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Exciplex-Enhanced Singlet Emission Efficiency of Non-Doped OLEDs Based on Derivatives of Tetrafluorophenylcarbazole and Tri/ tetraphenylethylene Exhibiting Aggregation Induced Emission Enhancement Galyna Sych, Jurate Simokaitiene, Oleksandr Bezvikonnyi, Uliana Tsiko, Dmytro Volyniuk, Dalius Gudeika, and Juozas Vidas Grazulevicius J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b03895 • Publication Date (Web): 01 Jun 2018 Downloaded from http://pubs.acs.org on June 1, 2018

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The Journal of Physical Chemistry

Exciplex-Enhanced Singlet Emission Efficiency of Non-Doped OLEDs Based on Derivatives of Tetrafluorophenylcarbazole and Tri/tetraphenylethylene Exhibiting Aggregation Induced Emission Enhancement Galyna Sych1, Jurate Simokaitiene1, Oleksandr Bezvikonnyi1, Uliana Tsiko2, Dmytro Volyniuk1, Dalius Gudeika1, Juozas V. Grazulevicius*1 1

Department of Polymer Chemistry and Technology, Kaunas University of Technology,

Radvilenu pl. 19, LT-50254, Kaunas, Lithuania; *e-mail: [email protected] 2

Faculty of Chemistry, Ivan Franko National University, Universytetska St., Lviv, 79000,

Ukraine

ACS Paragon Plus Environment

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ABSTRACT: Two derivatives of tetrafluorophenylcarbazole and tri/tetraphenylethylene displaying aggregation-induced emission enhancement were synthesized and investigated, by theoretical and experimental tools. The synthesized compounds exhibit efficient emission in solid state with fluorescence intensity maxima at 511 and 502 nm and photoluminescence quantum yields of 57 % and 27 %. They exhibit high thermal stability with 5% weight loss temperatures of 362 and 314 °C and glass-forming properties with glass-transition temperatures of 112 and 80 °C. Ionization potentials measured by photoelectron emission spectrometry were found to be comparable (5.83 eV and 5.87 eV). The layers of the compounds showed bipolar charge-transporting properties with balanced electron and hole mobilities reaching 10-3 cm2/Vs at high electric fields. Exciplex-host based OLEDs containing one or two emitting layers of tetraphenylethenyl-containing emitter were fabricated and showed by more than 50% higher external quantum efficiency as compared to that of the corresponding non-doped device. The best non-doped OLED containing the synthesized emitter showed turn-on voltage of 9.1 V, maximum brightness of 11800 cd/m2, maximum current efficiency of 4.5 cd/A and external quantum efficiency of ca. 1.7 %. The best modified device, with exciplex- based host layer showed turn-on voltage of 9.1 V, maximum brightness of 16300 cd/m2, maximum current efficiency of 7.3 cd/A and external quantum efficiency of ca. 2.6 %.


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Introduction Organic electroactive materials are of great demand for the applications in the devices of organic electronics, in which solid films of the materials are usually used 1. In the solid state, planar or disk-shape compounds often undergo strong intermolecular π−π stacking interactions

2

.

Relaxation of excited states in the solid films usually is of non-radiative nature, which results in the luminescence attenuation. As a result of aggregation cased quenching, discovered by Forster 3

, applications of solid organic emitters is rather restricted 4. The phenomenon that allows to

mitigate or totally to overcome solid state fluorescence quenching was firstly investigated by Tang et al. and termed as aggregation-induced emission (AIE) or aggregation induced emission enhancement (AIEE)

5,6

. The AIE approach has already found application in optoelectronic

devices 7, chemical sensors 8, biological probes 9. Restriction of intramolecular rotations 10, in the line with restriction of intramolecular vibrations 11, which can be generally named as restriction of intramolecular motions 12–15, are considered as the main working mechanisms of AIE or AIEE phenomena, which require non-planar conformational or vibrational rigidification of molecular structures

16

. Presence of the rotating systems, propeller-like rotors in the molecular structures

‘turn-on’ emission, since π−π stacking interactions are suppressed in the solid samples of AIEactive materials. Attachment of AIE-active moieties to the backbone of the emissive molecules is a successful approach for reducing of aggregation induced quenching and obtaining luminogens with high photoluminescence quantum yield (PLQY) in the solid state

17

. Such strategy allows

implementation of molecular aggregates in efficient non-doped organic light emitting diodes (OLEDs)

18–22

. Variety of AIE-active chromophores, containing stilbene, tri- and

tetraphenylethylene

23–26

, pentaphenylsilole

27–29

, benzophenone

30

and many others were

reported.


