Photofunctional Eu3+/Tb3+ hybrids through sulfoxide linkages: Coordination bonds construction, characterization and luminescence

Article abstract of DOI:10.1039/c0dt01793k

New kinds of organic-inorganic hybrid materials consisting of rare earth (Eu³⁺, Tb³⁺) complexes covalently bonded to a silica-based network have been obtained by a sol-gel approach. Three novel versatile molecular building blocks con

Full text of DOI:10.1039/c0dt01793k

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PAPER
Photofunctional Eu3+/Tb3+ hybrids through sulfoxide linkages: coordination
bonds construction, characterization and luminescence†
Lei Guo, Bing Yan* and Jin-Liang Liu
Received 18th December 2010, Accepted 1st March 2011
DOI: 10.1039/c0dt01793k
New kinds of organic–inorganic hybrid materials consisting of rare earth (Eu3+, Tb3+) complexes
covalently bonded to a silica-based network have been obtained by a sol–gel approach. Three novel
versatile molecular building blocks containing sulfoxide organic units have been synthesized by
methylene modification reaction, which are used as the ligands of rare earth ions and also as siloxane
network precursors. The obtained hybrids are characterized by chemical analysis and spectroscopic
methods such as FTIR and UV; XRD and SEM. Photoluminescence measurements on the prepared
hybrids were performed showing the intra-4fn emission in the visible (Eu3+, Tb3+) region and in all the
cases being sensitized by the sulfoxide ligands. The emission quantum efficiency and the Judd–Ofelt
intensity parameters of Eu3+ hybrid materials were also investigated in detail.
emitting process. In analogy to light-harvesting molecules found
Introduction
in natural photosynthetic systems, this process is called rare
earth luminescence sensitization, or antenna effect,2 and such
Rare earth ions are well known as an important class of emitting
centers in luminescent materials for their sharp and intense
complexes may be seen as efficient light-conversion molecular
devices (LCMDs).3 The b-diketonate complexes are the most
emission bands based on f–f electronic transitions and a wide
range of lifetimes suitable for various applications, which are due
popular and the most intensively investigated luminescent rare
earth coordination complexes.4 Their popularity is partially
because many different b-diketones are commercially available
and the synthesis of the corresponding rare earth complexes is
relatively easy, but also because of their excellent luminescence
properties.
to the fact that the emitting excited state and the ground state
have the same fn electronic configuration, and the shielding of the
4f electrons from interaction with their surrounding by the filled
5s2 and 5p6 orbitals.1 However, the direct RE3+ photoexcitation
is not very efficient, limiting the light output. This can be
improved with the consequent increase of the RE3+ luminescence
intensity through the design of rare earth complexes, in which
the ligands incorporate organic chromophores strongly bonded
to the 4f metal center. RE3+ ions form complexes with various
organic ligands, such as aromatic carboxylic acids, b-diketonates,
calixarenes and heterocyclic ligands, emitting efficiently in the
near-UV (Ce3+ and Gd3+), visible (Sm3+, Eu3+, Tb3+ and Dy3+), and
NIR (Nd3+, Er3+, Tm3+ and Yb3+) spectral regions. These organic
ligands typically present effective absorption cross-sections 104–
105 times higher and over a much broader spectral range than
the RE3+ corresponding ones, and the energy they absorb can be
transferred to nearby RE3+ ions, which in turn undergo a radiative
During the past years, there has been growing interest in the
development of new rare earth-based luminescent hybrid materials
that combine good mechanical, thermal, and chemical stability in
air with high room-temperature emission quantum yields.5 These
organic–inorganic hybrids processed by the sol–gel method are
considered to be good candidates to assemble such properties,
lending themselves to applications in displays and lighting devices.
The concept of organic–inorganic hybrids emerged in the last three
decades, with the advent of “soft” inorganic chemistry processes, in
particular the sol–gel route. However, the conventional and simple
doping method is unable to solve the problem of the quenching
effect of luminescent centers because only weak interactions
(such as hydrogen bonding, van der Waals forces, or weak static
effects) exist between organic and inorganic moieties. So another
appealing method has emerged which concerns covalently bonded
hybrids. The covalent bonding of rare earth complexes to the
inorganic matrix reduces the risk of leaching the complex out
from the matrix, and in general higher doping concentrations
can be reached such that the luminescent hybrid materials have
a better homogeneity.6 The design of new luminescent hybrid
systems based on covalently bonded rare earth complexes is at
present a very active research field.7
Department of Chemistry, Tongji University, Shanghai 200092, China.
