Family of diazapentalene chromophores and narrow-band-gap polymers: Synthesis, halochromism, halofluorism, and visible-near infrared photodetectivity

Article abstract of DOI:10.1021/cm300938s

A family of pyrrolo[3,4-c]pyrrol-1(2H)-one (PPO) and 2,5-diazapentalene (DAP) chromophores and DAP-containing polymers were synthesized, and their optical and electrochemical properties were studied. PPO and DAP chromophores are readily obtained by chemical transformation of the lactam unit in diketopyrrolopyrrole (DPP) dyes and can be used as versatile building blocks for construction of a variety of conjugated low-band-gap compounds and polymers. In comparison with the DPP chromophores, PPO and DAP chromophores have narrower energy gaps and low-lying HOMO levels in the order DPP > PPO > DAP. Interestingly, the PPO and DAP chromophores exhibit unique visible and near-infrared halochromic and halofluoric properties. The emission spectrum of DAP-containing polymer 6d covers the telecommunication window including the wavelength of 1310 nm. The photodetector with a device configuration of ITO/PEDOT:PSS (35 nm)/active layer (100 nm)/Al (100 nm) was fabricated using a blend of polymer 6d and [6,6]-phenyl-C₆₁-butyric acid methyl ester (PC₆₁BM) in a weight ratio of 1:3 as the active layer and exhibited photocurrent spectral response from 400 to 1000 nm and the detectivity in an order of 10¹¹ Jones at 800 nm.

Full text of DOI:10.1021/cm300938s

Article
pubs.acs.org/cm
Family of Diazapentalene Chromophores and Narrow-Band-Gap
Polymers: Synthesis, Halochromism, Halofluorism, and Visible−Near
Infrared Photodetectivity
Gang Qian, Ji Qi, James A. Davey, James S. Wright, and Zhi Yuan Wang*
†
‡,§
†
†
,†,‡
†
Department of Chemistry, Carleton University, 1125 Colonel By Drive, Ottawa, Ontario, Canada K1S 5B6
‡
State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences,
Changchun, China 130022
§
Graduate School of Chinese Academy of Sciences, Beijing, China 100039
S Supporting Information
*
ABSTRACT: A family of pyrrolo[3,4-c]pyrrol-1(2H)-one
PPO) and 2,5-diazapentalene (DAP) chromophores and
(
DAP-containing polymers were synthesized, and their optical
and electrochemical properties were studied. PPO and DAP
chromophores are readily obtained by chemical transformation
of the lactam unit in diketopyrrolopyrrole (DPP) dyes and can
be used as versatile building blocks for construction of a variety
of conjugated low-band-gap compounds and polymers. In
comparison with the DPP chromophores, PPO and DAP
chromophores have narrower energy gaps and low-lying
HOMO levels in the order DPP > PPO > DAP. Interestingly, the PPO and DAP chromophores exhibit unique visible and
near-infrared halochromic and halofluoric properties. The emission spectrum of DAP-containing polymer 6d covers the
telecommunication window including the wavelength of 1310 nm. The photodetector with a device configuration of ITO/
PEDOT:PSS (35 nm)/active layer (100 nm)/Al (100 nm) was fabricated using a blend of polymer 6d and [6,6]-phenyl-C -
6
1
butyric acid methyl ester (PC BM) in a weight ratio of 1:3 as the active layer and exhibited photocurrent spectral response from
6
1
11
4
00 to 1000 nm and the detectivity in an order of 10 Jones at 800 nm.
KEYWORDS: diazapentalene, near infrared, halochromism, halofluorism, photodetector
INTRODUCTION
The N-alkylation can improve solubility and alter molecular
■
4
14,15
3,16
stacking in the solid state. Thiophene, furan,
benzene,
The field of organic electronic devices, such as organic light-
emitting diodes (OLEDs), thin-film transistors (OTFTs), and
photovoltaic cells (OPVs), is characterized by fast-paced
progress and mainly driven by the discovery and availability
of key chromophores as building blocks. Diketopyrrolo[3,4-
c]pyrrole (DPP) is present as a key structural unit in an
important class of red pigments that has exceptional light,
5
17
18
thienothiophene, fluorene, carbazole, cyclopenta[2,1-b:3,4-
17
19
b′]dithiophene, dithieno[3,2-b:2′,3′-d]pyrrole, and other
groups have been introduced at the 3,6-positions, affording
many compounds and polymers with different band gaps and
some unique properties. However, a decrease in the band gap
of DPP compounds is always accompanied along by an increase
17
1
in the highest occupied molecular orbital (HOMO) level. For
solar cell applications, donor materials with a narrow band gap
and low-lying HOMO level are desirable because of their
weather, and heat stability, and is widely used as a building
block in the construction of a variety of compounds and
2
,3
polymers for device applications, in particular OLEDs,
4
−8
2,5,9−12
contribution to an increase in short-circuit current (J ) and
OPVs,
and OTFTs.
For example, OLED devices
sc
20,21
open-circuit voltage (Voc),
and improvement of the long-
based on DPP-fluorene copolymers emit orange light with an
external quantum efficiency of 0.45%. Bulk heterojunction
solar cells showing a power conversion efficiency of 4.4% for
3
term stability of organic optoelectronic devices. For photo-
detector applications, narrow-band-gap polymers can extend
photocurrent spectral response to the near infrared (NIR)
region, and low-lying HOMO level could lead to a high
DPP oligomers and 5.5% for DPP polymers have been
6
,8
reported,
which is comparable to the one based on
22
13
detectivity by decreasing the dark current. However, as in the
case of the development of DPP-based optoelectronic
regioregular poly(3-hexylthiophene). The hole mobility up
2
−1 −1
to 1.95 cm V
s
for the DPP polymer-based transistor has
5
been achieved.
In looking for other champion materials, structural
modifications on DPP derivatives have been done mainly at
the 3,6-positions and the nitrogen atom of the lactam moiety.
Received: March 24, 2012
Revised: May 31, 2012
Published: June 1, 2012
©
2012 American Chemical Society
2364
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Chemistry of Materials
Article
materials, the common methods to reduce the energy gap, such
as extension of conjugation length or introduction of strong
donor and acceptor groups, often lead to a rise in the HOMO
containing compounds and polymers could have a low band
gap and also low-lying HOMO levels.
