Linker Exchange via Migration along the Backbone in Metal-Organic Frameworks

Article abstract of DOI:10.1021/jacs.1c04804

In metal-organic frameworks (MOFs), organic linkers are subject to postsynthetic exchange (PSE) when new linkers reach sites of PSE by diffusion. Here, we show that during PSE, a bulky organic linker is able to penetrate narrow-window MOF crystals. The bulky linker migrates by continuously replacing the linkers gating the otherwise impassable windows and serially occupying an array of backbone sites, a mechanism we term through-backbone diffusion. A necessary consequence of this process is the accumulation of missing-linker defects along the diffusion trajectories. Using fluorescence intensity and lifetime imaging microscopy, we found a gradient of missing-linker defects from the crystal surface to the interior, consistent with the spatial progression of PSE. Our success in incorporating bulky functional groups via PSE extends the scope of MOFs that can be used to host sizable, sophisticated guest species, including large catalysts or biomolecules, which were previously deemed only incorporable into MOFs of very large windows.

Full text of DOI:10.1021/jacs.1c04804

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Linker Exchange via Migration along the Backbone in Metal−
Organic Frameworks
Nader Al Danaf, Waldemar Schrimpf, Patrick Hirschle, Don C. Lamb, Zhe Ji,* and Stefan Wuttke*
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ABSTRACT: In metal−organic frameworks (MOFs), organic linkers are subject to postsynthetic exchange (PSE) when new linkers
reach sites of PSE by diffusion. Here, we show that during PSE, a bulky organic linker is able to penetrate narrow-window MOF
crystals. The bulky linker migrates by continuously replacing the linkers gating the otherwise impassable windows and serially
occupying an array of backbone sites, a mechanism we term through-backbone diffusion. A necessary consequence of this process is
the accumulation of missing-linker defects along the diffusion trajectories. Using fluorescence intensity and lifetime imaging
microscopy, we found a gradient of missing-linker defects from the crystal surface to the interior, consistent with the spatial
progression of PSE. Our success in incorporating bulky functional groups via PSE extends the scope of MOFs that can be used to
host sizable, sophisticated guest species, including large catalysts or biomolecules, which were previously deemed only incorporable
into MOFs of very large windows.
must be a mechanism different from through-window diffusion
(Figure 1c) for PSE.
etal−organic frameworks (MOFs) are extended, porous
Mstructures built by connecting metal-containing clusters
with organic linkers.1 These rigid, sizable molecular building
units create interconnected pore space amenable to guest
incorporation and chemical functionalization, by, for instance,
using organic linkers bearing chemical moieties of interest.2,3
To incorporate a functionalized linker, a strategy termed
postsynthetic exchange (PSE) has been developed, presynthe-
sized MOFs are soaked in the solution of a new linker, which
replaces the original linker without altering the overall MOF
structures.4−13 Applying PSE enables the synthesis of
otherwise unattainable MOFs when de novo synthesis results
in linker decomposition or amorphous products.14
When performing PSE, it is a common practice to choose a
linker smaller than the window (i.e., pore aperture) of the
MOF, for fear that a larger linker will be blocked from
accessing the interior of MOF crystals. This is based on the
assumption that the new linker has to first diffuse through
windows before reaching the site of PSE at a distant pore.15
We sought to explore the possibility of performing PSE in a
prototypical MOF, UiO-67,16 using a bulky linker, biphenyl-
4,4′-dicarboxylic acid attached with rhodamine b (BPDC-RB,
Figure 1a), a fluorophore serving to report PSE progression.
RB is small enough to fit in the octahedral pore in UiO-67, yet
larger than the window (Figure 1b, Figure S5, and section S2
in the Supporting Information), through which diffusion
cannot occur (Figure S6). Surprisingly, we observed a
substantial amount of BPDC-RB incorporated into the interior
of UiO-67 crystals. This is not due to the windows enlarged by
missing-linkers (Figure S7); a low fraction of missing-linkers
(<0.2%) was found in the UiO-67 crystals that we synthesized
(Figure S7). Additionally, UiO-67 is a rather rigid MOF that
cannot undergo excessive pore swelling or breathing necessary
for accommodating incoming bulky linkers.17 Hence, there
We postulated that a bulky linker could migrate across a
narrow-window MOF by continuously replacing the linkers
gating the windows. When an original linker dissociates from
the MOF backbone under PSE conditions,18 it opens the
otherwise narrow window,19 allowing the new linker to move
its bulky group from one pore to the other and then reinsert
into the backbone (Figure 1d). The net result is that the new
linker migrates along the MOF backbone by one pore;
multiple rounds of these actions enable diffusion for a long
distance within the crystal. We term this process through-
backbone diffusion to distinguish it from the conventional
through-window diffusion.
