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Isomeric Thermally Activated Delayed Fluorescence Emitters for Color Purity-Improved Emission in Organic Light-Emitting Devices Dong-Yang Chen, Wei Liu, Cai-Jun Zheng, Kai Wang, Fan Li, Silu Tao, Xue-Mei Ou, and Xiao-Hong Zhang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b03954 • Publication Date (Web): 14 Jun 2016 Downloaded from http://pubs.acs.org on June 14, 2016

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ACS Applied Materials & Interfaces

Isomeric Thermally Activated Delayed Fluorescence Emitters for Color Purity-Improved Emission in Organic Light-Emitting Devices Dong-Yang Chen,†,‡ Wei Liu,‡,§ Cai-Jun Zheng,*,‡,ǁ Kai Wang,‡ Fan Li,‡ Si-Lu Tao,ǁ Xue-Mei Ou,†,‡ and Xiao-Hong Zhang,*,†,‡

†

Institute of Functional Nano & Soft Materials (FUNSOM), Jiangsu Key Laboratory for

Carbon-Based Functional Materials & Devices, Soochow University, Suzhou, Jiangsu, 215123, P. R. China ‡

Nano-organic Photoelectronic Laboratory and Key Laboratory of Photochemical Conversion and

Optoelectronic Materials, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing, 100190, P. R. China §

College of Materials Science and Engineering, Beijing Institute of Technology, Beijing, 100081, P.

R. China ǁ

School of Optoelectronic Information, University of Electronic Science and Technology of China

(UESTC), Chengdu, 610054, P.R. China

KEYWORDS: TADF, FWHM, color purity, OLEDs, isomeric compounds.

ABSTRACT: To improve the color purity of TADF emitters, two isomeric compounds, oPTC (5'-(phenoxazin-10-yl)-[1,1':3',1''-terphenyl]-2'-carbonitrile)

and

mPTC

(2'-(phenoxazin-10-yl)-[1,1':3',1''-terphenyl]-5'-carbonitrile), have been designed and synthesized with same skeleton but different molecular restrictions. Both two compounds exhibit similar HOMO, LUMO distributions and energy levels, photophysical properties in nonpolar cyclohexane solution, 1

ACS Paragon Plus Environment


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and high external quantum efficiencies (19.9% for oPTC and 17.4% for mPTC) in the devices. With the increased molecular space restriction induced by the additional phenyl substitutions at meta-position of the cyano-group from mPTC to oPTC, much weaker positive solvatochromic effect is observed for mPTC. And the color purity of emission from mPTC (FWHM of 86 nm) is also improved contrasted with that of oPTC (FWHM of 97 nm) in the devices. These results prove that increased restriction of the molecular structure is a simple and effective method to improve the color purity of the TADF emitters.

1. INTRODUCTION Organic light-emitting devices (OLEDs), as one of the most promising candidates for future large-area, flexible and economic displays and solid lighting sources, have attracted great interests in the past three decades.1-12 In the devices, the electrogenerated excitons are distributed as singlet and triplet excitons with a ratio of 1:3, resulting in that harvesting both singlet and triplet excitons is the basis of realizing high-efficiency OLEDs.3 Thus, phosphorescence OLEDs (PhOLEDs) have always been laid in the spotlight, because they can utilize both singlet and triplet excitons based on the strong spin-orbit coupling effect of heavy metal atoms.4-8 However, the high cost and potential pollution caused by heavy metals restrict the practical application of PhOLEDs.9-11 Since Adachi and his co-workers reported the device with a high external quantum efficiency (EQE) of 19.3% by employing a fluorescence emitter with thermally activated delayed fluorescence (TADF) characteristic in 2012, the new-generation OLEDs based on TADF emitters become the new focus of the OLED research.12 With an extremely small singlet-triplet splitting (∆EST), TADF emitter can realize efficient upconversion of non-radiative triplet excitons to radiative singlet excitons via reverse intersystem crossing (RISC) process.13-16 Thus, compared with PhOLEDs, the TADF OLEDs can also deliver a full exciton harvesting theoretically, but without heavy metals.18-21 2


