Photon Upconverted Emission Based on Dye-Sensitized Triplet

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Article Cite This: ACS Omega 2018, 3, 8529−8536

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Photon Upconverted Emission Based on Dye-Sensitized Triplet− Triplet Annihilation in Silica Sol−Gel System Hiromasa Nishikiori,*,†,‡,§ Masahiro Takeshita,‡ Yoshihiro Komatsu,† Hiroshi Satozono,∥ and Katsuya Teshima†,‡,§ Department of Materials Chemistry, Graduate School of Science and Technology, ‡Department of Environmental Science and Technology, Faculty of Engineering, and §Center for Energy and Environmental Science, Shinshu University, 4-17-1 Wakasato, Nagano 380-8553, Japan ∥ Central Research Laboratory, Hamamatsu Photonics K.K., 5000 Hirakuchi, Hamamatsu 434-8601, Shizuoka, Japan Downloaded via 185.50.250.53 on August 7, 2018 at 08:06:50 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

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S Supporting Information *

ABSTRACT: Photon upconverted emission based on dye-sensitized triplet−triplet annihilation was observed in silica gel systems containing Pt(II) octaethylporphyrin (triplet sensitizer) and 9,10-diphenylanthracene (singlet emitter). The triplet sensitizer was encapsulated and highly dispersed in the silica gels prepared by the sol−gel method. The singlet emitter was adsorbed on the silica gel pores accessible to the outside. Phosphorescence of the triplet sensitizer was partially quenched, and the singlet emission was enhanced with an increase in the concentration of the singlet emitter. The emission intensity increased in proportion to the approximate square of the irradiation power. The triplet energy transfer from some of the encapsulated triplet sensitizer molecules to the adsorbed singlet emitter molecules was observed in the silica gels followed by the triplet−triplet annihilation and upconverted singlet emission. The phosphorescence quenching and upconverted singlet emission were more significantly observed in the gel dried at a lower temperature (wetter gel). The wetter gel contained higher amounts of solvent and water molecules, in which the triplet sensitizer and singlet emitter should collide and then the sensitized emitters should collide between themselves during their excited-state lifetime. The photon upconversion process required the triplet sensitizer and singlet emitter molecules to be in an environment similar to the solvents.

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INTRODUCTION Dye-sensitized solar cells (DSSCs) are widely investigated worldwide as a new type of solar cell exhibiting high performance.1−3 The energy conversion efficiency of the DSSCs can be enhanced by utilizing unavailable near-infrared light. It is important that the higher photon energy is obtained due to the harvest dense energy, i.e., low entropy, and utilize the energy after transforming its type adapted to the situation.4 The photon upconverted emission based on the dye-sensitized triplet−triplet annihilation (TTA-UC) was investigated to convert a longer-wavelength light to a shorter-wavelength one having higher energy.5−7 This technique can be performed using certain organic molecules during irradiation by a noncoherent light source instead of laser light. Such functional molecules should be concentrated and immobilized on/in the solid particles/matrices to efficiently induce their energy transfer and practically utilize the energy.7−15 TTA-UC has been observed in the systems containing Pt(II) octaethylporphyrin (PtOEP, triplet sensitizer) and 9,10diphenylanthracene (DPA, singlet emitter).4−12,16−20 During this process, the triplet sensitizer molecules should not interact with each other because their aggregates act as an energy trap,8−11,16,20 whereas the triplet sensitizer molecules should © 2018 American Chemical Society

sensitize the emitter molecules to the triplet states and the resulting triplet molecules should interact with each other to be the emitter molecules in the singlet excited states. It has been reported that organic dye molecules can be separately encapsulated in the silica gel pores or silica network by the sol−gel method.21−24 It was postulated that the gel consists of amorphous, nanosized, and highly porous particle-like units (hereinafter called “particles”). The apparent specific surface area of the silica gel similar to the present samples was ca. 1000 m2 g−1.25 The silica gel had a hydrophilic surface due to the presence of one OH group per surface silicon atom. The rhodamine dyes encapsulated in the silica reached high concentrations without any undesirable dye aggregations during the sol−gel transition.24 The unique behavior of the molecules encapsulated in the silica gel is expected to produce their potential function. On the other hand, the molecule adsorption on the silica gels leads to aggregate formation on the surface.26−28 Received: May 23, 2018 Accepted: July 20, 2018 Published: August 1, 2018 8529

