New ladder-type conjugated polymer with broad absorption, high thermal stability, and low band gap

Article abstract of DOI:10.1002/pola.26234

New ladder-type π-conjugated polymer poly{6-alkyl-pyrrolo[3,2-b:4,5-b'] bis-[1,4]benzothiazine} (PPBBTZ) was synthesized through a simple method. PPBBTZ showed good solubility in organic solvents (such as chloroform, tetrahydrofuran, chlorobenzene, o-dichlorobenzene) and high thermal stability (decomposition temperatures up to 323 °C and 299 °C in nitrogen and air, respectively). The highest occupied molecular orbital and lowest unoccupied molecular orbital energy level estimated from electrochemistry result and absorption spectrum were -5.46 and -3.81 eV, respectively, with a band gap of 1.65 eV. More interestingly, PPBBTZ displayed broad absorption from ultraviolet to visible light regions (200-750 nm). These properties suggested PPBBTZ was a good candidate in opto-electronics.

Full text of DOI:10.1002/pola.26234

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New Ladder-Type Conjugated Polymer with Broad Absorption, High
Thermal Stability, and Low Band Gap
Shitao Wang,1 Wei Hong,1 Shendong Ren,1 Jie Li,1 Mao Wang,1,2 Xike Gao,1 Hongxiang Li1
1Laboratory of Materials Science, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, Shanghai 200032, China
2School of Materials Science and Engineering, East China University of Science and Technology, Shanghai 200237, China
Correspondence to: H. Li (E-mail: lhx@mail.sioc.ac.cn)
Received 18 May 2012; accepted 15 June 2012; published online
DOI: 10.1002/pola.26234
ABSTRACT: New ladder-type p-conjugated polymer poly{6-alkyl-
pyrrolo[3,2-b:4,5-b0]bis-[1,4]benzothiazine} (PPBBTZ) was syn-
thesized through a simple method. PPBBTZ showed good solu-
bility in organic solvents (such as chloroform, tetrahydrofuran,
chlorobenzene, o-dichlorobenzene) and high thermal stability
(decomposition temperatures up to 323 ꢀC and 299 ꢀC in nitro-
gen and air, respectively). The highest occupied molecular or-
bital and lowest unoccupied molecular orbital energy level
estimated from electrochemistry result and absorption spec-
trum were ꢁ5.46 and ꢁ3.81 eV, respectively, with a band gap
of 1.65 eV. More interestingly, PPBBTZ displayed broad absorp-
tion from ultraviolet to visible light regions (200–750 nm).
These properties suggested PPBBTZ was a good candidate in
C
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opto-electronics.
2012 Wiley Periodicals, Inc. J Polym Sci
Part A: Polym Chem 000: 000–000, 2012
KEYWORDS: heteroatom-containing polymers; high perform-
ance polymers; polyaromatics; thin film
INTRODUCTION Because of the unique structures, conju-
gated ladder-type polymers exhibited particular physico-
chemical and self-assembly properties and were found wide
applications in organic opto-electronic devices such as or-
ganic field-effect transistors (OFETs),1–3 organic photovoltaic
(OPV) devices,4 organic light-emitting diodes (OLED),5,6 non-
linear optical materials,7 and so on. For example, heteroaro-
matic ladder-type polymer poly(benzimidazobenzophenan-
throline) (BBL, chemical structure see Scheme 1)8,9
possessed high thermal stability and electrical conductivity.
Solution processed thin film transistors of BBL displayed
promising performance (electron mobility up to 0.1
cm2 Vꢁ1 sꢁ1 for single crystal transistors16), suggesting
PBBTZ is a good conjugated unit for organic electronics.
Considering the outstanding property of ladder-type poly-
mers, herein, a new kind of ladder-type polymer poly{6-
alkyl-pyrrolo[3,2-b:4,5-b0]bis-[1,4]benzothi-azine} (PPBBTZ)
(chemical structure see Scheme 1) was synthesized. Ther-
mogravimetric analysis (TGA) and differential scanning
calorimetry (DSC) analysis showed PPBBTZ exhibited high
thermal stability. UV–vis spectrum and electrochemistry
results illustrated PPBBTZ had broad absorption and was a
potential candidate for organic optoelectronics.
cm2 Vꢁ1 ꢁ1) under ambient condition. Conjugated ladder-
s
EXPERIMENTAL
type poly(p-phenylene) (LPPP, chemical structure see
Scheme 1)1,10 exhibited unique electronic properties (inten-
sive photo and electroluminescence, high charge carrier mo-
bility) and have been used as standard materials for OLED11
and optically pumped solid state lasers.12,13 Despite the out-
standing properties and wide applications, conjugated ladder
polymers reported in literature are still rare.14 So, it is
desirable and necessary to synthesize new type of conju-
gated ladder polymers and study their applications in opto-
electronics.
