A thermally activated delayed fluorescent material combining intra

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A thermally activated delayed fluorescent material combining intra- and intermolecular charge transfers Dongdong Zhang, Katsuaki Suzuki, Xiaozeng Song, Yoshimasa Wada, Shosei Kubo, Lian Duan, and Hironori Kaji ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b19428 • Publication Date (Web): 23 Jan 2019 Downloaded from http://pubs.acs.org on January 24, 2019

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ACS Applied Materials & Interfaces

A

thermally

activated

delayed

fluorescent

material combining intra- and intermolecular charge transfers Dong-dong Zhang,†‡ Katsuaki Suzuki,† Xiao-zeng Song,‡ Yoshimasa Wada,† Shosei Kubo,† Lian Duan,*‡ and Hironori Kaji*† †Institute

for Chemical Research, Kyoto University, Uji, Kyoto 6611-0011, Japan. E-mail:

[email protected] ‡Key

Lab of Organic Optoelectronics and Molecular Engineering of Ministry of Education,

Department of Chemistry, Tsinghua University, Beijing, 10086, P. R. China. E-mail: [email protected].

ABSTRACT A novel thermally activated delayed fluorescent (TADF) compound, 9-(3-((4,6diphenyl-1,3,5-triazin-2-yl)oxy)phenyl)-3,6-diphenyl-9H-carbazole (PhCz-o-Trz), with donor-σacceptor (D-σ-A) motif is developed. A flexible small space σ-junction is adopted to partly suppress the intramolecular charge transfer (intra-CT) while inversely enhancing the intermolecular CT (inter-CT) between D/A moieties, realizing the coexistence of both intra- and inter-CT in an amorphous aggregate. The coexistence of dual CTs increases the complexity of the singlet and triplet states mixing, enhancing the triplet-to-singlet spin-flip transition and thereby the TADF emission. Additionally, PhCz-o-Trz is evaluated not only as an emitter but also a sensitizing

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host for fluorescent and phosphorescent dopants, all realizing high efficiencies with alleviated efficiency roll-offs. These results shed light on the development of new TADF materials with dual CTs and may further deepen our understanding about TADF mechanisms.

KEYWORDS (a donor-σ-acceptor motif; intramolecular charge transfer; intramolecular charge transfer; a sensitizing host; thermally activated delayed fluorcence)

INTRODUCTION Since the pioneering work by Adachi et al. in 2012, pure-organic compounds with thermally activated delayed fluorescence (TADF) have truly revolutionized our concept about not only new generation emitters but also fantastic sensitizing hosts for fluorescent and phosphorescent dopants in organic light-emitting diodes (OLEDs).[1-6] A small singlet-triplet energy gap (ΔEST), generally less than a few hundred milli electron volts, is the most demanding parameter for TADF molecule, triggering efficient triplet-to-singlet spin-flip transition via endothermic reverse intersystem crossing (RISC) process.[7] Briefly, such small ΔEST can be resulted via spatial wave function separation of the highest occupied molecule orbital (HOMO) and the lowest unoccupied molecule orbital (LUMO).[8] To address this point, donor/acceptor (D/A) systems with intra- or intermolecular charge transfer (intra-CT or inter-CT) have been exploited.[9-12] Intra-CT usually originates from electronic coupling between D and A units in a single-molecule (intra-D/A) through the molecular π-system backbone while inter-CT derives from the appropriate combined bimolecular D/A system, usually referred to as exciplex. Interestingly, while materials with solely intra-CT or inter-CT have been reported a lot, there is no report on TADF emitters with coexistence


