N-Rich Porous Polymer with Isolated Tb3+-Ions Displays Unique Temperature Dependent Behavior through the Absence of Thermal Quenching

Article abstract of DOI:10.1002/chem.202002009

The challenge of measuring fast moving or small scale samples is based on the absence of contact between sample and sensor. Grafting lanthanides onto hybrid materials arises as one of the most promising accurate techniques to obtain noninvasive thermometers. In this work, a novel bipyridine based porous organic polymer (bpyDAT POP) was investigated as temperature sensor after grafting with Eu(acac)₃ and Tb(acac)₃ complexes. The bpyDAT POP successfully showed temperature-dependent behavior in the 10–310 K range, proving the potential of amorphous, porous organic frameworks. We observed unique temperature dependent behavior. More intriguingly, instead of the standard observed change in emission as a result of a change in temperature for both Eu³⁺ and Tb³⁺, the emission spectrum of Tb³⁺ remained constant. This work provides framework- and energy-based explanations for the observed phenomenon. The conjugation in the bpyDAT POP framework is interrupted, creating energetically isolated Tb³⁺ environments. Energy transfer from Tb³⁺ to Eu³⁺ is therefore absent, nor energy back transfer from Tb³⁺ to bpyDAT POP ligand (i.e. no thermal quenching) is detected.

Full text of DOI:10.1002/chem.202002009

Accepted Article
Accepted Article
Title: N-rich porous polymer with isolated Tb3+-ions displays unique
temperature dependent behavior through the absence of thermal
quenching
Authors: Flore Vanden Bussche, Anna Marta Kaczmarek, Savita
Veerapandian, Jonas Everaert, Maarten Debruyne, Sara
Abednatanzi, Rino Morent, Nathalie De Geyter, Veronique
Van Speybroeck, Pascal Van Der Voort, and Christian V.
Stevens
This manuscript has been accepted after peer review and appears as an
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To be cited as: Chem. Eur. J. 10.1002/chem.202002009
Link to VoR: https://doi.org/10.1002/chem.202002009
01/2020

10.1002/chem.202002009
Chemistry - A European Journal
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N-rich porous polymer with isolated Tb3+-ions displays unique
temperature dependent behavior through the absence of thermal
quenching
Flore Vanden Bussche,[a,b] Anna M. Kaczmarek,*[a] Savita K.P. Veerapandian,[c] Jonas Everaert,[b]
Maarten Debruyne, [b] Sara Abednatanzi, [a] Rino Morent, [c] Nathalie De Geyter, [c] Veronique Van
Speybroeck, [d] Pascal Van Der Voort, [a] Christian V. Stevens[b]
[a]
Prof. Dr. Anna M. Kaczmarek, ir. Flore Vanden Bussche, Sara Abednatanzi, Prof. Dr. Pascal Van Der Voort
Department of Chemistry
Ghent University
Krijgslaan 281 (S3), 9000, Ghent, Belgium
E-mail: Anna.Kaczmarek@ugent.be
[b]
ir. Flore Vanden Bussche, ir. Jonas Everaert, ir. Maarten Debruyne, Prof. Dr. ir. Christian V. Stevens
Department of Green Chemistry and Technology
Ghent University
Campus Coupure, Coupure Links 653, 9000 Ghent, Belgium
Savita k.P. Veerapandian, Prof . Dr. Rino Morent, Prof . Dr. Nathalie De Geyter
Department of Applied Physics
Ghent University
Sint-Pietersnieuwstraat 41 (B4), 9000, Ghent, Belgium
Prof. Dr. ir. Veronique Van Speybroeck
[c]
[d]
Center for Molecular Modeling (CMM)
Ghent University
Technologiepark 46, B-9052, Zwijnaarde, Belgium
Supporting information for this article is given via a link at the end of the document.
Abstract: The challenge of measuring fast moving or small scale
samples is based on the absence of contact betw een sample and
sensor. Grafting lanthanides onto hybrid materials arises as one of
the most promising accurate techniques to obtain noninvasive
thermometers. In this w ork, a novelbipyridine based Porous Organic
Polymer (bpyDATPOP) w as investigated as temperature sensorafter
grafting w ith Eu(acac)3 and Tb(acac)3 complexes. The bpyDAT POP
successfully showed temperature dependent behavior in the 10 - 310
K range, proving the potential of amorphous, porous organic
framew orks. More intriguingly, w e observed unique temperature
dependent behavior; instead of the standard observed change in
emission as a result of a change in temperature for both Eu3+ and Tb3+,
the emission spectrumof Tb3+ remained constant. This w orkprovides
framew ork- and energy-based explanations for the observed
phenomenon. The conjugation in the bpyDAT POP framew ork is
interrupted, creating energetically isolated Tb3+ environments. Energy
transfer fromTb3+ to Eu3+ is therefore absent, nor energy backtransfer
from Tb3+ to bpyDAT POP ligand (i.e. no thermal quenching) is
detected.
