Atomistic band gap engineering in donor-acceptor polymers

Article abstract of DOI:10.1021/ja208917m

We have synthesized a series of cyclopentadithiophene- benzochalcogenodiazole donor-acceptor (D-A) copolymers, wherein a single atom in the benzochalcogenodiazole unit is varied from sulfur to selenium to tellurium, which allows us to explicitly study sulfur to selenium to tellurium substitution in D-A copolymers for the first time. The synthesis of S- and Se-containing polymers is straightforward; however, Te-containing polymers must be prepared by postpolymerization single atom substitution. All of the polymers have the representative dual-band optical absorption profile, consisting of both a low- and high-energy optical transition. Optical spectroscopy reveals that heavy atom substitution leads to a red-shift in the low-energy transition, while the high-energy band remains relatively constant in energy. The red-shift in the low-energy transition leads to optical band gap values of 1.59, 1.46, and 1.06 eV for the S-, Se-, and Te-containing polymers, respectively. Additionally, the strength of the low-energy band decreases, while the high-energy band remains constant. These trends cannot be explained by the present D and A theory where optical properties are governed exclusively by the strength of D and A units. A series of optical spectroscopy experiments, solvatochromism studies, density functional theory (DFT) calculations, and time-dependent DFT calculations are used to understand these trends. The red-shift in low-energy absorption is likely due to both a decrease in ionization potential and an increase in bond length and decrease in acceptor aromaticity. The loss of intensity of the low-energy band is likely the result of a loss of electronegativity and the acceptor unit's ability to separate charge. Overall, in addition to the established theory that difference in electron density of the D and A units controls the band gap, single atom substitution at key positions can be used to control the band gap of D-A copolymers.

Full text of DOI:10.1021/ja208917m

Article
pubs.acs.org/JACS
Atomistic Band Gap Engineering in Donor−Acceptor Polymers
Gregory L. Gibson, Theresa M. McCormick, and Dwight S. Seferos*
Lash Miller Chemical Laboratories, Department of Chemistry, University of Toronto, 80 St. George Street, Toronto,
Ontario M5S 3H6, Canada
S
* Supporting Information
ABSTRACT: We have synthesized a series of cyclopentadi-
thiophene−benzochalcogenodiazole donor−acceptor (D−A)
copolymers, wherein a single atom in the benzochalcogeno-
diazole unit is varied from sulfur to selenium to tellurium, which
allows us to explicitly study sulfur to selenium to tellurium
substitution in D−A copolymers for the first time. The
synthesis of S- and Se-containing polymers is straightforward;
however, Te-containing polymers must be prepared by
postpolymerization single atom substitution. All of the
polymers have the representative dual-band optical absorption
profile, consisting of both a low- and high-energy optical transition. Optical spectroscopy reveals that heavy atom substitution
leads to a red-shift in the low-energy transition, while the high-energy band remains relatively constant in energy. The red-shift in
the low-energy transition leads to optical band gap values of 1.59, 1.46, and 1.06 eV for the S-, Se-, and Te-containing polymers,
respectively. Additionally, the strength of the low-energy band decreases, while the high-energy band remains constant. These
trends cannot be explained by the present D and A theory where optical properties are governed exclusively by the strength of D
and A units. A series of optical spectroscopy experiments, solvatochromism studies, density functional theory (DFT) calculations,
and time-dependent DFT calculations are used to understand these trends. The red-shift in low-energy absorption is likely due to
both a decrease in ionization potential and an increase in bond length and decrease in acceptor aromaticity. The loss of intensity
of the low-energy band is likely the result of a loss of electronegativity and the acceptor unit’s ability to separate charge. Overall,
in addition to the established theory that difference in electron density of the D and A units controls the band gap, single atom
substitution at key positions can be used to control the band gap of D−A copolymers.
absorption spectrum. This dual-band is responsible for broad
absorption characteristics. The origin of this dual-band,
however, is not fully understood.18 Two separate transitions,
that arise as the result of either (1) charge transfer between the
donor and acceptor units along with a π−π* transition that is
centered on the donor unit,15,29 or (2) a reorganization of
molecular orbitals to produce easily accessible low- and high-
lying energy levels spread across both donor and acceptor
units,30−32 are considered to give rise to the dual-band absorption
spectrum.
One of the challenges with determining structure−property
relationships in D−A polymers, and hence the origin of the
dual-band absorption, is that functional groups are used to
change and control electronic properties. A far more direct
means to understand structure−property relationships in D−A
polymers would be to carry out a study whereby a single atom
is systematically varied in either the donor or acceptor moiety
and properties are determined. To the best of our knowledge,
however, no such study has been reported in the literature.
Herein, we present a study in which the heteroatom of the
acceptor moiety of the fused-thiophene-containing D−A
polymer, poly(cyclopentadithiophene)benzothiadiazole
INTRODUCTION
■
In 1992 Wynberg and co-workers introduced the concept of
the donor−acceptor (D−A) approach to conjugated polymer
design.1,2 In the intervening years, this has become a widely
used method for preparing conjugated polymers with narrow
band gaps.3−18 The approach involves synthesizing a polymer
with a delocalized π-electron system that comprises alternating
electron-rich (donor) and electron-deficient (acceptor) repeat
units. The combination of high-lying HOMO levels (residing
on the donor units) and low-lying LUMO levels (residing on
the acceptor units) results in an overall narrow band gap for
the polymer. The band gap can be narrowed or widened on
the basis of the choice of donor and acceptor units, or more
specifically, the difference in electron density between the
donor and acceptor units along the polymer backbone. The
tunable nature of the D−A polymer approach is highly
desirable and has resulted in extensive exploration of these
materials with respect to light-harvesting and light-emitting
applications.19−22 Indeed, this approach has been used to create
a variety of chromophores that span the color spectrum.23,24
In the vein of tuning and controlling the band gap of D−A
polymers, researchers have focused on exchanging donor and
acceptor units for stronger or weaker ones,6,15,25,26 or altering
the donor to acceptor ratio within the polymer structure.23,27,28
One of the hallmarks of D−A polymers is a dual-band optical
Received: September 21, 2011
Published: November 30, 2011
© 2011 American Chemical Society
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1
(PCPDTBT), is systematically varied from sulfur to selenium
to tellurium. Using optical measurements as well as density
functional theory (DFT) calculations, we illustrate several
surprising trends in D−A behavior upon single atom
substitution in the acceptor unit, which cannot be explained
simply by the strength of D and A units. These trends are then
used to explain the dependence of optical properties on the
identity of the heteroatom in the acceptor structure. More
broadly, we are able to use these model systems to draw specific
conclusions with regard to the origin of the dual-band optical
absorption in general classes of D−A polymers.
determined from H NMR spectroscopy, involve treating the
iodinated precursor 3 with 5 equivalents of copper(I)thiophene
carboxylate in N,N′-dimethylformamide at 70 °C for 48 h. Puri-
fication by column chromatography using silica as the stationary
phase furnished the fused-thiophene, 4, in 60% yield.
