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meta-Substituted benzophenones as multifunctional electroactive materials for OLEDs
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meta-Substituted benzophenones as multifunctional electroactive materials for OLEDs R. Keruckiene a, J. Keruckas a, M. Cekaviciute a, D. Volyniuk a, P.-H. Lee b, T.-L. Chiu c, **, J.-H. Lee b, J.V. Grazulevicius a, * a b c
Department of Polymer Chemistry and Technology, Kaunas University of Technology, Radvilenu Rd. 19, LT-50254, Kaunas, Lithuania Graduate Institute of Photonics and Optoelectronics, National Taiwan University, No. 1, Sec. 4, Roosevelt Rd., Taipei, 10617, Taiwan, ROC Department of Photonics Engineering, Yuan Ze University, 135 Yuan-Tung Rd., Taoyuan, 32003, Taiwan, ROC
A R T I C L E I N F O
A B S T R A C T
Keywords: Acridan TADF Benzophenone
meta-Substituted benzophenones bearing tetrahydrocarbazole, phenoxazine and 2,7-ditert-butyl-9,9-dimethyla cridine moieties were synthesized as materials exhibiting both thermally activated delayed fluorescence and aggregation induced emission enhancement. The derivatives showed improved HOMO and LUMO separation compared to that of the corresponding derivatives with para-linkage which lead to the minimized singlet-triplet energy gap of ca. 0.04 eV. By employing the acridan-substituted benzophenone derivative as non-doped emissive layer in OLED, a green device with external quantum efficiency of 1.93% was obtained with the CIE coordinates of (0.37, 0.57). More efficient triplet population employment was achieved after doping the acridan-substituted benzophenone derivative with red phosphorescent emitter bis(1-phenyl-isoquinoline)(acetylacetonate)Iridium (III). Red phosphorescent device exhibited small efficiency roll-off and a relatively high external quantum effi ciency of 8.6%. External quantum efficiency of 7.60% was achieved for green device based on host:guest system using emitter 10-(4-(4,6-diphenyl-1,3,5-triazin-2-yl)phenyl)-10H-phenoxazine which exhibited thermally acti vated delayed fluorescence.
1. Introduction Organic light emitting diodes (OLEDs) have attracted considerable attention due to their applications in displays and lightning devices [1]. The search for suitable materials for these devices is still a subject of interest. To achieve better performance of OLEDs, it is of great impor tance to have a deep understanding in structure-properties relationship of organic emitters and charge-transporting materials. Materials exhib iting thermally activated delayed fluorescence (TADF) enable to achieve 100% of use of excitons through reverse intersystem crossing because of their low energy gap between the lowest singlet excited state S1 and triplet excited state T1 [2]. The strategy to achieve such characteristics is to employ both donor and acceptor moieties in a single molecular structure. The linking topology of chromophores should be also borne in mind as it influences the materials characteristics [3,4]. Benzophenone is known as a stable acceptor [5]. Para-substituted benzophenones bearing different donor substituents have been
investigated by several research groups [6,7]. Electroactive benzophe none derivatives with small energy gaps between their singlet and triplet excited states were used to achieve efficient full-colour delayed fluo rescence with the external quantum efficiency (EQE) of OLEDs up to 14.3% [8]. A series of TADF materials containing benzophenone and dimethydihydroacridine moieties were used in non-doped green OLED with EQE of 4.3% [9]. Benzophenone luminogens with both aggregation-induced emission and delayed fluorescence features (AIDF) were synthesized and characterized for imaging of living cells [10]. AIDF is advantageous as aggregation induced emission of non-doped emissive layers of devices can suppress high concentration annihilation and induce exciton harvesting up to 100% [11]. Non-doped OLEDs, based on emitters that simultaneously exhibit TADF and AIE, can reach EQE values of up to 14% [12–14]. In this work, we report on the synthesis and properties of electro active compounds bearing benzophenone moiety as an acceptor and tetrahydrocarbazole, phenoxazine and 2,7-ditert-butyl-9,9-
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (T.-L. Chiu), [email protected] (J.V. Grazulevicius). https://doi.org/10.1016/j.dyepig.2019.108058 Received 17 May 2019; Received in revised form 13 September 2019; Accepted 17 November 2019 Available online 20 November 2019 0143-7208/© 2019 Elsevier Ltd. All rights reserved.
Please cite this article as: R. Keruckiene, Dyes and Pigments, https://doi.org/10.1016/j.dyepig.2019.108058
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synthesis as illustrated in Scheme 1. NBS was used for the bromination of benzophenone in sulphuric acid. The addition of acetic acid was found to be useful for dissolving the resulting mono-brominated intermediate compound during the reaction course, thus, increasing the yield of 3,30 dibromobenzophenone (BPBr2) (87%). C–N bond formation was con ducted via the nucleophilic aromatic substitution reaction. BPBr2 and N-heterocyclic compounds reacted under the action of sodium tert-but oxide in DMF. The target compounds yielded in 27–65% and were found to be soluble in common organic solvents. Descriptions of the synthesis and characterization of the compounds are given in SI.
Scheme 1. Synthesis of benzophenone derivatives: (i) NBS, H2SO4, acetic acid, reflux, overnight; (ii) Heterocycle, Pd acetate, Na t-BuO, DMF, reflux, 24h.
2.2. Thermal properties
Table 1 Thermal characteristics.
dimethylacridine species as donors attached in meta-position. The donor moieties, i.e. tetrahydrocarbazole, phenoxazine and dimethydihy droacridine were chosen due to the favourable delocalization of HOMO orbitals and electron donating ability [15].
Morphological transitions and thermal stability of derivatives 1, 2 and 3 were investigated by DSC and TGA. The DSC and TGA thermo grams can be found in Supplementary Information (SI, Figs. S1 and S2). The temperatures of transitions are given in Table 1. The glass transition during DSC scans was detected only for the tetrahydrocarbazole-substituted benzophenone derivative 1, as it was isolated as amorphous powder, indicating its’ ability to form molecular glass. Benzophenone-based derivatives 2 and 3 showed double melting signals during DSC heating scans indicating polymorphism of their crystalline structures [16]. The crystallization signals during DSC cool ing scan were also observed for both of these compounds. The decom position temperature corresponding to 10% weight loss ranged from 218 to 389 � C. The overall higher thermal characteristic values of 2,7-ditert- butyl-9,9-dimethylacridine-substituted benzophenone derivative 3 can be explained by its’ much higher molar mass and increased rigidity caused by hydrogen bonding between oxygen atom and neighbouring hydrogen atom (O…H) of the acridan moiety [17].
