Anions make the difference: Conversion from zero- to one-dimensional structures and luminescent properties of lanthanide-based complexes

Article abstract of DOI:10.1039/c2ce25210d

Reaction of a newly designed amide type ligand, benzyl-N,N-bis[(2′- benzylaminofomyl)phenoxyl)ethyl]-amine (L) with lanthanide nitrates and picrates has produced two isostructural series of lanthanide coordination compounds, [Ln₂L₂(NO₃)₆(DMF)₂] _n [series 1, monoclinic P2₁/c, Ln = Pr (1), Sm (2), Eu (3), Tb (4), and Dy (5)] and [LnL(pic)₃(CH₃CH ₂OH)]_n [series 2, triclinic P1, Ln = Nd (6), Eu (7), Tb (8), and Er (9)]. Series 1 have zero-dimensional (0D) dinuclear rectangular macrocycle architectures, while series 2 exhibit one-dimensional (1D) chain patterns. Meanwhile, the two series of structures are further connected through weak intermolecular hydrogen bonds to yield three-dimensional (3D) supramolecular structures. The variations from zero- to one-dimensional coordination structures are attributed to different steric confinement of the nitrate and picrate anions. In addition, we have studied the luminescent properties of the nitrate and picrate complexes, and found that the picrate complexes give very weak luminescence due to coordinated ethanol molecules existing in complexes, which decrease the luminescence. So, herein the luminescent properties of only the Sm (2), Eu (3), Tb (4), and Dy (5) nitrate complexes are investigated in detail.

Full text of DOI:10.1039/c2ce25210d

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PAPER
Anions make the difference: conversion from zero- to one-dimensional
structures and luminescent properties of lanthanide-based complexes{
Xuhuan Yan, Zhenzhong Yan, Ye Zhang, Weisheng Liu, Yu Tang* and Minyu Tan
Received 14th February 2012, Accepted 25th April 2012
DOI: 10.1039/c2ce25210d
Reaction of a newly designed amide type ligand, benzyl-N,N-bis[(29-benzylaminofomyl)-
phenoxyl)ethyl]-amine (L) with lanthanide nitrates and picrates has produced two isostructural series
of lanthanide coordination compounds, [Ln2L2(NO3)6(DMF)2]n [series 1, monoclinic P21/c, Ln = Pr
¯
(1), Sm (2), Eu (3), Tb (4), and Dy (5)] and [LnL(pic)3(CH3CH2OH)]n [series 2, triclinic P1, Ln = Nd
(6), Eu (7), Tb (8), and Er (9)]. Series 1 have zero-dimensional (0D) dinuclear rectangular macrocycle
architectures, while series 2 exhibit one-dimensional (1D) chain patterns. Meanwhile, the two series of
structures are further connected through weak intermolecular hydrogen bonds to yield three-
dimensional (3D) supramolecular structures. The variations from zero- to one-dimensional
coordination structures are attributed to different steric confinement of the nitrate and picrate anions.
In addition, we have studied the luminescent properties of the nitrate and picrate complexes, and
found that the picrate complexes give very weak luminescence due to coordinated ethanol molecules
existing in complexes, which decrease the luminescence. So, herein the luminescent properties of only
the Sm (2), Eu (3), Tb (4), and Dy (5) nitrate complexes are investigated in detail.
lanthanide coordination polymers, though). The other advan-
Introduction
tages of lanthanides are their long excited-state lifetimes, high
chromaticities (especially, Sm(III), Eu(III), Tb(III), and Dy(III))
and brightest emission with some organic ligands, which are also
pertinent to applications in the domain of solid-state photonic
materials.2 On the other hand, amide type ligands have been
extensively applied in preparing lanthanide complexes. The
complexes possess novel coordination structures and exceptional
luminescent properties, because of their versatile coordination
modes, predictable patterns of hydrogen bonding donors and
acceptors and good light absorptivity.3
The preparation of coordination polymers has been provoking
considerable current interest during the past decades, owing to
their enormous variety of intriguing structural topologies as well
as great potential applications in the area of catalysis, gas
storage, separation.1 From the viewpoint of crystal engineering,
the key to coordination polymers with desired topologies and
physicochemical properties is to choose appropriate metal ions
and organic building blocks for an effective assembly. As
functional metal centers, lanthanide ions are helpful in the
formation of unusual molecular architectures due to their high
coordination numbers and variable coordination geometry
(which also make it difficult to control the preparation of
Anions also play an important role in preparing lanthanide
coordination polymers with fantastic coordination polymeric
architectures,3a,4 in addition to metal ions and organic ligands.