grow considerably thanks to device miniaturization and fast
technological advancements. The need for reliable and precise
temperature sensors is therefore very high. Noninvasive
temperature sensors - sensors w hich do not disrupt the medium
of interest, are indispensable for fast-moving or small scale
samples.4-5 Areas of interest are for example microelectronics,
microfluids and nanomedicine. Luminescent based noninvasive
sensors are one of the most promising accurate thermometers,
since their luminescence can depend on the temperature.6-7
These materials consist of trivalent lanthanide ions (Ln3+) w hich
possess follow ing unique and beneficial luminescence properties:
(i) narrow emission peaks ranging from the ultraviolet-visible to
near-infrared region (ii) relatively long decay times (iii) high
quantum yields and (iv) high photostability.8-9 The major dow nside
of these ions is low absorption coefficients, as a result of parity-
forbidden 4f-4f transitions. To improve their light absorption, Ln3+
ions are grafted onto organic-inorganic hybrid materials.10-11
These so called ‘antenna’ absorb the light and subsequently
transfer excitation to the metal ions, thereby improving their light
absorption. Various hybrid materials have been developed,
among them Metal Organic Framew orks (MOFs) and
coordination polymers are most extensively studied.12-15 The
presence of multiple luminescent centers in lanthanide-based
materials allow s their use as ratiometric temperature sensor.
These reliable sensors are independent of the concentration and
inhomogeneity of the luminescent centers in the material and of
fluctuations in the excitation source or detector. To elucidate and
discover interesting luminescent and sensing properties, the
Introduction
Given that temperature is a fundamental parameter that affects
the kinetics of virtually every process, temperature sensors are
ubiquitous and indispensable for correct processing, storage and
use of materials and products.1-3 They are applied for scientific
research as w ell as in the industry. The market is expected to
1
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study of novel classes of lanthanide-based materials is essential
and ongoing. Based on the success of organic-inorganic hybrid
antennas, purely organic substances have recently been
investigated as potential antenna for luminescence-based
temperature sensing. In 2020, some of us (A. M. Kaczmarek et
al.) w ere the first to successfully use a crystalline, porous organic
antenna (bipyridine-based Covalent Organic Framew ork (TpBpy
COF)), as a temperature sensor, in w hich the nitrogen rich
building blocks create an ideal complexation environment for
lanthanide ions.16 This material show ed unique and unusual
temperature dependent behavior w here no thermal quenching
w as observed for the terbium (Tb3+) ions as a result of the
absence of energy transfer from terbium (Tb3+) to europium (Eu3+).
Inspired by this result, w e developed a novel 6,6’-(2,2’-bipyridine-
5,5’-diyl)bis(1,3,5-triazine-2,4-diamine) (bpyDAT) based Porous
Organic Polymer (POP) and investigated the luminescent and
potential unique temperature dependent behavior after grafting of
the bpyPOP w ith Eu(acac)3 and Tb(acac)3 complexes. POPs are
renow ned for their lightw eight, high surface areas, excellent
stability and tunability. These features paved the w ay tow ards
polymer is formed via Schiff base condensation reaction in DMSO,
and subsequent attack of the imine double bond by primary
27
amines, resulting in the generation of an aminal (Figure 1).26,
The bpyDAT POP contains numerous N-moieties w hich form
excellent anchoring points for grafting of lanthanide complexes.
The chemical connectivity betw een the building blocks was
investigated by FTIR spectroscopy (Figure S2). Bands for NH2-
-1
stretching and deformation (1633 cm ) of the primary amine
groups in bpyDAT clearly disappeared in the spectrum of bpyDAT
POP. This indicates that all amine functionalities w ithin the
framew ork have reacted. The broad band at 3381 cm-1
corresponds to N-H aminal stretching vibrations of secondary
amines. The presence of a sharp band around 1570 cm-1 in both
linker and POP, indicates the triazine quadrant stretch.28 The
-1
corresponding band at 1658 cm indicates C=N stretching in the
bipyridine ring. The band at 1525 cm-1 demonstrates stretching
vibration of C=C bonds. Elemental analysis reveals C, H, N
content of 46.2, 3.6 and 28.9 % respectively. The calculated
experimental ratio (N/C
= 0.63) is in agreement w ith the
theoretical content (N/C = 0.64), and demonstrates that the
composition of the precursors is preserved during the
polymerization reaction. As expected, the bpyDAT POP is very
rich in nitrogen. The different binding states of nitrogen
functionalities are further verified via curve fitting of the high-
resolution XPS N1s spectrum (Figure 2). The peak at 399.55 eV
corresponds to NH linkages and the peak at 398.55 eV to the
imine groups of the triazine and bipyridine moieties. The PXRD
pattern show s a broad peak betw een 2θ values of 15 to 30 °,
consistent w ith the amorphous nature of the material (Figure S3).
