Lanthanide coordination compounds with 1H-benzimidazole-2-carboxylic acid: Syntheses, structures and spectroscopic properties

Article abstract of DOI:10.1039/c2ce26120k

A flexible multidentate ligand, 1H-benzimidazole-2-carboxylic acid, was synthesized to construct a series of lanthanide coordination polymers [Ln(HBIC)₃]_n (Ln = Eu (1), Tb (2), Gd (3), Pr (4), Nd (5); H₂BIC = 1H-benzimidazole-2-carboxylic acid) under hydrothermal conditions. All the compounds were fully characterized by elemental analysis, IR spectroscopy, single-crystal X-ray diffraction, thermal analysis and various spectroscopic techniques. Structural analyses reveal that they are isostructural and feature a 2D wave-like layer structure with distorted grids, in which the adjacent Ln³⁺ centers are bridged by the HBIC^- ligands with two kinds of new coordination modes and the adjacent HBIC^- ligands are tightly bound by two types of distinct intra-layer hydrogen bonds. The adjacent 2D layers are further interconnected by strong inter-layer hydrogen bond ring motifs R₂²(10) to generate a 3D supramolecular architecture. Optical studies indicate that the compounds 1, 2, 4 and 5 exhibit characteristic luminescence emission bands of the corresponding lanthanide ions in the visible or near-infrared regions at room temperature. In particular, compound 2 displays bright green luminescence in the solid state with a satisfactory ⁵D₄ lifetime of 1.2 ms and a high overall quantum yield of 31%, due to an ideal energy gap between the lowest triplet state energy level of H₂BIC ligand and the ⁵D₄ state energy level of Tb³⁺. The energy transfer mechanisms in compounds 1 and 2 were also described and discussed. The Royal Society of Chemistry.

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PAPER
Lanthanide coordination compounds with 1H-benzimidazole-2-carboxylic
acid: syntheses, structures and spectroscopic properties{
Zhengqiang Xia,a Qing Wei,*a Qi Yang,a Chengfang Qiao,ab Sanping Chen,*a Gang Xie,a Guochun Zhang,b
Chunsheng Zhoub and Shengli Gaoab
Received 10th July 2012, Accepted 26th September 2012
DOI: 10.1039/c2ce26120k
A flexible multidentate ligand, 1H-benzimidazole-2-carboxylic acid, was synthesized to construct a
series of lanthanide coordination polymers [Ln(HBIC)3]n (Ln = Eu (1), Tb (2), Gd (3), Pr (4), Nd (5);
H2BIC = 1H-benzimidazole-2-carboxylic acid) under hydrothermal conditions. All the compounds
were fully characterized by elemental analysis, IR spectroscopy, single-crystal X-ray diffraction,
thermal analysis and various spectroscopic techniques. Structural analyses reveal that they are
isostructural and feature a 2D wave-like layer structure with distorted grids, in which the adjacent
Ln3+ centers are bridged by the HBIC2 ligands with two kinds of new coordination modes and the
adjacent HBIC2 ligands are tightly bound by two types of distinct intra-layer hydrogen bonds. The
2
adjacent 2D layers are further interconnected by strong inter-layer hydrogen bond ring motifs R2 (10)
to generate a 3D supramolecular architecture. Optical studies indicate that the compounds 1, 2, 4 and
5 exhibit characteristic luminescence emission bands of the corresponding lanthanide ions in the
visible or near-infrared regions at room temperature. In particular, compound 2 displays bright green
5
luminescence in the solid state with a satisfactory D4 lifetime of 1.2 ms and a high overall quantum
yield of 31%, due to an ideal energy gap between the lowest triplet state energy level of H2BIC ligand
5
and the D4 state energy level of Tb3+. The energy transfer mechanisms in compounds 1 and 2 were
also described and discussed.
absorption coefficient (less than 10 M21 cm21) of the Laporte
forbidden f–f transitions.3 An effective approach to circumvent
Introduction
Over the past decade, the rational design and synthesis of
this problem is to efficiently transfer the energy from the ligand-
lanthanide-based metal–organic frameworks (Ln-MOFs) have
centered triplet excited states of an adjacent strongly absorbing
chromophore to the lanthanide metal centers.4 Therefore,
provoked great interest for their fascinating architectures and
potential technological application as functional materials.1
searching for suitable organic antenna chromophores with high
Especially, owing to their unique photophysical properties
absorption in the UV/near-UV spectral region is an attractive
(characteristic sharp emission and high color purity, long
task.
excited-state luminescence lifetimes up to milliseconds) as a
Given that lanthanide ions have high affinities for hard donor
atoms such as the oxygen of carboxylic groups5 and a delocalized
result of transitions within the partially-filled 4f shells of the
trivalent lanthanide ions, Ln-MOFs are excellent candidates for
the development of optical devices.2 However, the lanthanide
p-electric system could provide a strongly absorbing chromo-
phore.6 Aromatic multidentate ligands with oxygen or hybrid
ions usually give weak luminescence because of the low molar
oxygen–nitrogen atoms might be a good sensitizer to stimulate
lanthanide ion luminescence. Recently, we have focused on a
aKey Laboratory of Synthetic and Natural Functional Molecule Chemistry
of Ministry of Education, College of Chemistry and Materials Science,
Northwest University, Xi’an 710069, China.
multidentate ligand, 1H-benzimidazole-2-carboxylic acid
(H2BIC), in which the large p-conjugated system of the
benzimidazole moiety is expected as an effective chromophore
for lanthanide luminescence. The H2BIC ligand possesses four
potential metal binding sites and shows flexible and various
coordination modes (Scheme 1), wherein the oxygen atoms of the
carboxylic group could coordinate with the lanthanide ions in
terminal monodentate or diverse bridging motifs, or chelate Ln3+
with the help of the N atoms from the benzimidazole group.
Meanwhile, depending on the degree of deprotonation, the
H2BIC ligand can act as a hydrogen bond acceptor and donor to
E-mail: sanpingchen@126.com; weiqq@126.com; Fax: +86-29-88303798;
Tel: +86-29-88302604
bDepartment of Chemistry and Chemical Engineering, Shaanxi Key
Laboratory of Comprehensive Utilization of Tailing Resources, Shangluo
University, Shangluo 726000, China
{ Electronic supplementary information (ESI) available: 1H NMR
spectra of ligand, TGA and PXRD patterns, crystallographic details
and additional photophysical data. CCDC 874290 (H2BIC?2H2O),
874292 (1), 874291 (2), 874299 (3), 874293 (4), 874296 (5). For ESI and
crystallographic data in CIF or other electronic format see DOI: 10.1039/
c2ce26120k
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Scheme 1 Diverse coordination modes of H2BIC (M = metal).
