Nanoscale light-harvesting metal-organic frameworks

Article abstract of DOI:10.1002/anie.201007277

Highly crystalline nanoparticles of metal-organic frameworks with efficient light-harvesting properties were prepared by coordination-directed assembly (see picture). Functionalization of the ligands with long alkyl chains effectively stabilizes the nanoparticles and increases crystallinity. Different metal ions and ligands are organized into single nanoparticles and both organic acceptors and lanthanide ions can be co-sensitized within the frameworks. Copyright

Full text of DOI:10.1002/anie.201007277

DOI: 10.1002/anie.201007277
Metal–Organic Frameworks
Nanoscale Light-Harvesting Metal–Organic Frameworks**
Xuanjun Zhang, Mohamed Ali Ballem, Zhang-Jun Hu, Peder Bergman, and Kajsa Uvdal*
Artificial light-harvesting antenna materials have rapidly
gained growing interest in recent years because of their
applications in the design of sensors,[1] light-emitting diodes,[2]
and solar cells.[3] The long-range ordered organization of
donors and acceptors on the nano- to micrometer scale is
crucial for efficient Fꢀrster resonance energy transfer
(FRET) processes in these materials.[4,5] Various elegant
strategies have been developed to achieve organized multi-
chromophoric systems, such as organogels[6a] and hybrid
hydrogels,[6b] vesicles,[7] and biomolecule-based assemblies.[8,9]
Recently, novel approaches to host–guest light-harvesting
systems were achieved by loading dye molecules into a single
crystal zeolite[10] or periodic mesoporous organosilica.[11]
These organizations of dye molecules into long-range ordered
solids have proven to be very promising for attaining the
desired macroscopic properties. To date, however, the use of
nanocrystalline metal-organic frameworks (MOFs) as light-
harvesting materials is less explored. MOFs, also known as
coordination polymers that are assembled from organic
ligands and metal ions, are a very promising type of material
with a wide range of potential properties and applications
including gas sorption, catalysis, magnetism, fluorescence, and
nonlinear optics.[12] Recently, increasing interest has been
paid to the miniaturization of MOFs to the nanometer scale;
these miniaturized coordination polymers can overcome, to
some extent, the limited solution-based behavior of their
corresponding bulk materials.[13] The so-called nanoscale
coordination polymers[13] have potential applications such as
ion exchange,[14] multimodal bioimaging,[15] drug delivery, and
sensing.[16] Recent studies showed that some fluorescent
molecules confined in coordination polymer nanoparticles
by novel adaptive self-assembly or host–guest strategies
exhibit remarkably enhanced fluorescence and/or efficient
FRET.[17,18] Herein, we envisaged the use of nanoscale metal-
organic frameworks (NMOFs) as light harvesting antenna
materials because chromophores densely embedded within
the frameworks can increase the light absorption cross-
section while solution-based behavior of nanocrystals pro-
vides potential for further applications.
In light-harvesting systems, the energy-transfer efficiency
and optical properties always depend on the donor/acceptor
ratios. Compared with encapsulation by weak noncovalent
interactions, self-assembly by stronger metal–ligand complex-
ation can stabilize different components in the frameworks
and decrease the possibility of leakage, which is especially
important for sensors beased on FRET.[1] However, the
arrangement of different components into long-range ordered
frameworks is challenging.[13] Owing to different intermolec-
ular forces in the precursor solution, such as counter ionic
interactions, hydrogen bonding, and p–p interactions, coor-
dination polymers easily aggregate to form amorphous
particles. Recently, crystalline NMOFs have been prepared
by surfactant-assisted processes or by approaches combining
surfactants with hydrothermal techniques, microwave, or
ultrasonication.[15,19] However, further efforts are needed to
remove excess surfactant molecules encapsulated in the
porous structure. Herein, we report
a surfactant-free
method to create highly crystalline NMOFs. Direct function-
alization of ligands using long alkyl chains can effectively
stabilize lanthanide carboxylate nanocolloids. The affinity of
carboxylate groups for lanthanide ions, which have high
coordination numbers, is the driving force for the formation
of stable three-dimensional networks, while the long alkyl
chains in the ligands exclude the aggregation of the final
nanoparticles. Different lanthanide ions and energy donors
and acceptors can be rapidly organized into ordered frame-
works by this coordination-directed assembly process.
