Conducting thiophene-annulated azepine polymers

Article abstract of DOI:10.1021/ma100549w

We report the synthesis of annulated azepines, conjugated seven-membered ring systems with nitrogen, designed to undergo an electrochemically controlled bent-to-planar transformation driven by aromatization. A Pd-catalyzed double amination strategy enable

Full text of DOI:10.1021/ma100549w

Macromolecules 2010, 43, 5233–5237 5233
DOI: 10.1021/ma100549w
Conducting Thiophene-Annulated Azepine Polymers
Changsik Song,†,‡ D. Barney Walker,† and Timothy M. Swager*,†,‡
‡
†Department of Chemistry and Institute for Soldier Nanotechnologies, Massachusetts Institute of Technology,
Cambridge, Massachusetts 02139
Received March 10, 2010; Revised Manuscript Received May 12, 2010
ABSTRACT: We report the synthesis of annulated azepines, conjugated seven-membered ring systems with
nitrogen, designed to undergo an electrochemically controlled bent-to-planar transformation driven by
aromatization. A Pd-catalyzed double amination strategy enabled us to synthesize annulated azepines, which
are thermally and electrochemically stable even in highly oxidized states. Several methods of chemical and
electrochemical polymerization yielded azepine-based materials that were demonstrated to retain their redox
properties in the solid state under ambient conditions. Because of their redox stability and conductivity, these
polymers could find utility in actuating materials research.
Introduction
synthetically, they are expected to have low Se extrusion barri-
ers28 and would be likely to decompose on oxidation. Second,
The development of electrochemically driven actuating poly-
mers with a high degree of electrical to mechanical coupling is
predicted to enable a range of micro- and nanomechanical
devices, and several approaches are currently being pursued to
achieve this goal.1-10 Classically, the mechanical motion of
electroactive materials is driven by either the Coulombic inter-
action between electrodes (in field activated actuators) or the
reversible diffusion and intercalation of ions into the polymeric
matrix following electrochemical oxidation (in ion diffusion
activated actuators). Complementary to these approaches, sev-
eral research groups11-17 including our own18-22 have become
interested in developing materials in which a molecular-level
geometrical change leads to macroscopic actuation.
In pursuit of designing such polymers, we have developed a
number of systems containing moieties capable of extending and
contracting in response to an electrochemical stimulus. Although
our “molecular hinge” designs have predominantly been based on
functionalized calixarenes, annulated heteropines (conjugated,
heteroatom-bridged, seven-membered ring systems) also appear
to be good candidates for molecular actuators as the energy gained
from aromatization could be utilized to drive actuation. Being bent
in their neutral state (8π electrons), heteropines can be planarized
through oxidatively induced aromatization (6π electrons), leading
to a significant dimensional change in the plane of the hetero-
annulene ring. We have recently demonstrated the design and
synthesis of thiepin polymers;23 however, these materials were
shown to undergo desulfurization at high oxidation potentials.
We have thus turned our attention to other heteroepine structures
envisaged to have reversible redox properties. Herein we report on
the design and synthesis of electrochemically and thermally stable
azepine-based conjugated polymers.
heteroepines containing heavier group 15 elements, such as
phosphorus,29 are easily oxidized to higher oxidation states
(i.e., P5þ),30,31 which would preclude the desired aromaticity-
driven planarization. Lastly, the nitrogen-containing moiety
can be incorporated at a late stage in the synthesis utilizing
contemporary palladium-catalyzed amination reactions32-35
enabling the modular preparation of prototype compounds.
To synthesize the target azepine core, we designed the
periphery of the conjugated monomer units based on our
previously reported thiepin system (Figure 1a, I-III). Both
dithieno[2,3-b:30,20-f]azepines (type II) and isomeric dithieno-
[3,2-b:20,30-f]azepines (type III) fit our design specifications.
However, structures based on the type II design proved
difficult to access in significant quantities (vida infra), and
thus molecular modeling discussions are focused on varia-
tions of type III annulenes.
As expected, DFT calculations (B3LYP, 6-31G) suggested
that type III azepines would adopt a bent geometry in the
neutral state (Figure 1b and Supporting Information). How-
ever, it appeared that the degree of nonplanarity depended on
the steric bulk of the N-substituents (R0). In the molecular
models the distance between the outmost carbons (C3 and C10)
decreased, indicating a more bowed annulene (when R0 =
p-tolyl) compared with the unsubstituted analogue (where
R0 = H), suggesting that steric crowding could augment the
predisposition of the 8π electron azepine core to adopt a kinked
conformation. However, the substituent effect was also found in
the optimized geometries of the dicationic type III azepines.
