Synthesis and optoelectronic properties of symmetrical thiophene based 2,5-disubstiuted 1,3,4-oxadiazoles: Highly fluorescent materials for OLED applications

Article abstract of DOI:10.1007/s10895-011-0838-y

Symmetrical 2,5-disubstituted thiophene derivatives containing 1,3,4-oxadiazole moiety bearing different aromatic substitutions were synthesized by employing convenient and simple synthetic protocols using thiophene-2,5, dicarboxylic acid as a startingmaterial. The structures of these target molecules were established by their analytical and spectral data. The photophysical and electrochemical studies were carried out on these compounds and found that they exhibit good fluorescent properties with high quantum yield. Springer Science+Business Media, LLC 2009.

Full text of DOI:10.1007/s10895-011-0838-y

J Fluoresc (2011) 21:1515–1519
DOI 10.1007/s10895-011-0838-y
ORIGINAL PAPER
Synthesis and Optoelectronic Properties of Symmetrical
Thiophene Based 2,5-disubstiuted 1,3,4-oxadiazoles:
Highly Fluorescent Materials for OLED Applications
Shridhar I. Panchamukhi & Ningaraddi Belavagi &
Mohammed Hussain Rabinal & Imitiyaz Ahmed Khazi
Received: 22 September 2010 /Accepted: 10 January 2011 /Published online: 28 January 2011
#
Springer Science+Business Media, LLC 2011
Abstract Symmetrical 2,5-disubstituted thiophene deriva-
tives containing 1,3,4-oxadiazole moiety bearing different
aromatic substitutions were synthesized by employing conve-
nient and simple synthetic protocols using thiophene-2,5,
dicarboxylic acid as a starting material. The structures of these
target molecules were established by their analytical and
spectral data. The photophysical and electrochemical studies
were carried out on these compounds and found that they
exhibit good fluorescent properties with high quantum yield.
luminescence are highly desirable because of their wide
applicability in OLED’s to tune the emission colours in the
entire region of visible spectrum [5]. The existing organic
materials with blue light-emitting capability include styr-
ylarylenes [6], polyphenyls [7], perylenes [8], benzofurans
[9], indoles [10], thiophenes, etc. Derivatives of 1,3,4-
oxadiazoles are amongst the most widely studied classes of
electron-injection/hole-blocking materials due to their elec-
tron deficiency, high photoluminescence quantum yield, good
thermal, and chemical stabilities. It has been shown that 2-(4-
biphenylyl)-5-(4-tertbutylphenyl)-[1, 3, 4] oxadiazole (PBD)
functions very well as an excellent electron-transport material
(ETM) in multilayer OLEDs [11–17]. In the present
investigation we have reported the design and synthesis of
symmetrical 2,5-disubstituted thiophenic derivatives contain-
ing 1,3,4-oxadiazole moiety as blue emitting materials.
These oligomers are found to exhibit different emission
properties with different substitutions.
Keywords Symmetrical thiophene derivatives
.
2,5-disubstiuted 1,3,4-oxadiazoles Photolumiscence
.
High quantum yield OLED
Introduction
Conjugated molecules and polymers have been studied
extensively, not only to explore their fundamental optical
and electrical properties, but also to identify their potential
application for various devices, such as organic light
emitting diodes (OLED), organic field effect transistors
(OFETs) and photovoltaic cells [1–3]. Particularly, thio-
phene–based oligomers are promising building blocks for
these applications due to their unique optical and electrical
properties, as well as due to their ease of structural
modification [4]. Now a days, compounds with blue
Results and Discussion
The present work relates to novel oxadiazole compounds which
are useful not only as organic electroluminescent materials and
charge transporting materials, but also as fluorescent brighten-
ing agents. A relatively simple and efficient synthetic protocol
was employed for the synthesis of title compounds. Thiophene-
2,5-dicarboxylic acid was converted to the corresponding bis
methyl ester and then to the corresponding bis hydrazide by
treatment with hydrazine hydrate. 1H NMR spectra of
thiophene-2,5-dicarboxylic acid bishydrazide [18] displayed
peaks at 4.54 (s, br, 4H, NH2, D2O exchangeable), 7.63 (s,
2H, thiophenic protons),and 9.82 (s, br, 2H, NH, D2O
exchangeable). The compound 2 was further converted to
symmetrical 2,5-disubstitued thiophene1,3,4-oxadiazole deriv-
:
:
S. I. Panchamukhi N. Belavagi I. A. Khazi (*)
Department of Chemistry, Karnatak University,
Dharwad 58003, India
e-mail: drimkorgchem@gmail.com
M. H. Rabinal
Department of Physics, Karnatak University,
Dharwad 58003, India

