Synthesis of fully soluble azomethine-bridged ladder-type poly(p-phenylenes) by Bischler-Napieralski reaction

Article abstract of DOI:10.1021/ma1021117

The synthesis of fully soluble azomethine-bridged ladder-type poly(p-phenylenes) by Bischler-Napieralski reaction was investigated. 60 wt % NaH (0.96 g, 24 mmol) and 1-bromododecane (3.26 g, 13.1 mmol) were added into the solution of 2,7-dibromo-9H-carbazole, (3.27 g, 10.1 mmol), in DMF (40 mL). The mixture was stirred overnight at room temperature. The mixture was poured into water (200 mL), then the aqueous layer was extracted with diethyl ether, and the combined organic layers were dried over Na₂SO₄ and evaporated to dryness. The newly developed synthetic method allows the preparation of ladder polymers with good structural perfection. The synthesized ladder polymers were of high molecular weight and highly soluble in common organic solvents. The photophysical properties indicated that the phenanthridine-containing ladder-type polymers are effective pH sensor materials.

Full text of DOI:10.1021/ma1021117

1
0216 Macromolecules 2010, 43, 10216–10220
DOI: 10.1021/ma1021117
Synthesis of Fully Soluble Azomethine-Bridged
Ladder-Type Poly(p-phenylenes) by
Bischler-Napieralski Reaction
on Waters 410 system against polystyrene standards with
THF as an eluent. UV-vis absorption spectra were obtained
on a Shimadzu UV-vis spectrophotometer model UV-1601
PC. Fluorescence spectra were recorded on a Hitachi F-4500
fluorescence spectrophotometer. Absolute fluorescence
quantum yields of polymers were measured on a Fluoromax-4
fluorescence spectrophotometer. Elemental analyses were
performed on a Flash EA 1112 analyzer. Fourier transform
infrared (FT-IR) spectra were measured at room tempera-
ture using a Bruker Tensor 27 FT-IR spectrometer.
Yulan Chen, Weiguo Huang, Cuihong Li, and Zhishan Bo*
Institute of Polymer Chemistry and Physics, College of Chemistry,
Beijing Normal University, Beijing 100875, China
Received September 14, 2010
Revised Manuscript Received November 20, 2010
2
,7-Dibromo-9-dodecyl-9H-carbazole (2). 60 wt % NaH
0.96 g, 24 mmol) and 1-bromododecane (3.26 g, 13.1 mmol)
were added into the solution of 2,7-dibromo-9H-carbazole,
(3.27 g, 10.1 mmol), in DMF (40 mL). The mixture was
(
Introduction. In recent years, ladder-type conjugated polymers
have attracted tremendous scientific attention. The rigid
1
1
and planar main chain structures of ladder-type conjugated
polymers endow them with attractive physical properties
such as high carrier mobility and luminescence intensity.
stirred overnight at room temperature. Then the mixture was
poured into water (200 mL), the aqueous layer was extracted
with diethyl ether (2 ꢀ 40 mL), and the combined organic
layers were dried over Na SO and evaporated to dryness.
2
,3
Exploration of structurally defined conjugated ladder-type
polymers is thus highly required, owing to their potential
applications such as optoelectronics, microelectronics, and
chemical and biological sensors. During the past decade,
great effort has been made in the synthesis of ladder poly-
2
4
The crude product was purified by chromatography on silica
gel eluting with petroleum ether to afford 2 as a colorless
1
solid (4.9 g, 99%). H NMR (CDCl , 600 MHz): δ 7.75
3
3
(d, 2H, ArH), 7.41 (s, 2H, ArH), 7.22 (d, 2H, ArH), 4.04
mers. However, synthesis of conjugated ladder polymers,
(
t, 2H, CH ), 1.71 (m, 2H, CH ), 1.26-1.15 (m, 18H, CH ),
2
2
2
especially soluble and structurally perfect heteroaromatic
ladder polymers, remains a challenge. Although, Tour et al.
have reported the synthesis of azomethine-bridged ladder
polymers by Schiff base formation between alternating amine
and ketone functional groups. However, the known ladder
polymers synthesized by Schiff base formation reactions were
only soluble in acidic media such as CH Cl /TFA (3/2), usually
13
0
1
2
.79 (t, 3H, CH ). C NMR (CDCl , 150 MHz): δ 141.34,
3 3
22.51, 121.45, 121.26, 119.69, 111.99, 43.32, 31.94, 29.59,
9.51, 29.35, 28.77, 27.18, 22.72, 14.16. Anal. Calcd for
C H Br N: C, 58.43; H, 6.33; N, 2.84. Found: C, 58.43;
2
4
31
2
H, 6.24; N, 2.77.