When R0 =H, the oxidized annulene appears to have a com-
pletely flat conformation, whereas when R0 is more sterically
demanding (e.g., R0 =Me, p-tolyl), the geometry is distorted
slightly from a planar conformation. Nevertheless, based on our
calculations, oxidation of the annulated azepine should give rise
to a structural expansion of around 5% and could potentially
form the molecular basis for an actuating polymer.36
Results and Discussion
Design and Molecular Modeling of Annulated Azepine
Scaffold. We chose the nitrogen-containing azepine ring system
to form the core of our annulated system for the following
reasons. First, although related heteroepine systems with hea-
vier atoms than sulfur (such as selenium24-27) can be accessed
Synthesis of Thiophene-Annulated Azepines. We originally
attempted to synthesize the type II azepines using a ring
closing double amination strategy based on palladium-cat-
alyzed C-N bond formation as pioneered by Buchwald and
Hartwig.32-35 Although a similar approach has been pre-
viously employed to assemble dithienopyrrole derivatives,37
*To whom correspondence should be addressed. E-mail: tswager@
mit.edu.
r 2010 American Chemical Society
Published on Web 05/21/2010
pubs.acs.org/Macromolecules

5234 Macromolecules, Vol. 43, No. 12, 2010
Song et al.
Figure 1. (a) Illustration of the design of annulated azepines (II and III) based on previously reported thiophene structure (I). The thiophene units were
incorporated either side of the azepine ring to impart electrochemical stability and to allow for functionalization in the R-positions (C3 and C10).
(b) DFT optimization (B3LYP, 6-31G) of an annulated azepine (R = R0 = Me) in its neutral (top) and oxidized (bottom) states.
Scheme 1. Synthesis of Annulated Azepine Monomers and Conjugated Polymersa
a Reagents: (i) Pd2(dba)3 CHCl3, t-Bu3PH-BF4, NaOt-Bu, PhNH2, PhMe, 100 °C, 24 h, 29%; (ii) Pd2(dba)3 CHCl3, t-Bu3PH-BF4, NaOt-Bu,
3
3
ArNH2, PhMe, 100 °C, 24 h, 57-87% for 4a-c, 24% for 4d; (iii) N-chlorosuccinimide, CHCl3, AcOH, room temperature, 16 h, 57%; (iv) n-BuLi, THF,
-78 °C; then I2, room temperature, 16 h, 58-91%; (v) PdCl2(PPh3)2, 2-tributylstannylthiophene or 5-tributylstannyl-2,20-bithiophene, DMF, 80 °C,
16 h, 55/85%; (vi) Ni(cod)2, bipy, DMF/PhMe, 80 °C, 72 h, 66%; (vii) Pd(PPh3)4, 5,50-bis(trimethylstannyl)-2,20-bithiophene, PhMe, 80 °C, 72 h,
72%; (viii) FeCl3, CHCl3, room temperature, 15 h, 41%.
azepine-type compounds have not previously been prepared
using this methodology. After optimization of reaction con-
ditions we were eventually able to couple dibromide 1 with
aniline to form azepine 2 in 29% yield with the remainder of
the reaction mixture containing monoaminated product
alongside multiple polar impurities resulting from substrate
decomposition (Scheme 1, i).
This difficulty with the second ring-closing C-N bond forma-
tion is consistent with the findings of Hartwig and co-workers,
who report (1) diarylamines react much more slowly when
compared with N-methylaniline (the fastest) and aniline, (2)
3-bromothiophene reacts well with aniline but 2-bromothio-
phene does not, and (3) methyl substitution on the 3-position
further reduces the reactivity of 2-bromothiophenes.38 Fur-
thermore, in a subsequent study39 it is proposed that the
amination of five-membered heteroaryl halides is dominated
by the effectiveness of the reductive elimination step, which
is unproductive in the case of the thiophen-2-yl-palladium
amido complex. Thus, according to their findings, com-
pletion of the second amination is expected to be challenging.