Encouraged by this predication from calculation results,
several representative compounds and polymers (Figure 3)
containing PPO and DAP were designed, synthesized, and
characterized. By studying the chemical and photophysical
properties, the unique halochromic and halofluoric properties
of these new chromophores and polymers were also revealed.
Furthermore, the NIR photodetectors with photocurrent
spectral response up to 1000 nm were fabricated and
characterized.
2
3,24
level.
DPP has a planar, conjugated 8π-electron bicyclic ring and
contains the indigo-like, cross-conjugated vinylogous amidic
and vinylogous hydrazine units. Its lactim form has a bicyclic
2
,5-diazapentalene (DAP) unit (Figure 1). By chemical
EXPERIMENTAL SECTION
■
Materials. All chemicals and reagents were used as received from
commercial sources without purification. Solvents for chemical
synthesis were purified by distillation. Chemical reactions were carried
out under an argon atmosphere. 3,6-Di(thiophen-2-yl)pyrrolo[3,4-
2
6
c]pyrrole-1,4(2H,5H)-dithione (9), 2,5-bis-(2- ethylhexyl)-3,6-di-
2
7
thiophen-2-ylpyrrolo[3,4-c]pyrrole-1,4-dione (1), 2,2′-(9,9-dioctyl-
9H-fluorene-2,7-diyl)bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolane), 9-
28
Figure 1. Conversion of DPP and DTPP to other chromophores by
lactam-to-lactim transformation.
(2-ethylhexyl)-2,7-bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-
2
9
9
H-carbazole, 1-dodecyl-2,5-bis(5-(4,4,5,5-tetramethyl-1,3,2-dioxa-
30
borolan-2-yl)thiophen-2-yl)-1H-pyrrole, and 2,6-bis(trimethyltin)-
4
3
1
,8-bis(octyloxy)benzo[1,2-b:4,5-b′]dithiophene (14) were prepared
transformation of the lactam group in DPP to the lactim
derivatives, the two chromophores PPO (pyrrolo[3,4-c]pyrrol-
according to literature methods.
1
13
General Methods. H and C NMR spectra were recorded using
25
1
13
1
(2H)-one) and DAPO could be produced. By analogy,
a Bruker Avance 300 NMR spectrometer ( H NMR, 300 MHz; C
1
1
1
13
dithioketopyrrolopyrrole (DTPP), derived from DPP by
NMR, 75 MHz). H− H COSY and H− C HSQC were recorded
using a Bruker Avance 400 NMR spectrometer. Elemental analysis was
performed on a Vario EL Elementar Analysis Instrument. High-
resolution mass spectrometry (HRMS) was obtained from 7.0 T
actively shielded Fourier transform ion cyclotron resonance mass
spectrometers. Mass spectroscopy data (electrospray ionization, ESI)
were measured with a Micromass Quattro LC ESI mass spectrometer.
Mass spectroscopy data (matrix-assisted laser desorption ionization,
MALDI) were measured with Bruker Daltonics Reflex III MALDI-
2
6
thionation, could also yield the two new chromophores,
PPT (pyrrolo[3,4-c]pyrrol-1(2H)-thione) and DAP (Figure 1).
However, due to relative reactivity S-alkylation > N-alkylation >
O-alkylation, DPP, PPO, and DAP are preferably produced
during the alkylation of DPP and DTPP, respectively.
Therefore, PPO and DAP are potentially useful building blocks
for a variety of new optoelectronic compounds and polymers
that may show some interesting properties.
TOF (MALDI-time-of-flight). The number-average (M ) and weight-
n
The computational calculation was carried out on the three
chromophores DPP, PPO, and DAP. The energy gap of three
chromophores shows the order DPP > PPO > DAP, and their
HOMO levels gradually decease in order from DPP to PPO to
DAP (Figure 2). In comparison with DPP, lack of the donation
from lone-pair electrons on the nitrogen to the π-orbit of the
diene moiety may contribute to lowering the HOMO levels in
PPO and DAP. The calculation results imply that DAP-
average (M ) molecular weights of the polymers were determined by
w
gel-permeation chromatography (GPC) with a Waters 410 instrument
and polystyrene as standards and tetrahydrofuran (THF) as eluent.
Thermogravimetric analysis (TGA) and differential scanning calorim-
etry (DSC) were carried out in nitrogen with a heating rate of 10 °C/
min on a Hi-Res TGA 2950 thermogravimetric analyzer and a TA-
DSC Q100 (TA Instruments), respectively. Absorption and
fluorescence spectra were recorded with a Lambda 900 Perkin-Elmer
UV/vis/NIR spectrometer and PTI fluorescence system (near-infrared
region) or Shimadzu RF-1501 spectrofluorophotometer (visible
region), respectively. Cyclic voltammetry (CV) was performed using
a BAS 100B/W electrochemical workstation. Anhydrous dichloro-
methane was used as the solvent under an argon atmosphere, and 0.1
M of tetrabutylammonium hexafluorophosphate was used as the
supporting electrolyte. A platinum disk (2 mm diameter) was used as
working electrode, a platinum wire was used as counter electrode, and
a Ag/AgCl electrode was used as reference electrode. All measure-
ments were done using a scan rate of 50 mV/s at room temperature.
The energy levels were calculated relative to a ferrocenium/ferrocene
redox couple as an internal reference.
Computational Method. The structures were drawn with
GaussView 4. The geometries were initially optimized using the
3
2
AM1 method in the gas phase, followed by gas-phase B3LYP/6-
3
3,34
35
3
1+G(d)
optimization using Gaussian 09 running on an AMD
Aspire X3400 computer (AMD Athlon II quad core 645 processor, 3.1
GHz with 4GB RAM). The optimized geometries were checked for
negative vibrations. The HOMO and LUMO energies and coordinates
were taken from the Gaussian output, and the graphical oribtal plots
were generated with GaussView 4. The calculation time was reduced
by replacing the long alkyl chains with methyl groups.
Figure 2. Calculated energy levels and the frontier molecular orbital of
DPP and its derivatives. (Structures shown in Figure 1 with R = Me,
Ar = 2-thiophene).
