Solid-State, Near-Infrared to Visible Photon Upconversion via Triplet

Research Article Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Solid-State, Near-Infrared to Visible Photon Upconversion via Triplet−Triplet Annihilation of a Binary System Fabricated by Solution Casting Aizitiaili Abulikemu,*,†,⊥ Yusuke Sakagami,† Claire Heck,† Kenji Kamada,*,† Hikaru Sotome,‡ Hiroshi Miyasaka,‡ Daiki Kuzuhara,§ and Hiroko Yamada*,∥ †

IFMRI, National Institute of Advanced Industrial Science and Technology (AIST), Ikeda, Osaka 563-8577, Japan Division of Frontier Materials Science and Center for Advanced Interdisciplinary Research, Graduate School of Engineering Science, Osaka University, Toyonaka, Osaka 560-8531, Japan § Department of Physical Science and Materials Engineering, Iwate University, Morioka, Iwate 020-8551, Japan ∥ Graduate School of Materials Science, Nara Institute of Science and Technology (NAIST), Ikoma, Nara 630-0192, Japan

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‡

S Supporting Information *

ABSTRACT: Herein, triplet−triplet annihilation upconversion (TTAUC) from near-infrared (NIR, 785 nm) to visible (yellow, centered at 570 nm) regions has been demonstrated in the binary solid of condensed chromophores. Microparticles of the binary solid comprising rubrene as a matrix (emitter) and π-extended Pd-porphyrin as a dopant (sensitizer) in a mole ratio of 1000:1 were obtained by solution casting. Excitation intensity dependence and quantum yield (QY) of the upconverted emission were characterized for individual particles under a microscope and revealed a low threshold intensity (∼100 mW/cm2) as compared to the solution and moderate UC-QY (∼0.5%) in the NIR range. The factors contributing to the UC-QY were investigated by time-resolved and steady-state spectroscopies. It was found that the intersystem crossing of the sensitizer, triplet energy transfer, and TTA occurred efficiently in the binary solid, and the fluorescence QY of the emitter governed the UC-QY. KEYWORDS: photon upconversion, triplet−triplet annihilation, near-infrared to visible, binary solid, solution casting, intersystem crossing, triplet energy transfer

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INTRODUCTION Photon upconversion (UC) is the conversion of two lowenergy photons into a high-energy photon. There are several different mechanisms of UC. Among them, delayed fluorescence via triplet−triplet annihilation (TTA), abbreviated as TTA-UC, is the only one mechanism to date that can convert at the excitation intensity level of the sunlight (a few mW/ cm2). This feature of low-excitation intensity makes TTA-UC attractive for the application of spectral management of sunlight.1−8 Efficient use of the near-infrared (NIR) and infrared (IR) regions of sunlight is a challenging goal in the field of spectral management because most solar energy devices, such as photovoltaic and photocatalytic devices, have no or low efficiencies at NIR−IR wavelengths. Therefore, conversion of NIR or IR to the visible (vis) wavelengths operating at the sunlight intensity is required. Additionally, operation in the solid phase is preferable for the device applications. Thus, NIR-to-vis TTA-UC in a solid material is an important research goal for optimizing the use of the solar energy through boosting the effective efficiency of the energy devices. While TTA-UC from visible wavelengths has been studied extensively,9−12 there are very few reports of the NIR-to-vis © XXXX American Chemical Society

conversion. NIR-to-vis TTA-UC was pioneered by Baluschev and co-workers by using π-extended naphthylporphyrin as a sensitizer (λex = 700 nm).13,14 These researchers extended the excitation wavelength to 785 nm by fusing anthracene at the βposition of the porphyrin.15 Other π-extended porphyrins and phthalocyanines were also used for NIR excitation by other research groups.3,16 For the longer wavelength excitation, semiconductor quantum dots (QDs)17−22 and an Os complex for the direct S−T absorption23,24 have also been employed recently. Solid-state TTA-UC is the other important issue toward the device applications. For this purpose, dye-doped polymers,25−29 organogels,30−32 and condensed systems, where the emitter is used as the matrix,33 have been developed. In the condensed system, triplet exciton migration can mediate the excitation energy instead of molecular diffusion and lead to high efficiency of TTA. This is as expected because the phenomenon of TTA was first observed in molecular crystals;34 nevertheless, examples of TTA-UC in condensed Received: March 10, 2019 Accepted: May 15, 2019

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DOI: 10.1021/acsami.9b04148 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Figure 1. (a) Energy-level diagram and chemical structure of PdTPTAP (sensitizer) and rubrene (emitter) with their optimized geometries in vacuum by the B3LYP/6-31G(d) (Lanl2DZ for Pd) level of calculation (tert-butyl groups were omitted for PdTPTAP). (b) Absorption (red solid) and emission (fluorescence: black solid; phosphorescence: gray) spectra of the sensitizer or emitter in toluene (diluted solutions). The yellow area is the emission spectrum of the mixed solution of PdTPTAP (50 μM) and rubrene (1 mM) in toluene (Ar-bubbled) excited at 785 nm. (c) Photograph of the same mixed solution excited at 785 nm and (d) excitation-intensity (Iex) dependence of the upconverted emission (at 565 nm, filled circle) together with the theoretical fit with eq 1 (solid red line). The arrow shows the position of the threshold intensity obtained by the curve fit (53 W/cm2).

