Highly efficient blue OLED based on 9-anthracene-spirobenzofluorene derivatives as host materials

Article abstract of DOI:10.1039/c0jm00593b

A novel spiro[benzo[c]fluorene-7,9′-fluorene](SBFF)-based blue host material, 9-(10-phenylanthracene-9-yl)SBFF (BH-9PA), 9-(10-(naphthalene-1-yl) anthracene-9-yl)SBFF (BH-9NA) and 9-(10-(4-(naphthalene-1-yl)phenyl)anthracene- 9-yl)SBFF (BH-9NPA) were succ

Full text of DOI:10.1039/c0jm00593b

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PAPER
www.rsc.org/materials | Journal of Materials Chemistry
Highly efficient blue OLED based on 9-anthracene-spirobenzofluorene
derivatives as host materials
a
Myoung-Seon Gong, Hyun-Seok Leeb and Young-Min Jeonb
*
Received 3rd March 2010, Accepted 9th August 2010
DOI: 10.1039/c0jm00593b
A novel spiro[benzo[c]fluorene-7,90-fluorene](SBFF)-based blue host material, 9-(10-phenylanthracene-
9-yl)SBFF (BH-9PA), 9-(10-(naphthalene-1-yl)anthracene-9-yl)SBFF (BH-9NA) and 9-(10-(4-
(naphthalene-1-yl)phenyl)anthracene-9-yl)SBFF (BH-9NPA) were successfully prepared by reacting
9-bromo-SBFF (2) with 10-phenylanthracene-9-yl boronic acid, 10-(naphthalene-1-yl)anthracene-9-yl
boronic acid and 10-(4-(naphthalene-1-yl)phenyl)anthracene-9-yl boronic acid through the Suzuki
reaction, respectively. Bis[4-(di-p-N,N-diphenylamino)styryl]stilbene (DSA-Ph) and N,N-diphenyl-
N0,N0-dim-tolyl-SBFF-5,9-diamine (BD-6MDPA) were used as dopant materials. Blue OLEDs with the
configuration ITO/DNTPD/NPB/host : 5% dopant/Alq3/Al-LiF were prepared from the three host
materials doped with DSA-Ph and BD-6MDPA dopants and the device composed of BH-9PA doped
with DSA-Ph and BD-6MDPA showed blue EL spectra at 468 and 464 nm at 7 V and luminance
efficiencies of 7.03 and 6.60 cd Aꢀ1, respectively.
However the 2- and 5-positioned SBFFs are not sufficient to give
Introduction
full play to their characteristics regarding the luminescence effi-
Much effort has been devoted to the development of new amorphous
host and dopant materials possessing high morphological stability.1–7
ciency and color purity. Thus the aim was to improve the
performance of OLEDs host materials based on SBFF, as the
For example, Salbeck et al. have exploited spirobifluorene building
blocks to construct various oligomers.8 Carrier-transporting mate-
position of substitution and aryl substituent with a variety of
conjugation chains determine the color purity and luminescence
rials of spirobifluorene with a high glass transition temperature
efficiency for the OLEDs devices.
showed excellent nondispersive hole transporting and ambipolar
In this study, we report the synthesis of new host materials
carrier transporting properties.9,10 Incorporating aromatic substitu-
9-(10-phenylanthracene-9-yl) SBFF (BH-9PA), 9-(10-(naphtha-
ents would be a useful strategy to expand the application of spiro
lene-1-yl)anthracene-9-yl) SBFF (BH-9NA) and 9-(10-(4-(naph-
compounds. However, spiro compounds with versatile substituents,
thalene-1-yl)phenyl)anthracene-9-yl) SBFF (BH-9NPA) by
especiallyspirobifluorene withan asymmetricsubstitution pattern on
the fluorene units, have seldom been reported.11–13
substitution at the 9-position on the SBFF by a Suzuki reaction.
Various properties such as UV-vis absorption, photo-
Spiro-type derivatives containing fluorene and benzofluorene
luminescence, and electroluminescence including EL efficiency
as OLED fluorescent materials have received a great deal of
and color purity were evaluated.
attention because asymmetrical spiro compounds having
a naphthalene group not only can preserve the inherent charac-
teristics of a spiro compound such as morphological stability,
high glass transition temperature, and amorphous properties,
but also provide a variety of substituents on the aromatic ring of
spiro molecules and result in the formation of conjugation-
controlled OLED host and dopant materials.14–24
Many studies have dealt with spiro[benzo[c]fluorene-7,90-flu-
orene] (SBFF) as an organic electroluminescent host and dopant
material. Derivatives of 2-, 5- and both the 5- and 9- on the SBFF
by substituting the aromatic moieties with various conjugation
lengths were estimated. But the preparation of SBFF substituted
at only the 9-position is extremely difficulty due to the lower
reactivity than that of the naphthyl moiety to electrophilic
substitution reaction (bromination) as shown in Scheme 1.
Experimental
Materials and measurements
10-Phenylanthracene-9-yl boronic acid, 10-(naphthalene-1-
yl)anthracene-9-yl boronic acid and 10-(4-(naphthalene-1-
yl)phenyl)anthracene-9-yl boronic acid were synthesized
according to a previously-reported method.21 5,9-Dibromo-spi-
ro[fluorene-7,90-benzofluorene]19 and bis[4-(di-p-N,N-diphenyla-
mino)styryl]stilbene (DSA-Ph) was prepared by
a
previously-reported method.25 Tetrakis(triphenylphosphine)-
palladium(0), methyl 5-bromo-2-iodobenzoate, naphthalene-1-
boronic acid, phenylboronic acid, trimethyl borate, butyllithium
2.5 M solution in hexanes (Aldrich Chem. Co.), 2-bromobi-
phenyl (TCI Chem. Co.) methanesulfonic acid, hydrochloric
acid, potassium carbonate, sodium hydroxide, and sodium
bicarbonate were used without further purification. Tetrahy-
drofuran and toluene were distilled over sodium and calcium
hydride.
