Kinetics of Photon Upconversion in Ionic Liquids: Energy Transfer

Feb 6, 2013 - The efficiency of triplet-sensitized photon upconversion in ionic liquids was previously found to be dependent on the type of ionic liqu...
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Kinetics of Photon Upconversion in Ionic Liquids: Energy Transfer between Sensitizer and Emitter Molecules Yoichi Murakami,*,† Hitomi Kikuchi,‡ and Akio Kawai‡ †

Global Edge Institute, Tokyo Institute of Technology, 2-12-1-I1-15 Ookayama, Meguro-ku, Tokyo 152-8550, Japan Department of Chemistry, Graduate School of Science and Engineering, Tokyo Institute of Technology, 2-12-1-H89 Ookayama, Meguro-ku, Tokyo 152-8551, Japan

‡

S Supporting Information *

ABSTRACT: The efficiency of triplet-sensitized photon upconversion in ionic liquids was previously found to be dependent on the type of ionic liquid employed. The properties of the intermolecular energy transfer need to be understood in order to improve the upconversion efficiency. Here, we investigate the kinetics of the triplet energy transfer from the triplet sensitizing molecule to the emitter molecule where the latter is responsible for delayed upconversion fluorescence emission. The collision kinetics between the sensitizer and emitter molecules in imidazolium ionic liquids are investigated by systematically changing the alkyl chain length of the ionic liquid cation. Stern−Volmer analysis reveals unique diffusion behavior of the solute molecules in ionic liquids, and this observation is attributed to the microheterogeneity of the ionic liquids. Through time-resolved transient absorption measurements and determination of the triplet−triplet absorption coefficient of the sensitizer molecule used, we find that the quantum efficiency of the triplet energy transfer in the present system is sufficiently high (ca. 0.75) and independent of the type of ionic liquid. These findings show that the ionic liquid dependence of the upconversion efficiency arises from the later processes pertaining to the emitter molecule rather than the triplet energy transfer process.

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INTRODUCTION

While ionic liquid based upconverters have multiple engineering advantages, an unresolved question has existed: The magnitude of ΦUC has been found to be dependent on the type of ionic liquid employed.7 At least for the ionic liquids that are going to be employed in this report, it has been strongly implied from our experimental knowledge that this influence of ionic liquid on the ΦUC is not from extrinsic factors, such as by dioxygen or other type of quenchers, based on the following points. First, the values of ΦUC have been measured where the decay of the emitter triplets is dominated by second-order processes, including TTA rather than the first-order decay processes, such as spontaneous decay and quenching by residual dioxygen.7 Second, systematic tendencies have been observed according to the structure of ionic liquids, as some of which will be shown below. To address the origin of the observed differences in ΦUC, the elucidation of the energy transfer kinetics in the samples is essential. In particular, it is necessary to determine which process is more responsible for the observed variance of ΦUC, either the process of triplet energy transfer between the sensitizer molecule and the emitter molecule or the TTA process. The purpose of this report is to clarify, by timeresolved photophysical studies, the dependence of the efficiency

Photon upconversion (UC) based on triplet−triplet annihilation (TTA) between organic molecules is a promising light wavelength conversion technology, which can be applicable to low-intensity incoherent light, such as sunlight.1 This UC strategy, recently under active investigation,2−4 utilizes two kinds of molecules, one type of molecule that performs triplet sensitization (the “sensitizer”) and the other molecule that performs TTA and emission of an upconverted photon (the “emitter”). Since both the triplet sensitization and TTA are based on an electron-exchange mechanism induced by intermolecular collision,5,6 the host medium must allow for translational diffusion of the molecules in order to achieve a meaningful magnitude of upconversion quantum efficiency (ΦUC). Recently, we developed photon upconverters that employ ionic liquids for the host medium.7 Ionic liquids are roomtemperature molten salts that have negligibly small vapor pressures8 and high thermal stabilities,9 forming an emerging class of liquid with many potential applications.10 In addition to the nonvolatility and nonflammability that are beneficial for application, the negligible vapor pressures allow samples to be degassed under ultrahigh vacuum.7 This give rise to an efficient removal of dioxygen molecules that act as triplet quenchers and cause sample degradation. © 2013 American Chemical Society

