Structure-property correlation of halogen substituted benzothiazole crystals

Article abstract of DOI:10.1016/j.molstruc.2021.130765

We have synthesized 3 benzothiazole crystals (1–3) based on existing knowledge of combining flexibility and optical properties towards achieving applications for flexible optoelectronics. However, one crystal was found to be elastically bendable and was f

Full text of DOI:10.1016/j.molstruc.2021.130765

Journal of Molecular Structure 1243 (2021) 130765
Contents lists available at ScienceDirect
Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstr
Structure-property correlation of halogen substituted benzothiazole
crystals
Nipun P. Thekkeppata, Labhini Singlab, Srinu Tothadi c, Priyadip Dasa,
Angshuman Roy Choudhuryb, Soumyajit Ghosha,∗
a Department of Chemistry, SRM Institute of Science and Technology, Chennai 603 203, India
b Department of Chemical Sciences, Indian Institute of Science Education and Research, Mohali, Knowledge City, Sector 81, S. A. S. Nagar, Manauli PO,
Mohali. 140306. Punjab. India
c Organic Chemistry Division, CSIR-National Chemical Laboratory, Dr. Homi Bhabha Road, Pune 411 008, India
a r t i c l e i n f o
a b s t r a c t
Article history:
We have synthesized 3 benzothiazole crystals (1–3) based on existing knowledge of combining flexibility
and optical properties towards achieving applications for flexible optoelectronics. However, one crystal
was found to be elastically bendable and was found to comply necessary packing features for elasticity.
Other two crystals do not obey packing features for elasticity hence they are brittle in nature. Further,
Hirshfeld analysis illustrates that elastic crystal 1 possess more number of weak and dispersive interac-
tions compared to other crystals. These interactions were instrumental in invoking elasticity. Moreover,
crystals 1–3 were found to be fluorescent as well at specific excitation wavelengths. Therefore, among
these crystals, particularly crystal 1 is considered as more promising candidate for flexible optoelectron-
ics.
Received 20 March 2021
Revised 21 May 2021
Accepted 22 May 2021
Available online 28 May 2021
Keywords:
Benzothiazole crystals
Elasticity
Halogen bond
Mechanical properties
© 2021 Elsevier B.V. All rights reserved.
1. Introduction
slip stacked assembly in dense state due to π•••π interactions.
While stacked assembly of the molecules can give rise to enhanced
Nowadays, Flexible crystals have been a frequently researched
topic among the scientific communities owing to its promising
applications in flexible optoelectronics [1,2], optical waveguides
[3–5], sensors, flexible OLEDs etc. By systematic incorporation of
optical properties with flexibility make some of the crystalline ma-
terials suitable candidates for flexible optoelectronics. There are
several reports of flexible crystals such as elastically bendable
[6–8], plastically bendable [9–11] and helically twisted [12] and so
on. Each type of flexibility is associated with certain set of crys-
tal packing features along with presence of weak interactions such
as halogen bonding, hydrogen bonding etc. [13–16]. In general,
elastic bending of crystals are associated with criss-cross pack-
ing in isotropic structure and presence of weak and dispersive
interactions so called strain buffer or interaction buffer interac-
tions [17,18]. In order to combine flexibility with optical properties
suitable π conjugated molecules having reasonably good light ab-
sorbing or bright fluorescence can be chosen. Substituents of the
molecule can be fine-tuned in order to meet requisite criteria for
elasticity. Further, π conjugated molecules have tendency to form
fluorescence in solid state due to J-aggregates the substituents in
the periphery develop noncovalent interactions necessary for flexi-
ble behavior. Hence, crystal engineering can be gainfully employed
to design property based molecular crystals. Very recently, several
reports of fluorescent elastic organic crystals as optical waveguides
are known. For instance, Naumov et al. have shown in their re-
cent reports the dual-mode light transduction in optical waveg-
uides by using elastic flexible organic crystals [4]. Hayashi et al.
exhibited the role of elastic organic crystals comprising of fluo-
rescent π-conjugated molecules in mechanofluorochromism and
optical waveguides [5]. Zhang et al. showed optical waveguiding
organic single crystals exhibiting physical and chemical bending
features. However, some appropriate substituents may be intro-
duced at particular positions in order to fine tune the molecules
and should be apparently impervious to packing modes of crys-
tals. Therefore, perfect compatibility should be carefully taken into
consideration for designing of multiple stimuli responsive organic
bending crystals [19]. Recently, we have showed how micromanip-
ulation of organic crystals can led to microwaveguides [20]. Mi-
cromanipulation of flexible molecular crystals necessitates usage
in miniature devices. Therefore, the single organic crystal that can
withstand high mechanical flexibility similar to soft materials such
as elastomers, thin films, and polymers can be of prime importance
∗
Corresponding author.
E-mail addresses: soumyajitghosh89@gmail.com, soumyajs@srmist.edu.in (S.
Ghosh).
https://doi.org/10.1016/j.molstruc.2021.130765
0022-2860/© 2021 Elsevier B.V. All rights reserved.

