Molecular engineering for panchromatic absorbing oligothiophene donor-π-acceptor organic semiconductors

Article abstract of DOI:10.1016/j.tet.2012.09.009

A series of three novel organic semiconductor molecules based on the donor-π-bridge-acceptor (D-π-A) modular design have been designed, synthesised and characterised. These small organic molecules have a common donor (triphenylamine), π-bridge (an alkylat

Full text of DOI:10.1016/j.tet.2012.09.009

Tetrahedron 68 (2012) 9440e9447
Contents lists available at SciVerse ScienceDirect
Tetrahedron
journal homepage: www.elsevier.com/locate/tet
Molecular engineering for panchromatic absorbing oligothiophene
donore
p
eacceptor organic semiconductors
,
,
b
,
,
*
Akhil Gupta a b, Abdelselam Ali a, Th. Birendra Singh a, Ante Bilic d, Udo Bach a c, Richard A. Evans a
a CSIRO Materials Science and Engineering, CSIRO Future Manufacturing Flagship, Bag 10, Clayton South, 3169 Victoria, Australia
b Department of Materials Engineering, Faculty of Engineering, Monash University, Wellington Road, Clayton, 3800 Victoria, Australia
c Tech Fellow, The Melbourne Centre for Nanofabrication, 151 Wellington Road, Clayton, 3168 Victoria, Australia
d CSIRO Mathematics Informatics and Statistics, Bayview Avenue, Clayton, 3169 Victoria, Australia
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 30 May 2012
Received in revised form 14 August 2012
Accepted 3 September 2012
Available online 10 September 2012
A series of three novel organic semiconductor molecules based on the donorep-bridgeeacceptor
(De
p
eA) modular design have been designed, synthesised and characterised. These small organic
-bridge (an alkylated tetrathiophene) and various
molecules have a common donor (triphenylamine),
p
acceptors to provide tuning of optical and electronic properties. The examined acceptors were dicya-
novinylidene, cyanopyridone and oxoindenemalononitrile groups. These compounds were highly soluble
in common organic solvents, such as chloroform, toluene and chlorobenzene. As acceptor strength in-
creased from dicyanovinylidene to cyanopyridone to oxoindenemalononitrile groups, the wavelength of
the longest wavelength absorption maximum increased (518, 587, 619 nm (solution)), the respective
Keywords:
Donorepeacceptor
Organic semiconductors
Oligothiophene
Cyanopyridone
extinction coefficients increased (4.9ꢀ104, 6.3ꢀ104, 7.4ꢀ104
M
ꢁ1 cmꢁ1) and with increasing acceptor
strength the band gap narrowed (1.63, 1.42 1.38 eV). Panchromatic absorbance was observed for the
compounds comprising the cyanopyridone and oxoindenemalononitrile acceptors.
Crown Copyright Ó 2012 Published by Elsevier Ltd. All rights reserved.
1. Introduction
emitting diodes (OLEDs),7 electrochromics8 and organic field-effect
transistors (OFETs)9 make them ideal to use as
p-bridges in
There is continuing interest in the design of organic compounds
for semi-conducting or electro-optic materials.1 Some of the
products based on these organic semiconductors could include, but
are not limited to, cheap and stable solar cells, organic light-
emitting diodes and flexible displays.2 Such applications place
high demands on both the physical and electronic properties of the
material, including solution processability, appropriate energy
levels and charge mobility. Polymeric entities and small molecules
have been successfully used in these applications.
donoreacceptor small molecules. Small molecules can offer advan-
tages over polymeric materials in terms of ease of synthesis, in-
troduction of structural variation, purification, less batch to batch
variations and less end-group contamination.10
A large number of DeA molecules have been extensively in-
vestigated based on various molecular designs with recent work fo-
cused on OPV applications.11 Among them, the symmetrical AeDeA
or DeAeD or simply DepeA represents the most successful molec-
ular architectures. These designs allow intramolecular charge transfer
transition that broadens the absorption spectrum and narrows the
optical band gap. It isnot surprisingthat there is suchgrowing interest
in the development of small panchromatic organic molecules based
on various DeA combinations with the goal of obtaining broader ab-
sorption over the whole visible region, appropriate energy levels and
solution processability suitable for OPV devices.10 Because of these
Prior to their investigation as active components in solar cells,
non-linear semiconductor materials were primarily used to generate
electro-optical modulation in photorefractive applications for optical
data storage or optical switching.3 Such materials are typically based
on the ‘pushepull’ modular design that combine electron-donor and
electron-acceptor units connected via a
phene and oligothiophene based
p
-conjugated linker.4 Thio-
-conjugated materials have been
p
stated reasons, we are also interested in exploring the DepeA design
extensively studied due to their use as organic semiconductors.5
Their performance and electronic properties in donoreacceptor
(DeA) copolymers, for organic photovoltaics (OPVs),6 organic light-
principle in small organic molecules for their use in organic elec-
tronics. To achieve panchromatic dyes we combined highly electron
rich donor groups with highly electron deficient electron acceptor
groups in a De
facile synthesis and characterisation of three new molecules T1, T2
and T3 shown in Fig. 1. These molecules are based on De eA archi-
tecture, where we have chosen triphenylamine as common donor
peA architecture. In this study, we report the design,
p
* Corresponding author. Tel.: þ61 3 9545 2507; e-mail address: richard.evans@
csiro.au (R.A. Evans).
