Synthesis, photophysical and charge-transporting properties of a novel asymmetric indolo [3,2-b]carbazole derivative containing benzothiazole and diphenylamino moieties

Article abstract of DOI:10.1016/j.saa.2014.06.011

A novel asymmetric donor-π-donor-π-acceptor compound, 2-benzothiazolyl-8-diphenylamino-5,11-dihexylindolo[3,2-b]carbazole (BDDAICZ), has been successfully synthesized by introducing a benzothiazole moiety (as an electron-acceptor) and a diphenylamino moiety (as an electron-donor) to 2-position and 8-position of indolo[3,2-b]carbazole moiety (as a skeleton and an electron-donor), and characterized by elemental analysis, ¹H NMR, ¹³C NMR and MS. The thermal, electrochemical properties of BDDAICZ were characterized by thermogravimetric analysis combined with electrochemistry. The absorption and emission spectra of BDDAICZ was experimentally determined in several solvents and computed using density functional theory (DFT) and time-dependent density functional theory (TDDFT). The calculated absorption and emission wavelengths are coincident with the measured data. The ionization potential (IP), the electron affinity (EA) and reorganization energy of BDDAICZ were also investigated using density functional theory (DFT). Charge-transporting properties of BDDAICZ were characterized by OLEDs devices fabricated by using it as charge-transport layers. The results show that BDDAICZ has excellent thermal stability, electrochemical stability and hole-transporting properties, indicating its potential application as a hole-transporting material in OLEDs devices.

Full text of DOI:10.1016/j.saa.2014.06.011

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 133 (2014) 501–508
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Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Synthesis, photophysical and charge-transporting properties
of a novel asymmetric indolo [3,2-b]carbazole derivative containing
benzothiazole and diphenylamino moieties
a
b
b,
Heping Shi a, , Jiandong Yuan , Xiuqing Dong , Fangqin Cheng
⇑
⇑
a School of Chemistry and Chemical Engineering, Shanxi University, Taiyuan 030006, PR China
b State Environmental Protection Key Laboratory of Efficient Utilization Technology of Coal Waste Resources, Shanxi University, 92 Wucheng Road, Taiyuan 030006, Shanxi
Province, PR China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
ꢀ
ꢀ
ꢀ
ꢀ
A novel asymmetric indolo[3,2-
b]carbazole compound was
synthesized.
The compound contains
benzothiazole and diphenylamino
moieties.
The compound has strong
intramolecular charge transfer
character.
The compound can be served as an
excellent hole-transporting material
in OLEDs.
a r t i c l e i n f o
a b s t r a c t
Article history:
A novel asymmetric donor–p-donor–p-acceptor compound, 2-benzothiazolyl-8-diphenylamino-5,11-
Received 22 March 2014
Received in revised form 23 May 2014
Accepted 3 June 2014
dihexylindolo[3,2-b]carbazole (BDDAICZ), has been successfully synthesized by introducing a benzothi-
azole moiety (as an electron-acceptor) and a diphenylamino moiety (as an electron-donor) to 2-position
and 8-position of indolo[3,2-b]carbazole moiety (as a skeleton and an electron-donor), and characterized
by elemental analysis, 1H NMR, 13C NMR and MS. The thermal, electrochemical properties of BDDAICZ
were characterized by thermogravimetric analysis combined with electrochemistry. The absorption
and emission spectra of BDDAICZ was experimentally determined in several solvents and computed
using density functional theory (DFT) and time-dependent density functional theory (TDDFT). The calcu-
lated absorption and emission wavelengths are coincident with the measured data. The ionization poten-
tial (IP), the electron affinity (EA) and reorganization energy of BDDAICZ were also investigated using
density functional theory (DFT). Charge-transporting properties of BDDAICZ were characterized by
OLEDs devices fabricated by using it as charge-transport layers. The results show that BDDAICZ has
excellent thermal stability, electrochemical stability and hole-transporting properties, indicating its
potential application as a hole-transporting material in OLEDs devices.
Available online 13 June 2014
Keywords:
Synthesis
Indolo[3,2-b]carbazole
Benzothiazole
Diphenylamino
Photophysical properties
Charge-transporting properties
Ó 2014 Elsevier B.V. All rights reserved.
Introduction
Indolo[3,2-b]carbazole (ICZ) with a nitrogen atom as an elec-
⁄ Corresponding authors. Tel.: +86 351 7010588; fax: +86 351 7011688 (H. Shi).
Tel./fax: +86 351 7018813 (F. Cheng).
E-mail addresses: hepingshi@sxu.edu.cn (H. Shi), cfangqin@sxu.edu.cn
(F. Cheng).
tron donor is a promising heterocyclic compound. It can be easily
functionalized by covalently linked to various groups to achieve
different ICZ derivatives. The ICZ derivatives have the large planar
http://dx.doi.org/10.1016/j.saa.2014.06.011
1386-1425/Ó 2014 Elsevier B.V. All rights reserved.

