Tetraphenylthiophene-functionalized poly(n -isopropylacrylamide): Probing lcst with aggregation-induced emission

Article abstract of DOI:10.1021/ma201089j

A hydrophobic tetraphenylthiophene (TP) center with novel aggregation-induced emission (AIE) property was chemically linked to two poly(N-isoprppylacrylamide) (PNIPAM) chains to obtain thermoresponsive polymers to study the relationships between the lower critical solution transitions (LCSTs) and the AIE-operative fluorenscence (FL) emission. Three ethynyl-terminated PNIPAMs with different molecular weights were synthesized via controlled atom transfer radical polymerization (ATRP) using ethynyl-functionalized initiator. The PNIPAMs were then coupled with diazide-funtionalized TP (TPN₃) via click reaction to obtain the desired TP-embedded polymers of Px (x = 1, 2, and 3). All three polymers show AIE-property from their solution fluorescence behavior in THF/hexane mixtures. In the aqueous solution, the TP-center served as a fluorogenic probe that reveals the LCSTs of polymers and its relation to the degree of TP labeling in terms of polymer concentration. The thermoresponsiveness of Px was demonstrated by the complete emission quench when heated at temperatures above LCST. Dissociation of the TP aggregates above LCST is responsible for the emission quench, which was evaluated through the uses of transmittance measurement, dynamic light scattering, and ¹H NMR spectra.

Full text of DOI:10.1021/ma201089j

ARTICLE
pubs.acs.org/Macromolecules
Tetraphenylthiophene-Functionalized Poly(N-isopropylacrylamide):
Probing LCST with Aggregation-Induced Emission
Chung-Tin Lai, Rong-Hong Chien, Shiao-Wei Kuo, and Jin-Long Hong*
Department of Materials and Optoelectronic Science, National Sun Yat-Sen University, Kaohsiung 80424, Taiwan, Republic of China
S Supporting Information
b
ABSTRACT: A hydrophobic tetraphenylthiophene (TP) center
with novel aggregation-induced emission (AIE) property
was chemically linked to two poly(N-isoprppylacrylamide)
(PNIPAM) chains to obtain thermoresponsive polymers to study
the relationships between the lower critical solution transitions
(LCSTs) and the AIE-operative fluorenscence (FL) emission.
Three ethynyl-terminated PNIPAMs with different molecular
weights were synthesized via controlled atom transfer radical
polymerization (ATRP) using ethynyl-functionalized initiator.
The PNIPAMs were then coupled with diazide-funtionalized TP
(TPN3) via click reaction to obtain the desired TP-embedded
polymers of Px (x = 1, 2, and 3). All three polymers show AIE-
property from their solution fluorescence behavior in THF/hexane
mixtures. In the aqueous solution, the TP-center served as a fluorogenic probe that reveals the LCSTs of polymers and its relation to
the degree of TP labeling in terms of polymer concentration. The thermoresponsiveness of Px was demonstrated by the complete
emission quench when heated at temperatures above LCST. Dissociation of the TP aggregates above LCST is responsible for the
1
emission quench, which was evaluated through the uses of transmittance measurement, dynamic light scattering, and H NMR
spectra.
’ INTRODUCTION
dehydration of the PNIPAM chains triggers changes in the size and
shape of the micelles, often leading to macroscopic aggregation. The
phase transition temperature depends not only on the level of
hydrophobic incorporation and chemical structure but also on its
position on the chain. This effect can be traced to differences in the
structure of the micelles formed by the various HM-PNIPAMs in
cold water. Randomly modified HM-PNIPAMs adopt a loose
micellar conformation in which the hydrophobic groups are partly
exposed to water; the cloud point of their solutions is depressed
significantly compared to that of PNIPAM solutions.24 On the other
hand, polymers that carry a hydrophobic group at one chain end
tend to form coreÀshell structures in which the hydrophobic core is
insulated from the water by a brushlike corona of PNIPAM
chains.21,25 Like other associating polymers, such as hydrophobically
modified ethoxylated urethanes (HEUR) or telechelic alkyl end-
capped poly(ethylene oxides),26À31 telechelic HM-PNIPAMs form
flowerlike associates composing of loops of hydrated polymer chains
and both end groups entrapped in the micellar core. In a preparatory
dynamic light scattering measurement on the aqueous dilute
solutions of telechelic C18-PNIPAM-C18, individual flower micelles
collapse near LCST, forming colloidal state with an uneven segment
density distribution.29 Recently, Liu et al.,30 Geckeler et al.31 and
Thermoresponsive water-soluble poly(N-isopropylacrylamide)
(PNIPAM) in water displays a lower critical solution temperature
(LCST), which has been investigated by a diversity of experimental
techniques in the concentrated and dilute regimes.1À12 Some
possible models have been portrayed to account for the coil-to-
globule collapse of PNIPAM in water.13À15 The temperature-driven
conformational transformation of the single PNIPAM chain and the
macroscopic phase separation reflect rather subtle changes in
polymer/water hydrogen-bonded interactions, primarily the release
of water molecules from a polymer hydrophilic layer into bulk water.
Therefore, slight changes in the chemical composition of PNIPAM
are anticipated to have significant influences on the water/PNIPAM
phase diagram.
