High-Efficiency Narrow-Band Electro-Fluorescent Devices with Thermally Activated Delayed Fluorescence Sensitizers Combined Through-Bond and Through-Space Charge Transfers

Abstract

Open AccessCCS ChemistryRESEARCH ARTICLE1 Aug 2020High-Efficiency Narrow-Band Electro-Fluorescent Devices with Thermally Activated Delayed Fluorescence Sensitizers Combined Through-Bond and Through-Space Charge Transfers Chen Yin, Dongdong Zhang, Yuewei Zhang, Yang Lu, Rui Wang, Guomeng Li and Lian Duan Chen Yin Key Laboratory of Organic Optoelectronics and Molecular Engineering of Ministry of Education, Department of Chemistry, Tsinghua University, Beijing 100084 , Dongdong Zhang *Correspondence authors: E-mail Address: [email protected] E-mail Address: [email protected] Key Laboratory of Organic Optoelectronics and Molecular Engineering of Ministry of Education, Department of Chemistry, Tsinghua University, Beijing 100084 , Yuewei Zhang Key Laboratory of Organic Optoelectronics and Molecular Engineering of Ministry of Education, Department of Chemistry, Tsinghua University, Beijing 100084 , Yang Lu Key Laboratory of Organic Optoelectronics and Molecular Engineering of Ministry of Education, Department of Chemistry, Tsinghua University, Beijing 100084 , Rui Wang Key Laboratory of Organic Optoelectronics and Molecular Engineering of Ministry of Education, Department of Chemistry, Tsinghua University, Beijing 100084 , Guomeng Li Key Laboratory of Organic Optoelectronics and Molecular Engineering of Ministry of Education, Department of Chemistry, Tsinghua University, Beijing 100084 and Lian Duan *Correspondence authors: E-mail Address: [email protected] E-mail Address: [email protected] Key Laboratory of Organic Optoelectronics and Molecular Engineering of Ministry of Education, Department of Chemistry, Tsinghua University, Beijing 100084 Center for Flexible Electronics Technology, Tsinghua University, Beijing 100084 https://doi.org/10.31635/ccschem.020.202000243 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesTrack Citations ShareFacebookTwitterLinked InEmail Organic light-emitting diodes utilizing thermally activated delayed fluorescence sensitizers and multiple-resonance (MR) dopants may simultaneously offer high efficiencies and narrow-band emissions, but these devices still face intractable challenges with a lack of design rules for high-performance sensitizers. Here, sensitizers with ortho-arranged donor–acceptors on a (trifluoromethyl)benzene linker have been proposed, which not only facilitate relatively small molecular dipole moments but also combine through-bond and through-space charge transfers for fast reverse intersystem crossing (RISC). Their photophysical properties are further modulated by additional electron-deficient moieties to better understand the interplay between sensitizer and MR dopant. The highest maximum external quantum efficiency of 33.1% together with a full width at half maximum of 28 nm were obtained in the device utilizing the sensitizer featuring the fastest RISC though inferior emission spectra overlap (J) with MR-dopant absorption among all sensitizers. It has been shown that, other than a blue-shifted emission to enhance J, an ideal sensitizer should establish efficient photoluminescence and fast RISC to suppress exciton loss and annihilation, as well as a small dipole moment to maintain the narrow-band emission of the MR dopant. These findings pave the way for practical applications of all fluorescent devices with both high efficiency and high color purity. Download figure Download PowerPoint Introduction Bearing small singlet–triplet energy gaps (ΔESTs), materials with thermally activated delayed fluorescence (TADF) feature efficient triplet-to-singlet spin-flip transitions via reverse intersystem crossing (RISC) processes, thus providing great potential to harness all excitons formed under electrical excitation by noble metal-free organic materials.1 Therefore, TADF materials have been widely adopted in organic light-emitting diodes (OLEDs) as emitters or sensitizers.2–6 Particularly, bridged by an efficient Förster energy transfer (FET) process, TADF materials as sensitizers for conventional fluorescent dopants (CFDs), referred to as TADF-sensitized fluorescence (TSF), offers great potential for unity exciton utilization, relieved efficiency roll-off, and long device lifetime simultaneously.7–11 