Introduced Diphosphine Ligands for Both Singlet ... - ACS Publications

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Bluish-green Cu(I) Dimers Chelated with Thiophene Ring-introduced Diphosphine Ligands for Both Singlet and Triplet Harvesting in OLEDs Xiaoyue Li, Juanye Zhang, Zifeng Zhao, Xiao Yu, Peicheng Li, Yuhang Yao, Zhiwei Liu, Qiong-Hua Jin, Zuqiang Bian, Zheng-Hong Lu, and Chunhui Huang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b15897 • Publication Date (Web): 04 Jan 2019 Downloaded from http://pubs.acs.org on January 6, 2019

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ACS Applied Materials & Interfaces

Bluish-green Cu(I) Dimers Chelated with Thiophene Ring-introduced Diphosphine Ligands for Both Singlet and Triplet Harvesting in OLEDs Xiaoyue Li, Juanye Zhang, Zifeng Zhao, Xiao Yu, Peicheng Li, Yuhang Yao, Zhiwei Liu,* Qionghua Jin,* Zuqiang Bian, Zhenghong Lu*, Chunhui Huang Xiaoyue Li, Juanye Zhang, Zifeng Zhao, Yuhang Yao, Prof. Zhiwei Liu, Prof. Zuqiang Bian, Prof. Chunhui Huang Email: [email protected] Beijing National Laboratory for Molecular Sciences (BNLMS), State Key Laboratory of Rare Earth Materials Chemistry and Applications, Beijing Engineering Technology Research Centre of Active Display, College of Chemistry and Molecular Engineering, Peking University, Beijing, 100871, China Xiao Yu, Prof. Qionghua Jin Email: [email protected] Department of Chemistry, Capital Normal University, Beijing, 100048, China Peicheng Li, Prof. Zhenghong Lu Department of Materials Science and Engineering, University of Toronto, Toronto, M5G 3E4, Canada Email: [email protected] Keywords: copper complex, TADF, phosphorescence, Boltzmann fitting, photophysics, vacuum deposition, low-cost OLED Abstract: Two new Cu(I) dimers chelated with thiophene ring-introduced diphosphine ligands [Cu(μ2-I)dppt1]2 and [Cu(μ2-I)dppt2]2 (dppt1 = 3,4-bis(diphenylphosphino)thiophene, dppt2 = 2,3bis(diphenylphosphino)thiophene), have been prepared and studied in terms of photoluminescence and electroluminescence properties. Both two dimers exhibited two independent radiative decay pathways, which are equilibrated thermally at room temperature: one is thermally-activated delay fluorescence (TADF) via the first singlet excited state (S1) decay and the other is phosphorescence via the first triplet excited state (T1) decay. The dual emission mechanism for both singlet and triplet harvesting, as well as excellent photoluminescence properties such as bluish-green emission color (487 nm and 483 nm), short decay times (9.46 and 7.62 μs) and high photoluminescence quantum yields (69% and 86%) of the two Cu(I) dimers implies their potential to be highly efficient

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emitter molecules for organic light emitting diode (OLED) applications. As a result, the optimized OLEDs with [Cu(μ2-I)dppt2]2 showed the highest efficiency, exhibiting a current efficiency up to 32.2 cd A-1, a peak brightness of 3.67 × 103 cd m-2, as well as a maximum external quantum efficiency of 14.5%. INTRODUCTION Luminescent Cu(I) complexes have been extensively investigated as dye molecules in the field of semiconductors as well as organic light emitting diodes (OLEDs).1,2 In order to design suitable complexes to achieve high-efficiency OLEDs, Cu(I) complexes are demanded to be rigid by enhancing steric hindrance in ligand to minimize geometry distortion in the excited state. Therefore, photoluminescence quantum yield (PLQY) could be improved as a result of restricted non-radiation decay.3,4 Furthermore, the stability of Cu(I) complexes is also crucial to meet the requirement of different device fabrication technologies, such as vacuum deposition5, spincoating6-8 and ink-print9,10. Recently, plenty of Cu(I) complexes with high PLQYs have been demonstrated. However, not all Cu(I) complexes are well applicable in preparing high performance OLEDs because of the long emission decay time of hundred microseconds scale,11-17 which results in strong efficiency roll-off. One solution to this problem is to open up the singlet harvesting mechanism, where thermally-activated delayed fluorescence (TADF) is generated with dramatically shortened radiative emission decay time. To achieve so, the first singlet excited state (S1)  first triplet excited state (T1) energy gap (ΔEST) needs to be relatively small (< 0.1 eV) in the Cu(I) complexes1,18-21. H. Yersin group has always been making efforts to develop high efficiency Cu(I) compounds, among several of which had obtained unit PLQY with the singlet harvesting mechanism.22


