Tuning of the HOMO-LUMO gap of donor-substituted symmetrical and unsymmetrical benzothiadiazoles

Article abstract of DOI:10.1039/c4ob00629a

This article reports the design and synthesis of donor-substituted symmetrical and unsymmetrical benzothiadiazoles (BTDs) 5-12 of type D-π-A-D, D₁-π-A-D₂, D₁-A₁-A ₂-D₂, D-A₁-A

Full text of DOI:10.1039/c4ob00629a

Organic &
Biomolecular Chemistry
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Tuning of the HOMO–LUMO gap of donor-
substituted symmetrical and unsymmetrical
benzothiadiazoles†
Cite this: Org. Biomol. Chem., 2014,
12, 5448
Rajneesh Misra* and Prabhat Gautam
This article reports the design and synthesis of donor-substituted symmetrical and unsymmetrical benzo-
thiadiazoles (BTDs) 5–12 of type D–π–A–D, D1–π–A–D2, D1–A1–A2–D2, D–A1–A2–D and D–A1–A2–A1–D
by Ullmann, Suzuki and cycloaddition–retroelectrocyclization reactions. The photophysical, electro-
chemical and computational properties were studied and show substantial donor–acceptor interaction.
Their single photon absorption show strong charge transfer bands in the near-infrared (NIR) region and
the electrochemical reduction show multiple reduction waves. The optical HOMO–LUMO gap of BTDs
5–12 was found to be a function of the number and nature of the acceptors. Computational studies
reveal that strong cyano-based acceptors, dicyanoquinodimethane (DCNQ) and tetracyanobutadiene
(TCBD) lower the LUMO level in BTDs 7–12, which results in a low HOMO–LUMO gap compared to
acetylene linked BTDs 5 and 6. The BTDs with carbazole and single TCBD and DCNQ acceptors show
better thermal stability.
Received 25th March 2014,
Accepted 13th May 2014
DOI: 10.1039/c4ob00629a
www.rsc.org/obc
photovoltaics and non-linear optics.6 Diederich and coworkers
reacted TCNE and TCNQ with a variety of acetylenic donors
Introduction
π-Functional donor–acceptor (D–A) molecular systems are of and synthesized charge transfer chromophores via [2 + 2]
significant interest because of their potential applications cycloaddition–retroelectrocyclization process.7 Shoji et al.
in multi-photon absorption, organic field effect transistors reported the synthesis and properties of TCNE and TCNQ
(OFETs), organic light emitting diodes (OLEDs) and organic derivatives of azulene based molecular system.8 Li group
photovoltaics (OPVs).1 The electronic and photonic properties described the synthesis and nonlinear optical (NLO) properties
of the D–A system is a function of the HOMO–LUMO gap.2 The of N,N-dimethylaniline-substituted benzothiadiazole (BTD)
HOMO–LUMO gap in D–π-A systems can been tuned either by systems. Their [2 + 2] cycloaddition–retroelectrocyclization
altering the strength of D/A units or by varying the π-bridge.3 with TCNE and TCNQ resulted in a strong charge transfer (CT)
Literature reveals that a variety of donors (triphenylamine, molecular system.9
carbazole, etc.) have been attached to electron acceptors
Benzothiadiazole is a strong acceptor owing to its high elec-
(benzothiadiazole, diketopyrrolopyrrole, etc.) to generate low tron affinity and has been extensively utilized for the design of
HOMO–LUMO gap molecular systems.4 Our group is interested low HOMO–LUMO gap molecular systems.10 Recently we have
in the design and synthesis of D–A molecular systems for reported a series of low band gap symmetrical and unsymme-
various applications.5
trical ferrocenyl substituted BTDs.11 Triphenylamine and car-
The reaction of electron deficient tetracyanoethylene bazole are strong electron donors and their derivatives have
(TCNE) and 7,7,8,8-tetracyanoquinodimethane (TCNQ) with been explored for various optoelectronic applications.1f,12 We
electron rich acetylenic donors results in strong ICT chromo- were interested to explore the band gap of triphenylamine and
phores. The D–A systems incorporating these cyano-based carbazole substituted symmetrical and unsymmetrical BTDs
acceptors have proved to be efficient candidates in organic and to see the effect of acceptor strength on their photo-
physical properties. In this contribution, we wish to report
symmetrical and unsymmetrical donor–acceptor molecular
Department of Chemistry, Indian Institute of Technology, Indore 452 017, India.
