Facilitated synthesis of functional oligothiophenes for application in thin film devices and live cell imaging

Article abstract of DOI:10.1080/17415993.2013.807810

This paper describes recent developments in the synthesis of ultrapure functional oligothiophene-based materials taking advantage of enabling techniques such as microwave/ultrasound irradiation and chitosan-supported palladium catalysts. Examples showing how ultrapure oligothiophenes self-organize in order to optimize charge transport in thin film devices and fluorescence emission inside living cells are reported.

Full text of DOI:10.1080/17415993.2013.807810

Journal of Sulfur Chemistry, 2013
Vol. 34, No. 6, 627–637, http://dx.doi.org/10.1080/17415993.2013.807810
Facilitated synthesis of functional oligothiophenes for
application in thin film devices and live cell imaging†
Francesca DiMariaa and Giovanna Barbarellab*
aConsiglio Nazionale Ricerche, Laboratorio MIST.E-R, Via Gobetti 101, Bologna 40129, Italy; bCentro
Nazionale Ricerche, Istituto per la Sintesi Organica e la Fotoreattività,Via Gobetti 101, Bologna 40129, Italy
(Received 6 March 2013; final version received 9 May 2013)
This paper describes recent developments in the synthesis of ultrapure functional oligothiophene-based
materials taking advantage of enabling techniques such as microwave/ultrasound irradiation and chitosan-
supported palladium catalysts. Examples showing how ultrapure oligothiophenes self-organize in order to
optimize charge transport in thin film devices and fluorescence emission inside living cells are reported.
Keywords: oligothiophenes; synthesis; purity; fluorescence; charge transport
1. Introduction
Nanoscience and nanotechnologies rely on the synthesis of innovative materials. Among the
new materials successfully developed in the last few decades are thiophene-based oligomers
and polymers, which are still of great current interest from a scientific and technological point
of view.[1] Thiophene-based materials have numerous useful properties: they are electroactive
and fluorescent; are chemically stable and allow a great diversity in molecular structures and
functional properties; display ‘plasticity’ in adapting their geometry to the environment in the
solid state and in creating supramolecular self-assemblies; and have the capability to finely interact
with biologically relevant molecules such as intracellular proteins. They have been extensively
investigated for application in organic electronics and more recently in photovoltaics.[1, 2] Much
less attention has been devoted so far to the various possible biological applications of these
materials such as monitoring protein aggregates at the origin of severe diseases [3] or acting as
fluorophores for live cell imaging.[4]Their application in materials science requires that thiophene
*Corresponding author. Email: giovanna.barbarella@isof.cnr.it
†Article for the Special Issue in memory of Professor Alessandro Degl’Innocenti.
© 2013 Taylor & Francis