aDepartment of Nanobiomedical Science and WCU Research Center of
Nanobiomedical Science, Dankook University, Cheonan, Chungnam
330-714, Korea. E-mail: msgong@dankook.ac.kr; Fax: +82 41 5503431;
Tel: +82 41 5501476
The 1H-NMR and 13C-NMR spectra were recorded on Varian
bDepartment of Chemistry Dankook University, Chungnam 330-714,
Korea
Unity Inova (200 MHz) and Bruker Avance 500 (500 MHz)
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Scheme 1 Schematic routes to synthesize spiro[benzo[c]fluorene-7,90-fluorene] derivatives
spectrometers. The photoluminescence (PL) spectra were
recorded on a fluorescence spectrophotometer (Jasco FP-6500),
and the UV-vis spectra were obtained by means of a UV-vis
spectrophotometer (Shimadzu, UV-1601PC). The energy levels
were measured with a low-energy photo-electron spectrometer
(Riken-Keiki AC-2). The FT-IR spectra were obtained with
a Thermo Fisher Nicolet 850 spectrophotometer, and the
elemental analyses were performed using a CE Instrument
EA1110. The DSC measurements were performed on a SHI-
MADZU DSC-60 differential scanning calorimeter under
nitrogen at a heating rate of 10 ꢁC minꢀ1. The TGA measure-
ments were performed on a SHIMADZꢁU TGA-50 Thermogra-
vimetric analyzer at a heating rate of 5 C minꢀ1. The low- and
high-resolution mass spectra were recorded using a JEOL JMS-
AX505WA spectrometer in FAB mode.
6 h. The reaction mixture was cooled to room temperature and
acidified with 2 M HCl. The precipitate was filtered and recrys-
tallized from ethanol to give pure product as white crystals. Yield
96%. FT-IR (KBr, cmꢀ1) 3545, 3059, 3018, 1745 (aromatic–C–
H). 1H-NMR (200 MHz, CDCl3) 7.95 (d, J ¼ 2.0 Hz, 1H), 7.64–
7.78 (m, 3H), 7.04–7.40 (m, 6H), 11.0 (br, 1H). MS (FAB) m/z
327.99 [(M + 1)+]. Anal. Calcd for C17H11BrO2 (327.17): C,
62.41; H, 3.39; Br, 24.42; O, 9.78. Found: C, 62.42; H, 3.36%.
Synthesis of 9-bromo-7H-benzo[c]fluoren-7-one (3)
Compound 2 (10 g, 0.03 mol) was dissolved by portions in conc.
methanesulꢁfonic acid (300 mL). The red mixture was stirred for
24 h at 30 C. It was then poured into ice water. After the red
precipitate was filtered and washed with water, the resulting
powder was stirred in NaHCO3 solution for 3 h. The precipitate
was filtered again and washed with water until the pH was
neutral. The powdery product was recrystallized from AcOH to
give pure product as red crystals.
Yield 70%. FT-IR (KBr, cmꢀ1) 3059, 3018, 1760 (aromatic-C–
H). 1H-NMR (200 MHz, CDCl3, ppm) 8.37–8.41 (d, J ¼ 8.0 Hz,
1H), 7.23–7.90 (m, 8H). MS (FAB) m/z 309.98 [(M + 1)+]. Anal.
Calcd for C17H9BrO (309.16): C, 66.04; H, 2.93; Br, 25.85; O,
5.18. Found: C, 66.01; H, 2.91%.
Synthesis of methyl 5-bromo-2-(naphthalene-1-yl)benzoate (1)
A solution of methyl 5-bromo-2-iodobenzoate (10.0 g, 0.03 mol),
naphthalene-1-boronic acid (5.05 g, 0.03 mol) and tetrakis-
(triphenylphosphine)palladium(0) (1.7 g, 1.46 mmol) dissolved in
THF (100 mL) was stirred in a two-necked flask under a nitrogen
atmosphere for 30 min. To the above reaction mixture was added
a solution of potassium carbonate (1.63 g, 11.18 mmol) in
distilled water (50 mL) dropwise over a period of 20 min. The
ꢁ
resulting mixture was refluxed overnight at 80 C. The reaction
Synthesis of 9-bromospiro[benzo[c]fluorene-7,90-fluorene] (4)
mixture was extracted with dichloromethane and water. After
the organic layer was evaporated with a rotary evaporator, the
resulting powdery product was purified by column chromatog-
raphy from dichloromethane/n-hexane to give a white crystalline
solid.
Yield 90%. FT-IR (KBr, cmꢀ1) 3059, 3018, 2910, 2870, 1735
(aromatic–C–H). 1H-NMR (200 MHz, CDCl3, ppm) 8.14 (d, J ¼
2.0 Hz, 1H), 7.68–7.89 (m, 3H), 7.23–7.51 (m, 6H), 3.37 (s, 3H).
MS (FAB) m/z 342.01 [(M + 1)+]. Anal. Calcd for C18H13BrO2
(341.20): C, 63.36; H, 3.84; Br, 23.42; O, 9.38. Found: C, 63.26;
H, 3.81%.
Into a two-necked flask was placed 2-bromobiphenyl (7.88 g,
34.0 mmol) dissolved in THF (120 mL). The reaction flask was
ꢁ
cooled to ꢀ78 C and n-BuLi (2.5 M in hexane, 15.6 mL) was
added dropwise for 10 min. The whole solution was stirred at this
temperature for 1 h, followed by addition of a solution of 3
(8.06 g, 26.0 mmol) in THF (150 mL) under argon atmosphere.
The resulting mixture was gradually warmed to ambient
temperature and quenched by saturated aqueous NaHCO3
(90 mL). The mixture was extracted with dichloromethane. The
combined organic layers were dried over magnesium sulfate,
filtered, and evaporated under reduced pressure. The crude
residue was placed in another two-necked flask and dissolved in
acetic acid (250 mL). A catalytic amount of aqueous HCl (25 mL,
35 wt%) was then added and the whole solution was refluxed
for 12 h. After having been cooled to ambient temperature,
Synthesis of 5-bromo-2-(naphthalene-1-yl)benzoic acid (2)
After compound 1 (8.5 g, 25.0 mmol) and NaOH (1.2 g, 0.03 mol)
was dissolved in ethanol (100 mL), the mixture was refluxed for
10736 | J. Mater. Chem., 2010, 20, 10735–10746
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purification by silica gel chromatography using dichloro-
methane/n-hexane and gave a white powder.
