Blue Thermally Activated Delayed Fluorescence Polymers with

Nov 17, 2017 - Blue Thermally Activated Delayed Fluorescence Polymers with Nonconjugated Backbone and Through-Space Charge Transfer Effect...
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Communication Cite This: J. Am. Chem. Soc. 2017, 139, 17739−17742

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Blue Thermally Activated Delayed Fluorescence Polymers with Nonconjugated Backbone and Through-Space Charge Transfer Effect Shiyang Shao,† Jun Hu,†,‡ Xingdong Wang,† Lixiang Wang,*,† Xiabin Jing,† and Fosong Wang† †

State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, People’s Republic of China ‡ University of the Chinese Academy of Sciences, Beijing 100049, People’s Republic of China J. Am. Chem. Soc. 2017.139:17739-17742. Downloaded from pubs.acs.org by EASTERN KENTUCKY UNIV on 01/29/19. For personal use only.

S Supporting Information *

Chart 1. Architectures of TBCT- and TSCT-Based Polymers

ABSTRACT: We demonstrate novel molecular design for thermally activated delayed fluorescence (TADF) polymers based on a nonconjugated polyethylene backbone with through-space charge transfer effect between pendant electron donor (D) and acceptor (A) units. Different from conventional conjugated D−A polymers with throughbond charge transfer effect, the nonconjugated architecture avoids direct conjugation between D and A units, enabling blue emission. Meanwhile, spatial π−π interaction between the physically separated D and A units results in both small singlet−triplet energy splitting (0.019 eV) and high photoluminescence quantum yield (up to 60% in film state). The resulting polymer with 5 mol % acceptor unit gives efficient blue electroluminescence with Commission Internationale de l’Eclairage coordinates of (0.176, 0.269), together with a high external quantum efficiency of 12.1% and low efficiency roll-off of 4.9% (at 1000 cd m−2), which represents the first example of blue TADF nonconjugated polymer.

tends to induce a large red-shift of emission, undesired for realizing blue electroluminescence. Another challenge for development of blue TADF polymer lies in the contradiction between the small ΔEST and high photoluminescence quantum yield (PLQY): the sufficient separation of HOMO and LUMO required for small ΔEST could lead to small oscillator strength and thus low PLQY.15 Therefore, it is desirable to develop a strategy for control of charge transfer (CT) strength, reduction of ΔEST and enhancement of PLQY for blue TADF polymers. Here, we propose a novel concept for design of blue TADF polymers based on a nonconjugated polyethylene backbone with through-space charge transfer (TSCT) effect between pendant D and A units (Chart 1). In this motif, D and A units are physically separated, but meanwhile are spatially proximate, allowing through-space, rather than the through-bond charge transfer process to occur. This concept has the following advantages. First, the nonconjugated architecture avoids the strong electron coupling between D and A, favorable for realizing blue emission. Second, the physical separation of D and A would lead to a small overlap of HOMO and LUMO and thus a small ΔEST. Third, the electron clouds of D and A can communicate with each other through spatial CT interactions to enhance the radiative decay rate, therefore considerable PLQY can be expected. According to this strategy, we choose nonconjugated polyethylene as the backbone, 9,9-dimethyl-10-phenyl-acridan (Ac) or 9,9-bis(1,3ditert-butylphenyl)-10-phenyl-acridan (TBAc) as the pendant electron donor and 2,4,6-triphenyl-1,3,5-triazine (TRZ) as the pendant electron acceptor to construct polymers (Scheme 1a). Although Ac- and TBAc-based polymers contain arcidan as the electron donating units, the steric hindrance effect about the acridan unit may be different in the resulting polymers. Unlike the Ac-based polymers where acridan can get close to triazine

