Electropolymerizable 3Dπ-conjugated architectures with ethylenedioxythiophene (EDOT) end-groups as precursors of electroactive conjugated networks

Article abstract of DOI:10.1039/c0jm01873b

Three-dimensional conjugated architectures involving conjugated branches with terminal EDOT groups attached onto a bithiophene core twisted by ca. 90° by steric interactions have been synthesized by Stille coupling reactions. The UV-Vis absorption spectra recorded in solution show complex spectral features that depend on both the size and chemical structure of the main conjugated segment and of the conjugated side chains. Thanks to the fixation of the terminal EDOT groups, these compounds undergo straightforward and complete electropolymerization to produce stable electrode materials. The analysis of the electrochemical and optical properties of the polymers by cyclic voltammetry and spectroelectrochemistry suggests that the electrochemical coupling of the terminal EDOT groups leads to the formation of π-conjugated networks the electrochemical and optical properties of which can be tuned through the length and chemical composition of the oligomeric conjugated links.

Full text of DOI:10.1039/c0jm01873b

View Article Online / Journal Homepage / Table of Contents for this issue
PAPER
www.rsc.org/materials | Journal of Materials Chemistry
Electropolymerizable 3Dp-conjugated architectures with
ethylenedioxythiophene (EDOT) end-groups as precursors of electroactive
conjugated networks†
a
b
a
Flavia Piron,ab Philippe Leriche, Ion Grosu and Jean Roncali
*
Received 12th June 2010, Accepted 21st July 2010
DOI: 10.1039/c0jm01873b
Three-dimensional conjugated architectures involving conjugated branches with terminal EDOT
groups attached onto a bithiophene core twisted by ca. 90ꢀ by steric interactions have been synthesized
by Stille coupling reactions. The UV-Vis absorption spectra recorded in solution show complex spectral
features that depend on both the size and chemical structure of the main conjugated segment and of the
conjugated side chains. Thanks to the fixation of the terminal EDOT groups, these compounds undergo
straightforward and complete electropolymerization to produce stable electrode materials. The analysis
of the electrochemical and optical properties of the polymers by cyclic voltammetry and
spectroelectrochemistry suggests that the electrochemical coupling of the terminal EDOT groups leads
to the formation of p-conjugated networks the electrochemical and optical properties of which can be
tuned through the length and chemical composition of the oligomeric conjugated links.
heterojunction solar cells.17–19 Concurrently, several groups have
Introduction
recently reported the synthesis of the first examples of micro-
porous p-conjugated polymers.20–25 Although these materials
have been initially produced by transition-metal-catalyzed
coupling reactions,20–23 or by hyper-crosslinking of polyaniline or
Electrogenerated conducting polymers (ECP) have been a focus
of sustained interest for more than three decades.1–6While for
many years electrochemical polymerization has represented
a major route for the preparation of conducting polymers such as
poly(pyrrole)1 or poly(thiophene),2,3 with the development of the
chemical syntheses of conjugated polymers,4 the use of electro-
polymerization has progressively turned towards the synthesis of
functional electroactive polymers designed as electrode materials
for applications in energy, storage, electrocatalysis, or electro-
chemical sensors.4–7
polypyrrole,23,24
a first preparation of microporous poly-
(thienylene-arylenes) by oxidative polymerization of 3D conju-
gated architectures using iron trichloride in acetonitrile, has
recently been reported.25
In this context, it could be interesting to take advantage of the
simplicity and rapidity of electrochemical polymerization to
develop electrode materials based on p-conjugated networks by
electropolymerization of 3D precursors with multiple polymeri-
zation sites. A first step in this direction was reported a few years
ago with the study of the electrochemical polymerization of tetra-
terthienylsilane.26 However, the poor reactivity of terthiophene
cation radical and the lability of the thiophene–silicon bond
represented serious obstacles to the extension of this approach.
More recently we have reported preliminary results on the
successful electrochemical polymerization of a 3D precursor
consisting of a twisted bithiophene possessing four electro-
polymerizable EDOT groups (E4T2, Chart 1).27
Functional ECP of the first generation basically involved the
covalent fixation of functional groups onto an electro-
polymerizable pyrrole or thiophene monomer via a flexible
linker.1–4 The intensive research effort invested in the synthesis of
functional ECPs during more than two decades has led to a new
generation of precursors combining efficient electro-
polymerizability at low potential and low concentration, and
thus to the synthesis of advanced electrode materials containing
more complex functional groups.6–11 On the other hand, the
design of precursors containing multiple polymerization sites has
also contributed to the preparation of functional ECP with
improved sensitivity and stability under cycling.8–10
As a further extension of this strategy, we describe here the
synthesis and characterization of a series of 3D precursors built
by attaching conjugated branches of various size and composi-
tion onto a bithiophene core twisted by steric interactions. The
fixation of terminal EDOT groups at the end of each conjugated
branch takes advantage of the strong electron-donor properties,
reactivity, selectivity and self-structuring effect of the EDOT unit
to ensure efficient and complete electropolymerization at
moderate applied potentials.28
Recent years have seen the emergence of various classes of
molecular three-dimensional p-conjugated architectures
designed for multiple applications including molecular elec-
tronics,12 field-effect transistors,13 light-emitting materials for
electroluminescent devices,14–16 or donor materials for bulk
aGroup Linear Conjugated Systems, CNRS, MOLTECH, University of
Angers,
univ-angers.fr
2 Bd Lavoisier, F-49045, France. E-mail: jean.roncali@
These compounds have been electrochemically polymerized
and the electrochemical and optical properties of the resulting
ECP deposited on platinum and ITO electrodes have been
analyzed by cyclic voltammetry and NIR spectroelec-
trochemistry. The results provide a coherent picture, consistent
bFaculty of Chemistry and Chemical Engineering, Babes-Bolyai University,
Cluj-Napoca, 11 Arany Janos str, 400028 Cluj-Napoca, Romania
† CCDC reference number 773448. For crystallographic data in CIF or
other electronic format see DOI: 10.1039/c0jm01873b
10260 | J. Mater. Chem., 2010, 20, 10260–10268
This journal is ª The Royal Society of Chemistry 2010

View Article Online
Chart 1
with the formation of electroactive conjugated networks, in
and Pd(PPh3)4 catalyst (0.924 g, 0.8 mmol, 5%) were mixed in
100 mL of dry toluene and refluxed for 8 h under nitrogen. A
second portion of catalyst (1%) was added and the mixture was
refluxed for another 24 h. The mixture was washed with water,
extracted with toluene (20 ml), filtered over Celite and dried under
vacuum. The residue was purified by column chromatography
(diethyl ether) to generate 1.0 g of a yellow–orange solid. Yield:
69%, mp 215 ꢀC. 1H NMR (500 MHz, CDCl3) d/ppm: 7.58 (s, 2H),
6.24 (s, 2H), 6.13 (s, 2H), 4.32–4.33 (m, 4H), 4.23–4.25 (m, 4H),
4.17–4.18 (m, 4H), 4.14–4.15 (m, 4H). 13C NMR (125 MHz,
CDCl3) d/ppm: 114.78, 140.97, 137.95, 137.78, 135.55, 133.10,
126.23, 122.39, 112.29, 111.94, 99.13, 97.20, 64.98, 64.63, 64.57,
64.39. MS (MALDI-TOF): found 725.95; (HRMS): calc.
725.96389; found 725.96424.
which the electronic properties are determined by the length and
composition of the discrete oligomeric linkers formed by the
electrochemical coupling.
Experimental
General
1H and 13C NMR spectra were recorded on a Bruker AVANCE
DRX 500 spectrometer operating at 500.13 and 125.7 MHz and
Bruker AVANCE 300 spectrometer operating at 300 and
75 MHz; d are given in ppm (relative to TMS) and coupling
constants (J) in Hz. Mass spectra were recorded under EI mode
on a VG-Autospec mass spectrometer or under MALDI-TOF
mode on a MALDI-TOF-MS BIFLEX III Bruker Daltonics
spectrometer. UV-visible optical data were recorded with
a Perkin-Elmer Lambda 19 and Lambda 950 spectrophotome-
ters. Melting points were obtained from a Reichert–Jung
Thermovar hot-stage microscope apparatus and are uncorrected.
Column chromatography purifications were carried out on
Merck silica gel Si 60 (40–63 mm).
2,20,5,50-Tetrakis(3,4-ethylenedioxythiophene)-3,30-dithienyl-
2,20-bithiophene (E4T4)
Tetrabromo-3,30-dithienyl-2,20-bithiophene (0.210 g, 0.325 mmol),
2-(tributylstannyl)-3,4-(ethylenedioxy)thiophene (1.121 g,
2.6 mmol) and Pd(PPh3)4 catalyst (1.50 g, 0.13 mmol, 5%) were
mixed in 25 mL of dry toluene and refluxed for 8 h under
nitrogen. A second portion of catalyst (1%) was added and the
mixture was refluxed for another 48 h. The mixture was washed
with water, extracted with toluene (20 ml), filtered over Celite
and dried under vacuum. The residue was purified by column
chromatography (diethyl ether) to generate 90 mg of a yellow–
orange solid. Yield: 31%, mp 142 ꢀC (decomp.). 1H NMR
(500 MHz, CDCl3) d/ppm: 7.40 (s, 2H), 6.98 (d, 2H), 6.88 (d,
2H), 6.27 (s, 2H), 6.15 (s, 2H), 4.33–4.35 (m, 4H), 4.24–4.27 (m,
4H), 4.19–4.20 (m. 4H). 13C NMR (125 MHz, CDCl3) d/ppm:
141.81, 141.74, 138.20, 137.40, 136.29, 135.77, 135.45, 134.32,
125.37, 125.01, 123.29, 122.41, 112.28, 111.51, 97.54, 96.80,
Electrochemical experiments were performed with an EG&G
273 potentiostat in a standard three-electrode cell using platinum
electrodes and a saturated calomel reference electrode (SCE).
The solutions were degassed by argon bubbling and experiments
were carried under an argon blanket. The working electrode was
a 2 mm diameter Pt disk sealed in glass. Tetrabutylammonium
hexafluorophosphate (Fluka puriss) was used as received.
3,30,5,50-Tetrakis(3,4-ethylenedioxythiophene)-2,20-bithiophene
(E4T2)
3,30,5,50-Tetrabromo-2,20-bithiophene (0.964 g, 2 mmol), 2-(tri-
butylstannyl)-3,4-(ethylenedioxy)thiophene (6.899 g, 16 mmol)
This journal is ª The Royal Society of Chemistry 2010
J. Mater. Chem., 2010, 20, 10260–10268 | 10261

View Article Online
65.02, 64.85, 64.55 (2C). MS (MALDI-TOF): found 889.7;
(HRMS): calc. 889.93933; found 889.93901.
by column chromatography (diethyl ether followed by dichloro-
methane) and precipitated using a mixture of dichloromethane
and hexane, to generate 50 mg of an orange solid. Yield: 7%.
1H NMR (500 MHz, CDCl3) d/ppm: 7.42 (s, 1H), 6.99 (d, 1H),
6.90 (d, 1H), 6.31 (s, 1H), 6.25 (d, 1H), 4.38 (m, 6H), 4.26 (m,
6H), 4.23 (m, 1H), 4.20 (m, 1H). 13C NMR (125 MHz, CDCl3)
d/ppm: 141.34 (1C), 141.28 (1C), 137.86 (1C), 137.29 (2C), 137.09
(1C), 137.02 (2C), 136.96 (1C), 135.76 (1C), 135.48 (1C), 134.32
(1C), 125.61 (1C), 123.09 (1C), 110.26 (1C), 109.92 (1C), 109.81
(1C), 109.28 (1C), 108.43 (1C), 107.64 (1C), 98.23 (1C), 97.83
(1C), 65.20 (1C), 65.11 (2C), 65.02 (2C), 64.92 (1C), 64.70 (2C).
