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

Article abstract of DOI:10.1039/c9nj01100e

The imidazo[2,1-b][1,3,4]thiadiazole (ITD) ring is a fused, planar aromatic heterocyclic system consisting of four heteroatoms with a bridgehead nitrogen atom. In this study, three new donor-acceptor-donor (D-A-D)-type organic molecules (ThITD1-ThITD3) we

Full text of DOI:10.1039/c9nj01100e

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DOI: 10.1039/C9NJ01100E
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Effects of substituents on enriching optical
limiting action of novel imidazo[2,1-
b][1,3,4]thiadiazole fused thiophene based small
molecules
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Viprabha Kakekochia, Udaya Kumar D*a, Nikhil P Pb, Keloth Chandrasekharanb
a
Synthetic Organic 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.
*Contact email: udayaravi80@gmail.com, udayakumar@nitk.ac.in
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DOI: 10.1039/C9NJ01100E
Abstract
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Imidazo[2,1-b][1,3,4]thiadiazole (ITD) ring is a fused, planar aromatic heterocyclic system
consisting of four heteroatoms with a bridgehead nitrogen atom. In this work, three new
donor–acceptor–donor (D–A–D) type organic molecules (ThITD1–ThITD3) were designed
and synthesized, wherein ITD is an electron acceptor unit and thiophene/ phenyl moieties are
electron donor units that produce a D–A–D configuration. The thiophene–ITD core structure
comprises three different groups viz., thiophene–2–acetonitrile (ThITD1), phenylacetonitrile
(ThITD2) and rhodanine–3–acetic acid (ThITD3) and the effect of substituents on the
optical and electrochemical properties were discussed based on structural modifications. The
third–order nonlinear optical (NLO) properties analyzed by Z–scan technique revealed that
the molecules exhibit effective two photon absorption (TPA) with ThITD3 possessing
substantially higher effective TPA coefficient (βeff) than that of ThITD1 and ThITD2, which
is comparable to βeff values reported. The results unravel that ITD as an acceptor with an
appropriate π–linker will be a promising entrant in the field of optoelectronics/photonics.
Herein, the thiophene–ITD ring system for NLO applications is explored and reported for the
first time.
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Keywords
Thiophene, imidazo[2,1-b][1,3,4]thiadiazole, cyclic voltammetry, DFT, NLO, Z–scan.
1. Introduction
Organic semiconductors, especially π–conjugated small molecules, having well defined
molecular structure along with π–conjugated polymers are being spotlighted since couple of
1–3
4,5
decades
for their application in many optoelectronic and electronic devices
such as
organic photovoltaics, organic light emitting diodes (OLEDs) or electro chromic devices and
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optical limiters 6–14. The devices based on these materials have attracted widespreDaOdI:a10tt.1e0n39t/iCo9nNJ01100E
due to their flexibility, low cost and light weight compared to the conventional inorganic
devices 15,16. Simple modifications in synthetic route and functional groups lead to the
variations in optical, electronic and charge transport properties in these systems. Most of the
conjugated materials used in photovoltaic/optoelectronic industry are generally obtained by
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17–20
C–C coupling reactions viz., Suzuki, Heck, Stille and Kumada coupling reactions
which involve expensive transition metals as catalysts. Moreover, stringent reaction
conditions and tedious purification methods are followed to achieve the target compounds
which hinder the synthetic efficacy, thereby restricting the large–scale production and
commercialization 21–23. A clean, attractive, low-cost alternative route to synthesize
conjugated materials is the condensation chemistry 24,25, which can be performed without
catalyst at ambient conditions and most of the condensation reactions result water as the by–
product, which makes the purification of the product easier 26. Further, the reactions result in
high yield compared to that of C–C coupling reactions. The introduction of olefinic bond as
π–linker between the aromatic rings leads to the reduction in steric hindrance, improvement
in planarity and hence extends the π–conjugation 27. The presence of vinylic bond in
conjugated materials modulates the electrochemical properties such as light emitting
property, charge transfer ability, etc., 28–31. Similarly, introduction of cyano group as electron
withdrawing group lowers the lowest unoccupied molecular orbital (LUMO) level and also
increases the electrochemical stability of the conjugated material, which is desirable to
achieve high–performance optoelectronic devices 32. Despite of aforementioned benefits,
compounds containing cyano groups also exhibited promising performances on their usage in