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Application of bipolar charge-transporting compounds in OLEDs allow to enhance their efficiencies

31

. Heretofore, limited number of compounds exhibiting combination of AIEE and

efficient bipolar charge transport were reported

32

. Fluorescent AIE emitters with simple

chemical structure are required for fluorescent OLEDs which are characterized by much faster emission decays and higher stability in comparison to phosphorescent or TADF-based OLEDs. However, caused by the spin statistics 33, 5% limit of external quantum efficiency (EQE) cannot be overcame for non-doped singlet emission based devices even having perfect AIE-active fluorescent emitters with PLQYs of 100%. Utilizing emitters exhibiting thermally activated delayed fluorescence (TADF) phenomenon, TADF OLEDs without out-coupling were characterized by EQE of 25%

34

. However, such emitters usually require special molecular

design and appropriate hosts for electroluminescent device applications

35

. At the same time

intermolecular TADF was observed in the solid-state mixtures of different exciplex-forming donor and acceptor compounds 36. Exciplex-based emitters can be obtained at the interface of the layers of donating and accepting materials characterized by exciplex-forming properties 37. As a result, non-doped TADF OLEDs can be fabricated

38

. Unfortunately, PLQYs of most of 39–41

exciplex-based emitters are usually do not exceed 50% materials were utilized as hosts in highly efficient OLEDs

42

. Recently, exciplex-forming

. However, to our best knowledge,

reports on application of exciplex-forming hosts for AIE-active fluorescent emitters are absent in literature. The aim of this work was synthesis of highly efficient emitters and developing of approach for the enhancement of efficiency of non-doped fluorescent OLEDs based on AIE-active emitters. To be really-efficient, singlet emitters have to exhibit both AIEE and bipolar charge-transporting properties. Our approach is based on the employment of interface exciplexes which allow


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fabrication of non-doped OLEDs with the efficiency overcoming spin statistics limit for fluorescent devices. For the realization of this task, two compounds containing carbazolyl group, tetrafluorophenyl moiety and AIE-active tri/tetraphenylethylene moiety in a single molecule were designed. In the designed structures, carbazolyl substituted tetrafluorostyrene moiety is linked to the AIE unit with olefin rotor consisting of tri- or four- peripheral phenyl rings. Carbazole heterocycle was selected as electron-donating group, due to well-known thermal stability and hole-transporting properties of carbazole derivatives serve as electron-accepting moiety

44–46

43

, while tetrafluorostyrene

. To secure our object, appropriate highly efficient

exciplex-forming host for the developed emitters had to be selected. Thus, in this work we report on the synthesis and properties of two AIE-active emitters (PLQYs of which exceeded 50% in solid-state) and on selection of exciplex-forming host which allowed to fabricate fluorescent OLED EQE of which was by more than 50% higher relative to the corresponding non-doped fluorescent device.

Experimental Section Materials 2,3,4,5,6-Pentafluorostyrene, sodium tert-butoxide, 1-(4-bromophenyl)-1,2,2-triphenylethylene, 2-bromo-1,1,2-triphenylethylene, palladium acetate(II), tri(o-tolyl)phosphine were purchased from Sigma Aldrich and used as received. N,N-Dimethylformamide (DMF) was distilled and dried over molecular sieves (4A). Triethylamine was purified by distillation over CaH2. Other reagents and solvents were commercially purchased and were used as supplied. 9-(2, 3, 5, 6-Tetrafluoro-4-vinyl-phenyl)-9H-carbazole


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Sodium tert-butoxide (0.63 g, 6.6 mmol) was added to a glass flask containing carbazole (1g, 5.9 mmol) dissolved in DMSO (10 mL). The mixture was stirred at ambient temperature for 5 min. Then it was added to a solution of 2,3,4,5,6-pentafluorostyrene (1.39g, 7.2mmol) in DMSO (5 mL) dropwise under stirring. The solution was stirred for 3 hours. The reaction mixture was poured into water and extracted with EtOAc. The organic phase was dried over Na2SO4, filtered, and the solvent was removed. The crude product was purified by column chromatography on silica gel using hexane and dichloromethane as eluent mixture of solvents in a volume ratio of 5:1. After recrystallization from eluent mixture of solvents white crystals were obtained with the yield of 1.01g (57%); mp: 104-106 °C. 1

H NMR (400 MHz, CDCl3): δ 8.16 (d, J = 7.7 Hz, 2H, Ar), 7.52 – 7.43 (m, 2H, Ar), 7.37 (t, J =

7.5 Hz, 2H, Ar), 7.20 (d, J = 8.2 Hz, 2H, Ar), 6.84 (dd, J = 18.0, 11.9 Hz, 1H, CH), 6.28 (d, J = 18.0 Hz, 1H, CH), 5.86 (d, J = 11.9 Hz, 1H, CH)

13

C NMR (101 MHz, CDCl3): δ 139.98,

126.47, 124.72, 124.08, 122.06, 121.14, 120.53, 109.88. 19F NMR (376 MHz, CDCl3): δ -142.57 (td, J = 13.2, 4.3 Hz, 2F), -144.16 – -144.46 (m, 2F). m/z: cal. for C20H11F4N 342.3 [M+ H], found 342.6; 9-{2,3,5,6-Tetrafluoro-4-[2-(4-triphenylvinyl-phenyl)-phenyl}-9-H-carbazole (C4FS1) was synthesized by the procedure similar to that reported previously

47

. 1-(4-Bromophenyl)-1,2,2-

triphenylethylene (0.27 g, 0.65 mmol) and 9-(2, 3, 5, 6-tetrafluoro-4-vinyl-phenyl)-9H-carbazole (0.2 g, 0.59 mmol) were dissolved in 9 mL of dry DMF under argon atmosphere. Then, Pd(OAc)2 (0.003 g, 0.012 mmol ), P(o-tolyl)3 (0,004 g, 0.012 mmol) and triethylamine (7 mL) were added. The solution was stirred at 110°C f or 12 h. The reaction mixture was filtered through Celite. The filtrate was extracted with EtOAc/water, washed with saturated brine solution, dried over Na2SO4 and concentrated under reduced pressure. The crude product was