The anions can be divided into having two key effects upon the
structures of the final coordination polymers. Firstly, the anions,
that can coordinate to metal cations, obviously influence the
coordination sphere of the metal cations and hence the
construction of networks. Secondly, the size of the anions can
influence the long-range order and degree of interpenetration of
the lanthanide coordination polymers. Previously, we reported
two anion-induced chiral lanthanide-organic frameworks
assembled by an achiral amide type tripodal ligand.3a Inspired
by the aforementioned progress, we were prompted to design and
prepare a new conformationally flexible amide type ligand
benzyl-N,N-bis[(29-benzylaminofomyl)phenoxyl)ethyl]-amine (L)
(Scheme 1). The amide type ligands with several (generally
2–4) identical arms have been intensively studied,3a–i but ligands
with three incompletely identical arms such as ligand L here are
Key Laboratory of Nonferrous Metal Chemistry and Resources Utilization
of Gansu Province, State Key Laboratory of Applied Organic Chemistry
and College of Chemistry and Chemical Engineering Lanzhou University,
Lanzhou, 730000, P. R. China. E-mail: tangyu@lzu.edu.cn;
Fax: +86-931-8912582; Tel: +86-931-8912552
{ Electronic supplementary information (ESI) available: theoretical details
of ‘‘shape-measure’’ S, the intrinsic quantum yield of the Eu(III) complex,
tables of selected bond distances and angles, hydrogen bonds in crystal
packing of the complexes, the powder X-ray diffraction patterns of the Gd
and Tb (4) nitrate complexes, the intramolecular p–p stacking interactions in
compound 4, 2D supramolecular layer and 3D framework generated by
intermolecular hydrogen bonds, luminescence decay curves of the
complexes, excitation and emission spectra of the Tb(III) nitrate complex
in different solutions at room temperature, absorption spectra of the ligand,
phosphorescence spectrum of the compound [GdL(NO3)6]?H2O. CCDC
reference numbers 666992–666995, 788729, and 852907–852910. For ESI
and crystallographic data in CIF or other electronic format see DOI:
10.1039/c2ce25210d
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(concentration: 1.061023 mol L21, the excitation and emission
slit widths were 5.0 and 2.5 nm) and phosphorescence spectrum
of the Gd nitrate complex were obtained on a Hitachi F-4500
fluorescence spectrophotometer. UV/vis absorption spectra were
determined on a Varian UV-Cary100 spectrophotometer. The
quantum yields of the complexes were determined by an absolute
method using an integrating sphere (150 mm diameter, BaSO4
coating) from Edinburgh Instruments FLS920. The lifetime
measurement was measured on an Edinburgh Instruments
FLS920 Fluorescence Spectrometer with
OPOlette laser as the excitation source.
a Nd pumped
Scheme 1 A schematic illustration of the ligand L.
Preparation of the ligand L
rather rare.4c Moreover, the ligand L with two well flexible amide
type arms can construct discrete structures or coordination
polymers. We selected two different anions (small inorganic
nitrate and large organic picrate) for further comparison because
they have similar coordination modes (monodentate or bidentate
coordination mode, and the coordination sites are the oxygen
atoms). We expected to investigate how the anions influenced
supramolecular self-assembly of lanthanide complexes as well as
their luminescent properties. In this report, we constructed two
series of new coordination compounds with lanthanide nitrate and
picrate. The single crystal X-ray diffractions shows that the crystal
structures of [Ln2L2(NO3)6(DMF)2]n [Ln = Pr (1), Sm (2), Eu (3),
Tb (4), and Dy (5)] form interesting zero-dimensional (0D)
dinuclear [2+2] rectangular macrocyclic structures. However, the
structures of [LnL(pic)3(CH3CH2OH)]n [Ln = Nd (6), Eu (7), Tb
(8), and Er (9)] confirm one-dimensional (1D) linear coordination
polymers. Moreover, the solid state luminescent properties of the
complexes 2, 3, 4, and 5 at the room temperature were also
investigated in detail and correlated with the triplet energy level of
the designed ligand.
