Synthesis and characterization of near-infrared emissive BODIPY-based conjugated polymers

Article abstract of DOI:10.1055/s-0031-1290364

Three novel BODIPY-based conjugated polymers could be synthesized via palladium-catalyzed Sonogashira coupling reaction. Compared with BODIPY model derivatives, the polymers can emit in the range from deep-red to near-infrared region with emission spectral maxima at = 640-660 nm and exhibit moderate fluorescent quantum yield from 0.23 to 0.26. Density functional theory (DFT) calculations are performed on three polymers repeat units, and HOMO and LUMO energy levels of the conjugated polymers are estimated. Georg Thieme Verlag Stuttgart · New York.

Full text of DOI:10.1055/s-0031-1290364

LETTER
778
Synthesis and Characterization of Near-Infrared Emissive BODIPY-Based
Conjugated Polymers
NIR
E
missive BO
D
u
IPY-Based
a
Conjugated
n
Polymers zhao Wu, Xiao Ma, Jiemin Jiao, Yixiang Cheng,* Chengjian Zhu*
Key Lab of Mesoscopic Chemistry of MOE, School of Chemistry and Chemical Engineering, Nanjing University,
Nanjing 210093, P. R. of China
Fax +86(25)83317761; E-mail: yxcheng@nju.edu.cn; E-mail: cjzhu@nju.edu.cn
Received 29 November 2011
The synthesis procedures of the monomers and the conju-
gated polymers are outlined in Scheme 1. meso-Aryl-sub-
stituted BODIPY 1 dye was prepared by the starting
product 2,4-dimethylpyrrole in 33% overall yield accord-
Abstract: Three novel BODIPY-based conjugated polymers could
be synthesized via palladium-catalyzed Sonogashira coupling reac-
tion. Compared with BODIPY model derivatives, the polymers can
emit in the range from deep-red to near-infrared region with emis-
sion spectral maxima at l = 640–660 nm and exhibit moderate flu- ing to the literature.5a 2,6-Diiodotetramethyl BODIPY
orescent quantum yield from 0.23 to 0.26. Density functional theory
(DFT) calculations are performed on three polymers repeat units,
monomer 2 could be obtained by further iodization of
BODIPY dyes. The monomers 5, 6, and 10 were synthe-
and HOMO and LUMO energy levels of the conjugated polymers
sized according to the reported literature.12 The polymer-
are estimated.
ization reaction was carried out using the palladium-
Key words: organoboron dye, conjugated polymer, BODIPY,
near-infrared
catalyzed Sonogashira coupling reaction of p-phenylene-
ethynylene monomers with 2,6-diiodo-substituted BO-
DIPY monomer to afford dark blue polymers in over 50%
yields. The number average molecular weight (Mn) and
Luminescent organoboron dyes are of great interest for
the weight average molecular weight (Mw) of the polymer
not only chemistry and material science but also biologi-
are shown in Table 1. The gel permeation chromatogra-
cal science due to the excellent chemical and photostabil-
phy (GPC) results of the polymers show that three poly-
ity properties.1 4,4-Difluoro-4-borata-3a-azonia-4a-aza-s-
indacenes (BODIPY) have become the preferred fluoro-
mers have moderate molecular weight.
phores because of high fluorescent quantum yields, nar-
row absorption and sharp emission bands with high peak
intensities.2 The BODIPY-based derivatives have promis-
ing applications in fluorescence sensors toward heavy-
metal ions,3 pH,4 small molecules,5 live-cell imaging sys-
tems,6,2b and solar-cell materials.7 The tuning of optical
properties of the BODIPY moiety has been carried out by
systematic structural modification. Many strategies have
been investigated by functionalizing BODIPY cores at the
meso-, 2,6-, or 3,5-positions, including introducing some
aromatic groups to the BODIPY core or replacing pyrrole
unit by other linkers.8,2a,b A relatively new field is focused
on the BODIPY-based conjugated polymers for fluores-
cence-material dyes.9 So far, there have been few reports
on 2,6-functionalized BODIPY-based conjugated poly-
mers or copolymers for the near-infrared emissive materi-
al applications.10 In this paper, we synthesized three
polymers P1, P2, and P3 via palladium-catalyzed Sono-
gashira coupling reaction11 at 2,6-BODIPY core posi-
tions, and BODIPY dye was introduced into the main
chain backbone of the conjugated polymer. The resulting
polymers can emit in the range from the deep-red to the
near-infrared region with emission spectral maxima at l =
640–660 nm.
Table 1 Molecular Weights and Thermal Properties of Polymers
Polymer
P1
Td (°C)a
248
Mn (g/mol)b Mw (g/mol)b PDI
4240
5630
4020
7940
9650
7760
1.8
1.7
1.9
P2
234
P3
285
a Temperature of 5% weight loss measured by TGA in nitrogen.
b Molar mass (Mn, Mw) and polydispersity index (PDI) were deter-
mined by GPC in THF against polystyrene standards with UV detec-
tion set at absorption maxima.
The resulting polymers show excellent solubility in com-
mon organic solvents, such as THF, CHCl3, and CH2Cl2,
which is an important requirement for sensing or materials
applications. Thermogravimetric analyses (TGA) of the
polymers was carried out under a N2 atmosphere at a heat-
ing rate of 10 °C/min (Figure 1). The results show that the
polymers have relatively high thermal stability without
loss of weight before the temperature reached 230 °C.
Therefore, the herein described material can provide de-
sirable thermal property for practical applications as the
near-infrared emissive materials.
The absorption spectrum of the BODIPY 1 in CH2Cl2 so-
lution appear a strong S0–S1 (p–p*) transition at l = 506
and 550 nm and a much weaker broad band at a shorter
wavelength (l = 360 nm), but 3 shows larger redshift to
550 and 388 nm, which is attributed to the introduction of
SYNLETT 2012, 23, 778–782
Advanced online publication: 24.02.2012
DOI: 10.1055/s-0031-1290364; Art ID: W72911ST
x
x.
x
x.
2
0
1
2
© Georg Thieme Verlag Stuttgart · New York

