Diphenylaminofluorene-based donor-acceptor molecules for blue light-emitting diodes

Article abstract of DOI:10.1080/15421406.2012.689719

A series of blue fluorescent 2-diphenylaminofluorene-7-ylstyrene derivatives end-capped with electron-accepting groups, such as methylpyridine, qunoline, isoqunoline, phenanthridine were synthesized and characterized. To examine the electroluminescent properties of these molecules, multilayer devices were fabricated. The device D exhibited highly efficient blue emission with a luminous efficiency (LE), power efficiency (PE) and CIEx,y coordinates of 7.13 cd/A, 2.80 lm/W at 20mA/cm² and (0.148, 0.191) at 7.0V, respectively. In addition, the device A showed efficient deep-blue emission with a LE, PE and CIEx,y coordinates of 6.85 cd/A, 2.68 lm/W at 20mA/cm², and (0.147, 0.152) at 7.0V, respectively. Taylor and Francis Group, LLC.

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Diphenylaminofluorene-Based Donor-
Acceptor Molecules for Blue Light-
Emitting Diodes
Kum Hee Lee a , Suhyun Oh a , Seok Jae Lee b , Young Kwan Kim b &
Seung Soo Yoon a
a Department of Chemistry , Sungkyunkwan University , Suwon ,
440-746 , Korea
b Department of Information Display , Hongik University , Seoul ,
121-791 , Korea
Published online: 02 Aug 2012.
To cite this article: Kum Hee Lee , Suhyun Oh , Seok Jae Lee , Young Kwan Kim & Seung Soo Yoon
(2012) Diphenylaminofluorene-Based Donor-Acceptor Molecules for Blue Light-Emitting Diodes,
Molecular Crystals and Liquid Crystals, 563:1, 206-214, DOI: 10.1080/15421406.2012.689719
To link to this article: http://dx.doi.org/10.1080/15421406.2012.689719
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Mol. Cryst. Liq. Cryst., Vol. 563: pp. 206–214, 2012
Copyright © Taylor & Francis Group, LLC
ISSN: 1542-1406 print/1563-5287 online
DOI: 10.1080/15421406.2012.689719
Diphenylaminofluorene-Based Donor-Acceptor
Molecules for Blue Light-Emitting Diodes
KUM HEE LEE,1 SUHYUN OH,1 SEOK JAE LEE,2
YOUNG KWAN KIM,2,∗ AND SEUNG SOO YOON1,∗
1Department of Chemistry, Sungkyunkwan University, Suwon 440-746, Korea
2Department of Information Display, Hongik University, Seoul 121-791, Korea
A series of blue fluorescent 2-diphenylaminofluorene-7-ylstyrene derivatives end-capped
with electron-accepting groups, such as methylpyridine, qunoline, isoqunoline, phenan-
thridine were synthesized and characterized. To examine the electroluminescent prop-
erties of these molecules, multilayer devices were fabricated. The device D exhibited
highly efficient blue emission with a luminous efficiency (LE), power efficiency (PE) and
CIEx,y coordinates of 7.13 cd/A, 2.80 lm/W at 20 mA/cm2 and (0.148, 0.191) at 7.0 V,
respectively. In addition, the device A showed efficient deep-blue emission with a LE,
PE and CIEx,y coordinates of 6.85 cd/A, 2.68 lm/W at 20 mA/cm2, and (0.147, 0.152)
at 7.0 V, respectively.
Keywords Bluefluorescent materials; deep-blue emitting materials; diphenylaminoflu-
orene derivatives; organic light-emitting diode
Introduction
Full-color organic light-emitting diode (OLED) display applications require a set of pri-
mary RGB emitters with sufficiently high luminous efficiency and properly balanced color
chromaticity, as well as adequate operational stability. Recently, OLED doped with blue
emitters, such as anthracene, fluorene, carbazole and pyrene derivatives, were reported
[1–8]. Nevertheless, the efficiency, lifetime and color purity of these blue emitters should
be further improved.
Many emissive electron donor-acceptor type molecules have been reported, and EL
colors spanning the entire visible region, including green [9], yellow [10], and red [11], have
been achieved from OLEDs based on these materials. However, only a few electron donor-
acceptor type molecules have been developed as blue emitters in OLEDs [12–13]. There-
fore, more systematic studies of the structure-electroluminescent property relationships of
electron donor-acceptor type molecules will be essential for the design of next-generation
ambipolar emissive materials for OLEDs.