628 F. DiMaria and G. Barbarella
derivatives are prepared in a reproducible way in very pure form and in sufficient amount, that they
are synthesized in an environmentally safe way, and that their toxicity is accurately monitored.
Over the past few years the most investigated reactions for the preparation of thiophene mate-
rials have been the catalyzed cross-couplings between halogenated and metalated thienyls, which
allow the regioselective construction of the aromatic backbone starting from functionalized thio-
phene monomers. The most exploited cross-coupling reactions are palladium-catalyzed Stille
[5] and Suzuki–Miyaura [6] reactions. The Stille reaction consists in the coupling of weakly
nucleophilic thienyl stannanes with thienyl halogenides (Br, I), generally in the presence of
palladium(0) complexes. For the formation of the thienyl–thienyl linkage, the palladium-catalyzed
Suzuki–Miyaura reaction uses thienyl boronic acids or esters as nucleophiles in the presence of
electrophilic thienyl halides. The boron derivatives are less toxic than the tin derivatives and the
reaction is compatible with a great variety of functional groups and reaction conditions. Both reac-
tions are greatly dependent on the steric and electronic characteristics of reagents and catalysts and
require careful adjustment of experimental conditions. Alternative methods for thienyl–thienyl
bond formation have also been proposed [7] but they often do not posses the versatility of the Stille
and Suzuki–Miyaura reactions, which allow the preparation of highly regioregular compounds of
the most various sizes and functionalization types via stepwise addition of functionalized units.
In this paper, we describe how the combination of different enabling techniques [8] in the
Suzuki–Miyaura reaction in aqueous solvents leads to the expedient, rapid, cost-effective, and
environment friendly synthesis of ultrapure oligothiophenes from dimers to hexadecamers useful
for applications in supramolecular chemistry and chemical biology.
2. Enabling techniques
Among the various enabling techniques which have been described to speed up synthetic reac-
tions and ease workup and products separation,[8] we have employed microwave and ultrasound
irradiation and supported palladium catalysts.
2.1. Microwave and ultrasound irradiation
Replacing conventional heating with microwave heating leads to more homogeneous and higher
temperatures, faster reaction rates, higher yields, use of aqueous solvents, easy workup, and some-
times regio- and stereoselectivity of reactions.[9, 10] The last decade has witnessed an increasing
interest of organic chemists in microwave-assisted reactions, in both research laboratories and
industry, which has led to major improvements in equipment quality and parameters control, in
particular temperature and pressure. Microwave assistance has been explored in the synthesis of
thiophene oligomers in the solid state as well as in solution.[11, 12] Generally, in the presence
of microwaves, the formation of the thienyl–thienyl linkage takes place in a very short time,
sometimes minutes, and in high yields. The rate of the cross-coupling reaction is accelerated with
respect to that of halogen–metal exchange and, in consequence, much less undesired products are
formed. The yield of the Suzuki–Miyaura reaction depends on many tunable parameters related
to the electronic and steric characteristics of reagents and catalyst, solvent, base, and temperature
which are difficult to optimize. Increase of the reaction rate impressed by microwaves irradiation
allows a rapid screening of a wide range of conditions to identify the best combination of exper-
imental parameters. Once this is established, thiophene oligomers are rapidly obtained in high
yields and in mixtures that can easily be purified.
Ultrasound irradiation enhances the rate of chemical reactions and shortens the reaction times
via the process of acoustic cavitation.[13]When ultrasound irradiation is applied during a reaction,

Journal of Sulfur Chemistry 629
pressure and temperature changes displace the system from equilibrium, van der Waals forces
are unable to maintain intermolecular cohesion, and small cavities, that is, micro-bubbles, are
formed. Formation and collapse of these micrometer-scale bubbles constitute the phenomenon of
cavitation. Molecules inside the micro-bubbles experience high temperatures and pressures which
can lead to the breaking of the chemical bonds and the consequent return of short-living species
to the bulk liquid at room temperature favoring the reactivity. Recently, ultrasound irradiation has
been demonstrated to be very effective in the bromination of thiophene oligomers.[14]
2.2. Chitosan-supported Pd catalysts
The use of palladium catalysts supported by silicon or chitosan (a water-tolerant polysaccharide)
leads to the synthesis of highly pure thiophene oligomers, which are not contaminated by traces
of metals that can alter charge conduction and other functional properties.[15] Combining the use
of supported catalysts with microwave assistance in the Suzuki–Miyaura reaction resulted in a
marked acceleration of the cross-coupling reaction when using iodinated thienyls. [15] We have
now implemented the synthesis of a new chitosan-supported catalyst, named CHITCAT-Me, that is
characterized by the presence of an additional methyl group compared to the previously described
one. The combination of the new catalyst and microwave acceleration in aqueous solvents allows a
further reduction of the reaction times – from 1 h or more to few minutes – even when brominated
thienyls are used (Table 1). Moreover, as its unmethylated counterpart, this catalyst can be easily
removed from the reaction mixture by filtration and reused in consecutive reactions – up to 4
times – without substantial yield variations.
The molecular structure and the synthetic pattern for the preparation of CHICAT-Me using the
synthetic modalities already described for its unmethylated counterpart [15] is shown in Scheme 1.
Allthiopheneoligomers, frombi-toquinquethiophene, areobtainedinhighyieldfromiodinated
as well as brominated precursors in few minutes in aqueous EtOH or DMF. The conditions
employed also favor the rapid and high-yield formation of oligomers bearing a thiophene-S,S-
dioxide unit.
3. Synthesis of ultrapure thiophene oligomers
3.1. α, ω-Dialkylquaterthiophenes
Electronics based on organic materials as an alternative to the classical silicon technology has
rapidly progressed in the last few years and, owing to synthetic efforts, several polymers and
small molecules have been discovered displaying charge mobilities exceeding that of amorphous
silicon.[16] The main advantage in using electroactive organic materials is the compatibility with
solution processing and low-temperature vacuum evaporation techniques which are much less
expensive than the lithographic processes required by silicon technology.
Much interest has been devoted to thiophene oligomers for application in devices such as thin
film field-effect transistors (FETs), light-emitting diodes, light-emitting transistors, and photo-
voltaic devices.[1] They have also been investigated as useful materials for understanding the
relationship between the molecular structure and the functional properties and elucidating the
fundamental processes underlying charge transport and light emission in organic materials.[17]
Among the most investigated thiophene oligomers as active components in thin film FETs are
quaterthiophenes, which are excellent semiconducting materials.[18, 19] In particular, it has
been demonstrated that the purity of the materials employed is a stringent prerequisite to obtain
significantly good charge transport performance.

