Structure variation and luminescence properties of lanthanide complexes incorporating a naphthalene-derived chromophore featuring salicylamide pendant arms

Article abstract of DOI:10.1002/ejic.200701279

A new potentially bridging ligand containing two salicylamide pendant arms separated by a 2,3-dimethoxynaphthalene spacer has been prepared and its coordination chemistry with Ln^III ions has been investigated. An analysis of the presented crystal structures indicates that the diversity of these supramolecular structures is mainly dictated by the nature of the metal ions. These compounds represent good examples of tuning crystal structures arising from the flexibility of the ligands and the Ln contraction effect. Luminescence studies showed that the introduction of the methoxyl substituents on the naphthalene backbone lowers the triplet energy and considerably changes the luminescent behaviors of the Eu^III and Tb^III complexes, which is very different from the literature data on similar compounds. In the emission spectra of the Tb complex the ligand fluorescence remains relatively important because of the back-energy transfer from the Tb^III ion to the ligand, which to the best of our knowledge, may be the first example of salicylamide lanthanide complexes. Wiley-VCH Verlag GmbH & Co. KGaA, 2008.

Full text of DOI:10.1002/ejic.200701279

FULL PAPER
DOI: 10.1002/ejic.200701279
Structure Variation and Luminescence Properties of Lanthanide Complexes
Incorporating a Naphthalene-Derived Chromophore Featuring Salicylamide
Pendant Arms
Xue-Qin Song,[a,b] Wei-Sheng Liu,*[a] Wei Dou,[a] Ya-Wen Wang,[a] Jiang-Rong Zheng,[a]
and Zhi-Peng Zang[a]
Keywords: Lanthanide coordination polymers / Salicylamide / π–π Stacking / Hydrogen bonds / Luminescence properties
A new potentially bridging ligand containing two salicyl-
amide pendant arms separated by a 2,3-dimethoxynaphtha-
lene spacer has been prepared and its coordination chemistry
with LnIII ions has been investigated. An analysis of the pre-
sented crystal structures indicates that the diversity of these
supramolecular structures is mainly dictated by the nature of
the metal ions. These compounds represent good examples
of tuning crystal structures arising from the flexibility of the
ligands and the Ln contraction effect. Luminescence studies
showed that the introduction of the methoxyl substituents on
the naphthalene backbone lowers the triplet energy and con-
siderably changes the luminescent behaviors of the EuIII and
TbIII complexes, which is very different from the literature
data on similar compounds. In the emission spectra of the Tb
complex the ligand fluorescence remains relatively important
because of the back-energy transfer from the TbIII ion to the
ligand, which to the best of our knowledge, may be the first
example of salicylamide lanthanide complexes.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2008)
referred to as “molecular devices for wavelength conver-
sion”.[3] Furthermore, complexation with a ligand provides
lanthanides with a certain degree of protection from the
surrounding water, hence increasing their luminescence
quantum yields. The interest generated by the wide range
of applications of luminescent lanthanide complexes[4] ex-
plains why the design of efficient molecular edifices is still
an important research goal.
Introduction
Stimulated by potential applications in biochemical sen-
sors and fluoroimmunoassays,[1] there is intense current
interest being shown in the design of lanthanide complexes
that exhibit characteristic luminescence properties. The low
extinction coefficients (less than 10 dm3 mol–1 cm–1) of the
Laporte forbidden f–f transitions of lanthanides necessitates
the use of sensitized energy transfer from a suitable absorb-
ing chromophore to influence lanthanide luminescence.[2]
For this process to be efficient, suitable chromophores have
been used to act as antennae or sensitizers that can transfer
energy to the lanthanide ion indirectly (Scheme 1). In this
approach the lanthanide ion is linked by complexation with
a ligand that has an organic chromophore capable of ab-
sorbing light energy with a higher efficiency than the metal
itself. In a subsequent energy transfer process this chromo-
phore acts as a sensitizer of the lanthanide excited state,
which can subsequently be deactivated through lumines-
cence. Because of the large spectral gap between antennae
excitation (often UV light) and emitted photons (visible to
near IR), lanthanide-antenna conjugates have often been
Scheme 1. Sensitization of lanthanide excited states by energy
transfer from an antenna.
Recently a significant increase in the research of coordi-
nation polymers in which metal ion centers are bridged
through organic molecules has been observed, because of
the potential applications in optoelectronics, magnetism,
and catalysis.[5–12] Most of the described materials are based
on transition metal ions and have multidimensional struc-
tures with geometrically simple repeating units from the
templating effect of the transition metal ions, which gen-
erally show low coordination numbers. Only a few examples
of coordination polymers that incorporate lanthanide ions
[a] College of Chemistry and Chemical Engineering and State Key
Laboratory of Applied Organic Chemistry, Lanzhou University,
Lanzhou 730000, China
Fax: +86-931-8912582
E-mail: liuws@lzu.edu.cn
[b] School of Chemical and Biological Engineering, Lanzhou Jiao-
tong University,
Lanzhou 730070, China
Supporting information for this article is available on the
WWW under http://www.eurjic.org or from the author.
