Photoactive hybrid gelators based on a luminescent inorganic [Cu 4I4] cluster core

Article abstract of DOI:10.1002/chem.201303567

Luminescent clustogelator: Hybrid gelators with a [Cu₄I ₄] cluster core functionalized by cholesteryl derivatives allow solvent gelation through the formation of a globular network (see figure). The resulting gels display thermochromic and rigidochromic luminescence properties, the latter being used to probe the dynamics of the gelation phenomenon. Copyright

Full text of DOI:10.1002/chem.201303567

COMMUNICATION
DOI: 10.1002/chem.201303567
Photoactive Hybrid Gelators Based on a Luminescent Inorganic
[Cu4I4] Cluster Core
Quentin Benito,[a] Alexandre Fargues,[b] Alain Garcia,[b] Sꢀbastien Maron,[a]
Thierry Gacoin,[a] Jean-Pierre Boilot,[a] Sandrine Perruchas,*[a] and Franck Camerel*[c]
The formation of supramolecular self-assemblies of low-
molecular-weight gelators (LMWGs) is an attractive and el-
egant way to organize, through spontaneous aggregation,
small molecules at the micro- and nanoscale level into stim-
uli-responsive soft materials.[1] The discovery of new organo-
gels that are sensitive to external stimuli is very appealing
for tailored applications in biosciences, molecular electron-
ics, sensing, or as confined reaction media and templates for
well-defined inorganic materials.[2] Metal-based gelators
(metallogelators) have also emerged recently, affording
smart gels in which the metal plays a central role and leads
to new functional materials.[3] Introduction of metal centers
provides new properties such as magnetism,[4] catalytic activ-
ity[5], and luminescence,[6] properties that can be triggered
by external stimuli such as the temperature. Despite the in-
creasing number of gelation motifs being discovered, it re-
mains a challenge to reliably predict the gelation capability
of a compound, especially with nonconventional linear or
disc-shaped gelators. New gelation motifs are, however,
highly desirable from both fundamental and practical stand-
points. Few polymetallic coordination complexes, such as
Tetracopper(I) clusters formulated [Cu4X4L4] (X=Cl, Br,
I; L=organic ligand) are well known for their rich photo-
physical properties.[13] The molecular structure of these
cubane-type clusters is formed by four copper and four
halide atoms alternatively occupying the corners of a cube
(Figure 1). Their remarkable photoluminescence properties,
[8]
the linear Pt2LL’2,[7] cylindrical Pd2L2 and Ag3L3,[9] triangu-
Figure 1. Structure of the [Cu4I4L4] cubane clusters functionalized with
cholesteryl phosphine ligands, CUBCn (n=2, 5), and with (4-(diphenyl-
phosphino)phenyl)methanol phosphine, CUBC0.
[10]
lar M3L3 (M=Au, Cu, Ag), or Zn4L4 grid[11] compounds
(L=organic ligand), have been incorporated as cores in
LMWGs. Recently, polyoxometallate-containing supra-
molecular gels have also been reported but hybrid gelators
based on coordination clusters remain rare.[12]
especially their emission, are sensitive to environmental con-
ditions such as temperature,[14] rigidity of the medium[15], or
pressure,[16] and make these clusters particularly appealing
for the synthesis of stimuli-responsive materials exhibiting
original optical properties. First-row transition-metal com-
pounds are also attractive from an economical point of view
as they are less toxic and more readily available than noble
metal complexes. In this context, functionalization of tetra-
hedrally coordinated [Cu4I4L4] clusters with ligands able to
induce gelation should give rise to a new class of hybrid gel-
[a] Q. Benito, S. Maron, Dr. T. Gacoin, Prof. J.-P. Boilot,
Dr. S. Perruchas
Laboratoire de Physique de la Matiꢀre Condensꢁe – CNRS
Ecole Polytechnique, 91128 Palaiseau Cedex (France)
Fax : (+33)169334799
E-mail: sandrine.perruchas@polytechnique.edu
[b] Dr. A. Fargues, Dr. A. Garcia
CNRS – Univ. Bordeaux, ICMCB, UPR 9048
33600 Pessac (France)
AHCTUNGTREGaNNUN tors leading to materials with intriguing structural and pho-
tophysical properties.