,7-Dibromo-3,6-dinitro-9-dodecyl-9H-carbazole (3). A mix-
2
2
2
3
c-f
ture of 2 (7.4 g, 15.0 mmol), acetic acid (200 mL), and con-
centrated nitric acid (40 mL) was heated to reflux overnight.
After cooling, the precipitate was collected by filtration, and the
crude product was recrystallized from ethanol and dried under
insoluble in normal organic solvents.
In previous work, we have reported a facile synthesis
of 3,8-dibromo-substituted phenanthridine derivatives by
4
0
Bischler-Napieralski cyclization. A set of 4,4 -dibromo-2-
acylbiphenyls cyclized to 3,8-dibromophenanthridine mono-
mers with nearly quantitative yields (Scheme 1). Herein, we
apply this reaction to the synthesis of azomethine-bridged
ladder-type poly( p-phenylene)s. A new kind of poly(ladder-
type phenanthridine)s with good structural perfection has
been readily synthesized. To the best of our knowledge, this
is the first report on synthesis of fully soluble azomethine-
bridged ladder-type conjugated polymers. Preliminary photo-
physical property studies illustrated that this kind of ladder-
type polymer is a promising chemosensor material.
high vacuum to afford 3 as a slight yellow solid (5.4 g, 62%).
1
H NMR (CDCl , 600 MHz): δ 8.59 (s, 2H, ArH), 7.65 (s, 2H,
3
ArH), 4.20 (t, 2H, CH ), 1.78 (m, 2H, CH ), 1.26-1.13
2
2
13
(
m, 18H, CH ), 0.76 (t, 3H, CH ). C NMR (CDCl , 150 MHz):
2 3 3
δ 143.32, 143.13, 120.96, 119.51, 115.68, 113.90, 44.29, 31.88,
9.57, 29.51, 29.41, 29.30, 29.20, 28.79, 27.06, 22.67, 14.11.
Anal. Calcd for C H Br N O : C, 49.42; H, 5.01; N, 7.20.
2
24 29
2
3 4
Found: C, 49.45; H, 5.11; N, 6.89.
,6-Bis(dodecanamido)-2,7-dibromo-9-dodecyl-9H-carbazole (5).
To a solution of 3 (5.0 g, 8.6 mmol) in 200 mL of acetic acid
3
was added aqueous HCl (45 mL, 32%). Tin powder (10.0 g,
8
Experimental Section. Materials. Unless otherwise noted, all
chemicals were purchased from commercial suppliers and used
without further purification. Phosphoryl trichloride was freshly
distilled before use. Tetrahydrofuran (THF) was distilled over
sodium and benzophenone. Triethylamine (TEA) was distilled
4.0 mmol) was then added portionwise over 10 min, and the
reaction mixture was heated to reflux overnight. After cool-
ing, the mixture was poured into ice water (400 mL) and then
neutralized with aqueous NaOH solution (20%) until the
pH was 7. The precipitate was collected by filtration and
dried under high vacuum to give the crude product 4 as a
yellow solid. Because of the relative instability of the diamine
over CaH before use. All reactions were performed under an
2
atmosphere of nitrogen and monitored by thin layer chromatog-
raphy (TLC) with silica gel 60 F254 (Merck, 0.2 mm). Column
chromatography was carried out on silica gel (200-300 mesh).
The catalyst precursor Pd(PPh ) was prepared according
4 in air, the crude product of 4 was used for next step without
1
further purification. H NMR (CDCl
(s, br, 2H, NH), 7.40 (s, 2H, ArH), 7.34 (s, 2H, ArH), 3.92
, 400 MHz): δ 7.99
3
3
4
5
to the literature and stored in a Schlenk tube under nitrogen.
2
pinacol ester were prepared according to literature procedures.
