Highly conductive poly(phenylene thienylene)s: M-phenylene linkages are not always bad

Article abstract of DOI:10.1021/ma0501702

Two isomeric polymers, which contain meta- or para-phenylene linkages between conducting segments, have been synthesized and compared by electrochemical methods. The nonconjugated poly-(1,5-diacetoxy-m-phenylene tetrathienylene) (PMPT-OAc) showed similar

Full text of DOI:10.1021/ma0501702

Macromolecules 2005, 38, 4569-4576
4569
Highly Conductive Poly(phenylene thienylene)s: m-Phenylene Linkages
Are Not Always Bad
Changsik Song and Timothy M. Swager*
Department of Chemistry, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139
Received January 26, 2005; Revised Manuscript Received March 28, 2005
ABSTRACT: Two isomeric polymers, which contain meta- or para-phenylene linkages between conducting
segments, have been synthesized and compared by electrochemical methods. The nonconjugated poly-
(1,5-diacetoxy-m-phenylene tetrathienylene) (PMPT-OAc) showed similar electroactivity to the para-
isomer, poly(2,5-diacetoxy-p-phenylene tetrathienylene) (PPPT-OAc), in the cyclic voltammetry and in-
situ conductivity measurements. Spectroelectrochemistry showed a similar buildup of sub-band-gap
electronic transitions. Deacetylation of the polymers was performed successfully by reaction with
hydrazine, producing PMPT-OH and PPPT-OH. Both phenol-substituted polymers exhibited greater
electroactivity than the acetoxy-substituted polymers. In addition, the phenol substituents were found
to lower the “turn-on” potential in the conductivity-potential profile for PMPT-OH, but not for PPPT-
OH. An alkoxy-substituted PMPT-OMe shows electroactivity similar to that of PMPT-OAc in the cyclic
voltammetry, in-situ conductivity, and spectroelectrochemistry.
Scheme 1
Introduction
The demand for highly stable, highly conductive, and
easily processable conjugated polymers in organic elec-
tronic and optoelectronic applications has led to exten-
sive studies over the past two decades.1 Major advances
have been realized through novel design or synthetic
modifications of conjugated polymers. For example,
soluble and thus processable polythiophenes have been
achieved by incorporating flexible subtsituents such as
alkyl chains at the 3-position.2-4 Polythiophenes also
have been electronically tuned by aid of electron-
donating and/or electron-withdrawing substituents.4,5
Another approach is to modify the backbones by hybrid-
izing thienylenes with other conjugated molecules, such
as various types of phenylenes.6,7 However, in the design
of new highly electron/hole conductive polymers, meta-
linkages between conducting segments are generally
excluded because they interrupt conjugation. In fact,
meta-linkages have usually been introduced in order to
reduce conjugation in a tunable manner, for example,
in synthesizing polymers with blue emission.8 It appears
to be widely accepted that meta-linkages in polymers
are something to be avoided if one wishes to produce
systems with high conductivity that is generally associ-
ated with delocalized carriers.
Our group recently reported a calix[4]arene-based
conducting polymer,9 which is not fully conjugated but
instead contains electroactive segments between in-
sulating bridges. In such a system, phenol groups were
found to play a crucial role in the electropolymerization
and the conduction pathway. The polymer also demon-
strated an intriguing proton-dopable property, in which
the segments were proposed to fluctuate between p-
dihydroquinone-like and p-diquinone-like states (similar
to what is shown for the materials of interest here in
Scheme 1). Considering those features, phenol function-
alities could be useful for designing conducting polymer
sensors.
mented (i.e., those have a continuous interconneted
π-system) conducting polymers. We proposed that, when
strategically located, phenols would endow a nonconju-
gated meta-linked polymer with electroactive and con-
ductive properties that are similar to, or even better
than, those of a related para-isomer. In this study, two
isomeric phenol containing polymers were prepared:
poly(m-phenylene tetrathienylene)s (PMPTs) and poly-
(p-phenylene tetrathienylene)s (PPPTs) (Scheme 1). In
PMPTs, when they are oxidized, it is plausible that the
charges are localized on the phenolic oxygens. Upon
oxidation, the initially nonconjugated backbone becomes
highly delocalized with both aromatic and quinoid
structures in the same chain, a scheme that has been
utilized in the design of small-band-gap polymers.10 In
this work, a series of PMPTs and PPPTs were synthe-
sized and characterized by several electrochemical
methods. The in-situ conductivities of two isomeric
polymers were compared by using interdigitated micro-
electrodes,11 and their electronic transitions were also
compared by UV-vis spectroelectrochemistry.
Our present study was motivated to investigate the
potential of phenols as functional moieties in nonseg-
* Corresponding author. E-mail: tswager@mit.edu.
