Evidence for the contribution of sulfur-bromine intramolecular interactions to the self-rigidification of thiophene-based π-conjugated systems

Article abstract of DOI:10.1039/b802313a

Bithiophene associating 3,4-ethylenedioxythiophene and 3-bromothiophene, and the corresponding polymer exhibit self-rigidified structures of the conjugated backbones resulting from the association of S-Br and S-O non-bonded intramolecular interactions. Th

Full text of DOI:10.1039/b802313a

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LETTER
www.rsc.org/njc | New Journal of Chemistry
Evidence for the contribution of sulfur–bromine intramolecular
interactions to the self-rigidification of thiophene-based p-conjugated
systemsw
´
Noemie Hergue, Philippe Leriche, Philippe Blanchard, Magali Allain,
Nuria Gallego-Planas, Pierre Frere* and Jean Roncali
´
Received (in Montpellier, France) 11th February 2008, Accepted 4th April 2008
First published as an Advance Article on the web 7th May 2008
DOI: 10.1039/b802313a
Bithiophene associating 3,4-ethylenedioxythiophene and 3-bro-
mothiophene, and the corresponding polymer exhibit self-rigidi-
fied structures of the conjugated backbones resulting from the
association of S–Br and S–O non-bonded intramolecular
interactions.
oligomers in which the thiophene ring is associated with 3,4-
difluorothiophene8,9 or fluorophenylene units.8,10,11 Replacing
fluorine by a bulkier halogen atom, such as chlorine and
bromine, can be expected to induce steric repulsions and thus
to twist the conjugated system. Indeed, the crystal structures of
bithiophenes substituted by bromine at the 3 and 30 positions
revealed a large torsion between the thiophene cycles.12 Never-
theless, planar conformations have been observed for bithio-
phenes bearing only one bromine atom at the 3-position,13,14
indicating that the steric hindrance can be counterbalanced by
weak non-bonding interactions and by the deformability of
the thiophene ring.15 Recently, it has been demonstrated that
an electrodeposited film of poly(3-bromo-4-methoxythio-
phene) presented spectroscopic data characteristic of a rigid
planar structure without pointing out the origin of this
result.16 In order to analyze the structuring effect resulting
from the combination of bromine and oxygen atoms in
conjugated systems, we compare here the structural and
electronic properties of bithiophenes EDOT-3-bromothio-
phene 1 and EDOT-thiophene 2 and their corresponding
polymers (Chart 1).
The considerable research activity developed around func-
tional polymeric p-conjugated systems is motivated by the
high current interest in these materials in the field of plastic
electronics.1 Polythiophenes have been particularly investi-
gated as the most significant class of conjugated polymers2
and in particular poly(3,4-ethylenedioxythiophene) (PEDOT),
which has become one of the most popular polythiophene
derivatives.3,4 PEDOT presents a unique combination of
stability, moderate band gap, low oxidation potential and
optical transparency in the visible spectral region. These
properties result from the synergistic associations of the strong
donor effect of the ethylenedioxy groups with the propensity
of EDOT to develop noncovalent intramolecular S–O inter-
actions.5 The self-rigidification resulting from these intra-
molecular interactions makes EDOT an interesting building
block to develop copolymers promoting the planarization of
the p-conjugated systems. Even if in 3,4-ethylenedisulfa-
nylthiophene (EDST), the sulfur analogue of EDOT, S–S
steric repulsions induce torsions between the cycles in dimer
bis-EDST and polymer poly-EDST,6 it has been recently
demonstrated that the combination of EDOT and EDST
moieties leads to a quasi-planar conformation in bithiophene6
or terthiophene7 systems. The planar conformations are asso-
ciated with both S–O and S–S intramolecular interactions.
Therefore, the balance between the S–S steric repulsions and
non-bonding interactions can be favorably oriented for
strengthening the self-rigidification by the combination of
S–O and S–S interactions.
Compounds 1 and 2 were synthesized by Stille coupling
between the stannyl derivative of EDOT with 2-bromothio-
phene or 2,3-dibromothiophene in the presence of Pd(PPh3)4
(Scheme 1). By using 1 equivalent of stannyl derivative, the
coupling of EDOT on the 2-position of 2,3-dibromothiophene
was favored and led exclusively to compound 1 in 57% yield.
With 2-bromothiophene an excess of stannyl EDOT was used
to obtain compound 2 in 69% yield.17 Bithiophene 1 was
obtained as a crystalline solid while 2 was an oil.