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H. Shi et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 133 (2014) 501–508
and rigid conjugated structures together with remarkable photo-
physical properties, so these compounds have received much
attention. Many ICZ derivatives with outstanding properties such
as better morphological stability and thermal durability as well
as desirable charge-injecting and transporting properties have
been reported during the past decades. Ong and his co-workers
reported a series of ICZ derivatives, which could be served as excel-
lent hole-transporting materials and organic thin-film transistor
(OTFT) materials [1–4]. Tao et al. designed and synthesized several
ICZ derivatives, which can be used as excellent luminescence and
hole-transporting materials in the OLEDs [5–7]. Boudreault et al.
synthesized some new materials based on the ICZ framework,
which were particularly suitable for OFET [8,9]. Grazulevicius
and his co-workers developed various N,N-diarylated ICZ and these
compounds can be served as hole-transporting materials for light
emitting diodes [10–14]. Recently, Chen et al. reported two new
ICZ derivatives with multi-functionality, which were employed as
deep-blue emitters, hole-transporting materials and hosts to fabri-
cate organic light-emitting devices (OLEDs) [15].
At present, the studies on synthesis and application of asym-
metric indolo[3,2-b]carbazole derivatives are relatively limited
[16,17]. Therefore, further studies are still necessary. In the present
article, we report on the synthesis, structures and properties of a
novel asymmetric indolo[3,2-b]carbazole derivative bearing a ben-
zothiazole moiety as an electron-acceptor and a diphenylamino
moiety as an electron-donor. The combination of indolo[3,2-b ]car-
bazole with benzothiazole and diphenylamino was expected to
obtain new charge-transporting materials with excellent perfor-
mance. To the best of our knowledge, the study on this compound
has never been reported. The synthetic details are illustrated in
Scheme 1. The structure of the compound was characterized by
elemental analysis, 1H NMR, 13C NMR and MS. Its photophysical
properties were studied by UV–vis and fluorescence spectra,
electrochemical properties were studied by cyclic voltammetry
analysis, thermal property was studied by thermogravimetric
analysis and charge-transporting properties were characterized
by OLEDs devices fabricated by using it as charge-transport layers.
In addition, its geometry, molecular orbitals, electronic absorption
and emission spectra, charge-injection and transport properties
were determined by quantum chemical calculations.
Experimental section
Synthesis of BDDAICZ
All the reagents were purchased from commercial sources with-
out further purification. All the reactions were carried out using
Schlenk techniques under a nitrogen atmosphere. The synthesis
of compound was described in Scheme 1. Cyclohexane-1,
4-dione-bis(p-bromophenyl)-hydrazone (compound 1), 2,8-dib-
romoindolo[3,2-b]carbazole (compound 2) and 2,8-dibromo-5,
11-dihexylindolo[3,2-b]carbazole (compound 3) were synthesized
according the literature procedures [18].
Synthesis of cyclohexane-1,4-dione-bis (p-bromophenyl)-hy-drazone
(compound 1)
1,4-Cyclohexanedione (5.62 g, 50 mmol) was dissolved in abso-
lute ethanol (EtOH) (100 mL) and was added with stirring to a mix-
ture of powdered 4-bromophenylhydrazine hydrochloride (22.4 g,
100 mmol), sodium acetate (8.2 g, 100 mmol) and absolute ethanol
(200 mL) at room temperature. The mixture was quickly heated to
50 °C and kept stirring for 2 h, and then, the mixture was cooled to
room temperature and poured into ice water with stirring. A pre-
cipitate was filtered off and washed carefully with water to give
1 yield 66%.
Scheme 1. The synthetic route of BDDAICZ.

H. Shi et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 133 (2014) 501–508
503
The synthesis of 2,8-dibromoindolo[3,2-b]carbazole (compound 2)
Compound 1 (10 g, 22 mmol) was added into a mixture of AcOH
(130 mL) and H2SO4 (32 mL) at 0 °C and stirred for 5 min. The mix-
ture was heated up to 30 °C and kept stirring for 1 h. Afterward, the
mixture was further risen up to 60–70 °C and stirred for another
1 h. Then it was cooled down to room temperature and poured into
ice water with stirring. The greenish solid was filtered off and
washed with water and EtOH to neutral pH. Finally, dried and pure
2,8-dibromoindolo[3,2-b]carbazole (compound 2) (4.88 g, yield
53.5%) was achieved. Mp > 300 °C. 1H NMR (DMSOꢁd6) d 11.14
(2H, s, and N–H), and 7.1–8.24 (8H, m, and Ar–H).
to dryness. The crude compound was stirred for 15 min in ethanol
(30 mL) and then filtered. The pure product was obtained by col-
umn chromatography as
a yellow-green solid (0.53 g, yield:
83.2%). M.p.: 241.9–242.9 °C. 1H NMR (600 MHz, CDCl3) d 9.03 (s,
1H), 8.32 (d, J = 1.8 Hz, 1H), 8.20 (dd, J = 8.5, 1.4 Hz, 1H), 8.13 (d,
J = 7.6 Hz, 2H), 7.97 (s, 1H), 7.91 (d, J = 7.8 Hz, 1H), 7.56 (dd,
J = 8.6, 1.8 Hz, 1H), 7.51 (t, J = 7.6 Hz, 1H), 7.46 (d, J = 8.5 Hz, 1H),
7.38 (t, J = 7.5 Hz, 1H), 7.30 (d, J = 8.6 Hz, 1H), 4.44–4.38 (m, 4H),
1.96 (qd, J = 15.0, 7.5 Hz, 4H), 1.46 (dd, J = 13.8, 7.3 Hz, 4H),
1.39–1.30 (m, 8H), 0.89 (td, J = 7.2, 2.1 Hz, 6H). Anal. Calcd. for
C37H38BrN3S: C, 69.80%; H, 6.02%; N, 6.60%; Found: C, 69.12%; H,
5.96%; N, 6.54%.
