Strategies for the Molecular Design of Donor ... - ACS Publications

Article Cite This: Chem. Mater. 2018, 30, 857−863

pubs.acs.org/cm

Strategies for the Molecular Design of Donor−Acceptor-type Fluorescent Emitters for Efficient Deep Blue Organic Light Emitting Diodes Seung-Je Woo,† Youheon Kim,‡ Myeong-Jong Kim,§ Jang Yeol Baek,‡ Soon-Ki Kwon,*,§ Yun-Hi Kim,*,‡ and Jang-Joo Kim*,† †

Department of Materials Science and Engineering, RIAM, Seoul National University, Seoul 151-742, South Korea Department of Materials Engineering and Convergence Technology and ERI, Gyeongsang National University, Jinju 660-701, South Korea § Department of Chemistry and RIGET, Gyeongsang National University, Jinju 660-701, South Korea ‡

S Supporting Information *

ABSTRACT: We describe several strategies for the molecular design of high-efficiency blue fluorescent emitters. Asymmetric donor−acceptor (D−A) and symmetric A−D−A-type fluorescent emitters were designed with spiroacridine donors and diphenyltriazine acceptors. Substituting a toluene or xylene moiety for the phenyl group connecting the donor and acceptor and replacing the diphenylsilane group with a fluorene moiety resulted in a deeper blue emission without any losses in luminescence efficiency. On the basis of these substitutions, deep blue organic light emitting diodes (OLEDs) with Commission Internationale de L’Eclairage (CIE) coordinates of (0.149, 0.082) and an external quantum efficiency (EQE) of 7.7% were fabricated using a D−A-type emitter. Symmetrizing the D−A structure to an A−D−A structure increased the proportion of horizontally oriented emission dipoles in the organic film from 70 to 90%. OLEDs incorporating symmetric A−D−A-type emitters had EQEs as high as 8.5% due to increased outcoupling efficiencies and also showed deep blue emission with CIE coordinates of (0.142, 0.116). The molecular design strategies described herein can be applied to donor−acceptor-type fluorescent emitters for the fabrication of efficient deep blue OLEDs.

■

However, their extended π-conjugation makes them difficult to realize deep blue emission. Recently, D−A structures have been widely adopted in thermally activated delayed fluorescence (TADF) emitters to convert excitons from the triplet state to the singlet state via reverse intersystem crossing. However, attaining deep blue emission is challenging because of the bathochromic shift associated with intramolecular charge transfer. Blue TADF OLEDs with external quantum efficiencies (EQEs) close to 20%23 and higher than 20% have been realized,24,25 but deep blue TADF emitters that match the blue standard Commission Internationale de L’Eclairage (CIE) yvalue of the National Television System Committee (NTSC) still show EQEs lower than 10%, low maximum brightness, and large efficiency roll-offs.26,27 Future applications will require more efficient fluorescent deep blue emitters. We herein report strategies for the design of donor−acceptor-type fluorescent emitters to achieve pure blue emission and high efficiency at the same time.

INTRODUCTION Since the invention of organic light emitting diodes (OLEDs) by Ching Tang in 1987,1 their performance in display applications has improved significantly. Many electronics companies are now replacing liquid crystal displays (LCD) with OLED displays because of the superior display quality of the latter. Green and red phosphorescent OLEDs are particularly popular for use in displays because of their high efficiency and long operational lifetimes. Blue OLEDs typically incorporate conventional fluorescent emitters or triplet−triplet fusion (TTF) emitters, which combine good color quality with greater operational stability than that of blue phosphorescent OLEDs.2−7 However, the efficiency of blue conventional fluorescent or TTF emitters is much lower than that of phosphorescent emitters due to the theoretical limits of maximum internal quantum efficiency: 25% for conventional emitters and 62.5% for TTF emitters.8−22 A donor−acceptor (D−A) structure is often adopted for deep blue fluorescent OLEDs, especially for nondoped OLEDs, to improve charge balance and reduce driving voltage.8−14 The bipolar nature of D−A emitters reduces efficiency losses stemming from charge imbalances in emitting layers (EMLs). © 2018 American Chemical Society

Received: October 23, 2017 Revised: January 11, 2018 Published: January 11, 2018 857

DOI: 10.1021/acs.chemmater.7b04437 Chem. Mater. 2018, 30, 857−863

Article

Chemistry of Materials We designed five fluorescent emitters employing acridine or azasiline as a donor and diphenyltriazine as an acceptor (Figure 1a). DTPSAF, DTTSAF, and DTXSAF are asymmetric D−A-

Figure 1. (a) Molecular structures of DTPSAF, DTTSAF, DTXSAF, BDTPSAF, and BDTPDDA. (b) Absorption spectra of toluene solution and photoluminescence spectra of 10 wt % emitter:mCP host 30 nm films.

