Luminescent amphiphilic dendrimers with oligo(p-phenylene vinylene) core branches and oligo(ethylene oxide) terminal chains: Syntheses and stimuli-responsive properties

Article abstract of DOI:10.1039/b613278b

A class of new dendrimers consisting of hydrophobic oligo(p-phenylene vinylene) core branches and hydrophilic oligo(ethylene oxide) terminal chains was synthesized. These amphiphilic dendritic molecules are highly luminescent and exhibit critical micellization behaviors. They also show a lower critical solution temperature (LCST) in an aqueous medium. Both the critical micelle concentration (CMC) and LCST increased with increasing branch number and the ratio of the hydrophilic oligo(ethylene oxide) to hydrophobic oligo(p-phenylene vinylene) moieties. Besides, a temperature-dependent phase transition from a clear to cloudy aqueous solution was observed. The temperature-induced phase transition was also reflected by fluorescence quenching, ¹H NMR resonance peak broadening, and UV-vis absorption shift arising from the hydrophobic conjugated core. These changes in the phase structure and photophysical properties were demonstrated to be highly reversible, indicating some interesting stimuli-responsive behaviors. The Royal Society of Chemistry.

Full text of DOI:10.1039/b613278b

View Article Online / Journal Homepage / Table of Contents for this issue
PAPER
www.rsc.org/materials | Journal of Materials Chemistry
Luminescent amphiphilic dendrimers with oligo(p-phenylene vinylene)
core branches and oligo(ethylene oxide) terminal chains: syntheses and
stimuli-responsive properties{
Dong Wook Chang and Liming Dai*
Received 13th September 2006, Accepted 20th October 2006
First published as an Advance Article on the web 14th November 2006
DOI: 10.1039/b613278b
A class of new dendrimers consisting of hydrophobic oligo(p-phenylene vinylene) core
branches and hydrophilic oligo(ethylene oxide) terminal chains was synthesized. These
amphiphilic dendritic molecules are highly luminescent and exhibit critical micellization
behaviors. They also show a lower critical solution temperature (LCST) in an aqueous medium.
Both the critical micelle concentration (CMC) and LCST increased with increasing branch
number and the ratio of the hydrophilic oligo(ethylene oxide) to hydrophobic oligo(p-phenylene
vinylene) moieties. Besides, a temperature-dependent phase transition from a clear to cloudy
aqueous solution was observed. The temperature-induced phase transition was also reflected by
1
fluorescence quenching, H NMR resonance peak broadening, and UV–vis absorption shift
arising from the hydrophobic conjugated core. These changes in the phase structure and
photophysical properties were demonstrated to be highly reversible, indicating some interesting
stimuli-responsive behaviors.
instance,
a micellization–dissociation transition can be
Introduction
induced by concentration changes around the critical micelle
concentration (CMC).4,5
A stimuli-responsive molecule is one that changes its physical
structure and/or property in a controllable, reproducible, and
reversible manner in response to an external stimulus (e.g.
temperature, pH).1 These stimuli-responsive changes can be
utilized to create a large variety of smart devices, including
sensors, actuators, and controlled release systems, for various
practical applications.1 As illustrated by the temperature-
induced helix–coil transition of polypeptides,2 biomacro-
molecules can respond to small alterations in an external
variable with very dramatic conformational/property
changes. Although stimuli-responsive biomacromolecules
have attracted a great deal of interest for many years, it is
the recent effort to exploit synthetic stimuli-responsive
(macro)molecules.1 Among them, amphiphilic diblock
copolymers, such as poly(N-isopropylacrylamide)-block-poly-
(ethylene oxide), PNIPAM-b-PEO, can self-assemble into
shell–core architectures by changing temperature, solvent
composition, ionic strength, and/or pH.1 While both
PNIPAM and PEO blocks are hydrophilic and dissolve in
water at room temperature, PNIPAM becomes hydrophobic
and undergoes a ‘‘coil-to-globule’’ transition at temperatures
higher than 32 uC, leading to the formation of self-assembled
shell–core polymer nanoparticles consisting of a PNIPAM
core and a PEO shell.3 Apart from the temperature-induced
inter- or intra-molecular structure/phase changes, similar
changes can also occur in response to other stimuli. For
Dendrimers with a tree-like, monodispersed, three-dimen-
sional globular architecture5,6 would provide additional
advantages for constructing stimuli-responsive molecular
architectures.6–10 Just like certain stimuli-responsive amphi-
philic diblock copolymers, the amphiphilic dendritric mole-
cules with hydrophobic oligo(p-phenylene vinylene), OPV,
core branches and hydrophilic oligo(ethylene oxide), OEO,
terminal chains synthesized in the present study (Fig. 1) are
potential responsive materials in response to concentration
and/or temperature changes. Indeed, we found these amphi-
philic dendrimers showed a lower critical solution temperature
(LCST) and critical micellization behavior. As a consequence,
temperature- and concentration-responsive photophysical
property changes (e.g. UV–vis, PL) were observed for these
conjugated dendritic molecules. In this paper, we report the
synthesis and stimuli-responsive properties of these new
amphiphilic dendritic molecules (designated as: Den 20,
Den 30 and Den 40 in Fig. 1).