The surface area w as investigated via nitrogen sorption
measurement at 77 K (Figure 3). The isotherm show s reversible
type IV adsorption. A steep increase at a relative pressure of P/P0
< 0.1, suggests a permanent microporous material. The presence
of hysteresis in the desorption at P/P0 > 0.9, can probably be
ascribed to adsorbate condensation w ithin interparticle voids,
since this is observed in POPs w ith a similar structure.20-21, 23. The
fact that the isotherm does not close at the cavitation point of P/P0
= 0.42 is indicative for a slight sw elling of the polymer in liquid
nitrogen, a typical feature of some polymers and POPs. The
Brunauer-Emmett-Teller (BET) surface area is 505 m² g-1. The
thermal stability of the bpyDAT POP w as investigated via thermal
very diverse application
fields (gas storage/separation,
heterogeneous catalysis, chemical and bio-sensing, energy
storage and conversion).17-23 POPs differ from the crystalline
COFs in their amorphous nature. In this w ork, w e determine
w hether porous, amorphous materials are potential noninvasive
thermometers. Remarkably, w e also observe the peculiar
temperature dependent behavior (no thermal quenching) and
provide framew ork- and energy-based explanations.
Results and Discussion
The development of a novel temperature sensor involves three
main steps. First the required antenna, in w hich appropriate
lanthanide coordination pockets are present, is assembled and
characterized. Secondly, the lanthanide ions are grafted onto the
framew ork. Lastly, the luminescent and temperature dependent
properties are investigated. We selected 6,6’-(2,2’-bipyridine-5,5’-
diyl)bis(1,3,5-triazine-2,4-diamine)
(bpyDAT)
and
terephthalaldehyde as starting materials to synthesize a novel
bipyridine-triazine-based POP (bpyDAT POP). The porous
Figure 1. Synthesis of 6,6’-(2,2’-bipyridine-5,5’-diyl)bis(1,3,5-triazine-2,4-diamine) (bpyDAT) and bpyDATPOP.
2
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Figure 2. XPS (N1s) spectrum of bpyDATPOP showing the presence of NH
and C=N nitrogen binding states.
Figure 3. Nitrogen (N2) adsorption/desorption isotherm of bpy DATPOP
gravimetric analysis (Figure S4). The bpyDAT POP is stable up to
400 °C, implying its high thermal stability.
oxidation state of Eu cations. Furthermore, the peaks at 1154.87
eV and 1125.27 eV indicate the binding energies Eu(II) 3d3/2 and
Eu(II) 3d5/2 of the Eu2+ ions. The reduction of Eu3+ to Eu2+ probably
occurred during the measurement. In the Eu4d spectrum shown
in Figure S9, the same oxidation states are observed. The
characteristic peaks of Eu(III) 3d3/2 (142.47 eV) and Eu(III) 3d5/2
(136.67 eV) correspond to the multiplet structure of trivalent 4d4f6
configuration of Eu3+, the characteristic peaks of Eu(II) 3d3/2
(131.67 eV) and Eu(II) 3d5/2 (129.07 eV) to the divalent 4d4f7
configuration of Eu2+.30 In the Tb3d spectrum (Figure S10) of
Tb_acac@bpyDAT POP, the peaks at 1277.25 eV and 1242.55
eV correspond to Tb(III) 3d3/2 and Tb(III) 3d5/2 of Tb3+. In the 4d
spectrum (Figure S11), the peaks in the range of 147 - 152 eV
correspond to Tb3+. The presence of multiple energy bindings is
explained by multiplet effects. The multiplicity of final states
results from coupling a 4d core hole w ith an incomplete outer 4f
shell (4d-4f excitations) in low -energy excited emission spectra.31
The lanthanide content of the samples is determined using ICP-
OES (Table S1).
Lanthanide complexes
(Eu(acac)3 and Tb(acac)3) w ere
successfully grafted onto the framew ork through post-synthetic
modification, using an excess of lanthanide complexes (Figure 4).
The acac-ligand serves as a second antenna for the absorption
of light and it protects the lanthanides from quenching by H2O.
First, the bpyDAT POP grafted w ith only Eu3+ or Tb3+ complexes
w ere studied and after elucidating the luminescent properties of
the separate lanthanide ions onto bpyDAT POP, mixtures of Eu3+
/Tb3+ w ere grafted onto the POP. The grafting of the Eu3+(acac)3
complex w as proven by TEM-EDX S5. The ultrasound-assisted
sample preparation causes the presence of limited amount of dots
outside the sample. A homogeneous distribution of nitrogen and
europium throughout the sample is observed.
