Poly(alkyl thiophene-3-carboxylates). Synthesis, properties and electroluminescence studies of polythiophenes containing a carbonyl group directly attached to the ring

Article abstract of DOI:10.1039/a902504i

Improved synthetic methodology for poly(3-hexyl- and 3- octyloxycarbonylthiophene-2,5-diyl) (1a and 1b) is reported. M(n) = 6700 and 9400 (M(w)/M(n) = 2.5 and 3.2), λ(max) for fluorescence emission = 600 and 610 nm and λ(max) for electroluminescence = 600 and 615 nm, for 1a and 1b respectively. The ¹H NMR spectra required that pentads be considered to explain the spectra. That is the four nearest neighbours to a given ring influence the ¹H NMR spectrum. Electroluminescence efficiencies of 0.016% and 0.018% were observed for devices made from 1a and 1b, respectively. A bilayer device of ITO/poly(3-octylthiophene)/1b/A1 emitted at 646 nm, the same wavelength where poly(3-octylthiophene) itself emits. The efficiency was low but was an order of magnitude greater than for poly(3-octylthiophene) itself. Regioregular (HH-TT) poly(4,4'-bis(hexyl- and octyloxycarbonyl)[2,2'- bithiophene]-5,5'-diyl) (3a and 3b) were also prepared via the Ullmann reaction and M(n) = 7900 and 11000 respectively. Films of 3a and 3b were yellow in color and showed λ(max) = 377 and 381 nm respectively, about 55-80 nm blue shifted compared with 1a and 1b. This is due to the large rotational barrier in the HH dyads which reduces the effective conjugation length in 3a and 3b. 3a and 3b showed bright fluorescence and electroluminescence with emission of yellow light. Electroluminescence efficiencies were 8.5 x 10^-3% and 4.7 x 10^-3%, respectively.

Full text of DOI:10.1039/a902504i

J O U R N A L O F
Poly(alkyl thiophene-3-carboxylates). Synthesis, properties and
electroluminescence studies of polythiophenes containing a carbonyl
group directly attached to the ring
Martin Pomerantz,*a Yang Cheng,a Ramesh K. Kasimb and Ronald L. Elsenbaumer*ab
C H E M I S T R Y
aCenter for Advanced Polymer Research, Department of Chemistry and Biochemistry, Box 19065,
The University of Texas at Arlington, Arlington, Texas, 76019–0065 USA. E-mail:
pomerantz@uta.edu; elsenbaumer@uta.edu
bMaterials Science and Engineering Program, Box 19031, The University of Texas at Arlington,
Arlington, Texas, 76019–0031 USA
Received 29th March 1999, Accepted 20th May 1999
Improved synthetic methodology for poly(3-hexyl- and 3-octyloxycarbonylthiophene-2,5-diyl) (1a and 1b) is
reported. M =6700 and 9400 (M /M =2.5 and 3.2), l
for fluorescence emission=600 and 610 nm and l
for
9
9
9
n
w
n
max
max
electroluminescence=600 and 615 nm, for 1a and 1b respectively. The 1H NMR spectra required that pentads be
considered to explain the spectra. That is the four nearest neighbours to a given ring influence the 1H NMR
spectrum. Electroluminescence efficiencies of 0.016% and 0.018% were observed for devices made from 1a and 1b,
respectively. A bilayer device of ITO/poly(3-octylthiophene)/1b/Al emitted at 646 nm, the same wavelength where
poly(3-octylthiophene) itself emits. The efficiency was low but was an order of magnitude greater than for poly(3-
octylthiophene) itself. Regioregular (HH–TT) poly(4,4∞-bis(hexyl- and octyloxycarbonyl)[2,2∞-bithiophene]-5,5∞-
diyl) (3a and 3b) were also prepared via the Ullmann reaction and M =7900 and 11000 respectively. Films of 3a
9
n
and 3b were yellow in color and showed l =377 and 381 nm respectively, about 55–80 nm blue shifted compared
max
with 1a and 1b. This is due to the large rotational barrier in the HH dyads which reduces the effective conjugation
length in 3a and 3b. 3a and 3b showed bright fluorescence and electroluminescence with emission of yellow light.
Electroluminescence efficiencies were 8.5×10−3% and 4.7×10−3%, respectively.
ing alternating head-to-head (HH) and tail-to-tail (HT)
linkages.
Introduction
Since Burroughes et al. first demonstrated the electroluminesc-
O
C
O
C
ent properties of poly(p-phenylenevinylene) (PPV),1 electrolu-
minescent devices based on organic conjugated polymer thin
films have attracted much interest. Conjugated polymers show
unique optical and electronic properties, while still retaining
their polymeric materials properties, in particular ease of
processing.2 We recently reported on the synthesis of soluble
poly(3-hexyloxycarbonylthiophene-2,5-diyl) (1a) and poly(3-
octyloxycarbonylthiophene-2,5-diyl) (1b) from the corre-
sponding dibromo esters, hexyl and octyl 2,5-dibromothio-
phene-3-carboxylate (2a and 2b), using the Ullmann coupling
reaction employing 3 equivalents of copper metal (Scheme 1).3
OR
RO
n
S
S
3 a: R = C6H13
b: R = C8H17
Results and discussion
Regiorandom poly(3-hexyloxycarbonylthiophene-2,5-diyl) (1a)
and poly(3-octyloxycarbonylthiophene-2,5-diyl) (1b)
In that case the molecular weights, M , were about 3000–4000.
In this paper we report on an improved preparation of 1a
9
n
and 1b and on the properties of these polymers including their
electroluminescence behaviour. Further, we report on the
preparation and properties of poly(4,4∞-bis(hexyloxycar-
bonyl)[2,2∞-bithiophene]-5,5∞-diyl) (3a) and poly(4,4∞-bis(oc-
tyloxycarbonyl[2,2∞-bithiophene]-5,5∞-diyl) (3b), regioregular
variations of poly(3-hexyloxycarbonylthiophene-2,5-diyl) (1a)
and poly(3-octyloxycarbonylthiophene-2,5-diyl) (1b) contain-
Our recent report on the synthesis of poly(3-hexyloxycarbonyl-
thiophene-2,5-diyl) (1a) and poly(3-octyloxycarbonylthi-
ophene-2,5-diyl) (1b) shown in Scheme 1 involved the reaction
of the dibromo esters, hexyl and octyl 2,5-dibromothiophene-
3-carboxylate (2a and 2b), with three equivalents of freshly
prepared copper powder.3 We have now found, however, that
the use of four equivalents of copper provided 1a and 1b with
about double the molecular weights (determined by GPC with
polystyrene standards) in about 48–50% yield. The polymers
prepared this way were free flowing deep red powders. Table 1
CO2H
CO2R
Br
CO2H
Br
ii, iii
i
Br
Br
Table 1 Molecular weight, polydispersity and degree of polymerization
of 1a and 1b
S
S
S
2a,b
CO2R
a: R = C6H13
b: R = C8H17
iv
Polymer
M
M /M
Degree of polymerization
9
9
9
n
w
n
n
S
1a (4 equiv. Cu)
1a (3 equiv. Cu)
1b (4 equiv. Cu)
1b (3 equiv. Cu)
6600
3000
8800
3900
2.5
2.6
3.4
2.6
31
14
37
16
1a,b
Scheme 1 Reagents and conditions: i, Br , AcOH; ii, SOCl ; iii, ROH,
2
2
pyridine; iv, Cu, DMF, 7 days, 145–150 °C.
J. Mater. Chem., 1999, 9, 2155–2163
2155

4.13/4.14 are assigned to the head-to-tail (HT) and head-to-
head (HH) dyads, respectively. Furthermore, this assignment
was confirmed by preparation of a regioregular version of
these polymers containing only the HH and not the HT dyad
(vide infra). The ratio of the HH5HT dyads was 2.1551 and
2.1651 for 1a and 1b respectively. The greater amount of head-
to-head dyads is reasonable since it is known in the benzene
series that strong electron withdrawing groups activate the
ortho position in the Ullmann coupling reaction.6 Thus, in the
early stages of the reaction higher concentrations of HH dyads
would be formed.
