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Syntheses, Photoluminescence, and Electroluminescence of a Series of Sublimable Bipolar Cationic Cuprous Complexes with Thermally Activated Delayed Fluorescence Fuli Zhang,†,# Yuqiao Guan,‡,# Xulin Chen,§ Shuangshuang Wang,† Dong Liang,§ Yafei Feng,‡ Shufen Chen,*,‡ Suzhi Li,† Zhongyi Li,† Fuqiang Zhang,† Canzhong Lu,*,§ Guangxiu Cao,† and Bin Zhai*,† †

College of Chemistry and Chemical Engineering, Shangqiu Normal University, Shangqiu 476000, P. R. China Key Laboratory for Organic Electronics and Information Displays & Institute of Advanced Materials (IAM), Jiangsu National Synergetic Innovation Center for Advanced Materials (SICAM), Nanjing University of Posts and Telecommunications, 9 Wenyuan Road, Nanjing 210023, P. R. China § State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002, P. R. China ‡

S Supporting Information *

ABSTRACT: Three thermally activated delayed fluorescence cationic cuprous complexes [Cu(POP) (ECAF)]PF6 (1, POP = bis(2-diphenylphosphinophenyl)ether, ECAF = 9,9-bis(9ethylcarbazol-3-yl)-4,5-diazafluorene), [Cu(POP) (EHCAF)]PF6 (2, EHCAF = 9,9-bis(9-ethylhexylcarbazol-3-yl)-4,5diazafluorene), and [Cu(POP) (PCAF)]PF6 (3, PCAF = 9,9-bis(9-phenylcarbazaol-3-yl)-4,5-diazafluorene) with bipolar 4,5-diazafluorene ligand substituted by bis-carbazole have been successfully prepared, and their UV absorption, photoluminescent properties, and electrochemical behaviors were investigated. At room temperature, complexes 1, 2, and 3 exhibit efficient yellowish-green emission with peak maxima of 550, 549, and 556 nm, respectively, and lifetimes of 5.7 μs. In powder states, the quantum yields (ϕPL) of 22.4% for 1, 18.5% for 2, and 20.0% for 3, respectively, are found. These metal phosphors can be vacuum-evaporated and applied in the organic light-emitting diodes (OLEDs) of indium tin oxide/poly(3,4ethylenedioxy-thiophene):poly(styrenesulfonate) (40 nm)/4,4′,4″-tri(9-carbazoyl)triphenylamine (15 nm)/cuprous complexes (10 wt %): 1,3-bis(9-carbazolyl)benzene (30 nm)/1,3,5-tri[(3-pyridyl)-phen-3-yl]benzene (50 nm)/LiF (0.5 nm)/Al (100 nm). Complex 1-based device D1 achieved a maximum luminance of 11 010 cd m−2, a current efficiency of 47.03 cd A−1, and an external quantum efficiency of 14.81%. The high electroluminescence efficiencies of these complexes are assumed to be due to their good thermal stabilities and capture of both singlet and triplet excitons. The research presented here provides a powerful tool toward highly efficient and cheap OLED devices.

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INTRODUCTION

their application in bulk productions. Therefore, cheap and earth’s bountiful metal-based complexes are needed to be developed for producing low-cost flat panel displays and solidstate lighting based on OLEDs technology. Cuprous complexes are considered as promising alternatives to commonly used phosphorescent iridium complexes in applications of electroluminescent devices.30−33 First of all, precursor for synthesis of copper complex [Cu(MeCN)4]PF6 is nearly 10 times cheaper than iridium(III) chloride, which used as precursor for synthesis of iridium complex. Second, precious metal catalysts are always required for synthesis of the

Organic light-emitting diodes (OLEDs) have attracted much attention for their application in high-quality flat panel display technology and solid-state lighting, as they offer several advantages for being flexible, wider viewing angle, printable, wide operating temperature, high resolution, color temperature tenability, better contrast ratio, sharper image, lower power consumption, lightweight, and transparent.1−10 Since the influential and pioneering work in 1998,11,12 phosphorescent complexes containing heavy metallic atoms have been attracting more and more attention because of their capture of both singlet and triplet excitons achieving almost 100% internal quantum efficiency (IQE) in OLEDs.13−29 However, these transition metals are costly and not plentiful, which hinders © 2017 American Chemical Society

