Role of Excited‐State Structural Reorganization in Achieving Enhanced TADF in Molecular Aggregates for Efficient Cyan‐Green and Yellow OLEDs

Open AccessCCS ChemistryCOMMUNICATIONS2 Sep 2022Phosphonium-Based Ionic Thermally Activated Delayed Fluorescence Emitters for High-Performance Partially Solution-Processed Organic Light-Emitting Diodes Xu-Lin Chen, Xiao-Dong Tao, Ya-Shu Wang, Zhuangzhuang Wei, Lingyi Meng, Dong-Hai Zhang, Fu-Lin Lin and Can-Zhong Lu Xu-Lin Chen State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002 Xiamen Key Laboratory of Rare Earth Photoelectric Functional Materials, Xiamen Institute of Rare Earth Materials, Haixi Institutes, Chinese Academy of Sciences, Xiamen, Fujian 361021 Fujian Science & Technology Innovation Laboratory for Optoelectronic Information of China, Fuzhou, Fujian 350108 , Xiao-Dong Tao State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002 Xiamen Key Laboratory of Rare Earth Photoelectric Functional Materials, Xiamen Institute of Rare Earth Materials, Haixi Institutes, Chinese Academy of Sciences, Xiamen, Fujian 361021 , Ya-Shu Wang State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002 Xiamen Key Laboratory of Rare Earth Photoelectric Functional Materials, Xiamen Institute of Rare Earth Materials, Haixi Institutes, Chinese Academy of Sciences, Xiamen, Fujian 361021 , Zhuangzhuang Wei State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002 Xiamen Key Laboratory of Rare Earth Photoelectric Functional Materials, Xiamen Institute of Rare Earth Materials, Haixi Institutes, Chinese Academy of Sciences, Xiamen, Fujian 361021 , Lingyi Meng State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002 Xiamen Key Laboratory of Rare Earth Photoelectric Functional Materials, Xiamen Institute of Rare Earth Materials, Haixi Institutes, Chinese Academy of Sciences, Xiamen, Fujian 361021 , Dong-Hai Zhang State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002 Xiamen Key Laboratory of Rare Earth Photoelectric Functional Materials, Xiamen Institute of Rare Earth Materials, Haixi Institutes, Chinese Academy of Sciences, Xiamen, Fujian 361021 , Fu-Lin Lin State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002 Xiamen Key Laboratory of Rare Earth Photoelectric Functional Materials, Xiamen Institute of Rare Earth Materials, Haixi Institutes, Chinese Academy of Sciences, Xiamen, Fujian 361021 and Can-Zhong Lu *Corresponding author: E-mail Address: [email protected] State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002 Xiamen Key Laboratory of Rare Earth Photoelectric Functional Materials, Xiamen Institute of Rare Earth Materials, Haixi Institutes, Chinese Academy of Sciences, Xiamen, Fujian 361021 Fujian Science & Technology Innovation Laboratory for Optoelectronic Information of China, Fuzhou, Fujian 350108 https://doi.org/10.31635/ccschem.022.202202145 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail Ionic thermally activated delayed fluorescence (TADF) emitters are rarely investigated due to their poor photoluminescence and electroluminescence performance. Herein, highly efficient ionic TADF emitters with charged donor–acceptor (D–A+) and D–A+–D architectures are designed, innovatively based on the phosphonium cation electron acceptor. The symmetric D–A+–D compound in doped film exhibits a high photoluminescence quantum yield of 0.91 and a short emission lifetime of 1.43 microseconds. Partially solution-processed organic light-emitting diodes based on these ionic TADF emitters achieve a maximum external quantum efficiency (EQE) of 18.3% and a peak luminance of 14,532 candelas per square meter (cd/m2) and show a small efficiency roll-off of 7.1% (EQE = 17%) at a practical high luminance of 1000 cd/m2. These results demonstrate the high potential of phosphonium cations as promising electron acceptors to construct TADF emitters for high-performance electroluminescence devices. The current study opens up an appealing way for future exploitation of high-efficiency ionic TADF materials. Download figure Download PowerPoint Introduction Thermally activated delayed fluorescence (TADF) molecules have been developed as third-generation organic light-emitting diode (OLED) emitters, owing to their potential to realize an internal quantum efficiency of unity without using noble metals.1,2 While numerous highly efficient TADF-based OLEDs have been reported to date, some key challenges remain to be tackled for practical applications.3 For example, TADF-OLEDs usually suffer from severe efficiency roll-off during high-luminance operations owing to exciton annihilation.4–6 To prevent the formation of high-energy excitons and reduce efficiency roll-off in OLEDs, efficient TADF emitters with short emission lifetimes7–10 and fast reverse intersystem crossing (RISC)11–14 are highly desired. TADF emitters are generally composed of suitable electron-donor (D) and electron-acceptor (A) moieties that can create spatial separation of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO). Finding the ideal D and A species is the priority in order to exploit TADF molecules. Unlike electron donors that are almost limited to arylamine derivatives,15 a wide variety of electron acceptors based on main-group elements have been utilized to construct TADF emitters,16 including arylboron,17–22N-heterocycle,23–26 cyano,1,27,28 sulfone,29 carbonyl30–35 groups, and so on. To date, the vast majority of TADF materials are constructed based on neutral D and A moieties ( Supporting Information Figure S5). Charged luminescent materials (e.g., cyanines, rhodamines, ionic metal complexes, etc.) have been utilized in versatile photonic applications due to their ionic nature, diverse photophysical properties, and unique solubility. For instance, many ionic organic dyes have been ultilized as chemosensors in living systems because of their excellent photophysical properties, appreciable water solubility, and biocompatibility.36,37 Ionic transition metal complexes have been employed as emitters in light-emitting electrochemical cells.38 Ionic luminescent compounds are often overlooked in OLED applications, owing to their poor photoluminescence/electroluminescence (PL/EL) properties and inability to be processed via vacuum deposition. However, their intrinsic ionic nature and unique solubility may offer possibilities for developing multilayer solution-processed OLEDs. Recently, a few ionic TADF materials39–47 containing anion/cation moieties or D/A ion-pairs have revealed their potential for PL and EL applications. Nevertheless, it remains a formidable challenge to realize high-performance OLEDs based on ionic TADF emitters ( Supporting Information Table S8). Owing to its 3s23p3 valence shell, phosphorus favors valences ranging from 3 to 5 in organophosphorus compounds. The lone-pair electrons can make trivalent phosphorus atoms act as a donor in organophosphorus molecules, but oxidation to pentavalent phosphine oxide or arylation/alkylation to tetravalent phosphonium cations drastically changes its properties to a typical electron acceptor. Compared with phosphine oxide, which has usually been employed as a weak electron-deficient unit to build n-type and ambipolar host materials for OLEDs,48,49 the phosphonium cation is a noticeably stronger electron acceptor,50,51 offering appealing opportunities to yield visible-light emitters with strong intramolecular charge-transfer (ICT) character. For example, Koshevoy and coworkers52,53 recently reported the unique PL behaviors in solvents of a series of D–π–A fluorophores containing phosphonium cation acceptors. Inspired by the intrinsic electron-deficient property of phosphonium cations, we have selected tetraphenylphosphonium cation as acceptor and designed two ionic TADF materials, namely DMAC-TPP[PF6] and 2DMAC-TPP[PF6] (Figure 1a). These emitters exhibit efficient TADF in doped films with high photoluminescence quantum yields (PLQYs) and short emission lifetimes. Partially solution-processed OLEDs based on these emitters achieve high external quantum efficiencies (EQEs) with only minor efficiency roll-off, demonstrating the high potential of phosphonium cation electron acceptors for the construction of high-performance ionic TADF materials. Figure 1 | (a) Chemical structures; (b) frontier orbital distributions (left: DMAC-TPP[PF6]; right: DMAC-TPP[PF6]); (c) crystal structure (DMAC-TPP[PF6]). Download figure Download PowerPoint Results and Discussion The electronic properties of these compounds were investigated by density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations at the M06-2X/6-311G(d,p) level. As shown in Figure 1b, each compound shows spatially separated frontier molecular orbitals, with the HOMO predominantly distributed over the acridinyl unit and the LUMO mainly located on the remainder of the molecule. The TD-DFT calculations predict that the S1 and T1 states are characterized by HOMO–LUMO ICT transitions Supporting Information Figure S1, with S1–T1 energy difference (ΔEST) of 0.07 eV for DMAC-TPP[PF6] and 0.01 eV for 2DMAC-TPP[PF6], respectively. Owing to the symmetrical molecular structure, the excited states of 2DMAC-TPP[PF6] appear in pairs degenerately (S1/S2, T1/T2, T3/T4, etc.). The calculated energy levels of excited states and spin–orbit coupling (SOC) constants are summarized in Supporting Information Table S1. For 2DMAC-TPP[PF6], the total SOC between S1/S2 and T1/T2 states which have dominant CT exciton characters was calculated to be 0.145 cm−1, while the calculated SOC between S1/S2 and higher-lying triplet states, namely T3/T4 and T5/T6, reach up to 3.807 and 4.021 cm−1, respectively. The same situation exists in the case of DMAC-TPP[PF6] with SOCS1–T1, SOCS1–T2, and SOCS1–T3 of 0.270, 1.946, and 1.171 cm−1, respectively. The first-order mixing coefficient between singlet and triplet states is proportional to their spin–orbit interaction and inversely proportional to the energy gap between them.2 Considering the large energy gaps (over 0.5 eV) between the higher-lying triplet states and S1 states, we speculate that the small ΔEST rather than the higher-lying triplet states play a key role in spin-flip conversion between the S1 and T1 states. These tetraarylphosphonium salts were synthesized in high yields via nickel-catalyzed coupling reactions52,54 between triarylphosphines and aryl bromides and the subsequent anion-exchange reactions (see Supporting Information for details). These compounds are quite air stable in the solid state as well as in solution. The powders survive in ambient conditions without decomposition over at least several months. As depicted in Supporting Information Figure S2, DMAC-TPP[PF6] and 2DMAC-TPP[PF6] show excellent thermal stability with high decomposition temperatures (Td, 5% weight loss) of 399 and 394 °C, respectively. Atomic-force microscopy (AFM) images ( Supporting Information Figure S3) reveal that the spin-coated doped films of these materials (30 wt % in PYD2, the same as the OLED-emitting layers) show fairly smooth surfaces with small root-mean-square surface roughness (Rq) values of 0.246 and 0.256 nm for DMAC-TPP[PF6] and 2DMAC-TPP[PF6], respectively. The excellent thermal stability and high-quality film morphologies support EL device fabrication via solution processes. The crystal structure of DMAC-TPP[PF6] (Figure 1c) reveals that the phosphorus atom (sp3-hybridization) adopts tetrahedral geometry with tetrahedral angles of approximately 109°. The cationic moiety exhibits