Host–Guest Interactions in a Metal–Organic Framework Isoreticular Series for Molecular Photocatalytic CO2 Reduction

Article abstract of DOI:10.1002/anie.202102729

A strategy to improve homogeneous molecular catalyst stability, efficiency, and selectivity is the immobilization on supporting surfaces or within host matrices. Herein, we examine the co-immobilization of a CO₂ reduction catalyst [ReBr(CO)₃(4,4′-dcbpy)] and a photosensitizer [Ru(bpy)₂(5,5′-dcbpy)]Cl₂ using the isoreticular series of metal–organic frameworks (MOFs) UiO-66, -67, and -68. Specific host pore size choice enables distinct catalyst and photosensitizer spatial location—either at the outer MOF particle surface or inside the MOF cavities—affecting catalyst stability, electronic communication between reaction center and photosensitizer, and consequently the apparent catalytic rates. These results allow for a rational understanding of an optimized supramolecular layout of catalyst, photosensitizer, and host matrix.

Full text of DOI:10.1002/anie.202102729

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Accepted Article
Title: Host-Guest Interactions in Metal-Organic Framework Isoreticular
Series for Molecular Photocatalytic CO2 Reduction
Authors: Philip M. Stanley, Johanna Haimerl, Christopher Thomas,
Alexander Urstoeger, Michael Schuster, Natalia B. Shustova,
Angela Casini, Bernhard Rieger, Julien Warnan, and Roland
A. Fischer
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Host-Guest Interactions in Metal-Organic Framework Isoreticular
Series for Molecular Photocatalytic CO
2
Reduction
Philip M. Stanley,[a,b] Johanna Haimerl,[a,c] Christopher Thomas, Alexander Urstoeger, Michael
[b]
[d]
[d]
[c]
[e]
[b]
[a]
Schuster, Natalia B. Shustova, Angela Casini, Bernhard Rieger, Julien Warnan,* and Roland
A. Fischer*[a]
[a]
P. M. Stanley, J. Haimerl, Dr. J. Warnan*, Prof. Dr. R. A. Fischer*
Chair of Inorganic and Metal-Organic Chemistry, Department of Chemistry,
Technical University of Munich,
Lichtenbergstraße 4, Garching, Germany
E-mail: julien.warnan@tum.de, roland.fischer@tum.de
P. M. Stanley, C. Thomas, Prof. Dr. B. Rieger
WACKER-Chair of Macromolecular Chemistry, Department of Chemistry,
Technical University of Munich,
Lichtenbergstraße 4, Garching, Germany
J. Haimerl, Prof. Dr. N. B. Shustova
Department of Chemistry and Biochemistry,
University of South Carolina,
Columbia, South Carolina, USA
A. Urstoeger, Prof. Dr. M. Schuster
Division of Analytical Chemistry, Department of Chemistry
Technical University of Munich,
Lichtenbergstraße 4, Garching, Germany
Prof. Dr. A. Casini
[b]
[c]
[d]
[e]
Chair of Medicinal and Bioinorganic Chemistry, Department of Chemistry
Technical University of Munich,
Lichtenbergstraße 4, Garching, Germany
Abstract: A strategy to improve homogeneous molecular catalyst
stability, efficiency, and selectivity is immobilization on supporting
surfaces or within host matrices. Here, we examine the immobilization
of CO
2
3
reduction catalyst [ReBr(CO) (4,4’-dcbpy)] and photosensitizer
[
Ru(bpy)
2
(5,5’-dcbpy)]Cl using the isoreticular series of metal-organic
2
frameworks (MOFs) UiO-66, -67, and -68. Specific host pore size
choice enables distinct catalyst and photosensitizer spatial location –
either at the outer MOF particle surface or inside the MOF cavities –
affecting catalyst stability, electronic communication between reaction
centre and photosensitizer, and consequently the apparent catalytic
rates. These results allow for a rational understanding of optimized
supramolecular layout of catalyst, photosensitizer, and host matrix.