0040-4020/$ e see front matter Crown Copyright Ó 2012 Published by Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tet.2012.09.009

A. Gupta et al. / Tetrahedron 68 (2012) 9440e9447
9441
Fig. 1. Chemical structures of the new organic materials investigated in this work.
fragment, an alkylated tetrathiophene as a common conjugated
p-
knowledge, this is the first time that this type of examination has
bridge and different acceptors as a source of tunability of photo-
physical properties based on their different acceptor strengths. The
dicyanovinylidene acceptor in T1 has been commonly used in DeA
OPV devices12 and bithiophene chromophores.13 In order to optimise
the optoelectronic properties and possibly also solution processability
of small molecules, we examined two additional acceptors. We in-
corporated the cyanopyridone fragment as an acceptor group in T2
and oxoindenemalononitrile fragment as an acceptor group in T3 (see
Fig. 1). When compared with the commonly used dicyanovinylidene
group as an acceptor, the aromatizable cyanopyridione is an acceptor
of greater strength and oxoindenemalononitrile in-turn is an acceptor
of better strength than cyanopyridone.14
An additional potential advantage of these acceptor units is their
amenability to incorporate alkyl chains, which potentially improve
solubility of the materials if required. For example, we have chosen
the ethylhexyl group for tuning the solubility of T2. Similar sub-
stitutions could also be placed on the phenyl ring of the acceptor
fragment in T3, however the alkyl chains on the tetrathiophene
provided solubility for this study. These additional alkyl sub-
stitutions on the acceptor moieties not only enhance the solubility,
but also help for excellent film formation without crystallisation.
Intramolecular charge transfer (ICT) would be enhanced with the
use of an acceptor fragment with increased acceptor strength,
which in-turn would provide for greater light harvesting and per-
haps better positioning of energy levels. Thus we should observe
a bathochromic shift in absorbance while moving from T1 to T3. All
the materials, T1, T2 and T3, were synthesised via the Knoevenagel
condensation of aldehyde (3) with various active methylene groups
and their chemical structures were confirmed by 1H NMR spec-
troscopy and mass spectrometry. The molecules prepared in this
work were found to be highly soluble in a variety of conventional
organic solvents, such as dichloromethane, chloroform, chloro-
benzene and toluene, which is a feature that is desirable for the
fabrication of solution-processed organic semiconductor devices.
been reported with oligothiophenes.
2. Results and discussion
2.1. Optical properties
The optical properties of T1, T2 and T3 were investigated by
measuring their ultraviolet-visible (UVevis) absorption spectra in
chloroform solution (Fig. 2a) and in pristine spin-cast films. The
longest wavelength absorption maximum (lmax) exhibited by T1 was
at 518 nm with absorbance onset at 670 nm. T2 and T3 exhibited
their lmax at 587 nm and 619 nm with onsets at 770 nm and 798 nm,
respectively. Both the absorption maximum and extinction co-
efficient increased with increasing acceptor strength such that
lmax(T1)<lmax(T2)<lmax(T3) and
3 (T1)<
3
(T2)< (T3). This enhanced
3
profile allows a larger amount of the solar spectrum to be absorbed.
The bathochromic shift in absorbance maxima is clearly shown in
the solutions (Fig. 3). This is consistent with the idea that greater ICT
can be achieved with increasing acceptor strength. The use of the
tetrathiophene has also resulted in an additional absorption maxima
around 410e430 nm for all three compounds in solution and thin
films (Figs. 2a and 4, respectively). This provided greater absorption
of the entire visible light spectrum as compared to similar com-
pounds with fewer thiophenes. It is illustrated in Fig. 2b, which
shows the comparison between T2 that has four thiophenes (this
work) and T412 (see inset in Fig. 2b) that has only two thiophenes.
