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Exciplex Organic Light-Emitting Diodes with Nearly 20% External Quantum Efficiency: Effect of Intermolecular Steric Hindrance between the Donor and Acceptor Pair Tien-Lin Wu, Ssu-Yu Liao, Pei-Yun Huang, Zih-Siang Hong, ManPing Huang, Chih-Chun Lin, Mu-Jeng Cheng, and Chien-Hong Cheng ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b04365 • Publication Date (Web): 03 May 2019 Downloaded from http://pubs.acs.org on May 3, 2019

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ACS Applied Materials & Interfaces

Exciplex Organic Light-Emitting Diodes with Nearly 20% External Quantum Efficiency: Effect of Intermolecular Steric Hindrance between the Donor and Acceptor Pair Tien-Lin Wu,†,§ Ssu-Yu Liao,†,§ Pei-Yun Huang,† Zih-Siang Hong,‡ Man-Ping Huang,† Chih-Chun Lin,† Mu-Jeng Cheng,*,‡ Chien-Hong Cheng*,† †Department

of Chemistry, National Tsing Hua University, No. 101, Section 2, Kuang-Fu Road, Hsinchu, Taiwan. of Chemistry, National Cheng Kung University No. 1, Daxue Rd, East District Tainan, Taiwan. Keyword: intermolecular steric effect, exciplex emitter, thermally activated delayed fluorescence, organic light-emitting diode, donor-acceptor distance ‡Department

ABSTRACT: Exciplex emitters have emerged as an important class of thermally activated delayed fluorescence (TADF) materials for highly efficient OLEDs. A TADF exciplex emitter requires an intermolecular donor/acceptor pair. We have synthesized a bipolar donor-type material, DPSTPA, which was used to pair with known acceptor materials (2CzPN, 4CzIPN or CzDBA). The OLEDs based on the exciplex emitters DPSTPA/X, where X = 2CzPN and CzDBA give green and orange-red colors with record-high external quantum efficiencies (EQEs) of 19.0±0.6% and 14.6±0.4%, respectively. In contrast, the exciplex pair DPSTPA/4CzIPN gave very low photoluminescence quantum yield (PLQY) and EQE of the device. The DFT calculations indicate that the intermolecular distance between the donor and the acceptor plays a key factor for the PLQY and EQE. The observed low PLQY and the poor device performance for the DPSTPA/4CzIPN pair are probably due to the relative long distance between the DPSTPA and 4CzIPN in the thin film caused by the four congested carbazole (Cz) groups of 4CzIPN which effectively block the interaction of the nitrile acceptor with the triphenylamino donor of DPSTPA.

Introduction Thermally activated delayed fluorescence (TADF) materials have attracted great attention in organic lightemitting diodes (OLEDs) because they can harvest triplet excitons by up-conversion to the singlets and then fluorescence.1-3 In addition, they are rare-metal-free, pure organics, low cost and eco-friendly.4 The theoretical value of exciton utilization is 100% for TADF emitters,

and the EQE of the EL device could reach 20 ~ 30% without any out-coupling technologies.5 TADF can occur both intramolecularly and intermolecularly. For an intramolecular TADF emitter, the molecule should have both electron donating and accepting groups with suitable conformation that can generate a small energy gap (ΔEST) between the singlet and triplet state. The triplet state can undergo efficient up-conversion to the singlet state for fluorescence via reverse intersystem crossing (RISC) process due to the smaller ΔEST.6-7 For intermolecular TADF process, the RISC occurs between the two exciplex molecules8 via intermolecular chargetransfer transition.9-10 The transition occurs between the HOMO of donor molecules and the LUMO of acceptor molecules via Coulomb attraction.11 Exciplex-based applications have been extensively studied. The exciplex cohost devices with fluorescence12-15 or 16-19 phosphorescence dopant materials were well developed, and the device lifetimes of exciplex OLEDs was investigated and discussed for potential applications.20 In addition, WOLEDs constructed by all exciplex-based tandem device was also achieved.21-23 A pure-organic exciplex system has recently exhibited an amazing long-persistent luminescence (LPL) for the first time.24

