Thiophene-benzothiadiazole co-oligomers: Synthesis, optoelectronic properties, electrical characterization, and thin-film patterning

Article abstract of DOI:10.1002/adfm.200901424

Newly synthesized thiophene (T) and benzothiadiazole (B) co-oligomers of different size, alternation motifs, and alkyl substitution types are reported. Combined spectroscopic data, electrochemical analysis, and theoretical calculations show that the inser

Full text of DOI:10.1002/adfm.200901424

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Thiophene–Benzothiadiazole Co-Oligomers: Synthesis,
Optoelectronic Properties, Electrical Characterization,
and Thin-Film Patterning
By Manuela Melucci,* Laura Favaretto, Alberto Zanelli,
Massimiliano Cavallini, Alessandro Bongini, Piera Maccagnani, Paolo Ostoja,
Gwennaelle Derue, Roberto Lazzaroni, and Giovanna Barbarella
1. Introduction
Newly synthesized thiophene (T) and benzothiadiazole (B) co-oligomers of
Development of organic semiconductors
different size, alternation motifs, and alkyl substitution types are reported.
Combined spectroscopic data, electrochemical analysis, and theoretical
calculations show that the insertion of a single electron-deficient B unit into
the aromatic backbone strongly affects the LUMO energy level. The insertion
of additional B units has only a minor effect on the electronic properties. Cast
films of oligomers with two alternated B rings (B–T–B inner core) display
crystalline order. Bottom-contact FETs based on films cast on bare SiO2 show
hole-charge mobilities of 1 T 10ꢀ3–5 T 10ꢀ3 cm2 Vꢀ1sꢀ1 and Ion /Ioff ratios of
105–106. Solution-cast films of cyclohexyl-substituted compounds are
amorphous and do not show FET behavior. However, the lack of order
observed in these films can be overcome by nanorubbing and unconventional
wet lithography, which allow for fine control of structural order in thin
deposits.
with good thermal and chemical stability,
solution processibility, tailored morphol-
ogy, and high charge-carrier mobility is
essential for applications in organic electro-
nics and, more generally, in nanoscience.[1]
In this regard, design strategies allowing
for fine control of the HOMO–LUMO
energy levels in organic materials are a
crucial issue. Indeed, injection of holes and
electrons and absorption/emission proper-
ties depend on frontier-orbital energies and
corresponding energy gaps.[2] To this end,
alternation of electron-rich and electron-
deficient heterocycles in conjugated oligo-
mers and co-polymers to narrow the
bandgap and tune the HOMO–LUMO
energy levels has drawn much attention.
Otherkeyfactorsinoptimizingtheopticalandtransportproperties
of organic materials in thin films are the morphological
organization and the structural order at mesoscopic length
scales.[3]
[*] Dr. M. Melucci, L. Favaretto, Dr. A. Zanelli, Dr. G. Barbarella
`
Istituto per la Sintesi Organica e la Fotoreattivita (ISOF)
Benzothiadiazole (B) is an electron-deficient fused heterocycle
that, coupled to electron-rich partners, affords small bandgap
molecular and polymeric p-conjugated materials. Alternation of
thiophene (T) and B rings has been widely exploited for
applications in devices such as photovoltaic cells, light-emitting
diodes, and field-effect transistors. In fact, high ordering capability
in thin films and morphologies suitable for applications as active
layers in devices have been demonstrated.[4]
Herein, we report the synthesis of well-defined model BT
oligomers of different size and investigate the effect of the
number of B units on their optoelectronic properties by means
of cyclovoltammetry, theoretical calculations, and UV and
photoluminescence (PL) spectroscopy. The hole-transport
properties in the bottom-contact field-effect transistor (FET)
configuration were evaluated using solution-cast films as active
elements. Finally, the role of cyclohexyl substituents on both
electronic properties and morphology was investigated and
nanorubbing[5] and micro molding in capillaries (MIMIC)[6]
techniques were successfully applied to achieve control over
Consiglio Nazionale Ricerche
via P. Gobetti, 101, 40129 Bologna (Italy)
E-mail: mmelucci@isof.cnr.it
Dr. M. Cavallini
Istituto per i Materiali Nanostrutturati (ISMN)
Consiglio Nazionale Ricerche
via P. Gobetti 101, 40129 Bologna (Italy)
Prof. A. Bongini
`
Universita di Bologna
Dipartimento di Chimica ‘‘G. Ciamician’’
via F. Selmi, 2, 40122 Bologna (Italy)
Dr. P. Maccagnani, Dr. P. Ostoja
Istituto per la Microelettronica e i Microsistemi (IMM)
Consiglio Nazionale Ricerche
via P. Gobetti 101, 40129 Bologna (Italy)
Dr. G. Derue, Prof. R. Lazzaroni
´
Service de Chimie des Materiaux Nouveaux
Universite de Mons/Materia Nova
´
20, Place du Parc, B-7000 Mons (Belgium)
DOI: 10.1002/adfm.200901424
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Scheme 1. Synthetic route to TB pentamers and heptamers. i) Pd[AsPh3]4, toluene, 112 8C. Hex ¼ n-hexyl, Cy ¼ cyclohexyl. Y ¼ isolated yield.
molecular orientation and morphology in some of these
compounds.
decrease in solubility in common organic solvents was observed.
