Self-healing of optical functions by molecular in a swollen elastomer Mitsunori Saito, Tatsuya Nishimura, Kohei Sakiyama, and Sota Inagaki

Citation: AIP Advances 2, 042118 (2012); doi: 10.1063/1.4764292 View online: http://dx.doi.org/10.1063/1.4764292 View Table of Contents: http://aip.scitation.org/toc/adv/2/4 Published by the American Institute of Physics AIP ADVANCES 2, 042118 (2012)

Self-healing of optical functions by molecular metabolism in a swollen elastomer Mitsunori Saito,a Tatsuya Nishimura, Kohei Sakiyama, and Sota Inagaki Department of Electronics and Informatics, Ryukoku University, Seta, Otsu 520-2194, Japan (Received 5 June 2012; accepted 11 October 2012; published online 22 October 2012)

Optical functions of organic dyes, e.g., fluorescence or photochromism, tend to degrade by light irradiation, which causes a short lifetime of photonic devices. Self- healing of optical functions is attainable by metabolizing bleached molecules with nonirradiated ones. A polydimethylsiloxane elastomer provides a useful matrix for dye molecules, since its flexible structure with nano-sized intermolecular spaces al- lows dye from a reservoir to an operation region. Swelling the elastomer with a suitable promotes both dissolution and diffusion of dye molecules. This self-healing function was demonstrated by an experiment in which a photochromic elastomer exhibited improved durability against a repeated coloring-decoloring pro- cess. Copyright 2012 Author(s). This article is distributed under a Creative Commons Attribution 3.0 Unported License.[http://dx.doi.org/10.1063/1.4764292]

Organic dyes exhibit useful optical functions such as fluorescence, photochromism, and hole- burning. When compared to inorganic dyes, organic dyes have a drawback that their optical functions tend to degrade by light irradiation, excessive heating, or long-term preservation.1–6 The optical functions can be recovered by circulating a dye or diffusing dye molecules in a solvent, e.g., conventional dye lasers with a circulation pump or recent microfluidic devices.7–10 Use of dye , however, are not preferred for device fabrication, since liquids are difficult to handle. Dye-dispersed solids, therefore, have been synthesized with various polymers and glasses.11–17 A polydimethylsiloxane (PDMS) elastomer provides a suitable matrix for dispersing dye molecules, since 1) a high transmittance is attainable in the visible spectral range, 2) molding (nano-imprinting) is achievable at room , 3) chemical stability (nonreactivity) allows dispersion of various organic dyes, and 4) deformability facilitates optical tuning.7–10, 18–20 PDMS is also useful as a material for molecular transportation; i.e., its molecular linkage (bridging) is flexible enough to permit molecules and ions to flow through the nano-sized intermolecular spaces.21–24 The large diffusion coefficient has been used for drug delivery in the medical science fields, e.g., controlled release of steroid, glucose, or salicylic acid into biological tissues.25–28 This biomedical technique seems to be applicable to optical devices. That is, the molecular diffusion in PDMS will realize metabolism by which bleached dye molecules are replaced by fresh ones. Recently, the dye diffusion in PDMS has been utilized to improve durability of a dye-doped polymer laser.29 However, usable dye types are limited, since PDMS possesses a small for most organic dyes. In addition, the diffusion rate in PDMS is still insufficient to permit quick fluorescence recovery. The diffusion rate has to be increased if an unstable dye is to be used in a severe operation condition, e.g., irradiation of high-powered laser with a high repetition rate. In this study, we used a swollen PDMS elastomer to disperse an organic dye. As Fig. 1 shows, both hydrophilic and hydrophobic dyes could be dispersed in a PDMS elastomer that contained a suitable solvent, e.g., toluene (nonpolar solvent) or 2-propanol (polar solvent). These samples were prepared by mixing a dye solution with a PDMS oil (Shin-Etsu Chemical, KE-103) before solidification. Then a curing agent was added to the mixture, and solidification was complete in 8 h

aAuthor to whom correspondence should be addressed. Electronic mail: [email protected]

2158-3226/2012/2(4)/042118/7 2, 042118-1 C Author(s) 2012 042118-2 Saito et al. AIP Advances 2, 042118 (2012)

10 mm 10 mm Original

UV exposure

(a)(b) (c)(d) (e)

FIG. 1. (a–c) Photochromic dyes (diarylethene and spiropyran) that were dispersed in PDMS elastomers containing 1-vol% toluene. The molar ratio of these dyes was (a) 10:0, (b) 7:3, or (c) 0:10, and the dye in the samples was 0.1 mM. These elastomers, which were cured in polymer cells (10 mm square), exhibited (a) orange, (b) violet, or (c) blue color when exposed to ultraviolet light (365 nm). (d, e) Fluorescent dyes that were dispersed in PDMS elastomers (30×30×10 mm3). These elastomers contain (d) a 1-vol% toluene solution of dicyanomethylene (1 mM) or (e) a 5-vol% 2-propanol solution of rhodamine 6G (1 mM).

