A green solvent for operating highly efficient low-power photon upconversion in air

Jinsuo Ma a, Shuoran Chen *a, Changqing Ye *a, Mingzhu Li b, Teng Liu a, Xiaomei Wang *a and Yanlin Song b
aResearch Centre for Green Printing Nanophotonic Materials, Jiangsu Key Laboratory for Environmental Functional Materials, Institute of Chemistry, Biology and Materials Engineering, Suzhou University of Science and Technology, Suzhou 215009, P. R. China. E-mail: yechangqing@mail.usts.edu.cn
bKey Laboratory of Green Printing, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, P. R. China

Received 6th March 2019 , Accepted 23rd April 2019

First published on 26th April 2019


D-Limonene, obtained from the rind of citrus fruits, was demonstrated as a green solvent to realize air-stable and highly efficient triplet–triplet annihilation photon upconversion (TTA-UC). This natural low-toxic compound also contributed to noncoherent UC excited by a solar simulator in air, making TTA-UC materials promising candidates in solar energy and other practical applications. The rapid deoxygenating ability of D-limonene was thoroughly investigated. This system demonstrated very good UC performance for a fluid solution under ambient conditions. Besides, other eight types of terpene were also explored to enrich the alternatives for air-stable TTA-UC in protic and aprotic fluidic environments. This work provides a terpene-based protective platform for oxygen-sensitive TTA-UC applications ranging from life science to photonic devices.


Introduction

Photon upconversion (UC), which is a process that converts low-energy photons to higher ones, has the potential to break the thermodynamic efficiency limit of solar technologies such as photovoltaics and photocatalytic reactions.1–10 Triplet–triplet annihilation upconversion (TTA-UC) is well-known as one of the UC mechanisms of a relatively higher (>10%) quantum yield (QY) and lower (<10 mW cm−2) threshold excitation intensity (Ith).11 The TTA-UC process is a promising upconversion method that can be excited by noncoherent low-power illumination such as sunlight.12 The TTA-UC process typically employs organic and metalloorganic chromophores that are only soluble in organic solvents,13 and the contact among the chromophores is essential for the transfer and annihilation of triplet excitons.14 Consequently, numerous common organic solvents, such as toluene,7,15,16 chloroform17 and tetrahydrofuran,18 have been widely used in the TTA-UC systems for the last few decades. Moreover, to avoid triplet-state quenching by the dissolved oxygen molecules, anaerobic environments are needed, which requires time-consuming, multi-step and high-cost deoxygenating processes by bubbling inert gases or by repeated freeze–pump–thaw cycles.16,19–21 To simplify the operating process, researchers have been searching for innovative media with the aim of maximizing the efficiency, air stability and bio-safety of the TTA-UC method. For instance, some researchers used structured media to isolate the TTA-UC solution from external oxygen, such as microcapsules,10,13 nanoparticles,22,23 micelles,18,24 microemulsions25 and polymer gels.21,26,27 However, these architectures inevitably affect the mobility and the diffusion rate of the chromophores, reducing the TTA-UC efficiency significantly.20,28 In addition, certain types of oxygen isolation methods need extensive synthetic efforts for sample preparation. Recently, liquid media with photochemical deoxygenating ability have been developed to prevent oxygen quenching internally. Mongin et al. proposed polyethylene glycol (PEG, M = 200, 400 or 600 Da) containing oleic acid (OA) as promising media and achieved efficient ambient TTA-UC owing to the oxygen scavenging ability of OA and low permeability of oxygen in the highly viscous media.19 Dzebo et al. proposed commercially available thioethers and one thiol group could scavenge singlet oxygen in the TTA-UC system.29 Wan et al. recently reported that dimethylsulfoxide (DMSO) can be regarded as a deoxygenating solvent to solve the oxygen quenching problem in TTA-UC.30 Nevertheless, the strong polarity and high viscosity of these sulfoxides and cyclic ureas restrict the solubility of organic dyes and therefore limit the TTA-UC performance (QY < 8% and Ith > 177 mW cm−2) (Fig. S1, ESI). However, for practical solar energy conversion applications, when the sunlight is used as an excitation source for TTA-UC systems, Ith should be lower than the solar irradiance (the integration of AM1.5 solar spectrum gives only 1.54 mW cm−2 at the excitation wavelength of 532 ± 5 nm) (Fig. S2, ESI).31,32 Moreover, other deoxygenating liquid media including highly viscous soybean oil,16 oleic acid33 and Kolliphor EL18 have also been explored for air-stable TTA-UC. However, these bio-safe liquid media need toxic co-solvents to dissolve the TTA-UC dyes. Generally, the existing toxic co-solvents are difficult to remove completely, which cannot assure bio-safety in biological applications; moreover, the low UC efficiency limited by high viscosity is still a big issue. Accordingly, the straightforward approach for solving the oxygen quenching problem is to find a suitable solvent that can achieve air-stable, high-performance and bio-safe TTA-UC applied in fields ranging from energy conversion to biology.

