Energy transfer from quantum dots to metal-organic frameworks for enhanced light harvesting

Article abstract of DOI:10.1021/ja3097114

Because of their efficient energy-transport properties, porphyrin-based metal-organic frameworks (MOFs) are attractive compounds for solar photochemistry applications. However, their absorption bands provide limited coverage in the visible spectral range for light-harvesting applications. We report here the functionalization of porphyrin-based MOFs with CdSe/ZnS core/shell quantum dots (QDs) for the enhancement of light harvesting via energy transfer from the QDs to the MOFs. The broad absorption band of the QDs in the visible region offers greater coverage of the solar spectrum by QD-MOF hybrid structures. We show through time-resolved emission studies that photoexcitation of the QDs is followed by energy transfer to the MOFs with efficiencies of more than 80%. This sensitization approach can result in a >50% increase in the number of photons harvested by a single monolayer MOF structure with a monolayer of QDs on the surface of the MOF.

Full text of DOI:10.1021/ja3097114

Communication
pubs.acs.org/JACS
Energy Transfer from Quantum Dots to Metal−Organic Frameworks
for Enhanced Light Harvesting
Shengye Jin,†,§ Ho-Jin Son,‡,§ Omar K. Farha,‡ Gary P. Wiederrecht,*,† and Joseph T. Hupp*,‡
†Center for Nanoscale Materials, Argonne National Laboratory, 9700 South Cass Avenue, Argonne, Illinois 60439, United States
‡Department of Chemistry, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208, United States
S
* Supporting Information
and directional exciton migration suggest promising applica-
ABSTRACT: Because of their efficient energy-transport
properties, porphyrin-based metal−organic frameworks
(MOFs) are attractive compounds for solar photo-
chemistry applications. However, their absorption bands
provide limited coverage in the visible spectral range for
light-harvesting applications. We report here the function-
alization of porphyrin-based MOFs with CdSe/ZnS core/
shell quantum dots (QDs) for the enhancement of light
harvesting via energy transfer from the QDs to the MOFs.
The broad absorption band of the QDs in the visible
region offers greater coverage of the solar spectrum by
QD−MOF hybrid structures. We show through time-
resolved emission studies that photoexcitation of the QDs
is followed by energy transfer to the MOFs with
efficiencies of more than 80%. This sensitization approach
can result in a >50% increase in the number of photons
harvested by a single monolayer MOF structure with a
monolayer of QDs on the surface of the MOF.
tions of MOFs, superior to monomer dye molecules, as efficient
light-harvesting and energy-transport components in solar
energy conversion devices.
The efficiency of such a component depends in part on the
fraction of photons in the solar spectrum that MOFs can
absorb. However, few photoactive molecular building blocks of
MOFs are capable of efficiently absorbing photons across the
entire solar spectrum. For example, the UV−vis absorption
spectra of the zinc porphyrin molecules F-ZnP and DA-ZnP
used here to synthesize porphyrin-based MOFs are shown in
Figure 1a. Both exhibit Soret (λ = 422 nm for F-ZnP and 448
etal−organic frameworks (MOFs),1 which are hybrid
Mmaterials made from organic linkers and inorganic
nodes, have received extensive attention because of their
attractive chemical and physical properties, including tunable
and extraordinary porosity,2 long-distance internal energy
migration,3 and the capability of catalysis.4 These properties
lead to numerous potential applications of MOFs in gas
storage,5 sensing,6 chemical separation,7 drug delivery,8 and so
on. In addition to these attractive applications, we are
particularly interested in using MOFs as light-harvesting
antenna and photosynthesis-like ensembles for solar energy
conversion. A few important studies demonstrating the promise
of MOFs for conversion of solar energy to electrical or chemical
energy have been reported previously.9 For example, Lin,
Meyer, and co-workers have reported the fabrication and
energy-migration dynamics of metal−bipyridine-derived MOFs
for light harvesting.10 We reported the successful synthesis of
metalloporphyrin-derived MOFs for light harvesting and (dark)
catalysis.11 Since these metalloporphyrin molecules are
structurally similar to natural porphyrin-type light-harvesting
and photosynthesis pigments,12 porphyrin-based MOFs might
be expected to mimic important aspects of natural photo-
synthetic systems. Our previous studies have shown that in a
zinc porphyrin-based MOF the photogenerated exciton can
migrate over 10−30 porphyrin struts within its lifetime, with a
high anisotropy along a preferred direction.13 The long distance
Figure 1. (a) UV−vis absorption spectra (solid lines) of F-ZnP and
DA-ZnP in toluene and (b) absorption (solid lines) and emission
(dashed lines) spectra of CdSe/ZnS core/shell QDs of different sizes
(QD550 and QD620) in water. The inset in (a) shows the chemical
structures of F-ZnP and DA-ZnP. The inset in (b) compares the
normalized emission spectra of the QDs and the Q-band absorptions
of F-ZnP and DA-ZnP.
nm for DA-ZnP) and Q (λ = 555 nm for F-ZnP and 658 nm for
DA-ZnP) absorption bands, but their absorption spectra cover
only limited portions of the visible region.
