Nanoscale Ln(III)-Carboxylate coordination polymers (Ln = Gd, Eu, Yb): Temperature-controlled guest encapsulation and light harvesting

Article abstract of DOI:10.1021/ja102299b

We report the self-assembly of stable nanoscale coordination polymers (NCPs), which exhibit temperature-controlled guest encapsulation and release, as well as an efficient light-harvesting property. NCPs are obtained by coordination-directed organization of π-conjugated dicarboxylate (L1) and lanthanide metal ions Gd(III), Eu(III), and Yb(III) in a DMF system. Guest molecules trans-4-styryl-1-methylpyridiniumiodide (D1) and methylene blue (D2) can be encapsulated into NCPs, and the loading amounts can be controlled by changing reaction temperatures. Small angle X-ray diffraction (SAXRD) results reveal that the self-assembled discus-like NCPs exhibit long-range ordered structures, which remain unchanged after guest encapsulations. Experimental results reveal that the negatively charged local environment around the metal connector is the driving force for the encapsulation of cationic guests. The D1 molecules encapsulated in NCPs at 140 °C can be released gradually at room temperature in DMF. Guest-loaded NCPs exhibit efficient light harvesting with energy transfer from the framework to the guest D1 molecule, which is studied by photoluminescence and fluorescence lifetime decays. This coordination-directed encapsulation approach is general and should be extended to the fabrication of a wide range of multifunctional nanomaterials.

Full text of DOI:10.1021/ja102299b

Published on Web 07/08/2010
Nanoscale Ln(III)-Carboxylate Coordination Polymers
(Ln ) Gd, Eu, Yb): Temperature-Controlled Guest
Encapsulation and Light Harvesting
Xuanjun Zhang,† Mohamed Ali Ballem,‡ Maria Ahre´n,† Anke Suska,§
Peder Bergman,| and Kajsa Uvdal*,†
DiVisions of Molecular Surface Physics and Nanoscience, Nanostructured Materials, Applied
Physics, and Semiconductor Materials, Department of Physics, Chemistry, and Biology,
Linko¨ping UniVersity, 581 83 Linko¨ping, Sweden
Received March 18, 2010; E-mail: kajsa@ifm.liu.se
Abstract: We report the self-assembly of stable nanoscale coordination polymers (NCPs), which exhibit
temperature-controlled guest encapsulation and release, as well as an efficient light-harvesting property.
NCPs are obtained by coordination-directed organization of π-conjugated dicarboxylate (L1) and lanthanide
metal ions Gd(III), Eu(III), and Yb(III) in a DMF system. Guest molecules trans-4-styryl-1-methylpyridiniu-
miodide (D1) and methylene blue (D2) can be encapsulated into NCPs, and the loading amounts can be
controlled by changing reaction temperatures. Small angle X-ray diffraction (SAXRD) results reveal that
the self-assembled discus-like NCPs exhibit long-range ordered structures, which remain unchanged after
guest encapsulations. Experimental results reveal that the negatively charged local environment around
the metal connector is the driving force for the encapsulation of cationic guests. The D1 molecules
encapsulated in NCPs at 140 °C can be released gradually at room temperature in DMF. Guest-loaded
NCPs exhibit efficient light harvesting with energy transfer from the framework to the guest D1 molecule,
which is studied by photoluminescence and fluorescence lifetime decays. This coordination-directed
encapsulation approach is general and should be extended to the fabrication of a wide range of
multifunctional nanomaterials.
Introduction
molecules, peptide polymers, drugs, and small nanoparticles
havebeenloadedintoNCPsbycoordination-directedself-assembly12,13
The coordination polymers (aka metal-organic frameworks,
MOFs) have rapidly gained growing attention due to the
structure diversity and characteristics for a number of practical
applications, such as catalysis,1 gas storage,2 chiral transfer,3
luminescence,4 and nonlinear optics.5 However, the macroscopic
solid state nature of such materials show limited solution-based
behavior, which hinders further applications. Recently, scaling
down these materials has afforded an exciting new class of
highly tailorable materials known as nanoscale coordination
polymers (NCPs) or infinite coordination polymer particles
(ICPs).6-12 Compared with bulk coordination polymers, these
promising nanomaterials exhibit new possibilities for solution-
based applications, such as ion exchange,7 multimodal bioim-
aging,9 drug delivery,10 and sensing.10b Very recently, dye
and by elegant postsynthetic modifications.10c This combination
of inorganic and organic or even bioactive components into
single NCPs has made accessible an immense new area of
materials science that has extraordinary implications in the
development of multifunctional materials. However, the ground-
work for this emerging field has been laid only in the past few
(6) See recent reviews: (a) Lin, W.; Rieter, W. J.; Taylor, K. M. L. Angew.
