Radiochromic Hydrogen-Bonded Organic Frameworks for X-ray Detection

Article abstract of DOI:10.1002/chem.202101061

Porous materials have been investigated as efficient photochromic platforms for detecting hazardous radiation, while the utilization of hydrogen bonded organic frameworks (HOFs) in this field has remained intact. Herein, two HOFs were synthesized through

Full text of DOI:10.1002/chem.202101061

Accepted Article
Accepted Article
Title: Radiochromic Hydrogen-Bonded Organic Frameworks for X-Ray
Detection
Authors: Tian-Fu Liu, Mojtaba Khanpour, Wen-Zhou Deng, Zhi-Bin
Fang, Yu-Lin Li, Qi Yin, An-An Zhang, Farzaneh Rouhani, and
Ali Morsali
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To be cited as: Chem. Eur. J. 10.1002/chem.202101061
Link to VoR: https://doi.org/10.1002/chem.202101061
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10.1002/chem.202101061
Chemistry - A European Journal
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Radiochromic Hydrogen-Bonded Organic Frameworks for X-Ray
Detection
Mojtaba Khanpour [a and b], Wen-Zhou Deng [a], Zhi-Bin Fang [a], Yu-Lin Li [a], Qi Yin [a], An-An Zhang [a],
Farzaneh Rouhani [b], Ali Morsali [b], Tian-Fu Liu*[a and c]
[a]
Dr. Mojtaba Khanpour, Mr. Wen-Zhou Deng, Dr. Zhi-Bin Fang, Ms. Yu-Lin Li, Dr. Qi Yi, Mr. An-An Zhang, Prof. Tian-Fu Liu
State Key Laboratory of Structural Chemistry, Fujian Institute of Innovation, Fujian Institute of Research on the Structure of Matter, Chinese Academy of
Science, Fujian, Fuzhou, China.
E-mail: tfliu@fjirsm.ac.cn
[b]
[c]
Dr. Farzaneh Rouhani, Prof. Ali Morsali
Department of Chemistry, Faculty of Sciences, Tarbiat Modares University, P.O. Box 14115-175, Tehran, Iran
University of the Chinese Academy of Sciences, Fujian, Fuzhou, 350002, China.
Although photochromic behavior can be found in both inorganic[1c,
Abstract: Porous materials have been investigated as efficient
photochromic platforms for detecting hazardous radiation, while the
utilization of hydrogen bonded-organic frameworks (HOFs) in this field
has remained intact. Herein, two HOFs were synthesized through self-
assembly of tetratopic viologen ligand and formic acid (PFC-25, PFC-
26), as a new class of “all-organic” radiochromic smart materials,
opening a gate for HOFs in this desired field. PFC-26 is active upon
both X-rays and UV light, while PFC-25 is only active upon X-rays.
The same building block yet different radiochromic behaviors of PFC-
25 and PFC-26 allow us to gain a deep mechanistic understanding of
the factors that control the detection specificity. Theoretical and
experimental studies reveal that the degree of π-conjugation of
viologen ligand is highly related to the threshold energy of triggering
a charge transfer, therefore being a vital factor for the particularity of
radiochromic materials. Thanks to its convenient processibility,
nanoparticle size, and UV silence, PFC-25 can be further fabricated
into a portable naked-eye sensor for X-ray detection, which shows
obvious color change with the merits of high transmittance contrast,
good sensitivity (reproducible dose threshold of 3.5 Gy), and excellent
stability. The work exhibits the promising practical potentials of HOF
materials in photochromic technology.
