Article pubs.acs.org/cm
Isobenzofuranone- and Chromone-Based Blue Delayed Fluorescence Emitters with Low Efficiency Roll-Off in Organic Light-Emitting Diodes Jiyoung Lee,† Naoya Aizawa,† and Takuma Yasuda*,†,‡ †
INAMORI Frontier Research Center (IFRC), Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan Department of Applied Chemistry, Graduate School of Engineering, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan
‡
S Supporting Information *
ABSTRACT: Significant efforts have been devoted to the development of novel efficient blue-emitting molecules for organic light-emitting diode (OLED) applications. Blue organic emitters exhibiting thermally activated delayed fluorescence (TADF) have the potential to achieve ∼100% internal electroluminescence quantum efficiency in OLEDs. In this paper, we report a promising molecular design strategy for obtaining appropriate high singlet and triplet excited energies, short exciton lifetimes, and high quantum efficiencies in blue TADF emitters. We introduce isobenzofuranone and chromone containing a cyclic ketone or lactone moiety as effective acceptor building units to construct donor−acceptor TADF emitters. Owing to their small singlet−triplet energy splitting, properly contracted π-conjugation, and weakened intramolecular charge-transfer character, these new emitters display strong blue TADF emissions with high photoluminescence quantum yields (53−92%) and notably short TADF emission lifetimes (2.8−4.3 μs) in thin films. Blue TADF-OLEDs utilizing these emitters exhibit external electroluminescence quantum efficiencies of up to 16.2% and extremely low efficiency roll-off even at practical high luminance. The current findings open new avenues for designing practically usable high-performance blue TADF emitters with simple molecular structures.
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INTRODUCTION Since the seminal invention of efficient organic electroluminescence (EL) devices by Tang et al. in 1987,1 organic light-emitting diodes (OLEDs) have seen rapid advances toward practical applications in full-color flat-panel displays, solid-state lighting, and flexible wearable electronic devices.2−11 Development of high-efficiency organic luminophores displaying the three primary (RGB: red, green, and blue) emission colors is critical for future OLED technologies. The main factor determining the internal EL quantum efficiency (ηint) of OLEDs is the spin statistics upon recombination of holes and electrons under electrical excitation. In traditional long-lasting fluorescent OLEDs, only 25% singlet (S1) excitons can be exploited to produce EL emission, and the remaining 75% triplet (T1) excitons are typically lost through a nonradiative decay path, limiting ηint to 25%.5 One of the most viable strategies for maximizing ηint of OLEDs relies on the use of organometallic phosphorescent emitters incorporating heavy metals such as iridium or platinum. Such phosphorescent OLEDs can reach ηint values as high as ∼100% by harvesting both S1 and T1 excitons7,8,12 owing to fast intersystem crossing (ISC) via intense spin−orbit coupling induced by the heavy metal center. In this context, © 2017 American Chemical Society
numerous efforts have been devoted to the development of high-performance iridium- or platinum-containing phosphorescent emitters, and great success in terms of EL efficiency, color purity, and operational stability has been achieved for green and red phosphorescent OLEDs, which meet the criteria for commercial applications. Nevertheless, some issues still remain in current phosphorescent OLED technology. The main hurdle is the lack of efficient and stable deep-blue phosphorescent emitters, whose development lags behind their green and red counterparts because of the difficulty in designing stable and wide energy-gap phosphors possessing intrinsically high excited energy for metal-to-ligand chargetransfer transition.13−15 Moreover, the use of expensive precious metal elements is indispensable for high ηint in the existing phosphorescence systems. Thus, even with lower EL efficiencies, considerable attention is still directed to fully organic fluorophores, particularly for blue-emitting devices with high color purity and long-term stability. Received: August 9, 2017 Revised: August 28, 2017 Published: August 29, 2017 8012
DOI: 10.1021/acs.chemmater.7b03371 Chem. Mater. 2017, 29, 8012−8020
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Among the widely used arylamine-type D building units, spiroacridan derivatives are particularly effective and were reported to show superior TADF properties in our previous study.25,31−33,37 In contrast to the diverse variety of possible D units, A building units for highly efficient blue TADF emitters are still limited to benzonitriles,27 triazines,19,24,26,34 pyrimidines,32,33,38 sulfones,18,29,30 and phenylboranes,25,28,31,39 constraining the diversity of molecular design. Therefore, a more versatile design of A units becomes imperative for further expansion of the blue TADF family. Recently, we,17,37,40−42 Cheng et al.,43−45 Su and Tang et al.,46,47 and other groups35,48,49 explored TADF emitters bearing a carbonyl group such as benzophenone (BP) and xanthone (XT) derivatives as A units. OLEDs fabricated with BP- and XTbased TADF emitters showed excellent EL performance with maximum external quantum efficiencies (ηext) exceeding 18%. However, the BP- and XT-based emitters combining with (spiro)acridan-based donors displayed turquoise to green EL emissions centered around 490−510 nm with CIE(x,y) chromaticity coordinates of (0.18−0.26, 0.40−0.55),37,40 far from the deep-blue region. As can be seen from Figure 2, owing