Absorption spectra of T1, T2 and T3 in pristine films (equimolar
solutions of T1, T2 and T3 spun at 4000 rpm) were measured on
glass substrates (Fig. 4). Generally, for a given compound, the thin
film absorption spectrum will exhibit a bathochromic shift as
compared to its solution spectrum. This was observed for T1, T2 and
T3 in this study. The acceptor in T2 gave a lengthening of lmax by
70 nm as compared to T1 and further red shift of 36 nm beyond T2
was observed in case of T3. This strong red shift is attributed to the
This study builds upon our previous search for De
p
eA organic
more extended p-conjugation within the molecular backbone of T2
molecules12 by extending the conjugation length and increasing
the range of acceptor strengths. It is the study of the effect of dif-
ferent acceptors while all other molecular features are held con-
stant that is the primary aim of this work. To the best of our
and T3, together with the increased acceptor strength. This type of
control of altering the absorption profile through the use of an
acceptor with greater strength could be expected to lead to en-
hanced light harvesting. The performance of both T2 and T3 in the

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A. Gupta et al. / Tetrahedron 68 (2012) 9440e9447
1.2
1
T1 F
T2 F
T3 F
0.8
0.6
0.4
0.2
0
300 400 500 600 700 800 900
Wavelength (nm)
Fig. 4. Normalised UVevis absorption spectra of pristine films (F) of T1, T2 and T3
spin-casted from their chloroform solutions. Equimolar solutions of T1, T2 and T3 were
spun at 4000 rpm to give film thicknesses in the range 35e40 nm.
Table 1
Photo-physical properties of T1, T2 and T3
Dye
Absorption
(solution) labs
onset (nm)
Extinction
Absorption
(film) labs /
onset (nm)
Ebandgap/eVc
a
b
/
coefficient
(
3
/(Mꢁ1 cmꢁ1
)
T1
T2
T3
518/670
587/770
619/798
49,123
62,498
74,096
546/760
616/868
652/900
1.63
1.42
1.38
a
UVevis absorption in chloroform solution.
Longest wavelength of UVevis absorption maxima for spin-cast films from
b
chloroform solutions.
c
Optical band gap, calculated using the absorbance onset values from the UVevis
absorption spectra of thin films.
Fig. 2. a. UVevis absorption spectra of T1, T2 and T3 in chloroform solution. b. Com-
parative UVevis absorption spectra of T2 and T4 in chloroform solution.
further indicated the effect on lowest unoccupied molecular orbital
(LUMO) energy levels as we change the acceptors. Highest occupied
molecular orbital (HOMO) densities were evenly distributed on the
molecular backbone in all the materials, whereas LUMO densities
were localised mainly on the acceptor parts (Fig. 5). The stronger
the acceptor, the more tightly the orbital densities were held by
that acceptor fragment. This examination of the HOMO and LUMO
of these dyes indicates that the HOMOeLUMO excitation moves the
electron distribution from the phenyl amino unit (donor fragment)
to the corresponding acceptor functionality.
Experimentally, the HOMO energies of all the compounds were
estimated using photoelectron spectroscopy in air (PESA) and the
LUMO energies were calculated by adding the band gap de-
termined by the onset of thin film UVevis absorption to the
HOMO values. These PESA measurements were performed on thin
solid films to measure work functions corresponding to their
HOMO levels. The LUMO energy level of T2 was reduced by
0.11 eV in comparison to T1. Its HOMO level was further raised by
0.1 eV. Overall the band gap was reduced by 0.21 eV with the
insertion of cyanopyridone acceptor in T2. HOMO and LUMO en-
ergy levels of T3 were further affected, but the band gap of T3 was
decreased as a result of the different acceptor with better acceptor
strength, which possessed the higher extinction coefficient and
also lower band gap. Hence we were able to control the electro-
chemical properties and achieved low band gap materials with the
insertion of different acceptors. Our experimental findings fol-
lowed the theoretical calculations trend, which indicated that as
the acceptor strength increases, band gap decreases. Fig. 6 shows
the alignment of various energy levels measured on spin-casted
thin films of T1, T2 and T3.
Fig. 3. Equimolar (0.01 mM) chloroform solutions of T1 (left), T2 (centre) and T3
(right), showing increasing bathochromic shift of lmax with increasing acceptor
strength in the order T3>T2>T1.
neat film state could be described as panchromatic with their ab-
sorptions extending from UV, through the entire visible spectrum
and tailing into the near infra-red region (300e800 nm). The op-
toelectronic properties are summarised in Table 1.
