Using Ring-Opening Metathesis Polymerization of Norbornene To

Feb 16, 2018 - General Information, detailed synthesis of target polymers, AFM topographic images, TGA traces, UV–vis absorption spectra and PL spec...
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Article Cite This: Macromolecules XXXX, XXX, XXX−XXX

Using Ring-Opening Metathesis Polymerization of Norbornene To Construct Thermally Activated Delayed Fluorescence Polymers: High-Efficiency Blue Polymer Light-Emitting Diodes Xuan Zeng,†,# Jiajia Luo,†,# Tao Zhou,† Tianheng Chen,† Xiang Zhou,† Kailong Wu,† Yang Zou,‡ Guohua Xie,† Shaolong Gong,† and Chuluo Yang*,†,‡ †

Hubei Collaborative Innovation Center for Advanced Organic Chemical Materials, Hubei Key Lab on Organic and Polymeric Optoelectronic Materials, Department of Chemistry, Wuhan University, Wuhan 430072, People’s Republic of China ‡ Shenzhen Key Laboratory of Polymer Science and Technology, College of Materials Science and Engineering, Shenzhen University, Shenzhen 518060, People’s Republic of China S Supporting Information *

ABSTRACT: The exploitation of blue polymer emitters is of great importance for the application of solution-processed organic light-emitting diodes (OLEDs) in full color display. The highly efficient blue thermally activated delayed fluorescence (TADF) polymers are rarely reported up to now. Herein, we report an efficient approach to construct blue TADF polymers by ring-opening metathesis polymerization (ROMP) of norbornene. By side-chain engineering strategy, the polymers are endowed with distinct TADF features. By use of these blue polymeric emitters, the nondoped OLEDs achieved a maximum external quantum efficiency of 7.3% with the Commission Internationale de L’Eclairage (CIE) coordinates of (0.20, 0.29), which represents the state-of-the-art device performance for the TADF-based blue polymer light-emitting diodes (PLEDs).

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developed through the main-chain engineering strategy.13,14 Although, these main-chain-type TADF polymers have achieved impressive performance in polymer light-emitting diodes (PLEDs), the inevitable red-shift of emission caused by conjugation makes it difficult to obtain blue emission. To overcome this dilemma, our group successfully prepared a blue TADF polymer with emission peak at 478 nm by side-chain engineering strategy.15,16 Regardless of poor performance of this blue polymer in device, it shows us a feasible approach to design blue TADF polymers. In general, compared with the main-chain-type polymers, the side-chain-type TADF polymers are more likely to obtain blue emission by inheriting the characteristics of the side-chain blue TADF units. The key issue is to develop a polymer backbone with the suitable triplet energy level (ET). Up to now, most of the known conjugate polymers are not the ideal backbone for blue small molecule because their ET is too low to suppress triplet energy back transfer from emissive units to backbone which usually results in the poor performance of devices.17,18 Nonconjugated polymer backbones, in contrast, should be ideal candidates owing to their higher ET. From this point of view, Shao et al. demonstrated a pioneering work on blue TADF polymers by selecting nonconjugated polyethylene as the backbone, and an

INTRODUCTION Thermally activated delayed fluorescence (TADF) emitters have aroused persistent attention for their abilities to reach 100% theoretical internal quantum efficiency (IQE) without using noble metals.1,2 Along with a small energy gap (ΔEST) between the lowest excited singlet (S1) and triplet (T1) states, the triplet excitons can be efficiently upconverted into radiative singlet states by thermally aided reverse intersystem crossing (RISC). Up to now, numerous highly efficient TADF materials have been synthesized, which provide a potential alternative to phosphorescent emitters for organic light-emitting diodes (OLEDs). Especially, considering blue phosphorescent heavymetal complexes are troubled with several critical issues in cost, toxicity, and stability, hard endeavors have been made to develop blue TADF materials so as to overcome those challenge.3−6 Undoubtedly, the published works have achieved great breakthrough in improving efficiency and color purity. Large-scale manufacture of the devices, however, urgently requires solution processability to make fabrication process simpler, cheaper, and easily scalable.7−9 Therefore, the exploration of solution-processed blue TADF materials is of significant importance for cost-effective applications. Owing to the supreme solubility and morphological stability, polymers are very suitable for solution-processed OLED materials.10,11 In 2015, Nikolaenko et al. first fabricated a TADF polymer by imbedding TADF units into polymer backbone.12 Soon after, more TADF polymers have been © XXXX American Chemical Society

Received: December 14, 2017 Revised: February 4, 2018

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DOI: 10.1021/acs.macromol.7b02629 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules Scheme 1. Synthetic Route of the Target Polymers

