Novel donor-spacer-acceptor compound as the multifunctional component of exciplexes for efficient organic light-emitting diodes

ARTICLES SCIENCE CHINA Materials mater.scichina.com link.springer.com Published online 30 August 2021 | https://doi.org/10.1007/s40843-021-1752-7 Novel donor-spacer-acceptor compound as the multifunctional component of exciplexes for efficient organic light-emitting diodes Hao-Yu Yang1,2, Cai-Jun Zheng1*, Ming Zhang1,2, Jue-Wen Zhao1, Yi-Zhong Shi2, Chun-Peng Pu1, Hui Lin1, Si-Lu Tao1 and Xiao-Hong Zhang2* ABSTRACT By connecting tercarbazole (3Cz) and triphenyltriazine (TRz) units with a diphenyl ether group, we designed and synthesized a new donor-spacer-acceptor structure molecule 9'-(4-(4-(4,6-diphenyl-1,3,5-triazin-2-yl)phenoxy) phenyl)-9'H-9,3':6',9"-tercarbazole (3Cz-o-TRz) as the multifunctional component of exciplex emitters. As expected, the intermolecular charge-transfer transition is dominant for 3Cz-o-TRz because of the space-enough and conjugation-forbidden linkage of the diphenyl ether group, increasing the intrinsic characteristics of the 3Cz and TRz moieties in its single-molecule state. Accordingly, three common electron acceptors (4,6-bis(3,5-di(pyridin-3-yl)phenyl)-2-methylpyrimidine (B3PyMPm), 4,6-bis(3,5-di(pyridin-4-yl)phenyl)-2methylpyrimidine (B4PyMPm), and (1,3,5-triazine-2,4,6-triyl) tris(benzene-3,1-diyl)tris(diphenylphosphineoxide) (POT2T)) and three common electron donors (4,4'-(cyclohexane1,1-diyl)bis(N,N-di-p-tolylaniline) (TAPC), tris(4-(10H-phenoxazin-10-yl)phenyl)amine (TPA-PXZ), and N1-phenyl-N4, N4-bis(4-(phenyl(m-tolyl)amino)phenyl)-N1-(m-tolyl)benzene-1,4-diamine (m-MTDATA)) were chosen to construct six exciplex emitters with 3Cz-o-TRz. In the organic light-emitting diodes (OLEDs), the emission spectra of these exciplex emitters could be gradually modulated from 510 to 590 nm. In addition, TAPC:3Cz-o-TRz- and 3Cz-o-TRz:PO-T2T-based OLEDs achieved desirable performance with maximum external quantum efficiencies (EQEs) around 12%. Moreover, a simple tandem OLED containing TAPC:3Cz-o-TRz and 3Czo-TRz:PO-T2T emitters realized optimal performance with an ultralow turn-on voltage of 2.4 V and a maximum EQE of 14.1%. These results indicate the great potential of the donorspacer-acceptor structure compounds in developing exciplex emitters. Keywords: exciplex, donor-spacer-acceptor structure, intermolecular charge-transfer, organic light-emitting diode, multifunctional component INTRODUCTION Since Goushi et al. [1] reported the exciplex-based organic lightemitting diodes (OLEDs) with external quantum efficiencies (EQEs) exceeding the 5% theoretical limit of conventional fluorescent OLEDs in 2012, exciplex-based OLEDs have aroused great attention [2–9]. The exciplex is well known with an intermolecular charge-transfer (CT) transition between the highest occupied molecular orbital (HOMO) of an electrondonating molecule (D) and the lowest unoccupied molecular orbital (LUMO) of an electron-accepting molecule (A) [10]. With the frontier molecular orbitals (FMOs) located on different molecules, the intermolecular excited states would provide small exchange energies to exciplexes, resulting in extremely small singlet-triplet splittings (ΔEST). Thus, similar to single-molecule thermally activated delayed fluorescence (TADF) emitters, exciplex emitters can also improve the utilization of triplet excitons through the reverse intersystem crossing (RISC) process and achieve a theoretical internal quantum efficiency of 100% [11–14]. In addition, the D:A mixed exciplex emitters would naturally have good bipolar electrical conductivity, which optimizes device structures and decreases operating voltages [15–17]. Thus, developing high-performance exciplex emitters is important. In recent years, a new strategy has been proposed to improve the efficiency of exciplex emitters by introducing bipolar donoracceptor structure compounds as the constituting components [18–20]. In general, donor-acceptor structure molecules can form a CT transition by excitation, which would separate positive and negative charges. Thus, the donor and acceptor moieties in the donor-acceptor structure molecules are expected to possess more spatially separated charges than those without CT formation, which would lead to the formation of exciplexes [19]. To date, the electroluminescence (EL) performance of exciplex emitters has been significantly improved by employing donor-acceptor structure components [21–25]. However, current donor-acceptor structure molecules are purposefully developed for the emitters or hosts of OLEDs. They normally exhibit sole donor-dominated or acceptor-dominated properties because of the strong conjugation between the electron-donating and electron-accepting groups [26–29]. In 2015, Zhang et al. [30] attempted to use the typical donor-acceptor structure compound 10,10′-(sulfonylbis(4,1-phenylene))bis(9,9-dimethyl9,10-dihydroacridine) (DMAC-DPS) to construct exciplex emitters as the D and A components. The maximum EQEs of these exciplex emitters are fairly lopsided in the OLEDs, i.e., 9.1% with DMAC-DPS as the D component and 1.6% with DMAC-DPS as the A component. These results indicate that DMAC-DPS is donor-dominated and can only be used as the D 1 School of Optoelectronic Science and Engineering, University of Electronic Science and Technology of China, Chengdu 610054, China Institute of Functional Nano & Soft Materials (FUNSOM), Jiangsu Key Laboratory for Carbon-Based Functional Materials & Devices, Soochow University, Suzhou 215123, China * Corresponding authors (emails: (Zheng CJ); (Zhang XH)) 2 © Science China Press and Springer-Verlag GmbH Germany, part of Springer Nature 2021 1 ARTICLES SCIENCE CHINA Materials component to construct efficient exciplex emitters. Therefore, current donor-acceptor structure molecules obviously have their inherent limitation. As the components of exciplex emitters, the ideal donor-acceptor structure molecules should simultaneously maintain the intrinsic characteristics of electron-donating and electron-accepting moieties in the single-molecule state. In this work, we designed and synthesized a new molecule, 9′(4-(4-(4,6-diphenyl-1,3,5-triazin-2-yl)phenoxy)phenyl)-9′H9,3′:6′,9′′-tercarbazole (3Cz-o-TRz), by using a donor-spaceracceptor structure. In this molecule, the electron-donating group tercarbazole (3Cz) is connected with the electron-accepting group triphenyltriazine (TRz) through the diphenyl ether group. The former contributes to the separation of FMOs by appending secondary donor groups on a central carbazole to enlarge the delocalization of HOMOs, and the latter is a rigid planar acceptor moiety with a small steric hindrance, which can promote intermolecular interactions. With a space-enough and conjugation-forbidden diphenyl ether linkage, the intramolecular interaction between the 3Cz and TRz is efficiently suppressed [31]. Thus, 3Cz-o-TRz is supposed to maintain the intrinsic characteristics of the 3Cz and TRz moieties in the single-molecule state and serve as the multifunctional component of ex (...truncated)