Synthesis of π-Bridged Dually-Dopable Conjugated Polymers from Benzimidazole and Fluorene: Separating Sterics from Electronics

Article abstract of DOI:10.1021/acs.macromol.5b01174

We describe the synthesis and characterization of three new alternating copolymers containing fluorene and a dually dopable benzimidazole moiety. Poly(2-n-heptyl-benzimidazole-alt-9,9-di-n-octylfluorene) (PBIF), poly(2-n-heptyl-benzimidazole-vinylene-9,9-

Full text of DOI:10.1021/acs.macromol.5b01174

Article
pubs.acs.org/Macromolecules
Synthesis of π‑Bridged Dually-Dopable Conjugated Polymers from
Benzimidazole and Fluorene: Separating Sterics from Electronics
Jared D. Harris, Jiakai Liu, and Kenneth R. Carter*
Department of Polymer Science and Engineering, University of Massachusetts − Amherst, Conte Center for Polymer Research, 120
Governors Drive, Amherst, Massachusetts 01003, United States
*
S Supporting Information
ABSTRACT: We describe the synthesis and characterization
of three new alternating copolymers containing fluorene and a
dually dopable benzimidazole moiety. Poly(2-n-heptyl-benzi-
midazole-alt-9,9-di-n-octylfluorene) (PBIF), poly(2-n-heptyl-
benzimidazole-vinylene-9,9-di-n-octylfluorene) (PBIF-VL),
and poly(2-n-heptyl-benzimidazole-ethynylene-9,9-di-n-octyl-
fluorene) (PBIF-EL) were synthesized from Suzuki, Heck,
and Sonogashira cross-coupling reactions in reasonable yield.
The materials were characterized through ultraviolet−visible
spectroscopy, photoluminescence, and cyclic voltammetry. The
vinyl and ethynyl bridges in PBIF-VL and PBIF-EL were
incorporated to separate benzimidazole and fluorene units while retaining conjugation. This allowed us to differentiate between
steric and electronic contributions to the band gap (E ) changes that occur upon acid/base doping of these materials. We
g
demonstrate that the blue shift arising from acid doping PBIF is due to steric torsion, while the red shift found upon base doping
PBIF is due to both sterics and possibly an electronic effect. These findings are supported through the use of molecular
modeling.
INTRODUCTION
polyionomers’ stability and reversibility (from polyionomer to
neutral parent polymer) was demonstrated for PBIF. We
■
7
The potential for electronic devices made from relatively
inexpensive semiconducting polymers has inspired a large
research field (e.g., thin film transistors, photovoltaics, light
hypothesized that acid-doping would increase (lower) the
system’s electron affinity (LUMO) while base-doping would
decrease (raise) the ionization potential (HOMO). This
hypothesis came about by drawing simplified analogies between
our system and the inorganic Group IV semiconductors, where
p-doping effectively creates holes below the conduction band
1
−3
emitting diodes, etc.);
however, in comparison with their
traditional inorganic counterparts, polymeric semiconductors
suffer from a number of issues. These include polycrystalline to
amorphous morphologies complicating charge transport, poor
oxidative stability, higher batch-to-batch variability, generally
unipolar transport, and the difficulty of synthesizing a single
material that can selectively facilitate n- or p-channel trans-
(
∼LUMO), and n-doping adds electrons to the system in
electronic states above the valence band (∼HOMO). The
phenomena predicted by this modelwith respect to E are
g
8
1
,4−6
correct for the base-doped systems. Ranger et al. observed a
port.
In an attempt to address this final drawback, we have
decrease in optical E in base-doped poly(fluorene)s, while
g
chosen to investigate a class of conjugated polymers (CPs) that
fundamentally differs from the bulk of current research efforts.
We believe that the next generation of CPs will be based on
materials that can be systematically doped into either hole or
electron-transporting states.
Yamamoto and coworkers have observed similar phenomena in
9
base-doped poly(benzimidazole-alt-aryleneethynylene)s, poly-
1
0,11
(
2-alkylbenzimidazole-alt-thiophene)s,
and poly(2-heptyl-
12
benzimidazole-4,7-diyl). We demonstrated that this decrease
originates from an elevated HOMO in PBIF; however, in the
7
We recently reintroduced the possibility of dually dopable
conjugated polymers based on the benzimidazole (BI) moiety
acid-doping regime, E decreases have been observed in only
g
one of these systems, a poly(arylene-ethynylene-benzimida-
polymerized from the four and seven positions with fluorene
9
7
zole) copolymer.
(
PBIF). It is important to note that the doping as described
As found in our previous work, as well as others, the failure of
herein does not involve electronic reduction or oxidation of the
polymer. Charges originate from N−H bond cleavage or
formation at the imidazole nitrogen site(s). Although influence
of charge carrier flavor has not yet been demonstrated, this
system unveiled tremendous potential for manipulation of band
structures through simple postpolymerization acid/base chem-
istry to generate charged CPs (polyionomers). Additionally, the
acid-doping to decrease E is attributed to out-of-plane twisting
g
arising from the protonation of the BI moieties (Figure
Received: May 31, 2015
Revised: September 3, 2015
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.macromol.5b01174
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Macromolecules
Article
7
,11−13
1).
