Synthesis, photophysical, electrochemical and single-crystal x-ray diffraction study of (Z)-2-phenyl-3-(5-(4-(thiophen-2-yl)benzo[c][1,2,5]thiadiazol-7-yl)thiophen-2-yl)acrylonitrile

Article abstract of DOI:10.1080/00397911.2016.1185125

The optical characteristics, redox properties, thermogravimetric stability and single-crystal X-ray diffraction study of (Z)-2-phenyl-3-(5-(4-(thiophen-2-yl)benzo[c][1,2,5]thiadiazol-7-yl)thiophen-2-yl)acrylonitrile are examined using ultraviolet–visible spectrophotometry, cyclic voltammetry, thermal gravimetric analysis–diffraction scanning calorimetry analysis, single-crystal X-ray diffraction and density functional theory calculations. Evidently, the crystal structure of compound 6 is sustained by a number of weak nonconventional intermolecular forces of attraction such as C-H?…?N, C-H?…?π donor–acceptor interactions.

Full text of DOI:10.1080/00397911.2016.1185125

Synthetic Communications
An International Journal for Rapid Communication of Synthetic Organic
Chemistry
ISSN: 0039-7911 (Print) 1532-2432 (Online) Journal homepage: http://www.tandfonline.com/loi/lsyc20
Synthesis, Photophysical, Electrochemical
and Single Crystal X-Ray Diffraction Study of
(Z)-2-phenyl-3-(5-(4-(thiophen-2-yl)benzo[c]
[1,2,5]thiadiazol-7-yl)thiophen-2-yl)acrylonitrile
Viraj J. Bhanvadia, Heta A. Patel, Neha N. Sharma & Arun L. Patel
To cite this article: Viraj J. Bhanvadia, Heta A. Patel, Neha N. Sharma & Arun L. Patel (2016):
Synthesis, Photophysical, Electrochemical and Single Crystal X-Ray Diffraction Study of (Z)-2-
phenyl-3-(5-(4-(thiophen-2-yl)benzo[c][1,2,5]thiadiazol-7-yl)thiophen-2-yl)acrylonitrile, Synthetic
Communications, DOI: 10.1080/00397911.2016.1185125
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Date: 10 May 2016, At: 01:36

Synthesis, photophysical, electrochemical and single crystal X-ray diffraction study
of (Z)-2-phenyl-3-(5-(4-(thiophen-2-yl)benzo[c][1,2,5]thiadiazol-7-yl)thiophen-2-
yl)acrylonitrile
1
1
1
1,
Viraj J. Bhanvadia , Heta A. Patel , Neha N. Sharma , Arun L. Patel
1
Department of Chemistry, Faculty of Science, The Maharaja Sayajirao University of
Baroda, Vadodara, India
Address Correpondence to Arun L. Patel Department of Chemistry, Faculty of Science,
The Maharaja Sayajirao University of Baroda, Vadodara-390 002, India. Email:
arunpatel_5376@yahoo.co.in
Abstract
The optical characteristics, redox properties, thermogravimetric stability and single-
crystal
X-ray
diffraction
study
of
(Z)-2-phenyl-3-(5-(4-(thiophen-2-
yl)benzo[c][1,2,5]thiadiazol-7-yl)thiophen-2-yl)acrylonitrile are examined using UV-Vis
spectrophotometry, cyclic voltammetry, TGA-DSC analysis, single crystal X-ray
diffraction and density functional theory calculations. Evidently, the crystal structure of
compound 6 is sustained by a number of weak non-conventional intermolecular forces of
attraction such as C-H…N, C-H…π donor-acceptor interactions.
KEYWORDS: Benzothiadiazole, Single Crystal X-ray diffraction, Photophysical
properties
1

