Model compounds based on poly(p-phenylenevinyleneborane) and terthiophene: Investigating the p-n junction in diblock copolymers

Article abstract of DOI:10.1016/j.polymer.2013.05.008

Conjugated block copolymers represent a class of materials with potential applications in electronics and optoelectronics. Three block copolymer model compounds were made by first synthesizing 5-ethynyl-2,2′:5′, 2″-terthiophene, 5-(2-propynyl)-2,2′:5′,2″-terthiophene and 5-(3-butynyl)-2,2′:5′,2″-terthiophene and then performing hydroboration polymerization from the alkyne of the terthiophene. The impact of the connectivity between the polymer blocks of these compounds was investigated. Theoretical and experimental studies indicated that charge transfer was occurring within the all-conjugated copolymer model compound, and that electronic coupling decreased with increasing length of the linking bridge between the n-type and p-type materials. Preliminary efforts to synthesize the related regioregular poly(3-hexylthiophene) all-conjugated diblock copolymer and use this material as the active layer in photovoltaic cells are discussed.

Full text of DOI:10.1016/j.polymer.2013.05.008

Polymer 54 (2013) 3510e3520
Contents lists available at SciVerse ScienceDirect
Polymer
journal homepage: www.elsevier.com/locate/polymer
Model compounds based on poly(p-phenylenevinyleneborane) and
terthiophene: Investigating the pen junction in diblock copolymers
,
c
,
a
a
b
Diane M. Hinkens a , Qiliang Chen , Mahbube Khoda Siddiki , David Gosztola ,
*
Mark A. Tapsak c, Qiquan Qiao a, Malika Jeffries-EL d, Seth B. Darling b
,e,
**
a Electrical Engineering and Computer Science, South Dakota State University, United States
b Center for Nanoscale Materials, Argonne National Laboratory, United States
c Department of Chemistry and Biochemistry, Bloomsburg University of Pennsylvania, United States
d Department of Chemistry, Iowa State University, United States
e Institute for Molecular Engineering, The University of Chicago, United States
a r t i c l e i n f o
a b s t r a c t
Article history:
Conjugated block copolymers represent a class of materials with potential applications in electronics and
optoelectronics. Three block copolymer model compounds were made by first synthesizing 5-ethynyl-
2,20:50,200-terthiophene, 5-(2-propynyl)-2,20:50,200-terthiophene and 5-(3-butynyl)-2,20:50,200-terthio-
phene and then performing hydroboration polymerization from the alkyne of the terthiophene. The
impact of the connectivity between the polymer blocks of these compounds was investigated. Theo-
retical and experimental studies indicated that charge transfer was occurring within the all-conjugated
copolymer model compound, and that electronic coupling decreased with increasing length of the
linking bridge between the n-type and p-type materials. Preliminary efforts to synthesize the related
regioregular poly(3-hexylthiophene) all-conjugated diblock copolymer and use this material as the
active layer in photovoltaic cells are discussed.
Received 19 February 2013
Received in revised form
25 April 2013
Accepted 1 May 2013
Available online 11 May 2013
Keywords:
Organoborane
Block copolymer
Conjugated polymers
Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction
length while maintaining ordered and continuous pathways for
charge transport. For these reasons, fundamental studies using
Conjugated block copolymers (BCPs) are of great interest
because of their potential use in electronic and optoelectronic
technologies such as light emitting diodes and organic photovol-
taics (OPVs) [1,2]. The development of BCPs is an intriguing strategy
to explore the interplay of structure and function because of their
inherent ability to self-assemble into ordered morphologies. For
example, BCPs can limit the phase separation between the donor
and acceptor domains to be comparable to the exciton diffusion
block copolymers could provide valuable knowledge toward un-
derstanding the mechanisms of charge transfer, exciton diffusion
and other properties in organic electronics. The subject of block
copolymers for optoelectronics has been reviewed in the literature
[1e3], and only a brief summary is provided here.
Since first reports of the synthesis and characterization of P3HTe
polyacrylate and P3HTepolystyrene block copolymers [4], other
P3HT-containing block copolymers have been developed for use in
electronic applications. Boudouris et al. developed
a P3HT-
polylactide diblock copolymer that formed microphase separated
domains with a high degree of ordering [5]. Botiz and Darling sub-
sequently created self-assembled films of a similar polymer and then
selectively removed the polylactide polymer, thereby forming a
nanostructured vessel for C60 acceptor material [6,7]. Wu et al. used
ethynyl-terminated P3HT to synthesize polythiophene-polypeptide
copolymers and demonstrated their ability to self-assemble [8].
Sommer et al. and Zhang et al. reported donoreacceptor diblock
copolymers with poly(perylene bisimide acrylate) as the acceptor
block [9,10]. Zhang also included solar cell fabrication, obtaining 0.5%
power conversion efficiency (PCE) in the annealed copolymer devices
when thermal annealing was performed after fabrication of the
Abbreviations: TipB-ter0, 5-{2-[[2-(4-{2-[propenyl-(2,4,6-triisopropyl-phenyl)-
boranyl]-vinyl}-phenyl)-vinyl]-(2,4,6-triisopropyl-phenyl)-boranyl]-vinyl}-
[2,20;50,200]terthiophene; TipB-ter1, 5-{3-[[2-(4-{2-[propenyl-(2,4,6-triisopropyl-
phenyl)-boranyl]-vinyl}-phenyl)-vinyl]-(2,4,6-triisopropyl-phenyl)-boranyl]-allyl}-
[2,20;50,200]terthiophene; TipB-ter2, 5-{3-[[2-(4-{2-[propenyl-(2,4,6-triisopropyl-
phenyl)-boranyl]-vinyl}-phenyl)-vinyl]-(2,4,6-triisopropyl-phenyl)-boranyl]-
butenyl}-[2,20;50,200]terthiophene.
* Corresponding author. Department of Chemistry and Biochemistry, Bloomsburg
University of Pennsylvania, Bloomsburg, PA, United States. Tel.: þ1 951 533 9657.
** Corresponding author. Center for Nanoscale Materials, Argonne National Lab-
oratory, United States.
E-mail addresses: dhinkens@gmail.com, dhinkens@bloomu.edu (D.M. Hinkens),
darling@anl.gov (S.B. Darling).
0032-3861/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.polymer.2013.05.008

D.M. Hinkens et al. / Polymer 54 (2013) 3510e3520
3511
device, representing an improvement over the 0.4% PCE of the related
mixture. In the annealed BCPs, they found increased short-circuit
current, which could result from reorientation and enhanced or-
dering of the copolymer, thereby facilitating charge transport [10].
These are among highest reported efficiencies to date in a diblock
copolymer system.
