End-capping of conjugated thiophene-benzene aromatic systems

Article abstract of DOI:10.1016/j.tet.2010.08.073

The synthesis of end-capped thieno[3,2-f:4,5-f′]bis[1]benzothiophene was achieved from thiophene and 2,5-thiophenedicarboxaldehyde. Specifically, hexyl and dodecyl end-capping groups conferred reversible redox behavior as evidenced by cyclic voltammetry with oxidation potentials of 0.73 V versus Fc/Fc⁺ couple. An extensive spectrophotometric analysis is reported.

Full text of DOI:10.1016/j.tet.2010.08.073

Tetrahedron 66 (2010) 8778e8784
Contents lists available at ScienceDirect
Tetrahedron
journal homepage: www.elsevier.com/locate/tet
End-capping of conjugated thiopheneebenzene aromatic systems
,
b
b
b
Brigitte Wex a , Fadi M. Jradi , Digambara Patra , Bilal R. Kaafarani
*
a Department of Natural Sciences, Lebanese American University, PO Box 36, Byblos, Lebanon
b Department of Chemistry, American University of Beirut, Beirut 1107-2020, Lebanon
a r t i c l e i n f o
a b s t r a c t
The synthesis of end-capped thieno[3,2-f:4,5-f0]bis[1]benzothiophene was achieved from thiophene and
2,5-thiophenedicarboxaldehyde. Specifically, hexyl and dodecyl end-capping groups conferred reversible
redox behavior as evidenced by cyclic voltammetry with oxidation potentials of 0.73 V versus Fc/Fcþ
couple. An extensive spectrophotometric analysis is reported.
Article history:
Received 21 April 2010
Received in revised form 6 August 2010
Accepted 31 August 2010
Available online 9 September 2010
Ó 2010 Elsevier Ltd. All rights reserved.
Keywords:
Thieno[3,2-f:4,5-f 0]bis[1]benzothiophene
End-capping
Reversible redox behavior
1. Introduction
irreversible oxidation and possible polymerization of the material
during device operation.
Conjugated polycyclic aromatic systems are excellent candi-
dates for optoelectronic device applications and specifically are an
important resource of semiconducting materials in the de-
velopment of organic field-effect transistors (OFET).1e5 Conjugated
organic materials are attractive targets because they can be syn-
thesized on large scale, synthetically tailored to the desired HOMO
and/or LUMO level and processed at low temperature by wet and
dry methods. These properties make them suitable for application
on flexible, plastic substrates, and would potentially lower the
production costs over traditional, silicon-based FETs.6 OFETs find
application in displays, RFID tags and other modern electronic
devices.7 The development criteria for plastic electronics in OFET
applications are a high material stability under operating condi-
tions and good signal-to-noise ratio, as well as low turn-on voltages
and high charge-carrier mobility. Previously, we reported the ef-
fective four-step synthesis of two regioisomers of thieno[f:f0]bis[1]
benzothiophene (TBBT)8,9 comprising ring systems of alternating
benzene and thiophene units. These materials have shown prom-
ising properties in organic field-effect transistor applications,10
with excellent stability in air up to 340 ꢀC and a good oneoff cur-
rent ratio of >105.11 The syn isomer, thieno[3,2-f:4,5-f0]bis[1]ben-
zothiophene TBBTs 1, exhibited a high charge-carrier mobility of up
to 0.12 cm2/V s; however, displayed a strong hysteresis in the re-
verse sweep as observed in the output characteristic during device
operation coupled with a high threshold-voltage.11 One possible
source of the observed hysteresis and threshold-voltage shift is
2. Results and discussions
To improve the material’s properties, we have end-capped parent
compound 1 at the susceptible alpha-positions12 of the terminal
thiophene rings, since we attribute the undesired behavior in device
configuration during device operation to polymerization of the ma-
terial 1 at the labile alpha carbons next to sulfur on the terminal
thiophene rings.11 The target compounds 1a and 1b are depicted
in Figure 1. The target compounds carry alkyl chains on both alpha-
positions of the terminal thiophene units. The alkyl groups are hexyl
(1a) and dodecyl (1b). The synthetic approach for the hexyl derivative
1a is outlined below and requires the inclusion of the alkyl groups into
the starting building blocks 2a. Compound 1b was synthesized in
a similar approach. Both compounds, 2,3-dibromo-5-hexylthiophene
(1a) and 2,3-dibromo-5-dodeylthiophene (1b) are new compounds.
The synthesis of 1a from 2,3-dibromo-5-hexylthiophene (2a) is
depicted in Scheme 1 and is based on the synthetic approach of the
R
R
S
S
S
1
R = H
1a R = C6H13
1b R = C12H25
* Corresponding author. Tel.: þ961 70 131710; e-mail address: brigitte.wex@lau.
edu.lb (B. Wex).
Fig. 1. Structure of parent 1 and target compounds 1a and 1b.
0040-4020/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2010.08.073

B. Wex et al. / Tetrahedron 66 (2010) 8778e8784
8779
1. nBuLi
2.
