Near-infrared luminescent xerogel materials covalently bonded with ternary lanthanide [Er(III), Nd(III), Yb(III), Sm(III)] complexes

Article abstract of DOI:10.1039/b819644c

A β-diketone ligand 4,4,5,5,5-pentafluoro-1-(2-naphthyl)-1,3- butanedione (Hpfnp), which contains a pentafluoroalkyl chain, was synthesized as the main sensitizer for synthesizing new near-infrared (NIR) luminescent Ln(pfnp)₃phen (phen = 1,10-p

Full text of DOI:10.1039/b819644c

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www.rsc.org/dalton | Dalton Transactions
Near-infrared luminescent xerogel materials covalently bonded with ternary
lanthanide [Er(III), Nd(III), Yb(III), Sm(III)] complexes†
Jing Feng,a,b Jiang-Bo Yu,a Shu-Yan Song,a,b Li-Ning Sun,a Wei-Qiang Fan,a,b Xian-Min Guo,a,b Song Danga,b
and Hong-Jie Zhang*a
Received 5th November 2008, Accepted 8th January 2009
First published as an Advance Article on the web 12th February 2009
DOI: 10.1039/b819644c
A b-diketone ligand 4,4,5,5,5-pentafluoro-1-(2-naphthyl)-1,3-butanedione (Hpfnp), which contains a
pentafluoroalkyl chain, was synthesized as the main sensitizer for synthesizing new near-infrared (NIR)
luminescent Ln(pfnp)3phen (phen = 1,10-phenanthroline) (Ln = Er, Nd, Yb, Sm) complexes. At the
same time, a series of lanthanide complexes covalently bonded to xerogels by the ligand
5-(N,N-bis-3-(triethoxysilyl)propyl)ureyl-1,10-phenanthroline (phen-Si) were synthesized in situ via a
sol–gel process. [The obtained materials are denoted as xerogel-bonded Ln complexes (Ln = Er, Nd,
Yb, Sm).] The single crystal structures of the Ln(pfnp)3phen complexes were determined. The
properties of these complexes and the corresponding xerogel materials were investigated by
Fourier-transform infrared (FTIR), diffuse reflectance (DR), and field-emission scanning electron
microscopy (FE-SEM). After ligand-mediated excitation of these complexes and the corresponding
xerogel materials they all show the characteristic NIR luminescence of the corresponding Ln3+ ion. This
is attributed to efficient energy transfer from the ligands to the Ln3+ ion (the so-called “antenna effect”).
However, it is difficult to generate luminescence by direct
Introduction
excitation of lanthanide ions due to their poor absorption abilities.
Historically, most of the investigations in the field of luminescent
A useful way to sensitize the luminescence of lanthanide ions is to
lanthanide ions have been devoted to Eu(III) and Tb(III), which
employ organic chromophores as antennas for light absorption.9
emit in the visible spectral region.1 Recently, much attention
Weissman reported in 1942 that lanthanide luminescence can be
has been paid to near-infrared (NIR) luminescence of trivalent
improved by using an intramolecular energy transfer, the so-called
lanthanide ions,2 such as Yb(III), Er(III), Nd(III), due to their poten-
tial applications as biomolecule labels in luminescent bioassays,3
“antenna effect”.10 Commonly used ligands are b-diketones (1,3-
dikeones) and non-charged ligands like 1,10-phenanthroline. b-
as functional materials for optical telecommunication networks4
Diketones containing aromatic groups have strong absorption
and laser systems.5 For instance, the relative transparency of
over a large wavelength range and consequently have been targeted
human tissue at around 1000 nm suggests that in vivo luminescent
probes operating at this wavelength (Yb-based emission) could
to sensitize lanthanide luminescence.11 Additionally, they have
a strong ability to form adducts with lanthanide ions, and the
have diagnostic value.6 NIR luminescence from Er(III) proves
obtained complexes are stable enough for practical use. According
very useful when employed in telecommunication network optical
signal amplifiers.4a,7 Regarding the Nd(III) containing materials,
for a long time this metal has been found to have applications
to the literature reported previously, the replacement of C–H
bonds in a b-diketone ligand with lower-energy C–F oscillators
is able to lower the vibration energy of the ligand, which decreases
within laser systems.5a,8 Moreover, the sensitization of other
the energy loss caused by ligand vibration and enhances the
NIR-luminescent trivalent lanthanide ions, such as Sm(III) with
emission intensity of the lanthanide ion.12 To accomplish this
abundant emissions from its 4G5/2 excited state to lower manifolds,
goal, the ligand Hpfnp [4,4,5,5,5-pentafluoro-1-(2-naphthyl)-1,3-
is of high interest as supplementary emission wavelengths would
then also be available.
butanedione], was synthesized as the main sensitizer for synthe-
sizing the Ln(pfnp)3phen (Ln = Er, Nd, Yb, Sm) complexes.