Thus, we hypothesized that vinyl or ethynyl spacers
9,9-di-n-octylfluorene (4), and 2,7-diiodo-9,9-di-n-octylfluorene (5)
are described in the Supporting Information.
between the BI and fluorene units would alleviate this strain
2
,7-Divinyl-9,9-di-n-octylfluorene (6) was synthesized from 2,7-
diiodo-9,9-dioctylfluorene via the Stille route. 5 (0.6417 g, 0.999
mmol) was added to a cylindrical 50 mL Schlenk tube with a stir bar.
Pd(PPh3)4 (56.6 mg, 0.049 mmol) was then added under inert
conditions in a glovebox. Four mL of anhydrous, deoxygenated DMF
was then injected into the septum-sealed Schlenk flask with a syringe.
Tri-n-butyl(vinyl) tin (0.65 mL, 2.22 mmol) was then injected into the
reaction flask. The flask was purged of O by bubbling N through the
2
2
stirred mixture for 20 min. After 20 min the flask was kept under N2
and placed in a preheated oil bath (80 °C) for 45 min. After 45 min,
the black reaction mixture was allowed to cool to room temperature. It
was then poured into 50 mL of DI H O and extracted with 3 × 30 mL
2
ethyl acetate. The organic fractions were collected and washed with 3
×
30 mL H O, dried over MgSO , and gravity filtered. The resultant
pale-green volume was removed on a rotary evaporator. The oily green
2
4
residue was redissolved in DCM and absorbed to ∼2 g SiO before
2
purification by column chromatography using SiO as the medium and
2
hexanes as the eluent, resulting in a clear viscous oil that solidified
upon extended drying in vacuo and storing in a freezer (57.4%).
Furthermore, the product can be dissolved in a minimal amount of
hexanes and precipitated from methanol at −77 °C to yield white
Figure 1. Trimers of PBIF indicating the growing potential for
torsional strain as the system is increasingly protonated.
1
solids. H NMR (500 MHz, CDCl , Figure S7):δ 7.62 (d, 2H, J = 7.8
3
Hz, Ar−H), 7.39 (dd, 2H, J = 1.3, 7.9, Ar−H), 7.35 (s, 2H, Ar−H),
6
.80 (dd, 2H, J = 10.9, 17.6, internal vinyl), 5.80 (d, 2H, J = 17.10,
terminal vinyl), 5.26 (d, 2H, J = 10.9, terminal vinyl), 1.95 (m, 4H,
CH ), 1.00−1.22 (br, 20H, CH ), 0.81 (t, 6H, J = 7.2, CH ), 0.61 (br,
2 3
37.7, 136.7, 125.6, 120.8, 120.0, 113.4, 55.2, 40.7, 32.1, 30.4, 29.6,
and allow for monitoring of E narrowing through acid- and
g
2
2
3
base-doping. This hypothesis was supported by quantum-
chemical calculations using density functional theory (DFT)
13
4
1
H, CH ). C NMR (125 MHz, CDCl , Figure S8):δ 151.7, 141.1,
14
performed with the Gaussian 09 program suite. To this end
we have synthesized a set of three polymers: poly(2-n-heptyl-
benzimidazole-alt-9,9-di-n-octylfluorene) (PBIF), poly(2-n-
heptyl-benzimidazole-vinylene-9,9-di-n-octylfluorene) (PBIF-
VL), and poly(2-n-heptyl-benzimidazole-ethynylene-9,9-di-n-
octylfluorene) (PBIF-EL). Polymeric band structures were
determined experimentally using CV and UV−vis. Additionally,
the effects of chemical doping were quantified through optical
spectroscopies.
29.5, 24.0, 22.9, 14.4.
2,7-Diethynyl-9,9-di-n-octylfluorene (7) was synthesized as follows.
4 (0.573 g, 1.04 mmol), triphenylphosphine (recrystallized from
methanol; 0.017 g, 0.065 mmol), and stir bar were added to a 50 mL
two-necked flask. The flask was then loaded into an argon atmosphere
glovebox where Pd(PPh ) Cl (0.022 g, 0.031 mmol) and copper(I)
3
2
2
iodide (0.004 g, 0.021 mmol) were added. The flask was then sealed
with a septum and a condenser fitted with a septum. The flask was
removed from the glovebox and N was used to constantly flush the
2
system. Triethylamine (10 mL) and ethynyl trimethylsilane (1.2 mL,
8
.68 mmol) were added via syringe. The mixture was stirred and
EXPERIMENTAL SECTION
deoxygenated with N for 20 min. After 20 min, N was used to
■
2
2
Materials and Methods. All chemicals, solvents, and reagents
were used as received without further purification unless otherwise
noted. All materials were purchased from typical commercial suppliers.
