Indenofluorene-based blue fluorescent compounds and their application in highly efficient organic light-emitting diodes

Article abstract of DOI:10.1002/ejoc.201200117

Blue fluorescent compounds based on indenofluorene derivatives incorporating the (diphenylamino)arylvinyl group and di-tert-butylphenyl blocking units have been synthesized by Suzuki coupling and the Horner-Wadsworth-Emmons reaction. Their electroluminesc

Full text of DOI:10.1002/ejoc.201200117

FULL PAPER
DOI: 10.1002/ejoc.201200117
Indenofluorene-Based Blue Fluorescent Compounds and Their Application in
Highly Efficient Organic Light-Emitting Diodes
Kum Hee Lee,[a] Seul Ong Kim,[a] Sunwoo Kang,[a] Jin Yong Lee,[a] Kyoung Soo Yook,[b]
Jun Yeob Lee,*[b] and Seung Soo Yoon*[a]
Keywords: Photophysics / Luminescence / Organic light-emitting diodes / Fused-ring systems
Blue fluorescent compounds based on indenofluorene deriv- 12.2 cdA–1, a power efficiency of 6.08 lmW–1, and an external
atives incorporating the (diphenylamino)arylvinyl group and
di-tert-butylphenyl blocking units have been synthesized by
Suzuki coupling and the Horner–Wadsworth–Emmons reac-
tion. Their electroluminescence in multilayered OLEDs was
examined; the devices have the structures ITO/DNTPD/
NPB/MADN:blue dopant/Alq3/LiF/Al. A device with 7% 2-
(3,5-di-tert-butylphenyl)-8-(9,9-diethyl-2-(diphenylamino)-
fluoren-7-ylethenyl)-6,6,12,12-tetraethylindeno[1,2-b]fluo-
rene (3) as blue dopant showed a luminous efficiency of
quantum efficiency of 7.84% at 20 mAcm–1 with CIE (Com-
mission Internationale d’Énclairage) (x, y) coordinates of
(0.148, 0.241) at 8.5 V. A deep-blue OLED with CIE (x, y)
coordinates of (0.153, 0.128) doped with 2-(3,5-di-tert-but-
ylphenyl)-8-[9-(3,5-di-tert-butylphenyl)carbazol-3-ylethenyl]-
6,6,12,12-tetraethylindeno[1,2-b]fluorene (4) exhibited a lu-
minous efficiency of 3.25 cdA–1
1.74 lmW–1, and an external quantum efficiency of 2.93% at
20 mAcm–1
, a power efficiency of
.
stituents and moieties to control the HOMO and LUMO
energies of the indenofluorene core. Indenofluorenes have
unique geometries that allow the methylene bridges to be
either on the opposite side (indeno[1,2-b]fluorene) or on the
same side (indeno[2,1-b]fluorene) of the terphenyl core. Al-
though the electrochemical and photophysical properties of
indenofluorenes have been studied, the dependence of elec-
troluminescent properties on positional isomerism has not
yet been reported.[16]
In this paper we report the synthesis and electrolumines-
cent (EL) properties of a new series of indenofluorene de-
rivatives 1–5 with various aromatic groups at the terminal
2- and 8-positions. The indeno[1,2-b]fluorene derivatives 1–
4 have two methylene moieties at the C-6 and C-12 posi-
tions each alkylated with two ethyl groups. They have non-
coplanar structures, which effectively inhibits molecular ag-
gregation and excimer formation in the solid state. The in-
corporation of a bulky di-tert-butylphenyl group at the 8-
position could improve solubility and prevent molecular ag-
gregation and self-quenching. The attachment of electron-
rich groups such as (diphenylamino)phenylvinyl, (di-
phenylamino)biphenylvinyl, (diphenylamino)fluorenylvinyl,
and di-tert-butylphenylcarbazolylvinyl at the 2-position of
the indenofluorene core can control the energy bandgaps of
the dopants by increasing the HOMO energies, which could
make the compounds suitable for use as blue emitters in
OLEDs. These groups also enhance the hole transport
properties of 1–5 and thus lead to improved OLED effi-
ciency and stability. Compound 5, an isomer of compound
1, was designed to study the effects of structural changes in
the core of indenofluorene derivatives on the EL perform-
Introduction
Conjugated polycyclic aromatic compounds have optical
and electronic properties suitable for photonic and elec-
tronic devices such as organic light-emitting diodes
(OLEDs).[1–4] Fluorene derivatives are particularly attract-
ive light-emitting compounds for OLEDs due to their high
photoluminescence (PL) efficiency, wide bandgap, good sol-
