Organic Electronics 12 (2011) 202–209

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Organic Electronics

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Electrochromic and optical studies of solution processablebenzotriazole and fluorene containing copolymers

Emine Kaya a, Abidin Balan a, Derya Baran a, Ali Cirpan a,c,⇑, Levent Toppare a,b,c

a Department of Chemistry, Middle East Technical University, 06531 Ankara, Turkeyb Department of Biotechnology, Middle East Technical University, 06531 Ankara, Turkeyc Department of Polymer Science and Technology, Middle East Technical University, 06531 Ankara, Turkey

a r t i c l e i n f o a b s t r a c t

Article history:Received 4 October 2010Received in revised form 1 November 2010Accepted 3 November 2010Available online 20 November 2010

Keywords:BenzotriazoleFluoreneOptoelectronicConjugated copolymersDonor–acceptor

1566-1199/$ - see front matter � 2010 Elsevier B.Vdoi:10.1016/j.orgel.2010.11.001

⇑ Corresponding author at: Department of CheTechnical University, 06531 Ankara, Turkey. Tel.: +9

E-mail address: acirpan@metu.edu.tr (A. Cirpan)

2-Dodecyl benzotriazole and 9,9-dioctylfluorene containing alternating copolymerspoly((9,9-dioctylfluorene)-2,7-diyl-(2-dodecyl-benzo[1,2,3]triazole)) (P1), poly((9,9-dioc-tylfluorene)-2,7-diyl-(4,7-bis(thien-2-yl) 2-dodecyl-benzo[1,2,3]triazole)) (P2), poly((9,9-dioctylfluorene)-2,7-diyl-(4,7-bis(3-hexylthien-5-yl) 2-dodecyl-benzo[1,2,3]triazole)) (P3)were synthesized via Suzuki polycondensation. Electronic and optical properties of result-ing polymers showed that all polymers are both p and n dopable and have multicoloredelectrochromic states. HOMO and LUMO values, band gaps and spectral response of poly-mers upon externally applied potential and trifluoro acetic acid (TFA) addition were givenin detail.

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1. Introduction

Conjugated polymers (CPs) have been extensively stud-ied for their potentials as active materials in numerousindustrial applications namely, electrochromics (ECDs)[1], light emitting diodes (OLEDs) [2], photovoltaic devices(OPVs) [3], and thin film transistors (OFETs) [4]. They com-bine the physical properties of polymers with those ofsemiconductors to obtain unique and novel materials.Since the small structural modifications on polymer back-bone allow tuning of band gap and hence their optical andelectronic properties, there have been several methods toobtain polymers with desired properties [5]. However, foroptoelectronic applications, recent research interest havemainly focused on donor–acceptor (DA) type materials inwhich alternating electron rich and electron deficientgroups are both present on the backbone [6]. Electronicand optical properties of these types of polymers are tunedefficiently by intramolecular charge transfer (ICT) [7]. In

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mistry, Middle East0 3122105103.

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terms of electrochromic devices, the use of DA type CPsas active layers became more popular over the time dueto their variation of optical properties via structural alter-nations [8]. Possibility for applications of CPs to large andflexible display devices makes their development signifi-cant for future display technologies [9].

Poly-9,9-dialkylfluorenes (PFOs) and their copolymerswith different groups are considered as some of the mostpromising materials to be used in organic electronics andhave been examined quite extensively up to date [10].Combination of PFOs with an acceptor or a donor–acceptor–donor (DAD) group ensures that the band gapof the polymer can be modified according to device appli-cations [8]. Alkylation of fluorene provides high solubilityand stabilizes the carbon atom on 9-position; however,unsubstituted fluorenes form fluorenones which disturbeffective conjugation on polymer backbone. These PFOcopolymers, particularly benzothiadiazole (BTd) deriva-tives, were used in optoelectronic applications and insert-ing BTd units in PFO chains resulted in highly efficientOPVs, OLEDs and OFETs [11]. On the other hand, benzotri-azole (BTz) was introduced to the PFOs in few studieswhere it was used as internal molecular additive [12].

E. Kaya et al. / Organic Electronics 12 (2011) 202–209 203

However, properties of alternating copolymers with BTzgroups with fluorene are yet lack of scientific inventory.We expect that the use of BTz may bring an improvementon the several properties of such devices.

