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We experimentally evaluated the control of the mobility speed of organic chromophores in polydimethylsiloxane (PDMS) by developing novel chromophores with different molecular structures. As novel chromophores, PDMS-soluble coumarin- and pyrromethene-based chromophores were developed by attaching heptamethyltrisiloxane (HMTS). Then, the optical and mobility properties of these novel chromophores were evaluated. The investigation of mobility characteristics revealed that molecular weight and solubility both influence the diffusion coefficient. In the case of low solubility, molecular weight is the dominant factor in determining the diffusion coefficient. On the other hand, molecular solubility is the dominant factor at high solubility. To the best of our knowledge, this is the first investigation on controlling the diffusion coefficient in PDMS through the molecular weight and solubility.
© 2016 Optical Society of America
1. Introduction
Organic solid-state lasers are attractive for integrated applications [1, 2], such as lab-on-a-chip devices and waveguide lasers, because organic polymers simplify the fabrication process owing to their low glass transition temperatures, their low melting temperatures, and polymerization at room temperature. Moreover, organic polymers can provide additional functions to organic solid-state lasers owing to their flexibility and compatibility with other organic molecules. Various organic solid-state lasers, such as centimeter-order bulk lasers [3–6] and submillimeter-order waveguide lasers [7], have been previously reported. Research into the minimization of organic solid-state lasers was stimulated by the advances of fabrication technologies, including the photolithography method [8], micro-dispensing method [9], and ink-jet technique [10]. Recently, solid-state organic dye lasers have also been studied increasingly as micrometer-order lasers [10–13] that are also applicable to ultra-high-sensitivity sensors [14]. However, organic solid-state lasers have the drawback of short device lifetime due to the low durability of laser dyes that have been used. This issue is critical in the case of smaller devices like microcavity lasers, where there is a rapid lowering of the laser gain due to the small number of dyes fixed in the micro volume and no dye is exchanged. The gain lowering due to dye degradation occurs with a spectral shift of whispering-gallery modes (WGMs) in a microcavity laser [15]. This issue is critical for practical applications such as sensors, laser sources for optical communication, and optical signal processing. These applications use the spectral shift as a physical quantity or require stable spectra.
As a solution for dye degradation in organic solid-state lasers, novel laser dyes have been developed to enhance durability; this is a “chemical” approach. Then liquid-state dye lasers also exhibit a relatively high durability even in integrated optofluidic devices [16], as a “structural” approach. On the other hand, in a “physicochemical” approach, we previously suggested the degradation-recoverable solid-state dye laser using polydimethylsiloxane (PDMS), which provides mobility to laser dyes [17, 18]. The nanoporous structure of PDMS allows the diffusion of doped laser dyes. In fact, the orange degradation-recoverable solid-state polymer distributed feedback (DFB) laser based on pyrromethene-597-doped PDMS [17] showed an operation lifetime 20 times longer than that of a polymethylmethacrylate (PMMA)-based laser, which is a common plastic laser. After this first report, other groups reported studies on applications using the mobility of dye molecules in PDMS [19–24].
However, most of the laser dyes show poor solubility in PDMS. Of the commercial dyes, only some types of dipyrromethene boron difluoride (BODIPY, pyrromethene series) dyes show relatively high solubility in PDMS. To extend this approach over the lasing range, and to enhance their solubility in PDMS, previous dyes were altered by replacing or attaching compatible dimethylsiloxane (DMS) groups, such as dimethylsiloxane-yl, and trimethylsilyl groups. Using a developed fluorene-based chromophore, KIDL-F1 (9,9-di(1,1,3,3,5,5,7,7,9,9,9-undecamethylpentasiloxane-yl-propyl)fluorene), a blue degradation-recoverable solid-state polymer laser was developed [18]. The use of KIDL-F1 increased solubility. However, the mobility remained somewhat low because the molecule is relatively large owing to the attachment of both DMS groups. Moreover, the previous approach focused on the improvement of solubility in PDMS, but the mechanism of the change in the diffusion coefficient due to modification using siloxane groups has not been revealed thus far. Solving these issues would open the possibility for controlling the diffusion coefficient. The control of the diffusion coefficient would be significant and suitable for applications using molecular mobility such as microfluidic fields, which desires low diffusivity. Furthermore, controlling the mobility in PDMS can enable the further development of future advanced organic micro-devices or applications such as molecular frame design of molecules that act in PDMS, and degradation-recoverable solid-state polymer micro lasers which desire high diffusivity. Therefore, related researches on diffusivity in PDMS or micro channels of that have been previously conducted on some matters such as water, drugs, organic solvents and polymers, except for dyes in PDMS [25–29].
