Endowing imidazole derivatives with thermally activated delayed fluorescence and aggregation‐induced emission properties for highly efficient non‐doped organic light‐emitting diodes

The development and enrichment of high‐performance organic fluorophores that simultaneously possess thermally activated delayed fluorescence (TADF) and aggregation‐induced emission (AIE) properties is going pursued but remains a great challenge due to severe exciton quenching. Herein, a systematical investigation on imidazole moiety has successfully given rise to a series of highly efficient imidazole‐based TADF‐AIE luminogens for the first time. The attachment of two cyano functionalities on imidazole moiety can significantly increase the electron‐withdrawing ability, so as to realize TADF emissions with small singlet‐triplet energy gaps (ΔEST) values. Meanwhile, the installation of a steric hindrance group at N1 position of imidazole moiety can twist the geometry between imidazole and phenyl bridge, thus transforming imidazole derivative from an aggregation‐caused quenching emitter into an AIE luminogen. Consequently, the non‐doped organic light‐emitting diodes (OLEDs) utilizing these TADF‐AIE luminogens as emitters exhibit outstanding sky‐blue and green emissions, with external quantum efficiency (EQE) as high as 20.0% and low efficiency roll‐off (EQE at 1000 cd m−2, 16.1%). These values represent the state‐of‐the‐art performance for all imidazole‐based OLED devices, which illustrates the significant potential of imidazole derivatives in assembling high‐performance OLEDs.


INTRODUCTION
Since 1987, organic light-emitting diodes (OLEDs) have triggered a great deal of research and have been widely used in displays, such as mobile phone and television. [1] In OLEDs, to suppress aggregation-caused quenching (ACQ) effect, the emitting layer (EML) is always based on doped materials consisting of host and dopant. [2] This doped EML needs complicated co-evaporation process, which leads to high fabrication cost. As is known, due to the simplified fabrication process, non-doped OLEDs are considered as competitive candidates for future OLEDs applications. [3] However, up to now, the external quantum efficiency (EQE) of non-doped OLEDs is significantly lower than that of doped OLEDs. Especially at high exciton concentration, non-doped OLEDs face severe exciton quenching thus significant efficiency rolloff. [4] Recently, organic fluorophores those simultaneously possess thermally activated delayed fluorescence (TADF) and aggregation induced emission (AIE) properties have been widely used as emitters in high-performance non-doped OLEDs. [5] For one aspect, TADF materials can utilize both of singlet and triplet excitons owing to the small singlet and triplet energy gap (∆E ST ), thus potentially achieving 100% internal quantum efficiency, and meanwhile, TADF materials intrinsically possess bipolar transporting property that is beneficial for carrier recombination, thus potentially achieving high EQE and low efficiency rolloff. For the other aspect, AIE luminogens can efficiently overcome the drawbacks of ACQ effect in the aggregated states, thus significantly reducing exciton quenching in nondoped OLEDs. [6] Accordingly, using TADF-AIE luminogen as emitters, the EQEs of non-doped OLEDs have been rapidly improved. In 2020, our group has developed a novel F I G U R E 1 (A) The molecular design concept of imidazole-based thermally activated delayed fluorescence-aggregation-induced emission (TADF-AIE) luminogen. (B) Energy level of imidazole derivatives. (C) Molecular structures, calculated FMOs distributions, and energy levels of DMAC-CNIM, DMAC-CNIB and DMAC-CNBIM. (D) Synthetic routes to DMAC-CNIM, DMAC-CNIB, and DMAC-CNBIM heptagonal diimide acceptor (N-(4-(tert-butyl)phenyl)-1,1'biphenyl-2,2'-dicarboximide, BPI). [7] The well-balanced rotatability and rigidity of BPI skeleton endows BPI-based TADF-AIE luminogen with excellent external quantum efficiency (EQE) as high as 24.7% and impressively low efficiency roll-off as small as 1% at 1000 cd m −2 . Undoubtedly, the development and enrichment of high-performance TADF-AIE luminogens is ugently needed to promote future application of non-doped OLEDs.
Herein, we have demonstrated that the attachment of two cyano groups on imidazole moiety can significantly increase the electron-withdrawing ability, so as to achieve TADF property, and meanwhile the installation of a methyl or phenyl group at N1 position of imidazole moiety can twist the geometry, thus achieving AIE property (Figure 1A). As a result, a series of highly efficient imidazolebased TADF-AIE luminogens have been developed for the first time. The corresponding non-doped OLEDs devices exhibit outstanding EQE as high as 20% and low efficiency roll-off (EQE at 1000 cd m −2 , 16.1%), representing the state-of-the-art performance for all imidazole-based OLEDs (Table S2).