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Scheme 2. The synthetic route of the ligand L.
are known,[13–20] along with some examples of high-nu- examine whether the architecture is maintained across the
clearity lanthanide clusters.[21–24] However, these large ions lanthanides. To date, only a few reports are concerned with
achieve high and variable coordination numbers with ir- the diversity of Ln-based structures controlled by the radii
regular geometries, leading to potentially highly lumines- of Ln ions.[28] We report herein on (i) the diversity of the
cent coordination polymers with more complicated features. supramolecular structure of the lanthanide complexes with
To generate target coordination polymers by design, a judi- this new bridging ligand and (ii) the luminescence proper-
cious choice of ligands provides a way of controlling supra- ties of the resulting complexes formed with ions used in
molecular interactions. In general, ligands with certain fea- fluoroimmunoassays (Ln = Sm, Eu, Tb, Dy).
tures, such as rigidity and symmetry, are suitable for ob-
taining coordination polymers. In an attempt to optimize
lanthanide-centered emission in lanthanide-containing
polymers and explore the varied possibilities of lanthanide-
Results and Discussion
based coordination polymers, we are systematically study-
One of the important issues in determining the frame-
ing the structure and the photophysical properties of com- work structures is the geometry of the organic ligands. In
plexing agents capable of efficiently sensitizing EuIII and our study, the terminal coordinating donors are separated
TbIII emission while having a polymerizable moiety.[25,26] by the naphthalene ring at the para position, and two sali-
Along these lines, we have recently described the fluores- cylamide arms attached to naphthalene groups are rota-
cence properties of lanthanide complexes with naphthalene tionally free and are thus capable of adjusting to match the
bearing pendant arms of salicylamide moieties.[27] In order metal coordination preference. Such a divergent arrange-
to extend our work in this field we have explored a ligand ment would favor the formation of polymeric coordination
(Scheme 2) incorporating a naphthalene-derived chromo- polymers instead of discrete complexes. In addition, bulky
phore featuring salicylamide pendant arms, with the ad- coordination arms grafted on the 2,3-dimethoxyl-naphtha-
ditional interesting characteristics: (1) the high extinction lene backbone could result in intra-ligand twisting.
coefficients of the ligand in the near UV/Vis range provide
This bidentate spacer ligand, L, has been obtained in
effective absorption of the excitation energy, which pro- 78% yield by treating the picolylsalicylamide[29] with 1,4-
motes a more effective energy transfer to the lanthanide bis(bromomethyl)-2,3-dimethoxynaphthalene[30] in dry ace-
ion; (2) it also maintains flexibility because of the presence tone in the presence of an excess amount of K2CO3. The
1
of a –NHCH2– spacer between the aromatic ring that may new ligand gave satisfactory H NMR, IR, and mass spec-
bend to require its conformation to coordinate to metal tra, as well as elemental analyses. Treatment of Ln(NO3)3·
centers; (3) the presence of O and N atoms and aromatic 6H2O with the new ligand in an ethyl acetate/methanol
rings may form hydrogen bonds and π–π stacking interac- solution yields a series of complexes which, according to
tions to extend and stabilize the whole framework series. combustion analysis, correspond to the formulae [PrL-
Hence, diverse supramolecular structures may result from (NO3)3(H2O)CH3OH]·3/2H2O, [NdL(NO3)3(H2O)CH3OH]·
this interesting ligand.
1/2CH3OH,
SmL(NO3)3(CH3OH)
EuL(NO3)3(H2O),
The radii of the Ln cations decrease with increasing GdL(NO3)3(H2O), TbL(NO3)3(H2O), [DyL(NO3)3(H2O)2]·
atomic number, which imposes evident influences on the CH3OH, and ErL(NO3)3(H2O)2]·CH3OH. X-ray quality
coordination geometry and might lead to different types of crystals of all the complexes except TbL(NO3)3(H2O) were
structure and physical properties. Thus, it was interesting to obtained after several weeks from vapor diffusion from an
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Structure Variation and Luminescence Properties of Ln Complexes
ethyl acetate/methanol solution. Complexation of the arms bonding interactions exist between the adjacent chains of
through the carbonyl functions is reflected by the ν(C=O) the PrIII complex (Figure 3). The nitrate groups can interact
vibration, which is shifted towards lower energy by ca. with the oxygen atoms of the coordinated water forming
46 cm–1. The complexes are soluble in DMF, DMSO, and interesting two-dimensional supramolecular structures. The
methanol, slightly soluble in ethyl acetate, acetonitrile, and O···O distance [2.764(5) Å] and O–H···O angle (167.58°) are
acetone, but insoluble in CHCl3 and ether.
both within ranges of those reported for hydrogen bonds.
FT-IR Spectra Analyses
The characteristic bands of the carbonyl group belonging
to the free ligand are seen at 1657 cm–1 and shift to
1611 cm–1 upon complexation. Among the spectra of these
lanthanide complexes the main differences lie in the signals
at 1611 cm–1. The absence of a shoulder around this signal
for the PrIII to TbIII complexes indicates the complete coor-
dination of the ligand, while its presence in the DyIII and
ErIII complexes suggests the incomplete coordination of the
ligand, which may be attributed to strong hydrogen bond-
ing. Weak absorptions observed in the range 2900–
2950 cm–1 can be attributed to the υCH2 stretch of the li-
gand. The broad bands at ca. 3422 cm–1 are ascribed to the
vibration of the water ligands in the complexes.
Figure 1. ORTEP drawing of the PrIII complex showing the local
coordination environment of the crystallographically independent
PrIII center (thermal ellipsoids at 30% probability).
