Strategy Toward Tuning Emission of Star-Shaped Tetraphenylethene

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C: Plasmonics; Optical, Magnetic, and Hybrid Materials

Strategy Towards Tuning Emission of Star Shaped Tetraphenylethene Substituted Truxenes for Sky-Blue and Greenesh-White OLEDs Rekha Sharma, Dmytro Volyniuk, Charu Popli, Oleksandr Bezvikonnyi, Juozas Vidas Grazulevicius, and Rajneesh Misra J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b00777 • Publication Date (Web): 14 Jun 2018 Downloaded from http://pubs.acs.org on June 14, 2018

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The Journal of Physical Chemistry

Strategy Towards Tuning Emission of Star Shaped Tetraphenylethene Substituted Truxenes for Sky-Blue and Greenish-White OLEDs Rekha Sharma,a Dmytro Volyniuk,b Charu Popli,a Oleksandr Bezvikonnyi,b Juozas V. Grazulevicius,b Rajneesh Misra*a a

Department of Chemistry, Indian Institute of Technology Indore, 452017, India. Fax: +91 731 2361

482; Tel: +91 731 2438 754 *E-mail- [email protected] b

Department of Polymer Chemistry and Technology, Kaunas University of Technology, Radvilenu pl.

19, LT-50254, Kaunas, Lithuania; e-mail: [email protected]

Abstract Star shaped, C3-symmetric, tetraphenylethene (TPE) and 2,3,3-triphenyl acrylonitrile (TPAN) substituted truxenes 7, 8 and 9 were designed and synthesized by the Pd-catalyzed Suzuki and Sonogashira cross-coupling reactions. The TPE substituted truxenes 7 and 8 show aggregation-induced emission (AIE) behaviour whereas TPAN substituted truxene 9 shows aggregation-caused quenching (ACQ) effect in THF/water medium due to the π-π stacking. The computational calculation on truxenes 7–9 was performed, which reveals that, in truxene 9 the electron density transfers from truxene to TPAN. The truxenes 7–9 showed high thermal stability as the 10% weight loss temperature is more than 400 °C. The ionization potentials close to 6.0 eV were estimated for the solid samples of truxenes 7–9 by photoelectron emission spectrometry. Solid samples of the studied truxenes exhibited strong emission with high quantum yields (up to 47%). Electroluminescent properties of truxene derivatives 7–9 were investigated in solution-processed and vacuum-deposited organic light emitting diodes. Greenish-white non-doped organic light-emitting diodes with maximum brightness of 7000 cd/m2 and maximum external quantum efficiency of 3.8% were fabricated using truxene-cored compound 7 as fluorescent emitter.

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Introduction Global energy consumption may be significantly reduced by broad usage of white organic light-emitting diodes (WOLEDs) with healthy white electroluminescence for solid-state lighting.1 Due to the commonly accepted advantages such as mechanical flexibility, light weight, transparency, WOLEDs have great potential for illumination.2 It is a considerable challenge to develop WOLEDs with high-quality white electroluminescence since two (skyblue and orange) or three (blue, green and red) emitters producing excitons usually are utilised in multilayer WOLED structures in which the population of each colour excitons should be precisely controlled.3–6 It is difficult to get stable white electroluminescence at different voltages.7 In addition, WOLEDs are usually based on doped light-emitting layers due to the usage of emitters which suffer from aggregation induced quenching.8,9 Hence, such WOLEDs are rather complicated due to the application of several emitters and doping. In order to simplify WOLED structures, some interesting approaches were recently demonstrated including exciplex-based non-doped WOLEDs. For example, WOLED based on non-doped light-emitting layer containing small and simple organic compound which emitted exciton/electromer emission and formed interface exciplex/electroplex resulting in white electroluminescence was fabricated.10 Interface engineering method was demonstrated for the preparation of efficient non-doped WOLED by embedding an ultrathin layer of an ambipolar

red

emissive

compound

in

the

exciplex

formation

region.11

White

electroluminescence of these non-doped WOLEDs is based on the emission originating from two different organic materials. To obtain highly-efficient OLEDs of the simple structure, emitters with photoluminescence quantum yields (PLQY) close to 100% in solid-state are required. This requirement is practically very difficult to achieve for OLEDs containing non-doped light-emitting layers due to the aggregation caused quenching of emission observed for most organic emitters. To 2 ACS Paragon Plus Environment