systems of the type D–A1–A2–A1–D, D–π–A–D, D1–π–A–D2, D–A1–
A2–D and D1–A1–A2–D2. Triphenylamine substituted BTDs
3 and 4 were synthesized by the Pd-catalyzed Sonogashira cross-
coupling reaction of dibromo-BTD 2 with 4-ethynyltriphenyl-
amine (13). The unsymmetrical BTDs 5 and 6 were synthesized
E-mail: rajneeshmisra@iiti.ac.in; Fax: +91 7312361482; Tel: +91 7312438710
†Electronic supplementary information (ESI) available: Characterization data for
all the new compounds. Copies of 1H, 13C NMR, and HRMS spectra, fluore-
scence, electrochemical, thermogravimetric analysis and DFT calculation data of
new compounds. See DOI: 10.1039/c4ob00629a
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by Ullmann and Suzuki coupling reactions of BTD 3 with carb-
azole 14 and triphenylamine-4-boronic acid 15 respectively.
The BTDs 4, 5 and 6 were further subjected to a [2 + 2] cyclo-
addition–retroelectrocyclization reaction with tetracyanoethy-
lene (TCNE) and 7,7,8,8-tetracyanoquinodimethane (TCNQ),
which resulted in symmetrical and unsymmetrical BTDs 7–12.
Results and discussion
The synthesis of symmetrical and unsymmetrical benzothia-
diazoles 5–12 are shown in Schemes 2 and 3. The dibromo-
BTD 2 was synthesized by the bromination of BTD 1.13 The Pd-
catalyzed Sonogashira cross-coupling reaction of dibromo-BTD
2 with one equivalent of 4-ethynyltriphenylamine (13) resulted
Scheme 2 Synthesis of benzothiadiazoles 5 and 6.
in
[4-(7-bromo-benzo[1,2,5]thiadiazol-4-ylethynyl)-phenyl]-
diphenylamine (3) and 4,7-bis{2-[4-(N,N-diphenylamino)-
phenyl]ethynyl}-2,1,3-benzothiadiazole (4) in 50% and 30%
yields respectively (Scheme 1).14
The Ullmann and Suzuki coupling reaction of monobromo-
BTD 3 with carbazole (14) and triphenylamine-4-boronic acid
(15) resulted BTDs 5 and 6 in 40% and 65% yields respectively
(Scheme 2).15,16
The tetracyanobutadiene (TCBD) and dicyanoquinodi-
methane (DCNQ) linked BTDs 7–12 were synthesized via the
[2 + 2] cycloaddition–retroelectrocyclization reaction of the
BTDs 4–6 bearing acetylene linkage with tetracyanoethene
(TCNE) 16 and 7,7,8,8-tetracyanoquinodimethane (TCNQ) 17
(Scheme 3).9b The reaction of BTDs 5 and 6 with one equi-
valent of TCNE resulted BTDs 7 and 9 in 75% and 80% yields
respectively. The reaction of BTD 4 with two equivalents of
TCNE resulted BTD 11 in 85% yield. The reactions of 7,7,8,8-
tetracyanoquinodimethane (TCNQ) with the acetylene linked
BTDs 4, 5 and 6 were sluggish in nature.17 To overcome this
hurdle, the reactions were carried out under microwave
irradiation.18 The reaction of BTDs 5 and 6 with one equivalent
of TCNQ in 1,2-dichloroethane (DCE) at 100 °C under micro-
wave irradiation for 10 h resulted BTDs 8 and 10 in 70% and
65% yields respectively. The reaction of BTD 4 with two equi-
valents of TCNQ in DCE at 100 °C under microwave irradiation
for 20 h resulted BTD 12 in 50% yield. The reactions carried
out under microwave irradiation showed improved yields along
with a reduced time period.9b
The purification of BTDs 5–12 were achieved by column
chromatography. Benzothiadiazoles 5–12 were well character-
ized by 1H, 13C NMR, and HRMS techniques. The 1H NMR
spectra of unsymmetrical BTDs 5–10 show a characteristic
doublet between 8.30–7.66 ppm corresponding to two protons
of benzothiadiazole. The 1H NMR spectra of symmetrical BTDs
11 and 12 exhibit singlets for two benzothiadiazole protons at
8.07 ppm and 8.45 ppm respectively. Benzothiadiazoles 5–12
are readily soluble in common organic solvents such as chloro-
form, dichloromethane, toluene, tetrahydrofuran, acetone, etc.
