Synthesis, photophysics, and electrochemistry of thiophene-pyridine and thiophene-pyrimidine dyad comonomers

Article abstract of DOI:10.1139/V09-166

A series of new π-conjugated donor (D) and acceptor (A) dyad comonomers were prepared using Suzuki coupling protocols. The D-A comonomers consisting of thiophene/bithiophene as donors and pyridine/pyrimidine as acceptors were prepared to investigate their photophysical and electrochemical properties. The dyads were spectroscopically confirmed to be highly conjugated. This was further supported by the X-ray crystal structure of the bithophene-pyridine dyad that showed all the heterocycles to be coplanar. It was further found that the fluorescence yields (Φ_fl) of die dyads were highly dependent on the number of thiophenes. The bithiophene derivatives exhibited Φ_fl values ≥ 0.3 while die thiophene derivatives did not fluoresce. The suppressed fluorescence observed for me thiophene derivatives was due to tiieir higher triplet energy resulting in efficient intersystem crossing (ISC) to the triplet state with Φ_ISC ≥ 0.8. This was confirmed both by time-resolved and steady-state measurements. The singlet excited state of both thiophene and bithiophene dyads was deactivated solely by either fluorescence and (or) ISC. Owing to their donor and acceptor character, die dyads could be oxidized and reduced both electrochemically and photochemically to afford the radical cation and anion, respectively.

Full text of DOI:10.1139/V09-166

236
Synthesis, photophysics, and electrochemistry of
thiophene–pyridine and thiophene–pyrimidine
dyad comonomers
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Andreanne Bolduc, Stephane Dufresne, Garry S. Hanan, and W.G. Skene
Abstract: A series of new p-conjugated donor (D) and acceptor (A) dyad comonomers were prepared using Suzuki cou-
pling protocols. The D–A comonomers consisting of thiophene/bithiophene as donors and pyridine/pyrimidine as acceptors
were prepared to investigate their photophysical and electrochemical properties. The dyads were spectroscopically con-
firmed to be highly conjugated. This was further supported by the X-ray crystal structure of the bithophene–pyridine dyad
that showed all the heterocycles to be coplanar. It was further found that the fluorescence yields (Ffl) of the dyads were
highly dependent on the number of thiophenes. The bithiophene derivatives exhibited Ffl values ‡ 0.3 while the thiophene
derivatives did not fluoresce. The suppressed fluorescence observed for the thiophene derivatives was due to their higher
triplet energy resulting in efficient intersystem crossing (ISC) to the triplet state with FISC ‡ 0.8. This was confirmed both
by time-resolved and steady-state measurements. The singlet excited state of both thiophene and bithiophene dyads was
deactivated solely by either fluorescence and (or) ISC. Owing to their donor and acceptor character, the dyads could be
oxidized and reduced both electrochemically and photochemically to afford the radical cation and anion, respectively.
Key words: comonomer, thiophene, pyridine, pyrimidine, photophysics, fluorescence.
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Resume : A l’aide d’un couplage de Suzuki, une serie de comonomeres conjugues contenant une fonction donneur d’elec-
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trons (D) et une fonction accepteur d’electrons (A) ont ete prepares. Ces comonomeres, dits D–A, qui utilisent le thio-
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phene/bithiophene comme donneurs d’electrons et la pyridine/pyrimidine comme accepteurs d’electrons, ont ete
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synthetises dans le but d’analyser leurs proprietes photophysiques et electrochimiques. Il a ete evalue spectroscopiquement
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que les dimeres sont grandement conjugues. Ceci est d’ailleurs soutenu par la structure rayons-X du compose bithiophene–
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pyridine qui montre que tous les heterocycles sont coplanaires. Il a ete determine que les rendements quantiques de fluo-
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rescence (Ffl) des composes dependent du nombre de thiophenes presents dans la molecule. En effet, les derives du bithio-
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phene ont des Ffl ‡ 0,3 tandis que les derives du thiophene ne fluorescent pas. Cette absence de fluorescence est causee
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par la formation d’un etat triplet de haute energie par croisement intersysteme (ISC pour « intersystem crossing ») avec un
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FISC ‡ 0,8. L’etat singulet excite des dimeres de thiophene et de bithiophene est uniquement desactive par la fluorescence
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ou le ISC. Grace a leur caracteristique donneur/accepteur, les comonomeres ont pu etre oxydes et reduits electrochimique-
ment et chimiquement afin de former le radical cation et le radical anion.
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Mots-cles : comonomere, thiophene, pyridine, pyrimidine, photophysique, fluorescence.
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[Traduit par la Redaction]
atives often have undesirably high HOMO–LUMO energy
gaps (Eg) and oxidation potentials (Epa) that are unsuitable
for functional materials, as well as weak fluorescence.
Introduction
Conjugated materials have attracted much attention be-
cause of their electrochemical and photophysical properties
that are ideally suited for plastic electronics, including or-
ganic field effect transistor,1–4 photovoltaics,5,6 emitting de-
vices,7,8 and conducting materials,9,10 to name but a few.
Thiophenes have been widely used in these electronic appli-
cations owing to their low oxidation potential and their pos-
sibility of undergoing electropolymerization resulting in
conjugated polymers possessing extremely low oxidation po-
tentials. However, homopolymers of thiophene and its deriv-
The Eg and Epa can be modulated by copolymerizing thio-
phene with different electron-donating and -accepting mono-
mers. However, homopolymers are solely obtained if the
oxidation potentials of both the monomer and thiophene are
not closely matched. Although choosing monomers with Epa
that are closely matched to thiophene results in copolymers,
the Eg and Epa are similar to those of the homopolythio-
phene. This problem can be overcome with comonomers.
