A.U. Habeeba, M. Saravanan, T.C.S. Girisun et al.
Journal of Molecular Structure 1240 (2021) 130559
phase was extracted in methylene chloride and washed with
bis(4,4,5,5-tetramethyl-1,3,2-dioxoborolan-2-yl)-2-yl)-9H-carbazole
in toluene at highly inert condition. 4,7-Bis-5-dibromothiophene-
2-yl)benzo[c][1,2,5]thiadiazole was synthesized by conduct-
ing stannous coupling reaction between 4,7-Dithiophene-2-yl)
benzo[c][1,2,5]thiadiazole and stannylthiophene in DMF. PCTB
was synthesized by Suzuki coupling reaction of 9-Hexyl-3,6–
bis(4,4,5,5-tetramethyl-1,3,2-dioxoborolan-2-yl)-2-yl)-9H-carbazole
1
0% HCl, saturated Na HCO3, and then dried over MgSO . The
2
4
crude product was purified by column chromatography. FT-IR (KBr,
cm 1): ν 3436 (NH, H-bonding), 3020, 2915 (aromatic C-H), 1632
−
(
aromatic C=C), 1303 (aromatic CN), 938, 880, 811 (aromatic C-H
1
bending), 579 (C-S). H NMR (500 MHz, CDCl3, ppm) δ 8.28 (s,
H), 8.05 (d, J = 7.0 Hz, 2H), 7.91 (d, J = 7.0 Hz, 2H), 7.60 (d,
2
13
J = 3.7 Hz, 2H). C NMR (126 MHz, CDCl3, ppm) δ 153.15, 152.8,
41.70, 138.48, 132.48, 127.97, 127.33, 128.82, 114.09, 112.44. (See
supporting information, Figure S1, S6, S7).
and
4,7-Bis-5-dibromothiophene-2-yl)benzo[c][1,2,5]thiadiazole.
1
The high inert condition was provided to accomplish all coupling
reactions.
Infrared spectroscopy is performed to analyze the functional
groups in the synthesized compounds. The vibrational frequency
2
.5. 4,7-Bis-5-dibromothiophene-2-yl)benzo[c][1,2,5]thiadiazole
noticed in the specific ranges 2900-3100 cm 1, 1598-1632 cm
−
−1
,
(BDT-BT) (6)
−
−1
1
230-1310 cm 1, and 820-940 cm
corresponds to aromatic C-H
To a solution of compound 5 (200 mg, 0.68 mM) in chloro-
stretching, aromatic C=C stretching, aromatic C-N stretching and
form (15 ml) and acetic acid (10ml), NBS (121 mg, 4eq) was added
in portions upon maintaining the reaction temperature at 0°C.
Then, the reaction was maintained at room temperature under the
dark condition for 3 h. Afterward, the mixture was washed with
NaHCO3 solution and then the product was purified with column
aromatic C-H bending vibrations respectively. Specific bands 1378
cm 1 and 1330 cm
−
−1
are present in the FT-IR spectrum of HCOB,
which are the stretching frequencies of the B-C bond and B-O bond
respectively. Aliphatic chains in compounds 2, 3, and 7 show peaks
at 2800-2900 cm 1 due to stretching of alkyl C-H and 1400-1460
−
chromatography. FT-IR (KBr, cm 1): ν 3400 (b, NH), 2950 (aromatic
−
cm-1 correspond to alkyl bending vibrations. C-Br stretching vibra-
tions at 714 cm 1 and 713 cm are observed for HC and BDT-BT.
−
−1
CH), 2094 (C=N-S), 1667 (aromatic C=C), 1390, 1298 (aromatic
1
−1
CN), 850 (aromatic CH), 731 (C-Br), 614, 695 (C-S). H NMR (500
FT-IR band at 2094 cm corresponds to C-N-S stretching vibration
−1
MHz, CDCl3, ppm) δ 8.06 (d, J = 7.6 Hz, 2H), 7.87 (d, J = 3.2 Hz,
of benzothiadiazole group. Peaks in the range 540-690 cm
cor-
13
2
H), 7.17 (d, J = 4.0 Hz, 1H), 7.11 (d, J = 2.5 Hz, 1H). C NMR (126
responds to C-S vibrations, these peaks are specific in the spectra
of DTBT, BDT-BT, and PCTB.
MHz, CDCl3, ppm) δ 153.70, 12.79, 151.97, 138.55, 132.30, 127.61,
26.98, 126.45, 114.09, 111.94. (See supporting information, Figure
S1, S8,S9). ESI-MS calcd. for C14 H Br N S : 455.80. Found: 457.7
1
Proton NMR spectra of all compounds show aromatic proton in
the range 6.5-8.5 ppm. The -CH2 group in the alkyl chain bonded
to nitrogen of carbazole ring corresponds to the triplet peak at 4-
6
2
2 3
(
S15).
5
.5 ppm. The remaining 8 protons in the alkyl chain come as a
multiplet at 0.85 ppm. 13C NMR spectrum shows the peaks accord-
ing to the number of types of carbon in each compound.
