DOI: 10.1002/cphc.201300571

New Anthracene Derivatives as Triplet Acceptors forEfficient Green-to-Blue Low-Power UpconversionZuo-Qin Liang,[a] Bin Sun,[a] Chang-Qing Ye,[a] Xiao-Mei Wang,*[a] Xu-Tang Tao,[b]

Qin-Hua Wang,[c] Ping Ding,[a] Bao Wang,[a] and Jing-Jing Wang[a]

1. Introduction

Upconversion, that is, the processes of short-wavelength radia-tion from long-wavelength light sources, has been intensivelystudied due to its potential application in fields such as photo-voltaics,[1, 2] photocatalysis,[3] biological imaging and sensing,[4–7]

and photodynamic therapy of cancer.[8–10] A few techniques forupconversion have been well established, such as the two-photon absorption in organic molecules[11, 12] and the sequen-tial energy transfer upconversion in rare-earth ion-dopedglasses.[13, 14] Both need either a high power density (usually inthe order of kW cm�2 and MW cm�2) and/or a coherent excita-tion source. The extreme conditions limit the application ofthese upconversion techniques. Recently, a new method forupconversion, based on sensitized triplet–triplet annihilation(TTA), emerged as a promising wavelength-shifting technology.The excitation density required for TTA upconversion is quitelow. The energy density of a few mW cm�2 is sufficient to pro-duce efficient upconverted photons, and the excitation does

not need to be coherent. Thus, it is possible to use solar lightas the excitation source for TTA upconversion.

TTA upconvesion requires a triplet sensitizer and an acceptorto accomplish the cascade processes of light-harvesting, triple–triplet energy transfer (TTET), TTA, and upconverted fluores-cence emission. Over the past few years, tremendous advanceshave been made on the development of sensitizers by utilizingvarious heavy metal complexes, involving RuII polyimine com-plexes,[15–17] PtII/PdII porphyrin complexes,[18–22] PtII acetylidecomplexes,[23, 24] and cyclometalated IrIII complexes.[25] Many ex-periments indicate that triplet sensitizers with intense absorp-tion of visible light, and long triplet-excited-state lifetimes arehelpful to improve the TTA upconversion efficiency. Besides,some organic chromophores, which show intersystem crossingbetween the singlet state and the triplet state without heavymetal atoms, were selected as the triplet sensitizers.[27–29]

Compared to the development of triplet sensitizers, muchless attention has been paid to the development of triplet ac-ceptors.[30–33] To date, triplet acceptors are limited to a fewcommercially available compounds. In the process of TTA up-conversion, the sensitizer absorbs light and transfers it to theacceptor. Then, the singlet excited state of the acceptor decaysradiatively leading to upconversion fluorescence. Photophysi-cal properties of the acceptor thus also crucially influence theupconversion quantum yield. It is necessary for the develop-ment of new triplet acceptors to understand the common re-quirement for them. In this paper, we report three new anthra-cene derivatives, namely 2-chloro-9,10-dip-tolylanthracene(DTACl), 9,10-dip-tolylanthracene-2-carbonitrile (DTACN), and9,10-di(naphthalen-1-yl)anthracene-2-carbonitrile (DNACN), asorganic triplet acceptors for low-power upconversion.PdIIoctaethylporphyrin (PdOEP) was used as triplet sensitizer.The structures of the sensitizer and the acceptors employed in

Three new anthracene derivatives [2-chloro-9,10-dip-tolyl-anthracene (DTACl), 9,10-dip-tolylanthracene-2-carbonitrile(DTACN), and 9,10-di(naphthalen-1-yl)anthracene-2-carbonitrile(DNACN)] were synthesized as triplet acceptors for low-powerupconversion. Their linear absorption, single-photon-excitedfluorescence, and upconversion fluorescence properties werestudied. The acceptors exhibit high fluorescence yields in DMF.Selective excitation of the sensitizer PdIIoctaethylporphyrin(PdOEP) in solution containing DTACl, DTACN, or DNA-CN at

532 nm with an ultralow excitation power density of0.5 W cm�2 results in anti-Stokes blue emission. The maximumupconversion quantum yield (FUC = 17.4 %) was obtained forthe couple PdOEP/DTACl. In addition, the efficiency of the trip-let–triplet energy transfer process was quantitatively studiedby quenching experiments. Experimental results revealed thata highly effective acceptor for upconversion should combinehigh fluorescence quantum yields with efficient quenching ofthe sensitizer triplet.

