Synthesis and characterization of new triphenylamino-1,8-naphthalimides for organic light-emitting diode applications

Article abstract of DOI:10.1039/c4nj01785d

Three new triphenylamino naphthalimides (TPN1-3) were successfully synthesized using Suzuki cross-coupling reactions between N-phenyl-1,8-naphthalimide and triphenylamine precursors. In CHCl₃ solution phase, their maximum absorption wavelengths are around 430-433 nm, while the maximum emission wavelength was at 564-599 nm with quantum yields in the range of 0.34-0.55 in CHCl₃. In a solid thin-film state, where the packing force enhances the conjugated system and prevents molecular vibration, their absorption bands move towards the longer wavelengths and the emission peaks shift to shorter wavelengths. These compounds show excellent thermal stabilities with their 10% weight loss temperatures well above 350°C. The results from the electrochemical investigation using cyclic voltammetry agrees with the data from computational calculations using the Gaussian 09 program with B3LYP/6-31G(d,p) geometry optimization. When the multi-layered OLED device with a ITO/PEDOT:PSS/TPN1:CBP/BCP/LiF/Al structure was fabricated, a maximum brightness of 10404 cd m^-2 of yellowish green light at an applied voltage of 19 V and turn-on voltage of 5.8 V was observed. The effect of the solubility of these compounds on device performance was confirmed by the AFM images of the spin-casted thin films.

Full text of DOI:10.1039/c4nj01785d

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SYNTHESIS AND CHARACTERIZATIONS OF NEW TRIPHENYLAMINOꢀ1,8ꢀ
NAPHTHALIMIDES FOR ORGANIC LIGHTꢀEMITTING DIODE APPLICATION
Rungthiwa Arunchai,a Taweesak Sudyoadsuk,b Narid Prachumrak,b Supawadee
Namuangruk,c Vinich Promarak,d Mongkol Sukwattanasinitta and Paitoon Rashatasakhon,a*
a Organic Synthesis Research Unit, Department of Chemistry, Faculty of Science,
Chulalongkorn University, Bangkok, 10330, Thailand
b Department of Chemistry and Center of Excellence for Innovation in Chemistry, Faculty of
Science, Ubon Ratchathani University, Ubon Ratchathani, 34190, Thailand
c Nanoscale Simulation Laboratory, National Nanotechnology Center (NANOTEC), National
Science and Technology Development Agency (NSTDA), Klong Luang, Pathumthani, 12120,
Thailand
d School of Chemistry and Center of Excellence for Innovation in Chemistry, Institute of
Science, Suranaree University of Technology, Muang District, Nakorn Ratchasima, 30000,
Thailand
*Eꢀmail: paitoon.r@chula.ac.th
Tel: 02ꢀ218ꢀ7620
1

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Abstract
Three new triphenylamino naphthalimides (TPN1ꢀ3) are successfully synthesized by means
of Suzuki crossꢀcoupling between Nꢀphenylꢀ1,8ꢀnaphthalimide and triphenylamine
precursors. In CHCl3 solution phase, their maximum absorption wavelengths are around 430ꢀ
433 nm while the maximum emission wavelength at 564ꢀ599 nm with quantum yields in the
range of 0.34 to 0.55 in CHCl3. In solid thinꢀfilm state where the packing force enhances the
conjugated system and prevents the molecular vibration, their absorption bands move towards
longer wavelengths and the emission peaks shift to shorter wavelengths. These compounds
show excellent thermal stabilities with the 10% weight loss temperatures well above 350 °C.
Results from the electrochemical investigation by cyclic voltammetry agrees with the data
from computational calculations using Gaussian 09 program with B3LYP/6ꢀ31G(d,p)
geometry optimizations. When the multiꢀlayer OLED device of structure
ITO/PEDOT:PSS/TPN1:CBP/BCP/LiF/Al is fabricated, a maximum brightness of 10,404
cd/m2 of yellowish green light at an applied voltage of 19 V with a turnꢀon voltage of 5.8 V
are observed. The effect of solubility of these compounds on device performance is proven
by AFM images of the spinꢀcasted thin films.