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measurement) PtOEP and 20.0 mmol dm−3 DPA for 48 h separately. The diffuse reflection spectrum of the silica gel adsorbing PtOEP at 450−600 nm was broader than that of the silica gel encapsulating PtOEP, which was similar to the spectra in solvents. There is a possibility for the formation of higher amounts of the aggregates on the surface of the silica gel particles.30,31 The emission spectra of PtOEP and DPA indicated their phosphorescence and fluorescence, respectively. The phosphorescence of PtOEP adsorbed on the surface of the silica gel particles and encapsulated in the silica gel networks was observed at room temperature. The phosphorescence peak was located at 645 nm. The excitation spectral peak at 535 nm originated from the sensitizer absorption.8−12,16−20 The emission intensity of the PtOEP encapsulated in the silica gel networks was stronger than that adsorbed on the surface of the silica gel particles, although their concentrations were nearly equal (ca. 0.3 μmol g−1). This indicated that the molecular encapsulation efficiently shielded the molecules from oxygen molecules and enhanced their emission.32,33 In addition, the high dispersion of the molecules suppressed the interaction between the molecules (aggregation), which caused their deactivating energy transfer.8−11,16,20 The emission peak was located at 430 nm in the gels adsorbing the DPA. The related excitation peak at 375 nm originated from the DPA absorption.8−12,16−20 Figure S2 shows the emission decay curves of the PtOEP adsorbed on the surface of the silica gel particles and encapsulated in the silica gel networks. The decay curves were well fitted to a double exponential function. The fitting parameters of the emission decay curves are shown in Table S2. The shorter lifetime was much shorter than the values reported as the phosphorescence lifetime in similar systems8−10 and can be assigned to the deactivating energy transfer process due to the PtOEP aggregates. The longer lifetime was assigned to the relatively stable states causing the phosphorescence process. The lifetime values and their fractions were higher for the encapsulated state than for the adsorbed state. The activated-state stability was somewhat improved by encapsulating in the silica gels. The molecular encapsulation more efficiently shielded the molecules from oxygen molecules than the molecular adsorption. The encapsulated PtOEP molecules can more strongly interact with the DPA molecules adsorbed on the silica gel surface than with the atmospheric oxygen molecules. The phosphorescence lifetimes of PtOEP in the as-prepared and deaerated toluene solutions were reported to be 0.2 and 50 μs, respectively, indicating that the oxygen molecules quenched the phosphorescence.31 The lifetime observed in the present samples was shorter than that observed in the deaerated toluene solution. Therefore, some of the oxygen molecules on the surface still quenched the phosphorescence of the PtOEP encapsulated in the silica gels. This should be one of the factors lowering the emission efficiency. Figure 2 shows the dependence of the phosphorescence spectrum on the DPA concentration in the silica gel samples adsorbing and encapsulating PtOEP. The PtOEP and DPA adsorption on the silica gel was conducted by immersing in toluene solutions containing 60.0 μmol dm−3 PtOEP and 5.0− 20.0 mmol dm−3 DPA for 48 h. The concentration ranges of the adsorbed DPA were 75−220 and 94−340 μmol g−1 in the samples adsorbing and encapsulating PtOEP, respectively. The phosphorescence intensity of the PtOEP encapsulated in the silica gel without DPA was higher than that of the PtOEP

In this study, TTA-UC was attempted to be observed in the sol−gel silica systems containing PtOEP (triplet sensitizer) and DPA (singlet emitter). The triplet sensitizer was encapsulated and highly dispersed in the silica gels prepared by the sol−gel method. Therefore, the sensitizer molecules cannot easily interact with each other for their energy exchange. The singlet emitter was adsorbed from its toluene solution on the surface and pores of the silica gel particles accessible to the outside. Some of the encapsulated sensitizer molecules are expected to be close to the emitter molecules and interact with them. The emitter molecules can interact with each other. A schematic diagram of the interaction is shown in Scheme 1. The emission Scheme 1. Schematic Diagram of the Triplet Sensitizer Moleculesa and the Singlet Emitter Moleculesb

a

Encapsulated in the silica gel pores or silica network and badsorbed on the surface of the silica gel particles (particle-like units).

and excitation spectra of the moderately and thoroughly dried silica gel samples were observed using a fluorescence spectrophotometer to examine the influence of the solvents remaining in the gels on the emission process. The dependences of the emission peak intensities on the emitter concentration and irradiation light intensity were examined.

