Synthesis and Optoelectronic Properties of Thiophene Donor and Thiazole Acceptor Based Blue Fluorescent Conjugated Oligomers

Article abstract of DOI:10.1007/s10895-016-1838-8

We report on the synthesis and characterization of low band gap, blue light emitting and thermal stable conjugated oligomer by Wittig condensation. Thiophene and thiazole type of donor-acceptor based series of conjugated oligomers, Oligo-4,5-bis-[2-[5-[2-thiophene-2-yl-vinyl]thiophene-2-yl]-vinyl]-thiazole (OBTV-TZ) and Oligo-2,4,5-Tris–[2-[5-[2-thiophene-2-yl-vinyl]thiophene-2-yl]-vinyl]-thiazole (OTTV-TZ) were synthesized. These oligomers were confirmed by FT-IR and ¹H-NMR and LC/MS analysis. The effect of the number of thiophene rings on the optical, electrochemical, thermal and morphological properties of the oligomers were systematically investigated. Both oligomers were exhibited almost same absorption wavelength in methanol solution (λ_max?=?365?nm and 369?nm) which indicates both oligomers illustrate similar intra molecular charge transfer (ICT). In solid state, the oligomers were exhibited broadening peaks with higher onset absorptions (λ_max?=?600?nm and 580?nm). The photoluminescence absorption spectrum of the oligomers was observed at 433?nm and 434?nm respectively in methanol solution with blue emission. The electrochemical band gap (Egec) of the OBTV-TZ was 1.55?eV (low band gap) and OTTV-TZ was exhibited greater highest occupied molecular orbital (HOMO) value (E_HOMO?=??6.6?eV). Moreover morphological parameters of both oligomer film of 2D and 3D diagrams were observed by using AFM studies.

Full text of DOI:10.1007/s10895-016-1838-8

J Fluoresc
DOI 10.1007/s10895-016-1838-8
ORIGINAL ARTICLE
Synthesis and Optoelectronic Properties of Thiophene Donor
and Thiazole Acceptor Based Blue Fluorescent
Conjugated Oligomers
K. Mahesh1 & S. Karpagam1
Received: 25 March 2016 /Accepted: 9 May 2016
#
Springer Science+Business Media New York 2016
. .
Keywords Blue light emitting Conjugated oligomers
Abstract We report on the synthesis and characterization of
low band gap, blue light emitting and thermal stable conjugat-
ed oligomer by Wittig condensation. Thiophene and thiazole
type of donor-acceptor based series of conjugated oligomers,
Oligo-4,5-bis-[2-[5-[2-thiophene-2-yl-vinyl]thiophene-2-yl]-
vinyl]-thiazole (OBTV-TZ) and Oligo-2,4,5-Tris–[2-[5-[2-
thiophene-2-yl-vinyl]thiophene-2-yl]-vinyl]-thiazole (OTTV-
TZ) were synthesized. These oligomers were confirmed by
.
.
.
Thiophene Thiazole Internalchargetransfer Lowbandgap
Introduction
The design and synthesis of light emitting conjugated
oligomers have been much focus of intense research
over a last decade. Compared to oligomers, long chain
polymers are having some complications such as poly-
dispersity and purity. So it is mandatory to give more
attention for smaller systems of oligomers which pos-
sess well-defined spectroscopic and chemical character-
istics. The wide spread attention of the oligomers due to
their optical, morphological, electrochemical and electri-
cal properties [1]. These conjugated oligomeric materials
may be applied in various optoelectronic devices such
as organic photovoltaic devices (OPVs) [2] organic light
emitting diodes (OLEDs) [3] organic thin film transis-
tors (OTFTs) [4] and organic solar cells. These much
electronic properties of oligomers are mostly regulated
by delocalization of π-electrons in aromatic rings [1].