calculations also supported the good matching of the triplet energy levels [1.12 eV for PdTPTAP and 1.00 eV for rubrene, both at the B3LYP/6-31G(d) (Lanl2DZ for Pd) level in vacuum]. The triplet energy mismatch was only 0.03 eV in experiment and in 0.12 eV in calculation, suggesting that good TET could be expected from the sensitizer to the emitter. Interestingly, the optimized geometry showed that PdTPTAP had a saddle-like shape because of the steric hindrance with the substituents at the meso positions (Figure 1a). The TTA-UC for the PdTPTAP−rubrene pair was first assessed in a mixed solution. The mixed solution prepared under an argon environment showed yellow emission (peaked at 565 nm) by excitation at 785 nm with an irradiation intensity of 2.5 W/cm2 by a cw-laser diode (Figure 1c). The emission spectrum (Figure 1b) matched the fluorescence spectrum of rubrene, although the short-wavelength region was suppressed because of reabsorption by the high concentration of rubrene. The insertion of a shortcut filter (R68) in the excitation path did not affect the emission, proving that the emission was caused by NIR-excitation. The excitation intensity (Iex) dependence of the UC emission (Figure 1d) for the same solution showed the second-to-first-order transition, that is, typically for TTA-UC.11,41,42 Curve fitting with the theoretical equation for the UC emission intensity (IUC),12

solids are still rare, especially for the NIR-to-vis conversion except for the following works. Wu and co-workers have reported a multilayered structure of PbS QD and rubrene, which was fabricated by the vacuum deposition technique.35 Amemori et al. have demonstrated efficient TTA-UC of binary nanoparticles of the Os complex and rubrene fabricated by the reprecipitation technique.24 In this study, solid-state NIR-to-vis TTA-UC has been demonstrated in a binary solid with NIR-absorbing porphyrin. The binary solid was fabricated by the rapid drying casting technique33 because it is not only a simple method that requires no costly vacuum equipment but also can provide a good interface between the sensitizer and emitter for efficient triplet energy transfer (TET) in the solid phase. Microparticles of the binary solid were obtained by the casting technique and the UC properties of the individual microparticles were characterized under an optical microscope. The excitation intensity dependence and QY measurements of UC emission were performed for the individual microparticles. The elementary processes involved in TTA-UC were investigated by time-resolved and steady-state spectroscopies. The results clarified the bottleneck process that was responsible for limiting the UC quantum yield (UC-QY).

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RESULTS AND DISCUSSION NIR-to-Vis UC in Solution. As the NIR sensitizer, a πconjugation-extended metalloporphyrin (Pd-tetrakis(3,5-di-tbutyl-phenyl)tetraanthroporphyrin, PdTPTAP, Figure 1a) was employed. While PdTPTAP has the same framework as that reported by Yakutkin et al.,15 it has different in substituents at the meso positions. The synthetic route of PdTPTAP is summarized in the Experimental Section. PdTPTAP showed a sharp Q-band absorption peak at 778 nm and a wide transparent range of 500 to 700 nm (Figure 1b). The triplet energy level was determined by measuring the phosphorescence peak (1061 nm, 1.17 eV) in toluene. This triplet energy level was nearly the same as that of Yakutkin’s compound (1.12 eV15) and had good energy level matching with the triplet state of rubrene (1.14 eV16), used as the emitter for many NIR-to-vis systems.36−40 Quantum chemical

1 − 1 + 4Iex /Ith ji IUC = K jjjj1 + j 2Iex /Ith k

zyz zzIex zz {

(1)

(here, K is the instrumental factor) gave the threshold excitation intensity Ith, defined as the intensity at the crossing point of the second-order and first-order asymptotic lines, as ∼53 W/cm2. Based on this result, the UC-QY was measured at the excitation intensity (295 W/cm2) five times as high as Ith, to obtain the maximum (i.e., saturated) value of UC-QY. The value for the mixed solution was 2.2% and close to the reported value of Yakutkin’s compound after adjusting by a factor 2.15 UC in Binary Solid Fabricated by Casting. A binary solid of PdTPTAP−rubrene was fabricated by the rapid-drying B