All-conjugated block copolymers may be even more interesting
for their direct use as active layer materials. The first all-conjugated
BCP was reported by Jenekhe [11], and several other reports have
followed [12e16]. Recently, all-conjugated diblock and triblock co-
polymers have emerged that were also donoreacceptor systems and
were shown to assemble into nanostructured domains [14,17e24].
found a much improved solar cell performance when compared
with the corresponding polymer blends [37]. The spacer was
originally proposed by Aviram and Ratner as a non-conjugated
component between the donor and acceptor, which played the
role of tunneling barrier, preventing recombination [38]. Ng and Yu
designed a diblock oligomer that incorporated two different con-
jugated blocks with opposite electronic demands and showed a
diode rectifying effect, even with the two molecules directly
coupled (no spacer) [39].
Here, three terthiophene compounds with different alkynyl end
groups were synthesized (Scheme 1). Then hydroboration polymer-
ization was performed from the alkyne of the terthiophene com-
pounds, yielding copolymer model compounds with three different
pen junctions (Scheme 2). UVeVis absorption, cyclic voltammetry,
and transient absorption measurements of these model compounds
were obtained; supporting DFT calculations were also performed.
Using poly(phenylenevinyleneborane) as the n-type polymer
[40] and P3HT as the p-type polymer, initial attempts were made to
synthesize an all-conjugated donoreacceptor diblock copolymer
(Scheme 3) and use it as the active material in OPVs.
Although most
p-conjugated polymers are p-type (electron-
donor) semiconductors, when a borane is incorporated regularly in
the polymer backbone, the polymers are inherently electron defi-
cient. Chujo first synthesized polymers with boron in the main
chain and showed that the p-conjugation length was extended via
the vacant p-orbital of the boron atom [25,26], demonstrating their
potential to function as electron acceptors (n-type materials) [27e
30]. There has been increasing interest in the use of organoborane
polymers for electronic applications, including the use of organo-
boranes as pendant groups [31e36].
2. Experimental
Since the junction between the donor and acceptor polymer
blocks (pen junction) is the most likely location for intramolecular
exciton splitting, studying the pen junction could provide insight
into the problems of recombination, exciton diffusion and charge
generation in OPVs. Sun designed a block copolymer with a flexible
bridge (spacer) between the donor and acceptor molecules, and
Starting materials and reagents were purchased from Aldrich or
Acros Organics; Pd(PPh3)2Cl2 and Pd(PPh3)4 were purchased from
Strem Chemicals. An Innovative Technologies solvent purification
system was used to obtain dry solvents (toluene, THF and diethyl
ether). All reactions were run under an inert atmosphere of
TMS
K2CO3
S
Pd(PPh3)Cl2
S
S
S
THF/MeOH
(3:1)
S
S
THF/Et3N (4:1)
CuI, 65o/12h
S
S
S
Br
2
TMS
3
2eq NBS
TMS
BrMg
TMS
Br
S
CCl
4
reflux
SnBu3
Br
S
S
+
S
Br
CuBr, THF
S
4
5
TMS
KF, CH COOH
3
Pd(PPh3)4
DMF
S
S
DMF, 4h
S
S
S
S
reflux, 18h
6
7
Pd(PPh3)4
NBS
S
OH
SnBu3
Br
OH
+
S
toluene
reflux, 16h
Br
S
S
8
S
PPh , Br
3
2
OH
S
S
+
S
BrMg
S
TMS
CH Cl
rt, 16h
S
S
2
2
9
10
TMEDA
KF
DMF, 4h
S
S
Co(acac)
3
TMS
S
S
S
11
12
Scheme 1. Synthesis of alkynyl terthiophenes.

3512
D.M. Hinkens et al. / Polymer 54 (2013) 3510e3520
S
S
S
S
H
B
S
0.1 eq
n
S
tip
n
THF, rt
BH2
1
S
S
R
0.8 eq
S
THF, rt
n
B
m
tip
End group possibilities:
MeOH
BH
OMe
R =
B
tip
tip
S
tipB-ter0: n = no CH2
tipB-ter1: n = 1 CH2
tipB-ter2: n = 2 CH2
S
S
tripyl (tip)
Scheme 2. Synthesis of tipB-ter model compounds.
nitrogen or argon. NMR spectra were recorded on a Varian 300 or
400 MHz spectrometer. Chemical shifts are expressed in parts per
136.98, 136.73, 135.16, 133.60, 128.01, 125.01, 124.89, 124.41, 124.01,
123.32, 121.82, 100.32, 97.07, e0.24.
million (
d
) using residual solvent resonances for calibration.
5-ethynyl-2,20:50,20-terthiophene 3 [52]: To a solution of 2
(103.4 mg, 0.300 mmol) in THF/MeOH (v/v ¼ 3/1) was added
45.6 mg (0.33 mmol) of K2CO3. The mixture was stirred at room
temperature for 1 h. After removal of the solvent, the crude product
was loaded on a short plug of silica gel (w5 cm) and flushed with a
large amount of hexane. The pure product was obtained as yellow-
orange solid upon the evaporation of hexane (75.5 mg, 88%). The
product turned dark brown within a few hours and thus the
deprotection reaction was done immediately prior to use. 1H NMR
Tripylborane 1 [41e43], ethynyl-P3HT [44], 2,20-bithiophene
[45,46], 2-tributyltin-20-bithiophene [47,48], 2,20:50,200-terthio-
phene [49,50] and 5-bromo-2,20:50,200-terthiophene [45,50e52]
were synthesized according to literature procedures.
5-(trimethylsilylethynyl)-2,20:50,200-terthiophene 2 [52]: A so-
lution of 5-bromo-2,20:50,200-terthiophene (1.58 g, 4.8 mmol), tri-
methylsilylacetylene (TMSA) (0.90 mL, 5.8 mmol), Pd(PPh3)2Cl2
(0.144 mmol,101 mg, 3 mol%), and CuI (0.288 mmol, 55 mg, 6 mol%)
in a mixture of THF/Et3N (v/v ¼ 32/8) was degassed for 15 min and
then heated at 65 ꢀC for 12 h. The solvent was removed, affording a
brownish residue. The crude product was heated in hot hexane and
filtered two times and then recrystallized from hexane to afford
(400 MHz, CDCl3):
