Carbazole-pyrene-based organic emitters for electroluminescent device

Article abstract of DOI:10.1016/j.cplett.2005.04.019

Carbazole-pyrene-based compounds were synthesized and fully characterized. Their photoluminescence properties were studied. Fluorescence quantum yield was improved dramatically by inserting pyrene as electron-acceptor. Blending 9-(3-(2-(1-(2-(3-(9H-carbaz

Full text of DOI:10.1016/j.cplett.2005.04.019

Chemical Physics Letters 408 (2005) 169–173
www.elsevier.com/locate/cplett
Carbazole–pyrene-based organic emitters for
electroluminescent device
Yajun Xing , Xinjun Xu , Peng Zhang , Wenjing Tian c, Gui Yu , Ping Lu
,
a
b
c
b
a,
*
,
*
Yunqi Liu ,b, Daoben Zhu
b
*
a
Chemistry Department, College of Science, Zhejiang University, Yugu Road No. 38, Zhejiang 310027, PR China
b
Organic Solids Laboratory, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, PR China
c
Key Laboratory for Supramolecular Structure and Materials of Ministry of Education, Jilin University, Changchun 130012, PR China
Received 12 January 2005; in final form 19 March 2005
Available online 28 April 2005
Abstract
Carbazole–pyrene-based compounds were synthesized and fully characterized. Their photoluminescence properties were studied.
Fluorescence quantum yield was improved dramatically by inserting pyrene as electron-acceptor. Blending 9-(3-(2-(1-(2-(3-(9H-
carbazol-9-yl)-9-p-tolyl-9H-carbazol-6-yl)ethynyl)pyren-6-yl)ethynyl)-9-p-tolyl-9H-carbazol-6-yl)-9H-carbazole (16) with poly(N-
vinylcarbazole) (PVK), simplified electroluminescent (EL) device with green emission was fabricated. Maximum luminance reached
1000 cd/m2.
ꢀ 2005 Elsevier B.V. All rights reserved.
1. Introduction
excellent optical properties [9–11]. The other was the
multifunctional fluorophore, which was recently synthe-
sized and fabricated for simplified device architecture
[12–16]. For instance, stilbenoid molecules with pyrene
as an acceptor and phenothizine building block as donor
could be doped in a polymer blend for a single layer de-
vice [13]. Another simplified device structure was com-
posed of dye-dispersed poly(N-vinylcarbazole) (PVK)
as an emitter layer, which was doped with both hole-
transport material and electron-transport material
[5,6]. There still existed demand for searching more effi-
cient material in the simplified architecture. In this
Letter, carbazole–pyrene-based small molecules were
synthesized and fully characterized. For 15 and 16, car-
bazole was selected as the donor due to sufficient high
triplet energy and the hole-transport property. For in-
stance, carbazole derivatives used as donors were able
to emit red [17–19], green [8], and even blue in some
cases [20]. On the other hand, pyrene was chosen as
acceptor due to its good performance in solution [11]
and its electron-acceptor nature [12,21]. PVK was
Organic electroluminescence has attracted consider-
able attention since the first report of EL device based
on multilayered organic molecules [1]. Trends of organic
EL devices have been focused both on optimizations of
EL structure and on developing new optoelectronic
materials [2,3]. Different types of EL structures have
been, thereby, proposed to improve the quantum effi-
ciency, conversion efficiency, lifetime, and emission col-
ors [4–6]. Although very bright and efficient EL devices
have been achieved by using multilayer device structure,
from a manufacturing point of view, devices that have
fewer layers are much desirable because they simplify
the fabrication process. Meanwhile, organic compounds
with fascinating molecular structures have been de-
signed and synthesized to meet different requirements
[7,8]. One was the ethynylated acenes, which exhibited
*
Corresponding author. Fax: +86 571 87952543.
E-mail address: pinglu@zju.edu.cn (P. Lu).
0009-2614/$ - see front matter ꢀ 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.cplett.2005.04.019

