Eu(III)- and Tb(III)-coordination polymer luminescent thermometers constructed from a π-rich aromatic ligand exhibiting a high sensitivity

Article abstract of DOI:10.1016/j.dyepig.2018.10.055

A π-rich aromatic organic ligand, 4’-(3,5-dicarboxyphenyl)-2,2′,6′,2″-terpyridine (H₂-DCPTP), and four new lanthanide coordination polymers, {[Ln(DCPTP)(NO₃)]·(H₂O)}_n (Ln = Tb, Eu, Gd) and {[Tb_0.9064Eu_0.0936(DCPTP)(NO₃)]·(H₂O)}_n (termed Tb_0.9064Eu_0.0936-DCPTP), were prepared. Compound {[Ln(DCPTP)(NO₃)]·(H₂O)}_n is a 3D framework based on the dinuclear Ln₂(μ₂-COO)₂] units linked by the tridentate DCPTP^2? ligands. The H₂-DCPTP ligand with an appropriate triplet excited state confirmed by DFT calculations and experiments, is a light-harvesting ligand for efficiently sensitizing of Tb³⁺ and Eu³⁺ simultaneously, thus enhancing the overall luminescent properties of Tb_0.9064Eu_0.0936-DCPTP. The Tb_0.9064Eu_0.0936-DCPTP behaves as a high-performance luminescent thermometer ranging from 150 to 300 K with a large relative thermal sensitivity (S_m) of 21.5% K^?1 and a high energy transfer efficiency (η) of 84% at 300 K.

Full text of DOI:10.1016/j.dyepig.2018.10.055

Accepted Manuscript
Eu(III)- and Tb(III)-coordination polymer luminescent thermometers constructed from
a π-rich aromatic ligand exhibiting a high sensitivity
Xin-Wen Peng, Qing-Yan Liu, Hui-Hong Wang, Yu-Ling Wang
PII:
S0143-7208(18)31975-2
https://doi.org/10.1016/j.dyepig.2018.10.055
DYPI 7121
DOI:
Reference:
To appear in: Dyes and Pigments
Received Date: 6 September 2018
Revised Date: 22 October 2018
Accepted Date: 25 October 2018
Please cite this article as: Peng X-W, Liu Q-Y, Wang H-H, Wang Y-L, Eu(III)- and Tb(III)-coordination
polymer luminescent thermometers constructed from a π-rich aromatic ligand exhibiting a high
sensitivity, Dyes and Pigments (2018), doi: https://doi.org/10.1016/j.dyepig.2018.10.055.
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Graphical abstract
The 4'-(3,5-dicarboxyphenyl)-2,2',6',2''-terpyridine (H2-DCPTP) ligand serves as
a
light-harvesting ligand for efficiently sensitizing of Tb3+ and Eu3+ simultaneously, thus enhancing
the overall luminescence of Tb0.9064Eu0.0936-DCPTP, which behaves as a high-performance
luminescent thermometer with a high relative thermal sensitivity of Sm up to 21.5 % K‒1.

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Tb(III)-coordination polymer
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Eu(III)-
and
luminescent
thermometers constructed from a π-rich aromatic ligand exhibiting a
high sensitivity
Xin-Wen Peng, Qing-Yan Liu*, Hui-Hong Wang, Yu-Ling Wang*
College of Chemistry and Chemical Engineering, Key laboratory of functional small organic
molecule of ministry of education, Jiangxi Normal University, Nanchang, Jiangxi 330022, PR
China E-mail: qyliuchem@jxnu.edu.cn; ylwang@jxnu.edu.cn
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Dedicated to Professor Jin-Shun Huang on the occasion of his 80th birthday.