In this study, novel PDMS-soluble coumarin- and pyrromethene-based chromophores were developed to investigate diffusion coefficient control. The developed chromophores were synthesized by modifying the molecules by attaching heptamethyltrisiloxane-3-yl via alkylene chains. These developed chromophores had relatively low molecular weights owing to having the low-weight heptamethyltrisiloxane-3-yl instead of dimethylsiloxane chains. Then, investigation of mobility characteristics revealed that molecular weight and solubility both influence the diffusion coefficient. A high solubility increases the diffusion coefficient. At a low solubility, the influence of molecular weight on the diffusion coefficient becomes dominant. To the best of our knowledge, this is the first investigation on controlling the diffusion coefficient in PDMS through the molecular weight and solubility.
2. Synthesized dyes with heptamethyltrisiloxane
2.1 Novel PDMS-soluble chromophores
Novel two coumarin- and two pyrromethene-based chromophores that are soluble in PDMS were developed using previous chromophores with acceptable optical properties as basic molecular frameworks. It is desirable for the optical properties of the developed chromophores to be similar to those of the original chromophores. In the study, novel PDMS-soluble chromophores were developed by attaching heptamethyltrisiloxane with alkylene (m) chains (HMTS: 1,1,1,3,5,5,5-heptamethytrisiloxan-3-yl- (CH2)m), which improves the affinity for PDMS. The number of alkylene chains, m, can affect both the solubility and molecular mobility in PDMS. The alkylene chain lengths were chosen such that the solubility was acceptable and length was minimized, since long chains increase molecular weight and lower mobility, leading to the low durability of lasers. Compared to DMS chains [18], the adjustment using alkylene chains can be performed with only a slight increase in molecular weight. As shown in Fig. 1, four novel chromophores, KIDL-C1201 (7-{N-propyl-N-[3-(1,1,1,3,5,5,5-heptamethyltrisiloxane-3-yl)propyl]amino}-4-methylcoumarin), KIDL-C1511 (7-{N-propyl-N-[3-(1,1,1,3,5,5,5-heptamethyltrisiloxane-3-yl)propyl]amino}-4-trifluoromethyl-coumarin), KIDL-P5671 (1,3,5,7-tetramethyl-8-[10-(1,1,1,3,5,5,5-heptamethyltrisiloxane-3-yl)decyl]-2,6-diethylpyrromethene-difluoroborate complex), and KIDL-P5971 (1,3,5,7-tetramethyl-8-[10-(1,1,1,3,5,5,5-heptamethyltrisiloxane-3-yl)decyl]-2,6-di-tert-butyl-pyrromethene-difluoroborate complex), were developed from the original chromophores coumarin 120 (C440), coumarin 151 (C490), pyrromethene 567 (P567), and P597, respectively.
Fig. 1 Structures of the developed chromophores: (a) KIDL-C1201, (b) KIDL-C1511, (c) KIDL-P5671, and (d) KIDL-P5971.
2.2 Synthesis and NMR characterization
KIDL-C1511
A 100 mL four-necked round-bottom flask thoroughly flushed with nitrogen was charged with 7-(N-allyl-N-propylamino)-4-trifluoromethylcoumarin (693.4 mg, 2.22 mmol) in 3 mL of dry toluene, and a xylene solution of platinum-1,3-divinyl-1,1,3,3-tetramethyldisiloxane (1.0 × 10−2 mol%) was added to this solution. Then, 1,1,1,3,5,5,5-heptamethyltrisiloxane (551.4 mg, 2.48 mmol) was added dropwise to the flask over ten minutes at 25°C, and the mixture was stirred under nitrogen at 25°C for an hour. Conventional work-up and subsequent purification (preparative high-performance liquid chromatography (HPLC)) afforded the desired product as a yellow liquid. Yield: 844.6 mg.