R
Br
Br
R
Br
Br
OHC
CHO
S
S
S
3. NaCNBH3/ZnI2
S
R
S
3a R=C6H13 (88%)
3b R=C12H25 (72%)
2a R=C6H13
2b R=C12H25
OHC
CHO
1. nBuLi
2. DMF
R
R
R
R
S
3
S
S
S
Amberlyst-15
S
S
3a R=C6H13
3b R=C12H25
4a R=C6H13 (34%)
4b R=C12H25 (42%)
1a R=C6H13 (71%)
1b R=C12H25 (38%)
Scheme 1. Synthesis of target compounds 1a and 1b.
parent molecule 1.8,9 Compound 2a underwent selective lithiume
halogen exchange on position followed by reaction with
341 ꢀC in aerobic atmosphere, therefore only the dodecyl groups
have increased the stability of the material toward thermal de-
composition.8 The DSC and powder XRD measurements of 1a and
1b showed the absence of mesophase behavior in 1a and 1b.
Except a slight alteration in the peak positions, the absorption
spectra of 1a look similar in both polar (ethanol) and nonpolar
(cyclcohexane) environment providing three different peaks at
around 265 nm, 282 nm, and 293 nm, respectively, and a small
hump at around 328 nm as shown in Figure 4. In comparison to the
parent thieno[3,2-f:4,5-f0]bis[1]benzothiophene 1, a concentration
dependence study of 1a did not show any major change in the
spectra except a linear increase in absorbance with concentration.
Absorption spectra of 1b also showed a similar trend in various
solvent environments indicating the increase in chain length from
hexyl to dodecyl did not result in any change in the absorption
spectra.
2
2,5-thiophenedicarboxaldehyde to yield a dicarbinol, which was
immediately reduced using sodium cyanoboronhydride and zinc
iodide to yield 3a. A double-lithiumehalogen exchange followed by
treatment with dry DMF furnished the dialdehyde 4a. A Bradsher
reaction13 catalyzed by Amberlyst-15 in benzene under Deane
Stark conditions yielded the target compound 1a in 15% overall
yield. Target compound 1b was synthesized in
a similar
fashion starting from 2,3-dibromo-5-dodecylthiophene (2b). Due
to the extended alkyl chains, i.e., dodecyl, the lithiation reactions
leading up to target compound 1b were carried out in pure THF
due to the low solubility of the intermediates in chilled, dry ether.
The target compounds were purified by column chromatography,
characterized and passed elemental analysis. 2,5-Thiophenedi-
carboxaldehyde is commercially available, whereas the 2,3-
dibromo-5-alkylthiophenes were synthesized in an effective
approach starting from thiophene (Scheme 2). Compounds 1a and
1b showed marked solubility of 3 mg/mL in chlorobenzene, a com-
mon solvent for the wet processing of materials via spin coating.
The fluorescence excitation spectra of 1a and 1b provided in
Figure 5 look similar to absorption spectra suggesting that both S0
(ground state singlet) and S1 (first excited state singlet) have similar
electronic state and vibrational levels. The fluorescence spectra of
1. nBuLi
1. LDA
2. H2O
Br
2. C6H13
3. NBS
I
Br
R
S
S
3. NBS
Br
R
S
5
6a R=C6H13 (43%)
6b R=C12H25 (59%)
2a R=C6H13 (64%)
2b R=C12H25 (29%)
Scheme 2. Synthesis of 2,3-dibromo-5-alkylthiophene (2a, 2b).
The oxidation potential of compounds 1a and 1b was de-
termined in dichlorobenzene using tetrabutylammonium tetra-
fluoroborate as electrolyte, platinum electrodes as working and
reference electrode, and ferrocene/ferrocenium (Fc/Fcþ) couple as
internal standard. The observed oxidation potential is 0.732 (0.728)
V versus Fc/Fcþ couple for 1a (1b). The substitution of the parent
compound 1 with alkyl chains has therefore resulted in a potential
change of w0.1 V. The oxidation reactions of both 1a and 1b are
reversible and no polymerization on the electrode was observed,
Figure 2. Thermogravimetric analysis in an aerobic atmosphere of
the compounds revealed a high thermal stability up to 326 ꢀC for 1a
and 367 ꢀC for 1b, observed at 5% decomposition, respectively
(Fig. 3). Parent compound 1 shows onset of decomposition at
these compounds in various solvent environments mirror the ex-
citation spectra, but as expected positioned in the shorter energy
(longer wavelength) ranges, Figure 6. Variation of chain length also
did not change appreciably the fluorescence spectra of 1a or 1b.
Estimation of the Stokes shift was done by taking the difference
between the maxima of the absorption and emission spectra.14
Absorption maxima, emission maxima, and Stokes shifts in vari-
ous polar and nonpolar solvent environments for 1a and 1b are
estimated15 and summarized in Table 1. Spectral shifts can be
generally interpreted in terms of the solvents’ identity by the Lip-
perteMataga equation, which shows the relationship between the
Stokes shift of the compound in each solvent and the change in the
dipole moment of the fluorescence moiety upon excitation, and on

8780
B. Wex et al. / Tetrahedron 66 (2010) 8778e8784
Fig. 2. Cyclic voltammogram of 1a (left) and 1b (right).