Keywords: Triphenylamine, Naphthalimide, Suzuki crossꢀcoupling, OLED, Emissive
material
1. Introduction
Organic lightꢀemitting diodes (OLEDs) are considered to be the next generation of flat panel
displays after the LCD and plasma technologies due to their key advantages such as low
energy consumption, high brightness, light weight, and possibility for fabrication into large
and flexible displays [1]. Since the working principle of OLED involves the recombination of
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electron and hole in the layer of emissive material, the color of an OLED device thus depends
primarily on the electronic properties of this material [2]. Apart from the typical aluminumꢀ
hydroxyquinoline complex (Alq3) and some other coordination complexes of transition
metals [3], purely organic compounds such as pyrene derivatives [4] and tetraphenylsiloles
[5] have been reported as emissive materials. During our research project on design and
synthesis of novel OLED materials, we became interested in the combination of 1,8ꢀ
naphthalimide and triphenylamine motives into the same molecule since both of them have
distinctive photophysical properties. 1,8ꢀNaphthalimide derivatives have been generally used
as brilliant dyes in synthetic fiber [6] and fluorescent optical brighteners [7]. They have also
been used as electronꢀdeficient organic materials for OLEDs with high performance, good
light stability, and high fluorescent quantum yield [8]. When 1,8ꢀnaphthalimides are
substituted by an electronꢀdonating substituent at the 4ꢀposition, the fluorescent quantum
yields can be increased and the emission peaks can shift to a longer range [9]. On the other
hand, triphenylamine and its derivatives such as N,N’ꢀdi(1ꢀnaphthyl)ꢀN,N’ꢀdiphenylꢀ(1,1’ꢀ
biphenyl)ꢀ4,4’ꢀdiamine (NPB) are well known for their relatively easy oxidations into the
radicalꢀcation intermediates and have been frequently used as holeꢀtransporting materials in
OLED [10]. In this paper, we report our studies on synthesis and characterization of
triphenylaminoꢀNꢀphenylꢀ1,8ꢀnaphthalimides (TPN1ꢀTPN3, Fig. 1). The thermal and
electrochemical analyses, the computational calculation on HOMOꢀLUMO energy levels,
and the fabrication of these materials into OLED devices are reported herein.
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Fig.1 Structure of target compounds (TPN1ꢀTPN3).
2. Experimental
2.1 Chemical and instruments
All reagents including Pd catalyst, 4ꢀ(N,Nꢀdiphenylamino)ꢀphenylboronic acid, and
bis(pinacolato)diboron were purchased from Aldrich (USA). 4ꢀBromoꢀNꢀ(phenyl)ꢀ1,8ꢀ
naphthalimide [11], 4,4’ꢀ(dibromo)triphenylamine [12], 4,4’,4’’ꢀ(triiodo)triphenylamine [13],
and Nꢀphenylꢀ1,8ꢀnaphthalimideꢀ4ꢀboronic acid pinacol ester (A) [14] were prepared
according to the reference procedures. Thin layer chromatography (TLC) was performed on
aluminium sheets precoated with silica gel (Merck Kiesegel 60 F254) (Merck KgaA,
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Darmstadt, Germany). Column chromatography was performed on silica gel (Merck
Kieselgel 60G) (Merck KGaA, Darmstadt, Germany). All 1HꢀNMR spectra were determined
on Varian Mercury NMR spectrophotometer (Varian, USA) at 400 MHz with chemical shifts
reported as ppm in CDCl3. The 13CꢀNMR spectra were measured on Bruker Mercury NMR
spectrophotometer (Bruker, Germany) which equipped at 100 MHz with chemical shifts
reported as ppm in CDCl3. Mass spectra were recorded on a MicrOTOF high resolution mass
spectrometer (Bruker Daltonics). Absorption spectra were measured by a Shimadzu UVꢀ2550
UVꢀVis spectrophotometer. Fluorescence spectra were obtained from an Agilent technologies
Cary Eclipse spectrofluorometer. Fluorescent lifetimes were measured by a Horiba Jobin
Yvon FluoroMax 4 (Horiba Jobin Yvon, France) using the time correlated single photon
counting method with the following settings: excitation source, 370 nm NanoLED; time
range, 100 ns; peak preset 10,000 counts; repetition rate at 1 MHz; coaxial delay of 60 ns.