The b,b’-dichlorodiethylamine hydrochloride salt (1 mmol) and
potassium carbonate (2 mmol) were refluxed in acetone (25 cm3)
for 30 min, and then the benzyl bromide (1 mmol) was added to
the solution. The reaction mixture was refluxed for 12 h and the
hot solution was filtered off. The collected organic phase was
evaporated in a vacuum. Then the obtained product was added
to a mixture of N-benzylsalicylamide (2.0 mmol), potassium
carbonate (4 mmol) and dry dimethylformamide (DMF) (20
cm3) which was warmed to 90 uC, and the reaction mixture was
stirred at 90 uC for 12 h. After cooling down, the mixture was
poured into water (100 cm3). The resulted solid was treated with
column chromatography on silica gel [petroleum ether : ethyl
acetate (3 : 2)] to get the ligand L, yield 80%, m. p. 124–125 uC;
Anal. Calcd for C39H39O4N3: C, 76.32; H, 6.40; N, 6.85. Found:
1
C, 76.39; H, 6.18; N, 6.51. H NMR (CDCl3, 300MHz): 2.65–
2.69 (t, 4H), 3.47–3.49 (s, 2H), 3.88–3.92 (t, 4H), 4.60–4.62 (d,
4H), 6.63–7.38 (m, 23H), 8.21–8.27 (q, 2H); IR (KBr pellet,
cm21): 3330 m, 1636 s, 1600 m, 1531 m, 1488 m, 1449 m, 1307 m,
1237 m, 1106 w, 822 w, 753 s, 697 m.
Synthesis of the lanthanide complexes
[LnL(NO3)3]?H2O. An ethyl acetate solution (5 cm3) of
Ln(NO3)3?6H2O (Ln = Pr, Sm, Eu, Gd, Tb, and Dy) (0.1 mmol)
was added dropwise to a solution of 0.1 mmol ligand L in ethyl
acetate (5 cm3). The mixture was stirred at room temperature for
8 h, and then the precipitated solid complex was filtered, washed
with ethyl acetate, dried in vacuo over P4O10 for 48 h and
submitted for elemental analyses, yield: 62–75%. The six
complexes have similar IR spectra, of which the characteristic
bands have similar shifts, suggesting that they have a similar
coordination structure. IR (KBr pellet, cm21) for the Eu nitrate
complex as a representative example: 3362 m, 1614 s, 1477 m,
1304 m, 1239 m, 1112 w, 1029 w, 816 w, 754 s, 702 s .
Experimental section
Materials and instrumentation
N-benzylsalicylamide,5 b,b’-dichlorodiethylamine hydrochloride
salt,6 lanthanide nitrates7 and lanthanide picrates8 (CAUTION!
Although we have experienced no problems in handling picrate
compounds, these should be handled with great caution due to the
potential for explosion) were prepared according to the literature
methods, respectively. Other chemicals were obtained from
commercial sources and used without further purification. 1H
NMR spectrum was measured on a Varian Mercury-300B
spectrometer in d-chlorform solution with tetramethylsilane
[Si(CH3)4] as an internal standard. Elemental analyses were
determined on an Elementar Vario EL analyzer. The FT-IR
spectra were recorded on a Nicolet 360 FT-IR spectrometer
using KBr pellets in the range of 4000–400 cm21. Powder X-ray
diffraction patterns (PXRD) were determined with Rigaku-
Dmax 2400 diffractometer using Cu-Ka radiation. Mass spectra
were recorded on Bruker UHR-TOF maXis 4G Mass spectro-
meter. The solid state luminescence spectra of Sm (2), Eu (3), Tb
(4), and Dy (5) complexes (the excitation and emission slit widths
were 1.0 and 1.0 nm except for the Sm (2) complex, with the
excitation and emission slit widths being 2.5 and 1.0 nm),
solution luminescence spectra of the Tb nitrate complex
[LnL(pic)3]?nH2O (n = 1 or 2). A solution of Ln(pic)3?6H2O
(Ln = Nd, Eu, Tb, and Er) (0.1 mol) in ethanol (5 cm3) was
added to a solution of 0.1 mmol ligand L in ethanol (5 cm3). The
mixture was stirred at room temperature overnight and the
resulting yellow precipitate filtered off, washed with ethanol and
dried in vacuo over P4O10 for 48 h and submitted for elemental
analyses, yield: 60–77%. The four lanthanide picrate complexes
have similar IR spectra, suggesting that they are isostructural. IR
(KBr pellet, cm21) for Eu picrate complex as a representative
example: 3375 m, 1611 s, 1570 m, 1540 s, 1361 s, 1331 s, 1274 s,
1111 w, 703 s.
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Elemental analytical and IR (CLO) spectral data for the newly
synthesized lanthanide solid complexes are summarized in
Table 1.
stable in solution. The lanthanide picrate complexes are soluble
in DMF, DMSO, and acetone, slightly soluble in methanol,
ethanol, acetonitrile, but insoluble in CHCl3 and diethyl ether.
