A novel tetraphenylethene-carbazole type compound containing the dimesitylboron moiety: Aggregation-induced emission enhancement and electroluminescence properties

Article abstract of DOI:10.1039/c4ra01925c

A highly efficient blue light-emitting material based on tetraphenylethene as an aggregation-induced emission enhancement (AIEE) group, carbazole as a hole-transporting group and dimesitylboron as an electronic-transporting group was synthesized and utili

Full text of DOI:10.1039/c4ra01925c

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A novel tetraphenylethene–carbazole type
compound containing the dimesitylboron moiety:
aggregation-induced emission enhancement and
electroluminescence properties†
Cite this: RSC Adv., 2014, 4, 19418
Received 5th March 2014
Accepted 14th April 2014
ab
Jinwei Yang,a Xiuqing Dong,c Xiaohuan Wu,a Penghong Zhou,a
*
Heping Shi,
DOI: 10.1039/c4ra01925c
c
d
*
*
Fangqin Cheng and Martin M. F. Choi
www.rsc.org/advances
A highly efficient blue light-emitting material based on tetraphenyle- aggregation-caused quenching effect and was proved to be a
thene as an aggregation-induced emission enhancement (AIEE) group, thorny obstacle to the fabrication of efficient OLEDs devices.10,11
carbazole as a hole-transporting group and dimesitylboron as an Both the above-mentioned factors give rise to the rarity of blue
electronic-transporting group was synthesized and utilized in OLED light-emitting materials.
devices.
In 2001, Tang and co-workers reported a phenomenon,
termed aggregation-induced emission (AIE). They observed that
1-methyl-1,2,3,4,5-pentaphenylsilole is practically non-lumi-
nescent in solutions but becomes highly emissive when aggre-
gated.12 In the subsequent studies, they determined that
restriction of intramolecular rotation (RIR) in aggregation state
is the major impetus for AIE effect.13 Based on RIR mechanism,
Tang and other groups have discovered a number of AIE-active
molecules including siloles, cyanostilbenes, and tetraphenyl-
ethene (TPE) derivatives. Among these molecules, tetraphenyl-
ethene derivatives enjoy the advantages of facile synthesis and
outstanding AIE effect, and are thus considered as good elec-
troluminescence (EL) materials for fabrication of OLEDs
devices.14
Recently, Tang and co-workers reported an AIE-active
molecular combining TPE and dimesitylboron moiety, which is
a promising bifunctional EL material with excellent electron-
transporting and light-emitting properties.15 In their subse-
quent study, they synthesized three AIE-active molecules
incorporating TPE and carbazole moiety which are prospective
bifunctional EL materials with excellent hole-transporting and
light-emitting properties.16 Their results demonstrated that
compounds containing TPE and dimesitylboron as well as TPE
and carbazole moieties are attractive materials for organic EL
devices attributing to their efficient luminescence and transport
performance. Inspired by their results, we are interested in
synthesis of a new class of compounds possessing AIE, hole-
transporting and electronic-transporting groups. This type of
compounds is anticipated to show good thermal stability and
multifunctional property. To our knowledge, there are only few
reports on this type of compounds.
Since Tang's innovative work on organic light-emitting diodes
(OLEDs) in 1987,1 OLED devices have attracted considerable
interest owing to their potential applications in full-colour at-
panel displays and solid state lighting.2–5 In order to obtain
high-performance OLEDs, many efforts have been made to
develop various novel organic materials with desirable proper-
ties. To date, the photo-electronic properties of OLEDs devices
have been improved greatly. Great success has been made in
developing green and red organic light-emitting materials.
