Solution-phase microwave-assisted synthesis of unsubstituted and modified α-quinque- and sexithiophenes

Article abstract of DOI:10.1021/jo035723q

The facile synthesis of poorly soluble unsubstituted and modified α-quinque- and sexithiophenes under microwave irradiation in the liquid phase is described. The use of microwave irradiation allowed these compounds to be prepared in a few minutes and at high yields by means of the Suzuki cross-coupling reaction. Unsubstituted sexithiophene was obtained in 10 min via the one-pot borylation/Suzuki reaction, purified according to a very simple procedure, and isolated in 84% yield. The efficient synthesis of two new methylated quinque- and sexithiophenes displaying liquid crystalline properties is reported. A new microwave-assisted methodology for the conversion of aldehyde-terminated quinque- and sexithiophenes into the corresponding cyano derivatives is also described. The use of microwaves was extended to the Sonogashira coupling reaction and found to be very effective in the preparation of a quinquethiophene containing acetylenic spacers. The electronic and optical characterization of this compound is reported and discussed in relation to that of unsubstituted quinquethiophene.

Full text of DOI:10.1021/jo035723q

Solu tion -P h a se Micr ow a ve-Assisted Syn th esis of Un su bstitu ted
a n d Mod ified r-Qu in qu e- a n d Sexith iop h en es
M. Melucci,† G. Barbarella,*,† M. Zambianchi,† P. Di Pietro,† and A. Bongini‡
Consiglio Nazionale Ricerche (ISOF), Via Gobetti 101, 40129 Bologna, Italy, and Dipartimento di
Chimica “G. Ciamician”, Universita` di Bologna, Via Selmi 2, 40126 Bologna, Italy
barbarella@isof.cnr.it
Received November 24, 2003
The facile synthesis of poorly soluble unsubstituted and modified R-quinque- and sexithiophenes
under microwave irradiation in the liquid phase is described. The use of microwave irradiation
allowed these compounds to be prepared in a few minutes and at high yields by means of the Suzuki
cross-coupling reaction. Unsubstituted sexithiophene was obtained in 10 min via the one-pot
borylation/Suzuki reaction, purified according to a very simple procedure, and isolated in 84% yield.
The efficient synthesis of two new methylated quinque- and sexithiophenes displaying liquid
crystalline properties is reported. A new microwave-assisted methodology for the conversion of
aldehyde-terminated quinque- and sexithiophenes into the corresponding cyano derivatives is also
described. The use of microwaves was extended to the Sonogashira coupling reaction and found to
be very effective in the preparation of a quinquethiophene containing acetylenic spacers. The
electronic and optical characterization of this compound is reported and discussed in relation to
that of unsubstituted quinquethiophene.
Due to their relevant electrical and optical features,
oligothiophenes are among the most important and
widely studied organic molecular materials.1 Of main
interest are oligomers bearing five and six thiophene
rings that are important electroactive materials, capable
of self-assembly in highly ordered structures in the solid
state and giving high-performance organic thin film tran-
sistors (FET).2 Despite the simplicity of their molecular
structure, the preparation of quinque- and sexithiophenes
with a degree of purity suitable for electrical applications
requires many tedious purification steps to obtain them
free of byproducts and contaminants, i.e. to obtain
materials with reproducible electrical characteristics. To
improve the solubility and processability, modifications
of the structure by the grafting of substituents into the
aromatic backbone or by the introduction of spacers
between the thiophene rings are needed. However, the
self-alignment properties, morphology, and, in the end,
electrical performance of the oligomers are also modified
by the presence of substituents or spacers, in a way that
is difficult to predict. At present, understanding the
property/structure relationship relies on testing as many
modified molecular structures as possible, hence the
preparation of families of variously modified quinque-
and sexithiophenes. For this purpose, the development
of rapid and efficient synthetic methodologies to prepare
libraries of new materials, to test their optical and
electrical properties as a function of the molecular
structure, becomes a crucial task.
Currently, microwaves are applied in many fields of
organic synthesis to shorten reaction times, improve
reaction yields, and, more importantly, to provide envi-
ronmentally more sustainable methodologies.3 Studies
are underway to investigate the scalability of microwave-
assisted reactions from laboratory to industrial scale
without changing the laboratory-optimized reaction con-
ditions.4 Nevertheless, so far, only a few papers have been
published describing the improvements brought about by
the use of microwaves in the synthesis of thiophene-based
semiconductor materials.5 Recently, we have developed
a microwave-assisted, solvent-free methodology for the
synthesis of thiophene oligomers via the Suzuki reaction,
* Address correspondence to this author.
† Consiglio Nazionale Ricerche.
‡ Dipartimento di Chimica “G. Ciamician”.
(1) (a) Advanced Semiconductor and Organic Nano-Techniques,
Parts I, II and III; Morkoc, H., Ed.; Academic Press: New York, 2003.
(b) Fichou, D.; Ed. Hanbook of oligo and polythiophenes; Wiley-VCH:
New York, 1999. (c) Mu¨llen, K.; Wegner, G., Eds. Electronic Materials:
The Oligomer Approach; Wiley-VCH: New York, 1998. (d) Nalwa, H.
S., Ed. Handbook of Organic Conductive Molecules and Polymers; J .
Wiley & Sons: Chichester, UK, 1997.
(2) (a) Murphy, A. R.; Fre´chet, J . M. J .; Chang, P.; Lee, J .;
Subramanian, V. J . Am. Chem. Soc. 2004, 126, 1596-1597. (b)
Chesterfield, R. J .; Newman, C. R.; Pappenfus, T. M.; Ewbank, P. C.;
Haukaas, M. H.; Mann, K. R.; Miller, L. L.; Frisbie, C. D. Adv. Mater.
2003, 15, 1278-1282. (c) Facchetti, A.; Yoon, M. H.; Stern, C. L.; Katz,
H. E.; Marks, T. J . Angew. Chem., Int. Ed. 2003, 42, 3900-3903. (d)
Halik, M.; Klauk, H.; Zshieschang, U.; Schmid, G.; Ponomarenko, S.;
Kyrchmeyer, S. Adv. Mater. 2003, 15, 917-922. (e) Dimitrakopoulos,
C. D.; Malenfant, P. R. L. Adv. Mater. 2002, 14, 99-117. (f) Katz, H.
E.; Bao, Z. N.; Gilat, S. L. Acc. Chem. Res. 2001, 34, 359. (g) Garnier,
F.; Yassar, A.; Hajlaoui, R.; Horowitz, G.; Deloffre, F.; Servet, B.; Ries,
S.; Alnot, P. J . Am. Chem. Soc. 1993, 115, 8716-8721.
