Boron ketoiminate-based conjugated polymers with tunable AIE behaviours and their applications for cell imaging

Article abstract of DOI:10.1039/c5tb01262g

Three new boron ketoiminate-based conjugated polymers P1, P2, and P3 were designed and synthesized through the Sonogashira coupling reaction of 4,6-bis(4-bromophenyl)-2,2-difluoro-3-phenyl-2H-1,3,2-oxazaborinin-3-ium-2-uide (M1) with 1,4-diethynyl-2,5-bis

Full text of DOI:10.1039/c5tb01262g

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Boron ketoiminate-based conjugated polymers
with tunable AIE behaviours and their applications
for cell imaging†
Cite this:DOI: 10.1039/c5tb01262g
Chunhui Dai,a Dongliang Yang,b Wenjie Zhang,a Xiao Fu,b Qingmin Chen,a
Chengjian Zhu,a Yixiang Cheng*a and Lianhui Wang*b
Three new boron ketoiminate-based conjugated polymers P1, P2, and P3 were designed and synthesized
through the Sonogashira coupling reaction of 4,6-bis(4-bromophenyl)-2,2-difluoro-3-phenyl-2H-1,3,2-
oxazaborinin-3-ium-2-uide (M1) with 1,4-diethynyl-2,5-bis(octyloxy)benzene (M2), 3,6-diethynyl-9-octyl-
9H-carbazole (M3) and 3,7-diethynyl-10-octyl-10H-phenothiazine-S,S-dioxide (M4), respectively. All the
resulting polymers showed obvious aggregation-induced emission (AIE) behaviours. Interestingly, it was
found that a great difference in the electron-donating abilities of the D–A type polymer linkers can lead to
the unique AIE behaviour of the alternating polymers in the aggregate state, which provides us with a
practical strategy to design tunable AIE-active conjugated polymers. Most importantly, studies on MCF-7
breast cancer cell imaging revealed that the nanoparticles fabricated from the conjugated polymers could
serve as promising fluorescent probes with low cytotoxicity and high photostability.
Received 28th June 2015,
Accepted 21st July 2015
DOI: 10.1039/c5tb01262g
www.rsc.org/MaterialsB
encounter a notorious aggregation-caused quenching (ACQ)
effect in the aggregate state, which causes a significant decrease
Introduction
In the past few years, donor–acceptor (D–A) type conjugated in the fluorescence intensity in an aqueous system and thus
polymers (CPs) have received much attention due to their limits their applications for bioimaging.8 To overcome this
various optoelectronic applications for solar cells,1 thin film limitation, it is highly desirable to develop novel D–A CPs with
transistors (TFTs),2 field-effect transistors (FETs),3 electrochromic strong fluorescence emission in the aqueous solution. Currently,
devices,4 memory devices,5 etc. As one of the most promising aggregation-induced emission (AIE)-active polymer materials have
strategies to improve the optical properties of CPs in the organic attracted great interest. The luminogenic materials with AIE
electronic research field, the development of the D–A approach is characteristics are non-emissive or weakly fluorescent when dis-
mainly focused on the conjugated polymer with a delocalized solved in dilute solutions, but the fluorescence intensity greatly
p-electron system that comprises alternating electron-rich (donor) increases in their aggregation.9 Because of the highly emissive
and electron-deficient (acceptor) repeating units. The energy level efficiency when aggregated in water, AIE luminogens have been
of the polymer can be finely tuned through intramolecular charge used as powerful materials for biomedical applications.10
transfer (ICT) based on the dipole–dipole interactions between
Recently, more and more studies have demonstrated that
alternating donors and acceptors.6 Previously, our studies on the fluorophores with D–A systems can show AIE characteristics
D–p–A type CPs demonstrated that the incorporation of near- in highly polar aqueous media, the AIE-ICT luminogens can
infrared (NIR) emissive chromophores into p-conjugated polymer exhibit high solid emission efficiency and potential biological
backbones could give narrow band emissive polymers (even lower and optoelectronic device applications.11 Our group also recently
than 1.25 eV), which are potential materials for device-based found that the AIE properties of dicyanomethylene-1,4-dihydro-
applications.7 However, most of the D–A type compounds pyridine derivatives can be effectively tuned by intramolecular
D–p–A conjugated structure systems.12 In the present work, we
a Key Lab of Mesoscopic Chemistry of MOE, School of Chemistry and Chemical
Engineering, Nanjing University, Nanjing 210093, China.
E-mail: yxcheng@nju.edu.cn; Tel: +86-25-83686508
b Key Laboratory for Organic Electronics and Information Displays and Institute of
Advanced Materials, Nanjing University of Posts and Telecommunications,
Nanjing, 210023, China. E-mail: iamlhwang@njupt.edu.cn
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
c5tb01262g
designed and synthesized a series of boron ketoiminate-based
alternating D–p–A type CPs for live cell imaging (Scheme 1).