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J Fluoresc (2011) 21:1515–1519
O
O
O
O
i
S
S
H3C O
H2N
N
H
O CH3
N NH2
H
1
2
ii
Ar
Ar
O
O
S
N
N
N
N
3
Scheme 1 Synthesis of symmetrical thiophene based 2,5-disubstituted 1,3,4-oxadiazoles. i) Hydrazine Hydrate, reflux, 3 h. ii) R-COOH, POCl3
reflux 5–10 h. Ar = Thiophen-2-yl (3a), Furan-2-yl (3b), Naphthalen-2-yl (3c), 2-Styryl (3d), Antracen-9-yl (3e), Naphthalen-1-yl (3f).
atives by reacting with various commercially available
aromatic carboxylic acids in dry phosphorusoxychloride media
(Scheme 1). Formation of 1,3,4-oxadiazole moiety was
evident, by the absence of absorption bands at 3350, 3300,
3250 and 1725 cm−1 due to NH2, NH and -COOH functions
voltammetric measurements. HOMO energy levels were
À
Á
onset
ox
calculated using equation HOMO ¼ ꢀ E
þ 4:8 ðeVÞ.
The LUMO levels were estimated from HOMO values and
values of optical band gap according to equation: LUMO ¼
HOMO þ EgoptðeVÞ and were compared with those of
coumarin 440 (Table 2). These results revealed the facile
reversible redox behavior of the synthesized compounds and
hence they can be applied as bipolar transport materials for
electroluminescence applications. This in fact would facilitate
better sensitization of the dopants such as 2,5-disubstituted-
1,3,4-oxadiazole 3a–f employed for multi-layer OLED.
The fluorescence quantum yields were experimentally
determined by comparison with a standard dye Coumarin
440(C120) in ethanol [21]. The quantum yields (QY) of the
present samples were estimated by using the below given
equation.
1
respectively in the IR spectra of 3a–f. H NMR spectra of
these compounds exhibited the presence of two thiophene
protons at δ 7.6 ppm and aromatic protons of substituted acid
revealed the formation of product . 13C NMR spectra and
GCMS further confirmed the formation of symmetrical
thiophene based 2,5-disubstituted 1,3,4-oxadiazoles.
Photoluminescence spectra of compounds 3a–f were
measured in ethanol and Quantum yields [19–21] are
presented in (Table 1). The optical band gap value Egopt were
approximated from the onset of the low energy side of the
absorption spectra (λonset, solution) to the baseline [22]. Thus
obtained Egopt values were found to be in good agreement
with the theoretically calculated values obtained by using the
Gaussian 03 W software programme. Further HOMO-LUMO
energy levels of these molecules were calculated from cyclic
I ODR n2
QY ¼ QYR
nR2
IR OD
Table 1 Summary of physical measurements of п-conjugated molecules 3a–f
Compound
Reaction ime (h)
Product Yield (%)
Solution λemission (nm)
Full wave Half
maxima (nm)
QY
3a
4
5
8
6
7
8
–
72
65
88
80
79
85
–
422.75
422.19
438.06
436–53
492.67
420.11
435.06
82
90
86
92
86
50
65
0.51
0.47
0.41
0.38
0.16
0.13
0.98
3b
3c
3d
3e
3f
REF
QY Quantum yield (QY) in ethanol estimated using Coumarin 440 as standard