2.2. Electrochemical properties
To further understand the photo-physical properties, we per-
formed density functional theory (DFT) calculations on all the
materials using the Gaussian 09 suite of programs15 and B3LYP/6-
311þG(d,p)//B3LYP/6-31G(d) level of theory. DFT calculations
Cyclic-voltammetry experiments showed the presence of two
reversible oxidation potentials in all the materials, thus empha-
sising their suitability as donor components in organic semi-
conductor devices. But unlike absorption maxima, extinction

A. Gupta et al. / Tetrahedron 68 (2012) 9440e9447
9443
Fig. 5. HOMO and LUMO orbital densities as evaluated from DFT calculations.
Fig. 6. Energy level diagram depicting HOMO and LUMO levels of T1, T2 and T3, where HOMO levels of the dyes were measured using PESA of thin solid films on glass and LUMO
levels were calculated from the optical band gaps and HOMO levels (ELUMO¼EbandgapþEHOMO).

9444
A. Gupta et al. / Tetrahedron 68 (2012) 9440e9447
coefficient and band gap, there did not appear to be a systematic
relation between the onset of oxidation potentials and increasing
acceptor strengths. The respective cyclic-voltammograms (CVs) are
represented in Fig. 7.
microwave reactor. 2-(3-Oxo-2,3-dihydro-1H-inden-1-ylidene)
malononitrile was synthesised as per literature procedure.16
4.1.2. Spectroscopic measurements. Unless otherwise specified, all
1H and 13C NMR spectra were recorded using a Bruker AV400
spectrometer at 400 MHz and 100.6 MHz, respectively, or
a Bruker AV200 spectrometer at 200 MHz and 50 MHz, re-
6
T1
spectively. Chemical shifts (d) are measured in parts per million.
4
2
T2
T3
Thin Layer Chromatography (TLC) was performed using 0.25 mm
thick plates precoated with Merck Kieselgel 60 F254 silica gel, and
visualised using UV light (254 nm and 365 nm). Analytical HPLC
analyses were performed using a Waters 2695 alliance system
and Waters 2996 photodiode array detector scanning
190e700 nm using an Alltima HP C18 column at 30 ꢂC with 90%
acetonitrile and 10% tetrahydrofuran as mobile phase. Waters
Empower 2 data management system was used for data pro-
cessing. Melting points were measured using a Gallenkamp
MPD350 digital melting point apparatus and are uncorrected.
Electron impact (EI) mass spectra were carried out on a Ther-
moQuest MAT95XP mass spectrometer using ionisation energy of
70 eV and employing PerFluoroKerosene (PFK) as a reference
sample. Electrospray (ES) mass spectra were carried out on
a Thermo scientific Q-Exactive FTMS. UVevis absorption spectra
were recorded using a Hewlett Packard HP 8453 diode array
spectrometer. Fluorescence spectra were measured using a Per-
kineElmer LS50B fluorimeter.
0
-2
-4
-0.8
-0.4
0
Potential Vs /F+ in DCM (V)
0.4
0.8
F
C
C
Fig. 7. CVs of T1, T2 and T3 in dichloromethane solution containing 0.1 M Bu4NPF6; run
at a sweep rate of 50 mV/s, showing two reversible oxidation processes.
2.3. Charge mobility measurements
Considering these materials are potential candidates for OFETs
and OPVs, we examined the field effect mobility, me,h, of charge
carriers in devices fabricated using T1, T2 and T3. In this study,
bottom gate bottom contact OFETs were fabricated by depositing the
organic semiconductor from solution by spin casting onto a litho-
graphically patterned substrate (see Experimental for details). Of the
three compounds, T1 and T2 showed no transistor activity, thus in-
dicating poor charge transport. Only T3 showed charge mobility
with evidence of ability to conduct both electrons and holes. But the
charge mobility was low. It was found to be 3.19ꢀ10ꢁ6 cm2/V and
5.6ꢀ10ꢁ6 cm2/V for electrons and holes, respectively.
4.1.3. Electrochemical measurements. PESA measurements were
recorded using a Riken Keiki AC-2 PESA spectrometer with a power
setting of 5 nW and a power number of 0.5. Samples for PESA were
prepared on glass substrates. Electrochemical measurements were
carried out using a PowerLab ML160 potentiostat interfaced via
a PowerLab 4/20 controller to a PC running E-Chem For Windows
Ver. 1.5.2. The measurements were run in argon-purged dichloro-
methane with tetrabutylammonium hexafluorophosphate (0.1 M)
as the supporting electrolyte. The cyclic voltammograms were
recorded using a standard three electrode configuration with
a glassy carbon (2 mm diameter) working electrode, a platinum
wire counter electrode and a silver wire pseudo reference elec-
trode. The silver wire was cleaned in concentrated nitric acid and
then in concentrated hydrochloric acid to generate the Ag/AgCl
reference. Voltammograms were recorded with a sweep rate of
50e200 mV/s. All the potentials were referenced to E1/2 of the
ferrocene/ferrocenium redox couple.