We have also carried out the coupling reactions of 2a and
2b using Rieke nickel7–9 and, while the molecular weights of
1a and 1b obtained by this procedure were low (3300 and
7200 respectively), the ratio of the HH5HT dyads was 0.3251
and 0.3151. In this case it is known that aromatic couplings
with Rieke nickel are subject to steric effects and ortho
substituents hinder the coupling.7 Thus, in the early stages of
this reaction high concentrations of TT dyads would be formed
followed by HT dyads. In later stages of the reaction HH
coupling would be required and since there is considerable
steric hindrance, lower overall molecular weights are observed.
Since fluorescence is a necessary condition for electrolumi-
nescence, the fluorescence characteristics of the two polymers
1a and 1b from the Ullmann coupling reactions were examined.
The fluorescence emission maxima for 1a in THF solution and
as a film were 565 nm and 600 nm respectively while the
corresponding THF solution and film emission maxima for 1b
were 560 nm and 610 nm respectively.
Fig. 1 1H NMR spectra of poly(3-hexyloxycarbonylthiophene-2,5-
diyl) (1a).
compares the molecular weights (M ), polydispersity (M /M )
9
9
9
n
w
n
and the degree of polymerization. These higher molecular
weight polymers had better film forming characteristics than
the lower molecular weight materials.
Compounds 1a and 1b showed UV–VIS absorption maxima
at 410 and 419 nm for THF solutions and at 434 and 460 nm
for films cast from toluene. The 1H NMR spectra were quite
interesting. At 300 MHz the spectra clearly showed four sets
of peaks at d=7.92, 7.85, 7.66 and 7.60 for the ring hydrogen
[see Fig. 1 for the spectra of poly(3-hexyloxycarbonylthi-
ophene-2,5-diyl) (1a)]. However, the 500 MHz 1H NMR spec-
tra showed many more peaks, up to about 12–16 peaks and
shoulders (Fig. 1). It is well known in the case of many 3-
substituted polythiophenes such as poly(3-alkylthiophenes)
that there are four triads to be considered with respect to the
1H NMR spectra4,5 and so one should expect four peaks for
1a and 1b. The observation of many more than four peaks
suggests that pentads rather than triads must be considered.
This is because of the strongly electron withdrawing character
of the ester group which enables it to influence the 1H chemical
shift over a greater distance. Fig. 2 shows four pentads based
on a single configurational triad. Since there are four possible
triads there is a total of 16 pentads. As mentioned above we
see peaks or shoulders for 12–16 of them.
Regioregular poly(4,4∞-bis(hexyloxycarbonyl)[2,2∞-
bithiophene]-5,5∞-diyl) (3a) and poly(4,4∞-
bis(octyloxycarbonyl)[2,2∞-bithiophene]-5,5∞-diyl) (3b)
Preparation of regioregular variations of poly(3-hexyloxycar-
bonylthiophene-2,5-diyl) (1a) and poly(3-octyloxycarbonyl-
thiophene-2,5-diyl) (1b) were carried out as follows. Initially
an attempt was made to prepare completely head-to-tail (HT)
polymer employing Rieke zinc as was done by Rieke in the
case of regioregular poly(3-alkylthiophenes).10–12 To examine
the regiochemistry of the reaction of a 2,5-dibromothiophene
ester with Rieke zinc, methyl 2,5-dibromothiophene-3-carb-
oxylate was treated with Rieke zinc at −78 °C and the product
The 1H NMR spectra of 1a/1b showed two peaks at d=
4.30/4.31 and 4.13/4.14 which are assigned to the CH groups
2
attached to the oxygen in the esters. Based upon the assign-
ments of the CH groups attached to the thiophene rings in
was quenched with H O. Scheme 2 shows this reaction. The
2
2
poly(3-dodecylthiophene)4,5 the peaks at 4.30/4.31 and
ratio of products 554 was 86514, determined by integration
of the 1H NMR spectra, which indicates that the reaction was
not completely regioselective and so an alternate regioregular
polymer was prepared.
RO2C
CO2R
RO2C
S
S
The monomers chosen for these syntheses were dihexyl 5,5∞-
dibromo-2,2∞-bithiophene-4,4∞-dicarboxylate (6a) and dioctyl
5,5∞-dibromo-2,2∞-bithiophene-4,4∞-dicarboxylate (6b). The
synthesis of these monomers is shown in Scheme 3.
The preparation of the polymers, poly(4,4∞-bis(hexyloxycar-
bonyl)[2,2∞-bithiophene]-5,5∞-diyl) (3a) and poly(4,4∞-bis(oc-
tyloxycarbonyl)[2,2∞-bithiophene]-5,5∞-diyl) (3b), was carried
S
S
S
RO2C
CO2R
CO2R
CO2R
CO2R
RO2C
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
RO2C
CO2R
CO2R
CO2R
CO2R
CO2CH3
Rieke Zn
CO2CH3
CO2CH3
ZnBr
CO2R
CO2R
+
Br
Br
Br
BrZn
Br
THF
–78 °C
S
S
S
RO2C
CO2R
H2O
CO2CH3
RO2C
CO2CH3
+
Br
Br
S
S
RO2C
4
5
Fig. 2 4 Configurational pentads of 1a and 1b with a single central
triad.
Scheme 2 Reaction of methyl 2,5-dibromothiophene-3-carboxylate
with Rieke zinc.
2156
J. Mater. Chem., 1999, 9, 2155–2163

32% HT dyads. Similar observations involving regioregular
HH–TT poly(dialkylbithiophenes) have been made.13–15
We have recently shown by quantum mechanical
calculations on dimethyl 2,2∞-bithiophene-3,3∞-dicarboxylate,
dimethyl 2,2∞-bithiophene-3,4∞-dicarboxylate and dimethyl 2,2∞-
bithiophene-4,4∞-dicarboxylate that the rotational barrier for
the model head-to-head dimer is fairly large, 36 kJ mol−1
(8.55 kcal mol−1).16 Furthermore the rotation barriers for the
model head-to-tail and tail-to-tail dyads are quite low
[0.35 kJ mol−1 (0.084 kcal mol−1) for HT and 1.5 kJ mol−1
(0.37 kcal mol−1) for TT ].16 This means that in the polymers
at the HT and TT linkages the rings will be more coplanar
while at the HH linkages the rings will be considerably twisted.
The calculations indicate a thiophene–thiophene dihedral angle
of 77.4° in the lowest energy conformation of dimethyl 2,2∞-
bithiophene-3,3∞-dicarboxylate. Thus 3a and 3b with a greater
amount of HH dyads will absorb at shorter wavelengths than
1a and 1b. Generally, in most polythiophenes, the UV–VIS
absorption shows a significant bathochromic shift in going
from solution to the solid state (film);17 however, in the case
of 3a and 3b, there is essentially no shift. This is probably due
to the large HH rotational barrier16 which, even in the solid
state, can not be overcome to produce a more flattened and
conjugated system.17
CO2H
CO2H
ii, iii
CO2R
iv
i
Br
S
Br
S
S
S
RO2C
CO2R
RO2C
Br
CO2R
Br
v
S
S
S
a: R = C H
6
b: R = C68H13
17
Scheme 3 Reagents and conditions: i, 0.97 equiv. Br , AcOH; ii, SOCl ;
2
2
iii, ROH, pyridine; iv, NiBr , Zn, PPh , DMF, 80 °C, 24 h; v,
2
3
Br , CHCl .
2
3
Cu, DMF
2 days
145 °C
RO2C
Br
CO2R
Br
RO2C
CO2R
S
S
n
S
S
a: R = C6H13
b: R = C8H17
6a,b
3a,b
Scheme 4 Preparation of poly(4,4∞-bis(hexyloxycarbonyl)[2,2∞-bithio-
phene]-5,5∞-diyl) (3a) and poly(4,4∞-bis(octyloxycarbonyl)[2,2∞-bithio-
phene]-5,5∞-diyl) (3b).