The comonomer configuration consisting of a central aro-
Received 26 August 2009. Accepted 5 October 2009. Published on the NRC Research Press Web site at canjchem.nrc.ca on 12 February
2010.
This article is part of a Special Issue dedicated to Professor M. A. Winnik.
1
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A. Bolduc, S. Dufresne, G.S. Hanan, and W.G. Skene. Centre for Self-Assembled Chemical Structures, Departement de Chimie,
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Universite de Montreal, CP 6128, Succ. Centre-ville, Montreal, QC H3C 3J7, Canada.
1Corresponding author (e-mail: w.skene@umontreal.ca).
Can. J. Chem. 88: 236–246 (2010)
doi:10.1139/V09-166
Published by NRC Research Press

Bolduc et al.
237
Scheme 1. Synthetic scheme for 1 and 3 (top) and 2 and 4 (bottom).
Chart 1. Comonomers synthesized.
matic acceptor (A) core inserted between electron-rich do-
nors (D) simplifies the electropolymerization process that
would otherwise require two or three separate monomers to
obtain analogous alternating copolymers via other coupling
methods.11,12 The same polymer can also be obtained by
cathodic polymerization of a D–A–D comonomer.
Comonomers consisting of electron-deficient heterocycles,
such as pyridine, pyrazine, and pyrimidines, are also inter-
esting because they can coordinate with metals. This can be
done either prior to- or post-polymerization, affording metal-
containing polymers. Property tailoring for a given applica-
tion is possible by coordinating different metals. n-Doping
of the electron-deficient-containing compounds is further
possible resulting in the both p- and n-type polymers.
Unlike the polymerization of D–A–D or A–D–A comono-
mers that can be done in one step, the polymerization of
D–A comonomers requires two steps. This can either be
done by anodic dimerization followed by cathodic polymer-
ization or the reverse. The added advantage of the stepwise
polymerization of D–A comonomers is that the anodically
and cathodically coupled dimers can be characterized, and
the metal complexes can be isolated and characterized as
well. Structure–property relationships are therefore possible
in addition to tailoring the properties for a given application.
This is in contrast to their triad cousins that are insoluble
when polymerized, precluding their spectroscopic and mo-
lecular-weight characterization.
Our ongoing investigation of conjugated polymers13–19
and our previous endeavors in comonomer synthesis11,20
prompted us to prepare a series of p-rich and p-poor dyad
comonomers consisting of thiophenes and both pyridine and
pyrimidine. These compounds are of particular interest be-
cause they are expected to exhibit mutual p- and n-doping
properties allowing for selective stepwise anodic and catho-
dic coupling potentially leading to D–A–A–D and
A–D–D–A comonomer products, respectively. These can
then subsequently be electropolymerized. Furthermore, the
p-donor/acceptor arrangement provides the means to tune
the HOMO–LUMO energy levels and to tailor the spectro-
scopic and electrochemical properties.21–24
dyad comonomers along with their steady-state and time-
resolved photophysics and electrochemical characterization.
The combined effect of thiophene and bithiophene p-donors
coupled with pyridine and pyrimidine p-acceptors for como-
nomers is presented. Although such compounds are of interest
for their electrochemical properties, little work has examined
the photophysics for such dyads. Their structure–property re-
lationships and the understanding of their fluorescence deac-
tivation mechanisms are of interest for the design and
property tuning of new compounds. For these reasons, the
photophysical properties, including the emission yields and
excited-state deactivation pathways, of these new comono-
mers are studied. The investigation of such properties pro-
vides valuable information for determining the suitability of
such precursors for functional materials and for the design of
future compounds.
Synthesis
The comonomers 2 and 4 (Chart 1) were prepared by
standard Suzuki coupling.25,26 The 2-thiophene boronic acid
and 2-bithiophene boronic acid precursors required for Su-
zuki coupling were obtained by standard aryl anion forma-
tion with n-BuLi followed by nucleophilic addition to
triethylboronate ester starting from inexpensive thiophene
and bithiophene, respectively.27 The boronic acids were
Herein, we present the preparation of new p-donor/acceptor
Published by NRC Research Press

238
Can. J. Chem. Vol. 88, 2010
Table 1. Details of crystal structure determination for 4.
Fig. 1. X-ray structure of 4 and the numbering scheme adopted.
Formula
C13H9NS2
Mw (g/mol); F(000)
Crystal color and form
Crystal size (mm)
T (K); dcalcd. (g/cm3)
Crystal System
243.33 g/mol; 504
Yellow plate
0.15 ꢀ 0.07 ꢀ 0.05
150 (2); 1.407
Orthorhombic
Pna21
19.5336 (19)
10.1891 (10)
5.7716 (6)
Space group
˚
a (A)
˚
b (A)
˚
c (A)
a (8)
b (8)
g (8)
90.000
90.000
90.000
curs between S2–C5–C6–C7–C8 and the pyridine through
C12 with the two being separated by 3.665 A. C1 is also di-
˚
rected towards the N1–C9–C10–C11–C12–C13 ring, and
3
˚
V (A ); Z
1148.7 (2); 4
4.53–72.47; 0.852
8286/1783; 0.093
3.930
Semi-empirical
0.0581; 0.1260
0.0725; 0.1321
1.065
˚
they are separated by a distance of 3.427 A (Fig. 3).
q range (8); completeness
Reflections: collected/independent; Rint
m (mm–1)
Spectroscopy
Abs. Corr.
