Chinese Chemical Letters  2019, Vol. 30 Issue (11): 1955-1958   PDF    
Molecular engineering on all ortho-linked carbazole/oxadiazole hybrids toward highly-efficient thermally activated delayed fluorescence materials in OLEDs
Wenbo Yuana,1, Hannan Yangb,1, Mucan Zhanga,1, Die Hua, Shigang Wana, Zijing Lia, Changsheng Shib,*, Ning Sunb, Youtian Taoa,*, Wei Huanga,c     
a Key Lab for Flexible Electronics & Institute of Advanced Materials(IAM), Nanjing Tech University, Nanjing 211816, China;
b Department of Physics, Center for Optoelectronics Engineering Research, Yunnan University, Kunming 650091, China;
c Shaanxi Institute of Flexible Electronics(SIFE), North western Polytechnical University(NPU), Xi'an 710072, China
Abstract: The highest efficiency thermally activated delayed fluorescence (TADF) emitters in OLEDs are mostly based on twisted donor/acceptor (D/A) type organic molecules. Herein, we report the rational molecular design on twisted all ortho-linked carbazole/oxadiazole (Cz/OXD) hybrids with tunable D-A interactions by adjusting the numbers of donor/acceptor units and electron-donating abilities. Singlet-triplet energy bandgaps (ΔEST) are facilely tuned from~0.4, 0.15 to~0 eV in D-A, D-A-D to A-D-A type compounds. This variation correlates well with triplet-excited-state frontier orbital spatial separation efficiency. NonTADF feature with solid state photoluminescence quantum yield (PLQY) < 10% is observed in D-A type 2CzOXD and D-A-D type 4CzOXD. Owing to the extremely low ΔEST for efficient reverse intersystem crossing, strong TADF with PLQY of 71%-92% is achieved in A-D-A type 4CzDOXD and 4tCzDOXD. High external quantum efficiency from 19.4% to 22.6% is achieved in A-D-A typed 4CzDOXD and 4tCzDOXD.
Keywords: Oxadiazole    Carbazole    Donor-acceptor    Organic light-emitting diodes    Thermally activated delayed fluorescence    

Organic light-emitting diodes (OLEDs) have been widely developed in both academia and industry owing to their great potential applications in flat-panel displays and solid-state lightings [1-3]. According to the spin statistics rule, the ratio of electrogenerated singlet and triplet excitons in an OLED is 1:3 [4]. Therefore, the maximum internal quantum efficiency (IQE) of traditional fluorescence OLED is restricted to 25% due to the harvest of only singlet excitons. In addition, phosphorescent OLEDs (PHOLEDs) based on heavy-metal containing triplet emitters can reach a theoretical 100% IQE by the simultaneous utilization of all singlet and triplet excitons through intersystem crossing (ISC) [5]. However, the expensive noble metals and the lack of available pure blue phosphorescent emitters spur the exploration of new alternatives. Very recently, another emission mechanism of thermally activated delayed fluorescence (TADF) which can achieve 100% IQE without involving any precious heavy metals has attracted considerable attention [6-9].

In a TADF emitter, triplet state (T1) excitons could be efficiently up-converted into singlet (S1) excitons by endothermic reverse inter-system crossing (RISC) process due to the sufficiently low ∆EST [10-12]. It is reported that to reduce ∆EST, an effective spatial isolation simultaneously with a small portion of overlap between the ground state highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) on their corresponding hole- and electron-transport units is expected [13]. Such separation could be realized in twisted donor-acceptor (D-A) type molecules which induce twisted intramolecular charge transfer (ICT) transitions [14]. So far, the most efficient TADF emitters with the state-of-the-art external quantum efficiency (e.g., EQE > 20%) in OLEDs are mostly based on twisted D-A structured pure organic materials [15-24]. However, twisted D-A materials with efficient HOMO and LUMO separation may not always induce small ∆EST for TADF characteristics [25].

To distinguish effective TADF materials from diverse twisted D-A molecules with efficient HOMO and LUMO separation, in this work, we designed and synthesized a series of all twisted ortho-linked carbazole/oxadiazole (Cz/OXD) hybrid compounds. It is well known that carbazole is one of the most popular electron-donating building blocks to construct various OLED materials such as hole-transport, organic host as well as TADF materials. On the other hand, the oxadiazole-acceptor containing PBD and OXD7 are famous electron-transport materials in solution processed OLEDs [26, 27]. Besides, oxadiazole has been proved to be very effective for constructing D-A type bipolar transport host materials as well as TADF emitters [28-33].