Figure 1. (a) UV−vis absorption spectra and (b) PL spectra of the M2 and polymers in the neat film.

impressive performance was achieved.19 However, it is noteworthy that the polyvinyl-type backbone requires precise control during polymerization process to prevent implosion which no doubt increases the difficulty in synthesis. Therefore, seeking a superior method to fabricate nonconjugated backbone for blue TADF units is the key issue for the design of blue TADF polymers. In the past few years, the ring-opening metathesis polymerization (ROMP) has been widely used to fabricate nonconjugated polymers. As an ideal method for polymerization, the ROMP possesses various advantages in simplifying operation and controlling molecular weight, meanwhile, it is facile to introduce desired functional units into pendant groups.20−24 All above merits drive us to explore their application in fabricating blue TADF polymers. In this work, the nonconjugated polynorbornene was chosen as a backbone owing to its sufficiently high ET which could effectively confine the triplet excitons in TADF units. Considering the 9,9dimethylacridine−arylsulfone combinations have been used previously to obtain efficient RISC and blue small-molecule TADF OLEDs, the 10-(4-((4-(3-(4-methoxyphenyl)-9H-carbazol-9-yl)phenyl)sulfonyl)phenyl)-9,9-dimethyl-9,10-dihydroacridine (CSD) was designed and grafted to the backbone as the TADF units.25,26 The carbazole was also introduced into the side chain of the polymers to facilitate hole injection. As a result, by the side-chain engineering strategy, these blue polymers successfully inherited the characteristics of the blue TADF units. All the resulting polymers showed considerable molecular weight and good solubility in common organic solvents. To the best of our knowledge, this is the first report about using ROMP to fabricate TADF polymers. Utilizing these TADF polymers as the emitters, the nondoped PLED reached a maximum EQE of 7.3% with the Commission

Internationale de L’Eclairage (CIE) coordinates of (0.20, 0.29), which is one of the highest efficiency for blue PLEDs.

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RESULTS AND DISCUSSION As depicted in Scheme 1, the target polymers were synthesized by ring-opening metathesis polymerization of norbornene monomers (M1 and M2) in the presence of second-generation Grubbs catalyst, with the feed molar ratios of M1 and M2 being 95:5, 90:10, 85:15, and 80:20, respectively.27 Also, the polymer without M2 was synthesized for comparison. According to the feed molar ratio of M2 in the polymers, the polymers are titled as PBD-0, PBD-5, PBD-10, PBD-15, and PBD-20. The structures of these polymers were characterized by 1H NMR and 13C NMR spectroscopies, gel permeation chromatography, and elemental analysis. Calculated from 1H NMR spectroscopy, the final contents of M2 are ca. 2.4%, 7.1%, 11.1%, and 16.7% for PDB-5, PBD-10, PBD-15, and PBD-20, respectively. All the polymers were readily soluble in common organic solvents, such as chloroform, tetrahydrofuran, and chlorobenzene. Given that the morphology of films is of great importance to the device performance, the atomic force microscopy (AFM) was applied to explore the surface images of the polymers (see Figure S1 in the Supporting Information). Apparently, the four polymers show smooth and homogeneous film morphologies, and the values of root-mean-square (RMS) roughness range from 0.197 to 0.310 nm, which indicate their good film-forming abilities. Furthermore, the polymers show superior thermal stability with decomposition temperatures (Td, with 5% weight loss) of 371−405 °C (see Figure S2a), which benefited from their high molecular weights of polymers (Mn = 5.8 × 104−8.0 × 104). In differential scanning calorimetry (DSC), the glasstransition temperatures (Tg) of polymers are in the range 125− 156 °C (see Figure S2b), implying their similar amorphous B

DOI: 10.1021/acs.macromol.7b02629 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules

Figure 2. Temperature dependence of the transient PL decay spectra in the neat film for (a) PBD-5, (b) PBD-10, (c) PBD-15, and (d) PBD-20.