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To realize the key point of extremely small ∆ESTs for TADF emitters, minimizing the overlap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) in the molecules is essential.15,17-20 Thus, TADF emitters are generally constructed by linking the electron-donor (D) segment and electron-acceptor (A) segment with a nearly vertical dihedral angle.17,22 Such subvertical D-A structure can suppress the conjugation between D and A segments, and strictly isolate the HOMO and LUMO orbitals on D and A segments, respectively.17-20,22 Till now, nearly all of the reported TADF emitters are based on such design strategy, and successfully exhibit high efficiencies in the devices. However, without the restriction from the conjugation between D and A segments, the structural relaxations of TADF emitters become evident, resulting in increased Stokes shift and broadened emission spectra.23,24 The full-width at half-maximum (FWHM) of the emission spectra of the typical TADF emitters is close to 100 nm.25-28 To satisfy the broadcasting standards of color purity, OLED displays employ color filters or optical microcavities to filtrate the margin region of broad emission in commercial application, which will cause efficiency loss.32 Thus, it is impossible to realize high efficiencies in OLED displays with current TADF emitters.17,29-32 Most recently, Hatakeyama and his co-workers successfully utilized the multiple resonance effect to construct TADF emitters with extremely narrow FWHM. 32 However, this strategy is strictly confined to specific molecular structure and inapplicable for current D-A structure TADF emitters; and the OLEDs based on their new TADF emitters suffer serious efficiency roll-off and extremely low maximum luminance of about 200 cd m -2 . 32 Therefore, it is still essential to develop a simple and general method that can effectively decrease the emission FWHM of TADF emitters with D-A structures. To address this point, in this work, we designed two isomeric TADF emitters, 5'-(phenoxazin-10-yl)-[1,1':3',1''-terphenyl]-2'-carbonitrile

(oPTC)

and

2'-(phenoxazin-10-yl)-[1,1':3',1''-terphenyl]-5'-carbonitrile (mPTC). As shown in Scheme 1, two 3



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isomeric compounds are constructed on identical D-A skeleton with D segment of phenoxazine and A segment of benzonitrile. And the additional two phenyl groups are attached on different positions of benzonitrile to realize different molecular space restrictions. For oPTC, the phenyl groups are located on the ortho-position of cyano-group, providing no additional interaction between D and A segments; while for mPTC, the phenyl groups are located on the meta-position of the cyano-group for the purpose of further restricting the rotation of phenoxazine and benzonitrile segments. Due to identical D-A skeleton, two isomeric compounds exhibit similar HOMO, LUMO distributions and energy levels, and close photophysical properties in dilute cyclohexane solution. The two TADF emitters also show high maximum EQEs of 19.9% for oPTC and 17.4% for mPTC in the devices, indicating excellent and similar TADF characteristics of the two compounds. However, with the increased molecular space restriction from oPTC to mPTC, the positive solvatochromic effect is evidently decreased. And the device based on oPTC exhibits a green emission with a CIE coordinate of (0.22, 0.40), a peak at 500 nm and a FWHM of 97 nm; whereas the mPTC-based device exhibits a sky-blue emission with a CIE coordinate of (0.18, 0.32), a peak at 484 nm and a FWHM of 86 nm. These results indicate that increased molecular space restriction is a simple method to efficiently suppress the structural relaxation, and improve the color purity of the TADF emitters.

2. RESULTS AND DISCUSSION 2.1 Synthesis. The synthetic routes for oPTC and mPTC are depicted in Scheme 1. For oPTC, (5'-bromo-[1,1':3',1''-terphenyl]-2'-yl)methanamine was ﬁrst synthesized as the two benzene ring attached to the ortho-position of (4-bromophenyl)methanamine under the catalysis of Pd(OAc)2 and AgOAc. Then the resulting (5'-bromo-[1,1':3',1''-terphenyl]-2'-yl)methanamine was converted to 5'-bromo-[1,1':3',1''-terphenyl]-2'-carbonitrile

through

a

debydrochlorination

4


process

by

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trichloroisocyanuric (TCCA) and Et3N. Finally, oPTC was synthesized via the Pd(OAc)2 catalyzed cross-coupling reaction between 5'-bromo-[1,1':3',1''-terphenyl]-2'-carbonitrile and phenothiazine. While

for

mPTC,

2'-amino-[1,1':3',1''-terphenyl]-5'-carbonitrile

was

first

synthesized

as

phenylboronic acid react with 4-amino-3,5-dibromobenzonitrile via Suzuki reaction. 4-amino-3,5dibromobenzonitrile was converted to 2'-fluoro-[1,1':3',1''-terphenyl]-5'-carbonitrile via diazotization reaction. Finally, mPTC was synthesized via the NaH assisted cross-coupling reaction between 2'-fluoro-[1,1':3',1''-terphenyl]-5'-carbonitrile and phenothiazine. The chemical structures of oPTC and mPTC were fully characterized and confirmed via nuclear magnetic resonance (NMR) spectroscopy and mass spectrometry (MS). Moreover, two compounds were further purified by sublimation before they were used for device fabrication.

