Transparent and Nonflammable Ionogel Photon Upconverters and

Publication Date (Web): January 11, 2016. Copyright © 2016 American Chemical Society. *E-mail: [email protected]. Phone/Fax: +81-3-5734-38...
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Transparent and Nonflammable Ionogel Photon Upconverters and Their Solute Transport Properties Yoichi Murakami,*,† Yuki Himuro,† Toshiyuki Ito,†,§ Ryoutarou Morita,‡ Kazuki Niimi,‡ and Noriko Kiyoyanagi‡ †

Department of Mechanical Sciences and Engineering, Tokyo Institute of Technology, 2-12-1-I1-15 Ookayama, Meguro-ku, Tokyo 152-8552, Japan ‡ Nippon Kayaku Co., Ltd., 3-31-12 Shimo, Kita-ku, Tokyo 115-8588, Japan S Supporting Information *

ABSTRACT: Photon upconversion based on triplet−triplet annihilation (TTA-UC) is a technology to convert presently wasted sub-bandgap photons to usable higher-energy photons. In this paper, ionogel TTA-UC samples are first developed by gelatinizing ionic liquids containing triplet-sensitizing and light-emitting molecules using an ionic gelator, resulting in transparent and nonflammable ionogel photon upconverters. The photophysical properties of the ionogel samples are then investigated, and the results suggest that the effect of gelation on the diffusion of the solutes is negligibly small. To further examine this suggestion and acquire fundamental insight into the solute transport properties of the samples, the diffusion of charge-neutral solute species over much longer distances than microscopic interpolymer distances is measured by electrochemical potential-step chronoamperometry. The results reveal that the diffusion of solute species is not affected by gelation within the tested gelator concentration range, supporting our interpretation of the initial results of the photophysical investigations. Overall, our results show that the advantage of nonfluidity can be imparted to ionic-liquid-based photon upconverters without sacrificing molecular diffusion, optical transparency, and nonflammability.

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INTRODUCTION Photon upconversion (UC) is a technology to generate higherenergy photons from a larger number of lower energy photons. Through UC, presently wasted sub-bandgap photons in many solar energy conversion systems, such as photovoltaic devices and photocatalytic materials, are converted into usable photons possessing higher energies than their bandgaps, which enhances the solar utilization efficiency. Recently, a method of UC based on triplet−triplet annihilation (TTA) between organic molecules has been actively studied1−47 mainly because this method (denoted TTA-UC) is applicable to noncoherent and low-intensity light sources including sunlight.2,4,5,17,34,43 TTA-UC is achieved by combining two kinds of organic molecules, the “sensitizer” that performs photon absorption and triplet sensitization and the “emitter” that performs TTA and photon emission. In this article, meso-tetraphenyl-tetrabenzoporphyrin palladium (PdPh4TBP; Chart 1a) and perylene (Chart 1b) are used as the sensitizer and emitter, respectively. Scheme 1 shows the qualitative energy level diagram of TTAUC. First, the sensitizer in the ground (S0) state absorbs an incident photon and forms the lowest excited singlet (S1) state, which immediately forms the triplet (T1) state by intersystem crossing (ISC). When a sensitizer in the T1 state collides with an emitter molecule in the S0 state through their diffusive motion, the T1 energy is transferred from the sensitizer to the © 2016 American Chemical Society

emitter (triplet energy transfer; TET) based on the Dexter mechanism.48,49 Then, when two T1 emitter molecules collide with each other, TTA can occur with a certain probability14,36 to form one S0 emitter and one S1 emitter.48 Finally, the S1 emitter emits a fluorescence photon. Therefore, TTA-UC generates delayed fluorescence photons by TET and TTA. So far, the majority of TTA-UC studies have been carried out in organic solvents.1−3,5,10−15,17,19,20,26,29−33,40−42 This is partly because the use of organic solvents not only facilitates the dissolution of the organic sensitizer and emitter molecules, but also enhances their diffusive motions due to the low viscosity of such solvents. However, organic solvents are generally highly volatile and flammable, and affect many plastics, which have led to several concerns regarding their application. For example, organic solvents are generally not environmentally friendly because of their high vapor pressures and vapor toxicities, and not safe due to their ignitable and flammable character in air. Additionally, the fluid properties of organic solvents mean they can easily leak from containers, making the handling and sealing of samples difficult. Received: October 8, 2015 Revised: January 9, 2016 Published: January 11, 2016 748

