Heterometallic Actinide-Containing Photoresponsive Metal-Organic Frameworks: Dynamic and Static Tuning of Electronic Properties

Article abstract of DOI:10.1002/anie.202016826

Acquiring fundamental knowledge of properties of actinide-based materials is a necessary step to create new possibilities for addressing the current challenges in the nuclear energy and nuclear waste sectors. In this report, we established a photophysics–electronics correlation for actinide-containing metal-organic frameworks (An-MOFs) as a function of excitation wavelength, for the first time. A stepwise approach for dynamically modulating electronic properties was applied for the first time towards actinide-based heterometallic MOFs through integration of photochromic linkers. Optical cycling, modeling of density of states near the Fermi edge, conductivity measurements, and photoisomerization kinetics were employed to shed light on the process of tailoring optoelectronic properties of An-MOFs. Furthermore, the first photochromic MOF-based field-effect transistor, in which the field-effect response could be changed through light exposure, was constructed. As a demonstration, the change in current upon light exposure was sufficient to operate a two-LED fail-safe indicator circuit.

Full text of DOI:10.1002/anie.202016826

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How to cite: Angew. Chem. Int. Ed. 2021, 60, 8072–8080
International Edition:
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doi.org/10.1002/anie.202016826
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Metal-Organic Frameworks Hot Paper
Heterometallic Actinide-Containing Photoresponsive
Metal-Organic Frameworks: Dynamic and Static Tuning
of Electronic Properties
Corey R. Martin, Gabrielle A. Leith, Preecha Kittikhunnatham,
Kyoung Chul Park, Otega A. Ejegbavwo, Abhijai Mathur, Cameron R. Callahan,
Shelby L. Desmond, Myles R. Keener, Fiaz Ahmed, Shubham Pandey,
Mark D. Smith, Simon R. Phillpot, Andrew B. Greytak, and Natalia B. Shustova*
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Abstract: Acquiring fundamental knowledge of properties of
actinide-based materials is a necessary step to create new
possibilities for addressing the current challenges in the nuclear
energy and nuclear waste sectors. In this report, we established
a photophysics–electronics correlation for actinide-containing
metal-organic frameworks (An-MOFs) as a function of
excitation wavelength, for the first time. A stepwise approach
for dynamically modulating electronic properties was applied
for the first time towards actinide-based heterometallic MOFs
through integration of photochromic linkers. Optical cycling,
modeling of density of states near the Fermi edge, conductivity
measurements, and photoisomerization kinetics were em-
ployed to shed light on the process of tailoring optoelectronic
properties of An-MOFs. Furthermore, the first photochromic
MOF-based field-effect transistor, in which the field-effect
response could be changed through light exposure, was
constructed. As a demonstration, the change in current upon
light exposure was sufficient to operate a two-LED fail-safe
indicator circuit.
Introduction
Exploring optical, catalytic, and electronic properties of
actinide-containing metal-organic frameworks (An-MOFs) is
paramount to addressing the growing challenge of radioactive
[1–10]
waste accumulation.
However, unlocking the full poten-
tial of An-MOFs for future applications requires acquiring
extensive fundamental knowledge relative to their behavior,
which is an imperative first step, and thus, is the primary focus
Scheme 1. a) A schematic representation of the tunability of MOF
[11–22]
electronic properties as a function of stimuli-responsive capping linker
integration (blue and red spirals), f-block elements (teal and/or orange
spheres), and guest molecules (purple ovals). b) A schematic repre-
sentation of the MOF-based field-effect transistor. The inset shows an
optical image of drop-cast crystals between the electrodes. c) Two-LED
fail-safe circuit designed for visualization of electronic property control.
of the presented studies.
Hierarchical hybrids, such as
metal-organic frameworks (MOFs), allow for performing
studies of material properties in a stepwise manner by varying
one parameter at a time (Scheme 1a). For instance, they serve
as a multifaceted platform for the integration of a second or
third metal, guests, or organic linkers through various path-
ways (e.g., backbone, side group, or capping linker) with
the possibility for complete replacement of one linker by
electrons) could be the key to repurposing radionuclide waste
or even extend future applications of actinide-based hybrids
to the practical realm. Although there are clear benefits to
this approach, the majority of literature reports focus on
crystallographic aspects as the first step for understanding the
structure of actinide-hybrids, and only very recently have
[
23–26]
another.
