A low-bandgap conjugated polymer based on squaraine with strong two-photon absorption

Article abstract of DOI:10.1021/ma200386f

A new low-bandgap πjugated D-copolymer of squaraine and pyridopyrazine (P1) and a new small molecule squaraine-pyridopyrazine model compound (P2) were synthesized and compared. P1 and P2 exhibit strong NIR absorption and low bandgap (1-.3 eV). P2 in solut

Full text of DOI:10.1021/ma200386f

ARTICLE
pubs.acs.org/Macromolecules
A Low-Bandgap Conjugated Polymer Based on Squaraine with Strong
Two-Photon Absorption
Qinqin Shi,†,§ Wei-Qiang Chen,‡ Junfeng Xiang,† Xuan-Ming Duan,*,‡ and Xiaowei Zhan*,†
†Beijing National Laboratory for Molecular Sciences and Key Laboratory of Organic Solids, Institute of Chemistry,
Chinese Academy of Sciences, Beijing 100190, China
‡Laboratory of Organic Nanophotonics and Key Laboratory of Photochemical Convention and Functional Materials,
Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, China
§Graduate University of Chinese Academy of Sciences, Beijing 100049, China
S Supporting Information
b
ABSTRACT: A new low-bandgap π-conjugated DꢀA copoly-
mer of squaraine and pyridopyrazine (P1) and a new small
molecule squaraineꢀpyridopyrazine model compound (P2)
were synthesized and compared. P1 and P2 exhibit strong
NIR absorption and low bandgap (1ꢀ1.3 eV). P2 in solution
shows an intense and sharp absorption peak at 764 nm, while P1
in solution exhibits a red-shifted and broad absorption peak at
808 nm. The HOMO and LUMO levels of P1 were estimated to
be ꢀ5.02 and ꢀ4.15 eV, while the HOMO and LUMO levels of
P2 were estimated to be ꢀ5.27 and ꢀ3.22 eV, respectively.
Polymer P1 exhibits strong two-photon absorption (2PA) at
telecommunication wavelengths with 2PA cross sections per
repeat unit as high as 2300 GM, 3ꢀ5 times that for the small
molecule P2. The higher HOMO, the lower LUMO levels,
lower bandgap, red-shifted absorption, and stronger two-photon
absorption of P1 are attributed to higher degree of conjugation and
delocalization of π-electrons in the polymer.
’ INTRODUCTION
cross sections (δ). Schwoerer et al. reported that DꢀAꢀD
squaraine oligomers exhibited δ of 5000 GM (1 GM = 1 ꢁ
10ꢀ50 cm4 sꢀ1) at 820ꢀ890 nm.24 Later, Marder et al. reported
extended squaraine dyes with δ values as high as 33 000 GM,
which is the highest reported for squaraine-based dyes.25 Because
of substantial electronic coupling between the porphyrin and
squaraine moieties, a triad of porphyrinꢀsquaraineꢀporphyrin
also displayed a very high δ over a wide range of wavelengths.26,27
Since low-bandgap squaraine dyes are not suitable for the 2PA
application in optical limiting and photoinitiator, some indolic
squaraines with large optical bandgap were proposed, and δ of
450 GM was obtained.28,29
Squaraine dyes are generally synthesized by condensation of
electron-rich aromatic or heterocyclic compounds such as pyr-
role with squaric acid.1,2 Squaraines typically have a donor
(D)ꢀacceptor (A)ꢀdonor (D), resonance-stabilized zwitterionic
structure.3,4 Their main characteristic is the sharp and strong low-
energy absorption (ε > 105 L molꢀ1 cmꢀ1) frequently associated
with a strong fluorescence in the visible to near-IR (NIR) region in
solution.5 Squaraines are usually photostable due to the resonance
zwitterionic structure.4 Since squaraine dyes were first reported in
1965 by Triebs and Jacob,6 squaraines are widely used in the design
of a variety of electronic and photonic materials for applications such
as photoconductivity,7 data storage,8 light-emitting field-effect tran-
sistors,9 nonlinear optics,10 solar cells,11,12 fluorescence probes,13ꢀ17
and photodynamic therapy.18
Two-photon absorption (2PA) materials have attracted con-
siderable attention due to their potential applications in three-
dimensional (3D) data storage, 3D microfabrication, optical
limiting, optical pulse suppression, frequency up-converted las-
ing, bioimaging, and photodynamic therapy.19ꢀ23 Although squar-