The large number of conjugated polymers and organic
materials are displaying high fluorescent colours in the
visible region such as green, orange and red. Mostly
these three colours are well established in OLEDs. But
even, they have few drawbacks such as high driven volt-
age, commercial value and less stability [5]. To achieve
these problems and enhance properties of electrolumines-
cence devices there is a demand for stable and strong
blue-light emitting conjugated materials with great
photoluminescence (PL). The current organic materials
1
FT-IR and H-NMR and LC/MS analysis. The effect of the
number of thiophene rings on the optical, electrochemical,
thermal and morphological properties of the oligomers were
systematically investigated. Both oligomers were exhibited
almost same absorption wavelength in methanol solution
(λmax = 365 nm and 369 nm) which indicates both oligomers
illustrate similar intra molecular charge transfer (ICT). In solid
state, the oligomers were exhibited broadening peaks with
higher onset absorptions (λmax = 600 nm and 580 nm). The
photoluminescence absorption spectrum of the oligomers was
observed at 433 nm and 434 nm respectively in methanol
solution with blue emission. The electrochemical band gap
(Eegc ) of the OBTV-TZ was 1.55 eV (low band gap) and
OTTV-TZ was exhibited greater highest occupied molecular
orbital (HOMO) value (EHOMO = −6.6 eV). Moreover mor-
phological parameters of both oligomer film of 2D and 3D
diagrams were observed by using AFM studies.
* S. Karpagam
skarpagam80@yahoo.com
1
Organic Chemistry Division, School of Advanced Sciences, VIT
University, Vellore-14, Tamil Nadu, India

J Fluoresc
are combined with blue- light emitting efficiency in-
cludes polyphenylens [6], benzofurans [7], thiophenes [8],
indoles [9], styryl arylenes [10], perylenes [11]. Among these,
thiophenes are most widely studied classes of electron donors.
Owing to the fact that of thiophene unit containing conjugated
polymers are having excellent hole transporting ability, good
photoconductivity, good light-harvesting properties, relatively
strong inter chain interaction, excellent environmental and
thermal stability [12].
Experimental Section
Reagents and Solvents
Thiophene 2-carbaldehyde, Zn powder, POCl3, TiCl4, 2,4,5-
tri methyl thiazole, 4,5-di methyl thiazole, N-
bromosuccinimide, benzoylperoxide, sodium hydride,
triphenylphosphine, were purchased from Sigma Aldrich,
Bangalore, India. Solvents such as DCM, Ethyl acetate,
THF, DMF, CCl4, and Acetonitrile respectively were pur-
chased from Sd-fine chemicals, India. All solvents were dried
and distilled before the synthesis as per standard procedures.
Our main aim is lower band gap oligomers along
with blue light emitting fluorescence. The low band
gap (LBG) of the conjugated oligomers determined by
interaction between the electron donor and electron ac-
ceptor units [13]. These type of lower band gap donor-
acceptor conjugated oligomers are capable of absorbing
a broad range of solar photons, great photoluminescence
and also having a good hole mobility. There is need of
thiazole which is good electron acceptor to attach with
thiophene (donor) unit to produce low band gap.
Recently thiazole derivatives have been paid much at-
tention as strong withdrawing moieties used for OLEDs,
polymer solar cell, and light harvesting devices [14].
Thiazole derivatives has high electron transporting abil-
ity because of good electron delocalization with dynam-
ic π-stacking and also has fluorescent chromophore be-
cause it contains two electron withdrawing atoms of
carbon (C) and nitrogen (N) in one functional imine
(C = N) group [15]. This imine group influences the
interaction of the highest occupied molecular orbital
(HOMO) and lowest unoccupied molecular orbital
(LUMO) causes to reduce the band gap. These types
of fluorescence materials are also useful to interact be-
tween small probe molecules with nucleotide analogues
[16] and human serum proteins [17–20].
Characterization Methods
FT-IR analysis was studied through Thermo Nicolet 330 FT-IR
spectrometer with KBr pellets with midrange of (500–
4000 cm−1). The 1H NMR and 13C NMR spectra were analysed
by using a Bruker Advanced III 400 MHz spectrometer. The
UV-Visible absorption spectra were analyzed on Hitachi
U2910 spectra photometer. The PL emissions were performed
with Perkin elmer LS45 fluorescence spectrometer. The mor-
phological (AFM) images were obtained using nanosurf easy
scan II instrument. 0.10 mg/ml of oligomer sample was dis-
solved in methanol and spin coated onto clean glass substrate
using apex spin NXG-P1 instrument. Thermal stability was
performed on SDT Q600 V20.9 Build 20 instrument under
nitrogen flow rate 100 ml/min. The molecular weight of the
intermediate compounds were measured from Perkin Elmer
Calarus 680 GC-MS spectrometer and the oligomers were ob-
served from Shimadzu LC-MS –TOF-2010 mass spectrometry.
