One-step synthesis of ladder-type fused poly(benzopentalene) derivatives with tunable energy levels by variable substituents

Article abstract of DOI:10.1002/pola.24419

Novel ladder-type conjugated polymers, fused poly (benzopentalene) derivatives, were synthesized from the readily accessible 1,4-dibromo-2,5- diethynylbenzene derivatives by the Pd-catalyzed self-polycondensation in one-step with high yields. The low solu

Full text of DOI:10.1002/pola.24419

One-Step Synthesis of Ladder-Type Fused Poly(benzopentalene)
Derivatives with Tunable Energy Levels by Variable Substituents
DONG WANG,1 TSUYOSHI MICHINOBU1,2
1Global Edge Institute, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo 152-8550, Japan
2PRESTO, Japan Science and Technology Agency (JST), Japan
Received 22 August 2010; accepted 24 September 2010
DOI: 10.1002/pola.24419
Published online 18 November 2010 in Wiley Online Library (wileyonlinelibrary.com).
ABSTRACT: Novel ladder-type conjugated polymers, fused poly
(benzopentalene) derivatives, were synthesized from the read-
ily accessible 1,4-dibromo-2,5-diethynylbenzene derivatives by
the Pd-catalyzed self-polycondensation in one-step with high
yields. The low solubility of the ladder structure was suggested
when the triisopropylsilyl substituents were selected. However,
when longer alkyl chains were introduced into the peripheral
moieties, such as the dialkylanilino (DAA) and alkyloxyphenyl
groups, a high solubility was achieved and the number-aver-
age molecular weight (Mn) reached 18,000. The UV-Vis absorp-
tion spectral shapes of the polymers were similar to the
reported dibenzopentalene derivatives, except for the batho-
chromically shifted end absorptions. This result suggests an
extension of the p-conjugated systems due to the polymeriza-
tion. Moreover, the almost defect-free structure of the ladder-
type polymers was confirmed by the quantitative tetracyano-
ethylene (TCNE) addition to the DAA-activated alkynes. The
titration experiments of TCNE to the polymers revealed the
number of terminal alkynes, which enabled us to calculate the
molecular weight of the polymers. The calculated molecular
weight was consistent with that determined by GPC. After the
TCNE addition, the polymer band gaps reasonably decreased
as suggested by the UV-Vis-NIR absorption and electrochemi-
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cal measurements.
2010 Wiley Periodicals, Inc. J Polym Sci
Part A: Polym Chem 49: 72–81, 2011
KEYWORDS: conjugated polymers; electrochemistry; functionali-
zation of polymers; ladder-type polymers
INTRODUCTION Fully-conjugated ladder-type polymers have
received considerable attention because of their excellent
carrier mobility and luminescence properties for organic
electronic device applications.1 Conventional synthetic routes
to produce such ladder-type polymers mainly relies upon
the precursor polymer synthesis followed by the high-yield-
ing intramolecular cyclization.2,3 However, this multistep
procedure often encounters defect problems caused by the
unreacted moieties or side reactions during the cyclization
steps. Therefore, an efficient one-step approach, in which the
ladder structures are generated in a single reaction, is gener-
ally desired, but the number of successful examples is
limited.4
During the course of this study, we found that some aro-
matic monomers containing haloenyne structures, such as
the 3-alkyne-substituted-2,5-dibromothiophene derivatives,
do not provide the desired high molecular weight conjugated
polymers under the transition metal-catalyzed reductive
coupling conditions.7 A literature search suggested that the
reductive homocoupling proceeded to afford diarylpentalene
derivatives as the main product and biaryl derivatives as the
major byproduct (Scheme 2).8 Recently, Levi and Tilley
reported the optimized conditions using Pd catalysts, which
produces negligible biaryl derivatives and small amounts of
hydrodehalogenated byproducts.9 This prompted us to
design new bis-functionalized monomers containing two
haloeneyne units and apply the optimized reductive homo-
coupling conditions to the one-step synthesis of novel lad-
der-type polymers.10 Although the biaryl structures become
a defect of the ladder-type polymers, hydrodehalogenation
only results in the termination of the self-polycondensation.
We now report the successful one-step synthesis of ladder-
type fused poly(benzopentalene) derivatives and further
postfunctionalization by the quantitative addition reaction of
a small acceptor molecule, TCNE, to the terminal electron-
rich alkynes. The alkyne-TCNE reaction could not only
We recently started a project involving the postfunctionaliza-
tion of aromatic polymers based on click chemistry-type
addition reactions between electron-rich alkynes and strong
acceptor molecules, such as tetracyanoethylene (TCNE), for
tuning the polymer properties.5 When dialkylanilino or ferro-
cenyl groups are employed as the electron-donating group, a
[2þ2] cycloaddition followed by the ring opening reaction
proceeds in a quantitative yield under mild conditions
(Scheme 1).6 By using this reaction, the polymer energy lev-
els and thermal stability were excellently controlled.
Additional Supporting Information may be found in the online version of this article. Correspondence to: T. Michinobu (E-mail: michinobu.t.aa@
m.titech.ac.jp)
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Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 49, 72–81 (2011) 2010 Wiley Periodicals, Inc.
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cating the intrinsic low solubility of the fused poly(benzo-
pentalene) building block (Table 1). This problem was solved
by the introduction of long alkyl chains. Thus, the dihexade-
cylaniline (DHA)-appended monomers 1–3 afforded the cor-
responding ladder-type polymers P1–P3 with much higher
Mn values and narrow polydispersities (Mw/Mn) in about
90% yield (Table 1). The origin of the narrow polydisper-
sities is not clear, although a chain-growth type polymeri-
zation was recently reported for some conjugated polymer
syntheses.11 The polymerization time-monomer conversion
SCHEME 1 Click chemistry-type addition reaction between
TCNE and alkynes activated by electron-donating group (EDG).
change the polymer energy levels, but also quantify the
amount of alkynes in the polymers, which became evidence
for the absence of the undesired side reactions during the
polymerization.
