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Tetracyano-substituted spiro[fluorene-9,9′-xanthene] as electron acceptor for exciplex thermally activated delayed fluorescence
Journal of Molecular Structure 1196 (2019) 132e138
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Journal of Molecular Structure journal homepage: http://www.elsevier.com/locate/molstruc
Tetracyano-substituted spiro[fluorene-9,90 -xanthene] as electron acceptor for exciplex thermally activated delayed fluorescence Hong-Tao Cao a, 1, Chao-Shen Hong a, 1, Dan-Qing Ye a, Li-Hui Liu a, Ling-Hai Xie a, *, Shu-Fen Chen a, **, Chen Sun a, c, Sha-Sha Wang a, Hong-Mei Zhang a, Wei Huang a, b, *** a Center for Molecular Systems and Organic Devices (CMSOD), Key Laboratory for Organic Electronics and Information Displays & Institute of Advanced Materials (IAM), Jiangsu National Synergetic Innovation Center for Advanced Materials (SICAM), Nanjing University of Posts & Telecommunications (NUPT), 9 Wenyuan Road, Nanjing, 210023, PR China b Shaanxi Institute of Flexible Electronics (SIFE), Northwestern Polytechnical University (NPU), 127 West Youyi Road, Xi'an, 710072, Shaanxi, PR China c Madrid Institute for Advanced Studies in Nanoscience, IMDEA Nanociencia, Calle Faraday 9, Ciudad Universitaria de Cantoblanco, 28049, Spain
a r t i c l e i n f o
a b s t r a c t
Article history: Received 7 March 2019 Received in revised form 13 May 2019 Accepted 2 June 2019 Available online 21 June 2019
Herein, a tetracyano-substituted spiro[fluorene-9,90 -xanthene] (SFX) derivative, 2,30 ,60 ,7tetracarbonitrile-SFX (TCNSFX) was conveniently synthesized as electron acceptor for combining with tris(4-carbazoyl-9-ylphenyl)amine (TCTA) as electron donor to form exciplex emitter. Subsequently, exciplex-TADF was successfully realized with photoluminescence quantum yield of 29% and electroluminescence efficiency of 6.0 cd A1 in TCTA:TCNSFX. The exciplex-TADF performance of TCTA:TCNSFX is related to the stronger electron-accepting ability through tetracyano-substitution and large driving force in the exciplex emission. Although the emission efficiencies are slightly low, this work enriches the design system of electron acceptors for exciplex-TADF, simultaneously offering an efficient and convenient approach to developing low-cost materials and devices for TADF. © 2019 Published by Elsevier B.V.
Keywords: Spiro[fluorene-9,90 -xanthene] Tetracyano-substitution Electron acceptor Thermally activated delayed fluorescence Exciplex Organic light-emitting diode
1. Introduction Organic light-emitting diodes (OLEDs) have displayed a promising future because of their full-color, high-resolution, flat-panel displays and lighting techniques [1e6]. For luminescent materials in OLEDs, thermally activated delayed fluorescence (TADF)-based categories, harvesting both singlet and triplet excitons for light emission through efficient reverse intersystem crossing (RISC) from non-radiative triplet states (T1) to radiative singlet states (S1), have been designed and applied effectively in the past years [7e10]. So
* Corresponding author. ** Corresponding author. *** Corresponding author. Center for Molecular Systems and Organic Devices (CMSOD), Key Laboratory for Organic Electronics and Information Displays & Institute of Advanced Materials (IAM), Jiangsu National Synergetic Innovation Center for Advanced Materials (SICAM), Nanjing University of Posts & Telecommunications (NUPT), 9 Wenyuan Road, Nanjing, 210023, PR China. E-mail addresses: [email protected] (L.-H. Xie), [email protected] (S.-F. Chen), [email protected] (W. Huang). 1 These authors contributed equally to this work. https://doi.org/10.1016/j.molstruc.2019.06.003 0022-2860/© 2019 Published by Elsevier B.V.
far, high-efficiency OLEDs have been reported through utilizing pure TADF materials, which results in external quantum efficiencies (hext) above 20% [11e15]. In addition to pure TADF emitters, a new class of efficiency TADF-based OLEDs using exciplex emitters formed by blending electron-donating and electron-accepting molecules have been demonstrated [16e18]. This is due to the fact that the exciplex emitters can realize small DEST (0e0.05 eV) more easily since the electron and hole are located on two different molecules, giving small exchange energy [19e21]. Consequently, TADF contribution can be increased in exciplex-based OLEDs and theoretical 100% internal quantum efficiencies are favorably achieved as a result [17,22]. To date, a number of investigations on adopting exciplex emitters have been made to obtain TADF with promising luminescent properties [18,23e25]. For instance, Liu and co-workers employed 4,40 ,400 -tris[3-methylphenyl(phenyl)amino]triphenylamine (m-MTDATA) as electron donor and 1,3-bis(5-(4-(tert-butyl) phenyl)-1,3,4-oxadiazol-2-yl)benzene (OXD-7) as electron acceptor to successfully develop exciplex-TADF emitter, accompanied by an exciton utilization efficiency of 74.3% in the fabricated device [26]. Su et al. reported an exciplex system based on m-MTDATA as
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electron donor and 4,7-diphenyl-1,10-phenanthroline (Bphen) as electron acceptor in the obtained OLED, achieving a maximum external quantum efficiency (hext) of 7.79% and power efficiency (hp) of 12.97 lm W1 [27]. In 2016, Zhang and co-workers utilized an electron acceptor 2,4,6-tris[3-(diphenylphosphinyl)phenyl]1,3,5-triazine (PO-T2T) combined with a single-molecule TADF emitter 6-(9,9-dimethylacridin-10(9H)-yl)-3-methyl-1H-isochromen-1-one (MAC) to form exciplex emitter, achieving excellent maximum current efficiency (hc) of 52.1 cd A1, hp of 45.5 lm W1 and hext of 17.8% [28]. However, several problems are still remaining in exciplex-TADF although appealing performances have been realized. Firstly, most reported high-efficience exciplex emitters are mainly obtained through adopting oxadiazole, triarylboron, triazine, N-heterocycles and diphenylphosphine oxides groups as electron acceptors, which always suffer from complicated synthetic routes [17]. Secondly, obtaining clear molecular design methods of electron acceptors in exciplex-TADF is extremely urgent. Thus, it is of great requirement to develop novel design system of electron acceptor with convenient approach and investigate the structureproperty relationship for exciplex-TADF. Recently, we prepared two cyano-substituted spiro[fluorene9,90 -xanthene] (SFX) derivatives based on a convenient one-pot synthetic route and adopted them as electron acceptors to construct exciplex emitters [29,30]. Fortunately, efficient TADF is successfully achieved in the exciplex emitter formed by mixing the dicyano-substituted SFX derivative with tris(4-carbazoyl-9ylphenyl)amine (TCTA). Our results indicate that the cyanosubstitution in SFX can cause stronge electron-accepting ability and exciplex-TADF is thus obtained. Nevertheless, we discover that the reports on utilizing SFX or analogous spirocyclic skeleton to prepare electron acceptors in exciplex-TADF are still rare [24]. Consequently, we would like to make further structure modification in SFX to prepare electron acceptors, simultaneously evaluateing whether SFX can be used as an universal skeleton in designing electron acceptors for realizing exciplex-TADF. Herein, we design and synthesize a tetracyano-substituted SFX derivative, 2,30 ,60 ,7-tetracarbonitrile-SFX (TCNSFX, Scheme 1) as electron acceptor together with TCTA as electron donor to construct exciplex emitter. Encouragingly, TADF is then obtained in the TCTA:TCNSFX system accompanied by a maximum photoluminescence (PL) efficience of 29% and electroluminescence (EL) efficience of 6.0 cd A1. Despite the emission efficiencies are not very high, our results reveal that the SFX can be adopted as a convenient and universal skeleton in designing electron acceptors for achieving exciplexTADF.