The electronic absorption spectra of the benzothiadiazoles
(BTDs) 5–12 were recorded in dichloromethane (Fig. 1) and the
data are listed in Table 1. BTDs 5–12 exhibit strong absorption
band between 250–350 nm, corresponding to π→π*
transition.10,19
The absorption spectra of BTDs 5 and 6 exhibit a charge
transfer (CT) band at 464 nm and 473 nm respectively.1e,14b
The incorporation of the tetracyanobutadiene (TCBD) and
dicyanoquinodimethane (DCNQ) acceptor unit results in
multi-CT bands in BTDs 7–12.9b This indicates strong donor–
acceptor interaction in the BTDs 7–12 with TCBD and DCNQ
linkers. The presence of strong CT transition result in intense
color solution of BTDs 7–12 in dichloromethane (Fig. 2). The
trend observed in the optical HOMO–LUMO gap values exhibit
the order 5 > 6 > 7 > 9 > 11 > 8 > 10 > 12. This reveals that the
optical HOMO–LUMO gap values were significantly lowered in
the DCNQ linked BTDs followed by the TCBD and acetylene
linked BTDs. Incorporation of two TCBD and DCNQ linkages
in BTDs 11 and 12 results in lower optical HOMO–LUMO gap
values. Therefore the HOMO–LUMO gaps in these BTDs are a
function of acceptor strength.
Scheme 1 Synthesis of benzothiadiazoles 3 and 4.
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Scheme 3 Synthesis of benzothiadiazoles 7–12.
Fig. 1 Electronic absorption spectra of benzothiadiazoles 5–12 in dichloromethane at 1.0 × 10−5 M concentration.
The BTDs 5 and 6 exhibit broad fluorescence spectra in
In order to explore the effect of acceptors on the electronic
dichloromethane (Fig. S25; see ESI† for details).9a The fluore- structure and the HOMO–LUMO gap of the benzothiadiazoles
scence emission wavelengths λem for BTDs 5 and 6 were 5–12 density functional theory (DFT) calculations were
observed at 684 nm and 691 nm respectively (Table 1).
performed at the B3LYP/6-31G** level.20 The contours
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Table 1 Photophysical, electrochemical and thermal stability data of benzothiadiazoles 5–12
Photophysical data
Electrochemical datad
ε (M−1
λem
Optical gapb
(eV)
HOMO–LUMO gapc
(eV)
Eox
(V)
Ered
(V)
HOMO–LUMO gapd
(eV)
Td
a
a
f
λabs
Compound (nm)
cm−1
)
(nm)
(°C)
Ferrocene
5
—
—
0.38
0.91e
1.13e
0.84
—
−1.32
320
464
323
473
287
332
466
518
303
484
665
298
454
560
288
491
683
276
377
490
293
401
637
78 087
40 765
77 978
55 191
68 633
44 590
48 688
sh
81 366
57 868
51 092
46 557
33 497
33 770
87 541
56 994
56 612
56 010
49 125
60 218
60 054
60 163
51 366
684
691
—
2.24
352
165
373
2.20
2.19
1.68
2.44
2.42
2.46
6
7
−1.41
2.22
1.72
0.97e
1.18e
−0.36
−0.83
−1.03
−0.21
−0.35
−0.99
−0.43
−0.81
−1.02
−0.22
−0.36
−0.99
−0.15
−0.30
−0.96
−0.12
−0.22
−0.35
−0.45
−1.06
8
—
—
—
—
—
0.92e
1.34e
458
331
343
298
275
1.34
1.66
1.31
1.57
1.27
2.31
2.40
2.18
2.03
1.86
1.37
1.70
1.35
1.58
1.30
9
0.94e
1.15e
10
11
12
0.92e
1.07e
1.04e
0.91e
a Absorbance measured in dichloromethane at 1 × 10−5 M concentration; sh = shoulder; λabs: absorption wavelength; λem: emission wavelength; ε:
extinction coefficient. b Determined from onset wavelength of the UV/Vis absorption. c Calculated from theoretical study. d Recorded by cyclic
voltammetry, in 0.1 M solution of TBAPF6 in DCM at 100 mV s−1 scan rate versus SCE electrode, cyclic voltammetric HOMO–LUMO gap derived
from the difference between HOMO and LUMO energy levels.10a e Assigned by differential pulse voltammetry. f Decomposition temperatures for
5% weight loss under N2 atmosphere at heating rate of 10 °C min−1
.
BTDs 7–12 lowers the LUMO level, which leads to a low
HOMO–LUMO gap and red shift in the electronic absorption.
A time-dependent DFT calculation was performed on BTD 7
in dichloromethane to understand the absorption properties.
The contributions to the molecular orbitals in the UV/Vis
absorption were determined on the basis of their oscillator
strength ( f ). The TD-DFT calculation shows that, the lower
energy band at 490 nm in the absorption spectrum (Fig. S30†)
show a preferential contribution from HOMO → LUMO and
HOMO−1 → LUMO.