RESULTS AND DISCUSSION
Synthesis
Both symmetric and asymmetric bifunctional monomers
with various alkyne substituents were synthesized by the se-
quential Sonogashira cross-coupling and the silyl deprotec-
tion protocol. Starting from 1,4-dibromo-2,5-diiodobenzene,
the selective introduction of two equivalents of Me3SiCBCH
or iPr3SiCBCH into the iodo positions provided 5 or 4 in
about 90% yield, respectively (Scheme 3). The subsequent
silyl deprotection of 5 with K2CO3 followed by the Sonoga-
shira coupling with N,N-dihexadecyl-4-iodoaniline afforded a
symmetric monomer 1 in 78% yield. For the asymmetric
monomer synthesis, we first tried the partial deprotection of
5 by changing the K2CO3 amount and reaction time. How-
ever, all attempts were unsuccessful. Therefore, we went
back to the starting material, and one equivalent of
Me3SiCBCH was selectively reacted with 1,4-dibromo-2,5-
diiodobenzene under Sonogashira conditions to afford 7 in
25% yield. The further Sonogashira coupling of 7 with 4-
ethynyl-N,N-dihexadecylaniline successfully provided 8 in a
reasonably higher yield (63%). The subsequent silyl depro-
tection followed by Sonogashira coupling with the iodoben-
zene derivatives finally afforded the desired asymmetric
monomers 2 and 3 in 90% and 66% yields, respectively. All
the monomers were fully characterized by NMR, IR, MS spec-
tra, and elemental analysis.
All the bifunctional monomers 1–4 were subjected to the
self-polycondensation under the Pd-catalyzed conditions,
originally developed by Levi and Tilley9 (Scheme 4). In a
typical procedure, a mixture of the monomer, Pd2(dba)3,
P(tBu)3, Cs2COꢀ3, CsF, and hydroquinone in 1,4-dioxane was
heated to 135 C under N2, and the reaction time was deter-
mined to be 48 h based on the saturation of the molecular
weight increase (vide infra). When triisopropylsilyl groups
were used as the substituent, the number-average molecular
weight (Mn) of the obtained polymer P4 was only 2400, indi-
SCHEME 3 Synthesis of 1,4-dibromo-2,5-diethynylbenzene
monomers. (a) HCBCSiMe3, PdCl2(PPh3)2, CuI, iPr2NH, 5: 90%,
7: 25%. (b) K2CO3, CH3OH/THF, 100%. (c) N,N-dihexadecyl-4-
iodoaniline, Pd(PPh3)4, CuI, iPr2NH/THF, 78%. (d) 4-ethynyl-N,N-
dihexadecylaniline, Pd(PPh3)4, CuI, iPr2NH/THF, 63%. (e) 1-iodo-
4-nitrobenzene, Pd(PPh3)4, CuI, iPr2NH/THF, 90%. (f) 1-(hexade-
cyloxy)-4-iodobenzene, Pd(PPh3)4, CuI, iPr2NH/THF, 66%.
SCHEME 2 Reductive homocoupling of o-halogenoethynylben-
zene derivatives.
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
SCHEME 4 Synthesis of fused
poly(benzopentalene) derivatives.
(a) Pd2(dba)3, P(tBu)3, Cs2CO3, CsF,
hydroquinone, 1,4-dioxane, D.
efficiency was investigated for the symmetric monomer 1. the main chain were not clear in these spectra. The aromatic
The GPC profile suggested that a small amount of monomer protons of P4 corresponding to the main chain benzo pro-
remained after 24 h and the Mn and Mw/Mn of the polymer- tons were very broad. The alkyl chain peaks of all polymers
ized fraction were 6400 and 1.20, respectively [Fig. 1(a)]. were observed at the reasonable positions. The IR spectra of
When the polymerization time was increased to 48 h, all the all the polymers exhibited a weak alkyne vibrational peak in
monomers were converted into a higher molecular weight the range of 2203–2208 cmꢁ1, suggesting the presence of
polymer P1 with an Mn of 11000 and Mw/Mn of 1.23. The detectable alkyne amounts. A thermogravimetric analysis
UV-Vis-NIR spectral change from 1 to the produced polymers (TGA) revealed the high thermal stability with a 5% weight
P1 also suggested the bathochromic shift of the end absorp- loss temperature exceeding 280 ꢀC (Table 1 and Fig. 3).
tion (kend) with the increasing polymerization time. Thus, a
kend of 1 at 522 nm (2.38 eV) in THF shifted to 662 nm
(1.87 eV) and 815 nm (1.52 eV) after the polymerization
time of 24 and 48 h, respectively [Fig. 1(b)]. This dramatic
shift in the kend values clearly indicated the polymerization
progress or the extension of the effective conjugation length
of the polymer main chain.
All polymers showed good solubilities in the common
organic solvents, such as chloroform, 1,2-dichlorobenzene,
and THF, and they were characterized by 1H NMR and IR
spectroscopies. The 1H NMR spectrum of P1 displayed the
well-defined two sets of aromatic peaks at 6.53 and 7.33
ppm originating from the aniline moieties (Fig. 2). In con-
trast, P2 and P3 prepared from the asymmetric monomers
showed more complex peaks in the aromatic region,
although the phenylene ring protons of the side chains were
almost clearly observed. Unfortunately, the benzo protons of
TABLE 1 Summary of the Polymerization Results and
Thermal Properties
T
5%/ꢀCb
a
a
Polymer
Yield (%)
Mn
Mw/Mn
P1
P2
P3
P4
a
92
89
90
68
11,000
7,200
1.23
1.29
1.44
1.27
330
280
347
280
18,000
2,400
FIGURE 1 (a) GPC profiles detected by UV absorption at
254 nm (eluent: THF) and (b) normalized UV-Vis-NIR absorption
spectra (THF) of the polymerization products of 1 after 0, 24,
and 48 h.
Molecular weights determined by GPC (eluent: THF, calibrated by
polystyrene standards).
b
The 5% weightꢁl1oss temperatures determined by TGA at the heating
rate of 10 ꢀC min
.
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FIGURE 4 Normalized UV-Vis-NIR absorption spectra of the
ladder-type polymers in THF at 20 ꢀC.
