Photoswitchable Photon Upconversion from Turn-on Mode Fluorescent Diarylethenes
2020; Chinese Chemical Society; Volume: 3; Issue: 1 Linguagem: Inglês
10.31635/ccschem.020.202000227
ISSN2096-5745
AutoresJianlei Han, Jian Zhang, Tonghan Zhao, Minghua Liu, Pengfei Duan,
Tópico(s)Porphyrin and Phthalocyanine Chemistry
ResumoOpen AccessCCS ChemistryCOMMUNICATION1 Jan 2021Photoswitchable Photon Upconversion from Turn-on Mode Fluorescent Diarylethenes Jianlei Han, Jian Zhang, Tonghan Zhao, Minghua Liu and Pengfei Duan Jianlei Han CAS Center for Excellence in Nanoscience, CAS Key Laboratory of Nanosystem and Hierarchical Fabrication, National Center for Nanoscience and Technology (NCNST), Beijing 100190 , Jian Zhang *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] College of Chemistry, Chemical Engineering and Materials Science, Collaborative Innovation Center of Functionalized Probes for Chemical Imaging in Universities of Shandong, Key Laboratory of Molecular and Nano Probes, Ministry of Education, Institute of Molecular and Nano Science, Shandong Normal University, Jinan 250014 , Tonghan Zhao CAS Center for Excellence in Nanoscience, CAS Key Laboratory of Nanosystem and Hierarchical Fabrication, National Center for Nanoscience and Technology (NCNST), Beijing 100190 University of Chinese Academy of Sciences, Beijing 100049. , Minghua Liu CAS Center for Excellence in Nanoscience, CAS Key Laboratory of Nanosystem and Hierarchical Fabrication, National Center for Nanoscience and Technology (NCNST), Beijing 100190 Beijing National Laboratory for Molecular Science, CAS Key Laboratory of Colloid, Interface and Chemical Thermodynamics, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190 University of Chinese Academy of Sciences, Beijing 100049. and Pengfei Duan *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] CAS Center for Excellence in Nanoscience, CAS Key Laboratory of Nanosystem and Hierarchical Fabrication, National Center for Nanoscience and Technology (NCNST), Beijing 100190 University of Chinese Academy of Sciences, Beijing 100049. https://doi.org/10.31635/ccschem.020.202000227 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail Photon upconversion (UC) is one kind of anti-Stokes shift process, which generally requires a combination of two or more low-energy photons to produce one high-energy photon. However, another interesting anti-Stokes process, namely, single-photon-absorption-based upconversion (SPA-UC), can take place by exciting thermally excited vibrational-rotational energy levels of ground state to the first level of excited state. This phenomenon is less involved in organic systems due to the limitation of appropriate materials. Herein, for the first time, we have demonstrated a series of turn-on mode fluorescent diarylethenes, including Thiophene- and p-alkoxybromo-substituted phenyl-substituted derivatives ( DAES and DAEC4), which exhibited SPA-UC phenomenon in the closed-ring state. An anti-Stokes shift of 0.36 eV was observed in these molecules, which was the highest value accessible in such UC systems. A relatively high quantum efficiency (13.5%) was evaluated through a relative method by using methylene blue as a standard. Phonon-assisted or hot band absorption was in charge of the achievement of SPA-UC. Besides, the upconverted emission could be switched on/off by cyclization and cycloreversion reaction, regulated by UV–visible light. Thus, this unique finding of the SPA-UC phenomena in photoswitchable diarylethenes might enrich the development of new molecular engineering strategies for designing photofunctional materials. Download figure Download PowerPoint Introduction Photon upconversion (UC) is an anti-Stokes shift process, which emits a short-wavelength light when excited by long-wavelength light, and it has received tremendous attention in various fields due to its potential applications, including the construction of solar cells, display devices, artificial photosynthesis, photodynamic therapy, and bioimaging.1 There are several UC mechanisms that involve multiphoton-absorption process, such as two-photon absorption (TPA)2 (Scheme 1a), rare-earth element–doped nanoparticles, and triplet–triplet annihilation (TTA; Supporting Information Scheme S1).3–7 Additionally, another UC process, E-type delayed fluorescence has been discovered, called thermally activated delayed fluorescence (TADF),8 constituting an essential class of emitting materials in organic light-emitting diode (LED) devices, which exhibit 100% internal quantum yield. This UC process realizes thermally activated reverse intersystem