For migration from the crystal surface all the way to the
center, through-backbone diffusion requires PSE to occur in
every pore that the new linker passes through. Every time the
linker transits toward the next pore, it has to first dissociate
from the current site of the backbone, leaving behind a vacancy
(i.e., missing-linker defect). This eventually results in an array
of missing-linker defects along the diffusion trajectory (Figure
1d), provided that the defects are not healed by other free
linkers. In contrast, through-window diffusion creates only one
defect when the migrating linker departs from the backbone;
no more defects are created because it passes through windows
without PSE (Figure 1c).
Received: May 9, 2021
Published: July 6, 2021
https://doi.org/10.1021/jacs.1c04804
J. Am. Chem. Soc. 2021, 143, 10541−10546
© 2021 American Chemical Society
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Figure 1. (a) Molecular structures of BPDC and BPDC-RB. (b) Space-filling model of an octahedron unit in UiO-67, which comprises a BPDC-RB
linker, and a projection of BPDC-RB beneath a triangular window. (c) Structure illustration of through-window diffusion of a small linker and (d)
through-backbone diffusion of the bulky linker BPDC-RB in UiO-67. The horizontal arrows indicate the average direction of many diffusion events,
which follows the concentration gradient established from the crystal surface to its center. Color code for BPDC-RB: C, gray; O, red; S, yellow; N,
green. Color code for other UiO-67 components: Zr cluster, blue; C and O, off-white.
Here, we use fluorescence imaging to monitor the migration
of the bulky BPDC-RB linker across UiO-67 crystals and
observed that the PSE propagated from the surface to the
center of the crystals, establishing a concentration gradient of
the new linker. To investigate how the bulky linker can migrate
across the narrow-window MOF, we performed fluorescence
lifetime imaging microscopy (FLIM)20 to examine the change
in the chemical environment21−23 along the PSE gradient. The
lifetime was observed to decline toward the edge of the crystal,
indicating the accumulation of missing-linker defects during
PSE, a distinctive feature supporting the through-backbone
diffusion mechanism.
UiO-67 was synthesized as single crystals (∼50 μm), far
larger than the spatial resolution of conventional fluorescence
microscopy, therefore serving as a suitable object for our
studies. The obtained crystals were subjected to PSE through
incubation with BPDC-RB in a mixture of methanol (MeOH)
and N-N′-dimethylformamide (DMF) at a ratio of 80:20 (v/
v). We chose this solvent mixture (MeOH/DMF) because it
has been reported that MeOH facilitates linker substitution in
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UiO-6718 and DMF allows for high temperature incubation.
To investigate the effect of time and temperature on PSE,
incubation was performed at room temperature or 100 °C for
1 day or for 7 days (1d-RT, 1d-100 °C, 7d-RT, and 7d-100
°C). After PSE, we did not observe any change in crystal
morphology and crystallinity (Figure S9) or any photophysical
damage to the RB moiety (Figures S10−S12 and Table S1).
Fluorescence intensity imaging microscopy was employed to
measure the spatial distribution of BPDC-RB incorporated
inside UiO-67 crystals. These crystals, octahedral in shape
(Figure 2a), were oriented along the [111] direction, with
Figure 3. Fluorescence imaging of UiO-67 crystals after PSE in
MeOH/DMF: (a) fluorescence images taken at the surface and (b) 3
μm above the surface. The scale bar is 15 μm. (c) Fluorescence
intensity profile along lateral and (d) axial directions (error bars from
measurements on 8−10 crystals of UiO-67 per condition). The curves
recorded at t = 0 (before any PSE takes place) display the
autofluorescence of the pristine crystals (Figure S8).
Figure 2. (a) Schematic diagram of an octahedral crystal of UiO-67,
(b) crystal is oriented along the [111] direction, and (c) fluorescence
and lifetime images were collected both laterally and axially.
triangle and hexagonal cross sections displayed on the focal
plane (Figure 2b). The fluorescence intensity of RB was
mapped in the focal plane (lateral) and along the optical axis
(axial) (Figure 2c), both displaying a profile that peaks at the
surface and slowly decreases toward the center (Figure 3).