Fig. 2 Benzothiadiazoles 5–12 at
dichloromethane.
1 × M concentration in
10−5
The electrochemical properties of the BTDs 5–12 were
explored by cyclic voltammetry (CV) and differential pulse vol-
tammetry (DPV). All the measurements were performed in dry
dichloromethane (DCM) solution at room temperature using
tetrabutylammonium hexafluorophosphate (TBAPF6) as a sup-
porting electrolyte. The electrochemical data and cyclic voltam-
metric HOMO–LUMO gap values are listed in Table 1, and the
representative cyclic voltammograms are shown in Fig. 5.10a
BTDs 5–12 show irreversible oxidation waves attributed to the
donor aryl groups in the region 0.84 to 1.34 V. The BTDs 5 and
6 exhibit one reversible reduction wave corresponding to the
benzothiadiazole acceptor unit at −1.32 V and −1.41 V respec-
tively.11a The BTDs 7, 9 and 11 exhibit three reversible
reduction waves in the region of −0.15 V to −1.03 V. The first
of the HOMO, and LUMO of BTDs 5–12 are shown in
Fig. 3.
The HOMO is contributed by the aryl donors and the hydro-
carbon portion of the benzothiadiazole in BTDs 5 and 6.11b
The HOMO in BTDs 7, 8, 11 and 12 are delocalized over the
triphenylamine donor adjacent to the TCBD and DCNQ units.
On the other hand the HOMO in BTDs 9 and 10 is localized on
the triphenylamine donor directly linked to the benzothiadia-
zole core. The LUMO orbitals in BTDs 5–12 are concentrated
over the benzothiadiazole, TCBD and DCNQ acceptors units.
The energy level diagram of the frontier orbitals are shown in
Fig. 4. The presence of strong acceptors (TCBD and DCNQ) in
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Fig. 4 Energy diagram of the frontier orbitals of benzothiadiazole 5–12
estimated by DFT calculations.
and second reduction waves are attributed to the successive
one-electron reductions of the dicyanovinyl (DCV) groups of
the TCBD unit in BTDs 7 and 9.21 BTD 11 undergoes simul-
taneous electrochemical reduction of the two TCBD groups
and exhibits reduction waves at −0.15 and −0.30 V. The third
reduction wave in BTDs 7, 9 and 11 in the region of −0.96 V to
−1.03 V is assigned to the benzothiadiazole unit.11c BTDs
8 and 10 exhibit three reversible reduction wave in the region
of −0.21 V to −0.99 V corresponding to the electrochemical
reduction of the DCNQ and the benzothiadiazole acceptor
unit, whereas BTD 12 exhibit multiple reversible reduction
wave in the region of −0.12 to −1.06 V due to the two DCNQ
unit and the benzothiadiazole unit.9b The comparison of first
reduction potential of BTDs 5–12 reflects that the DCNQ
linked BTDs 8, 10 and 12 were easier to reduce than TCBD
linked BTDs 7, 9 and 11, which can be attributed to the strong
electron accepting nature of the DCNQ unit.22 The presence of
TCBD and DCNQ linkage facilitates the reduction of BTDs
7–12 compared to acetylene linked BTDs 5 and 6. The electro-
chemical and optical HOMO–LUMO gaps are in good agree-
ment and exhibit the same trend.
Fig. 3 HOMO and LUMO frontier orbitals of benzothiadiazoles 5–12 at
the B3LYP/6-31G** level for C, N, S, and H.
Fig. 5 Cyclic voltammograms of benzothiadiazoles 5 and 7 at 0.01 M concentration in 0.1 M TBAPF6 in dichloromethane recorded at a scan rate of
100 mV s−1
.
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these molecular systems which confirms the strong donor–
acceptor interaction. The thermal stability of the benzothiadia-
zoles can be enhanced by the incorporation of a planar carba-
zole donor. The presence of single TCBD or DCNQ improves
the thermal stability. The results obtained in this study will be
useful for the design and synthesis of materials with low
HOMO–LUMO gap and improved thermal stability for various
optoelectronic applications. These low HOMO–LUMO gap
molecular systems are potential candidate for organic photo-
voltaics. The study towards organic photovoltaic applications
of these BTDs are currently ongoing in our laboratory.
Experimental section
Chemicals were used as received unless otherwise indicated.
All the oxygen or moisture sensitive reactions were carried out
under argon/nitrogen atmosphere. 1H NMR spectra were
recorded using a 400 MHz spectrometer. The NMR spectra
Fig. 6 TGA plots of benzothiadiazoles 5, 6, 7 and 9 at a heating rate of
10 °C min−1, under a nitrogen atmosphere.