123.4, 123.0, 122.3, 120.4, 120.3, 66.5. FT-IR (cmꢀ1) 3059, 3040,
3012 (aromatic C–H). MS (FAB) m/z 669 [(M + 1)+]. Anal.
Calcd. for C53H32 (668.82): C, 95.18; H, 4.82. Found: C, 95.06;
H, 4.80%. UV-vis (THF): lmax (Absorption) ¼ 396 nm, lmax
(Emission) ¼ 426 nm.
Yield 95%. FT-IR (KBr, cmꢀ1) 3060, 3019 (aromatic-C–H).
1H-NMR (200 MHz, CDCl3, ppm) 8.70 (d, J ¼ 8.0 Hz, 1H), 8.23
(d, J ¼ 8.0 Hz, 1H), 7.83–7.89 (m, 3H), 7.49–7.73 (m, 4H), 7.34 (t,
J ¼ 8.0 Hz, 2H), 7.05 (t, J ¼ 8.0 Hz, 2H), 6.89 (s, 1H), 6.66–6.78
(m, 3H). MS (FAB) m/z 446.05 [(M + 1)+]. Anal. Calcd for
C29H17Br (445.35): C, 78.21; H, 3.85; Br, 17.94. Found: C, 78.18;
H, 3.83%.
Synthesis of 9-(10-(4-(naphthalene-1-yl)phenyl)anthracene-9-yl)-
spiro[benzo[c]fluorene-7,90-fluorene] (BH-9NPA)
A solution of 4 (4.51 g, 0.01 mol), 10-(4-(naphthalene-1-yl)phe-
nyl)anthracene-9-yl boronic acid (4.29 g, 0.01 mol) and tetra-
kis(triphenylphosphine)palladium(0) (0.59 g, 0.51 mmol)
dissolved in THF (100 mL) was stirred in a two-necked flask
under a nitrogen atmosphere for 30 min. To the above reaction
mixture was added a solution of potassium carbonate (1.63 g,
11.18 mmol) in distilled water (50 mL) dropwise over a period of
20 min. The resulting mixture was refluxed overnight at 80 ꢁC. The
reaction mixture was extracted with dichloromethane and water.
After the organic layer was evaporated with a rotary evaporator,
the resulting powdery product was purified by column chroma-
tography from dichlorometꢁhane/n-hexane to give an ivory
powder. Yield 83%. Mp 381 C. FT-IR (KBr, cmꢀ1) 3059, 3018
Synthesis of 9-(10-phenylanthracene-9-yl)spiro[benzo[c]fluorene-
7,90-fluorene] (BH-9PA)
A solution of 4 (5.07 g, 11.4 mmol), 10-phenylanthracene-9-yl
boronic acid (4.08 g, 13.7 mmol) and tetrakis(triphenyl-
phosphine)palladium(0) (0.66 g, 0.57 mmol) dissolved in THF
(100 mL) was stirred in a two-necked flask under a nitrogen
atmosphere for 30 min. To the above reaction mixture was added
a solution of potassium carbonate (1.63 g, 11.18 mmol) in
distilled water (50 mL) dropwise over a period of 20 min. The
ꢁ
resulting mixture was refluxed overnight at 80 C. The reaction
mixture was extracted with dichloromethane and water. After
the organic layer was evaporated with a rotary evaporator, the
resulting powdery product was purified by column chromato-
graphy from dichloromethane/n-hexane to give an ivory powder.
Yield 90%. Mp 351 ꢁC. FT-IR (KBr, cmꢀ1) 3060, 3018 (aromatic-
1
(aromatic-C–H). H-NMR (500 MHz, CDCl3, ppm) 8.93–8.96
(m, 2H), 8.68 (d, J ¼ 5.0 Hz, 1H), 6.85–8.06 (m, 33H). 13C-NMR
(500 MHz, CDCl3, ppm) 150.3, 148.3, 148.0, 147.6, 143.0, 142.4,
142.3, 140.9, 140.4, 139.8, 136.1, 134.3, 134.1, 132.1, 131.7, 130.1,
1239.8, 129.6, 129.5, 128.6, 128.2, 128.1, 127.8, 127.5, 127.4,
127.2, 127.1, 126.4, 126.1, 125.8, 125.6, 124.0, 123.7, 123.6, 122.9,
122.3, 120.5, 66.5. FT-IR (cmꢀ1) 3059, 3040, 3012 (aromatic C–
H). MS (FAB) m/z 746 [(M + 1)+]. Anal. Calcd. for C59H36
(744.92): C, 95.13; H, 4.87. Found: C, 95.09; H, 4.83%. UV-vis
(THF): lmax (Absorption) ¼ 396 nm, lmax (Emission) ¼ 432 nm.
1
C–H). H-NMR (500 MHz, CDCl3, ppm) 8.92–8.97 (m, 2H),
8.65 (d, J ¼ 5.0 Hz, 1H), 6.83–7.96 (m, 27H). 13C-NMR
(500 MHz, CDCl3, ppm) 150.3, 148.0, 147.6, 142.9, 142.3, 140.3,
139.2, 137.8, 137.3, 137.0, 136.4, 136.1, 134.3, 133.1, 131.5, 131.4,
129.9, 129.6, 129.5, 129.2 128.5, 128.2, 128.0, 127.7, 127.1, 127.0,
125.2, 125.1, 124.1, 124.0, 123.6, 123.5, 120.4, 120.3, 66.6. FT-IR
(cmꢀ1) 3059, 3040, 3012 (aromatic-C–H). MS (FAB) m/z 619 [(M
+ 1)+]. Anal. Calcd. for C49H30 (618.76): C, 95.11; H, 4.89.
Found: C, 94.99; H, 4.86%. UV-vis (THF): lmax (Absorption) ¼
395 nm, lmax (Emission) ¼ 428 nm.