With the monomer core in hand, we next carried out a
method to install alkyl chains at the bridge position of the
fused-thiophene system, which are essential for the solubility of
the final polymeric material. Based upon our previous
experiences, solubility is reduced for heavy chalcogen
analogs of conjugated polymers.36−38 As such, we chose 2-
(ethyl)hexyl side chains because these branched structures
prevent aggregation and improve solubility in common organic
solvents.12 As well, branched side chains disrupt the crystallinity
of the final material,39 which may allow us to investigate the
effects of single atom substitution on supramolecular inter-
actions. Accordingly, the bridge carbon in 4 was deprotonated
by treatment with potassium hydroxide, followed by treatment
with 1-bromo-2-ethylhexane in dimethyl sulfoxide to afford 5 in
58% yield. To afford bis-boronic ester 6 that is suitable for
Suzuki-coupling-polymerization, 5 was initially brominated at
the 5 and 5′ positions prior to treatment with n-butyllithium
and 2-isopropoxy-4,4,5,5-tetramethyl-1,3,2-dioxaborolane. The
Suzuki reagent 6 was found to be unstable when exposed to
silica gel, however, making purification of the final monomer
difficult. To circumvent these problems, 5 was purified on silica
gel and reacted directly with n-butyllithium followed by
addition of 2-isopropoxy-4,4,5,5-tetramethyl-1,3,2-dioxaboro-
lane, to produce 6, which was sufficiently pure after workup
to be used in subsequent polymerization reactions.
RESULTS
■
Synthesis. In order to study the effect of single atom
substitution on the PCPDTBT system, we carried out a
synthesis to prepare three chalcogen analogs of this polymer.
PCPDTBT is an important material, however, it can be very
challenging to prepare. The methodology for the synthesis of
PCPDTBT begins with the synthesis of the fused-thiophene
donor monomer (Scheme 1). Several syntheses of this core
Scheme 1. Synthesis of fused-thiophene monomer building
block
The acceptor molecules containing S, Se, and Te were
synthesized by following literature procedures.40,41 Suzuki
cross-coupling condensation polymerization was used to
copolymerize the S and the Se acceptors, individually, with
the fused-thiophene donor. This lead to 56% and 51% yields of
S- and Se-containing polymers (P1 and P2) respectively, after
extraction to remove catalyst and low molecular weight
polymers (Scheme 2). Copolymerization with the Te acceptor
(structurally analogous to S and Se acceptors in Scheme 2)
proved more difficult than the lighter analogs, even under mild
Pd-catalyzed conditions.38 As such, we conducted variable
temperature absorption experiments to determine the stability
of this monomer under typical polymerization conditions
(Figure S1, Supporting Information). These studies revealed
that this acceptor decomposes above 50 °C. Having determined
that heat could not be used during condensation polymer-
ization of this comonomer, several room temperature
conditions were attempted for copolymerization; however,
none of these conditions were successful at producing
sufficiently high molecular weight polymer and thus we chose
a unique approach to synthesize this macromolecule.
molecule have been published.33,34 Most routes involve
alkylation of the bridge carbon followed by functionalization
at the distal ends with appropriate groups for condensation
polymerization. In this work, we report a modified approach
that reduces the number of steps and the time required for
purification.
The first step in the synthesis of the fused-thiophene donor
monomer involves treating 3-bromothiophene with n-butyl-
lithium to form the 3-thienolithium species, and subsequent
treatment with thiophene-3-carboxaldehyde to join two thio-
phene rings by a bridge carbon at their respective 3-positions. The
aldehyde is reduced to an alcohol in the process (1). Further
reduction at the bridge carbon with LiAlH4 and AlCl3 affords 2 in
75% yield. The next step, forming the bond that fuses the
thiophene rings and forms the planar core, presents the greatest
synthetic challenge. This sequence has previously been carried out
by dibromination followed by a copper-catalyzed Ullmann
coupling reaction.34 In our case, we chose a unique copper
reagent, copper(I)thiophene carboxylate,35 which allows conven-
tional heating and eliminates the need for activated copper, which
can be difficult to use. Because copper(I)thiophene carboxylate
works best with iodinated aromatic substrates, N-iodosuccinamide
was used to first install iodine at the 2 and 2′ positions of 2,
affording 3, which is the precursor to the Ullmann coupling step.
The optimized conditions for the Ullmann coupling step, as
Specifically, we hypothesized that P3 could be obtained more
readily by a postpolymerization modification of analogs P1 or
P2. This methodology involves reducing either P1 or P2 to
afford diamine P4, followed by reoxidation to form the Te-
containing polymer. Experimentally, we determined that the Se-
containing polymer P2 is quantitatively reduced with lithium
aluminum hydride. Following that, reoxidation is accomplished
by treatment with TeCl4. The postpolymerization sequence was
verified by NMR spectroscopy (Figure 1) and Se-elemental
1
analysis (Table S1, Supporting Information). The H NMR
spectra of the aromatic region show clear shifting of the
polymer proton resonances upon reduction of the Se-polymer
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a
Scheme 2. Synthesis of D−A polymers P1−P3, Diamine Intermediate P4, and Re-oxidation of P4 back to P2
a
(*) Aliquat 336, K2CO3, Pd(PPh3)4, toluene, 80 °C, 72 h.
the effect of heavy atom substitution (Figure 2). Here, we
observe that the absorption maximum (λmax) of these acceptors
Figure 1. Aromatic region of the 1H NMR spectra of polymers P1−P4
in CDCl3 at 400 MHz.