10.1021/ma0501702 CCC: $30.25 © 2005 American Chemical Society
Published on Web 04/30/2005

4570 Song and Swager
Macromolecules, Vol. 38, No. 11, 2005
then washed with brine, dried over MgSO4, and evaporated
under reduced pressure. The resulting crude product was
purified by column chromatography (dichloromethane:hexane
Experimental Section
Instrumentation. NMR spectra were recorded on a Varian
Mercury-300, Bruker Advance-400, or Varian Inova-500 spec-
trometer. Chemical shifts are referenced to residual solvent
peaks. High-resolution mass spectra (HR-MS) were obtained
on a Bruker Daltonics APEX II 3T FT-ICR-MS. Electrochemi-
cal studies were carried out using an Autolab PGSTAT 10 or
PGSTAT 20 potentiostat (Eco Chemie) in a three-electrode cell
configuration consisting of a quasi-internal Ag wire reference
electrode (BioAnalytical Systems) submerged in 0.01 AgNO3/
0.1 M tetrabutylammonium hexafluorophosphate (TBAPF6) in
anhydrous CH3CN, a Pt button (1.6 mm in diameter), 5 µm
interdigitated Pt micro, ITO-coated glass (100 Ω sheet resis-
tance), or Au-coated poly(ethylene terephthalate) (PETE)
electrode as the working electrode, and a Pt coil or Pt gauze
as the counter electrode. The ferrocene/ferrocenium (Fc/Fc+)
redox couple was used as a reference. Half-wave potentials of
Fc/Fc+ were observed between 210 and 245 mV vs Ag/Ag+ in
CH2Cl2 and 80 mV in CH3CN. For the in-situ conductivity
measurements, polymer films were electrochemically deposited
on 5 µm interdigitated Pt microelectrodes and placed in a
monomer-free solution. Drain current measurements were
typically carried out at a 5 mV/s scan rate with a 40 mV offset
potential between the two working electrodes. The conductivity
was then calculated from the value of the drain current by
applying geometrical factors11 and also corrected with a known
material, poly(3-methylthiophene), at 60 S/cm. The polymer
film thickness for the conductivity calculation was measured
with a surface profilometer (Veeco Dektak 6M Stylus Profiler).
The baseline (A) of a bare interdigitated microelectrode was
first obtained. Next, the surface profile (B) of a given polymer
film on the electrode was measured. Several values of A and
B were taken to get averages of each. The thickness was
determined by subtracting the averaged value of A from the
averaged value of B. Absorption spectra for spectroelectro-
chemistry were obtained using a HP 8453 diode array spec-
trometer. FT-IR spectra of polymer films were taken using
Nicolet 8700 FT-IR spectrometer with a fixed 30° specular
reflectance accessory.
Materials. Spectroscopic grade CH2Cl2 and CH3CN were
purchased from Aldrich for electrochemistry. TBAPF6 was
recrystallized in ethanol prior to use. 4,6-Diiodobenzene-1,3-
diol,12 2,5-diiodobenzene-1,4-diol,13 and 5-tributylstannyl-2,2′-
bithiophene14 were prepared by literature methods. Anhydrous
DMF was purchased from Aldrich as Sure-Seal Bottles and
used as received. All other chemicals were of reagent grade
and used as received.
1,5-Diacetoxy-2,4-diiodobenzene (1b). In a 100 mL
round-bottom flask equipped with a stir bar were combined
4,6-diiodobenzene-1,3-diol (1.08 g, 3 mmol), acetic anhydride
(1.42 mL, 15 mmol), and 5 mL of pyridine. After being stirred
overnight at room temperature, the mixture was poured into
water and extracted with diethyl ether. The organic layer was
washed with brine, dried over MgSO4, and evaporated under
reduced pressure. The resulting crude product was purified
by column chromatography (ethyl acetate:hexane 1:1). Yield:
1.30 g of white solid (97%). 1H NMR (300 MHz, CDCl3) δ: 8.26
(s, 1H), 6.97 (s, 1H), 2.36 (s, 6H). 13C NMR (100 MHz, CDCl3)
δ: 168.1, 152.2, 147.7, 118.1, 88.1, 21.4. HR-MS (ESI): calcd
for C10H8I2O4 [M + Na]+, 468.8404; found, 468.8383.
1
1:15). Yield: 2.30 g of white solid (97%). H NMR (400 MHz,
CDCl3) δ: 8.04 (s, 1H), 6.40 (s, 1H), 1.07 (s, 18H), 0.28 (s, 12H).
13C NMR (100 MHz, CDCl3) δ: 156.4, 147.1, 109.5, 81.3, 26.0,
18.6, -3.8. HR-MS (ESI): calcd for C18H32I2O2Si2 [M + Na]+,
612.9922; found, 612.9895.
1,4-Bis(tert-butyldimethylsilanyloxy)-2,5-diiodoben-
zene. Similar to the synthesis of 1c except using 2,5-diiodo-
1
benzene-1,4-diol. Yield: 2.34 g of white solid (99%). H NMR
(400 MHz, CDCl3) δ: 7.18 (s, 2H), 1.05 (s, 18H), 0.26 (s, 12H).
13C NMR (100 MHz, CDCl3) δ: 150.4, 128.0, 89.7, 26.1, 18.5,
-3.9. HR-MS (ESI): calcd for C18H32I2O2Si2 [M + Na]+,
612.9922; found, 612.9904.
