Properties, theoretical study and crystal structure of 3-benzothiazole-9-ethyl carbazole

Article abstract of DOI:10.1002/bio.3095

The title compound of 3-benzothiazole-9-ethyl carbazole was synthesized by the reaction of 3-aldehyde-9-ethyl carbazole and 2-aminothiophenol. The compound was characterized by ¹H nuclear magnetic resonance (NMR) and mass spectrometry (MS). Its crystal structure was obtained and determined by single crystal X-ray diffraction. The results showed that the crystal belongs to the orthorhombic crystal system and the cell parameters of space group P2(1)2(1)2(1) were a?=?5.6626 (12) ?, b?=?12.606 (3) ?, c?=?22.639 (5) ?, α?=?90°, β?=?90°, γ?=?90°, V?=?1616.0 (6) ?³, Z?=?4, Dc?=?1.350?mg/m³. The UV–vis and fluorescence spectra were also studied preliminarily. The fluorescence spectra of the title compound with bovine serum albumin (BSA) showed that BSA could be marked with the compound and the stability constant between them was 0.82?×?10⁷?M^?1. Meanwhile, the crystal and molecule were theoretically surveyed by density functional tight-binding (DFTB). The results showed that there was an orbital overlap for lowest unoccupied molecular orbital (LUMO) between the neighbouring molecules for the crystal, which is different from the molecule structure. It was also showed that the crystal structure is a non-conductor. Copyright

Full text of DOI:10.1002/bio.3095

Research article
Received: 14 October 2015,
Revised: 17 December 2015,
Accepted: 21 December 2015
Published online in Wiley Online Library
(wileyonlinelibrary.com) DOI 10.1002/bio.3095
Properties, theoretical study and crystal
structure of 3-benzothiazole-9-ethyl carbazole
Yingchun Gu,a,b† Dayong Lin,c† Xuening Fei,a,c* Cuihong Wang,a Ling Li,a
Yalin Tangb* and Jianguo Zhoua
ABSTRACT: The title compound of 3-benzothiazole-9-ethyl carbazole was synthesized by the reaction of 3-aldehyde-9-ethyl
carbazole and 2-aminothiophenol. The compound was characterized by 1H nuclear magnetic resonance (NMR) and mass
spectrometry (MS). Its crystal structure was obtained and determined by single crystal X-ray diffraction. The results showed that
the crystal belongs to the orthorhombic crystal system and the cell parameters of space group P2(1)2(1)2(1) were a = 5.6626 (12)
Å, b = 12.606 (3) Å, c = 22.639 (5) Å, α = 90°, β = 90°, γ = 90°, V = 1616.0 (6) Å3, Z = 4, Dc = 1.350 mg/m3. The UV–vis and fluorescence
spectra were also studied preliminarily. The fluorescence spectra of the title compound with bovine serum albumin (BSA) showed
that BSA could be marked with the compound and the stability constant between them was 0.82 × 107 Mꢀ1. Meanwhile, the
crystal and molecule were theoretically surveyed by density functional tight-binding (DFTB). The results showed that there
was an orbital overlap for lowest unoccupied molecular orbital (LUMO) between the neighbouring molecules for the crystal,
which is different from the molecule structure. It was also showed that the crystal structure is a non-conductor. Copyright ©
2016 John Wiley & Sons, Ltd.
Keywords: 3-benzothiazole-9-ethyl carbazole; spectra properties; crystal structure; theoretical study; stability constant
Introduction
Experimental
Materials and methods
The carbazole nucleus can be easily functionalized at its 3, 6 and 9
positions and covalently linked with other molecular groups. Up to
now, carbazole derivatives have been widely utilized as a func-
tional building block in the fabrication of the organic medicine,
pesticide, material, etc. because of their excellent solubility, stabil-
ity, excellent hole-transporting capability (the electron-donating
properties) and biological activity (1–6). In addition, carbazole
and its derivatives can be used as a strong electro-donating
chromophore, which is the main structure of fluorescent dyes.
Recently, serious novel carbazole derivatives have been synthe-
sized as efficient emitting materials in organic devices.