Thanks to the Cerankowski and Taylor’s efforts of scientists
working in the LCST of poly(N-alkylacrylamides), such as
PNIPAM, can be raised or lowered via introduction of hydro-
philic or hydrophobic comonomers.16 Since then, there have
been numerous attempts to exploit this principle to create
“intelligent” devices and systems capable to respond reliably
and repetitively to temperature jumps.17,18 One approach is the
use of small number of long alkyl or perfluoroalkyl chains to
generate hydrophobically modified (HM) PNIPAM.19À23 The
hydrophobic groups drive the self-assembly of the polymers,
leading to the formation of polymeric micelles that exist as
isolated entities in dilute cold aqueous solutions. Upon heating,
Received: May 12, 2011
Revised:
June 23, 2011
Published: July 21, 2011
r
2011 American Chemical Society
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Scheme 1. Syntheses of Organic Compounds of TPBr, TPN3, BMP, and the Click Reaction between PNIPAMx and TPN3 To
Form Px Polymers
Yajima et al.32 investigated the aggregation behavior of fullerene-
(C60-) functionalized PNIPAM (C60-PNIPAM) in aqueous solu-
tion. The presence of highly hydrophobic C60 moieties leads to self-
assembled hybrid nanoparticles, which undergo thermo-induced
collapse/aggregation behavior due to the LCST phase transition of
PNIPAM chains.
and its derivatives exhibit interesting properties. With the
chemical framework of one central thiophene stator connected
by multiple peripheral aromatic rings, TP and TP-derivatives
were previously characterizedtobeAIE-activedue totherestricted
IMR. Compound TP and its derivatives can be readily prepared in
high yields through facile electrophilic substitutions51 (such as
bromination, acetylation, nitration, ..., etc.). With the chemical
readiness and the flexibility to prepare TP-derivatives, we thus
prepared the well-defined thermoresponsive PNIPAM with AIE-
active TP center (as Px, x = 1, 2 and 31) via the combination
of atom transfer radical polymerization (ATRP) and click
chemistry. We determined the temperature that triggers the
coil-to-globule collapse of the main chain and the following
aggregation and the thermodynamic values associated with the
conformation change during the LCST transition. DLS, TEM,
and temperature-dependent optical transmittance were em-
ployed to characterize the thermodynamic values associated with
the conformational changes.
In 2001, Tang’s groups discovered that one particular silole
molecule (1-methyl-1,2,3,4,5-pentaphenylsilole) emits strongly
in the aggregated or the solid state even though it is nonemissive
in the dilute solution.33 This interesting phenomenon was desig-
nated as “aggregation-induced emission” (AIE) to emphasize the
phenomenon that the originally nonluminescent solution of silole
can be induced to emit strongly when the corresponding nanoag-
gregates formed after introduction of the poor solvent water.
The AIE effect is rationalized as a result of restricted intramole-
cular rotation (IMR) of the phenyl peripheries against the central
silole core in the aggregate state. Since the discovery of silole
system, several AIE-active organic and polymeric materials34À52
Recently, small amounts of AIE-active tetraphenylethene (TPE)
comonomer were also incorporated into PNIPAM to study the
emission response toward LCST.45 The results indicated that
when heated above LCST, varied FL emission responses were
observed dependent on the degrees of TPE-labeling in the
PNIPAM polymers.
’ EXPERIMENTAL SECTION
Materials. Reagent grade benzyl chloride, sulfur powder, parafor-
maldehyde, hydrogen bromide solution (33% HBr solution in acetic
acid), sodium azide, propargylamine, R-bromoisobutyryl bromide,
triethyl amine, formic acid, tris(2-aminoethyl)amine, sodium hydroxide,
dichloromethane (DCM), magnesium sulfate and 2-propanol were
We had previously studied on certain AIE-active systems,50À52
among them, 2,3,4,5-tetra-phenylthiophene (TP, Scheme 1)
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purchased from Aldrich Chemical Co. and used directly without further
purification. CuBr (98%, Aldrich) was stirred overnight in acetic acid,
filtered and washed with ethanol and diethyl ether before dried in vacuo.
N-Isopropylacrylamide (NIPAM) was purchased from Acros and re-
crystallized from hexane. THF was refluxed over sodium/benzophenone
under nitrogen for more than 2À3 days before distillation to use. N,N-
Dimethylformamide (DMF) was refluxed over CaH2 under nitrogen for
5 h before distillation for use.
Syntheses of small molecules and polymer. Compound
TP53À55 and tris(2-dimethylaminoethyl)amine (Me6-TREN)56 were
prepared by the reported procedures. Other organic molecules and
polymers shown in Scheme 1 were prepared according to the detailed
procedures given below.
Preparation of 2-(4-Nitrophenyl)-3,4,5-triphenylthiophene (TPBr).
A mixture of TP (2 g, 5.15 mmol), paraformaldehyde (0.33 g, 11.0
mmol) and hydrogen bromide solution (33% HBr solution in acetic
acid, 25 mL) was heated and reacted at 90 °C for 36 h. After cooling to
room temperature, the precipitates were filtered, washed with water, and
dried in vacuo to afford pale white yellow solid. Final product (2.16 g,
73%) was obtained by recrystallization from toluene/hexane. Mp:
215 °C. IR (KBr pellet, cmÀ1): 3052, 3024, 2957, 2924, 2852, 1600,
1539, 1479, 1440, 1406, 1224, 1071, 1010, 825, 770, 701, 608 (Figure S4,
Supporting Information). 1H NMR (500 MHz, CDCl3): δ 6.77À7.21
(m, 18H, aromatic Hs), 4.36 (s, 4H, ÀCH2Br) (Figure S1, Supporting
Information): MS, m/e: calcd for C30H22Br2S, 571.98; found, 572.03
(M+). Anal. Calcd for C30H22Br2S: C, 62.73; H, 3.86; Br, 27.82; S, 5.58.