Figure 1. (a) Synthetic route for DPSTPA. (b) Single crystal structure of DPSTPA. ACS Paragon Plus Environment

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Although an exciplex emitter has the potential to give a high device EQEs > 20%, a serious issue of exciplex emitters is the low photoluminescence quantum yield (PLQY) compared with single-molecular TADF materials. Generally, the low PLQY leads to device external quantum efficiency (EQE) around 5~13%.8-10,13,25-26 Thus, the choice of exciplex materials to have an excellent exciplex pair becomes a top priority for exciplex OLED. A great advance had been the use of materials with high triplet state27 and with remote steric effect.26 To obtain more efficient exciplex OLEDs, bipolar donor material CN-Cz2 was employed for greenish-blue device with EQE of 16%.28 Then, a green device had achieved an EQE of 17.8% by introducing TADF donors (MAC) and acceptors (PO-T2T) having an additional RISC process.29 In this study, we designed a new bipolar donor-type material to match the TADF emitters, 2CzPN, 4CzIPN1 and CzDBA30 which act as the acceptor materials.

Results and Discussion Synthesis and Characterization. Figure 1 depicts the synthesis of bipolar material 2,2'-dimethyl-N,Ndiphenyl-4'-(phenylsulfonyl)-[1,1'-biphenyl]-4-amine (DPSTPA) via the Suzuki-coupling reaction of 4,4,5,5tetramethyl-2-(2-methyl-4-(phenylsulfonyl)phenyl)1,3,2-dioxaborolane with 4-bromo-3-methyl-N,Ndiphenylaniline according to a reported method.31 The product was purified by column chromatography and temperature-gradient vacuum sublimation to afford white powder in 85% yield. The material was characterized by 1H and 13C NMR spectra, highresolution mass and elemental analysis, In addition, the

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structure was confirmed by single-crystal X-ray diffraction analysis (Figure 1b). Thermogravimetric analysis (TGA) showed the decomposed temperature (Td, 5% weight loss) of 300 ºC. No glass transition temperature and crystallization temperature were found by the differential scanning calorimetry (Figure S2). The ultraviolet-visible (UV-vis) absorption and photoluminescence (PL) spectra are displayed in Figure S3. The maximum absorption band shows at 304 nm in toluene solution (10-5 M), while the emission band appears at 407 nm in toluene and at 426 nm in neat film. An energy gap (Eg) of 3.00 eV for DPSTPA was obtained from the edge of the absorption spectrum. In addition, the HOMO energy level of DPSTPA in neat film measured using photoelectron spectroscopy (Figure S4) is -5.48 eV and the LUMO level calculated from HOMO and Eg is 2.48 eV. We further determined the energy levels of S1 (3.34 eV) and T1 (3.07 eV) from the onset of fluorescence and phosphoresce spectra. The relative high triplet state of DPSTPA was caused by the methyl linker group,32-33 which lead to twisted conformation (dihedral angle 67º), the charge separation, and reduction of the overlap of HOMO and LUMO (Figure S5).34-35 Given the high triplet energy of DPSTPA, we try to use it as the host for the TADF emitter 2CzPN, which is known to emit sky blue light. 1 To our surprise, the light emitted from the device based on 2CzPN/DPSTPA as the emitting layer redshifted drastically (~540 nm). To find the reason behind this observation, we first examine the energy level diagrams and molecular structures of DPSTPA and 2CzPN. It is interesting to note that the HOMO and LUMO energy levels of DPSTPA are higher than the corresponding values of 2CzPN (Figure 2a). This can be

Figure 2. (a) Energy levels and structures of DPSTPA, 2CzPN, 4CzIPN and CzDBA. OLED performance of devices G, O1 and O2. (b) Current density and luminance vs. voltage characteristics. (c) External quantum efficiency vs luminance. (d) Electroluminescent spectra; inset: photos of device G (left) and O2 (right) under operation.