Merging the distinct advantages of MOFs (e.g.,
ultra-high porosity, crystallinity, robustness, and modularity)
with the characteristics of actinides (e.g., large cations and f-
[
*] C. R. Martin, G. A. Leith, Dr. P. Kittikhunnatham, K. C. Park,
Dr. O. A. Ejegbavwo, A. Mathur, C. R. Callahan, S. L. Desmond,
M. R. Keener, F. Ahmed, Dr. M. D. Smith, Prof. Dr. A. B. Greytak,
Prof. Dr. N. B. Shustova
studies providing a more fundamental glimpse into material
[27–30]
properties emerged.
This aspect highlights that the field
of actinide-containing metal-organic materials is a relatively
new direction that will grow substantially during the next
decade. This sector faces challenges such as the development
of safety protocols and material instability, but foreshadows
the possibility of solving challenges related to radioactive
waste accumulation through new directions focused on
Department of Chemistry and Biochemistry, University of South
Carolina
Columbia, SC 29208 (USA)
E-mail: shustova@sc.edu
Dr. S. Pandey
[31]
Department of Metallurgical and Materials Engineering, Colorado
School of Mines
Golden, CO 80401 (USA)
practical applications.
Herein, we tailor optoelectronic properties of heterome-
IV
tallic An-MOFs, M -MOFs (M = U and Th), through the
Prof. Dr. S. R. Phillpot
Department of Materials Science and Engineering, University of
Florida
design of heterometallic nodes, integration of stimuli-respon-
sive organic moieties, and inclusion of redox-active guests
(Scheme 1). Two types of guest molecules, iodine and 7,7,8,8-
Gainesville, FL 32611 (USA)
tetracyanoquinodimethane (TCNQ), have been selected for
tuning MOF electronic structures due to two different
reasons: synthetic limitations imposed by scaffold stability
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under:
https://doi.org/10.1002/anie.202016826.
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[32–38]
and redox activity as discussed below.
We consider static
2,2’-indoline]-4’,7’-diyl)dibenzoic acid (H TNDA, Figure 1),
2
and dynamic tailoring of electronic properties through second
metal integration (“irreversible” modifications, i.e., static)
and photochromic linker installation allowing alternation of
an electronic profile as a function of an excitation wavelength
that has a backbone consisting of three phenyl rings to match
[41,42]
the size of the installation pocket in the MOF matrix
and
terminal carboxylate groups that are necessary for coordina-
tion to the metal nodes. Selection of spiropyran-based
building blocks as a class of photochromic molecule for our
studies was due to the fact that they can undergo rapid
photoisomerization inside of porous frameworks (otherwise
(
i.e., dynamic). To the best of our knowledge, there are no
previous reports on dynamically controlled electronic behav-
ior of photoresponsive An-MOFs, as well as no reports on the
preparation of heterometallic photochromic MOFs in gen-
eral; therefore, it is a pressing goal to address this gap in
knowledge through a systematic approach that is presented in
this work. In particular, we harnessed the dynamic control of
electronic properties and established a photophysics–elec-
tronic structure relationship for photoresponsive An-MOFs
by analyzing changes in the electronic and absorption profiles
as a function of an excitation wavelength. We delved further
into tuning the electronic properties of photochromic MOFs
[
41,43]
inaccessible in the solid state),
leading to significant
[39,40]
dynamic changes in material performance.
merocyanine conversion can be induced by applying UV-
irradiation, and the reversible transformation occurs under
Spiropyran!
[44,45]
visible light.
The mechanism of spiropyran photoisome-
rization is based on a heterocyclic CÀO bond cleavage
followed by a cis-trans isomerization to form a highly colored
[44,45]
photoisomer, merocyanine (Figure 1).