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ABSTRACT: A π-rich aromatic organic ligand, 4'-(3,5-dicarboxyphenyl)-2,2',6',2''-terpyridine
(H2-DCPTP), and four new lanthanide coordination polymers, {[Ln(DCPTP)(NO3)]·(H2O)}n (Ln
= Tb, Eu, Gd) and {[Tb0.9064Eu0.0936(DCPTP)(NO3)]·(H2O)}n (termed Tb0.9064Eu0.0936-DCPTP),
were prepared. Compound {[Ln(DCPTP)(NO3)]·(H2O)}n is a 3D framework based on the
dinuclear Ln2(µ2-COO)2] units linked by the tridentate DCPTP2‒ ligands. The H2-DCPTP ligand
with an appropriate triplet excited state confirmed by DFT calculations and experiments, is a
light-harvesting ligand for efficiently sensitizing of Tb3+ and Eu3+ simultaneously, thus enhancing
the overall luminescent properties of Tb0.9064Eu0.0936-DCPTP. The Tb0.9064Eu0.0936-DCPTP behaves
as a high-performance luminescent thermometer ranging from 150 to 300 K with a large relative
thermal sensitivity (Sm) of 21.5 % K‒1 and a high energy transfer efficiency (η) of 84% at 300 K.
Keywords: Ratiometric fluorescent probe; luminescent thermometer; Eu(III)- and
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Tb(III)-coordination polymer; light-harvesting ligand.
1. Introduction
The lanthanide-based coordination polymers continue to attract intense attentions due to their
potential applications pertaining to the distinctive luminescence and magnetism derived from the
lanthanide centers with 4fn configuration [1‒7]. In particular, luminescent lanthanide coordination
polymers are highly sought after because of their diverse luminescent properties such as
characteristically sharp line emissions, high photoluminescence efficiency as well as long
luminescent lifetime, which have been developed for chemical sensors and light-emitting devices
[8‒15]. Recently, the lanthanide coordination polymers with the temperature-dependent emissions
have been developed for luminescent thermometers [16‒22]. Comparing to the conventional
temperature sensors, such luminescence-based thermometers own the distinct advantages of fast
response, non-invasive operation and high spatial resolution. Moreover, such thermometers can
work in strong electro-magnetic fields due to their inertness to strong electric-magnetic fields.
As is well-known, luminescent sensing based on relative emission intensity is more reliable and
powerful than through single-emission intensity. The accuracy of the sensing based on the
absolute intensity of an emission is often susceptible to external factors including the luminophore
quantity, excitation power, and the drifts of the optoelectronic systems. Moreover, the intensity
ratio for two emissions is almost a constant under a certain condition and thus additional
calibration of intensity is not necessary for such a ratiometric method, leading to desirable
ratiometric luminescent thermometers. A few mixed lanthanide coordination polymers
luminescent thermometers with Tb3+ and Eu3+-dual emissions have been reported recently [23‒27].
However, the relative thermal sensitivity of the Tb3+-Eu3+ luminescent thermometers is found to
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be lower than 10 % K−1 [28, 29]. Therefore, it is still a challenge to construct lanthanide
compounds for temperature sensing with high sensitivity and accurate sensing. As an efficient
sensitizer for the lanthanide ions, the organic ligand takes a decisive role in the construction of
lanthanide luminescent thermometers. To obtain high-performance luminescent thermometers with
Tb3+ and Eu3+ mixed metal ions, the Tb3+ and Eu3+ ions should be efficiently sensitized by the
organic ligand simultaneously. The organic ligand for such purpose must have an appropriate
triplet excited state energy between 22000 and 27000 cm−1 [1, 30, 31], matching the main
accepting energy levels of Tb3+ (5D4, 20500 cm−1) and Eu3+ (5D1, 19030 cm−1) [32].
In this contribution, an organic ligand with a terpyridine backbone and an isophthalic acid,
4'-(3,5-dicarboxyphenyl)-2,2',6',2''-terpyridine (H2-DCPTP), was designed for rational preparation
of luminescent thermometers. The H2-DCPTP with four carboxylate oxygens has a preferential
coordination to the oxophile lanthanide ions and the terpyridine moiety can chelate a lanthanide
ion, giving the robust lanthanide coordination polymers. In addition, the H2-DCPTP with
extensive aromatic π-systems is expected to be a light-harvesting ligand for efficiently sensitizing
of lanthanide ions, which is corroborated by the result of the time-dependent DFT calculations
(Fig. S1). The calculated triplet excited state energy for H2-DCPTP is 25217 cm−1, indicating the
H2-DCPTP is a suitable organic linker for construction of the Tb3+/Eu3+-coordination polymer
luminescent
thermometers.