1H-NMR (600 MHz, δ in CDCl3): 0.01 (s, 3H), 0.06–0.08 (m, 18H), 0.42–0.45 (m, 2H), 0.94 (t, 3H), 1.59–1.65 (m, 4H), 3.27–3.31 (m, 4H), 6.33 (s, 1H), 6.47 (d, 1H), 6.57 (dd, 1H), 7.44 (dd, 1H)
MS (EI) m/z: 533(M+), 518, 504, 284
KIDL-C1201
Following the same procedure, 7-{N-propyl-N-[3-(1,1,1,3,5,5,5-heptamethyltrisiloxane-3-yl) propyl]amino}-4-methylcoumarin was obtained from 7-(N-allyl-N-propylamino)-4-methylcoumarin as a faint yellow solid.
1H-NMR (600 MHz, δ in CDCl3): 0.01 (s, 3H), 0.06–0.08 (m, 18H), 0.41–0.45 (m, 2H), 0.93 (t, 3H), 1.58–1.64 (m, 4H), 2.30 (s, 3H), 3.25–3.30 (m, 4H), 5.90 (s, 1H), 6.44 (d, 1H), 6.53 (dd, 1H), 7.33 (d, 1H)
MS (EI) m/z: 479 (M+), 464, 450, 230
KIDL-P5671
Following the same procedure, 1,3,5,7-tetramethyl-8-[10-(1,1,1,3,5,5,5-heptamethyltrisiloxane-3-yl)decyl]-2,6-diethylpyrromethene-difluoroborate complex was obtained from 1,3,5,7-tetramethyl-8-(10-undecenoyl)-2,6-diethylpyrromethene-difluoroborate complex as an orange solid.
1H-NMR (600 MHz, δ in CDCl3): −0.02 (s, 3H), 0.07 (s, 18H), 0.41–0.45 (m, 2H), 1.03 (t, 6H), 1.22–1.38 (m, 12H), 1.47 (quint, 2H), 1.57–1.65 (m, 2H), 2.31 (s, 6H), 2.38 (q, 4H), 2.47 (s, 6H), 2.92–2.98 (m, 2H)
MALDI-TOFMS m/z: 664.5 (M+)
KIDL-P5971
Following the same procedure, 1,3,5,7-tetramethyl-8-[10-(1,1,1,3,5,5,5-heptamethyltrisiloxane-3-yl)decyl]-2,6-di-tert-butylpyrromethene-difluoroborate complex was obtained from 1,3,5,7-tetramethyl-8-(10-undecenoyl)-2,6-di-tert-butylpyrromethene-difluoroborate complex as a red-purple solid.
1H-NMR (600 MHz, δ in CDCl3): 0.00 (s, 3H), 0.08 (s, 18H), 1.24–1.50 (m, 34H), 1.59–1.68 (m, 2H), 2.52 (s, 6H), 2.68 (s, 6H), 2.98–3.06 (m, 2H)
MALDI-TOFMS m/z: 720.7 (M+)
3. Results and discussion
3.1 Absorption and emission cross sections of synthesized dyes
The absorption and emission cross sections were measured with dye-doped ethanol at room temperature using a spectrophotometer (V-630, JASCO Corp.) and fluorescence spectrofluorometer (FP-8200, JASCO Corp.), respectively. Figure 2 shows the absorption and emission cross sections of the developed chromophores. Table 1 shows the optical and solubility characteristics and the calculated sizes and dipole moments of the developed chromophores. Fluorescence lifetimes and quantum yields were measured by PL quantum yield spectrometer (Quantaurus-QY C13534-01, Hamamatsu Photonics Co., Japan). Solubility were experimentally measured at 25°C. Molecule sizes and dipole moments were estimated in UFF force field by Avogadro software. According to the data presented in Fig. 2(a) and Table 1, KIDL-C1201 is expected to be an efficient laser dye owing to its high quantum efficiency. A solubility of up to 40 mM was attained, despite the low innate solubility of the original chromophore, C440. According to Fig. 2(b) and Table 1, the high solubility of KIDL-C1511 of up to 130 mM is also useful for lasers, even with its relatively low quantum efficiency of 0.17. Actually, KIDL-C1511:PDMS laser based on the DFB structure shown in the Refs. 17 and 18 obtained a lasing with narrow spectrum as shown in the inset of Fig. 2(b). KIDL-P5671 is also soluble up to 5.0 mM and can be used in lasing owing to its high quantum efficiency and high emission cross section, as shown in Fig. 2(c) and Table 1. We note that the overlap of absorption and emission spectra could limit the lasing wavelength. Moreover, the absorptions at the reasonable pumping wavelengths of 355 (frequency-tripled Nd:YAG laser) and 532 nm (frequency-doubled Nd:YAG laser) are relatively low. Figure 2(d) shows that the wavelength of the emission peak of KIDL-P5971 is red-shifted compared to that of the original chromophore, P597, owing to its increased molecular weight. Table 1 shows that the quantum yield of KIDL-P5971 was relatively low. The solubility of KIDL-P5971 is twice as high as that of the original chromophore, P597.
