Preparation of 4-dicyanomethylene-2,6-distyryl-4H-pyran derivatives, their functional polystyrenes and study of their different aggregation induced emission behaviors

Article abstract of DOI:10.1039/c3tc32244k

Aggregated or solid state red fluorescent materials with efficient light emission are rare but have wide applications in optoelectronic and biological fields. Herein, two pyran derivatives, EtAmPy and EtOxPy with different charge transfers (CT), were designed as initiators and introduced to polystyrene chains via atom transfer radical polymerization (ATRP). The emission of EtAmPy was quenched both in highly polar solvents and in an aggregated state because of its strong CT and intermolecular interactions, respectively. After EtAmPy was introduced to normal polystyrene chains, the obtained functional PS emitted strong red light with a quantum yield of 37% in an aggregated state. Differently from EtAmPy, EtOxPy, possessing a weak CT and not being sensitive to the change of microenvironment, was AIE-active and displayed a large red shift in emission spectra in an aggregated state. Meanwhile, the functional PS initiated by EtOxPy showed obviously blue-shifted emission, compared to its initiator, in an aggregated state by reducing their intermolecular interactions. EtAmPy-functionalized polystyrene showed a successful application in cell imaging. Therefore, using the CT effect in molecular design and polymerization is beneficial to obtaining new red emission probes. The Royal Society of Chemistry.

Full text of DOI:10.1039/c3tc32244k

Journal of
Materials Chemistry C
View Article Online
View Journal | View Issue
PAPER
Preparation of 4-dicyanomethylene-2,6-distyryl-
4H-pyran derivatives, their functional polystyrenes
and study of their different aggregation induced
emission behaviors†
Cite this: J. Mater. Chem. C, 2014, 2,
2082
You-Hao Zhang,a Pei-Yang Gu,a Jie-Bo Zhou,b Yu-Jie Xu,b Wu Liu,a Qian-Feng Gu,a
a
a
a
a
*
*
Dong-Yun Chen, Na-Jun Li, Qing-Feng Xu and Jian-Mei Lu
Aggregated or solid state red fluorescent materials with efficient light emission are rare but have wide
applications in optoelectronic and biological fields. Herein, two pyran derivatives, EtAmPy and EtOxPy
with different charge transfers (CT), were designed as initiators and introduced to polystyrene chains via
atom transfer radical polymerization (ATRP). The emission of EtAmPy was quenched both in highly polar
solvents and in an aggregated state because of its strong CT and intermolecular interactions,
respectively. After EtAmPy was introduced to normal polystyrene chains, the obtained functional PS
emitted strong red light with a quantum yield of 37% in an aggregated state. Differently from EtAmPy,
EtOxPy, possessing a weak CT and not being sensitive to the change of microenvironment, was AIE-
active and displayed a large red shift in emission spectra in an aggregated state. Meanwhile, the
functional PS initiated by EtOxPy showed obviously blue-shifted emission, compared to its initiator, in an
aggregated state by reducing their intermolecular interactions. EtAmPy-functionalized polystyrene
showed a successful application in cell imaging. Therefore, using the CT effect in molecular design and
polymerization is beneficial to obtaining new red emission probes.
Received 18th November 2013
Accepted 12th December 2013
DOI: 10.1039/c3tc32244k
www.rsc.org/MaterialsC
interactions or effective intermolecular p-stacking could quench
their emission in an aggregated or solid state.31 One strategy to
Introduction
solve this problem is covalently attaching AIE units, such as tet-
raphenylethene (TPE) to the red luminophores.32–34 For example,
Tang's group has reported a DCM derivative, TPE-TPA-DCM,
possessing both AIE and intramolecular charge transfer (CT)
features.35 The nanoparticles (NPs) of this derivative exhibit effi-
cient FR/NIR uorescence, and have been applied in bioimaging
successfully.36 Another approach to preserve the intramolecular
CT effect in AIE systems is the effective wrapping of polar lumi-
nophores by polymer chains. Our previous work introduced a
single AIE initiator TPP-NI into the end of normal polymer
chains, such as polystyrene (PS), polymethyl methacrylate
(PMMA) and polyhydroxyethyl methacrylate (PHEMA) via ATRP to
obtain functional polymers.37 The dual photoresponse by intra-
molecular CT and AIE of the initiator is well-realized and even
enlarged aer polymerization, which indicates that the polymer
chains could play an important role in reducing the dipole–
dipole interactions between initiators and recover the emission
of initiators in an aggregated or solid state.38
The development of luminescent materials with efficient light
emission in an aggregated or solid state has attracted great
attention, due to their potential in organic light-emitting diodes
(OLEDs), chemosensors and biolabels.1–15 Since Professor Tang
and coworkers found the novel phenomenon of aggregation-
induced emission (AIE) in 2001, which could tackle the aggre-
gation-caused quenching (ACQ) problem and is not accompanied
by severe side effects, many groups have designed and synthe-
sized new AIE-active molecules, and investigated their aggregated
morphologies and the manipulation of their luminescence.16–27
To date, materials with efficient red emission in their aggregated
or solid state are still rare. Red luminophores are usually polar,
such as DCM (4-dicyanomethylene-2,6-distyryl-4H-pyran) or non-