INTRODUCTION
2
,1,3-Benzothiadiazole (BTD) derivatives are outstanding compounds due to their
electron-withdrawing properties and have been used as units of electron acceptors for
[
1]
conducting materials. BTD containing compounds are expected to afford well-ordered
crystal structures due to their highly polarized properties leading to intermolecular
[
2]
interactions such as heteroatom constants or π-π interactions. BTD derivatives are also
[
3]
known as efficient fluorophores. In addition to that polymers containing BTD units
[
4]
have been used as luminescent compounds in electroluminescence devices. The highly
electron-deficient BTD unit is one of the most popular building blocks in organic
electronics. BTD can conjugatively link with an electron-rich molecule to form low band
gap functional polymers or small molecules, and materials prepared in this manner
showed usefulness in organic light emitting diodes (OLEDs), dye sensitized solar cells
[5]
(DSSCs), light-harvesting and other optical or electronic functional devices.
Molecularly hybridized push−pull type materials, where the alternating arrangement of
electron rich and electron-deficient units along the π-conjugated backbone effectively
controls the frontier molecular orbitals. BTD is often the electron-deficient unit of choice
[
6]
for such materials. Moreover, to further fine-tune the frontier molecular orbital as well
as other important materials parameters such as solubility and crystallinity, chemical
[
7]
modifications of the BTD unit have attracted much interest.
Recently, BTD-based small molecule and its selenium analogue were showed good
[
8]
efficiency in heterojunction solar cells and photovoltaic cells, respectively.
2

Alternatively, BTD- based small molecules are becoming increasingly popular for
devising hybrid solar cells, because of high electron mobility and excellent chemical and
[
9a,b]
physical stability of inorganic semiconductors.
Recent studies have demonstrated that
formation of self-assembled monolayer of conjugated molecules on the surface of the
inorganic semiconductors can lead to the formation of interfacial dipoles, which in turn,
[
9c,d]
can improve the work function of substrate materials.
These type of small conjugated
molecules can act as interfacial modifiers (IMs), which enhances charge injection from
metals in to organic materials. J. Yu and co-workers have prepared and examined one of
such IMs, having cyano-acrylic acid as anchoring group and showed increase in electron
affinity at the polymer-inorganic semiconductor interface and formation of dipoles,
[
10]
which are oriented away from the semiconductor surface.
In this communication, we wish to report synthesis and photophysical study of
dithienobenzothiadiazole 4, dithienobenzooxadiazole 12 and a new conjugated compound
having cyano-benzylic anchoring group, (Z)-2-phenyl-3-(5-(4-(thiophen-2-
yl)benzo[c][1,2,5]thiadiazol-7-yl)thiophen-2-yl)acrylonitrile 6. These compounds were
successfully prepared by a multi-step synthetic route. All the molecules consisted of
donor–acceptor segments. The molecular structure of compound 6 was further confirmed
by single-crystal X-ray diffraction study. The thermal and electrochemical properties of
synthesized compound 6 were studied.
RESULTS AND DISCUSSION
3

BDT based novel highly conjugated molecule 6 is synthesized in five steps (scheme
[
10]
1
). In the first step of the synthesis ortho-phenylenediamine 1 was treated with thionyl
chloride to achieve 2,1,3- benzthiadiazole 2 in 36 % yield, which on further treated with
Br in HBr (48 % aqueous) to get 4,7-dibromo- 2,1,3-benzothiadizole 3 in 68 % yield.
2
Compound 3 was further treated with 2-tributylstannylthiophene in presence of
PdCl (PPh ) to get compound 4 in 49 % yield. Monoformylation of compound 4 gives
2
3 2
compound 5 in 56 % yield, which was subjected to Knoevenagel condensation reaction
with benzyl cyanide to get desired compound 6 in 62 % yield.
In addition, we have also synthesized oxadiazole based compound 12 from ortho-
[
11]
nitroaniline 7 in five step synthetic strategy (Scheme 2). Ortho-nitroaniline is
oxidatively cyclized to benzofuroxan 8 in excellent yield with commercial bleach, and
the latter is used as wet cake without drying. Compound 8 was deoxygenated to 2,1,3-
benzooxadiazole 9 with triethylphosphite. Compound 9 is purified by vacuum
sublimation. Bromination with molecular bromine in hydrobromic acid (48 % aqueous)
affords the tetrabromo compound 10 which is further treated with 1,8-
diazabicyclo[5.4.0]undec-7-ene (DBU) in tetrahydrofuran at room temperature for the
hydrodebromination. The resultant compound 11 is treated with 2-
tributylstannylthiophene in presence of PdCl (PPh ) to get compound 12 in 47 % yield.
2
3 2
SINGLE CRYSTAL X-RAY DIFFRACTION (SCXRD)
The molecular structure for compound 6 was elucidated by SCXRD (Figure S1). Crystals
of compound 6 was obtained from ethyl acetate (solvent). A suitable crystal was selected
4