Herein
three
compounds
with
formula
of
{[Ln(DCPTP)(NO3)]·(H2O)}n (Ln = Tb(1), Eu(2) and Gd(3)) and the Eu-doped Tb-compound of
{[Tb0.9064Eu0.0936(DCPTP)(NO3)]·(H2O)}n (termed Tb0.9064Eu0.0936-DCPTP) were presented.
Detailed luminescent behavior for these compounds were investigated. Temperature-dependent
luminescence of Tb0.9064Eu0.0936-DCPTP show the organic ligands with the extensive aromatic
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π-systems transfer the absorbed energy to the Tb3+ and Eu3+ ions efficiently, thus enhancing the
overall luminescent properties of Tb0.9064Eu0.0936-DCPTP. As a result, Tb0.9064Eu0.0936-DCPTP
behaves a high-performance luminescent thermometers displaying a large relative thermal
sensitivity and a high energy transfer efficiency.
2. Experimental
2.1. Materials and instrumentations
1H nuclear magnetic resonance (NMR) spectra were collected on a Bruker AVANCE 400
spectrometer. FT-IR (KBr pellets, cm–1) spectra were performed at a Perkin-Elmer Spectrum One
FT-IR spectrometer ranging from 400–4000 cm–1. Elemental analyses were carried out on an
Elementar Perkin-Elmer 2400CHN microanalyzer. Thermogravimetric analyses (TGA) were
carried out on a PE Diamond instrument under a N2 flow with a heating rate of 10 °C min–1.
Powder X-ray diffraction (PXRD) were measured on a Rigaku Miniflex II powder diffractometer
(Cu-Kα, λ = 1.5418 Å). Fluorescent spectra and luminescence lifetime were measured with
Edinburgh Instruments FLS980 fluorescence spectrometer equipped with an Oxford Instruments
liquid nitrogen flow cryostat. Inductively coupled plasma (ICP) (Ultima2, HORIBA Jobin Yvon)
spectra were used for determination of the metal concentration after the sample was dissolved in
concentrated nitric acid.
X-ray diffraction experiments were carried out on a Rigaku Oxford SuperNova diffractometer
with an Eos detector (Mo-Ka radiation, 0.71073 Å) and. The CrysAlisPro software package was
used for absorption correction and data reduction [33]. The structures were solved by the direct
methods using the SHELXTL [34] and refined by the full-matrix least-squares against F2 [35]. All
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atoms except for hydrogen atoms are refined with anisotropic thermal parameters. Aromatic
hydrogen atoms were placed in calculated positions. Hydrogen atoms bonded to oxygen atoms
couldn’t be located. The R1 and wR2 values are defined as R1 = Σ||Fo| – |Fc|| / Σ|Fo| and wR2 =
2
2 2
{Σ[w(Fo – Fc2)2] / Σ[w(Fo ) ]}1/2, respectively. The important crystallographic data is listed in
Table S1.
All chemicals used here were of reagent grade and obtained commercially. The
4'-(3,5-dicarboxyphenyl)-2,2',6',2''-terpyridine (H2-DCPTP) was synthesized in Scheme 1.
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Scheme 1 Preparation of H2-DCPTP ligand.
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2.2 Synthesis of H2-DCPTP ligand
2.2.1 Synthesis of 3-(3′,5′-dimethylphenyl)-1-(pyridin-2′-yl)prop-2-en-1-one (a).
All reactants including solvents for this reaction were cooled to 5 oC before use. In a flask, 2.50
g (18.7 mmol) of 3,5-dimethylbenzaldehyde and 1.20 g (33.6mmol) of NaOH were mixed with 90
mL methanol/water (v/v = 3:1) solution, followed by a 20 mL methanol solution of
2-acetylpyridine (18.0 mmol, 2.20 g). The mixture was stirred at 5 oC for 8 h to give yellow solid.
The yellow solid was washed with methanol and water (v/v = 1:4) solution and dried in vacuo at
room temperature; yield 4.18 g (95%). 1H NMR (CDCl3, ppm): δ = 8.76 (d, J = 4.0 Hz, 1H), 8.29
(d, J = 16.0 Hz, 1H), 8.20 (d, J = 8.0 Hz, 1H), 7.92 (s, 1H), 7.89 (m, J = 16.0 Hz, 1H), 7.50 (m, J
= 12.0 Hz, 1H), 7.35 (s, 2H), 7.05 (s, 1H), 2.35 (s, 6H).
