Letter Cite This: Org. Lett. XXXX, XXX, XXX−XXX
pubs.acs.org/OrgLett
Periphery Design of Macrocyclic Materials for Organic LightEmitting Devices with a Blue Phosphorescent Emitter Asami Yoshii,† Koki Ikemoto,‡ Tomoo Izumi,‡,§ Hideo Taka,‡,§ Hiroshi Kita,§ Sota Sato,‡ and Hiroyuki Isobe*,‡ †
Department of Chemistry, Tohoku University, Aoba-ku, Sendai 980-8578, Japan Department of Chemistry, The University of Tokyo, and JST, ERATO, Isobe Degenerate π-Integration Project, Hongo 7-3-1, Bunkyo-ku, Tokyo 113-0033, Japan § Konica Minolta, Ishikawa-cho, Hachioji 192-8505, Japan Downloaded via UNIV AUTONOMA DE COAHUILA on April 5, 2019 at 17:25:34 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.
‡
S Supporting Information *
ABSTRACT: Cyclo-meta-phenylenes were modified with trifluoromethyl groups at their periphery to create host materials suitable for use in blue phosphorescent organic light-emitting devices. The periphery design resulted in molecules with high triplet-state energies, which were required to support the blue emission from Ir phosphors. As a result, an external quantum efficiency of 9.9% was achieved. The most successful host, a pentameric congener, preferred CF-π/CH-π interactions in its crystalline packings, which could be beneficial for the host performance.
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conversion in the blue light region. Structure−performance relationship studies of congeners with different ring sizes demonstrated the importance of packing preferences with respect to the performance as host materials. We synthesized trifluoromethylated [n]CMP, nCF 3 [n]CMP, from commercially available 1,3-dibromo-5(trifluoromethyl)benzene. The one-pot Ni-mediated macrocyclization of dibromide produced nCF3-[n]CMP as a mixture (n = 5−11), and without recourse to chromatography, two major products with n = 5 and 6 were isolated (Scheme 1).2,3,7 The product distribution of macrocyclization was slightly affected by the CF3 substituents, and pentameric 5CF3[5]CMP (2) was obtained as the most abundant product in 24% yield.8 The simple synthesis was suitable for a large-scale reaction, and 1.18 g of 2 was obtained from a single reaction. The physical properties of nCF3-[n]CMP were examined to determine the fundamental properties required for OLED base materials (Figure 2; see also the Supporting Information). The absorption spectra of 2 and 3 in chloroform showed peaks at approximately 250 nm and confirmed the absence of absorption in the visible light region. The decomposition temperatures were determined to be 321 and 372 °C for 2 and 3, respectively, by thermogravimetric analysis. The ET levels were first determined to be 2.75 and 2.82 eV for 2 and 3, respectively, from phosphorescence in a 2-methyltetrahydrofuran (2-MeTHF) glass, which was higher than the value of 1
acrocyclic aromatic compounds (MACs) have begun to appear in the structural library of molecular materials.1 Modifications of MAC at their periphery not only diversify structural variations but also modulate the properties of cyclic conjugated π-systems. As electronic materials of MACs from concise syntheses, we recently developed [n]cyclo-metaphenylene ([n]CMP) congeners as fundamental molecular materials for organic light-emitting devices (OLEDs). When used as charge carrier-transporting materials, unsubstituted [n]CMP molecules possessed bipolar characteristics that enabled the transportation of both electrons and holes through the thin layers.2 Moreover, the introduction of bulky substituents at the periphery in the form of methylated [5]CMP [1 (Figure 1)] allowed for the development of a host material capable of a high-efficiency emission from a green Ir emitter.3,4 Although we further succeeded in fabricating a white phosphorescent OLED with a three-color blend of Ir emitters, an inferior electro-optical conversion with a blue Ir emitter was observed.5 For instance, when we doped an OLED of 1 with a blue Ir emitter, FIrpic,6 the external quantum efficiency (EQE) was only 5.8% (Figure 1b), which was much lower than the theoretical limit of 20−30%. Therefore, further modifications of the host materials are required to accommodate blue Ir emitters having a higher triplet-state energy (ET).2,3 We herein report the synthesis of periphery-modified [n]CMP congeners as the host material for the blue Ir emitter. Taking advantage of the simple syntheses of [n]CMP congeners via one-pot macrocylization,2,3 we introduced CF3 substituents in [5]CMP to find a superior electro-optical © XXXX American Chemical Society
Received: February 26, 2019
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DOI: 10.1021/acs.orglett.9b00717 Org. Lett. XXXX, XXX, XXX−XXX
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Organic Letters
Figure 2. Physical properties of [n]CMP congeners. (a) Absorption spectra (CHCl3, 25 °C, concentration of 6 × 10−6 M). (b) Decomposition temperatures (Td) and triplet-state energies (ET).
angles of approximately 30° (Figure S5). Interestingly, the effects of CF3 substituents appeared in a different manner between 2 and 3 over the crystalline packing structures. Thus, the intermolecular contacts of 2 and 3 were dominated by herringbone-type edge-to-face contacts and π-stack-type faceto-face contacts, respectively (Figure 3a,b). The Hirshfeld surfaces further allowed for in-depth analysis of the contacts (Figure 3c and Figures S6−S8).9 The herringbone-type
Figure 1. OLED with MAC materials. (a) [n]CMP materials. (b) Blue light-emitting OLED. ETL/HTL = 1. Cathode = Cs (1.5 nm)/ Al (100 nm). Anode = PEDOT:PSS (30 nm)/ITO (110 nm). EQE = external quantum efficiency. DV = driving voltage.