Found: C, 62.54; H, 4.05; S, 5.67.
Preparation of 3-Phenyl-5-(3,4,5-triphenylthiophen-2-yl)benzo[c]-
isoxazole (TPN3). Suspension of TPBr (1 g, 1.74 mmol) and sodium
azide (1.13 g, 17.4 mmol) in DMF (20 mL) was heated at 80 °C for 48 h.
Solvent was then removed from the reaction mixtures by vacuum
distillation at elevated temperature. The resultant solid was redissolved
in 30 mL dichloromethane and washed with 150 mL of saturated aq.
NaCl solution twice. The organic layer was then dried over MgSO4 and
concentrated to obtain 0.8 g of TPN3 (0.8 g. 92%) Mp: 229 °C. IR (KBr
pellet, cmÀ1): 3053, 3024, 2921, 2852, 2096, 1700, 1600, 1481, 1441,
1409, 1343, 1247, 1071, 820, 702 (Figure S4, Supporting Information).
1H NMR (500 MHz, CDCl3): δ 6.88À7.33 (m, 18H, aromatic Hs), 4.27
(s, 4H, ÀCH2N3) (Figure S2, Supporting Information). MS, m/e: calcd
for C30H22N6S, 498.16; found, 498.12 (M+). Anal. Calcd for C30H22N6S:
C, 72.27; H, 4.45; N, 16.86; S, 6.43. Found: C, 72.04; H, 5.22; N, 16.52;
S, 6.21.
Preparation of Propargyl 2-Bromo-2-methylpropionamide (BMP).
To an argon-blanketed solution of propargylamine (6.00 g, 5.45 mmol)
and triethylamine (11.4 mL, 8.18 mmol) in THF (300 mL) at iceÀwater
temperature, R-bromoisobutyryl bromide (12.5 g, 5.45 mmol) was
slowly added. The reaction mixtures were warmed to room temperature
and stirred overnight. The precipitate was filtered off and the solvent was
removed by vacuum distillation. The crude product was purified through
column chromatography (n-hexane/EtOAc, 4:1) to give I as a pale
yellow solid (7.84 g, 70.5%). Mp: 52 °C. IR (KBr pellet, cmÀ1): 3332,
3273, 3053, 2980, 2932, 2119, 1646, 1530, 1425, 1374, 1356, 1301, 1266,
1195, 1110, 1060, 1008, 910, 823, 697, 624, 581 (Figure S5, Supporting
Information). 1H NMR (500 MHz, CDCl3): δ 6.88 (broad, 1H, NH),
4.05 (dd, 2H, CCH2NH), 2.27 (t, 1H, CHtC), 1.98 (s, 6H, C-
(CH3)2Br) (Figure S3, Supporting Information). MS, m/e: calcd for
C7H10BrNO, 202.99; found, 202.46 (M+). Anal. Calcd for C7H10BrNO:
C, 41.20; H, 4.94; Br, 39.16; N, 6.86; O, 7.84. Found: C, 40.98; H, 5.56;
N, 6.59; O, 7.57.
Table 1. Click Reaction Results
polymer
na
nb
Mw (g/mol)c
Mn (g/mol)c (n)d
PDIc
PNIPAM1
PNIPAM1
PNIPAM1
P1
19
43
2900
6000
2600 (20)
5400 (41)
1.11
1.12
1.14
1.11
1.13
1.13
88
12800
5900
11200 (86)
5300 (40)
38
42
89
P2
85
12100
10700 (83)
P3
175
181
25500
22600 (174)
a Degree of polymerization calculated from the corresponding 1H
NMR (Figure 2). b Degree of polymerization calculated from the
calibration curve (Figure S7). c Determined by GPC (Figure 1); based
on polystyrene standard. d Degree of polymerization calculated from
Mn (GPC analysis).
times using the freezeÀpumpÀthaw cycle before vigorously stirred at
0 °C. After complete degassing, Me6TREN (23.0 mg; 0.1 mmol) was
injected into the solution via syringe. After 10 min, propargyl 2-bromo-
2-methylpropionamide (BMP; for PNIPAM1, 0.11 g (0.54 mmol);
for PNIPAM2, 55.1 mg (0.27 mmol); for PNIPAM3, 27.55 mg
(0.135 mmol)) was injected into the solution to initiate the reaction.
The mixture was at 0 °C for 48 h and then evaporated to dryness under
vacuum. The residue was diluted with tetrahydrofuran (THF) and then
passed through an alumina column to remove the copper catalyst. The
product was precipitated from diethyl ether three times and dried under
vacuum overnight at room temperature to obtain the polymer as a white
powder (PNIPAM1, 1.2 g, 89.6%). PNIPAM1: Tg = 108 °C (Figure S6,
Supporting Information); IR (KBr pellet, cmÀ1) 3447, 3297, 3083,
2974, 2935, 2877, 2134, 1650, 1547, 1459, 1389, 1367, 1171, 1131; 1H
NMR (500 MHz, CDCl3) δ 6.11À6.87 (broad, 2H, NHCO), 3.97
(broad, 1H, CH(CH3)2), 1.4À2.2 (broad, 3H, backbone Hs), 1.18
(broad, 6H, CH(CH3)2).