E-mail: byan@tongji.edu.cn
† Electronic supplementary information (ESI) available: Figures of in-
frared spectra of europium hybrid materials, phosphorescence spectra of
PPS and precursor PPSSi, ultraviolet-visible diffuse reflection absorption
spectra of the europium hybrid materials, and the emission decay curves
and the fit curves of the hybrid materials. Tables of assignments of
the main infrared absorption bands for organic ligands, silyated pre-
cursors and corresponding hybrid materials, and triple state energies of
organic ligands and their energy transfer with rare earth ions. See DOI:
10.1039/c0dt01793k
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In the present work we aim at gaining a deeper understanding
of the RE3+-based covalently bonded hybrids, especially the con-
struction of different functional bridges as the molecular building
blocks, which can display dual functions of both coordinating
to rare earth ions and allowing sol–gel processes to constitute
a covalent Si–O network.8 In this article, as a continuation of
our efforts to investigate the effects of new functional bridge
molecular and photoluminescent properties of such rare earth
hybrids, we report herein three novel sulfoxide functional bridge
ligands and the properties of the corresponding hybrids. Such
sulfoxide ligands are attractive because their flexibility and analogy
to the structure of the b-diketonate may offer good chelating
groups to coordinate with the rare earth ions, and can effectively
sensitize the luminescence of rare earth ions. This work provides an
effective pathway towards the design and construction of the novel
hybrids, and enriches the family of functional bridge structures by
incorporation of the different organic sulfoxide ligands.
Synthesis
of
bis(benzoylmethyl)
sulfoxide
(BBMS).
Bis(benzoylmethyl) sulfoxide was synthesized according to
the method of Ref. 10. Yield: 90%. mp: 110–111 ◦C. Anal. Calcd.
1
For C16H14O3S (%): C 67.1, H 4.93; Found: C 66.8, H 4.68. H
NMR (CDCl3, 400 MHz): d 4.43 (d, 2H, CH2), 4.77 (d, 2H,
CH2), 7.50 (t, 4H, m-C6H5), 7.63 (t, 2H, p-C6H5), 7.99 (d, 4H,
o-C6H5). 13C NMR (CDC13, 100 MHz): d 59.3 (CH2), 128.8
(m-C6H5), 128.9 (o-C6H5), 134.4 (p-C6H5), 136.1 (CC O), 192.4
(C O).
Synthesis of the precursors MSAPSi, PPSSi and BBMSSi. To
a solution of MSAP (4 mmol) in 10 mL of pyridine was added
8 mmol of NaH. Two hours later, TESPIC (8 mmol) dissolved
in 10 mL of pyridine was added dropwise with stirring, then the
mixture was refluxed at 70 ◦C for approximately 24 h under argon
in a covered flask. The solvent was removed in vacuum, and the
residue was obtained in a 90% yield.11: Anal. Calcd. For MSAPSi
(C29H52N2O10SSi2) (%): C 51.5, H 7.74, N 4.10; Found: C 50.8, H
7.48, N 4.02. 1H NMR (CDCl3, 400 MHz): d 0.61 (t, 4H, CH2Si),
1.21 (m, 18H, CH3), 1.60 (m, 4H, CH2CH2CH2), 3.13 (m, 4H,
NHCH2), 3.80 (m, 12H, SiOCH2), 7.45 (t, 2H, m-C6H5), 7.64 (t,
1H, p-C6H5), 7.95 (d, 2H, o-C6H5), 8.58 (d, 2H, NH). 13C NMR
(CDC13, 100 MHz): d 7.3 (CH2Si), 18.0 (CH2CH2CH2), 23.3
(CH2CH3), 42.7 (NHCH2), 58.2 (CH2CH3), 123.4 (CS O), 128.3
(m-C6H5), 128.5 (o-C6H5), 128.6 (p-C6H5), 135.7 (CC O), 150.0
(C O). The other precursor PPSSi and BBMSSi were synthesized
by the same manner from the reaction between TESPIC with PPS
and BBMS. Anal. Calcd. For PPSSi (C34H54N2O10SSi2) (%): C
Experimental section
Materials
The purity of rare earth oxide exceeds 99.99%. Rare earth nitrates
