Deep-Blue Thermally Activated Delayed Fluorescence Polymers for

Mar 8, 2019 - The exploitation of deep-blue polymeric emitters is of great importance for the application of solution-processed organic light-emitting...
0 downloads 0 Views 4MB Size
Download PDF
Article Cite This: Macromolecules XXXX, XXX, XXX−XXX

pubs.acs.org/Macromolecules

Deep-Blue Thermally Activated Delayed Fluorescence Polymers for Nondoped Solution-Processed Organic Light-Emitting Diodes Chensen Li,† Zhongjie Ren,*,† Xiaoli Sun,† Huihui Li,† and Shouke Yan*,†,‡ †

State Key Laboratory of Chemical Resource Engineering, College of Materials Science and Engineering, Beijing University of Chemical Technology, Beijing 100029, China ‡ Key Laboratory of Rubber-Plastics, Ministry of Education/Shandong Provincial Key Laboratory of Rubber-plastics, Qingdao University of Science & Technology, Qingdao 266042, China Macromolecules Downloaded from pubs.acs.org by EAST CAROLINA UNIV on 03/09/19. For personal use only.

S Supporting Information *

ABSTRACT: The exploitation of deep-blue polymeric emitters is of great importance for the application of solution-processed organic light-emitting diodes (OLEDs) in full color display. The highly efficient deep-blue thermally activated delayed fluorescence (TADF) polymers are rarely reported up to now. Herein, we developed a series of deep-blue TADF polymers to fabricate highly efficient nondoped solution-processed OLEDs. By incorporating appropriate host with high triplet energy and deep-blue emitter with high fluorescent efficiency, the polymers are endowed with distinct TADF features. Using these deep-blue polymeric emitters, the nondoped single polymer based OLEDs achieve a maximum external quantum efficiency of 5.3% with the Commission Internationale de L’Eclairage (CIE) coordinates of (0.15, 0.09), which represents the state-of-the-art device performance for the TADF-based deep-blue polymer light-emitting diodes (PLEDs).

■

EQE of 10.3% with CIE coordinates of (0.156, 0.063).5 However, those high-efficiency devices are fabricated by vacuum-deposition techniques, and the hosts together with emitters are used for emitting layer, which inevitably increase the complexity of fabrication and limit the utilization of OLED materials. In contrast, large-scale manufacture of OLEDs devices urgently requires solution processability to make fabrication process simpler and cheaper.7−9 Therefore, the exploration of solution-processed deep-blue TADF materials is of significant importance for cost-effective applications. Polymeric materials are considered as a kind of promising emissive material for solution-processed OLEDs.10−12 In view of the anticipated high efficiency and low cost, much attention has been paid for developing TADF polymers with different emissive colors.13−19 For example, Ding et al.17 designed a series of red-emitting polymers based on poly(fluorene-co-3,3′dimethyldiphenyl ether), revealing a bright red emission at 606 nm with a promising EQE of 5.6% and current efficiency of 10.3 cd/A. In addition, our group18 reported green-emitting TADF polymers with an polyethylene backbone and varying ratios of 2-(10H-phenothia-zin-10-yl)dibenzothiophene-S,Sdioxide as pendant TADF units, and the corresponding devices give a maximum EQE of 20.1% and CIE coordinates of (0.36, 0.55). Besides, Wang et al.19 designed a blue-emitting

INTRODUCTION Thermally activated delayed fluorescence (TADF) materials have aroused persistent attention due to their abilities to harvest 100% internal quantum efficiency (IQE) without noble metals.1,2 The triplet excitons can efficiently upconvert into radiative singlet states with the thermally aided reverse intersystem crossing (RISC) resulting from a small energy gap (ΔEST) between the lowest excited singlet (S1) and triplet (T1) states. Up to now, numerous highly efficient TADF materials with different emissive colors have been synthesized, which provide the potential alternatives to phosphorescent emitters for organic light-emitting diodes (OLEDs). Especially, considering deep-blue phosphorescent heavy-metal complexes are troubled with several critical issues concerning high cost, high toxicity and low stability, hard endeavors have been made to develop deep-blue TADF materials so as to overcome those problems.3−6 Undoubtedly, the published works have achieved great breakthrough in efficiency of deep-blue TADF OLEDs. For example, Adachi et al.4 synthesized highly efficient deepblue TADF emitters by tuning the electronic interaction between donor and acceptor units. The fabricated deep-blue OLEDs with these emitters together with (bis[2-(di(phenyl)phosphino)phenyl] ether oxide) host achieved high external quantum efficiency (EQE) over 19.2% with the Commission Internationale de L’Eclairage (CIE) coordinates of (0.148, 0.098). They further reported deep-blue TADF OLEDs based on emitter with the substituted carbazoles and benzonitrile, resulting in an electroluminescence peak at 428 nm and a high © XXXX American Chemical Society

Received: January 14, 2019 Revised: February 23, 2019

A

DOI: 10.1021/acs.macromol.9b00083 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules

Figure 1. (a) Molecular structures of PDT-1−3. (b) Cyclic voltammograms of PDT-1−3 in acetonitrile. (c) HOMO and LUMO distributions obtained by DFT simulations for three units of PDT-1−3.