˚
benefit the formation of stable supramolecular structures. On the
other hand, it was found that the presence of water molecules in
the first coordination sphere of the lanthanide ion is prone to
quench the emission due to the activation of nonradiative decay
pathways.7 So ancillary ligands with excellent chelating ability
such as oxalate are widely used to prevent solvent and water
molecules from entering the lanthanide coordination sphere.8
powder diffractometer (Cu Ka, 1.5418 A). Optical diffuse
reflectance and UV absorption studies were carried out with a
Shimadzu UV-2450 spectrophotometer. The solid-state photo-
luminescence analyses and lifetime measurements were performed
on an Edinburgh FLSP920 fluorescence spectrometer.
Synthesis of H2BIC
For the deprotonated HBIC2 or BIC22 ligands, the sufficiently
2-(Hydroxymethyl)benzimidazole. A mixture of o-phenylene-
diamine (16.2 g, 0.15 mol), sodium glycolate (35.2 g, 0.6 mol) in
300 mL HCl (4 mol L21) was stirred at 120 uC for 2 h. The reaction
mixture was cooled to room temperature and filtered. The filtrate
was adjusted to pH # 7 by saturated sodium hydroxide solution
and finally filtered to give a large amount of white microcrystals of
2-(hydroxymethyl)benzimidazole. Yield: 19.0 g (85.7%). M.p.:
131–132 uC. Anal. Calc. For C8H8N2O (Mr = 148.16): C, 64.85 H,
5.44 N, 18.91. Found: C, 64.22 H, 5.88 N, 19.01%. 1H NMR (400
MHz, DMSO-d6): d, ppm: 12.344 (s, 1 H), 7.484–7.491 (d, 2 H),
7.110–7.151 (m, 2 H), 5.744 (s, 1 H), 4.705 (s, 2H) (see Fig. S1,
ESI{). IR (KBr, v/cm21): 3270(m), 2918(s), 2850(s), 1623(w),
1580(w), 1490(m), 1452(s), 1439(s), 1380(m), 1351(m), 1274(m),
1207(m), 1106(w), 1055(s), 1034(s), 1002(s), 995(m), 958(w),
871(w), 828(w), 769(w), 750(s), 740(s), 639(w), 451(w).
22
strong chelate ability like C2O4
makes them excellent
candidates to form stable Ln-MOFs without coordinated water
or solvent molecules.9
It is worth noting that lanthanide coordination polymers
based on 1H-benzimidazole-2-carboxylic acid ligands have not
been explored.10 In this contribution, five new two-dimensional
Ln-MOFs based on H2BIC ligands, [Ln(HBIC)3]n (Ln = Eu (1),
Tb(2), Gd(3), Pr(4) and Nd(5)), were hydrothermally synthesized
and structurally characterized. The photophysical properties
and thermal stability of them were also investigated. To the best
of our knowledge, 1–5 are the first instances of Ln-MOFs
constructed with H2BIC ligands.
Experimental
Materials and analytical methods
1H-Benzimidazole-2-carboxylic acid (H2BIC). A mixture of 2-
(hydroxymethyl)benzimidazole (7.40 g, 0.05 mol) and sodium
hydroxide (4.00 g, 0.1 mol) in H2O (180 mL) was allowed to be
stirred at 100 uC for 2 h, to which potassium permanganate (11.9
g, 0.075 mol) was added while kept stirring at 100 uC for 10 h.
Then the reaction mixture was cooled to room temperature and
filtered, and the filtrate was acidified with dilute hydrochloric
acid to pH = 5. Upon being filtered off, washed with water, and
air-dried, a large quantity of yellow solid was obtained. Yield:
7.15 g (64.9%, based on o-phenylenediamine). M.p.: 166–167 uC.
Anal. Calc. For C8H6N2O2 (Mr = 162.15): C, 59.26 H, 3.73 N,
17.28. Found: C, 60.01 H, 3.81 N, 17.03%. 1H NMR (400 MHz,
DMSO-d6): d, ppm: 7.66–7.62 (m, 2 H), 7.37–7.34 (m, 2 H) (see
Fig. S2, ESI{). IR (KBr, v/cm21): 2923(m), 2842(m), 1653(s),
1530(m), 1495(s), 1388(s), 1352(s), 1279(w), 1151(w), 1020(w),
905(m), 847(m), 802(w), 766(m), 731(m), 635(m), 585(m). After
several days, colorless crystals of dihydrated 1H-benzimidazol-2-
carboxylate were obtained by slow evaporation of an ethanol
solution of H2BIC, which was determined by single X-ray
diffraction analysis (Fig. 1 and Table 1).10d
The 1H-benzimidazole-2-carboxylic acid (H2BIC) ligand was
prepared using o-phenylenediamine as the starting material by
oxidizing 2-(hydroxymethyl)benzimidazole in alkaline conditions
(Scheme 2). Other chemicals and reagents were obtained from
commercial sources and used without further purification.
Elemental analyses for C, H, and N were performed on an
Elementar Vario EL III analyzer. IR spectra were recorded on a
Tensor 27 spectrometer (Bruker Optics, Ettlingen, Germany) as
KBr pellets in the range of 4000–400 cm21. H NMR spectrum
1
was recorded on an INOVA-400 Varian 400 MHz instrument in
DMSO-d6 solution at room temperature. Chemical shifts are
reported in ppm relative to TMS. TG-DTG-DSC experiments
were carried out in a simulated air atmosphere (N2: 80 mL min21
,
O2: 20 mL min21) using a NETZSCH STA 449F3 equipment at a
heating rate of 10 uC min21 from 30 to 1000 uC, an empty Al2O3
crucible was used as a reference. Powder X-ray diffraction
(PXRD) data were recorded on a Bruker D8 ADVANCE X-ray
Synthesis of [Eu(HBIC)3]n (1)
A mixture of EuCl3?6H2O (0.0366 g, 0.1 mmol), H2BIC (0.0162
Scheme 2 Synthetic procedures for the ligand 1H-benzimidazole-2-
carboxylic acid.
g, 0.1 mmol), NaOH (0.0040 g, 0.1 mmol) and NaN3 (0.0065 g,
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Fig. 1 (a) Perspective view of H2BIC?2H2O. (b) The intermolecular H-bonding interactions in H2BIC?2H2O.
0.1 mmol) was dissolved in 6 mL distilled water. The resulting
Synthesis of [Tb(HBIC)3]n (2)
solution was stirred for about 30 min at room temperature,
sealed in a 10 mL Teflon-lined stainless steel autoclave, and
heated at 110 uC for 3 days under autogenous pressure. The
reaction system was then cooled to room temperature at a rate of
5 uC h21. Colorless flaky crystals of 1 were collected in 48% yield
(based on Eu). Anal. Calcd. for C24H15EuN6O6 (Mr = 635.38):
C, 45.37 H, 2.38 N, 13.23. Found: C, 45.81 H, 2.64 N, 12.99%.
IR (KBr, v/cm21): 3141(br, m), 1652(s), 1616(s), 1521(m),
1497(m), 1466(s), 1398(s), 1331(s), 1299(m), 1228(w), 1027(w),
988(w), 849(m), 821(m), 779(w), 739(s), 631(m), 580(m).
