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Unconventional molecular design approach of high efficiency deep blue thermally activated delayed fluorescent emitters using indolocarbazole as an acceptor Jeong-A Seo, Yirang Im, Si-Hyun Han, Chil Won Lee, and Jun Yeob Lee ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b09351 • Publication Date (Web): 05 Oct 2017 Downloaded from http://pubs.acs.org on October 6, 2017

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ACS Applied Materials & Interfaces

Unconventional molecular design approach of high efficiency deep blue thermally activated delayed fluorescent emitters using indolocarbazole as an acceptor Jeong-A Seo1+, Yirang Im1+, Si Hyun Han1, Chil Won Lee2, Jun Yeob Lee2* 1

School of Chemical Engineering, Sungkyunkwan University 2066, Seobu-ro, Jangan-gu, Seobu-ro, Suwon, Gyeonggi, 440-746, Korea 2 Department of Chemistry, Dankook University Cheonan, Chungcheongnam-do, Korea +Jeong-A Seo and Yirang Im contributed equally E-mail: [email protected] Keywords quantum efficiencydelayed fluorescenceindolocarbazoleblue deviceacceptor

Abstract Unconventional blue thermally activated delayed fluorescent emitters having an electron donating type indolocarbazole as an acceptor were developed by attaching carbazolylcarbazole or acridine donors to the indolocarbazole acceptor. Three compounds were derived from the indolocarbazole acceptor. The indolocarbazole-acridine combined products showed efficient delayed fluorescent behavior and a high quantum efficiency of 19.5% with a color coordinate of (0.15, 0.16) when they were evaluated as thermally activated delayed fluorescent emitters in deep blue fluorescent devices. This is the first demonstration of the use of electron donating carbazole-derived moieties as efficient acceptor units of blue thermally activated delayed fluorescent emitters.

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Introduction Delayed fluorescent emitters can provide higher quantum efficiency (QE) than conventional fluorescent emitters because of the supplementary contribution of triplet excitons to the fluorescent emission process by triplet-triplet fusion or by triplet up-conversion. The triplet up-conversion approach, which is also known as thermally activated delayed fluorescence (TADF), was used to maximize the triplet exciton usage in the fluorescent emission process.16

Therefore, recent research has focused on developing TADF emitters by molecular

engineering of the backbone structure and functional moieties within the molecules. Typical designs of TADF emitters involve coupling donor and acceptor moieties with or without aromatic connectors linking the two moieties.7-10 The donor and acceptor-type designs allow the emitters to have a small singlet-triplet energy gap for triplet up-conversion. Therefore, efficient TADF emission was observed for interconnected TADF molecules possessing both donor and acceptor moieties.11-14 The donor moieties are commonly carbazole or acridine derivatives due to their strong electron donating character,1, 9, 15-17 while triazine, cyanobenzene, and diphenylsulfone are used as acceptor moieties due to their strong electron accepting character.4, 10, 18-20 Electron deficient moieties derived from mostly sp2 N and sp2 O were used in the molecular design of the acceptor moieties in TADF emitters. Triazine, pyrimidine, and pyridine derivatives are examples of sp2 N-based acceptor moieties.13, 18 Other than the sp2 N-based organic moieties mentioned above, carbazole is another chemical moiety with the sp2 N unit in the molecular structure. Unlike the sp2 N units in other heteroaromatic moieties, the non-bonding electrons of the sp2 N in the carbazole are shared due to the aromatic nature of the carbazole unit. Nevertheless, carbazole-derived moieties have been known to have a weak electron accepting character. For instance, many carbazolederived compounds such as 1,3-bis(N-carbazolyl)benzene (mCP), 4,4′-bis(N-carbazolyl)-1,1′2 ACS Paragon Plus Environment