4]
and organic[5] materials, it is so far rather rare to find
radiochromic materials with satisfactory working stability and easy
processability for fabricating X-ray energy dependent and low X-
ray dose sensor. Hydrogen-bonded organic frameworks (HOFs)
is a type of materials based on pure organic ligands connecting
through H-bonding, which allows us to easily incorporate desired
photoactive moieties into the framework for preparing
photochromic materials.[6] Moreover, the unique features of high
crystalline and porosity, tunable structure, effortless regeneration
and processability not only allow for an understanding of the
photochromism mechanism but also facilitate the material
processing and manufacturing techniques. Viologen (N, N-
disubstituted bipyridinium) belongs to a large subgroup of
electron-transfer photochromic materials, which inspired us to
design and fabricate novel radiochromic HOF sensor using
viologen-based ligands.[1b, 7] Despite being applicable in this field,
the key challenges for viologen-based materials have yet to be
addressed, such as the detection specificity, switch speed, aerial
oxidation, agglomeration problem caused by π-π stacking, and
long-term stability. [8] Therefore, many studies focused on organic
synthesis to create molecules with desired functional groups and
suitable geometries to achieve UV/visible/x-ray light triggered
photochromism.[9] However, these requirements might be fulfilled
in HOFs through rational self-assembly of viologen moieties
without necessarily resorting to tedious organic synthesis.
Moreover, the precise structural information of HOFs allows us to
elucidate the vital factors that control the detection sensitivity and
specificity.
Introduction
The detection and identification of X-ray emission possess high
importance because of its extensive increasing demand in
medical diagnosis and treatment, lab analysis, astronomy,
industrial crack detection, and security checks.[1] A crucial
objective in the field of radiation detection is the ability to convert
the high-energy X-ray photons into other easily detectable
information or signal to indicate the exposure to X-ray
surroundings.[2] In this regard, it is vital to investigate novel
photochromic materials as ideal candidate for X-ray detection with
the merits of visualization, low-cost and simple apparatus.[1d, 3]
Herein, starting from a tetratopic viologen ligand with formate
counterion as electron donor, we synthesized two radiochromic
HOFs, PFC-25 and PFC-26 (PFC stands for Porous materials
from FJIRSM, CAS) as a new class of X-ray radiochromic sensor.
Interestingly, PFC-26 is active to both X-ray and UV-light while
1
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PFC-25 is only active to X-ray irradiation. The same building block
yet different behavior provides a good chance to investigate the
correlation between the crystal structure and photo-responsive
properties. Investigation results show that besides the electron
donor and acceptor distance[10], the degree of π-conjugation of
ligands has a sizable impact on the electron transfers and
therefore the photochromic behaviors. Thanks to the nanoparticle
size, easy processability and UV silence, PFC-25 was fabricated
into thin film as a sensitive sensor for x-ray detection. The
obtained film exhibits naked eye-detectable color change from
light yellow to green upon X-ray irradiation with obvious
transmittance contrast, good cycling stability, reversible
bleaching, and aerial stability.
to 3.445 Å for PFC-25 and from 2.49 to 3.16 Å for PFC-26,
respectively. Of special note, in both PFC-25 and PFC-26, we
cannot observe π-π stacking between the aromatic rings because
of the separation of viologen molecular planes.
Thermal and chemical stability test. The stabilityof PFC-25 and
PFC-26 under external stimuli such as thermal, photolytic,
chemical and electronic items is essential prerequisite for their
commercialization. Therefore, the chemical stability of PFC-25
and PFC-26 was investigated by immersing in different solvents
for 24 h (Figure 2b). Also, the thermal stability was confirmed by
with variable temperature PXRD, approving acceptable and
highly chemical / thermal stability (Figure S10). The porosity of
PFC-25 and PFC-26 was evaluated as another crucial item by
CO2 absorption measurement at 196 K (Figure 2a). The CO2
uptakes of 29.2 cm3/g for PFC-25 and 25.7 cm3/g for PFC-26,
showing the existenceof permanent porosity in structures with the
typical type-I CO2 adsorption isotherm. The relative low uptake
probably caused by the interpenetration of the 1D or 2D layered
structures.
Results and Discussion
Crystal Structure. Rhombus- shape crystals of PFC-25 and
PFC-26 HOFs were prepared by solvothermal reaction of 1,1′-
bis(3,5-bis(dicarboxylatophenyl)- 4,4′-bipyridinium dichloride
(ZV1) and formic acid in dimethylformamide (DEF) (Figure S3).
Crystallographic data demonstrated that PFC-25 (moiety formula
Photochromic Behaviors. Photochromic behaviors were
comparatively investigated for ZV-1, PFC-25 and PFC-26. As
seen in Figure S14 and S20, the initial powder of the ZV-1, PFC-
25 and PFC-26 display strong distinguishable UV absorption
peaks centered at 450 ,445 and 475 nm, respectively. After
irradiated by UV light, for ZV-1 and PFC-25 no obvious new
absorption peak were observed, while for PFC-26, two new
absorption peaks began to appear at 685 nm and 712 nm (less
than 2 second).