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overcome these difficulties, organic molecular systems exhibiting aggregation induced emission (AIE) have gained significant attention of the scientific community due to their application in the field of electroluminescent devices.12 Truxene (10,15-dihydro-5Hdiindeno[1,2-a;10,20-c]fluorene) is a planar, C3-symmetric polyaromatic hydrocarbon, which is a fusion of three fluorene moieties.13 The truxene scaffold is a potential building block for the construction of larger molecular architectures, due to its easy functionalization, rigid structure, and high thermal and chemical stability.14,15 Recent reports on truxene based donor–acceptor systems reveal that these molecular systems are potential candidates for application in two-photon absorption, organic light-emitting diodes (OLED), organic solar cells (OSC) and organic fluorescent probes.16-19 In addition, truxene-cored emitters can be good candidates in the search for compounds exhibiting white emission of a single molecule. Solid films of truxene-cored emitters are characterized by blue emission and by low-energy emission band which appears due to aggregation.20,21 Therefore, nature white emission can be obtained by mixing blue and orange emissions of exclusively truxene-based emitters. Mixing of blue and orange emissions is widely used approach in the design and fabrication of white OLEDs.22 Literature reveals that the conventional fluorophores suffer from aggregation-caused quenching (ACQ) effect. This problem can be overcome by introducing the concept of AIE.23,24 Tetraphenylethylene (TPE) and 2,3,3-triphenylacrylonitrile (TPAN) are propeller shaped AIE active molecules, which have the ability to promote aggregation induced emission in different fluorophores.25,26 The AIE active molecules are poorly emissive in solution but highly emissive in solid state.27 Our group is involved in the design and synthesis of TPE substituted donor-acceptor systems for their application in mechanochromism and optoelectronic devices.26 Recently, we have reported the AIE and mechanochromic studies of TPE substituted pyrenoimidazoles and 3 ACS Paragon Plus Environment


phenanthroimidazoles.28 To the best of our knowledge, there are no reports available on the TPE substituted truxenes. Therefore, in the context of our research on the TPE functionalized systems and to study the effect of AIE active TPE unit on the photonic properties of truxene core, the TPE and TPAN substituted truxene 7, 8 and 9 have been synthesised. Their photophysical and thermal properties were studied and DFT calculations were performed. It was shown that these compounds exhibit AIE effect resulting in high PLQY in solid state. Moreover, white emission was observed for the films of truxene 7 deposited by vacuum evaporation technique mostly due to the aggregation process. Depending on deposition method of light-emitting layers of the developed compounds, greenish-white and sky-blue OLEDs were fabricated by either wet or vacuum technologies. We note that simplified WOLEDs were fabricated using one non-doped light-emitting layer based on only one truxene-cored emitter exhibiting AIE effect. Experimental section: General: Chemicals were used as received unless otherwise indicated. All oxygen or moisture sensitive reactions were performed under nitrogen/argon atmosphere using standard Schlenk method. Triethylamine (TEA) was received from commercial source, and distilled on KOH prior to use. 1H NMR (400 MHz), and 13C NMR (100MHz) spectra were recorded on the Bruker Avance (III) 400 MHz, using CDCl3 as solvent. Tetramethylsilane (TMS) was used as reference for recording 1H (of residual proton; δ = 7.26 ppm), and 13C (δ = 77.0 ppm) spectra in CDCl3. UV-visible absorption spectra of all compounds in chlorofom were recorded on a Carry-100 Bio UV-visible Spectrophotometer. HRMS was recorded on BrukerDaltonics, micrO TOF-Q II mass spectrometer. Photoluminescence and UV spectra of dilute solutions and of solid-state films were recorded with the Edinburgh Instruments FLS980 and Aventes AvaSpec-2048XL spectrometers, respectively. Edinburgh Instruments FLS980 spectrometer and PicoQuant LDH-D-C-375 4 ACS Paragon Plus Environment