Operation region Reservoir region

Original 1 day 2 days 7days PDMS Dye (a) (b)

FIG. 2. (a) Penetration process of a dye solution in a PDMS elastomer. A toluene solution of dicyanomethylene (0.5 mM, 630 mm3) was poured on the PDMS elastomer (20 mm diameter, 20 mm height) in a glass vessel. The yellow solution gradually penetrated into the elastomer, and the entire sample was colored uniformly in seven days. (b) Schematic illustration of the molecular metabolism. The open and closed circles denote fresh and deteriorated molecules, respectively. at room temperature. In a PDMS elastomer with no solvent, dye molecules aggregated creating clusters or whiskers. Dye molecules could be dispersed into PDMS even after solidification, as shown in Fig. 2(a). An original PDMS elastomer, which contained no solvent, was fabricated in a glass vessel (inner diameter: 20 mm). The PDMS height was 20 mm, and accordingly its volume was 6300 mm2 (6.3 ml). Then a toluene solution of dicyanomethylene was put into the vessel. The dye concentration in the solution was 0.5 mM (5×10−4 mol/l) and the solution volume was 630 mm2, i.e., 1/10 of the PDMS volume. The solution, which stayed on the PDMS top just after the sample preparation, penetrated into the PDMS in several days; i.e., the yellow color of the dye extended to the lower portion. The entire sample was colored uniformly in seven days. A similar diffusion phenomenon was observed with fluorescent dyes in a vapor-transportation experiment, in which a polymer matrix was heated to high temperature (150–200 ◦C) to promote the diffusion.30 In comparison with this high-temperature process, the diffusion method has an advantage that it requires no sample heating that possibly destroys organic dye molecules. Swelling the PDMS elastomer seemed to promote the dye diffusion, since the penetration rate increased as the volume ratio of the solution increased. Of various examined, toluene was the most effective to promote the dye diffusion. This diffusion process can be utilized for metabolism of dye molecules. Figure 2(b) illustrates a model of a self-healable optical device, i.e., a swollen PDMS elastomer that contains dye molecules. When light irradiates the central region of the device (the operation region), dye molecules in that region exhibit an optical function, e.g., laser oscillation or photochromism, and degrade gradually. The degraded dye molecules (closed circles) diffuse out of this operation region, and reversely, fresh dye molecules (open circles) in the surrounding region diffuse into the operation region. In this manner the molecular metabolism extends the device lifetime. Photochromic dyes are more useful than fluorescent dyes for conducting experiments of the metabolism, since the molecular diffusion process can be traced easily by measuring a transmission spectrum. As Fig. 3 shows, for example, diarylethene exhibits an absorption band in the visible range when exposed to violet (or ultraviolet) light, and returns to the original state by exposure to green 042118-3 Saito et al. AIP Advances 2, 042118 (2012)

100 Original Green irradiation Violet Violet 80 60 Preservation 40 20 Violet irradiation Green Transmittance (%) 0 400 500 600 700 Wavelength (nm)