various organic electronics such as organic filed effect transistors (OFETs), OLEDs and
33–36
polymer
/small molecule photovoltaics 26,37. However, the defined molecular weight,
molecular structure, ease of synthesis and ease of purification of low molecular
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oligomers/small molecules make them superior to polymers 38. A great numbDeOrI: o10f.10s3m9/Ca9lNlJ01100E
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molecular materials with thiophene unit either as a central core or as end/peripheral group,
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explored as active materials in organic electronics 39–43
.
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Further, the meteoric rise of attention in the area of nonlinear optics leads to the
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development of novel materials with outstanding properties
. Since NLO effects could
manipulate the high energy laser beam to new optical frequencies, they are very useful in this
inspiring field. Organic molecular and polymeric materials have obtained considerable
interest as good NLO materials, as they are cheap, biodegradable, have high damage
threshold, excellent NLO responses, structural modification and ease of fabrication 45–47. The
presence of strong electron donor and acceptors increases the delocalization of π-electrons,
making the material highly polarizable and thus causing remarkable optical nonlinearities in
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such system . Due to these benefits, they are considered as potential candidates in various
applications such as high density optical data storage, optical communications, optical
limiters for eye and instrument safety and photo dynamic therapy for cancer treatment 49–54
.
Several organic materials with moderate to excellent NLO properties have been reported.
Among them, thiophene derivatives are found to be good candidates because of their
flexibility in structural modification and good semiconducting property when coupled with a
suitable electron acceptor 28,55. ITD is an efficient electron acceptor with a bridgehead
nitrogen atom, along with three other heteroatoms and two condensed heterocycles with
extended π–conjugations that make it planar and rigid heteroaromatic system 56. These
features anticipated to exhibit promising photo–physicochemical properties. There are plenty
of literatures that describe the pharmacological importance of ITD but there are no
reviews/literatures available which deal with the linear optical, electrochemical and NLO
properties of this moiety. The present study discloses the optoelectronic properties and
optical nonlinearity of three novel materials (ThITD1–ThITD3) containing thiophene as
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electron donor and ITD as electron acceptor moiety with the expectation thDaOt I:i1t0.1w03o9/uCl9dNJ01100E
produce good performance when used in optoelectronic devices and also this present study
focused on the evaluation of third order NLO properties such as nonlinear absorption
coefficient (βeff), nonlinear refractive index (η2) and third order nonlinear susceptibility (χ3) of
the novel molecules.
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2. Experimental
2.1 Materials and methods
All the reagents and starting materials used were procured from commercial vendors. The
chemicals used are, thiophene–2–carboxylic acid (Sigma Aldrich, 99%), thiosemicarbazide
(Alfa Aesar, 98%), phenacyl bromide (Spectrochem, 99%), thiophene–2–acetonitrile (Sigma
Aldrich, 97%), phenylacetonitrile (Spectrochem, 98%) and rhodanine–3–acetic acid (Alfa
Aesar, 98%). The solvents used were distilled and dried. All the reactions were carried out
under inert condition and the progress of the reaction was monitored using thin layer
chromatography (TLC) technique. All the intermediates and final compounds were purified
by column chromatography using an appropriate eluent.
2.2 Synthesis
2.2.1 Synthesis of 5-(thiophen-2-yl)-1,3,4-thiadiazol-2-amine (3)
Thiophene–2–carboxylic acid (1) (9.0 g, 70.23 mmol) and thiosemicarbazide (2) (6.4 g,
70.23 mmol) were taken in a clean round–bottomed (RB) flask, followed by the addition of
phosphorus oxychloride (POCl3) (10.0 mL). The mixture was refluxed for 2 h, later which,
the mixture was cooled to room temperature and quenched using ice cold water. It was then
basified with 20% NaOH solution and thus, obtained solid was filtered and dried. The crude
was purified by column chromatography (60–120 mesh silica) using dichloromethane as
eluent to get compound (3) as white colored solid (12.08 g, yield: 94%). ESI–MS (m/z) calcd.
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DOI: 10.1039/C9NJ01100E
for C6H5N3S2: 182.99, found: 184.0 [M+H]+; 1H NMR (400 MHz, DMSO–d6) δ (ppm): 7.61–
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7.60 (d, J = 6.0 Hz, 1H, Ar–H), 7.42 (s, 2H, Ar–NH2), 7.39–7.38 (d, J = 4.4 Hz, 1H, Ar–H),
