Experimental and theoretical study of three new benzothiazole-fused carbazole derivatives

Article abstract of DOI:10.1016/j.saa.2011.07.017

Three new D-π-A type compounds, each containing one benzothiazole ring as an electron acceptor and one N-ethylcarbazole group as electron donor, were synthesized and characterized by elemental analysis, NMR, MS and thermogravimetric analysis. The absorption and emission spectra of three compounds were experimentally determined in several solvents and were simultaneously computed using density functional theory (DFT) and time-dependent density functional theory (TDDFT). The calculated reorganization energy for hole and electron indicates that three compounds are in favor of hole transport than electron transport. The calculated absorption and emission wavelengths are well coincident with the measured data. The calculated lowest-lying absorption spectra can be mainly attributed to intramolecular charge transfer (ICT). And the calculated fluorescence spectra can be mainly described as originating from an excited state with intramolecular charge transfer (ICT) character. The results show that three compounds exhibited excellent thermal stability and high fluorescence quantum yields, indicating their potential applications as excellent optoelectronic material in optical field.

Full text of DOI:10.1016/j.saa.2011.07.017

Spectrochimica Acta Part A 81 (2011) 730–738
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Experimental and theoretical study of three new benzothiazole-fused carbazole
derivatives
He-ping Shia,∗, Lei Xua, Ying Chenga, Jing-yuan Hea, Jian-xin Daia,
Li-wen Xinga, Bai-quan Chenc, Li Fanga,b,∗∗
a School of Chemistry and Chemical Engineering, Shanxi University, Taiyuan 030006, PR China
b Engineering Research Center of Fine Chemicals, Ministry of Education, Taiyuan 030006, PR China
c Key Laboratory of Optoelectronic Materials Chemistry and Physics, Chinese Academy of Sciences, Fuzhou 350002, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 4 May 2011
Received in revised form 28 June 2011
Accepted 3 July 2011
Three new D–␲–A type compounds, each containing one benzothiazole ring as an electron acceptor and
one N-ethylcarbazole group as electron donor, were synthesized and characterized by elemental analy-
sis, NMR, MS and thermogravimetric analysis. The absorption and emission spectra of three compounds
were experimentally determined in several solvents and were simultaneously computed using density
functional theory (DFT) and time-dependent density functional theory (TDDFT). The calculated reorga-
nization energy for hole and electron indicates that three compounds are in favor of hole transport than
electron transport. The calculated absorption and emission wavelengths are well coincident with the
measured data. The calculated lowest-lying absorption spectra can be mainly attributed to intramolecu-
lar charge transfer (ICT). And the calculated fluorescence spectra can be mainly described as originating
from an excited state with intramolecular charge transfer (ICT) character. The results show that three
compounds exhibited excellent thermal stability and high fluorescence quantum yields, indicating their
potential applications as excellent optoelectronic material in optical field.
Keywords:
Benzothiazole
Carbazole
Absorption spectra
Emission spectra
TDDFT
© 2011 Elsevier B.V. All rights reserved.
1. Introduction
highlighted the design and investigations of carbazole derivatives
containing D–␲–A structure.
Carbazole is an important heterocyclic compound with a nitro-
gen atom as an electron donor. It has high thermal stability and
excellent photophysical property [1,2]. Because its ring is easily
functionalized and covalently linked to other molecules, a large
number of derivatives have been generated through chemical mod-
ification [3–8]. Owing to their great potential applications in many
fields, carbazole and its derivatives have been extensively investi-
gated during the past years [9–14].
The photophysical properties of carbazole can be improved
readily through derivatives. Thus, the development of new materi-
als with high-performance constitutes a very active area of research
[15–24].
Carbazole derivatives with D–␲–A structures have, in general,
a large electron delocalization length and the ability of experienc-
ing intramolecular charge transfer (ICT) from the electron donating
groups to electron accepting groups. In addition, the investigation
of carbazole derivatives with D–␲–A structure has been greatly
increased by the recent progresses in developing novel and efficient
active materials because of a series of new applications.
Recently, series novel carbazole derivatives with D–␲–A struc-
tures have been synthesized as efficient emitting materials in
organic devices. Many of these derivatives possess one or more
benzothiazole rings as the electron acceptors (A) and one carbazole
group as the electron donor (D). They have large electron delo-
calization lengths and are able to be experienced intramolecular
charge transfer (ICT) from the electron donor to the electron accep-
tors of the molecule. Their photophysical properties have also been
investigated in detail [25–28].
Up to now, the ꢀ-conjugated carbazole derivatives of the general
type D–␲–D, D–␲–A, D–␲–A–␲–D and A–␲–D–␲–A, where D and
A denote electron donating and accepting groups have been studied
widely by many research groups. A number of the literatures have
The carbazole derivatives with D–␲–A structure have great
potential applications. Therefore, the molecular design and syn-
thesis of novel carbazole derivatives with D–␲–A structure need
to be further developed by the structural modification, in order to
improve the properties and establish the relation between struc-
tures and properties.
