Impact of donor–acceptor alternation on optical power limiting behavior of H–Shaped thiophene–imidazo[2,1-b] [1,3,4]thiadiazole flanked conjugated oligomers

Article abstract of DOI:10.1016/j.dyepig.2019.108181

A new series of four D–A–D configured conjugated oligomers with H–type structure, possessing two thiophene–imidazo [2,1-b][1,3,4]thiadiazole branches and thiophene (TIT), thiophene–1,3,4-oxadiazole–thiophene (TITO), thiazolo [5,4-d]thiazole (TITz), phenyl

Full text of DOI:10.1016/j.dyepig.2019.108181

Journal Pre-proof
Impact of donor–acceptor alternation on optical power limiting behavior of H–Shaped
thiophene–imidazo[2,1-b] [1,3,4]thiadiazole flanked conjugated oligomers
Viprabha Kakekochi, P. Nikhil P, Keloth Chandrasekharan, Udaya Kumar D
PII:
S0143-7208(19)32587-2
DOI:
https://doi.org/10.1016/j.dyepig.2019.108181
DYPI 108181
Reference:
To appear in: Dyes and Pigments
Received Date: 5 November 2019
Revised Date: 26 December 2019
Accepted Date: 31 December 2019
Please cite this article as: Kakekochi V, Nikhil P P, Chandrasekharan K, Kumar D U, Impact of
donor–acceptor alternation on optical power limiting behavior of H–Shaped thiophene–imidazo[2,1-b]
[1,3,4]thiadiazole flanked conjugated oligomers, Dyes and Pigments (2020), doi: https://doi.org/10.1016/
j.dyepig.2019.108181.
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition
of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of
record. This version will undergo additional copyediting, typesetting and review before it is published
in its final form, but we are providing this version to give early visibility of the article. Please note that,
during the production process, errors may be discovered which could affect the content, and all legal
disclaimers that apply to the journal pertain.
© 2019 Published by Elsevier Ltd.

Impact of Donor–Acceptor Alternation on Optical Power Limiting
Behavior of H–Shaped Thiophene–Imidazo[2,1-b][1,3,4]Thiadiazole
Flanked Conjugated Oligomers
Viprabha Kakekochia, Nikhil P Pb, Keloth Chandrasekharanb, Udaya Kumar D*a.
a
Organic and Materials Chemistry Laboratory, Department of Chemistry, National Institute
of Technology Karnataka, Srinivasnagar 575 025, Mangaluru, India.
b
Laser and Nonlinear Optics Laboratory, Department of Physics, National Institute of
Technology Calicut, Kozhikode 673601, Kerala, India.
Corresponding Author
*Contact email: udayaravi80@gmail.com, udayakumar@nitk.ac.in
Keywords
Thiophene; HOMO–LUMO energy; Density functional calculations; UV/Vis spectroscopy;
Nonlinear optics; Two-photon absorption.
Highlights
•
Four novel H-shaped donor–acceptor conjugated oligomers were designed and
synthesized.
•
•
Structure-property relationship of the synthesized oligomers was studied in detail.
The influence of variation in the central core on third-order nonlinear optical
properties was also discussed in detail.
1

TOC / Graphical Abstract
Schematic of two photon absorption (TPA) of H–shaped thiophene–imidazo[2,1-
b][1,3,4]thiadiazole flanked conjugated oligomer TIPTz, implying the effective optical
limiting performance for sensors and eye protection.
Abstract
A new series of four D–A–D configured conjugated oligomers with H–type structure,
possessing two thiophene–imidazo[2,1-b][1,3,4]thiadiazole branches and thiophene (TIT),
thiophene–1,3,4-oxadiazole–thiophene (TITO), thiazolo[5,4-d]thiazole (TITz), phenyl–
thiazolo[5,4-d]thiazole–phenyl (TIPTz) units as central core moieties were efficiently
synthesized. These core moieties were specifically selected to increase the planarity, rigidity,
stability, extend the π–conjugation and to understand the influence of central core on
nonlinear absorption coefficient (βeff) and optical limiting behavior of the synthesized
oligomers. The structure-property relationships of these oligomers were established by the
optical absorption (UV–Vis), electrochemical (CV) and theoretical (DFT) studies. The
“effective two–photon absorption” of oligomers was confirmed by single–beam Z–scan
analysis. The exceptional increase in nonlinear response was achieved with the oligomers
TITO and TIPTz with nonlinear absorption coefficient (βeff) of 1.62 and 2.71×10−10 m W−1,
and limiting thresholds of 6.02 and 3.14 J cm−2, respectively, which suggest that these
oligomers could be potent materials for practical applications in laser photonics.
1. Introduction
Recently, materials possessing nonlinear optical (NLO) properties have gained high
importance because of their remarkable applications in bioimaging, telecommunication,
optical memory storage, all-optical switching, dynamic holography and harmonic generators
[1–8]. Among a wide category of NLO materials, π–conjugated organic molecules possess
2

specific advantages over the other materials such as, quick response, large molecular
hyperpolarizabilities, large NLO absorption coefficients, exceptional photo-thermal stability,
high structural flexibility, easy processing and low cost [9–12]. Moreover, the structures of
organic compounds can easily be modified by tuning the electron donors and/or acceptors
present in them, resulting in the desired NLO properties [13–16]. On the other hand, the
extension of conjugation in organic molecules either by appropriate π–linker or by
incorporating suitable donor/acceptor moieties enhances the intramolecular charge transfer
(ICT) between the electron donor and acceptor units, which significantly improves the NLO
response of organic materials [17,18].
Despite the numerous studies on impact of electron donors on optoelectronic properties, there
is a paucity of work on the effect of strong electron acceptors on the performance of donor
(D)–acceptor (A) (D–A) type materials. The one acceptor, that has immense applications in
optoelectronic industry is 1,3,4–oxadiazole [19]. Lately, 2,5–diaryl–1,3,4–oxadiazoles have
been explored as versatile electron transporting materials in optoelectronic field due to their
electron–deficient nature, resistance to oxidative degradation, high thermal and hydrolytic
stability. Further, they help in extending the conjugation when substituted in D–A type
systems [20,21]. In addition, the organic molecules with the fused heterocycles have gained
much interest as these molecules could increase the performance when applied in
optoelectronics. For an instance, thiazolo[5,4-d]thiazole (tztz), a rigid, fused heterocyclic
system having the coplanar structure with electron–withdrawing nitrogen atoms forming an
imine (C=N-) backbone, also acts as an efficient electron acceptor [22,23]. The small
molecules comprising tztz as an electron acceptor unit with either D–A–D or A–D–A
structure are utilized as potent optoelectronic/ photovoltaic materials in the recent past [24–
28]. However, the structure–property relationship of such molecules is yet to be understood.
In this regard, a series of four H–shaped conjugated oligomers TIT, TITO, TITz and TIPTz
with thiophene, thiophene–1,3,4–oxadiazole–thiophene, tztz and phenyl–tztz–phenyl units as
central cores, respectively and the thiophene–imidazo[2,1-b][1,3,4]thiadiazole (Th–ITD)
moiety at the periphery (Fig. 1) are designed and synthesized. The optoelectronic and the
third-order NLO properties of the peripheral Th–ITD moiety, were discussed in our previous
communication [29]. Herein, an attempt has been made to study the effect of structural
modifications on the optoelectronic properties of four new oligomers which differ in their
central core moieties. In particular, the optical, electrochemical and theoretical studies of
TIT, TITO, TITz and TIPTz are presented in the following sections.
3

