Synthesis and properties of novel hole-transporting materials with triphenylamine units

Article abstract of DOI:10.1016/j.cclet.2011.11.018

Two novel organic hole-transporting materials have been synthesized by combination of triphenylamines (TPA) via π-conjugated bonds using Wittig reaction. The structures were characterized by NMR, FT-IR and HRMS. The optical, electrochemical and thermal properties of the materials were studied in detail. The results show that these two compounds have blue emission, proper HOMO levels and high thermal stability. Furthermore, a quantum chemical calculation on electron distribution of the two compounds was performed, which suggests the current synthesized materials would be promising candidates for hole-transporting materials.

Full text of DOI:10.1016/j.cclet.2011.11.018

Available online at www.sciencedirect.com
Chinese Chemical Letters 23 (2012) 141–144
www.elsevier.com/locate/cclet
Synthesis and properties of novel hole-transporting materials with
triphenylamine units
a,b
Wen Zheng Gao a, Xiang Gao Li a, Shi Rong Wang a, , Hai Jun Lv
*
a School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China
b College of Chemical and Pharmaceutical Engineering, Hebei University of Science and Technology Shijiazhuang 050018, China
Received 12 July 2011
Available online 22 December 2011
Abstract
Two novel organic hole-transporting materials have been synthesized by combination of triphenylamines (TPA) via p-conjugated
bonds using Wittig reaction. The structures were characterized by NMR, FT-IR and HRMS. The optical, electrochemical and thermal
properties of the materials were studied in detail. The results show that these two compounds have blue emission, proper HOMO levels
andhigh thermalstability. Furthermore, a quantumchemical calculationon electrondistributionofthetwo compoundswas performed,
which suggests the current synthesized materials would be promising candidates for hole-transporting materials.
# 2011 Shi Rong Wang. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
Keywords: Wittig reaction; Triphenylamine; Hole-transporting materials; Quantum chemical calculation
In general, the hole-transporting layer in multi-layered organic light emitting diodes (OLEDs) plays the roles of
facilitating hole injection from the anode into organic layers, accepting holes and transporting the injected holes to the
emitting layer [1]. They should have proper energy level of highest occupied molecular orbital (HOMO) for efficient
hole injection and high mobility. They also ought to be of excellent solubility and can form homogenous thin films
with both morphological and thermal stability [2]. While a large number of organic materials exhibit good mobility,
there are some drawbacks such as poor stability and high driving voltage associated with these materials. So it is highly
desirable to design and synthesize new materials.
At present, triphenylamine (TPA) derivatives have been the most widely used hole-transporting materials because
of their high hole mobility [3]. The nitrogen atom in the triphenylamine bestows an electron-donating ability.
Moreover, Tokito et al. have found that the linear linkage is much more effective than the branch linkage in increasing
the Tg [4]. Herein, we designed and synthesized two new compounds by the combination of TPA units connected by p-
conjugate bonds. It was expected to obtain new hole-transporting materials with excellent performance.
1. Experimental
The designed synthetic route for the novel hole-transporting materials TM1 is depicted in Scheme 1.
Compounds 1–8 were synthesized according to previous literatures [5]. A mixture of 2:1 equiv of 4-[N,N-di
* Corresponding author.
E-mail address: wangshirong@tju.edu.cn (S.R. Wang).
1001-8417/$ – see front matter # 2011 Shi Rong Wang. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
doi:10.1016/j.cclet.2011.11.018