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RESULTS AND DISCUSSION Phosphorescence Quenching of the Triplet Sensitizer. The absorbance depicted in the UV−vis absorption spectra of the PtOEP and DPA solutions dispersing the silica gel powder samples decreased with an increase in time due to their adsorption on the silica particles. These adsorption data were fitted to a Langmuir model. Figure S1 shows the Langmuir isotherms of the PtOEP and DPA adsorption on the silica gel particles. Table S1 shows the fitting parameters (adsorption equilibrium constants, Kad, and adsorption capacities, Qmax) of the isotherms. On the other hand, the absorbance of PtOEP in the DPA solutions slightly increased, whereas the PtOEP-encapsulated gel samples were immersed in the solutions due to the desorption of the weakly adsorbed PtOEP. The amount of the desorbed PtOEP at 48 h was ca. 10% of the initial amount in the silica gels. Figure 1 shows the UV−vis diffuse reflection, excitation, and emission spectra of the triplet sensitizer, PtOEP, adsorbed on the surface of the silica gel particles and encapsulated in the silica gel networks, and the singlet emitter, DPA, adsorbed on the surface of the silica gel particles. The silica gel samples adsorbing PtOEP and DPA were prepared by immersing in the toluene solutions containing 60.0 μmol dm−3 (for emission measurements) and 0.300 mmol dm−3 (for diffuse reflection 8530

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Figure 1. (a) UV−vis diffuse reflection spectra of PtOEP (1) adsorbed on the surface of the silica gel particles and (2) encapsulated in the silica gel networks and DPA (3) adsorbed on the surface of the silica gel particles. (b) Excitation and emission spectra of the PtOEP (1) adsorbed on the surface of the silica gel particles and (2) encapsulated in the silica gel networks. (c) Excitation and emission spectra of the DPA adsorbed on the surface of the silica gel particles.

Figure 2. Dependence of the phosphorescence spectrum of PtOEP on the DPA concentration in (a) silica gel samples adsorbing PtOEP and DPA and (b) those encapsulating PtOEP and adsorbing DPA. The DPA was adsorbed from the toluene solutions containing (1) 0 mmol dm−3, (2) 5.0 mmol dm−3, (3) 10.0 mmol dm−3, and (4) 20.0 mmol dm−3 DPA. The phosphorescence spectra were observed upon 535 nm excitation.

contained low amounts of solvent molecules, such as toluene, ethanol, and water, in which the triplet sensitizer should collide with the singlet emitter in the excited-state lifetime of the triplet sensitizer. The phosphorescence quenching behavior indicated that the triplet sensitizer and singlet emitter molecules were in an environment similar to the solvents.5,6 The solvent molecules should be in the silica gel networks and on the surface of the silica particles. The phosphorescence quenching constant, KSV, of the PtOEP molecules encapsulated in the gel (5.8 × 103 g mol−1) was higher than that adsorbed on the silica gel (2.5 × 103 g mol−1). The encapsulated PtOEP

adsorbed on the silica gel without DPA, although their concentrations were nearly equal to 0.3 μmol g−1. The PtOEP concentration in these samples did not depend on the DPA concentration. The phosphorescence of PtOEP was gradually quenched with an increase in the concentration of DPA. Figure 3 shows the Stern−Volmer plot for the phosphorescence quenching in the samples. The y-axis indicates the ratio of the phosphorescence intensity observed without the singlet emitter (quencher, Q), Iem, to that observed with each concentration of the adsorbed quencher, Iem0, i.e., Iem0/Iem = 1 + KSV[Q]. KSV is a quenching constant. The result indicated that the gels 8531