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2. Experimental section 2.1. General information All reagents and solvents employed here were commercially available. The solvents for syntheses were freshly distilled over appropriate drying reagents. All experiments were performed under a nitrogen atmosphere by using standard Schlenk techniques. The intermediate product 2,30 ,60 ,7-tetrabromo-SFX (TBrSFX) was prepared by the unexpected one-pot method reported in our previous work [30], and the target product TCNSFX was obtained according to previously reported cyanozation method [31]. Subsequently, TBrSFX and TCNSFX were verified through 1H NMR and 13C NMR on a Bruker 400 MHz spectrometer in CDCl3 with tetramethylsilane as the interval standard. The molecular weights of them were tested on a Shimadzu GCMS 2010 PLUS. Element analyses were carried out on an Elementa Analysensysteme GmbH Vario EL III Instrument. 2.2. Synthesis 2.2.1. Synthesis of TBrSFX A mixture of 2,7-dibromo-9-fluorenone (5.07 g, 15.00 mmol) and 3-bromophenol (15.57 g, 90.00 mmol) in methanesulfonic acid (5.77 g, 60.00 mmol) was heated at 150 C under nitrogen for 8 h. After completion, the reaction mixture was vigorously stirred in ethanol to induce the precipitation of the crude product. Then, the precipitate was processed through drain filtration and washing with ethanol solution to obtain white solid TBrSFX in 75% yield (7.20 g, 11.11 mmol). 1H NMR (400 MHz, CDCl3, d [ppm]): 7.62e7.64 (d, J ¼ 8.0 Hz, 2H), 7.51e7.53 (d, J ¼ 8.0 Hz, 2H), 7.41 (s, 2H), 7.21 (s, 2H), 7.96e7.98 (d, J ¼ 8.0 Hz, 2H), 6.22e6.24 (d, J ¼ 8.0 Hz, 2H). GCMS (EI-m/z): 647.72. 2.2.2. Synthesis of TCNSFX A mixture of TBrSFX (0.50 g, 0.78 mmol), K4[Fe(CN)6]$3H2O (1.32 g, 3.12 mmol), sodium carbonate (0.08 g, 0.78 mmol) and Pd(OAc)2 (0.01 g, 0.03 mmol) in N,N-Dimethylformamide (DMF) (10 mL) was heated at 150 C under nitrogen for 24 h. The reaction mixture was then cooled down to room temperature and extracted with dichloromethane. The organic extract was dried with MgSO4 and concentrated through rotary evaporation. The crude product was purified by silica gel column chromatography using an petroleum ethereethyl acetate mixture (5:1, v/v) as eluent to afford a light yellow solid TCNSFX in 62% yield (0.21 g, 0.49 mmol). 1H NMR (400 MHz, CDCl3, d [ppm]): 8.00e8.02 (d, J ¼ 8.0 Hz, 2H), 7.81e7.83
Scheme 1. Synthetic routes of compound TCNSFX.
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(d, J ¼ 8.0 Hz, 2H), 7.65 (s, 2H), 7.44 (s, 2H), 7.17e7.19 (d, J ¼ 8.0 Hz, 2H), 6.41e6.43 (d, J ¼ 8.0 Hz, 2H). GC-MS (EI-m/z): 432.06. 2.3. Photophysical measurements Uv-vis absorption spectra were recorded on Shimadzu UV-3150 spectrometer. Emission spectra were using Shimadzu RF-530XPC luminescence spectrometer. The films of TCTA, TCNSFX and TCTA:TCNSFX (2:1, wt/wt) were prepared through vacuum evaporation. The fluorescence and phosphorescence spectra were mearsured at 77 K on a Hitachi F-4600 fluorescence spectrometer. The transient photoluminance decay characteristics and photoluminescence quantum yields (PLQYs, Fp) were tested using an Edinburgh Instruments FLS920 spectrometer. The temperaturedependent transient fluorescence decay characteristics were monitored with a TimeHarp 260 Time Correlated Single Photon Counting and Multichannel Scaling Board, and excitation at 355 nm was provided by a TEEM Photonics passive Q-switch Nd:YAG laser delivering 300 ps pulses at 75 Hz. 2.4. Electrochemical measurements Cyclic Voltammetric (CV) studies were conducted at room temperature on the CHI660E system in a typical three-electrode cell with a platinum sheet working electrode, a platinum wire counter electrode, and a silver/silver nitrate (Ag/Agþ) reference electrode. All electrochemical experiments were carried out under a nitrogen atmosphere at room temperature in an electrolyte solution of 0.1 M tetra-butylammonium hexafluorophosphate (Bu4NPF6) in CH2Cl2 at a sweeping rate of 0.1 V s1. According to the redox onset potentials of the CV measurements, the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of the materials are estimated based on the reference energy level of ferrocene (4.80 eV below the vacuum): HOMO/ LUMO ¼ e (Eox/Ered e 0.04 V) e 4.80 eV, where the value 0.04 V is for ferrocene vs Ag/Agþ. 2.5. Device fabrication and characterization Pre-cleaned indium-tin-oxide (ITO)-coated glass substrates (the sheet resistance is 20 U) were treated with UV-ozone for 15 min. A 45 nm thick Poly(3,4ethylenedioxythiophene):Poly(styrenesulfonate) (PEDOT: PSS) (Baytron P AI 4083) layer was spin-coated onto the ITO substrate and then baked at 120 C for 30 min in oven. Subsequently, 1,10bis(di-4-tolylaminophenyl) cyclohexane (TAPC), 4,40 ,400 -tris(N-carbazolyl)-triphenylamine (TCTA), TCNSFX, 1,3,5-tri[(3-pyridyl)phen-3-yl]benzene (TmPyPB), LiF, and Al were thermally evaporated on the PEDOT:PSS surface directly at a pressure of less than 4 104 Pa in the vacuum chamber. The emission area of the device is 4 4 mm2 as defined by the overlapping area of the anode and cathode. The current-voltage-brightness characteristics and EL spectra were measured with a Keithley 2400 source meter and a coupled PR655 spectroscan photometer. All the measurements were carried out at 298 K. 3. Results and discussion 3.1. Photophysical properties Herein, UV-vis absorption and emission spectra of the exciplex emitters together with the corresponding constituting molecules in solid films are firstly measured. Fig. 1 shows that normalized absorption spectrum of TCTA:TCNSFX displays similar pattern compared with those of TCTA and TCNSFX, indicating that no