2
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Chemistry of Materials
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Figure 3. Representative chromophores and polymers derived from DPP 1: PPO 2 and 4, DAP 3 and 5, and DAP polymers 6a−d. R = 2-ethylhexyl,
except for 6d (R = heptadecanyl).
Device Fabrication. The active layer solution comprised of
polymer 6d and [6,6]-phenyl-C -butyric acid methyl ester (PC BM)
the crude product was purified by column chromatography (silica gel,
dichloromethane/hexane =1/1 v/v) and the product was obtained as
61
61
1
at a weight ratio of 1:3 was dissolved in chlorobenzene and stirred at
0 °C for more than 48 h in the air to form a solution of 20 mg/mL.
red needle crystals. Yield: 1.9 g, 14%. H NMR (300 MHz, CDCl ) δ
3
3
(ppm): 8.49 (dd, 1H, J = 3.9 Hz, J = 0.9 Hz), 8.22 (dd, 1H, J = 3.9
1
2
1
Indium tin oxide (ITO) coated glass substrates were cleaned,
sequentially, by ultrasonic treatment in detergent at 90 °C for 3 h,
then washed with water and deionized water, dried under a stream of
air and then in an oven at 110 °C. Prior to use, the ITO-coated slides
were treated with UV-ozone for 15 min. A conducting film of poly(3,4-
ethylenedioxythiophene) doped with poly(styrenesulfonate) (PE-
DOT:PSS) was spin coated at a speed of 3000 rpm for 60 s, followed
by annealing at 120 °C for 30 min, forming a film of about 35 nm
thickness. The active layer solution was spin coated at a speed of 1000
rpm for 60 s and Al (100 nm) electrode was deposited by thermal
evaporation on top of the active layer. The devices were annealed at
Hz, J = 0.9 Hz), 7.70 (dd, 1H, J = 5.1 Hz, J = 0.9 Hz), 7.52 (dd, 1H, J
2 1 2 1
= 5.1 Hz, J = 0.9 Hz), 7.25 (dd, 1H, J = 3.9 Hz, J = 5.1 Hz), 7.22 (dd,
2
1
2
1H, J = 3.9 Hz, J = 5.1 Hz). 4.58−4.82 (m, 2H), 4.05−3.89 (m, 2H),
1
2
1
3
1.80 (br, 2H), 1.53−1.23 (m, 16H), 1.00−0.85 (m, 12H). C NMR
75 MHz, CDCl ) δ (ppm): 166.34, 161.70, 149.58, 142.42, 138.68,
(
3
1
7
2
34.86, 131.61, 131.53, 130.08, 130.00, 128.96, 127.96, 114.22, 111.12,
2.39, 45.48, 39.32, 39.08, 30.92, 30.73, 30.23, 29.07, 28.35, 24.08,
3.56, 23.02, 14.07, 14.02, 11.27, 10.49. MS (ESI): calcd., 524.25;
+
found, 525.25 [M+H ].
1
,4-Bis(2-ethylhexylthio)-3,6-di(thiophen-2-yl)pyrrolo[3,4-c]-
pyrrole (3). Compound 9 (1.7 g, 5.0 mmol), anhydrous K CO (2.8 g,
2
3
1
40 °C for 30 min, then taken out of the oven immediately to cool
2
0 mmol), 2-ethylhexyl bromide (2.3 mL, 12 mmol), and acetone
2
them quickly. The PD device area is 12 mm . All the procedures were
carried out in air except for the evaporation of Al electrode.
(100 mL) were added to a one-neck, round-bottomed flask. The
mixture was purged with Ar for 30 min and heated at 70 °C for 12 h.
After removal of the solvent under vacuum, the crude compound was
dissolved in dichloromethane and filtered. The filtrate was evaporated
to dryness, and the residue was purified by column chromatography
External Quantum Efficiency (EQE) and Detectivity Meas-
urement. EQE measurements were done using tungsten lamp (250
W) passing through a monochromator, which was chopped at 160 Hz
and focused on the active area of the device. The reference was a
calibrated Si photodiode with known EQE spectra. The photocurrent
from the device is converted to the voltage and measured with a lock-
in amplifier to eliminate background noise. The photodiode signal was
first amplified using a low-noise current amplifier and then detected by
a lock-in amplifier (SR830, Stanford Research Systems). Reverse bias
on the device was provided by Keithley 236 Source Measure Unit. The
detectivity was calculated according to the ref 36. If the shot noise
from the dark current is the major contribution, the detectivity (D*) is
calculated from the following equation:
(
silica gel, ethyl acetate/hexane = 1/9 v/v) to afford the product as
1
grayish blue solids. Yield: 2.2 g, 79%. H NMR (300 MHz, CDCl ) δ
3
(
(
1
1
3
ppm): 8.12 (d, 2H, J = 3.9 Hz), 7.64 (d, 2H, J = 5.0 Hz), 7.33−7.24
m, 2H), 3.65−3.45 (m, 4H), 1.96−1.74 (m, 2H), 1.66−1.25 (m,
13
6H), 1.11−0.81 (m, 12H). C NMR (75 MHz, CDCl ) δ (ppm):
3
66.95, 152.12, 139.24, 132.54, 132.40, 131.98, 128.96, 39.80, 36.45,
2.68, 28.97, 25.86, 23.03, 14.13, 11.05. MS (MALDI): calcd., 556.21;
+
found, 557.20 [M+H ].
3
,3′-(5,5′-(9,9-Dioctyl-9H-fluorene-2,7-diyl)bis(thiophene-5,2-
diyl))bis(2-(2-ethylhexyl)-4-(2-ethylhexyloxy)-6-(thiophen-2-yl)-
pyrrolo[3,4-c]pyrrol-1(2H)-one) (4a). Under argon atmosphere, a
solution of compound 8 (0.67 g, 1.1 mmol), 2,2′-(9,9-dioctyl-9H-
fluorene-2,7-diyl)bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolane) (0.30 g,
D* = R/(2qJd) = (J /L )/(2qJd)1/2
1
/2
ph
light
Synthesis. 2-(2-Ethylhexyl)-4-(2-ethylhexyloxy)-3,6-di(thiophen-
-yl)pyrrolo[3,4-c]pyrrol-1(2H)-one (2). This compound was synthe-
0
.46 mmol), aqueous 2 M potassium carbonate (8.5 mL, 17 mmol),
2
and Pd(PPh ) (11.2 mg, 0.01 mmol) in THF (17 mL) was stirred and
sized according to the same procedure as that for compound 1.