Synthetic Procedure
In this research we have synthesized and designed
two new low band gap blue emitting conjugated oligo-
mers namely OBTV-TZ and OTTV-TZ which consisted
repeating units of thiophene and thiazole linked directly
with ethylene linkage. These ethylene linkages change
the energy levels, increases the solubility, flexibility,
planarity of the π-conjugated system and also tuning
optoelectronic properties [21]. Actually these types of
conjugated molecules were synthesized based on Pd-
catalysed C-C coupling reactions with much cost such
as heck reaction and Suzuki reaction [22]. In this paper,
oligomers were synthesized without using metal catalyst
with low cost of Wittig condensation route. Moreover,
we investigated the surface morphology studies, optical,
electrochemical and thermal properties of these
oligomers.
Synthesis of Donor Molecule
Preparation of (E)-1,2-Bis(2-Thienyl)Ethylene (2) Zinc
powder (5.53 g, 84.70 mmol) was dissolved in freshly distilled
anhydrous THF (150 ml). To this stirred suspension TiCl4
(8.45 g, 44.58 mmol) was dropped under nitrogen atmosphere
at −10 °C. The resulting reaction mixture was refluxed under
nitrogen atmosphere for 60 min. Then, 2-formyl thiophene
(5 g, 44.58 mmol) was added drop wise. The resulting reaction
mixture was refluxed for 2 h. After cooling, reaction mixture
was diluted with water (300 ml) and extracted with diethyl
ether (60 ml × 3). Finally organic layer was washed with brine
solution and dried over Na2SO4. Resulting yellow colour
crude product was purified by column chromatography
through 60–120 mesh silica gel using hexane: ethyl acetate
as an eluent and obtained colourless crystalline compound.

J Fluoresc
1
Yield: 65.2 %, mp. 133 °C. H NMR (CDCl3, 400 MHz,
δppm): 7.21 (d, 2 H, thiophene H), 7.06 (dd, 2 H, thiophene
H), 7.05 (d, 2 H, thiophene H), 7.00 (s, 2 H, vinylene H). 13C
for long time and it was stable only at below 15 °C in DCM
solvent. This crude product was also used for further synthesis
without purification.
δ
NMR (400 MHz, CDCl3, ppm): 126.1, 121.45, 124.32,
142.41 (thiophene C), 127.62 (−CH = CH-). GC-MS calcu-
lated for C10H8S2 (m/z) = 192.01, found, 192.30.
Preparation of 4,5-(Tri Phenyl Phosphonium Di Bromo
Methyl) Thiazole (M2) In 100 ml round bottom flask, 4,5-
bis bromo methyl thiazole (4) (15 ml in DCM) and tri phenyl
phosphine (1 g) were dissolved in 30 ml of acetonitrile. Then
resulting suspension was stirred at room temperature. After
6 h white precipitation was formed, then filtered and washed
with diethylether. White colour solid was obtained. FT-IR
(KBr, ν/cm−1): 3435.2 (aromatic C-H stretch), 1737.8
(C = C bending), 1587.4 (−C = N- stretch), 1435.04 (aromatic
Preparation of (E)-1,2bis (5-Formyl-2-Thienyl)Ethylene
(M1) 8.3 ml (73.3 mmol) of DMF was slowly added to a
stirred solution of compound 1 (2.5 g, 13 mmol) in dichloro-
ethane (DCE) and cooled at 0 °C. Then, POCl3 (5.46 ml,
58.5 mmol) was dropped under protection of nitrogen and it
was refluxed for 15 h. Resulting suspension was cooled to
room temp, diluted with ice cold water and neutralised to PH
6–7 using saturated NaHCO3 solution. Then product was ex-
tracted with dichloromethane (DCM), after evaporation of
solvent crude product was washed with petroleum ether.
Resulting brown colour powder was obtained. Yield:
56.25 %, mp: 196–198 °C. FT-IR: (KBr, ν/cm−1): 1647.25
(−C = O), 1454.4 (C-C stretch in aromatic), 958.65 (trans-
vinylene). 1H NMR (CDCl3, 400 MHz, δppm): 7.20 (d, 2 H,
thiophene H), 7.62 (d, 2 H, thiophene H), 7.16 (s, 2 H,
vinylene H), 9.80 (s, 2 H, −CHO). 13C NMR (CDCl3,
1
C-C stretch), 746.4 (C-P), 688.5 (C-Br), 553.5 (P-Br). H
NMR (DMSO-d6, 400 MHz, δppm): 7.97–7.68 (m, all phenyl
H), 5.44 (s, 2 H, −CH2-), 5.41 (s, 2 H, −CH2-). 13C NMR
δ
(DMSO-d6, 400 MHz, ppm): 154.2, 154.1, 153.9, 153.8,
135.4, 135.3, 133.9, 133.8, 130.3, 130.2, 117.6, 116.7,
116.5, 116.4, 21.0, 14.1.