Figure 2. Absorption spectra of S, Se, Te acceptor molecules in
DMSO.
to the diamine intermediate and subsequent reoxidation to the
Te-polymer. Inductively coupled plasma atomic emission
spectroscopy (ICP-AES) reveals that 97% of the Se content
is removed upon conversion of P2 to P3 (Table S1, Supporting
Information). Additional evidence for this transformation is
available from absorption spectroscopy and is described in the
next section. Finally, in control experiments, reoxidation of P4
with selenium oxide produced a polymer that was spectroscopi-
cally identical to P2, and lends further support to the success of
the postpolymerization methodology (Figure S2 and S3,
Supporting Information). Absorption spectra of two additional
P2 samples, low (Mn = 2300 g/mol) and high (Mn = 6000 g/mol)
molecular weight, were compared and found to be nearly identical
(Figure S5, Supporting Information). Since P3 is prepared from
P2 we conclude that the optical properties of the polymers are in
the chain-length independent regime.
shifts to longer wavelengths (lower energy) as one moves from
S (315 nm) to Se (340 nm) to Te (404 nm). Not surprisingly,
these spectra resemble those of other π-conjugated small
molecules and do not possess the characteristic dual-band,
which is indicative of a D−A system.18
The shift in optical absorption on moving from S to Se to Te
is observed in the polymer absorption spectra (Figure 3). This
trend is further illustrated by the energy of the optical band gap
(measured from the absorption onset) of P1 (1.59 eV) when
compared to P2 (1.46 eV) when compared to P3 (1.06 eV).
The polymer spectra are more complicated than the small
molecules and, specifically, the characteristic D−A dual-band is
very apparent in polymers P1−P3. There are three general
aspects of these spectra that are worth pointing out. First, the
entire dual-band spectrum is shifted to a lower energy as
heavier chalcogens are substituted into the acceptor molecule.
Second, the low-energy band shifts by a greater extent than the
high-energy band. Third, the absorption coefficient (based on
the repeat unit molecular weight) of the low-energy band
Optical Properties. In initial attempts to produce the Te
polymer P3, the Te acceptor 4,7-dibromo-2,1,3-benzotellur-
adiazole was synthesized prior to conventional condensation
polymerization. Thus, we began our spectroscopic studies with
these three analogous acceptor molecules (S, Se, Te).
Absorption properties were measured in solution to determine
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spectrum of the representative polymer, P1 (Figure 5), which
has the most intense low-energy transition. A solution of the
Figure 3. Absorption spectra in chloroform of group 16 D−A
polymers, P1−P3.
decreases in intensity while the high-energy band remains
constant upon moving down the group. Additionally, it is
interesting that the diamine intermediate polymer P4 shows
only a single absorption band that appears at approximately the
same wavelength as the high-energy band in the D−A polymers
(Figure S3, Supporting Information). Comparison with P1−P3
provides strong support that the dual-band only arises in D−A
systems (the fused-thiophene and diamine units are energeti-
cally very similar, and both electron rich; see below).
It has been suggested in the literature that the low-energy
band of the D−A dual absorption is due to an intramolecular
charge transfer.15,29 This phenomenon should be evident in
solvatochromic experiments. That is, the more polar excited
state resulting from a charge transfer should be stabilized
relative to the less polar ground state in a solvent of greater
polarity. This excited state stability should result in a red-shift
of the emission spectra of the polymers in more polar solvents.
In order to identify a charge-transfer state, absorption and
emission experiments were conducted on representative
polymer P1 in solvents of increasing polarity (Figure 4).
Figure 5. Absorption spectra of solutions of S-containing polymer P1
of decreasing concentration: 3.7 × 10−5 M (red), 1.9 × 10−5
(green), 9.3 × 10−6 M (blue), 4.7 × 10−6 (black).
M
polymer was prepared and then diluted, and the absorption
spectrum of each solution was obtained. Here we observe that,
as the concentration of the solution decreases, the absorption
coefficient of the high-energy band remains relatively constant
while the low-energy charge transfer band decreases. This
indicates an intermolecular charge transfer also contributes to
this lower-energy absorption band; this is in addition to the
well-established intramolecular charge transfer discussed earlier.
Density Functional Theory Calculations. To comple-
ment the optical characterization, the geometry, and electronic
structure of small-molecule model dyads of the polymers P1−
P4 (termed M1−M4), consisting of a single donor and
acceptor pair with methyl groups in place of the 2-(ethyl)hexyl
side chains, were calculated using DFT and time-dependent
DFT (TD-DFT).43 The coordinates of the optimized geo-
metries (Supporting Information), the frontier orbitals involved
in the lowest-energy singlet transitions (Figure 6), and the
Figure 4. (Left) Absorption spectra of sulfur-containing P1 in
cyclohexane (blue), toluene (green), chloroform (red) and tetrahy-
drofuran (wine). (Right) Emission spectra of sulfur-containing P1 in
cyclohexane (blue), toluene (green), chloroform (red), and tetrahy-
drofuran (wine).
More specifically, the solvents tested, in order of increasing
dielectric constant42 are cyclohexane (ε = 2.0), toluene (2.4),
chloroform (4.8), and THF (7.5). The absorption spectra of P1
in the four solvents remain the same, with a λmax at approxi-
mately 700 nm. The emission spectra, however, are clearly
shifted to lower energy, with λem transitioning from 743 to
752 nm to 758 to 766 nm upon moving from cyclohexane to
toluene to chloroform to THF. The shift in the emission
indicates that this optical excited state is, indeed, a charge
transfer complex.
Figure 6. Frontier MO energy levels for model compounds M1−M4.
energies, orbitals, and oscillator strengths of the strongest
transitions (Table 1) were calculated using the Gaussian 09
program44 at the CAM-B3LYP45−47 level of theory with the
6-31G(d) basis set for C, H, N, and S, and the LAN2DZ basis
set48 for Se and Te. We will first describe the frontier orbitals of
In order to understand the changing absorption coefficient in
the dual-band spectrum on going from S to Se to Te, we
investigated the effects of concentration on the absorption
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down the series. The high-energy band is the second intense
singlet transition. This transition is HOMO to LUMO+1 and is
based solely on the fused-thiophene, shifting from 311 to 319
nm for M1 and M2, respectively. In the case of M3, however,
the high-energy band consists of two different transitions; both
are a combination of HOMO to LUMO+2 and HOMO−2 to
LUMO with wavelengths at 327 and 316 nm. The HOMO to
LUMO+2 is centered on the fused-thiophene, and the
HOMO−2 to LUMO is on the benzochalcogenodiazole. All
high-energy bands also shift to a lower energy going down the
period, although to a smaller extent than for the charge transfer
transitions.