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7.11–7.09 (m, 1H, Ar–H); C NMR (100 MHz, DMSO–d6) δ (ppm): 168.5, 151.0, 133.7,
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128.4, 128.0; Anal. calcd. for C6H5N3S2: C, 39.33; H, 2.75; N, 22.93; S, 34.99; found: C,
39.29; H, 2.78; N, 22.89; S, 34.97.
2.2.2 Synthesis of 6-phenyl-2-(thiophen-2-yl)imidazo[2,1-b][1,3,4]thiadiazole (4)
To a solution of (3) (6.0 g, 32.74 mmol) in ethanol (20 mL), phenacyl bromide (7.8 g,
39.29 mmol) was added and refluxed for 24 h. Excess of ethanol was removed under reduced
pressure, the resulting solid was suspended in water and refluxed for 4 h. After which, the
product was neutralized with 10% Na2CO3. The obtained solid (4) was filtered, dried and
purified by column chromatography using a mixture of petroleum ether (pet ether): ethyl
acetate (8:2, v/v) as eluent (8.25 g, yield: 89%). ESI–MS (m/z) calcd. for C14H9N3S2: 283.02,
found: 284.00 [M+H]+; 1H NMR (400 MHz, CDCl3) δ (ppm): 8.01 (s, 1H, Ar–H), 8.00–7.98
(d, J = 8.0 Hz, 1H, Ar–H), 7.83–7.81 (d, J = 8.0 Hz, 2H, Ar–H), 7.61–7.40 (m, 5H, Ar–H),
7.33–7.14 (m, 2H, Ar–H); 13C NMR (100 MHz, CDCl3) δ (ppm): 155.8, 146.1, 144.6, 133.1,
132.3, 129.9, 129.2, 128.8, 128.1, 127.8, 125.1, 109.4; Anal. calcd. for C14H9N3S2: C, 59.34;
H, 3.20; N, 14.83; S, 22.63; found: C, 59.25; H, 3.31; N, 14.76; S, 22.71.
2.2.3
Synthesis
of
6-phenyl-2-(thiophen-2-yl)imidazo[2,1-b][1,3,4]thiadiazole-5-
carbaldehyde (5)
A freshly distilled N,N–Dimethylformamide (DMF) (2.18 mL, 28.26 mmol) was taken in a
two neck RB flask and cooled to 0 to −5°C. To this POCl3 (2.64 mL, 28.26 mmol) was added
in drop wise and stirred for 15 min to get Vilsmeier salt. The solution of (4) (4.0 g, 14.13
mmol) in DMF was added to the formed Vilsmeier salt and the mixture was heated at 65°C
for 6 h, later which, the mixture was quenched using ice cold water. The obtained solid was
filtered, dried and purified by column chromatography using a mixture of ethyl acetate and
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hexane (3:7, v/v) as eluent to obtain (5) as yellow solid (3.78 g, yield: 86%). ESI–MS (m/z)
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calcd. for C15H9N3OS2: 311.02, found: 312.00 [M+H]+; H NMR (400 MHz, CDCl3) δ
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(ppm): 10.11 (s, 1H, –CHO), 7.92–7.90 (m, 2H, Ar–H), 7.66–7.60 (m, 2H, Ar–H), 7.54–7.45
(m, 3H, Ar–H), 7.19–7.17 (m, 1H, Ar–H); 13C NMR (100 MHz, CDCl3) δ (ppm): 177.4,
163.0, 157.8, 155.9, 149.9, 132.2, 131.6, 130.9, 129.9, 129.1, 128.8, 128.2, 125.1, 124.2;
Anal. calcd. for C15H9N3OS2: C, 57.86; H, 2.91; N, 13.50; S, 20.59; found: C, 57.79; H, 2.82;
N, 13.62; S, 20.67.
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2.2.4 Synthesis of (E)-3-(6-phenyl-2-(thiophen-2-yl)imidazo[2,1-b][1,3,4]thiadiazol-5-yl)-
2-(thiophen-2-yl)acrylonitrile (ThITD1)
To a freshly prepared solution of sodium ethoxide (89.3 mg, 8.11 mmol), thiophene–2–
acetonitrile (0.40 mL, 3.85 mmol) was added slowly with stirring. After 15 min of stirring,
compound (5) (1.0 g, 3.21 mmol) was added and the reaction mixture was stirred at room
temperature for 5 h. The precipitated solid was collected by filtration, washed with ethanol
and purified by column chromatography using a mixture of pet ether: ethyl acetate (9:1, v/v)
as eluent to get ThITD1 as yellow solid (0.97 g, yield: 73%). ESI–MS (m/z) calcd. for
C21H12N4S3: 416.02, found: 417.10 [M+H]+; 1H NMR (400 MHz, CDCl3) δ (ppm): 7.74–7.71
(m, 2H, Ar–H), 7.64–7.63 (m, 1H, Ar–H), 7.57–7.56 (d, J = 4.0 Hz, 1H, Ar–H), 7.51–7.47
(m, 2H, Ar–H), 7.44–7.41 (m, 3H, Ar–H), 7.34–7.33 (m, 1H, Ar–H), 7.17–7.15 (d, J = 8.0
Hz, 1H, Ar–H), 7.12–7.09 (dd, J = 12.0 Hz, 1H, Ar–H); 13C NMR (100 MHz, CDCl3) δ
(ppm): 155.4, 149.0, 147.3, 139.3, 130.5, 129.5, 128.9, 128.7, 128.3, 128.2, 128.1, 127.4,
126.6, 119.0, 116.3, 107.6; Anal. calcd. for C21H12N4S3: C, 60.55; H, 2.90; N, 13.45; S,
23.09; found: C, 60.61; H, 2.98; N, 13.36; S, 23.01.
2.2.5
Synthesis
of
(Z)-2-phenyl-3-(6-phenyl-2-(thiophen-2-yl)imidazo[2,1-
b][1,3,4]thiadiazol-5-yl)acrylonitrile (ThITD2)
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Compound ThITD2 was synthesized following the above procedure mentioneDdOiI:n10s.1e0c39t/iCo9nNJ01100E
2.2.4, in which the precursor used was phenylacetonitrile (0.44 mL, 3.85 mmol) with
compound (5) (1.0 g, 3.21 mmol) (1.00 g, yield: 76%). ESI–MS (m/z) calcd. for C23H14N4S2:
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410.07, found: 411.10 [M+H]+; H NMR (400 MHz, CDCl3) δ (ppm): 7.73–7.71 (d, J = 8.0
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Hz, 4H, Ar–H), 7.64–7.63 (m, 1H, Ar–H), 7.58–7.55 (m, 2H, Ar–H), 7.49–7.38 (m, 6H, Ar–
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H), 7.17–7.14 (m, 1H, Ar–H); C NMR (100 MHz, CDCl3) δ (ppm): 155.3, 149.0, 147.2,
134.2, 133.6, 132.3, 130.4, 129.4, 129.3, 129.1, 128.9, 128.6, 128.1, 126.2, 126.1, 119.5,