FIG. 3. Transmission spectra of a PDMS elastomer that contained 10-vol% toluene and 0.1-mM diarylethene. The transmit- tance was measured across the sample in a glass vessel (20 mm diameter). As the upper gray line shows, the original sample was transparent in the visible spectral range beyond 450 nm and accordingly exhibited a light-yellow color [Fig. 1(a)]. When a violet laser beam (405 nm, 0.15 mW/mm2) irradiated the entire sample for 2 min, an absorption band appeared at around 530 nm, and consequently, the sample color turned to orange. After stop of laser irradiation, the sample was preserved in darkness for 24 h. As the lower black line shows, no spectral change was observed during this preservation process. Then the entire sample was exposed to a green laser beam (532 nm, 0.15 mW/mm2) to decolor the sample. Consequently, the sample color recovered to light-yellow in 3 min, and the transmission spectrum overlapped the original one. light. Therefore molecular exchange between the exposed and unexposed portions can be evaluated quantitatively by spectral measurements. Diarylethene is more suitable than other photochromic dyes, e.g., spiropyran or azobenzene, because of the following advantages.31 First, the colored state is stable enough to allow a long-term experiment; i.e., as the lower black line in Fig. 3 shows, a thermal relaxation to the original transparent state (spontaneous decoloration) is negligible. Second, unlike the cis-trans deformation, photochromic isomerization of diarylethene induces little change in molecular shape, and accordingly, the diffusion process is thought to be affected little by whether the molecule is transparent or colored. Third, an absorption spectrum is independent of the polarity of the surrounding matrix. This property contrasts with that of spiropyran, whose absorption band shifts to a shorter wavelength as the matrix polarity increases.17 Samples were prepared by curing the mixed solution of PDMS oil and toluene in a glass vessel of 20 mm in diameter. The mixing ratio of PDMS and toluene was varied so that the volume ratio of toluene in the sample became 1, 20, 40, or 60%. A 0% mixture was not prepared, since the diarylethene solubility was low in the PDMS oil. Purchased powder of diarylethene (Tokyo Kasei Kogyo, B1535) was dissolved in toluene before creating the mixture. The dye concentration in the toluene solution was adjusted suitably so that the concentration in the entire sample (the swollen PDMS elastomer) became 0.1 mM. After adding the curing agent, the glass vessel was sealed off with a screw cap. Although the swollen PDMS stood alone without the need for an enclosure, it had to be sealed in the glass vessel to avoid toluene evaporation. Figure 4 shows the experimental setup for studying the dye diffusion process. The probe beam from a Xe lamp was focused by using a lens with an aperture so that the beam diameter became smaller than 1 mm in the sample. The transmitted beam was picked up by a fiber probe of a multi- channel spectrometer. A blank (reference) for the transmittance evaluation was a PDMS elastomer that was swollen with toluene at the same ratio as the dye-dispersed sample to be examined. Unlike the experiment of Fig. 3, in which a laser beam was expanded to irradiate the entire sample, the beam diameter of a violet laser diode (405 nm wavelength) was adjusted to ∼2 mm so that photochromism took place only in a region along the optical path. Laser irradiation continued for 30 s, and the transmission measurement was started just after stop of irradiation. The glass vessel was moved up and down, i.e., the probe beam position in the sample was changed, to measure the transmission spectra both inside and outside the irradiated region. Figures 5(a) shows a temporal change in the transmission spectra of the sample with 1-vol% toluene. These spectra were measured at the irradiated position (0 mm). The transmittance at 530 nm was ∼20% just after the stop of laser irradiation (0 min). As time passed, it increased gradually, indicating that colored molecules diffused out of the irradiated region. However, the diffusion was too slow to induce a transmission decrease at a distant position; i.e., as Fig. 5(b) 042118-4 Saito et al. AIP Advances 2, 042118 (2012)

Glass vessel Green Violet laser laser PDMS Aperture Laser beam Probe beam Glass plate Xe lamp Spectrometer

FIG. 4. Optical setup for observing diffusion of diarylethene in a swollen PDMS elastomer. The glass plate superposes the violet laser beam (2 mm diameter) and the probe light beam (<1 mm diameter) so that the latter passes within the irradiated portion in the sample. After stop of laser irradiation, the sample (the glass vessel) was lowered by 1–5 mm to measure the transmittance of nonirradiated portions. Dotted lines show a green laser and a mechanical shutter that were added in the experiment of Fig. 7. shows, no notable spectral change was observed at the position 2 mm distant from the irradiated position. The molecular diffusion was promoted by increasing the toluene ratio to 60 vol%; i.e., as Fig. 5(c) shows, the irradiated region became almost transparent in 60 min. On the other hand, an absorption band appeared at a position outside the irradiated region, as shown in Fig. 5(d). These temporal changes in the transmittance (530 nm) are summarized in Fig. 6(a). As the volume ratio of toluene increased, the dye diffusion became faster. Figure 6(b) shows the transmittance distribution after 60 min passage, which corresponds to the distribution of the colored dye molecules. In the sample with 1-vol% toluene, molecular diffusion is limited within 2 mm from the irradiated position. By contrast, dye molecules diffuse as far as 5 mm in the sample with 60-vol% toluene. When photochromic diarylethene molecules are exposed to strong light or high temperature, they occasionally take another molecular structure (isomer) that exhibits no photochromism.31 This phenomenon causes degradation of the device function. We conducted experiments to demonstrate the self-healing capability of the photochromic elastomer. Samples were the same as those used in the above experiments. As drawn with dotted lines in Fig. 4, a mechanical shutter with a mirror was inserted in the laser path, and a frequency-doubled Nd:YAG laser (532 nm) was placed opposite the violet laser. The shutter was controlled so that the violet and green laser beams irradiated the sample alternately every 1 min. The diameter and the power density of the laser beams were ∼2 mm and 0.07 mW/mm2, respectively. As the gray lines in Fig. 7 show, the sample transmittance at 530 nm decreased to ∼50% by the violet laser irradiation, and it returned to 100% by the green laser irradiation. This coloration-decoloration process was repeated 3000 times. Consequently, the photochromic spectral change became small in the sample with 1-vol% toluene, as shown with the thin lines in Fig. 7(a). This phenomenon was caused by degradation of diarylethene. By contrast, the degradation was less serious in the sample with 60-vol% toluene, as shown in Fig. 7(b). This fact indicates that the metabolism of dye molecules is effective to heal the sample degradation. Although the sample with 1-vol% toluene exhibited the degradation of photochromism, this fact does not mean the lack of the healing capability but indicates the slow healing rate in this sample. Actually, the photochromic function recovered by preserving the degraded sample in darkness for several days. If the operation conditions (duration, repetition rate, and power of laser irradiation) are milder than those of the current experiment, the self-healing function will be effective with this toluene-poor elastomer. As regards the sample with 60-vol% toluene, degraded molecules were assumed to diffuse over the entire sample during the experimental term, i.e., 4–5 days that were needed to complete the 3000-cycle processes. Consequently, the reservoir that contained a large amount of degraded molecules could not supply sufficient fresh molecules to the operation region. This is the reason that a slight degradation of the photochromic reaction was observed in Fig. 7(b). The durability against the repeated coloration process will be improved further by increasing the sample volume or the dye concentration. The self-healing function is based on the molecular diffusion in the swollen PDMS. Diffu- sion processes generally exhibit a temperature dependence that is characterized with activation .21–23, 30 In the current experiments, we kept room temperature at 30–34 ◦C. If the sample temperature is raised, the diffusion rate will increase and accordingly, the self-healing function will 042118-5 Saito et al. AIP Advances 2, 042118 (2012)