Table 1. Optical Properties of the TADF Polymers polymers

λem,max [nm]a/b

τp/ratio [ns]/[%]c

τd/ratio [μs]/[%]d

PLQYa/b

kp [106 s−1]e

kd [104 s−1]f

kISC [106 s−1]

kRISC [104 s−1]

PBD-5 PBD-10 PBD-15 PBD-20

466/453 467/457 467/461 467/464

25.2/53 26.4/52 27.0/50 27.6/49

5.4/47 5.9/48 5.8/50 6.1/51

0.40/0.38 0.47/0.42 0.56/0.55 0.40/0.34

8.0 8.3 10.2 6.0

3.3 3.4 4.7 2.8

3.8 4.0 5.1 3.1

6.2 6.5 9.4 5.6

Measured in toluene under argon condition. bMeasured in the neat film under argon condition. cThe lifetime and the ratio of the prompt fluorescence in the neat film. dThe lifetime and ratio of the delayed fluorescence in the neat film. eThe rate constant of prompt fluorescence process. f The rate constant of delayed fluorescence process. a

characteristics in the film state. All these results manifest that the polymers are suitable for solution-processed OLEDs because of their good film-forming abilities and thermal stabilities. Figure 1 shows the UV−vis absorption and fluorescence spectra of the M2 and polymers in the neat films. Obviously, the absorption peaks of PBD-5 to PBD-20 are similar to that of PBD-0, which reveals the M2 has little contribution to absorption at low loading content.28,29 The photoluminescent (PL) spectra of PBD-5 to PBD-20 in the neat films show distinctly single emission, with the emission peaks at 453, 457, 461, and 464 nm for PBD-5, PBD-10, PBD-15, and PBD-20, respectively, indicating the efficient energy transfer between the polymer backbone and side-chain TADF units even for the low ratio PBD-5. The slightly bathochromic shift in emission wavelength is due to the enhancive aggregation of TADF units with the increasing ratio of M2. Notably, all the polymers show emission peaks around 460 nm in the neat film, which is of great importance for the practical application of TADF-based OLEDs in full color display. Furthermore, to evaluate the triplet energy level of the backbone and TADF units, the phosphorescent spectra of PBD-0 and TADF moiety of CSD were collected at 77 K (see Figures S4 and S5). By taking the onset energy of phosphorescence as the transition energy of T1 to ground state, the ET were determined to be 2.95 and 2.82 eV for PBD-0 and CSD, respectively. As a result, the triplet energy

back transfer could be effectively suppressed owing to the significantly higher ET of backbone. To determine whether the TADF features exist in these polymers, their transient PL decay spectra in the neat films were investigated. As shown in Figure S6, PBD-5 to PBD-20 present second-order exponential decays with the prompt fluorescence lifetimes of 25.2−27.6 ns and the delayed fluorescence lifetimes of 5.4−6.1 μs in the neat films at room temperature. To further explore the origin of delayed fluorescence, we measured the transient PL decay spectra for PBD-5 to PBD-20 in the neat films from 100 to 300 K (Figure 2). Obviously, the intensities of the delayed fluorescence are monotonically enhanced with the rising temperatures, which indicates the reverse intersystem crossing (RISC) from the lowest triplet state to the lowest singlet state is accelerated by thermal activation. All the above results unambiguously confirm the TADF nature of these polymers. Moreover, the photoluminescence quantum yields (PLQYs) of the polymers in toluene (argon condition) and in the neat films are obtained (Table 1). The maximum PLQYs are achieved by PBD-15, which are 0.56 in toluene and 0.55 in the neat film. Accordingly, the rate constants of intersystem crossing (kISC) and reverse intersystem crossing (kRISC) were estimated and summarized in Table 1. The moderate kRISC values manifest that the triplet excitons can be efficiently harvested for delayed fluorescence.30,31 C

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Macromolecules In order to figure out the relationship between electronic and geometrical structures of the polymers, density functional theory (DFT) calculations at the B3LYP/6-31G(d) level were performed. As shown in Figure 3, the distributions of the

transformation of triplet excitons to radiative singlet excitons by RISC process.34 Therefore, these polymers successfully inherit the TADF features of the CSD units due to maintain the independence of the frontier molecular orbital distributions of the TADF units. To evaluate the performance of these polymers in electroluminescence (EL), solution-processed OLEDs with the structures of indium tin oxide (ITO)/poly(3,4-ethylenedioxythiophene)-doped poly(styrenesulfonate) (PEDOT:PSS) (30 nm)/PBD-x (x = 5, 10, 15, 20) (25 nm)/DPEPO (15 nm)/TmPyPB (45 nm)/8-hydroxyquinolinolatolithium (Liq) (1 nm)/Al (100 nm) (Figure 4a) were fabricated, where PEDOT:PSS and Liq serve as the hole- and electron-injection layer, respectively; 1,3,5-tri(m-pyrid-3-yl-phenyl)benzene (TmPyPB) acts as the electron transporting layer, and bis(2(diphenylphosphino)phenyl)ether oxide (DPEPO) acts as the exciton blocking layer due to its sufficiently high ET.35,36 Emitting material layers (EML) are PBD-5, PBD-10, PBD-15, and PBD-20 for devices A1−A4, respectively. The current density−voltage−luminance (J−V−L) characteristics, the EQEs versus current density curves, and the EL spectra for all these devices are shown in Figure 4c,d, and the key EL data are summarized in Table 2. Similar to the corresponding PL spectra in the neat film, devices A1−A4 exhibit blue EL emission from TADF units with the main peaks at 474, 478, 474, and 480 nm, respectively, suggesting the energy transfer from polymer backbone to emissive TADF side chain is complete at electrical excitation. Furthermore, as exhibited in Figure 4c, the current density−voltage characteristics are independent of the loading content of the TADF units, which implies the dominant EL mechanism is the Förster energy transfer from the backbone to the TADF units rather than direct charge trapping of the TADF units.15,27 Among these OLEDs, the maximum EQEs are 6.0%, 7.3%, 7.1%, and 6.7% for A1, A2, A3, and A4, respectively, and the maximum