3. Conclusions
In summary, we have shown that the variation of the acceptors
allows tuning of the electronic properties of a series of novel De
small organic molecules, T1, T2 and T3, that contained a common
triphenylamine donor and alkylated tetrathiophene -bridge. It was
peA
p
found that HOMOs were raised and LUMOs were lowered with the
increasing acceptor strengths resulting in both increased absorption
maxima and extinction coefficients in the order T1<T2<T3. The use
of an alkylated tetrathiophene unit not only provided excellent
solubility, but gave an additional absorption maximum at ca.
410e430 nm to help maximise absorption of the visible spectrum.
Greatest absorption of the visible spectrum was obtained when the
4.1.4. Device fabrication and characterisation of field effect tran-
sistors. A doped (N w3ꢀ1017 cmꢁ3) silicon (Si) wafer was used as
a substrate and as gate electrode. Discrete bottom contact OFETs
were fabricated on thermally grown smooth silicon dioxide (SiO2)
(230 nm). Interdigitated source and drain electrodes were photo-
lithographically patterned from a 50 nm sputtered gold layer.
tetrathiophene p-bridge was combined with the strongest acceptor,
as in dye T3. Panchromatic absorbance was observed for both the
dyes T2 and T3 with film absorbances beyond 800 nm observed. Our
results on the effect of acceptor type on the electronic properties of
oligothiophene derived donoreacceptor molecules can inform the
design of future molecules for organic electronics.
Channel length, L, of the devices was 20 mm and the channel width,
W, was 2 mm. The SiO2 layer was first cleaned with acetone, then
with 2-propanol and finally treated with UV ozone. Device fabri-
cation was completed by spin coating the organic semiconductor
layers at 1500 rpm in the glove box from solutions in chloroform.
The active layers consist of pristine compounds. Without any fur-
ther treatment, completed devices were transferred (under air-free
conditions) to another glove box fitted to probe the devices using
a probe station. Electrical measurements were carried out using
a Keithley 2612 dual channel SMU. The reproducibility of the OFET
preparation procedure was high with a confidence interval for the
extracted mobility of ꢃ15% of the values, which have been pre-
sented. The carrier mobility values presented in this work were
generally averaged from measurements done on at least four de-
vices on the same substrate.
4. Experimental section
4.1. General methods
4.1.1. Materials. All the reagents and chemicals used, unless other-
wise specified, were purchased from SigmaeAldrich Co. The solvents
used for reactions were obtained from Merck Speciality Chemicals
(Sydney, Australia) and were used after distillation. Knoevenagel
condensation reactions were completed in Biotage Initiator-60

A. Gupta et al. / Tetrahedron 68 (2012) 9440e9447
9445
4.2. Synthesis
temperature. The reaction mixture was heated to 80 ꢂC for 24 h
and reaction progress was monitored by thin layer chromatogra-
phy (hexane/ethyl acetate 9:1), which indicated the consumption
of starting aldehyde. The mixture was cooled to room tempera-
ture, filtered through Celite (2.00 g) and eluted with dichloro-
methane. The solvent was removed and the residue was purified
by flash chromatography eluted with 70% CHCl3/petroleum ether
(60e80 ꢂC) to afford the title compound 4 (800 mg, 70%) as an
orange solid; mp 186e190 ꢂC; Rf (70% CHCl3/petroleum ether
[60e80 ꢂC]) 0.29; IR (liquid film, cmꢁ1) 3063, 3034, 2952, 2925,
2855, 1663, 1590, 1491, 1456, 1327, 1279, 1241, 1153, 824; dH
(400 MHz, CDCl3) 9.81 (1H, s), 7.57 (1H, s), 7.44e7.42 (2H, m),
7.28e7.24 (4H, m), 7.20e7.19 (1H, m), 7.16e7.10 (6H, m), 7.06e7.01
(6H, m), 2.83e2.75 (4H, m), 1.74e1.62 (4H, m), 1.46e1.12 (12H, m),
0.94e0.84 (6H, m); dC (400 MHz, CDCl3) 182.7, 147.6, 142.4, 141.2,
141.1, 140.5, 140.4, 139.3, 139.2, 136.6, 135.8, 133.7, 129.5, 128.9,
128.5, 128.0, 126.5, 126.3, 125.5, 124.8, 124.7, 124.2, 123.7, 123.4,
31.8, 31.7, 30.6, 30.4, 29.8, 29.6, 29.4, 29.3, 22.8, 22.7, 14.3, 14.2;
HRMS (ESI): Mþ, found 769.2533. C47H47NO32S4 requires
769.2535.
