Thermally Activated Delayed Fluorescence Carbonyl Derivatives for

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Thermally Activated Delayed Fluorescence Carbonyl Derivatives for Organic Light Emitting Diodes with Extremely Narrow Full-Width at Half-Maximum Xing Li, Yizhong Shi, Kai Wang, Ming Zhang, Cai-Jun Zheng, Dian-Ming Sun, Gaole Dai, Xiao-Chun Fan, De-Qi Wang, Wei Liu, Yanqing Li, Jia Yu, Xue-Mei Ou, Chihaya Adachi, and Xiaohong Zhang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b19635 • Publication Date (Web): 20 Mar 2019 Downloaded from http://pubs.acs.org on March 20, 2019

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ACS Applied Materials & Interfaces

Thermally Activated Delayed Fluorescence Carbonyl Derivatives for Organic Light Emitting Diodes with Extremely Narrow Full-Width at Half-Maximum Xing Li, ‡, † Yi-Zhong Shi, ‡, † Kai Wang,*, † Ming Zhang,†, § Cai-Jun Zheng,*, § Dian-Ming Sun,† Gao-Le Dai, † Xiao-Chun Fan, † De-Qi Wang, †, § Wei Liu, † Yan-Qing Li, † Jia Yu, † XueMei Ou†, Chihaya Adachi, #,┴,║ and Xiao-Hong Zhang*,† †

Institute of Functional Nano & Soft Materials (FUNSOM), Jiangsu Key Laboratory for

Carbon-Based Functional Materials & Devices, Soochow University, Suzhou, Jiangsu 215123 (P.R. China) §

School of Optoelectronic Science and Engineering, University of Electronic Science and

Technology of China (UESTC), Chengdu, Sichuan 610054 (P.R. China) #Center

for Organic Photonics and Electronics Research (OPERA), Kyushu University, 744

Motooka, Nishi, Fukuoka 819-0395 (Japan) ┴Japan

Science and Technology Agency (JST), ERATO, Adachi Molecular Exciton

Engineering Project, 744 Motooka, Nishi, Fukuoka 819-0395 (Japan) ║International

Institute for Carbon Neutral Energy Research (WPI-I2CNER), Kyushu

University, 744 Motooka, Nishi, Fukuoka 819-0395 (Japan) Keywords:

Thermally activated delayed fluorescence, Resonance effect, Narrow full-width

at half-maximum, Organic light emitting diodes, Rigid framework Abstract: Two

novel

thermally

activated

delayed

fluorescence

(TADF)

emitters,

3-

phenylquinolino[3,2,1-de] acridine-5,9-dione (3-PhQAD) and 7-phenylquinolino[3,2,1-de] acridine-5,9-dione (7-PhQAD), were designed and synthesized based on a rigid quinolino[3,2,1-de] acridine-5,9-dione (QAD) framework. With the effective superimposed resonance effect from electron-deficient carbonyls and electron-rich nitrogen atom, both emitters realize significant TADF characteristics with small ΔESTs of 0.18 and 0.19 eV 1 ACS Paragon Plus Environment

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respectively. And molecular relaxations were dramatically suppressed for both emitters because of their conjugated structure. In the devices, 3-PhQAD realizes superior performance with a maximum EQE of 19.1% and a narrow full-width at half-maximum (FWHM) of 44 nm, while a maximum EQE of 18.7% and an extremely narrow FWHM of 34 nm are realized for 7PhQAD. These superior results reveal that apart from nitrogen and boron-aromatic systems, QAD framework can also act as a TADF matrix with effective resonance effect, and QAD derivatives are ideal candidates to develop TADF emitters with narrow FWHMs for practical applications. 1. Introduction Within recent decades, organic light emitting diodes (OLEDs) have been making great progress and emerging as the most promising technologies for full-color display and lighting applications. 1-7 In OLEDs, electrogenerated excitons can be characterized as singlet and triplet ones with an initial ratio of 1:3.8-10 Triplet excitons cannot be directly utilized via traditional fluorescence process and how to harvest all the excitons is crucial for OLEDs. Though noblemetal-contained phosphors can greatly accelerate the triplet excitons radiation process and solve above mentioned problem, their high costs limit their further applications in OLEDs.6, 11-13

The introduction of thermally activated delayed fluorescence (TADF) emitters is thus

considered as the most advanced approach since they can up-convert triplet excitons into singlet ones via the reverse intersystem crossing (RISC) process under the room temperature and realize a theoretically full exciton utilization with cheap pure organic compounds.

3, 14-24

To

activate the RISC channel, a small energy splitting (ΔEST) between the lowest singlet excited state (S1) and triplet excited state (T1) is essential. 14, 25-26 Currently, TADF emitters are almost all designed with a universal concept of combining electron-donor (D) and electron-acceptor (A) segments with various conjugation-restricted linkages. 24, 27-31 Obviously, such concept can easily isolate the highest occupied molecular orbitals (HOMOs) and lowest unoccupied 2 ACS Paragon Plus Environment

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molecular orbitals (LUMOs) in different molecular parts, and thus minimize the ΔESTs.