In addition, phen can serve as the synergistic agent, since an
aState Key Laboratory of Rare Earth Resource Utilization, Changchun
important issue in the design of lanthanide complexes is to prevent
water molecules from binding to lanthanide ions. With the help of
Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun,
130022, Jilin, China. E-mail: hongjie@ciac.jl.cn; Fax: +86-431-85698041;
resonant energy transfer, lanthanide ions in lanthanide complexes
may accept energy from the lowest triplet states of ligands.
Theoretically, efficient emissions corresponding to internal f–f
transitions of lanthanide ions are expected.
The practical applications of these NIR luminescent lanthanide
complexes are limited to a large extent by their poor thermal
stability and low mechanical strength. One solution is to incor-
porate lanthanide complexes in a host matrix.13 Sol–gel derived
hybrid materials have received great interest nowadays,1d,e,14 as they
Tel: +86-431-85262127
bGraduate School of the Chinese Academy of Sciences, Beijing, 100039,
P. R. China
† Electronic supplementary information (ESI) available: UV-vis absorp-
tion spectra of Hpfnp and phen ligands, excitation and emission spectra
of Er(pfnp)3phen, Nd(pfnp)3phen, Yb(pfnp)3phen and Sm(pfnp)3phen
complexes, the phosphorescence spectra of Gd(pfnp)3(H2O)2 complex
in DMF at 77 K, XRD patterns of Er(pfnp)3phen, Yb(pfnp)3phen and
Yb(pfnp)3phen simulated. CCDC reference numbers 699049–699052. For
ESI and crystallographic data in CIF or other electronic format see DOI:
10.1039/b819644c
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combine chemical and thermal stability, the mechanical strength
and the process flexibility of silica, together with the functional
characteristics of active organic molecules. Physical mixing of
the sol–gel materials and lanthanide complexes is easy to do,
however, the interactions between lanthanide complexes and silica
moieties in such sol–gel hybrid materials are so weak that the phase
separation, inhomogeneous dispersion, and optical quenching
of the dopants may occur.15 To overcome these shortcomings,
we prepared phen-Si [5-(N,N-bis-3-(triethoxysilyl)propyl)ureyl-
1,10-phenanthroline], which plays a double role, i.e. as a ligand
for lanthanide ions and an organosilane precursor to synthesize
the xerogel materials.16 Moreover, the in situ synthesis technique
reported by Qian et al. was selected.17 Thus, the NIR-luminescent
lanthanide b-diketone complexes were covalently attached to the
xerogel material and the above-mentioned problems of phase
separation and inhomogeneous dispersion between organic and
inorganic moieties can be avoided effectively.
Herein, we synthesize a b-diketone ligand (Hpfnp), followed
by synthesis of Ln(pfnp)3phen complexes and xerogel-bonded Ln
complex (Ln = Er, Nd, Yb, Sm) materials. The crystal structures of
the Ln(pfnp)3phen complexes are reported. The properties of the
Ln(pfnp)3phen complexes and the corresponding xerogel materials
were investigated by Fourier-transform infrared (FTIR), diffuse
reflectance (DR), field-emission scanning electron microscopy
(FE-SEM), and luminescence spectra.
Results and discussion
Crystal structures of the Ln(pfnp)3phen (Ln = Er, Nd, Yb, Sm)
complexes
The single crystal structures of a series of lanthanide complexes
containing Hpfnp and phen were studied. The single-crystal X-ray
diffraction studies revealed isostructural crystals (triclinic, Z =
4) for Ln(pfnp)3phen (Ln = Er, Nd, Yb, Sm) complexes whose
¯
crystals fall into the same P1 space group (also see Fig. S1, ESI†).