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except for the short wavelength edge that could probably be explained by the reabsorption process. As the cast solids of pure PdTPTAP or pure rubrene did not exhibit any visible emission under the same excitation condition at 785 nm, it was concluded that the yellow emission was due to the interactions taking place in the mixed binary solid. Excitation intensity dependence of the emission intensity (Figure 2e) revealed that the dependence was quadratic at weak excitation but was linear at strong excitation. This quadratic-to-linear transition is a characteristic feature for TTA-UC. From these facts, it was concluded that TTA-UC is responsible for the appearance of the emission in the binary solid of PdTPTAP−rubrene excited at 785 nm. The excitation intensity dependence was well reproduced by eq 1 (more data can be found in Figure S3). The Ith value obtained under air was scattered for 70−280 mW/cm2 (seven individual particles were studied) but gave an average of 115 mW/cm2. This value did not differ significantly from that obtained under an argon environment (Figure S4, Ith = 124 mW/cm2). These values for the cast particle were by 2 orders of magnitude lower than that of the mixed solutions (∼50 W/ cm2) and values obtained using a similar sensitizer.15 The observed low Ith of the binary solid was probably due to the high concentration of the sensitizer and the low molar ratio of the sensitizer to emitter.5,11 The UC-QY of individual particles was measured under the same microscope at 7 W/cm2, whose excitation intensity was much higher than the observed Ith value of the binary solid, so as to measure the saturated value of UC-QY. The value for the present system was estimated to be (0.5 ± 0.1)% as the average of more than 40 individual particles under air and (0.6 ± 0.1)% under argon (Figure S5 in Supporting Information). This result showed that UC-QY was not affected by the environment because the oxygen in air could not penetrate into the particles for the short time during the optical measurement. Exposing the samples to air for several weeks caused decolorization of the particles, primarily because of the degradation of rubrene.16,43 However, the sample was sealed under the Ar sealed condition, and the sample was stable and its UC-QY was unchanged for more than 140 days (Figure S6 in Supporting Information). Dynamics and QY of Elementary Processes of TTAUC. The UC-QY (ΦUC) in the present system can be

casting process from the mixed solution of PdTPTAP and rubrene (see the Experimental Section). After drying, numerous spherical microparticles were obtained mainly on the rim of the casted region (Figure 2a,b). Polarization

Figure 2. Upconverted emission from the binary solid of PdTPTAP− rubrene excited at 785 nm. (a) Appearance of the cast sample on slide glass irradiated at 785 nm via multimode optical fiber. (b) Transmission image and (c) UC emission image of microparticles of the binary solid. The scale bars are 50 μm. (d) UC emission spectrum of a single microparticle under aerated conditions at the intensity of 7 W/cm2 at the sample position (black solid curve filled with yellow, with a 785 nm notch filter) and normal fluorescence from rubrene neat solid excited at 532 nm (red dotted). (e) Typical excitation intensity dependence of the UC emission (at 570 nm) from a single microparticle (filled dots). The arrow indicates the threshold excitation intensity obtained from the theoretical fit (red curve).

microscopy (Figure S1) and scanning electron microscopy (Figure S2) clarified that these spherical particles were amorphous, indicating that the portion of the polycrystalline particles is small. Under the 785 nm illumination, the microparticles showed bright yellow emission under air (Figure 2c). The emission spectrum of individual particles was selectively measured by using a homemade microspectroscopic measurement system (Figure 2d), which gave spectra that were almost the same in shape to the fluorescence spectra of amorphous cast particles of pure rubrene excited at 532 nm

Figure 3. (a) Femtosecond transient absorption spectra of PdTPTAP in toluene. (b) Nanosecond transient absorption spectra of PdTPTAP in THF. Time profiles of transient absorption changes monitored at (c) 730 and (d) 510 nm [the same data as in panel (a)] with global fitting curves. C


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Figure 4. (a) Short-scale time profile of the UC emission from the binary solid of PdTPTAP−rubrene (red solid) excited by 800 nm ns-pulses, the IRF (black dotted), and the convolution of 11 ns decay (blue dashed) with IRF. All data are normalized at the peak. (b) Long-scale time profile of the normalized UC emission with various excitation intensities (dots) with the theoretical fits (dashed, see text for detail). The numbers in the figure refer to the excitation intensity in kW/cm2. (c) Normalized absorption spectra of the Q-band of PdTPTAP of an amorphous microparticle of the binary solid (i.e., in rubrene matrix; red solid), in THF solution (blue dashed), and of PdTPTAP powder (black dotted).

In an isotropic system, ΦTET can be estimated from the triplet lifetime of PdTPTAP with and without TET (τ and τ0, respectively) as follows.

factorized into the product of the QYs of the four elementary processes involved as follows, ΦUC = ΦISC ΦTET ΦTTA ΦFL

(2)