Table 1. Characterization Data for PDT-1−3

PDT-1 PDT-2 PDT-3

Mw/PDIa

Tgb (oC)

Tdc (oC)

λabsd (nm)

λPLd (nm)

Ege (eV)

HOMOf (eV)

LUMOg (eV)

S1h (eV)

T1h (eV)

ΔESTi (eV)

PLQYj (%)

kRISCk (105 s−1)

10200/1.39 11200/1.66 21200/1.73

174 178 194

386 387 391

284 286 288

441 444 445

3.72 3.72 3.72

−5.40 −5.38 −5.37

−1.67 −1.65 −1.64

3.21 3.16 3.15

3.17 3.13 3.11

0.04 0.03 0.04

42 54 46

4.07 4.43 4.18

Determined by gel permeation chromatography with polystyrene standards. bDetermined at a heating rate of 10 °C/min under a nitrogen atmosphere. cTemperature at 5% weight loss. dMeasured in neat films at room temperature. eOptical energy gap (Eg) deduced from the absorption onset in toluene solution. fCalculated according to EHOMO = −(E(onset,ox vs Fc+/Fc) + 5.1) by CV. gCalculated according to LUMO = HOMO + Eg. h Singlet and triplet energy was calculated from the onset wavelength of fluorescent and phosphorescent emission. iΔEST = S1 − T1. jAbsolute PL quantum yield in neat films determined by a calibrated integrating sphere in nitrogen; error ±2%. kThe constant rate of RISC. a

polymers as the emitting layers achieve an EQEmax of 5.3% with EL emission peak of 436 nm and CIE coordinates of (0.15, 0.09), which is one of the best efficiencies for deep-blue PLEDs with single polymer emitting layer.22

TADF polymer based on an insulting backbone with throughspace charge transfer (TSCT) effect between electron donor and acceptor pendants. A high EQE of 12.1% was achieved with CIE coordinates of (0.18, 0.27). It should be noted that the electroluminescence (EL) wavelength of most reported TADF polymers mainly covers 470−610 nm, but there are no studies of deep-blue emitting ones with an emission peak below 450 nm.13−19 Therefore, the development of efficient deep-blue TADF polymers is an important research goal. Herein, we developed a series of deep-blue TADF polymers to fabricate highly efficient nondoped solution-processed OLEDs. Specifically, as shown in Figure 1a, we have designed and synthesized a series of nonconjugated TADF polymers with polyethylene as the backbone, pendant 9,9-dimethyl-10phenylacridine (BDMAc)19 with high triplet energy (ET = 3.38 eV) and HOMO level of (−5.25 eV) as host unit, and 2-(9,9dimethylacridin-10-yl)-9,9-dimethylthioxanthene-S,S-dioxide (DMA-TXO2)20,21 with high PLQY (90%) and suitable HOMO level of (−5.50 eV) as deep-blue emitter. The high ET of host can suppress triplet energy back transfer from emissive pendant to host, which usually results in the poor performance of devices. Meanwhile, the shallow HOMO level of host could facilitate hole injection and transportation in the emissive layer. The resulting polymers with different ratios of emitters display obvious deep-blue emission with TADF features and high fluorescence quantum efficiency. The polymer light-emitting diodes (PLEDs) based on these

■

RESULTS AND DISCUSSION Synthesis and Characterization. The polymers PDT-1− 3 were prepared by radical copolymerization of 2-vinyl-9,9dimethyl-10-phenylacridine and 9,9-dimethyl-10-(3-vinylphenyl)-9,10-dihydroacridine with the molar feed ratios of 91:9, 88:12, and 86:14, respectively. The actual content of TADF unit, DMA-TXO2, in the copolymers estimated from elemental analysis data is ca. 9%, 13%, and 15% for PDT-1, PDT-2, and PDT-3, respectively. All polymers are readily soluble in chloroform, toluene, and tetrahydrofuran. The weight-average molecular weights (Mw) of PDT-1, PDT-2, and PDT-3 were determined to be 10.2, 11.2, and 21.2 kDa with PDIs of 1.39, 1.66, and 1.73, respectively (Table 1 and Figure S1). Their structures were confirmed by 1H NMR spectra (Figure S2). Typically, the chemical shift at ca. 8.3 ppm is assigned to the aromatic protons of dimethylthioxanthene-S,Sdioxide acceptor in TADF units, and 5.8−7.5 ppm is attributed to the protons of host and donors in TADF units, suggesting the successful copolymerization. Their thermal properties were investigated by differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). As shown in Table 1 and Figure S3, the decomposition temperatures (Td) with 5% B

DOI: 10.1021/acs.macromol.9b00083 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules

Figure 2. AFM topographic images of the solution-processed neat films for (a) PDT-1, (b) PDT-2, and (c) PDT-3.

Figure 3. UV−vis absorption and fluorescence spectra of PDT-1−3 in toluene (a) and in neat films (b). (c) Photoluminescence spectra of PDT-2 in different solvents at room temperature and the corresponding Lippert−Mataga plot (inset). (d) Phosphorescent spectra collected at 77 K of PDT-1−3.

weight loss under nitrogen are in the range 386−391 °C, showing their excellent thermal stability; meanwhile, the Td slightly increases with the decreased ratio of BDMAc units. A clear glass transition temperature (Tg) is observed for each polymer with value ranging from 174 to 194 °C. In addition, there are no exothermic peaks resulting from crystallization during the scan scopes for those polymers, suggesting an amorphous feature of all polymers. The high Td and Tg values favor long-term stability for practical applications in EL devices. The electrochemical behavior of three polymers was investigated by cyclic voltammetry (CV) in degassed anhydrous acetonitrile solution (Figure 1b). The CV curves show quasi-reversible oxidation and reduction processes for all polymers corresponding to electrochemical doping and dedoping during the potential sweeps. PDT-1−3 have similar oxidation and reduction potentials with the 9,9-dimethylacridine donor and dimethylthioxanthene-S,S-dioxide acceptor