nearly perpendicular D–A linkage with a dihedral angle of 81.4°, which can result in spatially well-separated frontier molecular orbitals and small ΔEST. Moreover, there exist significant intramolecular (Figure 1c) and intermolecular interactions ( Supporting Information Figure S4) in the lattice, which are expected not only to suppress nonradiative deactivation by rigidifying molecular conformation but also to facilitate the formation of high-quality thin films.55,56 The photophysical properties of these emitters were investigated in dichloromethane and 30 wt %-doped polymethyl methacrylate (PMMA) films. Both compounds exhibit similar absorption profiles, composed of two types of absorption bands (Figure 2a). The intense absorptions below 370 nm are assigned to the π–π* transition originating from the donor moieties while the much weaker absorption bands between 370 and 460 nm are attributed to the ICT transitions from the DMAC donor(s) to the tetraphenylphosphonium acceptor. From the onset of absorption spectra ( Supporting Information Figure S5), optical bandgaps (Eg) were calculated to be 2.76 and 2.74 eV for DMAC-TPP[PF6] and 2DMAC-TPP[PF6], respectively. The increased number of donors results in slightly red-shifted absorption bands. These compounds exhibit strong yellow emission (λmax = 563 and 567 nm, respectively) in degassed dichloromethane at room temperature. The broad and structureless PL spectra as well as their solvent-polarity-dependent behaviors ( Supporting Information Figure S14 and Table S4) confirm CT characteristics of the emissive states. The transient PL decay curves of the investigated emitters in dichloromethane before and after Ar bubbling were compared to verify the involvement of triplet states in the light-emitting process (Figure 2b). A conspicuous delayed decay with significantly increased intensity was observed after 15 min of Ar bubbling to remove dissolving oxygen which can quench the triplet excited states of emitters. This behavior clearly confirms the contribution of triplet states to the fluorescence processes. Remarkably, the delayed decay components of DMAC-TPP[PF6] and 2DMAC-TPP[PF6] in degassed dichloromethane were fitted with ultrashort single-exponential lifetimes of 600 and 549 ns ( Supporting Information Figures S9 and S10), respectively. Figure 2 | (a) Absorption and PL spectra measured in dichloromethane (c = 2 × 10−5 M) at room temperature; (b) transient PL decay curves in dichloromethane (c = 2 × 10−5 M) before/after Ar bubbling for 15 min at room temperature; (c) transient PL decay curves of 2DMAC-TPP[PF6] in 30 wt %-doped PMMA film at different temperatures; (d) time-resolved PL spectra in the 30 wt %-doped PMMA films at 77 K. Fluo.: fluorescence; Phos.: phosphorescence. The PL measurements were excited at 335 nm. Download figure Download PowerPoint The 30 wt %-doped PMMA films of DMAC-TPP[PF6] and 2DMAC-TPP[PF6] display bluish-green PL with emission maxima of 511 and 514 nm ( Supporting Information Figure S11 and Table 1) and PLQYs of 0.75 and 0.91, respectively. Compared with those recorded in dilute solution, for example, in dichloromethane, ethanol, and acetonitrile ( Supporting Information Figure S14 and Table S4), the PL spectra in 30 wt %-doped PMMA films significantly blue-shift. This behavior probably originates from two contributions. First, the solvation effect on the CT-excited states is considerably weakened in the doped nonpolar polymer films. Second, in the more rigid environment of the polymer films, intramolecular rotations and excited-state distortions are effectively restricted, thereby decreasing the vertical transition energies from the emissive ICT states (S1) to ground state (S0), resulting in significantly blue-shifted PL spectrum maxima. As shown in Figure 2c and Supporting Information Figure S12, temperature dependence of transient PL decay reveals that the intensity ratio of delayed fluorescence to prompt fluorescence increased when temperature was increased from 77 to 300 K. At ambient temperature (300 K), the observed emission originates from the S1 state, which is significantly populated via thermally-activated upconversion from the energetically lower-lying T1 state, demonstrating the TADF process. From onsets of the time-resolved PL spectra (fluorescence and phosphorescence spectra) taken at 77 K (Figure 2d and Supporting Information Figure S6), S1 and T1 energies and ΔEST are estimated to be 2.88, 2.82, and 0.06 eV for DMAC-TPP[PF6] and 2.86, 2.81, and 0.05 eV for 2DMAC-TPP[PF6], respectively. Such small ΔEST values would facilitate rapid upconversion of excitons from T1 to S1 through RISC process.57 At 300 K, the prompt fluorescence lifetimes (τPF) and delayed fluorescence lifetimes (τDF) were measured to be 16.6 ns and 1.67 μs for DMAC-TPP[PF6], and 18.2 ns and 1.43 μs for 2DMAC-TPP[PF6], respectively (see Supporting Information for detailed decay-curve fitting). The emission lifetimes are as short as those of representative phosphorescence Ir(III) complexes.58,59 Based on the PLQY and lifetime data, rate constants of the key photophysical processes were estimated using a previously reported method derived by Tsuchiya et al.60 (see Supporting Information for details). The radiative decay rate constants ( k r S ) from S1 to S0 are nearly the same and exceed 1.7 × 107 s−1 for both compounds, which are much higher than corresponding nonradiative rate constants ( k nr S ) (5.80 × 106 and 1.69 × 106 s−1 for DMAC-TPP[PF6] and 2DMAC-TPP[PF6], respectively). The ISC process of each compound (kISC = 2.75 × 107 s−1 for DMAC-TPP[PF6] and kISC = 3.51 × 107 s−1 for 2DMAC-TPP[PF6]) is much faster than its competitive processes, namely the radiative and nonradiative transition of the S1 state, implying that the initially generated singlet excitons in these compounds are significantly transformed to triplet excitons. Notably, the rate constants of RISC (kRISC) of DMAC-TPP[PF6] and 2DMAC-TPP[PF6] reach up to 1.55 × 106 and 2.05 × 106 s−1, respectively. Fast radiative transition together with fast RISC of these emitters result in efficient utilization of excitons and short exciton lifetimes.61 Table 1 | Photophysical Data of the Invesigated Compounds in 30 wt %-Doped PMMA Films at 300 K Compound λPLa (nm) ΦPLb ΦPF/ΦDFc τPF/τDFd (ns/μs) ES1/ET1/ΔESTe (eV) k r S / k nr S f (106 s−1) kISC/kRISCg (106 s−1) DMAC-TPP[PF6] 511 0.75 0.29/0.46 16.6/1.67 2.88/2.82/0.06 17.5/5.80 27.5/1.55 2DMAC-TPP[PF6] 514 0.91 0.31/0.60 18.2/1.43 2.86/2.81/0.05 17.0/1.69 35.1/2.05 aThe wavelength at PL maximum (excited at 335 nm). bOverall PLQY. cΦPF and ΦDF are the quantum yields of prompt fluorescence and delayed fluorescence, respectively. dτPF and τDF are the lifetimes of prompt fluorescence and delayed fluorescence, respectively. eEnergy levels of S1 and T1 state were estimated from the onsets of time-resolved PL spectra at 77 K ( Supporting Information Figure S6). f k r S and k nr S represent the radiative and nonradiative rate constants of S1 states, respectively. gkISC and kRISC refer to the rate constants of intersystem crossing (ISC) and reverse ISC, respectively. The film thickness of samples was 100 nm. To evaluate EL properties of these emitters, partially solution-processed OLEDs were fabricated with a device structure of indium tin oxide (ITO)│poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) (30 nm)│poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-(4,4′-(N-(4-sec-butylphenyl) diphenylamine)] (TFB) (10 nm)│PYD2: emitter (7:3, 40 nm)/m4PO (8 nm)│TPBi (45 nm)│LiF (1 nm)│Al (100 nm) (Figure 3a), where PEDOT:PSS, TFB, m4PO,62 TPBi, and LiF, act as the hole-injection, hole-transporting, hole-blocking, electron-transporting, and electron-injection layers, respectively ( Supporting Information Figure S16). PYD263 was selected as a host material due to its high triplet energy (T1 = 2.93 eV), which is conducive to confining triplet excitons of the investigated greenish-blue emitters (T1 = 2.81–2.82 eV), and due to its proper HOMO and LUMO energies (EHOMO/ELUMO = −5.7 eV/−2.3 eV) which fit the corresponding values of the emitters and adjacent layers for effective carrier transporting. The host–guest ratio was optimized ( Supporting Information Table S7 and Figures S17–S18). The results show that 30 wt % doped devices achieved the best device performance with efficient host to guest energy transfer. The HOMO and LUMO levels of DMAC-TPP[PF6] and 2DMAC-TPP[PF6] were estimated from the oxidation potentials and optical bandgaps ( Supporting Information Figures S15 and Tables S6). EL performances are shown in Figure 3b–d and Table 2. Figure 3 | (a) Energy-level diagram of the OLEDs; (b) current density–voltage–luminance characteristics; (c) EL spectra at various voltages; (d) EQE, current efficiency (CE) and power efficiency (PE) versus luminance characteristics. Download figure Download PowerPoint Table 2 | Summary of Device Performances Device (30 wt %-Doped) λELa (nm) Vonb (V) Lmaxc (cd/m2) EQEd (%) CEe (cd/A) PEf (lm/W) CIE1931g (x, y) DMAC-TPP[PF6] 508 5.0 9496 15.3/13.6/9.9 42.7/37.9/27.6 19.2/10.6/6.0 (0.25, 0.47) 2DMAC-TPP[PF6] 512 4.7 14532 18.3/17.0/13.2 53.4/49.5/38.5 26.0/15.2/9.3 (0.27, 0.49) aThe wavelength at EL maximum (recorded at 12 V). bTurn-on voltage at 1 cd/m2. cMaximum luminance. dEQE maximum value, value at 1000 cd/m2 and value at 5000 cd/m2. eCE maximum value, value at 1000 cd/m2, and value at 5000 cd/m2. fPE maximum value, value at 1000 cd/m2, and value at 5000 cd/m2. gCIE coordinates measured at 12 V. The 30 wt %-doped OLEDs employing DMAC-TPP[PF6] and 2DMAC-TPP[PF6] turned on at approximately 4.7 and 5.0 V, and exhibited bluish-green EL with emission maxima of 508 nm [Commission Internatinale de L'Eclairage (CIE) = 0.25, 0.47] and 512 nm (CIE = 0.27, 0.49), respectively. It is particularly worth mentioning that these devices exhibited perfect emission-color stability over a wide range of operating voltages (Figure 3c). The DMAC-TPP[PF6]- and 2DMAC-TPP[PF6]-based devices showed maximum EQEs of 15.3% and 18.3% and peak luminances of 9496 and 14532 cd/m2, respectively. The DMAC-TPP[PF6]-based device exhibits efficiency roll-offs of 11.1% (EQE = 13.6%) and 35.3% (EQE = 9.9%) at luminances of 1000 and 5000 cd/m2, respectively. Notably, the 2DMAC-TPP[PF6]-based device reached the maximum EQE at a luminance of 117 cd/m2 and showed low efficiency roll-offs of 7.1% (EQE = 17.0%) and 27.9% (EQE = 13.2%) at practical high luminances of 1000 and 5000 cd/m2, respectively. These device efficiencies and efficiency roll-offs represent the best device performance of ionic-TADF-emitter-based OLEDs hitherto and are comparable with those state-of-the-art partially solution-processed OLEDs based on neutral TADF emitters ( Supporting Information Table S8). The small efficiency roll-offs obtained at high luminances can mainly be attributed to the short-lived TADF emitters, which can alleviate the exciton annihilation in the emitting layers. Conclusion Tetraphenylphosphonium cation has been used as the electron acceptor to construct highly efficient ionic TADF emitters with D–A+ and D–A+–D architectures. These ionic TADF emitters, namely DMAC-TPP[PF6] and 2DMAC-TPP[PF6], show high PLQYs of 0.75 and 0.91 and short decay fluorescence lifetimes of 1.67 and 1.43 μs in doped films, respectively. High EL performance has been achieved in partially solution-processed OLEDs. 