HRMS: calc. 1449.9121; found 1449.9128
3,30,5,50-Tetrakis(2-thienyl)-2,20-bithiophene (T6)
3,30,5,50-Tetrabromo-2,20-bithiophene (2 g, 4.15 mmol), 2-(tri-
butylstannyl)thiophene (6.81 g, 18.26 mmol), and Pd(PPh3)4
catalyst (1.05 g, 0.91 mmol, 5%) were mixed in 50 mL of dry
toluene and refluxed for 12 h under nitrogen. The mixture was
washed with water, extracted with toluene (20 ml), filtered over
Celite and dried under vacuum. The residue was purified by
column chromatography (petroleum spirit–diethyl ether ¼ 2 : 1)
to generate 400 mg of a white solid. Yield: 20%. 1H NMR
(500 MHz, CDCl3) d/ppm: 7.42 (s, 2H), 7.28 (d, 2H), 7.24 (d,
2H), 7.14 (d, 2H), 7.08 (d, 2H), 7.04–7.05 (dd, 2H), 6.90–6.92 (dd,
2H). 13C NMR (125 MHz, CDCl3) d/ppm: 139.00, 137.12, 136.72,
136.42, 127.95, 127.03, 125.60, 125.30, 125.13, 124.44, 123.79.
MS (EI) 493.85.
2,5-Bis(3,4-ethylenedioxythiophene)-3,4-dibutylthiophene (6)
2,5-Dibromo-3,4-dibutylthiophene (6.15 g, 17.4 mmol), 5-(tri-
butylstannyl)-2,20-bis(3,4-ethylenedioxythiophene)
(22.5
g,
52.2 mmol) and Pd(PPh3)4 catalyst (3 g, 5%) were mixed in
500 mL of dry toluene and refluxed for 12 h under nitrogen. A
second portion of catalyst (1%) was added and the mixture was
refluxed for another 12 h. The mixture was filtered over silica and
Celite, and washed with toluene and dichloromethane. Solvents
were evaporated and the residuum was purified by chromatog-
raphy (petroleum spirit–dichloromethane 2 : 1) affording 6.7 g of
3,30,5,50-Tetrakis[bis(3,4-ethylenedioxythiophene)]-2,20-bithio-
phene (E8T2)
3,30,5,50-Tetrabromo-2,20-bithiophene (240.92 mg, 0.5 mmol), 5-
(tributylstannyl)-2,20-bis(3,4-(ethylenedioxy)thiophene) (1.71 g,
3 mmol) and Pd(PPh3)4 catalyst (150 mg, 5%) were mixed in
50 mL of dry toluene and refluxed for 12 h under nitrogen. A
second portion of catalyst (1%) and stannane were added and the
mixture was refluxed for another 12 h. The mixture was filtered
over a pad of silica and Celite, washed with toluene, diethyl ether,
dichloromethane and acetone. The toluene fraction contains
mostly 2,20-bis(3,4-ethylenedioxythiophene), the dichloro-
methane fraction contained the desired product while the acetone
fraction contained triphenylphosphine. After solvent evapora-
tion, the compound was purified by column chromatography
1
an oily liquid. Yield: 80%. H NMR (500 MHz, CDCl3) d/ppm:
6.36 (s, 1H), 4.26–4.24 (m, 2H), 4.23–4.20 (m, 2H), 2.67–2.64 (m,
2H), 1.53–1.49 (m, 2H), 1.40–1.36 (m, 2H), 0.93–0.90 (t, 3H). 13
C
NMR (125 MHz, CDCl3) d/ppm: 141.25, 140.74, 138.06, 127.47,
110.41, 98.80, 64.69, 64.40, 32.59, 27.80, 22.94, 13.80.
2-(2-Tributylstannyl-3,4-ethylenedioxythiophene)-5-(3,4-ethylene-
dioxythiophene)-3,4-dibutylthiophene (7)
To a solution of 2,5-bis(3,4-ethylenedioxythiophene)-3,4-dibu-
tylthiophene (3.35 g, 7 mmol) in 300 ml of dry THF, n-BuLi
(4.4 ml, 7 mmol) was added at ꢁ78 ꢀC under nitrogen and stirred
for 1 h. Tributylstannyl chloride (1.9 ml, 7 mmol) was added
dropwise at ꢁ40 ꢀC and the mixture was stirred overnight while
allowing to warm to room temperature. Solvent was removed,
extraction was carried out with ethyl acetate and the organic
layer was washed several times with sodium fluoride saturated
solution. After drying over magnesium sulfate, solvent was
removed affording 2.7 g of a brown liquid. Yield: 62%. 1H NMR
(500 MHz, CDCl3) d/ppm: 6.33 (s, 1H), 4.25–4.24 (m, 2H), 4.23–
4.21 (m,4H), 4.17–4.16 (m, 2H), 2.68–2.62 (m, 4H), 1.58–1.55 (m,
10H), 1.39–1.30 (m, 10H), 1.13–1.09 (m, 6H), 0.92–0.88 (m,
15H).
(dichloromethane) and precipitated using
a mixture of
dichloromethane and hexane, to generate 330 mg of an orange
1
solid. Yield: 51%. H NMR (500 MHz, CDCl3) d/ppm: 7.34 (s,
1H), 6.29 (s, 1H), 6.18 (s, 1H), 4.37–4.35 (m, 6H), 4.27–4.24 (m,
2H), 4.15–4.16 (m, 6H), 4.03–4.01 (m, 2H).13C NMR (125 MHz,
CDCl3) d/ppm: 141.37 (1C), 141.33 (1C), 137.49 (1C), 137.32
(1C), 137.21 (1C), 137.12 (1C), 136.91 (1C), 136.66 (1C), 134.75
(1C), 131.70 (1C), 128.54 (1C), 124.20 (1C), 110.30 (1C), 110.24
(1C), 109.91 (2C), 109.66 (1C), 107.90 (1C), 98.09 (1C), 96.97
(1C), 65.18 (1C), 65.03 (2C), 64.80 (1C), 64.73 (1C), 64.68 (2C),
64.47 (1C). HRMS: calc. 1285.93669; found 1285.9404.