(3) (a) Larhed, M.; Moberg, C.; Hallberg, A. Acc. Chem. Res. 2002,
35, 717-727. (b) Caddick, S. Tetrahedron 1995, 38, 10403-10432.
(4) Stadler, A.; Yousefi, B. H.; Dallinger, D.; Walla, P.; Van der
Eycken, E.; Kaval, N.; Kappe, C. O. Org. Proc. Res. Dev. 2003, 7, 707-
716.
(5) (a) Melucci, M.; Barbarella, G.; Sotgiu, G. J . Org. Chem. 2002,
67, 8877-8884. (b) Sotgiu, G.; Zambianchi, M.; Barbarella, G.; Botta,
C. Tetrahedron 2002, 58, 2245-2251.
10.1021/jo035723q CCC: $27.50 © 2004 American Chemical Society
Published on Web 06/05/2004
J . Org. Chem. 2004, 69, 4821-4828
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Melucci et al.
SCHEME 1
gation/filtration. Afterward the product was purified by
washing first with warm water, then dissolving in warm
dioxane and precipitating with a small volume of water.
A very good yield was also obtained in the preparation
of unsubstituted sexithiophene 9 (T6, Scheme 2) by the
one-pot borylation/Suzuki reaction of 5-bromoterthiophene
7 with commercial bis(pinacolato)diboron 8. T6 was
obtained in 10 min and the crude product was easily
purified, using a procedure similar to that described for
T5, by first centrifuging the suspension formed in
toluene, then washing the solid residue with warm water
and then with CH2Cl2. Afterward, centrifugation and
washing with warm dioxane afforded 9 in 84% isolated
yield. Subsequent sublimation with a coldfinger gave a
high-quality product. In the Supporting Information,
micrographs of the nematic liquid-crystalline phase of 9
are reported.
using PdCl2dppf/KF as the catalytic system (Scheme 1).5
This methodology is very effective in the preparation of
soluble oligothiophenes, but it cannot be applied in the
case of insoluble thiophene oligomers, such as unsubsti-
tuted sexithiophene, since the targeted products cannot
be separated from the aluminum oxide used as the solid
support in the solvent-free reaction.
In this paper we describe a novel, effective, microwave-
assisted, solution-phase methodology for the preparation
of poorly soluble unsubstituted and functionalized quinque-
and sexithiophenes by means of Suzuki6 and Sonogash-
ira7 cross-coupling reactions. We also show that the use
of microwaves allows for the facile and very rapid
transformation of aldehyde functionalized quinque- and
sexithiophenes into the corresponding cyano derivatives
without the use of expensive palladium catalysts or
dangerous cyanide salts.
II. Syn th esis a n d Ch a r a cter iza tion of Mod ified
r-Qu in qu e- a n d Sexith iop h en es. The procedure de-
scribed above was applied to the synthesis of scarcely
soluble quinque- and sexithiophenes substituted at both
terminal positions, R as well as â to sulfur.
Scheme 3 shows the preparation of the derivatives
dimethylated at both â-terminal positions. Dimethylated
quinquethiophene 11 was prepared from the reaction of
diiodoterthiophene 5 and thienyl boronic ester 10 for 10
min under the action of microwaves. Compound 11 was
slightly more soluble than T5 and it was obtained in 90%
isolated yield after recrystallization from toluene.
Resu lts a n d Discu ssion
I. Syn th esis of Un su bstitu ted r-Qu in qu eth ioph en e
(T5) a n d r-Sexith iop h en e (T6). To standardize the
solution-phase, microwave-assisted, Suzuki reaction con-
ditions, we tested the effectiveness of the PdCl2dppf/KF
catalytic system used in the solvent-free preparation of
soluble oligothiophenes5 in the synthesis of unsubstituted
R-quinquethiophene (T5, 6) from diiodoterthiophene 5
and thienyl boronic acid 3, using a 1:1 (v/v) toluene/
methanol mixture as the solvent (Scheme 2). The proce-
dure depicted in Scheme 2 afforded 6 very rapidly (10
min) and in a higher yield (isolated yield 85%) than the
solvent-free procedure (isolated yield 74%) previously
described,5 owing to the easier workup of the crude
product. Indeed, methanol evaporation during microwave
irradiation led to the formation of a fine suspension of
T5 in toluene, which could easily be isolated by centrifu-
Scheme 3 also illustrates the preparation of dimethyl-
ated sexithiophene 13 from dibromoquatertiofene 128 and
thienyl boronic ester 10. In this case, due to the low
solubility of dibromoquaterthiophene 12, the yield de-
pended greatly on the solvent used in the cross-coupling
reaction. By using a 1:1 (v/v) toluene:methanol mixture,
13 was obtained in only 20% isolated yield. However, the
yield increased to nearly 70% with DMF as the solvent,
in which 12 is more soluble, and increasing the temper-
ature to about 100 °C.
Both 11 and 13 displayed liquid-crystalline properties.
As an example, Figure 1 shows the micrographs, taken
under polarized light during the first cooling cycle, of
SCHEME 2. Syn th esis of Qu in qu eth iop h en e (T5, 6) a n d Sexith iop h en e (T6, 9) fr om Com m er cia l
Bith iop h en e (1)a
a
Reagents and conditions: (i) NIS, DMF, overnight, -20 °C; (ii) PdCl2dppf, basic alumina/KF, µv 5 min, max temp 80 °C; (iii) NIS,
CH2Cl2/AcOH, overnight, rt; (iv) PdCl2dppf, toluene/methanol, µv 10 min, max temp 70 °C; (v) NBS, DMF.
4822 J . Org. Chem., Vol. 69, No. 14, 2004

Synthesis of R-Quinque- and Sexithiophenes
SCHEME 3. Syn th esis of Qu in qu e- (11) a n d Sexith iop h en es (13) Meth yla ted a t Both Ter m in a l â-P osition sa
a
Reagents and conditions: (I) PdCl2dppf, KF, toluene/methanol, µν 10 min, max temp 70°C; (ii) PdCl2dppf, KF, DMF, µν 20 min, max
temp 100 °C.