achieved by increasing the aspect ratio of the emitting molecule along the direction of TDM, which results in higher outcoupling efficiency.28−30 Diphenyltriazine, which is an acceptor widely used for TADF emitters,31,32 was selected as the acceptor because of its planar molecular structure. By attaching diphenyltriazine to the donor moiety, the molecule’s aspect ratio can be easily tuned along the long molecular axis. Spiroacridine, which is a popular donor for green and blue TADF emitters,33,34 was selected as a donor because of its rigid structure and potential for deep blue emission. The spirocarbon or silicon connects the fluorene or diphenylsilane moiety to acridine, respectively, which makes the donor moiety rigid without increasing the length of πconjugation. The rigid structure reduces the degree of nonradiative decay through vibrational modes, leading to higher photoluminescence quantum yields (PLQY).18,35 The π-conjugation length does not extend between the acridine and spirofluorene or diphenylsilane moieties because it is blocked by an sp3-hybridized carbon or silicon atom. Due to the larger size of the silicon atom, the π-conjugation blocking effect is greater with silicon and a larger bandgap can be expected for BDTPDDA relative to that of BDTPSAF.36−38 The spirofluorene or diphenysilane moiety is orthogonal to the acridine donor plane, which facilitates deeper blue by inhibiting intermolecular interactions. Donor−acceptor connecting units are used to prevent the complete separation of highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO), providing sufficient oscillator strength for efficient radiative decay. The donor and acceptors are connected with paralinkage of the linker moieties to separately control the dihedral angle between donor and linker and between linker and acceptor. For the D−A-type emitters, phenyl, toluene, and xylene are used as linker moieties to shorten π-conjugation between the donor and acceptor for deeper blue emission.

type emitters, and BDTPSAF and BDTPDDA are symmetric A−D−A-type emitters. The para-linked donor−acceptor structure was used to control the emitting dipole moment (EDO) of emitters in the organic solid film. If both donor and acceptor moieties are connected within a molecule, then emission usually occurs via charge transfer. The S1→S0 transition dipole moment (TDM) points from the acceptor toward the donor, parallel to the molecular long axis, and the EDO follows the overall molecular orientation in the organic thin film. A high proportion of horizontal EDOs can be

RESULTS AND DISCUSSION DTPSAF, DTTSAF, DTXSAF, BDTPSAF, and BDTPDDA were prepared using various organic reactions such as bromination, Suzuki coupling reactions, and N-arylation. Structures of the synthesized compounds were confirmed by 1 H NMR, 13C NMR, and mass spectroscopy (see Supporting Information). Thermal stabilities were measured by thermogravimetric analyses (TGA) and differential scanning calorimetry (DSC) (Figures S1 and S2.) The decomposition temperatures

■

Table 1. Photophysical Properties of DTPSAF, DTTSAF, DTXSAF, BDTPSAF, and BDTPDDA experimental data material

HOMO/LUMO (eV)

DTPSAF DTTSAF DTXSAF BDTPSAF BDTPDDA

−5.51/-2.63 −5.55/-2.56 −5.52/-2.48 −5.46/-2.65 −5.45/-2.53

a

S1b

(eV) 2.76 2.80 2.82 2.74 2.76

calculation data

λPL (nm)

S1d

(eV)

fwhm (nm)

PLQY (%)

459 450 446 469 459

2.70 2.76 2.78 2.64 2.70

69 70 69 62 60

82 100 98 94 96

c

e

f

krg 8 −1

(10 s )

EDOh

3.13 3.23 3.13 3.85 3.85

70:30 70:30 74:26 90:10 90:10

S1, VAi (eV)

S1, VE (eV)

T1k (eV)

ΔESTl (eV)

fm

2.89 2.91 3.00 2.84 2.90

2.72 2.74 2.80 2.66 2.71

2.18 2.29 2.31 2.19 2.18

0.60 0.53 0.57 0.50 0.54

0.7388 0.5951 0.6451 1.6613 1.7740

j

a

HOMO levels measured by cyclic voltammetry (Figures S3). LUMO levels estimated by adding the optical band gaps to the HOMO levels. bPL peak S1 energy of toluene solution. cPL peak of 10 wt % emitter:mCP host film. dS1 energy of PL peak. eFull-width half-maximum of PL from 10 wt % emitter:mCP host film. fPhotoluminescence quantum yield of 10 wt % emitter:mCP host film. gRadiative decay rate constant measured from 10 wt % emitter:mCP host film. hEmitting dipole orientation of emitters (horizontal:vertical). iCalculated vertical absorption S1 energy. jCalculated vertical emission S1 energy. kCalculated relaxed T1 state energy. lCalculated adiabatic (relaxed) singlet−triplet energy gap (ΔEST). mCalculated oscillator strength of S1→S0 transition. Calculation was performed using LC-ωPBE functional with modified range-separation parameter (ω = 0.10). 858


Article

Chemistry of Materials

Figure 2. Optimized S1 state geometries and natural transition orbitals for S1→S0 transition of DTPSAF, DTTSAF, DTXSAF, BDTPSAF, and BDTPDDA. Dihedral angles of donor/linker and linker/acceptor for S1 geometries of DTPSAF, DTTSAF, and DTXSAF are indicated (dihedral angles for S0 geometries are in parentheses) (LC-ωPBE ω = 0.10, PBF toluene).

