Donor−σ–Acceptor Molecules for Green Thermally Activated Delayed

Letter pubs.acs.org/OrgLett

Donor−σ−Acceptor Molecules for Green Thermally Activated Delayed Fluorescence by Spatially Approaching Spiro Conformation Ya-Kun Wang, Sheng-Fan Wu, Yi Yuan, Si-Hua Li, Man-Keung Fung, Liang-Sheng Liao,* and Zuo-Quan Jiang* Jiangsu Key Laboratory for Carbon-Based Functional Materials & Devices, Institute of Functional Nano & Soft Materials (FUNSOM) & Collaborative Innovation Center of Suzhou Nano Science and Technology, Soochow University, Suzhou, Jiangsu 215123, P.R. China S Supporting Information *

ABSTRACT: By separating donor/acceptor with a σ linker while keeping them in contact through space interactions, new oxygen-bridged triphenylamine/fluorene-based donor−σ−acceptor (D−σ−A) type thermally activated delayed fluorescence (TADF) emitters are developed. X-ray structural analyses and time-dependent density functional theory reveal that tilted configuration of spiro skeleton, extended delocalization of the highest occupied molecular orbital (HOMO), and lowest triplet state of charge transfer property (3CT) play key roles in the TADF mechanism. OLEDs fabricated with these D−σ−A emitters achieved good external quantum efficiency of 20.4% and long operating durability of 18000 h at 100 cd m−2.

A

In this paper, we aimed to improve the efficiency of D−σ−Atype emitters by rational molecular modification. Two new molecules, OSTFCN and OSTFB, with a spiro core are proposed. For these two compounds, the rigidity and strong donor capacity of oxygen-bridged triphenylamine part greatly facilitate the HOMO-wide distribution and, thus, the enhancement of rate constant of fluorescence.19,20 Furthermore, the energies of the lowest triplet state of charge-transfer property (3CT) of these tilted molecules were greatly reduced, which has a big effect on minimizing ΔEst; this situation is quite different from the dominant lowest localized triplet state (3LE) in previouly reported D−σ−A systems. Noticeably, X-ray data reveal that the imperfect vertical configuration makes the donor part and acceptor part approach to each other, which is believed to be useful for enhancing the device performance.21 As expected, good device performance with current efficiency (CE)/power efficiency (PE)/EQE of 68.8 cd A−1/52.1 lm W 1− /20.4% peaked around 540 nm for OSTFCN is successfully achieved. This is believed to be the first sp3carbon-based D−σ−A-type TADF emitter exceeding 20% EQE. Additionally, a long lifetime of 18000 h at a practical brightness of 100 cd m−2 is successfully achieved as well. The synthetic routes of two target molecules are depicted in Scheme 1. The key intermediate10-(2-bromophenyl)-10Hphenoxazine was synthesized through Ullman reaction with commercially available starting materials.22 Then the spirointermediates were constructed by a nucleophilic addition reaction followed Friedel−Crafts cyclization. After appending different electron-withdrawing groups, the final products can be

s a viable alternative for phosphorescent materials, thermally activated delayed fluorescence (TADF) molecules have been in the spotlight because they can potentially achieve 100% internal quantum efficiency (IQE) without using any noble metals such as iridium and platinum.1−6 For the design strategy of TADF molecules, a D−π−A system (D: donor, A: acceptor) with twisted intramolecular charge transfer (TICT) characteristics is generally favored.7−9 In this respect, small energy splitting (ΔEst) between the low-lying singlet (S1) and triplet state (T1) to ensure effective reverse intersystem crossing (RISC) could be achieved by enlarging the D/A torsion angle.10−12 In the past five years, the TADF emitters following the D−π−A system have been studied extensively, and over 20% external quantum efficiencies (EQEs) have been widely achieved. Considering the requirement of D/A-separated conformation, it is reasonable to combine D/A blocks with an unconjugated linker; e.g., D and A are connected with a σ bond. In regard to bipolar characteristics, D−σ−A-type compounds were once utilized as host materials for phosphorescent OLEDs.13−15 Such D−σ−A-type molecules, however, have just a few examples in TADF emitters. Adachi et al. first reported a D−σ−A-type TADF emitter with a totally orthogonal spiro structure which achieved an EQE of 10.1%.16 By varying the fluorene to anthracenone, they further improved the EQE to 16.5%.17 Swager et al. also reported two triptycenebased TADF emitters for which the D/A are separated by two sp3 carbons while keeping in touch through homoconjugation of HOMO and LUMO. Using this strategy, they achieved an EQE around 10%.18 Obviously, the device performances of D−σ−A emitters lag far behind their D−π−A analogues. Additionally, their device lifespans are not reported. © 2017 American Chemical Society