SCHEME 4. Syn th esis of Ald eh yd e-Ter m in a ted Qu in qu e- (15) a n d Sexith iop h en es (18) a n d Th eir
Tr a n sfor m a tion to th e Cor r esp on d in g Cya n o Der iva tivesa
a
Reagents and conditions: (i) PdCl2dppf, KF, toluene/methanol, µν 10 min, max temp 70°C; (ii) NIS, CH2Cl2/AcOH, 2 h, rt, 76%; (iii)
PdCl2dppf, toluene/methanol, µν 20 min, max temp 70 °C; (iv) NH2OH‚HCl, MeSO2Cl, DMF, µν 20 min, 100 °C.
second endothermic peak centered at 209.00 °C (∆H )
64.021 J /g). The subsequent cooling run showed an
exothermic peak at 202.15 °C (∆H ) -60.695 J /g)
followed by a second exothermic peak at 162.48 °C (∆H
) -29.786 J /g). The first heating run of sexithiophene
13 (mp 257 °C) showed the presence of a first endother-
mic peak at 205.67 °C (∆H ) 7.355 J /g) followed by a
second endothermic peak at 266.67 °C (∆H ) 65.517 J /g).
The subsequent cooling run showed an exothermic peak
at 260.63 °C (∆H ) -62.215 J /g) followed by a second
exothermic peak at 155.30 °C (∆H ) -8.747 J /g).
However, while pentamer 11 could undergo several
heating and cooling runs reversibly, the low-enthalpy
transition of hexamer 13 became undetectable after the
third heating-cooling run. In the Supporting Information
we show a micrograph taken under polarized light of the
nematic phase of this compound during the first cooling
cycle.
F IGURE 1. Optical micrographs under polarized light of
dimethylated quinquethiophene 11 showing the birefringent
Schlieren texture and maltese crosses typical of nematic
phases.
pentamer 11, showing the characteristic texture and
maltese crosses of nematic mesophases. The first heating
and cooling runs of the differential scanning calorimetry
plots of 11 and 13 are reported in the Supporting
Information. These plots display charateristics very
similar to those reported for R,ω-dihexylquaterthiophene.9
The first heating run of quinquethiophene 11 (mp 201
°C) showed the presence of a first endothermic peak
centered at 188.33 °C (∆H ) 30.435 J /g) followed by a
Scheme 4 shows the preparation of aldehyde-termi-
nated quinque- and sexithiophenes 15 and 18 starting
from the commercially available 5-formyl-2-thiophene
boronic acid 14 and their transformation into the corre-
sponding cyano-terminated derivatives (19 and 20) under
microwave action.
J . Org. Chem, Vol. 69, No. 14, 2004 4823

Melucci et al.
TABLE 1. Ma xim u m Absor p tion (λm a x, n m ) a n d
SCHEME 5. Son oga sh ir a ’s Cou p lin g of Dibr om o
Bi- a n d Ter th iop h en e w ith 2-Eth yn ylth iop h en ea
Em ission (λP L, n m ) Wa velen gth s,a Mola r Absor p tion
Coefficien ts (E, m ol-1 cm -1),a a n d Ca lcu la ted b
HOMO-LUMO En er gies (EHOMO, ELUMO, eV) of TATTAT
(24) a n d TATTTAT (25) Com p a r ed to Con ven tion a l
Qu a ter - (T4) a n d Qu in qu eth iop h en e (T5)
compd
ꢀ
λmax
λPL
EHOMO ELUMO
TATTAT (24)
T4
TATTTAT (25) 63 500 422 (396)
T5 55 200 416 (390)
41 500 395a (381)b 478
-6.91
-6.95
-6.82
-6.84
-0.65
-0.50
-0.70
-0.59
31 500 391 (372)
480
516
510
a
a
b
Reagents and conditions: (i) Pd(PPh3)2Cl2, PPh3, CuI, (i-
In CH2Cl2. ZINDO/S on optimized ab initio HF/6-31G*
Pr)2NH, µν 5 min, 50 °C (24), and µν 20 min, 100°C (25).
geometries.
The diformyl quinquethiophene 15 was obtained in 10
min from diiodoterthiophene 5 in the presence of 4 mol
% of PdCl2dppf as the catalyst. After the usual work up
(centrifugation, filtration, and precipitation with water
from warm dioxane), the product was obtained in 84%
isolated yield. The diformyl sexithiophene 18 was ob-
tained in 88% isolated yield by the one-pot borylation-
Suzuki coupling of 5′′-bromo[2,2′;5′,2′′]terthiophene-5-
carbaldehyde 17, using 5 mol % of PdCl2dppf and
irradiating with microwaves for 20 min.
The yields in 15 and 18 are comparable to those
obtained by Wei et al.10 using the Stille coupling11 (88%
and 87% for 15 and 18, respectively) but the reaction
times are markedly reduced (from 4-6 h to 10-20 min).
Moreover, the microwave-assisted reactions were carried
out in air, while the Stille coupling needs an argon
atmosphere.
We found that when the aldehydic quinque- and
sexithiophene 15 and 18 were reacted with hydroxyl-
amine hydrochloride (NH2OH‚HCl) and methanesulfonyl
chloride (MeSO2Cl) in DMF12 under microwave irradia-
tion, they were transformed into the corrisponding cyano
derivatives 19 and 20 in high yields. After purification
by repeated washing/centrifugation steps of the crude
materials with warm water and CH2Cl2, we obtained 19
and 20 in 80% and 60% isolated yields, respectively. Since
boronic acid 14 is a commercial product, the pattern of
Scheme 4 is a useful alternative to the synthesis of cyano
derivates based on Stille coupling or on the exchange
reaction between a halogen atom and the nitrile group
of copper cyanide.13
acetylenic spacer. After several attempts, the synthesis
of TATTAT was accomplished in 5 min simply by
microwave irradiation of a solution of dibromobithiophene
21 and 2-ethynylthiophene 23 in (i-Pr)2NH (isolated yield
68%). In the absence of microwaves, using THF as the
solvent, the same reaction afforded 24 only in a 26%
yield, the self-coupling of 23 being the main process
taking place. The same reaction conditions proved to be
effective also in the preparation of the targeted quin-
quethiophene TATTTAT, 25, which was obtained in a
71% isolated yield. Contrary to quinque- and sexithio-
phenes 11 and 13, the first heating run of the DSC plot
of 25 showed only a single endothermic peak at 203.00
°C (∆H ) 99.213 J /g), while the subsequent cooling run
displayed a single exothermic peak at T ) 145.60 °C (∆H
) -86.295 J /g). Moreover, optical microscopy showed no
birefringence under polarized light for this compound
either on heating or cooling cycles. It is worth noting that,
contrary to compound 25, its dibutyl-terminated deriva-
tive displays liquid-crystalline properties.15
To have an insight into the effects of the introduction
of the acetylenic spacers into the aromatic backbone of
quinquethiophene, we ran the absorption and photolu-
minescence spectra of compounds 24 and 25 and carried
out theoretical calculations on their geometry and elec-
tronic and optical properties. All data were then com-
pared to those obtained for unsubstituted quater- and
quinquethiophene (T4 and T5) in the same conditions.