Preparation of Px (Click Reaction). A typical procedure for the click
reaction is as follows: DMF (15 mL) was placed in a three-neck flask and
degassed by bubbling argon for 1 h before the addition of CuBr (20 mg,
0.14 mmol), TPN3 (0.29 g, 0.59 mmol) and PNIPAM1 (0.15 g, 1.17
mmol). A solution of Me6TREN (46 mg; 0.2 mmol) in degassed DMF
(5 mL) was then introduced and the resultant transparent solution was
then heated at 80 °C under in argon atmosphere for 33 h. When the
mixture was cooled to room temperature, DMF was distilled off under
reduced pressure. The residue was diluted with tetrahydrofuran (THF)
and then passed through an alumina column to remove the copper
catalyst. The solid product was then precipitated from diethyl ether.
Further dissolution/precipitation procedures by THF/twice was re-
peated twice before vacuum drying to yield the final polymer as yellow
powder (P1, 1.23 g, 75%). P1: Tg = 138 °C (Figure S6, Supporting
Information); IR (KBr pellet, cmÀ1) 3447, 3297, 3083, 2974, 2935,
2877, 1650, 1547, 1459, 1389, 1367, 1171, 1131 (Figure S5, Supporting
Information); 1H NMR (500 MHz, CD2Cl2) δ 6.97À7.36 (broad,
aromatic Hs), 6.11À6.87 (broad, NHCO), 3.97 (broad, CH(CH3)2),
1.4À2.2 (broad, backbone Hs), 1.18 (broad, CH(CH3)2) (Figure 2).
The synthesis of P2 and P3 followed the same procedures as those
described for P1. The molecular parameters of the resultant polymers
are summarized in Table 1.
Characterization. 1H NMR spectra were recorded at various
temperatures on a Varian VXR-500 MHz instrument (resonance
frequency of 500 MHz) operated in the Fourier transform mode with
CD2Cl2, or CDCl3/D2O as the solvent. A VG Quattro GC/MS/MS/DS
instrument was used to determine the molar mass of the organic
molecules. The sample was charged into the rapidly moving gas and
converted into the corresponding ions, which were further separated
based on their mass-to-charge ratios (m/z). Molecular weight and mole-
cular weight distribution of polymers were determined from GPC using
Preparation of PNIPAM from ATRP. A 100 mL dried Schlenk flask
containing a magnetic stirrer bar was charged with CuBr (14.3 mg,
0.1 mmol) and NIPAM (1.35 g, 10.6 mmol). After filling the flask with
argon, 2-propanol (10 mL) was added and the solution was stirred for
another 10 min at room temperature. The mixture was degassed three
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Figure 1. GPC elution curves of PNIPAM1, PNIPAM2, PNIPAM3, P1,
P2, and P3 (RI detector).
Figure 2. 1H NMR spectra of P1, P2, and P3 (CD2Cl2).
a Waters 510 HPLC model equipped with a 410 differential refract-
ometer, a UV detector, and three Ultrastyragel columns (100, 500, and
1000 Å) connected in series. Polymer solution was eluted by THF with a
flow rate of 0.6 mL/min. A set of monodisperse polystyrene standards
covering molecular weight range of 103À106 g/mol was used for the
molecular weight calibration. The melting point (Mp) of the organic
molecules and the glass transition temperatures (Tg) of the polymers
were obtained from a TA Q-20 DSC calorimeter with a scan rate of
20 °C/min. FT-IR spectra were obtained on a Nicolet IR-200 spectro-
meter. Sample solution in THF was dropped on a KBr pellet and dried at
100 °C under vacuum to prepare the solid film for FT-IR analysis.
UVÀvis absorption and transmission spectra were recorded with an
Ocean Optics DT 1000 CE 376 spectrophotometer. The temperature-
variable experiment was carried out in a quartz cell with 1 cm Â1 cm
dimension after the solution was left at the preset temperature for 30 min
and the temperature was change with a step width of 1 °C. Temperature
was controlled by an HCS 302 hot-stage, upon which a Peltier cell from
OceanOptics was mounted.
Fluorescence (FL) emission spectra were obtained from a LabGuide
X350 fluorescence spectrophotometer using a 450W Xe lamp as the
continuous light source and a temperature controller. Fluorescence
quantum yields (Φf) of the polymer solutions (1 mg/mL) in solvent
mixtures of varied compositions were determined by comparison with a
quinine sulfate standard (10À5 M in 0.1 N H2SO4, Φf = 0.55). Particle
sizes of polymer aggregates in solution were measured by dynamic light
scattering (DLS) using a Brookhaven 90 plus spectrometer equipped
with a temperature controller. An argon ion laser operating at 658 nm
was used as light source. Cryo-transmission electron microscopy (cryo-
TEM) was used to determine the size of the polymer nanoparticles.