Solid films of polymers 3a and 3b showed bright orange
fluorescence under UV light, prompting examination of their
fluorescence properties. The peak emission wavelengths of
polymers 3a and 3b were independent of the excitation wave-
length, but the intensities were a function of the excitation
wavelength. The intensities reached a maximum when the
excitation wavelength was about 435 to 475 nm. Using an
excitation wavelength between 435 and 475 nm, THF solutions
of the two polymers 3a and 3b showed emission maxima at
530 and 540 nm, respectively, 20–35 nm shorter than poly(3-
hexyl- and 3-octyloxycarbonylthiophene-2,5-diyls) (1a and
1b). Both polymers 3a and 3b showed cast film emission at
560 nm, 40–50 nm shorter than 1a and 1b. This also suggests
that the conjugation lengths are shorter in the regioregular
HH–TT polymer even in the excited state as discussed above.
The 300 MHz 1H NMR spectra of poly(4,4∞-bis(hexyloxy-
carbonyl)[2,2∞-bithiophene]-5,5∞-diyl) (3a) and poly(4,4∞-
bis(octyloxycarbonyl)[2,2∞-bithiophene]-5,5∞-diyl) (3b) showed
only one main aromatic hydrogen peak at d=7.66 (Fig. 4). It
should be recalled that samples of 1a and 1b showed up to 16
peaks and shoulders in their 1H NMR spectra for the 16
different configurational pentads. Since 3a and 3b are both
made up of only a single pentad (Fig. 3) they show only the
single major peak. Furthermore, there is also one major triplet
out via the Ullmann reaction using 4 equivalents of copper
powder in DMF at 145 °C for 2 days as shown in Scheme 4.
Polymers 3a and 3b are regioregular, they contain alternating
HH and TT linkages and they contain only a single one of
the 16 possible pentads described above. This is shown in
Fig. 3.
GPC analysis of 3a and 3b provided the results shown in
Table 2. The molecular weights and average number of thio-
phene rings in a chain is comparable to what was observed in
polymers 1a and 1b. The UV–VIS absorption maxima for 3a
and 3b were 387 nm and 389 nm for THF solutions, respect-
ively, and 377 nm and 381 nm for cast films, respectively.
These values are unusual in that the film spectra are somewhat
blue shifted compared with the solution spectra. Generally
polythiophene film spectra are red shifted compared with
solution. These values are also considerably blue shifted rela-
tive to the values for 1a and 1b, 23–30 nm for the solution
spectra and 57–79 nm for the film spectra. This suggests that
there is less effective conjugation in 3a and 3b than in 1a and
1b. The origin of these UV–VIS spectral differences and the
shorter conjugation lengths is the result of there being more
HH linkages in the regioregular polymers 3a and 3b than in
1a and 1b. Polymers 3a and 3b have 100% HH dyads and
essentially no HT dyads while 1a and 1b have 68% HH and
at d=4.14 (–O–CH –) which corresponds to the HH dyads
2
and a very small peak (about 5%) at d=4.30 corresponding
to a very small impurity of the HT dyad (or possibly end
repeat unit
RO2C
RO2C
RO2C
S
S
S
S
S
CO2R
HH etc.
CO2R
HH
linkages:
TT
TT
Fig. 3 The single pentad of regioregular polymers 3a and 3b.
Table 2 Molecular weight, polydispersity and degree of polymerization
of 3a and 3b
Polymer
M
9
M /M
Number of thiophene rings
9
9
n
w
n
3a
3b
8100
8700
1.8
2.0
39
37
Fig. 4 300 MHz 1H NMR spectra of poly(4,4∞-bis(hexyloxycar-
bonyl)[2,2∞-bithiophene]-5,5∞-diyl) (3a).
J. Mater. Chem., 1999, 9, 2155–2163
2157

groups). In the 13C NMR spectra there is a single carbonyl
peak at d=162.2 (3a) and 162.1 (3b) and a single peak at d=
65.3 (both 3a and 3b) which corresponds to the carbon
attached to the oxygen (O–CH ). These peaks are either split
2
into four peaks or three peaks and a shoulder in 1a and 1b3
and, once again, this attests to the regioregular nature of the
polymers 3a and 3b.
Electroluminescence studies
In the literature, there is very little on the study of electron-
withdrawing group substituted polythiophenes for electrolumi-
nescent applications. Wudl and Heeger reported on the prep-
aration of poly(3,4-dicyanothiophene) and demonstrated a
bilayer polymer LED using it with MEH-PPV as the active
polymer. The devices were reported to show a quantum
efficiency of 0.1%. Since poly(3,4-dicyanothiophene) is not
soluble in common organic solvents, fabrication of devices
using this material proved extremely difficult.18 We have
successfully prepared LEDs from 1a, 1b, 3a and 3b using them
as the emitting material.
Fig. 5 Electroluminescence from an ITO/poly(3-hexyloxycarbonylthi-
ophene-2,5-diyl) (1a)/Al LED.
Electroluminescent devices were made by spin-coating the
polymer onto ITO (indium-tin oxide) coated glass and then
evaporatively depositing a layer of aluminum onto the polymer
layer. The electroluminescence spectra were acquired with a
Tracor-Northern 1710A Diode Array Spectrometer and the
emission efficiencies were measured using a silicon photodiode.
The electroluminescence % external efficiency, defined as the
number of photons emitted per 100 electrons injected, was
calculated as shown in eqn. (1), where p is the correction
factor for the flux per unit solid angle leaving the device
directly in the forward direction.19
Fig. 6 Current–voltage curve for an ITO/poly(3-hexyloxycarbonylthi-
ophene-2,5-diyl) (1a)/Al LED.
EL% external efficiency=
Light output (watts)×p
×100% (1)
Device current (amp)×Energy of photons
Poly(3-hexyloxycarbonylthiophene-2,5-diyl) (1a) produced
red–orange light with an emission maximum at around 600 nm
(Fig. 5). Poly(3-octyloxycarbonylthiophene-2,5-diyl) (1b) dis-
played an emission maximum at around 615 nm. The electrolu-
minescence efficiencies of 1a and 1b were 0.016% and 0.018%
respectively, which is hundreds of times higher than non-
regioregular poly(3-hexylthiophene) (7a) (5×10−5%), non-
regioregular poly(3-octylthiophene) (7b) (7×10−5%), regiore-
gular poly(3-hexylthiophene) (8) (1.5×10−4%)20 and regioreg-
ular poly[4-(octylphenyl)thiophene] (9) (2×10−4%).21 Fig. 6
shows the current–voltage plot for the LED made from poly(3-
hexyloxycarbonylthiophene-2,5-diyl) (1a) and it can be seen
that the turn-on voltage for passage of current is less than 4 V.
Fig. 7 Electroluminescence from an ITO/poly(octylthiophene)/1b/Al
LED.
inject electrons into the polymer, and consequently causes a
higher electroluminescent efficiency in the bilayer device than
in a device using poly(3-alkylthiophene) (7b) alone. This
increased electron affinity also allows 1b to function as a more
efficient electron transporting layer than 7b alone. Fig. 8 shows
the current–voltage plot for the LED made from ITO/
poly(octylthiophene)/poly(3-octyloxycarbonylthiophene-2,5-
diyl) (1b)/Al and it can be seen that the turn-on voltage for
passage of current is about 4–5 V with a sharp increase of the
current at about 9 V.
C8H17
C6H13
R
S
n
n
S
n
S
7a: R = C6H13
b: R = C8H17
8
9
A bilayer device was fabricated consisting of ITO/poly(3-
octylthiophene) (7b)/1b/Al. The electroluminescence emission
maximum was at 646 nm (Fig. 7) and the quantum efficiency
was 4.5×10−4%. Poly(3-octylthiophene) (7b) itself showed its
electroluminescence emission at 646 nm, but the efficiency was
only 5×10−5%. Thus, in the bilayer device the emission was
from 7b and electroluminescence efficiency was an order of
magnitude greater than for poly(3-octylthiophene) (7b) itself.
Polymer 1b with an electron-withdrawing ester group directly
attached to the ring has a high electron affinity. This affinity
reduces the barrier to electron injection, making it easier to
Fig. 8 Current–voltage curve for an ITO/poly(octylthiophene)/1b/Al
LED.