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laser (wavelength 374 nm) as the excitation source were used for recording photoluminescence (PL) decay curves of samples at room temperature. Fluorescence quantum yields were measured using an integrated sphere. Solid-state ionization potentials (IPPE) of the studied compounds were estimated by electron photoemission method in air. The measurements were done with setup which was similar to that described previously utilizing deep UV deuterium light source ASBN-D130-CM, CM110 1/8m monochromator, and 6517B Keithley electrometer.38 The studied compounds were tested in electroluminescent devices which were fabricated by both spin-coating and vacuum deposition techniques. The devices were fabricated on ITOcoated glass substrates (purchased from LUMTEC company) with a sheet resistance of 15 Ω/sq. In spin-coating processes, the layers were deposited at a spin speed of 1000 rpm for 60 s using spin coater SPIN150i placed in inert atmosphere of MB EcoVap4G glove box. In vacuum deposition processes, organic and metal layers were deposited under vacuum higher than 2×10−6 mBar exploiting vacuum equipment from Kurt J. Lesker in-built in MB EcoVap4G glove box. Density-voltage and luminance-voltage characteristics of the fabricated devices were recorded utilizing Keithley 6517B electrometer, certificated photodiode PH100-Si-HA-D0 together with the PC-Based Power and Energy Monitor 11SLINK

(purchased

from

STANDA

company)

and

Keithley

2400C

sourcemeter.

Electroluminescence (EL) spectra were recorded using an Aventes AvaSpec-2048XL spectrometer; while, device efficiencies were estimated from the luminance, current density, and EL spectra. Materials For

OLED

fabrication,

diphenyl-4-triphenylsilylphenyl-phosphineoxide

(TSPO1),

molybdenum trioxide (MoO3), N,N′-bis(naphthalen-1-yl)-N,N′-bis(phenyl)benzidine (NPB), 3-bis(9-carbazolyl)benzene (mCP), tris(4-carbazoyl-9-ylphenyl)amine (TCTA), and 2,2’,2’’-



(1,3,5-benzinetriyl)-tris(1-phenyl-1-H-benzimidazole) (TPBi) were used as received from Sigma-Aldrich and LUMTEC companies. Synthesis and Characterization The reactant 4, 5, 6 were synthesized according to known methods.31 Synthesis of truxenes 7 and 8. A solution of tri-iodotruxene 3 (200 mg, 0.16 mmol) and the corresponding TPE boronic ester (4.5 equivalent) in toluene–water 3 : 1 (60 mL) was deareated for 30 min with argon bubbling and then Pd(PPh3)4 (40 mg, 0.07 mmol) were added. The solution was deareated for further 5 min. The mixture was heated at 80 ºC for 48 h. The solvent was removed; the remaining residue was suspended in water (50 mL) and extracted with DCM (3 x 20 mL). The combined organic layers were dried (MgSO4) and concentrated in vacuum. The resulting crude product was purified by column chromatography on silica gel eluting with CH2Cl2/hexane (10%). The desired compounds obtained from the column was recrystalized from DCM/methanol to give compounds 7 and 8 in 70–75% yield. Synthesis of truxenes 9 A solution of tri-iodotruxene 3 (250 mg, 0.20 mmol) and the TPAN alkyne (4.5 equivalent) in toluene–triethylamine 5 : 1 (60 mL) was deareated for 30 min with argon bubbling and then Pd(dba)2 (40 mg, 0.07 mmol) and AsPh3 (170 mg, 0.55 mmol) were added. The solution was deareated for a further 5 min. The mixture was heated at 80 ºC for 48 h. The solvent was removed; the remaining residue was suspended in water (50 mL) and extracted with DCM (3 x 20 mL). The combined organic layers were dried (MgSO4) and concentrated in vacuum. The resulting crude product was purified by column chromatography on silica gel eluting with CH2Cl2/hexane (10%). The desired compounds obtained from the column was recrystalized from DCM/methanol to give compound 9 in 75% yield.