Received: July 30, 2016 Published: March 17, 2017 3742

DOI: 10.1021/acs.inorgchem.6b01847 Inorg. Chem. 2017, 56, 3742−3753

Article

Inorganic Chemistry

fabrication process of OLED devices.84,86−88 Therefore, it is highly desirable to design cationic complexes that can be deposited to form film by vacuum evaporation. To address the poor sublimability of the cationic complex, two solutions have been employed: (i) Employing bulky negative counterions. Qiu and co-workers developed a series of evaporable cationic iridium complexes with tetraphenylborate derivatives as the negative counterions and corresponding highly efficient vapor-processed devices.89,90 (ii) Introducing bulky ligands. Wong and co-workers reported two robust sublimable cationic iridium complexes bearing bulky diphenylamino- and fluorene-based 2-phenylpyridine ligands to fabricate yellow-emitting OLEDs by vacuum evaporation deposition.91 In our recently published article, we reported three sublimable yellow-emitting cationic iridium complexes with the bipolar ancillary ligands 9,9-bis(9-ethylcarbazol-3-yl)4,5-diazafluorene (ECAF), 9,9-bis(9-ethylhexylcarbazol-3-yl)4,5-diazafluorene (EHCAF), and 9,9-bis(9-phenylcarbazaol-3yl)-4,5-diazafluorene (PCAF) and corresponding OLEDs.92 These findings indicated that introducing bulky ligands or counteranions can weaken lattice energies and electrostatic interaction and tend to improve volatilities of these cationic complexes. To the best of our knowledge, other than the abovementioned reports based on cationic iridium complexes, there were no accounts on OLEDs using vacuum-sublimed cationic cuprous complexes as emitting dopants. Along this line of research, we developed a new class of yellowish-green emitting cuprous complexes [Cu(POP) (ECAF)]PF6 (1, POP = bis[2-(diphenylphosphino)phenyl]ether), [Cu(POP) (EHCAF)]PF6 (2), and [Cu(POP) (PCAF)]PF6 (3) employing ECAF, EHCAF, and PCAF as ancillary ligands, respectively. We propose that these complexes feature several unique features compared with other cationic POP-based cuprous compounds, which always contain smallsized diamine ligands.43,45,83,84,93,94 First, bulky ligands ECAF, EHCAF, and PCAF can weaken lattice energies and electrostatic interaction of their parental complexes and make them easier to form film by vacuum deposition. Second, these complexes possess bipolar charge-transporting abilities due to introducing hole-transporting carbazole and electron-transporting 4,5-diazafluorene moieties, which can balance charge carriers in the light-emitting layer in OLEDs to improve the efficiency of the device. These cationic cuprous complexes have been proved to be TADF emitters with high photoluminescent (PL) quantum yields at ambient temperature in solid state. Highly efficient OLEDs based on these cuprous complexes via vapor deposition are achieved.

cyclometalated ligand of iridium complex by cross-coupling reactions. Third, reaction conditions of synthesis of cuprous complex are much milder than those of iridium complex, which should be synthesized in anhydrous and anoxybiotic conditions. Cuprous complexes not only own the advantages of relative rich reserves and low costs but also offer exciting photoluminescence characteristics. For general cuprous complexes, their phosphorescence decay times corresponding to the triplet excited state (T1) → ground state (S0) transitions are much longer than those of fluorescence owing to very weak spin− orbit coupling (SOC).34−38 This kind of compound is unsuitable for OLED applications due to the severe efficiency roll-off.39 Fortunately, cuprous complexes can show thermally activated delayed fluorescence (TADF) with short radiative lifetimes at ambient temperature. This procedure is based on a pseudoequilibrium between T1 and singlet excited state (S1), attributed to a very rapid reverse intersystem crossing from T1 to S1 when adjusting the splitting gap (ΔEST) between the T1 and S1 states to small enough.30,40−48 Consequently, the radiative lifetimes are in the scope of microseconds, which are different from pure fluorescence or phosphorescence. These TADF cuprous complexes-based OLEDs present IQE of 100% by capturing both singlet and triplet excitons.30 Over the past decade, a large diversity of luminescent cuprous complexes with different structure motifs has been developed.33,49−53 As a typical d10 metal ion, Cu(I) always constructs two common coordination structures, namely, binuclear40,47,54−63 and mononuclear complexes, including four- and three-coordinated species.64−71 Among them, the luminescent four-coordinated mononuclear cuprous complexes have attracted particular interest owing to their ready synthesis and tunable light emission color.41,43−45,72−78 According to the characteristics of molecular structure, such cuprous complexes fall into two categories: (i) the neutral complexes with a neutral bidentate diphosphine ligand and a negative ligand and (ii) cationic complexes containing a neutral bidentate diphosphine ligand and a neutral ligand. The former class of cuprous complexes has good volatility and is suitable for fabricating highly efficient OLEDs via vacuum deposition. As far as we know, however, there are only a few examples in the literature of OLEDs processed from vacuum deposition using sublimable cuprous complexes.36,79−82 The state-of-the-art OLED based on neutral four-coordinated mononuclear cuprous complexes has achieved the maximum external quantum efficiency (EQE) of 17.7%.82 However, the choice of available negative ligand suitable for neutral cuprous complexes is narrow, and the synthesis of corresponding compounds is not easy. As for the latter case, cationic cuprous complexes can be readily synthesized with high yields under mild conditions. Nevertheless, cationic compounds generally have high sublimation temperature over their degradation temperature, and decomposition easily happens during thermal evaporation. The emitters of OLEDs based on these complexes were mostly fabricated using a spin-coating method.44,45,73,83−85 Zhang and co-workers reported a solution-processed OLED with [Cu(dnbp) (DPEPhos)]BF4 (dnbp = 2,9-di-n-butylphenanthroline, DPEPhos = bis[2-(diphenylphosphino)phenyl]ether) as dopant,83 which exhibits a high EQE of 15.0% and is comparable to those based on phosphorescent iridium complexes. Apart from this, the EQEs of other cationic [Cu(P^P)(N^N)]+-based devices by spin-coating method are just between ∼5%− 9%.44,45,73,83−85 Note that spin-coating method will accelerate ligand dissociation of cationic complexes in solution during the