Preparation of N,N0-diphenyl- N,N0-di-m-tolylspiro[fluorene-
7,90-benzofluorene]-5,9-diamine (BD-6MDPA)
Compound 5,9-dibromo-SBFF (5 g, 9.53 mmol), 3-methyl-
diphenylamine (24.79 mmol) and palladium acetate (0.150 g,
0.67 mmol) were dissolved in anhydrous toluene under nitrogen
atmosphere. To the reaction mixture was added a solution of tri-
t-butylphosphine (1 M, 0.42 g, 1.91 mmol) and potassium
t-butoxide (4.28 g, 38.15 mmol) dropwise slowly. The reaction
Synthesis of 9-(10-(naphthalene-1-yl)anthracene-9-yl)spiro[benzo-
[c]fluorene-7,90-fluorene] (BH-9NA)
A solution of 4 (5.01 g, 11.23 mmol), 10-(naphthalene-1-yl)an-
thracene-9-yl boronic acid (3.91 g, 11.23 mmol) and tetrakis-
(triphenylphosphine)palladium(0) (0.65 g, 0.56 mmol) dissolved
in THF (100 mL) was stirred in a two-necked flask under
a nitrogen atmosphere for 30 min. To the above reaction mixture
ꢁ
mixture was stirred for 12 h at 100 C. The mixture was diluted
with dichloromethane and washed with distilled water (150 mL)
three times. The organic layer was dried over anhydrous MgSO4
and evaporated in vacuo to give the crude product, which was
purified by column chromatography by n-hexane. The final
product was obtained, a yellow-green powder.
was added
a solution of potassium carbonate (1.63 g,
11.18 mmol) in distilled water (50 mL) dropwise over a period of
ꢁ
20 min. The resulting mixture was refluxed overnight at 80 C.
The reaction mixture was extracted with dichloromethane and
water. After the organic layer was evaporated with a rotary
evaporator, the resulting powdery product was purified by
column chromatography from dichloromethane/n-hexane to give
Yield 64%. Mp 248 ꢁC. 1H NMR (500 MHz, CDCl3) 8.77–8.75
(d, J ¼ 8.47 Hz, 1H, Ar-CH-naphthalene), 8.22–8.20 (d, J ¼
8.53 Hz, 1H, Ar-CH-naphthalene), 8.02–8.01 (d, J ¼ 8.45 Hz,
1H, Ar-CH-benzene), 7.69–7.67 (d, J ¼ 7.60 Hz, 2H, Ar-CH-
fluorene), 7.60–7.59 (t, J ¼ 8.00 Hz, 1H, Ar-CH-naphthalene),
7.39–7.38 (t, J ¼ 7.57 Hz, 1H, Ar-CH-naphthalene), 7.28–7.27 (t,
J ¼ 7.90 Hz, 2H, Ar-CH-fluorene), 7.12–7.10 (s, 2H, Ar-CH-
benzene), 7.12–7.10 (t, 4H, Ar-CH-benzene), 7.10–7.08 (d, 2H,
Ar-CH-fluorene), 7.04–7.02 (d, 2H, Ar-CH-benzene), 6.96–6.94
(d, 1H, Ar-CH-benzene), 6.90–6.89 (t, 4H, Ar-CH-benzene),
6.89–6.87 (d, 2H, Ar-CH-naphthalene), 6.80–6.78 (d, 2H,
an ivory powder. Yield 85%. Mp 339 ꢁC. FT-IR (KBr, cmꢀ1
)
3058, 3020 (aromatic–C–H). 1H-NMR (500 MHz, CDCl3, ppm)
8.93–8.99 (m, 2H), 8.67 (d, J ¼ 5.0 Hz, 1H), 6.72–8.04 (m, 29H).
13C-NMR (500 MHz, CDCl3, ppm) 150.3, 148.2, 148.0, 147.6,
143.0, 142.4, 142.3, 140.0, 138.7, 137.7, 137.4, 136.9, 136.4, 135.1,
134.3, 133.9, 133.8, 133.7, 130.7, 129.8, 129.6, 129.2, 128.2, 128.1,
127.8, 127.6, 127.2, 127.1, 126.1, 125.8, 125.3, 125.2, 124.1, 124.0,
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Scheme 2 Schematic routes of 9-bromospiro[benzo[c]fluorene-7,90-fluorene] (4)
Ar-CH-benzene), 6.74–6.72 (d, 2H, Ar-CH-benzene), 6.71–6.68
(d, 2H, Ar-CH-benzene), 6.62–6.60 (d, J ¼ 7.92 Hz, 1H, Ar-CH-
benzene), 6.57–6.57 (s, 1H, Ar-CH-benzene), 2.08–2.09 (s, 6H,
Ar-CH3). 13C NMR (CDCl3) 151.5, 148.1, 147.9, 147.8, 147.5,
147.4, 146.8, 142.7, 142.1, 139.0, 138.8, 136.9, 135.4, 131.9, 130.8,
129.2, 129.0, 128.8, 127.9, 127.8, 126.9, 126.1, 125.7, 124.5, 124.4,
124.1, 123.8, 123.6, 123.3, 123.0, 122.5, 122.1, 121.4, 121.3, 121.1,
120.4, 119.2, 118.6, 77.4, 77.2, 76.9, 66.4, 21.5, 21.4.
deposition of the metal cathode, LiF was deposited onto the
ꢀ
cathode was deposited at a rate of 1–5 A sꢀ1 with a thickness of
organic layers with a thickness of 10 A. A high-purity aluminium
ꢀ
ꢀ
2000 A as the top layer. After the device was fabricated, the
evaporation chamber was vented with nitrogen gas and the
device immediately transferred to a glove box. A thin epoxy
adhesive was applied from a syringe around the edge of a clean
cover glass. To complete the package, a clean cover glass was
placed on the top of the device. The epoxy resin was cured under
intense UV radiation for 3 min. The current–voltage character-
istics of the encapsulated devices were measured on a program-
mable electrometer having current and voltage sources (Source
Measure Unit, model Keithley 237). The luminance and EL
spectra were measured with a PR650 system (Photo Research
Co. Ltd.).