Solid-State Structure
Crystal Structure of Type I
Owing to the structural similarity of the PrIII and NdIII
complexes, the PrIII complex was selected for investigation
in detail. As illustrated in Figure 1, the asymmetric unit of
the PrIII complex consists of one crystallographically inde-
pendent PrIII center. The polyhedron around the PrIII ion
is a distorted, bicapped anti-square prism with two oxygen
atoms (O5, O6) from two carbonyl groups of two separate
ligands, six oxygen atoms from three bidentate nitrate
groups, one methanol molecule, and one aqua molecule
completing the decacoordinated sphere. The Pr–O bond
distances are in the range 2.387(8)–2.651(10) Å. The PrIII
centers were extended to form an infinite 1D-chain polymer
by the bridging connection of the ligand, L, stacking along
the crystallographic c axis (Figure 2). Although the ligands
have an “over and under” conformation, the chains are not
helical, rather each metal–ligand chain shows a clear zigzag
structure with a Pr–Pr–Pr internal angle of 88.64°. The
Pr···Pr separations between metals linked by the same
bridging ligand are ca. 17.6 Å. The separations between
alternate PrIII centers (on the same side of the zigzag) are
greater (ca. 24.6 Å). It is well known that the most impor-
tant driving forces in crystal engineering are coordination-
bonding and hydrogen-bonding interactions. Extended net-
works assembled by both coordination and hydrogen bonds
are one of the important strategies to construct supramolec-
ular networks.[31] In principle, higher dimensionality net-
works can be obtained by the assembly of lower dimension-
ality polymers (or molecules) via hydrogen-bonding interac-
tions. A series of such compounds has been reported pre-
Figure 2. View of the 1D chain of the PrIII complex along the a
axis (hydrogen atoms are omitted for clarity).
Figure 3. The 2D supramolecular structure of the PrIII complex
formed by coordinated water-mediated hydrogen bonds, which are
indicated with dashed lines (viewed along the b axis).
Crystal Structure of Type II
Once the ligand coordinates with SmIII and EuIII the co-
viously.[32] It is worth noting that significant hydrogen- ordination environment of the metal ions changes dramati-
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W.-S. Liu et al.
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cally and the coordination number decreases from 10 to 9 two adjacent chains are interlinked by two kinds of π–π
for the complexes of SmIII and EuIII. The difference may be stacking interactions. The dihedral angle between the ben-
ascribed to the larger ionic radius and lower electronic den- zene rings is 4.22° and the distance between the centroid
sity of PrIII and NdIII compared with SmIII and EuIII, which of each ring is 3.732 Å, while the aromatic π–π stacking
allows a higher coordination number for the former. The interaction between two parallel pyridine rings is situated
SmIII and EuIII complexes also have similar structures ex- at a distance of 3.869 Å (see Figure S2, Supporting Infor-
cept for the coordinated water molecule that occurs in the mation). In this way, the zigzag polymer chains are con-
EuIII complex instead of the coordinated methanol mole- nected together and constructed into a three-dimensional
cule in the SmIII complex. Hence, we selected the EuIII com- supramolecular structure (Figure 5).
plex for a detailed description. As shown in Figure 4, the
asymmetric unit contains one EuIII ion and two half ligands
(from two crystallographically independent ligands) such
that all the EuIII ions are crystallographically equivalent.
The europium complex of the ligand crystallizes in the mo-
noclinic P21/n space group. The coordination polyhedron
around the EuIII ion is a distorted, monocapped anti-square
prism. There is one water molecule and three bidentate ni-
trate anions completing the nonacoordinate sphere of the
europium ion, and the distance to the oxygen atom of the
water molecule is Eu–O(16) 2.383(6) Å. The other oxygen
atoms form bond distances between 2.322(7) and
2.528(7) Å. The ligands coordinate in the bidentate bridging
mode. This arrangement of two ligands around the euro-
pium ion leads to the structure of a one-dimensional zigzag
chain (see Figure S1, Supporting Information) with a Eu–
Eu–Eu internal angle of 91.67°. The Eu···Eu separations
Figure 5. The 3D supramolecular structure of the EuIII complex
between metals linked by the same bridging ligand are ca.
constructed by π–π stacking interactions (viewed along the c axis;
hydrogen atoms are omitted for clarity).
17.4 Å and the separations between alternate EuIII centers
(on the same side of the zigzag) are greater, ca. 25.0 Å,
which are very similar to those of the PrIII complex. It is
Crystal Structure of Type III
interesting to note that evidence for intermolecular hydro-
gen bonds was not found. Upon further careful investiga-
The molecular structure of colorless [GdL(NO3)3-
tion of the structure of the EuIII complex it can be seen that (H2O)]ϱ displays the following main features: (i) The gado-
linium complex crystallizes in the monoclinic P21/c space
group. The structure consists of neutral molecules in which
GdIII is nonacoordinated, bound on one side to six O atoms
from the three bidentate nitrate ions and one O atom from
the water molecule and on the other side by the two O
atoms from L (Figure 6). The coordination sphere of the
central GdIII ion is similar to that of the EuIII ion. The
coordinated salicylamide arms also adopt a trans–trans
conformation with respect to the naphthalene backbone.
Each ligand adopts a bridging mode, in which two oxygen
atoms (O5 and O6) act as chelating atoms bonded to crys-
tallographically independent gadolinium ions forming a 1D
zigzag chainlike framework (Figure 7) with a Gd–Gd–Gd
internal angle of 132.78°, and the Gd···Gd separations be-
tween metals linked by the same bridging ligand are ca.
18.23 Å. The separations between alternate GdIII centers
(on the same side of the zigzag) are larger, ca. 33.5 Å, which
is considerably different from that of type I and type II.