Recently, third-generation OLEDs based on metal-free thermally activated delayed fluorescence (TADF) emitters that can realize ∼100% exciton utilization have been recognized as attractive alternatives to phosphorescent systems.10,16−23 By virtue of the considerably small S1−T1 energy splitting (ΔEST), TADF emitters enable efficient upconversion of nonradiative T1 excitons to radiative S1 excitons via reverse intersystem crossing (RISC) and hence show highly efficient EL emission without the use of precious metals. Although numerous TADF emitters have been developed and tested as RGB-emitting OLEDs since 2012, there are only a few reports on highperforming blue TADF emitters which meet high external EL quantum efficiency (ηext) over 20% as well as suitable color purity (with Commission Internationale de l’Éclairage (CIE) chromaticity coordinates of x < 0.2 and y < 0.2).18,24−35 Hence, a rational design strategy for efficient blue TADF emitters is desired. Furthermore, most blue TADF-OLEDs reported so far suffer from severe efficiency roll-off at high current density. This roll-off stems from exciton annihilation processes, including triplet−triplet annihilation (TTA) and singlet−triplet annihilation (STA), which are primarily caused by the relatively long excited-state lifetimes of the blue TADF emitters.36 Accordingly, suppression of efficiency roll-off remains another important challenge for the production of high-performance blue TADF-OLEDs. In this paper, we report a novel series of high-performance blue TADF emitters (Figure 1) combining isobenzofuranone
Figure 1. Molecular structures of BF- and CM-based blue TADF emitters: MXAc-BF, MXAc-CM, and XAc-CM. Their common electron-donating and withdrawing substructures are displayed in red and blue, respectively.
(BF) or chromone (CM) as new electron-acceptor (A) units with spiro[acridan-9,9′-xanthene] electron-donor (D) units. These materials exhibit bright blue TADF emissions centered around 460−485 nm with high photoluminescence quantum yields (ΦPL = 53−92%) and very short delayed fluorescence lifetimes (τd = 2.8−4.3 μs) in doped thin films and nondoped neat films. OLEDs fabricated using these blue TADF emitters retain high EL efficiencies over a broad brightness range, demonstrating slight efficiency roll-off. This study will shed light on the design guidelines for high-performance blueemitting materials and devices that can promote the practical applications of TADF technology.
Figure 2. Comparison of intrinsic electronic properties of various carbonyl-appended aromatics, including anthraquinone (AQ), coumarin (CO), benzophenone (BP), xanthone (XT), chromone (CM), and isobenzofuranone (BF), calculated at the PBE0/6-31G(d) level.
to the relatively large π-conjugation systems and strong electron-accepting natures (or deep-lying LUMO energy levels) of BP and XT as well as the typical carbonyl-appended aromatic dyes (e.g., anthraquinone and coumarin), it is difficult to attain efficient deep-blue TADF emission when using them as the A building unit in combination with (spiro)acridan-based D units. Therefore, we designed BF and CM with contracted πconjugation as A units for present D−A systems with the intention of widening the HOMO−LUMO energy gap and raising the intrinsic T1 energy level for bluer TADF emission. To understand the electronic transition characteristics of MXAc-BF, MXAc-CM, and XAc-CM at the molecular level, the highest occupied and the lowest unoccupied natural transition orbitals (NTOs) for the S1 and T1 excitations were simulated using time-dependent density functional theory (TDDFT)
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RESULTS AND DISCUSSION Material Design and Synthesis. To minimize ΔEST and accelerate RISC, TADF emitters generally comprise pretwisted D−A π-electronic systems with strong intramolecular chargetransfer (ICT) characteristics, giving rise to a small spatial overlap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO).10,20−23 To simultaneously achieve deep-blue chromaticity and small ΔEST, the desired TADF emitters should be constructed by appropriate choice of weak D−A combinations. 8013
DOI: 10.1021/acs.chemmater.7b03371 Chem. Mater. 2017, 29, 8012−8020
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Chemistry of Materials calculations (Figure 3). All these molecules adopted nearly orthogonal D−A conformations in their optimized structures
Figure 4. (a) UV−vis absorption and PL spectra of MXAc-BF, MXAcCM, and XAc-CM in toluene (1 × 10−5 M). The inset represents a magnified view of the lower-energy ICT absorptions. (b) Photograph of PL emissions from the toluene solutions under UV irradiation at 365 nm.