An identical procedure with 1 was followed to prepare 2 except
EuCl3?6H2O was replaced by TbCl3?6H2O (0.0374 g, 0.1 mmol)
or Tb2(SO4)3?6H2O (0.0357 g, 0.05 mmol). Colorless flaky
crystals of 2 were collected in 54% yield (based on Tb). Anal.
Calcd. for C24H15TbN6O6 (Mr = 642.35): C, 44.88 H, 2.35 N,
13.08. Found: C, 44.61 H, 2.58 N, 13.01%. IR (KBr, v/cm21):
3128(br, m), 1650(s), 1611(s), 1528(m), 1499(m), 1467(s), 1399(s),
1335(s), 1308(m), 1235(w), 1025(w), 987(w), 860(m), 825(m),
777(w), 748(s), 638(m), 584(m).
Table 1 Crystal data and structure refinement summary for H2BIC?2H2O and compounds 1–5
H2BIC?2H2O
1
2
3
4
5
Empirical formula
Formula weight
Crystal system
Space group
C8H10N2O4
198.18
Monoclinic
P21/c
C24H15N6O6Eu
635.38
Monoclinic
P21/n
C24H15N6O6Tb
642.34
Monoclinic
P21/n
C24H15N6O6Gd
640.67
Monoclinic
P21/n
C24H15N6O6Pr
624.33
Monoclinic
P21/n
C24H15N6O6Nd
627.66
Monoclinic
P21/n
˚
a (A)
6.860(2)
7.368(2)
18.967(5)
90
109.738(9)
90
902.4(4)
4
1.459
296(2)
0.119
416
4373/1596
0.0750
1596/0/127
1.015
0.0696
0.2105
2
9.7575(10)
8.9776(9)
26.005(3)
90
91.113(1)
90
2277.5(4)
4
1.853
9.7089(14)
8.9276(13)
25.930(4)
90
91.090(2)
90
2247.1(6)
4
1.899
9.7434(11)
8.9656(10)
26.000(3)
90
91.179(2)
90
2270.8(4)
4
1.874
9.8425(7)
9.0698(7)
26.084(2)
90
91.216(1)
90
2327.9(3)
4
1.781
9.853(2)
9.073(2
26.152(6)
90
91.153(4)
90
2337.3(10)
4
1.784
˚
b (A)
˚
c (A)
a (u)
b (u)
c (u)
3
˚
V (A )
Z
Dc (g cm23
T (K)
)
296(2)
296(2)
296(2)
296(2)
296(2)
m (mm21
F(000)
)
2.810
3.204
2.977
2.147
2.276
1248
11 759/4461
0.0316
4461/0/334
1.097
0.0250
1256
10 687/3983
0.0452
3983/0/334
1.066
0.0410
1252
11 088/4012
0.0535
4012/0/334
1.033
0.0373
1232
11 382/4131
0.0329
4131/0/338
1.037
0.0290
1236
11 228/4141
0.0716
4141/0/334
1.009
0.0549
Reflections collected/unique
R(int)
Data/restraints/parameters
GOF on F2
R1 [I . 2s(I)]
a
b
wR2 (all data)
a
0.0785
0.1021
0.0774
0.0661
0.1249
R1 = g||Fo| 2 |Fc||/g|Fo|. wR2 = [gw(Fo 2 Fc2)2/gw[(Fo2)2]1/2
.
b
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Synthesis of [Gd(HBIC)3]n (3)
and many possible mechanisms could only be deduced by the
intuitive results. Therefore, according to the reaction conditions
and results of isolating 1–5, it is not difficult to find that the
An identical procedure with 1 was followed to prepare 3 except
EuCl3?6H2O was replaced by GdCl3?6H2O (0.0372 g, 0.1 mmol).
Colorless flaky crystals of 3 were collected in 61% yield (based on
Gd). Anal. Calcd. for C24H15GdN6O6 (Mr = 640.67): C, 44.99 H,
2.36 N, 13.12. Found: C, 45.21 H, 2.88 N, 12.92%. IR (KBr, v/
cm21): 3137(br, m), 1653(s), 1606(s), 1525(m), 1491(m), 1461(s),
1398(s), 1329(s), 1300(m), 1231(w), 1020(w), 986(w), 858(m),
820(m), 776(w), 743(s), 633(m), 585(m).
different anions of lanthanide salts (Cl2, SO4 or NO32) have
22
no effects on the formation of 1–5. Moreover, it should be noted
that the presence of a molar equivalent of NaN3 as the structure-
directing agent was a prerequisite for the preparation of 1–5.
Structural description
Fig. 1a depicts that the ligand 1H-benzimidazole-2-carboxylic
acid crystallizes as the dihydrate, and the organic molecule exists
in a zwitterionic form with the carboxyl group deprotonated and
the benzimidazole group protonated,10d which has a significant
advantage for the formation of abundant hydrogen bonds. As
shown in Fig. 1b, both the benzimidazole N atoms are involved
Synthesis of [Pr(HBIC)3]n (4)
An identical procedure with 1 was followed to prepare 4 except
EuCl3?6H2O was replaced by Pr(NO3)3?6H2O (0.0435 g, 0.1
mmol) or PrCl3?6H2O (0.0355 g, 0.1 mmol). Light-green flaky
crystals of 4 were collected in 33% yield (based on Pr). Anal.
Calcd. for C24H15PrN6O6 (Mr = 624.33): C, 46.17 H, 2.42 N,
13.46. Found: C, 46.67 H, 2.50 N, 13.98%. IR (KBr, v/cm21):
3133(br, m), 1657(s), 1609(s), 1531(m), 1500(m), 1463(s), 1401(s),
1325(s), 1302(m), 1238(w), 1027(w), 990(w), 854(m), 822(m),
779(w), 741(s), 635(m), 581(m).
…
in forming hydrogen bonds as donors: one gives N2–H2 O1
hydrogen bonds to connect two parallel adjacent
1H-benzimidazole-2-carboxylate molecules, yielding a centro-
2
symmetric hydrogen-bond motif R2 (10); and the other N atom
(N1) is attracted by O3 from a lattice water molecule to give a
…
intermolecular N1–H1 O3 hydrogen bond. Notably, the
carboxyl O atom (O2) exhibits a three-connected hydrogen-
bond motif, in which O2 is employed as accepter to form three
kinds of different hydrogen bonds with lattice water molecules
(Table S2, ESI{). In the crystal, molecules are packed into a
three-dimensional network (Fig. 2a) via these hydrogen bonds
and two distinct p–p stacking interactions between the benzimi-
dazole groups of adjacent molecules [centroid–centroid distances
Synthesis of [Nd(HBIC)3]n (5)
An identical procedure with 1 was followed to prepare 5 except
EuCl3?6H2O was replaced by Nd(NO3)3?6H2O (0.0438 g, 0.1
mmol). Light-purple flaky crystals of 5 were collected in 39%
yield (based on Nd). Anal. Calcd. for C24H15NdN6O6 (Mr =
627.66): C, 45.93 H, 2.41 N, 13.39. Found: C, 45.51 H, 2.44 N,
13.11%. IR (KBr, v/cm21): 3132(br, m), 1652(s), 1604(s),
1528(m), 1489(m), 1466(s), 1402(s), 1330(s), 1303(m), 1238(w),
1029(w), 988(w), 851(m), 823(m), 779(w), 741(s), 634(m), 580(m).