[c] Dr. F. Camerel
Here, we report on the first series of hybrid LMWGs
based on the [Cu4I4] cubane core functionalized by rational-
ly designed cholesteryl-based phosphine ligands. Cholesterol
derivatives are well-known gelation motifs and hydrogen-
bonding motifs have also been introduced to further stabi-
lize the molecular aggregation in solution.[17] The gels of
clusters obtained after cooling hot clear solutions present an
Laboratoire Matiꢀre Condensꢁe et Systꢀmes ꢂlectroactifs Institut des
Sciences Chimiques de Rennes
UMR 6226 CNRS – Universitꢁ de Rennes 1
Campus de Beaulieu, 35042 Rennes (France)
Fax : : (+33)223236631
E-mail: fcamerel@univ-rennes1.fr
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201303567.
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original nanosized globular structure, which is highly emis-
sive and exhibits luminescence thermochromism with a tem-
perature-dependent emission. Moreover, by taking advant-
age of the luminescence rigidochromism properties of the
cubane clusters, the gelation process was also monitored in
situ.
to form a gel, CUBC2 was revealed to be a good gelator of
cyclohexane (minimum gelation concentration, MGC=
12.5 gLꢀ1). Stable and robust gels (Figure 3a), displaying
a thermally reversible sol-to-gel phase transition, were ob-
Cholesteryl phosphine ligands were synthesized from (4-
(diphenylphosphino)phenyl)methanol[18] and cholesteryl
chloroformate by using classical organic reactions for pep-
tides syntheses. Two phosphine ligands were studied with
different spacer lengths between the triphenylphosphine
moiety and the cholesteryl group. The spacer varies with the
length of the alkyl chain (n=2, 5) between the ester and the
carbamate linkers (Figure 1). Reaction of CuI with the
ligand in dichloromethane afforded the corresponding
[Cu4I4L4] clusters, namely CUBCn (n=2, 5), as a white
powder in relatively good yields (>60%, see the Supporting
Information). For comparison, the CUBC0 cluster with the
benzyl alcohol precursor phosphine (Figure 1) was also syn-
thesized. Its molecular structure, determined by single-crys-
tal X-ray diffraction analysis, is represented in Figure 2 and
Figure 3. Gelation test in cyclohexane (20 gLꢀ1). The gel does not flow
when the vial is turned upside-down. Photos of CUBC2 gel at a) room
temperature in ambient light, b) room temperature under UV irradiation
at 312 nm (green emission), and c) after cooling in liquid nitrogen and
under UV irradiation at 312 nm (blue emission).
tained from hot cyclohexane solutions. The entire volume of
solvent was immobilized and could support its own weight
without collapsing. Surprisingly, with only three additional
methylene groups, CUBC5 gave only an insoluble suspen-
sion and failed to form a gel. This result highlights the sub-
tlety of the gelation process and the difficulty of prediction.
The colorless gel of CUBC2 appeared slightly scattering
under white light and isotropic between cross-polarizers
(Figure S2 in the Supporting Information). Upon UV irradi-
ation at 312 nm, the gel was highly luminescent, exhibiting
a green emission (Figure 3b). Epifluorescence microscopy
revealed the presence of a green luminescent cottony fiber-
like structure inside the solvent (Figure 4b). The thermo-
chromic luminescence properties of the gel were revealed
upon cooling down. Under the same UV excitation, the
room-temperature green emission becomes blue when the
sample is immersed in liquid nitrogen (Figure 3c).
To get more insight into the nature of the objects at the
origin of the gelation phenomenon, SEM and TEM analyses
were performed on CUBC2 xerogel obtained by freeze-
drying the corresponding gel. SEM images reveal the pres-
ence of a dense 3D network of short interlocked beaded-
like filaments with an average diameter of 40–60 nm (Fig-
ures 4a and S3 in the Supporting Information). TEM analy-
ses confirm this three-dimensional globular network (Figur-
es 4c and S4 in the Supporting Information), which is re-
sponsible for the gelation of the solvent. The formation of
this extended globular superstructure must arise from hier-
archical self-assembly processes in which nanospherical par-
ticles created at an early state of gel formation fused to
form such an unusual network.[20] The [Cu4I4] cubane struc-
ture and the tetrahedral arrangement of the cholesterol moi-
eties must prevent a facile packing of the clusters into ex-
Figure 2. Molecular structure of [Cu4I
cluster (CUBC0).