Figure 3. Frontier molecular orbital distributions for the CSD and CSD-4BD (repeat units of PBD-20) characterized by density functional theory calculations [B3LYP; 6-31G(d)].

frontier molecular orbital of CSD-4BD (a repeating unit of PBD-20) are similar to that of CSD. For the calculation model of CSD-4BD, the highest occupied molecular orbital (HOMO) is primarily localized on the electron-donating unit of 9,9dimethyl-9,10-dihydroacridine (DMAC), while the lowest unoccupied molecular orbital (LUMO) is mainly localized on the electron-accepting unit of diphenyl sulfone. The well spatial separation between HOMO and LUMO is conducive to small singlet−triplet splitting energy (ΔEST).32,33 Furthermore, by time-dependent DFT (TD-DFT) calculations, S1 and T1 for their optimized ground-state geometries were obtained, and the computational values of ΔEST are 0.01 eV for both CSD and CSD-4BD (see Table S1). Such small ΔEST is favorable for

Figure 4. (a) Energy level diagrams for the devices A1−A4. (b) Chemical structures of the materials used. (c) Current density−voltage−luminance curves for devices A1−A4. (d) External quantum efficiency versus current density curves for devices A1−A4. Inset: the normalized EL spectra of devices A1−A4 at 4 V. D

DOI: 10.1021/acs.macromol.7b02629 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules Table 2. Electroluminescent Properties of Devices A1−A4, B, and C device

emitter

V [V]a

λpeak [nm]b

EQE [%]c

CEmax. [cd A−1]d

A1 A2 A3 A4 B C

PBD-5 PBD-10 PBD-15 PBD-20 CzSi:CSD CSD

4.3 3.8 3.9 3.8 5.4 3.6

474 478 474 480 454 486

6.0/2.8 7.3/4.0 7.1/3.6 6.7/3.7 5.2/4.0 1.8/1.0

10.2 13.5 13.7 13.3 6.7 3.9

CIE (x, y)e (0.19, (0.20, (0.22, (0.18, (0.16, (0.19,

0.25) 0.29) 0.29) 0.30) 0.16) 0.32)

ηd [%]f 4.9 6.0 5.7 5.5

a The driving voltage at 1 cd m−2. bEL peak wavelength. cMaximum external quantum efficiency and the value at 100 cd m−2. dMaximum current efficiency. eThe Commission Internationale de L’Eclairage coordinates recorded at maximum EQE. fThe delayed components of EQE.

current efficiencies (CEs) are 10.2, 13.5, 13.7, and 13.3 cd A−1 for A1, A2, A3, and A4, respectively. Notably, such impressive performance was obtained in the nondoped devices, which is propitious to simplify the device fabrication and thus to cut the costs in practical application. Device A2 gives the highest EQE up to 7.3%, which probably ascribes to the moderate PLQY and kRISC of PBD-10 in comparison with other polymers. Comparatively, the performance of device A3 is slightly inferior to device A2, although PBD-15 is endowed with higher PLQY; most likely PBD-15 suffers from severer concentration quenching than that of PBD-10 in devices.37,38 It is noteworthy that all the devices A1−A4 achieve maximum EQE of above 5%, which demonstrate that delayed components make a contribution to the device performance. To quantitatively estimate the contribution of the different parts to device efficiencies, we calculated the prompt and delayed components of EQE (ηp and ηd); the detailed results are summarized in Table 2.39 Taking the device A2 as an example, the delayed component ηd is figured out to be 6.0%, which is the several times of prompt component (ηp = 1.3%). Similarly, the ηd of the rest devices are also much greater than ηp. It is obvious that the RISC process play a decisive role in promoting the device performance. Finally, two control devices with the same architecture as the device A2 but different EML were fabricated for comparison. The EMLs were CzSi:CSD (9:1) and pure CSD for devices B and C, respectively, where 9-(4-tert-butylphenyl)-3,6-bis(triphenylsilyl)-9H-carbazole (CzSi) was employed as the host for TADF small molecule (CSD). The J−V−L characteristics and EQEs versus current density curves for the control devices are displayed in Figure S7. The key EL data are summarized in Table 2. In detail, the maximum EQE of device A2 is apparently superior to that of device B (EQE = 5.2%) and device C (EQE = 1.8%). These results demonstrate PBD-10 is more suitable for solution-processed OLEDs, presumably due to the avoidance of phase separation and concentration quenching.40

