Synthesis and properties of dendritic emitters with a fluorinated starburst oxadiazole core and twisted carbazole dendrons

Article abstract of DOI:10.1021/ma1024957

Three generations of novel dendrimers (G1, G2, and G3) with an electron-deficient fluorinated starburst oxadiazole core and twisted carbazoles as dendrons were synthesized via aromatic nucleophilic substitution reaction at ambient temperature by the weak

Full text of DOI:10.1021/ma1024957

ARTICLE
pubs.acs.org/Macromolecules
Synthesis and Properties of Dendritic Emitters with a Fluorinated
Starburst Oxadiazole Core and Twisted Carbazole Dendrons
Zhen-Hua Zhao, Hao Jin, Yan-Xin Zhang, Zhihao Shen,* De-Chun Zou, and Xing-He Fan*
Beijing National Laboratory for Molecular Sciences, Key Laboratory of Polymer Chemistry and Physics of Ministry of Education,
College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China
S Supporting Information
b
ABSTRACT: Three generations of novel dendrimers (G1, G2,
and G3) with an electron-deficient fluorinated starburst oxa-
diazole core and twisted carbazoles as dendrons were synthe-
sized via aromatic nucleophilic substitution reaction at ambient
temperature by the weak base potassium fluoride with a high
selectivity in a nearly quantitative yield, which was attributed to
the high reactivity of the para fluorine atom activated by
1
oxadiazole and multiple fluorine atoms. H NMR, 19F NMR,
elemental analysis, and MALDI-TOF MS results confirmed the
designed chemical structures. Varying the generation to gradu-
ally modify the dendrons could gradiently modulate the HOMO-LUMO energy band gaps of Gn, which was verified by cyclic
voltammetry results. Photoluminescent spectra showed the dual fluorescence emission of the dendrimers in solutions, which
indicated that twist intramolecular charge transfer occurred between oxadiazole and carbazole separated by nonconjugated bonds.
The Gn-based devices with the configuration of ITO/PEDOT:PSS/Gn/TPBI/AlQ/LiF/Al displayed a stable green emission in
their electroluminescent spectra with a gradual increase in external quantum efficiency and current efficiency for higher-generation
dendrimers, owing to gradually improved charge transport. The device with G3 possessed the lowest turn-on voltage along with the
highest external quantum efficiency of 1.49%, maximum current efficiency of 4.6 cd/A, and a high luminance of 4882 cd/m2,
indicating that it had the best carrier balance.
’ INTRODUCTION
(EQE).24-26 Consequently, using different generations of large,
rigid dendrons and a nonplanar, for instance, starburst,27 core to
control charge transport and inhibit intermolecular interactions
is an ideal strategy to achieve desirable emitting dendrimers with
controlled charge transport. Fortunately, different generations of
3,6-cross-coupling carbazole dendrons28,29 as hole-transporting
materials have received much attention owing to their twisted
Organic light-emitting diodes (OLEDs) have received tre-
mendous research attention in both academic interests and
potential applications for full-color flat-panel displays because
of their advantages over more energy-consuming traditional
displays and lighting for addressing the energy crisis.1-3 To
achieve high power efficiency of electroluminescent (EL) devices,
one of the key factors is to balance charge injection from the two
electrodes and efficiently transport both holes and electrons within
the emitting layer.4-6 At present, using dendrimers7-9 in OLEDs
has received tremendous research attention.10-14 With their well-
defined structures, three-dimensional spherical architectures, and
site isolation effect, dendrimers possess excellent solution-proces-
sing capabilities and chemical advantages in tailoring multifunc-
tional groups to balance charge injection and transport.
However, the use of light-emitting dendrimers contain-
ing dendrons with controllable charge transport is still in its
infancy,11 even though there have been extensive research efforts
on charge-transporting dendrimers and dendrons.15,16 Further-
more, it remains a major challenge to effectively tune the charge
transport and device efficiency by simply changing the genera-
tion of dendrimers.17-23 Moreover, the flexibility of many highly
branched dendrons usually leads to poorly isolated cores suscep-
tible to strong intermolecular interactions, and the cores are
easily quenched, resulting in a low external quantum efficiency
configurations and gradiently tunable HOMO energy (EHOMO
)
levels.30 They are ideal dendrons to modulate the charge trans-
port by varying the generation.
