Iminodibenzyl-substituted distyrylarylenes as dopants for
blue and white organic light-emitting devices
Meng-Huan Ho
a,*, Chia-Ming Chang
a, Ta-Ya Chu
b,
Teng-Ming Chen
a, Chin H. Chen
caDepartment of Applied Chemistry, National Chiao Tung University, Hsinchu 300, Taiwan, ROC bDepartment of Electrophysics, National Chiao Tung University, Hsinchu 300, Taiwan, ROC
cDisplay Institute, Microelectronics and Information Systems Research Center, National Chiao Tung University, Hsinchu 300, Taiwan, ROC
Received 21 March 2007; received in revised form 9 July 2007; accepted 28 September 2007 Available online 9 October 2007
Abstract
A series of highly efficient blue materials based on iminodibenzyl-substituted distyrylarylene (IDB-series) fluorescent dyes using the concept of steric-compression have been designed and synthesized by means of a rigidized and over-sized ring. The steric-compression effect can shorten the effective conjugation length (chromophore) of the molecule and the added phenyl moiety in the core can alleviate the propensity for molecular aggregation. These materials also possess high glass transition temperature over 100C. The blue IDB-Ph device achieved a maximum external quantum efficiency of 4.8% with a Commission Internationale de l’Eclairage (CIEx,y) coordinate of (0.16, 0.28). When applied in two-element white OLED
system, the IDB-Ph doped device achieved a luminance efficiency of 11.0 cd/A with a CIEx,ycolor coordinate of (0.29, 0.36).
2007 Elsevier B.V. All rights reserved.
PACS: 78.55.Kz; 78.60.Fi; 85.60.Jb
Keywords: Iminodibenzyl; Distyrylarylenes; Fluorescent blue material; Organic electroluminescent device
1. Introduction
Recently, owing to their unique electrical and optical properties, various functional devices using organic materials have been developed in a variety of applied fields [1]. Since the initial work by Tang and Van Slyke[2], interest in organic light-emitting diodes (OLEDs) has been steadily growing. In par-ticular, OLEDs have been the subject of intensive
investigation because of their successful commer-cialization in various full-color displays[3,4]. Mate-rials development continues to play a pivotal role in this technology as OLED materials have to function often not only as a charge transporter but also an efficient light emitter. Morphology stability of vari-ous layers is another issue that needs to be addressed to assure sufficient long operational life-time for the devices[5,6].
To date, white organic light-emitting devices (WOLEDs) have drawn intensive studies due to their potential applications in full-color display 1566-1199/$ - see front matter 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.orgel.2007.09.006
* Corresponding author.
E-mail address:[email protected](M.-H. Ho).
Organic Electronics 9 (2008) 101–110
fabrication with color filters [7], in backlight for liquid crystal display as well as in solid-state ligh-tings[8]. One of the best known methods to achieve WOLEDs is the two-color system of sky blue and yellow or orange emission, which has been widely reported [9–11]. In this system, it has been shown that performance of WOLEDs can be improved sig-nificantly by adopting sky blue fluorescent materials with high efficiency, optimized color, and long oper-ational stability. To this end, there have been a number of efficient blue fluorescent dyes developed in the past several years [12–15], especially the di(styryl)- amine-based blue dopant, DSA-Ph [16]
and BUBD-1 [17] which, upon doping in the morphologically stable host material, 2-methyl-9,10-di(2-naphthyl)anthracene (MADN), their device performances achieved an EL efficiency of 9.7 cd/A with CIEx,y of (0.16, 0.32) and 13.2 cd/A
with CIEx,y of (0.16, 0.30) at 20 mA/cm2,
respectively.
In this paper, we disclose a newly designed series blue dopants based on the 7-membered N-hetero-cyclic core structure of iminodibenzyl-distyrylaryl-ene (IDB). It has been reported that when giminodibenzyl-distyrylaryl-eneral aromatic amino-substituents (e.g. diphenyl amine) of hole transport material are replaced with the imi-nodibenzyl groups, the thermal properties can be improved and the emission wavelength would be blue-shifted[18,19]. Hence, we decided to introduce the iminodibenzyl groups into the highly fluorescent distyrylarylene structure. From ab initio density functional theory (DFT) using B3LYP/6-31* level
of basis sets, we also found that the emission wave-length could be shifted to slightly deeper blue with increasing the number of phenyl moiety in the cen-ter core. We expect these new blue macen-terials will be potentially useful in producing a deep blue emis-sion with a properly matched host and a two-ele-ment white OLED system as well.