the range of 350−400 nm, which are assigned to the ICT transition associated with electron transfer from the acridan moiety to the BF or CM unit (see Figure 3a). Upon photoexcitation with UV light, these solutions emitted strong blue PL with emission peaks (λPL) ranging from 449 to 468 nm. In comparison with that of XAc-CM, the PL emission of MXAc-CM in toluene was red-shifted by ∼20 nm because of the enhanced electron-donating ability of dimethyl-substituted MXAc relative to nonmethylated XAc. Additionally, introducing dimethyl substituents in the acridan moiety also provided large influence on the absolute PL quantum yield (ΦPL) of the resulting D−A molecule; the ΦPL values in deoxygenated toluene solutions increased in the order XAc-CM (43%) < MXAc-BF (64%) < MXAc-CM (74%).50 To further investigate the photophysical and TADF properties in solid guest−host systems, doped thin films of three emitters (guest dopants) in solid host matrixes of bis(diphenylphosphoryl)dibenzo[b,d]furan (PPF)51 were prepared by varying the dopant concentrations in the range of 25− 100 wt %. Due to the sufficiently high T1 energy of PPF (ET = 3.1 eV), backward excited-energy transfer from the T1 states of the guest emitters to that of the host material is effectively suppressed in this system. Intriguingly, as presented in Figure 5a, thin films containing MXAc-BF and MXAc-CM retained considerably high ΦPL values (surpassing 70%) at high dopant concentrations even at 100 wt % (for neat films), revealing suppressed concentration quenching behavior. The terminal bulky xanthene moiety on the spiro-fused D unit and the pretwisted overall molecular structure are beneficial in inhibiting intermolecular electronic interactions, which usually cause excited-energy loss in the condensed state. The calculated spin densities of the T1 states of the BF- and CM-based molecules did not distribute on the terminal xanthene moiety (see Figure 3b), which may contribute to a reduction in the short-range electron-exchange interactions for T1 excitons between adjacent molecules.37 The detailed photophysical parameters obtained for the emitter:PPF doped thin films with different dopant concentrations are collected in Table 1. As exemplified in Figure 5b, the steady-state PL spectra of the 50 wt %-emitter:PPF doped films exhibited structureless intense blue emissions solely from the dopant emitters, similar to those in dilute solutions, even though their λPL positions were slightly red-shifted (within 15 nm) with respect to those in toluene solutions. From the
Figure 3. (a) Highest occupied and lowest unoccupied natural transition orbitals (HONTOs and LUNTOs) for the S1 state and arrangements of the S1 and T1 energy levels with oscillator strengths (f) for MXAc-BF, MXAc-CM, and XAc-CM. (b) Spin density distributions of their T1 states calculated at the PBE0/6-31G(d) level.
with dihedral angles of ∼88° between the BF (or CM) and the adjacent acridan moiety. Because of their highly twisted geometries and obvious ICT characters, the highest occupied NTOs are predominantly localized on the electron-donating acridan moiety but hardly on the peripheral spiro-fused xanthene moiety; the lowest unoccupied NTOs are thoroughly distributed over the electron-accepting BF or CM unit. This clear separation of NTOs results in small calculated ΔEST values of 0.04−0.10 eV, allowing for efficient RISC. Moreover, the calculated S1 energies are 2.65−2.84 eV, which are higher than that estimated for the corresponding green-emitting XTbased TADF molecule XAc-XT (2.58 eV).37 We can thus anticipate bluer TADF emissions from these BF- and CMbased molecules. MXAc-BF, MXAc-CM, and XAc-CM were synthesized by Buchwald−Hartwig reactions between 5-bromoisobenzofuranone or 7-bromo-2-methylchromone and the corresponding D units, i.e., spiro[2,7-dimethylacridan-9,9′-xanthene] (MXAc) or spiro[acridan-9,9′-xanthene] (XAc), using Pd(OAc)2/P(tBu)3HBF4 as the catalyst. All final products were purified using temperature-gradient vacuum sublimation after column chromatography. The chemical structures of these materials were ascertained using NMR spectroscopy, mass spectrometry, and elemental analysis (see the Experimental Section and Supporting Information for details). As revealed by thermogravimetric analysis, MXAc-BF, MXAc-CM, and XAc-CM had high thermal stability with decomposition temperatures (Td) of 363, 360, and 370 °C at 5% weight loss, which possibly resulted from the incorporation of robust spiro-fused D units. Photophysical Properties. Figure 4 depicts the UV−vis absorption and photoluminescence (PL) spectra of dilute solutions of MXAc-BF, MXAc-CM, and XAc-CM in toluene. All molecules showed relatively weak and broad absorptions in 8014
