Synthesis and optical properties of (thienylene)-[1,6-dithienylhexa-1,3,5-trienylene] copolymers

Article abstract of DOI:10.1039/b005290f

Two [thiophene-1,6-dithienylhexa-1,3,5-triene] copolymers were prepared by a palladium catalysed coupling reaction of a dibromothiophene derivative and a bis(tributylstannyl) derivative of a 1,6-dithienylhexa-1,3,5-triene unit. The electrochromism and the photoluminescence properties of the highly conjugated polymers were studied. In the solid state the polymers have strong photoluminescence bands at 2.0 eV for 9 and 1.95 eV for 8. Polymer 8 seems particularly promising for use as a red-light emitting diode, and the two polymers 8 and 9 exhibit red-pale blue electrochromism that makes them suitable for fabricating new devices.

Full text of DOI:10.1039/b005290f

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Synthesis and optical properties of (thienylene)–[1,6-dithienylhexa-
1,3,5-trienylene] copolymers
Franck Embert,a Jean-Pierre Le`re-Porte,a Joe¨l J. E. Moreau,*a Franc¸oise Serein-Spirau,a
Arieta Righib and Jean-Louis Sauvajolb
aHe´te´rochimie Mole´culaire et Macromole´culaire, UMR CNRS 076, E.N.S.C.M., 8, rue de
l’Ecole Normale, 34296 Montpellier, Cedex 05, France. E-mail: jmoreau@cit.enscm.fr
bGroupe de Dynamique des Phases Condense´es, UMR CNRS 5581, Universite´ Montpellier II,
Place Euge`ne Bataillon, 34095 Montpellier, Cedex 05, France
Received 3rd July 2000, Accepted 5th December 2000
First published as an Advance Article on the web 17th January 2001
Two [thiophene–1,6-dithienylhexa-1,3,5-triene] copolymers were prepared by a palladium catalysed coupling
reaction of a dibromothiophene derivative and a bis(tributylstannyl) derivative of a 1,6-dithienylhexa-1,3,5-
triene unit. The electrochromism and the photoluminescence properties of the highly conjugated polymers were
studied. In the solid state the polymers have strong photoluminescence bands at 2.0 eV for 9 and 1.95 eV for 8.
Polymer 8 seems particularly promising for use as a red-light emitting diode, and the two polymers 8 and 9
exhibit red–pale blue electrochromism that makes them suitable for fabricating new devices.
Conjugated organic oligomers and polymers are attractive
materials since a wide range of optical properties can result
from variations in their molecular structure. The tuning of
optical and electro-optical properties is of interest to elaborate
electrochromic or electroluminescent devices.1 Among these
materials, poly(para-phenylene vinylene) and its derivatives
have been extensively used for blue–green electroluminescent
devices but red light emitting materials are more scarce. For
electrochromic devices, conjugated polymers, especially those
based on polythiophenes2,3 such as poly(3,4-ethylenedioxythio-
phene),4 appeared particularly interesting because of their
transparency in the doped state. The tailoring of electro-
chromic properties is currently obtained upon blending at least
two polymers exhibiting different electrochromic properties.5
Our current interest in polymers with tunable electrooptical
properties led us to study the synthesis of well-defined
copolymers. We recently reported the synthesis of highly
conjugated thiophene–alkylthiophene6 and [thienylene–p-(2,5-
dialkoxy)phenylene]7 copolymers with electroluminescent and
electrochromic properties. The photophysics of a,v-diphenyl-
and -dithienyl oligomeric polyenes have been extensively
studied by theoretical and experimental approaches.8,9
Poly(p-phenylenehexa-1,3,5-trienylene) with a small number
of repeat units presented electronic spectroscopic data at
relatively high energy.10 After being doped, such molecular
structures offer distinct color contrasts corresponding to
polaronic radical cations and bipolaronic dication species.11,12
These molecular entities are presently sought after for their
optical power limiting properties12 and their incorporation in
the backbone of an organic conjugated polymer is of great
interest to produce materials with optical properties.
Pd(0) as the catalyst. It has already proved to be an efficient
route to regioregular poly(3-alkylthiophene)6,13,14 and to
alternating thienylene–1,4-dialkoxyphenylene copolymers.