Thus, only the crystal structure of Nd(pfnp)3phen will be described
in detail as an example. The crystal structure of Nd(pfnp)3phen
with the numbering scheme is displayed in Fig. 1a. Analysis of
the crystal structure indicates that the central Nd3+ ion is eight-
coordinate, with six oxygen atoms from three Hpfnp ligands
and two nitrogen atoms from the phen ligand. The coordination
geometry of the central Nd3+ ion is best described as a square
antiprism from the coordination site angles (shown in Fig. 1b). In
the b-diketone rings of the Ln(pfnp)3phen (Ln = Er, Nd, Yb, Sm)
complexes, the average distances for the C–C and C–O bonds are
shorter than a single bond but longer than a double bond. This
can be explained by the fact that there exists a conjugated structure
betweenthenaphthyl ring and the coordinated b-diketonate, which
leads to the delocalization of electron density of the coordinated
b-diketonate chelate ring. Also, the b-diketonate chelate ring itself
is a conjugated structure due to the delocalization of electron
density. Important experimental parameters for the structure
determinations are tabulated in Table 1, and the average Ln–O
and Ln–N bond lengths in the crystals are listed in Table 2. It can
be seen that the average Ln–O and Ln–N bond lengths decrease
from Nd to Yb for the Ln(pfnp)3phen (Ln = Er, Nd, Yb, Sm)
complexes due to the “lanthanide contraction”, as expected. As
described above, the central Ln3+ ion is completely surrounded by
Fig. 1 (a) ORTEP plot for Nd(pfnp)3phen with ellipsoids drawn at
the 30% probability level. Hydrogen atoms omitted for clarity. (b) Co-
ordination polyhedron of the central Nd3+ ion.
the anionic b-diketone ligand Hpfnp and the synergistic ligand
phen. Thus, the ligands (Hpfnp and phen) are able to absorb
energy and transfer the absorbed energy to the central Ln3+ ion.
The characteristic NIR luminescence of the Ln3+ ion (Ln = Er,
Nd, Yb, Sm) is expected.
Fourier-transform infrared (FTIR) and diffuse reflectance
(DR) spectra
The FTIR spectra of the Nd(pfnp)3phen complex and xerogel-
bonded Nd complex (the expected structure is outlined in
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Table 1 Crystal data and structure refinements for the Ln(pfnp)3phen (Ln = Er, Nd, Yb, Sm) complexes
Empirical formula
Formula weight
Temperature/K
C57H32F15N2O6Er
1293.11
293(2)
C57H32F15N2O6Nd
1270.09
293(2)
C57H32F15N2O6Yb
1298.89
293(2)
C57H32F15N2O6Sm
1276.20
293(2)
˚
Wavelength/A
0.71073
0.71073
0.71073
0.71073
Crystal system
Space group
Triclinic
P1
2
12.222(5)
20.080(5)
21.842(5)
89.163(5)
83.064(5)
84.338(5)
5295(3)
Triclinic
P1
2
Triclinic
P1
2
Triclinic
P1
2
¯
¯
¯
¯
Space group number
˚
a/A
12.346(5)
20.023(5)
21.938(5)
89.777(5)
82.019(5)
83.466(5)
5335(3)
4
1.581
1.078
2524
0.24 ¥ 0.22 ¥ 0.18
1.38 to 26.07
30313
12.159(5)
20.061(5)
21.769(5)
88.942(5)
83.288(5)
84.378(5)
5248(3)
4
1.644
1.889
2564
0.24 ¥ 0.20 ¥ 0.14
1.39 to 26.10
29729
12.633(3)
20.161(4)
21.541(4)
79.599(4)
86.203(4)
83.818(4)
5358.7(19)
4
1.582
1.200
2532
0.22 ¥ 0.18 ¥ 0.17
1.84 to 26.08
30412
˚
b/A
˚
c/A
a/◦
b/◦
g /◦
3
˚
Volume/A
Z
4
D
calcd./mg m-3
1.622
1.691
2556
Abs. coeff./mm-1
F(000)
Crystal size/mm
0.20 ¥ 0.16 ¥ 0.15
1.38 to 26.11
29900
20378(0.0836)
Full-matrix least-squares on F2
20378/28/1254
0.958
0.1014
0.1977
0.2589
0.2746
q range/◦
Reflns collected
Unique reflns (Rint
)
20614(0.0313)
20290(0.0522)
20680(0.0586)
Refinement method
Data/restraints/parameters
GOF on F2
20614/46/1459
1.010
0.0598
0.1263
0.1063
20290/45/1339
0.967
0.0857
0.1951
0.1837
20680/43/1439
0.969
0.0812
0.1384
0.2118
R1 [I>2s(I)]
wR2 [I>2s(I)]
R1 (all data)
wR2 (all data)
0.1517
0.2497
0.1877
Table 2 Average Ln–O and Ln–N bond lengths for the Ln(pfnp)3phen
(Ln = Er, Nd, Yb, Sm) complexes
which suggests the formation of Nd–O bonds in the xerogel-
=
bonded Nd complex. The bending vibration frequency of C N
has red-shifted in the xerogel-bonded Nd complex compared
with phen, suggesting the formation of a Nd–N bond.13f In
Fig. 2c, the formation of the Si–O–Si framework is evidenced
by the bands located at 1073 and 450 cm-1.18 The peak at 1526
cm-1, originating from the CONH group of phen-Si (Fig. 2b),
can also be observed in Fig. 2c, which could suggest that the
functionalized phen group in the framework remains intact. Thus,
we proposed that ternary lanthanide complexes [Ln(pfnp)3phen,
Ln = Er, Nd, Yb, Sm] were formed via in situ synthesis methods
and covalently bonded to the silica network in the corresponding
xerogels.