Chem., Int. Ed. 2009, 48, 650. (b) Spokoyny, A. M.; Kim, D.; Sumrein,
A.; Mirkin, C. A. Chem. Soc. ReV. 2009, 38, 1218. (c) Wang, X.;
McHale, R. Macromol. Rapid Commun. 2010, 31, 331.
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C. A. Angew. Chem., Int. Ed. 2006, 45, 5492.
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Nishiyabu, R.; Gondo, R.; Kimizuka, N. Chem.sEur. J. 2010, 16,
3604.
† Division of Molecular Surface Physics and Nanoscience.
‡ Division of Nanostructured Materials.
§ Division of Applied Physics.
| Division of Semiconductor Materials.
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W. J. Am. Chem. Soc. 2006, 128, 9024. (c) Taylor, K. M. L.; Jin, A.;
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A R T I C L E S
Zhang et al.
anions (such as CH3COO-, etc.) bind to the empty orbitals of
lanthanide ions in a neutral coordination polymer, the local
environment around Ln(III) will be negatively charged and
cationic guests are expected to balance the charges, leading to
their encapsulation in NCPs. To test this hypothesis, NCPs
derived from L1 and Ln(III) (Ln ) Gd, Eu, Yb) were prepared
and the encapsulations of cationic guests were investigated. We
choose trans-4-styryl-1-methylpyridiniumiodide (D1) and me-
thylene blue (D2) as guests, which are important fluorophores
in nonlinear optics and/or indicators in bioanalysis.18
Scheme 1. Molecular Structures of Ligand L1 and Guests
Encapsulated in NCPs
Results and Discussions
Self-Assembly and Characterization of Ln(III)-L1 NCPs.
Bare NCPs derived from the L1 and lanthanide ions without
guest encapsulation in the form of nanoparticles were initially
investigated. Here, we use L1 and Gd(III) as an example to
show the coordination-directed assembly of NCPs. With slow
addition of Gd(OAc)3 into a DMF solution of L1, the mixture
turned turbid gradually and colloids exhibiting bright blue
fluorescence formed after gentle stirring at room temperature
for 20 min. SEM analysis revealed that the particles were
relatively uniform disks with diameters of ca. 80-120 nm and
thicknesses of ca. 10-20 nm (Figure 1a). The NCPs colloid
(hereafter referred to as Gd-L1) was very stable. The material
was stored at room temperature for several months, and no
precipitate was observed. The solvent played a significant role
in the self-assembly process, and DMF was found as a good
medium for the stabilization of Gd-L1 NCPs. When DMSO
was used instead of DMF, aggregated spherical particles
precipitated rapidly. This may indicate that DMF acts as
stabilizer of the colloid particles. The presence of a coordinated
DMF molecule was further confirmed by an IR spectrum
(Supporting Information, Figure S1). The stretching mode
ν(CdO) of coordinated DMF in Gd-L1 NCPs peaked at 1659
cm-1. A shift of 13 cm-1 toward a lower wavelength number
compared with that of a free DMF molecule (1672 cm-1) is a
result of sCdO· · ·Gd binding.19 Thermogravimetric analysis
(TGA) showed a weight loss of ca. 6% from 50 to 400 °C,
followed by framework combustion (Figure S2). From these
results and elemental analysis, we can obtain a composition of
Gd (L1)1.5 ·DMF·H2O for the NCPs.
years. Hitherto, methods for controlling particle size and shape
are still fairly rudimentary.6b Issues like porosity control and
functional guest encapsulation and release have great potential
in the areas of drug delivery, medical diagnostics, and materials
science but pose a great challenge. This is to a large extent
owing to different molecular forces in the precursor solution
and general liability of the metal coordination numbers and
geometries, which make it very challenging to control hierarchi-
cal assemblies through a preferred packing of the building blocks
to give morphological and functional NCPs.