1,5-Dimethoxy-2,4-diiodobenzene (1d). In a 100 mL
round-bottom flask equipped with a stir bar were combined
4,6-diiodobenzene-1,3-diol (1.09 g, 3 mmol), K2CO3 (4.15 g, 30
mmol), and 30 mL of DMF under Ar. Iodomethane (0.936 mL,
15 mmol) was slowly added to the mixture, and it was stirred
for 6 h at room temperature. The mixture was diluted with
ethyl acetate, and the organic layer was washed with water
and brine, dried over MgSO4, and evaporated under reduced
pressure. The resulting crude product was filtered through a
pad of silica gel, eluding with dichloromethane. The solvent
was evaporated under reduced pressure, and the resulting
solid was further purified by recrystallization (dichloromethane,
hexane). Yield: 1.07 g of white solid (91%). 1H NMR (400 MHz,
CD2Cl2) δ: 8.03 (s, 1H), 6.40 (s, 1H), 3.88 (s, 6H). 13C NMR
(100 MHz, CD2Cl2) δ: 160.3, 147.3, 96.4, 75.5, 57.0. HR-MS
(ESI): calcd for C8H8I2O2 [M + Na]+, 412.8506; found, 412.8522.
1,5-Diacetoxy-2,4-bis(2,2′-bithiophen-5-yl)benzene (2b).
In a Schlenk tube equipped with a stir bar were combined 1b
(0.446 g, 1 mmol), Pd2(dba)3 (31 mg, 3 mol %), P(t-Bu3) (24
mg, 6.6 mol %), and 10 mL of toluene under Ar. To the mixture
was added 5-tributylstannyl-2,2′-bithiophene (1.37 g, 3 mmol)
and stirred for 48 h at 80 °C. The reaction mixture was then
cooled to room temperature, diluted with ethyl acetate, and
filtered through a pad of silica gel. The silica gel was
thoroughly washed with ethyl acetate, and the solvent was
evaporated under reduced pressure. The crude solid was
washed with diethyl ether and then further purified by
recrystallization (dichloromethane, hexane). Yield: 0.262 g of
1
pale yellow solid (50%). H NMR (300 MHz, CD2Cl2) δ: 7.91
(s, 1H), 7.32 (d, 2H, J ) 3.8 Hz), 7.29 (dd, 2H, J ) 5.3, 1.1
Hz), 7.25 (dd, 2H, J ) 3.6, 1.1 Hz), 7.21 (d, 2H, J ) 3.8 Hz),
7.07 (s, 1H), 7.06 (dd, 2H, J ) 5.3, 3.6 Hz), 2.36 (s, 6H). 13C
NMR (100 MHz, CD2Cl2) δ: 169.3, 146.6, 139.0, 137.4, 136.4,
129.5, 128.5, 127.7, 126.0, 125.4, 124.55, 124.52, 119.4, 21.8.
HR-MS (ESI): calcd for C26H18O4S4 [M + Na]+, 544.9980;
found, 544.9999.
1,5-Bis(tert-butyldimethylsilanyloxy)-2,4-bis(2,2′-bi-
thiophen-5-yl)benzene (2c). In a Schlenk tube equipped
with a stir bar were combined 1c (1.18 g, 2 mmol), Pd2(dba)3
(104 mg, 5 mol %), P(t-Bu3) (81 mg, 0.4 mmol), and 15 mL of
toluene under Ar. To the mixture was added 5-tributylstannyl-
2,2′-bithiophene (2.73 g, 6 mmol) and stirred for 48 h at 80
°C. The reaction mixture was then cooled to room temperature,
diluted with ethyl acetate, and filtered through a pad of silica
gel. The silica gel was thoroughly washed with ethyl acetate,
and the solvent was evaporated under reduced pressure. The
crude solid was washed with cold hexane and then fur-
ther purified by recrystallization (dichloromethane, hexane).
Yield: 0.872 g of yellow solid (65%). 1H NMR (400 MHz, CDCl3)
δ: 7.72 (s, 1H), 7.25 (d, 2H, J ) 3.6 Hz), 7.21 (dd, 2H, J ) 5.2,
1.2 Hz), 7.18 (dd, 2H, J ) 3.6, 1.2 Hz), 7.15 (d, 2H, J ) 3.6
Hz), 7.04 (dd, 2H, J ) 5.2, 3.6 Hz), 6.53 (s, 1H), 1.00 (s, 18H),
0.27 (s, 12H). 13C NMR (100 MHz, CDCl3) δ: 152.4, 138.7,
138.2, 136.4, 129.4, 128.0, 125.8, 124.1, 123.6, 123.2, 119.6,
111.5, 26.2, 18.8, -3.6. HR-MS (ESI): calcd for C34H42O2S4Si2
[M + H]+, 667.1679; found, 667.1683.