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emission should be a two-photon process. The triplet−triplet annihilation of the DPA molecules did not efficiently occur. The reason for this is presumed that the average distance of the triplet DPA molecules produced by the PtOEP sensitization was far from their diffusing distance to collide with each other in their triplet lifetime. There is only a slight possibility of back energy transfer from the DPA to PtOEP because the excitedstate lifetime of the DPA, at most several nanoseconds,8−10 is shorter than that of the PtOEP, at least 20 ns, as shown in Table S2. The photon upconversion efficiency should depend on the PtOEP concentration, which was necessarily lower than that of the DPA under the present conditions. The slopes were higher in the silica gel encapsulating the PtOEP than in that adsorbing the PtOEP. This depended on the effective PtOEP molecules without the deactivating energy transfer. Figure 7 shows the dependence of the DPA emission intensity on the irradiation power density, i.e., excitation light intensity, in the silica gel samples adsorbing and encapsulating the PtOEP. The graphs of the logarithmic axes are also shown. The slope values for the silica gel samples adsorbing and encapsulating the PtOEP were 2.1 and 2.0, respectively. Therefore, the emission intensity proportionally increased to the approximate square of the irradiation power, indicating that the emission was a quasi-two-photon process.9,10,17,18 The energy transfer from some of the encapsulated molecules to the adsorbed molecules and the TTA-UC were observed in the present silica gels. Influence of Solvents on Photon Upconverted Emission of the Singlet Emitter. The adsorption of the PtOEP-encapsulated silica gel for DPA was conducted by immersion in toluene solutions containing 5.0−20.0 mmol dm−3 DPA for 6 h. The phosphorescence intensity depended on the DPA concentration in the PtOEP-encapsulated silica gel samples dried at 25 and 60 °C. The PtOEP concentration in these samples was 0.28 ± 0.02 μmol g−1, independent of the DPA concentration. The concentration range of the adsorbed DPA was 60.0−220 μmol g−1 in the samples. Figure S4 shows the thermogravimetric and differential thermal analyses (TGDTA) curves of the silica gel samples dried at 25 and 60 °C. Almost all of the solvents were evaporated below 150 °C due to the weight losses and endothermic peaks. The amounts of the solvents remaining in the gels dried at 25 and 60 °C were estimated to be 8 and 4 wt % of the gels, respectively. The phosphorescence of PtOEP was gradually quenched with an increase in the concentration of DPA in the wetter gel. On the other hand, the phosphorescence was slightly quenched in the drier gel. Figure 8 shows the Stern−Volmer plot for the phosphorescence quenching in the samples dried at 25 and 60 °C. The phosphorescence quenching constants, KSV, in the wetter and dryer gels were 4.4 and 1.2 × 103 g mol−1, respectively. This indicated that the phosphorescence quenching more efficiently occurred in the wetter gels than in the dryer gels as expected. Figure 9 shows the dependence of the DPA emission intensity on the DPA concentration in the wetter and dryer silica gel samples encapsulating the PtOEP. The relative quantum efficiency of the emission is shown in Figure S5. This result indicated that the photon upconverted emission was observed in the silica gel systems. As the amounts of the solvents remaining in the wetter gel were twice as high as those in the dryer gel, the emission quantum efficiency of the former was 4 times higher than that of the latter. However, the emission efficiency was not high based on the linear

Figure 3. Stern−Volmer plot for the phosphorescence quenching in (1) silica gel samples adsorbing PtOEP and DPA and (2) those encapsulating PtOEP and adsorbing DPA.

molecules can more efficiently collide with the absorbed DPA molecules than the adsorbed PtOEP molecules in the present systems, which can induce their triplet-state quenching. It has been reported that certain organic molecules encapsulated in the silica gels transformed their structures during light irradiation as photochromic reactions.34,35 The encapsulated PtOEP molecules were presumed to be close to the adsorbed DPA molecules for energy transfer of the electron-exchange mechanism. Photon Upconverted Emission of the Singlet Emitter. Figure 4 shows the excitation and emission spectra of the silica

Figure 4. Excitation and emission spectra of the silica gel sample (1) adsorbing PtOEP and DPA and (2) encapsulating PtOEP and adsorbing DPA.

gel sample containing PtOEP and DPA. The emission peak was located at 430 nm in the silica gel samples adsorbing and encapsulating the PtOEP. The related excitation peak at 535 nm originated from the sensitizer absorption. The singlet emission was enhanced when the phosphorescence was quenched with an increase in the concentration of the singlet emitter.4−12,16−20 No emission was observed in the silica gel sample adsorbing only DPA upon 535 nm excitation. Figures 5 and 6 show the dependence of the DPA emission spectrum and intensity, respectively, on the DPA concentration in the silica gel samples adsorbing and encapsulating the PtOEP. The relative quantum efficiency of the emission is shown in Figure S3. This result indicated that photon upconverted emission was observed in the silica gel systems. However, the emission efficiency was not high, due to the linear relationship between the DPA emission intensity and the DPA concentration. The 8532

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Figure 5. Dependence of upconverted emission spectrum of DPA on DPA concentration in the silica gel samples (a) adsorbing PtOEP and DPA and (b) encapsulating PtOEP and adsorbing DPA. The DPA was adsorbed from toluene solutions containing (1) 0 mmol dm−3, (2) 5.0 mmol dm−3, (3) 10.0 mmol dm−3, and (4) 20.0 mmol dm−3 DPA. The emission spectra were observed upon 535 nm excitation.