ΦTET = 1 − τ /τ0

where ISC is intersystem crossing of the sensitizer, TET is triplet energy transfer from the sensitizer to the emitter, and FL is the fluorescence of the emitter (in amorphous solid). The QY of the total process, ΦUC, and the three factors in the right-hand side except ΦTTA, can be determined independently. Accordingly, QY of each process could be deduced. To obtain ΦISC, femtosecond transient absorption spectroscopy was applied to the direct elucidation of the dynamics of PdTPTAP in toluene (Figure 3a). Singlet-excited (S1) state peaks appeared at 570, 630, and 730 nm immediately after the excitation and were gradually replaced with the triplet (T1) state peaks at 510 and ∼670 nm within 10 ps. Transient absorption spectra at longer decay of 910 ps and nanosecond transient absorption measurement (Figure 3b) justified the assignment of the T1 state. The global curve-fit analysis of the decay of the S1 state at 730 nm (Figure 3c) and the corresponding rise of the T1 state at 510 nm (Figure 3d) gave 2.9 ps as the common lifetime (τS) for both decay and rise. This is the evidence that ISC occurs form the S1 state and generates T1 state of the sensitizer with the time scale of τS. This lifetime can be written as τS = 1/(kf + knr + kisc) where kf, knr, and kisc are the rate constants of the fluorescence (radiative) and nonradiative deactivations from the S1 state (S1 → S0) and of ISC (S1 → T1), respectively. This gives ΦISC = kisc τS and Φf,S = kf τS, where Φf,S is the FL-QY of PdTPTAP. The values of τS = 2.9 ps and Φf,S = 4.4 ± 0.2 × 10−4 in toluene obtained by the steady-state measurement gave kf = 1.5 × 108 s−1 and knr + kisc = 1/τS − kf = 3.5 × 1011 s−1. The radiative rate constant calculated from the absorption coefficient spectrum by the Strickler−Berg relation44 was 1.3 × 108 s−1 for PdTPTAP in toluene and showed good agreement with the value of kf. These values were much greater than ordinary knr of metalloporphyrin (e.g., knr = 5.1 × 108 s−1 for zinc tetraphenylporphyrin)45 and thus, the nonradiative deactivation pathway from S1 could be ignored in comparison. This gives ΦISC ≈ 1 − Φf,S ≈ 1.00.

(3)

The triplet lifetime of PdTPTAP without TET was determined to be τ0 = 16.2 μs by nanosecond transient absorption spectroscopy in tetrahydrofuran (THF) solution (Figure S7 in Supporting Information). On the other hand, the triplet state lifetime of the sensitizer with TET (τ) for the binary solid was determined by measuring the rise time of the UC emission intensity because the rise of the triplet state of the emitter corresponds to the decay of triplet state of the sensitizer via the TET process. The UC emission intensity is proportional to the square of the concentration of the triplet state of the emitter (see the Experimental Section). The time profile of UC emission of the microparticles excited by 800 nm, 13 ns laser pulses showed a fast rise and decay in tens of nanoseconds (Figure 4a). The convolution of a single exponential decay having a lifetime of 11 ns and instrument response function (IRF) could reproduce the overall shape of the rise and decay together with the delay of the peak against IRF, suggesting that the rise time of the UC emission was much faster than the time resolution of the system used. Unlike solution, the binary solid is considered to have nonuniformity inside and/or between the microparticles. Thus, we examined the possibility of the slower TET process because of nonuniformity with precise analysis of the decay of the UC emission intensity. The fast rise and decay were followed by a long decay that lasted for several tens of microseconds (Figure 4b). This part of the decay curves was analyzed by the Bachilo−Wiseman fit, IUC(t) = A{(1 − β)/ (exp(−k1t) − β)}2, where β = α/(k1 + α) and α = k2[3E*]0.46 The uni- and bimolecular decay of the emitter triplet (3E*), that is, triplet decay rate and TTA rate, are k1 and k2, respectively, and [3E*]0 is the initial concentration of the triplet emitter. Global fitting after 2 μs successfully reproduced all decay data and gave the triplet life time of rubrene as k1 = 16.3 μs. There was still a possibility of generating the rubrene triplet after the ns decay; however, the good agreement with the Bachilo−Wiseman fit, which does not consider the rise D


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ACS Applied Materials & Interfaces Scheme 1. Synthesis of PdTPTAP

process, meant that there was no significant contribution of the triplet rise at least after 2 μs. Based on this discussion, TET occurs faster than at least 2 μs and probably faster than 10 ns. Under the conventional estimation of τ = 2 μs or shorter, the TET-QY was estimated to be ΦTET = 88% or larger based on eq 3. The fast and efficient TET in the binary solid was supported by the absorption spectrum of the Q-band of PdTPTAP. The Q-band in the binary solid, that is, in the rubrene matrix, (Figure 4c), was as wide as in solution [20 nm in full width at half maximum (fwhm)], but much narrower than that of the powder (40 nm in fwhm). These results suggested that, unlike the powder, PdTPTAP molecules were molecularly disperse in the binary solid and did not form aggregates. Compared with ΦISC and ΦTET estimated above, the value of ΦFL in the solid was found to be much smaller. The ΦFL value in solution is known to be high, at least as high as 90% by our measurement (in THF) and 98−100% from previous reports.47−49 However, ΦFL = 1.6% for the amorphous microparticles of pure rubrene (without sensitizer) was in reasonable agreement with the previous report,50 and ΦFL = 1.5% for the microparticles of the binary solid (rubrene with sensitizer), determined under a microscope by using 532 nm excitation. From these results, the saturated value of ΦTTA is calculated to be ∼36% from eq 2. This value was comparable to the highest value of ΦTTA for rubrene (33%) in solution,51 showing that the TTA process occurred in the binary solid at least as efficiently as in solution.