OLED fabrication and measurement
Prior to device fabrication, ITO with a resistance of 12 U/, on
glass was patterned as an active area of 4 mm2 (2 mm ꢂ 2 mm).
The substrates were cleaned by sonication in deionized water,
boiled in IPA for 20 min, and dried with nitrogen. Finally, the
substrates were dry-cleaned using plasma treatment in an O2 and
Ar environment. Organic layers were deposited onto the
substrate sequentially by thermal evaporation from resistively
Results and discussion
heated alumina crucibles at a rate of 1.0 A sꢀ1. The thicknesses
ꢀ
Synthesis and characterization
of the N,N0-bis-[4-(di-m-tolylamino)phenyl]-N,N0-diphenylbi-
phenyl-4,40-diamine (DNTPD, HIL), bis[N-(1-naphthyl)-N-
phenyl]benzidine (NPB, HTL), host : 5% dopant (EML) and
aluminium tris(8-hydroxyquinoline) (Alq3, ETL) layers were
As the benzofluorene was composed of phenyl and naphthalene
moieties, it was difficult to prepare 9-bromo-SBFF by direct
bromination reaction of SBFF with bromine. The rate of
substitution reaction of bromine on the naphthalene ring was
ꢀ
about 600, 300, 200 and 200 A, respectively. Before the
Scheme 3 Schematic routes of BH-9 host materials.
10738 | J. Mater. Chem., 2010, 20, 10735–10746
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constants could not be distinguished in the 1H NMR spectrum of
three host materials and the chiral carbon peak was observed at
66.5 ppm in the 13C NMR spectra.
BH-9PA, BH-9NA and BH-9NPA were characterized by
1H-NMR, 13C-NMR and FT-IR spectroscopy. The results of
the elemental analysis and mass spectroscopy also supported the
formation of three host materials and matched well with the
calculated data.
Scheme 4 Preparation of dopant material BD-6MDPA
The dopant materials N,N0-diphenyl-N,N0-di-m-tolyl-SBFF-
5,9-diamine (BD-6MDPA) was prepared by amination reactions
faster than that of the phenyl ring. Thus, a multi-step synthesis
was indispensable to prepare 9-bromo-SBFF. First, the naph-
thalene group must be incorporated into the iodobromobenzene
core. The synthesis of 9-bromo-SBFF can only be achieved using
a typical four-step reaction: 1) introduction of a naphthyl group
to methyl 5-bromo-2-iodobenzoate by Suzuki reaction, 2)
hydrolysis of the ester group of methyl 5-bromo-2-(naphthalene-
1-yl)benzoate (1), 3) five-membered ring formation by acid
catalyzed cyclization of 5-bromo-2-(naphthalene-1-yl)benzoic
acid (2) and 4) spiro-formation of 2-bromobiphenyl with
9-bromo-7H-benzo[c]fluoren-7-one (3) via lithiation as shown in
Scheme 2.
SBFF-based blue host material, 9-(10-phenylanthracene-9-
yl)SBFF (BH-9PA), 9-(10-(naphthalene-1-yl)anthracene-9-yl)-
SBFF (BH-9NA) and 9-(10-(4-(naphthalene-1-yl)phenyl)an-
thracene-9-yl)SBFF (BH-9NPA) were successfully prepared by
reacting 9-bromo-SBFF (4) with 10-phenylanthracene-9-yl
boronic acid, 10-(naphthalene-1-yl)anthracene-9-yl boronic acid,
and 10-(4-(naphthalene-1-yl)phenyl)anthracene-9-yl boronic
acid using a palladium catalyst, as illustrated in Scheme 3. As the
acid-catalyzed cyclization reaction proceeded, the strong
hydroxyl peak of 5-bromo-2-(naphthalene-1-yl)benzoic acid in
the FT-IR spectrum disappeared. All of the proton peaks were
observed in the range of 6.71–8.93 ppm, of which the coupling
Table 1 UV absorption, PL, energy levels and thermal properties of
three host materials
Sample
Properties
BH-9PA BH-9NA BH-9NPA
Puritya
Thermal Analysis
%
99.9
351
393
252
395
2.96
400
428
51.8
99.8
339
413
261
396
2.96
401
426
49.9
99.9
381
435
250
396
2.95
401
432
54.1
Tm (ꢁC)
Td (ꢁC)
Tg (ꢁC)
Max (nm)
Bgb (eV)
Optical Analysis UV
Solid UV Max (nm)
PL
Max (nm)
FWHMc
(nm)
Solid PL Max (nm)
FWHMc
(nm)
444
58.24
441
58.36
449
39.62
QE
Electrical Analysis
Ffd
0.93
0.78
5.89
2.93
0.91
5.84
2.89
HOMO (eV) 5.87
LUMO (eV) 2.91
a
The purity of the samples were finally determined by high performance
liquid chromatography (HPLC) using the above prepared samples after
b
c
train sublimation.
Fluorescence quantum efficiency, relative to 9,10-diphenylanthracene
Bandgap.
Full width at half maximum.
d
Fig. 1 UV-vis and PL spectra of (a) BH-9PA, (b) BH-9NA and (c) BH-
in cyclohexane (Ff ¼ 0.90).
9NPA.
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of compounds 5,9-dibromo-SBFF with m-tolylphenylamine in
the presence of a palladium catalyst. The synthetic route to the
dopant material is described in Scheme 4.
times that of DPVBi (64 ꢁC) and much higher than that of
MADN (120 ꢁC). As a result, the amorphous glassy state films of
the three host materials were good candidates as EL materials.