Materials
2-Octyl-1-dodecanol was purchased from Aldrich Company.
2,5-Diamino-1,4-benzenedithiol Dihydrochloride was pur-
chased from TCI chemical and used without further purifica-
tion. Other reagents were commercially obtained. Pyrrole
and dimethylformamide (DMF) were distilled under reduced
pressure before use. Tetrahydrofuran (THF) was dried over
sodium/benzophenone ketyl and freshly distilled before use.
All reactions were conducted under purified nitrogen unless
otherwise noted.
Recently, we synthesized series of ladder-type small mole-
cules 6H-pyrrolo[3,2-b:4,5-b0]bis[1,4]benzothiazine deriva-
tives (PBBTZ).15 These materials exhibited excellent hole
mobility (0.34 cm2 Vꢁ1 sꢁ1 for thin film transistors, 3.6
Measurements
1H NMR and 13C NMR spectra were obtained in CDCl3 on
Varian Mercury (300 MHz or 400 MHz) instrument with
TMS as internal reference and peak multiplicity was
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tilled pyrrole (5.367 g, 80 mmol) was added dropwise within
15 min. The reaction mixture was maintained at that tempera-
ture for 1 h. After that, compound 1 (18.072 g, 50 mmol) was
added dropwise over 15 min. The reaction mixture was
allowed to warm to room temperature and stirred for 24 h.
The reaction solution was diluted with water and extracted
with PE. The PE solution was washed with water and brine
(200 mL), and dried with magnesium sulfate. PE was removed
and the resulted yellow oil was purified by column chroma-
tography (PE eluent) to yield compound 2 as clear oil (11.250
1
g, 60.0%). H NMR (300 MHz, CDCl3, d, ppm): 6.60 (t, J ¼ 1.8
Hz, 2H), 6.13 (t, J ¼ 1.8 Hz, 2H), 3.74 (d, J ¼ 6.9 Hz, 2H), 1.74
(m, 1H), 1.25 (m, 32H), 0.88 (t, J ¼ 6.9 Hz, 6H). 13C NMR (400
MHz, CDCl3, d, ppm): 121.02, 107.60, 53.53, 39.96, 31.93,
31.90, 31.40, 29.95, 29.64, 29.60, 29.56, 29.36, 29.31, 26.41,
22.70, 14.12. LRMS (EI) m/z: 347 (Mþ).
Synthesis of N-(2-Octyldodecyl)-2,3,4,5-tetrabromopyrrole, (3)
A 500-mL three-neck flask was charged with THF (200 mL)
and compound 2 (9.086 g, 24.3 mmol). The reaction solution
was cooled to 0 ꢀC. Then, N-bromosuccinimide (25.389 g,
136.9 mmol) was added. The reaction mixture was allowed to
warm to room temperature and stirred for 24 h. Solvent was
removed by rotary evaporation and the solid was extracted
with diethylether. The diethylether solution was washed with
water and brine, and dried with magnesium sulfate. Diethy-
lether was removed and the residue was purified by flash
chromatography on silica gel (PE as eluent) to afford com-
pound 3 as clear oil (15.930 g, 99.0%). 1H NMR (300 MHz,
CDCl3, d, ppm): 3.92 (d, J ¼ 7.8 Hz, 2H), 1.94 (m, 1H), 1.25 (m,
32H), 0.88(t, J ¼ 6.9 Hz, 6H). 13C NMR (400 MHz, CDCl3, d,
ppm): 103.51, 101.32, 54.32, 38.35, 31.92, 31.88, 31.09,
29.82, 29.62, 29.53, 29.48, 29.34, 29.27, 26.34, 22.69, 22.67,
14.13. LRMS (MALDI-TOF): m/z ¼ 664.4 (MþþH).
SCHEME 1 Structures of BBL, LPPP, PBBTZ, and PPBBTZ.
reported as follows: s, singlet; d, doublet; t, triplet; and m,
multiplet. Absorption spectra were measured on a U-3900
UV–vis spectrophotometer. Mass spectra (EI-MS) were per-
formed on a Shimadzu Qp-5050 A Spectrometer using an
electron impact ionization procedure (70 eV) and MALDI-
TOF spectra were performed on a Voyager-DE STR mass
spectrometer. Element analyses were performed on an Ele-
mentar Vario EL III element analyzer. Gel permeation chro-
matography (GPC) analyses were performed using a poly-
mer/THF solution and calibrated with polystyrene standards.