170
Y. Xing et al. / Chemical Physics Letters 408 (2005) 169–173
chosen as the matrix due to its hole transport property
and violet–blue emission [22].
2.2. Instrument
Solvents used were of spectrograde and purified by
distillation before use. The fluorescence spectra were
recorded on a Shimadzu RF-5301pc. The absorption
spectra were obtained on a Shimadzu UV-2450 spectro-
photometer. The quantum yields were calculated based
on 9,10-diphenyl-anthracene (DPA) (Uf = 0.95 in cyclo-
hexane) [28]. Chloroform solution was degassed with N2
for 40 min. Cyclic voltammetry experiments were per-
formed with a BAS 100 B/W electrochemical analyzer.
All measurements were carried out at room temperature
2. Experimental
2.1. Synthesis
Preparation routes to 13, 14, 15 and 16 were depicted
in Scheme 1. Triple bond was used as a bridge to link the
pyrene and carbazole unit. Symmetrical and asymmetri-
cal structures were designed for comparison of lumines-
cence properties. Sonogashira coupling reaction was
used to construct ethynyl bridged conjugated structures.
Compounds 1 [23], 2 [24], 6 [25], 7 [26], 8 [27], 9 [27], 10
[26] and 12 [11] were prepared by reference methods,
respectively.
with
a
conventional three-electrode configuration
consisting of a platinum working electrode, an auxil-
iary platinum electrode, and a non-aqueous Ag/Ag+
reference electrode. The solvent in this experiment
was DMF and the supporting electrolyte was
Scheme 1. Synthesis of 13, 14, 15 and 16: (a) K2CO3, Cu, 18-crow-6, 1-iodo-4-methylbenzene, DMF, N2, 170 ꢁC, 12 h; (b) KI, KIO3, acetic acid,
refluxed, 70 min; (c) K2CO3, Cu,18-crown-6, carbazole, DMF, N2, 170 ꢁC, 12 h; (d) 3-methyl-1-butyn-3-ol, CuI, Pd(PPh3)2Cl2, NEt3, 100 ꢁC, 24 h,
N2; (e) KOH, isopropyl alcohol, refluxed, 3 h, N2; (f) 2-chloro-2-methylpropane, AlCl3; (g) K2CO3, Cu,18-crown-6, 1,4-diiodobenzene, DMF,
170 ꢁC, N2, 12 h; (h) 3-methyl-1-butyn-3-ol, CuI, Pd(PPh3)2Cl2, NEt3, 100 ꢁC, 24 h, N2; (i) KOH, isopropyl alcohol, refluxed, 3 h, N2; (j) K2CO3,
Cu,18-crown-6, 1,4-diiodobenzene, DMF, 170 ꢁC, N2, 12 h; (k) 3-methyl-1-butyn-3-ol, CuI, Pd(PPh3)2Cl2, NEt3, 100 ꢁC, 12 h, N2; (l) KOH,
isopropyl alcohol, refluxed, 3 h, N2; (m) CuI, Pd(PPh3)2Cl2, NEt3, 100 ꢁC, 12 h, N2; (n) CuI, Pd(PPh3)2Cl2, NEt3/benzene: 3/5 (v/v), 100 ꢁC, 12 h, N2.

Y. Xing et al. / Chemical Physics Letters 408 (2005) 169–173
171
tetra-n-butylammonium perchlorate (TBAP). 1H and
13C NMR spectra were obtained on a Bruker AVANCE
DMX 500 spectrometer. 13C NMR spectra were ob-
tained with a broad band proton decoupling mode.
Spectra were run in CDCl3, (CD3)2SO or C6D6, and
internally referred to tetramethylsilane. EI (70 ev) was
applied to obtain mass spectra on HP 5989B (Hewlett–
Packard). MALDI-TOF-MS data were obtained from
IonSpec 4.7 T FTMS.
solubility and easier fabrication than those of 15. There-
fore, electrochemistry and electroluminescence studies
of 16 were carried out.
3.2. Electrochemistry studies
The cyclic voltammograms (CVs) of 16 is shown in
Fig. 1. A small amount of ferrocene was added as an
internal standard. During the process of anodic scan,
the peak at +0.70 V was observed against the reference
electrode, 16 exhibited oxidation reaction at +1.10 V.
This can be attributed to the electron-donating nature
of carbazole segment. Moreover, the oxidation is irre-
versible. HOMO and LUMO energy levels of 16
(À5.86 and À2.90 eV) were calculated from cyclic vol-
tammetry by À4.5 eVox/red [29,30], while À4.5 eVox/
red is referred to normal hydrogen electrode (NHE).
The reduction peak at 2.0 V was observed and it was
reversible. This implied that improvement of electron-
transport property of 16 was achieved by inserting pyr-
ene acceptor.
2.3. Preparation and characterization of LED device
Indium-tin oxide (ITO) coated glass with a sheet
resistance below 30 X/h was used as a substrate for
LEDs. The PVK layer doped with 16 was spin coated
from a chloroform solution onto the indium-tin-oxide
(ITO) glass substrates. Evaporation of the small organic
layers and the aluminum electrode were done at a vac-
uum of 2 · 10À4 Pa. The emitting area of the EL devices
was 4 mm2. The EL spectra were recorded on a Hitachi
F-4500 fluorescence spectrophotometer. Luminance was
measured with Photo research 650 spectrascan colorim-
eter. Current–voltage measurements were carried out
using a Hewlett–Packard 4140B semiconductor parame-
ter analyzer. All the measurements were performed un-
der ambient atmosphere at room temperature.
3.3. Electroluminescence studies
Since 16 is difficult to be evaporated onto the ITO
substrate, a single-layer electroluminescence device was
fabricated by doping 16 into PVK. We use PVK as the
host because its emission band is almost overlapped with
the absorption band of 16 (please see Fig. 5 in Support-
ing information), so an effective energy transfer may oc-
cur in this host–guest system. As shown in Fig. 2, 16 can
exhibit its own luminescence well, and green light emis-
sion at 500 nm was observed. The current–voltage–lumi-
nance (I–V–L) characteristics were given in Supporting
information. The single-layer device (ITO/PVK:16
(10:1, w/w)/Al) showed turn-on voltage at 8 V, the max-
imum luminance of 60 cd/m2 at 17 V, and the luminous
efficiency of 0.023 lm/W at 20 V.
3. Result and discussion
3.1. Photoluminescence studies
Absorption and emission spectra of the synthesized
compounds are given in Supporting Information. Their
optical properties are summarized in Table 1. By using
9,10-diphenyl-anthracene (DPA) as a reference, fluores-
cence quantum yields (Uf) of compounds 13–16 were ob-
tained. Compound 13 shows
a moderately high
fluorescence quantum yield with Uf = 0.61, whereas
lower quantum yield of 14 was found. This result may
arise from the twisted structure of 14 relative to 13.
When inserting pyrene acceptor, both Uf of 15 and 16
are improved significantly. Both 15 and 16 show extre-
mely high fluorescence quantum yield of nearly 100%.
In order to examine the potential application of these
new synthesized compounds, 16 was selected as the can-
didate for EL device fabrication due to its higher
60
16
50
40
30
20
10
0
Table 1
Optical properties of 13, 14, 15 and 16 (25 ꢁC, CHCl3)a
Compounds
kabs(max) (nm)
kex (nm)
kem (nm)
Uf
-10
13
14
15
16
a
358
373
412
422
350
350
350
350
422
433
448
460
0.61
0.02
1.18
1.03
-20
-2.5
-2.0
-1.5
-1.0
-0.5
0.0
0.5
1.0
1.5
Potential / V
Fig. 1. Cyclic voltammograms (CVs) of 16.
Quantum yields were calculated based on DPA.

