Asymmetric Thermally Activated Delayed Fluorescence Materials With Aggregation-Induced Emission for High-Efficiency Organic Light-Emitting Diodes

Introduction

Luminescent materials that are capable of thermally activated delayed fluorescent (TADF) have been widely investigated not only because of their great potential in utilizing theoretically 100% internal quantum efficiency (IQE) through the back transfer of non-radiative triplet exactions (75%) into radiative singlet excitons conferred by small single–triplet energy splitting (ΔE_ST) for efficient reverse intersystem crossing (RISC), but also due to their fundamental significance both in scientific investigations and technological applications of organic electronics (Uoyama et al., 2012; Tao et al., 2014; Etherington et al., 2016; Guo et al., 2018; Han et al., 2018; Kotadiya et al., 2019; Pershin et al., 2019; Zhang Y. L. et al., 2019). With flourish developments over the past few years, considerable attention has been devoted to designing and exploiting excellent TADF materials with the consideration of the following rational metrics (Park et al., 2016; Chen et al., 2017; Im et al., 2017; Wong and Zysman-Colman, 2017; Yang Z. et al., 2017; Zhang Y. et al., 2019): (i) the separated highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) distributions for enabling small ΔE_ST; (ii) slight overlap of frontier molecular orbital (FMO) for maintaining high photoluminescence quantum yield (PLQY); (iii) short exciton lifetimes to eliminate the concentration-induced quenching effect; (iv) reduced intermolecular interactions to alleviate aggregation quenching processes; and (v) acceptable thermal stability for long-term device operation and ease synthesis procedure for mass productions. Nonetheless, most of the reported TADF materials suffer from serious aggregation caused quenching (ACQ) phenomenon that obviously hinders their practical applications (Einzinger et al., 2017; Wei et al., 2017, 2019; Wong and Zysman-Colman, 2017; Gan et al., 2019). Therefore, it remains forbidden issues to construct prominent TADF materials because of the great challenge in obtaining such aforementioned features in a molecule simultaneously, especially for alleviating the serious ACQ (Aydemir et al., 2017).

Aggregation-induced emission (AIE) is a fascinating optical phenomenon with greatly enhanced luminescent efficiency in solid state, which has been increasingly emerging as a promising candidate in organic electronics, bio-electronics, and photonics (Huang et al., 2017; Mao et al., 2017; Tsujimoto et al., 2017; Yang J. et al., 2017; Chen et al., 2019). The implantation of AIE properties into TADF materials has demonstrated a possibility to suppress the ACQ of solid films (Lee et al., 2017; Zheng et al., 2019). Generally, the linkage of donor and acceptor units through the spiro- and/or twist structure to lessen the overlap of FMO and to inhibit the molecular aggregation has been proven to be a perspective strategy for achieving AIE-TADF materials. Following this guideline, an extensive collection of AIE-TADF materials has been designed and explored in fabricating efficient TADF organic light-emitting diodes (OLEDs). Recently, diphenyl ketone, which can not only serve as an electron-deficient core to construct charge transfer (CT) molecule with spatially separated HOMO and LUMO distributions through the incorporation of varieties of donor units for achieving a small ΔE_ST but can also be used as the twist and rotation center to reduce the interactions for alleviating the self-quenching effect of multiple molecules and to incorporate AIE for boosting emission of the resultant materials in solid state, has been regarded as one of the most key building blocks in constructing TADF materials with the AIE character (Guo et al., 2017; Huang et al., 2017). These impressive advantages have stimulated us to explore new diphenyl ketone-based TADF derivatives for fabricating high-performance OLEDs.

