Accurate tuning of rare earth metal-organic frameworks with unprecedented topology for white-light emission

Article abstract of DOI:10.1039/c9tc05471e

A series of isostructural rare-earth metal-organic frameworks (RE-MOFs), namely {[RE(L-X)(H₂O)]·2DMF}_n (UPC-38, RE = Eu, Tb, X = H, F, Cl, NH₂, CH₃, OCH₃; L = [1,1′:3′,1′′-terphenyl]-4,4′′,5′-tricarbo

Full text of DOI:10.1039/c9tc05471e

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Accurate tuning of rare earth metal–organic
frameworks with unprecedented topology
Cite this:DOI: 10.1039/c9tc05471e
for white-light emission†
Yutong Wang, ‡a Kai Zhang,‡a Xiaokang Wang,a Xuelian Xin,b Xiurong Zhang,a
ac
ac
ac
Weidong Fan,a Ben Xu,
Fangna Dai
*
and Daofeng Sun
*
A series of isostructural rare-earth metal–organic frameworks (RE-MOFs), namely {[RE(L-X)(H2O)]ꢀ2DMF}n
(UPC-38, RE = Eu, Tb, X = H, F, Cl, NH2, CH3, OCH3; L = [1,10:30,100-terphenyl]-4,400,50-tricarboxylic acid)
have been successfully synthesized under solvothermal conditions. Single-crystal X-ray diffraction analysis
shows that UPC-38 is an unprecedented three-dimensional (3D) (3,4,5)-c zkf topological framework with
one-dimensional (1D) chain secondary building units. Different functional groups can be attached to the
ligand (L), and the effects of the different functional groups on the fluorescence intensity, emission
wavelength and quantum yield of UPC-38 were systematically studied. It was found that the introduction of
an amino group causes the emission wavelength to red shift significantly, while that of other groups causes
slight effects on the emission wavelengths. UPC-38(Eu)-H and UPC-38(Tb)-H without modified functional
groups in these twelve crystals exhibit the highest quantum yield ratios of 16.64% and 28.06%, respectively.
Remarkably, by adjusting the ratio of Eu3+ to Tb3+ and the L-OCH3 ligand with the strongest blue
3+
emission,
a series of double-doped rare-earth metal–organic frameworks, UPC-38(EuxTb1ꢁx)-OCH3
(0 r x r 1), are successfully acquired. The calculated CIE coordinates of UPC-38(Eu)-OCH3 and
UPC-38(Tb)-OCH3 are (0.50, 0.28) and (0.23, 0.45), respectively. It is exciting that a white light emitting
material, UPC-38(Eu0.34Tb0.66)-OCH3, can be successfully obtained through careful tuning of the Eu3+/Tb3+
ratio in the framework. The CIE coordinate is (0.335, 0.325), which is very close to the standardized value of
(0.333, 0.333).
Received 7th October 2019,
Accepted 6th December 2019
DOI: 10.1039/c9tc05471e
rsc.li/materials-c
rare-earth (RE)-doped materials, because of the special lumi-
nescence properties.3 However, according to the Laporte rule
Introduction
In the past decade, white light emitting materials have been and screening effect, rare-earth ions have very weak emissions
extensively investigated and widely applied in illumination, when they are directly excited.4 The ‘‘antenna effect’’ produced
lasers, and displays.1 Especially in the field of illumination, by introducing organic chromophores is an effective way to
incandescent lamps have been gradually replaced by fluorescent increase the light absorption and enhance the luminescence
lamps and white light-emitting diodes (WLEDs) because they ability of rare earth ions.5
are more environmentally friendly, safe, efficient and energy-
More than half of rare earth elements with 4f electrons can
saving.2 Currently, almost all commercial phosphors are be stimulated and emit visible or near infrared light, for
example Nd3+, Sm3+, Eu3+, Tb3+, Dy3+, Er3+, Tm3+, Yb3+ and so
on, and the light covers the whole visible region.6 Moreover,
they emit three primary colors in nature: red, green and blue,
which enables them to theoretically regulate the luminescence
of rare earth complexes. According to RGB (Red, Green and
a College of Science, China University of Petroleum (East China), Qingdao,
Shandong 266580, China. E-mail: fndai@upc.edu.cn, dfsun@upc.edu.cn
b College of Public Health & Key Laboratory of Medicinal Chemistry and Molecular
Diagnosis, Ministry of Education, Hebei University, No. 342 Yuhuadonglu,
Blue) models, various colors of light in nature can be obtained
by adjusting red, green and blue light.7–10 However, controllable
doping of different rare earth ions to increase the antenna effect
remains a huge challenge.
