Blue Thermally Activated Delayed Fluorescence Polymers with

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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 J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.7b10257 • Publication Date (Web): 17 Nov 2017 Downloaded from http://pubs.acs.org on November 17, 2017

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Journal of the American Chemical Society

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,† Fosong Wang† †

State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, P. R. China ‡ University of the Chinese Academy of Sciences, Beijing 100039, P. R. China.

Supporting Information Placeholder ABSTRACT: We demonstrate a novel molecular design concept for thermally activated delayed fluorescence (TADF) polymers based on a nonconjugated polyethylene backbone with throughspace charge transfer effect between pendant electron donor (D) and electron acceptor (A) units. Different from conventional conjugated D-A polymers with through-bond charge transfer effect, the nonconjugated architecture avoids the direct conjugation between D and A units, enabling us to realize blue emission. Meanwhile, the 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 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.

Thermally activated delayed fluorescence (TADF) molecules1-6 have evolved rapidly as the new-generation materials for application 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), thereby providing an approach to reach 100% internal quantum efficiency (IQE) without the use of noble metal elements. Nowadays, TADF research has been mainly focused on small molecules that rely on vacuum-evaporation technologies. However, TADF polymers7-19 that are 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 yellow,8,10 and orange-red9 emission. Comparatively, the design of TADF polymer for blue electroluminescence (EL) has remained a great challenge, and is not reported yet.

Chart 1. Architectures of TBCT- and TSCT-based polymers.

undesired for realizing blue electroluminescence. Another challenge for the development of blue TADF polymer lies in the contradiction between the small ΔEST and high photoluminescence quantum yield (PLQY), that is, 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 the strategy for control of the charge transfer (CT) strength, reduction of ΔEST and enhancement of PLQY for blue TADF polymers. Scheme 1. Chemical structures of TSCT-based polymers (a) and the corresponding control polymers (b).

Similar as small TADF molecules, the 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 electron acceptor (A) are directly linked to each other through conjugated bonds (Chart 1). However, the strong through-bond charge transfer (TBCT) effect in the conjugated architecture tends to induce a large red-shift of emission, which is Table 1. Comparison of physical properties of P-Ac50-TRZ50, P-TBAc50-TRZ50 and the corresponding control polymers.

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Polymer

λem a (nm)

τp/τdb (ns, in N2)

τp/τdb (ns, in air)

PLQY c (%, in N2)

PLQY c (%, in air)

HOMOd (eV)

LUMOe (eV)

P-Ac50-TRZ50

489

24.3/1173.0

20.4/–

60

39

-5.26

-2.68

369/444

15.5/–

11.7/–

9

8

-5.30

-2.64

P-Ac

376

7.5/–

6.9/–

19

18

-5.30

–

P-TBAc

367

6.3/–

6.6/–

2

2

-5.39

–

P-TRZ

438

7.4/–

6.0/–

12

9

–

-2.67

P-TBAc50-TRZ50

Emission peaks tested in toluene (1 × 10−4 M) at 298 K. b Lifetimes of prompt emission (τp) and delayed emission (τd) in toluene at 298K in N2 or air. c PLQY measured in film state at 298K. d Obtained from the oxidation potential in CH2Cl2 solution. e Obtained from reduction potential in THF solution.

a

Figure 1. PL spectra of P-Ac50-TRZ50 (a) and P-TBAc50-TRZ50 (b) in toluene at 298K 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 298K. Here, we propose a novel concept for the 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 placed in such a way that they are physically separated, but meanwhile are spatially proximate to each other, allowing through-space, rather than the through-bond charge transfer process to occur. This design concept has the following advantages. First, the nonconjugated architecture avoids the strong electron coupling between D and A, which is 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 the above strategy, we choose nonconjugated polyethylene as the backbone, 9,9-dimethyl-10-phenylacridan (Ac) or 9,9-bis(1,3-di-tert-butylphenyl)-10-phenyl-acridan (TBAc) as the pendant electron donor, and 2,4,6-triphenyl-1,3,5triazine (TRZ) as the pendant electron acceptor to construct polymers (Scheme 1a). Although both 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-

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. based polymers where acridan can get close to triazine unit, in TBAc-based polymers the acridan unit is separated from the triazine unit by the steric 1,3-di-tert-butylphenyl groups. Therefore the influence of distance between the D and A units on the CT interactions of the resulting polymers can be explored. It is found that the Ac-based polymer shows 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 P-TBAc50TRZ50) and 95:5 mol% (P-Ac95-TRZ05 and P-TBAc95-TRZ05) to optimize their device efficiency. For comparison, control polymers containing only Ac, TBAc or TRZ units


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Table 2. Device Performance of the polymers.

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a

Polymer

Vona (V)

Lmaxb (cd m)

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

LE (cd A-1) c

EQE (%) d

Maximum value/at 100 cd m-2/at 1000 cd m-2 8.5/8.1/8.4 3.1/3.0/3.0 24.8/24.5/23.6 12.1/12.0/11.5 0.36/0.31/0.21/0.18/0.51/0.21/0.33/0.14/2.45/1.72/1.44/1.01/3.34/2.14/1.92/1.23/-

CIE (x,y) e @7V 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. b Maximum luminance. c Luminous efficiency. d External quantum efficiency; e CIE coordinates.