The positive-ion ESI mass spectra of the Eu picrate complex was
obtained in DMF–methanol solution. The ESI mass spectrum
displayed a dominant peak corresponding to [EuL(pic)3(CH3-
CH2OH)(DMF)H]+ (m/z 1569.5), which indicates that the
lanthanide picrate complexes are stable in solution. The
complexes have been well characterized by FT-IR spectroscopy
and elemental analyses. The IR spectrum of the free ligand
displays the characteristic absorption of a carbonyl group at
1636 cm21. In comparison with the free ligand, the absence of
the band at 1636 cm21, which is replaced by a new band at 1611–
1614 cm21 for the complexes, indicates that the oxygen atom of
the carbonyl group takes part in coordination to the lanthanide
ions. Accordingly, single-crystal X-ray structure analyses con-
firmed their structures. Unfortunately, it was not possible to
grow a suitable single crystal of the Gd nitrate complex. The
powder X-ray diffraction patterns for Gd and Tb (4) nitrate
complexes are similar, thus implying that they are isostructural
(Fig. S1, ESI{).
Single-crystal X-ray crystallography
Single crystals of the complexes were obtained from a mixed
solution of ethanol and DMF solution by slow evaporation for
about three weeks at room temperature. The X-ray single-crystal
data collections were performed using a Bruker SMART CCD
detector diffractometer using graphite-monochromatic Mo-Ka
radiation at 294 K. The structures were solved by direct methods
and refined by full-matrix least-squares on F2 using the
SHELXS-97 and SHELXL-97 programs.9 Anisotropic thermal
parameters were assigned to all non-hydrogen atoms. The
hydrogen atoms were placed in idealized positions and located
in the difference Fourier map. Standard DFIX restraints were
used for the distance of the disordered atoms, such as (C46 and
C47) for compound 2, (C35 and C10) for compound 3 and (H50
and O5) for compound 7. Crystallographic data as well as details
of data collection and refinement for these complexes are
summarized in Tables S1 and S2 (ESI{). Important bond lengths
bond angles are listed in Table S3 (ESI{). Hydrogen bonds in
crystal packing for the compounds 1–9 are listed in Table S4
(ESI{). CCDC 788729 (1), 666992 (2), 666993 (3), 666994 (4),
666995 (5), 852907 (6), 852908 (7), 852909 (8), and 852910 (9)
contain the supplementary crystallographic data for this paper.{
Crystal structures of 1–5
The single-crystal X-ray analyses of the complexes reveal that
they are isostructural dinuclear structures with the central metal
Pr, Sm, Eu, Tb, or Dy atom coordinated with nine donor atoms,
six of which belong to the three bidentate nitrate groups and two
oxygen atoms from carbonyl groups from two bridging ligands
and the remaining one to a carbonyl group of a DMF molecule.
The molecular structure of the complex 4 is shown in Fig. 1. The
crystal structure analyses provide direct evidence that the
complexes 1–5 form discrete [2+2] rectangular macrocycles
Results and discussion
Synthesis and characterization
Lanthanide nitrate complexes (Ln = Pr, Sm, Eu, Gd, Tb, and
Dy) were obtained by the reaction of lanthanide nitrates with L
in ethyl acetate, and lanthanide picrate complexes (Ln = Nd, Eu,
Tb, and Er) were synthesized in ethanol. The obtained complexes
are highly stable in air. The solid powders of the lanthanide
nitrate complexes are soluble in DMF, DMSO, methanol,
ethanol, and acetone, slightly soluble in acetonitrile, but
insoluble in CHCl3 and diethyl ether. To confirm whether the
lanthanide nitrate complexes are stable in solution, we obtained
the positive-ion ESI mass spectra of the Tb nitrate complex in
methanol–acetonitrile solution. The ESI mass spectrum dis-
played a dominant peak corresponding to the Ln2L2 species,
namely [Tb2L2(NO3)6(CH3OH)H]+ (m/z 1948.8). This observa-
tion strongly indicates that the lanthanide nitrate complexes are
˚
˚
whose sizes are 9.840 6 7.869 A (1), 9.797 6 7.830 A (2),
˚
˚
˚
˚
9.775 6 7.821 A (3), 9.745 6 7.805 A (4) and 9.729 A 6 7.783 A
(5), respectively.