Single crystals of 1, obtained by slow evaporation of
CHCl3–EtOH solution, have been analyzed by X-ray diffrac-
tion.w The structure of compound 1 shown in Fig. 1, reveals a
good planarity of the conjugated system with the two thio-
phene cycles in an anti conformation. The dihedral angle
Chalcogen–halogen interactions represent another type of
non-bonding interaction potentially useful for stabilizing the
planar conformation of p-conjugated systems. It has been
reported that sulfur–fluorine interactions contribute to rigidify
´
Universite d’Angers, CNRS, Laboratoire de Chimie et d’Ingenierie
Moleculaire d’Angers, CIMA UMR 6200, Groupe SCL, 2 boulevard
´
´
Lavoisier, 49045 Angers, France. E-mail: pierre.frere@univ-angers.fr;
Fax: +(33)241735405; Tel: +(33)241735063
w CCDC reference number 683797. For crystallographic data in CIF
or other electronic format see DOI: 10.1039/b802313a
Chart 1
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Scheme 1 Synthesis of bithiophenes 1 and 2.
between thienyl rings is less than 11. The observed dS1O1
=
2.834(3) A and dS2Br = 3.220(1) A distances are considerably
shorter than the sum of the van der Waals radii of sulfur and
oxygen (3.25 A) or sulfur and bromine (3.80 A), thus indicating
that both S–O and S–Br intramolecular interactions contribute
to the self-rigidification of the p-conjugated system of 1.
Fig. 2 shows the UV-Vis absorption spectra of compounds 1
and 2 in methylene chloride. The spectrum of 2 presents a
discernible vibronic fine structure with an absorption max-
imum (lmax) at 318 nm and shoulders at 300 nm and 330 nm
consistent with the establishment of a self-rigidified structure
due to intramolecular S–O interaction. In spite of the bulky Br
atom, compound 1 displays a better resolved fine structure
with the appearance of three distinct maxima at 303 nm,
316 nm and 331 nm. The three maxima are equally spaced
in energy by 1300–1400 cmꢁ1, which is consistent with a CQC
stretching mode in the heteroaromatic moieties, strongly
coupled to the electronic structure. Such behavior is indicative
of a more rigidified p-conjugated system for compound 1 than
for compound 2. Thus, in addition to the S–O interaction, the
non-bonding S–Br intramolecular interaction observed in the
solid state persists in solution and contributes to strengthening
the self-rigidification of the molecule.
Fig. 2 Electronic absorption spectra of 1 and 2 in CH2Cl2.
of the energy as a function of the dihedral angle y has been
calculated from y = 01 (anti conformer) to y = 1801 (syn
conformer). The comparison of the profile of the curves in
Fig. 3 for compounds 1 and 2 shows an analogous evolution
between 01 and 901 and a strong difference after 901. For
compound 2, a nearly planar cis conformer is stabilized at
1661, only 0.6 kcal molꢁ1 higher in energy than the planar
trans conformation. On the other hand, a minimum appears
for 1 at y = 1301 corresponding to a cis distorted conforma-
tion at 1.73 kcal molꢁ1 higher in energy that the planar trans
conformation. Due to strong steric Br–O repulsion the planar
cis conformer is strongly destabilized by about 7.3 kcal molꢁ1
.
For both compounds, minima are separated by a barrier at
y = 901 with a maximum energy around 3.0–3.3 kcal molꢁ1
with respect to the trans planar conformation. The planar
conformation adopted by compound 1, completely agrees with
the geometry obtained from the X-ray structure, with short
S–O (2.85 A) and S–Br (3.26 A) distances as expected for
intramolecular interactions. For compound 2 the S–O distance
is slightly longer than for 1, thus suggesting weaker S–O
intramolecular interactions. As indicated in Table 1, the slight
decrease in energy of the HOMO and LUMO levels of
compound 1 compared to 2 corresponds to a weak electron-
withdrawing effect of the bromine atom.
Theoretical calculations have been performed at the ab initio
density functional level with the Gaussian 03 package. Becke’s
three parameter gradient corrected functional (B3LYP) with a
polarized 6-31G(d,p) basis was used for all full geometry
optimization of the bithiophenes. The most stable optimized
conformations for the two compounds correspond to the
planar anti structures with dihedral angles between the thio-
phene cycles of less than 21.18,19 In order to study the influence
of the internal rotation between the two cycles, the evolution
The cyclic voltammograms (CV) of compounds 1 and 2
present irreversible anodic peaks at 1.2 V and 1.1 V,
Fig. 1 Molecular structure of compound 1; ellipsoids drawn at the
50% probability level. The intramolecular interactions are presented
by dotted lines.