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EXPERIMENTAL SECTION

Materials and Measurements. Unless otherwise stated, chemicals and reagents were used as received. 1H nuclear magnetic resonance (NMR) spectra were recorded on a JEOL JNM-ECA400 NMR spectrometer. Mass spectrometry was collected with a Thermo Electron Corporation Finnigan LTQ mass spectrometer. Elemental analysis was performed on an Elementar Vario EL CHN elemental analyzer. Thermogravimetric analysis (TGA) thermograms were obtained with STA 449 F3 (NETZSCH). Absorption and PL spectra were recorded with a UV−vis spectro-photometer (Agilent 8453) and a fluoro-spectrophotometer (Jobin Yvon, FluoroMax-3), respectively. The lifetime measurements of powder samples at different temperatures were measured on an Edinburgh Analytical instrument FLS920 with a picosecond laser diode. Absolute photoluminescence quantum yield (ΦPL) at room temperature was recorded using Hamamatsu (C9920−02) system for absolute PL quantum yield measurements. 3743


Article

Inorganic Chemistry Scheme 1. Synthetic Routes of Cuprous Complexes 1−3

sq−1 were cleaned by ultrasonic baths in organic solvents followed by UV−ozone treatment. The poly(3,4-ethylenedioxy-thiophene):poly(styrenesulfonate) (PEDOT:PSS) hole-injection layer was deposited by spin coating onto the substrate and heated at 120 °C for 30 min. A 4,4′,4″-tri(9-carbazoyl)triphenylamine (TCTA) film (15 nm) serving as a transporting material was deposited on the PEDOT:PSS-coated substrate. The emitter (10 wt %) and mCP host were coevaporated to form 30 nm emitting layer. After that, 1,3,5-tri[(3-pyridyl)-phen-3yl]benzene (TmPyPb; 50 nm), LiF (0.5 nm), and Al (100 nm) were evaporated, respectively. The luminance−voltage−current density characteristics were recorded with a PR655 spectrometer and a Keithley 2400 programmable voltage−current source. The current efficiency and EQE characteristics were calculated from electrical parameters, electroluminescence (EL) spectra, and luminance.

The film samples were deposited by doping the cuprous complex into poly(methyl methacrylate) (PMMA) or 1,3-bis(9-carbazolyl)benzene (mCP) using spin-coating onto a quartz glass slide and dried in vacuum for 1 h at 60 °C. Cyclic voltammetric experiments were performed with a Princeton Applied Research potentiostat/galvanostat model 283 voltammetric analyzer in CH3CN solutions (1 × 10−3 M) at a scan rate of 100 mV s−1 with a silver wire as the pseudoreference electrode, a platinum plate as the working electrode, and a platinum wire as the counter electrode. The supporting electrolyte was 0.1 M nBu4NPF6, and ferrocene was used as the internal standard. Before measurements, the solutions were bubbled with argon for 15 min. Synthesis of Diimine Ligands. Diimine ligands ECAF, EHCAF, and PCAF were prepared according to previously reported procedures.95 Synthesis and Structural Characterization of Cuprous Complexes. A mixture of [Cu(NCMe)4](PF6) (1.0 mmol) and POP (1.0 mmol) in dichloromethane (DCM; 10 mL) was stirred at room temperature for 2 h. After that, the corresponding diimine ligand ECAF, EHCAF, or PCAF (1.0 mmol) was added, and the resulting mixture was stirred for another 4 h. The crude product was precipitated with Et2O and then purified by column chromatography on silica gel (200−300 mesh) with CH2Cl2 /acetone (20:1 or 10:1) as the eluent, yielding a yellow powder. [Cu(POP) (ECAF)](PF6) (1): Yield: 82%. 1H NMR (400 MHz, deuterated dimethyl sulfoxide (DMSO-d6), δ): 8.49 (br, 3H), 7.98 (t, J = 8 Hz, 4H), 7.62−7.09 (m, 39H), 6.65 (s, 2H), 4.43 (s, 4H), 1.30 (s, 6H). Electrospray ionization mass spectrometry (ESI-MS) [m/z]: 1156.34 (M − PF6)+. Anal. found: C 69.25, H 4.42, N 4.27. Anal. Calcd for C75H58N4OP3F6Cu: C 69.20, H 4.49, N 4.30%. [Cu(POP) (EHCAF)](PF6) (2): Yield: 78%. 1H NMR (400 MHz, DMSO-d6, δ): 8.51 (br, 3H), 7.96 (t, J = 8 Hz, 4H), 7.54−7.13 (m, 39H), 6.65 (s, 2H), 4.25 (s, 4H), 1.94 (s, 2H), 1.29 (br, 16H), 0.85 (s, 6H), 0.73 (s, 6H). ESI-MS [m/z]: 1325.5 (M − PF6)+. Anal. found: C 71.14, H 5.59, N 3.75. Anal. Calcd for C87H82N4OP3F6Cu: C 71.08, H 5.62, N 3.81%. [Cu(POP) (PCAF)](PF6) (3): Yield: 73%. 1H NMR (400 MHz, DMSO-d6, δ): 8.50 (br, 3H), 8.11 (t, J = 8 Hz, 4H), 7.67−7.54 (m, 14H), 7.43−7.09 (m, 35H), 6.65 (s, 2H). ESI-MS [m/z]: 1252.3 (M − PF6)+. Anal. found: C 71.41, H 4.11, N 4.05. Anal. Calcd for C83H58N4OP3F6Cu: C 71.32, H 4.18, N 4.01%. Computational Methodology. Calculations on the electronic structures were performed using the Gaussian 09 program96 with the PBE97,98 density functional. The 6-31G(d, p)99 and Lanl2DZ100 basis sets were applied for the main-group atoms and Cu atom, respectively. The initial structures of complexes were constructed based on the previously reported crystal data with the similar coordination structures,45 and then they were optimized by setting singlet or triplet states at the above level, respectively. Vibrational analysis at the same level was performed to confirm that all the optimized structures do not have imaginary frequency. The visualization of molecular orbitals was accomplished using the GaussView program.101 Fabrication and Characterization of OLEDs. The prepatterned indium tin oxide (ITO) substrates with ITO sheet resistance of 10 Ω