Table 1 shows the maximum absorption and emission
wavelengths and molar absorption coefficients of 24 and
25 as well as T4 and T5, together with the λmax values
obtained from ZINDO/S calculations on optimized ab
initio HF/6-31G* geometries and the corresponding HOMO
and LUMO energies. The normalized absorption and
emission spectra of compounds 24 and 25 are reported
in the Supporting Information. The absorption and
emission spectra obtained for T4 and T5 were the same
as those already reported for these compounds.16 The
overall picture resulting from the data of Table 1 and
the absorption and photoluminescence spectra is that the
presence of the acetylenic spacers in 24 and 25 has very
little effect on the electronic and optical properties, which
remain very close to those of T4 and T5. It is worth
To prepare thiophene oligomers containing acetylenic
spacers, we also tested the use of microwaves in the
Sonogashira coupling reaction.7,14
The reaction conditions were optimized through the
synthesis of quaterthiophene TATTAT (24, Scheme 5, a
soluble compound) with the terminal thiophenes sepa-
rated by the inner bithiophene subsystem through an
(6) Suzuki, A. J . Organomet. Chem. 2002, 653, 83-90.
(7) Sonogashira, K. J . Organomet. Chem. 2002, 653, 46-49.
(8) Bauerle, P.; Wurthner, F.; Gotz, G.; Effemberg, F. Synthesis
1993, 1099-1103.
(9) Garnier, F.; Hajlaoui, R.; El Kassmi, A.; Horowitz, G.; Laigre,
L.; Porzio, W.; Armanini, M.; Provasoli, F. Chem. Mater. 1998, 10,
3334-3339.
(10) Wei, Y.; Yang, Y.; Yeh, J . M. Chem. Mater. 1996, 8, 2659-
2666.
(15) (a) Zhang, H.; Shiino, S.; Kanazawa, A.; Tsutsumi, O.; Shiono,
T.; Ikeda, T.; Nagase, Y. Synth. Met. 2002, 126, 11-18. (b) Zhang, H.;
Shiino, S.; Shishido, A.; Kanazawa, A.; Tsutsumi, O.; Shiono, T.; Ikeda,
T. Adv. Mater. 2000, 12, 1336-1339. (c) Ponomarenko, S.; Kirchmeyer,
S. J . Mater. Chem. 2002, 13, 197-202. (d) O’Neill, M.; Kelly, S. M.
Adv. Mater. 2003, 15, 1135-1146.
(11) (a) Kosugi, M.; Fugami, K. J . Organomet. Chem. 2002, 653, 50-
53. (b) Stille, J . K. Angew. Chem., Int. Ed. Engl. 1986, 25, 508-524.
(12) Sharghi, H.; Sarvari, M. H. Tetrahedron 2002, 58, 10323-
10328.
(13) Yassar, A.; Demanze, F.; J aafari, A.; El Idrissi, M.; Coupry, C.
Adv. Funct. Mater. 2002, 12, 699-708.
(14) Erde´lyi, M.; Gogoll, A. J . Org. Chem. 2001, 66, 4165-4169.
(16) Kanemitsu, Y.; Suzuki, K.; Masumoto, Y.; Tomiuchi, Y.; Shiraishi,
Y.; Kuroda, M. Phys. Rev. B 1994, 50, 2301-2305.
4824 J . Org. Chem., Vol. 69, No. 14, 2004

Synthesis of R-Quinque- and Sexithiophenes
F IGURE 2. ZINDO/S calculated HOMO and LUMO frontier orbitals of compounds 24 (TATTAT) and 25 (TATTTAT), using
optimized ab initio HF/6-31G* geometries.
noting that even the shape of the absorption and photo-
luminescence bands of the two compounds are very
similar to those already reported for T4 and T5, in
particular with regard to the presence of vibronic bands
in the PL spectra.16 Nevertheless, as expected for com-
pounds with larger molecular size, the molar absorption
coefficients of 24 and 25 are markedly greater than those
of T4 and T5.
The calculated electronic densities of the frontier
orbitals of 24 and 25 are shown in Figure 2. The
calculations show that in both compounds the terminal
TAT moieties, in which there are no steric interactions
between adjacent rings, are planar, while in the central
TT and TTT moieties the rings are tilted by 150° from
each other. The electronic densities of both frontier
orbitals, which are π in character, are extended over the
entire molecule, in agreement with the large molar
absorption coefficient values reported in Table 1. More-
over, ZINDO/S-C.I. calculations show that in both com-
pounds the maximum UV absorption band is essentially
due to a pure HOMO-LUMO transition, as in the case of
T4 and T5.
This was the case, in particular, of the preparation of
T6 which, being insoluble, is difficult to purify to the
degree that is useful for applications in thin film transis-
tors.19 The one-pot borylation/Suzuki reaction described
in Scheme 2 is very rapid and the centrifugation/washing
procedure for its purification is also rapid and expedient.