2,20,5,50-Tetrakis[bis(3,4-ethylenedioxythiophene)]-3,30-dithienyl-
2,20-bithiophene (E8T4)
2,20,5,50-Tetrakis[(3,4-ethylenedioxythiophene)-5-(3,4-ethylene-
dioxythiophene)-3,4-dibutylthiophene]-3,30-dithienyl-2,20-bithio-
phene (E8T6)
Tetrabromo-3,30-dithienyl-2,20-bithiophene (323.04 mg, 0.5 mmol),
5-(tributylstannyl)- 2,20-bis(3,4-ethylenedioxythiophene) (1.71 g,
3 mmol) and Pd(PPh3)4 catalyst (150 mg, 5%) were mixed in
50 mL of dry toluene and refluxed for 12 h under nitrogen. A
second portion of catalyst (1%) and stannane were added and the
mixture was refluxed for another 24 h. The mixture was filtered
over a pad of silica and Celite, and washed with toluene, diethyl
ether, dichloromethane and acetone. The toluene fraction
contains mostly 2,20-bis(3,4-ethylenedioxythiophene), while
dichloromethane and acetone fractions contained the desired
product. After solvent evaporation, the compound was purified
3,30,5,50-Tetrabromo-2,20-bithiophene (200 mg, 0.41 mmol),
compound 7 (1.9 g, 2.5 mmol), and Pd(PPh3)4 catalyst (150 g,
5%) were mixed in 100 mL of dry toluene and refluxed for 12 h
under nitrogen. A second portion of catalyst (1%) and stannane
were added and the mixture was refluxed for another 24 h period.
The mixture was filtered over a pad of silica and Celite, washed
with toluene, diethyl ether, dichloromethane and acetone. After
solvent evaporation, the product was purified by column
10262 | J. Mater. Chem., 2010, 20, 10260–10268
This journal is ª The Royal Society of Chemistry 2010

View Article Online
chromatography (diethyl ether first, followed by dichloro-
methane). A second chromatography was performed using
toluene as eluent to generate 500 mg of a red–brown solid. Yield:
1
58%. H NMR (500 MHz, CDCl3) d/ppm: 7.67 (s, 1H), 6.36 (s,
1H), 6.31 (s, 1H), 4.36–4.19 (m, 16H), 2.59–2.52 (m, 4H), 2.74–
2.64 (m, 4H), 1.56–1.19 (m, 16H), 0.96–0.77 (m, 12H). 13C NMR
(125 MHz, CDCl3) d/ppm: 141.34, 141.28, 137.86, 137.29,
137.09, 137.02, 136.96, 135.76, 125.61, 123.09, 109.92, 109.81,
108.43, 98.23, 97.83, 65.20, 65.11, 65.02, 64.92, 64.70. MS
(MALDI-TOF): calc. 2062.39; found 2062.29. Attempts to
obtain the HRMS were unsuccessful.
Crystal data and structure refinement for compound T6.
Empirical formula: C24H14S6, Mr ¼ 494.71, T ¼ 293(2) K, l ¼
˚
0.71073 A, monoclinic, space group P21, a ¼ 12.010(1), b ¼
ꢀ
3
˚
˚
6.0745(4), c ¼ 15.626(2) A, b ¼ 104.14(1) , V ¼ 1105.48(18) A ,
Z ¼ 2, Dc ¼ 1.486 g cmꢁ3, m ¼ 0.629 mmꢁ1, F(000) ¼ 508, crystal
size 0.62 ꢂ 0.15 ꢂ 0.08 mm, q range for data collection: 1.75–
26.04ꢀ, index ranges: ꢁ14 # h # 14, ꢁ7 # k # 7, ꢁ19 # l # 19,
reflections collected ¼ 15744, independent reflections ¼ 4289
[R(int) ¼ 0.0745], refinement method: full-matrix least-squares
on F2, data/restraints/parameters ¼ 4289/6/309, goodness-of-
method on F2 ¼ 1.022, final R indices [I > 2s(I)]: R1 ¼ 0.0462,
wR2 ¼ 0.1265; R indices (all data) R1 ¼ 0.0689, wR2 ¼ 0.1424;
ꢁ3
˚
largest diff. peak and hole: 0.421 and ꢁ0.434 e A
.
Scheme 1 Synthesis of the 3D precursors.
Results and discussion
Synthesis
accessibility of the reaction site in the case of the bulky Stille
reagent. This hypothesis is supported by the moderate yield of
E4T4 (31%) and the failure to synthesize E8T8 by coupling the
Stille reagent 7 with the tetrabromo compound 2.
The target compounds have been synthesized by fixation of
conjugated branches at the a,a0 and internal b,b0-position of
a bithiophene core. The steric hindrance to coplanarity caused by
the fixation of the two thiophene rings at the two internal
b-positions generates a dihedral twist angle close to 90ꢀ thus
making this system an interesting starting block to design various
tetrahedral p-conjugated architectures by varying the length and
the composition of the attached side branches.
In an attempt to extend the size of the starting twisted building
block, compound T6 has been synthesized by coupling
compound 1 with the Stille reagent of thiophene (Scheme 2).
A good quality single crystal of T6 was grown from a 2 : 1
petroleum ether–diethyl ether solution. The analysis of its crys-
tallographic structure by X-ray diffraction reveals a 80ꢀ dihedral
angle between the two thiophene rings forming the central 2,20-
bithiophene core (Fig. 1). This result confirms, in agreement with
previous results obtained on related compounds, that fixation of
2-thienyl substituents at the internal b-positions of a bithiophene
is a convenient method to generate a twisted structure in which
the two thiophene rings are almost perpendicular.19Unfortu-
nately all attempts to selectively brominate compound T6 at the
four free a-positions remained unsuccessful and this route was
not pursued further.
The synthetic work has been carried out using a few building
blocks, namely the tetrabromo bi-and quarter-thiophene 1 and 2
and the Stille reagents containing a variable number of EDOT
and thiophene units 3, 4 and 7 (Scheme 1).3,30,5,50-tetrabromo-
2,20-bithiophene (1)19,29 and tetrabromo-3,30-dithienyl-2,20-bithio-
phene19 (2) were synthesized using known procedures.