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2.2.2 Synthesis of (4′-(3,5-dimethylphenyl) -2,2′:6′,2″-terpyridine) (b).
Compound a (10.0 mmol, 2.37g), 2-acetylpyridine (10.0 mmol, 1.21 g), and KOH (30.0 mmol,
1.68 g) were mixed with 120 mL of ethanol followed by a 75 mL NH4OH solution (30%). The
resulting mixture was stirred at 60 °C for 1 d to give a pale yellow solid. The solid was
recrystallized from pyridine and water (v/v = 2:1) solvent, and dried in vacuo; yield 1.72 g (51%).
1H NMR (DMSO-d6, ppm): δ = 8.77 (d, J = 4.4 Hz, 2H), 8.69 (d, J = 10.0 Hz, 4H), 8.06 (m, J =
16.8 Hz, 2H), 7.55(d, J = 11.2 Hz, 4H), 7.17(s , 1H), 2.41(s, 6H).
2.2.3 Synthesis of 4'-(3,5-dicarboxyphenyl)-2,2',6',2''-terpyridine (H2-DCPTP).
1.21 g (3.6 mmol) of compound b and 3.40 g (21.6 mmol) of KMnO4 were mixed with 60 mL
pyridine and 30 mL water. The mixture was refluxed for 4 h. Then 3.40 g (21.6 mmol) of KMnO4,
10 mL water and 10 mL pyridine were added into the mixture, which was refluxed for another 3 h.
The resulting mixture was filtrated and the obtained precipitate was washed with water. The
combined filtrates were condensed under reduced pressure and then added by concentrated HCl to
give white precipitate. The white precipitate was recrystallized from solvent of pyridine/water (v/v
1
= 1:1) and dried in vacuo; yield 1.23g (86 %). H NMR (DMSO-d6, ppm): δ = 8.81 (d, J =4 .4,
2H), 8.74 (d, J = 8.4 Hz, 4H), 8.58 (s, 3H), 8.14 (m, J = 15.2 Hz, 2H), 7.64(m, J = 12.0 Hz, 2H).
2.3 Synthesis of {[Ln(DCPTP)(NO3)]·(H2O)}n
Lanthanide nitrate (Tb(NO3)3·6H2O (20.4 mg, 0.045 mmol), or Eu(NO3)3·6H2O (20.1 mg,
0.045 mmol), or Gd(NO3)3·6H2O (20.3 mg, 0.045 mmol )), H2-DCPTP (18.0 mg, 0.045 mmol ),
acetonitrile (4 mL), and HNO3 (0.5 mL, 1 M in H2O) were mixed. The resulting mixture was
introduced into a 25 mL Parr Teflon-lined stainless steel vessel, which was heated at 140 °C for 5
d. The mixture was cooled to room temperature to give crystals. Yield: 75% for Tb(1); 71% for
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Eu(2); 82% for Gd(3). Anal. Calcd for C23H15N4O8Tb (Mr = 634.31): C, 43.55; H, 2.38; N, 8.83%.
Found: C, 43.51; H, 2.43; N, 8.81%. Anal. Calcd for C23H15N4O8Eu (Mr = 627.35): C, 44.03; H,
2.41; N, 8.93%. Found: C, 44.02; H, 2.39; N, 8.91%. Anal. Calcd for C23H15N4O8Gd (Mr =
632.64): C, 43.67; H, 2.39; N, 8.86%. Found: C, 43.62; H, 2.42; N, 8.83%. IR spectrum (cm‒1):
for Tb(1), 3394 (m), 1627 (s), 1601 (m), 1586 (m), 1572 (m), 1541 (m), 1449 (s), 1385 (s), 1310
(m), 1165 (w), 1105 (w), 1038 (w), 1013 (w), 887 (w), 794 (w), 776 (w), 735 (w), 721 (m), 657
(w), 576 (w), 517 (w); for Eu(2), 3420 (m), 1628 (s), 1600 (m), 1586 (m), 1572 (m), 1541 (m),
1448 (s), 1383 (s), 1309 (m), 1164 (w), 1104 (w), 1013 (m), 793 (w), 734 (w), 720 (m), 705 (w),
657 (w), 637 (w), 573 (w); for Gd(3), 3404 (m), 1626 (s), 1600 (m), 1586 (m), 1572 (m), 1540 (m),
1448 (m), 1384 (s), 1308 (m), 1238 (w), 1164 (w), 1104 (m), 1038 (w), 1013 (m), 924 (w), 886
(w), 815 (w), 794 (w), 776 (w), 735 (w), 721 (m), 657 (w), 637 (w), 574 (w), 516 (w).