S1 and S0 states, resulting in a very short radiative decay time of about 3 ns without delayed emission (Figure S9). The transient PL of DTXSAF doped in various hosts were also measured and all of them showed very fast radiative decays without delayed fluorescence (Figure S10). No delayed fluorescence in the molecules even with the D−A structures can be supported by the large calculated ΔEST values. The radiative decay rate constants of the symmetric A−D−A-type emitters were 3.85 × 108 s−1, which are larger than those obtained from the asymmetric D−A emitters (Table 1). This can be explained by the larger overlap between the hole and electron wave function of the A−D−A-type emitters than that of the D−A emitters due to doubled donor−acceptor connections.44 The calculated oscillator strengths of the A−D−A emitters are also larger than those of the D−A emitters. Due to their fast radiative decay and rigid molecular structures, all of the designed emitters exhibited very high PLQYs. For example, DTTSAF attained a PLQY of unity. As expected from the vertical emission S1 energies obtained from TDDFT calculations, the emission spectra of the D−Atype emitters were blue-shifted following a change in the donor−acceptor connector unit from a phenyl moiety to a toluene or xylene moiety. The dihedral angle between the acridine moiety and the connecting units, and the dihedral angle between the connecting units and the diphenyltriazine moiety, increased as the phenyl connector was changed to a toluene or xylene due to steric hindrance by the methyl groups.45 The calculated dihedral angles for S1 and S0 geometries are indicated in Figure 2. The π-conjugation length of the LUMO was shortened by increasing the dihedral angle between the aromatic rings, resulting in higher LUMO levels and blue-shifted PL spectra.46−48 The emission spectra of the A−D−A-type emitters were also blue-shifted by substituting diphenylsilane for spirofluorene. The HOMO and LUMO levels were increased by 0.01 and 0.12 eV, respectively, resulting in an overall bandgap increase of 0.11 eV and an increase in the peak PL energy from 2.65 eV (469 nm) for BDTPSAF to 2.70 eV (459 nm) for BDTPDDA. Thus, deeper blue emission was realized without reducing PLQY and the radiative decay rate by employing two distinct design strategies: changing the phenyl connector to a toluene or xylene, and changing the fluorene moiety to a diphenylsilane. The full-width half-maximum (fwhm) of the PL emission from the A−D−A-type emitters was smaller than those of the

of DTPSAF, DTTSAF, DTXSAF, BDTPSAF, and BDTPDDA were 428, 374, 403, 463, and 600 °C, respectively. The melt transition temperatures of DTPSAF, DTTSAF, DTXSAF, BDTPSAF, and BDTPDDA were 344, 316, 289, 389, and 344 °C. The glass transition temperatures of DTPSAF, DTTSAF, DTXSAF, BDTPSAF, and BDTPDDA were 171, 162, 156, 200, and 259 °C, respectively. All compounds exhibited sufficiently high thermal stabilities. For PL measurements, high T1 (2.9 eV) N,N′-dicarbazolyl3,5-benzene (mCP) was used for the host of the emitters to confine triplet within the emitters as it could affect the delayed emission of the emitters. However, the PL measurement results showed that the emitters are conventional fluorescent materials, and high T1 of the host would not be necessary. The measured photophysical properties of emitters are listed in Table 1. Density functional theory (DFT) and time-dependent DFT (TDDFT) calculations were performed using the Schrodinger Materials Suite’s Jaguar to understand the photophysical properties of the emitters.39,40 Ground state geometries were optimized with B3LYP functional, and excited state geometries were optimized with CAM-B3LYP functional. Single point excitation energies and natural transition orbitals (NTOs) were calculated using LC-ωPBE functional with modified rangeseparation parameter (ω = 0.10), employing PBF (Poisson− Boltzmann Finite-element solver) solvent model with dielectric medium (toluene, ε = 2.3741).41 Optimized geometries and NTOs are shown in Figures 2 and S4, and calculation data are shown in Tables 1, S3, and S4. DFT/TDDFT calculations were also performed with B3LYP functional. Optimized ground state geometries, HOMO/LUMO frontier orbitals, NTOs, and excitation energies calculated with B3LYP are shown in Figures S5−7 and Table S1. For all five emitters, the electron wave function and hole wave function of NTOs are localized at the triazine acceptor moiety and the acridine donor moiety, respectively (Figures 2 and S4). NTO analyses revealed that the S0−S1 absorption and S1−S0 emission are mainly comprised of charge transfer (CT) transition. Strong solvatochromism of the PL in solution (Figure S8) also indicates that emission occurs from the charge transfer excited state.42,43 However, the hole wave function and electron wave function of the emitters’ NTOs are not completely separated. Instead, there is a slight overlap at the donor−acceptor connecting unit (phenyl, toluene, or xylene). This overlap generates sufficient oscillator strength between the 859


