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New Molecular Design Concurrently Providing Superior Pure Blue, Thermally Activated Delayed Fluorescence and Optical Out-Coupling Efficiencies P. Rajamalli,† N. Senthilkumar,† P.-Y. Huang,† C.-C. Ren-Wu,‡ H.-W. Lin,‡ and C.-H. Cheng*,† Departments of †Chemistry and ‡Materials Science and Engineering, National Tsing Hua University, Hsinchu 30013, Taiwan S Supporting Information *
Scheme 1. Synthesis of 2DPyM-mDTC and 3DPyM-pDTC
ABSTRACT: Simultaneous enhancement of out-coupling efficiency, internal quantum efficiency, and color purity in thermally activated delayed fluorescence (TADF) emitters is highly desired for the practical application of these materials. We designed and synthesized two isomeric TADF emitters, 2DPyM-mDTC and 3DPyM-pDTC, based on di(pyridinyl)methanone (DPyM) cores as the new electron-accepting units and di(tert-butyl)carbazole (DTC) as the electron-donating units. 3DPyM-pDTC, which is structurally nearly planar with a very small ΔEST, shows higher color purity, horizontal ratio, and quantum yield than 2DPyM-mDTC, which has a more flexible structure. An electroluminescence device based on 3DPyM-pDTC as the dopant emitter can reach an extremely high external quantum efficiency of 31.9% with a pure blue emission. This work also demonstrates a way to design materials with a high portion of horizontal molecular orientation to realize a highly efficient pure-blue device based on TADF emitters.
have been used to harvest light from both triplet and singlet excitons. To achieve efficient RISC, a very small singlet−triplet energy gap (ΔEST) is necessary. To obtain low ΔEST, earlier the molecules were designed to have a twisted structure. However, twisted molecules lead to structural relaxation, thus broadening and red-shifting the emission spectra.6d,7d,e As a result, a design strategy that employs rigid and linear (rod-like) structures for TADF materials is more attractive to achieve efficient, pureblue-emitting, TADF-based OLEDs. A second design consideration is to obtain emitters that are horizontally oriented in the film in order to raise the optical out-coupling efficiency of the OLED. Recent reports have revealed the significance of having emitting dipoles in OLED emitting layers oriented preferentially along the horizontal plane to improve the optical outcoupling.9 Thus, emitters showing high horizontal dipole ratios are highly desired to improve the efficiency of TADF OLED devices. To address this issue, we designed two isomeric TADF emitters, bis(6-(3,6-di-tert-butyl-9H-carbazol-9-yl)pyridin-2-yl)methanone (2DPyM-mDTC) and bis(6-(3,6-di-tert-butyl-9Hcarbazol-9-yl)pyridin-3-yl)methanone (3DPyM-pDTC). 2DPyM-mDTC contains a di(pyridin-2-yl)methanone (2DPyM) core and a meta electron-donating di(tert-butyl)carbazole (mDTC) substituent on each pyridine group (Scheme 1). Due to the meta connection between the keto and DTC groups, 2DPyM-mDTC shows partial folding, and the two DTC groups make the molecule very flexible. In contrast, 3DPyM-pDTC consists of a di(pyridin-3-yl)methanone (3DPyM) core and a para electron-donating di(tert-butyl)carbazole (pDTC) substituent. In the molecule, each side of the
O
rganic light-emitting diodes (OLEDs) have undergone significant advances and are now used in various flatpanel displays including large-screen televisions, smart phones, and smart watches.1 For OLEDs using conventional fluorescent dopants, the maximum internal quantum efficiency (IQE) is typically 25%, increasing to 100% for phosphorescent emitters.2−4 However, the need for noble metals such as Ir or Pt in phosphorescent emitters likely increases the device cost and may become an issue in terms of environmental sustainability. Further, although many blue phosphorescent materials were developed, they have either high Commission Internationale de l’Eclairage (CIE) coordinates (y coordinate is >0.25) or short device lifetimes and are not suitable for commercial use.5 Thus, the development of alternative, highly efficient blue-emitting materials to overcome these problems is needed. Recently, OLEDs employing metal-free thermally activated delayed fluorescence (TADF) emitters have emerged as a cheaper alternative to phosphorescent OLEDs.6,7 Although the number of TADF emitters has increased rapidly, only a few pure-blue TADF OLEDs with CIE coordinates of y < 0.2 and x + y < 0.35 are known; their efficiencies and related properties still need to be improved.8 TADF emitters can convert the lowest triplet excited state (T1) to the lowest singlet excited state (S1) through reverse intersystem crossing (RISC) and © 2017 American Chemical Society