Toluene 100 1 vol% 80 60 min 60 30 min 40 20 min

0 mm 10 min 20 0min (a) Transmittance (%) 0 400 500 600 700 Wavelength (nm) 100 0, 10, 20, 30, 60 min 80

2 mm 60 40 20 (b) Transmittance (%) 0 400 500 600 700 Wavelength (nm)

Toluene 100 60 min 60 vol% 80 30 min 60 10 min 20 min 40 0 mm 20 0min (c) Transmittance (%) 0 400 500 600 700 Wavelength (nm)

100 0min 80 10 20, 30, 60 min i

2 mm 60 40 20 (d) Transmittance (%) 0 400 500 600 700 Wavelength (nm)

FIG. 5. Transmission spectra that were measured at the irradiated (0 mm) and nonirradiated (2 mm) positions. The volume ratio of toluene in the sample was (a, b) 1 or (c, d) 60%. The diarylethene concentration was 0.1 mM. Numerals beside the spectra denote time after stop of violet laser irradiation. All spectra overlap with one another in (b).

be promoted. We will hereafter conduct an experiment to study the temperature dependence of the diffusion rate and its effect on the self-healing function. Concluding, a swollen PDMS elastomer is useful as a matrix for dispersing organic dyes, since efficient molecular diffusion replaces degraded dyes by fresh ones. This metabolic lifetime extension, which was confirmed with photochromic elastomers in the current experiment, will be applicable to other organic devices including dye lasers and illuminators. Although most industrial products today are designed on the basis of disposal parts and materials, a novel technological concept is needed 042118-6 Saito et al. AIP Advances 2, 042118 (2012)

100 60% 80 40% 60 20% 40 1% 0 mm 20 (a) Transmittance (%) 0 0 102030405060 Time (min)

100 60% 80 40% 60 1% 20% 5 mm

– 40 0 20 (b) Transmittance (%) 0 012345 Distance (mm)

FIG. 6. (a) Temporal change of the transmittance (530 nm wavelength) at the irradiated position (0 mm). The horizontal axis shows time after stop of violet laser irradiation. The numerals beside the curves denote the volume ratio of toluene in the samples. (b) Distribution of the transmittance (530 nm) that was measured 60 min after the stop of laser irradiation. The horizontal axis shows the distance from the irradiated position.