630 F. DiMaria and G. Barbarella
Table 1. Products, reagents and experimental conditionsa of the reaction of thienyl halogenides boronic acid/ester in
the presence of CHITCAT-Me.
Solvent
Microwaveb irradiation Yieldc
Entry
1
Starting material
Product
1:1(v/v)
(%) time (min)
(%)
EtOH/H2O
DMF/H2O
2
2
100
100
EtOH/H2O
DMF/H2O
6
6
23
30
2
3
EtOH/H2O
DMF/H2O
3
3
100
100
EtOH/H2O
DMF/H2O
6
6
23
22
EtOH/H2O
DMF/H2O
3
3
80
90
EtOH/H2O
DMF/H2O
6
6
68
88
4
EtOH/H2O
DMF/H2O
3
3
78
85
EtOH/H2O
DMF/H2O
2
2
30
35
5
EtOH/H2O
DMF/H2O
12
8
24
72
EtOH/H2O
DMF/H2O
8
3
72
86
6
7
EtOH/H2O
DMF/H2O
5
5
22
80
EtOH/H2O
DMF/H2O
4
4
71
81
(Continued)

Journal of Sulfur Chemistry 631
Table 1. Continued.
Starting
Solvent
1:1(v/v)
Microwaveb irradiation
(%) time (min)
Yieldc
(%)
Entry
material
Product
8
EtOH/H2O
DMF/H2O
4
4
60
80
a
Items 1-5, 7-8;
item 6; halide 1.0 equiv. boronic acid/ester 3 equiv. for monohalide, 4 equiv. for dihalide,
3.5 mol%, CHITCAT-Me, KF 3 equiv. per halide.
bFixed T = 140◦C (power 300W).
cGC conversion with respect to n-dodecane as internal standard.
Scheme 1. Molecular structure and synthetic pattern for the preparation of chitosan-supported
catalysts CHICAT-Me and CHITCAT.
The title compounds with R = CH3, C4H9, C6H13 – prepared according to the pattern reported
in Scheme 2, in highly pure form, metal free, and in yields varying from 75% to 87% – were tested
in thin film FETs prepared by vacuum sublimation on a poly(methylmethacrylate), transparent
support as gate dielectric. The FETs employed were bottom-gate top-contact devices with gold
contactsactingassourceanddrainelectrodes.Theactivelayer–madeofultrapurequaterthiophene
– was 10 nm thick. A comparative study of charge transport properties, film morphology, and
crystallinity as a function of the alkyl substituents was carried out. It was found that the length of
the terminal alkyl chains has a deep impact on the self-assembly process of thin films, hence on
charge transport properties.[20]
Scheme 2. Synthetic pattern for the preparation of α, ω-dimethyl; α, ω-di-n-butyl and
α, ω-di-n-hexyl quaterthiophenes using microwave irradiation and chitosan-supported CHIT-
CAT-Me catalyst as enabling techniques.

