Solution-processed efficient deep-blue fluorescent organic light-emitting diodes based on novel 9,10-diphenyl-anthracene derivatives

Article abstract of DOI:10.1039/c5ra00627a

A series of 9,10-diphenyl-anthracene derivatives bearing either benzimidazole or carbazole moieties as substituents were synthesized and characterized as blue emitters for organic light-emitting diodes (OLEDs). Their optical, electrochemical and thermal properties have been investigated, and their molecular structure-property relationships were evaluated. These compounds both exhibited a high glass-transition temperature (T_g ≥ 195 °C) and a high decomposition temperature (T_d ≥ 494 °C). The solution processed non-doped device using CAC as a fluorescence emitter showed a maximum luminance efficiency of 1.63 cd A^-1, a maximum power efficiency of 0.77 lm W^-1 and a maximum external quantum efficiency of 1.53%. By introducing 1,3-bis[4-tert-butylphenyl-1,3,4-oxadiazolyl] phenylene (OXD-7):polyvinylcarbazole (PVK) as host in the emitting layer, the doped deep-blue emitting device of CAC exhibited a turn-on voltage of 4.75 V, a maximum luminance efficiency of 3.03 cd A^-1, a maximum power efficiency of 1.64 lm W^-1 and a maximum external quantum efficiency of 2.81%. Our results demonstrate a promising approach to well-designed materials for use in deep-blue fluorescence OLED applications. This journal is

Full text of DOI:10.1039/c5ra00627a

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Page 1 of 35
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DOI: 10.1039/C5RA00627A
Solution-processed Efficient Deep-Blue Fluorescent Organic
Light-Emitting Diodes Based On Novel 9,10-diphenyl-anthracene
Derivatives
Zhaohang Zhang,a Wei Jiang,*a Xinxin Ban,a Min Yang,b Shanghui Ye,*b Bin Huang,a
and Yueming Sun,*a
a
School of Chemistry and Chemical Engineering, Southeast University, Nanjing,
210096, Jiangsu, P. R. China
b
Institute of Advanced Materials (IAM) Nanjing University of Posts &
Telecommunications (NUPT), 9 Wenyuan Road, Nanjing, 210023, Jiangsu, P. R.
China
Corresponding authors:
*eꢀmail: 101011462@seu.edu.cn (W. Jiang); eꢀmail: iamshye@njupt.edu.cn (S. Ye);
eꢀmail: sun@seu.edu.cn (Y. Sun).
1

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Abstract
A series of 9,10ꢀdiphenylꢀanthracene derivatives bearing either benzimidazole or
carbazole moieties as substituents materials were synthesized and characterized as
blue emitters for organic lightꢀemitting diodes (OLEDs). Their optical,
electrochemical and thermal properties have been investigated, and their molecular
structureꢀproperty relationships were evaluated. These compounds both exhibited high
glassꢀtransition temperature (Tg ≥ 195 °C) and high decomposition temperature (Td ≥
494 °C). The solution processed nonꢀdoped device using CAC for fluorescence
emitter showed a maximum luminance efficiency of 1.63 cd/A, a maximum power
efficiency of 0.77 lm/W and a maximum external quantum efficiency of 1.53 %. By
introducing
1,3ꢀbis[4ꢀtertꢀbutylphenylꢀ1,3,4ꢀoxadiazolyl]
phenylene
(OXDꢀ7) :Polyvinylcarbazole (PVK) as host in the emitting layer, the doped
deepꢀblue emitting device of CAC exhibited a turnꢀon voltage of 4.75 V, a maximum
luminance efficiency of 3.03 cd/A, a maximum power efficiency of 1.64 lm/W and a
maximum external quantum efficiency of 2.81 %. Our results demonstrate a
promising approach to wellꢀdesigned materials for use in deepꢀblue fluorescence
OLED applications.
Key words Anthracene, Carbazole, Benzimidazole, Solution processed, Blue
fluorescence
2

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Introduction
Organic light emitting diodes (OLEDs) have been extensively studied for
applications in fullꢀcolor flatꢀpanel displays and lighting sources owing to their ability
to be driven by low voltages, have high brightness, and be easily fabricated in large
area and thin film devices.1ꢀ5 The development of deepꢀblue, is of special significance
because such emitters could be utilized to generate light of other colors by energy
cascading to a lowerꢀenergy fluorescent.6 Recently, numerous materials have been
developed to be used as blue emitter, and some of them exhibited excellent
performance.7ꢀ9 The performance of blue electroluminescence (EL) devices needs to
be further improved in terms of efficiency, lifetime and color purity owing to the far
superior stabilities and device performances of green and red emitters. The synthesis
of blue fluorescence materials with multiple functions, such as high emissive
efficiency, transport properties and balanced charge injection, good morphological
and thermal stability, still remains one of the most challenging areas in this field.
