Syntheses, structures, tunable emission and white light emitting Eu 3+ and Tb3+ doped lanthanide metal-organic framework materials

Article abstract of DOI:10.1039/c3dt50315a

A series of novel lanthanide metal-organic frameworks, namely, {[Ln ₂(L)₂]·(H₂O)₃·(Me ₂NH₂)₂}_n (Ln = La (1), Ce (2), Pr (3), Nd (4), Sm (5), Eu (6), Gd (7), Tb (8), Ho (9), Er (10)), have been synthesized with hydro(solvo)thermal conditions based on a flexible multicarboxylic acid (H₄L = 5-(3,5-dicarboxybenzyloxy)isophthalic acid). Single crystal X-ray diffraction reveals that all of them are isomorphous and possess a (6,6) topological net with a Schlaefli symbol of {4 ⁸·6⁷}. Considering the blue-emission of compound 1 and the intense emission of the Eu and Tb compounds, we successfully construct isostructural Eu³⁺ doped Tb compounds whose color can be tuned easily by adjusting the different concentration of the doped ions, and we also obtained white light emitting materials through the doping of Eu and Tb ions in the La compounds.

Full text of DOI:10.1039/c3dt50315a

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Syntheses, structures, tunable emission and white
light emitting Eu3+ and Tb3+ doped lanthanide
Cite this: Dalton Trans., 2013, 42, 10579
metal–organic framework materials†
Ming-Li Ma, Can Ji and Shuang-Quan Zang*
A series of novel lanthanide metal–organic frameworks, namely, {[Ln2(L)2]·(H2O)3·(Me2NH2)2}n (Ln =
La (1), Ce (2), Pr (3), Nd (4), Sm (5), Eu (6), Gd (7), Tb (8), Ho (9), Er (10)), have been synthesized with
hydro(solvo)thermal conditions based on a flexible multicarboxylic acid (H4L = 5-(3,5-dicarboxybenzyloxy)-
isophthalic acid). Single crystal X-ray diffraction reveals that all of them are isomorphous and possess a
(6,6) topological net with a Schläfli symbol of {48·67}. Considering the blue-emission of compound 1 and
the intense emission of the Eu and Tb compounds, we successfully construct isostructural Eu3+ doped
Tb compounds whose color can be tuned easily by adjusting the different concentration of the doped
ions, and we also obtained white light emitting materials through the doping of Eu and Tb ions in the
La compounds.
Received 31st January 2013,
Accepted 13th May 2013
DOI: 10.1039/c3dt50315a
www.rsc.org/dalton
traditional molecular approach by using molecular organic
and coordination compounds to explore functional lumine-
Introduction
In the fields of supramolecular chemistry and crystal engineer- scent materials, the MOF approach has the advantage of colla-
ing, the self-assembly of metal ions and organic linkers in the borative interaction with each other among the periodically
luminescent metal–organic frameworks (MOFs) has provided located luminescent centers to direct new functionalities.
the bright promise to exploit functional luminescent MOF A great deal of luminescent MOFs have been obtained with
materials.1–5 Both organic moieties and inorganic moieties applications in chemical sensing, light-emitting devices and
can give a platform to generate luminescence, while metal– biomedicine in the past few years.6–10
ligand charge transfer related luminescence can add another
It is known that multicarboxylate ligands are good
dimensional luminescent functionality. Furthermore, some candidates in assembling coordination polymers with peculiar
guest molecules in MOFs can confer fluorescent properties structures and new functionalities, because multicarboxylate
too, including through exciplex formation. Compared with the groups can exhibit a rich variety of coordination modes.11,12
We selected 5-(3,5-dicarboxybenzyloxy)isophthalic acid (H4L)
as a ligand to design and synthesize 3D MOFs with the follow-
ing reasons. (i) Owing to two rigid benzene rings and a flexible
–OCH2– spacer, H4L possesses variable conformation. (ii) It is
a flexible aromatic carboxylate ligand with four carboxylate
groups, which is efficient for construct intriguing structures.
(iii) The flexible ligand has the potential to form a helical
coordination polymer and it can link metal clusters forming
an extended 3D network that has an interesting topology.13
During our research, the interesting ligand was also investi-
gated by Cao’s group.14 They mainly discuss the coordination
polymers’ structures and their basic fluorescent properties,
but we chiefly concentrate on the tunable emission and white
light emitting materials.