LETTER
NIR Emissive BODIPY-Based Conjugated Polymers
779
the acetylenyl group. Compared to BODIPY derivatives,
significant redshifts of three polymers can be observed in
the order of P2 > P1 > P3 (Table 2, Figure 2 and
Figure 3), which can be attribute to the extended p-conju-
gation of the polymers.
The monomers 1 and 3 can emit bright green and orange
light and both have good fluorescent quantum yields (j)
as high as 0.78 and 0.66, respectively. As to compound 2,
the introduction of the two iodo substituents to the dipyr-
romethene core of BODIPY results in a significant red-
shift (up to 30 and 38 nm) in the UV absorption and
fluorescent maxima. A much lower fluorescence quantum
yield was detected for 2 (j = 0.05), suggesting that the in-
tersystem-crossing efficiency from the lowest singlet ex-
cited state to the triplet state was enhanced by the internal
heavy-atom effect.13 Due to extended p-conjugation of
two acetylenyl groups at the 2,6-positions, the BODIPY 3
have large redshift and a little decrease of quantum yield
of fluorescence compared to the starting material BO-
DIPY 1 (Table 2). The emissive wavelengths of the three
polymers appear at 640, 652, and 660 nm, that is, they can
emit in the range from the deep-red to the near-infrared re-
gion. In addition, the conjugated polymers display slightly
broader absorption and emission peaks due to the exten-
sion of the p-conjugation structure and larger electron de-
localization (Figure 3), but they still possess desirable
chemical and photostability attributes, relatively high ab-
sorption coefficients and fluorescence quantum yields,
and contain narrow absorption and emission bands with
high peak intensities. What is more, the polymers exhibit
moderate fluorescent quantum yields P1 (0.22), P2 (23),
and P3 (0.26, Table 2) and were high enough in the range
from the deep-red to the near-infrared region.
Figure 1 TGA curve of the three conjugated polymers
In order to gain a further insight into the optical proper-
ties, molecular geometries, and molecular orbitals of
polymers, we performed density functional theory (DFT)
calculations on model compounds constituting the corre-
sponding repeat units. HOMO and LUMO energy levels
were estimated from the optimized geometry using the
Figure 2 UV-vis absorption spectra of 1, 2, 3, P1, P2, and P3
(1.0×10–5 mol⋅L–1 in CH2Cl2)
Table 2 UV-Vis Absorption and Emission Spectral Maxima and
Fluorescent Quantum Yields (j) of BODIPY Monomers and Poly-
mers (1.0×10–5 mol/L, CH2Cl2)
BODIPY
Absorption
(nm)
Emission
(nm)
Excitation Quantum
(nm)
507
536
543
645
620
621
yield (%)
1
506
536
543
608
606
610
510
548
550
660
652
640
78a
2
5a
3
66a
P1
P2
P3
22b
23b
29b
a Rhodamine B in EtOH as the reference fluorescence quantum yield
(j) = 0.65.14
Figure 3 Fluorescence spectra of 1, 2, 3, P1, P2, and P3
b Relative to ZnPc in DMF as the reference fluorescence quantum
yield (j) = 0.28.15
© Thieme Stuttgart · New York
Synlett 2012, 23, 778–782