Samples werepreparedaccordingtoprotocolsreportedelsewhere57,58 ona
JEOL JEM-2100 LaB6 microscope (Cs = 2.0 mm) operating at 200 kV.
Digital images were recorded on a Gatan Ultrascan 1000 CCD camera.
reaction procedure including bromomethylation of TP to obtain
TPBr and the following diazidation of TPBr to obtain the desired
diazide compound TPN3. On the other hand, the alkynyl-
terminated PNIPAMx polymers were prepared from ATRP of
NIPAM monomer by an alkynyl-functionalized initiator of
propargyl 2-bromo-2-methylpropionamide (BMP), which was
synthesized from the reaction of propargyl amine and 2-bromo-2-
methylpropanoyl bromide. Altogether, three polymers of PNIPAMx
(x = 1À3) with different molecular weights were used in this study
to proceed click reaction with TPN3 to result in thermoresponsive
homopolymers of P1, P2 and P3 with two hydrophilic PNIPAM
chains linked to the same hydrophobic TP-center.
Chemical structures of all reaction intermediates were con-
firmed from the 1H NMR, FTIR spectra (Figure S1ÀS5,
Supporting Information) and the elemental analysis (included
in Experimental Section). Infrared spectra can be primarily used
to demonstrate the success of click reaction: the characteristic
azide ÀN3 absorption of TPN3 and the ethynyl ÀCtC band of
PNIPAM1 are completely absent in the spectra of P1 (Figure S5,
Supporting Information). The molecular weight increases from
PNIPAMx to Px were also demonstrated from the correspond-
ing GPC traces in Figure 1. Here, we observed that all Px
polymers eluted at earlier time as compared to their respective
PNIPAMx precursors. The degree of polymerization (n) calcu-
lated from the number-average molecular weight (Mn) is listed in
Table 1, which suggests the well correlations between PNIPAMx
and Px. The molecular weights can be also evaluated from the
1
corresponding H NMR spectra in Figure 2. The integration
ratios between the proton He (ÀCH(CH3)2)) and the aromatic
protons can be used to formulate n values of P1, P2, and P3. The
results in Table 1 suggest that n values of Px calculated from the
1H NMR spectra are close to the ones evaluated from Mn, which
indicated the successful TP-labeling via the efficient click reac-
tion. According to the DSC thermograms (Figure S6, Supporting
Information), glass transition temperatures (Tgs) of the PNI-
PAMx and Px polymers are in the range 108À144 °C, dependent
on the molecular weights.
’ RESULTS AND DISCUSSION
Synthesis of TP-Embedded Water-Soluble Homopoly-
mers (P1, P2, and P3). As illustrated in Scheme 1, the highly
efficient click reaction59,60 between TPN3 and PNIPAMx was
used for the construction of TP-embedded polymers of P1, P2,
and P3. The diazide-based organic compound of TPN3 was
prepared from the TP53À55 compound through the two-step
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polymer aggregates with the average diameters of 175À265 nm
were detected in the THF/hexane mixtures with various hexane
contents.
The solution FL emission spectra in Figure 4A clearly demon-
strate the operative AIE properties of P1. Upon excitation at
320 nm, solution (1 mg/mL) of P1 in pure THF emitted weakly
but with the gradual hexane inclusion in the THF/hexane solvent
mixtures, the resultant solutions significantly developed their
emission intensities. For the 90 vol % hexane solution, more than
83-fold intensity gain was achieved when compared to P1 in pure
THF. In this case, the single fluorescent TP center in each of the
polymer chain is supposed to associate with other TP fluoro-
phores intermolecularly in order to exert the enhanced emission
in the aggregated domains. The solution FL emission spectra of
P2 and P3 in THF/hexane mixtures also confirmed the existence
of AIE effect in these two polymers.
The solution fluorescent quantum yields (Φf) of P1, P2, and
P3 solutions in THF/hexane mixtures were then measured and
summarized in Figure 4B. Solutions of P1 with low hexane (<60
vol %) content have low Φfs (∼0.0012) but beyond that, Φfs
increase abruptly to reach a final value of 0.23 for the 90 vol %-
hexane solution, which is ∼190 times of that for P1 in pure THF.
The lower degree of TP-labeling in P2 and P3 resulted in the
lower Φf values in comparison to P1 solution; nevertheless,
similar emission enhancements were observed for both when
hexane content in the solutions is over 60 vol %. Under UV-light
illumination, solutions of P1 in THF/hexane mixtures showed
the progressive development on the fluorescence with the
increasing hexane content in the solution (Figure 4C).
Self-Assembly of TP-Embedded Water-Soluble Homopo-
lymers (P1, P2, and P3) in Aqueous Solution. On the basis of
chemical intuition, TP-functionalized PNIPAM should self-as-
semble into micellar structures (Scheme 2) consisting of hydro-
phobic TP cores surrounded by the hydrophilic PNIPAM coronas
in the dilute aqueous solution at room temperature. The intimate
associations of TP fluorophores in the core domain should
enhance the aggregation of the TP fluorophores and in this case,
the FL emission intensity can be alternatively promoted due to
the AIE-operative emission. Relationship between the FL emis-
sion behavior and the self-assembled morphology is therefore
evaluated below.