2. Experimental information
2.1 Synthetic procedures
2.1.1 Synthesis of 5-(thiophen-2-yl)-1,3,4-thiadiazol-2-amine (3)
The intermediate (3) was synthesized following the procedure mentioned in the literature
[29].
2.1.2 Synthesis of 2-(thiophen-2-yl)imidazo[2,1-b][1,3,4]thiadiazol-6-ol (5)
The intermediate (5) was synthesized following the procedure mentioned in the literature [29]
using the precursors 3 (5 g, 27.28 mmol) and 4–hydroxy phenacyl chloride (5.58 g, 32.74
mmol). (off–white solid, 7.27 g, yield: 89%). ESI–MS (m/z) calcd. for C14H9N3OS2: 299.02,
found: 300.00 [M+H]+; 1H NMR (400 MHz, DMSO–d6) δ (ppm): 9.51 (s, 1H, –OH), 8.49 (s,
1H, Ar–H), 7.91–7.89 (m, 1H, Ar–H), 7.83–7.82 (m, 1H, Ar–H), 7.66 (d, J = 8.0 Hz, 2H, Ar–
H), 7.26–7.24 (m, 1H, Ar–H), 6.78 (d, J = 8.0 Hz, 2H, Ar–H); 13C NMR (100 MHz, DMSO–
d6) δ (ppm): 157.5, 155.3, 146.2, 143.9, 132.0, 131.5, 130.8, 129.1, 126.5, 125.0, 115.9,
109.4; Anal. calcd. for C14H9N3OS2: C, 56.17; H, 3.03; N,14.04; S, 21.42; found: C, 56.02;
H, 3.15; N,14.16; S, 21.47.
2.1.3 Synthesis of 6-(decyloxy)-2-(thiophen-2-yl)imidazo[2,1-b][1,3,4]thiadiazole (6)
The intermediate 5 (4 g, 13.36 mmol) and anhydrous potassium carbonate (5.53 g, 40.08
mmol) were added to N,N–dimethylformamide (DMF) (20 mL) and the mixture was stirred
at room temperature (RT) for 0.5 h. Later, 1–bromodecane (4.15 mL, 20.04 mmol) was added
slowly to the above mixture and heated at 80°C for 12 h. The reaction mixture was quenched
using cold distilled water. The obtained solid was filtered and purified using column
chromatography using a mixture of pet ether/ethyl acetate (9:1, v/v) as eluent to get (6) as a
yellow solid (5.34 g, yield: 91%). ESI–MS (m/z) calcd. for C24H29N3OS2: 439.18, found:
1
440.10 [M+H]+; H NMR (400 MHz, CDCl3) δ (ppm): 7.91 (s, 1H, Ar–H), 7.73 (d, J = 8.0
Hz, 2H, Ar–H), 7.54–7.52 (m, 2H, Ar–H), 7.15–7.13 (m, 1H, Ar–H), 6.94 (d, J = 8.0 Hz, 2H,
Ar–H), 3.98 (t, J = 6.8 Hz, 2H, –OCH2), 1.81–1.27 (m, 16H, –CH2), 0.87 (t, J = 7.2 Hz, 3H, –
CH3); 13C NMR (100 MHz, CDCl3) δ (ppm):158.9, 155.1, 146.4, 144.3, 132.5, 129.6, 128.9,
128.0, 126.3, 126.0, 114.7, 108.4, 68.0, 31.8, 29.5, 29.5, 29.4, 29.3, 29.2, 26.0, 22.6, 14.1;
Anal. calcd. for C24H29N3OS2: C, 65.57; H, 6.65; N,9.56; S, 14.58; found: C, 65.49; H, 6.67;
N,9.52; S, 14.64.
4

2.1.4 Synthesis of 6–(decyloxy)–2–(thiophen–2–yl)imidazo[2,1–b][1,3,4]thiadiazole–5–
carbaldehyde (7)
The intermediate (7) was synthesized following the procedure mentioned in the literature [29]
using a freshly distilled DMF (1.05 mL, 13.64 mmol), phosphorus oxychloride (POCl3) (1.27
mL, 13.64 mmol) and intermediate 6 (3 g, 6.82 mmol). (yellow solid, 2.74 g, yield: 86%).
ESI–MS (m/z) calcd. for C25H29N3O2S2: 467.17, found: 468.10 [M+H]+; 1H NMR (400 MHz,
CDCl3) δ (ppm): 10.09 (s, 1H,–CHO), 7.88 (d, J = 8.0 Hz, 2H, Ar–H), 7.64–7.59 (m, 2H,
Ar–H), 7.18–7.16 (m, 1H, Ar–H), 7.01 (d, J = 9.2 Hz, 2H, Ar–H), 4.02 (t, J = 6.8 Hz, 2H, –
13
OCH2), 1.81–1.27 (m, 16H, –CH2–), 0.87 (t, J = 7.2 Hz, 3H, –CH3); C NMR (100 MHz,
CDCl3) δ (ppm): 177.3, 160.7, 157.4, 155.8, 149.8, 131.6, 130.7, 130.4, 129.8, 128.1, 124.4,
123.6, 114.7, 68.1, 31.8, 29.5, 29.3, 29.1, 26.0, 22.6, 14.1; Anal. calcd. for C25H29N3O2S2: C,
64.21; H, 6.25; N,8.99; S, 13.71; found: C, 64.30; H, 6.19; N,8.92; S, 13.78.
The procedures for the synthesis of intermediates 9–21 are provided in ESI (section S 2.1–S
2. 10).
2.1.5
2,5–bis((E)–2–(6–(4–(decyloxy)phenyl)–2–(thiophen–2–yl)imidazo[2,1–
b][1,3,4]thiadiazol–5–yl)vinyl)thiophene (TIT)
The intermediates 10 (0.5 g, 0.52 mmol) and 7 (0.49 g, 1.05 mmol) were dissolved in a
mixture of chloroform and ethanol (1:1), to which, a freshly prepared solution of sodium
ethoxide (89.3 mg, 8.11 mmol in 3 mL of ethanol) was added drop wise in argon atmosphere
and the mixture was stirred at RT for 12 h. The excess of solvent was removed under reduced
pressure and the residue was purified by column chromatography using a mixture of pet
ether/ethyl acetate (8:2, v/v) to get the compound TIT as an orange colored solid (0.52 g,
yield: 83%). MALDI–TOF–MS (m/z) calcd. for C56H62N6O2S5: 1011.452, found: 1012.346
[M+H]+; 1H NMR (400 MHz, CDCl3) δ (ppm): 7.89 (s,1H, Ar–H), 7.85 (s,1H, Ar–H), 7.68(d,
J = 8.0 Hz, 4H, Ar–H), 7.60–7.56 (m, 4H, Ar–H), 7.19–7.16 (m, 2H, Ar–H), 7.11 (s, 1H, Ar–
H), 7.07 (s, 1H, Ar–H), 7.05–7.03 (m, 6H, Ar–H), 4.03 (t, J = 8.0 Hz, 4H, –OCH2), 1.86–
1.81 (m, 4H, –CH2–), 1.79–1.47 (m, 7H, –CH2–), 1.34–1.28 (m, 21H, –CH2–), 0.88 (t, J = 8.0
Hz, 6H, –CH3); 13C NMR (100 MHz, CDCl3) δ (ppm): 159.0, 154.8, 145.2, 145.1, 142.2,
132.6, 129.7, 129.4, 129.0, 128.0, 127.1, 126.7, 121.7, 121.5, 114.8, 113.7, 68.1, 31.9, 29.6,
29.5, 29.4, 29.3, 26.1, 22.6,14.1; Anal. calcd. for C56H62N6O2S5: C, 66.50; H, 6.18; N, 8.31;
S, 15.85; found: C, 66.58; H, 6.09; N, 8.43; S, 15.97.
5