Received: December 17, 2012 Published: February 6, 2013 2487

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This report employs five kinds of ionic liquids, [Cnmim][NTf2] (with n = 2, 4, 6, or 8) and [C4dmim][NTf2], where [Cnmim] = 1-alkyl-3-methylimidazolium, [C4dmim] = 1-butyl-2,3dimethylimidazolium, and [NTf2] = bis(trifluoromethylsulfonyl)amide. All of the ionic liquids were purchased from IoLiTec and stored under nitrogen. PdPh4TBP and perylene were purchased from Frontier Scientific and Sigma-Aldrich, respectively. All the ionic liquids were dried at 120 °C under vacuum for 4 h before use. The viscosities of the vacuum-dried ionic liquids were measured by a cone-plate rheometer with a thermoelectric temperature-controlled stage (Brookfield, R/S Plus) at 20 ± 0.2 °C. Scheme 1 shows the qualitative energy level diagram of the UC process, which proceeds as follows: First, an incident photon hν1 excites a sensitizer molecule in the ground state (S0) into the excited singlet state (S1), followed by an ultrafast intersystem crossing (ISC) to the triplet state (T1). The quantum yield of S1−T1 ISC of palladium porphyrins is almost unity.13 The triplet energy is then transferred to an emitter molecule through a collision-induced electron exchange based on the Dexter mechanism.5,6 A collision between two T1 emitter molecules leads to TTA, from which one blue shifted photon hν2 is emitted as delayed fluorescence. Hence, the upconversion efficiency is given by the product of the efficiencies of those processes4,11

of the sensitizer-to-emitter energy transfer on the type of ionic liquid and to determine the absolute magnitude in order to understand the nature of this process in the present system. So far, detailed kinetic studies of triplet-sensitized photon UC have been limited11,12 and especially the investigation on the triplet energy transfer process has been lacking. In previous kinetical studies,11,12 the quantum efficiency of the triplet energy transfer between the sensitizer and emitter molecules (ΦTET) was assumed to be unity. However, the importance of ΦTET in the triplet-sensitized UC has been recently pointed out.4 This provides further motivation to undertake this present study, in addition to the interest of the kinetics of photon UC in ionic liquids.

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EXPERIMENTAL SECTION Chart 1 shows the structures of the molecules used in this report. In the chart, (a) and (b) are the sensitizer (PdChart 1. Molecular Structures of (a) PdPh4TBP, (b) Perylene, (c) Ionic Liquid Cation, and (d) Ionic Liquid Anion

ΦUC = ΦTETΦTTA ΦF

(1)

where ΦTTA and ΦF denote the quantum efficiencies of the TTA process and the prompt fluorescence from the emitter molecule, respectively. We have confirmed that the ΦF of perylene is independent of the type of ionic liquid. Transient photoemission and transient absorption measurements were done using a home-built setup that consists of an optical parametric oscillator (OPO, Ekspla NT-242), a monochromator (Princeton SP-2300i), an arrayed CCD detector (Princeton PIXIS:100BR), and a photomultiplier tube (Hamamatsu H11461-03). The detail of the setup is shown in Figure S1 of the Supporting Information. The OPO was operated at 10 Hz, and the pulse duration was 4 ns. The wavelength for sample excitation was 630 nm for transient emission and either 578 or 630 nm for transient absorption measurements. The laser beam spot dimensions at the sample position were 0.8 mm (round) for transient emission and 1.5 mm × 2.5 mm (ellipse) for transient absorption measurements, as shown by Figure S2 (Supporting Information). A white light generated from an LED was passed through a band-pass filter to extract a specific wavelength (490 or 700 nm, fwhm = 10

tetraphenyltetrabenzoporphyrin, PdPh4TBP) and emitter (perylene) molecules, while (c) and (d) are the cation ([Cnmim]+, [Cndmim]+) and anion ([NTf2]−) of ionic liquids, respectively.