2158
J. Mater. Chem., 1999, 9, 2155–2163

Electrical conductivity studies
Attempts were made to study the electrical conductivity of
cast films of polymers 1a and 1b by doping with 0.1% solutions
of FeCl , Cu(ClO ) , NOPF (nitrosonium hexafluorophos-
3
4 2
6
phate), NOBF (nitrosonium tetrafluoroborate), DDQ (2,3-
4
dichloro-5,6-dicyanobenzo-1,4-quinone) and TCNQ (7,7,8,8-
tetracyanoquinodimethane) solutions in nitromethane at room
temperature for 10 to 15 min and with I vapour at room
2
temperature for 24 h. No color changes were observed for the
polymer films. The conductivity of the films before and after
doping was too low to be measured using our instrument
(<10−8 S cm−1). Thus poly(3-hexyloxycarbonylthiophene-
2,5-diyl) (1a) and poly(3-octyloxycarbonylthiophene-2,5-diyl)
(1b) could not be oxidatively p-doped. Attempts were also
made to reductively n-dope the polymers. The entire n-type
doping procedure was carried out in a dry box. All the solvents
were degassed (freeze–thaw) right before use. Polymer films
were immersed in a sodium naphthalide solution in THF for
1 to 2 min. The color of the film changed from orange to dark
brown–black. The polymer films became insoluble in THF.
Obviously, a reaction occurred between the polymer films
and the sodium naphthalide. However, the conductivity
measurements showed that the film did not conduct (s=
<10−8 S cm−1).
Fig. 9 Electroluminescence from an ITO/poly(4,4∞-bis(hexyloxycar-
bonyl)[2,2∞-bithiophene]-5,5∞-diyl) (3a)/Al LED.
The bilayer devices were prepared by spin-coating the
poly(3-octylthiophene) from toluene solvent, drying the film
and then spin-coating the poly(3-octyloxycarbonylthiophene-
2,5-diyl) (1b) from THF. The poly(3-octylthiophene)
employed was of high molecular weight, was much less soluble
in THF than toluene and dissolved in THF only very slowly.
Thus, the two layers were separate and discrete, but there may
have been a small amount of mixing of the polymers at
the interface.
The electroluminescence emission maxima for LEDs made
from 3a and 3b (ITO/3a or 3b/Al) were at 590 nm and 600 nm
for 3a (Fig. 9) and 3b and the quantum efficiencies were
8.5×10−3% and 4.7×10−3% for 3a and 3b respectively. Thus
the regioregular polymers 3a and 3b had electroluminescence
efficiencies about two to four times lower than those of 1a
and 1b. Polymers 3a and 3b have a homogeneous regioregular
structure which at first sight might appear to favour charge
transport. However, compared to polymers 1a and 1b, the
average conjugation length of polymers 3a and 3b is shorter
owing to increased steric interactions between adjacent thio-
phene rings causing a larger dihedral angle in the head-to-
head dyads. Therefore charge transport is hindered owing to
less efficient conjugation along the chains and this might be
the cause of the lower electroluminescence efficiency compared
with polymers 1a and 1b. In addition, there might also be a
somewhat higher barrier to charge injection. The devices based
on polymers 3a and 3b lasted only 20 to 30 min (for both 3a
and 3b devices, the operating voltage was 14 V), while the
devices fabricated from polymers 1a and 1b lasted 15 to 25 h
before failing (the operating voltages were 20 and 50 V for 1a
and 1b respectively). The current–voltage curve for an ITO/
poly(4,4∞-bis(hexyloxycarbonyl)[2,2∞-bithiophene]-5,5∞-diyl)
(3a)/Al LED is shown in Fig. 10 and it can be seen that the
turn-on voltage for passage of current is about 7–8 V with a
sharp increase of the current at about 12 V. This higher turn
on voltage is also consistent with shorter conjugation lengths
and the more twisted nature of the polymer backbone
compared to 1a.
Experimental
General
300 MHz NMR spectra were obtained on a Bruker MSL-300
spectrometer. 1H NMR spectra were recorded at 300 MHz
using TMS (d=0.00) as internal reference. 13C NMR spectra
were recorded at 75 MHz using 13CDCl (d=77.00) as internal
3
reference. All spectra were taken using CDCl as solvent and
3
reported in ppm relative to the internal references. 500 MHz
1H NMR spectra were obtained on a JEOL Eclipse 500
instrument or from the University of Michigan on a Bruker
AMX-500 spectrometer. Infrared spectra were recorded on a
JASCO FT/IR-410 Fourier Transform Infrared instrument,
using KBr pressed pellets (approximately 1 mg of sample in
100 mg of KBr) or neat liquid samples between NaCl plates
and are reported in cm−1 with a resolution of 4 cm−1. HPLC
was performed on a Waters HPLC system, using a Waters
Model 501 HPLC pump, a Lambda-Max Model 481 UV–VIS
detector (l=254 nm), Maxima 820 Chromatography
Software, and an Econosil C18 10U (10 mm) reversed-phase
column (250 mm×4.6 mm). Methanol was used as the eluent
with a flow rate of 1.0 mL min−1. Gel permeation chromatog-
raphy (GPC) was carried out on a Waters GPC system, using
a Waters Model 510 HPLC pump, a Model 490 multi-
wavelength detector (l=254 nm), Millennium 2010 Software,
˚
and a serial combination of 103, 104, and 105 A Ultrastyragel
columns. THF was used as the eluent with a flow rate of
1.0 mL min−1. The calibration curve was established using
polystyrene standards with a molecular weight range of 800
to 9×105 g mol−1. The molecular weights of polymer in THF
were measured relative to the polystyrene standards. UV–VIS
spectra were recorded on a Varian Cary 5E UV–VIS–NIR
spectrophotometer or a Perkin-Elmer Lambda Array 3840
UV–VIS
spectrophotometer.
Samples
were
either
polymer–THF solutions or polymer thin films cast on quartz
cuvettes from polymer–toluene solutions. Elemental analyses
were obtained on a Perkin-Elmer 2400 CHN elemental ana-
lyzer or determined by Texas Analytical Laboratories, Stafford,
Texas. Melting points were measured using a Thomas-Hoover
capillary melting point apparatus and were uncorrected.
Fluorescence determinations were performed on a Perkin-
Elmer Model 204 fluorescence spectrophotometer. All solvents
were purified and dried prior to use according to standard
procedures.22 All monomers were shown to be >99% pure by
Fig. 10 Current–voltage curve for an ITO/poly(4,4∞-bis(hexyloxycar-
bonyl)[2,2∞-bithiophene]-5,5∞-diyl) (3a)/Al LED.
J. Mater. Chem., 1999, 9, 2155–2163
2159

HPLC analysis. Cu and Zn powders were prepared by litera-
ture procedures.23 Electroluminescence spectra were recorded
using a Tracor-Northern 1710A Diode Array Spectrometer
and the emission efficiencies were measured using a silicon
photodiode.
room temperature for 12 h under argon. To the precipitated
black nickel powder alkyl 2,5-dibromothiophene-3-carboxylate
2a or 2b (2.0 mmol) in THF (5 mL) was added directly via a
syringe. The mixture was refluxed for 60 h under argon. The
reaction mixture was diluted with diethyl ether to 100 mL,
and then filtered to remove the metal powder. The dark red
organic phase was washed with water (3×50 mL) and dried
Syntheses
over MgSO . The ether was removed with a rotary evaporator,
4
Poly(alkyl thiophene-3-carboxylates) prepared by the
Ullmann reaction: general procedure. Alkyl 2,5-dibromothio-
phene-3-carboxylate 2a or 2b (27 mmol),3 6.86 g (0.108 mol)
of Cu powder, and 90 mL of dry DMF were put into a 250 mL
one-necked flask in a dry box. The flask was removed from
the dry box and equipped with a magnetic stirring bar and an
air cooled condenser capped with a drying tube. The polymeriz-
ation was carried out at 145 °C for 7 days. The mixture then
and a dark red solid polymer was obtained. The polymer was
extracted with methanol in a Soxhlet extractor for 48 h to
remove the low molecular weight material and, after drying
under vacuum at room temperature, a red solid was obtained.
