Synthesis and Electroluminescence of a Conjugated Polymer with

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Synthesis and Electroluminescence of a Conjugated Polymer with Thermally Activated Delayed Fluorescence⊥ Yunhui Zhu,†,‡ Yuewei Zhang,# Bing Yao,†,‡ Yanjie Wang,†,‡ Zilong Zhang,† Hongmei Zhan,† Baohua Zhang,*,† Zhiyuan Xie,† Yue Wang,*,# and Yanxiang Cheng*,† †

State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, P. R. China ‡ University of Chinese Academy of Sciences, Beijing 100039, P.R. China # State Key Lab of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun 130012, P. R. China S Supporting Information *

hermally activated delayed fluorescence (TADF) has emerged as a promising alternative for highly efficient organic light-emitting diodes (OLEDs), in which TADF emitters exhibit remarkable performance because they can harvest the triplet for delayed fluorescence through enhanced reverse intersystem crossing (RISC) process from the triplet to the singlet, thereby achieving 4-fold OLED performance enhancement relative to OLEDs based on traditional fluorescent emitters.1 In contrast to the state-of-the-art phosphorescence-type OLEDs, TADF-type OLEDs are economically favorable since TADF emitters are inexpensive organic molecules rather than expensive noble metal complexes.2 Furthermore, these organic compounds can be structurally modulated more easily than the noble metal complexes with cyclometalating ligands to achieve emission of various colors for display and lighting applications.3 The fundamental requirement for efficient TADF materials is a spatial separation between the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) to realize a small energy gap (ΔEST) between the lowest singlet (S1) and triplet (T1) states based on the donor− acceptor (D−A) molecular framework.1,3 These demands have been satisfied well for the design and synthesis of small molecular TADF materials. However, there seems a notable difficulty for the desired TADF-type conjugated polymers (CPs) because ΔEST is generally nearly constant at a high value of approximately 0.7 eV.4 An exception is that ΔEST can be tuned from 0.63 to 0.017 eV by incorporating a fused unit of triphenylene into the polymer backbone. However, additional emission from the triphenylene triplet is not observed in OLEDs using this material.5 Recently, an OLED with EQE of up to 10% has been fabricated using a TADF nonconjugated polymer as an emitter in which the “intermonomer TADF” emissive centers were embedded in the polymer backbone with a broken-conjugation high T1 flexible unit.6 The key characteristics for TADF emitters, including delayed emission, oxygenquenching and temperature dependency, were observed from the independent charge transfer (CT) emitting units in the polymer prepared using an ingenious block polymerization approach. Nevertheless, it is still a strong desire to develop CPs with efficient TADF because CPs face to low-cost solutionprocessed OLED products such as large-area, flexible display and lighting panels via roll-to-roll or printing manufacturing

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technology, after the simple structure solution-processed TADF-type OLEDs have been realized using the above nonconjugated polymer,6 doped small molecule systems7 and dendrimeric compounds8 as emitters. Here, we propose a flexible approach to tune the ΔEST of CPs, which has been demonstrated to be effective in polymers reported recently after submitting our manuscript.9 In this approach, only the donors are fixed in the backbone, and the acceptors are grafted in the side-chain (see Scheme 1). In contrast to general D−A type CPs, in which the donors and acceptors are connected to each other in the backbone, Scheme 1. Polymer Structuresa

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LKey: (a) General D-A type conjugated polymers, in which donors and acceptors are connected to each other in the backbone, resulting in large overlap between the HOMO and LUMO. (b) Designed conjugated polymers in which only donors are fixed in the backbone and acceptors are grafted in the side-chain. The HOMO and LUMO are sufficiently spatially separated to obtain a small ΔEST. (c) Polymer structures of PAPCC and PAPTC.

Received: February 28, 2016 Revised: May 24, 2016

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Macromolecules resulting in a large ΔEST and overlap between the HOMO and LUMO,4,10 the designed CPs remain a twisted D−A structure. Thus, the HOMO and LUMO are sufficiently spatially separated to obtain a small ΔEST and achieve the required up-conversion from T1 to S1 for both singlet and triplet harvest. Accordingly, two alternately conjugated polymers, PAPCC and PAPTC (see Scheme 1 for chemical structures), were synthesized by modified Suzuki coupling copolymerization using Pd[P(o-Tol)3]2Cl2 as a catalyst.11 In the polymers, the carbazole and 9,10-dihydroacridine derivative units, which are widely used in TADF molecular structures,1,3 were chosen as donors and were copolymerized via 3,6-positions to obtain a conjugated backbone with a high T1. This high T1 occurs because the energy of the triplet state containing carbazole groups is mainly determined by the poly(p-phenyl) structural species.12 The acceptors are phenyl units containing electronwithdrawing cyanogen or triazine groups, which are connected via the side chain through the nitrogen atom from the 9,10dihydroacridine group with a large dihedral angle to form a pretwisted intramolecular charge-transfer (TICT) structure.3,13 As shown below, photophysical and device characterizations confirmed the TADF emission properties of PAPTC. This is the first report of an all-conjugated polymer with highly efficient TADF. The absorption and photoluminescence (PL) spectra of the two polymers in toluene are shown in Figure 1. A broad and