In our case the second amination is an intramolecular
cyclization, which we suspect slightly mitigates some of the
above undesirable effects.
In light of this we began to investigate the synthesis of type
III dithieno[3,2-b:20,30-f]azepines. The double amination of
dibromide 3a to form azepine 4a was significantly more
effective using this approach, furnishing the desired azepine
in 57% yield. Using this methodology, we were also able to
access two other annulated N-arylazepines suitable for elec-
trochemical investigation (4b, 4c) in 82% and 87% yield,
respectively (Scheme 1, ii). In addition, we were able to
synthesize benzyl analogue 4d albeit only in 24% yield.40
Azepine monomer units suitable for polymerization were
synthesized either by addition of N-chlorosuccinimide, to access
5, or using a lithiation-iodine quenching sequence to obtain
compounds 6b-c. Of these, 6c was characterized by X-ray
crystallography (Figure 2). In the solid state the bent geometry
˚
of the neutral annulated azepine results in a distance of 6.41 A
between the outermost carbons, which is in good agreement
with our DFT calculations (see Supporting Information).
Cyclic Voltammograms of Annulated Azepines. Cyclic vol-
tammograms (CVs) of azepines 2, 4a, and 4d were recorded in

Article
Macromolecules, Vol. 43, No. 12, 2010 5235
Figure 2. X-ray crystal structureof6c. The ellipsoidprobability is50%,
and the hydrogens are omitted for clarity.
Figure 4. (a) Electropolymerization of 7 on a Pt button electrode in
CH2Cl2 under ambient conditions. The red dotted lines represent the
first scan. (b) CVs of films of P7 at different scan rates. (c) CV (dotted
lines) and in situ conductivity measurements (red solid lines) of films of
P7 on 5 μm interdigitated Pt microelectrodes in CH2Cl2. TBAPF6 (0.1
M) was used as a supporting electrolyte in all measurements.
higher than that of azepine 4a (0.19 V vs Fc/Fcþ). We suspect
that the type II annulation stabilizes the oxidized compound
more than type III annulations, a phenomenon we also ob-
served in structurally related thiepin systems.23 In the case
of benzylazepine 4d, the first oxidation E11/2 has shifted to a
lower potential (0.03 V vs Fc/Fcþ) compared with 4a. We
suspect that this difference is due to a slight inductive electron-
donating effect of the benzyl group’s CH2 fragment.41 Re-
markably, all three of the second oxidations occur at almost the
same potential, an observation that is not fully understood at
this time.
Electropolymerization of Extended Azepines. Electropoly-
merizable azepine 7 was synthesized using a Stille cross-
coupling reaction (Scheme 1, v), and a CH2Cl2 solution of the
compound was exposed to swept potential conditions in the
presence of 0.1 M TBAPF6 as a supporting electrolyte. Despite
the prediction of extended conjugated thiophenes to undergo
electropolymerization, it was necessary to scan to relatively high
oxidation potentials (∼1.15 V vs Fc/Fcþ) in order to initiate
polymer deposition on the platinum electrode (Figure 4a). We
were not able to observe any film growth when we scanned at the
low potential range (up to 0.98 V vs Fc/Fcþ, Supporting
Information). For this particular system it is conceivable that
initially developed radicals and charges are localized in the azepine
moiety and are consequently not available for monomer-
monomer coupling reactions. When a third oxidation peak
was developed and the polymer started to be deposited, it
became clear that P7 has two distinct electroactive regions: the
Figure 3. CVs of azepines 2 (a), 4a (b), and 4d (c) measured on Pt
button electrodes in CH2Cl2 with 0.1 M TBAPF6 as a supporting
electrolyte. The red lines represent the first scans.
order to investigate the effect on redox properties of (a) the
regiochemistry of the two thiophenes (type II vs type III aze-
pines, 2 vs 4a) and (b) the substituent on the azepine nitrogen
(R0 =Ph vs R0 =Bn, 4a vs 4d). Using a standard three-elec-
trode apparatus in CH2Cl2 with 0.1 M TBAPF6 as a supporting
electrolyte, reproducible CVs were obtained under ambient
conditions (Figure 3). In all three cases the CVs showed two
quasi-reversible one-electron redox waves. Interestingly, we
observed significant differences in the potential at which the
first (E11/2 =0.03-0.25 V vs Fc/Fcþ) but not the second oxi-
dation (E21/2=∼0.53 V vs Fc/Fcþ). The half-wave potential for
the first redox couple (E11/2) of azepin 2 (0.25 V) was slightly

5236 Macromolecules, Vol. 43, No. 12, 2010
Song et al.