Preparation of 2,4,5-(Tri Phenyl Phosphonium Tri Bromo
Methyl) Thiazole (M3) 2,4,5-Tris bromo methyl thiazole (6)
(20 ml in DCM) and tri phenyl phosphine (1.5 g) were dis-
solved in 40 ml of acetonitrile. Then resulting suspension was
stirred at room temperature for 6 h. White precipitation was
formed, filtered and washed with diethylether. FT-IR (KBr,
ν/cm−1): 3051.2 (aromatic C-H stretch), 1716.6 (C = C bend-
ing), 1580.6 (−C = N- stretch), 1430.0 (aromatic C-C stretch),
δ
400 MHz, ppm): 182.62 (−CHO), 150.26, 137.0, 128.28,
142.80 (thiophene C), 124.01 (−CH = CH-). GC-MS calcu-
lated for C12H8O2S2 (m/z) = 248, found, 248.80.
Synthesis of Acceptor Molecule
1
749.5 (C-P), 689.7 (C-Br), 550.1 (P-Br). H NMR (DMSO-
Preparation of 4,5-Bis-Bromo Methyl Thiazole (4) 4,5-
dimethylthiazole (2 g, 17.67 mmol), N-bromosuccinimide
(NBS) (6.2 g, 35.34 mmol) and benzoyl peroxide (BPO)
(20 mg) were dissolved in 100 ml of CCl4. The reaction
mixture was stirred at 70 °C for five hours. Formed
succinimide salt was removed by water washing, product
was extracted by DCM. Then solvent was removed under
room temperature at atmospheric pressure. The resulting
crude product was not stable at room temperature for long
time and it was stable at below 15 °C in DCM. So the
crude product was used for next synthesis without further
purification.
d6, 400 MHz, δppm): 7.99–7.69 (m, all phenyl H), 5.44 (s, 2
H, −CH2-), 5.38 (s, 2 H, −CH2-), 5.35 (s, 2 H, −CH2-). 13C
NMR (DMSO-d6, 400 MHz, δppm): 165.64, 165.61, 152.3,
152.2, 135.39, 135.36, 133.9, 133.8, 130.3, 130.2, 117.7,
116.8, 115.5, 115.4, 21.71, 18.56, 13.95.
Preparation of Oligomer OBTV-TZ Monomer 2 (M2)
(0.5 g, 0.57 mmol) was dissolved in 50 ml of THF. To this
60 % NaH (0.13 g, 5.72 mmol) was added pinch wise at −10 °C.
Then monomer 1 (M1) (0.28 g, 1.14 mmol) was added and the
reaction mixture was stirred at 70 °C for 24 h. After the reaction
period, ethyl acetate was added slowly to get precipitate and
filtered. Yellow coloured solid was obtained. FT-IR (KBr,
ν/cm−1): 3384 (aromatic C-H stretch), 3171 (=C-H stretch),
1637.9 (−C = N stretch), 1550.2 (C-C stretch in aromatic ring),
1338.6 (aromatic C = C), 1018.3 (=C-H bend), 925.9 (trans-
Preparation of 2,4,5-Tris Bromo Methyl Thiazole (6) 2,4,5-
tris methyl thiazole (2.5 g, 19.65 mmol), N-bromosuccinimide
(10.49 g, 58.96 mmol) and benzoyl peroxide (30 mg) were
dissolved in 150 ml of CCl4. Resulting mixture was stirred at
70 °C for 5 h. After cooling, succinimide salt was removed by
water washing and product was extracted by DCM (50 ml × 2).
To this organic layer washed with brine solution and dried over
Na2SO4. This product was also not stable at room temperature
1
δ
vinylene). H NMR (CH3OH-d4, 400 MHz, ppm): 8.44 (s,
thiazole H), 7.57–7.44 (m, all thiophene H nearer to N), 7.33–
7.19 (m, all thiophene H nearer to S), 7.03–6.87 (m, all vinylene
H). Anal. Calcd for C68H46N4S12: C 62.8; H 3.67; N 4.25; S
29.28 %. Found; C 62.25; H 4.62; N 4.52; S 28.61 %.