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of intra- and inter-CTs, of which the exploration may not only broaden the molecular design tactics but also deepen our understanding about TADF mechanisms. Recently, materials with D-σ-A structures have aroused extensive attention given their unique properties. Adachi et al. developed a series of TADF materials with D-σ-A motif, shortening the exciton lifetimes from intra-CT.[13] Later, Zhang et al. successfully proved the inter-CT for a compound with D and A moieties connected by a space-enough and conjugation-forbidden linkage.[14] It will be therefore theoretically proved that, for a molecule with D and A moieties, intra- and inter-CT should be competitive processes in the aggregate, which will provide the possibility of the coexistence of dual CTs. To achieve such goal, dedicated molecular structure has to be designed since in most cases the intra-CT usually dominates owing to the strong electronic coupling between D and A units through the molecular π-system backbones. Here,

we

developed

a

new

TADF

compound,

9-(3-((4,6-diphenyl-1,3,5-triazin-2-

yl)oxy)phenyl)-3,6-diphenyl-9H-carbazole (PhCz-o-Trz), with D-σ-A motif, utilizing an oxygen atom as the small space σ-junction to manipulate the competing dual CT processes. Consequently, the intra-CT was partly suppressed by the broken molecular π-system backbone while the interCT was enhanced, realizing the coexistence of both CTs. It is also found that dual CTs can increase the complexity of the singlet (S1) and triplet (T1) states mixing, thus facilitating the triplet-tosinglet spin-flip transitions. The compound was further evaluated as not only emitters but also sensitizing host for fluorescent and phosphorescent dopants, all realizing high efficiency with alleviated efficiency roll-off. Those findings here greatly testify the superiority of TADF compounds with dual CTs and also open an new avenue for the design of novel kind TADF materials.



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EXPERIMENTAL SECTION General Information: All reagents were purchased from commercial sources and used without further purification. Unless specially mentioned, all reactions were made under nitrogen with super dry solvents. With tetramethylsilane as the internal standard, the 1H NMR spectra of PhCz-o-Trz were measured at ambient temperature using a JEOL AL-600 MHz spectrometer. An ion trap mass spectrometer (Bruker Esquire) was adopted to measure the mass spectra of PhCz-o-Trz. A flash EA 1112 spectrometer was used to analysis elemental ratios of PhCz-o-Trz. For the UV-vis absorption spectra, an Agilent 8453 spectrophotometer was utilized while a transient spectrometer (Edinburg FL920P) was used to record the steady emission spectra and the transient photoluminescence characteristics. With a pulse laser (Vibrant Photonics 355II), the phosphorescent spectra in toluene was recorded by an Edinburgh Instruments LP920-KS fluorescence spectrophotometer with a 5 ms delay at an temperature of 77 K. The absolute photoluminescence quantum yields of the doped films fabricated by spin-coating were estimated by a Hamamatsu integrating sphere system (C9920-02G). With a platinum (Pt) disk working electrode, a Pt wire auxiliary electrode as well as a Ag/AgCl system reference electrode, the frontier energy levels of PhCz-o-Trz were recorded with an electrochemical workstation (Princeton Applied Research, Potentiostat/ Galvanostat Model 283), standardized against ferrocene/ferrocenium. A 0.1 M n-Bu4NPF6 was used as supporting electrolyte in super-dry dichloromethane (CH2Cl2) for the measurement of the oxidation potential while super-dry N,Ndimethylformamide (DMF) was used for the reduction potential with the same conditions. The scan rates for both are 100 mV s-1. Theoretical calculation: The geometrical and electronic properties of PhCz-o-Trz were investigated using density functional theory (DFT) with B3LYP/6‐31G* basis sets by


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Gaussian 09 software. While the time-dependent DFT with the same basis set was used to predict the singlet and triplet energies of the compounds. Gaussview 5.0. was used to visualize the contours of the energy levels distributions. Synthesis: The synthesis of 3-(3,6-diphenyl-9H-carbazol-9-yl)phenol: Under nitrogen atmosphere, a mixture of 3,6-diphenyl-9H-carbazole (319 mg, 1.0 mmol), 3-bromophenol (172 mg, 1.0 mmol), Pd2(dba)3·CHCl3 (23.3 mg, 0.0225 mmol), XPhos (40.6 mg, 0.0852 mmol) and tBuONa (237 mg, 2.46 mmol) in toluene (9 mL) was stirred under reflux for 12 h. After cooling to room temperature, the reaction mixture was extracted with CH2Cl2. The organic layer was washed with H2O and the fried over anhydrous MgSO4. After filtration and evaporation, the crude product was purified flash chromatography (silica gel, eluent: CH2Cl2: hexane) to provide white solid (yield: 75%). 1H