To address the problems mentioned above, we need to choose
a highly efficient reaction route or a highly reactive functional
group to synthesize novel dendritic emitters with well-defined,
rigid structures and controllable charge transport. Herein, in this
paper, we report the facile preparation of dendrimers by intro-
ducing starburst perfluorophenyl oxadiazole with highly reactive,
thus, functional C-F bonds and additional fluorine atoms which
facilitate electron transport.31-33 Notably, the C-F bond could
even react with phenols in a quantitative yield at ambient tem-
perature in the presence of a weak base potassium fluoride as the
catalyst.34 On the basis of the above considerations, we used the
Received: November 3, 2010
Revised:
January 12, 2011
Published: February 18, 2011
r
2011 American Chemical Society
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convergent strategy9 to synthesize dendrimers with carbazole
phenols transferring holes and starburst oxadiazole units trans-
ferring electrons. Meanwhile, the flexibility of this synthetic
strategy allowed us to systematically change the generation and
volume of the carbazole dendrons in the dendrimers and
gradually tune the HOMO-LUMO energy band gap, leading
to light-emitting dendrimers with tunable opto-electric and
electrochemical properties as well as excellent thermal stability.
Synthesis of the Core TPFOxd
5-(Perfluorophenyl)-2H-tetrazole. To a stirring solution of NaN3
(1.0 g, 15.3 mmol) in water, ZnCl2 (2.1 g, 15.3 mmol) and 2 mL of acetic
acid were added sequentially into a round-bottomed flask. After 10 min,
2,3,4,5,6-pentafluorobenzonitrile (3.0 g, 15.3 mmol) was added, and the
mixture was refluxed for 6 h. Then it was cooled to ambient temperature
and neutralized by HCl (2 mol/L) until pH = 1, followed by stirring for
another 30 min. The solid was collected and further washed by water to
give 2.5 g of white solid with a yield of 69%; mp =152-153 °C. 19F NMR
(d6-DMSO, 282 MHz, δ): -61.7 (m, 2F), -73.9 (m, 1F), -81.0 (m,
2F). MS (EIþ): calcd (M þ H)þ/z, 237.0; found (M þ H)þ/z, 237.0.
1,3,5-Tris(5-(perfluorophenyl)-1,3,4-oxadiazol-2-yl)benzene (TPFOxd).
To a stirring solution of 5-(perfluorophenyl)-2H-tetrazole (1.2 g, 5.1 mmol) in
30 mL of toluene, benzene-1,3,5-tricarbonyl trichloride (0.4 g, 1.6 mmol) and
pyridine (0.37 g, 4.8 mmol) were filled into a round-bottomed flask. After
being refluxed for 30 min, the mixture was cooled to ambient temperature and
filtered, followed by washing with methanol and water three times to afford
0.9 g of white solid with a yield of 92%; mp = 236-237 °C. IR (KBr) ν
’ EXPERIMENTAL SECTION
Materials. Benzene-1,3,5-tricarboxylic acid, boron tribromide
(BBr3), and tetrabutylammonium perchlorate [(C4H9)4NClO4] were
purchased from Acros Organics. 2,3,4,5,6-Pentafluorobenzonitrile was
purchased from Zhejiang Yongtai Chemical Products Inc. and used as
received. Anhydrous potassium fluoride was purchased from Beijing
Chemical Reagent Co. and treated at 200 °C overnight. Anhydrous N,
N0-dimethylformamide (DMF), toluene, and pyridine were carefully
treated before use. All other chemicals and reagents were purchased
from Beijing Chemical Reagent Co. and used as received.
(cm-1): 3071, 2131, 1657, 1521, 1496, 1096, 994, and 836. H NMR
1
(CDCl3, 300 MHz, δ): 9.09 (s, 3H); 13C NMR (CDCl3, 100 MHz, δ):
163.6, 155.7, 146.8-146.6, 145.2-145.1, 144.2-143.9, 142.6-142.3,
139.7-139.4, 137.2-136.8, 128.4, 125.8, 100.5, 100.2. 19F NMR
(CDCl3, 282 MHz, δ): -57.0 (m, 6F), -67.7 (m, 3F), -80.9 (m, 6F).
MS (EIþ): calcd Mþ/z, 780; found Mþ/z, 780. Anal. Calcd for C30H3F15-
N6O3: C, 46.17; N, 10.77; H, 0.39. Found: C, 46.05; N, 10.60; H, 0.60.