In this work, we report NCPs assembled from linear
π-conjugated ligand and lanthanide metal ions (Scheme 1) with
long-range ordered structures, which exhibit temperature-
controlled sizing, guest encapsulation and release, and an
efficient light-harvesting property. Fluorene derivatives are
known for exhibiting high fluorescence quantum yields and
excellent photostability.14 π-Conjugated dicarboxylate ligand L1
based on fluorene was synthesized by Sonogashira coupling,
which exhibits strong blue photoluminescence. We choose
lanthanide ions as metal connectors because of their high affinity
to carboxylate and nondetrimental nature to fluorescence. The
flexible coordination of lanthanide ions is expected to favor the
adaptive encapsulations of guests in the coordination polymers.8d,13
It is well-known that solvent molecules or small anions always
bind to metal ions when the main ligand cannot saturate the
high coordination number of lanthanide ions.15-17 If some
For the reaction performed at 20 °C, the particles maintain
diameters of ca. 80-120 nm after 20 min without further
growth. Increasing the temperature could result in larger particles
with a smoother surface, which likely is due to crystallization
of the coordination polymer at higher temperature. In addition,
the particle sizes could be tuned by changing the reaction time
at elevated temperatures. For example, discus-like NCPs (with
a thicker center and thinner edge) of 200-300 nm or 500-600
nm in diameters (Figure 1b and 1c) were obtained at 140 °C
with a reaction time of 5 and 20 min, respectively, while other
conditions remained unchanged (the particle growth could be
quenched by addition of cold ethanol).
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Keizer, A.; Drechsler, M.; Stuart, M. A. C. Angew. Chem., Int. Ed.
2008, 47, 4192. (b) Imaz, I.; Hernando, J.; Ruiz-Molina, D.; Maspoch,
D. Angew. Chem., Int. Ed. 2009, 48, 2325. (c) Champness, N. R.
Angew. Chem., Int. Ed. 2009, 48, 2274. (d) Zhang, X.; Chen, Z.-K.;
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Nishiyabu, R.; Aime´, C.; Gondo, R.; Noguchi, T.; Kimizuka, N.
Angew. Chem., Int. Ed. 2009, 48, 9465.
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M. Chem.sEur. J. 2002, 8, 2026. (b) Hill, R. J.; Long, D.-L.;
Champness, N. R.; Hubberstey, P.; Schro¨der, M. Acc. Chem. Res. 2005,
38, 337.
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K. Clin. Chim. Acta 2009, 404, 105.
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Ions: An Introductory OVerView. In Rare Earths; Saez Puche, R., Caro,
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Nanoscale Ln(III)-Carboxylate Coordination Polymers
A R T I C L E S
Figure 2. SEM and TEM images of Eu-L1 and Yb-L1 NCPs prepared
at 140 °C (10 min reaction): (a) SEM image of Eu-L1; (b) TEM image of
Eu-L1; (c) SEM image of Yb-L1; (d) TEM image of Yb-L1.
Figure 1. SEM images of Gd-L1 NCPs prepared in DMF at different
reaction conditions: (a) 20 °C for 20 min; (b) 140 °C for 5 min; (c) 140 °C
for 20 min. Inset in (a): photo images of Gd-L1 NCPs under daylight and
UV shine.
This approach to stable NCPs colloids can be extended to
the synthesis of NCPs with other lanthanide ions such as Eu(III)
and Yb(III) in a DMF system. Figure 2 showed the SEM and
TEM images of Eu-L1 and Yb-L1 NCPs prepared at 140 °C.
It is noted that all of these NCPs exhibit a discus-like
morphology, which likely resulted from a preferred arrangement
of the building blocks. Therefore, small-angle X-ray diffraction
(SAXRD) was performed to study this interesting self-assembled
nanostructure. The discus NCPs (Gd-L1, Eu-L1, and Yb-L1)
exhibit long-range ordered structures, as revealed by the
reflections in the 2θ range from 2° to 10° shown in Figure 3.