Scheme 1. Qualitative Energy Level Diagram of Upconversion Process between PdPh4TBP and Perylenea

a

ISC, intersystem crossing; TET, triplet energy transfer. 2488

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nm), and it was used as the probe light in the transient absorption measurements. The time-resolved intensities were measured with the photomultiplier tube. The output signal was processed with a 12 bit digitizer (Acqiris U1066A) and recorded by a PC. Quantitative measurements of the emission intensities were performed using the CCD by integrating the spectrum for a much longer time than the OPO pulse period. The sample preparation procedure was the same as in our previous report.7 Briefly, PdPh4TBP and perylene were dissolved into an ionic liquid with an intermediary of toluene. The toluene was subsequently removed by vacuum pumping. The ionic liquid containing these molecules was then brought into an argon-filled glovebox and degassed under ultrahigh vacuum overnight (more than 12 h) with a turbo molecular pump to remove oxygen molecules. The liquid was then injected into a quartz tube with a square cross section and sealed with metal alloy solder. The inner dimensions of the quartz tube was either 1 mm × 1 mm (for transient emission measurements) or 2 mm × 2 mm (for transient absorption measurements), as shown by Figure S2 (Supporting Information). Unless otherwise specified, the concentrations of the sensitizer (PdPh4TBP) and the emitter (perylene) in the sample were 1 × 10−5 and 3 × 10−3 M, respectively. Here, some fundamental aspects of the sample are briefly shown. Figure 1 shows the typical photoemission spectrum

Figure 2. (a) Time-resolved phosphorescence intensity at 800 nm from PdPh4TBP (thick curve) and UC fluorescence intensity at 475 nm from perylene (thin curves). (b) Dependence of the phosphorescence decay kinetics on the perylene concentration. The dashed curve is a single-exponential fit to the 0 mM signal. For all the cases, the solvent ionic liquid = [C4dmim][NTf2], [PdPh4TBP] = 1 × 10−5 M, and excitation wavelength = 630 nm.

constant is independent of the excitation pulse energy. The rising speed of the UC fluorescence intensity is also independent of the excitation pulse energy, as shown. The agreement between the rates of the decay of the phosphorescence intensity and the rise of the UC fluorescence intensity provides evidence that the UC emission is caused as a result of the energy transfer from triplet PdPh4TBP to perylene. Figure 2b shows the dependence of the phosphorescence decay on the perylene concentration. At [Perylene] = 0, the decay rate determined by a single exponential fit was 3920 s−1, which agrees with the reported phosphorescence decay rate of 3880 s−1.14 For [Perylene] > 0, the rate of the decay with the quenching process, denoted by k Tq(s) , increased with [Perylene], as shown by Figure 2b. Figure 3 shows the Stern−Volmer plot generated from the phosphorescence decay curves shown in Figure 2b. The linear

Figure 1. Photoemission spectrum (UC delayed fluorescence and phosphorescence) upon excitation at 632.8 nm (arrow). Inset is a photograph of the typical sample upconverting red incident light (632.8 nm, 10 mW, ca. 2 W/cm2) into blue emission.

upon excitation at 632.8 nm. The feature at ∼475 nm is upconverted delayed fluorescence from perylene, and the feature at ∼800 nm is phosphorescence from PdPh4TBP. The UC emission occurs as long as an incident light is absorbed by the Q-band of PdPh4TBP (whose optical absorption spectrum will be presented later). The inset of Figure 1 shows a photograph of a sample upconverting red incident continuouswave light (632.8 nm, ca. 2 W/cm2) into blue emission. Figure S3 in the Supporting Information shows a photograph of a sample taken 26 months after the sample was fabricated. Owing to the efficient elimination of oxygen molecules, the samples possess a high stability and a long lifetime.

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RESULTS AND DISCUSSION Quenching Kinetics of T1 Sensitizer by S0 Emitter. Figure 2a compares the time-resolved intensity signals of the phosphorescence at 800 nm (from PdPh4TBP) and the UC fluorescence at 475 nm (from perylene) acquired from the same sample. The phosphorescence intensity decay can be fit well by a single-exponential decay function, and the time

Figure 3. Stern−Volmer plot generated from the phosphorescence decay curves shown in Figure 2b. 2489