As shown in Fig. 1a, two ligands adopt a face-to-face
orientation and are joined together by two Tb(III) ions to
generate a discrete zero-dimensional (0D) binuclear rectangular
macrocycle structure. The unit consists of two independent
enantiomeric Tb(III) centers bridged by two ligands. As shown in
Fig. S2 (ESI{), two ligands are connected with each other by
intramolecular p–p interactions of the two benzene ring planes of
the arms which are parallel with each other with the centroid to
˚
centroid distance of 3.578 A, and two benzene rings of two arms
Table 1 Elemental analytical and IR spectral data of the complexes in solid state
Found (Calcd.)%
Complexes
C
H
N
IR (CLO) (cm21
)
[PrL(NO3)6]?2H2O
[SmL(NO3)3]?H2O
[EuL(NO3)3]?H2O
[GdL(NO3)3]?H2O
[TbL(NO3)3]?H2O
[DyL(NO3)3]?H2O
[NdL(pic)3]?H2O
[EuL(pic)3]?2H2O
[TbL(pic)3]?H2O
[ErL(pic)3]?H2O
48.12 (47.96)
48.75 (48.38)
48.45 (48.30)
48.40 (48.04)
47.97 (47.96)
47.59 (47.78)
46.92 (46.88)
46.21 (46.07)
46.50 (46.42)
46.20 (46.16)
4.30 (4.44)
4.44 (4.27)
4.43 (4.26)
4.47 (4.24)
4.40 (4.23)
4.34 (4.22)
3.33 (3.24)
3.08 (3.32)
3.27 (3.21)
3.08 (3.19)
8.25 (8.60)
1613
1613
1614
1613
1614
1614
1612
1611
1611
1612
8.06 (8.68)
9.01 (8.62)
9.00 (8.62)
9.01 (8.60)
8.09 (8.57)
11.17 (11.51)
11.40 (11.31)
11.08 (11.40)
10.98 (11.33)
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Er(III) are eight-coordinated in compounds 8, 9. They all possess
the same one-dimensional (1D) linear coordination polymeric
structures. As shown in Fig. 2, the Eu(III) ion in complex 7 has a
nine-coordinate environment, six of which are occupied by six
oxygen atoms from three bidentate picrate groups, two belong to
the oxygen atoms of carbonyl groups of two ligands and the
remaining one to coordinated ethanol molecule, forming a
distorted tricapped trigonal prism. The Tb(III) and Er(III) ions in
compounds 8 and 9 possess very similar coordination environ-
ments compared with those of Nd(III) and Eu(III), except that
one bidentate picrate group that occurs in 6 or 7 was replaced by
one monodentate picrate group in 8 or 9 (Fig. 3a). Eight-coordinate
complexes are known in a variety of coordination geometries for
which the assignment to the closest idealized polyhedron usually is
not straightforward. For the eight-coordinate geometry, the three
most common idealized coordination polyhedra correspond to the
Fig. 2 The local coordination environment of Eu(III) in compound 7.
(The hydrogen atoms are omitted for clarity.)
Fig. 1 (a) The molecular structure and atom labeling schemes of
compound 4. (b) An illustration of intermolecular hydrogen bonds
(yellow dashed lines) in the 3D supramolecular architectures in 4. (The
hydrogen atoms without formed hydrogen bonds and the benzyl groups
of the ligands are omitted for clarity.)
…
of each ligand interact with each other through C–H
p
interactions referred as a T-shape arrangement. The Tb(III) ions
have a TbO9 coordination environment, forming a distorted
˚
tricapped trigonal prism. The distance of Tb–Tb is 13.016 A. In
addition, the dinuclear complex was assembled into a three-
dimensional (3D) net-like supramolecular structure by inter-
molecular hydrogen bonds (Fig. 1b). Hydrogen bonding para-
meters of complex 4 are given in Table S4 (ESI{).
Crystal structures of 6–9
To explore the role of different anions in the self-assembly
process, picrate, with a larger size, was used instead of nitrate to
carry out the reaction. The single-crystal X-ray analyses reveal
that the coordination environments of Nd(III) and Eu(III) are
nine-coordinated in compounds 6, 7 and those of Tb(III) and
Fig. 3 (a) The local coordination environment of Tb(III) in compound
8. (b) A 1D linear coordination polymer chain of compound 8.
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bicapped trigonal prism (C2v), square antiprism (D4d), and trigonal
dodecahedron (D2d). In order to estimate the degree of the
distorted coordination structures of Tb(III) and Er(III) complexes,
we carried out calculations based on the ‘‘shape-measure’’ criterion
S.10 The lowest value of S for the three pairs represents the best fit
to the closest idealized geometry, and the resulting shape measure
calculations for the two complexes studied herein are summarized
in Table S5 (ESI{). It is readily apparent that the most common
matches for the two complexes were both the distorted bicapped
trigonal prism (C2v). Therefore, only [TbL(pic)3(CH3CH2OH)]n is
selected for investigation here in detail as a representative example.
Scheme 2 A schematic view of the variations from zero- to one-
dimensional structures.