FIGURE 2 1H NMR spectra of P1–P4 in CDCl3 at 20 ꢀC. Inset:
porting Information Fig. 1). These features are again similar
to those of the reported diarylpentalene monomers.8(c)
Enlarged aromatic region.
The redox properties of the P1–P4 thin films were investi-
gated using cyclic voltammetry (CV) in CH3CN with 0.1 M
(nC4H9)4NClO4 as the supporting electrolyte. All polymer
films showed irreversible redox behaviors and the observed
peak potentials are listed in Table 2. The CV of the P4 thin
film revealed the intrinsic electrochemical properties of the
poly(benzopentalene) framework, considering the neutral
feature of the silyl substituents. The oxidation and reduction
peaks of P4 were detected at 1.45 and ꢁ2.07 V, respectively
(Table 2 and Fig. 5). The electronic HOMO and LUMO levels
of P4 were roughly estimated to be ꢁ6.1 and ꢁ2.8 eV from
the onset oxidation and reduction potentials, respectively,
based on the assumption of Fcþ/Fc ¼ ꢁ4.80 eV. Replacement
of the silyl groups by electron-donating DHA moieties ele-
vated the HOMO levels. Thus, P1–P3 displayed the aniline-
centered well-defined oxidation peak at about 0.52–0.56 V.
These oxidation peaks suggested the corresponding HOMO
level of about ꢁ5.1 eV. Although P1 and P3 did not exhibit
any clear reduction peaks in the potential range expected
from the optical band gaps, P2 showed a very weak reduc-
tion peak at ꢁ1.69 V, originating from the electron-with-
drawing nitrophenyl group. The calculated electrochemical
band gap of P2 was in good agreement with the optical
band gap.
Electron-donating substituents tended to enhance the ther-
mal stability of the polymers. Thus, P1 and P3 with dialkyla-
nilino and alkyloxyphenyl donors showed about 50–70 ꢀC
higher T5% values than P2 with the nitrophenyl acceptors
and P4 without any donor groups.
UV-Vis-NIR Spectra and Electrochemistry
The UV-Vis-NIR absorption spectra of P1–P4 displayed
unfeatured broad bands in the low energy region, which are
ascribed to the extension of the conjugated main chain (Fig.
4). The positions of kmax and kend also depended on the sub-
stituents. The spectral shapes and positions are very similar
to the reported monomeric diarylpentalene derivatives.8(b,c)
It was reported that substitution of the electron donors pro-
duces a lower energy absorption, and such absorptions are
usually composed of weak vibrational peaks. Similar to this
report, the electron-donating DHA group tended to batho-
chromically shift the polymer spectra relative to P4, and the
extent of the shift increased in the order of P3<P2<P1.
Thus, the shift in kend from that of P4 (587 nm) is 94, 218,
and 244 nm for P3, P2, and P1, respectively. Despite the
fused polymer structures, all polymers were weakly fluores-
cent in THF (Table 2). There were mainly two peak maxima
(kems) in the range of 450–480 nm and 500–550 nm (Sup-
Alkyne-TCNE Reaction
MALDI-TOF MS is a powerful technique for determining
exact molecular weights, which helps us estimate the possi-
ble chemical structures of most polymers. MALDI-TOF MS
measurements of P1 were attempted by using various matri-
ces, and the best spectrum of P1 clearly exhibited the mole-
cular ionic peaks of the brominated and hydrogenated
terminal structures (Fig. 6). Based on the peak intensity,
hydrodehalogenation is the main termination process of the
polymerization. However, this result does not rule out the
presence of biaryl structural defects [chemical structure (b)
in Fig. 6] caused by a side reaction, although it was reported
that the Pd catalysts gave no isolable biaryl byproducts.9 To
indirectly prove the negligible amounts of such defects, the
reductive homocoupling of 4-[(2-bromophenyl)ethynyl]-N,N-
FIGURE 3 TGA curves of P1–P4 at the heating rate of 10 ꢀC
minꢁ1
.
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
TABLE 2 Optical and Electrochemical Properties of P1–P4
ox
peak
(V)b,c
red
kab
(nm)a
kem (nm)
[kex (nm)]a
kend (nm)
[eV]a
E
E
peak
Polymer
(V)b,c
P1
P2
P3
P4
a
369, 568, 652
352, 590
453, 500 [369]
478, 509 [352]
457, 549 [321]
452, 526 [270]
831 [1.49]
805 [1.54]
681 [1.82]
587 [2.11]
0.56
0.52
0.56
1.45
–
ꢁ1.69
–
321, 580
270, 515
ꢁ2.07
Measured in THF at 20 ꢀC. Emission spectra were measured at a concentration of 10ꢁ5 M/repeat unit.
b
Potentials versus Fcþ/Fc couple. Working electrode: glassy carbon electrode; counter electrode: Pt wire;
reference electrode: Ag/Agþ.
c
ox
peak
red
E
and E
represent the first oxidation peak top potential and the first reduction peak top potential,
peak
respectively, measured as thin films in CH3CN containing 0.1 M (nC4H9)4NClO4 at the scanning rate of 0.1
V sꢁ1 under flowing nitrogen.
dihexadecylaniline 10 was examined as a model reaction
the DHA-substituted alkynes reacted, while the other alkynes
remained unchanged under the employed conditions. Based
on this knowledge, a TCNE solution was stepwise added to
the polymer solutions and the UV-Vis-NIR spectral changes
were monitored. For the reaction completion, the 1,2-
dichlorobenzene solutions were heated to 170 ꢀC for 2–4 h,
a temperature that was determined from the TGA measure-
ments (vide supra). For P1–P3, the intensity of the longer
wavelength absorptions increased with the increasing
amount of the TCNE addition, suggesting the presence of
charge-transfer bands ascribed to the donor-acceptor
chromophore formation (Fig. 7 and Supporting Information
Fig. 2). The presence of the isosbestic points indicated no
side reactions. In contrast, the visible spectrum of P4 with-
out DHA subsituents did not change during the TCNE titra-
tion experiments. In the case of P1, the increase in the
670 nm peak intensity became saturated when the TCNE
addition amount was 57.4 nmol, corresponding to the DHA
substituted alkyne amount of 0.146 mmol gꢁ1 [Fig. 7(b)
(Scheme 5). After the reaction for 24 h under the same con-
ditions as shown in Scheme 4, the products were separated
by column chromatography on silica gel, yielding the desired
dibenzopentalene derivative 11 (yield 53%) and no isolable
amount of the biaryl product 12. Note that the low isolated
yield of 11 is ascribed to the chemical instability toward the
acidic silica gel due to the absence of substituents at the
high electron density positions. Assuming the similar chemi-
cal stability of 11 and 12, it is thought that the fused dimer
11 is the only main product.