intrinsic electronic interaction takes place on the ground state level. The emission spectrum of TCTA:TCNSFX presents red-shifted emission as compared to those of the constituting molecules. Fig. 1d reveals that the emission of TCTA:TCNSFX shows a maximum peak at 550 nm, which exhibits obvious red-shift compared to those of TCTA (403 nm) and TCNSFX (422 nm). These results imply that there is formation of new excited state in the mixed film TCTA:TCNSFX. According to the reported work [19,32], exciplex emission is considered to be occurring here under excitation because there are no new ground-state transitions in the absorption spectrum of the mixed film compared with those of the neat films. The relevant data described here have been collected in Table 1. Transient fluorescence decays of TCTA:TCNSFX film together with TCTA and TCNSFX pristine films were then measured. Fig. 2a and Table 1 reveal that the neat films of TCTA and TCNSFX exhibit two decay components with times of 0.9 and 7.2 ns, 3.5 and 16.1 ns, respectively. The measurements show that the TCTA:TCNSFX film displays a prompt fluorescence decay (t ¼ 2.1 ns) and a delayed fluorescence decay (t ¼ 219.0 ns) at 300 K. The observed delayed fluorescence is suggested to be the result from the recursive S1/S0 transition though successive upconversion of the triplet excitons, which is often observed in pure TADF emitters [9,33e35]. Moreover, temperature-dependent transient fluorescence decay measurements were performed to confirm TADF occurs in the TCTA:TCNSFX film. Fig. 2b and Table S1 (Supporting Information) show that the PL intensity of the delayed component for the TCTA:TCNSFX film is rising with the increase of temperature from 77 to 300 K. This result indicates that thermal activation energy for the delayed fluorescence exists in the exciplex emission from the TCTA:DCNSFX film [11,36]. The singletetriplet energy splitting property of the TCTA:TCNSFX mixed film together with TCTA and TCNSFX pristine films were investigated for better understanding of the TADF feature observed here. We measured the film-state fluorescence and phosphorescence spectra of TCTA, TCNSFX and TCTA:TCNSFX at 77 K, simultaneously presenting the corresponding results in Fig. 2 and Fig. S1. The DEST of TCTA, TCNSFX and TCTA:TCNSFX are then estimated to be 0.54, 0.64 and 0.04 eV, respectively, according to their onsets of its fluorescence and phosphorescence spectra. These results suggest that the TCTA:TCNSFX exhibits a smaller DEST which promotes an efficient RISC of triplet excitons, and TADF is achieved for TCTA:TCNSFX as a result. Morever, the exciplex formation situation was investigated for TCTA:TCNSFX because the exciplex-TADF of TCTA:TCNSFX film is directly related to its exciplex emission. According to the reported work [32,36e38], the driving force (DGcs) for exciplex formation in the solid state can be evaluated through a modified RehmeWeller equation [37,39], and this value (DGcs) is given approximately by the difference in the exciton energy (EA* or ED*) of the constituting molecules with the exciplex energy (Eexciplex). Herein, EA* and ED* represent the exciton energy of the acceptor and donor, respectively, and Eexciplex is the exciplex photon energy. Therefore, the Eexciplex of TCTA:TCNSFX is estimated to be 2.77 eV through subtracting the Ered value (2.01 eV) of TCNSFX from the Eox value (0.76 eV) of TCTA, which has been collected in Table S2. Using the S1 value (2.94 eV) of TCNSFX as the EA* value, the DGcs value is calculated to be 0.17 eV for TCTA:TCNSFX through subtracting the Eexciplex value (2.77 eV) from the EA* value (2.94 eV). The TCTA:TCNSFX film is thus believed to display exciplex emission, which have been confirmed in Fig. 1. It is well known that the exciplex-TADF generates based on the exciplex state. Hence, the TADF obtained in TCTA:TCNSFX is partially related to its larger driving force in exciplex emission. Table 1 shows that the PLQY of TCTA:TCNSFX film is measured to
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Fig. 1. Molecular structures of (a) TCTA and (b) TCNSFX; (c) UV-vis absorption and (d) PL spectra of deposited films for TCTA, TCNSFX and TCTA:TCNSFX, respectively.
Table 1 Photophysical properties of the mixed exciplex and relevant pristine films. Emitters
TCTA TCNSFX TCTA:TCNSFX a b c
lPL, maxa (nm)
403 422 550
Fpa (%)
16 22 29
S1b (eV)
3.20 2.94 2.25
T1b (eV)
2.66 2.30 2.21
DESTc (eV)
0.54 0.64 0.04
Decay timea
t (ns)
c2
0.9 (90%), 7.2 (10%) 3.5 (86%), 16.1 (14%) 219.0 (47%), 2.1 (26%), 14.0 (27%)
1.061 1.034 1.012
Measured in deposited films at 300 K. Estimated from high-energy peaks of film-state fluorescence spectra at 77 K. S1T1.