2.2 Theoretical calculations and electrochemical properties. To gain insight into the structure-property relationship of two isomeric compounds at the molecular level, DFT calculations were performed for oPTC and mPTC to study their HOMO and LUMO distributions. As shown in Figure 1, due to the large steric hindrance of phenoxazine segment,12,33-36 both two compounds exhibit subvertical D-A structure. The dihedral angels between phenoxazine and benzonitrile segments are 75.29º and 78.79º for oPTC and mPTC, respectively. Thus, in both two compounds, the electron distributions of LUMO are mainly located at benzonitrile moiety, and the HOMO is primarily confined at the phenoxazine region. There are only small overlaps between HOMO and LUMO on the core benzene ring for both oPTC and mPTC. The small overlaps insure both two compounds realizing small ∆EST. Cyclic voltammetry was used to investigate the electrochemical properties of the two isomeric compounds (shown in Figure S1). Based on nearly identical frontier orbitals distributions, the HOMO energy levels of oPTC and mPTC were measured to be similar values of -5.11 and -5.12 eV, 5



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respectively, which were obtained from the onset of oxidation curves with respect to that of ferrocene. Likewise, the LUMO energy levels of oPTC and mPTC were measured to be -2.85 and -2.84 eV respectively from the reduction curves.20,37 The nearly equal HOMO and LUMO energy levels of oPTC and mPTC indicate that our isomeric structures have little influence on the intrinsic energy levels of the two compounds.

2.3 Photophysical Properties. The photophysical properties of the two isomeric compounds were then investigated. Room-temperature ultraviolet-visible (UV-Vis) absorption and fluorescence spectra of oPTC and mPTC in diluted cyclohexane solution are illustrated in Figure 2a. Both of the two materials exhibit intramolecular charge-transfer (ICT) transition absorption in range of 350 to 450 nm as well as local excited transition absorption in range of 280 to 340 nm. The optical bandgaps calculated from the absorption edges for oPTC and mPTC are 2.80 and 2.77 eV, respectively. From the photoluminescence (PL) spectra, oPTC and mPTC present almost same fluorescence with peaks at 455 nm, and the FWHMs of 60 nm. These nearly identical behaviors in nonpolar solvent are also ascribed to little influence on the intrinsic energy levels from the two isomeric structures. Due to the ICT transition characteristic of the D-A compounds, the interaction between the D-A molecule in the excited state and the polar solvent is significant, and the induced structure relaxation will evidently lower the excited state energy levels of the D-A molecules, resulting in positive solvatochromism.38 As shown in Figure 2b and 2c, both two isomeric compounds exhibit significant

positive solvatochromic effect. The emission spectrum of oPTC exhibits a clear bathochromic shift from in nonpolar cyclohexane (peak at 455 nm, FWHM of 60 nm) to in higher polar toluene (peak at 510 nm, FWHM of 81 nm), ethyl acetate (peak at 550 nm, FWHM of 113 nm), and acetonitrile (peak at 615 nm, FWHM of 140 nm). Correspondingly, the bathochromic shift is observed for the 6


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emission spectrum of mPTC from in nonpolar cyclohexane (peak at 455 nm, FWHM of 60 nm) to in higher polar toluene (peak at 490 nm, FWHM of 74 nm), ethyl acetate (peak at 514 nm, FWHM of 82 nm), and acetonitrile (peak at 550 nm, FWHM of 107 nm). The Stokes shift of mPTC is evidently decreased compared with that of oPTC, which should be ascribed to the suppression of rotation relaxation in the mPTC molecules induced by the additional phenyl groups at meta-position. From the fluorescence and phosphorescence spectra of 6.5 wt% oPTC or mPTC doped m-bis(N-carbazolyl)benzene (mCP) film at 77 K (shown in Figure 2d), ∆ESTs of the two isomeric compounds are determined to be 0.02 eV for oPTC and 0.01 eV for mPTC by the difference between the peaks of the fluorescence spectra (oPTC: 511 nm, S1=2.43 eV; mPTC: 496 nm, S1=2.50 eV) and that of the phosphorescence spectra (oPTC: 515 nm, S1=2.41 eV; mPTC: 498 nm, S1=2.49 eV). Extremely small ∆ESTs should be ascribed to small overlaps between HOMO and LUMO of two compounds, and will benefit the up-conversion of triplet excitons to singlet excitons. To further prove the TADF property of two isomeric compounds, the transient decay characteristics of 6.5 wt% oPTC or mPTC doped mCP film were measured in vacuum (Figure S2). Composited film of oPTC exhibits a prompt decay with a lifetime of 14.2 ns in the time range of 100 ns and a delayed decay with a lifetime of 57.9 µs in the time range of 600 µs, whereas the mPTC-doped film displays a prompt lifetime of 10.5 ns and a delayed lifetime of 12.9 µs in the same time range. Moreover, with the increased temperature, both the delayed decays of oPTC and mPTC doped mCP film become more significant as presented in Figure 3. The enhanced delayed decays indicate that the RISC process from T1 state to S1 state is strengthened by increasing temperature, which confirms that oPTC and mPTC are TADF emitters. The PLQYs of 6.5 wt% oPTC or mPTC doped mCP film were obtained via integrating sphere measurements with high values of 46.6% and 54.9% respectively. The higher PLQY of mPTC should result from the stiff molecular structure which restrict the energy loss from molecular relaxation. 7