Anthracene has been intensively studied as an attractive starting material, due to
its unique chemical and electronꢀrich structure, low electronic band gap and strong
blue fluorescence, and also used as blue emitters in OLEDs.10ꢀ12 The anthracene can
be easily modified though directly introducing different functionalized blocks at the
9,10ꢀpositions because of the electronꢀrich properties and large pꢀconjugation system
which enhance the separated energy levels of HOMO and LUMO, as well as can be
used in solutionꢀprocessed devices. The phenyl rings and anthracene core were
applied as linker, which will reduce the substituent group interactions between
9,10ꢀpositions and solve the problems of huge bathochromic effect and low quantum
yield.13 Therefore, the devices containing diphenylanthracene as an emitter are
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expected to show higher electroluminescence efficiency, long lifetime, and deep blue
color. Furthermore, carbazoleꢀbased molecules have been widely used as blue
fluorescence OLEDs due to their high triplet energy and excellent holeꢀtransporting
properties.14ꢀ17 The introduction of the holeꢀtransporting carbazole moiety into a
molecule can significantly enhance the glassyꢀstate durability, thermal stability and
the electroꢀoptical properties of organic compounds.18ꢀ21 On the other hand, the
electronꢀtransporting materials are structurally more diverse such as
tris(8ꢀhydroxyquinolinato) aluminum(III) (Alq3),1 oxadiazole, triazole derivatives and
other nitrogenꢀcontaining electroꢀdeficient materials.22,23 Benzimidazole group was
introduced to the material as an electron acceptor due to its high electron affinity.24,25
In this paper, we synthesized a series of blue lighting materials containing
carbazole moiety and benzimidazole moiety of the 9,10ꢀdiphenylꢀanthracene
derivatives, 9,10ꢀbis(3',5'ꢀbis(1ꢀphenylꢀ1Hꢀbenzo[d]imidazolꢀ2ꢀyl)biphenylꢀ4ꢀyl)ꢀantꢀ
hracene (CAC), 9,10ꢀbis(3,6ꢀBis(3,6ꢀdiꢀtertꢀbutylꢀcarbazolꢀ9ꢀyl)ꢀcarbazole)biphenylꢀ
4ꢀyl)ꢀanthracene (BAB) and 9ꢀ[4ꢀ(3,6ꢀBis(3,6ꢀdiꢀtertꢀbutylcarbazolꢀ9ꢀyl)ꢀcarbazole)ꢀ
phenyl]ꢀ10ꢀ[4ꢀ(3',5'ꢀbis(1ꢀphenylꢀ1Hꢀbenzo[d]imidazolꢀ2ꢀyl))ꢀphenyl]ꢀanthracene (B
AC). As shown in the Scheme 1, it is particularly intriguing to compare the
electroluminescent properties of CAC, BAB and BAC, all possessing the same
molecular structure containing 9,10ꢀdiphenylꢀanthracene units. We only changed the
connecting substituent group of carbazole and/or benzimidazole. The color and energy
levels were modified by the phenyl bridge between the functionalized moieties and
the anthracene core. The thermal properties and photophyscial properties were fully
4

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investigated. The HOMO and LUMO values were measured by cyclic voltammetry.
In addition, a series of devices were fabricated to systematically study the emitting
properties of these 9,10ꢀdiphenylꢀanthracene derivatives. We achieved a current
efficiency of 3.03 cd/A, a maximum power efficiency of 1.64 lm/W, a maximum
external quantum efficiency of 2.81 % and a maximum brightness of 3490 cd/m2 with
a deepꢀblue CIE (0.16, 0.10) in a doped device for CAC.
Experimental
General
All reactants and solvents, unless otherwise stated, were purchased from
1
commercial sources and used as received. H NMR and 13C NMR spectra were
recorded on a Bruker ARX300 NMR spectrometer with tetramethylsilane as the
internal standard. Mass spectrometry was performed with a Thermo Electron
Corporation Finnigan LTQ mass spectrometer. Elemental analysis was performed on
an Elementar Vario EL CHN elemental analyzer. Absorption spectra were recorded
with a UVꢀvis spectrophotometer (SHIMADZU 2450) and PL spectra were recorded
with a fluorospectrophotometer (HORIBA, FL3ꢀ2iHR). Thermogravimetric analysis
(TGA) was performed using a Netzsch simultaneous thermal analyzer (STA) system
(STA 409PC) under dry nitrogen atmosphere at a heating rate of 10 °C/min.
Glassꢀtransition temperature was recorded by DSC at a heating rate of 10 °C/minwith
a thermal analysis instrument (DSC 2910 modulated calorimeter). Cyclic voltammetry
were performed on a Princeton Applied Research potentiostat/galvanostat model 283
voltammetric analyzer in CH2Cl2 solutions (10ꢀ3 M) (oxidation process) or acetonitrile
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solution (10ꢀ3 M) (reduction process) at a scan rate of 100 mV/s with a platinum plate
as the working electrode, a silver wire as the pseudoꢀreference electrode, and a
platinum wire as the counter electrode. The supporting electrolyte was
tetrabutylammonium hexafluorophosphate (0.1 M) and ferrocene was selected as the
internal standard. The solutions were bubbled with a constant argon flow for 10 min
before measurements.
Device fabrication and performance measurements
Blue OLED devices using CAC, BAB and BAC as EML with structure
ITO/PEDOT:PSS(30 nm)/EML(20 nm)/TPBI(35 nm)/Ca(10 nm):Ag(100 nm) and
ITO/PEDOT:PSS(30 nm)/PVK:OXDꢀ7(7:3):EML(5 wt%, 10 wt%, 15 wt%, 20
wt%)(20 nm)/TPBI(35 nm)/Ca(10 nm):Ag(100 nm) using Polyvinylcarbazole (PVK):
1,3ꢀbis[4ꢀtertꢀbutylphenylꢀ1,3,4ꢀoxadiazolyl]phenylene (OXDꢀ7) as mixed host were
fabricated and characterized as follows. In a general procedure, indiumꢀtin oxide
(ITO)ꢀcoated glass substrates were preꢀcleaned carefully and treated by UV ozone for
4
min.
A
30
nm
poly(3,4ꢀethylenedioxythiophene)
doped
with
poly(styreneꢀ4ꢀsulfonate)(PEDOT:PSS) aqueous solution was spin coated onto the
ITO substrate and baked at 210 °C for 10 min. The substrates were then taken into a
nitrogen glove box, where emitting layer was spin coated onto the PEDOT:PSS layer
from chlorobenzene solution and annealed at 120 °C for 30 min. The substrate was
then transferred into an evaporation chamber, where the TPBI was evaporated at an
evaporation rate of 1–2 Å/s under a pressure of 8 ×10ꢀ5 Pa and the Ca/Ag bilayer
cathode was evaporated at evaporation rates of 1 and 3 Å/s for Ca and Ag,
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respectively, under a pressure of 4 × 10ꢀ4 Pa. The currentꢀvoltage characteristics of
the devices were characterized with Keithley 2602 semiconductor characterization
system and the brightness characteristics of the devices was characterized with
Siꢀphotodiode characterization system. The electroluminescent spectra were collected
with a Photo Research PR655 Spectrophotometer. All measurements of the devices
were carried out in ambient atmosphere without further encapsulations.