˚
= 3.7602(10) and 3.6135(10) A] (Fig. 2b). It is of interest to note
that a helical chain generated by the hydrogen-bond interactions
between carboxyl oxygen atoms (O2) and lattice water molecules
is observed (Fig. 2c).
X-ray crystallography
X-ray analyses performed on single crystals of compounds 1–5
reveal an isostructural mode possessing one 2D wave-like layer
structure with distorted grids. Hence, only the structure of 1 is
described in detail. Compound 1 crystallizes in the monoclinic
space group P21/n, and the asymmetric unit consists of one
crystallographically independent Eu3+ ion and three HBIC2
ligands. As illustrated in Fig. 3a, the central Eu3+ cation is eight-
coordinated by five carboxyl oxygen atoms (O1, O3–O6) and
three nitrogen atoms (N1, N3 and N5) from five HBIC2 ligands,
resulting in a distorted bicapped triangle prism coordination
geometry (Fig. 3b). The Eu–O bond distances range from
Single-crystal X-ray diffraction data for H2BIC?2H2O and
compounds 1–5 were collected on a Bruker SMART APEXII
CCD diffractometer, equipped with graphite-monochromatized
˚
Mo Ka radiation (l = 0.71073 A) using v and Q scan mode. All
the structures were solved by direct methods and refined with
full-matrix least-squares refinements based on F2 using
SHELXS-97 and SHELXL-97.11 All non-hydrogen atoms were
refined anisotropically. Hydrogen atoms were placed in geome-
trically calculated positions. The detailed crystallographic data
and the structure refinement parameters of H2BIC?2H2O and1–5
are listed in Table 1. Selected bond distances, angles and
hydrogen bonds are given in Tables S1 and S2 (see ESI{).
˚
2.338(3) to 2.487(3) A with the mean of 2.415(3) A, which is
˚
˚
slightly shorter than the average Eu–N distance, 2.549(3) A. The
average O–Eu–O bond angle is 105.56(10)u, similar to that of N–
Eu–N [105.43(10)u], but much lager than the mean O–Eu–N
bond angle value, 94.87(11)u. All of the bond lengths and angles
in 1 are within normal ranges and comparable with those
observed in other reported Eu3+-carboxylates (Table S1, ESI{).13
Due to the effect of lanthanide contraction, the Ln3+ ions radii
Results and discussion
Synthesis
As well known, the hydro/solvothermal method used to be a
non-ignorable ‘‘hero’’ in the preparation of metal–organic
coordination frameworks with fascinating architectures and
function. It gives a more convenient and effective route to
synthesize high-quality single crystals of MOFs over other
methods.12 However, because of the limitation of unintuitive
reaction phenomenon and hard-to-monitor reaction process, the
research on the complex mechanism of reaction process in the
reaction vessel is still in the primary exploration phase at present,
are very close [Shannon’s ionic radii of Ln3+ in 1–5: 1.126 A
˚
3+
3+
(Pr3+), 1.109 A (Nd ), 1.066 A (Eu ), 1.053 A (Gd3+), and
˚
˚
˚
1.04 A (Tb3+)]. Comparing the geometries of isomorphic 1–5
˚
(Table 2), it is evident to find out that the differences on the Ln–
O/N bond lengths and correlative bond angles are very subtle,
and the variation of some geometry in 1–5 shows dependency on
the respective Shannon’s effective ionic radii. For example, the
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Fig. 2 (a) 3D supramolecular network of H2BIC?2H2O formed by intermolecular hydrogen-bonding and p–p stacking interactions. (b) The two
distinct p–p stacking interactions in H2BIC?2H2O. (c) Helical chain structure generated by the hydrogen-bond interactions between carboxyl oxygen
atoms and lattice water molecules.
Fig. 3 (a) Coordination environment of Eu3+ in 1, H atoms are omitted for clarity. Symmetry code: #1, 1/2 2 x, 21/2 + y, 1/2 2 z; #2, 3/2 2 x, 21/2
+ y, 1/2 2 z. (b) Coordination polyhedron geometry of Ln3+ in 1–5.
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Table 2 The geometry comparison for compounds 1–5
1 (Eu3+ 2 (Tb3+
)
)
3 (Gd3+
)
4 (Pr3+
)
5 (Nd3+
)
Ln–Oa (A)
˚
2.338(3)–2.487(3)
2.415(3)
2.519(3)–2.567(3)
2.549(3)
2.318(4)–2.467(4)
2.394(4)
2.493(6)–2.546(5)
2.526(5)
2.326(4)–2.480(4)
2.405(4)
2.514(5)–2.557(5)
2.541(5)
2.376(3)–2.524(3)
2.455(2)
2.593(3)–2.631(3)
2.612(3)
2.370(5)–2.522(5)
2.451(5)
2.586(7)–2.656(7)
2.618(7)
Ln–Ob (A)
˚
Ln–Na (A)
˚
Ln–Nb (A)
˚
O–Ln–Oa (u)
O–Ln–Ob (u)
O–Ln–Na (u)
O–Ln–Nb (u)
N–Ln–Na (u)
70.63(9)–157.00(10)
105.56(10)
63.63(9)–152.20(11)
94.87(11)
70.80(11)–152.32(11)
105.43(10)
70.36(15)–157.36(16)
105.57(15)
63.99(16)–152.26(16)
94.87(16)
71.00(18)–152.56(18)
105.67(17)
70.70(12)–156.80(14)
105.51(12)
64.09(13)–152.23(13)
94.90(13)
70.67(16)–152.53(15)
105.60(15)
70.64(8)–157.05(9)
105.65(8)
63.14(9)–152.29(9)
94.78(9)
70.02(10)–151.46(10)
104.53(10)
70.48(19)–157.2(2)
105.66(5)
62.97(19)–152.5(2)
94.79(6)
70.2(2)–151.7(2)
104.76(6)
N–Ln–Nb (u)
˚
˚
Grid size (A 6 A) 6.9561(5) 6 6.7012(5) 6.9219(8) 6 6.6662(8) 6.9443(7) 6 6.6949(6) 7.0167(4) 6 6.7600(4) 7.0234(12) 6 6.7663(11)
a
b
Range of bond length or bond angle. Average bond length or bond angle.
average length of Ln–O decreases along with reducing of the
Ln3+ radii (Ln–O: Pr3+ . Nd3+ . Eu3+ . Gd3+ . Tb3+), and
the average bond angle of N–Ln–N increases accordantly with
simplified as a four-connected 2D (4,4) network with the Eu3+
ions as nodes (Fig. 5b), in which each distorted grid has a size of
˚
˚
6.7012(5)
A 6 6.9561(5) A, slight larger than those in
the Ln3+ radii increasing (/N–Ln–N: Pr3+ , Nd3+ , Eu3+
,
compounds 2 [6.9219(8) 6 6.6662(8)] and 3 [6.9443(7) 6
6.6949(6)], and smaller than those in compounds 4 [7.0167(4)
6 6.7600(4)] and 5 [7.0234(12) 6 6.7663(11)] (Table 2).