aine dyes have a small detuning energy, the sharp and strong one-
photon absorption (1PA) band endows them with large 2PA
Polysquaraines were theoretically and experimentally consid-
ered as suitable candidates for low-bandgap semiconducting
polymers.30ꢀ32 Havinga et al. reported the first soluble squar-
aine-based polymer.33 Ajayaghosh et al. found that polysquar-
aines exhibited intrinsic conductivity of 5.3 ꢁ 10ꢀ4 S cmꢀ1 34ꢀ37
,
and they have also reported polysquaraines used for the detection
Received: February 19, 2011
Revised:
April 8, 2011
Published: April 15, 2011
r
2011 American Chemical Society
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coupling reaction between compounds 1 and 4 gave 3. At
compound 1/compound 0 = 1:1 (mole ratio), tiny 2 (ca. 5%)
and 4 (main product) were obtained. At compound 1/com-
pound 0 = 3:1 (mole ratio), 2 and 3 were obtained. At compound
1/compound 0 = 4:1 (mole ratio), 2 was the main product (60%
yield) (Scheme S1, Supporting Information). The final products
P1 and P2 were synthesized by condensation of pyridopyrazine-
substituted pyrrole species with squaric acid in refluxing
n-BuOH/benzene mixed solvent. The progress of the polymer-
ization was monitored by tracing the change in the absorption
spectrum of the reaction mixture at different time intervals
(Figure S4, Supporting Information). P1 and P2 are relatively
soluble in common organic solvents such as chloroform
and THF.
Molecular weights of polymer P1 were determined by gel
permeation chromatography (GPC) using polystyrene standards
as calibrants. The number-average molecular weight (Mn), weight-
average molecular weight (Mw), and polydispersity index (Mw/
Mn) of P1 were 6298, 11 005, and 1.75, respectively.
Absorption Spectra. Figure 2 shows UVꢀvisꢀNIR absorp-
tion spectra of P1 and P2 in chloroform solution and in thin film.
Figure S5 (Supporting Information) shows UV absorption
spectra of 2 and 3 in chloroform solution. Compound 3 exhibits
absorption maximum at 346 nm with a weak charge transfer band
at ca. 500 nm. Compound 2 exhibits a red-shifted absorption
maximum at 373 nm with a strong charge transfer band at ca.
547 nm due to increased conjugation. The small molecule P2 in
solution shows an intense and sharp absorption peak at 764 nm,
and the absorption of the film is broadened and red-shifted
remarkably, which is a typical phenomenon for squaraines
resulted from strong charge transfer interactions and tendency
to aggregation.45 Relative to 3, P2 in solution exhibits a huge red
shift (418 nm) of absorption maximum due to increased con-
jugation. Relative to P2, the polymer P1 in solution exhibits red-
shifted and broad absorption in visible and NIR region due to
high degree of conjugation and planarity of the polymer, which is
similar to other squaraine polymers.37 The optical bandgap of P1,
estimated from the absorption edge in films, is 0.97 eV, lower
than that of its small molecule counterpart (1.27 eV, Table 1).
The strong donorꢀacceptor interaction in these squaraines may
be responsible for the observed long wavelength optical absorp-
tion. No fluorescence in P1 and P2 was observed probably due to
strong intramolecular charge transfer between donor and
acceptor units.
Figure 1. Chemical structures of P1 and P2.
of cations.38,39 However, to our knowledge, there have been no
reports on polysquaraines for use in 2PA.
In recent years, fused-ring pyrazines have been proven to be
promising candidates for n-type semiconductors due to the
existence of electron-withdrawing imine nitrogen atoms, some
of which exhibit high electron mobilities.40ꢀ42 We reported that a
copolymer of porphyrin and 2,3-bis(4-trifluoromethylphenyl)-
pyrido[3,4-b]pyrazine exhibited strong two-photon absorption
with δ value of 3200 GM at telecommunication wavelengths.43
Here we report synthesis of a new low-bandgap π-conjugated
DꢀA copolymer of squaraine and pyridopyrazine (P1, Figure 1).