6
,7-Dibromo-9H-carbazole and 9,9-dioctylfluorene-2,7-diboronic
(t, 2H, CH ), 2.01 (m, 2H, CH ), 1.26-1.21 (m, 18H, CH ),
0.86 (t, 3H, CH ). To a dry THF solution (30 mL) of 4 and
2
2
2
7
3
1
13
Characterization. H and C NMR spectra were recorded
on a Bruker AV400 or an AV600 spectrometer. Gel permea-
tion chromatography (GPC) measurements were performed
triethylamine (15 mL) was added dropwise a dry THF
solution (35 mL) of dodecanoyl chloride (8.0 g, 36.5 mmol)
at 0 °C. After stirring at room temperature for 24 h, the solu-
tion was poured into water (80 mL) and the aqueous solution
was extracted with diethyl ether (2 ꢀ 50 mL). The combined
*Corresponding author. E-mail: zsbo@bnu.edu.cn.
pubs.acs.org/Macromolecules Published on Web 11/30/2010
r 2010 American Chemical Society

Communication
Macromolecules, Vol. 43, No. 24, 2010 10217
organic extracts were dried over anhydrous Na SO and
2
Polymer P2. A solution of P1 (0.059 g) and P O (1 g) in
2
4
5
evaporated to dryness. The residue was purified by chromatog-
raphy on silica gel eluting with CH Cl and recrystallization
from CH Cl /n-Hex to afford 5 as a colorless solid (1.75 g,
freshly distilled POCl
under N . The solvent was removed under high vacuum, the
2
residue was diluted with ethyl acetate (15 mL), and water
(30 mL) was added slowly. The aqueous layer was adjusted
to pH = 10 with NaOH solution (5 M) and then extracted
(12 mL) was stirred at reflux for 30 h
3
2
2
2
2
1
2
3%). H NMR (CDCl , 600 MHz): δ 8.54 (s, 2H, NH), 7.49
3
(
s, 2H, ArH), 7.30 (s, 2H, ArH), 3.94 (t, 2H, CH ), 2.37
2
(
t, 4H, CH ), 1.70 (m, 8H, CH ), 1.34-1.13 (m, 48H, CH ),
with CHCl
dried over anhydrous Na
3
(2 ꢀ 20 mL). The combined organic layers were
2
2
2
1
3
0
1
3
2
6
.77 (t, 9H, CH ). C NMR (CDCl , 150 MHz): δ 171.45,
2
SO . After the removal of most of
4
3
3
solvent, the residue was precipitated into methanol. The
resulted precipitate was collected by filtration and dried
under high vacuum to give P2 as a brown solid (0.045 g,
38.29, 127.33, 122.07, 115.87, 114.05, 112.00, 43.28, 37.74,
1.92, 29.63, 29.60, 29.55, 29.48, 29.45, 29.33, 28.75, 27.11,
5.81, 22.69, 14.12. Anal. Calcd for C H Br N O : C,
4
8
77
2
3
2
1
7
9%). H NMR (CDCl , 400 MHz): δ 9.11 (broad, 2H), 8.82
4.93; H, 8.74; N, 4.73. Found: C, 64.00; H, 8.44; N, 4.60.
3
þ
(
4
broad, 4H), 8.55 (broad, 2H), 4.82 (broad, 2H), 3.65 (broad,
H), 2.58 (broad, 2H), 2.32 (broad, 2H), 2.24 (m, 4H), 1.70
ESI HRMS: calcd for C H Br N O 885.43825; found
2
8
4
8
77
3
2
þ
86.44494 [M þ H] .
13
C
(
broad, 2H), 1.44-1.12 (m, 77H), 0.88 (broad, 12H).
Polymer P1. A mixture of 5 (0.10 g, 0.11 mmol),
,9-dioctylfluorene-2,7-diboronic pinacol ester (0.073 g,
NMR (CDCl , 100 MHz): δ 38.36, 38.31, 38.00, 33.67, 32.83,
9
0
3
3
2
1.06, 30.94, 30.71, 30.60, 30.56, 30.33, 30.26, 30.12, 27.99,
5.37, 23.59, 20.63, 15.01.
Results and Discussion. The synthetic route leading to
.11 mmol), NaHCO (0.38 g, 4.5 mmol), THF (12 mL),
3
toluene (4 mL), and H O (6 mL) was carefully degassed
2
before and after Pd(PPh ) (1.3 mg, 1.1 μmol) was added.