Recently, BTz containing DAD type polymers has beenreceived a great deal of attention in optoelectronic re-search due to their ease of synthesis and tremendous opti-cal and electronic properties [13]. DAD type polymers ofBTz with different donor units were shown to be multipur-pose materials for device applications such as ECDs andOPVs with promising results [14].

Here, we report the synthesis and characterizations ofbenzotriazole and PFO containing alternating copolymersP1, P2, and P3. Electronic doping in thin film formed byexternally applied potential revealed that polymers P2and P3 are great candidates for multicolored electrochro-mic device applications. Furthermore, oxidative doping ofpolymers in toluene resulted in multi-colored solution

Scheme 1. Synthetic pathw

phases due to varied oxidation states. Electronic and opti-cal properties of new alternating fluorene–BTz copolymersand comparison of results with those of parent polymers(PFO, PTBT, and PHTBT) were given in detail.

2. Results and discussion

2.1. Synthesis

Synthetic pathway for polymers; poly((9,9-dioctylflu-orene)-2,7-diyl-(2-dodecyl-benzo[1,2,3]triazole)) (P1), poly((9,9-dioctylfluorene)-2,7-diyl-(4,7-bis(thien-2-yl) 2-dode-cyl-benzo[1,2,3]triazole)) (P2), poly((9,9-dioctylfluorene)-2,7-diyl-(4,7-bis(3-hexylthien-5-yl) 2-dodecyl-benzo [1,2,3]triazole)) (P3) is outlined in Scheme 1. The synthesis of 4,7-bis(thien-2-yl) 2-dodecyl-benzo[1,2,3]triazole (TBT) [13a]and4,7-bis(3-hexylthien-5-yl) 2-dodecyl-benzo[1,2, 3]tria-zole (HTBT) [13b] was reported previously by our group.

ay for P1, P2, and P3.

204 E. Kaya et al. / Organic Electronics 12 (2011) 202–209

TBT and HTBT were brominated by N-bromosuccinimide(NBS) in chloroform (CHCl3). All polymers (P1, P2,and P3) were synthesized by Suzuki cross-coupling reac-tions [15]. Obtained polymers were purified by Soxhletextraction using acetone and structurally characterizedby 1H NMR. All polymers are highly soluble in common or-ganic solvents such as chloroform, dichloromethane, THF,and toluene. The average molecular weight (Mn) of P1,P2, and P3 determined by gel permeation chromatography(GPC) using polystyrene as the standards is 5900, 6900,and45,000 g/mol, respectively, with relatively low polydis-persity indexes. All the polymers showed excellent thermalstability with decomposition temperature (measured byTGA, 10 �C/min) of more than 350 �C. There was no transi-tion observed up to 300 �C.

2.2. Cyclic voltammetry

For electrochemical characterization, polymers weredissolved in chloroform to a concentration of 5 mg/mLand spray coated on indium tin oxide (ITO) coated glassslides. The polymer films on ITO were subjected to cyclicvoltammetry (CV) in ambient conditions to determine theredox potentials in 0.1 M tetrabutylammonium hexa-fluorophosphate (TBAPF6)/acetonitrile (ACN) solutions ata scan rate of 50 mV/s. The potentials were swept between1.5 and �2.4 V for P1, 1.4 and �2.3 for P2 and 1.4 V and

Fig. 1. Cyclic voltammograms for spray coated polymer films of P1, P2,and P3 in ACN/TBAPF6 solution at a scan rate of 50 mV/s.

Table 1Oxidation potentials and onset values for both p- and n-type doping, estimated HO

Oxidation potential (V) Reduction potential (V)

Eox Eonsetox

Ered Eonsetred

P1 1.43 1.19 �2.35 �1.78P2 1.16 0.96 �2.16 �1.60P3 1.25 0.98 �2.10 �1.70

a Calculated from CV.

�2.1 V for P3 versus Ag wire pseudo-reference electrode(50 mV versus Fc/Fc+). Initially, polymer films were sub-jected to a repeated cycling to obtain electrochemicallystable and reproducible doping–dedoping (Fig. 1).