d
7.23 (dd, J ¼ 5.1, 0.9 Hz, 1H), 7.18 (dd, J ¼ 3.8,
1.5 Hz, 1H), 7.13 (d, J ¼ 3.8 Hz, 1H), 7.08 (s, 2H), 7.03 (dd, J ¼ 5.1,
1.4 Hz, 1H), 7.01(d, J ¼ 3.8 Hz, 1H), 3.42 (s, 1H).
5-bromo-2-bromomethylthiophene 4 [53,54]: CCl4 (80 mL) was
added to NBS (28.5 g, 164 mmol). Methylthiophene (8.06 g,
82 mmol) was added slowly and the mixture was refluxed over-
night. The CCl4 was removed by reduced pressure, hexane was
added and the mixture was filtered through Celite. After removal of
1.36 g (82%) of a dark yellow solid. 1H NMR (400 MHz, CDCl3):
d 7.23
(dd, J ¼ 4.9, 0.7 Hz, 1H), 7.18 (dd, J ¼ 3.8, 0.7 Hz, 1H), 7.13 (d,
J ¼ 3.8 Hz, 1H), 7.07 (s, 2H), 7.02 (dd, J ¼ 4.9, 1.6 Hz, 1H), 7.00 (d,
J ¼ 3.8 Hz, 1H), 0.254 (s, 9H). 13C NMR (400 MHz, CDCl3):
d
138.48,
C6H13
excess:
C6H13
C6H13
tipBH2 (0.024 mmol)
R
Br
Br
S
n
S
n
THF, rt
Br
S
BH
tip
tipBH2 (1)
0.013 mmol
B
m
tip
P3HT-tipBPh
Scheme 3. Synthesis of P3HT-tipBPh.

D.M. Hinkens et al. / Polymer 54 (2013) 3510e3520
3513
the hexane, the pure product (12.76 g, 60.8%) was collected by
vacuum distillation (65 ꢀC/0.5 Torr). It should be noted that 5-
bromo-2-bromomethylthiophene 4 has a distinct smell and is a
ethyl acetate 7:3 to give 0.95 g (31%) of a yellow brown crystalline
solid. 1H NMR (400 MHz, CDCl3):
d
7.21 (dd, J ¼ 5.1 Hz, J ¼ 1.1 Hz,
1H), 7.16 (dd, J ¼ 3.7 Hz, J ¼ 1.1 Hz, 1H), 7.07 (d, J ¼ 3.8 Hz, 1H) 7.04e
7.01 (m, 3H), 6.79 (d, J ¼ 3.6 Hz, 1H), 3.89 (q, J ¼ 6.1 Hz, 2H), 3.06 (t,
J ¼ 6.2 Hz, 2H), 1.62 (t, J ¼ 6.0 Hz, 1H, OH). 13C NMR (400 MHz,
lachrymator. 1H NMR (400 MHz, CDCl3):
d
6.86 (d, J ¼ 3.9 Hz, 1H),
6.88 (d, J ¼ 3.8 Hz, 1H), 2.44 (s, 2H). 13C NMR (400 MHz, CDCl3):
d
142.21, 130.02, 128.49, 114.05, 26.42.
CDCl3): d 140.56, 137.38, 136.58, 136.12, 136.01, 128.08, 126.69,
5-bromo-2-[3-(trimethylsilyl)-2-propynyl]-thiophene 5 [55]: A
124.63, 124.50, 124.09, 123.84, 123.78, 63.54, 33.80.
1.5 M THF solution of 1-trimethylsilylethynylmagnesium bromide
(10 mmol) (which was prepared by reacting TMSA (10.0 mmol,
1.42 mL) in 3.6 mL of THF with 10.0 mL of 1M ethylmagmesium
bromide) was added to a stirred suspension of copper(I) bromide
(71.7 mg, 0.500 mmol) in THF (2 mL). A solution of 4 (2.56 g,
10.0 mmol) in THF (3 mL) was then added and the mixture was
refluxed for 2 h. The reaction mixture was poured into a cold satu-
rated NH4Cl aqueous solution, stirred for 0.5 h, and extracted with
Et2O. The organic extract was washed with water, dried, concen-
trated in vacuo and distilled (85 ꢀC/0.5 Torr) to give 1.96 g (71.7%) of a
clear liquid. MS: 272 (Mþ), 274 (Mþ2), 257, 259, 229, 231, 193, 89, 73.
5-(2-Bromoethyl)-2,20:50,200-terthiophene
10
[57]:
PPh3
(674 mg, 2.57 mmol) was dissolved in 5 mL CH2Cl2, and bromine
(411 mg, 2.57 mmol) in 5 mL CH2Cl2 was added dropwise, forming a
colorless solution. Hydroxyethylterthiophene
9
(751.6 mg,
2.57 mmol) in 10 mL CH2Cl2 was then added dropwise and the
mixture stirred overnight at room temperature. The mixture was
quenched with water (15 mL), separated, and the aqueous phase
was extracted with 2 ꢂ 15 mL CH2Cl2. The combined organic phase
was dried with MgSO4, filtered, and concentrated. The crude
product was purified by chromatography on silica gel using 5%
Et2O/hexanes) to afford 730 mg (78%) of a dark yellow solid. 1H
1H NMR (400 MHz, CDCl3):
d
6.70 (dt, J ¼ 3.7, 1.2 Hz, 1H), 6.87 (d,
NMR (300 MHz, CDCl3):
d
7.22 (dd, J ¼ 5.1, 1.1 Hz, 1H), 7.17 (dd,
J ¼ 3.7 Hz,1H), 3.71 (d, J ¼ 1.2 Hz, 2H), 0.19 (9H, s).13C NMR (400 MHz,
J ¼ 3.7,1.1 Hz,1H), 7.06 (d, J ¼ 3.8 Hz,1H), 7.02 (t, J ¼ 4.1 Hz, 3H), 6.80
(d, J ¼ 3.5, 1H), 3.59 (t, J ¼ 7.4 Hz, 2H), 3.35 (t, J ¼ 7.4 Hz, 2H).
5-[4-(trimethylsilyl)-3-butyn-1-yl]-2,20:50,200-terthiophene 11
[58] A THF solution of 1-trimethylsilylethynylmagnesium bromide
(0.8 mmol) was prepared by reacting TMSA (0.12 mL, 0.84 mmol)
CDCl3):
d 141.01, 129.78, 125.52, 110.42, 102.38, 87.78, 21.60, 0.133.
