Article Cite This: Chem. Mater. 2018, 30, 5005−5012
pubs.acs.org/cm
Chromenopyrazole-Based Bipolar Blue Host Materials for Highly Efficient Thermally Activated Delayed Fluorescence Organic LightEmitting Diodes Mallesham Godumala,† Suna Choi,† Seo Yeon Park,† Min Ju Cho,† Hyung Jong Kim,† Dae Hyun Ahn,‡ Ji Su Moon,‡ Jang Hyuk Kwon,*,‡ and Dong Hoon Choi*,† †
Department of Chemistry, Research Institute for Natural Sciences, Korea University, 145 Anam-Ro, Sungbuk-gu, Seoul 02841, South Korea ‡ Department of Information Display, Kyung Hee University, 26, Kyungheedae-ro, Dongdaemun-gu, Seoul 02447, South Korea Chem. Mater. 2018.30:5005-5012. Downloaded from pubs.acs.org by REGIS UNIV on 10/21/18. For personal use only.
S Supporting Information *
ABSTRACT: New electron-acceptor cores are necessary for developing highly efficient bipolar hosts, particularly for blue thermally activated delayed fluorescence (TADF) organic light-emitting diodes (OLEDs). Herein, chromenopyrazole (CP) was used for the first time as an electron-acceptor core to design and synthesize two novel blue bipolar hosts, viz., 8-(9H-carbazol-9yl)-3-methyl-1-phenylchromeno[4,3-c]pyrazol-4(1H)-one (CzCP) and 8-(9H[3,9′-bicarbazol]-9-yl)-3-methyl-1-phenylchromeno[4,3-c]pyrazol-4(1H)-one (2CzCP). The influence of donor strength on the photophysical, electrochemical, and electroluminescent performances was systematically investigated. CzCP and 2CzCP both maintain high triplet energy (∼3.0 eV), appropriate highest occupied and lowest unoccupied energy levels (HOMO/LUMO), and bipolar nature. Consequently, OLEDs containing CzCP as a host in the emissive layer exhibited state-of-the-art performance with external quantum efficiency of 27.9% and CIE color coordinates of (0.15, 0.21), thus achieving excellent performance among all reported blue host materials in TADF-OLEDs. This work highlights the importance of the CP unit in developing new host materials and paves the way for the realization of high-efficiency blue TADF-OLEDs.
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INTRODUCTION
Analogous to phosphorescent counterparts, TADF emitters should also be embedded in a suitable host(s) matrix to alleviate the detrimental self-quenching property, triplet− triplet annihilation, or triplet−polaron annihilation triggered by their long-lived triplet excitons. Host materials are often the majority in emissive layers (60−99%) and are particularly responsible for the turn-on voltage and power efficiency, thus demonstrating the influence of hosts on the overall device performance.9−13 In principle, the ideal TADF host materials must fulfill the following criteria: the triplet energy (ET) should be higher than that of the dopant to impede the reverse energy transfer from the dopant to host; the HOMO and LUMO energy levels should be suitable for efficient charge injection and transportation; small ΔEST should be maintained to enable the upconversion of accumulated triplet excitons to a singlet state for reducing the nonradiative triplet excitons decay, etc.9−13 From the perspective of materials design of the host materials, electron-acceptor (A) entities have not been explored extensively, thus highlighting the importance of their development. To date, blue TADF-OLEDs have been
Thermally activated delayed fluorescence (TADF) phenomenon has been considered as a research hotspot in the field of organic light-emitting diodes (OLEDs) based-on the promising study by Adachi et al., who reported an external quantum efficiency (EQE) exceeding 20%.1 Unlike conventional fluorescent emitters, TADF materials have the capability to harvest electrically excited both singlet and triplet excitons through reverse intersystem crossing (RISC). This is attributed to the small energy splitting between the lowest singlet and triplet excited energy levels (ΔEST) leading to an internal quantum efficiency of 100%.2−4 Therefore, small ΔEST is an essential property of TADF materials, which can be accomplished by controlling the spatial separation between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbitals (LUMO). Increasing the steric hindrance between the donor (D) and acceptor (A) entities or breaking conjugation in the molecular geometry are the most efficient pathways to achieve effective spatial separation of the frontier orbitals.5−8 TADF materials (both hosts and emitters) are composed of only organic entities, therfore the molecular design is unrestricted and more affordable. © 2018 American Chemical Society
Received: March 22, 2018 Revised: June 29, 2018 Published: July 2, 2018 5005
DOI: 10.1021/acs.chemmater.8b01207 Chem. Mater. 2018, 30, 5005−5012
Article
Chemistry of Materials Scheme 1. Synthetic Route of CzCP and 2CzCP: (i) K3PO4, Pd(OAc)2, P(t-Bu)3, Toluene, Reflux