Experimental
Materials and characterization
3,5-Dihydroxy benzaldehyde was used as received from
Alfa Aesar. Tetrahydrofuran (THF) and dimethylformamide
(DMF) from Aldrich were purified, according to the
standard procedure.11 Mesitylene, 1,2,4,5-tetramethyl benzene,
N-bromosuccinimide, azobisisobutyronitrile (AIBN), diethyl
azodicarboxylate (DEAD), tri(ethylene glycol) monomethyl
ether, triethyl phosphite, tetrachloromethane, and analytical
grade p-xylene were all used as received from Aldrich.
Departments of Chemical and Materials Engineering and Chemistry,
School of Engineering and UDRI, University of Dayton, 300 College
Park, Dayton, OH 45469, USA. E-mail: ldai@udayton.edu
{ The HTML version of this article has been enhanced with colour
images.
364 | J. Mater. Chem., 2007, 17, 364–371
This journal is ß The Royal Society of Chemistry 2007

View Article Online
Fig. 1 Molecular structures for amphiphilic dendrimers with oligo(p-phenylene vinylene) core branches and oligo(ethylene oxide) terminal chains.
Deuterium oxide containing 0.05 wt% 3-(trimethylsilyl)-1-
propanesulfonic acid sodium salt as reference for NMR studies
was obtained from Aldrich. 1H and 13C NMR spectra were
recorded on a Bruker Avance 300 spectrometer at 300 MHz.
UV–vis spectra were recorded on a Perkin-Elmer Lambda
900 UV/VIS/NIR spectrometer, while photoluminescence
emissions were measured on a Perkin-Elmer LS 55 spectro-
meter. An external circulation (Lauda RE 206) bath was used
to control temperatures (¡0.02 uC) for both the UV/VIS/NIR
and PL spectrometers. All the optical measurements were
carried out with the samples under argon and protected
from light as much as possible to avoid unnecessary photo-
oxidation. Surface tensions and the critical micelle concentra-
tion (CMC) were measured using a dataphysics DCAT 11
tensiometer at 25 uC with a Du-Nou¨y-ring. Atomic force
FT-IR (KBr, cm21): 2983, 2909, 1514, 1479, 1444, 1393, 1368,
1246, 1163, 1097, 1027, 965, 864.
1,3,5-Tris(diethoxyphosphinylmethyl)benzene (2)12,13. A mix-
ture of mesitylene (6.00 g, 49.92 mmol), N-bromosuccinimide
(26.95 g, 149.75 mmol) and a catalytic amount of AIBN
(20 mg) in dry tetrachloromethane (300 ml) was refluxed
overnight. After cooling, succinimide was removed by filtra-
tion and the organic phase was washed with water. Then,
the organic solvent was removed by rotary evaporation.
Subsequent recrystallization in petroleum ether led to a white
crystalline tribromo-compound (8.91 g, 24.96 mmol), which
was heated, together with an excess quantity of triethyl
phosphite (29.88 g, 240 mmol), for 5 h at 160 uC. Finally,
the excess triethyl phosphite was removed by distillation to
yield 2 as a yellowish viscous oil. (11.87 g, 45%). 1H NMR
(300 MHz, CDCl3, ppm) d: 1.23–1.28 (t, J = 7.1 Hz, 18H),
3.08–3.15 (d, J = 22.0 Hz, 6H), 3.97–4.05 (m, 12H), 7.14 (d, J =
2.5 Hz, 3H). 13C NMR (300 MHz, CDCl3) d: 132.0, 129.6,
61.9, 34.0, 32.2, 16.1. FT-IR (KBr, cm21): 2983, 2910, 1603,
1479, 1458, 1369, 1248, 1164, 1098, 1027, 963, 887, 783.
microscopic (AFM) images were recorded by
a silicon
cantilever in a tapping-mode on a Dimension 3100 AFM with
a Nanoscope 5 controller. Elemental analyses were conducted
by Atlantic Microlab Inc.
