H.-p. Shi et al. / Spectrochimica Acta Part A 93 (2012) 19–25
21
bath. A solution of NBS (8.77 g, 49.3 mmol) in DMF (50 mL) was
added dropwise. The reaction mixture was allowed to stir for 12 h
at room temperature. Then, the mixture poured into ice water,
and the white precipitate was collected by filtration to afford
6-bromo-9-ethylcarbazole-3-carbaldehyde (2). Yield: 82%. M.p.:
131.4–132.2 ◦C. 1H NMR (600 MHz, CDCl3) ı (ppm): 10.09 (s, 1H),
8.53 (d, J = 1.2 Hz, 1H), 8.25 (d, J = 1.9 Hz, 1H), 8.03 (dd, J = 8.5, 1.4 Hz,
1H), 7.61 (dd, J = 8.6, 1.9 Hz, 1H), 7.47 (d, J = 8.5 Hz, 1H), 7.33 (d,
J = 8.6 Hz, 1H), 4.38 (q, J = 7.3 Hz, 2H), 1.46 (t, J = 7.3 Hz, 3H).
The thermal stability of the compound was measured using
thermogravimetric analysis (TGA). The result reveals that the com-
pound exhibits excellent thermal stability up to 400 ◦C.
The ground-state geometry as well as its ionic structure of the
derivative were optimized at B3LYP level with 6-31G(d, p) basis set
[24,25]. The vibration frequencies and the frontier molecular orbital
characteristics were analyzed on the optimized structure at the
reorganization energy of compound BBECE were calculated by DFT
method based on the optimized geometry of the neutral and ionic
molecules. The excited-state geometry of compound BBECE was
optimized at the configuration interaction with single excitation
(CIS) level with 6-31G(d, p) basis set [26]. The absorption spectra
and the emission spectra of BBECE were carried out using time-
optimized ground state structures and the lowest singlet excited
state structures, respectively. Solvent effects were also taken into
account by using the polarized continuum model (PCM) [27,28]. All
calculations were carried out with the Gaussian03 program pack-
age [29]. All the calculations were performed using the advanced
computing facilities of supercomputing center of computer net-
work information center of Chinese Academy of Sciences.
2.3.3. (E)-1,2-bis(3-bromo-9-ethylcarbazol-6-yl)ethene (3)
6-Bromo-9-ethylcarbazole-3-carbaldehyde
(2)
(10 g,
33.1 mmol) and Zn powder (8.656 g, 132.4 mmol) were sus-
pended in anhydrous THF (200 mL) under N2. Titanium(IV)
chloride (7.28 mL, 66.2 mmol) was added dropwise to the reaction
mixture at −78 ◦C. Then, the cooling bath was removed, and the
mixture was refluxed for 10 h. After cooling to room temperature,
ice water (200 mL) was added and the reaction mixture was
stirred for a further 0.5 h. The mixture was extracted with CH2Cl2
and the combined organic extracts were dried over anhydrous
MgSO4, filtered, and concentrated on vacuum. The residue was
purified by flash chromatography on silica gel (CH2Cl2/hexane) to
afford (E)-1,2-bis(3-bromo-9-ethylcarbazol-6-yl)ethene (3). Yield:
75%. M.p.: 251.2–52.5 ◦C. 1H NMR (600 MHz, CDCl3) ı (ppm):
8.24 (d, J = 1.9 Hz, 2H), 8.19 (s, 2H), 7.73 (dd, J = 8.5, 1.1 Hz, 2H),
7.54 (dd, J = 8.6, 1.8 Hz, 2H), 7.39 (d, J = 8.5 Hz, 2H), 7.32 (s, 2H),
7.27 (d, J = 8.6 Hz, 2H), 4.34 (q, J = 7.2 Hz, 4H), 1.44 (t, J = 7.3 Hz,
6H).
4. Results and discussion
In this paper, we described methods for the preparation of a
carbazole dimer-based derivative with benzothiazole units. First,
9-ethylcarbazole-3-carbaldehyde (1) was obtained starting from
ethylation of carbazole followed by treating the resulting 9-
ethylcarbazole with POCl3 and DMF via Vilsmeier–Haack reaction
in 65% yield. Second, 6-bromo-9-ethylcarbazole-3-carbaldehyde
(2) was prepared in high yield by 9-ethylcarbazole-3-carbaldehyde
(1) reacted with NBS. Third, (E)-1,2-bis(3-bromo-9-ethylcarbazol-
6-yl)ethene (3) was synthesized by McMurry coupling reaction
under nitrogen. Then, (E)-1,2-bis(3-formyl-9-ethylcarbazol-6-
yl)ethene (4) was synthesized via lithiation of 3 with n-butyllithium
and reacted with DMF. Finally, (E)-1,2-bis(3-(benzothiazol-2-
condensation of 4 with 2-aminothiophenol in DMSO at 170 ◦C. All
of the new compounds were characterized by elemental analy-
sis, mass spectrometry and 1H NMR; further details are given in
Section 2.