P1 exhibits strong NIR absorption and large 2PA cross sections
at telecommunication wavelengths. We also compared this DꢀA
copolymer with a small molecule squaraineꢀpyridopyrazine model
compound (P2, Figure 1) and found 3ꢀ4 times 2PA enhancement
in P1 relative to its small molecule counterpart P2.
’ RESULTS AND DISCUSSION
Electrochemistry. Cyclic voltammograms of the squaraine
dyes P1 and P2 are illustrated in Figure 3, and the onset oxidation
and reduction potentials of the materials are summarized in
Table 2. Both compounds exhibit 1 oxidation peak and 3
reduction peaks. The potential of the Ag wire reference electrode
was calibrated by internal standard ferrocene (Ein = 0.48 V versus
Ag wire). The HOMO and LUMO energy levels were estimated
from the onset oxidation and reduction potentials, assuming the
absolute energy level of ferrocene/ferrocenium to be 4.8 eV
below vacuum.46 P1 has an estimated HOMO of ꢀ5.02 eV and
an estimated LUMO of ꢀ4.15 eV; P2 has an estimated HOMO
of ꢀ5.27 eV and an estimated LUMO of ꢀ3.22 eV. The higher
HOMO and lower LUMO of P1 indicates that the polymer is
easier to oxidize and reduce than P2 since the π-electrons are
more delocalized along the polymer backbone.
Synthesis and Characterization of P1 and P2. Synthetic
routes to the squaraine dyes P1 and P2 are shown in Scheme 1.
The 2-vinylpyrrole intermediate (1) was synthesized by
HornerꢀHammond condensation of 1-hexylpyrrole-2-carbaldehyde
with PPh3MeBr. The vinylpyrrole-disubstituted intermediate 2
was synthesized by Heck coupling reaction between compound 1
and 5,8-dibromo-2,3-bis(4-trifluoromethylphenyl)pyrido[3,4-
b]pyrazine (0). To our surprise, vinylpyrrole-monosubstituted
compound 3 was isolated in 38% yield as a byproduct. This
1
unexpected product was confirmed by HR-MS, H NMR, and
selective 1D NOE NMR (Figures S1ꢀS3, Supporting In-
formation). A similar phenomenon was reported in the literature.44
We have not yet fully explored this observation. We speculate
that trace HNMe2 (from hydrolysis of DMF) in the solvent DMF
reacts with compound 0 in the presence of the base to afford
5-dimethylamino-8-bromo-2,3-bis(4-trifluoromethylphenyl)pyrido-
[3,4-b]pyrazine (4, Scheme S1, Supporting Information). Heck
Two-Photon Absorption at NIR Wavelengths. As we know,
two-photon absorption (2PA) is a way of accessing a given
excited state by using photons of nearly half the energy (or nearly
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Scheme 1. Synthesis of Compounds P1 and P2
Table 1. Absorption Data of P1 and P2
solution
film
compd λmaxa (nm) εb (105 Mꢀ1 cmꢀ1
)
λmaxa (nm)
Egopt c (eV)
P1
P2
808
764
0.4
833
850
0.97
1.27
2.14
a Absorption maxima. b Molar extinction coefficient (P1 for repeat unit).
c Optical bandgap was obtained from Egopt = 1240/λonset (eV), wherein
λonset is wavelength of its onset absorption edge in the longer wavelength
direction.
acid, polymer P1, and small molecule P2. P1 and P2 exhibit
strong NIR absorption and low bandgap. Compared to P2, P1
exhibits higher HOMO and lower LUMO levels, red-shifted and
broader absorption spectra, and lower bandgap due to higher
degree of conjugation and delocalization of π-electrons. Polymer
P1 exhibits strong two-photon absorption at telecommunication
wavelengths with 2PA cross sections per repeat unit as high as
2300 GM, 3ꢀ5 times that for the small molecule P2 due to high
degree of conjugation and delocalization of π-electrons in P1.