To further verify the polymer structures and pursue the con-
trolled postfunctionalization procedure, acceptor addition to
the electron-rich alkynes in the polymers was evaluated. It is
known that alkynes activated by dialkylaniline donors
undergo a quantitative addition reaction with TCNE, furnish-
ing donor-acceptor chromophores with charge-transfer
bands in a low energy region.12 When the reactivity of TCNE
to the monomers 1–4 was examined, it was found that only
FIGURE 5 Cyclic voltammograms of
the polymer thin films (a) P1, (b) P2,
(c) P3, and (d) P4 in CH3CN with
0.1 M (nC4H9)4NClO4 at 20 ꢀC, at the
scanning rate of 0.1 V sꢁ1
.
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FIGURE 6 MALDI-TOF-MS spectrum of P1, prepared by the self-polycondensation of 1 for 24 h (matrix, dithranol), and the possible
chemical structures: (a) fully fused poly(benzopentalene) and (b) poly(benzopentalene) containing biaryl defects.
inset]. Considering that the reaction occurs at both terminals,
the molecular weight was calculated to be 13,700 g molꢁ1
Finally, the TCNE adducted polymers P5–P7 were prepared
from P1–P3, and their optical and electrochemical properties
were investigated (Supporting Information Scheme 1). As
shown in the UV-Vis-NIR spectra during the TCNE titration
experiments, the intensity of the longer wavelength absorp-
tion bands gradually increased and the kend values of P5–P7
bathochromically shifted compared to those of the corre-
sponding precursor polymers P1–P3 (Supporting Informa-
tion Table 1). The optical band gap of P5 was the narrowest,
reaching 1.15 eV. The CV of the P5–P7 thin films measured
in CH3CN containing 0.1 M (nC4H9)4NClO4 displayed a reduc-
tion peak ascribed to the newly formed 1,1,4,4-tetracyanobu-
tadiene moieties (Supporting Information Fig. 3). The
appearance of the lower reduction potentials suggests a
lowering of the polymer LUMO levels or enhancement of the
n-type character by the TCNE addition. The observed first
,
suggesting good agreement with that determined by GPC
and, consequently, an almost defect-free ladder structure.
Note that the exact determination of the specific alkyne
amounts in polymers has been elusive for a long time, since
the 13C NMR and IR spectroscopies do not meet this purpose
and elemental analysis usually encounters a problem due to
indefinite polymer terminal structures.13 Therefore, these
conventional methods were not able to elucidate the alkyne
amounts or the polymer structures in this case. The alkyne-
TCNE addition reaction is, to the best of our knowledge, the
first report on the perfect quantification of the specific
alkyne amounts in polymers. In a way similar to P1, the
DHA-activated alkynes in P2 and P3 were also estimated to
be 0.209 mmol gꢁ1 and 0.0962 mmol gꢁ1, respectively.14
SCHEME 5 Synthesis of dibenzopentalene derivatives. (a) Pd2(dba)3, P(tBu)3, Cs2CO3, CsF, hydroquinone, 1,4-dioxane, D.
ONE-STEP SYNTHESIS OF FUSED POLY(BENZOPENTALENE), WANG AND MICHINOBU
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
alkyne-acceptor addition reaction. In previous studies, this
reaction was employed as an effective postfunctional modifi-
cation method to introduce donor-acceptor chromophores
into various materials or enhance the n-type characters of
aromatic polymers.16 This study revealed the new utility of
this reaction, namely the exact quantification of specific
alkyne amounts in polymers. The synthetic and postfunc-
tional procedures can be extended to a variety of ladder-
type conjugated polymers composed of heterocyclic aromatic
rings and their organic device applications.
EXPERIMENTAL
General
Chemicals were purchased from Kanto, Tokyo Kasei, Wako,
and Aldrich and used as received. 1,4-Dioxane was distilled
under nitrogen from sodium metal. 1,4-Dibromo-2,5-diiodo-
benzene,17 [(2,5-dibromobenzene-1,4-diyl)diethyne-2,1-diyl]-
bis(trimethylsilane)18 5, N,N-dihexadecyl-4-iodoaniline,19 4-
ethynyl-N,N-dihexadecylaniline,19 and [(2,5-dibromobenzene-
1,4-diyl)diethyne-2,1-diyl]bis(tripropan-2-ylsilane)20 4 were
synthesized according to the literature methods. All reactions
were carried out in a dry reaction vessel by applying a posi-
tive pressure of dry N2 or Ar. Air or moisture-sensitive
liquids and solutions were transferred via syringe. 1H NMR
and 13C NMR spectra were measured on a JEOL model
AL300 spectrometer at 20 ꢀC. Chemical shifts are reported
in ppm downfield from SiMe4, using the solvent’s residual
signal as an internal reference. The resonance multiplicity is
described as s (singlet), d (doublet), and m (multiplet).