A similar
distribution behavior is expected for Tb3+ and mixed complex
grafting. The successfulgrafting is further proven by XRF (Figure
S6) and FTIR spectroscopy (Figure S7). The lanthanides are most
probably grafted onto different bipyridine units in the framew ork,
observed by the appearance of peaks in the FTIR spectra related
-1
to the acac-ligand (1400 cm , ν(C-H)) and a small shift in the
Luminescence study
-1
imine stretching band (1680 cm ). Bipyridine units are w ellknow n
The luminescent properties of the different materials w ere
investigated. First, the excitation-emission spectrum of the
pristine (ungrafted) bpyDAT POP w as recorded at room
temperature to get insight into the luminescent behavior of the
antenna (Figure S12). The excitation spectrum consisted of a
broad band from 250-425 nm, w ith a maximum at 372 nm. The
emission band is obtained w hen excited in this peak maximum. A
broad band is observed from 400-550 nm, w ith a maximum at 438
nm. Next, the room temperature photoluminescence study of the
Eu(acac)3 grafted bpyDAT POP (Eu_acac@bpyDAT POP)
revealed a broad band, originating from bpyDAT POP, in the
excitation spectrum from 275 nm to 425 nm, w ith a maximum at
347 nm (Figure 5). Due to grafting, the maximum has shifted
tow ards a low er w avelength. Upon excitation in this maximum, the
in thermometry-based literature as excellent ligands for anchoring
29
of Eu3+ and Tb3+ complexes.16,
The oxidation states of the
grafted Tb and Eu complexes w ere determined via XPS. In the 3d
spectrum of the Eu_acac@bpyDAT POP sample (Figure S8), the
observed peaks at 1164.67 eV and 1134.97 eV are characteristic
of Eu(III) 3d3/2 and Eu(III) 3d5/2 binding energies, confirming the 3+
emission spectrum show ed the characteristic 5D0
→
7F0-4
transition peaks of Eu3+. The transitions have been assigned to
the appropriate peaks in Table S2. The w eak broad band in the
emission spectrum (450-520 nm) resulted from incomplete
energy transfer from the POP ligand to the Eu3+ ions. The
luminescence decay time of Eu_acac@bpyDAT POP was
determined by fitting a biexponential function to the curve,
resulting in tw o decay times, t1 = 362 µs and t2 = 66 µs (Figure
S13). The presence of tw o decay times indicated tw o different
Figure 4. Synthesis of the Ln_acac @bpyDAT POP materials.
3
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Figure 5. Excitation-emission spectrum of Eu_acac@bpyDAT POP
(excited at 347 nm, observed at 614 nm) at room temperature. Picture in
the upper right corner of the sample when placed under UV lamp (at 302
nm excitation wavelength).
Figure 6. Excitation-emission spectrum of Tb_acac@bpyDAT POP
(excited at 326 nm, observed at 549 nm) at room temperature. Picture in
the upper right corner of the sample when placed under UV lamp (at 302
nm excitation wavelength).
coordination environments of the Eu3+ ions grafted to the POP. As
one decay time (t2) is significantly shorter, it must be present in an
unfavorable environment (e.g. in the presence of several w ater
molecules in its first coordination sphere). Subsequently, the
room temperature excitation-emission spectrum for Tb3+ grafted
bpyDAT POP (Tb_acac@bpyDAT POP) w as recorded (Figure 6)
and show ed a moderate broad ligand band from 275-350 nm, w ith
a maximum at 326 nm. After excitation in this maximum, the
thereby eliminating variations in the sample and equipment. The
different ratios, determined via ICP-OES, of Eu(acac)3 and
Tb(acac)3 are: 50 % Eu, 50 % Tb (1); 34 % Eu, 66 % Tb (2); 25 %
Eu, 75 % Tb (3). The resulting excitation-emission spectra are
presented in Figure 7 and their corresponding peak-transition
assignments are found in Table S4-6. Each graph depicts a
picture w ith the observed emission color under UV radiation (302
nm) in the upper right corner. The more Tb3+ present in the
material, the more the emission w avelengths fall in the green
region. The emission spectrum of 1 show s a higher Eu-to-Tb ratio,
resulting in incomplete energy transfer from the POP material to
the lanthanide ions. This is suggested by the w eak broad band in
the emission spectrum, and the presence of a sharp f-f Eu3+
transition peak in the excitation spectrum, like in the pure
Eu_acac@bpyDAT POP grafted sample. The tw o other grafted
materials show a strong Tb3+ peak (549 nm) and a w eaker Eu3+
peak (611 nm), w hereby efficient and complete energy transfer
from the ligand to the lanthanide ions is observed.
emission spectrum revealed the characteristic 5D4
→
7F6-3
transition peaks of Tb3+. The absence of a broad band in the
emission spectrum, suggested efficient energy transfer from the
ligand to the Tb3+-ions. This indicated the close existence of the
triplet energy levels of the POP and the accepting levels of the
Tb3+-ions. Table S3 connected the observed peaks w ith the
corresponding transitions. The decay time of this material was
determined by plotting a single exponential function, resulting in
t1 = 941 µs (Figure S14). The Eu_acac@bpyDAT POP material
yielded red emission, w hereas the Tb_acac@bpyDAT POP
material yielded green emission under UV excitation (302 nm).