Introduction
Catalysis will continue to be central to address global challenges
including rising energy consumption, environmental pressures,
and industrial chemical synthesis, promoting research toward
efficient systems.[1] In artificial photosynthesis based on molecular
catalysts significant progress has been made in the past decades,
however, metal complex instability under reaction conditions
remains challenging.[2,3] Immobilizing molecular catalysts from
homogeneous solution on support materials and providing
synergistic host environments are potential solutions toward
improved catalyst performance and recyclability.[2,4]
Metal-organic frameworks (MOFs) are particularly interesting
(
model) supports as their modular building principle offers a
myriad of topologies, cavity sizes, and molecular catalyst
inclusion capabilities.[5,6,7] Such MOF-based supramolecular host-
guest-systems have been extensively studied for catalytic
reactions from fine chemical synthesis to photocatalysis.[8,9] Solar
fuel generation strategies involving MOFs include hosting,
photoresponsive materials, encapsulation, and scaffolding.[10,11,12]
Figure 1. a Representation of integrated molecular photosystems (spheres) in
various assembly-controlling MOF topologies. b Structures of CO reduction
catalyst [ReBr(CO) (4,4’-dcbpy)] (1) and photosensitizer
Ru(bpy) (5,5’-dcbpy)]Cl (2). c Anchoring sites of 1 and 2 in the isoreticular
UiO (66, 67, 68) host series based on pore sizes and the respective MOF linkers.
2
3
[
2
2
1
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Alongside other hosting materials (e.g., micelles, particles,
covalent organic frameworks), MOF-based systems can display
an abundance of diversity in pore size, surface area and
topologies (Figure 1a) that deeply condition the electronic
communication efficiency between the (photo)electroactive
species.[13] Although few studies have started careful exploration
of MOF parameters on (photo)catalytic performance, e.g.,
promoting intermediates in engineered MOF pores, the
understanding of host-guest effects, specific anchoring sites, and
reactive center distances for molecular catalysts in solar fuel
production remains limited.[14,15] Herein, we rationally conceived a
host-guest system to correlate reactivity with spatial location in
MOF-entrapped molecular photosystems. Two distinct
approaches were employed: (i) specific surface modification
through grafting and/or entrapping of molecular photosystems,
and (ii) variation of average distance between catalysts and
photosensitizers via tuning their molecular ratio. This was
achieved through designing, synthesizing, and evaluating a
supramolecular photosystem/MOF series which systematically
differs threefold in microstructure enabled by varying MOF cavity
sizes (Figures 1b-c).
Figure 2. Immobilization of molecular complexes in MOFs. a Figurative 68
MOFs (octahedra) with dissolved complexes (spheres). b Exemplary ReRu-66
data: Supernatant UV-Vis spectroscopy over time of 66 (10.0 mg) and MeCN
(16 mL) with 1 (0.050 mM) and 2 (0.025 mM). c Time-absorption at 288 nm
during immobilization and during washing.
6 4 4
The chemically stable UiO MOF family, composed of Zr O (OH)
nodes and terephthalic acid derived expanded linkers forming
UiO-66, -67, and -68, was chosen as the model matrix.[16] These
MOFs exhibit a wide range of maximum pore diameters of 8.0,
Non-covalent immobilization of 1 and 2 was achieved by
immersing 66, 67, or 68 in an acetonitrile (MeCN) solution (details
in SI, p. S14) with a defined 2/1 ratio (Figure 2a, Tables S1-
3).[10,12,21] Loading was tracked by supernatant UV-Vis
spectroscopy, showing strong absorption decreases in all cases
reaching a plateau after 24 h (Figures 2b, S5-6). To verify stable
anchoring, the assemblies were then placed in pure MeCN and
no supernatant absorption was detected after 10 h, indicating no
complex leaching (Figures 2c and S6). Two further control
1
3.1, and 17.2 Å, respectively (Figure 1c), which enables
[
17]
systematic, pore size dependent, photocatalysis investigations.
As molecular photosystem components, the CO
catalyst [ReBr(CO) (4,4’-dcbpy)] (dcbpy dicarboxy-2,2’-
bipyridine) (1) and the photosensitizer [Ru(bpy) (5,5’-dcbpy)]Cl
bpy = 2,2’-bipyridine) (2) provide a well-studied benchmark
2
reduction
3
=
2
2
(
delivering modest homogeneous catalyst activity with a sacrificial
electron donor (SED).[18–20] Carboxyl groups on the dcbpy ligands
were chosen to anchor 1 and 2 at the MOF via its nodes and its
amine-modified linkers. The latter has shown stable anchoring
yielding colloidal systems where photoinduced electron transfers
from light-absorbing units to neighboring catalysts in presence of
a SED occur.[10,12,21] These rational host/guest choices allow us to
experiment sets, one with CO
2
H-free molecular complexes and
one with NH -free MOFs, suggested that complex acid groups are
2
essential for amine- and node-anchoring and MOF amines are
required for internal cavity hosting (SI p. S14, Figures S7-8).
