Strategy for Designing Electron Donors for Thermally Activated

Dec 22, 2014 - Thermally activated delayed fluorescence (TADF) emitters are promising dopants for organic light-emitting diodes, including those conta...
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Strategy for Designing Electron Donors for Thermally Activated Delayed Fluorescence Emitters Katsuyuki Shizu,†,‡,§ Hiroyuki Tanaka,† Motoyuki Uejima,∥ Tohru Sato,∥,⊥ Kazuyoshi Tanaka,∥ Hironori Kaji,‡ and Chihaya Adachi*,†,§,# †

Center for Organic Photonics and Electronics Research (OPERA) and #International Institute for Carbon Neutral Energy Research (WPI-I2CNER), Kyushu University, 744 Motooka, Nishi, Fukuoka 819-0395, Japan ‡ Institute for Chemical Research, Kyoto University, Uji, Kyoto 611-0011, Japan § Japan Science and Technology Agency (JST), ERATO, Adachi Molecular Exciton Engineering Project, 744 Motooka, Nishi, Fukuoka 819-0395, Japan ∥ Department of Molecular Engineering, Graduate School of Engineering and ⊥Unit of Elements Strategy Initiative for Catalysts & Batteries, Kyoto University, Kyoto 615-8510, Japan S Supporting Information *

ABSTRACT: Thermally activated delayed fluorescence (TADF) emitters are promising dopants for organic light-emitting diodes, including those containing highly twisted donor− acceptor-type structures. However, highly twisted structures limit the variety of chemical structures applicable as TADF emitters. We present a strategy for designing electron donors that can eliminate this requirement and increase the structural diversity of TADF emitters. Using this strategy, we developed an electron donor containing carbazolyl and diphenylamino groups by carefully controlling its electron-donating ability. By combining this donor with a quinoxaline-based acceptor, we obtained the efficient green TADF emitter, N3,N3,N6,N6-tetraphenyl-9-(4-(quinoxalin-6-yl)phenyl)-9H-carbazole-3,6-diamine (DACQ), without a highly twisted structure. DACQ exhibits high photoluminescence and electroluminescence efficiencies, comparable to those of a highly twisted TADF emitter containing the same electron-accepting unit. Quantum chemical calculations showed that the diphenylamino groups within the carbazolyl moiety effectively withdraw the HOMO distribution. This reduces the singlet−triplet energy gap, thus inducing TADF. The photophysical properties of TADF compounds depend on the twisting angle between the electron-donating and accepting units. Eliminating the highly twisted structure increases the diversity of potential TADF emitters and allows their photophysical properties to be controlled by changing the twisting angle.

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INTRODUCTION Thermally activated delayed fluorescence (TADF) emitters have attracted much attention because they can effectively convert triplet excitons into singlet excitons. TADF emitters can improve the electroluminescence (EL) efficiency of organic light-emitting diodes (OLEDs) using conventional carbonbased aromatic compounds. The EL efficiency of TADF-based OLEDs is now close to the theoretical maximum, and a paradigm shift from phosphorescence to TADF has occurred in OLED design.1 TADF emitters can realize high EL efficiency in the absence of rare metals such as platinum and iridium2−4 and so are promising alternatives to phosphorescent emitters. The optimum molecular design for TADF emitters is not well understood. Guidelines for rational molecular design are required for the practical application of TADF-based OLEDs, such as in flexible flat-panel displays5 and solid-state lighting.6,7 TADF emitters usually consist of electron-donating and -accepting moieties, and their excited states are of a chargetransfer (CT) character. The key process of TADF involves reverse intersystem crossing (RISC) from the lowest triplet © 2014 American Chemical Society

state (T1) to the lowest excited singlet state (S1) and radiative decay from S1 to the ground state (S0). Thus, TADF efficiency depends largely on the efficiency of S1 ← T1 RISC and S1 → S0 radiative decay. The RISC rate increases with a decreasing energy difference between S1 and T1 (ΔEST),8 so minimizing ΔEST more effectively generates TADF. Separating the spatial distribution of the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) is an effective approach for minimizing ΔEST when designing TADF emitters. By combining various electron donors and acceptors, we have developed TADF emitters with wellseparated HOMO and LUMO distributions and resulting small ΔEST values.9−21 The CT character of excited states of TADF emitters originates from electronic transition between their spatially separated HOMO and LUMO. Since the late 1990s, such intramolecular donor−acceptor compounds have been Received: November 5, 2014 Revised: December 17, 2014 Published: December 22, 2014 1291