Table 1. TD-DFT Calculated Transition Data for Model
Compounds M1−M4
a
wavelength/nm
(oscillator
strength)
wavelength/nm
(oscillator
strength)
b
b
transition
transition
M1 H→L (+93%)
M2 H→L (+92%)
M3 H→L (+92%)
425 (0.48)
445 (0.38)
458 (0.33)
H→L+1(+86%)
H→L+1(+80%)
311 (0.26)
319 (0.31)
327 (0.24)
H→L+2(+42%);
H-2→L(+33%)
H-2→L(+46%);
H→L
+2(+27%)
316 (0.23)
M4 H→L (+91%)
314 (0.53)
a
b
H = HOMO; L = LUMO. Coefficient percentage of orbitals involved
DISCUSSION
■
in the transition.
To our knowledge this is the first systematic study of S, Se, and
Te atom substitution in D−A polymers. The results above shed
significant light on the properties and fundamental mechanisms
of D−A polymers in general, and this will be discussed in this
section. The well-known polymer PCPDTBT (P1) was chosen
as a starting point because of its demonstrated potential for use
in organic electronic applications, particularly organic photo-
voltaic devices and field effect transistors.5 As well, it offered a
convenient location where single atom substitution could be
achieved using modular chemistry.
While the sulfur and selenium analogs (P1 and P2,
respectively) were relatively straightforward to synthesize, the
tellurium polymer (P3) proved much more difficult and
required the use of a novel postpolymerization synthetic
method. The dual-band absorption observed in the spectra of
P1−P3 is typical of the D−A polymer architecture (Figure 3).18
In the case of P3 the presence of the dual-band absorption
demonstrates that we have formed the heterocycle with
tellurium postpolymerization, because only the reconstruction
of an electron-deficient heterocycle would lead to this signature
absorption spectrum.
compounds M1−M3. All of these orbitals are very similar in
electronic distribution. The HOMO resides on the fused-
thiophene part of the molecule, while the LUMO is centered
on the benzochalcogenodiazole acceptor part of the molecule.
A slight destabilization of the HOMO and stabilization of the
LUMO result from changing the heteroatom in the
benzochalcogenodiazole from S to Se to Te, leading to an
overall narrowing of the calculated HOMO−LUMO gap.
To further corroborate these trends TD-DFT calculations
were carried out to model the ground-excited state transitions
in these polymers (Figure 7). Both the high-energy and
The synthesis of the benzochalcogenodiazole small mole-
cules, their subsequent polymerization, and the red-shifted
absorption upon substituting S for Se for Te demonstrate that
heavy atom substitution narrows the band gap of π-conjugated
molecules in general. This stands in contrast to studies of
the oxygen analog of P1. Optical properties change very little
when substituting O in place of S.49 In the case of the polymers
reported here, the λmax of the high-energy absorption band
remains relatively constant on moving from S to Se to Te,
shifting by only a few nanometers upon moving down group 16
(Figure 3). More remarkably, the λmax of the low-energy band
shifts significantly while its absorption coefficient decreases in
intensity upon moving from S to Se to Te. These spectra, taken
together with the electronic structure calculations, present at
least three fundamental issues worth addressing in D−A
polymers, namely: (a) the origin of the red-shift in the
absorbance spectra, (b) the origin of the peak intensity changes
on moving from S to Se to Te (high-energy band vs low-energy
band), and most importantly (c) how the present study offers
new insight into the D−A mechanism.
Figure 7. MO diagram of M1 on the left, the energy of the transitions
calculated from TD-DFT are shown with the arrows illustrating the
transition for both M1 and M2 as indicated. On the right the tellurium
model compound M3 is shown with the arrows indicating electron
transitions responsible for individual absorption bands. The thickness
of each band is representative of the contribution of the orbitals to the
transition.
We will first discuss (a), the origin of the red-shift in optical
properties for the series, P1−P3. One explanation is that the
low-energy transition is due to a charge transfer from the fused-
thiophene donor to the benzochalcogenodiazole acceptor. In all
three polymers this transition is HOMO to LUMO, as
supported by the molecular orbital calculations and optical
measurements. The calculations indicate two possible con-
tributions to the red-shift on going from P1 to P3. First, the
low-energy bands are seen in the TD-DFT results (Table 1). For
all three compounds, the lowest-energy singlet transition is a
charge transfer from the HOMO on the fused-thiophene to the
LUMO on the benzochalcogenodiazole. In changing the
heteroatom from S to Se to Te for M1 to M3 this transition
shifts from 425 to 445 nm to 448 nm, respectively. The
oscillator strength of this transition also decreases as one moves
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dipole moment of the ground state geometry is larger for M1
(3.27 D) than for M2 (2.82 D) and M3 (1.55 D). This same
trend is observed in the Mulliken charges on the chalcogenide
atom, 0.57, 0.58, 0.79. The increased positive charge agrees well
with the decrease in electronegativity upon going down the
group. In agreement with previous theoretical calculations,50
the lower ionization potential of the heavier atom leads to
destabilization of the occupied bonding MO related to the
LUMO. This stabilizes the LUMO of the polymer, thereby
narrowing the band gap and producing a red-shift in the
absorption spectrum as one moves from S to Se to Te. The
incorporation of heavier atoms into polythiophene analogues,
perhaps the most important class of conjugated polymers, has
been fuelled by the observation that the HOMO of such
polymers does not lie on the heavy atom, while the LUMO
does, enabling a narrowing of the band gap by heavy-atom
substitution.37,51,52
A second reason for the greater shift in the low-energy
transition is related to the degree of aromaticity found in the
acceptor unit. A longer E−N bond length (where E = S, Se, or
Te), produced upon single atom substitution, results in a
disruption of conjugation within the benzene ring, as shown by
the calculated bond lengths (Supporting Information). The
degree of bond length alternation in the benzene ring increases
in the order S to Se to Te, trending toward a decrease in
aromaticity which affects the conjugation of the acceptor unit
with the donor unit. This results in destabilization of the
HOMO, stabilization of the LUMO, and an overall decrease in
the band gap of the polymer. This is apparent not only in the
decreasing calculated energy of the LUMO, as discussed above,
which is concentrated on the benzochalcogenodiazole unit, but
also by a destabilization of the HOMO energy level (Figure 6).
A similar trend has been observed in thiophenes that have been
modified to stabilize their quinoid state.53,54 We further note
that the high-energy transition is centered entirely on the fused-
thiophene and is, therefore, not expected to shift as strongly.
This is consistent with the experimentally obtained absorption
spectra of the polymers.