polar but extensively p-conjugated, such as TPP(5,10,15,20-ter-
aphenylporphyrin).28–31
Either
attractive
dipole–dipole
aCollege of Chemistry, Chemical Engineering and Materials Science, Collaborative
Innovation Center of Suzhou Nano Science and Technology, Soochow University,
Suzhou, 215123, China. E-mail: lujm@suda.edu.cn; xuqingfeng@suda.edu.cn; Fax:
+86 0512 65880367; Tel: +86 0512 65880368
In this paper, we designed and synthesized two pyran deriva-
tives, 2-bromo-2-methyl-propionic acid 2-{[4-(2-{6-[2-(4-{[2-(2-
bromo-2-methyl-propionyloxy)-ethyl]-methyl-amino}-phenyl)-vinyl]-
4-dicyanomethylene-4H-pyran-2-yl}-vinyl)-phenyl]-methyl-amino}-
ethyl ester (abbreviated as EtAmPy) and 2-bromo-2-methyl-
bSchool of Radiation Medicine and Protection, Soochow University, Suzhou, 215123,
China
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c3tc32244k
2082 | J. Mater. Chem. C, 2014, 2, 2082–2088
This journal is © The Royal Society of Chemistry 2014

View Article Online
Paper
Journal of Materials Chemistry C
propionic acid 2-(4-{2-[6-(2-{4-[2-(2-bromo-2-methyl-propion-
yloxy)-ethoxy]-phenyl}-vinyl)-4-dicyanomethylene-4H-pyran-2-yl]-
vinyl}-phenoxy)-ethyl ester (abbreviated as EtOxPy), and intro-
duced two PS chains to the periphery of the initiators via
ATRP.39,40 As we know, the electron-donating ability of the
substituent in the para position of the styryl units inuences the
electronic interaction between the D–A pairs, and the electron-
donating ability of the ethyoxyl group is weaker than the
substituted amino.41 As a result, the CT of EtAmPy is stronger
than that of EtOxPy, as well as the intermolecular interactions,
which makes a difference to their sensitivity to the change of
microenvironment and their optical properties in solution and
in an aggregated state.42 On the other hand, the peripheral PS
chains could be used as building blocks to reduce the inter-
molecular interaction between initiators, and hence eliminate
the ACQ effect and amplify the aggregation-induced emission.
Thus, we have systematically studied the effect of PS chains on
the two initiators with different CT, as well as the effect of the
chain length on the emission of EtAmPy. Considering the bright
red aggregation emission of the functional PS initiated by
EtAmPy, we have prepared its NPs by using bovine serum
albumin (BSA) as a surfactant and applied it to cell imaging
(Scheme 1).43,44
Scheme 2 Synthetic routes for EtAmPy and EtOxPy.
white powder. Cyclohexanone and PMDETA were distilled
under vacuum. THF and TEA were dried over an appropriate
˚
agent and stored over 3 A molecular sieves before use. Styrene
was passed through a short column of aluminium oxide to
remove inhibitors prior to polymerization.
General characterization
1H NMR spectra were measured by an INOVA 400 MHz NMR
spectrometer and 13C NMR spectra were measured by an INOVA
300 MHz NMR spectrometer, CDCl3 or DMSO-d6 were used as
the solvent and tetramethylsilane (TMS) was used as the
internal standard at room temperature. Molecular weights and
the polydispersity indexes (PDIs) relative to PS were measured
using Waters1515 GPC with THF as a mobile phase at a ow rate
Experimental section
Materials
of 1 mL minꢀ1 and with a column temperature of 30 C. The
ꢁ
monomer conversions of polymerization were determined by a
gravimetric method. UV-vis absorption spectra were determined
on a Shimadzu RF540 spectrophotometer using a 1 cm square
quartz cell. Room temperature emission spectra were carried
out using an Edinburgh-920 uorescence spectra photometer
with a slit of 5 cm and the uorescent quantum yield (FF) in the
solution and aggregated states were estimated by using uo-
rescein (FF ¼ 79% in 0.1 mol Lꢀ1 NaOH) as standard. SEM
images were taken on a Hitachi S-4700 scanning electron
All the materials and reagents were used as purchased without
further purication except where otherwise stated. 2,6-
Dimethyl-g-pyrone, 2-(methylamino)ethanol, and N,N,N0,N0,N00-
penta-methyl-diethylenetriamine (PMDETA) were purchased
from Tokyo Chemical Industry Co., LTD. 4-Fluorobenzaldehyde
and 2-bromo-2-methylpropionyl bromide were purchased from
Energy Chemical. Malononitrile, 4-hydroxybenzaldehyde,
ethylene chlorohydrin, 18-crown-6, potassium carbonate,
piperidine, n-propanol, acetic anhydride, triethylamine (TEA),
cyclohexanone, copper(I) bromide (CuBr), dimethyl formamide
(DMF), dimethyl sulfoxide (DMSO), tetrahydrofuran (THF),
chloroform and styrene were purchased from Sinopharm
Chemical Reagent. CuBr was puried in acetic acid, washed
with ethanol and ether and dried under vacuum to afford a
Fig. 1 Emission spectra of (a) EtAmPy and (c) PSa in different polar
solvents. Concentration: 10 mM. Excitation wavelength: 479 nm.
Emission spectra of (b) EtOxPy and (d) PSd in different polar solvents.
Concentration: 10 mM. Excitation wavelength: 415 nm.
Scheme 1 Molecular structures of EtAmPy and EtOxPy.
This journal is © The Royal Society of Chemistry 2014
J. Mater. Chem. C, 2014, 2, 2082–2088 | 2083

View Article Online
Journal of Materials Chemistry C
Paper
Table 1 Optical properties of EtAmPy and PSa in different polar
solventsa
c
d
Solvents
labb (nm)
lem (nm)
FF (%)
Chloroform
THF
DMF
472e/466f
476/471
484/481
490/488
590/588
612/603
652/607
661/612
24.02/11.74
17.94/11.06
0.61/5.49
DMSO
0.42/5.02
a
b
Concentration: 10 mM. Excitation wavelength: 479 nm. Absorption
c
d
maximum.
Emission maximum.