Herein, boron ketoiminate was chosen as an AIE-active unit
with excellent optical properties.13 Importantly, it can also act
as an acceptor in the conjugated polymer backbone. To the best
of our knowledge, there has been little research on D–p–A type
boron ketoiminate-based conjugated polymers with tunable
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Table 1 Summary data for optical properties of P1–P3
In THF (1 Â 10À5 mol LÀ1 a
)
CPNsa
Polymer
labs (nm)
lPL (nm)
fb
lPL (nm)
fb
P1
P2
P3
234, 318, 418
234, 296, 410
234, 292, 386
459
553
460
o0.01
o0.01
o0.01
551
563
545
0.14
0.06
0.11
a
b
Excited at 405 nm. Quantum yields were determined using coumarin
153 in ethanol (F = 0.38) as the fluorescence reference.
peak at 318 nm, 296 nm and 292 nm, respectively. Another broad
peak emerged at 418 nm for P1, 410 nm for P2, and 386 nm for P3,
which could be regarded as a S0 - S1 (p–p*) transition. The longer
absorption bands of the three polymers are attributed to an
intramolecular charge transfer (ICT) band between donors and
acceptors.
Scheme 1 Chemical structures of the conjugated polymers.
AIE activities. Furthermore, the polymers were used to prepare
conjugated polymer nanoparticles (CPNs) by a simple repreci-
pitation method. The obtained CPN suspension showed high
photostability and low cytotoxicity and was successfully employed
for MCF-7 cancer cell imaging.
The AIE behaviours of P1–P3 were investigated in THF/water
mixtures at different water fractions ( fw). As shown in Fig. 2a,
P1 was almost non-emissive in THF solution and the intensities
had very small changes when the fw varied from 0 to 60%. When
the fw was 70% or higher, the intensity enhanced significantly
and reached nearly 27-fold greater than that in pure THF
solution (Fig. 2d). P2 in THF dilute solution also showed faint
PL signals peaking at 553 nm. When more water was added, the
emission enhanced with a red-shifted maximum emission
wavelength of 563 nm, which can be attributed to aggregate
formation. The emission intensity in 80% aqueous mixture
reached more than 9-fold higher than that of pure THF solution,
which was ascribed to the AIE characteristic. However, when the
fw reached 90%, the emission was badly quenched. The decreased
emission intensity can be ascribed to the twisted intramolecular
charge transfer (TICT) resulting from the gradually increasing
polarity of the solvent mixture, which is often seen in AIE-active
molecules with D–A structures.14,15 P3 showed similar AIE beha-
viors with P2. P3 exhibited a very weak emission in THF solution
with an emission peak at 460 nm. The emission increased slowly
as the fw progressively increased from 0 to 50%. Meanwhile,
the emission maximum red-shifted to around 550 nm. When
fw = 90%, the fluorescence intensity reached the maximum
(18-fold compared with that in pure solution) and the peak
Results and discussion
Synthesis and characterization
The detailed synthetic procedures of the monomers and conju-
gated polymers are illustrated in Scheme S1 (ESI 2†). To syn-
thesize the targeted conjugated polymers, we first synthesized the
corresponding monomer units. The monomer M1 was obtained
by the condensation reaction of aniline (1) with (Z)-1,3-bis(4-
bromophenyl)-3-hydroxyprop-2-en-1-one (2) in the presence of a
catalytic amount of p-toluenesulfonic acid, followed by coordina-
tion with trifluorobonetherate in the presence of triethylamine.
The monomers M2–M4 were synthesized using our previously
reported procedure (ESI 2†). The polymerization was accom-
plished by the Pd(II)-catalyzed Sonogashira coupling reaction of
M1 with M2, M3, and M4 in the presence of a catalytic amount
of [Pd(PPh3)4] and CuI, to afford P1, P2, and P3, respectively. In
a typical experiment, polymerization reactions were carried out
in a degassed mixture of anhydrous THF and anhydrous Et3N
under N2 at 80 1C for 24 h. The polymers were precipitated from
methanol, filtered, and dried in vacuum. The number average
molecular weight (Mn) was determined to be 8210 g molÀ1
(PDI = 2.49) for P1, 4220 g molÀ1 (PDI = 2.62) for P2, and
4750 g molÀ1 (PDI = 2.18) for P3 by GPC using a polystyrene
standard in THF. All the obtained conjugated polymers showed
good solubility in common organic solvents, such as THF,
toluene, CH2Cl2, and CHCl3, which can be attributed to the
flexible n-octyl substituents.