Substantial differ-
ences in properties of photoisomers arise from alternation of
a neutral charge and orthogonal geometry (spiropyran) and
4
+
4+
by comparing d- and f-block elements (Zr versus Th and
4
+
[44,45]
U ) for the first time. Furthermore, we gained insight into the
density of states (DOS) near the Fermi edge using theoretical
modeling that supports our experimental findings. The
presented studies also provide the first insight into photo-
isomerization rates within An-MOFs containing photores-
ponsive units. Lastly, we visualized our findings through
engineering the first photochromic MOF-based field-effect
transistor (FET) as well as its integration into a two-LED fail-
safe circuit.
a charge-separated planar zwitterion (merocyanine).
The photoresponsive H TNDA molecule was prepared
2
using a seven-step procedure (see Supporting Information for
synthetic details, Scheme S1). The spiropyran linker,
1
3
H TNDA, and all its precursors were characterized by
C
2
1
and H nuclear magnetic resonance (NMR) and Fourier-
transform infrared (FTIR) spectroscopies, and high-resolu-
tion mass spectrometry (Figures S1–S3). Crystals of H TNDA
2
and the ester precursor (Me TNDA) were analyzed using
2
single crystal X-ray diffraction (Table S1 and Figures S4 and
S5). To probe the possibility for integration of the prepared
[
23,26,30]
Results and Discussion
H TNDA linker, a postsynthetic installation procedure
2
was carried out: Zr-MOF was heated in a H TNDA solution
2
Due to safety specifications required for working with
radionuclides such as uranium and thorium, we performed
preliminary studies to determine reliable experimental con-
for three days (see Supporting Information for details).
Installation was possible due to the fact that Zr-MOF meets
the structural criteria: possessing unsaturated metal nodes
that are necessary for coordinative ligand installation and
IV
ditions using isostructural, but nonradioactive, Zr -based
[
30]
frameworks. Overall, coordinative immobilization of photo-
chromic moieties inside a framework requires: (i) accommo-
dation of structural rearrangements of the photoresponsive
unit accompanied with the photoisomerization process, (ii)
preservation of framework structural integrity upon irradi-
ation with light after several optical cycles, and (iii) a syntheti-
cally feasible pathway for integration of a photochromic unit
into a scaffold (e.g., through postsynthetic modification).
Moreover, the selected photochromic moiety should be
capable of fast photoisomerization inside a scaffold allowing
for dynamic control of photophysical behavior with the
having a suitable pocket size of 15.1 Š which correlates
with the length of the H TNDA linker (15.6 Š, the distance
2
between the centroids of the oxygen atoms of the terminal
carboxylate groups). As a result, two compositions with the
general formula M-L% (M = the metal and L = the amount of
2
À
TNDA linker installed), Zr-33% and Zr-65%, were pre-
pared (Figure S6). Our PXRD studies demonstrated that both
frameworks maintain structural integrity upon UV-irradia-
tion (Figures S7 and S8). Iodine and TCNQ were selected as
guest molecules to further tune the electronic properties of
the prepared MOFs. The rationale behind the guest choice
and experimental details are described below.
[39,40]
possibility of a pronounced effect on material properties.
Therefore, prior to working with heterometallic An-MOFs,
we initiated our studies using a topologically analogous Zr-
MOF, Zr (OH) O (Me BPDC) (Zr-MOF; H Me BPDC =
The selected actinide-containing framework, Th6-
(OH) O (Me BPDC) (Th-MOF), was prepared by heating
8
4
2
4
a solution of Th(NO ) and H Me BPDC in DMF (with the
6
8
4
2
4
2
2
3
4
2
2
[23]
2
,2’-dimethylbiphenyl-4,4’dicarboxylic acid),
that satisfies
addition of trifluoroacetic acid at 1208C for 3 days). Hetero-
the aforementioned criteria. In particular, Zr-MOF possesses
16 Š pores that can accommodate photoisomerization of
metallic Th4.77U1.23(OH) O (Me BPDC) (Th U-MOF) was
prepared by transmetallation of U (OH) O (Me BPDC) (U-
6 8 4 2 4
MOF) in a solution of Th(NO ) in DMF. X-ray photoelectron
3 4
spectroscopy (XPS) studies demonstrated that uranium has
8
4
2
4
5
ꢀ
the photoresponsive fragments. In addition, the metal nodes
of Zr (OH) O (Me BPDC) are unsaturated (i.e., containing
6
8
4
2
4
less than 12 linkers per metal node), allowing for postsyn-
thetic installation of the selected photochromic linker. As
a photoresponsive component, we have chosen a spiropyran-
based linker, 4,4’-(1’,3’,3’-trimethyl-6-nitrospiro[chromene-
a + 4 oxidation state in the parent monometallic U-MOF that
was preserved upon integration of Th (Figures S9 and S10),
IV
and the ratio of uranium to thorium was determined by
[24]
inductively coupled plasma mass spectrometry (ICP-MS).