Fig. 2 Absorption (blue) and emission (red) cross sections of (a) KIDL-C1201, (b) KIDL-C1511, (c) KIDL-P5671, and (d) KIDL-P5971. Inset of (b) shows lasing spectra from a DFB typed PDMS laser doped with KIDL-C1511.
Table 1. Optical and solubility characteristics, calculated molecule sizes, and dipole moments of novel chromophores and pyrromethene 597.
3.2 Mobility characteristics in PDMS
The diffusion coefficients of the developed chromophores were measured through their permeation into a PDMS film from the edge. Dye-doped DMS (SIM-360, Shin-Etsu Chemical Co., Ltd.) at a concentration of 3.0 mM was placed in a container, and a PDMS film with a thickness of 100 μm between two glass slides was placed in the solution. PDMS films were prepared by curing DMS (SIM-360, Shin-Etsu Chemical Co., Ltd.), and all PDMS samples were fabricated at the same time to be completely the same condition throughout the experiments for all dyes. The samples were kept for 24 h. The dye concentration distributions were measured using a scanned laser beam. Laser wavelengths of 355 and 532 nm were used for KIDL-C1201/C1511 and KIDL-P5671/5971, respectively. The laser intensity was reduced using a filter to avoid saturation and spot size was approximately 0.1–0.2 mm in diameter. Additionally, diffusion coefficients were obtained by fitting the measured concentration distributions with the one-dimensional diffusion equation.
First, the concentration distributions of the developed chromophores in PDMS films were measured after 24 h of diffusion. These concentration distributions were measured at 25°C, 50°C, and 75°C. Data from the 25°C and 75°C measurements are shown in Fig. 3. The lines in Fig. 3 are the result of fitting the experimental data (dots) to the one-dimensional diffusion equation. Additionally, Table 2 shows the diffusion lengths (1/e of normalized concentration) and diffusion coefficients of the developed chromophores at 25°C and 75°C.
Fig. 3 Concentration distributions measured via scanned laser beam absorption on PDMS films dyed with (a) KIDL-C1201, (b) KIDL-C1511, (c) KIDL-P5671, and (d) KIDL-P5971 through diffusion phenomena for 24 h at 25°C (black) and 75°C (red). Inset color bars show the photoluminescence views of the dyed PDMS films.
Table 2. Diffusion lengths and coefficients at 25°C and 75°C.
According to the data from Fig. 3 and Table 2, the measured permeation profiles of the developed chromophores agree well with the fitting by the diffusion equation. These diffusion lengths are relatively long at several millimeters, and can be utilized in various optical applications such as degradation-recoverable solid-state polymer micro lasers. Moreover, the obtained diffusion coefficients of the developed chromophores with HMTS are higher than that of previous chromophore KIDL-F1 developed with DMS groups. This indicates that a lower molecular weight lead to high mobility.
To summarize the data from Fig. 3 and Table 2, Arrhenius plots are shown in Fig. 4. Figure 4 shows mobility characteristics as a function of inverse of temperature for various logarithmic diffusion coefficients. The data for P597 and KIDL-F1 were taken from Ref. 18. These linear characteristics agree with previously published Arrhenius plots [18]. Although KIDL-F1 shows a low diffusion coefficient at 25°C (low temperature), its diffusion coefficient at high temperature is higher than those of other chromophores owing to the high temperature dependence. Specifically, the diffusion coefficient of KIDL-F1 exceeds those of KIDL-C1201, KIDL-C1511, KIDL-P5671, KIDL-P5971, and P597 at 193°C, 285°C, 108°C, 86°C, and 223°C, respectively. Although these mobility characteristics indicate the potential for controlling the diffusion coefficient by controlling the temperature, controlling the temperature in applications is challenging, especially in the integrated situations.
Fig. 4 Dependence of mobility characteristics on temperature of KIDL-C1201, KIDL-C1511, KIDL-P5671, KIDL-P5971, P597 [17], and KIDL-F1 [18].