hermally activated delayed fluorescence (TADF) molecules1−6 have evolved as next-generation materials in organic light-emitting diodes (OLEDs), because they are capable of utilizing triplet excitons through enhanced reverse intersystem crossing (RISC) process from the lowest triplet state (T1) to singlet state (S1), providing an approach to reach 100% internal quantum efficiency (IQE) without use of noble metal elements. TADF research has focused on small molecules that rely on vacuum-evaporation technologies. However, TADF polymers7−19 suitable for simple, low-cost and easily scalable solution processability are less developed. Moreover, most of TADF polymers are focused on green,7,11,14,16−18 yellow8,10 and orangered9 emission. Design of TADF polymer for blue electroluminescence (EL) has remained a challenge, and is not reported yet. Similar as small TADF molecules, design for TADF polymers requires spatial separation of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) to realize a small singlet−triplet energy splitting (ΔEST) for rapid RISC process. To meet this requirement, the majority of reported TADF polymers adopted the architecture where electron donor (D) and acceptor (A) are linked through conjugated bonds (Chart 1). However, the strong through-bond charge transfer (TBCT) effect in the conjugated architecture

T

© 2017 American Chemical Society

Received: September 26, 2017 Published: November 17, 2017 17739

DOI: 10.1021/jacs.7b10257 J. Am. Chem. Soc. 2017, 139, 17739−17742

Communication

Journal of the American Chemical Society Scheme 1. Chemical Structures of TSCT-Based Polymers (a) and the Corresponding Control Polymers (b)

Figure 1. PL spectra of P-Ac50-TRZ50 (a) and P-TBAc50-TRZ50 (b) in toluene at 298 K with a concentration of 1 × 10−4 M, λex = 310 nm; inset: PL images of the polymers under 365 nm UV light; and PL decay curves of P-Ac50-TRZ50 (c) and P-TBAc50-TRZ50 (d) in toluene under nitrogen and air at 298 K.

unit, in TBAc-based polymers the acridan unit is separated from the triazine unit by the steric 1,3-ditert-butylphenyl groups. Therefore, the influence of distance between the D and A units on the CT interactions of the resulting polymers can be explored. Ac-based polymer shows a distinct TSCT effect and TADF feature with both small ΔEST of 0.019 eV and high PLQY up to 60% in film state. In comparison, the TBAc-based counterpart exhibits no TSCT effect, giving only promote fluorescence emission. The resulting polymer with 95 mol % Ac and 5 mol % TRZ unit gives efficient blue electroluminescence with Commission Internationale de l’Eclairage (CIE) coordinates of (0.176, 0.269), together with an external quantum efficiency (EQE) of 12.1% for maximum value and 11.5% at 1000 cd m−2 (corresponding to a roll-off of 4.9%), which represents the first example of blue TADF nonconjugated polymer. The polymers were synthesized by free radical polymerization of the corresponding vinyl-functionalized acridan and triazine monomers using 2-azoisobutyronitrile as initiator and tetrahydrofuran (THF) as solvent (Scheme S1). The relative content of D and A units are tuned at 50:50 mol % (P-Ac50-TRZ50 and PTBAc50-TRZ50) and 95:5 mol % (P-Ac95-TRZ05 and PTBAc95-TRZ05) to optimize their device efficiency. For comparison, control polymers containing only Ac, TBAc or TRZ units (Scheme 1b) are also synthesized. The polymers exhibit typical number-average molecular weights (Mns) of 30− 100 K Da and polydispersity index (PDI) of 1.70−2.50. The decomposition temperatures (Tds) of the polymers are higher than 350 °C, and the glass transition temperatures (Tgs) are in the range of 190−260 °C, indicating their good thermal and morphological stability (Figure S1). All the polymers have excellent solubility in common organic solvents, such as toluene, chloroform, THF, etc., ensuring formation of high quality film through solution processes such as spin-coating and inkjet printing. PL spectra of the polymers in toluene at a concentration of 10−4 mol L−1 are shown in Figure 1a,b. P-Ac50-TRZ50 shows a broad featureless emission band with the maxima (λem,max) located at 489 nm, red-shifted relative to P-Ac (λem,max= 376 nm) and P-TRZ (λem,max= 438 nm). Additionally, this emission band shows strong dependence on solvent polarities. For example, the λem,max shifts from 463 nm in cyclohexane to 518 nm in THF (Figure S3). These observations are indicative of a distinct CT transition between the Ac and TRZ unit in P-Ac50-TRZ50. In comparison, P-TBAc50-TRZ50 shows two emission bands at 369 and 444 nm, which are close to those of P-TBAc (λem,max= 367 nm) and P-TRZ, respectively, implying no CT transition

