Preparation, photophysics, and electrochemistry of segmented comonomers consisting of thiophene and pyrimidine units: New monomers for hybrid copolymers

Article abstract of DOI:10.1021/jp075259j

An efficient coupling route to novel π-conjugated comonomers consisting of pyrimidine, thiophene, and bithiophene units was developed. The novel π-donor-acceptor-donor and π-donor-acceptor-acceptor-donor conjugated compounds were prepared by Suzuki heterocoupling and Ni(0)-mediated Ullman homocoupling reactions. Photophysical investigation of these alternating π-donor and acceptor compounds indicated that the deactivation of their singlet excited State proceeds predominately by fluorescence and results in high fluorescence quantum yields. Intersystem crossing to the triplet state was also present in ca. 10%. Quantification of the triplet manifold by laser flash photolysis further revealed that bithiophene ,produced its triplet state in only 31%. Cyclic voltammetry studies showed that the comonomers undergo both oxidation and reduction leading to their radical cations and radical anions, respectively. The radical cations are highly reactive and undergo anodic polymerization resulting in mutual p- and n-type dopable polymers. The extended conjugation resulting from polymer formation was confirmed by both absorbance and fluorescence spectroscopy and by GPC. Ruthenium binding with the conjugated homocoupled ligand was also found resulting in a hybrid alternating copolymer with significantly different spectroscopic and electrochemical properties relative to its metal-free counterpart.

Full text of DOI:10.1021/jp075259j

J. Phys. Chem. B 2007, 111, 11407-11418
11407
Preparation, Photophysics, and Electrochemistry of Segmented Comonomers Consisting of
Thiophene and Pyrimidine Units: New Monomers for Hybrid Copolymers
Ste´phane Dufresne,† Garry S. Hanan,‡ and W. G. Skene*,†
De´partement de Chimie, PaVillon J. A. Bombardier, UniVersite´ de Montre´al, CP 6128, succ. Centre-Ville,
Montre´al, Que´bec, H3C 3J7, Canada, and De´partement de Chimie, PaVillon Roger-Gaudry, UniVersite´ de
Montre´al, 2900 Edouard-Montpetit, Montre´al, Que´bec, H3T 1J4, Canada
ReceiVed: July 5, 2007
An efficient coupling route to novel π-conjugated comonomers consisting of pyrimidine, thiophene, and
bithiophene units was developed. The novel π-donor-acceptor-donor and π-donor-acceptor-acceptor-
donor conjugated compounds were prepared by Suzuki heterocoupling and Ni(0)-mediated Ullman
homocoupling reactions. Photophysical investigation of these alternating π-donor and acceptor compounds
indicated that the deactivation of their singlet excited state proceeds predominately by fluorescence and results
in high fluorescence quantum yields. Intersystem crossing to the triplet state was also present in ca. 10%.
Quantification of the triplet manifold by laser flash photolysis further revealed that bithiophene produced its
triplet state in only 31%. Cyclic voltammetry studies showed that the comonomers undergo both oxidation
and reduction leading to their radical cations and radical anions, respectively. The radical cations are highly
reactive and undergo anodic polymerization resulting in mutual p- and n-type dopable polymers. The extended
conjugation resulting from polymer formation was confirmed by both absorbance and fluorescence spectroscopy
and by GPC. Ruthenium binding with the conjugated homocoupled ligand was also found resulting in a
hybrid alternating copolymer with significantly different spectroscopic and electrochemical properties relative
to its metal-free counterpart.
Introduction
Comonomers consisting of electron-deficient heterocycles
such as pyridines and pyrimidines19 sandwiched between
electron-rich units further provide the means to tailor polymer
properties and they benefit from intramolecular charge transfer.
The π-deficient heterocycles can easily be reduced and polymers
derived from such comonomers are equally n-dopable. Incor-
poration of external π-rich units into the comonomer affords
polymers that can be both p- and n-doped. Such polymers are
of great interest because emitting devices constructed from them
require fewer layers and result in easier device assembly, in
addition to increased efficiency.20-22 Unfortunately, there are
only a limited number of polymers capable of sustaining both
oxidation and reduction with the capacity to act as mutual charge
and hole carriers; indeed, this shortage is due primarily to the
complicated synthesis of such polymers. An additional advan-
tage of polymers incorporating nitrogen-containing heterocycles
is their capacity to bind metal ions, leading to hybrid polymers.
The conductivity, photophysics, and redox potentials of these
hybrid polymers can be easily modulated by incorporating
different metals, thereby making them suitable for a variety of
applications including sensors17,23,24 for different anions and
small molecules and electronic uses including field effect
transducers,25 solar cells,26 and spintronics.27 An added benefit
of heterocyclic comonomers is that their electropolymerization
can occur in two ways as represented in Scheme 1. These
involve (1) electropolymerization of the pristine comonomer
followed by metal ion complexation into the resulting polymer
or (2) incorporating a metal ion into the comonomer to form a
metal-ligand complex followed by its polymerization.
Conjugated polymers have received much attention because
of their electrochemical and photophysical properties that are
ideal for the development of functional devices including field
effect transistors,1-3 light emitting diodes,4-10 and solar cells,11-13
to name but a few. Preparation of π-conjugated polymers is
well established including many powerful synthetic tools such
as Suzuki coupling14,15 and electropolymerization16-18 of het-
erocyclic monomers. However, polymers prepared via elec-
tropolymerization protocols often contain defects arising from
competing side reactions such as over oxidation, R,â-coupling,
and cross-linking, as well as other side reactions due to
mismatching of monomer redox potentials. A potentially at-
tractive solution to this problem involves the use of comonomers
because they offer the possibility to reduce such polymer defects
that otherwise would limit the degree of conjugation resulting
in inadequate properties. Comonomers contain the elements of
the copolymer structure such as a central aromatic acceptor core
inserted between electron-rich donors. This configuration sim-
plifies the electropolymerization process that would otherwise
require two or three separate monomers to obtain analogous
alternating copolymers via other coupling methods. The π-donor-
acceptor-donor relationship further permits tailoring the redox
potentials and other properties of both the monomer and the
resulting polymers to match those desired for a given applica-
tion.
* Address correspondence to this author. E-mail: w.skene@umontreal.ca.
† De´partement de Chimie, Pavillon J. A. Bombardier, Universite´ de
Our previous endeavors in comonomer synthesis28 and our
ongoing photophysical and electrochemical investigation of
conjugated polymers29-35 have prompted us to prepare a new
Montre´al.
‡ De´partement de Chimie, Pavillon Roger-Gaudry, Universite´ de Mon-
tre´al.
10.1021/jp075259j CCC: $37.00 © 2007 American Chemical Society
Published on Web 09/11/2007

11408 J. Phys. Chem. B, Vol. 111, No. 39, 2007
Dufresne et al.
SCHEME 1: Synthetic Representation of Possible Anodic Polymerization of Comonomers Either by Incorporating the
Metal Prior To or After Electropolymerization
CHART 1: Compounds Examined and Their Analogues
series of alternating π-rich and π-poor comonomers consisting
of thiophenes and pyrimidines. These compounds are of
particular interest because they are expected to exhibit the much
sought after mutual n- and p-doping properties. Furthermore,
the electron donor-acceptor arrangement leads to efficient
intramolecular charge transfer providing the means to reduce
the HOMO-LUMO energy levels and to tailor the spectroscopic
and electrochemical properties.14,36-38An additional advantage
of the nitrogen-containing heterocycles is that they can act as
ligands and coordinate metal ions leading to hybrid polymers.
Herein, we present the preparation of novel comonomers along
with their steady-state and time-resolved photophysics, elec-
trochemistry, and HOMO-LUMO energy gap characterization.