now, no one implemented it as a semiconductor although it exhibits excellent photoelectric properties, strong electronwithdrawing capability. Moreover, the CP- moiety has a facile synthetic protocol and contains various chemically reactive sites for easy functionalizations to tune its electronic properties. Herein, we introduce CP as an electron-acceptor for the first time and developed two novel blue hosts, CzCP and 2CzCP integrated with carbazole/biscarbazole as electron donors. OLEDs consisting of CzCP or 2CzCP as hosts and 9,9′,9″-(5-(4,6-diphenyl-1,3,5-triazin-2-yl)benzene-1,2,3-triyl)tris(9H-carbazole) (TCzTrz) as a TADF blue emitter23 exhibit significantly low Von of only 3.3 V, and the maximum EQE of 27.9% with CIE color coordinates of (0.15, 0.21) and 17.0% with CIE color coordinates of (0.17, 0.27), respectively. On the other hand, a controlled device with the leading host, i.e., DPEPO-based OLEDs exhibited a higher Von of 3.7 V with inferior EQE of 25.3%, which are comparable to those of the reported DPEPO-TCzTrz-based device.23
extensively developed; however, efficient and stable blue TADF-OLEDs with CIE coordinates of x + y < 0.40 have been less investigated.14−20 To obtain such pure-blue OLEDs, not only emitters but also host materials are responsible, as highly polar hosts will broaden and red-shift the electroluminescence spectrum of the emitter.21,22 Compared to emitters, suitable host materials for blue TADF-OLEDs are rare due to the prerequisites of high ET and large band gaps. Therefore, molecules like DPEPO, mCP, or mCBP (ΔEST between 0.7 and 1.0 eV), commonly used as hosts in bluephosphorescent emissive layers are also being utilized in TADF-OLED applications so far.23−25 Although these hosts achieved good results, some disadvantages like unipolar type behavior and uncomplementary energy levels restricted their widespread applications.26−29 Predominantly, the only holetransporting tendency of mCP and mCBP leads to chargerecombination near the interface between the emissive layer and the electron-transporting layer, which reduces the device efficiency. Therefore, to overcome the only hole transporting or electron transporting tendency of many familiar hosts, significant efforts have been devoted to develop new blue host materials with comparable hole and electron conductivities.9−13,30 For instance, Adachi et al. used benzimidazobenzothiazole as a new electron-acceptor core for developing two blue host materials 29Cz-BID-BT and 39Cz-BID-BT.9 By employing DPAC-TRZ as an emitter for both hosts, 29CzBID-BT showed the best EQE of 20.8% with CIE color coordinates of (0.16, 0.34) and a relatively high turn-on voltage (Von) of 4.0 V. Later, our group achieved an EQE as high as 25.7% by developing dibenzothiophene-based new host material, i.e., ZDN; however, these devices suffer from high Von values of 4.70 V.30 Xu and his co-workers reported phosphineoxide-based hosts via molecular engineering of DPEPO and achieved relatively lower Von of 2.8 V, but the EQE was only 23.0%.14 Although the same group recently demonstrated a lower Von of 2.5 V by developing a phosphanthrene-oxiderelated blue host material, i.e. DPDPO2A, with CIE color coordinates of (0.16, 0.23) by applying DMAC-DPS as the emitter; however, the maximum EQE was limited to 22.5%.10 These results manifest that the simultaneous realization of high EQE (>25.0%) and lower Von ( CzCP is consistent with the fact that 5007
DOI: 10.1021/acs.chemmater.8b01207 Chem. Mater. 2018, 30, 5005−5012
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Chemistry of Materials
Table 2. Photophysical Properties and Kinetic Parameters of TCzTrz-doped (10 wt %) CzCP, 2CzCP, and DPEPO Films Comp CzCP 2CzCP DPEPO
τp (ns)a 12.1 26.5 20.3
τd (μs)b 8.4 17.4 15.6
ΦPLc 1.00 0.82 0.98
Φpd 0.81 0.57 0.72
Φ de
kp (s−1)f
kd (s−1)g
0.19 0.25 0.26
8.26 × 10 3.77 × 107 4.93 × 107 7
kSr (s−1)h
1.19 × 10 5.75 × 104 6.41 × 104 5
6.69 × 10 2.14 × 107 3.55 × 107 7
kISC (s−1)i
kRISC (s−1)j
kTnr (s−1)k
1.57 × 10 1.64 × 107 1.38 × 107
1.47 × 10 5.95 × 104 8.27 × 104
0.00 2.38 × 104 4.58 × 103
7
5
a Prompt emission lifetimes. bDelayed emission lifetimes. cAbsolute PLQY measured with an integrating sphere. dPLQY of the prompt emission estimated according to the corresponding proportions in transient decay curve. ePLQY of the delayed emission estimated according to the corresponding proportions in transient decay curve. fPrompt emission rate constant. gDelayed emission rate constant. hRadiative decay rate constant of the singlet excited state. iIntersystem crossing rate constant. jReverse intersystem crossing rate constant. kNonradiative decay rate constant for triplet excited state.