This paper reports the synthesis and electroluminescent properties of four donor-
acceptor (D-A) type blue-emitting molecules containing 2-diphenylaminofluorene as a
∗Address correspondence to Prof. Young Kwan Kim, Department of Information Display,
Hongik University, Mapo-gu, Sangsu-dong, Seoul 121-791, Korea (ROK). Tel: (+82)2-320-1646;
Fax: (+82)2-3141-8928. E-mail: kimyk@hongik.ac.kr or S.S. Yoon, Department of Chemistry,
Sungkyunkwan University, Cheoncheon-dong, Jangan-gu, Suwon, 440-746, Korea (ROK). Tel: (+82)
31-290-5971; Fax: (+82) 31-290-7075. E-mail: ssyoon@skku.edu
206

Diphenylaminofluorene-Based Molecules for OLEDs
207
Figure 1. Synthesis of blue fluorescent materials (1–4).
electron donor and the various pyridine derivatives, including methylpyridine, qunoline,
isoqunoline, and phenanthridine, as electron acceptors. Figure 1 shows the molecular
structures of the 2-diphenylaminofluorene molecules, 7-(4-(6-methylpyridin-2-yl)styryl)-
9,9-diethyl-N,N-diphenyl-9H-fluoren-2-amine (1), 7-(4-(quinolin-2-yl)styryl)-9,9-diethyl-
N,N-diphenyl-9H-fluoren-2-amine
(2),
7-(4-(isoquinolin-1-yl)styryl)-9,9-diethyl-N,
N-diphenyl-9H-fluoren-2-amine (3), and 7-(4-(phenanthridin-6-yl)styryl)-9,9-diethyl-
N,N-diphenyl-9H-fluoren-2-amine (4). This paper reports the electron acceptor structure
property relationships in D-A molecules relevant to the photophysical properties,
HOMO/LUMO energy levels and solid-state electronic devices.
Experimental
Synthesis
All commercially available reagents were purchased from Aldrich and TCI and used with-
out further purification. (7-(Diphenylamino)-9,9-diethylfluoren-2-yl)methylphosphonate
[1], 4-(6-methylpyridin-2-yl)benzaldehyde [14], 4-(quinolin-2-yl)benzaldehyde [15], 4-
(isoquinolin-1-yl)benzaldehyde [15], and 4-(phenanthridin-6-yl)benzaldehyde [15] was

208
K. H. Lee et al.
synthesized by elsewhere reported procedures. The blue compounds were properly pu-
rified and dried. 1H and 13C NMR spectra were recorded on a Varian (Unity Inova 300Nb)
spectrometer. Fourier transform infrared (FT-IR) spectra were recorded using a Buker VER-
TEX70 FT-IR spectra spectrometer. Low- and high-resolution mass spectra were measured
using a JEOL JMS-AX505WA spectrometer in the Fast atom bombardment (FAB) mode
and a JMS-T100TD (AccuTOF-TLC) spectrometer in the positive-ion mode. The UV-Vis
absorption and photoluminescence spectra of these newly designed dopants were measured
in dichloromethane (10−5 M) using Sinco S-3100 and Amincobrowman series 2 lumines-
cence spectrometer. The fluorescent quantum yield were determined in dichloromethane
solution at 293K against 4,4ꢀ-bis(4-(diphenyl-amino)styryl)biphenyl (BDAVBi) as a ref-
erence (Ф = 0.86) [16]. The HOMO energy levels were measured with a low energy
photo-electron spectrometry (Riken-Keiki AC-2). LUMO energy levels were estimated by
subtracting the energy gap from the HOMO energy levels.
General procedure for the Horner-Wadsworth-Emmons reaction: To a mixture of phos-
phonate intermediate (0.82 mmol) and corresponding aldehyde (0.82 mmol) in 20 mL of
anhydrous THF at 0◦C was added dropwise 1.2 mL of 1.0 M KOt-Bu (1.2 mmol) in THF
under N2. The reaction mixture was stirred for 10 min at 0◦C, followed by 1 hr at room
temperature and quenched with water. The solution mixture was filtered with water. The
filtered solid was dissolved in CH2Cl2, dried over Na2SO4, filtered with silica gel, and
the solvent removed under reduced pressure to afford a crud product recrystallized from
CHCl3/EtOH, affording product (1–4).