hexahydrate were prepared by dissolving the corresponding oxides
in dilute nitric acid, then heating the solution appropriately. All
of the other reagents were analytically pure and solvents were
purified according to literature procedures.9
1
Synthetic procedures
55.3, H 7.36, N 3.81; Found: C 55.0, H 7.13, N 3.59. H NMR
(CDCl3, 400 MHz): d 0.63 (t, 4H, CH2Si), 1.21 (m, 18H, CH3),
1.85 (m, 4H, CH2CH2CH2), 3.11 (m, 4H, NHCH2), 3.73 (m, 12H,
SiOCH2), 7.29 (t, 4H, m-C6H5), 7.33 (t, 2H, p-C6H5), 7.52 (d,
4H, o-C6H5), 8.42 (d, 2H, NH). 13C NMR (CDC13, 100 MHz): d
7.5 (CH2Si), 17.3 (CH2CH2CH2), 23.0 (CH2CH3), 42.3 (NHCH2),
58.3 (CH2CH3), 119.2 (CS O), 124.3 (m-C6H5), 128.7 (o-C6H5),
130.6 (p-C6H5), 133.2 (CC O), 152.8 (C O). Anal. Calcd. For
BBMSSi (C56H98N4O19SSi4) (%): C 52.7, H 7.68, N 4.42; Found:
Synthesis of 2-(methylsulfinyl) acetophenone (MSAP). To
slurry of 3.7 g potassium tert-butoxide in 20 mL of dimethyl
sulfoxide (DMSO) was added 5.0 g of ethyl benzoate dropwise. The
◦
mixture was stirred vigorously at 70 C for 2 h and then poured
into ice water containing excess HCl. Product was extracted into
chloroform and recrystallized from ether-CHC13. Yield: 65%. mp:
84–85 ◦C. Anal. Calcd. For C9H10O2S (%): C 59.3, H 5.53; Found:
C 59.1, H 5.68. 1H NMR (CDCl3, 400 MHz): d 2.77 (s, 3H, CH3),
4.32 (m, 2H, CH2), 7.49 (t, 2H, m-C6H5), 7.62 (t, 1H, p-C6H5),
7.97 (d, 2H, o-C6H5). 13C NMR (CDC13, 100 MHz): d 39.7 (CH3),
62.3 (CH2), 129.0 (m-C6H5), 129.1 (o-C6H5), 134.6 (p-C6H5), 136.1
(CC O), 192.1 (C O).
1
C 52.5, H 7.40, N 4.25. H NMR (CDCl3, 400 MHz): d 0.62 (t,
8H, CH2Si), 1.21 (t, 36H, CH3), 1.62 (m, 8H, CH2CH2CH2), 3.16
(m, 8H, NHCH2), 3.79 (q, 24H, SiOCH2), 7.43 (t, 4H, m-C6H5),
7.66 (t, 2H, p-C6H5), 7.93(d, 4H, o-C6H5), 8.62 (t, 4H, NH). 13C
NMR (CDC13, 100 MHz) d 7.6 (CH2Si), 18.3 (CH2CH2CH2),
23.6 (CH2CH3), 43.0 (NHCH2), 58.5 (CH2CH3), 93.8 (CS O),
128.3 (m-C6H5), 128.6 (o-C6H5), 133.1 (p-C6H5), 136.0 (CC O),
149.9 (C O).
Synthesis of phenylphenacylsulfoxide (PPS). The thiophenol
was added to an ethanol solution of sodium hydroxide and
stirred at room temperature for one hour. Then the amount
of a-bromo acetophenone was added, and the mixture was
stirred overnight at room temperature. The mixture was poured
into water and extracted with ether and then, upon ethanol
recrystallization, white crystals were obtained. Yield: 80%. mp: 53
◦C. Phenylphenacyl sulfide was dissolved in acetone, and then 30%
hydrogen peroxide was added to it at once. The mixture was stirred
continuously at room temperature for 24 h. After the reaction
stopped, the mixture was extracted with ether. Then a white solid
was precipitated, filtered and dried in vacuum. yield: 80%. mp: 70–
71 ◦C. Anal. Calcd. For C14H12O2S (%): C 68.8, H 4.95; Found: C
68.8, H 5.10. 1H NMR (CDCl3, 400 MHz): d 4.34 (d, 2H, CH2),
7.48 (t, 4H, m-C6H5), 7.58 (t, 2H, p-C6H5), 7.89 (d, 4H, o-C6H5).
13C NMR (CDC13, 100 MHz): d 65.8 (CH2), 124.1 (m-C6H5),
129.1 (o-C6H5), 131.4 (p-C6H5), 133.9 (CC O), 191.0 (C O).
Synthesis of the hybrid materials. The precursor was dissolved
in N,N-dimethyl formamide (DMF) solvent, and a stoichiometric
amount of RE(NO3)3·6H2O was added into the solution with stir-
ring. After three hours, a stoichiometric amount of TEOS and H2O
was added into the mixed solution after the coordination reaction
was completed between precursors and rare earth ions. Then one
drop of dilute hydrochloric acid was added to promote hydrolysis.