Syntheses of the Dendrimers. G1. A mixture solution of TPFOxd
(0.18 g, 0.23 mmol), 4-(3,6-di-tert-butyl-9H-carbazol-9-yl)phenol (C1-
OH) (0.27 g, 0.72 mmol), and 0.10 g of anhydrous potassium fluoride in
DMF was stirred under argon at ambient temperature for 8 h. The
reaction solution was added dropwise into a mixture solvent of methanol
and distilled water (50 mL, v/v, 1/1). The resulting precipitate was
collected by filtration and thoroughly washed with methanol and
distilled water twice to give 0.39 g of white solid with a yield of 90%;
mp >300 °C, Td(5 wt %) = 449 °C, Tg = 196 °C. IR (KBr) ν (cm-1):
Measurements. 1H NMR and 13C NMR spectra were recorded at
ambient temperature on either a Bruker ARX 400 MHz spectrometer
(1H NMR 400 MHz and 13C NMR 100 MHz) or a Varian Mercury
300 MHz spectrometer (1H NMR 300 MHz and 13C NMR 75 MHz)
using tetramethylsilane as an internal reference. 19F NMR spectra were
recorded on a Varian Mercury 300 MHz (19F NMR 282 MHz) using
trifluoroacetic acid as an external standard. The chemical shifts were
reported in the ppm scale. FTIR spectra were recorded on a Bruker
Vector 22 Fourier transform infrared spectrometer. High-resolution
mass spectra (HRMS) were measured on a Bruker APEX IV Fourier
transform ion cyclotron resonance mass spectrometer by electrospray
ionization (ESI). The MALDI-TOF MS data were obtained using on
Bruker Daltonics Autoflex III MALDI-TOF spectrometer. Elemental
analysis was conducted by an Elementar Vario EL instrument
(Elementar Analysensysteme GmbH). UV-vis absorption spectra were
measured with a Perkin-Elmer lambda 35 spectrophotometer. Photo-
luminescent (PL) spectra were obtained using a Hitachi F-4500
fluorescence spectrophotometer. Thermogravimetric analysis (TGA)
was performed on a TA Q600 SDT instrument at a heating rate of
20 °C/min under nitrogen (100 mL/min). Differential scanning calori-
metry (DSC) was recorded with a TA Q100 thermal analyzer at a
heating rate of 10 °C/min under nitrogen.
1
3048, 2961, 1652, 1508, 1483, 1263, 1220, 996, and 845. H NMR
(CDCl3, 400 MHz, δ): 9.15 (s, 3H), 8.14 (s, 6H), 7.56 (m, 6H), 7.47 (m,
6H), 7.32-7.26 (m, 12H), 1.45 (s, 54H). 19F NMR (CDCl3, 282 MHz,
δ): -57.6 (m, 6F), -73.5 (m, 6F). HRMS (ESIþ): calcd (M þ Na)þ/z,
1856.6486; found (M þ Na)þ/z, 1856.6427. Anal. Calcd for
C108H87F12N9O6: C, 70.69; H, 4.78; N, 6.87. Found: C, 70.47, H,
4.87; N, 6.85.
G2. This compound was prepared according to the procedure for the
synthesis of G1 except that 4-(30,60-di-tert-butyl-6-(3,6-di-tert-butyl-9H-
carbazol-9-yl)- 9H-3,90-bicarbazol-9-yl) phenol (C2-OH) instead of
C1-OH was added, resulting in a yellow solid with a yield of 93%; mp
>300 °C, Td(5 wt %) = 479 °C, Tg = 306 °C. IR (KBr) ν (cm-1): 3047,
Cyclic voltammetry (CV) was conducted using a CHI 630C
(Shanghai Chenhua Instrument Co.) voltametric analyzer. The experi-
ments were performed at ambient temperature in CH2Cl2 solutions
containing 0.1 M tetrabutylammonium perchlorate as the supporting
electrolyte at a scanning rate of 100 mV/s, and the detailed procedures
could be found elsewhere.35 The energy levels were calculated using the
ferrocene (Fc) value of -4.8 eV with respect to the vacuum level, which
was defined as zero. Two drops of Fc (1 M in CH2Cl2) were added, and
the measured oxidation potential of Fc (vs Ag/AgCl) was 0.46 V.
Therefore, the EHOMO levels of the dendrimers could be calculated by
the equation EHOMO = -e[Uonset(ox) - U1/2,Fc þ 4.8 V] and the LUMO
1
2960, 1652, 1507, 1489, 1220, 996, and 809. H NMR (CDCl3, 400
MHz, δ): 9.15 (s, 3H), 8.24 (s, 6H), 8.16 (s, 12H), 7.76 (d, J = 8.4 Hz,
6H), 7.61 (m, 12H), 7.45 (d, J = 8.8 Hz, 12H), 7.39 (d, J = 8.4 Hz, 6H),
7.33 (d, J = 8.8 Hz, 12H), 1.45 (s, 108H). 19F NMR (CDCl3, 282 MHz,
δ): -57.4 (m, 6F), -73.3 (m, 6F). MALDI-TOF MS: calcd Mþ/z,
3161.4; found Mþ/z, 3161.8. Anal. Calcd for C204H177F12N15O6: C,
77.47; H, 5.64; N, 6.64. Found: C, 77.46; H, 5.97; N: 6.44.