NMR (600 MHz, CDCl3) δ = 8.39 (s, 2H), 7.73 (d, 4H), 7.68 (d, 2H), 7.53 (d, 2H), 7.49

(m, 5H), 7.36 (t, 2H), 7.20 (d, 1H), 7.10 (s, 1H), 6.96 (d, 1H), 5.00 (s, 1H); High‐resolution mass spectrometry (m/z): [M+H]+calculated for C45H30N4O, 412.1623; found, 412.1648. Elemental analysis calculated for C45H30N4O: C 87.56, H 5.14, N 3.40; found: C 87.56, H 5.15, N 3.39. The

synthesis

of

9-(3-((4,6-diphenyl-1,3,5-triazin-2-yl)oxy)phenyl)-3,6-diphenyl-9H-

carbazole: Under nitrogen atmosphere, 3-(3,6-diphenyl-9H-carbazol-9-yl)phenol (4.11 g, 10 mmol) in dehydrated DMF (20 mL) was added dropwise into a dehydrated DMF (20 mL) solution containing sodium hydride (60%, 0.60 g, 15 mmol) for 20 min and stirred for 1 h. Then, 2-chloro4,6-diphenyl-1,3,5-triazine (2.67 g, 10 mmol) in dehydrated DMF (20 mL) was added dropwise for 15 min. Then the solution was stirred for 12 h at 100 °C. After that, water (200 ml) was added



Page 6 of 22

into the solution and the precipitate was filtered and dried in vacuum. The product was purified by column chromatography on silica gel, resulted in the product PhCz-o-Trz (5.90 g, 9.2 mmol, 92%). 1H

NMR (600 MHz, CDCl3) δ = 8.606 (d, 2H), 8.373 (d, 2H), 7.752 (t, 1H), 7.701 (d, 4H),

7.623-7.596 (m, 6H), 7.562-7.550 (d, 2H), 7.548-7.514 (t, 4H), 7.485-7.459 (m, 5H), 7.345 (t, 2H); High‐resolution mass spectrometry (m/z): [M+H]+calculated for C45H30N4O, 643.2420; found, 643.2531. Elemental analysis calculated for C45H30N4O: C 84.09, H 4.70, N 8.72; found: C 84.07, H 4.71, N 8.70. Device fabrication and characterization: All organic materials were purified by a vacuum sublimation approach before device fabrication. And all the device fabrication processes as well as the device characterization were conducted under ambient laboratory conditions at room temperature. The ITO glass substrates were transferred to the deposition system after being carefully pre-cleaned. And then the organic layers were subsequently eaporated at a pressure of 5×10-5 Pa with a rate of 1.0-1.5 Å s-1. After deposition of all organic layers, the electron injection layer, LiF layer, and the cathode, aluminum layer, were thermally evaporated. To measure the device electrical properties, a Keithley 2400 source meter was adopted. And a PR650 spectrometer was utilized to record the device spectra and luminance under electrical excitation. RESULTS AND DISCUSSION Scheme 1 depcites that the aimed molecule, PhCz-o-Trz is readily systhesized in high yields from Buchwald-Hartwig reaction between 3,6-diphenyl-9H-carbazole and 3-bromophenol, followed by nucleophilic reaction with 2-chloro-4,6-diphenyl-1,3,5-triazine under strong basis. Here, 3,6-diphenyl-9H-carbazole (PhCz) and 4,6-diphenyl-1,3,5-triazine (Trz) were chosen as D