7-(4-(6-Methylpyridin-2-yl)styryl)-9,9-diethyl-N,N-diphenyl-9H-fluoren-2-amine
(1). Yield = 50%. 1H NMR (300 MHz, CDCl3): δ (ppm): 8.00 (d, J = 8.3 Hz, 2H), 7.63 (d,
J = 8.3 Hz, 4H), 7.59 (d, J = 5.4 Hz, 1H), 7.56–7.48 (m, 3H), 7.47 (s, 1H), 7.24–7.22 (m,
4H), 7.12 (d, J = 8.7 Hz, 6H), 7.10–6.99 (m, 4H), 2.64 (s, 3H), 1.99–1.92 (m, 4H), 0.38
(t, J = 7.3 Hz, 6H); 13C NMR (125 MHz, CDCl3): δ (ppm): 158.4, 156.5, 151.6, 150.4,
148.0, 147.3, 141.3, 138.6, 138.1, 136.9, 136.4, 135.5, 129.8, 129.3, 129.2, 127.3, 127.2,
126.8, 126.0, 123.9, 123.7, 122.5, 121.6, 120.6, 120.4, 119.4, 117.4, 56.1, 32.8, 24.8, 8.6;
MS (FAB+): 582 [M+]; HRMS (FAB+): [M+] calculated for C43H38N2, 582.3035, found:
582.3029.
7-(4-(Quinolin-2-yl)styryl)-9,9-diethyl-N,N-diphenyl-9H-fluoren-2-amine (2). Yield =
54%. 1H NMR (300 MHz, CDCl3): δ (ppm): 8.25–8.17 (m, 4H), 7.93 (d, J = 8.6 Hz, 1H),
7.83 (d, J = 8.1 Hz, 1H), 7.75 (d, J = 8.1 Hz, 1H), 7.71 (d, J = 8.2 Hz, 2H), 7.61 (t,
J = 7.8 Hz, 2H), 7.56 (d, J = 7.4 Hz, 2H), 7.49 (s, 1H), 7.29–7.24 (m, 4H), 7.14–7.11
(m, 6H), 7.06–6.99 (m, 4H), 2.01–1.93 (m, 4H), 0.39 (t, J = 7.3 Hz, 6H); 13C NMR
(125 MHz, CDCl3): δ (ppm): 151.7, 150.5, 148.0, 147.3, 141.4, 138.7, 138.4, 136.7, 135.4,
130.2, 129.7, 129.2, 127.9, 127.5, 127.4, 127.2, 127.0, 126.9, 126.3, 123.9, 123.7, 122.6,
120.7, 120.5, 119.3, 118.8, 56.1, 32.8, 8.6; MS (FAB+): 618 [M+]; HRMS (FAB+): [M+]
calculated for C46H38N2, 618.3035, found: 618.3031.
7-(4-(Isoquinolin-1-yl)styryl)-9,9-diethyl-N,N-diphenyl-9H-fluoren-2-amine
(3).
1
Yield = 53%. H NMR (300 MHz, CDCl3): δ (ppm): 8.63 (d, J = 5.7 Hz, 1H), 8.19 (d,
J = 8.4 Hz, 1H), 7.90 (d, J = 8.1 Hz, 1H), 7.72 (d, J = 2.6 Hz, 4H), 7.69–7.64 (m, 4H),
7.61–7.49 (m, 8H), 7.29–7.49 (m, 5H), 7.14–7.11 (m, 2H), 7.06–7.02 (m, 2H), 1.99–1.90
(m, 4H), 0.39 (t, J = 7.2 Hz, 6H); 13C NMR (125 MHz, CDCl3): δ (ppm): 151.7, 150.5,
148.3, 147.3, 142.3, 137.9, 136.9, 136.3, 135.4, 130.4, 130.1, 130.0, 129.2, 127.6, 127.2,
127.1, 126.7, 126.3, 126.1, 123.9, 123.7, 122.5, 120.7, 120.4, 119.9, 119.3, 56.1, 32.7, 8.6;
MS (FAB+): 618 [M+]; HRMS (FAB+): [M+] calculated for C46H38N2, 618.3035, found:
618.3028.