2.1.6
2,5–bis(5–((E)–2–(6–(4–(decyloxy)phenyl)–2–(thiophen–2–yl)imidazo[2,1–
b][1,3,4]thiadiazol–5–yl)vinyl)thiophen–2–yl)–1,3,4–oxadiazole (TITO)
Similar procedure was followed for the synthesis of TITO in which intermediate 16 (0.5 g,
0.63 mmol) was reacted with intermediate 7 (0.59 g, 1.26 mmol) (0.52 g, yield: 85%).
MALDI–TOF–MS (m/z) calcd. for C62H64N8O3S6: 1160.343, found:1159.100 [M]+; 1H NMR
(400 MHz, CDCl3) δ (ppm): 7.89 (s, 1H, Ar–H), 7.85 (s, 2H, Ar–H), 7.80–7.60 (m, 3H, Ar–
H), 7.57–7.48 (m, 2H, Ar–H), 7.42–7.39 (m, 3H, Ar–H), 7.22–7.15 (m, 3H, Ar–H), 7.14–
7.00 (m, 3H, Ar–H), 6.96–6.93 (m, 2H, Ar–H), 6.89–6.78 (m, 2H, Ar–H), 6.75–6.72 (m, 1H,
Ar–H), 4.06 (t, J = 8.0 Hz, 2H, –OCH2), 3.95 (t, J = 8.0 Hz, 2H, –OCH2), 1.86–1.83 (m, 4H,
–CH2–), 1.76–1.58 (m, 5H, –CH2–), 1.50–1.42 (m, 4H, –CH2–), 1.30–1.27 (m, 19H, –CH2–),
13
0.88 (t, J = 8.0 Hz, 6H, –CH3); C NMR (100 MHz, CDCl3) δ (ppm): 160.0, 159.9, 159.3,
158.9, 154.6, 148.3, 145.8, 144.8, 144.5, 132.6, 132.4, 130.4, 129.8, 129.5, 129.3, 128.9,
128.7, 128.5, 128.0, 127.8, 126.8, 126.3, 124.7, 121.1, 119.7, 118.0, 116.6, 114.9, 114.5,
68.1, 68.0, 31.8, 29.5, 29.3, 26.0, 22.6, 14.1; Anal. calcd. for C62H64N8O3S6: C, 64.11; H,
5.55; N, 9.65; S, 16.56; found: C, 64.23; H, 5.42; N, 9.58; S, 16.67.
2.1.7 Synthesis of 2,5–bis(6–(4–(decyloxy)phenyl)–2–(thiophen–2–yl)imidazo[2,1–
b][1,3,4]thiadiazol–5–yl)thiazolo[5,4–d]thiazole (TITz)
The oligomer TITz was synthesized following the procedure for intermediate (19) (ESI),
using the intermediate 7 (0.5 g, 1.06 mmol) and dithiooxamide (0.064 g, 0.53 mmol) (yellow
crystals, 0.44 g, yield: 41%). MALDI–TOF–MS (m/z) calcd. for C52H56N8O2S6: 1016.285,
found: 1016.989 [M+H]+; 1H NMR (400 MHz, CDCl3) δ (ppm): 7.96 (d, J = 8.0 Hz, 4H, Ar–
H), 7.65 (d, J = 4.0 Hz, 2H, Ar–H), 7.60 (d, J = 8.0 Hz, 2H, Ar–H), 7.20–7.18 (m, 2H, Ar–
H), 6.99 (d, J = 8.0 Hz, 4H, Ar–H), 4.03 (t, J = 8.0 Hz, 4H, –OCH2), 1.84–1.80 (m, 5H, –
CH2–), 1.50–1.46 (m, 4H, –CH2–), 1.38–1.28 (m, 23H, –CH2–), 0.87 (t, J = 8.0 Hz, 6H, –
CH3); 13C NMR (100 MHz, CDCl3) δ (ppm): 159.7, 156.2, 155.8, 147.2, 145.4, 132.2, 130.4,
130.2, 129.4, 129.2, 128.1, 125.7, 118.7, 114.2, 68.0, 31.8, 29.5, 29.4, 29.2, 26.0, 22.6, 14.0;
Anal. calcd. for C52H56N8O2S6: C, 61.39; H, 5.55; N, 11.01; S, 18.91; found: C, 61.48; H,
5.51; N, 11.13; S, 18.86.
2.1.8
Synthesis
of
2,5–bis(4–((E)–2–(6–(4–(decyloxy)phenyl)–2–(thiophen–2–
yl)imidazo[2,1–b][1,3,4]thiadiazol–5–yl)vinyl)phenyl)thiazolo[5,4–d]thiazole (TIPTz)
The compound TIPTz was synthesized following the procedure mentioned for TIT, using
intermediates 21 (0.5 g, 0.497 mmol) and 7 (0.46 g, 0.995 mmol) (0.49 g, yield: 82%).
1
MALDI–TOF–MS (m/z) calcd. for C68H68N8O2S6: 1221.706, found:1222.723 [M+H]+; H
6

NMR (400 MHz, CDCl3) δ (ppm): 7.97 (d, J = 8.0 Hz, 4H, Ar–H), 7.83 (s, 1H, Ar–H), 7.79
(s, 1H, Ar–H), 7.68 (d, J = 8.0 Hz, 4H, Ar–H), 7.60–7.58 (m, 7H, Ar–H), 7.40–7.36 (s, 3H,
Ar–H), 7.20–7.18 (m, 2H, Ar–H), 7.04 (d, J = 8.0 Hz, 4H, Ar–H), 4.04 (t, J = 8.0 Hz, 4H, –
OCH2), 1.87–1.80 (m, 4H, –CH2–), 1.52–1.46 (m, 5H, –CH2–), 1.31–1.25 (m, 23H, –CH2–),
0.89 (t, J = 8.0 Hz, 6H, –CH3); 13C NMR (100 MHz, CDCl3) δ (ppm): 159.1, 145.4, 132.6,
129.6, 129.5, 128.9, 128.4, 128.0, 126.8, 126.6, 125.8, 121.8, 115.6, 114.7, 114.4, 68.1, 31.8,
29.6, 29.5, 29.3, 29.2, 26.0, 22.6, 14.0; Anal. calcd. for C68H68N8O2S6: C, 66.85; H, 5.61; N,
9.17; S, 15.75; found: C, 66.93; H, 5.70; N, 9.06; S, 15.68.
The characterization details for the selected intermediates and final compounds are given in
ESI.
Fig. 1. Chemical structures of four oligomers TITz, TIT, TITO and TIPTz.
7