Figure 2. Absorption spectra of P1 (10ꢀ6 M of repeat unit) and P2
(10ꢀ6 M) in chloroform (s) and in thin film (f).
double the wavelength) of the corresponding one-photon transi-
tion. Since P1 and P2 in chloroform solution exhibit strong one-
photon absorption at 764 and 808 nm, respectively, they may
exhibit strong two-photon absorption at telecommunication
wavelengths (1300ꢀ1550 nm). The 2PA cross sections (δ) of
P1 and P2 were measured in the range of 1400ꢀ1640 nm in
CHCl3 by using the open-aperture Z-scan method. The typical
Z-scan traces of P1 and P2 exhibit a clear dip (Figure 4). 2PA
spectra of P1 and P2 in solution are shown in Figure 5. The 2PA
cross sections and corresponding wavelengths are summarized in
Table 3. Because of the extension of conjugation length, the 2PA
cross sections per repeat unit of polymer P1 are 1700ꢀ2300 GM,
3ꢀ5 times that for the small molecule P2.
’ EXPERIMENTAL SECTION
Materials. 1-Hexyl-1H-pyrrole-2-carbaldehyde25 and 5,8-dibromo-
2,3-bis(4-trifluoromethylphenyl)-pyrido[3,4-b]pyrazine42 were synthe-
sized according to the literature methods. Bio-Rad Bio-Beads S-X1 was
purchased from Bio-Rad and used without further purification, which is
neutral, porous styrene divinylbenzene beads for size exclusion chro-
matography of lipophilic polymers eluting with organic solvents. DMF
was refluxed and distilled from calcium hydride. THF was distilled from
sodium benzophenone under nitrogen before use. Unless stated other-
wise, other reagents were purchased from commercial sources and used
without further purification.
’ CONCLUSION
Characterization. The 1H and 13C NMR spectra were measured
on a Bruker AVANCE 400 MHz spectrometer using tetramethylsilane
In summary, we have synthesized two dicondensation pro-
ducts of pyridopyrazine-substituted pyrrole species with squaric
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Figure 3. Cyclic voltammograms for P1 and P2 in CH3CN/0.1 M [nBu4N]þ[PF6]ꢀ at 50 mV sꢀ1. The horizontal scale refers to an anodized Ag wire
pseudoreference electrode.
Table 2. Redox Potentials and Energy Levels of P1 and P2a
Eoxb (V)
Eredb (V)
Eoxc vs FeCp2þ/0 (V)
Eredc vs FeCp2þ/0 (V)
HOMOd (eV)
LUMOd (eV)
P1
P2
þ0.70
þ0.95
ꢀ0.17
ꢀ1.10
þ0.22
þ0.47
ꢀ0.65
ꢀ1.58
ꢀ5.02
ꢀ5.27
ꢀ4.15
ꢀ3.22
a Thin films on glassy-carbon electrode in CH3CN/0.1 M [nBu4N]þ[PF6]ꢀ at 50 mV sꢀ1
.
b Eox is the onset potentials versus anodized Ag wire
pseudoreference electrode corresponding to oxidations, while Ered is the onset potentials versus anodized Ag wire þp/s0eudoreference electrode
corresponding to reductions. c Eox vs FeCp2þ/0 = Eox ꢀ Ein, Ered vs FeCp2þ/0 = Ered ꢀ Ein. d HOMO = ꢀe(Eox vs FeCp2
þ 4.8) (eV), LUMO =
þ/0
ꢀe(Ered vs FeCp2
þ 4.8) (eV).
(TMS; δ = 0 ppm) as an internal standard. The selective 1D NOE NMR
and HSQC 2D NMR spectra were measured on a Bruker AVANCE
600 MHz spectrometer. Mass spectra were measured on a GCT-MS
micromass spectrometer using the electron impact (EI) mode or on a
Bruker Daltonics BIFLEX III MALDI-TOF Analyzer using MALDI
mode. Elemental analyses were carried out using a FLASH EA1112
elemental analyzer. Solution (chloroform) and thin-film (on quartz
substrate) UVꢀvisꢀNIR absorption spectra were recorded on a JASCO
V-570 spectrophotometer. Electrochemical measurements were carried
out under nitrogen on a deoxygenated solution of tetra-n-butylammo-
nium hexafluorophosphate (0.1 M) in acetonitrile using a computer-
controlled Zahner IM6e electrochemical workstation, a glassy-carbon
working electrode coated with sample films, a platinum-wire auxiliary
electrode, and an Ag wire anodized with AgCl as a pseudoreference
electrode. Potentials were referenced to the ferrocenium/ferrocene
(FeCp2þ/0) couple by using ferrocene as an internal standard. Thermo-
gravimetric analysis (TGA) measurements were performed on Shimad-
zu thermogravimetric analyzer (model DTG-60) under a nitrogen flow
at a heating rate of 10 °C minꢀ1. The gel permeation chromatography
(GPC) measurements were performed on a Waters 515 chromatograph
connected to a Waters 2414 refractive index detector, using THF as
eluent and polystyrene standards as calibrants, three Waters Styragel
columns (HT2, -3, -4) connected in series were used.