ubility, and excellent thermal and chemical stabilities.[5–10]
Because the optical, chemical, and electronic characteristics
of fluorene arise from its rigid and planar biphenyl moiety,
structurally related indenofluorene, with a planar and
longer conjugated p-terphenyl moiety, is also a promising
compound. Blue compounds containing indenofluorene
backbones instead of fluorene units have led to enhanced
OLED properties.[11–15] However, fluorescent molecules
based on indenofluorene can form aggregations or excimers
leading to an unwanted long-wavelength emission band and
decreased luminescent efficiency. The energy bandgap of an
indenofluorene core is also too large to be suitable for use
in blue organic light-emitting diodes. These problems are
mainly solved by the introduction of sterically bulky sub-
[a] Department of Chemistry, Sungkyunkwan University,
300 Cheoncheon-dong, Jangan-gu, Suwon, Gyeonggi, 440-746
Korea
Fax: +82-31-290-7075
E-mail: ssyoon@skku.edu
[b] Department of Polymer Science and Engineering,
Dankook University,
126, Jukjeon-dong, Suji-gu, Yongin, Gyeonggi, 448-701 Korea
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201200117.
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Indenofluorene-Based Blue Fluorescent Compounds
ance of devices. Herein, highly efficient blue OLEDs have and 13C NMR, FTIR, and low- and high-resolution mass
been demonstrated that incorporate these indenofluorene- spectroscopy. High-pressure liquid chromatography showed
based blue emitters as dopants.
that the compounds were all prepared with a purity of at
least 99.0%.
Figure 1 shows the UV/Vis absorption and PL spectra of
the blue dopant compounds in dilute CH2Cl2 solutions and
as thin films formed on quartz plates. Table 1 summarizes
their optical and photoluminescent properties. Figure 1 (a)
shows that the absorption spectrum of compound 2 is blue-
shifted by around 6 nm relative to the normalized optical
absorption spectra of compound 1 as a result of distorted
π conjugation at the inner biphenyl unit, which reduces the
length of the π conjugation of 2 relative to 1. Compound
3 shows a spectrum redshifted by around 4 nm relative to
compound 1 due to increased π conjugation at the inner
fluorene unit. Compound 4 shows a 10 nm hypsochromic
shift relative to compound 1 due to differences in the elec-
tronic properties of the triphenylamine and phenylcarbazole
units. The spectrum of 5 exhibits an absorption band hyp-
sochromically shifted by 51 nm relative to 1, which corrobo-
rates previous observations on the indeno[1,2-b]-
fluorene and indeno[2,1-b]fluorene cores.[16] Compared
with the indeno[1,2-b]fluorene core, π conjugation in in-
deno[2,1-b]fluorene is ineffective due to the unfavorable or-
Results and Discussion
The indenofluorene derivatives 1–5 were synthesized as
outlined in Scheme 1. Compound 10 was synthesized by an
established procedure[15,17] involving a Suzuki coupling re-
action, oxidation, Wolff–Kishner reduction, alkylation, and
bromination. It was monolithiated by using a stoichiomet-
ric amount of nBuLi in a lithium/halogen exchange reaction
at –78 °C, followed by reaction with DMF to afford the
monoformylated 11. The intermediate 12 was produced by
the Suzuki cross-coupling reaction of monoformylated 11
and 3,5-di-tert-butylphenylboronic acid in aqueous solu-
tion. Finally, the blue light-emitting compounds 1–4 were
produced in moderate yields (48–57%) by Horner–Wad-
sworth–Emmons reactions between the corresponding
phosphonates and intermediate 12. Compound 5, a posi-
tional isomer of compound 1, was produced similarly to
compound 1, starting from 1,3-dihydroindeno[2,1-b]fluo-
rene (13).[18] It was produced in 81% yield. The molecular
1
structures of all the compounds were characterized by H
Scheme 1. Synthesis of compounds 1–17. Reagents: (a) nBuLi, DMF, THF; (b) 3,5-(tBu)2C6H3B(OH)2, [Pd(PPh3)4], 2 m Na2CO3, ethanol,
toluene; (c) RCH2PO(OEt)2, tBuOK, THF; (d) nBuLi, bromoethane, THF; (e) Br2, FeCl3, CHCl3. See the Exp. Section for full details.