It is worth mentioning that the deprotonated carboxylate and
protonated benzimidazole N atom of the HBIC2 ligand make
the amphoteric HBIC2 ligand a good candidate to be involved in
the construction of strong hydrogen bonds as a donor or
acceptor. As can be seen from Fig. 6a, each grid in the 2D layer is
formed by four HBIC2 ligands bridging four Eu3+ centers with
Gd3+ , Tb3+). However, because the final formation of
lanthanide compounds may be influenced by many complicated
factors, not all the geometry variations in 1–5 are in line with the
consistent tendency, there are also some exceptions. The average
lengths of Ln–N in these compounds are 2.618(7), 2.549(3),
˚
2.541(5) and 2.526(5) A for compounds 5, 1, 3 and 2, steadily
decreasing from Nd3+ to Tb3+, respectively. Yet the average Pr–
˚
N distance [2.612(3) A] is shorter than the average Nd–N length,
which is not in good agreement with the radii of the two Ln3+
ions. Meanwhile, the situation of average O–Ln–N bond angle is
similar: /O–Pr–N [94.78(9)u] , /O–Nd–N [94.79(6)u] , /O–Eu–N
[94.87(11)u] , /O–Tb–N [94.87(16)u], just /O–Gd–N [94.90(13)u] .
Mode I, and the grid is further twisted by two types of distinct
#1
intra-layer hydrogen bonds [N2–H2 O5 = 2.718(4) A and
…
˚
#2
…
˚
N6–H6 O1 = 2.782(4) A. Symmetry code: #1, 2x + 3/2, y +
1/2,2z + 1/2; #2 2x + 1/2, y + 1/2, 2z + 1/2], where the adjacent
HBIC2 ligands are tightly bound together like the bis(chelating)-
bridging oxalate, thus resulting in a wave-like 2D layer.
Moreover, the HBIC2 ligands with Mode II are arranged face
to face between two adjacent layers, and the exposed carboxylic
O atoms and H atoms participate in constructing the strong
/
[94.87(16)u], while the average O–Ln–O bond angles in
1–5 seemingly look random in the changing tendency: /O–Nd–O
O–Tb–N
[105.66(5)u] . /O–Pr–O [105.65(8)u] . /O–Tb–O [105.57(15)u] .
/
[105.56(10)u] . /O–Gd–O [105.51(12)u].
O–Eu–O
In compound 1, all H2BIC ligands are present in the mono-
deprotonated form, HBIC2. Two kinds of new coordination
modes of HBIC2 (Modes I and II in Scheme 1) are first
observed. One is the HBIC2 ligand chelating one Eu3+ center
with one carboxyl oxygen atom and one imidazole nitrogen atom
first, and the other carboxyl oxygen atom further bridges
another Eu3+ ion, leading to a chelating-bridging m2-kN, O:
kO coordination fashion (Mode I); the other is the HBIC2
ligand chelating the Eu3+ center with a simple m1-kN, O
coordination fashion (Mode II). The adjacent Eu3+ centers are
bridged by the HBIC2 ligands in Mode I into 1D infinite chains
(Fig. 4), and these chains are further interweaved each other,
generating a 2D wave-like layered structure in the ab plane
(Fig. 5a). In this 2D network, each Eu3+ center is coordinated by
four HBIC2 ligands in Mode I, and the remaining two sites are
occupied by another HBIC2 ligand in Mode II. From the
viewpoint of network topology, the structure of 1 can be
2
#3
…
inter-layer hydrogen bond R2 (10) ring motifs [N4–H4 O2
˚
=
2.824(5) A, #3: 2x + 1, 2y + 1, 2z] (Fig. 6b), which finally
bridges the adjacent layers into a 3D supramolecular architecture
(Fig. 6c).
Thermal analysis
To examine the thermal stability of 1–5, the thermogravimetric
analyses for crystal samples of 1–5 were performed under a
simulated air atmosphere with a heating rate of 10 uC min21
from ambient temperature up to 1000 uC. As shown in Fig. S3,
ESI,{ TG analyses indicate that all these compounds have high
thermal stability and exhibit similar thermal behavior due to
their isostructural nature. Herein, only the thermal stability of 1
was taken as a representative example for discussion. The TG
curve depicted in Fig. 7 reveals that 1 remains stable up to 354.7
uC, which could be ascribed to the abundant intra-layer and
inter-layer hydrogen bonds occurring in framework 1. Then the
compound experiences almost one-step weight loss of 71.85%
from 354.7 to 528.3 uC, which is attributed to the thermal
decomposition of the organic components. The final mass of the
residue is 28.15% of the initial weight, which agrees well with
that expected value for the formation of a stoichiometric
quantity of Eu2O3 (calcd. 27.69%). The end product was
Fig. 4 1D infinite chain structure in compound 1.
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Fig. 5 (a) A wave-like 2D network in compound 1, H atoms are omitted for clarity. (b) A schematic view of the topology framework of 1.
Fig. 6 (a) Intra-layer hydrogen-bonding interactions in 1, the benzene rings of the benzimidazole groups are omitted for clarity. (b) Inter-layer
hydrogen-bonding interactions in 1. (c) 3D supramolecular architecture assembled via intra-layer and inter-layer hydrogen bonds in 1, where pink and
green dashed lines denote intra-layer and inter-layer hydrogen-bonding interactions, respectively.
identified as the body-centered cubic Eu2O3 by PXRD (Fig. S4,
ESI;{ JCPDS NO. 34-0392).
The DTG curve for 1 shows that the whole process of weight
loss consists of two sequential thermal decomposition processes,
and the sharp peaks located at 391.1 uC and 462.7 uC,
respectively, represent the fastest decomposition of the two
processes, which may caused by the two distinct binding fashions
between Eu3+ ions with ligands in 1. In addition, the broad
exothermic peak of the DSC curve shows that the compound
decomposes with a large heat of 7002 J g21
.
The DSC experiments were also carried out from room
temperature to 1000 uC under a simulated air atmosphere to
investigate the heat variation of the lanthanide compound
system. As illustrated in Fig. 8, the DSC curves of 1–5 are
similar, and each of them shows a broad downward band with
two consecutive exothermic peaks, corresponding to the two
sequential thermal decomposition processes. There are also some
significant differences on the heat release and the decomposition
Fig. 7 TG-DTG-DSC curves of compound 1 with a heating rate of
10 uC min21
.