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RESULTS AND DISCUSSION Preparation and Characterization of Compounds. Scheme 1 depicts the synthesis of the cationic cuprous complexes 1, 2, and 3. Before preparation of the final complexes, we easily synthesized the ligands ECAF, EHCAF, and PCAF from 4,5-diazafluoren-9-one and condensing carbazole derivatives in the presence of Eaton’s reagent.95 Note that although the cost of these ligands may be higher than that of some phosphorescent iridium complexes, title cuprous complexes are cheaper than phosphorescent iridium emitters due to the much lower cost of metal precursor [Cu(CH3CN)4]PF6 than that of iridium(III) chloride. Cuprous complexes 1, 2, and 3 were synthesized with yields of 82%, 78%, and 73%, respectively, by the reaction of [Cu(CH3CN)4]PF6, POP ligand, and corresponding 4,5-diazafluorene ligand with the molar ratio of 1:1:1. All these compounds were fully characterized by 1H NMR (Figures S1−S3, Supporting Information), ESI-MS, and elemental analysis. Thermal Stability. To examine their thermal stabilities, thermogravimetric analysis (TGA; Figure 1) and differential scanning calorimetry (DSC; Figure S4, Supporting Information) were performed under N2 atmosphere. As shown by TGA results it can be found that these complexes are thermally stable below 300 °C, and the decomposition temperatures (Td values: corresponding to 5% loss of weight) are 382 °C for 1, 375 °C for 2, and 384 °C for 3, respectively, which resemble those of previously reported sublimable neutral cuprous complexes.82 In addition, high glass transition temperatures (Tg values) are observed in complexes 2 (Tg = 123 °C) and 3 (Tg = 185 °C). More importantly, no glass transition of complex 1 was obtained, but its melting point (Tm = 249 °C) was observed. The results indicated that these cuprous complexes were very stable and can be formed film by vacuum deposition. 3744