[a] Dr. Z.-Q. Liang, B. Sun, Dr. C.-Q. Ye, Prof. X.-M. Wang, P. Ding, B. Wang,J.-J. WangJiangsu Key Laboratory for Environment Function MaterialsSchool of Chemistry, Biology and Material EngineeringSuzhou University of Science and TechnologySuzhou, 215009 (PR China)Fax: (+ 86) 512-68326615E-mail : wangxiaomei@mail.usts.edu.cn

[b] Prof. X.-T. TaoState Key Laboratory of Crystal Materials, Shandong UniversityJinan, 250100 (PR China)

[c] Prof. Q.-H. WangInstitute of Modern Optical TechnologiesKey Lab of Advanced Optical Manufacturing Technologies

of Jiangsu Province and Key Lab of Modern Optical Technologiesof Education Ministry of ChinaSoochow University, Suzhou, 215006 (PR China)

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the energy upconvesion system are shown in Figure 1. Theirlinear absorption, single-photon-excited fluorescence, and TTAupconversion fluorescence properties were studied. A high up-conversion quantum yield (FUC) of 17.4 % was obtained for the

couple PdOEP/DTACl upon excitation at 532 nm with an ultra-low excitation power density of 0.5 W cm�2. It was found thatfor an acceptor to be highly effective, it should combine highfluorescence quantum yield with efficient quenching of thesensitizer triplet.

2. Results and Discussion

The sensitizer PdOEP was prepared according to a method re-ported in the literature.[34] The synthetic strategies for the ac-ceptors (DTACl, DTACN, and DNACN) are depicted in Scheme 1.DTACl was obtained by the nucleophilic addition of lithiatedbromomethylbenzene to 2-chloroanthraquinone followed re-duction. DTACN was prepared by the Rosenmund–Brawn reac-tion between DTACl and CuCN in N-methyl pyrrolidone (NMP).The synthetic route to obtain DNACN was almost the same asthat of DTACN, with the only difference being the use of2-bromonaphthalene.

To investigate their photophysical properties, the absorptionand fluorescence spectra of PdOEP, DTACl, DTACN, and DNACNwere measured in DMF solution (1 � 10�6

m), as shown inFigure 2. The sensitizer PdOEP exhibits characteristic absorp-tion bands at 511 and 544 nm corresponding to the Q-bandand 354 nm, which corresponds to the Soret band. Upon exci-tation at 511 nm, PdOEP produces a singlet fluorescence at550 nm with a shoulder peak and long-lived triplet phosphor-escence at 663 nm. The absorption spectrum of DTACl showsthe characteristic vibronic pattern of an isolated anthracenegroup at about 359, 380, and 400 nm in DMF, as shown in Fig-ure 2 a. For DTACN and DNACN the characteristic vibronic pat-tern is observed as well, and their longest absorption bandsare centered at 416 nm. Compared with the longest absorptionband of DTACl, the bands of DTACN and DNACN are red-shift-ed by 16 nm. The redshift of the absorption spectra of DTACNand DNACN is due to the strong electron-withdrawing abilityof the cyano group. The fluorescence spectra of the three ac-ceptors are gradually red-shifted, as can be seen by their emis-sion bands, which are centered at 441 nm (DTACl), 446 nm(DTACN), and 452 nm (DNACN). The fluorescence quantumyields of the three acceptors in DMF were measured by using

quinine sulfate as standard. The fluorescence quantum yieldsof compounds DTACl, DTACN, and DNACN are found to be0.70, 0.53, and 0.65, respectively. Key photophysical data of theacceptors are listed in Table 1.