TGA was conducted with a PerkinElmer Pyris 1 TGA. Experi-
ments were performed on approximately 6–8 mg samples
ꢀ
heated in flowing nitrogen or air at a heating rate of 10 C/
min. DSC analysis were performed on ꢀa Perkin- Elmer Pyris
1 DSC in nitrogen at a scan rate of 10 C/min. Cyclic voltam-
metric measurements were performed in a conventional
three-electrode cell using a platinum working electrode, a
platinum wire counter electrode, and Ag/AgCl reference elec-
trode on a computer-controlled CH1610D instruments.
Synthesis of N-(2-Octyldodecyl)-3,4-dibromomaleimide, (4)
A three-neck flask equipped with an internal thermometer
was charged with 26 mL concentrated HNO3 (66–68%). The
Synthesis of 1-Bromo-2-octyldodecane, (1)
To a 250-mL three-neck reaction flask was added triphenyl
phosphine (26.230 g, 0.1 mol) and CH2Cl2 150 mL under
nitrogen. The reaction mixture was cooled to 0 ꢀC for 15
min and Br2 (16.0 g, 0.2 mol) was added dropwise. After
that, 2-octyl-1-dodecanol (30.408 g, 0.1 mol) was added
dropwise via an additional funnel over 30 min, the reaction
solution was stirred at room temperature for overnight.
CH2Cl2 was evaporated, and the residue was extracted with
petroleum ether (PE). The PE was removed via rotary evapo-
ration and the resulting crude yellow oil was purified by
flash chromatography on silica gel (PE eluent) to give com-
pound 1 as colorless oil (34.814 g, 96.3%). 1H NMR (300
MHz, CDCl3, d, ppm): 3.45 (d, J ¼ 4.8 Hz, 2H), 1.59 (m, 1H),
1.27 (m, 32H), 0.88 (t, J ¼ 6.9 Hz, 6H).
Synthesis of N-(2-Octyldodecyl)-pyrrole, (2)
Sodium hydride (95%) (2.094 g, 82.8 mmol) was added to a
DMF (200 mL) solution under ice bath. Then, freshly dis-
SCHEME 2 Synthetic route for monomer 4.
2
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SCHEME 3 Preparation of ladder polymer PPBBTZ.
ꢀ
solution was cooled to 0 C and then compound 3 (6.190 g,
9.33 mmol) dissolved in 10 mL CH2Cl2 was added dropwise
over a period of 20 min without allowing the temperature to
mopyrrole 3.19 We found that the dibromination step was
difficult to control in the first method, and byproducts such
as mono-substituted pyrrole 5 and 3,4-dibromopyrrole 7
were formed as shown in Scheme 2, which afforded low
yield of 4. Herein, compound 4 was prepared from 2,3,4,5-
tetrabromopyrrole. First, 4 equiv of NBS was added to a so-
ꢀ
rise above 35 C. After that, the mixture was stirred for 1 h
at 0 ꢀC and then poured into ice water. The aqueous phase
was extracted with CH2Cl2, and the combine organic phase
was washed with brine, dried over sodium sulfate. The sol-
vent was removed by rotary evaporation. The residue was
purified by flash chromatography on silica gel (PE: ethyl ace-
tate ¼ 100: 1) to give compound 4 as oil (3.987 g, 79.8%).
1H NMR (300 MHz, CDCl3, d, ppm): 3.49 (d, J ¼ 7.2 Hz, 2H),
1.76 (m, 1H), 1.25 (m, 32H), 0.88 (t, J ¼ 6.9 Hz, 6H). 13C
NMR (400 MHz, CDCl3, d, ppm): 164.13, 129.21, 43.90,
36.98, 31.90, 31.87, 31.25, 29.86, 29.61, 29.56, 29.51, 29.33,
29.26, 26.15, 22.67, 14.10. LRMS (EI) m/z: 535 (Mþ).