N.P. Thekkeppat, L. Singla, S. Tothadi et al.
Journal of Molecular Structure 1243 (2021) 130765
Fig. 1. Synthesized Crystals 1, 2 and 3.
for the design of future smart materials [21]. So far, the reports
on plasticity and elasticity of molecular crystals have been revolv-
ing around the supramolecular synthons like: (1) polyhalogenated
N-benzylideneanilines [22], (2) ester spacer based molecules, (3)
organic co-crystals [12], (4) naphthalene diimides derivatives, (5)
thiophenyl, (6) tetrafluoropyridyl based π-conjugated derivatives
(7) vanillin derivatives [23], (8) pyrimidine [24], (9) terephthala-
mide (10) trisubstituted haloimidazoles, (11), fluorobenzoic acid
derivative etc. and formation of these derivatives involved sub-
stitution of halogens or tedious synthetic routes. Benzothiazole is
well known compound for its optical properties as it is used as
a flurophore extensively and also as ligands for co-ordinating to-
wards metals like Iridium, Platinum etc. and studied for the in-
teractions in biological systems [17,25–28]. Herein, we have syn-
thesized three halogen substituted benzothiazole crystals based on
existing knowledge of combining flexibility with optical proper-
ties. However, we obtained one elastically bendable crystal 1 while
other two were brittle in nature. Structure-property correlation
study was done extensively. Crystal 1 was found to comply with
necessary packing features for elasticity whereas 2 and 3 do not
obey and hence are brittle in nature (Fig. 1).
2.3. Single crystal preparation
Some amount of synthesized substituted benzothiazole com-
pounds are dissolved in minimum amount of solvents (different
solvents such as ethanol, methanol, acetone, chloroform etc.) and
kept for crystallization in slow evaporation method in small coni-
cal flask. Long, good diffraction quality crystals were obtained after
4–5 days.
2.4. Powder X-ray diffraction
The PXRD patterns were collected on a Bruker (D8 Advance)
˚
diffractometer with a Cu Kα radiation (1.540 A). The tube volt-
age and amperage were set at 40 kV and 50 mA respectively. Each
powder sample was scanned between 5 and 50° 2θ with a step
size of 0.02° (Figure S1-S3). The instrument was previously cali-
brated using a silicon standard.
2.5. Single crystal X-ray crystallography
Single crystal X-ray diffraction data were collected using a
Rigaku XtaLABmini X-ray diffractometer equipped with Mercury
CCD detector with graphite monochromated Mo-Kα radiation
˚
(λ = 0.71073 A) using ω scans. The crystal structures were solved
2. Experimental section
by using ShelXT [29] and were refined using ShelXL97 [30] through
Olex2 suite [31]. All the hydrogen atoms were geometrically fixed
and refined using the riding model. Multi-scan method was em-
ployed for absorption correction.
2.1. Materials
2-Amino-4-Chloro benzenethiol, 4-Chloro benzaldehyde, 4-
Pyridine carboxaladehyde, 4–methoxy salicylaldehyde were pur-
chased from TCI, Alfa Aesar and used for synthesizing the required
molecules and recrystallized in different solvents. Commercially
available solvents were used as received without further purifica-
tion.
2.6. Melting point
Melting points of the synthesized crystals were measured using
a Digital Melting point apparatus VEEGO (Model: VMP-PM).
2.7. Differential scanning calorimetry (DSC)
2.2. Synthesis of benzothiazole derivatives
DSC experiments were conducted on a Netzsch Polyma 214 in-
strument by taking accurately weighed samples (2–3 mg) in a
sealed aluminum crucibles having a pin hole on lid and scanned
from 30 °C to 300 °C at a heating rate of 10 °C/min under a dry
nitrogen atmosphere (flow rate 100 mL/min).
A
series of crystals were synthesized by charging a vari-
ety of halogen substituted aldehyde (1.5 mmol), 2- amino-4-
chlorobenzenethiol (1.25 mmol), and DMSO (5 ml) in a round bot-
tom flask and stirred for minimum of 6 h at 60 °C. Reaction was
monitored by TLC. After the reaction water (50 ml) was charged
and extracted with ethyl acetate for three times. The combined or-
ganic layer was distilled under vacuum to get crude product. It was
purified by crystallization with acetone, methanol, ethanol and a
variety of solvents.
2.8. TGA/DTA studies
TGA/DTA studies were done on a Netzsch STA Regulus2500 by
loading a small amount of sample in an alumina crucible and
2