All the reagents and solvents were commercial and used without
further purification. Melting points were measured by using a melt-
ing point apparatus made by Beijing Tech. Instrument Co. UV–Vis
absorption spectra were recorded on a Perkin Elmer LAMBDA 35
spectrometer. Fluorescence spectra were recorded on a Hitachi
F-7000 spectrometer. Mass spectral analyses were obtained using
an electrospray ionization (ESI) mass spectrometer on MICROMASS
QUATTRO micro API (Waters Corporation, America). 1H NMR spec-
tra were recorded on a Bruker (400 MHz) spectrometer. Chemical
shifts were reported in parts per million (ppm) downfield from
Benzothiazole derivatives are widely used and are of good
potential in the preparation of organic photoconductors and other
fields because of the thick heterocyclic system and electron-
acceptor moieties and strong fluorescence effect (7,8). For this
reason, using benzothiazole ring as the electron acceptors (A)
and carbazole ring as the electron donor (D), researchers have
combined carbazole with benzothiazole to prepare many
carbazole–benzothiazole derivatives with different fluorescent
properties (9–13). Because of their large electron delocalization
lengths, they are used to experienced intramolecular charge trans-
fer (ITC) from A to D of the molecules.
So, it is very important for the applications of carbazoles as optical
materials to understand the relationship between optical properties
and molecule structure, especially the effect of π-conjugated struc-
ture on the optical properties. In order to understand the energy,
geometry structure and properties of carbazole–benzothiazole de-
rivative in detail, we report the crystal structures, theoretical
calculation and the spectra properties of 3-benzothiazole-9-ethyl
carbazole in this article.
* Correspondence to: Xuening Fei, Tianjin Chengjian University, 26 Jinjing Road,
Xiqing District, Tianjin China. Tel: +86 22 23773360. E-mail: xueningfei@126.
com
* Yalin Tang, Institute of Chemistry Chinese Academy of Sciences, Beijing,
100190, China. E-mail: tangyl@iccas.ac.cn
†
These authors contributed equally to this work and should be considered co-
first authors.
a
School of Science, Tianjin Chengjian University, Tianjin, 300384, China
b
Institute of Chemistry Chinese Academy of Sciences, Beijing, 100190, China
c
School of Environmental Science and Engineering, Tianjin University, Tianjin,
300072, China
Abbreviations: BSA, bovine serum albumin; DFTB, density functional tight-
binding; DOS, Disk Operation System; HOMO, highest occupied molecular or-
bital; HPLC, high pressure liquid chromatography; LUMO, lowest unoccupied
molecular orbital; TICT, twisted intramolecular charge transfer.
Luminescence 2016
Copyright © 2016 John Wiley & Sons, Ltd.

Y. Gu et al.
tetramethylsilane (TMS) using CDCl3 or DMSO-d6 as a solvent. The
chemical purity is analyzed by high pressure liquid chromatogra-
phy (HPLC) Lab Alliance PC4006A from the USA (flow rate:
1.0 mL/min; 4.6 mm × 250 mm, 5 μm, C18 reversed-phase column).
2-aminobenzenethiol was added. The mixture was slowly
heated to 333 K. The reaction was monitored by TCL (ethyl
acetate: petroleum ether =1:10) and after 4 h, the mixture
was cooled to room temperature. The resultant was
poured into a mixture of ice-water and faint yellow precip-
itate was obtained. After filtration, the crude product was
purified by silica-gel column chromatography to get the
title compound. M. p.: 142–145 °C, yield: 80%. Purity:
99.7% (HPLC, solution of 80% CH3OH and 20% H2O was
chosen as the mobile phase). The brown single crystal were
obtained by evaporation for petroleum ether and ethyl
acetate (4:1 = v/v) after a few days. 1H NMR (400 MHz,CDCl3)
δ:, 1.45–1.49 (t, J = 7.20 Hz, 3 H, 4.37–4.43 (m, 2 H), 7.28–7.38
(m, 2 H), 7.43–7.54 (m, 4 H), 7.91 (d, J = 8.00 Hz, 1 H), 8.08 (d,
J = 8.00 Hz, 1 H), 8.21 (d, J = 7.60 Hz, 2 H), 8.86 (s, 1 H). ESI-MS
(m/z): 329.9 [M++1].
Syntheses of 3-benzothiazole-9-ethyl carbazole
The title compound was synthesized according to the reference
(14) and the synthetic route was shown in Scheme 1.