780
Y. Wu et al.
LETTER
Pd(PPh3)4, CuI,
Et3N, TMSA
80 °C for 1 h
CHO
N
I2, HIO3
Et3N, BF3⋅OEt2
DDQ
H
I
I
EtOH, H2O
CH2Cl2
CH2Cl2
TFA
CH2Cl2
K2CO3, acetone
N
N
F
N
N
F
N
N
F
N
HN
B
B
B
F
F
F
3
2
1
OR1
OR1
Pd(PPh3)4,
CuI, Et3N,
TMSA
OR1
OR1
I
OR1
Me3Si
OH
OH
R1O
H
2
H
Cl(CH2CH2O)2OH
80 °C for 1 h
3
3
Pd(PPh3)4, CuI,
Et3N, 80 °C
for 24 h
N
N
F
n
Pd(PPh3)4, CuI,
Et3N, 80 °C for 1 h
K2CO3,
acetone
NaH, MeI
HIO3, I2
R1O
B
R1O
P1
F
OR1
OR1
6
4
5
OR1
2
Pd(PPh3)4, CuI,
Et3N, 80 °C for 24 h
N
N
F
n
B
R1O
F
P2
Cl
OR2
OMe
OMe
OH
OH
N
HCl
I
OR2
I
I
NaH, MeI
3
I2, HIO3
H2SO4
BBr3
I
K2CO3
I
I
N
N
F
Pd(PPh3)4, CuI,
Et3N, 80 °C for 24 h
n
OR2
10
B
R2O
OMe
OMe
OH
OH
F
P3
R1 = (CH2CH2O)2OMe
2 = CH2CH2NEt2
7
8
9
R
Scheme 1 Synthesis procedures for the BODIPY-based conjugated polymers
Figure 4 HOMO and LUMO surface plots for P1, P2, and P3
Synlett 2012, 23, 778–782
© Thieme Stuttgart · New York

LETTER
NIR Emissive BODIPY-Based Conjugated Polymers
781
(4) (a) Rurack, K.; Kollmannsberger, M.; Daub, J. Angew.
B3LYP functional and 6-31g basis set. The surface plots
for the model compounds are shown in Figure 4. The the-
oretical calculations show that LUMO and HOMO energy
levels of the three polymers were very close, which indi-
cates the similar optical band gaps of three polymers. The
LUMO of the model compounds is mainly located on the
boron di(iso)indomethene ligand, and the HOMO is local-
ized not only on the ligand but also on the p-phenylene-
ethynylene moieties. It can also be found from the calcu-
lation results that the band gaps of the model compounds
are in the order P2 < P1 < P3, and it is almost consistent
with the UV-vis absorption maxima of the polymers in the
order P2 > P1 > P3.
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In summary, three novel conjugated polymers incorporat-
ing BODIPY and p-phenylene-ethynylene moieties could
be synthesized via palladium-catalyzed Sonogashira cou-
pling reaction. The incorporation of BODIPY unit into the
conjugated polymer main chain backbone can lead to
strong fluorescence in deep-red to near-infrared region.
They are expected to be used as the potential fluorescence
materials.
(8) Ziessel, R.; Ulrich, G.; Harriman, A. New J. Chem. 2007, 31,
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(11) General Procedure for Polymerization via Sonogashira
Polymerization Reaction
Supporting Information for this article is available online at
http://www.thieme-connect.com/ejournals/toc/synlett.
Acknowledgment
This work was supported by the National Natural Science Founda-
tion of China (No. 21074054, 51173078, 21172106), National
Basic Research Program of China (2010CB923303)
References and Notes
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A 20 mL Schlenk flask was charged with monomers,
catalyst Pd(PPh3)4, (CuI), and a magnetic stirrer. A 1:1
mixture of degassed anhyd Et3N and anhyd toluene solvent
was then added to the monomers, and the reaction
temperature was raised to 80 °C. The polymerization was
continued for 72 h. Then the solvent was evaporated under
vacuum. The residue was dissolved in CH2Cl2 (100 mL) and
washed with H2O (3×). The organic layer was collected,
dried over anhyd Na2SO4, and filtered. The filtrate was
concentrated and added to MeOH to precipitate the polymer.
A dark powder was obtained by filtration, further purified
with MeOH, and then dried under vacuum for 24 h to afford
polymers.
P1
(2) (a) Ulrich, G.; Ziessel, R.; Harriman, A. Angew. Chem. Int.
Ed. 2008, 47, 1184. (b) Loudet, A.; Burgess, K. Chem. Rev.
2007, 107, 4891.
BODIPY dye 2 (57 mg, 0.1 mol), monomer 5 (103 mg, 0.1
mmol), Pd(PPh3)4 (12.0 mg, 0.01 mmol), and CuI (1.9 mg,
0.01mmol) were added to a mixture of toluene (3 mL) and
Et3N (3 mL). The mixture was stirred under N2 atmosphere
at 80 °C for 72 h, and then the solvent was evaporated under
vacuum. The residue was dissolved in CH2Cl2 (100 mL) and
washed with H2O (3×). The organic layer was collected,
dried over anhyd Na2SO4, and filtered. The filtrate was
concentrated and added to MeOH to precipitate the polymer.
A dark powder was obtained by filtration, further washed
with MeOH, and then dried under vacuum for 24 h to afford
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© Thieme Stuttgart · New York
Synlett 2012, 23, 778–782