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relationship indicates that the quenching process, or the process of the triplet energy transfer, follows pseudo-firstorder kinetics for the entire range of [Perylene] tested here. This also indicates that the molecules are dissolved in the ionic liquid without forming aggregates, because the formation of aggregates would cause the slope to be reduced with an increase of molecular concentration.15 The Stern−Volmer plots for other samples also exhibited clear linear relationships (Figure S4 in the Supporting Information). The quenching rate constant denoted by kq is obtained from the slope of the plot, as given below: k Tq(s) = k T(s) + kq[Perylene]

(2)

Here, kT(s) denotes the rate at [Perylene] = 0. Table 1 shows the values of kq determined for the samples made with the five ionic liquids. This table also shows the measured viscosities of those ionic liquids, denoted by η. Table 1. Ionic Liquid Viscosities and Rate Constants ionic liquid

ηa/mPa s

kqb/107 M−1 s−1

kTETc/107 M−1 s−1

[C2mim][NTf2] [C4mim][NTf2] [C6mim][NTf2] [C8mim][NTf2] [C4dmim][NTf2]

38 61 88 119 132

7.9 5.5 4.3 3.8 2.7

5.9 4.1 3.2 2.9 2.1

Figure 4. (a) Dependence of the kq determined from the Stern− Volmer analysis on the ionic liquid viscosity for the samples made with the five ionic liquids. (b) Dependence of the kq divided by the kdiff predicted from eq 3 on the ionic liquid viscosity.

Measured on ionic liquids without solute molecules at 20 ± 0.2 °C. Experimental uncertainty ∼ ±3%. cUncertainty ∼ ±8% that mostly arises from the uncertainty in ΦTET used for the derivation through eq 4. a b

hypothesize that it is more energetically favorable for the nonpolar solute molecules (PdPh4TBP and perylene) when they are in the nonpolar domain, which essentially agrees with the picture of ref 18. Assuming that this holds, while the absolute diffusivity decreases with increasing alkyl chain length, the magnitude of kq/kdiff is expected to increase with increasing alkyl chain length as a result of an increase in the volumetric fraction of the nonpolar domain in the ionic liquids. Another important aspect found in Figure 4b is that the data point for the sample made with [C4dmim][NTf2], where the Xposition of the cation is −CH3 instead of −H (Chart 1c), is not in line with this tendency. A previous theoretical investiagation21 has pointed out that the replacement of −H by −CH3 significantly lowers the entropy of this ionic liquid partly due to its stronger steric constraint onto the conformational freedom of the alkyl chain next to the −CH3, which explained the causes of the increased viscosity and the reduced melting point of this ionic liquid compared to those of [C4mim][NTf2]. Although a definite conclusion has not been obtained, we propose that the lower magnitude of kq/kdiff for this ionic liquid arises from a more ordered conformation of the alkyl chains constituting the nonpolar domain due to the lower entropy of this ionic liquid. Ionic Liquid Dependence of Triplet Energy Transfer Efficiency. With the knowledge of the fundamental characteristics of the present system obtained above, we investigate the quantum efficiency of the triplet energy transfer from PdPh4TBP to perylene, ΦTET, which is defined as

Ionic Liquid Dependence of Quenching Rate Constant. Figure 4a plots the values of kq for the five samples as a function of η based on the data in Table 1. The kq decreases as the viscosity increases, and this agrees with our expectation. On the other hand, a diffusion-controlled rate constant kdiff can be estimated according to Smoluchowski’s theory combined with the Stokes−Einstein approximation:5

kdiff =

8RT 3000η

(3)

Here, R and T denote the gas constant and the temperature, respectively. In Figure 4b, the ratio kq/kdiff is plotted as a function of η. For the samples made with [Cnmim][NTf2] (n = 2, 4, 6, 8), a clear tendency is found where the ratio kq/kdiff increases with increasing n. This tendency is thought to originate from the existence of a microscopic structure in the ionic liquids. So far, the existence of microscopic heterogeneity in ionic liquids has been indicated experimentally and theoretically.16−20 This heterogeneity has been explained to arise from the segregation of the alkyl chains into the mesoscopic nonpolar domains.16−20 Recently, Patra and Samanta18 investigated the diffusion behavior of probe molecules in imidazolium ionic liquids using fluorescence correlation spectroscopy combined with fluorescence lifetime measurements. These authors showed that neutral probe molecules exhibit a bimodal diffusion behavior where the slower component was assigned to the diffusion in the nonpolar domain and that the fraction of this component increases with the length of the alkyl chain.18 While these authors did not compare their diffusion coefficients with the values predicted by Smoluchowski’s theory (eq 3), the tendency of the kq/kdiff we have found in Figure 4b may share the same origin. We

k TET = ΦTETkq

(4)