Quantum chemical calculations
The geometrical and electronic properties of CAC, BAB and BAC were
performed with the Gaussian 03 program package.26 The calculation was optimized at
the B3LYP/6ꢀ31G(d) level of theory. The molecular orbitals were visualized using
Gaussview.
Materials
Compounds Synthesis: The benzimidazole 1 and 2 was prepared according to
published procedures.27 The carbazole dendrons 3 were synthesized according to the
literature methods.28,29 The compounds 4 was synthesized by already known reduction
procedure developed by Smet et al.30
Synthesis of 2,2'-(5-bromo-1,3-phenylene)bis(1-phenyl-1H-benzo[d]imidazole)
(1).The mixture of 5ꢀBromoꢀbenzeneꢀ1,3ꢀdicarbaldehyde (2.13 g, 10 mmol),
2ꢀAminodiphenylamine (4.6 g, 25 mmol) was dissolved in DMF (20 mL). A solution
of disodiummetabisulfite (3.8 g 20 mmol) in water (10 mL) was added slowly. The
mixture was stirred at 80 °C for 12 h. After cooling to room temperature, the solvent
was removed by suction filtration and the residue was extracted with dichloromethane.
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The product was then obtained by column chromatography on silica gel with
petroleum ether/ dichloromethane/ethyl acetate (5:5:1) as the eluent, to yield a white
1
solid (3.5 g, 65 %). HꢀNMR (500 MHz, CDCl3, δ): 7.87ꢀ7.85 (m, 4H), 7.53 (s, 1H),
7.51–7.44 (m, 6H), 7.34 (t, J = 7.6 Hz, 2H), 7.28 (d, J = 7.5 Hz, 2H), 7.19 (t, J = 6.9
Hz, 6H). 13CꢀNMR (300 MHz, CDCl3, δ): 149.74, 149.45, 137.12, 136.10, 133.35,
130.08, 129.09, 128.47, 127.34, 123.97, 123.43, 122.53, 119.99, 110.62. MS
(MALDIꢀTOF) [m/z]: calcd for C32H21N4Br, 540.1; found, 540.6. Anal. Calcd. for
C32H21N4Br: C, 70.99; H, 3.91; N 10.35. Found: C, 70.85; H, 3.94; N 10.26.
Synthesis of 2,2'-(5-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-1,3-phenylene)
bis(1-phenyl-1H-benzo[d]imidazole) (2). The mixture of 1 (1.03 g, 1.9 mmol),
bis(pinacolato)diboron (0.53 g, 2.1 mmol), Pd(dppf)Cl2 (0.06 g, 0.05 mmol),
anhydrous potassium acetate (0.4 g, 4 mmol), and dried DMF 20 mL was refluxed
under an nitrogen atmosphere for 24 h at 80 °C. After the reaction finished, the
reaction mixture was extracted with dichloromethane and washed with water. The
organic layer was dried by anhydrous MgSO4 and filtered. The product was isolated
by silica gel column chromatography using petroleum ether/ dichloromethane/ethyl
1
acetate (5:5:1) as eluent to afford a white solid (1.02 g, 76 %). H NMR (300 MHz,
CDCl3, δ): 8.20 (s, 2H), 7.88 (d, J = 7.9 Hz, 2H), 7.50 (s, 1H), 7.43ꢀ7.21 (m, 12H),
7.16 (d, J = 7.2 Hz, 4H), 1.27 (s, 12H). 13CꢀNMR (300 MHz, CDCl3, δ): 137.23,
129.84, 128.78, 127.46, 123.56, 119.76, 110.52, 84.09, 24.80. MS (MALDIꢀTOF)
[m/z]: calcd for C38H33BN4O2, 588.3; found, 588.6. Anal. Calcd. for C38H33BN4O2: C,
77.55; H, 5.65; N 9.52. Found: C, 77.45; H, 5.70; N 9.56.
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Synthesis of 9,10-bis(3',5'-bis(1-phenyl-1H-benzo[d]imidazol-2-yl)biphenyl-4-yl)-
anthracene (CAC). The mixture of 9,10ꢀBisꢀ(4ꢀbromoꢀphenyl)ꢀ9,10ꢀdihydroꢀanthraꢀ
cene (0.488 g, 1 mmol), 3 (1.8 g, 2.5 mmol), cuprous iodide (0.5 g, 2.63 mmol),
1,10ꢀPhenanthroline hydrate (0.5 g, 2.52 mmol), potassium carbonate (0.83 g, 6
mmol), and dried DMF 20 mL was refluxed under an nitrogen atmosphere for 24 h at
160 °C. After the reaction finished, the reaction mixture was extracted with
dichloromethane and washed with water. The product was isolated by silica gel
column chromatography using petroleum ether/ ethyl acetate (50:1) as eluent to afford
1
a yellow solid (1.05 g, 60 %). H NMR (300 MHz, CDCl3, δ): 8.32 (s, 4H), 8.19 (s,
8H), 8.06 (d, J=8.1 Hz, 4H), 7.98ꢀ7.88 (m, 12H), 7.73 (dd, J = 8.7, 1.8 Hz, 4H), 7.55
(q, J = 3.3 Hz, 4H), 7.51 (d, J = 1.5 Hz, 4H), 7.48 (d, J = 1.5 Hz, 4H), 7.4 (d, J = 8.4
Hz, 8H), 1.48 (s, 72H). 13CꢀNMR (300 MHz, CDCl3, δ): 142.13, 139.91, 139.66,
138.37 , 135.90, 132.70, 130.62, 129.58, 126.61, 126.40, 125.60, 125.19, 123.71,
123.09, 122.68, 118.94, 115.76, 110.83, 108.64, 34.26, 31.57. MS (MALDIꢀTOF)
[m/z]: calcd for C130H124N6, 1769.0; found, 1768.6. Anal. Calcd. for C130H124N6: C,
88.19; H, 7.06; N 4.75. Found: C, 88.05; H, 6.98; N 4.81.
