place in the device operation. Rapid circulation does not matter in dye lasers, since organic dye emits fluorescence within ~10 ns after excitation (before escaping from the optical path). In dye lasers, therefore, replacement of damaged molecules is achievable by circulating a dye solution [22]. Photochromic molecules, however, have to stay in the optical path until signal pulses arrive there (typically ~1 ms or longer depending on applications). Therefore a matrix with a suitable diffusion coefficient is needed so that photochromic molecules are replaced at a moderate rate. A large, flexible free-volume in PDMS is useful for allowing both molecular deformation and moderate diffusion [31].
Fig. 1 (a) Structures of transparent and colored diarylethene molecules. (b) Front and side (cross-sectional) views of the sample that was prepared for evaluation of the diffusion coefficient. A violet (or green) laser beam passed through the sample center, inducing photochromic coloration (or decoloration). Transmission spectra were measured at a distance x form the center.
In previous experiments, we achieved a rough estimation of the diffusion coefficient and demonstrated the effect of molecular circulation on the self-healing function of photochromic polymers [23, 24]. However, this estimation lacked accuracy, since experiments were conducted with small cylindrical samples for which the diffusion equation (Fick’s law) was difficult to solve (see Section 2) [32]. In addition, the self-healing function was still insufficient for preventing degradation of optical devices; i.e., in spite of the molecular replacement, the sample still suffered degradation after 3,000 photochromic cycles. In this study, we prepared thin, flat samples so that 2-D diffusion took place in the polymer. The diffusion coefficients were evaluated by comparing the experimental results with theoretical curves. On the basis of these results we prepared photochromic polymers, and examined their resistivity by repeating coloration-decoloration processes. There have been few reports on the lifetime of photochromic devices [23, 27, 30, 33], probably because their resistivity used to be too poor. In one of those conventional studies, photochromic processes of 14,000 cycles were demonstrated with diarylethene. For breaking this record, we continued photochromic coloration and decoloration for longer than 40 days until the repetition reached 30,000 cycles.
2. Diffusion coefficient
Figure 1(b) shows a sample that was prepared for evaluating the diffusion coefficient of dye molecules in a PDMS matrix. A toluene solution of diarylethene (Tokyo Kasei Kogyo, B1535) was mixed with PDMS oil that contained a curing agent (Shin-Etsu Chemical, KE-103). This mixture was enclosed in a sample cell consisting of two glass plates and an O ring (50 mm diameter, 3 mm width). The glass plates were pressed with aluminum plates and screws for sealing the sample. The curing agent promoted bridging reactions of PDMS molecules, and solidification was complete in 8 h at room temperature. The dye and toluene concentrations in the prepared samples were 10−4 mol/l and 1–60 vol%, respectively.
The entire sample was transparent after solidification. When a violet laser beam (405 nm wavelength, 0.2 mW, 2 mm diameter) was irradiated at the sample center, a red spot appeared and expanded as colored molecules diffused outward. Figure 2(a) shows transmission spectra that were measured at different positions in the sample after continuing the laser irradiation for 3 h. At the sample center (x = 0 mm), an absorption band was created by the laser beam. This absorption band was also visible at x = ± 2 mm, where no laser beam was irradiated. At distant positions (x = ± 8 mm), however, the sample remained transparent. Next, the reverse process was examined in the following method. The entire sample was colored uniformly by irradiating an expanded violet laser beam for 10 min. Then a green laser beam (532 nm, 0.2 mW, 1.5 mm diameter) was irradiated at the sample center to decolor the dye molecules. Consequently, the absorption band disappeared at the center (x = 0 mm), as shown in Fig. 2(b). When the green laser irradiation was continued for 3 h, the absorption band shrank at x = ± 2 mm, but no spectral change was visible at x = ± 8 mm.
Fig. 2 (a) Transmission spectra that were measured at different positions (x = 0, ± 2, or ± 8 mm) in the transparent sample (toluene: 60 vol%). A violet laser beam was irradiated at the sample center for 3 h. (b) Transmission spectra during the decoloration process. After the entire sample was colored with a violet laser beam, a green laser beam was irradiated at the center for 3 h. Measurements were conducted at room temperature (~30 °C).
The dye diffusion process was measured in this manner for the samples with a toluene concentration of 1 or 60 vol%. Figure 3 shows distributions of the optical density, –logT (T: transmittance at 530 nm), which corresponds to the concentration of colored molecules. In the coloration process, the optical density distribution decreased toward the outside, indicating an outward diffusion of colored molecules. The reverse distribution was visible in the decoloration process, which indicated that decolored molecules diffused outward and colored molecules diffused into the laser irradiation region. When the toluene concentration was 1 vol%, the distributions were limited within ~8 mm distance after the 15-h irradiation, as shown in Figs. 3(a) and 3(b). By increasing the toluene concentration to 60 vol%, the distributions extended beyond 12 mm.
