Two novel indolo[3,2-b]carbazole derivatives containing dimesitylboron moieties: Synthesis, photoluminescent and electroluminescent properties

Article abstract of DOI:10.1039/c4nj00140k

Two novel indolo[3,2-b]carbazole derivatives, 5,11-di(4′- dimesitylboronphenyl)indolo[3,2-b]carbazole (DDBICZ) and 2,8-dimesitylboron-5, 11-di(4′-dimesitylboronphenyl)indolo[3,2-b]carbazole (DDDBICZ), were synthesized by introducing two dimesitylboron groups and/or four dimesitylboron groups (as electron-acceptors) into the indolo[3,2-b]carbazole moiety (as an electron-donor). The structures of these two compounds were fully characterized by elemental analysis, mass spectrometry and proton nuclear magnetic resonance spectroscopy methods. Their thermal properties were studied by thermogravimetric analysis and differential scanning calorimetry. Their electrochemical and photophysical properties were studied by electrochemical methods, UV-vis absorption spectroscopy and fluorescence spectroscopy. The charge-transporting properties of DDBICZ and DDDBICZ were studied by fabricating single carrier devices using them as charge-transporting layers. The results reveal that DDBICZ and DDDBICZ have high thermal stability (the decomposition temperature of DDBICZ = 201 °C and the decomposition temperature of DDDBICZ = 210 °C) and good electrochemical and electron-transporting properties. Moreover, in order to examine the electroluminescent properties of DDBICZ and DDDBICZ, Device A and Device B were fabricated by using them as light-emitting layers, respectively. The turn-on voltage, maximum luminance and maximum luminance efficiency of Device A are 6.1 V, 5634 cd m^-2 and 2.96 cd A ^-1, whereas those of Device B are 3.6 V, 1363 cd m^-2 and 2.88 cd A^-1. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c4nj00140k

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Two novel indolo[3,2-b]carbazole derivatives
containing dimesitylboron moieties: synthesis,
photoluminescent and electroluminescent
properties†
Cite this: New J. Chem., 2014,
38, 2368
Heping Shi,*ab Jiandong Yuan,a Xiaohuan Wu,a Xiuqing Dong,c Li Fang,a
Yanqin Miao,d Hua Wang*d and Fangqin Cheng*c
Two novel indolo[3,2-b]carbazole derivatives, 5,11-di(40-dimesitylboronphenyl)indolo[3,2-b]carbazole
(DDBICZ) and 2,8-dimesitylboron-5,11-di(40-dimesitylboronphenyl)indolo[3,2-b]carbazole (DDDBICZ), were
synthesized by introducing two dimesitylboron groups and/or four dimesitylboron groups (as electron-
acceptors) into the indolo[3,2-b]carbazole moiety (as an electron-donor). The structures of these two
compounds were fully characterized by elemental analysis, mass spectrometry and proton nuclear
magnetic resonance spectroscopy methods. Their thermal properties were studied by thermogravimetric
analysis and differential scanning calorimetry. Their electrochemical and photophysical properties were
studied by electrochemical methods, UV-vis absorption spectroscopy and fluorescence spectroscopy. The
charge-transporting properties of DDBICZ and DDDBICZ were studied by fabricating single carrier devices
using them as charge-transporting layers. The results reveal that DDBICZ and DDDBICZ have high thermal
stability (the decomposition temperature of DDBICZ = 201 1C and the decomposition temperature of
DDDBICZ = 210 1C) and good electrochemical and electron-transporting properties. Moreover, in order to
examine the electroluminescent properties of DDBICZ and DDDBICZ, Device A and Device B were
fabricated by using them as light-emitting layers, respectively. The turn-on voltage, maximum luminance
and maximum luminance efficiency of Device A are 6.1 V, 5634 cd mꢀ2 and 2.96 cd Aꢀ1, whereas those
Received (in Montpellier, France)
28th January 2014,
Accepted 21st February 2014
DOI: 10.1039/c4nj00140k
of Device B are 3.6 V, 1363 cd mꢀ2 and 2.88 cd Aꢀ1
.
www.rsc.org/njc
develop various novel organic materials with desirable properties.8–10
Among these high-performance materials, indolo[3,2-b]carbazole
1. Introduction
Since the seminal work on organic light-emitting diodes (OLEDs) (ICZ) derivatives and boron-containing p-conjugated compounds
by Tang and VanSlyke,1 organic electroluminescent (EL) devices are the popular ones due to their excellent performance in EL
have gained enormous attention owing to their promising devices.
applications in solid-state lighting and full-color flat-panel
Indolo[3,2-b]carbazole (ICZ) derivatives have received much
display devices.2–7 With the purpose of improving the efficiency attention due to their large planar and rigid conjugated structures
and stability of the OLEDs, many efforts have been made to together with their remarkable photophysical properties. Many ICZ
derivatives with outstanding properties such as better morphological
a School of Chemistry and Chemical Engineering, Shanxi University,
92 Wucheng Road, Taiyuan 030006, Shanxi Province, China.
E-mail: hepingshi@sxu.edu.cn; Fax: +86-351-7011688; Tel: +86-351-7010588
b State Key Laboratory of Luminescent Materials and Devices, South China
University of Technology, Guangzhou, Guangdong, 510640, PR China
c State 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
stability and thermal durability as well as desirable charge-injecting
and transporting properties have been reported during the past few
decades. Ong and his co-workers reported a series of ICZ derivatives,
which could serve as excellent hole-transporting materials and
organic thin-film transistor (OTFT) materials.11–14 Tao et al.
designed and synthesized several ICZ derivatives, which can be
used as excellent luminescence and hole-transporting materials
in the OLEDs.15–17 Boudreault et al. synthesized some new
materials based on the ICZ framework, which were particularly
suitable for organic field-effect transistors (OFETs).18,19 Grazulevicius
and his co-workers developed various N,N-diarylated ICZ derivatives
and these compounds can serve as hole-transporting materials for
d Key Laboratory of Interface Science and Engineering in Advanced Materials,
Taiyuan University of Technology, Taiyuan 030024, Shanxi Province, China.