occurs. The film state PL spectra of the polymers show a similar trend where P-Ac50-TRZ50 gives a red-shifted CT emission at 490 nm relative to P-Ac (374 nm) and P-TRZ (452 nm), whereas P-TBAc50-TRZ50 shows an emission band that is close to PTRZ (see Figure S4). These results suggest TSCT occurs efficiently in P-Ac50-TRZ50, but is suppressed in P-TBAc50TRZ50 by introducing steric groups to separate the electrondonating acridan unit from the electron-accepting triazine unit. To probe the anticipated delayed fluorescence of the polymers, PL decay of the polymers was measured in nitrogen and air. As shown in Figure 1c and Table 1, under nitrogen, the solution of P-Ac50-TRZ50 in toluene displays distinctive delayed emission with a lifetime (τd) of 1173.0 ns in addition to a prompt emission with lifetime (τp) of 24.3 ns. The percentage of the delayed and prompt component is 13% and 87%, respectively. However, under air, the delayed component is not detectable, indicating that the delayed emission is arising from triplets which can be quenched by oxygen. Such results with prompt and delayed emissive components are consistent with TADF behavior and are also observed for P-Ac95-TRZ05, which shows τp of 36.3 ns and τd of 1279.4 ns under nitrogen (Figure S6). For the TBAc-based polymers, the delayed components were not detectable under nitrogen and air. For instance, PTBAc50-TRZ50 gives only promote fluorescence emission with τp of 6.3 ns under nitrogen and 6.6 ns under air (Figure 1d). To further explore the TADF character, ΔESTs of the polymers were first detected from the onset of fluorescence spectra at room temperature and phosphorescence spectra at 77 K in film state as reported.17,20 As shown in Figure S7, a ΔEST of 0.019 eV is obtained for P-Ac50-TRZ50, small enough to facilitate the rapid equilibration of the T1 and S1 states to give delayed emission under thermal activation.2 The small ΔEST is mainly ascribed to the separation of HOMO and LUMO distributions in P-Ac50TRZ50 as verified by the theoretical calculation results. As shown in Figure S8, the HOMO of the polymer model is localized on the acridan unit, whereas the LUMO is distributed mainly on the triazine unit. The well-separated frontier molecular orbitals lead to a small calculated ΔEST value of 0.0011 eV. Subsequently, PLQY of the polymer film was measured by an integration sphere, which was 60% for P-Ac50-TRZ50 under nitrogen. This value is impressive because it is comparable with the TBCTbased conjugated polymers (typically 40−70%).7,8,11,13,14 This result can be attributed to the through-space π−π interaction of 17740

DOI: 10.1021/jacs.7b10257 J. Am. Chem. Soc. 2017, 139, 17739−17742

Communication

Journal of the American Chemical Society

Table 1. Comparison of Physical Properties of P-Ac50-TRZ50, P-TBAc50-TRZ50 and the Corresponding Control Polymers Polymer

λema (nm)

τp/τdb (ns, in N2)

τp/τdb (ns, in air)

PLQYc (%, in N2)

PLQYc (%, in air)

HOMOd (eV)

LUMOe (eV)

P-Ac50-TRZ50 P-TBAc50-TRZ50 P-Ac P-TBAc P-TRZ

489 369/444 376 367 438

24.3/1173.0 15.5/− 7.5/− 6.3/− 7.4/−

20.4/− 11.7/− 6.9/− 6.6/− 6.0/−

60 9 19 2 12

39 8 18 2 9

−5.26 −5.30 −5.30 −5.39 −

−2.68 −2.64 − − −2.67

Emission peaks tested in toluene (1 × 10−4 M) at 298 K. bLifetimes of prompt emission (τp) and delayed emission (τd) in toluene at 298 K in N2 or air. cPLQY measured in film state at 298 K. dObtained from the oxidation potential in CH2Cl2 solution. eObtained from reduction potential in THF solution.

a

Table 2. Device Performance of the Polymers Maximum value/at 100 cd m−2/at 1000 cd m−2

a

Polymer

Vona (V)

Lmaxb (cd m)