All the new compounds, T1, T2 and T3 were synthesised and
characterised. The synthetic strategy is represented in Scheme 1.
4.2.1. Synthesis of 1-(2-ethylhexyl)-4-methyl-2.6-dioxo-1,2,5,6-
tetrahydropyridine-3-carbonitrile (1). 2-Ethylhexyl amine (38.7 g,
300 mmol) was added to a 500 mL round bottom flask and cooled
to 0 ꢂC. Ethyl cyanoacetate (28.3 g, 250 mmol) was added drop-
wise to this solution while maintaining the same reaction tem-
perature. The reaction was stirred overnight at room temperature.
On the following day a mixture of ethyl acetoacetate (39.5 g,
250 mmol) and pyridine (19.7 g, 250 mmol) was added drop-wise
and the reaction was heated to 100 ꢂC overnight. On the follow-
ing day the reaction was cooled and diluted with water before
acidifying with concentrated hydrochloric acid. The thick pre-
cipitate was collected by filtration. The precipitate was dried in air
before being recrystallised from ethyl acetate and ethanol to give 1
(32.0 g, 41%) as a pale pink solid. NMR revealed that the compound
existed in its enol form in DMSO; mp 211e214 ꢂC; IR (neat, cmꢁ1
)
2956, 2931, 2859, 2219, 1656, 1504, 1427, 1296, 1230; dH (400 MHz,
CD3SOCD3): 10.17 (1H, br s), 5.59 (1H, s), 3.83e3.73 (2H, m), 2.18
(3H, s), 1.77e1.68 (1H, m), 1.27e1.08 (8H, m), 0.82e0.77 (6H, m); dC
(400 MHz, CD3SOCD3): 161.4, 161.2, 158.7, 118.2, 92.6, 89.2, 44.9,
37.6, 30.6, 28.6, 24.0, 23.0, 21.1, 14.5,11.1; HRMS (EI): [MꢁH]þ, found
261.1599. C15H21N2O2 requires 261.1598.
4.2.5. Synthesis of 2-((5000-(4-(diphenylamino)phenyl)-3,3000-dihexyl-
[2,20:50,200:500,2000-quaterthiophen]-5-yl)methylene)malononitrile
(T1). Compound
4 (300 mg, 0.390 mmol) and malononitrile
(128 mg, 1.94 mmol) were placed in a microwave reactor tube
(10.0e20.0 mL) and dichloromethane (12 mL) was added to this
mixture followed by pyridine (0.154 g,1.95 mmol). The mixture was
heated at 75 ꢂC for 2 h in a microwave reactor (Biotage Initiator-60).
The progress of the reaction was followed by thin layer chroma-
tography (hexane/dichloromethane 8:2), which indicated the
consumption of the starting aldehyde. The dark blue reaction
mixture was cooled and methanol (20 mL) was added. The solid
that separated from the solution was collected by filtration and
washed with hot methanol (3ꢀ30 mL). Purification of the crude
product by flash chromatography (70% chloroform/petroleum ether
[60e80 ꢂC]) gave the title compound T1 (200 mg, 70%) as a deep
cherry solid; HPLC (10%THF/ACN) 94%; mp 122e124 ꢂC; Rf (70%
4.2.2. Synthesis of 3,3000-dihexyl-[2,20:50,200:500,2000-quaterthiophene]-5-
carbaldehyde (2). The substrate 3,3000-dihexyl-2,20:50,200:500,2000-qua-
terthiophene (420 mg, 0.840 mmol) was taken in a 100 mL RB flask
in ethylene dichloride (25 mL) and dimethylformamide (0.067 g,
0.920 mmol) was added to it. The resulting reaction mixture was
cooled to 0 ꢂC and POCl3 (0.230 mL, 2.52 mmol) was added drop-
wise. The reaction mixture was allowed to warm to room temper-
ature and heated at reflux overnight. The reaction mixture was
treated with a saturated solution of sodium acetate and the organic
layer was separated. The organic layer was washed with water fol-
lowed by brine, dried over anhydrous sodium sulfate and the solvent
evaporated in vacuo to give a crude dark yellow oil, which was pu-
rified by column chromatography on silica gel (10% ethyl acetate/
hexane) to give the title compound 2 (230 mg, 52%) as an orange oil.