1,4-Diacetoxy-2,5-diiodobenzene. Similar to the synthe-
sis of 1b except using 2,5-diiodobenzene-1,4-diol. Yield: 1.09
1
g of white solid (82%). H NMR (400 MHz, CDCl3) δ: 7.53 (s,
2H), 2.36 (s, 6H). 13C NMR (100 MHz, CDCl3) δ: 168.4, 149.6,
132.7, 90.2, 21.3. HR-MS (ESI): calcd for C10H8I2O4 [M + Na]+,
468.8404; found, 468.8416.
1,5-Bis(tert-butyldimethylsilanyloxy)-2,4-diiodoben-
zene (1c). In a 100 mL round-bottom flask equipped with a
stir bar were combined 4,6-diiodobenzene-1,3-diol (1.45 g, 4
mmol), TBDMSCl (1.8 g, 12 mmol), and 20 mL of anhydrous
DMF under Ar. After being stirred for 10 min, imidazole (1.38
g, 20 mmol) was added to the mixture, and it was stirred
overnight at room temperature. The mixture was poured into
water and extracted with diethyl ether. The organic layer was
1,5-Dimethoxy-2,4-bis(2,2′-bithiophen-5-yl)benzene (2d).
In a Schlenk tube equipped with a stir bar were combined 1d
(0.390 g, 1 mmol), Pd2(dba)3 (31 mg, 3 mol %), P(t-Bu3) (13
mg, 6.6 mol %), LiCl (170 mg, 4 mmol), and 10 mL of DMF

Macromolecules, Vol. 38, No. 11, 2005
Highly Conductive Poly(phenylene thienylene)s 4571
7.05 (dd, 2H, J ) 5.2, 3.6 Hz), 1.03 (s, 18H), 0.27 (s, 12H). 13
NMR (100 MHz, CDCl3) δ: 146.4, 138.6, 138.0, 137.2. 128.1,
126.4, 124.8, 124.4, 123.8, 123.5, 119.6, 26.3, 18.8, -3.5. HR-
MS (ESI): calcd for C34H42O2S4Si2 [M + H]+, 667.1679; found,
667.1706.
under Ar. To the mixture was added 5-tributylstannyl-2,2′-
bithiophene (1.37 g, 3 mmol) and stirred overnight at room
temperature. The reaction mixture was diluted with dichlo-
romethane and filtered through a pad of silica gel. Most of the
solvent was evaporated under reduced pressure. The residue
was diluted with ethyl acetate, washed with brine (×2), dried
over MgSO4, and evaporated under reduced pressure. The
resulting residue was precipitated with hexane, and the crude
solid was washed with copious amounts of hexane. The product
was further purified by recrystallization (dichloromethane,
C
Results and Discussion
Monomer Synthesis. The syntheses of the mono-
mers are outlined in Scheme 2. Attempts to synthesize
the meta-linked monomer 2a by Stille coupling between
4,6-diiodoresorcinol and 5-tri-n-butylstannyl-2,2′-bithio-
phene failed. Subsequently, the phenol groups were
protected with acetyl or TBDMS groups. Although pro-
tected monomers 2b,c could be obtained by Stille
coupling in good to high yields, deprotection to 2a was
complicated by its instability to air, giving black in-
tractable products. Therefore, removal of the protecting
group was carried out in postpolymerization modifica-
tion. For comparison, an O-alkyl version 2d, a nonsub-
stituted version 3, and para-linked isomers 4b,c were
synthesized under similar conditions.
1
hexane). Yield: 0.347 g of yellow solid (74%). H NMR (400
MHz, CD2Cl2) δ: 7.89 (s, 1H), 7.38 (d, 2H, J ) 3.8 Hz), 7.23-
7.25 (m, 4H), 7.18 (d, 2H, J ) 3.8 Hz), 7.05 (dd, 2H, J ) 5.1,
3.7 Hz), 6.64 (s, 1H), 4.00 (s, 6H). 13C NMR (100 MHz, CD2-
Cl2) δ: 156.7, 138.6, 138.2, 136.7, 128.4, 127.7, 125.6, 124.6,
124.0, 123.7, 116.4, 96.7, 56.4. HR-MS (ESI): calcd for
C24H18O2S4 [M]+, 466.0184; found, 466.0176.
1,3-Bis(2,2′-bithiophen-5-yl)benzene (3). In a Schlenk
tube equipped with a stir bar were combined 1,3-diiodobenzene
(0.337 g, 1 mmol), PdCl2(PPh3)2 (37 mg, 5 mol %), and 10 mL
of toluene under Ar. To the mixture was added 5-tributylstan-
nyl-2,2′-bithiophene (1.37 g, 3 mmol) and stirred overnight at
80 °C. The reaction mixture was then cooled to room temper-
ature, diluted with dichloromethane, and filtered through a
pad of silica gel. The solvent was evaporated under reduced
pressure, and the resulting residue was precipitated with
hexane. The crude solid was washed with hexane and fur-
ther purified by recrystallization (dichloromethane, hexane).