Diphenylaminofluorene-Based Molecules for OLEDs
209
7-(4-(Phenanthridin-6-yl)styryl)-9,9-diethyl-N,N-diphenyl-9H-fluoren-2-amine (4).
1
Yield = 38%. H NMR (300 MHz, CDCl3): δ (ppm): 8.73 (d, J = 8.3 Hz, 1H), 8.64
(dd, J = 0.9, 8.3 Hz, 1H), 8.26 (dd, J = 1.2, 8.0 Hz, 1H), 8.20 (d, J = 7.8 Hz, 1H), 7.89
(td, J = 1.2, 7.1 Hz, 1H), 7.79–7.63 (m, 8H), 7.61 (d, J = 7.8 Hz, 1H), 7.57–7.51 (m,
3H), 7.30–7.24 (m, 4H), 7.15–7.11 (m, 5H), 7.08–6.99 (m, 4H), 2.06–1.89 (m, 4H), 0.40
(t, J = 7.3 Hz, 6H); 13C NMR (125 MHz, CDCl3): δ (ppm): 160.9, 151.6, 150.5, 147.9,
147.3, 143.9, 141.4, 138.7, 138.1, 136.3, 135.4, 133.5, 130.6, 130.4, 130.3, 130.1, 129.2,
129.1, 128.9, 127.2, 127.1, 126.9, 126.4, 126.1, 125.2, 123.9, 123.7, 123.6, 122.5, 122.3,
121.9, 120.7, 120.4, 119.3, 56.1, 32.7, 8.6; MS (FAB+): 668 [M+]; HRMS (FAB+): [M+]
calculated for C50H40N2, 668.3191, found: 668.3188.
Fabrication of OLED and Measurements
For fabricating OLEDs, indium-tin-oxide (ITO) thin films coated on glass substrates were
used, 30 ꢀ/square of the sheet resistivity with a thickness of 100 nm. The ITO-coated
glass was cleaned in an ultrasonic bath by the following sequence: acetone, methyl alcohol,
distilled water, storage in isopropyl alcohol for 48 h, and dried by N2 gas gun. The substrates
were treated by O2 plasma under conditions of 2 × 10−2 Torr at 125 W for 2 min. All organic
materials and metals were deposited under high vacuum (5 × 10−7 Torr). The OLEDs were
fabricated in the following sequence: ITO/2-TNATA (60 nm)/NPB (20 nm)/2% dapant:
MADN (30 nm)/Alq3 (20 nm)/Liq (2 nm)/Al (100 nm). The current density (J), luminance
(L), luminous efficiency (LE), and the CIE chromaticity coordinates of the OLEDs were
measured with Keithly 2400, Chroma meter CS-1000A. Electroluminescence was measured
using a Roper Scientific Pro 300i.
Results and Discussion
Figure 1 shows the structures of the electron donor-acceptor (D-A) type fluorescent materi-
als. The (7-(diphenylamino)-9,9-diethylfluoren-2-yl)methylphosphonate [1] and aldehyde
intermediates, such as 4-(6-methylpyridin-2-yl)benzaldehyde [13], 4-(quinolin-2-yl)benzal
dehyde [14], 4-(isoquinolin-1-yl)benzaldehyde [14], and 4-(phenanthridin-6-yl)benzal
dehyde [14] were prepared according to the methods reported in the literature. The Horner-
Emmons reaction between 7-(Diphenylamino)-9,9-diethylfluoren-2-yl)methylphosphonate
and the corresponding aldehyde compounds afforded the blue-emitting materials (1–4) in
moderate yield. After conventional purification, such as column chromatography and re-
crystallization, these newly synthesized materials (1–4) were purified further by train subli-
mation under reduced pressure (<10−3 torr). The molecular structures of the blue-emitting
materials (1–4) werefully characterized by 1H- and 13C-nuclear magnetic resonance (NMR),
and low- and high-resolution mass spectroscopy.
Figure 2 presents the UV-Vis absorption and PL emission spectra of blue fluorescent
materials 1–4 in dichloromethane and on quartz plates. All compounds exhibited a charge-
transfer (CT) absorption band in the dichloromethane solution over the range, 385–395 nm.