Herein, to achieve the AIE-TADF materials, we designed and synthesized two emitters of SFCOCz and SFCODPAC with asymmetric D₁−A–D₂ architecture (Figure 1A) through the direct linkage of sulfurafluorene (SF)-modified ketone (CO) and arylamine of carbazole (Cz) or 9,9-diphenyl-9,10-dihydroacridine (DPAC). In this moiety, the D₁−A–D₂ can effectively render separated HOMO and LUMO distributions to guarantee a small ΔE_ST for the promotion of RISC process under thermal activation; moreover, the twisted and asymmetric molecular configuration can endow AIE and restrain intermolecular interaction of π-π stacking and/or aggregation to reduce ACQ in solid state (Aydemir et al., 2017; Wang et al., 2017; Zeng et al., 2018; Liu et al., 2019). These newly constructed AIE-TADF materials in D₁−A–D₂ skeleton can be easily prepared with high yields up to 68%. Strikingly, SFCOCz and SFCODPAC exhibited excellent TADF with small ΔE_ST of ~0.03 eV, high PLQY (Φ_PL) of ~73%, and efficient RISC rate and relatively short delayed fluorescence lifetime. High-performance non-doped and doped TADF OLEDs endowed by these emitters were fabricated, showing high luminance of 5,598 and 11,595 cd m⁻² and external quantum efficiencies (EQEs) of 7.5 and 15.9%, respectively. The ease of synthetic route, excellent optoelectronics, and high device performance make the SFCO-based asymmetric TADF emitters promising candidates in practical applications, conferring a new paradigm for next-generation organoelectronics.

FIGURE 1

Figure 1. (A) Schematical drawing of designed SFCO-based thermally activated delayed fluorescence (TADF) materials with asymmetric D₁−A–D₂ architecture. (B) ORTEP single-crystal structures of SFCOCz (CCDC 1964730) and SFCODPAC (CCDC 1964747).

Results and Discussion

Design, Synthesis, and Characterization

To prepare the asymmetric TADF materials exhibiting AIE trait, two molecules, namely (4-(9H-carbazol-9-yl)phenyl)(dibenzo[b,d]thiophen-2-yl)methanone (SFCOCz) and dibenzo[b,d]thiophen-2-yl(4-(9,9-diphenylacridin-10(9H)-yl)phenyl)methanone (SFCODPAC), were designed and synthesized in D₁−A–D₂ architecture through the direct connection of sulfurafluorene-tailored diphenyl ketone and donor of Cz (SFCOCz) or DPAC (SFCODPAC) by a conventional two-step procedure composed of Friedel–Crafts acylation and C–N coupling reaction (Figure 1A and Scheme S1, Figures S1–S4). As revealed by single-crystal X-ray diffraction (XRD) analysis, the dihedral angle between arylamine and SFCO in these two molecules is up to 79° (Figure 1B and Table S1). Such a highly twisted molecular conformation would not only be beneficial to reduce electron communications, ensuring an optimized HOMO and LUMO separation for acquiring a small ΔE_ST, but can also effectively suppress the molecular aggregations in solid state for eliminating ACQ (Wang et al., 2017). SFCOCz and SFCODPAC display high thermal stabilities (Figure S5), exposing the decomposition temperatures (T_d) of 386 and 429°C as revealed by the thermogravimetric (TGA) measurements and melting temperature (T_m) of 197 and 251°C as measured by differential scanning calorimetry (DSC) analyses. The slightly higher T_d and T_m of SFCODPAC than those of SFCOCz could be well-explained by its high molecular weight and rigid structure. In addition, the vacuum-evaporated thin films on glass substrates are amorphous and uniform with quite small root-mean-square roughness (RMS) of 0.307 and 0.173 nm for (Figure S6) SFCOCz and SFCODPAC. The excellent thermal and morphology stabilities of resultant AIE-TADF materials would be favored for vacuum-deposited device fabrication and long-term operation stability.