The d-spacing of the reflections and lattice constants calculated
from the SAXRD data were summarized in Table 1.20 The
sequence of reflections with relative positions 1, ꢀ3, ꢀ4 is
characteristic for a hexagonal structure.21
Figure 3. Small angle X-ray diffraction (SAXRD) data: (a) Gd-L1
prepared at 20 °C; (b) Gd-L1 prepared at 140 °C; (c) Eu-L1 prepared at
140 °C; (d) Yb-L1 prepared at 140 °C.
Temperature-Controlled Guest Encapsulations. As antici-
pated, the NCPs can encapsulate functional species. For
example, D1-loaded NCPs could be obtained by addition of
Gd(OAc)3 into the solution containing ligand L1 and D1 in DMF
via a one-step coordination-directed assembly process. However,
the guest loading amount was low (<1 mol % compared with
L1) at room temperature. Considering the stable colloids of bare
Gd-L1 NCPs formed in DMF, we reasoned that some DMF
molecules binding to Gd(III) might interfere with the coordina-
tion of acetate anions. As a solvent, DMF is expected to lose
(or reduce) the competitive strength of coordination at elevated
temperatures. Experimental results showed that the guest loading
amount increased with an increase of the temperature from 20
to 140 °C. As shown in Figure 4, by normalizing the absorption
peak that resulted from L1, the peak of D1 in NCPs at ∼480
nm increased apparently for the sample prepared at 140 °C.
This was further supported by the color changes of the samples
after purification (Figure 4, inset). The shoulder band of L1
that arises after encapsulation of D1 is most likely a result of
the slight aggregation of the particles. Since there is a long tail
of the absorption curve in the visible region, which is attributed
(20) If a is the lattice constant, the Bragg spacing of the first three orders
of reflection from the hexagonal packing pattern should be as follows:
d100 ) ꢀ3a/2, d110 ) a/2, d200 ) ꢀ3a/4; or d100:d110:d200 ) 1:0.577:
0.5.
(21) (a) Tchebotareva, N.; Yin, X.; Watson, M. D.; Samor`ı, P.; Rabe, J. P.;
Mu¨llen, K. J. Am. Chem. Soc. 2003, 125, 9734. (b) Chandrasekhar,
S.; Sadashiva, B. K.; Suresh, K. A. Pramana 1977, 9, 471. (c) Donnio,
B.; Heinrich, B.; Gulik-Krzywicki, T.; Delacroix, H.; Guillon, D.;
Bruce, D. W. Chem. Mater. 1997, 9, 2951. (d) Terazzi, E.; Be´nech,
J. M.; Rivera, J. P.; Bernardinelli, G.; Donnio, B.; Guillon, D.; Piguet,
C. Dalton 2003, 769. (e) Manickam, M.; Belloni, M.; Kumar, S.;
Varshney, S. K.; Rao, D. S. S.; Ashton, P. R.; Preece, J. A.; Spencer,
N. J. Mater. Chem. 2001, 11, 2790.
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A R T I C L E S
Zhang et al.
Table 1. SAXRD Data of the NCPs Prepared at 20 °C (Denoted with *) and 140 °C
Gd-L1*
Gd-L1
Eu-L1
Yb-L1
Gd-L1-D1
d100/nm
2.25
2.34
2.31
2.30
2.28
d110/nm
d200/nm
Lattice constant (a)/nm
d100:d110:d200
1.29
1.13
2.60
1:0.573:0.502
1.35
1.17
2.70
1:0.578:0.5
1.32
1.15
2.67
1:0.571:0.498
1.33
1.15
2.66
1:0.578:0.5
1.31
1.15
2.63
1:0.575:0.504
to Mie scattering caused by nanosized particles,22 a more
accurate loading amount was determined by UV-vis absorptions
(taking extinction coefficients into account) after digesting
Gd-L1-D1 NCPs using acetic acid. The D1 loading amount
based on L1 at 140 °C is ∼6.7 mol %, while that at 20 °C is
less than 1 mol %. This is, to our knowledge, the first report
showing that the guest loading amount in NCPs can be tuned
by the reaction temperature, which would be ascribed to the
high coordination flexibility of lanthanide ions under different
conditions.