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Figure 1. (left) Schematic representation of the synthetic pathways for development of photoresponsive monometallic and heterometallic
frameworks containing d- and f- elements pursued in the current work. (right) Building blocks used for construction or modification of
photochromic MOFs. The red and gray spheres in the metal nodes represent oxygen and carbon atoms, respectively.
Interestingly, the reverse transmetallation process (Th-to-U)
did not occur, allowing us to hypothesize that one of the
driving forces for solid-state metathesis to occur is the
oxidation of uranium (from + 4 to + 6), forcing its removal
from the metal node.
frameworks that underwent a digestion procedure using an
[41]
acidic treatment
H NMR spectroscopy to calculate the ratio of H TNDA to
and then we subsequently performed
1
2
H Me BPDC (see Supporting Information for details; Fig-
2
2
ures S17 and S18).
Both monometallic and heterometallic actinide-based
frameworks satisfy the aforementioned criteria for installa-
tion of a photochromic unit. Similar to the zirconium analog,
For spectroscopic characterization of the photoisomeriza-
tion process of the photoresponsive spiropyran moiety
coordinatively immobilized inside a framework, we used
[43]
metal nodes in Th-MOF and Th U-MOF are unsaturated,
diffuse reflectance (DR) spectroscopy. The spiropyran!
merocyanine conversion inside a framework upon 365-nm
excitation resulted in a bathochromic shift (lmax = 570 nm)
detected in the DR profile as shown in Figures S19–S23
(Supporting Information). The observed changes are in line
5
providing a pathway for H TNDA integration (Figure 2). The
2
pore size in the monometallic and heterometallic actinide-
based frameworks are close to 16 Š that is sufficient for
[41]
spiropyran!merocyanine photoisomerization. Initially, we
[44–49]
focused on installation of the photoresponsive H TNDA
with previous reports for spiropyran-based materials.
2
moiety into Th-MOF due to its higher stability in comparison
Using the collected DR data, we also performed a Tauc plot
analysis for evaluation of changes in the optical band gap
with heterometallic Th U-MOF. The integration of a photo-
5
[30,50–52]
chromic linker, H TNDA, inside a framework was performed
under the assumption of direct allowed transitions.
As
2
through heating of Th-MOF in a solution of H TNDA in
a result, by fitting the linear region of the Tauc plots to the
onset of the photon energy, we revealed that the optical band
gap narrows upon exposure to one minute of UV-irradiation
for photoresponsive frameworks (Figure 3, Figures S19–S23,
and Table S2). For instance, a reduction of the optical band
gap from 2.1 eV to 2.0 eV was detected for Th-65% upon in
situ UV-irradiation (Figure 3c). Evaluation of PXRD pat-
2
DMF at 758C and 858C for 3 days, resulting in formation of
Th-34% and Th-65%, respectively. Using the established
experimental conditions for monometallic Zr- and Th-frame-
work preparation, while also considering the difference in
framework stability, we were able to synthesize Th U-50%.
5
Our PXRD studies demonstrated that all An-MOFs maintain
structural integrity upon UV-irradiation (Figures S13–S16).