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FULL PAPER
bital interaction between the meta position of its central efficiency is highly dependent on the spectral overlap be-
benzene ring, which would lead to a shorter π-conjugation tween the emission of the host and the absorption of the
length in 5 than in 1. Figure 1 (a) shows overlapping be- dopant. Therefore all five blue compounds could effectively
tween the absorptions of the five blue dopants and the accept energy from the MADN host by Förster energy
emission of the common host compound 2-methyl-9,10- transfer, with the efficiencies of energy transfer ranked simi-
bis(2-naphthyl)anthracene (MADN). Overlapping of the larly to the extent of the overlap. Part b of Figure 1 shows
emission of MADN with the absorption spectra of the dop- the blue fluorescence of the compounds 1–5 to have maxi-
ants decreased as follows: 3Ͼ1Ͼ2Ͼ4Ͼ5. Energy-transfer mum emission wavelengths at 464, 469, 480, 429, and
492 nm, respectively. The band-gap energies range from
2.80 to 2.97 eV, narrower than the 3.0 eV exhibited by the
MADN host. The PL spectrum of 3 is redshifted relative
to that of 1, which suggests that its larger π-conjugated flu-
orene group reduces the HOMO–LUMO energy gap. The
distorted structure of compound 2 leads to breaks in the
conjugation along the framework and thus localization of
the π electrons, increasing its energy gap relative to com-
pound 3. Compound 4, which contains a carbazole unit,
shows a hypsochromic shift relative to compounds 1–3,
which contain triphenylamine, (diphenylamino)biphenyl,
and (diphenylamino)fluorene units, respectively. These re-
sults indicate that the carbazole moiety of the indeno-
fluorene derivative increases the bandgap energy. Interest-
ingly, compound 5, with an indeno[2,1-b]fluorene core, ex-
hibits absorption at a shorter wavelength but has a much
longer emission wavelength than compound 1 with an ind-
eno[1,2-b]fluorene core. This implies that the properties of
the excited states of compound 5 are very different to those
of compound 1. The fluorescent emission quantum yields
were determined by using BDAVBi (Φ = 0.86 in CH2Cl2)
as a standard and they decrease as follows: 3 (0.97)Ͼ1
(0.79)Ͼ4 (0.71)Ͼ2 (0.46)Ͼ5 (0.25). Compound 1 exhibits
a fluorescent emission quantum yield 3.20 times that of 5,
which indicates that the indeno[1,2-b]fluorene core im-
proves the emission quantum yield more than the ind-
eno[2,1-b]fluorene core.
The HOMO energies of the dopants were estimated by
using an AC-2 photoelectron spectrometer; the LUMO
levels were calculated by subtracting the corresponding op-
tical bandgap energies from the HOMO values. The
HOMO energies of compounds 1–5 were estimated to be
–5.34, –5.62, –5.48, –5.29, and –5.54 eV, respectively, and
their LUMO energies were calculated to be –2.49, –2.73,
–2.68, –2.32, and –2.61 eV, respectively. Figure 2 shows the
HOMO and LUMO energies of the blue fluorescent com-
pounds along with those of other compounds used in EL
devices: indium tin oxide (ITO), N,NЈ-diphenyl-N,NЈ-
bis[4-(phenyl-m-tolylamino)phenyl]biphenyl-4,4Ј-diamine
(DNTPD), N,NЈ-bis(1-naphthyl)-N,NЈ-diphenylbenzidine
(NPB),
MADN,
tris(8-hydroxyquinolinyl)aluminium
(Alq3), and LiF/Al.
To explore the observed photophysical properties of the
organic lighting-emitting diode compounds, density func-
tional theory calculations were carried out by using the
B3LYP hybrid functional with the 6-31G* basis set by using
the Gaussian 03 suite of programs.[19] The distributions of
the HOMOs and LUMOs are shown in Figure 3. The di-
hedral angles between the phenyl (cabazole/fluorene) and
indenofluorene moieties were calculated to be 16.15, 1.89,
Figure 1. (a) UV absorption and (b) PL spectra of compounds 1–5
and emission spectra of the common host MADN in dilute CH2Cl2
solution. (c) PL spectra of compounds 1–5 as thin films.
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Indenofluorene-Based Blue Fluorescent Compounds
Table 1. Photophysical data for compounds 1–5.