4ACHTUNGTRENNUG(PPh2CAHUTNGTERNN(GUN C6H4)CH2OH)4] cubane
consists of a distorted [Cu4I4] cube with each copper atom
coordinated by one phosphine ligand. All the clusters were
characterized by elemental analysis, FTIR, UV/Vis, 31P and
1H NMR spectroscopy (see the Supporting Information).
Coordination complexes based on copper(I) halides are
known to form a variety of structural motifs with the same
ligand.[19] In our case, the formation of the functionalized
[Cu4I4] core in CUBC2 and CUBC5 was ascertained by
solid-state 31P magic angle spinning (MAS) NMR spectros-
copy, which showed signals similar to the reference tetranu-
clear cluster CUBC0 (Figure S1 in the Supporting Informa-
tion). The formation of the cubane core is also confirmed by
the luminescence thermochromism properties of the clusters
(see below).
The gelation ability of CUBCn (n=2, 5) was evaluated in
various polar, nonpolar, protic, and nonprotic solvents by
the “stable to inversion of a test tube” method (Table S2 in
the Supporting Information). Whereas CUBC5 was unable
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Luminescent Gelators with a [Cu4I4] Cluster Core
COMMUNICATION
Figure 4. a) SEM image of CUBC2 xerogel deposited on silicon wafer. b) Drop of CUBC2 gel in cyclohexane (20 gLꢀ1) observed by optical microscopy
under UV irradiation at 340–380 nm (green emission). c) TEM photograph of a diluted xerogel of CUBC2 deposited on carbon-coated copper grid.
tended columnar aggregates as is commonly observed for
the formation of fibers or ribbons.
FTIR analyses revealed the role of hydrogen bonding in
the gel formation (see the Supporting Information). In
CHCl3, the two stretching uC bands at 1731 and 1712 cmꢀ1
=
O
are attributed to unbound ester and carbamate functions, re-
spectively, whereas the free NH functions are found at
3454 cmꢀ1. For the gel, xerogel, and solid, the presence of
a more marked shoulder at 1699 cmꢀ1 (uC O) and a new
=
band at 3351 cmꢀ1
ACHTUNGTRENNUNG(uNH), show that part of the carbonyl and
NH functions are engaged in hydrogen bonding (Figures S5
and S6 in the Supporting Information). Thus, the formation
of the globular aggregates responsible for gel formation is
likely directed by hydrogen bonds between the spacers and
van der Waals interactions between the cholesteryl groups
of the ligands. No p–p stacking of the phenyl groups is pres-
ent in the crystal structure of CUBC0, so this interaction
must be limited in the gel. Likewise, direct involvement of
the [Cu4I4] core in intermolecular interactions seems to be
unfavorable owing to the steric hindrance of the surround-
ing ligands. X-ray diffraction experiments performed on the
gels and xerogels confirm that the superstructure leading to
the gel formation is preserved upon freeze-drying (Figure S7
in the Supporting Information), in agreement with FTIR
analyses.
The photoluminescence spectra of the CUBC2 gel were
recorded from 290 K to 8 K (see the Supporting Informa-
tion, Figures S8–10 and corresponding data in Table S3). At
290 K, the emission spectrum displays a broad band cen-
tered at lmax =535 nm (Figure 5, lex =300 nm), correspond-
ing to the bright green emission observed in Figure 3b
(Fabsolute internal =7%; t1 =0.13 ms (97%), t2 =1.3 ms (3%) at
300 nm). By lowering the temperature, this band (LE for
low energy) progressively decreases in intensity with the
concomitant appearance of a new emission band at higher
Figure 5. Luminescence spectra of CUBC2 gel at 290, 125, and 8 K. Nor-
malized emission spectra (solid lines) have been recorded at lex =300 nm
and corresponding excitation spectra at lem =540 nm (dotted black and
middle grey lines) and at lem =425 nm (dashed middle grey and dotted
light grey lines).
energy lmax =435 nm (HE for high energy). At 125 K, the in-
tensities of the two bands are almost equal and the excita-
tion spectra recorded for both emissions are quite similar
with maxima at lmax =330 nm (Figure 5). Below 100 K, the
LE band is completely absent and only the HE band at
lmax =425 nm is visible, leading to the blue emission ob-
served for the sample cooled in liquid nitrogen (Figure 3c).