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influenced by α. For a small α, S1 → S0 radiative decay occurs efficiently, but S1 ← T1 RISC is suppressed because of the large ΔEST. For a large α, S1 → S0 radiative decay is suppressed, but S1 ← T1 RISC occurs efficiently because of the small ΔEST. This trade-off has led to the molecular design of TADF being largely focused on minimizing ΔEST and inducing S1 ← T1 RISC, while retaining a moderate S1 → S0 radiative decay rate. In the current study, we present a strategy for designing electron donors that simultaneously allow efficient S1 ← T1 RISC and S1 → S0 radiative decay. We report an electron donor containing carbazole and two diphenylamino groups (N3,N3,N6,N6-tetraphenyl-9H-carbazole-3,6-diamine, denoted DAC-II), which possesses controlled electron-donating ability. DAC-II is combined with a quinoxaline-based acceptor to give the green TADF emitter, N3,N3,N6,N6-tetraphenyl-9-(4(quinoxalin-6-yl)phenyl)-9H-carbazole-3,6-diamine (DACQ, Figure 1a). DACQ exhibits high photoluminescence and

reported to act as an effective emitter in electricity-to-light conversion and offer highly efficient electrochemiluminescence systems.22−24 A large twisting angle (α) between the electron-donating and -accepting units allows sufficient spatial separation of the HOMO and LUMO to realize a small ΔEST. Steric hindrance can be exploited to induce a large α. Examples of twisted TADF compounds include indolocarbazole,9,10 phenoxazine,11−13 phenothiazine,14 and spirofluorene derivatives.15−17 Phenoxazine is easily synthesized and has been used as an electrondonating unit for donor−acceptor-type TADF emitters. Phenoxazine has electron-donating N and O atoms and exhibits strong electron-donating ability. When an electronaccepting unit is introduced at the N atom position of phenoxazine, its central six-membered ring often induces a large α (approximately 80°) between phenoxazine and the electronaccepting unit. The resulting twisted structure prevents orbital interaction between fragment molecular orbitals of the electron-donating and accepting units, leading to a spatially well-separated HOMO and LUMO. However, requiring a large α limits the variety of structures applicable as TADF emitters. α significantly influences the photophysical properties of compounds containing low-lying CT excited states, such as TADF emitters, including the rates of S1 → S0 radiative and nonradiative decay, denoted kr and knr, respectively. kr and knr are related to the transition dipole moment (μ10) and vibronic coupling constants (V10m where m denotes the mth vibrational mode) between S0 and S1, respectively.25,26 kr increases with increasing |μ10|, while knr increases with increasing V10m. μ10 and V10m are expressed in terms of overlap density between the electronic wave functions of S0 and S1 (denoted ρ10).27,28 If x is a point in three-dimensional space, then μ10 is expressed as μ10 =

∫ ρ10 (x)(−ex)dx

(1)

where e is the elementary charge. V10m is expressed as V10 m =

∫ ρ10 (x)vm(x)dx

vm(x) = −∑ A

ZA (m) x − RA eA MA |x − RA|3

(2) Figure 1. Chemical structure, HOMO, and LUMO of (a) DACQ, (b) PXZQ, and (c) CZQ. HOMOs and LUMOs were calculated at the M06-2X/cc-pVDZ level of theory.

(3)

where ZA, MA, and RA are the charge, mass, and position of the Ath nucleus, respectively. eA(m) is a three-dimensional vector representing the relative displacement of the Ath nucleus for the mth vibrational mode. From eqs 1−3, both μ10 (kr) and V10m (knr) are controlled by ρ10. When HOMO−LUMO excitation dominates the S0−S1 transition, ρ10 is approximately equal to the HOMO−LUMO overlap density (ρHOMO−LUMO).29 In such cases, kr and knr are significantly influenced by ρHOMO−LUMO. For a TADF emitter with a small α, the orbital interaction between the fragment HOMO of the electron-donating moiety and fragment orbitals of the electronaccepting moiety is strong. Thus, the HOMO extends over the electron-accepting moiety. Similarly, the LUMO extends over the electron-donating moiety. Consequently, the spatial overlap between the HOMO and LUMO is large, resulting in a large ρHOMO−LUMO or ρ10. This large spatial overlap increases kr, and usually improves the luminescence efficiency. An overly large α will prevent S0-S1 transition and decrease the luminescence efficiency. TADF emitters exhibit a trade-off between the efficiency of S1 ← T1 RISC and S1 → S0 radiative decay, which is largely