2.4 Synthesis of {[Tb0.9064Eu0.0936(DCPTP)(NO3)]·(H2O)}n
The process for preparation was similar to that of Tb(1) with Tb(NO3)3·6H2O replaced by a
mixture of (Tb(NO3)3·6H2O (18.4 mg, 0.0405 mmol) and Eu(NO3)3·6H2O (2.0 mg, 0.0045 mmol).
Yield: 77%. IR spectrum (cm‒1): 3410 (m), 1628 (s), 1601 (m), 1587 (m), 1572 (m), 1540 (w),
1449 (s), 1384 (s), 1310 (w), 1238 (w), 1165 (m), 1105 (m), 1039 (w), 1013 (m), 925 (w), 886 (w),
794 (w), 776 (w), 735 (w), 721 (w), 657 (m), 637 (w), 574 (w), 516 (w).
3. Results and discussion
3.1 Syntheses
Reaction of H2-DCPTP and lanthanide salts in CH3CN solvent in the presence of HNO3
afforded the isomorphous coordination polymers, {[Ln(DCPTP)(NO3)]·(H2O)}n (Ln = Tb(1),
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Eu(2) and Gd(3)). Accordingly, the bimetallic Tb3+/Eu3+ compound of Tb0.9064Eu0.0936-DCPTP can
be readily prepared by using the initial molar ratio of 0.9:0.1 for Tb(NO3)3 and Eu(NO3)3 through
the similar preparation procedure. The actual molar ratio of Tb3+/Eu3+ (0.9064/0.0936) in the
resulting bimetallic coordination polymer, which was determined by inductively coupled plasma
spectroscopy, matches well with that of starting materials. As expected, the Eu-doped
Tb-compound of Tb0.9064Eu0.0936-DCPTP is isomorphous with the parent ones, which is confirmed
by their PXRD patterns (Fig. S2).
3.2 Crystal structures
For these isostructural compounds, structure Tb(1) is described representatively. There one Tb3+,
one DCPTP2− dianionic ligand, one nitrate anion, and one lattice water molecule are in the
asymmetric unit of Tb(1). Tb3+ ion is nine-coordinated by three N atoms of one DCPTP2− ligand,
four carboxylate O atoms (O1, O2A, O3C, and O4C) from three DCPTP2− ligands and two O
atoms of a NO3− group (Fig. 1a). The Tb–O bond distances vary from 2.286(4) to 2.540(4) Å and
Tb–N bond distances range from 2.531(6) to 2.577(6) Å. The nitrate anion chelates a Tb3+ ion
through its two O atoms with the third O atom uncoordinated. The DCPTP2− ligand binds four
metal ions through a chelating and a bidentate carboxylate groups, and a chelating terpyridyl
group (Fig. S3). Two bidentate carboxylate groups bridge two Tb3+ ions to generate a dinuclear
Tb2(µ2-COO)2] unit (Fig. 1b), which is connected by the DCPTP2− ligands to generate a 3D
framework (Fig. 2). Each dinuclear Tb2(µ2-COO)2] unit is surrounded by six DCPTP2− ligands
(Fig. 1b) and each DCPTP2− ligand connects three dinuclear Tb2(µ2-COO)2] units (Fig. S4). In this
way, the DCPTP2− ligand and the dinuclear Tb2(µ2-COO)2] unit define a 3- and 6-connected nodes,
respectively. Thus the 3D framework of Tb(1) is a 3,6-connected network with
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(42·6)2(44·62·87·102) topology (Fig. S5). When loss of the solvent water molecules, the main
framework can stable up to 400 °C (Fig. S6).