We also discuss the influence of the number of thiophene and
pyrimidine units on the photophysics and electrochemistry. The
comonomer electropolymerization and n- and p-doping behavior
of the resulting polymers are also investigated along with their
physical properties and the capacity of a polymer model to bind
ruthenium.
Results and Discussion
Synthesis. Synthesis of the novel comonomers in Chart 1
involved conventional Suzuki and Ullman couplings starting
from inexpensive and commercially available reagents according
to Scheme 2. The precursor 1, required for Suzuki coupling,
was prepared in moderate yields by nucleophilic aromatic
solvent ensures good homogeneity between the catalyst and
reagents. Interestingly, the reaction proceeded cleanly at 50 °C
with only 3 mol % of catalyst. Product selectivity between
mono- and bis-coupling to afford 4 and 6, respectively, could
be marginally controlled via different reagent stoichiometries.
For example, 4 was obtained in 71% with a 1:1 ratio of 1 and
2 while a 1:2 ratio afforded 86% of 6. The same trend in the
product distribution was observed with 3. The Ni(0)-mediated
Ullman homocoupling conversely proceeded in low yields to
provide 8 and 9 from 4 and 5, respectively.
Spectroscopy. The effect of the alternating π-donor and
acceptor groups and the number of thiophenes and pyrimidines
on the energy levels is evident from the spectroscopic data that
are tabulated in Table 1. A bathochromic shift of 50 nm relative
to the starting reagent 1 occurred with the substitution of one
chlorine by thiophene to afford 4. Even though unsubstituted
pyrimidine is a more suitable reference, 1 was chosen simply
chlorination of 4,6-dihydropyrimidine in the presence of tri-
ethylamine. This volatile base was selected instead of N,N-
dimethylaniline that is known to mediate this type of coupling
merely because excess base can be easily removed making for
easy purification of the product. However, this base is most
likely responsible for the observed moderate product yields since
similar reactions occurred in higher yields with N,N-dimethy-
laniline.39 The complementary 2-thiophene boronic acid (2) and
2-bithiophene boronic acid (3) precursors required for Suzuki
coupling were obtained by standard aryl anion formation with
n-BuLi followed by nucleophilic addition to triethylboronate
ester starting from inexpensive thiophene and bithiophene,
respectively. Even though the reagents for Suzuki coupling were
easily synthesized, the hetero aryl-aryl coupling proved
problematic with standard coupling protocols in toluene and
aqueous Na2CO3 with Pd(PPh3)4. The desired heterocoupling
occurred only in aqueous isopropanol most likely because this

New Monomers for Hybrid Copolymers
J. Phys. Chem. B, Vol. 111, No. 39, 2007 11409
SCHEME 2: Synthetic Routes Used for the Synthesis of Various π-Donor-Acceptor Comonomers
TABLE 1: Spectroscopic Values of the Various Comonomers Measured in Anhydrous Dichloromethane
d
f
j
abs.a
(nm)
ꢀλ
Em.b
(nm)
∆Ec
(eV)
Eg
kre
knr
τh
(ns)
Phosi
(nm)
ET
max
g
compd
(M-1 cm-1
)
(eV)
(ns-1
)
(ns-1
)
Φfl
(kJ mol-1
)
k
1
4
5
6
7
8
9
253
301
372
329
396
270/333
396
3 900
26 100
27 400
36 300
54 100
48 000
55 500
269
364
450
378
479
448
474
4.7
3.7
3.0
3.5
2.9
3.4
2.9
4.5
3.5
2.9
3.4
2.7
3.3
2.7
3 × 10-4
443
507
702
518
710
512/704
712
270
236
170
230
168
234/170
168
0.13
0.37
0.31
0.31
0.06
0.25
1.49
0.03
1.24
0.09
0.40
0.20
0.08
0.63
2.47
0.64
2.45
2.30
2.17
0.92
0.20
0.77
0.13
0.55
f
a Absorption maximum. b Fluorescence maximum. c Absolute HOMO-LUMO spectral difference. d Spectroscopic energy gap. e kr ) Φfl/τfl. knr
) kr(1 - Φfl)/Φfl. g Fluorescence quantum yields measured at λmax, relative to naphthalene,43,44 bithiophene,45 anthracene,43,44 or fluorescein46 depending
on λabs max. h Monoexponential fluorescence lifetime measured at maximum λem i Phosphorescence maximum measured in 1:4 methanol/ethanol
.
matrix at 77 K. j Triplet energy. k Fluorescence too weak to measure lifetime.
because it was the starting reagent used for the synthesis of the
compounds studied while also allowing for easy identification
of any residual starting material with the sensitive spectroscopic
methods used. Additionally, the influences of the degree of
conjugation and the electron donating and withdrawing groups
on the physical properties are possible with 1. The same trend
in the absorption shift as with 4 was also observed for 7. Further
coupling of an additional thiophene results in a 71 nm
bathochromic shift upon going from 4 to 5 and a similar 67 nm
shift from 6 to 7. Similarly, the fluorescence spectra also show
a bathochromic shift that correlates with the increased number
of thiophenes resulting from the increased degree of conjugation.
The emission is shifted by 100 nm with the addition of each
thiophene unit relative to 4. This trend confirms an increasing
degree of conjugation resulting from narrowing of the HOMO-
LUMO energy levels, which in turn are responsible for the
observed bathochromic shifts in the spectroscopic data. The
absence of vibronic transitions in the observed fluorescence
spectra suggests (Figure 1) that there are no defined transitions
in the singlet excited and the ground states and is consistent
with other oligothiophenes.40-42 The absolute energy difference
between the ground (HOMO) and excited (LUMO) states can
be calculated from the intercept of the normalized absorption
and emission spectra. The spectroscopically measured values
show a narrowing of the HOMO-LUMO energy gap when
going from 1 to both 4 and 5 and the same trend is observed
Figure 1. Normalized absorbance (9) and fluorescence (0) spectra
of 4 recorded in dichloromethane measured at room temperature and
the phosphorescence (b) measured in a 1:4 methanol/ethanol matrix
at 77 K.
upon going from 1 to 6 and 7. This is further supported by
narrowing of the energy levels determined from the absorption
onset whose values parallel those measured from the intercept
method and decrease from 4.5 to 2.7 eV. Accurate energy gaps
can be obtained from the absorption onset because little

11410 J. Phys. Chem. B, Vol. 111, No. 39, 2007
Dufresne et al.
difference between the absorbances in solution and in thin films
was observed. It can be concluded that the excited state energy
is lowered with the addition of each thiophene to the comono-
mer.
Given the spectral shifts and the increased degree of conjuga-
tion found for 4-7, significant bathochromic shifts for both the
absorption and fluorescence were expected for 8 and 9.