Syntheses
1,4-Bis(diethoxyphosphinylmethyl)benzene (1)12. A mixture
of p-xylene (3.00 g, 28.26 mmol), N-bromosuccinimide (10.17 g,
56.50 mmol) and a catalytic amount of AIBN (10 mg) in dry
tetrachloromethane (200 ml) was refluxed overnight. After
cooling, succinimide was removed by filtration and the organic
phase was washed with water. Then, the organic solvent was
removed by rotary evaporation. Subsequent recrystallization
in methanol led to a white crystalline dibromo-compound
(4.33 g, 16.39 mmol), which was heated, together with an
excess quantity of triethyl phosphite (15.36 g, 92.54 mmol), for
5 h at 160 uC. Finally, the excess triethyl phosphite was
removed by distillation to yield 1 as a white crystalline solid
(5.24 g, 49%). Mp: 74–75 uC,12 1H NMR (300 MHz, CDCl3,
ppm) d: 1.22–1.27 (t, J = 7.1 Hz, 12H), 3.10–3.17 (d, J =
20.2 Hz, 4H), 3.99–4.04 (m, 8H), 7.26 (s, 4H). 13C NMR
(300 MHz, CDCl3, ppm) d: 130.0, 129.8, 62.0, 34.1, 32.3, 16.2.
1,2,4,5-Tetrakis(diethoxyphosphinylmethyl)benzene (3)14. A
mixture of 1,2,4,5-tetrabromomethane (5.00 g, 37.25 mmol),
N-bromosuccinimide (26.82 g, 149 mmol) and a catalytic
amount of AIBN (15 mg) in dry tetrachloromethane (300 ml)
was refluxed overnight. After cooling, succinimide was
removed by filtration and the organic phase was washed with
water. Then, the organic solvent was removed by rotary
evaporation. Subsequent recrystallization in methanol led to
white crystalline tetrabromo-compound (7.54 g, 16.76 mmol),
which was heated, together with an excess quantity of triethyl
phosphate (38.21 g, 230 mmol), for 5 h at 160 uC. Finally, the
excess triethyl phosphite was removed by distillation to yield 3
1
as a colorless oil (9.35 g, 37%). H NMR (300 MHz, CDCl3,
ppm) d: 1.22–1.27 (t, J = 7.2 Hz, 24H), 3.36–3.40 (d, J =
20.0 Hz, 8H), 3.96–4.04 (m, 16H), 7.17 (s, 2H). 13C NMR
(300 MHz, CDCl3) d: 134.4, 129.6, 62.0, 31.5, 29.7, 16.3.
This journal is ß The Royal Society of Chemistry 2007
J. Mater. Chem., 2007, 17, 364–371 | 365

View Article Online
FT-IR (KBr, cm21): 2983, 2923, 1509, 1476, 1443, 1393, 1237,
1163, 1024, 964, 848, 792.
124.1, 105.5, 101.4, 71.9, 70.8, 70.6, 70.5, 70.0, 67.5, 59.0. FT-
IR (KBr, cm21): 2874, 1591, 1449, 1350, 1292, 1246, 1108, 968,
850. MALDI-TOF MS 1379.80 m/z [M + Na]+ (theory:
1356.72 m/z [M]+). Elemental analysis: calcd for C72H108O24, C
63.7, H 8.02, O 28.28; found, C 63.4, H 8.17, O 28.33%.
