Large stokes shift chiral polymers containing (R,R)-salen-based binuclear boron complex: Synthesis, characterization, and fluorescence properties

Article abstract of DOI:10.1016/j.polymer.2012.07.003

Two novel chiral fluorescence polymers P-1 and P-2 incorporating (R,R)-salen-based binuclear boron complex in the main chain backbone were synthesized by (R,R)-salen-based boron complex (M-1) with 9,9-dibutyl-2,7- diethynyl-9H-fluorene (M-2) and 1,4-dibutoxy-2,5-diethynylbenzene (M-3) via Pd-catalyzed Sonogashira coupling reaction, respectively. The chiral polymers were characterized by ¹H NMR, UV-vis spectroscopy, photoluminescence (PL), gel permeation chromatography (GPC), TGA, DSC, cyclic voltammetry (CV), and theoretical calculation using density-functional theory (DFT) method. P-1 and P-2 show anisotropic fluorescence property with emission spectral maxima at 474 nm and 514 nm, respectively. Both two chiral polymers can exhibit high fluorescence quantum yields (up to 44% and 52%) with large Stokes shifts over 90 nm. The DFT theoretical calculation of the polymer repeating units indicates that the optical band gaps of P-1 is higher than that of P-2, which was consistent with the CV determination results.

Full text of DOI:10.1016/j.polymer.2012.07.003

Polymer 53 (2012) 3894e3899
Contents lists available at SciVerse ScienceDirect
Polymer
journal homepage: www.elsevier.com/locate/polymer
Large stokes shift chiral polymers containing (R,R)-salen-based binuclear boron
complex: Synthesis, characterization, and fluorescence properties
Xiao Ma , Jiemin Jiao , Jing Yang , Xiaobo Huang , Yixiang Cheng ,*, Chengjian Zhu
a
a
b
a
a
a,
*
a
Key Lab of Mesoscopic Chemistry of MOE, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210093, China
College of Food Science and Light Industry, Nanjing University of Technology, Nanjing 210093, China
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 22 April 2012
Received in revised form
Two novel chiral fluorescence polymers P-1 and P-2 incorporating (R,R)-salen-based binuclear boron
complex in the main chain backbone were synthesized by (R,R)-salen-based boron complex (M-1) with
9
,9-dibutyl-2,7-diethynyl-9H-fluorene (M-2) and 1,4-dibutoxy-2,5-diethynylbenzene (M-3) via Pd-
27 June 2012
1
catalyzed Sonogashira coupling reaction, respectively. The chiral polymers were characterized by
H
Accepted 1 July 2012
NMR, UVevis spectroscopy, photoluminescence (PL), gel permeation chromatography (GPC), TGA, DSC,
cyclic voltammetry (CV), and theoretical calculation using density-functional theory (DFT) method. P-1
and P-2 show anisotropic fluorescence property with emission spectral maxima at 474 nm and 514 nm,
respectively. Both two chiral polymers can exhibit high fluorescence quantum yields (up to 44% and 52%)
with large Stokes shifts over 90 nm. The DFT theoretical calculation of the polymer repeating units
indicates that the optical band gaps of P-1 is higher than that of P-2, which was consistent with the CV
determination results.
Available online 7 July 2012
Keywords:
Chiral polymers
Anisotropic fluorescence
Stokes shifts
Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction
the notable advantages of such macromolecular dyes are the
homogeneity of the resulting colored material and the high
In the past few years, investigation on fluorescence materials of
organic fluorineeboron complexes has caused a considerable
amount of attention. In particular, BODIPY (4,4 -difluoro-4-bora-
migration fastness [18]. (R,R)-Salen, as a well-known building
block, has been widely used for constructing chelation-enhanced
fluorescence sensors for transition metal ion detection due to the
0
3
a,4a-diaza-s-indacene) is the best-known example due to high
2 2
potentially tetradentate N O donor [19e21]. Recently, Yoon’ group
absorption coefficients, narrow absorption and emission bands,
desirable chemical and photostability attributes, and high emission
quantum yields (F ) [1e4]. Not surprisingly, these excellent prop-
F
reported (R,R)-salen-based binuclear boron complexes incorpo-
rating pyrenyl group as a fluorophore for organic light-emitting
diodes (OLED) with the current efficiency of 0.15 cd/A. They also
found that these kind boron hybrid complexes could act as both an
electron transporting and light emitting material at the same time
[15]. Nevertheless, to the best of our knowledge, there has been no
report on synthesis and the anisotropic fluorescence emission of
chiral salen-based binuclear boron polymer complexes. In this
contribution, we first designed and synthesized two novel chiral
erties promote the exploration of these molecules for various
applications, such as ion sensing and signaling [5e9], biological
probe [10], solar cells [11e13], organ light-emitting diodes [14,15],
and so on. However, BODIPY and its derivatives exist many defects,
such as typical small Stokes shifts and high planarity, which can
lead to serious self-quenching in the solid state and limit their
potential application in organic electronics. Therefore, there is of
great interest in the development of a kind novel boron complex
with a well-designed structure to overcome the defects of BODIPY
as well as control the optical properties [14].
2
fluorescence polymers with salen-BF complex moieties in the
main chain backbone via Sonogashira polycondensation. The two
chiral polymers show evident anisotropic fluorescence feature with
high quantum efficiencies and large Stokes shifts.
Recently, much effort has been devoted to the covalent incor-
poration of organoboron dyes into a polymer main chain, side main,
or initial end [16,17]. As compared to small molecular counterparts,
2. Experimental
2.1. Measurements and materials
NMR spectra were obtained using a 300-Bruker spectrometer
with 300 MHz for H NMR and 75 MHz for C NMR. They were
*
Corresponding authors.
E-mail addresses: yxcheng@nju.edu.cn (Y. Cheng), cjzhu@nju.edu.cn (C. Zhu).
1
13
0
032-3861/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.polymer.2012.07.003