Now we will discuss (b), the origin of the peak intensity
changes on moving from S to Se to Te. We observe that the
absorption coefficient of the low-energy band decreases in
intensity upon moving from S to Se to Te, while the high-
energy band remains largely unaffected. This can be explained
by the decreasing electronegativity of the heteroatom. In order
to produce the low-energy absorption band, the acceptor unit
must be electron-deficient enough to produce a charge
separated state.18 As the electronegativity of the heteroatom
decreases upon moving down group 16, the acceptor unit’s
ability to separate charge, as well as stabilize a charge-separated
state, decreases. This reduction in ability to separate intra-
molecular charge leads to a decrease in the absorption
coefficient of the low-energy band when moving from S to
Se to Te.
Interestingly, a similar effect in the optical spectrum of the
S-containing polymer P1 was seen as the concentration of the
polymer in solution was decreased (Figure 5). Given that only
the intensity of low-energy charge transfer band is affected, this
suggests that there may be an intermolecular charge transfer
process at work as well as the intramolecular process previously
described. It follows that as the concentration of polymer
decreases, there are fewer opportunities for neighboring chains
to interact, which would be necessary for intermolecular charge
transfer to occur. This effect is more pronounced in P1 as the
relatively high electronegativity of S favors charge transfer, both
inter- and intramolecularly, to a greater extent than its heavier
chalcogen analogs. This effect is not observed in Se-
containing P2 nor in Te-containing P3 (Figure S4, Supporting
Information). The absence of concentration effects and
intermolecular charge transfer in P2 and P3 confirms our
hypothesis that the greater electronegativity of S relative to that
of Se and Te contributes to the relatively high intensity of the
charge transfer band in the spectrum of P1.
Now we will discuss (c), how the present study offers insight
into the D−A mechanism. Throughout the literature the notion
that a charge transfer state causes the low-energy D−A
transition is implied.17 Our DFT calculations predict a distinct
HOMO and LUMO for the polymer repeat unit that has a
charge transfer character. The majority contribution from the
donor unit on the HOMO and majority contribution from the
acceptor unit on the LUMO results in a significant shift of
electron density as an electron is promoted from the ground to
the excited state. The observation that the absorption
coefficient of the low-energy transition does not shift as the
S-containing polymer P1 is exposed to solvents of increasing
polarity is not surprising (Figure 4). It is expected that the
solvents of greater polarity will stabilize the more polar excited
state relative to the ground state. Thus, the red-shift observed
in the PL spectrum supports that a charge-transfer excited state
gives rise to the low-energy absorption.
The presence of a charge-transfer excited state highlights the
difference between the two basic models that describe these
D−A systems: (1) a low-lying charge transfer state results from
the push−pull interaction of the donor and acceptor units and
gives rise to the low-energy absorption band while a higher-
energy transition centered on the donor repeat unit gives rise to
the high-energy band in the absorption spectrum;15,17,29 and
(2) the incorporation of donor and acceptor units results in a
reorganization of molecular orbitals to give a HOMO and
LUMO that do not produce a charge transfer and retain less
character of either the donor or acceptor units.30−32 Our data
support a model involving a D−A charge transfer state
involving distinct MOs for the polymer.
It is also accepted throughout the literature that the source of
the narrow band gap in D−A polymers is the difference in
electron density between the donor and acceptor units.23 As
such, the absorption spectrum of a polymer can be shifted by
either adding electron density to the polymer chain through the
donor or removing electron density through the acceptor.17
Indeed, this has been shown to be the case in many systems,
most recently through the addition of Lewis acids to the
acceptor units.55,56 The observation that the band gap of
PCPDTBT analogs narrows as heavier atoms are substituted at
the heteroatom position is counterintuitive if the only factor
governing the location of the absorption is the distribution of
electron density in the main chain. If this were the case, one
would expect the strength of the D−A interaction to decrease
(resulting in a blue-shift of the absorption spectrum) as heavier
atoms are substituted. This is because the decreasing
electronegativity moving down the group should weaken the
electron-withdrawing ability of the acceptor going from S to Se
to Te. The opposite effect is observed, however, indicating that
another process must be at work in these systems. Given that
heavy atoms have demonstrated an ability to narrow the band
gap of all-donor homopolymers, it seems reasonable to suggest
that in the present case the lower ionization potential and loss
of aromaticity overcome the loss of electron-withdrawing ability
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Journal of the American Chemical Society
Article
dried with calcium hydride and distilled under nitrogen before use. n-
of the acceptor on moving down group 16. These results
indicate that the D−A polymer absorption is not governed
solely by the electron-rich or electron-poor nature of the
polymer chain. Further, this single atom substitution study
introduces another method for controlling the absorption
properties of D−A polymers through modular chemistry.
Butyllithium was titrated before each use. NMR spectra were recorded
1
on a Varian Mercury 400 spectrometer operating at 400 MHz for H
and 100 MHz for 13C. Chemical shifts are reported in ppm at ambient
temperature. 1H chemical shifts are referenced to the residual
protonated chloroform peak at 7.26 ppm. Absorption spectra were
recorded on a Varian Cary 5000 UV−vis−NIR spectrophotometer in
either DMSO or chloroform, as noted, at ∼0.01 mg/mL. Emission
spectra were recorded on a Photon Technology International
QuantaMaster 40-F NA spectrofluorometer in chloroform solution.
Polymer molecular weights were determined with a Viscotek HT-GPC
(1,2,4-trichlorobenzene, 140 °C) using Tosoh Bioscience LLC TSK-
GEL GMHHR-HT mixed-bed columns and narrow molecular weight
distribution polystyrene standards. Masses were determined on a
Waters GCT Premier ToF mass spectrometer (EI). ICP-AEOS samples
were digested in aqua regia for four days at 70 °C before being filtered
and diluted to 25 mL with deionized water. The samples were run on a
Perkin-Elmer model Optima7300DV ICP AEOS, and selenium levels
were quantified with National Institute of Standards and Technology
standards. The detection limit was 0.1 μg selenium/mL.