The molar ratio of RE(NO3)3·6H2O : precursor : TEOS : H2O was
1 : 3 : 6 : 24. The mixture was agitated magnetically to achieve a
◦
single phase after two days, and then it was aged at 70 C until
the onset of gelation in about one week. The gels were collected as
monolithic bulks and were ground into powdered materials for the
photophysical studies. The obtained hybrid materials are denoted
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as MSAPSi–RE, PPSSi–RE and BBMSSi–RE (RE = Eu3+ and
Tb3+), respectively.
Physical measurements
Melting points were measured on a XT4-100XA apparatus.
1H NMR and 13C NMR spectra were measured by a Bruker
AVANCE-400 spectrometer with tetramethylsilane (TMS) as
internal reference (CDCl3 as solvent). Fourier transform infrared
spectra were recorded on KBr disk using Nicolet model 5SXC
spectrometer in 4000– 400 cm-1 region. The ultraviolet absorp-
tion spectra (ethanol solution) and the ultraviolet-visible diffuse
reflection spectra of the powder samples were recorded by an
Agilent 8453 spectrophotometer and a BWS003 spectrophotome-
ter, respectively. The X-ray diffraction (XRD) measurements were
carried out on powdered samples via a BRUKER D8 diffrac-
tometer (40 mA/40kV), using monochromated Cu-Ka1 radiation
Fig. 1 Schemes of synthesis of the functional sulfoxide molecular bridges
and the predicted structure of the hybrid materials.
◦
◦
˚
(l = 1.54 A) over the 2q range of 10 to 70 . Thermogravimetric
analysis (TG) and differential scanning calorimetry (DSC) were
performed on a NETZSCH STA 449C with a heating rate of 10 ◦C
min-1 under a nitrogen atmosphere. Scanning electron microscope
(SEM) images were obtained with a Philips XL-30. Phosphores-
cent spectra (ethanol solution) of these ligands and precursors
were determined with a Perkin-Elmer LS-55 spectrophotometer
at 77 K. Luminescence excitation and emission spectra were
obtained on a SHIMADZU RF-5301 spectrofluorimeter at
room temperature. The slit width of excitation and emission
spectra were 3.0 nm. Luminescent lifetimes were determined on
an Edinburgh Instruments FLS 920 phosphorimeter, using a
450 W xenon lamp as the excitation source (pulse width, 3 ms)
with high dispersion, low stray light excitation and emission
300 mm monochromators of the Czerny–Turner Configuration
and a detector of single photon counting photomultiplier. The
excitation and emission wavelength for time decay curves were se-
lected from the corresponding luminescent spectra. The excitation
wavelength was selected by means of Andover Corporation optical
bandpass filters (10 2 nm).
Fig. 2 Infrared spectra of organic sulfoxide ligands and their correspond-
ing silylated precursors.
infrared absorption bands are shown in Table S1 (ESI†). For the
IR spectra of the precursors, the vibration absorption band of the
isocyanate group cannot be observed, indicating the completion
of the modification reactions of sulfoxide and TESPIC.13 The
vibrations of -CH2- at around 2950 cm-1 are taken place by a
strong broadband located at around 2975–2882 cm-1, which can be
designated to the methylene groups of TESPIC. At the same time,
the occurrence of peak corresponding to the stretching vibration
of grafted -NH group is also evidenced by the band located at
about 3340 cm-1.14
In addition, the complexation of RE3+ with the organic sulfoxide
ligands in all of the hybrids can be shown by infrared spectra
(Figure S1, ESI†). Compared with the IR spectra of the precursors,
the IR bands for n(C O) vibrations of hybrids are shifted to
lower frequency. This is ascribed to the complexation of the RE3+
ion with the oxygen atom of the C O. However, as most of the
peaks around 1000 cm-1 are rather broad, the assignment of the
Results and discussion
Chemical Characterizations of the Functional Sulfoxide Molecular
Bridges and Hybrid Materials
The synthetic scheme of the hybrid materials via the functional
sulfoxide molecular bridges covalently grafted to the inorganic
Si–O networks is shown in Fig. 1. The precursors are prepared
by the extraction of the methylene in sulfoxide ligands with the
1
electrophilic reagent 3-TESPIC.12 And H NMR and 13C NMR
spectra relative to the precursors are in full agreement with the
proposed structures. However, the exact structure of this kind
of non-crystalline hybrid materials is very difficult to prove and
it is hardly possible to solve the coordination behavior of rare
earth ions. But it can be predicted the main composition and
coordination effect of them according to the principle of rare
earth coordination chemistry and the functional groups of organic
ligands. These predictions have also been confirmed by infrared
spectra.