Hence, we listed four criteria that a suitable TTA-UC solvent needs to meet. (1) Oxygen scavenging ability;28 (2) low polarity,34 low viscosity and optical transparency;11 (3) non- or low-toxicity;16 (4) stable photophysical properties of upconverting dyes in the selected solvents.35 To our best knowledge, unfortunately, a solvent that fulfils all the above criteria has not been explored so far.

Here, inspired by nature, we demonstrated a type of natural solvent, terpenes, which can be used as a green solvent for operating air-stable and highly efficient TTA-UC. In nature, terpenes represent a class of cheap and abundant nonpolar substances with enormous potential for green chemistry.36 One of the typical “green” constituents, namely, D-limonene is abundantly present in the rind of oranges and related fruits,37 and it is recognized as a safe chemical by the U.S. Food and Drug Administration (FDA).36 Besides, non-polar D-limonene has similar molecular weight and chemical structure to substituted toluene; therefore, it possibly has the potential to replace toluene, which is commonly used as a TTA-UC solvent.38 Because of its low polarity, low viscosity and optical transparency, D-limonene is considered as the only bio-solvent capable of replacing nonpolar traditional solvents.36 We demonstrated that D-limonene can satisfy high performances of the TTA-UC process consisting of stable photophysical properties, high UC efficiency, low Ith and oxygen scavenging ability. In this work, we chose one of the classic dye pairs of sensitizer/emitter for the TTA-UC process: platinum(II) octaetylporphyrin (PtOEP) and 9,10-diphenylanthracene (DPA). It was easy to acquire a high sensitizer concentration (2 × 10−3 M) in D-limonene without any co-solvent. The result suggests that D-limonene has a unique advantage in the cases where high concentrations of dyes are required to ensure high absorption and a consequent stimulated observable emission at the excitation wavelength.11,39

Results and discussion

Fig. 1 shows the normalized absorption and steady-state photoluminescence (PL) spectra of DPA and PtOEP in toluene and D-limonene, respectively. The spectra match well with that of either DPA or PtOEP in the corresponding solutions, revealing that D-limonene does not interfere with their photophysical properties.
image file: c9cp01296f-f1.tif
Fig. 1 Normalized absorption and steady-state photoluminescence spectra of DPA (2 × 10−3 M) and PtOEP (1 × 10−5 M) in D-limonene (up) and toluene (down) at room temperature. Photoluminescence was excited at 355 nm.

The oxygen scavenging ability of the UC dye pair/D-limonene system is depicted in Scheme 1. In common solvents (toluene, THF and DMF), dissolved triplet oxygen (3O2) can readily quench the triplet excitons of the sensitizer and the annihilator by energy transfer (ET) to singlet oxygen (1O2*), which prevents the triplet–triplet energy transfer (TTET) and TTA processes (Scheme 1a). To eliminate the sustaining quenching ability of oxygen, terpene-based solvents were used to scavenge 1O2* by an oxidation reaction.40 When most of the dissolved oxygen molecules were scavenged, TTET and TTA occurred, yielding a UC emission under ambient conditions (Scheme 1b). In the air-saturated state, a strong blue UC emission was observed by the naked eye from the PtOEP/DPA/D-limonene solution upon irradiation by a 532 nm CW laser. However, it disappeared in toluene solution under the same conditions (Fig. 2a). Besides, an upconverted fluorescence signal at 430 nm was observed for the PtOEP/DPA/D-limonene solution under ambient conditions. In contrast, no corresponding emission was observed in toluene (Fig. 2b). It seems that TTA-UC is efficient in D-limonene solution under air conditions. Since oxygen is an effective quencher of the triplet excitons of the sensitizers,11 the phosphorescence lifetimes of the sensitizers decreased rapidly in the presence of dissolved oxygen and recovered readily in the deoxygenating solvent D-limonene. Therefore, we recorded the phosphorescence lifetime of PtOEP under N2 and air conditions (Fig. 2c). Experimentally, after the sample was exposed to air for ten hours, the lifetime of the sensitizer decreased dramatically. This apparent lifetime decrease occurred because quenching is an additional rate process that depopulates the excited state of PtOEP. Then, the same sample was measured under different irradiation times at 532 nm of a 2 mW laser (Fig. 2d). After 5, 10, and 20 s of irradiation, continuous lifetime increase for PtOEP could be observed. The highest lifetime value reached 9.88 μs, which was similar to that of the N2-bubbled D-limonene solution (12.73 μs). This indicated that most of the oxygen molecules were consumed by D-limonene rapidly.