The DR spectra of the Ln(pfnp)3phen complex and the xerogel-
bonded Ln complex (Ln = Er, Nd, Yb, Sm) are shown in
Fig. 3. All broad absorption bands in the UV region of the DR
curves were observed and can be assigned to electronic transitions
from the ground-state level (S0) to the excited level (S1) of the
organic ligands. In the visible and NIR region of these curves,
each absorption band corresponds to a characteristic transition
between two spin–orbit coupling levels of the corresponding
lanthanide ions, which can be assigned to the transitions from
the ground levels 4I15/2, 4I9/2, 2F7/2, and 6H5/2 to the higher energy
levels for erbium,19 neodymium,12b ytterbium,20 and samarium21
based complexes and xerogels, as marked in Fig. 3. It can be seen
that the Ln(pfnp)3phen complexes and the corresponding xerogel
materials possess a similar position for the absorption of the
ligands, which, together with the appearance of the characteristic
absorption of the Ln3+ ions (Ln = Er, Nd, Yb, Sm), suggests that
the lanthanide complexes were synthesized in situ in the xerogel-
bonded Ln complexes.
Yb(pfnp)3-
Nd(pfnp)3phen Sm(pfnp)3phen Er(pfnp)3phen phen
˚
˚
Ln–O/A 2.397
2.361
2.606
2.299
2.520
2.277
2.498
Ln–N/A 2.642
Scheme 1) are given as the representative patterns. Fig. 2 presents
the FTIR spectra of the Nd(pfnp)3phen complex, the phen-Si,
and the xerogel-bonded Nd complex. The peaks in the range
400–438 cm-1 corresponding to Nd–O vibrations in Fig. 2a
can also be seen in Fig. 2c (a very weak peak at 416 cm-1),
Fig. 2 FTIR spectra of the Nd(pfnp)3phen complex (a), the phen-Si (b)
and the xerogel-bonded Nd complex (c).
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Scheme 1 Synthesis procedure and proposed structure of the xerogel-bonded Ln complex (Ln = Er, Nd, Yb, Sm) materials.
Field-emission scanning electron microscopy (FE-SEM)
Photoluminescence studies
As a result of the synthesis process, transparent crack-free
monoliths, which have copied the shape of the reaction vessel,
were obtained (see Fig. 4, inset). The FE-SEM images of the
xerogel-bonded Ln complexes (Ln = Er, Nd, Yb, Sm) are similar
to one another, as presented by the representative patterns of the
xerogel-bonded Nd complex in Fig. 4. From the FE-SEM images
it can be seen that the xerogel material appears homogeneous,
and no sign of any phase separation was observed even when the
magnification was increased to 80 000. This can be explained by
the in situ synthesis technique we used and the strong covalent
bonds bridging between the organic and inorganic phases.
The excitation spectra of the lanthanide complexes and the
absorption spectra of the ligands Hpfnp and phen are shown in
Fig. S2 (see ESI†). It is worth noting that the absorption spectrum
of Hpfnp and the excitation spectra of the Ln(pfnp)3phen (Ln =
Er, Nd, Yb, Sm) complexes (monitored at the maximum emission
of the corresponding Ln3+ ion) are a better match than that of
phen and the excitation spectra, so Hpfnp is the main antenna
ligand transfering energy and sensitizing the Ln3+ ions.22
As shown in Fig. S2† and Fig. 5, a broad band ranging from
250 to 450 nm for the Er(pfnp)3phen complex and a broad band
ranging from 250 to 470 nm for the xerogel-bonded Er complex
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Fig. 3 DR spectra of the Ln(pfnp)3phen complexes and the xerogel-bonded Ln complex [Ln = Er (a), Nd (b), Yb (c), Sm (d)] materials.