X. Ma et al. / Polymer 53 (2012) 3894e3899
3895
Table 1
was removed, the residues were purified by silica gel column chro-
Polymerization results and characterization of P-1 and P-2.
matography using CH
2
Cl
2
as an eluent to afford 2 as a light yellow
a
ꢀ
25
D
25
D
1
M
n
(g/mol)
M
w
(g/mol)
PDI (M
w
/M
n
)
T
d
( C)
½
a
ꢁ
solid (0.27 g, 70%). ½
DMSO-d ):
6.92 (d, J ¼ 8.1 Hz, 2H), 4.45 (d, J ¼ 8.7 Hz, 2H), 2.17e2.08 (m, 2H),
a
ꢁ
¼ ꢂ190.0 (c 0.1, CH
2 2
Cl ). H NMR (300 MHz,
6
d
¼ 9.43 (s, 2H), 7.67e7.61 (m, 4H), 7.09e7.04 (m, 2H),
P-1
P-2
5840
7710
9730
11,240
1.67
1.46
350
325
þ110.0
þ150.0
13
a
1.97e1.83 (m, 4H), 1.55e1.45 (m, 2H). C NMR (75 MHz, DMSO-d
6
):
Temperature at 5% weight loss estimated using TGA under N2.
d
¼ 167.4,157.7,138.7,132.9,120.5,118.4,115.7, 33.9, 31.1, 24.6. MS (EI,
þ
m/z): 418 (M ). Anal. calcd for C20
20 2 4 2 2
H B F N O : C, 57.42; H, 4.78; N,
reported as parts per million (ppm) from the internal standard TMS.
UVevis spectra were obtained from a PerkineElmer Lambda 25
spectrometer. Fluorescence spectra were obtained from an RF-
6.70. Found: C, 57.35; H, 4.82; N, 6.75.
2.3. Synthesis of M-1
5
301PC spectrometer. The emission anisotropy spectra were
measured on FS920. MS was determined on a Micromass GCT.
Elemental analyses of C, H and N were performed on an Elementary
Vario MICRO analyzer. Thermogravimetric analyses (TGA) were
0
.5 mL Triethylamine and 1 mL BF $Et O were added to the
3 2
solution of 0.48 g compound 1 (1 mmol) in 20 mL toluene. The
mixture was stirred under N
ꢀ
2
for 2 h at 40 C. After the toluene was
performed on a PerkineElmer Pyris-1 instrument under N
sphere. Differential scanning calorimetry (DSC) was run on a Pyris-
DSC (PERKIN ELMER) shimadzu thermal analyst system. Cyclic
voltammetry (CV) measurements were performed on a BAS100W
with a three-electrode cell in a 0.1 M Bu NPF solution in CH Cl at
2
atmo-
removed, the residues were purified by silica gel column chroma-
tography using CH Cl as an eluent to afford M-1 as a light yellow
solid (0.38 g, 65%).½
2
a
2
1
2
5
1
ꢁ
¼ ꢂ216.0 (c 0.1, CH
2
Cl
¼ 9.43 (s, 2H), 7.81e7.77 (m, 4H), 6.94 (d, J ¼ 9.0 Hz,
H), 4.39e4.37 (m, 2H), 2.15e2.12 (m, 2H), 1.90e1.86 (m, 4H),
2
). H NMR (300 MHz,
D
6
DMSO-d ): d
4
6
2
2
2
1
1
a scanning rate of 50 mV/s. A gold disc electrode was used as
a working electrode, a platinum wire was used as a counter elec-
trode, and an AgCl/Ag electrode was used as a reference electrode.
After measurement, the reference electrode was calibrated with
13
.50e1.48 (m, 2H). C NMR (75 MHz, DMSO-d
6
): d 166.9, 156.8,
41.0, 134.3, 121.0, 117.3, 111.0, 46.1, 33.7, 24.5. MS (EI, m/z): 576
þ
(M
þ1). Anal. calcd for C20
18 2 2 4 2 2
H B Br F N O : C, 41.68; H, 3.13; N,
þ
4.69. Found: C, 41.60; H, 3.16; N, 4.59.
ferrocene (Fc) and the potential axis was corrected to Fc /Fc.
Molecular weight was determined by GPC with Waters-244 HPLC
pump and THF was used as solvent and relative to polystyrene
standards. All solvents and reagents were commercially available
A.R. grade.
2.4. Synthesis of P-1
A mixture of M-1 (48.0 mg, 0.1 mmol), M-2 (33.0 mg, 0.1 mmol),
Pd(PPh (5.8 mg, 0.005 mmol), CuI (1.0 mg, 0.005 mmol) were
dissolved in 6 mL THF and 6 mL Et N. The solution was stirred at
reflux for 2 d under N atmosphere. The mixture was cooled to
3 4
)
2.2. Synthesis of model compound 2
3
2
0
.5 mL Triethylamine and 1 mL BF
3
$Et
2
O were added to the
room temperature, and then was filtered through a short silica gel
column in methanol (30 mL) to precipitate the polymer. The
resulting polymer was filtered and washed with methanol several
solution of 0.32 g compound 1 (1 mmol) in 20 mL toluene. The
mixture was stirred under nitrogen for 2 h at 40 C. After the toluene
ꢀ
a
b
0.14
0.12
0.10
0.08
0.06
0.04
0.02
0.00
1.0
2
P-1
P-2
2
P-1
P-2
0.8
0.6
0.4
0.2
2
P-1
P-2
0.0
300
400
500
600
350
400
450
500
550
600
650
Wavelength (nm)
Wavelength (nm)
c
d
0
.10
.08
.06
600000
400000
200000
0
0
2
P-1
P-2
0
0.04
2
P-1
P-2
0.02
0.00
-200000
-0.02
250
300
350
400
450
400 420 440 460 480 500 520 540 560
Wavelength (nm)
Wavelength (nm)
Fig. 1. (a) UVevis absorption spectra and (b) normalized photoluminescence spectra and (c) CD spectra and (d) emission anisotropic spectra of 2, P-1 and P-2 in CH
2
Cl
2
ꢂ5
(
1.0 ꢃ 10 mol/L).