G3. This compound was prepared according to the procedure for the
synthesis of G1 except that 4-(6-(30,60-di-tert-butyl-6-(3,6-di-tert-butyl-
9H-carbazol-9-yl)-9H-3,90-bicarbazol-9-yl)-30,60-bis(3,6-di-tert-butyl-
9H-carbazol-9-yl)-9H-3,90-bicarbazol-9-yl)phenol (C3-OH) instead of
C1-OH was added, resulting in a yellow solid with a yield of 92%; mp
>300 °C, Td(5 wt %) = 445 °C, Tg = 367 °C. IR (KBr) ν (cm-1): 3047,
2959, 1651, 1490, 1223, 996, and 809. 1H NMR (CDCl3, 400 MHz, δ):
9.18 (s, 3H), 8.55 (s, 6H), 8.27 (s, 12H), 8.15 (s, 24H), 7.87-7.83 (m,
12H), 7.76 (m, 6H), 7.65-7.59 (m, 24H), 7.47-7.43 (m, 30H), 7.33
(m, 24H), 1.46 (s, 216H). 19F NMR (CDCl3, 282 MHz, δ): -57.3 (m,
6F), -73.2 (m, 6F). MALDI-TOF MS: calcd Mþ/z, 5820; found Mþ/z,
energy (ELUMO) levels could be estimated by the equation ELUMO
=
-e[Uonset(red) - U1/2,Fc þ 4.8 V], where U1/2,Fc standards for the half-
wave potential of Fc/Fcþ.
Device Fabrication and Characterization. The fabrication
process of the OLED devices followed a standard procedure which
was described elsewhere.35
Synthetic Procedures. Syntheses of the Dendrons. The synthe-
ses of the carbazole phenol precursors were prepared according to the
literatures or modified methods which can be found in the Supporting
Information, and the synthetic routes are outlined in Scheme S1 of the
Supporting Information.
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5820. Anal. Calcd for C396H357F12N27O6: C, 81.75; H, 6.18; N, 6.50.
Found: C, 81.48; H, 6.37; N, 6.40.
Scheme 1. Synthetic Procedures for Preparation of TPFOxd
and Dendrimers G1, G2, and G3
’ RESULTS AND DISCUSSION
Syntheses of the Dendrimers. For the syntheses of the
different generations of carbazole phenols as dendrons which
serve as the electron donors, the intermediate compounds C1-
OH, C2-OH, and C3-OH were synthesized using benzyl protec-
tion and Pd/C system rather than methyl or tosyl protecting
groups by Lu et al.29,36 The detailed synthetic procedures are
depicted in Scheme S1 of the Supporting Information.
We first attempted to synthesize the core, TPFOxd, using the
cyclization reaction through dehydration of bishydrazides by
phosphorus oxychloride (POC13).37-39 However, no starburst
oxadiazole was formed. Then we chose an alternative, effec-
tive route to synthesize the starburst oxadiazole derivative via a
tetrazole intermediate which was produced from nitrile with
azide.40-42 The detailed synthetic procedure is shown in
Scheme 1. The first step involved the formation of the aromatic
tetrazole, which was quantitatively generated by the correspond-
ing 2,3,4,5,6-pentafluorobenzonitrile with sodium azide. The key
step was to prepare a single pure product of the aromatic
tetrazole, which was synthesized with the classical route by using
zinc chloride or ammonium chloride with sodium azide. How-
ever, the para F of the 2,3,4,5,6-pentafluorobenzonitrile was very
easily replaced by azide, and the impurities could not be easily
removed by recrystallization. After adding 1-2 mL of acetic acid
in the azide solution and stirring for 10 min before the nitrile was
added, it could afford the pure white product by simple filtration.
Then the tetrazole was treated with benzene-1,3,5-tricarbonyl
trichloride, and the starburst oxadiazole derivative was obtained
via Huisgen intramolecular cyclization ring transformation with a
high yield and excellent purity without further purification.
Although the traditional solvent was pyridine,43 it did not work
in our system even when refluxed in pyridine for a few hours. We
chose anhydrous toluene as the reaction solvent and an equiva-
lent pyridine as the base to neutralize HCl which was released by
trichloride. 1H NMR, 13C NMR, 19F NMR, MS, and elemental
analysis results confirmed the designed chemical structure of the
core, TPFOxd.
Subsequently, the first-, second-, and third-generation den-
drimers Gn (n = 1, 2, 3) were synthesized by the convergent
growth method to join the presynthesized, different generations
of twisted carbazole dendrons and the core TPFOxd, as also
outlined in Scheme 1. Treating the TPFOxd core with phenols
via the aromatic nucleophilic substitution (SNAr) reaction gave
G1, G2, and G3 with an almost 100% yield. The para F atom with
respect to oxadiazole on the pentafluorophenyl was greatly
activated by the oxadiazole group and the other four fluorines,
and the C-F bond could even react with phenols in a quanti-
tative yield at ambient temperature in the presence of the weak
base potassium fluoride as the catalyst. Under the weakly basic
condition, phenols were converted to phenoxides, and HF was
converted to potassium hydrogen difluoride with KF. The
potassium hydrogen difluoride could be easily removed by
washing. The efficient nature of the reaction with few byproducts
allowed all Gn dendrimers to be purified by simple precipitation.