P1 film revealed reversible redox couple at positivepotentials pointed at 1.43 and 1.19 V versus Ag wirepseudo-reference electrode. Due to incorporation of elec-tron donating thiophene rings in P2 chains, polymer filmshowed lower redox potentials at (1.16 and 0.96 V) com-pared to those of P1 versus the same reference electrode(Fig. 1). The highest oxidation potential was observed forP1 among all as expected since it possesses electron poorstructure relative to P2 and P3.

True n-type doping properties of the polymers wereinvestigated by CV which also enabled the determinationof HOMO and LUMO levels for polymers electrochemically.Reversible doping/dedoping peaks confirmed that all poly-mers were both n-type dopable at negative potentials.Reduction of P2 film was achieved at �2.17 V and the cor-responding de-doping peak at �1.96 V which were �2.07/�1.8 V for P3 and �2.35/�1.96 V for P1.

Due to their dual properties, HOMO/LUMO energy lev-els were calculated precisely from the onset potentials ofp and n doping peaks from CV. The onset values were esti-mated by taking the intersection between the baseline andthe tangent line drawn to the increasing part of the cur-rent. The reference electrode and all the results were sub-sequently calibrated to Fc/Fc+ and the band energies werecalculated relative to the vacuum level considering thatthe value of NHE is �4.75 eV versus vacuum. Table 1 sum-marizes the electrochemical data for polymers P1, P2, andP3. As seen from table, higher onset potentials for P3 dueto sharper p and n doping peaks than those of P2 resultedin slightly higher HOMO energy and a wider electronicband gap. The electrochemical band gap values were calcu-lated from the onset potentials as 2.56 and 2.68 eV for P2and P3, respectively. Calculated electrochemical band gapswere comparatively higher than the ones estimated fromp–p* transition due to the creation of free ions by meansof applied potential [16].

In order to have a better understanding on structure–electronic properties relationship for PFO copolymers, theywere compared with previously reported parent polymersPFO, PTBT, and PHTBT in terms of their HOMO/LUMO en-ergy levels (Chart 1). p–p interaction between donor andacceptor determines the match between these units andplays a crucial role on the electronic properties of theresulting polymers since it influences the intramolecularcharge transfer in these types of polymers [7a]. Fig. 2shows that the addition of fluorene unit in PTBT did not

MO–LUMO energies and band gaps of synthesized polymers P1, P2, and P3.

Band gap (eV) Energy levelsa (eV)

Eecg Eop

g HOMO LUMO

2.97 2.55 �5.94 �2.972.56 2.16 �5.71 �3.152.68 2.24 �5.73 �3.05

Chart 1. Chemical structures of related polymers.

E. Kaya et al. / Organic Electronics 12 (2011) 202–209 205

significantly alter the electronic band gap of the resultingpolymer P2 (2.56 eV), but only lowered the HOMO–LUMOlevels ca. 0.1 eV. Considering the energy levels of P3, LUMOlevel was intact upon addition of PFO, however; HOMO ofthe polymer was lowered due to the contribution ofvalence band (VB) of PFO and resulted in a higher electro-chemical band gap (Fig. 2).

Although PHTBT has slightly lower band gap than PTBT,thiophene containing polymer P2 revealed narrower bandgap than that of its hexyl thiophene included analogue P3.This may be due to the steric interactions between the hex-yl groups and the adjacent fluorene units. The same trendalso is the case for benzothiadiazole bearing copolymers[18]. According to donor–acceptor approach, increasingdonor strength should have raised the HOMO level whilehaving less effect on LUMO energy. However, here bothHOMO and LUMO levels were affected by incorporationof electron rich thiophene and hexyl thiophene units to P1.