3,5-Di(methyltriglycoloxy)benzaldehyde (4)15. Diethyl azodi-
carboxylate (DEAD) (1.27 g, 7.06 mmol) in 5 ml dry THF was
added dropwise to the mixture of 3,5-dihydroxybenzaldehyde
(0.498 g, 3.53 mmol), tri(ethylene glycol) monomethyl ether
(1.22 g, 7.06 mmol) and triphenyl phosphine (1.76 g,
7.06 mmol) in 30 ml dry THF. The solution was stirred in
the dark at room temperature for 24 hours. After evaporation
of the solvent, the reaction mixture was purified by silica gel
column chromatography using ether–acetone (80 : 20) to yield
4 as a colorless oil (0.90 g, 59%). 1H NMR (300 MHz, CDCl3,
ppm) d: 3.38 (s, 6H), 3.56 (m, 4H), 3.67–3.75 (m, 12H), 3.87
(m, 4H), 4.16 (m, 4H), 6.76 (t, J = 2.4 Hz, 1H), 7.02 (d, J =
2.3 Hz, 2H), 9.88 (s, 1H). 13C NMR (300 MHz, CDCl3) d:
191.2, 160.2, 138.3, 107.9, 71.7, 70.6, 70.3, 70.1, 69.3, 67.6,
58.7. FT-IR (KBr, cm21): 2877, 1698, 1594, 1449, 1350, 1298,
1249, 1108, 947, 850.
1,2,4,5-Tetrakis(3,5-bis{2-[2-(2-methoxyethoxy)ethoxy]-
ethoxy}styryl)benzene (Den 40). 3 (0.40 g, 0.59 mmol) was
dissolved in 5 ml of dry DMF under high purity argon. Four
equivalents of NaH (60% dispersion in mineral oil) (93.80 mg,
2.35 mmol) and the aldehyde 4 (1.00 g, 2.35 mmol) were added
consecutively. The reaction mixture was refluxed for 1 hour.
After cooling to room temperature, it was neutralized by 1 N
HCl and extracted with CHCl3. The organic layer was washed
with brine and dried over MgSO4. The solvent was removed
by evaporation, and the residue was subjected to column
chromatography using ethyl acetate–methanol (80 : 20) to
produce Den 40 as a yellow viscous oil (0.39 g, 37%). 1H NMR
(300 MHz, CDCl3, ppm) d: 3.37 (s, 24H), 3.54 (m, 16H), 3.36–
3.76 (m, 48H), 3.86 (t, J = 4.8 Hz, 16H), 4.15 (t, J = 4.8 Hz,
16H), 6.46 (t, J = 2.2 Hz, 4H), 6.71 (d, J = 2.1 Hz, 8H), 7.00 (d,
J = 16.0 Hz, 4H), 7.39 (d, J = 16.0 Hz, 4H), 7.72 (s, 2H). 13C
NMR (300 MHz, CDCl3, ppm) d: 160.1, 139.3, 135.6, 131.7,
126.8, 105.8, 101.4, 71.9, 70.8, 70.6, 70.5, 69.7, 67.5, 59.0. FT-
IR (KBr, cm21): 2875, 1589, 1447, 1350, 1295, 1248, 1108, 960,
848. MALDI-TOF MS 1806.03 m/z [M + Na]+ (theory:
1783.95 m/z [M]+). Elemental analysis: calcd for C94H142O32, C
63.28, H 8.02, O 28.70; found, C 62.95, H 8.01, O 29.06%.
1,4-Bis(3,5-bis{2-[2-(2-methoxyethoxy)ethoxy]ethoxy}styryl)-
benzene (Den 20). 1 (0.14 g, 0.38 mmol) was dissolved in 5 ml
of dry DMF under high purity argon. Two equivalents of NaH
(60% dispersion in mineral oil) (30.00 mg, 0.76 mmol) and
3,5-di(methyltriglycoloxy)benzaldehyde 4 (0.33 g, 0.76 mmol)
were added consecutively. The reaction mixture was then
refluxed for 1 hour. After cooling to room temperature, it was
neutralized by 1 N HCl and extracted with CHCl3. The
organic layer was then washed with brine and dried over
MgSO4. After solvent evaporation, the residue was subjected
to column chromatography using ethyl acetate–methanol
(80 : 20) to produce Den 20 as a yellow viscous oil (0.19 g,
55%). 1H NMR (300 MHz, CDCl3, ppm) d: 3.38 (s, 12H), 3.56
(m, 8H), 3.64–3.78 (m, 24H), 3.87 (t, J = 4.8 Hz, 8H), 4.16 (t,
J = 4.8 Hz, 8H), 6.43 (t, J = 2.1 Hz, 2H), 6.69 (d, J = 2.1 Hz,
4H), 7.05 (d, J = 2.8 Hz, 4H), 7.49 (s, 4H). 13C NMR
(300 MHz, CDCl3, ppm) d: 160.0, 139.2, 136.5, 128.7, 128.5,
126.9, 105.5, 101.2, 71.9, 70.8, 70.5, 70.4, 69.7, 67.4, 59.0. FT-
IR (KBr, cm21): 2874, 1588, 1448, 1350, 1297, 1246, 1108, 960,
845. MALDI-TOF MS 953.60 m/z [M + Na]+ (theory: 930.50
m/z [M]+). Elemental analysis: calcd for C50H74O16, C 64.5, H
8.01, O 27.49; found, C 64.21 H 8.02 O 27.88%.