2,8-Dibromo-5,11-dihexyl-indolo[3,2-b]carbazole (compound 3)
To
a
solution of 2, 8-dibromoindolo[3,2-b]carbazole (2 g,
2-benzothiazolyl-8-diphenylamino-5, 11-dihexylindolo[3,2-
b]carbazole (BDDAICZ)
Compound 5 (185 mg, 0.29 mmol) and diphenylamino (196 mg,
1.16 mmol) were mixed with dry toluene (50 ml) in a two necked
4.8 mmol) in anhydrous THF (50 mL) in the round-bottomed flask,
NaH 0.35 g (14.4 mmol) was added under a nitrogen atmosphere
and the solution was cooled to 0 °C, then the reaction mixture
was heated to 55 °C for 6 h. The reaction mixture was cooled to
0 °C again, and 1-bromohexane (1.96 mL, 14 mmol) was added to
the reaction mixture slowly. The mixture was heated to 65 °C for
36 h, and then poured into water (250 mL) and the precipitate
was afforded. The precipitate was filtered and washed with ethanol
(100 mL). And the pure product was obtained by column chroma-
tography as a yellow solid (2.56 g, 91.5%). M.p.: 232.6–233.2 °C. 1H
NMR (600 MHz, CDCl3) d 8.30 (d, J = 1.8 Hz, 2H), 7.93 (s, 2H), 7.55
(dd, J = 1.8, 1.8 Hz, 2H), 7.26 (d, J = 7.2 Hz, 2H), 4.34 (t, J = 7.2 Hz,
4H), 1.91 (t, J = 7.2 Hz, 4H), 1.43 (s, 4H), 1.33–1.29 (m, 8H), 0.87
(t, J = 7.1 Hz, 6H).
round bottomed flash containing
a stir bar. The Pd(OAc)2
(10 mol%), t-Bu3P (10 mol%), and NaOBu-t (111 mg, 1.16 mmol)
were also added and the mixture was stirred under nitrogen at
80 °C for about 15 h. After cooling, the reaction was quenched by
adding water and then was extracted with CH2Cl2. The organic
layer was dried over anhydrous MgSO4 and evaporated under vac-
uum. The product was purified by silica gel column chromatogra-
phy using CH2Cl2/hexane (vol. ratio 1:2) as an eluant. Yield:
155 mg (74%) of yellow solid. M.p.: 220–222 °C. 1H NMR
(600 MHz, CDCl3) d 9.16 (s, 1H), 8.29 (dd, J = 17.0, 8.0 Hz, 2H),
8.24–8.13 (m, 1H), 8.00 (d, J = 17.9 Hz, 1H), 7.86 (d, J = 7.9 Hz,
2H), 7.54 (t, J = 7.6 Hz, 1H), 7.48–7.43 (m, 1H), 7.41 (t, J = 7.5 Hz,
2H), 7.33 (dd, J = 9.9, 7.6 Hz, 2H), 7.25–7.06 (m, 7H), 6.96 (s, 2H),
4.49–4.29 (m, 4H), 2.01–1.89 (m, 4H), 1.55–1.49 (m, 2H), 1.43–
1.29 (m, 10H), 0.90–0.84 (m, 6H). 13C NMR (151 MHz, CDCl3) d
173.69, 151.71, 147.04, 142.13, 141.86, 139.21, 138.12, 137.32,
131.95, 130.12, 129.13, 128.40, 126.60, 126.44, 126.17, 125.54,
124.34, 123.91, 121.91, 112.42, 111.85, 46.58, 34.58, 34.45, 32.57,
32.53, 30.02, 29.86, 25.50, 25.40, 16.92, 16.83. MS (m/z): 724.36
(M+). Anal. Calcd. for C49H48N4S: C, 81.18%; H, 6.67%; N, 7.73%;
Found: C, 80.96%; H, 6.6%; N, 7.65%.
8-bromo-5,11-dihexyl-indolo[3,2-b]carbazole-2-carbaldehyde
(compound 4)
Compound 3 (0.6 g, 1.03 mmol) was dissolved in anhydrous
THF (50 mL) under a nitrogen atmosphere. The solution was cooled
to ꢁ78 °C, and n-butyllithium (0.329 mL, 0.8 mmol) was added
into it dropwise. The reaction mixture was stirred for 1 h at
ꢁ78 °C, and the temperature of the reaction mixture was raised
to ꢁ50 °C. After being stirred for 0.5 h, the reaction mixture was
cooled to ꢁ78 °C again. A solution of DMF (1 mL, 12.9 mmol) was
added into dropwise at the temperature. Then the reaction mixture
was stirred for another 1 h. The reaction solution was allowed to be
warmed to room temperature and diluted with CH2Cl2. The organic
layer was washed with water, dried over MgSO4 and filtered and
evaporated under vacuum. The residue was purified by column
chromatography to afford 8-bromo-5, 11-dihexyl-indolo[3,2-
b]carbazole-2-carbaldehyde 4 as a yellow solid (0.39 g, 71.3%).