A further vacuum sublimation with a coldfinger affords
a high quality material for application in electronics, as
already seen for T5.17a
Methyl-terminated quinque- and sexithiophenes 11
and 13 were prepared in the hope of obtaining organic
semiconductors with charge transport properties as good
as those of their unsubstituted counterparts17,19 but more
soluble, hence suitable for application in modern stamp-
ing procedures.20 The rationale for grafting the methyls
at the terminal â-positions was the fact that methyl
groups in that position are expected to favor planar
geometries, as inferred from single-crystal, X-ray analysis
of a few methylated quaterthiophenes.21
Scheme 3 shows that, by means of microwave assis-
tence, 11 and 13 could be prepared rapidly and in high
yields. We found that both compounds possess liquid-
crystalline properties. Although a detailed analysis of the
liquid-crystalline properties of 11 and 13 is beyond the
objective of this paper, the initial experimental evidence
presented here (see Figure 1 and the Supporting Infor-
mation) indicates that both compounds have a great
tendency to self-organize in ordered structures, which is
promising for their electrical properties (currently under
Discu ssion
Our data show that the solution-phase, microwave-
assisted procedures described in this paper allow the
preparation of poorly soluble, unsubstituted, and modi-
fied R-quinque- and sexithiophenes very rapidly, with
great flexibility in experimental conditions, and in high
yields.
Unsubstituted T5 (6) and T6 (9)swhich are among the
compounds with the highest field-effect charge mobili-
ties,2 also characterized by high self-affinity, i.e., the
capability to display the same type of solid-state orga-
nization at different length scales17scould indeed be
prepared in 60-90% isolated yields, with reaction times
that were much shorter (minutes instead of hours) and
using purification procedures that were far simpler.18
(17) (a) Melucci, M.; Gazzano, M.; Barbarella, G.; Cavallini, M.;
Biscarini, F.; Maccagnani, P.; Ostoja, P. J . Am. Chem. Soc. 2003, 125,
10266-10274. (b) Viville, P.; Lazzaroni, R.; Bre´das, J . L.; Moretti, P.;
Samor`ı, P.; Biscarini, F. Adv. Mater. 1998, 10, 57-60.
(18) Merz, A.; Ellinger, F. Synthesis 1991, 462-464.
(19) Dodalabapur, A.; Torsi, L.; Katz, H. E. Science 1995, 268, 270-
273.
(20) Allard, S.; Braun, L.; Brehmer, M.; Zentel, R. Macromol. Chem.
Phys. 2003, 204, 68-75.
(21) (a) Barbarella, G.; Zambianchi, M.; Bongini, A.; Antolini, L. Adv.
Mater. 1992, 4, 282-285. (b) Marseglia, E. A.; Grepioni, F.; Tedesco,
E.; Braga, D. Mol. Cryst. Liq. Cryst. 2000, 348, 137-151.
J . Org. Chem, Vol. 69, No. 14, 2004 4825

Melucci et al.
investigation) and application in devices.15d It is worth
noting that among unsubstituted oligothiophenes only T6
is known to display liquid-crystalline properties,22 while
all the liquid-crystalline alkylated oligothiophenes re-
ported so far bear long alkyl chains grafted at the
terminal R positions.9,15a-c
in the thiophene rings and those of the sp-hybridized
carbon atoms of the acetylenic linkage. This mismatch
causes the increase of the energy gap, canceling out the
effect that should be expected from the introduction of
two π electrons in the conjugated system.26 Nevertheless,
as shown in Figure 2, the HOMO (as well as the LUMO)
electronic densities of 24 and 25 are extended over the
entire molecule and, in consequence, the molar absorp-
tion coefficients of these compounds are significantly
larger than those measured for T4 and T5. The molecular
shape of 24 and 25 being different from that of T4 and
T5, it can be predicted that the thin film morphology of
these compounds will be significantly different from that
of T4 and T5. Then these compounds are well suited for
studies of the effects of thin film morphology on charge
carrier mobility. Investigations in this direction are
currently under way.
As shown in Scheme 4, the poorly soluble aldehyde-
terminated sexithiophene, 18,23 was rapidly prepared in
a high yield by the one-pot borylation/Suzuki reaction,
starting from commercial bithiophene. The scheme shows
that the corresponding pentamer 15 was also prepared
in a high yield taking advantage of microwave assistance.
Moreover, by means of microwave assistance, both 15 and
18 were converted to the corresponding cyano-terminated
derivatives, 19 and 20, by using NH2OH‚HCl and MeSO2-
Cl in DMF as the solvent. As described by Sharghi et
al.,12 the reaction proceeds through the formation of an
aldoxime, which is the key intermediate of the reaction,
and the subsequent reaction with MeSO2Cl leads to the
formation of the cyano-terminated oligomers through the
elimination of MeSO3H. R-Cyano-substituted quinque-
and sexithiophenes are commonly prepared by cross-
coupling halothienyls with metallothienyls containing
cyano moieties or, alternatively, by transforming the
CHO functionalities into the corresponding cyano groups
by the use of cyanide salts in refluxing quinoline.24 The
procedure described in Scheme 4 is a rapid and conven-
ient alternative to these methodologies.
For the preparation of pentamer TATTTAT (25) the
microwave assistance in the Sonogashira coupling reac-
tion14 was tested. As shown in Scheme 5, by employing
the reacting amine as the solvent, the use of microwaves
not only allowed the targeted compound to be obtained
very rapidly but also allowed the formation of the
undesired homocoupling derivative of compound 23 to be
markedly reduced. We found that in the absence of
microwaves the formation of the homocoupling derivative
of 23 was the major (62% vs <5%) reaction product. These
results are of some interest in view of the fact that,
recently, ethynyl units have been widely employed to
build linear or dendritic π-conjugated nanostructures,25
in which the triple bonds play the role of wires connecting
the different aromatic moieties.
Con clu d in g Rem a r k s
The solution-phase, microwave-assisted methodology
described here for poorly soluble unsubstituted and
substituted quinque- and sexithiophenes, together with
the previously reported solvent-free procedure for the
preparation of soluble oligothiophenes,5 provides access
to a variety of new thiophene-based organic semiconduc-
tors and makes the preparation of the most performant
of already known thiophene semiconductors more ef-
ficient and expedient.
Exp er im en ta l Section
The characterization of compounds 2,27 5,28 6,18,5 7,8 9,8 12,8
15-18,10 19 and 20,13,24 21 and 22,8 and 2329 has already been
reported.
2,2′;5′,2′′-Ter th iop h en e, 4. The microwave oven reactor
was charged with 0.05 g (0.17 mmol) of 2, 0.055 g (0.43 mmol)
of 3, 6 mg (0.0069 mmol) of Pd catalyst, 80 mg (1.36 mmol) of
KF, and 240 mg of basic alumina. The mixture was stirred to
homogenize the reagents. After 5 min of irradiation a TLC on
silica gel with CH2Cl2/pentane 1:9 as eluent showed the
absence of starting materials. The solid crude was directly
charged on a cromatography column with the same conditions
used for TLC. After cromatography 42 mg of 4 was isolated
(60%). This reaction was repeated on a different scale, to obtain
4 in amounts of grams and with the same reaction yield.