[c]
λabs [nm][a]
λem [nm][a,b]
FWHM [nm][a,b]
ΦF
ΦF(film) [%][d]
ΔE [eV][e]
LUMO/HOMO [eV][f]
1
2
3
4
5
398
392
402
388
347
464/481
469/480
480/490
66/70
75/76
73/71
58/129
73/77
0.79
0.46
0.97
0.71
0.25
10
16
15
12
99
2.85
2.89
2.80
2.97
2.93
–2.49/–5.34
–2.73/–5.62
–2.68/–5.48
–2.32/–5.29
–2.61/–5.54
429,448/463,534
492/477
[a] Measured in CH2Cl2 solution. [b] Measured in film. [c] Values of ΦF were determined in CH2Cl2 solution at 298 K against BDAVBi
as reference (Φ = 0.86). [d] The fluorescent quantum yields of thin films were determined by using BDAVBi film as standard (ΦF = 30%
measured by the calibrated integration sphere system). [e] ΔE is the bandgap energy estimated from the intersection of the absorption
and photoluminescence spectra. [f] The HOMO energies were determined by using a low-energy photoelectron spectrometer (Riken-
Keiki, AC-2) and LUMO = HOMO + ΔE.
15.34, 16.62, and 10.21°, respectively. The HOMO electrons
are distributed over the (diphenylamino)phenyl(fluorenyl)-
vinylindenofluorene moiety in 1, 3, and 5, whereas in the
LUMO, the electrons are distributed over the phenyl(fluo-
renyl)vinylindenofluorene moiety. The HOMO of 2 has
electrons distributed over the (diphenylamino)biphenylvin-
ylindenofluorene moiety, whereas the electrons in the
LUMO are distributed over the biphenylvinylindenofluo-
rene. Compound 4 has electrons distributed over the cab-
azolylvinylindenofluorene moiety in the HOMO and over
the vinylindenofluorene moiety in the LUMO.
Figure 2. Energy-level diagram for compounds 1–5 used in OLED
fabrication.
Figure 3. HOMOs and LUMOs of 1–5 calculated at the B3LYP/6-31G* level of theory.
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J. Y. Lee, S. S. Yoon et al.
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The electronic transition properties of compounds 1–5 CIE coordinates with doping concentration; see part b of
were investigated by time-dependent DFT (TDDFT) calcu- Figure 4. All the devices showed similar behavior with re-
lations carried out on the DFT-optimized geometries. Only spect to their dopant concentrations, which suggests that
HOMO-to-LUMO transitions contributed to the electronic the di-tert-butylphenylated indenofluorenyl moieties effec-
transitions in all the compounds. The HOMO energies of tively prevent intermolecular interactions and concentra-
1–5 lie within a narrow range, as do their LUMO energies. tion quenching.
The excitation energy of 1 was calculated to be 2.8311 eV,
which corresponds to an absorption wavelength of
437.94 nm. The excitation energies of 2 and 3 were calcu-
lated to be 2.8225 and 2.7435 eV, respectively, which corre-
spond to longer absorption wavelengths than those shown
by 1, 439.27 and 451.91 nm, respectively. This difference is
due to the lengths of the chromophores. The HOMOs of 2
and 3 show better delocalized electron density than that of
1 (Figure 3). The excitation energy of 4 was calculated to
be 2.9751 eV (416.74 nm), which is slightly redshifted rela-
tive to that of 1. This is due to the HOMO in 1 showing
greater delocalized electron density than that of 4. The car-
bazole moiety appears to destabilize the HOMO and
LUMO more than the diphenylamine moiety. The exci-
tation energy of 5 was calculated to be 2.9549 eV, corre-
sponding to an absorption wavelength of 419.51 nm, which
is hypsochromically shifted relative to that of 1. This is due
to 5 having a more planar structure than 1, which leads to
increased delocalization of the electron density. These re-
sults are in good agreement with the experimentally ob-
served absorption wavelengths.
The electroluminescent properties of 1–5 were examined
by testing devices fabricated by thermal deposition with
configurations of ITO/DNTPD (60 nm)/NPB (30 nm)/dop-
ant (x%):MADN host (30 nm)/Alq3 (20 nm)/LiF (1.0 nm)/
Al (200 nm). DNTPD acts as a hole-injection layer, NPB
as a hole-transporting layer, MADN as host, and Alq3 as
an electron-transporting layer. The electroluminescent
properties of 1–5 were optimized by testing host/dopant
emitting systems incorporating 1–5 as blue dopants at con-
centrations of 3, 5, and 7% with MADN as the common
fluorescent host.