be 29%. Obviously, the PLQY of TCTA:TCNSFX is attributable to its exciplex-TADF feature even though this value is not very high. Fig. 2d shows that the TCTA:TCNSFX film possesses almost identical fluorescence and phosphorescence spectra at 77 K, which implies smaller singletetriplet energy splitting exists in this exciplex emission (Table 1). Furthermore, this situation is beneficial to achieving efficient RISC of triplet excitons in TCTA:TCNSFX film, which is similar to those of many reported exciplex-TADF emitters [25,36]. Whereas, Table 1 shows that the T1 (2.30 eV) of TCNSFX is close to that (2.21 eV) of TCTA:TCNSFX, which implies that there might be energy leakage occurring from TCTA:TCNSFX states to the T1 excited states of TCNSFX. More importantly, Fig. 2d depicts that the phosphorescence spectrum of TCTA:TCNSFX displays week peak around 450 nm, which possesses similar character and spectrum shape compared with the phosphorescence spectrum of TCNSFX. These results indicate that TCNSFX might have slight negative effect on the luminescent efficiency of TCTA:TCNSFX. As a result, the energy leakage maybe becomes a competitor to the emission process of TCTA:TCNSFX, and inevitably results in its lower PLQY. It should be noted that, TCNSFX is still presenting
interesting perspectives for developing favorable exciplex-TADF materials in the future even though the PLQY of TCTA:TCNSFX is not very high. 3.2. Electrochemical properties According to the reported work, the driving force for exciplex emission is directly related to the solution redox potentials of the constituting molecules. Additionally, the DGcs value is linear with e(Eox, A e Eox, D) [36]. Here, CV measurements of the constituting molecules TCTA and TDCNSFX were performed to study the exciplex emission of TCTA:TCNSFX. Table S2 reveals that compound TCNSFX displays smaller LUMO level (2.75 eV) than CNSFX (2.41 eV) that has been reported in our previous work [29]. This result implies that introduction of electron-withdrawing cyano groups into SFX framework induces effective lowering of the LUMO level energies and stronger electron-accepting ability, which has been proven by our early theory calculation results [40]. It is believed that the donor and acceptor strength play a leading role in the exciplex formation. Enhancing the acceptor strength of the
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Fig. 2. (a) Fluorescence decays of deposited films at 300 K for TCTA, TCNSFX and TCTA:TCNSFX; (b) temperature-dependent transient fluorescence decays of deposited film for TCTA:TCNSFX; film-state fluorescence and phosphorescence spectra at 77 K for (c) TCNSFX and (d) TCTA:TCNSFX.
acceptor will cause large charge separation in the molecule after excitation, which assists the exciplex formation [41]. Therefore, the stronger electron-accepting ability from the tetracyanosubstitution in TCNSFX results in larger driving force for exciplex emission of TCTA:DCNSFX, which indirectly promotes the formation of exciplex-TADF.
3.3. Electroluminescence properties To investigate the EL performance of the exciplex-TADF from TCTA:TCNSFX film, this exciplex system was then adopted as the emitting layers to fabricate OLED through vacuum evaporation method. The resultant device is named as A, which possesses a configuration of indium tin oxide (ITO)/PEDOT:PSS (2500 rpm)/ TAPC (40 nm)/TCTA:TCNSFX (2:1, wt/wt, 30 nm)/TmPyPB (50 nm)/ LiF (0.5 nm)/Al (100 nm), as shown in Fig. 3a. Chemical structures of the materials used here are exhibited in Fig. S2 (Supporting Information). The PEDOT:PSS, TAPC, TmPyPB and LiF are used as the hole injection layer (HIL), hole-transporting layer (HTL), electron transporting layer (ETL) and electron-injection layer (EIL), respectively. As displayed in Fig. 3b, device A presents the EL spectrum with a maximum at 564 nm, which is similar to its PL profile in Fig. 1. It indicates that the emission observed here occurs from the TCTA:TCNSFX film. Device A shows a CIE coordinate of (0.47, 0.51) under an applied voltage of 8 V. Fig. 3c exhibits the current densityevoltageeluminance characteristic of device A. It displays that a turn-on voltage (defined as the bias at a brightness of 1 cd m2) of 5.1 V and a low luminance of 653 cd m2 are achieved in this device.
The low luminance of device A is directly related to its exciplexTADF feature and slightly low PLQY. Table 2 has summarized the relevant EL performance of device A. In Fig. 3d, device A exhibits a maximum hc of 6.0 cd A1, hp of 2.7 lm W1 and hext of 2.3%, which are slightly lower than those (8.2 cd A1, 4.3 lm W1, 3.0%) of our reported work [29], respectively. Obviously, the slight low electroluminescence efficiency (2.3%) of device A are ascribed to the low PLQY of the TCTA:DCNSFX film, which is similar to that (2%) of reported exciplex-TADF [16]. Despite electroluminescence efficiency of device A is not high, TCNSFX still displays interesting exciplex-TADF feature for developing OLEDs. This low hext provides much room for performance improvement of exciplex-TADF in our future work. More importantly, these results also suggest that SFX can be used as an universal skeleton in designing electron acceptors for realizing exciplex-TADF. The preparation of other novel SFX-based electron acceptors for exciplex-TADF is thus expected in the future research. Therefore, our work enriches the design system of electron acceptors for exciplex-TADF and offers an efficient and convenient approach to developing low-cost materials and devices for TADF.
4. Conclusion In summary, we have prepared a tetracyano-substituted SFX derivative (TCNSFX) as electron acceptor with convenient and lowcost method. TCTA is selected as electron donor to form exciplex emitter. In TCTA:TCNSFX film, exciplex-TADF is successfully obtained with PL (29%) and EL efficiencies (6.0 cd A1, 2.7 lm W1). The slightly low EL efficiencies are related to the small PLQY in
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Fig. 3. (a) The general structure, (b) EL spectra (at 8 V), (c) current densityevoltage-luminance and (d) current efficiencyeluminanceepower efficiency characteristics for OLED A.
Table 2 Summary of electroluminescence performance for OLED A. Device
Vturn-ona (V)
Lmaxb (cd m2)
hcb (cd A1)
hpb (lm W1)
hextb (%)
lEL (nm)
CIE [(x, y), V]
A
5.0
653
6.0
2.7
2.3
564
(0.47, 0.51), 8.0
a b
Defined as the bias at a brightness of 1 cd m2. Maximum values of the device. Values after the comma are the voltages at which the data are collected.
TCTA:TCNSFX film. This work enriches the design system of electron acceptors for exciplex-TADF, simultaneously offering an efficient and convenient approach to developing low-cost materials and devices for TADF. Acknowledgements The authors gratefully acknowledge financial support from the National Foundation for Science and Technology Development of China (2015CB932203), the National Key Research and Development Program of China (2017YFB0404501), NSFC (61605090, 61604081, 51333007, 61705111, 61704091, 61274065 and 61705111), the Science Fund for Distinguished Young Scholars of Jiangsu Province of China (BK20160039), the Natural Science Foundation of Jiangsu Province (BK20170899), the National Postdoctoral Program for Innovative Talents (BX201700122), the Six Peak Talents Foundation of Jiangsu Province (XCL-CXTD-009), the China Scholarship Council (201608390023) and the Nanjing University of Posts and Telecommunications Scientific Foundation (NY215061) and the Open Foundation from Jilin University (IOSKL2017KF04). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.molstruc.2019.06.003.