The synthesis of 5 began with the 3,4-alkylation of thiophene
in dry THF by reaction of the octylmagnesium bromide with
3,4-dibromothiophene in the presence of a catalytic amount of
1,3-bis(diphenylphosphino)propanenickel(II) chloride. 3,4-
Dioctylthiophene (1) was then formylated with DMF in the
presence of phosphorus oxychloride in 1,2-dichloroethane as
the solvent. The 3,4-dioctylthiophene-2-carboxaldehyde (3)
obtained reacted with tetraethyl (E)-but-2-ene-1,4-diyldipho-
sphonate (4) in the presence of two equivalents of tBuOK in
THF to give 5 according to a Wittig–Horner reaction.
Compound 5 was treated at low temperature with butyllithium
in THF and distannylated with Bu3SnCl (Scheme 1).
The polymers 8 and 9 were synthesised by a polycondensa-
tion reaction using 2,5-dibromothiophene and 2,5-dibromo-3-
octylthiophene (7) respectively with an equimolar ratio of the
bis(tributylstannyl) derivative 6. This reaction gave THF or
CHCl3 soluble conjugated regular copolymers containing
alternating thienylene and (E,E,E)-1,6-bis(3’,4’-dioctyl-2’-
thienyl)hexa-1,3,5-triene (Scheme 2).
The spectroscopic data are consistent with the structure of 8
and 9 described in Scheme 1. The EDAX analysis of the two
isolated solid materials revealed only traces of Sn and Br and
the absence of Pd. The 1H NMR spectra of 8 and 9 showed two
groups of signals at 7.1 ppm (thienylene protons) and at
6.70 ppm (vinylene protons). It also showed at higher field
signals corresponding to the octyl substituents of the thienyl
rings. The polymers were soluble in organic solvents (THF,
CHCl3) and were analysed using gel permeation chromato-
graphy (GPC) on styragel columns calibrated with polystyrene
standards using THF as eluent. Average molecular weights Mw
in the range 3000–4000 and polydispersities Mw/Mn in the
range 1.4–1.8 were observed (Table 1). It indicates an average
degree of polymerisation (DPn) of only 4 or 5 of the chain unit
containing 18 conjugated carbon atoms.
Here, we report the synthesis of two well-defined conjugated
copolymers containing thienylene and 1,6-dithienylhexatriene
chain units. The electrochromism and the photoluminescence
of the prepared copolymers are discussed.
Results and discussion
1. Polymers synthesis
2. Optical properties
The synthesis of the two polymers was achieved using the Stille
coupling reaction between a 2,5-dibromothienyl substrate and
a bis(tributylstannyl) aromatic derivative in the presence of
The UV–Vis and the fluorescence spectra were recorded from
thin films on glass (Table 1). As with poly(p-phenylenehexa-
1,3,5-trienylene),10 the absorption of compounds 8 and 9 is at
718
J. Mater. Chem., 2001, 11, 718–722
This journal is # The Royal Society of Chemistry 2001
DOI: 10.1039/b005290f

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Scheme 1 Synthesis of (E,E,E)-1,6-bis(2’-thienyl-3’,4’-dioctyl-5’-tributylstannyl)hexa-1,3,5-triene (6).
Scheme 2 Synthesis of copolymers 8 and 9.
high energy in the range 450–466 nm. It is consistent with low
conjugation length probably due to the high flexibility of the
hexatrienyl system.
conjugation in 9 owing to the presence of the alkyl substituent.
The alkyl substituents induce some steric hindrance and cause
defects in the planar arrangement of the conjugated segments.
Related observations were made for head to head coupling
which leads to conjugation defects in polythiophene whereas
regioregular poly(alkylbithiophene)s exhibit extended conjuga-
tion.14 The absorption UV–Vis spectrum of 8 (lmax~466 nm),
with a higher wavelength value than 9 (lmax~450 nm), is
consistent with a lower conjugation in 9 compared with 8.