The amount of the installed linker was determined from
terns of Th-34%, Th-65%, and Th U-50% before and after
5
UV-irradiation revealed preservation of framework crystal-
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Figure 2. (top) A bar graph of conductivity data for Th-MOF, Th-65%,
Th U-50%, and I @Th U-50%. The gray bar correlates with conductiv-
5
2
5
ity values measured for the photoinactive framework (Th-MOF) and
red and blue bars represent pre- and post-UV irradiation (l =365 nm)
ex
of photochromic frameworks, respectively. (bottom) Single crystal X-ray
[
24]
structures of Th-MOF and Th U-MOF.
5
linity (Supporting Information, Figures S13–S16). To provide
further insight into changes in the electronic structure of
photoresponsive An-MOFs, theoretical calculations of the
[
43,53]
DOS
near the Fermi edge were employed. Using the
Vienna ab initio Simulation Package (VASP) and GGA-PBE
level of theory (+ U corrections for U), DOS calculations
were performed for frameworks containing spiropyran and
merocyanine linkers including simulation of the truncated
MOF models for M’-MOF, M’-MOF(spiropyran), and M’-
MOF(merocyanine), where M’ = Zr, Th, and Th/U (Fig-
ures S24–S30 and Table S3). Initially, we evaluated models of
spiropyran and merocyanine linkers themselves (Figure S24)
and then extended the DOS calculations to truncated models
of photochromic MOFs (Figures S25–S30). The electronic
structure calculations revealed that the frontier orbitals
nearest to the Fermi level for spiropyran- and merocyanine-
containing MOFs were localized on the spiropyran or
merocyanine linkers, respectively (Figure 3a). In particular,
the highest occupied molecular orbital (HOMO) was formed
by C(2p ) orbitals and hybridized by N(2p ) orbitals, and the
Figure 3. a) Total and partial DOS calculations of Th U-MOF and
5
photochromic Th U-MOF (spiropyran/merocyanine linkers). b) Nor-
5
malized optical and current cycling of photochromic Th-65% through
alternation of UV- (l =365 nm) and visible- (l =590 nm) irradiation.
ex
ex
Imax and Imin =the maximum and minimum current values, respectively;
Amax and Amin =the maximum and minimum absorption values (con-
verted from reflectance via the Kubelka–Munk function), respectively.
c) A Tauc analysis of the reflectance data for Th-65% before (red) and
after (blue) conversion to the merocyanine photoisomer by five
minutes of UV-irradiation (l =365 nm) showing a shift in the
ex
apparent band edge.
frameworks containing d- or f-block elements was associated
with the integrated photochromic linkers. Indeed, in the case
of Th U-MOF with spiropyran or merocyanine linkers, even
z
z
5
lowest unoccupied molecular orbital (LUMO) was mostly
contributed by O(2p ) orbitals and hybridized by N(2p ) and
with varying positions of the uranium cation in the metal node
(Supporting Information, Figure S30), the frontier orbitals
were identical to the ones found in photochromic Th- and Zr-
y
y
C(2p ) orbitals. The dominating character of the DOS of
y
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MOFs (Figure 3a). In contrast to the photochromic actinide
structures due to two different reasons: synthetic limitations
[32,33]
frameworks, the DOS near the Fermi edge of non-photo-
responsive heterometallic Th/U-MOFs originate mainly from
the uranium 5f-orbitals, whereas the DOS above and near the
Fermi edge (associated with the conduction band) are
composed of uranium and thorium 5f-orbitals. Thus, for the
first time, theoretical modeling allowed us to reveal a funda-
mental difference in electronic structures of non-photores-
ponsive and photochromic actinide-based MOFs.
imposed by scaffold stability and redox activity.
Iodine
doping in porous materials typically exploits a charge transfer
(CT) mechanism that introduces a labile charge transport
material that is affected by the microenvironment and redox
[55–60]
activity (I and TCNQ).