The UV-vis absorption spectra of the blue materials (1–4) overlapped well with the PL
emission spectra of a common blue host, MADN. This suggests that energy transfer be-
tween the MADN and these materials is quite efficient and MADN serves well as a host in
OLED devices using these materials as dopants. The emission spectra of the D-A molecules
increased in the order compounds 1 (495 nm) < 3 (520 nm) < 4 (525 nm) < 2 (529 nm).
The emission spectra of the D-A molecule showed a gradual red shift with increasing
π-conjugation length of the electron acceptor. For example, compared to compound 1 with

210
K. H. Lee et al.
Figure 2. (a) UV-Vis absorption spectra and (b) PL spectra of materials (1–4) (solution; closed
symbols and film; opened symbols).
the pyridine moiety, the observed red shifts in the emission spectra of compounds 2–4 may
arise from the extended π-conjugation in the electron accepting moieties. Compound 3 had
a shorter wavelength (520 nm) than compound 2 (529 nm), which may be rationalized from
the fact that the dihedral angle between the phenyl group and isoqunoline in compound 3 is
more distorted than that between phenyl group and quinoline in compound 2, resulting in
a wide band gap. Table 1 lists the photophysical properties of the D-A materials. Interest-
ingly, compounds 1–4 all showed a maximum emission wavelength that was blue-shifted
(3–30 nm) in the film compared to the solution. The emission quantum yield of these
materials was high (ꢁ = 0.50–0.79), suggesting that these materials have highly efficient
electroluminescent properties in OLED devices.
The highest occupied molecular orbital (HOMO) levels were measured using a pho-
toelectron spectrometer (Riken-Keiki AC-2), and the lowest unoccupied molecular orbital
(LUMO) levels were calculated by subtracting the corresponding optical band gap from
the HOMO values. The HOMO and LUMO energy levels of compounds 1–4 ranged from
−5.41 to −5.61 eV and −2.68 to −2.80 eV, respectively (Table 1). The calculated HOMO-
LUMO energy gaps for compounds 1–4 were 2.81, 2.73, 2.81 and 2.77 eV, respectively.
Figure 3 shows the HOMO and LUMO energy levels of the blue fluorescent materials,
Table 1. Physical properties of blue materials (1–4)
UVMax
(nm)[a]
PLMax
FWHM
HOMO/LUMO
(eV)[c]
Compound
(nm)[a]/[b]
(nm)[a]/[b]
Eg
Ф [d]
1
2
3
4
389
395
385
392
495/492
529/506
520/503
525/495
86/62
100/64
99/62
−5.58/−2.77
−5.41/−2.68
−5.61/−2.80
−5.56/−2.79
2.81
2.73
2.81
2.77
0.79
0.60
0.50
0.56
102/63
[a]Maximum absorption or emission wavelength in 1,2-dichloroethane (1 × 10−5 M). [b]Thin films.
[c]Obtained from AC-2 and UV-Vis absorption measurements. [d]Using BDAVBi as a standard; λex
360 nm (Ф = 0.86 in 1,2-dichloroethane).
=

Diphenylaminofluorene-Based Molecules for OLEDs
211
Figure 3. Energy level diagram of the materials used in devices.
along with those of other materials used in the electroluminescence devices, including ITO,
2-TNATA, NPB, MADN, Alq3 and Liq:Al.
To examine the EL properties of these materials, the devices were fabricated with the
following structure: ITO/2-TNATA (60 nm)/NPB (20 nm)/MADN: 2% dapant (30 nm)/Alq3
(20 nm)/Liq (2 nm)/Al (100 nm). Figure 5 shows the luminous efficiencies and luminance
of the devices, and the data is summarized in Table 2. Devices A-D showed high luminous
efficiencies of 6.85, 5.20, 6.90 and 7.13 cd/A, and power efficiencies of 2.68, 2.04, 2.80, and
2.74 lm/W at 20 mA/cm2, respectively. This suggests that the EL performance of devices
A–D is sensitive to the structural changes of the acceptor groups in the emitting layers. For
example, compared to device B using compound 2 with a quinoline end-capping group, the
luminous and power efficiency of device D using compound 4 with a phenanthridine end-
capping group increased by 37.1 and 34.3% at 20 mA/cm2, respectively. Interestingly, the
HOMO energy level of compound 2 was 0.15∼0.20 eV higher than that of compounds 1,
3 and 4. Presumably, this high HOMO energy level of dopant 2 and effective hole-trapping
in device B would reduce the EL efficiencies of device B compared to devices A, C and D.