782
Y. Wu et al.
LETTER
a yield of 59%. 1H NMR (300 MHz, CDCl3): d = 7.71–7.65
(br, 5 H), 7.57–7.42 (br, 6 H), 4.24–4.15 (br 12 H), 3.92–
3.84 (br, 12 H), 3.76–3.68 (br, 12 H), 3.52 (br, 6 H), 3.68–
3.65 (br, 12 H), 3.52 (br, 6 H), 3.37 (br, 6 H), 3.20 (br, 6 H),
2.70 (br, 6 H), 1.42 (br, 6 H). FT-IR (KBr): 3441, 2923,
2823, 1630, 1529, 1384, 1317, 1261, 1095, 1014, 862, 724,
535 cm–1.
P3
BODIPY dye 3 (37 mg, 0.1 mol) and monomer 10 (37 mg,
0.1 mmol) were added to a 5 mL Schlenk flask. Pd(PPh3)4
(12.0 mg, 0.01 mmol) and CuI (1.9 mg, 0.01 mmol) were
added to a mixture of toluene (3 mL) and Et3N (3 mL). The
mixture was stirred under N2 atmosphere at 80 °C for 72 h,
and then the solvent was evaporated under vacuum. The
polymerization was carried out according to the method for
polymer P1. A dark blue powder was obtained by filtration,
further washed with MeOH, and then dried under vacuum
for 24 h to afford a yield of 62%. 1H NMR (300 MHz,
CDCl3): d = 7.71–7.70 (br, 2 H), 7.53–7.54 (br, 5 H), 4.05–
4.02 (br, 4 H), 2.70–2.65 (br, 12 H), 2.05 (br, 3 H), 1.55–1.65
(br, 15 H), 1.28–1.27 (br, 6 H). FT-IR (KBr): 3438, 2924,
2363, 1637, 1527, 1384, 1315, 1261, 1097, 804 cm–1.
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3425.
P2
BODIPY dye 2 (57 mg, 0.1 mol) and monomer 6 (36 mg, 0.1
mmol) were added to a 5 mL Schlenk flask. Pd(PPh3)4 (12.0
mg, 0.01 mmol) and CuI (1.9 mg, 0.01 mmol) were added to
a mixture of toluene (3 mL) and Et3N (3 mL). The mixture
was stirred under N2 atmosphere at 80 °C for 72 h, and then
the solvent was evaporated under vacuum. The polymeriza-
tion was carried out according to the method for polymer P1.
A dark blue powder was obtained by filtration, further
washed with MeOH, and then dried under vacuum for 24 h
to afford a yield of 54%. 1H NMR (300 MHz, CDCl3): d =
7.55–7.51 (br, 5 H), 7.32–7.31 (br, 2 H), 4.17–4.15 (br, 12
H), 3.83–3.80 (br, 12 H), 3.65–3.64 (br, 12 H), 3.52–3.50
(br, 12 H), 3.39–3.40 (br, 18 H), 2.75–2.68 (br, 6 H), 1.55
(br, 6 H). FT-IR (KBr): 3445, 2922, 2853, 1530, 1399, 1315,
1272, 1237, 1182, 1011, 1107, 725, 586 cm–1.
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Synlett 2012, 23, 778–782
© Thieme Stuttgart · New York