The College of Chemistry and Molecular Engineering, Zhengzhou University,
Zhengzhou 450001, P. R. China. E-mail: zangsqzg@zzu.edu.cn;
Fax: (+86) 371 6778 0136
†Electronic supplementary information (ESI) available: Cell parameters and
structure refinement of LnL (Tables S1 and S2); selected bond distances and
angles for some of these compounds (Table S3); elemental analysis for the
codoped compounds (Table S4); the lifetime τ for the luminescence of Eu3+ in
the TbL:xEu3+ samples upon excitation at 324 nm (Table S5); the corresponding
CIE coordinates of the LaL: x% Tb, y% Eu excited at 363 nm, 365 nm, 367 nm,
369 nm (Table S6). Powder XRD patterns of the compounds (Fig. S1). Metal
helical chain generated by carboxylate units in 8 (Fig. S2); thermogravimetric
analyses (TGA) curve of polymers 1–10 (Fig. S3); excitation spectra (a, c) and
emission spectra (b, d) for the ligand H4L, compounds 1, 6 and 8 (Fig. S4 and
S5); excitation spectra (a, c) and emission spectra (b, d) for the compounds
Eu0.17Tb0.18La0.65L and Eu0.16Tb0.19La0.65L (Fig. S6). Emission spectra of
Solid state light emitting materials have attracted extensive
attention because of their applicational diversity in lighting,
automobiles, transportation, communication, imaging, agri-
culture and medicine,15–19 especially since the appearance of
EuxTbyLa1−x−yL excitation under 314 nm, 363 nm and 367 nm (Fig. S6–S9).
CCDC 922046 (7), 922047 (3), 922048 (5) 913881 (8) and 913882 (6). For ESI
and crystallographic data in CIF or other electronic format see DOI:
10.1039/c3dt50315a
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white light emitting diodes (WLEDs). Lanthanide ions are International de I’Eclairage (CIE) color coordinates were calcu-
widely used in luminescent materials because of their excel- lated on the basis of the international CIE standards.26
lent luminescence features such as long lifetime, large Stokes
shift and narrow-band, which are suitable for biomedical ana-
Preparation of 5-(3,5-dicarboxybenzyloxy)isophthalic acid
(H4L)
lyses, imaging, lighting, and display applications.20 Therefore,
lanthanide metal–organic frameworks (Ln–MOFs), in particu-
The ligand was prepared by the reaction of 5-bromomethyl-
lar display such interesting optical properties that can be used
isophthalic acid dimethyl ester27 with dimethyl-5-hydroxy-
in designing white-light emitting materials. The organic
benzene-1,3-dioate (Scheme 1).28 A mixture of 5-bromomethyl-
isophthalic acid dimethyl ester (1.00 g, 3.5 mmol), dimethyl-
5-hydroxybenzene-1,3-dioate (0.90 g, 4.3 mmol), K2CO3 (0.96 g,
6.9 mmol) and DMF (40 mL) were placed in a 100 mL round-
ligand, indeed, not only acts as a new building block for the
coordination networks, but also acts as an efficient sensitizer
for the Ln3+ ions via “antenna effect”.21 The study of tunable
white-light emitting metal–organic coordination polymers has
been developing, it is still a challenging task.20f,22–24 The
bottom flask and the reaction completed after the solution was
stirred at room temperature for 20 hours under a N2 atmos-
white-light emitting materials can be produced through mono-
phere. The mixture product was poured into H2O (100 mL),
chromatic, dichromatic, trichromatic, and tetrachromatic
approaches.25 The commonly reported one is the combination
and the aqueous solution was extracted by CH2Cl2
(2 × 100 mL). The combined CH2Cl2 layer was washed by satu-
of dichromatic components (blue and yellow), but the trichro-
rated NaHCO3 solution (3 × 100 mL), and 5% NaOH (50 mL)
matic approaches in which blue, green and red light emitting
solution, dried over Na2SO4, filtered and evaporated on a
components are incorporated into the resulting white light
rotary evaporator to give a white solid product 5-(3,5-dicarboxy-
emitting materials have been paid attention. The solid state
benzyloxy)isophthalic acid dimethyl ester. Then, the product
trichromatic white light emitting materials are typically
and NaOH (2.80 g, 0.07 mol) was dissolved in 50 mL of methanol
and 50 mL of water and refluxed overnight. The methanol was
removed by distillation under reduced pressure and the resulting
aqueous solution was acidified to pH ∼ 2.0 by 6 M HCI solution.
superior to others in terms of their good color rendering pro-
perties and high luminous efficiency.