When the sample is progressively warmed up to room tem-
perature (290 K), the green emission of the LE band is re-
covered, indicating a completely reversible phenomenon.
Note, the luminescence properties of CUBC2 in the xerogel
and in the powder are similar to the ones observed in the
gel state (Table S3 and Figures S8–10 in the Supporting In-
formation). The thermochromism observed is characteristic
of the [Cu4I4L4] copper iodide cubane clusters with two
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S. Perruchas, F. Camerel et al.
emission bands (LE and HE), the relative intensities of
which vary with temperature. Based on previous theoretical
and experimental studies, these bands are attributed to dif-
ed with confinement of the cluster in a dense and rigid 3D
globular network at the origin of gel formation. After
25 min (258C), the emission characteristics of the highly lu-
minescent gel remain stable with time, meaning that the gel-
3
ferent excited states.[21] The LE band is assigned to a CC
cluster-centered state (combination of an iodide-to-copper
charge transfer transition (XMCT) and a copper-centered
Cu d!s, p transition), whereas the HE band is attributed to
an iodide,copper-to-phosphine ligand charge-transfer transi-
tion (3X,MLCT). By exhibiting the typical thermochromic
luminescence of copper iodide clusters, this study also con-
firms that the [Cu4I4L4] cubane structure is preserved in the
gel, a fact that is also true in the cases of the xerogel and
powder.
ACTHUNGTNERNUNGaACHUTNGTRENtNNUG ion process is finished at this point under these experimen-
tal conditions. A shift in the emission wavelength is also ob-
served during gelation, changing from 580 to 535 nm. Ac-
cording to the rigidochromism phenomenon, this blue shift
is due to the confinement of the clusters in a more rigid en-
vironment leading to conformational states at higher energy
compared with a liquid state with fewer environmental con-
straints. The strong intensity increase (by a factor of 60) ob-
served during gelation can be attributed to limited nonradia-
tive phenomena through decreased fluxionality in the more
rigid gel state associated with enhanced scattering.[13b] The
effect of temperature on the luminescence changes observed
can be ruled out as cooling a solution of clusters lead to
a red shift of the emission.[23] Passing from an almost nonlu-
minescent liquid state to a brightly emissive gel state, the
cluster, thus, appears as a probe for the gelation phenomen-
on. This study confirms the hierarchical process of gel for-
mation occurring through the assembly of the clusters into
a globular rigid matrix, the driving forces of which are van
der Waals interactions and hydrogen bonds between the li-
gands.
To conclude, original hybrid gelators based on the [Cu4I4]
cubane cluster core have been obtained by the design of
specific phosphine ligands. The highly emissive gels exhibit
luminescence thermochromism properties. The formation of
a 3D dense globular network resulting from the aggregation
of nanoparticles is responsible for the observed gelation.
The hierarchical self-assembly process at the origin of the
nanoparticles formation is currently under investigation
(SAXS, rheology, etc.) to elucidate the cluster packing in
this superstructure. The specific structural and photophysical
properties of the gels are retained in the xerogels, giving
access to a new family of optically active materials of high
specific surface with potential sensing and catalytic applica-
tions. The rigidochromic luminescence properties of the
copper iodide clusters lead to an on/off system for which the
emission is exalted in the gel state. These clusters exhibiting
luminescent properties sensitive to the rigidity of the envi-
ronment appear as original probes to precisely study gela-
tion mechanisms and studies for this purpose are ongoing.
More generally, the present example paves the way for the
design of original functional materials based on nonconven-
tional copper halide metallogelators.
