The orange emission of single crystals and sol gels based on Sm3+ chelates

Article abstract of DOI:10.1016/j.jallcom.2007.04.088

The new type of Sm³⁺ chelates LnPhβ₃L with orange emission obtained as single crystals as well as incorporated into zirconia-glymo films can find application in electro-luminescent diodes. Absorption, emission and emission excitation spectra at 293, 77 and 4 K, as well as luminescence decay time measurements with different energy of the excitation lines were used to characterize the systems based on the Sm³⁺ l-phosphoro-azo derivative of β-diketone: (CCl₃C(O)NP(O)(OCH₃)₂) and dipyridine SmPhβ₃Ldip in the solid state, gel and solution. The dramatic increase of the luminescence intensities was a result of the efficient exchange energy transfer and increase of rigidity of the system under investigation.

Full text of DOI:10.1016/j.jallcom.2007.04.088

Journal of Alloys and Compounds 451 (2008) 94–98
The orange emission of single crystals and sol
gels based on Sm3+ chelates
a
J. Cybinska , J. Legendziewicza,∗, V. Trushb, R. Reisfeldc, T. Saraidarovc
´
a Faculty of Chemistry, University of Wrocław, Joliot-Curie 14, 50-383 Wrocław, Poland
b Department of Chemistry, Kiev State University, Vladimirskaja 64, 2520033 Kiev, Ukraine
c Department of Inorganic and Analytical Chemistry, The Hebrew University of Jerusalem, Givat-Ram, 91904 Jerusalem, Israel
Available online 13 April 2007
Abstract
The new type of Sm3+ chelates LnPh␤3L with orange emission obtained as single crystals as well as incorporated into zirconia-glymo films can
find application in electro-luminescent diodes. Absorption, emission and emission excitation spectra at 293, 77 and 4 K, as well as luminescence
decay time measurements with different energy of the excitation lines were used to characterize the systems based on the Sm3+ l-phosphoro-azo
derivative of ␤-diketone: (CCl3C(O)NP(O)(OCH3)2) and dipyridine SmPh␤3Ldip in the solid state, gel and solution. The dramatic increase of the
luminescence intensities was a result of the efficient exchange energy transfer and increase of rigidity of the system under investigation.
© 2007 Elsevier B.V. All rights reserved.
Keywords: ␤-Diketones; Samarium
1. Introduction
This paper is focused on the comparison of the optical prop-
erties of Sm3+ chelates LnPh␤3Ldip type as a single crystals as
well as incorporated into zirconia glasses. Basing on the model
described by Malta et al. for the energy transfer selection rules
and considering the possible channels of the energy transfer,
we analysed the mechanism responsible for enhancement of
luminescence efficiency.
The interest in the photophysical properties of lanthanide ion
complexes has grown considerably because such complexes can
be used as light conversion devices based on efficient energy
transfer thus overcoming the very small absorption coefficient
of the lanthanide ions [1,2]. The design of efficient lanthanide
complexes has become an important research goal, being pub-
lished by several research groups [1–5]. The rare earth ions with
␤-diketones and their derivatives can also find potential appli-
cation as NMR shifts reagents [5,6] as well as in bioscience [7].
Some important features of these organic fluorophores are their
large Stokes’ shift, narrow bands emission and long decay times
[7]. Among all trivalent lanthanide ions complexes the studies
were mainly focused on the europium and terbium ␤-diketones.
The Sm3+ systems have been less investigated because in gen-
eral present low emission intensity. To enhance luminescence
intensity, lanthanides ␤-diketones could be incorporated into sil-
ica or zirconia glasses using the sol gel technique, which is the
low temperature method for obtaining glass bulks or thin film
[5,8–10].
2. Experimental
The absorption spectra of single crystals were recorded in the 220–1600 nm
at 4 and 293 K using a Cary Varian 50 spectrometer range equipped with an
Oxford CF 1204 helium flow cryostat. The radiative transition probabilities
were calculated from absorption spectra.
The high-resolution luminescence spectra with a resolution of 0.01 nm were
recorded between 470 and 750 nm by using an Action Research Corporation
Spectra Pro 750 monochromator equipped with a Hamamatsu R 928 photomul-
tiplier as the detector. The emission excitation spectra at 77 and 293 K were
measured on a SLM AMINCO SPF-500 spectrofluorimeter equipped with a
300 W Xe-lamp and a liquid-N2 cooled cryostat.
The luminescence decay times at 293 and 77 K were measured using the
third harmonic of a Nd-YAG laser as the excitation source and oscilloscope
Tektronix TDS 3052 B.
The Raman spectra were recorded on a Nicolet Raman accessory attached
to a Nicolet Magna 860 spectrometer and were applied to assign the vibronic
lines in the electronic spectra.
∗
Corresponding author. Tel.: +48 71 3757300; fax: +48 71 3282348.
Intensities of the f–f transitions were calculated by integration of the
Gauss–Lorenz curve and transformed to the oscillator strength values according
E-mail address: jl@wchuwr.pl (J. Legendziewicz).
0925-8388/$ – see front matter © 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2007.04.088