image file: c9cp01296f-s1.tif
Scheme 1 (a) Mechanism of the quenching process of the triplet state sensitizer (3S*) and annihilator (3A*) by triplet oxygen (3O2) dissolved in ambient common solvents. The TTET and TTA processes were prevented and the energy was transferred to singlet oxygen (1O2*). (b) Mechanism of the oxygen scavenging ability of terpene-based solvents. The orange slice cartoons represent D-limonene, which can scavenge 1O2* in the system, leading to the TTET and TTA processes. Through the photochemical reaction from D-limonene and 1O2* to D-limonene oxide, UC process can be obtained under ambient conditions.

image file: c9cp01296f-f2.tif
Fig. 2 (a) Photograph of PtOEP (1 × 10−5 M) and DPA (1 × 10−3 M) in toluene and D-limonene under 532 nm CW laser irradiation. (b) UC emission spectra of PtOEP/DPA pair in toluene and D-limonene under air conditions. (c) The lifetime of PtOEP in D-limonene under air conditions and N2-bubbled conditions was measured at 645 nm under pulsed excitation at 532 nm (d) The aerobic lifetime of PtOEP in D-limonene solution under intermittent irradiation time at 532 nm (2 mW) was measured at 645 nm under pulsed excitation at 532 nm.

In order to explore the oxygen scavenging ability of D-limonene, the dynamic process of this reaction was tested. When a certain amount of air was continuously bubbled into the PtOEP/DPA/D-limonene solution at room temperature, the UC emission was suppressed immediately (Fig. 3a). After stopping the air-bubbling, the UC emission could quickly recover in less than 0.6 s under excitation by a 532 nm CW laser (64 mW cm−2) (Fig. 3b and c). Interestingly, the UC emission process could be repeated reversibly for many cycles, showing good photochemical stability under deoxygenating conditions (Fig. 3d). Importantly, this UC emission was surprisingly stable and remained almost unchanged in air for seven days (Fig. S3, ESI). All these results demonstrated that air-stable D-limonene could scavenge almost all the diffused oxygen molecules by the photochemical process under deoxygenating conditions.


image file: c9cp01296f-f3.tif
Fig. 3 (a) Dynamic oxygen scavenging process in a cycle of air-bubbled PtOEP/DPA/D-limonene solution under 532 nm CW laser irradiation. (b) Dynamic UC emission spectra of air-bubbled D-limonene under continuous irradiation of a green laser (please see Video S1, ESI). (c) Corresponding integral intensity of UC emission spectra. (d) UC emission intensity of the PtOEP/DPA/D-limonene solution in air-bubbled cycles.

The UC emission characteristics under air conditions were probed by steady-state luminescence spectroscopy. In Fig. 4a, it can be seen that the residual sensitizer phosphorescence at 645 nm is very weak, suggesting a TTET process from the triplet excited state of PtOEP produced by intersystem crossing to the excited triplet state of DPA. The efficiency of TTET between the sensitizer and the emitter (ΦTT-ET) can be determined by comparing the integrated sensitizer phosphorescence in the presence (IS/E) and the absence (IS) of the emitter under optimized bimolecular concentrations (PtOEP = 1 × 10−5 M, DPA = 2 × 10−3 M).23 Our analysis revealed ΦTT-ET of 0.99, reflecting a highly efficient transfer. In addition, high ΦTT-ET predicted the lifetime of 51 ns for PtOEP quenched by DPA at 645 nm, which was verified by the PL decay spectrum displayed in Fig. 4b. In fact, the experimental bimolecular energy transfer rate constant (kq = 6.49 × 109 M−1 s−1), reflecting the accessibility of the sensitizer to the emitter, was in excellent agreement with the theoretical value 7.72 × 109 M−1 s−1 near the largest possible value 1 × 1010 M−1 s−1 according to the Smoluchowski equation (please see the ESI for experimental details). The result suggested that the viscosity (0.95 cP) of D-limonene has negligible influence on the bimolecular energy transfer. Upconversion spectra with different incident power densities at 532 nm are shown in Fig. 4c. Considering non-negligible oxygen concentrations in ambient testing conditions, we investigated the value of the threshold excitation intensity (Ith) of the PtOEP/DPA/D-limonene solution in an air-tight quartz cell upon the photochemical deoxygenating ability of D-limonene under 532 nm CW laser irradiation (60 mW cm−2, irradiation time <10 s). Fig. 4d presents the double logarithm plots for the UC emission intensity of the PtOEP/DPA/D-limonene solution against the incident light power density. The Ith value of the solution is 1.22 mW cm−2, which is lower than that of the solar irradiance at the excitation wavelength (532 ± 5 nm, 1.54 mW cm−2). Furthermore, we obtained a high upconversion quantum yield value of 13.3% (upconversion efficiency is normalized to be 100% theoretical maximum) in D-limonene in air. Importantly, UC was also observed in D-limonene when the solution was excited by the noncoherent light from a solar simulator (Fig. 5a and b; please see Video S2, ESI). These results demonstrated that D-limonene is an excellent green deoxygenating solvent for practical air-intolerant solar energy harvesting/converting applications.