Interestingly, only marginal shifts were observed and the
spectroscopic properties for both 8 and 9 are similar to those
for 6 and 7, respectively. This is ascribed to the limited
conjugation between the two pyrimidines and illustrates the
segmented nature of the compounds in which the conjugation
is isolated to the thiophene-pyrimidine segments only. The
conjugation degree is further limited because substitution on
the pyrimidine occurs via the meta position and not via the para
position, which is know to promote extended electronic delo-
calization. Unfortunately, para substitution of the pyrimidine
eliminates a vital metal coordination site and further limits the
compounds as eventual hybrid polymers. Nonetheless, the meta
substitution of 6-9 affords the means to assess the donor-
acceptor capacity of the compounds and the effect of the
thiophene/pyrimidine moieties on the spectroscopic and elec-
trochemical properties. The limited delocalization extension over
the pyrimidine-pyrimidine segment owing to the meta substitu-
tion is further supported by the molar absorption coefficients
of 8 and 9 that are both ∼50 000 M-1 cm-1 and they are similar
to 7. This implies the pyrimidine units do not adopt a planar
configuration and that they are most likely twisted from planarity
leading to two isolated segments similar to biphenyl. A slight
twist from planarity is also expected to occur between the
thiophene-pyrimidine units supported by the crystal data (vide
infra). However, the 50 nm bathochromic shift corresponding
to a 40 kJ/mol lowering of the excited state energy for 7 versus
12 suggests the twisting between the dissimilar heterocycles is
not significant and that the compound is rather conjugated,
which can only occur as a result of coplanarity of the
thiophene-pyrimidines. Despite the deviations from coplanarity,
coordination of a metal center with the pyrimidine units of 8
and 9 is expected to reinforce a planar geometry leading to
increased degree of conjugation. This is expected to result in
significantly different spectroscopic properties compared to its
metal-free analogue.
Figure 2. Transient absorption spectra of 4 recorded in deaerated and
anhydrous dichloromethane at 5.4 (9) and 9.8 µs (O) after the laser
pulse at 266 nm. Inset: Typical triplet decay kinetic of 4 monitored at
370 nm.
relative to 12 suggest these two comonomers undergo ISC to
preferentially populate their triplet state. This manifold is
assumed to be formed inefficiently by the other compounds
reported in Chart 1 according to the calculated kr and knr values.
Formation of the triplet manifold is supported by the phospho-
rescence measurements at 77 K, where a bathochromic emission
relative to the fluorescence emission is seen according to Figure
1. Even though the low-temperature measurements artificially
increase this triplet manifold, it nonetheless confirms triplet
formation. The triplet energy (ET) values can also be derived
from the 0,0 band, from which all the compounds were
determined to exhibit roughly the same ET around 170 kJ/mol,
and these are comparable to those of 12.50,51 Unfortunately,
extensive information regarding the triplet state including
quantification of this manifold is not possible from the low-
temperature measurements.
Laser Flash Photolysis. Quantification of the triplet state is
possible by laser flash photolysis (LFP). Direct excitation of
all the comonomers yielded a transient absorption spectrum
similar to that represented in Figure 2. All the comonomers
investigated produced a triplet that was visible at 375 nm for
the thiophene containing comonomers while the bithiophene
derivatives exhibited a triplet-triplet absorption at 475 nm,
recorded in Table 2. The observed transients were assigned to
the triplet owing to their monoexponential decays, all of which
fell within the standard microsecond range of triplet lifetimes,
ranging between 5 and 15 µs. Conversely, deactivation of a
radical cation reactive intermediate would follow second-order
kinetics involving bimolecular radical cation coupling. Ad-
ditional confirmation of the triplet nature of the observed
transients is derived from the quenched signal with standard
triplet deactivators including oxygen, 1,3-cyclohexadiene, and
methylnaphthalene.
Time-resolved and steady-state fluorescence techniques were
used to further investigate the excited state properties of the
compounds. The fluorescence quantum yields (Φfl) and singlet
excited state lifetimes (τ) were obtained by these respective
methods. In all cases, the Φfl is greater than 0.1 with the
exception of 1 and 4. This is not surprising since halides are
known to deactivate efficiently the singlet excited state by
intersystem crossing (ISC) and populate the triplet state by heavy
atom induced spin-orbit coupling (SOC). The net effect is a
reduced fluorescence.47-49 High fluorescence quantum yields
were, however, measured for the remaining compounds. The
intense emission is unexpected owing to the thiophene unit that
is known to efficiently promote triplet formation resulting in
suppressed fluorescence.49 This phenomenon is consistent with
oligo- and polythiophenes with the exception of thiopheno
azomethines that deactivate their excited state by nonradiative
internal conversion (IC).29 Insight into the deactivation modes
responsible for the different fluorescence quantum yields is
derived from the excited state dynamics. The measured fluo-
rescence lifetimes were all monoexponential and are consistent
with unimolecular deactivation modes. Moreover, the similar
radiative (kr) and nonradiative (knr) rate constants of 4 and 6
Quantification of the produced triplets (ΦT) and their corre-
sponding molar absorption coefficients (ꢀTT) is possible accord-
ing to eq 1. These values are of interest not only because they
are unknown, but also because they provide important informa-
tion required to understand the excited states of the compounds
while permitting an assessment of the suitability of these
compounds for emitting applications. Quantification of the triplet
state relative to an actinometer (xanthone) was possible;
however, either ΦT or ꢀTT of the compound of interest must be
known in order to extract any viable information from eq 1.
Observing the growth of a common transient for both xanthone
and the comonomer of the study simplifies eq 1 such that ꢀTT

New Monomers for Hybrid Copolymers
J. Phys. Chem. B, Vol. 111, No. 39, 2007 11411
TABLE 2: Triplet State Properties of Various Comonomers Measured in Anhydrous and Deaerated Dichloromethane Excited
at 355 nm
a
b
d
e
f
λTT
τTT
(µs)
ko
kq (MeNP)
ꢀTT
(M-1 cm-1
kISC
kIC
λphos
(nm)
c
molecule
(nm)
(104 s-1
)
(106 M-1 s-1
)
)
ΦT
(107 s-1
)
(107 s-1
)
4g
5
6
7
8
370
470
380
475
370
470
385
5.7
10.7
12.3
9.5
6.9
14.1
1.5
18
9.3
8.1
11
14
7.1
67
507
702
518
710
512/704
712
601
6.5
33
10
13
11
15 000
9 000
12 500
13 500
17 500
29 500
0.05
0.22
0.14
0.11
0.11
0.31
2.0
34
5.7
4.8
5.1
36
1.2
91
3.7
33
16
78
9
12
a Triplet-triplet absorption maximum. b Molar absorption coefficient determined relative to xanthone in dichloromethane (ꢀTT ) 28 000 M-1
cm-1). c Triplet quantum yield measured against xanthone (ΦT ) 1) and methylnaphthalene in dichloromethane. d Intersystem crossing rate constant
calculated according to kISC ) ΦT/τfl. e Internal conversion rate constant calculated according to kIC ) ΦIC/τfl ) (1 - Φfl - ΦT)/τfl. f Phosphorescence
maximum measured at 77 K in 1:4 methanol/ethanol glass matrix. g Excited at 266 nm. Decomposes with laser radiation.
Figure 4. The maximum triplet of methylnaphthalene produced by
quenching the triplet of 5 (9), 6 (b), 7 (0), 8 (2), 9 (O), 12 (1), and
Figure 3. Rate constants of triplet decay for 8 measured in deaerated
xanthone ([) via energy transfer measured as a function of laser power
dichloromethane as a function of varying amounts of methylnaphtha-
in dichloromethane. Inset: Typical triplet of methylnaphthalene
lene. Inset: Change in triplet decay lifetime of 8 monitored at 370 nm
monitored at 420 nm obtained by quenching a triplet donor via energy
with 0 (0), 0.08 (b), 0.17 (O), and 0.42 mM (9) 2-methylnaphthalene.
transfer.
becomes unity. This is possible by using a triplet quencher in
350 nm allowing for triplet yield determination by laser flash
sufficient quantity such that it quenches 95% of the produced
photolysis by exciting at 355 nm.