TTA upconversion is strongly dependent on the experimen-tal conditions, such as the concentration of the acceptors.Thus, the relationship between upconversion intensity andconcentration of the acceptors was studied. Figure 3 showsthe dependence of upconversion fluorescence intensity on therelative concentration of the acceptor/sensitizer upon excita-tion at 532 nm (2.33 eV) with a power density of 0.5 W cm�2 indegassed DMF. For the PdOEP/DTACl system, a blue emissionappears at 444 nm (2.79 eV), resulting in a net energy shift of0.46 eV between the excitation wavelength and the emittedphotons. The upconversion fluorescence intensity continues toincrease with the concentration of DTACl up to a maximum of5 � 10�4

m, after which a small decrease in the upconvertedemission intensity was observed. At the same time, the emis-sion intensity of PdOEP at 663 nm gradually weakens. In con-trast, the emission of PdOEP at about 550 nm is not quenchedat all in the presence of DTACl. This is because the triplet excit-ed state, instead of the singlet excited state of the sensitizer, isinvolved in the TTET process. PdOEP/DTACN and PdOEP/

Figure 1. Chemical structures of the sensitizer and the acceptors.

Scheme 1. Synthetic routes to DTACl, DTACN, and DNACN.

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DNACN systems also exhibited a blue emission upon excitationat 532 nm, as shown in Figure 3 b and c. Their upconversionfluorescence spectra reveal the same trends as observed forthe PdOEP/DTACl system. A compilation of anti-Stokes energyshifts is shown in Table 1. Excitation at 532 nm of the sensitizeror the acceptors alone did not produce the blue emissionband in the range of 400–532 nm. The blue upconversion fluo-rescence is very similar to the fluorescence spectrum of thecorresponding acceptor. Therefore, the emission at 400–532 nm should be ascribed to the TTA upconversion process.

It is interesting to note that upon increasing the acceptors’concentration, the phosphorescence of PdOEP was slightlyquenched, but the upconverted fluorescence was enhancedsignificantly. According to the photophysics of the TTA upcon-version, the quenched phosphorescence emission area shouldbe at least twofold that of the upconverted fluorescence emis-sion.[23, 35] It was thus considered that non-emission of the sen-sitizer at the triplet excited state is involved in the TTET pro-cess. Zhao et al. have developed a series of organic sensitizers

based on boron-dipyrromethane (BODIPY) that showed non-emissive triplet excited states but effective upconversion capa-bility.[28, 29] Chow et al. reported the sensitizer 2,4,5,7-tetraiodo-6-hydroxy-3-fluorone (TIHF), which shows no phosphorescenceat room temperature but also ideal upconversion properties.[27]

Therefore, it is possible to use weakly phosphorescent or non-phosphorescent compounds astriplet sensitizers for TTA upcon-version, as long as the triplet ex-cited state of these compoundscan be efficiently populatedupon photoexcitation.

Figure 4 displays the depend-ence of the upconversion quan-tum yield on the concentrationof the acceptors at a constantconcentration (1 � 10�5

m) of the

Figure 2. Normalized linear absorption (a) and single-photon-excited fluores-cence (b) of PdOEP, DTACl, DTACN, and DNACN in DMF.

Table 1. Photophysical parameters of the acceptors DTACl, DTACN, and DNACN.

Compound labmax

[nm]lem

[nm]F[a] Ksv

[b] FUC[c]

[%]Eem�Eex

[eV][d]

DTACl 400 441 0.70 8.9 � 104 17.4 0.46DTACN 416 446 0.53 1.0 � 105 16.7 0.45DNACN 416 452 0.65 7.1 � 104 12.7 0.41

[a] Fluorescence quantum yields. [b] Stern–Volmer quenching constants. [c] Upconversion quantum yields.[d] em = emission peak; ex = excitation peak.

Figure 3. Dependence of upconversion fluorescence intensity on the relativeconcentrations of the acceptors DTACl (a), DTACN (b), and DNACN (c) atfixed concentration of the sensitizer PdOEP (1 � 10�5

m) upon excitation at532 nm with a power density of 0.5 W cm�2 in degassed DMF.

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sensitizer. The upconversion quantum yield measurements ofDTACl, DTACN, and DNACN in DMF were measured againstrhodamine 6G in methanol with an excitation wavelength of532 nm according to the literature.[35] The upconversion quan-tum yield of DTACl increases with increasing concentration.When the concentration is 5 � 10�4

m, the upconversion quan-tum yield reaches its maximum (17.4 %). Increasing the concen-tration further, leads to weaker quantum yields. This is becausethe increase in DTACl concentration renders the encounter ofthe sensitizer and the acceptor more likely, which enhancesthe yields of the energy transfer process.[36] However, when theconcentration of DTACl exceeds the threshold value of5 � 10�4

m, the upconversion fluorescence is partly suppresseddue to the concentration quenching effect. Similar results wereobtained across the DTACN and DNACN concentration profiles.The maximum upconversion quantum yields for DTACN andDNACN are 16.7 and 12.7 %, respectively.