2. Results and discussion
2.3. Theoretical calculations
2.1. Synthesis
DFT calculations were employed to gain an insight into the structureproperties relationship of benzophenone derivatives 1–3. The natural transition orbitals of singlet and triplet excited states are provided in
Compound a �
TM , C TGb, � C TCrc, � C TD-10 %d, � C a b c d e
1 e
91 – 312
2
3
118, 130 – 62 218
342, 348 – 269 389
Melting point observed at the first heating scan of DSC measurement. Glass transition from DSC curves. Crystallization from the cooling scan of DSC measurement. 10% weight loss temperature obtained from TG curves. Not detected.
The target compounds 1, 2 and 3 were obtained by two-step
Fig. 1. Theoretically calculated optimized geometries of compounds 1–3 at ground state. 2
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Supporting Information (SI, Fig. S3). In their optimized ground-state geometries, dihedral angles between phenyl rings of benzophenone unit and the respective donors were calculated to be 126� for 1, 123� for 2 and 119� for 3. The moderately twisted configurations determine separation of HOMO and LUMO (Fig. 1) and the favourable for TADF dihedral angles between donor and acceptor moieties which keep controlled conjuga tion and prevent intermolecular ð-ð stacking. On the other hand, as the D-A fragments do not adapt perpendicular position, it enables slight HOMO and LUMO overlap for efficient recombination and light emis sion. As shown in Fig. 1, LUMOs are located on the central electron-
Table 2 Electrochemical and photoelectrical characteristics of compounds 1–3. Eox vs.Fc, V IPCV, eV Eopt g , eV EACV, eV IPPE, eV HOMO, eV LUMO, eV
1
2
3
0.62 5.72 3.21 2.51 5.72 5.17 2.03
0.19 5.29 3.05 2.24 5.57 4.74 2.19
0.35 5.45 3.66 1.79 5.59 4.78 2.02
Fig. 2. Theoretical UV–Vis spectra, experimental UV–Vis spectra of the solutions and thin films of 1–3 (a, c, e); photoluminescence (λex ¼ 300 nm, at RT and 77 K) and phosphorescence (λex ¼ 300 nm) (at 77 K) spectra of dilute toluene solutions and thin films of derivatives 1–3 (b, d, f). 3
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Fig. 3. Fluorescence spectra of compound 3 in THF-water mixtures with different water fractions (wf in %) (a); temperature-dependant fluorescence spectra (b) and PL time decays (c) of the neat film of compound 3.
accepting benzophenone core, whereas HOMOs are mainly localized on the electron-donating peripheral substituents. The HOMO energy values (Table 2) show dependence on the donating characteristics of the substituents of benzophenone moiety in 1–3, whereas the LUMO values are comparable and characteristic to benzophenone derivatives. Electron-donating characteristics where investigated experimentally by employing two methods. Cyclic vol tammetry was used to determine the reversible oxidation up to 1 V. The ionization potential values were estimated from the oxidation onset potentials against ferrocene (Eox onset vs. Fc) and optical band gaps (Table 2). They range from 5.29 for compound 2 to 5.72 for compound 1. Ionization potentials of the solid samples of compounds 1–3 were estimated by photoelectron emission method (Fig. S4). The values were found to be in the same trend and were a little higher than those esti mated by cyclic voltammetry. Small differences in the values of ioni zation potentials obtained by employing different methods can be explained by different environments in the solution and the solid state. These values demonstrate similar electron-donating effect of tetrahy drocarbazole, phenoxazine and 2,7-ditert-butyl-9,9-dimethylacridine donor moieties on electron releasing energies. The ionization potential values of the derivatives of meta-substituted benzophenone (1–3) are similar to those of para-substituted benzophenone derivatives that have been described earlier [18].
2.4. Photophysical properties TD-DFT calculations were performed to gain insight into the elec tronic structures of the benzophenone derivatives in the excited states. The S1 state of all the derivatives has the oscillator strength of 0.0 indicating that transition S1→S0 is spin-forbidden due to quasiorthogonal geometry of the donor part on the acceptor core, and the charge-transfer (CT) character of S1 [19]. Theoretical UV spectrum (Fig. 2) of tetrahydrocarbazolyl-substituted benzophenone derivative 1 has two absorption bands. The band at 325 nm is mainly characterized by S0→S6 transition with the oscillator strength of 0.115 that takes place only on the benzophenone fragment (Fig. S5). The band at 230 nm is mainly characterized by S0→S18 transition with the oscillator strength of 0.260. This transition is from the whole molecule to benzo phenone unit. The theoretical UV–vis spectrum of derivative 2 has one absorption band with an overlapped shoulder. The band at ca. 220 nm is charac terized by a combination of various transitions towards several excited states (Fig. S2). This band is mainly characterized by S0→S32 transition with the oscillator strength of 0.369. It belongs solely to benzophenone unit. The theoretical UV–vis spectrum of derivative 3 has one band at ca. 240 nm. This band is mainly characterized by S0→S28 transition with the oscillator strength of 0.479. It is also a transition of solely on the benzophenone unit. 4
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intermolecular interaction in the solid state (Table 3). The 43 nm blueshift of solid-state fluorescence spectrum of 3 (PLQY value of 16%) with respect to that of the solution can be ascribed to the enhanced twisted intramolecular charge transfer (TICT) effect in solution [22]. TICT is induced as the molecule of 2,7-ditert-butyl-9,9-dimethylacridine-substi tuted benzophenone derivative 3 adopt a more twisted geometry and enhance charge transfer in solution rather that in solid state [23,24]. Large Stokes shift value (17757 cm 1) of compound 3 also coincide well with TICT as it signifies large changes in geometry upon excitation leading to energy losses during the relaxation process. AIEE properties. The absolute fluorescence quantum yields of the films of these derivatives are higher than those of the solutions, indi cating their aggregation-induced emission enhancement (AIEE) prop erty. AIEE was confirmed by investigating fluorescence of mixed water and THF solutions of compounds. PL spectra of the dispersions of com pound 3 in water/THF mixtures can be found in SI (Fig. 3a). When water concentration reached 40%, PL intensity is enhanced due to the for mation of aggregates. However, at 70% concentration of water PL in tensity slightly decreased. This phenomenon can be explained by the stabilization of charge transfer state [25]. The subsequent PL intensity increase (at 80% of water) can be attributed to aggregation of compound 3 in the mixture with higher water content. TADF property. Fluorescence decays of the films of 1–3 (Fig. S7) exhibited double-exponential behaviour. For the mathematical repre sentation of the decay curves, the following equation was used: F (t) ¼ A þ B1exp(–t/τ1) þ B2exp(–t/τ2); where τ1 and τ2 represent the time constants, and B1 and B2 represent the amplitudes of the fast and slow components, respectively [26]. Compounds 1 and 2 compounds showed the relaxation pathway with time constants in order of nanoseconds (Fig. S7). In contrast, compound 3 showed long-lived time component (in microsecond range) which attributed to delay fluorescence (Fig. S7). To investigate nature of that long lived emission of compound 3 in solid state, photophysical properties of compound 3 bearing 2,7-ditert-bu tyl-9,9-dimethylacridine moiety were further investigated. Due to those investigations, it is evident that compound 3 in solid state exhibits TADF. This claim is in agreement with (1) small singlet-triplet energy splitting (ΔEST) of ca. 0.04 eV which was obtained taking singlets and triplets from the edges of fluorescence (at 300 K) and phosphorescence (at 77 K) spectra of the tested layer, respectively (Fig. 3 b); (2) the in tensity of long-lived component gradually increased with the increase of temperature suggesting presence of TADF which is well-seen from the temperature-dependant time resolved PL measurements of neat film of compound 3 at the range from 77K to 300K (Fig. 3c); (3) the shape of PL spectra of compound 3 did not change through the temperature range (Fig. 3 b). The obtained results confirm that triplet states, which are readily quenched in the presence of oxygen, contribute to the radiative decay of this compound. This results in existence of prompt and delayed emissive components, and is consistent with TADF [27]. Considering the above presentented results, compound 3 in solid-state possesses a small energy gap (ΔEST) and a satisfactory delayed fluorescence lifetime, making it promising TADF material. The symmetrical D-A-D configu ration with meta-linkage results in the increase of efficiency of reverse intersystem crossing.