Figure 2. Current density versus voltage curves of the (a) hole-only and (b) electron-only devices for CzCP, 2CzCP and DPEPO.
The CzCP and 2CzCP in film states exhibit structureless emission with sharp peaks centered at 425 and 438 nm, respectively, and these peak positions are up to 15 nm higher than those of their dilute solutions in toluene, suggesting very small intermolecular interaction in the excited state (Figure 1a,b). 2CzCP has relatively larger conjugation length than CzCP, thus it exhibits a red-shifted emission compared to CzCP. The PL spectra in different polar solvents reveal significant red-shifts (30−64 nm) from nonpolar hexane to polar ethyl acetate solvents (Figure S3 and Table S1) attributed to the changes in their excited-state geometries.40 This geometry change leads to greater charge transfer/dipole− dipole interactions in a polar solvent. In particular, 2CzCP demonstrates predominant solvatochromic effects with a redshift of up to 64 nm owing to the enhanced push−pull interactions between the donor and acceptor entities in the excited state. Singlet (S1) and triplet energies (T1) of both hosts were calculated from the tangential lines of the initial wave in fluorescence and phosphorescence spectra in their film states, respectively (Figure 1c).22 The obtained S1/T1 energies of CzCP and 2CzCP were 3.43/2.98 and 3.44/2.99 eV, respectively and accordingly, the ΔEST values were calculated to be 0.45 eV for both hosts. For comparison, the phosphorescence spectra of the CP entity and carbazole were also obtained (Figure S4). However, the phosphorescence spectra of the two hosts are not similar to that of CP or carbazole, indicating that the triplet states of the hosts are the result of charge transfer state. Transient PL and Kinetic Parameters. Transient PL (TRPL) decay measurements were performed for the hosts in film states doped with TCzTrz to verify the TADF characteristics in nitrogen atmosphere at room temperature. Figure 1d reveals that both compounds involve a nanosecondorder decay component termed excited-state prompt lifetime (τp) and a microsecond-order decay component known as delayed lifetime (τd) (Table 2). τp is the resultant of the relaxation of excitons from the S1 to the S0 state, whereas τd is
due to the T1-to-S1, and thereafter S1-to-S0 transitions. These values were obtained by fitting a double-exponential decay mode I(t) = A1 exp(−t/τp) + A2 exp(−t/τd), where A1 and A2 are fitting parameters.22 The calculated τp/τd values for CzCP, 2CzCP, and DPEPO were 12.1 ns/8.4 μs, 26.5 ns/17.4 μs, and 20.3 ns/15.6 μs, respectively. Particularly, τd of CzCP is lower than those of 2CzCP (>2.0 times) and DPEPO (∼1.8 times), which is beneficial for accelerating the RISC and thus reducing the nonradiative decay of triplet excitons of the emitter. Furthermore, absolute photoluminescence quantum yields (PLQY) of 100%, 82%, and 98%, respectively, were acquired using the integrating sphere for the TCzTrz-doped films of CzCP, 2CzCP, and DPEPO. By utilizing the TRPL and absolute PLQY, the corresponding prompt (Φp)/delayed (Φd) components were ascertained to be 81%/19%, 57%/25%, and 72%/26%, for CzCP, 2CzCP, and DPEPO, respectively. For more in-depth understanding of the TADF mechanism, kinetic parameters such as the radiative decay rate constant of the singlet excited state (ksr), rate constant for intersystem crossing (ISC) from the singlet excited state to the triplet excited state (kISC), rate constant for RISC from the triplet excited state to the singlet excited state (kRISC), and nonradiative (nr) decay rate constant for the triplet excited state (kTnr) were calculated for the blended films (Table 2). In this regard, the nonradiative decay rate constant of the singlet excited state (kSnr) to be zero at room temperature was assumed.41 kp and kd are the prompt and