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not have a significant influence on the singlet excited state of the
free ligand. Nevertheless, compared with the H2BIC ligand, a
slight blue shift is discernible in the absorption maximum of the
five compounds, which could be attributable to the perturbation
induced by the metal ion coordination. The H2BIC ligand
displays three absorption bands at 269, 275, and 281 nm, which
are assigned to singlet–singlet 1p–p* absorptions of the aromatic
rings. The molar absorption coefficient values for compounds 1–
5 at about 273 nm of 3.35 6 104, 3.55 6 104, 3.02 6 104, 3.49 6
104 and 3.44 6 104 L mol21 cm21, respectively, are approxi-
mately three times as high as that of the ligand (1.11 6 104 at 275
nm), indicating the presence of three ligands in each compound,
this is also in good agreement with the crystallographic analysis.
The aforementioned features suggest that the H2BIC ligand has a
strong ability to absorb light.
Energy transfer between the H2BIC ligand and Ln3+ (Ln = Eu, Tb)
Fig. 8 DSC plots of compounds 1–5 with a heating rate of 10 uC min21
.
For lanthanide coordination compounds, the trivalent lantha-
nide ion luminescence generally needs to be stimulated by a
strong chromophore ligand via the ‘‘antenna effect’’, and the
details can be described as the following steps: the chromophore
ligand is first excited from ground state into the first excited
singlet state due to the UV absorption, immediately nonradiative
intersystem crossing of the ligand from the singlet to the triplet
state occurs; and the intramolecular energy is further transferred
from the ligand-centered triplet state to the excited 4f states of
lanthanide ion, followed by internal conversion to the lower
emitting states simultaneously; finally, the characteristic lantha-
nide emission is generated by the radiative transition from the
lanthanide ion emissive states to the lower different energy
states.14 So it can be seen that the intramolecular energy
migration efficiency from the organic ligand to the central
Ln3+ ions plays a key role in the luminescence property of the
lanthanide coordination compound.
peak temperature. As can be seen from Fig. 8, all the five
compounds display high heats, the maximum 2 (7697 J g21
)
outputs about 1330 J g21 more than the minimum 5 (6361 J g21).
Correspondingly, the greatest decomposition peak temperature
occurred in 2 (480.6 uC) is approximately 25 uC higher than that
in 5 (454.4 uC). It should be noted that the variation tendency of
both heat release and decomposition peak temperature are
consistent with that of the average Ln–N/O lengths, that is, the
decomposition heat and peak temperature decrease with the
increase of average Ln–N/O lengths. This finding confirms that
the shorter distance between Ln3+ ion and O/N atom, the greater
the heat yielded by thermo-decomposition.
UV-vis spectra
The UV-vis absorption spectra of the free ligand H2BIC and its
corresponding Ln3+ compounds 1–5 were recorded in DMSO
solution (c = 2 6 1025 M), as shown in Fig. 9. The identical
trends in absorption spectra of compounds 1–5 and the free
ligand, demonstrate that the coordination of the Ln3+ ion does
The choice of a suitable sensitizer is very important. According
to Reinhoudt’s empirical rule, the intersystem crossing process
becomes effective when energy gap DE [singlet energy level (1pp*)
– triplet energy level (3pp*)] is at least 5000 cm21 15
Thus, the
.
singlet and triplet energy levels of the H2BIC ligand should be
firstly determined. The 1pp* of H2BIC is estimated by reference to
the wavelength of the UV-vis higher absorption edge of Gd3+
compound 3, and the pertinent value is 286 nm (34 965 cm21) (see
Fig. 9). Because the first excited state of Gd3+ (about 32 000
cm21), 6P7/2, is too high to accept any energy from the first excited
triplet state of the ligand through intramolecular ligand-to-metal
energy transfer, the phosphorescence spectrum of the correspond-
ing Gd3+ compound could actually reveal the triplet energy level
of the ligand.16 As shown in Fig. S5, ESI,{ the triplet energy level
(3pp* = 24 630 cm21) of H2BIC is determined by the lower
wavelength emission edge (406 nm) from the low-temperature
phosphorescence spectrum of the Gd3+ compound 3.
Fig. 10 shows the diverse energy-level states of H2BIC, Eu3+
and Tb3+ and possible energy transfer pathways among them.
The energy gap for the H2BIC ligand DE (1pp* 2 3pp* = 10 335
cm21) is significantly higher than 5000 cm21, which suggests that
the intersystem crossing process is effective for the H2BIC
ligand.15,17 Further, the triplet energy level of H2BIC is also
Fig. 9 UV-vis absorption spectra of H2BIC and compounds 1–5 in
DMSO solution (2 6 1025 M).
5
obviously higher than the D0 level of Eu3+ (17 300 cm21) and
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Fig. 10 Schematic energy level diagrams and energy transfer mechanisms for compounds 1 and 2. S1 represents the first excited singlet state and T1
represents the first excited triplet state.
5D4 level of Tb3+ (20 500 cm21), giving rise to an advantageous
energy-transfer condition in 1 or 2. Of course, this does not mean
that the higher the better, and the energy gap between the lowest
series of sharp characteristic f–f transitions of the Eu3+ at 361
(7F0 A 5G6), 380 (7F0 A 5H4) and 394 nm (7F0,1 A 5L6) are also
evident, which can be reasonably assigned to the transitions
between the F0,1 and the 5L6 and D2,1 levels.20 The more
intense 4f absorption of Eu3+ suggests that the luminescence
sensitization via excitation of the H2BIC ligand is not very
promising in 1.
7
5
5
triplet state of the H2BIC ligand and the D0 state (Eu3+) or the
5D4 state (Tb3+) must be appropriate. The gap may be too large
or too small to get satisfactory luminescence efficiency; a large
gap always yields an inefficient energy transfer and a small gap is
beneficial to a back-energy transfer.18 Latva’s empirical rule
states that an optimal ligand-to-metal energy transfer process for
Ln3+ requires DE (3pp* 2 5DJ) = 2500–4000 cm21 for Eu3+ and
Under excitation at 394 nm, the ambient temperature emission
spectrum of 1 displays characteristic sharp emission bands of
Eu3+ in the spectral range of 570–750 nm. As shown in Fig. 11b,
5
2500–4500 cm21 for Tb3+ 19
Given that the energy gap between
.
the five anticipated transitions from the excited D0 state to the
7FJ ground-state multiplet are well resolved, that is, 5D0 A 7F0 at
5
the triplet state and D0 state of Eu3+ is 7330 cm21, the H2BIC
ligand is not in an ideal situation for the sensitization of Eu3+
luminescence. Whereas the energy gap between the triplet state
579 nm, 5D0 A 7F1 at 593 nm, 5D0 A 7F2 at 613 nm, 5D0 A 7F3
5
7
at 654 nm, and D0 A F4 at 700 nm.20a,20b,21 Compared with
others, the maximum intensities at 579 and 654 nm are much
5
and D4 state of Tb3+ is 4130 cm21, suggesting that the H2BIC
5
7
5
7
ligand could strongly sensitize Tb3+ emission in 2.