J Fluoresc
Preparation of Oligomer OTTV-TZ Monomer 3 (M3)
(0.5 g, 0.43 mmol) was dissolved in 50 ml of THF. To this
60 % NaH (0.15 g, 6.52 mmol) followed by Monomer 1 (M1)
(0.32 g, 1.30 mmol) was added. The reaction mixture was
stirred at 70 °C for 24 h. Precipitation was formed while slow
addition of ethyl acetate and then filtered to get a pale yellow
coloured solid. FT-IR (KBr, ν/cm−1): 3301 (aromatic C-H
stretch), 3188.8 (=C-H stretch), 1641 (−C = N stretch),
1556.8 (C-C stretch in aromatic ring), 1330.3 (aromatic
C = C), 1016 (=C-H bend), 933.9 (trans-vinylene). 1H NMR
(CH3OH-d4, 400 MHz, δppm): 7.86–7.79 (m, all thiophene H
between of thiazole S and N), 7.57–7.41 (m, all thiophene H
nearer to N), 7.33–7.18 (m, all thiophene H nearer to S), 7.05–
6.89 (m, all vinylene H). Anal. Calcd for C73H51N5S13: C
61.96; H 3.63; N 4.95; S 29.46 %. Found: C 60.45; H 4.71;
N 4.48, S 30.36 %.
mers and target oligomers were confirmed by FT-IR, 1H NMR
spectroscopic techniques. Firstly, the donor type of intermedi-
ate (2) was synthesized from 2-formyl thiophene (1) using Zn
powder, TiCl4 via mcmurry coupling. Formed vinyl linkage
1
was confirmed from H NMR spectroscopy. Signal was ap-
peared at 7.0 ppm (J = 14) which indicates the intermediate (2)
was in E- configuration and also appeared six thiophene pro-
ton signals at around 7.05–7.21 ppm. The monomer 1 (M1)
was synthesized from (E)-1,2-bis(2-thienyl)ethylene (2) in
presence of DMF and POCl3 by well-known Vilsmeier-
Haack reacton. The aldehyde protons were observed at
1647.2 cm−1 in FT-IR, 9.8 and 182 ppm was appeared in 1H
and C13 NMR spectrum which confirms the newly formed
formyl thiophene group. From the part of acceptor moiety,
bromo methyl thiazole was firstly synthesized and then con-
verted to monomers (M2 & M3) using triphenylphosphine.
The structure of M2 & M3 was confirmed by appearing the
sharp absorption bands at 746.4, 749.5 (C-P band), 688.5,
689.7 (C-Br stretching) in FT-IR and 7.99–7.69 ppm (phenyl
Results and Discussion
1
hydrogens of phosphonium salt) in H NMR spectra. The
Synthesis and Characterization of Monomers
and Oligomers
molecular weights of the intermediate and monomers were
well established using mass spectral analysis. The theo-
retical values was well agreement with the experimental
value which confirms the structure of monomers (M1,
M2 and M3).
The general synthetic scheme for monomers and two oligo-
mers are illustrated in scheme. 1. The structures of the mono-
TiCl4 /Zn
POCl3 / DMF
DCE, 85 oC, 15 hr
S
O
S
O
O
2
S
S
THF, 70 oC, 2 hr
S
1
2
Monomer-1 (M1)
Br
BrPh3P
BrPh3P
Monomer-2 (M2)
N
N
N
NBS / BPO
CCl4, 70 oC, 5 hr
PPh3
Br
CH3CN, 6 hr
S
3
S
4
S
Br
BrPh3P
BrPh3P
N
N
N
PPh3
NBS / BPO
CCl4, 70 oC, 5 hr
Br
Br
PPh3Br
CH3CN, 6 hr
S
S
S
Monomer-3 (M3)
6
5

J Fluoresc
n
S
n
S
Monomer-1
+
S
NaH
S
THF, 70 oC, 24hr
Monomer-2
S
N
OBTV-TZ
S
n
S
S
N
Monomer-1
+
S
NaH
n
S
THF, 70 oC, 24hr
Monomer-3
S
S
n
Z
-T
TV
OT
Fig. 1 FT-IR spectra of the oligomers OBTV-TZ and OTTV-TZ
Fig. 2 1H NMR spectra of OBTV-TZ

J Fluoresc
represents -C = N stretching. The sharp absorption peaks at
around 925.9 and 933.9 cm−1 which represents trans-vinylene
was predominant among the newly developed vinylene dou-
ble bonds. These IR values indicated for the successful reac-
tion of monomers. The 1H NMR spectrums of both oligomers
are shown in Figs. 2 and 3. The oligomer OBTV-TZ exhibited
down field peak at 8.44 ppm which indicates thiazole protons.