SEM images (Figure 5) showed that the D1-loaded
Gd-L1-D1 NCPs prepared at both 20 and 140 °C (with slight
aggregation) maintained the discus-like shape and the sizes are
comparable with those of their corresponding bare Gd-L1
NCPs. In addition, SAXRD analysis (Figure 5c) of Gd-L1-D1
NCPs exhibited very similar reflection peaks with bare Gd-L1
NCPs, which indicated that the guest doping did not influence
the main skeleton of the coordination polymers. It is interesting
that D1-loaded NCPs prepared at 140 °C can release the guest
gradually when dispersed in DMF at room temperature. Ap-
proximately 82% of the encapsulated D1 molecules can be
released after 30 h. It should be noted that the guest encapsula-
tion and release are reversible. The NCPs after release (collected
Figure 5. (a, b) SEM images of Gd-L1-D1 NCPs prepared by
coordination-directed assembly at 20 and 140 °C, respectively; (c) SAXRD
data of the Gd-L1-D1 sample shown in (b); (d) SEM image of
Gd-L1-D1 obtained by post-treatment of Gd-L1 using D1 at 140 °C.
All of the samples were prepared in DMF and washed with ethanol before
measurements.
by centrifugation) can re-encapsulate the guest molecules rapidly
(in 5 min) at 140 °C if they are redispersed into the supernatant
fluid of the reaction mixture. This novel mechanism of high-
temperature fast loading and room-temperature slow release has
great potential for drug delivery and controlled release systems.
As the NCPs can re-encapsulate after release, the guest
encapsulation is also expected by post-treatment of bare Gd-L1
NCPs with D1 at elevated temperatures. We found that, by the
addition of D1 into freshly prepared Gd-L1 colloids at
140 °C, D1 can be incorporated in 5 min while the particles
maintain the original morphology and size, as revealed by SEM
analysis (Figure 5d). The energy-dispersive X-ray (EDX)
analysis of Gd-L1-D1 nanoparticles prepared at 140 °C
revealed the loss of iodine (Supporting Information). This
indicated that the counteranion I- in D1 was exchanged by
another anion (most likely CH3COO-) in the self-assembly
process. The presence of the acetate group was supported by
two new peaks that arise at 1505 cm-1 and 1380 cm-1 in the
IR spectrum. In addition, the stretching mode ν(CdO) of
coordinated DMF that peaked at 1659 cm-1 disappeared due to
the exchange by the acetate anion. To further investigate the
temperature-controlled encapsulation mechanism, comparison
experiments were also performed by post-treatment of purified
Gd-L1 NCPs with D1 at 140 °C. Before post-treatment, the
particles were collected by centrifugation and washed with
ethanol, which was repeated four times to remove the free ligand
and acetate anions. The experimental results showed that purified
Gd-L1 NCPs could not encapsulate D1 molecules. These
experimental results indicate that the negatively charged local
environment around Ln(III) after binding of the acetate anion
Figure 4. Normalized UV-vis absorption spectra of NCPs dispersed in
ethanol: (a) Gd-L1 prepared at 20 °C; (b) Gd-L1-D1 prepared at 20 °C;
(c) Gd-L1-D1 prepared at 140 °C; (d) Gd-L1-D2 prepared at 20 °C;
(e) Gd-L1-D2 prepared at 140 °C. The dashed curves are the absorptions
of free D1 and D2 in ethanol, respectively. Inset: photograph of the
corresponding NCPs dispersed in ethanol.
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Nanoscale Ln(III)-Carboxylate Coordination Polymers
A R T I C L E S
Figure 7. Excitation spectra of Gd-L1-D1 NCPs prepared at 20 °C (a)
and 140 °C (b) after purification. The NCPs were dispersed in ethanol, and
the emissions were detected at 600 nm.