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and A units, respectively. The compound was sublimated before experiments and fully characterized by 1H NMR, mass spectrometry and elemental analysis. As above mentioned, D-σ-A motif has already been adopted to develop TADF materials, but only intra- or inter-CT observed with no both CTs. The plausible reason can be attributed to, on one hand, the rigid structures of the previously adopted σ-junction, such as sp3 carbon atom, hexafluoroisopropylidene etc., not facilitating the formation of intermolecular packing suitable for inter-CT; on the other hand, too large space between D and A units forbids the

intra-CT. It is

crucial to manipulate the molecular structures for the realization of both CTs. Here, the oxygen atom is utilized as the σ-junction, not only breaking the conjugation between PhCz and Trz, but also rendering the molecular backbone more flexibile for free rotation of D and A parts, thus enhancing the inter-CT. The relatively planar subunit structures also facilitate interactions of D and A units in adjacent molecules (inter-D/A) for inter-CT. Meanwhile, the oxygen atom is small in size and being attached directly on the 1,3,5-triazine, indicating that D and A units is still being spatially close and thus maintiaing the channel of intra-CT. In this scenario, the coexistence of dual CTs can be anticipated as illustrated in Figure 1. The occurence of both of intra- and interCT is verified by the experiments for dilute solutions, amorphous films with low doping concentration, and the doping concentration dependence (see below). Both the conformation geometries and the frontier molecular orbital (FMO) distributions were calculated using density functional theory (DFT) with the Gaussian 09 package at the B3LYP/6-31G(d) level while ΔEST of this compound was also calculated with time-dependent DFT at the same basis. Figure 1b clearly depicts that HOMO of PhCz-o-Trz is totally distributed on PhCz while LUMO on Trz, indicating that the FMO distribution is well separated by the σ-junction, which significantly suppresses the intra-D/A electronic coupling. The well separated FMOs also lead to an extremely



Page 8 of 22

small ΔEST only of 0.0071 eV with T1 of 2.9291 eV while S1 of 2.9362 eV for the S0-optimized structure. Besides, the oscillator strength (f) was calculated to be only 0.0052, much lower than common TADF emitters.[1-5] Since f stands for the transition possibility, the extremely small value indicates the suppressed intra-CT transition as we anticipated. The HOMO and LUMO energy levels of PhCz-o-Trz were experimentally obtained from the first oxidation and reduction potentials measured in dichloromethane (DCM) and N,Ndimethylformamide (DMF), respectively, by cyclic voltammetry (CV). The CV curves reveal both quai-reversible oxidation and reduction potential as shown in Figure S1, suggesting both stable radical cations and anions derived from the relatively stable Trz and PhCz moieties. The half-wave potentials E(1/2) were determined from the peaks of the oxidation and reduction curves to further calculate the LUMO and HOMO levels of PhCz-o-Trz, being -2.74 eV and -5.53 eV, respectively. The photoluminance (PL) properties of PhCz-o-Trz, involving absorption (Abs-), fluorescence (Fluor-) as well as phosphorescence (Phos-), were investigated in toluene firstly, where no intermolecular interaction exists. Generally, for the carbazole/ triazine derivatives with D-A structures, characteristic intra-CT absorption from HOMO to LUMO is expected in the range of 380-420 nm.[6,14] However, as can be seen from Figure 2a, PhCz-o-Trz only shows strong Abspeak at 280 nm and intra-CT Abs-bands were not found. This proves the suppressed intra-CT interaction in its ground state, owing to the alleviated intra-D/A electronic coupling by the σjunction as aforementioned. As shown in Figure S2, two emission peaks were observed for PhCzo-Trz in toluene before degassing, with one around 385 nm while the other around 456 nm, respectively. The shorter one agrees well with the emission spectrum of the donor moiety, assigned as emission from the localized-excited (LE) state of PhCz. The existence of the LE-emission also indicates the partial suppression of intra-CT in PhCz-o-Trz. After degassing (see Figure 2a), the