Thermal stability is significant for practical applications of were recorded at room temperature (298 K). Chemical shifts
organic chromophores. In order to have an elementary idea are reported in delta (δ) units, expressed in parts per million
about the thermal stability of BTDs 5–12 thermogravimetric (ppm) downfield from tetramethylsilane using residual proto-
analysis (TGA) were carried out at a heating rate of 10 °C nated solvent as an internal standard {CDCl3, 7.26 ppm;
min−1, under a nitrogen atmosphere (Fig. 6 and S29†) and the (CD3)2CO, 2.05 ppm}. 13C NMR spectra were recorded using a
decomposition temperatures (Td) for 5% weight loss are listed 100 MHz spectrometer. Chemical shifts are reported in delta
in Table 1. The BTD 6, 9 and 10 exhibit Td values at 165 °C, (δ) units, expressed in parts per million (ppm) downfield from
331 °C and 343 °C whereas BTDs 5, 7 and 8 exhibit Td values tetramethylsilane using the solvent as internal standard
at 352 °C, 373 °C and 458 °C respectively. The BTDs 11 and 12 {CDCl3, 77.0 ppm; (CD3)2CO, 29.8 ppm}. The 1H NMR splitting
with two TCBD and DCNQ units exhibit Td values above 298 °C patterns have been described as “s, singlet; d, doublet; and m,
and 275 °C respectively. The trend in thermal stability follows multiplet”. The 1H NMR signals are assigned as “HAR
”
the order 8 > 7 > 5 > 10 > 9 > 11 > 12 > 6. This indicates the fol- (1H NMR signals from Triphenylamine/Carbazole/DCNQ
lowing: (a) the carbazole substituted BTDs 5, 7 and 8 exhibit protons) and “HBTD” (1H NMR signals from benzothiadiazole
better thermal stability compared to their respective analogous protons). UV/Visible absorption spectra of all compounds were
triphenylamine substituted BTDs 6, 9 and 10. This may be due recorded in DCM. The density functional theory (DFT) calcu-
to the reduced conformational flexibility by the planar carba- lation were carried out at the B3LYP/6-31G** level for C, N, S,
zole unit.23 (b) The BTDs 7–10 with one TCBD or DCNQ unit H in the Gaussian 09 program. Cyclic voltammograms and
were thermally more stable compared to BTDs 11 and 12 with differential pulse voltammograms were recorded on electro-
two TCBD and DCNQ units.
chemical analyzer using glassy carbon as working electrode, Pt
The thermal stability results and observed trend may be wire as the counter electrode, and saturated calomel electrode
useful to improve the thermal stability of such type of D–A (SCE) as the reference electrode. The scan rate was 100 mV s−1
systems.
for CV. A solution of tetrabutylammonium hexafluoro-
phosphate (TBAPF6) in DCM (0.1 M) was employed as the sup-
porting electrolyte. DCM was freshly distilled from CaH2 prior
to use. All potentials were experimentally referenced against
the saturated calomel electrode couple. Under our conditions,
the Fc/Fc+ couple exhibited E° = 0.38 V versus SCE. HRMS was
recorded on TOF-Q mass spectrometer. All microwave reac-
tions were performed on CEM discover microwave instrument
(Model no. 908010). The method used to measure the
reaction temperature during microwave heating is external
sensor type.
Conclusion
In summary, a series of symmetrical and unsymmetrical
donor-substituted benzothiadiazoles were synthesized. The
number and nature of acceptor units perturbs the photonic
properties, HOMO–LUMO gap and thermal stability of
the benzothiadiazoles. The electronic absorption and compu-
tational calculation indicates substantial lowering of the
HOMO–LUMO gap by the incorporation of cyano-based dicyano-
quinodimethane (DCNQ) and tetracyanobutadiene (TCBD)
groups in the benzothiadiazoles. The TCBD and DCNQ linkage
Preparation of BTDs 3 and 4 by the Sonogashira coupling
reaction
of donor-substituted benzothiadiazole facilitates the reduction To a stirred solution of 4-ethynyltriphenylamine (3.7 mmol),
of the acceptor BTD unit and results in non-emissive nature of and dibromo-BTD 2 (3.7 mmol) in THF, and TEA (1 : 1, v/v)
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were added [PdCl2(PPh3)2] (100 mg, 0.14 mmol), and CuI 7.22–7.17 (m, 6H, HAR), 7.15–7.12 (m, 4H, HAR), 7.10–7.02 (m,
(20 mg, 0.1 mmol) under an argon flow at room temperature. 6H, HAR). 13C NMR (100 MHz, CDCl3, δ in ppm) 155.4, 153.2,
The reaction mixture was stirred for 12 h at 70 °C, and then 148.4, 148.3, 147.3, 147.1, 133.6, 132.9, 132.6, 130.4, 129.9,
cooled to room temperature. The solvent was then evaporated 129.41, 129.36, 126.7, 125.1, 125.0, 123.7, 123.4, 122.7, 122.0,
under reduced pressure, and the mixture was purified by SiO2 115.5, 115.4, 96.4, 85.0. HRMS (ESI-TOF, positive): m/z
chromatography with DCM–hexane (2 : 3, v/v) to obtain com- [M + H]+ calcd for C44H31N4S 647.2264, found 647.2262.