Received: April 16, 2017 Published: July 26, 2017 10948
DOI: 10.1021/jacs.7b03848 J. Am. Chem. Soc. 2017, 139, 10948−10951
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Journal of the American Chemical Society
eV. In addition, 3DPyM-pDTC shows a slightly lower twist angle (26°) between ketone and pyridine compared to that of 2DPyM-mDTC (31°). In both molecules, the HOMO is localized on the DTC unit and slightly extended to the pyridine unit. The LUMO is dispersed over the central DPyM unit owing to the electron-deficient nature of pyridine and the ketone. Here, the pyridine units are involved in both HOMO and LUMO distribution, which is important for enhancing the radiative decay. The main transitions along with oscillator strengths and contour plots of the occupied and unoccupied molecular orbitals of both crystal and geometry optimized structures are listed in Tables S1−S3. The crystal structure of 3DPyM-pDTC was determined to be nearly planar along the x and y directions. In addition, intramolecular H-bonding between the two pyridine nitrogen atoms and the proximal C−H bonds of the tert-butylcarbazole groups (Figure 1) with a CH···N of 2.5 Å was found. It is noteworthy that DFT optimization also predicted a similar structure and H-bonding (Figure S2). The presence of CH···N hydrogen-bonding should restrict the rotation between the donor and acceptor groups in the molecule and increase the photoluminescence quantum yield (PLQY) in the solid state. For 2DPyM-mDTC, although the crystals could not be obtained, DFT optimization also shows similar H-bonding. However, the molecule is predicted to be more flexible than 3DPyM-pDTC (Figure S2). In 2DPyM-mDTC, rotation of the C−C bond between the carbonyl and the pyridine groups will lead to a very different molecular shape due to the fact that the tert-butylcarbazole group is meta to the carbonyl group in the molecule. For 3DPyM-pDTC, the tert-butylcarbazole group is para to the carbonyl group; thus, it will lead to a nearly linear molecular shape, and similar C−C bond rotation is prevented. The ultraviolet−visible (UV−vis) absorption and steady-state photoluminescence (PL) emission spectra of these materials were measured (Figure S3). These materials exhibited a strong intramolecular charge-transfer absorption band. The absorption profiles of these compounds in various solvents vary little (Figure S4). Unlike the absorption spectra, the emission spectra are bathochromically shifted in polar solvents, with an emission maximum change from 464 nm in toluene to 535 nm in DCM for 3DPyM-pDTC. In contrast, 2DPyM-mDTC exhibits dual emissions in all solvents, attributed to a charge-transfer state (Figure S4) and a local excited state (short wavelength). Interestingly, 3DPyM-pDTC shows a small Stokes shift, 4492 cm−1, and narrow emission with fwhm of 3381 cm−1 (75 nm) in toluene solution (Table S4). Conversely, 2DPyM-mDTC shows a larger Stokes shift, 7577 cm−1, and broader emission fwhm of 3562 cm−1 (99 nm) due to the more flexible structure. The phosphorescence spectra measured at 77 K are also shown in Figure S1. All photophysical data of these emitters are summarized in Table S4. The HOMO levels of −5.63 and −5.76 eV for 2DPyM-mDTC and 3DPyM-pDTC, respectively, were measured by cyclic voltammetry (Figure S5), while the LUMO levels of −2.89 and −2.76 eV, respectively, were calculated from the equation HOMO − Eg (Table S4), where Eg is the singlet energy gap determined from the onset of the fluorescence spectrum. Both materials show high thermal stability, with thermal decomposition temperatures of 480 and 420 °C for 3DPyM-pDTC and 2DPyM-mDTC, respectively, determined by thermogravimetric analysis under a nitrogen atmosphere (Figure S6). To confirm the TADF property, the transient PL decay characteristics of these materials were measured at 10−5 M in
Figure 1. (Top) HOMO−LUMO distribution of 2DPyM-mDTC (a) and 3DPyM-pDTC (b). (Bottom) Crystal structure of 3DPyM-pDTC.