Results and discussion
Syntheses
As can be seen in Scheme 1, hydrophilic tri(ethylene oxide)
chains were introduced onto the benzaldehyde unit via the
Mitsunobu reaction using DEAD and triphenyl phospine.
For the syntheses of the dendritic cores (1–3, Scheme 1),
Wohl–Ziegler bromination of hydrocarbons and Arbusov
rearrangement reactions were performed (Scheme 1). Finally,
the Horner–Wadsworth–Emmons coupling reaction was used
to attach the aldehyde-functionalized pheriphery 4 onto the
core structures (1, 2, and 3), resulting in the formation of
Den 20, Den 30 and Den 40 (Scheme 1). These newly-
synthesized dendritic molecules showed strong fluorescent
emission which originated from the OPV core branches whilst
their oligo(ethylene oxide) terminal chains impart a good
solubility for the whole dendritic molecules in water and other
common organic solvents (e.g. ethanol, chloroform, THF).
1,3,5-Tris(3,5-bis{2[2-(2-methoxyethoxy)ethoxy]ethoxy}styr-
yl)benzene (Den 30). 2 (0.20 g, 0.40 mmol) was dissolved in 5 ml
of dry DMF under high purity argon. Three equivalents of
NaH (60% dispersion in mineral oil) (45.40 mg, 1.14 mmol)
and the aldehyde 4 (0.50 g, 1.10 mmol) were added con-
secutively. The reaction mixture was refluxed for 1 hour. After
cooling to room temperature, it was neutralized by 1 N HCl
and extracted with CHCl3. The organic layer was then washed
with brine and dried over MgSO4. After solvent evaporation,
the residue was subjected to column chromatography using
ethyl acetate–methanol (80 : 20) to produce Den 30 as a yellow
NMR studies
The good solubility of the resultant amphiphilic dendrimers
with oligo(p-phenylene vinylene) core branches and
oligo(ethylene oxide) terminal chains facilitates their charac-
terization by 1H NMR and 13C NMR measurements. As
expected, 4 exhibited 1H and 13C peaks at 9.88 and 191.2 ppm,
respectively, arising from proton and carbon atoms in the
aldehyde group. These magnetic resonances disappeared upon
successful coupling of the aldehyde focal point in 4 with the
phosphonate core reagents (1–3).
1
viscous oil (0.23 g, 44%). H NMR (300 MHz, CDCl3) d: 3.39
(s, 18H), 3.56 (m, 12H), 3.64–3.79 (m, 36H), 3.88 (t, J = 4.8 Hz,
12H), 4.17 (t, J = 4.8 Hz, 12H), 6.45 (t, J = 2.2 Hz, 3H), 6.73
(d, J = 2.1 Hz, 6H), 7.10 (s, 6H), 7.53 (s, 3H). 13C NMR
(300 MHz, CDCl3, ppm) d: 160.1, 139.1, 137.8, 129.3, 128.7,
366 | J. Mater. Chem., 2007, 17, 364–371
This journal is ß The Royal Society of Chemistry 2007

View Article Online
Scheme 1 Procedures for the synthesis of amphiphilic dendrimers with oligo(p-phenylene vinylene) core units and oligo(ethylene oxide)
terminal chains.
1H-NMR spectra of Den 30 in CDCl3 and D2O are shown in
Fig. 2. Peaks corresponding to all protons in the molecular
structure of Den 30 are seen in Fig. 2a as CDCl3 is a good
solvent for both the oligo(p-phenylene vinylene) core and the
oligo(ethylene oxide) terminal chains. In contrast, the corre-
sponding 1H-NMR spectrum in D2O (Fig. 2b) shows broad
and featureless magnetic resonances due to restricted chain
mobilities caused by the poor solubility of the hydrophobic
core in water. Similar NMR spectra were observed for Den 20
and Den 40. These results indicate possible micellization of the
amphiphilic dendrimers in water to minimize interactions
between the hydrophobic core and a polar aqueous medium.