Scheme 1. Synthesis of nCF3-[n]CMPa
a
Isolated yield after subsequent purification by recrystallization from PhCl.
(2.68 eV).3 Although the ET levels were lower in their powderform solid state (2.67 eV for 2 and 2.68 eV for 1), they were higher than the value of FIrpic (2.62 eV).6 Interestingly, the ET level of 3 was lowered by 0.14 eV in the solid state, which was larger than the reduction recorded for 2 (0.08 eV). This observation may correlate with the packing structures in the crystalline solid state (see below). The molecular and packing structures of 2 and 3 were determined by X-ray diffraction analysis of single crystals. The molecular structures of 2 and 3 did not deviate much from those of methylated congeners3 with averaged biaryl dihedral
Figure 3. Crystal structures of nCF3-[n]CMP. (a) Packing structures of 2. (b) Packing structures of 3 (PhCl). See Figure S9 for the data from PhBr. (c) Hirshfeld surfaces mapped with de parameters. See Figures S6−S8 for details. B
DOI: 10.1021/acs.orglett.9b00717 Org. Lett. XXXX, XXX, XXX−XXX
Letter
Organic Letters packing of 2 was mainly dictated by CH-π/CF-π contacts,10,11 whereas the dimeric pair of 3 was assembled by face-to-face πstacking. The dimeric pairs of 3 trapped solvent molecules in a sandwiched manner, and almost identical packing structures were observed for the crystal obtained from chlorobenzene (Figure 3) and bromobenzene (Figure S9). This observation may indicate preferable π-stack interactions among 3, which should decrease the ET level (Figure 2b).12,13 Finally, trifluoromethylated [n]CMP molecules were evaluated as the host material of the blue phosphorescent OLED.6 When we replaced methylated 1 with trifluoromethylated [n]CMP (2 and 3) in the OLED with FIrpic (see Figure 1), the blue emission from the emitter was commonly observed (Figure 4). More importantly, with 2 as the host, the EQE
layer (Figure S11), we observed a difference in their photoluminescence quantum yields (ϕ = 0.44 in 2, and ϕ = 0.21 in 3), which indeed showed the presence and consequence of the host−emitter interactions. In summary, we have designed and synthesized trifluoromethylated macrocycles for use in blue phosphorescent OLEDs. The periphery substituents played an important role in determining the performance as the host, which resulted in the improved EQE values of the blue FIrpic emitter. Structural changes from CH3 substituents to CF3 substituents seemed subtle but resulted in the improvement of EQE values from 5.8% to 9.9%. The results highlighted beneficial features of [n]CMP as wide-gap materials with its unique tolerance for periphery modifications. The subtle structural change from CH3 to CF3 was found to be effective in the design of the host materials for blue phosphorescent emitters,5 which also highlighted the uniqueness of MAC materials in comparison with noncyclic materials.14 The effects of the ring size were also crucial to the device performance, which suggested the important role of host−emitter interactions in the high EQE values. The results should inform further developments of MAC materials in the future.
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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.orglett.9b00717. Experimental details and crystallographic, NMR, theoretical, and device data (PDF) Accession Codes
CCDC 1898512−1898514 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing
[email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
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Figure 4. Blue phosphorescent OLED with 2 and 3 as the host. (a) Emission spectra and photographs of the device. See Figure 1 for the device configurations. (b) External quantum efficiency plotted as a function of current density. See Figure S10 for further details.
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
value was improved from 5.8% to 9.9%, which demonstrated favorable effects of trifluoromethylation at the periphery. When we used a larger congener, 3, as the host, the EQE value was also improved from that of 1. However, the EQE value was 6.3%, which was inferior to that of 2 as the host. The ET levels of 2 and 3 in their powder-form solid state were nearly identical (2.67 and 2.68 eV, respectively) and should not explain the difference in their performance. We suggest that host−emitter interactions may be the key for the performance of CMP as the host. As found in the crystal packing, CH-π/ CF-π interactions were dominant in the crystalline solid state of 2, and as was evident from the structure, the molecular surface of FIrpic was rich in CH and CF moieties. The CH-π/ CF-π interactions with 2 can thus facilitate favorable entrapments of the emitter during the vacuum deposition of 2/FIrpic on the OLED, which further results in favorable charge/energy transfers. On the other hand, hexameric 3 favors self-assembly in the solid state through π-stack interactions and thus may repel the FIrpic emitter during layer deposition. When we investigated the phosphorescence of FIrpic in a host
Koki Ikemoto: 0000-0003-4186-7156 Sota Sato: 0000-0002-7395-2112 Hiroyuki Isobe: 0000-0001-8907-0694 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This study is partly supported by JST ERATO (JPMJER1301) and KAKENHI (16K04864, 17H01033, 17K05772, and 18J10131). The authors thank the Aichi Synchrotron Radiation Center (2018N3008), SPring-8 (2017A1459), and KEK Photon Factory (2017G082) for the use of the X-ray diffraction instruments. A.Y. thanks JSPS for a predoctoral fellowship.
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DOI: 10.1021/acs.orglett.9b00717 Org. Lett. XXXX, XXX, XXX−XXX