Instrumentation. NMR spectra were recorded on either a Bruker
Ascend 500 (500 MHz) or a Bruker Avance 400 (400 MHz).
Chemical shifts were determined relative to residual peaks in the
blanket the mixture as it was heated to 50 °C for 20 h. The mixture
was allowed to cool to room temperature, and the volatiles were
removed under reduced pressure. The residue was dissolved/
suspended in diethyl ether and washed with 2 × 30 mL of DI water
and 2 × 30 mL of brine. The organic phase was dried over MgSO
the solvent removed resulting in an orange oil. The product was
purified via column chromatography using SiO as the separation
media and hexanes as eluent. The TMS-protected product was
and
4
15
deuterated solvent. NMR spectra are given in the Supporting
Information (Figures S1−S13). GPC was performed at 40 °C at a flow
rate of 1.0 mL/min using an Agilent 1260 series system equipped with
a refractive index (RI) detector, PL Gel 5 μm guard column, two 5 μm
analytical Mixed-C columns, and a 5 μm analytical Mixed-D column
2
1
obtained as a yellow oil. H NMR (400 MHz, CDCl
= 7.8 Hz, Ar−H), 7.49 (dd, 2H, J = 1.3, 7.8, Ar−H), 7.47 (s, 2H, Ar−
H), 1.94 (m, 4H, CH ), 1.31−0.99 (br, 20H, CH ), 0.83 (t, 6H, J = 7.1
Hz, CH ), 0.57 (br, 4H, CH ), 0.30 (s, 18H, Si(CH ). The product,
):δ 7.63 (d, 2H, J
3
2
2
(
Agilent) with THF as the eluent. UV−vis absorption in DMF
3
2
)
3 3
solution and solid-state were performed on a Cary 50 UV−vis
absorption spectrometer with 1 cm path-length quartz cuvettes or
quartz plates. Photoluminescence from solutions in DMF was
measured with a Cary Eclipse. Photoluminescence from thin films
was measured with a PerkinElmer LS-50B. Cyclic voltammetry was
carried out with a Bioanalytical Systems EC Epsilon potentiostat in an
(7), was obtained by stirring the oil with 2 mL of 1 M
tetrabutylammonium fluoride (in THF) at 80 °C in a 25 mL
septum-capped round-bottomed flask. After 24 h, the volatiles were
removed and the residue was dissolved in dichloromethane, washed
with water, and dried over MgSO
rotary evaporation and the resultant dark orange oil was purified on a
SiO column using hexanes as the eluent to afford 0.401 g of pale
orange oil (87.6%). H NMR (400 MHz, CDCl
. Dichloromethane was removed by
4
electrolyte solution of 0.1 M TBAPF in dry acetonitrile. A 3 mm
diameter glassy carbon work electrode (Bioanalytical Systems) was
2
6
1
, Figure S9):δ 7.63 (d,
3
employed alongside a platinum wire counter electrode (Bioanalytical
2H, J = 7.8 Hz, Ar−H), 7.49 (dd, 2H, J = 1.3, 7.8, Ar−H), 7.47 (s, 2H,
+
Systems) and a Ag/Ag reference electrode (Ag in 0.1 M AgCl
Ar−H), 3.15 (s, 2H, CH), 1.94 (m, 4H, CH ), 1.31−0.99 (br, 20H,
2
−1
13
solution, Bioanalytical Systems). All sweeps were done at 200 mV s
CH ), 0.83 (t, 6H, J = 7.1 Hz, CH ), 0.57 (br, 4H, CH ). C NMR
2
3
2
with a 2000 mV switching potential. Molecular modeling was
performed using the Gaussian 09 suite of programs.
(100 MHz, CHCl , Figure S10):δ 151.2, 141.1, 131.4, 126.7, 121.0,
3
14
120.1, 84.7, 77.5, 55.4, 40.4, 31.9, 30.1, 29.3, 23.8, 22.7, 14.2.
Poly(2-n-heptyl-benzimidazole-alt-9,9-di-n-octylfluorene) (PBIF)
was synthesized via Suzuki style polymerization. 1 (0.1871 g, 0.500
mmol), 9,9-di-n-octylfluorene-2,7-diboronic-acid bis(pinacol) ester
Synthesis. 4,7-Dibromo-1H-2-n-heptyl-benzo[d]imidazole (1) and
2
,7-diiodofluorene (3) were synthesized in accordance with the
7
,16
literature.
The syntheses of 2,7-dibromofluorene (2), 2,7-dibromo-
B
DOI: 10.1021/acs.macromol.5b01174
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Macromolecules
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a
Scheme 1. Syntheses of Monomers Used for Polymerization
a
Reported yields are for isolated products. i. Br /HBr; ii. NaBH , EtOH; iii. octanal, (NH ) Ce(NO ) , H O , acetonitrile, microwave; iv. Br , acetic
2
4
4
2
3
4
2
2
2
acid; v. I , HIO , H SO , CCl , acetic acid; vi. 1-bromooctane, 1:1 toluene/50 wt % NaOH ; vii. tri-n-butyl(vinyl) tin, Pd(PPh ) , DMF; viii.