S O group stretching modes can not be proved. Considering
the structure of these sulfoxide ligands and the principle of rare
earth coordination chemistry, we can infer that oxygen atom of
the S O group also takes part in the rare earth ions coordinative
environment.15 Besides, the spectra of hybrid materials show a
peak at 1384 cm-1 in the NO2 stretching region (1650–1250 cm-1),
suggesting the presence of nitrate ions. Furthermore, the spectra
of hybrid materials are dominated by the broad absorption band
at about 1040 cm-1 (n(Si–O–Si)), which indicated the formation
of siloxane bonds. The n(Si–C) vibration located in the 1200 cm-1
in the IR spectra of hybrid materials is consistent with the fact
Fig. 2 shows the IR spectra of three sulfoxide ligands and
their corresponding precursors, and the assignments of the main
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that no (Si–C) bond cleavage occurred during the hydrolysis and
condensation reactions.16
organic ligand or free RE nitrate, which is an indication of the
formation of true covalently-bonded molecular hybrid materials.
The thermal behavior of the material MSAPSi–Eu is examined
by TG and DSC (Fig. 5). A typical TG trace showed that the
obtained material has a good thermal stability, and there are
no weight losses up to 150 ◦C, showing a moderately improved
thermal stability compared with organic ligand MSAP whose
melting point is only 84 ◦C. The material decomposes in three
main steps between 25 ◦C and 1000 ◦C, leaving a residual mass of
53.58%. From the beginning to 150 ◦C, is possibly associated with
the loss of the coordinated H2O.19 The mass loss of 1.37% for this
thermal event is revealed by the TGA curve, and it is calculated that
there are approximately two molecular H2O in one complex. The
last two steps occurring in the range of 200–1000 ◦C are attributed
to the thermal degradation of the organosilicate framework,
involving Si–C, C–C, and C–N bond cleavage.20 Correspondingly,
the curves of DSC show the endothermic and exothermic peak
corresponding with the curve of TGA. At 196.8 ◦C, 463.6 ◦C,
there are obviously endothermic processes which are attributed
to the decomposition of precursor. Generally, the higher thermal
stability of the organic species in the hybrid is consistent with
the successful encapsulation of the ligand within the rigid silica
matrix.
The UV-visible absorption spectra (Fig. 3) of the organic
sulfoxide ligands and precursors were measured from their ethanol
solution. The accomplishment of the grafting reaction between
the ligand and the TESPIC can also be proved by the ultraviolet
spectra. From the spectra, the peak at about 250 nm is assigned to
the p–p* electronic transition of the aromatic rings in the sulfoxide
ligands, and there is a red shift between the organic ligands and the
precursors. This indicates that the conjugating system is influenced
by the introduction of the carbonyl group in TESPIC.17
Fig. 3 Ultraviolet absorption spectra of organic sulfoxide ligands and
their corresponding silylated precursors.
As determinedby XRDpatterns (Fig. 4)of the obtainedhybrids,
all the materials with 10◦ £ 2q £ 70◦ are totally amorphous. All
diffraction curves exhibit a single broad peak at about 22◦, which
is known as ‘amorphous hump’. According to the literature,18 for
the curve of hybrid materials with Si–O network, we can calculate
that the structural unit distance by Bragg’s law:
(1)
2dsinq = nl
Fig. 5 Thermogravimetric (TG) and differential scanning calorimetry
(DSC) curves of hybrid materials MSAPSi–Eu.
where d is the distance in the real space between the structural
units, l the wavelength of X-ray radiation (1.54 A), n is the
˚
diffraction series. Using the above equation, it is estimated
The scanning electron micrographs for the hybrid materials
prove that a homogeneous system is obtained. Because the rare
earth complexes are covalently linked into inorganic networks
by the strong Si–C bonds and a complicated huge molecule is
formed, they are composed quite uniformly via a self-assembly
process during the hydrolysis/polycondensation process so that
the inorganic and the organic phases can exhibit their distinct
properties together and no phase separation occurs.21 Fig. 6
shows the micrographs for (A) MSAPSi–Eu, (B) MSAPSi–
Tb, (C) PPSSi–Eu, (D) PPSSi–Tb, (E) BBMSSi–Eu, and (F)
BBMSSi–Tb, respectively. On the surface of those hybrids many
similar linear stripes can be observed, which are mostly owing
to the sol–gel treatment. Different hybrids derived from different
sulfoxide linkages and rare earth ions have little influence on the
micromorphology in this hybrid system. Besides, there are some
branches at the end of every linear stripe and the stripes will
continue to grow according to the directions of these branches. In
the formation of the whole hybrid systems, there exist two main
reaction processes, one is the cohydrolysis and copolycondensation
reactions between sulfoxide linkage organically modified silica
˚
that the structural unit distance is approximately 4.04 A. This
˚
value is similar to those reported for vitreous SiO2, i.e., 4.2 A.