Poly(3-hexyloxycarbonylthiophene-2,5-diyl) (1a from Rieke
Ni). 0.14 g (33%) of red polymer was obtained; 1H NMR
(CDCl ) d 8.18 (br, m), 8.04 (br, m), 7.93 (br, m), 7.91 (br,
3
was cooled, diluted with CHCl to about 1500 mL, and filtered
m), 7.90–7.80 (br, m), 7.60 (br, m), 7.59 (br), 7.58 (br, m)
3
to remove excess Cu powder. The orange–red organic phase
(there were also an additional 2 small br multiplet peaks at
7.66 and 7.64), 4.30 (br, m), 4.13 (br, m), 1.75 (br, s), 1.55
(br, s), 1.33 (br, m), 1.25 (br, m), 1.24 (br, m), 0.89 (br, m);
was washed with water (10×500 mL) and dried over MgSO .
4
The CHCl was removed with a rotary evaporator and a dark
3
red polymer was obtained. Soxhlet extraction with methanol
13C NMR (CDCl ) (1H decoupled) d 162.6, 162.5, 162.3,
3
was performed for 48 hours to remove the low molecular
weight material from the bulk polymer. After drying under
vacuum at room temperature, a red solid was obtained.
145–120 (at least 11 peaks), 65.6, 65.4, 65.2, 65.1, 65.0, 31.4,
29.7, 28.7, 28.6, 28.4, 25.7, 22.5, 14.0; UV–VIS (THF) l
max
420 nm (3.0 eV); film (cast from toluene) l
max
560 nm; film (cast from
457 nm (2.7 eV);
fluorescence emission (THF) l
max
605 nm. GPC analysis of the THF polymer
Poly(3-hexyloxycarbonylthiophene-2,5-diyl) (1a).3 2.9 g
(51%) of polymer 1a was obtained as a red solid. 300 MHz
toluene) l
max
solution indicated a number average molecular weight (M )
of 3300 g mol−1 with a polydispersity (M /M ) of 1.9.
9
n
1H NMR (CDCl ) d 7.93 (br, m), 7.91 (br, m), 7.87 (br, m),
9
9
3
w
n
7.85 (br, m), 7.66 (br, m), 7.59 (br, m), 7.58 (br, m), 7.56 (br,
m), 4.31 (br, t, J=7 Hz), 4.15 (br, t, J=6 Hz), 1.76 (br, m),
1.56 (br, m), 1.34 (br, m), 1.24 (br, s), 0.87 (br, m); 500 MHz
Poly(3-octyloxycarbonylthiophene-2,5-diyl) (1b from Rieke
Ni). 0.15 g (32%) of red polymer was obtained; 1H NMR
1H NMR (CDCl ) d 7.93 (br, m), 7.91 (br, m), 7.86 (br, m),
(CDCl ) d 8.18 (br, m), 8.03 (br, m), 7.93 (br, m), 7.86 (br,
3
3
7.85 (br, m), 7.65 (br, m), 7.59 (br, m), 7.57 (br, m), 7.56 (br,
m), 7.66 (br, m), 7.60 (br, m), 4.30 (br, s), 4.13 (br, s), 1.75
m), 4.30 (br, m), 4.14 (br, m), 1.75 (br, m), 1.56 (br, s), 1.39
(br, s), 1.33 (br, s), 1.24 (br, m), 0.87 (br, m); 13C NMR
(br, s), 1.55 (br, s), 1.27 (br, m), 1.22 (br, m), 0.86 (br, m);
13C NMR (CDCl ) (1H decoupled) d 162.6, 162.5, 162.3,
3
(CDCl ) (1H decoupled) d 162.6, 162.5, 162.2, 162.1, 145–123
145–120 (at least 17 peaks), 65.6, 65.4, 65.2, 65.0, 31.8, 29.2,
3
(at least 14 peaks), 65.5, 65.4, 65.2, 31.5, 28.5, 28.4, 25.7, 22.5,
28.6, 28.4, 26.0, 22.6, 14.1; UV–VIS (THF) l
max
475 nm (2.6 eV);
426 nm
14.0; IR (KBr) 3088, 2952, 2926, 2856, 1712, 1460, 1398, 1343,
(2.9 eV); film (cast from toluene) l
max
565 nm; film (cast from
1238, 1140, 987, 849, 770, 664 cm−1; UV–VIS (THF) l
fluorescence emission (THF) l
max
620 nm, shoulder at 670 nm. GPC analysis of
max
410 nm (3.0 eV); film (cast from toluene) l 434 nm (2.9 eV);
toluene) l
max
max
fluorescence emission (THF) l
max
600 nm. GPC analysis of the THF polymer
565 nm; film (cast from
the THF polymer solution indicated that this polymer had a
toluene) l
number average molecular weight (M ) 7200 g mol−1 with a
9
max
n
solution indicated that the polymer had a number average
polydispersity (M /M ) of 2.1.
9
9
w
n
molecular weight (M ) of 6600 g mol−1 with a polydispersity
9
n
(M /M ) of 2.5.
9
9
Generation of organozinc halides from methyl 2,5-dibromoth-
iophene-3-carboxylate and Rieke zinc. In a dry box, finely cut
lithium (0.10 g, 15 mmol) and a catalytic amount (10 mol%)
of naphthalene (0.19 g, 1.5 mmol) were weighed into a 50 mL
one-necked round bottomed flask equipped with a magnetic
stirring bar. The flask was sealed with a septum. Similarly,
w
n
Poly(3-octyloxycarbonylthiophene-2,5-diyl) (1b).3 2.83 g
(44%) of polymer was obtained as a red solid. 300 MHz 1H
NMR (CDCl ) d 7.93 (br, m), 7.91 (br, m), 7.87 (br, m), 7.85
3
(br, m), 7.66 (br, m), 7.59 (br, m), 4.30 (br, m), 4.14 (br, m),
1.75 (br, m), 1.55 (br, m), 1.22 (br, m), 0.87 (br, m); 500 MHz
anhydrous ZnCl (0.75 g, 5.5 mmol) was weighed into a 25 mL
2
1H NMR (CDCl ) d 7.93 (br, m), 7.91 (br, m), 7.86 (br, m),
one-necked round bottomed flask. 7.5 mL of THF were added
3
7.85 (br, m), 7.84 (br, m), 7.66 (br, m), 7.59 (br, m), 4.30 (br,
to the flask with the lithium and naphthalene. The solution
was dark green. 12.5 mL of THF were added to the flask with
m), 4.14 (br, m), 1.75 (br, m), 1.56 (br, m), 1.26 (br, m), 1.23
(br, m), 0.87 (br, m); 13C NMR (CDCl ) (1H decoupled) d
ZnCl and this was slowly transferred to the flask with the
3
2
162.6, 162.5, 162.3, 162.2, 145–120 (at least 17 peaks), 65.5,
lithium and naphthalene via syringe (about 1.5 h). The reaction
65.4, 65.2, 31.8, 29.2, 28.6, 28.5, 26.0, 22.6, 14.1; IR (KBr)
2962, 2924, 2846, 1712, 1654, 1462, 1400, 1346, 1258, 1198,
mixture was further stirred until the small lithium pieces were
consumed and black zinc powder was precipitated out. Methyl
2,5-dibromothiophene-3-carboxylate (1.5 g, 5.0 mmol in
10 mL of THF) was then added via syringe to the newly
prepared zinc at −78 °C. The mixture was stirred for 1 h at
this temperature and allowed to warm up to room temperature.
1150, 1122, 1095, 1021, 804, 770 cm−1; UV–VIS (THF) l
max
419 nm (3.0 eV); film (cast from toluene) l 460 nm (2.7 eV);
max
fluorescence emission (THF) l
max
610 nm. GPC analysis of the THF polymer
560 nm; film (cast from
toluene) l
max
solution indicated a number average molecular weight (M )
of 8800 g mol−1 with a polydispersity (M /M ) of 3.4.