Thermal properties
Optical properties and energy levels
For optoelectronic applications, the thermal stability of organic
materials is crucial to the stability and lifetime of the devices. The
degradation of organic optoelectronic devices depends on the
morphological changes resulting from the thermal stability of the
amorphous organic layer. A high glass transition temperature
(Tg) value indicates that the morphology of the material will not
easily be changed by the high temperature generated during the
operation of OLED devices, and is closely correlated with long
OLED device life-times.26,27 These morphological changes might
be promoted by rapid molecular motion near the Tg. The thermal
properties of the newly synthesized host materials were investi-
gated by DSC and TGA analysis in a nitrogen atmosphere. The
onset decomposition temperatures were 393, 413 and 435 ꢁC for
The UV and PL spectra of the synthesized compounds in CHCl3
solution and in thin films are shown in Table 1 and Fig. 1. In
solution, BH-9PA and BH-9NA had a UVmax of 395 and 396 and
a blue PLmax of about 428 and 426 nm. However, BH-9NPA had
a UVmax of 396 nm and a PLmax of 432 nm, indicating a red shift
of about 4–6 nm relative to BH-9PA and BH-9NA. The PL
spectra of the compounds in thin films, fabricated by vacuum
deposition, showed that BH-9PA and BH-9NA had a PLmax in
the blue region at 444 and 441 nm, and BH-6NPA had a PLmax at
449 nm, the latter again representing a red shift of about 8 nm.
All three synthesized compounds showed a PL wavelength that
was red shifted about 15 nm in solution compared to the film.
Since there is a p–p* interaction between molecules in a film, this
state typically shows red shifts relative to molecules in solution.
The red-shift of the emission observed in the solid thin film is
probably due to the difference in relative permittivity or dielectric
constant of the environment.28 The introduction of an anthra-
cenyl group to the 9-positions in spiro[benzo[c]fluorene-7,90-flu-
orene] (UVmax ¼ 379 nm and PLmax ¼ 444 nm) caused a little
blue-shift in the absorption and no shift in the PL spectrum even
though the conjugation length increased.
BH-9PA, BH-9NA and BH-9NPA, respectively. Table
1
summarizes the DSC and TGA data for the three host materials.
The purified samples of BH-9PA, BH-9NA and BH-9NPA
showed a melting point (Tm) of 351, 339 and 381 ꢁC after
sublimation, respectively. On the second heating, no melting
points were observed, even though it was given enough time to
cool in air. Once it became an amorphous solid, it did not revert
to the crystalline state at all. After the sample had cooled to room
temperature, a second DSC scan performed at 10 ꢁC minꢀ1
revealed a Tg at 240, 262 and 251 ꢁC, respectively, which are four
BD-6MDPA in solution and film showed UV-vis absorption
maxima of 408 nm and 416 nm, respectively. The PL emission
Fig. 2 HOMO and LUMO electronic density distributions of (a) BH-9PA, (b) BH-9NA, and (c) BH-9NPA, calculated at the DFT/B3LYP/6-31G* for
optimization and time dependent DFT (TDDFT) using Gaussian 03.
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Fig. 4 The device configuration and the chemical structure of the
materials used in the devices.
compounds was measured. All the compounds showed high PL
QE values: BH-9PA (0.93), BH-9NA (0.78), and BH-9NPA
(0.91). A more accurate QE comparison was possible for BH-
9PA and BH-9NA because they had identical spectra and PLmax
.
BH-9PA (side group, phenylanthracene) had a higher QE than
BH-9NA (side group, naphthylanthracene), even though the QE
of phenylanthracene is lower than that of naphthylanthracene;
this result emphasizes the importance of the side group.30
Moreover, from the structure obtained by molecular calculation,
the phenyl and naphthyl units incorporated at the 9- and 10-
positions of anthracene can have a torsion angle of 1–90, It seems
that BH-9PA deviated significantly from the coplanar confor-
mation. Particularly, naphthyl groups formed with the adjacent
anthracene group had a torsion angle of 90ꢁ, which allows the
anthracene unit and the naphthyl group to form independent
Fig. 3 Energy diagram of (a) BH-9PA : DSA-Ph, (b) BH-9PA : BH-
6MDPA, (c) BH-9NA : DSA-Ph, (d) BH-9NA : BH-6MDPA, (e) BH-
9NPA : DSA-Ph and (f) BH-9NPA : BH-6MDPA.
maximum of BD-6MDPA in solution and in solid film was
located at 468 and 487 nm, respectively.
To accurately interpret the EL efficiency of the compounds,
their relative PL quantum efficiencies (QE) in solution were
measured using the method described in the literature.29 The QE
of the reference material 9,10-diphenylanthracene in cyclohexane
was set at 0.90, and the relative QE of the synthesized
Fig. 5 EL spectra of the devices obtained from (a) BH-9PA and b-ADN
and (b) BH-9NA and BH-9NPA doped with various dopant materials.
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about 0.3 eV higher than that for MADN and b-ADN.32,36 The
optical energy bandgaps, as determined from the absorption
spectra of the present blue emitting films, were 2.96, 3.01 and
2.95 eV for BH-9PA, BH-9NA and BH-9NPA, respectively. The
energy gap for BD-6MDPA was calculated as 3.31 eV, which
corresponded to the absorption wavelengths of 416 nm
Electroluminescence devices
The EL properties of the BH-9PA, BH-9NA and BH-9NPA host
materials were examined by fabricating multilayer devices with
the following configuration: glass ITO anode/hole injection layer
(HIL)/hole transfer layer(HTL)/emitting layer (EML)/electron
transport layer (ETL)/electron injection layer (EIL)/Al cathode.
DNTPD was used as the HIL, NPB as the HTL, host : 5%
ꢀ
dopant as the EML and Alq3 as the ETL; A 10 A LiF layer was
Fig. 6 Current density-voltage characteristics of the device using (a)
BH-9PA and b-ADN and (b) BH-9NA and BH-9NPA doped with 5%
BD-6MDPA or DSA-Ph.
used as the EIL. The detailed device architecture and molecular
structure of DNTPD, NPB, and Alq3 are also shown in Fig. 4.
Fig. 5 shows the normalized EL spectra of a device composed
of BH-9PA, BH-9NA, BH-9NPA and b-ADN doped with BD-
6MDPA and DSA-Ph at 7 V. All the devices showed an intense
blue emission in the EL spectra at 464 nm for BD-6MDPA. The
maximum wavelength of the EL spectra varied significantly
according to the dopant material. The devices doped with DSA-
Ph showed a 4–5 nm longer wavelength in the EL spectra than
that of BD-6MDPA. The EL spectra of the OLEDs made using
BH-9PA, BH-9NA and BH-9NPA with the above structure at
7 V are shown in Fig. 4(a) and (b). The EL spectrum of the ITO/
DNTPD/NPB/host : 5% dopant/Alq3/LiF/Al device shows
a narrow FWHM of 47.6–57.3 nm and a lmax ¼ 444–469 nm.