(z-axis), so the local power density within the sample cell could be
changed under a constant laser power level. The transmitted laser beam
from the sample cell was then detected by the same power meter as used
for reference monitoring. The on-axis peak intensity of the incident
pulses at the focal point, I0, ranged from 41 to 46 GW cmꢀ2. Assuming a
Gaussian beam profile, the nonlinear absorption coefficient β can be
obtained by curve fitting to the observed open-aperture traces with eq 1.
βI0ð1 ꢀ eꢀR l
Þ
0
TðzÞ ¼ 1 ꢀ
ð1Þ
2
2R0ð1 þ ðz=z0Þ Þ
where R0 is the linear absorption coefficient, l the sample length, and z0
the diffraction length of the incident beam. We obtained the nonlinear
absorption coefficient β at the corresponding wavelength, where linear
absorption isnegligible, tosatisfy the conditionofR0 l , 1. When R0 l , 1,
eq 1 can be simplified as eq 2.
βI0l
TðzÞ ¼ 1 ꢀ
ð2Þ
2
2ð1 þ ðz=z0Þ Þ
The 2PA cross section δ of P1 and P2 (in units of 1 GM = 1 ꢁ 10ꢀ50 cm4
s
ꢀ1) can be determined by eq 3.
δNAd ꢁ 10ꢀ3
hν
β ¼
ð3Þ
2PA Measurement. The 2PA cross sections δ of P1 and P2 were
measured at 1520, 1560, and 1640 nm in CHCl3 by using the open-
aperture Z-scan method,47,48 with ∼120 fs pulses from an optical
parametric amplify (OPA) operating at a 1 kHz repetition rate generated
from a mode-locked Ti:sapphire femtosecond laser (Tsunami, Spectra-
Physics). The laser beam was divided into two parts. One was used as the
intensity reference and monitored by EPM 2000 power meter
(Coherent Inc.). The other was used for transmittance measurement;
the laser beam was focused by passing through lens (f = 15 cm) and
passed through a quartz cell with a sample thickness of 1 mm. The
position of the sample cell could be varied along the laser-beam direction
where NA is the Avogadro constant, d is the concentration of the repeated
unit of P1 and P2 in CHCl3 (the corresponding concentrations for P1 and
P2 are 2.8 ꢁ 10ꢀ4 and 1.6 ꢁ 10ꢀ4 M, respectively), h is the Planck
constant, and ν is the frequency of the incident laser beam.
1-Hexyl-2-vinyl-1H-pyrrole (1). To a 100 mL three-neck round-
bottom flask, dry THF (20 mL) was charged under N2 flow. NaOEt
(1.836 g, 27 mmol) and PPh3MeBr (8.9 g, 25 mmol) were added. The
mixture was stirred at room temperature for 3 h. Then a solution of
1-hexyl-1H-pyrrole-2-carbaldehyde (4.475 g, 25 mmol) in 10 mL of
THF was added dropwise. After the reaction mixture was stirred under
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Figure 5. 2PA spectra for P1 and P2 in the range of 1400ꢀ1640 nm.
Table 3. 2PA Cross Sections at Telecommunication Wave-
lengths for P1 (per Repeat Unit) and P2
δ (GM) at 1520 nm δ (GM) at 1560 nm δ (GM) at 1640 nm
P1
P2
1770
343
2271
679
2300
704
30 min. Compound 1 (265.7 mg, 1.5 mmol) in anhydrous DMF
(15 mL) was added directly, and the red mixture was stirred at
100 °C for 24 h. The mixture was cooled to room temperature, washed
with water, extracted with dichloromethane, and dried over MgSO4.