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Compound 7: white solid (yield: 70%) 1H NMR (400 MHz, CDCl3): δ (ppm) 8.3(d, 3H, J = 8.70 Hz), 7.587.68 (m, 6H), 7.53 (d, 6H, J = 7.50 Hz), 7.01-7.21 (m, 51H), 2.87-3.01 (m, 6H), 2.02-2.17 (m, 6H), 0.75-0.99 (m, 36H), 0.45-0.63 (m, 30H) ); 13C NMR (100 MHz, CDCl3): δ = 154.2, 145.0, 143.8, 142.7, 141.0, 140.6, 139.6, 138.8, 138.3, 138.0, 131.8, 131.4, 131.4, 131.3, 127.8, 127.7, 127.6, 126.4, 126.4, 126.0, 127.8, 124.7, 120.0, 55.0, 37.7, 31.5, 29.5, 23.9, 22.2, 13.8, HRMS (ESI) m/z, calcd for M+ (C141H144): 1838.1296; found: 1838.1608. Compound 8: white solid (yield: 75%) 1H NMR (400 MHz, CDCl3): δ (ppm) 7.96(d, 3H, J = 8.58 Hz), 6.72-7.51 (m, 51H), 2.57-2.79 (m, 6H), 1.54-1.69 (m, 6H), 0.59-1.00 (m, 66H) );

13

C NMR

(100 MHz, CDCl3): δ = 152.7, 144.9, 144.0, 143.9, 143.8, 141.7, 141.3, 140.5, 138.6, 137.9, 131.5, 131.4, 131.3, 129.9, 129.3, 129.0, 127.6, 127.5, 126.4, 126.3, 125.1, 123.9, 55.2, 36.5, 31.5, 29.7, 29.2, 23.6, 22.3, 13.9. HRMS (ESI) m/z, calcd for M+ (C123H132): 1610.0357; found: 1610.0355. Compound 9: yellow solid (yield: 72%) 1H NMR (400 MHz, CDCl3): δ (ppm) 8.33(d, 3H, J = 9.20 Hz), 7.52-7.63 (m, 6H), 7.39-7.51 (m, 20H), 7.18-7.34 (m, 16H), 7.04 (d, 6H, J = 7.36), 2.85-2.99 (m, 6H), 2.00-2.16 (m, 6H), 0.77-0.99 (m, 36H) ), 0.39-0.67 (m, 30H); 13C NMR (100 MHz, CDCl3): δ = 158.3, 153.6, 146.0, 140.4, 140.2, 138.9, 137.9, 134.6, 131.6, 130.8, 130.0, 129.9, 129.7, 129.2, 128.5, 128.4, 125.3, 124.5, 123.4, 120.7, 119.8, 110.9, 91.7, 89.3, 55.84, 36.9, 31.4, 29.6, 29.4, 23.9, 22.2, 13.8. HRMS (ESI) m/z, calcd for M+1 (C132H129N3): 1756.0214; found: 1757.0510. Results and discussion Synthesis



TPE and TPAN substituted truxenes 7, 8 and 9 were synthesized by the Pd-catalyzed Suzuki and Sonogashira cross-coupling reactions of tri-iodotruxene 3 with the corresponding boronic acid and alkyne (Scheme 2). The truxene core 1 was synthesized from 1-indanone in the presence of acetic acid and hydrochloric acid (Scheme 1).29 The alkylation reaction of the truxene 1 with bromo-hexane resulted in hexahexylated truxene 2.29 The iodination reaction of truxene 2 in the presence of HIO3 and I2 resulted in hexahexylated tri-iodo truxene 3 in 80% yield (Scheme 1).30

Scheme 1: Synthesis of tri-iodotruxene 3. Tetraphenylethene boronic ester (4),31 triphenylethene boronic ester (5), were synthesized by reported procedure from corresponding bromo-TPE.31 The TPAN-alkyne (6) was synthesized by the Sonogashira cross-coupling reaction of bromo-TPAN with trimethylsilylethyne, followed by deprotection of TMS group by K2CO3 in overall 69% yield.31 The possibility to study the effect of linking topology and origin of substituents on AIE properties of truxenes 7, 8 and 9 was explored by reacting different aryl ethenes (4, 5 and 6) with tri-iodo truxene 3.