Photophysical Properties

The photophysical profiles of asymmetric TADF molecules SFCOCz and SFCODPAC in dilute dichloromethane solution (CH₂Cl₂, 1 × 10⁻⁵ mol L⁻¹) and neat and doped films (Figure 2 and Figures S7–S11) were detailedly investigated by UV–visible absorption and photoluminescence (PL) spectra. SFCOCz and SFCODPAC imply that the n–π^* transition dominated absorption band peaked ~300 nm, and the CT band at ~350 nm originated from intramolecular CT (ICT) from arylamine to CO (Table 1 and Table S2) (Lee et al., 2017). The PL spectra of SFCOCz and SFCODPAC exhibit typical structureless ICT fluorescence bands located at 484 and 558 nm in CH₂Cl₂, respectively. The ICT characters were also demonstrated by the red-shifted emission bands with the increasing in solvent polarities (Figure S8). In film states, these two molecules reveal nearly the same absorption profiles to those of the solution, showing absorption peaks at 295 and 346 nm for SFCOCz and 297 and 359 nm for SFCODPAC. These findings verify the effective suppressing of molecule aggregation in the films. The optical bandgaps (^°ptE_gs), calculated by the onset edge of the absorption spectra, were 3.04 and 2.88 eV for SFCOCz and SFCODPAC, respectively. For the PL spectra, SFCOCz and SFCODPAC also show structureless ICT bands at ~450 and 492 nm with corresponding Φ_PL of ~22 and 73%, respectively. Notably, the Φ_PLs of SFCOCz and SFCODPAC in the solid state are up to ~3.7-fold that in the solution, indicating the obvious AIE characteristics. To further demonstrate the AIE properties of these two TADF molecules, the PL spectra in tetrahydrofuran (THF) with varied water fractions (f_w) were performed. SFCOCz and SFCODPAC exhibit intense emissions as the formation of nanoaggregates upon injecting a large amount of poor solvent of H₂O (fraction >80%) into THF solutions (Figures 2B–D), suggesting again the AIE features. This significantly enhanced photoluminescence in nanoaggregates should be due to the suppressed molecule rotation and motion that are highly active in the solution, thus blocking the non-radiative decay of excitons. In addition, we also explore the PL properties of SFCOCz (30 wt%) and SFCODPAC (30 wt%) doped films using 2,8-bis(diphenyl-phosphoryl)-dibenzo[b,d]thiophene (PPT) as the host material (Figure S9). Because of the strong polarity of PPT, the doped films showcase the red-shifted emission peaks with lower Φ_PL compared to their corresponding neat films (Méhes et al., 2014).

FIGURE 2

Figure 2. (A) Absorption and photoluminescence (PL) (excited at 320 nm) spectra of asymmetric thermally activated delayed fluorescence (TADF) molecules in films. PL spectra of (B) SFCOCz and (C) SFCODPAC in tetrahydrofuran (THF)/water mixtures with varied water fractions (f_w). (D) Plots of I/I₀ values vs. water fractions (f_w). I₀ is representing the integrated intensity in THF. Inset: photographs of the TADF molecules in THF/water mixture. (E,F) Temperature-dependent decay profiles of (E) SFCOCz and (F) SFCODPAC in doped film (30 wt% in PPT).

TABLE 1

Table 1. Optical, thermal, and electrical properties of SFCOCz and SFCODPAC.

To demonstrate their TADF properties, a set of experiments was carried out. We attempted to estimate the ΔE_ST of SFCOCz and SFCODPAC on the basis of the fluorescence spectra and phosphorescence spectra at 77 K. As shown in Figure S10, the ΔE_STs of SFCOCz and SFCODPAC in neat and (30 wt%) doped films were 0.17 and 0.03 eV in the neat films, and 0.21 and 0.02 eV in the doped films, respectively, potentially enabling the process of RISC by thermal activation. The transient photoluminescence profiles were also performed to understand the photophysical process of SFCOCz and SFCODPAC (Figures 2D,E and Figure S11). The double exponential lifetime decay curve was observed in both of SFCOCz and SFCODPAC in neat and doped films, showing a short nanosecond lifetime of prompt fluorescence (τ_PF) and a microsecond lifetime (τ_DF) of delayed emission (Table S2). Contributed by the small ΔE_ST, τ_DF of SFCODPAC were 0.22 and 0.84 μs in the neat and doped films, respectively, which is 1,513- and 24-fold smaller than those of SFCOCz. The temperature-dependent lifetime measurements of the doped films further present efficient proof of the TADF characteristics of these two materials. The delayed component afforded by the RISC was increased gradually, with the temperature increasing from 77 to 300 K (Figures 2E,F), obviously indicating the TADF trait. By means of the PLQY and lifetime, the calculated rate of RISC (Supplementary Material) are 0.1 × 10⁵ and 6.16 × 10⁶ s⁻¹ in neat films, and 1.3 × 10⁵ and 2.16 × 10⁶ s⁻¹ in doped films for SFCOCz and SFCODPAC, respectively. The greatly improved RISC of SFCODPAC could be attributed to its smaller ΔE_ST.