NMR of compound 2 matches with the reported literature.;17 Rf (10%
ethyl acetate/hexane) 0.45; dH (400 MHz, CDCl3) 9.85 (1H, s), 7.62
(1H, s), 7.24e7.19 (4H, m), 7.07e7.06 (1H, m), 6.98e6.97 (1H, m),
2.87e2.79 (4H, m),1.76e1.64 (4H, m), 1.48e1.30 (12H, m), 0.94e0.89
(6H, m).
CHCl3/petroleum ether [60e80 ꢂC]) 0.32; IR (liquid film, cmꢁ1
)
3063, 2954, 2926, 2857, 2220, 1591, 1567, 1491, 1413, 1326, 1273,
1179, 823, 788, 753, 696; dH (200 MHz, CDCl3) 7.68 (1H, s), 7.54 (1H,
s), 7.47e7.43 (2H, m), 7.32e7.24 (6H, m), 7.21e7.18 (2H, m),
7.14e7.10 (2H, m), 7.10e7.01 (7H, m), 2.88e2.75 (4H, m), 1.81e1.58
(4H, m), 1.51e1.22 (12H, m), 1.01e0.77 (6H, m); dC (200 MHz, CDCl3)
149.7, 147.5, 147.4, 143.7, 142.4, 141.6, 141.1, 140.7, 140.5, 136.9, 135.2,
132.5, 132.1, 129.3, 128.6, 127.7, 126.3, 126.1, 125.3, 125.0, 124.6,
124.2, 123.5, 123.2, 114.4, 113.4, 31.6, 31.5, 30.4, 29.9, 29.6, 29.3, 29.2,
29.1, 22.6, 22.5, 14.1, 14.0; HRMS (ESI): Mþ, found 817.2664.
C50H47N332S4 requires 817.2653.
4.2.3. Synthesis of 3,3000-dihexyl-5000-iodo-[2,20:50,200:500,2000-quaterthio-
phene]-5-carbaldehyde (3). Compound 2 (230 mg, 0.440 mmol) was
taken in a 100 mL RB flask in acetic acid/chloroform 1:1 (v/v) solvent
mixture (20 mL) and N-iodosuccinimide was added at room tem-
perature. The resulting reaction mixture was stirred overnight in the
dark. The solid that formed in the reaction was filtered off and
washed with hexane to obtain the title compound 3 (250 mg, 87%) as
a brick red solid. NMR spectrum of compound 3 is consistent with
NMR spectrum of the bromo analogue;17 dH (400 MHz, CDCl3) 9.86
(1H, s), 7.62 (1H, s), 7.24e7.23 (1H, m), 7.19e7.18 (2H, m), 7.11 (1H, s),
7.01e7.00 (1H, m), 2.87e2.83 (2H, m), 2.77e2.73 (2H, m), 1.76e1.60
(4H, m), 1.47e1.29 (12H, m), 0.94e0.89 (6H, m).
4.2.6. Synthesis of 5-((5000-(4-(diphenylamino)phenyl)-3,3000-dihexyl-
[2,20:50,200:500,2000-quaterthiophene]-5-yl)methylene)-1-(2-ethylhexyl)-
4-methyl-2,6-dioxo-1,2,5,6-tetrahydropyridine-3-carbonitrile
(T2). Compound
4 (300 mg, 0.390 mmol) and compound 1
(180 mg, 0.700 mmol) were placed in a microwave reactor tube
(10.0e20.0 mL) and dichloromethane (12 mL) was added to this
mixture followed by pyridine (50.0 mg, 0.633 mmol). The mixture
was heated at 75 ꢂC for 3 h in a microwave reactor (Biotage
Initiator-60). The progress of reaction was followed by thin layer
chromatography (hexane/ethyl acetate 8:2), which indicated the
consumption of starting aldehyde. The dark blue reaction mixture
was cooled and methanol (20 mL) was added. The separated solid
was collected by filtration and washed with hot methanol
(3ꢀ30 mL). Purification of the crude product by flash chromatog-
raphy (100% chloroform) gave the title compound T2 (360 mg, 91%)
as shiny bluish-black solid; HPLC (10%THF/ACN): 94%; mp
153e158 ꢂC; Rf (100% CHCl3) 0.37; IR (liquid film, cmꢁ1) 3063, 2954,
4.2.4. Synthesis of 5000-(4-(diphenylamino)phenyl)-3,3000-dihexyl-
[2,20:50,200:500,2000-quaterthiophene]-5-carbaldehyde (4). Compound
3
(1.00 g, 1.50 mmol), 4-(diphenylamino)phenylboronic acid
(660 mg, 2.30 mmol), sodium phosphate dodecahydrate (700 mg,
1.80 mmol) and 10% Pd(C) (200 mg) were added together in
a 250 mL RB flask and isopropanol (100 mL) was added at room

9446
A. Gupta et al. / Tetrahedron 68 (2012) 9440e9447
Scheme 1. Synthesis of compounds T1, T2 and T3: (i) 0 ꢂC to room temperature (ii) pyridine, 100 ꢂC (iii) dimethylformamide, POCl3, ethylene dichloride, 80 ꢂC (iv) N-iodosucci-
nimide, acetic acid/chloroform 1:1, room temperature (v) Pd(C) (10%), Na2PO4$12H2O, isopropanol, 80 ꢂC (vi) chloroform, pyridine, microwave, 75 ꢂC (vii) dichloromethane, pyr-
idine, microwave, 75 ꢂC (viii) dichloromethane, pyridine, microwave, 75 ꢂC.