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In the field of neutral Cu(I) complexes, research focus has been [Cu(N^N)2]+, [Cu(P^P)2]+, [Cu(N^N)(P^P)]+, and [Cu(P^N)(P^P)]+ compounds because of their high emission intensity as well as TADF characteristic18,23,24. With the elaboration of the hard and soft acids and bases (HSAB) theory, soft bases like phosphine can coordinate strongly with soft acidic CuI ion23,25-27, resulting in their good stability both in solution and during high-temperature sublimation. Several groups have reported typical examples such as [Cu2(μ-I)2(dppb)2] and [Cu2(μ-I)2(dpppy)2]28-29, where

dppb

and

dpppy

stands

for

1,2-bis(diphenylphosphino)benzene

and

2,3-

bis(diphenylphosphino)pyridine, respectively. Good stability, high PLQY, as well as regulatable emission color make these Cu(I) complexes competitive candidates in OLEDs. In this study, two dimeric Cu(I) complexes, [Cu(μ2-I)dppt1]2 and [Cu(μ2-I)dppt2]2 with two diphenylphosphine connected by thiophene were designed and synthesized. In the case of [Cu2(μI)2(dpppy)2], a red shift of emission occurred with the replacement of lower electron density pyridine ring. On the contrary, we demonstrated herein that blue-shift of the emission compared with classic dimer [Cu2(μ-I)2(dppb)2] was achieved by the introduction of higher electron density S-hetero-aromatic ring as the linker between two diphenylphosphine groups. Like reported counterparts such as Cu(I) and Ag(I) compounds with singlet harvesting mechanism30,31, these two Cu(I) dimers showed short decay lifetimes and high PLQYs at solid state as well. To deeply understand decay-path mechanism, a detailed photophysical study about TADF vs. phosphorescence balance was carried out. Electroluminescence behaviors of them were investigated, which benefits from their thermal stability and sublimation properties. EXPERIMENT SECTION General Procedures. Caution! All solvents and reagents used in this work are potentially hazard to human body and should be handled carefully as well as used in fume hood all the time with



proper protection. All starting materials are commercially available and were used as received, while solvents were purified with Na/NaH to remove H2O. Both organic ligands and Cu(I) dimers were purified by recrystallization and sublimation. Instruments and Measurements. The 1H and 31P NMR spectra were reported utilizing a Bruker400 MHz spectrometer. Chemical shifts were recorded in ppm units referred to the solvent residual, where δ(CHDCl2) is 5.32. Elemental analysis (C, H, and N) was conducted with an elemental analyser vario MICRO cube (Elementar Co). The UV-vis spectra were recorded on a Shimadzu UV-3600. The mass spectra were acquired on a Bruker Apex IV Fourier Transform Ion Cyclotron Resonance Mass Spectrometer. Photoluminescence spectra were analyzed on an Edinburgh FLS980 fluorescence spectrometer and corrected by detector response. PLQY measurements were performed utilizing a Hamamatsu C9920 system with xenon lamp as excitation source. Sample was fixed inside a calibrated integrating sphere and the emitted photons were collected by model C10027 photonic multichannel analyzer. Thermal gravimetric analysis was undertaken in a Q600 SDT instrument unit with 100 mL min-1 nitrogen atmosphere. The heating rate was set to be 10 °C min−1 from room temperature to 800 °C. Ultraviolet photoemission spectroscopy (UPS) was conducted on PHI 5500 Multi-technique system. The radiation was produced by He Iα (Ehv = 21.2 eV) emission line. The take-off angle was set to be 88 °. A bias of -15 V was applied during the measurement. Materials and Synthesis. The bis(diphenylphosphino)thiophene derivatives were synthesized with a moderate overall yield and their synthetic routes are shown in Figure 1. Two dimers [Cu(μ2I)dppt1]2 and [Cu(μ2-I)dppt2]2 were obtained from room temperature reaction between CuI powder and the corresponding ligand in toluene.


