Research Article www.acsami.org
Manipulating the Electronic Excited State Energies of PyrimidineBased Thermally Activated Delayed Fluorescence Emitters To Realize Efficient Deep-Blue Emission Ryutaro Komatsu, Tatsuya Ohsawa, Hisahiro Sasabe,* Kohei Nakao, Yuya Hayasaka, and Junji Kido* Department of Organic Materials Science, Graduate School of Organic Materials Science, Research Center for Organic Electronics (ROEL), and Frontier Center for Organic Materials (FROM), Yamagata University, 4-3-16 Jonan, Yonezawa, Yamagata 992-8510, Japan S Supporting Information *
ABSTRACT: The development of efficient and robust deep-blue emitters is one of the key issues in organic light-emitting devices (OLEDs) for environmentally friendly, large-area displays or general lighting. As a promising technology that realizes 100% conversion from electrons to photons, thermally activated delayed fluorescence (TADF) emitters have attracted considerable attention. However, only a handful of examples of deep-blue TADF emitters have been reported to date, and the emitters generally show large efficiency roll-off at practical luminance over several hundreds to thousands of cd m−2, most likely because of the long delayed fluorescent lifetime (τd). To overcome this problem, we molecularly manipulated the electronic excited state energies of pyrimidine-based TADF emitters to realize deep-blue emission and reduced τd. We then systematically investigated the relationships among the chemical structure, properties, and device performances. The resultant novel pyrimidine emitters, called Ac− XMHPMs (X = 1, 2, and 3), contain different numbers of bulky methyl substituents at acceptor moieties, increasing the excited singlet (ES) and triplet state (ET) energies. Among them, Ac−3MHPM, with a high ET of 2.95 eV, exhibited a high external quantum efficiency (ηext,max) of 18% and an ηext of 10% at 100 cd m−2 with Commission Internationale de l′Eclairage chromaticity coordinates of (0.16, 0.15). These efficiencies are among the highest values to date for deep-blue TADF OLEDs. Our molecular design strategy provides fundamental guidance to design novel deep-blue TADF emitters. KEYWORDS: organic light-emitting device, donor−acceptor system, solid-state emission, photochemistry, thermally activated delayed fluorescence (TADF), pyrimidine
1. INTRODUCTION Organic light-emitting devices (OLEDs) are some of most promising candidates for next-generation, ecofriendly, largearea displays and general lighting because of their outstanding features, such as lightness, thinness, large area, and flexibility when flexible substrates are used.1−7 The development of efficient and robust deep-blue emitters is one of the key issues in OLEDs. As a promising technology that realizes 100% conversion from electrons to photons, thermally activated delayed fluorescence (TADF) emitters have attracted considerable attention.8−18 However, only a handful of examples of deep-blue TADF emitters, which exhibit Commission Internationale de l’Eclairage chromaticity coordinates (CIE) of (x < 0.15, y < 0.15), have been reported to date.12−18 In addition, deep-blue TADF emitters generally show a large efficiency rolloff at practical luminance over several hundreds to thousands of cd m−2. One of the main reasons for this efficiency roll-off can be attributed to the relatively long delayed fluorescence lifetime (τd), on the order of several tens to hundreds of microseconds, © XXXX American Chemical Society
which causes quenching processes such as singlet−triplet annihilation (STA) and triplet−triplet annihilation (TTA) and reduces the efficiency at high brightness.19 To overcome this problem, reducing the energy difference (ΔEST) between the excited singlet (ES) and triplet state (ET) energies is critically important, as smaller ΔEST gives shorter τd. However, it is challenging to obtain a small ΔEST in a deep-blue emitter, particularly to realize a high ET of nearly 3.0 eV, due to the severe limitation of the molecular design. For example, the ET of biphenyl is reported to be 2.84 eV, and that of m-terphenyl is 2.81 eV; thus, these components themselves are difficult to use in deep-blue TADF emitters.20 Therefore, a novel strategy to increase ET is essential to obtain a deep-blue TADF emitter. For examples of deep-blue emitters with CIEx,y of (x < 0.15, y ∼ 0.15), Adachi and co-workers reported 10H-phenoxaborin Received: October 26, 2016 Accepted: January 16, 2017
A
DOI: 10.1021/acsami.6b13482 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces
Figure 1. (a) Chemical structures and (b) HOMO and LUMO distributions, energy levels, energy difference between HOMO and LUMO (EH−L), excited state energies (Es and ET), energy difference between the singlet and triplet excited states (ΔEST = ES − ET), and oscillator strength ( f) of the pyrimidine conjugate emitters used in this study.
and acridan units containing emitters with ηext,max = 20% and CIEx,y = (0.14, 0.16).13 Very recently, Hatakeyama and coworkers demonstrated efficient frontier molecular orbital (FMO) separation via the multiple resonance effect to generate an organoboron-based ultrapure blue emitter, DABNA-2, with ηext, max = 20% and CIEx,y = (0.12, 0.13). Surprisingly, DABNA2 exhibited a very narrow full width at half-maximum (fwhm) of 28 nm.17 In this work, we molecularly manipulated the electronic excited state energies of pyrimidine-based TADF emitters to realize deep-blue emission and reduced τd. Here we used a twisted molecular structure induced by the steric hindrance of methyl group(s) and systematically investigated the relationships among the chemical structure, properties, and device performance. The resulting novel pyrimidine emitters, called Ac−XMHPMs (X = 1, 2, and 3), contain different numbers of bulky methyl substituents at the acceptor moieties, resulting in increased ES and ET. Among them, Ac−3MHPM, with a high ET of 2.95 eV, exhibited a high ηext,max of 18% and a ηext of 10% at 100 cd m−2 with CIE chromaticity coordinates of (0.16, 0.15). These efficiencies are among the highest values to date for deep-blue TADF OLEDs.
2. RESULTS AND DISCUSSION 2.1. Molecular Design and Density Functional Theory (DFT) Calculation. In a previous study, we developed highly efficient blue to green TADF emitters and investigated the structure−property relationship by using different substituents at the 2-position of the diphenylpyrimidine skeleton.21,22 We were able to control the emission colors by changing the electron-donating property of the substituents. However, all molecules showed similar ET values of around 2.7 eV due to the 4,6-diphenylpyrimidine skeleton, which has a relatively long πconjugate system. To develop deep-blue TADF emitters, the ET values must be increased to obtain small ΔEST and realize the efficient up-conversion from triplets to singlets. Here, we used a twisted molecular structure induced by the steric hindrance of methyl group(s) and systematically investigated the structure− property relationship. We developed a series of pyrimidinebased blue TADF emitters, termed Ac−XMHPMs, with different numbers of methyl groups at different positions: Ac−1MHPM has one methyl group, Ac−2MHPM has two methyl groups, and Ac−3MHPM has three methyl groups (Figure 1a). First, we performed density functional theory (DFT) calculations to estimate the geometric structure, energy gap (Eg), ET, and ΔEST of each Ac−XMHPM (Figure 1b). The Ac−XMHPMs showed small ΔEST (