Unlike these molecular building blocks, semiconductor
quantum dots (QDs) have broad absorption spectra. They
Received: October 1, 2012
© XXXX American Chemical Society
A
dx.doi.org/10.1021/ja3097114 | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Communication
also feature large extinction coefficients, high luminescence
quantum yields, and excellent photostabilities. Their optical
spectra are systematically tunable via changes in their sizes.14
For example, the absorption and emission spectra of two sizes
of CdSe/ZnS core/shell QDs with emission peaks centered at
550 nm (named QD550) and 620 nm (named QD620) are
shown in Figure 1b. Compared with the porphyrin molecules,
QDs show much broader absorption spectra. These properties
make QDs an attractive class of light-harvesting materials.15
Huo, Farha, and co-workers have recently reported the
incorporation of CdTe QDs within the crystals of a zeolitic
imidazolate framework to introduce photoluminescence (PL)
properties in MOFs.16 However, the incorporation of QDs with
MOFs to enhance light harvesting has not been explored.
In this communication, we report a strategy of using
semiconductor QDs to enhance light harvesting by MOFs
through energy transfer from the QDs to the MOFs. The
strategy is illustrated in Figure 2. The CdSe/ZnS core/shell
QDs and the Q-band absorptions of the MOFs (Figure 1b
inset) indicate that resonant energy transfer from the QDs
(donor) to the MOFs (acceptor) is possible. To sensitize the
QDs on the external surfaces of the MOFs, the MOF samples
were first dried in an oven at 70 °C for 1 h and then exposed to
aqueous QD solutions (0.8 μM) for 1 h. The QD-sensitized
MOFs were finally dropped and dried on the surface of glass
coverslips. Typical TEM images of DA-MOFs before and after
QD sensitization (Figure S3) show the coating of QDs on the
surface of the QD−MOF hybrids. It is believed that the binding
of the QDs on the MOFs is due to amine−Zn coordination
(see Figure 2). The fluorescence lifetimes and emission spectra
of the QD−MOF complexes were obtained using a home-built
confocal microscope system coupled with a time-correlated
single-photon counting module, with λex = 400 nm (the
frequency-doubled output of our Ti:sapphire laser) at a power
of 30 nW. Details of the experimental setup are provided in the
SI and shown in Figure S4.
The strategy of QD-enhanced light harvesting by MOFs was
first examined by time-resolved fluorescence measurements.
The fluorescence decay curves of QD550- and QD620-
sensitized MOFs are shown in Figure 3. For measurements,
Figure 2. (a) Schematic diagram of a QD−MOF complex and (b)
optical microscopy image of a plate-shaped MOF particle. The surface
plane of the MOF contains the 1,2,4,5-tetrakis(4-carboxyphenyl)-
benzene moieties, and Zn metal centers connect the porphyrin blocks.
The number of porphyrin molecular layers determines the thickness of
the MOF particle. The CdSe/ZnS QDs are sensitized on the MOF
surface through the amine−Zn interaction. The QD−MOF hybrid is
able to harvest photons through energy transfer from the QDs to the
MOF even in spectral regions where the MOF has little absorption.
QDs are sensitized on the surface of the porphyrin-based
MOFs through the amine functional groups. The QDs are used
to harvest photons in the spectral region where the MOFs have
little absorption. The photon-generated excitons in the QDs are
then transferred to the MOFs through resonance energy
transfer. Such a QD−MOF hybrid can harvest photons well
beyond the absorption spectrum of the MOF. In this work, the
effectiveness of such a strategy was examined via time-resolved
fluorescence measurements, by which we found that the time
required for energy transfer from the QDs to the MOFs can be
1−2 ns with quantum efficiencies of >80%. Furthermore, the
observation of PL of MOFs due to the energy transfer from
QDs confirmed the light harvesting by the QD−MOF hybrids
even in the spectral regions where the MOFs have little
absorption.