However, it is still a challenge to design and synthesize blue
light-emitting materials of high efficiency, high colour purity,
and long operational time due to their wide energy gap. In other
words, the process of the injection of holes from the anode and
electrons from the cathode in blue light-emitting OLEDs devices
is difficult to balance.6–9 Moreover, the luminescence of many
organic luminophores is oen weakened or quenched at high
concentrations. This phenomenon is widely known as the
aSchool of Chemistry and Chemical Engineering, Shanxi University, 92 Wu Cheng
Road, Taiyuan 030006, Shanxi Province, China. E-mail: hepingshi@sxu.edu.cn; Fax:
+86-351-7011688; Tel: +86-351-7010588
bState Key Laboratory of Luminescent Materials and Devices, South China University of
Technology, Guangzhou 510640, Guangdong Province, China
cState Environmental Protection Key Laboratory of Efficient Utilization Technology of
Coal Waste Resources, Shanxi University, 92 Wucheng Road, Taiyuan 030006,
Shanxi Province, China. E-mail: cfangqin@sxu.edu.cn; Fax: +86-351-7018813; Tel:
+86-351-7018813
dPartner State Key Laboratory of Environmental and Biological Analysis, and
Department of Chemistry, Hong Kong Baptist University, 224 Waterloo Road,
Kowloon Tong, Hong Kong SAR, China. E-mail: mfchoi@hkbu.edu.hk; Fax: +852-
34117348; Tel: +852-34117839
Herein we rst report a novel compound based on tetra-
phenylethene (as aggregation-induced emission enhancement
(AIEE) group), carbazole (as hole-transporting group) and
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c4ra01925c
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dimesitylboron (as electronic-transporting group). This the excitation process. There is not much difference in the
compound, 3-dimesityboron-9-(4-(1,2,2-triphenylvinyl)phenyl)- spectral characteristics of DTPCZ in these solvents.
9H-carbazole (DTPCZ), was synthesized by introducing dimesi-
It has been reported that the AIE of some TPE derivatives in
tylboron and TPE moieties to the 3-position and 9-position of organic solvents could be induced in the presence of polar co-
carbazole, respectively. The resulting compound DTPCZ shows solvent.15 As such, the effect of water on AIE of DTPCZ in
unique AIEE as well as excellent EL, hole-transporting and different THF–water solvent mixtures was investigated. Herein,
electron-transporting properties. The OLEDs device using THF and water act as the solvent and non-solvent, respectively.
DTPCZ as the light emitting layer shows pure blue emission Fig. 1a depicts the photoluminescence (PL) spectra of DTPCZ in
with a luminance efficiency of 4.28 cd Aꢀ1
.
various THF–water solvent mixtures at an excitation wavelength
The target compound DTPCZ was obtained by a multi-step of 350 nm. DTPCZ displays very weak PL emission in pure THF
reaction. First, the key intermediate (3-bromo-9-(4-(1,2,2-tri- with a uorescence quantum yield (F) of 0.38% (the PL peak is
phenylvinyl)phenyl) carbazole) was synthesized via the modied at 425 nm). There is no signicant change in spectral charac-
Ullmann reaction of 1-(4-iodophenyl)-1,2,2-triphenylethene and teristics of DTPCZ when the water volume fraction (fw) of the
3-bromocarbazole in the presence of 18-crown-6, K2CO3 and CuI THF–water solvent mixtures is 0.0–60% v/v. However, the PL
ꢁ
catalyst system in DMF at 140 C with 19.4% yield, and then, peak shis to 475 nm (Table S1, ESI†) and the PL intensity
DTPCZ was obtained by reacting the key intermediate with increases drastically when (fw) is > 60% v/v. The higher the fw ¼
n-BuLi, followed with adding dimesitylboron uoride under N2 65–95% v/v, the stronger the emission intensity. When fw is 80%
at ꢀ78 ꢁC with 36% yield. The detailed synthetic route and v/v, a small increase in fw can promote a large increase in PL
1
chemical structure of DTPCZ conrmed by H and 13C nuclear intensity. When fw is 95% v/v, the PL intensity reaches the
magnetic resonance spectroscopy, mass spectrometry, and maximum. Fig. 1b depicts the plot of [(I/Io) ꢀ 1] against fw,
elemental analysis are presented in ESI.† Scheme 1 illustrates where Io and I are the PL intensities without and with water in
the synthetic route of DTPCZ.
the THF–water mixtures. It is obvious that water could induce
The photophysical properties of DTPCZ (5.0 mM) in various the PL of DTPCZ in THF–water solvent mixture, attributing to
solvent systems were initially studied. Fig. S1 (ESI)† depicts the the AIE of DTPCZ in high water content of solvent mixture.
absorption spectra of DTPCZ in n-hexane, dichloromethane,
The F of DTPCZ in fw ¼ 95% is 56%, which is enhanced by
tetrahydrofuran (THF), acetonitrile, and dimethylsulfoxide. Two 148 times in comparison to that in THF and the corresponding
absorption peaks at ca. 312 and 350 nm are observed in these PL peak intensity of DTPCZ at 475 nm increases by 256 times.