Compounds 8 and 13, having bulky substituents in the beta
position of the external thiophene rings, displayed much greater
solubility than their analogues with terminal alkyl chains.
2. Results and Discussion
2.2. Optical Properties and Cyclovoltammetry Data
2.1. Synthesis
UV–vis absorption and emission spectra of pentamers 3 and 6 and
heptamers 10 and 12 are shown in Figure 1a, and the maximum
absorption and emission wavelengths of all compounds are
summarized in Table1. TTBTT (6)and TTTBTTT (3) show thefirst
maximum at 360 nm and 380 nm, respectively, and at 327 nm and
358 nm for TBTBTand TTBTBTT, respectively. The second band is
located in the region 510–540 nm for all compounds. The spectra
show a redshift of the maximum absorption and emission
wavelength on going from pentamers to heptamers, indepen-
dently of the type of B–T alternation motif.
The synthetic route to T–B co-oligomers is outlined in Scheme 1.
TheStillecouplingreactionwasemployedtobuilduptheoligomer
backbone. Here, stannyl-derivatives 2, 5, and 7 were prepared
under conventional metallation conditions (BuLi/ Bu3SnCl, THF,
ꢀ78 8C).
Then, pentamers 3, 6, and 8 and heptamers 10, 12, and 13 were
synthesizedbyPd(0)-mediatedStillecouplingreactioninrefluxing
toluene, by using a catalytic amount (5 mol%) of in situ prepared
Pd[AsPh3]4 as the catalyst. Satisfactory yields were achieved in all
cases upon purification by chromatography and further crystal-
lization steps. On increasing the number of B units, a strong
Cyclohexyl-substituted oligomers show a blueshift when
compared to hexyl-terminated analogues (Table 1). This can be
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When comparing co-oligomers of the same size, only slight
differences can be found in the absorption features (e.g. TTBTTvs.
TBTBT or TTTBTTT vs. TTBTBTT). Moreover, in all cases,
insertion of B units results in a marked red shift of lmax with
respect to unsubstituted all-thiophene oligomers, as shown,
for example, by comparison of quinquethiophene TTTTT
(lmax ¼ 416 nm)[7] and co-oligomer TTBTT (lmax ¼ 510 nm).
This indicates that BT-based systems are characterized by
higher p-conjugation extent, in agreement with the greater
molecular rigidity and p-polarization already observed for
donor–acceptor–donor TBT-based systems.[8] Interestingly, the
observed redshift is higher than that promoted by insertion of a
central thienyl-S,S-dioxide electron deficient moiety (O) in
the backbone of unsubstituted oligothiophenes (TTOTT,
lmax ¼ 469 nm).[9] It is noteworthy that thienyl-S,S-dioxide inser-
tion is among the most effective strategies to affect the HOMO–
LUMO energy levels of oligothiophenes.[10] Table 1 also shows that
on increasing the co-oligomer size, a further redshift is observed,
in agreement with the trend observed for conventional oligothio-
phenes.[11] Here, the longer compounds TTTBTTT (12) and
TTBTBTT (10) show a red shift of lmax in the order of 80–90 nm
with respect to heptathiophene TTTTTTT (lmax ¼ 450 nm).[12]
Again, when compared to the insertion of thienyl-S,S-dioxide in
heptathiophene (TTTOTTT, lmax ¼ 505 nm),[13] the B system
appears much more effective in terms of redshifting lmax. On
going to photoluminescence spectra, 12 shows a greater Stokes
shift than 10 (139 vs. 116 nm) suggesting a higher degree of
planarity in the excited state for T–B–Tcompared to B–T–B inner
core based oligomers.