The FTꢀIR spectra were recorded between 400 cmꢀ1 to 4,000 cmꢀ1 in transmittance mode on a
Fourier Transform Infrared Spectrophotometer Impact 410 (Nicolet Instruments
Technologies, Inc, WI, USA). Cyclic voltammograms were obtained from an Autolab
voltammetry instrument. Elemental (CHN) analyses were performed on PerkinꢀElmer 2400
series II (PerkinꢀElmer, USA).
2.2 Synthesis
2.2.1 TPN1
Under a N2 atmosphere, a mixture of 4ꢀ(N,Nꢀdiphenylamino)ꢀphenylboronic acid (0.10 g,
0.36 mmol), 4ꢀbromoꢀNꢀ(phenyl)ꢀ1,8ꢀnaphthalimide (0.14 g, 0.41 mmol), and Pd(dppf)Cl2
(22 mg, 0.03 mmol) in toluene (10 mL) was stirred. Then an aqueous solution of K2CO3 (2 M
1 mL) was added via syringe. The reaction mixture was heated at 60 °C for 72 h. After
cooling to room temperature, the reaction was diluted with water and extracted with CH2Cl2.
The combined organic phase was dried over anhydrous MgSO4, filtered, and concentrated
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under reduced pressure. The crude product was purified by column chromatography using
CH2Cl2 as the eluent to afford TPN1 as yellow solid (0.15 g, 84%). IR (ATR, cmꢀ1): ν 3059,
1655, 1585, 1488, 1368, 1231. 1HꢀNMR (400 MHz, CDCl3, ppm) δ 8.71 – 8.65 (m, 2H), 8.48
(dd, J = 8.5 and 1.0 Hz, 1H), 7.76 (t, J = 7.9 Hz, 2H), 7.58 (dd, J = 10.3 and 4.7 Hz, 2H), 7.52
– 7.46 (m, 1H), 7.44 – 7.38 (m, 2H), 7.38 – 7.29 (m, 6H), 7.24 (ddd, J = 7.5, 5.2, and 2.5 Hz,
6H), 7.11 (t, J = 7.3 Hz, 2H). 13CꢀNMR (100 MHz, CDCl3, ppm) δ 164.5, 164.3, 148.5,
147.4, 147.2, 135.6, 133.2, 131.9, 131.6, 131.3, 130.8, 130.2, 129.5, 129.4, 129.3, 128.70,
128.65, 127.8, 126.8, 125.1, 123.7, 123.1, 122.5, 121.4. Anal. Calcd. for C36H24N2O2: C,
83.70; H, 4.68; N, 5.42. Found: C, 83.83; H, 4.59; N, 5.32. HRMS: m/z calcd for
C36H24N2O2: 516.1838; found: 539.1736 [M+Na]+
2.2.2 TPN2
This compound was prepared from 4,4’ꢀ(dibromo)triphenylamine (1 eq) and boronate ester A
(2.6 eq) using the same conditions for the synthesis of TPN1. The crude product was purified
by column chromatography using pure CH2Cl2 as the eluent to afford as yellow solid in 30%
1
yield. IR (ATR, cmꢀ1): ν 3059, 1655, 1585, 1488, 1368, 1231. HꢀNMR (400 MHz, CDCl3,
ppm) δ 8.70 (d, J = 7.5 Hz, 4H), 8.48 (t, J = 7.4 Hz, 2H), 7.79 (dt, J = 8.3 and 4.1 Hz, 4H),
7.58 (t, J = 7.4 Hz, 4H), 7.50 (dd, J = 8.0 and 3.7 Hz, 6H), 7.44 (d, J = 8.4 Hz, 1H), 7.37 (dt,
J = 8.4 and 5.5 Hz, 10H), 7.21 (dd, J = 8.0 and 6.5 Hz, 2H). 13CꢀNMR (100 MHz, CDCl3,
ppm) δ 164.5, 164.3, 147.9, 146.9, 135.5, 133.1, 133.0, 131.7, 131.3, 131.2, 131.1, 130.2,
129.8, 129.4, 129.3, 128.8, 128.7, 127.9, 126.9, 125.8, 124.5, 123.9, 123.6, 123.1, 121.6.