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longer wavelength emission was greatly enhanced and overlaid the LE one. Similar behaviours were also observed for previously reported D-σ-A structured molecules.[13] The broad and structure-less Fluor-spectrum shown in Figure 2a indicates that this emission intrinsically originates from the intra-CT states of PhCz-o-Trz. These results suggest that though being suppressed by the σ-junction, the intra-CT still exists to some extent as we expected. Contrarily, the Phos-spectrum of PhCz-o-Trz at 77 K with 5 ms delay was well-resolved and showed characteristic vibrational structures, thus indicating the LE characteristics of its lowest T1 in toluene. The calculated spin-density distribution (SDD) of T1 state shown in Figure 2a reveals that only PhCz contributes to the molecule T1 state, explaining its LE characteristics. The T1 energy determined from the highest energy peak of Phos-spectra is 2.80 eV, resulting in a ΔEST of 0.23 eV, combined with the S1 energy of 3.03 eV obtained from the onset of Fluor-spectra. Figure 2b shows the PL-properties of PhCz-o-Trz neat film. Interestingly, compared with the one in toluene, significant redshifted Fluor-spectrum peaked at 514 nm was observed for neat film. Since there is no new absorption band observed from the Abs-spectrum of the neat film, it suggests that no new species formed in the ground state. It is also worth noting that the Phos-spectrum of the neat film shows broad featureless emission, indicating that the T1 of PhCz-o-Trz in the neat film is intrinsically CT state, which is different from the LE one observed in toluene. This CT can be assigned to the inter-CT as clearly shown later. The S1 and T1 energy obtained from the onset of Fluor-spectrum and Phos-spectrum are 2.88 eV and 2.75 eV, respectively, leading to a ΔEST of 0.13 eV in neat film, which is relatively smaller than the one obtained in toluene. Theoretically, two plausible reasons may be responsible for the redshifted emission in neat film: one is the increased polarity in neat film while the other one is the inter-CT emission. To gain more insight, the PL-properties of PhCz-o-Trz doped in bis[2-(diphenylphosphino)phenyl] ether oxide



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(DPEPO) with different concentrations were analysed. As illustrated in Figure 2c, at low dopant concentration of 3 wt%, both emission peaks from the LE state of PhCz and the intra-CT state of PhCz-o-Trz were observed, which agree with the one in toluene before degassing. The LE emission gradually decreased with dopant concentration increasing and then vanished at dopant concentration of 20 wt%. The CT emission peaks, on the other hand, are 1) gradually redshifted from 460 nm to 478 nm with the dopant concentration increasing from 3 wt% to 30 wt%, 2) fixed at 478 nm from 30 wt% to 50 wt%, and 3) continued to be redshifted to finally 514 nm from 50 wt% to 100 wt%, as shown in Figure S3. Obviously, with dopant concentration gradually increasing from 20 wt%, the relative intensity of the inter-CT emission was gradually enhanced compared to the one of intra-CT emission at ~460 - 478 nm, which becomes the main peak for the neat film. On the basis of the above results, it can be assigned that the red-shift of the intra-CT emission from 460 nm to 478 nm as illustrated in Figure 2d, is induced by the increased polarity with the increasing dopant concentration from 3 wt% to 30 wt%. Under the excitation wavelength of 320 nm in this experiment, the dopant, PhCz-o-Trz, is photo-excited whereas the host matrix, DPEPO, is in the ground state. CT-type molecules in the excited state has much larger polarity compared to molecules in the ground state. But in the range of dopant concentrations from 30 wt% to 50 wt%, fixed intra-CT peaks at 478 nm were observed, indicating that the polarity increment is limited. The redshift from 478 nm to 514 nm is attributed to the increased intensity of the new emission peaks at 514 nm, deriving from the inter-CT emission between D and A units in adjacent molecules as depicted in Figure 2e. The formation of the exciplex emission between inter-D/A can be reflected by the emission of the mixed film of 2-chloro-4,6-diphenyl-1,3,5-triazine and 3,6diphenyl-9H-carbazole (1:1), which exhibited clearly exciplex-emission around 500 nm (Figure


