Water-dispersible nanospheres of hydrogen-bonded supramolecular polymers and their application for mimicking light-harvesting systems

Article abstract of DOI:10.1039/c3cc48618d

Water-dispersible nanospheres of hydrogen-bonded supramolecular polymers have been prepared for the first time by using the miniemulsion method. Nanospheres containing chromophores with high fluorescence quantum yields were fabricated to mimic the natural

Full text of DOI:10.1039/c3cc48618d

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Water-dispersible nanospheres of hydrogen-bonded
supramolecular polymers and their application for
mimicking light-harvesting systems†
Cite this:DOI: 10.1039/c3cc48618d
Received 12th November 2013,
Accepted 27th November 2013
Hui-Qing Peng,ab Jiang-Fei Xu,ab Yu-Zhe Chen,a Li-Zhu Wu,a Chen-Ho Tunga and
Qing-Zheng Yang*a
DOI: 10.1039/c3cc48618d
www.rsc.org/chemcomm
Water-dispersible nanospheres of hydrogen-bonded supramolecular and organic phase in the presence of a surfactant. The polymers
polymers have been prepared for the first time by using the mini- aggregate into nanospheres upon evaporation of the organic sol-
emulsion method. Nanospheres containing chromophores with high vent. Inspired by such a method, we envisioned that miniemulsions
fluorescence quantum yields were fabricated to mimic the natural could be utilized to prepare nanoparticles of hydrogen-bonded
light-harvesting system.
supramolecular polymers. Stable emulsified organic droplets may
provide a hydrophobic environment to stabilize hydrogen-bonded
Supramolecular polymers are arrays of small molecular-weight supramolecular polymers, which could aggregate into nanospheres.
monomers connected through reversible non-covalent linkages, Here we report a highly efficient preparation of nanospheres of
such as hydrogen bonds, hydrophobic interactions or other weak hydrogen-bonded supramolecular polymers in water based on
interactions.1 The highly directional quadruple hydrogen bonding miniemulsions (Scheme 1). We prepared nanospheres of four types
units of ureidopyrimidinone (UPy) are among the most used of UPy-based supramolecular polymers. The supramolecular poly-
building blocks in supramolecular polymer construction due to mers containing chromophores with high fluorescence quantum
their high association constant and synthetic accessibility.2 Impress- yields form nanospheres for efficient light harvesting. To the best of
ive progress has been achieved with these supramolecular polymers our knowledge, this work is the first example of water-dispersible
in the fabrication of functional materials.3 UPy-based monomers are nanospheres of hydrogen-bonded supramolecular polymers.
typically polymerized in apolar organic solvents, because polar
We synthesized monomers 1–4 containing two UPy moieties
solvents, such as water, weaken or eliminate the hydrogen bonding (ESI†). The formation of high-molecular-weight supramolecular
pairing.4 As a result, applications of UPy-based supramolecular polymers in chloroform from the monomers was confirmed by
polymers in life science or eco-friendly materials have been less NMR spectroscopy and viscometry.8 The 1H NMR spectra of 1–4 in
explored. Nanospheres are one of the most widely used shapes in CDCl3 clearly indicated dimerization of the UPy units, as the UPy
nano materials due to their good dispersibility and low surface N–H signals showed a large downfield shift (10.0–13.5 ppm) (Fig. S1,
energy. Nanospheres of UPy-based supramolecular polymers gener- ESI†). The viscosity measurements of CHCl3 solutions of 1–4 were
ated in the aqueous phase may find wide-ranging applications in
areas such as biomaterials and luminescent nanomaterials.
Supramolecular polymers share many properties with traditional
polymers but are also capable of rapid and reversible polymerization–
depolymerization.5 The methods for preparation of polymer nano-
particles with a desired architecture may also be suitable for
supramolecular polymers.6 Traditional polymers are fabricated as
nanospheres by the miniemulsion method.7 Homogeneous mini-
emulsions are easily formed by ultrasonication of a mixture of water
a Key Laboratory of Photochemical Conversion and Optoelectronic Materials,
Technical Institute of Physics and Chemistry, Chinese Academy of Sciences,
Beijing 100190, China. E-mail: qzyang@mail.ipc.ac.cn; Fax: +86 10-62554670
b University of the Chinese Academy of Sciences, Beijing 100049, China
Scheme 1 Chemical structures of monomer 1–4 and graphical repre-
sentation for preparation of water-dispersible nanospheres of hydrogen-
bonded supramolecular polymers.