With a doping concentration of 7%, devices 1C–5C,
doped with 1–5, respectively, exhibited blue emission at
8.5 V with Commission Internationale d’Énclairage (CIE)
and 7%).
coordinates of (0.146, 0.212), (0.149, 0.193), (0.151, 0.257),
Figure 4. (a) EL spectra of blue-emitting devices 1C–5C. (b) EL
spectra of devices 4A–4C at various doping concentration (3, 5,
(0.153, 0.128), and (0.216, 0.355), respectively [Figure 4 (a)].
The current density–voltage–luminance (J–V–L) charac-
Device 2C, the dopant of which contains a biphenyl spacer teristics of devices 1A–1C and the luminous efficiencies
group, shows blueshifted CIE coordinates relative to 1C, (LE), power efficiencies (PE), and external quantum effi-
those of 3C, the dopant of which contains a fluorenyl ciencies (EQE) of devices 1C–5C with respect to current
spacer group, are redshifted, 4C, the dopant of which con- density are shown in Figure 5 and summarized in Table 2.
tains a di-tert-butylphenylcarbazolylvinyl group rather than All the devices showed low turn-on voltages of 3.5–5.0 V
a (diphenylamino)arylvinyl group (1–3), shows blueshifted with stable blue emission from all over the surface. The de-
CIE coordinates due to the electronic effects of the tert- vices with 1–3 showed higher turn-on voltages than those
butylated carbazole moiety. Device 5C, which contains the with 4 or 5. The turn-on voltage was dependent on the
dopant with the indeno[2,1-b]fluorene core , shows signifi- charge-transport properties of the dopant and on the dif-
cantly redshifted CIE coordinates relative to the device 1C, ferent levels of charge trapping by the HOMO and LUMO
which contains the indeno[1,2-b]fluorene core, in good levels. The devices with 1–3 showed high EQEs (Ͼ5%) that
agreement with the PL spectra of the corresponding dop- nearly exceed the theoretical limits of fluorescent OLEDs.
ants. Normalized EL spectra of devices 4A–C, which are The high EQEs were attributed to the di-tert-butylphenyl-
fabricated with dopant 4 at concentrations of 3, 5, and 7%, ated indeno[1,2-b]fluorenyl cores with (diphenylamino)aryl-
respectively, show stable blue EL with little variation in the vinyl groups. The varying performances of the devices sug-
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Indenofluorene-Based Blue Fluorescent Compounds
gest that structural variation of the dopants’ spacer groups Devices with 2, containing biphenyl moieties as the spacer
and positional changes in the indenofluorene cores greatly groups, showed EQEs reduced by 6–14% relative to those
affect the performances of the solid-state devices. For exam- with 1 with phenyl spacer groups. Device 3A with fluorenyl
ple, changing the dopants’ spacer groups from phenyl to spacer units showed the best luminance of the devices with
fluorenyl moieties (1 vs. 3) increased the device’s EQE by indeno[1,2-b]fluorene core groups. The best luminous effi-
25–56%. Devices containing 3 showed the highest EQEs. ciency of 12.2 cdA–1, power efficiency of 6.08 lmW–1, and
Figure 5. (a) J-V-L data for devices 1A–1C. (b) Luminous efficiencies, (c) power efficiencies, and (d) external quantum efficiencies of the
devices 1C–5C as a function of current density.
Table 2. EL performance characteristics of devices 1–5.