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Scheme 2: Synthesis of TPE and TPAN substituted truxenes 7, 8 and 9 The reaction of tri-iodo truxene 3 with tetraphenylethene boronic ester (4), triphenylethene boronic ester (5) using Pd(PPh3)4 and TPAN-alkyne (6), under the catalytic system Pd(dba)2/AsPh3 resulted truxenes 7, 8 and 9 in 70%, 75% and 72% yields, respectively (Scheme 2). Truxenes 7, 8 and 9 are readily soluble in common organic solvents and were well characterized by 1H NMR, 13C NMR, and HRMS techniques Thermal properties Thermal stability is the significant requisite for practical applications of organic chromophores. The thermal properties of the TPE and TPAN substituted truxenes 7, 8 and 9 were explored using thermogravimetric analysis (TGA) at a heating rate of 10 °C min-1 under 9 ACS Paragon Plus Environment


nitrogen atmosphere (Figure 1). The truxenes 7, 8 and 9 showed 10% weight loss at 438 °C, 409 °C and 413 °C respectively. The TGA results indicate that these compounds are thermally stable enough to be used for OLED applications. The thermal stability of the TPE and TPAN substituted truxenes follow the order 7 > 9 > 8.

Figure 1. TGA plots of truxene 7–9

Theoretical calculations In order to gain insight into the electronic structures of truxenes and to understand the photophysical properties of the TPE and TPAN substituted truxenes 7, 8 and 9 the density functional theory (DFT) and time dependent density functional (TD-DFT) calculations were performed. The structures of 7, 8 and 9 were optimized using Gaussian 09W program at the B3LYP/6-31G** level. The TD-DFT calculations were carried out in tetrahydrofuran (THF) using the polarized continuum model (CPCM) of Gaussian 09 software. The 6-31G** basis set was used for all the calculations.32–34 The truxene core of TPE and TPAN substituted truxenes 7, 8 and 9 show planar geometry while TPE units show twisted geometry (Figure 2). The interplanar angles between the truxene core and the plane of TPE units in truxene 7 are 35.6°, 35.6° and 35.7°. In truxene 8 the dihedral angles between truxene core and TPE units are 46.9°, 46.7° and 46.8°. The large dihedral angles of truxenes 7 and 8 make the molecules highly twisted and AIE active. The 10 ACS Paragon Plus Environment

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truxene core of TPAN substituted truxene 9 and phenyl ring of TPAN are in the same plane, which reflects stronger electronic communication and gives sufficient space for planarization.31 This favours the ACQ and makes TPAN substituted truxene 9 AIE inactive.

7

8

9

Figure 2. The DFT optimized structures of the truxenes 7, 8 and 9 with Gaussian 09 at the B3LYP/6-31G** level of theory. The TD-DFT predicted vertical excitation energies for truxenes 7, 8 and 9 are shown in Figure 3 along with experimental UV-vis spectra and data are listed in Table 1. The major intense transition in the truxenes 7, 8 and 9 is π-π* in nature. The strong absorption bands calculated at CAM-B3LYP level are at 319 nm, and 326 nm for truxenes 7 and 8 respectively. The experimental values for these transitions are 283 nm for 7 and 8. The TPAN substituted truxene 9 show calculated band at 360 nm. The HOMO-LUMO energy level figure 4 reveals that electron density transfers from truxene core (donor) to TPAN (acceptor) unit.



Figure 3. The comparison of experimental and calculated (TD-DFT) at CAMB3LYP absorption spectrum the truxenes 7, 8 and 9 in THF solution.