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2.4 Thermal Properties. Thermal properties of oPTC and MPTC were measured by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) under nitrogen (Figure S3). The glass transition temperatures (Tg) are as high as 191 and 234 °C for oPTC and mPTC, respectively, which should be ascribed to the subvertical D-A structure of two compounds. The Tg values can be further improved with the even higher molecular space restriction. Such high Tg values of oPTC and mPTC will be beneficial to the stability of film’s morphology and reduce the phase separation rate of the guest-host system. The decomposition temperatures (Td, corresponding to 5% weight loss) of oPTC and mPTC are respectively 350 and 300 °C. The lower of Td mPTC might be caused by the contradiction between the increased restriction of the molecular structure and sharp intramolecular thermal vibration at high temperature. For better comparison of the two isomeric compounds, the key physical properties of oPTC and mPTC are also summarized in Table 1.

2.5 Electroluminescence Properties. To investigate the electroluminescence (EL) performance of the two isomeric compounds, we fabricated devices with the optimized structures of ITO/TAPC (40 nm)/TCTA (5nm)/mCP:6.5 wt% oPTC or mPTC (20 nm)/TmPyPb (35nm)/LiF (1 nm)/Al. In both devices, ITO (indium tin oxide) was

utilized

as

the

anode

and

LiF/Al

was

utilized

as

the

cathode;

TAPC

(1,1-Bis[4-[N,N-di(p-tolyl)amino]phenyl]cyclohexane) was the hole-transporting layer; TCTA (4,4’,4’’-tris(carbazol-9-yi)triphenylamine) (1,3,5-tri[(32pyridyl)-phen-3-yl]benzene)

was was

the the

exciton-blocking

layer,

electron-transporting,

and

TmPyPb

hole-blocking

and

exciton-blocking layer. As shown in Figure 4a, the turn-on voltages (at a brightness of 1 cd m-2) of two devices are 3.3 8


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and 3.2 V for oPTC and mPTC, respectively, while the device based on mPTC exhibits relatively higher current intensity and luminance. The difference should be ascribed to the increased molecular restriction of mPTC since the rigid structure is beneficial for carrier transport.39,40 The device based on oPTC exhibits the maximum current efficiency (CE) of 52.75 cd A-1, the maximum power efficiency (PE) of 46.01 lm W-1, and the maximum EQE of 19.9%. And the maximum efficiencies of mPTC-based device are 39.88 cd A-1 for CE, 35.78 lm W-1 for PE, and 17.4% for EQE. These high efficiencies prove the excellent TADF characteristics of the two compounds. As shown in Figure 4b, the EQE-luminance curves below 3000 cd m-2 are nearly identical for oPTC and mPTC, indicating close electro-optical conversion capability of two isomeric compounds. The efficiency roll-off of device based on mPTC is more serious beyond 3000 cd m-2, which is probably ascribed to the inferior stability of mPTC under high electric field, similar with the lower Td of mPTC. The EL spectra of two isomeric compounds are shown in Figure 4c. oPTC exhibits a green emission with a CIE coordinate (0.22, 0.40), a peak at 500 nm, and a FWHM of 97 nm in the device. Such nearly 100 nm wide FWHM is familiar to most TADF emitters reported before.20,25-28 Correspondingly, mPTC-based device shows a sky-blue emission with a CIE coordinate (0.18, 0.32), a peak at 484 nm, and a FWHM of 86 nm. The Stokes shift of mPTC is evidently smaller about 16 nm than that of oPTC, and the FWHM is 11 nm narrower comparing mPTC with oPTC in the devices. As listed in Table 2, compared with other reported sky-blue TADF emitters, mPTC exhibits significant narrower FWHM and decent maximum luminance in mCP-based device. By using DPEPO as the host material, the narrower FWHM characteristic of mPTC also exhibits in device (shown in Figure S4). The better color purity of mPTC indicates that the additional phenyl group at meta-position can effectively repress the structural relaxation in solid states. Beyond our work, two recently reported TADF emitters 2CzPN12 and DCzPN41 are also isomeric compounds. With stronger molecular restriction, 2CzPN shows an EL spectrum with a narrower FWHM about 80 nm in 9



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mCP-based device, whereas the FWHM of the emission from DCzPN is about 90 nm in mCP-based device. This accordant result proves the feasibility of the method to improve the color purity of TADF emitters by increasing the molecular space restriction.