crossing from the lowest triplet excited state to the lowest singlet excited state. Beyond all of these mechanisms, there is another type of unique anti-Stokes optical process, called single-photon-absorption-based upconversion (SPA-UC).9–13 By excitation of thermally excited vibrational-rotational energy levels of ground state to the first excited state, the emitter could produce upconverted emission (Scheme 1b and 1c). SPA-UC is less popular but has many advantages, including, noncoherent light with low intensity, unimolecular and single-photon absorption process, good oxygen tolerance, and high efficiency. A similar phenomenon named phonon-assisted UC in inorganic systems has been studied widely.14–16 However, to date, limited examples have been reported in organic systems due to the scarcity of such materials.9–13 Also, this type of anti-Stokes luminescence is highly temperature-dependent, because only the molecules in the thermally excited states could contribute to the luminescence. Nonetheless, due to the significant advantages of the emission, it has been used for imaging, laser cooling, and temperature sensing,17–19 focusing mainly on the investigation involving the use of rhodamine dyes. Scheme 1 | Upconversion mechanism for a two-photon absorption process (a) and a single-photon absorption process (b). (c) Potential energy diagrams of Stokes emission and anti-Stokes emission of DAES-C under the excitation of blue light and 635 nm laser, both of which could produce yellow emission. Download figure Download PowerPoint Diarylethene is one of the most important photoswitchable molecules and has been studied extensively because of its comprehensive and distinguished properties, such as excellent thermal stability, fatigue resistance, superior reversibility, and high-speed photoresponse.20–22 These diarylethene derivatives have been applied in various fields, including information storage, molecular switches, and logic gates.23–36 Recently, it was demonstrated that one type of diarylethene bearing sulfone unit showed intense emissive property in closed-ring state, while an open-ring state quenched the emission.37–39 Based on this fascinating characteristic, this type of emissive diarylethene has been used in super-resolution fluorescence microscopies, such as photoactivation localization microscopy (PALM) and stochastic optical reconstruction microscopy (STORM).40–42 However, the anti-Stokes shift photon UC of this diarylethene has never been reported. In this work, photon UC of a series of emissive diarylethene molecules was investigated for the first time. When the closed-ring state of diarylethene molecules was excited at 635 nm the molecules showed no absorption, while intense upconverted emission was observed. We validated that the observed photon UC process was induced by the hot band absorption of the closed-ring diarylethene (Scheme 1b and 1c). In addition, the excitation wavelength extended to 700 nm. Also, an anti-Stokes shift of 0.36 eV, the largest achievable value for SPA-UC, was obtained under 680 nm laser excitation. Further, we obtained a high SPA-UC quantum efficiency (13.5%), evaluated by using a standard dye as reference. This remarkably high SPA-UC characteristic offered advantages of good penetrability of long wavelength and outstanding performance of turn-on mode fluorescent diarylethenes, such as switching kinetics, photobleaching, and fatigue. We anticipated that the fabrication of this incredible SPA-UC could provide more opportunities for super-resolution fluorescence imaging and bioimaging. Results and Discussion We synthesized two kinds of emissive diarylethenes, according to a previously reported method (Scheme 2). The details of the synthetic procedures are described in Experimental Procedures in Supporting Information, with the resultant formation of Thiophene- and p-alkoxybromo-substituted phenyl-substituted derivatives ( DAES and DAEC4), which exhibited turn-on mode photoluminescence, based on the ring open–close cycle. Also, the emissive intermediate 1,2-bis(2-ethyl-6-iodo-1-benzothiophen-1,1-dioxide-3-yl) perfluoro-cyclopentene ( DAEI) was investigated. The absorption spectra of open-ring isomers and closed-ring isomers are shown in Supporting Information Figures S1–S3. The emission spectra data of closed-ring isomers and the photophysical data were collected and summarized in Supporting Information Table S1. The open-ring isomers were colorless in toluene at the concentration of 1.25 × 10−5 mol L−1. However, closed-ring isomers still existed even after purification by column chromatography, which