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3,4-bis(diphenylphosphino)thiophene (dppt1): n-BuLi droplet (2.4 M in n-hexane, 4.2 mL, 10.0 mmol, 1eq.) was added to 3,4-dibromothiophene solution (2.42 g, 10.0 mmol) in dry diethyl ether (10 mL) at -78 °C. The brownish mixture was then stirred at -78 °C for 30 min. After that, chlorodiphenylphosphine solution (Ph2PCl, 2.21 g, 10.0 mmol, 1 eq.) dispersed in dry diethyl ether (10 mL) was then added dropwise. The resulting mixture was then stirred for 1 hour at this temperature. Then the suspension naturally warm up to room temperature. The brownish solution was cooled down to -78 °C again, and the above steps were repeated with the same amount of nBuLi and Ph2PCl. 20 mL water was added to quench the reaction carefully. The organic phase was separated and CH2Cl2 (3 × 30 mL) was utilized to extract the aqueous phase. Combine the CH2Cl2 and ether phase and then dried them at atmosphere with sodium sulfate (Na2SO4). The solvent of filtrate then was moved out and evaporated under vacuum. The obtained product was glassy and yellowish solid after sublimation. Yield 58%. 1H NMR (400 MHz, CD2Cl2): δ (ppm) 7.33 (m, 20 H), 7.00 (d, J=2.0 Hz, 2 H); 31P NMR (162 MHz, CD2Cl2): δ (ppm) -19.46 (s). MS: m/z [MH+] = 453.1 2,3-bis(diphenylphosphino)thiophene (dppt2): The ligand was prepared using the similar procedure described for dppt1. Yield 33%. 1H NMR (400 MHz, CD2Cl2): δ (ppm) 7.57 (d, J=5.2 Hz, 1 H), 7.32 (m, 20 H), 6.87 (q, J=2.0 Hz, 1 H); 31P NMR (162 MHz, CD2Cl2): δ (ppm) -21.8 (d, J = 145 Hz). MS: m/z [MH+] = 453.1 [Cu(μ2-I)dppt1]2: CuI (0.286 g, 1.50 mmol) was dispersed into dppt1 solution in 15 mL toluene (0.679 g, 1.50 mmol, 1 eq). The mixture was stirred for 5 hours and generated precipitate at room temperature, then they was collected by using filtration. The grey-green powder was subsequently dealt with toluene, CH3CN, and CH3OH and dried at 100 °C under vacuum. The collected powder was sublimated in vacuum to afford yellow crystals. Yield: 0.670 g (0.525 mmol, 70%). 1H NMR



(400 MHz, CD2Cl2): δ (ppm) 7.25 (m, 44 H), 31P NMR (162 MHz, CD2Cl2): δ (ppm) -22.7 (s). Anal. Calcd. for C56H44Cu2I2P4S2: C, 52.31; H, 3.45. Found: C, 52.39; H, 3.37. MS: m/z [MH+] = 967.1, [Cu(dppt1)2]+. [Cu(μ2-I)dppt2]2: The dimer was prepared and purified using the same procedure with [Cu(μ2I)dppt1]2, where CuI (0.307 g, 1.61 mmol), dppt2 (0.729 g, 1.61 mmol, 1 eq.) and toluene (15 mL) were used. Yield: 0.770 g (0.604 mmol, 75%). 1H NMR (400 MHz, CD2Cl2): δ (ppm) 7.78 (d, J = 5.2 Hz, 2 H), 7.30 (m, 40 H), 6.99 (m, 2H); 31P NMR (400 MHz, CD2Cl2): δ (ppm) -24.9 (d). Anal. Calcd for C56H44Cu2I2P4S2: C, 52.31; H, 3.45. Found: C, 52.50; H, 3.42. MS: m/z [MH+] = 967.1, [Cu(dppt2)2]+. Single-Crystal X-ray Structural Analysis (XRD). XRD studies were conducted by using a Rigaku Mercury CCD diffractometer. The radiation used in the XRD analysis is the graphitemonochromated Mo Kα emission line (λ = 0.71069 Å). XRD data were collected by using the CrystalClear software. Structural refinements were conducted with SHELXL-97 or SHELXL2013 software. Non-H atoms were refined anisotropically, while H atoms were omitted by using the riding model. The crystallographic data collected for these dimer are provided in the Supporting Information (Table S1). The structures were submitted to the Cambridge Crystallographic Data Centre (CCDC 1818693 and 1824729 for [Cu(μ2-I)dppt1]2 and [Cu(μ2I)dppt2]2, respectively). Density functional theory (DFT) calculations. The DFT simulation were conducted with the Gaussian 09 software package32 using the hybrid functional B3LYP.33 The basis sets 6311G(d)34,35 (for H, C, S, P), and LANL2DZ (for Cu and I ) were applied to all calculations with an effective core potential (ECP)36. Data reclaimed from crystal structure measurements were used for the geometry optimizations as the starting input.