Article


the five emitters, which facilitates efficient energy transfer from the host to the emitters. Current density−voltage-luminance (J−V−L) curves, electroluminescence spectra, and current density−EQE curves of the five devices are shown in Figures 4a−c, respectively. Electroluminescent characteristics of the OLEDs are summarized in Table 2. Replacing the phenyl moiety with toluene or xylene in the D−A-type emitters lowered the CIE y-value in the EL spectra as expected from the PL spectra. The EQEs of the DTTSAF- and DTXSAF-doped OLEDs (6.2 and 7.7%, respectively) were higher than that of the DTPSAF-doped device (5.7%) due to the higher PLQYs of DTTSAF (100%) and DTXSAF (98%) over that of DTPSAF (82%). The device doped with DTXSAF had the deepest blue emission with CIE coordinates of (0.149, 0.082) at 500 cd/m2, almost reaching the NTSC blue CIE coordinate (0.14, 0.08). The EQE of the DTXSAF device (7.7%) was very high relative to those of conventional fluorescent blue OLEDs with CIE y < 0.085. OLEDs incorporating the A−D−A-type emitters had EQEs higher than those of the D−A-type emitters due to the greater horizontal EDO ratios of the former. EQEs of devices based on BDTPSAF and BDTPDAA were similar (8.5%) due to the similar PLQYs of BDTPSAF (94%) and BDTPDDA (96%) and the same horizontal EDO ratio. However, their EL spectra and CIE coordinates differed significantly. The EL peak wavelength of OLEDs based on BDTPDDA was 456 nm, which is a 12 nm blue shift from the EL peak of BDTPSAF at 468 nm. By replacing the fluorene moiety of DTPSAF with a diphenylsilane, the CIE y-value of BDTPDDA was impressively decreased from 0.188 to 0.115. The overall shape of the electroluminescence spectrum differed between the D−A-type and the A−D−A-type emitters. The EL and PL spectra of the A−D−A emitters contained a small shoulder peak that was not evident in the EL and PL spectra of the D−A-type emitters. The fwhm of the A−D−Atype emitters’ emission was ∼10 nm smaller than that of the D−A emission. Therefore, symmetrized D−A-type emitters are at an advantage in reproducing pure primary colors. We also fabricated OLEDs using BDTPDDA as the emitter with a higher doping ratio and in a mixed host EML. The current density−EQE curve and electroluminescence data of the resulting devices are shown in Figure 4d and Table 2. J−V− L curves and EL spectra are shown in Figure S12. OLEDs with 10 wt % DTPDDA showed higher maximum EQEs but showed larger efficiency roll-offs than the 20 wt % BDTPDDA devices. The 20 wt % BDTPDDA devices showed higher maximum luminance of 2650 and 3323 cd/m2, almost twice of that of 10 wt % devices. Due to concentration quenching and the aggregation of emitters, the emission spectra of the 20 wt % BDTPDDA devices were red-shifted and showed CIE y-values larger than those of the 10 wt % devices. Theoretical EQEmax values of the OLEDs were calculated by using a classical dipole model with measured PLQYs, PL spectra, and horizontal EDO ratios of emitter-doped films (Table 2.) Although the BDTPDDA-doped device had the highest EQE (8.5%), the EQEmax of the device did not reach the calculated EQEmax (9.59%) of the optimized device structure. This discrepancy seems to have originated from charge imbalance in the singlehost EML. A device incorporating a mCP:TSPO1 (1:1 wt %) mixed host in the EML doped with BDTPDDA was fabricated to see if the unexpectedly low experimental EQE was due to charge imbalance. The theoretical EQEmax of a device with the 10 wt % BDTPDDA:mCP:TSPO1 EML is 7.7%, which

D−A-type emitters. The electron wave function of the NTO is more delocalized over donor to triazine in the A−D−A-type emitters than in the D−A-type emitters. Thus, the donor-linker bond of the A−D−A-type emitters are more rigid in their excited state geometries than the D−A-type emitters, and intramolecular vibration and rotation are suppressed. Therefore, narrow spectra are expected in the A−D−A-type emitters.49 The featured spectra of the A−D−A-type molecules compared to the featureless spectra of the D−A-type molecules also indicate that the vibration and rotation are suppressed in the A−D−A-type molecules. The angle-dependent PL intensity of p-polarized light was measured from a film composed of 10 wt % emitter in mCP host. Measured data were fitted with an optical simulation using a classical dipole model and are shown in Figure S11. The EDOs were 70(horizontal):30(vertical), 70:30, and 74:26 for DTPSAF, DTTSAF, and DTXSAF, respectively, and 90:10 for the symmetric A−D−A-type emitters. BDTPDDA doped in an mCP:TSPO1 (1:1 wt %) mixed host exhibited the same molecular orientation as in a mCP host. Because the emitter doping ratio in the EML is only 10 wt %, the Tg of the emitting layer will be close to the Tg of the host molecule mCP (65 °C), and the Tg of the emitters are expected to have little effect on the EDO of the emitters.50,51 As shown in Figure 3, the S1→S0

Figure 3. Top view and side view of optimized S1 geometries and the S1→S0 transition dipole moments of DTPSAF and BDTPSAF.

TDM of the molecules direct toward the acridine donor from the diphenyltriazine acceptor, almost parallel to the long axis of the molecule. Hence, the orientation of the TDM follows the molecular orientation in the organic thin film. The A−D−Atype emitters are oriented more horizontally than the D−A emitters probably due to the elongated molecular structures, and it resulted in higher horizontal EDO ratio of the A−D−Atype emitters than the D−A-type emitters. Because the PLQYs of both emitter types were approximately the same, higher EQEs are expected for OLEDs incorporating the A−D−A-type emitters because of their higher outcoupling efficiencies. We fabricated OLEDs using all five emitters as dopants. The optimized device structure was as follows: ITO (75 nm)/6 wt % ReO3:mCP (45 nm)/mCP (x nm)/10 wt % emitter:mCP (15 nm)/TSPO1(15 nm)/6 wt % Rb2CO3:TSPO1 (35 nm)/Al (100 nm). The thickness (x) of the mCP layer was 10 nm for DTPSAF/DTTSAF/DTXSAF and 15 nm for BDTPSAF/ BDTPDDA. mCP was used as the HTL, and TSPO1 was used as the electron transporting layer. ReO3 and Rb2CO3 were used as charge injection dopants for the HTL and ETL, respectively. mCP was also used as the host of EML because of the large spectral overlap between the PL of mCP and the absorption of 860


Article


Figure 4. (a) Current density−voltage-luminescence curves of OLEDs. (b) Electroluminescence intensity and CIE 1931 coordinates (inset) of the OLEDs at 500 cd/m2. (c) Current density-EQE curves of the OLEDs. (d) EQE of mCP host EML and mCP:TSPO1 host EML BDTPDDA-doped OLEDs.