However, TSF–OLEDs still face intractable challenges of blocking the formation of triplet excitons on CFDs, of which the utilization is hindered by spin conservation. Quite recently, polycyclic compounds featuring multiple resonance (MR) have been widely developed as emitters, which can be a kind of ideal dopant for TSF–OLEDs.12 On one hand, the MR effect renders the separated localization of frontier molecular orbitals (FMOs) on different atoms, which can minimize bonding/antibonding characteristics and suppress vibrational coupling relaxation, consequently leading to strikingly small full width at half maximum (FWHM < 30 nm) values and large radiative decay rates (kr) in the range of 108–109 s−1 with nearly 100% photoluminescence quantum yield (PLQY). On the other hand, represented by polycyclic boron–nitrogen (B–N) emitters, some MR dopants also possess relatively small ΔEST values to trigger RISC though being slow, and can thus recycle triplets formed on them, guaranteeing 100% exciton utilization efficiency (Figure 1a). With deep blue B–N-type MR dopants, TSF devices exhibiting maximum external quantum efficiency (EQEmax) > 30% with FWHMs < 30 nm have been reported.13,14 Though the performances of related devices have been significantly improved, the interplay between sensitizer and MR dopant has not been thoroughly understood to date. Figure 1 | (a) The energy transfer process in TSF with MR dopant. (b) The molecular design strategy of sensitizers. Download figure Download PowerPoint To maximize the TSF process, a fast RISC process of a sensitizer should be satisfied together with an efficient FET from sensitizer to MR dopant, which requires a blue-shifted (higher energy) sensitizer emission compared with that of the MR dopant to guarantee efficient spectra overlap (J) with the dopant absorption.15 However, for most series of TADF materials, blue-shifted emission usually implies suppressed charge-transfer (CT) characteristics and, thus, a relatively large ΔEST for slow RISC.16 Moreover, owing to the high polarity induced by the CT nature of TADF sensitizers, the emission of MR dopant, which also features CT characteristics, will usually be red-shifted and broadened compared with that in conventional nonpolar hosts.12 Therefore, rational design of proper sensitizers to balance fast RISC and blue-shifted emission without broadening MR dopant spectra still faces formidable challenges, especially for green MR dopants, of which the performances largely lag behind than the blue ones.17 Here, a series of sensitizers for green MR dopants were developed with motifs of ortho-arranged donor (D)–acceptor (A) units on a (trifluoromethyl)benzene linker, of which the photophysical properties were further modulated by additional electron-deficit substituents on the D group, to study the interplay between sensitizer and dopant. Such motifs have been developed not only to facilitate relatively small molecular dipole moments but also to induce combined through-bond and through-space charge transfers (TBCT and TSCT) for efficient RISC. The optimized sensitizer showed a high RISC rate constant (kRISC) of 1.2 × 106 s−1 and PLQY of nearly 100% without significant concentration quenching. Based on this sensitizer, the corresponding green MR dopant-based device showed a EQEmax of 33.1% and small efficiency roll-off with EQE remaining at 27.0% under high brightness of 1000 cd/m2. By analyzing the photoluminescence (PL) and electro-luminescence (EL) characteristics of those sensitizers and corresponding devices, it was found that for an ideal TADF sensitizer, though a large J can guarantee the desired efficient FET process, a high PLQY and a fast RISC process are more preferable to suppress exciton loss and annihilation. Moreover, it was proven that sensitizers with small dipole moments were propitious to maintain the narrow-band EL emission of MR dopant, in which a small FWHM of only 28 nm was recorded in the sensitizing device. These findings provide not only an efficient sensitizer for green MR dopant to boost device performances but also provide guidance for rational design of ideal sensitizers. Experimental Methods Synthesis of sensitizers: Experimental details including synthesis pathway, cyclic voltammetry (CV) and NMR spectra can be found in the Supporting Information. Single-crystal characterization Single crystals were