17

However, due to the lack of restrictions on the conformational change, such concept would also lead evident molecular structural relaxations during excitations and result in large Stokes shifts. 32

As a consequence, these D-A structure TADF emitters typically exhibit broad and

structureless emissions in OLEDs with full-widths at half-maximum (FWHMs) as broad as 70100 nm. 32-33 And large FWHM values are detrimental to the color purity of TADF emitters and greatly delay their applications for high-resolution OLED display. 34 To address this issue, in 2016, Prof. Hatakeyama et. al. proposed a novel strategy to separate HOMO and LUMO by integrating the opposite resonance effect of the electron-withdrawing atoms and electron-donating atoms in one rigid aromatic framework.

35

Both their designed

emitters are based on nitrogen (N) and boron (B) atoms to produce different resonance and successfully realize TADF characteristic with maximum external quantum efficiency (EQE) of ~20% and remarkable cropped FWHM of ~30 nm in OLEDs. Currently, subject to the delicate atom selections (B and N atoms respectively as electron-withdrawing core and electrondonating moieties) in order to produce an effective aromatic system, the further attempts are still limited to the specific B, N-based framework with only slight variations.

36-38

Thus, it is

challengeable to further explore more practical frameworks with resonance effect for TADF emitters with narrow FWHMs. In this work, we identified a rigid framework quinolino[3,2,1-de] acridine-5,9-dione (QAD) as a TADF matrix with effective resonance effect and further developed two novel TADF compounds

3-phenylquinolino[3,2,1-de]

acridine-5,9-dione

(3-PhQAD)

and

7-

phenylquinolino[3,2,1-de] acridine-5,9-dione (7-PhQAD) based on QAD framework respectively with an additional phenyl ring as the tails attached on different sites. For both QAD derivatives, electron-donating nitrogen core is centered among three phenyl rings, while two carbonyl groups are connected with two phenyl rings at the ortho-positions relative to nitrogen 3 ACS Paragon Plus Environment


atom to act as the electron-withdrawing moieties as well as conjugated chains to restrain the molecular relaxation. 39 The superimposed resonance effect mainly results from carbonyls and nitrogen atom. As expected, the distributions of HOMOs and LUMOs are successfully separated in different sites in each phenyl rings of QAD frameworks. The additional phenyl tails only contribute the extension of HOMOs while the LUMOs are well confined in the QAD framework. Both 3-PhQAD and 7-PhQAD realize significant TADF characteristics with ΔESTs of 0.18 and 0.19 eV respectively. Moreover, due to the conjugated rigid molecular-frameworks, structural relaxations are significantly suppressed for two emitters. In toluene solution at room temperature, narrow and sharp emission spectra as well as extremely small Stokes shifts are obtained with an FWHM of 30 nm and a shift of 23 nm relative to the absorptions for 3-PhQAD, and an FWHM of 22 nm and a shift of 18 nm for 7-PhQAD, respectively. In OLEDs, 3-PhQAD realizes a maximum EQE of 19.1% and a narrow FWHM of 44 nm; while a maximum EQE of 18.7% and an extremely narrow FWHM of 34 nm are obtained for 7-PhQAD. These superior performances reveal that apart from B, N-aromatic systems, QAD framework can also act as TADF matrix based on resonance effect. It is expected our results would bring a new insight into exploiting novel TADF emitters with narrow FWHM for practical applications. 2. Results and Discussion The molecular structures of 3-PhQAD and 7-PhQAD are shown in Figure 1. Compared with previous B, N-aromatic systems, the rigid QAD framework is constructed by a sole electrondonating N atom as the core and electron-withdrawing carbonyl groups at the ortho-position to enforce the opposite resonance effect to the core. As a result, effective HOMO-LUMO separation can be also expected in QAD framework. Moreover, the introduction of phenyl ring on the QAD framework with 3 and 7-positision are expected to differ their molecular symmetry, which may disturb the distributions of the frontier molecular orbitals (FMO) on the QAD framework and eventually influence their relaxation process. 4 ACS Paragon Plus Environment