Polymers 8 and 9 exhibited a red–pale blue electrochromism,
associated with the redox behavior. The electrochromic
phenomenon was studied using a spectroelectrochemical cell
made in our laboratory (Fig. 2). It is a standard UV–Vis
spectroscopy cell in which a 7625 mm indium tin oxide (ITO)
glass plate as working electrode, a flexible Pt wire as the
counter electrode and an Ag wire as the reference electrode are
placed. A curved lead plate is pinched on to the top of the ITO
plate to improve the electrical contact. To avoid any contact
2.1. Electrochromic properties
Films of 8 and of 9 were formed on a platinum electrode from a
1023 M solution in CHCl3 and the deposits were studied by
cyclic voltammetry. The voltammograms were recorded
between 22 and z1 V (vs. SCE). Each polymer showed an
oxidation peak and a corresponding reduction one (Table 2
and Fig. 1). Polymers 8 and 9 appeared easier to oxidize than
polythiophene.15 Oxidation of 9 occurred at a higher potential
value than the oxidation of 8. This is indicative of a lower
Table 1 Selected data for polymers 8 and 9
Absorption
max/nm
Mw/Mn DPnb films on glass films on glass
Emission
l
lmax/nm
a
Polymers Mw
Table 2 Voltamperometry data of polymers 8 and 9
8
9
3019 1.8
4187 1.4
3.9
4.7
466
450
636
620
Polymers
Epox(vs. SCE)/V
Epred(vs. SCE)/V
aDetermined by GPC using polystyrene standards. bDegree of poly-
merisation.
8
9
0.77
0.93
0.74
0.81
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Fig. 3 Variation of the absorption spectrum of a film of 8 on an ITO
support as a function of the applied potential in the range 0–1 V.
Fig. 1 Cyclic voltamperometry of polymers 8 and 9.
between electrodes, two teflon blocks are placed at the bottom
and at the top of the cell. The sample is deposited on the ITO
glass support by slow evaporation of a 1024 M solution so that
it is crossed by the optical beam of the UV–Vis spectro-
photometer. The cell is filled with an electrolytic solution and
placed in the sample compartment of the spectrophotometer.
The three electrodes are then connected to a potentiostat
(Fig. 2).
Fig. 4 Variation of the absorption spectrum of a film of 9 on an ITO
support as a function of the applied potential in the range 0–1 V.
Films of 8 and 9 were deposited from (1024 M) solutions of
polymer in CHCl3, on an ITO glass support. The absorption
spectra were recorded for the films of polymer immersed in a
(0.1 M) solution of Bu4NPF6 as the supporting electrolyte in
CH3CN. The applied potential varied between 0 and 1 V. The
changes in the absorption spectra as a function of the applied
potential are shown in Fig. 3 for 8 and in Fig. 4 for 9. For the
two polymers, in the neutral form, the absorption spectra
showed a single band associated with an interband transition,
respectively at l~466 nm for 8 and l~450 nm for 9. Upon
oxidation, the absorption maximum, associated with the
appearance of polaronic and bipolaronic states, shifts to
higher wavelengths. The position of the absorption band of the
polymeric species could be correlated to the absorption of the
radical cation in the near infrared domain of the di-2’-
thienylhexa-1,3,5-triene unit.12 For each polymer studied (8
and 9), there is an isosbestic point in Fig. 3 and in Fig. 4
respectively, revealing that only two absorption bands are
involved. It is interesting to note that 8 and 9 have optical
properties tunable between one quasi transparent and one
opaque state. The electrochromic behavior appeared similar to
poly(3,4-ethylenedioxythiophene) (PEDOT).4 The low absorp-
tion of these polymers in the visible range in the oxidized state
makes them interesting for various uses,7 for example, as smart
windows and energy saving windows.16
2.2. Photoluminescence study
Emission properties of 8 and 9 have been studied in order to
evaluate their ability as materials suitable for electrolumines-
cent devices. The room temperature photoluminescence (PL)
bands recorded on thin films of 8 and 9 are displayed in Fig. 5a.
The profiles of these bands are similar. The maximum of the PL
band occurs at 2 eV (l~620 nm) for 9 and at 1.95 eV
(l~636 nm) for 8. A displacement to low energy with the
increase of the conjugation length was observed in thiophene-
based regioregular polymers.14 By analogy this shift is
attributed to a higher conjugation, and thus to a gap, smaller
in polymer 8 than in polymer 9. The temperature dependence of
the PL band does not reveal significant changes (Fig. 5b).