Fig. 3 Optical density distributions during the laser irradiation process of 15 h. The sample center (x = 0) was irradiated by (a, c) the violet or (b, d) green laser beam (2 or 1.5 mm diameter). The irradiation was paused momentarily for the spectral measurement at x = 0 (Δ, 3 h). The toluene concentration was (a, b) 1 or (c, d) 60 vol%. The black lines show the theoretical distributions that were calculated by assuming a diffusion coefficient of (a, b) 0.0001 or (c, d) 0.0015 mm2/s. The gray lines show initial distributions that were assumed corresponding to the laser beam diameter.
A temporal change of the dye concentration c(r, t) at a radius r ( = |x|) was evaluated theoretically on the basis of the 2-D diffusion equation [32]; i.e.,
where D denotes the diffusion coefficient. As an initial condition of the coloration process, we assumed a step distribution in which colored molecules are dispersed uniformly within the radius of the violet laser beam (r <1 mm). For the decoloration process, colored molecules were assumed to be dispersed uniformly outside the green laser beam (r >0.75 mm). The gray lines (0 h) in Fig. 3 show these initial distributions, whose height was determined suitably to fit the measured optical density. Numerical calculation was conducted for various diffusion coefficients by using the finite element method (mesh period: 0.1 mm, interval: 1 s). The black lines in Figs. 3(a) and 3(b) show theoretical values that were calculated with D = 0.0001 mm2/s. These theoretical curves fitted well to the measured values, and hence, the diffusion coefficient was evaluated to be 0.0001 mm2/s for the sample with 1% toluene. Similarly, the theoretical curves (the black lines) in Figs. 3(c) and 3(d), which were calculated with D = 0.0015 mm2/s, fitted well to the measured values. Hence, the diffusion coefficient was evaluated to be 0.0015 mm2/s for the sample with 60% toluene. This result predicts that the efficiency of the self-healing function will be enhanced fifteen-fold by swelling PDMS with toluene.3. Fading by repeated photochromic reactions
Optical degradation was examined by repeating coloration and decoloration processes. Samples were enclosed in a glass vessel of 20 mm diameter rather than the thin cell in Fig. 1, since a large reservoir volume was needed to replace damaged molecules in the optical path. As Fig. 4 shows, a sample was irradiated alternatively (duration: 1 or 60 s) by violet and green laser beams (0.6 mW/mm2) that were controlled by an electromagnetic shutter. Whereas the violet laser beam was 2 mm in diameter, the green laser beam was expanded to 4 mm so that colored molecules were decolored thoroughly in its path. A probe beam from a xenon lamp (1 mm diameter) passed through the irradiated portion and was detected by a multi-channel spectrometer (Hamamatsu, PMA-11). The sample transmittance was measured periodically during the repeated photochromic processes. Temperature was controlled between 30 and 34 °C throughout the experiment term.
Fig. 4 Optical setup for evaluating resistivity of the photochromic polymer. The violet laser beam (2 mm diameter) and the green laser beam (4 mm diameter) were aligned so that they passed through the same position in the sample. They irradiated the sample alternatively being controlled by a mechanical shutter and reflected by a glass plate. The probe beam was focused gently so that its diameter became smaller than the violet beam diameter in the sample.
Figure 5 shows the results for a sample of 20 mm diameter and 20 mm height (6 ml). The dye concentration was 10‒4 mol/l, and the toluene concentration was 1, 20, 40, or 60 vol%. When a sample with 1-vol% toluene was irradiated by the violet laser beam of 2 mW (0.6 mW/mm2), the transmittance at 530 nm decreased to 50% in 1 s, as shown by the lower gray line in Fig. 5(a). Then it recovered to 100% within 1 s by the green laser irradiation (8 mW, 0.6 mW/mm2), as shown by the upper gray line. This transmittance change was reproducible in the first several cycles, but faded gradually as the process was repeated. The transmittance difference between the colored and decolored states decreased to ~10% after 1,000 cycles (the black lines). As Fig. 5(b) shows, the optical degradation became moderate by increasing the toluene concentration to 60 vol%. Figure 5(c) shows the transmittance changes (530 nm) that were measured for samples with different toluene concentrations. Laser irradiation was paused for several seconds every 100 cycles in order to measure the transmittance, since otherwise the laser beam disturbed the spectral measurement. The transmittance of the colored state increased notably in 1,000 cycles, whereas that of the decolored state decreased slightly. As mentioned in Section 1, laser irradiation seemed to deform reactive isomers to non-reactive one with no photochromic function. The transmittance decrease of the decolored state is probably attributed to absorption by this non-reactive isomer. The transmittance increase of the colored state was caused by decrease in the number of reactive isomers. This fading phenomenon became less notable as the toluene concentration increased. These experimental results confirmed that the enhancement of the diffusion coefficient was effective to promote the self-healing function.