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S4), evidencing the possibility of inter-CT between donors and acceptors. The reduced LE and intra-CT emissions with the increased concentration also originate from the interactions of interD/A, with more D and A units participating into the formation of inter-CT. In the neat films, such inter-CT becomes the main emission mechanism, leading to the CT characteristic of the T1 state. Also, from the Fluor-spectra, it is observed that both the intra- and inter-CT emissions coexist from 20 wt% to nearly 100 wt%. To our best knowledge, this is the first time that a molecule can achieve intra- and inter-CT states in aggregate, facilely manipulated by tuning its concentrations. The measured PL-transient decay curves of the doped and neat films clearly showed prompt and delayed components, proving their significant TADF characteristics (Figure S5). Notably, those curves strongly depend on the dopant concentration. Similarly, as decipted in Figure 3a, the measured PL quantum yields (PLQYs) of the doped films were also found to depend on the doping concentrations. The highest PLQY about 68% was obtained for the DPEPO: 40 wt% PhCz-o-Trz film, which is larger than those of the film with 3 wt% PhCz-o-Trz (about 19%) and the neat film (about 43%). As aforementioned, due to the coexistence of the intra- and inter-CTs, the singlet and triplet energy levels varied with different dopant concentrations. The change of those energy levels not only varies the ΔEST of those films but also may change the RISC mechnisims. As illustrated in Figure 3b, in the 3 wt% doped film, the triplet-to-singlet upconversion is from LE triplet to the intra-CT singlet with a large ΔEST, thus leading to insufficient RISC and low PLQY. Naturally, the small f due to the well-separated FMO is also a part of reasons for low PLQY. On the other hand, at high dopant concentration, the energy of intra-CT singlet is slightly reduced, being more close to the LE triplet level. Also, the inter-CT singlet and triplet possess the similar or slightly lower energy with LE triplet (Figure 3b). It has been reported that the LE states and CT states with close energy levels facilitate the spin-vibronic coupling or the spin-orbit coupling.[15, 16] The



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TADF system with coexistence of intra- and inter-CTs in this study meets the condition; clearly enhances the triplet-to-singlet spin-flip transition as shown in Figure 3b. The enhanced RISC process may be reflected by the reduced lifetimes of the delayed part (τD) of the doped films with increasing doapnt concentrations as shown in Table S1. Compared with that of DPEPO: 20 wt% PhCz-o-Trz film, the τD of DPEPO: 40 wt% PhCz-o-Trz film is reduced. Since DPEPO: 40 wt% possess the higher PLQY, which rule out the influence of concentration quenching, the reduced τD can be explained by the enhanced RISC process. It is therefore reasonable to assume that for the films with intra- and inter-CT, the RISC process is more efficient, leading to high PLQYs. The reduced PLQYs of films with dopant concentration larger than 40 wt% may be attributed to the concentration quenching effect. It seems that the coexistence of both intra- and inter-CTs provides an alternative to tune the PL properties of TADF materials for better emitters, which deserves further continous reseach. The device with an emitting layer (EML) of DPEPO: PhCz-o-Trz was fabricated adopting structure of ITO/ TAPC (30 nm)/ TCTA (10 nm)/ mCP (10 nm)/ EML (30 nm)/ DPEPO (10 nm)/ Bphen (10 nm)/ LiF (0.5 nm)/ Al (150 nm). Where TAPC, TCTA, mCP and Bphen are 4,4′cyclohexylidenebis[N,N-bis(4-methylphenyl) benzenamine], tris(4-carbazoyl-9-ylphenyl)amine, 1,3-bis(9H-carbazol-9-yl)benzene

and

4,7-diphenyl-1,10-phenanthroline,

respectively.