N.P. Thekkeppat, L. Singla, S. Tothadi et al.
Journal of Molecular Structure 1243 (2021) 130765
Fig. 2. Snapshot of the bendable cycles of Crystal 1.
scanned from 30 °C to 600 °C at a heating rate of 20 °C/min under
a dry nitrogen atmosphere (flow rate 100 mL/min) (Figure S10).
imparting elasticity. The bonds can be broken or reformed easily
thereby invoking spring-like motion of the molecules with respect
to their thermodynamic positions. When the pressure gets released
the molecules come back to their original thermodynamic position
rendering elasticity [13,14,16]. In bent state in outer arc molecules
gets stretched out while in inner arc molecules gets compressed
leaving interfacial angle same during bending. As a result outer arc
increases and inner arc compresses to same extent [17,18,21,22].
Molecules located in middle arc do not change their mean position
invariably. When the applied stress exceeds the threshold limit the
crystal breaks. However, broken parts retain elasticity owing to in-
trinsic crystal packing. Elastic strain was estimated from the bent
crystal and was found to be approximately ~ 3% (Fig. 3a and Fig.
S5, ESI).
2.9. UV- Visible Spectrometry (UV–Vis)
Solution state UV–Vis absorption spectra of the crystals were
done by dissolving it in dimethyl sulfoxide (DMSO). These were
recorded by Agilent Cary 5000 spectrophotometer. The absorption
maxima (λmax) for Crystal 1, 2 and 3 were found out to be 307 nm,
351 nm and 297 nm respectively.
2.10. IR spectroscopy
Transmission infrared spectra of the solids were obtained using
a Fourier–transform infrared spectrometer (Schimadzu IR Tracer-
100 in KBr method. An averaging of 2000 scans was collected at
4 cm−1 resolution for each sample and the spectra were measured
Crystals of 2 were grown from methanol solvents as plate
shaped crystals (Dimension approx. 0.5 mm × 0.05 mm × 0.5 mm)
in monoclinic space group P21/c. 4-Methoxy salicylaldehyde and 4–
chloro-2-amino thiols were used as precursor materials for synthe-
sizing benzothiazole compounds of 2. In crystals of 2, in individual
molecule both benzothiazole ring and aromatic rings are coplanar
with respect to C–C (C7–C8) bond. Total six molecules are intercon-
over the range of 4000–500 cm−1
.
2.11. Fluorescence microscopy study
Some crystals were checked under LEICA DM6B fluorescence
microscope to capture optical images. This was checked with dif-
ferent ranges of excitation wavelengths and these were found to
be fluorescent (Figures S7-S9, ESI).
˚
˚
nected via Cl•••Cl (3.467 A) and C–H•••O (2.706 A) interactions to
form ring motif R44(54) (Fig. 4). Also two molecules are separately
connected via C–H•••O (2.588 A) interaction to form dimeric motif
R22(16). Further dimeric motifs are connected to another R44(54)
ring motif. Hence, this leads to interconnected network packing
with absence of slip planes [17,18].
˚
3. Results and discussion
As the crystal do not comply necessary packing features such
as criss-cross packing and presence of weak and dispersive interac-
tions they are brittle in nature. Moreover, the interactions are less
variant in nature (C–H•••O and Cl•••Cl) compared to interactions
in crystal 1. Packing clearly shows absence of slip plane or criss-
cross arrangement of molecules in lattice considered as prerequi-
site criteria for slippage or elastic flexibility (Fig. 5). The relatively