Fig. 3 Relative energy curves for the internal torsion of compounds 1
(black) and 2 (grey).
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Table 1 UV-Visa, electrochemical datab and calculated HOMO/
LUMO energy levels and gapc of compounds 1 and 2
electrodes. Fig. 5 shows the spectral series for polymers at
various potentials from the neutral to the fully oxidized states.
The spectrum of the neutral poly(2) presents a lmax at 505 nm
and a band gap of 1.82 eV estimated from the low energy
absorption onset. The spectrum of poly(1) shows a larger
absorption band with a lmax at 500 nm, a shoulder at
580 nm and a band gap of 1.72 eV. The absorption band
displays a discernible fine structure consistent with a more
rigid structure due to synergistic effects of the intramolecular
S–Br and S–O interactions. Oxidation of the two polymers
leads to a similar change in the optical spectra. When the
applied potential reaches the first oxidation wave, two bands
appear around 800 nm and 1400 nm which can be attributed to
the polaron state while at the second oxidation potential, a
large band with a maximum around 1000 nm appears which
should correspond to the formation of bipolaronic species.
In summary, the comparison of the electronic properties of
hybrid bithiophenes 1 and 2 shows that the presence of a
bromine atom at the 3-position of the thiophene ring leads to
the development of noncovalent intramolecular sulfur–
bromine interactions. Combined with the already well-estab-
lished sulfur–oxygen interactions observed in many EDOT-
containing compounds, bromine–sulfur interactions contri-
bute to strengthening the self-rigidification of the conjugated
system. The electropolymerization of the two precursors leads
to polymers with similar electronic properties which confirms
that the self-structuring effects persist in the films of polymers.
Compound lmax
l0–0
Eox
HOMO LUMO DE
1
2
a
316 nm 331 nm 1.22 V ꢁ5.3 eV ꢁ1.2 eV 4.10 eV
318 nm 330 nm 1.11 V ꢁ5.1 eV ꢁ1.0 eV 4.10 eV
b
5.10ꢁ5 M in CH2Cl2. 10ꢁ3 M in 0.1 M Bu4NPF6 in CH3CN,
reference Ag/AgCl, v = 100 mV sꢁ1
and gap are calculated by DFT methods (B3LYP/6-31g(d,p)).
.
HOMO/LUMO energy levels
c
respectively, corresponding to the oxidation of the compounds
into the corresponding radical cations. For compound 1 the
weakly electron-withdrawing bromine atom on the 3 position
leads to a slight increase of the oxidation potential corre-
sponding to a decrease of the HOMO level, in agreement with
theoretical calculations.
Application of recurrent potential scans to acetonitrile
solutions of compounds 1 and 2 in the presence of Bu4NPF6
as supporting electrolyte leads to the progressive development
of a new redox system at lower potential associated with the
electrodeposition of the polymers. The dissymmetry intro-
duced in the electronic structure of radical cations 1+ and
ꢂ
2+ should result in a different reactivity of the two free
ꢂ
a-positions. Thus, both polymers have probably a regularly
alternated polymeric structure, but the very low solubility of
the electrodeposited films did not allow characterization of
their structures.
The CV trace of poly(2) (Fig. 4 right) recorded in a mono-
mer-free electrolytic medium presents a broad reversible oxi-
dation wave with a maximum at ca. 0.78 V and a shoulder at
0.30 V which corresponds with the p-doping of the polymer.
The CV of poly(1) (Fig. 4 left) shows two anodic waves
peaking at 0.40 V and 0.95 V which can be attributed to the
oxidation to the polaron and bipolaron states. After a hundred
charging–discharging cycles, the CV of the polymers does not
present any decrease in the amount of exchanged charge, thus
indicating that the polymers are stable upon cycling.
The optical properties of the polymers have been analyzed
on a thin film electrodeposited on indium tin oxide (ITO)
Experimental
General
NMR spectra were recorded with a Bruker Advance DRX 500
(1H, 500.13 MHz and 13C, 125.75 MHz) instrument. Chemical
shifts are given in ppm relative to TMS. Cyclic voltammetry
was performed in CH3CN solutions purchased from SDS
(HPLC grade). Tetrabutylammonium hexafluorophosphate
(0.1 M) as supporting electrolyte was purchased from Acros
and used without purification. Solutions were de-aerated by
Fig. 4 CV traces of film of poly(1) (left) and poly(2) (right) deposited on a Pt disk (d = 1 mm) in 0.10 M Bu4NPF6 in CH3CN, scan rate
100 mV sꢁ1
.