The porphyrin molecules F-ZnP and DA-ZnP (Figure 1a
inset) and their corresponding MOFs (named F-MOF and DA-
MOF, respectively) were synthesized according to the methods
described in the Supporting Information (SI). The powder X-
ray diffraction (PXRD) patterns of the MOFs and space-filling
models of the structures are shown in Figures S1 and S2 in the
SI, respectively. The QD550 and QD620 CdSe/ZnS core/shell
QDs were purchased from Ocean Nanotech. These QDs were
coated by a monolayer of amphiphilic polymer with amine
functional groups and a monolayer of poly(ethylene glycol)
(PEG).17 The spectral overlaps between the emissions of the
Figure 3. Fluorescence decays of (a) QD550 and (b) QD620 on
various substrates (F-MOF, DA-MOF, and glass). Black solid lines are
fits of the decays according to eq S1.
specific optical filters before the detector were used to ensure
that the decay curves were constructed from only the photons
emitted from the QDs (see the SI and Figures S5 and S6 for
details). The lifetimes of both QD samples on both MOFs were
dramatically shorter than on glass. The shortened lifetimes of
the QDs could in principle be due to either charge transfer or
energy transfer between the QDs and the MOFs. Electron
transfer from the QDs to the MOFs is not energetically
allowed, as shown in the energy diagram in Figure S7. Charge
transfer from the MOFs to the QDs is also ruled out because
the fluorescence lifetimes of the MOFs were not changed by
the presence of the QDs. The possibility of energy transfer
from the QDs to free porphyrin molecules that may dissolve
from the MOFs was also excluded by our control experiments
(see Figure S8). We therefore attribute the shortened lifetimes
of the QDs to energy transfer from the QDs to the MOFs.
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Journal of the American Chemical Society
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The assignment to energy transfer is supported by the
spectral overlaps between the emissions of the QDs and the Q-
band absorptions of the MOFs (Figure 1b). For QD550
(Figure 3a), the lifetime of the QDs on F-MOF is shorter than
on DA-MOF, indicating faster energy-transfer dynamics on F-
MOF. This observation is consistent with the relatively large
spectral overlap between QD550 and F-MOF. In contrast to
QD550, QD620 offers a larger spectral overlap with DA-MOF,
hence leading to a shorter lifetime (faster energy transfer) on
DA-MOF than on F-MOF. Additionally, in the fluorescence
decay curves of DA-MOFs (Figure S9), the MOF excited
through energy transfer from QDs exhibits a much longer
kinetic rise time than the MOF excited directly by the laser,
indicating the generation of excitons. This confirms the energy
transfer from QDs to MOFs.
QD−MOF complexes were measured (Figure 4). At λex = 400
nm, PL for F-MOF was clearly observed (Figure 4a-1), while
To resolve the energy-transfer dynamics quantitatively, the
fluorescence decays in Figure 3 were fit with a biexponential
function (eq S1 in the SI), and the lifetimes of QDs on different
substrates were calculated according to eq S2. The average
lifetimes of QD550 and QD620 on glass were 8.0 and 11.4 ns,
respectively. The fluorescence lifetimes of the QDs on glass and
MOF are τglass = 1/(kr + knr) = 1/k0 and τMOF = 1/(k0 + keng),
respectively, in which kr and knr are the radiative and
nonradiative decay constants, respectively, and keng is the
energy-transfer rate constant. The quantum yield of energy
transfer is then given by ϕeng = keng/(k0 + keng). By comparison
of the lifetimes of QDs on glass and the MOFs, the energy-
transfer times (=1/keng) and ϕeng values for the QDs on the two
MOFs were calculated (Table 1). The energy-transfer times
Figure 4. Emission spectra of the MOFs and QD−MOF complexes at
λex = 400 nm: (a-1) F-MOF; (a-2) DA-MOF; (b-1) QD550−F-MOF;
(b-2) QD550−DA-MOF; (c-1) QD620−F-MOF; (c-2) QD620−DA-
MOF. The inset in (b-1) is an expanded view of the QD550 emission.
no PL signal was detected from DA-MOF (Figure 4a-2). These
observations are consistent with the absorption spectra of F-
ZnP and DA-ZnP (Figure 1a), which show that DA-ZnP is
hardly excited at 400 nm. For the QD−F-MOF complexes, the
emissions from both the QDs and F-MOF were detected
(Figure 4b-1 for QD550 and Figure 4c-1 for QD620). The
emissions of QDs correspond to the remaining fluorescence
signals after quenching by energy transfer to the MOF. Since F-
MOF itself has PL at λex = 400 nm, the PL from F-MOF due to
energy transfer is not resolvable in Figure 4b-1,c-1. In the PL
spectra of the QD−DA-MOF complexes (Figure 4b-2 for
QD550 and Figure 4c-2 for QD620), the DA-MOF emission
peak centered at 680 nm was observed in addition to the peaks
from the QDs. Since DA-MOF itself has no detectable PL
signal at λex = 400 nm (Figure 4a-2), the PL from DA-MOF in
the QD−DA-MOF complexes must be due to energy transfer
from the QDs. This confirms the light harvesting by the QD−
MOF hybrids through energy transfer from the QDs to the
MOF even at wavelengths where the MOF has little or no
absorption. In view of the low PL quantum yield of DA-MOF
(0.096), the DA-MOF emission peaks shown in Figure 4b-2,c-2
correspond to the harvesting of ∼2.2 × 105 and ∼4.6 × 105
photons/s by the QD550−DA-MOF and QD620−DA-MOF
hybrids, respectively, under our experimental conditions.