G1, G2, and G3 could be easily dissolved in common solvents,
such as dichloromethane, chloroform, ethyl acetate, toluene,
chlorobenzene, and so on. And the structures of G1, G2, and
1
G3 were confirmed using H NMR and 19F NMR, elemental
analysis, and HRMS or MALDI-TOF MS, etc.
19F NMR and 1H NMR Results. For the 19F NMR spectra in
Figure 1, the resonance peak of the para F of TPFOxd did not
appear in the spectra of G1, G2, and G3, indicating that the
replacement of the para F atoms by phenoxides had occurred,
which proceeded via the Meisenheimer complex.44,45 The
Meisenheimer complex was stabilized by the phenyl with
multiple fluorines and the neighboring electron-withdrawing
oxadiazole. At the same time, SNAr reaction took place at
ambient temperature, attributable to the exceptional reactivity
of the para F, and the low temperature prevented the ortho
F of the oxadiazole from being substituted. Because of the
large steric hindrance of the ortho F at ambient temperature,45
the para F reacted more quickly than the ortho F, even though
the ortho F was more electron-deficient. The chemical shift
of ortho F in G1, G2, and G3 only had a negligible change;
it changed from -57.6 ppm for G1 to -57.4 ppm for G2
and then to -57.3 ppm for G3, as shown in Figure 1. This
trend was similar to the hydrogen shift of the phenyl group in the
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Figure 1. 19F NMR spectra of G1, G2, G3, and TPFOxd.
Figure 3. DSC thermograms during the second heating processes of
the dendrimers.
Table 1. Electrochemical Properties of the Dendrimers Ob-
tained from CV Measurements
sample Uonset(red) (V)a Uonset(ox) (V)b Eg (eV) EHOMO (eV) ELUMO (eV)
G1
G2
G3
-1.43
-1.45
-1.45
1.13
0.97
0.94
2.56
2.42
2.39
-5.47
-5.31
-5.28
-2.91
-2.89
-2.89
a The onset voltage of the reduction process. b The onset voltage of the
oxidation process.
ppm for G3, which agreed well with the negligible change of the
19F NMR results, attributable to the negligible inducing effect of
different carbazole dendrons on the core TPFOxd. Furthermore,
these results also demonstrated that the electrons of the carba-
zole and oxadiazole had almost independent behaviors.
Thermal Properties of Dendrimers. Thermal properties of
G1, G2, and G3 were first investigated by TGA. The onset
temperatures at which 5% weight loss occurred were 448, 479,
and 445 °C for G1, G2, and G3, respectively. It indicated that the
Gn series had excellent thermal stability, which could be ascribed
to the presence of the thermally resistant fluorine element and
the oxadiazole with peripheral carbazoles. DSC results elucidated
that G1, G2, and G3 had high glass transition temperatures of
204, 306, and 367 °C, respectively, as observed during the second
heating process (Figure 3), which was attributed to the con-
siderably rigid structure. The increase in Tg with increasing
generation was owing to the increase in the molecular weight.46
Electrochemical Properties. We employed CV measure-
ments to explore the electrochemical behavior of the dendrimers.
The results are shown in Figure S1 of the Supporting Informa-
tion, and the detailed data are summarized in Table 1. The
oxidation peaks were originated from the peripheral carbazole
units of the electron-rich dendrons. G1 only underwent a single
reversible oxidation at 1.13 V, which involved a continuous one-
electron oxidation process of the three outer carbazoles. G2 and
G3 showed more complicated oxidation behaviors,47 which
involved two successive reversible oxidation processes. On the
basis of the decreased first onset oxidation potential of Gn (1.13,
0.97, and 0.94 V for G1, G2, and G3, respectively) with increasing
generation, the EHOMO levels were calculated to be -5.47, -
5.31, and -5.28 eV for G1, G2, and G3, respectively. The EHOMO
levels of the Gn dendrimers gradiently increased, and they were
modulated by increasing the generation of the carbazole
Figure 2. 1H NMR spectra (a) and structures (b) of C1-OH, C2-OH,
C3-OH, G1, G2, and G3.
core of G1, G2, and G3, which will be discussed in the following
section.
In the 1H NMR spectra, the resonance of the hydrogen at the
meta position with respect to the N atom in the carbazole units of
G3 (as shown in Figure 2) shifted from 8.15 ppm for c3 to 8.27
ppm for b3 and then to 8.55 ppm for a3, which suggested that the
electron density of carbazole gradually became lower from the
outermost units to the innermost one, attributable to the weak
inducing effect of the outer twisted carbazoles with an antiplanar
conformation.30 The corresponding electron density of the out-
ermost carbazole layer increased with increasing generation of
the dendrimers from G1 to G3, which resulted in increasing
EHOMO levels for the Gn dendrimers, in line with the CV results
discussed in the next section. In the meanwhile, the shift of the
hydrogens of the phenyl group in the core TPFOxd only had a
slight downfield shift from 9.15 ppm for G1 and G2 to 9.18
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Table 2. Maximum Photoluminescent Emission Wavelength
of the Dendrimers in Different Solvents and in Films
sample
CH2Cl2 (nm)
CHCl3 (nm)
toluene (nm)
film (nm)a
G1
G2
G3
556
578
596
510
539
553
485
490
532
486
502
505
a Photoluminescent wavelength of the Gn dendrimers in films spin-
coated from toluene solutions (10 mg/mL).