The detailed synthetic processes are shown below:
•
15 mL of anhydrous N,N-dimethylformamide (DMF) was added
slowly to 15.202 g of POCl3 (0.1 mol) under stirring in an ice bath
at room temperature until the solution became red. Then
3.155 g of 9-ethylcarbazole (16 mmol) dissolved in 25 mL
1,2-dichloroethane were added. The mixture was heated to
353 K and kept reacting at this temperature. The reaction was
monitored by thin layer chromatography (TCL) (ethyl acetate:
petroleum ether =1:10) and after 8 h, the mixture was cooled
to room temperature. The resultant was poured into a mixture
of ice-water and followed by extraction with dichloromethane
(3 × 100 mL). The resultant dichloromethane solution was
washed three times with 100 mL of water and dried over anhy-
drous magnesium sulphate. Afterwards the solvent was evapo-
rated under reduced pressure and the residue was dissolved in
a minimum amount of dichloromethane and then purified by
silica-gel column chromatography to get 3-formyl-9-ethyl carba-
zole. M. p.: 84–86 °C, yield: 50%. Purity: 99.8% (HPLC, a solution of
80% CH3OH and 20% H2O was chosen as the mobile phase).
560 mg (2.5 mmol) of 3-formyl-9-ethyl carbazole were
dissolvedin 25 mL of DMSO and then 0.27 mL (2.5 mmol) of
Structure determination and refinement
A brown crystal of the title compound was mounted on the
bottom of a glass fiber. X-ray diffraction data were collected on
a Bruker Smart-1000 CCD diffractometer equipped with a
graphite-monochromatized MoKα radiation (0.7170 Å) by using
an Ω/ψ scan mode in the range of 1.80 ≤ θ ≤ 27.88° (ꢀ7 ≤ h ≤ 7,
ꢀ16 ≤ k ≤ 15, ꢀ29 ≤ l ≤ 29) at 113 (2) K. In total, 16 575 reflections
were collected with 3858 unique ones (Rint = 0.0434). The struc-
ture was solved by direct methods and refined on F2 by full-
matrix least-squares procedure with SHELXS-97 and SHELXL-97
(15a). The crystal data and other structure refinement parame-
ters of the title compound (15b) are listed in Table 1.
•
Scheme 1. The synthetic route of 3-benzothiazole-9-ethyl carbazole.
Table 1. Crystal structure data for the compound
Empirical formula
C21 H16 N2 S
Formula weight
Wavelength
328.42
Temperature
Crystal system
a (Å)
c (Å)
113 (2) K
Orthorhombic
5.6626 (12)
22.639 (5)
90
1616.0 (6)
1.350
688
0.71073 A
P2 (1)2 (1)2 (1)
12.606 (3)
90
90
4
0.204
Space group
b (Å)
α (°)
γ (°)
Z
β (°)
Volume (Å3)
Calculated density (mg/m3)
F (000)
Absorption coefficient (mmꢀ1
Crystal size (mm)
)
0.20 × 0.18 × 0.12
ꢀ7 ≤ h ≤ 7, ꢀ16 ≤ k ≤ 15,
ꢀ29 ≤ l ≤ 29.
θ range for data collection (°)
1.80 to 27.88
Limiting indices
Reflections collected/unique
16575/3858
[R(int) = 0.0434]
3858/0/219
Refinement method
Full-matrix least-squares on F2
Data/restraints/parameters
Final R indices [I > 2σ(I)]
Max. and min. Transmission
Goodness-of-fit on F2
R indices (all data)
0.970
R1 = 0.0291, wR2 = 0.0750
0.9760 and 0.9604
R1 = 0.0311, wR2 = 0.0770
0.194 and ꢀ0.210
Largest diff. Peak and hole (e Åꢀ3
)
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Properties, theoretical study and crystal structure of 3-benzothiazole-9-ethyl carbazole
Results and discussion
Crystal structure
The selected bond lengths and bond angles are listed in Table 2,
and the selected torsion angles for non-hydrogen atoms are given
in Table 3. The molecular structure of the title compound is shown
in Fig. 1. The packing diagram of the title compound in a unit cell is
shown in Fig. 2.