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biphenyl (CBP), and 3,3-di(9H-carbazol-9-yl)biphenyl (mCBP) showed moderate electron transport properties.21-23 However, carbazole-based moieties could not be used as an acceptor in the molecular design of TADF emitters because the acceptor character is not as strong as that of other sp2 N-based heteroaromatic moieties. Even so, carbazole-derived moieties have potential for use as an acceptor unit if the non-bonding electrons of N are further shared by other aromatic units. In this work, we report a novel molecular design having a carbazole-derived moiety (i.e., indolocarbazole) as an acceptor unit in the molecular structure of TADF emitters. Three indolocarbazole acceptor-derived emitters were synthesized, and a design criterion was proposed for the indolocarbazole-based delayed fluorescent emitters. TADF emitters having an indolocarbazole acceptor-based molecular structure provided a high QE of 19.5% with a deep blue color coordinate of (0.15,0.16), which was comparable to that of the TADF emitters having typical acceptors. This work is the first demonstration of using an electron donating type carbazole-derived moiety as the acceptor in the molecular design of TADF emitters.

Results and discussion As described in the introduction, carbazole-derived moieties can also function as the acceptor moiety of the TADF emitter if the electron accepting character of the carbazole-derived unit is strengthened by proper molecular design. We hypothesized that the electron accepting character of the carbazole unit can be strengthened by expanding the aromatic structure of carbazole, and our approach in this work was to introduce highly aromatic indolocarbazole to expand the molecular structure. There have been several indolocarbazole moieties, but indolo[3,2,1-jk]carbazole (ICz) was selected as the electron accepting indolocarbazole moiety because only one nitrogen is included in the chemical structure.24-28 Non-bonding electrons in 3 ACS Paragon Plus Environment


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the nitrogen atom are shared by three aromatic units in ICz, indicating that the total electron density in the ICz unit would be less than that of other indolocarbazole units. The electron accepting character can be estimated based on the energy level of the lowest unoccupied molecular orbital (LUMO). Therefore, the LUMO values of several indolocarbazole moieties were calculated using the Gaussian 09 molecular simulation program. Table 1 shows the highest occupied molecular orbital (HOMO), LUMO, and the molecular structures of indolocarbazole moieties calculated using the B3LYP 6-31G basis set. The LUMO of the ICz moiety was deeper than that of other indolocarbazole moieties as shown in the molecular simulation results, and there was a LUMO level difference of about 0.25-0.57 eV between ICz and other indolocarbazole moieties. This implies that the ICz moiety has better electron accepting power than other indolocarbazole moieties. The increased electron accepting power was accompanied by weakened electron donating power, hypothesized to be due to the deeper HOMO level. Motivated by the molecular calculation results suggesting improved electron accepting properties of the ICz moiety, we designed three emitters with the ICz acceptor moiety by attaching different donor moieties to the ICz moiety. The chemical structures and synthetic processes of 2-(9H-[3,9'-bicarbazol]-9-yl)indolo[3,2,1-jk]carbazole (ICzCz), (9,9dimethylacridin-10(9H)-yl)indolo[3,2,1-jk]carbazole (ICzAc), and 2,5-bis(9,9dimethylacridin-10(9H)-yl)indolo[3,2,1-jk]carbazole (ICzDAc) emitters are presented in Scheme 1. ICzCz and ICzAc were prepared from 2-bromoindolo[3,2,1-jk]carbazole, while ICzDAc was prepared from 2,5-dibromoindolo[3,2,1-jk]carbazole by a Pd-catalyzed coupling reaction. Wet and dry purification of the synthesized crude products yielded pure final compounds identified by structural characterization using nuclear magnetic resonance spectroscopy and mass spectroscopy.