Figure 5. (a) CV of polymer P6b on a Pt button electrode (drop-cast from CHCl3 solution) in CH3CN with 0.1 M TBAPF6 as a supporting electrolyte
at a scan rate of100 mV/s. The red dotted line represents the first potential sweep. (b) Electronic absorption spectra of polymer P6b on ITO-coated glass
electrodes in CH3CN with 0.1 M TBAPF6 as a supporting electrolyte as a function of oxidation potential from 0.0 to 1.0 V vs Ag/Agþ.
two-electron redox region localized in the azepine moiety and
the pendant oligothiophene region. Subsequent analysis of scan
rate dependence on the deposited polymer thin films in mono-
mer-free CH2Cl2 electrolyte solution was demonstrated to be
almost linear up to a scan rate of 200 mV/s, indicating that once
P7 is formed it has reversible redox properties (Figure 4b).
In situ conductivity measurements revealed several intri-
guing features of P7 (Figure 4c). The drain current, which is
proportional to conductivity, has its first maximum at the
potential of the second oxidation. This bell-shaped conductivity
profile is commonly observed in segmented (partially con-
jugated) polymers18 and is indicative of a charge hopping
phenomenon, which in this case is highest when there are equal
number of singly and doubly oxidized azepine units. The latter
part of the profile is a result of the oligothiophene segments.
Hence, in the first oxidation, charges are strictly localized on
the azepine moieties of P7. At the second oxidation, the oligo-
thiophene portions start to be oxidized and the conductivity
increases again. Given the doubly charged nature of azepine
moieties, it is likely that at higher potentials the mobile carriers
are localized to the thiophene segments and that conduction is
dominated by interchain rather then intrachain transport.
Repeated measurements did not diminish the electrochemical
activity of P7 under the experimental conditions, highlighting
the stable nature of the azepine groups.
Chemical Synthesis, Redox Behavior, and Spectroelectro-
chemistry of Annulated Azepine Polymers. While electroche-
mical synthesis yielded material with promising electrical
properties, the resultant polymer proved difficult to manipulate
and characterize. Accordingly, we investigated various methods
of chemical polymerization in order to generate material suita-
ble ultimately for incorporation into devices. Using optimized
Yamamoto homopolymerization conditions, dichloride 5 was
converted into moderate molecular weight polymer P5 (Mn=
18.8 kDa, PDI=1.24) in 66% yield (Scheme 1, vi). We were also
able to convert 6b into conjugated polymer P6b (Mn=24.0 kDa,
PDI = 1.7) in 62% yield under Stille polymerization con-
ditions using 5,50-bis(trimethylstannyl)-2,20-bithiophene as a
comonomer (Scheme 1, vii). Following purification both poly-
mers were drop-cast onto Pt electrodes and investigated using
CV analysis in a 0.1 M CH3CN solution of TBAPF6. Under
these conditions polymer P5 displayed nonideal electroactivity
and appeared to become largely inactive after the first potential
cycle (see Supporting Information). Considering the high sta-
bility of the azepine unit, the loss of inactivity is likely due to a
loss of electrical contact with the working electrode. This
behavior may be a result of the larger strain imposed by having
every segment undergoing a large redox-induced geometric
change. In accord with this explanation, we observe two redox
couples in the CV of polymer P6b (Figure 5a) similar to the
electrochemical of parent compound 4a (Figure 3b). The first
oxidation peak for P6b was abnormally sharp, which implies
that the CH3CN did not swell the neutral polymer and thus
initially impeded the diffusion of ions and accompanying
solvent molecules. Once charged, the polymer is solvated by
the CH3CN, and ions diffused more readily into and out of the
polymer. After the initial electrochemical oxidation/reduction
cycle the CV adopted a more typical shape. Interestingly, during
the first scan the onset potential was higher and the integrated
current was larger when compared with successive scans. These
changes are likely related to the reorganization of the polymer’s
microstructure caused by the geometric changes and solvent
uptake/release during the initial potential cycle. The sharp
character of the first oxidation wave after the break in of the
film is consistent with the predicted geometric changes. Upon
reaching the initial oxidation, the strain associated with a
minority of azepine units adopting a planarized conformation
prevents oxidation. This inhibition to oxidation results in
increased current at higher potentials when the matrix relaxes.