Metal–Organic frameworks (MOFs) have been extensively
applied in recent years, such as gas storage/separation, hetero-
geneous catalysis, molecular magnetism, photoluminescence
Baoding 071000, China
c School of Materials Science and Engineering, China University of Petroleum
(East China), Qingdao, Shandong 266580, China
† Electronic supplementary information (ESI) available: Crystal data, ICP data,
quantum yield, PXRD, TGA curves, IR pattern, and PL spectra. CCDC 1951706–
1951710. For ESI and crystallographic data in CIF or other electronic format see
DOI: 10.1039/c9tc05471e
‡ Y. T. Wang and K. Zhang contributed equally to this work.
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and fluorescent probes.11–15 Particularly, rare-earth metal–organic and Tb3+, the luminescence characteristics of the RE-MOFs can
frameworks (RE-MOFs) show great application prospects in be fine-tuned to achieve white-light emission.
photoluminescence and fluorescent probes. Recently, some
RE-MOF materials with white light emitting properties have
been obtained through reasonable design.16 By carefully adjust-
Results and discussion
Crystal structure
ing the proportion of Eu3+, Tb3+, and Dy3+ ions, Yin et al.
obtained a ternary doped white light emitting RE-MOF.17 Wu
et al. realized a highly efficient white light emitting MOF Single-crystal X-ray diffraction analysis shows that UPC-38-H,
through packaging of Red–Green–Blue fluorescent dyes, and UPC-38-F, UPC-38-Cl, UPC-38-NH2, UPC-38-CH3, and UPC-38-OCH3
high color-rendering index values up to 92% and quantum are isostructural with the monoclinic space group I2/c. The
yields up to 26% were reported.18 A multicomponent MOF with crystal structure of UPC-38-F (Eu) is described representatively.
white light emission has been reported by Telfer et al., which The asymmetrical unit contains half deprotonated L-F3ꢁ ligand,
contains three controllable photoactive ligands.19 However, few half Eu3+ ion, and two coordinated water molecules. Each Eu3+
studies focusing on the generation of white-light emission ion adopts eight-coordination mode, which is coordinated by six
through combining the photo physical characteristics of the carboxylate oxygen atoms from five different ligands and two
organic ligand and metal ions have been reported, although water molecules (Fig. 1a). The spatial geometry of each Eu3+ ion
MOFs have the advantage of being an adjustable luminescent is distorted by the plane rotation of oxygen atoms (Fig. 1a).
material.
As shown in Fig. 1b, the carboxylate groups of the L-F3ꢁ ligand
It is well known that ligands with a rigid structure and adopt two coordination modes: chelating bidentate mode for the
p-electron conjugation system have strong fluorescence emis- group on the central benzene ring, and cis–cis bridging bidentate
sion properties.20 Moreover, the introduction of functional mode for the groups on the side benzene ring. Thus, each eight-
groups to aromatic hydrocarbons will affect the fluorescence coordinated Eu3+ ion is bridged by m2-Z1:Z1 p-benzoate carb-
intensity and wavelength. Thus, careful tuning of the organic oxylate groups from L-F3ꢁ ligands to form a one-dimensional
ligand and doping of RE ions in RE-MOFs are reasonable to (1D) rod-shaped secondary building unit (SBU) with a Eu–Eu
achieve white-light emission. Herein, we report the design distance of 4.927 Å (Fig. 1d). The 1D chain SBUs are further
and synthesis of a series of three-dimensional RE-MOFs linked by the backbone of the L-F3ꢁ ligands to form a 3D
{[RE(L-X)(H2O)]ꢀ2DMF}n (UPC-38, RE = Eu, Tb, X = H, F, Cl, framework structure (Fig. 1g). With the introduction of different
NH2, CH3, OCH3; L = [1,10:30,100-terphenyl]-4,400,50-tricarboxylic functional groups, the twist angle between the central benzene
acid) with the unprecedented zkf topology based on tricar- ring and the side benzene ring is changed (Fig. S1, ESI†).