E-mail: wanghua001@tyut.edu.cn
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
c4nj00140k
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OLEDs.20–24 Recently, Chen et al. reported two new ICZ derivatives an excellent hole-transporting material in OLEDs,28 and BDPA-2
with multifunctionality, which were employed as deep-blue emitters, and BDPA-3 can be used as both blue-emitting and electron-
hole-transporting materials and hosts to fabricate organic light- transporting materials in OLEDs.53 In addition, they investigated the
emitting devices.25
use of triarylboron-containing metal complexes as phosphorescent
Boron-containing p-conjugated compounds have emerged materials in OLEDs.59–63 Besides Wang and co-workers, many
during the past decade. They have high fluorescence quantum other research groups have devoted great efforts to explore
yields owing to the overlap between the vacant p-orbital of the triarylborane compounds for use in OLEDs. Among them,
boron center and the conjugated moieties,26–34 which make these Lambert and co-workers synthesized
a series of amino-
compounds a promising class of candidates for optoelectronic substituted triarylboranes (TABs) and investigated their electro-
materials. Generally, for the application of organoboron compounds chemical and photophysical properties.64 Later, they synthesized
in optoelectronic materials and devices, the boron center must polymers based on the carbazole-substituted TABs mentioned
resist nucleophilic attack and hydrolysis. Researchers have above.65 They found that the 3,6-linked polycarbazole is a
readily achieved this by functionalizing the boron center with promising candidate for application in OLEDs. Despite all those
bulky aryl substituents.35 Thus, triarylboron compounds containing efforts made by different groups, the field of boron-containing
a dimesitylboron group and/or several dimesitylboron groups have optoelectronic materials still remains largely unexplored.
been successfully developed as materials for nonlinear optics36,37
As mentioned above, compounds containing the ICZ moiety
and anion sensing38–47 and utilized as emitting and/or electron- and/or the dimesitylboron moiety are attractive materials for the
transporting materials for OLEDs.48–57 Shirota and co-workers first organic electroluminescent (EL) devices due to their efficient
reported the use of triarylboranes in OLEDs. They designed and luminescence and transport performance. Thus we became
synthesized a series of emitting materials with desired bipolar interested in incorporating the ICZ moiety and the dimesityl-
character containing a dimesitylboron group.50,51 Inspired by the boron moiety into a single molecule to obtain novel materials
work done by Shirota’s group, Wang and co-workers synthesized with excellent luminescent, hole-transporting and electron-
various donor–acceptor type triarylboranes as blue emitting transporting properties.
materials, including compounds based on BNPB,54,55 DPA and
In this paper, two new ICZ derivatives, 5,11-di(40-dimesityl-
AZAIN systems.53,58 They evaluated the performances of those boronphenyl)indolo[3,2-b]carbazole (DDBICZ) and 2,8-dimesityl-
compounds by fabricating EL devices. Their results showed that boron-5,11-di(40-dimesitylboronphenyl)indolo[3,2-b]carbazole
BNPB-2 can act as either a bifunctional electron-transporting (DDDBICZ), were synthesized by introducing dimesitylboron
and blue-emitting material in a simple double-layer EL device or groups into the ICZ moiety. The synthetic route is presented in
a trifunctional electron-transporting, hole-transporting and blue- Scheme 1. To the best of our knowledge, these compounds have
emitting material in a triple-layer EL device,54 BNPB-5 can act as never been reported before. The structures of DDBICZ and
Scheme 1 The synthetic routes for preparation of DDBICZ and DDDBICZ.
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DDDBICZ were characterized by 1H NMR, 13C NMR, MS and washed with water and EtOH to neutral pH. Finally, dry and
elemental analysis. The thermal, electrochemical, photoluminescent pure 2,8-dibromoindolo[3,2-b]carbazole (compound 4) (4.88 g,
1
properties were determined by thermogravimetric analysis yield 52.8%) was achieved. Mp 4 300 1C, H NMR (600 MHz
combined with electrochemistry, UV-vis absorption spectroscopy DMSO-d6) d (ppm): 11.14 (2H, s, and N–H) and 7.1–8.24 (8H, m,
and fluorescence spectroscopy. Furthermore, the electro- and Ar–H).
luminescent properties were characterized by the devices fabricated
using these compounds.
2.1.5. Synthesis of 5,11-di(40-bromophenyl)indolo[3,2-b]-
carbazole (compound 5). Indolo[3,2-b]carbazole (2.563 g,
10 mmol) and 1-bromo-4-iodobenzene (8.487 g, 30 mmol) were
dissolved in 80 mL of dry DMF under a nitrogen atmosphere.