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Fig. 1 Coordinated environment of Tb3+ ion (a) and the dinuclear Tb2(µ2-COO)2] unit (b) in Tb(1).
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Fig. 2 3D framework of Tb(1) (The nitrate anions and lattice water molecules are omitted for
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clarify).
3.2 Luminescent properties
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Upon excitation at 369 nm, the free H2-DCPTP ligand emits an emission centered at 434 nm in
the solid state, which was assigned to the intraligand π*−π electron transitions (Fig. S7). The solid
state luminescent emissions of Tb(1) and Eu(2) at room temperature are shown in Fig. S8 and S9,
respectively. Tb(1) displays an excitation band centered at 353 nm and no sharp peak of Tb3+ f−f
transition appears in the excitation spectrum, indicating the Tb3+ ions are excited via an effective
sensitized process involving the organic ligand. Tb(1) emits strong green emission with the
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characteristic peaks of Tb3+ (5D4 → FJ, J = 0 ‒ 6) at 490, 543, 584, 621, 650, 667, and 678 nm
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(Fig. S8), wherein the D4 → 7F5 transition (543 nm) has the most intense intensity. Additionally,
the emission of organic ligand is not detected in the emission spectrum of Tb(1), which indicates
an energy is transferred from organic ligands to Tb3+ ions efficiently. As case of Eu(2), a excitation
band centered at 361 nm overlapping with additional weak intra-4f6 sharp lines is observed,
indicating that the Eu3+ ions are mainly excited through a sensitized process but not via a direct
excitation process. Eu(2) exhibits typical bands arising from Eu3+ excited levels (5D0 → 7FJ, J = 0,
1, 2, 3, 4) at 580, 592, 616, 651, and 698 nm, after excited at 361 nm (Fig. S9). The intensity of
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the D0 → F2 emission, which is extremely sensitive to site symmetry, is much more intense
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→
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coordination environment.
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Fig. 3 Temperature-dependent emission spectra for Tb(1) between 77 K and 300 K (λex = 361 nm).
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Fig. 4 Temperature-dependent emission spectra for Eu(2) between 77 K and 300 K (λex = 361 nm).
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Fig. 5 Solid-state emission spectra of Tb0.9064Eu0.0936-DCPTP between 77 K and 300 K (λex = 361
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Temperature-dependent photoluminescence emission spectra of Tb(1) and Eu(2) between 77 K
and 300 K are presented in Fig. 3 and 4, respectively. For both compounds, the emission intensity
and lifetime decrease upon increasing temperature, as a result of the thermal activation of
nonradiative-decay. However, the emission intensity and lifetime of Tb3+ in Tb(1) diminish more
pronouncedly than those of Eu3+ in Eu(2) upon increasing temperature (Fig. S10). In particular for
the lifetime, lifetime of Eu3+ is almost unchanged in the test temperature range (Fig. S10b). Such
distinct temperature-dependent luminescent behavior for Tb(1) and Eu(2) can be attributed to the
different energy gaps between the triplet excited state of the ligand and the emitting levels of Tb3+
and Eu3+ ions. The triplet state energy level T1 for H2-DCPTP is 25445 cm‒1 (393 nm) estimated
from the phosphorescence spectrum of Gd(3) (λex = 372 nm) (Fig. S11). The experimental T1
value is consistent with the one (25217 cm−1) from DFT calculations. The energy gap between the
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T1 level and the D4 emitting level (20453 cm‒1) of Tb3+ [36] in Tb(1) is much smaller than the
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energy gap between T1 level and the D0 emitting level (17267 cm‒1) of Eu3+ [37] in Eu(2). As a
result, the thermally driven depopulation for Eu3+ emitting level is almost prohibited [38–40].