J. Cybin´ska et al. / Journal of Alloys and Compounds 451 (2008) 94–98
95
Fig. 1. Part of the molecular structure of SmPh␤3dip in crystal structure.
to the relation:
ꢀ
ꢀ
σ2
σ2
2.303mc2
P =
ε(σ) dσ = 4.33 × 10−9
ε(σ) dσ
Nπe2
σ1
σ1
3. The sample preparation
The phosphoro-azo derivatives of the ␤-diketones (HL) and
its sodium salts (NaL) were prepared according to the procedure
described earlier [11]. The relevant conditions for the prepara-
tions of the single crystals complex of LnPh␤3L are disclosed
in the patent procedure.
The zirconium thin films were obtained by hydrolysis of
zirconium n-tetrapropoxide. The procedure for thin films incor-
porating samarium complex was similar to that reported in the
literature [5,9,10].
4. Discussion
The SmPh␤3dip crystallizes in the space group C2/c. The
crystal structure of Sm(III) complex is identical with the pre-
viously reported europium compound and is created by the
monomericunits[5](Fig. 1). Thecoordinationnumberofsamar-
ium ion is eight. The central ion is bound by six oxygen atoms
of phosporo-azo derivatives of ␤-diketone and by two nitrogen
atoms of 2,2ꢀ-dypiridine.
The absorption spectra of the SmPh␤3dip single crystal
recorded at RT and 4 K are displayed on Fig. 2a and b. The spec-
6
tra consist of narrow bands assigned to transitions from H5/2
ground state to the higher energy multiplets. The Sm(III) ion has
Fig. 2. The absorption spectra of SmPh␤3dip single crystal at RT and 4 K.