2DMAC-TPP[PF6]-based device realized EQEmax of 18.3% and peak luminance of 14532 cd/m2. More importantly, the EQE values remain high with only tiny efficiency roll-off even at practical high luminances. Our results suggest that cationic acceptors are a promising choice for the design of high-performance TADF materials and that this research opens an avenue for designing new ionic TADF materials. Supporting Information Supporting Information is available and detailed and photophysical properties, of rate (see the Data at device fabrication and device performance and of is of to Information This research was as a result of a from the Key Research of the Chinese Academy of the Science of the Science of Fujian the Fujian Science and Technology Innovation Laboratory for Optoelectronic Information of and the Innovation of Xiamen and and the Research of Xiamen of a State and for Organic 2. 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Open AccessCCS ChemistryMINI REVIEW1 Aug 2020The Leap from Organic Light-Emitting Diodes to Organic Semiconductor Laser Diodes Chihaya Adachi and Atula S. D. Sandanayaka Chihaya Adachi *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] Center for Organic Photonics and Electronics Research, Kyushu University, Fukuoka 819-0395 Google Scholar More articles by this author and Atula S. D. Sandanayaka *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] Center for Organic Photonics and Electronics Research, Kyushu University, Fukuoka 819-0395 Department of Physical Sciences and Technologies, Faculty of Applied Sciences, Sabaragamuwa University of Sri Lanka, Belihuloya 70140 Google Scholar More articles by this author https://doi.org/10.31635/ccschem.020.202000327 SectionsAboutAbstractPDF ToolsAdd to favoritesTrack Citations ShareFacebookTwitterLinked InEmail In recent years, organic light-emitting device technology has expanded from organic light-emitting diodes (OLEDs) to organic semiconductor laser diodes (OSLDs) with the progress of sophisticated molecular and device architectural designs. In OLEDs, the development of thermally activated delayed fluorescence molecules has been intensively investigated recently. As a result, the internal quantum efficiency of OLEDs containing relatively simple aromatic compounds without precious metals has reached almost 100%. Furthermore, incorporating a distributed feedback resonator structure into the OLED architecture has yielded OSLDs that exhibit the features of current-pumped lasing. In this short review, the authors describe the recent paradigm shift from OLEDs to OSLDs, mainly from the perspective of materials innovation. Download figure Download PowerPoint Progress of Emitter Materials in Organic Light-Emitting Diodes In an organic light-emitting diode (OLED), electrons and holes are injected from the cathode and anode, respectively, into multiple organic layers with thicknesses of ∼100 nm and transported in these layers. The recombination of electrons and holes in the light-emitting layer generates excitons, which might deactivate radiatively, leading to light emission from the OLED. At the time of exciton generation, four eigenstates are formed from a combination of electrons and holes according to spin statistics (Figure 1).1 In this event, the excited singlet state with spin s = 0 is generated with a probability of 25% and the excited triplet states with s = 1 are generated with a probability of 75%. Singlet excitons are usually generated with a probability of almost 100% in the photoexcitation process, whereas triplet excitons are generated with a 75% probability in the electrical excitation process (Figure 2). Thus, achieving radiative deactivation of triplet excitons generated by electrical excitation is the key to realizing highly efficient OLEDs. However, most organic molecules are fluorescent materials that emit light from singlet excitons; emission from their triplet excited states is not usually observed at room temperature because of the competition of nonradiative deactivation. Therefore, triplet-state emission from fluorescent materials is typically limited to low temperatures, such as that of liquid nitrogen. OLED research started in the 1950s with single crystals of anthracene, which is a typical fluorescent molecule (first generation).2,3 Until around 1997, only fluorescent materials were used as light-emitting materials (Figure 3). It was then discovered that the excited triplet energy level of anthracene derivatives could be controlled systematically by introducing a wide variety of substituents, which led to the use of triplet–triplet upconversion (TTU) to raise their electroluminescence (EL) efficiency to higher than that of typical fluorescent molecules. At present, the external quantum efficiency (EQE) of TTU-based OLEDs is >10%, which exceeds the theoretical limit of fluorescence-based OLEDs, that is, the EQE = 5%. In addition, TTU-based emitters with durable molecular structures have been developed, resulted in their practical application in blue OLEDs. Figure 1 | Four eigenstates generated under current excitation. Statistically, the recombination of electrons and holes produces 25% excited singlets and 75% excited triplets. (a) Conceptual diagram of the four spin states. (b) Spin function. Download figure Download PowerPoint Since the early 1950s, it has been widely recognized from theoretical considerations that high EL efficiency can be obtained in OLEDs by using phosphorescence, which is direct luminescence from a triplet excited state. In the early 1990s, some phosphorescent materials such as keto-coumarin derivatives,4,5 Eu derivatives,6,7 and Tb derivatives7 were examined. However, the EQE of OLEDs containing these phosphorescent materials was much lower than that of fluorescent OLEDs. Then in the latter half of the 1990s, some organometallic complexes containing heavy metals such as Os, Au, Pt, and Ir were examined, aimed for OLED application. In fact, Ma and Che first demonstrated the feasibility of metal complexes to obtain high-efficiency OLEDs using Os(CN)2(PPh)3X,8 although their very first device showed a rather low EQE of <0.1%. This study initiated the examination of various luminescent materials, which revealed that PtOEP9 and Ir(ppy)310,11,12 showed great promise for use in OLEDs. Indeed, an internal quantum efficiency (IQE) of almost 100% was realized for OLEDs with Ir(ppy)3 derivatives and sophisticated device architectures,12 giving rise to second-generation luminescent materials. Then the molecular structure of Ir complexes was optimized considering device durability, resulting in current practical devices that operate in the green and red regions. However, Ir is inherently scarce and expensive. Furthermore, even after 15 years of research and development, it is still difficult to achieve highly stable blue phosphorescent OLEDs.13 Figure 3 | Progress of OLED light-emitting molecules: first generation (fluorescent molecules), second generation (phosphorescent molecules), and third generation (TADF). TTA is an extension of first-generation technology. Download figure Download PowerPoint In 2012, our research group reported a current-to-photon conversion efficiency of nearly 100% using advanced thermally activated delayed fluorescence (TADF) materials as third-generation luminescent materials,14 following our lead studies.15–17 To achieve efficient TADF, a small energy difference between the lowest singlet and triplet excited states (ΔEST) is needed to facilitate reverse intersystem crossing (RISC). In TADF, the RISC process is used as an emission light path (Figure 2). Moreover, the phenomenon of TADF itself was first confirmed in the 1930s,18 but the efficiency of upconversion was rather low, masking it as a possible OLED mechanism.19–21 However, focusing on precise molecular design with the aim of minimizing ΔEST has led to pure aromatic compounds with ΔEST as small as several hundreds of millielectronvolts with almost 100% upconversion efficiency. As a result, OLEDs with an IQE of 100% were realized.14 Figure 2 | Mechanisms of exciton generation under current excitation. (a) Conventional fluorescence and phosphorescence emission mechanisms under optical and electrical excitations. In case of fluorescence molecules, only 25% of electrically generated excitons contributes for light emission, while phosphorescence molecules can harvest 100% excitons for light emission via direct triplet exciton formation and indirect triplet formation through ISC. (b) TADF mechanism. In case of thermally activated delayed fluorescence (TADF) mechanism, both electrically generated singlet and triplet excitons contribute for prompt and delayed emissions, leading to 100% emission from the singlet state. ISC, intersystem crossing; RISC, reverse intersystem crossing; TADF, thermally activated delayed fluorescence; NRD, nonradiative decay process. Download figure Download PowerPoint So far, many reported TADF molecules comprise donor–acceptor (D–A) structures in which the electronic configurations of the ground and excited states are orthogonal to each other, like the n–π* transition but not π–π*. Thus, it is vital to understand the mechanism of effective spin upconversion in the TADF system. In the case of D–A-type TADF molecules, it has been well recognized that there are two major electronic states, such as locally excited (LE) and charge-transfer (CT) states, that form multiple energy levels depending on the molecular structures.14 A recent study clarified that LE and CT states could mix partially to form ψ(LE + CT) states. Upconversion from an excited triplet to an excited singlet state is a transition between different spin states, and according to the El-Sayed rule,22 a transition between triplet CT and singlet CT states or triplet LE and singlet LE states is a forbidden process when the wavefunctions of these states are composed of pure components. Thus, the transition between the same types of pure electronic states does not occur, but instead, as a mechanism to promote the triplet-to-singlet RISC transition, and a model was proposed in which the transition between the singlet CT and triplet CT states goes through an intermediate triplet LE transition state (Figure 4). Quantum chemical calculations have also revealed that in actual molecules pure CT and LE states do not exist, and in many cases, the electronic level is a mixture of CT and LE states.23–25 Furthermore, it has been pointed out that the presence of different CT levels, such as through-space and through-bond levels, plays an essential role in upconversion.26,27 Figure 4 | A possible mechanism of the electronic transition from the lowest triplet excited state to the lowest singlet excited state. Spin conversion from 3CT to 1CT occurs via 3LE. The CT state is based on the electronic transition from the donor site to the acceptor site in a molecule, and the LE state is the electronic state localized at the donor site. In practical devices, the mixing of CT and LE states occurs, promoting the RISC process. Download figure Download PowerPoint D–A compounds are considered the fundamental TADF structure for designing high-performance TADF molecules, and many such molecules have now been developed. It has also been clarified that high-performance TADF properties could be achieved using other novel molecular skeletons. In 2014, it was reported that an n–π*-type heptazine derivative without a D–A skeleton exhibited TADF properties.28 Although the photoluminescence quantum yield of guest–host thin films with the heptazine derivative was about 30%, its TADF lifetime