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Tetracyano-substituted spiro[fluorene-9,90 -xanthene] as electron acceptor for exciplex thermally activated delayed fluorescence Hong-Tao Cao a, 1, Chao-Shen Hong a, 1, Dan-Qing Ye a, Li-Hui Liu a, Ling-Hai Xie a, *, Shu-Fen Chen a, **, Chen Sun a, c, Sha-Sha Wang a, Hong-Mei Zhang a, Wei Huang a, b, *** a Center for Molecular Systems and Organic Devices (CMSOD), Key Laboratory for Organic Electronics and Information Displays & Institute of Advanced Materials (IAM), Jiangsu National Synergetic Innovation Center for Advanced Materials (SICAM), Nanjing University of Posts & Telecommunications (NUPT), 9 Wenyuan Road, Nanjing, 210023, PR China b Shaanxi Institute of Flexible Electronics (SIFE), Northwestern Polytechnical University (NPU), 127 West Youyi Road, Xi'an, 710072, Shaanxi, PR China c Madrid Institute for Advanced Studies in Nanoscience, IMDEA Nanociencia, Calle Faraday 9, Ciudad Universitaria de Cantoblanco, 28049, Spain
a r t i c l e i n f o
a b s t r a c t
Article history: Received 7 March 2019 Received in revised form 13 May 2019 Accepted 2 June 2019 Available online 21 June 2019
Herein, a tetracyano-substituted spiro[fluorene-9,90 -xanthene] (SFX) derivative, 2,30 ,60 ,7tetracarbonitrile-SFX (TCNSFX) was conveniently synthesized as electron acceptor for combining with tris(4-carbazoyl-9-ylphenyl)amine (TCTA) as electron donor to form exciplex emitter. Subsequently, exciplex-TADF was successfully realized with photoluminescence quantum yield of 29% and electroluminescence efficiency of 6.0 cd A1 in TCTA:TCNSFX. The exciplex-TADF performance of TCTA:TCNSFX is related to the stronger electron-accepting ability through tetracyano-substitution and large driving force in the exciplex emission. Although the emission efficiencies are slightly low, this work enriches the design system of electron acceptors for exciplex-TADF, simultaneously offering an efficient and convenient approach to developing low-cost materials and devices for TADF. © 2019 Published by Elsevier B.V.
Keywords: Spiro[fluorene-9,90 -xanthene] Tetracyano-substitution Electron acceptor Thermally activated delayed fluorescence Exciplex Organic light-emitting diode
1. Introduction Organic light-emitting diodes (OLEDs) have displayed a promising future because of their full-color, high-resolution, flat-panel displays and lighting techniques [1e6]. For luminescent materials in OLEDs, thermally activated delayed fluorescence (TADF)-based categories, harvesting both singlet and triplet excitons for light emission through efficient reverse intersystem crossing (RISC) from non-radiative triplet states (T1) to radiative singlet states (S1), have been designed and applied effectively in the past years [7e10]. So
* Corresponding author. ** Corresponding author. *** Corresponding author. Center for Molecular Systems and Organic Devices (CMSOD), Key Laboratory for Organic Electronics and Information Displays & Institute of Advanced Materials (IAM), Jiangsu National Synergetic Innovation Center for Advanced Materials (SICAM), Nanjing University of Posts & Telecommunications (NUPT), 9 Wenyuan Road, Nanjing, 210023, PR China. E-mail addresses: [email protected] (L.-H. Xie), [email protected] (S.-F. Chen), [email protected] (W. Huang). 1 These authors contributed equally to this work. https://doi.org/10.1016/j.molstruc.2019.06.003 0022-2860/© 2019 Published by Elsevier B.V.
far, high-efficiency OLEDs have been reported through utilizing pure TADF materials, which results in external quantum efficiencies (hext) above 20% [11e15]. In addition to pure TADF emitters, a new class of efficiency TADF-based OLEDs using exciplex emitters formed by blending electron-donating and electron-accepting molecules have been demonstrated [16e18]. This is due to the fact that the exciplex emitters can realize small DEST (0e0.05 eV) more easily since the electron and hole are located on two different molecules, giving small exchange energy [19e21]. Consequently, TADF contribution can be increased in exciplex-based OLEDs and theoretical 100% internal quantum efficiencies are favorably achieved as a result [17,22]. To date, a number of investigations on adopting exciplex emitters have been made to obtain TADF with promising luminescent properties [18,23e25]. For instance, Liu and co-workers employed 4,40 ,400 -tris[3-methylphenyl(phenyl)amino]triphenylamine (m-MTDATA) as electron donor and 1,3-bis(5-(4-(tert-butyl) phenyl)-1,3,4-oxadiazol-2-yl)benzene (OXD-7) as electron acceptor to successfully develop exciplex-TADF emitter, accompanied by an exciton utilization efficiency of 74.3% in the fabricated device [26]. Su et al. reported an exciplex system based on m-MTDATA as
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electron donor and 4,7-diphenyl-1,10-phenanthroline (Bphen) as electron acceptor in the obtained OLED, achieving a maximum external quantum efficiency (hext) of 7.79% and power efficiency (hp) of 12.97 lm W1 [27]. In 2016, Zhang and co-workers utilized an electron acceptor 2,4,6-tris[3-(diphenylphosphinyl)phenyl]1,3,5-triazine (PO-T2T) combined with a single-molecule TADF emitter 6-(9,9-dimethylacridin-10(9H)-yl)-3-methyl-1H-isochromen-1-one (MAC) to form exciplex emitter, achieving excellent maximum current efficiency (hc) of 52.1 cd A1, hp of 45.5 lm W1 and hext of 17.8% [28]. However, several problems are still remaining in exciplex-TADF although appealing performances have been realized. Firstly, most reported high-efficience exciplex emitters are mainly obtained through adopting oxadiazole, triarylboron, triazine, N-heterocycles and diphenylphosphine oxides groups as electron acceptors, which always suffer from complicated synthetic routes [17]. Secondly, obtaining clear molecular design methods of electron acceptors in exciplex-TADF is extremely urgent. Thus, it is of great requirement to develop novel design system of electron acceptor with convenient approach and investigate the structureproperty relationship for exciplex-TADF. Recently, we prepared two cyano-substituted spiro[fluorene9,90 -xanthene] (SFX) derivatives based on a convenient one-pot synthetic route and adopted them as electron acceptors to construct exciplex emitters [29,30]. Fortunately, efficient TADF is successfully achieved in the exciplex emitter formed by mixing the dicyano-substituted SFX derivative with tris(4-carbazoyl-9ylphenyl)amine (TCTA). Our results indicate that the cyanosubstitution in SFX can cause stronge electron-accepting ability and exciplex-TADF is thus obtained. Nevertheless, we discover that the reports on utilizing SFX or analogous spirocyclic skeleton to prepare electron acceptors in exciplex-TADF are still rare [24]. Consequently, we would like to make further structure modification in SFX to prepare electron acceptors, simultaneously evaluateing whether SFX can be used as an universal skeleton in designing electron acceptors for realizing exciplex-TADF. Herein, we design and synthesize a tetracyano-substituted SFX derivative, 2,30 ,60 ,7-tetracarbonitrile-SFX (TCNSFX, Scheme 1) as electron acceptor together with TCTA as electron donor to construct exciplex emitter. Encouragingly, TADF is then obtained in the TCTA:TCNSFX system accompanied by a maximum photoluminescence (PL) efficience of 29% and electroluminescence (EL) efficience of 6.0 cd A1. Despite the emission efficiencies are not very high, our results reveal that the SFX can be adopted as a convenient and universal skeleton in designing electron acceptors for achieving exciplexTADF.