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W.Z. Gao et al. / Chinese Chemical Letters 23 (2012) 141–144
H C CH
H C
CH
3
H C
CH
3
3
3
3
3
N
N
b
N
a
CHO
CH OH
2
1
CH PPh Br
2
3
2
H C
CH
3
3
H C
CH
H C
CH
H C
CH
3
3
3
3
3
3
H C
CH
3
3
N
N
N
N
f,g
a
c,d,e
N
b
CHO
CH OH
CH PPh Br
CHO
2
2
3
3
5
4
6
CH
3
N
2
CH
H C
3
3
N
N
CH
N
CH
N
3
3
CH
3
h,i,j
f,g
d,e
+
NH
CH
CH
3
3
TM 1
CH
I
3
OHC
CHO
8
7
N
CH
H C
3
3
6
N
N
CH
CH
TM 2
3
3
Scheme 1. Synthesis routes of TM1 and TM2. (a) NaBH4; (b) PPh3+HBrꢀ; (c) PPh3+CH3Brꢀ; (d) THF; (e) t-BuOK; (f) DMF; (g) POCl3; (h) CuCl;
(i) 1, 10-phenanthroline; (j) dimethylbenzene.
(4-tolyl)amino]benzyl(triphenyl)phosphonium bromide (compound 2) and 4-N,N-di(4-formylphenyl)tolylbenze-
namine (compound 8) were dissolved in freshly dried tetrahydrofuran (THF). The reaction system was cooled to
0 8C and then 2 equiv of potassium tertbutoxide which dissolved in dried THF was added dropwise under stirring.
The reaction mixture was stirred at room temperature for 6 h until compound 8 was consumed completely
monitored by thin-layer chromatography. THF was removed at reduced pressure and poured into water and
neutralized with dilute HCl and then extracted with dichloromethane. Target compounds were obtained as pure E
stereoisomer, after treatment of the Z/E mixture resulting from the Wittig condensation with a catalytic amount of
iodine under refluxing for 6 h in THF, then remanent iodine were removed by pouring into sodium hydroxide
solution (wt = 10%) and stirred for 2 h. After extracted with dichloromethane, the solvent was removed at reduced
pressure and the residues were chromatographed on a silica gel column (cyclohexane: ethyl acetate = 20:1 as
1
eluent) to give bright yellow solid. Yield 69.3%; IR (n (KBr)/cmꢀ1): 3026, 1590, 1500, 1310, 816; H NMR
(CDCl3, 500 MHz): d 2.326 (s, 12H), 2.342 (s, 3H), 6.933 (s, 4H), 6.986–7.112 (m, 28H), 7.323–7.365 (m, 8H);
ESI-HRMS (m/z) calcd. for C63H55N3(M+): 853.4396, Found: 853.4395.
TM2 was synthesized by using 1-{4-[N,N-di(4-tolyl)amino]phenyl}allyl(triphenyl)phosphonium bromide
(compound 6) and 4-N,N-di(4-formylphenyl)tolylbenzenamine (compound 8) according to the general method
described above. Yield 57.4%; IR (n (KBr)/cmꢀ1): 2910, 1597, 1500, 1322, 967, 807; 1H NMR (CDCl3, 500 MHz): d
2.313 (s, 12H), 2.328 (s, 3H), 6.640 (d, 2H, J = 15.5 Hz), 6.805 (d, 2H, J = 17.5 Hz), 6.948–7.072 (m, 34H), 7.291 (d,
6H, J = 7.5 Hz); ESI-HRMS (m/z) calcd. for C67H59N3(M+): 905.4709, found: 905.4706.
2. Results and discussion
The thermal properties of the two compounds were investigated by differential scanning calorimetry (DSC). The
two compounds show distinct Tg of 102.58 8C (TM1) and 114.68 8C (TM2), higher than that of the widely used hole-
transporting material N,N0-di(1-naphthyl)-N,N0-diphenyl-[1,10-biphenyl]-4,40-diamine (NPB) (98 8C) [6], which