2 ACS Paragon Plus Environment

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ACS Applied Materials & Interfaces Table 1. Comparison of OLED performance for each device comprising different exciplex emitters. Device

Exciplex emitter

Vd [V]b)

ηEQE [%]d)

ηCE [cd A-1]e)

ηPE [lm W-1]f)

λEL [nm]g)

CIE [x, y]i)

Reference

Ga) O1a) O2a) R1 R2 R3 R4 R5

DPSTPA/2CzPN DPSTPA/4CzIPN DPSTPA/CzDBA Cz-CN2/PO-T2T TAPC/DPTPCz MAC/PO-T2T TCTA/3P-T2T m-MTDATA/pCNBCzoCF3

3.0 3.1 2.9 2.3 2.7 2.4 2.0c) 2.8

19.0±0.6 3.8±0.1 14.6±0.4 16.0 15.4 17.8 7.8 9.4

59.9±2.2 8.3±0.2 29.6±1.1 37.8 45.7 52.1 23.6 18.2

62.7±2.2 7.5±0.2 31.0±1.2 47.5 47.9 45.5 26.0 12.1

544 590 592 490h) 510h) 516 550h) 572

(0.38, 0.55) (0.50, 0.48) (0.53, 0.46) (0.20, 0.40) (0.27, 0.52) (0.31, 0.55) (0.40, 0.55) (0.44, 0.44)

This work This work This work Ref 28 Ref 12 Ref 29 Ref 11a Ref 19

Device structure: ITO/TAPC (40 nm)/TCTA (20 nm)/exciplex emitter (30 nm)/3TPYMB (80 nm)/LiF (1 nm)/Al (100 nm). b) The turn-on voltage at a brightness of 1 cd m-2. c) Vd at which brightness became detectable. d) EQE, maximum external quantum efficiency. e) CE, maximum current efficiency. f) PE, maximum power efficiency. g) The EL wavelength at maximum intensity. h) Estimated value from the figures in the references. i) CIE 1931 coordinates. a)

rationalized by the stronger electron donating ability of triphenylamine in DPSTPA compared to the carbazole groups in 2CzPN and the lower electron accepting ability of diphenylsulfone in DPSTPA relative to 1,2dicyanobenzene in 2CzPN. Thus, the observed redshifted emission of the DPSTPA/2CzPN-based device is likely due to the formation of DPSTPA/2CzPN exciplex. The energy of exciplex was correlated to the energy gap between the LUMO of 2CzPN and HOMO of DPSTPA (Figure 2a). Performance of Exciplex OLEDs. To optimize the efficiency of the DPSTPA/2CzPN-based device, we fabricated a series of OLED devices with the following architecture: ITO/TAPC (40 nm)/TCTA (20 nm)/DPSTPA (75%): 2CzPN (25%) (30 nm)/3TPYMB (80 nm)/LiF (1 nm)/Al (100 nm), where N,N'-bis(1naphthyl)-N,N'-diphenyl-1,1'-biphenyl-4,4'-diamine (TAPC) is the hole transporting material, tris(4-(9Hcarbazol-9-yl)phenyl)amine (TCTA) is the electron blocker and tris(2,4,6-trimethyl-3-(pyridin-3yl)phenyl)borane (3TPYMB) acts as the electron transporter. The optimized weight ratio of DPSTPA to 2CzPN is 3:1. Device G fabricated with this weight ratio exhibits a maximum EQE (ηEQE) of 19.0±0.6%, current efficiency (ηCE) of 59.9±2.2 cd A-1 and power efficiency (ηPE) of 62.7±2.2 lm W-1 with a turn-on voltage of 3.0 V. The high device efficiency led us to explore the possibility of using DPSTPA to pair with other TADF materials including 4CzIPN and CzDBA for exciplex OLEDs. The exciplex ratio of 3:1 is used for all devices. The molecular structures in the device and the details of device fabrication are provided in Supporting Information. The electroluminescent properties including current density and luminance vs. voltage, EQE vs. voltage and the emission spectra, are shown in Figure 2 and summarized in Table 1. Current efficiency (CE) and power efficiency (PE) vs. luminance characteristics are shown in Figure S7. Surprisingly, device O1 shows a ηEQE of only 3.8±0.1% much less than the value of 19.0±0.6% for device G. The current and power efficiencies of 8.3±0.2 cd A-1 and 7.5±0.2 lm W-1 are also much lower than the corresponding values for device G. We then fabricated