Table 2. Electroluminescent Properties of OLEDs emitter DTPSAF DTTSAF DTXSAF BDTPSAF BDTPDDA BDTPDDA BDTPDDA BDTPDDA

EML of device 10 10 10 10 10 10 20 20

wt wt wt wt wt wt wt wt

% % % % % % % %

mCP mCP mCP mCP mCP mCP:TSPO1 mCP mCP:TSPO1

λELa (nm)

fwhmb (nm)

460 448 444 468 456 460 460 464

67 68 68 58 56 58 57 57

CIE500c (0.143, (0.147, (0.149, (0.139, (0.142, (0.140, (0.141, (0.140,

0.131) 0.096) 0.082) 0.189) 0.116) 0.135) 0.152) 0.172)

Vond (V)

PEmax/500/1000e (lm/W)

EQEmax/500/1000f (%)

calculated EQEmaxg (%)

4.1 4.1 4.1 3.8 3.5 3.3 3.3 3.3

6.9/1.4/− 4.8/1.3/− 5.2/0.82/− 10.7/4.0/3.3 8.4/2.9/2.3 9.0/3.0/2.2 9.2/4.9/4.1 10.8/4.7/3.9

5.7/2.8/− 6.2/3.4/− 7.7/2.7/− 8.5/5.3/4.8 8.5/5.3/4.6 7.7/4.8/4.1 7.9/6.6/6.1 7.6/6.0/5.5

6.43 7.37 7.78 9.51 9.59 7.68

a EL peak at 500 cd/m2. bFull width half-maximum at 500 cd/m2. cCIE 1931 coordinates at 500 cd/m2. dTurn on voltage at 1 cd/m2. ePower efficiency of maximum and at 500 and 1000 cd/m2. fExternal quantum efficiency of maximum and at 500 and 1000 cd/m2. gCalculated EQEmax by optical simulation.

applicable to donor−acceptor structured TADF emitters for efficient deep blue OLEDs.

matches the calculated EQEmax of 7.68%. This indicates that the difference between the calculated EQE and experimental EQE of the single-host EML device was due to charge imbalance in the EML. Although theoretical efficiency was achieved with the mCP:TSPO1 mixed-host device, the EQE of the device based on the mixed host was lower than that of the mCP single-host device due to the low PLQY of the mCP:TSPO1 host (71%) relative to that of the mCP host (96%). If a deep blue exciplex with efficient energy transfer and perfect charge balance is used as the host for BDTPDDA, the maximum efficiency of the emitter might be obtained.52 The molecular design strategies were successfully applied to donor−acceptor-type molecules comprised with triazine and spiroacridine moieties that are widely used in blue TADF emitters and we expect the molecular design strategies to be

■

CONCLUSIONS

In conclusion, we described molecule design strategies for the realization of highly efficient deep blue emission from donor− acceptor-type emitters. Changing the phenyl connector between the donor and acceptor to a toluene or xylene moiety and substituting the spirofluorene moiety with a diphenylsilane resulted in deeper blue emission without loss of luminescence efficiency. Furthermore, symmetrizing the D−A structure of the emitter, forming an A−D−A emitter, resulted in a high degree of molecular orientation with a horizontal EDO ratio of 90%. OLEDs with pure blue emission (CIE (0.149, 0.082)) and a high EQE of 7.7% were fabricated using DTXSAF. OLEDs with 861