Therefore, these materials obtained are amorphous and free of
any crystalline regions. In addition, none of the hybrid materials
contains measurable amounts of phases corresponding to the pure
Fig. 4 Selected XRD patterns of europium hybrid materials.
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kET = K Pda exp(-2 Rda/L) = K¢Fd(E) Ea(E) dE
(3)
and the other is the inverse energy transition by thermal deactiva-
tion mechanism:22
k(T) = A exp(-DE/RT)
(4)
Pda is the transition probability by the resonant exchange
interaction, and kET is the rate constant of the intramolecular
energy transfer. 2pZ2/R is a constant relative to the specific mutual
distance between the central RE3+ ion and its coordinated atoms
(oxygen or nitrogen). Fd (E) and Ea (E) are the experimental lumi-
nescence spectrum of energy donor (ligands) and the experimental
absorption spectrum of energy acceptor (RE3+), respectively, so
both of them represent the overlap spectrum of RE3+ cations. Rda
is the intramolecular distance between donor atoms and acceptor
atoms, and L is the van der Waals radius. Both Rda and L may
be considered to be constant in intramolecular transfer processes,
so kET is proportional to the overlap of Fd (E) and Ea (E). With
the decrease of the energy difference between the triplet state of
organic ligands and RE3+ excited levels, the overlap of Fd (E) and
Ea (E) is increased. On the other hand, the activation energy DE
is equal to the energy difference DE (Tr-RE3+) whereas from the
formula the inverse energy transfer rate constant k(T) increased
with decreasing DE (Tr-RE3+). Energy is usually transferred onto
RE3+ levels with higher energy than the emissive level; otherwise,
back energy transfer occurs, resulting in low quantum yields
and short, temperature-dependent lifetimes.6,24 From the Table S2
(ESI†), the triplet state energy of the precursors with different
sulfoxide ligands are similar, and we estimate the efficiency of the
energy transfer for all the hybrids of both series are similar too,
with respect to the value of the quantum efficiency. It also can
be deduced that sulfoxide ligands can sensitize the Eu3+ and Tb3+
ions.
UV-visible diffuse reflection absorption spectra are performed
on powdered hybrid materials. Figure S3 (ESI†) shows the
spectra of Eu3+ hybrid materials. It exhibits a broad absorption
band in the 200–500 nm region, which belongs to the whole
near-UV range. This absorption band corresponds to transition
from the ground state of the organic ligand to the first excited
state. It is more specifically attributed to p–p* transition of the
aromatic ring. Besides, it is also observed that the broad bands
overlap with the luminescence excitation spectra completely. It
can be predicted that the energy match between organic ligand
and RE3+ is appropriate so that the organic ligand can absorb
energy in the ultraviolet–visible range to transfer the energy to
the corresponding hybrid materials. This speculation has been
confirmed by the phosphorescence spectra and luminescence
spectra of these materials.
Fig. 6 Scanning electron micrograph (SEM) images for the hybrids:
(A) MSAPSi–Eu, (B) MSAPSi–Tb, (C) PPSSi–Eu, (D) PPSSi–Tb, (E)
BBMSSi–Eu, and (F) BBMSSi–Tb.
and TEOS, the other is the coordination reaction of rare earth
ions and ligands (sulfoxide linkage organically modified Si–O
network). The competition between the two reactions jointly
determines the microstructure of the final hybrid materials.
Certainly, the tendency to form a polymeric Si–O–Si network in
the cohydrolysis and copolycondensation reactions of sulfoxide
linkage organically modified silica and TEOS become the primary
tendency.