CONCLUSION
■
Three analogs of the well-known D−A polymer PCPDTBT
have been synthesized, including a novel Te analog, which
was prepared by a postpolymerization method involving
reduction of the Se polymer, isolation of an amine intermediate,
and reoxidation of the acceptor with tellurium(IV). By inserting
Se and Te into the S position of 2,1,3-benzothiadiazole, the
three polymers (P1−P3) allow for a systematic single atom
substitution study. Optical spectroscopy reveals that the
characteristic D−A dual-band is present in each of these
polymers and that heavy atom substitution leads to a red-shift
in the low-energy transition, while the high-energy band
remains relatively constant in energy. The red-shift in the low-
energy transition leads to optical band gap values of 1.59, 1.46,
and 1.06 eV for the S-, Se-, and Te-containing polymers,
respectively. In addition, the strength of the low-energy band
decreases while the high-energy band remains relatively
constant. A series of optical spectroscopy experiments,
solvatochromism studies, and DFT calculations were used to
understand the trends in the acquired spectra. The red-shift in
the low-energy absorption is likely due to both a decrease in the
ionization potential of the heavy atom and a decrease in
aromaticity as one moves from S to Se to Te. The decreasing
intensity of the low-energy band can be explained by the
decrease in the ability of the acceptor to separate and stabilize
charge, owing to the decreasing electronegativity of the
chalcogen atom. These results imply that the substitution of
heavy atoms into the acceptor can narrow the band gap of a
D−A polymer. This was not predicted on the basis of the
established theory that difference in electron density of the D
and A units controls the band gap of D−A polymers. This study
therefore introduces a new method for controlling the
absorption properties of D−A polymers through modular
chemistry.
Density Functional Theory Calculations. Geometry optimiza-
tions were performed for all model compounds (M1−M4) using the
Gaussian 09 program employing the Becke Three Parameter Hybrid
Functionals Lee−Yang−Parr (B3LYP) including Handy and co-
workers’ long-range corrected version of B3LYP using the
Coulombic-attenuating method (CAM-B3LYP). The standard
6-31G(d) basis set for C, H, N, S atoms, and LAN2DZ for Te and
Se atoms was used. Starting geometries were generated using
Gausview 05. A frequency calculation was performed on the optimized
geometries to ensure a local minimum was found. From the optimized
geometries time-dependent DFT (TD-DFT) calculations were carried
out with the same basis sets used for geometry optimizations; the first
20 singlet and triplet states were calculated.
Synthesis and Characterization. Di-3-thienylmethanol (1).
n-Butyllithium in hexanes (1.56 M, 14.5 mL, 22.2 mmol) was added
dropwise to a solution of 3-bromothiophene (2.10 mL, 22.1 mmol) in
dry diethyl ether (200 mL) at −78 °C. This mixture was allowed to stir
at −78 °C for 30 min before thiophene-3-carboxaldehyde (1.97 mL,
22.3 mmol) was added dropwise. This mixture was stirred overnight
and allowed to warm to room temperature. The mixture was washed
with 0.1 M HCl (2 × 75 mL), 0.1 M NaOH (2 × 75 mL), and brine
(2 × 75 mL). The organic layer was dried on magnesium sulfate, and
the solvent was removed by rotary evaporation to produce a light-
yellow oil that solidified on standing at room temperature (3.57 g, 82%
crude). The crude product was taken to the next reaction without
further purification. 1H NMR (400 MHz, CDCl3) δ: 7.28 (m, 2H), 7.18 (m,
2H), 7.02 (d of d, J = 8 Hz, 4 Hz, 2H), 5.90 (s, 1H), 2.82 (br s, 1H).
13C NMR (100 MHz, CDCl3) δ: 145.0, 126.4, 126.3, 121.8, 69.1.
EXPERIMENTAL SECTION
■
Di-3-thienylmethane (2). Aluminum chloride (8.76 g, 65.7 mmol)
was added to a suspension of lithium aluminum hydride (2.45 g, 64.6
mmol) in dry diethyl ether (250 mL) at 0 °C over 10 min. Di-3-
thienylmethanol was then added in portions over 10 min at 0 °C, and
the mixture was heated to reflux for 5 h. The mixture was diluted with
diethyl ether (150 mL) and quenched by adding 1 M HCl dropwise.
Once the bubbling had subsided, the mixture was washed with 1 M
HCl (150 mL), 1 M NaOH (150 mL), and brine (2 × 150 mL). All
aqueous washings were separately extracted once with diethyl ether
(75 mL), and all organic layers were combined and dried over
magnesium sulfate, and the solvent was removed under vacuum to
General Considerations. Unless stated otherwise, starting
materials were purchased and used as received. 3-Bromothiophene,
thiophene-3-carboxaldehyde, n-butyllithium (1.6 M in hexanes), and 2-
bromo-2-ethylhexane were purchased from Acros Organics. Lithium
aluminum hydride, aluminum(III)chloride, selenium(IV)oxide, and
tellurium(IV)chloride were purchased from Alfa Aesar. N-Iodosucci-
nimide, 2-isopropoxy-4,4,5,5-tetramethyl-1,3,2-dioxaborolane, Aliquat
336, potassium carbonate, tetrakis-triphenylphosphinepalladium(0),
and pyridine were purchased from Sigma-Aldrich. Potassium
hydroxide and magnesium sulfate were purchased from Fisher
Scientific. Copper(I)thiophene-2-carboxylate was purchased from
Frontier Scientific. 4,7-Dibromo-2,1,3-benzothiadiazole and 4,7-
dibromo-2,1,3-benzoselenadiazole were prepared according to pub-
lished procedures.40 Unless otherwise noted, all manipulations
involving air- or water-sensitive reagents were performed under an
atmosphere of dry nitrogen using conventional Schlenk techniques
and glassware or in an Innovative Technologies glovebox. Solvents
were sparged with nitrogen for 25 min and dried using an Innovative
Technologies solvent purification system. In the case of P3 absorption
experiments, chloroform was dried with calcium hydride and distilled
under nitrogen before use. Deuterated chloroform was purchased from
Cambridge Isotope Laboratories, and in the case of use with P3, was
1
produce di-3-thienylmethane (7.08 g, 75%) as a yellow oil. H NMR
(400 MHz, CDCl3) δ: 7.26 (d of d, J = 4 Hz, 2H), 6.94−6.98 (m, 4H),
4.01 (s, 2H). 13C NMR (100 MHz, CDCl3) δ: 141.0, 128.4, 125.6,
121.1, 31.1. HR-MS 180.0067 calc. 180.0060 found.