The TTET process between the sensitizer and the acceptorswas quantitatively studied by the photoluminescence quench-ing of the sensitizer PdOEP, with the acceptors as quenchers.The Stern–Volmer quenching curves were constructed, asshown in Figure 5. Stern–Volmer analysis yields quenching con-stants of 8.9 � 104, 1.0 � 105, and 7.1 � 104 for DTACl, DTACN,and DNACN, respectively. Thus, the efficiency of the TTET pro-

cess is DTACN>DTACl>DNACN. In contrast, the upconversionquantum yield follows the order DTACl>DTACN>DNACN. It isrevealed that the enhanced upconversion quantum yield doesnot simply result from the differences in quenching efficiency.The singlet excited state of the acceptor decays radiatively,leading to to upconversion fluorescence. Therefore, a good ac-ceptor should have a high fluorescence quantum yield.Measuring the fluorescence quantum yield, we found the fol-lowing order DTACl>DNACN>DTACN. It is believed thatDTACl’s increased fluorescence quantum yield with respect toDTACN is mainly responsible for the moderate increase in up-conversion efficiency. Compared to DNACN, the high upcon-version quantum yield of DTACN is proposed to be due to itsefficient quenching constants. These results manifest that largeStern–Volmer quenching constant and high fluorescence quan-tum yields for the acceptors are crucial for the improvement ofupconversion efficiency.

Selective excitation of PdOEP (1 � 10�5m) in the presence of

DNACN (1 � 10�4m) in degassed DMF with lex = 532 nm at vari-

ous incident excitation powers ranging from 1.03 to7.23 W cm�2 resulted in the upconverted fluorescence ofDNACN (Figure 6 a). Analysis of the sensitized upconverted in-tegrated emission intensity of DNACN as a function of the inci-dent light intensity is shown in Figure 6 b. These data werenormalized to the highest integrated emission intensity, as wellas to the highest incident power. The solid line passing

Figure 4. Dependence of the upconversion quantum yield on the relativeconcentration of the acceptors at a constant concentration (1 � 10�5

m) ofthe sensitizer upon excitation at 532 nm with a power density at 0.5 W cm�2.

Figure 5. Stern–Volmer analyses of the PdOEP triplet quenching by the ac-ceptors upon excitation at 532 nm with a power density at 0.5 W cm�2.

Figure 6. a) Upconverted emission intensity of DNACN following the selec-tive excitation of PdOEP (lex = 532 nm), measured as a function of incidentpower intensity. b) Normalized integrated emission intensity data from part(a), plotted as a function of the normalized incident power intensity of thelaser. Inset : Double-logarithmic plot of the normalized upconverted emissionof DNACN.

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through the data points represents the best quadratic fit (c2)to the data, illustrating the nonlinear nature of anti-Stokesemission process. The double logarithm plot in the inset ofFigure 6 b yields a straight line with a slope of 1.8 confirmingthe quadratic dependence of the sensitized annihilation pro-cess. The integrated upconverted fluorescence intensity ofDTACl and DTACN measured as a function of the incident laserpower was also evaluated. Their upconverted fluorescence isproportional to the square of the incident light power at532 nm.

The photon upconversion can be easily visualized in thePdOEP/DTACl, PdOEP/DTACN, and PdOEP/DNACN systems witha commercial green laser pointer (lex = 532 nm, peak power<5 mW). Figure 7 shows the photograph of the upconvertedfluorescence in degassed DMF solution containing PdOEP andDTACl. The naked eye image illustrates that low power isindeed realized in these chromophore mixtures.

