Efficient energy transfer between amphiphilic dendrimers with oligo(p-phenylenevinylene) core branches and oligo(ethylene oxide) termini in micelles

Article abstract of DOI:10.1002/pola.26356

The efficient fluorescence resonance energy transfer (FRET) between amphiphilic dendrimers with oligo(p-phenylenevinylene) core branches and oligo(ethylene oxide) termini have been observed in micelles. All dendrimers show the critical micelle concentrati

Full text of DOI:10.1002/pola.26356

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Efficient Energy Transfer Between Amphiphilic Dendrimers with
Oligo(p-phenylenevinylene) Core Branches and Oligo(ethylene oxide)
Termini in Micelles
Dong Wook Chang,1 Seo-Yoon Bae,2 Liming Dai,3 Jong-Beom Baek2
1Department of Chemical Systematic Engineering, Catholic University of Daegu, 13-13, Hayang, Gyeongbuk 712-702, South Korea
2Interdisciplinary School of Green Energy/Low-Dimensional Carbon Materials Center, Ulsan National Institute of Science and
Technology, 100, Banyeon, Ulsan 689-798, South Korea
3Department of Macromolecular Science and Engineering, Case Western Reserve University, 10900 Euclid Avenue,
Cleveland, Ohio 44106
Correspondence to: Liming Dai (E-mail: liming.dai@case.edu) or Jong-BeomBaek (E-mail: jbbaek@unist.ac.kr)
Received 5 July 2012; accepted 17 August 2012; published online 5 October 2012
DOI: 10.1002/pola.26356
ABSTRACT: The efficient fluorescence resonance energy trans-
fer (FRET) between amphiphilic dendrimers with oligo(p-phe-
nylenevinylene) core branches and oligo(ethylene oxide)
termini have been observed in micelles. All dendrimers
show the critical micelle concentration and lower critical so-
lution temperature as well as fluorescent emission. Tailoring
electronic structures of the conjugated amphiphiles for FRET
have been conveniently achieved by varying the branch
number and/or the conjugated core structure. The Stern-
Volmer constants (KSV) for FRET were found to be 4.51 ꢀ
10ꢁ5 and 8.78 10ꢁ5
for Den 30–40 and Den 50–40,
ꢀ
M
respectively. The effects external stimuli such as solvent and
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temperature on FRET have been also investigated.
2012
Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 51:
168–175, 2013
KEYWORDS: dendrimers; fluorescence; FRET; micelles; Stern-
Volmer constants
INTRODUCTION Fluorescence resonance energy transfer
(FRET) is a photophysical process, which involves the trans-
fer of excited state energy from the initially excited donor
(D) to an acceptor (A). A simultaneous quenching of donor
fluorescence and enhancement of acceptor fluorescence can
take place as a result of FRET.1,2 The Coulomb interaction
that leads to energy transfer is dipole-dipole interaction,
which is strongly dependent on the distance between
dipoles.3 In addition to the distance between donor and
acceptor, other factors also have significant contributions to
the FRET process. The most important requirement for effi-
cient energy transfer is resonance. The fluorescence spec-
trum of donor reveals the oscillation frequency of the exited
donor dipoles, which should match the frequency of elec-
tronic transition of acceptor from ground state to excited
state, that is, the absorption spectrum. If there is no reso-
nance between donor in excited state and acceptor in ground
state, the energy transfer is impossible.