Br
O
S
b
a
c
S
H2N NH
2
NH2
COOH
S
S
S
1
NH2
N N
3
OH
4
N
N
N
d
N
OH
OC10H21
S
S
S
5
N
S
N
6
CHO
N
N
OC10H21
S
7
N
8

Scheme 1. Synthetic pathways for the oligomers TITz, TIT, TITO and TIPTz. Reagents
and reaction conditions: (a) POCl3, 70°C, 0.5 h, yield: 94%; (b) C2H5OH, 80°C, 24 h, yield:
89%; (c) 1–Bromodecane, K2CO3, DMF, RT– 85°C, 12 h, yield: 91%; (d) POCl3, DMF, 0–
60°C, 6 h, yield: 86%; (e) NBS, AIBN, Benzene, 75°C, 2 h, yield: 64–74%; (f)
Triphenylphosphine, DMF, 85°C, 12 h, yield: 78–90%; (g) SOCl2, 0°C– RT, 4 h, yield: 94%;
(h) Hydrazine hydrate, triethylamine, NMP, RT, 8 h, yield: 94%; (i) POCl3, 75°C, 3 h, yield:
78%; (j) DMF, 160°C, 5 h, yield: 41–55%; (k) C2H5ONa, CHCl3, C2H5OH, RT, 12 h, yield:
82–85%.
9

3. Results and discussion
3.1 Synthesis
The thiophene substituted moiety, intermediate (5), obtained by the cyclization of
intermediate (3) with 4–hydroxy phenacyl chloride, was alkylated to yield intermediate (6).
In the alkylation step, the decyl chain was selected as alkylating agent to improve the
solubility in different organic solvents. Later, which was formylated via well-known
Vilsmeier-Haack reaction to achieve the intermediate (7). On the other hand, three different
Wittig salts (10, 16 and 21) with the different conjugations were synthesized using multistep
synthesis as mentioned in the Scheme 1. Finally, the intermediate (7) was reacted with
dithiooxamide and the Wittig salts 10, 16 and 21 to yield TITz, TIT, TITO and TIPTz,
1
respectively. All the intermediates and final molecules were characterized using H NMR,
13C NMR and mass spectral analyses (ESI).
3.2 Optical properties
The normalized UV–vis and PL spectra of TIT, TITO, TITz and TIPTz are shown in Fig. 2
and the corresponding spectral data are listed in Table 1. As it is seen in Fig. 2, there are
three absorption bands situated between 260–500 nm region for each of the four oligomers.
The absorption bands observed at higher energy regions (260–340 nm) are attributed to the
π–π* transitions of the aromatic conjugated skeleton, whereas, those at lower energy (400–
460 nm) region are ascribed to the ICT from the electron donor to the acceptor moieties. The
maximum absorption peaks (λmax) of TIT, TITO, TITz and TIPTz are observed at 429, 451,
408 and 438 nm, respectively. Moreover, the introduction of 1,3,4–oxadiazole, a strong
electron acceptor moiety, to the central core is considerably shifting the absorption maximum
of TITO to higher wavelength region due to stronger donor–acceptor interaction [30] when
compared to that of TIT, where, only thiophene moiety is present in the central core.
Similarly, there is a considerable red shift in the absorption maximum of TIPTz compared to
that of TITz due to the introduction of phenyl group (π–linker) to the central part, which
increases the donor–acceptor interaction in TIPTz, causing the extension of conjugation.
Further, the fluorescence emission spectra were recorded by exciting the oligomers at their
excitation wavelengths. All the four oligomers exhibited single emission peak in the 500–540
nm region, with the green light emission in solution state. The optical band gaps (Eg)
calculated from the intersection of normalized absorption and emission spectra were 2.63,
2.46, 2.66 and 2.55 eV for TIT, TITO, TITz and TIPTz, respectively. The extended π–
10

conjugation in TITO and TIPTz has considerably reduced the Eg of TITO and TIPTz
compared to that of TIT and TITz.
Fig. 2. Normalized UV−vis absorption and PL emission spectra of TIT, TITO, TITz and
TIPTz in chloroform (10−5 M).
Table 1 Summary of optical, electrochemical and theoretical data of TIT, TITO, TITz and
TIPTz.
Oligomers λmaxabs (nm) λplmax (nm) Eg (eV) Eoxonset(V) HOMO (eV) LUMO (eV)
429
451
408
438
512
540
503
537
2.63a
2.46d
2.46a
2.64d
2.66a
3.06d
2.55a
2.64d
0.54
0.66
0.60
0.55
−5.08b
−4.52d
−5.20b
−4.84d
−5.14b
−5.08d
−5.09b
−4.83d
−2.45c
−2.06d
−2.74c
−2.20d
−2.48c
−2.02d
−2.54c
−2.19d
TIT
TITO
TITz
TIPTz
a
Optical band gap calculated from the intersection of normalized absorption and
emission spectra.
11

Eoxonset Experimental onset oxidation potential vs SCE.
b Experimental values from CV using equation (1) with Fc/Fc+ as internal standard.
c Experimental values using equation (2).
d Theoretical results.
3.3 Electrochemical studies
Cyclic voltammetry (CV) was carried out to determine the energy levels of the oligomers.
Fig. 3a–d represent the cyclic voltammograms of TIT, TITO, TITz and TIPTz,
respectively. The onset oxidation potentials of the oligomers are at 0.54, 0.66, 0.60 and 0.55
V vs saturated calomel electrode (SCE) for TIT, TITO, TITz and TIPTz, respectively.
Using the equation (1), the highest occupied molecular orbital (HOMO) energy levels were
determined to be −5.08, −5.20, −5.14 and −5.09 eV for TIT, TITO, TITz and TIPTz,
respectively.
ox


EHOMO = − Eonset + 4.8eV − EFOC .................................................(1)


where, Eox
and EFOC are the onset of oxidation potentials of the oligomers and ferrocene
onset
(EFOC = 0.27 V vs SCE (Fig. S28, ESI)), respectively, −4.8 eV is the HOMO energy level of
ferrocene against vacuum.
Similarly, using the equation (2) the lowest unoccupied molecular orbital (LUMO) energy
levels of the molecules were calculated to be −2.45, −2.74, −2.48 and −2.54 eV for TIT,
TITO, TITz and TIPTz, respectively.
ELUMO = EHOMO + Eg.......................................................................(2)
There is a slight increase in the HOMO energy levels of TIT and TIPTz when compared to
that of TITO and TITz. This increment is attributed to the presence of thiophene in TIT and
phenyl group in TIPTz in the central core as electron donor and π–linker, respectively [31],
while, the decrease in the LUMO level of TITO compared to that of TIT, TITz and TIPTz
is due to the presence of strong electron withdrawing 1,3,4−oxadiazole moiety in TITO [32].
The results of CV studies and the values of energy levels are summarized in Table 1.
12