After elucidating the luminescent properties of the separate
lanthanide ions onto the bpyDAT POP, w e grafted different ratios
of Eu3+- and Tb3+-ions simultaneously onto the porous material to
investigate their potential use as a ratiometric temperature sensor.
This type of sensor relies on tw o discriminable transition peaks,
The decay times of the different samples are show n in Table S7;
the corresponding decay profiles are presented in Figures S15-
20. Standard luminescent behavior of Eu/Tb grafted materials is
mostly characterized by energy transfer of the ligand to Tb3+ and
subsequently to Eu3+ ions. This is caused by the proximity of the
triplet and accepting levels of the ligand and the ions. As a result
Figure 7. Room temperature emission-excitation spectra of the samples 1 (excited at 380 nm, observed at 614 nm), 2 (excited at 321 nm, observed at 549 nm) and 3
(excited at 322 nm, observed at 548 nm) respectively .
4
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5
of the energy transfer, the decay time of Tb3+-ions in mixed Eu/Tb
low est triplet level is w ell above the D4 accepting level of Tb3+. To
prove our hypothesis and to gain insight in the energy transfer
mechanism, w e unraveled the low est triplet level of bpyDAT POP
ligand. Therefore, the emission spectrum of bpyDAT POP grafted
w ith GdCl3 at 77 K in ethanol/methanol mixture (4:1) w as recorded.
The analysis is depicted in Figure S21. In gadolinium complexes,
the energy at the triplet level of the ligand is not transferred to the
Gd3+ metal, as the resonance level of Gd3+ lies far above the triplet
level.34 Subsequently, w e have recorded the decay time at its
maximum and observed microsecond range lifetimes, confirming
the band belongs to the triplet level of the material (Figure S22).
The triplet level of bpyDAT POP corresponds to the high-energy
edge of this band, the low est triplet level is therefore located at
samples
w ill decrease in comparison
to the pure
Tb_acac@bpyDAT POP sample and the decay time of Eu3+-ions
w ill increase since they receive energy.32-33 Remarkably, in our
samples the decay time of Tb3+ remains constant, w hile the decay
time of Eu3+ increases. The Tb3+ decay time is not decreasing,
suggesting the absence of energy transfer of Tb3+ to Eu3+ or
implying a long distance betw een the Eu3+ and Tb3+ ions. The
different decay times for Eu3+ arise from different coordination
environments. When looking at the bpyDAT POP structure, it can
be observed that the material is not fully conjugated, the
conjugation is interrupted at the aminal nodes. As a result,
electronic communication, and thus through-bond energy transfer
is rather limited, thereby explaining the observed phenomenon.
Materials 1 and 2 w ere further investigated for their temperature-
dependent luminescence properties. Since the Eu3+ peak in
material 3 is very w eak, it w as excluded from the temperature
study. Temperature dependent emission spectra w ere recorded
in the temperature range of 10 - 310 K for both samples (Figure
8). The corresponding integrated areas under the 614 nm (Eu)
and 548 nm (Tb) (1) and 612 nm and 549 nm (2) peaks as a
function of the temperature are show n in Figure S22. Intriguingly,
the intensity of the Tb3+ peak (548-549 nm) show s nearly no
fluctuations throughout the w hole temperature profile w hilst the
intensity of the Eu3+ peak (614-612 nm) clearly decreases upon
temperature increase (i.e. temperature dependent). This peculiar
temperature behavior w as observed only once before.16 To
explain this unique behavior, w e hypothesized the absence of Tb-
to-ligand energy back transfer, resulting in no temperature
quenching of Tb3+. This is possible if the energy of the ligand’s
-1
416 nm (24,040 cm ). The triplet level lies w ellabove the energy
5
level of the D4 accepting level of Tb3+, energy back transfer from
Tb3+ to the ligand is therefore absent. On the other hand, the high-
-1
excited levels of Eu3+ (400 nm, 25,000 cm ) are closer to the
ligand triplet level and allow energy back transfer, as can be seen
by the decrease in intensity as temperature increases. The
intensity decrease can therefore be attributed to thermal
quenching of the Eu3+ ions via the low est triplet level of the ligand.
This states the importance of appropriate ligand environment for
the luminescent temperature behavior.