Received: April 27, 2017 Published: June 7, 2017 3155

DOI: 10.1021/acs.orglett.7b01281 Org. Lett. 2017, 19, 3155−3158

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Organic Letters

between two oxygen-doped triphenylamines and shoulder-toshoulder packing mode between one molecule and another counterpart from an adjacent molecule. For the distance between the planes of two oxygen-doped OSTFCN, a short distance of ca. 3.2 Å is observed. By comparison, OSTFB crystallizes in a triclinic system with a P1(1) space group, of which the packing model is given in Figure 1b. Due to the existence of the bulky group, there are only two molecules in one full cell, and no obvious direct interaction is measured between them. The much-tilted skeleton of OSTF and short plane−plane distance of OSTFCN suggest both intra- and intermolecular interactions are possible. To verify the aforementioned discussion and prove that the tilted spiro skeleton is useful to achieve the large rate constant of fluorescence (kf) and its delayed part (kTADF), computational simulations were employed to investigate the electronic characteristics of both molecules. First, the So geometries of OSTFCN and OSTFB were performed at a B3LYP/6-31G(d) level for the optimization. As observed from the S0-optimized geometries (Figure S1a), HOMO and LUMO distributions in OSTFCN and OSTFB are localized on O-triphenylamine and fluorene, respectively. In addition, closely looking at the HOMO distributions reveals that they even penetrate into the fluorene. According to the discussion above, it is believed that the great product of HOMO can enhance the kf thus PLQY.20 Aiming to better understand the electronic characteristics, HOMO-1, HOMO-2, LUMO+1, and LUMO+2 distribution are also displayed in FigureS3. In accordance with the crystal structure, OSTFCN and OSTFB exhibit a tilted structure with an angle of inclination of 60° and 58°, respectively, and triplet spin density distribution (TSDD) results disclose that both OSTFCN and OSTFB show delocalized T1 states on the whole skeleton (Figure S1a). To understand the excited-state characteristics thoroughly, TDDFT was employed to investigate ΔEtt and ΔEst, and detailed information is collected in Figure S1b. Consequently, small ΔEtt/ΔEst of 0.02/0.09 eV for OSTFCN and 0.10/0.11 eV for OSTFB are calculated which are much smaller than similar references.23,24 To confirm the DFT simulations, photophysical properties are recorded. For compounds with weak to average intramolecular charge transfer, namely the mixed character of 1CT and 1LE, the emission spectrum in a low polar solvent like toluene displays the featured behavior.21 However, in our system, only structureless emission spectra were observed. In addition, significantly red-shifted emissions resulting from 1CT were observed with shifts as large as ∼50 nm for both compounds (Figure S2), which mostly stem from the strong oxygen-bridged triphenylamine part. It could be noted that other closed triphenylamine systems with sulfur or silicon atom as the bridge are also reported, but the merits of oxygen include stronger electron-donating ability because of the matched 2p lone-pair electrons and resistance for further oxidation.14,22,26 As for another important factor, PLQYs of the target molecules were estimated with integration sphere (IS). OSTFCN and OSTFB showed moderate PLQY in toluene of 48% and 39%, respectively. In order to confirm the triplet excited state did contribute to the rise of PLQY, we also measured the PLQYs after bubbling through argon. As expected, the PLQYs were increased to 76% and 69% for OSTFCN and OSTFB, respectively. These results indicated that the up conversion from triplet to singlet played key roles in