Synthesis of 9,10-bis(3,6-Bis(3,6-di-tert-butylcarbazol-9-yl)-carbazole)biphenyl-
4-yl)-anthracene (BAB). The mixture of 9,10ꢀBisꢀ(4ꢀbromoꢀphenyl)ꢀ9,10ꢀdihydroꢀ
anthracene (0.244 g, 0.5 mmol), 2 (0.7 g, 1.2 mmol), potassium carbonate (0.7 g, 5
mmol), terakis(triphenylphosphine)palladium (0.1 g, 0.08 mmol) was dissolved in 20
mL mixture solution of toluene: ethanol: water (6:3:1) and refluxed under nitrogen for
24 h at 80 °C. After the reaction finished, the reaction mixture was extracted with
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dichloromethane and washed with water. The product was isolated by silica gel
column chromatography using petroleum ether/ dichloromethane/ethyl acetate (5:5:1)
as eluent to afford a yellow solid (0.45 g, 72 %). 1H NMR (300 MHz, CDCl3, δ): 8.04
(s, 2H), 7.92 (d, J = 8.7 Hz, 8H), 7.68 (dd, J = 6.9, 3 Hz, 4H), 7.59ꢀ7.49 (m, 12H),
7.45 (d, J = 8.1 Hz, 4H), 7.35 (d, J = 7.5 Hz, 18H), 7.31 (d, J = 5.1 Hz, 10H).
13CꢀNMR (300 MHz, CDCl3, δ): 150.93, 140.32, 138.18, 138.15, 136.64, 136.31,
136.08, 131.23, 130.29, 129.58, 129.32, 128.82, 128.29, 127.12, 126.52, 126.37,
124.66, 123.16, 122.72, 119.54, 110.06. MS (MALDIꢀTOF) [m/z]: calcd for C90H58N8,
1250.48; found, 1250.82. Anal. Calcd. for C90H58N8: C, 86.37; H, 4.67; N 8.95. Found:
C, 86.22; H, 4.53; N 8.97.
Synthesis of 9-[4-(3,6-Bis(3,6-di-tert-butylcarbazol-9-yl)-carbazole)-phenyl]-10-
[4-(3',5'-bis(1-phenyl-1H-benzo[d]imidazol-2-yl))-phenyl]-anthracene (BAC). The
5 was synthesized by the same procedure like CAC through change the mixture ratio
of 9,10ꢀBisꢀ(4ꢀbromoꢀphenyl)ꢀ9,10ꢀdihydroꢀanthracene (1.22 g, 2.5 mmol) and 3
(0.72 g, 1 mmol). The 1.0 g mixture products of 5 and CAC, 2 (0.7 g, 1.2 mmol),
potassium carbonate (0.7 g, 5 mmol), terakis(triphenylphosphine)palladium (0.1 g,
0.08 mmol) were dissolved in 15 mL mixture solution of toluene: ethanol: water
(6:3:1) and refluxed under nitrogen for 24 h at 80 °C. After the reaction finished, the
reaction mixture was extracted with dichloromethane and washed with water. The
product was isolated by silica gel column chromatography using petroleum
ether/dichloromethane/ethyl acetate (5:5:1) as eluent to afford a yellow solid (0.7 g,
50 %). 1H NMR (300 MHz, CDCl3, δ): 8.31 (s, 2H), 8.18 (s, 4H), 8.03ꢀ7.83 (m, 14H),
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7.73 (t, J = 9.0 Hz, 4H), 7.57ꢀ7.48 (m, 15H), 7.41ꢀ7.30 (m, 16H), 1.48 (s, 36H).
13CꢀNMR (300 MHz, CDCl3, δ): 150.76, 142.11, 139.92, 139.68, 138.17, 136.55,
136.20, 132.71, 132.35, 131.25, 130.59, 129.62, 129.44, 128.99, 128.87, 128.83,
128.38, 127.13, 126.62, 126.56, 126.41, 126.21, 125.58, 125.04, 124.82, 124.68,
123.69, 123.29, 123.10, 122.89, 122.85, 122.67, 119.46, 118.92, 118.56, 115.75,
110.84, 110.11, 108.63, 34.26, 31.57. MS (MALDIꢀTOF) [m/z]: calcd for C110H91N7,
1509.7; found, 1509.2. Anal. Calcd. for C110H91N7: C, 87.44; H, 6.07; N 6.49. Found:
C, 87.57; H, 6.02; N 6.38.
Results and Discussion
Synthesis and characterization
The synthesis routes and chemical structures of CAC, BAB, and BAC are shown
in Scheme 1. Firstly, the reaction of 1ꢀfluoroꢀ2ꢀnitrobenzene and aniline in the
presence of potassium fluoride gave NꢀPhenylꢀoꢀphenylenediamine in 61 % yield,
followed by reduction of the nitro group with stannous chloride dihydrate (80 %). The
resulting diamine derivative reacted with 5ꢀBromoꢀbenzeneꢀ1,3ꢀdicarbaldehyde to
produce the 2,2'ꢀ(5ꢀbromoꢀ1,3ꢀphenylene)bis(1ꢀphenylꢀ1Hꢀbenzo[d]imidazole) (1) in
65%
yield.