To obtain more reliable luminescent thermometers based on two emissions with a ratiometric
method, the Eu-doped Tb-compound of Tb0.9064Eu0.0936-DCPTP was synthesized. The
Tb0.9064Eu0.0936-DCPTP displays characteristic emissions of Tb3+ and Eu3+ ions simultaneously
(Fig. S12), indicating the H2-DCPTP with an extensive π-conjugated system is a remarkable
sensitizer for sensitizing of Tb3+ and Eu3+ ions. Fig. 5 presents the emission spectra of
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Tb0.9064Eu0.0936-DCPTP from 77 to 300 K. The intensity of 5D0 → F2 (616 nm) transition of Eu3+
is enhanced significantly, while that of 5D4 → 7F5 transition (542 nm) of Tb3+ is diminished slowly
when increasing temperature (Fig. 6). At 77 K, the intensity of 5D4 → 7F5 (Tb3+) transition is about
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2.7 times more intense than that of 5D0 → F2 (Eu3+) transition (Fig. 7), which gives a green color
for the compound. In contrast, the intensity of D0 → F2 (Eu3+) transition is 19.6 times more
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intense than that of D4 → F5 (Tb3+) transition at 300 K, indicating the Eu3+ emission is
completely dominated in the high temperature region. Thus the temperature-dependent
luminescent colors are systematically tuned from green, via yellow to red with increasing
temperature (Fig. 8). The dramatic intensity changes of Tb3+- and Eu3+-emissions in opposite
directions suggests the Tb0.9064Eu0.0936-DCPTP is an excellent candidate for ratiometric
luminescent thermometers. As illustrated in Fig. 7, the emission intensity ratio (∆ = ITb/IEu)
5
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between D4 → F5 transition (Tb3+, 543 nm) and D0 → F2 transition (Eu3+, 616 nm) remains
roughly constant in the temperature range of 77 ‒ 150 K. The relationship between ∆ and
temperature (T) is well fitted with the exponential empirical equation (1) in the temperature of 150
‒ 300 K (Fig. 7 inset and Fig. S14)
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∆ = – 0.530 + 24.535e(– 0.013T) (1)
The correlation coefficient (R2) of 0.99652 is obtained from the fitting result. The relative thermal
sensitivity Sr (Sr = |∂∆/∂T|/∆) [41, 42] is used to characterize the performances of the thermometers.
As can be seen from the definition of Sr, the Sr value varies with the temperature. Thus the
maximum (Sm) value of Sr in the detection temperature range is proposed for comparing the
performances of the coordination compounds thermometers [28]. The Tb0.9064Eu0.0936-DCPTP has
a high Sm up to 21.5 % K‒1 at 300 K (Fig. S15), which is the highest value for the lanthanide
coordination polymers ratiometric luminescent thermometers (Table S2). Such results suggest the
Tb0.9064Eu0.0936-DCPTP is a potentially useful luminescent thermometer and further confirm the
present organic ligand is a very effective sensitizer for Tb3+ and Eu3+ ions. It is noted that there is a
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report claiming the Sm of 31 % K‒1 for the Tb0.95Eu0.05-5-hydroxy-1,2,4-benzenetricarboxylate very
recently [43]. However, its Sm value appears at low temperature of 4 K, which is different from the
present case and other thermometers (Table S2). Finally, the temperature-dependent emission
spectra for Tb0.9064Eu0.0936-DCPTP material are repeatable and reversible (Fig. S16), indicates it is
a robust lanthanide coordination polymer luminescent thermometer.
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Fig. 6 Integrated intensities of 5D4 → F5 (538‒555 nm) and 5D0 → F2 (605‒630 nm) emission
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bands versus temperature in Tb0.9064Eu0.0936-DCPTP.
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Fig. 7 Integrated intensity ratio between Tb3+ (543 nm) and Eu3+ (616 nm) versus temperature for
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Tb0.9064Eu0.0936-DCPTP (Inset: Fitted curve of the integrated intensity ratio for
Tb0.9064Eu0.0936-DCPTP from 150 to 300 K with equation (1).
A rapid decrease of lifetime for Tb3+ emission is observed in Tb0.9064Eu0.0936-DCPTP with
increasing temperature, especially between 150 and 300 K. The lifetime of Tb3+ emission is
decreased about 94% as the temperature increasing from 150 to 300 K (Fig. S13). While the
lifetime of the Eu3+emission decreases only slightly in Tb0.9064Eu0.0936-DCPTP with increasing the
temperature. Such observations indicate the nonradiative Tb3+-to-Eu3+ energy transfer occurs [44].