Considering that the FL-QY values of the neat solid of rubrene and the binary solid were close to each other, the former is the more probable cause in this case. SF, the reverse process of TTA, has been observed in the rubrene crystal53 because its energy levels are suitable for both TTA and SF. Improvement of the FL-QY is the key to enhancement of the overall UC-QY in the binary solid; thus, tuning of the molecular structure so as to lower the 1E* level relative to twice the 3E* level and/or the transferring the energy from 1E* to the secondary emitter with the competing SF process are possible strategies. Further studies are underway to improve the UC-QY of the solid-state NIR-to-vis TTA-UC system.

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EXPERIMENTAL SECTION

Materials. Rubrene (sublimed grade, 99.99%) from Sigma-Aldrich and IR-140 (C39H34Cl2N3S2 ClO4) from Exciton Inc. were used as received. Spectroscopic grade solvents were used for all purposes. Synthesis of PdTPTAP. The NIR sensitizer (PdTPTAP) was prepared from ZnTPTCAP (Scheme 1), synthesized by a previously reported method,54 as the starting material. To a solution of ZnTPTCAP (272 mg, 0.15 mmol) in CHCl3 (15 mL) was added TFA (1 mL) at room temperature. After stirring for 15 min, the reaction was quenched by water. The organic layer was washed with NaHCO3, water and brine, and dried over Na2SO4. The crude material was recrystallized from CHCl3/MeOH to give H2TPTCAP (195 mg, 0.11 mmol) in 74% yield as a brown solid. This compound was used for the next step without further purification. To a solution of H2TPTCAP (100 mg, 0.056 mmol) in CHCl3 (20 mL) was added Pd(OAc)2 (38 mg, 0.17 mmol) at room temperature. After stirring for 4 h, the reaction mixture was concentrated under a reduced pressure. The crude material was purified by column chromatography (CH2Cl 2) and recrystallization from CHCl3/MeOH to give PdTPTCAP (60 mg, 0.034 mmol) in 60% yield as a red solid. 1H NMR (400 MHz, CDCl3, mixed with stereoisomers): δ 8.40−7.29 (m, 36H), 3.95−3.88 (m, 8H, bridge head), 2.09−1.26 (m, 84H, bridge and t-Bu) ppm. PdTPTCAP was put in a microtube, and then it was heated in a glass tube oven at 310 °C for 1 h. The solid color was gradually changed from red to dark green. After cooling to room temperature, PdTPTAP was obtained quantitatively. 1H NMR (400 MHz, CDCl3): δ 8.26 (m, 4H), 8.23 (m, 8H), 8.21 (m, 8H), 8.04 (m, 8H), 7.78 (s, 8H), 7.44 (m, 8H), 1.53 (s, 72H) ppm. HRMS: m/z 1766.8258; calcd for C124H116N4Pd, 1766.8263. Fabrication of the Binary Solid by Casting. The binary solid of PdTPTAP−rubrene was fabricated by rapid-drying casting method33 from the mixed solution of PdTPTAP and rubrene on to a flat slide glass. For better wettability to the glass surface for fast evaporation, THF was used as the solvent. The concentration of the rubrene was 11 mM, close to its saturated concentration in THF. The concentration of PdTPTAP was 11 μM, to set the molar ratio of the sensitizer to emitter to 1:1000. Preparation of the solution and drop casting were performed in a glovebox filled with Ar (>99.99%). After casting, the sample was covered by a cover glass under different atmospheres (Ar and air) and then sealed with UV-curable adhesive (Norland optical adhesive #61). Ordinary (Bulk) Optical Measurements. The absorption spectra of the solutions were measured by a UV−vis spectropho-

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CONCLUSIONS In summary, solid-state NIR-to-vis (from 785 to ∼570 nm) TTA-UC was demonstrated in a binary solid that was fabricated by the rapid drying casting. The UC characteristics of the individual microparticles of the binary solid were evaluated by microscopic emission/absorption spectroscopies. The elementary processes involved in the TTA-UC were investigated by time-resolved and steady-state spectroscopies. The binary solid showed a lower threshold intensity (∼100 mW/cm2) as compared to the solution containing the same components because of the higher concentration of the chromophore. The very fast ISC and low FL-QY of the sensitizer (PdTPTAP) suggested an efficient ISC whose QY is almost unity. Nanosecond dynamics of the UC emission in the binary solid clarified the fast TET and its high QY of this step. However, the total UC-QY of ∼0.5% at the saturation excitation intensity was mostly limited by the FL-QY of rubrene in the solid state. Consequently, the TTA-QY was ∼36%, which was in reasonable agreement with the highest value in the solution system. The low value of FL-QY of rubrene in the solid phase could be explained by the deactivation of the generated emitter singlet (1E*) by singlet fission (SF) and back energy transfer to the sensitizer.52 E