Red-emitting diodes are presently sought after and it is
worth noting that 8 is a red-emitting polymer (l~636 nm).
Moreover, in the range of cathodic potentials, a reduction wave
was observed only for 8 at 21.7 V (vs. SCE) and revealed a high
electron affinity of 8. The observed value is compatible with the
extraction potential of magnesium (E‡(Mg2z/Mg)~22V) as
electrode material. The polymer 8 may therefore be of interest
to fabricate electroluminescent diodes. The realisation of light
emitting diodes with 8 is now in progress.
Experimental
Solvents were dried by distillation over phosphorus pentaoxide
(CHCl3), sodium benzophenone ketyl (toluene, THF, ether)
and CaH2 (hexane, DMF). 3,4-Dibromothiophene, 3-bromo-
thiophene, 1-bromooctane, 2,5-dibromothiophene and tribu-
tyltin chloride (Aldrich) were used without further purification.
NidpppCl2 and n-butyllithium in 2.5 M solution in hexanes
were purchased from Aldrich.
1H NMR spectra were recorded in CDCl3 at 200 MHz with a
Fig. 2 Electrochromic device.
720
J. Mater. Chem., 2001, 11, 718–722

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evaporated and the obtained oil was quickly eluted on a
5 cm long silica gel column with hexane. The solvent was
removed and 1 was isolated in 64% yield by distillation in
1
vacuo. Bp 145 ‡C/0.4 mmHg; H NMR (CDCl3) dH ppm: 0.95
(6 H, t, CH3), 1.34 (20 H, m, 5 CH2), 1.67 (4 H, m, bCH2), 2.55
(4 H, t, aCH2), 6.92 (2 H, s, Hthienyl); 13C NMR (CDCl3) dC
ppm: 14.16 (CH3), 22.76, 28.89, 29.37, 29.58, 29.7, 29.75, 31.9
(CH2, alkyl chain), 119.9 (Cthienyl C2, C3), 142.1 (Cthienyl C3,
C4); MS: m/z: 309 (Mzz1) (100%). Found: C, 77.80; H, 11.76.
C20H36S requires C, 77.85; H, 11.76%.
3-Octylthiophene (2)
This compound was prepared according to the literature.18
3,4-Dioctylthiophene-2-carboxaldehyde (3)
In a Schlenk tube under a nitrogen atmosphere 1,2-dichloro-
ethane (40 mL),
1 (7.07 g ,23 mmol) and DMF (2.1 g,
28.7 mmol) were successively introduced . The medium was
cooled to 0 ‡C and 4.4 g (28.7 mmol) of oxyphosphorus
chloride were added dropwise. After complete addition, the
medium was heated and refluxed for three hours. It was then
cooled to room temperature, poured into an aqueous acidic
(HCl 10%) solution and stirred for one hour. The organic phase
was extracted with dichloromethane, washed several times with
an aqueous sodium hydrogen carbonate solution, and dried
over magnesium sulfate. The solvent was evaporated and the
crude product was distilled under reduced pressure. Yield 97%,
bp 165 ‡C/0.3 mmHg; 1H NMR (CDCl3) dH ppm: 0.88 (6 H, m
CH3), 1.28 (20 H, m, 5 CH2), 1.55 (4 H, m, bCH2), 2.53 (2 H, t,
aCH2), 2.87 (2 H, t, aCH2), 7.33 (1 H, s, Hthienyl), 10.0 (1H, s,
Hald); 13C NMR (CDCl3) dC ppm: 14.09 (CH3), 22.65, 22.66,
27.03, 28.26, 29.19, 29.24, 29.32, 29.42, 29.47, 29.63, 29.71,
Fig. 5 Photoluminescence of polymers 8 and 9 recorded at: a) room
temperature; b) 10 K.