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purified by column chromatography on silica gel using hexane and dichloromethane as eluent mixture of solvents in a volume ratio of 10:1. After recrystallization from eluent mixture of solvents pale yellow crystals were obtained (0.23 g, 58%). mp: 223°C (DSC) 1

H NMR (400 MHz, CDCl3) δ 8.15 (d, J = 7.7 Hz, 2H, Ar), 7.53 (d, J = 16.8 Hz, 1H, CH), 7.49

– 7.43 (m, 2H, Ar), 7.38 – 7.32 (m, 4H, Ar), 7.22 – 7.02 (m, 20H, Ar, CH). 13C NMR (101 MHz, CDCl3) δ 145.04, 143.54, 143.45, 141.74, 140.27, 140.00, 137.99, 134.41, 131.94, 131.75, 131.36, 127.86, 127.79, 127.68, 126.71, 126.63, 126.60, 126.50, 126.43, 124.03, 121.08, 120.48, 118.49, 117.20, 112.85, 109.90. MS m/z: cal. for C46H29F4N 694.22 [M+ Na], found 694.73. Elemental analysis cal. for C46H29F4N: C, 82.25; H, 4.35; F, 11.31; N, 2.09. Found (%): C, 82.16; H, 4.41; F, 11.28; N, 2.15. 9-{2,3,5,6-Tetrafluoro-4-(3,4,4-triphenyl-buta-1,3-dienyl)-phenyl)-9-H-carbazole (C4FS2) C4FS2 was prepared by the similar procedure as C4FS1. 2-Bromo-1,1,2-triphenylethylene (0.26 g, 0.77 mmol), 9-(2, 3, 5, 6-tetrafluoro-4-vinyl-phenyl)-9H-carbazole (0.24g, 0.7mmol), Pd(OAc)2 (0,003g, 0.014mmol ), P(o-tolyl)3 (0,004 g, 0.014 mmol), triethylamine (9 mL) were stirred in dry DMF under argon at 120°C for 14 h. The mixture was filtered through Cellite and extracted with ethyl acetate and organic phase was washed with water and brine. The solution was dried over Na2SO4 and the solvent was removed. The crude product was purified by column chromatography using hexane/dichloromethane (10:1) as an eluent. Bright yellow crystals (0.26 g. 62%) were obtained after recrystallization from hexane. mp: 148°C (DSC) 1

H NMR (400 MHz, CDCl3) δ 8.13 (d, J = 7.7 Hz, 2H, Ar), 7.69 (d, J = 16.4 Hz, 1H, CH), 7.48

– 7.23 (m, 14H, Ar), 7.13 (d, J = 8.1 Hz, 2H, Ar), 7.10 – 7.05 (m, 3H, Ar), 6.97 – 6.93 (m, 2H, Ar), 6.36 (d, J = 16.4 Hz, 1H, CH).

13

C NMR (101 MHz, CDCl3) δ 146.77, 145.92, 145.07,

144.22, 143.42, 142.28, 141.76, 140.52, 140.00, 139.23, 138.72, 131.43, 131.11, 131.05, 128.22,


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128.10, 128.02, 127.45, 127.18, 126.84, 126.38, 123.97, 121.00, 120.45, 117.92, 117.00, 113.86, 109.85. MS m/z: cal. for C40H25F4N 596.63 [M+ H], found 596.79; cal. for C40H25F4N 618.63 [M+ Na], found 618.42. Elemental analysis cal. for C40H25F4N: C, 80.66; H, 4.23; F, 12.76; N, 2.3 5. Found (%): C, 80.59; H, 4.33; F, 12.70; N, 2.38.

Instrumentation 1H NMR and 13C NMR spectra were recorded with Bruker Avance III [400MHz (1H), 101MHz (13C)] spectrometers at room temperature. All the data are given as chemical shifts d (ppm) downfield from Si(CH3)4. Differential scanning calorimetry (DSC) measurements were done with Perkin Elmer DSC 8500 equipment at heating and cooling rates of 10oC/min in a nitrogen atmosphere. Thermogravimetric analysis (TGA) was carried out on a Perkin Elmer TGA 4000 apparatus in a nitrogen atmosphere at a heating rate of 20oC/min. Absorption spectra of dilute (10-5 M) solutions in toluene were recorded on a Perkin Elmer Lambda 35 spectrometer. Fluorescence spectra, fluorescence quantum yields (PLQY) and fluorescence decay curves of both toluene solution and solid samples were recorded by FLS980 fluorescence spectrometer. Cyclic voltammetry (CV) measurements were performed using a micro-Autolab III (Metrohm Autolab) potentiostat-galvanostat equipped with a standard three-electrode configuration. A three- electrode cell equipped with a glassy carbon working electrode, an Ag/Ag (0.01 M in anhydrous DMF) reference electrode and a Pt wire counter electrode were employed. The measurements were done in anhydrous dimethylformamide with 0.1 M of tetrabutylammonium hexafluorophosphate (Bu4N(PF)6) as the supporting electrolyte under nitrogen atmosphere at a scan rate of 0.1 V/s. The measurements were calibrated using ferrocene/ferrocenium (Fc) system, as an internal standard.