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ACS Applied Materials & Interfaces tometer (Shimadzu UV3150). The fluorescence spectra of the solution samples were recorded using a fluorescence spectrophotometer (Hitachi F-4500) and the phosphorescence spectrum of PdTPTAP in toluene was recorded with a photon-counting fluorescence spectrophotometer (Horiba Fluorolog-3). The FL-QY of the toluene solution of PdTPTAP was determined by a relative method based on the FL-QY of IR-140 in ethanol (Φf = 0.167 ± 0.010)55 and using a UV−vis spectrophotometer, a fluorescence spectrophotometer (JASCO FP8300), and a calibrated fiber spectrometer (Ocean Optics USB2000FLG) for the purpose. The absorbances of the sample and reference solution were adjusted to the same value at the excitation wavelength (785 nm). Upconverted Emission and QY Measurements of Solution. A 785 nm cw laser diode (Integrated Optics MatchBox785) was used as the excitation source through an OD 4 bandpass filter (800 ± 20 nm). The laser beam was loosely focused by a planoconvex lens (f = 300 mm) to irradiate the sample. A 2 mm quartz cuvette was placed at a 45° angle to the incident beam; the emission was detected through a 785 nm notch filter (OD 4) in an orthogonal configuration to probe the surface emission using the fiber spectrometer. To estimate the excitation intensity, the beam diameter at the sample position was measured by a CCD. The optical power was also recorded at the same position. Gaussian distribution was considered to estimate the excitation intensity at the sample position (188 W/cm2 for 1 mW incident laser power). To determine the UC-QY, the relative method was applied

ΦUC = 2ΦR

photodiode power sensor (Thorlabs S170C) and used to calculate the excitation intensity. For QY measurements of the individual microparticle, the relative method by using reference (eq 4) was also applied. Special attention had been paid for solid samples.33 First, the microscopic cell configuration was kept as same as possible: a cover glass was placed over slide glass with a spacer (80 μm) to keep the same thickness of glass plates and distance between them. The microparticle was located on the cover glass while the reference solution (vide infra) was filled between the cover and slide glasses. Of course, the microparticle is not uniformly filled the gap between the glasses. The shape and size differed one by one and the environment was gas (Ar or air, i.e., refractive index n ≈ 1.00), which may cause variation of scattering and collection angle. These problems were already examined by filling the gap with index-matching oil (n ≈ 1.404) and found to be not so critical33 at the shorter wavelength (430−500 nm). These effects are expected to be less significant than before because the excitation and emission wavelengths in this study were longer than previous ones. Another issue to be paid attention was the preparation of the reference sample for microscopic measurement. As for the reference solution, the ethylene glycol (EG) solution of IR-140 (100 μM) was used. EG did not evaporate during the measurement, giving no concentration change. The high concentration of the solution allowed to measure absorbance even with such short pathlength of the microscopic cell (80 μm). The EG solution was deaerated by Arbubbling before filling the cell. The prepared reference sample showed constant fluorescence intensity more than 120 min. The FL-QY of the 100 μM solution of IR-140/EG solution was Φf = 0.170 determined by the calibrated integration sphere (Hamamatsu Photonics C9920) with self-absorption correction.56 Further important issue was absorption measurement. The absorbance of microparticle was measured at the same position where the emission was measured for each sample. Because the obtained absorption spectrum was usually deformed by scattering, the scattering was subtracted by the extrapolation.33 Time-Resolved Emission Measurements. The time-resolved UC emission profiles of the cast sample were measured by pulse excitation with a ns-Ti:sapphire laser at 800 nm (10 ns, 1 kHz). The solid sample was irradiated with the laser intensity of 0.9 W/ cm2. The UC emission from the sample was detected by a Si-APD connected to a digital oscilloscope (500 MHz, 5 GS/s) through the excitation cut filter (HOYA CM-500) and a 700 nm short-pass filter (Thorlabs FES0700). The UC emission intensity (IUC) is proportional to the square of the concentration of triplet emitter [3E*] under the quasi-steady state assumption for the rate equation of the singlet excited state of the emitter, that is, d[1E*]/dt = φkTTA[3E*]2 − k1E[1E*] ≈ 0, where φ is the generation ratio of the singlet excited state of the emitter via the TTA process and kTTA is the second-order rate constant of TTA, k1E is the first-order rate constant for unimolecular deactivation of the emitter including both fluorescence and nonradiative paths, as IUC ∝ ΦFLk1E[1E*] = ΦFLφkTTA[3E*]2 where ΦFL is the FL-QY of the emitter as in eq 2. For the TET process (3S* + 1E → 1S + 3E*), the decay of the triplet state of the sensitizer, [3S*](t) ∝ exp(−τ−1t), and the rise part of the rise and decay of triplet state of the emitter, 1 − exp(−τ−1t), can express the common lifetime τ = (k3S + kTET[1E])−1 for the TET process (where k3S is the first-order rate constant of the unimolecular decay of the triplet sensitizer, kTET is the second-order rate constant of the TET, and [1E] is the ground state concentration of the emitter). Thus, the triplet lifetime of the sensitizer with TET (τ) can be determined as the rise time of the square root of the UC emission intensity. Transient Absorption Measurements. Transient absorption measurements in the femtosecond to picosecond time region were performed using a homemade setup. The excitation pulses (788 nm, 100 fs, 24 nJ/pulse) were provided by an optical parametric amplifier (Spectra-Physics TOPAS-Prime). The sample solution of PdTPTAP (60 μM) was set in a homemade rotation cell (path length: 1 mm) in a nitrogen atmosphere. Nanosecond transient absorption spectra were measured by using a commercial setup (Unisoku TSP-1000) with