Article


Bipolar Fluorophores for Nondoped Blue Organic Light-Emitting Diodes. Adv. Mater. 2008, 20, 3947−3952. (9) Liu, M.; Li, X. L.; Chen, D. C.; Xie, Z.; Cai, X.; Xie, G.; Liu, K.; Tang, J.; Su, S.-J.; Cao, Y. Study of Configuration Differentia and Highly Efficient, Deep-Blue, Organic Light-Emitting Diodes Based on Novel Naphtho[1,2- d ]imidazole Derivatives. Adv. Funct. Mater. 2015, 25, 5190−5198. (10) Tang, X.; Bai, Q.; Peng, Q.; Gao, Y.; Li, J.; Liu, Y.; Yao, L.; Lu, P.; Yang, B.; Ma, Y. Efficient Deep Blue Electroluminescence with an External Quantum Efficiency of 6.8% and CIE. Chem. Mater. 2015, 27, 7050−7057. (11) Liu, H.; Bai, Q.; Yao, L.; Zhang, H.; Xu, H.; Zhang, S.; Li, W.; Gao, Y.; Li, J.; Lu, P.; Wang, H.; Yang, B.; Ma, Y. Highly Efficient Near Ultraviolet Organic Light-emitting Diode Based on a Meta-linked Donor−acceptor Molecule. Chem. Sci. 2015, 6, 3797−3804. (12) Zhu, Z.; Chen, W.; Zhang, L.; Liu, X.; Tong, Q.; Lu, F.; Lee, C.S. A Pyridine Based Meta-linking Deep-blue Emitter With High Conjugation Extent and Electroluminescence Efficiencies. J. Mater. Chem. C 2016, 4, 6249−6255. (13) Hu, J.-Y.; Pu, Y.-J.; Satoh, F.; Kawata, S.; Katagiri, H.; Sasabe, H.; Kido, J. Bisanthracene-Based Donor−Acceptor-type LightEmitting Dopants: Highly Efficient Deep-Blue Emission in Organic Light-Emitting Devices. Adv. Funct. Mater. 2014, 24, 2064−2071. (14) Fisher, A. L.; Linton, K. E.; Kamtekar, K. T.; Pearson, C.; Bryce, M. R.; Petty, M. C. Efficient Deep-Blue Electroluminescence from an Ambipolar Fluorescent Emitter in a Single-Active-Layer Device. Chem. Mater. 2011, 23, 1640−1642. (15) Chercka, D.; Yoo, S.-J.; Baumgarten, M.; Kim, J.-J.; Müllen, K. Pyrene Based Materials for Exceptionally Deep Blue OLEDs. J. Mater. Chem. C 2014, 2, 9083−9086. (16) Kim, R.; Lee, S.; Kim, K.-H.; Lee, Y.-J.; Kwon, S.-K.; Kim, J.-J.; Kim, Y.-H. Extremely Deep Blue and Highly Efficient Non-doped Organic Light Emitting Diodes Using an Asymmetric Anthracene Derivative with a Xylene Unit. Chem. Commun. 2013, 49, 4664−4666. (17) Chou, P.-Y.; Chou, H.-H.; Chen, Y.-H.; Su, T.-H.; Liao, C.-Y.; Lin, H.-W.; Lin, W.-C.; Yen, H.-Y.; Chen, I.-C.; Cheng, C.-H. Efficient Delayed Fluorescence via Triplet−triplet Annihilation for Deep-blue Electroluminescence. Chem. Commun. 2014, 50, 6869−6971. (18) Park, H.; Lee, J.; Kang, I.; Chu, H. Y.; Lee, J.-I.; Kwon, S.-K.; Kim, Y.-H. Highly Rigid and Twisted Anthracene Derivatives: a Strategy for Deep Blue OLED Materials with Theoretical Limit Efficiency. J. Mater. Chem. 2012, 22, 2695−2700. (19) Chen, Y.-H.; Lin, C.-C.; Huang, M.-J.; Hung, K.; Wu, Y.-C.; Lin, W.-C.; Chen-Cheng, R.-W.; Lin, H.-W.; Cheng, C. H. Superior Upconversion Fluorescence Dopants for Highly Efficient Deep-Blue Electroluminescent. Chem. Sci. 2016, 7, 4044−4051. (20) Cha, S. J.; Han, N. S.; Song, J. K.; Park, S. R.; Jeon, Y. M.; Suh, M. C. Efficient Deep Blue Fluorescent Emitter Showing High External Quantum Efficiency. Dyes Pigm. 2015, 120, 200−207. (21) Kim, K.-H.; Baek, J. Y.; Cheon, C. W.; Moon, C.-K.; Sim, B.; Choi, M. Y.; Kim, J.-J.; Kim, Y.-H. Highly Efficient Non-doped Deep Blue Fluorescent Emitters with Horizontal Emitting Dipoles Using Interconnecting Units Between Chromophores. Chem. Commun. 2016, 52, 10956−10959. (22) Yang, X.; Xu, X.; Zhou, G. Recent Advances of the Emitters for High Performance Deep-blue Organic Light-emitting Diodes. J. Mater. Chem. C 2015, 3, 913−944. (23) Lee, I.; Yeob, J. Molecular Design of Deep Blue Fluorescent Emitters with 20% External Quantum Efficiency and Narrow Emission Spectrum. Org. Electron. 2016, 29, 160−164. (24) Liu, M.; Komatsu, R.; Cai, X.; Sasabe, H.; Kamata, T.; Nakao, K.; Liu, K.; Su, S.; Kido, J. Introduction of Twisted Backbone: A New Strategy to Achieve Efficient Blue Fluorescence Emitter with Delayed Emission. Adv. Opt. Mater. 2017, 5, 1700334. (25) Rajamalli, P.; Senthilkumar, N.; Huang, P.-Y.; Lin, H.-W.; Cheng, C.-H. New Molecular Design Concurrently Providing Superior Pure Blue, Thermally Activated Delayed Fluorescence and Optical Out-Coupling Efficiencies. J. Am. Chem. Soc. 2017, 139, 10948−10951.

less pure blue emission (CIE (0.142, 0.116)) but with a higher EQE of 8.5% were fabricated using BDTPDDA.

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.7b04437. Synthesis of the materials and fabrication of PL films and OLEDs; figures of TGA, DSC, CV, solvent PL, transient PL, NTOs, angle dependent PL, and JVL of devices (PDF)

■

AUTHOR INFORMATION

Corresponding Authors

*J.-J.K: [email protected]. *Y.H.K.: [email protected]. *S.K.K: [email protected]. ORCID

Yun-Hi Kim: 0000-0001-8856-4414 Jang-Joo Kim: 0000-0003-4926-2346 Author Contributions

J.-J.K. and Y.H.K. coordinated the collaborative work. Y.H.K. designed molecules. Y.K., J.Y.B., and M.-J.K. synthesized the emitters. S.-J.W. carried out the experiment, calculation, and analysis. S.-J.W. and J.-.J.K wrote the manuscript. S.-J.W. and Y.K. equally contributed to this work. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was supported by SNU-Industrial Strategic Technology Development Program (10079671) funded by the Ministry of Trade, Industry, & Energy (MOTIE, Korea) GNU-2015R1A2A1A10055620 funded by NRF, Korea.