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J. Cybin´ska et al. / Journal of Alloys and Compounds 451 (2008) 94–98
Table 1
Oscillator strength values (P × 108) of the f–f transitions for the SmPh␤3dip
single crystal at RT and 4 K
6H5/2
→
P × 108 293 K
P × 108 4 K
Range (nm)
6F1/2, 6H15/2
83.9862
77.5851
225.4082
296.5446
207.9782
27.3347
2.5138
37.9104
150.0577
166.2738
127.7130
120.3562
72.5448
3.2419
1700–1575
1575–1450
1450–1317
1317–1180
1150–1050
975–925
569–550
535–518
505–493
494–469
6F3/2
6F5/2
6F7/2
6F9/2
6F11/2
4G5/2
4F3/2
4.8751
1.4687
6.1584
4G7/2
14.6235
153.8770
53.6622
1.8027
29.1478
216.0506
4I11/2, 4M15/2, 4I9/2
58.9909
22.2767
3.0222
20.8652
513.6420
4I13/2
4F5/2
469–457
457–450
450–425
414–369
4I15/2, 4G9/2, 4M17/2
4M21/2, 4K11/2, 6P3/2
4F7/2
Fig. 3. The emission spectra of SmPh␤3dip single crystal at RT and 77 K.
,
4L13/2
83.5690
147.3613
77.1177
82.1248
396–385
385–369
be splitted into maximum number (J + 1/2) components. In the
4F9/2, 4K13/2, 4L17/2
6P7/2, 4D1/2
,
4
6
single crystal emission spectra apart from G5/2 → H5/2 tran-
sition, which consist of three components, all other bands are
not splitted into maximum number of Stark components. In the
(4D,6P)5/2, 4D3/2
4H7/2
147.9616
88.5572
34.9580
58.0559
25.9636
43.5105
369–358
358–354
349–344
4H11/2 . . . 4K15/2
6
4
6
4G5/2 → H7/2 and G5/2 → H9/2 transitions, the broadening
and overlapping of the bands was observed. The decrease the
temperature from 77 to 4 K most probably could lead to the
separation of all Stark components.
an odd-electron configuration (4f5), thus the maximum number
of the Stark components for any symmetry lower than cubic is
J + 1/2 dublet Kramers. The calculation of radiative transition
probabilities were done for all the transitions assigned in the
spectra and are collected in Table 1.
The luminescence decay curve for the SmPh␤3dip single
crystal was analysed and the decay times at RT as well as 77 K
were established. The decay functions were monoexponential.
The decay times are practically temperature independent and
they are equal to 60 and 80 ␮s at RT and 77 K, respectively.
These values are in good agreement with the data reported by
Malta for different kind of Sm(III) ␤-diketone with phosphine
oxides ligand [12].
It is expected that the lowering of the temperature leads to
decrease of the intensities, which is caused by depopulation of
the Stark components of 6H5/2 term splitted by crystal field and
by the effect of vibronic coupling. In the system under inves-
tigation, we observed decrease in the intensity of almost all
transitions, however, in some case the intensity increase but
the values of oscillator strengths remain in the same order of
The emission excitation spectra of the single crystals at 293
and 77 K exhibit some narrow lines of f–f transitions of Sm(III),
and in UV range broad ligand transitions (Fig. 4). This indicates
6
6
magnitude. The H5/2 → F1/2 is the hypersensitive transition
(obeying the selection rules ꢀJ = 2, ꢀL = 2) for Sm(III) ions. In
6
6
our compound this transition is overlapped by H5/2 → H15/2
transition, thus the detailed analysis of the spectra is rather dif-
ficult. The second transition, which obeys the selection rules for
6
the hypersensitive transitions is 6H5/2 → F9/2 one. The intensity
of this transition decrease with lowering temperature, indicating
no structural transformations.
The system under investigation shows a very efficient orange
luminescence as a single crystal, sol as well as incorpo-
rated into zirconia thin film. Fig. 3 presents the emission
spectra of SmPh␤3dip single crystals under the UV excita-
tion. The emission spectra of a SmPh␤3dip single crystal
4
6
at 293 and 77 K consist of four main bands G5/2 → H5/2
4
6
4
6
(565 nm), G5/2 → H7/2 (610 nm), G5/2 → H9/2 (650 nm),
4G5/2 → H11/2 (710 nm). The 4G5/2 → H3/2 transitions (in the
spectral range ∼530 nm) were not detected probably because of
concentration quenching. The highest relative intensity exhibits
the G5/2 → H9/2 transition magnetic dipole forbidden and
electric-dipole allowed. According to the X-ray data, the Sm(III)
ion occupies the low symmetry site, thus the transitions should
6
6
4
6
Fig. 4. The emission excitation spectra of SmPh␤3dip single crystal at RT and
77 K.