1
was
converted
to
the
aryboronic
ester
of
2,2'ꢀ(5ꢀ(4,4,5,5ꢀtetramethylꢀ1,3,2ꢀdioxaborolanꢀ2ꢀyl)ꢀ1,3ꢀphenylene)bis(1ꢀphenylꢀ1Hꢀ
benzo[d]imidazole) (2) catalyzed by Pd(dppf)Cl2 in the presence of CH3COOK in
DMF with
a
yield of 76 %. Secondly, the key intermediate
3,6ꢀbis(3,6ꢀdiꢀtertꢀbutylꢀcarbazolꢀ9ꢀyl)ꢀcarꢀbazole (3) was prepared via a fiveꢀstep
reaction with a total yield of 40%. Thirdly, the compounds 4 was synthesized by
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already known reduction procedure.30 Finally, the target molecules were synthesized
via a classic Ullmann reaction and Suzuki reaction respectively. All of the compounds
were purified by silica chromatography and recrystallization, producing very pure
1
powders. HꢀNMR, 13CꢀNMR, mass spectrometry, and elemental analysis were
employed to confirm the chemical structures of the aboveꢀmentioned compounds, as
described in the experimental section.
Thermal analysis
The thermal stabilities of the CAC, BAB and BAC were investigated by thermal
gravimetric analyses (TGA) and differential scanning calorimetry (DSC) under
nitrogen atmosphere at a heating rate of 10 °C/min. As shown in Fig. 1, TGA
indicates that CAC, BAB and BAC exhibit decomposition temperature (Td,
corresponding to 5% weight loss) as high as 508, 534 and 494 °C, respectively. The
clear glass transition temperature (Tg) values for BAB and BAC are 195 °C and
246 °C, respectively. No noticeable signals related to the crystallization or melting is
observed in the DSC curves. The Tg of CAC is not found in the DSC curves, which
suggests that CAC could form morphologically stable and uniform amorphous films.
As a result, the solutionꢀdeposited CAC ET layer has an excellent filmꢀforming
property which can be assigned to the favorable solubility of tertꢀbutyl. The excellent
thermal properties of these compounds will retain stable film morphological
properties during the device operation, which are also desirable for good performance
of OLEDs with high efficiency stability.
Electrochemical analysis
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The electrochemical properties of CAC, BAB and BAC were studied in solution
using cyclic voltammetry (CV) using tetrabutylammonium hexafluorophosphate
(TBAPF6) as the supporting electrolyte and ferrocene as the internal standard. As
shown in Fig. 2, during the anodic scan in dichloromethane, CAC and BAC exhibits
similar reversible oxidation process, which can be assigned to the oxidation of
electronꢀdonating carbazole moiety, with the onset potentials of 1.21 and 1.24 V,
respectively. No reduction waves were detected. The HOMO energy levels were
obtained according to the fellow equation: HOMO = ꢀ(4.8 + Eoox)eV, using ferrocene
as an internal standard (ꢀ4.8 eV below vacuum). On the basis of the onset potentials
for oxidation, we estimated the HOMO energy of CAC and BAC to be ꢀ5.23 and
ꢀ5.26 eV. These results reveal that the introduction of carbazole groups to the
anthracene core can lead to the raising of HOMO levels, which are in agreement with
the DFT calculations results. BAB and BAC exhibited reduction waves, arising from
their benzimidazole moiety, with onset potentials of ꢀ2.49, ꢀ1.92 V, respectively. These
together with absorption spectra were then used to obtain the LUMO energy levels.
Theoretical calculation
The threeꢀdimensional geometries and the frontier molecular orbital energy
levels of these anthracene derivatives were calculated using density functional theory
(DFT). The calculations of the HOMO and LUMO levels were depicted in Table 1.
The geometries of these compounds in Fig. 3 show that the carbazole, benzimidazole
and phenyl units are significantly twisted with the anthracene core, resulting in a
nonꢀplanar structure in each molecule. These geometrical characteristics can
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effectively prevent intermolecular interactions between πꢀsystems and thus suppress
molecular recrystallization and limit the effective conjugation length of these
molecules. As depicted in Fig. 3, The LUMO levels of all the compounds are
localized predominantly on the anthracene core, while the HOMO levels of CAC and
BAC are distributed over the electronꢀrich triꢀcarbazole fragment and BAB is
localized on the anthracene core, respectively. Compared with BAB, the HOMO
levels of CAC and BAC have increased as the increasing of the generation of the
carbazole with the calculated values of ꢀ5.04 eV and ꢀ5.00 eV, which are more
matching with the level of PEDOT:PSS so that reduce the injection of hole and
strengthen the device efficiency.
Photophysical properties
Fig. 4 depicts the UVꢀvis absorption (a) and photoluminescent (b) spectra of
CAC, BAB and BAC in CH2Cl2 and solid film. Table 1 summarized the maximal
absorption and PL emission wavelengths of the carbazole or/and benzimidazole
substituted anthracene derivatives. The absorption spectra of CAC and BAC exhibits
two absorption peaks at 242 nm and 298 nm which can be assigned to the πꢀπ* and
nꢀπ* transitions of the carbazole moiety respectively, while the absorption peak at
290ꢀ300 nm of BAB is attributed to the benzimidazole moieties and the peak at 263
nm can be due to the benzene ring characteristic absorption of BAB. The peaks at 242
nm and 263 nm of BAC indicated the small organic molecule with carbazole and
benzimidazole units was synthesized successfully. The common moderately weak
absorptions in the range from 350ꢀ400 nm can be assigned to the characteristic
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vibronic patterns πꢀπ* transition of anthracene.31 Upon UV excitation, the PL
spectrum of CAC, BAB and BAC have lost vibronic structure and the emission peaks
redꢀshifted to 435, 430, and 434 nm, respectively. The band gap was calculated from
the edge of the Uvꢀvis absorption peak, giving values of 3.00, 2.98, and 2.99 eV for
CAC, BAB and BAC, respectively. The film fluorescence quantum yields (ФF) of
CAC, BAB and BAC are 0.87, 0.59 and 0.70, respectively. The results indicates that
direct attachment of the carbazole substituent group to the 9,10ꢀdiphenylꢀanthracene
moiety enhanced the ФF of CAC and BAC.