device O2 using DPSTPA/CzDBA (30 nm) as the emitting layer. The device exhibits a superior ηEQE of 14.6±0.4%, ηCE of 29.6±1.1 cd A-1 and ηPE of 31.0±1.2 lm W-1 with a lower turn-on voltage at 2.9 V. The DPSTPA/2CzPNbased device G displays green emission at 544 nm with CIE (0.38, 0.55) totally different from the sky-blue color observed for the reported 2CzPN-based devices.1,31 Similarly, devices O1 and O2 gave orange emissions at

Figure 3. The absorption (black line), acceptor fluorescence (red line) in toluene, exciplex fluorescence spectra (blue line) at 300 K and phosphorescence spectra (violet line) at 77 K of (a) DPSTPA/2CzPN, (b) DPSTPA/4CzIPN and (c) DPSTPA/CzDBA thin films. The transient PL decay curves of (d) DPSTPA/2CzPN, (e) DPSTPA/4CzIPN and (f) DPSTPA/CzDBA thin films at 300 K; inset: prompt and delayed emission spectra. Red curves are instrument response factor (IRF).



Table 2. Photophysical properties of exciplex emitters Abs Fl Ph Exciplex emitter [nm]a) [nm]a) [nm]a) DPSTPA/2CzPN 296 540 534 DPSTPA/4CzIPN 290 596 608 DPSTPA/CzDBA 298 592 594

PLQY [%]b) 79 10 59

FWHM [nm]c) 113 137 112

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τp [ns]d) 152 161 195

τd [μs]d) 13.9 6.8 10.1

ΔEST [eV]e) 0.095 0.066 0.078

Absorption (Abs), fluorescence (Fl) maximum at 300 K and phosphorescence (Ph) maximum at 77 K in exciplex film. b) Absolute total PL quantum yield in a 50-nm-thick exciplex film (75% DPSTPA: 25% TADF emitter) using an integrating sphere. c) Full width at half maximum of PL. d) Lifetime of the prompt component and delayed component determined form the transient PL curves. e) The singlet energy (S1) and triplet energy (T1) level was calculated from the onset of the fluorescence and phosphorescence spectra of exciplex films, and the energy gap (ΔEST) between S1 and T1. a)

590 and 592 nm different from the original green emission of 4CzIPN and CzDBA. These exciplex-based devices show low maximum luminance of around 3500 cd m−2 with relative high efficiency roll-off (Figure 2c). These results were also observed in the reported exciplex-based devices.10 Furthermore, the commercially available hole-transporting material TPD was employed to replace DPSTPA. The HOMO/LUMO energy levels of TPD are -5.3/-2.3 eV, which appear slightly more electron donating compared to the corresponding values of DPSTPA. Devices T1 and T2 using TPD/2CzPN and TPD/CzDBA as the emitting layers, gave low device performance with ηEQE < 0.3% as shown in Figure S8 and were summarized in Table S2. In contrast, the bipolar molecule, DPSTPA, with twisted structure exhibited a relative high triplet level to prevent the exciplex energy27 from back transfer to the triplet states of the donor and acceptor. Thus, highly efficient exciplex emitters could be formed by combining DPSTPA with a TADF material 2CzPN or CzDBA via a dipoledipole interaction and the additional RISC process.29 It is worth noting that device G exhibited a record-high external quantum efficiency, compared with the reported exciplex-based OLEDs.9,29 Device O2 also showed superior performance among the orange-red TADF4 and exciplex9,36 OLEDs. Further, the wide bandwidth (110 nm) of the EL peak is an excellent candidate for the tandem WOLED application.