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Devices Fabrication and Characterization. All OLED devices were fabricated by thermal evaporation in the Sky tool system, the base pressure is 10−6 Pa. Before fabrication, the patterned indium tin oxide (ITO) substrate is cleaned (sonicated) by using Alconox soap, acetone, and methanol. Then the substrate went through UV ozone treatment for 15 min and was delivered to the system chamber immediately. The thickness of each layer and the evaporation rate of all the materials were monitored using quartz crystal monitors. Rate of deposition were maintained at 0.51 Å s-1 for organic materials and 5-10 Å s-1 for cathode, respectively. All optoelectronics characterization were performed at ambient environment. The current–voltage (I–V) and luminance–voltage (L–V) characteristics measurements were carried out by the HP4140B picoammeter and Minolta LS110 Luminance meter. The external quantum efficiency (EQE) were measured using a silicon photodiode and an integrating sphere. The electroluminescence (EL) spectra were recorded by the Ocean Optics USB4000 spectrometer. RESULTS AND DISSCUSSION Synthesis and Structural Characterization The studied Cu(I) dimers were obtained by reacting equally mole amounts of CuI with the respective chelated ligand (Fig. 1). Single crystals of the dimers were prepared by thermal sublimation in tubes. The chemical structures of the dimers were identified by 1H, 31P NMR, and single crystal X-ray structure analysis (XRD) as well (Fig. 2). In the 31P NMR spectra of the two dimers, singlet and double resonances at about δ −22.7 and −24.9 were observed, which is consistent with the expectation from the structures symmetry. However, the 1H NMR spectra in CD2Cl2 didn’t display well-resolved peaks. Herein, their purities were further characterized by high-resolution mass-spectra and elemental analysis. These dimers also displayed high thermal stability without degradation until 400 oC as shown by thermal gravimetric analysis (Fig. S1).



Fig. 1 Synthetic routes to [Cu(μ2-I)dppt1]2 and [Cu(μ2-I)dppt2]2.

Fig. 2 ORTEP drawings of [Cu(μ2-I)dppt1]2 (left) and [Cu(μ2-I)dppt2]2 (right), with 30% probability level of ellipsoids. Hydrogen atoms are not shown for clarity. Photophysical Properties The two Cu(I) dimers showed intense luminescence under UV excitation. We started with photophysics of dispersed molecular samples. The good stability of the two Cu(I) dimers in dichloromethane solution makes possibility for operators to prepare their poly(methyl methacrylate) (PMMA) films. Fig. 3a shows the UV-vis absorption spectra of the two Cu(I) dimers together with the corresponding ligands in PMMA films with concentration of 0.3 wt% at room


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temperature. [Cu(μ2-I)dppt1]2 and [Cu(μ2-I)dppt2]2 both showed significant absorption below 310 nm, which are characteristic π–π* transitions of the triarylphosphine ligand. Broad shoulders above 330 nm were also observed, which is due to the mixed CT (charge-transfer) transition from the Cu(I) cation to the diphosphine ligands, for the ligands only show absorption bands below 300 nm. This assignment is supported by DFT calculations on the two dimers. The highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) contour curves are drawn in Fig. 4 by applying ground-state calculation. Electron density in the HOMO levels are found to mainly distribute around the Cu(I) and halogen, while the LUMO levels mainly reside on π orbitals of the central thiophene rings. The significant orbital spatial separation is attributed to the special electronic influence of ligands, indicating that the transition possesses of HOMOLUMO showed significant charge transfer character. More importantly, charge transition occurs from the d orbitals of the CuI and the p orbitals of the iodine to the diphenylphosphine ligands with an excitation. Thus, this charge transfer can be attributed to (metal+halide)-to-ligand charge transfer ((M+X)LCT). By comparing the two dimers, it is found that the compound [Cu(μ2I)dppt1]2 showed higher energy absorption band edge than that of [Cu(μ2-I)dppt2]2. The trend of absorption is compelling consistent with the DFT results, that is the energy gap (ELUMO - EHOMO) of [Cu(μ2-I)dppt1]2 (3.14 eV) is higher than [Cu(μ2-I)dppt2]2 (3.03 eV).



Fig. 3 (a) UV−vis absorption of [Cu(μ2-I)dppt1]2 and [Cu(μ2-I)dppt2]2, together with the corresponding free ligands in PMMA films at 298 K. (b) Normalized emission and excitation spectra of [Cu(μ2-I)dppt1]2 and [Cu(μ2-I)dppt2]2 in PMMA films at 298 K. The emission spectra of [Cu(μ2-I)dppt1]2 and [Cu(μ2-I)dppt2]2 in PMMA films at 298 K are shown in Fig. 3b, with maximum emission intensity wavelength at 498 nm and 532 nm for [Cu(μ2I)dppt1]2 and [Cu(μ2-I)dppt2]2, respectively, suggesting that the location of sulfur atom in the linking group affects the electronic structure of the ligands and hence the Cu(I) dimers efficiently. The decay lifetimes (τ) of these two dimers were measured after laser excitation at 340 nm, which are 10.5 μs and 7.32 μs for [Cu(μ2-I)dppt1]2 and [Cu(μ2-I)dppt2]2, respectively.