The angles of C4-C5-C6-N1 (-178.54 (12)°), S1-C1-C2-C3 (179.02
(10)°), C7-S1-C1-C2 (-178.15 (13)°), C7-N1-C6-C5 (179.42 (12)°) show
that C4,C5,C6,C1,C2,C3,C7,N1,S1 are in the same plane. The angles
of N1-C7-C8-C9 (-2.18 (18)°), S(1)-C(7)-C(8)-C(19) (-2.92 (16)°) ,C(6)-N
(1)-C(7)-C(8) (-179.33 (11)°) , C(1)-S(1)-C(7)-C(8) (179.33 (10)°) show
that there is an angle of 2–3° between the thiazole ring and the
carbazole ring. The angle of C(9)-C(10)-C(17)-N(2) is 177.81 (11),
C(16)-N(2)-C(17)-C(18) is 177.49 (13), C(12)-C(11)-C(16)-N(2) is
-179.16 (11) respectively, indicating that C9, C10, C17, N2, C16,
C11 and C12 are in a plane.
Figure 1. X-ray crystal structure of the title compound.
So, the two aromatic rings are of an almost coplanar structure
linked by the single bond from the selected torsion angles. The
structure elucidation was achieved by X-ray diffraction, and proved
cleanly that the reaction occurs to form the title compound.
Theoretical calculation
In this study, the energy and geometry structure of the crystal and
molecule structure was calculated by density functional tight-
binding (DFTB) method and complemented by an empirical
London dispersion energy term (DFTB-D) method. The structures
Table 2. Selected bond length(Å) and bond angles (°)
Bond
Dist.
Bond
Dist.
Bond
C (3)-C (4)
C (8)-C (19)
C (10)-C (11)
C (20)-H(20 A)
C (20)-C (21)
C (21)-H(21 A)
Angle
Dist.
S (1)-C (1)
S (1)-C (7)
N (1)-C (7)
N (1)-C (6)
N (2)-C (17)
C (7)-C (8)
Angle
1.7304 (14)
1.7577 (13)
1.3002 (17)
1.3921 (16)
1.3802 (16)
1.4687 (17)
(°)
N (2)-C (16)
N (2)-C (20)
C (1)-C (2)
C (1)-C (6)
C (2)-C (3)
C (8)-C (9)
1.3881 (18)
1.4607 (17)
1.3982 (18)
1.4034 (18)
1.381 (2)
1.398 (2)
1.4112 (18)
1.4516 (17)
0.9900
1.518 (2)
0.9800
1.3963 (17)
(°)
Angle
(°)
C (1)-S (1)-C (7)
C (17)-N (2)-C (20)
C (2)-C (1)-S (1)
N (1)-C (6)-C (1)
N (1)-C (7)-S (1)
C (9)-C (8)-C (7)
C (9)-C (10)-C (11)
C (12)-C (11)-C (10)
89.08 (6)
C (7) -N (1)-C (6)
C (16)-N (2)-C (20)
C (6)-C (1)-S (1)
C (8)-C (7)-S (1)
C (19)-C (8)-C (7)
C (17)-C (10)-C (11)
C (16)-C (11)-C (10)
N (2)-C (16)-C (11)
110.60 (11)
125.40 (11)
109.51 (9)
119.86 (9)
120.20 (11)
106.22 (11)
106.52 (12)
109.36 (12)
C (17)-N (2)-C (16)
C (2)-C (1)-C (6)
N (1)-C (6)-C (5)
N (1)-C (7)-C (8)
N (2)-C (16)-C (15)
N (2)-C (17)-C (18)
N (2)-C (20)-C (21)
N (2)-C (17)-C (10)
108.54 (11)
121.54 (12)
125.19 (12)
124.49 (11)
128.44 (14)
128.71 (13)
111.48 (12)
109.36 (11)
125.62 (12)
128.93 (11)
115.15 (12)
115.65 (10)
119.85 (11)
134.23 (13)
133.97 (13)
Table 3. Selected torsion angles (°)
Angle
(°)
Angle
(°)
C (7)-S (1)-C (1)-C (2)
C (7)-N (1)-C (6)-C (5)
C (6)-N (1)-C (7)-C (8)
N (1)-C (7)-C (8)-C (9)
ꢀ178.15 (13)
179.42 (12)
-179.33 (11)
-2.18 (18)
176.97 (12)
-178.86 (11)
1.3 (2)
-176.89 (14)
-176.06 (14)
11.7 (2)
177.27 (13)
-2.65 (19)
C (7)-S (1)-C (1)-C (6)
C (1)-S (1)-C (7)-C (8)
S (1)-C (7)-C (8)-C (9)
0.39 (10)
179.33 (10)
177.93 (9)
-2.92 (16)
175.53 (13)
178.54 (14)
1.89 (19)
-175.49 (13)
-172.05 (12)
-179.16 (11)
175.82 (12)
-9.7 (2)
177.81 (11)
-176.16 (13)
-89.40 (17)
82.19 (16)
S (1)-C (7)-C (8)-C (19)
C (8)-C (9)-C (10)-C (11)
C (17)-C (10)-C (11)-C (12)
C (16)-C (11)-C (12)-C (13)
C (17)-N (2)-C (16)-C (15)
C (20)-N (2)-C (16)-C (11)
C (12)-C (11)-C (16)-N (2)
C (10)-C (11)-C (16)-C (15)