and PAPTC, respectively, indicating that the T1 states are locally excited (3LE) states,1c,3b which is also confirmed by the TD-DFT calculations of the T1 structure (Figure S4). The results show that the electron and hole distribution of the T1 states are both located on the donor units, i.e. the backbone of the polymers for PAPCC and PAPTC. Accordingly, the T1 energy values can be estimated from the maximum of the phosphorescence spectra, giving the ΔEST values of 0.37 eV for PAPCC and 0.13 eV for PAPTC. Time-dependent density functional theory (TD-DFT) calculations (with a repeat unit of 1 to 4, see Figure 2 and

Figure 2. HOMO (left) and LUMO (right) of three repeat units of polymer PAPTC calculated by the B3LYP/6-31G* method;.

Figure S1−S3 and Table S3 in the Supporting Information) support the above experimental observations. The HOMO of the two polymers is delocalized over the entire backbone, whereas the LUMO is localized on the pendant group of the acceptor. The similarities between the donor units of the two polymers results in similar 3LE values of 2.62−2.65 eV, whereas the differences in the LUMOs correspond to the different 1CT levels of 2.88 eV (PAPCC) and 2.76 eV (PAPTC). The ΔEST values derived from TD-DFT calculations were 0.23 and 0.14 eV for PAPCC and PAPTC, respectively, in good agreement with the experimental values. Here, the TADF in the ploymers is belived to occur through an internal conversion from the backbone centered 3LE state to the 3CT state followed by an ISC from the 3CT state to the 1CT state as Figure S5 shown, which is also observed in the TADF small molecules with lowlying 3LE states.1c,3b The up-conversion from T1 to S1 was confirmed by transient PL decay measurements of the two polymers in solution and in neat film. A long-lifetime delayed component was clearly detected in the transient decay spectra (Figure 3 and Figure S8)

Figure 1. Absorption and emission spectra of the PAPCC and PAPTC polymers in toluene at 300 K and their phosphorescence spectra (delayed by 1 ms) in toluene at 77 K.

strong absorption with a maximum at 340 nm corresponding to a π−π* transition was observed both in PAPCC and PAPTC. An additional weak absorption at long wavelength (ca. 425 nm) attributable to the charge-transfer (CT) transition was observed for PAPTC, implying a stronger D−A character in PAPTC than in PAPCC. This weak CT absorption is resulted from the nearly orthgonal D−A structure, like other TADF matrials with the similar structure.3a,b At room temperature, both PAPCC and PAPTC displayed broad, structureless PL spectra with emission peaks at 472 and 510 nm, respectively, corresponding to 1CT energy values of 2.92 eV for PAPCC and 2.65 eV for PAPTC as estimated from the onset of their emission spectra. This difference likely results from the different electronwithdrawing capabilities of the pendant groups. By contrast, the phosphorescence spectra of the two polymers at 77 K exhibted similar emissive patterns, with a maximum position at 486 or 493 nm with a shoulder at 516 or 522 nm for PAPCC

Figure 3. Transient PL decay characteristics of PATPC in toluene at room temperature. Black: in an air-equilibrated solution. Red: after bubbling with N2. B

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Figure 4. Left: Energy level diagram. Right: EQE-power efficiency-current density characteristics of the polymeric OLEDs based on PAPCC and PAPTC.