Fig. 3. TGA of 1a and 1b.
the other hand, the dependence of the energy of the dipole mo-
ment on the dielectric constant and refractive index of the solvents
being used.15e22 The LipperteMataga equation can be expressed as
follows:
ꢀ
ꢁ
2
3
ꢁ 1
n2 ꢁ 1
ð
mE
ꢁ
mGÞ2
vA ꢁ vF
¼
ꢁ
þ Constant
2n2 þ 1
a3
hc 2
3
þ 1
where vA and vF are the wavenumbers (cmꢁ1) of the absorbance
and fluorescence emission, respectively, which both determine the
Stokes shift, h is the Planck’s constant, c is the speed of light in
vacuum, a is the radius of the cavity in which the fluorophore re-
sides, mE and mG are the dipole moments in the excited and ground
states, respectively, 3 and n are the dielectric constant and the index
of refraction of the solvents, respectively.
Fig. 4. Absorption spectra of 1a and 1b in various solvents.
The LipperteMataga plot can be obtained by plotting the Stokes
shift versus the term in the brackets in the above equation, referred
to as the orientation polarizability (Df) of the solvent, which is the
result of both the mobility of the electrons in the solvent, and the
dipole moment of the solvent.
Stokes shift versus the orientation polarizability (Df) and ET30
values of the solvents used for 1a and 1b are shown in Figures 7 and
8, respectively. The Stokes shift varies with both orientation
polarizability (Df) and solvent polarity (ET30) in a rather systematic
linear fashion except for ET30 values of 1a giving the plots a scat-
tered appearance.
3
ꢁ 1
n2 ꢁ 1
D
f ¼
ꢁ
2n2 þ 1
2
3
þ 1
In the case of charge transfer in a molecule, the gross solvent
The fluorescence lifetimes, fluorescence quantum yield, and
radiative and nonradiative constants are also given in Table 2. The
polarity indicator scale such as ET30 is more applicable.23,24 Plots of

B. Wex et al. / Tetrahedron 66 (2010) 8778e8784
8781
Fig. 5. Fluorescence excitation spectra of 1a and 1b in various solvents.
Fig. 6. Fluorescence emission spectra of 1a and 1b in various solvents.
fluorescence lifetime studies for the 1a and 1b showed biexpo-
nential decays, one of them is a short one in picoseconds with
a higher population whereas the other one has a much longer
lifetime in the nanosecond time scale with a lower population. The
average fluorescence lifetime was calculated by taking the average
of shorter and longer components. There was no correlation of
average fluorescence lifetime with the polarity of the solvent for
these two compounds. The chain length also did not influence the
average fluorescence lifetime in a regular manner. The fluorescence
quantum yield was determined as stated in the Experimental sec-
tion. The fluorescence quantum yield was, in general, found to be
very poor and in most cases it was obtained as w0.01 for both 1a
and 1b, similar to the nonalkylated parent compound.25 The radi-
ative rate constant was calculated by the equation.15
Table 1
Absorption maxima, emission maxima, and Stokes shift 1a and 1b in various
solvents
(nm) Stokes shift (cmꢁ1
)
max
max
Compound Solvent
l
abs
(nm)
l
em
1a
Cyclohexane
Dichloromethane
Hexane
266
268
265
266
387
394
388
390
392
390
11,754
11,933
11,963
11,953
12,084
12,095
Tetrahydrofuran
Dimethylformamide 266
Ethanol
265
1b
Cyclohexane
Dichloromethane
Hexane
257
266
268
267
387
393
387
390
11,754
12,149
11,474
11,812
Tetrahydrofuran
kr
¼
=
ff sf
where kr is the radiative rate constant, ff is the fluorescence
quantum yield, and sf is the fluorescence lifetime, and the non-
radiative rate constant is evaluated as15
3. Experimental
3.1. General information
ꢂ
ꢃ
knr
¼
1=sf
ꢁ
kf
Chemicals and solvents were purchased from Acros and Aldrich.
Standard grade silica gel (60 A, 32e63
The calculated radiative rate constant is in the order of 106 sꢁ1
ꢀ
mm) and silica gel plates
compared to 107 sꢁ1 for the parent, nonalkylated molecule,25 while
(200 m) were purchased from Sorbent Technologies. Reactions
m
the nonradiative rate constants (Table 2) are in the order of 108 sꢁ1
,
that required anhydrous conditions were carried out under argon
in oven-dried glassware. A Bruker spectrometer was used to record
the NMR spectra. CDCl3 was the solvent for NMR and chemical
shifts relative to TMS at 0.00 ppm are reported in parts per million
similar to the parent, nonalkylated molecule.25
In summary, we report the preparation, electronic and spec-
troscopic characterization of two new materials for potential use in
organic field-effect transistor applications.