2. Experimental 2.1. Synthesis
The synthetic routes of iminodibenzyl-substi-tuted distyrylarylene derivatives (IDB-series) are shown in Scheme 1. The intermediate 4-(iminodib-enzyl)-benzoaldehyde was prepared by coupling iminodibenzyl (IDB) and 4-bromobenzene with a palladium-catalyzed aromatic amination reaction
[20]. After the reaction was completed, 9-phenyl-iminodibenzyl was purified by column
chroma-tography and then mixed with phosphoryl chloride in DMF at room temperature for 8 h under nitrogen
[21]. The mixture was quenched with sodium acetate and water to precipitate the gray solid and purified by recrystallization twice from ethanol to afford the intermediate as colorless crystal.
On the other hand, a mixture of p-xylylene dichloride and neat triethyl phosphite was heated at 200C for 24 h under nitrogen. After cooling to room temperature, the reaction mixture was puri-fied by bulb to bulb distillation to afford tetraethyl p-(xylylene)diphosphonate [22]. The tetraethyl biphenyl-4,40-diylbis(methylene)diphosphonate was
synthesized from 4,40-bis(chloromethyl)-biphenyl
with same procedure. The tetraethyl triphenyl-4,400-diybis(methylene)diphosphonate was
synthe-sized by adding aqueous K2CO3(2.0 M, 20 mL) to Scheme 1. Synthetic routes of IDB-series materials and structure of DSA-Ph.
a solution of diethyl-4-bromobenzyl phosphonate (4.5 mmol) and 1,4-benzenediboronic acid (1.9 mmol) in toluene (60 mL) and ethanol (10 mL). The mixture was degassed and tetrakis(triphenyl-phosphine) palladium (3.9 mol%) was added in one portion under an atmosphere of N2 and then
heated under reflux for 24 h[23]. After the solution cooled, the solvent was evaporated under vacuum and the product was extracted with ethyl acetate. The organic solution was washed with water several times and dried with anhydrous MgSO4, followed
by recrystallization from ethanol.
Finally, the IDB-series materials were readily synthesized by Horner–Wadsworth–Emmons reac-tion according to a known procedure [24]. To a solution of 4-(iminodibenzyl)-benzoaldehyde (2.1 mmol) and the appropriate phosphonate (1 mmol) in DMF cooled in an ice bath, sodium tert-butoxide (1.5 mmol) was added and stirred at 25C for 20 h. The mixture was then poured into water, and the precipitated product was collected and washed with methanol. The crude product was purified by chro-matography to give pure IDB-series material as a yellow solid. The final products were purified by temperature gradient sublimation before using in subsequent studies.
tetraethyl p-(xylylene)diphosphonate. 1H NMR (300 MHz, CDCl3): d/ppm 1.19 (t, 12H), 3.08 (d,
4H), 3.99-4.03 (m, 8H), 7.23 (s, 4H).
tetraethyl biphenyl-4,40
-diylbis(methylene)diphos-phonate. 1H NMR (300 MHz, CDCl3): d/ppm 1.22
(t, 12 H), 3.14 (d, 4H), 4.00–4.04 (m, 8H), 7.35 (d, 4H), 7.51 (d, 4H).