133
2. Experimental section 2.1. General information All reagents and solvents employed here were commercially available. The solvents for syntheses were freshly distilled over appropriate drying reagents. All experiments were performed under a nitrogen atmosphere by using standard Schlenk techniques. The intermediate product 2,30 ,60 ,7-tetrabromo-SFX (TBrSFX) was prepared by the unexpected one-pot method reported in our previous work [30], and the target product TCNSFX was obtained according to previously reported cyanozation method [31]. Subsequently, TBrSFX and TCNSFX were verified through 1H NMR and 13C NMR on a Bruker 400 MHz spectrometer in CDCl3 with tetramethylsilane as the interval standard. The molecular weights of them were tested on a Shimadzu GCMS 2010 PLUS. Element analyses were carried out on an Elementa Analysensysteme GmbH Vario EL III Instrument. 2.2. Synthesis 2.2.1. Synthesis of TBrSFX A mixture of 2,7-dibromo-9-fluorenone (5.07 g, 15.00 mmol) and 3-bromophenol (15.57 g, 90.00 mmol) in methanesulfonic acid (5.77 g, 60.00 mmol) was heated at 150 C under nitrogen for 8 h. After completion, the reaction mixture was vigorously stirred in ethanol to induce the precipitation of the crude product. Then, the precipitate was processed through drain filtration and washing with ethanol solution to obtain white solid TBrSFX in 75% yield (7.20 g, 11.11 mmol). 1H NMR (400 MHz, CDCl3, d [ppm]): 7.62e7.64 (d, J ¼ 8.0 Hz, 2H), 7.51e7.53 (d, J ¼ 8.0 Hz, 2H), 7.41 (s, 2H), 7.21 (s, 2H), 7.96e7.98 (d, J ¼ 8.0 Hz, 2H), 6.22e6.24 (d, J ¼ 8.0 Hz, 2H). GCMS (EI-m/z): 647.72. 2.2.2. Synthesis of TCNSFX A mixture of TBrSFX (0.50 g, 0.78 mmol), K4[Fe(CN)6]$3H2O (1.32 g, 3.12 mmol), sodium carbonate (0.08 g, 0.78 mmol) and Pd(OAc)2 (0.01 g, 0.03 mmol) in N,N-Dimethylformamide (DMF) (10 mL) was heated at 150 C under nitrogen for 24 h. The reaction mixture was then cooled down to room temperature and extracted with dichloromethane. The organic extract was dried with MgSO4 and concentrated through rotary evaporation. The crude product was purified by silica gel column chromatography using an petroleum ethereethyl acetate mixture (5:1, v/v) as eluent to afford a light yellow solid TCNSFX in 62% yield (0.21 g, 0.49 mmol). 1H NMR (400 MHz, CDCl3, d [ppm]): 8.00e8.02 (d, J ¼ 8.0 Hz, 2H), 7.81e7.83
Scheme 1. Synthetic routes of compound TCNSFX.
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(d, J ¼ 8.0 Hz, 2H), 7.65 (s, 2H), 7.44 (s, 2H), 7.17e7.19 (d, J ¼ 8.0 Hz, 2H), 6.41e6.43 (d, J ¼ 8.0 Hz, 2H). GC-MS (EI-m/z): 432.06. 2.3. Photophysical measurements Uv-vis absorption spectra were recorded on Shimadzu UV-3150 spectrometer. Emission spectra were using Shimadzu RF-530XPC luminescence spectrometer. The films of TCTA, TCNSFX and TCTA:TCNSFX (2:1, wt/wt) were prepared through vacuum evaporation. The fluorescence and phosphorescence spectra were mearsured at 77 K on a Hitachi F-4600 fluorescence spectrometer. The transient photoluminance decay characteristics and photoluminescence quantum yields (PLQYs, Fp) were tested using an Edinburgh Instruments FLS920 spectrometer. The temperaturedependent transient fluorescence decay characteristics were monitored with a TimeHarp 260 Time Correlated Single Photon Counting and Multichannel Scaling Board, and excitation at 355 nm was provided by a TEEM Photonics passive Q-switch Nd:YAG laser delivering 300 ps pulses at 75 Hz. 2.4. Electrochemical measurements Cyclic Voltammetric (CV) studies were conducted at room temperature on the CHI660E system in a typical three-electrode cell with a platinum sheet working electrode, a platinum wire counter electrode, and a silver/silver nitrate (Ag/Agþ) reference electrode. All electrochemical experiments were carried out under a nitrogen atmosphere at room temperature in an electrolyte solution of 0.1 M tetra-butylammonium hexafluorophosphate (Bu4NPF6) in CH2Cl2 at a sweeping rate of 0.1 V s1. According to the redox onset potentials of the CV measurements, the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of the materials are estimated based on the reference energy level of ferrocene (4.80 eV below the vacuum): HOMO/ LUMO ¼ e (Eox/Ered e 0.04 V) e 4.80 eV, where the value 0.04 V is for ferrocene vs Ag/Agþ. 2.5. Device fabrication and characterization Pre-cleaned indium-tin-oxide (ITO)-coated glass substrates (the sheet resistance is 20 U) were treated with UV-ozone for 15 min. A 45 nm thick Poly(3,4ethylenedioxythiophene):Poly(styrenesulfonate) (PEDOT: PSS) (Baytron P AI 4083) layer was spin-coated onto the ITO substrate and then baked at 120 C for 30 min in oven. Subsequently, 1,10bis(di-4-tolylaminophenyl) cyclohexane (TAPC), 4,40 ,400 -tris(N-carbazolyl)-triphenylamine (TCTA), TCNSFX, 1,3,5-tri[(3-pyridyl)phen-3-yl]benzene (TmPyPB), LiF, and Al were thermally evaporated on the PEDOT:PSS surface directly at a pressure of less than 4 104 Pa in the vacuum chamber. The emission area of the device is 4 4 mm2 as defined by the overlapping area of the anode and cathode. The current-voltage-brightness characteristics and EL spectra were measured with a Keithley 2400 source meter and a coupled PR655 spectroscan photometer. All the measurements were carried out at 298 K. 3. Results and discussion 3.1. Photophysical properties Herein, UV-vis absorption and emission spectra of the exciplex emitters together with the corresponding constituting molecules in solid films are firstly measured. Fig. 1 shows that normalized absorption spectrum of TCTA:TCNSFX displays similar pattern compared with those of TCTA and TCNSFX, indicating that no