632 F. DiMaria and G. Barbarella
Figure 1. (a) Supramolecular self-assembly as determined from low-angle thin-film X-ray diffraction, (b) film mor-
phology obtained by atomic force microscopy for a film thickness value of 1 nm, and (c) current/voltage output of a
thin-film FET having α, ω-di-n-hexyl quaterthiophene as the active material.
The di-n-hexyl quaterthiophene displays the best charge mobility (0.09 cm2 V−1 s−1) as a result
of the interplay of superior supramolecular organization and film morphology (Figure 1).
However, the charge transport properties do not systematically improve by increasing the
alkyl chain length. Indeed, di-n-butyl-quaterthiophene, having an intermediate chain length of
the terminal groups, displays the lowest charge mobility. Also in this case the supramolecu-
lar arrangement plays a key role. Indeed, while X-ray thin film diffraction of di-n-hexyl- and
dimethyl quaterthiophenes shows reflection peaks typical of well-organized and crystalline mate-
rials with the molecules aligned with their long axis perpendicular to the substrate, for di-n-butyl
quaterthiophene no diffraction peaks were observed indicating the lack of crystalline order, an
arrangement disfavoring charge transport.
3.2. Sulfur overrich octa- and hexadecathiophenes
With the aid of microwave and ultrasound irradiation in the Suzuki–Miyaura reaction, we
have synthesized thiophene octamer and hexadecamer characterized by the repetition of the
3,3ꢀ-bis(hexylthio)-2,2ꢀ-bithiophene.[21, 22]
In addition to the fact that long alkyl chains favor solubility and processability, the rationale
behind the choice of these oligomers – that we named ‘sulfur-overrich’ because of the presence
of one extra sulfur per ring attached to one β carbon – was to understand the effect of intra- and
intermolecular S· · ·S interactions on the aggregation modalities and the functional properties. The
regiochemistry of substitution was of the type called ‘head–head’, a substitution pattern generally
avoided since it induces backbone distortions causing a loss of π–π overlap and the formation of
amorphous films. In our case, however, the loss of π conjugation is compensated by the donating
effect of the β-sulfurs and the aggregation properties into crystalline structures are favored by
intra- and intermolecular S· · ·S interactions.
The synthesis was carried out by doubling the oligomer size via sequential selective mono-
brominations using N-bromosuccinimide (NBS) and one-pot borylation/cross-coupling reactions
using 4,4,5,5-tetramethyl-1,3,2-dioxaborolane in the presence of a palladium catalyst, Pd(dppf)2,
according to the pattern described in Scheme 3.
The octamer had the right balance of shape, size, and conformation and the appropriate S· · ·S
intra- and intermolecular interactions to self-assemble on surfaces in the form of crystalline
microfibers that were conductive and electroactive.[21] Most of the fibers were helical in shape
as shown in Figure 2. The figure also displays the intense circular dichroism (CD) signal of the
film where helical fibers were spontaneously formed on glass.
Since the octamer does not contain chiral centers, no CD signal is observed in solution. Thus,
the CD signal observed in film is the result of molecular and supramolecular chirality in the
solid state. In other words, it is the result of two contributions: one, due to molecular chirality