2
precisely study CO reduction through 1/2 loading variations on
Table 1. Assembly ICP-MS, loading calculation, and BET data.
MOF outer particle surfaces’ or inside the cavities, in relation to
host pore diameter and molecular size of 1 and 2.
66-based
67-based
68-based
Loading (nmol·mgMOF−1)
ReRu-MOF[a]
Results and discussion
Molecules
1 and 2 were synthesized from literature and
59.5 ± 5.4
58.4 ± 0.8
73.0 ± 4.3
47.1 ± 1.0
93.0 ± 3.9
63.3 ± 1.3
[
18,22]
characterized (Supporting Information, SI; Figure S1).
1’s
−
reduction potential, E(1/1 ), is −0.94 V vs saturated calomel
Calculated max.
surface loading[b]
[20]
electrode (VSCE). Light excitation of 2 yields the triplet excited
3
−
state with E( 2*/2 ) = 1.07 VSCE allowing oxidation of triethanol-
Total pore
loading (%)
/ (surface)
11.4 ± 0.2[c]
17.0 ± 0.2[d]
amine (TEOA) (E(TEOA /TEOA) = 0.59 VSCE) used as a SED.[23–
+
[b]
2
5]
−
As E(2/2 ) ≈ −1 VSCE, exergonic electron transfer to 1 is
possible triggering CO
UiO-66-NH (66), UiO-67-NH
synthesized following modified literature procedures (SI).
2
reduction.[
24–26]
Amine-modified
(67) and UiO-68-NH (68) were
BET area (m )
2
·g−1
2
2
2
Pristine MOF
Re-MOF
959.7 ± 3.9
294.4 ± 3.9
337.6 ± 0.6
1755.7 ± 3.7
1550.8 ± 2.6
2406.7 ± 4.8
1384.5 ± 6.0
[15,27]
The obtained samples showed powder X-ray diffraction (PXRD)
reflexes matching simulated patterns from single crystals,
confirming crystallinity (Figure S2). Density-functional theory
ReRu-MOF
1538.8 ± 4.0
(RMOF 0.4)
287.1 ± 0.8
(RMOF 2.0)
(RMOF 2.7)
(
1
DFT) calculations on 1 and 2 yielded van der Waals spheres of
2.0 and 14.5 Å, respectively – larger than the maximum pore
diameter of 66, but smaller than 68, with 67 in between (Figures
b-c, S3-4).
[
6
a] Average max. from ICP-MS, full data in Tables S4-6. [b] See SI. [c] for Re-
7. [d] for ReRu-68(RMOF 2.0).
1
2
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linkers and increased node spacing compared to 66. Similar PSD
decreases for both Re-67 and ReRu-67(RMOF 0.4) are consistent
with the internal surface being the main contributor to BET surface
area. 68-based MOFs enable simultaneous 1 and 2 entrapment
inside the scaffold in-line with its bigger pore size diameter and
apparent from the substantial
N
2
uptake decrease and
unsymmetrical PSD reduction for ReRu-68(RMOF 2.0). We note
that Re-68 displays a surprisingly large BET surface area
reduction in comparison to Re-67, despite similar loadings, which
potentially results from having both tetrahedral and octahedral
cavities loaded in 68.