Fig. 5 Excitation (EX, lem = 1533 nm) and emission (EM, lex = 413 nm)
spectra of the xerogel-bonded Er complex.
absorption band, characteristic Er3+ ion emissions were observed
for the Er(pfnp)3phen complex (Fig. S3, ESI†) and the xerogel-
bonded Er complex (Fig. 5), respectively. Both of the emission
spectra cover a wide spectral range with the emission maxima
centered at 1533 nm. The emission obtained is attributed to the
typical I13/2 → I15/2 transition of the Er3+ ion. For many years,
Er-doped materials with a high bandwidth and optical gain were
of great interest in optical communications technology, because
the I13/2 → I15/2 transition around 1540 nm matches one of the
fiber low-loss windows.23 To enable a wide gain bandwidth for
optical amplification, a broad emission band is desirable.24 For
the xerogel-bonded Er complex, the full width at half maximum
(FWHM) of the 1533 nm emission band is 72 nm. Compared with
that of the Er(pfnp)3phen complex (59 nm) and the other Er-doped
4
4
Fig. 4 FE-SEM images of the xerogel-bonded Nd complex (a) with a
magnification of 20 000 and (b) with a magnification of 80 000. The inset
shows the photo of the transparent xerogel-bonded Nd complex.
4
4
were observed. The two broad bands can be attributed to the light
absorption of the Hpfnp and phen ligands, which is in agreement
with the DR spectra (Fig. 3a). Upon excitation of the ligands’
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materials,25 such a broad spectrum enables a wide gain bandwidth
for optical amplification.
Meanwhile, the excitation and emission spectra of the
Ln(pfnp)3phen complexes and the xerogel-bonded Ln complexes
(Ln = Nd, Yb, Sm) were obtained as shown in Fig. S2, Fig. S3,†
and Fig. 6–8, respectively. After ligand-mediated excitation, all
of the emission spectra of the Ln(pfnp)3phen complexes and the
corresponding xerogels show the characteristic NIR luminescence
of the corresponding lanthanide ions. For the Nd(pfnp)3phen
complex and the xerogel-bonded Nd complex, the emission at
4
around 1059 nm (4F3/2 → I11/2) is the strongest, which is potentially
applicable for laser systems. Moreover, the emission band at
1332 nm offers the opportunity to develop new materials which
are suitable for optical amplifiers operating at 1.3 mm.26 For the
emission spectra of the Yb(pfnp)3phen complex and the xerogel-
bonded Yb complex, the Yb3+ ion emits in the range 910–1145 nm
and 900–1150 nm, respectively, with a sharp peak around 978 nm
assigned to the F5/2 → F7/2 transition of the Yb3+ ion broader
vibronic components at longer wavelength.27 Similar splitting has
been reported in previous literature.28 This may be the splitting
of the energy levels of the Yb3+ ion as a consequence of ligand
field effects.24a The Yb3+ ion plays an important role in laser
emission because of its very simple f–f energy level structure. There
is no excited-state absorption on reducing the effective laser cross-
Fig. 8 Excitation (EX, lem = 950 nm) and emission (EM, lex = 372 nm)
spectra of the xerogel-bonded Sm complex. Transitions start from the 4G5/2
level.
2
2
section, no up-conversion, no concentration quenching, and no
absorption in the visible range. The intense Yb3+ ion absorption
lines are well suited for laser diode pumping in this range and
the smaller Stokes shift between absorption and emission reduces
the thermal loading of the material during laser operation.29 These
properties of the Yb3+ ion and the obtained emission make xerogel-
bonded Yb complexes very important for various photonic appli-
cations in ionic crystals and glasses.30 Additionally, the relative
transparency of human tissue at approximately 1000 nm suggests
that in vivo luminescent probes operating at this wavelength of
Yb-based emission could have diagnostic value.6 Compared with
the NIR luminescence of Er(III), Nd(III), and Yb(III), that of
Sm(III) is less well studied, although this situation is being
4
remedied.28b,c,31 The G5/2
→
6F5/2 transition at 950 nm in the
emission spectra of the Sm(pfnp)3phen complex and the xerogel-
bonded Sm complex is the most intense.