(ppm) on the
d scale. Elemental analyses were performed at

8782
B. Wex et al. / Tetrahedron 66 (2010) 8778e8784
Fig. 7. Stokes shift versus solvent polarity scale (orientation polarizability,
ET30 scale (bottom)) for 1a.
Df (top);
Fig. 8. Stokes shift versus solvent polarity scale (orientation polarizability,
ET30 scale (bottom)) for 1b.
Df (top);
Table 2
Fluorescence lifetime, average fluorescence lifetime, fluorescence quantum yield, radiative and nonradiative rate constants of 1a and 1b in various solvents at 25 ꢀC
Compound
Solvent
s1 (B1) in ns
s
2 (B2) in ns
c2
s
av in ns
f
kr/106 sꢁ1
knr/108 sꢁ1
1a
Cyclohexane
Dichloromethane
Hexane
Tetrahydrofuran
Dimethylformamide
Ethanol
0.474 (79%)
2.59 (13%)
0.758 (77%)
0.341 (79%)
0.371 (72%)
0.933 (81%)
3.58 (21%)
1.70
1.25
1.83
1.96
1.97
1.60
2.03
6.56
2.22
1.64
1.93
2.18
w0.01
w0.01
w0.01
w0.01
d
4.93
1.52
4.51
6.11
d
4.88
1.51
4.46
6.05
d
10.53 (87%)
3.68 (23%)
2.93 (21%)
3.48 (28%)
3.42 (19%)
d
d
d
1b
Cyclohexane
Dichloromethane
Hexane
1.00 (83%)
0.667 (83%)
0.612 (83%)
0.652 (86%)
5.44 (17%)
2.24 (17%)
2.07 (17%)
2.27 (14%)
1.94
1.63
1.94
1.82
3.22
1.45
1.34
1.46
w0.01
w0.02
w0.01
w0.01
3.11
13.76
7.46
3.07
6.74
7.38
6.78
Tetrahydrofuran
6.84
Atlantic Microlab Inc., Norcross, GA. Diisopropylamine was freshly
distilled from NaH before use.
NMR (CDCl3, 300 MHz):
(6H, m), 2.76 (2H, t, J¼7.8 Hz), 6.52 (1H, d, J¼3.6 Hz), 6.83 (1H, d,
J¼3.6 Hz).
d
0.88 (3H, t, J¼6.9 Hz), 1.62 (2H, m), 1.30
3.2. Synthesis
3.2.3. 4-Bromo-2-hexylthiophene. Synthesized according to
a
3.2.1. 2-Hexylthiophene. Synthesized according to a literature pro-
modified literature procedure.27 Diisopropylamine (16.88 mL,
120.14 mmol) was cooled to ꢁ78 ꢀC in a solution of THF (70 mL).
nBuLi (50 mL, 120.12 mmol) was dropwise added and the reaction
mixture stirred for 5 min. The thus generated LDA was transferred
via cannula to a solution of 2-bromo-5-hexylthiophene in THF
(100 mL) at ꢁ78 ꢀC. The reaction mixture was stirred for 30 min
and then water was added at the same temperature. After warming
to room temperature, THF was removed under vacuum and the
aqueous phase extracted with ether (2ꢂ200 mL). The organic layer
cedure.26 Yield: 70% (lit.26 96%) of a clear oil. 1H NMR (CDCl3,
300 MHz):
d
0.88 (3H, t, J¼4.8 Hz), 1.30 (6H, m), 1.67 (2H, m), 2.82
(2H, t, J¼7.2 Hz), 6.77 (1H, m), 6.90 (1H, dd, J¼5.2, 3.6 Hz), 7.09 (1H,
dd, J¼5.2, 1.2 Hz). 13C NMR (CDCl3, 75 MHz):
d 14.1, 22.6, 28.8, 29.9,
31.6, 31.8, 122.7, 123.9, 126.6, 145.9.
3.2.2. 2-Bromo-5-hexylthiophene 6a. Synthesized according to
a literature procedure.26 Yield: 61% (lit.26 80%) of a clear oil. 1H

B. Wex et al. / Tetrahedron 66 (2010) 8778e8784
8783
is then washed with brine (50 mL) and water (50 mL), before drying
NaCNBH3 (3.49 g, 55.7 mmol) were added and the reaction mixture
was stirred overnight. A color change from orange to pink to yellow
was observed. The reaction mixture was then filtered through
Celite, washed with aq satd NH4Cl, dried over MgSO4, and the
solvent evaporated. Column chromatography using hexanes yiel-
ded 3.80 g (88%) of a yellow oil that was used immediately in the
next step without further purification due to instability. 1H NMR
over MgSO4 (anhyd). Purification using vacuum distillation yielded
24.12 g (89%) (lit.27 64%). 1H NMR (CDCl3, 300 MHz):
d 0.89 (3H, t,
J¼6.9 Hz), 1.31 (6H, m), 1.64 (2H, m), 2.77 (2H, t, J¼7.7 Hz), 6.69
(1H; d, J¼1.5 Hz), 7.00 (1H, d, J¼1.5 Hz). 13C NMR (CDCl3, 75 MHz):
d
14.1, 22.6, 28.7, 30.0, 31.3, 31.5, 108.9, 120.0, 126.7, 147.2.