C (20)-N (2)-C (17)-C (18)
C (9)-C (10)-C (17)-N (2)
N (2)-C (17)-C (18)-C (19)
C (17)-N (2)-C (20)-C (21)
C (16)-N (2)-C (20)-C (21)
N (1)-C (7)-C (8)-C (19)
C (7)-C (8)-C (9)-C (10)
C (9)-C (10)-C (11)-C (12)
C (9)-C (10)-C (11)-C (16)
C (10)-C (11)-C (12)-C (13)
C (20)-N (2)-C (16)-C (15)
C (14)-C (15)-C (16)-N (2)
C (12)-C (11)-C (16)-C (15)
C (16)-N (2)-C (17)-C (18)
C (20)-N (2)-C (17)-C (10)
C (11)-C (10)-C (17)-C (18)
C (7)-C (8)-C (19)-C (18)
177.49 (13)
172.29 (12)
-178.09 (12)
-179.86 (12)
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Y. Gu et al.
as the data reported previously (21). But for the crystal, the
θ = 177.3°, indicating that there is a large deformation and a distor-
tion between the benzothiazole ring and carbazole ring to form a
stable interaction. The results also indicate that the crystal struc-
ture is almost planar.
Frontier molecular orbitals. The frontier molecular orbitals, in-
cluding the highest occupied molecular orbital (HOMO) and the
lowest unoccupied molecular orbital (LUMO), can provide a rea-
sonable qualitative indication of the excitation properties and
the ability of electron or hole transport. Information about the
HOMO and LUMO distributions of the crystal and molecule in
gas phase respectively are shown in Fig. 5.
For 3-benzothiazole-9-ethyl carbazole in gas phase, we found
that the HOMO remains on the carbazole ring moiety with similar
distributions to that of previous theoretical studies of dipolar
molecules (11, 22), while LUMO is localized on the benzothiazole
ring and little on the ꢀC2H5 group, which indicated that the
electron was transferred from the ꢀC2H5 group to the carbazole
ring. For the crystal, The HOMO is similar to that of single molecule,
while there is an orbital overlap for LUMO between the
neighbouring molecules, meaning that there is a significant
interaction between them.
Net charge. Meanwhile, the net charge of the molecule and the
crystal structure were also calculated and some important atom
and their net charges are shown in Table 4.
Figure 2. A packing diagram for the crystal compound.
It is seen that when the molecule was formed to be crystral
structure, the net charges of the atoms changed. For example,
the net charges of atoms 1, 3, 4, 13 and 14 on benzothiazole de-
creased, which indicated that the electron transfer occurred.
The title charges of the atoms on the benzothiazole rings
(except H) is ꢀ0.40359 in the molecule and ꢀ0.41758 in the
crystal structure, title charges of the atoms on the carbazole
rings (except H) in the molecule is ꢀ0.58681 and ꢀ0.64787 in
the crystal structure, which indicated that the negative charges
on the both ring (except H) increased in the crystal structure.
were optimized using the conjugate gradient algorithm, and the
force converge criterion and the charge convergence criterion
were set to 10ꢀ4 a.u. and 10ꢀ5 electrons, respectively. All the
calculations were performed using the DFTB+ 1.2 program
(16–21). To make a comparison, we optimized both the crystal of
3-benzothiazole-9-ethyl carbazole and the molecular structure in
gas phase. For the crystal, the lattice parameters were set to be
20.8470 × 8.4679 × 20.8720 for the lengths. The crystal cell and
single molecule structures are shown in Fig. 3.