2
3
2
4
4
(aq)
3 4
ethynyl trimethylsilane, Pd(PPh ) Cl , PPh , CuI, TEA; ix. tetrabutylammonium fluoride, THF.
3
2
2
3
(
0.3213 g, 0.500 mmol), Aliquat 336 (4 drops), and a stir bar were
1.01 (br, 34H), 0.89 (m, 3H), 0.79 (m, 6H). GPC (PS standards,
−1
−1
added to a 25 mL two-necked round-bottomed flask. Pd(PPh ) (49.1
CHCl portion) M : 9200 g mol ; M : 11 800 g mol Đ: 1.28.
3
4
3
n
w
mg, 0.042 mmol) was added to the flask under an inert atmosphere in
a glovebox. The flask was fitted with septa and a condenser in the
glovebox. Upon removal from the glovebox, deoxygenated toluene (10
Poly(2-n-heptyl-benzimidazole-ethynylene-9,9-di-n-octylfluorene)
(PBIF-EL) was synthesized from 1 and 7 via a Sonogashira style
polymerization. 1 (0.1719 g, 0.459 mmol), 7 (0.2016 g, 0.459 mmol),
mL) and 2 M Na CO
(2.0 mL) were added via syringes. The
PPh
bar were added to a 25 mL two-necked round-bottomed flask.
Pd(PPh ) Cl (17.0 mg, 0.0242 mmol) and CuI (6.0 mg, 0.0315
3
(recrystallized from methanol, 12.8 mg, 0.0488 mmol), and a stir
2
3(aq)
mixture was then stirred as it was purged with bubbling N for 20 min.
2
After 20 min, N was used to blanket the reaction as it was heated to
3
2
2
2
mmol) were added to the flask under inert conditions in a glovebox.
The flask was then fitted with septa and a condenser before removing
from the glovebox. Ten mL of a deoxygenated 2:1 v/v mixture of
toluene/triethylamine was injected into the flask via syringe. The
1
00 °C for 48 h. A small portion of bromobenzene was then dissolved
in ∼2 mL of deoxygenated toluene and injected into the reaction as a
polymer end-capping agent. After an additional 2 h, a small portion
phenyl boronic acid was similarly dissolved in ∼2 mL deoxygenated
toluene and injected into the reaction mixture. This was allowed to
react for 2 h before precipitating the polymer into ∼400 mL of stirred
acidic (∼2 to 3 drops of concentrated HCl) methanol (0 °C). The
precipitate was isolated by vacuum filtration and further purified by
sequential Soxhlet extraction with methanol, hexanes, and CHCl . H
NMR (500 MHz, CDCl , Figure S11):δ 9.13 (s, 1H), 8.28 (bs, 1H)
mixture was further deoxygenated by bubbling N for 20 min with
2
stirring. After 20 min, N was used to blanket the reaction as it was
2
heated to 90 °C for 48 h. A small portion of bromobenzene was then
dissolved in ∼2 mL of deoxygenated toluene and injected into the
reaction as a polymer end-capping agent. After an additional 2 h, a
small portion of ethynyl benzene was similarly dissolved in ∼2 mL of
deoxygenated toluene and injected into the reaction mixture. This was
allowed to react for 2 h before precipitating the polymer into ∼400 mL
of stirred acidic (∼2 to 3 drops concentrated HCl) methanol (0 °C).
The precipitate was isolated by vacuum filtration and further purified
1
3
3
7.98 (m, 3H), 7.66 (m, 3H), 7.48 (bs, 1H), 3.00 (m, 2H), 2.10 (m,
4H), 1.93 (m, 2H), 1.54−1.06 (br, 34H), 0.92 (m, 3H, J = 6.1 Hz),
0.82 (t, 6H, J = 6.3 Hz). GPC (PS standards, CHCl portion) M :
1
3
n
−1
−1
0 200 g mol ; M : 13 800 g mol Đ: 1.34.
w
by sequential Soxhlet extraction with methanol, hexanes, and CHCl .
3
Poly(2-n-heptyl-benzimidazole-vinylene-9,9-di-n-octylfluorene)
1
H NMR (500 MHz, CDCl , Figure S13): δ 9.79 (bs, 1H), 7.51 (m,
3
(PBIF-VL) was synthesized from 1 and 6 using a Heck style
8
0
8
H), 2.98 (br, 2H), 1.95 (br, 4H), 1.86 (br, 2H), 1.44−0.95 (br, 28H),
polymerization. 1 (0.1594 g, 0.426 mmol), 6 (0.1887 g, 0.426 mmol),
and a stir bar were added to a 25 mL Schlenk tube. The tube was then
transferred to a dry, inert glovebox where Pd(OAc) (5.9 mg, 0.026
.81 (m, 9H), 0.58 (br, 4H). GPC (PS standards, CHCl portion) M :
3
n
−1
−1
200 g mol ; M : 11 400 g mol Đ: 1.38.
w
2
mmol) and P(o-tol) (recrystallized from methanol, 40.8 mg, 0.134
3
RESULTS AND DISCUSSION
Synthesis. The benzimidazole monomer was synthesized in
a previously discussed three-step route (Scheme 1).