2,2′:5′′,2′′:5′′,2′′′:5′′′,2′′′′-Qu in qu eth iop h en e, 6. First 0.1 g
(0.2 mmol) of 5 and 8 mg (0.01 mmol) of Pd catalyst were
dissolved in 5 mL of toluene. Then a methanol solution (5 mL)
of 0.132 g (1 mmol) of boronic acid 3 and 0.093 g (1.6 mmol) of
KF was added. The mixture was irradiated for 10 min at 70
°C giving a suspension of orange powder. After centrifugation
the crude product was washed twice with warm water,
centrifuged again, and washed with CH2Cl2. The residue was
first dissolved in warm dioxane and then precipitated with a
small volume of water, affording 70 mg of 6 (85% of yield).
The data reported in Table 1 show that compound
TATTTAT (25) and its shorter homologue TATTAT (24)
have optical gaps which are very similar to those of
quinque- (T5) and quaterthiophene (T4), respectively.
This is in agreement with the fact that the presence of a
triple bond between thiophene rings leads to an incom-
plete electron delocalization, owing to the mismatch of
the electronic orbitals of the sp2-hybridized carbon atoms
(22) (a) Taliani, C.; Zamboni, R.; Ruani, G.; Rossini, S.; Lazzaroni,
R. J . Mol. Electron. 1990, 6, 225-228. (b) Scandola, M.; Finelli, L.;
Bongini, A.; Barbarella, G.; Sotgiu, G.; Zambianchi, M. Macromol.
Chem. Phys. 2001, 202, 1878-1882.
(23) (a) Wie, Y.; Wang, B.; Wang, W.; Tian, J . Tetrahedron Lett.
1995, 36, 665-668. (b) Destri, S.; Mascherpa, M.; Porzio, W. Synth.
Met. 1995, 69, 287-288.
(24) Barclay, T. M.; Cordes, A. W.; MacKinnon, C. D.; Oakley, R.
T.; Reed, R. W. Chem. Mater 1997, 9, 981-990.
(25) (a) Gonzalo Rodriguez, J .; Esquivias, J .; Lafuente, A.; Diaz, C.
J . Org. Chem. 2003, 68, 8120-8128. (b) Nielsen, M. B.; Utesch, N. F.;
Moonen, N. N. P.; Boudon, C.; Gisselbrecht, J . P.; Concilio, S.; Piotto,
S. P.; Seiler, P.; Gu¨nter, P.; Gross, M.; Diederich, F. Chem. Eur. J .
2002, 8, 3601-3613. (c) Tovar, J . D.; Swager, T. M. J . Organomet.
Chem. 2002, 653, 215-222.
(26) (a) Geisler, T.; Petersen, J . C.; Bjornholm, T.; Fischer, E.;
Larsen, J .; Dehu, C.; Bredas, J . L.; Tormos, G. V.; Nugara, P. N.; Cava,
M. P.; Metzeger, R. M. J . Phys. Chem. 1994, 98, 10102-10111. (b)
Samuel, I. D. W.; Ledoux, I.; Delporte, C.; Pearson, D. L.; Tour, J . M.
Chem. Mater. 1996, 8, 819-821.
(27) (a) Field, J . S.; Haines, R. J .; Lakoba, E. I.; Sosabowski, M. H.
J . Chem. Soc., Perkin Trans. 2001, 3352-3360. (d) Pappenfus, T. M.;
Mann, K. R. Org. Lett. 2002, 4, 3043-3046.
(28) Hotta, S.; Kimura, H.; Lee, S. A.; Tamaki, T. J . Heterocycl.
Chem. 2000, 37, 281-286.
(29) Rossi, R.; Carpita, A.; Veracini, C. A. Tethrahedron 1985, 41,
1919-1929.
4826 J . Org. Chem., Vol. 69, No. 14, 2004

Synthesis of R-Quinque- and Sexithiophenes
This reaction was repeated on a different scale, to obtain 6 in
powder corresponding to the characteristics reported in the
amounts of 0.5 g and with same reaction yield.
literature for compound 15.
2,2′:5′,2′′:5′′,2′′′:5′′′,2′′′′:5′′′′,2′′′′′-Sexith iop h en e, 9. First 50
mg (0.15 mmol) of 7, 19 mg (0.07 mmol) of bispinacolatodiboron
8, and 4 mg (0.006 mmol) of PdCl2dppf were introduced in the
microwave oven reactor and dissolved in 2 mL of toluene, then
42 mg (0.73 mmol) of KF dissolved in 2 mL of MeOH was
added. The mixture was irradiated for 10 min at 70 °C then
the resulting suspension was centrifugated. The product
obtained was washed twice with warm water to eliminate
inorganic byproducts, then it was centrifuged again, washed
with warm CH2Cl2, and finally washed with warm dioxane and
32 mg of 9 (84% yield) was obtained.
5′′-Iod o-[2,2′;5′,2′′]ter th iop h en e-5-ca r ba ld eh yd e, 17. To
a solution of 0.1 g (0.36 mmol) of 16 dissolved in 3.5 mL of
CH2Cl2 was added 3.5 mL of CH3COOH. The mixture was
shielded from light and cooled in an ice bath and 0.1 g (0.398
mmol) of NIS was added stepwise. After 2 h the solvent was
evaporated and the crude was washed twice with MeOH.
Compound 17 was isolated as an orange powder (0.11 g, 76%).