These devices have a narrow FWHM, which is required for
a monochromic light source. On the bases of the narrow and
similar FWHM and between the PL and EL spectra, we conclude
that the blue EL is due to the intrinsic emission of BH-9PA, BH-
9NA and BH-9NPA, with very little or no emission interference
from the dopant.
conjugation systems. The torsional angle between the phenyl
group and the adjacent anthracene was 83.72ꢁ for compound
BH-9PA. The relationship between the quantum yield of the
fluorescence and molecular structure is determined to a large
extent by the structural dependence of the competing photo-
physical and photochemical processes. Thus for most rigid
aromatic compounds fluorescence is easy to observe, with
quantum yields in the range 1 > FF > 0.01. For example, it is well
known that in BH-9NA a twist angle greater than 90ꢁ can
diminish or even suppress the conjugation and therefore limit the
fluorescence.31,32
Density functional theory (DFT) calculations have also been
performed to characterize the three-dimensional geometries and
the frontier molecular orbital energy levels of BH-9PA, BH-
9NA, and BH-9NPA at the B3LYP/6-31G* level33–35 by using the
Gaussian 03 program. The HOMO–LUMO energy gaps for BH-
9PA, the calculated geometries of BH-9PA, BH-9NA, and BH-
9NPA in Fig. 2 show that the fluorene and benzofluorene are
significantly twisted against each other because of the spiro
center, resulting in a non-coplanar structure in each molecule.
These geometrical characteristics can effectively prevent inter-
molecular interactions between p-systems and, thus, suppress
molecular recrystallization, which improves the morphological
stability of these molecules. The electron densities of the HOMOs
and LUMOs are mostly localized on the anthracene unit,
implying that the absorption and emission processes can only be
attributed to the p–p* transition centered at the anthracene unit.
The energy levels of three host and two dopant materials used
to fabricate OLEDs in the present study are shown in Fig. 3(a)–
(f) and Table 1. A low-energy photoelectron spectrometer was
used to obtain information on the HOMO energies of the dopant
materials and to examine the charge injection barriers. The
HOMO energy levels of dopant materials were determined to be
5.87, 5.89 and 5.84 eV for BH-9PA, BH-9NA and BH-9NPA,
respectively, by low-energy photoelectron spectrometer. These
HOMO values were different from the HOMO values of other
anthracene derivatives, 5.5–5.6 eV, such as MADN.33 In partic-
ular, BH-9PA, BH-9NA and BH-9NPA had HOMO values
Fig. 6 and 7 show the current density-voltage and luminance-
voltage characteristics of four deep-blue fluorescent devices.
Fig. 7 Luminance-voltage characteristics of the device using (a) BH-
9PA and b-ADN and (b) BH-9NA and BH-9NPA doped with 5% BD-
6MDPA or DSA-Ph.
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Table 2 EL properties of the devices obtained from BH-9PA, BH-9NA and BH-9NPA doped with DSA-Ph and BD-6MDPA dopant materials
Device
BH-9PA
None
BH-9NA
DSA-Ph BD-6MDPA None
BH-9NPA
b-ADN
Properties
DSA-Ph BD-6MDPA DSA-Ph BD-6MDPA DSA-Ph BD-6MDPA
EL (V)
l
max (nm) 448
468
464
444
464
464
469
464
50.0
4.29
4.89
(5 V)
1.92
3.27
(4.5 V)
2147
0.14
464
464
mA cmꢀ2
24.88
2.35
2.40
(6 V)
1.05
1.34
21.51
6.88
7.03
(6 V)
3.09
4.16
21.64
6.52
6.60
(6.5 V)
2.93
3.81
(5 V)
1412
0.13
0.15
5.35
51.09
1.82
2.06
(4.5 V) (5.5 V)
0.81
1.44
51.1
6.01
6.39
48.75
5.18
5.63
(5.5 V)
2.32
3.70
(4.5 V)
2528
0.14
0.15
4.24
40.0
4.80
5.24
(5 V)
2.16
3.40
(4.5 V)
1920
0.16
0.22
2.94
3.1
28.27
6.01
6.01
(7 V)
2.69
3.60
(5 V)
1700
0.15
0.22
3.79
3.83
(8.5 V)
29.15
6.40
6.40
(7 V)
2.87
3.80
(5 V)
1866
0.13
0.17
4.96
5.07
(8 V)
a
cd Aꢀ1
cd Aꢀ1
b
a
lm Wꢀ1
lm Wꢀ1
2.69
4.01
b
(5.5 V) (5 V)
(4.5 V) (4.5 V)
cd mꢀ2
CIE-x
CIE-y
584.9
0.15
0.10
1481
0.15
0.21
4.49
4.53
928.2
0.15
0.09
2.05
2.29
(5 V)
3075
0.15
0.21
3.83
3.99
(6 V)
0.17
3.26
3.58
(5.5 V)
EQE (%)a 2.54
EQE (%)b 2.57
5.39
(6.5 V)
4.52
(5.5 V)
(6.5 V) (6 V)
(5.5 V)
a
b
Values at 7 V. values at a highest peak.
Fig. 8 Efficiency-current density characteristics of the device using (a) BH-9PA and b-ADN and (b) BH-9NA and BH-9NPA doped with 5% BD-
6MDPA or DSA-Ph.
Devices with BH-9PA, BH-9NA and BH-9NPA as the emitting
layer had similar I–V characteristics to the device with b-ADN as
the emitting layer. The threshold voltage for luminescence is
about 4.5 V, at which the electrons and holes appear to reach the
light-emitting layer; in all the devices, no light emission occurs
below this voltage as shown in Fig. 6(a) and 6(b). In the case of
the devices using BH-9PA and b-ADN doped with 5% BD-
6MDPA, the maximum brightness of 7551 and 6294 cd mꢀ2
occur at approximately 9.0 V as shown in Fig. 7(a). All the
devices exhibited very low current density before the turn-on
voltage because of the absence of a shunt resistance. All the
OLEDs made using BH-9NA and BH-9NPA showed good
performance for the two dopant materials DSA-Ph and BD-
6MDPA, as detailed in Fig. 7 (b) and Table 2.