was extremely short (about 250 ns). Furthermore, Hatakeyama et al.29–33 proposed a separation mechanism of the highest occupied and lowest unoccupied molecular orbitals using the charge-resonance effect, which yielded a high-performance TADF molecule. Since this molecule had a rigid molecular skeleton, it showed a very narrow emission spectrum with a full width at half maximum (FWHM) of 27 nm, making it an excellent candidate for display applications. Currently, the molecular skeletons of TADF materials include D–A type, charge-resonance type, multiple heterocycles utilizing the n–π* excited state, and proton transfer molecules.32 Therefore, a wide variety of molecular skeletons could be used to realize TADF, and it is expected that further molecular designs would be developed in the future. In this way, OLED research started with fluorescent molecules, progressed to the development of room-temperature phosphorescent molecules, and then rapidly evolved to focus on TADF molecules. Besides, very recent studies have demonstrated some novel conceptual light-emitting materials based on organic radical and organic–inorganic perovskite materials, which use triplet-to-triplet,33 doublet-to-doublet,34 and band-to-band transitions,35,36 respectively. Indeed, various developments are being made because of the high degree of freedom in the molecular design of organic molecules. Active Molecules for Organic Lasers Another attractive feature of organic light-emitting molecules is their ability to amplify light; that is, laser action. Since the first reports of lasing from organic materials using Eu complexes by Sorokin, Lankard, and Schafer more than 50 years ago,37–41 various molecular skeletons have been developed for this purpose. Research has centered on styrylamine-, coumarin-, and cyanine-based materials, keeping their application to liquid dye lasers in mind, and the number of such lasing materials exceeds tens of thousands.42 Especially since 1995, the development of materials for solid-state waveguide thin-film lasers has progressed along with that of OLED light-emitting molecules, and various molecular skeletons exhibiting low lasing thresholds have been reported.43–58 Figure 5 summarizes the lasing/amplified spontaneous emission (ASE) threshold of representative laser materials in solid films. It has been recognized that stilbene and fluorene units in both small molecules and polymers provide excellent lasing behaviors, indicating all possessing rigid backbones with high photoluminescent quantum yield (PLQY) and radiative decay rates. Actually, some reports have aimed to develop current injection lasers using organic materials.59,60 Because organic molecules exhibit strong concentration quenching, a thin solid film consisting of a few mol % of the laser molecules dispersed in a host matrix, that is, guest–host system, is used in such current injection lasers. Figure 5 | Correlation between the molecular structures of organic laser molecules and thresholds of ASE and lasing. Download figure Download PowerPoint Of these various molecular skeletons, it has been reported that laser molecules with a stilbene skeleton exhibit a low threshold value for ASE and lasing.61,62 In particular, 4,4′-bis[(N-carbazole)styryl]biphenyl (BSB-Cz) showed an ASE oscillation wavelength (λASE) of 461 nm in a thin-film waveguide structure with 6 wt % BSB-Cz: 4,4′-Bis(N-carbazolyl)-1,1′-biphenyl CBP as the active layer and an ASE threshold (Eth) of 0.32 ± 0.1 μJ/cm2, which is extremely low (Figure 6).61 The fluorescence lifetime (τf) of this thin film was short (∼1.0 ns), its fluorescence quantum yield (Φf) reached almost 100%, and its radiative deactivation rate constant (kr) was large (1 × 109 s−1). Because Φf and τf of this film did not show temperature dependence from 5 to 300 K, nonradiative deactivation was suppressed entirely even at room temperature. λASE of BSB-Cz occurs near the 0–1 transition in its fluorescence emission spectrum, which suggests the slight self-absorption of the 0–0 transition. Here, λASE is discussed based on kr, the stimulated emission cross section (σem), and the absorption cross section (σABS). kr (kr = ΦPL/τf) is calculated from τf and the emission quantum efficiency (ΦPL) of each codeposited thin film. σem is calculated using the following formula,63,64 σ em ( λ ) = λ 4 E f ( λ ) 8 π n 2 ( λ ) c τ f (1) n f = ∫ E f ( λ ) d λ (2) Figure 6 | Laser oscillation characteristics and optical properties of a 6 wt % BSB-Cz:CBP thin film as an active layer. (a) Chemical structures of BSB-Cz and CBP as an active emitter and host, respectively. (b) Temperature dependence of the emission quantum efficiency and emission lifetime of the thin film. (c) Lasing oscillation spectrum. (d) Excitation power dependence of emission intensity. The threshold is around 0.32 μJ/cm2. Download figure Download PowerPoint In Eq. (1), Ef(λ) is the quantum yield distribution, and n is the refractive index. σABS54 was calculated using Eq. (3), in which n = 1.8. σ ABS , Sol ( λ ) = 1000 ɛ ( λ ) ln 10 N A (3)where ɛ(λ) is the molar extinction coefficient, and NA is Avogadro’s number. In the 6 wt % BSB-Cz:CBP thin film, a high value of σem = 2.7 × 10−16 cm2 was obtained. Furthermore, the effective stimulated emission cross section (σemeff) is the difference between σem and the cross section related to a loss (σABS and the singlet and triplet excited-state absorption cross sections, σSS and σTT, respectively), and is given by Eq. (4). σ emeff = σ em − ( σ ABS + σ SS + σ TT ) (4) Figure 7 shows the spectra of σeff and σABS and the excited-state absorption spectrum of a 6 wt % BSB-Cz:CBP coevaporated thin film. In BSB-Cz, the singlet excited-state absorption, triplet excited-state absorption, and ground-state absorption spectra do not have a large overlap with λASE. Thus, the 6 wt % BSB-Cz:CBP codeposited thin film has a high kr (i.e., a large σem), σABS as small as <10−19 cm2 at λASE, and an excited-state absorption. The absence of these absorptions provides a very large σemeff, leading to a very low Eth. Figure 7 | (a) Ground-state absorption spectrum (solid red line), fluorescence spectrum, laser oscillation spectrum (solid blue line), S–S absorption spectrum (blue circles), and T–T absorption spectrum (orange circles) of BSB-Cz. (b) Energy-level diagram of BSB-Cz. Download figure Download PowerPoint Laser Oscillation Characteristics Under Optical Excitation As described earlier, BSB-Cz is suitable for optical amplification because of its high Φf, low probability of intersystem crossing, and the absence of overlapping excited-state absorption in the λASE region.66 For laser oscillation, it is necessary to introduce an optical resonator structure; however, in an amorphous organic thin film with a thickness of several hundred nanometers, it is difficult to form an end face like in the case of an inorganic semiconductor crystal with a distributed Bragg reflector structure. Thus, for organic thin-film lasers, it is ideal for forming a distributed feedback (DFB) resonator structure, which could outcouple the emission perpendicular to the longitudinal direction of the device. Among DFB resonator structures, the mixed-order DFB structure, which has a primary feedback region that produces strong optical feedback and a secondary Bragg scattering region that allows light extraction, is suitable for organic thin-film lasers. In a DFB resonator structure, the Bragg condition is given by Eq. (5), m λ Bragg = 2 n eff Λ (5)where m is the diffraction order, λBragg is the Bragg wavelength, neff is the effective refractive index of the gain medium, and Λ is the grating period. Laser oscillation occurs when this condition is satisfied.25 Using the reported values of neff and λBragg for BSB-Cz, the optimum Λ for m = 1 and 2 in DFB laser devices are 140 and 280 nm, respectively. Figure 8 shows a DFB grating observed by scanning electron microscopy (SEM). The DFB grating was designed to possess a depth of 65 ± 5 nm and Λ of 140 ± 5 and 280 ± 5 nm. The primary and secondary DFB grating lengths were approximately 15.12 and 10.08 µm, respectively. By forming a 200 nm-thick BSB-Cz film on the grating by vacuum deposition, the surface morphology of the organic layer possessed a lattice structure with a surface modulation depth of 20–30 nm. Figure 9 shows the oscillation characteristics of a mixed-order DFB device under optical excitation. With increasing excitation intensity, the FWHM decreased remarkably, and at Eth = ∼0.2 μJ/cm2, laser oscillation occurred from the vicinity of the stopband at the central oscillation wavelength of 481 nm. In this mixed-order-type DFB structure, compared with those of devices with ASE and second-order DFB structures, Eth was decreased by about 1/3 and 1/2, respectively, demonstrating the superior performance of the mixed-order-type DFB structure. These results confirmed the light confinement effect of the mixed-order DFB structure with BSB-Cz. Figure 8 | (a) Schematic of a mixed-order DFB structure with first- and second-order gratings. (b, c) SEM images of the DFB structure with a 140-nm primary structure and 280-nm secondary structure. Download figure Download PowerPoint The limited overlap of the excited-state absorption, ground-state absorption, and emission spectra of BSB-Cz suggest the possibility to realize quasi-continuous-wave (qCW) laser oscillation. Figure 10 shows the qCW laser oscillation characteristics of a device with BSB-Cz. Continuous laser action was obtained even at a high frequency of 80 MHz. Besides, continuous laser action was observed even with a long pulse excitation of 800 µs to 30 ms.55 The optical gain and loss coefficient estimated from the ASE characteristics of the doped film (optical waveguide structure with a thickness of 200 nm) using the variable stripe method were 40 and 3 cm−1, respectively. These results confirmed that BSB-Cz is an attractive candidate for qCW lasers able to operate even under long-pulsed light excitation. Figure 10 | Quasi-CW lasing characteristics of mixed-order DFB structures. Streak images of the oscillation state when (a) the excitation frequency was changed from 0.01 to 80 MHz, and (b) the pulse width was 30 ms and 800 (c) Excitation dependence of the lasing threshold The doped film exhibited a lower Eth than that of the film. continuous Download figure Download PowerPoint of Organic Semiconductor Laser With the of fluorescent molecules, phosphorescent molecules, and TADF molecules, OLEDs, of conversion with the IQE of 100%, are now At the same the of an organic semiconductor laser diode has long been a in organic semiconductor proposed an using an Eu in the active layer in but 30 years have since organic semiconductor laser that by current excitation is expected to have great because of its low and wavelength from the to the Furthermore, such organic semiconductor lasers are attractive for use at the of organic such as in optical on Figure 9 | (a) Laser oscillation characteristics of the mixed-order DFB structure. (b) of oscillation and FWHM on excitation intensity. (c) of emission around the with the calculated Download figure Download PowerPoint In our research group reported of lasing by current The device structure was based on that of a OLED. To electrical the primary and secondary DFB structures in the optical resonator and a fluorescent thin film of BSB-Cz as the organic semiconductor active layer were between an thin and cathode (Figure In this organic amorphous thin-film a thin-film