boxylic acid ligands with different functional groups (ꢁH, ꢁF, We systematically studied the effect of steric hindrance on the
ꢁCl, ꢁNH2, ꢁCH3 and ꢁOCH3). Through careful selection structure control of multinuclear RE-MOFs in our previous
of the organic ligand and doping of different ratios of Eu3+ work.21 In contrast, here they tend to form non-multinuclear
Fig. 1 (a) The coordination environments of Eu3+ ions and the distortion of the cubic geometry through a rotation of the plane formed by oxygen atoms
O10, O20, O30 and O40; (b) the coordination environments of L-F; (c) the simplified diagram of ligands; (d and e) 1D chain secondary building units; (f) the
simplified diagram of SBUs; (g and h) view of the 3D open framework along the c-axis; (i) (3,4,5)-c zkf topological frameworks.
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clusters when there are no templates in the reaction system.
The rod-shaped SBUs have better compatibility, which can well
tolerate the twist of the ligands. UPC-38 has an unprecedented
topology with a 3, 4, 5-connected net, in which carboxylate C
atoms on the rod define diamond ladders (Fig. 1f), and the
ligands are simplified to a three-link point (Fig. 1c). The net in
this case has the Reticular Chemistry Structure Resource (RCSR)
symbol zkf and is shown in Fig. 1i.
Solid-state luminescence properties
H3L-H, H3L-F, H3L-Cl, H3L-NH2, H3L-CH3 and H3L-OCH3 at
room temperature exhibit an emission peak at 429, 427, 425,
473, 426 and 425 nm (lex = 330 nm), respectively (Fig. 3a). These
blue emissions could be ascribed to the intra ligand n–p or
p–p* electron transition. Compared with that of H3L-H, the
emission peak of H3L-NH2 with an inserted amino group was
red-shifted. For UPC-38(Eu)-X and UPC-38(Tb)-X (X = H, F, Cl,
NH2, CH3, OCH3), the characteristic bands of Eu3+ and Tb3+
ions appeared. As shown in Fig. 2b, UPC-38(Eu)-X shows four
emission bands at 592, 615, 652, and 698 nm, which are
Fig. 3 (a) Solid-state emission spectra of UPC-38(Tb)-OCH3; (b) solid-
state emission spectra of UPC-38(Eu)-OCH3; (c) solid-state emission
spectra of HL-OCH3; (d) solid-state emission spectra of UPC-38(Eu0.34Tb0.66)-
OCH3 (the insets show the corresponding luminescent materials under UV-light
irradiation at 365 nm and the CIE coordinates at 330 nm).
5
7
(0.503, 0.278) and red emission (lex = 330 nm) (Fig. 3b). The
emission wavelength of H3L-OCH3 locates just in the blue
region. The CIE coordinate is calculated to be (0.158, 0.032),
corresponding to blue emission (lex = 330 nm) (Fig. 3c).
This makes it possible to provide white-light emission through
careful tuning of the ratios of Eu3+ and Tb3+ ions in the
framework.
attributed to the characteristic D0 - FJ (J = 1–4) transitions
of Eu3+ ions. The isolated coordination geometry of Eu3+ metal
ions leads to a strong 5D0 - 7F4 peak due to the distorted non-
cubic configuration,22 but 5D0
these chain SBUs (Fig. 1a).
-
7F2 emission dominates in
As shown in Fig. 2c, UPC-38(Tb)-X shows four emission
bands at 490, 545, 586, and 622 nm, which are attributed to
the characteristic 5D4 - 7FJ (J = 3–6) transitions of the Tb3+ ion.
With the introduction of functional groups, the quantum yield
Tunable emission of bimetallic doped EuxTb1ꢁx MOFs
drastically changed. Compared with that of UPC-38(Eu/Tb)-X All GRB colors can be obtained based on red, green and blue.
(X = F, Cl, NH2, CH3, OCH3), the highest quantum yields of The aforementioned results clearly demonstrate that isomorphic
UPC-38(Eu)-H and UPC-38(Tb)-H are 16.64% and 28.06%, UPC-38 can produce all the three primary colors. Thus, in theory,
respectively. The lowest quantum yields of UPC-38(Eu)-Cl and color adjustment can be achieved by adjusting the ratio of
UPC-38(Tb)-Cl are 2.98% and 4.89%, respectively (Table S2, Eu3+/Tb3+. Therefore, bimetallic doped UPC-38(EuxTb1–x)-OCH3
ESI†).