Then powdered potassium carbonate (11.040 g, 80 mmol), Cu
(2.921 g, 46 mmol) and 18-crown-6 (1.056 g, 4 mmol) were
added into it. The mixture was stirred for 30 min at room
2. Experimental
2.1. Synthesis of compounds
The synthesis of compounds cyclohexane-1,4-dione-diphenyl hydra- temperature and then heated up slowly to 150 1C and kept for
zone (1), cyclohexane-1,4-dione-bis( p-bromophenyl)-hydrazone (2), 48 h. Afterwards, the reaction mixture was cooled down to room
indolo[3,2-b]carbazole (3) and 2,8-dibromoindolo[3,2-b]carbazole temperature and poured into ice water with stirring 1 h. The
(4) was performed according to or slightly modified literature precipitate was collected and recrystallized with EtOH and
procedures.66
dried finally. The residue was purified by flash chromatography
2.1.1 Synthesis of cyclohexane-1,4-dione-diphenyl hydrazone on silica gel (CH2Cl2/hexane) to afford compound 5. Yield: 50%,
(compound 1). To a solution of 1,4-cyclohexanedione (5 g, Mp: 4300 1C, 1H NMR (600 MHz CDCl3) d (ppm): 8.118 (2H, d,
44.5 mmol) in absolute EtOH (100 mL) were added 90 mL of J = 7.8), 8.033–7.992 (3H, m, J = 7.2), 7.805 (3H, d, J = 8.4), 7.575
absolute ethyl alcohol and 0.1 mL of acetic acid. The reaction (3H, d, J = 8.4), 7.359–7.384 (5H, m, J = 9.0), 7.273 (1H, d, J = 7.2),
mixture was stirred for 20 min, and then 10.73 mL of phenyl- 6.981 (1H, s).
hydrazine was added dropwise. The reaction mixture was heated
2.1.6 Synthesis of 2,8-dibromo-5,11-di(40-bromophenyl)indolo-
to 35 1C and kept at the same temperature for 30 min and then [3,2-b]carbazole (compound 6). To a solution of 2,8-dibromoindolo-
cooled. The precipitate was slowly filtered off, washed carefully [3,2-b]carbazole (2.76 g, 6.7 mmol) in anhydrous DMF (80 mL) in
with water and dried to give cyclohexane-1,4-dione-diphenyl the round-bottomed flask, 1-bromo-4-iodobenzene (5.64 g,
hydrazone (7.13, 54.9%) as a yellow powder.
20 mmol) was added under a nitrogen atmosphere. Then powdered
2.1.2 Synthesis of cyclohexane-1,4-dione-bis( p-bromophenyl)- potassium carbonate (7.36 g, 53 mmol), CuI (0.38 g, 2 mmol) and
hydrazone (compound 2). 1,4-Cyclohexanedione (5.62 g, 50 mmol) 18-crown-6 (0.704 g, 2.7 mmol) were added into it. The mixture was
was dissolved in absolute ethanol (EtOH) (100 mL) and was added heated up slowly to 170 1C after stirring for 30 min at room
with stirring to a mixture of powdered 4-bromophenylhydrazine temperature and kept for 48 h. Then, the reaction mixture was
hydrochloride (22.4 g, 100 mmol), sodium acetate (8.2 g, 100 mmol) cooled down to room temperature and poured into ice water with
and absolute ethanol (200 mL) at room temperature. The mixture stirring for 1 h. The precipitate was collected and recrystallized with
was quickly heated to 50 1C, and then the temperature was cooled EtOH and dried. The crude product was purified by flash chromato-
to 0 1C. The precipitate was filtered off and washed carefully with graphy on silica gel (CH2Cl2/hexane) to afford compound 6. Yield:
1
water to give 2 (yield 59%).
28.7%, Mp: 4300 1C, H NMR (600 MHz CDCl3) d (ppm): 8.674
2.1.3. Synthesis of indolo[3,2-b]carbazole (compound 3). (1H, s), 8.375–8.363 (2H, t, J = 4.2), 8.074 (1H, d, J = 7.8), 7.918 (2H,
Cyclohexane-1,4-dione-bisphenyl hydrazone (10 g, 0.036 mmol) d, J = 7.8), 7.714 (4H, t, J = 7.8), 7.606–7.513 (3H, m, J = 7.8), 7.349
was added portionwise to a mixture of AcOH and H2SO4 (4 : 1, (2H, d, J = 9.0), 7.240 (1H, d, J = 9.0).
50 mL) with stirring at 10 1C. Thereafter, the mixture was
2.1.7. Synthesis of 5,11-di(40-dimesitylboronphenyl)indolo-
heated to 30 1C and kept for 30 min. Afterwards, the tempera- [3,2-b]carbazole (DDBICZ). Compound 5 (0.846 g, 1.5 mmol)
ture was increased slowly to 60–70 1C and the mixture changed was suspended in anhydrous THF (60 mL) under a nitrogen
colour from bright raspberry to grey-green. After half an hour, atmosphere and the solution was cooled to ꢀ78 1C with
the reaction mixture was refluxed for another 30 min with the stirring. To this solution, butyllithium (1.7 mL, 2.2 M in
colour changing to yellow. Then the mixture was allowed to hexanes) was injected and the resulting solution was stirred
cool to room temperature and the precipitate was slowly filtered for 30 min at this temperature. Afterwards, the reaction
off and was washed carefully with water and EtOH, and dried to solution was raised to room temperature and kept so for 6 h.
give the target compound (5.74 g, 62.3%) as a yellow solid.