9
The mixture was then poured into a saturated aqueous NH Cl
n
4
9
9
solution (10 mL) and extracted with diethyl ether (3×20 mL).
w
n
The organic phase was washed with saturated NaCl
Poly(alkyl thiophene-3-carboxylates) prepared by Rieke
nickel reaction: general procedure. A 25 mL one-necked flask
(3×20 mL) and dried over MgSO . The diethyl ether was
removed with a rotary evaporator. Naphthalene was removed
4
was charged with NiI (1.563 g, 4.994 mmol), freshly cut
lithium (0.080 g, 11 mmol), naphthalene (0.064 g, 0.50 mmol)
and THF (10 mL), and the mixture was stirred vigorously at
from the mixture using column chromatography (silica gel,
1×8 inch) with ethyl acetate–hexane (1575) as eluent and the
resultant products methyl 2-bromothiophene-3-carboxylate (4)
2
2160
J. Mater. Chem., 1999, 9, 2155–2163

and methyl 2-bromothiophene-4-carboxylate (5) were
reaction mixture was cooled, diluted with CHCl to 50 mL,
and then filtered to remove the metal powder. The organic
phase was washed with water (3×30 mL) and dried over
3
obtained. 1H NMR (CDCl ) d 7.99 (d, J=1.5 Hz), 7.47 (d,
3
J=1.5 Hz), 7.36 (d, J=5 Hz), 7.24 (d, J=5 Hz), 3.89 (s),
3.86 (s).
MgSO . The CHCl was removed with a rotary evaporator.
4
3
Cold MeOH was added and the precipitate which formed was
filtered to give a brown–yellow crude solid product which was
purified as described below.
2-Bromothiophene-4-carboxylic acid. 2-Bromothiophene-4-
carboxylic acid was prepared essentially as described in the
literature.24 A solution of 23 g (0.14 mol) of bromine in
113 mL of glacial acetic acid was slowly added to a stirred
solution of 19 g (0.15 mol) of thiophene-3-carboxylic acid in
175 mL of glacial acetic acid at room temperature. The mixture
was stirred for 15 minutes and then poured into 1 L of cold
water (ice bath). The mixture was filtered and a light yellow
solid was obtained. The crude product was recrystallized from
1300 mL of water. 15 g (48%) of product 5 was obtained as
white needles: mp 140–141 °C (lit.,24 117–118 °C; lit.,25
139–140 °C); Anal. Calcd. for C H O SBr: C, 29.01; H, 1.46;
Dihexyl 2,2∞-bithiophene-4,4∞-dicarboxylate. The crude
product (0.21 g) was purified using column chromatography
(silica gel, 1×8 inch) with ethyl acetate–hexane (1550) as
eluent to give 0.18 g (43%) of a pale yellow solid product;
purity (HPLC) >99%; mp 60–61 °C. Anal. Calcd. for
C H O S : C, 62.53; H, 7.16; Found: C, 62.87; H, 7.22%;
22 30 4 2
1H NMR d 8.00 (d, J=1 Hz, 1H), 7.58 (d, J=1 Hz, 1H),
4.28 (t, J=7 Hz, 2H), 1.75 (quintet, J=7 Hz, 2H), 1.45–1.30
(m, 6H), 0.91 (t, J=7 Hz, 3H); 13C NMR (CDCl ) (1H
5
3 2
3
Found: C, 28.71; H, 1.13%; 1H NMR (CDCl ) d 10.27 (very
br s, 1H), 8.12 (d, J=1 Hz, 1H), 7.51 (d, J=1 Hz, 1H), 13C
NMR (CDCl ) (1H decoupled) d 166.8, 135.8, 133.1, 130.4,
113.4.
decoupled) d 162.4, 136.9, 134.6, 131.7, 124.6, 65.1, 31.4, 28.6,
3
25.6, 22.5, 14.0; IR (KBr) 3116, 3096, 3062, 2952, 2928, 2871,
2851, 1704, 1523, 1467, 1428, 1397, 1357, 1224, 1176, 1097,
1038, 984, 964, 908, 868, 844, 775, 751, 622, 467 cm−1;
3
UV–VIS (THF) l
9.9×103), two shoulders at 297 nm (e=9.7×103) and 312 nm
(e=9.4×103).
243 nm (e=1.4×104), 303 nm (e=
max
Alkyl 2-bromothiophene-4-carboxylates: general procedure.
8.28 g (40.0 mmol) of 2-bromothiophene-4-carboxylic acid
and 100 mL of SOCl were refluxed for 6 hours. The excess
2
SOCl was removed under vacuum (water aspirator) to pro-
2
Dioctyl 2,2∞-Bithiophene-4,4∞-dicarboxylate. The crude
product (0.28 g) was purified using column chromatography
(silica gel, 1×8 inch) with ethyl acetate–hexane (1550) as
eluent to give 0.24 g (50%) of a pale yellow solid product;
purity (HPLC) >99%; mp 69–70 °C. Anal. Calcd. for
duce a light yellow solid. Then, 0.40 mol of ROH and 8 mL
of dry pyridine were added to the flask dropwise through an
addition funnel. The mixture was stirred for 6 h at 80 °C,
cooled, poured into a 250 mL beaker with 60 g of ice and
80 mL of 1 M HCl, and stirred for a while. The solution was
washed with diethyl ether (4×300 mL) and the organic phase
was then washed with saturated NaHCO (3×300 mL) and
dried with K CO . The ethyl ether was removed with a rotary
evaporator and a light yellow crude product was obtained and
purified as described below.
C H O S : C, 65.24; H, 8.00; Found: C, 64.97; H, 7.78%;
26 38 4 2
1H NMR d 8.00 (d, J=1 Hz, 1H), 7.58 (d, J=1 Hz, 1H),
3
4.28 (t, J=7 Hz, 2H), 1.75 (quintet, J=7 Hz, 2H), 1.45–1.30
2
3
(m, 10H), 0.89 (t, J=7 Hz, 3H); 13C NMR (CDCl ) (1H
3
decoupled) d 162.4, 136.9, 134.6, 131.7, 124.6, 65.1, 31.8, 29.2,
28.7, 26.0, 22.6, 14.1; IR (KBr) 3121, 3060, 2953, 2915, 2870,
2847, 1709, 1523, 1475, 1403, 1363, 1228, 1173, 1102, 1035,
1018, 972, 941, 864, 845, 800, 772, 734, 621, 486 cm−1;
Hexyl 2-bromothiophene-3-carboxylate. The crude product
was distilled under vacuum to give 6.87 g (59%) of product as
UV–VIS (THF) l
9.7×103), two shoulders at 300 nm (e=9.6×103) and 310 nm
(e=9.3×103).
243 nm (e=1.4×104), 303 nm (e=
a
pale yellow liquid; purity (HPLC) about 99%; bp
max
75–76 °C/0.05 mm Hg. Anal. Calcd. for C H O SBr: C,
45.37; H, 5.19; Found: C, 45.41; H, 5.03%; 1H NMR d 7.98
11 15 2
(d, J=1 Hz, 1H), 7.46 (d, J=1 Hz, 1H), 4.25 (t, J=7 Hz,
2H), 1.72 (quintet, J=7 Hz, 2H), 1.45–1.30 (m, 6H), 0.90 (t,
Dialkyl 5,5∞-dibromo-2,2∞-bithiophene-4,4∞-dicarboxylates (6a
and 6b): general procedure. A solution of 0.144 g (0.90 mmol)
J=7 Hz, 3H); 13C NMR (CDCl ) (1H decoupled) d 161.6,
3
of bromine in 6 mL of CHCl was added slowly to a stirred
134.2, 133.6, 130.2, 112.8, 65.1, 31.4, 28.6, 25.6, 22.5, 14.0; IR
3
solution of 0.300 mmol of dihexyl or dioctyl 2,2∞-bithiophene-
(neat) 3108, 2959, 2929, 2858, 1719, 1528, 1467, 1422, 1378,
1227, 1175, 1094, 983, 927, 849, 735, 660 cm−1.
Octyl 2-bromothiophene-3-carboxylate. The crude product
was distilled under vacuum to give 8.29 g (65%) of product as
4,4∞-dicarboxylate and 6 mL of CHCl in a 25 mL round-
3
bottomed flask at room temperature. The mixture was warmed
to 60 °C for an additional 10 h and then poured into 50 mL
of cold water (ice bath), stirred for a while and Na SO was
2
3
added to react with excess bromine. The solution was washed
a
pale yellow liquid; purity (HPLC): >99%; bp
with CHCl (3×50 mL), and the organic phase was then
91–94 °C/0.05 mm Hg. Anal. Calcd. for C H O SBr: C,
3
13 19 2
washed with water (3×30 mL) followed by saturated NaCl
48.91; H, 6.00; Found: C, 48.74; H, 5.88; 1H NMR d 7.98 (d,
(2×50 mL) and dried over MgSO . The CHCl was removed
J=1 Hz, 1H), 7.46 (d, J=1 Hz, 1H), 4.25 (t, J=7 Hz, 2H),
1.72 (quintet, J=7 Hz, 2H), 1.45–1.30 (m, 10H), 0.88 (t, J=
4
3
with a rotary evaporator and a light yellow crude solid was
obtained and purified as described below.