The molecular weight of PCTB polymer was estimated by GPC
in THF medium against polystyrene standards. The average molec-
ular weight (Mn) and polydispersity index (PDI) of PCTB were
found to be 6440 and 2.23 respectively (Fig S16).
2
.6. Poly-4-(5-(9-Hexyl-9H-carbazol-2-yl)thiophen-2-yl)
benzo[c][1,2,5]thiadiazole (PCTB) (7)
Monomer 4,7-bis-(5-bromothiophene-2-yl)
benzo[c][1,2,5]
thiadiazole and 9-hexyl-3,6-(4,4,5,5-tetramethyl-1,3,2-dioxoboro
lan-2-yl)-9H-carbazole were dissolved in 15 ml THF and 5 ml
water. Pd(0)(dpph)4 1 mol% was added along with 8.5 eq. K CO .
2
3
The reaction tube was sealed with nitrogen and heated to 85°C.
After 48 h the reaction mixture was allowed to precipitate in
cold methanol. Then, soxhlet extraction of the crude product
was carried out in hexane and chloroform respectively. The final
3.2. Photophysical properties: UV-visible absorption and fluorescence
emission
UV visible spectra of the prepared compounds are depicted in
Fig. 2. UV-vis spectroscopy gives insights into the electronic excita-
tion from the valence band to the conduction band, generally tran-
sition between the highest occupied molecular orbital (HOMO) and
the lowest unoccupied molecular orbital (LUMO) as a result of en-
ergy absorption by the chromophore and the possibility of charge
transfer within the molecules. In the given UV-Vis spectra, 9-hexyl-
9H-carbazole (HC) absorbs UV-radiation at 347 and 364 nm. These
product was collected and further characterized. FT-IR (KBr, cm 1):
−
ν 3436 (NH), 2938 (s, aromatic C-H), 2858 (aliphatic C-H), 1606
(
aromatic C=C), 1460 (alkyl CH bending), 1247, 1022 (aromatic
1
C-N), 823 (aromatic CH), 797 (alkyl C-H). H NMR (500 MHz,
CDCl3, ppm) δ 8.62 (s, 2H), 8.53 (s, 1H), 8.06 (d, J = 12.2 Hz, 2H),
7
.97 (d, J = 13.2 Hz, 2H), 7.79 (d, J = 7.3 Hz, 2H), 7.63 (d, J = 7.8
Hz, 2H), 4.89 – 4.85 (m, 2H), 2.23 – 1.77 (m, 2H), 1.27 – 1.07 (m,
J = 36.1 Hz, 6H), 0.83 – 0.76 (m, 3H). 13C NMR (126 MHz, CDCl3,
ppm) δ 158.61, 126.99, 120.57, 120.05, 122.89, 122.84, 120.78,
∗
high-energy peaks are the result of π - π transition of an electron
to the high-lying LUMO orbital of HC. The other two compounds 4,
7- dithiophene-2-yl) benzo[c][1,2,5] thiadiazole (DTBT) and poly-4-
(5-(9-hexyl-9H-carbazol-2-yl)thiophen-2-yl)benzo[c][1,2,5] thiadia-
zole (PCTB) shows a broad absorption peaks at 359 nm and 448
nm respectively. A substantial change in wavelength of absorp-
tion maxima is found for PCTB as compared to HC and DTBT. This
bathochromic shift is due to the extended π-electron conjugation
in PCTB. According to molecular orbital (MO) theory, on increasing
the length of π-electron conjugation the electronic energy levels
of chromophore become more close together, and the energy re-
quired to produce a transition from lower energy level to higher
energy level decreases. PCTB is comprised of carbazole-thiophene
and benzothiadiazole moieties, all are aromatic conjugated frag-
ments. So the π-electron cloud delocalization is higher for PCTB
and this extended π electron conjugation shifts the peaks to a
higher wavelength of 448 nm. The compound PCTB is a polynu-
clear aromatic conjugated chain with a strong π-electron density,
1
20.17, 108.17, 37.79, 29.73, 29.59, 29.38, 28.14, 13.92. GPC (THF)
4
3
Mw =1.44 × 10 , Mn =6.44 × 10 , Polydispersity Index=2.23. (See
supporting information Figure S1, S10, S11, S16). COSY spectrum is
included to specify the coupling in the polymer, coupling peaks
for a single unit is marked in the COSY spectrum (S14).
3
. Results and Discussion
3
.1. Physicochemical characterization
The synthesis scheme of all compounds is given in Fig. 1.
FT-IR, NMR, and HRMS data are given along with the proce-
dure. Spectra are given in supporting information (Fig S1-S15).
3
,6-dibromo-N-hexyl carbazole was synthesized by hexylation
of 3,6-dibromo-9H-carbazole in a basic DMSO medium. Miyau-
raborylation reaction was conducted to synthesize 9-Hexyl-3,6-
3