Here, kTET denotes the rate of the energy transfer. The determination of ΦTET is important for elucidating not only whether this process is a bottleneck for the entire process but 2490

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also whether the aforementioned dependence of ΦUC on the type of ionic liquid arises from this process. This investigation is performed by means of transient absorption experiments. Figure 5 shows a typical transient absorption signal monitored at 490 nm upon excitation at 630 nm. At 490 nm,

5c, we plot the dependence of the magnitudes of the fast and slow components on the monitoring wavelength to check their T-T absorption spectrum, where all the points were acquired upon excitation at 630 nm. The shape of the spectrum that we have attributed to perylene agrees well with the reported shape of the T-T absorption spectrum of perylene,22 which further corroborates the attributions of the fast and slow components in Figure 5. To analyze the observed decay kinetics, the transient absorption signals are fitted by the following function: ⎛ ⎛ t⎞ t⎞ ⎛ t ⎞ ΔO.D.(t ) = A exp⎜ − ⎟ + B exp⎜1 − ⎟exp⎜ − ⎟ τ1 ⎠ ⎝ τ2 ⎠ ⎝ ⎝ τ1 ⎠ (5)

The first term represents the decay of the triplet population in PdPh4TBP with the time constant of τ1. The first factor in the second term represents the creation of triplet population in perylene, which also proceeds at the same time constant τ1. The second factor in the second term denotes the decay of the triplet perylene that proceeds at a much longer time constant of τ2. (Note: While the second term should have been written in the form of a deconvolution integral of these two factors, it can be simplified to this form because τ2 ≫ τ1.) The values of τ1 were obtained from the phosphorescence decay curves at 800 nm because their signal-to-noise ratio (S/N) is much higher than the S/N of transient absorption decay curves. The values of τ2 were obtained by a single-exponential fit to the transient absorption signals for the time range of 5τ1 ≤ t ≤ 25τ1. From the fitting by eq 5, we have obtained for each sample the value of B/A, which is proportional to ΦTET. In Figure 6, the values of A/B determined for the samples made with the five ionic liquids are plotted against the

Figure 5. Typical transient absorption decay signals monitored at 490 nm upon excitation at 630 nm, presented for (a) a short time range, (b) a longer time range, and (c) a further longer time range with a comparison to the square root of the UC fluorescence intensity. The solvent ionic liquid was [C2mim][NTf2], and the excitation pulse energy was 160 μJ (with a 2.5 × 1.5 mm2 spot size on the sample; see Figure S2 of the Supporting Information). The inset of panel (c) shows the wavelength dependence of the magnitudes of the fast (circle) and slow (square) components, which have been attributed to PdPh4TBP and perylene, respectively. Figure 6. Values of A/B determined from the fitting to the transient absorption decay curves by eq 5, plotted against the excitation pulse energy.

the triplet−triplet (T-T) absorption coefficient (εT) of perylene takes its maximum.22 Around this wavelength, PdPh4TBP also has some εT. Figure 5a shows that there is a fast decay component immediately after the excitation. This is followed by another decay component that is much slower, as shown by Figure 5b. Since the decay time constant of the fast component always agrees well with that of the phosphorescence from PdPh4TBP, we assign this component to be the decay of the triplet population in PdPh4TBP. The slower component shown in Figure 5b is attributed to the T-T absorption of perylene. In Figure 5c, we compared the same transient absorption signal with the square root of the emission intensity of the UC fluorescence at 475 nm measured from the same sample, and their agreement supports this attribution. In the inset of Figure

excitation pulse energy. This figure shows that the ΦTET is independent of the type of ionic liquid and also of the excitation pulse energy within the certainty of our experiment. While this result has revealed an important aspect of the present system, to further understand the nature of this process, the absolute magnitude of ΦTET has to be clarified. To do this, however, the value of εT of PdPh4TBP has to be known. Determination of εT of PdPh4TBP. To determine the εT of PdPh4TBP, we need a reference whose εT is already established. We have chosen C60 for this, because (i) the triplet absorption properties have been thoroughly studied by many 2491