was also observed by UV–vis spectrometry ( Supporting Information Figures S2 and S3). Upon irradiation with the UV 365 nm lamp, the colorless solution of DAEI turned to yellow coloration, while the solution of DAES and DAEC4 changed from colorless to orange, because of the formation of the closed-ring isomers. Scheme 2 | Synthesis of series of diarylethenes from thionaphthene as starting material. Download figure Download PowerPoint In addition, strong fluorescence was observed with a short lifetime of a few nanoseconds ( Supporting Information Table S1 and Figure S4). Figure 1a shows normalized absorption and emission spectra of DAES closed-ring isomers ( DAES-C) in toluene and dimethyl sulfoxide (DMSO). The solvatochromic properties of turn-on mode fluorescent diarylethene have been reported elsewhere.43–44 The emission color was solvent polarity dependent due to the intramolecular charge transfer (ICT) property of this kind of diarylethene, composed of electron-donating thiophene rings at both ends and electron-accepting benzothiophene 1,1-dioxide groups in the center. When the solvent (e.g., DMSO) polarity increased, the UV–vis spectrum displayed moderate bathochromic shift, accompanied by the blurring of the vibronic bands at ∼ 526 nm, as shown in Figure 1a. The solution color changed from orange in toluene to red in DMSO, while luminescence in DMSO showed even larger Stokes-shifts. The luminescence of DAES-C exhibited strong yellow color in toluene and red color in DMSO with fluorescence quantum yield of 0.87 and 0.49, respectively (Figure 1b). The transformation from open-ring isomer to closed-ring isomer was monitored by UV–vis and emission spectra (Figure 1c). Upon irradiation with 365 nm UV light, DAES-O immediately converted to DAES-C (Figure 1c), accompanied by an increase of emission intensity. The photocyclization reaction progressed rapidly in less than 1 min. On the other hand, a photobleaching process, which took place during the DAES-C to DAES-O transformation, occurred at a very slow pace, compared with the photocyclization reaction.37,41,45 The photobleaching conversion was achieved via irradiation of DAES-C under 532 nm laser at high excitation power of ∼ 59 W cm−2 ( Supporting Information Figure S5a and b), A longer time (> 20 min) was required for the transformation from the closed-ring form to the open-ring form; meanwhile, the ring-opening reaction of DAEI-C took place rapidly with a blue LED lamp irradiation ( Supporting Information Figure S6a and b). Further, photoswitching reactions were conducted repeatedly by alternating the irradiation of UV 365 nm lamp and 532 nm laser. After five cycles, there was no appreciable alteration of the absorption and emission intensities, indicating an excellent reversibility (Figure 1d and Supporting Information Figure S5). Accordingly, the stability and fatigue resistance paved the way for the practical application of such materials. Figure 1 | (a) Normalized absorption and emission spectra of DAES-C in toluene and DMSO. (b) Photographs of DAES-C in toluene and DMSO under day light and UV 365 nm lamp. (c) Absorption and fluorescence spectral changes of DAES-O in toluene under the irradiation of 365 nm light. (d) Photoswitching of DAES in toluene monitored at 490 nm by alternating irradiation of UV 365 nm light and visible light (532 nm laser). ([DAES] = 1.25 × 10−5 mol L−1, emission spectra were recorded at the excitation of 490 nm). Download figure Download PowerPoint When the DAES-C solution was irradiated by a 635 nm continuous-wave (CW) laser, orange luminescence was observed ( Supporting Information Figure S7 insert picture). The emission spectrum excited at 635 nm matched very well with that of downconversion spectrum excited at 490 nm ( Supporting Information Figure S7). This kind of emission was assigned to the S1 to S0 emission, even though the light excitation energy was 1.95 eV, which was lower than the emission maximum of 2.18 eV, implying an energy UC process. To exclude the possibility of an artificial signal, a 532 nm long-pass filter was placed between the excitation light and the sample, and the emission spectra showed no change at all. The emission profile of 635 nm laser is provided in the Supporting Information Figure S8, and the emission spectrum of the 635 nm excitation with the 532 nm long-pass filter is shown in Supporting Information Figure S9. Next, we confirmed the validity of the UC process of DAES-C excited by 635 nm laser. Figure 2a shows the emission spectra of DAES-C in toluene excited by 635 nm laser with different incident power density. The