3.2.4. 2,3-Dibromo-5-hexylthiophene 2a. 4-Bromo-2-hexylthiophene
(24.10 g, 97.49 mmol) was dissolved in DMF and cooled to 0 ꢀC. A
solution of NBS (17.49 g, 98.27 mmol) in DMF was dropwise added
under darkness. The icebath was removed and the reaction mixture
stirred until GC showed complete conversion (7 h). Then, the re-
action mixture was poured on water containing few crystals of so-
dium thiosulfate and the aqueous layer extracted with CHCl3. The
organic layer was twice washed with water and dried over Na2SO4
(anhyd). After removal of the solvent, a dark oil remained, which
yielded 22.98 g (72%) (lit.29 93%) of a yellow oil after vacuum dis-
(CDCl3, 300 MHz):
m), 2.69 (4H, t, J¼7.8 Hz), 4.15 (4H, s), 6.59 (2H, t, J¼0.6 Hz), 6.68
(2H, s). 13C NMR (CDCl3, 75 MHz):
14.0, 22.5, 28.7, 29.8, 30.2, 31.1,
d
0.88 (6H, t, J¼6.9 Hz), 1.29 (12H, m), 1.60 (4H,
d
31.5, 107.9, 125.1, 126.5, 134.7, 140.7, 144.5.
3.2.10. 2,20-[2,5-Thiophenediylbis(methylene)]bis[5-hexylthiophene-
3-carboxaldehyde] 4a. nBuLi (4.36 mL, 10.0 mmol) was chilled
to ꢁ78 ꢀC in ether. 2,5-Bis(3-bromo-5-hexyl-2-thien-2-ylmethyl)
thiophene (2.75 g, 4.56 mmol) was added dropwise as a solution in
ether. Ten minutes after addition, degassed DMF (774 mL,
tillation. Bp 130e134 ꢀC/2 Torr. 1H NMR (CDCl3, 300 MHz):
d
0.89
10.0 mmol) was added and the reaction mixture stirred for 1 h. The
mixture was poured into ice-cold satd NH4Cl and extracted into
ether (200 mL). The combined organic layers were washed with
water before drying over MgSO4 (anhyd). Column chromatography
using a gradient of 0e10% ethyl acetate in hexanes yields a pink oil
(3H, t, J¼6.9 Hz), 1.30 (6H, m), 1.62 (2H, m), 2.71 (2H, t, J¼7.7 Hz),
6.60 (1H, t, J¼0.9 Hz). 13C NMR (CDCl3, 75 MHz):
d 14.0, 22.5, 28.6,
30.5, 31.0, 31.5, 107.4, 112.8, 126.9, 147.3. Anal. Calcd C, 36.83; H, 4.33.
Found: C, 37.22; H, 4.40.
(0.77 g, 34%). 1H NMR (CDCl3, 300 MHz):
d
0.88 (6H, t, J¼6.9 Hz),
1.29 (12H, m), 1.60 (4H, m), 2.72 (4H, t, J¼7.6 Hz), 4.57 (4H, s), 6.70
(2H, s), 7.05 (2H, s), 9.97 (2H, s). 13C NMR (CDCl3, 75 MHz):
14.0,
3.2.5. 2-Dodecylthiophene29. Synthesized in a similar fashion as
2-hexylthiophene. Bp 134 ꢀC/2 Torr (lit.28 178e182 ꢀC/13 Torr).
d
Yield: 64% (lit.29 80%). 1H NMR (CDCl3, 300 MHz):
d
0.88 (3H, t,
22.5, 28.5, 28.7, 29.8, 31.1, 31.5, 123.8, 125.6,136.6, 141.0,144.4, 152.1,
184.5. Anal. Calcd C, 67.16; H, 7.25. Found: C, 67.14; H, 7.01.
J¼6.9 Hz), 1.26 (18H, m), 1.67 (2H, m), 2.82 (2H, t, J¼8.1 Hz), 6.77
(1H, m), 6.90 (1H, dd, J¼5.1, 3.3 Hz), 7.09 (1H, dd, J¼5.1, 1.2 Hz). 13C
NMR (CDCl3, 75 MHz):
d
14.1, 22.7, 29.1, 29.4, 29.6, 29.7, 29.9, 31.8,
3.2.11. 2,7-Dihexylthieno[3,2-f:4,5-f0]bis[1]benzothiophene
1a.
31.9, 122.7, 123.9, 126.6, 145.9.