3. Conclusions

The linear absorption, single-photon-excited fluorescence andTTA upconversion fluorescence properties of three acceptorshave been studied. These acceptors exhibited high fluores-cence yields in DMF. The upconversion properties of PdOEP/DTACl, PdOEP/DTACN, and PdOEP/DNACN were investigated asa function of acceptor concentration. Selective excitation ofthe dye cocktail at 532 nm with ultralow excitation power den-sity of 0.5 W cm�2 resulted in anti-Stokes blue emission fromthe acceptors. The highest FUC = 17.4 % was obtained for thecouple PdOEP/DTACl. In addition, the efficiency of the TTETprocess was quantitatively studied by quenching experiments.It has been found that upconversion quantum yield does notsimply result from the differences in quenching efficiency.Highly effective acceptors for upconversion should combinehigh fluorescence quantum yields with efficient quenching ofthe sensitizer triplet. It is important to note that the green-to-blue photon upconversion can be easily visualized in the threesensitizer/acceptor systems investigated here with a commer-cial green laser pointer.

Experimental Section

All reagents and materials used for the synthesis were commercialavailable. Dry tetrahydrofuran (THF) was freshly distilled oversodium prior to use. All other reagents were used as received. The1H NMR and 13C NMR spectra were recorded at 25 8C on a BrukerAvance 400 MHz spectrometer. Elemental analyses were performedon an Elementar Vario EL-III instrument. UV/Vis absorption spectrawere recorded on a Varian Cary 50 spectrophotometer. Fluores-cence measurements were carried out with a Hitachi F-4500 fluo-rescence spectrometer equipped with a 150 W Xe lamp.

A diode-pumped solid-state laser (emission wavelength: 532 nm)was used as the excitation source for the TTA upconversion. Thelaser power was measured with a photodiode detector. For the up-conversion experiments, the mixed solution of the triplet sensitizerand the acceptor was degassed for at least 30 min with N2. Thenthe solution was excited with the laser. The upconverted fluores-cence was observed with a PR655 SpectraScan colorimeter.

Synthesis of 2-chloro-9,10-dip-tolyl-9,10-dihydroanthracene-9,10-diol : Under a N2 atmosphere at �78 8C, 1.6 m tBuLi (25.0 mL,62.0 mmol) was added dropwise to a mixture of 1-bromo-4-meth-ylbenzene (5.50 mL, 41.0 mmol) and 40 mL dried THF. After the ad-dition, stirring was continued for 30 min. At �40~�50 8C, a solu-tion of 2-chloroanthraquinone (5.00 g, 30.8 mmol) in 60 mL THFwas injected through a syringe. The turbid solution changed fromwhite to wine red. The mixture was allowed to warm to room tem-perature and maintained for 24 h. The reaction mixture was addedto a saturated aqueous solution of NH4Cl (120 mL). The organic sol-vent was evaporated, and the suspension was extracted with ethylacetate. The organic extracts were washed by water and brine, anddried by anhydrous magnesium sulfate. Removing of the solventsafforded a yellow oil, which was used without further purification.

Synthesis of 2-chloro-9,10-dip-tolylanthracene (DTACl): 2-chloro-9,10-dip-tolyl-9,10-dihydroanthracene-9,10-diol (4.05 g, 9.0 mmol),sodium phosphinate monohydrate (16.6 g, 157 mmol), and potassi-um iodide (15.8 g, 95.0 mmol) in acetic acid (40 mL) were refluxedat 120 8C for 3 h. After the mixture was cooled, water was addedto it. The crude solid product was filtered and washed with water.The crude product was recrystallized using acetic acid to givea light yellow solid. Yield (3.27 g, 50.6 %). 1H NMR (CDCl3, 400 MHz)d= 2.54 (d, 6 H, J = 3.2 Hz), 7.20–7.23 (m, 1 H), 7.31–7.43 (m, 10 H),7.65–7.71 ppm (m, 4 H). 13C NMR (CDCl3, 100 MHz) d= 21.39, 21.41,125.18, 125.23, 125.47, 125.99, 127.02, 127.13, 128.21, 129.00,129.20, 129.30, 130.13, 130.39, 130.68, 131.01, 131.10, 131.11,135.28, 135.50, 136.39, 137.34, 137.38, 137.47 ppm. Elemental anal-ysis calc. for C28H21Cl: C, 85.59; H, 5.39; Cl, 9.02. Found: C, 85.62; H,5.52; Cl, 8.86.