3. CONCLUSION In summary, to develop the strategy for improving the color purity of TADF emitters, two isomeric compounds, oPTC and mPTC, which have identical D-A skeleton but two additional phenyl groups attached on different positions to realize different molecular space restrictions, have been designed and synthesized. With the same D-A framework, the two compounds exhibit similar frontier orbitals distributions and energy levels, close photophysical properties in dilute cyclohexane solution, and even close electro-optical conversion capabilities in the devices. However, with the increased molecular space restriction induced by the additional phenyl group at meta-position, mPTC exhibits much weaker positive solvatochromic effect compared with oPTC. And the color purity of emission from mPTC (FWHM of 86 nm) is also evidently improved contrasted with that of oPTC (FWHM of 97 nm) in the devices. These results prove that increased restriction of the molecular structure is a simple but effective method to improve the color purity of the TADF emitters.

4. EXPERIMENTAL SECTION 4.1 General Methods. All of the reagents and solvents utilized for synthesis were procured from J&K and used without further purification. All of the reactions were performed under inert atmosphere. The 1H-NMR and 13

C-NMR spectra were measured by AVANCZ 500 spectrometers at 298 K by utilizing deuterated

acetone as solvents and tetramethylsilane (TMS) as a standard. MS data were obtained via Finnigan 10


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4021C gas chromatography mass spectrometry instrument. UV-vis spectra and PL spectra were obtained via Hitachi UV-vis spectrophotometer U-3010 and a Hitachi fluorescence spectrometer F-4600, respectively. Transient PL measurements were measured on Edinburgh Instruments FLS920 spectrometer.

Cyclic

voltammetry

measurements

were

measured

on

a

CHI660E

electrochemical analyzer with 0.1 M Bu 4 NPF 6 as a supporting electrolyte, a saturated calomel electrode(SCE) as a reference electrode, a Pt disk as a working electrode, and a scan rate of 10 mV/s. Calibrated against a ferrocene/ferrocenium redox couple. The photoluminescence quantum yield of doped solid film was obtained via QY-2000 fluorescence spectrometer and estimated via an F-3018 integrating sphere under ambient atmosphere. Moreover, DSC measurements were acquired using a NETZSCH DSC204 instrument at a heating rate of 10 °C min-1 from 20 to 420 °C in a N2 atmosphere. TGA measurements were acquired using a TAQ 500 thermogravimeter by measuring the weight loss of the specimens while heating them at a rate of 10 °C min-1 from 25 to 800 °C in a N2 atmosphere.

4.2 Synthesis. All commercially available reagents and chemicals were used without further purification Synthesis of (5'-bromo-[1,1':3',1''-terphenyl]-2'-yl)methanamine (O2). A mixture of 10 mmol (1.86 g) 4-bromobenzylamine, 50 mmol (55 mL) indobenzene, 50 mmol (3.8 mL) trifluoroacetic acid, 20 mmol (3.8 g) silver acetate and 0.5 mmol (0.11 g) palladium diacetate were added into a clean 100 mL flask and refluxed in 120 oC oil bath for 4 h under nitrogen ambience. After cooling down the products were washed by saturated sodium carbonate solution and extracted by dichloromethane and dried over Na2SO4. The organic phase was spin dry under vacuum to afford O2 as brown solid (2.02g, yield 60%). After verified via MS-EI, the gross products were used for next reaction directly. 1

HNMR (400 MHz, CD3COCD3, δ, ppm) 8.25 (s, 2H), 7.52-7.54 (m, 6H), 7.42-7.38 (d, 4H, J=8.75 11