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Scheme 1 illustrates the synthetic routes of 3-PhQAD and 7-PhQAD. First, the intermediate dimethyl 2,2'-([1,1'-biphenyl]-4-ylazanediyl)dibenzoate was synthesized via the Ullmann coupling reaction between [1,1'-biphenyl]-4-amine and 2-iodobenzoic acid methyl ester and then following hydrolysis reaction with sodium hydroxide in dioxane/H2O (1:1) mixture. Finally, the target molecules were obtained via the Friedel–Crafts acylation induced cyclization reaction with an assistance of thionyl chloride and aluminum chloride. Owing to the different cyclization directions, 3-PhQAD and 7-PhQAD can be obtained simultaneously as isomers, separated and purified with yields of 53% and 27% by silica gel chromatography before further utilizations. The chemical structures of the intermediates and target compounds were fully characterized and confirmed by nuclear magnetic resonance (NMR) spectroscopy, mass spectrometry (MS) and elemental analysis (EA). Theoretical calculations were firstly carried out to simulate the molecular structures and electronic transition properties. As shown in Figure S1, on ground state (S0), all the atoms in initial QAD framework share an extended π-conjugation helical structure with C2-symmetry owing to the steric exclusion of hydrogen atoms from the bay area. Figure S2a exhibits the FMO distributions of a sole QAD framework. HOMO originates from the electron-rich N core and LUMO predominantly locates on the carbonyl groups.39 Because of the superimposing opposite resonance effect of the electron-donating and electron-withdrawing groups, the distributions of FMO are well separated in interphase sites of the phenyl rings, suggesting it is an ideal framework with resonance effect. Compared with the sole QAD framework, 7-PhQAD still remains C2-symmetry, because the added phenyl ring passes through the symmetry axis of QAD framework. While 3-PhQAD with the phenyl group attached on the QAD framework at 3-position is an asymmetric structure. It is expected that 3-PhQAD will have more complicated energy level splittings in various conformational forms with different energies than 7-PhQAD, which may result in a broader FWHM in its emission spectrum. 40 The FMOs of 3-PhQAD and 5 ACS Paragon Plus Environment


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7-PhQAD are illustrated in Figure 1a. Through there are evident HOMOs extended to additional phenyl rings, which may give rise to a slight intramolecular charge transfer (ICT) character mixed, their FMO distributions on QAD parts remain similar with the initial QAD framework, suggesting the resonance effect is remained for both emitters. The calculated ΔEST of both 3PhQAD and 7-PhQAD are respectively estimated to be 0.55 and 0.60 eV, which are similar with initial QAD framework (0.57 eV) and the reported B, N-based emitter.35 Figure 1b illustrates a typical transition process in molecular potential energy surfaces. Accompanying more structural relaxations upon excitation, the optimized geometry of S0 state avoidably shifts to an optimized one on S1 state, and the geometric difference between S0 and S1 states will significantly influence the Stokes shift and FWHM value of emission spectra. Thus, to elucidate the molecular relaxation process upon excitation, we further calculated and obtained the corresponding optimal geometries of the excited state. As seen in Figure S1, comparing the geometries on S0 states, almost no conformation change can be observed in QAD derivatives during excitation because of the rigid constructions, indicating that QAD is also an ideal framework to restrict molecular relaxation. The most recognized differences for these two emitters are characterized from the free rotated C-C bridges between QAD and phenyl tails. The variations of the dihedrals and the bond lengths during exactions are calculated to be within 10° and 0.012 Å for 7-PhQAD while within slightly higher values of 13° and 0.022 Å for 3PhQAD, respectively. Furthermore, by comparing the optimized geometries between S0 and S1 states, the root mean square deviation (RMSD) of 3-PhQAD is calculated to be 0.021 Å larger than that of 7-PhQAD (seen in Figure 1a). These results reveal that 3-PhQAD would suffer larger structural relaxation than 7-PhQAD because of its breaking symmetry. In particular, the estimated reorganization energies (the potential energy differences between G0 and GV in Figure 1b), which have crucial impacts on the resulting spectral shapes and FWHMs,

6 ACS Paragon Plus Environment

41

are

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calculated to be 0.22 eV for 3-PhQAD and 0.08 eV for 7-PhQAD, further revealing 7-PhQAD would have a narrower FWHM than 3-PhQAD. Photophysical properties of both QAD derivatives were then characterized. As shown in Figure 2a and b, in diluted toluene at room temperature, both 3-PhQAD and 7-PhQAD exhibit sharp and strong absorption bands with nearly identical shapes in the range higher than 360 nm (peaked at 443 nm for 3-PhQAD and 446 nm for 7-PhQAD, respectively), which should be ascribed to their same QAD framework. Well consistent with the behavior of the core QAD moiety, mirror-images can also be observed in their emission spectra with peaks at 466 nm for 3-PhQAD and 464 nm for 7-PhQAD, respectively.39 In particular, comparing the corresponding absorption peaks, the Stokes shift of 7-PhQAD can be estimated to be 900 cm-1, which is 178 cm-1 smaller than that of for 3-PhQAD (1078 cm-1). Moreover, the FWHMs of emission spectra are respectively estimated to be as narrow as 22 nm for 7-PhQAD and a slightly broader value of 30 nm for 3-PhQAD. These results are well consistent with our theoretical prediction, further proving molecular relaxations are strongly restricted because of the conjugated structures. To further explore the excitation transition characteristics, we further measured the emission spectra of 3-PhQAD and 7-PhQAD in different solvents at room temperature. As shown in Figure 2c, 2d and summarized in Table S2, with environmental polarity increased from nonpolar hexane to the highest polar acetonitrile, typical solvatochromic effect can be observed for both emitters. In particular, the spectra exhibit gradually featureless variations with increasing environmental polarity. These results further identified both emitters as strongly mixed transitions of local excited and ICT states, which can induce considerable oscillator strengths, beneficial to fluorescence process.42 Meanwhile, as shown in Figure S6, both 3-PhQAD and 7PhQAD process linear relationship of the Stokes shift as a function of solvent polarity, suggesting their emissions are both from single emitting states. 43