Side view and dihedral angles of conformer. The side view
and dihedral angles of conformer of 3-benzothiazole-9-ethyl car-
bazole in crystal and gas phase were also calculated (Fig. 4). We
can see that the structure torsion occurred when the molecule
formed crystal structure. The bond angles and dihedral angle
changed. The torsional energy curves indicate that the dihedral an-
gle of the molecule is more stable for θ = 179.9°, which is the same
The density of states of the crystal. The Disk Operation System
(DOS) of the crystal was also calculated and is shown in Fig. 6. The
energy shown by the dashed line is the Fermi energy.
We can see that there is a wide band gap (ΔE = 1.6 eV), which
indicated that the crystal structure is a non-conductor. The two
peaks of the wide pseudogap indicated strong covalent bonding
in the crystal molecule.
(a)
(b)
Figure 3. Optimized structures of the ground state of (a) crystal cell; and (b) single molecule.
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Properties, theoretical study and crystal structure of 3-benzothiazole-9-ethyl carbazole
Figure 4. Side view, bond angles and dihedral angle of (a) crystal; and (b) molecule.
Figure 5. (a) HOMO and (b) LUMO diagrams of the crystal.
Spectral properties
Table 4. Net charges of some atoms
Spectral properties in different solvents. The UV–Vis
and fluorescence spectra were measured. The sample was mea-
sured at room temperature in the range of 300–400 nm in the
UV–Vis spectrum and the fluorescence spectrum was excited at
295 nm in the range of 350–550 nm
The UV–Vis and fluorescence spectra in different solvents were
studied and the results are shown in Fig. 7 and Table 5.
The compound has two absorption peaks at ca. 307 nm and ca.
345 nm respectively in different solvents. The peak at ca. 307 nm is
originated from the π–π* transition of the conjugated system in
the carbazole moiety and the peak is at ca. 345 nm from the π
donator–acceptor structure formed by the benzothiadiazole
moiety.
The absorption peak shifted blue and the absorption intensity
enhanced with the higher polarity of the solvent. The energy of
the antibonding orbital and bonding orbital of the molecular
both reduced with the polarity of the solvent. Because the polar-
ity of the ground state is low, the energy of bonding orbital re-
duced less, resulting in the energy difference between the
antibonding orbital and bonding orbital enlarged and the ab-
sorbing peak shifted blue.
Figure 7(b) and Table 5 showed that compound has different
maximum emission wavelength (λem) in different solvents, and it
obtained the largest λem in protonic solvent (methanol). In the
aprotic solvents, it has the largest λem in DMSO, which is because
DMSO has the maximum polarity. The fluorescence peak shifted
Atom
Net charge
in molecule
Net charge in
crystal structure
Change
of charge
1
2
3
4
5
7
9
11
13
14
15
16
18
19
20
22
24
26
28
29
30
32
ꢀ0.11215019
-0.30231969
-0.01599018
0.02536549
-0.08646399
-0.09100618
-0.07775094
-0.11444038
0.16165198
0.20951352
-0.01399070
-0.07273426
-0.04184887
-0.03812864
-0.06855822
-0.10357274
-0.06554401
-0.14078787
0.08932942
0.09239369
-0.14131108
-0.08205945
ꢀ0.09103415
-0.30805490
-0.00580141
0.01933700
-0.09094260
-0.10178281
-0.08726501
-0.11740788
0.15879021
0.20658310
-0.01751352
-0.08511343
-0.05033290
-0.04781313
-0.08047183
-0.11279878
-0.07901990
-0.14320401
0.08640278
0.09383171
-0.13192956
-0.07990573
Decrease
Increase
Decrease
Decrease
Increase
Increase
Increase
Increase
Decrease
Decrease
Increase
Increase
Increase
Increase
Increase
Increase
Increase
Increase
Decrease
Increase
Decrease
Decrease
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Y. Gu et al.
Table 5. The solvent properties and fluorescent characters of
compounds
Solvent
Methanol Ethanol DCM
DMF
DMSO
nD20a
1.33
1.70
32.7
0.59
345
1.36
1.69
24.5
1.20
344
1.42
1.60
9.08
0.44
345
410
65
1.43
3.82
38.3
0.92
345
417
72
1.48
3.96
47.2
2.00
341
μb
εc
cPd
λ max/nm
λ em/nm
Stokes shift
424
79
419
75
421
80
a
Refractive index.