comonomer and xanthone triplets. Methylnaphthalene (MeNp)
is an ideal triplet quencher because of its low-lying triplet that
∆AbsunknownΦrefꢀref
Φunknown
)
(1)
quenches most triplets by energy transfer (ET), resulting in a
visible triplet at 420 nm. The low-lying triplet results normally
in rapid exothermic triplet quenching by ET, therefore only a
small amount of the quencher is required. The amount of
quencher required to deactivate 95% of the produced triplets
can be calculated empirically according to ko > 20kq, where ko
is the rate constant for triplet decay in the absence of quencher
and kq is the intermolecular triplet rate constant for quenching
with methylnaphthalene. The required kq values for the different
comonomers are determined from the slope of Figure 3, which
are obtained by observing the change in the triplet rate constant
in the presence of varying amounts of quencher, Q (inset Figure
3), according to kobs ) ko + kq[Q]. The calculated rate constants
reported in Table 2 are slower than diffusion controlled
processes (1010 M-1 s-1) implying the energy transfer quenching
process is endothermic. This implies the ET of the comonomers
∆Absrefꢀunkown
comonomer
9
hυ8 comonomer*
(2)
(3)
ISC
comonomer * 8 3comonomer
3comonomer + MeNp
9
ET8 comonomer + 3MeNp (4)
As shown in Figure 1, direct excitation of the comonomers
generates their singlet excited state (2) that undergoes ISC to
then form the triplet state (3). This manifold is then quenched
quantitatively by methylnaphthalene resulting in its triplet
formation (4) visible at 420 nm (inset Figure 4). Comparing
the maximum signal of triplet methylnaphthalene, produced
under similar conditions for both xanthone and the comonomers
as a function of laser power, provides the representative plots
shown in Figure 4. The triplet quantum yields are derived from
the ratio of the slopes of xanthone and the comonomers
according to (1) by the widely accepted method of relative
actinometry with methylnaphthalene.44,53 The ꢀTT for the various
comonomers were calculated from the ratio of the slopes
obtained by observing the amount of xanthone triplet (ΦT ) 1
and ꢀTT ) 28 000 M-1 cm-1) measured at 630 nm relative to
the triplet comonomers observed at their corresponding maxi-
is lower than that of methylnaphthalene (379 kJ mol-1 52
and
)
corroborates the spectroscopic values derived from the low-
temperature measurements. The required kq values could be
calculated for all the compounds except 4, which decomposed
under the photolysis conditions at 266 nm. The photoinstability
of 4 was confirmed by steady-state irradiation at 254 nm
resulting in dramatic changes in the ground state absorption (see
the Supporting Information). Conversely, the remaining com-
pounds from Chart 1 exhibited high photostability at 254 and

11412 J. Phys. Chem. B, Vol. 111, No. 39, 2007
Dufresne et al.
TABLE 3: Cyclic Voltammetrya Data of Various
Thiophene-Pyrimidine Comonomers Measured in
Anhydrous Dichloromethane
b
Epa
(V)
Epc
(V)
HOMO
(eV)
LUMO
(eV)
Eg
compd
(eV)
c
1
4
5
6
7
8
1.2
1.3
1.5
1.2
1.4
1.6
1.5
1.4
1.5
-
5.3
5.5
5.7
5.3
5.4
5.7
5.6
5.7
5.7
0.8d
2.0d
2.5
1.9d
2.3
2.4
2.5
2.5
2.3
4.5e
3.5e
3.2
3.4
3.1
3.3
3.1
3.3
3.7
c
-
-1.5
c
-
-1.7
-1.6
-1.5
-1.9
-2.3
9
10f
11g
a Values reported against SCE. b Electrochemical energy gap cal-
culated from the difference between the oxidation and reduction
potential onsets. c Undefined reduction peak. d Approximated from the
spectroscopically measured HOMO-LUMO energy gap (∆E) from
Table 1. e HOMO-LUMO energy gap measured spectroscopically.
f Literature value.22 g Literature value.59
Figure 5. Cyclic voltammogram of 7 (9) and its polymer (O) obtained
by anodic polymerization.
process was observed for all the compounds, corresponding to
oxidation of the thiophene unit to form a radical cation. Even
through the specific thiophene unit that undergoes oxidation
cannot be unambiguously assigned, the irreversible oxidation
process consistently occurred at ca. 1.2 V for all the compounds
reported in Table 3. Similarly, a one-electron reduction process
was observed for all the compounds and is ascribed to the radical
anion formation located on the pyrimidine moiety. The measured
oxidation potentials are higher than those for similar thiophene
compounds16,28 in part owing to the electron-deficient pyrimi-
dine. The nitrogen-containing heterocycle is responsible for
increasing the ionization potential of the compounds and further
confirms the strong donor-acceptor nature of these compounds
in which the electron-deficient pyrimidine exerts its influence
on the thiophene core. By comparison, the oxidation potential
of 8 is 250 mV higher than its bipyridine analogue 10,
illustrating the difficulty in oxidizing the comonomer as a result
of the additional nitrogen. Conversely, the reduction of 8 is much
easier as evidence by the 200 mV lower reduction potential
relative to 10.22 The same trend of more difficult reduction and
easier oxidation holds true for 6 and 7 relative to their pyridine
analogue 11. The difference in the redox potentials for these
compounds is a result of the additional nitrogen that perturbs
the HOMO and LUMO energy levels owing to its electron-
deficient character.
mum as a function of laser power. The triplet molar absorption
coefficients were determined from such plots by using the
calculated ΦT from Table 2 according to eq 1.54-56 The
calculated values in Table 2 show the ꢀTT values for all the
compounds are greater than 12 000 M-1 cm-1 confirming the
conjugated nature of the transients. The similar ꢀTT observed
for all the compounds corroborates the segmented degree of
conjugation due to the meta substitution for the compounds since
higher values would be observed for 8 and 9 if these were more
conjugated than the other comonomers.
The measured ΦT range from 0.1 to 0.2 with the exception
of 5. The lower value observed for 5 relative to the other
comonomers is consistent with its high fluorescence quantum
yield. This result suggests there are only two possible deactiva-
tion modes for these compounds: (i) fluorescence and (ii) ISC.
The low values for 6-9 are, however, surprising since thiophene-
containing compounds are known to have high triplet quantum
yields.45,49 To validate our LFP results, we measured the ΦT
for 12. Our measured value of 0.31 is in agreement with other
spectroscopic studies57,58 and differs from the previously
measured values by photoacoustic calorimetry.47 The discrep-
ancy between these values can be in part due to the photoa-
coustic method, whose accuracy relies heavily on the triplet
lifetime being longer than the detector’s response time. This
notwithstanding, the low yield of triplet formation for the
comonomers implies that their major pathway for deactivation
occurs predominately by fluorescence according to the following
energy conversation equation: Φfl + ΦISC + ΦIC ≈ 1, where
IC is the nonradiative deactivation by internal conversion. From
the combined fluorescence and ΦT data from Tables 1 and 2,
respectively, deactivation by IC occurs in only a small amount
according to the above energy conversion equation. The
exception is 8 where IC plays a major role in dissipating the
singlet excited state energy according to the above energy
conservation equation and is further supported by the large knr.
The observed long-lived singlet excited state and high fluores-
cence yields concomitant with the high singlet state energies
make the comonomers ideal ligands for metal complexes and
they are expected to exhibit light harvesting properties making
them suitable for solar cell applications. Furthermore, the pristine
blue emission displayed by the comonomers further makes them
suitable candidates for use in emitting devices.
Determination of the HOMO and LUMO energy levels is
possible from the oxidation and reduction potentials, respec-
ox
tively. The oxidation onset (E ) provides the ionization
onset
ox
potential (IP) according to IP ) E (SCE) + 4.4, where
onset
E
onset(SCE) is the oxidation potential onset in volts versus the
SCE electrode. Similarly, the LUMO energy level can be
calculated from the electron affinity (EA) according to the
red
onset
red
onset
reduction potential onset (E ) by EA ) E (SCE) + 4.4.