Infrared spectra (IR) were recorded on a JASCO FT/IR-4100
spectrometer. Elemental analyses were performed on a
PerkinElmer 2400-Series II CHNS/O analyzer. Gel permeation
chromatography (GPC) was measured on a Shodex system
equipped with polystyrene gel columns using THF as an elu-
ent after calibration with standard polystyrenes. MALDI-TOF-
MS spectra were measured on a Shimadzu AXIMA-CFR mass
spectrometer. The operation was performed at an accelerat-
ing potential of 20 kV by a linear positive ion mode with
dithranol as a matrix. UV/Vis spectra were recorded in a
quartz cuvette on a JASCO V-670 spectrophotometer. Fluores-
cence spectra were measured on a JASCO FP6500 spectro-
photometer. Thermogravimetric analysis (TGA) was carried
out on a Seiko SII TG/DTA 6200 under nitrogen flow at the
scanning rate of 10 ꢀC minꢁꢀ 1. Electrochemistry measure-
ments were carried out at 20 C in a classical three-electrode
cell. The working and auxiliary electrodes were a glassy
carbon disk electrode (2 mm in diameter) and a platinum
wire, respectively. The reference electrode was Ag/Agþ/
CH3CN/(nC4H9)4NClO4. All potentials were referenced to the
ferricinium/ferrocene (Fcþ/Fc) couple used as an internal
standard.
FIGURE 7 (a) TCNE addition reaction to the electron-rich
alkynes at the polymer terminals and (b) the UV-Vis-NIR spec-
tral change of P1 (1.20 g Lꢁ1; initial volume: 0.327 mL in a
1
mm cuvette) in 1,2-dichlorobenzene during the titration
experiment with a 5.52 mM TCNE solution in 1,2-dichloro-
ethane at 170 ꢀC. Inset: Plots of TCNE addition amount versus
absorbance increase at 670 nm.
oxidation peak potentials of P5 and P7 anodically shifted
relative to those of the corresponding precursor polymers
P1 and P3, respectively, implying the efficient intramolecular
donor-acceptor interactions.15 In contrast, the first oxidation
peak potential of P6 originally substituted by both the DHA-
donor and nitrophenyl acceptor moieties cathodically shifted
relative to that of the precursor polymer P2, and the calcu-
lated polymer energy level was shallower than those of P5
and P7. Overall, the electrochemical band gaps of all the
TCNE adducted polymers were consistent with the optical
band gaps.
COLCLUSIONS
We have developed a one-step synthetic route to ladder-type
conjugated polymers, that is, fused poly(benzopentalene)
derivatives, by the Pd-catalyzed self-polycondensation. The
intrinsic low solubility of the polymer main chain was
improved by the introduction of long alkyl chains at the
polymer peripheral positions. Thus, the detailed optical and
electrochemical properties of the polymers were investi-
gated, and it was found that the ladder-type polymers pos-
sess properties similar to the monomeric dibenzopentalene
derivatives, and such properties can be tunable by the
peripheral electron-donating and accepting substituents.
Moreover, the synthesis of the almost defect-free planar
structures was definitely proved by the click chemistry-type
1,4-Dibromo-2,5-diethynylbenzene (6)
To a solution of [(2,5-dibromobenzene-1,4-diyl)diethyne-2,1-
diyl]bis(trimethylsilane)17 5 (1.28 g, 3.00 mmol) in THF/
CH3OH 3:1 (16 mL), K2CO3 (2.5 g, 18 mmol) was added. The
reaction mixture was stirred at r.t. for 5 h. After evaporation
of the solvents, the mixture was purified by column
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chromatography (SiO2, CH2Cl2) to give 6 (0.82 g, 96%) as a
white solid.
CH2Cl2, and filtered through a plug of silica gel. The solvent
was removed in vacuo and the crude product was purified
by column chromatography (SiO2, hexane/CH2Cl2 100:3) to
give 8 (0.34 g, 63%) as a yellow solid.
1H NMR (CDCl3, 300 MHz): d ¼ 3.50 (s, 2 H), 7.72 ppm (s, 2
H); 13C NMR (CDCl3, 75 MHz): d ¼ 80.3, 84.7, 123.7, 126.0,
137.0 ppm.
1H NMR (CDCl3, 300 MHz): d ¼ 0.28 (s, 9 H), 0.88 (m, 6 H),
1.26 (m, 52 H), 1.54 (m, 4 H), 3.28 (m, 4 H), 6.57 (d, J ¼ 9.0
Hz, 2 H), 7.38 (d, J ¼ 9.0 Hz, 2 H), 7.67 (s, 1 H), 7.68 ppm
(s, 1 H); 13C NMR (CDCl3, 75 MHz): d ¼ -0.27, 14.1, 22.7,
27.11, 27.19, 29.36, 29.50, 29.60, 29.64, 29.66, 29.71, 31.9,
51.0, 85.3, 99.4, 101.8, 102.1, 107.4, 111.2, 123.0, 123.7,
124.8, 127.9, 133.2, 135.2, 136.3, 148.6 ppm; FTIR (KBr): m
¼ 2923, 2851, 2200, 1607, 1521, 1465, 1400, 1250, 1061,
861, 811, 760 cmꢁ1; MALDI-TOF-MS (dithranol): m/z: calcd
for C51H81Br2NSi: 893.45 g molꢁ1, found: 895.7 g molꢁ1
[MH]þ; elemental analysis calcd (%) for C51H81Br2NSi
(896.09): C 68.36, H 9.11, N 1.56; found: C 68.27, H 9.14,
N 1.54.
4,40-[(2,5-Dibromobenzene-1,4-diyl)diethyne-
2,1-diyl]bis(N,N-dihexadecylaniline) (1)
In a 500 mL round bottom flask, 6 (0.57 g, 2.0 mmol) and
N,N-dihexadecyl-4-iodoaniline (2.94 g, 4.40 mmol) were dis-
solved in iPr2NH/THF 1:1 (200 mL). After the solution was
purged with bubbling Ar for 30 min, Pd(PPh3)4 (139 mg,
0.120 mmol) and CuI (46 mg, 0.24 mmol) were added. The
reaction mixture was then stirred at r.t. for 12 h under Ar.
The mixture was concentrated, rediluted with CH2Cl2, and fil-
tered through a plug of silica gel. The solvent was removed
in vacuo and the crude product was purified by column
chromatography (SiO2, hexane/CH2Cl2 10:1) to give
(2.13 g, 78%) as a yellow solid.