4(C26H18N2O8)·8(CHO2)·8(CH2O2)
and
sum
formula
C120H100N8O68) crystallized in the monoclinic system with the C2/c
space group. The neighboring viologen moieties are connected
by two formate anions and two formic acids through hydrogen
bonds, forming an interpenetrating ladder-like and corrugated
one-dimensional (1D) chain, which are parallel packing in AA
mode (Figure 1). In contrast, PFC-26 (moiety formula
C26H18N2O8·2(CHO2) and sum formula C28H20N2O12) crystallized
in the triclinic P1 space group in which neighboring viologen
moieties are connected with one formate anions through
hydrogen bonds, forming a 2D sql network (Figure 1). To gain a
better insight into PFC-25 and PFC-26, the crystal structure of
PFC-25 revealed that there are two crystallographic independent
formic acid (one protonated and one deprotonated) and half of the
viologen molecules in the asymmetric unit, while in the
asymmetric unit of PFC-26, two crystallographic independent
formate anions and two independent halves of the viologen
molecules can be observed (Figure S4). Interestingly, this study
can help to get better understandings of HOFs self-assembly,
exhibiting as the concentration of formic acid increased, the
structure undergoes distortion form a system with the higher
symmetry from P1 in PFC-26 to C2/c in PFC-25 (Table S1). As
seen in Figure S5 and listed in Table S3, the detailed hydrogen
bond lengths of D−H···A in these two structures range from 2.44
The PXRD pattern demonstrated acceptable crystallinity as same
as before irradiated upon UV (Figure S12) as an acceptable
evidence for their radiation-stability. As next step to monitoring the
radiochromic properties, ZV-1, PFC-25 and PFC-26 irradiated
upon different X-ray sources. Notably, ZV-1 is silent upon X-ray
irradiation (both Cu-Kα and Mo-Kα), giving belief to the requisite
of the ordered framework for boosting of the radiochromic process
(Figure S20). As shown in Figure 3a, the single crystal of PFC-25
presents X-ray induced color change from light yellow to green (in
90 s). Also, the same trend can be seen for PFC-26 with the faster
response to both X-ray sources (22 s). Compared with the state-
of-the-art (Table S6), PFC-25 and PFC-26 show moderate
coloration time but relative fast bleaching time. The different
radiochromic behavior of these two HOFs could be
a
consequence of changes in the framework’s environment for
creating radical and the stabilization of the radical form.[11]
2
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Figure 1. Crystal structures of PFC-25 and PFC-26, showing viologen ligands and their hydrogen-bonding interactions between neighboring carboxylate groups of
formic acid modules, extended packing, and simplified schematic view, respectively.
Figure 2. (a) CO2 isotherms of PFC-25 and PFC-26 at 196 K (Solid symbols presents adsorption and empty ones presents desorption) (b) PXRD patterns of PFC-
25 at different solvents for 24 hours.
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features[2c, 12] of PFC-25 and PFC-26 may open up an avenue for
HOFs as new energy dependent X-ray-induced radiochromic
materials with visually distinguishable response to both soft and
hard X-ray at room temperature. The recorded IR spectrum and
PXRD pattern before and after irradiation upon X-ray have
demonstrated the high stability of PFC-25 and 26 against X-ray
radiation during the coloration process (Figure S12 and S21).
We provided further explorations with regard to this point, whether
the radiochromic behaviors our pristine powdery HOFs only
depend on energy or the intensity of X-ray play as another pivotal
factor. By fixing the irradiation power and changing the irradiation
time, or inversely, fixing the irradiation time and changing the
irradiation power, can reveal if the coloration behavior is intensity-
dependent.[12] Experimental results exhibited that PFC-25 upon
irradiation by Mo-Kα X-rays for 2 h with different power verified the
same color change (Figure S17). Also, the same result can be
observed for the color change of PFC-25 upon Cu-Kα X-ray
irradiation (Figure S18). Moreover, the time-dependent UV-Vis
Under constant power for PFC-25 irradiated by Cu-Kα X-rays
showing constant position and shape of absorption peak (Figure
8a). The same result can obtain with changing the X-ray source
to Al-Kα X-ray and constant power (150 W) with the different
irradiation time (Figure S19). Therefore, it can be concluded that
the radiochromic behavior of PFC-25 is mostly connected with the
energy than the intensity of X-rays.