As

illustrated in Figure 3c, the device electroluminance spectra of the devices are in corresponding to the PL ones, with the inter-CT emission increasing with increased dopant concentrations. The current density-voltages characters shown in Figure 3d clearly reveal that with the dopant concentration increasing, higher current density can be obtained, indicating that PhCz-o-Trz benefits charge transporting in the EML. A maximum external quantum efficiency (EQEmax) of 8.5% was observed for the device with dopant concentration of 40 wt% (Figure 3e), which


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remained 8.3% at 1000 cd/m2. Devices with DPEPO: 20% PhCz-o-Trz and PhCz-o-Trz neat film as the EMLs were also fabriated, only showing EQEmax of 4.4% and 3.7%, respectively, owing to their low PLQYs. Although PhCz-o-Trz is not a TADF emitter with good efficiencies, the findings here may be used to design TADF materials in the future. Also, based on D-σ-A motif, by varying the D and A units as well as σ-junction, TADF materials with inter-CT or intra-CT or both CTs have been developed, showing the great possiblitiy of such motif. It is believed that with more molecules developed, more unique properties and better device performances an be anticipated. Besides being emitters, TADF materials have been widely utilized as the hosts for all kinds of dopants, including conventional fluorescent dopant and phosphorescent dopant and TADF dopant, known as TADF-sensitizing emissions.[6] Such concept have significantly promoted the performances of OLEDs. Bearing efficient TADF process, exciplex-forming materials as the hosts have been widely exploited, yet suffering from ternary-evaporated EMLs; emitter, donor-host, and acceptor-host.[17, 18] Here, being an exciplex-type single-molecular compound in neat film, PhCzo-Trz was also evaluated as the versatile sensitizing host with

even simple device structures of

ITO/ TAPC (30 nm)/ TCTA (10 nm)/ EMLs (30 nm)/ Bphen (30 nm)/ LiF (0.5 nm)/ Al (150 nm), as

illustrated

in

Figure

4a.

Here,

N9,N9,N10,N10-tetrakis(4-(2-phenylpropan-2-

yl)phenyl)anthracene-9,10-diamine (PhtBuPAD), tris(2-phenylpyridine) iridium(III) (Ir(ppy)3) and

9-(4-(4,6-diphenyl-1,3,5-triazin-2-yl)phenyl)-N3,N3,N6,N6-tetraphenyl-9H-carbazole-3,6-

diamine (DACT-II) were chosen as green conventional fluorescent dopant and phosphorescent dopant and TADF dopant with concentration of 1 wt%, 10 wt% and 10 wt%, respectively. Figure 4b depicts the electroluminance (EL) spectra of the OLEDs, with Commission Internationale de L'Eclairage (CIE) coordinates of (0.34, 0.60), (0.34, 0.61) and (0.37, 0.54) for PhtBuPAD, Ir(ppy)3 and DACT-II, respectively. The EQE versus brightness characteristics were shown in Figure 4c



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and the device performances were summarized in Table 1. Interestingly, for the device with conventional fluorescent emitter, PhtBuPAD, EQEmax of 11.9% was achieved. The high device performances can be attributed to the TADF-sensitizing fluorescence emission, in which process, the triplet of the host can be upconverted into the singlet ones and then being transferred to the fluorescent emitters to emit through the long-range Förster energy transfer (FRET).[6] Also, the device efficiency roll-off is greatly suppressed, with EQE of 11.7% at 5000 cd/m2. To our knowledge, OLEDs with exciplex hosts to sensitize the conventional fluorescence usually suffer from significant efficiency roll-off. The reduced efficiency roll-off here testify the potential of PhCz-o-Trz as an exciplex-type single-molecular host. For the device with Ir(ppy)3 as emitter, an extremely EQEmax of 27.1% was observed, which remained at 26.6% even at 5000 cd/m2, even outperforming devices using exciplex hosts for the same emitter. The high efficiency and the extremely low roll-off can attributed, on one hand, the balanced charge injection and charge transporting abilities of PhCz-o-Trz, on the other hand, the enhanced FRET significantly reduce the triplet exciton concentration in the emitting layers, leading to the reduced exciton annihilations.[6] Besides, owing to the similar reasons, the device with DACT-II as emitter also realized a high EQEmax of 20.2% with reduced efficiency roll-off. Those results testify the possibility of such compounds as versatile single-molecule exciplex-type sensitizing hosts, facilitating high performances with easy device fabrication procedure.[17,

18]

Furthermore,

possibility also exist that with proper molecular design, single-molecule white emission can also be anticipated with TADF emission.