J. Cybin´ska et al. / Journal of Alloys and Compounds 451 (2008) 94–98
97
For the dipole–dipole, dipole–2λ pole (λ = 2, 4 and 6) and
exchange mechanisms one can use the following expressions
for the transfer rate WET proposed by Malta et al. [13]:
ꢁ
2π
h¯
e2SL
(2J + 1)GR6L
WET
WET
WET
=
=
=
F
ΩλedꢁαꢀJꢀ||U(λ)||αJꢂ2
λ=2,4,6
2π e2SL
ꢁ
F
γλꢁαꢀJꢀ||U(λ)||αJꢂ2
h¯ (2J + 1)G
λ=2,4,6
8π e2(1 − σ0)2
ꢁ
2
FꢁαꢀJꢀ||S||αJꢂ
4
3h¯
(2J + 1)RL
m
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢃ
ꢄ
2
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢁ
φ
μ (k)s (k) φꢀ
ꢂ
z
m
ꢂ
k
It is worth noting that for the large U(4), U(6) matrix elements
most probably the 4I13/2 level takes part in the E–T process. It is
also important that the energy of the ligand Sstate (31,000 cm−1
)
is localized in the range of the Sm(III) levels with very low
U(λ) matrix elements, thus other direct S–Sm(III) states only
slightlyaffectthefinalquantumyieldsofthesensibilizedSm(III)
emission.
Fig. 5. The absorption and emission excitation spectra of SmPh␤3dip single
crystal at RT.
that most of the excitation energy is absorbed by the ligand and
then the energy is transfered to Sm(III) ion levels and the orange
emission occurs. It is well known that the efficiency of lumines-
cence in lanthanide ␤-diketones depends on the energy levels
of organic ligand and metal ions. The data reported previous
by Legendziewicz et al. for EuPh␤3dip and TbPh␤3dip single
crystals show that the energy transfer in the same kind of com-
pound occurs mainly from singlet and triplet states of dipyridine
ligand [5]. Since the efficiency of emission depends on energy
of excitation, therefore also in this chelate sensitization of the
emission is due to dipyridine states. It is worth to notice that
the intensity of the ligand bands in UV region for SmPh␤3dip
is lower than in EuPh␤3dip UV spectra. According to the calcu-
lation of the probability of E–T in europium systems by Malta
et al., the C–T state should decrease the efficiency of energy
transfer. In consequence, for Sm(III) the E–T should be more
efficient because the C–T state not take part in the energy trans-
fer mechanism. Thus, the other mechanism is responsible for the
decrease of emission intensity in SmPh␤3dip complex, which
could be explained by comparison of UV absorption and excita-
tion spectra. Fig. 5 displays absorption and excitation spectra of
SmPh␤3dip chelate in 200–500 nm range. Basing on the results
presented in Fig. 5, we can conclude that the surface quenching
in Sm (III) chelate is similar as in Eu(III) chelate spectra, but
because the energy of triplet state is almost in resonance with
4F5/2 state of Sm(III) and the energy gap between T–4G5/2 state
islarge, themultiphononrelaxationshouldfadtheemittinglevel,
moreover, we should take into account the back energy trans-
fer from 4F5/2 → T state. On the other hand, from two possible
mechanisms proposed by Malta for energy transfer in the case of
Sm(III) the exchange mechanism seems to be operative, whereas
the dipole–dipole and dipole–2λ pole mechanism are less
important.
Fig. 6 shows the emission spectra of the crystal under inves-
tigation in the form of sol obtained by conventional sol–gel
methods. The bands are much broader, but the main features
of the spectra remain the same as for the single crystals,
which allow us to conclude that the structure of the complex
was not destroyed during the process of the sol gel prepara-
tion. The situation is different when we compare the emission
spectra of SmPh␤3dip single crystals with the spectra of a com-
pound incorporated into zirconia glasses (Fig. 7). In the single
4
6
crystal spectra, the electronic dipole transition G5/2 → H9/2
band is more intensive than the magnetic dipole transition
4G5/2 → H7/2. In the spectra of a compound incorporated in
6
zirconia-glymo films both transitions are overlapped. The max-
6
imum of this broad band peaks at 610 nm 4G5/2 → H7/2, which
may suggest some structural transformation.
Fig. 6. Emission spectra of single crystal and sol of Sm␤3dip single crystal
dissolve into acetone or methanol, respectively.

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J. Cybin´ska et al. / Journal of Alloys and Compounds 451 (2008) 94–98
Acknowledgement
Financial support from the Polish Committee for Scien-
tific Research (KBN, grant no. 20401331/0289) is gratefully
acknowledged.
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crystal incorporating into Zr-glymo film.
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The effective energy transfer from ligand T and S states
strongly sensibilizes the orange emission of Sm(III) ion.
The decay times are in the range of tens microseconds and
almost temperature independent.
In the single crystal spectra, the electronic dipole transition
6
4G5/2 → H9/2 is more intensive than the magnetic dipole transi-
6
tion 4G5/2 → H7/2. In the spectra of the compound incorporated
into zirconia-glymo films both transitions are overlapped. The
6
maximum of this broad band peaks at 610 nm (4G5/2 → H7/2),
which may suggest some structural transformation.