5-[3-(trimethylsilyl)-2-propynyl]-2,20:50,200-terthiophene
6
[48]: To a 40 mL solution of toluene containing 0.462 g (0.400 mmol)
of Pd(PPh3)4 was added 5 (2.05 g, 7.05 mmol) and 2-(tributyl-
stannyl)bithiophene (3.21 g, 7.05 mmol). After degassing for 15 min,
the mixture was refluxed overnight. After evaporation of toluene,
the residue was chromatographed on silica gel with 5% ether/hex-
ane and recrystallized from hexane to give 0.401 g (14.2%) of a
with
a THF solution of ethylmagnesium bromide (0.8 mL,
0.8 mmol). The THF was evaporated under reduced pressure for
5 min to give a solid. Immediately, TMEDA (0.6 mL) was added to
the flask to furnish a suspension. Cobalt(III) acetylacetonate
(28.5 mg, 0.080 mmol) was placed in a 25-mL flask and was heated
under vacuum for 5 min. Anhydrous TMEDA (0.4 mL) was added
and the mixture was stirred for 3 min at room temperature. It was
then added to bromoterthiophene (71.1 mg, 0.22 mmol) via a sy-
ringe. The suspension of 1-trimethylsilylethynylmagnesium bro-
mide was next added to the mixture at room temperature with a
syringe with stirring for 15 min after which the mixture turned
brown. The mixture was poured into a saturated NH4Cl solution,
extracted with hexane and ether (50 mL ꢂ 2) and dried over MgSO4,
filtered and concentrated to give 115 mg of yellow crystals (38.6%).
yellow brown crystalline solid.1H NMR (400 MHz, CDCl3):
d 7.21 (dd,
J ¼ 5.1,1.2 Hz,1H), 7.16 (dd, J ¼ 3.6,1.1 Hz,1H), 7.06 (d, J ¼ 3.8 Hz,1H),
7.04e7.01(m, 2H), 7.00(d, J ¼ 3.8 Hz,1H), 6.86 (dt, J ¼ 3.6,1.1 Hz,1H),
3.78 (d, J ¼ 1.0 Hz, 2H), 0.17 (s, 9H). 13C NMR (400 MHz, CDCl3):
d
138.81,137.38,136.56,136.19,136.07,128.07,126.04,124.63,124.49,
124.18, 123.85, 123.60, 102.87, 87.48, 21.54, 0.196.
5-(2-propynyl)-2,20:50,200-terthiophene 7 [55,56]: Trimethylsi-
lylpropargylterthiophene 6 (72 mg, 0.20 mmol) was dissolved in
5 mL DMF. Then 1.5 equiv of KF.2H2O (28 mg) and 1.5 equiv of acetic
acid (17 mL) was added and it was stirred for 24 h at room temper-
ature, then treated with an excess of 3N HCl and extracted with
hexane. The hexane extract was washed with 3N HCl and then
washed with saturated NaHCO3, water and dried with MgSO4. The
hexane was removed by reduced pressure and the residue was
recrystallized from hexane to give 51 mg (89%) of a brownish yellow
crystalline solid. MS: 286(Mþ), 287(Mþ1), 253, 127. 1H NMR
MS: 372 (Mþ), 261, 207, 73. 1H NMR (300 MHz, CDCl3):
d 7.22 (dd,
J ¼ 5.1, 1.1 Hz, 1H), 7.16 (dd, J ¼ 3.6, 1.1 Hz, 1H), 7.06 (d, J ¼ 3.8 Hz,
1H), 7.03-6.98 (m, 3H), 6.76 (d, J ¼ 3.6, 1H), 3.02 (t, J ¼ 7.4 Hz, 2H),
2.57 (t, J ¼ 7.4 Hz, 2H), 0.168 (s, 9H).
5-(3-butynyl)-2,20:50,200-terthiophene 12 [55]: The trimethylsi-
lylbutynylterthiophene 11 (74.5 mg, 0.20 mmol) was dissolved in
5.0 mL of DMF and KF$2H2O (28.2 mg, 0.30 mmol) was added. The
mixture was stirred and the reaction was monitored by GCMS for
quantitative removal of the TMS group. After 3 h at room tempera-
ture, the reaction was complete and the mixture was treated with an
excess of 3N HCl and extracted with hexane. The hexane extract was
washed with 3N HCl and then washed with saturated NaHCO3, water
and dried with MgSO4. The hexane was removed by reduced pres-
sure and the residue was recrystallized from hexane to give 54 mg
(90%) of a dark yellow solid. MS: 300(Mþ), 301(Mþ1), 261, 131.
General procedure for the hydroboration polymerization of
alkynyl terthiophene: The procedure with ethynylterthiophene,
giving tipB-ter0 is representative. Tripylborane 1 (1.0 mmol,
217 mg) was dissolved in 1 mL THF. Ethynylterthiophene 3
(0.10 mmol, 28.0 mg) in 1 mL THF was added with stirring for ½
hour. Then diethynylbenzene (0.80 mmol, 101 mg) in 2 mL THF was
added with stirring for ½ hour and stirring continued overnight.
After removal of the solvent, dry methanol was added to the dark
orange residue, it was filtered and washed with methanol, then
reprecipitated with THF/methanol. The precipitate was dissolved in
toluene and was run quickly through a small silica gel column. The
orange fraction was collected and 217 mg of a bright orange shiny
(400 MHz, CDCl3):
d
7.22 (dd, J ¼ 5.1, 1.1 Hz, 1H), 7.17 (dd, J ¼ 3.7,
1.1 Hz, 1H), 7.06 (d, J ¼ 3.8 Hz, 1H), 7.04e6.99(m, 3H), 6.88 (dd,
J ¼ 3.6,1.1 Hz,1H), 3.75 (dd, J ¼ 2.6,1.1 Hz, 2H), 2.24 (t, J ¼ 2.7 Hz,1H).
2-bromo-5-(2-hydroxyethyl)thiophene 8 [56]: To a solution of
5.21 g (0.041 mol) of 2-(2-thienyl)ethanol in 50 mL of toluene was
added stepwise 7.23 g (0.041 mol) of NBS at ꢁ20 ꢀC. The mixture
was stirred at room temperature overnight and then treated with
10% KOH and washed with brine. The toluene phase was dried with
MgSO4, then filtered and the solvent was removed with reduced
pressure. The crude product was chromatographed on silica gel
(ethyl acetate/petroleum ether 7:3) and isolated as a colorless
liquid (6.58 g, 80%). 1H NMR (400 MHz, CDCl3):
d
6.90 (d, J ¼ 3.7 Hz,
1H), 6.63 (d, J ¼ 3.7 Hz,1H), 3.83 (t, J ¼ 6.2 Hz, 2H), 3.00 (t, J ¼ 6.2 Hz,
2H), 1.70 (s, 1H, OH); 13C NMR (400 MHz, CDCl3):
125.9, 109.7, 62.8, 33.5.
d
142.7, 129.5,
5-(2-Hydroxyethyl)-2,20:50,200-terthiophene
9
[46,56]: To
a
50 mL solution of toluene containing 0.578 g (0.500 mmol) of
Pd(PPh3)4 was added 8 (2.06 g, 10 mmol) and 2-(tributylstannyl)
bithiophene (4.55 g, 10 mmol). After degassing for 15 min, the
mixture was refluxed overnight. After evaporation of toluene, the
residue was chromatographed on silica gel with petroleum ether/

3514
D.M. Hinkens et al. / Polymer 54 (2013) 3510e3520
solid was collected after evaporation of the solvent. GPC: Mw 5419/
Mn 3,656, 1H NMR (400 MHz, CDCl3):
7.13e7.71 (8H, AreH and Be
systems were calibrated with polystyrene standards having a mo-
lecular weight range of 3000 to 600,000 g/mol.
d
CH ¼ CH; 2H, thiophene), 6.92e7.06 (2H, AreH, (tripyl) and 5H,
thiophene), 6.77 (d, J ¼ 18.0 Hz, 1H, vinyl, thiophene), 3.70 (eOMe
end group), 3.02e2.81 (m, 2H, CH(tripyl)), 2.41e2.55 (m, 1H,
CH(tripyl)), 1.18e1.36 (18H, CH3(tripyl)); 11B NMR (400 MHz,
Matrix-assisted laser desorption ionization time-of-flight
(MALDI-TOF) mass spectrometry was performed on a Bruker Re-
flex III spectrometer. MALDI-TOF samples were prepared by drop-
casting a THF solution of polymer with terthiophene as a matrix.