well-organized structures to efficiently facilitate energy
transfer.8–12 In particular, FRET in micelles has been widely
investigated due not only to their biological significance
associated with the similarity between the micelles and bio-
logical membranes but also to the possible use of FRET as
probes for micelle/membrane studies and biomedical imag-
ing. Much effort has been focused on multicomponent micel-
lar systems consisting of various host molecules such as sur-
factants, dendrimers, and guest molecules like dyes.13–17
Although, p-conjugated molecules have several advantages
for FRET,18–20 only a few of studies have been reported on
energy transfer between p-conjugated amphiphiles in aque-
ous medium as far as we are aware.21,22 In most cases,
totally different p-conjugated structures have often been
used as donor and acceptor to realize FRET. In these
approaches, multiple and complex synthetic steps are
required and the FRET responses are somewhat
complicated.21,22
FRET in various confined geometries, such as micelles,
vesicles, clays, and zeolite, are specifically interesting.4–7 This
is because these systems can provide a high local concentra-
tion of chromophores and enhanced spatial proximity
between donor and acceptor molecules associated with their
Herein, we report efficient FRET between amphiphilic conju-
gated dendrimers in aqueous medium, Den 30 (donor-1),
Den 50 (donor-2), and Den 40 (acceptor), with oligo(p-phe-
nylenevinylene) (OPV) cores and oligo(ethylene oxide) ter-
mini in micellar structure (Fig. 1). This strategy allows us to
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FIGURE 1 (a) Chemical structures of Den 30, Den 50, and Den 40 and (b) schematic diagram of the formation of mixed micelle
between donor and acceptor.
tailor electronic structures of the conjugated amphiphiles by
using simple synthetic approaches like variation of the
branch number and/or the conjugated core structure. The
amphiphilic nature and structural similarity could easily pro-
mote the formation of a micelle-like confined geometry
between donor and acceptor to further enhance FRET. As a
result, efficient FRET has been achieved between these con-
jugated amphiphiles, although their spectral overlapping is
not so significant. Furthermore, we have also investigated
their unique FRET responses to external stimuli, such as sol-
vents and temperature.
of Den 30, Den 50, and Den 40 are 1.01 ꢀ 10ꢁ6, 0.78 ꢀ
10ꢁ6, and 2.14 ꢀ 10ꢁ6 M, respectively (Table 1). They also
show typical lower critical solution temperature (LCST)
behavior as the solubility of triethylene oxide end groups
decreases with increasing temperature.24
EXPERIMENTAL
Materials and Equipments
The synthesis and characterization of Den 30, Den 40, and
3 have been previously reported.24 The detailed synthetic
procedures of Den 50 are shown in Scheme 1. Hydrophobic
dendritic core 1 and 2 were synthesized following the litera-
ture procedure.26,27 All reagents and solvents were used as
received from Aldrich. 1H and 13C NMR spectra were
recorded on a Varian VNMRS spectrometer at 600 and 150
MHz, respectively. UV–vis spectra were recorded on a Perkin-
Elmer Lambda 900 UV–vis-NIR spectrometer, while photolu-
minescence (PL) emissions were measured on a Perkin-
Elmer LS 55 spectrometer. An external circulation (Lauda RE
206) bath was used to control temperatures (60.02 ꢂC) for
PL spectrometers. Atomic force microscopy (AFM) analysis
was conducted with a Veeco Multimode V using a silicone
cantilever in a tapping mode. The samples of mixed micelles
with Den 30 and Den 40 for AFM measurement were pre-
pared by drop casting of aqueous solution on mica. The
We have previously demonstrated that amphiphilic dendrons
and dendrimers (Den 30 and Den 40) with OPV core
branches and oligo(ethylene oxide) termini have unique sol-
vatochromic and stimuli responsive properties.23,24 Den 50
is newly synthesized and the detailed synthetic procedures
were shown in Scheme 1. Above the critical micelle concen-
tration (CMC) in aqueous solvent, these amphiphilic den-
drons can form aggregated structures.23,24 The CMC values
TABLE 1 Optical Properties and Critical Micelle Concentration
(CMC) of Den 30, Den 50, and Den 40 in Aqueous Medium
Quantum
Yield (%)c
UV–visa (nm,
e:molꢁ1 cmꢁ1
PLa,b
(nm)
CMC
(10ꢁ6 M)
)
Water
THF
Den 30
Den 50
Den 40
a
310, 38650
328, 40740
330, 44450
400
430
460
0.05
0.10
0.20
0.35
0.72
0.27
1.01
0.78
2.14
Obtained from aqueous solution.
Excitation wavelength is 310 nm.
b
c
SCHEME 1 Procedures for synthesis of amphiphilic dendrimer
(Den 50).