Herein, combined with theoretical calculations, we designed and synthesized a series of carbazole/oxadiazole (Cz/OXD) hybrid com-pounds by connecting all carbazole donor units to the ortho-position of phenyl substituted 1, 3, 4-oxadiazole acceptors. The all ortho-linked Cz/OXD hybrids demonstrated relatively efficient ground state (S0) HOMO and LUMO separation. Differently, the D-A type 2CzOXD showed large portion (2, 5-diphenyl-1, 3, 4-oxadiazole) of triplet excited state highest occupied natural transition orbitals (HONTO) and lowest unoccupied natural transition orbitals (LUNTO) overlap, and the D-A-D type 4CzOXD exhibited moderate overlap (2, 5-diphenyl) at triplet excited state, while the A-D-A structured 4CzDOXD and 4tCzDOXD presented the most efficient separation. Thus, ∆EST could be facilely tuned from ~0.4, 0.15 to ~0 eV in D-A, D-A-D to A-D-A type compounds. Non-TADF characteristics with solid-state photoluminescence quantum yield (PLQY) < 10% was found in D-A and D-A-D type 2CzOXD and 4CzOXD due to large ∆EST, while strong TADF with PLQY up to 92% was achieved in A-D-A type 4CzDOXD and 4tCzDOXD. Varying donor property from Cz to 3, 6-tert-butyl carbazole (tCz) improved the device EQE from 19.4% to 20.8% and 20.3% to 22.6% when with different organic hosts.

Fig. 1 shows the chemical structures of the new carbazole/ oxadiazole hybrid compounds. Distorted ortho-linkage was employed between all carbazole donor and oxadiazole acceptor for twisted molecular conformation. The four compounds could be favorably synthesized through a simple step catalyst-free C-N coupling reaction by using carbazole and the corresponding electron withdrawing oxadiazole activated fluorophores as starting materials [28, 34, 35]. Dual-OXD activated 4CzDOXD and 4tCzDOXD showed satisfactory high yields over 90%, while mono-OXD substituted 2CzOXD and 4CzOXD exhibited lower yields of 60 and 45%, respectively (Scheme S1 in Supporting information). All new compounds were fully characterized by 1H NMR, 13C NMR, mass spectrometry and elemental analysis (Fig. S1 in Supporting information).

From density functional theory (DFT) calculations of the optimized 3D geometries, the ortho-linkage between carbazole and the phenyl ring of diphenyl-1, 3, 4-oxadiazole resulted in large twisted angles of 58.4°-70.2°, which significantly reduced the conjugation between donor and acceptor. As shown in Fig. 1, the ground state (S0) orbital distributions indicated relatively efficient separation between HOMO and LUMO, with a small portion of overlap at the phenyl ring neighboring to both N-carbazole and 1, 3, 4-oxadiazoles for all compounds. Besides, the HOMO and LUMO were mainly located on carbazole donor and phenyl-substituted oxadiazole acceptors, respectively. At triplet excited state, separation efficiency of the highest occupied natural transition orbitals (HONTO) and lowest unoccupied natural transition orbitals (LUNTO) between the electron-donating and accepting units followed in the orders of 4CzDOXD and 4tCzDOXD > 4CzOXD > > 2CzOXD. The A-D-A type 4CzDOXD and 4tCzDOXD only exhibited tiny overlap at the central phenyl, and D-A-D structured 4CzOXD displayed moderate overlap at the two phenyl rings of 2, 5-biphenyl-1, 3, 4-oxadiazole, while the D-A type 2CzOXD demonstrated a large portion of overlap spreading the whole acceptor of 2, 5-biphenyl-1, 3, 4-oxadiazole. The efficient separation of HONTO and LUNTO suggested strong twisted intramolecular charge transfer at triplet state. Therefore, potential TADF characteristic was supposed to achieve in both 4CzDOXD and 4tCzDOXD.

Fig. 1. Chemical structures, optimized 3D geometries comprising dihedral angles and dipole moments (μ) and ground state (S0) HOMO/LUMO and triplet excited state (T1) HONTO/LUNTO distributions.