Figure 4 shows the electron density distribution of the HOMO and LUMO of the TPE and TPAN substituted truxenes 7, 8 and 9. The HOMO and LUMO in TPE substituted truxenes 7 and 8 are delocalized over the truxene and TPE unit. The TPAN substituted truxene 9 shows HOMO delocalized over the truxene core while the LUMO is exclusively located on the TPAN units, which indicates charge transfer from HOMO to LUMO. The HOMO-LUMO gap is lower for TPAN substituted truxene 9 as compared to truxene 7 and 8, due to the incorporation of TPAN alkyne as a strong acceptor. Table 1. Computed vertical transition energies, Oscillator strengths (f) and major contributions for truxenes 7–9. 12 ACS Paragon Plus Environment

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Compound

TD-DFT/ CAM-B3LYP (THF) fa

Major contribution (%)

2.88

HOMO−2→LUMO+1(18%)

λmax 7

319 nm

HOMO−1→LUMO (10%) HOMO−1→LUMO+2 (11%) HOMO→LUMO+2 (14%)

325 nm

1.88

HOMO−2→LUMO (17%) HOMO−1→LUMO+2 (15%) HOMO→LUMO (10%) HOMO→LUMO+2 (11%)

8

326 nm

1.89

HOMO−2→LUMO (18%) HOMO−1→LUMO+1 (16%) HOMO-1→LUMO+2 (25%) HOMO→LUMO (17%)

329 nm

0.85

HOMO-2→LUMO+1 (18%) HOMO-1→LUMO (18%) HOMO→LUMO+1 (18%) HOMO→LUMO+2 (23%)

9

361 nm

3.30

HOMO−2→LUMO (24%) HOMO→LUMO (11%), HOMO→LUMO+2 (14%) HOMO-1→LUMO (8%)

360 nm

2.86

HOMO-2→LUMO+1 (24%) HOMO-1→LUMO+2 (17%) HOMO-4→LUMO+2 (4%) HOMO-1→LUMO (5%)

a

f is Oscillator strengths



Figure 4. The energy level diagram of the frontier molecular orbitals of the TPE and TPAN substituted truxenes 7, 8 and 9 using B3LYP/6-31G(d,p) level of DFT theory. Photophysical properties The electronic absorption spectra of the dilute solutions of TPE and TPAN substituted truxenes 7, 8 and 9 in tetrahydrofuran (THF) are shown in Figure 5 and the data are listed in Table 2. The truxenes 7, 8 and 9 show strong absorption band between 283–285 nm respectively, with high molar extinction coefficient, corresponding to π→π* transition.35,36 The low intensity band between 346–370 nm is caused by intramolecular charge transfer from TPE and TPAN to the truxene core. The emission properties of the TPE and TPAN substituted truxenes 7, 8 and 9 were studied by steady state and time-resolved fluorescence techniques. Their emission spectra are shown in Figure 5b. The TPE substituted truxene 7 exhibit fluorescence maximum at 487 nm, truxene 8 shows band at 512 nm and truxene 9 shows fluorescence maximum at 424 nm (Figure 5b). The dilute solutions of truxenes 7, 8 and 9 show fluorescence quantum yields lower than 1% (Table 2).


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(a)

(b)

Figure 5. (a) Normalized electronic absorption and (b) fluorescence spectra of the solutions of truxenes 7, 8, and 9 in THF at RT.

Figure 6. UV-vis and PL spectra of the dilute toluene solutions and of the layers of the studied compounds. The PL spectra of the frozen THF solutions of truxenes 7, 8 and 9 showed emission maxima at 440, 448 and 491 nm respectively which were blue shifted in comparison to those of THF solutions recorded at room temperature (Figure S1 and Table 2). This observation shows that the emission of THF solutions of truxenes 7, 8 and 9 is only stabilized by the weak electronic contribution of the static solvent shell when THF becomes solid and geometric relaxation of 15 ACS Paragon Plus Environment


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the solvent is no longer possible at 77 K.37 As a result, blue shifts of the spectra recorded at 77 K were observed relative to those recorded the room temperature. Differences between FL maxima of the frozen THF solutions of truxenes 7, 8 and 9 and PL maxima of solid-state films recorded at room temperature can be explained by the polarity effect (Figure 6 and S1b). Additionally, the red-shift of fluorescence maxima of the solid-state films can occur due to the intermolecular interactions of truxenes 7, 8 and 9 displaying the aggregation processes. Table 2. Photophysical and Thermal properties of truxenes 7, 8 and 9 Compounds

7

8

9

343 522

Strategy Toward Tuning Emission of Star-Shaped Tetraphenylethene

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