The stannylated derivatives of EDOT (3) and bis-EDOT
(4) were prepared according to the previously published
methods.30,31 The terthienyl compound 6 was synthesized in 80%
yield by Stille coupling of compound 3 with 2,5-dibromo-3,4-
dibutylthiophene (5). This latter compound was prepared in 90%
yield by bromination of 3,4-dibutylthiophene with NBS in acetic
acid–chloroform. Treatment of terthienyl6 with butyllithium and
tin tributyl chloride gave the stannylated compound 7 in 62%
yield.
All target compounds E4T2, E4T4, E8T2, E8T4 and E8T6 were
synthesized by a fourfold Stille coupling reaction between the
tetrabromo compounds 1 and 2 and the appropriate Stille
reagent, 3, 4 or 7. The final compounds were obtained in
moderate to good yields except for E8T4 which was isolated in
7% yield only. This low yield was attributed to the difficult
Scheme 2 Synthesis of compound T6.
This journal is ª The Royal Society of Chemistry 2010
J. Mater. Chem., 2010, 20, 10260–10268 | 10263

View Article Online
Fig. 1 Crystallographic structure of compound T6.
UV-Vis absorption spectroscopy
Fig. 2 shows the UV-Vis absorption spectra of the five compounds
recorded in methylene chloride solutions. All compounds present
a complex UV-Vis spectrum in which two main absorption
features can be distinguished around 300 nm and in the 370–
400 nm region (Table 1). These two maxima are consistent with
the coexistence of two different conjugation pathways in the 3D
structure. The band at the longest wavelength can be assigned to
the longest conjugated segment in the molecule, while the short
wavelength band can be attributed to the side chain.
For compounds E4T2 and E4T4 the longest a–a0 linked
conjugated chain (LCC) corresponds to the EDOT–thiophene–
thiophene–EDOT (ETTE) sequence. Because of the large twist
angle (q) in the middle of the structure one can consider that the
effective conjugation length (ECL) corresponds to the half of the
LCC due to complete interruption of conjugation achieved for
q ¼ 90ꢀ. The ECL corresponds to the ET sequence for E4T2 and
E4T4, EET for E8T2 and ETET for E8T6 (Table 1).
Fig. 2 UV-Vis absorption spectra in methylene chloride. Top: E4T2
(---), E4T4 (—); middle: E8T2, (---) E8T4 (—); bottom E8T6.
Table 1 UV-Vis absorption data for 3D compounds in methylene
chloride solution
In fact the 300 nm value of lmax for E4T2 and E4T4 is consistent
with this conclusion and comparison with theETTE molecule
(lmax ¼ 412 nm),30 reveals a considerable hypsochromic shift, in
agreement with the large torsion angle in the middle of the four-
ring system.
Compound
lmax/nm
LCC
ECL
E4T2
E4T4
E8T2
E8T4
E8T6
310, 370
300, 370
390
416
390
ETTE
ETTE
EETTEE
EETTEE
ETETTETE
TE
TE
TEE
TEE
TETE
The introduction of an additional thiophene ring in the side
chain achieved in compound E4T4 produces an increase of the
intensity of the 370 nm band. This phenomenon could be related
to an enhancement of the absorption coefficient of the chromo-
phore due to an increase of the electron-donor effect of the side-
chain on the main conjugated chain.
Increasing the length of the LCC to a six ring system (EET-
TEE) in E8T2 and E8T4 leads as expected to a bathochromic shift
of lmax to 390 and 416 nm, respectively, while the spectrum
10264 | J. Mater. Chem., 2010, 20, 10260–10268
This journal is ª The Royal Society of Chemistry 2010

View Article Online
exhibits essentially a single absorption band. Again, comparison
of these values to those of hybrid EDOT–thiophene oligomers of
comparable chain length reveals a considerable blue shift due to
the shortened ECL created by the twisted structure.30,31 On the
other hand, the 26 nm red shift of lmax between E8T2 and E8T4
can be attributed to the combined effects of a stronger electron-
donor effect of the branch and a slight decrease of the steric
interactions and hence of the dihedral twist angle when replacing
EDOT by thiophene at the internal b-positions of the median
bithiophene core. Furthermore, the fact that E8T2 and E8T6
show the same lmax suggests that besides an extension of the
conjugated system, the 3,4-dibutylthiophene rings induce addi-
tional steric interactions and distortions that limit the ECL in the
branches. Finally, keeping in mind the large dihedral angle in the
middle of the LCC, the optical features of the largest 3D mole-
cules are consistent with the presence of four conjugated
branches of rather comparable ECL, unlike the smaller systems
E4T2 and E4T4.
Electrochemistry
The electrochemical polymerization of E4T2, E4T4, E8T4 and
E8T6 has been analyzed by cyclic voltammetry in acetonitrile,
dichloromethane or a mixture of the two solvents, in the presence
of 0.10 M tetrabutylammonium hexafluorophosphate as sup-
porting electrolyte. Compound E8T2 was not investigated
because the obtained quantity was insufficient for a complete
electrochemical and spectroelectrochemical analysis.
The cyclic voltammogram resulting from the application of
a single potential scan to millimolar solutions of the 3D
compounds shows a first anodic peak potential (Epa) corres-
ponding to oxidation of the system to the cation radical state. As
already discussed, the ECL corresponds to the half of the LCC
sequence namely ET for E4T2 and E4T4, EET for E8T4 and
ETET for E8T6. The Epa values in Table 2 are consistent with this
conclusion. E4T2 and E4T4 present very similar Epa values at 0.85
and 0.87 V, respectively. The CV of E8T4 shows a similar Epa
value, with however an onset for oxidation at lower potential
suggesting the possible co-existence of different molecular
conformations. Finally E8T6, presents the lowest Epa value of
0.65 V which is consistent with the longest ECL (TETE).
Furthermore, the oxidation process becomes clearly reversible
whereas a second oxidation peak assigned to the dication state of
the TETE chain is observed around 1.00 V. These results are in
good agreement with the Epa values of 0.56 and 0.90 V for the
two oxidation processes of the TETE oligomer.31
Fig. 3 Potentiodynamic electropolymerization of the 3D precursors.