DOI: 10.1021/acs.jpcb.5b09880 J. Phys. Chem. B 2016, 120, 748−755

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The Journal of Physical Chemistry B

To address these concerns from a different approach, we previously developed TTA-UC samples using ionic liquids (ILs) as the medium.16 ILs are transparent, colorless molten salts possessing several advantages such as practical nonvolatility, nonflammability, and relatively wide electrochemical windows.50,51 We elucidated the kinetics of TTA-UC in ILs regarding the TET27 and TTA28,36 processes. However, to fully address the aforementioned concerns, the fluidity of ILs has to be suppressed. In this article, gelatinized photon upconverters in the form of ionogels are first introduced, and their fundamental physical properties are presented. Then, their photophysical properties are measured, and the results are discussed. Finally, to explain the results of the photophysical characterization and acquire fundamental insight into the solute transport properties of the ionogels, the diffusion of solute species is investigated for diffusion distances much longer than the microscopic distance between adjacent polymer chains in the ionogel using a time-resolved electrochemical technique.

Chart 1. Molecular Structures of (a) PdPh4TBP, (b) Perylene, (c) [C4dmim][NTf2], and (d) CDBA6·NTf2

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EXPERIMENTAL METHODS Materials. PdPh4TBP and perylene were purchased from Frontier Scientific and Sigma-Aldrich, respectively, and used without further purification. Concentrations of PdPh4TBP and perylene in the UC samples were 1.6 × 10−5 and 4.5 × 10−3 M, respectively. For the IL, 1-butyl-2,3-dimethylimidazolium bis(trifluoromethylsulfonyl)amide ([C4dmim][NTf2]; Chart 1c) supplied from IoLiTec was used. The IL was purified according to the method we developed previously36 and vacuum-dried at 120 °C for 4 h to remove moisture prior to use. During vacuum drying, the height of the IL in a glass vial was set 3 mm or lower to facilitate the escape of moisture from the IL. The vacuum was generated using an oil-free pump (Edwards, nXDS15i) to ensure that it was clean. The water content in the IL after these processes was found to be less than 50 ppm according to the result of Karl Fischer titration.36 Ionogels were formed by gelatinizing the IL using a polymeric salt (CDBA6·NTf2; Chart 1d) recently developed by Nagasawa et al.52 As shown in Chart 1d, CDBA6·NTf2 is an ionic gelator that possesses the same anion as that of the IL. CDBA6·NTf2 was synthesized according to the method described in ref 52. The chemical structure of the product was confirmed from a 1H NMR spectrum acquired using an NMR spectrometer (JEOL, JNM-ECS400) and its agreement with the 1H NMR information reported in ref 52.

To address the concerns regarding the volatility and fluidity of organic solvents, researchers have embedded sensitizer and emitter molecules in solid or quasisolid polymer matrices,24 such as viscous polymers,6 rigid polymers,7,8 and soft polymers like polyurethane,4,9,13,22,25 or encapsulated them in polymer spheres or microcapsules.18,21,23,38 However, because these polymer matrices and capsule materials are essentially hydrocarbons, such samples are still flammable in air and emit fumes after their ignition. Therefore, this issue should also be addressed. Additionally, some reports observed weak or weakened UC fluorescence from their solid or quasisolid samples at room temperature,7−9,26 which was ascribed to the lowered solute diffusion in the solidified forms. Recently, two groups (Weder and co-workers46 and Schmidt and co-workers47) reported transparent organogel photon upconverters fabricated by gelation of an organic solvent using covalently cross-linked poly(vinyl alcohol) and a sorbitol gelator, respectively. While these samples successfully suppressed fluidity of organic solvents, they are still flammable and possess volatility because of the use of organic solvents. It is notable that Schmidt and co-workers47 showed that gelation of the organic solvent did not affect TTA-UC by comparison of the TTA-UC of a sample prepared with a single gelator concentration (0.3% w/v) with one prepared without gelator.