To measure spectroelectrochemical properties, we drop-cast
azepine polymer P6b onto an indium-tin oxide (ITO) coated
glass electrode. Figure 5b illustrates the in situ measurements
of UV-vis spectra following increased oxidation levels in
CH3CN: the black line represents the absorption at 0 V, the
red line at 0.6 V, and the blue line at 1.0 V (all vs Ag/Agþ).
There are two regimes in the potential-dependent optical
spectra that reveal the development of subgap absorptions.
One is from 0 to 0.6 V and the other is from 0.6 to 1.0 V,
and these roughly correlate with the first and second azepine
oxidations, respectively. In the first regime, the original band-
gap absorption decreased and new absorption increased in the
subgap region. The new absorptions appear broad and fea-
tureless, implying that the polymer has a very delocalized
electronic structure. However, the absorptions are more dis-
tinct in the second oxidation regime. It is an interesting feature
that the original bandgap region retains a considerable amount
of absorption while most polythiophene derivatives show
considerable depletion of these absorptions on oxidation.42
We attempted to measure in situ conductivities from the
azepine-incorporated polymer P6b by drop-casting the poly-
mer on an interdigitated microelectrode. Unfortunately, we
were not able to detect any drain current signals. This was
most likely due to high contact resistance with the electrode
as an absolute electrical conductivity of 1.5 S/cm was mea-
sured using a four-point probe and a drop-cast film of
oxidized P6b (doping by saturation with iodine vapor).
In contrast, we were able to measure the in situ conducti-
vity of chemically polymerized P7, which was prepared by

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Macromolecules, Vol. 43, No. 12, 2010 5237
FeCl3-mediated polymerization (Scheme 1, viii). Although
P7 showed poor solubility in common organic solvents and
was thus intractable for conventional polymer characteriza-
tion, a drop-cast slurry of the polymer in CHCl3 onto an
interdigitated microelectrode produced a functional device.
In this case the CV and the in situ conductivity measured for
chemically prepared P7 (Supporting Information) have very
similar features (the reversible two electron redox sweep and
the bell-shaped conductivity curve) to the electrochemically
prepared material (Figure 4c). It was not possible to measure
the absolute electrical conductivity of the polymer using a
four-point probe as the material was not readily formulated
into a homogeneous film. We are currently pursuing free-
standing films of azepine-incorporated polymers for electro-
chemical actuation testing.
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In summary, we have designed and synthesized several structu-
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Buchwald-Hartwig coupling to fuse the seven-membered ring in
the key synthetic step. In contrast to previously reported thiepin
systems, dithieno[3,2-b:20,30-f]azepines were found to be very
stable even in highly oxidized states. Azepine polymers with a
molecular weight between 18 and 24 kDa were accessed by both
chemical and electrochemical means, and two of these materials
were proved to display redox stable behavior analogous to their
parent monomers in the solid state. Because of their high electro-
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Acknowledgment. We thank Dr. Peter Muller for collecting
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supported by the U.S. Army through the Institute for Soldier
Nanotechnologies under contract with the U.S. Army Research
Office. We are grateful to the European Union for the Marie
Curie International Outgoing Fellowship to D.B.W.
(36) DFT calculations also suggest that the one-electron oxidation of
azepines (4n - 1 state) gives rise to a slight expansion toward a
planar conformation, the degree of which is smaller than that of
two -electron oxidation (4n - 2 state). This conformational change
is probably due to the delocalization of the charge generated,
not the aromatic stabilization, and seems to be true for most
4n-electron conjugated cyclic systems, including thiepins and
[8]annulenes. Similar geometric rearrangements are found to occur
as well in simple N-functionalized 3-aminothiophenes. See: Heth,
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Supporting Information Available: Detailed experimental
procedures for the synthesis of all compounds; crystallographic
data for 6c; geometry optimizations (DFT calculations) of
annulated azepines; CVs of P5 in CH2Cl2 and CH3CN; CV
and in situ conductivity measured for chemically prepared P7;
1H and 13C NMR’s of all compounds. This material is available
free of charge via the Internet at http://pubs.acs.org.
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