Fig. 4 Diagrams and shapes of the molecular orbital related to the calculated transition of the two Cu(I) dimers [Cu(μ2-I)dppt1]2 (left) and [Cu(μ2-I)dppt2]2 (right). The photophysical properties of the two Cu(I) dimers in powder at 298 K and 77 K were also studied. At room temperature, the two dimers emitted efficient bluish-green emission, with maximum emission peak at 487 nm for [Cu(μ2-I)dppt1]2 and 483 nm for [Cu(μ2-I)dppt2]2, respectively. The emission maximums are blue-shifted by 11 nm and 49 nm as compared to those of their PMMA films. These observations can be explained by changes of the molecular geometry


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of the Cu(I) dimers which take place after MLCT excitation, a flattening distortion of the molecular structure occurs more easily in a less rigid PMMA environment37. High PLQYs were measured from crushed powder, 69% for [Cu(μ2-I)dppt1]2 and 86% for [Cu(μ2-I)dppt2]2. The sterically bulky diphenylphosphino substituents affect the extend of geometry reorganization in the excited state. Consequently, high PLQYs were perceived due to suppression of the non-radiative decay rate. The decay lifetimes (9.46 μs and 7.62 μs) are almost the same with those of their PMMA films, indicating that the molecular aggregation and crystal packing does not change the photoluminescence mechanism of the two Cu(I) dimers. At 77 K, a large increase in the decay lifetime is accompanied by red-shifted emission spectra, i.e. the dimers [Cu(μ2-I)dppt1]2 and [Cu(μ2-I)dppt2]2 showed decay lifetime of 59.3 μs and 116 μs, and maximum emission wavelength at 500 nm and 494 nm, respectively. All the four broad emission spectra showed no vibronic progressions (Fig. 5), implying that the emission generates from the CT states7,21,38. The PLQYs increased significantly up to 81% and 96% for [Cu(μ2-I)dppt1]2 and [Cu(μ2-I)dppt2]2 at 77 K, respectively. From the equations kr = ΦPL/τ and ΦPL = kr/(kr + knr)39, where kr, ΦPL, τ, and knr stands for radiative constant, PLQY, decay lifetime, and nonradiative constant, respectively. The change of PLQYs under different temperatures are mainly attributed to the change of radiative rate constants (Table 1). When the temperature decreases from 298 K to 77 K, the kr drops by a factor of 5.32 and 10.8 for [Cu(μ2-I)dppt1]2 and [Cu(μ2-I)dppt2]2, respectively. The observed changes in the spectral shift of the emission and radiative rate at 298 K to 77 K may be elaborated by the emission mechanism of TADF.



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Fig. 5 Normalized emission and excitation spectra of [Cu(μ2-I)dppt1]2 and [Cu(μ2-I)dppt2]2 in powder at 77 K and 298 K. The emission spectra were detected at an excitation wavelength of at 340 nm, while the excitation spectra were recorded at λem = 490 nm.

Table. 1 Representative photophysical data of [Cu(μ2-I)dppt1]2 and [Cu(μ2-I)dppt2]2 in powder. 77 K Dimers [Cu(μ2-I)dppt1]2 [Cu(μ2-I)dppt2]2

λem [nm] 500 494

ΦPL [%] 81 96

τ [μs] 59.3 116

kr [104 s-1] 1.37 0.828

knr [103 s-1] 3.20 0.345

λem [nm] 487 483

ΦPL [%] 69 86

298 K τ kr [μs] [104 s-1] 9.46 7.29 7.62 8.92

knr [104 s-1] 3.28 1.45

TADF Mechanism The detailed photoluminescence properties are understood by studying the observed decay lifetime in powder state at temperature varied from 4 K to 298 K. As the consequence of rigid steric of diphosphine ligands, the excited states of both Cu(I) dimers undergo a very small geometry distortion in solid state,28,40-43 which leads to a lowered excited state energy. So that the energy couldn’t be transferred from the excited molecule to a neighbor one because the process of this resonance condition is not fulfilled. High PLQYs and mono-exponential decay behavior between 77 K and 298 K support that no obvious concentration quenching exists in the two Cu(I)


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dimers. The decay time of both dimers barely changes and a plateau is observed in the range of 20 K ≤ T ≤60 K. The corresponding emission is assigned to the T1→S0. Interestingly, the decay curves became distinctly nonexponential with the temperature going down successively. For example, the decay of [Cu(μ2-I)dppt2]2 can be described by a triexponential decay function with individual components of 68.8 μs, 147 μs, and 381 μs at 4 K. This change is most likely related to the zero field splitting (ZFS)44, which results three substates of the lowest triplet state T1 at extremely low temperature. Other longer decay can be also perceived for [Cu(μ2-I)dppt1]2 at 4 K, but decay around 60 μs is still dominant with proportion over 90 %. The decay profiles of two Cu(I) dimers at 298 K, 77 K, 38 K, and 4K are illustrated in Fig. S2. The temperature dependent decay lifetime result is depicted in Fig. 6 for both Cu(I) dimers. According to TADF mechanism, the decay from two inter-convertible excited states are combined balance. Applying an assumption that, there is a rapid thermal equilibrium between the S1 and T1 states, the decay lifetimes and their relationship with temperature can be expressed by the following equation: (1)