The effect of the number of heterounits in the comonomer
on the spectroscopic properties can be seen from the spec-
troscopic data in Table 2. Both the absorption and fluores-
cence maxima of 3 and 4 are bathochromically shifted
relative to 1 and 2. The bathochromic shifts imply an in-
creased degree of conjugation resulting from the bithiophene
unit relative to thiophene. The increased degree of conjuga-
tion of the bithiophene dyad relative to the thiophene is also
evident from the Eg, which is lower for 3 and 4 than for 1
and 2. It is noteworthy that the Eg is larger for pyridine-
containing dyads compared with their pyrimidine analogues,
owing to the more electron deficient nature of diazines,
which perturbs the LUMO level.
Functional materials with high fluorescence yields are of
interest for emitting devices. High fluorescence yields are
also desired for efficient ligand-to-metal energy transfer.
For these reasons, we examined the fluorescence quantum
yields for the comonomer dyads. Both the bithiophene dyads
are highly fluorescent relative to their thiophene analogues.
High fluorescence yields were confirmed by measuring the
absolute emissions with an integrating sphere. The advant-
age of the integrating sphere is that references with similar
absorption, emissions, and emission yields to the compounds
of study are not required. Even though the fluorescence val-
ues measured for 1 and 2 are below the detection limit of
the integrating sphere, accurate fluorescence measurements
of 3 and 4 are possible. It can be concluded that 1 and 2
are nonfluorescent. The difference in fluorescence between
the two sets of dyads implies two different deactivation
pathways of the singlet excited state are present. Although
both the thiophene and bithiophene dyads exhibit unimolec-
ular fluorescence decays of approximately < 2 ns, an effi-
cient deactivation mode competes with fluorescence for 1
and 2, resulting in suppressed fluorescence.
R1(F); wR(F2) [I > 2s (I)]
R1(F); wR(F2) (all data)
GoF(F2)
Max. residual e– density
0.284 e A
–
–3
˚
coupled with a chloropyridine by Suzuki coupling in aque-
ous isopropanol, and this solvent led to homogenous reac-
tion mixtures resulting in higher yields.11 The reaction
proceeded cleanly at 50 8C with only 1 mol% of catalyst.
Conversely, 1 and 3 (Chart 1) were less straightforward to
prepare, since the required 4-halogenopyrimidine commer-
cially precursor is unavailable and the 2- and the 5-substi-
tuted derivatives are expensive. The required pyrimidine
precursors for the synthesis of 1 and 3 were therefore pre-
pared by a Michael reaction according to Scheme 1. This
route unfortunately required a slow ring-closing step requir-
ing 7 days for completion.28–30
Crystallography
Confirmation of the formation of 4 was obtained by
X-ray diffraction (Table 1). Suitable crystals for analysis
were obtained from the slow evaporation of 4 in dichloro-
methane. The resolved structure is shown in Fig. 1. The re-
solved structure shows disorder of the terminal thiophene
consisting of in inversion of the thiophene. The thiophene
(S1–C1–C2–C3–C4) weights are 76% for the structural
representation with the two thiophene adopting an anti con-
formation. The remaining 24% is attributed to the minor
isomer in which the two thiophenes are syn. It should be
noted that the heterocycles are nearly coplanar, contribu-
ting in part to its high degree of conjugation (vide infra).
The torsion angle between the pyridine and the central thi-
ophene is 5.388 while that of two thiophenes is 9.758, and
is consistent with similar structures.11
Deactivation of the excited state by intramolecular charge
transfer (ICT) can lead to suppressed fluorescence. ICT
often results from charge separation occurring on separate
donor–acceptor moieties. Charge separation can be visual-
ized from the molecular orbitals, which can be accurately
calculated by theoretical means. The HOMO and LUMOs
of the comonomers were therefore investigated to afford in-
sight into the different excited-state deactivation modes. As
The molecules of 4 in the crystal lattice align along the
a–b axis in a zig-zag fashion as shown in Fig. 2. This ar-
rangement is in part a result of non-conventional H-bonding
occurring between C11–H and N1. The distance between
C11 and N1 is 3.443 A, which is within a suitable distance
for such a weak interaction. Face-edge p-stacking also oc-
˚
Published by NRC Research Press

Bolduc et al.
239
Fig. 2. The three-dimensional network of 4 illustrating the zig-zag configuration along the a–b axis.
Fig. 3. The interactions involved in the lattice of 4. The disorder is omitted for clarity. (i) 0.5 + x, 0.5 + y, z; (ii) 1.5 – x, 0.5 + y, 0.5 + z;
(iii) 1.5 – x, –0.5 + y, 0.5 + z.
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240
Can. J. Chem. Vol. 88, 2010
Table 2. Comonomer spectroscopic properties measured in anhydrous and deaerated dichloromethane.
DGet (kJ/mol) N,N-
DGet (kJ/mol)
Dimethylanilineb
Benzoquinoneb
a
Compound
labs (nm)
306
320
365
354
DE (eV)
3.7
3.8
3.1
3.2
Eg (eV)
3.3
3.7
2.9
3.1
lem (nm)
380
356
433
420
Ffl
tfl (ns)
2.6
5.3
1.9
6.2
1
2
3
4
<0.01 (0)
0.04 (0.08)
0.66 (0.86)
0.15 (0.30)
–79
–69
–2
–223
–213
–175
–174
–11
aValues in parentheses were measured using a calibrated integrating sphere.
bCalculated energetics of photoinduced electron transfer (DGet) between the different comonomers and electron donors and acceptors.
Fig. 4. HOMO (left) and LUMO (right) energy levels calculated via DFT of 4 (A), 3 (B), and 2 (C).
seen in Fig. 4, the HOMO and LUMO orbitals calculated
from DFT single-point energies from the X-ray crystal coor-
dinates are not isolated on one heterocycles and they are
evenly distributed across the heterocycles regardless of the
dyad. ICT is therefore not responsible for the different fluo-
rescence yields despite the p-donor/acceptor character.