(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 oC, and the glass transition temperatures (Tgs) are in the range of 190–260 oC, indicating their good thermal and morphological stability (Figure S1, Supporting Information). All the polymers have excellent solubility in common organic solvents, such as toluene, chloroform, THF, etc., ensuring the formation of high quality film through solution processes such as spin-coating and ink-jet printing. PL spectra of the polymers in toluene at a concentration of 10–4 mol L–1 are shown in Figure 1a–b. It is found that P-Ac50-TRZ50 shows a broad featureless emission band with the maxima (λem,max) located at 489 nm, which is 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 nm and 444 nm, which are close to those of P-TBAc (λem,max= 367 nm) and P-TRZ, respectively, implying that no CT transition occurs in this case. 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 (463 nm) and P-TRZ (452 nm), while P-TBAc50TRZ50 shows an emission band that is close to P-TRZ (see Figure S4). These results suggest that TSCT occurs efficiently in P-Ac50TRZ50, but is suppressed in P-TBAc50-TRZ50 by introducing steric groups to separate the electron-donating acridan unit from the electron-accepting triazine unit. To probe the anticipated delayed fluorescence of the polymers, PL decay of the polymers were measured in nitrogen and air. As shown in Figure 1c and Table 1, under nitrogen, the solution of PAc50-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 calculated to be 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). While for the TBAc-based polymers, the delayed components were not detectable under both nitrogen and air. For instance, P-TBAc50-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, ΔEST 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 previously.17, 20 As shown in Figure S7, a ΔEST of 0.019 eV is obtained for P-Ac50-TRZ50, which is 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 PAc50-TRZ50 as verified by the theoretical calculation results. As can be seen 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 very small calculated ΔEST value of 0.0011 eV. Subsequently, PLQY of the polymer film was measured by an integration sphere, which was found to be 60% for P-Ac50-TRZ50 under nitrogen. This value is very impressive because it is even comparable with the TBCT-based conjugated polymers (typically 40–70%)7,8,11,13,14. This result can be attributed to the through-space π−π interaction of the spatially proximate D and A units, which is favorable for enhancing the radiative decay rate according to the Franck-Condon principle.21 This deduction is verified by the fact that P-TBAc50-TRZ50 with spatially separated D and A units gives a low PLQY of 9% under the same condition. Therefore, it is reasonable to conclude that the TSCT-based nonconjugated polymer provides us 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 PAc95-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 the 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). It is found that the mixture film exhibits a similar longwavelength CT emission which contains both prompt and delayed component as the polymer. However, the intensity of the CT emission is very weak in the mixture, accompanied by a low PLQY of 13% (Figure S9). These results indicate that 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(styrene sulfonate)) serves as the hole-injection layer, while TSPO1 (diphenyl(4-(triphenylsilyl)phenyl)phos-



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phine oxide)22 and TmPyPB (1,3,5-tri(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 nm with CIE coordinates of (0.222, 0.428). In comparison, P-Ac95-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 V to 8 V. Interestingly, we note that 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 that the intermolecular charge transfer between D and A is not as effective as intramolecular charge transfer. As for P-TBAc95-TRZ05, complicated profile with several long-wavelength emission bands at 545 nm and 635 nm were observed, with the origin not clear yet. Figure 2b–d show the current density – voltage and luminance – voltage characteristics, as well as the luminance dependence of EQEs for the devices. The device performance is summarized in Table 2. We first note that 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-Ac95TRZ05), the maximum EQE increases from 3.1% to 12.1%, probably because that 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, it is found that the device efficiency of P-Ac95-TRZ05 is significantly higher than that of P-TBAc95-TRZ05. For instance, the maximum EQE of PAc95-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. Since the theoretical EQE for OLEDs is generally expressed as EQE = IQE × ηout = γ × ηST × ηPL × ηout

( 1)

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 P-Ac95TRZ05 can be calculated to be 94.9% assuming a ηout of 25%.24 This value is nearly four times that of conventional fluorescent materials (ηST=25%), confirming the contributions of the triplets for radiative excitons. Third, we find that the maximum EQE of PAc95-TRZ05 is also much higher than that of the P-Ac/P-TRZ (95/5) blend (1.92%). This result supports that the 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 that the TSCT-based TADF polymers show very low efficiency roll-off at high luminance. At the luminance of 100 m-2 and 1000 cd m-2, the EQE of PAc95-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

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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 believe this work will open up a new way for us—from conjugated polymers with through-bond charge transfer effect to nonconjugated polymers with through-space 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 strategy25, or enhancement of the planarity of the donor/acceptor units,20 would be promising to achieve this goal.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website, which includes the synthesis and characterization details, computational results, photophysical and electrochemical properties, thermal properties, and device fabrication procedures.

AUTHOR INFORMATION Corresponding Author *[email protected]

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT We acknowledge financial support from the National Natural Science Foundation of China (Nos. 51573182, 51203149 and 91333205) and the 973 Project (No. 2015CB655000).

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Blue Thermally Activated Delayed Fluorescence Polymers with

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