Based on the consideration above, recently, we have
engaged in the coordination polymers research using H4L and
The white solid was precipitated and filtered. The solid was
lanthanide centers to prepare new materials with interesting
dried under vacuum overnight. Yield: 0.68 g, 80%. Elemental
structural topology and excellent photoluminescent properties.
analysis (%): calcd for C17H12O9: C, 56.67; H, 3.36; O, 39.97.
Herein, we present a series of novel lanthanide metal–organic
1
Found: C, 56.29; H, 3.41; O, 40.30. H NMR (300 MHz, DMSO):
frameworks {[Ln2(L)2]·(H2O)3·(Me2NH2)2}n (we term this MOF
δ 8.452 (s, 1H), 8.344 (d, 2H), 8.110 (s, 1H), 7.818 (d, 2H), 5.542
as LnL, Ln = La (1), Ce (2), Pr (3), Nd (4), Sm (5), Eu (6), Gd (7),
(s, 2H). IR (KBr, cm−1): 3364 (m), 3097 (w), 2989 (w), 1606
Tb (8), Ho (9), Er (10)). We successfully obtained functional
luminescent MOF materials by doping isostructural Eu3+ into
(ms), 1597 (m), 1558 (m), 1465 (w), 1410 (ms), 1306 (w), 1274
Tb3+ compounds and changing the doped Eu3+ concentration
(s), 1138 (ms), 1068 (ms), 919 (w), 757 (ms), 692 (w), 617 (w).
Synthesis of {[Ln2(L)2]·(H2O)3·(Me2NH2)2}n. (Ln = La (1),
Ce (2), Pr (3), Nd (4), Sm (5), Eu (6), Gd (7), Tb (8), Ho (9),
Er (10)). A mixture of Ln(NO3)3·6H2O (0.1 mmol), H4L
(0.05 mmol), DMF (1 mL) and H2O (1.5 mL) were placed in a
16 mL Teflon lined stainless steel container, which was kept at
140 °C for 3 d. After being cooled to room temperature, the
to tune the emission color, and we also obtained white-light
emitting materials through the doping of Eu and Tb ions in
the La compounds.
Experimental
prism-shaped crystals were obtained by filtration, and washed
Materials and physical measurements
with DMF and H2O.
{[La2(L)2]·(H2O)3·(Me2NH2)2}n (1). Elemental analysis (%):
calcd for C38H38La2N2O21: C, 40.15; H, 3.37; N, 2.46. Found:
C, 39.49; H, 3.54; N, 2.35. IR (KBr, cm−1): 3424 (m), 3060 (w),
2783 (m), 1628 (ms), 1587 (m), 1558 (m), 1453 (w), 1367 (s),
1333 (s), 1261 (m), 1103 (m), 912 (m), 794 (m), 717 (s), 656 (w).
{[Ce2(L)2]·(H2O)3·(Me2NH2)2}n (2). Elemental analysis (%):
calcd for C38H38Ce2N2O21: C, 40.07; H, 3.36; N, 2.46. Found: C,
All reagents and solvents employed in the present work were of
analytical grade as obtained from commercial sources and
used without further purification. Elemental analysis for C, H,
and N were performed on a Perkin-Elmer 240 elemental analy-
zer. The FT-IR spectra were recorded from KBr pellets in the
range from 4000 to 400 cm−1 on a Nicolet NEXUS 470-FTIR
spectrometer. Thermal analysis was performed on an SDT
2960 thermal analyzer from room temperature to 800 °C with a
heating rate of 20 °C min−1 under nitrogen flow. Powder X-ray
diffraction (PXRD) of these compounds were measured at
293 K on a Rigaku D/max-3B diffractometer (Mo-Kα, λ =
0.71073 Å). Luminescence spectra for the solid samples were
recorded on a Hitachi 850 fluorescence spectrophotometer.
Inductively coupled plasma spectroscopy (ICP) was performed
on a Thermo ICAP 6500 DUO spectrometer. The Commission Scheme 1 The synthesis of ligand H4L.