Pump-probe ultrafast TA spectroscopy experiments were carried
out at the Center for Nanoscale Materials (CNM) at Argonne National
Laboratory using an amplified Ti:sapphire laser system (Spectra
Physics, Spitfire-Pro) and an automated data acquisition system (Ul-
trafast Systems, Helios). The amplifier was seeded with the 120 fs
output from the oscillator (Spectra Physics, Tsunami) and was oper-
ated at 1.66 kHz. The output from the amplifier was split with 90%
used to pump an optical parametric amplifier (Topas, Light Conver-
sion) and the 10% used to generate a visible continuum (440e
780 nm) probe by focusing into a thin sapphire plate. The excitation
CDCl3):
tipB-ter1: shiny yellow solid, 455 mg. GPC: Mw 4270/Mn 2700.
1H NMR (400 MHz, CDCl3):
7.13e7.71 (8H, AreH and BeCH]CH;
d 45.8 (br).
d
2H, thiophene), 6.92e7.06 (2H, AreH, (tripyl); 5H, thiophene), 5.54
(BeCH]CH), 3.78-3.58 (CH2, propargyl, and eOMe end groups),
2.42e2.98 (3H, CH(tripyl)), 1.11e1.32 (18H, CH3(tripyl)).
tipB-ter2: dark yellow solid, 78 mg. GPC: Mw 5750/Mn 2630. 1H
NMR (400 MHz, CDCl3): d 7.15e7.64 (8H, AreH and BeCH]CH; 2H,
thiophene), 6.85e7.13 (2H, AreH, (tripyl); 5H, thiophene), 5.90e
6.34 (BeCH]CH), 3.78e3.45 (eOMe end groups), 2.72e2.98 (m,
1H, CH(tripyl)) and 4H, CH2(ethyl)), 2.39e2.52(m, 2H, CH(tripyl)),
1.11e1.32 (18H, CH3(tripyl)).
(pump) wavelength was 350 nm and the pump pulse energy was 1 mJ.
Molecular modeling of the three model compounds was per-
formed using hybrid density functional theory (DFT). The calcula-
tions were done on terthiophene connected to a tetramer of the
phenyleneborane and were carried out using Gaussian’09 [60] on a
28 TFlop computer cluster at Argonne National Laboratory;
GaussView 5 was used for visualization. For ground state optimi-
zations, the B3LYP/3-21G* level was used. Molecular orbitals were
calculated as a single point ground state energy B3LYP/6-311G(d,p)
with tight convergence, and UVeVis calculations were based on
single point energy TD-SCF/PBE1PBE/6-31Gþ(d,p) gas phase cal-
culations with tight convergence.
Polythiophene-b-poly(phenylenevinyleneborane)
P3HT-
tipBPh: Ethynyl-P3HT [44] (50 mg, 0.013 mmol, Mn 3900, MALDI-
TOF MS) was dissolved in THF (50 mL). Tripylborane (154.6 mg,
0.715 mmol) was weighed in the glove box, dissolved in 3 mL THF,
and from a gas-tight syringe, 0.1 mL was slowly added to the P3HT
and stirred for 10 min. This was to ensure that all the ethynyl
groups from the P3HT were reacted. Diethynylbenzene (81.2 mg,
0.64 mmol) was dissolved in 2.9 mL of THF and taken up into a
syringe. A large excess (5ꢂ) of the tripylborane and dieth-
ynylbenzene were then added to the mixture at the same rate over
a 2-h period. The mixture was stirred overnight. Most of the THF
was removed, and the thick solution was precipitated with dry
methanol and filtered, then washed several times with dry meth-
anol to obtain a dark purple residue. The residue was dissolved in
toluene and run through a silica gel column, and the red fraction
(75 mg), which contained the polymer, was collected. 1H NMR
For photovoltaic cell fabrication, indium tin oxide (ITO)-coated
glass slides of 1.5 ꢂ 1.5 cm2 were used as the conducting positive
electrode. The ITO layer was partially etched and the glass sub-
strates (Delta Technologies Ltd. with resistivity 8e12
U per square)
were cleaned with detergent and sonicated in deionized water,
acetone, and isopropanol. The substrates were dried under N2 flux
and finally were treated for 20 min in a plasma surface decon-
tamination system (Harrick Plasma) connected to an O2 gas source.
An aqueous PEDOT:PSS (0.5 w/v%, Baytron PH 500, HC Starck Inc.)
(400 MHz, CDCl3):
d 7.02e7.64 (8H, AreH and BeCH]CH), 6.95e
7.03 (2H, AreH, tripyl) 6.93 (s, CH, thiophene), 5.91e6.78 (BeCH]
CH), 3.83e3.46 (eOMe end groups), 2.76e3.01 (m, 1H, CH(tripyl)),
2.86e2.74 (br, 2H CH2), 2.41e2.53(m, 2H, CH(tripyl)), 1.74e1.67 (br,
2H, CH2), 1.48e1.39 (br, 2H, CH2) 1.19e1.23 (br, 2H, CH2), 1.04e1.38
(18H and 4H, CH3(tripyl) and CH2), 0.95e0.87 (br, CH3).
solution was passed through a 0.45 mm filter and then spin-coated
from aqueous solution at 5000 rpm. The substrate was dried for
20 min at 120 ꢀC on a hot plate.
UVeVis measurements were performed in THF with a Varian/
Cary Eclipse UVeVIS Spectrometer or an Agilent 8352 UVevisible
spectrophotometer. A Varian/Cary Eclipse fluorescence spectro-
photometer and a PerkineElmer LS-55 fluorimeter were used for
fluorescence measurements.