7 Hz, 3H); 13C NMR (CDCl ) (1H decoupled) d 161.6, 134.2,
3
133.6, 130.2, 112.8, 65.1, 31.7, 29.2, 28.6, 26.0, 22.6, 14.0; IR
(neat) 3109, 2954, 2926, 2855, 1721, 1528, 1467, 1422, 1405,
Dihexyl
5,5∞-dibromo-2,2∞-bithiophene-4,4∞-dicarboxylate
1378, 1229, 1175, 1095, 983, 959, 923, 849, 735, 660 cm−1.
Dialkyl 2,2∞-bithiophene-4,4∞-dicarboxylates: general pro-
cedure. In a 10 mL one-necked flask were placed 22 mg
(6a). The crude product (0.12 g) was purified using column
chromatography (silica gel, 1×8 inch) with ethyl acetate–hex-
ane (15100) as eluent to give 0.104 g (60%) of a white solid
product; purity (HPLC) >99%; mp 87–88 °C. Anal. Calcd.
for C H O S Br : C, 45.53; H, 4.86; Found: C, 45.37; H,
(0.10 mmol) of NiBr , 0.20 g (0.76 mmol) of triphenylphos-
2
22 28 4 2
2
phine, 0.20 g (3.1 mmol) of zinc and 1 mL of DMF under
4.82%; 1H NMR d 7.36 (s, 1H), 4.30 (t, J=7 Hz, 2H), 1.76
argon and the mixture was stirred at 50 °C for 30 min. The
color of the reaction mixture changed from orange to reddish
brown and then 2.00 mmol of hexyl or octyl 2-bromothio-
phene-3-carboxylate in 1 mL of DMF was added via a syringe.
The mixture was warmed to 80 °C and stirred for 24 h. The
(quintet, J=7 Hz, 2H), 1.45–1.30 (m, 6H), 0.91 (t, J=7 Hz,
3H); 13C NMR (CDCl ) (1H decoupled) d 161.5, 135.2, 132.2,
3
125.9, 119.1, 65.5, 31.4, 28.6, 25.7, 22.5, 14.0; IR (KBr) 3095,
3066, 2954, 2927, 2890, 2869, 1729, 1522, 1479, 1423, 1318,
1221, 1159, 1060, 1017, 974, 919, 890, 852, 828, 766, 721, 553,
J. Mater. Chem., 1999, 9, 2155–2163
2161

477, 445 cm−1; UV–VIS (THF) l
324 nm (e=1.4×104).
243 nm (e=1.6×104),
6 Hz) (there was also an additional small br triplet peak at
4.29, J=8 Hz), 1.55 (br, m), 1.23 (br, m), 0.86 (br, t, J=
7 Hz) (there were also three additional small br multiplets at
max
Dioctyl
5,5∞-dibromo-2,2∞-bithiophene-4,4∞-dicarboxylate
1.75, 1.43, 0.89); 13C NMR (CDCl ) (1H decoupled) d 162.1,
3
(6b). The crude product (0.17 g) was purified using column
chromatography (silica gel, 1×8 inch) with ethyl acetate–hex-
ane (15100) as eluent to give 0.095 g (50%) of a white product;
purity (HPLC) >99%; mp 93–94 °C. Anal. Calcd. for
C H O S Br : C, 49.06; H, 5.70; Found: C, 48.80; H, 5.60%;
138.0, 136.0, 133.0, 126.5, 65.3, 31.8, 29.3, 28.5, 26.0, 22.6,
14.1; IR (KBr) 2955, 2925, 2855, 1713, 1465, 1398, 1345, 1215,
1132, 978, 846, 767, 673 cm−1; UV–VIS (THF) l
389 nm
382 nm (3.2 eV); fluo-
max
(3.2 eV); film (cast from toluene) l
rescence emission solution (THF) l
max
max
540 nm; film (cast from
26 36 4 2
2
1H NMR 7.36 (s, 1H), 4.30 (t, J=7 Hz, 2H), 1.76 (quintet,
toluene) l
560 nm. GPC analysis indicated a number
max
J=7 Hz, 2H), 1.50–1.20 (m, 10H), 0.89 (t, J=7 Hz, 3H); 13C
NMR (CDCl ) (1H decoupled) d 161.5, 135.2, 132.1, 125.9,
119.1, 65.5, 31.8, 29.7, 29.2, 28.6, 26.0, 22.6, 14.1; IR (KBr)
average molecular weight (M ) of 11000 g mol−1 with a
9
n
polydispersity (M /M ) of 2.3.
9
9
3
w
n
3092, 3062, 2956, 2925, 2892, 2849, 1729, 1521, 1479, 1423,
1382, 1319, 1222, 1159, 1081, 1026, 975, 918, 828, 766, 717,
Device fabrication
Sputtered ITO coated glass plates, 300×300×1.1 mm size,
were purchased from Photran Corporation. The ITO film
thickness and surface roughness were measured to be 50–60 nm
and 2–5 nm respectively. The surface resistivity was specified
to be 10 V per sq. The glass plates were cut into
25×25×1.1 mm pieces with a diamond scribe for device
fabrication. Substrate preparation and device fabrication steps
such as photolithography and metal electrode deposition were
performed inside Class 10 and Class 100 clean rooms. Polymer
films were cast in an argon filled inert atmosphere glove box.
477 cm−1; UV–VIS (THF) l
243 nm (e=1.6×104), 323 nm
max
(e=1.4×104).
Poly(4,4∞-bis(alkoxycarbonyl)[2,2∞-bithiophene]-5,5∞-diyl)s
(3a and 3b): general procedure. 0.156 mmol of dialkyl 5,5∞-
dibromo-2,2∞-bithiophene-4,4∞-dicarboxylate 6a or 6b, 40 mg
(0.63 mmol) of Cu powder, and 0.5 mL of dry DMF were
charged into a 5 mL one-necked flask equipped with a magnetic
stirring bar and a long air condenser capped with a drying
tube. The mixture was stirred at 145 °C for 2 days, cooled,
diluted with CHCl to 100 mL, and then filtered to remove
excess Cu powder. The organic phase was washed with water
3
LED Characterization
(5×50 mL) followed by saturated NaCl (2×50 mL) and dried
Contacts to positive and negative electrodes were formed using
conductive silver epoxy paint purchased from Delta
Technologies. The contacts were dried for at least 8 h before
making measurements. Current–voltage characteristics of
devices were measured using a Keithley 236 Current/Voltage
Source-Measure unit. A single device of area 1×1 mm was
used for this purpose. Current–voltage sweeps were collected
using the ‘step mode’ with a step height of 0.5 V and a time-
period of 2 s. EL spectra were recorded using a Tracor-
Northern 1710A diode-array spectrometer using a single device
of area 4×4 mm. Light intensity measurements were made
with a calibrated silicon photodiode attached to a Newport
1830-C power meter. The measured values were multiplied by
p to correct for the flux per unit solid angle leaving the device
directly in the forward direction. Film thickness was measured
using a Tencor-Northern Alpha Step profilometer.
over MgSO . The excess CHCl was removed with a rotary
4
3
evaporator, and
a solid yellow–orange compound was
obtained. Soxhlet extraction with methanol was carried out
for 48 hours to remove the low molecular weight material.
After drying under vacuum at room temperature, an orange
solid was obtained.
Poly(4,4∞-bis(hexyloxycarbonyl)[2,2∞-bithiophene]-5,5∞-diyl)
(3a). 30 mg (41%) of an orange solid compound was obtained.