units (Figure S4), respectively, indicating the centers of electrochemical oxidation and reduction processes are located on the DMA-TXO2 units. A slight decrease in oxidation potential is observed when BDMAc is added; i.e., the polymers are easier to oxidize than DMA-TXO2. To predict the TADF feature of PDT-1−3, the distributions of HOMO and LUMO were calculated by density functional theory (DFT) simulation with the B3LYP 6-311G(d,p) level. Specifically, single TADF unit and double host units were chosen for study purposes. As shown in Figure 1c, typically, the LUMO and HOMO of PDT-1−3 are mainly localized on the electron acceptor of dimethylthioxanthene-S,S-dioxide and the electron donor of 9,9-dimethylacridine, respectively. The low overlap density distribution between the electronic wave functions of the ground state decreases the ΔEST, resulting in a strong intramolecular charge transfer (ICT).23 The film-forming ability and film morphology of solutionprocessed films of PDT-1−3 were investigated by atomic force C

DOI: 10.1021/acs.macromol.9b00083 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules

Figure 4. Transient PL decay (excited at 300 nm) curves from 100 to 300 K for (a) PDT-1, (b) PDT-2, and (c) PDT-3. Steady state fluorescence spectra in the film states under air and vacuum for (a) PDT-1, (b) PDT-2, and (c) PDT-3.

microscopy (AFM). The films were spin-coated from 10 mg mL−1 toluene solution with 2000 rpm and then annealed for 30 min at 120 °C. As shown in Figure 2, the AFM height images display smooth and homogeneous morphology with small root-mean-square (RMS) roughness values of 0.326 nm for PDT-1, 0.314 nm for PDT-2, and 0.308 nm for PDT-3. It is free of particle aggregation or phase separation, suggesting the good film-forming ability. Therefore, the polymeric films are indeed suitable for light-emitting layers of OLEDs. Basic Photophysical Properties. Figures 3a,b show the UV−vis absorption and photoluminescence (PL) spectra of PDT-1−3. The absorption spectra in toluene and neat films mainly show π−π* transitions of the 9,9-dimethylacridine moiety, which reveals the DMA-TXO2 has little contribution to absorption because of low loading contents. As shown in Figures 3a,b, the PL spectra of PDT-1−3 in toluene display a weak emission peak at 360 nm from the host and a strong peak at 444 nm from TADF units (Figures S5 and S6), which can be ascribed to the insufficient energy transfer from the host to TADF units. In contrast, PL spectra of polymers in the neat films show distinct single emission without characteristic structure, and the emission peaks locate at 442, 444, and 445 nm for PDT-1, PDT-2, and PDT-3, respectively, indicating the efficient energy transfer in the film states between the host and TADF units. The slight bathochromic shift of emission wavelength from PDT-1 to PDT-3 is due to the enhancive aggregation of TADF units with the increasing ratio of DMATXO2. Solvatochromic studies reveal that the maximum emission peaks of the polymers are sensitive to solvent polarity (Figure 3c), which can measure the characteristics of the excited state of charge-transfer excited (CT) or low-lying local excited (LE).24 Typically, PDT-2 displays an obvious red-shift of PL peaks with increased solvent polarity. Maximum PL peak (λmax) locates at 436 nm in n-hexane, 444 nm in toluene, 463

nm in tetrahydrofuran, 473 nm in dichloromethane, and 489 nm in dimethylformamide (see Figure 3c). The relatively conspicuous solvatochromic effect observed for the Lippert− Mataga plot (νabs − νem against polarity of solvent) exhibits a slope of ∼6364 cm−1 for PDT-2 (the inset of Figure 3c), suggesting a large dipole moment of the emissive excited state and strong CT characteristics in the excited state.25 A similar phenomenon can be observed for PDT-1 with a Lippert− Mataga plot slope of ∼6473 cm−1 and PDT-3 with a slope of ∼7008 cm−1 (Figure S7). The higher content of TADF units for PDT-3 may cause a larger increase in dipole moment of the emissive excited state and more CT characteristics in the excited state. Furthermore, to evaluate the triplet energy level of polymers, the phosphorescent spectra of PDT-1−3 were collected at 77 K (Figure 3d). By taking the onset energy of phosphorescence as the transition energy of T1 to ground state, the ET was determined to be 3.17, 3.13, and 3.11 eV for PDT1, PDT-2, and PDT-3, respectively. As a result, the triplet energy back-transfer could be effectively suppressed because of the significantly higher ET (3.38 eV) of the host. The calculated values of ΔEST are ≤0.04 eV for PDT-1−3, and such a small ΔEST is favorable for transformation of triplet excitons to radiative singlet excitons by the RISC process.26,27 Delayed Fluorescence Properties. To study the origin of delayed fluorescence, the transient PL decay spectra for PDT1−3 were measured in the neat films from 100 to 300 K. As shown in Figures 4a−c, all polymers present second-order exponential decays with the prompt fluorescence (PF) lifetimes of 25.2−27.8 ns and the delayed fluorescence (DF) lifetimes of 5.11−5.87 μs in the neat films at 300 K. Typically, the lifetimes and ratios of the DF are monotonically enhanced from 2.34 μs (48.8%) to 5.87 μs (59.4%) with the rising temperature for PDT-2, which indicates the RISC from T1 to S1 is accelerated by thermal activation. Also, PDT-1 and PDT3 show similar trends when the temperature increases from D

DOI: 10.1021/acs.macromol.9b00083 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules

Figure 5. (a) Energy-level diagram and structure of OLED devices. (b) Current density−voltage−luminance (J−V−L) curves of PDT-1−3. (d) CIE coordinates of PDT-1−3 at 100 cd/m2. (d) EQE and power efficiency vs luminance for PDT-1−3.