intrinsic electronic interaction takes place on the ground state level. The emission spectrum of TCTA:TCNSFX presents red-shifted emission as compared to those of the constituting molecules. Fig. 1d reveals that the emission of TCTA:TCNSFX shows a maximum peak at 550 nm, which exhibits obvious red-shift compared to those of TCTA (403 nm) and TCNSFX (422 nm). These results imply that there is formation of new excited state in the mixed film TCTA:TCNSFX. According to the reported work [19,32], exciplex emission is considered to be occurring here under excitation because there are no new ground-state transitions in the absorption spectrum of the mixed film compared with those of the neat films. The relevant data described here have been collected in Table 1. Transient fluorescence decays of TCTA:TCNSFX film together with TCTA and TCNSFX pristine films were then measured. Fig. 2a and Table 1 reveal that the neat films of TCTA and TCNSFX exhibit two decay components with times of 0.9 and 7.2 ns, 3.5 and 16.1 ns, respectively. The measurements show that the TCTA:TCNSFX film displays a prompt fluorescence decay (t ¼ 2.1 ns) and a delayed fluorescence decay (t ¼ 219.0 ns) at 300 K. The observed delayed fluorescence is suggested to be the result from the recursive S1/S0 transition though successive upconversion of the triplet excitons, which is often observed in pure TADF emitters [9,33e35]. Moreover, temperature-dependent transient fluorescence decay measurements were performed to confirm TADF occurs in the TCTA:TCNSFX film. Fig. 2b and Table S1 (Supporting Information) show that the PL intensity of the delayed component for the TCTA:TCNSFX film is rising with the increase of temperature from 77 to 300 K. This result indicates that thermal activation energy for the delayed fluorescence exists in the exciplex emission from the TCTA:DCNSFX film [11,36]. The singletetriplet energy splitting property of the TCTA:TCNSFX mixed film together with TCTA and TCNSFX pristine films were investigated for better understanding of the TADF feature observed here. We measured the film-state fluorescence and phosphorescence spectra of TCTA, TCNSFX and TCTA:TCNSFX at 77 K, simultaneously presenting the corresponding results in Fig. 2 and Fig. S1. The DEST of TCTA, TCNSFX and TCTA:TCNSFX are then estimated to be 0.54, 0.64 and 0.04 eV, respectively, according to their onsets of its fluorescence and phosphorescence spectra. These results suggest that the TCTA:TCNSFX exhibits a smaller DEST which promotes an efficient RISC of triplet excitons, and TADF is achieved for TCTA:TCNSFX as a result. Morever, the exciplex formation situation was investigated for TCTA:TCNSFX because the exciplex-TADF of TCTA:TCNSFX film is directly related to its exciplex emission. According to the reported work [32,36e38], the driving force (DGcs) for exciplex formation in the solid state can be evaluated through a modified RehmeWeller equation [37,39], and this value (DGcs) is given approximately by the difference in the exciton energy (EA* or ED*) of the constituting molecules with the exciplex energy (Eexciplex). Herein, EA* and ED* represent the exciton energy of the acceptor and donor, respectively, and Eexciplex is the exciplex photon energy. Therefore, the Eexciplex of TCTA:TCNSFX is estimated to be 2.77 eV through subtracting the Ered value (2.01 eV) of TCNSFX from the Eox value (0.76 eV) of TCTA, which has been collected in Table S2. Using the S1 value (2.94 eV) of TCNSFX as the EA* value, the DGcs value is calculated to be 0.17 eV for TCTA:TCNSFX through subtracting the Eexciplex value (2.77 eV) from the EA* value (2.94 eV). The TCTA:TCNSFX film is thus believed to display exciplex emission, which have been confirmed in Fig. 1. It is well known that the exciplex-TADF generates based on the exciplex state. Hence, the TADF obtained in TCTA:TCNSFX is partially related to its larger driving force in exciplex emission. Table 1 shows that the PLQY of TCTA:TCNSFX film is measured to
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Fig. 1. Molecular structures of (a) TCTA and (b) TCNSFX; (c) UV-vis absorption and (d) PL spectra of deposited films for TCTA, TCNSFX and TCTA:TCNSFX, respectively.
Table 1 Photophysical properties of the mixed exciplex and relevant pristine films. Emitters
TCTA TCNSFX TCTA:TCNSFX a b c
lPL, maxa (nm)
403 422 550
Fpa (%)
16 22 29
S1b (eV)
3.20 2.94 2.25
T1b (eV)
2.66 2.30 2.21
DESTc (eV)
0.54 0.64 0.04
Decay timea
t (ns)
c2
0.9 (90%), 7.2 (10%) 3.5 (86%), 16.1 (14%) 219.0 (47%), 2.1 (26%), 14.0 (27%)
1.061 1.034 1.012
Measured in deposited films at 300 K. Estimated from high-energy peaks of film-state fluorescence spectra at 77 K. S1T1.