Device
Doping [%] Von [V] ELmax [nm] L [cd/m2][a]
LE [cd/A][b,c]
PE [lm/W][b,c] EQE [%][b,c]
CIE (x, y)[d]
1A
1 (3)
1 (5)
1 (7)
2 (3)
2 (5)
2 (7)
3 (3)
3 (5)
3 (7)
4 (3)
4 (5)
4 (7)
5 (3)
5 (5)
5 (7)
4.5
4.5
4.5
5.0
5.0
5.0
5.0
4.5
4.5
4.0
4.0
4.0
4.0
3.5
3.5
4.0
453/481
454/482
455/483
451/479
451/474
452/480
459/488
460/489
461/491
438/461
437/463
437/462
507
10740
10260
10480
7541
7863
11580
15290
13420
22410
8767
8887
7952
7394
9490
9114
954
8.06/7.81
8.44/8.22
9.19/8.92
7.00/7.00
7.84/7.72
7.84/7.84
12.6/12.2
11.3/11.1
12.2/11.5
2.98/2.97
3.13/3.07
3.25/3.25
2.21/2.21
1.99/1.90
3.68/1.99
1.4/1.1
4.41/4.10
4.62/4.31
4.82/4.60
3.98/3.47
4.11/3.83
4.12/3.89
6.69/6.08
5.04/5.04
6.18/5.95
2.13/1.59
1.77/1.70
1.82/1.74
1.59/1.28
1.18/1.12
3.71/1.15
1.0/0.5
5.96/5.70
6.17/6.00
6.40/6.12
5.34/5.28
5.73/5.55
5.98/5.69
8.21/7.84
7.14/6.30
7.55/7.01
2.87/2.81
2.82/2.69
2.96/2.93
0.91/0.91
0.89/0.81
1.37/0.84
1.6/1.5
(0.145, 0.194)
(0.145, 0.198)
(0.146, 0.212)
(0.149, 0.183)
(0.150, 0.194)
(0.149, 0.193)
(0.148, 0.241)
(0.149, 0.250)
(0.151, 0.257)
(0.153, 0.121)
(0.153, 0.133)
(0.153, 0.128)
(0.222, 0.376)
(0.217, 0.362)
(0.216, 0.355)
(0.15, 0.08)
1B
1C
2A
2B
2C
3A
3B
3C
4A
4B
4C
5A
5B
508
507
442
5C
MADN only
[a] Maximum luminance 8.5 V. [b] Maximum values. [c] At 20 mA/cm2. [d] Commission Internationale d’Énclairage (CIE) coordinates at
8.5 V.
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the highest EQE of 7.84% at 20 mAcm–2 were obtained
with device 3A. Devices with dopant 3 showed more effec-
tive energy transfer from the MADN host to the dopant
than devices with 1, 2, 4, and 5 due to the good spectral
overlap between the emission of the MADN host and the
dopant’s absorption; see Figure 1 (a). Dopant 3 also exhib-
ited the highest fluorescent emission quantum yield (0.97).
These factors likely contributed to the improved EL effi-
ciencies of the devices fabricated with 3. The low quantum
yield of compound 5 (0.25) and the poor overlap of its ab-
sorption spectrum with the emission spectrum of the
MADN host likely contributes to the reduced EL efficienc-
ies of devices 5A–5C. These results suggest that effective
Förster energy transfer between the host and dopant and
the high fluorescent emission quantum yields of the dop-
ants lead to highly efficient blue OLEDs. The EL efficienc-
ies of the devices were not significantly reduced by increas-
ing the doping to 7%. These observations indicate that
tetraethylated indenofluorene derivatives containing a (di-
phenylamino)arylvinyl group and di-tert-butylphenyl block-
ing units reduce molecular aggregation in the emitting layer,
preventing concentration quenching.
were determined from the intersections of the absorption and pho-
toluminescence spectra. LUMO energies were calculated by sub-
tracting the corresponding optical bandgap energies from the
HOMO values.
Materials and Syntheses: Solvents for organic synthesis were rea-
gent- or HPLC-grade. Only HPLC-grade solvents were used in
photophysical spectroscopy and fluorescence quantum yield mea-
surements. THF was dried with sodium benzophenone ketyl anion
radicals and distilled under a dry nitrogen immediately prior to use.
Commercially available reagents were used without further purifi-
cation unless otherwise stated. Diethyl 4-(diphenylamino)benzyl-
phosphonate (6),[22] diethyl [4Ј-(diphenylamino)biphenyl-4-yl]meth-
ylphosphonate (7),[23] diethyl (7-diphenylamino-9,9-diethylfluoren-
2-yl)methylphosphonate (8),[24] diethyl [9-(3,5-di-tert-butylphenyl)-
9H-carbazolyl]methylphosphonate (9),[25] 2,8-dibromo-6,6,12,12-
tetraethylindeno[1,2-b]fluorene (10),[15] and 1,3-dihydroindeno[2,1-
b]fluorene (13)[18] were prepared as reported elsewhere.