electroluminescence efficiencies, which are comparable to those of the phenoxazine-based highly twisted TADF emitter, 10-(4-(quinoxalin-2-yl)phenyl)-10H-phenoxazine (PXZQ, Figure 1b). Exploiting DAC-II as an electron-donating unit eliminates the reliance on highly twisted structures, and allows for flexible molecular design of TADF emitters. The photophysical properties of TADF emitters vary with α, so can be controlled by changing α. These findings open the pathway to efficient TADF emitters with a wide variety of chemical structures.

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RESULTS AND DISCUSSION Quantum chemical calculations were performed using the Gaussian 09 program package.30 The S0 geometries of DACQ and PXZQ were optimized at the M06-2X/cc-pVDZ level of theory.31,32 The electronic configurations of S1 and T1 of these compounds were computed with time-dependent density functional theory33,34 (TD-DFT) and the M06-2X/cc-pVDZ method, using the optimized S0 geometries (denoted as TD1292

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Figure 2. Overlap densities (ρ10) of (a) DACQ and (b) PXZQ, calculated at the TD-M06-2X/cc-pVDZ//M06-2X/cc-pVDZ level of theory. The isosurface value for ρ10 is 5 × 10−4 a.u. (yellow: positive; blue: negative).

Figure 3. Vibronic coupling constants (V10m) for (a) DACQ and (b) PXZQ, calculated at the TD-M06-2X/cc-pVDZ//M06-2X/cc-pVDZ level of theory. Vibrational modes with the largest V10m are shown in molecular structures.

donating moieties: one carbazolyl and two diphenylamino moieties. The outer diphenylamino moieties induce the separation of the HOMO and LUMO of DACQ by withdrawing the HOMO in the opposite direction to the electron acceptor, as indicated by green arrows in Figure 1a. This makes DAC-II a promising donor for TADF emitters. The α of DACQ is small, but the electron-donating ability of DACII is sufficiently strong to localize the HOMO on it. Thus, the resulting spatial overlap between the HOMO and LUMO is small, leading to a sufficiently small ΔEST to generate TADF. The spatial overlap between the HOMO and LUMO is large in the absence of the diphenylamino groups, and TADF would not be observed. To demonstrate this, we performed quantum chemical calculations on the compound consisting of carbazole and quinoxaline (CZQ, Figure 1c). The HOMO of CZQ is widely distributed over the acceptor moiety, especially on the phenyl ring connecting carbazole and quinoxaline (HOMO distribution within the orange circled region in Figure 1c). Consequently, although the LUMO distribution of CZQ is

M06-2X/cc-pVDZ//M06-2X/cc-pVDZ). The HOMO− LUMO excitation dominates the S0-S1 transition of DACQ and PXZQ (Table S1, Supporting Information), so ρ10 is approximately equal to ρHOMO−LUMO. In the calculation of ρ10, μ10, and V10m, electronic configurations with configuration interaction coefficients larger than 1.0 × 10−4 were included. The M06-2X functional was selected because by taking solvent effects into account within the polarizable continuum model35−37 it well reproduces the emission wavelengths of DACQ in toluene solution (Table S2, Supporting Information). For PXZQ, however, it overestimates the emission wavelength in toluene solution. Figure 1 shows the chemical structure, HOMO, and LUMO of DACQ and PXZQ. The quinoxaline-based electron acceptor forms CT excited states when combined with aromatic amines.38 α was calculated to be 50.0 and 77.9° for DACQ and PXZQ, respectively. PXZQ thus has a highly twisted structure. DAC-II, the donor unit of DACQ, has a strong electron-donating ability because it contains three electron1293