Secondly, we explored other factors that determine mobility speed besides temperature. The dependence of mobility characteristics on molecular weight at 25°C, 50°C, and 75°C are shown in Fig. 5. The data for P597 and KIDL-F1 were taken from Ref. 18. According to Fig. 5, a low molecular weight is correlated with a high diffusion coefficient at all temperatures. In other words, light chromophores can rapidly diffuse in PDMS. Then, by examining on the coumarin-based chromophores (KIDL-C1201 and KIDL-C1511) and pyrromethene-based chromophores (KIDL-P5671, KIDL-P5971, and P597), the linearity of the mobility characteristics can be confirmed. Therefore, the molecular weight is an important factor in determining the diffusion coefficient. However, KIDL-C1201 and KIDL-C1511 showed opposite relationships between molecular weight and mobility characteristics.
Fig. 5 Dependence of mobility on molecular weight of KIDL-C1201, KIDL-C1511, KIDL-P5671, KIDL-P5971, P597 [17], and KIDL-F1 [18].
To analyze this issue, we focused on the solubility of chromophores in PDMS. Figure 6(a) shows the dependence of mobility characteristics on solubility at 25°C. According to Fig. 6(a), a high solubility of over approximately 7.5 mM is correlated with a high diffusion coefficient. On the other hand, this correlation is negative at under approximately 7.5 mM. This characteristic can indicate that the solubility is one factor that determines the diffusion coefficient. In other words, a high solubility increases the diffusion coefficient. Meanwhile, the influence of the molecular weight on the diffusion coefficient becomes dominant at a low solubility. In fact, KIDL-F1 in Fig. 6(a) shows a large diminution in the diffusion coefficient owing to the high molecular weight.
Fig. 6 (a) Dependence of mobility characteristics on solubility of KIDL-C1201, KIDL-C1511, KIDL-P5671, KIDL-P5971, P597 [17], and KIDL-F1 [18]. (b) Dependence of mobility characteristics on solubility normalized by molecular weight and size for these chromophores.
According to Fig. 5 and Fig. 6(a), it was found that two factors of molecular weight and solubility can give dominant influence to define the diffusion coefficient of a dye molecule in PDMS. However, fine linearity or correlation was not obtained due to the mixed influence of two factors or difference of molecular family in both cases. Therefore, we extracted the only influence of solubility on diffusion characteristics from Fig. 6(a) to split factors. Figure 6(b) shows the dependence of mobility characteristics on the solubility normalized by the cubes of molecular weight and size at 25°C. A relatively high sample correlation coefficient of 0.79 was obtained. This indicates that the normalized solubility, which reduced the influence of molecular weight and size, is correlated with a high diffusion coefficient. Therefore, the molecular weight and solubility are important factors that determine the diffusion coefficient of a dye molecule in PDMS. Furthermore, focusing on the fact that the cubes of molecular weight and size are inversely proportional to the diffusion coefficient, it can be suggested that the dependence on molecular weight is due to space filling.
4. Conclusions
In this study, novel PDMS-soluble coumarin- and pyrromethene-based chromophores were developed to investigate diffusion coefficient control. The developed chromophores were synthesized by modifying the molecules by attaching heptamethyltrisiloxane-3-yl via alkylene chains. These developed chromophores had relatively low molecular weights owing to having the low-weight heptamethyltrisiloxane-3-yl, instead of dimethylsiloxane chains. First, the optical and mobility characteristics of these chromophores were investigated. Compared to the previously developed KIDL-F1 with compatible DMS groups, the mobility of the developed chromophores was improved. Then, investigation of the mobility characteristics revealed that molecular weight and solubility influence the diffusion coefficient. A high solubility increases the diffusion coefficient. Meanwhile, the influence of molecular weight on the diffusion coefficient becomes dominant at a low solubility. To the best of our knowledge, this is the first investigation on the control of the diffusion coefficient in PDMS through the molecular weight and solubility. This control of the diffusion coefficient is significant and suitable for applications using molecular mobility such as microfluidic fields. Furthermore, controlling the mobility in PDMS opens possibilities for future advanced organic micro-devices or applications such as molecular frame design of molecules that act in PDMS and degradation-recoverable solid-state polymer micro lasers.
Funding
Japan Society for the Promotion of Science (JSPS) KAKENHI (16K17531).
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