Figure 6. Dependence of upconverted emission intensity of DPA on DPA concentration in the silica gel samples (1) adsorbing PtOEP and DPA and (2) encapsulating PtOEP and adsorbing DPA.

Figure 8. Stern−Volmer plot for the phosphorescence quenching of samples dried at (1) 25 °C and (2) 60 °C.

relationship between the DPA emission intensity and the DPA concentration, although the emission should be a two-photon process. The triplet−triplet annihilation of the DPA molecules did not efficiently occur because the average distance of the triplet DPA molecules was far from their diffusing distance to collide with each other in their triplet lifetime, similar to the above-mentioned systems. The slopes were higher in the wetter gel than in the dryer gel. Figure 10 shows the dependence of the DPA emission intensity on the irradiation power in the wetter and dryer silica gel samples encapsulating PtOEP. The graphs of the

logarithmic axes are also shown. The slope values for the wetter and dryer gels were 2.4 and 2.6, respectively. Therefore, the emission can be regarded as a quasi-two-photon process in the wetter silica gels. The energy transfer from some of the encapsulated molecules to the adsorbed molecules and the TTA-UC were observed in the present silica gels containing low amounts of the solvent molecules such as toluene, ethanol, and water. The energy transfer process for the triplet sensitization and triplet−triplet annihilation process required the collision of the component molecules in an environment similar to the solvents during their activated-state lifetime.

Figure 7. Dependence of the singlet emission intensity on the excitation light intensity in the silica gel samples (1) adsorbing PtOEP and DPA and (2) encapsulating PtOEP and adsorbing DPA. 8533

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was enhanced with an increase in the concentration of the singlet emitter. The emission intensity proportionally increased to the approximate square of the irradiation power. The energy transfer from some of the encapsulated molecules to the adsorbed molecules and the TTA-UC were consequently observed in the present silica gels. The phosphorescence quenching and TTA-UC were more significantly observed in the wetter gel than in the dryer gel. The wetter gel contained higher amounts of solvent and water molecules, in which the triplet sensitization and TTA processes proceeded during the triplet-state lifetime. These processes required that the sensitizer and singlet emitter molecules were in an environment similar to the solvents for the collision of the components. Silica gel containing much higher amounts of solvent molecules in the gel networks and on the surface of the particles promises to lead to a much higher efficiency of the TTA-UC.

Figure 9. Dependence of upconverted emission intensity of DPA on the DPA concentration of samples dried at (1) 25 °C and (2) 60 °C.

Finally, Figure S6 shows the phosphorescence decay curves of the wetter silica gel sample encapsulating PtOEP and that encapsulating PtOEP and adsorbing DPA, as well as the emission rise and decay curve of that encapsulating PtOEP and adsorbing DPA. The decay components were well fitted to a double exponential function. The fitting parameters of the emission time profiles are shown in Table S3. The phosphorescence lifetime values were shorter in the DPAcontaining sample than in the DPA-free sample due to the quenching energy transfer. The rise time of the emission was nearly equal to the average of the shorter and longer lifetime values of the phosphorescence in the DPA-containing sample. The photon upconverted emission based on the dye-sensitized triplet−triplet annihilation was clearly observed in the silica sol−gel system, although its efficiency was not very high.