Yield: 0.350 g of bright yellow solid (86%). 1H NMR (400 MHz,
CD2Cl2) δ: 7.83 (pseudo-t, 1H, J ) 1.7 Hz), 7.54 (dd, 2H, J )
7.7, 1.7 Hz), 7.41 (t, 1H, J ) 7.7 Hz), 7.33 (d, 2H, J ) 3.8 Hz),
7.27 (dd, 2H, J ) 5.1, 1.1 Hz), 7.25 (dd, 2H, J ) 3.7, 1.1 Hz),
7.20 (d, 2H, J ) 3.8 Hz), 7.06 (dd, 2H, J ) 5.1, 3.7 Hz). 13C
NMR (100 MHz, CD2Cl2) δ: 142.9, 137.7, 137.6, 135.2, 130.1,
128.5, 125.2, 125.1, 124.8, 124.3, 123.0. HR-MS (ESI): calcd
for C22H14S4 [M + H]+, 407.0051; found, 407.0042.
Electropolymerization.15 All monomers were
polymerized via oxidative coupling under swept poten-
tial conditions. Electropolymerizations were performed
in CH2Cl2 solutions containing ca. 2 mM of monomers
and 0.1 M of TBAPF6 as a supporting electrolyte. In
most cases, CH2Cl2 proved to be a good solvent for
electropolymerization. However, when monomers were
too soluble in CH2Cl2 (2c or 4c) or when the working
electrode had a large surface area (ITO-coated glass
electrode or gold-coated PETE), a significant amount of
the resulting polymer failed to be deposited onto the
electrode. This could be attributed to the solubility of
initially coupled oligomers. This effect was especially
evident when O-TBDMS versions (2c or 4c) were
polymerized in CH2Cl2. It was observed that oxidized
oligomers diffused away from the electrode surface. This
problem was overcome by adding CH3CN, a poor sol-
vent. Depending on the solubility of each monomer, a
1:1 to 1:3 mixture of CH2Cl2:CH3CN was used for
electropolymerizations.
Figure 1 shows the cyclic voltammograms (CVs)
during the polymerization of 2b and 4b. The initial
oxidation of monomer 2b occurred at a potential slightly
higher than 4b, reflecting the nonconjugated nature of
the meta-linkage. For both materials, all subsequent
scans displayed much lower potential oxidation onsets,
thereby indicating that electroactive polymer had de-
posited onto the electrode. The oxidation peak potential
of poly(2b) (PMPT-OAc) and poly(4b) (PPPT-OAc) were
minimally different. Table 1 summarizes the electro-
chemical results for monomers (2b-d, 3, 4b,c) and their
polymers. As expected, the electron-donating effect of
methoxy groups results in a lower oxidation potential
for monomer 2d. The O-TBDMS versions 2c and 4c also
oxidized at low potentials for the same reason.
1,4-Diacetoxy-2,5-bis(2,2′-bithiophen-5-yl)benzene (4b).
In a Schlenk tube equipped with a stir bar were combined 1,4-
diacetoxy-2,5-diiodobenzene (0.446 g, 1 mmol), PdCl2(PPh3)2
(37 mg, 5 mol %), and 10 mL of toluene under Ar. To the
mixture was added 5-tributylstannyl-2,2′-bithiophene (1.37 g,
3 mmol) and stirred overnight at 80 °C. The product precipi-
tated as the reaction progressed. The reaction mixture was
then cooled to room temperature. The product was filtered out
and washed thoroughly with diethyl ether. The crude product
was then redissolved with dichloromethane and passed through
a pad of silica gel. The silica gel was thoroughly washed with
dichloromethane, and the solvent was evaporated under
reduced pressure. The resulting solid was then further purified
by recrystallization (dichloromethane, hexane). Yield: 0.429
1
g of pale yellow solid (82%). H NMR (500 MHz, THF-d8) δ:
7.60 (s, 2H), 7.43 (d, 2H, J ) 3.8 Hz), 7.37 (m, 2H), 7.29 (m,
2H), 7.23 (d, 2H, J ) 3.8 Hz), 7.04 (dd, 2H, J ) 5.0, 3.5 Hz),
2.38 (s, 6H). 13C NMR (125 MHz, THF-d8) δ: 169.0, 145.5,
139.7, 138.0, 136.9, 128.9, 128.2, 127.6, 126.0, 124.92, 124.86,
124.1, 21.5. HR-MS (ESI): calcd for C26H18O4S4 [M + Na]+,
544.9980; found, 544.9992.