LE (cd A−1)c

EQE (%)d

CIE (x,y)e @ 7 V

P-Ac50-TRZ50 P-Ac95-TRZ05 P-TBAc50-TRZ50 P-TBAc95-TRZ05 P-Ac/P-TRZ (50:50) P-Ac/P-TRZ (95:5)

3.4 3.2 3.8 5.8 4.0 4.6

7989 6150 325 140 421 532

8.5/8.1/8.4 24.8/24.5/23.6 0.36/0.31/− 0.51/0.21/− 2.45/1.72/− 3.34/2.14/−

3.1/3.0/3.0 12.1/12.0/11.5 0.21/0.18/− 0.33/0.14/− 1.44/1.01/− 1.92/1.23/−

0.222, 0.428 0.176, 0.269 0.235, 0.243 0.207, 0.196 0.197, 0.211 0.185, 0.169

Voltage at 1 cd m−2. bMaximum luminance. cLuminous efficiency. dExternal quantum efficiency. eCIE coordinates.

nm with CIE coordinates of (0.222, 0.428). In comparison, PAc95-TRZ05 exhibits blue-shifted EL emission at 472 nm, and the CIE coordinates are moved to (0.176, 0.269). The emission is almost bias-independent when the driving voltage changes from 5 to 8 V. Interestingly, we note emission from the Ac unit is not observed although the TRZ content is as low as 5 mol %. In contrast, for the P-Ac/P-TRZ (95/5) device, intense emission from Ac unit is observed at the same voltages. This observation is reasonable considering the intermolecular charge transfer between D and A is not as effective as intramolecular charge transfer. As for P-TBAc95-TRZ05, complicated profiles with several long-wavelength emission bands at 545 and 635 nm were observed, with the origin not clear yet. Figure 2b−d shows current density−voltage and luminance− voltage characteristics, as well as luminance dependence of EQEs for the devices. The device performance is summarized in Table 2. We first note the efficiencies of the polymers are sensitive to the TRZ content. For instance, as the TRZ content decreases from 50 mol % (P-Ac50-TRZ50) to 5 mol % (P-Ac95-TRZ05), the maximum EQE increases from 3.1% to 12.1%, probably

the spatially proximate D and A units, favorable for enhancing the radiative decay rate according to the Franck−Condon principle.21 This deduction is verified by the fact that PTBAc50-TRZ50 with spatially separated D and A units gives a low PLQY of 9% under the same condition. Therefore, it is reasonable to conclude the TSCT-based nonconjugated polymer provides an exquisite motif to control the degree of electron cloud overlap between D and A to reach the balance between small ΔEST and high PLQY. Importantly, the balance is maintained for P-Ac95-TRZ05 with a low acceptor content of 5 mol %, which shows a ΔEST of 0.021 eV and film state PLQY of 51% under nitrogen. This finding is favorable for us to optimize device performance of the polymers through varying the donor/ acceptor content ratios. Finally, to gain deeper insight into the emission behavior of the polymers, comparison of the PL properties of P-Ac50-TRZ50 and the mixture of monomeric units MC-Ac and MC-TRZ with the same mole ratio (1:1) was carried out (see Figure S8 for chemical structure). The mixture film exhibits a similar long-wavelength CT emission that contains both prompt and delayed component as the polymer. However, the intensity of the CT emission is weak in the mixture, accompanied by a low PLQY of 13% (Figure S9). These results indicate the polymer backbone plays an important role in enhancing the CT transition and achieving high PL efficiency for this donor/acceptor system. To investigate the EL performance, polymer OLEDs were fabricated with device configuration of ITO/PEDOT:PSS (40 nm)/polymer (40 nm)/TSPO1 (8 nm)/TmPyPB(42 nm)/LiF (1 nm)/Al (100 nm) (Figure S11). In the devices, PEDOT:PSS (poly(3,4-ethylenedioxythiophene:poly(styrenesulfonate)) serves as the hole-injection layer, whereas TSPO1 (diphenyl(4(triphenylsilyl)phenyl)phosphine oxide)22 and TmPyPB (1,3,5tri(m-pyrid-3-yl-phenyl) benzene)23 act as the exciton blocking layer and the electron-transporting layer, respectively. For comparison, devices containing the physical blends of P-Ac and P-TRZ with mole ratios of 50:50 and 95:5 (denoted as P-Ac/ P-TRZ (50/50) and P-Ac/P-TRZ (95/5), respectively) as the emissive layers were also fabricated. As shown in Table 2 and Figure S12, P-Ac50-TRZ50 shows EL emission peaked at 497