˚
The Tb–O distances are between 2.232(4) and 2.584(5) A, which
numbers of lanthanide ions were reduced from nine in 6 and 7 to
eight in 8 and 9 under the same conditions, which is attributed to
lanthanide contraction effects.
are comparable to the corresponding Tb–O bond lengths found in
related complexes.3b,11 Each ligand binds to two Tb(III) ions using
its two oxygen atoms of the amide groups. So the whole structure
consists of an infinite array of Tb(III) ions bridged by bidentate
ligands and a one-dimensional (1D) linear coordination polymer
formed (Fig. 3b). The weak intermolecular hydrogen bonds C(40)–
Luminescent properties of the compounds 2–5
Lanthanide complexes are known for their photoluminescent
properties. The luminescence of lanthanide complexes is
produced through three steps. First, exciting-light energy is
absorbed by the organic ligands attached to Ln(III), energy
is then transferred into one or several excited states of the central
Ln(III) ions, and, finally, the Ln(III) emits characteristic and
narrow light. In the second step, efficient energy transfer from
ligand to Ln(III) is one of key factors to achieve characteristic
lanthanide luminescence.13 We have studied the luminescent
properties of the nitrate and picrate complexes, and we found
that the picrate complexes give very weak luminescence. We
presume that this phenomenon might be due to coordinated
ethanol molecules existing in lanthanide picrate complexes,
which decrease the luminescence intensity and overall lumines-
cence quantum yields, as the thermal oscillator of the hydroxyl
group of ethanol molecules will consume some excitation energy
absorbed by ‘‘antenna’’ ligands.14 So, we herein only discuss the
luminescent properties of compounds 2–5 in detail.
…
…
H(40) O(15) and C(60)–H(60) O(4) in complex 8 link the 1D
chains into a two-dimensional (2D) layer supramolecular struc-
ture, as shown in Fig. S3a (ESI{). In addition, the layers are linked
…
by weak intermolecular hydrogen bonds C(35)–H(35A) O(10),
…
…
…
C(37)–H(37) O(16), C(51)–H(51) O(1) and C(58)–H(58B) O(23)
to form an infinite three-dimensional (3D) supramolecular
structure (Fig. S3b, ESI{). The single-crystal X-ray analysis reveals
that a coordinated ethanol molecule exists in the lanthanide picrate
complex, which may quench lanthanide luminescence.
Anion influence on structures of lanthanide coordination
compounds
For further careful investigation of the self-assembly between
Ln(NO3)3?6H2O, Ln(pic)3?6H2O and the ligands, nine lantha-
nide coordination compounds were isolated. Herein,
[Tb2L2(NO3)6(DMF)2]n (4) and [TbL(pic)3(CH3CH2OH)]n (8)
are representatively discussed in detail. Calculation using
PLATON13 reveals that the nitrates and picrates account for
about 12.7 and 41.7% of the crystal volumes in compounds 4 and
8, respectively. A deeper view of compound 4 indicates that the
angle of O(9)–Tb(1)–O(11) is 80.5u, which is relatively easy to
form into the 2 : 2 mode dinuclear metallomacrocycle through
the connectivity of two ligand bridges; however, in compound 8,
the angle of O(20)–Tb(1)–O(21) is 135.4u, which could not easily
form a 2 : 2 mode complex owing to the larger steric confine-
ment of the picrates. (O(9), O(11), O(20) and O(21) are
coordinated oxygen atoms of carbonyl groups of the ligands.)
Therefore, it forms a one-dimensional (1D) chain pattern
through opening one ring. A better insight into the nature of
the involved structures can be achieved from Scheme 2. Under
the same conditions of the solution, a DMF molecule was
coordinated to 4 but an ethanol one to 8. The volumes of DMF
and ethanol molecules calculated by PLATON12 are 10.7 and
2.2%, respectively. We speculated that the variations of
coordinated solvent molecules from the DMF in 4 to ethanol
in 8 are also attributed to the different steric confinement of
nitrates and picrates. So we unequivocally presume that the size
of the anions plays a prominent role in the architecture of
lanthanide coordination compounds. Besides this, with the
increasing atomic number of the lanthanide, the coordination
Excited by the absorption band at 323 nm, the ‘‘free’’ ligand
exhibits broad emission bands (lmax = 438 nm) in the solid state.
The excitation and luminescence emission spectra of the zero-
dimensional dinuclear complexes 2–5 in the solid state (Fig. 4a–
d) were recorded at room temperature, and the relevant
photophysical data are summarized in Table 2. For 2, the
combined excitation and emission spectra are presented in
Fig. 4a. The excitation spectrum was obtained by monitoring the
4
596 nm line of the G5/2A6H7/2 emission. The emission intensity
is the weakest, but two characteristic bands can still be observed,
4
which are attributed to G5/2A6HJ (J = 5/2, 7/2) transitions.