Table 2. Characterization Data for PDT-1−3 PDT-1 PDT-2 PDT-3

Vona (V)

Lmaxb (cd/m2)

CEmaxc (cd/A)

PEmaxd (lm/W)

EQEmaxe (%)

EQE100f (%)

λpeakg (nm)

CIE8Vh (x, y)

4.8 5.0 5.0

787 993 1008

6.3 8.7 6.9

2.7 3.9 3.1

3.9 5.3 4.4

3.5 4.9 4.1

434 436 438

(0.15, 0.08) (0.15, 0.09) (0.17, 0.14)

a The voltage at 1 cd m−2. bMaximum luminance. cMaximum current efficiency. dMaximum power efficiency. eMaximum external quantum efficiency. fExternal quantum efficiency at 100 cd m−2. gEL spectra peak at 8 V. hCIE coordinates at 8 V.

which is consistent with their PLQY and transient PL decay spectra. Electroluminescent Properties. To evaluate the electroluminescent performance of these polymers, solution-processed OLEDs with the architecture of indium tin oxide (ITO)/poly(3,4-ethylenedioxythiophene)-doped poly(styrenesulfonate) (PEDOT:PSS) (40 nm)/PDT-x (x = 1, 2, 3) (40 nm)/TmPyPB (40 nm)/LiF (1 nm)/Al (100 nm) (Figure 5a) were fabricated, where PEDOT:PSS and LiF serve as the holeand electron-injection layers, respectively, and 1,3,5-tri(mpyrid-3-yl-phenyl)benzene (TmPyPB) acts as the electron transporting layer. PDT-1, PDT-2, and PDT-3 are emitting layers (EML) for devices. The current density−voltage−luminance (J−V−L) characteristics, the EQEs versus luminance curves, and the EL spectra for all these devices are shown in Figures 5b−d, and the key EL data are summarized in Table 2. Similar to the corresponding PL spectra of PDT-1−3 in the neat film, the devices exhibit deep-blue EL emission (Figure S9) with the main peaks at 434, 436, and 438 nm with the CIE coordinates of (0.15, 0.08), (0.15, 0.09), and (0.17, 0.14), respectively, suggesting the complete energy transfer from polymer hosts to the emissive TADF under electrical excitation. As shown in Figure 5c, PDT1,2 is bluer than PDT-3. Furthermore, as exhibited in Figure 5b, J−V characteristics are independent of the loading contents of the TADF units, which implies the dominant EL mechanism is the energy transfer from the polymer host to the TADF unit rather than direct charge injection in the TADF units.30,31

100 to 300 K, and the delayed lifetimes and ratios elevate from 2.09 μs (50.7%) to 5.79 μs (57.5%) for PDT-1 and from 2.99 μs (46.4%) to 5.11 μs (55.7%) for PDT-3, respectively. Those results unambiguously confirm the TADF nature of PDT-1−3 that comes from DMA-TXO2 units (Figure S8). Moreover, the photoluminescence quantum yields (PLQYs) of PDT-1−3 in the neat films are obtained (Table 1). The maximum PLQYs are achieved by PDT-2, which is 54% in the neat film under a nitrogen atmosphere. Accordingly, the rate constants of reverse intersystem crossing (kRISC) were estimated and summarized in Table 1 and Table S1. The values of PDT-1, PDT-2, and PDT3 were calculated to be 4.1 × 106, 4.4 × 106, and 4.2 × 106 s−1, respectively. The relatively high kRISC values of them manifest that the triplet excitons can be efficiently upconverted from triplet to singlet to harvest both prompt and delayed emissions.28 Another signature characteristics of TADF is quenching of PL intensity in the presence of oxygen due to the triplet is sensitive to oxygen.29 To detect DF ratio in overall emission, steady-state PL spectra of PDT-1−3 in the neat films with and without air were investigated. As the atmosphere changes from air to vacuum, the DF component increases because oxygen can quench triplet excitons. It is obvious that the steady-state fluorescence spectra of PDT-1−3 in a vacuum display stronger emission compared with those in air condition (Figures 4d−f). The calculated emission ratio of DF in the overall emission is 33.6% for PDT-1, 40.8% for PDT-2, and 37.5% for PDT-3, E

DOI: 10.1021/acs.macromol.9b00083 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules Table 3. PL and EL Performances of the Reported TADF Polymers with Blue Emission a

PDT-2 P9a PBD-10a P-Ac95-TRZ05a PCzDP-10b

PLpeakc (nm)

PLQYd (%)

τDFe (μs)

ELpeakf (nm)

EQEmaxg (%)

CIEh (x, y)

ref

444 494 457 489 500

54 33.6 47 60 74

5.87 6.08 5.9 1.28 3.0

436 498 478 472 496

5.3 4.0 7.3 12.1 16.1

(0.15, 0.09) (0.22, 0.37) (0.20, 0.29) (0.176, 0.269) (0.24, 0.40)