Photoluminescence studies
The low-temperature phosphorescence spectra of organic sulfox-
ide ligand PPS and precursor PPSSi are recorded in Figure S2
(ESI†). The lowest triplet state energy levels and energy differences
(DE) between the triplet state energy levels and the resonant
emitting energy levels of the central RE3+ (RE = Eu, Tb) are shown
in Table S2 (ESI†). From the Figure S2, a broad phosphorescence
band, which corresponds to the triplet state emission of the ligand
and precursor, is observed and the blue shift occurs between PPS
and PPSSi. According to the energy transfer and intramolecular
energy mechanism,19 the intramolecular energy transfer efficiency
from organic ligands to the central RE3+ is the most important
factor to determine the luminescence properties of rare earth
complexes. This process is referred to as luminescence sensitization
or antenna effect and is quite intricate since several mechanisms
may be involved:22 One is from the triplet state energy of organic
ligands to the resonant energy level by Dexter’s resonant exchange
interaction theory:23
The luminescence properties of the hybrids have been in-
vestigated at room temperature. The organic–inorganic hybrids
containing rare earth ions give out a bright red (for Eu3+) or
green (for Tb3+) emission when irradiated under a UV lamp. The
excitation and emission spectra of the obtained hybrids are shown
in Fig. 7 and 8, respectively. For the europium hybrid materials,
the excitation spectrum exhibits a broad band ranging from 250 to
400 nm and no f–f transitions could be observed. The broad band
can be mainly ascribed to transitions involving the p–p* states
of the chelate ring of the organic ligand.23 The higher relative
ꢀ
Pda = (2pZ2/R) Fd(E) Ea(E) dE
(2)
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Table 1 Lifetime (t, ms), quantum efficiency (h), radiative (Ar, s-1) and
nonradiative (Anr, s-1) transition probabilities of the 5D0 level and number
of water molecules coordinated to the Eu3+ ion (nw) for europium hybrid
materials
Hybrids
MSAPSi–Eu
PPSSi–Eu
BBMSSi–Eu
t (ms)
386.0
3.45
252.4
2338.3
11.0
~2
400.8
3.88
276.2
2218.8
11.1
~2
572.4
2.43
220.9
1526.1
12.6
~1.5
3.66
I
02/I01
Ar (s-1)
Anr (s-1)
h (%)
nw
X2 (¥ 10-20 cm2)
X4 (¥ 10-20 cm2)
5.21
0.35
5.85
0.35
0.61
Fig. 7 Excitation and emission spectra of the europium hybrids.
by a single exponential function: ln[S(t)/S0] = -k1t = -t/t, which
corroborates that all the rare earth ions occupy the same average
local environment within each of the hybrids. From the emission
spectra, it can be suggested that there exist weak emission bands
for the residual emission of the aromatic groups of organically
modified silane host, so we have tried fitting the curves to a double
exponential decay, but there is no improvement in fit when a double
exponential is applied. The resulting lifetime data of europium
hybrids are given in Table 1. For the Tb3+-containing materials, the
obtained lifetime values are 334.9, 325.0, 437.4 ms for MSAPSi–
Tb, PPSSi–Tb, BBMSSi–Tb, respectively.
Further, the radiative (Ar) and nonradiative (Anr) transition
rates of the 5D0 level and the quantum efficiency (h) of the
europium hybrids, h = Ar/(Ar + Anr) are estimated by using a
procedure based on the room temperature emission spectrum and
5D0 lifetime values.26 The radiative contribution may be calculated
from the relative intensities of the 5D0 - 7F0–4 transitions (the 5D0–
7F5,6 branching ratios are neglected due to their poor relative
Fig. 8 Excitation and emission spectra of the terbium hybrids.
intensity of the ligand excited state reveals that the rare earth
sensitization process is the more efficient excitation path for the
hybrid material.25 The typical photoluminescence spectra of the
europium hybrids recorded by excitation at 366 nm are shown
in Fig. 7. All of the spectra show the characteristic narrow band
emission of Eu3+ ions, which indicates that the surrounding ligands
absorb and transfer energy efficiently to the chelated rare earth
ions. Five narrow emission peaks centered at 577, 590, 614, 649,
5
intensity with respect to that of the remaining D0–7F0–4 lines).
5
The D0–F1 transition does not depend on the local ligand field
and thus may be used as a reference for the whole spectrum,
in vacuo A01 ª 50 s-1.27 The values found for h, Ar and Anr of
Eu3+ hybrids are gathered in Table 1. From the Table 1, is can be
seen that similar h (5D0) values for different functional sulfoxide
precursor are observed. Comparing the h (5D0) values of the Eu3+
hybrids with those previously reported in our work, the quantum
efficiencies of functional sulfoxide hybrid systems are much higher
than the hybrid materials with functional calix[4]arene organic
ingredients (below 5%),27 or those with functionalized triazole
(below 4.5%).27 On the basis of these results, we may presume
that the functional sulfoxide precursor can sensitize luminescence
of rare earth ions and easily provide stable and efficient hybrid
materials.
5
and 695 nm, of Eu3+. D0–7FJ (J = 0–4) are observed, with the
5
hypersensitive transition D0–7F2 as the most prominent group.