Figure 3. (a) The color change of a single crystal PFC-25 upon X-ray irradiation
with different powers, Cu-Kα X-ray (λ = 1.54178Å). (b) Photos of the time-
dependent color change of tableted samples of PFC-25 upon Cu-Kα X-ray (λ
=1.54056 Å, 8.0478 keV) from the Rigaku Desktop Miniflex PXRD (powered at
450 W) and Al-Kα X-ray (λ = 8.357 Å; power, 150 W) from ThermoFisher
ESCALAB250 X-ray photoelectron spectrometer at room temperature.) (c) UV-
vis diffuse-reflectance spectra of PFC-25 and PFC-26 upon X-ray irradiation by
Al/Cu-Kα X-rays.
The Relation between Structure and Photochromic
Behaviors.
It has been well established that photochromic phenomena can
be assigned to the occurrence of electron transfer between ionic,
or even non-ionic, N-heterocyclic aromatic ligand and lone pairs
of electrons from either bonded or unbonded molecules (such as
carboxylate and phosphate), which generate radical species with
different colors. It is noticeable that the preferred electron transfer
pathway can be observed if meeting the essential factors such as
distance and angle between donor and acceptor (bipyridinium
core), also the dihedral angle between two pyridyl groups of the
bipyridinium rings. Besides aforesaid items, the hydrogen bonds
between pyridinium C−H and donor act as auxiliary factors. The
optimal distances reflected in all reported structures are 3.4- 4 Å
for UV and till ~ 4.5 Å for X-ray to achieve electron transfer. In
PFC-25 and 26, both the electron-rich carboxyphenyl groups and
formates can act as competent electron donor sites, and the
distances between O atoms from carboxylate groups to the
electron-deficient N+ centers (2.964~3.689 Å for PFC-25 and
3.115~4.00 Å for PFC-26, see Table S4) fall into the range
capable of triggering a photochromic behavior upon UV or X-ray.
To gain better insight into radiochromic mechanism of PFC-25 and
PFC-26, the single crystal X-ray diffraction analyses (after and
before irradiating) was performed, indicating the photochromic
transformations only involve electron transfer because no obvious
skeleton changes was observed (Table S2). Interestingly, PFC-25
shows energy-dependent color change toward X-ray irradiations.
Upon irradiation by hard X-ray (Cu-Kα, λ = 1.54178 Å, 8.045 keV),
the color of PFC-25 changes from yellow to dark green (saturated
after 15 min). In contrast, the soft X-ray irradiation with decreased
energy (Al-Kα, λ = 8.357 Å, 1.4866 KeV) gave rise to a light green
(saturated after 3min, Figure 3b). Besides, UV/Vis diffuse
reflectance spectra for PFC-25 revealed a redshift from 520 nm
for Al-Kα X-rays to 574 nm for Cu-Kα (Figure 3c). The similar
redshift was also observed for PFC-26 from 525 nm for Al-Kα X-
rays to 579 nm for Cu-Kα X-rays. Such energy-dependent
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Figure 4. The selected distances between the O atoms from formate or
carboxyphenyl groups to the N atoms from bipyridinium (N···O (Å)) in (a) PFC-
25 and (b, c) PFC-26.
However, previous studies show that longer distance usually
gives rise to x-ray induced photochromism due to the required
higher threshold energy and vice versa[10a, 12-13]. This trend
obviously conflicts with the phenomena observed in this work.
PFC-25 with a shorter electron-transfer pathway can only be
triggered by high energy input of X-ray, while PFC-26 with a
longer electron transfer pathway can be triggered by not only x-
ray but also low energy of UV light. Therefore, we cannot see a
clear correlation between the distance and the different
photochromic behaviors. To understand the mechanism and
figure out the vital role in determining the photochromic properties
in these two structures, we resort to theoretical calculation for
more information.