Fig. 3. Cyclic voltammograms of (a)TIT, (b) TITO, (c) TITz and (d) TIPTz recorded in
CH3CN solutions containing 0.1 M Bu4N(ClO4) as supporting electrolyte at a scan rate of 100
mV s−1 using SCE as reference electrode, Pt wire as counter electrode and material coated
glassy carbon as working electrode. The inset shows enlarged image of the oxidation peak in
the anodic region.
3.4 Theoretical studies
To gain further insight of the molecular orbital distribution and excited state transition, DFT
and TD−DFT calculations were carried out. The long alkoxy chains were considered as
methoxy groups in order to reduce the calculation time. As shown in the Fig. 4, both the
HOMO and LUMO electrons are mainly populated on vinylic linkage and central part of the
system in TIT, TITO, TITz and TIPTz. The HOMO distribution is further extended on the
imidazole and decyloxy substituted phenyl ring of the terminal groups, whereas, the LUMO
is mostly distributed on thiophene and imidazo[2,1-b][1,3,4]thiadiazole ring of the peripheral
group and also, distribution in the central core of all the four oligomers is seen. This type of
separated spatial distribution suggests the balanced charge transfer process within the system
[33]. The theoretical HOMO/LUMO values are given in Table 1. Further, the simulated
absorption spectra of TIT, TITO, TITz and TIPTz are shown in the Fig. 5. There exist two
absorption bands, the one at lower energy region corresponds to charge transfer transition and
the other at higher energy region is due to π–π* transition. In general, the tiny discrepancy is
13

attributed to the theoretical overestimation. Nevertheless, the trend remains same and
theoretical results are in good agreement with the experimental results.
The presence of long decyloxy chain causes the twisting of the decyloxy substituted phenyl
group by an angle of ~32° from thiophene–imidazo[2,1-b][1,3,4]thiadiazole plane of
peripheral Th–ITD moiety in all the four oligomers. The dihedral angle between the central
core and the peripheral moiety is 9.8°, 9.5° and 10.4° in TIT, TITO and TIPTz, respectively.
The presence of vinylic bond between the central core and the peripheral moiety in TIT,
TITO and TIPTz reduces the steric hindrance between the central core and the peripheral
moiety which is confirmed by the least dihedral angles (Fig. S30), whereas, in the case of
TITz the two peripheral Th–ITD moieties are present orthogonal to each other with a
dihedral angle of 36° between the central core and the peripheral moiety. This is due to the
steric effects of imidazo[2,1-b][1,3,4]thiadiazole and tztz moieties, resulting from the direct
bond present between them (Fig. S31). In all the four oligomers, the Th–ITD plane is present
90° to the plane of central unit, giving a H–shaped structure. As it is seen from the optimized
geometry of TITO and TIPTz, the higher degree of conjugation decreases the twisting due to
steric effects between the side groups and the central unit, providing a perfect H–shaped
structure. The dihedral angles and the optimized geometries are shown in Fig. S29.
14

Fig. 4. Optimized geometry and HOMO/LUMO electronic distributions of TIT, TITO, TITz
and TIPTz.
Fig. 5. Simulated absorption spectra of TIT, TITO, TITz and TIPTz.
3.5 Thermal properties
Thermo gravimetric analysis (TGA) was carried out to measure the thermal stability of the
oligomers. Fig. 6 represents the graph of weight loss as a function of temperature of TIT,
TITO, TITz and TIPTz. As it is seen from the plot, the onset of decomposition temperature
(Td) corresponding to a 5% weight loss for TIT and TITO are 370 and 384°C, respectively
and that corresponding to a 20% weight loss for TITz and TIPTz are 378 and 348°C,
respectively. Further, there is a minor decomposition at 183 and 115°C observed for TITz
and TIPTz, which is attributed to the decomposition of central units of TITz and TIPTz,
respectively. Between TIT and TITO, TITO exhibits higher Td, which is ascribed to the
extended D–A–D structure with the presence of thermally stable 2,5–disubstituted–1,3,4–
oxadiazole in the central unit. However, the higher Td of TITz compared to that of TIPTz is
due to the direct attachment of central tztz unit to the partially rigid imidazo[2,1-
b][1,3,4]thiadiazole moiety of the terminal group in TITz, making the oligomer structurally
more rigid compared to TIPTz. All the four oligomers exhibited good thermal stability,
which revealed the suitability of these materials for practical application in optoelectronics.
15

Fig. 6. TGA plots of TIT, TITO, TITz and TIPTz.
3.6 Third–order NLO studies
3.6.1 Nonlinear absorption (NLA) properties
In the present study, Z–scan with the single–beam technique was used to investigate the
third–order NLO coefficients of the synthesized thiophene–based oligomers[34]. The open
aperture (OA) and the closed aperture (CA) Z–scan patterns of the oligomers in chloroform
solution were measured at a wavelength of 532 nm laser beam with a pulse width of 7 ns. The
oligomers were excited at an energy of 50 µJ (on–axis intensity of 1.386 GW cm−2) with a
linear transmittance maintained at 70–72% (experimental details are given in ESI, section
S4). The OA Z–scan traces of the oligomers TIT, TITO, TITz and TIPTz are shown in the
Fig. 7a–d, respectively. As shown in the Fig. 7a–d, there is a decrease in the transmittance as
the samples were moved near to the focus and reaches a minimum value at the focus,
displaying a deep transmittance trough near the focal plane which is the signature of reverse
saturable absorption (RSA). Further, on the nanosecond time scale this RSA is combined
with two photon absorption (TPA) and excited state absorption (ESA), collectively called as
“effective TPA” process [35,36]. The experimental results are matching well with the
theoretical model for effective TPA process in all the samples. Thus, the effective nonlinear
absorption coefficient (α(I)), the nonlinear propagation equation (dI/dz), the normalized
transmittance (T(z)) and finally, the imaginary part of third order nonlinear susceptibility (im
χ(3)) for open aperture configuration were determined using the equations S1–S4,
16

respectively. The NLA parameters were obtained by fitting the experimental data to the
theoretical model using equation S1 and S3. All the calculated NLA coefficients are listed in
Table 2.
The numerically estimated effective TPA coefficient (βeff) values are 0.52×10−10, 1.62×10−10
,
1.55×10−10 and 2.71×10−10 m W−1 for TIT, TITO, TITz and TIPTz, respectively. These
obtained βeff values are very much closer and predominantly higher than that reported in the
literature (Table 2). For example, under the similar condition, in our previous
communication, the Th–ITD based derivatives which exhibited excellent NLO properties
with βeff value of 1.55×10−10 m W−1 for one of the molecules is reported [29]. The NLO
properties of quinacridone derivatives were studied by Jia et al. [37], which showed notable
NLO responses with βeff of 0.76×10−10 m W−1. Gowda et al. [38,39] synthesized 3,4-
ethylenedioxythiophene (EDOT) based bent-core and hockey stick like liquid crystals and
studied the effect of metal nanoparticles (gold/silver), dispersed in phthalocyanine discotic
liquid crystal matrix, which exhibited large optical nonlinear properties with βeff of 0.56 and
2.5×10−10 m W−1, respectively. Edappadikkunnummal et al. [40] synthesized phenothiazine–
gold nanoparticles by laser ablation method and studied the third−order NLO properties,
which exhibited significant enhancement in NLA with βeff of 1.7×10−10 m W−1. Gopi et al.
[41] synthesized new quinoxaline based push–pull molecules with 1,3-indandione as acceptor
and investigated the NLO properties of the molecule, which showed an effective TPA with
very high βeff of 2.0×10−10 m W−1. Zhang et al. [42] studied the NLO and optical limiting
properties of graphene oxide–Fe3O4 hybrid material, which showed an enhanced βeff of
2.6×10−10 m W−1. Therefore, all the aforementioned studies involve uses of one or the other
dopant for the enhancement of nonlinear response but the observed phenomenal increment in
the nonlinear responses in the present study without any dopants, of the synthesized
oligomers suggests that these oligomers would serve as effective optical limiters for sensors
and eye protection.
17