As described above, the obtained characteristic NIR lumi-
nescence of the Ln3+ ions is undoubtedly attributable to the
contribution of the intramolecular energy transfer from the
ligands, since Ln3+ ions have no absorption at the corresponding
excitation wavelength. This verifies that the lanthanide complexes
have been synthesized in situ in the xerogel-bonded Ln complexes,
which is in agreement with the preliminary conclusion obtained
from the FTIR and DR spectra. The triplet state energy level of
Hpfnp is 19 685 cm-1, found by examining the phosphorescence
spectrum of the Gd(pfnp)3(H2O)2 complex in DMF solution at
77 K (see Fig. S4†).32 According to Dexter’s theory,33 it can
match well with the 4f levels of the Ln3+ ions mentioned above
(see Fig. S5†), and finally obtain the efficient NIR luminescence
of the corresponding Ln3+ ion. However, the emission features
of the xerogel-bonded Ln complex (Ln = Er, Nd, Yb, Sm)
materials are different from those found for the corresponding
Ln(pfnp)3phen complexes, which may be due to the Ln3+ ions
being coordinated to the modified phen ligand and the different
local environments of the Ln3+ ions in the xerogel materials.
Furthermore, time-resolved measurements on the Ln(pfnp)3phen
complexes and the xerogel-bonded Ln complex (Ln = Er, Nd,
Yb, Sm) materials were carried out at room temperature (RT) by
using an excitation wavelength of 355 nm and monitored around
the most intense emission line of their corresponding emission
Fig. 6 Excitation (EX, lem = 1059 nm) and emission (EM, lex = 386 nm)
spectra of the xerogel-bonded Nd complex.
Fig. 7 Excitation (EX, lem = 978 nm) and emission (EM, lex = 376 nm)
spectra of the xerogel-bonded Yb complex.
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spectra. The luminescence decays of the Ln(pfnp)3phen complexes
are fitted well by a single-exponential function, confirming that
the Ln3+ ions occupy the same average local environment within
with anhydrous ethanol. The concentration of the Ln3+ (Ln = Er,
Nd, Yb, Sm, Gd) ions was determined by titration with a standard
ethylenediaminetetraacetic acid (EDTA) aqueous solution.
Fourier-transform infrared (FTIR) spectra were measured
within the 4000–400 cm-1 wavenumber range using a Perkin-
Elmer model 580B IR spectrophotometer with the KBr pellet
technique and operating in the transmittance mode. The diffuse
reflectance (DR) measurements were recorded on a SHIMADZU
UV-3600 spectrophotometer. NMR spectra were recorded on a
Bruker DRX 400 spectrometer. Element analyses were performed
using a Vario Element Analyzer. Field-emission scanning electron
microscopy (FE-SEM) images were obtained with a XL30 ESEM
FEG microscope. The luminescence excitation and emission
spectra were recorded on a HORIBA Jobin Yvon FluoroLog-
3 spectrofluorometer equipped with a 450 W Xe-lamp as an
excitation source and a liquid-nitrogen-cooled R5509-72 PMT
as a detector. The time-resolved measurements were performed
by using the third harmonic (355 nm) of a Spectra-physics
Nd:YAG laser with a 5 ns pulse width and 5 mJ of energy per
pulse as a source, and the NIR emission lines were dispersed
by the emission monochromator of the HORIBA Jobin Yvon
FluoroLog-3 equipped with a liquid-nitrogen-cooled R5509-72
PMT, and the data were analysed with a LeCroy WaveRunner 6100
1GHz Oscilloscope. The luminescence lifetimes were calculated
with the Origin 7.0 software package. All the above measurements
were performed at RT. The low-temperature phosphorescence
spectrum of the Gd(pfnp)3(H2O)2 complex was measured on a
Hitachi F-4500 spectrophotometer at liquid nitrogen temperature
(77 K).
4
4
each sample.34 The corresponding lifetimes of the I13/2 → I15/2
4
transition of Er(pfnp)3phen, the F3/2
→
4I11/2 transition of
2
2
Nd(pfnp)3phen, the F5/2 → F7/2 transition of Yb(pfnp)3phen,
and the 4G5/2 → F5/2 transition of Sm(pfnp)3phen are 1.78, 0.99,
6
11.01, 51.84 ms, respectively. The decay curve for the xerogel-
bonded Er complex is singly exponential, and the lifetime of
4
4
the I13/2 → I15/2 transition is 1.24 ms. The decay curves for the
xerogel-bonded Ln complex (Ln = Nd, Yb, Sm) materials are
fitted by double-exponential functions. The lifetimes of the 4F3/2
→
4I11/2 transition of the xerogel-bonded Nd complex are 0.34 ms
2
2
(48.02%) and 85 ns (51.98%). The lifetimes of the F5/2 → F7/2
transition of the xerogel-bonded Yb complex are 0.85 ms (32.07%)
6
and 4.87 ms (67.93%). The lifetimes of the 4G5/2 → F5/2 transition
of the xerogel-bonded Sm complex are 4.59 ms (36.96%) and 20 ms
(63.04%).