Fig. 2a and Fig. S3 (Supporting information) showed the UV-vis absorption and PL spectra of the four compounds in neat film and dilute CH2Cl2 solution. The relatively intense absorption below 350 nm was attributed the localized n-π* and π-π* transitions, while absorption tails > 350 nm for all four compounds were ascribed to twisted ICT transition from carbazole donors to phenyl-substituted oxadiazole acceptors. Both ICT absorption and PL for the two mono-OXD derivatives were blue-shifted than the dual-OXD hybrids. 2CzOXD and 4CzOXD depicted deep-blue fluores-cence with emission peaks at 427 nm and 436 nm in film, and PLQY of only 9% and 6% in solid powder, respectively. However, 4CzDOXD and 4tCzDOXD demonstrated strong yellowish-green emission with peaks at 544 and 548 nm, and the solid PLQY greatly improved to 92% and 71%, respectively. The singlet (S1) and triplet (T1) energy measured from the 77 K fluorescence and phosphorescence spectra (Fig. 2b) are listed in Table S1 (Supporting information). D-A structured 2CzOXD exhibited the largest ∆EST of ~0.40 eV, followed by ~0.15 eV of D-A-D structured 4CzOXD, while the A-D-A type 4CzDOXD and 4tCzDOXD demonstrated the lowest ∆EST of ~0 eV, which was beneficial to effective RISC process. The trend was in very good agreement with the extent of triplet excited state HONTO and LUNTO separation. The most efficient spatial separa-tion in 4CzDOXD and 4tCzDOXD resulted in the lowest ∆EST, and the most overlap in 2CzOXD leads to the largest ∆EST.

Fig. 2. (a) Normalized UV-vis absorption (up) and PL spectra (down), (b) fluorescence (up) and phosphorescence (down) spectra at 77 K and (c) transient decay curves of compounds 2CzOXD, 4CzOXD, 4CzDOXD and 4tCzDOXD in neat film (inserted: 2CzOXD and 4CzOXD in PMMA doped film (5%)).

2CzOXD and 4CzOXD only displayed short-lived transient PL decay (Fig. 2c) with lifetime fitted at 5.38 and 6.15 ns, respectively, indicating their conventional fluorescence behavior. On the other hand, both 4CzDOXD and 4tCzDOXD demonstrated two components of PL decay, with the fast/slow decay lifetimes at 23.2 ns/ 1.405 μs and 18.9 ns/0.956 μs, respectively, which was undoubtedly ascribed to the prompt/delayed fluorescence. Moreover, the delayed decay components displayed notable temperature depen-dence (Fig. S4 in Supporting information). Together with the increased PLQY from 14% to 54% for 4CzDOXD and 18% to 38% for 4tCzDOXD at ambient to deoxygenated toluene solutions, the TADF feature of 4CzDOXD and 4tCzDOXD could be further illustrated.

To get insight into the radiative and non-radiative decay processes for the two A-D-A type TADF emitters, a series of related photophysical rate constants were estimated according to a previously reported method [36] by using the solid state PLQY and lifetime data obtained at 300 K (Table S2 in Supporting information). Thus, the fluorescence radiative decay (κF) and intersystem crossing (κISC) rate constants were determined to be 1.39 × 107 and 2.91 × 107 s-1 for 4CzDOXD as well as 9.36 × 106 and 4.38 × 107 s-1 for 4tCzDOXD, respectively. The extremely small ∆EST of nearly 0 eV account for the fast κRISC of 1.94 × 106 and 2.93 × 106 s-1 for the two compounds, which indicated an effective RISC process. Our results revealed that excellent TADF property and high-efficiency photoluminescence can be achieved based on the A-D-A type structural design rather than D-A and D-A-D when oxadiazole was used as the acceptor unit.