This implies that the ligand in the GdIII complex has a more
extended conformation. (ii) The two salicylamide arms
form fold-back angles of about 72.69 and 33.01° with the
naphthalene ring. We also note that the naphthalene moie-
ties are slightly puckered with CCC triangles folded by 2°
with respect to the naphthalene mean plane. (iii) The two
methoxyl substituents on C2 and C5 lying on the same side
Figure 4. ORTEP drawing of the EuIII complex showing the local
coordination environment of the crystallographically independent
EuIII center (thermal ellipsoids at 30% probability).
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Structure Variation and Luminescence Properties of Ln Complexes
of the naphthalene ring also adopt a trans–trans arrange- P21/n. The metal ion is ninefold coordinated and sur-
ment to minimize the intramolecular methoxyl–methoxyl rounded by three bidentate nitrate groups, two water mole-
repulsion. (iv) The one-dimensional zigzag chain is self-as- cules, and one ligand. The bond distances of Dy–O are in
sembled through hydrogen bonds giving rise to a 3D MOF the range 2.318(6)–2.488(8) Å. The DyIII coordination poly-
with hexagonal, chair-conformational cavities (see Fig- hedron is fairly distorted and best described as a mono-
ure S3 and S4, Supporting Information). Provided that nei- capped anti-square prism with an O atom from the nitrate
ther water nor other molecules or ions are present in these group lying in the capping position similar to the arrange-
cavities their dimensions are worth noting. The inner dia- ment observed above. The [DyL(NO3)3(H2O)2] units self-
meter of each hexagonal pore has been evaluated taking assemble via intermolecular O16–H41···O6 and O17–
into account the van der Waals radii of the involved H44···O6 hydrogen-bonding interactions where water mole-
atoms.[33] Particularly, the shorter distance shown in Fig- cules act as an O–H hydrogen-bond donor to the carbonyl
ure S4 corresponds to an effective free pore opening of O atoms forming a zigzag chain (Figure 9). These chains
about 3.4 Å. This value agrees well with the total free vol- are further threaded by O16–H42···O18, O17–H43···N2,
ume of 170.1 Å3 per cell (ca. 3.9% of the crystal volume) and O18–H18···N4 interchain hydrogen-bonding interac-
determined by using the PLATON program.[34]
tions leading to a three-dimensional (3D) supermolecular
structure (Figure 10). In addition, the intramolecular N–
H···O hydrogen bonds, forming among the carbonyl oxy-
gens, amide nitrogen atoms, and aqua ligands, play a vital
role in determining the crystal packing and in the construc-
tion of the extended 3D supramolecular network of the
DyIII complex.
Figure 6. ORTEP drawing of the GdIII complex showing the local
coordination environment of the crystallographically independent
GdIII center (thermal ellipsoids at 30% probability).
Figure 8. ORTEP drawing of the DyIII complex showing the local
coordination environment of the crystallographically independent
DyIII center (thermal ellipsoids at 30% probability; crystallization
water molecules are omitted for clarity).
Figure 7. View of the 1D zigzag chain of the GdIII complex along
the a axis (hydrogen atoms are omitted for clarity).
Crystal Structure of Type IV
Figure 9. The 1D zigzag chain of the DyIII complex self-assembled
by hydrogen bonds between coordinated water and uncoordinated
carbonyl oxygen atoms, which are indicated with dashed lines
(viewed along the a axis; hydrogen atoms, except those forming
hydrogen bonds, are omitted for clarity).
Even though the radius of DyIII is only 0.026 Å smaller
than that of GdIII, the introduction of Dy during the syn-
thesis results in quite a different product. The ligand coor-
dinates to a single metal center in a monodentate mode, in
contrast to the di-monodentate bridging coordination mode
Many lanthanide complexes based on the same ligand
observed for the PrIII to the EuIII complexes, possibly be- usually have similar building blocks due to similar valence
cause the combination of the bulkier ligand and the smaller shell electron configurations.[35] However, polymers con-
ionic radius results in a higher degree of steric crowding. structed from PrIII to GdIII and mononuclear DyIII and
DyIII and ErIII complexes are also isomorphous, therefore, ErIII complexes with the new ligand present different arrays.
only the structure of the DyIII complex will be discussed in The only possible explanation for this change might be the
detail as a representative. A representative molecular struc- templating effect resulting from the different lanthanide
ture plot of the DyIII complex is shown in Figure 8. The ions due to all the complexes crystallized under exactly the
DyIII complex crystallizes in the monoclinic space group same conditions, such as solvent system, temperature, con-
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W.-S. Liu et al.
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group. Indeed, even though the band at 325 nm has the low-
est intensity in the absorption spectrum, it has a high inten-
sity in the excitation spectrum. Complexation with the lan-
thanide ion induces minor red shifts in the energy of the
transitions and a small variation in their intensity (Fig-
ure 11 and Table 1). We note that molar absorption coeffi-
cients at the maximum of the ligand-centered band for these
lanthanide complexes are large (ca. 15000 Lmol–1 cm–1), a
favorable condition for an efficient antenna effect.
It is well known that the energy transfer from ligand ex-
cited states to the resonance states of the LnIII ion in the
complexes can occur in different ways. The favored simpli-
fied mechanism involves ligand excitation by the absorption
Figure 10. The 3D supramolecular structure of the DyIII complex
with methanol oxygen atoms as hydrogen-bond donors and ac-
ceptors, which are indicated with dashed lines (viewed along the b
axis; hydrogen atoms, except those forming hydrogen bonds, are of ultraviolet energy to an excited singlet state, followed by
omitted for clarity).
energy migration via nonradiactive intersystem crossing to
a ligand triplet state [E1 = E(S1 – T)] and energy transfer
from the triplet state to a resonance state of the LnIII ion,
centration, and metal-to-ligand ratio. The ligand conforma-
tion in these coordination polymers extends due to the lan-
thanide contraction effect. These results not only imply that
the similar metal ions could be separated via self-assembly
reactions with appropriate organic ligands but illustrate the
caution that one must take in extending the results obtained
for one complex to the rest of the lanthanide series. As
demonstrated above, complexes of lanthanide ions having
the same ligand throughout the series may preserve the
same atomic coordination, but need not be isomorphic.
from which the emission occurs [E2 = E(T) – E].[36]
The ligand has multiple aromatic rings with a semi-rigid
skeleton structure, so it is a strong fluorescence substance.