The fluorescence and phosphorescence spectra of both QAD derivatives 2 wt% doped in 1,3di-9-carbazolylbenzene (mCP) films at 77 K are illustrated in Figure 3. From the onset of corresponding fluorescence and phosphorescence spectra, the energy levels of S1 and T1 states are estimated to be 2.60 and 2.42 eV for 3-PhQAD and 2.63 and 2.44 eV for 7-PhQAD, respectively. The slight lower S1 energy level of 3-PhQAD can be ascribed to its more obvious molecular relaxations during excitations. Accordingly, the ΔESTs are calculated to be 0.18 eV for 3-PhQAD and 0.19 eV for 7-PhQAD, respectively, enough to activate effective RISC process. Specifically, both fluorescence spectra at 77 K contain the extra emission, which should be attributed to the strong influence of their phosphorescence at such low temperature and cannot be observed at room temperature as shown in Figure 2. Photoluminescence (PL) quantum yields (PLQYs) of 2 wt% both QAD derivatives doped mCP films were measured via integrating sphere measurements under nitrogen. By exciting at 300 nm, high PLQY values of 73% for 3-PhQAD and 68% for 7-PhQAD were obtained, respectively. To further confirm their TADF properties, temperature-dependent transient PL delayed decays were further measured. Both emitters exhibit short prompt lifetimes of ~5.5 ns at room temperature. Meanwhile, significant delay components can be observed for both compounds as shown in Figure S7. With temperature gradually increased from 77 to 300 K, the intensities of delay components are observed significantly improved. As listed in Table S3, the nearly negligible delayed components at 200 K might be because relatively large ΔESTs of two emitters require high environmental temperature to activate RISC process. These results characterize both compounds as TADF emitters. In particular, at room temperature, both emitters exhibit rather long delayed lifetime of 250 μs for 3-PhQAD and 474 μs for 7-PhQAD, respectively, which might also be caused by their relatively large ΔESTs. Accordingly, the main kinetic parameters of both compounds were estimated as seen in Table S4. Their rate constants of singlet radiative processes (𝑘Sr) are significantly larger than most of reported TADF emitters, suggesting their 8 ACS Paragon Plus Environment

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effective fluorescence process because of the high oscillator strengths of the adiabatic S1 states. Meanwhile, though the rate constants of RISC processes (kRISC) of two emitters are evidently lower than most of reported TADF emitters, their rigid frameworks successfully restrict the competitive energy loss process for triplet excitons, remaining triplet excitons in RISC channels and resulting in effective TADF processes. The electrochemical properties of QAD derivatives were investigated by cyclic voltammetry (CV) measurement (shown in Figure S4) and the key physical properties were summarized in Table 1. From the onsets of oxidation and reduction curves with respect to that of ferrocene, the HOMO and LUMO energy levels are thus calculated to be -6.20 and -3.38 eV for 3-PhQAD, and -6.19 eV and -3.40 eV for 7-PhQAD, respectively. Their nearly identical HOMO and LUMO energy levels should be attributed to their same component framework. To confirm the thermal stabilities of both QAD derivatives, thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) were carried out under nitrogen atmosphere (seen in Figure S4). Both emitters show high thermal stabilities with decomposition temperatures (Td, corresponding to 5% weight loss) of 391 and 399 ℃, respectively. Moreover, within the whole measurement range from 25-300 ℃, there are no obvious glass transitions observed for both compounds. These results further confirm they are suitable to be used for OLEDs via vacuum evaporation. To evaluate their electroluminance (EL) performance, the optimized OLEDs were finally fabricated by applying 3-PhQAD and 7-PhQAD as the dopants with a device structure of ITO/TAPC (35 nm) /TCTA (10 nm)/mCP: x wt% dopant (20 nm)/TmPyPB (40 nm)/LiF (1 nm)/Al, in which the ITO is indium tin oxide as the anode, while LiF/Al is performed as the cathode; TAPC represents 1,1-bis[4-[N,N-di(p-tolyl)-amino]phenyl]cyclohexane, as the hole transport layer; TCTA is 4,4',4''-tris(carbazol-9-yl)-triphenylamine, as the hole injecting and electron blocking layer; TmPyPB is 1,3,5-tri[(3-pyridyl)-phen-3-yl]benzene as the electron 9 ACS Paragon Plus Environment