The sums of O 2p states of 4O atoms from carboxyphenyl groups
as well as that of 4 O atoms from free formate were extracted and
shown in Figure 5c and 5f. Results show that the contribution of
the O atoms from carboxyphenyl contributes to the valence band
near the Fermi level is more significant than those from formic
acid, an indicative of the electron mainly transferring from the
carboxyphenyl groups to the pyridinium rings for both PFC-25 and
PFC-26. This can also be clearly seen from the wave functions of
VBM and CBM of PFC-25 and PFC-26 in Figure S25, where the
HOCO mainly distributed on the carboxyphenyl groups and the
LOCO mainly distributed on the bipyridinium rings. With these
calculated results, when we further look into the detailed crystal
structures of PFC-25 and PFC-26, dihedral angle as another
essential factor seems to be more related to the different
photochromic behaviors. The dihedral angles between two
pyridinium rings are 36.79ο and 0.0ο for PFC-25 and 26,
respectively. It’s known that a larger π-conjugated system can
raise the highest occupied crystalline orbital (HOCO) energy and
lower the lowest unoccupied crystalline orbital (LOCO) energy.
Accordingly, PFC-26 with a planar conformation can achieve the
electron transfer with less threshold energy compared to PFC-25
wherein twisted viologen ligand was observed. Therefore, PFC-
26 can generate radical via both UV and X-ray irradiation, while
PFC-25 can only be activated by high energy X-ray source. These
phenomena are also consistent with the DFT calculation that
PFC-25 has a larger direct band gap of 2.22 eV than that of PFC-
26 (1.80 eV, indirect band gap).
Theoretical Study. The band structures of the two compounds
(PFC-25 and PFC-26) are plotted in Figure 5a and 5d. Based on
DFT calculation, the partial density of states (PDOS) of the former
shown in Figure 5 indicates that the C 2p and N 2p electron states
contribute mostly to the conduction band minimum (CBM),
whereas the valence band maximum (VBM) is primarily
comprised of C 2p and O 2p electrons (Figure 5b). Similarly, for
PFC-26, mixed C 2p and N 2p electron states predominate the
conduction band bottom, and valence band top is occupied by C
2p and O 2p electrons (Figure 5e).
Radical Characterization. ESR (electron spin resonance)
measurements were employed to identify the generated active
species. As seen in Figure 6a, before irradiation, the light-yellow
samples belong to PFC-25 and PFC-26 are ESR-silent, while
PFC-25 upon X-ray irradiation and PFC-26 upon UV light and X-
ray irradiation at room temperature exhibit strong and sharp
resonance signals with g values of 2.0046, 2.0042, 2.0049,
respectively. These values are very close to that of a viologen
radical (2.0023) reported in other studies [20]. Therefore, we
speculate that x-ray irradiation generates photo-induced electron,
which transfers from carboxylate groups (HOCO) to pyridinium
rings (LUCO) and produces viologen radicals. This speculation is
supported by the X-ray photoelectron spectroscopy (XPS)
characterization (Figure 6a-c). We should note that owing to the
fast response time of PFC-25 upon X-ray source, we cannot get
XPS data before irradiation. Therefore, all comparison carried out
between amorphous ligands and PFC-25 after irradiation.
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significant shift towards a lower N 1s binding energy was
observed due to the exposure of X-ray source. Meanwhile, an
additional peak around 401.0 eV owing to receiving electron by
nitrogen and converted to radical (N0) can be observed (Figure
6a). On other side, the total signal for the binding energy of O 1s
in PFC-25 is 1.04 eV higher than ligand owing to the decrease of
electron density caused by the X-ray irradiation (Figure 6c). These
obtained results for XPS indicate the electron transfer from the
carboxyphenyl groups to the pyridinium rings.