Fluorescent quantum yield
estimated using uorescein (FF ¼ 79% in 0.1 mol Lꢀ1 NaOH) as
e
f
standard. Characteristic values of EtAmPy. Characteristic values
of PSa.
Fig. 3 (a) Emission spectra of PSa in the THF–water mixtures. (b) Plots
of the quantum yields of PSa, PSb, PSc versus water fraction (fw) in the
THF–water mixtures. Concentration: 10 mM. Excitation wavelength:
479 nm. (c) Emission spectra of PSd in the THF–water mixtures. (d)
Plots of quantum yield and maximum wavelength of PSd versus water
fraction (fw) in the THF–water mixtures. Concentration: 10 mM.
Excitation wavelength: 415 nm.
Table 2 Absorption and emission characteristics of EtAmPy, EtOxPy,
PSa, PSb, PSc, PSd in solution and suspensiona
b
c
Sample
lab (nm)
lem (nm)
FFd (%)
EtAmPy
PSa
PSb
PSc
EtOxPy
PSd
476e/528f
471/—g
468/—
465/—
416/422
406/—
612/636
603/591
602/590
594/585
515/648
508/520
17.94/4.72
11.06/18.50
12.13/22.39
16.66/37.36
0.16/2.07
Fig. 2 (a) Emission spectra of EtAmPy in the THF–water mixtures. (b)
Plots of quantum yield and maximum wavelength of EtAmPy versus
water fraction (fw) in the THF–water mixture. Concentration: 10 mM.
Excitation wavelength: 479 nm. (c) Emission spectra of EtOxPy in the
THF–water mixtures. (d) Plots of quantum yield and maximum
wavelength of EtOxPy versus water fraction (fw) in the THF–water
mixture. Concentration: 10 mM. Excitation wavelength: 415 nm.
0.67/4.98
a
b
Concentration in solution and suspension: 10 mM. Absorption
c
d
maximum.
Emission maximum.
Fluorescent quantum yield
estimated using uorescein (FF ¼ 79% in 0.1 mol Lꢀ1 NaOH) as
e
f
standard. Characteristic values in pure THF solvent. Characteristic
g
values in THF–water (1 : 9, v/v) suspension. Not determined in
suspension.
microscope. Thermogravimetric analysis (TGA) was conducted
on a TA Inꢀst1ruments Dynamic TGA 2950 at a heating rate of
10 ꢁC min
and under an N2 ow rate of 50 mL minꢀ1
.
Differential scanning calorimetry (DSC) analysis was performed
on a Shimadzu DSC860A 85 instrument. Dynamic light scat-
tering (DLS) measurements were carried out at 25 ꢁC using a
Zetasizer Nano-ZS from Malvern Instruments equipped with a
633 nm He–Ne laser using back-scattering detection.
Synthesis of 2-[2,6-bis-(2-{4-[(2-hydroxy-ethyl)-methyl-amino]-
phenyl}-vinyl)-pyran-4-ylidene]-malononitrile (1)
2,6-(dimethyl-4H-pyran-4-ylidene)malononitrile (30 mmol, 5.16 g),
4-[(2-hydroxy-ethyl)-methyl-amino]-benzaldehyde (70 mmol,
12.53 g) and 10 drops of piperidine were dissolved in 50 mL of
n-propanol and heated to reux for 24 h. Aer the complete
Fig. 4 (a) SEM image of the PSa-BSA NPs and (b) size distribution of
the particles measured by DLS analysis. Scale bar is 1 mm.
conversion of 2,6-(dimethyl-4H-pyran-4-ylidene)malononitrile, the 7.71–7.56 (m, 5H), 7.02 (d, J ¼ 15.8 Hz, 2H), 6.76 (d, J ¼ 8.4 Hz,
reaction mixture was cooled and ltered. The precipitate was 3H), 6.65 (s, 2H), 4.74 (s, 2H), 3.58 (d, J ¼ 5.6 Hz, 4H), 3.48 (s, 4H),
washed with 40 mL of ethanol and recrystallized three times 3.02 (s, 6H). 13C NMR (300 MHz, DMSO-d6) d (ppm) 53.57, 54.36,
from a DMF–ethanol mixture (1 : 10 by volume). Yield: 53% 58.63, 105.22, 111.94, 113.32, 116.86, 122.56, 130.45, 138.78,
(15.9 mmol, 7.85 g). 1H NMR (400 MHz, DMSO-d6) d (ppm) 151.13, 156.22, 160.30.
2084 | J. Mater. Chem. C, 2014, 2, 2082–2088
This journal is © The Royal Society of Chemistry 2014

View Article Online
Paper
Journal of Materials Chemistry C
Polymer synthesis
All of the ATRP reactions were carried out following a typical
experimental procedure: initiator (0.02 mmol), CuBr
(0.06 mmol, 8.64 mg), PMDETA (0.12 mmol, 20.76 mg), cyclo-
hexanone (1 mL), and styrene (10 mmol, 1.04 g) were mixed in a
round-bottomed ask. The ask was sealed and cycled four
times between vacuum and nitrogen. The polymerization took
Fig. 5 CLSM images of HeLa cells incubated with BSA NPs at 37 ꢁC for
3 hours. (a) Bright field. (b) Excited with 488 nm laser. (c) Overlay of (a)
and (b). Scale bar is 20 mm.
ꢁ
place at 70 C for a certain amount of time under a nitrogen
atmosphere. Samples, taken at regular intervals using a syringe
for conversion and molecular weight analysis, were diluted with
THF, passed through a short column of basic aluminium
oxide to remove the copper ions, and precipitated in a large
amount of methanol. The product was ltered and dried under
vacuum.