𝜏𝑜𝑏𝑠 =

Δ𝐸𝑆𝑇 1 1 + exp ( ― ) 3 𝑘𝐵𝑇 Δ𝐸𝑆𝑇 1 1 + exp ( ― ) 𝑘𝐵𝑇 𝜏(𝑇1) 3𝜏(𝑆1)

where τobs represents the decay lifetime of Cu(I) dimer recorded under each certain temperature, kB is the Boltzmann’s constant, ΔEST is the energy separation (activation energy) between the singlet state S1 and the triplet state T1. τ(S1) and τ(T1) are the emission decay lifetimes of these two states in the absence of thermalization, respectively. These three parameters can be determined by non-liner fitting equation (1) to the measured lifetime data (τobs). As shown in Fig. 6, the red fitted curve matches well with the experimental results. The decay lifetime of the prompt fluorescence



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τ(S1) are 182 ns and 299 ns, the phosphorescence τ(T1) are 58.8 μs and 126 μs, and the energy separation (ΔEST) are 586 cm-1 and 362 cm-1 for [Cu(μ2-I)dppt1]2 and [Cu(μ2-I)dppt2]2, respectively. The short decay lifetimes of τ(S1) substantiate their singlet state feature. Therefore, the observed decay lifetime at 298 K of 9.46 and 7.62 μs represent an S1→S0 radiative process fed by the long-lifetime triplet reservoir. The observed delayed fluorescence indicates that the reverse intersystem crossing (RISC) from T1 to S1 state is literally faster than the spontaneous phosphorescence decay processes. The fitted ΔEST values are much smaller unlike other heavymetal dimers, such as Pd(II) or Pt(II) complexes with ΔEST values over 103 cm-1.45 Moreover, fitted ΔEST are in good agreement with the determined values from the onsets wavelength of emission spectra at 298 K and 4 K. The small ΔEST can be attributed to the separation of molecular orbitals which is involved in the CT transition: the Cu2I2 core moiety and the diphosphine ligand moiety. This separation achieves thermal equilibrium between the two MLCT excited states. With this fast equilibrium, the emissions of the Cu(I) dimers originate from both S1 and T1 states, showing both TADF and phosphorescence characters at the same time. To further understand the temperature dependence of the ratio of the TADF (reversed from T1 to S1) and phosphorescence (originating from T1) intensities with respect to the total intensity Itot, we used the ΦPL values obtained at 298 K and 77 K to estimate the relative contributions from TADF and phosphorescence individually. Assuming that the PLQYs of singlet, i.e. ΦPL (S1) and the PLQYs of triplet, i.e. ΦPL (T1) remain constant at varied temperatures and the population of both states follows a Boltzmann distribution, so we can obtain the following equations: (2) 𝐼(𝑇1) 𝐼𝑡𝑜𝑡

=

𝐼(𝑇1) 𝐼(𝑆1) + 𝐼(𝑇1)

= [1 +

𝜙𝑃𝐿(𝑆1)𝜏(𝑇1)𝑔(𝑆1)

(

Δ𝐸𝑆𝑇

)

exp ― ] 𝑘𝐵𝑇 𝜙𝑃𝐿(𝑇1)𝜏(𝑆1)𝑔(𝑇1)

(3)


―1

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𝐼(𝑆1) 𝐼𝑡𝑜𝑡

= 1 ― [1 +

𝜙𝑃𝐿(𝑆1)𝜏(𝑇1)𝑔(𝑆1)

(

exp ―

𝜙𝑃𝐿(𝑇1)𝜏(𝑆1)𝑔(𝑇1)

Δ𝐸𝑆𝑇 𝑘𝐵𝑇

)

―1

]

Where g(S1) = 1 and g(T1) = 3 are the degeneracy factors for the S1 and the T1 states, I (S1) and I (T1) represent the intensity of S1, T1, respectively. Use the fitting parameters from eq. (1) and observed PLQY values in eq. (2) and (3), we can plot the emission intensities percentage of two radiation paths at different temperatures in Fig. 6. Only phosphorescence (T1) occurs at low temperature (T < 60 K) for both two Cu(I) dimers. The intensity originated from the T1 state decreases with the temperature warming up, as well as the intensity attributed to the S1 state (TADF) increases. When the temperature goes to 298 K, [Cu(μ2-I)dppt2]2 mainly exhibits TADF emissions and the proportion is 96%. Meanwhile, TADF contributes just 84% of total PL intensity in [Cu(μ2I)dppt1]2. This consequence is consistent with ΔEST order of the two Cu(I) dimers ([Cu(μ2-I)dppt2]2 < [Cu(μ2-I)dppt1]2), indicating [Cu(μ2-I)dppt2]2 is a better efficient TADF material. Table 2 summarized the fitting parameters and TADF characterization of these two Cu(I) dimers.