(0 r x r 1) was obtained by doping Eu3+/Tb3+ in different
Considering comprehensively, we choose UPC-38-OCH3 as proportions. As shown in the PXRD patterns (Fig. S9, ESI†),
the representative research object. As shown in Fig. 3a, under a bimetallic doped UPC-38(EuxTb1ꢁx)-OCH3 are isostructural
365 nm UV lamp, UPC-38(Tb)-OCH3 shows green light. The CIE to the corresponding UPC-38. The relative molar ratios of
coordinate is calculated to be (0.226, 0.448), corresponding to Eu3+/Tb3+ calculated through the ICP measurements are listed
green emission (lex = 330 nm). UPC-38(Eu)-OCH3 shows red in Table S9 (ESI†). As shown in Fig. 4b, these bimetallic complexes
light under a 365 nm UV lamp, with the CIE coordinate of demonstrate the characteristic emissions of both Eu3+ and Tb3+
Fig. 2 (a) The solid-state fluorescence of L-X; (b) the solid-state fluorescence of UPC-38(Eu)-X; (c) the solid-state fluorescence of UPC-38(Tb)-X
(X = H, F, Cl, NH2, CH3, OCH3; lex = 330 nm, at room temperature).
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Scheme 1 Schematic of the energy-transfer process of UPC-
38(Eu0.34Tb0.66)-OCH3.
from the low temperature phosphorescence spectrum (Fig. S31,
ESI†). On the basis of Reinhoudt’s empirical rule, when the
energy gap DE (S1 ꢁ T1) surpasses the limit value of 5000 cmꢁ1
,
the intersystem crossing (ISC) process will be effective.23
Therefore, the characteristic luminescence emissions of UPC-
38(Eu0.34Tb0.66)-OCH3 indicated the occurrence of the antenna
effect. That is, the light was first absorbed by the ligands,
followed by ISC, and then antenna transfer from T1 to f levels
of RE3+, leading to f–f emissions (Scheme 1).
Fig. 4 (a) The UPC-38(EuxTb1ꢁx)-OCH3 luminescent materials under
UV-light irradiation at 365 nm (1ꢁ11); (b) solid-state emission spectra
of UPC-38(EuxTb1ꢁx)-OCH3 (lex = 330 nm); (c) the CIE coordinates of
UPC-38(EuxTb1ꢁx)-OCH3 (lex = 330 nm); (d) solid-state emission spectra
of UPC-38(Eu0.34Tb0.66)-OCH3 at different excitation wavelengths; (e) the
CIE coordinates of UPC-38(Eu0.34Tb0.66)-OCH3 at different excitation
wavelengths.
In order to test its white light emission performance, as
phosphor, UPC-38(Eu0.34Tb0.66)-OCH3 is coated on the surface
of a commercially available ultraviolet LED, to make a WLED.
The resultant WLED exhibits bright white light at an applied
voltage of 3.0 V (Fig. 5). On the basis of these results, it is clear
that UPC-38(Eu0.34Tb0.66)-OCH3 has potential for practical light-
ing applications.
ions. With the increase of Eu3+, the emission intensity of Eu3+ ions
5
7
5
7
from D0 - F2 increases and that of Tb3+ ions from D4 - F5
decreases. The color of UPC-38(EuxTb1ꢁx)-OCH3 is from green to
yellow, white, pink and finally to red under a 365 nm UV lamp
(Fig. 4a). The CIE coordinates cross the green-red region (Fig. 4c).
As shown in Fig. 4c, the CIE coordinates of these doped MOFs are
closest to the white light region when the proportions of Eu3+ are
20%, 30% and 40%. Hence, the emission wavelengths of these
doped MOFs were measured at different excitation wavelengths.
The results show that the sample with 30% doping of Eu3+ has the
ability to emit white light, and its emission intensity decreases
with the increase of the excitation wavelength (Fig. 4d). When
the excitation wavelength is 335 nm, UPC-38(Eu0.34Tb0.66)-OCH3
exhibits white light emission, and its CIE coordinates are (0.335,
0.325), closest to the standard values (0.333, 0.333) (Tables S3–S8,
ESI†). Therefore, by the precise adjustment of the Eu3+/Tb3+ ratios,
the emission colors of this series of doped complexes can be
systematically tuned (Fig. S14–S19, ESI†). The quantum yield
of UPC-38(Eu0.34Tb0.66)-OCH3 is 13.96%, which is between
UPC-38(Eu)-OCH3 (9.24%) and UPC-38(Tb)-OCH3 (24.37%).