Fig. 7. OA Z–scan curves of (a) TIT, (b) TITO, (c) TITz and (d) TIPTz at 50 µJ.
3.6.2 Optical limiting studies
An optical limiter (OL) is a material which exhibits linear transmittance at low input
intensities and becomes slightly opaque at high input intensities. An optical limiter keeps the
output transmitted fluence below some specified maximum value regardless of the magnitude
of the input powers. The materials which exhibit at least one of the nonlinear optical
processes viz., NLA, nonlinear refraction (NLR), nonlinear scattering (NLS), optically
induced phase transitions and photo refraction (PR) show optical limiting action. The optical
limiting devices found applications most importantly in the protection of human eyes and
sensitive optical sensors/components from laser damage. Further, there are other potential
applications such as laser power regulation, restoration of signal levels in optical data
transmission and logic systems.
In order to explain the optical limiting properties of the present oligomers, graphs of input
fluence versus the normalized transmission of pulse intensity of TIT, TITO, TITz and
TIPTz were plotted and is shown Fig. 8a–d, respectively. The onset values of limiting action
(the value of input fluence at which the transmission of output pulse intensity starts
decreasing) are 0.349, 0.231, 0.270 and 0.151 J cm−2 for TIT, TITO, TITz and TIPTz,
respectively and the limiting threshold (LT) values (the value of input fluence at which the
18

output pulse intensity becomes half of the initial value) are 6.02, 4.51 and 3.14 J cm−2 for
TITO, TITz and TIPTz, respectively. Such a low onset and LT values of the synthesized
oligomers infer that these materials would be powerful candidates in optical power limiting
devices. The oligomer TIT did not show LT, as the output transmittance did not reach 50 %
of its initial value at the input fluence used.
Fig. 8. Optical power limiting studies of (a) TIT, (b) TITO, (c) TITz and (d) TIPTz at 50
µJ.
Table 3 Comparison of NLA and NLR parameters of TIT, TITO, TITz and TIPTz with
similar work reported.
Sample
βeff (×10−10
ɳ2 (esu)
χ3 (esu)
(10−12
References
mW-1)
)
TIT
TITO
TITz
0.52
1.62
1.55
2.71
4.1637×10−11
6.4593×10−11
5.8669×10−11
8.4294×10−11
1.69375
5.16077
4.95886
8.55506
This work
This work
This work
This work
TIPTz
19

1.55
6.57×10−11
4.90
[29]
0.76
0.56
6.27×10−19
4.423
-
[37]
[39]
-
Phenothiazine–Gold
(Au) nanocomposite
1.74
2.5
8.0×10−11
-
-
[40]
[38]
phthalocyanine–0.5%
gold nanoparticles
matrix
-
2.0
2.6
-
-
-
-
[41]
[42]
Graphene oxide–
Fe3O4
3.6.3 Nonlinear refraction (NLR) properties
The CA Z–scan was performed to determine both the magnitude and sign of the NLR. The
pure NLR trace was obtained by dividing CA data by OA data to avoid the contribution of
NLA and the corresponding plots of TIT, TITO, TITz and TIPTz are shown in Fig. 9a–d,
respectively. All the four oligomers show negative nonlinearity and exhibit peak-valley
pattern which are associated with the self-defocusing nature of the oligomers. The normalized
transmittance (T) at CA condition, the real part of third order nonlinear susceptibility (real
χ(3)) and the nonlinear refractive index (η2) were fitted using equations S5–S7, respectively.
The data of NLR coefficients are summarized in Table 2.
20

Fig. 9. CA Z–scan curves of (a) TIT, (b) TITO, (c) TITz and (d) TIPTz at 50 µJ.
In the present study, introduction of strong electron acceptor groups such as 1,3,4–oxadiazole
(TITO) and thiazolo[5,4–d]thiazole (TIPTz), between the electron rich groups (thiophene in
TITO and phenyl in TIPTz) at the central position of conjugated backbone ameliorated the
planarity of the oligomers which extended the π–electron delocalization over the entire
conjugated system and improved the effective charge transfer between the donor and acceptor
groups which in turn increased the polarizability of the oligomers. And the cyclic groups
(ITD) present at the peripheral position of the oligomers have improved the rigidity and
strengthened the molecular backbone [43]. As a result, the oligomers TITO and TIPTz
exhibited predominant increment in the NLO properties (enhanced βeff of the order 10−10
,
considerably low onset of limiting action and LT) compared to that of TIT and TITz. The
enhanced conjugation facilitated by D–A–D framework of all the four oligomers substantially
improved their NLO characteristics, as confirmed by Z–scan analysis.
21

4. Conclusion
In summary, four new D–A–D type, multi-heterocyclic conjugated oligomers were
synthesized and characterized. The influence of incorporation of different donor–acceptor
groups in the central unit on the optoelectronic properties and the energy levels of the
molecules is confirmed. The red shift of the absorption and emission maxima of TITO and
TIPTz, is due to the presence of 1,3,4–oxadiazole and thiazolo[5,4–d]thiazole units in TITO
and TIPTz, respectively. Similarly, the increase in the HOMO level of TIT and TIPTz, is
due to the electron rich thiophene and phenyl groups (π–spacer) present in the central position
of the TIT and TIPTz, respectively. And all the four oligomers exhibited high thermal
stability of >350ºC. All the four oligomers exhibited an effective two photon absorption with
tremendous increment in the βeff of the order of 10−10. The TITO and TIPTz exhibited
improved ICT and high polarization with high βeff of 1.62 and 2.71×10−10 m W−1, very low
onset of limiting of 0.231 and 0.151 J cm−2 and limiting thresholds of 6.02 and 3.14 J cm−2,
respectively. The attractive NLO responses of the TITO and TIPTz proved that these
oligomers could be excellent candidates for all-optical limiting and opto-electronic device
applications.
Associated content
Supporting Information.
Includes materials and methods, synthetic procedures of intermediates 9–21, characterization
details, spectral characterizations of selected intermediates and final oligomers, experimental
set up of Z–scan measurements and the equations used for the theoretical fitting of Z–scan
experimental data.
Conflicts of interest
None.
Acknowledgement
Authors are thankful to SERB (SB/FT/CS-145/2012), DST INDIA for financial support and
department of Chemical Engineering, NITK, Surathkal for TGA facility.
22