General Procedure for the Horner–Wadsworth–Emmons Reactions:
Potassium tert-butoxide (2.3 mmol) was added to a solution of 2-
(3,5-di-tert-butylphenyl)-8-formyl-6,6,12,12-tetraethylindeno[1,2-b]-
fluorene (1.5 mmol) and the corresponding phosphonate
(1.5 mmol) in THF in an ice bath under nitrogen. The reaction
mixture was stirred for 15 min at 0 °C for 1 h at room temperature
and then quenched with water. The mixture was extracted with
ethyl acetate and washed twice with water. The combined organic
layers were dried with MgSO4 and the solvent was removed under
reduced pressure to afford a crude product that was purified by
silica gel column chromatography and subsequent recrystallization
from CH2Cl2/EtOH.
Conclusions
Blue fluorescent indenofluorene derivatives containing
(diphenylamino)arylvinyl groups and di-tert-butylphenyl
blocking units exhibit high EL performances. The device
with 3% of 3 as blue dopant shows a luminous efficiency
of 12.2 cdA–1, a power efficiency of 6.08 lmW–1, and an
external quantum efficiency of 7.84% at 20 mAcm–1. Its
CIE coordinates are (0.148, 0.241) at 8.5 V. A deep-blue
OLED with CIE coordinates (0.153, 0.128) at 8.5 V was
prepared by using compound 4 as the dopant. It exhibits
a luminous efficiency of 3.25 cdA–1, a power efficiency of
Device Fabrication and Measurement: The blue devices had configu-
rations of indium tin oxide (ITO, 150 nm)/N,NЈ-diphenyl-N,NЈ-
bis[4-(phenyl-m-tolylamino)phenyl]biphenyl-4,4Ј-diamine (DNTPD,
60 nm)/N,NЈ-bis(1-naphthyl)-N,NЈ-diphenylbenzidine
(NPB,
30 nm)/MADN:dopants (30 nm)/tris(8-hydroxyquinolinyl)alumin-
ium (Alq3, 20 nm)/LiF (1.0 nm)/Al (200 nm). All organic com-
pounds, except the dopants, were deposited at 1 Å/s. Device current
(J)–voltage (V)–luminance (L) characteristics and electroluminesc-
ence (EL) spectra were measured by using a Keithley 2400 measure-
1.74 lmW–1, and an external quantum efficiency of 2.93% ment unit and CS 1000A spectrophotometer, respectively.
at 20 mAcm–1. Di-tert-butylphenylated indeno[1,2-b]fluor-
enyl derivatives were shown to possess properties suitable
for use in blue-emitting OLEDs.
Supporting Information (see footnote on the first page of this arti-
cle): Detailed experimental procedures, characterization data, 1H
and 13C NMR spectra for all new compounds, low-energy photo-
electron spectra of compounds 1–5.
Experimental Section
Acknowledgments
General Methods: 1H and 13C NMR spectra were recorded in
CDCl3 with a Varian spectrometer (Unity Inova 300Nb or Unity
Inova 500Nb). FTIR spectra were recorded with a Bruker VER-
TEX70 or TENSOR27 spectrometer. Low- and high-resolution
mass spectra were measured by using a JEOL JMS-AX505WA
spectrometer in FAB mode. UV/Vis absorption spectra of the blue-
emitting compounds in dichloromethane (10–5 m) in quartz cuvettes
(1.0 cm paths) were acquired by using a Sinco S-3100 spectropho-
tometer at 293 K. Photoluminescence spectra were measured with
an Amincobrowman series 2 luminescence spectrometer. Fluores-
cence quantum yields were determined in nitrogen-degassed di-
chloromethane at 293 K against a BDAVBi reference (Φ = 0.86)[20]
and as films formed on quartz plates (50 nm) using BDAVBi as a
standard (Φf = 30% measured by the calibrated integration sphere
system).[21] HOMO energies were determined by using a low-energy
photoelectron spectrometer (Riken-Keiki, AC-2). Bandgap energies
This research was supported by the Korean Ministry of Education,
Science and Technology, National Research Foundation (NRF)
(Basic Science Research Program) (20110004655). J. Y. L. acknowl-
edges financial support from the Gyeonggi Province (GRRC pro-
gram Dankook2011-B01: Materials Development for High-Effi-
ciency Solid-State Lighting) and from the Development of Core
Technologies for Organic Materials Applicable to OLED Lighting
with High Color Rendering Index by the Ministry of Knowledge
and Economy (MKE).
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Published Online: April 10, 2012
Eur. J. Org. Chem. 2012, 2748–2755
© 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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