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Figure 4. Transient PL decay curves for (a) 6 wt % DACQ:DPEPO and (b) 6 wt % PXZQ:DPEPO films, measured at 50−300 K. Prompt and delayed fluorescence spectra for (c) 6 wt % DACQ:DPEPO and (d) 6 wt % PXZQ:DPEPO films, measured at 300 K.

transition alone. The calculated |μ10| of DACQ and PXZQ were 1.296 and 0.033 a.u., respectively (corresponding to 3.295 and 0.085 D, respectively). Hence, electronic excitations other than the HOMO−LUMO excitation contributes more to |μ10| in DACQ than in PXZQ, suggesting that the contribution from the mixing of locally excited states is larger for DACQ than for PXZQ. Thus, reducing α has an additional effect of increasing the mixing of local excitations with HOMO−LUMO excitation and increasing |μ10|. Like |μ10|, V10m increases with increasing ρ10. However, V10m is less sensitive to ρ10 than |μ10|. Figure 3a,b shows V10m of DACQ and PXZQ calculated on the basis of the TD-M06-2X/ cc-pVDZ//M06-2X/cc-pVDZ results. V10m values of DACQ are larger than those of PXZQ because ρ10 is larger for DACQ than for PXZQ. The difference in V10m values is small compared with that in |μ10|. For example, V10m values of DACQ are of the same order of magnitude as those of PXZQ, while the |μ10| of DACQ is approximately 70 times larger than that of PXZQ. The fact that |μ10| is more sensitive to ρ10 than V10m can be rephrased as kr being more sensitive to α than knr. This suggests that by decreasing α and promoting radiative decay, nonradiative decay can be suppressed in TADF emitters. Vibrational modes with the largest V10m (indicated with arrows) are shown in Figure 3a,b. For DACQ, a mode containing a C− C stretching vibration of the central phenyl ring has the largest V10m, reflecting that ρ10 of DACQ has a large value on the phenyl ring. For PXZQ, an out-of-plane bending mode localized on quinoxaline has the largest V10m, and stretching modes of quinoxaline have small V10m. This is because the ρ10 distribution on the quinoxaline moiety is predominantly localized on atoms. To investigate the photophysical properties of DACQ and PXZQ in a solid-state host matrix, 6 wt % DACQ-doped bis(2(diphenylphosphino)phenyl)ether oxide (DPEPO) and 6 wt % PXZQ-doped DPEPO films were fabricated by vacuum codeposition. The photoluminescence quantum yields (PLQYs, ΦPL) of DACQ- and PXZQ-doped films measured at room temperature in air are 74 and 66%, respectively. Under nitrogen flow, these increase to 84 and 86%, respectively. The

almost identical with that of DACQ, because the HOMO extends over the acceptor moiety, CZQ is expected to have a larger ΔEST than DACQ. In fact, at the TD-M06-2X/ccpVDZ//M06-2X/cc-pVDZ level, ΔEST of CZQ was calculated to be 0.77 eV, which is much larger than that of DACQ (0.60 eV), suggesting that S1 ← T1 RISC would be suppressed without the diphenylamino groups. For PXZQ, steric repulsion between the H atoms of phenoxazine and electron-accepting moiety (H atoms within the green circled region in Figure 1b) is responsible for the large α. The twisted structure prevents orbital interaction between fragment molecular orbitals of electron-donating and accepting moieties, leading to the well-separated HOMO and LUMO. The HOMO is strongly localized on the donor moiety, and the LUMO is strongly localized on the acceptor moiety, as shown in Figure 1b. The spatial separation of the HOMO and LUMO provides a weak exchange interaction between the electrons in the HOMO and LUMO. This results in a sufficiently small ΔEST to induce efficient S1 ← T1 RISC and generate TADF. Quantum chemical calculations reveal that the small α of DACQ is responsible for its large ρ10 and |μ10|. Figure 2 shows the calculated ρ10 based on the TD-M06-2X/cc-pVDZ//M062X/cc-pVDZ results for DACQ and PXZQ. ρ10 is larger for DACQ than for PXZQ because the smaller α of DACQ results in its larger ρHOMO−LUMO. For DACQ, ρ10 is distributed over the entire molecule, from the outmost phenyl rings of the quinoxaline and diphenylamino moieties. For PXZQ, the ρ10 distribution is limited to the central ring of the phenoxazine moiety, the phenyl ring connecting phenoxazine and quinoxaline, and the pyrazine ring of quinoxaline. |μ10| of DACQ and PXZQ were calculated to be 2.427 and 0.036 a.u., respectively (corresponding to 6.169 and 0.091 D, respectively). The larger |μ10| of DACQ results from the larger ρ10 distribution. The small α thus gives a large ρ10 and |μ10|, suggesting that the kr of TADF emitters can be increased by decreasing the α. To investigate intrinsic contribution from mixing of locally excited states to |μ10|, |μ10| was calculated by assuming that S1 is a pure CT state; in other words, S1 consists of the HOMO−LUMO 1294

