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Light scattering and luminescence studies of M(CF3SO3)x- polyether complexes containing trivalent cations
SOLID STATE
Solid State Ionics 72 (1994) 165-171 North-Holland
IONICS Light scattering and luminescence studies of M (CF3SO 3 )xpolyether complexes containing trivalent cations G. Petersen, A. Brodin, L.M. Torell 1 Department of Physics, Chalmers Universityof Technology, S-412 96 Gothenburg, Sweden
M. Smith Department of Chemistry, Universidade do Minho, Largo do Pafo, 4 719 Braga Codex, Portugal
Raman scattering has been used to study ionic interactions in trivalent M (CF3SO 3)3-PEO complexes, M = Eu3+, Nd 3+ and Ce3÷. Both solvated ions and cation-anion pairs/multiplets were found, and, in the case of Nd 3+ and Ce3+ complexes, the spectra suggest more complicated configurations of combined cation-anion-polyethercoordinations. The findings are discussed in terms of the cationic charge density. Intense luminescence was observed in the Eu3+ complexes which converted blue laser light into red radiation. The well defined luminescence spectra support the concept of several cationic sites in these complexes.
1. Introduction
2. Experimental
It has been shown that trivalent salts can form complexes with poly (ethylene oxide) and that reasonable ionic conductivities are o b t a i n e d [ 1-4 ]. Little is however known about the b e h a v i o r o f the solvated ions on the molecular level in these systems. In the present study we have investigated i o n - p o l y mer and i o n - i o n interactions in P E O - M (CF3SO3) 3, where M = Eu 3+, N d 3+ and Ce 3+, respectively, from the molecular vibrational dynamics using R a m a n scattering techniques. The much studied m o n o v a lent PEO-LiCF3SO3 complex has also been included for R a m a n investigations for comparisons with the trivalent systems. The Eu3+-complexes are found to be strongly fluorescent in the o r a n g e - r e d wavelength range [ 1 ], a fact which m a y have consequences for these systems as a new group o f optical materials, namely as flexible phosphors. F o r the Eu 3+ complexes we therefore also investigated the frequency range for electronic transitions to see whether the luminescence spectrum o f Eu 3+ can be used as a probe o f its local coordinations.
Low molecular weight poly (ethylene oxide) with an average molecular weight o f 400 and CH3-endc a p p e d (Polysciences) was used. By endcapping the chains, anion coordination to the chain ends could be avoided. This allows for using the low molecular weight complexes, which are easy to handle spectroscopically, as model systems for the high molecular weight complexes o f interest in technical applications [5]. The triflate salts M(CF3SO3)3, M being Li +, Eu 3+, Nd 3+, and Ce 3+ respectively, and the PEO glyme were dried separately under vacuum for several days. The p o l y m e r was then degassed on a vacuum-line by freeze-drying techniques. Samples o f concentration O: M ~ 40:1 (O: M being the ether oxygen to salt ratio) were prepared in a glovebox by dissolving the salt directly into the p o l y m e r on a hot plate ( T ~ 350 K ) . The solutions were then poured into quartz sample cells, again degassed, and thereafter sealed under vacuum. Narrow sample cells o f cylindrical shape (inner diameter ~ 2 m m ) were used to m i n i m i s e the sample volume and thereby the fluorescent background o f the R a m a n spectra, see below. The 488 nm line o f an a r g o n - i o n laser was focused into the sample cell situated inside the
To whom all correspondence should be addressed.
0167-2738/94/$ 07.00 © 1994 Elsevier Science B.V. All rights reserved.
166
G. Petersen et al. I M(CF~SO3)~-polyethercomplexes
cryostat/thermostat. A Spex double monochromator (model 1403) with 1800 lines/mm was used to resolve the spectra (resolution ~ 2 c m - ~ ) . The spectra were recorded by either a C C D (_charge _coupled _device) or a photomultiplier with typical recording times of 20 s and 5 min, respectively. Every spectrum is the result of accumulation and averaging of ~ 20 recordings.
3. Results and discussion 3.1. R a m a n spectra o f (PEO)-M(CF~SO~)x complexes
In fig. 1 the Raman spectra of Li +, Eu 3+, N d 3+ a n d Ce 3+ containing salt-polymer complexes are
presented over a wide frequency range (0-1500 cm-~). The spectra generally contain considerable background which spans the whole frequency range studied and increases with increasing frequency shift. A general observation is that the purer the salt, the smaller is the spectral background intensity. In the spectra presented in fig. 1 the background has been subtracted. The sharp modes in the spectra of the various complexes in fig. 1 (star marked in the figure) are due to the internal vibrations o f C F 3 S O 3 which is the
J!
E
i
¢~ =
I
',!