RESULTS AND DISCUSSION
The molecular structures of the designed cyan-substituted imidazole acceptors (CNIM, CNIB, and CNBIM) and the corresponding TADF materials (DMAC-CNIM, DMAC-CNIB, and DMAC-CNBIM) are illustrated in Figure 1B,C. The density functional theory (DFT) and time-dependent DFT calculation with the B3LYP function and the 6-31+G(d) basis are used to optimize the geometry in the ground state and calculate the energy level alignment of the frontier molecular orbitals. As shown in Figure 1B, introducing two cyan groups on imidazole and benzimidazole groups F I G U R E 2 Crystal structures with dihedral angles (drawn in green) and packing patterns with intermolecular interaction distances (drawn in green) of (A) DMAC-CNIM, (B) DMAC-CNIB, and (C) DMAC-CNBIM can significantly decrease the lowest unoccupied molecular orbital (LUMO) energy levels, which indicates the largely enhanced electron-withdrawing ability of cyan-substituted imidazole groups. Incorporation with 9,9-dimethyl-9,10dihydroacridine (DMAC) donors, the target molecules all exhibit well-separated highest occupied molecular orbital (HOMO) and LUMO distributions, resulting in the small singlet-triplet energy gaps (ΔE ST ) of 0.034 eV, 0.009 eV, 0.007 eV for DMAC-CNIM, DMAC-CNIB, and DMAC-CNBIM, respectively ( Figure 1C). The synthetic routes to DMAC-CNIM, DMAC-CNIB, and DMAC-CNBIM are shown in Figure 1D. Firstly, the condensation reactions between diaminomaleonitrile derivatives and 4-(9,9-dimethylacridin-10(9H)-yl)benzaldehyde are carried out to rapidly construct dicyanoimidazole skeletons (DMAC-CNIH and DMAC-CNBIH). Then, the N-methylation and N-phenylation reactions are successfully used to afford the target molecules (DMAC-CNIM, DMAC-CNIB, and DMAC-CNBIM) in good yields.
The molecular structures and the packing patterns of DMAC-CNIM, DMAC-CNIB, and DMAC-CNBIM are investigated by their crystal analysis, as shown in Figure 2. [8] The three molecules exhibit similar molecular geometries with large dihedral angles between phenyl bridges and DMAC donors (80.1 • , 87.8 • , and 83.2 • for DMAC-CNIM, DMAC-CNIB, and DMAC-CNBIM, respectively) and relatively smaller dihedral angels between phenyl bridges and imidazole acceptors (36.2 • , 24.2 • , and 46.5 • for DMAC-CNIM, DMAC-CNIB, and DMAC-CNBIM, respectively). The highly twisted conformation is beneficial to separate the HOMO and LUMO distributions, thus potentially leading to small ΔE ST values, as calculated ( Figure 1C). Furthermore, the deeper crystal analysis reveals that the packing patterns of three isomers are considerably different. In DMAC-CNIB crystal, the phenyl group on N1 position brings significant steric hindrance to inhibit the formation of continuous imidazole packing frameworks. Therefore, only isolated π-π bonding interactions between imidazole skeletons (3.64 Å) are found. This packing mode is more favorable for the transporting of holes than that of electrons ( Figure S1). As for DMAC-CNIM, the phenyl group on N1 position is replaced by methyl group, which is found to form strong C-H⋅⋅⋅π bonding interaction (2.93 Å) with cyano group. Accordingly, the DMAC-CNIM molecules pack together to form a 3D supramolecular framework via π-π bonding interaction between imidazole skeletons and C-H⋅⋅⋅π bonding interactions between methyl and cyano groups. This packing mode significantly narrows the gap between electron and hole transporting properties, which leads to a relatively more balanced bipolar transporting property of DMAC-CNIM than that of DMAC-CNIB ( Figure  S1). Moreover, a more compact packing mode that possesses C-H⋅⋅⋅N hydrogen bonding interaction (3.32 Å) besides ππ and C-H⋅⋅⋅π bonding interactions (2.83 Å) is found in DMAC-CNBIM crystal. Crucially, this packing mode gives a well-balanced hole and electron transporting properties (Figure S1), which is beneficial to assemble high-performance non-doped OLED devices. [7,9] The TADF and AIE properties of DMAC-CNIM, DMAC-CNIB, and DMAC-CNBIM are further investigated, and the performances are summarized in Table 1. As shown in ultraviolet (UV)-via absorption spectra ( Figure 3A), the three molecules all exhibit absorption peaks at long wavelength region from 350 nm to 450 nm, which are attributed to the charge-transfer transitions from DMAC donors to imidazole acceptors. Compared with DMAC-CNIM, DMAC-CNIB with a phenyl substitution on N1 position and DMAC-CNBIM with a fused phenyl group on imidazole skeleton both show significantly red-shifted emissions, which is mainly attributed to the more extended LUMO distributions when phenyl groups are introduced ( Figure 1C). According to the onset wavelengths of room-temperature photoluminescence spectra (RTPL) and low-temperature photoluminescence spectra (LTPL) measured at 77 K after 10 ms decay, the singlets energies (E S ) and triplet energies (E T ) of DMAC-CNIM, DMAC-CNIB and DMAC-CNBIM are measured and their ΔE ST values are further calculated to be 0.11 eV, 0.06 eV and 0.06 eV, respectively. The small ΔE ST values demonstrate that DMAC-CNIM, DMAC-CNIB, and DMAC-CNBIM all possess TADF properties. Subsequently, the AIE properties of these compounds are investigated by measuring the RTPL spectra of these compounds in mixed tetrahydrofuran (THF)/water solutions with different water fraction (f w ) from 0% to 90% ( Figure 3B). To gain insight into the AIE mechanism, DMAC-CNIH is also synthesized for comparison. In the absence of a steric hindrance group at the N1 position of imidazole moiety, DMAC-CNIH prefers  Abbreviations: HOMO, highest occupied molecular orbital; LUMO, lowest unoccupied molecular orbital. a Decomposition temperature corresponding to 5% weight loss. DMAC-CNIH tends to decompose rather than evaporate at a rather low temperature ( Figure S3A). b Measured in dry dimethylformamide solution (1 × 10 −3 M). The reduction potential of DMAC-CNIH solution was not detected possibly due to the reactive hydrogen atom at N1 position of imidazole moiety ( Figure S3B). c Measured in neat film ( Figure S4). d Measured in toluene solution with a concentration of 1 × 10 −5 mol L −1 . e Estimated according to the onset wavelength of room-temperature photoluminescence (RTPL) spectra. f Estimated according to the onset wavelength of low-temperature photoluminescence (LTPL) spectra. g Calculated from S 1 and T 1 . h Measured in toluene solution and neat film.