Conclusions
Based on a synthesized b-diketone (Hpfnp), Ln(pfnp)3phen
complexes and the transparent homogeneous NIR-luminescent
xerogels covalently bonded with ternary lanthanide (Ln = Er, Nd,
Yb, Sm) complexes have successfully been prepared by using an
in situ synthesis technique. The structures of the Ln(pfnp)3phen
complexes have been determined by single-crystal X-ray diffrac-
tion, and the properties of the Ln(pfnp)3phen complexes and
the corresponding xerogels were also investigated. The complexes
have successfully been synthesized in situ in the xerogels. After
ligand-mediated excitation, all of the emission spectra of the
Ln(pfnp)3phen complexes and the corresponding xerogels show
the characteristic NIR luminescence of the corresponding Ln3+
ions through intramolecular energy transfer from the ligands to
the Ln3+ ions. Of importance here is the observation of efficient
NIR luminescence of the Sm3+ ion, which has rarely been explored.
The excellent NIR properties provide access to NIR-luminescent
xerogel materials in optical applications, such as in amplifiers,
lasers, and in vivo luminescent probes.
Syntheses
Synthesis of the ligand Hpfnp. The synthesis procedure of
the ligand Hpfnp was similar to that described by our group
for the ligand 4,4,5,5,6,6,6-heptafluoro-1-(2-naphthyl)hexane-1,3-
dione (Hhfnh).35 A modified method of a typical Claisen con-
densation procedure was used as follows. Sodium metal thread
(2.76 g, 0.12 mol) was added to dry anhydrous ethanol (150 mL).
The reaction mixture was stirred for 30 min at RT, after which 2¢-
acetonaphthone (17.02 g, 0.1 mol) and ethyl pentafluorobutyrate
(18.0 mL, 0.12 mol) were added. The mixture was then stirred for
48 h at RT. The resulting mixture was acidified to pH = 2–3 using
hydrochloric acid (2 M solution), and the solvent was removed
under reduced pressure at 70 ◦C. Dried CH2Cl2 was added to
the resulting mixture and stirred for 10 min. The mixture was
filtered and washed with dried CH2Cl2 several times. The filtrate
was collected, and the solvent was removed under reduced pressure
to obtain a maroon oil, which was purified by chromatography on
a silica gel column with hexane as the eluent to get a maroon liquid
(25.91 g, 0.082 mol, yield of 82% based on 2¢-acetonaphthone). 1H
NMR (DMSO, 400 MHz): d, 15.20 (broad, 1H), 8.47 (s, 1H), 8.04
(d, 1H, J = 8.8, 1.6 Hz), 8.01 (d, 1H, J = 8.8 Hz), 8.00 (d, 1H, J =
8.0 Hz), 7.91–7.87 (m, 2H), 7.61 (td, 1H, J = 8.0, 1.6 Hz), 7.56 (td,
1H, J = 8.0, 1.6 Hz), 7.26 (s, 1H). 19F NMR (DMSO, 400 MHz):
d, -46.21 (s, CF2), -5.57 (s, CF3). Anal. calcd for C15H9F5O2: C,
56.97%; H, 2.87%. Found: C, 56.87%; H, 2.79%.