obtained during vacuum sublimation and analyzed by a RIGAKU (Tokyo, Japan)RAXIS-RAPID diffractometer equipped with a graphite monochromator Mo-Kα radiation (λ = 1.54184 Å) at 173 K. The structures were solved by direct methods and refined with a full-matrix least-squares technique based on F2 with the SHELXL-97 (Germany) crystallographic software package. The corresponding Cambridge Crystallographic Data Centre (CCDC) reference numbers (1993227 for 9-(2-(4,6-diphenyl-1,3,5-triazin-2-yl)-4-(trifluoromethyl)phenyl)-3-phenyl-9H-carbazole [CTPCF3]; 1993228 for 4-(9-(2-(4,6-diphenyl-1,3,5-triazin-2-yl)-4-(trifluoromethyl) phenyl)-9H-carbazol-3-yl)benzonitrile [CNCTPCF3]; and 1993229 for 3-(4,6-diphenyl-1,3,5-triazin-2-yl)-9-(2-(4,6-diphenyl-1,3,5-triazin-2-yl)-4-(trifluoromethyl)phenyl)-9H-carbazole [TCTPCF3]) and the data can be obtained free of charge from CCDC. Theoretical calculations The geometric and electronic properties of sensitizers were performed with the Gaussian 09 (Pittsburgh, Pennsylvania, USA) program package by means of B3LYP with the 6-31G(d) atomic basis set. The singlet and triplet states were calculated using time-dependent density functional theory (TD-DFT) calculations with B3LYP/6-31g(d). The molecular orbitals were visualized using a Gaussview program (Pittsburgh, Pennsylvania, USA), and the contributions of TBCT and TSCT were calculated with Multiwfn program (China).18 Optical characterization of organic thin films UV–Vis absorption spectra were recorded using an Agilent 8453 (Palo Alto, California, USA) spectrophotometer. PL quantum efficiency was measured by an absolute PL quantum yield measurement system (C9920-02; Hamamatsu Photonics, Shizuoka, Japan) in air atmosphere with an excitation wavelength of 360 nm. The PL spectrum and PL transient decay curves of the films were measured using a transient spectrometer (Edinburgh FL920P, Edinburgh, Britain). The concentrations of all three molecules in toluene solution for UV–Vis absorption and PL spectra was 10−5 M. All organic films for characterization were prepared through spin-coating on clean quartz substrates by DCM solution at room temperature. Device fabrication and measurement Before device fabrication, the indium-tin oxide (ITO) glass substrates were carefully precleaned. Then the samples were transferred to the deposition system. The devices were prepared in vacuum at a pressure of 5 × 10−5 Torr. All of the organic materials used were purified using a vacuum sublimation approach. All organic materials were thermally evaporated at a rate of 1.0 Å s−1. The forward-viewing electrical characteristics of the devices were measured by an absolute EL quantum yield measurement system (C9920-02; Hamamatsu Photonics) at room temperature under ambient laboratory conditions. For measurement of the transient electro-luminance characteristics, short-pulse excitation with a pulse width of 15 µs was generated using an Agilent 8114A pulse generator. The decay curves of the devices were detected using an Edinburg FL920P transient spectrometer. Results and Discussion The targeted molecules, CTPCF3, CNCTPCF3, and TCTPCF3, are shown in Figure 1b. All three compounds were synthesized via two simple steps including Suzuki coupling reactions and nucleophilic substitution of carbazole derivatives, and then carefully characterized by NMR, mass spectroscopy, elemental analysis, and single-crystal X-ray diffraction. Clearly, all molecules followed a motif of ortho-arranged D–A units on a (trifluoromethyl)benzene linker with 4,6-diphenyl-1,3,5-triazin-2-yl (Trz) moiety and 3-position-substituted carbazole units employed as the A and D units, respectively. Such motifs were adopted, on one hand, to reduce the spatial distance of the D–A unit for a small dipole moment to relieve the influence of high polarity on MR-dopant emission; and, on the other hand, to separate the distribution of FMOs for small ΔESTs via highly twisted structures. Moreover, an added advantage can be anticipated that such D–A arranged units may induce combined CT pathways, including TSCT and TBCT, which has been demonstrated to be beneficial for enhanced RISC processes.19,20 Generally, it is noticed that for such structures, TSCT will be predominant while TBCT is hindered by limited FMOs overlap on the aryl linker owing to the highly twisted structure.21 In addition, the limited FMOs overlap is unfavorable