Synthesis of 2-(2,6-bis-{2-[4-(2-hydroxy-ethoxy)-phenyl]-vinyl}-
pyran-4-ylidene)-malononitrile (2)
The procedure was similar to that for the preparation of 1. Yield:
55% (16.5 mmol, 7.72 g). 1H NMR (400 MHz, DMSO-d6) d (ppm)
7.74 (t, J ¼ 11.3 Hz, 6H), 7.25 (d, J ¼ 16.1 Hz, 2H), 7.04 (d, J ¼
8.2 Hz, 4H), 6.81 (s, 2H), 4.92 (t, J ¼ 5.0 Hz, 2H), 4.06 (s, 4H), 3.74
(d, J ¼ 4.5 Hz, 4H). 13C NMR (300 MHz, DMSO-d6) d (ppm) 56.16,
59.95, 70.17, 106.70, 115.33, 116.18, 117.03, 128.04, 130.37,
137.91, 156.48, 159.61, 160.88.
Preparation of BSA NPs loaded with functional PS or EtAmPy
To a solution of BSA (10 mg) in physiological saline (5 mL) was
added dropwise 1.3 mL of 1 mg mLꢀ1 functional PS (Mn ¼ 5320)
or EtAmPy in pure THF. The mixture was vigorous stirred at
ꢁ
25 C for 24 hours. Then the mixture was evaporated to thor-
oughly remove the remaining THF on a rotary evaporator under
vacuum. The BSA NPs suspension was ltered through a
0.45 mm microlter and le for 12 hours to stabilize before use.
Synthesis of EtAmPy
Compound 1 (10 mmol, 4.94 g) and dried TEA (80 mmol, 8.08 g)
were dissolved in 200 mL of anhydrous THF and cooled with ice.
Aer the solution was cooled below 0 ꢁC, 2-bromo-2-methyl-
propionyl bromide (60 mmol, 13.80 g), dissolved in 20 mL of
anhydrous THF, was added dropwise. The mixture was stirred
for another hour at 0 ꢁC and then stirred overnight at room
temperature. The reaction mixture was ltered and the ltrate
was poured into cold water. The precipitate was ltered and
recrystallized twice from ethyl acetate. Yield: 69% (6.9 mmol,
5.46 g). 1H NMR (400 MHz, DMSO-d6) d (ppm) 7.70–7.62 (m,
6H), 7.08 (d, J ¼ 16.0 Hz, 2H), 6.82 (d, J ¼ 8.5 Hz, 4H), 6.69 (s,
2H), 4.32 (s, 4H), 3.77 (s, 4H), 3.04 (s, 6H), 1.81 (s, 12H). 13C
NMR (300 MHz, DMSO-d6) d (ppm) 30.87, 39.15, 50.35, 54.19,
57.76, 64.22, 105.74, 112.55, 114.16, 117.01, 123.49, 130.68,
138.92, 151.05, 156.57, 160.51, 171.53. m.p. ¼ 180 ꢂ 0.5 ꢁC.
Anal. calcd for C38H40Br2N4O5: N, 7.07%, C, 57.57%, H, 5.05%;
found, N, 7.06%, C, 57.67%, H, 5.13%. LC-MS (m/z): [M + Na]+
calcd for C38H40Br2N4NaO5, 813.1258; found, 813.1251
(Scheme 2).
Results and discussion
Polymer characterization
Polymerizations of styrene were processed by using EtAmPy (or
EtOxPy) as the initiator, CuBr/PMDETA as a catalyst system, and
cyclohexanone as a solvent. A proper ratio was selected:
[initiator]0 : [CuBr]0 : [PMDETA]0 : [styrene]0 ¼ 1 : 3 : 6 : 500,
[styrene]0 ¼ 10.0 mol Lꢀ1. The kinetic plot of polymerization is
presented in Fig. S1-1.† The linearity of the semi-logarithmic
plot of ln([M]0/[M]) versus time indicated that the polymeriza-
tion was rst-order with respect to the monomer and that the
concentration of the growing radicals remained constant. The
number average molecular weight (Mn) increased linearly with
the conversion, and the PDIs of the polymers were relatively
narrow in all cases (Mw/Mn ¼ 1.20–1.40) (Fig. S1-2, Table S1†).
All of these results suggested that the polymerization was well-
controlled.
To study the effect of different chain lengths of the PS on the
properties of EtAmPy, we chose three polymers with different
molecular weights (Mn ¼ 5320, 9220, 14 740), and named them
PSa, PSb and PSc, respectively. We also synthesized PSd (Mn ¼
6390, PDI ¼ 1.35) to further research the impact of the PS chains
on the properties of EtOxPy with weaker CT.
Synthesis of EtOxPy
The procedure was similar to that for the preparation of
EtAmPy. Yield: 73% (7.3 mmol, 5.59 g). 1H NMR (400 MHz,
DMSO-d6) d (ppm) 7.77 (t, J ¼ 11.6 Hz, 5H), 7.28 (d, J ¼ 16.1 Hz,
2H), 7.09 (d, J ¼ 8.4 Hz, 4H), 6.83 (s, 2H), 4.50 (s, 4H), 4.33 (s,
4H), 1.90 (s, 12H). 13C NMR (300 MHz, DMSO-d6) d (ppm) 30.92,
Thermal stability
56.52, 57.69, 64.86, 66.48, 107.13, 115.81, 116.40, 117.64, The thermal properties of the polymers were evaluated by TGA
128.78, 130.63, 138.06, 156.81, 159.87, 160.61, 171.50. M.p. ¼ and DSC under nitrogen atmosphere, as shown in Fig. S9.† The
173 ꢂ 0.5 ꢁC. Anal. calcd for C36H34Br2N2O7: N, 3.66%, C, 5% weight-loss temperatures (Td) of PSa, PSb, PSc and PSd were
ꢁ
ꢁ
ꢁ
ꢁ
56.40%, H, 4.44%; found, N, 3.34%, C, 55.84%, H, 4.57%. LC- 298 C, 324 C, 333 C and 326 C, respectively, and the glass
MS (m/z): [M + Na]+ calcd for C36H34Br2N2NaO7, 787.0625; transition temperatures (Tg) of them were found to be 97 ꢁC,
ꢁ
100 C, 103 ꢁC and 100 ꢁC, respectively.
found, 787.0622.