2,20-[2,5-Thiophenediylbis-(methylene)]bis[5-hexylthiophene-3-
carboxaldehyde] 4a was dissolved in dry benzene and Amberlyst-
15 (0.200 g) was added. The reaction mixture was stirred and
heated for 2 days under DeaneStark conditions. After cooling,
dichloromethane was added and the mixture was filtered through
a cotton plug. After washing with satd NH4Cl (aq), the mixture was
dried over MgSO4 (anhyd) and the solvent evaporated leaving
a reddish-brown solid. Purification by column chromatography (2%
ethyl acetate/hexanes) yielded a white solid 0.489 g (71%). Mp
224e225 ꢀC. IR nmax 3050 (w), 2956, 2927 (s), 2873, 2849, 1466 (s),
1462, 1437, 1421, 1259, 1223, 1130, 1048, 884 (s), 858, 835, 725,
3.2.6. 2-Bromo-5-dodecylthiophene 6b.29. Synthesized in similar
fashion as 2-bromo-5-hexylthiophene. Yield: 92% (lit.29 87%). Bp
162 ꢀC/2 Torr (lit.29 120e125 ꢀC/0.001 Torr). 1H NMR (CDCl3,
300 MHz):
d
0.88 (3H, t, J¼6.9 Hz), 1.26 (18H, m), 1.62 (2H, m), 2.73
(2H, t, J¼7.5 Hz), 6.53 (1H, m), 6.83 (1H, d, J¼3.6 Hz). 13C NMR
(CDCl3, 75 MHz):
d 14.1, 22.7, 29.0, 29.3, 29.5, 29.6, 30.3, 31.4, 31.9,
108.5, 124.3, 129.4, 147.7.
3.2.7. 4-Bromo-2-dodecylthiophene28. Synthesized in similar fash-
ion as 4-bromo-2-hexylthiophene. Product was used in next step
613 cmꢁ1. 1H NMR (CDCl3, 300 MHz):
(12H, m),1.78 (4H, m), 2.94 (4H, t, J¼7.5 Hz), 7.12 (2H, s), 8.14 (2H, s),
8.43 (2H, s). 13C NMR (CDCl3, 75 MHz):
14.0, 22.6, 28.8, 30.9, 31.0,
d
0.90 (6H, t, J¼7.2 Hz), 1.33
without further purification. 1H NMR (CDCl3, 300 MHz):
d 0.88 (3H,
t, J¼6.9 Hz), 1.26 (18H, m), 1.64 (2H, m), 2.77 (2H, m), 6.69 (1H, m),
d
7.00 (1H, d, J¼1.2 Hz). 13C NMR (CDCl3, 75 MHz):
d
14.1, 22.7, 29.0,
31.6, 114.5, 115.6, 120.2, 133.2, 135.5, 137.9, 138.9, 146.7. Anal. Calcd
C, 72.36; H, 6.94. Found: C, 72.34, H, 6.96.
29.3, 29.5, 29.6, 30.0, 31.33, 31.9, 108.9, 120.0, 126.7, 147.2. Anal.
Calcd C, 58.00; H, 8.21. Found: C, 58.14; H, 7.95.
3.2.12. 2,5-Bis(3-bromo-5-dodecyl-2-thien-2-ylmethyl)thiophene
3b. Synthesized in a similar procedure as the hexyl analog 3. Yield:
5.04 g(72%)ofayellowoil usedwithoutfurtherpurification.1HNMR
3.2.8. 2,3-Dibromo-5-dodecylthiophene28 2b. Synthesized in a sim-
ilar fashion as 2,3-dibromo-5-hexylthiophene. Yield (two steps):
29%. Bp 169e172 ꢀC/0.02 Torr. 1H NMR (CDCl3, 300 MHz):
d
0.88
(CDCl3, 300 MHz):
d
0.88 (6H, t, J¼6.9 Hz),1.29 (36H, m),1.58 (4H, m),
(3H, t, J¼6.9 Hz), 1.26 (18H, m), 1.61 (2H, m), 2.71 (2H, m), 6.60 (1H,
2.69 (4H, t, J¼7.6 Hz), 4.15 (4H, s), 6.59 (2H, s), 6.68 (2H, s). 13C NMR
t, J¼0.9 Hz). 13C NMR (CDCl3, 75 MHz):
d
14.1, 22.7, 29.0, 29.3, 29.5,
(CDCl3, 75 MHz): d 14.1, 22.7, 29.0, 29.3, 29.4, 29.5, 29.62, 29.63, 29.7,
29.6, 30.4, 31.0, 31.9, 107.4, 112.8, 126.9, 147.3. Anal. Calcd C, 46.84;
H, 6.93; Br, 38.95; S, 7.82. Found: C, 46.88; H, 6.21; Br, 38.97; S, 7.86.
29.8, 30.1, 31.1, 31.9, 107.9, 125.1, 126.5, 134.7, 140.7, 144.5.