Table 3 Photophysical characteristics of compounds 1–3. Compound 1 2 3
Toluene solution/thin film €eAbs, nm
€eFL, nm
Stokes shift, cm
293, 350 325, 373 285
488/537 389, 404/430 577/534
8080 1103 17757
1
QY, % 3/20 12/15 6/16
λAbs are wavelengths of absorption maxima; λFL are wavelengths of emission maxima; Stokes shift ¼ λFL–λAbs; QY is fluorescence quantum yield.
Experimental UV spectra of the derivatives 1–3 (Fig. 3) are consistent with the theoretical ones. Energy band gap values determined from the edges of experimental UV spectra of the solutions of derivatives 1–3 increase almost linearly with the ionization potential values discussed earlier. This is very important for the optimization of hole injection into the active layers, and for balancing electron and hole currents in the multilayer OLEDs. The photophysical characteristics determined from the UV–Vis absorption, photoluminescence and phosphorescence spectra are given in Table 3. The transition pathways between the moieties of the molecules in the first excited and triplet states do not differ (Fig. S3). The transfer from HONTO to LUNTO goes from donor fragment towards the central benzophenone fragment indicating intramolecular charge transfer. Such € ST spatial distribution leads to the small theoretically calculated AE values of 0.02–0.06 eV and suggests that excitons can be harvested by T1→S1 upconversion. The experimental photophysical properties of the compounds 1–3 are determined by their geometrical structures in the excited states (Fig. 2). The broad and unstructured emission band of the solution of 1 and 3 in toluene at room temperature is observed with the intensity maximum at ca. 488 nm (PLQY of 3%) and 577 nm (PLQY of 6%) (Table 3). The strongly red-shifted fluorescence bands with the intensity maxima at ca. 138 nm and 292 nm with respect to absorption peaks, is characteristic to charge-transfer (CT) transition [20]. The vibrationally-structured FL spectrum of the solution of 2 in toluene show intensity maxima at 389 and 404 nm with PLQY of 12%. In order to determine the origin of the emission, the fluorescence decay curves of derivatives 1–3 were recorded and analysed (Fig. S6). Dilute toluene solutions of benzophenone derivatives exhibited single-exponential de cays in nanosecond (ns) range. After degassing the toluene solutions only, the PL lifetime of the compounds 2 and 3 increased from 4.8 ns to 7.2 ns up to 5.9 ns and 8.9 ns, and from 0.3 ns to 4.5 ns up to 0.9 ns and 5.4 ns, respectively. The similar emission lifetime of both air-equilibrated and degassed toluene solutions observed for compound 1 showed that the emission originated exclusively from local excitation fluorescence (1LE). At low temperature (77 K), photoluminescence spectra of the de rivatives in THF solutions remain similar to their respective room tem perature spectra (Fig. 2 b, d, f). The triplet energy values determined from the onset of the phosphorescence spectra of the compounds 1–3 range from 2.65 to 3.02 eV. The highest triplet energy was observed for tetrahydrocarbazolyl-substituted benzophenone (1). The experimental ΔEST values the dilute THF solutions of 2 and 3 are 0.57 eV and 0.49 eV, respectively. The relatively high ΔEST values of 2 and 3 indicate the forbidding for efficient reverse intersystem crossing of the triplet exci tons in THF solution (Table 3). The meta-linkage in the molecular structure crucially suppresses the π-conjugation which results in a better separation of HOMO and LUMO and much stronger charge-transfer character leading to a smaller singlet triplet splitting. In this case the position of the donor affects the amount of π-π* and n-π* mixing and the charge-transfer strength in the molecules [21]. Fluorescence spectra of thin films of compounds 1–3 are broad and unstructured (Fig. 2 b,d,f). The 49 nm and 26 nm red-shifts of the emission spectra of 1 and 2 (PLQY values of 20% and 15% respectively) with respect to the spectra of the solutions may be caused by
2.5. Device performance Compound 3 as TADF emitter may find applications in efficient nondoped OLEDs due to its TADF and AIEE properties. On the other hand, efficient doped OLEDs based on compound 3 as TADF host may be constructed taking into account its HOMO/LUMO and triplet energy levels. To prove these considerations, following experiments were pro vided. The device configuration, shown in Fig. 4, contained a 50 nm layer of 1,1-bis[N,N-di(4-tolyl)aminophenyl]cyclohexane (TAPC), a 10 nm layer of 1,3-bis(N-carbazolyl)benzene (mCP), a 30 nm EML, a 50 nm layer of bathophenanthroline (Bphen), a 1 nm layer of lithium fluoride (LiF), and a 100 nm layer of aluminum (Al). The layers of TAPC, mCP, 5
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system is also plotted in Fig. 4. In particular, the 7% Ir(piq)2acac doping ratio was selected taking into account the electroluminescent spectra of devices (Fig. S8). Absence of emission from compound 3 showed the sufficient energy transfer from compound 3 to Ir(piq)2acac. Fig. 5 shows the EO properties of TADF and phosphorescent OLEDs. Their characteristics such as driving voltage, maximum current effi ciency (CEmax), maximum power efficiency (PEmax), maximum external quantum efficiency (EQEmax), CIE 1931 color coordinates are summa rized in Table 4. Fig. 5a displays J-V and L-V curves. Phosphorescent device exhibited the favourable J-V and L-V performances, indicating that Ir(piq)2acac benefited from charge carrier transport, carrier recombination, exciton formation and energy transfer. Fig. 5b illustrates efficiency performances of OLEDs at various current densities. The de vice with pristine compound 3 as EML showed higher CE and PE values at low current density, but efficiency roll-off increased with increasing current density. Its CEmax of 11.8 cd/A, PEmax of 9.3 lm/W and EQEmax of 1.93% were recorded at a very low current density of 60 μA/cm2. By doping PXZ-TRZ with compound 3 as the EML, an efficient green TADF device showed a CEmax of 31.1 cd/A, PEmax of 28.7 lm/W and EQEmax of 7.6%. In particular, this device performed the highest brightness closed to 30,000 cd/m2 at 10 V, the greater brightness is prospective after in crease the driving voltage. The ultimate red phosphorescent device exhibited a favourable efficiency performance with a small efficiency roll-off and a relatively higher EQE of 8.6. The intrinsic TADF emission
Fig. 4. OLED structure with compound 3 for TADF and phosphorescent emis sion mechanisms.
