New Molecular Design Concurrently Providing Superior Pure Blue

3DPyM-pDTC shows a slightly lower twist angle (26°) between ketone and ... The crystal structure of 3DPyM-pDTC was determined and shows a nearly plana...
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New Molecular Design Concurrently Providing Superior Pure Blue Thermally Activated Delayed Fluorescent and Optical Outcoupling Efficiencies Pachaiyappan Rajamalli, Natarajan Senthilkumar, Pei-Yun Huang, Chen-Cheng Ren-Wu, Hao-Wu Lin, and Chien-Hong Cheng J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.7b03848 • Publication Date (Web): 26 Jul 2017 Downloaded from http://pubs.acs.org on July 26, 2017

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New Molecular Design Concurrently Providing Superior Pure Blue Thermally Activated Delayed Fluorescent and Optical Outcoupling Efficiencies P. Rajamalli,† N. Senthilkumar,† P.-Y Huang,† C.-C. Ren-Wu,‡ H.-W. Lin‡ and C.-H. Cheng*,† †

Department of Chemistry, National Tsing Hua University, Hsinchu 30013, Taiwan. Department of Materials Science and Engineering, National Tsing Hua University, Hsinchu 30013, Taiwan. Supporting Information Placeholder

‡

ABSTRACT: Simultaneous enhancement of outcoupling efficiency, internal quantum efficiency and color purity in thermally activated delayed fluorescence (TADF) emitters are highly desired for the practical application of these materials. Here, we designed and synthesized two isomeric TADF emitters, 2DPyMmDTC and 3DPyM-pDTC, based on di(pyridin-2-yl)methanone (DPyM) cores as the new electron-accepting units and di-tertbutyl-carbazole (DTC) as the electron-donating units. 3DPyMpDTC, which is structurally nearly planar with a very small ∆EST, shows higher color purity, horizontal ratio, and quantum yield compared with 2DPyM-mDTC having a more flexible structure. The electroluminescence device based on 3DPyM-pDTC as the dopant emitter can reach an extremely high external quantum efficiency (EQE) of 31.9% with a pure blue emission. Moreover, this work 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 emitter.

Organic light-emitting diode (OLED) has shown significant progress and are now used in various flat-panel displays including large-screen televisions, smart phones and smart watches.1 For the OLEDs using conventional fluorescent dopants, the maximum internal quantum efficiency (IQE) is typically 25%, and this value increases to 100% for phosphorescent emitters.2-4 However, the need for noble metals such as iridium or platinum 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 more than 0.25) or short device lifetimes and are not suitable to be used commercially.5 Therefore, to develop alternative highly efficient blue emitting materials to overcome the problems is urgently wanted. Recently, OLEDs employing metal-free TADF emitters have emerged as a cheaper alternative for phosphorescent organic light emitting diodes (PhOLEDs).6,7 Although the number of TADF emitters increase rapidly, only a few reports on 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 have been used to harvest light from both triplet and singlet excitons. To achieve efficient RISC, a very small singlet-triplet energy gap (∆EST) is necessary. In

order to obtain low ∆EST, earlier the molecules were designed to have a twisted structure. However, twisted molecules lead to structural relaxation, thus broaden and red-shift the emission spectra. 6i,7d,e As a result, a design strategy that employs rigid and linear (rod-like) structures for TADF materials is more attractive to achieve pure blue efficient TADF based OLEDs. A second design consideration is to obtain emitters that are horizontally oriented in the film to raise the optical outcoupling 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 Therefore, emitters showing high horizontal dipole ratios are highly desired to improve the efficiency of TADF OLED devices. To address this issue, here, we designed two isomeric TADF emitters, bis(6-(3,6-di-tert-butyl-9H-carbazol-9-yl)pyridin-2yl)methanone (2DPyM-mDTC) and bis(6-(3,6-di-tert-butyl-9Hcarbazol-9-yl)pyridin-3-yl)methanone (3DPyM-pDTC). As shown in Scheme 1, 2DPyM-mDTC contains a di(pyridin-2yl)methanone (2DPyM) core and a meta electron-donating di(tbutyl)carbazole (mDTC) substituent on each pyridine group. Due to the meta connection between the keto and DTC groups, 2DPyM-mDTC shows partial folding and the two DTC group making the molecule very flexible. In contrast, 3DPyM-pDTC consists of a di(pyridin-3-yl)methanone (3DPyM) core and a para electron-donating di(t-butyl)carbazole (pDTC) substituent. In the molecule, each side of the DTC and pyridine group and the center keto group form two rigid linear axes. Different molecular shapes and physical properties for these two isomers as displayed in Scheme 1 and Figure 1 are resulted because of the different positions on the pyridine group connecting to the keto and to 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 EQE over 31% with CIE coordinates at (0.14, 0.18) using 3DPypDTC 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 successfully 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 emitter.