Cyclic voltammograms (CVs) of pentamers TTBTT, TBTBT, and
TyTyBTyTy are shown in Figure 1b. The CVof TTBTT (6, solid line
in Fig. 1b) shows one reduction and two oxidation quasi-reversible
waves. The reduction and oxidation peak potentials are located at
ꢀ1.26 V, 0.88 V, and 1.21 V, respectively. TBTBT (3, red line in
Fig. 1b) shows a first quasi-reversible oxidation wave with the
maximum at a potential just 0.03 V more positive than that for
the corresponding oxidation of 6, but the second oxidation wave
appears irreversible withthe maximumabout 0.17 Vmore positive
than that of6. On theotherhand, TyTyBTyTy (8, blue linein Fig. 1b)
shows only one oxidation wave with the peak potential 0.29–0.26 V
more positive than that of 6 and 3, as expected, because of the
distortioneffectofthebulkycyclohexylgroupswhichdecreasesthe
overall conjugation extent.
Figure 1. a) Normalized absorption and emission spectra of compounds
3, 6, 10, and 12 (green, black, red, and blue lines, respectively) in CH2Cl2.
b) CVs on Pt electrode in CH2Cl2 0.1 mol Lꢀ1 (C4H9)4NClO4 at 0.2 V sꢀ1
and room temperature of TTBTT (6,
), TBTBT (3, ) and TyTyBTyTy
(8, ) 1.5 mmol ꢁ Lꢀ1. Inset: peak current versus the square root of the scan
rate.
ascribed to the distortion effects of the bulky cyclohexyl groups,
which reduce the molecule overall planarity and the p-electron
conjugation. However, the Stokes shift values are comparable to
those of hexyl-terminated systems, indicating that cyclohexyl
substituents do not prevent planarization in the excited state.
In the cathodic region, the presence of two B groups induces the
risingofa secondreversible reductionwave withthepeakpotential
Table 1. Absorption and emission wavelengths and CV data of compounds 3, 6, 8, 10, 12, and 13.
Compound
lmax [nm] [a]
lPL [nm]
Eg [eV] [b]
E0 [V] [c]
E0 [V] [c]
E0red [V] [c]
i.p. [eV] [d]
e.a. [eV] [d]
ox1
ox2
TBTBT, 3
520
510
540
533
480
519
624
656
656
672
621
657
2.08
2.07
1.96
1.95
2.21
2.03
0.86
0.83
0.68
0.77
1.04
0.93
1.37
1.16
ꢀ1.21 [f]
ꢀ1.21
ꢀ1.13
ꢀ1.16
ꢀ1.24
ꢀ1.18
ꢀ5.54
ꢀ5.51
ꢀ5.36
ꢀ5.45
ꢀ5.72
ꢀ5.61
ꢀ3.47
ꢀ3.47
ꢀ3.55
ꢀ3.52
ꢀ3.44
ꢀ3.50
TTBTT, 6
TTBTBTT, 10
TTTBTTT, 12
TyTyBTyTy, 8
TyTyTBTTyTy, 13
1.06 [e]
1.00 [e]
– [g]
1.10
[a] In CH2Cl2. [b] Energy-gap evaluated as Eg ¼ 1240 lꢀ1onset. [c] E0 is the average value between the peak potential and the related reverse one referred to
SCE. [d] i.p. ¼ e(4.68 þ E0ox1) and e.a. ¼ e(4.68 þ E0red) [15]. [e] peak potential, the process is irreversible, [f] TBTBT shows a second quasi-reversible
reduction wave at ꢀ1.35 V. [g] Non-detectable.
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0.15 V more negative than the first one, suggesting that the radical
anions are localized on the B groups that interact only because of
electrostatic repulsion. In contrast with oxidation, the reduction
wave of TyTyBTyTy, 8 (ꢀ1.29 V) approaches the potentials
of both TTBTT (6) and TBTBT (3), confirming that the outer
thiophene rings do not affect the energy of the LUMO orbital,
which is mostly localized on the B units (see theoretical
calculations in Fig. 2a).
The insets of Figure1b show the peak current density versus the
square root of the scan rate. All the peaks corresponding to first
oxidation and reduction fit to straight lines, showing that the redox
processes are driven by the diffusion in solution. The trends for
the second oxidation peaks are similar but the correlation is not as
good. The slopes of those straight lines are comparable, except
that of TyTyBTyTy (8) oxidation which is higher, even with respect
to that of the reduction. This suggests the contribution of two
simultaneous monoelectronic oxidations, rather than a higher
diffusion coefficient that mismatches the bigger hindrance of
TyTyBTyTy. CVs of heptamers 10, 12, and 13 showed the same
trend observed for the corresponding pentamers and are reported
in the Supporting Information (Fig. S8). Data in Table 1 shows that
the reduction potentials of TTBTBTT (10) and TTTBTTT (12) are
0.08–0.05 V less negative than those of TBTBT (3) and TTBTT (6),
in agreement with the increase of the chain length. On the other
hand, the two oxidation waves appear irreversible[14] and partially
overlapped. The oxidation potentials result less positive than
those of the shorter co-oligomers with a decrease related to the
inverse number of adjacent thiophenes. Indeed, the decrease from
single thiophene terminal to bithiophene is 0.08 V but that from
bithiophene to terthiophene is only 0.03 V. The steric hindrance
of the cyclohexane group undoes this shift in TyTyTBTTyTy, 13.