1,4-Bis(tert-butyldimethylsilanyloxy)-2,5-bis(2,2′-bi-
thiophen-5-yl)benzene (4c). In a Schlenk tube equipped
with a stir bar were combined 1,4-bis(tert-butyldimethylsila-
nyloxy)-2,5-diiodobenzene (0.177 g, 0.3 mmol), PdCl2(PPh3)2
(11 mg, 5 mol %), and 3 mL of toluene under Ar. To the mixture
was added 5-tributylstannyl-2,2′-bithiophene (0.341 g, 0.75
mmol) and stirred for 20 h at 80 °C. The product precipitated
as the reaction progressed. The reaction mixture was then
cooled to room temperature. The product was filtered out and
washed thoroughly with ethyl acetate. The crude product was
then redissolved with dichloromethane and passed through a
pad of silica gel. The silica gel was thoroughly washed with
dichloromethane, and the solvent was evaporated under
reduced pressure. The resulting solid was then further purified
by recrystallization (dichloromethane, hexane). Yield: 0.194
g of pale yellow solid (97%). 1H NMR (400 MHz, CDCl3) δ: 7.32
(d, 2H, J ) 3.8 Hz), 7.23 (dd, 2H, J ) 5.2, 1.0 Hz), 7.21 (dd,
2H, J ) 3.6, 1.0 Hz), 7.16 (d, 2H, J ) 3.8 Hz), 7.15 (s, 2H),
Deprotection To Give Free -OH Groups. PMPT-
OAc and PPPT-OAc were chosen to generate free -OH
versions, PMPT-OH and PPPT-OH, respectively, be-
cause the acetyl groups can be easily removed by
addition of hydrazine. As-grown polymer films on
several types of electrodes were exposed to hydrazine
vapor, under which the films immediately became
crimson in color. After ca. 15 min, the films were washed
with copious amounts of MeOH and then with CH2Cl2.
Figures 2 and 3 show the FT-IR and UV-vis spectra,
respectively, of films before and after exposure to
hydrazine. It is clearly observed that the strong CdO

4572 Song and Swager
Macromolecules, Vol. 38, No. 11, 2005
Scheme 2a
a
(i) Acetic anhydride, pyridine, RT. (1b); TBDMSCl, imidazole, DMF, RT. (1c); MeI, K2CO3, DMF, RT. (1d) (ii) 5-Tri-n-
butylstannyl-2,2′-bithiophene, Pd cat., toluene or DMF, 80 °C or RT. (iii) Electropolymerization under swept potential conditions
in CH2Cl2 containing ca. 2 mM of monomers and 0.1 M TBAPF6 as a supporting electrolyte. (iv) Hydrazine, RT.
Table 1. Electrochemical Results: Monomer Oxidation
Potentials (Ea,m) and Polymer Oxidation/Reduction
Potentials (Ea,p/Ec,p
)
monomer
Ea,m (V)a
>0.8
0.57
Ea,p (V)
Ec,p (V)
2bb
2cc
2dd
3b
0.41, 0.68
0.12, >1.0
0.37, 0.61
0.05, 0.33
0.14
0.47
0.81
0.69
0.48
0.01, 0.41
>1.0
-0.10
4bb
4cc
0.20, 0.36, 0.72
0.74
-0.18, 0.18
0.41
a All potentials measured vs Fc/Fc+ at scan rate 100 mV/s.
b Performed in CH2Cl2. c Performed in a mixed solvent of CH2Cl2
and CH3CN. d Performed in CH3CN.
exception of the first oxidation peak. Interestingly for
PPPT-OAc, we observed a well-resolved low-potential
redox couple, similar to those reported for polythiophenes
with long alkoxy substituents.5,6 We also observed that
this redox couple was more pronounced at low scan rates
and thin films. This is most evident for the reduction
cycle of PPPT-OAc, which suggests that it is more
sensitive to ion diffusion or that a structural relaxation
has occurred in the oxidized state. However, after the
first oxidation, the electrochemistry of both O-Ac poly-
mers became similar, which suggests that the lone pairs
of the acetoxy groups in PMPT-OAc are capable of
contributing to the delocalization suggested in Scheme
1.
Deprotection of the acetyl groups changed the elec-
trochemistry of the polymers dramatically, especially for
PMPT-OAc. For PPPT-OH we observed a broader elec-
troactivity and a loss of the low-potential redox activity
shown for PPPT-OAc. The oxidation onset potential of
PPPT-OH remained approximately the same as that of
PPPT-OAc. In contrast, the oxidation onset potential of
PMPT-OH shifted to lower potential, and the polymer’s
Figure 1. Electropolymerization (100 mV/s) of 2b (left) and
4b (right) on Pt button electrodes in CH2Cl2 with 0.1 M
TBAPF6 as a supporting electrolyte. Dotted lines represent the
first scan.
vibration disappeared after exposure to hydrazine. The
weaker than expected C-H and O-H stretching vibra-
tions in spectra C and D in Figure 2 are a result of the
thin nature of the films grown from PPPT-OAc and
PPPT-OH, which results in broad and weak signals. The
hydrazine reaction did not significantly affect the UV-
vis absorption (Figure 3); hence, we conclude the acetyl
groups were successfully removed to leave free -OH
without degradation of the polymer structure.
Polymer Electrochemistry Comparisons of Meta
vs Para. Figure 4 compares the CVs of the PMPTs and
the PPPTs. We observed two broad, poorly resolved
oxidation and reduction peaks in both PMPT-OAc and
PPPT-OAc, and their CVs were quite similar, with the

Macromolecules, Vol. 38, No. 11, 2005
Highly Conductive Poly(phenylene thienylene)s 4573
Figure 2. Specular reflectance FT-IR spectra of PMPT-OH (B) and PPPT-OH (D). The strong CdO stretchings (arrows) of PMPT-
OAc (A) and PPPT-OAc (C) disappeared when exposed to hydrazine. The spectra were measured from films deposited onto gold-
coated PETE.
measurements of those polymers were performed in a
glovebox under a nitrogen atmosphere.