delayed fluorescence decay rate constants obtained using their corresponding prompt and delayed lifetimes (kp = 1/τp and kd = 1/τd). In contrast to 2CzCp and DPEPO blend films, the CzCP:TCzTrz film showed higher kRISC (1.8 and 2.5 times higher than those of DPEPO and 2CzCP, respectively), indicating faster up-conversion of the lowest triplet excitons to the lowest singlet excited state. The higher kRISC could be attributed to its stabilized S1 state by the host environment. The ΔEST values of TCzTrz in different host environments are almost similar to be 0.22 eV (Figure S5). Figure S5 shows that 5008
DOI: 10.1021/acs.chemmater.8b01207 Chem. Mater. 2018, 30, 5005−5012
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Figure 3. Characteristics of TADF-OLED devices for TCzTrz using CzCP, 2CzCP, and DPEPO as hosts: (a) EL spectra at a brightness of 1000 cd m−2 (inset shows the chemical structure of TCzTrz). (b) Current density−voltage−luminance (J−V−L). (c) EQE versus luminance. (d) Current efficiency and power efficiency versus luminance plots.
Table 3. CzCP-, 2CzCP-, and DPEPO-Based TADF-OLED Performance Data Host
Von (V)a
λmax (nm)b
Lmax (cd m−2)c
CE (cd A−1)d
PE (lm W−1)e
EQE (%)f
CIE (x, y)g
CzCP 2CzCP DPEPO
3.3 3.3 3.7
470 478 473
5192 6290 2152
40.5 27.0 36.8
36.3 23.0 28.9
27.9, 23.3, 14.5 17.0, 16.5, 15.1 25.3, 19.8, 11.1
(0.15, 0.21) (0.17, 0.27) (0.16, 0.24)
Turn-on voltage at a brightness of 1 cd m−2. bMaximum EL wavelength at 1000 cd m−2. cMaximum luminance. dMaximum current efficiency. Maximum power efficiency. fExternal quantum efficiency at maximum value, 100 and 500 cd m−2. gCommission Internationale de l’Elcairage at 1000 cd m−2.
a e
the PL spectrum of TCzTrz is red-shifted in the doped system compared to that of the neat film. The intermolecular interactions between the TADF emitter and host material are mainly responsible for the stabilization of S1 state of the emitter. Consequently, the nonradiative decay rate of triplet excitons (kTnr) approaches zero for CzCP-based films. Thus, the TCzTrz in the CzCP-hosted environment can radiate 100% of triplet excitons through RISC into the singlet state, whereas in 2CzCP or DPEPO hosts, triplet excitons are wasted to some extent as nonradiative decay, which is disadvantageous for high performance of OLEDs. Single Carrier Devices. The CP entity has been introduced in this study for the first time as an electronacceptor core in OLED applications, it is worthwhile to examine the structure−property relationship by measuring the carrier transport property of the newly developed hosts. Holeonly devices (HOD) with the configuration of ITO/TAPC (20 nm)/Host (50 nm)/TAPC (20 nm)/Al (100 nm) and electron-only devices (EOD) with the configuration of ITO/ TPBi (20 nm)/Host (50 nm)/TPBi (20 nm)/LiF (1.5 nm)/ Al (100 nm) were fabricated for both hosts, and were compared with DPEPO (Figure 2). Thin layers of 4,4′cyclohexylidenebis[N,N-bis(4-methylphenyl)benzenamine] (TAPC, stacked between the host and Al) in HOD and of 2,2′,2″-(1,3,5-benzinetriyl)-tris(1-phenyl-1-H-benzimidazole) (TPBi, stacked between the ITO anode and host layers) in EOD are used to prevent electron and hole injection from the
cathode and anode, respectively. The hole current density of 2CzCP was higher than that of CzCP at a fixed bias, indicating that the hole injection and transporting capabilities of 2CzCP were greater than those of CzCP owing to the stronger donating capability of the biscarbazole entity. In contrast, CzCP exhibits superior electron current, suggesting dominant electron injection and transporting capabilities compared to 2CzCP. Meanwhile, the hole and electron current densities of both hosts are significantly higher than those of DPEPO, implying facile charge injection and transportation owing to their bipolar nature. Particularly, the electron current