ꢀ
lution of N-(2-octyldodecyl)pyrrole at 0 C to afford 3. 3 was
easily purified by column chromatography. Then, 4 was pre-
pared by oxidation of 3 with concentrated HNO3 under ice-
bath for about 1 h. The total yield of these two steps was
about 78%.
PPBBTZ was synthesized by reacting 4 with 2,5-diamino-
1,4-benzenedithiol dihydrochloride in refluxed acetic acid.
The formation of PPBBTZ was through two steps, one was
the nucleophilic substitution of ASH with ABr which was
evidenced by the quick color changes of reaction solution
from light yellow to red. The other was the ring close reac-
tion between ANH2 group and C¼¼O, proved by the color
change from red to dark when the reaction solution was
heated to reflux. PPBBTZ was purified by reprecipitation,
Soxhlet extraction, and polymer gel chromatography. The
number-average molecular weight was 15.8 kDa with a poly-
dispersity of 4.9 determined from GPC using polystyrene as
the standard in THF solution.
Synthesis of Polymer of PPBBTZ
A three-neck, 50-mL reaction flask equipped with a reflux
condenser were charged with dry acetic acid (12 mL), 2,5-
diamino-1,4-benzenedithiol dihydrochloride (0.246 g, 1.0
mmol) and anhydrous sodium acetate (0.165 g, 2.0 mmol).
The reaction mixture was stirred for 1 h at room tempera-
ture. After that, compound 4 (0.536 g, 1.0 mmol) dissolved
in dry acetic acid (12 mL) was added and the solution
turned to reddish color in a few minutes. The reaction mix-
ture was heated to reflux for 48 h and the color of the solu-
tion changed to black gradually. The reaction mixture was
cooled to room temperature and the solvent was removed
by rotary evaporation. The residue was extracted with THF
and filtered. THF solution was concentrated to a minimum
amount and then poured into methanol. The black precipi-
tate was collected and purified by Soxhlet extraction with
methanol, acetone, PE, and THF, then further purified
through polymer gel chromatography, affording PPBBTZ as
black solid (0.154 g, 30.2%). 1H NMR (300 MHz, CDCl3, d,
ppm): 7.61 (s, 2H), 3.75 (br, 2H), 1.64 (br, 1H), 1.23–1.35
(br, 32H), 0.85 (br s, 6H). Anal. Calcd for C30H43N3S2: C,
70.68; H, 8.50; N, 8.24. Found: C, 69.91; H, 8.54; N, 6.91.
RESULTS AND DISCUSSION
The synthetic route of monomer 4 and ladder polymer
PPBBTZ were shown in Schemes 2 and 3. There are two
reported methods to prepare monomer 4. One is from 2,5-
dibromopyrrole 617, 18 and the other is from 2,3,4,5-tetrabro-
FIGURE 1 TGA curves of PPBBTZ in N2 and air at the heating
rate of 10 ꢀC/min.
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TABLE 1 Optical, Electrochemical Data, and Energy Levels of
PPBBTZ
Solution
Film
ox
onset
kmax
kmax
konset Egopt
E
HOMO LUMO
(nm)
(nm)
(nm) (eV)a (eV)b (eV)c
(eV)d
PPBBTZ 584
561, 587 753
1.65 0.75 ꢁ5.46 ꢁ3.81
a
Estimated from thin film absorption onset and calculated by the equa-
tion: 1240/konset
.
b
Measured by cyclic voltammetry.
c
ox
EHOMO ¼ ꢁ (Eonset þ 4.80 ꢁ 0.09) (eV).
d
opt
ELUMO ¼ EHOMO ꢁ Eg
.
Figure 3 exhibited the absorption spectra of PPBBTZ in
dilute chloroform solution and on thin film. Compared with
that of PBBTZ, the maximum absorption peak and initial
absorption of PPBBTZ in solution were red shifted ꢂ96 nm
and 236 nm respectively, which was raised from the
enlarged conjugation length of PPBBTZ. The absorption of
PPBBTZ thin film was slightly red-shifted compared with
that of solution, indicating PPBBTZ formed J-type aggrega-
tion in the solid state.20 The optical energy gap (Eg) was
1.65 eV estimated from the initial absorption on thin film.