Temperature dependent NMR measurements in D2O (Fig. 3)
added substance to the above conclusion. As can be seen in
Fig. 3, the resonance intensities of aromatic core protons over
6–7 ppm gradually decreased with increasing temperature. At
high temperatures, the OEO terminal chains became less
soluble in water, causing the hydrophobic OPV core branches
to be surrounded by a more rigid shell so that the relaxation
time of the aromatic core protons decreased significantly to
diminish the resonance signals from the core units.16 This
result is consistent with other thermo-responsive materials
like poly-N-isopropylacrylamide (PNIPAM) that shows
similar LCST behavior.16,17 Similar results were observed for
Den 20 and Den 40.
Temperature-induced phase transition
Closely related to the NMR results, a temperature-induced
transition from a clear solution to a turbid one was observed
for all of the amphiphilic dendrimers in an aqueous medium
at a concentration above the critical micelle concentration
(CMC), as exemplified by Fig. 4 for an aqueous solution of
Den 30 upon changing the solution temperature from 25 to
75 uC. The CMC values for Den 20, Den 30, and Den 40
were measured, by the surface tension method with a platinum
ring, to be 0.56, 1.37, and 3.81 mg L21, respectively. These
CMC values are consistent with those reported for some other
amphiphilic dendritic macromolecules and copolymers,18,19
though they are much lower than those of certain low
This journal is ß The Royal Society of Chemistry 2007
J. Mater. Chem., 2007, 17, 364–371 | 367

View Article Online
Fig. 4 Temperature-induced reversible micellization behavior of
Den 30 in water at 25 uC (left) and at 75 uC (right). The concentration
Fig. 2 1H NMR spectra of Den 30 in (a) CDCl3 and (b) D2O at 298 K.
Inset in (a) shows the chemical unit structure of Den 30. The
is 2.7 g L21
.
concentration of Den 30 is 2.7 g L21
.
scattering. Den 20 and Den 40 exhibited similar temperature-
induced phase transition behaviors with a slightly varied
LCST, depending on their chemical structure. In all cases, the
transition is very fast (within seconds) and highly reversible.
The LCSTs (or cloud points) for all of the amphiphilic
dendrimers were determined on the Lambda 900 UV/VIS/NIR
spectrometer as the temperature at which optical transmittance
of the dendrimer solution sharply reduced (below 1% of the
original value). The optical transmittance at 600 nm was used
for the cloud point measurements because the transmittance
change at this wavelength was clearly noticeable. As can be
seen in Fig. 5a, the cloud points of Den 20, Den 30, and Den 40
at a constant molar concentration of 15.57 mM are 23.7 uC,
32.2 uC and 49.5 uC, respectively. The observed increase in the
cloud points along the series Den 20, Den 30, and Den 40
indicates, once again, a better solvency for the amphiphilic
dendrimers with a higher ratio of hydrophilic OEO/hydro-
phobic OPV moieties. In comparison with poly(ethylene
oxide)-poly (propylene oxide) block copolymers, these newly-
synthesized amphiphilic dendrimers showed lower phase
transition temperatures with a stronger temperature depen-
dence due to the shorter oligo(ethylene oxide) terminal
chains.22,23 The concentration-dependence given in Fig. 5b
shows that the solution concentration also affects the phase
transition temperature of the amphiphilic dendrimers in an
aqueous medium, which decreased with increasing solution
concentration.
Fig. 3 1H NMR spectra of Den 30 at (a) 300 K, (b) 303 K, (c) 306 K,
and (d) 309 K, respectively, in D2O at 2.7 g L21. Numbers in
the figure represent the apparent integration ratios between the
aromatic (6–7 ppm) and aliphatic (3–4 ppm) protons for each of the
temperatures.
molecular weight surfactants (e.g. 2.30 g L21 for sodium
dodecyl sulfate, SDS).18 All of the CMCs increased with
increasing branch number and the ratio of the hydrophilic
OEO/hydrophobic OPV moieties in the series from Den 20
through Den 30 to Den 40.
The temperature-induced phase transition seen in Fig. 4 is
attributable to the fact that the oligo(ethylene oxide) terminal
chains undergo a polar–nonpolar transition20,21 at the lower
critical solution temperature (LCST) of 32.2 uC (vide infra).