Herein, we present a series of different p-conjugated
dicarboxylate ligands with differing side-chain lengths
(Scheme 1). Nanoparticles derived from these ligands were
studied and those prepared from H2L1 were chosen as a
starting point (Figure 1). Well-defined nanocolloids of coor-
dination polymers were prepared by addition of [Ln(OAc)3]
(Ln = Gd, Eu, Yb) to a DMF solution of H2L1 and were left
to react at 1408C for ten minutes. The material is very stably
dispersed in DMF; no precipitate was observed after standing
of the dispersion at room temperature for six months.
Elemental analysis and IR spectra (see Figure S4 in the
Supporting Information) reveal that the ligand is deproton-
ated to form the neutral coordination polymers Ln2(L1)3·-
(DMF)x·(H2O)y (x, y = 1–2), which are in the following
abbreviated as Ln–L1. SEM and TEM reveal that the
particles have discuslike shape with thicker centers and
sharp edges. Representative SEM and TEM images of Eu–L1
nanoparticles are shown in Figure 1a. The discus particles
have average diameters of approximately 200–300 nm and
center thickness of approximately 30–60 nm. High-resolution
TEM (HRTEM) analysis reveals that the discuslike particles
exhibit a long-range ordered structure. As shown in Fig-
ure 1b, highly ordered nanoscale channels can clearly be seen
from the side view of a stand-up Eu–L1 particle. The
[*] Dr. X. Zhang, M. A. Ballem, Z.-J. Hu, Prof. Dr. P. Bergman,
Prof. Dr. K. Uvdal
Department of Physics, Chemistry, and Biology
Linkꢀping University, 58183 Linkꢀping (Sweden)
Fax: (+46)13-28-8969
E-mail: kajsa@ifm.liu.se
[**] We acknowledge the support from the Swedish Foundation for
Strategic Research (SSF) within the Nano-X program (Grant No.
SSF [A3 05:204]) and from VINNOVA with the program Innovations
for future health, Multifunctional Nanoprobes for Biomedical
Visualization DNr: 2008-03011.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201007277.
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ray diffraction (XRD) analysis (Figure 1c). The sharp dif-
fraction peaks indicate that the as-obtained nanoparticles are
highly crystalline. The fine reflexions are attributed to a
hexagonal structure.[21] A diagram of standard hexagonal
packing is shown in Figure 1c (inset). The d100 spacing of Eu–
L1 measured from XRD is consistent with that obtained from
the HRTEM image. The influence of ligand structures on the
final LnIII–carboxylate nanoparticles was systematically stud-
ied. Experimental results reveal that longer alkyl chains
(H2L3, H2L7, and H2L8) and more alkyl groups (H2L1) in the
ligands make nanoparticles more crystalline and lead to less
aggregation (see the Supporting Information).
Different lanthanide ions can be organized by this
coordination-directed assembly strategy. The discus morphol-
ogy does not change when two or three different kinds of
metal ions are used as connectors. For example, stable Eu–
Gd–Yb–L1 colloids were obtained by addition of premixed
[Ln(OAc)3] (Ln = Eu, Gd, Yb, with molar ratios of 1:1:1) to a
DMF solution that contains
Scheme 1. Chemical structures of the ligands with different p-conjuga-
tion lengths and side chains.
H2L1 at 1408C. TEM
images reveal that the Gd–
Eu–Yb–L1
nanoparticles
are relatively uniform with
diameters of approximately
200 nm. XRD data reveal
that the sample has a highly
ordered hexagonal struc-
ture (Figure 1c). Seven fine
reflexions (d spacings:
3.365, 1.948, 1.672, 1.265,
1.121, 0.968, 0.923 nm) cor-
respond to the squared
spacing ratios 1, 3, 4, 7, 9,
12, 13, and to the indexing
(hk) = (10), (11), (20), (21),
(30), (22), (31), respec-
tively.[21] HRTEM images
are shown in Figure 1d.