The energy gap (Eg) between the two levels is the difference
between the EA and IP. From the calculated values reported in
Table 3, it can be concluded that the HOMO and the LUMO
energy levels vary by 39 and 58 kJ/mol, respectively. The
oxidation potential onsets taken together with the spectroscopic
data provide an accurate representation of the influence of the
adjacent electron-rich and electron-poor aryl groups on the
energy levels and the electronic effect, schematically depicted
in Scheme 3. It is obvious from this scheme that the adjacent
aryl units affect the LUMO more than the HOMO energy level.
As expected, an adjacent pyrimidine decreases the LUMO level
more than an adjacent thiophene because of its electron-
Electrochemistry. The thiophene-pyrimidine’s capacity to
undergo oxidation is evident from the cyclic voltammetric
measurements represented in Figure 5. Only a one-electron

New Monomers for Hybrid Copolymers
J. Phys. Chem. B, Vol. 111, No. 39, 2007 11413
SCHEME 3: Schematic Representation of the HOMO-
LUMO Energy Gaps Calculated According To the
Oxidation and Reduction Onset Potentials, Respectively
Figure 6. HOMO (left) and LUMO (right) of 6 calculated by DFT-
B3LYP with the 6-31g* basis set using the crystal data as the optimized
geometry.
anodic potential that are otherwise gained from the increased
degree of conjugation occurring from extended delocalization.
Even though the deposited products on the ITO electrode exhibit
higher oxidation potentials relative to their corresponding
comonomers, their polymeric nature is supported by the
spectroscopic data. A bathochromic shift between 80 and 150
nm in the absorption spectra occurs for all polymers relative to
their corresponding comonomers. Similarly, fluorescence ba-
thochromic shifts corresponding to lowering of the excited state
energy by 26-84 kJ/mol occur, leading to HOMO-LUMO
energy gaps for the homocoupled products that are lower than
their corresponding comonomers. The net decrease in the energy
levels is a result in the increased degree of conjugation that
can only come from polymer formation. Both the absorption
and the fluorescence bathochromic shifts of the polymers relative
to their corresponding comonomer decrease less upon going
from 6 to 9 owing to the limited degree of conjugation from
the meta substitution. The same decreasing trend was also
observed for the absolute HOMO-LUMO energy difference
in the series 6 to 9. This trend further demonstrates that the
degree of conjugation is confined to the thiophene-thiophene-
pyrimidine portion of the molecule for 7 and 9 and it does not
extend across adjacent pyrimidines. Similarly, the conjugation
is confined to the thiophene-pyrimidine segment for 6 and 8.
The spectroscopic data and the bathochromic trend nonetheless
provide evidence that R-R homocoupled products are obtained
by anodic polymerization.
Anodic coupling of 4 and 5 were additionally examined to
further confirm that 6-9 afforded homocoupled products via
radical cation coupling of the terminal thiophenes. Since these
two compounds possess only one vacant terminal site that is
capable of sustaining homocoupling, anodic R-R coupling of
the resulting radical cations would result only in soluble dimers.
Conversely, undesired R-â coupling would lead to insoluble
and unconjugated polymers deposited as discolored films on
the ITO electrode. The absence of insoluble material deposited
on the ITO electrode after anodic coupling of 4 and 5 in addition
to no visible changes in the absorption and fluorescence spectra
of the transparent electrode further confirm that these com-
pounds do not undergo any decomposition reactions or any
unwanted R-â coupling defects. Undeniable proof of anodic
homocoupling of 6-9 affording higher ordered polymers is
derived from the large-scale anodic polymerization of 8. A large
surface area mesh electrode was used to obtain sufficient
material for conventional molecular weight characterization. A
molecular weight of 900 g/mol and a polydispersity index of
1.1 were measured by conventional GPC for only the soluble
fraction (oligo-8). Even though the calculated average molecular
weight corresponds to a product with 12 aromatic units, this
represents a lower limit for the molecular weight of the soluble
coupled products, while the molecular weight of the bulk
insoluble material deposited on the ITO electrode by similar
anodic polymerization is much higher.29,31,35,61 It is clear from
Table 4 that the absorption and the fluorescence spectra for the
soluble oligo-8 are bathochromically shifted by approximately
withdrawing capacity. Conversely, the HOMO level is influ-
enced by the electron-rich thiophene. The increased degree of
conjugation moreover decreases the energy difference between
the HOMO and LUMO energy levels. Confirmation of the
pyrimidine-deficient character and the electronic-rich nature of
thiophene are evidenced in Figure 6. This figure shows the
orbital densities calculated by DFT-B3LYP for 6, using the
6-31g* basis set from our crystallographic data. It is clear from
this figure that the LUMO density is concentrated predominately
on the nitrogen centers while the HOMO is localized on the
thiophene. This allows for intramolecular charge transfer
between the electron-poor and the electron-rich segments and
further contributes to the donor-acceptor charge transfer and
narrowing of the HOMO-LUMO energy levels. The charge
transfer further promotes the distribution of the electronic density
across the entire conjugated compound resulting in broad
absorption and emissions spectra.60 This is supported by the
absence of discrete vibronic transitions observed in the absorp-
tion and fluorescence spectra. Since the LUMO and the HOMO
levels are controlled by the pyrimidine and the thiophene units,
respectively, tailoring of the oxidation and the reduction
potentials is possible by adjusting the modular segments. This
affords the means to customize the electronic properties for a
given application.
Electropolymerization. The irreversible reduction of the
radical cation observed with scan rates ranging from 100 to 1000
mV/s confirms the reactive nature of this transient. Its reactivity
was exploited to promote anodic polymerization via homocou-
pling according to two methods: (i) by applying a constant
potential 100 mV more positive than the observed oxidation
potential and (ii) by repeated cycling of the potential between
0 and 1700 mV. Polymer deposition onto the platinum working
electrode was possible with the former method. Electrochemical
and spectroscopic characterization of the resulting polymer
deposited onto the electrode was, however, problematic. Anodic
polymerization of the comonomers was subsequently done on
a transparent ITO electrode to circumvent these short comings.
Anodic polymerization of 6-9 on ITO afforded the results
recorded in Table 4. A significant reduction in the oxidation
potential is normally associated with polymers obtained by
anodic polymerization as a result of the high degree of
conjugation occurring from R-R homocoupling. However, the
electron-withdrawing pyrimidine leads to acceptor terminated
segments with an A-D-D-A arrangement for P6 and P8 and a
A-D-D-D-D-A configuration for the polymers P7 and P9. The
strong electron acceptor group in both cases counter affects any
HOMO-LUMO energy reduction and any lowering of the

11414 J. Phys. Chem. B, Vol. 111, No. 39, 2007
Dufresne et al.
TABLE 4: Electrochemical and Spectroscopic Data of Polymers Deposited on an ITO Electrode by Anodic Polymerization
abs.b
(nm)
Em.c
(nm)
Epa
(V)
Epc
(V)
HOMO
(eV)
LUMO
(eV)
Eg(echem.)d
∆E(spec.)e
polymera
P6
P7
P8
(eV)
(eV)
475
533
479
484
378
513
503
565
506
508
427
606
1.4
1.9
2.1
2.0
1.5
1.3
-1.1
-0.5
-0.6
-0.8
-1.5
-1.3
5.7
6.2
6.4
6.3
5.8
5.6
3.2
3.8
3.6
3.5
2.8
3.0
2.5
2.4
2.8
2.8
3.0
2.6
2.5
2.2
2.5
2.5
3.1
2.3
P9
oligo-8f
Ru-oligo-8f
a Thin film measurements were done directly on the ITO electrode. b Absorption maximum. c Fluorescence maximum. d HOMO-LUMO energy
gap calculated from the oxidation and reduction onset differences. e Absolute HOMO-LUMO difference taken from the absorption and fluorescence
spectral intercept. f Measured in dichloromethane.