Excited by the absorption band at 336 nm, the ligand dis-
plays two strong luminescence bands, one at 365 nm as-
1
signed to a ππ* state, while the other at 456 nm, which is
3
much weaker, corresponds to an emission from the ππ*
state. The ligand-centered luminescence in the GdIII com-
plex (Figure S5) basically displays the same features ob-
served for the free ligand with the singlet band slightly blue-
shifted (maximum around 362 nm), thus confirming the
suitability of this ligand as a sensitizer for EuIII and TbIII
luminescence. In order to both obtain additional infor-
mation on excited states of the systems studied and to esti-
mate the potential of their luminescent applications, the
Photophysical Properties of Ligand L and of the Complexes
in the Solid State
Photophysical Properties of the Ligand
The investigated ligand presents two strong π–π* absorp- phosphorescence spectra of the gadolinium complex was
tions at 251 and 290 nm, which are attributed to π–π* li- also considered (Figure S6). The triplet state of the ligand,
gand transitions centered on the pyridine and salicylamide which is located at 21930 cm–1 is affected by complexation
group, respectively. The other relatively weak band centered in the GdIII complex and sustains a red shift (–1354 cm–1)
at 325 nm can be assigned to the substituted naphthalene up to 20576 cm–1 at 77 K in methanol solution. Introduc-
Figure 11. Absorption spectra of the ligand L (black dotted line) and its EuIII complex (black solid line).
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Structure Variation and Luminescence Properties of Ln Complexes
Table 1. Absorption in ethyl acetate solution (1ϫ10–5 molL–1) and emission features in the solid state of L and its SmIII, EuIII, TbIII
,
and DyIII complexes.
Absorption
λmax [nm]
Fluorescence
Φ (%)
εϫ104 [Lmol–1 cm–1]
λex [nm]
λem [nm]
RFI [a.u.]
L
251
290
325
251
290
325
252
290
325
251
291
324
1.27
1.28
0.18
1.55
1.57
0.22
1.49
1.51
0.21
1.50
1.53
0.19
336
325
325
325
365
456
1700
170.2
–
–
[SmL(NO3)3(CH3OH)]ϱ
[EuL(NO3)3(H2O)]ϱ
[TbL(NO3)3(H2O)]ϱ
564
597
20.35
24.46
–
6
581
594
618
492
546
585
621
484
575
38.33
132.7
450.8
438.4
1216
76.10
13.49
88.03
70.63
0.23
0.14
243
428
[DyL(NO3)3(H2O)2]·CH3OH
251
290
325
1.47
1.49
0.18
325
–
25
Figure 12. Emission spectrum of L and its EuIII complex in the solid state. Black dotted line: emission spectra of ligand at room temp.,
λex = 336 nm; black solid line: emission spectra of the Eu complex at room temp., λex = 325 nm, excitation and emission slit widths:
2.5 nm.
tion of the methoxyl group to the naphthalene backbone SmIII, EuIII, TbIII, and DyIII complexes, respectively. The
lowers the energy of the triplet state by about 3520 cm–1.[27] value of 3276 cm–1 is near to optimal for the high lumines-
The relatively low energy of the triplet state of the ligand cence intensity of europium complexes according to the
may be explained by the large delocalization of the π elec- published data.[37] The absence of any residual lumines-
trons upon methoxyl substitution. It does not appear that cence of excited ligand states for the Eu complex is evidence
the terminal effect is a likely factor for the observed result, of the effective energy transfer from the excited ligand states
since measurements of the pyridine terminal and benzene to the EuIII resonance levels and is in line with this estima-
terminal (20491 cm–1) give a phosphorescence band in al- tion (Figure 12).
most the same range. The triplet states are much too low in
energy to act as sensitizers of the lowest energy excited state
of GdIII [E(6P7/2) = 32000 cm–1]. The energy gap E1 is rela-
Luminescence Properties of the 1:1 Nitrato Complexes in
the Solid State
tively large (about 5467 cm–1), which points to an effective
intersystem crossing process. To estimate the luminescence
efficiency for other lanthanide complexes the energy gap E2
In order to examine the ability of the ligand to act as an
was also calculated on the basis of the values obtained. The antenna group for sensitized luminescence from lantha-
E2 values are ca. 3276, 2676, 76, and –524 cm–1 for the nides, we measured the luminescence properties of four of
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W.-S. Liu et al.