For cyclic voltammetry (CV) measurements, the three electrode
system consisted of a glass-carbonelectrodecoated withpolymer film
as the working electrode, platinum wire as the counter electrode and
Ag/Agþ as the reference electrode. The voltammograms were recor-
ded at 100 mV sꢁ1 in CH2Cl2 containing Bu4N[PF6] (0.1M) as the
supportingelectrolyte. The scans werereferenced after theaddition of
a small amount of ferrocene as internal standard and are reported
relative to the Fc/Fcþ couple. HOMO energy levels were calculated by
the equation HOMO ¼ ꢁ½Eox ꢁ E1=2ðFc=FcþÞ þ 4:8ꢃeV, where Eox
was the onset oxidation potential versus SCE [59]. LUMO levels were
calculated similarly.
Gel permeation chromatography (GPC) was performed in THF
(35 ꢀC) at a flow rate of 1.0 mL/min) using a Waters system comprised
of a Waters HPLC 515 pump, a Waters 2410 RI detector and two
10 mm by 250 mm columns. The first column was a DuPont In-
struments Zorbax S-06 and the second was a Jordi-Gel DVB mixed
bed column. GPC for tipB-ter0 was performed using a Viscotek
GPCmax with VE 2001 GPC solvent/sample module, a 2600 UV de-
tector, a TDA 305 triple detector array and three columns, including
In the glove box, a layer of copolymer P3HT-tipBPh was
deposited on top of the PEDOT:PSS layer by spin-coating
(1200 rpm) from anhydrous DCB (30 mg/mL) or a 1:1 mixture of
P3HT and tipBPh, 20 mg/mL in DCB. A calcium layer (20 nm), which
facilitates electron collection at the organic/metal electrode, and an
aluminum layer (85 nm) were deposited on the polymer films by
thermal evaporation at 2 ꢂ 10ꢁ6 mbar. The active area of the de-
vices was w0.16 cm2. All current voltage (IeV) characteristics of the
devices were measured using a Keithley 2400 source meter under
A.M. 1.5 illumination (100 mW/cm2) from a Xenon light source.
Measurements were calibrated using an NREL-measured mono-Si
photodetector (Hamamatsu S1133-14), which produces 1.638 mA of
current at A.M. 1.5 illumination. Short circuit current density (JSC),
open circuit voltage (VOC), and fill factor (FF) were obtained from
the current densityevoltage (JeV) curves. The conversion efficiency
was calculated by
power density; and FF is given by FF ¼ Jmax ꢂ Vmax/JSC ꢂ VOC, where
Jmax ꢂ Vmax is the maximum output power density of the solar cell.
h
¼ JSC ꢂ VOC ꢂ FF/Pin, where Pin is the incident
3. Results and discussion
3.1. Synthesis of the model compounds
Terthiophene was made by reacting dibromothiophene with
tributylstannylthiophene in the presence of Pd(PPh3)4 in DMF
one PLgel 5 mm mixed-D and two PLgel 5 mm mixed-C columns. Both

D.M. Hinkens et al. / Polymer 54 (2013) 3510e3520
3515
(Stille coupling) [49,50]. A Sonagashira coupling reaction with 5-
bromo-2,20:50,20-terthiophene and TMSA provided good yields of
the TMS-protected ethynylterthiophene 2 (Scheme 1) [52]. The
TMS group was removed with standard deprotecting conditions
and the product was used immediately [61].
Synthesis of propargylterthiophene was difficult due to the
acidity of the non-conjugated carbon between the alkyne and ter-
thiophene. The final synthesis started with 2-methylthiophene,
which was dibrominated with NBS in CCl4 and the product was
purified by distillation [53,54]. The TMS-protected ethynyl
Grignard was then used in the presence of copper bromide to give
the protected propargylthiophene 5 [55]. Tributylstannyl-2,20-
bithiophene [47,62] was coupled to the propargylthiophene using
Stille conditions to give 6 [46,62]. When KF was used under neutral
conditions to remove the TMS group the allene was obtained, but
when acetic acid was added as a buffer, propargylterthiophene 7
was obtained [63].
For 3-butynylterthiophene, 2-(2-hydroxyethyl)thiophene was
brominated with NBS [56]. Stille conditions were then used to
couple tributylstannylbithiophene to the hydroxyethylthiophene to
give 9. The hydroxy group was transformed to a bromide using
bromine and triphenylphosphine [57]. Finally a cobalt coupling
method was used to add TMSA to the 2-(2-bromoethyl)terthio-
phene 10 to give the TMS-protected butynylterthiophene 11 [58].
The TMS group was efficiently removed with KF under standard
conditions.
The deprotected terthiophene was dissolved in THF and tri-
pylborane was added, the mixture was stirred for ½ hour, and
then the dialkyne was added slowly with stirring continued
overnight (Scheme 2). After repeated precipitation with meth-
anol from THF, the product was dissolved in toluene and run
quickly through a small silica gel column with toluene. Following
precipitation with methanol, it was expected that most
endgroups were methoxy-, although other possibilities exist (see
Scheme 2).
non-conjugated link could signify that the alkyl groups prevent
charge transfer from the terthiophene to the borane moieties.
It has been established that organoborane polymers show a
strong bathochromic shift relative to the monomer, suggesting
effective p-
p
overlap [40]. This red shift of absorption represents the
-conjugated system containing the boron atom and
extension of the
p
is accompanied by a lowering of the LUMO [28,64]. A bathochromic
effect was also observed in anthracene derivatives when more boryl
anthracene moieties were added, indicating effective p-conjugation
to the external moieties [65]. Unusually large Stokes shifts were
found for organoborane heteroaromatic polymers containing thio-
phene and furan units in the main chain, which was likely due to
increasingly pronounced charge transfer character [31,66]
The observed additional absorption peak at 470 nm in tipB-ter0
could be a marker of charge transfer, or the result of cross-
conjugation between the terthiophene and the boron polymer
because it is in addition to the effects of extension of conjugation in
the borane polymer itself (absorption of the borane homopolymer
is observed at around 400 nm) [40]. In related work, Pammer, et al.
observed bands at 330 and 470 nm in polythiophene with dime-
sitylvinylborane side groups [67]. In this system, the band at
470 nm represented extended conjugation with expansion of the
molecular system, while the band at 330 nm was assigned to charge
transfer as confirmed by its suppression upon addition of CN- or F-.
Similarly, a polythiophene system with directly attached boryl
groups showed a strong absorption at 412 nm with an additional
peak at 340 nm, which resulted from excitation to a cross-
conjugated state of bithiophene with two mesitylborane moieties
[68]. Looking back at the UVeVis of tipB-ter0, there is evidence of
another band at w335 nm that would corroborate the findings of
Jäkle and coworkers discussed above, but it overlaps with the broad
absorption from the organoborane polymer.