Meta vs Para in-Situ Conductivity Measure-
ment. It is not surprising, considering the similarity of
the CVs, that we find the conductivity of the meta-linked
PMPT-OAc is of the same order of magnitude as the
para-linked PPPT-OAc (Figure 5). In the potential-
conductivity profile of PMPT-OAc, the conductivity
increased rapidly from ca. 0 V and reached a plateau
above ca. 0.3 V (vs Fc/Fc+). The conductivity profile of
PPPT-OAc behaved in a similar manner. This conduc-
tivity plateau results from the limit of an interchain
charge hopping process.16,17 As in the case of poly-
thiophenes, the interchain charge transport in PMPTs
and PPPTs operates due to the proximity of the polymer
backbones, providing multichannel electronic connectiv-
ity. It would appear that the meta-linkages do not
hamper the conduction pathway in this system.
The shapes of potential-conductivity profile of the
-OH versions, PMPT-OH and PPPT-OH, resembled
their acetate versions (Figure 6). In PPPT-OH, the drain
current, which is proportional to conductivity, “turns on”
at almost the same potential as PPPT-OAc. In contrast,
the drain current onset is at a much lower potential for
PMPT-OH. As mentioned above, the free -OH appears
to facilitate the formation of extended p-diquinone
states.
Meta vs Para Spectroelectrochemistry. UV-vis
spectroelectrochemical studies further reveal that meta-
linkages produce delocalized electronic structures when
the polymers are oxidized. Figure 7 shows in-situ
measurements of the UV-vis absorption of polymers
Figure 3. UV-vis spectra of PMPT-OH (left; solid line) and
PPPT-OH (right; solid line). Dotted lines represent UV-vis
absorption of PMPT-OAc (left) and PPPT-OAc (right). The
spectra were measured from films deposited onto ITO-coated
glass.
CV was much broader than the acetate derivative. This
latter feature is attributed to the formation of extended
p-diquinone states that are more favored in the depro-
tected form because of the superior ability of the -OH
group to stabilize positive charges. Although the free
-OH subsituents seemed to improve the polymers’
electroactivity, oxidized PMPT-OH and PPPT-OH were
not stable under ambient conditions. All electrochemical

4574 Song and Swager
Macromolecules, Vol. 38, No. 11, 2005
Figure 5. CVs (dotted line) and in-situ conductivity measure-
ments (5 mV/s, offset potential of 40 mV; solid line) of PMPT-
OAc (top) and PPPT-OAc (bottom) on 5 µm interdigitated Pt
microelectrodes in CH2Cl2 with 0.1 M TBAPF6 as a supporting
electrolyte.
Figure 4. (top) CVs of PMPT-OAc (left) and PPPT-OAc (right)
at different scan rates: (a) 200, (b) 100, (c) 50, (d) 25, and (e)
10 mV/s. (bottom) CVs of PMPT-OH (left) and PPPT-OH
(right). Dotted line represents the CV before exposure to
hydrazine. All measurements were carried out in CH2Cl2 with
0.1 M TBAPF6 onto Pt button electrodes.
deposited onto ITO-coated glass electrodes. Because of
the instability of the oxidized PMPT-OH and PPPT-OH,
only the acetate versions were measured. The UV-vis
spectra of PPPT-OAc show a decrease of the original
band-gap transition and the buildup of intragap energy
states, which appear very similar to those observed for
alkoxy-substituted poly(phenylene bisthienylene)s.6 This
matches well to polaron-bipolaron model18 of charge-
delocalized π-platforms. It should be noted that even in
the nonconjugated PMPT-OAc similar electronic states
develop and more delocalized energy states build up at
lower applied potentials.
Substituent Effects in PMPTs. To further inves-
tigate the electronic properties of these systems, an
alkoxy-substituted derivative, PMPT-OMe, and a non-
substituted derivative, PMPT-H, were studied for com-
parison (Figure 8). As the scan rate increased, the peak
potentials of PMPT-OMe shifted minimally as shown
in Figure 8 (left), while a significant shift of anodic and
cathodic peak potentials was observed in PMPT-H. We
observed two oxidation and reduction peaks in the CV
of PMPT-OMe similar to that discussed for PMPT-OAc
(Figure 4). The oxidation in PMPT-H occurred at a
higher potential than PMPT-OMe due to the absence
of charge-stabilizing substituents, and peak potentials
shifted significantly at fast scan rates. At higher po-
tentials (>1.0 V vs Fc/Fc+), PMPT-H was unstable, and
hence we were unable to show a peak in the CV. When
PMPT-OMe and PMPT-H are compared to PMPT-OH,
Figure 6. Drain current profiles (5 mV/s, offset potential of
40 mV; solid line) of PMPT-OH and PPPT-OH on 5 µm
interdigitated Pt microelectrodes in CH2Cl2 with 0.1 M TBAPF6
as a supporting electrolyte. The absolute conductivity is
proportional to drain current.11 For comparison, drain current
profiles of PMPT-OAc (left, dotted line) and PPPT-OAc (right,
dotted line) are also plotted.
it is apparent that phenol functionalities enhance the
electroactivity of the polymer.