For instance, it has been reported
2
that iodine doping in porous materials resulted in enhanced
conductivity due to I -ligand interactions (e.g., CT complex
2
[61,62]
formation),
a Grotthuss-like charge transport mecha-
[63–65]
To establish a photophysics-electronics correlation, we
combined time-resolved DR spectroscopic studies with con-
ductivity measurements. For the latter, we used a home-built
in situ 2-contact probe pressed-pellet setup (2C3PS) that
allowed for simultaneous irradiation of An-MOFs while
measuring the flow of current at a sweep of applied voltages
nism,
or through metal oxidation in framework no-
[
55,66]
4+
4+
des.
In the presented studies, Th and Zr in metal
nodes cannot be further oxidized, and therefore, conductivity
[61,62]
enhancement likely originates from I -ligand interactions.
2
In a similar vein, TCNQ is a well-studied electron acceptor
and introduction of which commonly results in the formation
[
43]
[67–69]
(
details in the Supporting Information). For DR measure-
of CT complexes.
A number of literature reports have
ments in this instance, the sample background was initially
subtracted in order to visualize only the changes resulting
from the spiropyran photoisomerization process. A mounted
demonstrated that incorporation of TCNQ in MOFs can
[56,57,67]
enhance conductivity.
In our report, we used TCNQ as
4+
4+
a guest in MOFs with Th and Zr metal nodes that are
resistant to further oxidation and as such, the guest-guest or
ligand-guest interactions would likely be the primary mode of
conductivity enhancement. Experimental measurements for
TCNQ-integrated Th-MOF (2.4 TCNQ molecules per pore
determined using the digestion procedure followed by
high-powered LED (l = 365 nm) was used to irradiate the
ex
samples in situ for five minutes prior to measurements, and
then the sample underwent photoinduced attenuation for five
minutes (see Supporting Information for more details).
Similarly, for electronic measurements, a sample was loaded
into the 2C3PS, and a constant voltage of 1 V was applied.
Alternating in situ irradiation with 365-nm and 590-nm light
1
H NMR spectroscopic analysis) revealed a 66-fold increase
in conductivity compared to the parent Th-MOF, which is in
line with the data obtained for TCNQ-integrated Zr-MOF
(
irradiation time for both wavelengths was five minutes) was
[70]
used to induce reversible photoisomerization of a photochro-
mic moiety. This procedure was performed for several optical
cycles, and the rates were extrapolated by fitting the data with
and expected based on literature reports. We also antici-
pated that changes between photoisomers with vastly differ-
ent dipole moments upon irradiation could also affect CT
[54]
[41,44]
a first-order rate equation (Figures 3b and S31). The rate of
processes,
and changes in conductivity could reveal it.
optical attenuation for Th-34% (i.e., merocyanine!spiro-
Therefore, we carried out experiments to measure the
conductivity of photochromic actinide frameworks containing
redox-active guest moieties (TCNQ@Th-65%). As a result,
we were able to modulate the conductivity value by 55%
upon alternating excitation wavelength. Expansion of our
trials on TCNQ inclusion into the heterometallic actinide
framework was limited by the framework stability. Therefore,
we had to select an alternative synthetic route that required
less rigorous conditions for guest inclusion. We chose iodine
since its incorporation could be performed through iodine
vapor exposure, in contrast to rigorous heating (758C) for
3 days required for TCNQ immobilization. Specifically, an
uncapped 0.5-dram vial (containing 8.0 mg of washed MOF)
was placed inside a 20-mL vial that was charged with iodine
crystals, sealed, and left undisturbed for 72 hours. The
quantity of iodine adsorbed was determined by ICP-MS after
an extensive washing procedure (Table S5). Conductivity
pyran conversion inside the MOF) was estimated to be 3.9 ”
À1 À1
1
0
s
which is in line with the measured rate of the
À1 À1
spiropyran moiety integrated inside Zr-33% (1.53 ” 10 s ).