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Photoelectron emission spectra for vacuum deposited layers of the studied compounds C4FS1 and C4FS2 were recorded as reported previously

48,49

. The samples for photoelectron emission

measurements were vaccum-deposited using fluorine doped tin oxide (FTO) coated glass slides as substrates. Photoelectron emission spectra were recorded in air exploiting setup which included ASBN-D130-CM deep UV deuterium light source, CM110 1/8m monochromator and 6517B Keithley electrometer. Charge transporting properties of vacuum deposited layers of C4FS1 and C4FS2 were studied by time-of-flight (TOF) method. Applying positive or negative voltages (U) to an opticallytransparent electrode of TOF samples, hole or electron transport properties of C4FS1 and C4FS2 were estimated, respectively. The samples with the structure ITO/ thick layer of C4FS1 or C4FS2/Al were fabricated depositing organic layers under vacuum of ca. 2×10−6 mBar. EKSPLA NL300 laser (excitation wavelength of 355 nm) was used as the excitation source. 6517B electrometer (Keithley) as high voltage source and TDS 3032C oscilloscope (Tektronix) for recording the photocurrent transients under the different electric fields for holes and electrons were employed in TOF measurements. Equation µ=d2/(U×ttr) was utilized to calculate charge mobilities having transit times (ttr) from the photocurrent transients and thicknesses of the layers (d). C4FS1 and C4FS2 were tested as emitters in non-doped electroluminescent devices which were fabricated by vacuum deposition. ITO-coated glass substrates with a sheet resistance of 15 Ω/sq were pre-patterned for getting seven independent devices with area of 6 mm2. Densityvoltage and luminance-voltage characteristics were taken by Keithley 2400C source meter, certificated photodiode PH100-Si-HA-D0 together with the PC-Based Power and Energy Monitor 11S-LINK (from STANDA). Electroluminescence (EL) spectra were recorded using the


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Aventes AvaSpec-2048XL spectrometer. Device efficiencies were calculated having the luminance, current density, and EL spectra.

Results and Discussion Synthesis Compounds C4FS1 and C4FS2 were synthesized according to the synthetic route outlined in Scheme 1. The first step was nucleophilic substitution reaction of fluorine atom by carbazole aromatic heterocycle

50

with the further Heck coupling of vinyl group with tri-/tetraphenyl

ethylene bromides as the final step. Chemical structures of the compounds were characterized by 1

H and

13

C NMR spectroscopies, mass spectrometry and elemental analysis. The synthesized

compounds showed good solubility in organic solvents, such as acetone, THF, toluene, chloroform.

Scheme 1. Synthetic scheme of C4FS1 and C4FS2. Conditions: (i) 9-H carbazole, NaOtBu, DMF, 3h; (ii) Pd(OAc)2, P(o-tolyl)3, TEA, K2CO3, DMF, 12h.

Thermal properties Behaviour of C4FS1 and C4FS2 under heating was investigated by TGA and DSC under a nitrogen atmosphere. TGA and DSC curves of the compounds are shown in Fig. 1a and Fig. 1b.


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Figure 1. TGA (a) and DSC (b) curves of C4FS1 and C4FS2.

TGA revealed that compounds C4FS1 and C4FS2 exhibit high thermal stability, which is typical for carbazole containing materials 51–53. Their 5% weight loss temperatures (Td) were found to be 362 and 314 °C respectively (Fig. 1a). Very low carbonized residuals observed for C4FS1 and C4FS2 show that sublimation could be the main reason of weight loss. Higher 5% weight loss temperature C4FS1 can apparently be explained by its higher molecular weight, which predetermines stronger intermolecular interaction. C4FS1 and C4FS2 were obtained after the synthesis as crystalline substances. They showed endothermic melting signals in the first heating scans at 223 and 148 °C respectively (Table 1). In the cooling scans the samples did not show crystallization and in the following second heating scans they showed glass transitions at 112 and 80 °C respectively. C4FS2 having by one phenyl group less as compared to C4FS1 showed both lower melting point and lower glass transition temperature. Despite of lower molar mass molecular glass of C4FS2 exhibited higher morphological stability relative to that of C4FS1. In


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the second DSC heating scan C4FS2 showed only glass transition signal, while C4FS1 after the glass transition signal showed crystallization and melting transitions.

Table 1. Thermal Characteristics of С4FS1 and C4FS2. Compounds

TD a, °C

Tg b, °C

Tm b, °C

C4FS1

362

112

223

C4FS2

314

80

148

TD is 5% weight loss temperatures, scan rate 20 °C/min, nitrogen atmosphere atmosphere.

Tg is glass transition

temperature, scan rate 10 °C/min, nitrogen atmosphere. Tm is melting temperature. a

Estimated from TGA.

b

Estimated from DSC.

Photophysical properties UV-VIS absorption and fluorescence spectra of dilute solutions in toluene and of solid films of compounds C4FS1 and C4FS2 are shown Fig. 2a and Fig. 2b. Photophysical data for the compounds are summarized in Table 2.