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The green and blue TADF emitters, 2DPyM-mDTC and 3DPyM-pDTC were synthesized by the Ullman reaction of ditert-butylcarbazole 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 described in 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 ∆EST, geometry, the energy gap and the 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 3DPyM-pDTC with an energy gap of 2.9 eV. However, 2DPyM-mDTC, shows a folded molecular structure with an energy gap of 2.6 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 di-t-butyl carbazole unit, and slightly extended to pyridine unit (Figure 1). The LUMO is dispersed over the central di(pyridinyl)methanone (DPyM) unit owing to the electron-deficient nature of pyridine and ketone unit. 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 Table S1-S3.

Scheme 1. Synthesis of 2DPyM-mDTC and 3DPyM-pDTC O O

N Cu, K2CO3

N Br

N

N N

HN

N

p-xylene, 150 °C,12 h Br

2DPyM-mDBr 2DPyM-mDTC

O O Cu, K2CO3

N Br

N

3DPyM-pDBr

HN Br

N N

N

p-xylene, 150 °C,12 h

N

3DPyM-pDTC

The crystal structure of 3DPyM-pDTC was determined and shows a nearly planar structure along the x and y direction. In addition, intramolecular H-bonding between the two pyridine nitrogen atoms and the proximal C−H bonds of the t-butyl carbazole 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 donors and acceptor groups in the molecule and increase the PL quantum yield in the solid state. For 2DPyM-mDTC, although the crystals could not be obtained, the DFT optimization also shows similar H-bonding. However, the molecule is predicted to be more flexible than 3DPyM-pDTC (Figure S2). In 2DPyM-mDTC, the rotation of the C−C bond between the carbonyl and the pyridine groups will lead to very different molecular shape due to the fact that the t-butyl carbazole group is meta to the carbonyl group in the molecule. For 3DPyM-pDTC, the t-butyl carbazole group is para to the carbonyl group, therefore it lead to nearly linear in molecular shape and similar C−C bond rotation is prevented.

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Figure 1. HOMO-LUMO distribution of 2DPyM-mDTC (a) and 3DPyM-pDTC (b); crystal structure of 3DPyM-pDTC (bottom).

The ultraviolet-visible (UV-vis) absorption and steady-state photoluminescence (PL) emission spectra of these materials were measured and shown in Figure S3. These materials exhibited a strong intramolecular charge transfer (ICT) absorption band. The absorption profiles of these compounds in various solvents vary little and are shown Figure S4. In contrast to the absorption spectra, the emission spectra are bathochromically shifted in polar solvent, 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 (see 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 were measured at 77 K are also shown in Figure S1. All photophysical data of these emitters are summarized in Table S4. The HOMO levels -5.63 eV 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 eV and -2.76 eV, respectively were calculated from the equation HOMO-Eg (Table S4), where the Eg is the singlet energy gap and determined from the onset of fluorescence spectrum. Both materials show high thermal stability with the thermal decomposition temperature of 480 °C and 420 °C for 3DPyM-pDTC and 2DPyM-mDTC, respectively, determined by thermogravimetric analysis (TGA) under a nitrogen atmosphere (Figure S6). To confirm the TADF property, the transient PL decay characteristic of these materials are measured at 10-5 M in toluene solution under vacuum and are shown in Figure S7. The transient decay curve of 3DPyM-pDTC shows two exponential decays with the prompt and delayed fluorescence lifetime of 26.4 ns and 0.27 µs, respectively. The transient decay curve of 2DPyM-mDTC shows three exponential decays with the two prompt and delayed fluorescence lifetime of 2.8 and 22.3 ns and 0.34 µs, respectively. The results support that these materials possesses TADF property.7 To study the photophysical properties in the thin-film state, 3DPyM-pDTC is co-doped with the host material in order to avoid concentration quenching. The host selected is 3,3’bis(carbazol-9-yl)-1,1’-biphenyl (mCBP) due to the high triplet energy of 3.0 eV suitable for this blue fluorescent emitter.

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The absolute photoluminescence quantum yield measured using an integrating sphere under the N2 atmosphere of the co-doped films are 59% and 98% for 2DPyM-mDTC and 3DPyM-pDTC, respectively. The ∆EST 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) from the mCBP co-doped thin films. The low ∆EST values support that these materials exhibit TADF property in mCBP thin film with effective RISC. The transient PL profile (Figure 2a) of 3DPyM-pDTC in mCBP at 300 K consists of fast and slow components with the lifetimes of 8 ns and 10 µs ascribing to the prompt fluorescence and TADF, respectively. Using the PLQY and decay times, we calculated the 3DPyM-pDTC rate constants according to the reported method.6i,j The rate constants kISC and kRISC were estimated to 1.8 x 107 s-1 and 1.3 x 105 s-1, respectively. Fast RISC is achieved mainly due to very low ∆EST (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 temperature from 200 to 300 K. As shown in Figure 2a, the relative delayed PL intensities of 3DPyMpDTC 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 S9) and coincide with each other well. The results further confirms that 3DPyM-pDTC is a TADF emitter.6

Intensity, a.u.

a) 1

3DPyM-pDTC 200 K 3DPyM-pDTC 250 K 3DPyM-pDTC 300 K

0.1

0.01

1E-3 0

10

20

30

Time, µs

b) Normalized PL Instensity (a.u.)