Modulation of electronic properties upon alternation of 365-
nm and 590-nm excitation wavelengths was explored by
monitoring changes in conductivity. Indeed, for both mono-
metallic and heterometallic frameworks containing d- or f-
block elements, integration of a photoresponsive unit allowed
us to study conductivity as a function of an excitation
wavelength. As a control experiment, we performed studies
on the parent scaffolds (i.e., non-photochromic MOFs) and,
as expected, no changes in electronic properties were
detected. Although integration of spiropyran moieties inside
frameworks allowed us to dynamically control electronic
behavior of the material, a significant “static” conductivity
enhancement was achieved through integration of a second
4+
actinide metal, U inside Th-MOF. Indeed, a 31-fold increase
measurements on an iodine-doped framework, I @Th-MOF,
2
in conductivity value was measured for Th U-MOF in
showed a significant enhancement in conductivity: 50-fold for
I2@Th-MOF compared to that of Th-MOF. Dynamic modu-
lation of the electronic properties could also be achieved
through spiropyran-linker integration (Table S4). Notably, all
measurements were performed on bulk powders. Similar to
the monometallic Th-containing framework, integration of
5
comparison with the parent Th-MOF. The measurements of
the U-MOF were challenging due to instability of the sample,
even under anaerobic conditions. Integration of spiropyran
units inside Th U-MOF also allowed us to observe conduc-
5
tivity modulation (Figure 2 and Table S4). An additional
avenue for tailoring electronic behavior of actinide-contain-
ing scaffolds relies on guest integration inside a scaffold. As
mentioned in the introduction, two types of guest molecules,
iodine and TCNQ, were selected to tune MOF electronic
iodine into heterometallic Th U-50% led to changes in
5
electronic properties that were detected through conductivity
enhancement of I @Th U-50% versus Th U-MOF (Ta-
2
5
5
ble S4).
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To visualize the differences in electronic behavior of
2À
photoresponsive MOFs (from TNDA isomerization), we
built a FET (Scheme 1b) using our MOF as the semiconduct-
ing layer. To prepare the FET, the MOF was deposited
between the electrodes on a home-built chip (Scheme 1b, see
Supporting Information for details) by drop-casting a DMF
suspension of MOF crystals. Due to the safety protocols and
limitations associated with Th- and U-containing frameworks,
we utilized a Zr-MOF as a model to illustrate the optoelec-
tronic behavior of photochromic frameworks. A p-type gate
voltage response was observed for the FET prepared with
TCNQ@Zr-65% (drain voltage = 5 V). Interestingly, the
drain current could be modulated with applied gate voltages
and by UV excitation. Specifically, sweeps of gate voltage
(
5
À10 V to 5 V) were measured in the dark, and after
minutes of UV excitation, we observed a shift to a less
negative threshold voltage and increased transconductance
following UV exposure (Figure 4).
We exploited the concept of tunable conductivity ob-
served in photochromic MOFs and facilitated a guest-induced
field-effect response, each of which has previously been
[43,70]
considered independently in other MOFs,
to build a “fail-
safe” circuit powered by our TCNQ@Zr-65%-FET (Sche-
me 1c). As a model system to detect and report hazardous
conditions, the circuit has a constantly illuminated red LED
(
indicating flow of current to a target device) and a “switch-
able” green LED that toggles as a function of UV stimulus
Figures 4a and b). Specifically, for a low total drain current,
(
most of the current flows through the red LED due to its
smaller forward threshold voltage. As the drain current
increases, the voltage drop across the red LED and its series
resistor causes the green LED to come into forward
conduction, so that the ratio of the light intensity varies
sharply with the total current. We verified the operation of the
circuit by separately measuring the current–voltage curves of
the two LEDs (Figures 4a and S46) and then connected it in
series with the TCNQ@Zr-65%-FET. The circuit showed
constant illumination of a lower biased red LED under a set
gate voltage (À5 V); however, upon 365-nm irradiation, the
green LED lit up, indicating electronic property modulation
under 365-nm irradiation. Importantly, the change in drain
current was sufficiently large to drive noticeable changes in
the output of the LED circuit with no further amplification
required. This proof-of-concept study portends the possibility
for orthogonal control of electronic devices by using photo-
chromic MOFs as their active elements, responding both to an
applied electric field and light.