This journal is © The Royal Society of Chemistry 2014
J. Mater. Chem. C, 2014, 2, 2082–2088 | 2085

View Article Online
Journal of Materials Chemistry C
Paper
in its emission spectra (Fig. 1(d)) in different polar solvents just
like EtOxPy, and the FF values remained unchanged in the
region of 0.70% because of its weak CT.
Solvent effect
EtAmPy is a kind of DCM derivative with a typical donor–p–
acceptor (D–p–A) structure, which exhibits an ultra-fast CT.30,45
To conrm the existence of CT in the luminophore, we
measured the absorption and emission spectra of EtAmPy in
different polar solvents, as shown in Fig. S10(a)† and 1(a),
respectively. In the DMSO solution, EtAmPy showed multiple
absorption bands at 370 nm and 490 nm, which may be
deduced to a localized aromatic p–p* transition and CT tran-
sition, respectively. When the solvent polarity decreased, the
absorption maximum underwent a blue-shi from 484 nm in
DMF to 472 nm in chloroform. EtAmPy exhibited a very brilliant
Aggregation behavior
To investigate the aggregation behavior of EtAmPy, the
absorption and PL spectra of the dye in the THF–water mixtures
with different water fractions were measured and are shown in
Fig. S12† and 2(a). When EtAmPy was molecularly dissolved in
pure THF solution, its emission spectrum peaked at 612 nm
with a FF value of 17.94%. The emission was dramatically
quenched as fw in the solvent mixture and progressively
increased from 0 to 50%. Meanwhile, the emission maximum
was red-shied to around 645 nm with a FF of 1.26%, which
arised from the CT effect with the increase of the solvent
polarity. When more water was added (fw > 50 vol%), the
solvating power of the mixture was worsened to such an extent
that the luminogen molecules began to aggregate. The peak
wavelength was blue-shied to 609 nm in 70% aqueous mixture
and the FF value increased to 10.65%, which may be attributed
to the fact that the less polar environment inside the nano-
aggregates alleviated the CT effect. On the other hand, the
formation of the aggregate may rigidify and planarize the
conformation and extend the conjugation of the dyes. As a
result, when the fw reached 90%, the peak wavelength was red-
shied to 636 nm and the FF value decreased to 4.72%. More-
over, an attractive dipole–dipole interaction of the polarized
EtAmPy dyes facilitated the formation of detrimental excimeric
species in an aggregated or solid state, which led to the
ACQ effect.36
Contrary to the EtAmPy suffering from the ACQ effect,
EtOxPy was AIE-active. The emission spectra of EtOxPy in the
THF–water mixture with different water fractions are shown in
Fig. 2(c). When dissolved in pure THF, its emission maximum
was at 515 nm with a FF as low as 0.16%. When the fw in the
solvent mixture was increased from 0 to 60%, its emission was
intensied a little with a FF of 1.25% in 60% aqueous mixture.
As a large amount of water (fw > 70%) was added into THF, a
level-off tail was observed in the visible region of the absorption
spectra, indicating the nanosized luminogen aggregates were
formed. Unlike EtAmPy, the aggregates of EtOxPy displayed a
dual uorescence emission characterized by a short band at
545 nm and a new long band at 648 nm. With the addition of
water, the short band emission gradually reduced and the long
band signicantly intensied. Its FF in 90% aqueous mixture
was 2.07%, more than 12-fold higher than that of pure THF
solution, which was the AIE characteristic. Moreover, the
emission maximum underwent a red shi as large as 133 nm
from pure THF solution to suspension in 90% aqueous mixture.
This may be attributed to the strong intermolecular interactions
between the EtOxPy molecules aer aggregation.49
uorescence in chloroform (FF ¼ 24.02%) and THF (FF
¼
17.94%) solution with a maximum emission wavelength at 590
nm and 612 nm, respectively, while its emission was so weak
that it could be hardly observed in highly polar solvents, such as
DMF (FF ¼ 0.61%) and DMSO (FF ¼ 0.42%). The difference of
the emission maxima could be as large as 71 nm. The decreased
photoluminescence (PL) intensity may be attributed to the
transformation of the excited state from the locally excited (LE)
state to the intramolecular CT state, as the dye's intramolecular
CT state was susceptible to various nonradiative quenching
processes in highly polar solvents.46,47 Compared to EtAmPy, the
emission maximum and FF of EtOxPy suffered a much smaller
change when the solvent polarity increased. The emission
maximum underwent a red shi from 514 nm in chloroform to
542 nm in DMSO, as shown in Fig. 1(b). The difference of the
emission maxima of EtOxPy was smaller than that of EtAmPy,
suggesting the CT of EtOxPy was weaker.48 The spatial distri-
butions of the highest occupied molecular orbital (HOMO) and
the lowest unoccupied molecular orbital (LUMO) levels of the
two initiators are shown in Fig. S22.† The dipole moment of
EtAmPy (14.5 Debye) was larger than that of EtOxPy
(12.7 Debye), which could further illustrate the stronger CT of
EtAmPy than that of EtOxPy. The emission of EtOxPy in
different polar solvents was very weak with a FF below 0.50%,
which may be due to the fact that the weak pull–push interac-
tion between the ethyoxyl group (donor) and cyano (acceptor)
endowed EtOxPy with such a exible conformation that the
phenyl and pyranyl rings may rotate upon the axes of the
olenic double bonds and the dye was nonemissive in dilute
solutions (Table 1).41
The absorption and emission spectra of PSa in different
polar solvents were measured to conrm the preservation of the
CT effect in polymers, as shown in Fig. S10(b)† and 1(c). The
absorption pattern of PSa was similar to that of its initiator.