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The electronic molecular orbitals of the three emitters were calculated because the HOMO and LUMO distribution is critical to understanding the TADF behavior of the emitters. In the molecular orbital pictures in Figure 1, the HOMO and LUMO were dominated by the donor moiety and ICz moiety, respectively. The LUMO was found only in the ICz moiety, and no electron cloud was observed in the donor moieties. This is caused by the electron accepting properties of the ICz moieties, as described in the HOMO/LUMO picture of the ICz moiety. The HOMO essentially dispersed in the donor moiety. In the ICzAc and ICzDAc, the HOMO and LUMO showed weak overlap in the ICz moiety because of the perpendicular orientation of the acridine unit to the ICz plane due to steric hindrance. The molecular orbital picture confirmed that the ICz moiety behaved as an acceptor when it was combined with the acridine and carbazolylcarbazole donors. The simulation results suggest that HOMO/LUMO separation occurred for small singlettriplet energy splitting (EST). Therefore, the singlet and triplet energy of the emitters was measured by analyzing the photoluminescence (PL) emission of solid films of the emitters dispersed in polystyrene at a doping concentration of 1%. The onset points of the room and low temperature PL emission spectra in Figure 2 were used to calculate the singlet and triplet energy because the origin of the singlet emission is charge transfer (CT) emission, as manifested by the solvatochromic effect at different solvents with dissimilar polarity (Figure 3). In the case of phosphorescence, ICzCz and ICzAc showed emission spectra mainly from low triplet excited state, but ICzDAc exhibited emission from CT triplet excited state. This may be due to strong donor-acceptor character of ICzDAc due to two acridine donor units. Measured singlet and triplet energy values of the three emitters are shown in Table 2. These results suggest that ICzAc and ICzDAc might be better than ICzCz as a TADF emitter because of their small EST. The small EST values of ICzAc (0.20 eV) and ICzDAc (0.17 eV) emitters were attributed to the strong electron donating power of the acridine donor, 5 ACS Paragon Plus Environment


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while the relatively large EST of ICzCz was because of the relatively weak electron donating power of carbazolylcarbazole, which also increases the singlet energy of ICzCz. The order of singlet energy was ICzCz (3.27 eV)>ICzAc (3.10 eV)>ICzDAc (3.05 eV). The strong CT character introduced by the acridine moiety decreased the singlet energy of ICzAc and ICzDAc. We expected ICzAc and ICzDAc to be efficient TADF emitters based on the EST and singlet energy. Ultraviolet-visible (UV-vis) absorption spectra of the TADF emitters were added in Figure 4. In the UV-vis absorption spectra, main absorption peaks appeared at a wavelength below 330 nm and weak absorption peaks were additionally observed at a long wavelength above 330 nm. TADF character of the ICzCz, ICzAc, and ICzDAc emitters was analyzed by transient PL experiment of the compounds (Figure 5). The ICzCz emitter showed only fast decaying prompt fluorescence component without any delayed component, while ICzAc and ICzDAc showed strong delayed component in addition to the prompt component. This suggests that the ICzAc and ICzDAc are TADF emitters which can convert triplet excitons to singlet excitons by up-conversion process. The delayed fluorescence lifetimes of ICzAc and ICzDAc were 9.86, and 8.46 s, respectively. It was rather short in the ICzDAc emitter by contribution of two dimethylacridine donors which increased the donor strength. The TADF character and blue PL emission of the ICzCz, ICzAc, and ICzDAc emitters motivated the development of high efficiency blue TADF devices. The three emitters were doped in a common bis{2-[di(phenyl)phosphino]phenyl}ether oxide (DPEPO) host to harvest blue singlet and triplet excitons for singlet radiative transition. The triplet energy of the DPEPO host was over 3.0 eV, which is appropriate for the three blue emitters.29 Current density (J) and luminance (L) data of the blue TADF devices doped with ICzCz, ICzAc, and ICzDAc emitters were recorded as a function of driving voltage (Figure 6). The J of the blue TADF devices was correlated with the HOMO level of the emitters because the 6 ACS Paragon Plus Environment