Mp 182 °C; MS m/e 402 [M•+]; λmax (CH2Cl2) 405.11 nm; 1H
NMR (CDCl3, TMS/ppm) δ 9.86, (s, 1H), 7.67 (d, J ) 4 Hz,
1H), 7.24 (d, J ) 4, 1H), 7.13 (d, J ) 4.0 Hz, 1H), 7.07 (d, J )
3.6 Hz, 1H), 6.89, (d, J ) 3.6 Hz, 1H); 13C NMR (CDCl3, TMS/
ppm) δ 182.5, 146.5, 142.3, 141.8, 137.9, 137.7, 137.3, 126.9,
125.8, 125.1, 124.2. Anal. Calcd for C13H7S3OI: C, 38.81; H,
1.75. Found: C, 38.75; H, 1.76.
4,4,5,5-Tet r a m et h yl-2-(4-m et h ylt h iop h en -2-yl)[1,3,2]-
d ioxa bor ola n e, 10. To a stirred solution of 2-bromo-4-
methylthiophene (1.0 g, 5.6 mmol) in fresh distilled THF (43
mL) was added 2.76 mL of 2.5 M n-BuLi in hexane (6.7 mmol)
dropwise at -78 °C under nitrogen atmosphere. After 10 min
the flask was warmed to 0 °C and left at this temperature for
30 min. Then, after cooling the solution to -78 °C, 2.5 mL (12
mmol) of 2-isopropoxy-4,4,5,5-tetramethyl-1,3,2-dioxaborolane
was added by syringe and the reaction allowed to warm to
room temperature. After 12 h of stirring the gray cloudy
mixture was poured into water and extracted with ether. The
organic phase was washed with brine and dried over MgSO4.
After flash chromatography on silica gel with petroleum ether/
Et2O (9:1) 1.13 g of 10 was obtained as a yellow oil (yield 90%).
MS m/e 224 [M•+]. 1H NMR (CDCl3) δ 1.34 (s, 12H), 2.29 (s,
3H), 7.20 (s, 1H), 7.46 (s, 1H). 13C NMR (CDCl3) δ 15.03, 24.70,
29.67, 83.94, 128.12, 138.94, 139.43 ppm.
5′,5′′′′′-Difor m yl-2,2′:5′,2′′:5′′,2′′′:5′′′,2′′′′:5′′′′,2′′′′′-se xi-
th iop h en e, 18. The procedure was the same as used for the
preparation of 15, starting from 0.1 g (0.25 mmol) of 17 and
30 mg (0.18 mmol) of 8. After 10 min of irradiation at 70 °C a
further 0.5 equiv of 8 and 2 mol % of PdCl2dppf were added
and the mixture was irradiated for an additional 10 min. The
precipitate obtained was centrifuged and washed twice with
warm MeOH, centrifuged again, and washed twice with
toluene and then twice with CH2Cl2 yielding 60 mg (88%)
corresponding to the characteristics reported in the literature
for compound 18.10
Gen er a l P r oced u r es for Micr ow a ve-Assisted Tr a sfor -
m a tion of Ald eh yd e En d -Ca p p ed Oligom er s in to th e
Cor r esp on d in g Cya n o En d -Ca p p ed On es (19, 20). Alde-
hyde end-capped oligomers 15 and 18 (1 equiv), NH2OH‚HCl
(8 equiv), MeSO2Cl (2.4 equiv), and DMF (0.5 mL) were
introduced into the microwave oven reactor and irradiated for
20 min at 100 °C. The crude obtained was washed with warm
water, centrifuged, and then washed with CH2Cl2 to give cyano
end-capped oligomers 19 and 2013,24 in 80% and 60% yields,
respectively.
4,4′′′′-Dim e t h yl-2,2′:5′′,2′′:5′′,2′′′:5′′′,2′′′′-q u in q u e t h io-
p h en e, 11. The microwave oven reactor was charged with 10
mL of toluene, 50 mg (0.1 mmol) of 2,2′′′-diiodoterthiophene
(5), and 3 mg (0.004 mmol) of PdCl2(dppf). Then a solution of
KF (46 mg, 0.8 mmol) in MeOH (10 mL) was added at room
temperature. Finally, 112 mg (0.5 mmol) of boronic ester 10
was added at once. After 10 min of irradiation at 70 °C, the
solvent was evaporated under reduced pressure and the crude
product was purified by repeated washing steps with warm
H2O and then crystallized from toluene yielding 11 in 90%
yield as an orange solid. Mp 231 °C. MS (m/e) 440 [M•+]; λmax
(CH2Cl2) 421.10 nm. 1H NMR (CS2/CDCl3 ) δ 2.62 (s, 3H), 6.80
(m, 1H), 6.99 (m, 1H), 7.06 (m, 3H). 13C NMR (CS2/CDCl3) δ
138.2, 136.7, 136.6, 135.9, 135.5, 125.9, 124.2, 124.1, 124.0,
120.0, 15.7. Anal. Calcd for C22H16S5: C, 59.96; H, 3.66.
Found: C, 60.04; H, 3.67.
5,5′-Bis(2-th ien yleth yn yl)-2,2′-bith iop h en e, 24 (TAT-
TAT). The microwave oven reactor was charged with 165 mg
(0.50 mmol) of 21,8 130 mg (1.20 mmol) of 2-ethynylthiophene
23,28 28 mg (0.04 mmol) of Pd(PPh3)2Cl2, 31 mg (0.12 mmol)
of triphenylphosphine, 30 mg (0.16 mmol) of copper(I) iodide,
and 2.02 g (2.80 mL, 20 mmol) of diisopropylamine. The
mixture was irradiated for 5 min at 50 °C. After cooling, the
crude product was poured into water (15 mL) and extracted
with dichloromethane (3 × 15 mL). The combined extracts
were centrifuged for 3 min at 3000 rpm and the isolated solid
recrystallized from toluene to afford 128 mg (68% yield) of 24
as a deep chrome yellow solid. Mp 165 °C; MS m/e 378 [M•+];
4,4′′′′′-Dim e t h yl-2,2′:5′,2′′:5′′,2′′′:5′′′,2′′′′:5′′′′,2′′′′′-se xi-
th iop h en e, 13. The microwave oven reactor was charged with
0.125 g (0.26 mmol) of dibromoquaterthiophene8 12 and 11 mg
(0.013 mmol) of PdCl2(dppf) dissolved in 4 mL of DMF. Then
0.286 g (1.3 mmol) of 10 and 0.075 g (1.3 mmol) of KF were
added and the mixture was irradiated for 20 min at 100 °C.
The red suspension obtained was centrifuged and the precipi-
tate washed twice with warm water and centrifuged again.