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Hz) 7.20 (d, 2H, J=5.45 Hz), 4.36 (s, 2H). MS (EI) m/z: [M]+ calculated for C19H16NBr 337.05, found 337.0513. Synthesis of 5'-bromo-[1,1':3',1''-terphenyl]-2'-carbonitrile (O3). 5 mmol (1.69g) O1 and 10 mmol (2.32 g) trichloroisocyanuric acid were dissolved by 10 mL DMF and stirred for half a hour before adding 3 mL of triethylamine dropwise under 0 oC. Then the temperature recovered to room temperature and stirred for 4 h. The mixture was washed by saturated ammonium chloride solution and extracted by dichloromethane. After removing solvent by rotary evaporation, the gross product was purified by column chromatography on silica gel (eluent: hexane/CH2Cl2 = 4:1) to attain 5'-bromo- [1,1':3',1''-terphenyl]-2'-carbonitrile O3 (1.50 g, yield 90%) as a yellow solid. 1HNMR (400 MHz, CD3COCD3, δ, ppm) 7.64 (s, 2H), 7.52-7.54 (m, 6H), 7.42-7.38 (d, 4H, J=8.68 Hz). MS (EI) m/z: [M]+ calculated for C19H12NBr, 333.02 found 333.0219. Synthesis of 5'-(phenoxazin-10-yl)-[1,1':3',1''-terphenyl]-2'-carbonitrile (oPTC). A mixture of 4 mmol (1.33 g) O3, 5 mmol (0.92 g) phenoxazine, 12 mmol (1.66 g) K2CO3, 0.12 mmol Pd(OAc)2, 0.44mmol (0.09 g) Tri-t-butylphosphine and 10 mL toluene were added into a clean 100mL flask and refluxed in 95 oC oil bath for 24 h under nitrogen ambience. After cooling down the products were washed by saturated ammonium chloride solution and extracted by dichloromethane. The solvent was volatilized by rotary evaporation and gross product was purified by column chromatography on silica gel (eluent: hexane/CH2Cl2 = 2:1) to attain oPTC (1.05 g, yield 60%) as gold power. 1HNMR (400 MHz, CD3COCD3, δ, ppm) 6.25-6.35 (dd, 2H, J=5.86 Hz), 6.70-6.73 (m, 2H), 6.74-6.77(m, 4H) 7.49-7.59 (m, 6H), 7.70(s, 2H), 7.72-7.79 (d, 4H, J=7.84 Hz). 13C NMR (100 MHz, CD3COCD3, δ, ppm): 115.24, 119.04, 119.21, 120.84, 127.40, 128.86, 133.82, 134.19, 134.32, 135.97, 138.63, 143.22, 148.39, 149.27, 155.02. MS (EI) m/z: [M]+ calculated for C31H20N2O 436.16, found 436.1622. Synthesis of 2'-amino-[1,1':3',1''-terphenyl]-5'-carbonitrile (M2): A mixture of 10 mmol (2.75 g) 12


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4-amino-3,5-dibromobenzonitrile (M1), 24 mmol (2.88 g) phenylboronic acid, 0.5 mmol (0.56 g) Pd(PPh3)4 and 60 mmol (8.34 g) K2CO3 were added into a clean 100 mL flask. The composed solvent (toluene: 40 mL, H2O: 30 mL, ethanol: 15 mL) were injected into the flask after evacuated air and inflated with nitrogen. The system was stirred at 90 oC for 8 h before washed with saturated ammonium chloride solution and extracted by dichloromethane. The solvent was volatilized by rotary evaporation and gross product was purified by column chromatography on silica gel (eluent: hexane/CH2Cl2 = 4:1) to attain M2 (2.29 g, yield 85%) as white powder. 1HNMR (400 MHz, CD3COCD3, δ, ppm):6.55 (S, 2H) 7.40-7.50 (m, 6H), 7.60-7.70 (d, 4H, J=8.67 Hz), 7.85-7.95 (d, 2H, J=4.84 Hz) MS (EI) m/z: [M]+ calculated for C19H14N2 270.12, found 270.1221. Synthesis of 2'-fluoro-[1,1':3',1''-terphenyl]-5'-carbonitrile (M3). 10 mmol (2.71 g) of M2 were dissolved in 20 mL concentrated hydrochloric acid in 100 mL flask and 10 mmol (0.69 g) of NaNO2 were dissolved in 10 mL H2O before dropwise into the flask in 0 oC. After the system stirred for 0.5 h, 20 mmol (2.21 g) of NaBF4 were added into the mixture and stirred for 0.5 h then filtrate the deposit. The deposit was dehydrated in vacuum in 24 h before dissolved in toluene and heated in 120 o

C for 8 h for thermal decomposition. After cooling down, the mixture was washed with saturated

ammonium chloride solution and extracted by dichloromethane. The solvent was removed by rotary evaporation and gross product was purified by column chromatography on silica gel (eluent: hexane/CH2Cl2 = 2:1) to attain M3 (1.64g, yield 60%).