Figure 5. Photoluminescent spectra of the Gn dendrimers and the core
in CH2Cl2 solutions (1 ꢀ 10-6 mol/L) (a) and those of the Gn
dendrimers in toluene solutions (1 ꢀ 10-6 mol/L) (b).
were calculated to be all in the vicinity of -2.90 eV. These results
indicated that the outer carbazoles had little influence on the
electrochemical behavior of oxadiazole, as TPFOxd exhibited
ELUMO = -(-1.42 þ 4.34) = -2.92 (eV), almost unchanged
after the para F atoms were substituted in the Gn dendrimers.
From the HOMO and LUMO energy levels, the band gaps (Eg’s)
were estimated to be 2.56, 2.42, and 2.39 eV for G1, G2, and G3,
respectively, which were gradually modified by varying the
generation of the outer dendrons.
Photophysical Properties. The UV-vis absorption spectra
of the dendrimers in CHCl3 solutions were measured, as shown
in Figure 4a. The spectra of G1, G2, and G3 displayed two
distinct absorption bands centered at 345 and 290 nm, which
Figure 4. UV-vis absorption spectra of the Gn dendrimers and the
core (a) and photoluminescent spectra of the Gn dendrimers (b) in
CHCl3 solutions (1 ꢀ 10-6 mol/L) and photoluminescent spectra of
G2 in CH2Cl2, CHCl3, and toluene (1 ꢀ 10-6 mol/L) (c).
dendrons, in line with the H NMR results discussed in the
above section.
During the negative scanning in CH2Cl2 solutions, an irrever-
sible reduction process started from -1.43 to -1.45 V for G1 to
G3, which corresponded to the reduction process of the electron-
deficient oxadiazole units. Similarly, ELUMO levels of the dendrimers
1
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Figure 8. Luminance-current density-voltage characteristics of
devices a with the configuration of ITO/PEDOT:PSS/Gn/TPBI
(15 nm)/LiF (1 nm)/Al (100 nm).
Figure 6. An estimated energy diagram.
the normal emission, could be attributed to carbazole (donor,
402 nm). Considering the rigid, bulky nature of the dendrimers,
we assigned the long-wavelength emission (F2) to the intramo-
lecular charge-transfer (ICT) emission.48 Taking into account of
the rotation of the electron-donor and electron-acceptor systems
linked by C-O bonds, we reasoned that the F2 band was a
twisted intramolecular charge-transfer (TICT) emission.47,49,50
The change of the solvent polarity led to a large Stokes shift, as
expected: the maximum emission wavelength of 578 nm was
longer for the spectrum in CH2Cl2 which has a higher polarity
than CHCl3, while the emission blue-shifted to 490 nm in
toluene with a lower polarity (Figure 4c).
Figure 5 illustrates the photoluminescent spectra of Gn in
CH2Cl2 and toluene solutions, and the detailed data are also
summarized in Table 2. As expected with the effect of solvent
polarity, the higher polarity, the longer emission wavelength. For
example, the emission wavelength was 556, 578, and 596 nm in
CH2Cl2 for G1, G2, and G3, respectively, while it blue-shifted to
480, 502, and 530 nm for G1, G2, and G3, respectively, when the
solvent was changed to toluene with a lower polarity. The low
intensity of the F2 band of G3 relative to those of G1 and G2
(Figures 4b and 5a) in polar solvents could be explained by the
solvation effect which did not favor the twisted intermolecular
charge transfer.49,50 On the other hand, such an effect favored the
F2 band (TICT) of the dendrimers in the nonpolar solvent
toluene. Further, as the generation of outer dendron increased,
the distance between the donor (carbazole) and acceptor
(oxdiazole) was larger, resulting in the red shift of the F2 band
for G3 compared with that for G1 and G2.
In addition, from photoluminescent spectra of the Gn den-
drimers in films (Figure S2 of Supporting Information) and in
solutions (Figure 5b), the full widths at half-maximum (fwhm) of
the spectra were narrower in the films than in corresponding
toluene solutions, and no aggregation peak appeared in the spectra
of films, probably ascribable to the nonplanar rigid carbazole with a
67° twist dihedral30 as well as the difficulty in a close packing and
suppressed intermolecular interactions, both of which were caused
by the starburst oxadiazoles of the dendrimers.