M.p.: 230–231.5 °C. 1H NMR (600 MHz, CDCl3) d 10.11 (s, 1H),
8.72 (d, J = 0.9 Hz, 1H), 8.31 (d, J = 1.7 Hz, 1H), 8.06 (s, 1H), 8.01
(dd, J = 8.4, 1.3 Hz, 1H), 7.98 (s, 1H), 7.57 (dd, J = 8.6, 1.8 Hz, 1H),
7.46 (d, J = 8.5 Hz, 1H), 7.30 (d, J = 8.6 Hz, 1H), 4.39 (dt, J = 22.3,
7.3 Hz, 4H), 1.99–1.90 (m, 4H), 1.48–1.41 (m, 4H), 1.38–1.29 (m,
8H), 0.88 (td, J = 7.1, 2.9 Hz, 6H). 13C NMR (151 MHz, CDCl3) d
191.54, 145.24, 141.90, 141.84, 136.73, 136.37, 128.45, 127.80,
126.14, 123.58, 123.28, 122.98, 122.66, 122.62, 120.31, 118.26,
108.59, 108.40, 99.57, 99.27, 43.64, 43.45, 31.65, 31.56, 28.82,
28.76, 27.09, 26.99, 22.56, 22.52, 13.98, 13.95. MS (m/z): 530.15
(M+). Anal. Calcd. for C31H35BrN2O: C, 70.05%; H, 6.64%; N, 5.27%;
Found: C, 69.81%; H, 6.57%; N, 5.22%.
Measurement and characterization
Melting points were determined with a WRS-1B melting point
detector. All the NMR spectra were measured with a Bruker Avance
600 MHz NMR spectrometer. Thermogravimetric analyses were
performed with a TA TGA 2050 thermogravimetric analyzer under
nitrogen atmosphere with a heating rate of 20 °C/min from room
temperature to 600 °C. Elemental analyses were performed with
an Elementar Analysensysteme (GmbH). Mass spectra were
recorded with the LC–MS system consisted of a Waters 1525 pump
and a Micromass ZQ4000 singlequadrupole mass spectrometer
detector (Waters). Cyclic voltammetry experiments were carried
out with a CHI-600C electrochemical analyzer. The measurements
were performed with a conventional three-electrode configuration
consisting of a glassy carbon working electrode, a platinum-disk
auxiliary electrode and an Ag/AgCl reference electrode. The scan
speed of the measurements is 50 mV/s. UV–vis absorption spectra
were obtained from a Shimadzu UV-2450 spectrophotometer.
Fluorescence spectra were taken from a Shimadzu RF-5301PC fluo-
rescence spectrometer. All the spectral experiments were carried
out at room temperature.
8-(Benzothiazol-2-yl)-2-bromo-5,11-dihexyl-indolo[3,2-b]carbazole
(compound 5)
The mixture of
4 (0.531 g, 1 mmol), 2-aminothiophenol
(0.16 mL, 1.5 mmol) and DMSO (50 mL) was heated to 180 °C in
an oil bath, and held at the temperature for 12 h. Subsequently,
the reaction mixture was cooled to room temperature, then poured
into water and extracted with CH2Cl2 (3 ꢂ 30 mL). The combined
organic layer was dried over anhydrous MgSO4 and evaporated
Device fabrication and measurement
The multilayer OLEDs were fabricated by the vacuum-deposi-
tion method. Organic layers were deposited by high-vacuum
(5 ꢂ 10ꢁ4 Pa) thermal evaporation onto a glass (3 cm ꢂ 4 cm)

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H. Shi et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 133 (2014) 501–508
substrate pre-coated with an indium tin oxide (ITO) layer. The ITO
surface was cleaned ultrasonically sequentially with acetone,
methanol, and deionized water and then it was treated with UV-
ozone. All organic layers were sequentially deposited. Thermal
deposition rates for organic materials, LiF and Al were 0.5, 0.5
and 1 Å sꢁ1, respectively. The active area of the devices was
12 mm2. The current-characteristics of the OLEDs devices were
measured with Keithley 2400 Source Meter. All measurements
were done at room temperature under ambient conditions.
The molecular structure and frontier orbitals of BDDAICZ
The molecular structures of BDDAICZ in different polar solvents
were optimized using DFT method at the DFT/B3LYP/6-31G (d, p)
level with the polarized continuum model (PCM). The optimized
geometry of BDDAICZ shows that the indolo[3,2-b]carbazole and
benzothiazole moieties are of planar structure and the diphenyl-
amino group forms propeller-like conformation originated from
the trigonal nitrogen center as well as molecular structures have
only slight differences in different polar solvents. The optimized
structure in THF solvent is shown in Fig. 1. The values of the
parameters are listed in Table 1.