10580 | Dalton Trans., 2013, 42, 10579–10586
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39.41; H, 3.54; N, 2.37. IR (KBr, cm−1): 3418 (m), 3074 (w), monochromatized Mo Kα radiation (λ = 0.71073 Å) at room
2782 (w), 1628 (m), 1587 (w), 1548 (m), 1453 (m), 1376 (s), 1261 temperature using the ω-scan technique. Lorentz polarization
(m), 1130 (w), 1103 (w), 913 (m), 795 (s), 717(s), 620 (w).
and absorption corrections were applied. The structures were
{[Pr2(L)2]·(H2O)3·(Me2NH2)2}n (3). Elemental analysis (%): solved by direct methods with SHELXS-9730 and refined with
calcd for C38H38Pr2N2O21: C, 40.01; H, 3.36; N, 2.45. Found: C, the full-matrix least-squares technique using the SHELXL-9731
39.36; H, 3.56; N, 2.35. IR (KBr, cm−1): 3423 (m), 3052 (w), program. All nonhydrogen atoms were refined anisotropically.
2780 (m), 1629 (ms), 1587 (ms), 1558 (m), 1450 (w), 1375 (s), The hydrogen atoms of the ligands were included in the struc-
1261 (ms), 1131 (w), 1024 (s), 912 (ms), 783 (s), 717(s), 619 (w).
ture factor calculation at idealized positions by using a riding
{[Nd2(L)2]·(H2O)3·(Me2NH2)2}n (4). Elemental analysis (%): model and refined isotropically. The hydrogen atoms of the
calcd for C38H38Nd2N2O21: C, 39.78; H, 3.34; N, 2.44. Found: C, solvent water molecules and dimethylammonium cations were
39.11; H, 3.52; N, 2.37. IR (KBr, cm−1): 3416 (m), 3075 (m), located from the difference Fourier maps, then restrained at
2979 (w), 1630 (m), 1588 (w), 1547 (ms), 1454 (m), 1376 (s), fixed positions and refined isotropically. Analytical expressions
1262 (m), 1146 (w), 1025 (m), 913 (ms), 784 (s), 718 (s), of neutral atom scattering factors were employed, and
620 (m).
anomalous dispersion corrections were incorporated. Cell
{[Sm2(L)2]·(H2O)3·(Me2NH2)2}n (5). Elemental analysis (%): parameters of these compounds are listed in Table S1† and
calcd for C38H38Sm2N2O21: C, 39.36; H, 3.30; N, 2.41. Found: selected bond distances and angles for compounds 3, 5, 6, 7
C, 38.71; H, 3.57; N, 2.32. IR (KBr, cm−1): 3415 (m), 3075 (m), and 8 are listed in Table S3.†
2962 (w), 1629 (m), 1588 (w), 1558 (m), 1454 (w), 1376 (s), 1261
(ms), 1131 (m), 1024 (s), 912 (m), 784 (s), 717 (s), 619 (m).
{[Eu2(L)2]·(H2O)3·(Me2NH2)2}n (6). Elemental analysis (%):
Results and discussion
Structure description
calcd for C38H38Eu2N2O21: C, 39.25; H, 3.29; N, 2.40. Found:
C, 38.96; H, 3.32; N, 2.27. IR (KBr, cm−1): 3422 (m), 3077 (w),
2779 (w), 1633 (ms), 1589 (ms), 1558 (m), 1456 (w), 1377 (s), Compounds 1–10 were obtained under the same conditions by
1263 (ms), 1132 (w), 1026 (m), 914 (m), 784 (m), 718 (s), the mild hydro(solvo)thermal reactions of Ln(NO3)3·6H2O with
620 (w).
H4L in a solution of DMF and water which have the better
{[Gd2(L)2]·(H2O)3·(Me2NH2)2}n (7). Elemental analysis (%): solubility at 140 °C. Single crystal X-ray diffraction analyses
calcd for C38H38Gd2N2O21: C, 38.90; H, 3.26; N, 2.38. Found: C, reveal that all the compounds are isomorphous, crystallizing
38.27; H, 3.49; N, 2.31. IR (KBr, cm−1): 3405 (m), 3077 (w), in the orthorhombic space group C2221 and exhibit a 3D
2786 (w), 1634 (m), 1591 (w), 1548 (m), 1457 (m), 1378 (s), 1264 framework, hence only the structure of compound 8 will be
(m), 1147 (w), 1026 (s), 914 (m), 785 (s), 718 (s), 621 (w).