Then the red precipitate was washed with warm CH2Cl2,
centrifuged, and washed twice with warm toluene. Sexithio-
phene 13 was obtained as an intense red powder (95 mg, 70%
yield). Mp 257 °C MS (m/e) 522 [M•+]; λmax (CH2Cl2) 430.94
1
λmax (CH2Cl2) 395 nm; λPL (CH2Cl2) 478 nm. H NMR (CDCl3,
3
4
TMS/ppm) δ 7.33 (dd, J ) 5.2 Hz, J ) 1.2 Hz, 2H), 7.29 (dd,
3J ) 3.6 Hz, J ) 1.2 Hz, 2H), 7.17 (d, J ) 4.0 Hz, 2H), 7.08
4
3
3
3
3
(d, J ) 4.0 Hz, 2H), 7.02, (dd, J ) 5.2 Hz, J ) 3.6 Hz, 2H);
13C NMR (CDCl3, TMS/ppm) δ 138.1, 132.8, 132.2, 127.8, 127.1,
124.0, 122.5, 122.1, 87.7, 86.0. Anal. Calcd for C20H10S4: C,
63.46; H, 2.66. Found: C, 63.37; H, 2.68.
5,5′′-Bis(2-t h ien ylet h yn yl)-2,2′:5′,2′′-t er t h iop h en e, 25
(TATTTAT). Compound 25 was synthesized by using the
same method as was used for the synthesis of compound 24.
The starting material in this case was 5,5′′-bisbromo-2,2′:5′,2′′-
terthiophene 22.8 The reaction took 20 min of microwave
irradiation at 100 °C to be completed. Pure title compound 25
was obtained in 71% yield as a bright orange solid. Mp 200
°C; MS m/e 460 [M•+]; λmax (CH2Cl2) 421 nm; λPL(CH2Cl2) 517
nm; 1H NMR (CDCl3, TMS/ppm) δ 7.33, (dd, 3J ) 5.2 Hz, 4J )
1
nm. H NMR (CS2/CDCl3) δ 2.18 (s, 6H), 7.05 (m, 8), 6.80 (br
s, 4H). Anal. Calcd for C26H18S6: C, 59.73; H, 3.47. Found: C,
59.61; H, 3.46.
5′,5′′′′-D i fo r m y l-2,2′:5′′,2′′:5′′,2′′′:5′′′,2′′′′-q u i n q u e -
th iop h en e, 15. To a solution of 0.1 g (0.25 mmol) of 5 and 5
mg (0.006 mmol) of PdCl2(dppf) in 8 mL of toluene was added
a solution of 0.156 g (1 mmol) of thiophene boronic acid 14
and 0.093 g (1.6 mmol) of KF on 8 mL of methanol. The
mixture was irradiated at 70 °C for 10 min then the dark-red
suspension obtained was centrifuged and the precipitate
washed twice with warm water, centrifuged again, and finally
washed twice with CH2Cl2. The solid was then crystallized
from dioxane-water to give 80 mg (85% yield) of a bright red
3
4
1.2 Hz, 2H), 7.29 (dd, J ) 4.0 Hz, J ) 1.2 Hz, 2H), 7.18 (d,
3J ) 4.0 Hz, 2H), 7.11 (s, 2H), 7.08 (d, J ) 4.0 Hz, 2H), 7.02,
3
(dd, 3J ) 5.2 Hz, 3J ) 4.0 Hz, 2H); 13C NMR (CDCl3, TMS/
ppm) δ 138.6, 136.1, 133.0, 132.3, 127.86, 127.2, 125.0, 123.7,
122.8, 121.9, 87.7, 86.1. Anal. Calcd for C24H12S5: C, 62.57;
H, 2.63. Found: C, 62.72; H, 2.64.
J . Org. Chem, Vol. 69, No. 14, 2004 4827

Melucci et al.
Th eor etica l Ca lcu la tion s. Ab initio calculations were
performed with the Gaussian98 series of programs.30 The
geometries were fully optimized by standard gradient tech-
niques and the critical points checked by frequency analysis.
Optimized coordinates are reported as Supporting Information.
Frontier orbitals and UV transitions were calculated by
ZINDO/S-C.I. (8 × 8) single-point calculations on ab initio
geometries with the HyperChem integrated package.31
Ack n ow led gm en t. This work was partially sup-
ported by the project Nanodispositivi molecolari (FIRB,
RBNE01YSR8). Thanks are due to Dr. Martino Colonna
(UNIBO) for DSC measurements and to Dr. Mario Benzi
(ISOF-CNR) for the synthesis of compound 5.
(30) Frisch, M. J .; Trucks, G.; Schlegel, H. B.; Scuseria, G. E.; Robb,
M. A.; Cheeseman, J . R.; Zakrzewski, V. G.; Montgomery, J . A., J r.;
Stratmann, R. E.; Burant, J .; Dapprich, S.; Millam, J . M.; Daniels, A.
D.; Kudin, K. N.; Strain, M. C.; Farkas, O.; Tomasi, J .; Barone, V.;
Cossi, M.; Cammi, R.; Mennucci, B.; Pomelli, C.; Adamo, C.; Clifford,
S.; Ochterski, J .; Petersson, G. A.; Ayala, P. Y.; Cui, Q.; Morokuma,
K.; Malick, D. K.; Rabuck, A.; Raghavachari, K.; Foresman, J . B.;
Cioslowski, J .; Ortiz, J . V.; Baboul, A. G.; Stefanov, B. B.; Liu, G.;
Liashenko, A.; Piskorz, P.; Komaromi, I.; Gomperts, R.; Martin, R. L.;
Fox, D. J .; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.;
Gonzalez, C.; Challacombe, M.; Gill, P. M. W.; J ohnson, B.; Chen, W.;
Wong, M. W.; Andres, J . L.; Gonzalez, C.; Head-Gordon, M.; Replogle,
E. S.; Pople, J . A. Gaussian 98, Revision A.7; Gaussian, Inc.: Pitts-
burgh, PA, 1998.
Su p p or tin g In for m a tion Ava ila ble: Micrographs of the
1
nematic mesophase of compound 9, H and 13C NMR spectra
of compound 10, DSC plots of compound 11, DSC plots and
micrographs of the nematic phase of compound 13, normalized
absorption and photoluminescence spectra of compounds 24
and 25, and optimized Cartesian coordinates of compounds 24
and 25. This material is available free of charge via the
Internet at http://pubs.acs.org.
J O035723Q
(31) HyperChem, rel 7.0; HyperCube: Waterloo, Ontario, Canada.
4828 J . Org. Chem., Vol. 69, No. 14, 2004