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frequency factors; τp and τd are the lifetimes of the prompt and delayed decay components, respectively. While the prompt component with τp of 24−33 ns corresponds to conventional fluorescence (S1 → ground state, S0), the delayed component with τd of 2.8−4.3 μs originates from TADF involving ISC and RISC processes, followed by emission decay (S1 → T1 → S1 → S0). Their delayed decay components gradually intensified with increasing temperature from 10 to 300 K (Supporting Information), testifying the typical TADF feature. It is worth noting that the TADF lifetimes for all new BF- and CM-based emitters were remarkably shorter than those of most existing blue TADF emitters, including 1,2-bis(carbazole-9-yl)-4,5dicyanobenzene10 (2CzPN; τd = 280 μs in the PPF host). Their reduced τd could be attributed to the accelerated RISC (T1 → S1 upconversion) process. To gain deeper insight into exciton upconversion through RISC, we evaluated the rate constants for RISC (kRISC) using eq 1:10 kRISC =
Φd k pkd Φp kISC
(1)
where Φp and Φd are fractional quantum yields of the prompt and delayed fluorescence components, respectively, which can be derived from each transient PL decay curve (see Figure 5c); kp and kd are the rate constants of prompt and delayed fluorescence, respectively, which can be experimentally determined by equations kp = 1/τp and kd = 1/τd, respectively; and kISC is the rate constant of ISC (S1 → T1), which can be estimated by kISC = (1 − Φp)kp. The calculated kRISC values for MXAc-BF and MXAc-CM (7.5 × 105 and 6.6 × 105 s−1, respectively) are slightly higher than that of XAc-CM (4.9 × 105 s−1), presumably due to their smaller ΔEST values. More importantly, kRISC values of these three emitters are 2 orders of magnitude higher than that of 2CzPN (kRISC ∼ 103 s−1). This result unambiguously indicates that RISC can be drastically accelerated in BF- and CM-based blue TADF molecules featuring a simple cyclic ketone or ester (lactone) constituent. Electroluminescence Performance. Blue TADF emitters that can simultaneously achieve high ΦPL, large kRISC, and low
Figure 5. (a) Dependence of overall PL quantum yields (ΦPL) on dopant concentration in emitter:PPF doped thin films; the inset shows the molecular structure of PPF. (b) Steady-state PL spectra and (c) transient PL decay profiles of 50 wt %-emitter:PPF doped thin films measured at 300 K.
energy differences between the onsets of the respective timeresolved fluorescence and phosphorescence spectra measured at 10 K, ΔEST values were experimentally determined to be 0.08 eV for MX-Ac-BF and MXAc-CM and 0.11 eV for XAc-CM (Table 1 and Supporting Information), which are in accord with the TDDFT results (see Figure 3a). Figure 5c depicts transient PL decay curves of 50 wt %-emitter:PPF doped films at room temperature (300 K), which can be fitted by two exponential decay components as IPL(t) = A1exp(−t/τp) + A2exp(−t/τd), where A1 and A2 are
Table 1. Photophysical Data for BF- and CM-Based Blue TADF Emitters in Thin Films emitter MXAc-BF
MXAc-CM
XAc-CM
conca (wt %)
λPLb (nm)
ΦPLc (%)
Φpd (%)
Φdd (%)
τpe (ns)
τde (μs)
kRISCf (105 s−1)
HOMOg (eV)
LUMOh (eV)
ESi (eV)
ETi (eV)
ΔESTj (eV)
25 50 75 100 25 50 75 100 25 50 75 100
472 478 478 478 480 484 483 482 454 462 461 461
92 88 80 78 79 77 74 71 76 74 68 53
27 27 27 27 35 35 35 35 32 32 32 32
65 61 53 51 44 42 39 36 44 42 36 22
24 24 24 24 33 33 33 33 25 25 25 25
4.3 4.1 3.9 3.8 2.9 2.8 2.8 2.8 4.0 3.9 3.7 2.8
7.7 7.5 6.9 6.8 6.7 6.6 6.1 5.7 5.1 4.9 4.5 3.6
−5.44
−2.48
2.98
2.90
0.08
−5.56
−2.77
2.95
2.87
0.08
−5.73
−2.87
3.11
3.00
0.11
a Dopant concentration in emitter:PPF codeposited thin films. bPL emission maximum measured at 300 K. cAbsolute PL quantum yield evaluated using an integrating sphere under N2 at 300 K. dFractional quantum yields for prompt fluorescence (Φp) and delayed fluorescence (Φd) components; Φp + Φd = ΦPL. ePL lifetimes for prompt fluorescence (τp) and delayed fluorescence (τd) components measured under N2 at 300 K. f Rate constant of RISC (T1 → S1). gDetermined using photoelectron yield spectroscopy in a neat film. hLUMO = HOMO + Eg, in which the optical energy gap (Eg) was derived from the absorption onset of the neat film. iLowest excited singlet (ES) and triplet (ET) energies estimated from onset wavelengths of the time-resolved fluorescence and phosphorescence spectra, respectively, for 50 wt %-emitter:PPF doped film at 10 K. jSinglet− triplet energy splitting determined experimentally using ΔEST = ES − ET.