∗
Corresponding author. Tel.: +86 351 7018094; fax: +86 351 7011688.
Corresponding author at: School of Chemistry and Chemical Engineering, Shanxi
∗∗
University, Taiyuan 030006, PR China. Tel.: +86 351 7018094; fax: +86 351 7011688.
E-mail addresses: hepingshi@sxu.edu.cn (H.-p. Shi), fangli@sxu.edu.cn (L. Fang).
1386-1425/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2011.07.017

H.-p. Shi et al. / Spectrochimica Acta Part A 81 (2011) 730–738
731
For this reason, we report the synthesis, structures and spec-
tral properties of three novel D–␲–A type carbazole derivatives
containing an electron donating carbazole and an electron with-
drawing benzothiazole in this paper. We also attempt to elucidate
the structure–property correlation of these molecules using exper-
imental and theoretical methods. Our results show that these
carbazole derivatives exhibited excellent thermal stability and
eminent photophysical properties as well as could served as
useful photoluminescence materials with blue-light emission.
The results will increase the knowledge to design and synthe-
size novel carbazole derivatives with excellent photophysical
properties.
(10 mmol), 2.82 g 1-bromo-4-iodobenzene (10 mmol), 0.53 g 18-
crown-6 (2.0 mmol), 6.07 g K2CO3 (44 mmol) and 1.47 g Cu
(23 mmol) were dissolved in 60 ml DMF in a 100 ml flask under
nitrogen atmosphere. The mixture was refluxed for 24 h under
nitrogen atmosphere. After cooling to room temperature, the
mixture was poured into ice-water and extracted with CH2Cl2
(3 × 50 ml). The combined organic layer was dried with anhydrous
Na2SO4 and evaporated to dryness. The compound was obtained
as a white solid by crystallized from ethanol (2.45 g, yield: 76%).
m.p. 142–144 ◦C [30]. 1H NMR (CDCl3, 300 MHz): ıppm 8.17–8.20
(d, 2H), 7.76–7.87 (d, 2H), 7.49–7.60 (d, 2H), 7.43–7.48 (t, 2H),
7.41–7.43 (d, 2H), 7.34–7.39 (t, 2H). MS (M+): 321.0351. Anal. Calcd.
for C18H12NBr: C, 67.10%; H, 3.75%; N, 4.35%. Found C, 66.93%; H,
3.71%; N, 4.31%.
2. Experimental
2.4.1.2. Synthesis of 4-(9H-carbazol-9-yl)benzaldehyde (2). To
a
2.1. Reagents
solution of 0.90 g 9-(4-bromo-phenyl)-9H-carbazole (1) (2.8 mmol)
in 30 ml anhydrous Et2O was added 1.34 ml of 2.5 M n-BuLi in
hexane (3.4 mmol) at −78 ◦C under nitrogen atmosphere. After
the reaction mixture was warmed to 0 ◦C, 0.26 ml DMF(3.4 mmol)
was added at −78 ◦C. The resulting mixture was allowed to warm
to room temperature and stirred at room temperature overnight.
Then the mixture was poured into ice water and 5% HCl (aq.)
(5 ml) was added and the two layers were separated. The aque-
ous layer was extracted with Et2O (3 × 20 ml) and the combined
organic layer was washed with brine (15 ml), dried with MgSO4
and evaporated under reduced pressure, a yellow power was
obtained by crystallized from ethanol (0.39 g, yield:51%). m.p.
148–150 ◦C [31]. 1H NMR (300 MHz, CDCl3) ıppm 10.12–10.20
(s, 1H), 8.12–8.16 (m, 4H), 7.78–7.86 (d, 2H), 7.49–7.63 (d, 2H),
7.32–7.44 (t, 2H), 7.19–7.30 (t, 2H). MS(M+) 271.0997. Anal. Calc. for
9-H-carbazole and 1-bromo-4-iodobenzene were pur-
chased from Alfa Aesar and used without further purifications,
2-aminothiophenol,
2-methylbenzothiazole
and
2-p-
tolylbenzothiazole were purchased from Aacros, and other
reagents were purchased from Beijing Chemical plants, DMF,
DMSO, Et2O, THF, toluene, CCl4, etc. were purified according to
standard methods used before. All reactions were carried out
under an inert atmosphere of nitrogen.
2.2. Apparatus
Melting points were determined on an X-5 melting point
detector and uncorrected. All NMR spectra were measured on
a Bruker DLX 300 MHz spectromenter with CDCl3 or DMSO as
solvent. 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).
UV–vis spectra were obtained on a shimadzu UV-2450 spec-
trophotometers. Fluorescence spectra were obtained on a F-4500
fluorothotometer. Fluorecence quantum yields were determined
using the a standard actinometry method. Quinine sulfate was used
in the actinometer with a known fluorescence quantum yield of
0.55 in 0.1 mol/L sulphuric acid, the sample was excited at 350 nm.
The fluorescence decay curves were recorded with time-correlated
single photon counting (TCSPC) technique using a commercially
available Edinburgh Instruments.