Optical properties
The photophysical properties of P1–P3 in THF solutions and
aggregated states (10 mM) were measured using UV-vis absorp-
tion and photoluminescence (PL) spectroscopy, and the rele-
vant data are summarized in Table 1. As shown in Fig. 1, P1, P2,
and P3 in THF solutions all exhibited a strong absorption peak
at 234 nm, which could be ascribed to the strong S0 - S2 (p–p*)
transition. In addition, P1, P2, and P3 showed a weak shoulder Fig. 1 UV/vis absorption spectra of P1–P3 in THF solution (10 mM).
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Fig. 2 Fluorescence spectra of (a) P1, (b) P2, and (c) P3 in THF/water mixtures with different water fractions and the plots of I/I0 of (d) P1, (e) P2, and (f) P3
versus the volume fraction of water in THF/water mixtures, where I0 and I are the emission intensities in pure THF solution and the THF/water mixture,
respectively. Photographs of solutions of (g) P1, (h) P2, and (i) P3 in THF/water mixtures with different water fractions taken under UV illumination. The
concentration of all solutions and suspensions is 10 mM, excited at 405 nm.
blue-shifted to 545 nm. With the increase of the water fraction, exerted an influence on the emission wavelength and the
the aggregates of P3 may possess a conformation of a twisted efficiency of the luminogens in aggregated states.
conjugated backbone, which leads to a decrease of the effective
Preparation and characterization of CPNs
conjugation length, thus resulting in an unusual blue-shifted
emission.15e When fw = 100%, the intensity decreased in a small P1–P3 NPs were prepared by using the reprecipitation method
range. Finally, when fw = 100%, the enhancement of PL inten- as described previously.16 In brief, the conjugated polymer in
sity was only 6-fold for P2 (Fig. 2e) and 17-fold for P3 (Fig. 2f). dilute THF solution was injected rapidly into Milli-Q water
These results implied that the D–A interactions in P1–P3 under ultrasonication. The CPN suspension was obtained after
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vacuum removal of THF. The particle size and the morphology Electrochemical properties
of the CPNs were characterized by transmission electron micro-
To determine the frontier orbital levels, the electrochemical
properties of all the conjugated polymers were evaluated using
cyclic voltammetry. The cyclic voltammograms were recorded
in anhydrous CH2Cl2 solution with 0.1 M tetrabutylammonium
hexafluorophosphate (Bu4NPF6) as the supporting electrolyte at
a scan rate of 50 mV sÀ1. A platinum wire was used as the
counter electrode and an Ag/AgCl electrode as the reference
electrode, and redox potentials were calibrated using the ferrocene/
ferrocenium (Fc/Fc+) redox couple. The CV curves of P1–P3 are
shown in Fig. 3, and the onset oxidation and reduction potentials
along with the estimated HOMO and LUMO energy levels are
summarized in Table 2. As a result, the HOMO/LUMO energy levels
of the polymers, estimated from the onset oxidation/reduction
potentials, were À5.82/À3.86 eV for P1, À5.77/À3.83 eV for P2,
and À5.78/À3.78 eV for P3, and thus the corresponding electro-
chemical bandgaps of P1, P2, and P3 were determined to be
1.96, 1.94, and 2.00 eV, respectively.17
scopy (TEM) and dynamic light scattering (DLS) (Fig. S4 in
ESI 9†). The TEM images illustrate that the CPNs exhibit a
spherical shape with particle diameters of B63 nm, B55 nm,
and B57 nm for P1, P2 and P3, respectively. As indicated by
DLS, the average diameters of the CPNs are 79.5 nm, 69.5 nm,
and 71.7 nm for P1, P2 and P3, respectively. The CPNs disperse
homogeneously in water without aggregation. The fluorescence
quantum yields were determined to be 0.14, 0.06, and 0.11 for
the NP suspensions of P1, P2, and P3, respectively. In addition,
to better understand the emission dynamics of CPNs and the
fluorescence decay process, the fluorescence lifetimes of CPN
suspensions were measured by a time resolved technique. The
fluorescence emission lifetime of CPNs are determined to be
2.09 ns for P1 NPs, 5.20 ns for P2 NPs, and 2.04 ns for P3 NPs,
with the mean square deviation w2 (CHISQ) values ranging from
1.053 to 1.145 (Fig. S5 and S6 in ESI 10†).
Fig. 3 Cyclic voltammograms of P1–P3 in deoxygenated CH2Cl2 solution containing Bu4NPF6 (0.1 M).