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Figure 2. Normalized UV-Vis absorption and fluorescence spectra of 10-5 M toluene solution(solid lines) and thin films (dash and dot lines) of compounds C4FS1 (a) and C4FS2 (b); Fluorescence decay

curves of solid films of C4FS1 and C4FS2 (c). (λex=360 nm).

The lowest energy absorption peaks of the dilute solutions of C4FS1 and C4FS2 were observed at 367 nm and 360 nm respectively. Due to the similar chemical structure, absorption spectra of the compounds are similar, but the lowest energy absorption band of the solution of C4FS1 is slightly red-shifted with respect of the corresponding band of C4FS2 owing to the prolonged molecular conjugation. For both of the compounds, absorption spectra of neat films are slightly


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red-shifted (by 9-10 nm) with respect to the spectra of the solutions, presumably due to formation of aggregates and occurrence of the intermolecular interactions in the solid state.

Table 2. Photophysical Characteristics of C4FS1 and C4FS2.

Compounds

λPlmax,tol,a nm

λPlmax, b nm

Stokes shift a/b , nm

ηPL a/b ,%

C4FS1

498

511

131/137

0.3/57

C4FS2

445

502

85/135

0.1/28

λPlmax

τ1b, ns 2.08 (χ2=1.276) 2.16 (χ2=1.141)

is wavelength of emission maximum; ηPL is photoluminescence

quantum yield; τ1/τ2 – photoluminescence lifetimes a

Dilute toluene solution (10-5M).

b

Solid film.

Dilute toluene solutions of C4FS1 and C4FS2 exhibited fluorescence in violet-blue range of spectrum with the intensity peaks at 498 nm and 445 nm respectively, while neat solid films showed emission intensity maxima at 511 and 502 nm respectively. Emission band of the solution of C4FS1 showed significant bathochromic shift of 53 nm in comparison with that of the solution of C4FS2, while in the solid state this shift is much smaller (9 nm). Considerably shorter Stokes shift observed for the solution of C4FS2 can apparently be explained by the shorter relaxation time in the excited state due to the presence of less bulky and less rigid triphenylbutadienyl substituent. For the deeper understanding of the photophysical processes, photoluminescence decay measurements and of the solid samples were carried out. Fluorescence decay curves of both the compounds C4FS1 and C4FS2 in solid-state revealed monoexponential decay profiles (Fig. 2c). The Fluorescence lifetimes of the solid samples of C4FS1 and C4FS2 were found to be of 2.08 ns and 2.16 ns (Table 1), respectively.


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Both C4FS1 and C4FS2 were found to be soluble in typical organic solvents, while insoluble in water. To study AIEE properties of the compounds, fluorescence spectra of the compounds dispersed in the THF:water systems were estimated. The solution of C4FS1 and C4FS2 in THF showed very week emission, due to the intramolecular rotations and vibrations of phenyl rotors against the ethylene stator. Nevertheless, with the addition of water formation of molecular aggregates resulting in enhancement of the fluorescence intensity was observed (Fig. 3a). Fluorescence intensity started to grow dramatically when the content of water reached ca. 90% (Fig. 3c, Fig. 3d). Bathochromic shift of fluorescence maxima in the emission pattern, with the increment of water fraction can be explained by the gradual increase of polarity of the media, with the addition of polar water (Fig. 3a). Twisted conformation of incorporated multiphenyl ethylene unit linked to carbazolyl tetrafluorostyrene moiety prevents photoluminescence quenching in the solid state.


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Figure 3. PL spectra of C4FS1 (a) and C4FS2 (b) dispersed in THF:water mixtures with the different water content (fw); photoluminescence intensity versus water fraction in THF:H2O mixtures(d, c);

Photoluminescence quantum yields (PLQY) of toluene solutions of compound C4FS1 and C4FS2 were estimated to be 0.3% and 0.1% respectively, while those of the solid samples were found to be 57% and 28% respectively. Lower PLQY of C4FS2 can be assigned to its less twisted conformation compared to that of C4FS1, and thus higher possibility of intermolecular interactions.

Theoretical Calculations


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Theoretical geometry optimizations of the molecules in electronic ground state as well as distribution of electron density were performed with Gaussian 16 program using density functional (DFT) method with the 6-311G(d) basis set in vacuum

54

. HOMO was found to be

located over the entire molecule for both molecules, while LUMO is mainly located on electronaccepting tetrafluorostyrene and partially on the multi-phenyl ethylene scaffold as a result of planar tetrafluorophenyl˗C=C˗ tri/tetraphenylethylene manifold, that can be noticed from the top and side view of the optimized structures (Fig. 4).

Figure 4. Optimized geometries and HOMO / LUMO energy levels calculated using B3LYP/6311g basis set in Gaussian 09program. Eteorg (energy gap) = |LUMO˗HOMO|.