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major hole carrying pathway in the DPEPO-based emitting layer is the emitter rather than the DPEPO host due to the deep HOMO level (-6.81 eV) of DPEPO.30-31 Electrochemical measurement data (Figure 7) showed that the HOMO/LUMO levels of ICzCz, ICzAc, and ICzDAc were -5.89/-2.87, -5.82/-2.79, and -5.82/-2.82 eV, respectively, from the onset voltage of the oxidation and reduction scan data. Therefore, hole injection was facilitated in the ICzAc and ICzDAc devices due to the shallow HOMO level, while it was retarded in the ICzCz device because of the relatively deep HOMO level of the ICzCz emitter as shown in the energy level diagram of the blue TADF devices in Figure 8. The hole only devices of emitting layer were also fabricated to compare the hole density in the blue TADF devices. The hole current density-voltage curves of ICzCz, ICzAc, and ICzDAc hole only devices were presented in Figure 9. The hole current density of ICzAc and ICzDAc devices was higher than that of ICzCz device, confirming that the high J is due to high hole current density. The J values of the blue devices dictated the L of the devices; therefore, the same trend was evident for L as a function of voltage. QE data of the blue TADF devices are plotted as a function of L in Figure 10. Maximum QEs of the ICzCz, ICzAc, and ICzDAc devices were 2.3, 13.7, and 19.5%, respectively. These values correlated well with the delayed fluorescent performance of the three emitters. PL quantum yield was measured in film with oxygen and nitrogen bubbling. Although the PL quantum yield of ICzCz, ICzAc, and ICzDAc doped film was 0.40, 0.32, and 0.49 under oxygen, the PL quantum yield was increased to 0.53, 0.95, and 0.96 under nitrogen. The improvement of PL quantum yield under nitrogen is large, which demonstrates that triplet excitons of ICzAc and ICzDAc are related to the PL emission by TADF process. The PLQYs of the prompt fluorescence/delayed fluorescence were 0.40/0.13, 0.32/0.63, and 0.49/0.47, respectively. The efficient delayed fluorescence behavior of the ICzAc and ICzDAc emitters also projected from the transient PL decay (Figure 5) and small EST, resulting in the high 7 ACS Paragon Plus Environment


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QEs of 13.7% and 19.5% in the ICzAc and ICzDAc devices, respectively. However, the poor delayed fluorescence character due to the large EST in the ICzCz emitter resulted in the low QE of 2.3% in the ICzCz device. Therefore, the strong acridine donor was more effective than the relatively weak carbazolylcarbazole donor in harvesting triplet excitons via the triplet to singlet up-conversion process. The QE was optimized at 10% doping concentration and it was reduced at high doping concentration by concentration quenching effect (Table 3). The QE of the ICz acceptor-derived blue TADF devices was comparable to that of the state of the art deep blue TADF devices, indicating that the ICz acceptor can be a substitute for well-known acceptors such as diphenylsulfone and benzophenone.32-33 The deep blue emission of the ICzCz, ICzAc, and ICzDAc devices was identified in the electroluminescence (EL) spectra of the blue TADF devices in Figure 11. Peak wavelengths of the EL spectra of the ICzCz, ICzAc, and ICzDAc devices were 416, 454, and 462 nm, respectively. The main emission peak appeared in the deep blue emission range of the emission spectra. Color coordinates of the ICzCz, ICzAc, and ICzDAc devices were (0.17, 0.04), (0.15, 0.09), and (0.15, 0.16), respectively. Deep blue emission color and high QE were simultaneously achieved in the ICzDAc device. For the ICzAc device, a very deep blue emission color with a y color coordinate below 0.10 was demonstrated in addition to the high QE of 13.7%. The color coordinate and QE of the ICzDAc device were similar to those of the state of the art pure deep blue 9-([1,1'-biphenyl]-3-yl)-N,N,5,11-tetraphenyl-5,9-dihydro5,9-diaza-13b-boranaphtho [3,2,1-de]anthracen-3-amine (DABNA-2) device.34 Full width at half maximum (FWHM) values of the ICzCz, ICzAc, and ICzDAc devices were 49, 56, and 58 nm, respectively. The FWHM of the ICz-derived emitters was below 60 nm. This low FWHM was attributed to the rigid planar structure of the ICz acceptor, which restricts molecular motion and vibration of the molecules. The FWHM values were correlated with donor strength because the latter determines the degree of charge transfer. Therefore, the ICz 8 ACS Paragon Plus Environment

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acceptor effectively induced TADF behavior, increased QE, and reduced the FWHM of TADF emitters. Detailed device data are summarized in Table 3.