low PLQY and large ΔEST. This is the highest efficiency ever reported for polymer-based6,9 or nondoped solution-processed TADF emitters.8 More interestingly, unexpected low turn-on voltages (at a luminance of 1 cd m−2) were observed. In particular, in the PAPTC device (2.6 V), the voltage was nearly identical to the optical band gap of 2.56 eV. This effect can be attributed to at least two factors: 1) the suitable HOMO position (−5.33 eV) reduces the hole injection barrier from PEDOT:PSS (φ = −5.2 eV) (Figure S10 and Table S1 in the Supporting Information); 2) the nondoped device structure avoids the energy loss from the host-dopant doping effect (e.g., charge trapping and scattering).16 Thanks to the low driving voltage characteristics, the PAPTC device realizes high power efficiency (max. 37.1 lm W−1). The J0 values (the current density at the half-maximum of the EQE) of this device was 21.2 mA cm−2 (at a luminance of ca. 4400 cd m−2), distinctly higher than that in a very recent report (ca. 0.5 mA cm−2) based on a nonconjugated TADF polymer,6 despite lower than that of state-of-the-art fluorescent and phosphorescent OLEDs.17 In summary, a conjugated polymer PAPTC with efficient TADF was prepared via an alternate copolymerization approach of carbazole and 9,10-dihydroacridine derivative monomers. Photophysical studies and performances of the EL devices demonstrated that the backbone/side-chain strategy, in which the donor and acceptor units are incorporated into the backbone and side-chain, respectively, is effective for achieving a TADF polymer. In the structure, the HOMO and the LUMO are sufficiently spatially separated to obtain a small ΔEST, while maintaining a high T1 level via linkage of carbzole at the 3,6-positions. Correspondingly, a maximum EQE of 12.6% with an emission peak at 521 nm was realized in the solution-processed polymeric OLED. TADF behavior and PLQY were more efficient for PAPTC, which contains an electron-withdrawing triazine group, than for its PAPCC counterpart featuring cyanogen group, indicating that the LUMO energy level and delocalization of the side-chain species should be designed precisely. Attempts to achieve other emitting-color TADF-type CPs using this strategy are in progress.

of the O2-free solutions of PAPCC and PAPTC at room temperature. This component was nearly absent in the airequilibrated solutions due to the oxygen quenching effect on triplet. The relatively short decay lifetimes of 0.32 and 0.23 μs (see Table S2) for the long-lifetime component indicate that an efficient TADF is facilitated due to the small ΔEST in PAPCC and PAPTC. Meanwhile, the proportion of the delayed component increased from 0.12 (PAPCC) to 0.42 (PAPTC) because of the smaller ΔEST of PAPTC compared to PAPCC, which is also consistent with the measured photoluminescence quantum yield (PLQY) results. The PLQY of PAPCC in toluene solution was only 9% regardless of the removal of O2. By contrast, the PLQY of PAPTC in toluene solution was 22% in air and increased to 40% after bubbling with N2 to remove O2. The data unambiguously indicate that the triplet of the PAPTC sample can be efficiently transferred back to the singlet to produce fluorescence via RISC in O2-free toluene solution. The relatively higher PLQY of PAPTC can be ascribed to the larger delocalization of the LUMO in the acceptor units.1f,3a Moreover, the transient PL behavior of PAPTC also displays the distinct temperature dependence, the same as in the typical TADF materials,1 that is, a much higher proportion of the longlifetime decay component at higher temperature (see Figure S12). To further confirm the TADF character of the synthesized CPs, polymeric OLEDs were fabricated with a device structure of ITO/PEDOT:PSS (50 nm)/PAPTC or PAPCC (40 nm)/ TmPyPB (50 nm)/LiF (1 nm)/Al (100 nm) (see Supporting Information for the fabricated details). A thermal evaporated TmPyPB14 was used as an efficient electron transport and exciton confining layer to fully reveal the intrinsic electroluminescent (EL) performance of these two polymers by eliminating uncontrolled cathode interface charge/exciton quenching in devices.15 Examination of the EL spectra from both devices verified that emission only origineted from the emissive polymers, although a small shift was observed compared to their PL spectra (Figure S13 in the Supporting Information), consistent with the literature.3 As shown in Figure 4 (also Figure S14 and Table S4 in the Supporting Information), the maximum EQE of the PAPTC and PAPCC devices reached 12.6% and 1.3% at luminance of 180 cd m−2 and 6 cd m−2, respectively. The distinctly high EQE level for PAPTC was ascribed to the small ΔE ST (0.13 eV), consequently, efficient TADF emission, and high PLQY in contrast to the distinctly low EQE for PAPCC (1.3%) due to its C

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.6b00430. Details of the polymer synthesis routes and characterization, DFT calculations, electrochemical measurements, device fabrication, and measurements (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*(B.Z.) E-mail: [email protected]. *(Yu.W.) E-mail: [email protected]. *(Y.C.) E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the 973 Project (2015CB655001) and the National Natural Science Foundation of China (Nos. 51473162, 51325303, 21174141, 51303172, and 21404101).

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DEDICATION Dedicated to Professor Dr. Max Herberhold on the occasion of his 80th birthday ⊥

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DOI: 10.1021/acs.macromol.6b00430 Macromolecules XXXX, XXX, XXX−XXX