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simulation isotropic (θ=0.67) simulation horizontal (θ=1) Experimental mCBP:2DPyM-mDTC (θ=0.76) Experimental mCBP:3DPyM-pDTC (θ=0.85)

1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0 0

20

40

60

80

Angle (degree)

Figure 2. a) Transient PL characteristics of co-doped thin film (mCBP:3DPyM-pDTC (7%)) at various temperature, b) 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 nm and 464 nm, respectively. The green solid line (∆) represents the experimental data with the horizontal transition dipole ratio of 0.76 and the vertical transition dipole ratio of 0.24 for 2DPyM-mDTC; and the blue solid line (∇) represents the experimental data with the horizontal transition dipole ratio of 0.85 and the vertical transition dipole ratio of 0.15 for 3DPyM-pDTC.

simulate the outcoupling efficiency.9 The angle-dependent PL intensity is consistent with horizontal transition dipole ratios (Θ) of 0.76 and 0.85 for 2DPyM-mDTC and 3DPyM-pDTC, respectively as shown in Figure 2b, 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 3DPyM-pDTC has a flat planar structure along the x and y direction as shown in 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. Two devices A and B with the following structures: 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 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, were fabricated. In the devices, N,N'-bis(1-naphthyl)-N,N'diphenyl-1,1'-biphenyl-4,4'-diamine (NPB) acts as a hole injection material and 1,1-bis[4-[N,N'-di(p-tolyl)amino]phenyl] cyclohexane (TAPC) as a hole-transporting material.10 3,3'-Bis(Ncarbazolyl)-1,1'-biphenyl (mCBP) as a host and exciton blocker, (oxybis(2,1-phenylene))-bis(diphenylphosphine oxide) (DPEPO) is an exciton blocker11 and 1,3,5-tri(m-pyrid-3-yl-phenyl)benzene (TmPyPb) is the electron-transporting material.12 LiF and Al were used as the electron injection layer and the cathode, respectively. The molecular structures used in the devices are shown in Figure S11. The EL properties of devices A and B are shown in Figure 3, Figure S12 and summarized in Table 1. As shown in Figure 3, the quantum efficiency-luminance curves reveal a maximum EQE of 12.8% for device A and 31.9% for device B. 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 level. The EQE of device B is much higher than those of the reported blue TADF devices (EQE~10% and 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 the phosphorescent blue OLEDs.3a The EL spectrum of device B exhibits a pure blue emission with the maximum at 464 nm and the CIE coordinates of (0.14, 0.18) as shown in 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 EML. 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 wellknown FIrpic-based devices.15

The orientation of the transition dipole moment in the codoped mCBP films containing 7 wt% 3DPyM-pDTC and 7 wt% 2DPyM-mDTC, respectively, were also measured using the angledependent p-polarized PL spectra on a fused glass substrate to

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pDTC, thermograms of the 2DPyM-mDTC and 3DPyM-pDTC, phosphorescence and fluorescence spectra, current and power efficiency vs luminance. 10

E.Q.E.

Device A Device B

AUTHOR INFORMATION

Corresponding Author

1.0 Device A Device B

0.8

1

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E-mail: [email protected]

0.6 89 nm

62 nm

0.4

ACKNOWLEDGMENT

0.2 0.0 400

500

600

700

Wavelength, nm

0.1

10

100

1000

10000

2

Luminance, cd/m

Figure 3. The 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 the 2DPyM-mDTC (Device A) and 3DPyM-pDTC (Device B).a,b Devicea

Vd (V)b

(cd/m2, V)

L

EQE (%, V)

A

3.5

3178 (13.5)

12.8 (4.0)

B

3.7

9670 (12.5)

31.9 (4.0)

CE (cd/A, PE (lm/W, λmax V) V) (nm)

31.8 (4.0) 37.6 (4.0)

FWHM (nm)

29.5 (4.0)

506

89

37.3 (4.0)

464

62

a Device configuration for A: ITO/NPB (30 nm)/TAPC (20 nm)/mCBP: 2DPyMmDTC (7 wt%) (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 wt%) (30 nm)/DPEPO (5 nm)/TmPyPb (60 nm)/LiF (0.8 nm)/Al (100 nm); b Vd, The operating voltage at a brightness of 1 cd/m2; maximum luminance (L); maximum external quantum efficiency (EQE); maximum current efficiency (CE); maximum power efficiency (PE); λmax, the wavelength where the EL spectrum has the highest intensity; and full width at half maximum (FWHM).

The transient electroluminescence decay is measured for Device B at room temperature to confirm the TADF property of the device under electrical excitation. Figure S13 shows that in the EL transient plot, the predominant delayed electroluminescence components of device last for several tens of microseconds. 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(tbutyl)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 a FWHM of 62 nm and CIE of (0.14, 0.18) suitable blue emitter for practical application. 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.

ASSOCIATED CONTENT Supporting Information Synthesis procedure, DFT results, ultraviolet-visible (UV-vis) absorption and steady-state photoluminescence (PL) emission spectra, cyclic voltammograms of 2DPyM-mDTC and 3DPyM-

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 number: u32chc04) for providing the computing time.

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