W.Z. Gao et al. / Chinese Chemical Letters 23 (2012) 141–144
143
1.2
1.0
0.8
0.6
0.4
0.2
0.0
1.2
1.0
0.8
0.6
0.4
0.2
0.0
1.2
1.0
0.8
0.6
0.4
0.2
0.0
1.2
1.0
0.8
0.6
0.4
0.2
0.0
PL
UV
UV PL
PL of TM 1
PL of TM 2
Abs of TM 1
Abs of TM 2
PL of TM 1
PL of TM 2
Abs of TM 1
Abs of TM 2
a
b
200
300
400
500
600
700
800
200
300
400
500
600
700
800
Wavelength(nm)
Wavelength(nm)
Fig. 1. UV–vis absorption and PL spectra of TM1 and TM2 (a) in THF solution; (b) in thin film.
means that homogeneous and stable amorphous thin films of the compounds could be prepared due to the thermal and
morphological stability of the two compounds.
Fig. 1 shows the UV–vis absorption and photoluminescence emission spectra of TM1 and TM2, in THF solution
and thin film, respectively. The spectral data are summarized in Table 1. The absorption in the range of 302–307 nm
and 397–434 nm could be ascribed to the n–p* transition of the TPA moiety and the intramolecular charge transfer of
p–p* respectively [8]; In solution, the absorption of TM2 caused by intramolecular charge transfer show an obvious
red shift (about 22 nm) compared to TM1, which could be due to the extension of the conjugated length. The UV–vis
absorption spectra of the compounds are similar in both solution and thin film, indicating the absence of aggregation in
the thin film which is beneficial to the working life of the device.
For the emission spectra in THF solution, the maximum emission peaks are 460 nm and 482 nm for TM1, TM2
respectively, corresponding to blue light emission. Similarly, as the result of the extension of the conjugated length, the
emission peaks of TM2 show a red-shift (22 nm) compared to TM1.
Cyclic voltammetry (CV) was used to assess the ionization potentials (hole-injecting ability) and the
electrochemical properties of the compounds. CV was carried out on an IM6e electrochemistry workstation with a
three-electrode configuration in 0.10 mol/L etrabutylammonium perchlorate (Bu4NClO4) in chloroform as supporting
electrolyte with a scanning rate of 100 mV/s at room temperature. Platinum disc and platinum wire were used as
working and counter electrode, respectively, and a saturated calomel electrode (SCE) as the reference electrode. The
results are shown in Table 1. The pair of anodic/cathodic peaks is located at 1.08/0.78 Vand 1.16/0.83 V for TM1 and
TM2 respectively, which could be attributed to the gain/loss of electrons of nitrogen atom within TPA units. On the
other hand, the HOMO levels of two molecules are TM1 (ꢀ5.22 eV) and TM2 (ꢀ5.24 eV), higher than that of NPB
(ꢀ5.40 eV) [6], which means that the energy barrier between the interface of ITO and derivatives is smaller than that of
ITO and NPB, thus it results the better hole injection from ITO to derivatives than that from ITO to NPB, and the joule
heat produced at the interface during device operation become much lower.
In order to investigate the electron distribution of the HOMO and LUMO energy levels of these compounds,
quantum-chemical calculations were carried out through the Gaussian 03 program at the B3LYP/6-31G* level [9]. The
Table 1
Optical, electrochemical and thermal properties of TM1 and TM2.
HOMO (eV)c
Eg (eV)d
opt
LUMO (eV)e
Tg (8C)f
a
b
lmax
PLmax
Sol (nm)
Film (nm)
Sol (nm)
Film (nm)
TM1
TM2
NPB
302, 412
304, 434
307, 397
303, 425
460
482
462
493
ꢀ5.24
ꢀ5.22
ꢀ5.4
2.29
2.13
ꢀ2.95
ꢀ3.09
114.68
102.58
98
a
Aborption spectra were recorded in the 10ꢀ5 mol/L THF solution and in thin film on quarts.
b
c
d
e
f
Emission spectra were recorded in the 10ꢀ5 mol/L THF solution and in thin film, excited at absorption maxima.
OX
HOMOðeVÞ ¼ ꢀðeE
þ 4:4 eVÞ [7].
onset
Egopt(eV) = 1240/lonset
.
jLUMOj ¼ jHOMOj ꢀ Egopt
:
DSC in N2.

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W.Z. Gao et al. / Chinese Chemical Letters 23 (2012) 141–144
Table 2
Frontier on the molecular orbitals of TM1 and TM2.
TM1
TM2
HOMO
LUMO
HOMO
LUMO
results of the quantum-chemical calculations are shown in Table 2. Because electronic excitation from the HOMO to
the LUMO produces the first excited open-shell singlet state, the orbital features presented in Table 2 might provide
further insight into the distinct photophysical properties. Comparison of the electron distribution in the frontier on the
molecular orbitals reveals that the HOMO and LUMO are both localized on the triphenylamine donor moiety,
dispersed with the expansion of p-conjugate bonds. It also indicates that the conjugated length of the derivatives could
be increased by introduction of TPA units, which will be favorable for high hole transporting efficiently. The increased
conjugated length led to enlarged energy gap and hence red shift in the UV absorption, which is consistent with the
experiment results (Fig. 1).
In summary, two novel hole-transporting materials based on triphenylamine derivatives have been synthesized and
characterized. Target compounds show Tg at 102.58 8C and 114.68 8C for each other. The photoluminescence spectra
of the two compounds exhibit a maximum at 460 nm and 482 nm in solution, while 462 nm and 493 nm in film. And
the HOMO levels of two molecules are ꢀ5.22 eVand ꢀ5.24 eV. Therefore they can be a potential alternative material
to NPB. Future studies on the application of these compounds as photovoltaic materials are now in progress.
Acknowledgments
The authors gratefully thank the National Natural Science Foundation of China (No. 21176180) and Research Fund
for the Doctoral Program of Higher Education of China (No. 20100032110021) for the financial support.
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