image file: c9cp01296f-f4.tif
Fig. 4 (a) Photoluminescence spectra of D-limonene solution of PtOEP and PtOEP/DPA upon excitation at 532 nm. The inset picture shows the D-limonene solution of PtOEP under ambient irradiation at 532 nm (right) and the upconverted blue emission of PtOEP/DPA/D-limonene solution under 532 nm excitation (left). (b) PL decay at 645 nm of the PtOEP/DPA pair (blue square) in D-limonene, the tail fit (red line). (c) Emission spectra of ambient PtOEP/DPA/D-limonene solution acquired upon irradiation with a 50 mW green laser (532 nm) whose power density was controlled by a neutral density filter. (d) Double logarithm plots of the upconverted emission intensity as a function of incident power.

image file: c9cp01296f-f5.tif
Fig. 5 (a) A picture of green-to-blue UC PtOEP/DPA/D-limonene solution excited with noncoherent light seen without 520 nm short-pass filter in air conditions. (b) The picture of (a) seen through a 520 nm short-pass filter in air conditions. (c) Photoluminescence spectrum of PtTPBP/BPEA in a natural solvent (squalene) upon excitation at 635 nm in air. (d) Integrated UC intensity of selected natural solvents in air and inert gas (PtTPBP: 1 × 10−5 M, BPEA: 1 × 10−3 M).

In nature, singlet oxygen is a common damaging agent to all living organisms, which generate large amounts of compounds such as vitamin C41 to resist the singlet oxygen and protect themselves.42 These compounds also provide us a huge treasure trove to explore eco-efficient and bio-safe alternative solvents for operating highly efficient TTA-UC. Two major categories of natural deoxygenating solvents consisting of protic solvents and aprotic solvents were explored (Chart S1, ESI). We used another TTA-UC dye pair consisting of platinum(II)-tetraphenyltetrabenzoporphyrin (PtTPBP)/9,10-bisphenylethynylanthracene (BPEA) under air conditions (please see Video S3, ESI). Fig. 5c shows the UC spectrum of the PtTPBP/BPEA bimolecular system in a selected natural deoxygenating solvent (squalene) under irradiation at 635 nm. Beyond our expectations, both protic and aprotic solvents exhibited a good and stable performance of UC in air (Fig. 5d and Fig. S4, ESI). These results suggest that such natural terpene-based solvents afford a general platform to search for alternative organic solvents that can be used in ambient TTA-UC-based aprotic/protic fluidic environments.

Conclusions

In conclusion, we demonstrated that oxygen-sensitive TTA-UC can be efficiently operated in purely terpene-based organic solvents under air atmosphere. In the case of D-limonene, optimum performances of TTA-UC in air were achieved, revealing stable photophysical properties, high QY (13.3%) and lower Ith (1.22 mW cm−2, lower than the solar irradiance intensity). These findings represent a concrete advancement in the development of green deoxygenating solvents for realizing highly efficient TTA-UC in air. Their employment must not be limited to air-intolerant TTA-UC-based energy conversion fields, and they must also be exploited in the wide field of biological applications.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by National Natural Science Foundation of China (Grant No. 51603141, 51873145, 51673143); Natural Science Foundation of Jiangsu Province-Excellent Youth Foundation (BK20170065), Natural Science Foundation of Jiangsu Province (BK20160358), Natural Science Foundation of the Higher Education Institutions of Jiangsu Province (17KJA430016, 18KJB430024), 5th 333 High-level Talents Training Project of Jiangsu Province (No. BRA2018340), Six Talent Summits Project of Jiangsu Province (No. XCL-79); Qing Lan Project.

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Footnote

Electronic supplementary information (ESI) available. See DOI: 10.1039/c9cp01296f

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