Bphen, LiF and Al were respectively deposited as 1st hole transporting layer, 2nd hole transporting layer, electron transporting layer, electron injection layer, and cathode. In the EML, the TADF and phosphorescent emission mechanisms were investigated using a pristine compound 3 layer and two doped layers respectively with a 10% and 7% doping ratio of green TADF emitter 10-(4-(4,6-diphenyl-1,3,5-triazin-2-yl)phenyl)10H-phenoxazine (PXZ-TRZ) and red phosphorescent emitter bis(1phenyl-isoquinoline)(acetylacetonate)Iridium(III) (Ir(piq)2acac). The energy transfer diagram of these three emission mechanisms in EML
Fig. 5. (a) J-L-V curves, (b) efficiency-current density curves, (c) emission spectra at 8 V, (d) TREL signals of intrinsic, TADF and phosphorescent devices. Table 4 Characteristics of intrinsic, TADF and phosphorescent devices. Device
Driving voltage (V)a
Luminance (nits)b
CEmax (cd A 1)
PEmax (lm W 1)
EQEmax (%)c
CIE(x, y)c
Decay time (μs)
Intrinsic TADF Phosphorescent
7.56 5.82 5.19
114 5775 1094
11.8 31.1 5.6
9.3 28.7 5.3
1.93 7.60 8.60
(0.37, 0.57) (0.38, 0.58) (0.68, 0.32)
3.21 2.82 2.75
a b c
Operation voltage recorded under current density of 10 mA/cm.2. Luminance recorded under driving voltage is 6 V. CIE coordinates at EQEmax. 6
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of compound 3 was green with a peak at 536 nm (Fig. 5c). It was very close to PL emission peak recorded at 534 nm (Fig. 2f). Low EQE of the green TADF device with pristine compound 3 can be explained the low solid-state PLQY (Table 3). It also implies that most of exciton energy was hindered by the singlet emission. The emission spectrum of PXZTRZ device was very similar to that of compound 3, showing a nar rower emission band. Notably, this PXZ-TRZ device exhibited very efficient energy transfer from host to TADF emitter no mater singlet and triplet excitons. Additionally, in phosphorescent device, the red emitter Ir(piq)2acac was added to transform more exciton energy to the emis sion, which was red with a peak at 628 nm. The implication in energy transfer of excitons can be also deduced from the transient electrolu minescent (TREL) signals of these three devices shown in Fig. 5d and their fitting exponential decay time are shown in Table 4. The TADF device with the EML of pristine compound 3 exhibited longer expo nential decay time of approximately 3.21 μs, belonging to the delayed fluorescence. Obviously, employing the PXZ-TRZ and Ir(piq)2acac dopant of EML, the shorter exponential decay time of ca. 2.82 and 2.75 μs was observed in phosphorescent device TREL signal. This observation shows, that the blind exciton energy of compound 3 is effectively transferred to PXZ-TRZ and Ir(piq)2acac.
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3. Conclusions We developed a series of derivatives of meta-substituted benzophe none bearing tetrahydrocarbazole, phenoxazine or 2,7-ditert-butyl-9,9dimethylacridine donor moieties. The derivatives showed improved HOMO and LUMO separation compared to that of the corresponding derivatives with para-linkage which lead to the minimized singlet-triplet energy gap of ca. 0.04 eV, and the emission energy spanning widely in UV - (deep) blue region (2.65–3.46 eV). Ionization potentials of the solid samples of the derivatives were found to be in the range of 5.57–5.72 eV. Photoluminescence quantum yields of non-doped solid films of the compounds ranged from 15% to 20% which were up to 7-fold higher than those observed for the solutions in low-polarity solvent toluene. Acridan-based emitter showed a combination of aggregation induced emission enhancement and delayed fluorescence. By employing the layer of this emitter as non-doped emissive layer in OLED, a green device with EQE of 1.93% was obtained with the CIE coordinates of (0.37, 0.57). The more efficient triplet population employment was achieved after doping this acridan-substituted benzophenone derivative with red emitter bis(1-phenyl-isoquinoline)(acetylacetonate)Iridium(III). Red phosphorescent device exhibited a favourable efficiency performance with a small efficiency roll-off and a relatively higher EQE of 8.6%. External quantum efficiency of 7.60% was achieved for green device based on host:guest system using emitter 10-(4-(4,6-diphenyl-1,3,5-tri azin-2-yl)phenyl)-10H-phenoxazine which exhibited thermally acti vated delayed fluorescence. Declarations of competing interest None. Acknowledgment This project has received funding from European Social Fund (proj ect No 09.3.3-LMT-K-712-01-0140) under grant agreement with the Research Council of Lithuania. P.-H. Lee, T.-L. Chiu and J.-H. Lee aknowledge Taiwanese Ministry of Science and Technology for research funding (Grant No. MOST 106-2923-E-155-002-MY3S). Special thanks goes to dr. Jurate Simokaitiene for DSC and TGA measurements and Matas Guzauskas for the help with photophysical measurements. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. 7
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[23] Chua MH, Zhou H, Lin TT, Wu J, Xu J. Triphenylethylenyl-based donor–acceptor–donor molecules: studies on structural and optical properties and AIE properties for cyanide detection. J Mater Chem C 2017;5:12194–203. https:// doi.org/10.1039/C7TC04400C. [24] Meng F-Y, Zhou Y-L, Zou H-H, Zeng M-H, Liang H. Solution and solid-state photoluminescence of bis{succinatobis[2,6-bis(2-benzimidazolyl)] lead(II)}: a comment on the lone-pair stereochemistry and its effect on supramolecular interaction. J Mol Struct 2009;920:238–41. https://doi.org/10.1016/J. MOLSTRUC.2008.11.023.