C19H13ON: C, 84.13%; H, 4.80%; N, 5.17%. Found: C, 84.40%; H, 4.77%;
N, 5.12%.
2.4.1.3. Synthesis of 9-(4-(benzothiazol-2-yl)phenyl)-9H-carbazole
(3). A mixture of the corresponding 0.41 g 4-(9H-carbazol-
9-yl)benzaldehyde (2) (1.50 mmol), 0.20 ml 2-aminothiophenol
(1.87 mmol), and 30 ml DMSO was heated in an oil bath to 120 ◦C,
held at that temperature for 12 h, then poured into water and
extracted with CH2Cl2 (3 × 10 ml). The combined organic layer
was dried with anhydrous Na2SO4 and evaporated to dryness.
The crude compound was stirred for 15 min in 30 ml of boil-
ing ethanol, then the solution was cooled, and the pure product
was isolated by filtration. (0.48 g, yield: 86%). m.p. 190–193 ◦C.
1H NMR (300 MHz, CDCl3) ıppm 8.11–8.17 (m, 2H), 7.82–7.97
(m, 4H), 7.72–7.75 (d, 2H), 7.50–7.59 (d, 2H), 7.41–7.46 (d, 2H),
7.25–7.34 (m, 4H). MS(M+) 376.1382. Anal. Calc. for C25H16N2S:
C, 79.78%; H, 4.26%; N, 7.45%. Found: C, 80.02%; H, 4.12%;
N, 7.27%.
2.3. Procedures
2.4.2. Synthesis of
9-(4-(2-(benzothiazol-2-yl)vinyl)phenyl)-9H-carbazole (4)
2.4.2.1. Synthesis of 1-(4-(9H-carbazol-9-yl)phenyl)-2-
The synthesis of these compounds were described in Section
2.4. All spectral experiments were carried out at room tem-
perature. The concentration of compounds is 1.0 × 10−5 mol/L
in spectral experiments. All emission spectra were cor-
rected. Excitation and emission slits width were both set at
2.5 nm.
(benzothiazol-2-yl)ethanol. Under nitrogen, to a solution of 0.16 ml
(1.2 mmol) of 2-methylbenzothiazole in 10 ml of anhydrous THF
at −78 ◦C was added dropwise, 0.56 ml (1.4 mmol) of 2.5 M n-BuLi
in hexane. The mixture was stirred at −78 ◦C for 30 min, then
0.27 g (1.0 mmol) of 4-(9H-carbazol-9-yl)benzaldehyde in 5 ml of
anhydrous THF was added dropwise. The reaction was stirred at
−78 ◦C for 1 h, then allowed to warm to ambient temperature
and stirred overnight. Next, 0.20 ml of acetic acid was added, and
the mixture was diluted with 50 ml of CH2Cl2. The solution was
dried over MgSO4, filtered, and concentrated in vacuo. A yellow
power was obtained by recyrstallized from ethanol. (0.38 g, yield:
75%). m.p. 176.5–178 ◦C. 1H NMR (300 MHz, CDCl3) ıppm 7.88–7.94
(d, 2H), 7.70–7.72 (d, 1H), 7.52–7.59 (d, 1H), 7.50–7.51 (d, 2H),
2.4. Synthesis and characterization of the compounds
The synthetic route of three compounds is shown in Scheme 1.
2.4.1. Synthesis of 9-(4-(benzothiazol-2-yl)phenyl)-9H-carbazole
(3)
2.4.1.1. Synthesis of 9-(4-bromo-phenyl)-9H-carbazole (1). 9-(4-
bromo-phenyl)-9H-carbazole (1) was synthesized following a
literature method [29].
A mixture of 1.67 g 9-H-carbazole

732
H.-p. Shi et al. / Spectrochimica Acta Part A 81 (2011) 730–738
Scheme 1. The synthetic route of three compounds.
7.40–7.43 (m, 2H), 7.27–7.30 (m, 6H), 7.26 (m, 2H), 5.44–5.46 (m,
1H), 3.55–3.57 (m, 2H), 2.84–2.85 (s, 1H). MS(M+) 420.1356. Anal.
Calc. for C27H20N2OS: C, 77.12%; H, 4.79%; N, 6.66%. Found: C,
77.20%; H, 4.84%; N, 6.72%.
2.4.3.2. Synthesis of 4-(2-benzothiazolyl)- benzyl triphenyl phospho-
nium bromide. Under nitrogen atmosphere, a mixture of compound
3.04 g 2-(4-(bromomethyl)phenyl)benzothiazole (10 mmol), 2.62 g
triphenyl phosphine (10 mmol) and 50 ml of freshly distilled CHCl3
was heated to refluxed for 24 h. On cooling, the solvent was
removed under reduced pressure gave the white solid which was
recrystallized from Et2O (4.98 g, yield: 88%). m.p. 295–297 ◦C. 1H
NMR (300 MHz, CDCl3): ıppm 7.98–8.08 (d, 3H), 7.72–7.84 (d, 1H),
7.48–7.70 (m, 15H) 7.43–7.45 (m, 3H), 7.26–7.33 (t, 1H), 5.72–5.77
(s, 2H) MS(M+): 566.0217 Anal. Calc. for C32H25BrNSP: C, 67.84%;
H, 4.42%; N, 2.47%. Found: C, 67.79%; H, 4.40%; N, 2.53%.