CONCLUSIONS


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In summary, we developed a TADF compound with a D-σ-A motif, with the oxygen-bridged σ-junction not only partly suppressing the intra-CT of intra-D/A through the molecule backbone, but also providing flexibility to the molecular backbone, both facilitating the formation of interCT of inter-D/A. Consequently, by tuning the compound concentrations in the aggregate, coexistence of the intra-CT and inter-CT was observed, facilitating the triplet-to-singlet spin-flip transition. The device with DPEPO: 40% PhCz-o-Trz as EML provided EQEmax of 8.5%. Furthermore, devices with PhCz-o-Trz as the single-host for dopants with fluorescence, phosphorescence and TADF were evaluated, realizing EQEmaxs of 11.9%, 27.1% and 20.2%, respectively. Our results here will shed new light on developing novel TADF materials with tuneable intra- and inter-CT states, enabling not only TADF emitters with dual CT states but also single-molecule exciplex-type hosts with easy device fabrication procedure, and may also deepen our understanding about TADF mechanisms. FIGURES

Scheme 1. The synthesis process of PhCz-o-Trz.



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Figure 1. a) The molecular structure of PhCz-o-Trz. b) The HOMO and LUMO distributions of the optimized ground state calculated by B3LYP/ 6-31G (d) level.

Figure 2. (a) The Abs-, Fluor- and Phos-spectra of PhCz-o-Trz in toluene with a concentration of 1×10-5 mol/L with degassing. (b) The Abs-, Fluor- and Phos-spectra of PhCz-o-Trz neat film. (c) The Fluor-spectra of DPEPO: PhCz-o-Trz doped films. (d) The diagram for the intra-CT. (e) The diagram for the coexistence of intra- and inter-CT. The PL spectra were recorded with excitation wavelength of 300 nm.


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Figure 3. (a) The PLQYs of the DPEPO: PhCz-o-Trz with different concentrations. (b) The diagrams of the emission processes with intra-CT only or dual CTs. (c) The electro-luminance spectra of the devices with PhCz-o-Trz as emitters. (d) The current density-voltage-brightness characteristics of the device with PhCz-o-Trz as emitters. (e) The EQE-brightness characteristic of the device with PhCz-o-Trz as emitters.



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Figure 4. (a) The energy diagram of the devices and the molecular structures utilized. PhtBuPAD, Ir(ppy)3 and DACT-II are adopted emitters in the devices. (b) The EL-spectra of the devices with different emitters. (c) The EQE-brightness characteristics of devices with different emitters. Table 1: the device performances. emitter PhtBuPAD Ir(ppy)3 DACT-II

100 cd/m2 3.54 3.50 3.88

Voltage (V) 1000 cd/m2 4.06 4.02 4.50

5000 cd/m2 4.94 4.79 5.60

Max 11.9 27.1 20.2

EQE (%) 1000 cd/m2 11.1 24.4 19.0

5000 cd/m2 11.7 26.6 16.0

Power efficiency (lm/W) 1000 5000 Max cd/m2 cd/m2 26.9 24.4 21.3 67.6 63.5 58.6 38.8 36.9 25.0

CIE (x, y) (0.34, 0.60) (0.34, 0.61) (0.37, 0.54)

Note: a) T90 at initial luminance of 5000 cd/m2.

AUTHOR INFORMATION Corresponding Author *Correspondence to:

E-mail address: [email protected]; [email protected].

Fax: +86 10 62795137; Tel: +86 10 62782197 Notes The authors declare no competing financial interests. ACKNOWLEDGMENT This work was supported by a JSPS KAKENHI Grant Nos. 17H01231 and 17J09631. Computation time was provided by the Super Computer System, Institute for Chemical Research, Kyoto University. REFERENCES


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A thermally activated delayed fluorescent material combining intra

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