Although the PL intensity of PSa was gradually weakened with
the solvent polarity increasing, the emission was quenched a
little in highly polar solvents (DMF and DMSO), which was
different from its initiator. Moreover, the difference of emission
maximum of PSa in chloroform and DMSO was 24 nm, much
smaller than that of EtAmPy. These phenomena may be
ascribed to the fact that non-polar PS chains could reduce the
polarization of the initiators induced by solvents and hence the
intermolecular interaction between initiators and solvents was
weakened. In terms of PSd, no obvious difference was exhibited
The absorption and emission behaviors were similar for the
three PS chains with different molecular weights. The emission
spectra of PSa in THF–water with different water fractions are
shown in Fig. 3(a). When dissolved in THF, the absorption
maximum was at 471 nm. With the addition of water, the
2086 | J. Mater. Chem. C, 2014, 2, 2082–2088
This journal is © The Royal Society of Chemistry 2014

View Article Online
Paper
Journal of Materials Chemistry C
absorption spectra exhibited light-scattering effects in the short prepare the NPs (abbreviated as EtAmPy-BSA NPs, Fig. S20†) in
band suggesting the formation of aggregates, which was the same conditions as we prepared the PSa-BSA NPs to
conrmed by SEM (Fig. S18†). Aer a “small” amount of water compare their differences. As shown in Fig. S21 and Table S2,†
was added into THF (fw < 20%), the PL intensity of PSa was the emission of EtAmPy-BSA NPs in physiological saline
slightly decreased because of the transformation to the intra- suspension was almost quenched with a FF value as low as
molecular CT state resulting from the gradually increasing 1.18%. In contrast to EtAmPy-BSA NPs, PSa-BSA NPs emitted
polarity of the solvent mixture. Aer that, the emission was bright red aggregation emission whose FF value was as high as
intensied and the emission maximum was progressively blue- 13.34%. Considering this advantage of PSa-BSA NPs, we applied
shied to 591 nm (fw ¼ 90%) due to the weakened microenvi- PSa-BSA NPs in cell imaging. Fig. 4(a) is an SEM image of the
ronmental polarity near the initiators by aggregated non-polar obtained PSa-BSA NPs, exhibiting uniform morphology and
PS chains. From the molecular solution in pure THF to the good monodispersity. The DLS analysis (Fig. 4(b)) also showed
aggregate suspension in 90% aqueous solution, the PL intensity that the PSa-BSA NPs had an average diameter of about
was enhanced 1.85-fold with a FF increment from 11.06% to 146.9 nm and a narrow size distribution (PDI ¼ 0.113), which
18.50%. This may be attributed to the fact that the aggregated corresponded well to the SEM result. The potential cell imaging
polymer protected the initiators from intermolecular interac- application of the uorescent PSa-BSA NPs was examined by
tions, which led to the emission quenching of EtAmPy in an confocal laser scanning microscopy (CLSM) with HeLa cells.
aggregated state. Similar results were observed for PSb and PSc, The confocal images were taken upon excitation at 488 nm. As
as shown in Fig. S14 and S15.† The FF values of the three shown in Fig. 5, bright red uorescence could be easily observed
polymers in 90% aqueous suspension are all much higher than aer the cells were incubated with PSa-BSA NPs at 37 ꢁC for
those of the pure THF solution (Fig. 3(b)). Evidently, all three 3 hours. Apart from the strongly uorescent areas, some dark
polymers were aggregation-enhanced emission (AEE)-active. areas were also found in the cells, which could be the location of
More interestingly, as the molecular weights increased, the the cell nucleus. These phenomena indicated that the PSa-BSA
emission maximum (fw ¼ 90%) showed a blue-shi to 590 nm NPs could be taken up by cells through nonspecic endocytosis
and 585 nm for PSb and PSc, respectively. Meanwhile, the and were distributed mainly in the cytoplasm.50,51
aggregate became more emissive, and the FF of PSb and PSc in
90% aqueous solution increased from 22.39% to 37.36%, which
further manifested that the longer polymer chains could reduce
Conclusion
the probability of the intermolecular interactions between In this work, we have synthesized two pyran derivatives with
initiators with a much higher efficiency (Table 2).
different CT and introduced two PS chains to the periphery of
To further study the impact of the PS chains on the uo- initiators via ATRP. EtAmPy, possessing a strong CT, emits a
rescent behavior of EtOxPy whose CT was weaker than EtAmPy, brilliant red light in relatively polar solvents (chloroform and
we measured the absorption and emission spectra of PSd in the THF), but its emission is quenched in highly polar solvents
THF–water mixtures. The absorption pattern of PSd was similar (DMF and DMSO) and the aggregated state because of the CT
to its initiator. However, the emission of PSd in pure THF and ACQ effect, respectively. Aer introduction to the PS chains,
solution peaked at 508 nm with a FF of 0.67%. When the water the CT of EtAmPy is alleviated by the polymer chains and the
was added, the emission was progressively enhanced and the FF emission is “turned-on” in the aggregated or solid state. The FF
increased to 4.98% in 90% aqueous mixture, accompanied by a values of the polymers in 90% aqueous suspension are all much
slight red shi of 12 nm in the emission maximum. Compared higher than those of their pure THF solution. In addition, the
to EtOxPy, the emission spectra of PSd only showed a single longer the polymer chains are, the more enhanced the emission
short band both in pure THF solution and suspension in the is. Taking advantage of the bright red emission of the functional
THF–water mixtures. The long band at 648 nm of EtOxPy in 90% PS initiated by EtAmPy, its NPs are modied by BSA and applied
aqueous mixture did not appear in the emission spectra of PSd. to cell imaging successfully. Compared to EtAmPy, the CT of
This indicated that the PS chains may function as a kind of solid EtOxPy is much weaker and it exhibits the typical AIE
solvent which could separate the initiators from strong inter- phenomenon with a red shi as large as 133 nm in emission
molecular interactions aer aggregation, and restrict the spectra due to the intermolecular interactions between the dye
intramolecular rotations of the pyranyl and phenyl rings of molecules in an aggregated state. However, the PS chains could
EtOxPy upon the axes of the olenic double bonds.41,49 Thus, separate EtOxPy from intermolecular interactions and restrict
PSd was also AIE-active, just like EtOxPy, but only emitted a their intramolecular rotations. As a result, the functional poly-
green light in an aggregated state.