3896
X. Ma et al. / Polymer 53 (2012) 3894e3899
Table 2
Table 3
ꢂ5
Summary of optical properties of 2, P-1 and P-2 (1.0 ꢃ 10 mol/L, CH
2
2
Cl ).
Electrochemical data of P-1 and P-2, and calculated HOMO, LUMO energy values.
b
From CVa
UVevis
l
ex
l
em
Stokes
F
F
Anisotropy r
From calculation
HOMOe LUMOe
opt
(eV)a
(nm) (nm) shift
onset
ox
b
HOMOc
[eV]
onset
red
[V]
b
LUMOc
[eV]
d
e
l
max
l
onset
E
g
E
E
E
g
E
g
(nm)
(
nm) (nm)
[V]
[eV] [eV]
[eV]
[eV]
2
353
392
437
491
3.16
2.84
2.53
355
400
410
435
474
514
82
94
117
0.57 <0.01
P-1 0.59
P-2 0.54
ꢂ5.39
ꢂ5.34
ꢂ1.97
ꢂ1.89
ꢂ2.83
ꢂ2.91
2.56 ꢂ5.74
2.43 ꢂ5.48
ꢂ2.54
ꢂ2.46
3.20
3.02
P-1 380
P-2 397
0.44
0.52
0.08
0.08
a
þ
The ferrocence couple (Fc /Fc) was used as the internal reference and under our
a
þ
Estimated from the absorption onset (E
g
¼ 1240/
Quantum yields were determined with 9,10-diphenylanthracene as fluores-
cence reference in CHCl
¼ 0.91).
lonset).
experimental conditions, E(Fc /Fc) ¼ 0.42 V vs Ag/AgCl.
b
b
E
ox and Ered determined from the onset potentials of the oxidation and reduction
3
(
F
waves.
c
onset
ox
onset
HOMO ¼ ꢂ(E
þ 4.80) eV and LUMO ¼ ꢂ(Ered þ 4.80) eV.
d
e
E
g
¼ LUMO ꢂ HOMO.
DFT quantum mechanical calculations (B3LYP/6-31G*).
times. Further purification could be conducted by dissolving the
polymer in THF to precipitate in methanol again. The polymer was
dried in vacuum to give 41.5 mg as a yellow solid in 56% yield. GPC
Sonogashira reaction in the presence of a catalytic amount of
Pd(PPh and CuI with Et N under N . The molecular weights data
2
D
5
3
)
4
3
2
results: M
w
¼ 9730, M
). H NMR (300 MHz, CDCl
.76e7.64 (m, 6H), 7.60e7.46(m, 4H), 6.98e6.80 (m, 2H), 3.44e3.32
m, 2H), 2.10e1.86 (m, 10H), 1.20e1.02 (m, 6H), 0.67e0.50 (br, 10H).
Anal. Calcd for (C45 : C, 73.01; H, 5.68; N, 3.90. Found:
C, 72.97; H, 5.75; N, 3.92.
n
¼ 5840, PDI ¼ 1.67. ½
a
ꢁ
¼ þ110.0 (c 0.1,
1
of P-1 and P-2 are listed in Table 1, indicating moderate molecular
weights. P-1 and P-2 were air-stable solids and soluble in common
organic solvents including THF, CHCl , and CH Cl , which can be
3 2 2
attributed to the nonplanarity of the twisted polymer backbone and
the flexible n-butoxy substituents. According to thermal analysis of
the polymers (Figure S1), P-1 and P-2 exhibit high thermal stability
CH
2
Cl
2
3
):
d
¼ 8.29e8.19 (m, 2H),
7
(
42 2 4 2 2 n
H B F N O )
ꢀ
ꢀ
2
with 5% weight loss temperatures at 350 C and 325 C under N ,
2
.5. Synthesis of P-2
respectively. Through differential scanning calorimetry (DSC)
measurements, neither endo- nor exothermic processes were
observed, suggesting that these polymers have very rigid chain
structures [28,29]. Therefore, the resulting two chiral polymers can
provide a desirable thermal property for fluorescence materials.
P-2 was synthesized by using the same procedure as for the