3.2.13. 2,20-[2,5-Thiophenediylbis(methylene)]bis[5-dodecylth-
iophene-3-carboxaldehyde] 4b. Synthesized in a similar approach as
hexyl analog 4. Due to the lowsolubility in the chilled ether, THF was
used as solvent for the lithiation. Yield: 0.888 g (42%) of a pink oil.1H
3.2.9. 2,5-Bis(3-bromo-5-hexyl-2-thien-2-ylmethyl)thiophene
3a. 2,3-Dibromo-5-hexylthiophene (4.65 g, 14.3 mmol) was dis-
solved in 5:2 ether/THF and purged with argon. At ꢁ78 ꢀC, nBuLi
(17.12 mmol) was dropwise added and stirred for 10 min. Then,
a solution of 2,5-thiophenedicarboxaldehyde (1.00 g, 7.13 mmol) in
THF, chilled to ꢁ78 ꢀC, was added and the reaction mixture allowed
to warm to room temperature. After addition of water, the aqueous
layer was extracted with ether and the combined organic layers
were washed with satd NH4Cl, and water and dried over MgSO4
(anhyd). Evaporation of the solvents yielded a yellow liquid, which
was dissolved in 1,2-dichloroethane. ZnI2 (3.43 g, 10.7 mmol) and
NMR (CDCl3, 300 MHz):
(4H, m), 2.71 (4H, t, J¼7.2 Hz), 4.56 (4H, s), 6.70 (2H, s), 7.05 (2H, s),
9.97 (2H, s). 13C NMR (CDCl3, 75 MHz):
14.1, 22.7, 28.5, 29.0, 29.27,
d
0.88 (6H, t, J¼6.9 Hz), 1.33 (36H, m), 1.60
d
29.3, 29.5, 29.6, 29.8, 31.1, 31.9,123.9,125.6,136.6,141.0,144.4,152.1,
184.4. Anal. Calcd C, 71.80; H, 9.04. Found: C, 71.59; H, 9.11.
3.2.14. 2,7-Didodecylthieno[3,2-f:4,5-f0]bis[1]benzothiophene
(1b). Synthesized in a similar procedure as the hexyl analog 1a. Mp

8784
B. Wex et al. / Tetrahedron 66 (2010) 8778e8784
174e175 ꢀC. Yield: 108 mg (38%). IR nmax 3054 (w), 2955, 2914 (s),
2873, 2847, 1466, 1455, 1436, 884 (s), 859, 840, 721, 614 cmꢁ1 1H
NMR (CDCl3, 300 MHz):
0.88 (6H, t, J¼6.6 Hz), 1.26 (36H, m), 1.78
of the American University of Beirut, and the Lebanese National
Council for Scientific Research (CNRS). The authors are grateful for
this support.
.
d
(4H, m), 2.93 (4H, t, J¼7.2 Hz), 7.11 (2H, s), 8.14 (2H, s), 8.43 (2H, s).
13C NMR (CDCl3, 75 MHz): 14.1, 22.7, 29.1, 29.35, 29.37, 29.5, 29.6,
29.7, 31.0, 31.9, 114.6, 115.6, 120.1, 133.1, 135.4, 137.8, 138.8, 146.7.
Anal. Calcd C, 75.89; H, 8.92. Found: C, 75.71; H, 8.90.
Supplementary data
Supplementary data associated with this article can be found in
the online version at doi:10.1016/j.tet.2010.08.073.
3.3. Spectroscopic measurement
3.3.1. Absorption spectroscopy. A JASCO V-570 UVeVISeNIR Spec-
trophotometer was used for acquisition of absorption spectra;
a Jobin-Yvon-Horiba Fluorolog III spectrofluorimeter was used for
fluorescence measurements with a 100 W Xenon lamp as excita-
tion source, and a R-928 detector (V¼950 V).
References and notes
1. Thin Film Transistors; Kagan, C. R., Andry, P., Eds.; Marcel Dekker: New York, NY,
2003.
2. Hains, A. W.; Ramanan, C.; Irwin, M. D.; Liu, J.; Wasielewski, M. R.; Marks, T. J.
ACS Appl. Mater. Interfaces 2010, 2, 175e185.
3. Helgesen, M.; Krebs, F. C. Macromolecules 2010, 43, 1253e1260.
4. Shinamura, S.; Miyazaki, E.; Takimiya, K. J. Org. Chem. 2010, 75, 1228e1234.
5. Gao, P.; Cho, D.; Yang, X.; Enkelmann, V.; Baumgarten, M.; Mullen, K. Chem.d
Eur. J. 2010, 16, 5119e5128 and references therein.
6. Horowitz, G. J. Mater. Res. 2004, 19, 1946e1962.
7. Singh, T. B.; Sariciftci, N. S. Annu. Rev. Mater. Res. 2006, 36, 199e230.
8. Wex, B.; Kaafarani, B. R.; Kirschbaum, K.; Neckers, D. C. J. Org. Chem. 2005, 70,
4502e4505.
9. Wex, B.; Kaafarani, B. R.; Neckers, D. C. J. Org. Chem. 2004, 69, 2197e2199.
10. Coropceanu, V.; Kwon, O.; Wex, B.; Kaafarani, B. R.; Gruhn, N. E.; Durivage, J. C.;
Neckers, D. C.; Brédas, J. L. Chem.dEur. J. 2006, 12, 2073e2080.