We noticed that progression of PSE is highly dependent on
the incubation temperature. A significant increase in
fluorescence was observed when performing PSE at 100 °C
as compared to RT (Figure 3a−d). At RT, a longer reaction
time led to more PSE on the surface but was not effective in
promoting PSE in the crystal interior. In contrast, longer
reaction times at 100 °C resulted in more BPDC-RB
penetrating into the crystals. The solvent dependence of PSE
was tested by using DMF as an alternative solvent. For DMF,
PSE was found sluggish and required high temperature to
achieve any noticeable progress (Figure S14). The heteroge-
neous distribution of BPDC-RB, which concentrates on the
surface and declines toward the crystal interior, reveals that
linker diffusion becomes the rate-limiting step of PSE. This
step, unlike the free diffusion of linkers in solution and in large-
window MOFs, likely involves a high energy barrier, indicated
by the dependence of PSE progression on temperature, time
(weekly scale), and solvent. In fact, the diffusion of RB alone is
prohibited inside UiO-67. After soaking UiO-67 crystals with
RB in DMF for 7 days, only fluorescence emission from the
crystal surface was observed (Figure S6), indicating that RB
itself cannot diffuse through the window, let alone the bulkier
BPDC-RB. Alternatively, through-backbone diffusion becomes
a plausible mechanism to account for the slow penetration of
BPDC-RB.
To elucidate the mechanism of linker migration, we
employed FLIM, a technique used to acquire spatially resolved
fluorescence lifetime information. In the phasor approach, the
obtained fluorescence lifetime decay (Figure 4a) is trans-
formed into Fourier space followed by a graphical translation
into a phasor plot,24,25 with long lifetimes near (0.5, 0.5) and
short ones near (1, 0) (Figure 4b, section S1.6 in the
Supporting Information). In a previous study, we established a
correlation between the fluorescence lifetime and local defects
in UiO-67: more defects result in shorter lifetime.26 Thus, by
using BPDC-RB as both a PSE participant and a fluorescence
lifetime reporter, we investigated the formation and spatial
distribution of defects accompanying the progression of PSE.
The measured lifetime values vary from 0.65 ns (high defect
level) to 2.91 ns (low defect level) (Figures S16−S20). A
lifetime of 2.91 ns is even larger than the lifetime of BPDC-RB
in solvents (2.1−2.3 ns) (Table S1), suggesting suppression of
nonradiative pathways and stabilization of fluorescence in the
MOF.
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Figure 4. Fluorescence lifetime analysis of UiO-67 crystals after PSE in MeOH/DMF: (a) fluorescence lifetime decay, (b) phasor plot of lifetime,
and (c) FLIM images of UiO-67 crystals. The scale bar is 15 μm.
For PSE conducted for 1 day at 100 °C, the FLIM results
displayed significant differences in lifetime between the crystal
surface and interior (Figure 4c(i)). The lifetimes measured at
the surface (1.36 0.22 ns) are shorter than those inside the
crystals. The FLIM images taken at 2 μm above surface
showed a lifetime of 2.14 0.20 ns at the core while the edge
exhibited the same short lifetime observed for the surface
plane. By shifting the focal plane deeper (4 μm above surface),
even longer lifetimes (2.55 0.15 ns) were observed in the
crystal core, establishing a gradient of lifetime increasing from
the edge to the center. Interestingly, such a gradient in lifetime
is analogous to the previously measured fluorescence profile,
which exhibits a decay in fluorescence intensity from the
outside toward inside of the crystal. This correlation suggests
that the progression of PSE is accompanied by the generation
of missing-linker defects, i.e., PSE leaves defects in its wake.
There are two possible origins for the missing-linker defects.
The first possibility is that MeOH, a PSE solvent, can replace
the original BPDC linker in UiO-67 by coordinating to the
zirconium clusters. However, this process alone cannot
establish and maintain a defect gradient more than 1 day,
because the diffusion of methanol is largely unimpeded due to
its small size. This claim is supported by previous observations
that the incorporation of linker-substituting MeOH into UiO-
67 plateaued within 1 day, even at 40 °C.18 An alternative
explanation for the observed gradient of missing-linker defects
would be that these defects are the product of PSE according
to the through-backbone diffusion mechanism. This mecha-
nism agrees well with our observation that there are more
defects on the surface where many through-backbone
trajectories start from and fewer defects in the core where
only a few PSE events reach this depth.