Conclusions In conclusion, novel deep blue TADF emitters having a carbazole type ICz as an acceptor were developed by attaching carbazolylcarbazole or acridine donors to the ICz acceptor. Three compounds were derived from the ICz acceptor, and the acridine-modified ICzDAc emitter showed a deep blue color coordinate of (0.15, 0.16) with a high QE of 19.5% and a FWHM of 58 nm. Moreover, another acridine-modified ICzAc exhibited a very deep blue color coordinate of (0.15, 0.09) with a high QE of 13.7% and a small FWHM of 56 nm. This is the first demonstration of using an electron donating carbazole-derived moiety as a high efficiency acceptor unit in deep blue TADF emitters. Overall, our data show that the indolocarbazole type ICz moiety can be effectively used as an acceptor in deep blue TADF materials.

Experimental General information 2-Bromoindolo[3,2,1-jk]carbazole and 9H-3,9'-bicarbazole were purchased from INCO Co. Potassium-tert-butoxide was purchased from Tokyo Chemical Industry Co.,LTD. 9,9Dimethyl-9,10-dihydroacridine, tris-tert-butylphosphine and palladium acetate were purchased from P&H Co. These chemical were used without further purification. Toluene was distilled over sodium and calcium hydride. General analysis method of the compounds is described in previous work.15,35



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Synthesis 2-(9H-[3,9'-bicarbazol]-9-yl)indolo[3,2,1-jk]carbazole (ICzCz) 2-Bromoindolo[3,2,1jk]carbazole(1 g, 3.12 mmol), 9H-3,9'-bicarbazole(0.85 g, 2.62 mmol), palladium acetate(0.21 g, 0.94 mmol), tris-tert-butylphosphine(0.95 g, 4.69 mmol), potassium-tertbutoxide(0.53 g, 4.69 mmol) were added in a two-neck round-bottomed flask filled with a dry toluene under a nitrogen atmosphere. The mixture was refluxed for 12h and then washed distilled water and methylene chloride. The mixture was purified by column chromatography on silica gel using a mixed eluent of ethyl acetate and n-hexane (1:4). A highly pure product was obtained in 42% yield. Tg : 103 oC Tm : 327 oC Td : 357 oC (5% weight loss) 1H NMR (400 MHZ, CDCl3): δ 8.36 (s, 1H), 8.27 (s, 1H), 8.19 (d, 5H, J=22.80 Hz), 8.01 (d, 2H, J=8.00 Hz), 7.66 (t, 2H, J=7.80 Hz), 7.55 (s, 2H), 7.52-7.41 (m, 8H), 7.38-7.29 (m, 3H). C NMR (100 MHZ, CDCl3): δ(ppm): 143.31, 143.06, 142.10, 141.74, 139.55, 132.57,

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129.84, 127.77, 126.81, 125.99, 125.69, 124.19, 123.72, 123.25, 122.84, 122.40, 120.71, 120.41, 120.30, 119.97, 119.70, 119.44, 112.65, 111.04, 110.37, 110.01 MS (APCI) m/z 572.1857 [(M+H)+]. Anal. Calcd for C42H25N3: C, 88.24; H, 4.41; N, 7.35. Found: C, 88.24; H, 4.44; N, 7.29.

2-(9,9-Dimethylacridin-10(9H)-yl)indolo[3,2,1-jk]carbazole (ICzAc) ICzAc was synthesized according to the procedure used for the synthesis of ICzCz except that 9,9dimethyl-9,10-dihydroacridine was used instead of 9H-3,9'-bicarbazole. Eluent of the column chromatography was a mixed solvent of methylene chloride and n-hexane (1:4) and synthetic yield was 51%.