density of the two new hosts is significantly higher than that of DPEPO, indicating that the CP moiety is more efficient for electron injection and transportation. Electroluminescent (EL) Properties. The higher ET, appropriate HOMO and LUMO energy levels, and balanced charge transport properties of CzCP and 2CzCP enable their application as blue host materials in TADF-OLEDs. A recognized TADF blue emitter, TCzTrz23 was selected as an emissive material because of the good overlap of the absorption spectra of TCzTrz with the PL emission spectra of the hosts (Figure S6), which facilitate efficient Förster resonance energy transfer from host to the dopant.42 Moreover, it’s rather lower triplet energy level (T1 of 2.80 eV) preventing the energy back-transfer from dopant to host, and nearly 100% PLQY in host environment (film state) are the additional advantages. The OLED device configuration was 5009
DOI: 10.1021/acs.chemmater.8b01207 Chem. Mater. 2018, 30, 5005−5012
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2CzCP as hosts resulted in Von as low as ∼3.3 V, while using DPEPO as a host in the emissive layer produced higher Von (4.6 V) because of its deeper HOMO level. The maximum EQE of 21.1% with CIE color coordinates of (0.16, 0.26) was realized for devices using CzCP as the host material in the emissive layer, whereas the standard DPEPO-based device displayed an EQE of only 14.7% with CIE color coordinates of (0.17, 0.28). All the corresponding device performance data are given in Table S2. These outcomes clearly evidenced that the newly synthesized hosts are superior in terms of efficiency and roll-off behavior compared with DPEPO. Figure 4 and Table S3 illustrate the OLED performances of highly efficient TADF blue host materials reported thus
as follows; ITO (50 nm)/HATCN (7 nm)/TAPC (50 nm)/ DCDPA (10 nm)/Host (CzCP, CzCPCz, or DPEPO):TCzTrz (25 nm, 10 wt %)/TSPO1 (5 nm)/TPBi (25 nm)/LiF (1.5 nm)/Al (100 nm). To confirm our results, familiar devices with DPEPO as the host material were fabricated with the identical device configuration. All the OLED device performances are shown in Figure 3 and their collected data are presented in Table 3. The energy level diagram of all the devices and the materials used in this study are shown in Figure S7. The EL spectrum of CzCP (Figure 3a) shows the deepest blue peak at 470 nm and CIE color coordinates of (0.15, 0.21), whereas 2CzCP and DPEPO displayed peaks at 478 and 473 nm with CIE coordinates of (0.17, 0.27) and (0.16, 0.24), respectively. This phenomenon can be explained using single carrier devices. From the single carrier devices results, we could assume different exciton recombination zones for the TADF devices. Current density values measured in the single-carrier devices reveal a higher electron conductivity than hole conductivity for devices using CzCP as the host in the emissive layer. Hence, the exciton recombination zone would be formed near the HTL. Thus, blue-shifted emission was expected in devices using CzCP as a host in the emissive layer. The EL emission in the 470−478 nm range, which apparently corresponds to the peak of only dopant emissive material without any host emission, indicated that effective charge transfer occurred from the host to the dopant and excitons were confined only to the dopant. Furthermore, the EL spectra of all the devices remain unchanged at different applied voltages (3.5−10 V, Figure S8), indicated excellent EL stability and the absence of exciplex or electroplex emission, which adversely affect the device efficiency and color purity. Figure 3b shows that the Von (at a luminance of 1 cd m−2) of CzCP and 2CzCP (3.3 V) are significantly lower than that of DPEPO (3.7 V), presumably because of the shallower HOMO and deeper LUMO levels, which facilitate charge injection from their adjacent layers. The