Scheme 1. Qualitative Energy Level Diagram of the Photon Upconversion (UC) Processa

a

ISC, intersystem crossing; TET, triplet energy transfer; TTA, triplet−triplet annihilation; solid arrows, radiative processes; dashed arrows, nonradiative processes. 749

DOI: 10.1021/acs.jpcb.5b09880 J. Phys. Chem. B 2016, 120, 748−755

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The Journal of Physical Chemistry B Sample Preparation. First, gelator stock solutions were prepared by adding CDBA6·NTf2 (3.4−25.2 mg depending on the target concentration) to 1:1 v/v mixtures of [C4dmim][NTf2] and methanol (170−280 μL each) in a glass vial. In this report, stock solutions with three concentrations (10, 30, and 60 g/L) were used to make samples with different final gelator concentrations. After each glass vial was closed with a screw cap, the mixture inside was ultrasonically homogenized for 10− 30 min to give a uniform, transparent, and colorless liquid. The vials were stored with the screw cap tightly closed until use to prevent evaporation of methanol. Sample preparation is described in detail in the Supporting Information for the specific case of a final gelator concentration of 7 g/L. Briefly, IL (242−270 μL depending on the target concentration) was added to a glass vial (capacity: 8 mL), and then toluene solutions of the sensitizer and emitter molecules were added to it. After the resulting mixture was mechanically and ultrasonically homogenized, the toluene was removed under vacuum using a scroll pump. These procedures are the same as those used in our previous reports.16,27,28,36 Then, gelator stock solution (51−116 μL) was added, and the mixture was homogenized again by the same method. After the methanol in the mixture was removed by vacuum pumping for 2 h, the vial was transferred into a vacuum-type glovebox (UNICO, UN-650F) that contained an atmosphere of freshly replaced argon gas (purity: over 99.998%). Inside the glovebox, the vial was heated to 80 °C, which is well above the gel−sol transition temperature of the ionogel for gelator concentrations lower than 20 g/L,52 for 10 min to uniformly melt the mixture. Then, part of the mixture was quickly taken and injected into containers used for measurements. In this report, quartz tubes with a square cross section (inner dimensions: 1 × 1 mm2 or 2 × 2 mm2; one end was closed by a flame) and quartz cuvettes (Starna, Type 53/Q/1; optical path length: 1 mm) were used for photoemission and optical absorption measurements, respectively. Subsequently, the samples were pumped under high vacuum (10−4 to 10−5 Pa) for 21−22 h in a vacuum chamber installed inside the same glovebox. Finally, in the glovebox where the atmosphere had been freshly replaced with argon gas again, the open end of the quartz tube was sealed using metal alloy solder. The gelation of the samples after the pumping under high vacuum was confirmed by checking each sample remaining in its glass vial. Near-infrared optical absorption spectra of the samples revealed that the residual amounts of toluene and methanol in them were below the detection limit (i.e., lower than the spectrum noise level), and were therefore considered negligibly small. Optical Measurements. Optical absorption measurements were performed using an ultraviolet−visible−near-infrared spectrophotometer (Shimadzu, UV-3600). Two kinds of photoemission measurements were carried out. The first was steady-state measurements of photoluminescence spectra, where the sample was irradiated by a continuous-wave laser with a wavelength of 633 nm. Spectra were acquired by a thermoelectrically cooled CCD (Princeton Instruments, PIXIS:100BR) mounted after a monochromator (Princeton Instruments, SP-2300). The second was time-resolved measurements of photoemission intensity carried out with short light pulses (pulse duration, 4 ns; repetition rate, 10 Hz; wavelength, 630 nm) generated by an optical parametric oscillator (EKSPLA, NT-242). Typically, the measured emission intensity has an accompanying uncertainty of ca. 10%, which needs to be