for a high oscillator strength (f) and PL properties. To modulate the combined CTs, a trifluoromethyl (CF3) group on the phenyl bridge was elaborately adopted as the secondary A to permit more distribution of the lowest unoccupied molecular orbital (LUMO) on the aryl linker to enhance FMOs overlap. Besides, to get further understanding about the interplay between sensitizer and dopant, substituents on the 3-position carbazolyl (Cz) unit were introduced to tune the photophysical properties of the sensitizers. It is anticipated that the electron-deficit 4-cyanophenyl and 4,6-diphenyl-1,3,5-triazin-2-yl (Trz) units can reduce the electron-donating ability of the Cz unit to realize blue-shifted emission and enlarged J values. The single-crystal structures of all three compounds are depicted in Figure 2. Dihedral angles between Trz/Cz planar and the phenyl bridge are 33.55°/77.53°, 45.23°/74.78°, and 43.85°/61.34° for CTPCF3, CNCTPCF3, and TCTPCF3, respectively. Theoretically, substitutions on Cz units should show limited influence on those torsional angles, hence the difference may arise from the specific molecular interaction in single crystals. As shown in Supporting Information Figure S1, different to CTPCF3 crystal without obvious molecular interaction, CNCTPCF3 exhibited two types of hydrogen bonds formed between C-H...F and C-H...N with distances of 2.585 and 2.918 Å, respectively. While for TCTPCF3, besides F···H hydrogen bonds with a distance of 2.425 Å, strong π–π interactions between neighboring Trz substituents on Cz units was recorded with a distance of 3.240 Å. Moreover, the crystal structures confirmed the close atom packing between D and A units, with distances in the range of 2.85 –3.81 Å, making TSCT possible.20 Figure 2 | Single-crystal structures, FMO distribution, and calculated energy levels of CTPCF3, CNTPCF3, and TCTPCF3. Download figure Download PowerPoint The molecular geometries of those materials were also calculated by DFT at the B3LYP/6-31G(d) level. As expected, without intermolecular interactions, all compounds showed similar dihedral angles between Trz/Cz planes and the phenyl bridge, being 36.67°/68.59°, 37.08°/68.68°, and 36.78°/65.53° for CTPCF3, CNCTPCF3, and TCTPCF3, respectively. As illustrated in Figure 2, for all compounds, their highest occupied molecular orbitals (HOMOs) were mainly distributed on the carbazole moiety with some extended to the substituents on the 3-positions and the aryl bridge. While LUMOs were largely extended to the phenyl bridge besides the distribution on Trz unit, owing to the existence of the electron-deficit CF3 group. In this way, increased HOMO–LUMO overlap on the phenyl can be anticipated, not only enhancing TBCT process but also facilitating large f values, which were calculated to be 0.0196, 0.0266, and 0.0410 for CTPCF3, CNCTPCF3, and TCTPCF3, respectively. Besides, given that the HOMO and LUMO were spatially close to each other, TSCT was also likely. The integrals of HOMO–LUMO overlaps contributed through space/through aryl bridge can be conveniently calculated with Multiwfn program, which were 0.149/0.072 for CTPCF3, 0.165/0.078 for CNCTPCF3, and 0.180/0.085 for TCTPCF3, respectively. The proportions of TSCT/TBCT in the S1 state can be further characterized by integrating the transition densities localized on/not on the aryl bridge, being 67.5%/32.5%, 68.3%/31.7%, and 67.6%/32.4% for CTPCF3, CNCTPCF3, and TCTPCF3, respectively. To emphasize the importance of the existence of CF3, the HOMO–LUMO overlaps contributed through space/through aryl bridge of 9-(2-(4,6-diphenyl-1,3,5-triazin-2-yl)phenyl)-3-phenyl-9H-carbazole were also calculated to be 0.113/0.052. It was noticed that without the CF3 group, the HOMO–LUMO overlap would be weakened on the aryl bridge, thus leading to a lower f value of 0.0124. Those results suggested the critical role of CF3 in modulating the combined CTs in those materials. The energy levels of HOMO–LUMO, as well as singlets and triplets, were also calculated, showing that, compared with CTPCF3, the other two compounds exhibited both deeper energy levels and larger singlet and triplet energies, which indicated expected blue-shifted emissions. The reason can be attributed to the weakened electron-donating ability of Cz units with the electron-deficit substituents on the 3-position. In addition, larger ΔESTs than that