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Figure 6. (a) Energy level diagram for the blue TADF-OLEDs employing MXAc-BF, MXAc-CM, and XAc-CM as emitters. (b) EL spectra measured at 10 mA cm−2 (left) with corresponding emission colors on the CIE 1931 chromaticity diagram and photos of blue EL emissions from the devices with 50 wt % doped films as the EML. (c) Current density−voltage−luminance (J−V−L) characteristics and (d) external EL quantum efficiency (ηext) vs L plots for the blue TADF-OLEDs fabricated with different dopant concentrations.
voltage were observed in the range of 4−8 V. The XAc-CMbased device achieved EL emission with a peak at the shortest wavelength (λEL = 462 nm) and the CIE coordinates (0.15, 0.19), corresponding to deep-blue light. The EL spectrum of the MXAc-BF-based device peaked at 478 nm with the CIE coordinates (0.17, 0.29), and its pattern was nearly identical to that of the corresponding MXAc-CM-based device. Figures 6c and d show current density−voltage−luminance (J−V−L) and external EL quantum efficiency−luminance (ηext−L) characteristics, respectively, of TADF-OLEDs fabricated by varying the dopant concentration, and their key EL parameters are compiled in Table 2. Intriguingly, the MXAc-BF-based devices turn on at the lowest bias voltage of 3.0 V and perform well with high maximum ηext of 16−17% and maximum power efficiencies of 31−33 lm W −1 , irrespective of dopant concentration (even for the nondoped EML). In addition, the efficiency roll-offs of the MXAc-BF-based devices are remarkably low, retaining high ηext over 12% even when driven at a high practical luminance of 1000 cd m−2 (for display applications). Meanwhile, devices with 50 wt % dopant (MXAc-CM and XAc-BF) concentration were found to outperform those with other concentrations (25 and 100 wt %) from an overall perspective. With 50 wt % dopant concentration, the MXAc-CM- and XAc-CM-based devices
concentration quenching are particularly attractive for developing nondoped, host-free blue TADF-OLEDs in addition to heavily doped devices. To evaluate the EL performances of BFand CM-based blue TADF emitters, multilayer OLEDs were fabricated using the emitter:PPF doped films with various dopant concentrations as well as nondoped neat films as emission layers (EMLs). As illustrated in Figure 6a, the device configuration was indium−tin−oxide (ITO, 100 nm)/HATCN (10 nm)/α-NPD (40 nm)/CCP (5 nm)/EML (20 nm)/ PPF (10 nm)/TPBi (30 nm)/Liq (1 nm)/Al (80 nm). In this architecture, HAT-CN (2,3,6,7,10,11-hexacyano-1,4,5,8,9,12hexaazatriphenylene) and α-NPD (4,4′-bis-[N-(1-naphthyl)N-phenylamino]-1,1′-biphenyl) act as hole-injection layer and hole-transporting layer (HTL), respectively, whereas TPBi (1,3,5-tris(N-phenylbenzimidazol-2-yl)benzene) and Liq (8hydroxyquinoline lithium) serve as electron-transporting layer (ETL) and electron-injection material, respectively. Additionally, to suppress unfavorable exciton quenching at the HTL/ EML and EML/ETL interfaces, thin layers of CCP25 (9phenyl-3,9′-bicarbazole) and PPF51 with high ET of 3.0 and 3.1 eV, respectively, were incorporated as exciton-blocking layers. The EL spectra of the fabricated devices contained pure emissions from the incorporated TADF emitters, as exhibited in Figure 6b; no significant changes in spectra with applied 8016
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in Figure 7, the curves fit using eq 4 are in reasonable agreement with the experimental ηext−J data, demonstrating
Table 2. EL Performance of Blue TADF-OLEDs emittera
MXAc-BF
MXAc-CM
XAc-CM
λELb (nm) Vonc (V) ηext,maxd (%) ηext,100e (%) ηext,1000e (%) ηpf (lm W1−) CIEg (x, y)
478 3.0 16.2 15.7 12.0 31.5 (0.17, 0.29)
478 3.3 15.0 14.6 11.1 25.7 (0.16, 0.29)
462 3.6 12.1 11.6 7.1 14.7 (0.15, 0.19)
a Device configuration: ITO/HAT-CN (10 nm)/α-NPD (40 nm)/ CCP (5 nm)/50 wt %-emitter:PPF (20 nm)/PPF (10 nm)/TPBi (30 nm)/Liq (1 nm)/Al (100 nm). bEL emission maximum at 10 mA cm−2. cTurn-on voltage at a luminance over 1 cd m−2. dMaximum external EL quantum efficiency. eExternal EL quantum efficiency driven at 100 and 1000 cd m−2. fMaximum power efficiency. g Commission Internationale de l’É clairage (CIE) chromaticity coordinates recorded at 10 mA cm−2.