Experimental section
Materials and apparatus
Tetraethoxysilane (TEOS, Aldrich), 3-(triethoxysilyl)propyl iso-
cyanate (TCI), ethyl pentafluoropropionate (98%, Aldrich), 2¢-
acetonaphthone (98%, Acros), fuming nitric acid and anhydrous
ethanol were used as received. Sodium metal (≥98%, AR) and
1,10-phenanthroline monohydrate (phen·H2O, 99%, AR) were
bought from Beijing Fine Chemical Co. (Beijing, China). The
solvent chloroform (CHCl3) was used after desiccation with
anhydrous calcium chloride. Erbium oxide (Er2O3, 99.99%),
neodymium oxide (Nd2O3, 99.99%), ytterbium oxide (Yb2O3,
99.99%), samarium oxide (Sm2O3, 99.99%), and gadolinium oxide
(Gd2O3, 99.99%) were purchased from Yue Long Chemical Plant
(Shanghai, China). LnCl3 (Ln = Er, Nd, Yb, Sm, Gd) ethanol
solutions were prepared as follows: Ln2O3 (Ln = Er, Nd, Yb, Sm,
Gd) was dissolved in concentrated hydrochloric acid (HCl), and
the HCl was removed by evaporation. The residue was dissolved
Synthesis of the near-infrared luminescent xerogel-bonded
ternary Ln complexes [Ln = Er, Nd, Yb, Sm]. The starting
reagent 5-amino-1,10-phenanthroline (denoted as phen-NH2) was
2412 | Dalton Trans., 2009, 2406–2414
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prepared according to the procedure described in the literature.36
phen-Si was synthesized by the reaction of phen-NH2 and 3-
(triethoxysilyl)propyl isocyanate in CHCl3.37 Then phen-Si was
dissolved in ethanol, and TEOS and deionized water (acidified
with HCl, pH = 2) were added under stirring. An appropriate
amount of Hpfnp and LnCl3 (Ln = Er, Nd, Yb, Sm) ethanol
solution were introduced into the starting solution consecutively.
The molar composition of the original synthetic mixture was 0.01
phen-Si : 1.0 TEOS : 4.0 H2O : 0.01 Ln3+ : 0.03 Hpfnp. The
mixed solution was stirred for 4 h at RT to ensure homogeneous
mixing and a single phase was achieved, and then transferred into
a plastic container. The precursor solution converted to a wet gel
after several days of gelation at 45 ◦C and then was continuously
dried to obtain a transparent monolithic xerogel. The lanthanide
complexes were supposed to be synthesized in situ during the
corresponding sol to monolithic xerogel conversion accompanied
with the evaporation of HCl, respectively. Before the luminescence
measurements were made, the xerogel materials were dried under
vacuum for 24 h at 80 ◦C. The obtained xerogel materials were
denoted as xerogel-bonded Ln complexes (Ln = Er, Nd, Yb, Sm).
Synthesis of the Ln(pfnp)3phen (Ln = Er, Nd, Yb, Sm) complexes.
An appropriate amount of 1.0 mol L-1 sodium hydroxide solution
was added dropwise to the Hpfnp and phen ethanol solution
under stirring to adjust the pH value to approximately 8–9. The
LnCl3 ethanol solution was added dropwise into the above mixture
under stirring with the molar ratio of Ln3+–Hpfnp–phen being
1 : 3 : 1. The mixture was heated under reflux for 6 h and then
cooled to RT. After an appropriate amount of water was added,
the precipitates were collected by filtration, washed with water
and ethanol, and dried overnight at 70 ◦C under vacuum. The
Ln(pfnp)3phen complexes were recrystallized from ethanol. Block
crystals suitable for X-ray single-crystal structural determination
were grown from the mother liquor at RT.
Ln(pfnp)3phen (Ln = Er, Nd, Yb, Sm) complexes were refined
anisotropically. The hydrogen atoms were included using a riding
model. All calculations were performed using the SHELXL-
97 crystallographic software package. Crystallographic data and
structural refinements for the Ln(pfnp)3phen (Ln = Er, Nd, Yb,
Sm) complexes are summarized in Table 1. CCDC reference
numbers 699049 [for Er(pfnp)3phen], 699050 [Nd(pfnp)3phen],
699052 [Yb(pfnp)3phen], and 699051 [Sm(pfnp)3phen].†
Acknowledgements
The authors are grateful for financial aid from the National
Natural Science Foundation of China (Grant No. 20631040, and
20771099) and the MOST of China (Grant No. 2006CB601103,
2006DFA42610).
Notes and references
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Elemental analysis. For Er(pfnp)3phen (pale pink), anal. calcd:
C, 52.94%; H, 2.49%; N, 2.17%. Found: C, 52.88%; H, 2.47%;
N, 2.12%. For Nd(pfnp)3phen (blue), anal. calcd: C, 53.90%; H,
2.54%; N, 2.21%. Found: C, 53.86%; H, 2.51%; N, 2.17%. For
Yb(pfnp)3phen (colorless), anal. calcd: C, 52.71%; H, 2.48%; N,
2.16%. Found: C, 52.68%; H, 2.44%; N, 2.13%. For Sm(pfnp)3phen
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