Theoretical calculation
The frontier molecular orbitals, three highest occupied molecu-
lar orbitals (HOMOꢁ2, HOMOꢁ1 and HOMO) and six lowest unoc-
cupied molecular orbitals (LUMO, LUMO+1, LUMO+2, LUMO+3,
LUMO+4, LUMO+5) of BDDAICZ were analyzed based on the opti-
mized structures in different polar solvents. The diagrams of these
orbitals in THF solvent are placed in Fig. 2. As shown in Fig. 2, The
HOMO is localized mainly on diphenylamino moiety, the HOMOꢁ1
is localized mainly on diphenylamino and indolo[3,2-b]carbazole
moieties and the HOMOꢁ2 is localized mainly on diphenylamino
moiety, while the LUMO is localized mainly on benzothiazole moi-
ety, the LUMO+1 is localized mainly on indolo[3,2-b]carbazole
moiety and the LUMO+5 is localized mainly on diphenylamino
moiety. Therefore, the electronic transition from the ground state
to the first excited state mainly involves the intramolecular charge
transfer (ICT) from the diphenylamino moiety (as electron-donor)
and indolo[3,2-b]carbazole moiety (as electron-donor) to
benzothiazole moiety (as electron-acceptor). Generally, holes and
electrons in OLEDs are transferred through the individual HOMO
and LUMO levels [25]. Fig. 2 shows that BDDAICZ exhibits almost
complete separation of LUMO and HOMO. The almost complete
localization of HOMO and LUMO means that the HOMO to LUMO
transition becomes a typical charge transfer transition, this is
essential for the efficient hole and electron transport [26]. The cal-
culated HOMO energy level, LUMO energy level and HOMO–LUMO
gap are ꢁ4.72 to ꢁ4.78 eV, ꢁ1.44 to ꢁ1.50 eV and 3.25–3.29 eV in
different polar solvents. These data are collected in Table 2.
The neutral and ionic geometries of BDDAICZ in ground-state
were optimized by density functional theory (DFT) method at
B3LYP level with 6-31G (d, p) basis set [19,20]. The frontier
molecular orbital characteristics were analyzed on the optimized
structures at the same level. The ionization potential (IP), electron
affinity (EA), reorganization energy and HOMO–LUMO gap of
BDDAICZ were also calculated by DFT method based on the opti-
mized geometry of the neutral and ionic molecules. The excited-
state geometry of BDDAICZ was optimized using time-dependent
density functional theory (TDDFT) method at B3LYP level with
6-31G (d, p) basis set [21]. The absorption spectra and the emission
spectra of this compound were carried out using TDDFT method
based on the optimized ground state structures and the lowest
singlet excited-state structures, respectively. Solvent effects were
also taken into account in these calculations by using the polarized
continuum model (PCM) [22,23]. All calculations were carried out
with the Gaussian09 program package using the advanced com-
puting facilities of supercomputing center of computer network
information center of Chinese Academy of Sciences [24].
Results and discussion
Synthesis and thermal properties of BDDAICZ
Scheme 1 Outlines the synthesis of BDDAICZ. Firstly, compound
1 was prepared via the reaction of 1,4-Cyclohexanedione with
4-bromophenylhydrazine, then compound 2 was synthesized by
the Fischer reaction of compound 1 in a mixture of acetic acid
and H2SO4 at 65 °C in high yield. Secondly, compound 3 was
obtained via the reaction of indolo[3,2-b]carbazole, NaH and
1-bromohexane in 91.5% yield. Thirdly, compound 4 was obtained
as the key intermediate for the whole procedure via the bromine–
lithium exchange reactions of compound 3 with n-BuLi and react-
ing with DMF under nitrogen atmosphere in 71.3% yield. After that,
compound 5 was prepared in 83.2% yield by the condensation
reaction of compound 4 with 2-aminothiophenol in DMSO at
180 °C. Finally, the compound BDDAICZ was synthesized via the
Photophysical properties of BDDAICZ
UV–vis spectra of BDDAICZ
UV–vis absorption spectra of BDDAICZ in various solvents of
different polarity (1.0 ꢂ 10ꢁ5 M) are presented in Fig. 3. The
Buchwald–Hartwig coupling reaction of compound
5 with
diphenylamino under nitrogen in the presence of Pd(OAC)2,
t-Bu3P and NaOBu-t as a catalytic system in 74% yield. Structure
of BDDAICZ was confirmed by NMR, MS and elemental analysis.
Further details are given in the experimental section. BDDAICZ is
soluble in many of the common organic solvents such as dichloro-
methane, chloroform, dimethylformamide, tetrahydrofuran, tolu-
ene and dimethylsulfoxide. The thermal properties of BDDAICZ
were investigated by thermogravimetric analysis (TGA) under a
nitrogen atmosphere. The TGA curve of BDDAICZ is shown in
Fig. S1. Fig. S1 is placed in electronic supplementary information
(ESI). The TGA result of BDDAICZ show 5% weight loss temperature
is higher than 193 °C, indicating that BDDAICZ has good thermal
stability and should be adequate for most optoelectronic device
applications.
Fig. 1. The optimized geometry of BDDAICZ in THF solvent.

H. Shi et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 133 (2014) 501–508
505
Table 1
the indolo[3,2-b]carbazole backbone and the low-energy absorp-
tion band is attributed to
pꢃ transition of the intramolecular
charge transfer (ICT) from the diphenylamino moiety (as an elec-
tron-donor) and indolo[3,2-b]carbazole (as an electron-donor) to
benzothiazole moiety (as an electron-acceptor) [27].