discussed in detail. Selected bond distances and angles for
{[Tb2(L)2]·(H2O)3·(Me2NH2)2}n (8). Elemental analysis (%): compounds 3, 5, 6, 7 and 8, which are the most suitable for
calcd for C38H38Tb2N2O21: C, 38.79; H, 3.26; N, 2.38. Found: C, single-crystal X-ray analyses, are listed in Table S3.† The asym-
38.15; H, 3.47; N, 2.32. IR (KBr, cm−1): 3423 (m), 3076 (w), metric unit contains two crystallographically independent Tb3+
2779 (w), 1635 (ms), 1549 (ms), 1457 (m), 1331 (s), 1264 (m), (Tb1, Tb2) ions, two L4− ligands, two free dimethylammonium
1105 (w), 1026 (m), 914 (m), 785 (s), 718 (s), 619 (w).
cations that result from the hydrolysis of DMF under the reac-
{[Ho2(L)2]·(H2O)3·(Me2NH2)2}n (9). Elemental analysis (%): tion conditions,32 and three free water molecules. Each L4−
calcd for C38H38Ho2N2O21: C, 38.40; H, 3.22; N, 2.35. Found: C, ligand links six Tb(III) ions with the μ6-η1:η1:η1:η1:η1:η1:η1:η1
37.80; H, 3.42; N, 2.27. IR (KBr, cm−1): 3423 (m), 3060 (w), coordination mode, the two phenyl rings are nonplanar and
2783 (w), 1636 (ms), 1587 (ms), 1558 (m), 1456 (w), 1333 (s), the dihedral angle between the phenyl rings is approximately
1265 (m), 1106 (m), 1028 (s), 912 (m), 784 (m), 718 (s), 615 (w).
90°. As depicted in Fig. 1a, both of the Tb atoms are eight-
{[Er2(L)2]·(H2O)3·(Me2NH2)2}n (10). Elemental analysis (%): coordinated with a distorted bicapped trigonal prism, which
calcd for C38H38Er2N2O21: C, 38.24; H, 3.21; N, 2.34. Found: C, are completed by four oxygen atoms from two chelating carb-
37.61; H, 3.39; N, 2.26. IR (KBr, cm−1): 3419 (m), 2987 (w), oxyl groups and four bridging oxygen atoms. The range of
2779 (w), 1637 (ms), 1585 (ms), 1557 (m), 1457 (w), 1380 (s), Tb–O bond lengths fall in 2.261(10)–2.481(10) Å and all in the
1266 (w), 1134 (w), 1076 (m), 914 (m), 785 (m), 717 (s), 618 (w).
reasonable range.33 The Tb(1)O8 and Tb(2)O8 square pyramids
As for isostructural Ln analogies, Eu3+ doped or Eu3+/Tb3+ are connected to each other via two carboxyl groups to form
codoped compounds adopt the same methods as are men- a dinuclear cluster with the Tb⋯Tb distance of 5.5070 Å
tioned above just by loading the corresponding Ln(NO3)3·6H2O (Fig. S2†). Interestingly, there are three types of right-handed
as the starting materials in stoichiometric ratios. Their purities helices in this structure. As described in Fig. 1b, the coordi-
were confirmed by single crystal XRD (cell parameters, nation of the carboxyl groups and Tb ions form two kinds
Table S1†), powder XRD (Fig. 1e and S1†), and elemental analy- of right-handed (Tb–L)n helical chains with the pitch of
sis of C, H, N (Table S4†).
16.57(7) Å and the [Tb2O16] fragments are connected together
by the oxygen atoms in the carboxyl groups of the L4− ligand
to form another helical chain along the c direction with the
pitch of 16.57(7) Å. These helical chains with the same chiral-
X-Ray crystallography
Single-crystal X-ray analyses were conducted on
a
Bruker SMART APEX CCD diffractometer29 using graphite- ity are alternately arranged along the c axis to exhibit a 3D
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Fig. 1 (a) Coordination environment of the Tb(III) ion in 8 with hydrogen atoms and solvent molecules omitted for clarity. Symmetry codes: #1 x, −y, 1 − z; #2 −0.5
+ x, 0.5 − y, 1 − z; #3 −0.5 + x, −0.5 + y, z; #4 1 − x, y, 1.5 − z; #5 1.5 − x, −0.5 + y, 1.5 − z. (b) Three adjacent homochiral helical chains along the c-axis. (c) Projection
view of the 3D framework along the c-axis, exhibiting 1D helical channels. (d) Schematic representation of 8 with a Schläfli symbol of {48·67}. (e) Powder XRD pat-
terns of the whole family compounds.