2.4.2.2. Synthesis
of
9-(4-(2-(benzothiazol-2-yl)vinyl)phenyl)-
9H-carbazole (4). To
a
mixture of 0.38 g (0.90 mmol) of
1-(4-(9H-carbazol-9-yl)phenyl)-2-(benzothiazol-2-yl)ethanol
and 0.02 g (0.09 mmol) of p-toluenesulfonic acid monohydrate
was added 20 ml of toluene. The reaction was heated at reflux
for 3 h, then cooled to ambient temperature and concentrated in
vacuo. The crude product was recyrstallized from ethanol to give a
brown yellow power. (0.18 g, yield: 51%). m.p. 195.3–196.8 ◦C. 1H
NMR (300 MHz, CDCl3) ıppm 8.15–8.17 (d, 2H), 8.03–8.04 (d, 1H),
7.89–7.90 (d, 1H), 7.82–7.83 (d, 2H), 7.63–7.66 (m, 2H), 7.48–7.51
(m, 4H), 7.41–7.47 (m, 4H), 7.30–7.32 (t, 2H) MS(M+) 402.1197.
Anal. Calc. for C27H18N2S: C, 80.57%; H, 4.51%; N, 6.96%. Found: C,
80.49%; H, 4.53%; N, 6.88%.
2.4.3.3. Synthesis of 9-(4-(4-(benzothiazol-2-yl)styryl)phenyl)-9H-
carbazole (5). Under nitrogen atmosphere, a stirred slurry of
1.00 g 4-(2-benzothiazolyl)-benzyl triphenyl phosphonium bro-
mide (1.77 mmol) in 30 ml anhydrous THF was cooled to −78 ◦C,
0.70 ml of 2.5 M n-BuLi in hexane (1.77 mmol) was added and the
mixture was warmed to room temperature over 2 h. Then a solu-
tion of 0.48 g 4-(9H-carbazol-9-yl)benzaldehyde (2) (1.77 mmol) in
10 ml THF was added dropwise with vigorous stirring at −78 ◦C.
The reaction was warmed to room temperature and stirred for
12 h. Then the THF was evaporated at reduced pressure and the
residue was separated by column chromatography (petroleum
ether/methylene chloride 1:3) to give a yellow power.(0.20 g, yield:
24%). m.p. 263–265 ◦C. 1H NMR (300 MHz, CDCl3): ıppm 8.11–8.13
(m, 1H), 7.94–7.82 (m, 1H), 7.69–7.71 (m, 2H), 7.60–7.62 (m, 4H),
7.51–7.53 (d, 2H), 7.42–7.45 (d, 2H), 7.38–7.40 (m, 2H), 7.26–7.36
(d, 2H), 7.21–7.24 (m, 4H), 7.15–7.18 (m, 2H). MS(M+): 478.1528.
Anal. Calc. for C33H22N2S: C, 82.85%; H, 4.60%; N, 5.86%. Found: C,
82.91%; H, 4.58%; N, 5.77%.
2.4.3. Synthesis of
9-(4-(4-(benzothiazol-2-yl)styryl)phenyl)-9H-carbazole (5)
2.4.3.1. Synthesis
of
2-(4-(bromomethyl)phenyl)benzothiazole
[27]. 2.71 g 2-p-Tolylbenzothiazole (12 mmol) and 2.14 g N-
bromosuccinimide (NBS) (12 mmol) were dissolved in 50 ml of
CCl4. The solution was heated under reflux for 4 h. The precipitated
succinimide was filtered, and the solvent was evaporated from the
solution. The remaining gray oil was recrystallized from ethanol.
The obtained white crystalline powder was dried in vacuum
(3.43 g, 94%). m.p. 132–133 ◦C [27]. 1H NMR (300 MHz, CDCl3):
ıppm 8.08–8.13 (d, 3H), 7.92–7.99 (d, 1H), 7.53–7.76 (m, 3H),
7.42–7.50 (t, 1H), 4.55–4.62 (s, 2H). MS(M+): 302.9717 Anal. Calc.
for C14H10BrNS: C, 55.45%; H, 3.30%; N, 4.62%. Found: C, 55.38%; H,
3.40%; N, 4.57%.
The thermal stability of three compounds was measured using
thermogravimetric analysis (TGA). The result reveals that three

H.-p. Shi et al. / Spectrochimica Acta Part A 81 (2011) 730–738
733
styryl)phenyl)-9H-carbazole (5) was resulted by the Wittig
condensation of 4-(9H-carbazol-9-yl)benzaldehyde (2) with cor-
responding phosphonium salt. All of these new compounds were
characterized by mass spectrometry and 1H NMR spectroscopy,
further details are given in Section 2.