Prolonged PSE for 7 days at 100 °C resulted in a higher level
of fluorescence quenching; a shorter lifetime of 0.90 0.21 ns
was found. Interestingly, a homogeneous distribution of short
lifetime was observed both at the surface and deeply in the
interior of the crystals (Figure 4c(ii)). This is in contrast to the
lifetime gradient observed for 1d-100 °C and matches the
trend of PSE progression: the BPDC-RB concentration
increases throughout the whole crystal when PSE is prolonged
to 7 days (Figure 3c,d). Nevertheless, compared with the
interior, the surface still has a higher level of PSE, but its defect
level does not increase correspondingly. This might result from
an equilibrium between defect generation and healing at high
levels of PSE; replaced BPDC linkers can be reincorporated
and thus heal missing-linker defects during their migration
toward the outside, the opposite direction of BPDC-RB
migration. This hypothesis is supported by a careful
investigation of the FLIM data, which exhibits slightly higher
lifetime at the surfaces compared to the interior (Figure S20).
We further investigated PSE at RT and found long lifetimes
(Figures S16−S18) and hence low concentrations of defects,
consistent with the low level of BPDC-RB incorporation.
Extension of PSE from 1 day to 7 days led to a moderate
decrease in lifetime. Likewise, PSE conducted in pure DMF
resulted in a similar yet less pronounced temperature
dependence: higher fluorescence lifetimes were observed at
RT as compared to 100 °C, the latter showing a lifetime
gradient from outside toward inside (Figures S21−S25). We
argue that PSE at RT is in general unproductive because it
failed to initiate the dissociation of BPDC, an energetically
expensive step yet a prerequisite for the subsequent through-
backbone diffusion of BPDC-RB. Once this energy barrier is
overcome and the migration of BPDC-RB is initiated, more
and more missing-linker defects will be created. These defects
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can act to accelerate the following PSE events by opening the
narrow windows, making the whole process autocatalytic. This
highlights the importance of finding the optimal conditions for
preforming PSE with bulky linkers.
Our studies of PSE using BPDC-RB present a unique
scenario where only through-backbone diffusion is possible.
For linkers smaller than the MOF window, we envision that
both through-backbone diffusion and through-window dif-
fusion can occur. PSE will proceed via the mechanism that is
most efficient. We expect that PSE via through-backbone
diffusion is applicable to many other MOFs, allowing for the
incorporation of sizable, sophisticated functional groups.
NanoScience Munich (CeNS), and the LMU innovative
BioImaging Network.
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/jacs.1c04804.
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Zhe Ji − Department of Chemistry, University of California-
Berkeley, Berkeley, California 94720, United States; Present
Address: Department of Chemistry, Stanford University,
Stanford, CA 94305; orcid.org/0000-0002-8532-333X;
Email: zhej@stanford.edu
Stefan Wuttke − Department of Chemistry and Center for
NanoScience, University of Munich, 81377 Munich,
Germany; BCMaterials (Basque Center for Materials,
Applications & Nanostructures), 48940 Leioa, Spain;
IKERBASQUE, Basque Foundation for Science, 48009
Bilbao, Spain; orcid.org/0000-0002-6344-5782;
Email: stefan.wuttke@bcmaterials.net
Authors
Nader Al Danaf − Department of Chemistry and Center for
NanoScience, University of Munich, 81377 Munich, Germany
Waldemar Schrimpf − Department of Chemistry and Center
for NanoScience, University of Munich, 81377 Munich,
Germany
Patrick Hirschle − Department of Chemistry and Center for
NanoScience, University of Munich, 81377 Munich, Germany
Don C. Lamb − Department of Chemistry and Center for
NanoScience, University of Munich, 81377 Munich,
Germany; orcid.org/0000-0002-0232-1903
Complete contact information is available at:
https://pubs.acs.org/10.1021/jacs.1c04804
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
■
We thank O. M. Yaghi (University of California, Berkeley) for
providing resources, mentorship, support, and helpful dis-
cussions. We thank H. Wang (University of California,
Berkeley) for help with the NMR data analysis. We are
grateful for financial support from the Deutsche Forschungs-
gemeinschaft (DFG) and the SFB1032 (Project-B3, D.C.L.).
We also thankfully acknowledge the support of the Excellence
Cluster Nanosystems Initiative Munich (NIM), the Center for
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