117.2, 113.5; Anal. calcd. for C23H14N4S2: C, 67.29; H, 3.44; N, 13.65; S, 15.62; found: C,
67.35; H, 3.53; N, 13.55; S, 15.51.
2.2.6 Synthesis of (E)-2-(4-oxo-5-((6-phenyl-2-(thiophen-2-yl)imidazo[2,1-b][1,3,4]
thiadiazol-5-yl)methylene)-2-thioxothiazolidin-3-yl)acetic acid (ThITD3)
A mixture of intermediate (5) (1.0 g, 3.21 mmol) and rhodanine–3–acetic acid (0.61 g, 3.21
mmol) was taken in acetonitrile (10 mL). Piperidine (0.5 mL) was added to the mixture and
refluxed for 8 h. After which, the solvent was evaporated and to the obtained residue distilled
water (20 mL) was added and neutralized using 10% HCl. The suspended solid was filtered,
dried and purified by column chromatography using a mixture of pet ether: ethyl acetate (6:4,
1
v/v) as eluent to get (ThITD3) as orange solid (1.08 g, yield: 70%). H NMR (400 MHz,
DMSO–d6) δ (ppm): 13.00 (s, 1H, –COOH), 8.03–8.02 (m, 1H, Ar–H), 7.96–7.95 (m, 1H,
Ar–H), 7.76 (s, 1H, Ar–H), 7.69–7.67 (m, 2H, Ar–H), 7.58–7.49 (m, 3H, Ar–H), 7.31–7.29
(m, 1H, Ar–H), 4.70 (s, 1H, –CH2). 13C NMR (100 MHz, DMSO–d6) δ (ppm): 194.77,
167.93, 167.47, 157.83, 153.29, 149.74, 144.53, 133.24, 132.32, 131.45, 129.92, 129.50,
129.27, 119.44, 119.16, 118.31, 45.70; Anal. calcd. for C20H12N4O3S4: C, 49.57; H, 2.50; N,
11.56; S, 26.46; found: C, 49.46; H, 2.58; N, 11.64; S, 26.38.
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DOI: 10.1039/C9NJ01100E
S
N
S
(i)
(ii)
N
4
S
NH2
NH2
COOH
H2N
N
S
S
1
S
N
H
N N
3
2
8
9
(iv)
ThITD1
CHO
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N
S
N
5
(iii)
(v)
ThITD2
ThITD3
S
N
(vi)
HOOC
S
S
N
O
S
NC
N
NC
N
S
N
N
N
N
S
S
N
S
N
S
S
N
ThITD1
ThITD2
ThITD3
Scheme 1. Synthetic route and molecular structures of molecules ThITD1–ThITD3.
Reaction conditions: (i) POCl3, 75°C, 2 h; (ii) Phenacyl bromide, ethanol, 85–90°C, 24 h; (iii)
POCl3, DMF 60–65°C, 6 h; (iv) 2-(thiophen-2-yl)acetonitrile, NaOC2H5, C2H5OH, RT; (v) 2-
phenylacetonitrile, NaOC2H5, C2H5OH, RT; (vi) Rhodanine-3-acetic acid, piperidine, 80–
85°C, 8 h.
2.3 Characterization details
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ESI–Mass (Waters micro mass Q-Tofmicro), H NMR (Bruker, 400 MHz) and C NMR
(Bruker, 100 MHz) spectral techniques were utilized to elucidate the structures of all the
intermediates and final compounds. A flash EA-112 CHNS analyzer (Thermo electron
corporation) was used for the elemental analysis. The UV–Vis spectral data were collected
from Analytik jena SPECORD S600 spectrometer and photoluminescence (PL) spectra were
recorded using a Fluoromax Horiba Jobin Yuan spectrometer. IVIUM (Vertex-V55610)
electrochemical work station was used to carryout cyclic voltammetric (CV) measurements
of the molecules using three electrode system wherein the molecules were dissolved in
chloroform and drop casted on glassy carbon electrode which was taken as working
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electrode, Platinum (Pt) wire as a counter electrode and saturated calomel electrode (SCE) as
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a reference electrode and the electrolyte used was 0.1 M solution of tetrabutylammonium
perchlorate (TBAP) in acetonitrile. Ferrocene was used as an internal standard for all the
electrochemical measurements and the scan rate used was set at 100 mV s−1. Gaussian 09
software with B3LYP functional 6-311+G (d,p) level basis set was used for the density
functional theory (DFT) and time–dependent density functional theory (TDDFT) studies of
the molecules. Thermogravimetric analysis (TGA) were carried out using Seiko Instruments
TGA/DTA Exstar 6300, Japan at a heating rate of 10°C min‒1 under argon atmosphere.
2.4 Z–scan analysis
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To experiment the application of the samples in optoelectronics, NLO studies were
57
performed by Z–scan technique under nano second excitation . Using this technique both
nonlinear absorption and nonlinear refractive index parameters were measured which finally
provided χ3 value. The nonlinear absorption (NLA) and optical limiting (OL) parameters
were obtained from open aperture (OA) Z–scan analysis whereas nonlinear optical refraction
(NLR) parameters were obtained from closed aperture (CA) Z–scan analysis. In OA Z–scan
analysis the optical transmittance of the sample was recorded as a function of input intensity.
The schematic representation of the Z–scan set up is shown in the Fig. S18 (ESI). The sample
solutions were made to translate across the focus of a convex lens (focal length 150 mm) in
the direction of laser beam (z axis from -z to +z) in predetermined steps using a translational
stage which was controlled by a computer program.
The maximum intensity of the samples was observed at the focus which decreases equally
on either side of the focus. The output transmittance from the sample at each position was
collected by a detector which was then plotted against position of the samples. The NLA