The mixture was heated to reflux and stirred under nitro-
gen for 96 h. The reaction mixture was extracted with
3
4
azomethine-bridged ladder-type poly(p-phenylene)s is shown
in Scheme 2. 3,6-Bis(dodecanamido)-2,7-dibromo-9-dodecyl-
9
H-carbazole is the key monomer, whose synthesis is rather
CHCl (3 ꢀ 30 mL), and the combined organic layers were
3
straightforward. Starting from 2,7-dibromo-9H-carbazole,
its reaction with 1-bromododecane under NaH/DMF con-
ditions gave 2,7-dibromo-9-dodecyl-9H-carbazole 2 in a
yield of 99%. Nitration of 2 with a mixture of concentrated
nitric acid and acetic acid under reflux conditions afforded
dried over anhydrous Na SO . After the removal of most
4
2
of solvent, the residue was precipitated into methanol. The
resulted precipitate was collected by filtration and dried under
vacuum to give P1 as a grayish solid (0.096 g, 76%). H NMR
1
(
(
CDCl , 400 MHz): δ 8.90 (broad, 2H), 7.92 (broad, 2H), 7.53
3
2
,7-dibromo-3,6-dinitro-9-dodecyl-9H-carbazole 3 in a 62%
broad, 4H), 7.34 (broad, 2H), 7.18 (broad, 2H), 4.30 (broad,
þ
yield. Subsequent reduction of the nitro groups with Sn/H
furnished the corresponding diamine 4. Diamine 4 is not
very stable in air, so the crude 4 was used for the next step
without further purification. The reaction of 4 with dodecanoyl
chloride in a solvent mixture of THF and TEA furnished 3,6-
bis(dodecanamido)-2,7-dibromo-9-dodecyl-9H-carbazole (5) in
a total yield of 23%. Except for compound 4, all other inter-
2
H), 2.24 (broad, 4H), 2.09 (broad, 2H), 1.91 (broad, 2H), 1.70
(m, 4H), 1.63 (broad, 2H), 1.44-1.14 (m, 77H), 0.88 (broad,
1
3
6
1
1
2
2
H), 0.82 (broad, 6H). C NMR (CDCl , 100 MHz): δ 171.17,
3
51.53, 140.12, 138.67, 132.97, 128.76, 126.83, 124.22, 122.53,
20.31, 115.80, 109.89, 55.49, 40.36, 37.63, 31.92, 31.81, 30.22,
9.68, 29.64, 29.54, 29.42, 29.36, 29.21, 29.17, 29.12, 27.40,
5.77, 22.69, 22.64, 14.12, 14.06.
1
13
C
mediates were unambiguously characterized by H and
NMR spectroscopy, elemental analysis, or high-resolution
mass spectroscopy.
The synthesis of azomethine-bridged ladder-type poly-
Scheme 1. Facile Synthesis of 3,8-Dibromophenanthridine Derivatives
by Bischler-Napieralski Cyclization
(
p-phenylene)s involves two key steps: (1) Suzuki-Miyaura-
Schl u€ ter polycondensation (SMSPC) was used to prepare the
single-chain carbazole-alt-fluorene polymer P1 bearing later-
al dodecanamide chains on each carbazole repeating unit,
Scheme 2. Synthetic Route to Azomethine-Bridged Ladder-Type Poly(p-phenylene)s

1
0218 Macromolecules, Vol. 43, No. 24, 2010
Communication
1
Figure 1. H NMR spectra of single-stranded polymer P1 and ladder
polymer P2 in aromatic region.
Figure 2. FT-IR spectra of the polymers P1 and P2.
and (2) Bischler-Napieralski cyclization was used to achieve
azomethine-bridged ladder-type poly( p-phenylene)s P2.
SMSPC of dibromo monomer 5 and 9,9-dioctylfluorene-
In the FT-IR spectrum of the single-stranded polymer P1,
-
1
the sharp absorption peak at 3423 cm is due to the stretch-
ing mode of the free NH of amido groups, the broad absorp-
-
1
2,7-diboronic pinacol ester 6 was carried out in a biphasic
mixture of THF and aqueous NaHCO3 solution with
tion peak at 3269 cm is due to the stretching mode of the
hydrogen bonding NH of amide groups, and the absorption
-1
Pd(PPh ) as the catalyst precursor afforded P1 in a yield
3
peaks at 1689 and 1663 cm are the corresponding the CdO
stretching vibrations of the free and hydrogen-bonding
amido groups, respectively. In the FT-IR spectrum of the
ladder-type polymer P2, the characteristic absorptions
related to amido groups disappeared completely, indicating
that a complete cyclization has been achieved.