Photophysical Properties of Exciplex. To look into the insight of the exciplex formation, we measured the absorption and emission spectra, time-resolved PL and PLQY of all exciplex thin films (75% DPSTPA: 25% TADF materials) and the results are displayed in Figure 3 and summarized in Table 2. Figures 3a-c show the absorption and emission spectra of the thin film and also the emission of the corresponding TADF material. The UV-vis absorption spectra of the three exciplex films are broad with the maximum at around 290~298 nm, and the emission spectra also show broad peaks at 540, 596 and 592 nm for DPSTPA/2CzPN, DPSTPA/4CzIPN and DPSTPA/CzDBA, respectively. The emission peaks correspond to 2.30, 2.08 and 2.10 eV and are close to the energy difference between the HOMO energy level of the donor material (DPSTPA) and the LUMO energy levels of the acceptor materials. Furthermore, compared to the emission of the corresponding TADF emitters, 2CzPN, 4CzIPN and CzDBA, all exciplex emissions are redshifted. The full width at half maximum (FWHM) of the exciplex emission bands are 113, 137 and 112 nm, (or 0.47, 0.46 and 0.39 eV) for DPSTPA/2CzPN, DPSTPA/4CzIPN, and DPSTPA/CzDBA, respectively. These broad bands appear to be characteristic of exciplex emission. The phosphorescence of each exciplex film was obtained at 77 K, and the ΔEST which were determined by the onset of exciplex fluorescence and phosphorescence were 0.095, 0.066 and 0.078 eV for DPSTPA/2CzPN, DPSTPA/4CzIPN and DPSTPA/CzDBA, respectively. The

Figure 4. The optimized structures of the three donor-acceptor complexes. The electron-center and hole-center are marked by red and blue dots, respectively. The distance between the two centers is marked by a dashed line.


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ACS Applied Materials & Interfaces transient PL decay curves of the three thin films and the emission spectra of the prompt and delayed components were shown in Figure 3d-f. The lifetimes for the prompt (τp) and delayed (τd) parts of the exciplex emitters were obtained by fitting the observed PL transient data. The calculated τp is around 152~195 ns and τd is 13.9, 6.8 and 10.1 μs, for DPSTPA/2CzPN, DPSTPA/4CzIPN and DPSTPA/CzDBA, respectively. The τd data correlate well with the trend of ΔEST. The emissions from prompt and delayed components are displayed in the inset of Figures 3d-f revealing that the delayed emissions (at 10 μs) are slightly red-shifted compared to the prompt ones.8,13,37 The temperature-dependent-transient PL curves of these exciplex emitters were measured at temperatures from 77 to 300 K (Figure S9). By using the prompt and delayed fluorescent QYs and the corresponding lifetimes measured from the transient PL curves as summarized in Table S3, we calculated the rate constants, kp, kd, kIC, kISC, and kRISC of these three exciplexes. The DPSTPA/4CzIPN exciplex with the lowest PLQY exhibits the slowest kd of 1.47 × 104 and highest non-radiative kIC of 2.12 × 106 s-1. The kRISC values were determined as 4.49 × 104, 0.15 ×104, and 3.45 × 104 s−1 for DPSTPA/2CzPN, DPSTPA/4CzIPN, and DPSTPA/CzDBA, respectively. The kRISC values are consistent with device performance of these TADF exciplexes. The photophysical data of the exciplex films are summarized in Supporting Information. Quantum Mechanical Calculations. Density functional theory, B3LYP/6-31G*, were then performed to examine the structures of the three donor/acceptor complexes. Since before illumination both donor and acceptor need to be charged, we first calculated the -1 charged states of 2CzPN, CzDBA and 4CzIPN (acceptors) and +1 charged state of DPSTPA (donor) to obtain the electrostatic potentials and spin distributions (Figure S10, S11). This information was then used to construct the initial structures of the three donor-acceptor complexes for geometry optimization by allowing the negatively charged part of 2CzPN, CzDBA and 4CzIPN to be close to the positively charged part of DPSTPA, and also to define the distance between the hole and electron centers in the complexes (Re-h). After analyzing those structures (Figure 4), we found that Re-hs are 5.61 and 5.93 Å for DPSTPA/2CzPN and DPSTPA/CzDBA, respectively, but much longer (6.85 Å) for DPSTPA/4CzIPN. The corresponding exciplex states, where the acceptor molecule has a total charge of -1 and the donor of +1, were simulated approximately using the newly developed energy decomposition analysis (EDA) scheme by allowing electron density polarization inside each of the two molecules but not the charge transfer between them during geometry optimizations.38-40 Similarly, we found that the Re-hs were 5.39 and 5.29 Å for DPSTPA/2CzPN and DPSTPA/CzDBA, respectively, but still longer (6.73 Å) for DPSTPA/4CzIPN.