Dipole moment.
Dielectric constant.
b
c
Figure 6. DOS of the crystal.
red and the fluorescence intensity reduced with the higher polar-
ity. We further explored the relationship between dipole moment
of the solvents and λem. We can see that the λem and Stokes shift
both increased with the increasing dipole moment and dielectric
constant. The possible reason can be explained by twisted intra-
molecular charge transfer (TICT). The solvent with the larger polar-
ity could result in the reduction of energy difference between
excited state and ground state, which as a result led to a red-shift
in the compound (22–26).
Labeling of bovine serum albumin (BSA) protein. BSA was
added into the compound solutions at the concentration of
10ꢀ5 mol/L to give compound–BSA solutions with protein concen-
trations of 0 μM, 0.18 μM, 0.54 μM, 0.72 μM, 1.44 μM or 1.62 μM,
the fluorescence emission spectra results are shown in Fig. 9.
The fluorescence intensity increases with increasing BSA con-
centration, accompanied by a blue shift of the λem. The increase
in intensity is because the hydrogen bond between the molecule
and BSA (the amino and thiol) made the molecule inlayed into
the helical structure of BSA. The protection from the BSA reduced
the polarization of the solvent, and made a fluorescence enhance-
ment. With the increase of protein concentration, the α-helix struc-
ture changed from a stretching state to a folding state and steric
hindrance to the molecule made it easier to form a rigid flat
structure (27). The blue shift of the λem maybe caused by the re-
duced ground state energy of the title compound caused by the
interaction between the compound and protein.
Spectral properties of compound 6 at different pH. The
UV–vis and fluorescence spectra were also studied with the con-
centration of 10ꢀ6 M in aqueous solution with different pH and
the results are shown in Fig. 8.
From Fig. 8, we can see that the absorbing intensity increased
until the pH was 7 and decreased after that. The fluorescence in-
tensity increased with the increases of the pH value, which is differ-
ent from absorbing intensity. There is always a λem at ca. 437 nm
and λmax at ca. 355 nm regardless of the pH value of the solutions.
Interestingly, a new peak was observed at ca. 375 nm. The intensity
ratio of peaks at ca. 355 nm and ca. 375 increased when the pH
value increased. The possible explanation is that under acid condi-
tions, there is protonation of N on the benzothiazole ring. With the
decrease in the pH value, there is more protonation occurring,
resulting in the relative intensity increase at ca. 375 nm.
To quantify the interaction between the compounds and BSA, we
determined the stability constants (Ks). The stability constants (Ks)
were determined according to a method in the literature (28,29).
The interaction between the dye (D) and the protein (P) expressed
as D + P = DP can be quantified by the stability constant Ks:
½DPꢁ
Ks
¼
½Dꢁ½Pꢁ
a
b
Figure 7. The UV–Vis (a) and fluorescence (b) spectra of the title compound with the concentration of 10ꢀ6 M in MeOH.
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Properties, theoretical study and crystal structure of 3-benzothiazole-9-ethyl carbazole
a
b
Figure 8. The UV–Vis (a) and fluorescence (b) spectra of the title compound with the concentration of 10ꢀ6 M in water at different pH.
the neighbouring molecules for the crystal, which were different
from those of the molecule structure. The UV–vis and fluorescence
spectra were also studied in different solvents and pH. The results
showed that the synthesized compound was sensitive to both sol-
vent and pH value. The protein labeling experiment and the stabil-
ity constant result showed that BSA could be marked by the
compound with enhanced fluorescence intensity and the stability
constant between them was 0.82 × 107 Mꢀ1. The findings sug-
gested that the title compound could be used in protein labeling,
and will give guidance to study the design, structure and proper-
ties of other carbazole compound with benzothiazole moieties.
Acknowledgements
The present study was financially supported by the National Natu-
ral Science Foundation of China ( grant nos. 51578360, 21502137,
11447222) and Technology Development Foundation Plan Project
of Tianjin Colleges (20130505).
Figure 9. Fluorescence of the compound–BSA solution with different BSA
concentrations.
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