TABLE 5: Details of Crystal Structure Determination for 6
CHART 2: Possible Heteroleptic and Homoleptic
Ruthenium Complexes of Comonomer 8, Respectively
formula
C12H8N2S2
MW; F(000)
244.32; 1008
yellow plate
0.28 × 0.24 × 0.04
150(2); 1.450
monoclinic
P21/c
crystal color and form
crystal size (mm3)
T (K); dcalc. (g/cm3)
crystal system
space group
unit cell:
a (Å)
19.194(4)
b (Å)
c (Å)
11.297(2)
10.366(2)
R (deg)
90.000
95.35(3)
90.000
2237.8(8); 8
2.31-71.88; 0.982
26249/4310; 0.041
4.065
â (deg)
γ (deg)
V (Å3); Z
θ range (deg); completeness
reflections: collected / independent; Rint
µ (mm-1
abs corr
)
50 nm relative to 8. This results in a 0.3 eV narrowing of the
HOMO-LUMO energy gap that can result only from an
enhanced degree of conjugation arising from R-R homo-
coupling via the terminal thiophenes. Given the more pro-
nounced bathochromic shifts of 100 nm of P8 relative to oligo-8
concomitant with the lower HOMO-LUMO energy gap of 0.6
eV observed for P8, it can be confirmed that the insoluble film
deposited on the ITO possesses a higher degree of conjugation
and therefore is indeed a polymer with DP > 10. The
spectroscopic data provide additional evidence that P8 is a
conjugated polymer with substantially higher molecular weight
than oligo-8. It can thus be concluded that the anodic polym-
erization of 6-9 occurs by R-R type coupling via the
unsubstituted 5-thiophene position leading to conjugated poly-
mers.
Since pyrimidines can coordinate with metal ions, we
examined the possibility of forming a ruthenium complex with
8 to afford hybrid polymers. Such polymers can be achieved
by two possible routes: either polymerization followed by metal
complexation or direct polymerization of the metal-ligand
complex depicted in Scheme 1. We first examined the latter
method using ruthenium heteroleptic and homoleptic complexes
13 and 14 shown in Chart 2. These two compounds were
examined because their physical properties are well studied and
therefore would allow for easy investigation of anodic polym-
erization leading to hybrid alternating copolymers. Unfortu-
nately, anodic polymerization of both ruthenium complexes
failed to yield any detectable polymers despite countless efforts
with various reaction conditions and is consistent with other
reports.23,24,62 The absence of homocoupled products is most
likely a result of the steric hindrance of the terminal thiophenes
and the incorrect geometry of the metal-ligand complex
required for R-R coupling. Electropolymerization of the
comonomer followed by metal ion coordination was alterna-
tively pursued. Oligo-8 was examined for metal coordination
because its metal complex readily lends itself to subsequent
photophysical and electrochemical characterization owing to its
semiempirical
0.0390; 0.1088
0.0451; 0.1123
1.061
R1(F); wR(F2) [I > 2σ(I)]
R1(F); wR(F2) (all data)
GoF(F2)
max residual e- density
0.300 e-‚Å-3
solubility in most organic solvents, contrary to P8. Complexation
of RuCl3 with the pyrimidine oligo-8 ligand in dichloromethane
at room temperature led to significant spectroscopic and
electrochemical changes. Even though a weak metal-to-ligand
charge-transfer (MLCT) band was observed at 513 nm, char-
acteristic of Ru(II) complexes,63,64 strong evidence supporting
Ru-oligo-8 complexation is the intense emission at 606 nm, also
indicative of Ru(II) complex formation.65-67 The observed
emission is shifted bathochromically by 169 nm from that of
the free oligo-8 corresponding uniquely to the Ru-oligo-8
complex. The sufficiently different absorption and emission
changes associated with metal ion complexation compared to
the free ligand and Ru(III) confirm formation of the Ru-oligo-8
complex. The voltammogram of the Ru complex revealed that
both the oxidation and the reduction potentials are decreased
by 200 mV relative to oligo-8 while no redox processes
corresponding to the uncomplexed oligo-8 were observed. The
spectroscopic and electrochemical data prove unprecedented
metal ion complexation postpolymerization is possible. The
resulting physical properties of the hybrid polymers are com-
parable to those of highly conjugated metal-free polymers of
high molecular weight. Therefore, hybrid polymers that are
metal-containing conjugated materials provide the means to
tailor the spectroscopic and electrochemical properties of
polymers.
Crystallography. Suitable crystals for X-ray diffraction were
obtained by slow evaporation of 6 in dichloromethane. As shown
in Figure 7, the structure of 6 consists of two different aryl
units: two thiophenes and one pyrimidine. The molecule appears
to be almost perfectly symmetric, with two crystallographically
different molecules in the unit cell. The major difference

New Monomers for Hybrid Copolymers
J. Phys. Chem. B, Vol. 111, No. 39, 2007 11415
The measured distances for these interactions are within the
limit of the sum of van der Waals radii calculated to be 3.00
and 2.75 Å, respectively. The aryl-aryl and the donor-acceptor
interactions contribute to a close-packed crystal structure
arrangement.
Conclusion
A synthesis of novel conjugated thiophene-pyrimidine
comonomers was presented. The singlet excited state of these
novel π-donor-acceptor-donor and π-donor-acceptor-ac-
ceptor-donor conjugated comonomers preferentially deactivate
by radiative means leading to high fluorescence quantum yields,
confirmed by laser flash photolysis in which the triplet state
was formed in only 10%. Both the spectroscopic and the
electrochemical properties are dependent upon the number of
π-donors and acceptors in the comonomer. Anodic polymeri-
zation on ITO plates afforded conjugated polymers that exhibit
narrow HOMO-LUMO energy gaps and their absorption and
fluorescence spectra are bathochromically shifted relative to their
corresponding comonomers. The pendant pyrimidines from the
polymer backbone act as potential π-acceptor sites for metals
such as ruthenium, ultimately leading to conjugated polymers
with incorporated metals providing new properties with the
potential for functional devices such as field effect transistors
and photovoltaic devices.25
Figure 7. ORTEP representation of the crystal structure of 6 (top)
with the atom numbering scheme. Schematic representation of 6 seen
along the a-c plane (bottom).
Experimental Section
Materials and General Experimental Details. All reagents
were commercially available from Aldrich and were used as
received unless otherwise stated. Anhydrous and deaerated
solvents were obtained via a Glass Contour solvent purification
system. Isopropanol was dried over activated molecular sieves
and stored under nitrogen prior to use. 1H and 13C NMR spectra
were recorded on a Bruker 400 spectrometer with the appropriate
deuterated solvents.
Spectroscopic Measurements. Absorption measurements
were done on a Cary-500 spectrometer and fluorescence studies
were carried out on an Edinburgh Instruments FLS-920 fluo-
rimeter after deaerating the samples thoroughly with nitrogen
for 20 min. Fluorescence quantum yields were measured at 10-5
M by exciting the corresponding compounds near the maximum
absorption wavelength in spectroscopic grade dichloromethane
using relative actinometry by exciting the references at the same
wavelength. Naphthalene (φ275nm ) 0.21),44 bithiophene (φ303nm
) 0.013),45 anthracene (φ356nm ) 0.26),44 or fluorescein (φ494nm
) 0.92)46 were used as fluorescence references. The actinometer
absorbances, and those of the compounds, were matched at the
excitation wavelength to within 5%. Since the fluorescence
quantum yields of naphthalene and anthracene in dichlo-
romethane are unknown, these values were determined relative
to the reported values in cyclohexane (φcyclohexane ) 0.23) and
dichloromethane (φethanol ) 0.27), respectively.43,44 Phospho-
rescence measurements were done on a Cary Eclipse at 77 K,
using a 4:1 mixture of ethanol/methanol. Steady-state irradiations
were performed with a Luzchem photoreactor, using 8 lamps
emitting at 254 nm and irradiating for 12 h at room temperature.