PXRD, IR, PL and thermogravimetric analysis of UPC-38-(H, F,
Cl, NH2, CH3, OCH3)
The phase purity of different Eu3+/Tb3+ concentration doped
UPC-38(EuxTb1ꢁx)-OCH3 powders and different ligands UPC-
38(Eu/Tb)-X (X = H, F, Cl, NH2, CH3, OCH3) was identified by
the X-ray powder diffraction patterns shown in Fig. S3–S10
(ESI†). Photoluminescence tests of different solvents are shown
in Fig. S12 and S13 (ESI†). We found that only nitrobenzene has
Energy-transfer process
Photoluminescence is caused by electron transitions and
energy transfer. In order to study the process of energy transfer
in UPC-38, the singlet (S1) state of the ligand is estimated to be
30 769 cmꢁ1 by UV-vis absorbance (Fig. S30, ESI†), and the
triplet (T1) state of UPC-38-Gd is calculated to be 19 455 cmꢁ1
Fig. 5 (a) An illuminating 5 mm ultraviolet LED (not turned on); (b) the
same LED turned on; (c) the same LED coated with a thin layer of the
sample of UPC-38(Eu0.34Tb0.66)-OCH3 (not turned on); (d) the coated
LED was turned on and illuminates bright white light.
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a strong quenching effect on UPC-38. The infrared spectra (KBr, cmꢁ1): 3406(m), 2925(w), 1659(s), 1585(s), 1540(s), 1412(s),
are tested and show a specific functional group absorption 1183(w), 1095(m), 1011(w), 857(m), 782(s), 711(w), 658(w).
vibration peak (IR, Fig. S20–S25, ESI†). The thermal stability of
Synthesis of UPC-38(Eu)-F. The procedure was the same
UPC-38 was determined by thermogravimetric analysis (TGA, as that for UPC-38(Eu)-H, except for H3L-H instead of H3L-F.
Fig. S26–S29, ESI†) and X-ray powder diffraction patterns UPC-38(Eu)-F was obtained in 67.5% yield. Anal. calcd for
(Fig. S11, ESI†). The initial weight loss before 120 1C is
C13.5H14Eu0.5F0.5NO5: C, 45.57; H, 3.96; N, 3.93. Found: C,
attributed to the removal of the solvent molecules, whereas 45.46; H, 3.68; N, 4.06. IR data (KBr, cmꢁ1): 3382(s), 2934(w),
the weight loss from 150 to 200 1C can be attributed to the 2866(w), 1660(s), 1585(s), 1412(s), 1096(m), 917(m), 868(m),
removal of strongly coordinated DMF cations trapped inside 786(s), 724(s), 662(m), 552(m).
the pores. The crystal structure can be maintained at 200 1C.
Synthesis of UPC-38(Eu)-Cl. The procedure was the same
The decomposition of UPC-38 starts at 550 1C, leading to the as that for UPC-38(Eu)-H, except for H3L-H instead of H3L-Cl.
removal of organic linkers and structural collapse.
UPC-38(Eu)-Cl was obtained in 70.8% yield. Anal. calcd for
C
21H14ClEuO8: C, 43.35; H, 2.42. Found: C, 44.06; H, 2.28. IR
data (KBr, cmꢁ1): 3245(s), 2928(w), 2872(w), 1950(w), 1660(s),
1543(m), 1406(s), 1255(w), 1096(m), 1034(m), 910(m), 779(s),
662(m), 593(m), 538(w).
Conclusions
In conclusion, a series of isomorphic three-dimensional rare earth
metal–organic frameworks were synthesized and their photo-
luminescence properties were systematically studied. Single-
crystal X-ray diffraction analysis shows that UPC-38 is an unpre-
cedented 3D (3,4,5)-c zkf topological framework with 1D rod-
shaped SBUs. Through precise tuning of the ratio of Eu3+/Tb3+
ions in the framework, a white light emitting material UPC-
38(Eu0.34Tb0.66)-OCH3 was successfully obtained. The WLED
covered with the UPC-38(Eu0.34Tb0.66)-OCH3 material emits bright
white light. It is clear that UPC-38(Eu0.34Tb0.66)-OCH3 has potential
for practical lighting applications. The present work provides a
promising tri-component approach to the design of tunable color
and solid state white-light emitting MOF materials, which could
find applications in displays and lighting in the future.