Anal. Calcd. for C54H33N3O4: C, 82.32; H, 4.22; N, 5.33. Found: C, 83.55; H, 4.39; N, 5.12.
HRMS: m/z calcd for C54H33N3O4: 787.2471; found: 810.2368 [M+Na]+.
2.2.3 TPN3
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This compound was prepared from 4,4’,4’’ꢀ(triiodo)triphenylamine (1 eq) and boronate ester
A (9.0 eq) using the same conditions for the synthesis of TPN1. The crude product was
purified by column chromatography using pure CH2Cl2 to CH2Cl2/EtOAc (20/1) as the eluent
to afford as orange solid in 56% yield. IR (ATR, cmꢀ1): ν 3059, 1655, 1585, 1488, 1368,
1231. 1HꢀNMR (400 MHz, CDCl3, ppm) δ 8.71 (d, 6H), 8.51 (d, J = 7.7 Hz, 3H), 7.82 (d, J =
13
7.3 Hz, 6H), 7.50 (m, br, 21H), 7.35 (d, J = 7.2 Hz, 6H). CꢀNMR (100 MHz, CDCl3, ppm)
δ 164.4, 164.2, 147.6, 146.6, 135.4, 133.9, 132.9, 131.7, 131.3, 131.2, 130.2, 129.4, 129.3,
128.74, 128.66, 127.9, 126.9, 124.5, 123.2, 122.4, 121.8. Anal. Calcd. for C72H42N4O6: C,
81.65; H, 4.00; N, 5.29. Found: C, 81.90; H, 4.14; N, 5.11. HRMS: m/z calcd for
C72H42N4O6: 1058.3104; found: 1081.3005 [M+Na]+.
2.3 Electrochemical analysis
In the CV experiments, platinum wire was used as a counter electrode, Ag/AgNO3 as a
reference electrode and platinum as a working electrode. Ferrocene was used as an external
standard. Tetra-nꢀbutylammonium hexafluorophosphate (TBAPF6) was used as an electrolyte
in 20% MeCN in CH2Cl2. The concentration of electrolyte was 0.1 M, and the concentration
of TPN1, TPN2 and TPN3 was 1.0 mM and a scan rate was 50 mV/s. The HOMO energy
levels were calculated from the onset of oxidation potential (Eonset) according to a formula,
HOMO = ꢀ(Eonset – EFc/Fc+ + 4.8). The LUMO energy levels were calculated by subtracting the
HOMO energy levels with the band gaps energy estimated from the onset of UV–Vis
absorption.
2.4 Quantum chemical calculation method
The ground state geometries of the compounds were optimized by density functional theory
(DFT) at B3LYP/6ꢀ31G(d,p) method in gas phase without any constraint. The highest
occupied molecular orbital (HOMO), lowest occupied molecular orbital (LUMO), and energy
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difference between HOMO and LUMO (ꢁHꢀL) were analyzed at the ground state. The lowest
vertical excitation energy (Eex) from ground to excited state was calculated by timeꢀ
dependent DFT at TDꢀB3LYP/6ꢀ31G(d,p) level of theory. All calculations were performed
by Gaussian 09 program package [15].
3. Results and Discussion
3.1 Synthesis of TPN1-TPN3
The synthesis of our target compounds relies on the Suzuki crossꢀcoupling reactions (Scheme
1). Such reaction between the readily accessible 4ꢀbromoꢀNꢀ(phenyl)ꢀ1,8ꢀnaphthalimide and
the commercially available 4ꢀ(N,Nꢀdiphenylamino)ꢀphenylboronic acid under the catalysis of
(dppf)PdCl2 provided TPN1 in excellent yield of 84%. The conversion of 4ꢀbromoꢀNꢀ
(phenyl)ꢀ1,8ꢀnaphthalimide into the pinacol boronate ester A was carried out using the
procedure reported in the literature [14]. The Suzuki reaction between A and 4,4’ꢀ
(dibromo)triphenylamine or 4,4’,4’’ꢀ(triiodo)triphenylamine afforded TPN2 or TPN3 in
moderate yields. The characterization by IR, NMR, and HRꢀMS confirmed the successful
synthesis of these target compounds.