Bruker AC 200 spectrometer, using residual CHCl3
(d~7.27 ppm) as the internal standard. 13C NMR spectra
were obtained at 50.32 MHz with a Bruker AC 200 using
CDCl3 (d~77.70 ppm) as the internal standard. IR spectra
were recorded with a Perkin-Elmer 1000 spectrometer. Melting
points were measured with an Electrothermal 9100 apparatus.
Absorption spectra were recorded on a MC2 Safas spectro-
meter and emission spectra on a SLM-Aminco MC 200
spectrometer. Fluorescence quantum yields were measured by
the standard procedure.17 GPC measurements were carried out
on a Waters chromatograph equipped with HR2 and HR3
Styragel columns. Mass spectra (FAB) were obtained using a
JEOL JMS-DX 300 apparatus. Cyclic voltammetry was carried
out on an EGG potentiostat connected to a Kipp & Zonen
tracer. Electrochromism measurements were made from a three
electrode (ITO, Pt, Ag) spectroelectrochemical cell connected
to an EGG 362 potentiostat. EDAX measurements were
carried out using a Cambridge 360-stereoscan with an EDS-
Link AN 10000 attachment. Raman spectra were recorded
using a Jobin-Yvon T64000 spectrometer equipped with a
liquid nitrogen cooled CCD detector. The measurements were
performed in the back scattering configuration using an Ar-ion
laser (Spectra Physics 2000) with 514.5 nm exciting radiation.
All spectra were realized under vacuum to avoid degradation of
the sample. The samples were maintained in a cryostat (Air
Liquide) where the temperature could be varied between 5 and
300 K. To minimize the local heating the power density was
29.80, 31.84, 31.36, 31.93, 130.23 (Cthienyl, C5), 138.18 (Cthienyl
,
C4), 144.39 (Cthienyl, C3), 151.50 (Cthienyl, C2), 182.60 (Cald);
HRMS (FABz): calculated for C21H37OS: m/z 337.2565;
observed 337.2608. Found: C, 74.84; H, 10.63. C21H36OS
requires C, 74.94; H, 10.78%.
Tetraethyl (E)-but-2-ene-1,4-diyldiphosphonate (4)
This compound was prepared according to the literature.19
(E,E,E)-1,6-Bis(3’,4’-dioctyl-2’-thienyl)hexa-1,3,5-triene (5)
Into a three-necked flask equipped with a condenser and a
dropping funnel
4 (3.34 g, 8.66 mmol) and 3 (6.45 g,
kept below 5 mW mm22
.
19.08 mmol) in 100 mL of dry THF were introduced under
nitrogen. From the funnel was added dropwise a solution of
2.62 g (23.4 mmol) of potassium tert-butoxide in 80 mL of dry
THF. The mixture became first yellow then black. After
complete addition the medium was stirred at room temperature
for 12 hours. The solution was then poured into 250 mL of
distilled water at 0 ‡C and stirred for one hour. A solid phase
appeared, and was filtered and dissolved in dichloromethane.
This organic phase was dried over MgSO4, filtered and
concentrated in vacuo. The obtained oil was chromatographed
over a silica gel column using hexane–ethyl acetate (95 : 5)
mixture as the eluent. A yellow powder was obtained in 85%
yield after the solvent was removed. Mp 47 ‡C; 1H NMR
(CDCl3) dH ppm: 0.89 (12 H, t), 1.28 (40 H, m), 2.50 (8 H, m),
6.44 (2 H, m), 6.68 (2 H, m), 6.74 (2 H, s); 13C NMR (CDCl3)
dC ppm: 14.15, 22.72, 26.98, 29.04, 29.32, 29.46, 29.53, 29.63,
29.73, 29.80, 31.08, 31.94, 118.36, 124.22, 128.07, 132.72,
137.14, 140.11. Absorption: lmax((1024 M) CHCl3 solution)
400 nm. MS: m/z: 692 (Mzz1) (100%). Found: C, 79.55; H,
10.63. C46H76S2 requires C, 79.79; H, 11.05%.
3,4-Dioctylthiophene (1)
Into a three-necked flask equipped with a magnetic stirrer,
dropping funnel and condenser 7.1 g (293 mmol) of
a
magnesium powder and 5 mL of dried THF were introduced
under a nitrogen atmosphere. A few drops of pure 1-
bromooctane (43.75 mL, 226 mmol) were added from the
funnel to the medium. As soon as the reaction started the
remaining 1-bromooctane was diluted in 150 mL of dry THF
and added dropwise. After complete addition, the medium was
heated and refluxed for 3 hours.