Fig. 5 (a, b) Transmission spectra of the colored and decolored states. The violet and green laser beams (0.6 mW/mm2) were irradiated alternatively every 1 s. The gray spectra were measured in the first photochromic cycle, and the black spectra were measured after 1,000 cycles. The sample contained diarylethene of 10−4 mol/l and toluene of (a) 1 or (b) 60 vol%. (c) Transmittances (530 nm) of the colored and decolored states that were measured during the repeated laser irradiation processes (irradiation duration: 1 s). The toluene concentration was 1 (●), 20 (□), 40 (▲), or 60 vol% (○). (d) Transmittance change during the slow photochromic process in which the violet and green laser beams of 0.06 mW/mm2 were irradiated alternatively every 60 s.
4. Improvement of the resistivity
In the experiments above, the diffusion rate was still insufficient to replace damaged molecules thoroughly. Photochemical damage was usually induced by the violet laser [27, 30], which irradiated 4 × 1015 photons every cycle (2 mW, 1 s). It was assumed that damaged molecules could be replaced thoroughly if photons were irradiated more slowly. The violet laser power was therefore reduced to 0.2 mW in the next experiment. The coloration rate became slower by reducing the laser power; i.e., the transmittance decreased to 50% in ~30 s and thereafter stopped decreasing, since the photochromic isomerization reached the equilibrium. (The transmittance in the equilibrium mainly depends on the dye concentration and the sample length. The laser power density affects the coloration rate or the response time.) The violet laser of 0.2 mW (0.06 mW/mm2) was therefore irradiated for 60 s to ensure that the transmittance decreased to the equilibrium level (50%). Consequently, 2 × 1016 photons were irradiated in a single cycle (irradiation rate: 4 × 1014 photons/s). The green laser was also attenuated to 0.8 mW (0.06 mW/mm2) and irradiated for 60 s until decoloration was complete. This alternative irradiation process was repeated for several days, during which process the transmission spectra were measured occasionally. Figure 5(d) shows the transmittances (530 nm) of the colored and decolored states. As regards a sample with 1% toluene (●), degradation occurred before 1,000 cycles, since the molecular diffusion rate was insufficient to replace damaged molecules. This result indicated that the degradation was unavoidable by only decreasing the laser power. By contrast, a sample with 60% toluene (○) exhibited little change until 1,500 cycles. The degradation between 2,000th and 3,000th cycles was probably caused by a shortage of fresh molecules in the reservoir region (around the optical path). The number of dye molecules in the sample has to be increased in order to solve this problem.
For further improving the resistivity, the dye concentration was doubled and the sample volume was increased eightfold, i.e., 2 × 10−4 mol/l and 50 ml (diameter: 35 mm, height: 50 mm). The toluene concentration was also increased to 65 vol% for both enhancing diffusivity and avoiding dye precipitation. First, each of the laser beams (0.6 mW/mm2) was irradiated for 1 s. It took about an hour (67 min) to repeat this process 2,000 times, since a single cycle needed 2 s. Figure 6(a) shows spectra that were measured before and after the 2,000 photochromic cycles. Durability was improved notably in comparison with the small sample [Fig. 5(a)]. As regards the colored state, the spectrum after 2,000 cycles (the lower black line) overlapped the original one (the lower gray line), although the transmittance of the decolored state decreased slightly. After the irradiation was paused for 1 h, however, the spectrum of the decolored state recovered to the original level, as shown by the upper dashed line. Figure 6(b) shows the effect of the irradiation pausing. When the laser irradiation was repeated continuously 18,000 cycles (●), the transmittance at 530 nm increased by 15% in the colored states and decreased by 5% in the decolored states. As the black circles show, this fading rate was halved, when the irradiation was paused for 1 h every 2,000 cycles. The transmittance difference between the colored and decolored states, which had been 50% in the first cycle, still retained a contrast of ~30% after 30,000 cycles. This result revealed that the lifetime of the photochromic polymer could be extended by setting a restoration time occasionally even if it was exposed to a strong laser beam.