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Scheme 1 Synthesis of TPN1ꢀ3
3.2 Photophysical properties
The absorption and emission properties of TPN1ꢀ3 in both CHCl3 solution and spinꢀcasted
thin film were investigated and summarized in Table 1. In solution phase, each compound
exhibited a characteristic πꢀπ* transition as observed by the absorption peak around 312ꢀ326
nm, along with a charge transfer between the naphthalimide(s) and triphenylamine group
evidenced by the absorption peak around 430ꢀ433 nm (Fig. 2). Therefore, the molar
extinction coefficient of the latter peak increases with the numbers of naphthalimide unit. In
the solid state, the absorption spectra of these compounds also have the same pattern as in
solution phase (Fig.3). However, the maximum absorption wavelengths appear at longer
ranges (316ꢀ336 and 435ꢀ437 nm) due to more efficient πꢀconjugation between the
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naphthalimide and triphenylamine groups in the solid state, which may cause by the restricted
molecular vibration and rotation.
Table 1 Photophysical properties of TPN1ꢀ3 in CHCl3 solutions and spinꢀcasted thin films.
CHCl3 solution
Thin film
Abs
Emis
Abs
Emis
Cmpd.
TPN1
TPN2
TPN3
λmax
Log ε
λmax
τave
kr
knr
λmax
λmax
Φb
(nm) (Mꢀ1cmꢀ1) (nm) a
(ns)c (nsꢀ1)d (nsꢀ1)e
(nm)
316
435
326
435
336
437
(nm)
312
433
322
431
326
430
4.37
4.03
4.57
4.36
4.65
4.51
599
580
564
0.34 7.14 0.0476 0.0924
0.49 6.58 0.0745 0.0775
555
551
550
0.55 6.11 0.0900 0.0736
a
b
Excited at the shorter maximum absorption wavelength. Quantum yield determined in
CHCl3 solution (A<0.1) at room temperature with the excitation wavelength at the longer
absorption maxima of each compound. Fluorescein in 0.1 M NaOH(aq) (ΦF = 0.91) was used
c
as the standard. Average fluorescent decay times. d Radiative rate estimated from Φ/ τave. e
Nonꢀradiative rate estimated from (1ꢀΦ)/ τave.
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Fig. 2 Normalized absorption and emission spectra of TPN 1ꢀ3 in CHCl3 (15 ꢂM). The
excitation wavelengths were at 430 nm.
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Fig. 3 Normalized absorption and emission spectra of spinꢀcasted film of TPN 1ꢀ3. The
excitation wavelengths were at 435 nm.
The normalized fluorescence spectra in solution phase are also shown in Fig. 2. TPN1,
TPN2, and TPN3 exhibited a maximum emission wavelength at 599, 580, and 564 nm,
respectively. With the increase in the number of naphthalimide groups, the fluorescence
emission peaks are gradually blueꢀshifted due to a decrease in electron delocalization
between the donor and the acceptor. The quantum yields of these compounds increase with
the number of naphthalimide units as TPN3 has the highest quantum yield of 0.55. The data
from average fluorescent decay time also suggested that excited TPN3 possesses the fastest
radiative rate (kr) and the slowest nonꢀradiative rates (knr). As for the emission spectra of thinꢀ
film, the packing force and the restricted structural rotation in the solid state shifted all of the
photoluminescent spectra towards shorter wavelengths at 550ꢀ555 nm (Fig. 3).