FSAR PR nS2 FR A SPSnR 2

(4)

where F is the integrated emission intensity, A is the absorbance measured separately by a UV−vis spectrophotometer, n is the refractive index, and P is excitation power, with subscripts of S and R referring to the sample and reference, respectively. The factor of 2 in the equation is required to scale the full conversion as unity. For the reference, again, the same solution of IR-140 in ethanol55 (10 μM) was used as shown in the previous paragraph (ordinary (bulk) optical measurements). Upconverted Emission and QY Measurements of Individual Microparticles under an Optical Microscope. The measurements of upconverted emission and its QY of individual microparticles were done by using a homemade microscopic spectroscopic setup.33 For NIR excitation, we built a new setup that had the same configuration but was different in optics for the longer wavelength. The excitation beam from a 785 nm cw laser diode (Integrated Optics MatchBox785) spectrally cleaned through an OD 4 bandpass filter (800 ± 20 nm) was introduced into an excitation port of an inverted optical microscope (Olympus IX73). The beam was loosely focused in order to collimate the beam at the sample position and to uniformly excite the sample through a 20× objective lens (NA 0.45). The excitation beam diameter was 700 μm (measured by an NIR-enhanced CCD camera connected to the microscope), covering the most view of the microscope. On the other hand, the detection area of emission was located at the center of the view with the diameter of 5 μm (measured by the knife edge method), much smaller than the size of microparticles (∼50 μm). We select a single microparticle so that the emission from the central part of the microparticle was detected. The variation of the excitation intensity within the detection area was less than 0.5%, calculated from the diameters and the Gaussian distribution of the intensity. The emission from the sample was collected by the same objective lens and detected by a fiber-coupled spectrometer (Ocean Optics USB2000FLG) after cutting the scattering of the excitation light by using a fluorescent cube equipped with a notch dichroic beam splitter (Semrock NFD01-785) and a detection-side notch filter (centered at 785 nm, bandwidth 25 nm, OD 4). The spectral correction function of all related optics was obtained and confirmed to give a properly corrected emission spectrum, which is important for QY measurement. Excitation power at the sample surface was measured by a calibrated microscope F


Research Article

ACS Applied Materials & Interfaces second harmonics (532 nm, 11 ns, 200 μJ/pulse) of a Q-switch Nd:YAG laser (10 Hz) as the light source. The deaerated (Arbubbled) THF solution (2 μM) in 1 cm cuvette was used for the experiment.