The singlet excited state deactivation pathways of the co-
monomers were further investigated using various quenchers.
In all cases, the comonomers were self-quenched with diffu-
sion-controlled kinetics in acetonitrile (2 ꢀ 1010 M–1 s–1).31
All spectroscopic measurements were subsequently done at
extremely low concentrations (10–6 mol/L) to avoid self-
quenching. An electron acceptor (benzoquinone) and donor
(N,N’-dimethylaniline) were added to determine whether the
comonomers could undergo photoinduced electron transfer.
Interestingly, all the comonomers studied were quenched at
diffusion-controlled limits by both benzoquinone and N,N’-
dimethylaniline (Fig. 5). This implies that the comonomers
are capable of generating both radical cations and radicals.
This is in contrast to thiophene derivatives that can generate
only radical cations while pyridine and pyridimine afford
uniquely radical anions. The photoinduced electron-transfer
mechanism leading to radical ion intermediates is further
supported by the exergonic DGet values calculated from the
Rehm–Weller equation, shown in Table 2.31 The dual
quenching confirms that the comonomers can potentially be
both oxidized and reduced while their increased degree of
conjugation relative to their discrete monomers afford the
means to tailor the spectroscopic and electrochemical prop-
erties.
Laser flash photolysis
The donor–acceptor character of the comonomers can
lead to radical ions upon photoirradiation, as observed by
the fluorescence quenching with the electron donor and ac-
Published by NRC Research Press

Bolduc et al.
241
Fig. 5. Static Stern–Volmer analysis of 3 as a function of benzo-
quinone. Inset: Decrease in fluorescence intensity of 3 with benzo-
quinone addition in deaerated acetonitrile.
Fig. 6. Kinetic decay of triplet 4 monitored at 490 nm after excita-
tion at 355 nm. Inset: Transient absorption spectra of 4 monitored
at 16 ms (&) and 27 ms (*) after the laser pulse of 355 nm.
ceptor. Laser flash photolysis (LFP) was used for spectro-
scopically detecting these intermediates in addition to any
triplets, either of which could be responsible for the reduced
fluorescence of 1 and 2. The transient absorption spectra for
the dyads studied are found in the inset of Fig. 6. Both the
thiophene and bithiophene comonomers exhibited transient
spectra whose absorption maxima are dependent on the
number of thiophenes. The unimolecular decay (Fig. 6) im-
plies that the observed transients are triplets. Triplet assign-
ment was further confirmed by the fast quenching of the
transients with known triplet quenchers; oxygen, 1,3-cyclo-
hexadiene, b-carotene, and 2-methylnaphthalene (Table 3).
The unimolecular decay precludes the formation of radical
ions that can be produced by intramolecular photoinduced
electron transfer. The observed transients were further as-
signed to triplets, since they were not quenched by radical
ion generators, benzoquinone, or N,N-dimethylaminopyri-
dine.
methylnaphthalene (MN) is added to both the reference and
comonomer samples. In this case, MN quenches the como-
nomer and xanthone triplet by energy transfer to produce
triplet MN that is observed at 420 nm. MN is the quencher
of choice because its triplet cannot be produced by direct
excitation. More importantly, its triplet can be detected by
LFP and its absorption does not overlap with any of the co-
monomers or with xanthone. Equation [2] is valid if all the
triplets produced within the laser pulse are rapidly quenched
by energy transfer to produce the triplet methylnaphthalene.
The amount of MN required for deactivating at least 95% of
the comonomer and xanthone triplets can be calculated by
the quenching rate constant derived from Fig. 7.
The FTT of the comonomers can be derived from the
slope of the maximum triplet signal as a function of laser
power relative to xanthone (FTT = 1; Fig. 8) according to
the simplified eq. [2] by observing the common triplet MN.
The comonomer 3TT can similarly be calculated from the
slope of the maximum signal observed at the comonomer
absorption maximum as a function of laser power versus
xanthone in the absence of MN. The calculated triplet values
found in Table 4 show that the 3TT are larger for the bithio-
phene dyads than for the thiophene comonomers. The larger
3TT implies the bithiophene triplets exhibit a higher degree
of conjugation than 1 and 2.
It is also evident from Table 3 that the comonomers pro-
duce appreciable amounts of triplet. From the energy con-
servation (1 = SFx = Ffl + FISC + FIC) and the
spectroscopic data, it can be concluded that the comonomers
deactivate their singlet excited state predominately by two
pathways: either fluorescence or intersystem crossing (ISC)
to form their triplet state. In the case of the thiophene como-
nomers, the singlet excited manifold is deactivated predomi-
nately by ISC, supported by the fluorescence and LFP data.
This is most likely a result of their higher triplet level favor-
ing ISC. The higher triplet energy of 1 and 2 is confirmed
by the phosphorescence emissions that are significantly hyp-
sochromically shifted relative to 3 and 4 (Table 4 and
Quantification of the triplet state is possible by LFP ac-
cording to eq. [1]
DAbsx ꢂ Fref ꢂ 3x
½1ꢁ
Fx ¼
DAbsref ꢂ 3ref
where FTT is the quantum yield of triplet formation, 3TT
is the triplet–triplet molar absorption coefficient, DAbs is
the signal intensity monitored at the maximum triplet ab-
sorption as a function of laser power, ref refers to the acti-
nometer, and x refers to the corresponding comonomer.
Either FTT or 3TT can be derived by direct comparison of
the comonomer triplet signal to the reference, xanthone.
Equation [1] can be simplified to eq. [2] for calculating the
triplet comonomer yield without knowing 3TT, which is not
normally precisely known.