homochiral framework (Fig. 1c). The 3D framework is anionic, min−1. Thermograms show similar profiles and are provided
the dimethylammonium cations and water molecules reside in as Fig. S3,† so only the compound 8 will be discussed. The
its 1D pores. A better insight into the nature of the involved curve exhibits two major losses in the temperature range of
framework can be achieved by the application of a topological 50–570 °C, the first weight loss of 12.94% is close to the calcu-
approach. As described above, each Tb(III) ion is linked by six lated value (12.42%), corresponding to the loss of the solvent
L4− ligands and each L4− ligand links six Tb(III) ions, therefore, molecules in the temperature range of 50–382 °C, the removal
both Tb(III) centers and L4− ligands can be viewed as six- of the organic components (56.49%) occurred in the temp-
connected nodes, and the whole network can be extended to erature range of 382–570 °C, which is in agreement with the
an unusual 3D (6,6)-connected msw framework with a Schläfli calculated value of 56.03%. The final residues are composed of
symbol of {48·67}, as displayed in Fig. 1d.
Tb2O3.
Thermogravimetric analysis
Luminescence properties
The thermal stability of compounds 1–10 was examined by The luminescence properties of the solid-state lighting appli-
thermogravimetric analyses (TGA) in the temperature range of cations are very interesting, especially since the ligands
30–800 °C under a flow of nitrogen with a heating rate of 10 °C themselves display luminescence within these materials. The
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Fig. 2 Emission spectra of compounds 6 and 8.
emission and excitation spectra measured at room temp-
erature displayed in Fig. 2 and Fig. S4, S5.† The powders are
H4L, compounds 1, 6 and 8. The excitation and emission
spectra of compound 1 are similar to those of the ligand,
in fact, there is no f–f transition for compound 1, but a π–π*
transition for the H4L ligand. The emission spectrum of com-
pound 1 consists of a broad band with a maximum at
about 447 nm when excited at 340 nm, which exhibits a slight
red-shift compared with the strongest emission of H4L. The
phenomenon may possibly be attributed to the coordination
of the L4− ligands to La3+ ions, which increases the rigidity
of the ligands with the intramolecular interactions, and
decreases the symmetry of the ligands, thus reducing the loss
of energy by radiationless decay of the intraligand emission
excited state.34 Compound 6 shows emission peaks at 579,
Fig. 3 PL emission spectra of TbL:xEu solid samples, λex = 324 nm (top). CIE
chromaticity diagram for TbL:xEu (x = 0.1% for A, 0.3% for B, 0.6% for C, 1.6%
for D, 4.5% for E, 8.2% for F, 19.1% for G, 49.3% for H) nanobelts (bottom).
the most important luminescent activators, which are attrac-
tive in visible luminescent materials owing to their strong red
and green emissions. So we can make them cocrystallize
with tunable optical activity. In addition, Eu3+ and Tb3+ have
the same charge and similar ionic radii, hence Eu3+ can dope
easily into the Tb3+ compounds and the corresponding emis-
sion color tuning is decided by the concentration of the Eu3+
ions, through the energy-transfer process. A variety of partially
substituted Tb3+/Eu3+ compounds were prepared, shown as
TbL:xEu. We can judge whether the different percent compo-
sitions would result in different colors by photoluminescence
properties testing in each sample.
588, 618, 656, 704 nm arising from the 5D0
→
7FJ ( J = 0–4)
transitions of the Eu3+ ions upon excitation at 320 nm
(Fig. 2a), which yields intense red luminescence in the solid
state. Upon excitation at 324 nm, compound 8 presents an
intense green luminescence at room temperature, because of
7
the characteristic transitions of 5D4 → FJ ( J = 3–6) (Fig. 2b).
The absence of the ligand-based emission in the fluorescence
spectra of compounds 6 and 8 suggests that the energy trans-
fer from the ligands to the lanthanide centers is very effective.
Since the whole family are isostructural, it is expected that
we can easily tune their emissions by doping methods.
Among the lanthanide ions, the Eu3+ and Tb3+ ions are two of
As shown in Fig. 3, a series of codoped compounds exhibit
characteristic fluorescence peaks at 488, 542, 590, 618 and
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704 nm. Along with the increase of Eu3+ concentration, we can
see that the luminescence intensity of the Tb3+ ions decreases,
while the luminescence intensity of the Eu3+ ions increases.