Figure 6. PL spectra of L1 (a), L1 + D1 (b), and L1 + D1 + Gd(OAc)3
prepared at 20 °C (c) and 140 °C (d) in DMF. All of the samples have the
same concentrations of L1; samples b, c, and d have the same concentration
of D1. The dotted line is a UV-vis spectrum of D1 in DMF. Inset: photo
images of samples (a) and (d) under UV light (365 nm).
is the driving force for the encapsulation of cationic guests. The
temperature-controlled coordination-directed encapsulation mech-
anism is also suitable for other functional guests, such as D2
(Figure 4d and 4e). The dye loading amount of D2 is ∼2.8 mol%
at 140 °C. The relatively lower loading concentration of D2 is
most likely due to the larger size of the three fused aromatic
rings of the D2 molecule compared to the linear D1 molecule.
Light-Harvesting Property. The D1-loaded NCPs exhibit an
efficient light-harvesting effect. In the present system, L1
exhibits bright blue-violet emission (Φ ) 0.98 in DMF) while
D1 exhibits very weak emission due to efficient intramolecular
charge transfer (ICT). The good overlap between the emission
spectrum of L1 (donor) and the absorption of D1 (acceptor)
favors the Fo¨rster-type energy transfer between them (Figure
6). However, when L1 is mixed with D1 in DMF with a molar
ratio of 1:2, the strong emission of L1 was quenched but no
energy transfer emission could be observed. This indicates there
is interaction between L1 and D1 and the quenching of the
emission is most likely due to electron transfer or formation of
nonfluorescent complexes.23 Greatly enhanced emission at ∼580
nm appeared after further addition of Gd(OAc)3 (Figure 6, inset).
The excitation wavelength for the spectra here was 365 nm (the
optimal excitation for L1 but not for D1). It should be noted
that even though the samples have the same concentrations of
L1, D1, and Gd(OAc)3, respectively, the NCPs prepared at
140 °C (Figure 6, curve d) exhibit apparently enhanced orange
emission compared with the NCPs that were prepared at 20 °C
(Figure 6, curve c). This indicates that the arrangement of D1
in the nanometer-scaled matrix favors light harvesting.
Figure 8. Fluorescence decay profiles (Ex: 380 nm) of L1 monitored at
420 nm (a-420); Gd-L1-D1 sample prepared at 20 °C monitored at 420
nm (b-420) and 580 nm (b-580); and Gd-L1-D1 sample prepared at
140 °C monitored at 420 nm (c-420) and 580 nm (c-580). All of the samples
were dissolved or dispersed in ethanol.
fluorescence decay was also investigated to study the energy
transfer process. As shown in Figure 8, the fluorescence lifetime
of the free ligand L1 detected at 420 nm (a-420) is 0.88 ns,
which is shortened to 0.162 ns in Gd-L1-D1 NCPs prepared
at 20 °C (b-420), and even shorter 0.026 ns in the Gd-L1-D1
NCPs prepared at 140 °C (c-420). The last value is close to the
time resolution of the detection system. The lifetimes of the
emission of the energy transfer, detected at 580 nm, are longer
and on the order of 2.2 ns for the Gd-L1-D1 NCPs prepared
at both 20 °C (b-580) and 140 °C (c-580). There is a small but
clearly observed delay in the buildup of the decay at 580 nm
showing that they are not directly excited by the laser pulse.
The long-range ordered organization of photophysically active
units at the supramolecular level on the nano- to micrometer
scale is important in energy-transfer processes in photosynthetic
systems, as well as electronic devices based on organic
compounds.24,25 Linear π-conjugated systems are especially
relevant for this purpose, which play crucial roles in organic
electronic devices, as the charge-transfer properties are strongly
The energy transfer was supported by the excitation spectra
(Figure 7) of purified Gd-L1-D1 NCPs prepared at 20 and
140 °C. The experiment results revealed that the emissions from
the D1 in NCPs are strongly enhanced by energy transfer from
L1 in the framework under excitation at 365 nm compared to
direct excitation of the D1 dye at 480 nm. Time-resolved
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E.; Rieger, J.; Weiss, H. Angew. Chem., Int. Ed. 1999, 38, 2188.
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G. C. J. Am. Chem. Soc. 2006, 128, 1188. (c) Kaur, P.; Wu, M.;
Anzaldi, L.; Waldeck, D. H.; Xue, C.; Liu, H. Langmuir 2007, 23,
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Langmuir 2005, 21, 1687. (e) Xu, Q.-H.; Moses, D.; Heeger, A. J.