properties of the samples were measured by numerically fitting the recorded data to the
theoretical model. To obtain the NLR parameters using CA Z–Scan, a small aperture was
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placed in front of the signal detector. Thus, obtained transmittance is affected by both NLA
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as well as NLR and is susceptible to phase distortion. Division of CA output from OA output
results in pure NLR part. All the experiments were carried out in the “single shot” mode.
Sufficient time intervals were provided between successive pulses to avoid accumulative
thermal effects in the sample. The NLO studies were performed using a Q– switched Nd–
YAG laser Quanta Ray INDI–40 (Austin, USA) (7 ns pulse width) working at a wavelength
of 532 nm with a frequency of 10 Hz. The linear transmittance of all the molecules was fixed
between 70–80% at the excitation wavelength when taken in a 1 mm cuvette.
3. Results and discussion
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3.1 Synthesis and structural characterization
The synthetic route and molecular structures of ThITD1–ThITD3 are shown in the scheme
1. The synthesis of molecules ThITD1–ThITD3 includes four steps. In the first step, the
intermediate (3) was synthesized from the reaction between commercially available
thiophene–2–carboxylic acid and thiosemicarbazide in the presence of POCl3 with the yield
of 94%. The intermediate (4) was synthesized by the reaction between (3) and phenacyl
bromide followed by cyclization, which was then subjected to Vilsmeier–Hack formylation to
obtain intermediate (5). In the final step, precursor (5) was subjected to well–known
Knoevenagel condensation with active methylene compounds viz., thiophene–2–acetonitrile,
phenylacetonitrile and rhodanine–3–acetic acid to achieve the target compounds ThITD1–
ThITD3, respectively. All the synthesized intermediates and final compounds were purified
using either recrystallization or column chromatography. All the intermediates and final
compounds were characterized using 1H NMR, 13C NMR and mass spectroscopic techniques
(see ESI for details).
3.2 Optical properties
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Fig. 1 a and b show the absorption and PL spectra of ThITD1–ThITD3, respectively and
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the corresponding spectral data are listed in Table 1. All the spectra were recorded in solution
state (1×10−5 M) in which chloroform was used as solvent. The absorption spectra exhibit two
separate bands, one is around 250–270 nm which is ascribed to the π–π* electronic
excitations and the other, in the range of 340–413 nm, is attributed to intramolecular charge
transfer (ICT) from donor (thiophene, phenyl) to the acceptor (ITD) unit 58. The large
bathochromic shift in ThITD3 compared to ThITD1 and ThITD2 is due to the strong
electron withdrawing nature of rhodanine–3–acetic acid which is further leading to the
extension of conjugation 59,60. The absorption onset and the corresponding optical band gaps
are summarized in Table 1. The determined optical band gaps to be 2.83, 2.95 and 2.72 eV
for ThITD1–ThITD3, respectively. The emission spectra exhibit the maximum emission for
ThITD1–ThITD3 at 483, 480 and 509 nm, respectively upon exciting at their corresponding
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λ max, establishing their luminescence property.
1.2
(b)
(a)
ThITD1
ThITD2
ThITD3
1.0
0.8
0.6
0.4
0.2
0.0
ThITD1
ThITD2
ThITD3
1.0
0.8
0.6
0.4
0.2
0.0
450
500
550
600
650
300
400
500
600
700
Wavelength (nm)
Wavelength (nm)
Fig. 1 (a) Normalized UV–vis absorption spectra (b) PL spectra of ThITD1–ThITD3 in
chloroform (1×10−5 M).
Table 1 UV–vis absorption and fluorescence emission data of molecules ThITD1–ThITD3
in chloroform (1×10−5 M).
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Molecules λ abs (nm)
λ pl max (nm) λcedge(nm)/ Egopt (eV)
264, 385a
371b
483
480
509
438/ 2.83
419/ 2.95
455/ 2.72
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ThITD1
ThITD2
ThITD3
254, 338a
360b
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245, 411a
430b
a Experimentally determined values from UV–vis absorption spectra
b values derived theoretically from TD–DFT studies
λ pl max= λ max from PL spectra
c Determined from the intersection between the tangent drawn to the absorption edge and the
abscissa, the optical bandgap is calculated using the equation Eg = 1240/λedge
.
3.3 Electrochemical studies
Cyclic voltammetric measurements were performed to investigate electrochemical
properties, which further quantify the energies of molecular orbitals of the molecules. Fig. 2
a, b and c show the cyclic voltammograms of ThITD1–ThITD3, respectively. Considering
the onset oxidation potential, energy of highest occupied molecular orbital (HOMO) level
(EHOMO) was calculated using the equation (1) 61,
ox