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(6) Gray, V.; Dzebo, D.; Abrahamsson, M.; Albinsson, B.; MothPoulsen, K. Triplet-Triplet Annihilation Photon-upconversion: Towards Solar Energy Applications. Phys. Chem. Chem. Phys. 2014, 16, 10345−10352. (7) Zhou, J.; Liu, Q.; Feng, W.; Sun, Y.; Li, F. Upconversion Luminescent Materials: Advances and Applications. Chem. Rev. 2015, 115, 395−465. (8) Ye, C.; Zhou, L.; Wang, X.; Liang, Z. Photon Upconversion: From Two-photon Absorption (TPA) to Triplet-Triplet Annihilation (TTA). Phys. Chem. Chem. Phys. 2016, 18, 10818−10835. (9) Kozlov, D. V.; Castellano, F. N. Anti-Stokes Delayed Fluorescence From Metal-organic Bichromophores. Chem. Commun. 2004, 1, 2860−2861. (10) Islangulov, R. R.; Kozlov, D. V.; Castellano, F. N. Low Power Upconversion Using MLCT Sensitizers. Chem. Commun. 2005, 1, 3776. (11) Monguzzi, A.; Mezyk, J.; Scotognella, F.; Tubino, R.; Meinardi, F. Upconversion-induced Fluorescence in Multicomponent Systems: Steady-state Excitation Power Threshold. Phys. Rev. B 2008, 78, 195112. (12) Murakami, Y. Photochemical Photon Upconverters with Ionic Liquids. Chem. Phys. Lett. 2011, 516, 56−61. (13) Baluschev, S.; Yakutkin, V.; Miteva, T.; Avlasevich, Y.; Chernov, S.; Aleshchenkov, S.; Nelles, G.; Cheprakov, A.; Yasuda, A.; Müllen, K.; Wegner, G. Blue-green Up-conversion: Noncoherent Excitation by NIR Light. Angew. Chem., Int. Ed. 2007, 46, 7693−7696. (14) Baluschev, S.; Yakutkin, V.; Miteva, T.; Wegner, G.; Roberts, T.; Nelles, G.; Yasuda, a.; Chernov, S.; Aleshchenkov, S.; Cheprakov, a. A General Approach for Non-coherently Excited Annihilation Upconversion: Transforming the Solar-spectrum. New J. Phys. 2008, 10, 013007. (15) Yakutkin, V.; Aleshchenkov, S.; Chernov, S.; Miteva, T.; Nelles, G.; Cheprakov, A.; Baluschev, S. Towards the IR Limit of the TripletTriplet Annihilation-supported Up-conversion: Tetraanthraporphyrin. Chem. - Eur. J. 2008, 14, 9846−9850. (16) Singh-Rachford, T. N.; Castellano, F. N. Pd(II) Phthalocyanine-Sensitized Triplet−Triplet Annihilation from Rubrene. J. Phys. Chem. A 2008, 112, 3550−3556. (17) Mongin, C.; Garakyaraghi, S.; Razgoniaeva, N.; Zamkov, M.; Castellano, F. N. Direct Observation of Triplet Energy Transfer from Semiconductor Nanocrystals. Science 2016, 351, 369−372. (18) Li, X.; Fast, A.; Huang, Z.; Fishman, D. A.; Tang, M. L. Complementary lock-and-key Ligand Binding of a Triplet Transmitter to a Nanocrystal Photosensitizer. Angew. Chem., Int. Ed. 2017, 56, 5598−5602. (19) Okumura, K.; Mase, K.; Yanai, N.; Kimizuka, N. Employing Core-shell Quantum Dots as Triplet Sensitizers for Photon Upconversion. Chem. - Eur. J. 2016, 22, 7721−7726. (20) Mahboub, M.; Huang, Z.; Tang, M. L. Efficient Infrared-tovisible Upconversion with Subsolar Irradiance. Nano Lett. 2016, 16, 7169−7175. (21) Huang, Z.; Simpson, D. E.; Mahboub, M.; Li, X.; Tang, M. L. Ligand Enhanced Upconversion of Near-infrared Photons with Nanocrystal Light Absorbers. Chem. Sci. 2016, 7, 4101−4104. (22) Huang, Z.; Li, X.; Mahboub, M.; Hanson, K. M.; Nichols, V. M.; Le, H.; Tang, M. L.; Bardeen, C. J. Hybrid Molecule-nanocrystal Photon Upconversion Across the Visible and Near-infrared. Nano Lett. 2015, 15, 5552−5557. (23) Sasaki, Y.; Amemori, S.; Kouno, H.; Yanai, N.; Kimizuka, N. Near infrared-to-blue photon upconversion by exploiting direct S-T absorption of a molecular sensitizer. J. Mater. Chem. C 2017, 5, 5063− 5067. (24) Amemori, S.; Sasaki, Y.; Yanai, N.; Kimizuka, N. Near-Infraredto-Visible Photon Upconversion Sensitized by a Metal Complex with Spin-Forbidden yet Strong S0-T1 Absorption. J. Am. Chem. Soc. 2016, 138, 8702−8705. (25) Monguzzi, A.; Mauri, M.; Frigoli, M.; Pedrini, J.; Simonutti, R.; Larpent, C.; Vaccaro, G.; Sassi, M.; Meinardi, F. Unraveling Triplet Excitons Photophysics in Hyper-cross-linked Polymeric Nano-

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.9b04148. Crossed Nicole microphotograph; SEM images; excitation-intensity dependence of emission intensity in air and Ar; histogram of the UC-QY; long-term stability of UC-QY; and microsecond transient absorption change of the sensitizer (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (A.A.). *E-mail: [email protected] (K.K.). *E-mail: [email protected] (H.Y.). ORCID

Kenji Kamada: 0000-0002-7431-5254 Hiroshi Miyasaka: 0000-0002-6020-6591 Daiki Kuzuhara: 0000-0001-7948-8501 Hiroko Yamada: 0000-0002-2138-5902 Present Address ⊥

Innovation Center for Organic Electronics, Yamagata University, Yonezawa, Yamagata 992-0119, Japan.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by a Grant-in-Aid for Scientific Research on Innovative Areas “Photosynergetics” (JP26107002 for H.M. and JP26107004 for K.K.) from MEXT, Japan and Grant-in-Aid for Scientific Research JP16H02286 (H.Y.) from JSPS. We gratefully acknowledge the valuable discussions with Dr. Toshiko Mizokuro, ESPRIT, AIST. We also thank Dr. Takeyuki Uchida Kansai Center, AIST for the SEM observations and Prof. Naoto Tamai, Kwansei Gakuin University for allowing us to use the photon counting fluorescence spectrophotometer and measure the phosphorescence spectrum of the sensitizer.

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REFERENCES

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Solid-State, Near-Infrared to Visible Photon Upconversion via Triplet

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