Copper iodide clusters can also display luminescence rig-
idochromic properties with a change in the emission wave-
length and intensity depending on the rigidity of the local
environment.[13b] This phenomenon is attributed to the varia-
3
ble extent of the molecular distortions of the CC state rela-
tive to the ground state, an effect that is influenced by con-
straints imposed by the medium.[22] Upon excitation, strong
geometrical relaxation of the 3CC state occurs owing to elec-
tron redistribution associated with large distortions of the
[Cu4I4] core that is revealed by a large Stokes shift. This
effect is commonly observed as a blue shift of the emission
along with a large increase in intensity when solutions of
clusters are solidified.[15] Here, this property has been ex-
ploited in order to probe the gelation process of CUBC2
during the sol–gel transition. A suspension of CUBC2 in cy-
clohexane (20 gLꢀ1) was heated at 658C until a clear solu-
tion was obtained and the emission spectra were recorded
upon cooling down (Figure 6). At the beginning of the cool-
ing process, a gradual increase of the emission intensity is
observed and is likely attributed to a growing process in
which the clusters form semirigid spherical nanoparticles.
Note that the luminescence of a CHCl3 solution of the clus-
ter at the same concentration (20 gLꢀ1) at room tempera-
ture is too weak to be measured. This means that aggregates
of clusters are already present in cyclohexane solution even
at 658C. After 11 min (308C), the observed sudden and
rapid increase of the luminescence intensity can be associat-
Experimental Section
Experimental procedures for the synthesis of the ligands and clusters,
characterization data, additional microscopy images of the xerogels, and
temperature-dependent spectroscopic studies of the xerogels and pow-
ders are available in the Supporting Information. A color version of
Figure 4 is also available in the Supporting Information. CCDC-953833
contains the supplementary crystallographic data for this paper
(CUBC0). These data can be obtained free of charge from The Cam-
Figure 6. Time-dependent emission wavelength and intensity measured
upon cooling a hot solution of CUBC2 at 20 gLꢀ1 in cyclohexane, to
room temperature (lex =300 nm).
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Luminescent Gelators with a [Cu4I4] Cluster Core
COMMUNICATION
bridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_
;request/cif.
[10] a) A. Kishimura, T. Yamashita, T. Aida, J. Am. Chem. Soc. 2005,
127, 179–183; b) A. Kishimura, T. Yamashita, K. Yamaguchi, T.
Aida, Nature Mater. 2005, 4, 546–549; c) S. A. Joshi, N. D. Kulkarni,
Chem. Commun. 2009, 2341–2343.
[11] J. G. Hardy, X. Cao, J. Harrowfield, J.-M. Lehn, New J. Chem. 2012,
36, 668–673.
[12] a) B. Liu, J. Yang, M. Yang, Y. Wang, N. Xia, Z. Zhang, P. Zheng,
W. Wang, I. Lieberwirth, C. Kuebel, Soft Matter 2011, 7, 2317–2320;
b) L. Zhang, B. Marzec, R. Clꢁrac, Y. Chen, H. Zhang, W. Schmitt,
Chem. Commun. 2013, 49, 66–68.
[13] a) A. Barbieri, G. Accorsi, N. Armaroli, Chem. Commun. 2008,
2185–2193; b) P. C. Ford, E. Cariati, J. Bourasssa, Chem. Rev. 1999,
99, 3625–3627; c) V. W.-W. Yam, K. K.-W. Lo, Chem. Soc. Rev. 1999,
28, 323–334.
Acknowledgements
The authors thank the CNRS and Ecole Polytechnique for funding. Q.B.
thanks the DGA for his PhD fellowship. F.C. thanks the University of
Rennes 1 and Rennes Metropole for their financial support. C. Mꢁriadec
is gratefully acknowledged for help with XRD measurements. G. Clavier
and A. Brosseau are thanked for the lifetime measurements and M.
Poggi for the TEM analysis. G. Nocton and E. Nicolas are also gratefully
acknowledged for their help with single-crystal XRD experiments.
[14] a) M. Radjaipour, D. Oelkrug, Ber. Bunsen-Ges. Phys. Chem. 1978,
82, 159; b) C. Tard, S. Perruchas, S. Maron, X. F. Le Goff, F. Guillen,
A. Garcia, J. Vigneron, A. Etcheberry, T. Gacoin, J.-P. Boilot, Chem.
Mater. 2008, 20, 7010.
[15] D. Tran, J. L. Bourassa, P. C. Ford, Inorg. Chem. 1997, 36, 439.
[16] S. Perruchas, X. F. Le Goff, S. Maron, I. Maurin, F. Guillen, A.