EHOMO   Eonset  4.8eV  EFOC ..........................................................................(1)


where, Eox
and EFOC are the onset oxidation potentials of the samples and ferrocene,
onset
respectively, −4.8 eV is the HOMO energy level of ferrocene against vacuum.
Further, LUMO levels were obtained from electrochemically measured HOMO energy and
spectroscopically determined band gap, using the equation (2)
ELUMO  EHOMO  Eg ................................................................................................(2)
where, Eg = 1240/λedge
.
The HOMO energy levels are at −5.66, −5.77 and −5.83 eV whereas LUMO energy levels
are at −2.83, −2.82 and −3.11 eV for ThITD1, ThITD2 and ThITD3, respectively. The
significant reduction in the LUMO level is mainly attributed to the presence of strong
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compared to that of cyano groups in ThITD1 and ThITD2. The experimentally obtained
HOMO and LUMO values from CV are in agreement with the theoretically determined
HOMO and LUMO values (Table 2).
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(a)
ThITD1
0.05
0.00
-0.05
-0.10
-0.15
-0.20
-2
-1
0
1
2
Potential (V vs SCE)
0.08
0.04
ThITD2
(b)
0.00
-0.04
-0.08
-2
-1
0
1
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Potential (V vs SCE)
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ThITD3
(c)
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0.0
-0.1
-0.2
-2
-1
0
1
2
Potential (V vs SCE)
Fig. 2 Cyclic voltammograms of (a) ThITD1, (b) ThITD2 and (c) ThITD3.
Table 2 Electrochemical and DFT data of ThITD1–ThITD3
Molecules
Eox
(V vs SCE)
1.18
HOMO (eV)
LUMO (eV)
onset
ThITD1
-5.66a
-5.96b
-5.77a
-6.03b
-5.83a
-6.21b
-2.83a
-2.61b
-2.82a
-2.54b
-3.11a
-3.19b
ThITD2
ThITD3
1.29
1.35
a Experimentally determined values from CV, b values derived theoretically.
3.4 DFT studies
The electronic properties of the molecules ThITD1–ThITD3 were investigated
theoretically using DFT calculations. Fig. 3 depicts the optimized geometry and frontier
orbital distribution of the molecules ThITD1–ThITD3. The estimated HOMO and LUMO
energy levels are listed in Table 2 along with the experimental results. The HOMO and
LUMO levels were obtained using Avogadro software (Version 1.2.0). The effect of
thiophene functioning as electron donor appended with ITD and cyano/ rhodanine–3–acetic
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acid group as acceptor species governs the effective HOMO orbital distribution. This
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distribution is commonly observed in the three molecules ThITD1–ThITD3. The nature of
electronic distribution in molecule ThITD1 and ThITD2 pertaining to LUMO portrays a π–
π* type of transition, whereas in ThITD3 electron density is mostly localized on the ITD and
rhodanine units. Such a type of distribution observed in the system effectively facilitates
charge transfer transition between the acceptor group and the entire molecular network. The
effect of rhodanine–3–acetic acid moiety which imparts stronger electron withdrawing nature
in orbital distribution at LUMO level of ThITD3, were not observed in the case of ThITD1
and ThITD2, possessing a cyano group as electron acceptor.
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Also, the optimized structures of the compounds in Fig 3 reveal that after introducing
double bonds the dihedral angle between ITD and thiophene -2-acetonitrile plane is 36.5° and
that of ITD and benzene plane is 29.3° in ThITD1 whereas dihedral angle between ITD and
phenyl acetonitrile plane is 39.5° and that of ITD and benzene plane is 30.2° in ThITD2
while dihedral angle between ITD and rhodanine–3–acetic acid plane is 23.0° and that of ITD
and benzene plane is 42.4° in ThITD3. The steric hindrance caused by rhodanine moiety due
to the lesser dihedral angle of 23.0° between ITD and rhodanine–3–acetic acid plane
increases the dihedral angle of ITD and benzene plane in ThITD3 to maintain the most stable
conformation. The dihedral angles between ITD plane and the substituted groups, ITD plane
and phenyl group are listed in Table S1 and the planes are represented in Fig S19.
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Fig. 3 Ground state optimized geometry and molecular orbitals (HOMO and LUMO) of
ThITD1–ThITD3.
TD-DFT calculations of ThITD1–ThITD3 were carried out to estimate the electronic
excited state transitions and to corroborate with the experimental absorption transitions. The
simulated absorption spectra of ThITD1–ThITD3 is shown in Fig 4. The theoretical
absorption maxima (λmax) for ThITD1–ThITD3 are at 371, 360 and 430 nm, respectively
(Table 1). The experimental and theoretical values differ by 14, 22 and 19 nm for ThITD1,
ThITD2 and ThITD3, respectively. This energy difference can be attributed to the fact that
there exist many conformers in the solution state whereas such conformers may not be
considered for the calculation 62. However, the results obtained by theoretical simulations are
in concordant with the experimental results. Further, the spectral data exhibit two bands,
wherein, the higher energy band corresponds to π–π* transition and the lower energy band
corresponds to charge transfer transition. Overall, the trend remains the same for both
theoretical and experimental calculations but the trifling inconsistencies observed in energy
transition is due to the theoretical overestimation 63.
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ThITD1
ThITD2
ThITD3
1.0
0.8
0.6
0.4
0.2
0.0
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200
300
400
500
600
Wavelength (nm)
Fig. 4 Simulated absorption spectra of ThITD1–ThITD3.
3.5 Thermal properties
TGA were carried out to study the thermal stability of ThITD1–ThITD3. As shown in the
Fig. 5, ThITD1–ThITD3 showed satisfactory thermal stability. The decomposition
temperatures (Td) for ThITD1, ThITD2 and ThITD3 are 330, 357 and 346°C, respectively.
The lower Td of molecules ThITD1 and ThITD3 than that of ThITD2 is due to the presence
of heterocyclic rings attached with Th–ITD core moiety in ThITD1 and ThITD3. The results
showed that the molecules could be of relevant materials for electronic/photonic applications.
100
ThITD1
ThITD2
ThITD3
80
60
40
20
100
200
300
400
500
600
Temperature (oC)
Fig. 5 TGA curves of ThITD1–ThITD3.
3.6 Third–order nonlinear optical properties
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3.6.1 Nonlinear absorption studies
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Fig. 6 a, b and c show the open aperture Z–scan curves of the molecules ThITD1–
ThITD3, respectively in DMF solutions. The molecules ThITD1–ThITD3 were excited
using the intensity of 1.386 GWcm−2 (50 μJ). From the plots it is observed that the
transmittance is minimum at the focus (z=0) and all the transmission curves are symmetric
about the focus. Thus, the absorption is dependent on the excitation intensity. The molecules
ThITD1–ThITD3 showed two photon absorption (TPA) along with reverse saturable
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absorption (RSA), thereby the net effect becomes the “effective TPA” process . The solid
line indicates the theoretical fitting and the symbol indicates the experimental data. In all the
cases, the data are found to be matching well with the theoretical model for excited state
absorption (ESA). Thus, the effective nonlinear absorption coefficient α(I) (including both
TPA and ESA) can be written as 65,
o
1 I Is
(I) 
 eff I...........................................................................................(3)
where, α0 is the linear absorption coefficient at the excitation wavelength, Is is the saturation
intensity, I is the input laser intensity, βeff is the effective TPA coefficient.
The rate of change in the intensity of a beam as it passes through a medium is given by,