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3.3 Thermal properties
The thermal properties of all compounds were determined by thermogravimetric analysis
o
(TGA) and differential scanning calorimetry (DSC) with a heating rate of 10 C/min under
nitrogen atmosphere. The TGA curves (Fig. 4a) revealed that all TPN1, TPN2, and TPN3
were thermally stable with high 10% weight loss temperatures (Td 10) at 398, 453 and 527oC,
respectively. This trend in thermal stabilities might be attributed to the increasing molecular
weight. Since the morphological changes might be promoted by rapid molecular motion at
higher temperature, amorphous materials with high glass transition temperature (Tg) are
usually preferred for OLED materials. From the DSC experiment on TPN1 (Fig. 4b), one
°
sharp endothermic peak at 264 C due to melting temperature (
Tm) and no glass transition
temperature suggested that TPN1 is a crystalline material. For both TPN2 and TPN3, there
were endothermic baseline shifts which indicated glass transition temperature at 254 °C. The
melting temperature of TPN2 and TPN3 were at 385 and 477 °C, respectively. These abilities
to form molecular glasses with high thermal stabilities are highly desirable for optoelectronic
materials casted into thin films by vacuum evaporation or solution casting processes.
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Fig.4 a) TGA thermograms and b) second DSC traces of TPN1, TPN2 and TPN3
Table 2 Thermal properties of TPN1ꢀ3.
a
b o
Compounds
TPN1
Tg (oC)
Tma (oC)
264
Td ( C)
ꢀ
398
453
527
254
254
385
TPN2
477
TPN3
a Obtained from DSC measurements on the second heating cycle with a heating rate of 10
oC/min under N2. b10% weight loss temperature obtained from TGA measurement with a heat
rate of 10 oC/min under N2.
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3.4 Electrochemical analysis
Table 3 The experimental and calculated electrochemical properties of TPN1ꢀ3.
Calculated datae
Experimental data
λex
Eex
Cmpd.
Eg
Eonset
(V)b
HOMO LUMO
ꢁHꢀL HOMO LUMO
(nm)f (eV)f
(eV)a
(eV)c
(eV)d
(eV)g
(eV)
(eV)
2.48
2.52
2.54
0.51
0.53
0.55
ꢀ5.21
ꢀ5.23
ꢀ5.25
ꢀ2.73
ꢀ2.71
ꢀ2.71
487
481
467
2.54
2.58
2.65
2.92
2.97
3.04
ꢀ5.22
ꢀ5.42
ꢀ5.58
ꢀ2.30
ꢀ2.45
ꢀ2.53
TPN1
TPN2
TPN3
a The optical energy gap estimated from the onset of the absorption spectra (Eg = 1240/λonset).
b
c
Onset oxidation potential estimated from the cyclic voltammogram. Estimated by the
d
empirical equation: HOMO = ꢀ(Eonset – EFc/Fc+ + 4.8). Estimated from LUMO = HOMO +
e
f
Eg. Geometry optimizations were done by B3LYP/6ꢀ31G(d,p) method. Lowest excitation
energy (the first absorption peak) were calculated by TDꢀB3LYP/6ꢀ31G(d,p) method (no
g
solvent is included). ꢁHꢀL is the energy difference between HOMO and LUMO calculated
by TDꢀB3LYP/6ꢀ31G(d,p) method.