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Tg : 168 oC Tm : 299 oC Td : 452 oC (5% weight loss) 1H NMR (400 MHZ, CDCl3): δ (ppm): 8.10 (d, 2H, J=7.60 Hz), 7.99 (t, 4H, J=9.20 Hz), 7.61 (t, 2H, J=7.80 Hz), 7.53-7.44 (m, 2H), 7.38 (t, 2H, J=7.60 Hz), 6.95-6.88 (m, 4H), 6.33-6.28 (m, 2H), 1.78 (s, 6H). C NMR (100 MHZ, CDCl3): δ (ppm): 142.99, 142.11, 139.37, 136.51, 130.11, 129.96,

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127.51, 126.45, 125.42, 123.67, 123.16, 122.28, 120.56, 120.30, 114.64, 112.56, 36.22, 31.82. MS (APCI) m/z 657.5500 [(M+H)+]. Anal. Calcd for C33H24N2: C, 88.36; H, 5.39; N, 6.25. Found: C, 87.89; H, 5.50; N, 6.32.

2,5-Bis(9,9-dimethylacridin-10(9H)-yl)indolo[3,2,1-jk]carbazole (ICzDAc) ICzDAc was synthesized according to the procedure used for the synthesis of ICzCz except that 9,9dimethyl-9,10-dihydroacridine was used instead of 9H-3,9'-bicarbazole and starting material was 2,5-dibromoindolo[3,2,1-jk]carbazole(0.6 g, 1.50 mmol) instead of 2-bromoindolo[3,2,1jk]carbazole. Eluent of the column chromatography was a mixed solvent of methylene chloride and n-hexane (1:1) and synthetic yield was 48%. Tg: no detection Td : 448 oC (5% weight loss) 1H NMR (400 MHZ, CDCl3): δ 8.23 (d, 1H, J=8.40 Hz), 8.16-8.02 (m, 5H), 7.68 (t, 1H, J=7.60 Hz), 7.60 (d, 1H, J=10.40 Hz), 7.53-7.43 (m, 5H), 7.00-6.91 (m, 8H), 6.41(d, 2H, J=9.60 Hz), 6.35-6.30 (m, 2H), 1.76 (d, 12H, J=10.40 Hz). C NMR (100 MHZ, CDCl3): δ (ppm): 143.55, 142.04, 141.46, 139.31, 138.58, 136.89,

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135.53, 132.21, 130.54, 130.25, 130.19, 129.99, 127.79, 126.75, 126.55, 126.45, 125.42, 125.39, 123.86, 123.58, 122.71, 120.78, 120.65, 119.86, 114.57, 114.43, 114.29, 112.63, 36.20, 36.18, 31.64, 31.48



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MS (APCI) m/z 656.5500 [(M+H)+]. Anal. Calcd for C48H37N3: C, 87.91; H, 5.69; N, 6.41. Found: C, 87.16; H, 5.80; N, 6.44.

Device fabrication TADF OLEDs with a device structure of ITO (50 nm)/PEDOT:PSS (60 nm)/TAPC (4,4′Cyclohexylidenebis[N,N-bis(4-methylphenyl)benzenamine]) (20 nm)/mCP (1,3-Bis(Ncarbazolyl)benzene) (10 nm)/DPEPO:TADF dopant (25 nm)/TSPO1 (diphenyl-4triphenylsilylphenyl-phosphineoxide) (5 nm)/TPBi (2,2′,2"-(1,3,5-Benzinetriyl)-tris(1phenyl-1-H-benzimidazole)) (30 nm)/LiF (1.5 nm)/Al (200 nm). TADF dopant materials were ICzAc, ICzCz and ICzDAc doped at a doping concentration of 10%. Hole only devices were fabricated to compare hole current density dependence on the TADF emitters. The hole only device structure was ITO/PEDOT:PSS (60 nm)/TAPC (20 nm)/mCP (10 nm)/DPEPO:TADF dopant (25 nm)/TAPC (5 nm)/Al (200 nm).