maximum EQE/current efficiency (CE)/power efficiency (PE) for devices using CzCP or 2CzCP as the host material in the emissive layer were 27.9%/40.5 cd A−1/36.3 lm W−1 and 17.0%/27.0 cd A−1/ 23.0 lm W−1, respectively, whereas the corresponding values of DPEPO-based devices were 25.3%/36.8 cd A−1/28.9 lm W−1. In addition, the maximum brightness of the devices using CzCP and 2CzCP as hosts was nearly 2.4−2.9 times higher than that of DPEPO, ascribed to their efficient charge transport. The highest PLQY of 100%, high kRISC, and kTnr approaching zero are the main factors responsible for the best performance of CzCP. Figure 3c displays that the EQEs were still 23.3% and 14.5% (CzCP), 16.5% and 15.1% (2CzCP), and 19.8% and 11.1% (DPEPO) at the practical brightnesses of 100 and 500 cd m−2, respectively. Although the 2CzCP-based device showed inferior performance, it exhibited extremely low roll-off behaviors of only 3.0% and 11.2% at the brightness of 100 and 500 cd m−2, respectively. The maximum luminance and small efficiency roll-off behavior of 2CzCP could be attributed to the best charge balance. The plots of CE and PE against luminance are shown in Figure 3d. To confirm the general applicability of CzCP as a highperformance host material for various blue TADF emitters, we fabricated devices using 2CzPN as another blue TADF dopant with the same configuration as those of TCzTrz-based devices (Figure S9), and the tendency was correlated with the results obtained from the TCzTrz-based devices. Using CzCP and
Figure 4. Maximum EQE values of the reported highly efficient blue hosts in TADF-OLEDs. The corresponding CIE coordinates are shown in parentheses.
far.9−11,14,30,43These results proved that CzCP realized the best performance with moderately lower Von in the reported TADF blue host materials. Conversely, it is worth mentioning that although the EQE of the CzCP-based device is little inferior to that of the recently developed TADF blue emitter 3DPyMpDTC using a known host, i.e., mCBP, other important factors like Von are significantly reduced and current efficiency is enhanced.25
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CONCLUSION Two novel bipolar hosts, viz. CzCP and 2CzCP, with carbazole/biscarbazole as the electron-donor and a CP unit as the electron-acceptor core were synthesized in a facile manner and well characterized. The twisted geometry of the two new hosts not only retained their ET near 3.0 eV but also resulted in smaller ΔEST owing to the spatial separation of the HOMO and LUMO energy levels. Consequently, OLEDs fabricated with TCzTrz as a blue TADF emitter and CzCP as a host exhibited outstanding performance with EQE up to 27.9%, without using any out-coupling enhancement techniques, whereas devices fabricated with DPEPO exhibited EQEs limited to 25.3%. Besides, the CzCP-based devices showed lower turn-on voltage than DPEPO-based devices due to the shallower HOMO and deeper LUMO levels, enabling charge injection and transportation, accompanied by reduced efficiency roll-off behavior ascribed to their bipolar nature. The excellent performance of CzCP is likely due to the higher RISC rate and higher PLQY. This is the first report of CP5010
DOI: 10.1021/acs.chemmater.8b01207 Chem. Mater. 2018, 30, 5005−5012
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Chemistry of Materials *D. H. Choi. E-mail:
[email protected].
based host materials that produced good performance in blue TADF-OLEDs. Hence, we believe that the electron-deficient CP core is a versatile electron acceptor in OLED applications and this study can promote the development of D−A type TADF blue host materials.
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ORCID
Jang Hyuk Kwon: 0000-0002-1743-1486 Dong Hoon Choi: 0000-0002-3165-0597 Notes
The authors declare no competing financial interest.