Even w ith stable emission of Tb3+ upon changing the temperature,
the material can be investigated as temperature sensor. For a
ratiometric temperature sensor, the ratio of the peak intensities
(IEu/ITb) is determinative. Since the intensities of europium do
Figure 9. A. Plot representing ∆ as a function of temperature (equation 3)
for 2. The points show the experimental delta values. The solid line
corresponds to the best fit of the experimental values. B. Relative thermal
sensitivity as a function of temperature (10 - 310 K). The solid line is a
guide for the eyes.
Figure 8. Emission maps recorded between 10 - 310 K for A. sample 1 and B.
sample 2.
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decrease, w e w ere able to calculate the ratio of the integrated
areas under the peaks (I614/I548). All calculations w ere performed
using the TeSen softw are tool.25 The data points could be w ell
fitted (R² = 0.99637) using equation (3) (Experimental section) for
sample 2 yielding ∆0 = 1.2615 ± 0.0002, α = 7.5203 ± 0.0052, ∆퐸
-1
= 248 ± 0.160 cm . The fitting of equation (3) w hen calculated
from the maxima of the peaks (R² = 0.9916) yielded ∆0 = 1.7448
-1
± 0.0003, α = 17,192 ± 0.009, ∆퐸 = 282 ± 0.121 cm (Figure 9A).
Applying equation (4), the relative sensitivity w as represented as
a function of temperature (Figure 9B). The maximum relative
sensitivity w as calculated to be 1 % K-1 (125 K), w hen using the
data from the maxima of the peaks. Furthermore, the temperature
uncertainty w as calculated using equation (5), δT < 0.25 K in the
temperature range 60 - 310 K confirming good temperature
sensor behavior (Figure 10A). The repeatability of the
thermometer w as determined using equation (6), revealing an
excellent repeatability of 98.5 % (Figure 10B). The temperature
dependent luminescent properties of
1 are presented and
analyzed in the Supporting Information (Figure S24-25). The
sensor operates w ith moderate sensitivity in a similar temperature
range, as expected since the ratio I614/I548 only varies in the
numerator.
In Table 1, w e compared the maximum relative sensitivity of our
temperature sensor w ith Eu/Tb grafted materials that operate in a
similar temperature range and can conclude our sensor shows
comparable sensitivity w ith most of the reported temperature
sensors. Our maximum sensitivity can how ever not compete w ith
the highest reported sensitivity, obtained in organic-inorganic
MOF-based antenna.
Conclusion
Figure 10. A. Calculated temperature uncertainty δT in the range 60 - 310 K
(the values below 60 K were high and are therefore omitted for clarity), and
B. repeatability for sample 2.
In an effort to develop a temperature sensor, based on a novel
bpyDAT POP material, w e discovered peculiar luminescent
temperature behavior w hen anchoring a mixture of 33% Eu3+ and
67% Tb3+ onto the bpyDAT POP material. The material can be
used as temperature sensor in the range of 10 to 310 K w ith
moderate sensitivity (Sr = 1.00 % K-1). After demonstrating the
potential of crystalline porous organic framew orks (COFs) in
temperature sensing, w e now prove amorphous porous organic
polymers (POPs) are potential thermometers. More intriguingly,
the unique temperature dependent behavior observed in the
TpBpy COF, recently investigated by our group, w as also
detected in the bpyDAT POP, thereby becoming the second
material w ith this peculiar feature. This w orkprovides framew ork-
and energy-based explanations for the observed phenomenon.
The conjugation in the framew ork is interrupted at the aminal
nodes, creating energetically isolated Tb3+ environments. Energy
transfer from Tb3+ to Eu3+ is therefore absent, nor energy back
transfer from Tb3+ to bpyDAT POP ligand (i.e. no thermal
quenching) is detected due to the large energy gap betw een the
low est triplet level of the bpyDAT POP ligand and the 5D4 energy
level (Tb3+). The interruption of the conjugation and the bpy units
are present in both TpBPyCOF and bpyDAT POP. In this w ay, an
‘energy island’ is obtained, resulting in a unique temperature
dependent luminescent emission behavior w hen the energy is
released through luminescence. The Tb3+ emission spectrum
remains stable throughout the investigated temperature range,
w hile the emission spectrum of Eu3+ does show temperature
dependency. For future research, it is recommended to explore
fully conjugated porous organic framew orks (POPs – amorphous),
or covalent organic framew orks (COFs – crystalline) as potential,
organic temperature sensors w hen aiming at high thermal
sensitivity.
Table 1. Comparison of the relative sensitivity Sr (Sm) for various reported Eu/Tb materials operating in a similar temperature regime.