References
[1] Fischer SA, Cramer CJ, Govind N. Excited-State Absorption from Real-Time Time-
Dependent Density Functional Theory: Optical Limiting in Zinc Phthalocyanine. J Phys
Chem Lett 2016;7:1387–91. https://doi.org/10.1021/acs.jpclett.6b00282.
[2] He GS, Zhu J, Baev A, Samoć M, Frattarelli DL, Watanabe N, et al. Twisted π-System
Chromophores for All-Optical Switching. J Am Chem Soc 2011;133:6675–80.
https://doi.org/10.1021/ja1113112.
[3] Li D, Zhang Q, Wang X, Li S, Zhou H, Wu J, et al. Self-assembly of a series of
thiocyanate complexes with high two-photon absorbing active in near-IR range and
bioimaging
applications.
Dyes
Pigments
2015;120:175–83.
https://doi.org/10.1016/j.dyepig.2015.03.039.
[4] Zheng Z, Zhang Q, Yu Z, Yang M, Zhou H, Wu J, et al. Four new two-photon
absorbing imidazo[4,5-f]1,10-phenanthroline dye derivatives with different dipole
moment orientation based on different groups: synthesis, optical characterization and
bioimaging. J Mater Chem C 2012;1:822–30. https://doi.org/10.1039/C2TC00175F.
[5] Jia J, Wang W, Cui Y, Dong H, Luo C, Li Y, et al. New quinazolinone derivatives and
its nonlinear optical properties. Chem Phys Lett 2018;708:201–209.
[6] Champagne B, Plaquet A, Pozzo J-L, Rodriguez V, Castet F. Nonlinear Optical
Molecular Switches as Selective Cation Sensors. J Am Chem Soc 2012;134:8101–3.
https://doi.org/10.1021/ja302395f.
[7] Qin ZB, Wen YQ, Shang YL, Song YL, Wan YZ. Study of an organic nonlinear optical
material for nanoscale data storage by scanning tunneling microscope. Appl Phys A
2007;87:277–80. https://doi.org/10.1007/s00339-006-3827-2.
[8] Liu H, Rua AJ, Vasquez O, Vikhnin VS, Fernandez LFE, Fonseca F, et al. Optical and
Nonlinear Optical Response of Light Sensor Thin Films. Sensors 2005;5:185–98.
https://doi.org/10.3390/s5040185.
[9] Sun J, Wang G, Liu C, Shi Y, Zhao M. Synthesis of four pyrene-containing chalcone
derivatives: Achieving excellent third-order nonlinear optical properties by optimizing
halopyridines. Opt Laser Technol 2019;109:600–607.
[10] Marder SR. Organic nonlinear optical materials: where we have been and where we are
going. Chem Commun 2006;0:131–4. https://doi.org/10.1039/B512646K.
[11] Hu C, Chen Z, Xiao H, Zhen Z, Liu X, Bo S. Synthesis and characterization of a novel
indoline based nonlinear optical chromophore with excellent electro-optic activity and
23

high thermal stability by modifying the π-conjugated bridges. J Mater Chem C
2017;5:5111–8. https://doi.org/10.1039/C7TC00735C.
[12] Kakekochi V, Kumar D U, P NP, Chandrasekharan K. An investigation on
photophysical and third–order nonlinear optical properties of novel thermally–stable
thiophene–imidazo [2,1-b][1,3,4] thiadiazole based azomethines. Dyes Pigments
2019;167:216–24. https://doi.org/10.1016/j.dyepig.2019.04.033.
[13] Muhammad S, Al-Sehemi AG, Su Z, Xu H, Irfan A, Chaudhry AR. First principles
study for the key electronic, optical and nonlinear optical properties of novel donor-
acceptor
chalcones.
J
Mol
Graph
Model
2017;72:58–69.
https://doi.org/10.1016/j.jmgm.2016.12.009.
[14] Xu L, Zhang D, Zhou Y, Zheng Y, Cao L, Jiang X-F, et al. 4-N, N-bis(4-
methoxylphenyl) aniline substituted anthraquinone: X-ray crystal structures, theoretical
calculations and third-order nonlinear optical properties. Opt Mater 2017;70:131–7.
https://doi.org/10.1016/j.optmat.2017.05.025.
[15] Ju C, Li X, Yang G, Yuan C, Semin S, Feng Y, et al. Polymorph dependent linear and
nonlinear optical properties of naphthalenyl functionalized fluorenones. Dyes Pigments
2019;166:272–82. https://doi.org/10.1016/j.dyepig.2019.03.030.
[16] Ju C, Zhou Y, Semin S, Yang G, Tinnemans P, Duan Y, et al. Solvent dependent linear
and nonlinear optical properties of triphenylamine unit incorporated difluoroboron β-
diketonate
complexes.
Dyes
Pigments
2019;162:776–85.
https://doi.org/10.1016/j.dyepig.2018.11.002.
[17] Coe BJ, Rusanova D, Joshi VD, Sánchez S, Vávra J, Khobragade D, et al. Helquat
Dyes: Helicene-like Push–Pull Systems with Large Second-Order Nonlinear Optical
Responses. J Org Chem 2016;81:1912–20. https://doi.org/10.1021/acs.joc.5b02692.
[18] Zhan X, Zhang J, Tang S, Lin Y, Zhao M, Yang J, et al. Pyrene fused perylene diimides:
synthesis, characterization and applications in organic field-effect transistors and optical
limiting
with
high
performance.
Chem
Commun
2015;51:7156–9.
https://doi.org/10.1039/C5CC00966A.
[19] Cristiano R, Santos DMP de O, Gallardo H. Synthesis and characterization of low
molecular mass luminescent liquid crystalline materials with 1,3,4 oxadiazole units.
Liq Cryst 2005;32:7–14. https://doi.org/10.1080/02678290412331329288.
[20] Li Q, Cui L-S, Zhong C, Yuan X-D, Dong S-C, Jiang Z-Q, et al. Synthesis of new
bipolar host materials based on 1,2,4-oxadiazole for blue phosphorescent OLEDs. Dyes
Pigments 2014;101:142–9. https://doi.org/10.1016/j.dyepig.2013.09.029.
24