and data were collected using CuKα (λ = 1.5418 Å) radiation on an Xcalibur- Eos-
Gemini diffractometer. The crystal was kept at 293 K during data collection. The
[
12]
structure was solved and refined using Olex2. The structure was solved with the
[
13]
Superflip structure solution program using Charge Flipping and refined with the
[
14]
ShelXL refinement package using Least Squares minimization. Table S1 summarize
the crystal structure and refinement data whereas Table S2 and Table S3 summarizes
selected bond lengths and bond angles, respectively.
X-ray structure of compound 6 clearly suggest that it crystallizes in monoclinic system
with space group P2 /c. Asymmetric unit of this contains a full molecules and there is
1
four such molecules present in the unit cell as shown in the following Figure 2. The unit
cell parameters for this are: a = 16.9905 (7); b = 7.2712 (3), c = 16.5005 (7) and α =
9
0.00, β = 107.130 (4) and ϒ = 90.00.
Notably, the presence of various polar sub-units in the molecular framework of
compound 6 induces conformational changes in the molecule that apparently changes the
nature and number of donor-acceptors sites. For instance, all the heterocyclic as well
carbocyclic rings present in compound 6 are not co-planar, making differential dihedral
angles with one-another ca a dihedral angle between the least squares planes drawn
through peripheral thiophene ring and adjacent benzothiadiazole ring is estimated 24.24°,
whereas a dihedral angle between the least squares planes drawn through internal
thiophene ring and adjacent benzothiadiazole ring is estimated 23.18° and vice versa. In
fact, the crystal structure of compound 6 is sustained by a number of weak non-
5

conventional intermolecular forces of attraction such as C-H…N, C-H…π donor-acceptor
interactions as shown in the following Figure 1.
Interestingly, C-H…N donor-acceptor interaction limits the growth of molecular packing
along the c-axis by forming a closed contact. However, C-H…π donor-acceptor
interactions arranged the molecules of 6 along the b-axis and forming a zig-zag
networking as shown in Figure 2.
PHOTOPHYSICAL AND ELECTROCHEMICAL PROPERTIES OF
COMPOUND 4, 6 AND 12
The photophysical properties of compounds 4, 6 and 12 were studied. The absorption and
emission spectra of 4, 6 and 12 were carried out in methanol (Figure 3). All these
compounds possess the dual-band nature; one lower energy band is ranging in between
*
2
50-340 nm which is due to π-π transition of the conjugated backbones, while second
higher energy band is ranging from 400-520 nm due charge transfer transition between
D→A. Compound 4, 12 and 6 shows absorption maxima at 441, 436, 463 nm with stokes
shifts of 159, 146, 142. The lower energy charge transfer leads to red-shift (bathochromic
shift) while going from 4 (441 nm) to 6 (463 nm).
The band-gap calculation for all three compounds 4, 6 and 12 was carried out using
[
15]
Density Functional Theory as well as electronic spectra. As can be seen in Table 1,
there is slight difference in Eband-gap (Theoratical) and Eband-gap (Optical) for compound 4,
6
and 12.
6