Figure 7. Electroluminescent spectra of devices a with the configuration
of ITO/PEDOT:PSS/Gn/TPBI (15 nm)/LiF (1 nm)/Al (100 nm) at a
bias voltage of 8 V.
were assigned to the carbazole moiety (345 nm) and oxadiazole
(290 nm, overlapping with the carbazole absorption). In the
absorption spectra of G1, G2, and G3, the cutoff absorption
shifted from 445 to 448 nm. The end absorption had no
significant red shift with increasing polarity of the solvent. This
was attributed to the steric hindrance and the nonconjugated
ether bond between carbazole and oxadiazole, and both of these
factors resulted in minimized electronic coupling of the two
chromophores in the ground states.
The photoluminescent spectra of the Gn dendrimers in
CHCl3 solutions were measured at ambient temperature, as
shown in Figure 4b. The detailed data are listed in Table 2. At
a first glance, the photoluminescent spectrum of G2 in the
CHCl3 solution exhibited distinct dual luminescence with max-
ima located at 402 nm (F1 band, locally excited state) and
539 nm (F2 band). The spectra of G2 in other solvents also
displayed dual luminescence, so did those of G1 and G3 in
different solvents. As the concentration was increased from 1 ꢀ
10-6 to 1 ꢀ 10-4 mol/L, no aggregation bands appeared, and the
emission maxima barely shifted. The F1 band, generally known as
Electroluminescent Properties of LED Devices. On the
basis of their nonconjugated structures and redox behaviors,
the dendrimers were expected to facilitate the transport of
electrons and holes in OLED devices. We fabricated devices
with appropriate configurations to match up corresponding energy
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Table 3. Electroluminescent Performances of Devices a with the Configuration of ITO/PEDOT: PSS/Gn/TPBI (15 nm)/LiF
(1 nm)/Al (100 nm)
sample
Uonset (V)
Lmax (cd/m2)
ηImax (cd/A)
ηLmax (lm/W)
CIE coordinate
ηextmax (%)
λ (nm)
G1
G2
G3
5.6
5.2
5.3
2191
2420
2307
0.346
0.596
0.459
0.688
1.138
0.955
(0.23, 0.44)
(0.30, 0.56)
(0.27,0.55)
0.260
0.355
0.310
501
526
517
Table 4. Electroluminescent Performances of Devices b with the Configuration of ITO/PEDOT:PSS/Gn/TPBI (15 nm)/AlQ
(30 nm)/LiF (1 nm)/Al (100 nm)
sample
Uonset (V)
Lmax (cd/m2)
ηImax (cd/A)
ηLmax (lm/W)
CIE coordinate
ηextmax (%)
λ (nm)
G1
G2
G3
6.3
5.6
5.2
4758
4252
4882
1.018
3.560
4.600
0.532
2.240
2.640
(0.23, 0.45)
(0.30, 0.56)
(0.26, 0.57)
0.350
0.911
1.491
501
522
514
levels (Figure 6). First, devices a with a configuration of ITO/
PEDOT:PSS/Gn/TPBI/LiF/Al were fabricated. The devices a
with G1, G2, and G3 had low turn-on voltages (at 1 cd/m2) of
5.6, 5.2, and 5.3 V, respectively. The electroluminescent spectra
in Figure 7 showed that the major peaks fell in the green-light
region. The corresponding electroluminescent spectra were
similar to those of casting films (Figure S2 in Supporting
Information) with much narrower emission bands, of which
the main one could be attributed to the F2 band, in comparison
with the photoluminescent spectra of solutions (Figure 5b),
suggesting that there was no aggregation in the devices. Char-
acteristics of current density and luminance versus bias voltage
(J-V-L) for the devices a with the dendrimers are depicted in
Figure 8, and the detailed data are also listed in Table 3. The
device with G2 had the maximum luminance of 2420 cd/m2, and
the EQE was 0.35%. In fact, this device possessed the best
performance compared with those containing G1 and G3 in both
maximum luminance and efficiency, and the lower turn-on
voltages could be attributed to the better carrier balance of G2.
To further demonstrate the advantages of these multifunc-
tional dendrimers, we fabricated devices b containing G1, G2,
and G3 with the configuration of ITO/PEDOT:PSS/Gn/TPBI/
AlQ/LiF/Al. In these devices, the Gn dendrimers functioned as
both emitting materials and hole transporting materials, and
the AlQ layer was introduced between the TPBI layer and the
cathode to act as the electron-transporting layer because the
ELUMO level of AlQ was in between those of TPBI and aluminum.
From the energy level diagram in Figure 6, the introduction of
the AlQ layer led to a smaller energy gap between the cathode
and the TPBI layer, resulting in the reduction of the electron
injection barrier from the cathode to the TPBI layer. As the
generation of the carbazole dendrons increased, the hole trans-
port increased, with the turn-on voltage decreasing from 6.3 to
5.2 V, along with improved current efficiency, luminous effi-
ciency, and EQE, as summarized in Table 4. The results in
Tables 3 and 4 suggested that after the AlQ layer was introduced,
there was a better carrier balance in the devices.