Scheme 1. Synthetic Approach and Yield to the Target Materials OSTFCN and OSTFB

obtained in good yields. The detailed synthetic procedures are depicted in the Supporting Information (SI). Single-crystal X-ray crystallographic analysis is applied to confirm the structure of target molecules and investigate their packing modes. As Figure 1a shows, the tetrahedral sp3-

Figure 1. (a) Single-crystal structures and (b) packing models of OSTFCN and OSTFB.25

hybridized carbon between the fluorene- and oxygen-doped triphenylamine adopts a tetrahedral conformation. Unlike the classic orthogonal structure of spiro-type conjugative materials, the 10-phenyl-10H-phenoxazinemoiety tentatively inclines toward the fluorene part with an angle of inclination around 60° for both molecules (deformation angle around 30°). By appending different electron-withdrawing groups, OSTFCN and OSTFB show different crystal systems and packing behaviors. For OSTFCN, it is found to crystallize in a monoclinic system, with a C2/c space group. In the crystal lattice, the OSTFCN molecules form alternately interembedded columns along the b axis, with oxygen-doped triphenylamine moieties acting as a connection center. The OSTFCN adopts two stacking models with face-to-face packing behavior 3156

DOI: 10.1021/acs.orglett.7b01281 Org. Lett. 2017, 19, 3155−3158

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Organic Letters enhancing the PLQY. Furthermore, the film PLQY of OSTFCN and OSTFB were also up to 60% and 52%. Then low-temperature phosphorescent spectra (Phos) were tested to clarify the nature of triplet energy of OSTFCN and OSTFB. As shown in Figure 2b, both target molecules tend to

energy calculated from their HOMO energy and band gap is −2.41 eV/−2.58 eV for OSTFCN and OSTFB (Table S1). Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) of OSTFCN and OSTFB reveal their high thermal stabilities with Td (5 wt % loss) > 370 °C and Tg (glass transition temperatures) > 180 °C (Figure S5). Actually, these excellent thermal stabilities can be ascribed to the conservation of spiro-type core structures, which is a typical merit of the rigid and spiro structure.27 In order to assess the performance of OSTFCN and OSTFB as emitters in OLEDs, devices were fabricated with a configuration of ITO/HAT-CN (10 nm)/TAPC (35 nm)/ TCTA (10 nm)/mCP (5 nm)/mCP: 10 wt % guest (20 nm)/ TmPyPB (45 nm)/Liq(1 nm)/Al (120 nm). As expected, better performance was successfully delivered when OSTFCN and OSTFB were used as TADF emitters. As Figure 3a showed,

Figure 2. (a) UV−vis absorption, photoluminance (PL), and phosphorescent spectra of OSTFCN and OSTFB at 77 K. (b) Time-resolved PL decay curves of OSTFCN and OSTFB doped into mCP.

have structureless characters with Phos behavior of OSTFCN showing a more prominent Gaussian band shape than that of OSTFB. This indicates that the ΔEtt of OSTFCN may be smaller than that of OSTFB and that 3CT behavior is more dominant in OSTFB, which are in accordance with the simulation results. The absorptions of target molecules in film show typical behavior of D−σ−A compounds (Figure 2a and Figure S7). As speculated from the onset of fluorescence recorded at 300 K and Phos recorded at 77 K, S1 and T1 of OSTFCN (2.63/2.54 eV) and OSTFB (2.58/2.47 eV) were estimated, resulting in ΔEst as small as 0.09 eV for OSTFCN and 0.11 eV for OSTFB. To better comprehend the nature of the excited states, transient PL characteristics of OSTFCN/OSTFB and their temperature dependence were recorded. Parts c and d of Figure 2 show the transient PL decay curves of OSTFCN/ OSTFB:mCP codeposited film. As the results show, twocomponent emission decay behavior is observed for OSTFCN and OSTFB, which contain a nanosecond-scale prompt constituent (39 ns/63 ns for OSTFCN/OSTFB) and a microsecond-scale delayed component (158 μs/180 μs for OSTFCN/OSTFB) at 300 K. When the temperature was elevated from 150 to 300 K, the intensity of delayed components for both OSTFCN and OSTFB increased as well, which can be ascribed to the more efficient reverse intersystem crossing from T1 to S1. Compared with OSTFB, OSTFCN show more obvious temperature-dependence behavior, and at higher temperature (300 K), OSTFCN also accounts about 80% of total quantum yield while OSTFB only takes up about 60%. Ultraviolet photoelectron spectroscopy (UPS) was then applied to determine their HOMO energy levels (Figure S4). As both HOMOs mainly localize on the oxygen doped spiroarylamine part, there is little energy difference between the two molecules where it was estimated to be −5.53 eV/−5.64 eV OSTFCN and OSTFB, respectively. In addition, LUMO