Quinine sulfate was used as a standard reference with F ¼ 0.51%
in 0.1 N sulfuric acid.25
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energy transfer studies were done by taking the emission
spectra of the solution at a series of acceptor concentration
and comparing steady state PL intensities. The mixed
micelles with various ratios of donor/acceptor were pre-
pared via two step procedure. First, the concentrated stock
solution of donor and acceptor are made in THF, which is
good solvent for all amphiphiles. The stock solutions were
protected from ambient light until use. The aliquots of donor
and acceptor solutions were mixed to get homogenous phase
of donor and acceptor in normal quartz cuvette with a 1 cm
path length and 3.5 mL volume. Then predetermined large
amount of water (95–99, v/v %) was injected into homoge-
neous THF solution with vigorous stirring for several
minutes. The mixtures were directly used for UV–vis and PL
measurement. The excitation wavelength for PL measure-
ment was 310 nm. For temperature variation experiments,
cuvettes were sealed with PTFE cover to minimized evapora-
tion of solvents.
refluxed for 1 h. After cooling to room temperature, it was
neutralized by 1N HCl and extracted with CHCl3. The organic
layer was then washed with brine and dried over MgSO4. Af-
ter solvent evaporation, the residue was subjected to column
chromatography using ethyl acetate/methanol (80/20) to
1
produce Den 50 as yellow viscous oil (0.45 g, 54% yield). H
NMR (300 MHz, CDCl3, ppm) d: 3.39 (s, 18H), 3.56 (m, 12H),
3.66–3.78 (m, 36H), 3.88 (t, J ¼ 4.8 Hz, 12H), 4.17 (t, J ¼ 4.8
Hz, 12H), 6.45 (t, J ¼ 2.1 Hz, 3H), 6.73 (d, J ¼ 1.9 Hz, 6H),
7.11 (d, J ¼ 5.5 Hz, 6H), 7.62 (d, J ¼ 8.4 Hz, 6 H), 7.73 (d,
J ¼ 8.4 Hz, 6H), and 7.83 (s, 3H). 13C NMR (300 MHz, CDCl3,
ppm) d: 160.1, 141.9, 140.2, 139.2, 136.6, 128.9, 128.6,
127.6, 127.1, 105.6, 101.4, 71.9, 70.9, 70.7, 70.6, 69.7, 67.5,
and 59.0. MALDI-TOF MS 1607.89 m/z [M þ Na]þ (calcd:
1584.82 m/z [M]þ). ELEM. ANAL.: calcd for C90H120O24, C 68.2,
H 7.6, O 24.2; found, C 68.2 H 7.6 O 24.3.
RESULTS AND DISCUSSION
Synthesis
Synthesis
As can be seen in Scheme 1, Wohl-Ziegler bromination of
hydrocarbons and Arbusov rearrangement reactions were
performed for the synthesis of the dendritic core 2. The
spectroscopic results were well matched with literature
reports.26,27 Finally, the Honer-Wadsworth-Emmons coupling
reaction was used to attach the aldehyde-functionalized pe-
riphery 3 onto the core 2, resulting in the formation of Den
50 (Scheme 1). These newly synthesized Den 50 also
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 den-
dritic molecules in water and other common organic solvents
(e.g. ethanol, chloroform, and THF).
1,3,5-Tris(p-tolyl)benzene (1)
A mixture of 4-methylacetophenone (67.09 g, 0.50 mol) and
trifluoromethanesulfonic acid (0.5 mL) in 350 mL anhydrous
toluene was refluxed with Dean-Stark trap for the removal of
water as a byproduct.26 After 20 h, an additional 0.5 mL of
trifluoromethanesulfonic acid added and then reflux contin-
ued for 60 h. The solvent was concentrated and subsequent
cooling to room temperature led to products 1. (24.44 g,
42% yield) 1H NMR (300 MHz, CDCl3, ppm) d: 2.42 (s, 9H),
7.28 (d, J ¼ 7.9 Hz, 6H), 7.60 (d, J ¼ 7.9 Hz, 6H), and 7.73
(s, 3H). 13C NMR (300 MHz, CDCl3, ppm) d: 142.3, 138.5,
137.4, 129.7, 127.3, 124.7, and 21.3.