4
of 76%. The single-stranded polymer P1 was converted to
the corresponding azomethine-bridged double-stranded ladder-
type poly( p-phenylene)s P2 in a yield of 79% under the
optimized Bischler-Napieralski reaction conditions, using
POCl as the solvent and P O as the catalyst at refluxing
3
2
5
condition. With five lateral long alkyl chains on each repeat-
ing unit, both the single-stranded polymer P1 and the ladder-
type polymer P2 are readily soluble in common organic
solvents such as chloroform and THF, making the charac-
terization of their structures very convenient. H and C NMR
spectra of the two polymers P1 and P2 confirmed the
expected structures.
The molecular weights of two polymers were measured by
gel permeation chromatography (GPC) against polystyrene
standards with THF as an eluent. The number-average molec-
Photophysical properties of single-stranded polymer P1
and double-stranded polymer P2 in THF and chloroform
solutions and films were investigated by UV-vis absorption
and photoluminescent (PL) spectroscopy. It is worth to note
that the ladder-type polymer P2 has an alternating donor-
acceptor structure in the polymer main chain. The normal-
ized UV-vis absorption and photoluminescent (PL) spectra
of single-stranded polymer P1 and P2 in dilute THF solu-
tions are shown in Figure 3a. P1 exhibited a strong feature-
less absorption in the ultraviolet region ranging from 320 to
410 nm with a maximum at 355 nm and a broad featureless
emission band peaked at 441 nm. Ladder polymer P2 showed
well-resolved strong absorption in the ultraviolet region
ranging from 300 to 450 nm with three peaks at 391, 403,
and 416 nm and two shoulders at 365 and 428 nm, indicating
the existence of a highly defined electronic structure that is
low in defects. P2 displayed a well-resolved PL spectrum
with the emission maximum at 496 nm and a shoulder at
about 470 nm. Molar absorption coefficients of P1 and P2
are 69 117 and 12 524, respectively. Compared with the
single-stranded polymer P1, significant bathochromic shift-
ing of the absorption and emission spectra of ladder polymer
P2 has been found, which suggests that the effective con-
jugation length has been extended after the polymer was
transformed to ladder structure. The normalized UV-vis
absorption and PL spectra of P1 and P2 in chloroform
solutions are shown in Figure 3b. The absorption and
emission spectra of P1 in chloroform solution are quite
similar to those in THF solution. In chloroform solution,
the ladder-type polymer P2 displayed a broad absorption in
the range 300-450 nm with three peaks at 368, 391, and 416
nm and a shoulder at 432 nm. In chloroform solution, P2
showed a broad emission band in the range 400-600 nm with
three peaks located at 432, 464, and 493 nm. In comparison
with the emission spectrum in THF solution, the emission
spectrum of P2 in chloroform solution is much broader and
slightly hypsochromically shifted. The above results illustrated
1
13
ular weight (M ) and the weight-average molecular weight
n
(
ring closure M and M of ladder-type polymer P2 decreased
M ) of P1 are 20.0 and 32.0 kDa, respectively, whereas after
w
n
w
to 16.0 and 23.0 kDa, respectively. GPC is known not to be a
good method to determine the actual molecular weight of
rodlike polymers, and the molecular weights determined by
GPC are only a relative measurement of the hydrodynamic
volume of polymers. The difference in the elution curves also
maybe due to the different interaction with the column
material.
1
b
In our previous paper, we have proved that Bischler-
Napieralski cyclization is a very effective route to prepare
phenanthridine derivatives, and in many cases quantitative
7
yield can be achieved. To verify that the polymer-based
analogous ring closure reaction also goes to completion, HNMR
1
and FT-IR spectroscopy investigations have been carried
out. The assigned H NMR spectra of the single-stranded
1
polymer P1 and the ladder-type polymer P2 are shown in
Figure 1 for a comparison. In the aromatic region, the single-
stranded polymer P1 shows five broad signals peaked at 8.90,
7
polymer P2 shows only three broad peaks peaked at 9.09,
.92, 7.53, 7.34, and 7.18 ppm, whereas the ladder-type
1
.80, and 8.51 ppm. In the C NMR spectrum of P2, the
3
8
disappearance of the carbonyl signal also indicates that
the ring closure reaction is complete. The FT-IR spectra of
the single-stranded precursor polymer P1 and the ladder-
type polymer P2 are shown in Figure 2 for a comparison.