Journal of Sulfur Chemistry 633
Scheme 3. Microwave (MW)- and ultrasound (US)-assisted synthesis of sulfur overrich octamer
and hexadecamer. Solvent: THF/water (1:1 v/v); R = C6H13; isolated yields.
induced by the freezing of the molecule on the surface, and the other, due to the supramolecular
organization achieved in the film. The presence of the CD signal and its inversion from one film
to the other is probably due to the restrictions to rotation around the thienyl–thienyl interring
bond which causes the molecule to loose all symmetry elements (atropisomerism). Once the first
molecule is deposited on the surface in the form of one or the other enantiomer, the remaining
molecules deposit in the same way causing chiral amplification.
The two different CD signals displayed in Figure 2, reversed in sign, come from two different
samples deposited on glass. In agreement with the interpretation given above, the helicity hand-
edness is random. The thickness of the films does not exceed 2 μm, hence the intensity of the CD
signal is remarkable.
The thiophene fibers described above are the first examples of conjugate functional fibers
displaying the same supramolecular arrangement no matter what the nature of the support on
which they were deposited was. Their remarkable thermodynamic stability was ascribed to the
S· · ·S intra- and intermolecular interactions related to the regiochemistry of substitution and
defining a novel ‘recognition algorithm’.[21]
Finally, it must be noted that micrometer-sized fibers, bridging the gap between the microscopic
and macroscoping world, are the necessary step toward a bottom-up self-organizing organic
electronics.
The hexadecamer was one of the longest thiophene oligomers ever prepared and was char-
acterized by the largest molar absorption coefficient reported so far for thiophene oligomers
(95.069 cm−1 mol−1). Its UV–VIS spectrum and redox potentials were very similar to those of
poly(3-hexylthiophene), P3HT. However, contrary to P3HT, the sulfur overrich hexadecamer was
very soluble and, being a monodisperse compound, much easier to re-prepare with exactly the
same characteristics, including material’s purity.

634 F. DiMaria and G. Barbarella
Figure 2. Bottom: Scanning electron microscopy image of helical microfibers spontaneously formed on glass by sulfur
overrich octathiophene. Top: Absorption and CD spectra of two different films of the same compound on glass showing
CD signals of opposite sign. Scale bar: 25 μm.
Figure 3. (a) Absorption spectrum and (b) current density/wavelength output of the bulk heterojunction photovoltaic
cell with a blend of hexadecamer:PCBM as the active element. The efficiency of light–current conversion of the blend
was 1.49%. (c) Sketch of the solar cell embodying the blend of hexadecamer and PCBM as the active element.
The hexadecamer was also an electroactive compound and a charge mobility value of
10−5/10−4 cm2 V−1s−1 was reproducibly measured in FET devices.[22]
Owing to the HOMO and LUMO energy values similar to those of P3HT, the hexadecamer
could be employed in a solar cell (Figure 3).
A bulk heterojunction solar cell with the hexadecamer as electron donor and a fullerene deriva-
tive, namely 1-(3-methoxycarbonyl)propyl-1-phenyl[6,6]-methanofullerene (PCBM), as electron
acceptor afforded a reproducible light-energy conversion efficiency of 1.49% without any attempt
to optimize device fabrication or processing conditions. Figure 3 shows a sketch of the solar cell,
the absorption spectrum of a 1:1 (w:w) blend of the hexadecamer and PCBM, and the current
density of the device as a function of the wavelength.