DTC and pyridine group and the center keto group form two rigid linear axes. Different molecular shapes and physical properties result for these two isomers (see Scheme 1 and Figure 1), because of the different positions on the pyridine group connecting to the keto and carbazole substituents. 3DPyM-pDTC appears to have a nearly planar and more rigid structure, while 2DPyM-mDTC has a flexible and partially folded structure. Thus, 3DPy-pDTC gives a narrow, true blue emission (464 nm) with a full width at half-maximum (fwhm) of 62 nm, while 2DPyM-mDTC shows a broader green emission (506 nm) with fwhm of 89 nm. The OLED shows an external quantum efficiency (EQE) over 31%, with CIE coordinates at (0.14, 0.18), using 3DPy-pDTC as the dopant. On the other hand, for the 2DPyM-mDTC-based device, an EQE of 12.8% with CIE coordinates at (0.23, 0.47) was obtained. Moreover, the efficiency roll-offs are also very different. Importantly, this work demonstrates a way to design materials with a high portion of horizontal molecular orientation with narrow emission to realize a highly efficient pure-blue device based on TADF emitters. The green and blue TADF emitters, 2DPyM-mDTC and 3DPyM-pDTC, were synthesized by Ullman reaction of DTC with bis(6-bromopyridin-2-yl)methanone (2DPyM-mDBr) and with bis(6-bromopyridin-3-yl)methanone (3DPyM-pDBr) in 72% and 84% yields, respectively (Scheme 1). The molecules were fully characterized by NMR, mass, and single-crystal X-ray diffraction analysis. The detailed procedure for the synthesis of these materials and their characterization data are given in the Supporting Information. We conducted the density functional theory (DFT) calculation using Gaussian 09 software and Becke’s three-parameter nonlocal density functional employing a Lee−Yang−Parr functional (B3LYP) with the 6-31G* basis set to estimate the ΔEST, geometry, energy gap, and HOMO and LUMO of 2DPyM-mDTC and 3DPyM-pDTC. Figure 1 reveals the optimized molecular structures for the two compounds. A nearly planar structure was found for 3DPyMpDTC with an energy gap of 2.9 eV. However, 2DPyM-mDTC shows a folded molecular structure with an energy gap of 2.6 10949
DOI: 10.1021/jacs.7b03848 J. Am. Chem. Soc. 2017, 139, 10948−10951
Communication
Journal of the American Chemical Society toluene solution under vacuum (Figure S7). The transient decay curve of 3DPyM-pDTC shows two exponential decays, with prompt and delayed fluorescence lifetimes of 26.4 ns and 0.27 μs, respectively. The transient decay curve of 2DPyMmDTC shows three exponential decays, with two prompt and one delayed fluorescence lifetimes of 2.8 and 22.3 ns and 0.34 μs, respectively. The results support that these materials possesses the TADF property.7 To study the photophysical properties in the thin-film state, 3DPyM-pDTC is co-doped with the host material to avoid concentration quenching. The absolute PLQY values measured using an integrating sphere under an N2 atmosphere for the co-doped films are 59% and 98% for 2DPyM-mDTC and 3DPyM-pDTC, respectively. The ΔEST was estimated from the onset of the fluorescence and phosphorescence emission spectra to be 0.11 and 0.02 eV (Figure S8 