considered when emission intensities from different samples are compared. The uncertainty is mainly caused by slight variance in the positioning of the sample quartz tube held in the sample holder, which is designed to be located at a focal point of a lens toward the monochromator. This variance mainly arises from the variation of the outer dimensions of the fused-quartz tube (supplied by VitroCom) depending on the lot and position along its length. Electrochemical Measurements. The diffusion of solute species in the samples was investigated by electrochemical potential-step chronoamperometry53 using charge-neutral ferrocene (Fc) as a probe solute species. The measurements were carried out using a potentiostat/galvanostat (Princeton Applied Research, VersaSTAT 4) and hermetically sealed microelectrochemical cell (Princeton Applied Research, TVB7) equipped with a glassy-carbon-disc working electrode (diameter: 3 mm), coiled platinum-wire counter electrode, and platinum-disc quasireference electrode (diameter: 1 mm). Because the surface area of the counter electrode was ca. 14 times larger than that of the working electrode, the diffusion was not limited by the counter electrode. The concentration of Fc was 4.3 ± 0.2 × 10−3 M for all electrochemical measurements (i.e., ±5% uncertainty in the concentration). The sample preparation and hermetic sealing of the electrochemical cell were performed in the glovebox freshly filled with argon gas. The platinum quasireference electrode was calibrated using the Fc/Fc+ redox potential determined from a cyclic voltammogram. All electrochemical measurements were carried out under quiescent conditions and starting from an opencircuit state.

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RESULTS AND DISCUSSION The number of times that experiments were performed to confirm their reproducibility is summarized in the Supporting Information. Basic Properties of Samples. Figure 1 shows some basic properties of the developed ionogel photon upconverter,

Figure 1. Ionogel photon upconverter prepared using a gelator concentration of 7 g/L. These panels show the (a) optical transparency, (b) mechanical stability upon inversion, (c) upconversion of incident red light (633 nm, ca. 10 mW) to blue light, (d) emission and optical absorption spectra, and (e) demonstration of the nonflammability by direct exposure to a flame for 3 min. In all photographs, the thickness of the gel in the glass container was 6 mm. Wavelength-dependent sensitivities of the monochromator grating and CCD have been corrected in the emission spectrum. The optical path length in absorption spectrum was 1 mm. 750