At 150 K, the lifetimes of Tb3+ emission in Tb0.9064Eu0.0936-DCPTP (1.04 ms) (Fig. S13) and in
pure Tb(1) (1.16 ms) (Fig. S10b) are similar. In contrast, the lifetime of Tb3+ emission in
Tb0.9064Eu0.0936-DCPTP decreases to 0.04 ms at 300 K, which is about one-tenth of that for pure
Tb(1) at 300 K (0.41 ms). The energy migration between Tb3+ centers is easier than that of from
Tb3+ to Eu3+
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Fig. 8 The CIE chromaticity diagram showing the luminescence color of Tb0.9064Eu0.0936-DCPTP
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at different temperatures.
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Fig.
9
Energy transform efficiency from Tb3+ to Eu3+ versus temperature in
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Tb0.9064Eu0.0936-DCPTP.
because of the larger energy gap between Tb3+ to Eu3+ energy levels. When temperature higher
than 150 K, the abrupt decrease of lifetime for Tb3+ emission in Tb0.9064Eu0.0936-DCPTP indicates
the energy migration occurred between Tb3+ centers, ultimately resulting in Tb3+ to Eu3+ energy
transfer. However, because of the small variations in the energy levels for Tb3+ center, the energy
migration between neighboring Tb3+ centers is hampered at lower temperature. Thus in the
temperature range from 77 to150 K, the slight changes in the intensity and lifetime of Tb3+
emission in Tb0.9064Eu0.0936-DCPTP are not unreasonable. With further increasing the temperature,
the energy in the excited state of Tb3+ has more opportunity to transfer to a Tb3+ ion neighboring a
Eu3+ center, followed by subsequent energy transfer to Eu3+. As a result, the observation of
enhancement of Eu3+ emission and recession of Tb3+ emission in the temperature range 150−300
K can be explained by the phonon-assisted Förster transfer mechanism [45]. The energy transfer
efficiency (η) from Tb3+ and Eu3+ ions can be calculated from the equation of η = 1 ‒ τ/τ0 (τ and τ0
represent the lifetimes of Tb3+ with and without Eu3+ acceptor, respectively) [46]. The
temperature-dependent η for Tb0.9064Eu0.0936-DCPTP is presented in Fig. 9, which displays the
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energy transfer efficiency from Tb3+ to Eu3+ is low and almost unchanged in the range of 77‒150
K. Then the energy transfer efficiency is dramatically enhanced on going from 150 to 300 K, thus
resulting in the increase of the intensity of Eu3+-emission at the cost of the quenching of
Tb3+-emission upon temperature increasing. The η value at 300 K is about 84%, which is the
highest value for the Eu3+/Tb3+mixed coordination polymers
4. Conclusions
In summary, a H2-DCPTP ligand with π-rich conjugated systems and four lanthanide
compounds including the mixed lanthanide compound constructed of H2-DCPTP ligand have been
prepared and characterized. These isostructural compounds have a 3D framework featuring the
dinuclear lanthanide-carboxylate units. The Tb0.9064Eu0.0936-DCPTP serving as a ratiometric
luminescent thermometer was evaluated detailedly in terms of luminescence intensity and lifetime.
The result shows that Tb0.9064Eu0.0936-DCPTP can be developed as a high-performance temperature
sensor in the temperature range of 150−300 K, with higher relative sensitivity and larger energy
transfer efficiency than those of previously reported Eu3+/Tb3+ coordination polymers luminescent
thermometer. This work demonstrates a strategy for designing of robust luminescent thermometers
with high sensitivity and large energy transfer efficiency.
Acknowledgements
This work was supported by the NSF of China (Grants 21661014, 21561015 and 21861020),
the NSF of Jiangxi Province (Grant 20171ACB20008 and 20181BAB203001). We thank
Professor Xingfa Gao for the DFT calculations
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Appendix A. Supplementary data
Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.dyepig.
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Highlights
•
•
•
•
{[Ln(DCPTP)(NO3)](H2O)}n and {[Tb0.9064Eu0.0936(DCPTP)(NO3)](H2O)}n were presented.
The H2-DCPTP ligand is a light-harvesting ligand for efficiently sensitizing of Tb3+ and Eu3+.
Tb0.9064Eu0.0936-DCPTP behaves as a high-performance luminescent thermometer.
Luminescent thermometer with a high relative thermal sensitivity of Sm up to 21.5 % K‒1.