More interestingly, PPBBTZ displayed a very broad range of
absorption, from ultraviolet to visible light regions (200–750
nm). Organic semiconductors with broad and intense absorp-
tion across the solar spectrum are usually good cabdidates
for high efficiency OPV.21 Considering the good solubility and
high thermostability, PPBBTZ might find applications in OPV
devices.
FIGURE 2 DSC curves of PPBBTZ in N2 at the heating/cooling
rate of 10 ꢀC/min.
Conjugated ladder-type polymers usually display poor solu-
bility in organic solvents, which hinders their applications in
organic opto-electronics. The solubility of PPBBTZ was
tested by dissolving 5 mg of PPBBTZ in 1 mL of organic sol-
vents at room temperature. Experimental results showed,
due to the long branched alkyl chains, PPBBTZ displayed
high solubility in normal organic solvents such as chloro-
form, THF, chlorobenzene, and dichlorobenzene. The good
solubility guaranteed the applications of PPBBTZ in solution
processable organic opto-electronics.
The thermal properties of PPBBTZ were examined by TGA
and DSC. Figure 1 illustrates a typical TGA curves of
PPBBTZ under nitrogen and air atmospheres. The 5%
weight loss temperature in nitrogen and air were 323 ꢀC
The highest occupied molecular orbital (HOMO) and the low-
est unoccupied molecular orbital (LUMO) energy levels are
important parameters for organic opto-electronic materials.
The HOMO energy level of PPBBTZ was determined by
cyclic voltammogram (CV) and the results are summarized
ꢀ
and 299 C, respectively. The DSC results of PPBBTZ (Fig. 2)
showed no peaks were observed at the first and second
heatiꢀng/cooling cycles in the range of room temperature to
280 C. The TGA and DSC results proved PPBBTZ possessed
high thermal stability.
FIGURE 4 CV of PPBBTZ film on platinum electrode measured
FIGURE 3 UV–vis absorption spectra of PPBBTZ in chloroform
solution (1 ꢃ 10ꢁ5 molLꢁ1) and thin film (quartz substrate).
in 0.1 M Bu4NPF6-acetonitrile solution at a scan rate of 50
mVsꢁ1
.
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€
2 Hertel, D.; Scherf, U.; Bassler, H. Adv. Mater. 1998, 10,
1119–1122.
in Table 1. The CV was conducted in an anhydrous acetoni-
trile solution using 0.1 M Bu4NPF6 as a supporting electro-
lyte and under a nitrogen atmosphere. An irreversible oxida-
tion peak was observed in the CV of PPBBTZ (Fig. 4), and
the onset oxidation potential was 0.75 eV. The HOMO energy
levels calculated from the onset oxidation potentials was
ꢁ5.46 eV (The onset oxidation potential of ferrocene meas-
ured under the same condition was 0.09 eV, and the HOMO
energy level of ferrocene was ꢁ4.8 eV). Combining with the
HOMO–LUMO energy gap estimated from the absorption
spectrum, the LUMO energy level of PPBBTZ was ꢁ3.81 eV.
The HOMO–LUMO energy level suggested PPBBTZ was a typ-
ical candidate for opto-electronics.
3 Durban, M. M.; Kazarinoff, P. D.; Segawa, Y.; Luscombe, C. K.
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In summary, a new kind of ladder-type conjugated polymer
PPBBTZ was synthesized in a simple method. PPBBTZ
exhibited good solubility in organic solvents and high ther-
mal stability both in nitrogen and air. The HOMO and LUMO
energy levels of PPBBTZ estimated from cyclic voltammetry
and absorption spectrum were ꢁ5.46 and ꢁ3.81 eV, respec-
tively. More interestingly, PPBBTZ displayed broad range of
absorption from 200 to 750 nm both in solution and on film.
These properties suggested PPBBTZ has potential applica-
tions in OPV and OFET. Device fabrication and characteriza-
tion of PPBBTZ are currently under progress in our
laboratory.
12 Stagira, S.; Zavelani-Rossi, M.; Nisoli, M.; DeSilvestri, S.;
Lanzani, G.; Zenz, C.; Mataloni, P.; Leising, G. Appl. Phys. Lett.
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ACKNOWLEDGMENTS
4437–4447.
This work was supported by National Natural Sciences Founda-
tion of China (21190031, 21002119) and National Basic
Research Program of China (2011CB808405).
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