Electrochemical, photoelectrical and charge-transporting properties Ionization potentials, electron affinities and charge mobilities are the key parameters of organic electroactive materials used for the fabrication of optoelectronic devices. Cyclic voltammograms recorded for dimethylformamide solutions containing 0.1 M of tetrabutylammonium hexafluorophosphate (Bu4NPF6) as the supporting electrolyte are presented in Fig. 5a. For compounds C4FS1 and C4FS2 the irreversible oxidation waves, conceivable due to the active


17


unsubstituted C-3- and C-6 positions of carbazolyl moieties were detected

Page 18 of 37

55

. The oxidation

peaks were observed at 1.01 and 1.08 V for C4FS1 and C4FS2 respectively. The oxidation potentials determined from the onsets of the oxidation parts of the voltamperograms were found to be 0.83 and 0.95 V respectively. Irreversible reduction peaks were detected at -2.29 for C4FS1 and at 2.21 for C4FS2. The reduction potentials for C4FS1 and C4FS2 as the onsets of the reduction parts were estimated to be -2.10 and 2.01 V respectively.

Figure 5. a) Cyclic voltammograms of C4FS1 and C4FS2 recorded at a scan rate of 50 mV s˗1. b) Photoelectron emission spectra of the films of compounds C4FS1 and C4FS2.

Since C4FS1 and C4FS2 were designed for solid-state optoelectronic applications, energy levels of the layers of C4FS1 and C4FS2 were important to evaluate. Solid-state ionization potentials IPPE of the vacuum deposited films determined from photoelectron photoemission spectra were found to be comparable, i.e. 5.83eV for C4FS1 and 5.87eV C4FS2 (Fig. 5b). These values correlate with the values of ionization potentials (IPCV) determined by CV which were found to be 5.63eV and 5.75 eV respectively, and with theoretically calculated ones which were estimated to be 5.67eV and 5.73eV respectively. Optical band gaps (Eoptg) found for the films of C4FS1


18

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and C4FS2 from the onsets of the lowest energy absorption bands (2.81 eV and 2.89 eV) as well as electrochemical band gaps (2.93 eV and 2.96 eV) were found to be consistent with those obtained by theoretical calculations (3.18 eV and 3.28 eV) and showed the same tendency. С4FS2 exhibited slightly higher bandgap than C4FS1 (Table 3). Finally, solid-state electron affinities (EAPE) of 3.01 and 2.98 eV for C4FS1 and C4FS2 were estimated using question EA= IPPE-Eg and the optical band-gap energies. EAPE values were also found to be in good agreement with those obtained by electrochemical and theoretical techniques.

Table 3. Electrochemical and Photoelectrical Properties of C4FS1 and C4FS2. Compound

Eox a V

Ered a V

IPPE b, eV

IPCV c, eV

EACV c, eV

HOMO d eV

LUMO d eV

Eelcg,e eV

Eoptg,film,f eV

Eteorgg eV

C4FS1

0.83

-2.10

5.83

5.63

2.71

5.67

2.49

2.93

2.81

3.18

C4FS2

0.95

-2.01

5.87

5.75

2.79

5.73

2.45

2.96

2.89

3.28

a

Onset of the first oxidation/ reduction potential.

b

Solid-state ionization potential, measured by photoelectron emission

spectrometry. Determined by the formula IPCV =|e|(4.8 + Eox), EACV = |e|(4.8 + Ered). d Theoretically calculated with B3LYP/6c

311G level. e Electrochemical band gap, estimated using the approximation Eelcg = IPCV ˗ EACV. f Optical band gap, determined from the onset of the last absorption peak using the equation Eoptg = 1240/ λabsfilm. g Theoretical band gap Ecalcg = │HOMO – LUMO│.

To show the potential of C4FS1 and C4FS2 as organic semiconductors for optoelectronic applications, charge-transporting properties of the vacuum-deposited layers of C4FS1 and C4FS2 were studied by time of flight technique. Both the compounds showed bipolar charge transport. Time-of-flight current transient curves for holes and electrons recorded at the different electric fields are shown in Fig. 6a. The compounds showed low-dispersity transport of holes and electrons. Electric field dependencies of charge mobilities of the layers of C4FS1 and C4FS2 are presented in Fig. 6b. Compound C4FS1 containing tetraphenyl ethylene moiety


19


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exhibited more efficient and more balanced charge transport than compound C4FS2 having triphenyl ethylene moiety. C4FS1 showed slightly more efficient transport of electrons than that of holes while C4FS2 showed the opposite phenomenon. Hole mobilities in the layer of C4FS2 exceeded electron mobilities. In the layer of C4FS1 hole-drift mobility reached the value of 2.33×10˗3 cm2/Vs at electric field of 6.67×105 V cm˗1 while electron mobility was found to be 1.55×10˗3 cm2/Vs. For the layer of C4FS1 hole mobility of 1.42×10˗3 cm2/Vs and electron mobility of 5.78×10˗4 cm2/Vs were recorded at the same electric field electric field.

Figure 6. a) Time-of-flight current transients for holes and electrons for the layers of C4FS1 and C4FS2. b) Electric field dependencies of hole and electron drift mobilities for the vacuumdeposited layers of compounds C4FS1 and C4FS2 recorded at room temperature.