transport layer; mCP is selected as the host since it not only has high triplet energy which would confine the triplet excitons within the guest emitters, but also features low molecular polarity and high carrier mobility which favor the superior performance of OLEDs. The doping ratios were optimized as 2 wt% for both emitters to avoid broadening FWHM due to excimer formation and obtain the best performance for both emitters (detailed shown in Table S4 and Figure S9). Figure 4a plots the EL spectra of both optimized devices. 3-PhQAD-based OLED shows sky-blue emission with a peak at 480 nm, corresponding to a Commission Internationale de L’Eclairage (CIE) coordinate of (0.13, 0.32); while the 7-PhQAD-based one exhibits a blue emission with a peak at 472 nm, corresponding to a CIE coordinate of (0.12, 0.24). The slight EL variations are well consistent with their corresponding PL spectra. More importantly, owing to the conjugated rigid framework of QAD, both emitters exhibit significantly narrower FWHMs than conventional TADF ones44-49 and even comparable those with the B, N-based framework38 (detailed seen in Table S6). In particular, the FWHM of asymmetric 3-PhQAD is estimated to be 44 nm while symmetric 7-PhQAD is further narrowed to an extremely small FWHM of 34 nm. This obvious improvement should be attributed to suppressed molecular relaxation of C2-symmetric 7-PhQAD. The EL performances of the optimized devices based on both QAD derivatives as well as the core QAD are summarized in Table 2. As shown in Figure 4c, a maximum EQE of 18.7% is obtained for 7-PhQAD, corresponding to a maximum power efficiency (PE) of 28.2 lm W-1 and a maximum current efficiency (CE) of 28.8 cd A-1; while 3-PhQAD-based device shows a slightly higher efficiency with a maximum EQE of 19.1%, a maximum PE of 32.9 lm W-1 and CE of 33.5 cd A-1, which is similar with that of the sole core QAD.39 Moreover, Both emitters exhibit similar device results tendency with different doping ratios (seen in Table S4) and well consistent with their PLQY values. Both devices successfully realize excellent performance 10 ACS Paragon Plus Environment

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approaching the theoretical EQE limits of ~20%, further proving the feasibility of generating effective resonance effect in a conjugated helicoid to obtain superior TADF characteristics. More importantly, thanks to the much stable electrochemical properties of QAD-framework, 3PhQAD and 7-PhQAD-based devices reach high maximum luminance of 4975 and 2944 cd m-2, respectively, which is sufficient for practical utilizations and evidently higher than sole QAD.39 Meanwhile, it should be noted that the devices based on both compounds suffer serious efficiency roll-off at high driving voltages comparing those based on lasted reported blue TADF emitters43-48 (seen Table S6). To figure out the reason, we carried out triplet-triplet annihilation (TTA) and singlet exciton-polaron annihilation (SPA) model50 to simulated EQE-current density curves. As shown in Figure S8, for both emitters, the TTA models are well fitted experimental curves with low driving voltages, suggesting it is the determined exciton loss channel. But under high driving voltages, experimental curves are between those estimated from SPA and TTA models, suggesting they both play significant roles in efficiency roll-off. Thus, it is highly desired to further suppress such kinds of bimolecular quenching processes to improve their performances for practical utilizations. 3. Conclusion In conclusion, we identified a rigid framework QAD as a TADF matrix with effective resonance effect, and further designed and synthesized two novel TADF emitters 3-PhQAD and 7-PhQAD. Thanks to the effective superimposed resonance effect from electron-deficient carbonyls and electron-rich nitrogen atom, both 3-PhQAD and 7-PhQAD realize significant TADF characteristics with small ΔESTs of 0.18 and 0.19 eV respectively. Moreover, molecular relaxations were dramatically suppressed because of their conjugated rigid molecular-skeleton. In OLEDs, 3-PhQAD realizes superior performance with a maximum EQE of 19.1% and a narrow FWHM of 44 nm, while a maximum EQE of 18.7% and an extremely narrow FWHM 34 nm are realized for 7-PhQAD. These results provide QAD derivatives as an ideal candidate 11 ACS Paragon Plus Environment