All thiophene protons of both oligomers show multiple signals
in between of 7.57–7.19 ppm. The signal formed at 7.03–6.76
due to trans-vinylene protons. The low molecular weight of
the OBTV-TZ and OTTV-TZ was measured from mass spec-
trometry which was found to be 1330 m/z and 1398 m/z. The
reason behind that wittig formed product usually oligomers
were having low molecular weight [24].
Fig. 3 1H NMR spectra of OTTV-TZ
Optical Characterization
Characterization of Oligomers
Absorption Studies
The synthetic procedures of the both oligomers (OBTV-
TZ, OTTV-TZ) were sketched in Scheme 1. The present
oligomers were prepared from the thiophene
dicarbaldehyde monomer (M1) and thiazole phosphoni-
um salt monomers (M2 & M3) in dry THF solvent using
well-known Wittig condensation route. Sodium hydride was
selected as a base for deprotonation of phosphonium salt to
prepare oligomer.
The oligomers were well soluble with water and methanol
without introducing any solubilizing group in thiophene and
thiazole moiety [23]. The structures of both oligomers were
verified by FT-IR and NMR. The FT-IR spectra demonstrated
(Fig. 1) that both oligomers (OBTV-TZ & OTTV-TZ) showed
characteristic absorption bands formed at 3384, 3301 cm−1
which indicates heterocyclic aromatic C-H stretching, 3171,
3188 indicates =C-H- stretching and 1637, 1641 cm−1
The optical and photo physical properties of both oligomers
were investigated by UV-visible spectroscopy and their opti-
cal absorption spectra were measured in methanol solution
(5 mg mL−1) and thin films, which are shown in Fig. 4.
Both oligomers displayed identical absorption peaks. This is
the recognisable feature in donor-acceptor oligomers. Both
oligomers were exhibited two types of absorption peaks,
The first one is higher energy absorption peak (weak band)
which was appeared at lower wavelength side. In absorption
spectra it was observed at 244 to 297 nm, which is due to π-π*
electronic transition of the conjugation. Second one is lower
energy absorption peak (strong band) appeared at higher
wavelength side. In absorption spectra it was observed at
300 to 420 nm which is responsible for the strong internal
charge transfer (ICT) between donor (thiophene) and acceptor
Fig. 5 Fluorescence spectra of the both oligomers in methanol solution.
Colour of the OBTV-TZ and OTTV-TZ solutions under a visible light b
under UV-Lamp (λmax = 365 nm)
Fig. 4 Absorption spectra of the both oligomers in methanol solution and
solid state

J Fluoresc
Fig. 6 Three-dimensional
fluorescence spectra of both
oligomers
(thiazole) when compared with literature based benzo
thiadiazole based co-oligomer [5].
intramolecular charge transfer among thiophene (donor) and
thiazole (acceptor) moieties after photo excitation. Both olig-
omers showing pure fluorescence emission spectra without
any shoulder peaks because of one kind of chromophore.
Both oligomers exhibit almost similar emission values which
indicate same conjugation length between thiophene and thi-
azole segments. These two oligomers were showed
bathochromic shift when compared with thiophene based
2,5-di sustituted 1,3,4-oxazoles [27]. Moreover we in-
vestigated the three-dimensional fluorescence spectrosco-
py of both oligomers which is depicted in Fig. 6. The
fluorescence parameters including peak intensity, peak
locations, and fluorescence intensities were identified from
3D fluorescence spectra, which could be useful for quantita-
tive analysis [28].
The thin film absorption spectra of both oligomers were
bathochromic shifted when compared with their solution ab-
sorption spectra. This demonstrate that the thin film states was
shown strong intra or inter chain molecular interactions. Both
oligomers were depicted a single absorption broadening
peaks. In solid state π-π* transition peak was absent due to
carbon-carbon bonds rotation was restricted leads to form ag-
gregation or π-π stacking.
The optical band gap values of both oligomers were calcu-
lated from the absorption onset of the thin film according to
the equation Eogpt = 1240/absorption onset [25]. The optical
band gap values was observed as 2.0 and 2.13 eV
which was calculated from their absorption onset values
600 and 580 nm respectively. These broadened peaks
absorption onset values are greater than our previously report-
ed carbazole and quinoline based donor-acceptor conjugated
polymer (λonset = 460 nm) [26].