Electroluminescent properties Non-doped solid films of C4FS1 or C4FS2 are characterized by high PLQY values, relatively high bipolar charge mobilities, and appropriate energy levels for hole and electron injection which all are required for OLED applications. To demonstrate the potential of C4FS1 or C4FS2 in OLED applications, non-doped light-emitting layers of the compounds were tested in OLEDs with

the

simple

structures

of

ITO/MoO3(1nm)/NPB(60nm)/


C4FS1

or

C4FS2

20

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(30nm)/TPBi(60nm)/LiF(0.5nm)/Al (device A and device B respectively). The equilibrium energy diagram of the devices is presented in Fig. 7. Low injection barriers for holes and electrons between the device interfaces are observed. The determined by photoelectron spectroscopy ionization potentials of 5.83 and 5.87 eV (HOMO of solid samples) and estimated using the optical bandgap electron affinities of 3.02 and 2.98 eV (LUMO of solid samples) for C4FS1 or C4FS2 were used to design the device structures. Good charge injection properties for the devices A and B were expected using hole and electron transporting layers N,N′-di(1naphthyl)-N,N′-diphenyl-(1,1′-biphenyl)-4,4′-diamine (NPB) and 2,2’,2’’-(1,3,5-benzinetriyl)tris(1-phenyl-1-H-benzimidazole) (TPBi). Molybdenum trioxide (MoO3) and lithium fluoride (LiF) vacuum-deposited films were utilized as hole and electron-injecting layers, respectively. The layers of indium tin oxide (ITO) and aluminium (Al) played their usual roles of anode and cathode respectively.

Figure 7. Equilibrium energy diagrams for devices A-F. The values of IPPE and EAPE, estimated for solid films of C4FS1 or C4FS2, were used for the device design.

The intensity maxima of EL spectra were observed at 496 and 499 nm respectively at different external voltages (Fig. 8a, Table 4). These maxima were slightly blue shifted on 15 and 3 nm in


21


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comparison to the intensity maxima (511 and 502 nm) of the corresponding PL spectra of neat solid films C4FS1 and C4FS2, respectively (Fig. 2a,b, Table 2). The slight differences between PL and EL spectra of neat solid films C4FS1 and C4FS2 and devices A and B can be apparently explained by different applied optical and electrical excitation sources, respectively. For devices A and B, very intensive electroluminescence (EL) was observed at high voltages achieving maximum brightness of 12500 and 3200 cd/m2 at 7.5 V, respectively (Fig. 8b, Table 4). EL of both devices A and B are characterized by greenish-blue light with CIE coordinates of (0.185, 0.38) and (0.218, 0.367). Good charge injection properties, relatively low turn-on voltages of 3.8 V were observed for both devices A and B (Table 3). Mainly caused by fluorescent nature of EL, rather low maximum external quantum efficiencies (EQEmax) of 1.08 and 0.34 % were achieved for devices A and B, respectively. These values are lower than theoretical maximum (5%) for fluorescent devices without out-coupling mainly due to the relative low PLQYs of C4FS1 or C4FS2 in solid state as well as due to the lower than 100% charge recombination probability.


22

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Figure 8. a) EL spectra of C4FS1 and C4FS2 based devices A and B recorded at different voltages; b) Current density and brightness versus voltage; and c) Current and external quantum efficiencies versus brightness for devices A and B. Because of higher PLQY of C4FS1 in solid state and because of higher efficiency of device A in comparison to C4FS2 and device B, C4FS1 was selected for the fabrication of the optimized devices. For device C having the structure ITO/NPB(50nm)/TCTA(15nm) / C4FS1 (20nm)/TPBi(65nm)/LiF(0.5nm)/Al (Fig. 7), higher EQEmax of 1.7 % was achieved in comparison to the device A based on the same emitter C4FS1 (Table 4). Higher efficiency of device C is apparently mainly related to the increase of charge recombination probability. Despite of such optimization, EQEmax of device C is much lower than that of phosphorescent and ACS Paragon Plus Environment

23


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TADF OLEDs. Indeed, because of the 25% limit of the excitons generation probability for fluorescent devices, OLEDs based on many efficient fluorescent emitters suffer from low EQE. Therefore, the approach based on an interface exciplex-based host exhibiting TADF was used for the further increase of EQE of non-doped devices based on C4FS1. Taking into account bipolar charge-transporting property of C4FS1, light-emitting layer of device C was further modified placing

the

layers

3-bis(9-carbazolyl)benzene

(mCP)

and

2,4,6-tris[3-

(diphenylphosphinyl)phenyl]-1,3,5-triazine (PO-T2T) between two layers of C4FS1 (Fig. 7, Device D). Note, that a solid-state mixture of PO-T2T and (mCP) is characterized by exciplex emission

of

TADF

nature

ITO/NPB(50nm)/TCTA(15nm)/

56

.

As

a

C4FS1

result,

the

device

D

with

the

(10nm)/mCP(5nm)/POT2T(5nm)/

structure C4FS1

(10nm)/TPBi(65nm)/LiF(0.5nm)/Al was fabricated. Due to the HOMO and LUMO energy levels of mCP and PO-T2T (Fig. 7), a charge recombination zone in device D is predicted to be at the mCP/PO-T2T interface. As a result, both singlet and triplet excitones can be harvested at the mCP/PO-T2T interface in the device D due to TADF of the interface exciplex mCP/PO-T2T. This consideration is supported by higher EQEmax (2.4 %) observed for the device D in comparison to that of device C.