a
b
c
4
Total
3
2
Total
C(2p)
N(2p)
O(2p)
H(1s)
N(2p)
O(2p)
O(2p)
1
0
-1
-2
-3
-4
C|C2Y2
V2
G
G
M2D|D2 AG|L2
G
0
100
200
0
100 200
Density of states
Density of states
d
e
f
4
3
a
b
Total
Total
C(2p)
N(2p)
O(2p)
H(1s)
N(2p)
O(2p)
O(2p)
2
1
0
-1
-2
-3
-4
T2|U2
V2
G
C|U G Z|R2
G
G
0
50
0
50
Density of states
Density of states
c
d
Figure 5. The computed electrical structures of PFC-25 and PFC-26. In the
band diagrams of (a) PFC-25 and (d) PFC-26, the dotted horizontal lines denote
the Fermi level. The filled orange circle represents CBM and the violet circle
refers to VBM. The PDOS of (b) PFC-25 and (e) PFC-26. Sum of some selected
atoms in (c) PFC-25 and (f) PFC-26. Cyan line refers to 2p states of four O
atoms from carboxyphenyl groups, and the orange line refers to 2p states of
four O atoms from formic acids.
Figure 6. (a-c) XPS core-level spectra of PFC-25 and ligand ZV-1 (d) ESR
curves of PFC-25, PFC-26 upon X-ray (Cu-Kα) and UV light irradiation, PFC-25
and PFC-26 after decoloration.
The core energy level spectra of N 1s for the amorphous ligands
shows a peak of 401.3 eV assignable to the positively charged
nitrogen (N+) without detectable color change during the XPS
measurements. The coloration of the reported X-ray and UV-
induced photochromic viologen-based materials is mainly caused
by the electron transfer from O- donor atom to pyridinium N atom,
which is confirmed by X-ray photoelectron spectroscopy (XPS).
For PFC-25, the color change from light yellow to dark green with
significant shift towards a lower N 1s binding energy was
observed due to the exposure of X-ray source. Meanwhile, an
additional peak around 401.0 eV owing to receiving electron by
nitrogen and converted to radical (N0) can be observed (Figure
6a). On other side, the total signal for the binding energy of O 1s
in PFC-25 is 1.04 eV higher than ligand owing to the decrease of
electron density caused by the X-ray irradiation (Figure 6c). These
obtained results for XPS indicate the electron transfer from the
carboxyphenyl groups to the pyridinium rings.
Fabrication of Portable Sensor for X-Ray Detection.
With the eye-detectable color change, silence to UV irradiation,
nanoparticle size accessible, and easy processability, PFC-25
can be fabricated into a sensitive thin film sensor, showing the
application potentials for X-ray energy dependence and low X-ray
dose detection. For economic and convenient utilizing, the
obtained thin film should maintain the specificity as the powdery
sample but with upgraded repeatability, portability and easy
fabricating process. For these reasons, poly (N-isopropyl
acrylamide) (PNIPAM) microgels and highly transparent poly
acrylamide (PAM) hydrogel have been selected for fabricating thin
film. PNIPAM microgels were then embedded into the PAM
hydrogel in deionized (DI) water with the addition of as-prepared
PFC-25 nanometric particle (∼25 to 100 nm) as the radiochromic
part. As a result, the solution was loaded on the clean glass slides
and left for reaction to be complete at room temperature for 18
hours.[14] SEM images shown in Figure 7 illustrate the uniform
topography of the obtained film with the thickness of 25.66 μm
and the calculated direct band gap ~ 2.25 eV as important factor
for optical behavior (Figure 7d). The obtained sensor shows good
The coloration of the reported X-ray and UV-induced
photochromic viologen-based materials is mainly caused by the
electron transfer from O- donor atom to pyridinium N atom, which
is confirmed by X-ray photoelectron spectroscopy (XPS). For
PFC-25, the color change from light yellow to dark green with
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crystallinity as indicated by PXRD results. Interestingly, during the
PXRD measurement, the exact position X-ray light passing
through the transparent thin film was clearly indicated by showing
a noticeable green spot-on site and varying along with the rotation
of anode generator (Figure S26). As the X-ray irradiation time
further increases, the transmittance gradually decreases due to
the rise of radical concentration.
Figure 7. (a- b) SEM photographs of PFC-25 nano particles, (c) cross sectional
view of the thin film, (d) PFC-25 thin film before (up) and after (down) X-ray
irradiation upon Cu-Kα X-ray (λ =1.54056 Å, 8.0478 keV, powered at 450 W).