Thus, from the structures of the ground states and exciplex states, we found that the Re-h of DPSTPA/4CzIPN is indeed much longer. This is expected to render the electron transfer process of the exciplex less probable and the emission less efficient. The larger Re-h observed in DPSTPA/4CzIPN is likely due to the steric effect caused by the carbazole between the two CN groups. To verify the assumption, we substituted the carbazole group with a hydrogen. The Re-hs were reduced to 5.39 Å for the ground state and 5.24 Å for the exciplex state, which are close to the numbers for DPSTPA/2CzPN and DPSTPA/CzDBA. Analysis of Exciplex Formation. The formation of exciplex emitters was explained in the schemes 29 shown in Figure S12. All the three acceptors, 2CzPN, 4CzIPN and CzDBA are known TADF molecules and the exciplex pairs of these three molecules with DPSTPA also show TADF property. Efficient exciplex emitters were formed for the pairs DPSTPA/2CzPN and DPSTPA/CzDBA. One of the reasons is that the T1 states of the donor and acceptor molecules are higher than that of the exciplex state. The high T1 of the molecules prevents the quenching of exciplex energy by the triplets of the donor and the acceptor.27 Additionally, the facile RISC of the exciplex gives high PLQYs of 79% and 59%, respectively.29 A key reason for the low exciplex efficiency of DPSTPA/4CzIPN relative to the other pairs is probably the very bulky structure of 4CzIPN which prevents the donor DPSTPA to get close to acceptor in 4CzIPN. Thus, a much longer separation between the donor and acceptor in the DPSTPA/4CzIPN thin film would be expected leading to a weaker interaction and low PLQY of the DPSTPA/4CzIPN film (only 10%). Both the PLQY and device performance become the lowest among the three exciplex pairs. Figure S13 exhibited a weaker emission likely from DPSTPA in the PL spectrum of the DPSTPA/4CzIPN film. This also reveals the inefficient formation of the exciplex. As a result, a bulky acceptor is not recommended for efficient exciplex formation, because they result in inefficient exciplex formation and poor performance of 3.8±0.1% EQE in device O1.

Conclusion In summary, we have designed and synthesized a versatile bipolar material DPSTPA for use as a donortype material in the generation of efficient exciplex emitters. Three well-known TADF emitters were selected as the acceptors to pair with DPSTPA. Two of these pairs are proved to be highly efficient. The DPSTPA/2CzPN-paired green device G has approached 20% EQE and is the record-high efficiency among exciplex-type OLEDs. The orange-red device O2 based on DPSTPA/CzDBA as the exciplex also had achieved a superior EQE of 14.6±0.4% among the orange-red intermolecular or intramolecular TADF OLEDs. Importantly, the DFT modeling of the exciplex pair



provides a guideline for the material selection. We believed this strategy is valuable for making efficient exciplex-based OLEDs in the future.