† Electronic supplementary information (ESI) available: Experimental details,
1H NMR, specific viscosity, SEM, TEM, DLS, FT-IR, absorption and fluorescence
measurements. See DOI: 10.1039/c3cc48618d
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consistent with the results of the NMR studies (Fig. S3, ESI†). The
double logarithmic plots of specific viscosity vs. concentration of 2
was linear with a slope of 1.08 at a concentration of o33 mM,
indicating the presence of solutes with concentration-independent
size (e.g., oligomers). A slope of 3.11 was observed at >33 mM
indicating an increase in the degree of polymerization of 2 with
concentration. Similar to 2, a critical polymerization concentration
Scheme 2 Synthesis of compound benzyl 2.
was also observed at 100 mM for 3.3b In contrast to 2 and 3, the dimerization of UPy moieties through the quadruple hydrogen
slope of the graph for 1 and 4 was constant at 1.67 and 2.11, bonding is the prerequisite for the formation of the nanospheres.
respectively, over the whole concentration range, indicating the
propensity of 1 and 4 to polymerize even at low concentrations.
We speculated that the hydrogen bonding of the supramolecular
polymers was protected by a hydrophobic microenvironment pro-
We prepared nanospheres of hydrogen-bonded supramolecular vided by emulsified organic droplets during preparation in water. As
polymers in water by the miniemulsion method. A 200 mL stock a result, the random-coil supramolecular polymers aggregate into
solution of 1–4 in CHCl3 (25 mg mLÀ1) was quickly added to an nanospheres under the influence of hydrophobic interactions. It
aqueous solution of the cetyl trimethyl ammonium bromide (CTAB) should be noted that our nanospheres made from supramolecular
surfactant (10 mL, cCTAB = 0.9 mM). We sonicated the resulting polymers have smaller size and better defined structures than most
solution for 25 minutes to emulsify the organic phase. After three of those obtained from common covalent polymers by using the
cycles of centrifuge–wash with water, we obtained stable water- miniemulsion method,7 which may reflect the dynamic character of
dispersible nanospheres as confirmed by electron microscopy and supramolecular polymers. The dynamic formation and breaking of
dynamic light scattering (DLS). As illustrated in Fig. 1, scanning hydrogen bonds adjust the length and configuration of the poly-
electron microscopy (SEM) and transmission electron microscopy mers, which may lower the energy of the system especially the
(TEM) images indicated regular shape and uniform size of the well- surface energy. Finally the polymers aggregate into the stable
dispersed nanospheres of 1 with an average diameter of B60 nm. nanospheres with desired shape and size.
The average hydrodynamic diameter estimated from DLS was
We studied the influence of surfactants and the concentration
B109 nm (Fig. S4, ESI†). The size of nanospheres formed from 3 of monomers on properties of such nanospheres. Both anionic
and 4 were similar to that from 1, and 2 formed slightly larger (e.g., sodium dodecyl sulfate) and nonionic surfactants (e.g., Pluro-
nanospheres than 1 (Fig. S5, Table S1, ESI†).
nic F-127) yielded nanospheres whose shape and size distribution
Because water lowers the dimerization constant of UPy consider- were similar to those of the nanospheres prepared with CTAB as the
ably by competing with hydrogen-bond donor and acceptor sites emulsifier, indicating that the method is not restricted to a parti-
of these units, the importance of hydrogen bonding in the for- cular surfactant type (Fig. S8, ESI†). On the other hand, the average
mation of nanospheres from 1–4 is not obvious. The presence of size of the nanospheres was sensitive to the initial monomer
peaks at 3212 and 3147 cmÀ1 (Fig. S6, ESI†) in the FT-IR spectra of concentration. As the monomer concentration increased from 5 to
the nanospheres suggests the formation of the self-complementary 100 mg mLÀ1, the hydrodynamic diameters measured by DLS
hydrogen bonds.2a To further establish the importance of such increased from 49 nm to 237 nm (Fig. S9, ESI†). This correlation
self-complementary hydrogen bonding for the formation of the enables us to fabricate nanospheres of different sizes by adjusting
nanospheres, we synthesized derivatives of 2 and 4 (benzyl 2 and the monomer concentration.
benzyl 4, Scheme 2 and Scheme S1, ESI†) in which one of the
To illustrate the utility of the described method we fabricated
hydrogen-bonding acceptor sites was blocked.9 The formation of nanospheres to mimic the natural light-harvesting systems. Construc-
our supramolecular polymers is based on the UPy dimerization. tion of artificial light-harvesting systems is important due to their
The absence of the UPy N–H signals between 10.0 and 13.5 ppm potential applications in solar cells, photocatalysts and optical sen-
1
in the H NMR spectrum suggests that the UPy dimers are not sors.10 Although impressive progress has been made using directed
formed (Fig. S2, ESI†). As a result, these derivatives did not assemble synthesis, the multiple steps needed to prepare complex architectures
into supramolecular polymers. The aggregates formed from benzyl limit the reach of such approaches. Preparation of a highly efficient
2 and benzyl 4 under identical conditions were disorderly (Fig. S7, antenna system by self-assembly from readily available building
ESI†). Such results prove that supramolecular polymerization by blocks can avoid such complex synthesis and separation.11 We chose
green emissive chromophore 9-(4-methoxyphenylethynyl)-10-(4-UPy-
phenylethynyl) anthracene (5) derivatized with a UPy moiety (Fig. 2) as
the energy acceptor since its absorption has a significant overlap with
the emission of energy donor 4 (Fig. S10, ESI†). We constructed a
light-harvesting system from the copolymer of 4 and 5 (Fig. 3).