In order to gain a detailed insight into the nature of the UV–vis
absorption of BDDAICZ observed experimentally, we computed
singlet–singlet electronic transition in different polar solvents
based on the optimized geometries of the ground state of BDDAICZ
using time-dependent DFT method at the B3LYP/6-31G (d, p) level
with the polarized continuum model (PCM). The computed data
are collected in Table S1 (ESI). On the basis of the calculated verti-
cal excited energy and their corresponding oscillator strengths, the
continuous electronic absorption spectrum was simulated with the
help of SWIZARD software. The simulated electronic absorption
spectrum of BDDAICZ is shown in Fig. S2 (ESI). As seen in Fig. S2
and Table S1, The absorption band at about 320–430 nm is derived
Selected bond length and dihedral angle of the optimized structures of BDDAICZ in its
ground state and excited state in THF solvent.
p–
Parameters
The ground state
The excited state
Bond length (Å)
C1–C85
C18–N102
C33–N102
C34–N102
C85–N88
1.46362
1.43141
1.41772
1.41760
1.30217
1.79549
1.45878
1.40480
1.42125
1.42219
1.30635
1.80384
C85–S83
Dihedral angle (°)
C2–C1–C85–N88
C2–C1–C85–S83
C6–C1–C85–N88
C6–C1–C85–S83
C17–C18–N102–C33
C17–C18–N102–C34
C19–C18–N102–C33
179.72130
ꢁ0.25640
178.50773
ꢁ1.61660
ꢁ0.10353
ꢁ1.25204
179.91878
53.77483
178.62363
34.20806
ꢁ125.17283
ꢁ145.73058
ꢁ125.99306
ꢁ145.83987
from the
p
–
p
⁄ transitions of intramolecular charge transfer (ICT)
C19–C18–N102–C34
55.05928
34.22149
from the diphenylamino moiety (as an electron-donor) and indo-
lo[3,2-b]carbazole moiety (as an electron-donor) to benzothiazole
moiety (as an electron-acceptor) owing to the lower-energy
electronic transition from HOMO to LUMO, HOMO to LUMO+1,
HOMOꢁ2 to LUMO and HOMOꢁ2 to LUMO+1. The absorption
UV–vis absorption spectra data are listed in Table 3. As depicted in
Fig. 3, the absorption spectra of BDDAICZ show two similar
absorption bands, a high-energy absorption band at 290–320 nm
and a low-energy absorption band at 320–430 nm, in which the
bands at 290–320 nm are ascribed to the
p–
pꢃ transitions of the
conjugated molecular backbone owing to the higher-energy
electronic transition from HOMO to LUMO+5. The calculated
results are consistent well with the experimental results.
high-energy absorption band is attributed to
p–
pꢃ transition of
LUMO+3
LUMO+4
LUMO+5
LUMO+2
HOMO
LUMO
LUMO+1
HOMO-2
HOMO-1
Fig. 2. The frontier molecular orbitals of BDDAICZ in the ground state in THF solvent.

506
H. Shi et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 133 (2014) 501–508
Table 2
Hex
HOMO and LUMO energies level, and HOMO–LUMO gap of BDDAICZ obtained from
DFT/B3LYP/6-31G (d, p) calculation in different solvents and experimental measure-
ment in Acetonitrile.
1.0
0.8
0.6
0.4
0.2
0.0
CH Cl
2
2
THF
CH CN
3
Methods
Solvents
HOMO
(eV)
LUMO
(eV)
DE
(eV)
DMSO
Theoretical calculation
Hex
ꢁ4.72
ꢁ1.47
ꢁ1.49
ꢁ1.44
ꢁ1.5
3.25
3.27
3.29
3.28
3.28
Dichloromethane ꢁ4.76
THF
ꢁ4.73
ꢁ4.78
ꢁ4.78
Acetonitrile
DMSO
ꢁ1.5
Experimental
measurement
Acetonitrile
ꢁ5.02
ꢁ2.03
2.99
400
450
500
550
600
Wavelength/nm
Fig. 4. Fluorescence spectra of BDDAICZ in different solvents. Excited at 350 nm.
0.35
0.30
0.25
0.20
0.15
0.10
0.05
HEX
CH2Cl2
THF
CH3CN
DMSO
energy from the excited states to the ground states. The computed
data are collected in Table S2 (ESI). On the basis of the calculated
vertical transitions energy and their corresponding oscillator
strengths, the continuous electronic emission spectrum was simu-
lated with the help of SWIZARD software. The simulated emission
spectra of BDDAICZ are shown in Fig. S3 (ESI). As can be seen in
Fig. S3, the simulated maximum emission wavelengths of BDDA-
ICZ are around 439–516 nm in different polar solvents. The emis-
sion wavelengths are mainly originated from the intramolecular
charge transfer (ICT) from the diphenylamino moiety (as an elec-
tron-donor) and indolo[3,2-b]carbazole (as an electron-donor) to
benzothiazole moiety (as an electron-acceptor) owing to the
0.00
300
350
400
450
Wavelength/nm
Fig. 3. UV–vis absorption spectra of BDDAICZ in different solvents.
p
–
pꢃ transition from HOMO to LUMO and the molecular backbone
owing to the
p
–
pꢃ transition from HOMO to LUMO+1. The simu-
lated emission spectra are in relatively good agreement with the
Fluorescence spectra of BDDAICZ
experimental fluorescence spectra.