The fluorene monomers were synthesized following
literature procedures (Scheme 1).
mmol) were added. The flask was sealed under inert conditions with a
septum and removed from the glovebox. Deoxygenated 5:2 DMF/
TEA (v:v, 7 mL) was added to the reaction flask via syringe. The flask
■
7
,17
was then deoxygenated further by bubbling N through the mixture for
2
16,18
2
0 min. N was used to blanket the reaction as it was heated to 90 °C
2
First, fluorene was
for 48 h. A small portion of bromobenzene was then dissolved in ∼2
mL of deoxygenated DMF and injected into the reaction as a polymer
end-capping agent. After an additional 2 h, a small portion of styrene
was similarly dissolved in ∼2 mL of deoxygenated DMF and injected
into the reaction mixture. This was allowed to react for 2 h before
precipitating the polymer into ∼400 mL of stirred acidic (∼2 to 3
drops concentrated HCl) methanol (0 °C). The precipitate was
brominated in the presence of Br and acetic acid. The product
2
(
2) was readily precipitated and recrystallized in high yield. 2
−
and 3 were alkylated through a S 2 reaction, where OH
N
deprotonates at the 9 position, leaving a highly nucleophilic
carbon, which subsequently attacks the electrophilic carbon 1 of
1
-bromooctane. The product could be purified by flash column
chromatography using hexanes as the eluent or recrystallization.
We found the column to be challenging considering the small
isolated by vacuum filtration and further purified by sequential Soxhlet
1
extraction with methanol, hexanes, and CHCl . H NMR (500 MHz,
3
C D O, Figure S12): δ 10.85 (s, 1H), 8.17 (m, 2H), 7.75−7.37 (m,
ΔR between the product and the monoalkylated defect. These
f
4
8
6
H), 5.38 (m, 2H), 2.97 (m, 2H), 2.10 (m, 4H), 1.95 (m, 2H), 1.58−
spots often overlapped when monitoring fractions with TLC,
C
DOI: 10.1021/acs.macromol.5b01174
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a
Scheme 2. Synthesis of PBIF, PBIF-VL, and PBIF-EL
a
i. Pd(PPh ) , Na CO , toluene; ii. Pd(OAc) , P(o-tol) 5:2 DMF/TEA; iii. Pd(PPh ) Cl , PPh , CuI, 1:1 toluene/TEA.
3
4
2
3(aq)
2
3
3
2
2
3
and thus if conversion was high enough, recrystallization was
often easier.
products due to the high level of separation between interacting
benzoidal protons in comparison with PBIF. We expected the
vinyl protons of PBIF-VL to obstruct planarity to a lesser
degree than the benzoidal protons of PBIF, placing its expected
torsion angles in-between that of PBIF and PBIF-EL. Density
functional theory (DFT), using the B3LYP functional and the
6-31G(d,p) basis set, was employed to determine the torsion
angles of the lowest energy confirmations on a set of trimers T,
T-VL, and T-EL (Figures 2 and 3, Tables 2 and 3) in the spirit
of PBIF, PBIF-VL, and PBIF-EL. Charged trimers are denoted
as either T(+) or T(−) to represent acid- and base-doped
products. The alkyl substituents were substituted with methyl
groups to reduce computational costs.
Results from the modeling support our hypotheses when
comparing the neutral trimers. The torsion angles are most
severe in T on the N−H side of the benzimidazole moiety
(42.92 and 32.00°). This indicates that the N−H proton does
interact with the neighboring fluorene protons, inducing
torsion. (It is important to note that the model does not
account for tautomerization between the two N sites, which is
known to be rapid in solution based on the unification of
aromatic proton resonances in the benzimidazole monomer).
For the π-spaced derivatives, we chose to report the torsion
angles between the BI ring and the π bridge (carbons 3, 4, 9,
and 10) in addition to the angles between the BI and fluorene
rings. In T-VL, there is noticeable relaxation of torsion angles,
particularly on the Schiff base side of BI where the ring torsion
is only 1.45° and the BI-vinyl torsion is 0.98°. The N−H side of
BI remained relatively strained with a high BI-vinyl torsion
angle (18.55°); the vinyl proton on carbon 10 further pushes
the fluorene out of the BI plane elevating the ring torsion to
2
,7-Divinyl-9,9-di-n-octylfluorene (6) was synthesized first
19
using a Stille coupling with 2,7-dibromo-9,9-di-n-octylfluor-
ene (4), but yields were lower than expected (∼50%). Thus, a
route employing 2,7-diiodo-9,9-di-n-octylfluorene (5) was
employed, but yields improved minimally. 2,7-Diethynyl-9,9-
di-n-octylfluorene (7) was synthesized in two steps; first, the
Sonogashira reaction was used to couple 2,7-dibromo-9,9-di-n-
octylfluorene (4) to ethynyltrimethylsilane in high yields. This
product was easily purified by column chromatography, then
quantitatively deprotected by tetrabutylammonium fluoride.