Insummary, theCVvaluesinTable1showthattheinsertionofa
B ring leads to a marked increase in electron affinity with respect to
a conventional oligothiophene of the same size. For instance,
replacing a Tring with a B ring in a end-capped quinquethiophene
taken as reference system (TTTTT)[16] shifts the reduction about
0.9 V toward less negative potentials (Ered ¼ ꢀ2.13 V for TTTTTvs.
ꢀ1.21 for 3 and 6).[16] However, the first oxidation shifts only of
about 0.05 V toward less positive potentials, reduce the overall
energygapofTTBTT(6)of0.9 eVwithrespecttothemodelTTTTT
(Eox ¼ 0.92 V[16]). Finally, comparison of B with the electron-
deficient thienyl-S,S-dioxide unit (O), which shifts the reduction of
TTOTT[9] of about 1V with respect to TTTTT,[16] indicates that B is
lessefficient than O in increasing theelectron affinity of thiophene
oligomers. However, contrary to thienyl-S,S-dioxide, B does not
increase the oxidation potential, leading to a comparable
electrochemical energy gap for pentamers TTBTT and TTOTT[9]
(2.14 vs. 2.21 eV). To gain some insight into the electronic
properties of BT based co-oligomers, the electronic densities of
the frontier molecular orbitals and their energy values were
determined by semiempirical calculations. Trimers BTB and
TBT and all-thiophene oligomers of comparable size were also
considered as reference compounds. Figure 2 is representative of
all compounds and compares the electron densities of pentamers
TTBTT (6) and TBTBT (3) with those of TTTTT (electron densities
of heptamers are reported in Fig. S9 of the Supporting
Information). In the figure, the calculated HOMO and LUMO
energy values are also reported. It can be seen that in all cases the
HOMO orbitals are homogeneously distributed all over the
Figure 2. a) Calculated electronic densities of the frontier molecular
orbitals of pentamers TTBTT, TBTBT and quinquethiophene TTTTT.
b) HOMO–LUMO energy values calculated by semiempirical calculations
for all compounds. Values for thiophene oligomers of comparable size are
given for comparison.
molecule. In contrast, the LUMO orbitals in BTco-oligomers show
a higher density on the benzothiadiazole group(s), in agreement
with what has been observed upon the insertion of thienyl-S,S-
dioxide units.[10,13,17] The calculations also show that the insertion
ofasingleBunithasonlyaminoreffectontheHOMOenergylevel
of thiophene oligomers, while it strongly shifts the LUMO energy
levels toward less negative values, that is, closer to that of the
benzothiadiazole ring (ꢀ1.66 eV). Increasing the length of the
oligomer backbone by adding one or two terminal thiophene rings
to trimers TBTandBTB(Fig. 2b) leads to substantial changes in the
HOMO energy but only minor changes in the LUMO energy,
consistent withthe localization of theLUMO orbital on theBunits.
Inagreementwithelectrochemicaldata,theinsertionofasecondB
unit leads to a further stabilization of the LUMO, by about
ꢀ0.14 eV. This effect isasstrong as thatof the addition of an extra T
unit on the HOMO energy (e.g., TTBTT HOMO ¼ ꢀ6.54 eV;
TTTBTTT HOMO ¼ ꢀ6.40 eV).
2.3. Thin-Film Morphology, Electrical Characterization, and
Patterning
Thin films of all compounds were prepared by drop casting 20 mL
of a 1 g Lꢀ1 (CHCl3 ortoluene)solution onglass. The samples were
investigated by atomic-force microscopy (AFM) in intermittent
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Figure 4. Optical micrographs of nanorubbed thin film of 13 deposited on
glass. a) Image in bright field and b) corresponding image between crossed
polars. The crossed polars are oriented as shown by the arrows. The inset
shows a detail of about 10 ꢂ 10 mm2 rubbed zone.
showed charge mobility of mFETh ¼ 5 ꢂ 10ꢀ3 cm2
V
ꢀ1sꢀ1 and
1 ꢂ 10ꢀ3 cm2 Vꢀ1sꢀ1, calculated from the saturation regime, and
high Ion/Ioff ratios of 106 and 105, respectively. A current–voltage
plotand asemilogarithmic plot ID versusVGS forcompound 10 are
shown in Figure 4c and d while the corresponding curves for
compound 3 are reported in the Supporting Information
(Fig. S10).