1

HNMR (400 MHz, CD3COCD3, δ, ppm):

7.44-7.58 (m, 6H), 7.65-7.74 (d, 4H, J=8.58 Hz), 7.90-7.98 (d, 2H, J=5.68 Hz). MS (EI) m/z: [M]+ calculated for C19H12FN 273.10, found 273.1011. Synthesis of 2'-(phenoxazin-10-yl)-[1,1':3',1''-terphenyl]-5'-carbonitrile (mPTC). 10 mmol of NaH were added into 100 mL flask in 0 oC and evacuated and inflated with nitrogen, the 6 mmol of 10H-phenoxazine were dissolved in 10 mL of DMF before dropwise into the flask and stirred for 0.5 h. Then 5 mmol (1.37 g) of M3 which dissolved in 10 mL of DMF were injected into the mixture. 13



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The mixture was stirred in room temperature for 0.5 h then heating to 60 oC for 4 h. The reaction was terminated by adding 1 mL of water and the liquids were washed by saturated ammonium chloride solution and extracted by dichloromethane. The solvent was volatilized by rotary evaporation and gross product was purified by column chromatography on silica gel (eluent: hexane/CH2Cl2 = 2:1) to attain mPTC (1.00 g, yield 50%) as green power. 1HNMR (400 MHz, CD3COCD3, δ, ppm): 6.01-6.08 (dd, 2H), 6.36-6.45 (dd, 2H, J=8.56 Hz), 6.45-6.59 (m, 4H), 7.25-7.33 (t, 6H), 7.37-7.43 (m, 6H), 8.02 (s, 2H).

13

C NMR (100 MHz, CD3COCD3, δ, ppm):

118.50, 118.71, 120.35, 122.94, 126.85, 128.41, 133.44, 133.50, 133.75, 137.40, 140.51, 142.24, 142.47, 148.05, 150.86. MS (EI) m/z: [M]+ calculated d for C31H20N2O 436.16, found 436.1618. 4.3 Device Fabrication and Measurement. Standard ITO-coated glasses with a sheet resistance of 30 Ω per square were adopted as substrates for device fabrication. The glasses were cleaned by sonication in acetone for twice, ethanol for twice, and ultrapure water for 5 min before dried on oven at 120 °C for 10 h. For better carrier mobility, the glasses were treated with UV-ozone for 5 min and then conveyed to a thermal deposition system at vacuum approximately 1×10-6 Torr. Organic layers were thermally evaporated onto the ITO glasses at rate of 1-2 Å s-1 consecutively. The cathode was combined with the LiF deposited at a rate of 0.1 Å s-1 and the Al metal deposited at rate of 10 Å s-1. Spectrascan PR650 photometer was utilized for EL luminance, CIE color coordinates, and spectra measurement. The current-voltage-Luminance characteristics were measured via Keithley 2400 Source Meter as the voltage varied from 3 to 10 V with 0.5 V intervals. EQE was determined from the current density, luminance, and EL spectrum, presuming a Lambertian distribution for EL spectrum.

ASSOCIATED CONTENT

14


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Supporting Information

Cyclic voltammograms of oPTC and mPTC, transient PL decay curve of 6.5 wt% oPTC or mPTC doped mCP film, TGA and DSC thermograms of oPTC and mPTC, and EL spectra of oPTC and mPTC on OLED device based on DPEPO are supplied as Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] *E-mail: [email protected]

Notes The authors declare no competing financial interest.

Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant No. 51373190, 51533005), the Beijing Nova Program (Grant No. Z14110001814067), Collaborative Innovation Center of Suzhou Nano Science & Technology, the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD) and Qing Lan Project, P. R. China.

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(43) Kim, M.; Jeon, S. K.; Hwang, S. H.; Lee, S. S.; Yu, E.; Lee, J. Y. Highly Efficient and Color Tunable Thermally Activated Delayed Fluorescent Emitters Using a “Twin Emitter” Molecular Design. Chem. Commun. 2016, 52, 339-342.

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Page 22 of 27 N

N H 2N

H2N AgOAc,Pd(OAC) 2

TCCA

CF3COOH Br

Pd(OAc)2 ,TTBP

Et3 N I

O1

Br

Br

O2

K2CO 3

N O

O3

oPTC N

N

N

N

NaH/DMSO

Pd(PPh3)4 , K 2CO3 Br

Br NH2

M1

H 2O/Toluen OH B OH

1: NaNO2/HCl 2: NaBF4 , N NH2

F

M2

M3

O

mPTC

Scheme 1. Synthetic routes and molecular structures of oPTC and mPTC.