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the complexes with L (Table 1). Excitation spectra of the that commonly observed for aromatic ligands with similar
lanthanide complexes in the solid state (Ln = Sm, Eu, Tb, excited triplet-state levels suggesting that one or more non-
or Dy) display only one ligand-centered band with a maxi- radiative pathways assist in the deactivation of the excited
mum at ca 30769 cm–1, indicating that the LǞLn energy state and shorten the observed lifetimes. It is known from
transfer goes through the triplet state and suggesting that X-ray structural data that L does not encapsulate the entire
the geometry of the ligand should be very similar regardless metal leaving one site ligated with water. Earlier work has
of the metal ion considered. The excellent agreement be- established that a weak vibronic coupling between lantha-
tween the absorption and excitation spectra unequivocally nides and OH oscillators of coordinated water molecules
shows that in all of the complexes studied the lanthanide provides a facile path for radiationless de-excitation of the
ions are excited by ligand-metal intersystem energy transfer metal ion.[39] Thus, the lifetimes observed are expected to
from the sensitized substituted naphthalene chromophore.
be shorter because of the direct coordination of water onto
5
For the EuIII ion, the energy transfer to the upper DJ the lanthanide ion. It is important to note that the inter-
levels (J = 1–3) seems to be most efficient when the ligand layer π–π stacking interactions between the pyridyl rings
triplet state is approximately 500 cm–1 more energetic than and benzene rings of the ligands may favor the reduction
the specific acceptor level concerned, excitation energy is of the energy of the π–π* transition to some extent and
5
then transferred to the D0 level, which is the predominant thus help the photoluminescence.[40] On the other hand, a
emitting state. The emission peaks of the EuIII complex at quenching mechanism through bonded water molecules
5
579, 592, and 619 nm can be assigned to D0 Ǟ7FJ (J = 0, and a low-lying charge-transfer state, from the low re-
1, 2) transitions of the EuIII ion, respectively, as shown in duction potential of the EuIII ion and the presence of the
5
Figure 12. The intensity of the D0 Ǟ7F0 transition is par- electron pairs of the aliphatic nitrogens, may account for
ticularly large, pointing to a Cn (or Cnv) point group, which the relatively low quantum yield. The direct observation of
is validated by X-ray analysis of the EuIII complex. Notably, LMCT transitions in the absorption spectra of the EuIII
the 5D0 Ǟ7F2 transition is much more intense than the complex is not possible because of their much smaller ex-
5D0 Ǟ7F1 transition, the intensity ratio of the EuIII com- tinction coefficients compared with those of the ligand-cen-
plex for I(5D Ǟ7F )/I(5D Ǟ7F ) indicates the absence of an in- tered bands.
0
2
0
1
version center at the EuIII site. This is also in agreement
When TbIII is introduced into the edifices, the lumines-
with the result of single-crystal X-ray analysis. No obvious cence spectra display a mixture of ligand-centred band
emission bands from the ligands are observed, indicating emission and metal-centred line emissions (Figure 13). Two
that the ligands transfer the excitation energy efficiently to intense emission bands at 490 and 544 nm correspond to
5
the EuIII ion. The fluorescence quantum yield, Φ, of the 5D4 Ǟ7F6 and D4 Ǟ7F5 transitions, respectively, while the
5
europium nitrate complex in spectroscopic grade ethyl ace- weaker emission bands at 584 nm originate from D4 Ǟ7F4
tate (concentration: 1.0ϫ10–5 molL–1) was found to be transitions and the typical emission band at 621 nm corre-
0.23% with quinine sulfate as reference.[38] The room-tem- sponds to 5D4 Ǟ7F3 transitions. The ligand fluorescence re-
perature solid-state phosphorescence lifetime of the sample mains relatively important and may be a result of the energy
was determined to be 243 µs. This lifetime is shorter than back transfer from the TbIII ion to the ligand. A large in-
Figure 13. Emission spectrum of L and its TbIII complex in the solid state. Dotted line: emission spectra of ligand at room temp.; solid
line: emission spectra of the Tb complex at room temp.; short dash dotted line: emission spectra of the Tb complex at 77 K, λex = 325 nm,
excitation and emission slit widths: 2.5 nm.
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Eur. J. Inorg. Chem. 2008, 1901–1912

Structure Variation and Luminescence Properties of Ln Complexes
crease in the luminescent intensity ratio between the TbIII be 76 cm–1 and, consequently, is greatly lower than the
1
5D4 Ǟ7F5 transition and the ligand π–π* transition (from threshold value. No transition corresponding to the proper
2.68 to 58.8) is observed when the temperature is lowered TbIII absorption levels, especially those located at 485 nm
to 77 K. This indicates that thermally activated back-energy and between 340 and 380 nm, is observable in the excitation
transfer, which in particular is observed for the TbIII com- spectra. This unequivocally shows that an energy transfer
plexes of the bipyridine-based ligands,[41,42] is rather ef- process from the substituted naphthalene chromophore to
ficient in the TbIII complex. This result is consistent with the metal ion is the only photophysical pathway leading to
the work of Latva et al., who suggested that a fast and observable luminescence in the sample. The fluorescence
irreversible energy transfer to the TbIII ion occurs when quantum yield, Φ, of the terbium nitrate complex in
an energy gap, E2 [E(T) – E(5D4)], is greater than spectroscopic grade ethyl acetate (concentration:
1850 cm–1.[42] In the TbIII complex, this energy gap, mea- 1.0ϫ10–5 molL–1) was found to be 0.14% with eosin as the
sured as usual from the ligand phosphorescence spectrum reference[43] at room temperature. The room-temperature
of the GdIII complex [E(T) = 20576 cm–1], is estimated to phosphorescence lifetime of the terbium complex was deter-
Figure 14. Emission spectrum of the SmIII complex in the solid state at room temp.; λex = 325 nm, excitation and emission slit widths:
2.5 nm.
Figure 15. Emission spectrum of the DyIII complex in the solid state at room temp.; λex = 325 nm, excitation and emission slit widths:
2.5 nm.
Eur. J. Inorg. Chem. 2008, 1901–1912
© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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1909

W.-S. Liu et al.