Compound 7 (7.4 g, 25 mmol), K CO (13.8 g, 100 mmol), and
3 4
heated at reflux for 18 h. After being cooled to room temperature, the
reaction mixture was extracted with dichloromethane, washed with
2
3
dimethylformamide (DMF; 180 mL) were added to a 500-mL, three-
neck round-bottomed flask. The reaction mixture was purged with
argon for 30 min and heated to 120 °C for 1 h. 2-Ethylhexyl bromide
brine and dried over anhydrous Na SO . The crude product was
2 4
purified by column chromatography (silica gel, dichloromethane/
(
12.5 mL, 62.5 mmol) was added dropwise via syringe, and the
hexane =1/1 v/v). The product was obtained as brown solids. Yield:
1
reaction mixture was stirred for 5 h at 130 °C. The mixture was cooled
down to room temperature and poured into 200 mL of water. Then,
the mixture was extracted with dichloromethane (100 mL) three
times, and the combined organic phase was washed with brine and
0.33 g, 50%. H NMR (300 MHz, CDCl
) δ (ppm): 8.43 (d, 2H, J =
3
4.2 Hz), 8.22 (dd, 2H, J
= 3.6 Hz, J
= 0.9 Hz), 7.74−7.69 (m, 6H),
1
2
7.55 (d, 2H, J = 3.9 Hz), 7.28−7.24 (m, 4H), 4.59−4.56 (m, 4H),
3.40−3.97 (m, 4H), 2.10−2.00 (m, 4H), 1.90−1.70 (m, 4H), 1.60−
dried over MgSO . After removal of the solvent by rotary evaporation,
0.60 (m, 86H). 13C NMR (75 MHz, CDCl ) δ (ppm): 161.64, 152.01,
4
3
2
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Chemistry of Materials
Article
1
1
3
2
49.89, 140.94, 134.79, 133.31, 133.17, 131.53, 130.08, 128.00, 125.28,
25.08, 120.32, 120.22, 72.57, 55.47, 45.59, 40.42, 39.37, 39.11, 31.74,
0.74, 30.28, 29.99, 29.20, 29.13, 28.38, 24.12, 23.79, 23.61, 23.06,
23.54, 23.04, 14.07, 14.02, 11.25, 10.47. MS (ESI): calcd., 602.16;
found, 603.19 [M+H ].
+
1,4-Bis(5-bromothiophen-2-yl)-3,6-bis(2-ethylhexylthio) pyrrolo-
[3,4-c]pyrrole (10). Compound 3 (1.0 g, 1.8 mmol) was dissolved
in dry chloroform (50 mL), and the solution was purged with argon
for 30 min. NBS (0.71 g, 4.0 mmol) was added in a few portions, and
the solution was stirred at room temperature in the dark overnight.
Chloroform was removed by evaporation, and the crude product was
2.57, 14.12, 14.05, 11.32, 10.54. MS (MALDI): calcd., 1434.80;
+
found, 1435.80 [M+H ].
3
,3′-(5,5′-(9-(2-Ethylhexyl)-9H-carbazole-2,7-diyl)bis(thiophene-
5
,2-diyl))bis(2-(2-ethylhexyl)-4-(2-ethylhexyloxy)-6-(thiophen-2-yl)-
pyrrolo[3,4-c]pyrrol-1(2H)-one) (4b). Compound 4b was synthesized
1
according to the same procedure for 4a. Yield: 0.18 g, 27%. H NMR
washed with methanol. The pure product was obtained as dark purple
(
300 MHz, CDCl ) δ (ppm): 8.55 (d, 2H, J = 3.9 Hz), 8.23 (dd, 2H,
1
3
solids. Yield: 0.82 g, 63%. H NMR (300 MHz, CDCl ) δ (ppm): 7.82
3
J = 3.9 Hz, J = 1.2 Hz), 8.08 (d, 2H, J = 8.1 Hz), 7.71−7.69 (m, 4H),
1
2
(d, 2H, J = 4.1 Hz), 7.23 (d, 2H, J = 4.1 Hz), 3.60−3.41 (m, 4H),
7
7
4
1
1
9
1
3
1
.64 (dd, 2H, J = 8.1 Hz, J = 1.2 Hz), 7.57 (d, 2H, J = 4.2 Hz), 7.28−
13
1
2
1.89−1.73 (m, 2H), 1.61−1.27 (m, 16H), 1.06−0.87 (m, 12H).
C
.24 (m, 2H), 4.58−4.56 (m, 4H), 4.35−4.20 (m, 2H), 4.00−3.90 (m,
H), 2.20−2.10 (m, 1H), 1.90−1.70 (m, 4H), 1.6−1.2 (m, 40H),
NMR (75 MHz, CDCl ) δ (ppm): 166.71, 151.01, 140.78, 132.56,
3
1
32.16, 121.07, 39.79, 36.59, 32.68, 29.05, 25.83, 23.07, 14.18, 11.03.
13
.05−0.80 (m, 30H). C NMR (75 MHz, CDCl ) δ (ppm): 166.35,
3
Elemental analysis for C H Br N S : calcd., C, 50.42; H, 5.36; N,
30
38
2
2 4
61.56, 150.32, 149.29, 142.07, 141.56, 137.65, 134.69, 133.31, 131.
3, 131. 33, 130.15, 127.95, 125.29, 122.77, 120.74, 117.78, 114.41,
11.30, 106.19, 72.39, 47.29, 45.53, 39.43, 39.36, 39.12, 30.95, 30.76,
0.28, 29.12, 28.93, 28.39, 24.66, 24.12, 23.60, 23.13, 23.08, 14.20,
3
.92; found, C, 50.22; H, 5.29; N, 3.67. HRMS: Calcd., 714.70412;
found, 715.03891.
1
,4-Bis(heptadecan-9-ylthio)-3,6-di(thiophen-2-yl)pyrrolo[3,4-c]-
pyrrole (11). This compound was synthesized according to the same
4.14, 14.06, 11.32, 11.06, 10.54. MS (MALDI): calcd., 1323.67;
procedure for compound 3 just by replacing 2-ethylhexyl bromide to
+
found, 1324.67 [M+H ].