Laser Flash Photolysis Measurements (LFP). Laser flash
photolysis experiments were done on a Luzchem mini-LFP
system excited at 355 nm with the third harmonic from a
continuum YAG:Nd Sure-lite laser with the exception of 4 that
was excited at 266 nm owing to an absence of absorption at
355 nm. The selected excitation wavelength was 355 nm for
every analysis and the solutions were prepared with absorbances
around 0.3 at this wavelength. The triplet quantum yields (ΦT)
Figure 8. Crystal packing of 6 showing eight molecules per unit cell
and the π-stacking occurring between alternating thiophene-pyrimidine
units.
between the two molecules is the 3° deviation in the torsion
angle between the planes of the aromatic units. Although the
aryl units are coplanar, there exists a slight twist between the
different aryl units as illustrated in Figure 7. The mean angles
between the planes of the two terminal thiophenes relative to
the central pyrimidine plane are 19.7(1)° and 6.1(1)° for one of
the molecules in the lattice. Mean angles of 15.9(1)° and 3.2-
(1)° were found for the corresponding mean planes for the other
molecule in the crystal lattice. The observed deviation from
planarity is required to prevent steric hindrance between H13-
H16 and H16-H19. The distance between these hydrogens is
approximately 2.35 Å compared to 2.40 Å for the sum of the
van der Waals radii. The aryl group twisting limits the degree
of conjugation as evidenced by the spectroscopic data. There-
fore, no increase in the degree of conjugation is possible with
4 aryl units such as for 8 or 9 owing also to the meta substitution
effect.
Two π-interactions take place between two molecules within
the lattice. π-Stacking occurs between the electron-poor pyri-
midine and the electron-rich thiophene. These interactions take
place between the same molecules within the lattice, represented
in Figure 8. The distance between the aryl group is 3.89(1) Å.
In addition to the aryl π-stacking, weak hydrogen-bonding
interactions between S‚‚‚H-C and N‚‚‚H-C were also found.

11416 J. Phys. Chem. B, Vol. 111, No. 39, 2007
Dufresne et al.
and molar absorption coefficients (ꢀTT) were measured by
relative actinometry according to known methods,34 using
xanthone in dichloromethane as the reference (ΦT ) 1 and ꢀTT
) 28 000). Since the required ΦT and ꢀTT values of xanthone
in dichloromethane are unknown, these were measured relative
Synthesis. 4,6-Dichloropyrimidine (1). Triethylamine (6.34
mL, 46 mmol) and phosphorus oxychloride (13.0 mL, 182
mmol) were mixed dropwise in a 50 mL round-bottomed flask.
Then 4,6-dihydroxypyrimidine (3.00 g, 26.8 mmol) was added
dropwise. The reaction was refluxed for 1 h and then poured
onto crushed ice. The precipitate was filtered and then purified
by flash chromatography eluted with hexanes/ethyl acetate (50/
50% v/v) to yield the title compound as a light yellow solid
to the reported values for xanthone in benzene (ꢀTT
)
5 300).54,56,68
Electrochemical Measurements. Cyclic voltammetric mea-
surements were performed on a Bio Analytical Systems EC
Epsilon potentiostat with scan rates varying from 100 to 1000
mV/s. The compounds of study were dissolved in anhydrous
and deaerated dichloromethane at 10-5 M with 0.1 M NBu4-
PF6. Either a glassy carbon electrode or a platinum electrode
was employed as the working electrode while a platinum wire
was used as the auxiliary electrode. A saturated Ag/AgCl
electrode was used as the reference electrode and the values
were corrected to SCE in dichloromethane by using ferrocene
as an internal reference (0.46 mV).69
1
(1.74 g, 44%). Mp 58-60 °C. H NMR (CDCl3) δ 8.90 (s,
1H), 7.85 (s, 1H). 13C NMR (CDCl3) δ 162.5, 159.2, 122.4.
HRMS(+) calcd for [C4H2N2Cl + H]+ 148.96733, found
148.96723.
Thiophen-2-yl-2-boronic Acid (2). Thiophene (5.31 g, 63
mmol) and 2.5 M n-BuLi in hexanes (26.5 mL, 66 mmol) were
mixed in THF (100 mL) at 0 °C under nitrogen. Trimethylborate
(21.5 mL, 189 mmol) was added dropwise to the solution at
-70 °C. The temperature was raised gradually and the mixture
was then kept at room temperature for 1 h. A 10% H2SO4 (50
mL) solution was added followed by dichloromethane (50 mL).
The organic layer was extracted and poured into 1 M NaOH
(50 mL). The basic aqueous layer was extracted and 1 M HCl
was added until pH 1-2 was obtained. This acidic layer was
extracted with dichloromethane (50 mL) to afford a white-gray
product (3.45 g, 43%). The compound was used without any
further purification. Mp 137-139 °C. 1H NMR (acetone-d6) δ
7.71 (d, 1H, J ) 4.4 Hz), 7.31 (s, 1H), 7.18 (t, 1H, J ) 4.1
Hz), 5.79 (s, 2H). 13C NMR (acetone-d6) δ 136.1, 131.7, 128.4,
128.3.
ITO plates were purchased from Delta Technologies and were
coated on one surface with RS ) 5-10 Ω. Rectangular shapes
were cut to serve as the working electrode and they were used
after cleaning according to standard protocols involving washing
with water, acetone, and dichloromethane for 20 min each
followed by ultrasonication. The cross-coupled products were
electrochemically deposited onto the ITO Corning 1737 plates
by 50 repeated anodic scans from 0 to 1.4 V at 100 mV/s.
Alternatively, the anodic polymerization was done by applying
an anodic potential 100 mV greater than the corresponding
radical cation and held at that potential for 10 min. A final
potential of -100 mV was applied for 2 min to ensure the
resulting products were in the neutral form.
5-(Thiophen-2-yl)thiophen-2-yl-2-boronic Acid (3). Bithiophene
(3.00 g, 18 mmol) and 2.5 M n-BuLi in hexanes (7.56 mL, 19
mmol) were mixed together in THF (50 mL) at 0 °C under
nitrogen. Trimethylborate (6.15 mL, 54 mmol) was added
dropwise to the solution while keeping the temperature at
-70 °C. The mixture was allowed to warm to room temperature
and then stirred for 1 h before 10% H2SO4 (50 mL) was added.
An extraction with dichloromethane was done before 1 M NaOH
(50 mL) was added. The basic aqueous layer was extracted
before 1 M HCl was added to give pH 1-2. A final extraction
of the acidic layer with dichloromethane (50 mL) gave a white-
greenish product (3.06 g, 81%). The compound was used
without further purification. Mp 159-161 °C. 1H NMR
(acetone-d6) δ 7.62 (d, 1H, J ) 3.6 Hz), 7.45 (dd, 1H, J ) 5.2
and 1.1 Hz), 7.40 (s, 2H), 7.33 (dd, 1H, J ) 3.7 and 1.2 Hz),
7.32 (d, 1H, J ) 3.6 Hz), 7.10 (dd, 1H, J ) 5.2 and 3.6 Hz).