Figure 3. Morphology and electrical performance in FET configuration of
cast film (from toluene) on bare SiO2 of compound 10. a) Optical
microscopy image with cross polarizers showing a polycrystalline film
and b) corresponding AFM image. c) Current–voltage plot for a bottom-
contact FET device at VG ¼ 0 V, ꢀ10 V, ꢀ20 V, ꢀ30 V, VT ¼ ꢀ5 V. d) Semi-
logarithmic plot of ID versus VGS at VDS ¼ ꢀ10 V.
The electrical properties of compounds 6 (TTBTT)[4b,c] and 12
(TTTBTTT)[4b] with only one B unit, evaluated under the same
experimental conditions, have already been reported.
Interestingly, pentamer 6 (TTBTT), showed charge mobility
mFETh ¼ 4 ꢂ 10ꢀ4 cm2
V
ꢀ1sꢀ1 (Ion/Ioff ¼ 103), while no FET
behaviour was observed for drop-cast films of heptamer 12.
These results indicate that increased B-unit number could
substantially improve the hole transport in BT co-oligomers. It
is likely that the enhanced rigidity[4a,8] and dipole-induced
intermolecular stacking[4f] promoted by the acceptor–donor–
acceptor alternated motif favour intermolecular overlap and
improve the charge-transport capabilities in oligomers containing
the B–T–B motif.
It is worth noting that enhanced performances can be expected
for our B/T co-oligomers upon tailored-device optimization.
Indeed, higher mobility values on self-assembled monolayer
(SAM)-modified SiO2 substrate in top contact FETs have been
reported for compound 6.[4c]
contact mode and by optical microscopy. Thin films of hexyl-
terminated oligomers showed birefringent structures (Fig. 3a)
independent of the number of B units and co-oligomer size.
Observation with crossed polar optical microscopy showed the
typical optically anisotropic material behavior: the background did
not transmit light, while the crystals appeared coloured under
crossed polars, with colours ranging from violet to yellow
depending on local thickness. By rotating the crystal orientation
to the polarised light, the crystals extinguish only in some portions
of the structure. This behavior suggests that each structure is
formed by several domains with different orientation. Hole-
charge-transport capabilities were investigated for such crystalline
materials in a bottom contact–FET configuration. The devices
realized by using drop-cast film on bare SiO2 (2 mg mLꢀ1, toluene,
room temperature) of compounds 3 (TBTBT) and 10 (TTBTBTT)
In contrast to the previously described BTco-oligomers, no FET
behaviour was found for cyclohexyl-substituted 8 and 13 under our
experimental conditions. For these compounds, amorphous films
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withnoparticular opticalsignaturewereobserved, thisbeinglikely
due to the bulky cyclohexyl groups, which could prevent self-
organization. However, local ordering in those amorphous films
was successfully promoted by nanorubbing. Nanorubbing con-
sists of applying a unidirectional constraint on a surface with the
tip of an AFM operated in contact mode. The high precision of the
positioning of the tip and the force exerted makes it a powerful tool
for controlled surface modification on the micro- or nanoscale.
Nanorubbing has been recently used to locally align liquid
crystals[5c] and conjugated polymers.[5a,b,d] In thin films of
cyclohexyl-substituted compounds 8 and 13, the optical properties
of the rubbed zones are clearly different, while the AFM images do
not reveal any significant morphological modification.
As shown in Figure 4, rubbed zones exhibit a clear contrast in
bright-field optical microscopy and appear colored when observed
with crossed polars. These zones show the typical behavior of
optical anisotropic materials: a complete extinction of the
nanorubbed areas is observed when the polarization direction is
either parallel or perpendicular to the rubbing direction, while the
highest brightness is observed when the polarization directions
are rotated 458 to the rubbing directions. This behavior suggests
that the surface orientation induced initially by nanorubbing at the
surface (in polymers, rubbing typically induces alignment over a
depth of 10–20 nm) propagates through the entire thickness of the
film. This result is particularly relevant because it can be obtained
only by rubbing; in fact the simple application of pressure (for
example by imprinting)[18] does not induce any relevant effect in
the optical properties of 8 and 13 thin films.
Figure 5. a) Scheme of MIMIC method [19]. b) Optical micrograph of
micrometric stripes of 8 fabricated by MIMIC on glass taken with crossed
polars oriented along the axes of the image.