Then the solution was cooled to ꢀ78 1C again and dimesityl-
2.1.4. Synthesis of 2,8-dibromoindolo[3,2-b]carbazole boron fluoride (1.005 g, 3.75 mmol) was quickly added into the
(compound 4). Compound 2 (10 g, 22 mmol) was added into stirred reaction mixture under a nitrogen atmosphere. The
a mixture of AcOH (130 mL) and H2SO4 (32 mL) at 0 1C and mixture was allowed to warm to room temperature with stirring
stirred for 5 min. The mixture was heated up to 30 1C and kept for 48 h. Afterwards, the solvent was removed. The residue was
stirring for 1 h. Afterward, the temperature of the mixture was diluted with CH2Cl2 and the organic layer was collected. The
further increased up to 60–70 1C and stirred for another 1 h. combined organic phase was evaporated in vacuo and the
Then it was cooled down to room temperature and poured into residue was washed carefully with Et2O and EtOH, filtered off
ice water with stirring. The greenish solid was filtered off and and dried to get the crude product, and the crude product was
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purified by flash chromatography on silica gel (CH2Cl2/hexane) electrochemical analyser. The CV measurements were carried
to obtain DDBICZ (0.464 g, 34%) as a light yellow solid. 1H NMR out with a conventional three-electrode configuration consisting
(600 MHz CDCl3) d (ppm): 8.146 (4H, dd, J = 19.8, 22.2), 7.815 of a glassy carbon working electrode, a platinum-disk auxiliary
(4H, d, J = 7.8), 7.700 (4H, d, J = 4.2), 7.527 (2H, d, J = 4.2), 7.415 electrode and a Ag/AgCl reference electrode, and the scan speed
(4H, t, J = 7.2), 6.892 (8H, s), 2.345 (12H, s), 2.150 (24H, s). 13C was 50 mV sꢀ1. UV-vis absorption spectra were acquired using a
NMR (600 MHz CDCl3) d (ppm): 144.612, 144.521, 144.480, Shimadzu UV-2450 spectrophotometer. Fluorescence spectra
143.787, 141.773, 141.081, 139.551, 132.900, 132.793, 130.239, were obtained using a Hitachi F-4500 spectrofluorometer.
130.018, 129.050, 128.983, 128.904, 128.800, 126.797, 126.672, Fluorescence quantum yield was determined by using quinine
123.211, 123.159, 123.107, 122.707, 122.657, 122.611, 122.100, sulfate as the reference. All measurements were performed at
112.690, 112.644, 103.106, 102.973, 102.888, 26.454, 24.119. MS room temperature.
(m/z): 904.6 (M+). Anal. Calcd for C66H62B2N2: C, 87.61%; H,
2.3 Device fabrication and testing
6.91%; N, 3.10%. Found: C, 87.69%; H, 6.87%; N, 3.14%.
2.1.8 Synthesis of 2,8-dimesitylboron-5,11-di(40-dimesityl-
boronphenyl)indolo[3,2-b]carbazole (DDDBICZ). Compound 6
(0.5 g, 0.69 mmol) was dissolved in anhydrous THF (60 mL)
under a nitrogen atmosphere and the resulting solution was
cooled to ꢀ78 1C with stirring. Then, n-butyllithium (5.3 mL,
1.3 M in hexanes) was added to the stirred solution at this
temperature. After 30 min, the reaction solution was raised to
room temperature and kept so for 6 h. Then, the solution was
cooled to ꢀ78 1C again and dimesitylboron fluoride (2.22 g,
8.28 mmol) was quickly added into the stirred reaction mixture
under a nitrogen atmosphere. The mixture was allowed to
warm to room temperature with stirring for 48 h. Afterwards,
the solvent was evaporated and the residue was extracted with
water and CH2Cl2. The organic layer was collected and evapo-
rated in vacuo, and then the residue was washed carefully with
Et2O and EtOH, filtered off and dried to get the crude product.
Finally, the crude product was purified by flash chromatogra-
phy on silica gel (CH2Cl2/hexane) to obtain DDDBICZ (0.268 g,
28%) as a light yellow solid. 1H NMR (600 MHz DMSO) d (ppm):
8.335 (2H, s), 8.068 (2H, s), 7.802 (4H, d, J = 7.8), 7.651 (4H, d, J =
8.4), 7.589 (2H, d, J = 8.4), 7.368 (2H, d, J = 8.4), 6.874 (8H, s),
6.842 (8H, s), 2.346 (12H, s), 2.327 (12H, s), 2.104 (24H, s), 2.049
(24H, s). 13C NMR (600 MHz DMSO) d (ppm): 145.15, 144.533,
143.847, 143.795, 143.725, 141.759, 141.227, 141.045, 140.184,
138.772, 133.096, 132.766, 131.982, 131.213, 131.035, 130.427,
129.231, 127.019, 126.571, 112.056, 103.872, 32.563, 28.493,
26.489, 26.405, 24.095. MS (m/z): 1401.5 (M+). Anal. Calcd for
The multilayer OLEDs were fabricated by the vacuum-deposition
method. Organic layers were deposited by high-vacuum (5 ꢁ
10ꢀ4 Pa) thermal evaporation onto a glass (3 cm ꢁ 4 cm) substrate
pre-coated with an indium tin oxide (ITO) layer. N,N-bis(naphthyl)-
N,N-bis(phenyl)benzidine (NPB), 4,4-cyclohexylidene-bis[N,N-bis-
(4-methylphenyl)benzenamine] (TAPC), DDBICZ/DDDBICZ,
2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP), 4,7-diphenyl-
1,10-phenanthroline (BPhen) and LiF/Al were used as the
hole-transporting layer, electron-blocking layer, emitting layer,
hole-blocking layer, electron-transporting layer and cathode,
respectively. All organic layers were sequentially deposited.