DOI: 10.1021/acs.jpcb.5b09880 J. Phys. Chem. B 2016, 120, 748−755

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transition from the “weak excitation regime” where a bimolecular encounter between T1 emitter molecules is the limiting factor, to the “strong excitation regime” where this encounter is no longer the limiting factor because of the sufficient population of T1 emitter molecules.54,55 Figure 2 reveals that the dependence of IUC on Iexct for these two samples is the same within experimental uncertainty. This result suggests that the diffusive motion of the solute molecules is unaffected by the gelator polymer in the sample. This suggestion is different from our intuition, so this point is investigated further in the following section. Here, a threshold intensity (denoted Ith) is defined as the excitation intensity corresponding to the intersection of a line extrapolated from the low Iexct limit with the quadratic slope with a line extrapolated from the high Iexct limit with the linear slope in Figure 2. For these samples, Ith is ca. 0.01−0.02 W/ cm2. It is noted that this Ith is much smaller than that in our initial report (Ith ∼ 1 W/cm2)16 on IL-based samples made with the same sensitizer and emitter species. We attribute this difference to the fact that the oxygen management in our glovebox at the time of our initial report16 was less rigorous. For example, we used rubber gloves with relatively high oxygen permeability in our initial report, while here (and in our recent work, ref 36) we used butyl gloves with much lower oxygen permeability. The triplet lifetime of the emitter, denoted τT, is shortened by residual oxygen molecules (O2). In the present samples, τT of perylene is ca. 4 ms regardless of the gelator concentration (Figure S1 of the Supporting Information), which is the same as that measured in our recent report36 in the same IL (i.e., [C4dmim][NTf2]). Because Ith is inversely proportional to the square of τT,54 the large Ith in our initial report16 can be explained on the basis of such a possibility, which was unnoticed at that time. In fact, a remarkable decrease in Ith was recently observed by lowering residual O2 in a sample.55 It is also noted that because the UC quantum efficiency (ΦUC) measured under strong excitation conditions (denoted ΦUC(strg)) is unaffected by the magnitude of Ith, ΦUC(strg) determined previously16,36 have not been affected by this issue. (Note: In addition, ILs were purified before sample preparation in this article and our recent report36 while the ILs in the initial report16 were used without purification.) Because Ith strongly depends on the absorbance of a sample at the excitation wavelength,55 Ith could be further decreased by increasing the concentration of PdPh4TBP. Dependence of UC Emission Intensity on Gelator Concentration. Figure 3 shows the dependence of IUC on gelator concentration measured under strong excitation conditions (ca. 2 W/cm2). The samples used here had the same absorbance at the excitation wavelength (within 5% variance, which, although minor, has been corrected in this figure). Therefore, the ordinate of Figure 3 is proportional to ΦUC(strg), where ΦUC(strg) for the case of 0 g/L is ca. 10% based on the definition that the maximum ΦUC is 100%.16,36 This figure reveals that ΦUC(strg) is not affected by the gelator concentration at least within the tested concentration range. To support this interpretation, it is mentioned that τT in the present samples is independent of the gelator concentration, as shown in Figure S1 of the Supporting Information. Recently, we found that ΦUC(strg) strongly depends on the viscosity of the IL (Figure 3 of ref 36), and this dependence has been attributed to the influence of solvent viscosity on the kinetics of the TTA process through its effect on the dynamics

exemplifying a sample prepared using a gelator concentration of 7 g/L (0.49 wt %). This gel sample with a thickness of 6 mm is optically transparent (Figure 1a) and mechanically stable under inversion (Figure 1b). The optical transparency agrees with a previous observation by Nagasawa et al.52 that ionogels prepared with CDBA6·NTf2 are transparent and colorless for gelator concentrations of 10 g/L or lower.52 This sample functions as a red-to-blue photon upconverter (Figure 1c), and its optical absorption and emission spectra are displayed in Figure 1d. We confirmed that the sample did not ignite nor generate eye-recognizable fumes during and after exposure to a flame for 3 min (Figure 1e), demonstrating its thermal stability and nonflammability. This makes it, to the best of our knowledge, the first gel-form TTA-UC sample that simultaneously realizes transparency and nonflammability. Regarding the dependence of sample morphology on gelator concentration, the samples with concentrations lower than 5 g/ L were either a fluidic sol or physically weak gel that broke upon inversion. Conversely, those with gelator concentrations higher than 15 g/L showed weak light scattering because of the emergence of slight clouding, which was noticed when the glass vial was observed from the side. Therefore, for the present purpose, we consider a gelator concentration of 7 g/L to be optimal. Dependence of UC Emission Intensity on Excitation Intensity. Figure 2 shows the dependence of the UC

Figure 2. Dependence of the UC fluorescence intensity on excitation intensity measured for an ionogel sample (○; gelator concentration: 7 g/L) and ionic liquid sample prepared without the gelator (□). The excitation wavelength was 633 nm.

fluorescence intensity (IUC) on the excitation intensity (Iexct) plotted in double logarithmic scale measured of an ionogel sample prepared with a gelator concentration of 7 g/L and an IL sample prepared without the gelator (0 g/L). The ordinate (IUC) represents the magnitude of the wavelength-integrated CCD signal. Because the measurements of these two samples were carried out at the same time using the same optical and instrumental settings, the ordinate values for these two samples are quantitatively comparable. The slopes of the plots changed from 2 (quadratic) to 1 (linear) with increasing Iexct; this is the 751