mer becomes emissive in an aggregated state. In summary, we
have successfully utilized polymer chains to tune the optical
properties of the luminophores, and studied the interaction
between the polymer chains and initiators with different CT.
Cell imaging
Considering the bright red aggregation emission of PSa, we
prepared its NPs (abbreviated as PSa-BSA NPs) by using BSA as a
surfactant. Amphiphilic BSA could interact with PSa through
Acknowledgements
hydrophobic interactions and change the surface to be hydro- The authors graciously thank the Chinese Natural Science Foun-
philic in aqueous solution. We also used EtAmPy and BSA to dation (21071105, 21371128 and 21336005), Chinese-Singapore
This journal is © The Royal Society of Chemistry 2014
J. Mater. Chem. C, 2014, 2, 2082–2088 | 2087

View Article Online
Journal of Materials Chemistry C
Paper
Joint Project (2012DFG41900), the Specialized Research Fund for 26 X. J. Diao, W. Li, J. Yu, X. J. Wang, X. J. Zhang, Y. L. Yang,
the Doctoral Program of Higher Education of China (grant no.
20113201130003).
F. F. An, Z. Liu and X. H. Zhang, Nanoscale, 2012, 4, 5373.
27 Z. L. Wang, B. Xu, L. Zhang, J. B. Zhang, T. H. Ma,
J. B. Zhang, X. Q. Fu and W. J. Tian, Nanoscale, 2013, 5, 2065.
28 C. T. Chen, Chem. Mater., 2004, 16, 4389.
29 M. Vanjinathan, H. C. Lin and A. S. Nasar, J. Polym. Sci., Part
A: Polym. Chem., 2012, 50, 3806.
Notes and references
1 M. Sameiro and T. Goncalves, Chem. Rev., 2009, 109, 190.
2 Y. Suzuki and K. Yokoyama, J. Am. Chem. Soc., 2005, 127, 30 Z. Q. Guo, W. H. Zhu and H. Tian, Chem. Commun., 2012, 48,
17799. 6073.
3 Y. H. Li, Y. Q. Wu, J. Chang, M. Chen, R. Liu and F. Y. Li, 31 B. J. Jung, J. I. Lee, H. Y. Chu, L. M. Do, J. Lee and H. K. Shim,
Chem. Commun., 2013, 49, 11335.
J. Mater. Chem., 2005, 15, 2470.
4 M. Faisal, Y. Hong, J. Liu, Y. Yu, J. W. Y. Lam, A. J. Qin, P. Lu 32 W. Z. Yuan, P. Lu, S. M. Chen, J. W. Y. Lam, Z. M. Wang,
and B. Z. Tang, Chem.–Eur. J., 2010, 16, 4266.
5 Y. Dong, B. Xu, J. Zhang, X. Tan, L. Wang, J. Chen, H. Lv,
Y. Liu, H. S. Kwok, Y. G. Ma and B. Z. Tang, Adv. Mater.,
2010, 22, 2159.
S. Wen, B. Li, L. Ye, B. Zou and W. Tian, Angew. Chem., Int. 33 Z. J. Zhao, P. Lu, J. W. Y. Lam, Z. M. Wang, C. Y. K. Chan,
Ed., 2012, 51, 10782.
H. H. Y. Sung, I. D. Williams, Y. G. Ma and B. Z. Tang,
Chem. Sci., 2011, 2, 672.
34 Z. J. Zhao, S. M. Chen, J. W. Y. Lam, P. Lu, Y. C. Zhong,
K. S. Wong, H. S. Kwok and B. Z. Tang, Chem. Commun.,
2010, 46, 2221.
6 Z. Luo, X. Yuan, Y. Yu, Q. Zhang, D. T. Leong, J. Y. Lee and
J. Xie, J. Am. Chem. Soc., 2012, 134, 16662.
7 Y. Kubota, S. Tanaka, K. Funabiki and M. Matsui, Org. Lett.,
2012, 14, 4682.
8 T. He, X. Tao, J. Yang, D. Guo, H. Xia, J. Jia and M. Jiang, 35 J. L. Geng, K. Li, D. Ding, X. H. Zhang, W. Qin, J. Z. Liu,
Chem. Commun., 2011, 47, 2907. B. T. Tang and B. Liu, Small, 2012, 8, 3655.
9 Y. Liu, Z. Wang, G. Zhang, W. Zhang, D. Zhang and X. Jiang, 36 W. Qin, D. Ding, J. Z. Liu, W. Z. Yuan, Y. Hu, B. Liu and
Analyst, 2012, 137, 4654. B. Z. Tang, Adv. Funct. Mater., 2012, 22, 771.