1
9
-bromoheptadecane. Yield: 40%. H NMR (300 MHz, CDCl ) δ
3
3
,3′-(5′,5″-(1-Dodecyl-1H-pyrrole-2,5-diyl)bis(2,2′-bithiophene-
(
ppm): 8.09 (dd, 2H, J = 3.6 Hz, J = 0.9 Hz), 7.62 (dd, 2H, J = 4.8
1
2
1
5
′,5-diyl))bis(2-(2-ethylhexyl)-4-(2-ethylhexyloxy)-6-(thiophen-2-yl)-
Hz, J = 0.9 Hz), 7.29−7.26 (m, 2H), 4.39−4.30 (m, 2H), 2.10−1.20
2
pyrrolo[3,4-c]pyrrol-1(2H)-one) (4c). Compound 4c was synthesized
(
m, 56H), 0.95−0.80 (m, 12H).
1
according to the same procedure for 4a. Yield: 0.28 g, 40%. H NMR
1
,4-Bis(5-bromothiophen-2-yl)-3,6-bis(heptadecan-9-ylthio)-
(
300 MHz, CDCl ) δ (ppm): 8.43 (d, 2H, J = 3.9 Hz), 8.21 (dd, 2H,
3
pyrrolo[3,4-c]pyrrole (12). This compound was synthesized according
to the same procedure for compound 10. Yield: 26%. H NMR (300
J = 3.9 Hz, J = 1.2 Hz), 7.70 (dd, 2H, J = 5.1 Hz, J = 1.2 Hz), 7.32−
1
1
2
1
2
7
4
1
.23 (m, 6H), 7.03 (d, 2H, J = 3.9 Hz), 6.43 (s, 2H), 4.60−4.45 (m,
H), 4.30−4.20 (m, 2H), 4.03−3.90 (m, 4H), 1.90−1.70 (m, 4H),
MHz, CDCl ) δ (ppm): 7.82 (d, 2H, J = 3.0 Hz), 7.23 (d, 2H, J = 3.9
3
Hz), 4.34−4.29 (m, 2H), 2.20−1.20 (m, 56H), 0.90−0.88 (m, 12H).
13
.60−1.20 (m, 52H), 1.00−0.80 (m, 27H). C NMR (75 MHz,
+
MS (MALDI): calcd., 964.3; found, 983.5 [M+H O+H ].
2
CDCl ) δ (ppm): 166.32, 161.58, 148.87, 141.78, 141.74, 137.31,
3
Compound 3-Me. In a 25 mL round-bottomed flask, compound 3
1
1
3
2
36.85, 134.95, 134.73, 132.89, 131.56, 131.43, 130.12, 128.95, 127.98,
26.28, 125.55, 124.94, 114.48, 111.57, 72.39, 45.55, 39.34, 39.11,
1.94, 31.09, 30.75, 30.26, 29.67, 29.57, 29.44, 29.38, 29.11, 28.98,
(
28 mg, 0.05 mmol) was dissolved in 5 mL of anhydrous
dichloromethane, and the solution was degassed for 15 min.
Trimethyloxonium tetrafluoroborate (22 mg, 0.15 mmol) was added,
and the mixture was stirred at room temperature for 2 h under argon.
The mixture was filtrated, and the precipitate was washed with
dichloromethane. The filtrate was collected, and after removal of
8.37, 26.39, 24.11, 23.59, 23.05, 22.70, 14.12, 14.05, 11.30, 10.52. MS
+
(MALDI): calcd., 1443.68; found, 1444.69 [M+H ].
1
,4-Bis(5-(6-ethoxynaphthalen-2-yl)thiophen-2-yl)-3,6-bis(2-
ethylhexylthio)pyrrolo[3,4-c]pyrrole (5). In a 25-mL, round-bottomed
flask, compound 10 (0.25 g, 0.35 mmol), 6-ethoxynaphthalen-2-
ylboronic acid (13) (0.22 g, 1.0 mmol), and Aliquat 336 (0.07 g, 0.17
mmol) were dissolved in 5 mL of anhydrous toluene, and the resulting
solution was degassed for 15 min. A degassed 2.0 molar solution of
potassium phosphate (3 mL) was added, and the mixture was degassed
for another 5 min. Tetrakis(triphenylphosphine)palladium (7.7 mg,
solvent, the product was obtained as golden solid with no need for
1
further purification. Yield: 29 mg (90%). H NMR (300 MHz, CDCl )
3
δ (ppm): 8.48 (d, 2H, J = 3.9 Hz), 8.07−7.94 (m, 2H), 7.46 (br, 2H),
3
0
.77−3.74 (m, 4H), 1.91−1.87 (m, 2H), 1.57−1.38 (m, 19H), 1.01−
.93 (m, 12H).
Typical Synthetic Procedure for Polymers 6a−c. Under argon
atmosphere, a mixture of 9,9-dioctyl-2,7-bis(trimethyleneboronate)-
fluorene (0.279 g, 0.5 mmol) and compound 10 (0.357 g, 0.5 mmol),
Pd(PPh ) , and aqueous 2 M potassium carbonate in THF was heated
6
.7 μmol) was added, and the reaction mixture was stirred and heated
to 90 °C under argon for 24 h. The mixture was cooled to room
temperature and then poured into water. The precipitated solid was
collected by vacuum filtration and washed with water, methanol, and
3
4
under reflux for 36 h. After being cooled to room temperature, the
reaction mixture was extracted with dichloromethane (3 × 50 mL),
washed with brine, and dried over anhydrous Na SO . The solution
dichloromethane. The product was obtained as brown solids. Yield:
1
0
4
4
1
.24 g, 77%. H NMR (300 MHz, CDCl ) δ (ppm): 8.12−8.10 (m,
2
4
3
was concentrated and precipitated in methanol. The crude polymer
was collected by filtration and washed in a Soxhlet apparatus with
methanol, acetone, and hexane consecutively until the washings were
colorless. Finally, the polymer was placed in a Soxhlet trap and
extracted by chloroform. The chloroform solution was condensed to
10 mL under the reduced pressure and then precipitated in 250 mL of
H), 7.82−7.75 (m, 6H), 7.62−7.61 (m, 2H), 7.22−7.15 (m, 4H),
.21 (t, 4H, J = 6.9 Hz), 3.67−3.54 (m, 4H), 2.00−1.80 (m, 2H),
.70−1.20 (m, 22H), 1.10−0.80 (m, 12H). HRMS: calcd., 896.35376;
+
found, 897.36731 [M+H ].