Both are due to enhancing the probability of the energy trans-
fer from Tb3+ to Eu3+. The luminescence intensity of Eu3+
reaches a maximum when the Eu3+ concentration is 4.5%, and
then begins to decrease due to the concentration quenching
effects.35 This effect can be confirmed by the fluorescent
dynamics of Eu3+, as shown in Table S5,† from which a faster
fluorescence decay of the 5D0 state of Eu3+ in TbL:0.24%Eu
(63.26 µs) and TbL:8.2%Eu (81.44 µs) is observed than that in
TbL:4.5%Eu (100.07 µs). All the results above indicate an
efficient energy transfer from Tb3+ to Eu3+. Also, such energy
transfer behaviour shows that the as-synthesized TbL:xEu is
not a mixture of TbL and EuL, but a coordination polymer, in
which Eu3+ has successfully been incorporated into the TbL
lattice. Otherwise, the Tb3+ → Eu3+ energy transfer could not
occur in the separated phases. Lastly, the luminescence of the
Tb3+ almost disappears when the Eu3+ concentration increases
to 19.1%. Therefore, the photoluminescence colors of the Eu3+
doped Tb3+ compounds can be tuned from green to green-
yellow, yellow, orange, red-orange, and the homologous CIE
chromaticity coordinates change from (0.378, 0.556) to (0.640,
0.342) by doping with different Eu3+ concentrations (Fig. 3).
This study can be applied to tuning luminescent properties in
the visible region by selecting the appropriate metal–organic
host materials and activator ions.
Considering the compounds 1, 6, and 8, exhibiting the
primary colors (blue, red and green) and isostructural, it is
expected that we can obtain white light emission through the
doping of Eu3+ and Tb3+ ions into the La compound. This
would be of interest owing to the white light emitting com-
pounds having extensive applications in general lighting
sources. A series of codoped compounds with different doping
ratios were obtained. The emission and excitation spectra of
the codoped compounds are listed in Fig. S6.† The emission
spectra of these compounds exhibit characteristic emissions of
the Eu3+/Tb3+ ions36 excited at 314 nm (Fig. S7†). If the wave-
length of the excitation is increased to about 363 nm, we can
obtain red/green light of the Eu3+/Tb3+ ions and blue light
resulting from the H4L ligand (Fig. S8†). The emission spectra
of these compounds excited at 367 nm are listed in Fig. S9.†
The corresponding CIE chromaticity coordinates are listed
in Table S6.† We can see that the color of the codoped
compounds can be systematically tuned through different
combinations of the amount of Eu3+ and Tb3+ ions. These
observations also indicate that the energy transfer is incom-
plete from the La3+ host to doped Ln3+ ions excited at about
363 nm, which is the most significant feature to realize the
white light emission in the codoped system. When the emis-
sion peaks at 448, 542 and 618 nm are comparable in intensity
(Fig. 4a), the white light emission can be readily produced.
Accordingly, the best combination of Eu3+ and Tb3+ concen-
Fig. 4 (a) PL emission spectra of the Eu/Tb doped La compound (λex = 367 nm,
solid samples). (b) CIE chromaticity diagram for Eu0.17Tb0.18La0.65L (A) and
Eu0.16Tb0.19La0.65L (B).
(0.332, 0.338), respectively; both are very close to the coordi-
nates for pure white-light (0.333, 0.333), thus ideal for the
white-light emission according to the 1931 CIE coordinate
diagram (Fig. 4b).
Conclusions
tration in the codoped compounds as a result of the white- In summary, a series of novel Ln–MOFs have been synthesized
light emission is Eu0.17Tb0.18La0.65L and Eu0.16Tb0.19La0.65 under hydro(solvo)thermal conditions. This doped lanthanide
with the corresponding CIE coordinates (0.330, 0.334) and metal–organic framework approach can be applied to generate
L
10584 | Dalton Trans., 2013, 42, 10579–10586
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tunable and white light emitting materials. The distinctive
organic linkers provide the basic light emitting colors from
their π–π* and metal–ligand charge transfer transitions. Due to
the strong chemical similarities of the lanthanide(III) ions, the
self-assembly of lanthanide ions and organic linkers forming
the luminescent metal–organic frameworks (Ln–MOFs) has
provided the bright promise to exploit functional MOF
materials. Further investigations on this domain are underway.
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We gratefully acknowledge financial support by the National
Natural Science Foundation of China (No. 20901070), the
China Postdoctoral Science Foundation (Grant 20090460859)
and Zhengzhou University (P. R. China).
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