FULL PAPER
mined to be 428 µs. The moderate quantum yield and life- made possible by the amide functional group as well as the
time may be tentatively attributed to a poor energy match pyridyl group of the ligand are also significant structure
between the donating level of the ligand and the accepting directing factors. The introduction of the methoxyl substit-
level of the TbIII ion and the high-energy vibrational modes uents on the naphthalene backbone lowers the triplet en-
of one coordinated water molecule on quenching the ex- ergy by about 3520 cm–1, leading to a better match with the
5
5
cited state.
acceptor D0 state of the europium ion than the D4 state
In order to demonstrate the capability of antenna-modi- of terbium. It is worth noting that we first observed the
fied ligands as sensitizers for lanthanides other than the back-energy transfer phenomenon in the TbIII complex of
commonly used europium and terbium, we extended our salicylamide derivatives. The behavior of the Eu and Tb
work to SmIII and DyIII. The excitation spectra registered complexes, which is very different from the literature data
at the maximum of the SmIII or DyIII emissions are in line for similar compounds, shows that the physical properties
with previous observations for the europium and terbium of the lanthanide edifices may be easily tuned by changing
complexes. Upon excitation at λex = 325 nm, typical emis- the nature of the backbone properties.[25–27] In conclusion,
sion spectra of Sm and Dy were detected (Figures 14 and this work not only demonstrates that the supramolecular
15). In the case of SmIII, this corresponds to emission lines structure of lanthanide complexes with a naphthalene-de-
at 564 and 597 nm, which are assigned to 4G5/2 Ǟ6HJ tran- rived chromophore featuring salicylamide pendant arms
sitions, with J = 5/2 and 7/2, respectively. The relatively can be tuned by the nature of metal ions, but also offers
4
strong peak is the G5/2 Ǟ6H7/2 transition at 597 nm. The interesting perspectives for the development of efficient lu-
emission spectrum of Dy is composed of two intense lines minescent stains.
at 484 and 577 nm, corresponding to 4F9/2 Ǟ6HJ transitions
with J = 15/2 and 13/2, respectively. Generally, the lumines-
cence spectra of SmIII and DyIII were quite weak. Quantum
yields are expected to be very small and their measurements
Experimental Section
Synthesis and Characterizations: Solvents and chemicals were used
without further purification. Acetone was treated with K2CO3 and
distilled prior to use. Lanthanide nitrate was prepared from the
oxides (99.99%) and nitric acid.[46] 1,4-Bis(bromomethyl)-2,3-di-
methoxynaphthalene and picolylsalicylamide were synthesized as
described previously.[29,30] Ligand L was prepared using the method
similar to that used by Zhao et al.[27] with 1,4-Bis(bromomethyl)-
2,3-dimethoxynaphthalene instead of 1,4-Bis(chloromethyl)-naph-
thalene and picolylsalicylamide instead of benzylsalicylamide.
were therefore omitted. The higher sensitivity of samarium-
and dysprosium-derived luminescence, in comparison with
europium or terbium luminescence, can be explained by the
energy gap between the excited luminescent states and the
highest J levels of the ground states of these lanthanides.
This energy gap is about 7500 cm–1 for SmIII and 7800 cm–1
for DyIII, that is, considerably smaller than that for EuIII
and TbIII ions with values around 12300 cm–1 and
14800 cm–1, respectively.[44] Since OH oscillators act inde-
pendently (Evib = 3657 cm–1; symmetric OH stretching
mode),[45] it can be estimated that, in the case of samarium
and dysprosium, fewer water molecules are needed to
bridge the gap than is the case for europium and terbium
(ca. 2 vs. 3–4). Lifetime measurements for both the Sm and
Dy complexes are found to be 6 and 25 µs, respectively,
based on a single-exponential fit of the data. This higher
water sensitivity explains why lifetimes of DyIII or SmIII lu-
minescence are generally much shorter than those observed
1
Yield 1.042 g (78%); m.p. 161.7 °C. H NMR (300 MHz, CDCl3,
25 °C): δ = 3.89 (s, 6 H), 4.43 (d, J = 5.4 Hz, 4 H, NHCH2), 5.68
(s, 4 H, OCH2), 6.91 (d, J = 9.9 Hz, 4 H, Ar), 7.15 (t, 2 H, Ar),
7.24–7.56 (m, 8 H, Ar), 7.99–8.02 (m, 4 H, Ar), 8.25–8.27 (m, 2 H,
Ar), 8.55 (t, 2 H, NH) ppm. IR (KBr, ν=): ν = 3415 (s, NH), 1657
˜
˜
(s, C=O), 1597 (m), 1537 (m), 1469 (m), 1298 (m), 1220 (s), 991 (s),
755 (s) cm–1. Analytical data L (668.3): calcd. C 71.84, H 5.43, N
8.38; found C 71.69, H 5.45, N 8.42. ES: m/z = 669.3 [M + H].
Complexes: A solution of Ln(NO3)3·6H2O (Ln = Pr, Nd, Sm, Eu,
Gd, Tb, Dy, and Er) (0.1 mmol) in a minimum amount of ethyl
acetate was added dropwise to a solution of ligand L (0.1 mmol)
in a minimum amount of ethyl acetate. The mixture was stirred at
room temperature for 4 h. Methanol (1 cm3) was then added and
the solution was filtered. Upon standing the filtrate at room tem-
perature for several weeks, {[PrL(NO3)3(H2O)CH3OH]·3/2H2O}ϱ,
with EuIII or TbIII
.