pound 3 as orange solid (0.89 g; yield 50%). Further elution UV/Vis (DCM) λmax (ε [M−1 cm−1]) 323 (77 978), 473 (55 191).
with DCM–hexane (3 : 2) gave compound 4 as deep red solid
General procedure for the preparation of BTDs 7 and 9
(0.74 g; yield 30%). The 1H NMR and 13C NMR data were
consistent with earlier reports.14
TCNE (39.0 mg, 0.30 mmol) was added to a solution of com-
pound 5/6 (0.30 mmol) in DCM (50 mL). The mixture
was refluxed at 40 °C for 8 h. The solvent was removed
Preparation of BTDs 5
Carbazole 14 (0.67 g, 4.0 mmol), bromo-BTD 3 (1.45 g, in vacuo, and the product was purified by SiO2 column
3.0 mmol), anhydrous potassium carbonate (1.66 g, chromatography with DCM as the eluent to yield 7/9 as a dark
12.0 mmol), cupric sulfate (91 mg, 0.63 mmol), and 1,2- colored solid.
dichlorobenzene (10 mL) were added to a round bottom flask,
2-(4-(Diphenylamino)phenyl)-3-(4-(9H-carbazol-9-yl)benzo[c]-
degassed, and flushed with N2. The reaction mixture was [1,2,5]thiadiazol-7-yl)buta-1,3-diene-1,1,4,4-tetracarbonitrile
heated at 180 °C for 2 days, and then cooled to room tempera- (7). Black solid (0.16 mg, yield: 75%). Rf = 0.30 (SiO2, hexane–
ture. After that dichloromethane and water were added. The DCM, 1 : 9). Mp 184.0–185.0 °C. 1H NMR (400 MHz, (CDCl3,
organic phase was washed with water and then dried over δ in ppm) 8.30 (d, 1H, J = 7.8, HBTD), 8.18–8.16 (m, 2H, HAR),
Na2SO4. After removal of the solvent, the residue was 8.02 (d, 1H, J = 7.8, HBTD), 7.77–7.74 (m, 2H, HAR), 7.44–7.36
purified by SiO2 column chromatography, using DCM–hexane (m, 8H, HAR), 7.30–7.28 (m, 3H, HAR), 7.24–7.21 (m, 5H, HAR),
(1 : 1) mixture as eluent to afford compound 5 (685 mg, 40% 6.70–6.93 (m, 2H, HAR). 13C NMR (100 MHz, CDCl3, δ in ppm)
yield). 164.0, 163.7, 153.7, 152.7, 150.8, 144.6, 140.2, 136.0, 133.4,
N-(4-(2-(4-(9H-Carbazol-9-yl)benzo[c][1,2,5]thiadiazol-7-yl)- 132.3, 130.1, 126.9, 126.6, 126.3, 125.6, 124.7, 123.3, 122.2,
ethynyl)phenyl)-N-phenylbenzenamine
(5). Orange
solid 121.8, 120.6, 118.1, 113.6, 113.0, 111.8, 111.0, 110.9, 91.9,
(685 mg, yield: 40%). Rf = 0.45 (SiO2, hexane–DCM, 1 : 1). Mp 79.6. HRMS (ESI-TOF, positive): m/z [M + Na]+ calcd for
220.5–221.5 °C. 1H NMR (400 MHz, ((CD3)2CO, δ in ppm) C44H24N8SNa 719.1731, found 719.1737. UV/Vis (DCM) λmax
8.26–8.24 (m, 2H, HAR), 8.13 (d, 1H, J = 7.5 Hz, HBTD), 8.02 (d, (ε [M−1 cm−1]) 287 (68 633), 466 (48 688), 518 (sh).