Article

Inorganic Chemistry

changed with increasing polarity from CH2Cl2, acetone, to acetonitrile, the PL spectra (Figure S5, Supporting Information) in solutions for these complexes showed large deviation in maximum photoluminescence peaks, consistent with the assignment of predominant CT characters for the emissive excited states. These complexes exhibit strong yellowish-green (peaks at 543, 538, and 546 nm for complexes 1, 2, and 3, respectively) emission in CH2Cl2 glasses at 77 K, which show large hypsochromic shifts (70−78 nm) by comparison with their emission spectra at 300 K. This can be observed from some other reported cuprous complexes41,84,85,106−108 due to the rigidity of the CH2Cl2 glass at 77 K, which strongly suppresses the configuration changes upon photoexcitation. So, the excited states at 77 K are supposed to preserve a molecular geometry with the same as that of the ground state. The PL spectra of 1, 2, and 3 at both 300 and 77 K in the solid state are depicted in Figure 2b−d. At 300 K, these three complexes emit efficient yellowish-green light with major emission peaks at 550, 549, and 556 nm, respectively, and lifetimes of 5.7 μs. At 77 K, complexes 1, 2, and 3 exhibit slightly lower energy emission with the peaks at 568, 560, and 564 nm and lifetimes of 343, 317, and 363 μs, respectively. The considerable difference of emissions and lifetimes between 300 and 77 K indicates that the luminescence of these complexes in the solid state might originate from two different excited states in thermal equilibrium, that is, S1 and T1.40,43−45,79,82 Such a behavior can be attributed to a process called TADF.36,41,43−45,60,79,82 When doped in 5 wt % PMMA films, the emission peaks of these complexes at 300 K (528, 525, and 528 nm for complexes 1, 2, and 3, respectively) are shifted to higher energy levels than those in CH2Cl2 matrix at 77 K by 13−18 nm. Presumably, the configuration of the emissive excited state in amorphous films can be more effectively suppressed to flatten than that in CH2Cl2 glass at 77 K, because PMMA film at room temperature is more rigid than CH2Cl2 glass. In powder states at 300 K, the ΦPL of 22.4% for 1, 18.5%

Figure 1. TGA curves of complexes 1−3.

Photophysical Properties. The UV−vis absorption and PL spectra of complexes 1, 2, and 3 in CH2Cl2 (5 × 10−5 M) are shown in Figure 2, and the photophysical data are summarized in Table 1. The intense absorption bands in the ultraviolet region at ∼300 nm are ascribed to spin-allowed intraligand 1LC (π→π*) transition of both the diimine ligand and the POP ligand. The absorption bands at lower energies after 300 nm attributed to ligand-to-ligand charge-transfer (1LLCT) transitions, according to density functional theory (DFT) calculation. The homogeneous absorption profiles for the complexes 1, 2, and 3 indicated the similar vibrational and electronic structures of S0 and S1. As shown in Figure 2a, complexes 1, 2, and 3 all exhibit red (peak at ∼616 nm) emission in degassed CH2Cl2 at 300 K. From the large Stokes shifts of emission peaks and structureless emission spectra at ambient temperature, it means that the luminescent excited states of the three complexes have predominantly CT characters and undergo a structural change from the tetrahedral-like ground state (d10) to the flattened excited state (d9) upon excitation.102−105 As the solvent was

Figure 2. UV−vis absorption and PL spectra of complexes 1−3 in CH2Cl2 solution (a) and PL spectra of cuprous complexes 1 (b), 2 (c), and 3 (d) in powder state and doped PMMA film. 3745


Article

Inorganic Chemistry Table 1. Photophysical Characteristics of Cuprous Complexes 1−3 solution (in degassed CH2Cl2) 1 2 3

powder, 300 K

PMMA film

powder, 77 K

λabs, nm

λem, nm

λem, 77 K/nm

λem, nm

Φem

τ, μs

λem, nm

Φem

τ, μs

λem, nm

Φem

231/288/354 231/288/354 243/291/348

616 616 616

543 538 546

550 549 556

22.4% 18.5% 20.0%

5.7 5.7 5.7

568 560 564

19.3% 16.2% 19.4%

343 317 363

528 525 528

33.1% 31.7% 31.5%

Figure 3. Emission decay time of complexes 1 (a), 2 (b), and 3 (c) in powder state vs temperature. The solid line represents a fit curve according to TADF eq 1. (d) Energy level diagram for the low-lying states of complex 1 in which τ(S1), τ(TADF), τ(T1), ISC, RISC, and ΔEST represent prompt fluorescence decay time, delayed fluorescence decay time, phosphorescence decay time, intersystem crossing, reverse intersystem crossing, and the energy gap between the S1 state and T1 state.

Table 2. Energy Levels and Gaps of of Cuprous Complexes 1−3 Td,a °C

ΔEg,b eV

HOMO,c eV

LUMO,d eV

S1,e eV

T1,f eV

ΔEST,g eV

ΔEST,h eV

382 375 384

3.47 3.53 3.55

−6.13 −6.12 −6.16

−2.66 −2.59 −2.61

2.48 2.49 2.46

2.40 2.40 2.42

0.08 0.09 0.04

0.09 0.09 0.09

1 2 3 a

Decomposition temperatures of 5% weight loss temperature. bEstimated from the onset wavelengths of the absorption spectra measured in CH2Cl2. Calculated from the oxidation potentials of cyclic voltammetry. dCalculated from the HOMO and cooperating with the optical band gaps ΔEg value. e Estimated from the high-energy flank at 50% of the emission maximum intensity measured at 298 K as powders. fEstimated from the high-energy flank at 50% of the emission maximum intensity measured at 77 K as powders. gCalculated from S1 and T1 of complexes as powders. hFitted from the TADF equation. c

for 2, and 20.0% for 3, respectively, are found. In PMMA film, these complexes show better ΦPL (33.1% for 1, 31.7% for 2, and 31.5% for 3, respectively) due to the great suppression of nonradiative transition from rotations and vibrations in rigid polymer. In the temperature range of 77−300 K, more decay times of these complexes (Figure S6, Supporting Information) were examined to gain deeper understanding of the emission characteristics in the solid state. For thermally equilibrated states (S1 and T1), the temperature dependence of the emission decay time can be expressed as in eq 1.30,41,66,109 1