The distance between two
adjacent pore centers is
approximately 3.3–3.5 nm,
which is comparable to that
of Eu–L1. The presence of
three kinds of lanthanides
was confirmed by energy-
dispersive analysis of X-rays
(EDAX). Figure 1e shows
the EDAX data obtained by
using STEM mode; the data
Figure 1. a) SEM and TEM (inset) images of Eu–L1. b) HRTEM side view of a representative Eu–L1 particle
showing the ordered nanochannels. c) XRD data of Eu–L1 (black) and Eu–Gd–Yb–L1 (blue) nanoparticles.
Inset: Schematic illustration showing the relationship between the lattice constant a and d100 of the hexagonal
packed pattern. d) HRTEM image of an Eu–Gd–Yb–L1 nanoparticle, inset: magnified view of the hexagonal
packed pores. e) EDAX data measured by STEM mode. The data is collected from the region marked with a
red square.
hexagonally packed pores are visualized directly by HRTEM
analysis of a particle lying flat on the copper grid (see
Figure S5 in the Supporting Information). The d100 spacing of
Eu–L1 measured from HRTEM images is approximately
3.3 nm. To date, although numerous porous coordination
polymers have been reported, the direct imaging of the
porous structure by HRTEM was only achieved for some very
rare examples of MOFs that were prepared by hydrothermal
synthesis,[20a,b] or at a surfactant-assisted supercritical condi-
tion.[20c] The nanoparticles were further characterized by X-
was collected from the section marked with red color. This
result reveals that all three kinds of lanthanides are organized
into single nanoparticles. The atomic ratio between the three
elements is 8.6:8.0:8.9 based on EDAX result, which roughly
is consistent with the feed ratio in the synthesis.
Different ligands can also be organized by this coordina-
tion-directed assembly process. Efficient light-harvesting
antenna can be achieved by doping the framework of Ln–
L1 nanoparticles with H2L2. Illustrations that show the
preparation of the light-harvesting NMOF and an energy
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Figure 2. Synthesis and light harvesting study of multicomponent nanoparticles: a) Illustration showing the preparation of Gd–L1–L2 multi-
component nanoparticles. b) Schematic representation of the energy transfer in long-range ordered MOFs. c) Normalized absorption and
emission spectra of H2L1 and H2L2 in DMF. d) Emission spectra of Gd–L1, Gd–L1 mixed with 2 mol% Gd–L2, Gd–(L1+2 mol% L2), and Gd–L2
nanoparticles. The gray curve is the excitation spectrum of Gd–(L1+2 mol% L2) nanoparticles; inset (from left to right): photoimages of Gd–L1,
Gd–L2, and Gd–L1–L2 nanoparticles in daylight (upper row) and under an ultraviolet lamp (365 nm; bottom row). e) Time-resolved fluorescence
decay of Gd–L1, Gd–L2, and Gd–(L1+2 mol% L2). Inset: photoimage of Gd–(L1+2 mol% L2) nanoparticles under excitation by laser (370 nm).
transfer (ET) scheme are shown in Figure 2a,b. Our strategy
is based on the following factors: firstly, the fluorescence
spectrum of H2L1 and the S1 absorption band of H2L2
overlap well, which favors the Fꢀrster-type energy trans-
fer[5–11] between them (Figure 2c); secondly, H2L1 and H2L2
have the same molecular lengths (ca. 3.5 nm), hence the
molecular ordering is not seriously influenced when small
amounts of L2 are added to Ln–L1 frameworks. To rule out
the emission from EuIII and YbIII, we use GdIII as connector to
fabricate light-harvesting nanoparticles.
Stable colloids were obtained by addition of [Gd-
(OAc)3·4H2O] to a DMF solution that contains H2L1
premixed with H2L2 (2 mol%) at 1408C. Nanoparticles with
different H2L2 doping concentrations were also investigated.