Experimental Section General Information. All the experimental operations were performed under nitrogen; the equipment was dried in an oven at 110 °C for several hours. Normal chemicals and reagents were purchased from commercial providers without further purification. Tetrahydrofuran, toluene and o-xylene were distilled over sodium particles. NMR spectra using CHCl3/CDCl3 (δ 7.24/77.0 ppm) were recorded with a Varian Mercury 400 spectrometer, and high-resolution mass (HRMS) was performed on JMS-T200GC AccuTOF GCx in EI source mode. The single crystal suitable for X-ray diffraction was grown by sublimation. The X-ray diffraction was carried out on a Bruker X8 APEX X-ray diffractometer with Mo Kα radiation (l = 0.71073 Å) and the structure was solved by SHELX 97 program. Elemental analyses were performed using an Elementar vario EL cube microanalyzer. UV-vis spectra were taken using a Hitachi U-3300 model, while the room and low temperature PL spectra were recorded using a Hitachi F7000 fluorescence spectrophotometer. The timeresolved emission spectra were observed on an Edinburgh Instruments spectrometer (FLS980). The temperature dependent experiments were performed with an Oxford Optistat DN2 cryostat. The photoluminescence quantum efficiency of thin film (50 nm) was determined with a calibrated integrating sphere under a nitrogen atmosphere. The HOMO levels of DPSTPA in neat film were determined by a Riken Keiki AC-2 photoelectron spectrometer in ambient conditions with a UV source. The glass transition temperatures were determined by DSC at a heating rate of 10 °C min-1 from 25 to 300 °C under nitrogen using a TA Instrument Q10 instrument. The decomposition temperature corresponding to 5% weight loss was conducted on a PerkinElmer Pyris 1 TGA thermal analyzer. Synthesis of 4-bromo-3-methyl-N,Ndiphenylaniline. Diphenylamine (1.7 g, 10 mmol), copper(I) iodide (0.02 g, 0.1 mmol), and sodium tertbutoxide (1.44 g, 15 mmol) were placed in a two-necks round bottle. 2-bromo-5-iodotoluene (1.4 ml, 10 mmol), trans-1,2-diaminocyclohexane (0.11 g, 1 mmol) and 1,4dioxane (15 ml) were added. The resulting solution was stirred for 48 h at 110 °C. After the reaction was completed and cooled to room temperature, the mixture was filtered through a flash column and washed with dichloromethane. The resulting filtrate was concentrated under reduced pressure to give the crude product. The mixture was purified by column chromatography (CH2Cl2/n-hexane, 1:5) to afford white powder (2.5 g, 74% yield). 1H NMR (400 MHz, CDCl3): δ 7.33 (d, J = 8.4 Hz, 1 H), 7.25-7.21 (m, 4 H), 7.05-6.98 (m, 6 H), 6.93 (d, J = 2.4 Hz, 1 H), 6.75 (dd, J = 8.4, 2.4 Hz, 1