and Table S4) for the mCBP co-doped thin films. The low ΔEST values support that these materials exhibit the TADF property in mCBP thin films with effective RISC. The transient PL profile (Figure S9a) of 3DPyM-pDTC in mCBP at 300 K consists of fast and slow components with lifetimes of 8 ns and 10 μs, ascribed to the prompt fluorescence and TADF, respectively. Using the PLQY and decay times, we calculated the rate constants of 3DPyM-pDTC according to the reported method.6d,e The rate constants kISC and kRISC were estimated to be 1.8 × 107 and 1.3 × 105 s−1, respectively. Fast RISC is achieved mainly due to very low ΔEST (0.02 eV) and the heteroatoms in the acceptor unit, which enhance the coupling between singlet and triplet state.7f,i To further confirm the TADF mechanism, the transient PL decay was measured at temperatures from 200 to 300 K. As shown in Figure S9a, the relative delayed PL intensities of 3DPyM-pDTC increase from 200 to 300 K. Moreover, the prompt and delayed spectra (at 10 μs delay time) of the co-doped thin film at room temperature were measured (Figure S9b) and coincide with each other well. The results further confirm that 3DPyM-pDTC is a TADF emitter.6 The orientation of the transition dipole moment in the codoped mCBP films containing 7 wt% 3DPyM-pDTC and 7 wt% 2DPyM-mDTC, respectively, was also measured using the angle-dependent p-polarized PL spectra on a fused glass substrate to simulate the out-coupling efficiency.9 The angledependent PL intensity is consistent with horizontal transition dipole ratios (Θ) of 0.76 and 0.85 for 2DPyM-mDTC and 3DPyM-pDTC, respectively (Figure 2), where Θ = 100% for fully horizontal dipoles and Θ = 67% for isotropic dipole orientation. The transition dipole moment of the molecule is calculated using DFT and TD-DFT to elucidate the orientation of the 3DPyM-pDTC. The calculation showed that the 3DPyMpDTC has a flat planar structure along the x and y directions (Figure S10). This shape of the molecule typically enables stacking parallel to the substrate.9d Next we investigated the electroluminescence (EL) properties of these two TADF emitters.10−12 Two devices were fabricated with the following structures: (A) ITO/NPB (30 nm)/TAPC (20 nm)/mCBP:2DPyM-mDTC (7 w%) (30 nm)/ TmPyPb (60 nm)/LiF (0.8 nm)/Al (100 nm) and (B) ITO/ NPB (30 nm)/TAPC (20 nm)/mCBP (10 nm)/mCBP: 3DPyM-pDTC (7 w%) (30 nm)/DPEPO (5 nm)/TmPyPb (60 nm)/LiF (0.8 nm)/Al (100 nm), respectively. The molecular structures used in the devices are shown in Figure S11. The EL properties of devices A and B are shown in Figures 3 and S12 and summarized in Table 1.
Figure 2. Angle-dependent PL intensity of the p-polarized light from a 30 nm thick film composed of mCBP:(7 wt% 2DPyM-mDTC) and mCBP:(7 wt% 3DPyM-pDTC) at 506 and 464 nm, respectively. The green solid line (△) represents the experimental data for 2DPyMmDTC with the horizontal to vertical transition dipole ratio of 0.76:0.24; the blue solid line (▽) is for 3DPyM-pDTC with the corresponding ratio of 0.85:0.15.
Figure 3. EL characteristic plots of devices A and B: external quantum efficiency vs luminance. Inset: Electroluminescence spectra of devices A and B.