2 2
All materials showed CO uptake that behaves similarly as in N -
based experiments (Figure S11). Solid-state UV-Vis
spectroscopy of complex-containing MOFs displayed additional
bands matching 1 and 2 (Figure S12), supporting retained
molecular integrity, albeit with potentially modified photosensitizer
absorption properties.[28] Further, attenuated total reflectance
infrared (ATR-IR) spectra for all 1-loaded MOFs displayed bands
−1
3
at 1917 and 2025 cm characteristic of the Re(CO) moiety,
highlighting the catalyst’s molecular integrity (Figures 3c and
S13).[10,29] Thermal gravimetric analysis revealed earlier
degradation on-sets for functionalized MOFs, attributed to the
loaded complexes (Figure S14). Scanning electron microscopy
(
SEM) visualized particles, showing comparable surfaces and
sizes pre- and post-immobilization, suggesting no aggregation
(
Figures 3d, S15-20). This is in-line with calculated crystalline
domain sizes and hydrodynamic radii (Figures S21-22).[
30]
Calculating MOF surface areas from SEM particle sizes (Figure
3
d) with DFT-optimized complexes gave an estimated maximum
outer surface loading (Table 1). For 66 this matched experimental
values, while actual 67 and 68 provided higher uptake, supporting
internal anchoring (Tables S4-6). Further calculations modeled
molecular guest interactions with tetrahedral and octahedral pore
types and pore loadings (pages S3-4 and Figures S23-27). 1 is
hosted by octahedral pores for 67, by tetra- and octahedral
cavities for 68, while 2 is exclusively loaded into 68’s octahedral
pores. Together with ICP-MS values this suggested that 34% of
octahedral pores are occupied in Re-67, corresponding to 11%
total framework pores (Table 1). For ReRu-68(RMOF 2.0) the total
loading increases to 17%, with up to 38% of octahedral pores
occupied by 2 for photosensitizer-rich ReRu-68(RMOF 3.4).
Figure 3. Assembly characterization. 66 (bottom), 67 (middle), 68 (top) with
pristine MOFs (green), Re-MOF (red), ReRu-66(RMOF 2.7, blue), ReRu-67(RMOF
0.4, blue), ReRu-68(RMOF 2.0, blue), and corresponding post-catalysis ReRu-
MOFs (purple). a N adsorption isotherms at 77 K. b Calculated pore size
2
−1
distributions. c ATR-IR spectra from 1810 to 2075 cm of MOF assemblies and
pure 1 (black). d Particle size histograms from SEM. Bottom to top: Re-66,
ReRu-66(RMOF 2.7), Re-67, ReRu-67(RMOF 0.4), Re-68, ReRu-68(RMOF 2.0).
1
-loaded, and 1- and 2-loaded MOF assemblies, denoted as Re-
MOFs and ReRu-MOFs, respectively, were further characterized
thoroughly, with the main findings discussed below (more in SI).
Precise MOF-entrapped ratio of 2/1 (RMOF, Eq. 1) was determined
by inductively-coupled-plasma mass spectrometry (ICP-MS)
through Ru and Re quantification yielding average maximum
MOF metal loadings increasing from 66 to 67 to 68 (Tables 1, S4-
Confocal microscopy images were recorded for 2 and 2-loaded
6
6- and 68-samples to investigate their spatial luminescence
behavior (details in SI, Figures S28-29). While surface-
immobilized dye on 66 provided reduced luminescence lifetime
estimations compared to pristine 2 with an average photon arrival
time (AAT) of 4.13 ± 0.03 and 4.75 ± 0.02 ns, respectively, 68-
entrapped photosensitizer yielded further lifetime reduction with
an AAT of 3.67 ± 0.05 ns (Figure S28). Additional experiments on
larger 68 crystals (2 µm) enabled spatially resolved luminescence
imaging, clearly demonstrating shorter lifetimes from within the
crystal than on the surface (Figure S29) and indicating
entrapment-induced quenching mechanisms.
Having shown well-defined assembly structures and
compositions within the UiO series (Figure 4a), we systematically
2
investigated photocatalytic CO reduction performance and RMOF
impact on turnover numbers (TONs), compared to homogeneous
conditions. MOF samples in MeCN/TEOA (20/1 v/v) suspension
6
, Figure S9).
ꢅꢆꢇꢈ ꢅꢆRuper mg of MOF
ꢀꢁꢂꢃ ꢄ ꢄ
ꢈ
ꢈ
Eq. 1
ꢅꢆꢉꢈ ꢅꢆReper mg of MOF
PXRD data showed MOF crystallinity retention after molecular
immobilization (Figure S2). N gas adsorption experiments
2
displayed significant uptake differences within the series,
following isoreticular linker expansion,[6,7] as well as a decrease in
all cases upon immobilizing 1 and 2 (Table 1, Figures 3a and S10).