solvents, corresponding to p / p* transitions of skeleton and These data strongly indicate the prominent aggregation-
intramolecular charge transfer (ICT), respectively. The absorp- induced emission enhancement (AIEE) characteristics of
tion data are displayed in Table S1 (ESI).† The uorescence DTPCZ, which can be attributed to the RIR process in the
spectra of DTPCZ were depicted in Fig. S2 (ESI).† With the aggregate formation which populates the radioactive decay of
increasing polarity of the solvents, a bathochromic shi of the excitons and makes the luminogen uoresce strong.17
70 nm ranging from 383 nm (in hexane) to 453 nm (in CH3CN) DTPCZ in THF–water mixtures (fw ¼ 65–95% v/v) are macro-
was observed. Such a distinct solvatochromism indicated that scopically homogenous with no sign of precipitates, thereby
intramolecular charge transfer from the tetraphenylethene– suggesting the aggregates are nano-dimensional. The
carbazole moiety to dimesitylboron moiety takes place during morphology of these nano-aggregrates was investigated by SEM
Scheme 1 Synthetic route of DTPCZ.
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slightly on the backbone of carbazole and TPE. Thus, the elec-
tronic transition from the ground state to the rst excited state
mainly involves the ICT from the backbone of carbazole and
TPE as the electron-donor to the dimesitylboron moiety as the
electron-acceptor.
The thermal properties of DTPCZ were investigated by
thermal gravimetric analysis (TGA) and differential scanning
calorimetry (DSC), as displayed in Fig. S6 and S7 (ESI),†
respectively. The deꢁcomposition temperature (Td based on 5%
Fig. 1 (a) PL spectra of DTPCZ in various water contents of THF–water weight loss) at 241 C, the glass transition temperature (Tg) at
ꢁ
ꢁ
mixtures at excitation 350 nm. (b) Plot of (I/Io) ꢀ 1 against water volume
fraction (fw), where Io and I are the PL intensities without and with
water in the THF–water mixtures. The intensity was monitored at
excitation/emission wavelengths of 350/475 nm.
123 C and the melting temperature (Tm) at 202 C were deter-
mined from the TGA and DSC curves, respectively. These data
are summarized in Table S1 (ESI).† Our results indicate that
DTPCZ have excellent thermal properties which should be
adequate for optoelectronic device applications.
The electrochemical property of DTPCZ was investigated by
cyclic voltammetry. Fig. S8 (ESI)† depicts the cyclic voltammo-
gram (CV) of DTPCZ which exhibits two reversible oxidation and
reduction peaks. The oxidation peak at 0.76 V is attributing to
the oxidation of carbazole moiety and the other oxidation peak
at 1.35 V is from the oxidation of TPE moiety. The two reversible
reduction peaks at ꢀ0.897 and ꢀ0.475 V are corresponding to
the reduction of triarylboron and TPE moieties, respectively.
The CV curves remain unchanged under multiple successive
potential scans, indicating the excellent redox stability of
DTPCZ. The HOMO and LUMO energy level and energy band
gap (Eg) are determined from the CV curves and summarized in
Table S1 (ESI).† The energy band gap is in consistent with the
ICT band (vide supra) of DTPCZ.
In order to evaluate the potential application of DTPCZ as
luminescent material in EL device, a multi-layer device (Device
A) with the conguration of indium tin oxide (ITO)/N,N0-di-1-
naphthyl-N,N0-diphenylbenzidine (NPB) (60 nm)/DTPCZ
(20 nm)/2,20,200-(1,3,5-benzinetriyl)tris(1-phenyl-1-H-benzimid-
azole) (TPBi) (10 nm)/tris(8-hydroxyquinolinolato) aluminium
(Alq3) (30 nm)/LiF (1 nm)/Al (200 nm) was fabricated by vacuum
deposition, in which NPB and Alq3 functioned as the hole-
transporting layer and the electron-transporting layer, respec-
tively. TPBi and DTPCZ worked as the hole-blocking layer and
the emitting layer, respectively. Fig. 2a depicts the EL spectra of
Device A at driving voltages of 6.0–16.0 V, which remain
unchanged when the driving voltage increases from 6.0 to
16.0 V. The EL spectrum has a peak maximum of 464 nm and
CIE coordinates (0.18, 0.21) which is identical to the PL spec-
trum of the solid thin lm of DTPCZ. Fig. 2b and c display the
current density–voltage–luminance curve and luminance effi-
ciency–current density curve of Device A, respectively. Device A
exhibits good performance with a turn-on voltage of 6.0 V, a
maximum luminance of 4624 cd mꢀ2 at 16.2 V and a maximum
luminescent efficiency of 4.28 cd Aꢀ1 at 10.5 V. Table S3 (ESI)†
summarizes the EL properties of DTPCZ. The data indicate that
DTPCZ is a promising blue light emitting material for fabrica-
tion of OLEDs. Although the maximum luminance and lumi-
nance efficiency of the EL device of DTPCZ might not be as high
as in some of the blue EL devices reported before,15,16 the novel
compound is promising for application in OLEDs as emitting
and TEM. Fig. S3 (ESI)† shows the SEM image of the nano-
aggregrates of DTPCZ obtained from THF–water (1 : 9) mixture.