Conclusions
{[NdL(NO3)3(H2O)CH3OH]·1/2CH3OH}ϱ,
OH)]ϱ, [EuL(NO3)3(H2O)]ϱ, [GdL(NO3)3(H2O)]ϱ, [DyL(NO3)3-
thalene-bridged space ligand is a useful building block in (H2O)2]·CH3OH, and [ErL(NO3)3(H2O)2]·CH3OH were obtained
[SmL(NO3)3(CH3-
We have demonstrated here that the new bidentate naph-
in 32–58% yield (based on Ln). The elemental analyses showed
that the metal complexes obtained are composed of a 1:1 ratio of
Ln to ligand. This ratio was later unequivocally confirmed by X-
ray crystallographic analyses. Data for {[PrL(NO3)3(H2O)CH3OH]·
3/2H2O}ϱ, C41H45N7O18.5Pr (1072): calcd. C 45.91, H 4.23, N 9.14,
the construction of polymeric and discrete lanthanide com-
plexes. It is interesting to note that the two salicylamide
arms of L are somewhat flexible and rotational around the
naphthalene group in order to meet the geometrical prefer-
ence of the metal centers and the structural variation may
be a result of the different coordination features of the lan-
thanide ions. In the structures of the PrIII to GdIII com-
plexes, L acts as a di-monodentate bridging ligand to con-
nect two metal ions, but acts as a monodentate ligand in the
Pr 13.14; found C 45.72, H 4.21, N 9.17, Pr 13.08. IR (KBr): ν =
˜
3378 (br. s), 2924 (w), 1611 (s, C=O), 1562 (m), 1457 (m), 1309 (s),
1226 (m), 1110 (m), 1028 (m), 755 (m) cm–1. Data for {[NdL(NO3)3-
(H2O)CH3OH]·1/2CH3OH}ϱ, C41.50H44N7NdO17.50 (1063): calcd.
C 46.80, H 4.16, N 9.21, Nd 13.54; found C 46.57, H 4.15, N 9.25,
complexes of DyIII and ErIII. Hydrogen-bonding pathways Nd 13.48. IR (KBr): ν = 3352 (br. s), 2928 (m), 1613 (s, C=O),
˜
1910
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Eur. J. Inorg. Chem. 2008, 1901–1912

Structure Variation and Luminescence Properties of Ln Complexes
1564 (m), 1456 (m), 1307 (s), 755 (w) cm–1. Data for [SmL(NO3)3- Supporting Information (see footnote on the first page of this arti-
(CH3OH)]ϱ, C41H40N7O16Sm (1037): calcd. C 47.48, H 3.89, N
9.45, Sm 14.50; found C 47.59, H 3.90, N 9.43, Sm 14.43. IR (KBr):
cle): Crystal data and structure refinement, the view of the 1D
chain of the EuIII complex, the 3D supramolecular structure of the
EuIII complex showing π–π stacking interactions, the 3D supramo-
lecular structure of the GdIII complex, a space-filling diagram of
ν = 3348 (br. s), 2922 (m), 1611 (s, C=O), 1452 (m), 1308 (s), 1222
˜
(m), 1110 (m), 1038 (m), 752 (w) cm–1. Data for [EuL(NO3)3-
(H2O)]ϱ, C40H38EuN7O16 (1025): calcd. C 46.88, H 3.74, Eu 14.83, the nanosized holes in the GdIII complex constructed by intermo-
N 9.57; found C 47.06, H 3.71, Eu 14.77, N 9.53. IR (KBr): ν =
lecular hydrogen bonds, fluorescence of the Gd complex at room
˜
3422 (br. s), 2922 (w), 1611 (s, C=O), 1457 (m), 1304 (s), 1222 temperature and phosphorescence spectra of the Gd complex at
(m), 752 (w) cm–1. Data for [GdL(NO3)3(H2O)]ϱ, C40H38GdN7O16 77 K (Table S1, Figures S1–S6, see also the footnote on the first
(1030): calcd. C 46.64, H 3.72, Gd 15.27, N 9.52; found C 46.42,
page of this article).
H 3.71, Gd 15.21, N 9.48. IR (KBr): ν = 3350 (br. s), 2928 (m),
˜
1611 (s, C=O), 1480 (m), 1294 (s), 1227 (m), 754 (w) cm–1. Data
for [TbL(NO3)3(H2O)]ϱ, C40H38N7O16Tb (1032): C 46.57, H 3.71,
N 9.50, Tb 15.40; found C 46.38, H 3.69, N 9.46, Tb 15.34. IR
Acknowledgments
This work was supported by the National Natural Science Founda-
tion of China (Grants 20771048, 20431010, 20621091, and
J0630962).
(KBr): ν = 3378 (br. s), 2927 (m), 1611 (s, C=O), 1452 (m), 1308
˜
(s), 1221 (m), 752 (w) cm–1. Data for [DyL(NO3)3(H2O)2]·CH3OH,
C41H44DyN7O18 (1085): C 45.37, H 4.09, Dy 14.97, N 9.03; found
C 45.59, H 4.07, Dy 14.92, N 9.07. IR (KBr): ν = 3348 (br. s), 2922
˜
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equipped with quartz cuvettes of 1 cm path length. An excitation
slit of 2.5 nm and an emission slit of 2.5 nm were used for the
measurements in the solid state. The 77 K solid-state emission spec-
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inside a quartz-walled optical Dewar flask filled with liquid nitro-
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corded with solution samples loaded into a quartz tube inside a
quartz-walled optical Dewar flask filled with liquid nitrogen in the
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Received: November 29, 2007
Published Online: February 27, 2008
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