dI
dz
o
2
 
I  eff I ......................................................................................(4)
I






1
Is
where, z is the propagation distance within the sample.
Dividing output intensity by input intensity and normalizing it with linear transmittance
yielded the normalized transmittance 66 and is given by the equation

 2
1


T z 
 
ln 1 q z e d............................................................................(5)
0  



 q z
0   
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1 e L
o
z2
z02

and Leff 







where,
q z   I L
0  
1
eff
0
eff
0
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As shown in the Fig. 6 a, b and c, the experimental data are in a good agreement with the
theoretical simulation and thus the estimated βeff values are 0.49×10−10, 1.05×10−10 and
1.55×10−10 m W−1 for molecules ThITD1–ThITD3, respectively. Under similar excitation
conditions many NLO materials have shown effective NLA coefficient in the order of 10−10
m W−1. The βeff values obtained in the present study are comparable and better than that of
many other organic NLO materials available in the literature (Table 3). Therefore, these
materials could act as better optical limiters for high energy devices.
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1.2
ThITD1
(a)
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0.8
-24000 -16000 -8000
0
8000
16000 24000
ThITD2
Position (microns)
1.1
1.0
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0.8
0.7
0.6
(b)
-24000 -16000 -8000
0
8000
16000 24000
Position (microns)
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1.2
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(c)
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-24000 -16000 -8000
0
8000
16000 24000
Position (microns)
Fig. 6 OA Z–scan curves of (a) ThITD1, (b) ThITD2 and (c) ThITD3 at 50 μJ.
3.6.2 Optical limiting studies
The optical limiters are the materials which exhibit constant transmission at lower input
energy and reduced transmission at higher input energy. These materials find their
importance in laser safety devices, mode locking, pulse compression etc. After certain critical
value of the input energy, these materials show the limiting action because of NLA and NLR
properties of the materials at the threshold value, which ultimately leads to the decrease in the
transmittance. The optical limiting is due to the processes such as TPA, ESA, RSA, optically
induced heating, nonlinear scattering and free carrier absorption that take place in these types
of materials upon irradiation of laser beam 44
.
The graphs of transmittance versus input fluence which are shown in Fig. 7 a, b and c
reveal the optical limiting studies of molecules ThITD1–ThITD3. The onset values (input
fluence at which the transmittance starts decreasing) are at 1.60, 0.78 and 0.66 J cm−2 for
ThITD1, ThITD2 and ThITD3, respectively. Thus, the molecules could act as protective
layers in devices which involve intense laser beam.
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1.1
1.0
0.9
0.8
ThITD1
(a)
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Input intensity (J/cm2)
1.1
1.0
0.9
0.8
0.7
0.6
ThITD2
(b)
0.07
0.7
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14
Input intensity (J/cm2)
(c)
ThITD3
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14
Input intensity (J/cm2)
Fig. 7 Optical limiting studies of (a) ThITD1, (b) ThITD2 and (c) ThITD3 at 50 μJ.
Table 3 NLO parameters of ThITD1–ThITD3 and few comparisons
Sample
Is
βeff
ɳ2
χ3
1012 (Wm-2)
10−10 (mW-1)
10−11 (esu)
10−12 (esu)
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ThITD1
ThITD2
ThITD3
(present study)
Ref. 44
1.39
1.50
1.59
0.49
1.05
1.55
2.09
5.40
6.57
1.63
3.29
4.90
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1.78
1.74
1.41
1.60
0.56
8.0
-
Ref. 67
-
-
-
-
4.65
Ref. 68
-
-
-
Ref. 69
0.98
3.6.3 Nonlinear refraction studies
Fig. 8 a, b and c represent normalized closed by open Z–scan curves of the molecules
ThITD1–ThITD3. The graphs show a change in the transmission with a characteristic peak,
followed by a null and a valley as the sample is moved from the input lens towards the output
lens through the focus. This is the signature of negative nonlinearity. Due to this effect the
molecules ThITD1–ThITD3 behave like diverging lens when the contact is made with the
incoming beam which results in the defocusing of the beam.
The normalized transmittance (T) at closed aperture condition is given by the equation (6) 57,