2.3. Spectral properties

Neutral P1, P2, and P3 films were spray coated on ITOcoated glass slides from a solution of 3 mg/mL in chloro-form. UV–vis–NIR spectra were recorded for correspondingpolymer films in 0.1 M TBAPF6/ACN solution upon gradu-ally increased external bias. Coated films were subjected

Fig. 2. HOMO/LUMO levels and band gaps of related polymers. Values for PFO

to subsequent redox cycling by CV to obtain electrochem-ical stability prior to spectroelectrochemical analysis. Fig. 4exhibits that kmax values for the p–p* transitions for P2 andP3 were centered at 503 and 460 nm, respectively. Substi-tution of 9,90-dioctyl fluorene units resulted in blue shiftedabsorption for polymers P2 and P3 compared to their pre-viously studied parent polymers PTBT and PHTBT. Step-wise oxidation of the polymer films by applied externalpotentials resulted in a decrease in the p–p* transitionsand their lower energy transitions intensified at 715 and1560 nm for both P2 and P3. These absorbance bands innear-IR (NIR) region indicate the formation of lower energycharge carriers such as polarons and bipolarons. Visiblepolaronic absorbance for P2 and P3 allowed detection ofdifferent colored states upon stepwise oxidations. How-ever, their electron poor homologue P1 revealed blueshifted dominant wavelengths both in visible and in NIRregions (Fig. 3) which were consistent with electrochemi-cal data. Electron deficient structure of P1 complicatedthe formation of positive charge carriers on the polymerbackbone upon oxidative doping. Thus, the neutral stateabsorption in the visible region (430 nm) did not reveal asignificant depletion as P2 and P3 but hypsochromicallyshifted to 380 nm. Although the oxidation potential forP1 was determined as 1.43 V, spectroelectrochemical datawere recorded up to 1.3 V since at higher potentials nospectral change was observed. This indicates the limitedformation of polaronic and bipolaronic bands for P1(Fig. 3).

Since neutral state absorptions for P2 and P3 are ataround 400 nm and lower energy transition emerged at700 nm, partially oxidized polymer films revealed greencolor for which these two absorption maxima are crucial.Further oxidation of polymer films resulted in blue colordue to diminished p–p* transition intensity.

Highly soluble polymers P2 and P3 were dissolved intoluene (8 � 10�3 mg/mL) and oxidatively doped by thestepwise addition of trifluoroacetic acid (TFA). TFA addi-tion to solutions was continued until no spectral changewas observed. As seen from the spectra (Figs. 4 and 5), ini-tially neutral P2 and P3 have absorption maxima at 475and 446 nm, respectively, in solution. Upon addition of

[17], PTBT [13a], and PHTBT [13b] were taken from previous reports.

Fig. 4. (A) UV–vis–NIR spectra of P2 in film form (left) and in toluene (right). (a) 0.5, (b) 0.8, (c)1.0, (d) 1.2, and (e) �2.16 V and (a0) 0%, (b0) 5%, (c0) 10%, (d0)20%, (e0) 30%, (f0) 40% (g0) 50% TFA (v:v) and (h0) dedoped form with 1 mL TEA. (B) UV–vis–NIR spectra of P3 in film form (left) and in toluene (right). (a) 0.5,(b) 0.8, (c) 1.0, (d) 1.2, (e) �2.0, and (f) �2.10 V and (a0) 0%, (b0) 5%, (c0) 10%, (d0) 20%, and (e0) 30% TFA (v:v).

Fig. 3. Electrochemical p-type doping electronic absorption spectra of P1 between 0.0 and 1.3 V with 0.1 V potential intervals.

206 E. Kaya et al. / Organic Electronics 12 (2011) 202–209

Fig. 5. Kinetic switching results in both visible and NIR regions for P1 (right) and P2 (left).

E. Kaya et al. / Organic Electronics 12 (2011) 202–209 207

5% TFA (v:v) new absorption bands emerged at 750 and1550 nm which are accompanied by a decrease in neutralstate absorptions. The sharp peaks for the p–p* transitionsresulted in wider band gaps for polymers in solution thanin film due to trapped dopant ions and the more densepolymer morphology in film form. The oxidative dopingalso resulted in color change in solutions. Neutral solutionsof both polymers were yellow however upon the additionof TFA, the solution turned into green and blue as is shownas inset picture in Fig. 41 for P2 and P3, respectively.

After doping, 1 mL of triethyl amine (TEA) was added tosolutions which resulted in immediate appearance of theinitial colors for both polymer solutions. This indicatesthe reversibility of solution doping process by adding TFAand pH responsive characters of polymers. It is worth not-ing that spectral responses of the polymers to stepwiseoxidation were identical in both solution and thin filmforms regardless of the doping or dopant type. Any coloror spectral change was not observed by TFA addition toP1 although reversible doping (both p and n) andde-doping were possible in thin film form electrochemi-cally. This obviously indicates the effect of the presenceof thiophene unit on solution doping process. Oxidationstarts on electron rich thiophene heterocycle prior tocharge transport along the conjugated polymer backbone(Fig. 4B).