Theoretical and Electrochemical Investigations

As shown in Figure 3, the spatial distributions of HOMOs and LUMOs of SFCOCz and SFCODPAC were clearly observed (Leitl et al., 2014). The HOMOs are largely located on the electron-donating (D₂) unit of Cz for SFCOCz and DPAC for SFCODPAC, while the LUMOs are mainly concentrated on CO and slightly located on the SF unit, showing an overlap extent (I_H/L) of 32.2 and 10.9%, respectively (Chen et al., 2015). The discrete HOMOs and LUMOs distribution in the asymmetric D₁−A–D₂ materials would be favorable for realizing small ΔE_ST. The simulated ΔE_STs were 0.38 and 0.01 eV for SFCOCz and SFCODPAC in the monomeric state. The decreased ΔE_ST of SFCODPAC may be originated from the combined effect of the strong electron-donating ability of DPAC and large dihedral angle between CO and DPAC for more separated FMO distributions. The FMO energy levels of these two molecules were analyzed by the cyclic voltammetric (CV) curves. According to the oxidation onset at 0.94 and 0.76 V, the HOMOs were evaluated to be −5.70 and −5.52 eV for SFCOCz and SFCODPAC, respectively. By means of optical bandgaps and HOMOs, the LUMOs were speculated to be −2.66 and −2.64 eV. To reveal the electron transition components of excited states in SFCOCz and SFCODPAC, the natural transition orbital (NTO) analyses were also carried out (Li et al., 2018). The highest occupied NTO (HONTO) and the lowest unoccupied NTO (LUNTO) distributions of SFCOCz and SFCODPAC at excited states are almost identical to their corresponding ground-state FMOs. The HONTOs of these two molecules were primarily dominated by the donor moiety; the LUNTO were largely assigned on the CO core with slight extension on the SF unit. The overlap extent of HONTO and LUNTO is 41.5 and 16.2% on singlet state (I_s), and 65.7 and 22.2% on triplet state (I_T) for SFCOCz and SFCODPAC, respectively. The overlap at the excited state should play an important role in guaranteeing high PLQY.

FIGURE 3

Figure 3. Density functional theory (DFT) calculated frontier molecular orbital (FMO) distributions, natural transition orbital (NTO) analyses, and I_H/L, I_S, and I_T of asymmetric thermally activated delayed fluorescence (TADF) molecules of SFCOCz and SFCODPAC.