tetraethyl biphenyl-4,400 -diylbis(methylene)diphos-phonate.1H NMR (300 MHz, CDCl3): d/ppm 1.24 (t, 12H), 3.19 (d, 4H), 4.03–4.08 (m, 8H), 7.23– 7.64 (m, 12H). IDB-Ph. 1H NMR (300 MHz, CDCl3): d/ppm 3.00 (s, 8H), 6.56 (d, 4H), 6.82–6.97 (m, 4H), 7.21–7.43 (m, 24H). FAB-MS: m/z = 668 (M+). Anal. for C50H40N2: Calcd: C, 89.78; H, 6.03; N,
4.19. Found: C, 89.14; H, 5.83; N, 3.66.
IDB-biPh. 1H NMR (300 MHz, CDCl3): d/ppm
3.00 (s, 8H), 6.59 (d, 4H), 6.89–7.08 (m, 8H), 7.23–7.60 (m, 24H). FAB-MS: m/z = 744 (M+). Anal. for C56H44N2: Calcd: C, 90.29; H, 5.95; N,
3.76. Found: C, 89.72; H, 5.82; N, 3.46. IDB-triPh.1H NMR (300 MHz, CDCl3): d/ppm 3.00 (s, 8H), 6.56(d, 4H), 6.87–7.06 (m, 8H), 7.20– 7.66 (m, 28H). FAB-MS: m/z = 820 (M+). Anal. for C62H48N2: Calcd: C, 90.70; H, 5.89; N, 3.41. Found: C, 89.91; H, 5.83; N, 2.95.
2.2. Characterization of material properties
All IDB-series materials were further purified via train sublimation and fully characterized with satis-factory spectroscopic data. UV–Vis and solution photoluminescence spectra were recorded in toluene by Hewlett Packard 8453 and Acton Research Spec-tra Pro-150, respectively. Electrochemical properties were studied by cyclic voltammetry using CHI 604 A. The energy gap can be calculated from the edge of UV–Vis absorption peak. Melting points (Tm), Glass transition temperatures (Tg) of the
respective compounds were measured by differential scanning calorimetry (DSC) under nitrogen atmo-sphere using a SEIKO SSC 5200 DSC Computer/ thermal analyzer.
2.3. Geometry optimization
The ground-state structures of IDB-series materials were optimized by using ab initio density functional theory (DFT) with the B3LYP (Becke three-parameter Lee–Yang–Parr) [25,26] exchange correlation function with 6-31G* basis sets, in
Gaussian 03 program[27]. 2.4. Device fabrication
The structures of blue and white devices and materials applied in this study are shown inFig. 1. In the device fabrication, CFx, N,N0
-bis-(1-naphthyl)-N,N0-diphenyl,1,10-biphenyl-4,40-diamine
(NPB), tris(8-quinolinolato)aluminium (Alq3), and
LiF were used as hole injection[28], hole transport, electron transport and electron injection materials, respectively. The emitting layer (EML) of blue-doped devices is composed of blue IDB dopants doped in the stable blue host material of MADN with each optimized concentration. In two-element WOLED device, the sky-blue light emission was generated by doping 7% highly fluorescent IDB-Ph into MADN while the yellow emission was derived from the doping 4% 2,8-di(t-butyl)-5,11-di [4-(t-butyl)phenyl]-6,12-diphenylnaphthacene (TBRb)
[29]in NPB to obtain a white emission.
After a routine cleaning procedure, the indium-tin-oxide (ITO)-coated glass was loaded on the grounded electrode of a parallel-plate plasma reac-tor, pretreated by oxygen plasma, and then coated with a polymerized fluorocarbon film. Devices were fabricated under the base vacuum of about 106torr in a thin-film evaporation coater following
a published protocol [30]. In the evaporation of EML, the fluorescent dopant was co-deposited with host molecule at its optimal molar ratio. After the thermal deposition of the organic layers and with-out a vacuum break, the ultra thin layer of 1 nm of LiF followed by 200 nm of Al was deposited through a patterned shadow mask on top of the organic layers using separately controlled sources to complete the cathode. All devices were hermeti-cally sealed prior to testing. The active area of the EL device, defined by the overlap of the ITO and the cathode electrodes, was 9 mm2. The current– voltage–luminance characteristics of the devices were measured with a diode array rapid scan system using a Photo Research PR650 spectrophotometer and a computer-controlled programmable dc source. The device lifetime measurements were per-formed in a glove box at a constant drive current density of 20 mA/cm2.