13C NMR (acetone-d6) δ 143.1, 137.7, 137.1, 128.5, 125.4,
125.3, 125.2, 124.5. HRMS(+) calcd for [C8H7BO2S2 + H]+
211.0053, found 211.00489.
Sufficient material of the cross-coupled oligo-8 derived from
8 required for standard polymer characterization methods was
obtained according to the same experimental procedures de-
scribed above with the exception of the working electrode. The
standard platinum wire working electrode was replaced by a
large area platinum mesh gauze. Several milligrams of 8 were
dissolved in 75 mL of anhydrous dichloromethane and a
potential 100 mV greater than its radical cation (i.e., 1.7 V)
was applied for 20 min under a blanket of nitrogen followed
by the usual 2 min at -100 mV. The solvent was removed under
reduced pressure and the resulting oligo-8 was dissolved in a
mixture of AcOEt:Et2O (50:50) where the undesired NBu4PF6
electrolyte precipitated and was removed by filtration.
Molecular Weight Analysis of Oligo-8. The molecular
weight was determined by standard gel permeation chromatog-
raphy relative to polystyrene standards, using a Breeze system
from Waters equipped with a 717 plus autosampler, a 1525
Binary HPLC pump, and a 2410 refractive index detector. Three
Styragel columns HR3, HR4, and HR6 (7.8 × 300 mm2) in
series (from Waters) at 33 °C were used to resolve the sample
in THF with a flow rate of 1 mL/min. A calibration curve
required to interpolate the molecular weight of the sample was
created by using 10 polystyrene standards from Shodex. An
average number molecular weight (Mn) of 900 g/mol and a
polydispersity index of 1.1 were determined for oligo-8.
4-Chloro-6-(thiophen-2-yl)pyrimidine (4). 2 (0.564 g, 4.4
mmol) and 1 (0.653 g, 4.4 mmol) were dissolved in deoxygen-
ated isopropanol (20 mL) to which was added 2 M aqueous
Na2CO3 (5 mL). Tetrakis(triphenylphosphine)palladium
(0.261 g, 0.24 mmol) was added to the solution, which was
then refluxed for 16 h. The solution was extracted with
dichloromethane and the organic layer was then purified by flash
chromatography eluted with neat hexanes then the polarity was
increased with hexanes/ethyl acetate (90/10% v/v) to yield a
light yellowish solid (0.451 g, 52%). Mp 105-107 °C. 1H NMR
(CDCl3) δ 8.89 (s, 1H), 7.79 (dd, 1H, J ) 3.8 and 0.9 Hz),
7.59 (dd, 1H, J ) 5.0 and 0.9 Hz), 7.58 (s, 1H), 7.18 (dd, 1H,
J ) 5.0 and 3.8 Hz). 13C NMR (CDCl3) δ 162.0, 160.8, 159.3,
140.9, 131.8, 129.1, 128.9, 115.5. HRMS(+) calcd for [C8H5-
ClN2S + H]+ 196.99347, found 196.99418.
Crystal Structure Determination. Diffraction data for 6
were collected on a Bruker Smart 6000 diffractometer, using
graphite-monochromatized Cu KR radiation with 1.54178 Å.
The structure was solved by using direct methods (SHELXS97).
All non-hydrogen atoms were refined based on Fobs2
(SHELXS97), while the hydrogen atoms were refined on
calculated positions with fixed isotropic U, using riding model
techniques.
4-Chloro-6-(5-(thiophen-2-yl)thiophen-2-yl)pyrimidine (5). In
deoxygenated isopropanol (40 mL) along with 2 M aqueous

New Monomers for Hybrid Copolymers
J. Phys. Chem. B, Vol. 111, No. 39, 2007 11417
Na2CO3 (5 mL) were dissolved 3 (1 g, 4.8 mmol) and 1 (0.563
g, 3.8 mmol). Tetrakis(triphenylphosphine)palladium (0.300 g,
0.28 mmol) was added to the solution, which was then warmed
to 50 °C. This temperature was maintained for 15 h. The product
was extracted with dichloromethane and the organic layer was
purified by flash chromatography with hexanes/ethyl acetate (90/
10% v/v) to yield a yellow-orange solid (0.750 g, 71%). Mp
dust (0.120 g, 1.82 mmol) was added and the blue solution was
warmed to 60 °C until it turned red. Then 5 (0.400 g, 1.44 mmol)
was added to the mixture and the reaction was poured into 10%
NH4OH (50 mL) after an additional 4 h of stirring. The
precipitate was then washed with ethyl acetate and dichlo-
romethane. The product was isolated as an orange powder
1
(0.150 g, 43%). Mp 290 dec °C. H NMR (CDCl3) δ 9.27 (s,
1
2H), 8.73 (s, 2H), 7.93 (d, 2H, J ) 4.0 Hz), 7.37 (d, 2H, J )
3.5 Hz), 7.35 (d, 2H, J ) 5.1 Hz), 7.31 (d, 2H, J ) 3.9 Hz),
7.11 (dd, 2H, J ) 4.8 and 3.8 Hz). HRMS(+) calcd for
[C24H14N4S4 + H]+ 487.01744, found 487.01788.
153-155 °C. H NMR (CDCl3) δ 8.90 (s, 1H), 7.72 (d, 1H, J
) 4.0 Hz), 7.58 (s, 1H), 7.34 (d, 2H, J ) 4.3 Hz), 7.26 (d, 1H,
J ) 4.0 Hz), 7.09 (dd, 1H, J ) 4.2 Hz). 13C NMR (CDCl3) δ
161.9, 160.4, 159.3, 143.9, 138.8, 136.8, 129.8, 128.6, 126.5,
125.6, 125.3, 115.1. HRMS(+) calcd for [C12H7ClN2S2 + H]+
278.98110, found 278.98119.
Acknowledgment. The authors acknowledge financial sup-
port from the Natural Sciences and Engineering Research
Council Canada, Fonds de Recherche sur la Nature et les
Technologies, and the Canada Foundation for Innovation. Prof.
D. Zargarian is thanked for helpful discussions and S.D. extends
appreciation to the Universite´ de Montre´al for a graduate
scholarship. Johnson Matthey PLC is thanked for the gift of
RuCl3.
4,6-Di(thiophen-2-yl)pyrimidine (6). In deoxygenated ethanol
(20 mL) and 2 M Na2CO3 (5 mL) in distilled water were
dissolved 2 (0.346 g, 2.70 mmol) and 1 (0.200 g, 1.35 mmol).
Pd(PPh3)4 (0.300 g, 0.27 mmol) was added to the solution, which
was then refluxed for 6 h. The product was extracted with
dichloromethane and the organic layer was then purified by flash
chromatography eluted first with neat hexanes then the polarity
was increased with hexanes/ethyl acetate (90/10% v/v) to yield
the product as a yellow solid (0.283 g, 86%). Mp 143-145 °C.
1H NMR (CDCl3) δ 9.07 (d, 1H, J ) 1.3 Hz), 7.85 (dd, 2H, J
) 3.7 and 1.0 Hz), 7.81 (d, 1H, J ) 1.4 Hz), 7.56 (d, 2H, J )
5.0 and 1.1 Hz), 7.20 (dd, 2H, J ) 5.0 and 3.7 Hz). 13C NMR
(CDCl3) δ 159.6, 159.5, 142.6, 130.7, 128.9, 127.9, 109.2.
HRMS(+) calcd for [C12H8N2S2 + H]+ 245.02017, found
245.02078.
Supporting Information Available: 1H and 13C NMR
spectra of compounds 1-9 and the absorption, fluorescence,
and cyclic voltammograms of 1 and 4-9. This material is
available free of charge via the Internet at http://pubs.acs.org.
References and Notes
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