24

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Figure 9. a) Current density and brightness versus voltage; and b) Current and external quantum efficiencies versus brightness for C4FS1-based devices C, D, and E. c) PL spectrum of the solid mixture of mCP and PO-T2T, EL spectra of C4FS1-based devices C, D, and E recorded at 10 V.


25


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Taking into account that exciplexes can be formed even in the presence of some distance between donor and acceptor layers

57

, 5 nm-thick layer of emitter C4FS1 was placed in the

region of exciplex-formation of device D. As a result, device E with the structure ITO/NPB(50nm)/TCTA(15nm)/ C4FS1 (10nm)/mCP(5nm)/ C4FS1 (5 nm)/POT2T(5nm) / C4FS1 (10nm)/TPBi(65nm)/LiF(0.5nm)/Al was fabricated. EQEmax of device E was found to be of 2.6 % which is a little higher than that of device D (Fig. 9b). EL spectra of the C4FS1-based devices C, D, and E were found to be very similar to that of C4FS1-based device A which showed emission of C4FS1 (Fig. 9c). No influence of mCP/PO-T2T interface exciplex emission on EL spectra of studied devices was observed. Blue spectral shifts in the electroluminescenes of exciplex-based OLEDs with increasing of external voltages were previously observed due to the electron–hole coulomb attraction 58, 59. Noting, EL spectra of the devices D and E with exciplexbased host were not sensitive to applied voltages due to the effective energy transfer from the host mCP/PO-T2T to the emitter C4FS1 (Fig. S1). Brightness of devices C, D, E exceeded 10000 cd/m2 at high voltages (Fig. 9a, Table 4). Devices D and E are characterized by exciplex-enhanced efficiency demonstrating the potential of the proposed approach. We note that this approach of efficiency enhancement of fluorescent non-doped OLEDs may be utilized for many other effective fluorescent emitters including those exhibiting AIEE effect. EQEmax of exciplex-based device E was increased by more than 50% relative to that of device C.


26

Page 27 of 37

Table 4. Characteristics of OLEDs.

Device

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Structure

Von, V

ITO/MoO3/NPB/ 3.8 C4FS1/TPBi/LiF/Al ITO/MoO3/NPB/C4FS2 B 3.8 /TPBi/LiF/Al ITO/NPB/TCTA/ C4FS1 C 9.1 /TPBi/LiF/Al ITO/NPB/TCTA/ C4FS1 D /mCP/POT2T/ C4FS1 8.1 /TPBi/LiF/Al ITO/NPB/TCTA/ C4FS1 E /mCP/ C4FS1 /POT2T/ 9.1 C4FS1 /TPBi/LiF/Al a CIE for EL spectrum recorded at 10 V. A

Max. brightness, cd/m2

CEmax,

EQEmax %

λmaxEL, nm

CIE, (x, y)

12500

2.8

1.08

496

0.185, 0.38

3200

0.8

0.34

499

0.218, 0.367

11800

4.5

1.7

496

0.202, 0.399

14100

6.2

2.2

496

0.194, 0.379

16300

7.3

2.6

498

0.205, 0.404

Conclusions Derivatives of 4-carbazolyl-substituted tetrafluorostyrene and tri/tetraphenyl ethylene were synthesized and characterised as materials exhibiting both aggregation induced emission enhancement and bipolar charge-transporting properties. The difference in the molecular architecture of simply one phenyl ring can considerably change properties of compounds. The solid sample of compound having tetraphenylethylene moiety is characterized by twice higher photoluminescence quantum yield of 57% than the layer of compound having triphenylethylene moiety (27%). Tetraphenylethenyl-substituted compound exhibited red-shifted emission maxima (500 nm for solution and 514 nm for thin film) relative to triphenylethenyl-substituted counterpart (442nm for solution and 503 nm for thin film. Tetraphenylethenyl-substituted derivative demonstrated high thermal stability with 5% weight loss temperature of 361 °C and high glass transition temperature of 113°C. It showed relatively balanced bipolar charge transport with hole-drift mobility of 2.33×10˗3 cm2/Vs and elctron mobility of 1.55×10˗3 cm2/Vs at electric


27


Page 28 of 37

field of 6.67×105 V cm˗1. Non-doped OLEDs employing the synthesized compounds as emitters were fabricated and exhibited green electroluminescence. Exciplex-host based OLEDs containing one or two emitting layers of tetraphenylethenyl-containing emitter were fabricated and showed by more than 50% higher external quantum efficiency relative to that of the corresponding non-doped device. The best non-doped device based tetraphenylethenylcontaining emitter showed turn-on voltage of 9.1 V, maximum brightness of 11800 cd/m2, maximum current efficiency of 4.5 cd/A and external quantum efficiency of ca. 1.7 %. The best modified device, containing exciplex-forming host showed turn-on voltage of 9.1 V, maximum brightness of 16300 cd/m2 (at.. V), maximum current efficiency of 7.3 cd/A and external quantum efficiency of ca. 2.6 %.

Acknowledgements This research was funded by the European Social Fund according to the activity ‘Improvement of researchers’ qualification by implementing world-class R&D projects’ of Measure

No.

09.3.3-LMT-K-712

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