preparation of P-1. A mixture of M-1 (48.0 mg, 0.1 mmol), M-3
27.0 mg, 0.1 mmol), Pd(PPh
1.0 mg, 0.005 mmol) in 6 mL THF and 6 mL Et
to give P-2 (40 mg, 58%) as a yellow solid. GPC results: M
(
(
3
)
4
(5.8 mg, 0.005 mmol) and CuI
N was polymerized
¼ 11,240,
3
w
3
.2. Photophysical properties
2
5
D
1
M
n
¼ 7710, PDI ¼ 1.46. ½
a
ꢁ
¼ þ150.0 (c 0.1, CH
2
Cl
¼ 8.26e8.19 (m, 2H), 7.48e7.29 (m, 4H),
.05e6.80 (m, 4H), 4.07e3.85 (m, 4H), 3.41e3.31 (m, 2H), 2.05e1.40
m, 16H), 1.05e0.70 (m, 6H). Analytical Calculated Value for
: C, 66.71; H, 5.56; N, 4.10. Found: C, 66.75; H,
2
). H NMR
(
7
(
(
5
3
300 MHz, CDCl ): d
The absorption and emission spectra of model compound 2 and
ꢂ5
two chiral polymers P-1 and P-2 (1.0 ꢃ 10 mol/L corresponding to
salen-based binuclear boron complex unit in CH
Fig.1. The optical data are summarized inTable 2. As shown in Fig.1a,
the absorption wavelengths max of P-1, and P-2 show 27 and 44 nm
of red-shift compared with the model compound 2, which can be
attributed to the effective * conjugated segment of 2 and phenyl
2 2
Cl ) are shown in
38 38 2 4 2 4 n
C H B F N O )
.51; N, 4.15.
l
3
. Results and discussion
p-p
or fluorenyl groups via the ethynyl linker in the repeating unit of
chiral polymers. This conclusion can be also drawn from the energy
of the optical band gap (measured from the absorption onset) of 2
(3.16 eV) when compared to P-1 (2.84 eV) when compared to P-2
(2.53 eV) [30]. In addition, the difference of P-1 and P-2 on electronic
spectra also indicates that p-dibutoxyphenyl moiety has stronger
electron donating effect than that of fluorenyl moiety [31]. In the
photoluminescence spectra (Fig. 1b), the model compound 2
exhibitsstrongfluorescencecenteredat 435nmwith the fluorescent
quantumyield as highas 0.57 and a largeStokesshiftof 82 nm. When
3
.1. Synthesis and feature of the chiral polymers
The synthesis procedures of 2, M-1, M-2, M-3 and the chiral
polymers are outlined in Scheme 1. Compound 1 [22], Compound 3
23,24], 9,9-dibutyl-2,7-diethynyl-9H-fluorene (M-2) [25,26], and
,4-dibutoxy-2,5-diethynylbenzene (M-3) [27] were synthesized
[
1
according to previously published procedures. M-2 and M-3 could
be served as the monomers for the synthesis of the chiral polymers.
The polymerization was accomplished by palladium-catalyzed
-
2.5 -2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 2.0
-2.5 -2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 2.0
+
+
Potential vs Fc /Fc (V)
Potential vs Fc /Fc (V)
Fig. 2. Cyclic voltammograms of P-1 (left) and P-2 (right) in dry 1 mM CH
2
Cl
2
solution with 0.1 M tetrabutylammonium hexafluorophosphate as electrolyte. Three-electrode
electrochemical cell was used with a gold disc electrode as working electrode, a platinum wire as counter electrode and an AgCl/Ag (in 3 M aq. KCl) as a reference electrode.
Scan rate ¼ 50 mV/s.