4.1. Structures of the compounds
The optimized geometrical parameters for the three compounds
in the ground state are compiled in Table S1. Table S1 is placed
in supporting information. and the optimized geometries of these
compounds in the ground state are shown in Fig. 2. According to
the data listed in Table S1, we can see three compounds possess
similar molecular structures. Each compound contains two planar
moiety in its molecular structure, one planar moiety is carbazole
ring and the other is the rest of the molecular. The dihedral angles
C4–N7–C14–C15 and C4–N7–C14–C19 of the two planar moiety
are −52.24◦ and 127.74◦, 52.11◦ and −127.71◦ as well as 53.08◦
and −126.69◦ in compound (3), compound (4) and compound (5),
respectively. It is concluded that three compounds show the dis-
torted geometrical structures.
Fig. 1. The molecular structures as calculation models.
4.2. Frontier molecular orbitals of three compounds in the ground
state
compounds exhibits excellent thermal stability up to 250 ◦C, 280 ◦C
and 350 ◦C, respectively.
The frontier molecular orbitals are the most important indi-
cation in determining the spectral properties and the ability of
electron or hole transport of molecules [38]. In general, extended
conjugation decreases the gap between the HOMO and LUMO, thus
causing bathochromic shift in UV–vis absorbtion.
3. Calculation method and models
The ground-state geometries as well as their ionic structures
of three carbazole derivatives were optimized at B3LYP level with
6-31G(d, p) basis set [32,33]. The vibration frequencies and 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 them were calculated by DFT method based on the optimized
geometry of the neutral and ionic molecules. The excited-state
geometries of three derivatives were optimized at the configura-
tion interaction with single excitation (CIS) level with 6-31G(d,
p) basis set [34]. The absorption spectra and the emission spectra
of three derivatives were carried out using time-dependent den-
sity functional theory (TD-DFT) method based on the optimized
ground state structures and the lowest singlet excited-state struc-
tures, respectively. Solvent effects were also taken into account
by using the polarized continuum model (PCM) [35,36]. All cal-
culations were carried out with the Gaussian03 program package
[37]. All the calculations were performed using the advanced com-
puting facilities of supercomputing center of computer network
information center of Chinese Academy of Sciences. The molecular
structures as calculation models are shown in Fig. 1.
The contour plots of HOMO and LUMO of the three compounds
were exhibited in Fig. 3. It can be seen from Fig. 3 that the elec-
tion density of HOMO is mainly localized on the carbazole ring of
each compound and it is ␲-bonding orbitals. And the election den-
sity of LUMO is mainly localized on the benzothiazole ring of each
compound and it is ␲*-bonding orbitals.
The energies of the HOMO and LUMO as well as the
HOMO–LUMO energy gap of three compounds are given in Table 1.
As shown in Table 1, with the increase of conjugation, the energy of
highest occupied molecular orbital (HOMO) is increased, while the
energy of lowest unoccupied molecular orbital (LUMO) is reduced.
The energy gap of three compounds was reduced with the increase
of conjugation. The electronic transition from the ground state to
the excited state is mainly about an electron flowing from the car-
bazole ring of each compound to the benzothiazole ring of each
compound, which belongs to ␲–␲* transition.
4.3. Charge injection and transport properties of three
compounds in their ground states
The charge injection and transport ability are important param-
eters for OLEDs. In general, ionization potential and electronic
affinity are used to evaluate the energy barrier for injection holes
and electrons. Normally, the lower IP value of the hole-transport
layer (HTL), the easier the entrance of holes from ITO to HTL; and
the higher EA value of the electron transport layer (ETL), the easier
the entrance of electrons from cathode to ETL. In order to fur-
ther understand charge injection and transport properties of three
compounds, we calculated their ionization potentials, electronic
affinities. The calculated results are listed in Table 2. As shown in
Table 2, the compound (5) has lower IP value, it is easier to trans-
port hole than other compounds and the compound (5) has higher
EA value, it is easier to transport electron than other compounds.
These results are consistent with the indication from the energies
analysis of their HOMOs and LUMOs.
4. Results and discussion
In this paper, we described methods for the preparation of
carbazole derivatives. First, 4-(9H-carbazol-9-yl)benzaldehyde (2)
was synthesized as the key intermediate for the whole procedure
via a two step reaction as shown in Scheme 1. This compound
(2) served as the central core. Second, 9-(4-(benzothiazol-2-
yl)phenyl)-9H-carbazole (3) was prepared in high yield by stirring
2-aminothiophenol and 4-(9H-carbazol-9-yl)benzaldehyde (2) in
DMSO at 120 ◦C.
Then 9-(4-(2-(benzothiazol-2-yl)vinyl)phenyl)-9H-carbazole
(4) was synthesized from the reaction between 2-methyl-
benzothiazole and 4-(9H-carbazol-9-yl)benzaldehyde (2) via
lithiation with n-butyllithium. Finally, 9-(4-(4-(benzothiazol-2-yl)

734
H.-p. Shi et al. / Spectrochimica Acta Part A 81 (2011) 730–738
Fig. 2. The optimized geometries of three compounds in the ground state.