The interaction between the polar oligo(ethylene oxide)
terminal chains and water molecules is favorable below the
LCST, whereas the oligo(ethylene oxide) terminal chains
become non-polar above the LCST due to their conforma-
tional changes with temperature.21 At high temperatures,
therefore, water is a poor solvent for the oligo(ethylene oxide)
terminal chains and micellization occurred to cause light
Photophysical properties
The above temperature-dependent phase behavior, together
with interesting optoelectronic properties intrinsically
associated with the oligo(p-phenylene vinylene)1 cores in
these amphiphilic dendrimers, prompted us to investigate the
possible stimuli-responsive photophyscial properties of Den 20,
Den 30, and Den 40. Fig. 6a shows optical absorption spectra
for aqueous solutions of Den 20, Den 30, and Den 40 above the
368 | J. Mater. Chem., 2007, 17, 364–371
This journal is ß The Royal Society of Chemistry 2007

View Article Online
Fig. 5 (a). Temperature-dependence of light transmittance for aqueous solutions of Den 20, Den 30, and Den 40 with a molar concentration of
15.57 mM. (b) Concentration-dependence of the LCSTs for aqueous solutions of Den 20, Den 30, and Den 40.
Fig. 6 Temperature-dependent (a) UV–vis and (b) PL spectra of: Den 20, Den 30, and Den 40 in water at 20 uC (solid line) and 75 uC (dotted line).
Excitation wavelength: 310 nm. The concentrations are 20 mM and 0.003 mM for UV–vis and PL, respectively—note that a higher concentration
was used for the UV–vis measurements because of the much lower sensitivity for the UV–vis signal than for that of PL.
CMC at 20 and 75 uC. As can be seen in Fig. 6a, all of the
dendrimer solutions showed a relatively well-defined optical
absorption band over 300–400 nm below their respective
phase transition temperature at 20 uC. Above the phase
transition temperature, however, all the optical absorption
bands red-shifted and became broad, most probably
indicating a micellization-induced increase in the effective
conjugation length through intermolecular overlapping of
the oligo(p-phenylene vinylene) cores. This indication was
supported by the corresponding intermolecular fluorescence
quenching with increasing temperature, as shown in Fig. 6b,
though the observed fluorescence quenching could also be
partially caused by light scattering associated with the micellar
structures.
This journal is ß The Royal Society of Chemistry 2007
J. Mater. Chem., 2007, 17, 364–371 | 369

View Article Online
Fig. 7 Temperature-dependent PL spectra of Den 30 in an aqueous medium: around CMC (1.8 mg L21): (a) heating, (b) cooling.
We have also investigated the reversibility of the tempera-
ture-induced changes in the photoluminescence emissions for
Den 20, Den 30, and Den 40. As expected, there was a
monotonic decrease in PL intensity by heating up an aqueous
solution of Den 30 above the CMC (Fig. 7a). The spectro-
scopic changes shown in Fig. 7a are fully reversible upon
cooling the same Den 30 solution (Fig. 7b). The changes seen
in Fig. 7a and b can be reproduced with many heating–cooling
cycles. Den 20 and Den 40 showed similar temperature-
responsive PL intensity changes.
one macromolecule. Due to the amphiphilic properties arising
from the large difference in hydrophobicity/hydrophilicity
between the two constituent components, unique tempera-
ture-/concentration-responsive phase transition and photo-
physical property changes were observed. These amphiphilic
dendrimers exhibited a lower critical solution temperature
(LCST) and the critical micellization (CMC) behavior, as
evidenced by UV–vis, PL spectroscopic and 1H NMR
measurements. AFM images showed spherical aggregates
and featureless surface for solution-cast thin films from
aqueous solutions of the amphiphilic dendrimers with
solution concentrations above and below CMC, respectively.
These highly fluorescent, stimuli-responsive amphiphilic
dendrimers possess unusual phase structures and photo-
physical properties which are attractive for a wide range of
potential applications, including sensing, photoswitching, and
controlled drug delivery systems.
AFM studies
The concentration-dependent micellization of the amphiphilic
dendrimers was further investigated by AFM imaging. As seen
in Fig. 8a, spherical micelles about 80 nm in diameter with a
well-defined structure formed from the aqueous solution of
Den 30 above the CMC after air-drying. In contrast, there is no
detectable aggregated structure observable for the correspond-
ing sample below the CMC (Fig. 8b).
Acknowledgements
This work was supported by AFOSR (FA9550-06-1-0384),
WBI, AFRL/ML. The authors thank Dr Xiaojun Li and
Dr Qiwen Zhang for their help with AFM measurements.