be 29%. Obviously, the PLQY of TCTA:TCNSFX is attributable to its exciplex-TADF feature even though this value is not very high. Fig. 2d shows that the TCTA:TCNSFX film possesses almost identical fluorescence and phosphorescence spectra at 77 K, which implies smaller singletetriplet energy splitting exists in this exciplex emission (Table 1). Furthermore, this situation is beneficial to achieving efficient RISC of triplet excitons in TCTA:TCNSFX film, which is similar to those of many reported exciplex-TADF emitters [25,36]. Whereas, Table 1 shows that the T1 (2.30 eV) of TCNSFX is close to that (2.21 eV) of TCTA:TCNSFX, which implies that there might be energy leakage occurring from TCTA:TCNSFX states to the T1 excited states of TCNSFX. More importantly, Fig. 2d depicts that the phosphorescence spectrum of TCTA:TCNSFX displays week peak around 450 nm, which possesses similar character and spectrum shape compared with the phosphorescence spectrum of TCNSFX. These results indicate that TCNSFX might have slight negative effect on the luminescent efficiency of TCTA:TCNSFX. As a result, the energy leakage maybe becomes a competitor to the emission process of TCTA:TCNSFX, and inevitably results in its lower PLQY. It should be noted that, TCNSFX is still presenting
interesting perspectives for developing favorable exciplex-TADF materials in the future even though the PLQY of TCTA:TCNSFX is not very high. 3.2. Electrochemical properties According to the reported work, the driving force for exciplex emission is directly related to the solution redox potentials of the constituting molecules. Additionally, the DGcs value is linear with e(Eox, A e Eox, D) [36]. Here, CV measurements of the constituting molecules TCTA and TDCNSFX were performed to study the exciplex emission of TCTA:TCNSFX. Table S2 reveals that compound TCNSFX displays smaller LUMO level (2.75 eV) than CNSFX (2.41 eV) that has been reported in our previous work [29]. This result implies that introduction of electron-withdrawing cyano groups into SFX framework induces effective lowering of the LUMO level energies and stronger electron-accepting ability, which has been proven by our early theory calculation results [40]. It is believed that the donor and acceptor strength play a leading role in the exciplex formation. Enhancing the acceptor strength of the
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Fig. 2. (a) Fluorescence decays of deposited films at 300 K for TCTA, TCNSFX and TCTA:TCNSFX; (b) temperature-dependent transient fluorescence decays of deposited film for TCTA:TCNSFX; film-state fluorescence and phosphorescence spectra at 77 K for (c) TCNSFX and (d) TCTA:TCNSFX.
acceptor will cause large charge separation in the molecule after excitation, which assists the exciplex formation [41]. Therefore, the stronger electron-accepting ability from the tetracyanosubstitution in TCNSFX results in larger driving force for exciplex emission of TCTA:DCNSFX, which indirectly promotes the formation of exciplex-TADF.
3.3. Electroluminescence properties To investigate the EL performance of the exciplex-TADF from TCTA:TCNSFX film, this exciplex system was then adopted as the emitting layers to fabricate OLED through vacuum evaporation method. The resultant device is named as A, which possesses a configuration of indium tin oxide (ITO)/PEDOT:PSS (2500 rpm)/ TAPC (40 nm)/TCTA:TCNSFX (2:1, wt/wt, 30 nm)/TmPyPB (50 nm)/ LiF (0.5 nm)/Al (100 nm), as shown in Fig. 3a. Chemical structures of the materials used here are exhibited in Fig. S2 (Supporting Information). The PEDOT:PSS, TAPC, TmPyPB and LiF are used as the hole injection layer (HIL), hole-transporting layer (HTL), electron transporting layer (ETL) and electron-injection layer (EIL), respectively. As displayed in Fig. 3b, device A presents the EL spectrum with a maximum at 564 nm, which is similar to its PL profile in Fig. 1. It indicates that the emission observed here occurs from the TCTA:TCNSFX film. Device A shows a CIE coordinate of (0.47, 0.51) under an applied voltage of 8 V. Fig. 3c exhibits the current densityevoltageeluminance characteristic of device A. It displays that a turn-on voltage (defined as the bias at a brightness of 1 cd m2) of 5.1 V and a low luminance of 653 cd m2 are achieved in this device.
The low luminance of device A is directly related to its exciplexTADF feature and slightly low PLQY. Table 2 has summarized the relevant EL performance of device A. In Fig. 3d, device A exhibits a maximum hc of 6.0 cd A1, hp of 2.7 lm W1 and hext of 2.3%, which are slightly lower than those (8.2 cd A1, 4.3 lm W1, 3.0%) of our reported work [29], respectively. Obviously, the slight low electroluminescence efficiency (2.3%) of device A are ascribed to the low PLQY of the TCTA:DCNSFX film, which is similar to that (2%) of reported exciplex-TADF [16]. Despite electroluminescence efficiency of device A is not high, TCNSFX still displays interesting exciplex-TADF feature for developing OLEDs. This low hext provides much room for performance improvement of exciplex-TADF in our future work. More importantly, these results also suggest that SFX can be used as an universal skeleton in designing electron acceptors for realizing exciplex-TADF. The preparation of other novel SFX-based electron acceptors for exciplex-TADF is thus expected in the future research. Therefore, our work enriches the design system of electron acceptors for exciplex-TADF and offers an efficient and convenient approach to developing low-cost materials and devices for TADF.
4. Conclusion In summary, we have prepared a tetracyano-substituted SFX derivative (TCNSFX) as electron acceptor with convenient and lowcost method. TCTA is selected as electron donor to form exciplex emitter. In TCTA:TCNSFX film, exciplex-TADF is successfully obtained with PL (29%) and EL efficiencies (6.0 cd A1, 2.7 lm W1). The slightly low EL efficiencies are related to the small PLQY in
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Fig. 3. (a) The general structure, (b) EL spectra (at 8 V), (c) current densityevoltage-luminance and (d) current efficiencyeluminanceepower efficiency characteristics for OLED A.
Table 2 Summary of electroluminescence performance for OLED A. Device
Vturn-ona (V)
Lmaxb (cd m2)
hcb (cd A1)
hpb (lm W1)
hextb (%)
lEL (nm)
CIE [(x, y), V]
A
5.0
653
6.0
2.7
2.3
564
(0.47, 0.51), 8.0
a b
Defined as the bias at a brightness of 1 cd m2. Maximum values of the device. Values after the comma are the voltages at which the data are collected.
TCTA:TCNSFX film. This work enriches the design system of electron acceptors for exciplex-TADF, simultaneously offering an efficient and convenient approach to developing low-cost materials and devices for TADF. Acknowledgements The authors gratefully acknowledge financial support from the National Foundation for Science and Technology Development of China (2015CB932203), the National Key Research and Development Program of China (2017YFB0404501), NSFC (61605090, 61604081, 51333007, 61705111, 61704091, 61274065 and 61705111), the Science Fund for Distinguished Young Scholars of Jiangsu Province of China (BK20160039), the Natural Science Foundation of Jiangsu Province (BK20170899), the National Postdoctoral Program for Innovative Talents (BX201700122), the Six Peak Talents Foundation of Jiangsu Province (XCL-CXTD-009), the China Scholarship Council (201608390023) and the Nanjing University of Posts and Telecommunications Scientific Foundation (NY215061) and the Open Foundation from Jilin University (IOSKL2017KF04). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.molstruc.2019.06.003.
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