Determined from the onset of quasi-reversible oxidation curves from cyclic voltammetry (CV) measurement (Fig. S5 in Supporting information), the HOMO level was measured to be -5.65, -5.51, -5.66 and -5.53 eV, while the LUMO levels calculated from the difference between HOMO and optical bandgap was estimated to be -2.30, -2.25, -2.96 and -2.72 eV for 2CzOXD, 4CzOXD, 4CzDOXD and 4tCzDOXD, respectively. The trend for these values was in good agreement with the theoretical calculations. Due to the double electron withdrawing OXD units, the LUMO for the A-D-A compounds was lowered than the mono-OXD derivatives. The thermal properties of were examined by thermal gravimetric analysis (TGA) and differential scanning calorimetry (DSC). All compounds exhibited favorable thermal stability with decomposi-tion temperature (Td, 5% weight loss) over 400 ℃. The value for 4CzDOXD and 4tCzDOXD which will be employed in the OLED devices was as high as 469 ℃ and 475 ℃ owing to their rigid bulky molecular skeleton (Fig. S6 in Supporting information). Unfortunately, the melting point (Tm) and glass transition temperature (Tg) was not observed from DSC measurement.

Inspired by the excellent TADF characteristics of 4CzDOXD and 4tCzDOXD emitters, vacuum-deposited TADF OLEDs using different organic host materials of 4, 4'-bis(N-carbazolyl)-1, 1'-biphenyl (CBP) and 2, 5-bis(2-(9H-carbazole-9-yl)phenyl)-1, 3, 4-oxadiazole (o-CzOXD) were fabricated with the configuration of ITO/MoO3/ TAPC (40 nm)/CBP or o-CzOXD: emitter (10 wt%, 20 nm)/TmPyPB (40 nm)/LiF/Al. The chemical structures for related organic materials and device energy level diagrams are shown in Fig. S7 (Supporting information). The device fabrication details including the doping concentration dependent experiments (Fig. S8 in Supporting information) are given in Supporting information. The luminance-voltage- current density (L-V-J) curves are shown in Fig. 3a and the key device data are summarized in Table 1. The electroluminescence (EL) spectra (Fig. 3b) of all devices revealed bright green emission peaking at around 520 and 530 nm for 4CzDOXD and 4tCzDOXD, respectively, which was in agreement with their PL spectra. All devices showed outstanding device efficiency with maximum EQE approaching to or even exceeding 20% (Fig. 3c). It is noted that the tert-butyl substituted 4tCzDOXD emitter presented higher EL efficiency than 4CzDOXD either with CBP or o-CzOXD host, which might be contributed from the more suitable D-A interaction with stronger electron-donating 3, 6-tert-butyl carbazole instead of weaker carbazole in twisted D-A TADF materials. The best EL efficiency was attained in o-CzOXD:4tCzDOXD based device D, with maximum CE of 74.5 cd/A and EQE of 22.6%. We believe the EL performance could be further improved by carefully optimizing device conditions for more efficient and balanced charge injection and transport.

Fig. 3. (a) L-V-J characteristics, (b) normalized electroluminescent (EL) spectra, (c) current efficiency (CE), power efficiency (PE) and external quantum efficiency (EQE) versus luminance curves of devices A-D.

Table 1
Summary of key EL data for devices A-D.

In summary, four all ortho-linked carbazole/oxadiazole hybrids have been designed and synthesized through a simple catalyst free C-N coupling reaction. Through judicious molecular engineering by adjusting the numbers of the donor and acceptor units, the singlet-triplet energy bandgaps have been facilely tuned in a wide range from 0.4 eV to 0.15 eV, and further to ~0 eV for D-A, D-A-D and A-D-A structured compounds. DFT calculations have proved that the lower ∆EST could be achieved in a more efficient frontier orbital separation at the triplet excited state. The less separation of HONTO and LUNTO at triplet state for twisted D-A type 2CzOXD and D-A-D type 4CzOXD induced weak conventional fluorescence with solid state PLQY < 10%, while efficient separation for A-D-A type 4CzDOXD and 4tCzDOXD resulted in outstanding TADF property with PLQY up to 92%. Maximum EQE of 20.3% and 22.6% have been attained for 4CzDOXD and 4tCzDOXD, respectively. In combine with theoretical calculations, our work provides guidance on the design of highly efficient TADF materials.


We thank the National Natural Science Foundation of China (Nos. 91833304, 61805211), National Key Research and Develop-ment Program of China for the Joint Research Program between China and European Union (No. 2016YFE0112000), the Natural Science Foundation of Jiangsu Province (Nos. BK20160042 and XYDXX-026) and the Foundation for the Author of National Excellent Doctoral Dissertation of China FANEDD (No. 201436).

Appendix A. Supplementary data

Supplementary material related to this article can be found, in the online version, at doi:

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