F I G U R E 3 (A)
Normalized UV/vis absorption spectra, photoluminescence spectra measured at room-temperature (room-temperature photoluminescence [RTPL]) and photoluminescence spectra measured at 77 K after 10 ms decay (low-temperature photoluminescence

CONCLUSIONS
In conclusions, a series of novel and highly efficient imidazole-based luminogens have been developed and systematically investigated. Among the all, DMAC-CNBIM, which features a methyl-sustituted imidazole skeleton with a fused phthalonitrile group as the acceptor, acquires a significant TADF property and a mostly pronounced AIE property. Moreover, the unique packing pattern also endows DMAC-CNBIM with a well-balanced bipolar transporting property. As a whole, the corresponding non-doped OLEDs device utilizing DMAC-CNBIM as the emitter represents the state-of-the-art performance for all imidazole-based OLEDs. This work exemplifies the great potential of cyano-fused  strategy in TADF design and also the significant substitution effect on ACQ/AIE properties.

A C K N O W L E D G M E N T S
We acknowledge financial support from the National NSF of China (grant numbers: 22031007 and 22005204) and the Sichuan Science and Technology Program (grant numbers: 2020YJ0245 and 2020YJ0302). We also thank the Comprehensive Training Platform Specialized Laboratory, College of Chemistry, Sichuan University.

C O N F L I C T O F I N T E R E S T
The authors declare that there is no conflict of interest.

D ATA AVA I L A B I L I T Y S TAT E M E N T
Supporting data are available from authors upon reasonable request.