[25] Huang M, Ye S, Xu K, Zhou J, Liu J, Zhu X, et al. Whole-rainbow-color organic solid fluorophores from subtle modification of thiazolo[5,4-b]thieno[3,2-e]pyridines (TTPs). J Mater Chem C 2017;5:3456–60. https://doi.org/10.1039/C7TC00513J. [26] Yang P, Yuan Z, Yang J, Zhang A, Cao Y, Jiang Q, et al. Science as art: self-assembly of hybrid SiO 2 -coated nanocrystals. CrystEngComm 2011;13:1814–20. https:// doi.org/10.1039/C0CE00350F. [27] Wu K, Wang Z, Zhan L, Zhong C, Gong S, Xie G, et al. Realizing highly efficient solution-processed homojunction-like sky-blue OLEDs by using thermally activated delayed fluorescent emitters featuring an aggregation-induced emission property. J Phys Chem Lett 2018;9:1547–53. https://doi.org/10.1021/acs.jpclett.8b00344.
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Dyes and Pigments journal homepage: http://www.elsevier.com/locate/dyepig
meta-Substituted benzophenones as multifunctional electroactive materials for OLEDs R. Keruckiene a, J. Keruckas a, M. Cekaviciute a, D. Volyniuk a, P.-H. Lee b, T.-L. Chiu c, **, J.-H. Lee b, J.V. Grazulevicius a, * a b c
Department of Polymer Chemistry and Technology, Kaunas University of Technology, Radvilenu Rd. 19, LT-50254, Kaunas, Lithuania Graduate Institute of Photonics and Optoelectronics, National Taiwan University, No. 1, Sec. 4, Roosevelt Rd., Taipei, 10617, Taiwan, ROC Department of Photonics Engineering, Yuan Ze University, 135 Yuan-Tung Rd., Taoyuan, 32003, Taiwan, ROC
A R T I C L E I N F O
A B S T R A C T
Keywords: Acridan TADF Benzophenone
meta-Substituted benzophenones bearing tetrahydrocarbazole, phenoxazine and 2,7-ditert-butyl-9,9-dimethyla cridine moieties were synthesized as materials exhibiting both thermally activated delayed fluorescence and aggregation induced emission enhancement. The derivatives showed improved HOMO and LUMO separation compared to that of the corresponding derivatives with para-linkage which lead to the minimized singlet-triplet energy gap of ca. 0.04 eV. By employing the acridan-substituted benzophenone derivative as non-doped emissive layer in OLED, a green device with external quantum efficiency of 1.93% was obtained with the CIE coordinates of (0.37, 0.57). More efficient triplet population employment was achieved after doping the acridan-substituted benzophenone derivative with red phosphorescent emitter bis(1-phenyl-isoquinoline)(acetylacetonate)Iridium (III). Red phosphorescent device exhibited small efficiency roll-off and a relatively high external quantum effi ciency of 8.6%. External quantum efficiency of 7.60% was achieved for green device based on host:guest system using emitter 10-(4-(4,6-diphenyl-1,3,5-triazin-2-yl)phenyl)-10H-phenoxazine which exhibited thermally acti vated delayed fluorescence.
1. Introduction Organic light emitting diodes (OLEDs) have attracted considerable attention due to their applications in displays and lightning devices [1]. The search for suitable materials for these devices is still a subject of interest. To achieve better performance of OLEDs, it is of great impor tance to have a deep understanding in structure-properties relationship of organic emitters and charge-transporting materials. Materials exhib iting thermally activated delayed fluorescence (TADF) enable to achieve 100% of use of excitons through reverse intersystem crossing because of their low energy gap between the lowest singlet excited state S1 and triplet excited state T1 [2]. The strategy to achieve such characteristics is to employ both donor and acceptor moieties in a single molecular structure. The linking topology of chromophores should be also borne in mind as it influences the materials characteristics [3,4]. Benzophenone is known as a stable acceptor [5]. Para-substituted benzophenones bearing different donor substituents have been
investigated by several research groups [6,7]. Electroactive benzophe none derivatives with small energy gaps between their singlet and triplet excited states were used to achieve efficient full-colour delayed fluo rescence with the external quantum efficiency (EQE) of OLEDs up to 14.3% [8]. A series of TADF materials containing benzophenone and dimethydihydroacridine moieties were used in non-doped green OLED with EQE of 4.3% [9]. Benzophenone luminogens with both aggregation-induced emission and delayed fluorescence features (AIDF) were synthesized and characterized for imaging of living cells [10]. AIDF is advantageous as aggregation induced emission of non-doped emissive layers of devices can suppress high concentration annihilation and induce exciton harvesting up to 100% [11]. Non-doped OLEDs, based on emitters that simultaneously exhibit TADF and AIE, can reach EQE values of up to 14% [12–14]. In this work, we report on the synthesis and properties of electro active compounds bearing benzophenone moiety as an acceptor and tetrahydrocarbazole, phenoxazine and 2,7-ditert-butyl-9,9-
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (T.-L. Chiu), [email protected] (J.V. Grazulevicius). https://doi.org/10.1016/j.dyepig.2019.108058 Received 17 May 2019; Received in revised form 13 September 2019; Accepted 17 November 2019 Available online 20 November 2019 0143-7208/© 2019 Elsevier Ltd. All rights reserved.
Please cite this article as: R. Keruckiene, Dyes and Pigments, https://doi.org/10.1016/j.dyepig.2019.108058
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synthesis as illustrated in Scheme 1. NBS was used for the bromination of benzophenone in sulphuric acid. The addition of acetic acid was found to be useful for dissolving the resulting mono-brominated intermediate compound during the reaction course, thus, increasing the yield of 3,30 dibromobenzophenone (BPBr2) (87%). C–N bond formation was con ducted via the nucleophilic aromatic substitution reaction. BPBr2 and N-heterocyclic compounds reacted under the action of sodium tert-but oxide in DMF. The target compounds yielded in 27–65% and were found to be soluble in common organic solvents. Descriptions of the synthesis and characterization of the compounds are given in SI.
Scheme 1. Synthesis of benzophenone derivatives: (i) NBS, H2SO4, acetic acid, reflux, overnight; (ii) Heterocycle, Pd acetate, Na t-BuO, DMF, reflux, 24h.
2.2. Thermal properties
Table 1 Thermal characteristics.
dimethylacridine species as donors attached in meta-position. The donor moieties, i.e. tetrahydrocarbazole, phenoxazine and dimethydihy droacridine were chosen due to the favourable delocalization of HOMO orbitals and electron donating ability [15].