Measurements Basic chemical analysis and photophysical characterization methods are described in supporting information.33,36 . Transient PL and time resolved PL data were obtained using an optical measurement system equipped with a pulsed Nd-YAG laser (355 nm) and an intensified charge-coupled device (ICCD) detector. Cyclic voltametry (CV) measurement of organic compounds was carried out in acetonitrile solution with tetrabutylammonium perchlorate at 0.1 M concentration. Ag/AgCl was used as the reference electrode and Pt was the counter electrode. Ferrocene was used as the standard material.

Acknowledgements


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This work was supported by Basic Science Research Program (NRF-2016R1A2B3008845) through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT, and Future Planning.

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List of tables Table 1. Molecular structure and HOMO and LUMO distribution of the several indolocarbazole moieties calculated by Gaussian software using a basis set of B3LYP 6-31G basis set. Table 2. Photophysical properties of ICzCz, ICzAc and ICzDAc emitters. Table 3. Summarized device performances of ICzCz, ICzAc and ICzDAc devices.


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Table 1

Donor

Chemical structure

Molecular orbital distribution HOMO LUMO

5,12dihydroindolo[3,2a]carbazole 5,11dihydroindolo[3,2b]carbazole 5,7-dihydroindolo[2,3b]carbazole 11,12dihydroindolo[2,3a]carbazole indolo[3,2,1jk]carbazole


HOMO [eV]

LUMO [eV]

-5.11

-0.68

-4.91

-1.00

-5.04

-0.71

-5.10

-0.79

-5.56

-1.25


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Table 2 λabs (nm)a

λPL (nm) solb / filmc

ES / ET (eV)

HOMO / LUMO (eV)

ΦF / ΦTADF / ΦPLf

241, 267, 288, 400 / 400 3.27 / 2.89 -5.89 / -2.87 0.40d / 0.13/ 0.53e 344 244, 269, 278, ICzAc 439 / 441 3.10 / 2.90 -5.82 / -2.79 0.32d / 0.62/ 0.95e 288, 368 245, 271, 279, ICzDAc 449 / 446 3.05 / 2.88 -5.82 / -2.82 0.49d / 0.47/ 0.96e 289, 367 a measured in THF solution; bmeasured in toluene solution; c1 wt%-doped film in a PS; dunder ICzCz

oxygen; eunder nitrogen ; f10 wt%-doped film in DPEPO ; ΦF : PLQY of prompt fluorescence ; ΦTADF : PLQY of thermally activated delayed fluorescence

18

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Table 3

Devices

ICzCz

ICzAc

ICzDAc a

Doping concentration (%)

Voltagea (V)

Current densitya (mA/cm2)

Color coordinate (x, y)

EQEmax (%)

CEmax (cd/A)

PEmax (lm/W)

10 20 30 10 20 30 10 20 30

11.0b 8.9 8.5 5.5 4.8 4.6 4.8 4.5 4.5

17.5b 17.1 17.6 1.9 1.1 1.2 0.5 0.4 0.5

(0.17, 0.04) (0.20, 0.08) (0.19, 0.08) (0.15, 0.09) (0.15, 0.13) (0.15, 0.16) (0.15, 0.16) (0.15, 0.20) (0.15, 0.22)