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Figure 5. (a) PL spectra for 6 wt % DACQ:DPEPO and 6 wt % PXZQ:DPEPO films, measured at 300 K. (b) External quantum efficiency and current density characteristics for OLEDs containing DACQ and PXZQ as dopants. The inset shows EL spectra for the OLEDs.

This is true for PXZQ. The Φp and Φd of TADF emitters thus depend largely on their kr values (|μ10|), which are influenced by their ρHOMO−LUMO distributions. This result suggests that controlling the ρHOMO−LUMO is essential for controlling photophysical properties of TADF emitters. To evaluate the potential of DACQ and PXZQ as OLED dopants, OLEDs containing DACQ and PXZQ were fabricated with vacuum deposition. The configuration of the OLEDs is ITO (100 nm)/N,N′-diphenyl-N,N′-bis(1-naphthyl)-1,10-biphenyl-4,4′-diamine (α-NPD) (35 nm)/6 wt % TADF emitter: DPEPO (15 nm)/TPBi (65 nm)/LiF (0.8 nm)/Al (100 nm). Figure 5b shows EQE−current density curves. The maximum EQEs of the DACQ- and PXZQ-based devices are 12.8 and 10.4%, respectively, at a current density of 0.02 mA/cm2. These are higher than EQEs exhibited by conventional fluorescent emitters (5−7.5%). This suggests that TADF significantly contributes to the EQE of the TADF-based OLEDs. Figure 5b (inset) shows EL spectra for the OLEDs. The EL spectrum of the DACQ-based OLED is blue-shifted relative to that for the PXZQ-based OLED, reflecting the PL spectra of the doped films (Figure 5a). The theoretical maximum EQE (ηEQE) for TADF-based OLEDs is calculated from9

transient PL characteristics of the doped films indicate that DACQ and PXZQ emit TADF in the DPEPO host. Parts a and b of Figure 4 show transient PL decay curves for DACQ and PXZQ, respectively. The intense emission and long tails result from prompt and delayed fluorescence, respectively. The contributions of prompt and delayed fluorescence (denoted Φp and Φd, respectively) were determined from Figure 4a,b. For DACQ at 300 K, Φp = 75.6 and Φd = 8.4%, respectively. For PXZQ at 300 K, Φp = 67.5 and Φd = 18.5%, respectively. Φd increases with increasing temperature for both compounds (Figure 4a and b). The delayed fluorescence spectra are in sufficiently good agreement with the delayed fluorescence spectra (Figure 4c and d), so the delayed components are attributed to TADF. For PXZQ, the Φd at 300 K is smaller than that at 250 K. This suggests that nonradiative decay is dominant at 300 K. Figure 5a shows PL spectra of the doped films measured at room temperature. The spectrum for DACQ is blue-shifted relative to that for PXZQ. The peak positions for DACQ and PXZQ are 526 and 541 nm, respectively. DACQ and PXZQ exhibit different photophysical properties, depending on the degree of spatial overlap between their HOMO and LUMO. DACQ exhibits a faster kr than PXZQ because DACQ has a larger |μ10|. From the transient PL decay curves up to 100 ns, the lifetimes of the prompt components (τp) of DACQ and PXZQ are determined to be 4.7 and 11.0 ns, respectively. The respective kr values are calculated to be 2.1 × 108 and 9.1 × 107 s−1. From the transient PL decays of the delayed components, the lifetimes of TADF (τd) of DACQ and PXZQ are determined to be 260 and 1200 μs, respectively. This suggests that S1 ← T1 RISC is faster in DACQ than in PXZQ. From the blue edges of the fluorescence and phosphorescence spectra of the doped films, the ΔEST values of DACQ and PXZQ are estimated to be 0.08 and 0.19 eV, respectively. The smaller ΔEST of DACQ leads to its faster S1 ← T1 RISC and shorter τd. DACQ thus exhibits faster S1 → S0 radiative decay and S1 ← T1 RISC than PXZQ. The different photophysical properties of DACQ and PXZQ lead to their different Φp and Φd values. Φd is smaller for DACQ than for PXZQ, though their ΦPL values are comparable. For DACQ, the fast kr means that most S1 excitons generated by photoexcitation are decayed radiatively, leading to a small occupation of T1 excitons. In such cases, Φd is small even if TADF effectively occurs via S1 ← T1 RISC. In contrast, when kr is slow, T1 excitons can be efficiently produced via S1 → T1 ISC before the S1 excitons are radiatively decayed. In such cases, Φd is large when T1 → S1 RISC overcomes the competitive nonradiative T1 → S0 decay process.