Absorption spectra for P2 revealed isosbestic point inthin film form which confirms the coexistence of discretechromophores which is not taking place in solution. How-ever, P3 showed isosbestic point both in film and in solu-tion indicating well-separated chromophores. That alsoeased the detection of different colors for P3 solutionsupon TFA addition in toluene.

The electrochemical stability as well as percent trans-mittance changes can be monitored by successive potentialcycling of the polymer films in monomer-free solutions.Optical studies were carried out for polymers P1 and P2in ACN/TBAPF6 solution in order to observe the changes

1 For interpretation of color in Fig. 4, the reader is referred to the webversion of this article.

between their neutral and fully oxidized states uponapplied external bias within 5 s time intervals. Percenttransmittance changes were recorded for P2 at its domi-nant wavelengths at 485 and 1500 nm. The polymer filmrevealed 22% optical contrast with a switching time of3.5 s in visible region and 45% transmittance change(3.5 s) was recorded between doped and de-doped statesin NIR region (Fig. 5). Although the switching times wererelatively slow, the polymer film was stable upon repeti-tive cycles. Due to the high rigidity in P3 films which stemsfrom the long alkyl chains on both fluorene, thiophene andbenzotriazole units, reasonable kinetic values could not beacquired. Since P1 revealed a blue shift for high energytransition upon oxidation, it showed relatively high opticalcontrast for the one observed at 431 nm (Fig. 5).

3. Conclusion

Multicolored electrochromic alternating PFO and BTzcopolymers P1, P2 and P3 were synthesized and investi-gated in terms of their electronic and optical properties.Convenient HOMO and LUMO levels, band gaps, and strongabsorptions in the visible region suggest that they shouldbe tested for various optoelectronic applications. Solutiondoping of P2 and P3 revealed multicolored solutions whichmake them pH responsive chromophores that may be usedin different applications in due course. Reported propertiesprove that fluorene and BTz-based copolymers are multi-purpose materials which will be subject of further researchinterest in near future.

4. Experimental

4.1. General

All chemicals were used without further purification asthey were purchased from commercial sources. All reac-tions were carried out under argon atmosphere unlessotherwise mentioned. Electrochemical studies were per-formed in a three-electrode cell consisting of an indiumtin oxide doped glass slide (ITO) as the working electrode,

208 E. Kaya et al. / Organic Electronics 12 (2011) 202–209

platinum wire as the counter electrode, and Ag wire as thereference electrode under ambient conditions using a Vol-talab 50 potentiostat. The value of normal hydrogen elec-trode (NHE) was taken as �4.75 eV [19]. 1H and 13C NMRspectra were recorded in CDCl3 on Bruker SpectrospinAvance DPX-400 Spectrometer. Chemical shifts were givenin ppm downfield from tetramethylsilane. Varian Cary5000 UV–vis spectrophotometer was used to perform thespectroelectrochemical studies of the polymer. Averagemolecular weight was determined by gel permeation chro-matography (GPC) using a Polymer Laboratories GPC 220.Differential scanning calorimetry (DSC) and thermalgravimetric analysis (TGA) were performed on PerkinElmer Diamond DSC and Perkin Elmer Pyris 1 TGA. Trifluo-roacetic acid (TFA) used for solution doping was 99%.2-Dodecylbenzotriazole [13], 4,7-dibromo-2-dodecylbenzotriazole [13], tributyl(thiophen-2-yl) stannane[20], tributyl(4-hexylthiophen-2-yl)stannane [21], 4,7-bis(thien-2-yl) 2-dodecyl-benzo[1,2,3]triazole (TBT) [13a],4,7-bis(3-hexylthien-5-yl) 2-dodecyl-benzo[1,2,3]triazole(HTBT) [13b] were synthesized according to previouslypublished procedures.