Purification by column chromatography (silica gel, CH2Cl2:hexanes =
1:1) afforded a purple solid (100 mg, 26%). 1H NMR (400 MHz,
CDCl3): δ 8.95 (s, 1H), 8.26 (d, J = 15.5 Hz, 1H), 8.12 (d, J = 15.7 Hz,
1H), 7.82 (d, J = 16.3 Hz, 1H), 7.78 (d, J = 7.7 Hz,, 4H), 7.72 (d, J =
7.7 Hz,, 4H), 7.63 (d, J = 16.3 Hz, 1H), 6.84 (s, 1H), 6.81 (s, 1H), 6.74
(s, 1H), 6.70 (s, 1H), 6.24 (s, 1H), 6.22 (s, 1H), 4.11 (t, J = 7.3 Hz, 2H),
3.99 (t, J = 7.3 Hz, 2H), 1.83 (m, 2H), 1.73 (m, 2H), 1.31 (m, 6H), 1.24
(m, 6H), 0.88 (t, J = 6.3 Hz, 3H), 0.85 (t, J = 5.9 Hz, 3H). HRMS (EI):
m/z cacld for C45H45F6N5: 769.3579, found: 769.3588. Anal. Calcd for
C45H45F6N5: C, 70.20; H, 5.89; N, 9.10. Found: C, 69.83; H, 5.76; N,
8.97%. This compound was further confirmed by HSQC 2D NMR
(Figure S6, Supporting Information).
Figure 4. Z-scan traces (circle) of P1 (2.8 ꢁ 10ꢀ4 M per repeat unit)
and P2 (1.6 ꢁ 10ꢀ4 M) in CHCl3 in a 1 mm cell at (a) 1640 nm
(I0 = 41.0 GW cmꢀ2), (b) 1560 nm (I0 = 44.0 GW cmꢀ2), and
(c) 1520 nm (I0 = 46.0 GW cmꢀ2) with a theoretical fit assuming a
2PA process (solid line).
reflux for 15 h, the THF was removed under vacuum and the residue was
suspended in CH2Cl2 and filtered. The filtrate was extracted with 50 mL
of NaHSO3 solution (15 g of NaHSO3 in water), 50 mL of Na2CO3
solution (10 g of Na2CO3 in water), and 50 mL of H2O. After that the
organic phase was dried over Na2SO4, and the solvent was removed
under vacuum. Purification by column chromatography (silica gel, hexanes)
afforded colorless oil (3.1 g, 70%). 1H NMR (400 MHz, CDCl3): δ 6.63
(s, 1H), 6.58 (t, J = 8.8 Hz, 1H), 6.40 (t, J = 1.7 Hz, 1H), 6.12 (t, J =
2.9 Hz, 1H), 5.53 (d, J = 17.3 Hz, 1H), 5.06 (d, J = 11.2 Hz, 1H), 3.89 (t,
J = 7.3 Hz, 2H), 1.71 (m, 2H), 1.31 (m, 6H), 0.90 (t, J = 6.3 Hz, 3H).
HRMS (EI): m/z cacld for C12H19N: 177.1517, found: 177.1514. Anal.
Calcd for C12H19N: C, 81.30; H, 10.80; N, 7.90. Found: C, 81.02; H,
10.77; N, 7.77%.
5,8-Bis((E)-2-(1-hexyl-1H-pyrrol-2-yl)vinyl)-2,3-bis(4-(trifluoro-
methyl)phenyl)pyrido[3,4-b]pyrazine (2). To a 50 mL three-
neck flask, 5,8-dibromo-2,3-bis(4-trifluoromethylphenyl)pyrido[3,4-b]-
pyrazine (287.45 mg, 0.5 mmol), K2CO3 (1.034 g, 7.5 mmol), Bu4NBr
(240.9 mg, 0.75 mmol), LiCl (31.5 mg, 0.75 mmol), and Pd(OAc)2
(16.8 mg, 0.075 mmol) were added and deoxygenated with nitrogen for
(E)-8-(2-(1-Hexyl-1H-pyrrol-2-yl)vinyl)-5-dimethylamino)-
2,3-bis(4-(trifluoromethyl)phenyl)pyrido[3,4-b]pyrazine (3).