■

REFERENCES

(1) Tang, C. W.; VanSlyke, S. A. Organic Electroluminescent Diodes. Appl. Phys. Lett. 1987, 51, 913−915. (2) Wen, S. W.; Lee, M.-T.; Chen, C. H. Recent Development of Blue Fluorescent OLED Materials and Devices. J. Disp. Technol. 2005, 1, 90−99. (3) Chen, W.-C.; Lee, C.-S.; Tong, Q.-X. Blue-emitting Organic Electrofluorescence Materials: Progress and Prospective. J. Mater. Chem. C 2015, 3, 10957−10963. (4) Giebink, N. C.; D’Andrade, B. W.; Weaver, M. S.; MacKenzie, P. B.; Brown, J. J.; Thompson, M. E.; Forrest, S. R. Intrinsic Luminance Loss in Phosphorescent Small-molecule Organic Light Emitting Devices due to Bimolecular Annihilation Reactions. J. Appl. Phys. 2008, 103, 044509. (5) Lee, J.-H.; Shin, H.; Kim, J.-M.; Kim, K.-H.; Kim, J.-J. ExciplexForming Co-Host-Based Red Phosphorescent Organic Light-Emitting Diodes with Long Operational Stability and High Efficiency. ACS Appl. Mater. Interfaces 2017, 9, 3277−3281. (6) Kwong, R. C.; Nugent, M. R.; Michalski, L.; Ngo, T.; Rajan, K.; Tung, Y. J.; Weaver, M. S.; Zhou, T. X.; Hack, M.; Thompson, M. E.; Forrest, S. R.; Brown, J. J. High Operational Stability of Electrophosphorescent Devices. Appl. Phys. Lett. 2002, 81, 162−164. (7) Chwang, A. B.; Kwong, R. C.; Brown, J. J. Graded Mixed-layer Organic Light-Emitting Devices Graded Mixed-Layer Organic Lightemitting Devices. Appl. Phys. Lett. 2002, 80, 725−727. (8) Lin, S.-L.; Chan, L. H.; Lee, R. H.; Yen, M. Y.; Kuo, W. J.; Chen, C. T.; Jeng, R. J. Highly Efficient Carbazole- p -Dimesitylborane 862


Article

Chemistry of Materials (26) Zhang, Q.; Li, J.; Shizu, K.; Huang, S.; Hirata, S.; Miyazaki, H.; Adachi, C. Design of Efficient Thermally Activated Delayed Fluorescence Materials for Pure Blue Organic Light Emitting Diodes. J. Am. Chem. Soc. 2012, 134, 14706−14709. (27) Liu, M.; Seino, Y.; Chen, D.; Inomata, S.; Su, S.; Sasabe, H.; Kido, J. Blue Thermally Activated Delayed Fluorescence Materials Based on Bis(phenylsulfonyl)benzene Derivatives. Chem. Commun. 2015, 51, 16353−16356. (28) Oh, C. S.; Moon, C.-K.; Jeong, M.C.; Huh, J.-S.; Kim, J.-J.; Lee, J. Y. Relationship Between Molecular Structure and Dipole Orientation of Thermally Activated Delayed Fluorescent Emitters. Org. Electron. 2017, 42, 337−342. (29) Yokoyama, D. Molecular Orientation in Small-molecule Organic Light-emitting Diodes. J. Mater. Chem. 2011, 21, 19187−19202. (30) Komino, T.; Sagara, Y.; Tanaka, H.; Oki, Y.; Nakamura, N.; Fujimoto, H.; Adachi, C. Electroluminescence from Completely Horizontally Oriented Dye Molecules. Appl. Phys. Lett. 2016, 108, 241106. (31) Tanaka, H.; Shizu, K.; Miyazaki, H.; Adachi, C. Efficient Green Thermally Activated Delayed Fluorescence (TADF) from a Phenoxazine−Triphenyltriazine (PXZ − TRZ) Derivative. Chem. Commun. 2012, 48, 11392−11394. (32) Endo, A.; Sato, K.; Yoshimura, K.; Kai, T.; Kawada, A.; Miyazaki, H.; Adachi, C. Efficient Up-conversion of Triplet Excitons into a Singlet State and its Application for Organic Light Emitting Diodes. Appl. Phys. Lett. 2011, 98, 83302. (33) Mehes, G.; Nomura, H.; Zhang, Q.; Nakagawa, T.; Adachi, C. Enhanced Electroluminescence Efficiency in a Spiro-Acridine Derivative through Thermally Activated Delayed Fluorescence. Angew. Chem., Int. Ed. 2012, 51, 11311−11315. (34) Lin, T.; Chatterjee, T.; Tsai, W.; Lee, W.; Wu, M.; Jiao, M.; Pan, K.; Yi, C.; Chung, C.; Wong, K. T. Sky-Blue Organic Light Emitting Diode with 37% External Quantum Efficiency Using Thermally Activated Delayed Fluorescence from Spiroacridine-Triazine Hybrid. Adv. Mater. 2016, 28, 6976−6983. (35) Li, Y.; Wang, Z.; Li, X.; Xie, G.; Chen, D.; Wang, Y. F.; Lo, C. C.; Lien, A.; Peng, J.; Cao, Y.; Su, S. J. Highly Efficient Spiro[fluorene9,9 ′-thioxanthene] Core Derived Blue Emitters and Fluorescent/ Phosphorescent Hybrid White Organic Light-Emitting Diodes. Chem. Mater. 2015, 27, 1100−1109. (36) Ren, X.; Li, J.; Holmes, R. J.; Djurovich, P. I.; Forrest, S. R.; Thompson, M. E. Ultrahigh Energy Gap Hosts in Deep Blue Organic Electrophosphorescent Devices. Chem. Mater. 2004, 16, 4743−4747. (37) Xiao, L.; Su, S. J.; Agata, Y.; Lan, H.; Kido, J. Nearly 100% Internal Quantum Efficiency in an Organic Blue-Light Electrophosphorescent Device Using a Weak Electron Transporting Material with a Wide Energy Gap. Adv. Mater. 2009, 21, 1271−1274. (38) Sun, J. W.; Baek, J. Y.; Kim, K.-H.; Moon, C.-K.; Lee, J.-H.; Kwon, S.-K.; Kim, Y.-H.; Kim, J.-J. Thermally Activated Delayed Fluorescence from Azasiline Based Intramolecular Charge-Transfer Emitter (DTPDDA) and a Highly Efficient Blue Light Emitting Diode. Chem. Mater. 2015, 27, 6675−6681. (39) Jaguar, version 9.3; Schrodinger, Inc.: New York, NY, 2016. (40) Bochevarov, A. D.; Harder, E.; Hughes, T. F.; Greenwood, J. R.; Braden, D. A.; Philipp, D. M.; Rinaldo, D.; Halls, M. D.; Zhang, J.; Friesner, R. A. Jaguar: A High-Performance Quantum Chemistry Software Program with Strengths in Life and Materials Sciences. Int. J. Quantum Chem. 2013, 113, 2110−2142. (41) Sun, H.; Zhong, C.; Brédas, J.-L. Reliable Prediction with Tuned Range-Separated Functionals of the Singlet-Triplet Gap in Organic Emitters for Thermally Activated Delayed Fluorescence. J. Chem. Theory Comput. 2015, 11, 3851−3858. (42) Slama-Schwok, A.; Blanchard-Desce, M.; Lehn, J.-M. Intramolecular Charge Transfer in Donor-Acceptor Molecules. J. Phys. Chem. 1990, 94, 3894−3902. (43) Borowicz, P.; Herbich, J.; Kapturkiewicz, A.; Opallo, M.; Nowacki, J. Radiative and Nonradiative Electron Transfer in Donor− acceptor Phenoxazine and Phenothiazine Derivatives. Chem. Phys. 1999, 249, 49−62.