DOI: 10.1021/acs.jpcb.5b09880 J. Phys. Chem. B 2016, 120, 748−755

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Figure 3. Gelator concentration dependence of the UC fluorescence intensity normalized relative to that measured for a sample prepared without the gelator (0 g/L). Error bar for the vertical axis is ±10%; the reason for this uncertainty is provided in the Experimental Section. The excitation wavelength was 633 nm, and the power was 10 mW (ca. 2 W/cm2).

of emitter molecules.36 Therefore, comparison of that finding (dependence of ΦUC(strg) on solvent viscosity)36 with the present finding (Figure 3; independence of ΦUC(strg) from gelator concentration) implies that the influence of the gelator polymer on the dynamic motions of the solute molecules in the present samples is negligibly small. This deduction agrees with the results in Figure 2. Solute Transport Properties. The solute transport properties of the samples are investigated to further examine the negligible influence of gelator concentration on solute motions. During this investigation, we found that the diffusioncontrolled bimolecular rate constant between the sensitizer (PdPh4TBP) and emitter (perylene) molecules, kq [M−1 s−1], is independent of the gelator concentration; the results and pertinent discussion are provided in the Supporting Information (Figures S2 and S3). To understand the results presented in Figures 2 and 3, the solute transport properties of the ionogel samples over much larger diffusion distances than the dimensions of the microscopic space between the gelator polymer chains (denoted d; see also Figure S3 in the Supporting Information and the related description) have to be investigated. It is mentioned here that Nagasawa and colleagues, who originally developed CDBA6·NTf2 as a gelator for ILs, observed that the ionic conductivity of the ionogel measured by an alternating current (ac) impedance technique did not noticeably decrease with increasing gelator concentration.52,56 These authors attributed this observation to the low gelator concentration required for gelation and the fact that this gelator itself is an electrolyte.56 Although the diffusion distances of the involved ions in their ac impedance measurements were unknown and the measurements were carried out on solvent ions (i.e., the ions that constitute the solvent medium), their results also motivated us to carry out the present investigation. Solute transport properties were measured using electrochemical potential-step chronoamperometry. We employed charge-neutral Fc as probe solute species, because Fc and its oxidized cation (Fc+) are electrochemically stable and exhibit good Nernstian behavior.53 Figure 4a depicts a cyclic voltammogram of the Fc/Fc+ redox reaction in the ionogel. The reaction occurs inside the electrochemical windows of both

Figure 4. (a) Cyclic voltammograms measured for ionogels with and without Fc at a scan rate of 10 mV/s. Black curve: Cyclic voltammogram of the Fc/Fc+ redox reaction in the ionogel prepared with gelator and Fc concentrations of 10 g/L and 4.3 ± 0.2 × 10−3 M, respectively. Red curve: Baseline voltammogram of the reference sample (gelator concentration: 10 g/L) without Fc. (b) Cottrell plot obtained after stepwise application of 0.344 ± 0.01 V vs Fc/Fc+ at t = 0. The sample was prepared with the same gelator and Fc concentrations as those used in part a. The asterisk indicates the point at which the current range automatically set by our potentiostat/ galvanostat was switched to the lower range, after which the signal-tonoise ratio increased. The straight red line is a least-squares fit to the data points between t = 30 and 60 s calculated using eq 1.

the IL and gelator. Cyclic voltammograms measured for samples prepared with different gelator concentrations, including the case of no gelator (0 g/L), are shown in Figure S4 in the Supporting Information. In the chronoamperometry measurements, the initial state was at the open-circuit potential, which was −0.27 ± 0.1 V versus Fc/Fc+. At the start time (t = 0), a stepwise potential of 0.344 ± 0.01 V versus Fc/Fc+ was applied to the working electrode. From the results in Figure 4a, this applied potential should cause diffusion-controlled oxidation. For t > 0, the oxidation current (i) decreases with increasing t because of the depletion of Fc near the electrode. As long as the diffusion length is sufficiently smaller than the electrode diameter, the diffusion can be regarded as one-dimensional (1D), and the Cottrell equation below should hold between i and t:53 ⎛ nFACD1/2 ⎞ ⎟t −1/2 i = −⎜ 1/2 ⎠ ⎝ π