Electroluminescence properties
To investigate the EL performance of CAC, BAC and BAB, nonꢀdoped
fluorescent OLEDs device A and doped fluorescent OLEDs device B were fabricated
with a configuration of ITO/PEDOT:PSS(30 nm)/EML(20 nm)/TPBI(35 nm)/Ca(10
nm):Ag(100 nm) and ITO/PEDOT:PSS(30 nm)/PVK:OXDꢀ7(7:3):EML(5 wt%, 10
wt%, 15 wt%, 20 wt%)(20 nm)/TPBI(35 nm)/Ca(10 nm):Ag(100 nm), respectively.
The materials structures and the energy level diagrams of the devices are show in Fig.
5. In these devices, indium tin oxide (ITO) was used as the anode,
poly(3,4ꢀethylenedioxythiophene) : poly(styreneꢀ4ꢀsulfonate) (PEDOT:PSS) as the
hole
injection
layer,
the
electronꢀtransporting
material
1,3ꢀbis[4ꢀtertꢀbutylphenylꢀ1,3,4ꢀoxadiazolyl]phenylene (OXDꢀ7) mixed into the
holeꢀtransporting material Polyvinylcarbazole (PVK) as host in the emitting layer,
1,3,5ꢀtri(phenylꢀ2ꢀbenzimidazolyl)benzene (TPBi) as electronꢀtransporting and
holeꢀblocking layer, and Ca:Ag as the electron injecting layer. All of the important
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performance parameters of these OLEDs are summarized in Table 2. The
electroluminescent (EL) spectra and current densityꢀvoltageꢀluminance (JꢀVꢀL)
characteristics of the devices are shown in Fig. 6 and Fig.7.
Fig. 6 (a) presents the current densityꢀvoltageꢀluminance (JꢀVꢀL) curves of CAC,
BAB and BAC based device A, which turned on at 4.91, 5.80 and 5.49 V
(corresponding to 1cd/m2) and have a maximum brightness (Lmax) of 3983, 400 and
2945 cd/m2, respectively. As revealed in Fig. 6 (b), the device A based on CAC shows
better performance with the maximum luminance efficiency (ηc, max) of 1.63 cd/A, the
maximum power efficiency (ηp, max) of 0.77 lm/W and the maximum external quantum
efficiency (ηext, max) of 1.53 %. In the contrast, the device A of BAB and BAC shows
the maximum luminance efficiency of only 0.11 cd/A and 0.61 cd/A because of the
deep HOMO result in the unmatched level with PEDOT:PSS and unbalanced of the
hole and electron. The low turnꢀon voltage indicates an efficient and balanced charge
injection, transport and recombination, most likely due to the HOMO energy levels of
CAC being better matched with the PEDOT:PSS and the holeꢀtransporting carbazole
moiety increasing carrier balance in the emitting layer may also contribute to the high
performance. The excellent performance is also probably due to the excellent thermal
stability of the CAC, which significantly enhances the capability of forming stable
amorphous thin films. The unsatisfactory performance of BAB based device A on
account of the BAB bad solubility in CH2Cl2 among these molecules. Fig. 6 (c)
reveals the identical electroluminescent spectra with deepꢀblue emission wavelength
at 444, 448 and 448 nm of device A, respectively. The result also implies that CAC,
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BAB and BAC should be a potential candidate for stable deepꢀblue fluorescent
OLEDs materials.
To further improve the performance of the device, CAC, BAB and BAC were
doped in PVK:OXDꢀ7 (7:3) as emitting layer, respectively. We fabricated device B
with an architecture of ITO/PEDOT:PSS(30 nm)/PVK:OXDꢀ7(7:3):EML(5 wt%, 10
wt%, 15 wt%, 20 wt%)(20 nm)/TPBI(35 nm)/Ca(10 nm):Ag(100 nm). The devices’
performances are presented in Fig. 7 with the key parameters summarized in Table 2.
Device B doped with BAB shows the highest turnꢀon voltage than CAC and BAC,
which decreased with the increase of BAB doping concentration. The maximum
electrochemical properties value of device B with 5 % doping concentration is similar
to that of the device B with 10 % doping concentration, while device B with 15 %
doping concentration shows higher efficiency over all current density range. For
instance, the device B with 15 % doping concentration displays the ηc, max of 0.67 cd/A
and the ηext, max of 0.60 %, while the 5 %, 10 % and 20 % doping concentration
devices show the ηc, max of 0.55, 0.57 and 0.42 cd/A, and the ηext, max of 0.50, 0.52 and
0.38 %, respectively. Furthermore, device B doped CAC and BAC have the similar
trends on the doping concentration change and the OLEDs performance. For example,
the device B doped BAC with 15 % doping concentration shows the ηc, max, ηp, max and
ηext, max of 1.00 cd/A, 0.36 lm/W and 0.73 %, which displays the best performance
during four various doping concentration. The devices of compounds with the
holeꢀtransporting carbazole moiety, CAC and BAC, show better performance than the
BAB devices. Especially, the device B doped CAC has the best performance among
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these diverse devices. For instance, the device B doped CAC with 15 % doping
concentration shows the ηc, max, ηp, max and ηext, max of 3.03 cd/A, 1.64 lm/W and 2.81 %
better than all other devices. The device also has a low turnꢀon voltage of 4.75 V and a
high Lmax of 3490 cd/m2. Device B of CAC shows the best electroluminescent
characteristics can be attributed to the highest film fluorescence quantum efficiency
(ФF) of CAC. This result is also consistent with the view that the introduction of the
holeꢀtransporting carbazole moiety into the fluorescent materials can balance charge
injection, transport and recombination more efficiently which also contribute to the
high performance. TPBI as electron transport layer can effective transmit electron so
that the device contain BAB or BAC with electronꢀtransporting benzimidazole
moieties have weak capacity to balance the exciton recombination in the emitting
layer. The electronꢀtransporting material OXDꢀ7 and the holeꢀtransporting material
PVK as host with appropriate mixing ratio in the emitting layer can enhance the
exciton transport and recombination, which contribute to lowering the turnꢀon voltage,
increasing the maximum brightness and improving the performance of deepꢀblue
fluorescent OLEDs materials. As shown in table 2, the CIE coordinates are (0.16,
0.10), (0.17, 0.11) and (0.16, 0.11) for CAC, BAB and BAC, respectively, which were
measured in device B with 15 % doping concentration. Fig. 7 (c) reveals the identical
electroluminescent spectra with deepꢀblue emission wavelength at 448, 436 and 440
nm of device B, respectively. Furthermore, no additional emission coming from host
materials was observed, indicative of efficient energy transfer from the host to the
emitters.