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Zhang et al.
influenced by the long-range ordering of the conjugated
chromophores.25,26 Compared with the inefficient and costly
covalent syntheses of large molecular arrays, self-assembly or
supramolecular organization provides a facile mechanism for
assembling large numbers of molecules into structures that
can bridge length scales from nanometers to macroscopic
dimensions.24,27-29 Various elegant strategies have been fol-
lowed to achieve organized multichromophoric light-harvest-
ing antennae, such as organogels,28 loading chromophores
into microporous and mesoporous host materials29 or orga-
nosilica structures,30 dye-doped DNA nanofibers,31 and other
biomolecule-templated assemblies.32 NCPs based light-
harvesting materials reported here have considerable advan-
tages for the design of a range of applications, such as
photoreaction and light emitting with tunable emission
wavelengths as the coordination-directed assembly can
incorporate a variety of photoactive materials. For example,
the energy transfer between L1 and D1 can be quenched by
coloading D1 and methylene blue (D2) into NCPs. This
provides possibilities for the construction of more compli-
cated photofunctional nanomaterials.
fabrication of a wide range of multifunctional nanomaterials
by judicious selection of the ligands, metal connectors, guests,
and reaction mediums.
Experimental Section
Materials and Methods. All of the chemicals were purchased
from Sigma-Aldrich and used without further purification. The
deionized (D.I.) water was generated using a Millipore Milli-Q
system (Billerica, MA). MALDI-TOF (matrix-assisted laser de-
sorption ionization time-of-flight) analysis was performed on a
Bruker Autoflex II spectrometer. NMR analysis was performed on
Bruker 300 and Bruker 400 instruments. UV-vis absorptions
spectra of the NCPs were collected at room temperature on a UV-
2450 UV-vis-NIR spectrophotometer with the samples dissolved/
dispersed in ethanol. To determine the guest loading concentration,
NCPs were digested using acetic acid and monitored by UV-vis
absorption taking extinction coefficients (measured in acetic acid)
into account. Fluorescence measurements were performed with a
Fluoromax-4 spectrofluorometer at room temperature. The quantum
efficiency (Φfl) of L1 in a dilute DMF solution was measured using
quinine sulfate in 0.1 mol L-1 sulfuric acid as a standard. IR spectra
were collected on a Bruker Vertex 70 spectrometer (using KBr
pellets) in the range 400-4000 cm-1 (the magnified view from
700 to 1900 cm-1 was shown in Figure S1). Small angle X-ray
diffraction (SAXRD) measurements of the NCPs were performed
using powdered samples on a Philips PW 1729 powder X-ray
diffractometer (Cu KR radiation) over a 2θ range from 0.8° to 10°,
and the data from 2° to 10° were shown in Figure 3. SEM images
were obtained on an LEO 1550 FEG scanning electron microscope.
Transmission electron microscopy (TEM) and EDX analysis were
performed with an FEI Tecnai G2 microscope. The time-resolved
fluorescence decay was investigated using 380 nm picosecond laser
excitation and detected with a 0.5 m spectrometer equipped with a
synchroscan streak camera. The time resolution is determined by
the dispersion in the spectrometer and is typically 20 ps. The NCPs
for fluorescence lifetime measurements were measured in an ethanol
system. Before measurements, the NCPs were collected by centri-
fuge and redispersed in ethanol, which was repeated for four times
to remove free molecules and metal ions.
Conclusions
In summary, we have demonstrated the coordination-directed
organization of π-conjugated molecules into stable nanoparticle
colloids with long-range ordered structures. These nanoscale
coordination polymers exhibit temperature-controlled particle
sizing, guest encapsulation, and release. The negatively charged
local environment around the metal connector is the driving
force for the encapsulation of cationic guests. Guest-loaded
nanoparticles exhibit an efficient light-harvesting property. We
envisage that this coordination-directed organization and en-
capsulation approach is general and should be extended to the
(26) (a) Hoeben, F. J. M.; Jonkheijm, P.; Meijer, E. W.; Schenning,
A. P. H. J. Chem. ReV. 2005, 105, 1491. (b) Schenning, A. P. H. J.;
Meijer, E. W. Chem. Commun. 2005, 3245.