Thermal deposition rates for organic materials, LiF and Al were
0.5, 0.5 and 1 Å sꢀ1, respectively. The active area of the devices
was 12 mm2. The electroluminescent spectra were measured
using a Hitachi MPF-4 spectrofluorometer. The voltage–current
density (V–J) characteristics of OLEDs were recorded using a
Keithley 2400 Source Meter. The characterization of brightness–
current–voltage (B–I–V) were measured with a 3645 DC power
supply combined with a 1980A spot photometer and were
recorded simultaneously. All measurements were done at room
temperature under ambient conditions.
2.4. Theoretical calculations
The ground-state geometries of DDBICZ and DDDBICZ were
optimized at the B3LYP level with the 6-31G(d,p) basis set.67–69
Their vibration frequencies and frontier molecular orbital
characteristics were analysed on the optimized structure at the
same level. Solvent effect was also taken into account by using
the polarised continuum model.70,71 All calculations were carried
out using the Gaussian 03 program package72 and performed in
the Supercomputing Centre of Computer Network Information
Centre of the Chinese Academy of Sciences.
C
102H104B4N2: C, 87.43%; H, 7.48%; N, 2.00%. Found: C,
87.37%; H, 7.51%; N, 2.05%.
2.2 Measurement and characterization
Melting points were determined using an X-5 melting point
detector. All NMR spectra were recorded on a Bruker 600 MHz
spectrometer. Thermogravimetric analysis (TGA) was per-
formed using a TGA 2050 thermogravimetric analyser under a
N2 atmosphere with a heating rate of 10 1C minꢀ1 from room
temperature to 750 1C. Differential scanning calorimetry (DSC) was
3. Results and discussion
3.1 Synthesis
performed using a Q2000 DSC differential scanning calorimeter As shown in Scheme 1, the syntheses of indolo[3,2-b]carbazole
under a N2 atmosphere with a heating rate of 10 1C minꢀ1 from derivatives comprising dimesitylboron moieties were carried
room temperature to 250 1C. Elemental analyses were performed out using a multi-step synthetic route. Firstly, compound 1
using an Element Analysis System. Mass spectra were recorded was synthesized via the reaction of 1,4-cyclohexanedione with
using a LC-MS system consisting of a Waters 1525 pump and a phenylhydrazine. Then compound 3 was prepared by the Fischer
Micromass ZQ4000 single quadrupole mass spectrometer detector. reaction of compound 1 in a mixture of acetic acid and sulfuric
Cyclic voltammetry (CV) was performed using a CHI-600C acid at 65 1C in high yield. Secondly, the intermediate compound
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5 was prepared by the reaction of indolo[3,2-b]carbazole with DSC curves of DDBICZ and DDDBICZ are shown in Fig. 1 and 2,
1-bromo-4-iodobenzene in the presence of Cu, 18-crown-6 and respectively. The decomposition temperatures (Td) were
potassium carbonate at 170 1C with 50% yield. Finally, DDBICZ observed for DDBICZ (Td = 201 1C) and DDDBICZ (Td
=
was synthesized by the reaction of compound 5 with n-BuLi and 210 1C) by TGA analyses. The glass transition temperatures
dimesitylboron fluoride at ꢀ78 1C with 34% yield. The synthetic (Tg) were observed for DDBICZ (Tg = 145 1C) and DDDBICZ (Tg =
route for preparation of DDDBICZ was similar to that of DDBICZ. 162 1C) by DSC analyses. These data are also collected in
DDDBICZ was obtained in 28% yield. DDBICZ and DDDBICZ Table 1. The high Td and Tg of DDBICZ and DDDBICZ can be
were characterized by 1H NMR, 13C NMR, MS and elemental attributed to their large molecular mass, rigid indolo[3,2-b]carbazole
analysis. The 1H NMR spectra of DDBICZ and DDDBICZ cores and non-planar dimesitylboron peripheries. Our results
displayed downfield peaks at 7.00–8.48 ppm, which were indicate that these two compounds have excellent thermal
assigned to the protons of the 5,11-diphenylindolo[3,2-b]carbazole properties which should be adequate for most optoelectronic
moiety. The downfield peaks at 6.59–6.89 ppm and the upfield peaks device applications.
at 2.09 and 2.37 ppm were assigned to the protons of dimesitylboron
moieties. The 13C NMR, mass spectrometry and elemental analysis
3.3 Theoretical calculations
were consistent with the desired structures. Further details are
given in the Experimental section. DDBICZ and DDDBICZ are
soluble in many common organic solvents such as dichloro-
methane, chloroform, DMF, THF, toluene and DMSO.
Theoretical calculations can provide a reasonable qualitative
indication of the excitation and emission properties of a
compound.73 The ground-state geometries of DDBICZ and
DDDBICZ were optimized at the B3LYP level with the 6-31G(d,p)
basis set. The vibration frequencies and the frontier molecular
orbital characteristics were analyzed on the optimized structures
3.2 Thermal properties
The thermal properties of DDBICZ and DDDBICZ were investigated at the same level. The molecular structure of DDBICZ contains
by thermogravimetric analysis (TGA) and differential scanning a 5,11-diphenylindolo[3,2-b]carbazole unit as the core and two
calorimetry (DSC) under a nitrogen atmosphere. The TGA and dimesitylboron groups as the terminals. As shown in Fig. 3, the
Fig. 1 TGA curves of DDBICZ and DDDBICZ.
Fig. 2 DSC curves of DDBICZ and DDDBICZ.