Synthesis of 9,10-dip-tolylanthracene-2-carbonitrile (DTACN):A three-necked flask was charged with DTACl (0.72 g, 1.80 mmol),CuCN (0.97 g, 10.8 mmol), and NMP (30 mL) under N2 atmosphere.After the reaction mixture was refluxed for 24 h, the mixture wasallowed to cool to 70 8C. Then FeCl3 (5.80 g, 35.8 mmol) in concen-trated hydrochloric acid (10 mL) was added. The mixture wasstirred at 70 8C for 3 h. The reaction mixture was filtered, and theresidue was washed by water and dried under vacuum. The crudeproduct was purified by flash column chromatography over silicausing petroleum/dichloromethane (v/v= 6:1) as eluent to givea yellowlish green solid. Yield (0.46 g, 66.7 %). 1H NMR (CDCl3,400 MHz) d= 2.56 (d, 6 H, J = 3.6 Hz), 7.32–7.45 (m, 12 H), 7.74–7.80(m, 2 H), 8.16 ppm (s, 1 H). 13C NMR (CDCl3, 100 MHz) d= 21.40,108.23, 119.74, 123.80, 126.00, 126.59, 127.20, 127.50, 128.51,128.60, 129.34, 129.44, 129.92, 130.85, 131.01, 131.02, 131.86,

Figure 7. Photographs of the emission of PdOEP alone and the upconvertedfluorescence of PdOEP/DTACl. the excitation source was a commercial greenlaser pointer (lex = 532 nm).

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134.39, 134.87, 134.89, 137.68, 137.73, 137.98, 138.97 ppm. Elemen-tal analysis calc. for C29H21N: C, 90.86; H, 5.48; N, 3.65. Found: C,91.01; H, 5.57; N, 3.54.

Synthesis of 9,10-di(naphthalen-1-yl)anthracene-2-carbonitrile(DNACN): Analogous procedure as for the preparation of DTACN,but starting from 2-bromonaphthalene (6.40 g, 31.0 mmol). DNACNwas obtained as a yellow solid. Yield (0.32 g, 65.3 %). 1H NMR(CDCl3, 400 MHz) d= 7.34–7.44 (m, 3 H), 7.58-7.66 (m, 6 H), 7.76-7.83(m, 3 H), 7.93–7.98 (m, 4 H), 8.03–8.07 (m, 2 H), 8.12 (t, J = 8.0 Hz,2 H), 8.18 ppm (s, 1 H). 13C NMR (CDCl3, 100 MHz) d= 29.70, 53.41,108.61, 119.52, 124.14, 126.29, 126.61, 126.76, 126.88, 127.26,127.55, 127.98, 128.02, 128.11, 128.16, 128.35, 128.48, 128.56,128.65, 128.95, 129.08, 129.98, 130.26, 130.32, 130.96, 131.91,132.95, 133.05, 133.33, 133.39, 134.77, 134.91, 135.39, 137.76 ppm.Elemental analysis calc. for C35H21N: C, 92.28; H, 4.65; N, 3.07.Found: C, 92.54; H, 4.72; N, 2.74.

Acknowledgements

The authors are grateful to the National Natural Science Founda-tion of China (50973077, 51273141), the Natural Science Founda-tion of JiangSu Province Education Committee (11KJA430003),the Project of Person with Ability of Jiangsu Province (2010-xcl-015), the Priority Academic Program Development of JiangsuHigher Education Institutions (PAPD), the Project of Science andTechnology of Suzhou (SYG201204), and the Opening Project ofKey Lab of Advanced Optical Manufacturing Technologies ofJiangsu Province (KJS1102) for financial supports.

Keywords: anthracene · fluorescence · Stern–Volmerquenching constants · triplet acceptors · upconversion

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CHEMPHYSCHEMARTICLES www.chemphyschem.org

ARTICLES

Z.-Q. Liang, B. Sun, C.-Q. Ye, X.-M. Wang,*X.-T. Tao, Q.-H. Wang, P. Ding, B. Wang,J.-J. Wang

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New Anthracene Derivatives as TripletAcceptors for Efficient Green-to-BlueLow-Power Upconversion

Three against three: Organic acceptorsare used for triplet–triplet-annihilationupconversion and upconversion quan-tum yields of up to 17.4 % are observed.The efficient triplet–triplet-annihilationupconversion is attributed to an effi-cient quenching of the sensitizer tripletand the high fluorescence quantumyields.

� 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim ChemPhysChem 0000, 00, 1 – 7 &7&

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