Morphological transitions and thermal stability of derivatives 1, 2 and 3 were investigated by DSC and TGA. The DSC and TGA thermo grams can be found in Supplementary Information (SI, Figs. S1 and S2). The temperatures of transitions are given in Table 1. The glass transition during DSC scans was detected only for the tetrahydrocarbazole-substituted benzophenone derivative 1, as it was isolated as amorphous powder, indicating its’ ability to form molecular glass. Benzophenone-based derivatives 2 and 3 showed double melting signals during DSC heating scans indicating polymorphism of their crystalline structures [16]. The crystallization signals during DSC cool ing scan were also observed for both of these compounds. The decom position temperature corresponding to 10% weight loss ranged from 218 to 389 � C. The overall higher thermal characteristic values of 2,7-ditert- butyl-9,9-dimethylacridine-substituted benzophenone derivative 3 can be explained by its’ much higher molar mass and increased rigidity caused by hydrogen bonding between oxygen atom and neighbouring hydrogen atom (O…H) of the acridan moiety [17].
2. Results and discussion
2.3. Theoretical calculations
2.1. Synthesis
DFT calculations were employed to gain an insight into the structureproperties relationship of benzophenone derivatives 1–3. The natural transition orbitals of singlet and triplet excited states are provided in
Compound a �
TM , C TGb, � C TCrc, � C TD-10 %d, � C a b c d e
1 e
91 – 312
2
3
118, 130 – 62 218
342, 348 – 269 389
Melting point observed at the first heating scan of DSC measurement. Glass transition from DSC curves. Crystallization from the cooling scan of DSC measurement. 10% weight loss temperature obtained from TG curves. Not detected.
The target compounds 1, 2 and 3 were obtained by two-step
Fig. 1. Theoretically calculated optimized geometries of compounds 1–3 at ground state. 2
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Supporting Information (SI, Fig. S3). In their optimized ground-state geometries, dihedral angles between phenyl rings of benzophenone unit and the respective donors were calculated to be 126� for 1, 123� for 2 and 119� for 3. The moderately twisted configurations determine separation of HOMO and LUMO (Fig. 1) and the favourable for TADF dihedral angles between donor and acceptor moieties which keep controlled conjuga tion and prevent intermolecular ð-ð stacking. On the other hand, as the D-A fragments do not adapt perpendicular position, it enables slight HOMO and LUMO overlap for efficient recombination and light emis sion. As shown in Fig. 1, LUMOs are located on the central electron-
Table 2 Electrochemical and photoelectrical characteristics of compounds 1–3. Eox vs.Fc, V IPCV, eV Eopt g , eV EACV, eV IPPE, eV HOMO, eV LUMO, eV
1
2
3
0.62 5.72 3.21 2.51 5.72 5.17 2.03
0.19 5.29 3.05 2.24 5.57 4.74 2.19
0.35 5.45 3.66 1.79 5.59 4.78 2.02
Fig. 2. Theoretical UV–Vis spectra, experimental UV–Vis spectra of the solutions and thin films of 1–3 (a, c, e); photoluminescence (λex ¼ 300 nm, at RT and 77 K) and phosphorescence (λex ¼ 300 nm) (at 77 K) spectra of dilute toluene solutions and thin films of derivatives 1–3 (b, d, f). 3
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Fig. 3. Fluorescence spectra of compound 3 in THF-water mixtures with different water fractions (wf in %) (a); temperature-dependant fluorescence spectra (b) and PL time decays (c) of the neat film of compound 3.
accepting benzophenone core, whereas HOMOs are mainly localized on the electron-donating peripheral substituents. The HOMO energy values (Table 2) show dependence on the donating characteristics of the substituents of benzophenone moiety in 1–3, whereas the LUMO values are comparable and characteristic to benzophenone derivatives. Electron-donating characteristics where investigated experimentally by employing two methods. Cyclic vol tammetry was used to determine the reversible oxidation up to 1 V. The ionization potential values were estimated from the oxidation onset potentials against ferrocene (Eox onset vs. Fc) and optical band gaps (Table 2). They range from 5.29 for compound 2 to 5.72 for compound 1. Ionization potentials of the solid samples of compounds 1–3 were estimated by photoelectron emission method (Fig. S4). The values were found to be in the same trend and were a little higher than those esti mated by cyclic voltammetry. Small differences in the values of ioni zation potentials obtained by employing different methods can be explained by different environments in the solution and the solid state. These values demonstrate similar electron-donating effect of tetrahy drocarbazole, phenoxazine and 2,7-ditert-butyl-9,9-dimethylacridine donor moieties on electron releasing energies. The ionization potential values of the derivatives of meta-substituted benzophenone (1–3) are similar to those of para-substituted benzophenone derivatives that have been described earlier [18].
2.4. Photophysical properties TD-DFT calculations were performed to gain insight into the elec tronic structures of the benzophenone derivatives in the excited states. The S1 state of all the derivatives has the oscillator strength of 0.0 indicating that transition S1→S0 is spin-forbidden due to quasiorthogonal geometry of the donor part on the acceptor core, and the charge-transfer (CT) character of S1 [19]. Theoretical UV spectrum (Fig. 2) of tetrahydrocarbazolyl-substituted benzophenone derivative 1 has two absorption bands. The band at 325 nm is mainly characterized by S0→S6 transition with the oscillator strength of 0.115 that takes place only on the benzophenone fragment (Fig. S5). The band at 230 nm is mainly characterized by S0→S18 transition with the oscillator strength of 0.260. This transition is from the whole molecule to benzo phenone unit. The theoretical UV–vis spectrum of derivative 2 has one absorption band with an overlapped shoulder. The band at ca. 220 nm is charac terized by a combination of various transitions towards several excited states (Fig. S2). This band is mainly characterized by S0→S32 transition with the oscillator strength of 0.369. It belongs solely to benzophenone unit. The theoretical UV–vis spectrum of derivative 3 has one band at ca. 240 nm. This band is mainly characterized by S0→S28 transition with the oscillator strength of 0.479. It is also a transition of solely on the benzophenone unit. 4
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intermolecular interaction in the solid state (Table 3). The 43 nm blueshift of solid-state fluorescence spectrum of 3 (PLQY value of 16%) with respect to that of the solution can be ascribed to the enhanced twisted intramolecular charge transfer (TICT) effect in solution [22]. TICT is induced as the molecule of 2,7-ditert-butyl-9,9-dimethylacridine-substi tuted benzophenone derivative 3 adopt a more twisted geometry and enhance charge transfer in solution rather that in solid state [23,24]. Large Stokes shift value (17757 cm 1) of compound 3 also coincide well with TICT as it signifies large changes in geometry upon excitation leading to energy losses during the relaxation process. AIEE properties. The absolute fluorescence quantum yields of the films of these derivatives are higher than those of the solutions, indi cating their aggregation-induced emission enhancement (AIEE) prop erty. AIEE was confirmed by investigating fluorescence of mixed water and THF solutions of compounds. PL spectra of the dispersions of com pound 3 in water/THF mixtures can be found in SI (Fig. 3a). When water concentration reached 40%, PL intensity is enhanced due to the for mation of aggregates. However, at 70% concentration of water PL in tensity slightly decreased. This phenomenon can be explained by the stabilization of charge transfer state [25]. The subsequent PL intensity increase (at 80% of water) can be attributed to aggregation of compound 3 in the mixture with higher water content. TADF property. Fluorescence decays of the films of 1–3 (Fig. S7) exhibited double-exponential behaviour. For the mathematical repre sentation of the decay curves, the following equation was used: F (t) ¼ A þ B1exp(–t/τ1) þ B2exp(–t/τ2); where τ1 and τ2 represent the time constants, and B1 and B2 represent the amplitudes of the