The oligomers OBTV-TZ and OTTV-TZ were not exhibit-
ed their emission in film and solid state. The reason may be
due to the physical constraint in the solid state restricts their
intermolecular rotations. This was blocked the non-
radioactive channels and populating the radioactive decay
thus making the oligomer and polymer non-emissive [29].
The photo luminescence of both oligomers could be
favourable under solution state than film state.
Stokes shift is the difference between positions of the ab-
sorption and emission spectroscopy. Both absorption and
emission energies were depend on the unique characteristics
of particular oligomeric material structure. If the stokes shift is
Photoluminescence Studies
The fluorescence spectra for both oligomers in methanol are
shown in Fig. 5. OBTV-TZ and OTTV-TZ oligomers exhib-
ited blue light emission (Fig. 5b) with wavelength maxima at
433 nm and 434 nm respectively. It is highly expected that
blue fluorescence is due to defined conjugation and fast
Fig. 7 Cyclic voltammogram of
both oligomers films on ITO glass
in 0.1 M Bu4NPF6, CH3CN
solution with a scan rate 50 mV/s

J Fluoresc
According to that, the electrochemical band gaps (Egec) of
the both oligomers were calculated through following empir-
ical equation [31].
EHOMO ¼ −ðEox þ 4:4Þ
ELUMO ¼ −ðEred þ 4:4Þ
Eg ¼ EHOMO − ELUMO
The electrochemical potential and energy levels of OBTV-
TZ and OTTV-TZ are shown in Table. 1. The onset oxidation
(Eox) and reduction (Ered) potentials of OBTV-TZ are 1.05 V
and −0.5 V respectively. The HOMO and LUMO energy
levels of OBTV-TZ are calculated as 5.45 and −3.9 eVrespec-
tively and electrochemical energy band gap is 1.55 eV. This
was lower compared to optical band gap which was measured
from UV solid absorption spectra (2.00 eV). The difference
between the optical band gap and electrochemical band gap is
0.45 eV. This may be accredited to the creation of free ions in
cyclic voltammetry in electrochemical experiments which dif-
ferentiate with solid UV absorption maxima. OBTV-TZ was
also observed low band gap (Egec = 1.55 eV) oligomer when
compared with dithiazole and benzo di thiophene based con-
Fig. 8 Schematic diagram of HOMO/LUMO energy levels of OBTV-TZ,
OTTV-TZ and comparison with PC61BM
very small, the overlapping of emission and absorption spectra
will be more. Then luminescent efficiency will decrease in
device because the emitting light will be self-absorbed by
the material and the luminescent efficiency will be decreased
[30]. In this study, stokes shift of OBTV-TZ and OTTV-TZ
were found to have 85.4 and 82 nm. The longer stokes shift
was observed in OBTV-TZ (85.4 nm) illustrate that the more
structural adjustment among the ground state and excited
state. So it was advised as best oligomeric material for organic
light emitting diodes.
ec
jugated polymer (Eg = 2.86 eV) [14] and thieno pyrazine-
based copolymers (Egec = 1.84 eV, Egec = 2.24 eV) [32]. These
types of low band gap oligomers will be worked as good
efficiency in the area of photovoltaic application.
The onset oxidation (Eox) potential of OTTV-TZ is
2.2 V. The HOMO value of OTTV-TZ is estimated to
be −6.6 eV. This HOMO value is higher than thieno [3–4-b]
pyrazine based polymer [33]. The LUMO level was calculated
to be −4.5 eV according to the equation ELUMO = EHOMO
+
Electrochemical Properties
Eogpt [34]. The higher HOMO level (−6.6 V) of OTTV-TZ is
The electro chemical studies of both oligomers were investi-
gated by the cyclic voltammetry (CV). The HOMO and
LUMO energy values of the conjugated oligomers were cal-
culated from oxidation and reduction potential values. The
cyclic voltammograms of both oligomers and schematic dia-
gram of HOMO/LUMO energy levels are depicted in Figs. 7
and 8. The CV is performed in 0.1 mol/L of tetra butyl am-
monium hexa fluoro phosphate (Bu4NPF6) in acetonitrile so-
lution at scan rate 50 mV/S using platinum wire as a counter
electrode and Ag/AgCl as reference electrode. The potentials
were internally calculated using the Fc/Fc+ redox couple.
due to higher number of electron donating moieties
(p-doped) respect to thiazole acceptor (n-doped). These types
of highest HOMO value oligomers are acceptable for oxida-
tive stable devices.