Self-assembly behavior of TP-functionalized PNIPAMs in
water was then investigated through their FL emission beha-
vior. Primarily, it is safe to say that the FL emissions resolved
for all Px polymers (Figure 5A) are due to the inherent TP unit
since no FL emission can be detected for the aqueous PN-
IPAM1 solution. The FL emission spectra of P1, P2, and P3
in water all resemble to those obtained in the THF/hexane
mixtures (Figure 4A) and consist of one broad emission centered
at 405 nm. The emission intensity shown in Figure 5A is in the
order of P1 > P2 > P3 and is therefore in line with the degree of
TP-labeling in Px. The maximum peak intensities at 405 nm from
other concentration ranges were monitored and were summar-
ized in Figure 5B, which shows the trend of increasing emission
intensity with the increasing polymer concentration for all three
polymer solutions. Aggregations of the AIE-active TP fluoro-
phores in the micelles are supposed to develop with increasing
polymer concentration, leading to the enhanced emissions at
higher concentrations. Primary evaluation in Figure 5B indicates
that the peak intensity is proportional to the concentration of Px
in water. In addition, the three traces in Figure 5B merge into a
single linear curve in Figure 5C if intensity was plotted against the
Figure 3. Absorption spectra of THF solutions of PNIPAM1 (1 mg/
mL), TP (10À5 M), TP (10À5 M) in PNIPAM1 (1 mg/mL), and P1
(1 mg/mL) measured at 25 °C.
The presence of TP moiety in P1, P2, and P3 can be further
verified from the UVÀvis spectroscopy. Absorption spectrum of
TP compound included in Figure 3 was used as standard to
characterize the absorption spectrum of P1 polymer. Small
molecule of TP in THF exhibits the characteristic absorption
peak at 320 nm, which is essentially absent in the absorption
spectrum of PNIPAM1. With the presence of TP fluorophore, P1
exhibits the same absorption band as mixtures of TP and
PNIPAM do. The calibration curve (Figure S7, Supporting
Information) based on the UVÀvis absorption data of TP
molecule in THF can be used to determine the degrees of TP
labeling in P1, P2, and P3. According to BeerÀLambert law, the
molar absorption coefficient (ε) from the calibration curve is
0.0927 L/mol  cm. The TP content in μmol/g of polymer can
be determined by dividing the absorption of a polymer solution
of known concentration in g/L by the molar absorption coeffi-
cient of TP in L/mol  cm. The TP content of the polymer can
then be easily converted into a degree of polymerization. In
consistent with the order of the molecular weight of the applied
PNIPAMx, the degrees of TP labeling in P1, P2, and P3 are 2.33,
1.11, and 0.55%, respectively. As illustrated in Table 1, the resultant
n values based on the UVÀvis absorption spectra are well corre-
lated with those from the 1H NMR spectra and GPC, too.
AIE Effect. The AIE-active TP center facilitate the fluores-
cence of the Px polymers in the solution aggregated state. By
adding poor solvent (hexane) to solution of P1 in good solvent
(THF), the polymer aggregates formed should have stronger
emission intensity if AIE effect comes into operation. Before-
hand, solutions of P1 in THF/hexane mixtures were vigorously
stirred to ensure uniform dispersion of the aggregates thus
formed upon hexane inclusion. All the resultant mixture solu-
tions (1 mg/mL) are macroscopically homogeneous but visually
opaque, indicating the presence of nanoaggregate in the solvent
mixtures. The resultant nanoggregates are quite stable in the
solvent mixtures without visible mass precipitations after stand-
ing for at least one month. Formations of nanoparticles can
be confirmed by the particle size measurements from DLS
(Figure S8, Supporting Information). No signal is recorded from
the polymer solution in pure THF, indicating that polymer P1 is
genuinely dissolved in the good solvent of THF. On the contrary,
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Figure 4. (A) FL emission spectra of P1 (1 mg/mL) in THF/hexane mixtures with different hexane contents (measured at 25 °C and λex = 320 nm),
(B) the relative quantum yields of Px in THF/hexane mixtures, and (C) photographs of P1 (1 mg/mL) in THF/hexane mixtures.
Scheme 2. Schematic Representation of the CoreÀShell
involved nanoparticles increases with the molecular weight of the
Micellar Structure of the TP-Embedded PNIPAM in Aqueous
Solution
polymer; that is, P3 has the highest Dh value of 43 nm in
comparison to 28 nm for P2 and 22 nm for P3. Comparative
experiment on the PNIPAM1 solution resulted in no detectable
DLS signal; therefore, formation of nanoparticles in the Px
solutions is due to the small amounts of the hydrophobic TP
core embedded in the hydrophilic PNIPAM chain segments.
Presumably, the nanoassemblies referred here are micelles with
TP core surrounded by the extended PNIPAM segments served
as the coronas (cf. the micellar structure in Scheme 2). The
preferable πÀπ interactions of the phenyl rotors of the TP
fluorophores are the driving forces contributing to the formation
of nanoaggregates of multiple-polymer chains.
Cryo-TEM was applied to examine the actual morphologies of
the TP-containing Px nanoparticles in the dilute aqueous solu-
tion with the same concentration of 1 mg/mL. The cryo-TEM
image of the aqueous P1 solution in Figure 6B revealed the
presence of spherical nanoparticles with diameters in the range of
18 À27 nm. The results are in reasonable agreement with that
determined by DLS (Figure 6A). The cryo-TEM images of P2
and P3 polymers (Figure S9A and 9B, Supporting Information)
are also consistent with the DLS results with the detected
particles’ diameters in the ranges 23À34 and 33À51 nm, respec-
tively. The observed dimensions are basically dependent on the
molecular weights of the Px samples.