robust interactions in the lattice do not facilitate easy breaking or
reformation of weak interactions under applied stress. Hence, over-
all crystal packing and noncovalent interactions are not enough to
endow elasticity in crystals of 2. Therefore, the crystals are not
compliant in nature similar in line of 1 [13–18].
Crystals of 1 were grown from methanol solvent as long acic-
ular crystals (Dimension approx. 10 mm × 0.09 mm × 0.074 mm)
¯
having triclinic space group P1. The crystals were found to be elas-
tically bendable when pressure is applied from any of the side
faces. When the long needle crystal of 1 was held with forceps and
pressure was applied from opposite sides with needle the crystal
can be quickly bent and regains original shape upon pressure re-
traction. With increasing applied pressure it shows propensity to
bend further and eventually half loop can be constructed as shown
in Fig. 2. This bending is under the elastic limit and when the pres-
sure exceeds limit it ultimately breaks down. The process can be
repeated several times reflecting reversible and non-fatigue elastic
bendability as it is evident from snapshots in Fig. 2. (Supporting
video)
Crystals of 3 were grown as long needles (Dimension approx.
5 mm × 0.08 mm × 0.12 mm) from methanol, ethanol etc. The
crystals were grown in monoclinic P21/c space group. Molecules
In order to establish structure-property correlation face index-
ing was performed for the crystal. It has been noted bendable side
˚
˚
are connected mainly by C–H•••S (3.032 A), C–H•••N (2.962 A)
˚
and C–H•••π (2.895 A) to form zigzag tape along b axis. Further,
neighboring tapes are connected via type I Cl•••Cl (3.434 A) and
¯ ¯
¯
¯
faces are assigned as (011/011) and (011/011) (Fig. S6, ESI). Crystal
˚
packing analysis shows two molecules form dimeric motif via C–
˚
C–H•••N (2.797 A) to form corrugated sheet in bc plane. Overall
˚
H•••O (2.992 A) interactions. Further dimeric motifs are connected
structure is isotropic and corrugated pattern is clearly visible from
side faces along b and c axes (Fig. 6). Crystals do not comply pack-
ing feature similar to 1 and hence are brittle in nature (Fig. S4, ESI).
Lesser number of weak and dispersive interactions such as Cl•••Cl
interactions and presence of relatively stronger interactions such as
C–H•••S hinders spring like motion of the molecules under applied
stress [17,18].
˚
˚
by C–H•••Cl (3.087 A) and Cl•••S (3.598 A) to form zigzag tapes.
˚
Within the tape there is presence of type II Cl•••Cl (3.996 A) which
plays sustainability within the tape. Neighbouring tapes are con-
˚
nected by mainly two interactions Cl•••S (3.766 A) and C–H•••Cl
˚
˚
(3.233 A, 3.22 A) to form corrugated network. Criss-cross packing
¯
view is clearly evident from (011) and (011) side faces (Fig. 3).
From packing analysis it is quite clear that the crystal possess req-
uisite criteria for elastic bending, i) absence of slip planes with
isotropic criss-cross packing and ii) presence of multiple weak and
dispersive interactions such as Cl•••Cl, Cl•••S, C–H•••Cl and C–
H•••O etc. These restorative interactions play significant role in
Hirshfeld analysis can be used to define space of the molecule
and gives some insights of the type and number of noncovalent
interactions present in them. The marked white area indicates the
area of separation between neighbouring atoms equivalent to sum
3