Molecular mechanics (MM2) energy minimization simulations of compounds 1–4
were carried out using ChemDraw to explain the observed differences in electroluminescent
properties of devices A–D at the molecular level of the dopant materials. Figure 4 shows the
energy-minimized structures of compounds 1–4. The calculated dihedral angles between the
phenyl and methylpyridyl (qunolinyl) moiety of compounds 1 and 2 were 0.54 and 2.30◦,
respectively. However, the angles between the phenyl and isoqunolinyl (phenanthridyl)
Table 2. EL performance characteristic of devices (A–D)
L[a]
Von
LE[c]/[d]
(cd/A)
PE[c]/[d]
(lm/W)
EL[e]
(nm)/fwhm
[b]
Dopant
(%)
(cd/m2)
(V)
CIE[e](x,y)
A
B
C
D
1
2
3
4
7522
6785
6308
6441
4.0
4.5
4.5
4.5
6.85/6.85 2.68/3.64
5.20/5.23 2.04/2.33
6.90/6.99 2.74/3.15
7.13/7.16 2.80/3.36
460/48
464/56
464/64
466/64
(0.147, 0.152)
(0.154, 0.156)
(0.148, 0.179)
(0.148, 0.191)
[a]Value of the luminance at 11 V. [b]Turn-on voltage at 1.0 cd/m2. [c]Value measured at 20 mA/cm2.
[d]Maximum values. [e]Value measured at 7.0 V.

212
K. H. Lee et al.
Figure 4. Energy-minimized structure of materials (1–4) by MM2 calculation.
moieties of compounds 3 and 4 were 41.6 and 43.7◦, respectively. These results suggest
that the molecular aggregation between compounds 1 and 2 is more effective than that
between compounds 3 and 4. Accordingly, devices A and B using compounds 1 and 2
would show lower EL efficiency through concentration quenching compared to devices C
and D using compounds 3 and 4. Presumably, the higher quantum yield of compound 1 (ꢁ =
0.79) than compound 2 (ꢁ = 0.60) plays an important role in the improved EL efficiency
of device A using compound 1 compared to device B using compound 2.
As shown in Figure 6, devices A-D emitted blue electroluminescence in the blue
region at 460, 464, 466, and 464 nm, respectively. The CIE coordinates of devices A-D
Figure 5. (a) J-V-L characteristics of devices and (b) luminous efficiency and power efficiency vs.
current density relationship of devices A–D.

Diphenylaminofluorene-Based Molecules for OLEDs
213
Figure 6. EL spectra of blue OLEDs at 7.0 V.
were (0.147, 0.152), (0.154, 0.156), (0.148, 0.179) and (0.148, 0.191), respectively. Device
A using compound 1 showed deep blue emission with a Commission Internationale de
l’Eclairage (CIEy) coordinates of approximately 0.15 at 7.0 V. Given that the electron
donor moiety in compounds 1–4 is the same, the observed blue shift in device A may be
due to the less π-conjugation of the electron acceptor moiety of compound 1 in the emitting
layer of device A.
Conclusions
This paper reported a molecular design strategy of combining a 2-diphenylaminofluorene-
7-ylstyrene as an electron donor and pyridine derivatives as an electron acceptor to give the
new electron donor-acceptor type blue materials. In particular, device D exhibited highly
efficient blue emission with a luminous efficiency, power efficiency and CIEx,y coordinates
of 7.13 cd/A, 2.80 lm/W at 20 mA/cm2 and (x = 0.148, y = 0.191) at 7.0 V, respectively. In
addition, device A showed efficient deep-blue emission with a luminous efficiency, power
efficiency and CIEx,y coordinates of 6.85 cd/A, 2.68 lm/W at 20 mA/cm2 and (x = 0.147,
y = 0.152) at 7.0 V, respectively. This clearly suggests that the materials based on the
donor-acceptor structures have excellent properties suitable for blue-emitting materials in
OLEDs
Acknowledgment
This research was supported by Basic Science Research Program through the NRF funded
by the Ministry of Education, Science and Technology (20110004655).
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