based on resonance effect for stable TADF emitters with narrow FWHMs in practical applications. Moreover, due to its helical structure, QAD framework derivatives can be further expected to have a great potential in chiral OLED applications. 4. Experimental Procedures Synthesis. The solvents were purified by a conventional procedure and distilled under dry argon before using. Synthesis of dimethyl 2,2'-([1,1'-biphenyl]-4-ylazanediyl)dibenzoate. A mixture of 20 mmol (3.38 g) [1,1'-biphenyl]-4-amine, 41 mmol (10.74 g) 2-iodobenzoic acid methyl ester, 6 mmol (0.38 g) finely powdered copper, 4 mmol (0.76 g) copper(I)iodide, 60 mmol (8.29 g) potassium carbonate and 80 mL argon-purged dibutyl ether were added into a clean 250 mL flask and refluxed in 150 °C for 24 h under nitrogen ambience. After cooled to room temperature, the solvent was distilled off under reduced pressure and the residue filtered through celite with 200 mL EtOAc and 100 mL DCM. The combined organic phases are concentrated in vacuo using a rotavap and the crude product was further purified by column chromatography on silica gel using 1:2 hexane/dichloromethane as the eluent, followed by re-crystallized from hexane to yield 3.94 g (45%). M.P. = 123 oC. 1H NMR (600 MHz, CDCl3) δ 7.69 (dd, J = 7.7 Hz, 2H), 7.53 (d, J = 7.5 Hz, 2H), 7.47 – 7.36 (m, 6H), 7.28 (t, J = 7.4 Hz, 1H), 7.24 (d, J = 8.1 Hz, 2H), 7.18 (t, J = 7.5 Hz, 2H), 6.82 (d, J = 8.5 Hz, 2H), 3.41 (s, 6H). 13C NMR (151 MHz, CDCl3) δ 167.77, 147.94, 146.25, 140.53, 134.05, 132.71, 130.97, 128.76, 128.69, 127.98, 127.39, 126.67, 126.42, 124.18, 120.70, 51.83. MS (EI) m/z: [M]+: calcd 437.16; found 437.32. Anal. Calc. for C28H23NO4: C, 76.87; H, 5.30; N, 3.20; O, 14.63. Found: C, 76.60; H, 5.42; N, 3.15; O, 14.83. Synthesis of 2,2'-([1,1'-biphenyl]-4-ylazanediyl)dibenzoic acid. The dimethyl 2,2'-([1,1'biphenyl]-4-ylazanediyl)dibenzoate (9 mmol, 3.94 g) was hydrolyzed by dissolution in 60 mL 12 ACS Paragon Plus Environment

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of dioxane/H2O (1:1) mixture, addition of sodium hydroxide (180 mmol, 7.20 g), the reaction was heated at 105 °C for 12 h and then acidified with dilute hydrochloric acid to pH~2-3. The resulting solid was removed by filtration, washed with water, over dried (3.51 g, 95% yield) and used without further purification. M.P. = 254 oC. 1H NMR (600 MHz, DMSO-d6) δ 12.68 (s, 2H), 7.70 (dd, J = 7.7, 1.4 Hz, 2H), 7.54 (t, J = 8.1 Hz, 4H), 7.44–7.36 (m, 5H), 7.30–7.22 (m, 6H), 6.56 (d, J = 8.7 Hz, 2H). 13C NMR (151 MHz, DMSO-d6) δ 168.24, 148.43, 145.82, 140.73, 140.12, 133.30, 131.91, 131.13, 129.93, 129.30, 127.24, 126.93, 126.16, 125.40, 118.89, 55.31. MS (EI) m/z: [M]+: calcd 409.13; found 409.25. Anal. Calc. for C26H19NO4: C, 76.27; H, 4.68; N, 3.42; O, 15.63. Found: C, 76.07; H, 4.80; N, 3.39; O, 15.63. Synthesis

of

3-phenylquinolino[3,2,1-de]acridine-5,9-dione

(3-PhQAD)

and

7-

phenylquinolino[3,2,1-de]acridine-5,9-dione (7-PhQAD). A mixture of 2,2'-([1,1'-biphenyl]4-ylazanediyl)dibenzoic acid (409 mg, 1 mmol), thionyl chloride (0.73 mL, 10 mmol) and two drops of DMF was refluxed in 20 mL dry dichloromethane for 5 h. Then aluminum chloride (800 mg, 6 mmol) was added and the reaction refluxed for an additional 12 h. When cooled to the room temperature, the dilute hydrochloric acid was added dropwise to the reaction mixture at 0 oC and stirred for 10 min. The products were washed by saturated ammonium chloride solution and extracted with dichloromethane. The combined organic phases are concentrated in vacuo using a rotavap and the crude product was purified by column chromatography on silica gel using 1:4 hexane/dichloromethane as the eluent, followed by re-crystallized from hexane to yield 200 mg (53%) 3-PhQAD and 100 mg (27%) 7-PhQAD. 3-PhQAD: M.P. = 228 oC. 1H NMR (600 MHz, CDCl3) δ 8.77 (t, J = 7.7 Hz, 2H), 8.73 (s, 1H), 8.51 (d, J = 7.9 Hz, 1H), 8.21 (d, J = 8.8 Hz, 1H), 8.19 (d, J = 8.6 Hz, 1H), 7.94 (d, J = 8.7 Hz, 1H), 7.75 (d, J = 7.6 Hz, 2H), 7.72 (d, J = 8.2 Hz, 1H), 7.66 (t, J = 7.6 Hz, 1H), 7.52 (t, J = 7.4 Hz, 3H), 7.43 (t, J = 7.4 Hz, 1H). 13C NMR (151 MHz, CDCl3) δ 178.52, 178.46, 145.93, 13 ACS Paragon Plus Environment