Unconventional lithography was also successfully employed for
the patterning of compound 8. In particular, we used MIMIC to
obtain a micrometric pattern of 8 (therefore optically accessible)
with controlled size and position. In MIMIC, a polydimethylsilox-
ane mold is placed on to a flat surface to effectively form
microchannels (Fig. 5a1).[6,19]
effect on the LUMO energy level of the resulting co-oligomer.
Addition of further electron-accepting B units spaced by thienyl
rings has a negligible effect on both electron affinity and
morphology. Polycrystalline cast films were observed for hexyl-
terminated co-oligomers, independently of the molecular
structure and number of B units. However, initial electrical
measurements showed improved electrical performances for both
pentamers and heptamers with two B inner units. The insertion of
bulky b-cyclohexyl substitution greatly increases the solubility of
the longer oligomers but affects the overall degree of planarity,
hence the extent of p-conjugation. As a result, blueshifts of the
maximum absorption and emission in cast films are observed
compared to terminal-substituted co-oligomers. Solution-cast
films of cyclohexyl-substituted compounds are amorphous.
However, the lack of order in thin films of these compounds
can be overcome by nanorubbing and MIMIC techniques, to
obtain ordered structures. We believe that the results presented
herein will be useful to the design of BT-based materials with
tailored electronic properties and enhanced charge-transport
capabilities. Detailed electrical characterization (p–n type) of all
compounds is currently under way.
When the solution is poured at the open end of the stamp, the
liquid fills the microchannels by capillary action (Fig. 5a2). After
the complete evaporation of the solvent (24 hours in air at room
temperature) the stamp is removed and the sample characterized
byAFMandopticalmicroscopy. AFMstudiesofthestructuresof 8,
printed by MIMIC on glass, reveal the formation of micrometric
stripes of ꢃ2-mm width, coherent with the size of the stamp.
Optical images obtained by polarized microscopy (Fig. 5b) show
the typical behavior of optically anisotropic materials exhibiting
birefringence. In particular, the m-stripes appear homogeneously
colored; this indicates that their thickness is almost constant over
the entire stripe. The m-stripes extinguish in four positions at
intervals of 908. Thus, we can deduce that the confined deposition
by MIMIC has induced a coherent, long-range order along the
direction of the stripes.
Nanorubbing and MIMIC experiments on the highly soluble
cyclohexyl substituted co-oligomers 8 and 13 show that, despite the
intrinsic tendency to give amorphous cast films, good local
ordering at micrometer scale can be achieved by means of
appropriate patterning techniques.
4. Experimental
Synthesis: Bis-4,7-(50-hexyl-2,20-bithien-5yl)-2,1,3-benzothiadiazole TTBTT,
6, and bis-4,7-(500-hexyl-2:20, 50:200-terthien-5yl)-2,1,3-benzothiadiazole
TTTBTTT, 12, were synthesized as described elsewhere [4b,c].
3. Conclusions
In summary, our data show that insertion of a benzothiadiazole
unit (B)as inner unit in theoligothiophene backbone hasa marked
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General Procedure for Stille Cross Coupling: Pd[AsPh3]4, generated in
situ, was prepared by a refluxing solution of triphenylarsine AsPh3 and
tris(benzylidene acetone) dipalladium(0)chloroform adduct Pd2dba3
(5 mol% of Pd) in dry toluene under nitrogen. Then a toluene solution
of bromo/dibromoderivative (1 equiv.) was added at reflux temperature
and finally the proper stannyl derivative (1.1/2.2 equiv.) added dropwise.
After 6 hours, the solution was cooled to room temperature and the crude
product was chromatographed on silica gel or alumina. Finally, crystal-
lization from toluene afforded the target compounds. In the case of
compound 1, the reaction was carried out under microwave assistance.
4-(5-Hexyl-thien-2-yl)-7-bromo-2,1,3-benzothiadiazole, 1: A microwave-
oven reactor was charged with Pd2dba3 (16 mg, 15 mmol), AsPh3 (37 mg,
0.12 mmol), and 16 mL of toluene, and the mixture was irradiated until it
turned black. Then, dibromo-2,1,3-benzothiadiazole (0.147 g, 0.0005 mol)
in 4 mL of toluene was introduced and the mixture was irradiated for
1 minute at 90 8C. Finally, 2-hexyl-5-tributhylstannylthiophene in 5 mL of
toluene was added dropwise and the resulting mixture was irradiated for an
additional 1 h. The solvent was evaporated and the crude chromatographed
7.04 (d, J ¼ 5.6 Hz, 2 H,), 2.60 (m, 4H), 1.50 (m, 40H). 13C NMR (100 MHz,
CDCl3, TMS): d ¼ 152.5, 148.9, 148.1, 139.1, 137.8, 137.0, 128.2, 127.4,
127.1, 126.5, 125.9, 125.5, 125.1, 124.3, 123.6, 38.3, 38.2, 34.4, 34.3, 26.6,
26.0.; lmax (CH2Cl2) ¼ 519 nm; MS (70 eV, EI): m/z 956 (M.þ). Anal. calcd
for C54H56N2S7: C, 67.74; H, 5.90. Found: C, 67.82; H, 6.01.