structure is to a high of for effective current and the thickness of the organic active layer was limited to nm. Furthermore, to form with the the cathode of the organic active layer was with and a layer was on the of the organic active layer to achieve Figure | Schematic diagram of the structure of a was obtained using BSB-Cz on the cathode and layer on the cathode Download figure Download PowerPoint Conventional OLEDs are based on a to and electrons and holes and the generated excitons in the light-emitting layer. This structure to achieve high and the EL emission efficiency of OLEDs did not to current of about 1 However, an injection of 1 is needed for laser oscillation. At such a high current and various exciton deactivation by the Therefore, it is necessary to use a structure containing to both and deactivate excitons in the of the light-emitting layer. current revealed that the of both electrons and holes in the BSB-Cz layer were about Thus, that there are at the between the and the organic the recombination site be near the of the BSB-Cz emission layer. In fact, the characteristics of the showed a constant EQE to a high current of without A organic molecules under current excitation is the presence of and radical absorption. Because many organic molecules have a absorption spectrum in the radical state, the absorption by overlap with the oscillation wavelength laser oscillation. BSB-Cz shows strong absorption around to 1000 nm, which does not overlap with the emission spectrum near nm. Therefore, BSB-Cz the overlap of the absorption from the ground state, excited singlet absorption, excited triplet absorption, and absorption, the to achieve excellent performance as a laser molecule for current excitation. A laser was using a 6 wt % BSB-Cz:CBP codeposited thin film as an active and as an optical resonator structure suitable for an first- and second-order DFB structures were into an OLED device to Figure shows the laser oscillation characteristics of the device under the current excitation. a current of about a narrow and emission were obtained. A in FWHM was observed with a and a width of nm or was obtained. The current threshold was almost to the threshold value estimated under The efficiency under current excitation was which was the same as that under optical excitation In the efficiency of the device without a metal was which suggests that the loss by the metal was Figure | Lasing characteristics of a current-pumped (a) of the oscillation spectrum near the threshold on current (b) of oscillation and FWHM on current Download figure Download PowerPoint the of laser oscillation, the current a very short device of OLEDs has been by by and and chemical of light-emitting molecules using device However, a very high current compared with that used for OLEDs. the yield of OSLDs was because of the of device DFB In the along with the of the laser the mechanism of by the of the excited triplet states be BSB-Cz is to because of its which is relatively to it has been confirmed that the of triplet into organic semiconductor laser structures the of lasing In the in to the triplet exciton deactivation mechanism, it is necessary to the

Open AccessCCS ChemistryCOMMUNICATIONS2 Sep 2022Towards Efficient Blue Delayed-Fluorescence Molecules by Modulating Torsion Angle Between Electron Donor and Acceptor Jinke Chen, Xing Wu, Hao Liu, Nuoling Qiu, Zhangshan Liu, Dezhi Yang, Dongge Ma, Ben Zhong Tang and Zujin Zhao Jinke Chen State Key Laboratory of Luminescent Materials and Devices, Guangdong Provincial Key Laboratory of Luminescence from Molecular Aggregates, South China University of Technology, Guangzhou 510640 , Xing Wu State Key Laboratory of Luminescent Materials and Devices, Guangdong Provincial Key Laboratory of Luminescence from Molecular Aggregates, South China University of Technology, Guangzhou 510640 , Hao Liu State Key Laboratory of Luminescent Materials and Devices, Guangdong Provincial Key Laboratory of Luminescence from Molecular Aggregates, South China University of Technology, Guangzhou 510640 , Nuoling Qiu State Key Laboratory of Luminescent Materials and Devices, Guangdong Provincial Key Laboratory of Luminescence from Molecular Aggregates, South China University of Technology, Guangzhou 510640 , Zhangshan Liu State Key Laboratory of Luminescent Materials and Devices, Guangdong Provincial Key Laboratory of Luminescence from Molecular Aggregates, South China University of Technology, Guangzhou 510640 , Dezhi Yang State Key Laboratory of Luminescent Materials and Devices, Guangdong Provincial Key Laboratory of Luminescence from Molecular Aggregates, South China University of Technology, Guangzhou 510640 , Dongge Ma State Key Laboratory of Luminescent Materials and Devices, Guangdong Provincial Key Laboratory of Luminescence from Molecular Aggregates, South China University of Technology, Guangzhou 510640 , Ben Zhong Tang School of Science and Engineering, Shenzhen Institute of Aggregate Science and Technology, The Chinese University of Hong Kong, Shenzhen, Guangdong 518172 AIE Institute, Guangzhou Development District, Huangpu, Guangzhou 510530 and Zujin Zhao *Corresponding author: E-mail Address: [email protected] State Key Laboratory of Luminescent Materials and Devices, Guangdong Provincial Key Laboratory of Luminescence from Molecular Aggregates, South China University of Technology, Guangzhou 510640 https://doi.org/10.31635/ccschem.022.202202196 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail Constructing blue thermally activated delayed-fluorescence materials for high-performance organic light-emitting diodes (OLEDs) remains challenging due to the intrinsically strong intramolecular charge transfer nature of the nearly orthogonal connection of electron donor (D) and acceptor (A), which results in long-wavelength emission. Herein, an effective delayed-fluorescence design strategy of modulating D–A torsion angles is proposed and efficient sky-blue, pure-blue, and deep-blue delayed-fluorescence molecules consisting of a xanthenone acceptor and carbazole-based donors are created by decreasing the torsion angles. They exhibit strong delayed fluorescence with high photoluminescence quantum yields of 85–94% in doped films, and their delayed-fluorescence lifetimes are elongated from 1.0 to 27.6 μs as the torsion angles decrease. These molecules can function as excellent emitters in OLEDs, providing efficient electroluminescence peaking at 442 nm (CIEx,y = 0.15, 0.08), 462 nm (CIEx,y = 0.15, 0.18), and 482 nm (CIEx,y = 0.17, 0.30) with state-of-the-art external quantum efficiencies of up to 22.2%, 33.7%, and 32.1%, respectively, demonstrating the proposed molecular design for efficient blue delayed-fluorescence molecules is successful and promising. Download figure Download PowerPoint Introduction Efficient blue organic luminescent materials are highly desired because they are one of the fundamental elements of the three primary colors in organic light-emitting diodes (OLEDs).1–7 Organic fluorescence molecules with blue emission, which are employed as the first-generation luminescent materials in OLEDs, can be readily designed, but only 25% of the electro-generated excitons under electrical excitation are used, leading to low external quantum efficiency with an upper limit of 5–7.5%.8–10 Several strategies, such as triplet–triplet fusion11–13 and hybridized local and charge-transfer excited states,14–17 have been proposed to enhance triplet exciton utilization of fluorescence molecules, but full exciton harvesting remains difficult. Second-generation noble-metal-containing phosphorescence materials have been developed, which can reach unity exciton utilization by converting singlet excitons to triplet excitons via intersystem crossing based on heavy-atom induced large spin–orbit coupling (SOC). But, because of the intrinsic metal-to-ligand charge-transfer (CT) characteristics, pure blue emissions are hardly achieved in most phosphorescence materials, and the long lifetimes of triplet excitons result in poor stability of these materials in OLEDs.18–21 After decades of continuous research, purely organic thermally activated delayed-fluorescence (TADF) molecules have been invented and are currently emerging as the third-generation luminescent materials for the fabrication of high-performance OLEDs, thanks to the advantages of easy molecular design, high exciton utilization, noble metal-free structures, and so on.1,22–28 The reported TADF molecules generally have a highly twisted conformation, consisting of an electron donor (D) and acceptor (A) connected in a nearly perpendicular manner, to minimize the exchange integral between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO). Thus, the energy split (ΔEST) between the lowest singlet excited (S1) state and the lowest triplet excited (T1) state can be reduced to allow fast reverse intersystem crossing (RISC), which results in the occurrence of delayed fluorescence.3,29–31 Although numerous efficient sky-blue to red TADF molecules have been successfully explored based on this design method,24,31–37 deep-blue to blue TADF molecules are still challenging because these nearly orthogonal D–A systems are inevitably accompanied by a strong intramolecular charge-transfer (ICT) effect that causes redshifted emissions.38–41 Weakening the ICT effect by choosing weak D and A groups3,42–44 or designing a through-space CT framework45,46 can to some extent shift the emission peaks to the short-wavelength region, but the corresponding electroluminescence (EL) efficiencies are often unsatisfactory. Therefore, modulating the torsion angles of proper D and A groups could be a promising strategy to explore high-efficiency deep-blue and pure-blue TADF molecules. As a proof of concept, we wish to report an effective design of blue luminescent molecules based on a xanthenone (XT) acceptor and two carbazole (Cz) donors (Figure 1a–c). XT is selected as electron acceptor because of its relatively weak electron-withdrawing nature, high structural rigidity, and ability to promote RISC by enlarging SOC stemming from the n−π* transition of the carbonyl group.40 A previous study demonstrated the ability to tune the color of blue TADF emitters by the introduction of methyl substituents.47 Here, the torsion angles between XT and Cz are tuned progressively by introducing methyl groups at the 1 and 8 positions of Cz, and the strength of the D–A interaction is further optimized by modification at the 3 and 6 positions of Cz with electron-donating tert-butyl groups. We found that the ICT effect is weakened sequentially as the torsion angles between XT and Cz diminishes, gradually blueshifting the emissions from 2MCz-XT to MCz-XT and then to Cz-XT. The introduction of tert-butyl groups can enhance the ICT effect, leading to moderately redshifted emission of 2TBCz-XT relative to Cz-XT. Meanwhile, all these molecules exhibit apparent delayed fluorescence, while the lifetimes of the delayed fluorescence are closely associated with the torsion angles and the strength of the ICT effect. By adopting these new molecules as emitters, highly efficient deep-blue, pure-blue, and sky-blue OLEDs with EL peaks at 442, 462, and 482 nm and outstanding maximum external quantum efficiencies (ηext,maxs) of 22.2%, 33.7%, and 32.1%, respectively, are obtained. These impressive EL performances demonstrate the significance of modulating torsion angles in the design of blue TADF emitters. Figure 1 | (a) Molecular design