For 66, pore size distributions (PSDs) revealed that the two pore
types decreased by the same volume (Figure 3b). This coverage
renders the underlying network harder to access and blocks both
pore openings similarly. In contrast, 67-based PSDs showed an
unsymmetrical decrease for different pores upon immobilizing 1
in Re-67 that remained identical for ReRu-67(RMOF 0.4). This is
rationalized as the smaller 1 enters the pores, while 2 remains on
the outer surface without fully blocking 67’s pores due to longer
2
were saturated with CO and irradiated at 450 nm (SI) under
vigorous stirring.
3
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ReRu-68 assemblies with RMOF > 2.0 delivered TONs comparable
to homogeneous conditions, however over 8 h instead of 1.5 h
(
Figure 4b). Here RMOF had the strongest impact, as TONs
gradually decreased from ~10 to ~2 with lower RMOF. As both
complexes load inside the MOF, RMOF > 2.0 ensures sufficient 2
close to 1 on average for efficient CO reduction (Figures 4a,c).
2
Post-catalysis analysis conducted on ReRu-68 samples showed
retention of MOF crystallinity and 1’s integrity (Figures 3c, S2, S21,
S22), but substantial Ru leaching (Table S9), suggesting
photosensitizer degradation as a main deactivation source.[10]
Consequently, post-catalysis UiO samples were subjected to
further immobilization of 2 and more catalysis cycles. Only ReRu-
6
2
8 samples showed revived activities reaching ~15 TONs after a
nd
cycle. This process was triggered for another two cycles,
yielding final accumulated TONs of ~19 after 25 h (Table S10),
highlighting 1’s stabilization inside the scaffold (Figure 4a) and
internal anchoring benefits compared to homogenous conditions
and surface anchoring, however coupled to a lower apparent
turnover frequency. Control experiments where ReRu-68(RMOF
2
.0) was pre-incubated for 2 h in a CO
solution without irradiation yielded comparable CO evolution rates
Figure S31), suggesting that initial SED diffusion is not limiting.
2
-saturated MeCN/TEOA
(
Nonetheless, SED replenishment and 2’s degradation products
may contribute to declining rates as slower reaction rates for host-
guest photosystems were previously attributed to reaction
environment change or transport limitations.[10,33,35] Additionally,
luminescence quenching from pore-entrapment (Figures S28-29)
lessens bimolecular electron transfer probabilities potentially
resulting in hindered catalysis kinetics.
Figure 4. a Schematic concept behind catalytic performance differences. b
Accumulated TON vs time plot for 66 (bottom), 67 (middle), 68 (top) with pristine
MOFs (green), Re-MOF (red), ReRu-MOF (blue) with best-performing RMOF
Finally, replacing TEOA by 1,3-dimethyl-2-phenyl-2,3-dihydro-
shown, homogeneous 1 and 2 (black). c First cycle TON vs RMOF
.