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research groups,23−25 (ii) the triple quantum yield is ≅ 1,25 and (iii) the ground state optical absorption spectrum has a moderate overlap with that of PdPh4TBP. We prepared a PdPh4TBP solution in [C4dmim][NTf2] and a C60 solution in toluene so that their absorbances at the excitation wavelength (578 nm) coincided, as shown by Figure 7. Their concentrations were 5 × 10−5 M and ca. 9.2 × 10−4 M,

Figure 7. Optical absorption spectra of the samples used for the determination of εT of PdPh4TBP. The optical path length was 2 mm. The spectra have been subtracted by the baseline (cuvette + solvent). The arrow indicates the common wavelength (578 nm) used to excite these samples.

respectively. The C60 solution was bubbled with argon just before the transient absorption measurement, and the absorption spectrum (Figure 7) was obtained just after the bubbling treatment. Both liquids were sealed in the quartz tubes with a 2 mm × 2 mm inner dimension, as mentioned earlier. Figure 8a shows the relationship between the “ΔO.D. at t = 0” obtained from the transient absorption signals of the C60 solution monitored at 700 nm and the excitation pulse energy. While the relationship is mostly linear over the entire range of excitation pulse energies, a slight photosaturation at higher pulse energies is observed. We adopt the value at 40 μJ because the linearity is assured. The corresponding transient absorption signal is shown in Figure 8b. Figure 9a shows the relationship between the values of “ΔO.D at t = 0” obtained from the transient absorption signals of the PdPh4TBP solution at 490 nm and the excitation pulse energy. The value at 40 μJ is to be adopted also for this case; however, non-negligible photosaturation is recognized in this data point. The influence of this photosaturation is later corrected by using the intensity saturation data of the phosphorescence of PdPh4TBP measured from the same sample, which is plotted on the right axis of Figure 9a. The corresponding transient absorption signal is shown in Figure 9b. The value of εT of PdPh4TBP is derived from the following relationship: εT(PdPh4TBP) =

ΔO.D.(PdPh4TBP) × A(C60) ΔO.D.(C60) × A(PdPh4TBP) × fPS

Figure 8. (a) Dependence of the “ΔO.D. at time 0” of the C60 solution at 700 nm on the excitation pulse energy. The optical absorption spectrum of this sample is shown in Figure 7. The arrow indicates the data point adopted for analysis. (b) The transient absorption decay curve of the C60 solution at 40 μJ.

All the “ΔO.D.” in eq 6 are at t = 0. Two research groups have independently reported the same value of εT(C60), which is 6250 M−1 cm−1 at 700 nm in benzene.24,25 By adopting this value, the εT(PdPh4TBP) at 490 nm has been determined to be 26 900 M−1 cm−1. Determination of ΦTET. With the value of εT(PdPh4TBP) determined, the values of ΦTET are calculated from B εT(PdPh4TBP) ΦTET = A εT(Perylene) (7) where the values of A/B are in Figure 6. The εT of perylene has been reported to be 14 300 M−1 cm−1 in nonpolar solvents (at 490 nm) and 13 400 M−1 cm−1 in polar solvents (at 485 nm).26 Imidazolium-based ionic liquids have been known to be polar solvents whose polarity is close to methanol.27−29 However, since it is still unclear whether ionic liquids act as a polar or nonpolar environment for perylene molecules at the microscopic level and since these two values are close to each other, here we simply take their mean value, 13 850 M−1 cm−1. Figure 10 shows the values of ΦTET calculated from eq 7 for the samples made with the five ionic liquids plotted against the excitation pulse energy. The value is 0.75 ± 0.05 for all the samples. The reason for this relatively high value is considered as follows: The T1 energy level of perylene in polar and nonpolar solvents has been reported to be 1.56 and 1.54 eV, respectively.26 On the other hand, the T1 level of PdPh4TBP has recently been reported to be 1.57 eV, which was