τobs =

(

1 + 3 exp − 1 τ(T) 1

+

ΔEST kBT

)

(− )

1 exp 2τ(S1)

In this equation, kB and T represent the Boltzmann constant and the absolute temperature, respectively. τ(S1) and τ(T1) are the individual decay times of the lowest singlet and triplet state, respectively, in the absence of thermalization. ΔEST is the energy separation between the states S1 and T1. For a fit of this equation to the experimental data (Figure 3), the parameters were acquired with values of τ(S1) = 125 ns, τ(T1) = 334 μs, and ΔEST = 0.09 eV for 1, τ(S1) = 105 ns, τ(T1) = 310 μs, and ΔEST = 0.09 eV for 2, and τ(S1) = 132 ns, τ(T1) = 356 μs, and ΔEST = 0.09 eV for 3. The fitted τ(T1) values are very close to the values (343 μs for 1, 317 μs for 2, and 363 μs for 3) observed at 77 K. And the ΔEST values (Table 2), except for 3, exactly matched the energy differences estimated from the highenergy flank at 50% of the emission maximum intensity at 77 and 300 K.60 The small ΔEST can greatly accelerate an efficient up-conversion and result in a dominating TADF at room

ΔEST kBT

(1) 3746


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Inorganic Chemistry

Figure 4. Photoluminescence intensities of complexes 1 (a), 2 (b), and 3 (c) stemming from S1 (TADF) and T1 (phosphorescence) vs the total intensity Itot = I(S1) + I(T1) in dependence of temperature according to eqs 4 and 5.

Electrochemical Properties. Electrochemical properties of complexes 1, 2, and 3 were investigated by cyclic voltammetry. As shown in Figure 5, the oxidation potentials of complexes 1,

temperature. Accordingly, this TADF compound can harvest both singlet and triplet excitons for the generation of light in electroluminescent devices. To fully visualize the individual devotions of phosphorescence and TADF further, we denote the TADF intensity originating from the singlet state S1 as I(S1), the phosphorescence intensity from the triplet state T1 as I(T1), and the total emission intensity as Itot, which is the sum total of I(S1) and I(T1). Accordingly, the percentage of I(S1) and I(T1) relative to Itot can be expressed as the eqs 2−5.44,46 I(S1) = α N(S1)kr(S1) = α N(S1)⌀PL(S1)τ(S1)−1

(2)

−1 I(T) 1 = α N(T) 1 k r(T) 1 = α N(T) 1 ⌀PL (T) 1 τ(T) 1

(3)

I(T) I(T) 1 1 = IWL I(S1) + I(T) 1 −1 ⎡ ⎛ ΔEST ⎞⎤ ⌀PL(S1)τ(T) 1 g (S1) = ⎢1 + exp⎜ − ⎟⎥ ⎢⎣ ⌀PL(T) ⎝ kBT ⎠⎥⎦ 1 τ(S1)g (T) 1

I(S1) Itot

⎡ ⌀ (S )τ(T)g (S ) ⎛ ΔE ⎞⎤−1 PL 1 1 1 ST ⎟⎟⎥ exp⎜⎜ − ⎢1| ⎢⎣ ⌀PL(T) T k (S ) (T) g τ ⎝ ⎠⎥⎦ B 1 1 1

Figure 5. Cyclic voltammograms of complexes 1−3 in CH3CN solution.

(4)

2, and 3 in CH3CN solution are 1.33, 1.32, and 1.36 V, respectively, which are different from those of other POP-based cuprous complexes,84,87,110 in which the oxidation arises primarily at the cuprous ion. The highest occupied molecular orbitals (HOMOs, −6.13 eV for 1, −6.12 eV for 2, and −6.16 eV for 3, respectively) of the three complexes were calculated from the oxidation potentials. However, for cuprous complexes, the calculation of the lowest unoccupied molecular orbitals (LUMOs) from reduction potential is infeasible due to the dissociation during the reduction of the complexes. If we assume that the first absorption of the spectra of these complexes is mainly attributed to a HOMO to LUMO transition, then we can determine their LUMOs from EHOMO and cooperating with the optical band gaps ΔEg value, which is obtained from the UV absorption edge according to the literature.111 LUMOs (−2.66 eV for 1, −2.59 eV for 2, and

(5)