Taking the QD620−DA-MOF complex with the highest ϕeng
as an example, the enhancement in MOF light harvesting can
be quantitatively estimated through calculations (see the SI for
the details). For a DA-MOF presenting a single external
monolayer of porphyrin building blocks, a monolayer of
QD620 enables the MOF to harvest ∼51% more photons
under one-sun conditions. Considering that the exciton can
migrate at least 10 net porphyrin struts in DA-MOF along
preferred directions (this value limits the thickness of the MOF
Table 1. Average Lifetimes, Energy-Transfer Times, and
Quantum Yields of QD550 and QD620 Sensitized on F-
MOF and DA-MOF
F-MOF
DA-MOF
τave (ns) 1/keng (ns)
ϕeng
τave (ns) 1/keng (ns)
ϕeng
QD550
QD620
1.4
5.2
1.7
9.6
82%
54%
2.7
1.8
4.1
2.1
66%
84%
from QD550 to F-MOF and DA-MOF were 1.7 and 4.1 ns,
respectively, and those for QD620 were 9.6 and 2.1 ns,
respectively. The ϕeng values were as high as 82% for QD550 on
F-MOF and 84% for QD620 on DA-MOF. Thus, efficient
energy transfer from the QDs to the MOFs clearly is possible.
We hypothesized that the sensitization of QDs on MOFs is
spatially facilitated by coordination of available Zn metal
centers on the external surfaces of the MOFs by the amine
functional groups on the surfaces of the QDs (Figure 2). To
confirm this interaction between the QDs and MOFs, the
fluorescence decays of similar CdSe/ZnS QDs without surface
amine functional groups were also measured on F-MOF. These
QDs have an emission spectrum and luminescence quantum
yield very similar to those of QD550. The samples were
prepared using the same procedures as mentioned above, and
the QDs were believed to be physically attached to the MOF
surface. The energy-transfer time for these QDs on F-MOF was
calculated to be 8.0 ns (Figure S10), which is much greater than
the time of 1.7 ns for QD550 on the same MOF. This
comparison suggests that the amine functional groups may help
the QDs to be closely attached on the MOF surface through
amine−Zn interactions, leading to a higher energy-transfer rate.
To examine further the light harvesting by MOFs through
energy transfer from QDs, the PL spectra of the MOFs and
to ensure efficient energy delivery in practical applications),13
a
more realistic enhancement capacity of 5−10% is estimated for
DA-MOFs with a thickness of 10 porphyrin layers. One would
expect more efficient energy transfer from QDs to DA-MOF
and hence a greater enhancement of light harvesting if the QD
C
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Journal of the American Chemical Society
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W. Chem. Soc. Rev. 2009, 38, 1248. (c) Ma, L.; Falkowski, J. M.; Abney,
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overlap. Such tuning could be achieved simply by changing the
size of the QDs. QD550 and QD620 were particularly selected
in this study for the convenience of detection in the separation
of the PL from the QDs and that from the MOFs.
In summary, we report here a QD-based strategy for
enhancing light harvesting of appropriately designed MOFs.
With CdSe/ZnS core/shell QDs chemically bound to their
exterior surfaces, F-MOF and DA-MOF are able to function as
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ASSOCIATED CONTENT
■
S
* Supporting Information
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setup and fluorescence lifetime measurements, fitting parame-
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AUTHOR INFORMATION
■
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Corresponding Author
wiederrecht@anl.gov; j-hupp@northwestern.edu
Author Contributions
§S.J. and H.-J.S. contributed equally.
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Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
J.T.H. gratefully acknowledges support from the U.S. Depart-
ment of Energy, Office of Science, Office of Basic Energy
Sciences (DOE-BES) under Grant DE-FG02-87ER13808
(work involving the synthesis of molecular materials). S.J.
acknowledges support from the ANSER Center, an Energy
Frontier Research Center funded by DOE-BES under Award
DE-SC0001059 (work involving QDs and/or time-resolved
fluorescence spectroscopy). Use of the Center for Nanoscale
Materials and support of G.P.W. was funded by DOE-BES
through Contract DE-AC02- 06CH11357.
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