11. Wex, B.; Kaafarani, B. R.; Schroeder, R.; Majewski, L. A.; Burckel, P.; Grell, M.;
Neckers, D. C. J. Mater. Chem. 2006, 16, 1121e1124.
3.3.2. Fluorescence lifetime measurement. A Jobin-Yvon-Horiba
Fluorolog III spectrofluorimeter with a pulsed diode laser of exci-
tation wavelength 282 nm served to determine the fluorescence
lifetime, whereas the decay data was analyzed using Data Analysis
}
Software and decay fits with values of c
2 between 0.99 and 1.5 were
considered to be within the acceptable range.
3.3.3. Determination of the fluorescence quantum yield. The fluo-
rescence quantum yield was determined under dilute conditions
(w100 nM) to avoid self-quenching of fluorophores and inner fil-
ter effects. Anthracence in ethanol was used as a standard to de-
termine the fluorescence quantum yield of the unknown samples.
The excitation wavelength used for the reference and standard
sample was 280 nm for 1a and 330 nm for 1b. The fluorescence
quantum yield of 1a and 1b was determined using the following
equation:30
12. Handbook of Oligo- and Polythiophenes; Fichou, D., Ed.; Wiley-VCH: Weinheim,
1999.
13. Saraf, S. D.; Ahmad, I. J. Math. Sci. 1974, 1, 75e123.
14. Barrash-Shiftan, N.; Brauer, B.; Pines, E. J. Phys. Org. Chem. 1998, 11, 743e750.
15. Lakowicz, J. R. Principles of Fluorescence Spectroscopy; Kluwer Academic,
Plenum: New York, NY, 1999 and references therein.
16. Degheili, J. A.; Moustafa, R. M.; Patra, D.; Kaafarani, B. R. J. Phys. Chem. A 2009,
113, 1244e1249.
17. Kalyanasundaram, K.; Thomas, J. K. J. Am. Chem. Soc. 1977, 99, 2039e2044.
18. Lippert, E. Z. Elektrochem. Angew. P. 1957, 61, 962e975.
19. Mataga, N.; Kaifu, Y.; Koizumi, M. Bull. Chem. Soc. Jpn. 1956, 29, 465e470.
20. Moustafa, R. M.; Degheili, J. A.; Patra, D.; Kaafarani, B. R. J. Phys. Chem. A 2009,
113, 1235e1243.
n2unk
n2std
Funk Astd
Fstd Aunk
funk
¼
fstd
ꢂ
ꢂ
ꢂ
21. Shyamala, T.; Sankararaman, S.; Mishra, A. K. Chem. Phys. 2006, 330, 469e477.
22. Subuddhi, U.; Haldar, S.; Sankararaman, S.; Mishra, A. K. Photochem. Photobiol.
Sci. 2006, 5, 459e466.
23. Reichardt, C. Solvents and Solvent Effects in Organic Chemistry, 2nd ed.; VCH
Verlagsgesellschaft: Weinheim, 1988.
where ‘
f
unk’ and ‘fStd’ are the quantum yield of the sample of
interest and that of the reference sample, respectively. The quan-
tum yield of anthracene in ethanol was taken to be 0.31.22 ‘Funk’ and
‘Fstd’ are the integrated intensities of the emission spectrum of the
analyte and that of the reference sample, respectively. ‘A’ corre-
sponds to the optical density of the samples taken at the excitation
wavelength and finally ‘n’ is the refractive index of the solvents
used. In the above equation, standard (std) refers to anthracene in
ethanol, and unknown (unk) refers to 1a (1b) molecules.
24. Reichardt, C. Chem. Rev. 1994, 94, 2319e2358.
25. Wex, B.; Kaafarani, B. R.; Danilov, E. O.; Neckers, D. C. J. Phys. Chem. A 2006, 110,
13754e13758.
26. Xia, C.; Fan, X.; Locklin, J.; Advincula, R. C. Org. Lett. 2002, 4, 2067e2070.
27. Brusso, J. L.; Hirst, O. D.; Dadvand, A.; Ganesan, S.; Cicoira, F.; Robertson, C. M.;
Oakley, R. T.; Rosei, F.; Perepichka, D. F. Chem. Mater. 2008, 20, 2484e2494.
28. Miyazaki, E.; Kaku, A.; Mori, H.; Iwatani, M.; Takimiya, K. J. Mater. Chem. 2009,
19, 5913e5915; Miyazaki, E. Novel Porphyrazine Derivative and Intermediate
thereof, Method for Production of Novel Porphyrazine Derivative and In-
termediate thereof, and Use of Porphyrazine Derivative. World Patent WO
2008/108442 A1 application, September 12, 2008.
Acknowledgements
€
€
29. Bauerle, P.; Würthner, F.; Gotz, G.; Effenberger, F. Synthesis 1993, 1099e1103.
30. Parker, C. A. Photoluminescence of Solutions with Applications to Photochemistry
and Analytical Chemistry; Elsevier: Amsterdam, 1968.
This work was supported by the University Research Committee
of the Lebanese American University, the University Research Board