N.P. Thekkeppat, L. Singla, S. Tothadi et al.
Journal of Molecular Structure 1243 (2021) 130765
Fig. 3. (a) Crystal of 1 can be bent into half loop. (b) Crystal packing view perpendicular to (011), (0–11) and (100) faces. Criss-cross packing view is clearly evident from
bendable side faces.
Fig. 4. Crystal packing of 2 shows ring motifs R44(54) and R22(16).
Table 1
of van der Waal’s radii and can be estimated by H•••H interactions.
Crystallographic Information Table.
Blue regions indicate no interaction between neighbouring atoms
because of their wide separation from each other. Red and blue
region in the shape index represent the surface area surrounding
acceptor and donor atoms in the molecule. C•••C contacts denote
π•••π distance between consecutive molecules. From Hirshfeld
surface analysis it is quite evident that crystal 2 and 3 lacks sev-
eral weak and dispersive interactions compared to 1. From relative
percentage composition of weak interactions it is clear that crystal
1 possess more number of interactions resulting an important role
in rendering elasticity (Fig. 7). As weak and dispersive interactions
act as “strain buffer” interactions, numbers as well as strength
of these interactions play as decisive factor in invoking elasticity.
More number of relatively weaker interactions in 1 can be bro-
ken or reformed easily allowing easy movement of molecules un-
der applied stress. Hence, number and type of weak interactions
play significant role in deciding elastic bending behavior (Table 1
and Fig. 8).
Compounds
1
C
2
3
Formula
13H7Cl2NS
C14H10ClNO2S
291.74
298.0
C12H7ClN2S
246.7
Formula weight
T/K
280.17
298.0
298.0
Crystal system
Space group
Triclinic
Monoclinic
P 21/c
Monoclinic
P 21/c
¯
P 1
˚
a/A
3.8994(9)
15.242(3)
23.426(8)
86.67(3)
89.72(2)
84.376(19)
1383.3(6)
2
14.1643(10)
5.8445(3)
15.3897(11)
90
3.9298(3)
11.6031(8)
23.5663(18)
90
˚
b/A
˚
c/A
α/o
β/o
γ /o
94.022(6)
90
90.921(15)
90
3
˚
Volume/A
1270.87(14)
4
1074.43(14)
4
Z
ρ, Mg.cm–3
μ /mm−1
1.345
1.525
1.525
0.596
0.460
0.518
Reflections collected
Independent Reflections
Rint
10,152
4837
7182
12,412
3518
4203
0.0454
0.985
0.0216
1.032
0.0332
0.924
GOF
Final R[I >2σ]
R1/ wR2
0.0596
0.1272
2,033,995
0.0453
0.1089
1,987,126
0.0457
0.1045
1,987,127
3.2. Thermal analysis
CCDC Number.
3.2.1. Diffrential scanning calorimetry
A detailed DSC experiments were carried out of all the above
crystals to study their thermal behavior. Crystals (1–3) are show-
ing characteristic melting endotherms and their melting points are
tabulated below (Table 2). The melting transition temperatures of
all the crystals were different from either of the individual compo-
nents confirming the formation of new phase. Further, the absence
4