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EXPERIMENTAL SECTION Sample Preparation. PtOEP (Sigma-Aldrich), DPA, ethanol, hydrochloric acid, tetraethyl orthosilicate (TEOS), and toluene (Wako Chemicals, S grade) were used without further purification. The water was deionized and distilled using a Yamato WG23 distiller. PtOEP was dissolved in ethanol at 0.100 mmol dm−3 for the sol−gel reaction. The starting solution of the sol−gel system contained 5.25 cm3 of PtOEP in an ethanol solution, 5.00 cm3 of TEOS, and 1.65 cm3 of 1.00 mmol dm−3 hydrochloric acid as the catalyst. The system without PtOEP was also prepared. The solutions were stirred during the addition, thoroughly stirred for an additional 5 min, and then poured into individual polypropylene vials (50 cm3). The vials were closed with a holed cover and kept in a thermostated oven at 60 °C for 3 days. The resulting gel samples were ground in mortars. The silica gel samples without PtOEP (0.100 g) were immersed in a toluene solution containing 0.300 mmol dm−3 PtOEP and 20.0 mmol dm−3 DPA for 1−6 and 48 h or in toluene solutions containing 60.0 μmol dm−3 PtOEP and 5.0− 20.0 mmol dm−3 DPA for 48 h to adsorb the PtOEP and DPA. The PtOEP-encapsulated samples (0.100 g) were immersed in a toluene solution of 20 mmol dm−3 DPA for 1−6 h or in toluene solutions of 5.0−20.0 mmol dm−3 DPA for 48 h to adsorb the DPA. The two-way preparations provided nearly the same amount of PtOEP molecules encapsulated in the silica gels or adsorbed on the surface and pores of the silica gel particles (ca. 0.3 μmol g−1, 1.8 × 10−4 nm−2 on the suppose of

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CONCLUSIONS TTA-UC was observed in the sol−gel silica systems containing PtOEP (triplet sensitizer) and DPA (singlet emitter). The triplet sensitizer was encapsulated and highly dispersed in the silica gels prepared by the sol−gel method. The singlet emitter was adsorbed from its toluene solution on the silica gel pores accessible to the outside. The emission and excitation spectra of the moderately and thoroughly dried silica gel samples were observed. The encapsulated triplet sensitizer emitted phosphorescence more efficiently than the sensitizer adsorbed on the silica gel pores due to the low deactivating energy transfer. Dependences of the emission peak intensities on the emitter concentration and irradiation light intensity were evaluated. The phosphorescence was quenched, and the singlet emission

Figure 10. Dependence of the singlet emission intensity on the excitation light intensity in (1) wetter and (2) dryer silica gel systems containing PtOEP and DPA. 8534

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the specific surface area, 1000 m2 g−1 25). These samples were dried at 25 °C for 1 day. The other PtOEP-encapsulated samples (0.100 g) were immersed in toluene solutions of 5.0− 20.0 mmol dm−3 DPA for 6 h. These samples were dried at 25 and 60 °C for 1 day to examine the influence of the remaining solvents. Measurements. The UV−vis absorption spectra and emission spectra were observed using a Shimadzu UV-3150 spectrophotometer and a Shimadzu RF-5300 fluorescence spectrophotometer, respectively. A sharp cut filter was used during the excitation for the emission spectral measurements to cut light with a wavelength shorter than the excitation wavelength, such as the second-order light. The powder samples were diluted with barium sulfate (Wako Chemicals, 1st grade) for the diffuse reflection spectroscopy using the UV−vis spectrophotometer. The powder samples (20.0 mg) were placed between two glass plates with a window area of 1.5 cm2 to observe their emission spectra. The relative emission quantum efficiency was estimated using the absorption and emission intensities of the samples compared to those of an ethanol solution of 1.00 μmol dm−3 DPA.29 The irradiation light power was changed by neutral-density filters and measured using a Coherent LabMaX-TOP laser power/energy meter. Thermogravimetric and differential thermal analyses (TGDTA) of the silica gel samples dried at 25 and 60 °C were conducted using a Rigaku Thermo Plus EVOII TG8120 thermal analyzer. The emission decay curves were measured using a timecorrelated single photon counting system. The samples were excited using a green light-emitting diode (LED) (Toyoda Gosei E1L51-3GC02) as pulse light source with a peak wavelength at 525 nm. The detection of single photons was conducted using a process memory (NAIG E-562), an analogto-digital converter (NAIG E-551), a time-to-amplitude converter (Canberra model 2043), a photomultiplier (Hamamatsu R3896), and a monochromator (Bunkoukeiki MC-25). The green LED was driven by a self-made pulse generator. The curves were analyzed by applying an iterative reconvolution of the instrument response function with an exponential decay model.

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curve of that encapsulating PtOEP and adsorbing DPA (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +81-26-269-5536. Fax: +81-26-269-5531. ORCID

Hiromasa Nishikiori: 0000-0002-6398-310X Katsuya Teshima: 0000-0002-5784-5157 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Division of Instrumental Research, Research Center for Supports to Advanced Science, is acknowledged for providing instrumental facilities.