gave ηext values of up to 15.0 and 12.1%, respectively, with lower efficiency roll-off. The lower EL efficiencies of the MXAcCM- and XAc-CM-based devices can be attributed to their lower ΦPL (or Φd) values than those of MXAc-BF in the thin films (see Figure 5a and Table 1). Analysis of Efficiency Roll-Off. In general, efficiency rolloff is serious for TADF-OLEDs, particularly for blue-emitting devices. Because of the long lifetime of electrogenerated T1 excitons ranging from several tens of microseconds to milliseconds, triplet−triplet annihilation (TTA)36,52 is typically a major factor causing efficiency roll-off in TADF-OLEDs. Another possible mechanism, singlet−triplet annihilation (STA),36 can also influence roll-off behavior. Considering TTA and STA, the singlet and triplet exciton densities (NS and NT, respectively) in TADF-OLEDs upon electrical excitation can be expressed as53,54 dNS = −(k rS + kISC)NS + kRISCNT − k STANSNT dt J + αk TTANT2 + 4de
(2)
dNT = kISCNS − (kRISC + k nrT)NT dt 3J − (1 + α)k TTANT2 + 4de
(3)
Figure 7. (a) ηext−J characteristics of TADF-OLEDs employing MXAc-BF, MXAc-CM, XAc-CM, and 2CzPN (for reference) as emitters. The solid lines are the fitted results based on the TTA−STA model. (b) Triplet exciton density (NT) as a function of J for each device, calculated using the eqs 2 and 3
that roll-off in these TADF-OLEDs was primarily caused by TTA and STA processes. In spite of their suppressed roll-off characteristics, both kTTA and kSTA values estimated for the BFand CM-based devices were comparable or slightly larger than those obtained for 2CzPN (kTTA ∼ 1 × 10−14 cm3 s−1 and kSTA ∼ 2 × 10−11 cm3 s−1; Supporting Information). Given similar krS and kISC values for all these materials (1−3 × 107 s−1 and 2− 3 × 107 s−1, respectively; Supporting Information), it is assumed that kRISC is the most prominent factor affecting the degree of device efficiency roll-off. As described above, the kRISC values of MXAc-BF, MXAc-CM, and XAc-CM were two orders of magnitude larger than that of 2CzPN. Thus, for the current BF- and CM-based devices, the larger kRISC makes it possible to decrease NT upon electrical excitation (Figure 7b) and suppress the competing exciton deactivation processes to realize extremely low efficiency roll-off in blue TADF-OLEDs.
where krS state, knrT
is the rate constant for radiative decay from the S1 is the rate constant for nonradiative decay from the T1 state, kTTA and kSTA are the rate constants of TTA and STA, respectively, α is the singlet exciton production ratio via TTA (α = 0.25), J is the injected current density, d is the recombination zone thickness (assumed 10 nm), and e is the elementary charge. For steady-state conditions (i.e., dNS/dt = 0 and dNT/dt = 0), ηext can be described as ηext(J ) = ηext,max
NS(t = ∞ , J ) NS0
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CONCLUSIONS In this study, isobenzofuranone and chromone bearing an electron-withdrawing cyclic ketone or lactone moiety were introduced for the first time as the effective acceptor building units for producing efficient blue TADF emitters. Owing to their rather small ΔEST (0.08−0.11 eV), properly contracted πconjugation, and weakened ICT character, these emitters successfully achieved blue TADF emissions (λPL = 461−484 nm) with simultaneous high PL quantum yields (ΦPL = 53− 92%) and notably short TADF emission lifetimes (τd = 2.8−4.3 μs) in thin films. Blue TADF-OLEDs fabricated using these emitters exhibited impressive ηext of up to 16.2%. While the maximum ηext values were still limited by their moderate ΦPL values in heavily doped (or neat) films, remarkably reduced efficiency roll-off was attained in these devices, retaining ηext as
(4)
where ηext,max denotes the maximum external EL quantum efficiency and NS0 represents singlet exciton density without TTA and STA (i.e., kTTA = kSTA = 0). To analyze the difference in roll-off characteristics, this TTA−STA model was applied to the present BF- and CM-based devices as well as the 2CzPNbased reference device with the same configuration. As shown 8017
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Chemistry of Materials high as 12.0% at practical high luminance (1000 cd m−2). Excellent device performance with reduced roll-off characteristics possibly resulted from their high RISC rates (kRISC > 105 s−1), which contributed to suppressing the competing exciton deactivation processes such as TTA and STA under electrical excitation. By utilizing the molecular design concept presented here and further enhancing intrinsic PL properties, outstanding deep-blue TADF emitters can be rationally designed and synthesized for future OLED applications.