3. Results and discussion 3.1. Luminescence in solution
The photo-physical, electrochemical and thermal properties of DSA-Ph and IDB-series materials are
summarized inTable 1.Fig. 2compares the absorp-tion and photoluminescence (PL) spectra of DSA-Ph and IDB-series materials in toluene. The features of the lowest absorption band and fluorescence of IDB-Ph are very similar to those of DSA-Ph, except they are blue-shifted. The emission wavelength of IDB-Ph is 449 nm which is blue-shifted around 9 nm with respect to that of DSA-Ph. From the con-formational structures of DSA-Ph and IDB-Ph optimized by the density functional theory method using B3LYP/6-31* level of basis sets shown in Fig. 3, we found that the orthogonality between the distyrylarylene core and the phenyl group of iminodibenzyl substituent is higher than that between the core and phenyl group of diphenyl-amine substituent in the ground state. We attributed that the phenomenon is due to the rigid imino-dibenzyl substituent which would increase the steric strain and cause the iminodibenzyl moiety to twist slightly out of the plane defined by the p–p conjugation of the distyrylarylene core. For example, the angle between the stilbene core and the phenyl group of iminodibenzyl (66) is higher than that between the core and phenyl group of diphenylamine (42). As a result, the effect of steric-compression would be the reason for the Fig. 1. Structures of MADN, TBRb, blue and white devices.
Table 1
The photo-physical, electrochemical and thermal properties of DSA-Ph, IDB-Ph, IDB-biPh and IDB-triPh
Material kabs,max(nm) kem,max(nm) HOMO (eV) LUMO (eV) Band gap (eV) Tg(C) Tm(C)
DSA-Ph 410 458 5.4 2.7 2.7 89 172
IDB-Ph 408 449 5.2 2.4 2.75 119 325
IDB-biPh 402 447 5.2 2.4 2.75 131 335
hypsochromic-shifted emission wavelength of IDB-Ph as compared to DSA-IDB-Ph.
It was also found that IDB-biPh and IDB-triPh (with emission kmax 447 nm and 443 nm,
respec-tively) have a slightly blue-shifted emission wave-length with respect to IDB-Ph, which means the optical spectra can be slightly shifted to shorter wavelength with the increasing phenyl moiety in the molecular core. This hypsochromic-shifted phe-nomenon can also be rationalized from the confor-mational structures of IDB-series materials shown inFig. 3, in which the added phenyl moiety appears to enlarge the twist angle between two (styryl)imino-dibenzyl chromophores and further decrease the p– pconjugation. As a result, the effective conjugation length (chromophore) of the molecule is slightly
shortened and causes the emission wavelength to deeper blue region.
3.2. Luminescence in the solid state
In order to investigate the energy-transfer between host material (MADN) and dopant (IDB-series materials), we measured the solid-state emis-sion spectra of various doping concentration of IDB materials doped in MADN thin films (excited with 400 nm, kex,max of MADN). As shown in
Fig. 4a, the IDB-Ph emission can be clearly
observed at 5% doping concentration and the emis-sion of MADN around 430 nm essentially quenched confirming that the Fo¨rster energy-transfer from MADN to IDB-Ph is efficient. Moreover, when Fig. 2. Normalized absorption, photoluminescence (PL) spectra of DSA-Ph and IDB-series materials.
doping concentration is up to 7%, the IDB-Ph emis-sion intensity would be decreased and the intensity of long wavelength shoulder tends to grow with increasing doping concentration and appears to be even higher than that of the main peak. This phenomenon is primarily due to the molecular aggregation propensity of IDB-Ph with the flat stilbene-based center core, especially at high doping concentration.
The emission spectra of IDB-biPh/MADN and IDB-triPh/MADN thin films reveal the same results as there are also efficient Fo¨rster energy-transfer between IDB-biPh/MADN and IDB-triPh/MADN,
respectively. Fig. 4b depicts the normalized solid-state emission spectra of IDB-series materials/ MADN thin films. The solid-state emission spectra were similar to those recorded in toluene solution shown in Fig. 2. It is observed that the solid-state emission spectra become broader and the intensity of long wavelength shoulder become higher when compared with the solution PL spectra. Interest-ingly, the intensity of long wavelength shoulder and the full width at half maximum (FWHM) of solid-state emission spectra can be decreased with the increasing phenyl moiety in the molecular core. (The FWHM of solid-state emission of IDB-series Fig. 4. (a) Solid state emission spectra of IDB-Ph/MADN thin films. (b) Normalized solid state emission spectra of IDB-series/MADN thin films.