Anal. Calcd. for C H O S : C, 62.83; H, 6.71; S, 15.25;
22 28 4 2
Found: C, 62.51; H, 6.74; S, 15.20%; 300 MHz 1H NMR 7.65
(br, s) (there were also three additional small br singlet peaks
at 8.04, 7.63, 7.61), 4.14 (br, t, J=6 Hz) (there was also an
additional small br triplet peak at 4.29, J=7 Hz), 1.56 (br,
m), 1.24 (br, m), 0.86 (br, t, J=6 Hz); 500 MHz 1H NMR
(CDCl ) d 7.65 (br, s) (there were also two additional small
3
br singlet peaks at 7.64 and 7.63 and two additional small br
Metal electrode deposition
doublet peaks at 8.03 and 7.61, both with J=1 Hz), 4.14 (br,
t, J=6 Hz) (there was also an additional small br triplet peak
at 4.29, J=8 Hz), 1.55 (br, t, J=6 Hz), 1.23 (br, m), 0.86 (br,
t, J=6 Hz) (there were also four additional small br multiplet
Thin metal films for the negative electrodes were deposited by
vacuum evaporation which was carried out in a Veeco Thermal
Evaporator under high vacuum of the order of 1×10−6 to
8×10−7 torr. Aluminum, purchased from Cerac Metals, Inc.,
was 99.999% pure.
peaks at 1.75, 1.34, 0.91, 0.83); 13C NMR (CDCl ) (1H
3
decoupled) d 162.2, 138.1, 136.0, 133.0, 126.6, 65.3, 31.5, 28.5,
25.7, 22.6, 14.0; IR (KBr) 3082, 2959, 2926, 2855, 1713, 1465,
1399, 1345, 1218, 1139, 985, 846, 771 cm−1; UV–VIS (THF)
(3.3 eV); fluorescence emission: solution (THF) l
Conclusions
l
387 nm (3.2 eV); film (cast from toluene) l
377 nm
530 nm;
max
max
max
An improved synthesis of poly(3-hexyl- and 3-octyloxycarbon-
ylthiophene-2,5-diyl) (1a and 1b) has been worked out. It
provides material with molecular weights, M =6700 and 9400
film (cast from toluene) l
560 nm. GPC analysis indicated
max
a number average molecular weight (M ) of 7900 g mol−1 with
9
9
n
n
a polydispersity (M /M ) of 1.6.
(M /M =2.5 and 3.2), l =434 and 460 nm for films cast
9
9
9
9
w
n
w
n
max
for fluorescence emission (film)=600 and
from toluene, l
max
Poly(4,4∞-bis(octyloxycarbonyl)[2,2∞-bithiophene]-5,5∞-diyl)
(3b). 33 mg (44%) of an orange solid compound was obtained.
610 nm and for electroluminescence l =600 and 615 nm,
for 1a and 1b respectively. The 1H NMR spectra required that
max
Anal. Calcd. for (C H O S ): C, 65.51; H, 7.61; S, 13.45;
pentads be considered to explain the spectra. That is the four
nearest neighbours to a given ring influence the 1H NMR
chemical shift of the hydrogen in that ring.
Electroluminescence efficiencies of 0.016% and 0.018% were
observed for devices made from 1a and 1b respectively. A
bilayer device of ITO/poly(3-octylthiophene)/1b/Al emitted at
646 nm, where poly(3-octylthiophene) emits. The efficiency
was 4.5×10−4% and this was an order of magnitude greater
than that for poly(3-octylthiophene) itself (5×10−5%).
26 36 4 2
Found: C, 65.26; H, 7.68; S, 13.24%; 300 MHz 1H NMR d
7.65 (br, s) (there were also three additional small br singlet
peaks at 8.04, 7.63, 7.61), 4.14 (br, t, J=6 Hz) (there was also
an additional small br triplet peak at 4.29, J=6 Hz), 1.55 (br,
m), 1.23 (br, m), 0.86 (br, t, J=5 Hz); 500 MHz 1H NMR
(CDCl ) d 7.65 (br, s) (there were also four additional small
3
br singlet peaks at 8.04, 7.64, 7.63, 7.61 and four additional
very small br singlet peaks between 7.4–7.6), 4.14 (br, t, J=
2162
J. Mater. Chem., 1999, 9, 2155–2163

5
6
7
M. Sato and H. Morii, Macromolecules, 1991, 24, 1196.
Regioregular (HH–TT) poly(4,4∞-bis(hexyl- and octyloxycar-
bonyl)[2,2∞-bithiophene]-5,5∞-diyl) (3a and 3b) were also pre-
pared via the Ullmann reaction and provided material whose
J. Forrest, J. Chem. Soc., 1960, 592.
H. Matsumoto, S. Inaba and R. D. Rieke, J. Org. Chem., 1983,
48, 840.
molecular weights were M =7900 and 11000, respectively.
9
8
9
S. Inaba, H. Matsumoto and R. D. Rieke, Tetrahedron Lett., 1982,
n
Films of 3a and 3b were yellow in color and showed l
=
23, 4215.
max
S. Inaba, H. Matsumoto and R. D. Rieke, J. Org. Chem., 1984,
49, 2093.
377 and 381 nm, respectively, about 55–80 nm blue shifted
compared with 1a and 1b. This is due to the large rotational
barrier in the HH dyads. Polymers 3a and 3b showed bright
fluorescence and electroluminescence at 590 nm and 600 nm,
respectively. The electroluminescence efficiencies were
8.5×10−3% and 4.7×10−3% for 3a and 3b, respectively. Many
of the differences in the behavior of 1a and 1b compared with
3a and 3b were ascribed to the greater number of head-to-
head linkages in the regioregular polymers 3a and 3b which
gives rise to shorter conjugation lengths.
10 T. Chen and R. D. Rieke, J. Am. Chem. Soc., 1992, 114, 10087.
11 T. Chen and R. D. Rieke, Synth. Met., 1993, 60, 175.
12 T. Chen, X. Wu and R. D. Rieke, J. Am. Chem. Soc., 1995,
117, 233.
13 R. M. S. Maior, K. Hinkelmann, H. Eckert and F. Wudl,
Macromolecules, 1990, 23, 1268.
14 M. Zagorska and B. Krische, Polymer, 1990, 31, 1379.
15 M. Zagorska, I. Kulszewicz-Bajer, A. Pron, L. Firlej, P. Bernier
and M. Galtier, Synth. Met., 1991, 45, 385.
16 M. Pomerantz and Y. Cheng, Tetrahedron Lett., 1999, 40, 3317.
17 J. R. Reynolds and M. Pomerantz, in Electroresponsive Molecular
and Polymeric Systems, ed. T. A. Skotheim, Dekker, New York,
1991, Vol. 2, ch. 4, p. 187.
18 F. Hide, Y. Greenwald, F. Wudl and A. J. Heeger, Synth. Met.,
1997, 85, 1255.
19 N. C. Greenham, R. H. Friend and D. D. C. Bradley, Adv. Mater.,
1994, 6, 491.
20 F. Chen, P. G. Mehta, L. Takiff and R. D. McCullough, J. Mater.
Chem., 1996, 6, 1763.
21 M. Berggren, G. Gustafsson, O. Inganas, M. R. Andersson,
We wish to thank the Texas Advanced Technology Program
(Grant No. 003656–009) and the Robert A. Welch Foundation
(Grant Nos. Y-0684, Y-1407 and Y-1291) for financial support
and the National Science Foundation (Grant No. CHE
9601771) for funds for the purchase of a 500 MHz NMR
spectrometer. We also wish to thank Drs Haitao Cheng and
Chris Kojiro, University of Michigan, for several 500 MHz
NMR spectra and Professor Zoltan Schelly for the use of the
Tracor-Northern 1710A diode-array spectrometer.
¨
O. Wennerstrom and T. Hjerberg, Appl. Phys. Lett., 1994, 65,
¨
1489.
22 M. Casey, J. Leonard, B. Lygo and G. Procter, Advanced Practical
Organic Chemistry, Chapman & Hall, New York, 1990.
23 B. S. Furniss, A. J. Hannaford, V. Rogers, P. W. G. Smith and
A. R. Tatchell, Vogel’s Textbook of Practical Organic Chemistry,
4th edn., Longman Group, London, UK, 1978, p. 285.
24 E. Campaigne and R. C. Bourgeois, J. Am. Chem. Soc., 1954,
76, 2445.
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4
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Paper 9/02504I
J. Mater. Chem., 1999, 9, 2155–2163
2163