Fig. 6 Transmittance change of the sample with a large volume (50 ml) and a high dye- concentration (2 × 10−4 mol/l). The violet and green laser beams of (a, b) 0.6 or (c, d) 0.06 mW/mm2 were irradiated alternatively every (a, b) 1 or (c, d) 60 s. The gray lines show the original transmission spectra. The solid and dashed lines in (a) show the spectra that were measured in the 2,000th cycle and 1 h after that. The black and gray circles in (b) show the transmittances (530 nm) during the repetition processes with no or occasional restoration times. The dotted and solid spectra in (c) were measured after 10,000 and 30,000 cycles. The circles in (d) show the transmittance change (530 nm) during the 30,000 cycles.
Next, the slow photochromic process was examined by irradiating the laser beams of 0.06 mW/mm2 for 60 s. The sample was irradiated continuously with no restoration time. It took longer than 40 days (1,000 h) to complete 30,000 cycles, since a single cycle needed 2 min. The gray lines in Fig. 6(c) show the transmission spectra in the first cycle. The transmittance of the colored state (in the equilibrium) decreased to below 40% (530 nm), which was lower than that of the small sample [Fig. 5(d)] since both the sample length and dye concentration increased. As the dotted lines show, the transmittance decreased in the short wavelength range when the repetition exceeded 10,000 cycles. In the long wavelength range (>530 nm), however, the transmittance of the colored state changed little even after 30,000 cycles (the lower black line). As Fig. 6(d) shows, the transmittance change (530 nm) was only 2% in the colored state and 7% in the decolored state. The large reservoir region proved to be effective for healing the optical function.
5. Discussion
In the degradation processes shown in Figs. 5(c), 5(d), and 6(b), the transmittance decreased in the decolored state and increased in the colored state. In the final experiment shown in Fig. 6(d), however, the transmittance decreased in both colored and decolored states. As mentioned in Section 3, the transmittance decrease in the decolored state is attributed to the absorption by the non-reactive isomer. Although this absorption also affects the transmittance in the colored state, the decrease in the number of active isomers usually affects more notably, leading to the transmittance increase in the colored state. When the molecular replacement is rapid enough (the final experiment), however, a sufficient number of active isomers exist in the optical path, and hence, the transmittance in the colored state decreases slightly due to the absorption by the non-reactive isomer.
As regards the small sample [Fig. 5], the transmittance decreased to ~50% by either 1- or 60-s irradiation, since the photochromic reaction reached the equilibrium. That is, the transmittance stopped decreasing in ~1 s when the laser power was 2 mW, and in ~30 s when 0.2 mW. By contrast, the equilibrium transmittance of the large sample [Fig. 6] was lower than 40% because of the longer optical length and higher dye-concentration. In this sample the violet laser became weak near the exit side, since the transmittance was low at 405 nm, as shown in Figs. 6(a) and 6(c). When the laser power was 2 mW, therefore, the transmittance did not reach the equilibrium level in 1 s and became ~50% on the midway [Fig. 6(a)].
As this example indicates, transmittance of photochromic devices is affected by various parameters, and hence, a suitable device structure has to be designed to attain an efficient optical function. An optical switch, for example, requires 0% transmittance in the off-state. For decreasing the transmittance, either the dye concentration or the optical length has to be increased. By doing so, however, the violet laser (the control light) is absorbed too heavily to reach the exit end of the device. If this is the case, the violet laser has to be irradiated from the side of the signal path (the green laser path). In this manner, the device structure depends on required functions, and therefore, degradation by the repeated operation has to be evaluated for each structure depending on operation conditions. Whatever the device structure is, the self-healing function of the swollen PDMS is thought to be effective to extend the device lifetime.
6. Conclusion
Optical damage of photochromic diarylethene was healed by dispersing it in a swollen PDMS matrix, since the matrix allowed replacement of damaged and fresh molecules owing to its high diffusivity. The diffusivity of the dye molecules was enhanced fifteen-fold (0.0015 mm2/s) by swelling PDMS with toluene. Excellent resistivity of this polymer was demonstrated by repeating coloration-decoloration processes 30,000 cycles for a period of 40 days. This self-healing function of the PDMS matrix will be useful for extending the lifetime of optical devices that are based on organic dyes.
Acknowledgment
The authors thank Prof. K. Uchida of Ryukoku University for useful suggestions on photochromic dyes. This research was supported by Japan Society for the Promotion of Science.
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