1
4-[(2,5-Dibromo-4-ethynylphenyl)ethynyl]-
N,N-dihexadecylaniline (9)
1H NMR (CDCl3, 300 MHz): d ¼ 0.88 (m, 12 H), 1.27 (s, 104
H), 1.58 (m, 8 H), 3.28 (m, 8 H), 6.57 (d, J ¼ 8.7 Hz, 4 H),
7.39 (d, J ¼ 8.7 Hz, 4 H), 7.69 ppm (s, 2 H); 13C NMR
(CDCl3, 75 MHz): d ¼ 14.1, 27.1, 27.2, 29.36, 29.49, 29.60,
29.65, 29.67, 29.70, 31.9, 51.0, 85.4, 98.5, 107.7, 111.1, 123.1,
126.2, 133.2, 135.2, 148.4 ppm; FTIR (KBr): m ¼ 2920, 2851,
2211, 1604, 1522, 1466, 1402, 1372, 1299, 1190, 1138,
1057, 880, 810, 721, 528 cmꢁ1; UV-Vis (1,2-dichloroben-
zene): k ¼ 428 nm; MALDI-TOF-MS (dithranol): m/z: calcd
for C86H142Br2N2: 1360.95 g molꢁ1, found: 1361.5 g molꢁ1
[MH]þ; elemental analysis calcd (%) for C86H142Br2N2
(1363.87): C 75.73, H 10.49, N 2.05; found: C 75.05, H
10.64, N 2.00.
To a solution of 8 (0.25 g, 0.28 mmol) in THF/CH3OH 3:1
(4 mL), K2CO3 (0.12 g, 0.84 mmol) was added. The reaction
mixture was stirred at r.t. for 5 h. After evaporation of the
solvents, the mixture was purified by column chromatogra-
phy (SiO2, CH2Cl2) to give 9 (0.23 g, 100%) as a yellow
solid.
1H NMR (CDCl3, 300 MHz): d ¼ 0.89 (m, 6 H), 1.27 (m, 52
H), 1.58 (m, 4 H), 3.28 (m, 4 H), 3.46 (s, 1 H), 6.57 (d, J ¼
8.4 Hz, 2 H), 7.39 (d, J ¼ 8.7 Hz, 2 H), 7.69 (s, 1 H), 7.71
ppm (s, 1 H); 13C NMR (CDCl3, 75 MHz): d ¼ 14.1, 22.7,
27.11, 27.20, 29.36, 29.50, 29.60, 29.66, 29.69, 31.9, 51.0, 80.8,
83.6, 85.2, 99.7, 107.4, 111.2, 123.1, 123.6, 123.8, 128.5,
133.3, 135.3, 136.8, 148.6 ppm; FTIR(KBr): m ¼ 3290, 2924,
2852, 2197, 1606, 1571, 1521, 1465, 1369, 1063, 888, 812
[(2,5-Dibromo-4-iodophenyl)ethynyl]
(trimethyl)silane (7)
cmꢁ1
48H73Br2N: 821.41 g molꢁ1, found: 823.8 g molꢁ1 [MH]þ.
;
MALDI-TOF-MS (dithranol): m/z: calcd for
In a 500 mL round bottom flask, 1,4-dibromo-2,5-diiodoben-
zene (6.39 g, 13.1 mmol) was dissolved in iPr2NH/THF 1:1
(280 mL). After the solution was purged with bubbling Ar
for 30 min, trimethylsilylacetylene (1.29 g, 13.1 mmol),
PdCl2(PPh3)2 (180 mg, 0.256 mmol), and CuI (100 mg, 0.525
mmol) were added. The reaction mixture was then stirred at
r.t. for 12 h under Ar. The mixture was concentrated, redi-
luted with CH2Cl2, and filtered through a plug of silica gel.
The solvent was removed in vacuo and the crude product
was purified by column chromatography (SiO2, hexane) to
give 7 (1.50 g, 25%) as a yellow solid.
C
4-({2,5-Dibromo-4-[(4-nitrophenyl)ethynyl]phenyl}
ethynyl)-N,N-dihexadecylaniline (2)
In a 100 mL round bottom flask, 9 (0.165 g, 0.200 mmol)
and 1-iodo-4-nitrobenzene (0.075 g, 0.30 mmol) were dis-
solved in iPr2NH/THF 1:1 (6 mL). After the solution was
purged with bubbling Ar for 30 min, Pd(PPh3)4 (8 mg, 0.007
mmol) and CuI (2.5 mg, 0.013 mmol) were added. The reac-
tion mixture was then stirred at r.t. for 12 h under Ar. The
mixture was concentrated, rediluted with CH2Cl2, and filtered
through a plug of silica gel. The solvent was removed in
vacuo and the crude product was purified by column chro-
matography (SiO2, hexane/CH2Cl2 9:1) to give 2 (0.17 g,
90%) as a red solid.
1H NMR (CDCl3, 300 MHz): d ¼ 0.27 (m, 9 H), 7.69 (s, 1 H),
8.04 ppm (s, 1 H).
4-({2,5-Dibromo-4-[(trimethylsilyl)ethynyl]phenyl}
ethynyl)-N,N-dihexadecylaniline (8)
In a 100 mL round bottom flask, 7 (0.275 g, 0.600 mmol)
and N,N-dihexadecyl-4-iodoaniline (0.379 g, 0.670 mmol)
were dissolved in iPr2NH/THF 1:1 (14 mL). After the solu-
tion was purged with bubbling Ar for 30 min, Pd(PPh3)4
(13.8 mg, 0.0120 mmol) and CuI (4.6 mg, 0.024 mmol) were
added. The reaction mixture was then stirred at r.t. for 12 h
under Ar. The mixture was concentrated, rediluted with
1H NMR (CDCl3, 300 MHz): d ¼ 0.87 (m, 6 H), 1.26 (m, 52
H), 1.54 (m, 4 H), 3.29 (m, 4 H), 6.58 (d, J ¼ 8.7 Hz, 2 H),
7.40 (d, J ¼ 8.4 Hz, 2 H), 7.71 (d, J ¼ 8.7 Hz, 2 H), 7.72 (s, 1
H), 7.77 (s, 1 H), 8.24 ppm (d, J ¼ 8.7 Hz, 2 H); 13C NMR
(CDCl3, 75 MHz):d ¼ 14.1, 15.6, 22.7, 27.1, 27.2, 29.36,
29.60, 29.66, 29.69, 31.9, 51.0, 85.4, 91.9, 93.5, 100.4, 107.2,
111.2, 123.2, 123.71, 123.70, 123.9, 128.8, 129.4, 132.4,
ONE-STEP SYNTHESIS OF FUSED POLY(BENZOPENTALENE), WANG AND MICHINOBU
79

JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
133.3, 135.4, 136.2, 147.4, 148.7 ppm; FTIR (KBr): m ¼
2918, 2850, 2200, 1607, 1543, 1512, 1466, 1340, 884,
810 cmꢁ1; UV-Vis (1,2-dichlorobenzene): k ¼ 435, 331 nm;
MALDI-TOF-MS (dithranol): m/z: calcd for C54H76Br2N2O2:
942.43 g molꢁ1, found: 943.4 g molꢁ1 [MH]þ; elemental
analysis calcd (%) for C54H76Br2N2O2 (945.00): C 68.63, H
8.11, N 2.96; found: C 68.65, H 8.25, N 2.90.