Consequently, the additional absorption band at 470 nm is likely
the result of extended conjugation between the terthiophene
p-
system and the organoborane polymer as well as a marker of direct
connectivity between the boron and the terthiophene systems. GPC
measurements of tipB-ter0 with a UVeVis PDA detector demon-
strate this connectivity because the same GPC peak profile was
observed at 378 nm and 470 nm (Fig S1 and Fig S2). Solid state UVe
Vis measurements were also completed by spin coating a solution
of tipB-ter0 onto a glass slide. The solid state spectra are similar to
the solution UVeVis, demonstrating that the solution data are
likely representative of the solid state properties (Fig S3).
3.2. UVeVis and fluorescence measurements of the model compounds
The UVeVis and fluorescence spectra of the three copolymer
model compounds were similar, except tipB-ter0 showed an
additional absorption band at 470 nm and an additional fluores-
cence band at 525 nm (Fig. 1). That the extra absorbance and
fluorescence was not present in either of the compounds with a
Fig. 1. UVeVisible absorption (left) and fluorescence emission (350 nm excitation) spectra of copolymer model compounds, terthiophene and tripylborane polymer. The spectra
have been normalized.

3516
D.M. Hinkens et al. / Polymer 54 (2013) 3510e3520
Table 1
3.3. DFT calculations of the model compounds
Calculated energy levels (DFT) and transitions (TD-SCF), and experimental absorp-
tion data of model compounds. All energies are given in electron Volts (eV).
The molecular orbitals and electronic states involved in the optical
transitions were investigated with DFT calculations. The structures of
the calculated compounds were the same as the synthesized com-
pounds, except that the calculated compounds had four boron
monomers, and based on GPC analysis, the synthesized compounds
had an average of about ten boron monomers. In all three copolymer
model compounds, the highest occupied molecular orbitals (HOMO)s
HOMO
LUMO
LUMOþ1
Transitions
(Calcd)a
Transitions
(Exp)b
tipB-ter0
tipB-ter1
tipB-ter2
ꢁ5.43
ꢁ5.30
ꢁ5.29
ꢁ2.96
ꢁ3.01
ꢁ3.00
ꢁ2.71
ꢁ2.66
ꢁ2.65
2.75, 2.67, 2.42
2.70
2.70
3.1, 2.79
3.1
3.1
a
Electronic transitions with significant oscillator strengths (f > 0.11) based on
TD-SCF calculations. Other orbitals involved in these transitions are detailed in
Figure S2 and Figure S3.
resided on the terthiophene
p-system, with similar orbital
energies: ꢁ5.43 eV, ꢁ5.30 eV, ꢁ5.29 eV for tipB-ter0, tipB-ter1 and
tipB-ter2, respectively (Fig. 2, Table 1). For tipB-ter0, there is spillover
from the terthiophene onto the boron atom. This would be stabilizing
and could therefore account for the slightly lower HOMO energy of
tipB-ter0. In the other two model compounds, this spillover was
blocked by the alkyl group(s).
In the LUMOs of all three model compounds, the electron den-
sity was spread across both the terthiophene and the organoborane
moiety (except for the perpendicular tip group), and not on the
alkyl groups (See Fig. 2). The next higher orbital (LUMOþ1) is
where the differences between tipB-ter0 and the other two model
compounds become most significant. In tipB-ter0, the LUMOþ1
shows cross conjugation from the terthiophene to the empty p-
orbital on boron, whereas in the other two model compounds, all
the electron density in the LUMOþ1 resides only on the organo-
borane moiety.
b
Based on the estimated absorption maxima from Fig. 1.
ter2. The larger HOMOeLUMO energy gap for tipB-ter0 is an un-
expected result because the longest wavelength absorption of tipB-
ter0 is significantly lower in energy than for the other two model
compounds.
TD-SCF calculations reveal the reason for this result. The elec-
tronic transition to the first excited state of tipB-ter0 is primarily
based on excitation from the HOMO to the LUMO with an addi-
tional smaller component from the HOMO to the LUMOþ1
(Figure S4), both of which involve some charge transfer from the
terthiophene to the borane polymer chain. For the other two model
compounds only the S0 / S2 electronic transition shows significant
intensity. This transition occurs primarily from the HOMOe1 to the
LUMO (Figure S5), so the reason that these compounds absorb at
higher energy is because the absorption is to the second excited
state, a higher energy transition [32]. This HOMO-1 to LUMO
excitation is also present in tipB-ter0 and corresponds to the
S0 / S3 transition. In conclusion, while the higher HOMOeLUMO
gap is seemingly inconsistent with the longer wavelength
To summarize the energy differences, while the orbital energies
of tipB-ter1 and tipB-ter2 are nearly the same, for tipB-ter0, the
HOMO is 0.13 eV lower, the LUMO is 0.05 eV higher and the
LUMOþ1 is 0.06 eV lower (see Table 1). The calculated HOMOe
LUMO gap is 2.5 eV, about 0.2 eV larger than tipB-ter1 and tipB-
Fig. 2. HOMO, LUMO and LUMOþ1 of tipB-ter model compounds. Generated with Gaussview (scaling radii 75%, isovalue 0.02).

D.M. Hinkens et al. / Polymer 54 (2013) 3510e3520
3517
absorption of tipB-ter0, the TD-SCF calculations show that for tipB-
ter1 and tipB-ter2, the lowest energy transition involves HOMO-1
to LUMO (not HOMO to LUMO) excitation. The calculations pre-
dict a lmax at 458 nm (Figure S6). In comparison, for the experi-
mental UVeVis results, the lmax is at 400 nm, and this consistent
difference could be due to intermolecular charge transfer phe-
nomena not captured by modeling individual molecules in the gas
phase or to inherent problems associated in modeling charge
transfer processes using TDDFT methods.
3.4. Cyclic voltammetry and transient absorption measurements of
the model compounds
The experimental HOMOeLUMO gaps were determined using
oxidation and reduction curves that were obtained from cyclic
voltammetry measurements (Table 2), and match the values from
calculations quite well. The gaps are similar to those found by Li
et al., where in polythiophene with side group borylation the op-
tical gap was lowered from 2.70 with silicon groups to 2.36 with
boron groups [68].
In order to better understand the complex exciton dissociation
processes at the donoreacceptor interface that affect photovoltaic
performance, the photophysical and electronic properties of these
materials needed to be further studied. Transient absorption
measurements were performed on the three linker compounds
(Fig. 3), as well as on the constituent parts: terthiophene and the
organoborane polymer. The transient data show complex spectral
evolution over time due to the competing interconversion be-
tween excited singlet and triplet states, and charge transfer and
recombination. A complete analysis of the transient data is beyond
the scope of this paper but a qualitative examination clearly
supports the idea that photoinduced charge transfer occurs in
some of the copolymer model compounds. As reported by Paa
et al. [69], the transient absorption measurements of terthiophene
excited at 350 nm exhibited strong singlet excited state absorption
at 600 nm, which evolved into triplet state absorption at 470 nm
with a time constant of ca. 175 ps. However, in analogous mea-
surements of the compounds having either direct linkage or a one-
carbon non-conjugated spacer (tipB-ter0 and tipB-ter1, respec-
tively), neither the singlet nor the triplet state absorption signa-
tures were detected. The terthiophene singlet and triplet peaks
were observed in tipB-ter2, which has the longer, two-carbon
bridge. These results are consistent with charge transfer occur-
ring in the direct and one-carbon linked compounds and not in
the two-carbon bridged compound (Fig. 3). These data are in
agreement with a theoretical study of electron transport through a
conjugated-saturated hydrocarbon molecular wire showing that
the rectification performance was correlated to the length of the
saturated barrier [70].