1H, J = 7.5 Hz, HBTD), 7.58 (d, 2H, J = 8.8 Hz, HAR), 7.41–7.28
2-(4-(Diphenylamino)phenyl)-3-(4-(4-(diphenylamino)phenyl)-
(m, 10H, HAR), 7.18–7.14 (m, 6H, HAR), 7.06–7.03 (m, 2H, HAR). benzo[c][1,2,5]thiadiazol-7-yl)buta-1,3-diene-1,1,4,4-tetracarbo-
13C NMR (100 MHz, CDCl3, δ in ppm) 156.0, 151.2, 148.7, nitrile (9). Black solid (0.19 g, yield: 80%). Rf = 0.30 (SiO2,
147.0, 141.0, 133.0, 132.1, 129.7, 129.5, 127.7, 126.0, 125.2, hexane–DCM, 1 : 9). mp 171.0–173.0 °C. 1H NMR (400 MHz,
124.0, 123.9, 121.8, 120.6, 120.4, 117.2, 114.8, 110.4, 97.7, (CDCl3, δ in ppm) 8.15 (d, 1H, J = 7.8, HBTD), 7.96–7.92 (m, 2H,
84.4. HRMS (ESI-TOF, positive): m/z [M
+
Na]+ calcd
HAR), 7.81 (d, 1H, J = 7.8, HBTD), 7.70–7.66 (m, 2H, HAR),
for C38H24N4SNa, 591.1614, found 591.1615. UV/Vis (DCM) 7.39–7.31 (m, 8H, HAR), 7.26–7.11 (m, 14H, HAR), 6.91–6.87 (m,
λmax (ε [M−1 cm−1]) 320 (78 087), 464 (40 765).
2H, HAR). 13C NMR (100 MHz, CDCl3, δ in ppm) 164.5, 164.4,
153.4, 153.3, 152.1, 149.8, 146.8, 144.7, 140.2, 133.5, 132.2,
130.7, 130.0, 129.55, 128.1, 126.9, 126.4, 125.7, 125.4, 124.3,
Preparation of BTDs 6
To a stirred mixture of triphenylamine-4-boronic acid 15 122.4, 122.3, 121.4, 118.0, 113.7, 113.0, 112.1, 111.3, 90.1, 79.7.
(1.45 g, 5.0 mmol), bromo-BTD 3 (2.41 g, 5.0 mmol), toluene HRMS (ESI-TOF, positive): m/z [M
Na]+ calcd for
+
(5 ml), THF (5 ml) and H2O (1 ml) were added [Pd(PPh3)4] C50H30N8SNa 797.2206, found 797.2204; UV/Vis (DCM) λmax
(0.075 g, 0.75 mmol), Na2CO3 (0.73 g, 5 mmol). The reaction (ε [M−1 cm−1]) 298 (46 557), 454 (33 497), 560 (33 700).
mixture was heated at reflux for 24 h and then water was
Preparation of BTD 11
added to quench the reaction. The product was extracted with
diethyl ether. The organic layer was collected, dried over anhy- TCNE (77 mg, 0.60 mmol) was added to a solution of com-
drous Na2SO4 and evaporated under vacuum. The solid was pound 4 (0.30 mmol) in DCM (50 mL) at room temperature.
adsorbed on silica gel and purified by column chromato- The mixture was refluxed at 40 °C for 16 h. The solvent was
graphy, using a DCM–hexane (1 : 1) mixture as eluent to afford removed in vacuo, and the product was purified by SiO2 chrom-
compound 6 (2.1 g, 65%).
atography with DCM as the eluent to yield compound 11 as a
N-(4-(2-(4-(4-(Diphenylamino)phenyl)benzo[c][1,2,5]thiadiazol- dark colored solid.
7-yl)ethynyl)phenyl)-N-phenylbenzenamine (6). Red solid
3,3′-(Benzo[c][1,2,5]thiadiazole-4,7-diyl)bis[2-{4-(dimethyl-
(2.1 g, yield: 65%). Rf = 0.50 (SiO2, hexane–DCM, 1 : 1). Mp amino)-phenyl}buta-1,3-diene-1,1,4,4-tetracarbonitrile
(11).