1,3,5-Tris(p-(diethoxyphosphonylmethyl)phenyl)benzene (2)
A mixture of 1 (5.00 g, 14.35 mmol), N-bromosuccinimide
(7.66 g, 43.04 mmol) and catalytic amount of AIBN (10 mg)
in dry tetrachloromethane (200 mL) was refluxed for over-
night.27 After cooling, succinimide was removed by filtration
and the organic phase was washed with water. Then, the or-
ganic solvent was removed by rotary evaporation. Subse-
quent recrystallization in methanol with small amount of
chloroform led to crystalline tribromo-compound (3.61 g,
6.16 mmol), which was heated, together with an excess
quantity of triethyl phosphite (8.19 g, 49.28 mmol), for 5 h
The Spectroscopic Study
The basic photophysical properties of amphiphilic den-
drimers were studied in aqueous medium by using UV–vis
and PL spectroscopy [Fig. 2(a)] and their numerical values
were summarized in Table 1. As observed in Figure 2(a), the
increase in the number of branches from Den 30 to Den 40
leads to bathochromic shifts for both absorption and fluores-
cence peaks. Photoexcitation of pure solutions of Den 30
and Den 40 at kex ¼ 310 nm revealed the intense fluores-
cence peak maxima at kem ¼ 400 and 460 nm, respectively.
The spectral overlap between fluorescence of Den 30
(donor-1) and absorption of Den 40 (acceptor), which is
indispensable for FRET, was also observed [Fig. 2(a)]. As
shown in Figure 2(b), donor and acceptor in a mixture solu-
tion (designated as: Den 30–40) in aqueous medium
showed no detectable change in their respective spectro-
scopic features. Thus, the electronic absorption spectra of
donor in aqueous solution were not affected by the presence
of acceptor or vice versa. However, fluorescence spectra of
the mixed micelles of Den 30–40 excited at 310 nm in an
aqueous medium showed a significant decrease in donor
luminescence at 400 nm with a concomitant increase in the
emission of acceptor [Fig. 2(c)]. This result is an indication
of FRET from donors to acceptor. On the contrary, in case of
THF solution in which both donor and acceptor are well
ꢂ
at 160 C. Finally, the excess triethyl phosphite was removed
by distillation to yield 2 as yellowish viscous oil. (4.67 g,
43% yield). 1H NMR (300 MHz, CDCl3, ppm) d: 1.28 (t, J ¼
7.0 Hz, 18 H), 3.22 (d, J ¼ 21.9 Hz, 6H), 4.06 (m, 12H), 7.42
(dd, J ¼ 8.2, 2.4 Hz, 6H), 7.66 (d, J ¼ 7.7 Hz, 6H), 7.76 (s,
3H) 13C NMR (300 MHz, CDCl3, ppm) d:141.8, 139.6, 139.5,
131.0, 130.3, 127.4, 124.8, 62.2, 62.1, 34.3, 32.5, and 16.4.
1,3,5-Tris(4-{3,5-bis{2-[2-(2-methoxyethoxy)ethoxy]
ethoxy}styryl}phenyl)benzene (Den 50)
The compound 2 (0.40 g, 0.53 mmol) was dissolved in 5 mL
of dry DMF under high purity argon. Two equivalent of NaH
(60% dispersion in mineral oil; 0.13 g, 3.17 mmol) and 3,5-
di(methyltriglycoloxy)benzaldehyde 3 (0.68 g, 1.59 mmol)
were added consecutively. The reaction mixture was then
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FIGURE 2 (a) Normalized UV–vis and PL spectra of Den 30 and Den 40 in water, (b) UV–vis spectra of Den 30, Den 40, and Den 30-
40 in water. PL spectra of Den 30, Den 40, and Den 30-40 in (c) water and (d) THF. The concentration of Den 30 and Den 40 are
6.65 ꢀ 10ꢁ6 and 5.13 ꢀ 10ꢁ6 M, respectively.
soluble into molecular level, the emission of donor was not
altered by the presence of acceptor, revealing that no energy
transfer from donor to acceptor occurred in THF [Fig. 2(d)].