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under reduced pressure. The product was purified by column chromatography on silica gel (eluent: hexane/CHCl3 = 1/1, v/v) and dried under vacuum to produce a light yellow solid (yield = 3.8 g, 68%). 1H NMR (400 MHz, CDCl3): δ 8.04 (d, J = 8.4 Hz, 1H), 7.62 (d, J = 2.0 Hz, 1H), 7.50 (dd, J = 8.4 Hz, 2.0 Hz, 1H), 6.17 (s, 1H), 2.38 (s, 3H). MS (MALDI-TOF): m/z calcd 237.96 [M]+; found 238.37. Synthesis of MXAc-BF. To a solution of 5-bromophthalide (2.70 g, 12.7 mmol) and spiro[2,7-dimethylacridan-9,9′-xanthene] (4.76 g, 12.7 mmol) in dry toluene (150 mL) were added Pd(OAc)2 (0.09 g, 0.40 mmol), P(t-Bu)3HBF4 (0.55 g, 1.90 mmol), and K2CO3 (5.26 g, 38.1 mmol). The mixture was stirred for 5 days at 90 °C under N2. After cooling to room temperature, the reaction mixture was poured into water and then extracted with CHCl3. The combined organic layers were washed with water and dried over anhydrous Na2SO4. After filtration and evaporation, the product was purified by column chromatography on silica gel (eluent: hexane/CHCl3/ethyl acetate = 7/2/1, v/v), recrystallized from CHCl3/hexane, and dried under vacuum to afford MXAc-BF as a light yellow solid (yield = 3.79 g, 59%). Mp 343 °C. 1H NMR (500 MHz, CDCl3): δ 8.24 (d, J = 7.9 Hz, 1H), 7.63 (dd, J = 7.9, 1.5 Hz, 1H), 7.60 (s, 1H), 7.20−7.16 (m, 4H), 7.11 (dd, J = 7.9 Hz, 1.5 Hz, 2H), 6.96 (td, J = 6.9 Hz, 2.4 Hz, 2H), 6.68 (dd, J = 8.5 Hz, 1.7 Hz, 2H), 6.64 (d, J = 1.7 Hz, 2H), 6.09 (d, J = 8.5 Hz, 2H), 5.46 (s, 2H), 2.02 (s, 6H). 13C NMR (125 MHz, CDCl3): δ 170.05, 149.40, 148.48, 147.55, 136.63, 132.88, 132.79, 131.79, 131.22, 130.48, 129.71, 128.67, 127.89, 127.61, 125.67, 125.61, 123.64, 116.01, 113.73, 69.36, 44.63, 20.47. MS (MALDI-TOF): m/z calcd 507.18 [M]+; found 507.21. Anal. Calcd (%) for C38H25NO2: C 82.82, H 4.96, N 2.76; found: C 82.74, H 4.93, N 2.82. Synthesis of MXAc-CM. The syntheses of MXAc-CM and XAc-CM are outlined in Scheme 1. To a solution of Br-CM (1.00 g, 4.18 mmol) and spiro[2,7-dimethylacridan-9,9′-xanthene] (1.57 g, 4.18 mmol) in dry toluene (50 mL) were added Pd(OAc)2 (0.03 g, 0.13 mmol), P(tBu)3HBF4 (0.18 g, 0.62 mmol), and K2CO3 (1.73 g, 12.5 mmol). The mixture was stirred for 5 days at 90 °C under N2. After cooling to room temperature, the reaction mixture was poured into water and then extracted with CHCl3. The combined organic layers were washed with water and dried over anhydrous Na2SO4. After filtration and evaporation, the product was purified by column chromatography on silica gel (eluent: hexane/CHCl3/ethyl acetate = 5/2/1, v/v), recrystallized from CHCl3/hexane, and dried under vacuum to afford MXAc-CM as a light yellow solid (yield = 1.16 g, 52%). Mp 306 °C. 1 H NMR (500 MHz, CDCl3): δ 8.48 (d, J = 8.3 Hz, 1H), 7.56 (d, J = 1.8 Hz, 1H), 7.45 (dd, J = 8.3 Hz, 1.8 Hz, 1H), 7.22−7.16 (m, 4H), 7.13 (dd, J = 7.9 Hz, 1.5 Hz, 2H), 6.97 (td, J = 7.0 Hz, 2.2 Hz, 2H), 6.68 (dd, J = 8.4 Hz, 1.5 Hz, 2H), 6.63 (d, J = 1.5 Hz, 2H), 6.28 (s, 1H), 6.16 (d, J = 8.4 Hz, 2H), 2.45 (s, 3H), 2.01 (s, 6H). 13C NMR (125 MHz, CDCl3): δ 177.55, 166.53, 158.09, 148.47, 146.52, 136.54, 132.69, 131.85, 131.25, 130.36, 129.65, 128.69, 128.27, 127.89, 127.56, 123.66, 123.41, 120.99, 115.95, 113.81, 110.99, 44.63, 20.63, 20.48. MS (MALDI-TOF): m/z calcd 533.20 [M]+; found 532.90. Anal. Calcd (%) for C38H23NO2: C 83.28, H 5.10, N 2.62; found: C 83.36, H 5.02, N 2.66. Synthesis of XAc-CM. XAc-CM was synthesized according to the same procedure as described above for the synthesis of MXAc-CM, except that spiro[acridane-9,9′-xanthene] (1.74 g, 5.00 mmol) was used as the reactant instead of spiro[2,7-dimethylacridan-9,9′xanthene] (see Scheme 1), yielding XAc-CM as a light yellow solid (yield = 1.05 g, 50%). Mp 366 °C. 1H NMR (500 MHz, CDCl3): δ 8.51 (d, J = 8.3 Hz, 1H), 7.59 (d, J = 1.8 Hz, 1H), 7.48 (dd, J = 8.3 Hz, 1.8 Hz, 1H), 7.21−7.16 (m, 4H), 7.14 (dd, J = 7.6 Hz, 1.3 Hz, 2H), 6.97 (td, J = 7.0 Hz, 2.2 Hz, 2H), 6.91−6.87 (m, 4H), 6.72 (td, J = 7.6 Hz, 1.2 Hz, 2H), 6.30−6.25 (m, 3H), 2.45 (s, 3H). 13C NMR (125 MHz, CDCl3): δ 177.48, 166.59, 158.13, 148.48, 145.94, 138.44, 132.54, 131.80, 131.19, 129.86, 128.85, 128.21, 127.70, 127.04, 123.68, 123.59, 121.45, 120.98, 116.03, 113.93, 111.03, 44.64, 20.64. MS (MALDI-TOF): m/z calcd 505.17 [M]+; found 504.99. Anal. Calcd (%) for C29H23NO2: C 83.15, H 4.59, N 2.77; found: C 83.11, H 4.56, N 2.80.