3
-(5-Bromothiophen-2-yl)-2-(2-ethylhexyl)-4-(2-ethylhexyloxy)-6-
thiophen-2-yl)pyrrolo[3,4-c]pyrrol-1(2H)-one (8). Compound 2
0.22 g, 0.42 mmol) was dissolved in dry chloroform (30 mL), and
the solution was purged with argon for 30 min. N-Bromosuccinimide
NBS) (0.16 g, 0.92 mmol) was added in a few portions, and the
(
(
methanol. The fiber-like polymer 6a was collected by filtration and
1
dried in vacuum. Yield: 0.40 g, 85%. H NMR (300 MHz, CDCl
) δ
3
(
(ppm): 8.17−8.16 (m, 2H), 7.95−7.61 (m, 8H), 3.67−3.59 (m, 4H),
mixture was stirred at room temperature in the dark overnight.
Chloroform was removed by evaporation, and the crude product was
purified by column chromatography (silica gel, dichloromethane/
2.09−2.07 (m, 4H), 1.93−1.92 (m, 2H), 1.6−0.6 (m, 58H). GPC: M
n
= 11 kDa, M = 18 kDa, PDI = 1.6.
w
1
Polymer 6b. Dark blue fiber; Yield: 0.34 g, 70%. H NMR (300
hexane =1/1 v/v). The product was obtained as dark red solid. Yield:
MHz, CDCl ) δ (ppm): 8.20−8.10 (m,4H), 7.80−7.40 (m, 6H), 4.67
3
1
0
3
1
2
.11 g, 45%. H NMR (300 MHz, CDCl ) δ (ppm): 8.24 (d, 1H, J =
(br, 1H), 3.65 (br, 4H), 2.43−2.37 (br, 2H), 2.10−1.80 (m, 4H),
1.80−0.80 (m, 58H). GPC: M = 14 kDa, M = 28 kDa, PDI = 2.0.
3
.9 Hz), 8.19 (d, 1H, J = 3.9 Hz), 7.72 (d, 1H, J = 4.8 Hz), 7.25 (m,
H), 7.16 (d, 1H, J = 4.2 Hz), 4.55−4.45 (m, 2H), 3.97−3.88 (m,
H), 1.81−1.70 (m, 2H), 1.50−1.10 (m, 16H), 1.05−0.80 (m, 12H).
n
w
1
Polymer 6c. Dark blue fiber; Yield: 0.33 g, 65%. H NMR (300
MHz, CDCl ) δ (ppm): 8.07−8.03 (m, 2H), 7.40−7.34 (m, 4H),
3
1
3
C NMR (75 MHz, CDCl ) δ (ppm): 166.10, 161.59, 148.12, 143.02,
7.10−6.90 (m, 2H), 6.36−6.33 (m, 2H), 4.41−4.38 (m, 1H), 3.66−
3.51 (m, 4H), 2.00−1.80 (m, 4H), 1.70−1.10 (m, 42H), 1.10−0.80
(m, 18H). GPC: M = 12 kDa, M = 26 kDa, PDI = 2.2.
3
1
1
40.19, 135.15, 131.96, 131.86, 131.30, 129.93, 128.05, 117.95, 114.04,
11.13, 72.48, 45.52, 39.29, 39.07, 30.72, 30.22, 29.05, 28.33, 24.06,
n
w
2
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Chemistry of Materials
Article
a
Scheme 1. Synthesis of Monomers and PPO, DAP-based Compounds, and Polymers
a
Reagents and conditions: (i) Pd(PPh ) , K CO (aq, 2M), THF; (ii) Pd(PPh ) , Aliguat 336, K PO (aq, 2M), toluene; (iii) Pd(PPh ) , toluene.
3
4
2
3
3
4
3
4
3 4
Polymer 6d. Under argon atmosphere, a mixture of compound 12
0.193 g, 0.2 mmol), 2,6-bis(trimethyltin)-4,8-bis(octyloxy)benzo[1,2-
Thermal and Optical Properties. The thermal properties
of 4a−c and polymers 6a−d were evaluated by differential
scanning calorimetry (DSC) and thermogravimetric analysis
(
b:4,5-b′]dithiophene (14) (0.155 g, 0.2 mmol), and Pd(PPh ) (11
3
4
mg, 9.8 μmol) in toluene (5 mL) was heated at reflux for 36 h. After
being cooled to room temperature, the reaction mixture was extracted
with dichloromethane, washed with brine, and dried over anhydrous
Na SO . The solution was concentrated and precipitated in methanol.
(
TGA) (Figure S1, Supporting Information). The TGA study
indicates that both the PPO-based compounds and DAP-based
polymers possess good thermal stability in nitrogen with a
decomposition temperature (5% weight loss) up to 250 °C.
2
4
The product was collected by filtration and dried in vacuum. Polymer
1
6
d was obtained as dark greenish blue fiber. Yield: 0.15 g, 60%. H
Compound 4a exhibits crystallization (T ) and melting
c
NMR (300 MHz, CDCl ) δ (ppm): 8.10−8.00 (m, 2H), 7.75−40 (m,
transitions (T ) at 95 and 149 °C, respectively. Polymer 6a
3
m
4
1
H), 4.70−4.20 (m, 6H), 2.20−1.80 (m, 12H), 1.80−1.10 (m, 68H),
has a clear glass-transition temperature (T ) at 107 °C.
g
.10−0.70 (m, 18H). GPC: M = 23 kDa, M = 83 kDa, PDI = 3.6.
n
w
The absorption and emission maxima of DPP 1 are at 546
and 567 nm, respectively (Figure 4). In comparison, the
absorption onset of PPO 2 is red-shifted about 25 nm and the
emission shifts to 600 nm (Table 1). Similarly, the absorption
onset of 3 is red-shifted about 70 nm relative to DPP 1 and 45
nm relative to PPO 2. Extension of π-conjugation of PPO
chromophore leads to further red-shift in both absorption and
emission spectra, as compounds 4a−c having the fluorene,
carbazole, and thiophene-pyrrole-thiophene units display a red-
shift of 80 nm in absorption and 30−50 nm in emission relative
to 2 (Figure S2, Supporting Information; Table 1). In
comparison with DAP 3, the absorption maxima of compound
RESULTS AND DISCUSSION
■
Design and Synthesis. The chemical structures of the
PPO and DAP-based compounds and polymers are illustrated
in Figure 3, and the synthetic routes are outlined in Scheme 1.