materials are 1.32· 105cm1, 1.43· 105cm1, 1.47· 105cm1, respectively.) It was also found that the optimal doping concentration of IDB-biPh and IDB-triPh is 7% which is higher than that of IDB-Ph. Based on these results, we conclude that the introduction of added phenyl moiety can enlarge the twist angle in the center core and thus can alle-viate the propensity for molecular aggregation. Therefore, the added phenyl moiety in the center core can be expected to further improve the color purity of blue-doped devices.
3.3. Electrochemical and thermal properties
The HOMO energy level of each material can be obtained by cyclic voltammetry and the energy-gap can be calculated from the edge of UV–Vis absorp-tion peak. The measured HOMO energy levels of IDB-series materials are about 5.1–5.2 eV and are smaller than that of DSA-Ph (5.4 eV). This phe-nomenon can be rationalized by the strong donor characteristic of iminodibenzyl group, which has a small ionization potential (Ip) [31] and oxidation
potential [32] and consequently causes the LUMO energy level of IDB-series dopants are much decreased to 2.3–2.4 eV.
We used differential scanning calorimetry (DSC) to investigate the thermal properties of IDB-series materials. The thermal characteristics of IDB-series materials are also summarized inTable 1. The glass transition temperature (Tg) of IDB-Ph, IDB-biPh
and IDB-triPh are found at 119C, 131 C and 137C, respectively, which are all much higher than that of DSA-Ph (89C). This result indicates that the steric iminodibenzyl substituent and the added phenyl moiety would not only affect the emission wavelength but also improve the material thermal stability. As a result, these IDB-series materials can form amorphous thin films that are more stable than that of DSA-Ph, and they are more promising in terms of their thermal stability for application in OLEDs.
3.4. Blue device performance
The EL efficiency of the undoped MADN is 1.5 cd/A at 20 mA/cm2with a CIEx,ycolor
coordi-nate of (0.15, 0.10). When doped with IDB-Ph, IDB-biPh and IDB-triPh at their optimal doping concentrations of 5%, 7% and 7%, the EL efficien-cies are increased to 9.1, 6.3, and 3.7 cd/A with CIEx,y color coordinate of (0.16, 0.28), (0.15, 0.24)
and (0.13, 0.20), respectively. Their optimal doping levels are consistent with those of their correspond-ing solid-state thin film fluorescence yield. The over-all EL performances of the new blue dopants doped devices are summarized inTable 2. The device per-formance indicates that the IDB-series materials are useful in producing blue OLED devices with high efficiency. The maximum external quantum effi-ciency (EQE) of IDB-Ph doped device is close to the theoretical limit of 4.8% and the half-decay life-time (t1/2) is 700 h with an initial brightness of
1976 cd/m2 monitored in a dry box. Assuming the scalable law of Coulombic degradation[29]for driv-ing at L0of 100 cd/m
2
, the half-decay lifetime (t1/2)
of the IDB-Ph doped device is projected to be over 13,000 h.
Fig. 5shows the normalized EL spectra of these blue-doped devices. The EL spectra peak and FWHM of dopants Ph, biPh and
IDB-triPh are 461 nm, 460 nm, 456 nm and
1.39· 105
cm1, 1.47· 105
cm1, 1.67· 105
cm1, respectively, that are in good agreement with their corresponding solid-state emission spectra. Most importantly, the intensity of long wavelength shoul-der is indeed suppressed with the increasing number of phenyl moiety at the molecular core which fur-ther improves the CIE y value to deeper blue (from 0.28 to 0.20).