this work 32 33 19 13

a f

Nondoped device. bDoped device. cPhotoluminescence emission peak. dPhotoluminescence quantum yield. eDelayed emission lifetime. Electroluminescence emission peak. gMaximum external quantum efficiency. hCIE coordinates. = 150 mm). Differential scanning calorimetry (DSC) was performed on a TA Q2000 differential scanning calorimeter at a heating rate of 10 °C min−1 from 30 to 300 °C under a nitrogen atmosphere. The glass transition temperature (Tg) was determined from the second heating scan. Thermogravimetric analysis (TGA) was undertaken with a METTLER TOLEDO TGA/DSC 1/1100SF instrument. The thermal stability of the samples under a nitrogen atmosphere was determined by measuring their weight loss while heating at a rate of 10 °C min−1 from 40 to 800 °C. Cyclic voltammetry (CV) was performed in nitrogen-purged acetonitrile at room temperature with a CHI voltammetric analyzer. Tetrabutylammonium hexafluorophosphate (TBAPF6, 0.1 M) was used as the supporting electrolyte. The conventional three-electrode configuration consists of a glassy carbon working electrode, a platinum wire auxiliary electrode, and an Ag/AgNO3 pseudoreference electrode with ferrocenium−ferrocene (Fc+/Fc) as the internal standard. Cyclic voltammograms were obtained at scan rate of 100 mV s−1. The onset potential was determined from the intersection of two tangents drawn at the rising and background current of the cyclic voltammogram. Gel permeation chromatography (GPC) analysis was performed on a Waters 5152410 system using polystyrene standards as molecular weight references and tetrahydrofuran (THF) as the eluent. The morphologies of the polymer films coated on the glass substrate were measured using atomic force microscopy (Agilent-5500 AFM) under tapping mode. The quantum-chemical calculations reported here were performed with the Gaussian 09 program. Device Fabrication and Characterization. The hole-injection material PEDOT:PSS (Al 4083) and electron-transporting and holeblocking material TmPyPB were obtained from commercial sources. ITO-coated glass with a sheet resistance of 10 Ω/sq was used as the substrate. Before device fabrication, the ITO-coated glass substrate was thoroughly cleaned in ultrasonic bath of tetrahydrofuran, isopropyl alcohol, detergent, deionized water, and isopropyl alcohol and treated with O2 plasma for 10 min in sequence. PEDOT:PSS was then spin-coated onto the clean ITO substrate as a hole-injection layer. Next a polymer in chlorobenzene was spin-coated (10 mg/mL; 2000 rpm) to form a ca. 40 nm thick emissive layer and then annealed at 80 °C for 30 min to remove the residual solvent. Finally, a 40 nm thick electron-transporting layer of TmPyPB was vacuum deposited, and a cathode composed of a 1 nm thick layer of liquid and aluminum (100 nm) was sequentially deposited through shadow masking with an array of 3 mm × 3 mm openings under a pressure of 10−5 Torr. Deposition rates are 1−2 Å s−1 for organic materials, 0.1 Å s−1 for LiF, and 6 Å s−1 for Al. Electroluminescence (EL) spectra were recorded by an optical analyzer (Photo Research PR745). The current density− voltage−luminance (J−V−L) characteristics of the devices were measured using a Keithley 2400 source meter and Konica Minolta chromameter (CS-200). The EL spectra were recorded using a JYSPEX CCD3000 spectrometer. The EQE values were calculated from the luminance, current density, and electroluminescent spectrum according to previously reported methods.34 All measurements were performed at room temperature under ambient conditions. Synthesis of 2-(9,9-Dimethylacridin-10-yl)-8-vinyl Dimethylthioxanthene-S,S-Dioxide (M1). 2-Bromo-8-vinyl-9,9-dimethylthioxanthene-S,S-dioxide (181 mg, 0.5 mmol, 1 equiv) and 9,9dimethyl-10H-acridine (105 mg, 0.5 mmol, 1 equiv) were evacuated for 30 min in a two-neck 100 mL round-bottomed flask fitted with a reflux condenser. The flask was backfilled with argon, and dry toluene

Among these OLEDs, the maximum EQEs (EQEmax) are 3.9%, 5.3%, and 4.4% for PDT-1, PDT-2, and PDT-3, respectively. The maximum current efficiencies (CEs) are 6.3, 8.7, and 6.9 cd A−1 for PDT-1, PDT-2, and PDT-3, respectively (Figure S9). Specifically, the PDT-2-based device gives the highest EQEmax up to 5.3%, which probably ascribes to the highest PLQY and kRISC among these polymers. Such comprehensive performance is outstanding among the state-of-the-art blueemitting TADF polymer-based OLEDs (Table 3). Notably, those impressive performances obtained in the nondoped solution-processed devices are in favor of simplifying the device fabrication and reduce the costs in practical application.

■

CONCLUSION In summary, we have designed and synthesized a series of deep-blue TADF polymers in which 2-(9,9-dimethylacridin-10yl)-9,9-dimethylthioxanthene-S,S-dioxide units emit deep-blue emission, while 9,9-dimethyl-10-phenylacridine units act as a host. These polymers show good film-forming ability and superior thermal stability due to their high molecular weights. Owing to the high ET (3.38 eV) of the host, the triplet energy back-transfer from the TADF pendant to the polymer host can be effectively suppressed. In addition, an efficient RISC process and delayed fluorescence are obtained, and thus highly efficient deep-blue TADF emission can be observed. It is noteworthy that the nondoped OLEDs fabricated by the solution process with the target polymers as emitters achieve a maximum EQE of 5.3% and a deep-blue emission at 436 nm with CIE coordinate of (0.15, 0.09), which is the first example of TADFbased deep-blue polymeric OLEDs. This work provides a promising approach to develop highly efficient deep-blue TADF polymers for the solution-processed light-emitting devices.