As a result of competitive N- and O-alkylations, PPO 2 was
obtained in 14% yield along with N-alkylated DPP 1 (Scheme
1). Bromination of 2 yielded only monobrominated PPO, and
subsequent Suzuki cross-coupling reaction afforded three PPO
compounds 4a−c.
A large family of DAP-containing compounds can be readily
derived in high yields from the corresponding DTPP
compounds. As an example, alkylation of DTPP gave
exclusively DAP 3. Bromination at the two terminal thiophene
rings of 3 yielded a dibromo compound that is useful as a
building block for constructing a variety of DAP-containing
compounds (e.g., 5) and as a monomer for making many
conjugated polymers (e.g., 6a−d) by the Suzuki or Stille cross-
coupling reaction. The fluorene, carbazole, and other electron-
donating moieties were introduced in compounds 4 and
polymers 6 in order to tune the optical and electrochemical
properties.
5
and polymer 6a are red-shifted by 120 and 160 nm (Figure
4b). Polymerization allows for further band gap tuning and
realization of other unique optoelectronic properties of the
resulting polymers through incorporation of many other
conjugated moieties (Figure S3 and Table S1, Supporting
Information). Among all the representative compounds and
polymers, polymer 6d absorbs at the longest wavelength at 807
nm as a result of incorporation of a benzodithiophene moiety in
the conjugated polymer backbone. Similar to compounds 3 and
5, polymers 6a−d are not fluorescent, presumably due to the
presence of thio-lactim group.
2
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Chemistry of Materials
Article
Table 1. Absorption, Fluorescence, and Energy Gaps
max
PL
λ
abs
λ
HOMO LUMO
Eg
max
a
b
(
nm)
Log ε (nm)
Φ (%)
(eV)
(eV)
(eV)
f
c
1
2
3
4
4
4
5
6
6
2
3
4
546
527
560
611
613
610
682
723
807
542
628
651
4.48
4.44
4.54
5.35
5.36
5.29
4.85
5.03
4.57
4.43
4.65
5.18
567
600
2.5
5.31
5.34
5.45
5.24
5.12
4.91
5.20
5.17
5.03
5.43
5.54
5.30
3.13
3.30
3.70
3.54
3.60
3.60
3.79
3.84
3.84
3.45
3.85
2.18
2.04
1.75
1.70
1.52
1.31
1.41
1.34
1.19
1.98
1.69
c
2.9
d
ND
ND
e
a
b
c
647
643
628
ND
ND
ND
612
660
710
16
e
42
e
1.3
ND
ND
ND
a
d
+
+
c
-H
-H
a-
1.5
c
20
d
f
g
ND
3.58
1.72
+
+
H
f
g
4
4
b-
H
652
662
5.20
5.10
706
653
ND
ND
5.35
5.00
3.66
1.69
f
g
c-
H
3.48
1.52
+
+
h
f
g
5
6
-H
a-
744
883
4.87
4.74
863
974
4.0
5.40
5.46
3.87
1.53
h
f
g
1.8
4.16
1.30
+
H
h
f
g
6
d-
H
969
4.60
1200
<1
5.18
4.10
1.08
+
a
In chloroform (10−5 M). In chloroform (10−4 M). Fluorescence
b
c
37
quantum yield relative to rhodamin 6G (Φ = 0.86 in methanol).
f
d
e
ND: not detectable. Relative to methylene blue (Φ = 0.03 in
f
3
7
f
g
methanol). Calculated from HOMO and E . Optical energy gap.
g
h
38
Relative to IR-125 (Φ = 0.13 in DMSO).
f
Figure 4. Absorption (a) and (b) and normalized fluorescence (c)
spectra of representative DPP, PPO, and DAP chromophores,
polymers, and their corresponding protonated salts in chloroform.
+
+
Figure 5. Energy level of compounds 1, 2, 2-H , 3, and 3-H . (R = 2-
ethylhexyl, Ar = 2-thiophene).
Electrochemical Properties. The electrochemical proper-
ties are investigated by cyclic voltammetry (CV), and the data
are summarized in Table 1. They are all electrochemically active
and have two or three quasi-reversible reduction waves in cyclic
voltammogram. The first reduction potentials are from the
reduction of acceptor cores, such as DPP, PPO, or DAP units.
The LUMO levels are decreased from compound 1 to 3 due to
the presence of a stronger acceptor (Figure 5). The reversible
oxidation waves were observed (Figure S9, Supporting
Information), and the first one may be attributed to the
oxidation of the donors such as the thiophene. It is interesting
to note that the stronger acceptor not only narrows the band
gap, but also lowers the HOMO levels. These results are highly
consistent with the computational calculation. When comparing
each pair of the DPP-and DAP-based compounds and
polymers, the latter always shows a decrease in energy gap
and a drop in the HOMO levels at the same time (Table S2,
Supporting Information), which is unique and desirable for
designing donor materials in organic bulkheterojunction (BHJ)
incorporation of stronger electron donor (6d vs 6a) effectively
decreases the band gap.
Halochromic and Halofluoric Properties. Besides the
changes in band gap and the HOMO levels, since the lactim
nitrogen in DAP is more basic than the lactam nitrogen in DPP,
3
9,40
the acid−base reaction can occur.
In a test, when
camphorsulfonic acid (CSA) was added to DPP 1 in
chloroform, its absorption spectrum did not show any change,
indicating its chemical inertness to acid (Figure S4, Supporting
Information). In contrast to DPP, PPO 2 can be readily
39
protonated by CSA or other mineral and organic acids, and
+
the absorption and emission maxima of 2-H were red-shifted
about 15 nm (Figure 4, Table 1). The HOMO and LUMO
levels of 2-H+ are lowered by 0.09 and 0.15 those of 2,
respectively, due to the decrease of electron density of the
41
entire π-conjugated system (Figure 5). Interestingly, when
compound 3 was protonated, the absorption at 560 nm
decreased and a new, intense peak at 630 nm appeared (Figure
6a). Visually, the color changes from purple for 3 to dark cyan
2
0,21
+
solar cells.