Toluene 100 1 vol% 3000 80 1 60 Violet Violet 40 20 (a) Transmittance (%) 0 Green 400 500 600 700 Wavelength (nm)

Toluene 100 60 vol% 80 3000 60 Violet Violet 40 20 1 (b) Transmittance (%) 0 Green 400 500 600 700 Wavelength (nm)

FIG. 7. Spectral change by the repeated coloration-decoloration processes. Samples were the PDMS elastomers containing (a) 1- or (b) 60-vol% toluene. Diarylethene of 0.1 mM was dispersed in the samples. The samples were colored and decolored repeatedly by irradiating the violet and green laser beams alternately every 1 min. The diameter and the power density of both laser beams were 2 mm and 0.7 mW/mm2, respectively. The thick and thin lines show the spectra that were measured at the first cycle and after 3000 cycles, respectively. 042118-7 Saito et al. AIP Advances 2, 042118 (2012) for saving resources and energy. The self-healable metabolic materials will provide a solution to this problem. This research was supported by Grant-in-Aid for Scientific Research from Japan Society for the Promotion of Science. 1 Y. Kobayashi, Y. Kurokawa, Y. Imai, and S. Muto, J. Non-Cryst. Solids 105, 198 (1988). 2 M. D. Barnes, K. C. Ng, W. B. Whitten, and J. M. Ramsey, Anal. Chem. 65, 2360 (1993). 3 M. Faloss, M. Canva, P. Georges, A. Brun, F. Chaput, and J.-P. Boilot, Appl. Opt. 36, 6760 (1997). 4 I. G. Kytina, V. G. Kytin, and K. Lips, Appl. Phys. Lett. 84, 4902 (2004). 5 M. Hanazawa, R. Sumiya, Y. Horikawa, and M. Irie, J. Chem. Soc. Chem. Commun., 206 (1992). 6 A. Tork, F. Boudreault, M. Roberge, A. M. Ritcey, R. A. Lessard, and T. V. Galstian, Appl. Opt. 40, 1180 (2001). 7 S. R. Quake and A. Scherer, Science 290, 1536 (2000). 8 J. C. McDonald and G. M. Whitesides, Acc. Chem. Res. 35, 491 (2002). 9 M. Gersborg-Hansen, S. Balslev, N. A. Mortensen, and A. Kristensen, Appl. Phys. Lett. 90, 143501 (2007). 10 W. Song, A. E. Vasdekis, Z. Li, and D. Psaltis, Appl. Phys. Lett. 94, 161110 (2009). 11 B. H. Soffer and B. B. McFarland, Appl. Phys. Lett. 10, 266 (1967). 12 R. Reisfield and G. Seybold, Chimia 44, 295 (1990). 13 R. E. Hermes, T. H. Allik, S. Chandra, and J. A. Hutchinson, Appl. Phys. Lett. 63, 877 (1993). 14 F. Buchholtz, A. Zelichenok, and V. Krongauz, Macromolecules 26, 906 (1993). 15 D. Avnir, Acc. Chem. Res. 28, 328 (1995). 16 B. Lebeau and C. Sanchez, Curr. Opin. Solid State Mater. Sci. 4, 11 (1999). 17 M. Saito, Y. Tsubokura, N. Ota, and A. Fujiuchi, Appl. Phys. Lett. 91, 061114 (2007). 18 Z. Li, Z. Zhang, T. Emery, A. Scherer, and D. Psaltis, Opt. Express 14, 696 (2006). 19 M. Saito, H. Shimatani, and H. Naruhashi, Opt. Express 16, 11915 (2008). 20 H. Nakashima and M. Irie, Macromol. Rapid Commun. 18, 625 (1997). 21 W. L. Robb, Ann. NY Acad. Sci. 146, 119 (1968). 22 J. A. Barrie and D. Machin, J. Macromol. Sci. B 3, 645 (1969). 23 J. M. Watson and M. G. Baron, J. Membrane Sci. 110, 47 (1996). 24 A. Hahn, S. Gunter,¨ P. Wagener, and S. Barcikowski, J. Mater. Chem. 21, 10287 (2011). 25 T. J. Roseman, J. Pharm. Sci. 61, 46 (1972). 26 P. Lopour and V. Janatova,´ Biomaterials 16, 633 (1995). 27 A. G. Andreopoulos and M. Plytaria, J. Biomater. Appl. 12, 258 (1998). 28 R. K. Malcolm, S. D. McCullagh, A. D. Woolfson, S. P. Gorman, D. S. Jones, and J. Cuddy, J. Control. Rel. 97, 313 (2004). 29 H. Yoshioka, Y. Yang, H. Watanabe, and Y. Oki, Opt. Express 20, 4690 (2012). 30 H. Mochizuki, T. Mizokuro, N. Tanigaki, X. Mo, and T. Hiraga, Appl. Phys. Lett. 85, 5155 (2004). 31 K. Uchida and M. Irie, Organic Photochemistry and Photobiology, 2nd Edition (CRC Press, Boca Raton, 2004), Chapter 35.