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Table 1 Physical properties of compounds DDBICZ and DDDBICZ
HOMO (eV)
Theo
LUMO (eV)
Theo
Eg (eV)
Compounds
labs (nm)
lem (nm)
F
Exp
Exp
Theo
Exp
Td (1C)
Tg (1C)
DDBICZ
337
398
345
382
474
0.52
201
210
145
ꢀ5.20
ꢀ5.34
ꢀ5.27
ꢀ5.28
ꢀ1.97
ꢀ2.02
ꢀ2.38
ꢀ2.32
3.23
3.32
2.89
2.96
DDDBICZ
453
0.48
162
l
abs, absorption peak maximum; lem, emission peak maximum in film; F, fluorescent quantum yield in THF; HOMO: highest occupied molecular
orbital; LUMO: lowest unoccupied molecular orbital; Eg, energy gap between HOMO and LUMO; Td, decomposition temperature; Tg, glass
transition temperature
of LUMO and HOMO, which is essential for the efficient hole
and electron transport.75 The calculated HOMO and LUMO
energy levels and the energy gap of DDBICZ are ꢀ5.20, ꢀ1.97
and 3.23 eV (Table 1), while those of DDDBICZ are ꢀ5.34, ꢀ2.02 eV
and 3.32 eV (Table 1), respectively. The theoretical HOMO and
LUMO levels of DDBICZ and DDDBICZ are higher than the
experimental ones, the HOMO–LUMO energy gaps of DDBICZ
and DDDBICZ in theory are ca. 0.34 eV and 0.36 eV larger than
those of the optical energy bands obtained from UV-vis absorption
measurements, respectively.
3.4 Photophysical properties
Fig. 4a shows the absorption spectra of DDBICZ in various
solvents (1.0 ꢁ 10ꢀ5 M). The absorption spectra data are
summarized in Table 1. As depicted in Fig. 4a, DDBICZ exhibits
two major absorption bands at 290–350 and 350–429 nm,
respectively. The absorption peak maximum (labs) of 337 nm
corresponds to the p–p* electronic transition of the indolo[3,2-b]-
carbazole skeleton and the labs of 398 nm corresponds to intra-
molecular charge transfer (ICT) from the indolo[3,2-b]carbazole
Fig. 3 HOMO and LUMO diagrams of DDBICZ and DDDBICZ.
core to dimesitylboron terminals. Fig. 4b shows the absorption
spectra of DDDBICZ in various solvents (1.0 ꢁ 10ꢀ5 M). The
absorption spectra data are also summarized in Table 1. As shown
indolo[3,2-b]carbazole moiety has planar structure, the two in Fig. 4b, the absorption spectra of DDDBICZ are similar to those
phenyl groups present a twisted structure relative to this plane of DDBICZ in various solvents. The optical energy band gap of
and the two mesityl groups form propeller-like conformations DDBICZ calculated from the absorption band edge of the absorp-
originating from the trigonal boron center. The highest occupied tion spectrum is approximately 2.89 eV, while that of DDDBICZ is
molecular orbital (HOMO) and the lowest unoccupied molecular approximately 2.96 eV. The normalized fluorescence spectra of
orbital (LUMO) of DDBICZ were also calculated at the DFT/ DDBICZ in various solvents (1.0 ꢁ 10ꢀ5 M) are shown in Fig. 5a
B3LYP/6-31G(d,p) level using the polarized continuum model for and the data are also collected in Table 1. As shown in Fig. 5a,
geometry optimization. Coordinates of DDBICZ are collected in DDBICZ exhibits strong fluorescence emission bands at 390–
Table S1 (ESI†). The HOMO of DDBICZ is localized mainly in the 670 nm in various solvents. The emission peak maximums (lem
)
indolo[3,2-b]carbazole moiety, while the LUMO is localized in the exhibit a red-shift of 115 nm ranging from 425 nm (in hexane) to
dimesitylborylphenyl moiety (Fig. 1a). The molecular structure of 540 nm (in DMSO) with the increasing polarity of the solvents,
DDDBICZ contains a 5,11-diphenylindolo[3,2-b]carbazole unit as which is assigned to p–p* transition of the intramolecular charge
the core and four dimesitylboron groups as the terminals. The transfer (ICT) from the indolo[3,2-b]carbazole core to the dimesi-
geometry of DDDBICZ at the ground state was optimized using tylboron terminals in the excitation process. The fluorescence
the same method. Coordinates of DDDBICZ are collected in spectra of DDDBICZ show similar characteristics to those of
Table S2 (ESI†). Fig. 1b shows that the geometry of DDDBICZ is DDBICZ in various solvents (Fig. 5b). The emission bands are
similar to DDBICZ and the distributions of the HOMO and the located at 390–660 nm with a red-shift of 95 nm ranging from
LUMO of DDDBICZ are similar to those of DDBICZ, respectively. 423 nm (in hexane) to 518 nm (in DMSO). The fluorescence
Generally, holes and electrons in OLEDs are transferred through quantum yields (F) of DDBICZ and DDDBICZ in several solvents
the individual HOMO and LUMO levels.74 Fig. 1a and b show (1.0 ꢁ 10ꢀ6 M) were measured by using quinine bisulfate in
that DDBICZ and DDDBICZ exhibit almost complete separation 0.10 M sulfuric acid as the reference76 and collected in Table 1.
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Fig. 4 The UV-vis absorption spectra of DDBICZ and DDDBICZ in several solvents.
Fig. 5 The normalized fluorescence spectra of DDBICZ and DDDBICZ in several solvents.