TADF-based blue PLEDs. All in all, this work provides a promising strategy to develop highly efficient blue TADF polymers for the solution-processed light-emitting devices.

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EXPERIMENTAL SECTION

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ASSOCIATED CONTENT

General Polymerization Procedure. The polymerization was performed according to general procedures of ring-opening metathesis polymerization (ROMP). Under argon, different ratios of M1 and M2, second-generation Grubbs catalyst, 5 mL of tetrahydrofuran were added into 50 mL flask at room temperature. After stirring in the dark for an hour, a small amount of ethyl vinyl ether was added into flask to inhibit the polymerizations. Several minutes later, the whole mixture was dropwise added into a mixture solvent (250 mL of MeOH and 30 mL of acetone) for sedimentation. 10 mL of H2O2 (5%) was added into the precipitated polymer in CH2Cl2 and stirred for 5 h at room temperature. After that, the combined organic phases were dried over Na2SO4 and purified by a short column chromatography (silica, CH2Cl2). After sedimentation in a mixture solvent (250 mL of methyl alcohol and 30 mL of acetone) twice, the precipitated polymer was washed with acetone in a Soxhlet apparatus for 3 days and continuously washed with n-hexane for another 3 days. After that, the residue was dried under vacuum at room temperature to provide target polymers. Device Fabrication and Measurement. The ITO-coated glass was cleaned with acetone and ethanol in an ultrasonic bath first. Ultraviolet light-ozone treatment was then conducted with a UVozone surface processor PL16 series (SenLights Corporation). PEDOT:PSS was spin-coated onto ITO surface under 3000 rpm and then baked at 120 °C to remove the residual water. Afterward, the substrate was transferred into a N2-filled glovebox, and the polymer emitting layer was spin-coated under 1500 and 1000 rpm for emitting layer using small molecules, followed with a baking at 50 °C. The exciton blocking layer (DPEPO), the electron transporting layer (TmPyPB), the electron injecting layer (Liq), and aluminum cathode were consecutively evaporated in a vacuum chamber under 10−6 mbar. The as-fabricated device was measured in ambient air without any encapsulation. The electroluminescence properties were measured with a Photoresearch SpectraScan PR735 spectrometer and a Keithley 2400 direct-current source meter. The external quantum efficiency was calculated by assuming a Lambertion emission profile.

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CONCLUSIONS In conclusion, a series of blue TADF polymers have been successfully fabricated by the ring-opening metathesis polymerization. To the best of our knowledge, the ROMP was first employed to synthesize TADF polymers. All polymers show good film-forming ability and superior thermal stability ascribe to their high molecular weights. Owing to the high ET (2.95 eV) of the backbone, the triplet energy back transfer from the TADF units to the polymer backbone can be effectively suppressed. Noteworthily, the nondoped OLEDs were fabricated by solution process with the target polymers as emitters, and a maximum EQE of 7.3% was achieved, which represents the state-of-the-art device performance for the

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.7b02629. General Information, detailed synthesis of target polymers, AFM topographic images, TGA traces, UV− vis absorption spectra and PL spectra, transient PL decay spectra, theoretical and experimental data of CSD and PBD-20, and electroluminescent characteristics of devices B and C (PDF) E

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Macromolecules

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (C.Y.). ORCID

Guohua Xie: 0000-0003-0764-7889 Shaolong Gong: 0000-0002-1166-9047 Chuluo Yang: 0000-0001-9337-3460 Author Contributions #

X.Z. and J.L. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the grants from the National Natural Science Foundation of China (Nos. 91433201, 61575146, and 51573141), the National Basic Research Program of China (973 Programs 2015CB655002), the National Key Research Program (2016YFB0401002), and the Innovative Research Group of Hubei Province (No. 2015CFA014).

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DOI: 10.1021/acs.macromol.7b02629 Macromolecules XXXX, XXX, XXX−XXX