Extension of π-conjugation (6a vs 3) or
for 3-H upon addition of CSA. However, only one of the two
2
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Chemistry of Materials
Article
In contrast to nonemissive compound 3, the protonated salt
-H is highly fluorescent, exhibiting a bright red emission at
+
3
6
60 nm and higher emission efficiency (20%) than DPP 1
(
Figure 6b, Table 1). Narrow-band gap polymers 6a and 6d are
basically nonfluorescent. Protonation effectively induces the
+
NIR fluorescence turn-on, as polymer 6a-H emits at 970 nm
+
(
Figures 4c and 6c) and 6d-H emits at the telecommunication
wavelength of 1300 nm (Figure 4c). The cation-induced
fluorescence or halofluorism is unique for a certain type of
42−44
heterocyclic chromophores
and potentially applicable for
“
fluorescence turn-on” probing. These narrow-band gap
polymers represent the first example of NIR halochromic and
halofluoric materials. As expected, the basic lactim nitrogen
could be methylated as well, and the salt 3-Me, obtained by
reaction of 3 with trimethyloxonium tetrafluoroborate (Scheme
45
2
), had the enhanced emission (Figure S7, Supporting
Scheme 2. Methylation of Compound 3 with
Trimethyloxonium Tetrafluoroborate
Information). By complexation with a Lewis acid, the polymer
6
d-BCl adduct emits in the NIR region similar to the emission
3
+
of 6d-H (Figure S8, Supporting Information).
Photodetector and Detectivity. To demonstrate the
potential applications of DAP-based materials, polymer 6d with
a narrow band gap of 1.19 eV was used to fabricate polymer
photodetector devices. The device configuration is ITO/
PEDOT:PSS (35 nm)/active layer (100 nm)/Al (100 nm),
the active layer comprises a blend of polymer 6d and [6,6]-
phenyl-C -butyric acid methyl ester (PC BM) in a weight
6
1
61
−9
2
ratio of 1:3. The dark current density was 3.27 × 10 (A/cm )
at bias of −0.1 V (Figure S10, Supporting Information) and
even lower than that of the reported high-detectivity photo-
detector based on a narrow-band gap polymer, which may be
attributed to, at least in part, the lower HOMO level of polymer
22
Figure 6. Changes in absorption of 3 (a) and proton-induced
halochromic fluorescence spectra of 3 (b) and 6a (c) in chloroform
6
d. The photocurrent spectral response covered a wide range
+
upon gradual addition of CSA. Inset: photographs of 3, 6a and 3-H ,
from 400 nm to 1000 nm, matching well with the absorption
profile of the film of polymer 6d and PC BM mixture (Figure
+
6
a-H in chloroform. Photos in (b) are taken under UV illumination.
61
7
). The EQE and detectivity were calculated at 800 nm (points
1
1
A and B in Figure 7d) as 4.0% and 1.6 × 10 Jones,
respectively. The relatively low EQE value may be due to the
large domain size of the active layer (about 100 nm, Figure S10,
Supporting Information) and the close LUMO levels of 6d and
PC61BM, which could lead to insufficient exciton dissociation.
At the wavelengths from 400 to 850 nm, the device exhibited a
photodetectivity greater than 1011 Jones, being comparable to
basic nitrogen atoms in 3 is protonated, as evident by the
presence of several isosbestic points at about 335, 447, 529, and
593 nm in the titration spectra and the Job’s plot (Figure S5,
Supporting Information). The halochromic property was
confirmed by adding triethylamine (TEA) to 3-H+ and
recovering the absorption spectrum of 3 (Figure S6, Supporting
+
46,47
Information). Similar to the case of 2-H , the energy levels of
previously reported polymer photodetectors.
+
compound 3-H also decreased about 0.1 eV. As expected,
In conclusion, by simple chemical transformation of readily
available DPP derivatives, a new family of DAP chromophores
and polymers are obtained. DAP-based materials have smaller
energy gaps and lower HOMO levels relative to DPP analogs
and display unique acid-induced halochromic fluorescence in
the NIR spectral region. Further band gap tuning is likely to
afford suitable DAP polymers for applications in NIR
optoelectronic devices such as NIR photodetectors and NIR
OLEDs.
compound 5 and polymers 6 are also halochromic. Polymers
+
+
6
a-H and 6d-H absorb in the NIR spectral region at 883 nm
(
160-nm red-shift relative to 6a) and 700−1100 nm,
respectively (Figure 4b). In fact, polymers 6 are halochromic
upon protonation by mineral acids (e.g., HCl and H SO ),
2
4
organic acids (e.g., camphorsulfonic acid and CH COOH),
3
Lewis acid (e.g., BCl ), and even phenols (e.g., 2-
3
chlorophenol).
2
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Chemistry of Materials
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Figure 7. (a) Absorption spectrum of film of 6d and 6d with PC BM
61
(
1:3 weight ratio). (b) Schematic layout of the ITO/PEDOT:PSS/
active layer/Al photodetector on a glass substrate. (c) Current−
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ASSOCIATED CONTENT
■
(
*
S
Supporting Information
J. Sensors 2010, 10, 6488.
Detailed experimental procedures, DSC and TGA traces,
absorption spectra, cyclic voltammograms, and NMR spectra.
This material is available free of charge via the Internet at
http://pubs.acs.org.
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AUTHOR INFORMATION
■
(27) Tamayo, A. B.; Tantiwiwat, M.; Walker, B.; Nguyen, T.-Q. J.
Corresponding Author
Phys. Chem. C 2008, 112, 15543.
*Email: wayne_wang@carleton.ca.
(28) Yu, W.-L.; Pei, J.; Cao, Y.; Huang, W.; Heeger, A. J. Chem.
Notes
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The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
■
This work was supported by the Natural Sciences and
Engineering Research Council of Canada and National Natural
Science Foundation of China (Grant Nos. 21134005,
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