As shown in Table 1, DDBICZ and DDDBICZ have relatively high reduction peak are observed within the entire electrochemical
fluorescence quantum yields, revealing their potential application window of acetonitrile. The reversible oxidation peak at around
as excellent optoelectronic materials in the optical field.
+1.12 V is assigned to the oxidation of indolo[3,2-b]carbazole,
whereas the reversible reduction peak at around ꢀ1.23 V is
attributed to the electron injection into the vacant p-orbital of
3.5 Electrochemical properties
The electrochemical properties of DDBICZ and DDDBICZ were dimesitylboron. The CVs of DDDBICZ are displayed in Fig. 6b.
studied by cyclic voltammetry (CV) with Ag/AgCl as the reference As shown in Fig. 6b, its reversible oxidation and reduction peak
electrode. The measurement was conducted in 1.0 mM DDBICZ is at +1.18 V and ꢀ1.25 V, respectively. The electrochemical
and/or DDDBICZ in acetonitrile containing 0.10 M tetrabutyl- behaviors of DDDBICZ are similar to those of DDBICZ.
ammonium perchlorate as the supporting electrolyte under The HOMO energy level can be calculated with the empirical
N2 atmosphere. The CVs of DDBICZ are displayed in Fig. 6a. equation: HOMO = ꢀ(Eox + 4.40) eV, where Eox is the onset
As shown in Fig. 6a, a reversible oxidation peak and one reversible oxidation potential.77 The Eox of DDBICZ is 0.87 and the HOMO
Fig. 6 The cyclic voltammogram curves of DDBICZ and DDDBICZ.
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energy level of DDBICZ is ꢀ5.27 eV. The Eg is estimated to be devices fabricated by using DDBICZ (Fig. 7b). Our results are in
2.89 eV by the absorption edge of the absorption spectrum of good agreement with the results obtained by Lin et al.80 Thus, these
DDBICZ. The LUMO energy level of DDBICZ is thus ꢀ2.38 eV two compounds are promising candidates as electron-transporting
which is calculated from the HOMO energy level and Eg. materials.
The HOMO, LUMO and Eg of DDBICZ are very close to the
3.7 Electroluminescent properties
theoretical values (Table 1). Similar to DDBICZ, the HOMO,
LUMO and Eg of DDDBICZ are also very close to the theoretical
values (Table 1). The LUMO levels of DDBICZ and DDDBICZ are
as low as those of two common electron-transporting materials,
1,3,5-tris(1-phenyl-1H-benzimidazol-2-yl)benzene (2.39 eV) and
2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP) (2.44 eV),
indicating that these two compounds have small barrier of
electron transport and can be used as effective electron-
transporting materials for OLEDs.78 Furthermore, the CV
curves of the compounds remained unchanged under multiple
successive potential scans, indicating that the compounds have
excellent redox properties.79
In order to evaluate DDBICZ and/or DDDBICZ as potential
luminescent materials for their application in OLEDs, two
multilayer organic electroluminescence (EL) devices with the
configuration of ITO/NPB (50 nm)/DDBICZ (50 nm)/Bphen
(20 nm)/LiF (0.5 nm)/A1 (Device A) and ITO/NPB (50 nm)/
DDDBICZ (50 nm)/Bphen (20 nm)/LiF (0.5 nm)/A1 (Device B)
were fabricated, where NPB and Bphen were used as the hole-
transporting layer and the electron-transporting layer, respec-
tively. The EL spectra of Device A measured at different voltages
are shown in Fig. S1 (ESI†), which are in good agreement with
those of DDBICZ films. (The PL spectra of DDBICZ films are
displayed in Fig. S2 (ESI†).) The EL spectra of Device A exhibit
the same pure blue emissions at different voltages with a
3.6 Charge-transporting properties
In order to further demonstrate the charge-transporting properties maximum at 472 nm and CIE coordinates of x = 0.22 and y =
of DDBICZ and DDDBICZ, several single carrier devices were 0.32. The current density–voltage–luminance curves of Device A are
prepared. First, two hole-only devices, ITO/TAPC (8 nm)/DDBICZ shown in Fig. 8a and the luminance efficiency–current density
(30 nm)/TAPC (8 nm)/Al and ITO/TAPC (8 nm)/DDDBICZ (30 nm)/ curve of Device A is shown in Fig. 8b. The electroluminescent data
TAPC (8 nm)/Al, were prepared to examine the hole-transporting for Device A are summarized in Table 2. As shown in Table 2, the
capability of DDBICZ and DDDBICZ, respectively. In which TAPC turn-on voltage, maximum luminance and maximum luminance
was used to block the electron from the cathode because of its efficiency of Device A are 6.1 V, 5634 cd mꢀ2 and 2.96 cd Aꢀ1
,
high LUMO level. Then, another two electron-only devices, ITO/ respectively. The EL spectra of Device B measured at different
BCP (8 nm)/DDBICZ (30 nm)/BCP (8 nm)/LiF (1 nm)/Al and ITO/ voltages are shown in Fig. S3 (ESI†), which are in good agreement
BCP (8 nm)/DDDBICZ (30 nm)/BCP (8 nm)/LiF (1 nm)/Al, were with those of DDDBICZ films. (The PL spectra of DDDBICZ films
prepared to examine the electron-transporting capability of are displayed in Fig. S4 (ESI†).) The EL spectra of Device B exhibit
DDBICZ and DDDBICZ, respectively. In which BCP was used the same pure blue emissions at different voltages with a max-
to block the hole from the anode because of its low HOMO imum at 464 nm and CIE coordinates of x = 0.16 and y = 0.19. The
level. Fig. 7 depicts the current-characteristics of the four current density–voltage–luminance curves of Device B are shown in
devices as a function of the average electric field. Both devices Fig. 9a and the luminance efficiency–current density curve for
fabricated by using DDBICZ can conduct rather significant Device B is shown in Fig. 9b. The electroluminescent data for
currents, suggesting that DDBICZ has hole- and electron- Device B are also summarized in Table 2. As shown in Table 2, the
transporting capabilities. Under the same average electric field, turn-on voltage, maximum luminance and maximum luminance
the electron-only device conducts higher currents than the efficiency of Device B are 3.6 V, 1363 cd mꢀ2 and 2.88 cd Aꢀ1
,
hole-only device, indicating that DDBICZ may possess stronger respectively. Although the maximum luminance and luminance
electron-transporting ability despite its bipolar capability (Fig. 7a). efficiency of the EL devices of DDBICZ and DDDBICZ might not
The devices based on DDDBICZ show similar performance to the be as high as in some of the blue EL devices reported
Fig. 7 The current characteristics as a function of the average electric field for hole-only and electron-only devices of DDBICZ and DDDBICZ.