The particles kept the discus-like morphology and smooth
surface even if the MOF was doped with 5 mol% of H2L2, as
revealed by SEM measurements (see Figure S6 in the
Supporting Information). In the absence of [Gd(OAc)3], the
strong fluorescence of H2L1 (1 ꢁ 10À6 m in DMF) was not even
quenched after addition of 10 mol% of H2L2 (see Figure S7
in the Supporting Information). In contrast, efficient light
harvesting was observed for Gd–L1 nanoparticles with
2 mol% of L2 in the frameworks. When the multicomponent
nanoparticles are excited at 360 nm (the absorption maximum
of donor L1), apparent quenching could be seen for the strong
emission of Gd–L1 (F = 0.78), and the concomitant increase
in the acceptor emission reached a maximum at about 550 nm
(Figure 2d), thus indicating excitation energy transfer from
L1 to L2. The multicomponent Gd–L1–L2 nanoparticles
exhibit bright white emission (Figure 2d) as the emission
covers most of the visible region (overall F = 0.31).
The energy transfer is supported by the excitation
spectrum of Gd–L1–L2 nanoparticles dispersed in DMF. As
shown in Figure 2d, there is only one band at around 370 nm
in the excitation spectrum, which indicates that the long-
wavelength emission band comes from energy transfer rather
than direct excitation of the H2L2 at 440 nm. Although H2L2
(F = 0.08) has an additional absorption band at about 360 nm,
the emission from direct excitation of L2 at 360 nm is
negligible (Figure 2d, orange curve) because the final con-
centration of L2 for the measurement in Gd–L1–L2 nano-
particles is very low (2 ꢁ 10À8 m). A comparison experiment
was carried out, in which the ester of L2 (compound 3 in
Scheme S1 in the Supporting Information) was used as
acceptor instead of H2L2. Experimental results showed that
there is no efficient FRET observed although ester 3 has
similar absorbance and emission properties; because of the
lack of carboxylate groups, ester 3 cannot be assembled into
the framework by a coordination-directed process. The
tunable emission might also result from physically mixed
Gd–L1 and Gd–L2 nanoparticles. To rule out this possibility,
another comparison experiment was carried out by mixing
Gd–L1 and 2 mol% Gd–L2 nanoparticles (based on ligand
concentrations). As shown in Figure 2d (brown curve), a
slight decrease of the emission intensity of Gd–L1 occurred
after addition of 2 mol% Gd–L2, but no apparent energy
transfer emission was observed. These results indicate that
organization of the donor and the acceptor into long-range
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ordered structures can effectively promote the energy migra-
the first example of co-sensitization of both the organic
acceptor and the metal ion by the same donor in NMOFs.
In conclusion, we have demonstrated a surfactant-free
method to create highly crystalline MOF nanoparticles with
efficient light-harvesting properties. Direct modification of
the ligands by using hydrophobic alkyl chains has been proven
to effectively stabilize the particles. Different metal ions,
donors, and acceptors have been incorporated into the
frameworks by one-step coordination-directed assembly.
Experimental results clearly show that the optical properties
of NMOFs can be enhanced and tuned by chemical manip-
ulation of the inorganic and organic components in the
frameworks. The nanocolloids can form stable dispersions
that are not disassembled by common organic solvents and
thus provide the possibility of spincoating for further
applications. This strategy may encourage efforts toward
multicomponent photofunctional nanomaterials.
tion. Time-resolved fluorescence decay of Gd–L1 nanopar-
ticles dispersed in DMF (Figure 2e) is 0.79 ns. This lifetime is
shortened to 0.33 ns by doping of H2L2, accompanied by a fast
rise in the decay profile of L2 emission. The lifetime of energy
transfer emission in Gd–L1–L2 nanoparticles detected at
550 nm is longer (5.06 ns) compared to free H2L2 (4.79 ns).
These results again indicate energy transfer from L1 to L2
within the frameworks.