The efficiency of the device composed of BH-9PA doped with
DSA-Ph and BD-6MDPA was much higher than that of the
device with undoped BH-9PA because of the improved electron
and hole balance in the device; its luminescence efficiency (LE)
and external quantum efficiency (EQE) reaches 6.52 cd Aꢀ1 and
5.35%, respectively, with CIE coordinates of (0.13, 0.15) at
a current density of 21.64 mA cmꢀ2 at 7 V as shown in Fig. 8(a),
Fig. 9(a) and Fig. 10(a). The device doped with DSA-Ph showed
an EQE of 4.49% and LE of 6.88 cd Aꢀ1. However, the EL
emission of OLEDs with the BH-9PA : 5% DSA-Ph shows a blue
spectrum with CIE coordinate (0.15, 0.21), as a result of the sky-
blue dopant. Pure deep-blue EL was obtained from the device
made with only BH-9PA with CIE coordinates of (0.15, 0.10),
which is obviously a deep-blue emission, as shown in Fig. 10(a).
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To the best of our knowledge, this is one of the highest EQEs in
deep-blue OLEDs.36,37 Although the CIE value of the BH-9PA
device was slightly different from that of the b-ADN device, its
luminance efficiency and power efficiency were higher than that
of the b-ADN device. The BH-9PA device also had an excellent
FWHM value (50 nm) despite its asymmetric structure. This
value was lower than for b-ADN (53 nm) because the bulky spiro
group effectively prevented p–p* stacking interactions between
molecules.
The device with BH-9NA : 5% BD-6MDPA as EML shows
the maximum an LE of 5.63 cd Aꢀ1 and power efficiency (PE)
of 3.70 lm Wꢀ1 with EQE 4.52% at a current density of
48.75 mA cmꢀ2 (5.5 V, 485.2 cd mꢀ2) as shown in Fig. 8(b) and
Fig. 9(b). The CIE coordinates of the device (0.14, 0.15) are very
close to a deep-blue color as shown in Fig. 10(b). Although the
device that used BH-9NA : DSA-Ph as the emitting layer showed
EQE 3.38%, it had a sky blue color coordinate of (0.15, 0.21) as
summarized in Table 2. Thus the efficiency of the BH-9PA device
was higher than that of the BH-9NA device, which is identical to
the PL QE result. In comparison with BH-9NA, BH-9PA
composed of a phenylanthracenyl group in the side part showed
increase in cd Aꢀ1 and lm Wꢀ1 resulted from reduction in the
energy barrier inside layers of devices due to the formation of
relatively narrow band-gap. As a result, easier movement of hole
and electron brings a lower operating voltage and higher cd Aꢀ1
Fig. 9 Efficiency-voltage characteristics of the device using (a) BH-9PA
and b-ADN and (b) BH-9NA and BH-9NPA doped with 5% BD-
6MDPA or DSA-Ph.
and lm Wꢀ1
.
A maximum luminescence of 6562 cd mꢀ2 occurs at 194.5 mA
cmꢀ2 for the device with BH-9NPA : 5% BD-6MDPA. The fine
structure of the EL spectrum of the EL spectrum, with a peak at
464 nm and CIE coordinates of x ¼ 0.14, y ¼ 0.17, is consistent
with the PL spectra. There is a slight broadening in the EL
spectra, as compared with a solid PL spectrum, and a shoulder at
481 nm for of BH-9NPA : 5% DSA-Ph is present, which is not
observed in either the solution or solid-state emission spectra of
pure BH-9NPA at room temperature. The maximum efficiency
reaches 5.23 cd Aꢀ1 (3.09% EQE) with a PE of 3.39 lm Wꢀ1 at
39.94 mA cmꢀ2, which is comparable to other fluorescent blue-
light-emitting anthracene and spirobifluorene derivatives.
Although BH-9NPA did not yield a better blue-light-emitting
material than b-ADN and the other two host materials, it has
also been previously studied with the aim of device optimization
using the other dopants, HTL and ETL.
Fig. 11 shows the absorption spectrum of BD-6MDPA and the
PL spectrum of BH-9PA and BH-9NPA. It can be seen from
Fig. 11 and its inset that the energy band diagram of BH-9PA
and BH-9NPA overlaps with a major portion of the absorption
spectrum of BD-6MDPA. This indicates that the BD-6MDPA
dopant can effectively accept energy from the host by Forster-
type energy or function as a direct recombination center because
of higher HOMO level. Especially the efficient energy transfer
from the BD-6MDPA to BH-4PA occurs between the absorption
of the dopant and emission of host material.
Among the synthesized compounds, the lifetime of BH-9PA,
which showed the best EL efficiency in this series, was measured
as the operational lifetime (t1/2) of the blue devices under initial
luminance of 5000 cd mꢀ2 at a constant current. Its lifetime
(137 h) was more than that of b-ADN : 5% BD-1 (128 h), which
can be explained by the more stable morphology of the series of
BH-9 host materials as shown in Fig. 12.
Fig. 10 The CIE 1931 coordinates diagram of device obtained from (a)
BH-9PA and b-ADN and (b) BH-9NA and BH-9NPA doped with 5%
BD-6MDPA or DSA-Ph.
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Fig. 11 UV-vis absorption spectrum of BD-6MDPA and PL spectra of the BH-9PA, BH-9NA and BH-9NPA.
Acknowledgements
This research was supported by WCU (World Class University)
program through the National Research Foundation of Korea
funded by the Ministry of Education, Science and Technology
(R31-10069).
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9NA and BH-9NPA were prepared to construct a blue, organic
light-emitting diode. The devices made using BH-9PA, BH-9NA
and BH-9NPA as the host and BD-6MDPA as the dopant
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