Top: 10 mM E4T4 in 0.10 M Bu4NPF6–CH3CN. Middle: 1 mM E8T4 in
0.10 M Bu4NPF6–CH2Cl2. Bottom: 1 mM E8T6 in 0.10 M Bu4NPF6–
CH2Cl2, scan rate 100 mV sꢁ1
.
Electropolymerization of the four compounds was performed
in potentiostatic and potentiodynamic conditions. Application
of repetitive potential scans to electrolytic solutions of the
precursors, leads to the progressive development of a broad
redox system typical of the electrodeposition of an electroactive
material in the 0.20–0.80 V potential region. This new redox
system is essentially structureless except for E8T4 for which two
sub-components are clearly distinguished around 0.50 and
0.80 V. Despite the sterically constrained structure of the
precursors, a straightforward electropolymerization is observed
in every case as could be expected from the presence of highly
reactive EDOT end groups.28,33
Table 2 Cyclic voltammetric data for the 3D precursors and corre-
sponding polymers in 0.10 M Bu4NPF6–CH3CN, scan rate 100 mV sꢁ1
Reference SCE
.
Electropolymerization was also performed in potentiostatic
conditions at an applied potential slightly higher than the anodic
peak potential determined by single scan CV (Table 2). This
slight overpotential was necessary to obtain homogeneous
polymer films. The CV of the resulting polymers were then
analyzed in acetonitrile in the presence of 0.10 M Bu4NPF6 as
supporting electrolyte (Fig. 4).
Epa
Compound Epa/V (polymer)/V Qd/mC cmꢁ2 Qr/Qd ECL (max.)
E4T2
E4T4
E8T4
E8T6
PEDOT
0.85
0.87
0.86
0.65
1.49
0.65
0.69
0.50, 0.75
0.62
0.04
33
20
13
16
0.336 TEET
0.225 TEET
0.667 TEEEET
0.684 TETEETET
0.080
Comparison of the ratio of the charge reversibly exchanged
upon redox cycling of the polymers (Qr) to the electrodeposition
132
This journal is ª The Royal Society of Chemistry 2010
J. Mater. Chem., 2010, 20, 10260–10268 | 10265

View Article Online
Scheme 3 Coupling possibilities of the E4T2 precursor.
horizontal branch leads to a TEET sequence which represents the
longest possible conjugated segment. A homo-coupling of two E
side groups will form an EE segment (and not a TEET, because
the fixation of the EDOT at the b-position of thiophene with
a large twist angle interrupts the conjugation) while finally
a hetero-coupling between a ‘‘horizontal’’ TE and a ‘‘vertical’’ E
branch will generate a TEE segment (Scheme 3). A similar
analysis shows that for E4T4 all couplings produce a TEET
sequence, while for E8T4 and E8T6 the longest conjugated
sequences are TEEEET and TETEETET, respectively. Further
considerations show that when the chemical composition of the
side chain is similar to that of the half of the LCC polymerization
should produce a network with quasi-monodisperse conjugated
linkers.
Fig. 4 Cyclic voltammograms recorded in 0.10 M Bu4NPF6–CH3CN,
scan rate 100 mV sꢁ1. Top: poly(E4T4); Middle: poly(E8T4); Bottom:
poly(E8T6). The polymers were deposited in potentiostatic conditions
using the Qd values indicated in Table 2.
Examination of the Epa values for the various polymers shows
a good agreement with the conclusions of this analysis. As
expected, the polymers derived from E4T2 and E4T4 show very
similar Epa values of 0.65 and 0.69 V. A slightly lower value is
found for poly(E8T6), for which the steric hindrance to planarity
caused by the 3,4-dibutylthiophene units limits the ECL in spite
of a longer LCC. Finally the low Epa value of poly(E8T4) and the
two reversible oxidation waves in the CV are consistent with the
formation of the highly conjugated EEEE sequence which
combines the strong electron-donor effect of EDOT with the self-
rigidification associated with intramolecular sulfur–oxygen
interactions.28,32
charge (Qd) shows that Qr/Qd is approximately four times larger
for E4T2 than for poly(EDOT) used as reference and eight times
larger for E8T4 and E8T6. These results are in good agreement
with the expected larger amount of electroactive material
produced with a given Qd for the polymers obtained from the 3D
precursors.
Electropolymerization of these 3D precursors is expected to
produce conjugated networks by coupling of the terminal EDOT
units. However, the ca. 90ꢀ dihedral angle in the middle of the
LCL divides it into two quasi-independent conjugated segments
that define the ECL. On the other hand, the lack of coplanarity of
the side chains with the LCL, limits p-electron delocalization and
hence electronic interactions between the side chains and the
LCL. Consequently these side chains can be roughly considered
as independent. In this context, the material produced by elec-
trochemical coupling of the terminal EDOT units can be viewed
as a ‘‘poly-oligomeric system’’ containing discrete conjugated
segments resulting from the duplication of the elemental conju-
gated segments of the starting pseudo-tetrahedral structure.
Scheme 3 shows the various possibilities of coupling taking
E4T2 as a representative example. The coupling of the TE
Whereas these results clearly show that all compounds
undergo straightforward electropolymerization, a major ques-
tion concerns the completion of the process. In other words are
all EDOT end groups effectively involved in the electro-
polymerization process to produce the expected electroactive
conjugated network?
Previous work has shown that the electrooxidation of short-
chain oligothiophenes leads to the electrodeposition of materials
that contain significant amounts of short-chain oligomers and of
unreacted starting material.34–37 This point has been demon-
strated by extraction of the electrodeposited material,35 or by the
analysis of the electrochemical and optical properties of elec-
trodeposited films immersed in a monomer-free electrolytic
10266 | J. Mater. Chem., 2010, 20, 10260–10268
This journal is ª The Royal Society of Chemistry 2010

View Article Online
Fig. 5 CV of the polymers electrodeposited on platinum electrodes
recorded in 0.10 Bu4NPF6–CH3CN, with a progressive increase of the
upper potential limit. Top: poly(E8T6), (0.0 to +1.80 V). Bottom: poly-
(E8T4) (0.0 to +2.00 V, scan rate 100 mV sꢁ1
.