Bis(2-iodothien-3-yl)methane (3). N-Iodosuccinimide (18.0 g,
80.0 mmol) was added to a solution of di-3-thienylmethane (7.03 g,
39.0 mmol) in dry DMF (250 mL) at 0 °C over 45 min. This mixture
was stirred overnight with warming to room temperature. Following
stirring, 0.1 M HCl (200 mL) was added, and the mixture was
extracted with diethyl ether (3 × 150 mL). The ether layers were com-
bined and washed with water (2 × 150 mL) and brine (2 × 150 mL)
545
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Journal of the American Chemical Society
Article
and dried over magnesium sulfate, and the solvent was removed under
vacuum to produce bis(2-iodo-3-yl)methane (12.3 g, 73%). H NMR
(400 MHz, CDCl3) δ: 7.37 (d, J = 8 Hz, 2H), 6.67 (d, J = 8 Hz, 2H),
3.84 (s, 2H). 13C NMR (100 MHz, CDCl3) δ: 144.2, 130.7, 128.3,
75.0, 35.0. HR-MS 431.8000 calc. 431.7999 found.
under vacuum for 72 h before being poured into methanol (30 mL)
and filtered through a Soxhlet thimble. The filtered solid was extracted
with methanol, hexanes, and chloroform until each solvent in the
extraction chamber was clear and colorless. The solvent was removed
from the chloroform extract to yield the title polymer as a blue powder
(0.022 g, 0.042 mmol, 56%). 1H NMR (400 MHz, CDCl3) δ: 8.14 (m,
2H), 7.88 (br s, 2H), 2.07 (br, 4H), 1.02 (br, 18H), 0.88 (br, 6H),
0.68 (br,6H). GPC: Mn = 14 107 g/mol, Mw = 47 188 g/mol, PD
I = 3.35.
Poly[4,4-bis(2-ethylhexyl)cyclopenta[2,1-b;3,4-b′]dithiophene-
2,6-diyl-alt-2,1,3-benzoselenadiazole-4,7-diyl] (P2). This polymer
was synthesized from 2,6-bis(4,4,5,5-tetramethyl-1,3,2-dioxaboralan-
2-yl)-4,4-di(2-ethylhexyl)-4H-cyclopenta-[2,1-b:3,4-b′]dithiophene
(0.300 g, 0.454 mmol) 1,2,-dibromo-2,1,3-benzoselenadiazole (0.154 g,
0.454 mmol), tetrakis-triphenylphosphinepalladium(0) (0.052 g, 0.045
mmol) in toluene (10 mL) in a manner that is analogous to P1 to
yield P2 as a dark-green powder (0.062 g, 0.11 mmol, 51%). 1H NMR
(400 MHz, CDCl3) δ: 8.02 (br m, 2H), 7.79 (br s, 2H), 2.05 (br s,
4H), 1.02 (br s, 16H), 0.88 (br s, 2H), 0.68 (br s, 12H). GPC: Mn =
5022 g/mol, Mw = 11 016 g/mol, PDI = 2.19.
1
4H-Cyclopenta[2,1-b:3,4-b′]dithiophene (4). Dry DMF (250 mL)
was added by cannula to bis(2-iodothien-3-yl)methane (14.9 g, 34.3
mmol). Copper(I)thiophene-2-carboxylate (32.7 g, 171 mmol) was
added, and the mixture was stirred at 70 °C for 48 h. The mixture was
diluted with ethyl acetate (40 mL) and filtered through a plug of silica.
The ethyl acetate was removed from the mixture by rotary
evaporation, and brine (200 mL) was added to the organic mixture.
This brine/DMF mixture was extracted with hexanes (3 × 200 mL),
and the combined organic layers were washed with brine (3 × 200 mL)
and dried over magnesium sulfate, and the solvent was removed under
vacuum to produce a brown oil which was purified by column
chromatography on silica gel (pentane) to yield the title compound
(3.64 g, 20.4 mmol, 60%) as a white solid. 1H NMR (400 MHz,
CDCl3) δ: 7.19 (d, J = 4 Hz, 2H), 7.10 (d, J = 4 Hz, 2H), 3.56 (s, 2H).
13C NMR (100 MHz, CDCl3) δ: 149.9, 138.9, 124.7, 123.2, 31.8. HR-
Poly[4,4-bis(2-ethylhexyl)cyclopenta[2,1-b;3,4-b′]dithiophene-
2,6-diyl-alt-2,3-diaminobenzo-1,4-diyl] (P4). Poly[4,4-bis(2-
ethylhexyl)cyclopenta[2,1-b;3,4-b′]dithiophene-2,6-diyl-alt-2,1,3-ben-
zoselenadiazole-4,7-diyl] (P2, 0.222 g, 0.380 mmol) was placed in a
100-mL Schlenk flask under nitrogen with dry THF (25 mL). Lithium
aluminum hydride (0.150 g, 3.95 mmol) was added under nitrogen
at 0 °C, and the resulting mixture was stirred at room temperature for
16 h. The mixture was poured into methanol (50 mL) and filtered
through a Soxhlet thimble. The filtered solid was extracted with
methanol, hexanes, and chloroform until each solvent in the extraction
chamber was clear and colorless. The solvent was removed from the
chloroform extract to yield the title polymer as a red solid (0.140 g,
0.275 mmol, 72%). 1H NMR (400 MHz, CDCl3) δ: 7.11 (s, 2H), 6.94
(s, 2H), 3.94 (br s, 4H), 1.96 (br s, 4H), 1.03 (br s, 18H), 0.79 (br s,
6H), 0.67 (br s, 6H).
Poly[4,4-bis(2-ethylhexyl)cyclopenta[2,1-b;3,4-b′]dithiophene-
2,6-diyl-alt-2,1,3-benzotellurodiazole-4,7-diyl] (P3). Poly[4,4-bis(2-
ethylhexyl)cyclopenta[2,1-b;3,4-b′]dithiophene-2,6-diyl-alt-2,3-diami-
nobenzo-4,7-diyl] (P4, 0.053 g, 0.11 mmol) was placed in a 10-mL
Schlenk flask under nitrogen with dry pyridine (5 mL). Tellurium(IV)-
chloride (0.036 g, 0.12 mmol) was added under nitrogen, and the
resulting mixture was stirred at room temperature for 16 h. This
mixture was poured out into dry methanol (15 mL), and the solid was
collected by filtration, washed with dry methanol (30 mL), and dried
under vacuum to give the title polymer as a dark-green solid (0.036 g,
0.057 mmol, 54%). 1H NMR (400 MHz, CDCl3) δ: 7.83 (s, 2H), 7.71
(s, 2H), 2.02 (br s, 4H), 1.03 (br s, 16H), 0.88 (br s, 2H), 0.69 (m, 12H).