UC emission was enhanced with an increase in excitation power, while linear proportionality was calculated between emission intensity and the laser power (Figure 2b), indicating a single-photon absorption process. Thus, the UC intensity should be proportional to the amount of emitters. Four samples with varying DAES-C concentrations were tested ( Supporting Information Figure S10) with similar excitation power density. We found that the higher the concentration, the stronger the SPA-UC emission. Supporting Information Figure S11 shows emission photograph of the four samples excited by 635 nm laser at a large power density, which demonstrate clearly that the more the excited molecules, the stronger the UC emission. Subsequently, we examined the photochromic properties such as reversibility and stability of DAES-C excited by 635 nm laser. By exciting the sample with 365 nm UV light, the UC emission intensity increased as the cyclization reaction proceeded (Figure 2c). UC emission reversibility was conducted based on downconversion emission. The results shown in Figure 2d reveal no significant decay in fluorescence intensity even after five cycles of alternating UV 365 nm light and 532 nm laser irradiation. Additionally, the UC emission at 570 nm rarely changed by the continuous irradiation of 635 nm laser for as long as 240 min ( Supporting Information Figure S12), which indicated that the 635 nm light could not have initiated the reversal of the ring-opening reaction. Figure 2 | (a) Photon upconversion (UC) emission spectra of DAES-C with different incident power density of 635 nm laser (the sharp peaks correspond to the scattered exciting light). (b) Dependence of UC emission intensity at 570 nm on the incident power density. (c) UC emission spectra of DAES-O in toluene excited by 635 nm CW laser with different irradiation time of 365 nm UV light (the excitation power density is 1.21 W cm−2). (d) Photoswitchable UC emission at 570 nm of DAES in toluene with the irradiation of 365 nm light and 532 nm laser (inset shows emission pictures of open-ring isomer and closed-ring isomer excited by 635 nm laser). ([DAES] = 5 × 10−4 mol L−1). Download figure Download PowerPoint There are some anti-Stokes shift mechanisms in organic systems such as two-photon absorption (TPA), triplet–triplet annihilation (TTA), thermally activated delayed fluorescence (TADF), and single-photon absorption (SPA). We considered the possibility of any of these phenomena occurring in our established emissive diarylethene derivatives system. We ruled out all but the last possibility by claiming the following: (1) First, TTA was excluded because of the absence of sensitizer and aerated condition (vide infra). Another mechanism called cooperative energy pooling46 was ruled out for the same reason. (2) It is well-known that excitation power about MW cm-2 or even higher was needed for the TPA UC and ultrafast laser was needed for the excitation. Therefore, there was very limited possibility of an involvement of a TPA process, as the orange luminescence was observed by naked eye with a relatively low excitation power (as low as 8 mW cm−2) by a 635 nm CW laser. Moreover, linear relationship was obtained between anti-Stokes emission intensity and the laser power density of 635 nm (Figure 2b), which excluded a TPA mechanism, since TPA is a quadratic process. (3) TADF was also excluded because TADF process involves excited triplet, exhibiting a long-lived lifetime. Nevertheless, in our present system, the lifetime of UC emission was just 2.85 ns (Figure 3c), not longer than that of the normal fluorescence excited by 377 nm ( Supporting Information Table S2). On the other hand, excited triplet is oxygen-sensitive, thus, might quench triplet exciton and subsequently quench the UC emission. Accordingly, the Supporting Information Figure S13 shows the UC spectra of DAES-C in deaerated and aerated toluene solution, respectively, which displays no observable intensity change, as well as the UC lifetime (Figure 3c and Supporting Information Table S2) under the same excitation intensity. Figure 3 | (a) Temperature-dependent UC emission spectra of DAES-C excited by 635 nm laser at various temperatures. (b) Dependence of PL intensity at 570 nm on temperature excited by 635 nm laser. ([DAES] = 5 × 10−4 mol L−1 in toluene). (c) UC emission lifetime of DAES-C detected at 570 nm in deaerated and aerated toluene excited by 635 nm. ([DAES] = 5 × 10−4 mol L−1 for both deaerated and aerated toluene). Download figure Download PowerPoint (4) Since the energy of the exciting light, which is 0.23 eV