Here, α is a constant, being equal in both eqs 2 and 3. For this crude estimate, it is assumed that the quantum yields ΦPL(S1) and ΦPL(T1) do not depend on the temperature, which can be measured at 298 and 77 K, respectively. Assuming that the populations of both S1 and T1 adhere to the Boltzmann distribution, the relative intensities I(S1)/Itot and I(T1)/Itot can be indicated as eqs 4 and 5, in which g(S1) = 1 and g(T1) = 3 are the degeneracy factors for S1 and T1, respectively. We obtained the curves depicted in Figure 4 using these equations. As shown in Figure 4, at 300 K, the TADF comprises 97% of the total emission for complexes 1, 2, and 3, which are among the highest values reported for TADF cuprous complexes.32,44,53,58,60,73 Obviously, these cuprous complexes are all highly efficient TADF materials at ambient temperature. 3747


Article

Inorganic Chemistry −2.61 eV for 3, respectively) of these complexes were obtained using the formula ELUMO = EHOMO − ΔEg.44,45,73,112 Theoretical Calculations. To gain better insight into the photophysical behavior of these cationic cuprous complexes, quantum chemical calculations were performed (see Experimental Section). Energetics data and calculated bond lengths and angles (Table S1) of these complexes in S0 and T1 are provided in the Supporting Information. The Cu(I) center in all three complexes in S0 has a distorted tetrahedral geometry formed by two phosphorus atoms from POP ligand and two nitrogen atoms from diamine ligand. The bond angles of N5− Cu−N14 and P2−Cu−P3 are in the range of 81.360−81.503° and 117.273−118.028°, respectively, which are consistent with previously studied cuprous complexes with the similar ligands.44,73,113 Cu−N and Cu−P bond lengths localize in a region of 2.2362−2.2600 Å and 2.3644−2.3762 Å, respectively, which are slightly longer than those of other cuprous complexes114,115 due to the steric hindrance of the large biscarbazole substituents on the diimine ligands. Figure 6 presents

TmPyPb (50 nm)/LiF (0.5 nm)/Al (100 nm) were fabricated. Because of the good volatility of these complexes, the lightemitting layers of these devices were fabricated by vacuum deposition. Figure 7 shows the energy-level diagram of the

Figure 7. Device architecture and molecular structures for materials investigated.

devices, as well as the molecular structures of the materials used. Obviously, the LUMO levels of complexes 1, 2, and 3 are close to or within that (−2.6 eV) of the host mCP, while the HOMO levels for complexes 1, 2, and 3 are comparable to −6.1 eV of mCP, indicating that mCP acts as a suitable host for above three complexes. The EL characteristics of each device are depicted in Figure 8, and the key EL data are summarized in Table 3. As illustrated in Figure 8a, devices D1, D2, and D3 emit yellowish-green light with CIE coordinates of (0.37, 0.55), (0.38, 0.55), and (0.38, 0.54), respectively. The emission at 373 nm from host mCP is completely absent, suggesting the complete energy transfer from the mCP to the cuprous complexes and effective exciton recombination and confinement on the dopants. Meanwhile, the EL spectral characteristics of devices are very similar to those of PL spectra in powder at 300 K, indicating that electroluminescence originated from the thermally generated singlet excited state of these complexes. As illustrated in Figure 8b, device D1 with complex 1 as the dopant exhibits a lower injection current density, which results from the relatively high LUMO level of complex 1. This may be beneficial for trapping electrons and confining them into the emitting layer. Although a low injection current, device D1 exhibits an optimal EL performance with the maximum luminance, current efficiency, and EQE of 11 010 cd m−2 (at 11 V), 47.03 cd A−1 (at 9 V), and 14.81% (at 9 V), far higher than 5152 cd m−2 (at 11 V), 35.61 cd A−1 (at 8 V), and 11.17% (at 8 V) for D2 and 5242 cd m−2 (at 9 V), 21.33 cd A−1 (at 6 V), and 6.67% (at 6 V) for D3 (Figure 8b,c). Note that such high EQE values of these devices are associated with significantly increasing PLQYs of cuprous complexes 10% doped in mCP thin films, which are 89% for complex 1, 85% for complex 2, and 77% for complex 3, respectively. Low current efficiencies and EQEs in D2 and D3 with complexes 2 and 3 as an emitter may be attributed to an inefficient electron confinement within the emitting layers of D2 and D3 due to a similar LUMO level for host mCP (−2.6 eV), complex 2 (−2.59 eV), and complex 3 (−2.61 eV). Note that the highest EL perfomances in this paper are among the highest levels of ever reported devices based on cuprous complexes,32,81−83 and these high efficiencies are considered to be owing to the harvesting of both singlet and triplet excitons characteristics of cuprous complexes 1−3.30,41 As depicted in Figure 8c,d, the EL

Figure 6. Isodensity plots of the frontier orbitals of complexes 1−3. All of the MO surfaces correspond to an isocontour value of |Ψ| = 0.03.