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Conclusion
In summary, we have designed and synthesized three novel materials CAC, BAB and
BAC for solutionꢀprocessed deepꢀblue fluorescent OLEDs. A series of devices based
on these derivatives exhibit their excellent thermal and electrochemical stability,
charge transport character, solubility and filmꢀforming property. The calculated
geometries of CAC and BAC show that the carbazole units are significantly twisted
against the anthracene core, effectively suppressing molecular recrystallization and
limit the extent of conjugation, which improves the morphological stability of the thin
film. The holeꢀtransporting carbazole moiety increasing carrier balance in the emitting
layer may also contribute to the high performance. Herein, we fabricated
solutionꢀprocessed deepꢀblue fluorescence OLEDs with CAC as a dopant, and the
device showed low turnꢀon voltage of 4.75 V, high efficiency of 3.03 cd/A, 1.64 lm/W,
and a maximum external quantum efficiency of 2.81 %. According to these
characteristics, these materials, especially CAC, have sufficient potential for use in
deepꢀblue fluorescence OLED applications.
Acknowledgments
The authors thank the National basic Research Program China (2013CB932902) and
National Natural Science Foundation of China (51103023, 21173042) for their
financial support.
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References
1 C. W. Tang and S. A. Vanslyke, Appl. Phys. Lett., 1987, 51, 913.
2 S. R. Forrest, Nature, 2004, 428, 911.
3 B. W. D’Andrade and S. R. Forrest, Adv. Mater., 2004, 16, 1585.
4 C. C. Wu, Y. T. Lin, K. T. Wong, R. T. Chen and Y. Y. Chien, Adv. Mater., 2004, 16,
61.
5 A. J. Heeger, Angew. Chem., Int. Ed., 2001, 40, 2591.
6 Z. Q. Jiang, Z. Y. Liu, C. L. Yang, C. Zhong, J. G. Qin, G. Yu and Y. Q. Liu, Adv.
Funct. Mater., 2009, 19, 3987.
7 M. R. Zhu and C. L. Yang, Chem. Soc. Rev., 2013, 42, 4963.
8 C. Fan, Y. Li, C. Yang, H. Wu, J. Qin and Y. Cao, Chem. Mater., 2012, 24, 4581.
9 H. Uoyama, K. Goushi, K. Shizu, H. Nomura and C. Adachi, Nature, 2012, 492,
234.
10 K. Danel, T. H. Huang, J. T. Lin, Y.T. Tao and C.H. Chuen, Chem. Mater., 2002, 14,
3860.
11 C. H. Wu, C. H. Chien, F. M. Hsu, P. I. Shih and C. F Shu., J. Mater. Chem., 2009,
19, 1464.
12 C. H. Chien, C. K. Chen, F. M. Hsu, C. F. Shu, P. T. Chou and C. H. Lai, Adv.
Funct. Mater., 2009, 19, 560.
13 J. Huang, J. H. Su, X. Li, M. K. Lam, K. M. Fuang, H. H. Fan, K. W. Cheah, C. H.
Chen and H. Tian, J. Mater. Chem., 2011, 21, 2957.
14 M. H. Tsai, H. W. Lin, H. C. Su, T. H. Ke, C. C. Wu, F. C. Fang, Y. L. Liao, K. T.
Wong and C. I. Wu, Adv. Mater., 2006, 18, 1216.
15 S. O. Jeon, K. S. Yook, C. W. Joo and J. Y. Lee, Adv. Mater., 2011, 22, 1872.
16 W. Jiang, L. Duan, J. Qiao, L. D. Wang and Y. Qiu, Org. Lett., 2011, 13, 3146.
17 Y. Tao, Q. Wang, C. Yang, Q. Wang, Z. Zhang, T. Zou, J. Qin and D. Ma, Angew.
Chem., Int. Ed., 2008, 47, 8104.
18 W. J. Kuo, S. L. Lin, S. D. Chen, C. P. Chang, R. H. Lee and R. J. Jeng, Thin Solid
Films, 2008, 516, 4145.
19 I. Cho, S. H. Kim, J. H. Kim, S. Park and S. Y. Park, J. Mater. Chem., 2012, 22,
123.
20 P. Kundu, K. R. J. Thomas, J. T. Lin, Y. T. Tao and C. H. Chien, Adv. Funct. Mater.,
2003, 13, 445.
21 K. R. J. Thomas, J. T. Lin, Y. T. Tao and C. W. Ko, J. Am. Chem. Soc., 2001, 123,
9404.
22 G. Hughes and M. R. Bryce, J. Mater. Chem., 2005, 15, 94.
23 J. H. Kim, D. Y. Yoon, J. W. Kim and J. J. Kim, Synth. Met., 2007, 157, 743.
24 Y. Shirota and H. Kageyama, Chem. Rev., 2007, 107, 953.
25 C. H. Chen, W. S. Huang, M. Y. Lai, W. C. Tsao, J. T. Lin, Y. H. Wu, T. H. Ke, L. Y.
Chen and C. C. Wu, Adv. Func. Mater., 2009, 19, 2661.