Fig. 6 Temperature dependence of the decay lifetime for [Cu(μ2-I)dppt1]2 (a) and [Cu(μ2-I)dppt2]2 (b). The photoluminescence intensities of [Cu(μ2-I)dppt1]2 (c) and [Cu(μ2-I)dppt2]2 (d) originating from the first excited singlet state S1 (TADF, red line) and triplet state T1 (phosphorescence, black line).

Table. 2 Fitting parameters and TADF characterization of [Cu(μ2-I)dppt1]2 and [Cu(μ2-I)dppt2]2 in powder. 4K 298 K τ (DF)b) ΔEST c) ΔEST a) Onset b) Onset b) -1 [μs] [cm ] [cm-1] [nm] [nm] [Cu(μ2-I)dppt1]2 182 58.8 9.46 448 436 597 586 [Cu(μ2-I)dppt2]2 299 126 7.62 433 426 369 362 a) From the Boltzmann distribution fitting, b) measured by powder sample, c) estimated from the Dimers

τ (S1)a) [ns]

τ (T1)a) [μs]

onset wavelength of emission spectra at 298 K and 4 K. Electroluminescence Performance In the assortment of OLEDs, different functional materials have different carrier mobility and energy levels. The optimization of molecular species and physical architecture affect triplet exciton recombination area and device performance directly. In order to design the device structure reasonably and select the functional materials rationally, UPS was carried out to measure the


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HOMO energy levels of the two Cu(I) dimers in the film state. Experimental UPS spectra of the two Cu(I) dimers are shown in Fig. 7. The secondary electron cut-off edge (ECutoff) and HOMO band edge are also indicated in Fig. 7. Thus, work function (ΦWF) and HOMO level of dimer can be calculated.46

Fig. 7 Experimental UPS spectra of 10 nm vacuum deposited [Cu(μ2-I)dppt1]2 (left) and [Cu(μ2I)dppt2]2 (right) films. The HOMO levels of the studied dimers are 5.12 eV and 5.18 eV for [Cu(μ2-I)dppt1]2 and [Cu(μ2-I)dppt2]2, respectively, which are calculated from their UPS data by equations of HOMO = ΦWF +EH and ΦWF = hv - ECutoff, the calculation results were listed in Table. S2. Based on the energy level data of the two Cu(I) dimers, we screened materials and investigated a variety of device structures to fabricate doped OLEDs. Firstly we tried a very classical structure ITO/MoO3 (1 nm)/mCP (35 nm)/host:Cu(I) dimers (10 wt.%, 20 nm)/TPBi (65 nm)/LiF (1 nm)/Al (100 nm). Based on the results of device performance then we further optimized the host material and thickness of each layer. The structure of each kind of device and optimization procedure are elaborated in Supporting Information. Several representative device data were selected for reporting in Table S3. Among these devices, the best overall performance is achieved in the structure of ITO/MoO3 (1 nm)/mCP:MoO3 (20 wt.%, 10 nm)/mCP (40 nm)/mCP:Cu(I) dimer (10



wt.%, 20 nm)/DPEPO (10 nm)/TPBi (40 nm)/LiF (1 nm)/Al (100 nm), where the Cu(I) dimer is either [Cu(μ2-I)dppt1]2 or [Cu(μ2-I)dppt2]2. In the OLEDs, mCP (ET = 2.9 eV) serves as the host materials due to its proper energy level and excellent charge transport ability, mCP is served as the hole transporting material and TPBi (ET = 2.6 eV) the electron transport materials. DPEPO acts as a step between the light emitting layer and TPBi due to its proper LUMO energy level, which can reduce the electronic transmission barriers.47 Moreover, LiF is the electron injection material, and MoO3 was the hole injection material to modify the ITO’s work function.48,49 A 10 nm mCP doped with MoO3 was deposited on top of MoO3 before a pure mCP layer, in order to further facilitate the hole injection and transport. Fig. 8 shows the energy level diagrams and thickness of each functional layers. Chemical structures of each material are stated in Supporting Information (Fig. S3).