DAbsx ꢂ Fref
½2ꢁ
Fx ¼
DAbsref ꢂ Fx
Equation [2] is valid if both the reference and the como-
nomer produce a common triplet. This is the case when 2-
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242
Can. J. Chem. Vol. 88, 2010
Table 3. Triplet-quenching rate constants for the different comonomers with various quenchers measured in anhydrous
and deaerated acetonitrile.
Quencher
1 (109 M–1 s–1)
2 (109 M–1 s–1)
3 (109 M–1 s–1)
4 (109 M–1 s–1)
1.3-Cyclohexadiene
b-Carotene
2-Methylnaphthalene
Self-quenching
4.5
4.8
4.5
3.8
0.2
5.5
0.004
8.4
0.4
2.2
0.003
19.9
0.3
4.3
0.001
16.4
Fig. 7. Variation of triplet rate constant of 1 with the addition of 2-
methylnaphthalene measured at 390 nm. Inset: Effect of triplet
transient of 1 monitored at 390 nm with 0 (black), 26 mmol/L
(red), and 53 mmol/L (green) 2-methylnaphthalene.
sion.32,33 The steady-state and time-resolved data collec-
tively confirm that the fluorescence can be tuned as a
function of comonomer structure and that the excited-state
energy can be channeled to the emitting manifolds. Tuning
of the spectroscopic properties as a function of structure is
therefore possible.
Electrochemistry
Cyclic voltammetry of each comonomer was done to de-
termine their redox potentials. All the comonomers exhibited
both oxidation and reduction potentials corresponding to the
radical cation and anion, respectively. The electrochemical
data also demonstrate the inherent dual n- and p-type char-
acter of the comonomers. This behaviour is corroborated by
the fluorescence quenching data. The HOMO and LUMO
values, and the corresponding energy gaps can be calculated
from the measured redox potentials by standard methods
leading to the values reported in Table 5.23 It is evident
from the calculated values that the p-donating thiophene
group affects the LUMO of the p-accepting nitrogen-con-
taining heterocycles. This is supported by the less-negative
reductive potentials (Epc) of 1–4 compared with pyridine
(Epc = –2.6 V) and pyrimidine (Epc = –2.4 V).34,35 Con-
versely, the oxidation potentials (Epa) of the comonomers
are unchanged compared to their thiophene (Epa = 1.6 V)36
and bithiophene (Epa = 1.3 V) monomers.37 This is a result
of the p-accepting character of the adjacent nitrogen hetero-
cycle, further confirming that the comonomers are conju-
gated.
Fig. 8. Maximum absorption of triplet 2-methylnaphthalene as a
function of laser power produced upon energy transfer from
*
xanthone (&), 1 (*), 2 ( ), 3 (&), and 4 (~) monitored at
420 nm in anhydrous and deaerated acetonitrile.
Both the reduction and oxidation processes of the como-
nomers are not reversible, as seen in Fig. 10. This is ex-
pected, since the radical ions are highly reactive and can
cross-couple to form their corresponding dimers.38 Electro-
chemical dimerization using a large-surface-area mesh elec-
trode was used to afford sufficient quantities of dimers for
characterization. Unfortunately, only products insoluble in
standard organic solvents were obtained. This is most likely
a result of multiple radical ion coupling on the same como-
nomer. This nonetheless confirms that the donor–acceptor
dyads are electrochemically active and that D–A–A–D and
A–D–D–A products are possible.
Conclusion
A series of new donor–acceptor dyad comonomers were
prepared. The spectroscopic and electrochemical properties
can be modulated as a function of structure. The comono-
mers can both be photochemically and electrochemically
oxidized and reduced resulting in radical ions. This dual ac-
tive behaviour is a result of incorporating both p-donors and
p-acceptors to form the comonomers. The singlet excited
state was found to deactivate exclusively by either fluores-
Fig. 9). The increased degree of conjugation resulting in
splitting of the singlet–triplet energy levels is consistent
with previous reports and further confirms that deactivation
does not occur from intramolecular photoinduced electron
transfer or other radiative means, such as internal conver-
Published by NRC Research Press

Bolduc et al.
243
Table 4. Triplet properties of the different comonomers measured in anhydrous and deaerated acetonitrile.
Compound
lTT (nm)
390
380
490
490
tTT (ms)
4.8
7.9
12.3
11.2
k0 (s–1) (10–5)
3TT (M–1 cm–1)
4000
FTT
0.79
0.83
0.30
0.86
lphos (nm)a
522
534
700
tphos (ms)a
1
2
3
4
2.1
1.3
0.8
0.9
190
71
6
8000
15 000
20 000
703
5
aMeasured in a 4:1 ethanol/methanol matrix at 77 K.
Fig. 9. Normalized absorbance (~) and fluorescence (*) of 1
measured in dichloromethane at room temperature and phosphores-
cence (&) measured at 77 K in 4:1 ethanol/methanol glass matrix.
dichloromethane (FTT = 1, 3TT = 28 000 mol/L–1 s–1) and
were determined using standard techniques.39–42
Crystal structure determination
Diffraction data for 4 was collected on a Bruker Smart
6000 diffractometer using graphite-monochromatized Cu Ka
˚
radiation with 1.54178 A. The structures were solved by di-
rect methods (SHELXS97). Disorder was found in the termi-
nal thiophene and consisted of a flip of this heterounit. The
weight of each isomer was optimized. All non-hydrogen
atoms were refined based on Fobs2 (SHELXS97), while hy-
drogen atoms were refined on calculated positions with
fixed isotropic U, using riding model techniques.