Reoxidation of P4 to Poly[4,4-bis(2-ethylhexyl)cyclopenta[2,1-
b;3,4-b′]dithiophene-2,6-diyl-alt-2,1,3-benzoselenadiazole-4,7-diyl]
(P2). Poly[4,4-bis(2-ethylhexyl)cyclopenta[2,1-b;3,4-b′]dithiophene-
2,6-diyl-alt-2,3-diaminobenzo-4,7-diyl] (P4, 0.023 g, 0.044 mmol) was
placed in a 10-mL Schlenk flask under nitrogen with dry pyridine (5 mL).
Selenium(IV)oxide (0.0060 g, 0.054 mmol) was added under nitrogen,
and the resulting mixture was stirred at room temperature for 16 h. This
mixture was poured into methanol (15 mL), and the solid was collected
by filtration, washed with methanol (30 mL), and dried under vacuum to
give the title polymer as a green solid (0.015 g, 0.026 mmol, 59%). GPC:
Mn = 6363 g/mol, Mw = 11 747 g/mol, PDI = 1.85.
MS 177.9911 calc. 177.9906 found.
4,4-Di(2-ethylhexyl)-4H-cyclopenta[2,1-b:3,4-b′]dithiophene
(5). 1-Bromo-2-ethylhexane (7.30 mL, 40.8 mmol) was added to a
solution of 4H-cyclopenta[2,1-b:3,4-b′]dithiophene (3.52 g, 19.8
mmol) in DMSO (80 mL), powdered KOH (3.65 g, 65.0 mmol),
and KI (0.47 g, 2.8 mmol) under air. This mixture was allowed to stir
at room temperature overnight and then poured out into water and
extracted with diethyl ether (3 × 100 mL). The organic extracts were
combined and dried over magnesium sulfate, and the solvent was
removed under vacuum to give a brown oil which was purified by
column chromatography on silica gel (hexane) to yield 4,4-di(2-
ethylhexyl)-4H-cyclopenta[2,1-b:3,4-b′]dithiophene (4.58 g, 11.4
mmol, 58%) as a yellow oil . 1H NMR (400 MHz, CDCl3) δ: 7.10 (d,
J = 4 Hz, 2H), 6.92 (m, 2H), 1.85 (m, 2H), 0.90 (m, 18H), 0.75 (t,
J = 4 Hz, 2H), 0.58 (t, J = 4 Hz, 2H). 13C NMR (100 MHz, CDCl3) δ:
157.8, 137.0, 124.1, 122.5, 53.4, 43.4, 35.2, 32.1, 29.0, 28.8, 23.0, 14.3,
10.8. HR-MS 402.2415 calc. 402.2405 found.
2,6-Bis(4,4,5,5-tetramethyl-1,3,2-dioxaboralan-2-yl)-4,4-di(2-eth-
ylhexyl)-4H-cyclopenta-[2,1-b:3,4-b′]dithiophene (6). A solution of
4,4-di(2-ethylhexyl)-4H-cyclopenta[2,1-b:3,4-b′]dithiophene (0.420 g,
1.04 mmol) in THF (15 mL) was cooled to −78 °C, and n-
butyllithium in hexanes (1.6 M, 1.50 mL, 2.40 mmol) was added
dropwise. This mixture was allowed to stir at −78 °C for one hour
followed by warming to room temperature and stirring for a further
2 h. The mixture was subsequently cooled again to −78 °C, and
2-isopropoxy-4,4,5,5-tetramethyl-1,3,2-dioxaborolane (0.45 mL, 2.2
mmol) was added. This mixture was allowed to warm to room
temperature with stirring overnight and subsequently poured into
water (50 mL) and extracted with hexanes (3 × 50 mL). The organic
layers were combined and washed with brine (3 × 50 mL) and dried
on magnesium sulfate, and the solvent was removed under vacuum.
The produced oil was further dried on a vacuum line for 24 h,
1
producing a sticky orange product (622 mg, 0.941 mmol, 91%). H
NMR (400 MHz, CDCl3) δ: 7.44 (t, 32H, J = 8 Hz), 1.84 (m, 4H),
1.34 (s, 24H), 0.96 (m, 18H), 0.73 (m, 6H), 0.58 (m, 6H). 13C NMR
(100 MHz, CDCl3) δ: 161.2, 144.3, 132.0, 84.1, 52.9, 43.4, 35.3, 34.1,
31.8, 28.5, 27.8, 25.0, 23.0, 14.3, 10.8. HR-MS (EI) m/z, calcd for
C37H60B2O4S2 (M+): 655.4210; found: 655.4197.
Poly[4,4-bis(2-ethylhexyl)cyclopenta[2,1-b;3,4-b′]dithiophene-
2,6-diyl-alt-2,1,3-benzothiadiazole-4,7-diyl] (P1). In a nitrogen-filled
glovebox, 2,6-bis(4,4,5,5-tetramethyl-1,3,2-dioxaboralan-2-yl)-4,4-di(2-
ethylhexyl)-4H-cyclopenta-[2,1-b:3,4-b′]dithiophene (0.133 g, 0.201
mmol), 1,2,-dibromo-2,1,3-benzothiadiazole (0.060 g, 0.20 mmol),
tetrakis-triphenylphosphinepalladium(0) (0.032 g, 0.028 mmol), and
toluene (8 mL) were sealed inside a 50-mL reaction bomb. The
reaction bomb was brought out of the glovebox and placed under
nitrogen. Aliquat 336 (3 drops) and bubble-degassed aqueous
potassium carbonate (2.0 M, 0.45 mL, 0.90 mmol) were added. The
reaction bomb was sealed again, and the entire mixture was degassed
by three freeze−pump−thaw cycles. The mixture was heated to 80 °C
ASSOCIATED CONTENT
■
S
* Supporting Information
Additional absorption and NMR spectra as well as
supplemental calculation data; complete ref 44. This material
is available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
dseferos@chem.utoronto.ca
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Journal of the American Chemical Society
Article
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ACKNOWLEDGMENTS
■
This work was supported by the University of Toronto, NSERC,
the CFI, and the Ontario Research Fund. We are grateful to
Professor Ignacio Vargas-Baca and Dr. Anthony Cozzolino
(McMaster University) for assistance with the synthesis of 4,
7-dibromo-2,1,3-benzotelluradiazole. G.L.G. thanks the Xerox
Research Centre of Canada for a Graduate Award.
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