strategy. (b) Chemical structures of the new molecules with calculated torsion angles and (c) crystal structures of Cz-XT and 2MCz-XT with observed torsion angles. (d) Distributions of HOMOs and LUMOs and the calculated energy splits (ΔESTs) of the new molecules. Download figure Download PowerPoint Results and Discussion The target molecules Cz-XT, MCz-XT, 2MCz-XT, and 2TBCz-XT were facilely synthesized in good yields by palladium-catalyzed Buchwald–Hartwig C–N coupling reactions of 3,6-dibromoxanthen-9-one with Cz and Cz derivatives ( Supporting Information Scheme S1). The molecular structures were characterized by 1H NMR and 13C NMR ( Supporting Information Figures S1–S4) and high-resolution mass spectrometry with satisfactory results. They are thermally and morphologically stable with high decomposition temperatures of 397–459 °C and high glass-transition temperatures over 210 °C, as determined by thermogravimetry analysis and differential scanning calorimetry measurements, respectively ( Supporting Information Figure S5). Their electrochemical properties were measured by cyclic voltammetry using ferrocene as the calibration compound ( Supporting Information Figure S6). The experimental HOMO and LUMO energy levels of Cz-XT, MCz-XT, 2MCz-XT, and 2TBCz-XT are calculated to be −5.69 and −2.92; −5.66 and−2.93; −5.56 and −2.94; and −5.60 and −2.92 eV, respectively. Single crystals of Cz-XT and 2MCz-XT were obtained from a mixture of n-hexane and dichloromethane via slow solvent evaporation. Single-crystal X-ray crystallography analysis reveals that 2MCz-XT adopts a highly twisted D–A connection with large torsion angles of 83° and 91°, due to the severe steric hindrance imposed by the two methyl groups at the 1 and 8 positions of Cz. In contrast, Cz-XT shows a more planar molecular conformation, in which the torsion angles are decreased to 37° and 40°, indicating Cz-XT has a better π-conjugation between Cz and XT than 2MCz-XT. The optimized structures and molecular orbitals of these new molecules were calculated employing a density functional theory (DFT) method.48 As depicted in Figure 1d and Supporting Information Figure S7, the optimized geometry of 2MCz-XT has a similar highly twisted conformation to its crystal structure, with large torsion angles of 83°–84° between XT and Cz. However, MCz-XT and Cz-XT show gradually decreased torsion angles of 70° and 51°–52° due to the reduced steric hindrance. Similar molecular geometry is simulated for 2TBCz-XT compared with Cz-XT. The electron clouds of the HOMOs and LUMOs of these molecules are primarily distributed on Cz and XT, respectively. Due to the highly twisted molecular geometry, 2MCz-XT has the highest degree of separation between the HOMO and LUMO, which leads to the smallest ΔEST of 0.01 eV. The ΔEST of MCz-XT is increased to 0.12 eV due to decreased torsion angles. Cz-XT and 2TBCz-XT have overlapping HOMOs and LUMOs because of the relatively planar conformation. Thus, they have much larger ΔESTs of 0.23 and 0.21 eV than 2MCz-XT and MCz-XT. As displayed in Figure 2a, Cz-XT and 2TBCz-XT have strong absorption maxima at 365 and 384 nm in tetrahydrofuran (THF) solution, which are mainly comprised of the π–π* transitions. MCz-XT and 2MCz-XT have relatively weak absorption maxima at 362 and 368 nm, associated with the ICT states. 2MCz-XT exhibits a green photoluminescence (PL) peak located at 501 nm in THF solution, whereas the PL peaks are blueshifted progressively to 481 nm for MCz-XT and 459 nm for Cz-XT (Figure 2b) due to the weakened ICT effect. The PL peak of 2TBCz-XT is redshifted to 481 nm, which is ascribed to the strengthened ICT effect due to the presence of the tert-butyl groups. To evaluate the ICT effect, the PL spectra of the four molecules in various solvents are tested ( Supporting Information Figure S8). The spectral displacements gradually increase from Cz-XT (60 nm) to MCz-XT (63 nm) and then to 2MCz-XT (71 nm), in good agreement with the increased dihedral angles and strengthened ICT effect. 2TBCz-XT exhibits a larger spectral displacement of 70 nm than Cz-XT (60 nm) because of a stronger ICT effect. These solvation effects further validate that both enlarging D–A dihedral angles and introducing electron-donating groups strengthen the ICT effect of the molecules. When doped in (diphenylphosphoryl)-dibenzo[b,d]-furan (PPF) host at a concentration of 15 wt %, Cz-XT shows a PL peak at 459 nm, similar to that in THF solution, whereas MCz-XT and 2MCz-XT have redshifted PL peaks at 466 and 483 nm, respectively. The PL peak of 2TBCz-XT is located at 468 nm, which is redshifted by 9 nm compared with that of Cz-XT. The photoluminescence quantum yields (ΦPLs) of these molecules in doped films are in the range of 85–94%, higher than those in THF solution (Table 1). The molecular motions are active in solution, largely dissipating the excited-state energy and thus leading to low ΦPL values. But in the doped films, the intramolecular motions of the molecules are greatly suppressed so that the nonradiative dissipation pathways are blocked, accounting for the significantly improved ΦPL values.16,35 Figure 2 | (a) Absorption and (b) photoluminescence (PL) spectra of the new luminogens in THF solutions (10−5 M) and in doped films with a doping concentration of 15 wt % in PPF. Temperature-dependent transient PL decay spectra of (c) Cz-XT, (d) MCz-XT, (e) 2MCz-XT, and (f) 2TBCz-XT doped in PPF host with a doping concentration of 15 wt %, measured under nitrogen. Download figure Download PowerPoint Table 1 | Photophysical Properties of the New Molecules Solutiona Doped Filmb λabs (nm) λem (nm) ΦPLc (%) λem (nm) ΦPLc (%) τdelayedd (μs) Rdelayede (%) kFf (×107 s−1) kICg (×107 s−1) kRISCh (×105 s−1) ΔESTi (eV) Cz-XT 365 459 47 459 85 27.6 64 9.9 1.7 1.0 0.15 MCz-XT 362 481 52 466 86 11.3 51 7.0 1.1 1.8 0.04 2MCz-XT 368 501 47 483 91 1.0 60 1.1 0.1 25.0 0.01 2TBCz-XT 384 481 71 468 94 17.0 50 12.4 0.8 1.2 0.10 aMeasured in THF solution (10−5 M) at room temperature. bVacuum-deposited on a quartz substrate with a doping concentration of 15 wt % in PPF. cPhotoluminescence quantum yield (ΦPL) determined by a calibrated integrating sphere under nitrogen at room temperature. dDelayed fluorescence lifetime (τdelayed) evaluated at 300 K under nitrogen. eRatio of delayed component. fFluorescence decay rate. g Internal conversion decay rate from S1 to S0. hRate constant of RISC process. iEstimated from the high-energy onsets of fluorescence and phosphorescence spectra at 77 K. From the onsets of fluorescence and phosphorescence spectra of doped films ( Supporting Information Figure S9), the experimental ΔESTs of these molecules are calculated to be 0.01–0.15 eV, which are small enough to ensure the occurrence of RISC and thus delayed fluorescence (Figure 2c–f). By progressively reducing the torsion angles between Cz and XT, the ΔEST increases from 0.01 eV of 2MCz-XT to 0.04 eV of MCz-XT and to 0.15 eV of Cz-XT. The ΔEST of 2TBCz-XT is 0.10 eV, smaller than that of Cz-XT, although both molecules adopt nearly identical molecular conformations. These results demonstrate that enlarging the torsion angles and strengthening the ICT effect between D–A groups are conducive to achieving a small ΔEST. Because of the smaller ΔEST, 2MCz-XT exhibits a shorter delayed-fluorescence lifetime (τdelayed) of 1.0 μs and faster RISC, rate constant (kRISC) of 2.5 × 106 s−1, than Cz-XT (27.6 μs, 1.0 × 105 s−1) and MCz-XT (11.3 μs, 1.8 × 105 s−1). Compared with Cz-XT, 2TBCz-XT displays faster RISC, corresponding to a shorter τdelayed of 17.0 μs and a larger kRISC of 1.2 × 105 s−1 (Table 1). The temperature-dependent transient PL decay spectra indicate that Cz-XT and 2TBCz-XT have greatly promoted RISC with apparently enhanced delayed components (Rdelayeds) at high temperatures ( Supporting Information Table S1). However, the change in delayed fluorescence of 2MCz-XT by increasing temperature is obviously diminished, and its τdelayed and Rdelayed vary slightly from 77 to 300 K. These results manifest that the very small ΔEST allows 2MCz-XT to enjoy fast RISC even at low temperatures, while the large ΔESTs make Cz-XT and 2TBCz-XT more dependent on the thermal activation for sufficient RISC. The energy levels of Cz-based donors, XT acceptor, and the new molecules are measured from the phosphorescence spectra and shown in Supporting Information Figure S10. Generally, the locally excited triplet (3LE) energy levels of the donors are close to the 1CT states of MCz-XT, 2MCz-XT, and 2TBCz-XT, whereas the 3LE energy level of XT is close to the 1CT state of Cz-XT. For 2MCz-XT, the 3LE energy level of the donor is close to both 1CT and 3CT states, which may contribute to the fastest RISC and most efficient delayed fluorescence.49 Furthermore, the time-dependent DFT method is employed to gain insights into the RISC in these blue molecules. The natural transition orbital analysis reveals that the S1 and T1 states of the four molecules are dominated by CT transition, whereas the second triplet excited (T2) states are energetically close to the T1 states with LE transition characteristics ( Supporting Information Figures S11 and S12). The different transition natures of S1 and T2 are favorable for RISC.50,51 Furthermore, the calculated SOC matrix elements are also considerable between S1 and T2. These results suggest T2 is involved in RISC and facilitates the occurrence of delayed fluorescence in these molecules, which could be important for Cz-XT and 2TBCz-XT, who have small torsion angles and relatively large ΔESTs. To evaluate the EL performances of these blue molecules, doped OLEDs are fabricated with the configuration of indium tin oxide (ITO)/hexaazatriphenylenehexacabonitrile (HATCN) (5 nm)/1,10-bis(di-4-tolylaminophenyl)cyclohexane (TAPC) (50 nm)/tris[4-(carbazol-9-yl)phenyl]amine (TcTa) (5 nm)/1,3-di(carbazol-9-yl)benzene (mCP) (5 nm)/emitting layer (EML) (20 nm)/PPF or diphenyl-4-triphenylsilylphenyl-phosphine oxide (DPEPO) (5 nm)/3,3′-[5′-[3-(3-pyridinyl)phenyl][1,1′:3′,1″-terphenyl]-3,3″-diyl]bispyridine (TmPyPB) (30 nm)/lithium fluoride (LiF) (1 nm)/Al (Figure 3a–d), where the doped films of these molecules in PPF or DPEPO hosts with varied doping concentrations of 10, 15, and 20 wt % work as EMLs, HATCN and LiF serve as hole- and electron-injection layers, respectively, TAPC and TmPyPB perform as hole- and electron-transporting layers, respectively, TcTa serves as electron-blocking layer, PPF and DPEPO work as hole-blocking layers, and mCP functions as exciton-blocking layer. The key performance data of all the devices with corresponding configurations are summarized in Supporting Information Figures S13–S20 and Tables S2–S5. In general, these devices turn on at low voltages of 2.7–3.6 V and radiate strong light in deep-blue to sky-blue regions. In comparison with Cz-XT, MCz-XT and 2MCz-XT exhibit apparently redshifted EL emissions, and 2TBCz-XT shows redder EL emission than Cz-XT (Table 2). These EL behaviors are consistent with their PL behaviors. Cz-XT and MCz-XT have better EL efficiencies in the DPEPO host, whereas 2MCz-XT and 2TBCz-XT give better EL efficiencies in the PPF host. Whether in PPF or DPEPO, the EL spectra remain stable with minor redshifts less than 8 nm, but the maximum luminance (Lmax) is enhanced greatly by increasing doping concentrations from 10 to 20 wt %. Figure 3 | (a) Energy level diagram and chemical structures of the functional layers. (b) EL spectra at 4 V. (c) Plots of luminance–voltage–current