3.5. Two-element white OLED device performance We also introduced the new sky-blue emitter, IDB-Ph, into the white OLED structure incorporat-ing a dual-layered emittincorporat-ing layer (EML) of blue and yellow to compose the white emission additively. Within the device structure, NPB doped with TBRb was used as the yellow emission layer. The perfor-mances of IDB-Ph doped WOLED are summarized
Table 2
Performance of IDB-series doped blue devices and two-element WOLED devices at 20 mA/cm2
Blue dopant Voltage (V) Yied (cd/A) Pow. Eff. (lm/W) EQE (%) CIEx,y Blue device IDB-Ph 5.9 9.1 4.9 4.8 (0.16, 0.28) IDB-biPh 5.9 6.3 3.3 3.7 (0.15, 0.24) IDB-triPh 6.8 3.7 1.7 2.5 (0.13, 0.20) Two-element WOLEDs IDB-Ph 6.5 11.0 5.3 4.8 (0.29, 0.36)
inTable 2. The IDB-Ph doped WOLED can achieve an EL efficiency of 11.0 cd/A and 5.3 lm/W with a CIEx,y color coordinate of (0.29, 0.36) which are
all better than the reported DSA-Ph doped WOLED [11]. The inset of Fig. 6 shows the EL spectra of IDB-Ph deoped WOLED, it covers a wild range of visible region, clearly indicating the emis-sions of IDB-Ph and TBRb with a dominant peak at 464, 488, and 564 nm, respectively. It is evident that there is no obvious EL color shift with
increased driving currents from 20 mA/cm2 to 200 mA/cm2. Fig. 6 shows the operational lifetime of the IDB-Ph doped WOLED under a constant current density of 20 mA/m2 monitored in a dry box. The t80 (the time for the luminance to drop
to 80% of initial luminance) and initial luminance (L0) is 420 h with an initial brightness of 2198 cd/m2
monitored in a dry box. Assuming the scalable law of Coulombic degradation [29] for driving at L0 of 100 cd/m2, the half-decay lifetime (t1/2) of Fig. 5. Normalized EL spectra of IDB-series doped blue devices at 20 mA/cm2.
Fig. 6. Device operational stability of the IDB-Ph doped WOLED. Inset: The normalized EL spectra of IDB-Ph doped WOLED at various current densities.
the IDB-Ph doped device is projected to be over 42,000 h.
These results indicate that the two-element WOLED performance can really be significantly enahnced both in device efficiency and CIEx,ycolor
coordinates by replacing DSA-Ph to IDB-Ph. The improved result is due to the new blue emitter, IDB-Ph, which emits deeper blue light with higher efficiency and better thermal stability than that of DSA-Ph. Therefore, when this new blue emitter was used in a two-element WOLED system, it can achieve a high EQE of 4.8% and generate a more balanced white CIEx,ycolor coordinates.
4. Conclusions
By molecular engineering of the di(styryl)amine-based structure, we have designed and synthesized a series of highly efficient blue dopants based on the iminodibenzyl-substituted distyrylarylene (IDB-series) compounds. The steric-compression effect can shorten the effective conjugation length (chromophore) of the molecule and the added phe-nyl moiety in the core can alleviate the propensity for molecular aggregation. These materials also pos-sess high glass transition temperature over 100C. When doped in the stable blue host material, MADN, the maximum external quantum efficiency of IDB-Ph doped device is close to the theoretical limit of 4.8% with a CIEx,y color coordinate of
(0.16, 0.28). When IDB-Ph was used in a two-ele-ment WOLED system, the doped device achieved a luminance efficiency of 11.0 cd/A at 20 mA/cm2 with a CIEx,ycolor coordinate of (0.29, 0.36). The
white device achieved a half-decay lifetime (t1/2) of
42,000 h at an initial brightness of 100 cd/m2. Acknowledgements
This work was supported by grants from Chung-hwa Picture Tubes, Ltd. (CPT) of Taoyuan, Tai-wan. and National Science Council of TaiTai-wan. We thank e-Ray Optoelectronics Technology Co., Ltd. of Taiwan for generously supplying some of the OLED materials studied in this work.
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![Fig. 3. Conformational Structures of DSA-Ph and IDB-series materials [DFT with B3LYP/6-31G(d)].](https://thumb-ap.123doks.com/thumbv2/9libinfo/7645310.138734/5.816.111.703.487.680/fig-conformational-structures-dsa-idb-series-materials-dft.webp)