Electroluminescent Performance of OLEDs

To further elucidate the feasibility of asymmetric D₁−A–D₂ molecules in constructing high-performance thermal-evaporated devices, the non-doped (TA-TB) and doped (TC-TD) TADF OLED of SFCOCz (TA, TC) and SFCODPAC (TB, TD) were successfully fabricated using the following configurations (Figure 4A): ITO/MoO₃ (30 nm)/4,4′-bis[N-(1-naphthyl)-N-phenylamino]-1,1′-biphenyl (NPB) (50 nm)/1,3-bis(carbazol-9-yl)benzene (mCP) (5 nm)/EML (20 nm)/PPT (5 nm)/bathophenanthroline (BPhen) (5 nm)/LiF (1 nm)/Al (100 nm). In these devices, NPB and BPhen were hole- and electron-transporting layers, and mCP and PPT were exciton-blocking layers. TA-TD exhibited pure electroluminescence (EL) spectra inherited from their neat and doped films at different driving voltages, showing maximum emission peaks at 451, 482, 468, and 485 nm with corresponding Commission International de l'Eclairage (CIE) coordinates of (0.16, 0.11), (0.20, 0.37), (0.15, 0.18), and (0.19, 0.37), respectively (Table S3). Compared to SFCOCz-based TA and TC, SFCODPAC-endowed TADF OLEDs of TB and TD revealed slightly decreased driving voltages (V_ons) of 4.4 and 3.8 V, respectively. In addition to the decreased V_on, TB and TD also demonstrated improved luminance up to 5,598 and 11,595 cd m⁻² than those of TA and TC. The low V_ons and high luminance of TB and TD should be attributed to its high HOMO and PLQYs of SFCODPAC for the promotion of hole injection from the adjacent layer and the generation of excitons. Not surprisingly, TADF OLEDs based on SFCODPAC show much better device efficiencies with current efficiencies (CEs) of 16.8 and 35.6 cd/A, power efficiencies (PEs) of 9.1 and 29.8 lm/W, and EQEs of 7.5 and 15.9% for TB and TD, respectively (Figures 4B,C and Table S3). There values are comparable to the best results of non-doped and doped TADF OLEDs based on AIE-type TADF emitters (Table S4). In addition, TD displays acceptable device performance with 29.1 and 21.0 cd A⁻¹ for CE, 11.8 and 5.6 lm W⁻¹ for PE, and 13.1 and 9.4% for EQE at 100 and 1,000 cd m⁻², respectively. Although the non-doped films suggest enhanced PLQYs, the poor charge transport properties in neat films induced by highly twisted asymmetric molecular skeleton should be responsible for the decreased device performance of non-doped TADF OLEDs.

FIGURE 4

Figure 4. (A) Device configuration and chemical structures of the adopted materials, (B) luminance (solid symbols)–current density (open symbols)–voltage inset: electroluminescence (EL) spectra of thermally activated delayed fluorescence organic light-emitting diodes (TADF OLEDs) at 6.0 V, and (C) efficiencies–luminescence curves of the non-doped and doped TADF OLEDs.

Conclusions

In summary, we have succeeded in designing and developing two asymmetric TADF molecules of SFCOCz and SFCODPAC in the D₁−A–D₂ skeleton through a simple and effective procedure in high yields. The resultant TADF molecules exhibit small ΔE_ST, short delayed lifetimes, and robust AIE characteristics with high PLQY up to 73%. More impressively, benefiting from the excellent optoelectronic properties, the non-doped and doped TADF OLEDs conferred by SFCODPAC display high efficiencies with peak EQEs of 7.5 and 15.9%, accompanied with the CIE coordinates of (0.20, 0.37) and (0.19, 0.37), respectively. Our work here provides a delicate molecular design strategy for the construction of asymmetric AIE-type TADF emitters, and clearly manifests the significant advance of the combined TADF and AIE features in exploiting high-performance organic emitters.

Data Availability Statement

The datasets generated for this study can be found in the Cambridge Structural Database under the following identifiers: CCDC/1964730, CCDC/1964747.

Author Contributions

HuaL, YZ, YT, WH, and RC conceived the experiment. HuaL, YZ, YJ, and HuiL performed the characterizations of optoelectronic properties. YD, YT, and ML designed and fabricated the devices. QY and PL contributed the theoretical simulations. HuaL, YZ, YJ, and YT wrote the manuscript. All authors contributed to the scientific discussion.

Funding

This work was financially supported in part by the National Natural Science Foundation of China (21604039, 21704042, 61875090, 91833306, and 51873159), the Six Talent Plan of Jiangsu Province (XCL-049), 1311 Talents Program of Nanjing University of Posts and Telecommunications (Dingshan), China Postdoctoral Science Foundation project (2018M642284), and Nanjing University of Posts and Telecommunications Start-up Fund (NUPTSF) (NY219007 and NY217140).

Conflict of Interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Supplementary Material

The Supplementary Material for this article can be found online at: https://www.frontiersin.org/articles/10.3389/fchem.2020.00049/full#supplementary-material

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