This also indicated that the Eu3+ local environment is characterized
by a low symmetry group without an inversion centre. Further
evidence of a single Eu3+ local environment is given by the presence
5
of a single line for the non-degenerate D0–7F0 transition. Fig. 8
illustrates typical photoluminescence spectra of the terbium hybrid
materials. Similarly to the luminescent features of Eu3+ hybrids,
5
the excitation spectrum monitored with the Tb3+. The D4–7F5
(545 nm) emission exclusively consists of a broad excited band,
indicating an energy transfer from the ligands to the central Tb3+
ions. The emission spectrum obtained after excitation at 324 nm
contains four characteristic emission bands at 488 (5D4–7F6), 543
(5D4–7F5), 581 (5D4–7F4), and 619 (5D4–7F3) nm, with the 5D4–7F5
green emission as the most prominent peak.
Moreover, the variations in the h and Anr values may be
rationalized in terms of the number of water molecules coor-
dinated to the Eu3+ ions (nw) based on the empirical formula
nw = 1.11 ¥ (Anr-0.31).28 The results obtained for hybrids indicate
about 2 water molecules in the first Eu3+ coordination sphere,
which is in agreement with the result of the thermogravimetric
analysis. However, the coordinated water molecules produce severe
vibration of the hydroxyl group, resulting in the large nonradiative
transition and decreasing the luminescence efficiency.
In order to further quantify the different photoluminescence
features of the hybrids, the lifetime values of the 5D0 (Eu3+)
5
and D4 (Tb3+) excited states were measured on the basis of the
emission decay curves monitored within the more intense Eu3+
(5D0–7F2) and Tb3+ (5D4–7F5) transitions, respectively. Both of
the decay curves (Figure S4 and Figure S5†) can be well-fitted
From the emission spectra shown in Fig. 6, we have determined
the Judd–Ofelt Parameters for the Eu3+ hybrids. In the case
5
of Eu3+, the pure magnetic dipole transition D0–7F1 does not
4938 | Dalton Trans., 2011, 40, 4933–4940
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depend on the host environment (and can thus be used as an
intensity standard for comparison of spectra) while the 5D0–
7F2,4,6 induced electric dipole transitions depend only on the
corresponding doubly reduced matrix elements.29 So, the X2, X4
and X6 parameters for the 5D0–7F2,4,6 transitions, representing
the square of the charge displacement due to the induced
electric dipole transition, may be regarded as phenomenological
parameters that characterise the radiative transition probabilities
within the ground state configuration.
sulfoxide functional bridge ligands, prepared by the extraction
of the methylene in sulfoxide ligands with the electrophilic reagent
TESPIC, were used as the ligands of rare earth ions and also as
the siloxane network precursors. The chemical characterizations
and photoluminescence properties of the hybrids are studied.
The hybrids exhibit the characteristic luminescence of Eu3+ and
Tb3+ ions, in all the cases being sensitized by the sulfoxide
ligands. The distinction of the luminescent intensity parameters
for the obtained hybrids with different sulfoxide precursors is not
apparent. Furthermore, the current molecular design method can
be conveniently applied to other ligands and to different modified
alkoxysilanes, and the desired properties can be tailored by an
appropriate choice of the different precursors.
The spontaneous emission probability, A, of the transition is
related to its dipole strength according to eqn (5).
4
3
A = [64p g /3h(2J+1)][n(n2+2)2SED/9 + n3SMD
]
(5)
where g is the average transition energy in cm-1, h is Planck’s
constant (6.63 ¥ 10-27 erg s), 2J+1 is the degeneracy of the initial
state (1 for 5D0). SED and SMD are the electric and magnetic dipole
strengths (in esu2 cm2), respectively. n(n2+2)2/9 is the Lorentz local
field correction term and the average refractive index n = 1.5.
Acknowledgements
This work is supported by the National Natural Science Founda-
tion of China (20971100) and Program for New Century Excellent
Talents in University (NCET-08-0398).
5
7
The transitions from D0 to F0,3,5 (J = 0, 3, 5) are forbidden
both in magnetic and induced electric dipole schemes (SED and
7
SMD are zero). The transition to F1 (J = 1) is the only magnetic
dipole transition, and has no electric dipole contribution. As is
already mentioned, magnetic dipole transitions in rare earth ions
are practically independent of the ion’s surroundings, and can
be well calculated by theory (SMD = 9.6 ¥ 10-42 esu2 cm2 = 9.6 ¥
10-6 debye2).30 The remaining transitions (J = 2, 4, 6) are purely of
induced electric dipole nature. According to the Judd–Ofelt theory,
the strength of all induced dipole transitions (absorption and
emission) of a rare earth ion in a certain matrix can be calculated
on basis of only three parameters Xl, using eqn (6).
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