Electrochemical measurements
Cyclic voltammetry measurements were recorded with a
Bio Analytical Systems Ec Epsilon potentiostat. The com-
pounds of study were dissolved in anhydrous and deaerated
dichloromethane with 0.1 mol/L TBAPF6. A platinum elec-
trode was employed as the working electrode, while a plati-
num wire was used as the auxiliary electrode. The reference
electrode was a saturated Ag/AgCl electrode.
cence or intersystem crossing. The deactivation pathway can
be tailored by adjusting the triplet energy level, which in
turn, is controlled by the degree of conjugation and the num-
ber of thiophenes. These novel comonomers are ideal candi-
dates for emitting applications owing to their high
fluorescence yields.
Syntheses
Thiophen-2-ylboronic acid
Thiophene (998 mg, 11 mmol) and butyl lithium
2.5 mol/L (5 mL, 12 mmol) were added to anhydrous THF
(25 mL) at 0 8C under nitrogen. Trimethylborate (4.05 mL,
36 mmol) was added dropwise to the solution at –78 8C.
The temperature was raised to room temperature, and the
solution was stirred for 1 h before 10% H2SO4 (20 mL) was
added. The product was extracted with dichloromethane
(20 mL), and the organic layer was then washed with
3 mol/L NaOH (50 mL). The pH of the aqueous layer was
adjusted to 1 using 3.2 mol/L HCl, then it was washed with
dichloromethane (60 mL). The product was isolated as a
yellow-orange solid (840 mg, 55%) and was used without
Experimental
Analyses
Spectroscopic measurements
Absorbance measurements were recorded on a Cary-500
spectrometer, while fluorescence measurements were per-
formed with an Edinburgh Instruments FLS-920 fluorimeter
after deaerating the sample for 20 min with nitrogen. Fluo-
rescence quantum yields were measured at low sample con-
centration in dichloromethane by exciting the compounds
close to their maximum absorption wavelength and by com-
paring the emission to bithiophene (Ffl = 0.013) at the same
wavelength. Phosphorescence measurements were done on a
Cary Eclipse at 77 K in a 4:1 ethanol/methanol solvent.
1
additional purification. Mp 137–139 8C. H NMR (acetone-
3
d6): d = 7.71 (d, 1H, J = 4.4 Hz), 7.31 (s, 1H), 7.18 (t, 1H,
3J = 4.1 Hz), 5.79 (s, 2H). 13C NMR (acetone-d6): d = 136.1,
131.7, 128.4, 128.3.
1-(2,2’-Bithiophen-5-yl)ethanone (5)
To anhydrous THF (20 mL) was added 2,2’-bithiophene
(1.72 g, 10 mmol), and the resolution solution was cooled
to –78 8C. n-Butyl lithium (10 mol/L, 1 mL, 10 mmol) was
added under nitrogen, then the temperature was raised to
room temperature and the mixture was allowed to stir for
1 h. N,N-Dimethylacetamide (0.93 mL, 10 mmol) was
added, and the reaction was stirred for an additional 2 h at
room temperature. The reaction was quenched with water
Laser flash photolysis (LFP) measurements
Laser flash photolysis experiments were done on a Luz-
chem mini-LFP system excited at 355 nm with the third har-
monic of a Nd-YAG laser. The solutions were prepared with
absorbances between 0.3 and 0.5 at the excitation wave-
length. Triplet-state quantum yield and triplet–triplet molar
absorption coefficients were measured against xanthone in
Published by NRC Research Press

244
Can. J. Chem. Vol. 88, 2010
Table 5. Cyclic voltammetry data obtained in 1 mol/L TBAPF6 in anhydrous and deaerated dichloro-
methane.
Compound
Epa (V)
1.5
1.7
1.4
1.5
Epc (V)
–1.4
–1.6
–1.6
–1.6
HOMO (eV)
LUMO (eV)
Eg (eV)
2.9
3.3
2.7
2.7
1
2
3
4
5.9
6.1
5.8
5.9
3.0
2.8
3.1
3.2
Fig. 10. Cyclic voltammogram of 4 measured in anhydrous and
deaerated dichloromethane with TBAPF6 at 100 mV/s.
added. The mixture was then refluxed for 16 h. The solvent
was then evaporated, the solid washed with dichlorome-
thane, and then filtered. The filtrate was purified by flash
chromatography using hexanes/ethyl acetate (70:30, v/v) to
afford the product as a yellow powder (541 mg, 31%).
1
Mp 54–56 8C. H NMR (acetone-d6): d = 9.05 (s, 1H), 8.74
3
3
5
(d, 1H, J = 5.2 Hz), 7.99 (dd, 1H, J = 4 Hz, J = 1.2 Hz),
3
5
3
7.88 (dd, 1H, J = 5.2 Hz, J = 1.2 Hz), 7.75 (dd, 1H, J =
5
3
4.8 Hz, J = 0.8 Hz), 7.23 (dd, 1H, J = 3.6 Hz and 1.2 Hz).
13C NMR (acetone-d6): d = 160.8, 160.5, 159.4, 144.1,
132.5, 130.5, 130.0 117.1. HR-MS(+) calculated for
[C8H6N2S + H]+: 163.03245; found: 163.03303.