Figure 3. Device performance of (a) OSTFCN and (b) OSTFB.

high CE/PE/EQE of 68.8 cd A−1/52.1 lm W1−/20.4% (530 nm) for OSTFCN-based devices were achieved. To further confirm the great potential of this tilted skeleton, another acceptor (bis(2,4,6-trimethylphenyl)borane) was utilized to replace the cyano group. Within expectations, excellent device performance with CE/PE/EQE of 62.2 cd A−1/58.6 lm W1−/ 18.8% (550 nm) was also delivered (Figure 3b). Additionally, efficiency roll-offs of OSTF-based emitters are also minimal and among the best results in D−σ−A-type TADF emitters. Besides the excellent efficiency, by carefully combining the relatively stable transport and host materials with device configuration of HAT-CN/NPB/mCBP/mCBP: 15% OSTFCN/T2T/Alq3/Liq/Al, the operational lifetime LT50 (the time to reach 50% of initial luminance) of the device using OSTFCN as an emitter stands at 180 h at the initial luminance (L0) of 500 cd m−2 and 4500 h at L0 of 100 cd m−2 (Figure S6a). Furthermore, the operating time recorded at an initial brightness of 1000 cd m−2 can be 180 h and around 18000 h at a practical luminance of 100 cd m−2 when a thin Liq layer is inserted between T2T and emitter (Figure S6b). However, because of the mismatched host/transport materials and undesired triplet quenching, the efficiency would be inevitably lowered.28,29 We believe that the high efficiency and good stability for our TADF emitters could be achieved simultaneously with suitable host/transport materials. In summary, we propose a rational design strategy to construct efficient D−σ−A-type TADF emitters. Thanks to the tilted configuration, multiple inter- and intramolecular inter3157

DOI: 10.1021/acs.orglett.7b01281 Org. Lett. 2017, 19, 3155−3158

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Organic Letters actions between D/A, which play key roles in reducing ΔEst and obtaining high PLQY, have been observed. Moreover, both ΔEst and ΔEtt have been obviously minimized compared to all other orthogonal spiro structures, resulting in a lowest chargetransfer triplet state (3CT) of target molecules. Also, the rigid tilted skeleton greatly facilitates the spread of HOMO distribution and thus kf. Consequently, an excellent EQE of 20.4% is first achieved in the D−σ−A system. Additionally, a long lifetime of 18000 h at a luminance of 100 cd m−2 is also reached, indicating D−σ−A-type molecules could be another effective choice for TADF emitters in the future.

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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/acs.orglett.7b01281. X-ray data for compound OSTFCN (CIF) X-ray data for compound OSTFB (CIF) Experimental details, NMR spectra, UV−vis spectra, fluorescence spectra, UPS, device stability, and singlecrystal X-ray diffraction data (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Liang-Sheng Liao: 0000-0002-2352-9666 Zuo-Quan Jiang: 0000-0003-4447-2408 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 (Grant Nos. 61575136 and 21572152). This project is also funded by the Collaborative Innovation Center of Suzhou Nano Science and Technology (Nano-CIC) and by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD).

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REFERENCES

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DOI: 10.1021/acs.orglett.7b01281 Org. Lett. 2017, 19, 3155−3158