Ultraviolet and visible (UV–vis) absorption and emission spectra were
recorded by using a Perkin–Elmer Lambda 20 and a Perkin–Elmer LS 50 B
spectrometer, respectively (CH2Cl2 solution, conc. about 10ꢀ5 M).
Redox potentials were determined at room temperature in CH2Cl2
(Merck, Uvasol, distilled over P2O5 and stored under Ar pressure) with
0.1 mol Lꢀ1 (C4H9)4NClO4 (Fluka, puriss. crystallized from CH3OH and
vacuum dried). Cyclic voltammetry (CV) was performed at several scan
rates in the range 0.02–0.2 V sꢀ1 after Ar bubbling in a three compartment
glass electrochemical cell under Ar pressure, by using an AMEL
electrochemical system model 5000. Working electrode was semi-spherical
Pt (area 0.05 cm2), reference electrode was saturated calomel electrode
(SCE) and auxiliary electrode was a Pt wire. The potential of SCE with the
liquid junction to CH2Cl2 0.1mol Lꢀ1 (C4H9)4NClO4 resulted ꢀ0.475 V
versus ferrocene/ferricinium redox couple [20].
on Al2O3 using
a solution of Et2O (5%) and petroleum ether
(95%) ! Et2O. Compound 1 was obtained as an orange powder in 60%
Calculations were carried out by HyperChem integrated package [21].
Frontier orbitals were calculated by ZINDO/S calculations on PM3
optimized geometry for the metyl end-capped molecules taken as
simplified models.
The nanorubbing process was performed with a Nanoscope IIIa AFM
microscope working in contact mode. The tips are made of antimony-
doped silicon with a spring constant of 2 ꢄ 0.5 N/m [22]. Typically 10
consecutive rubbing scans were carried out with the AFM tip in order to
induce a sufficient deformation on the deposits. The loading force applied
by the tip on the film during nanorubbing is approximately 200 nN, as
calculated from the approach–retract curves.
yield (0.11 g).
m.p. 45 8C; 1H NMR (400 MHz, CDCl3 TMS): d ¼ 7.91 (d, J ¼ 3.6 Hz,
1H), 7.80 (d, J ¼ 7.6Hz, 1H), 7.61 (d, J ¼ 7.6 Hz, 1H), 6.86 (d, 1H,
J ¼ 3.6 Hz), 2.87 (m, 2H), 1.74 (m, 2H), 1.34 (m, 6H), 0.90 (m, 3H). 13C
NMR (100 MHz, CDCl3, TMS): d ¼ 153.8, 151.8, 148.6, 135.9, 132.3, 128.1,
127.5, 125.4, 125.1, 111.5, 31.6, 30.3, 29.7, 28.8, 22.6, 14.1; lmax
(CH2Cl2) ¼ 424 nm; MS (70 eV, EI): m/z 380-382 (M.þ).
Bis-2,5-[4-(5(-hexyl-thieno) 2,1,3-benzothiadiazole-7yl]-thiophene, TBTBT,
3: Deep purple powder in 65% of yield. m.p. 182 8C; 1H NMR (400 MHz,
CDCl3 þ CS2, TMS): d ¼ 8.21 (s, 2H), 7.97 (d, J ¼ 4Hz, 2H), 7.93 (d,
J ¼ 7.6 Hz, 2H), 7.80 (d, J ¼ 7.6 Hz, 2H), 6.86 (d, J ¼ 4 Hz, 2H), 2.90 (m,
4H), 1.76 (m, 4H), 1.38 (m, 12H), 0.93 (m, 6H). 13C NMR (100 MHz,
CDCl3 þ CS2, TMS): d ¼ 152.5, 147.9, 146.7, 140.3, 136.7, 128.3, 127.8,
126.4, 125.6, 125.3, 125.0, 124.8, 31.7, 31.6, 30.4, 28.9, 22.7, 14.2; lmax
(CH2Cl2) ¼ 520 nm; MS (70 eV, EI): m/z 683 (M.þ); Anal. calcd for
C36H36N4S5: C, 63.12; H, 5.30. Found: C, 63.21; H, 5.39.
A Nikon Eclipse 80i optical microscope was used for optical
measurements. The images were recorded with a digital color camera
¨
Nikon Coolpix 5400. Glass substrate were furnished by Knittel glaser and
were washed with Acetone spectroscopic grade (Aldrich) before use.