(a)

(c)

(b)

(d)

Figure 1. Molecular structures and calculated spatial distributions of the HOMO and LUMO energy densities. a) LUMO and b) HOMO of oPTC; c) LUMO and d) HOMO of mPTC. 22


Page 23 of 27

a)

0.8

0.6

0.4

CYH Tol EAC MeCN

1.0

Normalized Intensity

Normalized intensity

b)

oPTC-UV mPTC-UV oTPC-FL mTPC-FL

1.0

0.8 0.6 0.4 0.2

0.2

0.0

0.0 300

400

500

600

400

700

450

Wavelength (nm)

0.8

550

600

650

700

0.6 0.4 0.2

OTPC-FL OTPC-PL MTPC-FL MTPC-PL

1.0



d)

CYH Tol EAC MeCN

1.0

500

Wavelength (nm)

c)

0.8

0.6

0.4

0.2

0.0

0.0

400

450

500

550

600

650

700

400

450

500

Wavelength (nm)

550

600

650

Wavelength (nm)

Figure 2. a) Normalized UV-vis absorption and fluorescence spectra for oPTC and mPTC in cyclohexane at room temperature. Fluorescence spectra of b) oPTC and c) mPTC in different solvent. d) Normalized fluorescence and phosphorescence spectra of 6.5 wt% oPTC or mPTC doped mCP thin solid film at 77 K. 4

a)

4 b) 10

10

T=100K T=200K T=300K

3

PL Intensity ( arb. unit)

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


10

2

10

1

10

3

10

2

10

1

10

0

0

10

T=100K T=200K T=300K

10

0

100

200

300

400

500

600

0

100

200

300

400

500

600

Time (µs)

Time (µs)

Figure 3. Transient PL decay curve for delayed emission of 6.5 wt% a) oPTC and b) mPTC doped mCP film and in time range of 600 µs at different temperatures. (Excitation wavelength was 300 nm.) 23


ACS Applied Materials & Interfaces b) 150

Luminance ( cd m-2)

100 3

10

50 2

10

0 1

10

3

4

5

6

Current density ( mA cm-2)

oPTC mPTC

4

10

25

External Quantum Efficiency (%)

a)

oPTC mPTC

20

15

10

5

0 100

7

1000

10000

Luminance (cd m-2)

Voltage (V)

c) oPTC-EL mPTC-EL

1.0


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Page 24 of 27

0.8 0.6 0.4 0.2 0.0 400

450

500

550

600

650

700

750

Wavelength (nm)

Figure 4. a) Current density-voltage-luminance characteristics; b) The EQE-luminance characteristics; and c) The EL spectra of the devices based on oPTC and mPTC.

24


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Table 1. Summary of key physical properties of oPTC and mPTC. Compounds

λ abs a) (nm)

λ P L b) (nm)

E g c) (eV)

FWHM d) (nm)

∆E ST e) (eV)

PLQY f) (%)

HOMO/ LUMO g) (eV)

T d /T g h) ( o C)

oPTC mPTC

398/310 405/320

455 455

2.80 2.77

60 60

0.02 0.01

46.6 54.9

-5.11/-2.85 -5.12/-2.84

350/191 300/234

a)

Determined from UV-vis absorption spectra measured in cyclohexane solution; Determined from PL spectra measured in cyclohexane solution; c) Calculated from the initiate absorb position in UV-vis absorption spectra in cyclohexane; d) Calculated from PL spectra measured in cyclohexane solution; e) ∆E ST calculated as ∆E ST = S 1 -T 1 ; f) Measured by integrating sphere with 6.5 wt% oPTC or mPTC doped mCP film; g) HOMO was determined from the onset of oxidation potential with respect to ferrocence; LUMO was determined from the onset of reduction potential with respect to ferrocenium; h) T d : decomposition temperature, corresponding to 5% weight loss; T g : glass transition temperature.

b)

25



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Page 26 of 27

Table 2. Summary of sky blue OLED performance based on TADF emitter Compounds

References

FWHM (nm)

Peak (nm)

CIE coordinate

maximum luminance (cd m-2 )

mPTC PPZ-4TPT CC2TA 2PXZ-TAZ DTC-pBPSB CPC 34TCzPN 2a

this work Ref. [27] Ref. [26] Ref. [28] Ref. [42] Ref. [20] Ref. [43] Ref. [36]

86 105±5 115±5 100±5 100±5 105±5 98±5 100±5

484 495 490 456 480 485 475 487

(0.18,0.32) N/A N/A (0.16,0.15) (0.18,0.19) (0.19,0.32) (0.17,0.29) (0.19,0.35)

5800 800 N/A N/A N/A 4000 1000 6000

26


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ToC figure

27


Isomeric Thermally Activated Delayed ... - ACS Publications

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