X. Ma et al. / Polymer 53 (2012) 3894e3899
3897
Fig. 3. The HOMO and LUMO energy levels of the repeating unit (left: P-1; right: P-2) by the DFT calculation.
model molecule 2 is introduced into the polymer backbone, a red-
shift of 39 nm for P-1 and 79 nm for P-2 are observed, centered at
main chain backbone (insert Fig.1b). Significantly, P-1 and P-2 retain
the large Stokes shifts and high fluorescence quantum yields. This
might be attributed to an excited-state intramolecular charge
transfer (ICT) between the donor and acceptor in the polymers
474 nm for P-1 and 514 nm for P-2. P-1 and P-2 can emit bright blue
and green light due to the extended -electronic structure in the
p
Scheme 1. Synthetic procedures for model compound 2, M-2, M-3, P-1 and P-2.

3898
X. Ma et al. / Polymer 53 (2012) 3894e3899
[
32,33]. It is noteworthy that the Stokes shift of P-2 is greater than
backbone by palladium-catalyzed Sonogashira polymerization. The
resulting chiral polymers can exhibit great anisotropic values (0.08)
and display large Stokes shifts (94e117 nm) with high fluorescent
quantum yields, which are expected to be used as the potential fluo-
rescence materials. Further studies in the structural optimization and
stronger anisotropic fluorescence emission are currently in progress.
that of P-1 (117 nm and 94 nm, respectively).
Fig. 1c illustrates the CD spectra of model compound 2, P-1 and
P-2. P-1 and P-2 have similar CD signals due to their similar chiral
architecture. Chiral model compound 2 displays negative Cotton
effects at the wavelengths of 284 and 366 nm. However, P-1
exhibits positive Cotton effect with red shifts of 13 and 17 nm, and
P-2 appears 9 and 16 nm red shifts, respectively. In addition, the
chiral polymers have the longer wavelength CD effect at about
Acknowledgments
4
10 nm, which is attributed to the extended conjugated structure in
This work was supported by the National Natural Science
Foundation of China (No. 21074054, 51173078, 21172106), National
Basic Research Program of China (2010CB923303) and Zhejiang
Provincial Natural Science Foundation (No. Y4110141).
the repeating unit. To obtain further insight into their photo-
physical properties, we also explored the steady-state emission
anisotropy spectra of 2, P-1 and P-2 in CH Cl solution through the
2 2
emission bands. As shown in Fig. 1d, the emission anisotropy, r, of 2
is almost zero in the range from 400 to 460 nm, demonstrating that
the chiral model small molecule 2 rotates very fast in the diluted
solution. When the chiral model molecule 2 is introduced into the
polymer backbone, free rotation of the whole chiral polymers is
effectively limited due to the increase of the macromolecule and
dimension structure, leading to that the measured steady state
anisotropy greatly increases [34]. The anisotropic values of chiral
polymers P-1 and P-2 were found to be as high as 0.08. Such an
obviously surprising result suggests that a new enhanced polari-
zation channel is effective in the polymer structure [35], which is
potential application for circularly polarized light emission devices.
Appendix A. Supplementary material
Supplementary data related to this article can be found online at
http://dx.doi.org/10.1016/j.polymer.2012.07.003.
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2
1

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