(44) Sagara, Y.; Shizu, K.; Tanaka, H.; Miyazaki, H.; Goushi, K.; Kaji, H.; Adachi, C. Highly Efficient Thermally Activated Delayed Fluorescence Emitters with a Small Singlet-Triplet Energy Gap and Large Oscillator Strength. Chem. Lett. 2015, 44, 360−362. (45) Cui, L.; Nomura, H.; Geng, Y.; Kim, J. U.; Nakanotani, H.; Adachi, C. Controlling Singlet−Triplet Energy Splitting for Deep-Blue Thermally Activated Delayed Fluorescence Emitters. Angew. Chem. 2017, 129, 1593−1597. (46) Komatsu, R.; Ohsawa, T.; Sasabe, H.; Nakao, K.; Hayasaka, Y.; Kido, J. Manipulating the Electronic Excited State Energies of Pyrimidine-Based Thermally Activated Delayed Fluorescence Emitters to Realize Efficient Deep-Blue Emission. ACS Appl. Mater. Interfaces 2017, 9, 4742−4749. (47) Schrögel, P.; Tomkevičienė, A.; Strohriegl, P.; Hoffmann, S. T.; Köhler, A.; Lennartz, C. A Series of CBP-derivatives as Host Materials for Blue Phosphorescent Organic Light-Emitting Diodes. J. Mater. Chem. 2011, 21, 2266−2273. (48) Sonar, P.; Singh, S. P.; Leclère, P.; Surin, M.; Lazzaroni, R.; Lin, T. T.; Dodabalapur, A.; Sellinger, A. Synthesis, Characterization and Comparative Study of Thiophene−Benzothiadiazole Based Donor− acceptor−donor (D−A−D) Materials. J. Mater. Chem. 2009, 19, 3228−3237. (49) Im, Y.; Kim, M.; Cho, Y. J.; Seo, J.; Yook, K. S.; Lee, J. Y. Molecular Design Strategy of Organic Thermally Activated Delayed Fluorescence Emitters. Chem. Mater. 2017, 29, 1946−1963. (50) Mayr, C.; Brütting, W. Control of Molecular Dye Orientation in Organic Luminescent Films by the Glass Transition Temperature fo the Host Material. Chem. Mater. 2015, 27, 2759−2762. (51) Jiang, J.; Walters, D. M.; Zhou, D.; Ediger, M. D. Substrate Temperature Controls Molecular Orientation in Two-component Vapor-deposited Glasses. Soft Matter 2016, 12, 3265−3270. (52) Park, Y.-S.; Lee, S.; Kim, K.-H.; Kim, S.-Y.; Lee, J.-H.; Kim, J.-J. Exciplex-Forming Co-host for Organic Light-Emitting Diodes with Ultimate Efficiency. Adv. Funct. Mater. 2013, 23, 4914−4920.

863


Strategies for the Molecular Design of Donor ... - ACS Publications

Recommend Documents