Temperature-Programming Self-Assembly of TP-Em-
bedded PNIPAMs. As suggested above, TP-embedded PNI-
PAMs formed coreÀshell micelles in water at room temperature.
By heating to temperature above LCST, the originally extended
hydrophilic PNIPAM chains were converted to a more hydrophobic
TP content in the aqueous concentration; therefore, the fluo-
rescence signal of the aggregated TPs is directly related to the TP
molar concentration.
Dynamic light scattering (DLS) was then used to measure
the dimensions of the involved micelles in water. Figure 6 shows
the size distribution curves of the nanoparticles involved in the
dilute (1 mg/mL) solutions of P1, P2, and P3 at room tempera-
ture. Clearly, the average hydrodynamic diameter (Dhs) of the
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Figure 5. (A) FL emission spectra of aqueous solutions of polymers P1ÀP3. Concentration of Px: 1 mg/mL. Temperature: 25 °C. Excitation
wavelength: 320 nm. (B) FL emission intensities of P1ÀP3 at 405 nm vs solution concentration and (C) plot of peak intensity as a function of the TP
concentration in umol/L from (B). Temperature: 25 °C. Excitation wavelength: 320 nm.
Figure 6. (A) Histograms of hydrodynamic diameters of P1, P2, and P3 (1 mg/mL) in water and (B) cryo-TEM image of micellar structure of P1
in water.
chain state due to the dissociation of hydrogen-bonds and further
aggregation should proceed with the reduced hydrophilicity of
the polymer chains. At elevated temperatures above the LCST of
PNIPAM, the originally formed micelles should undergo struc-
tural transformations due to the collapse/aggregation of the
PNIPAM segments. The varied responses of the hydrophobic TP
core and the hydrophilic PNIPAM shell at high temperatures
should affect the resultant FL emission behavior and are the
subject to be discussed below. The thermoresponsive behavior
of the aqueous Px solutions can be investigated by the
combined instrumentations including temperature-dependent
FL emission spectra, optical transmittance, DLS, and 1H NMR
spectroscopy.
Aqueous solution of P1 is transparent at room temperature
(left, Figure 7A) but becomes highly opaque (right, Figure 7A)
when heated to 40 °C. This visual difference can be reversibly
manipulated by heating and cooling cycle. The variations of solution
transparency with temperature can be traced by transmittance
measurement. Here, optical transmittance at 700 nm is beyond
both the absorption and the emission ranges of the TP fluoro-
phore and was used to locate LCST without the possible inter-
ference by the UVÀvis absorption and the FL emission from the
TP core. The temperature-dependent optical transmittance of the
aqueous P1 solution was used as representative to correlate with
the average hydrodynamic diameter (Dh) measured in the same
temperature ranges (Figure 7B). The resultant transmittance
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Figure 7. (A) Photograph showing an aqueous solution of P1 at room temperature (left) and at 40 °C (right) and (B) transmittance (blue line +
symbol) and hydrodynamic diameter (orange line + symbol) vs temperature for the aqueous P1 (1 mg/mL) solution (monitored at 700 nm).
decreased when heated to above 28 °C and became nil at >32 °C.
The sudden transmittance drop in the temperature ranges of 28
and 32 °C indicates that further aggregations of the originally
formed micelles proceeded, forming large nanoparticles capable
to scatter incident light to result in opaqueness of the solutions at
high temperatures. The self-assembled aggregations above LCST
was evidenced by the corresponding Dhs measured in the same
temperature ranges: the resolved Dhs are quiet constant
(∼22 nm) below 29 °C but beyond that, the corresponding
Dhs increase drastically to the highest value of 275 nm at 33 °C
before reaching the constant value of 250 nm at 40 °C. The high
Dhs (>250 nm) detected above LCST are essentially different
from the micelle dimensions (∼22 nm) observed below LCST.
The initial micelles formed at low temperatures therefore under-
went further aggregation to form large entities when heated.
Optical transmittance and DLS were also conducted on the
aqueous P2 and P3 (Figure S10A and 10B, Supporting In-
formation) solutions in the same temperature ranges. Both of
the P2 and the P3 solutions exhibited the same responses toward
temperatures as the P1 solution behaved. The LCSTs thus
located by transmittance measurement are 29 °C for P1,
30.1 °C for P2, and 31.3 °C for P3, respectively. The introduction
of hydrophobic TP central groups should lower the miscibility of
the polymer in water as a result of unfavorable interactions
between water and the hydrophobic TP center. In addition, the
mixing entropy of the polymer chains is reduced due to the
increase of the apparent molecular weight with the introduction
of TP core. Both factors favor phase separation, so that the
resolved LCSTs tend to shift downward. The factors leading to
the early phase separation are larger in polymer with higher
hydrophobicity, which was reflected in the observed lower LCST
of the P1 polymer as compared to P2 (or P3) polymer with lower
hydrophobicity.
were summarized in Figure 8A. The initial emission intensity
resolved here follows the order of P1 > P2 > P3, which is
consistent with the degree of TP-labeling in all three polymers.