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REFERENCES

(1) O’Regan, B.; Grätzel, M. A Low-Cost, High-Efficiency Solar Cell Based on Dye-Sensitized Colloidal TiO2 Films. Nature 1991, 353, 737−740. (2) Nazeeruddin, M. K.; Kay, A.; Rodicio, I.; Hamphry-Baker, R.; Müeller, E.; Liska, P.; Vlachopoulos, N.; Grätzel, M. Conversion of Light to Electricity by cis-X2Bis(2,2′-bipyridyl-4,4′-dicarboxylate)ruthenium(II) Charge-Transfer Sensitizers (X = Cl−, Br−, I-, CN−, and SCN−) on Nanocrystalline TiO2 Electrodes. J. Am. Chem. Soc. 1993, 115, 6382−6390. (3) Grätzel, M. Dye-Sensitized Solar Cells. J. Photochem. Photobiol., C 2003, 4, 145−153. (4) Zhou, J.; Liu, Q.; Feng, W.; Sun, Y.; Li, F. Upconversion Luminescent Materials: Advances and Applications. Chem. Rev. 2015, 115, 395−465. (5) Singh-Rachford, T. N.; Castellano, F. N. Photon Upconversion Based on Sensitized Triplet−Triplet Annihilation. Coord. Chem. Rev. 2010, 254, 2560−2573. (6) Zhao, J.; Ji, S.; Guo, H. Triplet−Triplet Annihilation Based Upconversion: from Triplet Sensitizers and Triplet Acceptors to Upconversion Quantum Yields. RSC Adv. 2011, 1, 937−950. (7) Gray, V.; Moth-Poulsen, K.; Albinsson, B.; Abrahamsson, M. Towards Efficient Solid-State Triplet−Triplet Annihilation Based Photon Upconversion: Supramolecular, Macromolecular and SelfAssembled Systems. Coord. Chem. Rev. 2018, 362, 54−71. (8) Lissau, J. S.; Gardner, J. M.; Morandeira, A. Photon Upconversion on Dye-Sensitized Nanostructured ZrO2 Films. J. Phys. Chem. C 2011, 115, 23226−23232. (9) Lissau, J. S.; Nauroozi, D.; Santoni, M.-P.; Ott, S.; Gardner, J. M.; Morandeira, A. Anchoring Energy Acceptors to Nanostructured ZrO2 Enhances Photon Upconversion by Sensitized Triplet−Triplet Annihilation Under Simulated Solar Flux. J. Phys. Chem. C 2013, 117, 14493−14501. (10) Lissau, J. S.; Nauroozi, D.; Santoni, M.-P.; Ott, S.; Gardner, J. M.; Morandeira, A. Photon Upconversion from Chemically Bound Triplet Sensitizers and Emitters on Mesoporous ZrO2: Implications for Solar Energy Conversion. J. Phys. Chem. C 2015, 119, 25792− 25806. (11) Massaro, G.; Gentili, P. L.; Ambrogi, V.; Nocchetti, M.; Marmottini, F.; Ortica, F.; Latterini, L. Triplet-Triplet Annihilation Based Upconversion in Silica Matrices. Microporous Mesoporous Mater. 2017, 246, 120−129. (12) Fang, J.; Wang, W.; Zhu, C.; Fang, L.; Jin, J.; Ni, Y.; Lu, C.; Xu, Z. CdS/Pt Photocatalytic Activity Boosted by High-Energetic Photons Based on Efficient Triplet−Triplet Annihilation Upconversion. Appl. Catal., B 2017, 217, 100−107. (13) Kwon, O. S.; Kim, J.-H.; Cho, J. K.; Kim, J.-H. Triplet−Triplet Annihilation Upconversion in CdS-Decorated SiO2 Nanocapsules for

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsomega.8b01107. Langmuir isotherms and fitting parameters of the PtOEP and DPA adsorption on the silica gel particles; emission decay curves, fitting parameters of the emission decay curves, and fitted double exponential function curves of the PtOEP adsorbed on the surface of the silica gel particles and encapsulated in the silica gel networks; TGDTA curves of the silica gel samples dried at 25 and 60 °C; DPA concentration dependence of the relative quantum efficiency of the upconverted emission in the samples dried at 25 and 60 °C; and phosphorescence decay curves and fitting parameters of the emission decay curves of the wetter silica gel sample encapsulating PtOEP and that encapsulating PtOEP and adsorbing DPA, as well as upconverted emission rise and decay 8535

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DOI: 10.1021/acsomega.8b01107 ACS Omega 2018, 3, 8529−8536