Compound 3 exhibits red luminescence, which arises from the
5D0A7F2 transition of Eu(III) ion, as shown in Fig. 4b. The
excitation spectrum of complex 3 shows that the excitation
5
5
5
5
energy is mainly absorbed by the D4, G0-4, L6, D2,1 levels of
the central Eu(III) ion (see Fig. 4b).15 The weak ligand-localized
(S0AS1) excitation band proves that luminescence sensitization
via direct excitation of the ligands is not promising. There are
five characteristic peaks of Eu(III) shown in the emission
spectrum which are attributed to 5D0A7F0 (579 nm), 5D0A
5
5
5
7F1 (592 nm), D0A7F2 (614 nm), D0A7F3 (651 nm), D0A7F4
(691 nm). The intensity of the 5D0A7F2 transition (electric
dipole) is stronger than that of the 5D0A7F1 transition (magnetic
dipole), and it indicates that the coordination environment of the
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Fig. 4 Room-temperature excitation and emission spectra of (a) 2 (lex = 329 nm), (b) 3 (lex = 396 nm), (c) 4 (lex = 327 nm), and (d) 5 (lex = 327 nm) in
the solid state.
complex. The emission intensity of 4 is stronger than that of the
other three under the same conditions. There are four
characteristic peaks shown in Fig. 4c, which result from the
5D4A7FJ (J = 6, 5, 4, 3) transitions. For 5, there are two
characteristic bands that can be seen in the emission spectrum,
which are attributed to transitions of 483 nm (4F9/2A6H15/2), and
572 nm (4F9/2A6H13/2) (Fig. 4d).
The overall luminescence quantum yields, Woverall, of 2–5 in
the solid state were found to be 1.1, 2.4, 50, and 7.0% using an
integrating sphere when excited at 329, 327, 327, and 327 nm,
respectively. The luminescence lifetime values of 2–5 were
determined to be 0.047 ¡ 0.001, 1.072 ¡ 0.001, 1.346 ¡
0.001, and 0.050 ¡ 0.001 ms from the luminescent decay profiles
at room temperature by fitting with a monoexponential curve
(Figs S4–S7, ESI{), indicating the presence of single chemical
environment around the emitting Ln(III) ion.
Table 2 Photophysical characterization of compounds 2–5
lex lem
Woverall
(%)
Compounds (nm) (nm) Assignment
t (ms)
2
3
329
396
562
596
579
592
614
651
691
490
545
582
620
483
572
4G5/2A6H5/2
4G5/2A6H7/2
5D0A7F0
5D0A7F1
5D0A7F2
5D0A7F3
5D0A7F4
5D4A7F6
5D4A7F5
5D4A7F4
5D4A7F3
0.047 ¡ 0.001
1.072 ¡ 0.001
1.1
2.4
4
5
327
327
1.346 ¡ 0.001 50
4F9/2A6H15/2 0.050 ¡ 0.001
4F9/2A6H13/2
7.0
Comparing the relative intensity and overall luminescence
quantum yield of four complexes, we could find that they follow
the trend 4 . 5 . 3 . 2, which means that the energy transfer
from the ligands to Eu(III) and Sm(III) is less effective than that
to Tb(III) and Dy(III). This behaviour can be explicated by the
ligand-to-metal energy migration pathways discussed below. In
addition, the luminescence excitation and emission spectra of the
Eu(III) ion is devoid of an inversion center,16 which is consistent
with the crystallographic analyses. For 4, the excitation spectrum
exhibits a broad excitation band (BEB) between 280 and 400 nm,
which is associated with the electronic transitions of the ligand
(Fig. 4c). This proves that the luminescence sensitization via
excitation of the ligand is effective in the case of the Tb(III)
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Tb(III) nitrate complex in methanol, ethanol, acetone, DMF and
DMSO solutions were recorded at room temperature (Fig. S8,
ESI{). The characteristic emission bands of Tb(III) can be
observed in the emission spectra, which indicated that the
lanthanide nitrate complexes survive in solution.
Conclusions
As a continuing project in our laboratory about the development
of lanthanide complexes generated from various amide type
ligands and lanthanide salts, we successfully designed and
synthesized ten new lanthanide supramolecular architectures
based on a new amide type ligand L (L = benzyl-N,N-bis[(29-
benzylaminofomyl)phenoxyl)ethyl]-amine) in this report. The
single-crystal X-ray diffraction analyses reveal that the com-
plexes [Ln2L2(NO3)6(DMF)2]n are isostructural zero-dimen-
sional (0D) dinuclear rectangular macrocycle structures, while
the compounds [LnL(pic)3(CH3CH2OH)]n exhibit one-dimen-
sional (1D) chain coordination polymers. The variations from
zero- to one-dimensional coordination structures are attributed
to different steric confinement of the nitrate and picrate anions.