153.0–154.0 °C. 1H NMR (400 MHz, (CDCl3, δ in ppm) Black solid (0.24 g, yield: 85%). Rf = 0.20 (SiO2, hexane–DCM,
1
7.90–7.86 (m, 2H, HAR), 7.83 (d, 1H, J = 7.3, HBTD), 7.67 (d, 1H, 1 : 9). Mp >300 °C; H NMR (400 MHz, (CDCl3, δ in ppm) 8.07
J = 7.3, HBTD), 7.53–7.50 (m, 2H, HAR), 7.32–7.27 (m, 8H, HAR), (s, 2H, HBTD), 7.64–7.60 (m, 4H, HAR), 7.40–7.35 (m, 8H, HAR),
5454 | Org. Biomol. Chem., 2014, 12, 5448–5457
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7.27–7.26 (m, 1H, HAR), 7.25–7.23 (m, 3H, HAR), 7.18–7.16 (m,
2,2′-(Benzo[c][1,2,5]thiadiazole-4,7-diylbis{2-[4-(dicyanomethyl-
8H, HAR), 6.88–6.86 (m, 4H, HAR). 13C NMR (100 MHz, CDCl3, ene)-cyclohexa-2,5-dien-1-ylidene]-2-[4-(dimethylamino)phenyl]-
δ in ppm) 162.6, 162.0, 154.0, 151.0, 144.3, 131.9, 131.0, 130.1, eth-1-yl-1-ylidene})dimalononitrile (12). Black solid (0.16 g,
129.2, 126.9, 126.8, 120.6, 118.1, 113.4, 113.3, 111.0, 110.4, yield: 50%). Rf = 0.10 (SiO2, DCM). Mp 204.0–205.0 °C; 1H
94.6, 79.5. HRMS (ESI-TOF, positive): m/z [M + Na]+ calcd for NMR (400 MHz, (CD3)2CO, δ in ppm) 8.45 (s, 2H, HBTD),
C58H30N12SNa 949.2329, found 949.2327. UV/Vis (DCM) λmax 8.01–7.98 (m, 2H, HAR), 7.58–7.56 (m, 2H, HAR), 7.41–7.28 (m,
(ε [M−1 cm−1]) 276 (56 010), 377 (49 125), 490 (60 218).
16H, HAR), 7.22–7.18 (m, 4H, HAR), 7.10–7.08 (m, 8H, HAR),
6.79 (d, 4H, J = 8.8 Hz, HAR). 13C NMR (100 MHz, (CD3)2CO,
δ in ppm) 166.7, 154.9, 152.3, 152.1, 151.6, 146.5, 137.3, 137.2,
136.2, 134.9, 132.7, 132.4, 130.7, 127.2, 126.7, 126.4, 126.0,
119.7, 114.8, 113.4, 113.1, 94.7, 74.9. HRMS (ESI-TOF, positive):
m/z [M + Na]+ calcd for C70H38N12SNa 1101.2955, found
1101.2957. UV/Vis (DCM) λmax (ε [M−1 cm−1]) 293 (60 054), 401
(60 163), 637 (51 366).
General procedure for the preparation of BTDs 8 and 10
TCNQ (61 mg, 0.30 mmol) was added to a solution of 5/6
(0.30 mmol) in 1,2-dichloroethane (DCE) (4 mL) in a micro-
wave tube. The mixture was reacted under microwave con-
ditions at 100 °C for 10 h. The solvent was removed in vacuo,
and the product was purified by SiO2 chromatography with
DCM–ethylacetate (9 : 1) as the eluent to yield compound 8/10
as a dark colored solid.
Acknowledgements
2-[4-(3,3-Dicyano-1-{4-(diphenylamino)phenyl}-2-{7-(9H-carb-
azol-9-yl)benzo[c][1,2,5]thiadiazol-4-yl}allylidene)cyclohexa-2,5-
dien-1-ylidene]malononitrile (8). Black solid (0.16 g, yield:
70%). Rf = 0.20 (SiO2, DCM). Mp 208.5–209.5 °C. 1H NMR
(400 MHz, (CDCl3, δ in ppm) 8.17–8.15 (m, 2H, HAR), 8.04 (d,
1H, J = 7.5, HBTD), 7.92 (d, 1H, J = 7.5, HBTD), 7.51–7.48 (m, 1H,
RM thanks CSIR, and DST, New Delhi for financial
support.
Notes and references
H
AR), 7.42–7.27 (m, 12H, HAR), 7.23–7.13 (m, 9H, HAR),
6.94–6.90 (m, 2H, HAR); 13C NMR (100 MHz, CDCl3, δ in ppm)
167.4, 154.9, 152.9, 151.6, 150.8, 145.3, 140.3, 135.7, 134.9,
134.7, 134.4, 132.4, 129.9, 126.8, 126.62, 126.56, 126.52, 126.2,
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[M + Na]+ calcd for C50H28N8SNa 795.2050, found 795.2047.
UV/Vis (DCM) λmax (ε [M−1 cm−1]) 303 (81 366), 484 (57 868),
665 (51 092).
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