z
4
zo
T z, 1
 o 
o........................................................(6)
2




2







z
z









1
 9
zo
zo


where ∆φ0 is the on–axis phase shift at z=0.
The real part of third order nonlinear susceptibility is given by,
3
Real   2no2ocn2...............................................................................................(7)
2
where n0 is the linear refractive index, c velocity of light, ɛ0 is the permittivity of free space
and the nonlinear refractive index is thus given by,
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cn0o
n esu 
2 
.......................................................................................D..O..I.:.1.0(.810)39/C9NJ01100E

80 2Io Leff
6
7
8
ThITD1
(a)
1.2
1.1
1.0
0.9
0.8
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-20
-10
0
10
20
Position (µm)
1.6
1.4
1.2
1.0
0.8
0.6
(b)
ThITD2
-20
-10
0
10
20
30
Position (µm)
1.6
1.4
1.2
1.0
0.8
0.6
0.4
(c)
ThITD3
-20
-10
0
10
20
Position (µm)
Fig. 8 CA Z–scan curves of (a) ThITD1, (b) ThITD2 and (c) ThITD3 at 50 μJ.
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In the present study, the introduction of imidazo[2,1-b][1,3,4]thiadiazole as acceptor unit in
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between high electron donating thiophene and phenyl moieties improve the rigidity and
planarity of the molecules thereby has increased the strength of D–A–D backbone which in
turn enhanced the extent of delocalization in the entire molecule. Thus, the synthesized
molecules ThITD1–ThITD3 exhibited substantially improved optical nonlinearity when
subjected to the Z–scan analysis, which is due to the presence of donor–acceptor arrangement
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and π–delocalization in them 70,71
.
Further, the stronger electron accepting nature and polarity of rhodanine–3–acetic acid
moiety in ThITD3 than that of cyano groups in ThITD1 and ThITD2 increased the rigidity,
hyperpolarizability and conjugation length in ThITD3, which resulted in larger NLO
properties and improved βeff and χ3 values of ThITD3 compared to those of ThITD1 and
ThITD2. The comparison of obtained NLO parameters of ThITD1–ThITD3 with a few of
the similar works reported is presented in Table 3 and it is understood that the molecules
studied would be of potential candidates for optical limiters in photonic devices.
4. Conclusions
Three novel conjugated molecules (ThITD1–ThITD3) with thiophene as the donor and
imidazo[2,1-b][1,3,4]thiadiazole as an acceptor unit were designed and synthesized via
simple Knoevenagel condensation reaction. The chemical structures of ThITD1–ThITD3
1
were confirmed by H NMR, 13C NMR and mass spectroscopic techniques. The high
absorption maxima and low LUMO level of ThITD3 compared to that of ThITD1 and
ThITD2 are due to the strong electron withdrawing ability of rhodanine–3–acetic acid
present in ThITD3. The third–order nonlinear optical studies confirmed the effective two
photon absorption of the molecules, exhibiting a large third–order nonlinearity with high βeff
of the order of 10−10 m W−1 and optical limiting property. The higher βeff value of ThITD3
compared to that of ThITD1 and ThITD2 is confirmed to be originated from the strong
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electron accepting nature of rhodanine– 3–acetic acid moiety which also resulted in rigViidewitAyrticle Online
DOI: 10.1039/C9NJ01100E
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and enhanced π–delocalization in ThITD3. The results unveiled that the molecule ThITD3
could provide a good choice of optical limiters in laser photonics and also advance its
development in other optoelectronic/photovoltaic devices.
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Associated content
Supporting Information
1H NMR, 13C NMR and ESI-MS characterizations of molecules ThITD1–ThITD3 and
experimental set up of Z–scan measurements.
Conflicts of interest
There are no conflicts to declare.
Acknowledgment
Authors are thankful to SERB (SB/FT/CS-145/2012), DST INDIA for financial support, Dr.
Reddy’s Institute of Life Sciences and DST-PURSE Laboratory, Mangalore University for
analytical facilities.
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