Cyclo-Voltammetry Experiment Data For Compound 6
The electrochemical properties of compound 6 was studied in which ferrocene was used
as an internal standard. According to Figure 4, cyclic voltammogram exhibit an
irreversible oxidation curve with no reduction wave deducted. The HOMO was then
calculated from the oxidation potential (E ), according to the formula of EHOMO = –
oxi
[
(E –E ) + 4.8 eV], where E denotes the measured oxidation potential of ferrocene
oxi Fc Fc
and 4.8 eV is the absolute oxidation potential value of ferrocence under vacuum.
Calculated value of EHOMO for compound 6 is –5.86 eV and value of ELUMO is –3.59 eV
which is calculated from the formula ELUMO = [EHOMO + EOptical].
o
All the compounds 4, 6 and 12 showed reasonably good thermal stabilities up to 450 C.
The thermogravimetric analysis shows high stability of compound 6 (Figure 5). As the
o
DTA curve shows phase transition was observed at 172.6 C which is also in accordance
o
with melting point of compound 6 (173-175 C). It shows thermal decomposition at 429.3
o
C as suggested by DTG curve.
CONCLUSION
In this short communication we have report synthesis of (Z)-2-phenyl-3-(5-(4-(thiophen-
2
-yl)benzo[c][1,2,5]thiadiazol-7-yl)thiophen-2-yl)acrylonitrile. The synthesized
1
13
compound was well characterized by ( H and C) NMR, Mass spectra, IR spectral data
and single crystal X-ray diffraction analysis data. This novel compound was screened for
its band-gap properties using absorption and emission spectral data and cyclic-
voltammogram. Experimental value of band-gap was compared with theoretical band-
7

gap. Thermal stability was studied using thermogravimmetric analysis. In summary, the
synthesized new class of donor–acceptor compound featuring benzothiadiazole electron-
accepting unit, displays distinct optical, electrochemical, and thermal properties. The
experimental data shows that compound 6 could be a potential candidate as an interfacial
modifier for Photovoltaic Devices.
EXPERIMENTAL
All the chemicals were reagent grade and used as purchased. Moisture-sensitive reactions
were performed under an inert atmosphere of dry nitrogen with dried solvents. Reactions
were monitored by TLC analysis using Merck 60 F254 aluminium coated plates and the
spots were visualized under UV light. Column chromatography was carried out on Silica
gel (60-140 mesh). All melting points were determined using Thiele’s tube using paraffin
oil and are uncorrected. IR spectra were recorded on a Shimadzu Prestige 21
spectrometer. Mass spectra were recorded on Thermo-Fischer DSQ II GCMS instrument.
NMR spectra were recorded on a BrukerAvance-III 400 spectrometer in CDCl3.
Diffraction data were collected using CuKα (λ = 1.5418 Å) radiation on an Xcalibur, Eos,
Gemini diffractometer. CV data were obtained with CH Instruments model of CHI 600C
with three electrode (glassy carbon as the working electrode, platinum as the counter
electrode, and non-aqueous Ag/AgNO as the reference electrode) cells in anhydrous
3
THF solution containing 0.1 M tetra-n-butylammoniumhexafluorophosphate at a scan
−
1
rate of 100 mV s under N atmosphere. DFT calculations were performed using
2
[
15]
Gaussian 09 program with B3LYP functional and 6-311G (++) basis set.
8

Compounds 2, 3, 4, 5 And 6
[10]
Compounds 2, 3, 4 and 5 were prepared by the corresponding literature methods.
o
o
Compound 2: Yield: 36 %; mp: 43-45 C. Compound 3: yield: 68 %; mp:191-194 C.
4
,7-Dithien-2-Yl-2,1,3-Benzothiadiazole 4
4
,7-dibromo 2,1,3-benzothiadiazole (1 g, 3.4 mmol) is taken in dry THF (25 mL). Add to
this 2-tributylstannylthiophene (2.37 mL, 7.48 mmol) and PdCl (PPh ) (0.11924 g, 0.17
2
3 2
mmol) and reflux for 10-12 hrs under nitrogen atmosphere. After cooling the reaction
mixture, quench with water and extracted in EtOAc. Purification is done using column
chromatography on alumina using EtOAc : Pet ether (1 : 20) as eluent and further
recrystallized in pet ether.
o
Yield: 49 %; mp: 123-124 C. IR (KBr): 2978, 2885, 2389, 1725, 1562, 1481, 1013, 823,
1
7
72, 692. H NMR (400 MHz, CDCl ): δ 7.22-7.24 (dd, J = 5.2, 4 Hz, 2H), 7.47-7.48
3
1
3
(
dd, J = 5.2, 1.2 Hz, 2H), 7.88 (s, 2H), 8.13-8.14 (dd, J = 3.6, 1.2 Hz, 2H). C NMR: δ
25.8, 126.0, 126.8, 127.5, 128.0, 139.3, 152.6. Mass (EI) m/z: 300 (M+, 100 %), 299
90 %), 81 (15 %), 68 (22 %).
1
(
5
-(4-(Thiophen-2-Yl)Benzo[C][1,2,5]Thiadiazol-7-Yl)Thiophene-2-Carbaldehyde 5
Take DMF (0.39 mL, 5 mmol) in round bottom flask under nitrogen atmosphere and cool
°
to 0 C. To this solution add POCl (0.47 mL, 5 mmol) dropwise and stir it for 15 minutes
3
at room temperature. After this add compound 4 (0.150 g, 0.5 mmol in 20 mL of DCM)
and refluxed the reaction mixture for 17 h. After cooling add sodium acetate (50 mL) and
9