Preparation of NCPs. All of the bared NCPs were prepared by
a similar method using L1 and different lanthanide acetates. Here,
we use the preparation of Gd-L1 as an example: L1 (0.06 mmol,
37.3 mg) was dissolved in DMF (20 mL), and a Gd(OAc)3 ·4H2O
(0.04 mmol, 16.2 mg) solution dissolved in DMF (10 mL) was
added dropwise under gentle stirring. The white colloid was stirred
for 20 min and maintained at room temperature for different
characterizations. SEM analysis revealed the particles are plate-
shaped with diameters of 80-120 nm and thicknesses of 10-20
nm. NCPs with diameters of 500-600 nm can be obtained by
increasing reaction temperatures to 140 °C with a reaction time of
20 min. Anal. Calcd (%) for GdL11.5 ·DMF·H2O: C, 68.73; H, 5.90;
N, 1.19. Found (%): C, 68.70, H, 5.70; N, 0.80.
(27) (a) Cui, H.; Pashuck, T.; Velichko, Y. S.; Weigand, S. J.; Cheetham,
A. G.; Newcomb, C. J.; Stupp, S. I. Science 2010, 327, 555. (b) Abbel,
R.; van der Weegen, R.; Meijer, E. W.; Schenning, A. P. H. J. Chem.
Commun. 2009, 169.
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43, 1229. (b) Ajayaghosh, A.; Praveen, V. K. Acc. Chem. Res. 2007,
40, 644. (c) Ajayaghosh, A.; Praveen, V. K.; Vijayakumar, C.; George,
S. J. Angew. Chem., Int. Ed. 2007, 46, 6260. (d) Ajayaghosh, A.;
Praveen, V. K.; Srinivasan, S.; Varghese, R. AdV. Mater. 2007, 19,
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Ajayaghosh, A. AdV. Mater. 2009, 21, 2059.
Preparation of Gd-L1-D1 NCPs with Different D1
Loading Amounts. All of the guest-loaded NCPs were prepared
by a similar method using L1 and different lanthanide acetates in
the presence of guest molecules. Here, we use the preparation of
Gd-L1-D1 as an example: L1 (0.06 mmol, 37.3 mg) and D1 (0.12
mmol, 47.3 mg) were dissolved in DMF (30 mL), and a
Gd(OAc)3 ·4H2O (0.04 mmol, 16.2 mg) solution dissolved in DMF
(10 mL) was added dropwise under gentle stirring. The mixture
was stirred for 20 min (at 20, 80, or 140 °C) and then collected by
centrifugation and washed with ethanol four times to remove free
L1 and D1. The D1-loaded NCPs can release guests gradually in
DMF at room temperature but are stable in ethanol (does not release
D1 when it is dispersed in ethanol).
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Preparation of Gd-L1-D1 NCPs by Post-treatment of
Gd-L1. L1 (0.06 mmol, 37.3 mg) was dissolved in DMF (20 mL)
at 140 °C, and a Gd(OAc)3 ·4H2O (0.04 mmol, 16.2 mg) solution
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Nanoscale Ln(III)-Carboxylate Coordination Polymers
A R T I C L E S
dissolved in DMF (10 mL) was added dropwise under stirring. The
white colloid was stirred for 20 min at 140 °C, a solution of D1
(0.12 mmol, 47.3 mg) in DMF (10 mL) was added, and the mixture
was stirred for 5 min. The red powder was collected by centrifuga-
tion and washed with ethanol to remove free L1 and D1.
Shankara Narayanan for helpful discussions and Mr. Thomas
Lingefelt for SEM measurement.
Supporting Information Available: Experimental details,
synthetic procedures for the ligand, characterization data, and
complete refs 13a and 25b. This material is available free of
charge via the Internet at http://pubs.acs.org.
Acknowledgment. We acknowledge the support from the
Swedish Foundation for Strategic Research (SSF) within the
Nano-X program (Grant No. SSF [A3 05:204]). We thank Dr.
JA102299B
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