1
H-benzo[d]imidazole (BIH) as an innoxious SED that maintains
pore diffusion (maximum molecular diameter of BIH = 10.6 Å)
enabled higher molecular stability and activity.[12,26] ReRu-
While particle scattering is expected to impact overall absorption,
it is comparable within the series due to similar particle sizes. CO
6
6(RMOF 2.7) deactivates within 5 h with final TONs of 419 ± 31,
clearly outperforming corresponding homogeneous conditions
with BIH (TONs = 182 ± 15) (Table S7). ReRu-67 samples
showed limited reactivity, while ReRu-68(RMOF 2.0) combines
reactivity and catalyst stabilization reaching TONs of 506 ± 29
after two 24 h cycles (Table S10). These results further confirm
and H
detected in all runs. CO
labelled CO
produced only 13CO (Figure S30). Control
2
formation was monitored via gas chromatography, with no
H
2
2
was the sole source of CO as 13C-
2
experiments, including pristine MOFs, no SED, or no irradiation
yielded no detectable CO (Table S7). All ReRu-66 assemblies
showed rapid CO evolution reaching ~16 TONs and deactivating
after 1.5 h, due to 1’s established instability under reaction
the TEOA-based experiments with the overall prolonged higher
activity indicating system limitation by TEOA radicals.[10,31]
This performance compares well to state-of-the-art colloidal MOF
conditions, further observed from Re(CO)
3
IR band
[9]
systems with TONs in the mid-100s to low 1000s. In a broader
disappearance (Figure 3c).[31,32] This is superior to homogeneous
TONs with 1 and 2, and 2-free Re-66 (TONs ~11 and ~5,
respectively) (Figure 4b, Tables S7-8), ascribed to efficient
electron transfers between molecular species in direct proximity
on 66’s surface. ReRu-66 samples displayed limited RMOF impact
further suggesting electronic communication from 2 to 1 is not
performance limiting, but rather 1’s instability (Figure 4c). In sharp
contrast, Re-67 and ReRu-67 assemblies yielded marginal CO
formation over 24 h irradiation with a tenuous RMOF impact. For
Re-67, and although theoretically possible as TEOA’s maximum
molecular diameters of 8.6 Å is smaller than 67’s pore size, this is
ascribed to limited TEOA diffusion, reducing efficiency as shown
previous reports on immobilized Re catalysts.[10,12,33,34]
context, our systems are competitive to dye-sensitized TiO
semiconductor particles with surface-anchored
2
a
ReCl(CO)
3
(bpy)-derivative, which reached TONs of 435 in
[36]
organic solvents with BIH. Similarly, hosting analogues of 1 and
2
within organosilica nanotubes yielded TONs ~20 with
[34]
DMF/TEOA under 450 nm irradiation.
Conclusion
As porous matrices are widely employed to host molecular
catalysts, understanding their interactions and correlating
reactivity with guest location is key. Thus, we designed the
isoreticular MOF series that specifically allows for molecule
anchoring to occur on particle surfaces’ and/or inside the cavities
with different photosystem ratios. Prepared assemblies showed
strikingly differing photophysical and photocatalytic behaviors,
from partially quenched luminescence upon dye confinement, to
rapid CO evolution and catalyst deactivation, over encumbered
electronic communication, to lower reaction rates paired with
Results with ReRu-67 samples are in-line with disabled electron
transfer between distant complexes due to the surface-anchoring
of 2 and entrapping of 1. This is supported by decreasing TONs
with higher RMOF values, as the probability of having both dye and
catalyst surface-anchored decreases with excess 2. Re-68 and
4
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[
15] M. Kaposi, M. Cokoja, C. H. Hutterer, S. A. Hauser, T. Kaposi, F.
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Dalton Trans. 2015, 44, 15976.
catalyst shielding and recyclability. These findings show that the
guest anchoring site (inside vs outside) and microenvironment
design (pore size) has distinct advantages and drawbacks that
require a rarely discussed fine tuning. They also shed light on
adequacy between the molecular photosystem and MOF host.
For the latter, intrinsic structures and guest distances have effects
on activity, providing a concept for MOF-based heterogeneous
catalyst development. Future studies could investigate covalent
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activity, as well as cage environment fine-tuning to strengthen
productive directional charge transfer while suppressing
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Our results highlight that host design is paramount, with
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transferrable insights should advance efficient applications at the
interface of porous host and molecular catalysis research.
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[
P.M.S. thanks the Chemical Industry Fonds for a PhD stipend and
Katia Rodewald for SEM. The authors thank Kathrin
Kollmannsberger and Lisa Semrau for proof reading. The
assistance of Heike Glauner, Alexander Fahrner, and Karl-Heinz
Körtje from Leica Microsystems for confocal microscopy is greatly
appreciated. There is no conflict of interest to declare. The
German Research Foundation (DFG) Priority Program 1928
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Keywords: Metal-organic frameworks • Solar fuel production •
Molecular catalysis • Hybrid materials • Host-guest systems
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Angewandte Chemie International Edition
10.1002/anie.202102729
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Entry for the Table of Contents
The search for more efficient and stable catalysts often yields hybrid host-guest systems. This work rationally engineered a series of
metal-organic frameworks as hosts and selectively positioned guests, a CO reduction catalyst and a photosensitizer, on the surface
2
or inside the pores for photocatalytic solar fuel production. The results obtained help understand host-guest-interactions and provide
transferrable criteria for material design.
6
This article is protected by copyright. All rights reserved.