■

EXPERIMENTAL SECTION

Materials. All reactants (Adamas-beta) were purchased from Adamas Reagent, Ltd., without further purification, and all solvents were supplied by Beijing Chemical Reagent Co., Ltd. Anhydrous and deoxygenated solvents were obtained by distillation over a sodium benzophenone complex. Characterization. 1H NMR and 13C NMR spectra were recorded on a Bruker AV400 spectrometer. Chemical shifts (δ) are given in parts per million (ppm) relative to tetramethylsilane (TMS; δ = 0) as the internal reference. 1H NMR spectra data are reported as chemical shift, relative integral, multiplicity (s = singlet, d = doublet, m = multiplet), coupling constant (J in Hz), and assignment. Elemental analyses of carbon, hydrogen, nitrogen, and sulfur were performed on a Vario EL cube. UV/vis absorption spectra were recorded on a Hitachi U-2910 spectrophotometer. PL spectra were recorded on a Hitachi F-7000 fluorescence spectrophotometer. The temperature dependence of transient PL decay curves in films and PL spectra in vacuum/air were determined using a spectrometer (FLS980) from Edinburgh Instruments Limited. The fluorescence quantum yields of solid films were measured on a FLS980 with an integrating sphere (φ F

DOI: 10.1021/acs.macromol.9b00083 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules (15 mL) was added. The reaction mixture was bubbled with argon for 30 min, then Pd2(dba)3·CHCl3 (13 mg, 13 μmol, 0.05 equiv) and HPtBu3BF4 (7 mg, 25 μmol, 0.1 equiv) were added, and the reaction mixture was bubbled with argon for a further 30 min. NaOtBu (70 mg, 0.73 mmol, 3 equiv) was added under a high flow of argon, and the reaction was then heated to 107 °C with stirring for 21 h. At the end of the reaction, the solvent was removed under reduced pressure, and the crude mixture was purified by silica gel column chromatography eluting with 70:30 v/v CH2Cl2:hexane increasing to 90% CH2Cl2 in 10% increments. Removal of solvent under reduced pressure gave the product as a white solid (370 mg, 75%). 1H NMR (400 MHz, CDCl3): δ 8.38 (d, J = 8.3 Hz, 1H), 8.11 (d, J = 8.2 Hz, 1H), 8.02 (d, J = 1.5 Hz, 1H), 7.96 (d, J = 1.8 Hz, 1H), 7.82 (dd, J = 1.3 Hz, 1H), 7.63 (dd, J = 1.8 Hz, 1H), 7.54 (dd, J = 1.6 Hz, 2H), 7.00 (d, J = 1.5 Hz, 1H), 6.98 (d, J = 1.8 Hz, 1H), 6.96 (d, J = 1.4 Hz, 1H), 6.94 (dd, J = 1.3 Hz, 2H), 6.23 (d, J = 1.2 Hz, 1H), 6.17 (d, J = 1.1 Hz, 1H), 6.15 (d, J = 17.5 Hz, 1H), 5.53 (d, J = 11.3 Hz, 1H), 1.88 (s, 6H), 1.65 (s, 6H). 13C NMR (100 MHz, CDCl3): δ 150.3, 147.2, 146.6, 143.2, 141.3, 137.7, 136.7, 134.2, 132.0, 131.4, 131.1, 130.3, 128.7, 127.6, 127.5, 126.3, 125.9, 125.5, 125.1, 122.1, 118.2, 115.2, 40.5, 36.7, 31.4, 30.8. MS-ESI+ m/z (%): 492.1991 ([M + H]+, 100%). Anal. Calcd for C32H29NO2S: C, 78.18; H, 5.95; N, 2.85; O,6.51; S,6.52. Found: C, 78.20; H, 5.94; N, 2.86; S, 6.51. Synthesis of 9,9-Dimethyl-10-(3-vinylphenyl)-9,10-dihydroacridine (M2) According to Ref 19. 1H NMR (400 MHz, CDCl3): δ 7.61−7.50 (m, 2H), 7.45 (dd, J = 7.5, 1.7 Hz, 2H), 7.40−7.36 (m, 1H), 7.25−7.20 (m, 1H), 6.99−6.89 (m, 4H), 6.76 (dd, J = 17.6, 10.9 Hz, 1H), 6.29 (dd, J = 8.0, 1.4 Hz, 2H), 5.79 (d, J = 17.5 Hz, 1H), 5.31 (d, J = 11.0 Hz, 1H), 1.69 (s, 6H). General Synthetic Procedure for the Copolymers. A mixture of AIBN (5 mg, 0.03 mmol), toluene (8.0 mL)/THF (20 mL), and different ratios of 2-(9,9-dimethylacridin-10-yl)-8-vinyldimethylthioxanthene-S,S-dioxide (M1) and 9,9-dimethyl-10-(3-vinylphenyl)9,10-dihydroacridine (M2) were placed in an ampule, which was cooled, degassed, and sealed in a vacuum. After stirring at 60 °C for 20 h, the reaction mixture was poured into a large excess of methanol. The white polymer was obtained by filtration and then was dried in a vacuum. The polymer was fractionated by Soxhlet extraction using hexane. PDT-1: M1 (73.2 mg, 0.15 mmol) and M2 (467 mg, 1.5 mmol) were used in the polymerization (yield: 81%). Elemental analysis. Found: C 86.1; H 6.11, N 4.33, S 0.85%. PDT-2: M1 (97.6 mg, 0.2 mmol) and M2 (467 mg, 1.5 mmol) were used in the polymerization (yield: 87%). Elemental analysis. Found: C 83.5; H 6.59, N 3.68, S 1.26%. PDT-3: M1 (97.6 mg, 0.2 mmol) and M2 (373 mg, 1.2 mmol) were used in the polymerization (yield: 83%). Elemental analysis. Found: C 85.99; H 6.05, N 4.17, S 1.27%.