The charge mobility in organic materials is often described by a
hopping model. The charge transport rate can be approximated by
the Marcus electron-transfer theory with Eq. (1) [39].
Table 1
ꢁ
ꢂ
HOMO and LUMO energy, and HOMO–LUMO energy gap of three compounds
obtained from DFT/B3LYP/6-31G(d, p) calculation (eV).
4ꢀ2
h
1
ꢂhole/electron
khole/electron
=
t2 exp
−
ꢀ
4kBT
4ꢀꢂhole/electronkBT
Compounds
HOMO
LUMO
ꢁEHOMO–LUMO
(1)
Compound 3
Compound 4
Compound 5
−5.415
−5.361
−5.279
−1.769
−2.095
−2.095
3.646
3.266
3.184
where h is Planck’s constant and kB is Boltzmann’s constants, T is
the temperature, t is the electronic transfer integral between the
donor and acceptor molecules, and ꢂhole/electron is the reorganiza-
tion energy for hole or electron transfer between both molecules.
Therefore, the transport process is determined by two important
molecular parameters: the reorganization energy (ꢂ) which needs
to be small for an efficient charge-transport and the intermolecular
hole or electron transfer integral (t) which describes the strength
of the electronic coupling between adjacent molecules [40].
Table 2
Ionization potentials, electron affinities and reorganization energy of three com-
pounds obtained from DFT/B3LYP/6-31G(d, p) calculation (eV).
Compounds
IPv
IPa
ꢂHol
EAv
EAa
ꢂEle
Compound 3
Compound 4
Compound 5
6.698
6.567
6.371
6.635
6.483
6.279
0.128
0.171
0.186
0.403
0.780
0.928
0.596
0.980
1.098
0.415
0.411
0.347

H.-p. Shi et al. / Spectrochimica Acta Part A 81 (2011) 730–738
735
Fig. 3. The contour plots of HOMOs and LUMOs of three compounds in the ground state.
The reorganization energy for hole transfer, ꢂhole, is the sum of
two contributions, ꢂ+1 + ꢂ+2, that are defined as
ꢂhole exhibits lower value than corresponding ꢂelectron. It indicates
that three compounds are in favor of hole transport than elec-
tron transport, thus these compounds are potential hole transport
materials.
ꢂ+1 = E0(M+) − E0(M)
ꢂ+2 = E1(M) − E1(M+)
(2)
(3)
where E0(M+) and E0(M) represent the energies of the neutral
molecule at the cation geometry and at the optimal ground-state
geometry, respectively. E1(M) and E1(M+) represent the energy of
the charged state at the neutral geometry and optimal cation geom-
etry, respectively.
Similarly, the reorganization energy for electron transfer,
ꢂelectron, is also the sum of two contributions, ꢂ−1 + ꢂ−2, that are
defined as
4.4. UV–vis spectra of three compounds
The UV–vis absorption spectra of three compounds have been
studied in various solvents of different polarity and the spectral
data have been collected in Table 3. The spectra are shown in Fig. 4.
As indicated in the absorption spectra, three compounds exhibit
two absorption bands at 266–303 and 300–400 nm due to ␲–␲*
electronic transitions, which are almost the same in the differ-
ent polar solvents meaning the independence of UV absorption to
the solvent polarity. The low-energy broad band at 300–400 nm
assigned to an intramolecular charge transfer (CT) band from the
carbazole ring of each compound to the benzothiazole ring of each
compound in all solvents.
In order to gain a detailed insight into the nature of the UV–vis
absorption of three compounds observed experimentally, we com-
puted singlet–singlet electronic transition in gas phase and in
different polar solvents based on the optimized geometries of
the ground state of three compounds, using time-dependent DFT
ꢂ
= E0(M−) − E0(M)
= E1(M) − E1(M−)
(4)
−1
−2
ꢂ
(5)
where E0(M−) and E0(M) represent the energies of the neutral
molecule at the anion geometry and at the optimal ground-state
geometry, respectively. E1(M) and E1(M−) represent the energy
of the charged state at the neutral geometry and optimal anion
geometry, respectively.