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H), 2.26 (s, 3 H). 13C NMR (100 MHz, CDCl3) : δ 147.5, 147.1, 138.6, 132.8, 129.3, 126.0, 124.2, 123.0, 123.0, 117.7, 23.0. Synthesis of DPSTPA. To a stirred solution of 4bromo-3-methyl-N,N-diphenylaniline (0.92 g, 2.73 mmol), 4,4,5,5-tetramethyl-2-(2-methyl-4(phenylsulfonyl)phenyl)-1,3,2-dioxaborolane (1.08 g, 3 mmol), K2CO3 (1.45 g, 10.5 mmol), and Pd(PPh3)4 (0.32 g, 0.27 mmol), and nitrogen-bubbled toluene (90 mL), ethanol (23 mL) and water (23 mL) were added under a nitrogen atmosphere. The mixture was stirred at 80 ℃ for 12 h. After the reaction was completed and cooled to room temperature, the mixture was filtered through a Celite pad, and the filtrate was extracted with dichloromethane. The combined organic layer was dried by magnesium sulfate and concentrated under reduced pressure to give the crude product. The crude material was purified by column chromatography (CH2Cl2/nhexane, 1:2) to give the expected white powder. Further purification by temperature-gradient vacuum sublimation afforded pure DPSTPA (1.14 g, 85% yield) for chemical analysis, photophysical measurements, and fabrication of the OLED devices. Melting point: 237 ºC. 1H NMR (400 MHz, CDCl3): δ 7.99-7.97 (m, 2 H), 7.81 (s, 1 H), 7.75 (dd, J = 7.9, 1.9 Hz, 1 H), 7.57-7.50 (m, 3 H), 7.277.23 (m, 5 H), 7.10 (d, J = 8.2 Hz, 4 H), 7.01 (t, J = 7.3 Hz, 2 H), 6.95 (d, J = 2.0 Hz, 1 H), 6.90-6.87 (m, 1 H), 6.83 (d, J = 8.2 Hz, 1 H), 2.14 (s, 3 H), 1.87 (s, 3 H). 13C NMR (100 MHz, CDCl3) : δ 147.7, 147.4, 146.9, 141.8, 139.9, 138.1, 136.3, 133.7, 133.1, 130.7, 129.4, 129.3, 129.3, 128.8, 127.7, 124.8, 124.6, 124.5, 122.9, 120.8, 20.0, 19.9. HRMS(EI+): [M+] calcd. for C32H27NO2S, 489.1762; found, 489.1757. Anal. Calcd. for C32H27NO2S: C, 78.50; H, 5.56; N, 2.86; O, 6.54; S, 6.55. Found: C, 78.40; H, 5.65; N, 2.91; O, 6.66; S, 6.35. Organic electroluminescent device fabrication. The EL devices with the configuration of ITO/TAPC (40 nm)/TCTA (20 nm)/75% DPSTPA：25% TADF emitter (30 nm)/3TPYMB (80 nm)/LiF (1 nm)/Al (100 nm) were constructed by thermal evaporation onto a clean glass pre-coated with a layer of indium tin oxide (ITO) with a sheet resistance of 30 Ω/square at a pressure under 10-6 Torr. The evaporation rate for organic materials and Al layer were around 0.1~0.2 nm s-1 and 0.3~0.4 nm/s, respectively. And the rate for LiF and dopant were 0.005~0.01 nm s-1. The effective area of the emitting diode was 9.00 mm2. Characterization of current, voltage and luminance and electroluminescent spectra were recorded by using a Keithley 2400 source meter and a Konica Minolta CS2000A spectroradiometer. External quantum efficiencies and power efficiencies were determined by employing the assumption of Lambertian devices. All devices were encapsulated in a glove box and the measurements were performed at room temperature. Theoretical method. Quantum mechanical calculations were performed using the B3LYP/6-31G*


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ACS Applied Materials & Interfaces level of theory. We calculated the -1 charged states of the three acceptors (2CzPN, CzDBA, and 4CzIPN) and +1 charged state of the donor (DPSTPA) to obtain the charge (based on their electrostatic potentials) and spin distributions for those molecules. The obtained charge distributions (Figure S10) were then used to construct the initial structures of the three donor-acceptor complexes for the subsequent geometry optimizations by letting the negatively charged part of 2CzPN, CzDBA, and 4CzIPN to be close to the positively charged part of DPSTPA. The obtained spin distributions (Figure S11) were used to define the hole and electron centers and also the distance between the two centers (Re-h).

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI:. Synthetic route for DPSTPA (Scheme S1-S2); ORTEP drawing (Figure S1); Thermal properties (Figure S2) PL spectra (Figure S3); Photoelectron spectra (Figure S4); DFT of HOMO/LUMO distribution (Figure S5); molecular structures (Figure S6); Device performance (Figure S7-S8); temperature-dependent transient PL curves (Figure S9); Electrostatic potentials (Figure S10); Spin density distribution (Figure S11); Energy transfer scheme (Figure S12); PL spectra of Exciplex (Figure S13); 1H NMR and 13C NMR spectra (Figure S14-S17); crystal data and structure refinement (Tables S1); OLED performance (Table S2); summarized photophysical data and rate constants (Table S3) X-ray X-ray crystallographic files for DPSTPA (CIF). AUTHOR INFORMTION

Corresponding Author *[email protected] *[email protected]

Author Contributions §(T.-L.W.

and S-Y.L.) These authors contributed equally.

Notes The authors declare no competing financial interest. ACKNOWLEDGMENT We thank the Ministry of Science and Technology of Taiwan (MOST 107-2113-M-007-004, MOST 107-3017-F-007-002) and the Ministry of Education, Taiwan, for support of this research, and the National Center for High-Performance Computing of Taiwan for providing computing time.

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