Material
cycEu-phTb
Range (K)
10 - 200
77 - 297
1 - 290
Sm (% K-1)
1.85
Ref
35
36
15
37
12
14
38
39
Eu0.5Tb0.5(L)1@PMMA
Tb0.95Eu0.05HL
0.46
31 (4 K)
Tb0.95Eu0.05(btb)
10 - 320
10 - 325
60 - 360
100 - 450
40 - 300
2.85 (14 K)
5.96 (25 K)
0.71 (60 K)
0.11 (450 K)
0.17 (300 K)
[(Tb0.914Eu0.086)2(pda)3(H2O)](H2O)2
Eu,TbPOM@MOF
[Tb0.3Eu0.7(d-cam)(Himdc)2·(H2O)2]3
Tb0.7Eu0.3L MOF
16
16
Tb0.4Eu0.6TpBpyCOF
Tb0.05Eu0.95TpBpyCOF
10 - 360
10 - 360
10 - 310
1.02 (110 K)
1.40 (160 K)
1.00 (125 K)
6
Eu0.34Tb0.66_acac@bpyDAT POP (2)
This work
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10.1002/chem.202002009
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Synthesis of the lanthanide bpyPOP DAT. The Ln-grafted sampleswere
prepared by weighing 2 mg of the bpyPOP DAT sample. Next, 10 times
molar amounts(based on the theoretical number of coordination sites) of
the lanthanide acetylacetone complexes, in appropriate ratios (Ln = Eu,
Tb), were weighed. All productswere placed in a Pyrex tube and 5 mL of
ethanol was added. The tube was sealed well and placed on a heating
blockset at 80 °C for 24 h. The next day the sampleswere cooled to room
temperature, collected by centrifugation and washed several times with
ethanol before drying at 80 °C in air.
Experimental Section
Materials All chemicalswere purchased from Merckand TCI Europe, and
were used asreceived. Thesolventsused during preparationwere reagent
grade and were not purified further.
Synthesis and characterization of the bpyDAT POP
Luminescence and temperature sensor study. The luminescence of
solid POP samples was studied. Solid powdered samples were put
between quartz plates (Starna cuvettes for powdered samples, type
20/C/Q/0.2). Luminescence measurements were performed on an
Edinburgh InstrumentsFLSP920UV-vis-NIR spectrometer setup. A 450W
xenon lampwasused asthe steady state excitation source.Luminescence
decay timeswere recorded using a 60 W pulsed Xe lamp, operating at a
frequency of 100 Hz. A Hamamatsu R928P photomultiplier tubewasused
to detect the emission signals in the near UV to visible range. All of the
luminescence measurementswere recorded at room temperature unless
indicated otherwise. In order to compare the measurements, the same
amounts of powders were used as well as the same settings for each
measurement (same slit size, step, and dwell time). All emission spectra
in the manuscript have been corrected for detector response. The
luminescence decay curvesof the sampleswere measured when excited
into the maximum of the broad ligand band and monitored at the
appropriate wavelength (strongest peakof the givenlanthanide). All of the
decay curvescould be well fitted usingeither a single exponential function
or a biexponential function.
The bpyDAT POP was obtained in a one-step synthesis procedure. The
monomer 6,6’-(2,2’-bipyridine-5,5’-diyl)bis(1,3,5-triazine-2,4-diamine) was
prepared according to an optimized literature procedure.24 The synthesis
and characterizationcan be found inthe ESI. The polymerization reaction
was performed in dry glassware. In a 50 mL flask, 0.8 mmol 6,6’-(2,2’-
bipyridine-5,5’-diyl)bis(1,3,5-triazine-2,4-diamine) and
1.6
mmol
terephthalaldehyde were dissolved under stirring in 15 mL anhydrous
DMSO. The temperature was slowly increased towards 160 °C under an
argon atmosphere, and thereaction mixturewasrefluxed for 72 h. A grey
precipitation is observed. After cooling down, the precipitate was filtered
off and rinsed with 50 mL water. To fully removethe unreactedmonomer,
the collected grey powder was further washed with a MeOH/THF (1:1)
mixture using a Soxhlet apparatus. The resulting off-white powder was
collectedby filtrationandwashed with acetone.The final product wasdried
under vacuum for 48h at 150°C, to afford bpyDAT POP in67 % yield. The
porous polymer was characterized using multiple techniques, including
Fourier Transform Infrared Spectrophotometer (FTIR) spectroscopy,
thermogravimetric analysis (TGA), elemental analysis, X-ray
photoelectron spectroscopy (XPS), powder X-ray diffraction (PXRD)
analysis, Brunauer-Emmett-Teller (BET) surface area and transmission
electron microscopy-energy-dispersive X-ray spectroscopy (TEM/EDX).