fast and slow components, respectively [26]. Compounds 1 and 2 compounds showed the relaxation pathway with time constants in order of nanoseconds (Fig. S7). In contrast, compound 3 showed long-lived time component (in microsecond range) which attributed to delay fluorescence (Fig. S7). To investigate nature of that long lived emission of compound 3 in solid state, photophysical properties of compound 3 bearing 2,7-ditert-bu tyl-9,9-dimethylacridine moiety were further investigated. Due to those investigations, it is evident that compound 3 in solid state exhibits TADF. This claim is in agreement with (1) small singlet-triplet energy splitting (ΔEST) of ca. 0.04 eV which was obtained taking singlets and triplets from the edges of fluorescence (at 300 K) and phosphorescence (at 77 K) spectra of the tested layer, respectively (Fig. 3 b); (2) the in tensity of long-lived component gradually increased with the increase of temperature suggesting presence of TADF which is well-seen from the temperature-dependant time resolved PL measurements of neat film of compound 3 at the range from 77K to 300K (Fig. 3c); (3) the shape of PL spectra of compound 3 did not change through the temperature range (Fig. 3 b). The obtained results confirm that triplet states, which are readily quenched in the presence of oxygen, contribute to the radiative decay of this compound. This results in existence of prompt and delayed emissive components, and is consistent with TADF [27]. Considering the above presentented results, compound 3 in solid-state possesses a small energy gap (ΔEST) and a satisfactory delayed fluorescence lifetime, making it promising TADF material. The symmetrical D-A-D configu ration with meta-linkage results in the increase of efficiency of reverse intersystem crossing.
Table 3 Photophysical characteristics of compounds 1–3. Compound 1 2 3
Toluene solution/thin film €eAbs, nm
€eFL, nm
Stokes shift, cm
293, 350 325, 373 285
488/537 389, 404/430 577/534
8080 1103 17757
1
QY, % 3/20 12/15 6/16
λAbs are wavelengths of absorption maxima; λFL are wavelengths of emission maxima; Stokes shift ¼ λFL–λAbs; QY is fluorescence quantum yield.
Experimental UV spectra of the derivatives 1–3 (Fig. 3) are consistent with the theoretical ones. Energy band gap values determined from the edges of experimental UV spectra of the solutions of derivatives 1–3 increase almost linearly with the ionization potential values discussed earlier. This is very important for the optimization of hole injection into the active layers, and for balancing electron and hole currents in the multilayer OLEDs. The photophysical characteristics determined from the UV–Vis absorption, photoluminescence and phosphorescence spectra are given in Table 3. The transition pathways between the moieties of the molecules in the first excited and triplet states do not differ (Fig. S3). The transfer from HONTO to LUNTO goes from donor fragment towards the central benzophenone fragment indicating intramolecular charge transfer. Such € ST spatial distribution leads to the small theoretically calculated AE values of 0.02–0.06 eV and suggests that excitons can be harvested by T1→S1 upconversion. The experimental photophysical properties of the compounds 1–3 are determined by their geometrical structures in the excited states (Fig. 2). The broad and unstructured emission band of the solution of 1 and 3 in toluene at room temperature is observed with the intensity maximum at ca. 488 nm (PLQY of 3%) and 577 nm (PLQY of 6%) (Table 3). The strongly red-shifted fluorescence bands with the intensity maxima at ca. 138 nm and 292 nm with respect to absorption peaks, is characteristic to charge-transfer (CT) transition [20]. The vibrationally-structured FL spectrum of the solution of 2 in toluene show intensity maxima at 389 and 404 nm with PLQY of 12%. In order to determine the origin of the emission, the fluorescence decay curves of derivatives 1–3 were recorded and analysed (Fig. S6). Dilute toluene solutions of benzophenone derivatives exhibited single-exponential de cays in nanosecond (ns) range. After degassing the toluene solutions only, the PL lifetime of the compounds 2 and 3 increased from 4.8 ns to 7.2 ns up to 5.9 ns and 8.9 ns, and from 0.3 ns to 4.5 ns up to 0.9 ns and 5.4 ns, respectively. The similar emission lifetime of both air-equilibrated and degassed toluene solutions observed for compound 1 showed that the emission originated exclusively from local excitation fluorescence (1LE). At low temperature (77 K), photoluminescence spectra of the de rivatives in THF solutions remain similar to their respective room tem perature spectra (Fig. 2 b, d, f). The triplet energy values determined from the onset of the phosphorescence spectra of the compounds 1–3 range from 2.65 to 3.02 eV. The highest triplet energy was observed for tetrahydrocarbazolyl-substituted benzophenone (1). The experimental ΔEST values the dilute THF solutions of 2 and 3 are 0.57 eV and 0.49 eV, respectively. The relatively high ΔEST values of 2 and 3 indicate the forbidding for efficient reverse intersystem crossing of the triplet exci tons in THF solution (Table 3). The meta-linkage in the molecular structure crucially suppresses the π-conjugation which results in a better separation of HOMO and LUMO and much stronger charge-transfer character leading to a smaller singlet triplet splitting. In this case the position of the donor affects the amount of π-π* and n-π* mixing and the charge-transfer strength in the molecules [21]. Fluorescence spectra of thin films of compounds 1–3 are broad and unstructured (Fig. 2 b,d,f). The 49 nm and 26 nm red-shifts of the emission spectra of 1 and 2 (PLQY values of 20% and 15% respectively) with respect to the spectra of the solutions may be caused by
2.5. Device performance Compound 3 as TADF emitter may find applications in efficient nondoped OLEDs due to its TADF and AIEE properties. On the other hand, efficient doped OLEDs based on compound 3 as TADF host may be constructed taking into account its HOMO/LUMO and triplet energy levels. To prove these considerations, following experiments were pro vided. The device configuration, shown in Fig. 4, contained a 50 nm layer of 1,1-bis[N,N-di(4-tolyl)aminophenyl]cyclohexane (TAPC), a 10 nm layer of 1,3-bis(N-carbazolyl)benzene (mCP), a 30 nm EML, a 50 nm layer of bathophenanthroline (Bphen), a 1 nm layer of lithium fluoride (LiF), and a 100 nm layer of aluminum (Al). The layers of TAPC, mCP, 5
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system is also plotted in Fig. 4. In particular, the 7% Ir(piq)2acac doping ratio was selected taking into account the electroluminescent spectra of devices (Fig. S8). Absence of emission from compound 3 showed the sufficient energy transfer from compound 3 to Ir(piq)2acac. Fig. 5 shows the EO properties of TADF and phosphorescent OLEDs. Their characteristics such as driving voltage, maximum current effi ciency (CEmax), maximum power efficiency (PEmax), maximum external quantum efficiency (EQEmax), CIE 1931 color coordinates are summa rized in Table 4. Fig. 5a displays J-V and L-V curves. Phosphorescent device exhibited the favourable J-V and L-V performances, indicating that Ir(piq)2acac benefited from charge carrier transport, carrier recombination, exciton formation and energy transfer. Fig. 5b illustrates efficiency performances of OLEDs at various current densities. The de vice with pristine compound 3 as EML showed higher CE and PE values at low current density, but efficiency roll-off increased with increasing current density. Its CEmax of 11.8 cd/A, PEmax of 9.3 lm/W and EQEmax of 1.93% were recorded at a very low current density of 60 μA/cm2. By doping PXZ-TRZ with compound 3 as the EML, an efficient green TADF device showed a CEmax of 31.1 cd/A, PEmax of 28.7 lm/W and EQEmax of 7.6%. In particular, this device performed the highest brightness closed to 30,000 cd/m2 at 10 V, the greater brightness is prospective after in crease the driving voltage. The ultimate red phosphorescent device exhibited a favourable efficiency performance with a small efficiency roll-off and a relatively higher EQE of 8.6. The intrinsic TADF emission
Fig. 4. OLED structure with compound 3 for TADF and phosphorescent emis sion mechanisms.