N.P. Thekkeppat, L. Singla, S. Tothadi et al.
Journal of Molecular Structure 1243 (2021) 130765
Fig. 5. Crystal packing view of 2 along a axis (a), b axis (b) and c axis (c).
Fig. 6. Crystal packing view of 3 along a axis (a), b axis (b) and c axis (c).
5

N.P. Thekkeppat, L. Singla, S. Tothadi et al.
Journal of Molecular Structure 1243 (2021) 130765
Fig. 7. (a) and (b) Hirshfeld surface analysis and percentage contributions of interactions in crystal 1. (c) and (d) Hirshfeld surface analysis and percentage contributions of
interactions in crystal 2. (e) and (f) Hirshfeld surface analysis and percentage contributions of interactions in crystal 3.
of any other endotherm excludes the formation of solvate or hy-
drate. Also there is no phase transition during course of heating.
The thermogram shows the stability of the crystal and points out
there is no weight loss upto melting of each form (Fig. 9).
3.3. FTIR analysis
Powdered samples were examined for FTIR spectrum using KBr
pellet where the sample is mixed thoroughly with KBr powder and
made as a pellet and analysis were done. Fig. 10a shows FTIR spec-
6

N.P. Thekkeppat, L. Singla, S. Tothadi et al.
Journal of Molecular Structure 1243 (2021) 130765
Fig. 8. Number of inter molecular interactions (close contacts) are more in 1 (a) compare to 2 (b) and 3 (c).
Table 2
Melting points of the resultant
crystals.
Crystal
Melting Point ( °C)
1
2
3
155.9
228.8
182.8
trum for crystals from 4000 to 2000 cm−1. In Crystal 2, a broad
peak around region of 3200 cm−1 confirms the presence of –OH
bond. Fig. 10b for crystal 1 shows aromatic C–H stretching peak
appears at 2980 cm−1 as a sharp peak with high intensity, C = C
vibrational mode appears at 1434 cm−1 as sharp peak and C–C
stretching frequency comes at around 1092 cm−1. There is a pres-
ence of C–S stretching vibrational mode at 623 cm−1 and C = N
mode at 1646 cm−1. Halogen substitution gives the C–Cl stretch-
ing peak at 831 cm−1. This shows sharp characteristic peak of O–
CH3 stretching mode at 1203 cm−1 for crystal 2. Aromatic C–H
bond stretching vibrational mode is showing a high intense peak
in 1881 cm−1. C = C stretching vibrational mode appears at 1433
cm−1 and C–C stretching mode comes around 1200 cm−1. There is
a presence of C–S stretching vibrational mode at 643 cm−1 and a
weak intense peak corresponding to C = N at 1646 cm−1. Halogen
substitution gives the C–Cl stretching peak at 808 cm−1. Similarly
this is also evident for crystal 3. Aromatic C–H stretching vibra-
tional mode appears as high intense peak in 2900 cm−1 and C = C
stretching vibrational mode appears at 1436 cm−1. Here also there
is a presence of C–S stretching vibrational mode at 633 cm−1 and
C = N weak intense peak at 1652 cm−1. Halogen substitution gives
the C–Cl stretching peak at 820 cm−1 [32,33,34] (Table 3).
Fig. 9. The endotherm of the DSC corresponds to the melting of the respective crys-
tals (1–3).
Table 3
FTIR frequency table.
1
2
3
C-H stretching
C = N
1872
1646
623
831
–
1881
1646
643
1732
1652
633
820
–
C-S
–
C
Cl
808
O-H
3425
1203
–
O
CH3 strectching
–
–
7

N.P. Thekkeppat, L. Singla, S. Tothadi et al.
Journal of Molecular Structure 1243 (2021) 130765
Fig. 10. The FT-IR spectrum of Crystal (a) 4000 to 2000cm−1 and (b) 2000 to 400cm−1
.
4. Conclusion
Acknowledgment
We have synthesized three halogen substituted benzothiazole
crystals based on existing knowledge of combining flexibility with
optical properties. However, we got one elastically bendable crystal
1 while other two were brittle in nature. Structure-property corre-
lation study shows crystal 1 was found to comply with necessary
packing features for elasticity whereas 2 and 3 do not comply and
hence are brittle in nature. Furthermore, Hirshfeld analysis reflects
the fact that more number of weak and dispersive interactions in
crystal 1 is instrumental in pertaining elasticity. Thermal and FTIR
analysis were done extensively.
S. G. thanks Science and Engineering Research Board, Depart-
ment of Science and Technology, Government of India for Core Re-
search Grant (File No: CRG/2020/000885). N. P. T. thanks SRM In-
stitute of Science and Technology for fellowship. S. T. thanks CSIR-
HRDG for award of senior research associateship under scientist
pools scheme of CSIR (Pool No. 9119-A). L. S. thanks IISER Mo-
hali for research fellowship (SRF) and A. R. C. thanks IISER Mohali
for providing access to the departmental XtaLabmini X-ray diffrac-
tometer.
Supplementary materials
Supplementary material associated with this article can be
found, in the online version, at doi:10.1016/j.molstruc.2021.130765.
Credit author statement
Dr. Soumyajit Ghosh the corresponding author designed the
work and analysed the results obtained from several experiments
and wrote the manuscript.
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cial interests or personal relationships that could have appeared to
influence the work reported in this paper.
8

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