EXPERIMENTAL SECTION
Materials and Methods. Commercially available reagents and solvents were used as received unless otherwise noted. 5Bromoisobenzofuranone (or 5-bromophthalide) was purchased from Tokyo Chemical Industry Co., Ltd. (TCI). Spiro[2,7-dimethylacridan9,9′-xanthene] (MXAc),33 spiro[acridan-9,9′-xanthene] (XAc),25 2,8bis(diphenylphosphoryl)dibenzo[b,d]furan (PPF),51 and 9-phenyl3,9′-bicarbazole (CCP)25 were prepared according to the literature procedures. 2,3,6,7,10,11-Hexacyano-1,4,5,8,9,12-hexaazatriphenylene (HAT-CN) was donated by the Nippon Soda Co., Ltd. and purified using vacuum sublimation before use. Other OLED materials were purchased from e-Ray Optoelectronics Technology Co., Ltd. and used for the device fabrication without further purification. NMR spectra were recorded on an Avance III 500 or III 400 spectrometer (Bruker). 1H and 13C NMR chemical shifts were determined relative to the signals of tetramethylsilane (δ = 0.00) and CDCl3 (δ = 77.0), respectively, as internal standards. Matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF) mass spectra were collected on an Autoflex III spectrometer (Bruker Daltonics) using dithranol as a matrix. Elemental analyses were carried out using an MT-5 analyzer (Yanaco). Density functional theory (DFT) calculations were performed using the Gaussian 09 program package. The molecular geometries in the ground state were optimized using the PBE0 functional with the 6-31G(d) basis set in the gas phase. The lowest singlet and triplet excited states were calculated using the optimized structures with time-dependent DFT (TDDFT) at the same level. Synthesis of 7-Bromo-2-methylchromone (Br-CM). Br-CM was synthesized according to Scheme 1. To a stirred suspension of sodium
Scheme 1. Synthesis of the CM-Based Blue TADF Emitters
hydride (2.3 g, 95 mmol) in dry THF (10 mL) was added dropwise a solution of 4-bromo-2-hydroxyacetophenone (5.1 g, 24 mmol) and ethyl acetate (5.1 g, 58 mmol) in dry THF (10 mL) under N2. The mixture was allowed to react for 10 min at room temperature and was then quenched by pouring into a large amount of ice water. After acidification with aqueous HCl to pH ∼6, the product was extracted with ethyl acetate. The combined organic layers were washed with water and dried over anhydrous MgSO4. After filtration and evaporation, a crude product of 1-(4-bromo-2-hydroxyphenyl)butan1,3-dione was obtained, which was used in the next step without further purification. This compound was dissolved in a mixture of methanol (30 mL) and conc HCl (36%, 1 mL). After being stirred for 14 h at room temperature, the reaction mixture was concentrated 8018
DOI: 10.1021/acs.chemmater.7b03371 Chem. Mater. 2017, 29, 8012−8020
Article
Chemistry of Materials Photophysical Measurements. UV−vis absorption and PL spectra were measured with a V-670 spectrometer (Jasco) and a FP8600 spectrophotometer (Jasco), respectively, using degassed spectral grade solvents. The absolute PL quantum yields were determined using an ILF-835 integrating sphere system (Jasco). The transient PL decay measurements were carried out using a C11367 Quantaurus-tau fluorescence lifetime spectrometer (Hamamatsu Photonics; λ = 340 nm, pulse width = 100 ps, and repetition rate = 20 Hz) under N2, and a C9300 streak camera (Hamamatsu Photonics) with an N2 gas laser (λ = 337 nm, pulse width = 500 ps, and repetition rate = 20 Hz) under vacuum (