In terms of above calculated model, the calculated reorganiza-
tion energy for hole and electron are also listed in Table 2. Where

736
H.-p. Shi et al. / Spectrochimica Acta Part A 81 (2011) 730–738
0.30
0.25
0.20
0.15
0.10
0.05
0.00
10000
Hex
Hex
CH Cl
CH Cl
2
2
2
2
8000
6000
4000
2000
0
CHCl
CHCl
3
3
Acetone
Acetone
CH CN
CH CN
3
3
CH OH
3
CH OH
3
DMSO
DMSO
400
450
500
550
600
350
300
350
400
450
Wavelength(nm)
Compound (3)
Wavelength(nm)
compound (3)
1800
1600
1400
1200
1000
800
600
400
200
0
0.2
0.1
Hex
2
CHCl
CH Cl
Hex
2
3
CH Cl
2
2
Acetone
CHCl
CH CN
3
3
CH OH
3
Acetone
CH CN
DMSO
3
CH OH
3
DMSO
400
450
500
550
600
650
0.0
0.3
Wavelength(nm)
300
350
400
450
compound (4)
Wavelength(nm)
Compound (4)
10000
8000
6000
4000
2000
0
Hex
CH Cl
2
2
Hex
CHCl
3
CH Cl
Acetone
CH CN
2
2
3
3
CHCl
3
CH OH
DMSO
Acetone
CH CN
CH OH
0.2
0.1
0.0
3
3
DMSO
400
450
500
550
600
650
Wavelength(nm)
compound (5)
300
350
400
450
Wavelength(nm)
Compound (5)
Fig. 5. Emission spectra of three compounds in different polar solvents.
Fig. 4. The UV–vis spectra of three compounds in different polar solvents.
information. As seen in Fig. S1 and Table S2, the electronic tran-
sitions are of ␲␲* type. The calculated S0 → S1 excitation energy,
oscillator strength of three compounds in different polar solvents
method at the B3LYP/6-31G (d, p) level. The computed data of
vertical electronic transitions of S0 → S1, S0 → S2 and S0 → S3 are
collected in Table S2. Table S2 is placed in supporting information.
On the basis of the calculated vertical excited energy and their cor-
responding oscillator strengths, the continuous absorption spectra
were simulated with the help of SWIZARD software with the width
at half-height of 2500 cm−1. The simulated absorption spectra of
three compounds are shown in Fig. S1. Fig. S1 is placed in supporting
are 25,480–25,661 cm−1, 0.5277–0.5586 and 23,167–23,396 cm−1
,
0.7812–0.8460 as well as 22,795–23,016 cm−1, 1.2464–1.3942,
respectively. All the electronic transitions herein are strongly
allowed. The calculated S0 → S1 vertical excitation energy data of
three compounds are lower than the experimental value in differ-
ent polar solvents. This is due to that the solvent effect is taken
into account, but the role of the solvent effect has not been well

H.-p. Shi et al. / Spectrochimica Acta Part A 81 (2011) 730–738
737
Table 3
UV–vis spectra data and fluorescence spectra of three compounds in several solvents.
Compounds
Solvents
UV wavelengths (␭ nm)
FL wavelengths (␭ nm)
Stokes shift (cm−1
)
Hexane
343
343
343
342
341
341
345
339
342
341
341
341
340
343
357
364
363
362
361
361
371
394
436
427
445
463
470
453
422
466
459
481
493
498
500
438
468
460
474
497
499
498
3774
6219
5736
6766
7728
8049
6911
5802
7781
7540
8536
9042
9332
8788
5180
6106
5809
6527
7581
7661
6874
Compound 3
Dichloromethane
Chloroform
Acetone
Acetonitrile
Methanol
DMSO
Hexane
Compound 4
Compound 5
Dichloromethane
Chloroform
Acetone
Acetonitrile
Methanol
DMSO
Hexane
Dichloromethane
Chloroform
Acetone
Acetonitrile
Methanol
DMSO
reflected. However, the change trends of the absorption spectra
in different solvents could be well reflected. Therefore, we can
know that this calculation method is imperfect in the calculation
of the solvent effect on the absorption spectra, and it needs further
improvement.
the continuous emission spectra were simulated with the help of
SWIZARD software with the width at half-height of 2500 cm−1
.
The simulated emission spectra of three compounds are shown in
Fig. S2. Fig. S2 is placed in supporting information. As can be seen in
Fig. S2 and Table S2, the electronic transitions are of the ␲–␲* type.
The calculated S1 → S0 emission energy, oscillator strength of three
compounds in different polar solvents are 23,166–23,318 cm−1
,
4.5. Fluorescence spectra of three compounds
0.8633–0.9095 and 20,798–20,985 cm−1, 1.1756–1.2396 as well as
20,523–20,732 cm−1, 1.6607–1.7945, respectively. All these elec-
tronic transitions are strongly allowed. The simulated maximum
emission wavelengths of three compounds are 432, 481 and 487 nm
in different polar solvents. The simulated emission spectra are
in relatively good agreement with the experimental fluorescence
spectra of three compounds. But the maximum FL wavelength of
compounds not show red-shift in polar solvents. This is due to
that the solvent effect is taken into account, but the polarity of the
solvents on the emission spectra have not been well reflected. How-
ever, the change trends of the emission spectra in different solvents
could be well reflected. Therefore, we can know that this calcula-
tion method is imperfect in the calculation of the solvent effect on
the emission spectra, and it needs further improvement. Simulation
emission spectra show three compounds in different polar solvents
can emit blue light and it might be potential luminescent materials
with blue light emission.