139.69, 139.14, 138.75, 137.92, 132.94, 132.72, 131.17, 129.06, 128.03, 127.82, 126.91, 126.59, 126.39, 125.41, 125.20, 123.58, 123.39, 123.37, 120.84, 120.29. MS (EI) m/z: [M]+: calcd 373.11 found 373.19. Anal. Calc. for C26H15NO2: C, 83.63; H, 4.05; N, 3.75; O, 8.57. Found: C, 83.56; H, 4.09; N, 3.79; O, 8.56.

7-PhQAD: M.P. = 338 oC. 1H NMR of (600 MHz, CDCl3) δ 8.99 (s, 2H), 8.52 (d, J = 7.9 Hz, 2H), 8.16 (d, J = 8.5 Hz, 2H), 7.82 (d, J = 7.5 Hz, 2H), 7.71 (t, J = 7.7 Hz, 2H), 7.57–7.47 (m, 4H), 7.43 (t, J = 7.3 Hz, 1H). 13C NMR (151 MHz, CDCl3) δ 178.63, 145.97, 139.71, 138.38, 136.59, 132.74, 130.81, 129.12, 128.14, 127.91, 127.12, 126.41, 125.22, 123.87, 120.22. MS (EI) m/z: [M]+: calcd 373.11 found 373.21. Anal.

Calc. for C26H15NO2: C, 83.63; H, 4.05; N, 3.75; O, 8.57. Found: C, 83.55; H, 4.09; N, 3.80; O, 8.56. Associated Content Supporting Information. Experimental details including general information, device fabrication and measurement, theoretical calculations, cyclic voltammograms, TGA spectra and NMR spectra. This material is available free of charge via the Internet at http://pubs.acs.org.AUTHOR INFORMATION. Corresponding Author * E-mail: [email protected] (X.-H.Z.); E-mail: [email protected] (C.-J.Z.); E-mail: [email protected] (K.W.). Notes The authors declare no competing financial interest. Author Contributions 14 ACS Paragon Plus Environment

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Page 15 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


‡ These authors contributed equally. Acknowledgments This work was supported by the National Key Research &Development Program of China (grant number 2016YFB0401002), the National Natural Science Foundation of China (grant numbers 51533005, 51821002, 51773029, 51373190), the China Postdoctoral Science Foundation (grant number 2018M642307, 2016M590498), the Jiangsu Planned Projects for Postdoctoral Research Funds (grant number 1601241C), Collaborative Innovation Center of Suzhou Nano Science &Technology, the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD), the 111 Project and Joint International Research Laboratory of Carbon-Based Functional Materials and Devices. References (1) Tang, C. W.; Vanslyke, S. A., Organic Electroluminescent Diodes. Appl. Phys. Lett. 1987, 51, 913-915. (2) Burroughes, J. H.; Bradley, D. D. C.; Brown, A. R.; Marks, R. N.; Mackay, K.; Friend, R. H.; Burn, P. L.; Holmes, A. B., Light-Emitting-Diodes Based on Conjugated Polymers. Nature 1990, 347, 539-541. (3) Uoyama, H.; Goushi, K.; Shizu, K.; Nomura, H.; Adachi, C., Highly Efficient Organic Light-Emitting Diodes from Delayed Fluorescence. Nature 2012, 492, 234-238. (4) Reineke, S.; Lindner, F.; Schwartz, G.; Seidler, N.; Walzer, K.; Lussem, B.; Leo, K., White Organic Light-Emitting Diodes with Fluorescent Tube Efficiency. Nature 2009, 459, 234-238. (5) Sun, Y.; Giebink, N. C.; Kanno, H.; Ma, B.; Thompson, M. E.; Forrest, S. R., Management of Singlet and Triplet Excitons for Efficient White Organic Light-Emitting Devices. Nature 2006, 440, 908-912.



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Page 23 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


I +

O

O

O O

(a)

N O

(b)

N HO

O

O

OH

O

NH2

O

95%

45%

N

(c)

O

3-PhQAD, 53%

N

or O

O

7-PhQAD, 27%

Scheme 1. Synthetic routes of 3-PhQAD and 7-PhQAD. (a) Cu, CuI, K2CO3, n-Bu2O, 150 oC, 24h; (b) NaOH, H2O/dioxane (1:1), reflux for 12 h and then HCl; (c) SOCl2, CH2Cl2, DMF (cat.), reflux for 5 h and then AlCl3, reflux 12 h. Isolated yield.