Bphen, LiF and Al were respectively deposited as 1st hole transporting layer, 2nd hole transporting layer, electron transporting layer, electron injection layer, and cathode. In the EML, the TADF and phosphorescent emission mechanisms were investigated using a pristine compound 3 layer and two doped layers respectively with a 10% and 7% doping ratio of green TADF emitter 10-(4-(4,6-diphenyl-1,3,5-triazin-2-yl)phenyl)10H-phenoxazine (PXZ-TRZ) and red phosphorescent emitter bis(1phenyl-isoquinoline)(acetylacetonate)Iridium(III) (Ir(piq)2acac). The energy transfer diagram of these three emission mechanisms in EML
Fig. 5. (a) J-L-V curves, (b) efficiency-current density curves, (c) emission spectra at 8 V, (d) TREL signals of intrinsic, TADF and phosphorescent devices. Table 4 Characteristics of intrinsic, TADF and phosphorescent devices. Device
Driving voltage (V)a
Luminance (nits)b
CEmax (cd A 1)
PEmax (lm W 1)
EQEmax (%)c
CIE(x, y)c
Decay time (μs)
Intrinsic TADF Phosphorescent
7.56 5.82 5.19
114 5775 1094
11.8 31.1 5.6
9.3 28.7 5.3
1.93 7.60 8.60
(0.37, 0.57) (0.38, 0.58) (0.68, 0.32)
3.21 2.82 2.75
a b c
Operation voltage recorded under current density of 10 mA/cm.2. Luminance recorded under driving voltage is 6 V. CIE coordinates at EQEmax. 6
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of compound 3 was green with a peak at 536 nm (Fig. 5c). It was very close to PL emission peak recorded at 534 nm (Fig. 2f). Low EQE of the green TADF device with pristine compound 3 can be explained the low solid-state PLQY (Table 3). It also implies that most of exciton energy was hindered by the singlet emission. The emission spectrum of PXZTRZ device was very similar to that of compound 3, showing a nar rower emission band. Notably, this PXZ-TRZ device exhibited very efficient energy transfer from host to TADF emitter no mater singlet and triplet excitons. Additionally, in phosphorescent device, the red emitter Ir(piq)2acac was added to transform more exciton energy to the emis sion, which was red with a peak at 628 nm. The implication in energy transfer of excitons can be also deduced from the transient electrolu minescent (TREL) signals of these three devices shown in Fig. 5d and their fitting exponential decay time are shown in Table 4. The TADF device with the EML of pristine compound 3 exhibited longer expo nential decay time of approximately 3.21 μs, belonging to the delayed fluorescence. Obviously, employing the PXZ-TRZ and Ir(piq)2acac dopant of EML, the shorter exponential decay time of ca. 2.82 and 2.75 μs was observed in phosphorescent device TREL signal. This observation shows, that the blind exciton energy of compound 3 is effectively transferred to PXZ-TRZ and Ir(piq)2acac.
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3. Conclusions We developed a series of derivatives of meta-substituted benzophe none bearing tetrahydrocarbazole, phenoxazine or 2,7-ditert-butyl-9,9dimethylacridine donor moieties. The derivatives showed improved HOMO and LUMO separation compared to that of the corresponding derivatives with para-linkage which lead to the minimized singlet-triplet energy gap of ca. 0.04 eV, and the emission energy spanning widely in UV - (deep) blue region (2.65–3.46 eV). Ionization potentials of the solid samples of the derivatives were found to be in the range of 5.57–5.72 eV. Photoluminescence quantum yields of non-doped solid films of the compounds ranged from 15% to 20% which were up to 7-fold higher than those observed for the solutions in low-polarity solvent toluene. Acridan-based emitter showed a combination of aggregation induced emission enhancement and delayed fluorescence. By employing the layer of this emitter as non-doped emissive layer in OLED, a green device with EQE of 1.93% was obtained with the CIE coordinates of (0.37, 0.57). The more efficient triplet population employment was achieved after doping this acridan-substituted benzophenone derivative with red emitter bis(1-phenyl-isoquinoline)(acetylacetonate)Iridium(III). Red phosphorescent device exhibited a favourable efficiency performance with a small efficiency roll-off and a relatively higher EQE of 8.6%. External quantum efficiency of 7.60% was achieved for green device based on host:guest system using emitter 10-(4-(4,6-diphenyl-1,3,5-tri azin-2-yl)phenyl)-10H-phenoxazine which exhibited thermally acti vated delayed fluorescence. Declarations of competing interest None. Acknowledgment This project has received funding from European Social Fund (proj ect No 09.3.3-LMT-K-712-01-0140) under grant agreement with the Research Council of Lithuania. P.-H. Lee, T.-L. Chiu and J.-H. Lee aknowledge Taiwanese Ministry of Science and Technology for research funding (Grant No. MOST 106-2923-E-155-002-MY3S). Special thanks goes to dr. Jurate Simokaitiene for DSC and TGA measurements and Matas Guzauskas for the help with photophysical measurements. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. 7
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