lower than that of the emission maximum, is too low to induce a transition from the lowest vibration–rotation levels of S0 state, absorption could take place only from higher vibration–rotation levels of S0 state to one or more vibration–rotation levels of S1 state. This means that hot band absorption should be responsible for the occurrence of SPA-UC. Hence, the populations of the vibration–rotation levels of the ground state were determined by a Boltzmann distribution, which is a temperature-dependent phenomenon. The temperature dependence of SPA-UC emission intensity was conducted from 200 to 300 K in toluene, as shown in Figure 3a. The emission below 230 K was very weak. At the temperature > 230 K, the emission peak became more and more prominent, and the UC intensity increased exponentially (Figure 3b). By fitting the data into the Arrhenius plot I = A exp(−ΔE/kT), where k is the Boltzmann constant, T is the absolute temperature, and I is the UC intensity,47,48 a ΔE of ∼ 0.2 eV was obtained, which corresponded to an energy difference between emission wavelength and excitation wavelength. In the SPA-UC process, this would be the energy difference between the lowest vibrational level (v0) and the high vibrational level. This value is in reasonable agreement with the difference between the excitation energy of 635 nm (1.95 eV) and the energy of the 0–0 transition from S1 to S0 (570 nm, 2.18 eV), determined to be 0.23 eV. Therefore, it is more likely that electrons at high vibration–rotation levels of the ground state absorbing 635 nm photons induced a transition to S1 state, enabling radiative decay from the S1 state to S0 state, thereby, emitting higher energy photons. DAEI and DAEC4 ( Supporting Information Figures S1 and S3) were also used to verify hot band absorption UC of such materials. Perhaps the energy gap was too large (0.49 eV lower than the emission maximum; Table 1) for DAEI to produce a UC emission, while DAEC4, possessing similar energy gap with DAES, exhibited much better performance ( Supporting Information Figure S14). Table 1 | Anti-Stokes Shift between S1 State and 635 nm of Fluorescent Diarylethenes λS1a/nm Eb/eV ΔEc/eV DAEI-Cd 508 2.44 0.49 DAES-Cd 567 2.19 0.23 DAES-Cd 567 2.19 0.36e DAES-Cf 610 2.03 0.08 aMaximum emission wavelength was used to define the energy of S1 state. bEnergy of S1 state. cEnergy gap between S1 state and 635 nm (1.95 eV). d5×10−4 mol L−1 in toluene. eEnergy gap between S1 state and 680 nm (1.82 eV). f5×10−4 mol L−1 in DMSO. Very recently, supracence phenomena of rhodamine B was reported showing a very sharp anti-Stokes shift emission. The extra energy was ascribed to couplings of the absorbed and emitted photon to molecular phonons,49 while hot band absorption of rhodamine compounds were being well investigated.9–13 Both the phonon-assisted UC and the hot band absorption could be assigned to the Urbach rule.50–52 Generally, in organic system, hot band absorption has been widely accepted as a popular theory. The solvatochromic properties of emissive diarylethenes have been reported, 43,44 these previous reports led us to suggest that the narrowed energy gap in polar solvent could result in the bathochromic shift of the emission wavelength (Figure 4a and 4e). Due to the ICT property, the emission quantum yield of DAES-C in DMSO (0.49) was much lower than that in toluene (0.87), as shown in Figure 4c. Indeed the emission intensity is stronger in toluene than that in DMSO under the same excitation condition. The UV–vis spectra of DAES in toluene and DMSO showed almost the same absorbance at 360 nm ( Supporting Information Figure S15). Thus, the emission intensity difference resulted mainly from the difference between the quantum yields other than the differences between the excitation conditions. On the contrary, the SPA-UC emission exhibited totally opposite phenomenon; the brightness of the SPA-UC emission in DMSO was much stronger than that in toluene (Figure 4d and 4f; the UC behavior in DMSO see Supporting Information Figures S16 and S17). Even though, the absorption in DMSO showed redshift, compared with that in toluene, there was still no obvious absorption at 635 nm. These results indicated that the narrow energy gap could facilitate the hot band absorption process, which is totally different from observations made from the normal downconversion emission. We confirmed further a deduction by testing the SPA-UC emission of DAEI-C molecule. As expected, due to the wide energy gap of DAEI-C, the SPA-UC emission was silent (Table 1). Figure 4b illustrates the SPA-UC behavior with different