medium and subjected to repetitive voltammetric scans with an
increasingly positive potential limit. Thus, application of this
procedure to poly(terthiophenes) produces a negative shift of the
oxidation wave and a red shift of lmax due to increase of the mean
conjugation length resulting from the coupling of unreacted
short-chain oligomers trapped in the electrodeposited mate-
rial.36,37 Furthermore, these modifications are accompanied by
a significant increase of the conductivity of the material.37
In order to confirm the complete electropolymerization of the
3D precursors, electrodes modified by the electrodeposited
polymers have been immersed in an electrolytic medium and
subjected to continuous redox cycling with application of
a progressively increasing positive potential limit. After each
incremental increase of the potential limit, the CV was recorded
within the initial potential window in order to detect any even-
tual modification of shape or amount of exchanged charge. Fig. 5
shows as representative examples the results of such experiments
for poly(E8T2) and poly(E8T6). In fact, for all electrodeposited
polymers, no significant modification of the CV was observed
until the applied potential reaches the limit where the polymer
begins to degrade (ca. 1.60 V). This behavior provides a strong
support to the absence of unreacted EDOT groups in the poly-
mer films and to the conclusion that all EDOT end-groups have
been coupled during the initial electropolymerization process, as
could be anticipated from the well-known high reactivity of
EDOT.28,33
Fig. 6 Spectroelectrochemistry of the polymers on ITO at various
applied potentials. Top: poly(E4T4). Bottom: poly(E8T4). Electrolytic
medium 0.10 M Bu4NPF6–CH3CN, reference SCE.
distribution of conjugated segments associated with the speci-
ficity of the coupling process (Scheme 3). Application of positive
potentials corresponding to the maximum oxidation of the
polymer, namely +0.80 V (see the CV in Fig. 4), to the film
produces the bleaching of the visible absorption band together
with the concomitant development of two broad absorption
bands around 620 and 1300 nm corresponding to the two tran-
sitions associated with the polaronic state. The neutral polymer
appears red and turns gradually blue upon oxidation. Interest-
ingly, upon application of more positive potentials these two
bands do not merge into the single transition corresponding to
the bipolaron state. This result suggests that the mean conjuga-
tion length in the polymer network is too short to accommodate
a bipolaron state.
A quite different situation is observed for poly(E8T4). The
spectrum of the neutral state shows an absorption maximum at
510 nm, significantly red shifted compared to the previous one.
This value agrees well with the larger ECL already suggested by
electrochemical data. Application of increasingly positive
potentials produces the progressive appearance of the two
polaronic transitions around 750 and 1500 nm while the intensity
of these bands begins to decrease for potential values above
0.60 V with an evolution towards a single bipolaronic transition.
This easy access to the bipolaron state is in good agreement with
the two reversible oxidation steps observed in the CV.
The optical properties of the polymers have been analyzed on
thin film electrodeposited on ITO-coated glass electrodes. Fig. 6
shows the UV-Vis-NIR spectra of polymers recorded in situ at
various applied potentials. The spectrum of the fully undoped
E4T4 is very similar to the already reported E4T2 one27 and shows
a lmax at 470 nm with an absorption onset at 640 nm corres-
ponding to a band gap of 2.00 eV. The small width of the
absorption band is consistent with the expected narrow
The behavior of poly(E8T6) (not shown) is rather similar to
that of poly(E8T4). The spectrum of the neutral state presents
a lmax at 446 nm, this blue shift which indicates a shorter ECL
than the previous polymers can be explained by the distortions
This journal is ª The Royal Society of Chemistry 2010
J. Mater. Chem., 2010, 20, 10260–10268 | 10267

View Article Online
introduced in the conjugated branches by the steric effects of the
3,4-dibutylthiophene unit. Application of more positive potential
leads to a decrease of the intensity this band with the concomi-
tant development of two bands at ca. 750 and 1500 nm. Upon
application of rather high potentials (1.20–1.30 V) the intensity
of these two bands progressively decreases and they begin to
merge into a single broad band with a maximum around 1000 nm
which suggests the formation of a bipolaronic state.
References
1 A. F. Diaz, K. Kanazawa and G. P. Gardini, J. Chem. Soc., Chem.
Commun., 1979, 635.
2 G. Tourillon and F. Garnier, J. Electroanal. Chem., 1982, 135, 173.
3 J. Roncali, Chem. Rev., 1992, 92, 711.
4 Handbook of Conductive Polymers, ed. J. R. Reynolds, T. Skotheim
and R. Elsenbaumer, Marcel Dekker Inc., New York, 1997.
5 A. Deronzier and J. C. Moutet, Coord. Chem. Rev., 1996, 147, 339.
6 J. Roncali, J. Mater. Chem., 1999, 9, 1875.
7 D. T. McQuade, A. E. Pullen and T. M. Swager, Chem. Rev., 2000,
100, 2537.
ꢀ
8 I. Perepichka, E. Levillain, M. Salle and J. Roncali, Chem. Mater.,
2002, 14, 449.
9 B. Jousselme, P. Blanchard, E. Levillain, R. de Bettignies and
J. Roncali, Macromolecules, 2003, 36, 3020.
10 B. Jousselme, P. Blanchard, M. Oc¸afrain, M. Allain, E. Levillain and
J. Roncali, J. Mater. Chem., 2004, 14, 421.
It could seem surprising that despite a short ECL poly(E8T6)
can be oxidized to the bipolaronic state. This phenomenon can be
explained by considering the extended LCC of this polymer
(TETEETET). It is well known that oxidation of polyaromatic
systems to the polaron state produces a transition from an
aromatic to a quinoid structure in which the adjacent thiophene
rings are locked into a rigid planar geometry. Furthermore, in the
case of sterically hindered systems, the deformability of the
thiophene ring38 can contribute to accommodate this transition.
Therefore, once the extended TETEETET sequence is oxidized
to the polaronic state, a planarization occurs that makes easier
the subsequent oxidation to the bipolaron state. However, due to
the strong steric constraint, this geometrical transition requires
additional energy which explains the overpotential needed to
observe the optical signature of the bipolaron state.