We also show herein another kind of light-harvesting
antenna, in which the acceptor ligand (H2L4) and EuIII in
multicomponent nanoparticles can be co-sensitized by the
same donor H2L3 (Figure 3). Coordination-directed assembly
of GdIII and H2L3 with 1% mol of H2L4 doping results in an
efficient light harvesting (Figure 3b). The FRET process is
Experimental Section
Ligand H2L1 was synthesized by a previously reported method.[18b]
The synthesis of the ligands H2L2–H2L8 is described in the Support-
ing Information. Synthesis of Eu–L1 nanoparticles: H2L1 (0.06 mmol)
was dissolved in DMF (25 mL) and heated to 1408C. A [Eu-
(OAc)3·xH2O] (0.04 mmol) solution in DMF (10 mL) was added
dropwise under stirring. The white colloids were stirred for 10 min
and were left to cool to room temperature. The nanoparticles for the
measurements were collected by centrifugation after addition of
ethanol/water (25 mL, 1:1), were washed thoroughly with ethanol and
dried in vacuum. Elemental analysis calcd (%) for [Eu–L11.5
(DMF)1.1· (H2O)1.7]: C 77.5, H 7.9, N 0.7; found (%): C 77.7, H 8.0,
0.8. Other nanoparticles such as Gd–L1 and Yb–L1 were
·
N
synthesized by the same method using [Gd(OAc)3·xH2O] or [Yb-
(OAc)3·xH2O] instead of [Eu(OAc)3·xH2O]. For the synthesis of
multicomponent nanoparticles such as Eu–Gd–Yb–L1, Gd–L1–L2,
and Gd–Eu–L3–L4, premixed ligands and/or [Ln(OAc)3] (Ln = Gd,
Eu, Yb) with appropriate molar ratios were used instead of a single
ligand or lanthanide acetate.
UV/Vis absorption spectra were collected on a UV-2450 UV/Vis/
NIR spectrophotometer. Fluorescence measurements were per-
formed with a Fluoromax-4 spectrofluorometer at room temperature.
The quantum efficiency (Ffl) of the ligands and nanoparticles in dilute
DMF solution was measured using quinine sulfate in 0.1molLÀ1
sulfuric acid and Coumarin-102 in ethanol as standards. Elemental
analysis was performed using a flash 2000 series CHN analyzer. IR
spectra were collected on a Bruker vector 33 Fourier transform
infrared spectrophotometer (using KBr pellets) in the range 400–
4000 cmÀ1. X-ray diffraction (XRD) measurements of the nano-
particles were performed using powdered samples on a Philips PW
Figure 3. a) Normalized UV/Vis absorption and emission spectra of
H2L3 and H2L4 in DMF. b) Excitation and emission spectra of Gd–
(L3+1% mol L4) nanoparticles. c) Excitation and emission spectra of
Eu–L3 nanoparticles. d) Excitation (detected at 590 nm) and emission
spectra of GdIII0.95–EuIII0.05–L31.47–L40.03 multicomponent nanoparticles
dispersed in DMF.
very similar to that of the Gd–L1–L2 system. Interestingly,
very efficient sensitization of EuIII is also observed in Eu–L3
nanoparticles. After coordination with EuIII, the strong
emission of H2L3 is quenched and red emission of EuIII is
sensitized (Figure 3c). These interesting results inspired us to
investigate the optical properties of multicomponent nano-
particles. Figure 3d showed the excitation and emission
spectra of GdIII0.95–EuIII0.05–L31.47–L40.03 nanoparticles. The
multiband emissions (with bands at 420, 530, and 612 nm)
cover the entire visible region, although the intensities are not
well balanced. Excitation spectra monitored at different
emission wavelengths from 390 to 700 nm exhibit very similar
characteristics to those shown in Figure 3b and c, indicative of
co-sensitization of both L4 and EuIII by the same donor L3.
The multiband emissions from these multicomponent nano-
particles could potentially be applied as barcodes[22] and
sensors based on FRET. To the best of our knowledge, this is
1729 powder X-ray diffractometer (Cu
radiation) over 2q ranges
Ka
from 0.88 to 108 and the data from 28 to 148 were shown in Figure 1e.
SEM images were taken on a LEO 1550 FEG scanning electron
microscope. TEM and EDX analysis were performed with a FEI
Tecnai G2 microscope. The time-resolved fluorescence decay (mea-
sured in DMF medium) was investigated by using 370 nm picosecond
laser excitation and was 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.
Received: November 19, 2010
Revised: March 9, 2011
Published online: May 9, 2011
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Keywords: lanthanides · light harvesting ·
metal-organic frameworks · nanocolloids · nanostructures
.
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