Fig. 4 shows the steady-state fluorescence spectra for BDDAICZ
in several different polarity solvents (1.0 ꢂ 10ꢁ5 M). Fluorescence
spectra data were listed in Table 3. As shown in Fig. 4 and Table
3, BDDAICZ possesses mainly a broad emission band in several sol-
vents except in hexane, which is mainly originated from overlap of
According to the published work [28], solvatochromic shifts
may be described by the state and excited dipole moments in
different polar solvents. In order to gain insight into the nature
of significant solvatochromism, we computed the state and excited
dipole moments of BDDAICZ in different polar solvents. The data
are listed Table 4. Table 4 show BDDAICZ has the large difference
between the excited- and state-dipole moments. The large
difference indicates the strong intramolecular charge transfer and
significant solvatochromism. Our results are similar with those of
the reported work [29–31].
the p–
pꢃ transition of the conjugated molecular backbone and the
intramolecular charge transfer (ICT) from the diphenylamino moi-
ety to the benzothiazole moiety. In addition, BDDAICZ presents
significant solvatochromism and a bathochromic shift of 45 nm
ranging from 453 nm (in hexane) to 498 nm (in DMSO) with the
increasing polarity of the solvents. Such a remarkably solvatochro-
mism suggests that the intramolecular charge transfer (ICT) from
the diphenylamino moiety to the benzothiazole moiety takes place
in the excitation process and the polar solvents are propitious to
the ICT process.
In order to gain insight into the nature of the fluorescence emis-
sion observed for BDDAICZ, we optimized its geometries of the
excited states using TDDFT method at the B3LYP/6-31G (d, p) level
with the polarized continuum model (PCM). The data of optimized
structure in THF solvent are also listed in Table 1. Based on the
optimized structures, we calculated their electronic transitions
The fluorescence quantum yields and lifetimes of BDDAICZ
The fluorescence quantum yields (U) of BDDAICZ were
measured in the several solvents using quinine bisulfate in
0.1 mol/L sulfuric acid as a standard, which were carried out at
room temperature. The fluorescence quantum yield was calculated
from Eq. (1) [32].
ꢀ
ꢁ
2
Fs Ar nr
/s ¼ /
ð1Þ
r Fr As ns
Table 3
Photophysical properties of BDDAICZ in several solvents.
Solvents
UV wavelength, k (nm)
FL wavelength, k (nm)
Stokes shift (cmꢁ1
)
Fluorescence quantum yields (U)
Fluorescence lifetime, s (ns)
Hex
307, 346
311, 350
309, 350
307, 347
311, 351
454
483
468
492
499
10546, 6875
11450, 7867
10994, 7203
12248, 8493
12114, 8450
0.102
0.016
0.084
0.017
0.085
1.72, 4.91
1.52, 7.47
1.16, 7.43
2.72, 7.51
2.75, 7.63
Dichloromethane
THF
Acetonitrile
DMSO

H. Shi et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 133 (2014) 501–508
507
where U is the fluorescence quantum yield, F is the integration of
Table 5
the emission intensities, A is the absorbance at the excitation wave-
length, n is refractive index of the solution and the subscripts ‘‘r’’
and ‘‘s’’ denote reference and sample, respectively. The data are
summarized in Table 3. The fluorescence decay behaviors of BDDA-
ICZ were also studied in the several solvents. Typical fluorescence
decay curve of BDDAICZ are displayed in Fig. S4 (ESI). It is easily
found that the fluorescence decay curves in several solvents are
multi-exponential model. The fluorescence lifetimes are also col-
lected in Table 3.
Ionization potential (IP), electron affinity (EA) and reorganization energy of BDDAICZ
obtained from DFT/B3LYP/6-31G (d, p) calculation (eV) in different solvents.
Solvents
IPv
IPa
kHol
EAv
EAa
kEle
Hex
5.152
4.763
4.785
4.682
4.676
5.026
4.607
4.631
4.519
4.512
0.282
0.287
0.285
0.296
0.297
0.835
1.454
1.356
1.625
1.639
0.939
1.612
1.566
1.789
1.804
0.317
0.327
0.325
0.333
0.333
Dichloromethane
THF
Acetonitrile
DMSO
Electrochemical properties of BDDAICZ
Hole-only Device
Electron-only Device
The electrochemical properties of BDDAICZ were explored
through cyclic voltammetry (CV) in 1.0 ꢂ 10ꢁ3 M MeCN solution.
The CVs were performed using the Ag/AgCl as the reference elec-
trode and 0.1 mol/L of tetrabutylammonium perchlorate in anhy-
drous MeCN as the supporting electrolyte under a nitrogen
atmosphere. The CV curve is shown in Fig. S5 (ESI). Onset oxidation
potential is observed at 0.62 V. Thus, the HOMO and LUMO energy
levels was calculated by using the empirical formula
HOMO = ꢁ(Eox + 4.40) from the onset of the first oxidation [33].