The polymers PBIF, PBIF-VL, and PBIF-EL were readily
synthesized using the appropriate Pd(0)-catalyzed coupling
reactions (Scheme 2). The products were purified by
precipitation and Soxhlet extraction. The chloroform fraction
was used for further experiments. GPC against PS standards in
THF indicates respectable molecular weights were achieved for
each product (Table 1). DSC was used in an attempt to define
Table 1. Summary of Synthetic Data for PBIF, PBIF-VL, and
PBIF-EL
a
M /M (kg/mol)
xn/xw
Đ
yield (%)
n
w
PBIF
hexanes
8.4/14.4
14/24
17/23
1.72
1.34
54.1
21.3
chloroform
PBIF-VL
hexanes
10.2/13.8
3.7/4.6
5/7
1.29
1.28
63.3
20.7
chloroform
PBIF-EL
hexanes
9.2/11.8
14/18
6.6/9.3
10/14
13/18
1.41
1.38
26.4
35.7
3
0.61°. The torsion angles found in T-EL are effectively zero,
chloroform
8.2/11.4
indicating sufficient spacing between BI and fluorene.
Upon deprotonation, torsion angles were considerably
reduced for both T and T-VL. This indicates that steric
a
Molar masses of PBIF, PBIF-VL, and PBIF-EL product fractions as
determined by GPC against PS standards.
relaxation may play a role in the E reduction observed in the
g
a T for all of the products, but no discernible transition could
be identified. This may be due to the modest molecular weight
of the products.
Molecular Modeling. Molecular modeling was used to
predict the torsion angles between benzimidazole and fluorene
moieties for the polymer products. We hypothesized that
PBIF-EL would have the lowest torsion angles of the three
base-doped regime of PBIF and PBIF-VL; however, in the case
of T-EL, the torsion angles are essentially unfazed by
deprotonation, implying that sterics play little role in the Eg
decrease observed by doping PBIF-EL (Table 4). Similarly,
protonation of benzimidazole caused an increase in torsion
angle for both T and T-VL but effectively no change in T-EL.
The protonation is felt more strongly by T-VL, where the BI-
g
D
DOI: 10.1021/acs.macromol.5b01174
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Figure 2. Trimers depicting the modeled structures T, T-VL, and T-EL.
fluorene ring torsion angle increases 32.83° (on the newly
protonated side of the ring) compared with a 12.19° increase
for T.
Approaching these results from the viewpoint that all spectral
changes induced by doping arise from steric origins, one would
thin solid film. Similar conclusions may be drawn about PBIF-
EL, considering the relatively large shift in λem going from
solution to solid-state photoluminescence (52 nm, see Figure
S14). This results in a sky-blue solution appearing green in
the solid state.
20
expect PBIF and PBIF-VL to experience decreases in E upon
The doping of polymer solutions was achievable in both
DMAc and DMF; these solvents provide sufficient solvation for
both the neutral and poly(ionomeric) products. Doping by
exposing solid thin films to acidic vapor was avoided for this
particular study due to the relatively low control in such a
setup. The quantity of polymer and dopant concentration can
be more accurately known in a solution-doped system than
vapor-doping. Acid doped products were obtained by the
addition of 1 M TFA (in DMF) in 10 μL increments to
g
base-doping and increases in E upon acid-doping. Meanwhile,
g
PBIF-EL would not experience any changes in its band
structure; however, as discussed later, this is not what is
observed.
Optical Spectroscopies. Beginning with the UV−vis of
each neutral parent polymer, it is easy to observe the increase in
conjugation length experienced by the π-bridged PBIF-VL and
PBIF-EL. These two products’ λmax values are shifted 48 and 9
nm, respectively, bathochromically in DMF solution relative to
their unbridged counterpart, PBIF (Figure 4). Additionally, the
π-bridged products’ λmax values and onsets shift to lower
energies when spin-coated into a thin film on quartz. (The
polymers’ solubility in DMF was too low, and thus films were
spun from 1:1 THF/toluene solutions.) PBIF shows no
significant difference between solution and solid state
absorbance, impling a relatively high level of disorder in the
solid state; however, both PBIF-VL and PBIF-EL display
bathochromic shifts in the solid state, 433 → 453 nm and 393
polymer solutions (∼2−4 μg/mL) under N (these progessions
2
may be viewed in the SI, Figures S15−S17). Figure 5A shows
the neutral (solid) and fully acid-doped (dash, judged by UV−
vis and PL) UV−vis spectra of all three products. As
anticipated, PBIF shifted hypsochromically upon doping into
PBIF(+). This observation matches our previous studies on a
7
similar product. As the molecular modeling discussed above
suggests, this is likely due to steric torsion, resulting in a
decreased effective conjugation length. Surprisingly, PBIF-VL
and PBIF-EL display almost no change in band edge or λmax
.