■

Xiaoli Sun: 0000-0002-5477-0401 Huihui Li: 0000-0001-5745-4079 Shouke Yan: 0000-0003-1627-341X Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The financial support of the National Natural Science Foundation of China under Grant 51521062 is gratefully acknowledged.

■

(1) Uoyama, H.; Goushi, K.; Shizu, K.; Nomura, H.; Adachi, C. Highly Efficient Organic Light-Emitting Diodes from Delayed Fluorescence. Nature 2012, 492, 234−238. (2) Liu, Y.; Li, C.; Ren, Z.; Yan, S.; Bryce, M. R. All-organic thermally activated delayed fluorescence materials for organic lightemitting diodes. Nat. Rev. Mater. 2018, 3, 18020. (3) Cai, X.; Gao, B.; Li, X.-L.; Cao, Y.; Su, S.-J. Singlet-Triplet Splitting Energy Management via Acceptor Substitution: Complanation Molecular Design for Deep-Blue Thermally Activated Delayed Fluorescence Emitters and Organic Light-Emitting Diodes Application. Adv. Funct. Mater. 2016, 26, 8042−8052. (4) Cui, L. S.; Nomura, H.; Geng, Y.; Kim, J. U.; Nakanotani, H.; Adachi, C. Controlling Singlet-Triplet Energy Splitting for Deep-Blue Thermally Activated Delayed Fluorescence Emitters. Angew. Chem., Int. Ed. 2017, 56, 1571−1575. (5) Chan, C.-Y.; Cui, L.-S.; Kim, J. U.; Nakanotani, H.; Adachi, C. Rational Molecular Design for Deep-Blue Thermally Activated Delayed Fluorescence Emitters. Adv. Funct. Mater. 2018, 28, 1706023. (6) Wada, Y.; Kubo, S.; Kaji, H. Adamantyl Substitution Strategy for Realizing Solution-Processable Thermally Stable Deep-Blue Thermally Activated Delayed Fluorescence Materials. Adv. Mater. 2018, 30, 1705641. (7) Wu, H.; Ying, L.; Yang, W.; Cao, Y. Progress and Perspective of Polymer White Light-Emitting Devices and Materials. Chem. Soc. Rev. 2009, 38, 3391−3400. (8) Huang, T.; Jiang, W.; Duan, L. Recent progress in solution processable TADF materials for organic light-emitting diodes. J. Mater. Chem. C 2018, 6, 5577−5596. (9) Zou, Y.; Gong, S.; Xie, G.; Yang, C. Design Strategy for SolutionProcessable Thermally Activated Delayed Fluorescence Emitters and Their Applications in Organic Light-Emitting Diodes. Adv. Opt. Mater. 2018, 6, 1800568. (10) Sekine, C.; Tsubata, Y.; Yamada, T.; Kitano, M.; Doi, S. Recent Progress of High Performance Polymer OLED and OPV Materials for Organic printed Electronics. Sci. Technol. Adv. Mater. 2014, 15, 034203. (11) Wei, Q.; Ge, Z.; Voit, B. Thermally Activated Delayed Fluorescent Polymers: Structures, Properties, and Applications in OLED Devices. Macromol. Rapid Commun. 2019, 40, 1800570. (12) Nikolaenko, A. E.; Cass, M.; Bourcet, F.; Mohamad, D.; Roberts, M. Thermally Activated Delayed Fluorescence in Polymers: A New Route toward Highly Efficient Solution Processable OLEDs. Adv. Mater. 2015, 27, 7236−7240. (13) Xie, G.; Luo, J.; Huang, M.; Chen, T.; Wu, K.; Gong, S.; Yang, C. Inheriting the Characteristics of TADF Small Molecule by SideChain Engineering Strategy to Enable Bluish-Green Polymers with High PLQYs up to 74% and External Quantum Efficiency over 16% in Light-Emitting Diodes. Adv. Mater. 2017, 29, 1604223. (14) Nobuyasu, R. S.; Ren, Z.; Griffiths, G. C.; Batsanov, A. S.; Data, P.; Yan, S.; Monkman, A. P.; Bryce, M. R.; Dias, F. B. Rational Design of TADF Polymers Using a Donor-Acceptor Monomer with Enhanced TADF Efficiency Induced by the Energy Alignment of Charge Transfer and Local Triplet Excited States. Adv. Opt. Mater. 2016, 4, 597−607.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.9b00083. Experimental procedures including materials, characterization as well as device fabrication and materials synthesis, and results and discussion; figures of TGA, DSC, PL, CV, 1H NMR, transient PL decay spectra, and current efficiency−luminance curves and EL spectra of devices (PDF)

■

REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (Z.R.). *E-mail: [email protected] (S.Y.). ORCID