The experimental and computationally calculated electrochemical properties are summarized
in Table 3. The cyclic voltammograms acquired by the anodic scans at room temperature
exhibit one reversible oxidation peak for each compound, which might be attributed to the
electronꢀdonating ability of triphenylamine moiety. The HOMO energy levels were obtained
from the onset oxidation potential (Eonset) observed on the cyclic voltammograms, and the
LUMO energy levels were estimated from the Eg and HOMO data. The Eonset were influenced
and shifted toward more positive potentials with the increase of naphthalimide units. This
data indicated the stabilization of the HOMO level due to the electronꢀwithdrawing
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characteristic of naphthalimide group. It is noted that when the number of naphthalimides is
increased, the measured HOMO is stabilized from ꢀ5.21 eV, ꢀ5.23 eV, and ꢀ5.25 eV while the
LUMO is increased from ꢀ2.73 eV, ꢀ2.71, and ꢀ2.71 eV for TPN1, TPN2, and TPN3,
respectively. These cause the wider band gap which increases as the order; 2.48 eV (TPN1) <
2.52 eV (TPN2) < 2.54 eV (TPN3). The tendencies of the experimental Eg, HOMO, and
LUMO are well consistent with the calculated values as shown in Table 3. In calculations,
both ꢁHꢀL and Eex are widely used to qualitatively determine the experimental Eg in relatively
order. As shown, the Eex is more relatively accurate than the ꢁHꢀL since it is calculated by the
TDꢀDFT which provided the excitation directly. The orbital plots show exclusive electron
localizations at the triphenylamine pendant for the HOMOs, and at the napthalimide
fragments for the LUMOs (Fig. 5). Since these HOMO and LUMO levels are larger than the
work functions of ITO and LiF/Al (Fig. 6), these materials can be fabricated into OLED
devices using these electrodes. In addition, there is also a possibility of using the
commercially available PEDOT:PSS as the holeꢀtransporting layer and BCP as the holeꢀ
blocker.
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Fig. 5 Frontier orbital plots for TPN1ꢀ3.
Fig. 6 Band diagrams of ITO, PEDOT:PSS, TPN1, TPN2, TPN3, BCP and LiF/Al.
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3.5 OLED device fabrication
For the preliminary investigation on material performances and their electroluminescence,
five nonꢀoptimized OLED devices were fabricated between the ITO and LiF/Al electrodes
using PEDOTꢀPSS as the holeꢀinjection layer (Fig. 7). The attempt to fabricate singleꢀlayer
device using TPN3 as emissive layer led to poor device performances (Device 1, Table 4),
which might results from an improper barrier for electron migration at the interface between
emissive layer and the LiF/Al electrode and a poor filmꢀforming ability of TPN3 due to its
low solubility. When a layer of holeꢀblocking material, dimethylꢀ4,7ꢀdiphenylꢀ1,10ꢀ
phenanthroline (BCP), was incorporated (Device 2), slightly improved device performances
such as higher maximum luminance were achieved. These devices showed a yellowꢀorange
luminescence with featureless patterns, emission peaks around 586 nm (Fig. 8). The EL
spectra were redꢀshifted (∼36 nm) from the thinꢀfilm PL spectrum with large full width at
half maxima (FWHM) of 151 nm, indicating that the EL originates from the emission of the
excimer species. This is due to the flat molecular structure of TPN3.
We thus fabricated multilayer Device 3ꢀ5 using TPN3ꢀ1ꢀdoped 4,4’ꢀbis(Nꢀcarbazolyl)ꢀ1,1’ꢀ
biphenyl (CBP) as the emissive layer (EML). Under the applied voltages, all fabricated
OLEDs (Devices 3ꢀ5) emit a bright yellowish green emission with featureless patterns,
emission peaks around 530ꢀ533 nm, and small FWHM of 75ꢀ95 nm (Fig. 8 and Table 4).
The EL spectra of devices 3−5 were identical to their thinꢀfilm PL spectra, indicating that the
EL purely originates from their corresponding EML. No emission from the host BCP
suggesting an efficient energy transfer from the host BCP to TPN3ꢀ1. Furthermore, no
emission shoulder at a longer wavelength resulting from the excimer and exciplex species
formed at the interface of the EML and BCP layers is detected. Moreover, stable emission is
obtained from all devices, and the EL spectra did not change over the entire driven voltages.
Among these diodes, Device 5, which has 6% TPN1ꢀdoped 4,4’ꢀbis(Nꢀcarbazolyl)ꢀ1,1’ꢀ
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biphenyl (CBP) as the emissive layer, offers the best performance with a maximum
luminance of 10,404 cd/m2 at 19 V, a turnꢀon voltage of 5.8 V, device luminance efficiency
(ηmax) of 3.77 cd/A and EQE of 1.11% (Fig. 9). This device also showed a current density of
410 mA/m2 and emitted a yellowish green light with the CIE coordinates at (0.295, 0.600).