Synthesis of UPC-38(Eu)-NH2. The procedure was the same
as that for UPC-38(Eu)-H, except for H3L-H instead of H3L-NH2.
UPC-38(Eu)-NH2 was obtained in 75.8% yield. Anal. calcd for
C
13.5H15Eu0.5N1.5O5: C, 45.77; H, 4.26; N, 5.93. Found: C, 45.37;
H, 4.21; N, 5.64. IR data (KBr, cmꢁ1): 3334(s), 2928(m), 2563(w),
2170(w), 1950(w), 1592(s), 1412(s), 1365(s), 1090(m), 868(m),
779(s), 738(m), 662(m), 545(w).
Synthesis of UPC-38(Eu)-CH3. The procedure was the same
as that for UPC-38(Eu)-H, except for H3L-H instead of H3L-CH3.
UPC-38(Eu)-CH3 was obtained in 76.4% yield. Anal. calcd for
C
14H15.5Eu0.5NO5: C, 47.53; H, 4.41; N, 3.96. Found: C, 47.66; H,
4.49; N, 4.02. IR data (KBr, cmꢁ1): 3251(s), 2928(m), 2866(w),
2177(w), 1943(w), 1585(s), 1406(s), 1178(m), 1096(m), 868(m),
786(s), 724(m), 662(m), 545(w).
Synthesis of UPC-38(Eu)-OCH3. The procedure was the same
as that for UPC-38(Eu)-H, except for H3L-H instead of H3L-
OCH3. UPC-38(Eu)-OCH3 was obtained in 78.8% yield. Anal.
calcd for C14.5H14Eu0.5NO5.5: C, 47.55; H, 3.85; N, 3.82. Found:
C, 47.06; H, 3.24; N, 4.03. IR data (KBr, cmꢁ1): 3382(s), 2927(w),
2872(w), 1950(w), 1660(s), 1537(s), 1406(s), 1227(m), 1089(m),
1006(m), 868(m), 786(s), 724(s), 662(m), 545(w).
Synthesis of doped bimetallic UPC-38(EuxTb1ꢁx)-OCH3 MOFs.
Microcrystalline powders of the codoped bimetallic complex UPC-
38(EuxTb1ꢁx)-OCH3 were synthesized according to similar proce-
dures mentioned above only with changing the stoichiometric
ratios of RE(NO3)3ꢀ6H2O. Powder X-ray diffraction (PXRD) proved
the isostructural structures of the doped complexes and the
UPC-38 complex. Inductively coupled plasma (ICP) spectroscopy
was executed to determine the relative molar ratios within the
doped MOFs.
Experimental
Materials and measurements
All chemical reagents were obtained from commercial sources
and can be used without further purification. H3L-X (X = H, F, Cl,
NH2, CH3, OCH3) were synthesized through the procedures in
1
ESI† Section 1. H NMR spectra were obtained using an Inova
500 MHz spectrometer. Single crystal X-ray diffraction experi-
ments were carried out using a SuperNova diffractometer
equipped with mirror Cu-Ka radiation (l = 1.54184 Å) and an
Eos CCD detector under 150 K. Powder X-ray diffraction (PXRD)
was carried out using a Bruker D8-Focus Bragg–Brentano X-ray
Powder Diffractometer equipped with
a Cu sealed tube
(l = 1.54178 Å) at 40 kV and 40 mA. The photoluminescence
spectra were measured using a Hitachi F7000 fluorescence spec-
trometer. The absolute fluorescence quantum yields were mea-
sured using an integrating sphere.
Conflicts of interest
Synthesis of UPC-38
The authors declare no competing financial interest.
Synthesis of UPC-38(Eu)-H. A mixture of H3L-H (10.0 mg,
0.02 mmol) and Eu(NO3)3ꢀ6H2O (20.0 mg, 0.04 mmol) was
dissolved in a mixed solvent of DMF (1 mL) and H2O (1 mL)
in a 10 mL vial and heated at 100 1C for 24 h. Then, the mixture
Acknowledgements
was slowly cooled to room temperature over 3 h. The crystals This work was financially supported by the National Natural
were collected with 79.8% yield based on the H3L-H. IR data Science Foundation of China (NSFC Grant No. 21875285,
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