→
407 nm λmaxes, respectively (Figure 4). The bathochromic
By our simple theory previously described, we expected a
shift in PBIF-VL is likely due to aggregation effects seen across
many rigid rod conjugated polymers. Aggregation tends to lead
toward planarization of the backbone, extending the π
conjugation length. PBIF-VL also shows considerable red-
shifting between solution and thin film photoluminescence
narrowing of the E due to an increase in electron affinity for
g
these products considering they are relatively free from steric
considerations. The experimental results can be explained
through the use of donor−acceptor theory wherein a system’s
HOMO can be approximated by the donor’s HOMO, while its
2
(Figure S14). This results in a green solution forming a yellow
LUMO may be estimated by acceptor’s LUMO. Here
E
DOI: 10.1021/acs.macromol.5b01174
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a
Table 2. Torsion Angles between BI and Fluorene for T
φ1
φ2
T(−)
T
18.01
32.00
44.19
18.03
42.92
44.19
T(+)
a
Determined using DFT at the B3LYP level with the 6-31G(d,p) basis
set.
Table 3. Torsion Angles between Marked Carbon Atoms in
a
T-VL and T-EL
φ1
,2−5,6
φ3,4−5,6
1.48
φ7,8−9,10
1.46
φ7,8−11,12
4.09
T-VL(−)
T-VL
3.78
1.45
34.28
0.13
0.01
0.22
0.98
18.55
20.29
0.94
30.61
29.64
0.15
T-VL(+)
T-EL(−)
T-EL
20.28
0.83
0.03
0.01
0.04
T-EL(+)
1.75
1.32
0.23
a
Determined using DFT at the B3LYP level with the 6-31G(d,p) basis
set.
Table 4. Spectral Data for Each of the Products and Their
Acid- (+) and Base- (−) Doped Analogs
a
a
a
λmax (nm)
λonset (nm)
optical E (eV)
g
PBIF
384
368
443
449
431
488
393
394
419
424
411
481
505
503
550
443
444
481
2.92
3.02
2.58
2.45
2.46
2.25
2.80
2.79
2.58
PBIF(+)
PBIF(−)
PBIF-VL
PBIF-VL(+)
PBIF-VL(−)
PBIF-EL
PBIF-EL (+)
PBIF-EL(−)
a
Measured in dilute (2−4 μg/mL) anhydrous DMF.
Figure 4. Solution (dashed) and solid-state (solid) normalized UV−
vis absorption spectra of PBIF (black), PBIF-VL (red), and PBIF-EL
(blue). All solution spectra are from dilute DMF solutions. Films were
spin-cast from 5 mg/mL solutions in 1:1 tetrahydrofuran/toluene.
Figure 3. Optimized (DFT/B3LYP/6-31G(d,p)) structures of T, T-
VL, and T-EL and their doped forms.
calculations employing an adapted Su−Schreiffer−Heeger
π-bridged products shows little change in the fundamental
emission modes (Figure S18).
2
1,22
Hamiltonian
indicate that the LUMO is largely controlled
by fluorene, while the HOMO is dominated by the BI moiety
Figure S19). Thus, increasing the electron affinity of the BI
unit does little to effect the system’s overall LUMO. PL of the
As expected, E values were reduced for all three products in
their poly(anionic), base-doped states (Figure 5B, Table 4).
These spectra were obtained in the same fashion as previously
g
(
F
DOI: 10.1021/acs.macromol.5b01174
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Macromolecules
Article
Figure 5. (A) Normalized solution UV−vis spectra of neutral (solid) and acid-doped (dashed) PBIF (black), PBIF-VL (red), and PBIF-EL (blue)
in DMF. (B) Normalized solution UV−vis spectra of neutral (solid) and base-doped (dashed) PBIF (black), PBIF-VL (red), and PBIF-EL (blue) in
DMF.
described for the acid-doped materials through the addition of a
50 mM NaOEt solution (these progessions may be viewed in
the SI, Figures S15−S17). We observed 0.34, 0.20, and 0.22 eV
reductions in E for PBIF, PBIF-VL, and PBIF-EL, respectively
g
These decreases are significant and represent a powerful
postpolymerization modification of the systems’ E . The
g
bathochromic shifts in the π-bridged products very likely
have little influence from steric relaxation. The reduction in Eg
is notably lower in these systems (∼0.1 eV). We suggest that
this discrepancy roughly quantifies the reduction in E going
g
from PBIF to PBIF(−) due to steric relaxation. We ascribe the
remaining ∼0.2 eV as being due to an electronic effect
stemming from the increased electron density on the imidazole
moiety.
Base doping also results in significant changes to the PL
spectra of each product. PBIF(−) emits green light at ∼73%
the intensity of PBIF with major transitions occurring at 478,
Figure 6. Representative voltammograms of PBIF (black), PBIF-VL
red), and PBIF-EL (blue) thin films on a glassy carbon electrode (3
mm diameter). Each sweep was performed at 200 mV/s, and its
(
current normalized to 1 at its switching potential (2 V).