10 V. Bhalla, A. Gupta and M. Kumar, Org. Lett., 2012, 14, 3112. 37 P. Y. Gu, C. J. Lu, F. L. Ye, J. F. Ge, Q. F. Xu, Z. J. Hu, N. J. Li
11 A. J. Qin, J. W. Y. Lam and B. Z. Tang, Chem. Soc. Rev., 2010,
and J. M. Lu, Chem. Commun., 2012, 48, 10234.
38 Y. X. Tao, Q. F. Xu, N. J. Li, J. M. Lu, L. H. Wang and
X. W. Xue, Polymer, 2011, 52, 4261.
39, 2522.
12 A. J. Qin, J. W. Y. Lam and B. Z. Tang, Macromolecules, 2010,
43, 8693.
39 S. H. Lee, M. Ouchi and M. Sawamoto, J. Polym. Sci., Part A:
Polym. Chem., 2013, 51, 4498.
40 K. Nakatani, K. Terashima, M. Ouchi and M. Sawamoto,
Macromolecules, 2010, 43, 8910.
41 H. Tong, Y. N. Hong, Y. Q. Dong, Y. Ren, M. Haussler,
J. W. Y. Lam, K. S. Wong and B. Z. Tang, J. Phys. Chem. B,
2007, 111, 2000.
13 S. Mukherjee and P. Thilagar, Chem. Commun., 2013, 49,
7292.
14 J. F. Sun, Y. Lu, L. Wang, D. D. Cheng, Y. J. Sun and
X. S. Zeng, Polym. Chem., 2013, 4, 4045.
15 A. Qin, J. W. Y. Lam and B. Z. Tang, Prog. Polym. Sci., 2012,
37, 182.
16 Y. N. Hong, J. W. Y. Lam and B. Z. Tang, Chem. Soc. Rev., 42 S. Hecht and J. M. J. Frechet, Angew. Chem., Int. Ed., 2001, 40,
2011, 40, 5361.
74.
17 S. J. Yoon, J. W. Chung, J. Gierschner, K. S. Kim, M. G. Choi, 43 J. Liu, D. Ding, J. L. Geng and B. Liu, Polym. Chem., 2012, 3,
D. H. Kim and S. Y. Park, J. Am. Chem. Soc., 2010, 132, 13675. 1567.
18 B. K. An, S. K. Kwon, S. D. Jung and S. Y. Park, J. Am. Chem. 44 X. Q. Zhang, X. Y. Zhang, S. Q. Wang, M. Y. Liu, L. Tao and
Soc., 2002, 124, 14410. Y. Wei, Nanoscale, 2013, 5, 147.
19 Y. Liu, X. T. Tao, F. Z. Wang, J. H. Shi, J. L. Sun, W. T. Yu, 45 X. F. Xu, R. Q. Zhang, Z. X. Cao and Q. E. Zhang, J. Theor.
Y. Ren, D. C. Zou and M. H. Jiang, J. Phys. Chem. C, 2007,
111, 6544.
Comput. Chem., 2008, 7, 719.
46 R. R. Hu, E. Lager, A. Aguilar-Aguilar, J. Z. Liu, J. W. Y. Lam,
H. H. Y. Sung, I. D. Williams, Y. C. Zhong, K. S. Wong,
E. Pena-Cabrera and B. Z. Tang, J. Phys. Chem. C, 2009,
113, 15845.
47 M. Mao, M. G. Ren and Q. H. Song, Chem.–Eur. J., 2012, 18,
15512.
20 Y. Liu, X. T. Tao, F. Z. Wang, X. N. Dang, D. C. Zou, Y. Ren
and M. H. Jiang, J. Phys. Chem. C, 2008, 112, 3975.
21 Z. G. Chi, X. Q. Zhang, B. J. Xu, X. Zhou, C. P. Ma, Y. Zhang,
S. W. Liu and J. R. Xu, Chem. Soc. Rev., 2012, 41, 3878.
22 B. Chen, G. Yu, X. Li, Y. B. Ding, C. Wang, Z. W. Liu and
Y. S. Xie, J. Mater. Chem. C, 2013, 1, 7409.
48 K. Y. Jung and Y. S. Kim, Mol. Cryst. Liq. Cryst., 2011, 538, 45.
23 G. F. Zhang, M. P. Aldred, W. L. Gong, C. Li and M. Q. Zhu, 49 H. Tong, Y. Q. Dong, Y. Ren, M. Haussler, Y. N. Hong,
Chem. Commun., 2012, 48, 7711.
24 F. F. Wang, J. Y. Wen, L. Y. Huang, J. J. Huang and J. Ouyang,
Chem. Commun., 2012, 48, 7395.
J. W. Y. Lam, H. H. Y. Sung, I. D. Williams, H. S. KwoK
and B. Z. Tang, Chem. Phys. Lett., 2006, 428, 326.
50 X. Y. Zhang, X. Q. Zhang, B. Yang, M. Y. Liu, W. Y. Liu,
Y. W. Chen and Y. Wei, Polym. Chem., 2013, 4, 4317.
25 Q. Zeng, Z. Li, Y. Q. Dong, C. A. Di, A. J. Qin, Y. N. Hong, L. Ji,
Z. C. Zhu, C. K. W. Jim, G. Yu, Q. Q. Li, Z. A. Li, Y. Q. Liu, 51 X. Q. Zhang, M. Y. Liu, B. Yang, X. Y. Zhang, Z. G. Chi,
J. G. Qin and B. Z. Tang, Chem. Commun., 2007, 70.
S. W. Liu, J. R. Xu and Y. Wei, Polym. Chem., 2013, 4, 5060.
2088 | J. Mater. Chem. C, 2014, 2, 2082–2088
This journal is © The Royal Society of Chemistry 2014