The kinetic investigation of PFC-25 sensor was calculated by
plotting a curve of diffuse reflection intensity at 525 nm vs. time
(Figure 8b). The data points in 0−750 s range follow the linear
relationship (R2 = 0.992) and exhibits a first-order reaction with
the kinetic constant of 1.66×10-2 S-1. It should be highlighted that
in viologen-based materials based on previous studies and our
HOF, the multi-pathway for electron transfer should play an
essential role for exhibiting a visually color change upon different
X-ray sources. Herein, the calculated kinetic value suggests the
preferred through-space mechanism of PFC-25.[15] To measure
the X-ray sensitivity of PFC-25 sensor, the X-ray dose was
calculated by linearly changing the tube current, from 2 to 15 mA,
while keeping the tube voltage constant (V = 40 KV) or by
changing the tube voltage, from 20 KV to 40 KV, while maintaining
the tube current constant (15 mA) (Table S4). Interestingly, the
results exhibited that this sensor can be an applicable candidate
for the purpose of visually detecting X-ray doses (threshold
dosage ~ 3.5 Gy). The X-ray generated and colored MV+o could
be changed back to the pristine colorless MV2+ by exposing the
green thin film to air in half-hour (Figure S24). On the other hands,
PFC-25 shows excellent cycling stability which almost maintains
its original transmittance change after seven times of bleaching
and irradiation. The obtained thin film sensor shows good aerial
and thermal stability, which retains its crystallinity in water,
different organic solvents, and various temperature, indicating
practicability for various applications (Figure S7 to S10).
Figure 8. (a) Time-dependent UV-Vis reflectance of PFC-25 thin film upon X-
ray irradiation (Cu Kα (λ = 1.54178 Å, 450 W)), (b) the solid-state kinetic
investigation of PFC-25 thin film at 525 nm
Conclusion
In conclusion, for the first time, two novels crystalline HOFs as a
new class of X-ray energy dependence and low X-ray dose
sensor have been prepared through self-assembly of cationic
tetratopic viologen ligand with formate counterion. PFC-26 is
sensitive to both X-ray and UV light due to the narrow band gap
and low electron transfer threshold energy, while PFC-25
specifically response to X-ray irradiation and show energy
dependence. With these two materials constructed by the same
building block but self-assembled in different ways, we clearly
demonstrate that the viologen ligand with the larger conjugation
system can ease the electron transfer process, while the twisted
geometry of ligand usually required high threshold energy to
trigger the photochromism. These results provided a horizon that
can design a new class of radiochromic materials with required
detecting specificity, exhibiting good stability upon X-ray
irradiation, also can guide to investigate the mainly unreliable
experimental effects of X-ray irradiation. To demonstrate the
application potential of PFC-25, a radiochromic sensor using
transparent hydrogel was fabricated. The obtained sensor shows
the features of powdery sample with apparent transmittance
change, good cycling, aerial, thermal and chemical stability.
Although there is still so much more to learn regarding the intrinsic
7
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10.1002/chem.202101061
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radiochromic behavior of HOFs, these studied HOFs may hold
sizable promise for promoting the wide application of energy
dependent X- ray sensing technology and low-dose dosimetry at
room temperature.
Conflict of Interest
There is no conflict of interest to report.
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Funding Information
National Key Research and Development Program of China
(grant no. 2018YFA0208600), Foreign Young Talents Program
(grant no QN20200120001). CAS-Iranian Vice-Presidency for
Science
and
Technology
Research
Projects
(121835KYSB20200034), Iran national science foundation for
international joint research project (INSF) (NO.99009822),
National Natural Science Foundation of China (grant no.
21871267, 21802142), Strategic Priority Research Program of the
Chinese Academy of Sciences (XDB20000000) and Tarbiat
Modares University.
[6]
Keywords: Sensor. X-ray Detection. Hydrogen-Bonded Organic
Frameworks. Sensor. Radiochromism.
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For the first time in this work, HOFs were explored as radiochromic materials responsive to X-ray irradiation, opening a new avenue
for HOFs applications. The HOFs reported here have the same building block yet different radiochromic behaviors: one is sensitive to
both X-ray and UV light, while the other one is specifically responsive to X-ray light. These phenomena provide a precious chance to
understand the vital factors that account for the detection specificity of radiochromic materials.
9
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