weaker, because their corresponding D0 A F0 and D0 A F3
transitions are both strictly forbidden in magnetic and electric
dipole schemes.14c,21c The 5D0
A
7F1 transition is relatively
stronger, which is a magnetic dipole. The 5D0 A 7F0 and 5D0 A
7F1 transitions consist of only one peak at 579 nm and three split
peaks at 593 nm, respectively, which suggests the existence of a
single chemical environment around the Eu3+ ion in 1.16a,22 It is
Photophysical property
Photoluminescence properties of 1 and 2
Considering that the H2BIC ligand is an adequate light-harvest-
ing chromophore, and expected to be used for sensitizing
lanthanide ion luminescence. The solid-state luminescence
properties of 1–2 were studied at room temperature. The solid-
state excitation and emission spectra of the Eu3+ compound 1
recorded at room temperature are displayed in Fig. 11. The
excitation spectrum (Fig. 11a) monitored around the intense 5D0
A 7F2 transition of the Eu3+ exhibits a broad and relatively weak
band between 250 and 350 nm (lmax = 331 nm), which can be
assigned to the p–p* electronic transition of the H2BIC ligand. A
clear that the hypersensitive 5D0
A
7F2 transition (electric
dipole) shows the highest intensity at 613 nm, pointing to a
highly polarizable chemical environment around the Eu3+ ion.
The luminescent intensity ratio of the (5D0 A 7F2)/(5D0 A 7F1) is
11.53, which indicates that compound 1 exhibits bright red light
emission and the Eu3+ ion in it is not located in a site with
inversion center symmetry.23
Fig. 12 summarizes the room-temperature excitation and
emission spectra of Tb3+ compound 2 in the solid state. The
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Fig. 11 Room-temperature excitation (a) and emission (b) spectra for 1 (lex = 394 nm) with emissions monitored at approximately 613 nm.
excitation spectrum of 2 (Fig. 12a) monitored around the
Furthermore, the emission band from the H2BIC ligand is not
detected, implying efficient energy transfer from the ligand to the
central Tb3+ ion in 2, which is further confirmed by the very high
hypersensitive D4 A F5 transition (545 nm) of Tb3+ reveals a
broad band between 280 and 350 nm with a maximum at
approximately 333 nm, which can be attributable to the p–p*
electronic transition of the H2BIC ligand. Because this broad
band overlaps the absorption spectrum of the H2BIC ligand in
the region of 280–350 nm, the energy transfer from the ligand to
Tb3+ is operative. In addition, a series of faint f–f transitions 7F5
5
7
emission intensity of Tb3+
.
For a deeper research on the photophysical behaviors of the
compounds 1–2, the lifetime values for the 5D0 level of 1 and 5D4
level of 2 (tobs) were measured at ambient temperature based on
the corresponding luminescent decay profiles by fitting with
monoexponential functions (Fig. S6 and S7, ESI{), and the
relevant values are summarized in Table 3. The typical decay
profiles indicate that single chemical environments exist around
the emitting Eu3+/Tb3+ ions. The ideal luminescence lifetimes for
1 (0.952 ms) and 2 (1.207 ms) could be due to the strong
chelating coordination ability and rational steric stabilization of
the HBIC2 ligand, which effectively prevented the solvent guest
molecules from entering the coordination sphere of the
lanthanide ion and greatly reduced the energy loss caused by
the O–H vibrations.
5
7
5
7
5
A L9 (359 nm), F5 A L10 (369 nm) and F5 A G6 (378 nm)
are also observed in the excitation spectrum of 2,24 which are
much weaker than the absorptions owing to the organic ligands.
Apparently, luminescence sensitization via ligand excitation is
significantly more efficient than the direct excitation of the Tb3+
absorption level.
Being excited by UV light (lex = 333 nm), the solid-state
emission spectrum of 2 recorded at room temperature (Fig. 12b)
exhibits the characteristic narrow emission bands of Tb3+, which
results from deactivation of the 5D4 excited state to the
7
corresponding ground state FJ (J = 6, 5, 4, 3) of the Tb3+ ion
In addition, to clearly explain the sensitization efficiency of the
H2BIC ligand (Wsen), it should be necessary to determine the
overall luminescence quantum yields (Woverall) of 1 and 2 first. In
compliance with the method described by Bril et al.,18b,25 the
centered at 489, 545, 584, and 621 nm.20a–d The most intense
emission centered at 545 nm represents the hypersensitive 5D4 A
7F5 transition, which shows the strongest green emission of 2.
Fig. 12 Room-temperature excitation (a) and emission (b) spectra for 2 (lex = 333 nm) with emissions monitored at approximately 545 nm.
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ꢀ
ꢁ
Table 3 The radiative (ARAD) and nonradiative (ANR) decay rates,
5D0/5D4 lifetime (tobs), intrinsic quantum yield (WLn), radiative lifetimes
(tRAD), energy transfer efficiency (Wsen), and overall quantum yield
(Woverall) for compounds 1–2 in the solid state
1
ITOT
ARAD
~
~AMD,0n3
(4)
tRAD
IMD
Photophysical parameters Compound 1 (Eu3+
)
Compound 2 (Tb3+
)
In comparison with the complicated measurement and
calculation of Eu3+ compound 1, the intrinsic quantum yield
of Tb3+ (WTb) in 2 can be directly estimated by means of eqn (5)
using the assumption that the decay process at 77 K in
deuterated solvents is purely radiative (Fig. S9, ESI{).29
ARAD (s21
)
343
706
952 ¡ 1
2911 ¡ 1
33
—
—
ANR (s21
tobs (ms)
)
1207 ¡ 1
1380 ¡ 1
87
36
31 ¡ 3
tRAD (ms)
WLn (%)
Wsen (%)
8
2 ¡ 1
tobs (298 K)
Woverall (%)
WTb
~
(5)
tRAD (77 K)
overall quantum yields of 1 and 2 were respectively estimated to
be 2% and 31% through the following expression:
The correlative photophysical parameters of compounds 1–2
are presented in Table 3, the lower sensitization efficiency of the
ligand (gsen = 8%) in 1 indicates that the energy transfer from the
first excited triplet state of ligand to the excited 5D0 level of Eu3+
is not an optimal process. It is also evident from Table 3 that the
overall luminescence quantum yield of 2 is much higher than that
of 1, though both of them have satisfactory luminescence
lifetime. This could be due to the weak sensitization efficiency
of the H2BIC ligand and smaller intrinsic quantum yield of the
Eu3+ ion in 1. On the other hand, because of the smaller energy
gap between the ligand triplet and Tb3+ ion excited states (4130
cm21), the Tb3+ compound 2 exhibits good luminescence
efficiency with a overall quantum yield of 31% in the solid state,
thus rendering it as a promising candidate for use in various
photonic applications.