J Fluoresc (2011) 21:1515–1519
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recorded on a Nicolet −5700 FT-IR spectrophotometer.1H
and 13C NMR spectra were recorded on a Brucker AM 300-
MHz spectrometer (at 300 and 75 MHz, respectively) with
SiMe4 as the internal standard in dimethyl sulfoxide
(DMSO-d6) and CDCl3. Mass spectra were recorded on
GCMS Schimadzu Japan QP-2010S. UV and fluorescence
spectra were recorded on a UV-3310-UV–VIS Hitachi
spectrophotometer and Hitachi F-7000 Fluorescence spectrom-
eter (300–700 nm). The quantum Yield (QY) measurements
were carried out at room temperature in Ethanol solutions,
using coumarin 440 as standard reference (QY=0.98).
Table 2 Energy values of HOMO and LUMO of 3a–f
HOMO (eV)b
LUMO(eV)b
onset
ox
E
(eV)a
Egopt
(eV)
Compound
3a
0.39
−5.19
−5.27
−5.58
−5.52
−5.54
−5.59
−5.45
−2.06
−2.25
−2.32
−2.35
−2.59
−2.39
−2.29
3.13
3.02
3.26
3.17
2.95
3.22
3.16
3b
3c
0.47
0.78
0.72
0.74
0.79
0.65
3d
3e
3f
REF
a Oxidation potential relative to Ag/AgCl electrode
ꢀ
ꢁ
À
Á
Synthesis of 2, 5-disubstitued-[1,3,4]-oxadiazole
b
onset
HOMO ¼ ꢀ E
þ 4:8 (eV), LUMO ¼ HOMO þ Egopt (eV)
ox
REF=Coumarin 440
2,5-Thiophenedicarboxylic acid bishydrazide (2), [18]
(7.6 g, 50 mmol) and aromatic acids (7.5 g, 50 mmol)
were dissolved in 50 ml POCl3. The mixture was refluxed
under stirring for 5–10 h. After cooling to room tempera-
ture, the mixture was poured into ice water carefully. The
solid that separated was collected by filtration and then
crude product was subjected to column chromatography
(hexane: ethyl acetate) to get the product, 2,5-disubstitued
[1,3,4]oxadiazole, in good yield.
Where QY is quantum yield, I is intensity, OD is optical
density and n is refractive index. In the above equation
subscript R refers to the standard taken. The concentration of
the samples was selected to obtain the optical density of 0.1
at excitation wave length. The quantum yield were measured
at an emission wavelength of 435 nm using Coumarin 440
(C120) as standard. Figure 1 illustrates the UV absorption
and fluorescence spectra of 3a–f compared to coumarins
440 indicating their potential for OLED applications.
2-(thiophen-2-yl)-5-(5-(5-(thiophen-2-yl)-1,3,4-oxadizol-2yl)
thiophen-2yl)-1,3,4-oxadiazole (3a)
Experimental
Yield 72%, m.p 201–203 °C, IR (KBr) cm−1: 3100, 2923,
2853, 1621, 1589, 1020.1H NMR (300 MHz, δ, DMSO-d6):
7.33 (m, 4 H), 7.92 (d, J=3.6 Hz, 2 H), 7.98 (d, J=4.8 Hz,
2 H). 13C NMR (75 MHz, δ, DMSO-d6):125.12, 129.54,
131.63, 133.34, 161.56. MS m/z: 384.9 (m+). Anal. Calcu-
lated for C16H8N4O2S3 (%): C, 49.98; H, 2.10; N, 14.57; S,
25.02 Found: C, 50.06; H, 2.14; N, 14.61; S, 25.10.
General
Melting points were determined by open capillary method
and are uncorrected. All the compounds were analyzed
satisfactorily for C H N & S. IR spectra (KBr disc) were
3a
3b
3a
3b
1.0
1.0
3c
3d
3e
3c
3d
3e
0.8
0.8
3f
3f
REF
REF
0.6
0.6
0.4
0.2
0.0
0.4
0.2
0.0
400
450
500
550
600
650
300
400
500
600
700
Wavelength(nm)
Wavelength (nm)
Fig. 1 UV absorption and Photoluminescence spectra of 3a–f in
ethanol 2-(thiophen-2-yl)-5-(5-(5-(thiophen-2-yl)-1,3,4-oxadizol-2yl)
thiophen-2yl)-1,3,4-oxadia- zole(3a), 2-(furan-2-yl)-5-(5-(5-(furan-2-
yl)-1,3,4-oxadiazol-2-yl)thiophen-2-yl)-1,3,4-oxadiazole(3b), 2-(5-(5-
(naphthalen-2-yl)-1,3,4-oxadiazol-2-yl)thiophen-2-yl)-5-(naphthalen-
3-yl)-1,3,4-oxadiazole(3c), 2-styryl-5-(5-(5-styryl-1,3,4-oxadiazol-2-
yl)thiop- hen-2-yl)-1,3,4-oxadiazole(3d), 2-(anthracen-9-yl)-5-(5-(5-
(anthracen-9-yl)-1,3,4-oxadia- zol-2-yl)thiophen-2-yl)-1,3,4-oxadia-
zole (3e), 2-(naphthalen-1-yl)-5-(5-(5-(naphthalen-1-yl)-1,3,4-oxadia-
zol-2-yl)thiophen-2-yl)-1,3,4-oxadiazole(3f), REF: Coumarin 440

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J Fluoresc (2011) 21:1515–1519
Acknowledgements The authors thanks the Karnatak University,
Dharwad, and University Grants Commission (UGC), New Delhi,
India, for financial support and thankful to the University Science
Instruments Centre, Karnatak University, Dharwad, for providing the
spectral data.
2-(furan-2-yl)-5-(5-(5-(furan-2-yl)-1,3,4-oxadiazol-2-yl)
thiophen-2-yl)-1,3,4-oxadiazole (3b)
Yield 65%,m.p: 207–209 °C. IR (KBr) cm−1: 3048, 2919,
2850, 1628, 1590, 1021.1H NMR (300 MHz, δ, DMSO-d6),
6.86 (m, 4 H), 7.49 (d, J=2.6 Hz, 2 H), 8.12 (d, J=1 Hz, 2 H).
Anal. Calculated for C16H8N4O4S (%): C, 54.54; H, 2.29; N,
15.90; S, 9.10, Found: C, 54.60; H, 2.41; N, 15.98; S, 9.14.
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Yield 88%, m.p: 168–170 °C. IR (KBr) cm−1: 3049, 2923,
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1
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