The steady-state fluorescence spectra of three compounds were
measured in different polar solvents and were shown in Fig. 5. Their
spectral data were collected in Table 3. The emission spectra of
three compounds consist of one broad band except in n-hexane,
where some structure could be observed. This band can be assigned
to the S1 → S0 electronic transition. The spectra of three compounds
show a large stokes shift in polar solvents, accompanied by losing
of the fine structure and an increase in the fluorescence half-width.
As can be seen in Fig. 5 and Table 3, with the solvent changes
from n-hexane to acetonitrile (ACN), the maximum emission wave-
lengths of three compounds were redshifted from 393 to 461 nm,
427 to 496 nm and 436 to 497 nm, respectively. The stocks shifts
for three compounds are larger in polar solvents than in non-polar
solvents showing a energetic stabilization of the excited state in
polar solvents.
In order to gain insight into the nature of the fluorescence emis-
sion observed for three compounds, the geometries of the first
excited singlet state (S1) were optimized for three compounds.
The optimized geometrical parameters for three compounds in
the first excited state are also compiled in Table S1. According to
the data listed in Table S1, we can see these compounds possess
similar molecular structures in the first excited state and in the
ground state, respectively. The dihedral angles C4–N7–C14–C15
and C4–N7–C14–C19 are −48.41◦ and 131.36◦, 50.90◦ and −129.23◦
as well as 54.82◦ and −125.19◦ in compound (3), compound (4) and
compound (5), respectively.
4.6. molecular photophysical properties
The fluorescence quantum yields of three compounds were
measured in several organic solvents at room temperature by a
relative method using quinine bisulfate in 0.1 M sulphuric acid as a
standard. The fluorescence quantum yield was calculated from Eq.
(6) [41].
ꢃ
ꢄ
2
Fs Ar nr
ꢃs = ꢃr
(6)
The optimized geometries of three compounds in the first
excited state (S1) were used as input to calculate singlet–singlet
electronic transition using time-dependent DFT method at the
B3LYP/6-31G(d, p) level in gas phase and in different polar solvents,
respectively, yielding the vertical electronic transitions energy of
S1 → S0. The computed data are collected in Table S3. Table S3 is
placed in supporting information. On the basis of the calculated ver-
tical excited energy and their corresponding oscillator strengths,
Fr As ns
In the above equation, Ф is the fluorescence quantum yield,
F is the integration of the emission intensities, n is the index of
refraction of the solution, and A is the absorbance at the excitation
wavelength, the subscripts “r” and “s” denote the reference and
unknown samples, respectively. The fluorescence quantum yields
(Ф) were collected in Table 4.

738
H.-p. Shi et al. / Spectrochimica Acta Part A 81 (2011) 730–738
Table 4
The fluorescence quantum yields (Ф) and the fluorescence lifetimes (ꢄ, ns) of three compounds in different polar solvents.
Solvents
Fluorescence quantum yields (Ф)
Fluorescence lifetime (ꢄ, ns)
Compound 3
Compound 4
Compound 5
Compound 3
Compound 4
Compound 5
Hexane
0.34
0.54
0.56
0.42
0.37
0.19
0.32
0.14
0.22
0.17
0.22
0.12
0.08
0.17
0.19
0.46
0.44
0.43
0.38
0.36
0.32
3.66
3.98
4.11
4.15
4.15
2.12
4.13
2.43
2.68
2.76
2.69
2.74
1.70
2.73
1.49
1.65
1.67
1.75
1.73
1.29
1.80
Dichloromethane
Chloroform
Acetone
Acetonitrile
Methanol
DMSO
The fluorescence decay behaviors of three compounds were also
studied in several solvents. The decay profile of compound (3) was
shown in Fig. S3 as representative. Fig. S3 is placed in supporting
information. The fluorescence lifetimes (ꢄ, ns) were collected in
Table 4. For three compounds, the fluorescence decay curves were
mono-exponential model.
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were synthesized and characterized by elemental analysis, NMR,
MS and thermogravimetric analysis. The calculated absorption
and emission wavelengths are well coincident with the measured
data. The calculated reorganization energy for hole and electron
indicates that three compounds are in favor of hole transport than
electron transport. The calculated lowest-lying absorption spectra
can be mainly attributed to intramolecular charge transfer (ICT).
And the calculated fluorescence spectra can be mainly described
as originating from an excited state with intramolecular charge
transfer (ICT) character. The results show that three compounds
exhibited excellent thermal stability and high fluorescence quan-
tum yields, indicating their potential application as excellent
optoelectronic material in optical field. Such studies are currently
under way in our laboratory, and the results will be released soon.
Acknowledgements
This work was supported by Shanxi Province scientific and
technological project (No. 20100321085) and Fund of Key Labo-
ratory of Optoelectronic Materials Chemistry and Physics, Chinese
Academy of Sciences (No. 2011KL004). All the calculations have
been performed using the advanced computing facilities of super-
computing center of computer network information center of
Chinese Academy of Sciences. All the authors express their deep
thanks.
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Appendix A. Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.saa.2011.07.017.
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