Table 2 Electrochemical properties and calculated HOMO and LUMO energy levels of P1–P3
From CVa
From calculation
HOMO [eV]
b
b
c
d
Polymer
Eox,onset [V]
HOMO [eV]
Ered,onset [V]
LUMO [eV]
Eg [eV]
LUMO [eV]
Eg [eV]
P1
P2
P3
1.02
0.97
0.98
À5.82
À5.77
À5.78
À0.94
À0.97
À1.02
À3.86
À3.83
À3.78
1.96
1.94
2.00
À5.76
À5.37
À5.72
À2.49
À2.31
À2.47
3.27
3.06
3.25
a
b
The ferrocence couple (Fc/Fc+) was used as the internal reference and under our experimental conditions, E(Fc+/Fc) = 0.42 V vs. Ag/AgCl. Eox
c
and Ered determined from the onset potentials of the oxidation and reduction waves, respectively. Optical band gap (Eg), Eg = LUMO À HOMO.
d
DFT quantum mechanical calculations (B3LYP/6-31G*). HOMO = À(Eox + 4.8) eV and LUMO = À(Ered + 4.8) eV.
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Fig. 4 Optimized model compounds from P1–P3 for DFT calculations and molecular orbital diagrams for the LUMO and the HOMO of models 1–3.
Molecular orbital calculations
To assess the influence of the donor on the frontier orbital
energies, DFT calculations were performed on the three optimized
model molecules constituting the corresponding repeating units
shown in Fig. 4 at the B3LYP/6-31g(2d,p) level using the Gaussian
09 A.02 package.18 In the calculations, the long alkyl chains on the
polymer backbones were replaced with methyl groups. The calcu-
lated LUMO/HOMO energy levels of models 1–3 are À2.49/À5.76,
À2.31/À5.37, and À2.47/À5.72, respectively. The LUMO of the
model compounds is mainly centred on the boron-ketoiminate
unit because it is an acceptor. Compared to models 1 and 3, model 2
shows the highest HOMO level, suggesting that the carbazole unit
has a higher electron-donating ability compared to benzene and
phenothiazine-S,S-dioxide units. Because of the strong D–A inter-
action between the boron ketoiminate unit and the carbazole group,
the HOMO–LUMO band gap of model 2 (3.06 eV) is smaller than
that of model 1 (3.27 eV) and model 3 (3.25 eV), which is consistent
with the electrochemical results discussed above.
Fig. 5 (a–c) Metabolic viability of MCF-7 cells after incubation with P1–P3
NP suspensions at different concentrations for 48 h. (d) Photostability of
P1–P3 NPs upon continuous laser excitation at 405 nm with a laser power
of 5 mW for 0–10 min. I0 is the initial fluorescence intensity and I is the
fluorescence intensity of each sample at different time intervals after
continuous laser scanning.
Cytotoxicity and photostability
To evaluate the cytotoxicity of P1–P3 NPs, the metabolic viability
of MCF-7 cells after incubation with CPNs was investigated at As shown in Fig. 5d, upon continuous excitation at 405 nm (5 mW)
different nanoparticle suspension concentrations. Fig. 5a–c shows for 10 min, the PL intensity loss of P1, P2, and P3 NPs is only 10%,
the cytotoxicity results of P1–P3 NPs upon incubation with the 14%, and 12%, respectively. This observation indicates that all the
suspension at 0 to 40 mM for 48 h, respectively. The cell viabilities CPNs have good photostability and are promising in bioimaging.
remain above 90% upon incubation with NPs for 48 h at a
Cell imaging application of CPNs
concentration of 40 mM, indicating the low cytotoxicity of the
probes, which is ideal for bioimaging. Excellent photostability in Inspired by the excellent photophysical performance of P1–P3 NP
water is crucial for bioimaging because high photostability can suspensions, we subsequently applied them in cell imaging. The
effectively resist photobleaching and allows the imaging process in vitro cellular imaging was obtained by CLSM using MCF-7
to last for a long period. We evaluated the photostability of P1–P3 cancer cells as the example cell line and fluorescence imaging was
NPs by using CLSM and analysed the intensity changes upon performed upon excitation at 405 nm. The fluorescence images
incubation with PBS buffer solution at 37 1C for 0–10 minutes. of MCF-7 cancer cells were obtained after treatment with B4 mM
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Fig. 6 Confocal laser scanning microscopy (CLSM) images of MCF-7 cells
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cells, which demonstrates that the CPNs could be effectively
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observed on the cell membrane, which is attributed to the large
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In summary, we have designed and synthesized a series of AIE-
active D–A type conjugated polymers incorporating the boron
ketoiminate unit as the acceptor. The emission of these poly-
mers can be tuned by introducing monomers with different
electron-donating abilities. Furthermore, these polymer nano-
particles can successfully serve as promising fluorescent probes
for MCF-7 breast cancer cell imaging due to low cytotoxicity and
high photostability.
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Acknowledgements
This work was supported by the National Natural Science Founda-
tion of China (21174061, 51173078, 21172106, and 21474048) and
open project of Beijing National Laboratory for Molecular Sciences.
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