For FET fabrication, heavily doped h100i n–Si wafers were used as
substrates, and 150-nm thick layer of SiO2 (grown by thermal oxidation)
was used as the gate dielectric. Au (80 nm) was evaporated and
photolithographically defined to obtain the electrodes for gate, source
and drain. The source–drain electrodes have a circular structure, with a
channel width of 1880 mm and a channel length of 40 mm. Thin films were
prepared by casting deposited on top of the substrates. The electrical
measurements were performed using a computer controlled parametric
characterization system at room temperature under atmospheric pressure
conditions.
Bis-4,7-(3,3(-dicyclohexyl-2,2(-bithien-5yl)-2,1,3-benzothiadiazole,
TyTyBTyTy, 8: Chromatography performed on standard alumina using
pentane!3% Et2O, followed by crystallization from toluene, afforded
compound 8 as a red powder (48% yield). M.p. > 2008; 1H NMR
(400 MHz, CDCl3, TMS): d ¼ 8.13 ppm (s, 2 H), 7.85 ppm (s, 2H), 7.35 (d,
J ¼ 5.6 Hz, 2H), 7.05 (d, J ¼ 5.2 Hz, 2H), 2.64 (m, 4H), 1.50 (m, 40H). 13C
NMR (100 MHz, CDCl3, TMS: d ¼ 152.6, 149.0, 148.0, 139.2, 129.5, 127.3,
127.3, 126.5, 126.0, 125.7, 125.4, 38.3, 38.2, 34.5, 34.4, 26.6, 26.5, 26.0;
lmax (CH2Cl2) ¼ 480 nm. MS (70 eV, EI): m/z 791 (M. ꢀ1). Anal. calcd for
C46H52N2S5: C, 69.65; H, 6.61. Found: C, 69.57; H, 6.53.
4-(5’-hexyl-2,2(-bithien-5yl)-7-(bromo)-2,1,3-benzothiadiazole, 9: deep
orange powder, 40% yield, m.p. 101 8C; 1H NMR (400 MHz, CDCl3,
TMS): d ¼ 8.00 (d, J ¼ 4Hz, 1H), 7.82 (d, J ¼ 7.6Hz, 1H), 7.66 (d, J ¼ 7.6 Hz,
1H), 7.16 (d, J ¼ 4 Hz, 1H), 7.09 (d, J ¼ 3.2Hz, 1H), 6.72 (d, J ¼ 3.2 Hz,
1H), 2.81 (m, 2H), 1.70 (m, 2H), 1.34 (m, 6H), 0.90 (m, 3H). 13C NMR
(100 MHz, CDCl3 þ CS2, TMS): d ¼ 151.6, 150.1, 146.3, 139.9, 136.4, 134.3,
132.3, 128.8, 126.9,125.1, 125.0, 124.0, 123.8, 111.9, 31.6, 30.2, 29.7, 28.8,
22.6, 14.1; lmax (CH2Cl2) ¼ 459 nm; MS (70 eV, EI): m/z 462-464 (M.þ).
Bis-2,5-[4-(500-hexyl-2,2(-bithien-5(yl)-2,1,3-benzothiadiazole-7yl]-
thiophene, TTBTBTT, 10: Black solid, 80% yield. m.p. 230 8C; 1H NMR
(400 MHz, CDCl3 þ CS2, TMS): d ¼ 8.25 (s, 2H), 8.07 (d, J ¼ 4.0 Hz, 2H),
7.96 (d, J ¼ 7.6Hz, 2H), 7.85 (d, J ¼ 7.6 Hz, 2H), 7.15 (d, J ¼ 4.0 Hz, 2H),
7.07 (d, J ¼ 3.6 Hz, 2H), 6.70 (d, J ¼ 3.6 Hz, 2H) 2.83 (m, 4H), 1.73 (m,
4H), 1.38 (m, 12H), 0.93 (m, 6H); lmax (CH2Cl2) ¼ 540 nm; MS (70 eV,
EI): m/z 848 (M.þ). Anal. calcd for C44H40N4S7: C, 62.23; H, 4.75. Found: C,
62.32; H, 4.86.
Acknowledgements
This work was partially supported by projects FIRB RBNE03S7XZ_005
(SYNERGY), PRRIITT-Misura 4.A PROMINER and (MC) ESF-EURYI
DYMOT. Research in Mons was supported by the Science Policy Office
of the Belgian Federal Government (PAI 6/27) and FNRS-FRFC. Supporting
Information is available online from Wiley InterScience or from the author.
Received: June 30, 2009
Published online: January 7, 2010
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