1146, 1116, 1043, 1026, 812, 750, 722 cmꢁ1; MALDI-TOF-
MS (dithranol): m/z: calcd for C46H74BrN: 720.99 g molꢁ1
found: 719.3 g molꢁ1 [M-H]þ.
,
4,40-Indeno[2,1-a]indene-5,10-diylbis
(N,N-dihexadecylaniline) (11)
In a 20 mL round bottom flask, hydroquinone (22 mg, 0.20
mmol), Cs2CO3 (65 mg, 0.20 mmol), CsF (33 mg, 0.22
mmol), P(tBu)3 (1.2 mg, 0.0060 mmol), and Pd2(dba)3 (1.4
mg, 0.0016 mmol) were placed under nitrogen. A solution of
10 (0.072 g, 0.1 mmol) in 1,4-dioxane (1 mL) was trans-
ferred into the flask, and the suspension was immediately
heated to 135 ꢀC for 24 h. After cooling to r.t., the reaction
mixture was diluted with toluene (10 mL), filtered through
celite, and then evaporated to dryness to yield a crude red
solid. A single TLC spot was isolated by column chromato-
graphy (SiO2, hexane/CH2Cl2 8:2). The isolated dark red
solid was further purified by a recycling HPLC to yield 11
(0.034 g, 53%) as a main product and a trace amount of
byproduct with the same molecular weight as 11 (possible
chemical structure 12).
4-[(2,5-Dibromo-4-{[4-(hexadecyloxy)phenyl]ethynyl}
phenyl)ethynyl]-N,N-dihexadecylaniline (3)
In a 100 mL round bottom flask, 9 (0.165 g, 0.200 mmol)
and 1-(hexadecyloxy)-4-iodobenzene (0.107 g, 0.241 mmol)
were dissolved in iPr2NH/THF 1:1 (8 mL). After the solution
was purged with bubbling Ar for 30 min, Pd(PPh3)4 (7 mg,
0.006 mmol) and CuI (3 mg, 0.02 mmol) were added. The
reaction mixture was then stirred at r.t. for 12 h under
Ar. The mixture was concentrated, rediluted with CH2Cl2,
and filtered through a plug of silica gel. The solvent was
removed in vacuo and the crude product was purified by
column chromatography (SiO2, hexane/CH2Cl2 10:1) to give
3 (0.15 g, 66%) as a yellow solid.
1H NMR (CDCl3, 300 MHz): d ¼ 0.89 (m, 9 H), 1.27 (s, 76
H), 1.44 (m, 2 H), 1.56 (m, 4 H), 1.79 (m, 2 H), 3.28 (m, 4
H), 3.97 (m, 2 H), 6.57 (d, J ¼ 9.0 Hz, 2 H), 6.88 (d, J ¼ 9.0
Hz, 2 H), 7.39 (d, J ¼ 8.7 Hz, 2 H), 7.49 (d, J ¼ 9.0 Hz, 2 H),
7.71 (s, 1 H), 7.72 ppm (s, 1 H); 13C NMR (CDCl3, 75 MHz):
d ¼ 14.1, 22.7, 26.0, 27.1, 27.2, 29.15, 29.37, 29.50, 29.56,
29.59, 29.61, 29.66, 29.71, 31.9, 51.0, 68.1, 85.4, 85.9, 96.5,
99.0, 107.5, 111.1, 114.2, 114.6, 123.1, 123.4, 125.4, 127.1,
133.2, 133.3, 135.2, 135.6, 148.5, 159.8 ppm; FTIR (KBr):
m ¼ 2954, 2922, 2850, 2215, 1606, 1571, 1522, 1468, 1413,
1H NMR (CDCl3, 300 MHz): d ¼ 0.88 (m, 12 H), 1.26 (s, 104
H), 1.65 (m, 8 H), 3.33 (m, 8 H), 6.73 (d, J ¼ 9.0 Hz, 4 H),
6.86 (m, 4 H), 7.16 (m, 2 H), 7.36 (m, 2 H), 7.57 ppm (d, J ¼
8.4 Hz, 4 H); 13C NMR (CDCl3, 75 MHz): d ¼ 14.1, 22.7, 27.3,
27.9, 29.4-29.7, 31.9, 51.9, 110.9, 120.6, 121.7, 122.5, 126.6,
126.7, 130.1, 136.0, 139.8, 140.6, 148.2, 149.6 ppm; FTIR
(KBr): m ¼ 2920, 2850, 1602, 1516, 1468, 1430, 1404, 1372,
1346, 1313, 1299, 1269, 1250, 1229, 1195, 1152, 1136,
1089, 833, 812, 746, 721 cmꢁ1; UV-Vis (1,2-dichloroben-
zene): k ¼ 386, 400, 528 nm; MALDI-TOF-MS (dithranol):
m/z: calcd for C92H148N2: 1281.16 g molꢁ1, found: 1282 g
molꢁ1 [M]þ.