Garcia, T. Gacoin, J.-P. Boilot, J. Am. Chem. Soc. 2010, 132, 10967.
[17] A. Ajayaghosh, C. Vijayakumar, R. Varghese, S. J. George, Angew.
Chem. 2006, 118, 470–474; Angew. Chem. Int. Ed. 2006, 45, 456–
460.
Keywords: cluster compounds · gels · metallogelators ·
rigidochromic luminescence · thermochromic luminescence
[1] R. G. Weiss, P. Terech, Molecular Gels, Materials With Self-Assem-
bled Fibrillar Networks, Springer, Dordrecht, 2006.
[2] S. Banerjee, R. K. Das, U. Maitra, J. Mater. Chem. 2009, 19, 6649–
6687.
[3] M.-O. M. Piepenbrock, G. O. Lloyd, N. Clarke, J. W. Steed, Chem.
Rev. 2010, 110, 1960–2004.
[18] a) O. Herd, A. Hessler, M. Hingst, M. Tepper, O. Stelzer, J. Organo-
met. Chem. 1996, 522, 69–76; b) R. A. Leising, J. J. Grzybowski,
K. J. Takeuchi, Inorg. Chem. 1988, 27, 1020–1025.
[4] a) O. Roubeau, A. Colin, V. Schmitt, R. Clꢁrac, Angew. Chem. 2004,
116, 3345–3348; Angew. Chem. Int. Ed. 2004, 43, 3283–3286; b) R.
Perochon, P. Davidson, S. Rouziꢀre, F. Camerel, L. Piekara-Sady, T.
Guizouarn, M. Fourmiguꢁ, J. Mater. Chem. 2011, 21, 1416–1422.
[5] a) Y. R. Liu, L. He, J. Zhang, X. Wang, C. Y. Su, Chem. Mater. 2009,
21, 557–563; b) Y. He, Z. Bian, C. Kang, Y. Cheng, L. Gao, Chem.
Commun. 2010, 46, 3532–3534.
[6] a) F. Camerel, L. Bonardi, M. Schmutz, R. Ziessel, J. Am. Chem.
Soc. 2006, 128, 4548–4549; b) V. K.-M. Au, N. Zhu, V. W.-W. Yam,
Inorg. Chem. 2012, 51, 7537.
[19] R. Peng, M. Li, D. Li, Coord. Chem. Rev. 2010, 254, 1–18.
[20] L. Yang, L. Luo, S. Zhang, X. Su, J. Lan, C.-T. Chen, J. You, Chem.
Commun. 2010, 46, 3938–3940.
[21] S. Perruchas, C. Tard, X. F. Le Goff, A. Fargues, A. Garcia, S.
Kahlal, J.-Y. Saillard, T. Gacoin, J.-P. Boilot, Inorg. Chem. 2011, 50,
10682.
[22] Upon excitation, strong geometrical relaxation of the 3CC state
occurs owing to electron redistribution associated with large distor-
tions of the [Cu4I4] core that is revealed by the large Stokes shift.
[23] a) K. R. Kyle, C. K. Ryu, J. A. DiBenedetto, P. C. Ford, J. Am.
Chem. Soc. 1991, 113, 2954; b) I. Roppolo, E. Celasco, A. Fargues,
A. Garcia, A. Revaux, G. Dantelle, F. Maroun, T. Gacoin, J.-P.
Boilot, M. Sangermano, S. Perruchas, J. Mater. Chem. 2011, 21,
19106.
[7] a) W. Lu, Y.-Y. Law, J. Han, S. S.-Y. Chui, D.-L. Ma, N. Zhu, C.-M.
Che, Chem. Asian J. 2008, 3, 59–69; b) J. Zhang, X.-D. Xu, L.-J.
Chen, Q. Luo, N.-W. Wu, D.-X. Wang, X.-L. Zhao, H.-B. Yang, Or-
ganometallics 2011, 30, 4032–4038.
[8] T. Naota, H. Koori, J. Am. Chem. Soc. 2005, 127, 9324–9325.
[9] X. Wei, X. Li, Z. Yang, J. Lan, G. Gao, Y. Xue, J. You, Chem. Sci.
2012, 3, 359–363.
Received: September 9, 2013
Published online: October 21, 2013
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