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Fig. 8 Current density–voltage–luminance curves and current density–luminance efficiency curve of Device A.
and proton nuclear magnetic resonance spectroscopy methods.
Their thermal, electrochemical and photophysical properties
were studied by thermogravimetric analysis combined with
differential scanning calorimetry, electrochemistry, UV-vis
absorption spectroscopy and fluorescence spectroscopy. The
charge-transporting properties of DDBICZ and DDDBICZ were
studied by fabricating single carrier devices using them as
charge-transporting layers. The results reveal that DDBICZ
and DDDBICZ have high thermal stability (the decomposition
temperature of DDBICZ = 201 1C and the decomposition
temperature of DDDBICZ = 210 1C) and good electrochemical
and electron-transporting properties. Moreover, in order to
examine the electroluminescent properties of DDBICZ and
DDDBICZ, Device A and Device B were fabricated by using
them as light-emitting layers, respectively. The turn-on voltage,
maximum luminance and maximum luminance efficiency of
Device A are 6.1 V, 5634 cd mꢀ2 and 2.96 cd Aꢀ1, whereas those
of Device B are 3.6 V, 2036 cd mꢀ2 and 2.88 cd Aꢀ1. Our work
demonstrates that DDBICZ and DDDBICZ are promising blue
emitting materials with potential application in producing
OLED devices.
Table 2 Electroluminescent characteristics of Devices A and B
Lmax (cd mꢀ2
(at voltage)
)
ZL,max (cd Aꢀ1
)
(at voltage)
Devices
lEL (nm)
Von
A
B
472
464
6.1
3.6
5634 (at 10 V)
1363 (at 10.5 V)
2.96 (at 8 V)
2.88 (at 5.1 V)
l
EL, electroluminescence emission peak; Von, turn-on voltage at 1 cd mꢀ2
Lmax, maximum luminance; ZL,max, maximum luminance efficiency.
;
before,28,51,53,54,81 these two novel compounds are promising
for application in OLEDs as emitting materials due to their
excellent thermal, electrochemical and charge-transporting
properties. In addition, these two materials can be utilized to
make white OLED devices by integrating with other materials,
and we have obtained some preliminary results. The further
large-scale application study of these two materials is currently
in progress.
4. Conclusion
In summary, two novel indolo[3,2-b]carbazole derivatives, 5,11-
di(40-dimesitylboronphenyl)indolo[3,2-b]carbazole (DDBICZ) and
2,8-dimesitylboron-5,11-di(40-dimesitylboronphenyl)indolo[3,2-b]-
carbazole (DDDBICZ) were synthesized by introducing two
dimesitylboron groups and/or four dimesitylboron groups (as
Acknowledgements
electron-acceptors) into the indolo[3,2-b]carbazole moiety (as This work was supported by the Natural Science Foundation of
electron-donor). The structures of these two compounds were Shanxi Province (2013011013-1); Open Fund of the State Key
fully characterized by elemental analysis, mass spectrometry Laboratory of Luminescent Materials and Devices, South China
Fig. 9 Current density–voltage–luminance curves and current density–luminance efficiency curve of Device B.
2376 | New J. Chem., 2014, 38, 2368--2378
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19 P. T. Boudreault, A. A. Virkar, Z. Bao and M. Leclerc, Org.
Electron., 2010, 11, 1649–1659.
20 M. Kirkus, J. V. Grazulevicius, S. Grigalevicius, R. Gub,
W. Dehaen and V. Jankauskas, Eur. Polym. J., 2009, 45,
410–417.
University of Technology (2014-skllmd-09); Scientific and Techno-
logical Innovation Programs of Higher Education Institutions in
Shanxi Province (2012005); Program for Changjiang Scholar and
Innovation Research Team in University (IRT0972); International
Science & Technology Cooperation Program of China (2012-
DFR50460); National Natural Scientific Foundation of China
(21101111, 61205179, 61307030, 61307029, and 21376144); Shanxi
Provincial Key Innovative Research Team in Science and Techno-
logy (2012041011); Shanxi Scholarship Council of China (2012-006);
and 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.
21 D. Velasco, V. Jankauskas, J. Stumbraite, J. V. Grazulevicius
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23 M. Kirkus, J. Simokaitiene, J. V. Grazulevicius and
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24 J. Simokaitiene, E. Stanislovaityte, J. V. Grazulevicius,
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