εT(C60) (6)

Here, εT(PdPh4TBP) and ΔO.D.(PdPh4TBP) are at 490 nm, and εT(C60) and ΔO.D.(C60) are at 700 nm. A(PdPh4TBP) and A(C60) are the absorbances at the excitation wavelength (578 nm; see Figure 7). The f PS is the correction factor to the photosaturation of PdPh4TBP ( f PS = 0.928 at 40 μJ, from Figure 9a). 2492

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kTET are estimated from eq 4 and listed in Table 1. Finally, while we have relied on the value of εT(C60) = 6250 M−1 cm−1 at 700 nm based on refs 24 and 25, we note that, if the value reported in ref 23 (εT(C60) = 7400 M−1 cm−1 at 700 nm) is employed, the magnitude of ΦTET would be higher (∼0.89).

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CONCLUSION We have investigated the kinetics of the triplet energy transfer from the sensitizer molecule (PdPh4TBP) to the emitter molecule (perylene) in order to clarify the triplet sensitization process of photon upconversion in ionic liquids. The linearity of the Stern−Volmer plots, which were generated from the time-resolved intensity measurements of the phosphorescence of PdPh4TBP, has confirmed that the process of triplet energy transfer obeys pseudo-first-order kinetics and solute molecules are dissolved in the ionic liquids without forming aggregates. With this confirmation, the dependence of the rate constant for this process (kq) on the type of ionic liquid was investigated, from which the ratio of the kq to the kdiff predicted by eq 3 has been found to increase with increasing alkyl chain length of the [Cnmim]+ cation (Figure 4). This finding has unveiled a unique aspect of using ionic liquids, which have been proposed to be microstructured fluids, for triplet-sensitized upconversion. The quantum efficiencies of the triplet energy transfer (ΦTET) for different ionic liquids were investigated by transient absorption measurements, from which ΦTET was found to be independent of the ionic liquid type (Figure 6). This indicates that the strong ΦUC dependence on ionic liquid type7 is not reliant on triplet energy transfer. In addition, we have determined the absolute magnitudes of the ΦTET (Figure 10) to be relatively large (ca. 0.75) and attribute this high value to the close matching of the T1 levels of the sensitizer and emitter molecules in the present system. These findings show that the ionic liquid dependence of ΦUC arises from the ΦTTA in eq 1, suggesting that a strategy of increasing this efficiency should be conceived in order to increase the ΦUC.

Figure 9. (a) Dependences of the “ΔO.D. at time 0” of the PdPh4TBP solution at 490 nm (circles) and the phosphorescence emission intensity from the identical sample measured by the CCD (squares) on the excitation pulse energy. The optical absorption spectrum of this sample is shown in Figure 7. The arrow indicates the data point adopted for analysis. (b) The transient absorption decay curve of the PdPh4TBP solution at 40 μJ.

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ASSOCIATED CONTENT

S Supporting Information *

Details of the measurement setup, laser beam profiles at the sample position, dimensions of the sample quartz tubes, sample photograph taken 26 months after the fabrication, and Stern− Volmer plots for the samples made with the other four ionic liquids. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

Figure 10. Plot of the values of ΦTET determined from eq 7 using the A/B values shown in Figure 6.

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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determined from the phosphorescence spectrum at 77 K in glass toluene.30 Since the spectrum (Figure 3 of ref 30) was still broad even at 77 K, the last digit of the value (1.57 eV) may not be certain. Even if this is taken into account, the matching between the T1 energy levels of PdPh4TBP and perylene is regarded as good. This close-matching of the T1 energies could give rise to resonant energy transfer between the T1 levels of PdPh4TBP and perylene, which may explain the relatively high value of ΦTET ∼ 0.75. With the ΦTET determined, the values of

ACKNOWLEDGMENTS This work was supported by a Grant-in-Aid for Young Scientists A (no. 23686035) and a Grant-in-Aid for Scientific Research (no. 24350004) from the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan. The authors thank Ryan Gresback of Tokyo Inst. Tech. for the refinement of English and expressions. 2493

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dx.doi.org/10.1021/jp3124082 | J. Phys. Chem. B 2013, 117, 2487−2494