505, and 551 nm. PBIF-VL(−) also retains vibronic structure,
albeit at only ∼56% the intensity of PBIF-VL, with major
modes at 542 and 582 nm giving a yellow solution. PBIF-
EL(−) suffers from considerable quenching (∼26% the
intensity of PBIF-EL) but retains the characteristic red shift
with its λem at 478 nm. This results in a faintly green solution.
Electrochemistry. CV was used to determine the oxidation
potential of polymeric thin films drop cast from dichloro-
methane directly onto a 3 mm glassy carbon electrode. CV was
attempted for the doped species, but reliable sweeps were not
attainable. Each polymer showed multiple oxidation peaks;
however, only PBIF displayed any reversibility with a small
reduction peak observable on the return sweep. From the onset
potentials, Eonset, the EHOMO was determined for each material
based on an external calibration of the Ag/AgCl electrode with
the ferrocene/ferrocenium redox couple (assumed to be −4.8
Table 5. Summary of Band Structure Data
optical E
E
HOMO
b
LUMOc
g
onset
b
a
(
eV)
(V)
(−eV)
(−eV)
PBIF
2.86
2.36
2.71
1.23
0.84
1.39
5.59
5.20
5.75
2.73
2.84
3.04
PBIF-VL
PBIF-EL
a
Determined by the absorption onset of thin films spun onto quartz
b
plates. An average of at least three distinct films measured by CV
against a Ag/AgCl ref electrode and externally calibrated against the
c
ferrocene/ferrocenium redox couple. Estimated by the addition of the
optical E to the HOMO measured by CV.
g
more promising candidate as a p-channel material while
5
retaining oxidative stability. The electron-withdrawing nature
23
of PBIF-EL’s sp hybridized carbons is likely the cause for its
deeper EHOMO. ELUMO was estimated by adding each materials’
eV relative to vacuum). The following equation was used
⎛
⎞
⎟
eV
J ⎠
optical E
g
to their EHOMO.
EHOMO = [(Eonset − E1/2fc) + 4.8](−q)⎜
⎝
CONCLUSIONS
■
where Eonset is the onset potential of the analyte, E1/2 fc is the
measured half-wave potential of the ferrocene/ferrocenium
redox couple, q is the elementary charge, and eV/J is a
conversion factor. The reported Eonset values are the average of
at least three distinct films to mitigate the variability of CV.
EHOMO was determined to be −5.5 , −5.2 , and −5.7 eV for
Through the synthesis and acid/base doping of PBIF-VL and
PBIF-EL we have confirmed that the spectral effects observed
from the protonic doping of benzimidazole containing
conjugated polymers arise from a combination of steric and
electronic effects. The effect of acid doping for this particular
system appears to be of a strictly steric nature due to the
fluorene unit’s relatively low-lying LUMO. Base doping of
9
0
5
PBIF, PBIF-VL, and PBIF-EL, respectively (Figure 6, Table
). The measured value for PBIF agrees well with the
5
PBIF leads to a 0.34 eV reduction in E ; we believe that ∼0.1
g
7
previously reported value (−5.54 eV) and implies good
oxidative stability. The shallower E of PBIF-VL makes it a
eV of that reduction is due to steric relaxation considering each
of the π-spaced polymers demonstrated a ∼0.2 eV decrease in
HOMO
G
DOI: 10.1021/acs.macromol.5b01174
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Macromolecules
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Eg upon base doping. These findings suggest that the electronic
structure of benzimidazole containing polymers can be easily
tailored through simple postpolymerization methods leading to
highly functional semiconducting materials. Future studies
involving the preparation and characterization of stable, doped
thin films are underway.
N.; Millam, J. M.; Klene, M.; Knox, J. E.; Cross, J. B.; Bakken, V.;
Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.;
Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Martin, R. L.;
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ASSOCIATED CONTENT
Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.macro-
mol.5b01174.
■
62, 7512−7515.
*
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(
1H and 13C NMR spectra, PL spectra of the neutral
polymers in DMF solution and thin films, UV−vis and
PL spectra detailing the incremental doping of each
material, PL spectra comparing the neutral materials
directly to their doped analogs, and a density of states
diagram, as predicted by aSSH theory are available free of
charge via the Internet. (PDF)
(
K. R. J. Polym. Sci., Part A: Polym. Chem. 2011, 49, 3004−3013.
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AUTHOR INFORMATION
■
(23) Cardona, C. M.; Li, W.; Kaifer, A. E.; Stockdale, D.; Bazan, G. C.
Corresponding Author
Adv. Mater. 2011, 23, 2367−2371.
*E-mail: krcarter@polysci.umass.edu.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank the National Science Foundation (CHE-1213072)
for financial support. J.D.H. thanks the National Science
Foundation Graduate Research Fellowship Program for support
(GRFP-451512).
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DOI: 10.1021/acs.macromol.5b01174
Macromolecules XXXX, XXX, XXX−XXX