The transient absorption spectrum of the all-conjugated tipB-
ter0 showed a new peak centered at w540 nm, which may be a
signature of a state resulting from charge transfer. This possibility
will require further study through spectroelectrochemical and
other measurements for firm assignment [18].
Fig. 3. Transient absorption spectroscopy data. a) TipB-ter2 and terthiophene spectra
both exhibit peaks for the singlet (at 2 ps) and triplet (at 2 ns) excited state, demon-
strating the electronic isolation in the former. b) Spectra for TipB-ter1 are similar to
those for tipBPh and exhibit no evidence for terthiophene triplet character. c) Transient
absorption measurements taken at excitation 350 nm show a new peak forming in
tipB-ter0 that might be a charge transfer state.
Table 2
HOMOeLUMO gaps of model compounds calculated from cyclic voltammetry
measurements and compared to the calculated EHOMOeLUMO. All energies are given
in electron Volts (eV).
Transient absorption measurements of the organoborane ho-
mopolymer (tipBPh) showed slow decay for the first excited state
(exciton) of the polymer, which is desirable, because a long-lived
excited state provides more time for charge separation to occur.
The fluorescence lifetime of this tripylorganoborane polymer was
EHOMO
ELUMO
EHOMOeLUMO
EHOMOeLUMO
(calculated)
(experimental)a
TipB-ter0
TipB-ter1
TipB-ter2
ꢁ5.4
ꢁ5.2
ꢁ5.2
ꢁ3.0
3.0
ꢁ3.1
2.4
2.2
2.1
2.47
2.29
2.29
measured to be
Figure S7).
G
¼ 0.39 ns þ 0.85 ns, c2 ¼ 0.964 (THF 1 mg/mL,
a
Calculated from cyclic voltammetry measurements.

3518
D.M. Hinkens et al. / Polymer 54 (2013) 3510e3520
Fig. 6. NMR of P3HT-tipBPh copolymer (lower), tipBPh (middle) and P3HT (top) with
integration of two peaks (1.0:1.2), each representing 2 protons in tipBPh and P3HT.
Fig. 4. UVeVis (left) and fluorescence (excitation at 350) of the copolymer P3HT-
tipBPh in THF.
Oxidation and reduction peaks were obtained from cyclic vol-
tammetry measurements (Fig. 5), and the HOMO and LUMO were
calculated (EHOMO ¼ ꢁ5.1 eV; ELUMO ¼ ꢁ3.0 eV; HOMOeLUMO
gap ¼ 2.1 eV). The very small wave near 0.6 V is P3HT, showing a
trace amount of P3HT impurity. The starting MW of the P3HT was
3900 by MALDI-MS. GPC of P3HT was 9360 Mn/14,000 Mw (1.5) and
of the copolymer was 13,809 Mn/44,905 Mw (3.2) (Fig S8). Due to
difficulties in obtaining MALDI mass spectra, representative spectra
of the copolymer were not obtained. The NMR is shown in Fig. 6
3.5. Synthesis and studies of the P3HT-tipBPh diblock copolymer
Attempts were then made to synthesize the all-conjugated
diblock copolymer related to tipB-ter0, which was of interest
because of the charge transfer band it exhibited (Scheme 3). P3HT
was synthesized and endcapped with ethyne [44], and was sub-
sequently characterized using GPC, NMR and MALDI-TOF MS. This
compound was then dissolved in THF, and tripylborane was added
to react with the alkyne groups. This was followed by a large excess
of diethynylbenzene and tripylborane (1M in THF) added at the
same rate over a 2-h period. The mixture was stirred overnight and
precipitated with methanol. The filtered copolymer was dissolved
in toluene and run through a silica gel column, and the red fraction
containing the copolymer was collected. UVeVis measurements
showed the same band of absorbance at 470 nm that was found in
tipB-ter0, (Fig. 4) indicating the presence of the diblock copolymer.
The homopolymers P3HT and tipBPh absorb at 450 nm and 375e
425 nm, respectively, so the absorbance at 470 nm in the P3HT-
tipBPh mixture must be indicative of the presence of the diblock
copolymer. As was discussed with tipB-ter0, the additional band of
absorption is the result of extended conjugation resulting from the
direct connection of the terthiophene to the organoborane poly-
mer, possibly indicating charge transfer at the pen junction.
and two isolated peaks (d 2.47 tripyl, 2H, CH and 1.70 hexyl, 2H,
CH2) were integrated to give a ratio of 1:1.2. The aryl region (7.2e
7.4 ppm) demonstrates the presence of the organoborane polymer,
but some of these resonances could be due to homopolymer. The
MS weight of 3900 for P3HT represents 23 thiophene monomers
and this was used with the ratio from the NMR to give another MW
estimate of w10,000 for the diblock copolymer (18 monomers
(344 g/monomer) of the borane). From the UV data (peak at
470 nm), the presence of the diblock copolymer in this hybrid
mixture was confirmed, but the presence of small amounts of tri-
block copolymer or the possibility of organoborane homopolymer
and P3HT fragments cannot be excluded.
In order to further evaluate the optoelectronic properties of
these novel materials, the diblock copolymer hybrid mixture was
fabricated as the active layer in solar cells via spin casting from a
30 mg/mL solution in dichlorobenzene (DCB). As has been observed
with organoborane acceptor OPV systems previously [71], the open
Fig. 5. Cyclic voltammograms of P3HT-tipBPh; CH2Cl2/0.1M Bu4N[PF6], 100 mV sꢁ1; reported relative to the Fc/Fcþ couple.

D.M. Hinkens et al. / Polymer 54 (2013) 3510e3520
3519
circuit voltage was quite high (1.1 V), but the short circuit current
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Funding sources
This material is based upon work supported by the National
Science Foundation under CHE-0836082. This work was supported
in part by the MRSEC Program of the National Science Foundation
under Award #DMR-0212302. Use of the Center for Nanoscale
Materials at Argonne National Laboratory was supported by the U.
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Appendix A. Supplementary data
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.polymer.2013.05.008.

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