the isodensity plots of the frontier molecular orbitals for complexes 1, 2, and 3. The HOMOs of these complexes were all composed of bis-carbazole π orbitals distributed among ECAF, EHCAF, and PCAF ligands, while their LUMOs were mainly located on the 4,5-diazafluorene units and phenyl groups of POP ligands. This kind of distribution is very different from those of other previously reported cuprous complexes, in which the HOMOs reside on the Cu(I) and the POP ligand, while the LUMOs are mainly distributed on the diimine ligands.45,109,114,116 The pronounced spatial separation of these frontier orbitals can result in a small exchange energy between them and thus a small ΔEST value.45,60,109 Note that changes of geometry in T1 compared with that in S0 are not obvious, which can be seen in Table S1. For example, the variation scopes of bond lengths and angles are in the range of 0.0087−0.0496 Å and 0.02−2.61°, respectively. Electroluminescent Properties of OLED Devices. Because of the outstanding advantages of complexes 1, 2, and 3 with high thermal stability and photoluminescence quantum yields (PLQYs), the highly efficient evaporated OLEDs are expected. To evaluate their EL performance, devices D1, D2, and D3 using complexes 1, 2, and 3 as the emitters, respectively, with the structure of ITO/PEDOT:PSS (40 nm)/TCTA (15 nm)/1, 2, or 3 (10 wt %):mCP (30 nm)/ 3748


Article

Inorganic Chemistry

Figure 8. (a) EL spectrum; (b) L−V−J curves; (c) ηc−J−ηext curves; (d) ηp−J curves.

Table 3. EL Performances of the Devices D1, D2, and D3 device

Vturn‑on,a V

Lmaxb (voltage), cd m−2 (V)

ηc, maxc (voltage), cd A−1 (V)

ηp, maxd (voltage), lm W−1 (V)

ηext, maxe, % (V)

λmax, nm

CIE (x, y)

D1 D2 D3

5.2 5.6 5.3

11010 (11.0) 5152 (11.0) 5242 (9.0)

47.03 (9.0) 35.61 (8.0) 21.33 (6.0)

21.62 (8.0) 14.48 (8.0) 12.07 (7.0)

14.81 (9.0) 11.17 (8.0) 6.67 (6.0)

544 544 544

(0.37, 0.55) (0.38, 0.55) (0.38, 0.54)

Vturn‑on: turn-on voltage recorded at a luminance of 1 cd m−2. bLmax: maximum luminance. cηc, max: maximum current efficiency. dηp, max: maximum power efficiency. eηext, max: maximum external quantum efficiency.

a

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efficiencies of devices decrease gradually with a further increase of current density and luminance. This phenomenon is the socalled EL efficiency roll-off and can be attributed to the triplet− triplet annihilation (TTA)117 originating from the long lifetime of triplet excited state and triplet−polaron quenching (TPQ).118 The better performances in devices based on this kind of complex are expected to be obtained via rational molecular design strategy and further device optimization. This work provides guidelines for developing highly efficient OLEDs using vacuum-sublimable bipolar cationic cuprous phosphors with thermally activated delayed fluorescence.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b01847. 1 H NMR spectra of complexes 1, 2, and 3; DSC thermograms, PL spectra in degassed acetone and acetonitrile solutions and transient PL in solid powder in the temperature range of 77−300 K for complexes 1, 2, and 3; energetics, Cartesian coordinate, selected calculated bond length and angles of complexes 1, 2, and 3 (PDF)

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CONCLUSION

In summary, three TADF cationic cuprous complexes 1, 2, and 3 with bipolar 4,5-diazafluorene ligand substituted by biscarbazole have been successfully prepared. These metal phosphors can be vacuum-evaporated without decomposition and applied in the OLEDs of ITO/PEDOT:PSS (40 nm)/ TCTA (15 nm)/cuprous complexes (10 wt %):mCP (30 nm)/ TmPyPb (50 nm)/LiF (0.5 nm)/Al (100 nm). A maximum luminance of 11 010 cd m−2, a current efficiency of 47.03 cd A−1, and an EQE of 14.81% are realized in device D1 based on complex 1. The high EL efficiencies of these complexes are assumed to be due to their good thermal stabilities and harvesting both singlet and triplet excitons. The research work presented here paves a new way toward highly efficient and cheap OLED devices.

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. (S.-F.C.) *E-mail: [email protected]. (C.-Z.L.) *E-mail: [email protected]. (B.Z.) ORCID

Bin Zhai: 0000-0002-2866-1121 Author Contributions #

Both authors contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge financial support from the Ministry of Science and Technology (973 Project, Grant No. 3749


Article

Inorganic Chemistry

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2015CB932202) and National Natural Science Foundation of China (Grant Nos. 21501117, 21371114, 21401126, 61274065, and 21571123), the Program for Science and Technology Innovation Talents in Universities of Henan Province (2012HASTIT031), the Scientific and Technological Projects of Science and Technology Department of Henan Province (Grant Nos. 152102210338 and 122102210255), and Ordinary University Graduate Student Practical Innovation Projects of Jiangsu Province (Grant No. SJLX15_0390) for financial support. We also thank Prof. H. Wu of Shanxi Normal Univ. for the theoretical calculations.

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