Figure 1. (a) Chemical structures, calculated distributions of HOMOs and LUMOs and the geometry comparisons between optimized S0 (red) and S1 (blue) states of 3-PhQAD and 7PhQAD, respectively, and (b) their transition process modes.



(b) 23 nm

1.0

Abs. Em.

0.8 0.6 0.4 0.2

18 nm

1.0

Normalized Intensity(a.u.)

Normalized Intensity(a.u.)

(a)

Abs. Em.

0.8 0.6 0.4 0.2 0.0

0.0 400

450

500

550

400

600

450

500

550

600

Wavelength(nm)

Wavelength(nm) (d)

(c) 1.0

Hex Bu2O

0.8

EA CH3CN DMF

0.6 0.4

1.0

Normalized Intensity

Normalized Intensity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 27

Hex Bu2O

0.8

EA CH3CN DMF

0.6 0.4 0.2

0.2

0.0

0.0 450

500

550

600

450

650

500

550

600

650

Wavelength(nm)

Wavelength(nm)

Figure 2. Normalized absorption and emission spectra of (a) 3-PhQAD and (b) 7-PhQAD in dilute toluene measured at room temperature; Normalized emission spectra of (c) 3-PhQAD and (d) 7-PhQAD in hexane (Hex), butyl ether (Bu2O), ethyl acetate (EA), N, Ndimethylformamide (DMF) and acetonitrile (CH3CN).


Page 25 of 27

(b)

(a) 1.0

Fluo. Phos.

Normalized Intensity (a.u.)

Normalized Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.8 0.6 0.4 0.2 0.0 450

500

550

600

650

700

Fluo. Phos.

1.0 0.8 0.6 0.4 0.2 0.0 450

500

550

600

650

700

Wavelength(nm)

Wavelength (nm)

Figure 3. Fluorescence and phosphorescence spectra of (a) 3-PhQAD and (b) 7-PhQAD 2 wt% doped mCP films measured at 77 K. Table 1. Summary of key parameters of 3-PhQAD and 7-PhQAD. In Toluene @ r.t. Emitters

Doped in mCP film @77 K

PLQY f HOMO/LUMO g [%] [eV]

λabs.a λem. a Stokes Shift b. [nm] [nm] [cm-1]

λfluo.c S1 d [nm] [eV]

λphos.c [nm]

T1 d [eV]

ΔEST e [eV]

3-PhQAD

443 466

1078

478 2.60

512

2.42

0.18

73

-6.20/-3.38

7-PhQAD

446 464

900

472 2.63

509

2.44

0.19

68

-6.19/-3.40

aEstimated

from the maxima of spectra; bStokes Shift = 1/λfluo.-1/λabs.; cOnset positions of the fluorescence and phosphorescence spectra of 3-PhQAD and 7-PhQAD 2 wt% doped mCP films measured at 77 K; dS1=1241/ λfluo. and T1=1241/λphos.; eΔEST= S1- T1; fPLQYs of 3-PhQAD and 7-PhQAD 2 wt% doped mCP films under the nitrogen condition; gHOMO energy levels are determined from the onset of oxidation curves with respect to that of ferrocene in dichloromethane (DCM), while LUMO energy levels are determined from the onset of reduction curves with respect to that of ferrocene in DCM.



(a)

1.0

Luminance (cd m-2)

0.6

0.4

0.2

102

450

500

550

600

650

700

750

100

101 50 10

0

10-1

0.0 400

150

103

Current density (mA cm-2)

0.8

EL Intensity (a.u.)

(b)

3-PhQAD 7-PhQAD

0 3.0

3.5

4.0

4.5

5.0

5.5

6.0

6.5

7.0

Voltage (V)

Wavelength (nm)

Power Eiffciency (lm W-1)

(c) 100

10

10 1

1 10-1

100

101

102

103

External Quantum Efficiency (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 27

0.1

Luminescence (cd m-2)

Figure 4. (a) EL spectra, (b) current density–voltage–luminance characteristics, and (c) PE– EQE–luminance characteristics of the OLEDs based on 3-PhQAD and 7-PhQAD. Table 2. Summary of EL performance of QAD-based TADF emitters. Emitters

Peak [nm]

FWHM [nm]

CE [cd A-1]

PE [lm W-1]

EQE [%]

CIE (x,y)

Max. Luminance [cd m-2]

3-PhQAD

480

44

33.5

32.9

19.1

(0.13,0.32)

4975

7-PhQAD

472

34

28.8

28.2

18.7

(0.12,0.24)

2944

QAD39

468

39

26.2

31.6

19.4

(0.13,0.18)

≈1100


Page 27 of 27

SYNOPSIS TOC

O

1.0

EL Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


N O

0.5

FWHM= 44 /34 nm

0.0 400

450

500

550

600

650

Wavelength (nm)


700

750

Thermally Activated Delayed Fluorescence Carbonyl Derivatives for

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