ELUMO = EHOMO ꢁ Eg, and the optical energy gaps were derived from
the lowest energy absorption onset in the absorption spectra,
which is 2.99 eV. The HOMO and LUMO energies and energy differ-
ence ELUMO–HOMO of BDDAICZ are listed in Table 2. As shown in
Table 2, the calculated HOMO and LUMO levels in different polar
solvents are higher than the experimental values. The calculated
HOMO–LUMO gaps in different polar solvents slightly deviate from
those obtained from the experimental results (ca. 0.26–0.30 eV). As
we all know, the values of HOMO and LUMO usually determine the
hole and electron injection of the materials which is in accord with
the work function values of cathode and anode, and the hole-
transporting material (HTM) with the smaller negative value of
HOMO should lose their electrons more easily, while the electron-
transporting material (ETM) with larger negative value of LUMO
should accept electrons more easily. It can be seen from Table 2
that the HOMO level of BDDAICZ matches well with the work
function of the indium tin oxides (ITOs) from ꢁ4.8 to ꢁ5.1 eV. This
implies that BDDAICZ is promising as hole injection and transport
materials in OLEDs [34].
103
102
101
1x108
2x108
3x108
4x108
Average Electric Field (V/m)
Fig. 5. The current characteristics as a function of the average electric field for hole-
only and electron-only devices.
IP value [37,38] and a small hole reorganization energy compared
with electron reorganization energy in different polar solvents,
suggesting its potential application as the hole-transport material.
To further explore the application of BDDAICZ as the charge-
transporting material in the OLEDs, two single carrier devices were
prepared. First, a hole-only device, with the configuration of ITO/
NPB (30 nm)/BDDAICZ (30 nm)/NPB (30 nm)/Al, was prepared to
examine the hole-transporting capability of BDDAICZ. In which
NPB (N,N0-bis-(1-naphthyl)-N,N0-diphenyl-1,10-biphenyl-4,40-dia-
mine) was used to block the electron from the cathode because
the LUMO energy level of NPB is high enough to block electron
injection. Then the other electron-only device, ITO/Alq3 (30 nm)/
BDDAICZ (30 nm)/Alq3 (30 nm)/LiF (1 nm)/Al, was fabricated to
examine the electron-transporting capability of this compound.
In which Alq3 (tris(8-quinolinolato)aluminum) as was used to
block the hole from the anode because of its low HOMO level.
Fig. 5 depicts the current-characteristics of the two devices as a
function of the average electric field. As shown in Fig. 5, the
hole-only device could conduct more significant currents than
the electron-only device, indicating that BDDAICZ possesses the
excellent hole-transporting property and it is a promising candi-
date as hole-transport material for OLEDs devices. The results of
BDDAICZ as the charge-transporting material in the OLEDs are
consistent with those of theoretical calculation. Further study on
its application in OLEDs is being carried out in our laboratory.
Charge-transporting properties of BDDAICZ
The charge injection and transport ability are important param-
eters for OLEDs. In general, the lower the ionization potential (IP),
the easier the entrance of holes from ITO to hole-transport layer
(HTL). The higher the electronic affinity (EA), the easier the
entrance of electrons from cathode to electron-transport layer
(ETL). And the charge transport rate can be approximated by the
Marcus electron-transfer theory, which is mainly decided by
hole/electron reorganization energy according to this theory
[35,36]. In order to evaluate the charge-injection and transport
properties, the ionization potential (IP), electronic affinity (EA)
and hole/electron reorganization energy of BDDAICZ are calculated
and collected in Table 5. As shown in Table 5, BDDAICZ has lower
Conclusion
Table 4
In summary, a novel asymmetric compound, 2-benzothiazolyl-
8-diphenylamino-5,11-dihexylindolo[3,2-b]carbazole (BDDAICZ),
was synthesized and characterized by elemental analysis, 1H
Ground and excited state dipole moments of BDDAICZ computed in different polar
solvents.
NMR, 13C NMR and MS. BDDAICZ belongs to donor–
p-donor–p-
Solvents
Hex
Dichloromethane THF
Acetonitrile DMSO
Dipole moments
(ground state)
Dipole moments
(excited state)
0.9749 1.1328
1.5224 2.1102
1.1179 1.1855
2.0595 3.2663
1.1903
3.2815
acceptor system comprising an indolo[3,2-b]carbazole moiety
(as a skeleton and an electron donor), a diphenylamino moiety
(as an electron donor) and a benzothiazole moiety (as an electron
acceptor). The thermal, electrochemical, photophysical and

508
H. Shi et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 133 (2014) 501–508
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charge-transporting properties of BDDAICZ were characterized by
the combination of experimental and theoretical methods. The
results show that BDDAICZ has excellent thermal stability, electro-
chemical stability and hole-transporting properties, indicating its
potential application as a hole-transporting material in OLEDs.
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This work was supported by Natural Science Foundation of
Shanxi Province (No. 2013011013-1), Scientific and Technological
Innovation Programs of Higher Education Institutions in Shanxi
Province (No. 2012005) and National Natural Scientific Foundation
of China (No. 21376144) and Fund of Key Laboratory of Optoelec-
tronic Materials Chemistry and Physics, Chinese Academy of
Sciences (No. 2008DP173016). All the calculations have been per-
formed with the advanced computing facilities of supercomputing
centre of computer network information centre of Chinese
Academy of Sciences. All the authors express their deep thanks.
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Appendix A. Supplementary material
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.saa.2014.06.011.
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