Communication
Macromolecules, Vol. 43, No. 24, 2010 10219
-
6
Figure 4. (a) UV-vis absorption spectra of P2 in THF (6 ꢀ 10 M for
-
7
repeating units) at various concentration of TFA: [TFA] = 0, 8 ꢀ 10
,
-
6
-5
-4
-3
-2
-2
8
1
ꢀ 10 , 8 ꢀ 10 , 8 ꢀ 10 , 8 ꢀ 10 , 1.2 ꢀ 10 , 1.6 ꢀ 10 , 4.8 ꢀ
- -1 -6
2
0
, 3.2 ꢀ 10 M. (b) PL spectra P2 in THF (6 ꢀ 10 M for repeating
-7 -6
units) at various concentration of TFA: [TFA] = 0, 8 ꢀ 10 , 8 ꢀ 10
,
-
5
-4
-3
-2
-2
-2
8
1
3
ꢀ 10 , 8 ꢀ 10 , 8 ꢀ 10 , 1.2 ꢀ 10 , 1.6 ꢀ 10 , 4.8 ꢀ 10 , 3.2 ꢀ
-
1
0
M and the neutralization with TEA. The excitation wavelength is
91 nm.
and deprotonation processes are reversible. The UV-vis
absorption spectra of ladder polymer P2 by titration with
trifluoroacetic acid (TFA) are shown in Figure 4a. With the
-
6
addition of TFA to a solution of P2 in THF (6 ꢀ 10 M), the
absorption spectrum of the protonated ladder polymer became
broader, red-shifted, and featureless with the absorption
maximum located at 410 nm. For the emission spectra, the
intensity of peaks at 470 and 296 nm decreased with the
addition of TFA, and a new featureless weak green emission
band peaking at 545 nm appeared gradually with the dis-
appearance of the two blue emission peaks. The protonated
ladder polymer can be easily deprotonated by the addition of
triethylamine (TEA). After the addition of a droplet of TEA
to the protonated ladder polymer solution, the long wave-
length green emission completely disappear with partial
recovery of the blue emission. These preliminary optic
property investigations revealed that azomethine-bridged
ladder-type poly( p-phenylene)s are promising protic acid
sensitive materials.
Figure 3. Normalized UV absorption and photoluminescent spectra of
P1 and P2 in THF solutions (a), CHCl solutions (b), and films (c).
3
that P2 has less intramolecular interaction between donor
and acceptor units in the ground state, whereas the emission
spectra of P2 displayed remarkable solvent dependence
behaviors. The absolute quantum efficiencies in highly dilute
THF and chloroform solutions are 30% and 24% for P1 and
1
1% and 10% for P2, respectively. The normalized UV-vis
absorption and emission spectra of P1 and P2 in films are
shown in Figure 3c. Compared to its behaviors in solution,
the absorption and emission spectra of P1 were only slightly
bathochromically shifted. For ladder polymer P2, both the
absorption and emission spectra became featureless and
bathochromically shifted for 32 and 107 nm, respectively.
This result indicated that the planar ladder polymer P2
formed aggregation in solid films.
Conclusions. In conclusion, we have developed a facile
approach to synthesize azomethine-bridged ladder-type
poly( p-phenylene)s. The newly developed synthetic method
allows the preparation of ladder polymers with good struc-
tural perfection. The ladder polymers we synthesized are of
The nitrogen atom on the ladder polymer backbone can be
The protonation
3
e,8
easily protonated and deprotonated.

1
0220 Macromolecules, Vol. 43, No. 24, 2010
Communication
high molecular weight and highly soluble in common organic
solvents. Photophysical property studies indicated that the
phenanthridine-containing ladder-type polymers are pro-
mising pH sensor materials.
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Acknowledgment. Financial support by the NSF of China
(
Grants 20774099, 20834006, and 50821062), the 863 Program
Grant 2008AA05Z425), and the National Basic Research Program
(
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