1362, 1289, 1250, 1174, 1058, 1028, 827, 812, 531 cmꢁ1
;
UV-Vis (1,2-dichlorobenzene): k ¼ 411 nm; MALDI-TOF-MS
(dithranol): m/z: calcd for C70H109Br2NO: 1140.43 g molꢁ1
found: 1140 g molꢁ1 [M]þ; elemental analysis calcd (%) for
70H109Br2NO (1140.43): C 73.72, H 9.63, N 1.23; found: C
,
General Polymerization Procedure
C
In a 20 mL round bottom flask, hydroquinone (2.00 eq),
Cs2CO3 (2.00 eq), CsF (2.20 eq), P(tBu)3 (0.06 eq), and
Pd2(dba)3 (0.016 eq) were placed under nitrogen. A solution
of 1,4-dibromo-2,5-diethynylbenzene derivatives in 1,4-diox-
ane (0.1 M) was transferred into the flask, and the suspen-
sion was immediately heated to 135 ꢀC. After cooling to r.t.,
the reaction mixture was poured to CH3OH. The precipitate
was collected and dried in vacuo.
73.91, H 9.76, N 1.22.
4-[(2-Bromophenyl)ethynyl]-N,N-dihexadecylaniline (10)
In a 100 mL round bottom flask, 1-bromo-2-iodobenzene
(0.113 g, 0.399 mmol) and 4-ethynyl-N,N-dihexadecylaniline
(0.226 g, 0.399 mmol) were dissolved in iPr2NH/THF 1:1
(10 mL). After the solution was purged with bubbling Ar for
30 min, Pd(PPh3)4 (14 mg, 0.012 mmol) and CuI (6 mg, 0.03
mmol) were added. The reaction mixture was then stirred at
r.t. for 12 h under Ar. The mixture was concentrated, redi-
luted with CH2Cl2, and filtered through a plug of silica gel.
The solvent was removed in vacuo and the crude product
was purified by column chromatography (SiO2, hexane/
CH2Cl2 8:2) to give 10 (0.15 g, 52%) as a liquid.
P1. Yield 92% (125 mg); 1H NMR (CDCl3, 300 MHz): d ¼
0.88 (m), 1.26 (s), 1.58 (br s), 3.24 (br s), 6.53 (br s), 7.33
ppm (br s); FTIR (KBr):m ¼ 2923, 2853, 2203, 1606, 1519,
1466, 1400, 1368, 1193, 813, 720 cmꢁ1
.
1
P2. Yield 89% (70 mg); H NMR (CDCl3, 300 MHz): d ¼ 0.87
(m), 1.25 (s), 1.60 (br s), 3.26 (br s), 6.54 (br s), 7.33 (br s),
7.65 (br s), 8.20 ppm (br s); FTIR (KBr): m ¼ 2923, 2851,
2203, 1604, 1517, 1466, 1401, 1342, 1190, 1109, 853, 815,
1H NMR (CDCl3, 300 MHz): d ¼ 0.89 (m, 6 H), 1.27 (s, 52
H), 1.56 (m, 4 H), 3.28 (m, 4 H), 6.57 (d, J ¼ 8.7 Hz, 2 H),
7.11 (m, 1 H), 7.25 (m, 1 H), 7.41 (d, J ¼ 8.4 Hz, 2 H), 7.51
(m, 1 H), 7.58 ppm (m, 1 H); 13C NMR (CDCl3, 75 MHz): d ¼
14.1, 22.7, 27.1, 29.37, 29.51, 29.61, 29.66, 29.68, 29.70, 31.9,
50.9, 86.0, 95.8, 108.1, 111.1, 125.2, 126.4, 126.9, 128.3,
132.3, 132.7, 133.0, 148.1 ppm; FTIR (KBr): m ¼ 2925, 2851,
2212, 1607, 1584, 1521, 1467, 1433, 1401, 1369, 1197,
721 cmꢁ1
.
1
P3. Yield 90% (60 mg); H NMR (CDCl3, 300 MHz): d ¼ 0.87
(m), 1.25 (s), 1.44 (br s), 1.68 (br s), 2.04 (br s), 3.25 (br s),
3.97 (br s), 6.53 (br s), 6.79 (br s), 7.26 ppm (br s); FTIR
(KBr): m ¼ 2925, 2864, 2208, 2153, 1719, 1603, 1506, 1464,
80
WILEYONLINELIBRARY.COM/JOURNAL/JPOLA

ARTICLE
7 The high molecular weight polythiophenes have never been
obtained, see: Zhou, E. J.; Hou, J. H.; Yang, C. H.; Li, Y. F. J
Polym Sci Part A 2006, 44, 2206–2214.
1383, 1366, 1196, 1072, 1017, 996, 883, 830, 813,
678 cmꢁ1
.
1
P4. Yield 68% (30 mg); H NMR (CDCl3, 300 MHz): d ¼ 1.25
(s), 1.67 (br s), 7.6 ppm (br s); FTIR (KBr):m ¼ 2919, 2852,
2203, 1605, 1510, 1467, 1399, 1369, 1285, 1248, 1174, 830,
8 (a) Chakraborty, M.; Tessier, C. A.; Youngs, W. J. J Org
Chem 1999, 64, 2947–2949; (b) Kawase, T.; Konishi, A.; Hirao,
A.; Matsumoto, K.; Kurata, H.; Kubo, T. Chem Eur J 2009, 15,
2653–2661; (c) Konishi, A.; Fujiwara, T.; Ogawa, N.; Hirao, Y.;
Matsumoto, K.; Kurata, H.; Kubo, T.; Kitamura, C.; Kawase, T.
Chem Lett 2010, 39, 300–301.
813, 721 cmꢁ1
.
This work was supported, in part, by a Grant-in-Aid for Scien-
tific Research and Special Coordination Funds for Promoting
Science and Technology from MEXT, Japan, the Murata Science
Foundation, and the Ogasawara Foundation for the Promotion
of Science and Engineering.
9 Levi, Z. U.; Tilley, T. D. J Am Chem Soc 2009, 131, 2796–2797.
10 A similar approach was reported for the 2-bromo-stilbenes
as reactive units, see: Wang, H. B.; Schaffner-Hamann, C.;
Marchioni, F.; Wudl, F. Adv Mater 2007, 19, 558–560.
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