⎡ Φd ⎤ ⎥γη ηEQE = ⎢0.25Φp + {0.75 + 0.25(1 − Φp)} ⎢⎣ 1 − Φp ⎥⎦ out (4)

where γ is the charge recombination factor and ηout is the light out-coupling efficiency, respectively. γηout depends on the device structure, while Φp and Φd are characteristic for an emitting layer. γ ranges from 0 to 1, and ηout is typically 0.2− 0.3. Hence, γηout = 0.2−0.3 when the charge balance is fully optimized (γ = 1). Calculating ηEQE is useful to evaluate the potential use of the TADF emitter as a dopant for OLEDs. When the experimental EQE is much lower than ηEQE obtained by setting γηout to be 0.2, there is much room for optimizing the device structure and improving EQE. When the experimental EQE is comparable to or larger than ηEQE with γηout = 0.2, the device structure is well optimized. We estimated γηout such that ηEQE reproduces the experimental EQE. The calculated ηEQE for DACQ/PXZQ is in good agreement with the experimental value, assuming that γηout is equal to 0.27/0.16. Because γηout of the PXZQ-based OLED is smaller than 0.2, there is room to further improve its EQE by optimizing the device structure. In contrast, for DACQ, the device structure is well optimized. The prompt (ηp) and delayed components (ηd) of ηEQE are calculated from13 1295