2-Thiophene-2-yl-pyridine (2)
To isopropanol (40 mL) was added a 2 mol/L Na2CO3
solution in water (5 mL). Thiophene-2-yl-2-boronic acid
(459 mg, 36 mmol) and 2-chloropyridine (408 mg,
36 mmol) were then added. Tetrakis (triphenylphosphine)
palladium (101 mg, 0.36 mmol) was added under nitrogen
under the cover of light. The temperature was raised to
50 8C, and the reaction was stirred at this temperature for
17 h. The product was extracted with dichloromethane and
then purified by flash chromatography using hexanes/ethyl
acetate (95:5, v/v) followed by 10% ethyl acetate. The prod-
uct was isolated as a yellow solid (268 mg, 46%). Mp 56–
(50 mL), and the organic layer was extracted with dichloro-
methane (60 mL). The product was purified by flash chro-
matography eluted with hexanes/ethyl acetate (90:10, v/v) to
1
give a yellow powder (451 mg, 21%). H NMR (acetone-
3
3
5
d6): d = 7.80 (d, 1H, J = 4), 7.56 (dd, 1H, J = 5.2, J =
1
3
58 8C. H NMR (acetone-d6): d = 8.52 (d, 1H, J = 4.8),
3
5
3
1.2), 7.46 (dd, 1H, J = 3.6, J = 0.8), 7.34 (d, 1H, J = 4),
7.14 (dd, 1H, J = 5.2 and 4), 2.52 (s, 3H).
3
5
3
7.85 (dd, 1H, J = 6.8, J = 0.8), 7.79 (td, 1H, J = 7.2,
3
5J = 1.6), 7.74 (dd, 1H, J = 4, J = 1.2), 7.54 (dd, 1H, J =
3
5
3
5.2, J = 1.2), 7.24 (dd, 1H, 3J = 4.8, J = 1.2), 7.14 (dd, 1H,
3J = 4 and 1.2). 13C NMR (acetone-d6): d = 154.4, 151.3,
147.0, 138.7, 129.9, 126.5, 123.9, 120.3. HR-MS(+) calcu-
lated for [C9H7NS + H]+: 162.03719; found: 162.03711.
5
5
(2,2’-Bithiophen-5-yl)-3-(dimethylamino)prop-2-en-1-one
(6)
To a 2:1 ethanol/dichloromethane solution (3 mL) was
added 5 (110 mg, 0.5 mmol), and the solution was then re-
fluxed for 1 h. N,N-Dimethylformamide dimethyl acetal
(225 mg, 1.9 mmol) was added, and the reaction was re-
fluxed for 17 h. The reaction was cooled to room tempera-
ture, and then hexanes were added and the solution was left
to cool in the freezer overnight. The precipitate was filtered
to afford the product as an orange solid (102 mg, 75%). The
compound was used without any further purification. 1H
4-(2,2’) Bithiophenyl-5-yl-pyrimidine (3)
In ethanol (25 mL) was added 1-(2,2’) bithophenyl-5-yl-3-
dimethylamino-propenone (178 mg, 0.67 mmol), and the
solution was refluxed for 1 h. Formamidine acetate was
then added (227 mg, 2 mmol), and the reaction was stirred
for 10 min followed by the addition of sodium (57 mg,
2.5 mmol) in ethanol (2 mL). The slurry was then refluxed
for 7 days. The solvent was evaporated, the product was
taken up in dichloromethane, and the remaining inorganic
salts were filtered. The product was purified by flash chro-
matography using hexanes/ethyl acetate (95:5, v/v) to afford
the product as a bright yellow solid (144 mg, 87%).
3
NMR (acetone-d6): d = 7.68 (d, 1H, J = 12.4), 7.65 (d, 1H,
3
5
3J = 3.6), 7.49 (dd, 1H, J = 5.2, J = 1.2), 7.39 (dd, 1H,
3J = 3.6, J = 0.8), 7.27 (d, 1H, J = 4), 7.13 (dd, 1H, J =
5.2 and 3.6), 5.79 (d, 1H, J = 12), 3.21 (s, 3H), 2.98 (s,
5
3
3
3
3H).
1
4-(Thiophen-2-yl)pyrimidine (1)
Mp 113–115 8C. H NMR (acetone-d6): d = 9.06 (d, 1H,
3
3
3J = 1.6), 8.74 (d, 1H, J = 5.2), 7.96 (d, 1H, J = 4), 7.90
To absolute ethanol (25 mL) was addded 6 (2.00 g,
11 mmol). The solution was brought to reflux after which
time formamidine acetate (3.45 g, 33 mmol) was added,
and the solution was refluxed for an additional 10 min. So-
dium (0.77 g, 33 mmol) in absolute ethanol (2 mL) was then
3
5
3
5
(dd, 1H, J= 5.6, J = 1.6), 7.53 (dd, 1H, J = 5.2, J = 1.2),
3
5
3
7.45 (dd, 1H, J = 3.6, J = 0.8), 7.39 (d, 1H, J = 4), 7.15
(dd, 1H, J = 3.6 and 1.6). 13C NMR (acetone-d6): d = 160.8,
3
160.1, 159.3, 143.7, 142.4, 131.0, 130.3, 128.0, 126.9,
Published by NRC Research Press

Bolduc et al.
245
126.8, 116.8. HR-MS(+) calculated for [C12H8N2S2 + H]+:
245.02017; found: 245.02088.
Met. 2006, 156 (7-8), 582. doi:10.1016/j.synthmet.2006.02.
005.
(2) Dell’Aquila, A.; Mastrorilli, P.; Nobile, C. F.; Romanazzi,
G.; Suranna, G. P.; Torsi, L.; Tanese, M. C.; Acierno, D.;
Amendola, E.; Morales, P. J. Mater. Chem. 2006, 16 (12),
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Adv. Mater. 2005, 17 (19), 2281. doi:10.1002/adma.