A. Gupta et al. / Tetrahedron 68 (2012) 9440e9447
9447
2928, 2859, 2215, 1686, 1638, 1592, 1542, 1492, 1407, 1383, 1295,
1177, 822, 782, 696; dH (400 MHz, CD2Cl2) 7.87 (1H, s), 7.65 (1H, s),
7.49e7.45 (3H, m), 7.33e7.27 (6H, m), 7.12e7.05 (10H, m),
3.95e3.92 (2H, m), 2.89 (2H, t, J 7.92 Hz), 2.83 (2H, t, J 7.92 Hz), 2.62
(3H, s), 1.90e1.82 (8H, m), 1.48e1.30 (17H, m), 0.94e0.91 (12H, m);
dC (200 MHz, CDCl3) 163.2, 160.9, 158.0, 149.4, 148.3, 147.5, 147.4,
143.7,142.3,141.1,140.5,140.5,136.9,135.4,134.8,133.7,129.3,128.7,
127.7,126.3,126.1,125.3, 124.9,124.5,124.3,123.5,123.2, 116.2,115.1,
103.5, 43.9, 37.4, 31.6, 31.6, 30.5, 30.4, 29.8, 29.7, 29.3, 29.2, 29.1,
28.4, 23.8, 23.1, 22.6, 22.5, 18.8, 14.1, 14.1, 14.0, 10.5; LRMS (EI):
[MþH]þ, found 1014.1; HRMS (ESI): Mþ, found 1013.4114.
C62H67N3O322S4 requires 1013.4111.
The use of the NCI National Facility supercomputers at the ANU is
gratefully acknowledged.
References and notes
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hexyl-[2,20:50,200:500,2000-quaterthiophene]-5-yl)methylene)-3-oxo-2,3-
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8.69 (1H, s), 8.62e8.58 (1H, m), 7.91e7.85 (1H, m), 7.71e7.67 (2H,
m), 7.56e7.54 (1H, m), 7.44e7.25 (7H, m), 7.15e6.95 (12H, m),
2.82e2.72 (4H, m), 1.81e1.59 (4H, m), 1.52e1.19 (12H, m),
1.04e0.76 (6H, m); dC (200 MHz, CDCl3) 188.2, 160.1, 148.5, 147.8,
147.4, 147.3, 142.2, 140.9, 140.6, 140.5, 139.9, 137.1, 136.9, 136.8,
135.3, 134.9, 134.3, 133.7, 129.3, 129.1, 128.7, 127.7, 126.3, 125.9,
125.3, 125.2, 124.8, 124.6, 124.2, 123.5, 123.4, 123.2, 122.0, 114.6,
69.1, 31.7, 31.5, 30.3, 29.7, 29.5, 29.3, 29.2, 22.6, 14.1, 14.0; LRMS
(ESI): Mþ, found 945.21; HRMS (ESI): Mþ, found 945.2910.
C59H51N3O32S4 requires 945.2911.
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Acknowledgements
This research was funded through the Flexible Electronics
Theme of the CSIRO Future Manufacturing Flagship and was also
supported by the Victorian Organic Solar Cell Consortium (Victo-
rian Department of Primary Industries, Sustainable Energy Re-
search and Development Grant, Victorian Department of Business
and Innovation, Victoria’s Science Agenda Grant, Victorian De-
partment of Primary Industries and the Australian Solar Institute).
A.B. thanks the CSIRO for support through the Julius Career Award.
€
A. D.; Farkas, O.; Foresman, J. B.; Ortiz, J. V.; Cioslowski, J.; Fox, D. J. Gaussian
09, Revision C.01; Gaussian: Wallingford CT, 2010.
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