4.2. Synthesis of 4,7-bis(5-bromothien-2-yl)-2-dodecylbenzo[1,2,3]triazole (7)

Two hundred milligrams of TBT (0.44 mmol) and190 mg of N-bromosuccinimide (1.07 mmol) were stirredin 100 mL of CHCl3 at room temperature by preventingthe mixture from light exposure. After 12 h, solvent wasremoved under reduced pressure and crude product wasfiltered over silica by CHCl3 to obtain 250 mg (93%) 7 as yel-low solid. 1H NMR (400 MHz, CDCl3, d): 7.72 (d, J = 5.6 Hz,2H), 7.44 (s, 2H), 7.04 (d, J = 6.0 Hz, 2H), 4.72 (t, J = 7.0 Hz,2H), 2.10 (m, 2H), 1.32–1.17 (m,18H), 0.80 (t, J = 6.9 Hz,3H); 13C NMR (100 MHz, CDCl3-d6, d): 144.4, 142.2, 140.4,127.3, 125.8, 123.9, 123.0, 57.1, 32.2, 30.3, 29.9, 29.8,29.7, 29.6, 29.5, 29.3, 26.9, 22.9, 14.4. MS (m/z): 608 [M+].

4.3. Synthesis of 4,7-bis(5-bromo-4-hexylthien-2-yl)-2-dodecylbenzo[1,2,3]triazole (8)

Bromination of HTBT was performed with the sameprocedure explained for TBT above. Two hundred milli-grams of HTBT (0.32 mmol) and 138 mg of N-bromosuc-cinimide (0.77 mmol) were used to obtain 235 mg(94.4%) 8 as yellow solid. 1H NMR (400 MHz, CDCl3, d):7.69 (s, 2H), 7.41 (s, 2H), 4.77 (t, J = 8.0 Hz, 2H),2.61(t, J = 8.0 Hz, 4H) 2.16 (m, 2H), 1.65 (m, 4H),1.41–1.24 (m, 26H), 0.91–0.85 (m, 9H); 13C NMR(100 MHz, CDCl3, d): 143.1, 141.7, 139.2, 127.5, 123.0,122.0, 110.1, 56.8, 32.0, 31.7, 30.0, 29.8, 29.7, 29.6, 29.5,29.4, 29.1, 29.0, 26.7, 22.7 14.1. MS (m/z): 778 [M+].

4.4. General route for synthesis of P1, P2, and P3

Dibromo compound (1 mol equivalent), 2,20-(9,9-dioc-tyl-9H-fluorene-2,7-diyl)bis(1,3,2-dioxaborinane) (1 molequivalent), Pd(PPh3)4 (5 mol%), and tetrabutylammoniumiodide (N(Bu)4I, 1 mol%) were added into a degassedmixture of potassium carbonate (K2CO3, 2 M in H2O),

toluene (3:2 toluene:water). The mixture was refluxed at115–120 �C for 48 h under argon. Solvent was removedand extracted with CHCl3:H2O twice to remove alkali solu-tion. Combined organic layers were dried over anhydrousMgSO4 and evaporated under reduced pressure. The crudeproduct was washed with methanol and Soxhlet extractedby acetone to obtain corresponding polymer. P1: 1H NMR(400 MHz, CDCl3, d): 8.20 (benzotriazole), 8.1 (fluorene),7.9 (fluorene), 7.7 (fluorene), 4.8 (N–CH2), 2.2 (C–CH2),1.6–0.7 (pendant alkyl chains). GPC: Mn: 5900, Mw:10,600, PDI: 1.8, P2: 1H NMR (400 MHz, CDCl3, d): 8.1 (ben-zotriazole), 8.8–8.5 (fluorene, thiophene), 7.4 (thiophene),4.8 (N–CH2), 2.2 (C–CH2), 1.6–0.7 (pendant alkyl chains).GPC: Mn: 6900, Mw: 17,500, PDI: 2.5, P3: 1H NMR(400 MHz, CDCl3, d): 8.00 (benzotriazole), 7.8 (fluorene),7.6 (fluorene), 7.4 (fluorene, thiophene), 4.9 (N–CH2), 2.8(CH2), 2.2 (C–CH2), 1.8–0.7 (pendant alkyl chains). GPC:Mn: 45,000, Mw: 183,000 PDI: 4.1.

Acknowledgements

The authors thank TUBA for financial support.

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