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(2) Baldo, M. A.; O’Brien, D. F.; You, Y.; Shoustikov, A.; Sibley, S.; Thompson, M. E.; Forrest, S. R. Highly Efficient Phosphorescent Emission from Organic Electroluminescent Devices. Nature 1998, 395, 151−154. (3) Baldo, M. A.; Thompson, M. E.; Forrest, S. R. Phosphorescent Materials for Application to Organic Light Emitting Devices. Pure Appl. Chem. 1999, 71, 2095−2106. (4) Adachi, C.; Baldo, M. A.; Thompson, M. E.; Forrest, S. R. Nearly 100% Internal Phosphorescence Efficiency in an Organic LightEmitting Device. J. Appl. Phys. 2001, 90, 5048−5051. (5) Forrest, S. R. The Path to Ubiquitous and Low-Cost Organic Electronic Appliances on Plastic. Nature 2004, 428, 911−918. (6) Reineke, S.; Thomschke, M.; Lüssem, B.; Leo, K. White Organic Light-Emitting Diodes: Status and Perspective. Rev. Mod. Phys. 2013, 85, 1245−1293. (7) Sasabe, H.; Kido, J. Development of High Performance OLEDs for General Lighting. J. Mater. Chem. C 2013, 1, 1699−1707. (8) Berberan-Santos, M. N.; Garcia, J. M. M. Unusually Strong Delayed Fluorescence of C70. J. Am. Chem. Soc. 1996, 118, 9391− 9394. (9) 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, 083302. (10) Sato, K.; Shizu, K.; Yoshimura, K.; Kawada, A.; Miyazaki, H.; Adachi, C. Organic Luminescent Molecule with Energetically Equivalent Singlet and Triplet Excited States for Organic LightEmitting Diodes. Phys. Rev. Lett. 2013, 110, 247401. (11) 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. (12) Tanaka, H.; Shizu, K.; Nakanotani, H.; Adachi, C. Twisted Intramolecular Charge Transfer State for Long-Wavelength Thermally Activated Delayed Fluorescence. Chem. Mater. 2013, 25, 3766−3771. (13) Lee, J.; Shizu, K.; Tanaka, H.; Nomura, H.; Yasuda, T.; Adachi, C. Oxadiazole- and Triazole-Based Highly-Efficient Thermally Activated Delayed Fluorescence Emitters for Organic Light-Emitting Diodes. J. Mater. Chem. C 2013, 1, 4599−4604. (14) Tanaka, H.; Shizu, K.; Nakanotani, H.; Adachi, C. Dual Intramolecular Charge-Transfer Fluorescence Derived from a Phenothiazine-Triphenyltriazine Derivative. J. Phys. Chem. C 2014, 118, 15985−15994. (15) Nakagawa, T.; Ku, S.-Y.; Wong, K.-T.; Adachi, C. Electroluminescence Based on Thermally Activated Delayed Fluorescence Generated by a Spirobifluorene Donor-Acceptor Structure. Chem. Commun. 2012, 48, 9580−9582. (16) Méhes, 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. (17) Nasu, K.; Nakagawa, T.; Nomura, H.; Lin, C.-J.; Cheng, C.-H.; Tseng, M.-R.; Yasuda, T.; Adachi, C. A Highly Luminescent SpiroAnthracenone-Based Organic Light-Emitting Diode Exhibiting Thermally Activated Delayed Fluorescence. Chem. Commun. 2013, 49, 10385−10387. (18) 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. (19) Lee, S. Y.; Yasuda, T.; Nomura, H.; Adachi, C. High-Efficiency Organic Light-Emitting Diodes Utilizing Thermally Activated Delayed Fluorescence from Triazine-Based Donor-Acceptor Hybrid Molecules. Appl. Phys. Lett. 2012, 101, 093306−093304. (20) Li, J.; Nakagawa, T.; MacDonald, J.; Zhang, Q.; Nomura, H.; Miyazaki, H.; Adachi, C. Highly Efficient Organic Light-Emitting Diode Based on a Hidden Thermally Activated Delayed Fluorescence Channel in a Heptazine Derivative. Adv. Mater. 2013, 25, 3319−3323.

(5)

and ηd = {0.75 + 0.25(1 − Φp)}

Φd γη 1 − Φp out

(6)

The calculated ηd values for the DACQ- and PXZQ-based OLEDs are 7.5 and 7.6%, respectively, suggesting that TADF significantly contributes to the ηEQE values. The efficient generation of TADF is responsible for the high EQEs of the TADF-based OLEDs.

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CONCLUSIONS We presented a strategy for designing electron donors for TADF emitters and used it to develop an electron donor containing carbazole and diphenylamino groups (DAC-II). The DAC-II-based TADF emitter, DACQ, exhibited efficient green TADF. Quantum chemical calculations and transient PL decay measurements revealed that DAC-II simultaneously induced S1 ← T1 RISC and S1 → S0 radiative decay in DACQ, resulting in efficient TADF. The photophysical properties of TADF compounds varied depending on the twisting angle between the electron-donating and accepting units. The use of DAC-II as an electron-donating unit eliminates the requirement of highly twisted molecular structures in TADF emitters and allows their photophysical properties to be controlled by changing the twisting angle.

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ASSOCIATED CONTENT

S Supporting Information *

Synthetic procedure and characterization data of DACQ and PXZQ. Quantum chemical calculations of DACQ and PXZQ in toluene solution. CCDC 948738 for PXZQ. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +81-92-802-6920. Fax: +81-92-802-6921. Author Contributions

K.S., H.K., and C.A. designed the research. K.S., M.U., T.S., and K.T. carried out theoretical calculations. H.T. and H.K. synthesized the compounds. K.S. and H.T. performed the experimental work. K.S. wrote the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was funded by the Japan Society for the Promotion of Science (JSPS) through the “Funding Program for World-Leading Innovative R&D on Science and Technology (FIRST Program)”. Computations were partly carried out using computer facilities at the Research Institute for Information Technology, Kyushu University, and the Academic Center for Computing and Media Studies (ACCMS), Kyoto University.

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