The octylmagnesium bromide was added at 0 ‡C to a
solution of 23.58 g of 3,4-dibromothiophene (97.4 mmol) and
0.525 g of 1,3-bis(diphenylphosphino)propanenickel(II) chlor-
ide in 100 mL of dry THF. After the addition was completed,
the mixture was refluxed for four hours at least, then cooled to
room temperature. Magnesium salts were precipitated with a
large excess of hexane. After filtration, the solvent was
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(E,E,E)-1,6-Bis(3’,4’-dioctyl-5’-tributylstannyl-2’-thienyl)hexa-
1,3,5-triene (6)
Copolymer 9
For the preparation of copolymer 9, 7 (0.001 mol, 0.402 g) and
6 (0.001 mol, 0.354 g) were reacted (Yield 90%) (0.797 g).
(CDCl3) dH ppm: 0.92 (15 H, t, 5 CH3), 1.36 (60 H, m, 30 CH2),
2.65 (10 H, t, 5 CH2), 6.70 (6 H, m, CHvinyl), 7.15 (1 H, s,
CHthienyl); nmax(KBr)/cm21: 2925, 2854, 1609, 1465, 1377, 985,
873, 722. Found: C, 71.58; H, 9.18%. EDAX analysis: Sn/S:
0.1; Br/S: 0.13. Absorption: lmax (film on glass) 450 nm.
Emission: lmax (film on glass) 620 nm.
Under a nitrogen atmosphere, 5 (2 g, 2.89 mmol) and dry THF
(50 mL) were introduced into a Schlenk tube. The medium was
cooled to 280 ‡C and 2.54 mL (6.34 mmol) of a solution of n-
butyllithium in hexane (2.5 M) were added. After the reaction
mixture was stirred at 280 ‡C for one hour, Bu3SnCl (2.07 g,
6.34 mmol) was added dropwise and the solution was left under
stirring for eight hours. After hydrolysis, the organic phase was
extracted three times with 50 mL of ether, dried with
magnesium sulfate, filtered and concentrated in vacuo. The
crude oil obtained in 95% yield is used without purification. 1H
NMR (CDCl3) dH ppm: 0.91 (30 H, m), 1.37 (84 H, m), 2.50 (8
H, m), 6.45 (2 H, m), 6.66 (4 H, d); 13C NMR (CDCl3) dC ppm:
28 signals between 32.65 and 10.95, 123.99, 128.20, 131.25,
132.69, 140.98, 142.83, 150.95. Absorption: lmax((10-4 M)
CHCl3 solution) 418 nm.
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233.
Copolymer 8
In a Schlenk tube equipped with a condenser, Pd(PPh3)4
(0.0115 g, 1025 mol) in 10 mL of a THF–DMF (50 : 50)
mixture was dissolved under a nitrogen atmosphere and at
room temperature. Compound 7 (0.001 mol, 0.402 g) and
0.001 mol (0.242 g) of 2,5-dibromothiophene were introduced
by a syringe. The reaction mixture was stirred at 80 ‡C during
three days. Heating was then stopped and a red precipitate
formed upon the addition of acetone. The powder was filtered,
washed with acetone and dried in vacuo to give 0.697 g of 8
(Yield 90%). 1H NMR (CDCl3) dH ppm: 0.90 (12 H, t, 4 CH3),
1.33 (48 H, m, 24 CH2), 2.63 (8 H, t, 4 CH2), 6.65 (6 H, m,
CHvinyl), 7.13 (2 H, d, CHthienyl); nmax(KBr)/cm21: 2924, 2360,
1608, 1465, 1375, 984, 790. Found: C, 69.95; H, 8.78%. EDAX
analysis: Sn/S: 0.1; Br/S: 0.08. Absorption: lmax (film on glass)
466 nm. Emission: lmax (film on glass) 636 nm.
722
J. Mater. Chem., 2001, 11, 718–722