Table 1. EL Performances of 2DPyM-mDTC (Device A) and 3DPyM-pDTC (Device B)a Vd (V) device 3.5 device 3.7
EQE
CE
PE
λmax
fwhm
(%, V)
(cd/A, V)
(lm/W, V)
(nm)
(nm)
3178, 13.5
12.8, 4.0
31.8, 4.0
29.5, 4.0
506
89
9670, 12.5
31.9, 4.0
37.6, 4.0
37.3, 4.0
464
62
L 2
(cd/m , V) A B
a
See text for device configurations. Vd, operating voltage at a brightness of 1 cd/m2; L, maximum luminance; EQE, maximum external quantum efficiency; CE, maximum current efficiency; PE, maximum power efficiency; λmax, the wavelength where the EL spectrum has the highest intensity; and fwhm is the full width at halfmaximum.
Quantum efficiency−luminance curves reveal a maximum EQE of 12.8% for device A and 31.9% for device B (Figure 3). The EQEs of device A at 100 and 500 cd/m2 decrease to 4.3 and 1.9%, respectively. Fortunately, device B shows a much lower efficiency roll-off, and the EQEs at 100 and 500 cd/m2 remain at 26.1 and 20.1%, respectively. Thus, device B shows both high-efficiency and low-efficiency roll-off at practical brightness levels. The EQE of device B is much higher than those of the reported blue TADF devices (EQE ≈ 10% and 10950
DOI: 10.1021/jacs.7b03848 J. Am. Chem. Soc. 2017, 139, 10948−10951
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8.7%).11,13 To the best of our knowledge, there is no report of a blue TADF OLED with an EQE greater than 31%, although EQEs greater than 30% have been reported for sky-blue and green TADF devices.9a,14 The current and power efficiencies of 47.7 cd/A and 37.3 lm/W, respectively, for device B are higher than those of the phosphorescent blue OLEDs.3a The EL spectrum of device B exhibits a pure blue emission with a maximum at 464 nm and CIE coordinates of (0.14, 0.18) (Figure 3). In addition, the EL emission is only from the dopant 3DPyM-pDTC, indicating a complete energy transfer from the mCBP host to the dopant emitter and hole and electron recombination only in the emitting layer. Notably, the fwhm of the EL spectrum of device B is 62 nm narrower than that of device A (∼89 nm).7b Further, the emission from device B is bluer than the phosphorescent emission from the well-known FIrpic-based devices.15 The transient EL decay is measured for device B at room temperature to confirm the TADF property of the device under electrical excitation. In the EL transient plot, the predominant delayed EL components of the device last for several tens of microseconds (Figure S13). The TADF process dominates the device emission under electrical excitation and is due to the facile RISC from T1 to S1 of the emitter (3DPyM-pDTC). In summary, we have designed and synthesized two isomeric compounds, 2DPyM-mDTC and 3DPyM-pDTC, containing a central keto group, two pyridine rings, and two di(tertbutyl)carbazolyl units. The connection of the three functional groups is the key for the PL and EL properties of these two isomers. 3DPyM-pDTC, with a nearly planar structure, shows a high Θ of 85%, very high PLQY of 98%, and EQE of 31% compared to those of 2DPyM-mDTC, with a more flexible structure. In addition, 3DPyM-pDTC shows a narrow-band true blue emission with fwhm of 62 nm and CIE of (0.14, 0.18), making it a suitable blue emitter for practical applications. These results prove that increased restriction of the molecular structure could be an effective method to enhance the outcoupling efficiency, EQE, color purity, and reduced roll-off of the TADF devices.
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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/jacs.7b03848. Experimental details and characterization data, including Tables S1−S4 and Figures S1−S13 (PDF) X-ray crystallographic data for 3DPyM-pDTC (CIF)
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AUTHOR INFORMATION
Corresponding Author
*
[email protected] ORCID
C.-H. Cheng: 0000-0003-3838-6845 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We thank the Ministry of Science and Technology of Republic of China (MOST 104-2633-M-007-001) for support of this research and the National Center for High-Performance Computing of Taiwan (account no. u32chc04) for providing computing time. 10951
DOI: 10.1021/jacs.7b03848 J. Am. Chem. Soc. 2017, 139, 10948−10951