DTPCZ is highly aggregrated to form some nanoparticles with
diameters of less than 100 nm. The inset of Fig. S3† displays the
TEM image of the nano-aggregrates of DTPCZ. Spherical
nanoparticles of diameter ca. 50–100 nm is clearly observed.
These results conrm that DTPCZ could form nano-aggregrates
in high water contents of solvent mixtures and these nano-
aggregrates possess strong PL emission. DTPCZ does not form
nano-aggregrates such as in organic solvents and so no or very
weak PL is found. Thin lm of DTPCZ was fabricated on a glass
substrate and its PL property was taken. Fig. S4 (ESI)† displays
the PL spectrum of thin lm of DTPCZ at an excitation wave-
length of 350 nm. A strong blue emission band at 464 nm (Table
S1, ESI†) is observed which is similar to that of the nano-
aggregates of DTPCZ. Our results demonstrate that once the
DTPCZ molecules are aggregated, the PL properties of DTPCZ
will be produced and the PL emission band will depend on the
particle size and/or polydispersity of the nano-aggregates.
To obtain better insight on geometrical structure and elec-
tronic properties of DTPCZ was studied with DFT/B3LYP/6-31G
(d,p) method by using the polarized continuum model (PCM).
Coordinates of DDBICZ from DFT calculation are deposited in
Table S2 (ESI).† The optimal structure and the highest occupied
molecular orbital (HOMO) and lowest unoccupied molecular
orbital (LUMO) distributions of DTPCZ are shown in Fig. S5
(ESI).† The optimal geometry shows that the TPE moiety is
twisted from the plane of the carbazole and the dimesitylboron
moiety forms propeller-like conformation at the 3-position of
carbazole. We infer that the intramolecular rotation (IMR)
process of the TPE unit and dimesitylboron moiety of DTPCZ
can effectively consume the excited state energy, resulting in
non-radiative decay process of the luminogens in the solution
state, and when DTPCZ is aggregated as nano-suspensions or
solid lms, the intramolecular rotations are impeded and the
propeller-like conguration hinder the emission quenching
caused by intermolecular p–p stacking interaction, thus
endowing the aggregates with intense emission.
The electron density of the HOMO is localized on the back-
bone of carbazole and TPE while the electron density of the
LUMO is localized mainly on the dimesitylboron moiety and
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Fig. 2 (a) Electroluminescent spectra, (b) current density–voltage–luminance curves and (c) current density–luminance efficiency curve of
Device A.
material due to its excellent thermal, electrochemical and 10 (a) S. W. Thomas III, G. D. Joly and T. M. Swager, Chem. Rev.,
charge-transporting properties.
In summary, a novel compound (DTPCZ) with AIEE behav-
iour through combination of TPE, carbazole and dimesi-
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This work was supported by the Natural Science Foundation 14 (a) Y. Dong, J. W. Y. Lam, A. Qin, J. Liu, Z. Li, B. Z. Tang,
of Shanxi Province (2013011013-1); Open Fund of the State Key
Laboratory of Luminescent Materials and Devices, South China
University of Technology (2014-skllmd-09); Scientic and
Technological Innovation Programs of Higher Education Insti-
tutions in Shanxi Province (2012005); Hundred Talent Pro-
gramme of Shanxi Province; National Natural Scientic
Foundation of China (no. 21376144); Fund of Key Laboratory of
Optoelectronic Materials Chemistry and Physics, Chinese
Academy of Sciences (2008DP173016). The authors express their
sincere thanks to the Advanced Computing Facilities of the
Supercomputing Centre of Computer Network Information
Centre of Chinese Academy of Sciences for all the theoretical
calculations.
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