The same procedure as 2 to afford a red solid (120 mg, 38%). 1H NMR
(400 MHz, CDCl3): δ 8.51 (s, 1H), 7.78 (d, J = 7.9 Hz, 2H), 7.63
(m, 6H), 7.47 (dd, J = 21.2 Hz, 4.8 Hz, 2H), 6.68 (s, 1H), 6.57 (s, 1H),
6.18 (s, 1H), 3.96 (t, J = 7.1 Hz, 2H), 3.59 (s, 6H), 1.71 (m, 2H), 1.23
(m, 6H), 0.83 (t, J = 6.4 Hz, 3H). The structure was further certified by
selective 1D NOE NMR (Figures S1ꢀ3, Supporting Information) and
HSQC 2D NMR(Figure S7, SupportingInformation). HRMS (EI): m/z
cacld for C35H33F6N5: 637.2640, found: 637.2648. Anal. Calcd for
C35H33F6N5: C, 65.92; H, 5.22; N, 10.98. Found: C, 65.53; H, 5.14; N,
10.88%.
Polymer P1. To a 50 mL three-neck flask, squaric acid (17.4 mg,
0.15 mmol) and compound 2 (115 mg, 0.15 mmol) were added. A
mixture of n-butanol (4 mL) and benzene (8 mL) was added, and the
reaction mixture was refluxed under a nitrogen atmosphere for 11 h.
After the reaction mixture was cooled to room temperature, the polymer
was precipitated by addition of 100 mL of methanol. Finally, the polymer
was purified by size exclusion column chromatography over Bio-Rad
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Bio-Beads S-X1 eluting with chloroform. The polymer was recovered as
a black solid from the chloroform fraction by rotary evaporation (70 mg,
55%). 1H NMR (400 MHz, CD2Cl2): δ 8.20 (br, 1H), 7.65 (br, 12H),
7.10 (br, 4H), 4.13 (br, 2H), 3.78 (br, 2H), 2.33 (br, 2H), 2.02 (br, 2H),
1.55ꢀ1.26 (br, 12H), 0.88 (br, 6H) (Figure S8, Supporting In-
formation). GPC: Mn, 6298; Mw, 11 005; Mw/Mn, 1.75. Anal. Calcd
for (C49H43F6N5O2)n: C, 69.41; H, 5.11; N, 8.26. Found: C, 64.14; H,
6.06; N, 7.31%.
Compound P2. To a 50 mL three-neck flask, squaric acid (8.12 mg,
0.07 mmol) and compound 3 (127.45 mg, 0.2 mmol) were added. A
mixture of n-butanol (2 mL) and benzene (4 mL) was added, and the
reaction mixture was refluxed under a nitrogen atmosphere for 11 h. The
mixture was cooled to room temperature, washed with water, and
extracted with dichloromethane, and the organic phase was dried over
MgSO4. Purification by column chromatography (silica gel, CH2Cl2)
afforded a green solid (70 mg, 74%). 1H NMR (400 MHz, CDCl3): δ
8.55 (br, 2H), 7.87 (br, 4H), 7.72ꢀ7.64 (br, 12H), 7.52 (br, 4H), 7.01
(br, 4H), 4.85 (br, 4H), 3.67 (br, 12H), 1.85 (m, 4H), 1.40 (m, 12H),
0.86 (m, 6H) (Figure S9, Supporting Information). MS (MALDI): m/z
1352 (Mþ). Anal. Calcd for C74H64F12N10O2: C, 65.67; H, 4.77; N,
10.35. Found: C, 65.33; H, 4.74; N, 10.18%.
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’ ASSOCIATED CONTENT
S
Supporting Information. Effect of compound 1:com-
b
1
pound 0 mole ratio on products; H NMR, selective 1D NOE
and HSQC NMR spectra of 3; HSQC spectrum of 2; 1H NMR
spectra of P1 and P2; absorption spectra of polysquaraine
varying with reaction time; absorption spectra of compounds 2
and 3 in chloroform solution. This material is available free of
charge via the Internet at http://pubs.acs.org.
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’ AUTHOR INFORMATION
Corresponding Author
*E-mail: xwzhan@iccas.ac.cn (X.Z.), xmduan@mail.ipc.ac.cn
(X.-M.D.).
’ ACKNOWLEDGMENT
This work was supported by NSFC (Grants 21025418 and
21021091), 973 Project (Grant 2011CB808401), and the Chinese
Academy of Sciences.
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