stir reaction mixture for 2 h at room temperature. Extract the compound with DCM and
purify by column chromatography on silica gel using EtOAc : Pet ether (2 : 10) as eluent.
°
-1
Yield: 56 %; mp: 168-170 C. IR (KBr, cm ): 2983, 2888, 2410, 1655, 1440, 1236, 834,
1
8
1
18; H NMR (400 MHz, CDCl ): δ 7.25-7.27 (m, 1H), 7.53-7.54 (dd, J = 5.2, 1.2 Hz,
3
H), 7.86-7.88 (d, J = 4.4 Hz, 1H), 7.9-8.0 (d, J = 7.6 Hz, 1H), 8.02-8.04 (d, J = 7.6
Hz, 1H), 8.19-8.20 (dd, J = 3.6, 0.8 Hz, 1H), 8.23-8.24 (d, J = 4 Hz, 1H), 10.0 (s, 1H).
1
3
C NMR: δ 124.3, 125.2, 127.4, 127.8, 127.9, 128.0, 128.2, 128.4, 136.8, 138.8, 143.3,
1
48.6, 152.4, 152.46, 183.
(Z)-2-Phenyl-3-(5-(4-(Thiophen-2-Yl)Benzo[C][1,2,5]Thiadiazol-7-Yl)Thiophen-2-
Yl)Acrylonitrile 6
Take mixture of compound 5 (0.1 g, 0.305 mmol) and benzyl cyanide (0.038 g, 0.32
mmol) in 30 mL dry ethanol. To this reaction mixture add slowly solution of Na metal
(0.031 g, 1.53 mmol) in 10 mL dry ethanol. Reflux the reaction mixture for 10-12 h.
After completion of reaction distill out the excess of ethanol. The crude product is
purified by column chromatography on silica gel using EtOAc : Pet ether (1 : 4) as
eluent.
°
-1
Yield: 62 %; mp: 173-175 C. IR (KBr, cm ): 2983, 2897, 2390, 2289, 1583, 1480, 1439,
1
8
1
28, 801, 756, 720, 690; H NMR (400 MHz, CDCl ): δ 7.23-7.25 (m, 1H), 7.38-7.42 (m,
3
H), 7.45-7.51 (m, 3H), 7.68-7.70 (m, 3H), 7.73-7.74 (d, J = 4 Hz, 1H), 7.89-7.91 (dd, J
=
7.6, 0.8 Hz, 1H), 7.97-7.99 (dd, J = 7.2, 0.8 Hz, 1H), 8.15-8.16 (d, J = 4 Hz, 1H), 8.21-
1
0