Surface Morphology Analysis
The morphology of both oligomers was investigated by AFM.
This technique is important and useful for characterization of
high resolution images of the surface of organic thin films.
The thin films of both oligomers were coated on glass
Table 1 Electrochemical
property of both oligomers
ox
onset
EEgc (eV)
EOg pt (eV)
red
onset
Oligomers
HOMO (eV)
LUMO (eV)
E
(V)
E
(V)
OBTV-TZ
OTTV-TZ
1.05
2.2
−0.5
0.1(cal from EOg pt
−5.45
−6.6
−3.9
−4.5
1.55
2.13
2.0
2.1
)

J Fluoresc
Fig. 9 AFM images of oligomers
OBTV-TZ and OTTV-TZ
substrate using spin coating technique. After evaporation
of solvent it shows mainly three interactions those are
molecule-solvent, molecule-surface and molecule-
molecule interactions.
[35]. Higher surface roughness of the oligomers leads to
increase the photo current, internal light scattering light
absorption and internal series resistance of the device
[36, 37].
The surface morphology has been measured for two olig-
omers which are depicted in Fig. 9. The oligomer OBTV-TZ
displayed rod shaped particle with size of width and length
was measured as 1.36 μm and 6.36 μm. OTTV-TZ displayed
micrometers sized long strip like structure that width was
2.5 μm and length was 8.5 μm.
AFM topography was also used to calculate the surface
roughness (Sq), root mean square (RMS), average surface
roughness (Sa), maximum peak height (Sp), peak-peak height
(Sy), and maximum peak valley depth. These all surface mea-
surements of both oligomers were depicted in Table. 2.
The RMS surface roughness of OBTV-TZ and OTTV-
TZ were noticed to be 13.6 nm and 10.6 nm respective-
ly. These values were higher than thermally annealed and cast
films of PPV with dithenyl(thienothiadiazole) copolymer
Thermal Analysis
The thermal properties of OBTV-TZ and OTTV-TZ oligomers
were characterised by thermogravimetric analysis at a temper-
ature ramp rate of 20 °C min−1 under nitrogen atmosphere
(Fig. 10). Both oligomers showed different curve shapes and
thermal behaviour. OTTV-TZ has shown good thermal stabil-
ity than OBTV-TZ. The degradation temperature (Td) with
13 % and 9 % weight loss at 350 °C was noticed for OBTV-
TZ and OTTV-TZ. Obviously thermal stability of OTTV-TZ \
Both oligomers had higher thermal stability than dodecyl
thieno[3,4-b]thiophene-2-carboxylate conjugated polymer
(Td = 350 °C with 25 % weight loss) [38]. Final degradation
temperatures of the oligomers were found to be 490 and
Table 2 Surface morphology measurement of both oligomers
Oligomers
Average surface
roughness (Sa) (nm)
RMS roughness
(Sq) (nm)
Peak-peak
height (Sy) (nm)
Maximum peak
height (Sp) (nm)
Maximum peak
valley depth (Sv) (nm)
Maximum peak
valley (Sp-Sv) (nm)
OBTV-TZ
OTTV-TZ
7.2
4.5
13.6
10.6
211.3
310.9
183.4
238.6
−27.8
−72.3
211.2
310.9

J Fluoresc
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We have successfully synthesized and characterized two ther-
mally stable donor-acceptor conjugated oligomers OBTV-TZ
and OTTV-TZ using Wittig condensation. We have achieved
good solubility with methanol without introducing solubiliz-
ing groups. The optical band gaps of oligomers were 2.0 eV
and 2.13 eV which was calculated from solid absorption on-
set. Both oligomers exhibited stronger blue light emission
maxima with 433 and 434 nm wavelength. The optical and
electrochemical characters of the both oligomers imply that
the introduction of electron withdrawing thiazole and electron
donating thiophene which both reduces the optical and elec-
trochemical band gap of the oligomers (1.55 eV). Oligomer
film displayed rod shaped particles and long strip like particles
with lower surface roughness. Both oligomers were expressed
good thermal stability with final decomposition temperature
around 500 °C with 40–45 % weight loss. The result of both
conjugated oligomers will be promised for future OLEDs and
photovoltaic applications.
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Acknowledgments This work was financially supported by DST-
SERB, New Delhi (Grant no. SB/FT/CS-140/2013). The authors are
grateful to DST/VIT-FIST, VIT University for providing instru-
mental facilities.