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2-(2,2’) Bithiophenyl-5-yl-pyridine (4)
To ethanol (25 mL) was added 2 mol/L Na2CO3 (5 mL)
followed by (2,2’) bithiophenyl boronic dimethyl ester
(1.5 g, 7.27 mmol) and 2-chloropyridine (1.65 g,
14.54 mmol) under nitrogen. Tetrakis (triphenylphosphine)
palladium (822 mg, 72.7 mmol) was then added under nitro-
gen and in the absence of light. The reaction mixture was
heated to 50 8C for 16 h. The product was extracted with
dichloromethane and was purified by flash chromatography
using hexanes and increasing the polarity with hexanes/ethyl
acetate (90:10, v/v). The product was isolated as a pale or-
1
ange solid (656 mg, 40%). Mp 124–126 8C. H NMR (ace-
3
3
tone-d6): d = 8.53 (d, 1H, J = 4.8), 7.83 (d, 1H, J = 8),
´
(6) Segura, J. L.; Martın, N.; Guldi, D. M. Chem. Soc. Rev.
3
5
3
7.81 (td, 1H, J = 7.6, J = 2), 7.68 (d, 1H, J = 3.6), 7.46
2005, 34 (1), 31. doi:10.1039/b402417f. PMID:15643488.
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11:1<15::AID-ADFM15>3.0.CO;2-A.
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2007, 111 (39), 11407. doi:10.1021/jp075259j. PMID:
17845027.
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Polyhedron 2007, 26 (17), 4929. doi:10.1016/j.poly.2007.06.
025.
3
5
3
5
(dd, 1H, J = 5.2, J = 1.2), 7.37 (dd, 1H, J = 3.6, J = 1.2),
3
3
5
7.29 (d, 1H, J = 3.6), 7.25 (dd, 1H, J = 4.8, J = 0.8), 7.11
(dd, 1H, J = 3.6 and 1.2). 13C NMR (acetone-d6): d = 154.0,
3
151.3, 145.8, 140.9, 139.1, 138.7, 130.1, 127.4, 127.0,
126.428, 126.0, 124.0, 120.2. HR-MS(+) calculated for
[C13H9NS2 + H]+: 244.02492; found: 244.02546.
Anodic dimerization
The comonomer 2 (20 mg, 0.12 mmol) was dissolved in
anhydrous dichloromethane (12 mL) under nitrogen.
FeCl3ꢂ6H20 (166 mg, 0.61 mmol) was added, and the mix-
ture was refluxed for 24 h. After refluxing, an insoluble pre-
cipitate formed. The soluble product was extracted with
dichloromethane, the solvent evaporated, and taken up into
THF for MS analysis.
`
(13) Bourgeaux, M.; Perez Guarın, S. A.; Skene, W. G. J. Mater.
Supplementary data
Chem. 2007, 17, 972. doi:10.1039/b615325a.
´
`
Supplementary data for this article are available on the
journal Web site (canjchem.nrc.ca) or may be purchased
from the Depository of Unpublished Data, Document Deliv-
ery, CISTI, National Research Council Canada, Ottawa, ON
K1A 0R6, Canada. DUD 5336. For more information on ob-
taining material, refer to cisti-icist.nrc-cnrc.gc.ca/cms/
unpub_e.shtml. CCDC 753448 contains the X-ray data in
CIF format for this manuscript. These data can be obtained,
free of charge, via www.ccdc.cam.ac.uk/conts/retrieving.
html (Or from the Cambridge Crystallographic Data Centre,
12 Union Road, Cambridge CB2 1EZ, UK; fax +44 1223
336033; or deposit@ccdc.cam.ac.uk).
(14) Perez Guarın, S. A.; Tsang, D.; Skene, W. G. N. J. Chem.
2007, 31 (2), 210. doi:10.1039/b611060f.
(15) Guarın, S. A.; Bourgeaux, M.; Dufresne, S.; Skene, W. G. J.
Org. Chem. 2007, 72 (7), 2631. doi:10.1021/jo070100o.
PMID:17343421.
(16) Dufresne, S.; Bourgeaux, M.; Skene, W. G. J. Mater. Chem.
2007, 17 (12), 1166. doi:10.1039/b616379c.
(17) Bourgeaux, M.; Skene, W. G. Macromolecules 2007, 40 (6),
1792. doi:10.1021/ma070292p.
`
´
`
(18) Perez Guarın, S. A.; Dufresne, S.; Tsang, D.; Sylla, A.;
Skene, W. G. J. Mater. Chem. 2007, 17, 2801. doi:10.1039/
b618098a.
´
`
(19) Perez Guarın, S. A.; Skene, W. G. Mater. Lett. 2007, 61
(29), 5102. doi:10.1016/j.matlet.2007.04.015.
Acknowledgments
(20) Hansford, K. A.; Perez Guarin, S. A.; Skene, W. G.; Lubell,
W. D. J. Org. Chem. 2005, 70 (20), 7996. doi:10.1021/
jo0510888. PMID:16277320.
(21) Guldi, D. M. R.; Rahman, G. M. A.; Zerbetto, F.; Prato, M.
Acc. Chem. Res. 2005, 38 (11), 871. doi:10.1021/ar040238i.
PMID:16285709.
(22) Galand, E. M.; Kim, Y.-G.; Mwaura, J. K.; Jones, A. G.;
McCarley, T. D.; Shrotriya, V.; Yang, Y.; Reynolds, J. R.
Macromolecules 2006, 39 (26), 9132. doi:10.1021/
ma061935o.
The authors acknowledge financial support from the Nat-
ural Sciences and Engineering Research Council of Canada
(NSERC), le Fonds de Recherche sur la Nature et les Tech-
nologies, Centre for Self-Assembled Chemical Structures,
and additional equipment funding from the Canada Founda-
tion for Innovation (CFI). Both S.D. and A.B. thank NSERC
Canada for graduate and undergraduate scholarships, respec-
tively.
(23) Zhu, Y.; Champion, R. D.; Jenekhe, S. A. Macromolecules
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