26 Gaussian 03 (Revision B.05), Gaussian, Inc., Wallingford CT, 2004.
27 W. Jiang, J. N. Tang, X. X. Ban, Y. M. Sun, L. Duan and Y. Qiu, Org. Lett., 2014,
16, 5346.
20

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RSC Advances
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28 H. Zhang, S. Wang, Y. Li, B. Zhang, C. Du, X. Wan and Y. Chen, Tetrahedron,
2009, 6, 4455.
29 K. Albrecht and K. Yamamoto, J. Am. Chem. Soc., 2009, 131, 2244.
30 M. Smet, J. V. Dijk and W. Dehaen, Tetrahedron, 1999, 55, 7859.
31 I. B. Berlan, Handbook of fluorescence spectra of aromatic molecules. 2nd ed.
New York: Academic, 1971.
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Scheme 1 Synthetic routes toward CAC, BAB, and BAC.
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[a]
[b]
[b]
[c]
[c]
[d]
Compd. Tg/Td
λabs
λem
λem
ФF
Eg
(eV)
Eg Calcd [e]
HOMO/
LUMO[e]
(eV)
HOMO/
LUMO[f]
(eV)
(°C)
(nm)
(nm) (nm)
(eV)
CAC
BAB
BAC
ꢀ/508
195/534
242/298/351/376/396
263/298/358/377/397
435 439
430 439
0.87 3.00
0.59 2.97
0.70 2.98
3.01
3.47
3.15
ꢀ5.04/ꢀ2.03 ꢀ5.23/ꢀ2.25
ꢀ5.11/ꢀ1.64 ꢀ5.87/ꢀ2.88
ꢀ5.00/ꢀ1.85 ꢀ5.26/ꢀ2.26
246/494 242/262/298/356/377/397 434 442
[a] Measured by DSC/TGA at a heating rate of 10 °C min–1.[b] Measured in CH2Cl2.[c] Measured in thin film on quartz
substrate. [d] Estimated from the optical absorption edge, Eg = 1240/λonset.[e] Obtained from quantum calculations by using
B3LYP/6ꢀ31G(d).[f] Calculated by HOMO = –(4.8 + EOXonset), and LUMO = HOMO+ Eg, where EOX
is the onset
onset
potential of the oxidation.
Table 1 Physical data of the materials.
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[a]
[b]
[c]
[d]
[e]
Device
emitter
Von
Lmax
ηc,max
ηp,max
ηext,ma
(%)
Devices@100cd/m2
ηc ηp ηext
Devices@1000cd/m2
ηc ηp ηext
CIE
(V) (cd/m2) (cd/A) (lm/w)
(x, y)[f]
(cd/A) (lm/w) (%) (cd/A) (lm/w) (%)
A
CAC
BAB
BAC
4.91
5.80
5.49
3983
400
1.63
0.11
0.61
1.53
1.95
3.03
1.96
0.55
0.57
0.67
0.42
0.68
0.75
1.00
0.74
0.77
0.04
0.27
0.60
0.89
1.64
1.03
0.20
0.22
0.23
0.14
0.20
0.28
0.36
0.26
1.53
0.10
0.47
1.41
1.80
2.81
1.81
0.50
0.52
0.60
0.38
0.50
0.55
0.73
0.53
1.35
0.10
0.60
1.23
1.94
3.01
1.95
0.48
0.67
0.57
0.39
0.43
0.75
1.00
0.73
0.69
0.04
0.27
0.55
0.85
1.39
0.90
0.14
0.21
0.20
0.14
0.15
0.26
0.33
0.26
1.26 1.46
0.09
0.61
ꢀ
1.37
ꢀ
(0.16, 0.10)
(0.22, 0.22)
(0.18, 0.16)
(0.16, 0.10)
(0.17, 0.12)
(0.16, 0.10)
(0.16, 0.10)
(0.16, 0.09)
(0.18, 0.13)
(0.17, 0.11)
(0.17, 0.11)
(0.16, 0.10)
(0.17, 0.11)
(0.16, 0.11)
(0.16, 0.11)
ꢀ
2945
2826
3476
3490
2970
874
1212
1425
998
1608
1359
1711
1983
0.46 0.40
1.14 1.27
1.79 1.68
2.78 2.22
1.80 1.48
0.13
0.39
0.54
0.71
0.49
ꢀ
0.11
0.12
ꢀ
0.15
0.11
0.16
0.15
0.31
1.17
1.56
2.05
1.37
ꢀ
0.40
0.40
ꢀ
0.44
0.32
0.48
0.41
B[g] CAC 5% 4.92
CAC 10% 4.89
CAC 15% 4.75
CAC 20% 4.30
BAB 5% 7.64
BAB 10% 7.36
BAB 15% 6.42
BAB 20% 7.01
BAC 5% 6.40
BAC 10% 6.68
BAC 15% 6.97
BAC 20% 6.99
0.43
ꢀ
0.60 0.44
0.52 0.44
0.35
ꢀ
0.31 0.61
0.54 0.44
0.72 0.66
0.53 0.57
[a]Recorded at 1 cd/m2.[b]Maximum luminance at applied voltage.[c]Maximum current efficiency.[d] Maximum power efficiency. [e]Maximum
external quantum efficiency.[f]Data were measured at 10 V.[g]CAC, BAB and BAC doped in PVK:OXDꢀ7, respectively.
Table 2 Electroluminescent performance.
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Fig. 1 (a) TGA traces of CAC, BAB and BAC recorded at a heating rate of 10 °C
minꢀ1. (b) DSC measurement recorded at a heating rate of 10 °C minꢀ1.
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Fig. 2 Electrochemical properties of CAC, BAB and BAC.
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Fig. 3 Optimized geometries and calculated HOMO and LUMO density maps for CAC, BAB and BAC according to DFT calculations at the
B3LYP/6ꢀ31* level.
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Fig. 4 Normalized absorption (a) and emission (b) spectra of CAC, BAB and BAC in
dilute dichloromethane solution and solid film.
29