Zhongjie Ren: 0000-0002-7981-4431 G

DOI: 10.1021/acs.macromol.9b00083 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules (15) Yang, Y.; Wang, S.; Zhu, Y.; Wang, Y.; Zhan, H.; Cheng, Y. Thermally Activated Delayed Fluorescence Conjugated Polymers with Backbone-Donor/Pendant-Acceptor Architecture for Nondoped OLEDs with High External Quantum Efficiency and Low Roll-Off. Adv. Funct. Mater. 2018, 28, 1706916. (16) Li, C.; Wang, Y.; Sun, D.; Li, H.; Sun, X.; Ma, D.; Ren, Z.; Yan, S. Thermally Activated Delayed Fluorescence Pendant Copolymers with Electron- and Hole-Transporting Spacers. ACS Appl. Mater. Interfaces 2018, 10, 5731−5739. (17) Yang, Y.; Zhao, L.; Wang, S.; Ding, J.; Wang, L. Red-Emitting Thermally Activated Delayed Fluorescence Polymers with Poly(fluorene-co-3,3′-dimethyl diphenyl ether) as the Backbone. Macromolecules 2018, 51, 9933−9942. (18) Ren, Z.; Nobuyasu, R. S.; Dias, F. B.; Monkman, A. P.; Yan, S.; Bryce, M. R. Pendant Homopolymer and Copolymers as SolutionProcessable Thermally Activated Delayed Fluorescence Materials for Organic Light-Emitting Diodes. Macromolecules 2016, 49, 5452− 5460. (19) Shao, S.; Hu, J.; Wang, X.; Wang, L.; Jing, X.; Wang, F. Blue Thermally Activated Delayed Fluorescence Polymers with Nonconjugated Backbone and Through-Space Charge Transfer Effect. J. Am. Chem. Soc. 2017, 139, 17739−17742. (20) Dos Santos, P. L.; Ward, J. S.; Bryce, M. R.; Monkman, A. P. Using Guest-Host Interactions To Optimize the Efficiency of TADF OLEDs. J. Phys. Chem. Lett. 2016, 7, 3341−3346. (21) Lee, I.; Lee, J. Y. Molecular design of deep blue fluorescent emitters with 20% external quantum efficiency and narrow emission spectrum. Org. Electron. 2016, 29, 160−164. (22) Yang, X.; Xu, X.; Zhou, G. Recent advances of the emitters for high performance deep-blue organic light-emitting diodes. J. Mater. Chem. C 2015, 3, 913−944. (23) Ziegler, T.; Rauk, A.; Baerends, E. J. On the calculation of multiplet energies by the Hartree-Fock-Slater method. Theor. Chim. Acta 1977, 43, 261−271. (24) Dias, F. B.; Bourdakos, K. N.; Jankus, V.; Moss, K. C.; Kamtekar, K. T.; Bhalla, V.; Santos, J.; Bryce, M. R.; Monkman, A. P. Triplet harvesting with 100% efficiency by way of thermally activated delayed fluorescence in charge transfer OLED emitters. Adv. Mater. 2013, 25, 3707−3714. (25) Liu, Y.; Wang, Y.; Li, C.; Ren, Z.; Ma, D.; Yan, S. Efficient Thermally Activated Delayed Fluorescence Conjugated Polymeric Emitters with Tunable Nature of Excited States Regulated via Carbazole Derivatives for Solution-processed OLEDs. Macromolecules 2018, 51, 4615−4623. (26) Lee, D. R.; Choi, J. M.; Lee, C. W.; Lee, J. Y. Ideal molecular design of blue thermally activated delayed fluorescent emitter for high efficiency, small singlet-triplet energy splitting, low efficiency roll-off, and long lifetime. ACS Appl. Mater. Interfaces 2016, 8, 23190−23196. (27) Zhu, Y.; Zhang, Y.; Yao, B.; Wang, Y.; Zhang, Z.; Zhan, H.; Zhang, B.; Xie, Z.; Wang, Y.; Cheng, Y. Synthesis and electroluminescence of a conjugated polymer with thermally activated delayed fluorescence. Macromolecules 2016, 49, 4373−4377. (28) Kono, H.; Lin, S. H.; Schlag, E. W. On the role of lowfrequency modes in the energy or temperature dependence of intersystem crossing. Chem. Phys. Lett. 1988, 145, 280−285. (29) Endo, A.; Sato, K.; Yoshimura, K.; Kai, T.; Kawada, A.; Miyazaki; Adachi, C. Efficient up-conversion of triplet excitons into a singlet state and its application for organic light emitting diodes. Appl. Phys. Lett. 2011, 98, 083302. (30) Cai, M.; Zhang, D.; Xu, J.; Hong, X.; Zhao, C.; Song, X.; Qiu, Y.; Kaji, H.; Duan, L. Unveiling the Role of Langevin and TrapAssisted Recombination in Long Lifespan OLEDs Employing Thermally Activated Delayed Fluorophores. ACS Appl. Mater. Interfaces 2019, 11, 1096. (31) Zhang, D.; Song, X.; Cai, M.; Kaji, H.; Duan, L. Versatile Indolocarbazole-Isomer Derivatives as Highly Emissive Emitters and Ideal Hosts for Thermally Activated Delayed Fluorescent OLEDs with Alleviated Efficiency Roll-Off. Adv. Mater. 2018, 30, 1705406.

(32) Luo, J.; Xie, G.; Gong, S.; Chen, T.; Yang, C. Creating a Thermally Activated Delayed Fluorescence Channel in a Single Polymer System to Enhance Exciton Utilization Efficiency for Bluishgreen Electroluminescence. Chem. Commun. 2016, 52, 2292−2295. (33) Zeng, X.; Luo, J.; Zhou, T.; Wu, K.; Zou, Y.; Xie, G.; Gong, S.; Yang, C. Using Ring-Opening Metathesis Polymerization of Norbornene to Construct Thermally Activated Delayed Fluorescence Polymers: High-Efficiency Blue Polymer Light-Emitting Diodes. Macromolecules 2018, 51, 1598−1604. (34) Forrest, S. R.; Bradley, D. D. C.; Thompson, M. E. Measuring the Efficiency of Organic Light-Emitting Devices. Adv. Mater. 2003, 15, 1043−1048.

H

DOI: 10.1021/acs.macromol.9b00083 Macromolecules XXXX, XXX, XXX−XXX