These results indicate that the formation of aggregated (e.g.,
in water) structures between donor and acceptor is the key
factor for efficient FRET, which depends strongly on the dis-
tance between donor and acceptor. Similar results were
obtained between Den 50 (donor-2) and Den 40 (acceptor)
except absorption and emission of Den 50 is more red-
shifted than that of Den 30 (Fig. 3). In Den 50, the core of
amphiphilic dendrimer has been changed into a more rigid
structure (Scheme 1).
PL0/PL ¼ 1 þ KSV[Q],21,22,28 where PL0 and PL represent
the fluorescence intensity of the donor in the absence of
acceptor and in the presence of acceptor, respectively, and
[Q] is the acceptor concentration. The quenching rate con-
stant, KSV, could be calculated from the Stern-Volmer rela-
tion, showing that linear dependence was observed at lower
acceptor concentration and it was saturated at higher con-
centration [Fig. 4(c)]. A linear region in Stern-Volmer plots
indicated that only one type of quenching could occur in
mixtures.2 From the slope of the linear region in Stern-
Volmer plots, the quenching rate constant KSV values of 4.51
and 8.78 ꢀ 105 Mꢁ1 were calculated for Den 30–40 and
Den 50–40, respectively. The higher KSV value in FRET
observed for Den 50 can be attributed to higher quantum
efficiencies and more hydrophobic conjugated core lowering
CMC. The efficiency of energy transfer can be also deter-
mined from the following relationship, U ¼ 1 ꢁ PL/PL0,13
where PL0 and PL represent the fluorescence intensity of
the donor in the absence and presence of acceptor, respec-
tively. As shown in Figure 4(d), the efficiencies of energy
transfer for Den 30-40 and Den 50-40, can be reached up
to 59 and 75%, respectively. This high energy-transfer effi-
ciency of Den 50-40 is comparable with that of diblock con-
jugated polymer in film containing donor and acceptor in
Effect of Concentration on FRET
The efficiency of energy transfer is also dependent on molar
ratio of donor and acceptor. Mixed micelles with various do-
nor/acceptor ratios were prepared. As can be seen in Figure
4(a), the fluorescence of Den 30 (donor-1) was gradually
decreased with increasing Den 40 (acceptor) concentrations,
showing an isobestic point near 420 nm [Fig. 4(a)]. As
shown in Figure 3(b), similar results were also observed in
Den 50 (donor-2) and Den 40 (acceptor). Typically, the con-
centration of acceptor was increased from 0 to 3.41 ꢀ 10ꢁ6
M with the fixed concentration of donors at 8.60 ꢀ 10ꢁ6 M.
Stern-Volmer analysis was a useful method of presenting
data for the energy transfer according to following equation,
one molecule.19 In addition, Forster’s radius (Ro) can be cal-
€
culated from a few experimental parameters.29
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FIGURE 3 (a) Normalized UV–vis and PL spectra of Den 50 and Den 40 in water, (b) UV–vis spectra of Den 50, Den 40, and Den 50-
40 in water. PL spectra of Den 50, Den 40, and Den 50-40 in (c) water and (d) THF. The concentration of Den 50 and Den 40 are
6.67 and 5.13 ꢀ 10ꢁ6 M, respectively.
FIGURE 4 PL spectra of (a) Den 30-40, (b) Den 50-40, and (c) Stern-Volmer plot of Den 30-40 and Den 50-40 with different concen-
tration of Den 40 (acceptor). (d) Plots of energy transfer efficiency (f) of Den 30-40 and Den 50-40. The concentrations of Den 30
and Den 50 were fixed at 8.60 ꢀ 10ꢁ6 M and the concentration of Den 40 was varied from 0 to 6.00 ꢀ 10ꢁ6 M.

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FIGURE 5 PL spectra of (a) Den 30-40 and (b) Den 50-40 in water with a consecutive adding of THF. The concentration of Den 30,
Den 50, and Den 40 were 8.60 ꢀ 10ꢁ6, 8.60 ꢀ 10ꢁ6, and 5.26 ꢀ 10ꢁ6 M, respectively. AFM images of mixed micelles, Den 30-40, (c)
before and (d) after addition of THF. The scanning areas of both are 10 ꢀ 10 mm. Insets in Figure (c, d) represent the related PL
emission spectra. The concentration of Den 30 and Den 40 for AFM images are 0.53 and 0.22 mM, respectively.