energy gap caused by changes in solvent polarity. Figure 4 | Simplified Jablonski diagram of Stokes emission and anti-Stokes emission in toluene and DMSO excited by 490 nm blue light (a) and by 635 nm red light (b). Emission spectra of DAES in toluene and DMSO excited by 360 nm CW laser (c, excitation power is 26 mW cm−2) and 635 nm CW laser (d, excitation power is 1.2 W cm−2). (e) Normalized PL spectra of the samples in toluene and DMSO excited by 360 nm laser from Figure 4c. (f) Anti-Stokes emission photographs of DAES in toluene and DMSO excited by 635 nm CW laser at the excitation power of 641 mW cm−2 (the concentration of DAES is 5×10−4 mol L−1 both in toluene and in DMSO). Download figure Download PowerPoint At the same time, Irie et al.45 have reported that cyclization reaction could take place after absorbing very weak hot bands, which is longer than the 0–0 transition of open-ring isomer. Thus, it seems that the closed-ring isomers could also be excited by a hot band absorption process, which promotes electrons to excited state and gives anti-Stokes emission other than cycloreversion reaction. These reports indicated the abnormally large population of higher vibration–rotation levels of these materials at room temperature, not only for the open-ring isomers but also for the closed-ring isomers, although the origin of the effect is still not clear at present. Only minimal amount of DAES molecules could interact with the 635 nm laser. However, exceptionally strong UC emission was observed directly by naked eyes, indicating an extraordinary high UC emission yield. Since the absorption beyond 600 nm is extremely weak, a high concentration of 2 × 10−2 mol L−1 was used. The tested absorbance at 635 nm was 0.001, which enabled us to evaluate the SPA-UC efficiency by using a relative method with methyl blue as reference ( Supporting Information Figure S18). A UC efficiency of ∼ 13.5% was obtained, which is a considerably large value in UC systems. Although the absorption of DAES at 635 nm was not high, the SPA-UC quantum yield was relatively high. Since the theoretical value of UC efficiency should be 100%, there is still plenty of room for the future work. We recorded excitation spectra to explore the excitation boundary of SPA-UC by setting the emission wavelength at 570 nm ( Supporting Information Figures S19–S21). It was apparent that the SPA-UC emission was achievable by excitation from 600 to 700 nm, which is a really long band, compared with the absorption spectrum, indicating the population at higher vibration–rotation levels. To verify the possibility of long-wavelength excitation, we also tested the SPA-UC emission by applying a 680 nm CW laser. Compared with the UC behavior at 635 nm excitation, we observed relatively weak SPA-UC emission of DAES in both toluene and DMSO ( Supporting Information Figures S22 and S23) due to the larger energy gap, and the largest anti-Stokes shift in this SPA-UC system with 0.36 eV was recorded. Conclusions We report here a series of UC systems, constructed as turn-on mode fluorescent diarylethenes, and anti-Stokes emissions of closed-ring isomers based on single-photon absorption was demonstrated for the first time in these emissive diarylethenes. Our findings suggest that hot band absorption or phonon-assisted absorption might be the mechanism for the SPA-UC. By taking the advantages of diarylethenes, UC emission could be switched by UV and visible lights. Thus, the integration of SPA-UC and photochromism could provide a new frontier in photochemistry and physics. We envisaged that these remarkable properties would be applicable widely to the design of photofunctional materials. Supporting Information Supporting Information is available. Conflict of Interest There is no conflict of interest to report. Acknowledgments This work was supported by National Natural Science Foundation of China (51673050, 91856115, 21905065, and 21907060); the Ministry of Science and Technology of the People's Republic of China (2017YFA0206600 and 2016YFA0203400); P.D. thanks for the support of "New Hundred Talent Program" research fund of the Chinese Academy of Sciences. J.H. thanks for the support of China Postdoctoral Science Foundation (BX20180082 and 2019M650603). References 1. Zhou J.; Liu Q.; Feng W.; Sun Y.; Li F.Upconversion Luminescent Materials: Advances and Applications.Chem. Rev.2015, 115, 395–465. Google Scholar 2. Ye C.; Zhou L.; Wang X.; Liang Z.Photon Upconversion: From Two-Photon Absorption (TPA) to Triplet-Triplet Annihilation (TTA).Phys. Chem. 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