Device 4 using 6% TPN2ꢀdoped CBP as the EML showed a slightly lower device
performance with device luminance efficiency (ηmax) of 2.49 cd/A and EQE of 0.73%, while
device 3 using 6% TPN3ꢀdoped CBP as the EML exhibited a poor device performance.
Because the electrochemical properties of TPN1ꢀ3 are relatively similar, it was postulated
that the efficiency differences in Device 3 to Device 5 might be related to the solubility of
these compounds the solvents used in the spinꢀcasting process ꢀ CHCl3 and toluene (4:1 v/v).
The AFM analysis of casted films revealed that the TPN1ꢀdoped CBP could form a
homogeneous thin film (Fig. 10), whereas the TPN3ꢀdoped CBP film consisted of particles
from 0.45 ꢂm or larger. The aggregation of TPN3 may prohibit the exciton migration, and
resulted in the poor device performance.
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Fig. 7 Structures of Device 1ꢀ5
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Fig. 8 Plots of a) EL spectra of the OLEDs (devices 1ꢀ5).
Fig. 9 a) Current densityꢀluminanceꢀvoltage (JꢀVꢀL) and b) Efficiencyꢀcurrent density (ηꢀI)
characteristics of the OLEDs (devices 1ꢀ5).
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Table 4 Electroluminescent properties of Devices 1ꢀ5.
a
c
e
Device
Von
Lmax (V)b
ηlum
Jd
EQEmax
PEf
CIEg
λ
EL /FWHM (nm)
586/151
586/151
533/95
1
2
3
4
5
4.0
3.8
5.5
6.2
5.8
133 (9.6)
323 (10.4)
877 (14.2)
5898 (15.8)
0.02/7.6V
0.04/8.2V
0.93/10.8V
2.49/12.0V
985
1446
128
0.006/7.6V
0.01/8.2V
0.009/6.6V
0.02/1.0V
0.32/8.4V
0.68/11.2V
0.96/11.4V
0.484, 0.477
0.487, 0.486
0.258, 0.575
0.319, 0.604
0.295, 0.600
0.27/10.8V
0.73/12.0V
1.11/14.0V
531/76
352
530/75
10404 (19.0) 3.77/14.0V
410
a Turnꢀon voltage (V). b Maximum luminance (cd/m2) (at applied potential V). c Luminance efficiency (cd/A). d Current density (mA/m2). e
Maximum external quantum efficiency (% at V). f Power efficiency (lm/W at V). g Commission International d’Eclairage coordinates (x, y).
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Fig. 10 AFM images of thin films of CBP doped with TPN1, TPN2, and TPN3
4. Conclusion
Three new triphenylamine derivatives substituted by different numbers of Nꢀ
phenylnaphthalimide units were successfully synthesized by means of Suzuki coupling in
good to moderate yields. These compounds exhibited absorption maxima around 430 nm and
emission maxima around 564ꢀ599 nm in CHCl3 solution. The Stoke’s shifts were narrower
for the spectra of these compounds in solid state as a result of solid packing. With the 10%
weight loss temperatures above 350 °C and glassꢀtransition temperature at 254 °C, these
compounds showed excellent thermal stability suitable for application as OLED materials.
The electrochemical properties were examined by cyclic voltammetry and the data were in
good agreement with those obtained from the computational calculations using Gaussian 09
program. The experimental HOMO level of the three compounds were around 5.2 eV while
the LUMO levels were around 2.7 eV. When TPN1ꢀdoped BCP was used as a holeꢀ
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transporting
material
in
the
OLED
device
of
structure
ITO/PEDOT:PSS/TPN1:CBP/BCP/LiF/Al, the yellowish green light with a maximum
brightness of 10,404 cd/m2 of at an applied voltage of 19 V were observed.
Acknowledgements
This research was supported by the Ratchadaphiseksomphot Endowment Fund (2013) of
Chulalongkorn University (CUꢀ56ꢀ425ꢀAM). RA thanks the Development and Promotion of
Science and Technology Talents Project from the Royal Thai Government for her
scholarship. PR thanks Dr.Somboon Sahasithiwat of the National Metal and Materials
Technology Center for the determination of fluorescent lifetimes.
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