ꢀ
ꢁꢀ
ꢁ
1{rst
1{rx
Ax
Woverall
~
Wst
(1)
Ast
where rx and rst represent the diffuse reflectance of the
compounds (with respect to a fixed wavelength) and the standard
phosphor, respectively (Fig. S8, ESI{), and BaSO4 was used as a
reflecting standard to acquire absolute intensity values. Ax and
Ast are the areas under the compounds and the standard
emission spectra, respectively. Wst is the quantum yield of the
standard phosphor, peprylene (purchased from Aldrich), whose
emission spectrum consists of an intense broad band with a
maximum at around 580 nm and a constant W value (98%) for
excitation wavelength at 436 nm.26 The final calculated Woverall
value for each sample was taken from the average of three
measurement values, and the corresponding test errors were
estimated to be ¡10%.25b,c
In summary, the coordinated solvent molecules, the conjuga-
tion effect of chromophores and/or co-ligands, and the energy
gap between the ligand triplet state and the lowest-lying excited
state of Ln3+ ion play cooperative roles in the photoluminescence
properties of Ln-MOFs, as evidenced by some monomeric Ln-
MOFs (Table S3, ESI{).
As is well-known, when the internal process in the system
(usually treated as a ‘‘black box’’) is not considered explicitly, the
overall quantum yield (Woverall) for a lanthanide compound can
be defined as the product of the intrinsic quantum yield of Ln3+
ion (WLn) and the efficiency of energy transfer from the ligand to
Ln3+ ion (Wsen),18a,27 which can be depicted as:
Photoluminescence properties of 4 and 5
Compared with the extensively studied europium and terbium
compounds, less effort has been devoted to the investigation of
Nd3+/Pr3+-containing compounds. Actually, they also exhibit
good luminescent properties in both visible and near-infrared
(800–1400 nm) regions, and especially the advantages of signal
transmittance of near-infrared radiation enable widespread
application in areas such as laser systems, medical diagnosis
and telecommunications.2c,30 Therefore, the photoluminescence
properties of 4 and 5 were investigated in the solid state at room
temperature. As shown in Fig. 13, compound 5 displays the
characteristic emission bands of Nd3+ ion in the near-infrared
region upon 354 nm radiation excitation: two moderate-intensity
Woverall = WsenWLn
(2)
Hence, WLn can be used for calculating the sensitization
efficiency of the ligand (Wsen). Eqn (3) shows the calculating
formula of WLn, in which ARAD, ANR, tobs and tRAD correspond
to the radiative decay rate, nonradiative decay rate, 5D0/5D4
lifetimes and radiative lifetimes, respectively.
ꢀ
ꢁ
ARAD
tobs
WLn
~
~
(3)
A
RADzANR
tRAD
4
For Eu3+ compound 1, the ARAD, ANR and tRAD are finally
calculated to be 343 s21, 706 s21 and 2911 ms via eqn (4), here,
emission bands at 869 and 893 nm (4F3/2 A I9/2), a very strong
4
emission band at 1060 nm (4F3/2 A I11/2), and a series of weak
bands around 1329 nm (4F3/2 A 4I13/2). The splitting of the 4F3/2
A 4I9/2 transition band is mainly due to the ‘‘crystal field effect’’
of the Nd3+ ion located at a C1 site.31 The decay curves are well
fitted with a monoexponential function. The lifetime of the
5
7
the energy of the D0 A F1 transition (MD) and its oscillator
strength are assumed to be constant, and AMD,0 (14.65 s21
represents the spontaneous emission probability of the 5D0
)
A
7F1 transition in vacuo,6c,28 n (1.5) is the average refractive index
of the medium,14b ITOT/IMD represents the ratio of the total area
of the corrected Eu3+ emission spectrum to the area of the 5D0 A
4
excited F3/2 state of Nd3+ ion at room temperature for 5 was
measured to be 1.21 ms (Fig. S11, ESI{). Because Pr3+ can show
emission lines originating from three different levels (3P0, 1D2
and 1G4) which span the visible and NIR regions, the
5
7F1 band. Combined with the measured D0 lifetime value (952
ms), WEu of compound 1 is finally determined to be 33%.
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Conclusions
In this paper, a series of lanthanide frameworks involving
1H-benzimidazole-2-carboxylic acid ligands (1–5) have been
hydrothermally synthesized and structurally characterized for
the first time. The isostructural compounds 1–5 exhibit 2D (4,4)
networks with distorted grids constructed by zwitterionic HBIC2
ligands with two kinds of unprecedented coordination modes.
The thermal analyses indicate that their frameworks exhibit good
thermal stability and release a large amount of heat during the
decompositions. The detailed optical properties investigations
reveal that: 1 and 2 exhibit remarkable lanthanide-centered
luminescence emissions in the visible region showing lumines-
cence lifetimes at millisecond order and intrinsic quantum yields
of 33% and 87%, respectively; compound 5 emits strong
characteristic luminescence in the near-infrared region. The
results suggest that the strong chelating-bridging ligand HBIC2,
which could efficiently occupy the first coordination sphere of
the lanthanide ion, is an excellent chromophore for constructing
luminescence materials. In addition, the more efficient energy
Fig. 13 Solid-state emission spectrum for 5 (lex = 354 nm) in the near-
infrared region at room temperature.
luminescence spectrum of Pr3+ compound is much more complex
in comparison to that of other Ln3+ systems, leading to far fewer
studies of Pr3+ emission spectra.32 Fig. 14 depicts the room
temperature emission spectrum of 4. Unlike 5, no characteristic
emission bands of Pr3+ in the NIR region were detected, whereas
compound 4 exhibits a series of characteristic emission bands for
Pr3+ ion in the visible region with a 399 nm excitation source.
The emissions at 462 and 479 nm can be respectively assigned to
1I6 A 3H4 and 3P1 A 3H4 transitions. A series of emission bands
centered at approximately 499, 545, and 614 nm correspond to
the 3P0 A 3HJ (J = 4, 5 and 6, respectively) transitions. The two
5
migration [DE (3pp* 2 D4) = 4130 cm21] in 2 gives rise to a
larger overall quantum yield of 31% over a poor value of 2% in 1,
demonstrating that the suitable energy gap between the ligand
triplet state and the lowest-lying excited state of Ln3+ ion plays a
crucial role in the efficient sensitization of Ln3+ ion emission,
and the HBIC2 ligand is preferable for the sensitization of Tb3+
-
centered luminescence.
Acknowledgements
1
This work was supported by the National Natural Science
Foundation of China (Nos. 21073142, 21173168 and 21127004),
the Natural Science Foundation of the Department of Education
of Shaanxi Province (Nos. 2010JK882, 2010JQ2007, 11JK0578
and 11JS110).
peaks observed at 597 and 604 nm are attributed to the D2 A
3H4 transition, and the weak signal with a maxima at 643 nm can
be referred to the 3P0
A
3F2 transition.33 All the observed
transitions in Fig. 14 suggest that the triplet state of the H2BIC
ligand is located in a suitable level and there is enough
intramolecular energy transferred from the ligand-centered
1
3
3
1
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