of CTPCF3 were also recorded owing to the more significant HOMO–LUMO overlap in CNCTPCF3 and TCTPCF3 as discussed earlier. The molecular dipole moments were also calculated, showing that CTPCF3 and TCTPCF3 exhibited relatively small values of 1.601 and 1.797 Debye, respectively. It has been recognized that the molecular dipole moment is closely related to the distance of the positive and negative charge centers. For the ortho-arranged D–A motif, the distance should be short for small molecular dipole moments, as in the cases of CTPCF3 and TCTPCF3. Interestingly, it was noted that CNCTPCF3 exhibited a much higher value of 5.611 Debye. To explain this, the spatial orientation of the dipole moment for all compounds are provided in Supporting Information Figure S2, revealing an opposite orientation of CNCTPCF3 compared with the other two compounds. This can be attributed to the strong electron-withdrawing ability of the cyano unit with more electrons on the Cz plane withdrawn to the phenyl plane. As a result, a relatively long distance between positive and negative charge centers could be anticipated, leading to high dipole moment values. The photophysical properties, including UV–Vis absorption and PL spectra, were recorded in toluene solution with a sample concentration of 10−5 M, as depicted in Figure 3 and Table 1. For all three molecules, besides the similar absorption peaks around 290 nm, other main absorption peaks around 345, 323, and 347 nm were also observed for CTPCF3, CNCTPCF3, and TCTPCF3, respectively, which were influenced by the specific substituents with different electronic properties. Those absorptions should arise from the n–π* or π–π*transition of carbazole moieties.21 It was also noted that for all three compounds, though being weak, broad absorptions with peaks around 400 nm were recorded, as depicted in Supporting Information Figure S3. Interestingly, for emitters with single TSCT, no CT absorption was observed in previous studies.22 Therefore, this broad absorption band could be attributed to the TBCT owing to the effective HOMO–LUMO overlap on the aryl linker. All emitters showed wide structureless emission spectra with peaking at 494, 475, and 468 nm for CTPCF3, CNCTPCF3, and TCTPCF3, respectively, suggesting that they were intrinsically CT-type emissions. In terms of the phosphorescence spectra, well-resolved emission with characteristic vibrational structures were observed for all three compounds, evidenced by the locally excited-type emission. Therefore, singlet energies, determined from the onset of fluorescence peak, and the triplet energies, defined by peaks values with the highest energy of phosphorescence, were 2.76/2.72, 2.84/2.56, and 2.91/2.85 eV for CTPCF3, CNCTPCF3, and TCTPCF3, respectively, which together decided the corresponding ΔESTs of 0.04, 0.28, and 0.06 eV. The different triplet energies of those materials may be explained by the different spin-density distributions of their triplet states as illustrated in Supporting Information Figure S4. For CTPCF3 and TCTPCF3, the triplet states were distributed on both D and A units, whereas for CNCTPCF3, the triplet state was predominantly localized on the D unit. Transient decay curves were further measured, as illustrated in Figure 3c, showing clear prompt and delayed components with lifetimes of 32.06 ns/3.21 μs, 21.94 ns/6.04 μs, and 23.73 ns/2.52 μs for CTPCF3, CNCTPCF3, and TCTPCF3, respectively, showing TADF characters as expected. Moreover, PLQYs of 0.96, 0.67, and 0.65 were recorded with corresponding prompt parts of 0.21, 0.23, and 0.28 for CTPCF3, CNCTPCF3, and TCTPCF3, respectively. Consequently, the krs/kRISCs can be calculated, being 6.5 × 106 s−1/1.4 × 106 s−1, 10 × 106 s−1/0.41 × 106 s−1, and 12 × 106 s−1/0.50 × 106 s−1 for CTPCF3, CNCTPCF3, and TCTPCF3, respectively.23 The higher krs of CNCTPCF3 and TCTPCF3 than that of CTPCF3 agreed well with the theoretical f values. For CTPCF3, it showed the highest kRISC among these molecules owing to its small ΔEST. In addition, PLQYs of pristine films of those materials were also measured with 0.90, 0.64, and 0.60 for CTPCF3, CNCTPCF3, and TCTPCF3, respectively, suggesting that the concentration quenching effects in those materials were greatly suppressed by the highly twisted structures. Figure 3 | (a) The absorption, fluorescence, and phosphorescence spectra of sensitizers