ꢀ
11 G. Trippe, E. Levillain, F. Le Derf, J. Lyskawa, M. Mazari,
J. Roncali, A. Gorgues and M. Salle, Chem.–Eur. J., 2004, 10, 6497.
ꢀ
12 J. Tour, Acc. Chem. Res., 2000, 33, 791.
13 A. Zen, A. Bilge, F. Galbrecht, R. Alle, K. Meerholtz, J. Grenzer,
D. Neher, U. Scherf and T. Farrel, J. Am. Chem. Soc., 2006, 128,
3914.
14 S. Wang, W. J. Oldham, Jr., R. A. Hudack, Jr. and G. Bazan, J. Am.
Chem. Soc., 2000, 122, 5695.
15 T. P. I. Saragi, T. Spehr, A. Siebert, T. Fuhrmann-Lieker and
J. Salbeck, Chem. Rev., 2007, 107, 1011.
16 C. Poriel, J.-J. Liang, J. Raukt-Berthelot, F. Barriere, N. Cocherel,
A. M. Z. Slavin, D. Horhant, M. Virboul, G. Alcaraz,
N. Audebrand, L. Vignau, N. Huby, G. Wantz and L. Hirsh,
Chem.–Eur. J., 2007, 13, 10055.
Conclusion
17 J. Roncali, P. Leriche and A. Cravino, Adv. Mater., 2007, 19, 2045.
18 S. Roquet, R. de Bettignies, P. Leriche and J. Roncali, J. Mater.
Chem., 2006, 16, 3040.
Three-dimensional p-conjugated systems based on a twisted 2,20-
bithiophenic core derivatized with various conjugated branches
containing terminal EDOT units have been synthesized by Stille
coupling reactions. X-Ray diffraction and UV-Vis spectroscopic
data are consistent with the expected 3D quasi-tetrahedral
geometry. Cyclic voltammetry shows that all compounds can be
oxidized into the corresponding cation radical at moderate
potential. The presence of the highly reactive terminal EDOT
groups confers on these structures a good aptitude for electro-
chemical polymerization and all compounds lead to the electro-
deposition of electroactive materials. The electrochemical and
optical properties of the resulting polymers have been investi-
gated by cyclic voltammetry and spectroelectrochemistry. The
results show that the electrochemical behavior is correlated with
the size and the nature of the conjugated branches and consistent
with a complete electrochemical coupling of all terminal EDOT
groups leading to the formation of conjugated networks con-
taining discrete conjugated oligomeric linkers with low poly-
dispersity. These first results pave the way toward the
development of a new generation of electroactive conjugated
materials of potentially interest for applications that require
a high surface-to-volume ratio such as catalysis, electrochemical
energy storage or sensors.
ꢁ
19 S. Karpe, A. Cravino, P. Frere, M. Allain, G. Mabon and J. Roncali,
Adv. Funct. Mater., 2007, 17, 1163.
20 J.-X. Jiang, F. Su, A. Trewin, C. D. Wood, N. L. Campbell, H. Niu,
C. Dickinson, A. Y. Ganin, M. J. Rosseinsky, Y. Z. Khimyak and
A. I. Cooper, Angew. Chem., Int. Ed., 2007, 46, 8574.
21 J. X. Jiang, F. Su, A. Trewin, C. D. Wood, H. Niu, J. T. A. Jones,
Y. Z. Khimyak and A. I. Cooper, J. Am. Chem. Soc., 2008, 130, 7710.
22 J. Weber and A. Thomas, J. Am.Chem. Soc., 2008, 130, 6334.
ꢀ
23 J. Germain, J. M. J. Frechet and F. Svec, J. Mater. Chem., 2007, 17,
4989.
24 J. Germain, J. M. J. Frechet and F. Svec, Chem. Commun., 2009, 1526.
ꢀ
25 J. Schmidt, J. Weber, J. D. Epping, M. Antonietti and A. Thomas,
Adv. Mater., 2009, 21, 702.
26 J. Roncali, C. Thobie-Gautier, H. Brisset, J. F. Favard and A. Guy,
J. Electroanal. Chem., 1995, 381, 257.
27 F. Piron, P. Leriche, G. Mabon, I. Grosu and J. Roncali, Electrochem.
Commun., 2008, 10, 1427.
ꢁ
28 J. Roncali, P. Blanchard and P. Frere, J. Mater. Chem., 2005, 15,
1589.
29 H. S. O. Chan, S.-C. Ng, S.-H. Seowa and M. J. G. Moderscheim,
J. Mater. Chem., 1992, 2, 1135.
ꢁ
30 M. Turbiez, P. Frere, P. Blanchard and J. Roncali, Tetrahedron Lett.,
2000, 41, 5521.
31 M. Turbiez, P. Frere, M. Allain, C. Videlot, J. Ackerman and
ꢁ
J. Roncali, Chem.–Eur. J., 2005, 11, 3742.
ꢁ
32 M. Turbiez, P. Frere and J. Roncali, J. Org. Chem., 2003, 68, 5357.
33 L. Groenendaal, G. Zotti, P.-H. Aubert, S. M. Waybright and
J. R. Reynolds, Adv. Mater., 2003, 15, 855.
34 J. Roncali, F. Garnier, R. Garreau and M. Lemaire, Synth. Met.,
1986, 15, 323.
35 J. Roncali, M. Lemaire, R. Garreau and F. Garnier, Synth. Met.,
1987, 18, 139.
Work aiming at the analysis of the gas sorption properties of
these materials and at the molecular engineering of the size,
rigidity and functionalization of the internal nanopores are now
underway and will be reported in future publications.
36 J. Roncali, M. Jubault and A. Gorgues, Chem. Mater., 1993, 5,
1456.
37 G. Zotti, S. Zecchin, G. Schiavon, A. Berlin, L. Huchet and
J. Roncali, J. Electroanal. Chem., 2001, 504, 64.
38 G. Barbarella, M. Zambianchi, A. Bongini and L. Antolini, Adv.
Mater., 1993, 5, 834.
Acknowledgements
ꢀ
The authors thank M. Lagree for mass spectrometry analyses
and M. Allain for X-ray diffraction studies.
10268 | J. Mater. Chem., 2010, 20, 10260–10268
This journal is ª The Royal Society of Chemistry 2010