Z
1
ð8:8 ꢀ 10ꢁ25Þj2UD
dm
v4
investigated the effects of solvent on FRET by a consecutive
addition of THF droplets to a pre-formed aqueous solution
of mixed micelle. As the amount of THF increased, the sup-
pressed fluorescence of donors by FRET at 400 nm was
gradually restored to their original intensity with an isobes-
tic points at 450 nm [Fig. 5(a, b)]. These results showed that
the solvent polarity has a strong influence on the aggregated
structure of amphiphilic dendrimers and the efficiency of
energy transfer. AFM images shown in Figure 5(c, d) con-
firmed the changes in micellar structures of mixed micelles,
Den 30-40, upon addition of THF. When the mixed micelles
were generated, in which the emission from acceptor is
prominent via efficient FRET, AFM image showed the globu-
lar micelles with bigger size [Fig. 5(c)]. However, the mixed
micelles were completely disappeared, when THF was added
until the emission of donor was almost restored [Fig. 5(d)].
R6o ¼
fDðmÞeAðmÞ
n4
0
The integral, known as the overlap integral, is a measure of
the spectral overlap of the fluorescence of donor and the
absorption spectrum of acceptor. The m represents units of
frequency in wavenumber (cmꢁ1). The eA(m) is the molar
decadic extinction coefficient and the n is the refractive
index of solvent. The K2 is a constant that reflects the rela-
tive orientation of the electronic dipole. A value of 2/3 is
normally taken for molecules that are rotating much faster
than the energy transfer rate. From above equation, the criti-
cal radius of Den 30-40 and Den 50-40 mixtures are 2.64
and 2.83 nm, respectively.
Cosolvent Effect on FRET
Effects of Temperature on FRET
FRET in micelle is quite sensitive to solvents, which can
cause drastic changes in their aggregation state. Thus, we
can expect that the change of solvent polarity can also make
large differences in micelle structures of aggregates in solu-
tion. THF is a good solvent for all the amphiphiles studied in
this work and can well separate donors from acceptors. We
All amphiphilic dendrimers show unique LCST behavior in
aqueous medium due to a polar–nonpolar transition of tri-
ethylene oxide periphery at elevated temperature, which
results in the diminished solubility of it with increasing tem-
perature.24 Both solutions of individual and mixed micelles
in aqueous solution become turbid above cloud point as a
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FIGURE 6 (a) Reversible LCST behavior of mixed micelles composed of Den 30 and Den 40. The concentrations of Den 30 and
Den 40 are 0.53 and 0.22 mM, respectively. Fluorescence spectra of Den 30-40 in aqueous medium during (b) heating and (c) cool-
ing. The concentrations of Den 30 and Den 40 were 8.60 ꢀ 10ꢁ6 and 6.00 ꢀ 10ꢁ6 M, respectively.
result of aggregation [Fig. 6(a)]. The fluorescence of amphi-
philic dendrimers was quenched at elevated temperature,
because nonradiative decays were developed by the forma-
tion of aggregates.24 To study the stability of mixed micelles,
repeated heat and cooling treatments in aqueous solution
were performed, followed by fluorescence measurements.
Den 30-40 showed a sharp decrease in the fluorescence
peak centered at 460 nm with increasing temperature; how-
ever, the PL intensity is almost recovered with decreasing
temperature [Fig. 6(b, c)]. The similar trend has also been
obtained from Den 50-40 (data not shown). Thus, the mixed
micelles composed of amphiphilic dendrimers, Den 30, Den
50, and Den 40, are relatively stable at repeated heating and
cooling processes. The persistence of FRET during the
repeated heating-cooling cycles indicates that the micellar
structure remained unchanged with individual constituent
chains changing their conformations in response to the tem-
perature change.
study provides important insights into the design of new
conjugated macromolecules with controllable molecular and
micellar structures, and hence tunable FRET for potential for
biomedical applications (e.g., as optical probes for membrane
studies and biomedical imaging).
ACKNOWLEDGMENTS
The authors acknowledge financial support from the World
Class University (WCU) program supported by National
Research Foundation and Ministry of Education, Science and
Technology of Korea.
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