measured in toluene. (b) The PL decay curves of sensitizers in toluene after degassing. (c) The absorption spectrum of 2F-BN and emission spectra of mCPBP and sensitizers. (d) The emission spectra of the doped films with different dopant concentrations in mCPBP: 10 wt % CTPCF3 or only mCPBP. (e) The PL decay curves of doped films with different dopant concentrations in mCPBP: 10 wt % CTPCF3 or only mCPBP. (f) The PLQYs of the doped films with different dopant concentrations in mCPBP: 10 wt % sensitizer, and the inserted is the emission spectra of doped films with 1 wt % 2F-BN. Download figure Download PowerPoint Table 1 | The Photophysical and of Sensitizers S1 ΔEST HOMO LUMO kRISC f CTPCF3 32.06 CNCTPCF3 0.28 21.94 TCTPCF3 2.85 0.06 23.73 0.0410 Table 2 | Device Device EQE FWHM 1000 1000 1000 CTPCF3 28 CNCTPCF3 30 TCTPCF3 28 mCPBP To the performances of those materials as sensitizers, a green MR dopant, was which green emission peaking at nm with a small FWHM of nm and a high PLQY of in Figure the absorption spectrum of 2F-BN and emission of sensitizers, showing significant overlap between the MR dopant and all three sensitizers. For ideal an efficient FET from TADF sensitizer to is not only facilitating exciton utilization efficiency but also showing to reduce the exciton lifetimes for suppressed exciton To the FET FET defined as an intermolecular distance at which the energy transfer rate constant is to the decay rate constant of the pristine donor without were calculated, being and Å for CTPCF3, CNCTPCF3, and TCTPCF3, The blue-shifted of CNCTPCF3 and TCTPCF3 than CTPCF3 to higher and which the molecular design strategy to enhance the FET process between sensitizers and dopant via group The emission spectra of 10 wt % sensitizers: × wt % 2F-BN were measured and illustrated in Figure and Supporting Information Figure In all with the dopant concentration the emission of sensitizers were to the enhanced FET dopant concentration was sensitizer emission was recorded, suggesting energy transfer in all three for mCPBP: 1 2F-BN without sensitizer, clear emission was given the spectra of dopant and The PL decay curves of those doped films were and depicted in Figure and Supporting Information Figure For the reference obtained from mCPBP: 1 wt % 2F-BN group, a long and of delayed were recorded, owing to the kRISC and efficient of 2F-BN. sensitizers were lifetimes with increased proportions of delayed parts were owing to the sensitizing process, that the specific of emission was closely related with the sensitizers in this In addition, with the increased dopant an enhanced FET process from sensitizers to dopants can be anticipated, in both delayed lifetimes and in dopant emission. The PLQYs of those doped films were further measured, showing increased values with increased dopant concentrations in CNCTPCF3 and TCTPCF3 doped owing to the FET Interestingly, from the high PLQY of CTPCF3, at a dopant a PLQY of nearly 100% was a dopant concentration of 1 wt % was the sensitizer emission was and the PLQYs were and for CTPCF3, CNCTPCF3, and TCTPCF3 doped respectively. The relatively PLQYs of CNCTPCF3 and TCTPCF3 doped films can be attributed to the significant decay process of sensitizers by the PLQYs of sensitizers Therefore, sensitizers with high PLQYs show blocking the exciton loss in sensitizing To the EL performances of those sensitizers, devices with structures of 30 10 5 10 wt % sensitizers: 1 wt % 2F-BN 30 nm) were devices only mCPBP as single were also with a highly optimized dopant concentration of 3 wt % to guarantee energy energy of each functional as well as the molecular structures, are depicted in Figure Clearly, for all three sensitizers, their and LUMOs were close to that of and respectively, small energy for both and Moreover, charge on 2F-BN should be well suppressed to its LUMO and dopant it can be that charge would predominantly on the sensitizers. EL spectra are illustrated in Figure It was noted that the reference device showed a red-shifted spectrum peaking at nm with a larger FWHM of nm compared with the sensitizing devices, which can be to the high dopant sensitizer, the main energy transfer process from triplet to that of dopant was interactions that high dopant concentration to reduce the intermolecular distances for energy high dopant concentration was

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