Journal of Sulfur Chemistry 635
3.3. Dithienothiophene-S,S-dioxide fluorophores
Currently, fluorescence microscopy is rapidly developing in the field of chemical biology owing to
the discovery of innovative classes of fluorescent probes (fluorophores) such as green fluorescent
proteins [23] or quantum dots.[24] Several classes of small molecule organic fluorophores have
also been developed, among them thiophene-based fluorophores.[4, 25] Since even tiny amounts
of undesired contaminants may lead to a drop of the quantum yield – namely the number of
photons emitted per photons adsorbed – the fluorophores prepared via organic synthesis must be
obtained with the maximum degree of purity.
With the assistance of ultrasounds and microwaves, we have synthesized a few classes of
efficient thiophene-based fluorophores.[25] Recently, we have focused our interest on thiophene-
based fluorophores capable of spontaneously crossing the membrane of live cells and reveal
cellular processes detectable via fluorescence techniques.[4] Scheme 4 reports the synthesis of
the most intriguing thiophene-based fluorophore that we have been able to engineer so far, namely
2,6-diphenyl-3,5-dimethyl-dithieno[3,2-b:2ꢀ,3ꢀ-d]thiophene-4,4-dioxide(DTTO).Whenobtained
in very pure form, the green fluorescent DTTO displays a quantum efficiency on the order of 90%.
Scheme 4. Microwave (MW)- and ultrasound (US)-assisted synthesis of DTTO: (i)
bis(tri-n-butyltin) sulfide, Pd(PPh3)4, toluene, 130◦C, 90%; (ii) n-BuLi, CuCl2, ethyl ether, 0◦C,
50%; (iii) 3-chloroperbenzoic acid, CH2Cl2, 70%; (iv) 2 equiv. of NBS, CH3COOH/CH2Cl2, US,
80%; (v) tributyl(phenyl)stannane, 5% Pd(PPh3)4, toluene, MW, 80◦C, 95%. Isolated yields.
Treatment of living NIH 3T3 mouse embryonic fibroblast cells with DTTO dissolved in phys-
iological solution led first to rapid and uniform fluorescent staining of the cytoplasm of the cells,
without low or undetectable labeling of the nucleus. Then bundles of green fluorescent fibrils
of micrometer length and sub-micrometric diameter started to be formed and be progressively
accumulated into the cell-conditioned medium (Figure 4). Finally, the fluorescent fibers were
extruded into the extracellular matrix.[4]
During the entire process, cell viability and proliferation activity remained unaltered while
fluorescence was transmitted from mother to daughter cell. Persistence of fluorescence up to 10
days was observed.
Laser scanning confocal microscopy images of independent cell cultures examined at different
times revealed that the fluorescent fibrils were synthesized inside the cells and then released
as unique supramolecular assemblies through the cell plasma membrane. By means of various
experimental data supported by theoretical calculations, it was found that collagen type-I was a
structural component of the backbone of the fluorescent fibrils.[4] The protein, which is the main
product of the functional activity of fibroblasts, was able to incorporate the dye via molecular
recognition between DTTO and the hydroxyproline component of procollagen polypeptide chains
and formation of multiple hydrogen bondings. It is known that the structural hallmark of collagen
is a rod-like triple helix with a core domain involving the repeating amino acid triplet Gly-X-Y,
where Gly is the smallest amino acid glycine and X and Y are most frequently an l-proline and

636 F. DiMaria and G. Barbarella
Figure 4. Green fluorescent micrometer-sized fibers formed inside living mouse fibroblasts upon treatment with DTTO
in physiological solution and extruded into the extracellular matrix. (a) Laser scanning confocal microscopy image, (b)
overlay of image A with that obtained by light transmission optical microscopy showing the underlying cells. Images
obtained after 24 h of treatment of the fibroblasts with DTTO and extensive washing to eliminate the free fluorophore.
Scale bar: 10 μm.
a 4(R)-hydroxy-l-proline residue, respectively. It has been reported that each glycine amide-NH
forms a hydrogen bond with the X-position amide carbonyl on an adjacent strand, whereas the
X- andY-positions stabilize the triple helix by stereoelectronic effects and water-bridged hydrogen
bonds. Therefore, it was conceivable that for its peculiar properties, such as the presence of
oxygen atoms in the central position and its molecular symmetry, DTTO could have some affinity
for interacting with collagen, characterized by a quaternary structure and inherently folded via
self-assembly.
4. Conclusions
Preparing functional oligothiophenes in very pure form by means of rapid and environmentally
friendly procedures is crucial for the application of these compounds in materials chemistry as
semiconductor and fluorescent compounds for electronic and optoelectronic devices as well as for
application in cell imaging and monitoring of intracellular processes. The presence of by-products
originated by demetalation or dehalogenation of the starting materials or by homo-coupling or
boron–halogenexchangesidereactionsorbythepresenceofcatalystresiduesishighlydetrimental
for their performance in the different fields, altering charge conduction and fluorescence efficiency.
We have demonstrated that the combination of microwave and ultrasound irradiation and/or
the use of chitosan-supported catalysts in Suzuki–Miyaura coupling reactions allows the rapid
and high-yield preparation of ultrapure oligothiophenes at the best of their functional properties,
that is, displaying excellent charge transport mobility and fluorescence efficiency.
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