2.3 1.8 1.6 13.7 12.7 8.7 19.5 17.3 14.0

0.7 0.6 0.8 11.8 13.9 11.0 24.7 25.7 22.0

0.5 0.5 0.7 10.6 14.1 9.9 22.2 23.0 20.0

at 100 cd/m2 bat 86.8 cd/m2

19



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List of figures Scheme 1. Synthetic scheme of ICzCz, ICzAc, and ICzDAc. Figure 1. HOMO and LUMO distribution of ICzCz, ICzAc, and ICzDAc. Figure 2. Solid PL and low temperature PL spectra of ICzCz, ICzAc, and ICzDAc Figure 3. Solution PL spectra of ICzCz (a), ICzAc (b), and ICzDAc (c) in various solvents. Figure 4. UV-vis absorption spectra of ICzCz, ICzAc, and ICzDAc. Figure 5. Transient PL decay curves of TADF emitters doped in the DPEPO host at a doping concentration of 10%. Figure 6. Current density-voltage-luminance plot of the blue TADF devices. Figure 7. CV scan data of IczAc (a), IczCz (b), and IczDAc (c). Figure 8. Device structure and energy level diagram of blue TADF devices. Figure 9. Current density-voltage curves of hole only devices of the TADF emitter doped DPEPO emitting layer. Figure 10. Quantum efficiency-luminance plot of the blue TADF devices. Figure 11. EL spectra of the blue TADF devices.

20


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Scheme 1

21



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Page 22 of 33

Figure 1

ICzCz

ICzAc

ICzDAc

LUMO

HOMO

Optimized Geometry

67°

90°

22

90°


90°

Page 23 of 33

Figure 2

ICzCz (Solid PL)

1

ICzCz (Low T PL)

Intensity (arb. unit)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


ICzAc (Solid PL)

0.8

ICzAc (Low T PL) ICzDAc (Solid PL)

0.6

ICzDAc (Low T PL)

0.4 0.2 0 350

450

550

650

Wavelength (nm)

23



Figure 3


(a)

700 600

CycloHex

500

Toluene

400

THF

300 200 100

0 350

450

550

650

Wavelength (nm)

(b)

800


700

CycloHex

600 Toluene

500 THF

400 300 200 100 0 350

450

550

650

Wavelength (nm)

(c) Intensity (arb. unit)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

700 600

CycloHex

500

Toluene

400

THF

300 200

100 0 350

450

550

650

Wavelength (nm)

24


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Figure 4

1

ICzCz ICzAc

0.8

ICzDAc

Intensity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.6

0.4

0.2

0 200

250

300

350

400

450

Wavelength (nm)

25



Figure 5

1

ICzAc ICzCz

ICzDAc

0.1

Intensity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

0.01

0.001

0.0001

0

30

60

90

120

150

180

Second (μs)

26


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Figure 6 40

10000

ICzCz

1000

ICzAc

30

100

ICzDAc

25

10 20 1 15 0.1

10

Luminance (cd/m2)

35

Current density (mA/cm2)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.01

5 0

0.001 0

2

4

6

8

10

Voltage (V)

27



Figure 7

Current (arb. unit)

(a)

-4

-3

-2

-1

0

1

2

3

1

2

3

1

2

3

Voltage (V) Current (arb. unit)

(b)

-4

-3

-2

-1

0

Voltage (V) (c)

Current (arb. unit)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

-4

-3

-2

-1

0

Voltage (V)

28


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Figure 8

-2.4

-2.51 -2.87

-2.79

-2.52

-2.8

-2.82

LiF/Al

-2.0

ICzDAc

-5.82

-5.82

TPBi

ICzAc

-5.89

TSPO1

ICzCz

-5.1

mCP

-5.1

TAPC

PEDOT:PSS

-2.9

ITO

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


-5.5 -6.1

-6.1

-6.81

-6.79

29



Figure 9

100

Current density (mA/cm2)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ICzCz

80

ICzAc ICzDAc

60

40

20

0

0

2

4

6

8

10

Voltage (V)

30


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Figure 10

ICzCz

100

Quantum efficiency (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


ICzAc ICzDAc

10

1

0.1 1

10

100

Luminance

1000

(cd/m2)

31



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 11

32


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Table of Contents

ICzCz

100

Quantum efficiency (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


ICzAc ICzDAc

10

1

0.1 1

10

100

Luminance (cd/m2)

33


1000

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