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Fig. 5 Electronic absorption spectra of films of poly(1) (left) and poly(2) (right) electrodeposited on ITO.
nitrogen bubbling prior to each experiment, each of which was
run under a nitrogen atmosphere. Experiments were done in a
one-compartment cell equipped with a platinum working
microelectrode (F = 1 mm) and a platinum wire counter
electrode. An Ag/AgCl electrode checked against the ferro-
cene/ferricinium couple (Fc/Fc+) before and after each experi-
ment was used as reference. Electrochemical experiments were
carried out with a PAR 273 potentiostat with positive feed-
back.
EDOT-thiophene 2. Stille coupling was done with the above
procedure by using 2-bromothiophene (0.81 g, 5 mmol) and
the stannic derivative (7.5 mmol, 1.5 equivalents), and
Pd(PPh3)4 (5% mol). The product was purified by chromato-
graphy on silica gel (CH2Cl2–PE 1 : 1) to give 2 as a colourless
oil (770 mg, 69%).
2: 1H NMR (CDCl3): d = 4.24 (m, 2H), 4.33 (m, 2H), 6.21
(s, 1H), 7.01 (dd, 3J = 5.1 Hz, 3J = 3.7 Hz, 1H), 7.20 (d, 3J =
5.1 Hz, 1H), 7.22 (d, 3J = 3.7 Hz, 1H); elemental analysis
calculated (%) for C10H8O2S2: C 53.55, H 3.60; found: C
53.82, H 3.49.
Procedure for Stille coupling
Preparation of the stannyl derivative of EDOT. To a solution
containing EDOT dissolved in dry THF at ꢁ78 1C under an
inert atmosphere (N2), an equivalent of n-BuLi (2.5 M or
1.6 M in hexane) was added dropwise. The mixture was stirred
for 1 h at the addition temperature. Then, tributyltin chloride
was added dropwise and the mixture was stirred at the same
Crystal data and structure refinement for compound 1w
Data collections were performed at 293 K on a STOE IPDS
diffractometer equipped with a graphite monochromator uti-
lizing MoKa radiation (l = 0.71073 A). The structure was
solved by direct methods using SIR9220 and refined on F2 by a
full-matrix least-squares method, using SHELXL9721. Non-
hydrogen atoms were refined anisotropically and absorption
was corrected by a Gaussian technique. The H atoms were
found by Fourier difference synthesis.
1
temperature for h before allowing to warm at room tem-
2
perature. After dilution with diethyl ether, the organic phase
was successively washed using a saturated solution of NaH-
CO3 then water. After drying over MgSO4, the solvent was
evaporated and the stannic derivative was used without other
purification in the Stille coupling reactions.
C10H7BrO2S2, Mw = 303.19, crystal size 0.77 ꢃ 0.19 ꢃ 0.14
mm3, orthorhombic, P212121 a = 4.1115(4) A, b = 14.688(1) A,
c = 17.247(2) A, V = 1041.6(2) A3, Z = 4, rcalc = 1.933 g
cmꢁ3, 6104 reflections collected in the 2.4–261 y range, 1988
independent reflections (Rint = 0.04) from which 1738 with I 4
2s(I) converged to R = 0.0292 and wR2 (all data) = 0.0721 with
EDOT-3-bromothiophene 1. A mixture of stannyl derivative
(3.56 g, 8.2 mmol) and 2,3-dibromothiophene (1.98 g, 8.2
mmol) and Pd(PPh3)4 (5% mol) was heated in anhydrous
toluene at 80 1C for 16 h under an inert atmosphere. After
concentration, the residue was dissolved in CH2Cl2. The
organic phase was washed twice with a saturated solution of
NaHCO3 and then with water. After drying over MgSO4 and
evaporating the solvent, the product was purified by chroma-
tography on silica gel (CH2Cl2–petroleum ether (PE) 1 : 1) to
give 1 as a pale yellow solid (1.41 g, 57%).
164 parameters, GOF = 0.975, ꢁ0.417 o Dr o 0.478 e Aꢁ3
.
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936 | New J. Chem., 2008, 32, 932–936