(1)

Here, n is the number of electrons involved in the reaction (1 in the present case), F is the Faraday constant, A is the electrode area, C is the bulk concentration of reductant (4.3 ± 752

DOI: 10.1021/acs.jpcb.5b09880 J. Phys. Chem. B 2016, 120, 748−755

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The Journal of Physical Chemistry B 0.2 × 10−3 M in the present case), and D is the diffusion coefficient. Figure 4b displays the Cottrell plot, where the current density (i/A) is plotted for the inverse square of time (t−1/2) for the ionogel sample prepared with the same Fc and gelator concentrations as those used in Figure 4a. This plot has been subtracted by the baseline signal obtained from a reference sample prepared without Fc at the same gelator concentration. The magnitude of the baseline was ca. 6%−8% of the raw (i.e., as-measured) signal including that for the sample without gelator (0 g/L), as shown in Figure S5 in the Supporting Information. Figure S5 illustrates the Cottrell plots for samples with various gelator concentrations including their raw and baseline signals. The solid red line in Figure 4b is the least-squares fit for the data points between t = 30 and 60 s calculated using eq 1. The agreement confirms that the diffusion of Fc was 1D and diffusion controlled.53 From the slope of the plot in Figure 4b, D of Fc was determined to be 1.52 ± 0.23 × 10−7 cm2/s in the sample. The time range of 30 ≤ t ≤ 60 s was chosen for the fitting because this range corresponds to the diffusion length, defined by δ ≡ Dt , of 21 ≤ δ ≤ 30 μm based on the D determined above, and thus the magnitude of δ is sufficiently larger than d, which is on the order of tens of nanometers (see the Supporting Information). Figure 5 shows the magnitudes of D determined for samples prepared with different gelator concentrations (0, 7, 10, 14, and

Finally, as mentioned above, Nagasawa et al.52,56 observed that the ionic conductivity of an ionogel did not noticeably decrease with increasing gelator concentration, and they ascribed this observation to (i) the relatively low gelator concentration required for the gelation of ILs and to (ii) the fact that this gelator itself is an electrolyte.56 In the present report, however, neither the gelator nor the IL participated in the chronoamperometry, because the measurements were performed inside their electrochemical windows (Figure 4a). Therefore, our results suggest that reason i above is plausible, while reason ii is not essential in the present study, although detailed reasoning behind this point remains to be elucidated. Nevertheless, the physical properties of the present ionogel samples clarified above provide beneficial insights that support their use as a medium for TTA-UC.

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CONCLUSIONS

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ASSOCIATED CONTENT

The problem of the fluidity of IL-based photon upconverters16,27,28,36 was resolved by gelatinizing the system using a polymeric salt. The developed ionogel sample is optically transparent and mechanically stable, and functions as a photon upconverter. The high thermal stability of the sample was confirmed by exposure to a flame, which was attributed to its high fraction (>99 wt %) of IL. Therefore, the developed photon upconverters simultaneously achieved transparency, nonflammability, and nonfluidity. The dependence of IUC on Iexct measured for the ionogel sample quantitatively agreed with that for a normal IL sample prepared without the gelator. Additionally, IUC measured under strong excitation conditions was independent of the gelator concentration. These results show that the gelator does not affect TTA-UC and suggest that the influence of the gelation on the diffusional motion of the solute species in the present samples was negligibly small. Chronoamperometry measurements of the diffusion of solute species over distances much longer than the microscopic interpolymer distances indicated that the gelation of the IL does not slow the diffusion of solute species, at least within the tested range of gelator concentration. Although the detailed reason for this has yet to be clarified, we presently attribute this unique transport property to the low gelator concentration (