Figure 4. a) Current-voltage curves of green and red (with 1.0 kW
resistor) LEDs used to construct our two-LED fail-safe circuit and
color-coded accordingly. The orange curve is the sum of the green and
red curves. The black squares correspond to the data of the optical
images (right) of the LED dyad powered at varying applied voltages.
b) Circuit diagram for our two-LED fail-safe circuit with integrated
MOF-based FET. c) Plots of drain current against gate voltage for
TCNQ@Zr-65% in the dark (red) and under constant UV irradiation
(
blue, l =365 nm). The inset shows Zr-MOF in the dark (red) and
ex
under constant UV irradiation (blue, l =365 nm).
ex
To confirm that the observed changes were due to the
2À
photoresponsive moieties (TNDA ), we performed control
experiments with Zr-MOF, Zr-65%, and TCNQ@Zr-MOF.
As expected, utilization of Zr-MOF did not result in any
changes to the current during modulation of the gate voltage
stricted exclusively to applied gate voltage and not to UV-
irradiation. Thus, these experiments demonstrated that cur-
rent could only be simultaneously controlled via light and an
applied gate voltage using photochromic frameworks.
(
À10 V to 5 V) in the dark or under UV-irradiation (Fig-
ure 4c). On the other hand, Zr-65% showed changes in
current as a function of UV excitation, but there was no field Conclusion
effect observed, regardless of applied gate voltage. The
TCNQ-containing framework, TCNQ@Zr-MOF, showed
a discernible field effect upon applying a range of gate
voltages; however, the gate-dependence behavior was re-
The aforementioned results demonstrated the first prep-
aration of photochromic monometallic and heterometallic
actinide-containing crystalline scaffolds. Tunability of their
8
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Angewandte
Research Articles
Chemie
electronic properties can be realized by harnessing frame-
work modularity through a stepwise approach (e.g., integra-
tion of photoswitches, secondary metal integration, and guest
incorporation). Notably, the relationship between photochro-
mic An-MOFs photophysical properties and electronic struc-
ture as a function of excitation wavelength was revealed for
the first time. Furthermore, the presented studies provide the
first insight into photoisomerization rates within An-MOFs
containing photoresponsive units. This work also sheds light
on the distinct effect that d- and f-block elements have on the
electronic structure of crystalline scaffolds. Theoretical anal-
ysis probing DOS near the Fermi edge of photoresponsive
An-MOFs indicated that the photochromic linker contribu-
tion dominated in contrast to non-photochromic actinide-
based frameworks, in which DOS near the Fermi edge
originate mainly from the actinide 5f-orbitals. To visualize
the observed dynamic control of electronic properties, we
designed a FET device for orthogonal control of drain current
due to incorporation of photochromic scaffolds. Thus, this
proof-of-concept study showcases a novel way to utilize
a photochromic MOF to control the FET drain current.
Furthermore, we were able to design a two-LED fail-safe
circuit utilizing photochromic MOFs as the FETs’ semi-
conducting layer. To summarize, only the surface of synthetic
methodologies, photophysics, and theoretical modeling of
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Acknowledgements
This work was supported by the NSF CAREER Award
(
DMR-1553634). This work was also supported as part of the
Center for Hierarchical Wasteform Materials (CHWM), an
Energy Frontier Research Center funded by the U.S. Depart-
ment of Energy, Office of Science, Basic Energy Sciences at
the University of South Carolina under award# DE-
SC0016574. N.B.S. thanks the Cottrell Scholar Award from
the Research Corporation for Science Advancement, Sloan
Research Fellowship provided by Alfred P. Sloan Foundation,
Camille Dreyfus Teaching-Scholar Award provided by Henry
and Camille Dreyfus Foundation, and the Hans Fischer
Fellowship. We also acknowledge USCꢀs XPS user facility, as
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Conflict of interest
The authors declare no conflict of interest.
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Keywords: actinides · heterometallic ·
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Manuscript received: December 18, 2020
Accepted manuscript online: January 15, 2021
Version of record online: February 24, 2021
[
8
080 www.angewandte.org
ꢀ 2021 Wiley-VCH GmbH
Angew. Chem. Int. Ed. 2021, 60, 8072 – 8080