We have also studied the solid state luminescent properties of the
nitrate and picrate complexes, and found that the picrate
complexes give very weak luminescence due to coordinated
ethanol molecules decreasing the luminescence. so we only
investigated the luminescent properties of the lanthanide nitrate
complexes in this report. Under the excitation, the four
complexes 2–5 can all show the characteristic luminescent
emissions of Sm(III), Eu(III), Tb(III) or Dy(III). The relative
intensity and overall luminescence quantum yield changes in the
order of 4 . 5 . 3 . 2, which shows the ligands can effectively
sensitize the luminescence of Tb(III) and Dy(III) rather than
Eu(III) and Sm(III). In addition, the ligand-to-metal energy
migration pathways studies also make it rational. This work is
hoped to afford inspiration for the construction of lanthanide-
based photonic devices.
Energy transfer between the ligand and Ln(III)
To elucidate the energy migration pathways in these lanthanide
complexes, it was necessary to determine the singlet and triplet
energy levels of the ligand, which were estimated by referring to
their wavelengths of UV-vis absorbance edges and the lower
wavelength emission edges of the corresponding phosphores-
cence spectra. The wavelength of absorbance edge of the ligand
L is 318 nm (Fig. S9, ESI{), indicating that the singlet energy
level (S1) of the ligand is 31 446 cm21. A triplet excited state T1,
which is localized on one ligand only and is independent of the
lanthanide nature.17 In order to acquire the triplet excited state
T1 of the ligand, the phosphorescence spectrum of the compound
[GdL(NO3)6]?H2O was measured at 77 K in a methanol–ethanol
mixture (v : v = 1 : 1) (Fig. S10, ESI{),18 and the triplet state
energy level T1 of the ligand is 24 331 cm21. It is interesting to
note that the triplet state energy level T1 of the newly designed
4
ligand lies well above the lowest excited resonance level G5/2 of
Sm(III) (17 900 cm21), 5D0 of Eu(III) (17 300 cm21), 5D4 of
4
Tb(III) (20 500 cm21) and F9/2 of Dy(III) (21 000 cm21),
therefore indicating that the ligand can act as an antenna for
the photosensitization of Sm(III), Eu(III), Tb(III) or Dy(III) ions.
According to Reinhoudt’s empirical rule,19 an efficient ligand-to-
metal energy transfer requires good S1–T1 intersystem crossing,
which becomes effective when the energy difference between
these states is at least 5000 cm21, and the energy gap between S1
and T1 for the ligand L amounts to 7115 cm21. Thus, this newly
designed ligand has relatively good intersystem crossing effi-
ciency. On the other hand, an intramolecular energy transfer
from the triplet state level of the ligand to the resonance level of
the Ln(III) ion is one of the most important processes having
influence on the Ln(III) luminescence. It is therefore necessary
that there exists a suitable energy gap between the ligand-
centered triplet state and the lanthanide ion emissive states.
Latva’s empirical rule17,20 states that an optimal ligand-to-metal
energy transfer process for Eu(III) needs DE = E(T1) 2 E(5D0) =
2500–4000 cm21 and for Tb(III) DE = E(T1) 2 E(5D4) = 2500–
4500 cm21. The energy gaps between ligand and lanthanide-
centered levels are 3831 cm21 (DE = T1–5D4) for Tb(III) and
3331 cm21 (DE = T1–4F9/2) for Dy(III), which facilitates efficient
energy transfer.17b By contrast, the energy gaps between ligand
and lanthanide-centered levels are 7031 cm21 for Eu(III) and
6431 cm21 for Sm(III), respectively. Following the method
described in the literature,21 it is possible to evaluate that the
sensitization efficiency of the ligand (gsens) for Eu(III) complex is
10.4% on the basis of the luminescence data (emission spectrum
and 5D0 lifetime) (Table S6, ESI{). The poor sensitization
efficiency of the ligand (gsens) indicated luminescence sensitiza-
tion of the Eu(III) complex vis excitation of the ligand is not
efficient, which is mainly because of the larger energy gap
Acknowledgements
This work was financially supported by the National Natural
Science Foundation of China (Project 20931003, 21071068) and
the Fundamental Research Funds for the Central Universities
(Project lzujbky-2012-k08).
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