[21] Hughes G, R. Bryce M. Electron-transporting materials for organic electroluminescent
and electrophosphorescent devices. Mater Chem 2005;15:94–107.
https://doi.org/10.1039/B413249C.
J
[22] Nazim M, Ameen S, Akhtar MS, Lee Y-S, Shin H-S. Novel thiazolothiazole based
linear chromophore for small molecule organic solar cells. Chem Phys Lett
2013;574:89–93. https://doi.org/10.1016/j.cplett.2013.04.060.
[23] Cheng P, Shi Q, Lin Y, Li Y, Zhan X. Evolved structure of thiazolothiazole based small
molecules towards enhanced efficiency in organic solar cells. Org Electron
2013;14:599–606. https://doi.org/10.1016/j.orgel.2012.11.026.
[24] Shi Q, Cheng P, Li Y, Zhan X. A Solution Processable D-A-D Molecule based on
Thiazolothiazole for High Performance Organic Solar Cells. Adv Energy Mater
2012;2:63–7. https://doi.org/10.1002/aenm.201100505.
[25] Dutta P, Park H, Lee W-H, Kang I-N, Lee S-H. A crystalline D-π-A organic small
molecule with naphtho[1,2-b:5,6-b′]dithiophene-core for solution processed organic
solar cells. Org Electron 2012;13:3183–94. https://doi.org/10.1016/j.orgel.2012.09.019.
[26] Kyu Lee S, Min Cho J, Goo Y, Suk Shin W, Lee J-C, Lee W-H, et al. Synthesis and
characterization of a thiazolo[5,4- d ]thiazole-based copolymer for high performance
polymer
solar
cells.
Chem
Commun
2011;47:1791–3.
https://doi.org/10.1039/C0CC03036H.
[27] Cheng C, Yu C, Guo Y, Chen H, Fang Y, Yu G, et al. A diketopyrrolopyrrole –
thiazolothiazole copolymer for high performance organic field-effect transistors. Chem
Commun 2013;49:1998–2000. https://doi.org/10.1039/C2CC38811A.
[28] Peng Q, Peng J-B, Kang ET, Neoh KG, Cao Y. Synthesis and Electroluminescent
Properties
of
Copolymers
Based
on
Fluorene
and
2,5-Di(2-
hexyloxyphenyl)thiazolothiazole.
https://doi.org/10.1021/ma050378n.
Macromolecules
2005;38:7292–8.
[29] Kakekochi V, D UK, P NP, Chandrasekharan K. Effects of substituents on the
enrichment of the optical limiting action of novel imidazo[2,1-b][1,3,4]thiadiazole fused
thiophene-based
small
molecules.
New
J
Chem
2019;43:9232–42.
https://doi.org/10.1039/C9NJ01100E.
[30] Roncali J. Molecular Engineering of the Band Gap of π-Conjugated Systems: Facing
Technological Applications. Macromol Rapid Commun 2007;28:1761–75.
https://doi.org/10.1002/marc.200700345.
25

[31] Zhou H, Yang L, Stoneking S, You W. A Weak Donor−Strong Acceptor Strategy to
Design Ideal Polymers for Organic Solar Cells. ACS Appl Mater Interfaces
2010;2:1377–83. https://doi.org/10.1021/am1000344.
[32] Randell NM, Radford CL, Yang J, Quinn J, Hou D, Li Y, et al. Effect of Acceptor Unit
Length and Planarity on the Optoelectronic Properties of Isoindigo–Thiophene Donor–
Acceptor
Polymers.
Chem
Mater
2018;30:4864–73.
https://doi.org/10.1021/acs.chemmater.8b02535.
[33] Kothavale S, Sekar N. Novel pyrazino-phenanthroline based rigid donor-π-acceptor
compounds: A detail study of optical properties, acidochromism, solvatochromism and
structure-property
relationship.
Dyes
Pigments
2017;136:31–45.
https://doi.org/10.1016/j.dyepig.2016.08.032.
[34] Sheik-Bahae M, Said AA, Wei T-, Hagan DJ, Stryland EWV. Sensitive measurement of
optical nonlinearities using a single beam. IEEE J Quantum Electron 1990;26:760–9.
https://doi.org/10.1109/3.53394.
[35] Poornesh P, Umesh G, Hegde PK, Manjunatha MG, Manjunatha KB, Adhikari AV.
Studies on third-order nonlinear optical properties and reverse saturable absorption in
polythiophene/poly (methylmethacrylate) composites. Appl Phys B 2009;97:117–24.
https://doi.org/10.1007/s00340-009-3635-4.
[36] Vishnumurthy KA, Adhikari AV, Sunitha MS, Mary KAA, Philip R. Design and
synthesis of a new thiophene based donor–acceptor type conjugated polymer with large
third order nonlinear optical activity. Synth Met 2011;161:1699–706.
https://doi.org/10.1016/j.synthmet.2011.06.007.
[37] Jia J, Li Y, Wang W, Luo C, Han L, Li Y, et al. New quinacridone derivatives:
Structure-function relationship exploration to enhance third-order nonlinear optical
responses. Dyes Pigments 2017;146:251–262.
[38] Gowda AN, Kumar M, Thomas AR, Philip R, Kumar S. Self-Assembly of Silver and
Gold Nanoparticles in a Metal-Free Phthalocyanine Liquid Crystalline Matrix:
Structural, Thermal, Electrical and Nonlinear Optical Characterization. ChemistrySelect
2016;1:1361–70. https://doi.org/10.1002/slct.201600122.
[39] Gowda A, Jacob L, Joy N, Philip R, Pratibha R, Kumar S. Thermal and nonlinear
optical studies of newly synthesized EDOT based bent-core and hockey-stick like liquid
crystals. New J Chem 2018;42:2047–2057.
[40] Edappadikkunnummal S, Nherakkayyil SN, Kuttippurath V, Chalil DM, Desai NR,
Keloth C. Surface Plasmon Assisted Enhancement in the Nonlinear Optical Properties
26

of Phenothiazine by Gold Nanoparticle. J Phys Chem C 2017;121:26976–86.
https://doi.org/10.1021/acs.jpcc.7b06528.
[41] Gopi V, Subbiahraj S, Chemmanghattu K, Ramamurthy PC. 2,3-di(2-furyl) quinoxaline
bearing 3 -ethyl rhodanine and 1,3 indandione based heteroaromatic conjugated T-
shaped push -pull chromophores: Design, synthesis, photophysical and non-linear
optical
investigations.
Dyes
Pigments
2020;173:107887.
https://doi.org/10.1016/j.dyepig.2019.107887.
[42] Zhang X-L, Zhao X, Liu Z-B, Shi S, Zhou W-Y, Tian J-G, et al. Nonlinear optical and
optical limiting properties of graphene oxide–Fe3O4hybrid material. J Opt
2011;13:075202. https://doi.org/10.1088/2040-8978/13/7/075202.
[43] Yu LP, Dalton LR. Synthesis and characterization of new electroactive polymers. Synth
Met 1989;29:463–70. https://doi.org/10.1016/0379-6779(89)90334-2.
27

Please make the following change:
Please replace “Table 3” with Table 2 and the caption for Table 2 (which was written as
Table 3 in page no:18, line:1059|) is as follows
Table 2 Comparison of NLA and NLR parameters of TIT, TITO, TITz and TIPTz with
similar work reported.
Sample
βeff (×10−10
ɳ2 (esu)
χ3 (esu)
(10−12
References
mW-1)
)
TIT
TITO
TITz
0.52
1.62
1.55
2.71
4.1637×10−11
6.4593×10−11
5.8669×10−11
8.4294×10−11
1.69375
5.16077
4.95886
8.55506
This work
This work
This work
This work
TIPTz
1.55
6.57×10−11
4.90
[29]
0.76
0.56
6.27×10−19
4.423
-
[37]
[39]
O
O
-
S
C4H9
C4H9
Phenothiazine–Gold
(Au) nanocomposite
1.74
2.5
8.0×10−11
-
-
[40]
[38]
phthalocyanine–0.5%
gold nanoparticles
matrix
-
2.0
2.6
-
-
-
-
[41]
[42]
Graphene oxide–

Fe3O4