172
Y. Xing et al. / Chemical Physics Letters 408 (2005) 169–173
2.31
2.90
2.70
PL
EL
Al
4.28
ITO
4.7
5.80
5.86
6.20
Fig. 4. Relative energy alignments in the device ITO/PVK:16 (10:1,
w/w)/TPBI/Al.
300
400
500
600
700
800
Wavelength /nm
Fig. 2. The EL spectrum of 16 (ITO/PVK:16 (10:1, w/w)/Al), and
photoluminescence (PL) spectrum in solid state of 16.
a
b
c
It is known that in PVK holes are dominant and elec-
trons are minor, leading to the unbalance of electrons
and holes. Therefore, the EL performances of the sin-
gle-layer device are not satisfying. To improve the device
performance, an additional electron-transporting layer
(1,3,5-tris-(phenyl-2-benzimidazole)-benzene
(TPBI),
whose structure was shown in Supporting information)
was deposited by vacuum thermal evaporation under
3 · 10À4 Pa. (ITO/PVK: 16 (10:1 w/w) (60 nm)/TPBI
(30 nm)/Al (100 nm)). The I–V–L characteristics are
shown in Fig. 3. Device performance was improved
without variety at the emission wavelength [31]. Physical
performance appeared to be improved: turn-on voltage
11 V, maximum luminance reached 1000 cd/m2, external
quantum efficiency was found to 0.85% at 15.5 V, and
luminous efficiency was 1.1 lm/W at 15.5 V. Analysis
of HOMO and LUMO energy levels of layers for this
device was shown in Fig. 4. HOMO energy level of 16
(5.86 eV) lies between those of ITO (4.70 eV) or PVK
250
300
350
400
450
500
550
Wavelength / nm
Fig. 5. PL emission spectra of PVK (a), TPBI (b) and absorption
spectrum of 16 (c).
(5.80 eV) [32] and that of TPBI (6.20 eV) [33], while
the LUMO energy level of 16 (2.90 eV) lies under those
of TPBI (2.70 eV) or PVK (2.31 eV). There is a barrier
of 0.4 eV for holes to migrate from PVK to TPBI layer.
Thus, under the present configuration (ITO/PVK:16
(10:1 w/w) (60 nm)/TPBI (30 nm)/Al (100 nm)), TPBI
acts as a electron-transporter and a hole blocker and
charge recombination is confined to PVK layer. On
the other hand, an efficiency energy transfer [3,33] was
approached from the host PVK (404 nm) [34] to the
guest 16 due to the wide range overlap (Fig. 5). Such
an energy transfer is consistent with the relative energy
levels due to the suitable HOMO and LUMO energy
levels [35] of 16.
200
1000
150
800
600
100
400
50
200
4. Conclusions
0
0
30
0
5
10
15
20
25
In this Letter, carbazole–pyrene-based compounds
were synthesized and fully characterized. Their photolu-
minescence properties were studied. Emission efficiency
was improved dramatically by inserting pyrene as
Voltage (V)
Fig. 3. I–V–L characteristics of the device (ITO/PVK:16 (10:1, w/w)/
TPBI/Al).

Y. Xing et al. / Chemical Physics Letters 408 (2005) 169–173
173
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Appendix A. Supplementary data
Supplementary data associated with this article can
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j.cplett.2005.04.019.
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