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EXPERIMENTAL SECTION
Synthesis of 8-(9H-carbazol-9-yl)-3-methyl-1phenylchromeno[4,3-c]pyrazol-4(1H)-one (CzCP). A mixture of compound Br-CP (1.0 g, 2.8 mmol), carbazole (0.52 g, 3.1 mmol), and K3PO4 (1.8 g, 8.4 mmol) in toluene (30 mL) was purged with nitrogen, and heated to reflux for 15 min. Then, the mixture was cooled to 60 °C, palladium(II) acetate (19 mg, 0.08 mmol) and tritert-butylphosphine (68 μL, 0.17 mmol, 50 wt % in xylene) were added, and the temperature was raised to reflux for 8 h. Subsequently, the mixture was cooled to room temperature, filtered, and washed with methylene chloride (MC) (60 mL). The filtrate was concentrated and the resulting brown colored solid was purified using column chromatography with 1:1 v/v ratio of MC and hexane as the eluent. The subsequent solid was further purified by precipitation in a mixture of MC and hexane solvents to obtain a white powder (1.15 g, 93% yield). 1H NMR (500 MHz, CDCl3, δ): 8.09 (d, J = 7.6 Hz, 2H), 7.69−7.66 (m, 1H), 7.65−7.63 (m, 1H), 7.56−7.53 (m, 2H), 7.48−7.43 (m, 2H), 7.41−7.36 (ddd, J = 8.2, 7.1, 1.2 Hz, 2H), 7.36−7.32 (tt, J = 7.5, 1.2 Hz, 1H), 7.31 (d, J = 2.1 Hz, 1H), 7.30−7.27 (m, 2H), 7.24 (d, J = 7.9 Hz, 2H), 2.73 (s, 3H). 13C NMR (125 MHz, CDCl3, δ): 157.62, 151.69, 151.04, 141.11, 140.18, 138.91, 133.47, 130.38, 130.04, 129.26, 126.50, 126.01, 123.47, 120.46, 120.39, 120.34, 119.61, 112.97, 109.29, 106.57, 12.93. MS (MALDI-TOF) [m/z]: Calcd for C29H19N3O2, 441.15; Found, 459.22 (M + H2O)+. Anal. Calcd for C29H19N3O2: C, 78.90; H, 4.34; N, 9.52. Found: C, 78.82; H, 4.39; N, 9.58. Synthesis of 8-(9H-[3,9′-bicarbazol]-9-yl)-3-methyl-1phenylchromeno[4,3-c]pyrazol-4(1H)-one (2CzCP). Reaction conditions similar to those used for the synthesis of CzCP were employed, using Br-CP (1.5 g, 4.2 mmol), 9H-3,9′-bicarbazole (1.54 g, 4.7 mmol), K3PO4 (2.7 g, 12.7 mmol), palladium(II) acetate (28 mg, 0.13 mmol), and tritert-butylphosphine (102 μL, 0.25 mmol, 50 wt % in xylene) to give 2CzCP as a white solid (2.2 g, 86% yield). 1H NMR (500 MHz, CDCl3, δ): 8.23 (d, J = 1.8 Hz, 1H), 8.19 (d, J = 7.9 Hz, 2H), 8.05 (d, J = 7.6 Hz, 1H), 7.77 (dd, J = 8.7, 2.3 Hz, 1H), 7.70 (d, J = 8.9 Hz, 1H), 7.60−7.56 (m, 2H), 7.53−7.48 (m, 3H), 7.47− 7.39 (m, 6H), 7.36 (s, 1H), 7.35−7.28 (m, 5H), 2.74 (s, 3H). 13C NMR (125 MHz, CDCl3, δ): 157.54, 151.84, 151.10, 141.72, 141.00, 140.80, 139.15, 138.96, 133.13, 130.37, 130.34, 130.13, 129.09, 126.77, 126.58, 125.88, 125.51, 124.58, 123.14, 123.02, 120.94, 120.68, 120.38, 120.24, 119.80, 119.74, 119.55, 113.10, 110.46, 109.66, 109.58, 106.62, 12.94. MS (MALDI-TOF) [m/z]: Calcd for C41H26N4O2, 606.21; Found, 623.15 (M + H2O)+. Anal. Calcd for C41H26N4O2: C, 81.17; H, 4.32; N, 9.24. Found: C, 81.28; H, 4.41; N, 9.18.
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ACKNOWLEDGMENTS This work was supported by the National Research Foundation of Korea (NRF2015R1A2A1A05001876) and by the Key Research Institute Program (NRF20100020209). D. H. Choi particularly thanks for the support from LG display Co. Limited (2017−2018). It was also supported by a Korea University Grant (2017-2018).
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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/acs.chemmater.8b01207. Synthetic procedure, thermal analysis data, cyclic voltammograms, photophysical properties, kinetic parameters, device energy level diagram, fabrication of single carrier devices, OLED performance, comparison data reported for host materials, and 1H NMR spectra (PDF)
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REFERENCES
AUTHOR INFORMATION
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*J. H. Kwon. E-mail:
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DOI: 10.1021/acs.chemmater.8b01207 Chem. Mater. 2018, 30, 5005−5012
Article
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