1
3
8
1
.22 (d, J = 4 Hz, 1H). C NMR: δ 108.2, 118.3, 124.8, 125.5, 125.7, 126.6, 127.1,
27.4, 127.9, 128.1, 129.0, 129.1, 133.7, 133.8, 133.9, 138.5, 139.1, 143.7, 152.5, 152.6.
Mass (EI) m/z: 427 (M+, 88 %), 426 (59 %), 400 (70 %), 399 (68 %), 368 (43 %), 298
63 %), 133 (39 %), 128 (35 %).
(
Compounds 9, 10 And 11
These compounds were prepared by the corresponding literature method. Compound 9:
[
11]
o
o
Yield: 20 %; mp: 50-51 C. Compound 10: yield: 46 %; mp: 141-143 C. Compound 11:
o
Yield: 70 %; mp: 111-113 C.
4
,7-Di(Thiophen-2-Yl)Benzo[C][1,2,5]Oxadiazole 12
Take compound 11 (0.2 g, 0.719 mmol), 2-tributylstannylthiophene (0.55 mL, 1.73
mmol) and PdCl (PPh ) (0.025 g, 0.03595 mmol) in dry THF (20 mL) under nitrogen
2
3 2
atmosphere. Reflux the reaction mixture for 10-12 h. After completion of the reaction,
quenched the reaction mixture in water and extract it with EtOAc. The crude product was
purified the compound by column chromatography on alumina using EtOAc : Pet ether (1
o
[11]
-1
:
20) as eluent. Yield: 47 %; mp: 111-113 C . IR (KBr, cm ): 2394, 1663, 1592, 1536,
1
1
2
2
430, 1216, 825, 711; H NMR (400 MHz, CDCl ): δ 7.22-7.24 (dd, J = 4.8, 3.6 Hz,
3
H), 7.46-7.48 (dd, J = 5.2, 1.2 Hz, 2H), 7.63 (s, 2H), 8.13-8.14 (dd, J = 3.6, 0.8 Hz,
1
3
H). C NMR: δ 122.1, 126.3, 126.9, 128.6, 128.7, 137.9, 147.8. Mass (EI) m/z: 284
+
(
M , 100 %), 283 (64 %), 254 (35 %), 253 (26 %), 90 (22 %), 69 (36 %).
SUPPLEMENTARY DATA
1
1

Crystallographic data for compound 6 has been deposited in the Cambridge
Crystallographic Data Center, CCDC No. 1431814. These data can be obtained free of
charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html, or from the Cambridge
Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44)
1
223 336 033; or e-mail: deposit@ccdc.cam.ac.uk.
ACKNOWLEDGMENTS
ALP acknowledges GUJCOST, Gandhinagar, Gujarat (India) for the financial support
Project No. GUJCOST/MRP/2014-15/408). One of the authors, VJB acknowledges
(
Solvay Specialities India Pvt Ltd and UGC-BSR for providing fellowship. The authors
would like to acknowledge Dr S S Zade, Associate Professor, IISER, Kolkata for DFT
calculations.
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Appl. Cryst. 2009, 42, 339-341.
[
(
4
[
[
[
[
13]. Palatinus, L.; Chapuis, G. J. Appl. Cryst. 2007, 40, 786-790.
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1
4

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013, 5, 12460–12468.
2
1
5

Table 1. Absorption and Electrochemical properties of compound 4, 6 and 12
Compound Wavelength (UV) Wavelength
nm
(Fluorescence) nm
nm
600
EBand-gap (eV)
Optical
EBand-gap (eV)
Theoretical
onset
max
[
[
16]
16]
4
6
1
253, 300, 441
379, 463
508
2.44
2.27
2.42
2.69
2.80
2.62
605
582
548
513
2
229, 290, 436
1
6

Scheme 1. Synthesis of (Z)-2-phenyl-3-(5-(4-(thiophen-2-yl)benzo[c][1,2,5]thiadiazol-7-
yl)thiophen-2-yl)acrylonitrile
1
7

Scheme 2. Synthesis of dithienobenzooxadiazole
1
8

Figure 1. Intermolecular force of attraction in compound 6
1
9

Figure 2. Zig-zag networking formed by C-H... π donor-acceptor interactions in
compound 6
2
0

Figure 3. (a) Absorption spectra and (b) Emission spectra of 4, 6 and 12 (Red, Black and
-7
-6
-7
Blue solid lines) measured in methanol containing 3.33 × 10 , 3.33 × 10 , 6.67 × 10
moles respectively. Photographs of 4, 6 and 12 in methanol under 365 nm UV lamp.
2
1

Figure 4. Cyclic voltammogram of compound 6 in anhydrous THF containing 0.1 M
tetrabutylammoniumhexafluorophosphate as a supporting electrolyte with glassy carbon
as working electrode, platinum as counter electrode and non-aqueous Ag/AgNO as
3
reference electrode
2
2

Figure 5. Thermogravimetric analysis of compound 6
2
3