Because 5 contains a single UPy moiety, it terminates a supramole-
cular macrochain of chromophores 4. The presence of 5 did not
affect the shape and size distribution of the nanospheres within
molar ratios of 4 to 5 from 352 : 1 to 44 : 1 (Fig. 4).
The energy transfer from the donor to the acceptor was con-
firmed by steady state and time-resolved fluorescence spectroscopy.
Fig. 1 (a) SEM and (b) TEM images of nanospheres prepared from supra-
molecular polymers of 1.
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To evaluate the light-harvesting ability of the system, we studied
the ‘‘antenna effect’’, a widely used empirical parameter, which is
defined to determine the degree of the amplified acceptor emission
assisted by the donor excitation.11a,b In our system, the antenna effect
was calculated according to eqn (1):
Antenna effect = IA(lex = 375 nm)/IA(lex = 445 nm) (1)
where IA(lex = 375 nm) and IA(lex = 445 nm) are the fluorescence
intensities of 5 upon excitation of the donor at 375 nm and direct
excitation of the acceptor at 445 nm, respectively. With different molar
ratios between the donor and acceptor, the acceptor emission ampli-
fied significantly as the ratio of the donor increased (Fig. S13, Table S2,
ESI†). A value of 11 fold emission enhancement was obtained for
nanospheres of copolymers containing 44 : 1 molar ratio of donor to
acceptor. With donor to acceptor ratios of 58 : 1, 88 : 1, and 176 : 1, the
antenna effects resulted in 13, 20, and 29 fold enhancement of
acceptor fluorescence, respectively. Eventually, the energy transfer
was saturated at one acceptor per 352 donors, when the maximum
acceptor emission amplification reached a factor of 35, which is much
higher than that of other artificial light harvesting systems.11a,b
In summary, we have described a highly efficient method to
prepare water dispersible nanospheres of hydrogen-bonded supra-
molecular polymers with well-defined shape and size by the mini-
emulsion method. This method is not restricted to a particular
surfactant type. The size of the nanospheres was controlled by
adjusting the monomer concentration in the precursor solution.
We used this new method to construct brightly fluorescent light-
harvesting nanospheres, from supramolecular copolymers contain-
ing an energy donor and acceptor. In contrast to their covalent
counterparts, nanospheres of supramolecular polymers are formed
from low-molecular-weight molecules and their composition and
functions are easily tunable. These features may offer great oppor-
tunities in bioapplications. Such an efficient fabrication of nano-
spheres of hydrogen-bonded supramolecular polymers also opens
up new opportunities for wide applications of UPy based supra-
molecular polymers in areas such as biomaterials and opto-
electronic materials.
Fig. 2 Fluorescence spectra of nanospheres dispersed in water with
different molar ratios between donor (D) and acceptor (A). [D] = 49.7 mM.
[D] to [A] molar ratio is 352 : 1, 176: 1, 88: 1, 58 : 1, 44: 1 from bottom to top.
lex = 375 nm. Inset: chemical structure of 5.
Fig. 3 Schematic illustrations of the energy transfer among chromo-
phores densely organized in the nanospheres. Left: nanospheres prepared
from supramolecular polymers of 4. Right: nanospheres prepared from
supramolecular copolymers of 88 : 1 of 4 to 5. Inset: photographs of
nanospheres in water under UV light (lex = 365 nm).
As shown in Fig. 2, an increase of the acceptor-to-donor molar ratio
from 1/352 to 1/44 lowered the intensity of the donor emission at
430 nm while enhancing that of the acceptor at 496 nm when the
donor was selectively excited at 375 nm. The excitation spectrum of
the acceptor was nearly identical to the absorption spectrum of 4,
indicating that the donor contributed directly to the acceptor
emission (Fig. S11, ESI†). Time-resolved fluorescence measurements
clearly show that the fluorescence decay of chromophore 4 accel-
erates after the co-assembly of acceptor 5, indicating an efficient
energy transfer (Fig. S12, ESI†).
The change of fluorescence was easily visualized by a varia-
tion in the emission color from the bright blue emission of the
donor to the bright green emission of the acceptor (Fig. 3),
indicating that the unique optical properties of the nanosphere
generated from small monomers could be tuned via convenient
backbone copolymerization. Such fluorescent nanospheres
may have potential applications for in vitro and in vivo fluores-
cence imaging.
We are grateful for the financial support from the 973 Program
(2013CB933800, 2013CB834505), the National Natural Science Foun-
dation of China (21072202, 21222210, 91027041) and the Chinese
Academy of Sciences (100 Talents Program).
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