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(a)

Fig. 8 (a) Energy-level schematics of the various layers in both devices, (b) Electroluminescence spectra of both devices at 12 V, (c) J–V–L characteristics of both devices, (d) PE–L–EQE characteristics of both devices. Electroluminescence (EL) spectra of devices with 10 wt.% Cu(I) dimers are shown in Fig. 8b. The excitons are recombined and confined in the emitting layer to exhibit pure strong green emissions with a peak wavelength at 515 nm for [Cu(μ2-I)dppt1]2 and 540 nm for [Cu(μ2-I)dppt2]2, which are consistant with neat films PL.(Fig, S4). This red shift of EL spectra in comparison to powder sample and PMMA sample is also associated to interaction with matrix environment and distortion after electronic excitation40,43,44,50. No emission from mCP was observed, implying that energy transfer is sufficient from host material to Cu(I) dimers. The maximam emissions are stable and show no shift under different driving voltages (Fig. S5). From the current



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density−voltage−luminance (J−V−L) and power efficiency-current dengsity-external quantum efficiency (PE–L–EQE) characteristics (Fig. 8c and 8d), the [Cu(μ2-I)dppt1]2 and [Cu(μ2-I)dppt2]2 based devices showed a same turn-on voltage of 4.4 V, a maximum EQE of 7.44 % and 14.5 %, and a maximum power efficiency (PE) of 11.7 and 21.2 lm W-1, respectively. Table. 3 illustrates the perfprmace of these two devices, including EL emission peaks and luminances as well. Table. 3 Summarized key parameters of devices with [Cu(μ2-I)dppt1]2 and [Cu(μ2-I)dppt2]2. Vona)

CEmaxb)/CE100c)

EQEmaxd)/EQE100e)

λmaxf)

Luminance

[V]

[cd A-1]

[%]

[nm]

[cd m-2]

[Cu(μ2-I)dppt1]2

4.4

26.3/23.0

7.44/6.23

515

1.38 × 103

[Cu(μ2-I)dppt2]2

4.4

32.2/27.5

14.5/12.7

540

3.67 × 103

Emitter

a) Turn on voltage is recorded when the luminance is 1 cd m-2 as a reference point; b) Maximum current efficiency; c) current efficiency at 100 cd m-2; d) Maximum EQE; e) EQE at 100 cd m-2; f) Maximum luminance. CONCLUSIONS Inspired by the outstanding performance of [Cu(P^P)2]+ type Cu(I) dimers, we designed and synthesized two new bluish-green Cu(I) dimers [Cu(μ2-I)dppt1]2 and [Cu(μ2-I)dppt2]2 with high PLQYs up to 69% and 86%, respectively. Replacement of the benzene ring with thiophene heterocycle results in a blue shift of emission spectra while maintaining good thermal stability of these Cu(I) dimers. The characterization of photoluminescence and DFT calculation indicated that, the position of the sulfur atom on the heterocycle relative to the diphenylphosphine groups affects the distribution of molecular orbitals. The radiative processes of [Cu(μ2-I)dppt1]2 and [Cu(μ2I)dppt2]2 can be assigned to (M+X)LCT character like most Cu(I) complexes. The energy gap and decay lifetimes of S1 and T1 were characterized to study the thermal equilibrium between the


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1(M+X)LCT

and 3(M+X)LCT excited states. These Cu(I) dimers exhibited highly efficient TADF

with short emissive decay times at room temperature while only phosphorescence were observed at low temperature, which is a consequence of narrow gaps between up-lying singlet state and triplet state (ΔEST). Detailed investigation of the decay behaviors and Boltzmann fitting at different temperatures were carried out to estimated independent prompt fluorescence, long lifetime phosphorescence, and ΔEST. Applying [Cu(μ2-I)dppt2]2 to vapor-deposited OLEDs as emitters, we obtained highly efficient device with EQE and current efficiency up to 14.5% and 32.2 cd A-1, respectively. In contrast, device based on [Cu(μ2-I)dppt1]2 in the same structure exhibited relatively inferior performances, which may arise from lower TADF proportion and PLQY at room temperature. The encouraging results demonstrate that stable and low-cost Cu(I) dimers are very promising emitters for highly efficient OLEDs. ASSOCIATED CONTENT Supporting information. Synthetic details, X-ray crystallographic data (CIF), density functional theory calculations, thermogravimetric analysis, chemical structure of commercial materials used in OLED fabrication, and a table of experimental data. All materials are available free of charge from author or via the Internet. AUTHOR INFORMATION Corresponding authors [email protected] [email protected] [email protected] ACKNOWLEDGEMENT



Authors from Peking University received the financial support from the National Natural Science Foundation of China (21201011), National Basic Research Program of China (No. 2014CB643802). Authors from Capital Normal University received the financial support from the Beijing Natural Science Foundation (No. 2172017). Author Xiaoyue Li gratefully acknowledges the scholarship from the China Scholarship Council (No. 201706010063).


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