density and (d) external quantum efficiency–luminance of the OLEDs based on the new luminogens. EML, doped films of the new molecules in PPF or DPEPO hosts. Download figure Download PowerPoint Table 2 | EL Performances of the Doped OLEDs Based on the New Molecules Emitter Von (V) ηC (cd A−1) ηP (lm W−1) ηext (%) Lmax (cd m−2) CIE (x, y) λEL (nm) Maximum Value/at 100/at 500 cd m−2 Cz-XT 3.5 15.9/10.3/5.8 13.9/7.5/3.5 22.2/14.4/8.0 1989 (0.15, 0.08) 442 MCz-XT 3.5 29.7/26.4/21.6 25.9/19.3/13.8 24.0/21.3/17.5 5751 (0.15, 0.15) 460 2MCz-XT 3.0 64.4/57.6/53.3 65.2/47.6/38.9 32.1/28.7/26.6 42250 (0.17, 0.30) 482 2TBCz-XT 2.9 47.9/38.6/31.9 50.2/33.7/24.5 33.7/27.1/22.4 20370 (0.15, 0.18) 462 Abbreviations:Von, turn-on voltage at 1 cd m−2; ηC, current efficiency; ηP, power efficiency; ηext, external quantum efficiency; Lmax, maximum luminance; CIE, Commission Internationale de I'Eclairage coordinates; λEL,EL peak. Cz-XT radiates deep blue-light with an EL peak at 442 nm, Commission Internationale de l'Eclairage (CIE) color coordinates of (0.15, 0.08), and an Lmax of 1989 cd m−2 in DPEPO host at a doping concentration of 10 wt %. The full width at half maxima value of the EL spectrum is 63 nm, similar to that of the PL spectrum (64 nm). The maximum current efficiency (ηC,max), maximum power efficiency (ηP,max), and ηext,max are 15.9 cd A−1, 13.9 lm W−1, and 22.2%, respectively. More importantly, 2TBCz-XT shows pure-blue light with an EL peak at 462 nm (CIEx,y = 0.15, 0.18) and a Lmax of 20370 cd m−2 in PPF host at a doping concentration of 20 wt %. The ηC,max, ηP,max, and ηext,max are 47.9 cd A−1, 50.2 lm W−1, and 33.7%, respectively. The device comprised of 2MCz-XT in PPF host at a doping concentration of 10 wt % displays sky-blue light with an EL peak at 482 nm (CIEx,y = 0.17, 0.30) and provides an ηext,max of 32.1% similar to that of 2TBCz-XT ( Supporting Information Tables S2–S5). In addition to the efficient RISC that ensures nearly full exciton utilization and excellent ΦPLs of 91% (2MCz-XT) and 94% (2TBCz-XT), the high horizontal orientation ratios of 78.0% and 84.0% of 2MCz-XT and 2TBCz-XT (Figure 4a,b), respectively, account for the outstanding ηext,maxs exceeding 30%.7,24,25,34,35,52,53 To the best of our knowledge, these impressive ηext,maxs demonstrate Cz-XT and 2TBCz-XT are among the currently reported state-of-the-art deep-blue and pure-blue TADF materials ( Supporting Information Table S6). Figure 4 | PL of (a) 2MCz-XT and (b) 2TBCz-XT in doped Download figure Download PowerPoint TADF molecules are in highly twisted D–A structures, which to efficient blue emissions because of a strong ICT effect. To this we a and effective strategy of modulating torsion angles of D–A groups for the of efficient blue delayed-fluorescence materials, based on a of deep-blue and pure-blue luminescent molecules consisting of Cz donor and XT By gradually decreasing the torsion the PL peak of Cz-XT is apparently blueshifted relative to those of MCz-XT and 2MCz-XT, accompanied by increased ΔEST and elongated By the electron-donating ability of Cz via the introduction of tert-butyl 2TBCz-XT shows a moderately redshifted PL and its ΔEST and τdelayed smaller and respectively, in comparison with Cz-XT. Although Cz-XT and 2TBCz-XT have more planar structures and larger they strong deep-blue and pure-blue delayed fluorescence in doped films with excellent ΦPLs of and respectively. The EL emissions of these molecules the as the PL emissions, thus achieving blue high-performance The device using Cz-XT as radiates deep-blue light with an EL peak at 442 nm (CIEx,y = 0.15, 0.08) and a high ηext,max of more efficient OLEDs are achieved by adopting 2TBCz-XT and 2MCz-XT as emitters, providing pure-blue and sky-blue light peaking at 462 nm (CIEx,y = 0.15, 0.18) and 482 nm (CIEx,y = 0.17, 0.30) with outstanding ηext,maxs of and 32.1%, respectively. These OLEDs are among the current state-of-the-art TADF OLEDs with similar which may the of efficient blue organic luminescent materials by of D–A torsion Supporting Information Supporting Information is and and fabrication and crystal data of Cz-XT and 2MCz-XT, analysis and differential scanning calorimetry cyclic transient PL decay fluorescence and phosphorescence and device performance of The of study is by the Science of China the Science of Guangdong and the State Key of Luminescent Materials and Devices, South China University of TADF Emitter and Efficient and Doped OLEDs with Wu Chen Ma Zhao Tang OLEDs with and by a of Efficient Materials for Blue Organic Efficient in Organic Yang Efficient by for Chen Wu of Blue and and Their OLEDs with Efficient Blue Based on and from and Their to Organic for Blue Organic Efficient Blue Based on Single and Organic The Key of Chen Wu for Efficient

Aggregation induced intermolecular charge transfer in simple nonconjugated donor–acceptor system

This presentation reports our recent studies on the understanding of spin-dependent processes in TADF (Thermally Activated Delayed Fluorescence) light-emitting materials based on magneto-optical studies. Recently, we have performed magneto-optical studies on TADF light-emitting molecules (DMAC-TRZ) by using magneto-photoluminescence (magneto-PL). Our magneto-PL studies provide the first evidence that the TADF is a spin-dependent process occurring in charge-transfer states. Essentially, the key spin-dependent process, namely spin mixing, necessarily required to activate the TADF, is determined by the competition between two critical parameters: (i) exchange interaction which functions as a resistance force to the TADF and (ii) spin-orbital coupling which acts as a driving force to the TADF. Therefore, controlling the exchange interaction and spin-orbital coupling becomes a critical issue in the development of highly efficient TADF light-emitting materials. By using magneto-PL studies, we further found that, doping soluble magnetic nanoparticles (surface-modified Fe3O4) can conveniently change the exchange interaction and spin-orbital coupling, and consequently alters the TADF rate. At low doping concentrations, the spin-orbital coupling is enhanced, leading to an increase on TADF rate. However, at high doping concentrations, the exchange interaction is increased, causing a decrease on the TADF rate. Furthermore, we studied the polarization effects of spin mixing in liquid TADF materials by using various solvents with different polarities. We observed that increasing the host polarization can directly weaken the spin mixing and leads to a decrease on the TADF rate. This experimental observation indicates that host polarization can weaken the spin-orbital coupling and thus decreases the driving force to TADF. Clearly, the magneto-PL studies provide an insightful understanding on the spin-dependent process to control the TADF rate in organic light-emitting materials.

Thermally activated delayed fluorescence (TADF) is an excellent way to convert originally dark triplet excitons into light.1 To realize the conversion, minimization of the energy gap between S1 and T1, called ΔE ST, has been effective.1,2 Actually, reverse intersystem crossing (RISC) becomes possible by the small ΔE ST.1-4 However, the rate constant of RISC (k RISC) is relatively small compared to those of competing processes, such as radiative and non-radiative decays, resulting that RISC is the rate-limiting process in TADF systems. Therefore, the acceleration of RISC is a key factor to obtain further excellent TADF-based organic light-emitting diodes (OLEDs).The small k RISC originates from small spin-orbit coupling (SOC). The S1 and T1 of TADF molecules tend to have charge transfer (CT) character, because the HOMO and LUMO are well-separated to make the ΔE ST small. The intersystem crossing (ISC) and RISC between CT-type S1 (1CT) and CT-type T1 (3CT) are spin-forbidden.5 This problem can be solved by intervening locally excited (LE) state(s) between 1CT and 3CT. Recently, we successfully achieved very fast RISC with a k RISC exceeding 107 s-1 by a new material design concept, named “tilted face-to-face alignment with optimal distance” (tFFO)6-8 by realizing the excellent energy level matching of 1CT, 3CT, and 3LE with sufficient SOC between 1CT and 3LE.In the presentation, I will first talk about detailed analysis of a TADF molecule, MA-TA.8-10 The performance is very high; the experimentally-obtained photoluminescence quantum yield (PLQY) and maximum external quantum efficiency (EQE) were 100% and 23.9%, respectively. The results seem to be contradictory to the above, because donor and acceptor (D-A) segments are perpendicular with each other resulting that both the S1 and T1 are CT-type, and higher-lying 3LE states cannot participate in the RISC process. Also, highly PLQY of 100% cannot be expected from the perpendicular structure. Our detailed quantum chemical analysis revealed that the dynamic or static D-A fluctuation was the key trigger of both the fast RISC and high PLQY. Such direct 3CT→1CT RISC-based OLEDs without using any 3LEs have an advantage that various kinds of hosts having different polarities can be used without sacrificing the device performance, that is, high EQE and blue-shifted emission can be realized simultaneously.Secondly, I will talk about the development of a high-throughput material screening method by quantitatively predicting rate constants of all relevant electronic transitions. TADF molecules have been designed through the calculations of ΔE ST and oscillator strength to realize effective RISC and following radiative decay simultaneously. However, in addition to ΔE ST, SOC is also another key factor to control k RISC as described above. Not only radiative but also non-radiative decays should be considered to obtain high PLQYs. We here propose a new method to predict TADF performance more precisely.11 Our method based on the Fermi golden rule enables us to theoretically predict relevant rate constants for all types of electronic transitions in an emitter molecule with reasonable computational cost. We applied the method to benzophenone; the calculated rate constants quantitatively agreed with the experimental ones. We are now extending this method to various TADF molecules and singlet fission systems.12-14 We express sincere thanks to my group members. This work was supported by JSPS KAKENHI Grant Numbers JP20H05840 (Grant-in-Aid for Transformative Research Areas, “Dynamic Exciton”). Computation time and NMR measurements were supported by the international Joint Usage/Research Centre at the Institute for Chemical Research, Kyoto University, Japan.1 H. Uoyama, K. Goushi, K. Shizu, H. Nomura & C. Adachi Nature 492, 234 (2012).2 H. Kaji et al. Nat. Commun. 6, 8476 (2015).3 Z. Yang et al. Chem. Soc. Rev. 46, 915 (2017).4 M. Y. Wong & E. Zysman-Colman Adv. Mater. 29, 1605444 (2017).5 M. A. El-Sayed J. Chem. Phys. 38, 2834 (1963).6 Y. Wada, H. Nakagawa, S. Matsumoto, Y. Wakisaka & H. Kaji Nat. Photon. 14, 643 (2020).7 Y. Kusakabe, Y. Wada, H. Nakagawa, K. Shizu & H. Kaji Front. Chem. 8, 530 (2020).8 H. Imahori, Y. Kobori & H. Kaji Acc. Chem. Res. 2, 501 (2021).9 Y. Wada, Y. Wakisaka & H. Kaji ChemPhysChem 22, 625 (2021).10 Y. Wada, K. Shizu & H. Kaji J. Phys. Chem. A 125, 4534 (2021).11 K. Shizu & H. Kaji J. Phys. Chem. A 125, 9000 (2021).12 K. Shizu, C. Adachi & H. Kaji J. Phys. Chem. A 124, 3641 (2020).13 K. Shizu, C. Adachi & H. Kaji Bull. Chem. Soc. Jpn. 93, 1305 (2020).14 K. Shizu, C. Adachi & H. Kaji ACS Omega 6, 2638 (2021).