Figure 2. EL spectra of P-Ac95-TRZ05 at various driving voltages (a), current density−voltage (b) and luminance−voltage characteristics (c) of the devices and (d) EQEs of the devices as a function of luminance. 17741

DOI: 10.1021/jacs.7b10257 J. Am. Chem. Soc. 2017, 139, 17739−17742

Communication

Journal of the American Chemical Society ORCID

because concentration quenching of the excitons is suppressed and the electron leakage from the emissive layer to the anode is inhibited at low TRZ content. Second, the device efficiency of PAc95-TRZ05 is significantly higher than that of P-TBAc95TRZ05. For instance, the maximum EQE of P-Ac95-TRZ05 (12.1%) is ∼37 times that of P-TBAc95-TRZ05 (0.33%). The high EQE of P-Ac95-TRZ05 is in accordance with its capability to utilize triplet excitons through TADF effect. Because the theoretical EQE for OLEDs is generally expressed as EQE = IQE × ηout = γ × ηST × ηPL × ηout

Lixiang Wang: 0000-0002-4676-1927 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge financial support from the National Natural Science Foundation of China (Nos. 51573182, 51203149 and 91333205), the National Key Research and Development Program (2017YFB0404402 and 2016YFB0401301), and the 973 Project (No. 2015CB655000).

(1)

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where ηout is the light out-coupling efficiency, γ is the charge balance factor (ideally γ = 1), ηST is the fraction of radiative excitons and ηPL is PLQY of the emissive material. The ηST of PAc95-TRZ05 can be 94.9% assuming a ηout of 25%.24 This value is nearly 4 times that of conventional fluorescent materials (ηST = 25%), confirming the contributions of the triplets for radiative excitons. Third, the maximum EQE of P-Ac95-TRZ05 is also much higher than that of the P-Ac/P-TRZ (95/5) blend (1.92%). This result supports that intramolecular through-space charge transfer, rather than the intermolecular charge transfer, plays the key role in reaching high efficiency for P-Ac95-TRZ05. Finally, it is worth noting the TSCT-based TADF polymers show low efficiency roll-off at high luminance. At the luminance of 100 m−2 and 1000 cd m−2, the EQE of P-Ac95-TRZ05 is maintained at 12.0% and 11.5%, respectively, corresponding to a roll-off of only 0.8% and 4.9% relative to the maximum value. This result is encouraging considering that the small efficiency roll off at high luminances is much desired for the practical display and lighting applications. In summary, we have proposed a novel design concept for blue TADF polymers based on a nonconjugated polyethylene backbone with through-space charge transfer effect between pendant electron-donating acridan unit and electron-accepting triazine unit. The polymer exhibits distinct delayed fluorescence with τd of 1173.0 ns in the absence of oxygen, with a small ΔEST of 0.019 eV and a promising PLQY up to 60% in film state. The resulting polymer with 5 mol % acceptor unit displays blue electroluminescence with CIE coordinates of (0.176, 0.269), together with a high external quantum efficiency of 12.1% and low efficiency roll-off of 4.9% (at 1000 cd m−2), which represents the first example of blue TADF nonconjugated polymer for solution-processed OLEDs. We postulate this work will open up a new way for us, from conjugated polymers with through-bond charge transfer effect to nonconjugated polymers with throughspace charge transfer effect, to develop blue TADF polymers. Future efforts on further improving the PL and EL efficiencies of these polymers are still needed, and approaches like TADF assisted fluorescence strategy,25 or enhancement of the planarity of the donor/acceptor units,20 would be promising to achieve this goal.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.7b10257.

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REFERENCES

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DOI: 10.1021/jacs.7b10257 J. Am. Chem. Soc. 2017, 139, 17739−17742