IR spectra with a S/N-ratio of 30,000:1 were obtainedfrom samplesin neat
form with a Quest ATR (Attenuated Total Reflectance) accessory with
푡
(
)
( )
퐼 = 퐴 exp −
1
휏
푡
푡
(
)
(
)
퐼 = 퐴1 exp −
+ 퐴2 exp −
(2)
휏1
휏2
Where I represents the luminescence intensity of a transition and A, A1
and A2 are constants, t is time and τ, τ1 and τ2 are the luminescence
lifetimes.
diamond crystal puck using
a Shimadzu IRAFFINITY-1S Fourier
Transform Infrared Spectrophotometer (FTIR). A TA Netzsch Model STA
449 F3 Jupiter system was used to perform a thermogravimetric analysis
(TGA) under air flow (100 mL/min). For CHNS elemental analysis, a
Thermo Flash 2000 thermal analyzer (Thermo Scientific) isused with V2O5
as catalyst and methionine as standard. XPS measurements were
performed using a PHI Versaprobe II spectrometer equipped with a
monochromatic Al Kα X-ray source (hνꢀ=ꢀ1486.6ꢀeV) operating with a beam
size of 100ꢀμm and a power of 25.8ꢀW. All measurementswere conducted
at a pressure of at least 10-6 Pa and photoelectrons were detected by a
hemispherical analyzer positioned at an angle of 45° with respect to the
normal of the sample surface. Individual high-resolutionC1s, N1s, Eu3d,
Eu4d, Tb3d and Tb4d spectra were recorded with a pass energy of 23.5
eV (step size = 0.1 eV). Binding energieswere calibrated with respect to
the C−C/C−H peak of the C1sspectrum at 285.0 eV. Deconvolution of the
high-resolution N1s, Eu4d and Tb4d spectra were carried out using
Gaussian-Lorentzian peak shapes (80–100% Gaussian). PXRD pattern
was collected on a Thermo Scientific ARL X’Tra diffractometer, operated
at 40 kV, 30 mA using Cu Kα radiation (λ = 1.5406 Å). The specific BET
surface area was measured on a Belsorp Mini II apparatusat 77 K. Prior
to analysis, the sample was dried at 100°C under vacuum for 24 h.
HAADF-STEM and the EDX mappinganalysiswasperformed using JEOL
JEM-2200FS High-ResolutionSTEM equipped with an EDX spectrometer
with a spatial resolution of 0.13 nm, image lens spherical aberration
corrector, electron energy lossspectrometer (filter) and an emission field
gun (FEG) operating at200 keV. X-ray fluorescence (XRF) wasmeasured
on Rigaku Nex CG. The metal loading wasdetermined using anICP-OES
Optime 8000atomic emission spectrometer.
The temperature-measurements were performed using an ARS closed
cycle cryostat at a temperature range between 10 - 310 K. For the
conversion of the intensity into the temperature, the thermometric
parameter Δ was employed. Here, 훼 = W0/WR is the ratio between the
nonradiative (W0 at T = 0 K) and radiative (WR) rate, ∆퐸 is the activation
energy for the nonradiative channel, and ∆0 isthe thermometric parameter
at T = 0 K.2-3, 25
퐼1
∆
0
∆ =
=
(3)
∆퐸
푘퐵푇
퐼2
(
)
1 + 훼 exp −
The relative temperature sensitivity S푟 was calculated using the following
equation:
1 휕∆
|
|
S푟 = 100%×
(4)
∆ 휕푇
S푟 indicatesthe relative change of thethermometric parameter per degree
of temperature change (% K-1). Compared with S푎, S푟 has the important
advantage of being independent of the nature of the thermometer and
allowsdirect and quantitativecomparison of different materials.
Another important parameter when assessing the performance of a
thermometer isthe temperature uncertainty 훿T. It can be determined using
the following equation2-3
:
Synthesis and characterization of the lanthanide bpyPOP DAT
temperature sensor
1 δ∆
δT =
(5)
푆푟
∆
7
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10.1002/chem.202002009
Chemistry - A European Journal
FULL PAPER
where δ∆ is the relative error in the determination of the thermometric
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parameter. The repeatability of the thermometer wascalculatedemploying
the equation2-3
:
|
|
푚푎푥 ∆푐 − ∆푖
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R = 1 −
(6)
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FVB, MDB, VVS, PVDV, CS acknow ledge the Research Board of
Ghent University (BOF GOA20177000303) for funding. AMK
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– Flanders (FWO) for funding. SA
acknow ledges the financial support from the Ghent University
BOF doctoral grant 01D04318. The authors w ould like to thank
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Sven Herremans (SGS) for his help w ith ICP-OES analyses and
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9
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Entry for the Table of Contents
A novel bipyridine-based Porous Organic Polymer (bpyDAT POP) is investigated as temperature sensor after grafting w ith Eu(acac)3
and Tb(acac)3 complexes. The material successfully demonstrates the potential of amorphous, porous polymers in temperature sensing.
Furthermore, peculiar temperature dependent behavior is observed through the interruption of conjugation in the framew ork and the
absence of thermal quenching.
10
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