Figure 9. Electroluminescent spectra of G3 in device b under different
voltages.
voltage was increased from 6 to 15 V (Figure 9), ascribable to the
stability of oxadiazole and carbazole as well as the high glass
transition temperatures of the dendrimers.
Therefore, the properties of devices a and b with the Gn
dendrimers as the emitting layer indicated that G2 itself had a
better balance of charge carriers when compared with G1 and G3.
On the other hand, when an electron-transporting layer AlQ was
added, high-performance devices containing G1, G2, and G3
could be obtained, and the performance of the device containing
G3 was superior compared with those of the devices containing
the two other dendrimers G1 and G2.
Considering the moderate quantum efficiencies of the Gn
series, we could change the content of carbazole in the periphery
to adjust the charge transfer. Taking advantage of the reactivity of
the ortho fluorines with respect to oxadiazole, we could also
introduce more oxadiazoles in the ortho positions of the starburst
core. Further research on optimizing the chemical structures to
adjust the balance of carriers and device architectures to improve
efficiency is in progress.
Furthermore, the device containing G3 had a superior perfor-
mance due to the better balance of charge carriers in the device
compared with those having G1 and G2. The maximum lumi-
nance was 4882 cd/m2, luminous efficiency was 2.64 lm/W, and
the EQE was 1.49%. All the devices had a green emission (Figure
S3 in Supporting Information) similar to devices a, indicative of a
stable emission region with varying generation. It is worth noting
that the OLED devices remained quite stable when the bias
’ CONCLUSIONS
In summary, we introduced highly active C-F bonds to
synthesize a series of novel dendrimers (G1, G2, and G3)
containing a starburst oxadiazole core and twisted carbazole
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dendrons via SNAr reaction at ambient temperature by the weak
base potassium fluoride with a high selectivity in a nearly
quantitative yield. TGA results demonstrated high thermal
stability owing to the presence of the rigid carbazole, multiple
fluorines, and oxadiazole in the dendrimers. CV measurements
along with 1H NMR results indicated that the dendrimers had a
gradient change of the LUMO-HOMO energy band gap due to
gradual chemical modification. UV-vis absorption spectra
showed that the dendrimers had almost no red shift due to the
separation of the donor and the acceptor by the nonconjugated
ether bonds. Photoluminescent spectra showed the dual fluor-
escence emission of the dendrimers in solutions, which indicated
that twist intramolecular charge transfer occurred between
oxadiazole and carbazole. Electroluminescent spectra showed a
single-peak emission in the region of green light in the devices a
and b with the configurations of ITO/PEDOT/Gn/TPBI/LiF/
Al and ITO/PEDOT/Gn/TPBI/AlQ/LiF/Al, respectively. In
the devices a, G2 had the best performance which could be
attributed to a better carrier balance of the dendrimer itself. On
the contrary, for devices b with an additional AlQ layer to
transport electrons, the external quantum efficiency and max-
imum current efficiency gradually increased, attributable to
better hole transport with increasing generation of the carbazole
dendrons. The devices had low turn-on voltages. The third-
generation dendrimer G3 had the highest EQE of 1.49%,
maximum current efficiency of 4.6 cd/A, and power efficiency
of 2.6 lm/W, accompanied by a high luminance of 4800 cd/m2,
which suggested that G3 had the best balance with AlQ in the
device b. Because of the remarkable ability for controlling
properties, we expect that the method described in this work
will be of important value for preparation of high-performance
OLEDs.
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’ ASSOCIATED CONTENT
S
Supporting Information. Syntheses of intermediates 1
b
to 17, cyclic voltammograms of the Gn dendrimers, photolumi-
nescent spectra of the Gn dendrimers in films, and electrolumi-
nescent spectra of devices b. This material is available free of
charge via the Internet at http://pubs.acs.org.
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’ AUTHOR INFORMATION
(31) Sakamoto, Y.; Suzuki, T.; Miura, A.; Fujikawa, H.; Tokito, S.;
Taga, Y. J. Am. Chem. Soc. 2000, 122, 1832–1833.
(32) Heidenhain, S. B.; Sakamoto, Y.; Suzuki, T.; Miura, A.; Fujikawa,
H.; Mori, T.; Tokito, S.; Taga, Y. J. Am. Chem. Soc. 2000, 122, 10240–
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Corresponding Author
*E-mail: fanxh@pku.edu.cn (X.-H.F.); zshen@pku.edu.cn (Z.S.).
’ ACKNOWLEDGMENT
(33) Facchetti, A.; Yoon, M.-H.; Stern, C. L.; Katz, H. E.; Marks, T. J.
Financial support from the National Natural Science Founda-
tion of China (Grant 20974002) and Beijing Municipal Natural
Science Foundation (Key Project from the Science and Tech-
nology Development Plans of the Beijing Municipal Commis-
sion of Education) is gratefully acknowledged.
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