2.3.4. (E)-1,2-bis(3-formyl-9-ethylcarbazol-6-yl)ethene (4)
3 (1.72 g, 3 mmol) was dissolved in anhydrous THF (40 mL)
under N2 at −78 ◦C. To this solution, n-BuLi (3.3 mmol, 1.6 M solu-
tion in hexanes) was injected. The resulting mixture was allowed
to stir for 4 h. Then dry DMF (0.31 mL, 4 mmol) was added drop-
wise and the reaction was stirred at room temperature for 12 h.
The mixture was quenched with aqueous NH4Cl and extracted
with CH2Cl2. The organic layer was washed with water and dried
over anhydrous MgSO4. After the solvents were removed, the
crude product was recrystallized from MeOH to yield (E)-1,2-bis(3-
formyl-9-ethylcarbazol-6-yl)ethene (4) as a yellow powder. Yield:
70%. 1H NMR (600 MHz, CDCl3) ı (ppm): 10.13 (s, 2H), 8.66 (s,
2H), 8.34 (s, 2H), 8.03 (dd, J = 8.5, 1.2 Hz, 2H), 7.79 (dd, J = 8.4,
1.2 Hz, 2H), 7.51–7.45 (m, 4H), 7.39 (s, 2H), 4.43 (q, J = 7.3 Hz, 4H),
1.50 (t, J = 7.3 Hz, 6H). MS (m/z): 470.2005 (M+). Anal. calcd. for
C32H23N2O2: C, 81.68%; H, 5.57%; N, 5.95%. Found: C, 81.30%; H,
5.37%; N, 5.72%.
2.3.5. (E)-1,2-bis(3-(benzothiazol-2-yl)-9-ethylcarbazol-6-yl)
ethene (BBECE)
4.1. Molecular structure
A mixture of 0.71 g 4 (1.50 mmol), 0.20 ml 2-aminothiophenol
(1.71 mmol), and 30 ml DMSO was heated to 170 ◦C for 12 h. The
reaction mixture was cooled to room temperature and poured into
water. The organic component was extracted with CH2Cl2. The
combined organic layers were washed with water and dried over
MgSO4. Evaporation of the solvent gave a residue that was puri-
fied column chromatography on silica gel (CH2Cl2/hexane) to give
BBECE. Yield: 40%. 1H NMR (600 MHz, DMSO-d6) ı (ppm): 9.00
(s, 2H), 8.72 (s, 2H), 8.23 (d, J = 9.8 Hz, 2H), 8.17 (d, J = 7.9 Hz, 2H),
8.07 (d, J = 7.3 Hz, 2H), 7.81 (d, J = 9.1 Hz, 2H), 7.73 (d, J = 8.3 Hz, 2H),
7.60–7.54 (m, 4H), 7.50–7.42 (m, 4H), 4.55 (q, J = 7.2 Hz, 4H), 1.40 (t,
J = 6.9 Hz, 6H). 13C NMR (600 MHz, DMSO-d6) ı (ppm): 176.1, 156.5,
145.4, 144.6, 142.8, 134.9, 131.2, 129.8, 129.0, 126.9, 125.1, 124.7,
124.1, 112.2, 110.8, 47.2, 23.1. MS (m/z): 680.2132 (M+). Anal. calcd.
for C44H32N4S2: C, 77.62%; H, 4.74%; N, 8.23%. Found: C, 77.48%; H,
4.52%; N, 8.17%.
in the ground state was optimized using the DFT/B3LYP/6-31G(d,
p) method. The optimized structure is shown in Fig. 1. The values of
the parameters are listed in Table 1. As can be seen from the data of
Table 1, BBECE has a planar geometrical structure and contains two
carbazole rings as electron-donating groups and two benzothiazole
rings as electron-accepting groups. Two carbazole rings are conju-
gated by double bonds as the rigid core and two benzothiazole rings
are linked to the core through the 3,3ꢀ positions as the terminal. The
dihedral angles C4–C1–C2–C3, C5–C4–C1–C2 and C1–C2–C3–C10
of the two carbazole rings planar moiety are 180◦, −179.63◦
and 179.72◦, respectively. The dihedral angles C21–C22–C34–N35,
C21–C22–C34–S38, C25–C26–C33–N39 and C25–C26–C33–S42 of
the carbazole rings planar moiety and benzothiazole rings planar
moiety are −0.86◦, 179.10◦, 0.83◦ and −179.14◦, respectively.