Thermally activated delayed fluorescence (TADF) is now an established way to convert inhelently dark triplet excitons into light through reverse intersystem crossing (RISC).1-4 To realize the RISC process, minimizing the energy gap (ΔE ST) between S1 and T1 is considered to be effective theoretically, and this has been realized experimentally. At the beginning of the research, RISC had been the rate-limiting process in TADF, although RISC was possible. In such situation, we have shown that acceleration of RISC has now become possible through a variety of methods.5-7 Although excellent TADF materials and highly efficient TADF-based organic light-emitting diodes (OLEDs) have been realized as described above, the fundamental understanding is still insufficient. In the presentation, we first show our recently-proposed method for quantitatively predicting rate constants of all electronic transitions relevant to light emission.8-10 The method proposed here has successfully predicted all rate constants quantitatively and therefore can predict TADF performance precisely. The method also allows quantitative predictions of dynamics (time evolution) of experimentally-obtained rate constants for radiative, non-radiative, intersystem crossing, and RISC as well as exciton population dynamics.11 Such an approach will contribute significantly to the fundamental science of exciton dynamics.We express sincere thanks to my group members. This work was supported by JSPS KAKENHI Grant Numbers JP20H05840 (Grant-in-Aid for Transformative Research Areas, “Dynamic Exciton”). Computation time and NMR measurements were supported by the international Joint Usage/Research Centre at the Institute for Chemical Research, Kyoto University, Japan. Uoyama, H.; Goushi, K.; Shizu, K.; Nomura, H.; Adachi, C., Highly efficient organic light-emitting diodes from delayed fluorescence. Nature 2012, 492, 234. Kaji, H., et al., Purely organic electroluminescent material realizing 100% conversion from electricity to light. Nat. Commun. 2015, 6, 8476. Yang, Z., et al., Recent advances in organic thermally activated delayed fluorescence materials. Chem. Soc. Rev. 2017, 46, 915. Wong, M. Y.; Zysman-Colman, E., Purely organic thermally activated delayed fluorescence materials for organic light-emitting diodes. Adv. Mater. 2017, 29, 1605444. Wada, Y.; Nakagawa, H.; Matsumoto, S.; Wakisaka, Y.; Kaji, H., Organic light emitters exhibiting very fast reverse intersystem crossing. Nat. Photon. 2020, 14, 643. Ren, Y.; Wada, Y.; Suzuki, K.; Kusakabe, Y.; Geldsetzer, J.; Kaji, H., Efficient blue thermally activated delayed fluorescence emitters showingvery fast reverse intersystem crossing. Appl. Phys. Express 2021, 14, 071003. Wada, Y.; Wakisaka, Y.; Kaji, H., Efficient direct reverse intersystem crossing between charge transfer‐type singlet and triplet states in a purely organic molecule. ChemPhysChem 2021, 22, 625. Shizu, K.; Kaji, H., Theoretical determination of rate constants from excited states: Application to benzophenone. J. Phys. Chem. A 2021, 125, 9000. Shizu, K.; Kaji, H., Comprehensive understanding of multiple resonance thermally activated delayed fluorescence through quantum chemistry calculations. Commun. Chem. 2022, 5. Shizu, K.; Ren, Y. X.; Kaji, H., Promoting reverse intersystem crossing in thermally activated delayed fluorescence via the heavy-atom effect. J. Phys. Chem. A 2023, 127, 439.Shizu, K.; Kaji, H., submitted.

The photophysical properties of six types of carbazole benzonitrile (CzBN) derivatives are investigated in different solvents to examine the thermally activated delayed fluorescence (TADF) activation via reducing the energy gap between the singlet charge-transfer and triplet locally excited states, ΔE ST(LE) . Relative to the ΔE ST(LE) values for the CzBN derivatives in the low polarity solvent toluene ( ϵ ∼ 2 ), a reduction of ΔE ST(LE) for the CzBN derivatives in the polar solvent acetonitrile ( ϵ ∼ 37 ) was confirmed while maintaining fairly constant Δ E ST values. Notably, TADF activation was observed in acetonitrile for some CzBN derivatives that are TADF inactive in toluene. A numerical analysis of various rate constants revealed the cause of TADF activation as an increase in the reverse intersystem crossing rate and a suppression of the non-radiative decay rate of the triplet states. The positive effect of ΔE ST(LE) was limited, however, as an excessive decrease in ΔE ST(LE) facilitates the nonradiative deactivation of the triplet states, leading to a loss of the TADF efficiency. This paper shows that ΔE ST(LE) provides a measure of TADF activation and that appropriate regulation of ΔE ST(LE) is required to achieve high TADF efficiency.

Simple Double Hetero[5]helicenes Realize Highly Efficient and Narrowband Circularly Polarized Organic Light-Emitting Diodes

Aggregation-induced thermally activated delayed fluorescence (TADF) phenomena have attracted extensive attention recently. In this paper, several theoretical models including monomer, dimer, and complex are used for the explanation of the luminescent properties of (R)-5-(9H-carbazol-9-yl)-2-(1,2,3,4-tetrahydronaphthalen-1-yl)isoindoline-1,3-dione ((R)-ImNCz), which was recently reported [Chemical Engineering Journal 418 129167 (2021)]. The polarizable continuum model (PCM) and the combined quantum mechanics and molecular mechanics (QM/MM) method are adopted in simulation of the property of the molecule in the gas phase, solvated in acetonitrile and in aggregation states. It is found that large spin–orbit coupling (SOC) constants and a smaller energy gap between the first singlet excited state and the first triplet excited state (ΔE st) in prism-like single crystals (SCp-form) are responsible for the TADF of (R)-lmNCz, while no TADF is found in block-like single crystals (SCb-form) with a larger ΔE st. The multiple ultralong phosphorescence (UOP) peaks in the spectrum are of complex origins, and they are related not only to ImNCz but also to a minor amount of impurities (ImNBd) in the crystal prepared in the laboratory. The dimer has similar phosphorescence emission wavelengths to the (R)-lmNCz-SCp monomers. The complex composed of (R)-lmNCz and (R)-lmNBd contributes to the phosphorescent emission peak at about 600 nm, and the phosphorescent emission peak at about 650 nm is generated by (R)-lmNBd. This indicates that the impurity could also contribute to emission in molecular crystals. The present calculations clarify the relationship between the molecular aggregation and the light-emitting properties of the TADF emitters and will therefore be helpful for the design of potentially more useful TADF emitters.

Radical-based light-emitting diodes are an innovative type of organic light-emitting diodes (OLEDs), which adopt luminescent radicals as emitters, aiming at improving the external quantum efficienc...

Organic light-emitting devices (OLEDs) are already widely used for common applications like OLED TVs or smartphone displays. Nevertheless, it is still a challenge for both science and industry to develop OLED systems for lighting applications that combine true-color white light, high efficiencies and high brightness at the same time. Since white emission in OLEDs is usually a combination of two or more different emitters with individual colors it is necessary that all included systems are efficient. It has been shown that the concept of thermally activated delayed fluorescence (TADF) allows to synthesize very efficient light-emitting molecules with various emission colors. In our work, we use the sky-blue TADF emitter 9-[2,3,4,5-tetra(carbazol-9-yl)-6-(trifluoromethyl)phenyl]carbazole (5CzCF3Ph) with an emission maximum at a wavelength of 495 nm in thin films, reaching a photoluminescence quantum yield of 70 %. In an OLED, the emitter delivered up to 18 % external quantum efficiency (EQE). This is beyond the theoretical limit of conventional fluorescent OLEDs. To achieve warm-white emission, we combine the sky-blue emission of 5CzCF3Ph with the red emission of the common phosphorescent emitter Ir(MDQ)2(acac) within one emission layer. Due to the very broad blue emission (FWHM ~ 95 nm), a dedicated deep blue emitter becomes obsolete and it is possible to tune the combined two-color spectrum in such a way, that a high color rendering index of over 80 and correlated color temperatures about 2800 K can be obtained by this strategy. EQEs of up to 17 % and luminous efficacies of 16 lm/W have been measured for the hybrid white OLEDs. This two-color concept paves the way towards future utilization of TADF emitters in lighting applications by simplifying the required sequence of organic layers inside the OLED. In our approach, the excitons are formed mostly on the TADF emitter itself. To achieve a suitable amount of red light for the white emission, it is necessary to enable efficient exciton transition pathways between 5CzCF3Ph and Ir(MDQ)2(acac). Due to the variety of potential local and charge-transfer excited states in the emitter system, there are several probable scenarios for the energy transfer. Utilizing time-correlated single photon counting (TCSPC) with a wavelength-sensitive detection, we study the exciton decay of both the TADF prompt and delayed fluorescence as well as the phosphorescent emission channel in detail. With this technique, we deliver a thorough investigation of the exciton transfer and exchange mechanisms in the emitter system of our warm-white hybrid OLEDs.