Figure 9 shows the in-situ conductivity measurements
of the PMPT-OMe and the PMPT-H. The conductivity-
potential profile of PMPT-OMe resembles that of PMPT-
OAc and appears to be limited by the interchain charge
hopping. However, in PMPT-H, the onset is shifted to

Macromolecules, Vol. 38, No. 11, 2005
Highly Conductive Poly(phenylene thienylene)s 4575
Figure 7. Electronic absorption spectra of PMPT-OAc (top)
and PPPT-OAc (bottom) on ITO-coated glass electrodes in CH2-
Cl2 with 0.1 M TBAPF6 as a supporting electrolyte. The UV-
vis spectra are plotted as a function of oxidation potential from
0.0 V (b) to 1.0 V (9) vs Ag/Ag+ (0.01 M).
Figure 9. CVs (dotted line) and in-situ conductivity measure-
ments (5 mV/s, offset potential of 40 mV; solid line) of PMPT-
OMe (top) and PMPT-H (bottom) on 5 µm interdigitated Pt
microelectrodes in CH2Cl2 with 0.1 M TBAPF6 as a supporting
electrolyte.
Figure 8. CVs of PMPT-OMe (left) and PMPT-H (right) in
CH3CN and CH2Cl2, respectively, with 0.1 M TBAPF6 as a
supporting electrolyte at different scan rates: (a) 200, (b) 100,
(c) 50, (d) 25, and (e) 10 mV/s.
the significantly higher potential, and the conductivity-
potential profile passes a peak at ca. 0.55 V and then
increases again, finally reaching a plateau at above 0.8
V (vs Fc/Fc+). Although there might be a different
regime at low potential, or at a low doping level, we
suspect that the conductivity of PMPT-H is also gov-
erned by the interchain charge hopping at a high doping
level. The maximum conductivity of PMPT-OMe and
PMPT-OAc was consistently determined to be ap-
proximately 2-fold greater than that of PMPT-H.
Spectroelectrochemical studies (Figure 10) reveal
approximately the same buildup of energy states as the
doping level increases. The UV-vis absorption spectra
of PMPT-OMe and PMPT-OAc are similar at the similar
oxidation level. Here again, we observed the stabilizing
effect of alkoxy functionalities on positively doped states.
In PMPT-H, a similar but retarded development of
intergap transitions was observed. In addition, the
Figure 10. Electronic absorption spectra of PMPT-OMe (top)
and PMPT-H (bottom) on ITO-coated glass electrodes in CH2-
Cl2 with 0.1 M TBAPF6 as a supporting electrolyte, as a
function of oxidation potential from 0.0 V (b) to 1.0 V (9) vs
Ag/Ag+ (0.01 M).
shape of intergap transitions of PMPT-H was different
from that of PMPT-OMe or PMPT-OAc (Figure 7, top).

4576 Song and Swager
Macromolecules, Vol. 38, No. 11, 2005
(5) Zotti, G.; Gallazzi, M. C.; Zerbi, G.; Meille, S. V. Synth. Met.
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We suspect that without the alkoxy or phenolic substit-
uents delocalized energy states would not be observed
in PMPTs at a moderate doping level.
(6) Child, A. D.; Sankaran, B.; Larmat, F.; Reynolds, J. R.
Macromolecules 1995, 28, 6571.
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Conclusion
We have found that meta-linked polymers can be
rendered as electroactive as para-linked isomers by
strategic positioning of phenolic substituents. The maxi-
mum conductivities of both meta- and para-linked
polymers in in-situ measurements were similar to each
other but appeared to be limited by an interchain charge
transport. Spectroelectrochemical studies demonstrated
that there were very delocalized energy states in both
systems when they were oxidized. Free -OH substitu-
ents showed a marked effect on the meta-linked polymer
over that of the acetoxy or methoxy substituents,
resulting in much lower onset potential in a conductiv-
ity-potential profile and broad electroactivity of the
polymer.
(9) Yu, H.-h.; Xu, B.; Swager, T. M. J. Am. Chem. Soc. 2003,
125, 1142.
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W.-C.; Liu, C.-L.; Yen, C.-T.; Tsai, F.-C.; Tonzola, C. J.; Olson,
N.; Jenekhe, S. A. Macromolecules 2004, 37, 5959.
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Acknowledgment. We are thankful for funding
(13) Zhou, Q.; Swager, T. M. J. Am. Chem. Soc. 1995, 117, 12593.
(14) Zhu, S. S.; Swager, T. M. J. Am. Chem. Soc. 1997, 119, 12568.
(15) Chemical polymerization with FeCl3 was attempted but
produced insoluble materials that could not be characterized.
(16) Electroactive Polymer Electrochemistry; Lyons, M. E. G., Ed.;
Plenum: New York, 1994.
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from DARPA and the Office of Naval Research.
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