References
1 L. Dai, Intelligent Macromolecules for Smart Device: From
Materials Synthesis to Device Applications, Springer-Verlag,
Berlin, 2004.
2 C. R. Cantor and P. R. Schimmel, Biophysical Chemistry,
W.H. Freeman and Company, New York, 1980.
3 H. G. Schild, Prog. Polym. Sci., 1992, 17, 163.
4 C. Tanford, The hydrophobic effect: formation of micelles and
biological membranes, Wiley, New York, 1973.
5 F. Zeng and S. C. Zimmerman, Chem. Rev., 1997, 97, 1681.
6 G. R. Newkome, G. R. Baker, M. J. Saunders, P. S. Russo,
V. K. Gupta, Z. Q. Yao, J. E. Miller and K. Bouillion, J. Chem.
Soc., Chem. Commun., 1986, 752.
Fig. 8 AFM images for samples from aqueous solutions of Den 30
with different concentrations (a) above CMC (1.88 g L21) and (b)
below CMC (0.5 mg L21). Scanning area: 10 mm 6 10 mm.
Conclusions
7 (a) A. P. H. J. Schennig, C. E. Roman, J. W. Weener,
M. W. P. L. Baars, S. J. Gaast and E. W. Meijer, J. Am. Chem.
Soc., 1998, 120, 8199; (b) T. Aathimanikandan, E. N. Savariar and
S. Thayumanavan, J. Am. Chem. Soc., 2005, 127, 14922.
8 B. K. Cho, A. Jain, J. Nieberle, S. Mahajan, U. Wiesner,
S. M. Gruner, S. Turk and H. J. Rader, Macromolecules, 2004,
37, 4227.
We have synthesized a class of new amphiphilic dendrimers
consisting of oligo(p-phenylene vinylene) core branches and
oligo(ethylene oxide) terminal chains. These dendrimers
combine the hydrophobic conjugated core branches with the
hydrophilic soluble oligo(ethylene oxide) terminal chains in
370 | J. Mater. Chem., 2007, 17, 364–371
This journal is ß The Royal Society of Chemistry 2007

View Article Online
9 B. B. Wang, X. Zhang, X. R. Jia, Z. C. Li, Y. Ji, L. Yang and
Y. Wei, J. Am. Chem. Soc., 2004, 126, 15180.
16 A. Larsson, D. Kuckling and M. Scho¨nhoff, Colloids Surf., A,
2001, 190, 185.
10 (a) G. R. Newkome, Z. Q. Yao, G. R. Baker, V. K. Gupta,
P. S. Russo and M. J. Saunders, J. Am. Chem. Soc., 1986, 108, 849;
(b) S. K. Mohanty, S. Thirunavukarasu, S. Baskaran and
A. K. Mishra, Macromolecules, 2004, 37, 5364.
17 M. Iijima and Y. Nahasaki, J. Polym. Sci., Part A: Polym. Chem.,
2006, 44, 1457.
18 Y. Chang, C. Park, T. Kim and C. Kim, Langmuir, 2005, 21,
4334.
11 D. D. Perrin and W. L. F. Armarego, Purification of Laboratory
Chemicals, Pergamon Press, New York, 1988.
12 M. J. Plater and T. Jackson, Tetrahedron, 2003, 59, 4673.
13 H. Meier and M. Lehman, Angew. Chem., Int. Ed., 1998, 110, 643.
14 J. Gerald, U. Holzenkamp and H. Meier, Eur. J. Org. Chem., 2001,
66, 2757.
19 Y. Nagasaki, T. Okada, C. Scholz, M. Iijima, M. Kato and
K. Kataoka, Macromolecules, 1998, 31, 1473.
20 G. Karlstrom, J. Phys. Chem., 1985, 89, 4962.
21 P. Linse, Macromolecules, 1993, 26, 4437.
22 M. Malmsten and B. Lindman, Macromolecules, 1992, 25,
5446.
15 L. Ding, D. W. Chang, L. Dai, T. Ji, S. Li, J. Lu, Y. Tao,
D. Delozier and J. Connel, Macromolecules, 2005, 38, 9389.
23 V. Sivakumar, N. S. Elamprakash and S. Thayumanavan, J. Am.
Chem. Soc., 2005, 127, 14922.
This journal is ß The Royal Society of Chemistry 2007
J. Mater. Chem., 2007, 17, 364–371 | 371