When heated between 20 to 29 °C, the aqueous P1 solution
emitted with constant intensity; however, further heating beyond
29 °C resulted in the sudden emission slump until the complete
emission quench at 34.4 °C. Similar responses were observed for
the aqueous P2 and P3 solutions. The apparent emission
reductions were also observed for the aqueous P2 and P3
solutions. The large emission reduction should be correlated
with the self-assembly aggregation process at temperatures above
LCST and can be further verified by the transmittance measure-
ments below.
Since the maximum absorbance of the polymer solutions is
located at 320 nm (cf. Figure 3), the optical transmittance at
320 nm was monitored in order to find out the plausible reason for
the emission quench at high temperatures. The resultant trans-
mittances of P1, P2, and P3 measured in between 20 to 40 °C were
summarized in Figure 8B. As expected, the initial optical
transmittance at 20 °C follows the same order of P3 > P2 >
P1, which is consistent with the TP content in the polymers and
suggests that the TP-core is the real absorber for the 320 nm light.
When heated above 30 °C, the transmittance slumped abruptly
and reached almost zero before 34 °C. The apparent reduction
on the transmittance at 320 nm is due to the onset of efficient Mie
scattering caused by the large aggregates formed at high tem-
peratures. As suggested by Figure 7B (and Figure S10), further
aggregation of the originally formed micelles resulted in large
aggregates with the average Dhs > 250 nm, which happen to be
the effective dimensions capable to cause serious Mie scattering
on the incident 320 nm light.
To collect more information about chain conformation of Px
1
and to gain more insight into the thermal transitions, the H
Emission of the AIE-active TP core in Px is considered to
correlate with the extent of aggregation in the nanoparticles
formed in the solutions. Therefore, the temperature-program-
ming emission study should provide valuable information on
the structural variations during heating. The maximum emiss-
ion peak intensity of P1, P2 and P3 solutions at 405 nm was
measured at varied temperatures from 20 to 43 °C and the results
NMR spectra are measured in solvents of different affinities and
at different temperatures. When in good solvent of CD2Cl2, the
1H NMR spectrum (Figure 9A) of P1 polymer contains reso-
nance signals of all the protons in the TP fluorophore and the
PNIPAM chain. In contrast, we failed to observe the aromatic
proton (as Hjs) signals of the TP unit of P1 in D2O (Figure 9B).
In D2O, the hydrophobic TP moieties self-assemble to constitute
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Figure 8. Effects of temperature on (A) the FL emission intensity (excitation wavelength =320 nm) and (B) the optical transmittance of the aqueous
P1, P2 and P3 (1 mg/mL) solutions (monitored at 320 nm).
Figure 9. 1H NMR spectra of P1 in (A) CD2Cl2 at 25 °C and (BÀF) in D2O at various temperatures.
the central cores of the micelles. The molecular motions of the
TP units in the aggregated core are so restricted that their
motions can no longer be detected by the 1H NMR in this case.
The congested environment in the aggregated core deactivates
nonradiative decay channels leading to energy loss, which
explains the observed strong FL emission for the dilute aqueous
P1 solution (Figure 5A).
within the large nanaparticles are supposed to be largely re-
stricted. As indicated by Figure 9BÀF, the molecular rotations of
the main (protons Ha and Hb) and the side (protons He) chain
segments of the PNIPAM are so highly hampered that they
gradually lost the resonance resolutions with increasing tem-
peratures. Same experimental results were observed in the
1
temperature-programming H NMR spectra (Figure S11, Sup-
In the temperature ranges of 28 and 34 °C, dissociations of
hydrogen-bond interactions result in dehydration and the fol-
lowing phase separation to form compact aggregates, which can
porting Information) of P2 and P3 in D2O. Again, we failed to
observe the aromatic proton signals in the high temperature
ranges. At high temperature, PNIPAM is not soluble in water
and it interacts more strongly with itself and the TP unit,
resulting in a breakup of the TP aggregates. The TP unit was
attached to PNIPAM segment whose motion was strongly
1
be detected by the H NMR spectra recorded at temperatures
above LCST (Figure 9BÀF). At high temperatures, the molec-
ular motions of the PNIPAM chains and the TP fluorophores
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restricted, therefore, the motion of the isolated TPs was also
restricted and their signal was not seen by the H NMR. The
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molecular motions of the PNIPAM and the TP units are highly
restricted; however, the breakup of the TP aggregates resulted in the
disappearance of the AIE-oriented emission at high temperature.
’ CONCLUSIONS
(11) Zhou, K.; Lu, Y.; Li, J.; Shen, L.; Zhang, G.; Xie, Z.; Wu, C.
Thermoresponsive water-soluble PNIPAMs containing single
AIE-active TP center were prepared via the combination of atom
transfer radical polymerization (ATRP) and click chemistry.
Alkynyl-terminated PNIPAMx and diazide-functinalized TPN3
precursor were prepared first. The target polymers Px were then
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resulted in the dissociations of the TP aggregates and the
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Supporting Information. All supporting data including
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the sample characterizations (1H NMR, FTIR spectra, DSC
thermogram), calibration curve, cryo-TEM images, DLS, trans-
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’ AUTHOR INFORMATION
Corresponding Author
*E-mail: jlhong@mail.nsysu.edu.tw. Telephone: +886-7-5252000
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We appreciate the financial support from the National Science
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