' i
1000
500
Raman
shift
(era - 1 )
Fig. 1. Raman spectra ofM(CF3SO)x-(PEO) (CH3-endcapped,
MW~400) complexes of salt concentrations M:O~I:40, M = Li +, Ce3+, Eu3+ and Nd 3+, respectively.
c o m m o n anion in all the studied complexes. The corresponding frequencies are given in table 1 and they are assigned according to the recent study by Shriver et al. [6]. It can be seen in table 1 that while the various complexes in all cases display the same set of vibrational modes for the anion significant differences in their frequencies and bandshapes arc demonstrated. Thus the triflate anions in the saltpolymer complexes are influenced by the cations and slightly differently for different cations. In the low frequency region the most intense line is the C-S stretching mode, the frequency of which changes from 313 c m - J to ~ 3 2 0 cm-~ as the valency of the cation increases from monovalent Li + to trivalent Eu 3+, Nd 3+ or Ce 3+. From literature data the 313 cm ~ mode is recognised as the vibration of a free (solvated) anion whereas the frequency increases for anions coordinated by cations in pairs or triplets [7,8]. Almost as intense as the C-S-stretch is the SOy-rocking mode at ~ 349 cm-~ observed for all the investigated salt-polymer complexes and attributed to solvated anions. In the case of the trivalent systems the mode is splitted with a smaller intensity contribution on the high frequency side. Thus the low frequency region indicates the presence of both solvated ions and ions coordinated in pairs a n d / o r multiplets in the trivalent complexes. The 500-800 c m - ~region is the range of the bending modes. The intensities of the symmetric and asymmetric SOy bending modes at about 639 and 520 c m - ~, respectively, are very weak and cannot be used for any closer analysis. The symmetric CF3 bending is strongest in intensity in this range. It is observed as a splitted mode for all the investigated complexes. The low frequency component is assigned to solvated ions [7,8] and occurs at about the same frequency ~ 755 cm-~ independent of cationic valency. The higher frequency component is stronger in intensity and is attributed to ion pairs/multiplets [ 7,8 ]. It shifts from ~ 758 c m - ~ in the Li + complex to about 763-766 cm ~ in the trivalent complexes. The band of the antisymmetric CF3 bending mode. though considerably weaker in intensity, shows in all cases splitted behavior; a low frequency component at ~ 574 c m - ~, identified as the flee ion mode, and a mode at ~ 582 cm ~ due to ion-ion coordinated configurations. The high frequency region is dominated by the S()3
G. Petersen et al. / M(CF~SO3)x-polyether complexes
167
Table 1 Comparison of (CF3SO 3 ) - bands and assignments for M (CF3SO 3 )x-PEO complexes. O: M = 40: 1.
Li
~(CS)
p(SO3)
0as(CF3)
0~(CF3)
~(SO3)
313
347
574 581
753 758
I032 1041 1051
solvated ions ion pairs multiplets
319
347 350
574 583
756 763
1021 1032 1041
ion-ion-solvent solvated ions ion pairs, multiplets
320
349 361
571 581
766
1019 1032 1039
ion-ion-solvent solvated ions ion pairs, multiplets
320
349 357
574 582
755 763
1019 1032 1039
ion-ion-solvent solvated ions ion pairs, multiplets
Eu
Nd
Ce
symmetric stretching band. This is the most intense feature of the Raman spectra and it was used for a detailed analysis. The bandshape revealed in some cases a complicated multicomponent contour. The component at ~ 1032 cm -J, attributed to free (solvated) ions, was present in all the complexes and its relative intensity was used as a measure of the concentration of solvated species. The behavior of the SO3 symmetric stretch is discussed for the various complexes below.
3.2. Ion-polymer and ion-ion interactions in PEOM(CFsSOs)x complexes In fig. 2 we show Raman spectra of the SO ~- symmetric stretch for the various complexes. Measurements were performed over a temperature range of 275-350 K and in fig. 2 the spectra obtained at the lowest and highest temperatures are demonstrated. The bandprofile was analysed in terms of a multicomponent band. In the case of the Li + complex a two-component profile with intensity peaks at 1032 c m - ~ and 1041 c m - l fitted well the spectrum at low temperatures. The two modes are assigned to vibrations of solvated triflate anions respectively anions coordinated by cations in pair configurations, (the assignment is based on a series of previous papers reviewed in ref. [5]). As temperature increases a third component at ~ 1 0 5 0 cm -~ is growing in strength. In accordance with previous observations from various monovalent (Li + and Na +) salts in
T=275
K
T=350
Li
Li
,., Eu
j
Ce
,2\
/j
,
~
=
K
Eu
'~"
.,: ,
- --
,
Ce
Nd
1020
1;60 Flaman
I
Shift
1020
-
10160
(cm "1 )
Fig. 2. Temperature evolution of the SO;- symmetric stretch band of M(CF3SO3)x-(PEO) (CH3-endcapped) complexes, M = L i ÷, Eu 3+, Nd 3+ and Ce 3÷, respectively.
polyethers the mode is assigned to vibrations ofa triflare ion in a multiplet configuration of anions-cations. By comparing the temperature dependence of the intensities of the respective modes in fig. 2 it can
168
G. Petersen et al. / M(CFsSO3)x-polyether complexes
be seen that as the Li + complex is heated the concentration of solvated ions decreases and that of the associated ions (pairs, triplets, etc.) increases as expected [ 5 ]. For the trivalent complexes a more complicated band structure is revealed, see fig. 1. The bandprofile has been analysed for the minimum number of modes to obtain a fit consistent within the experimental accuracy to the spectrum. The mode at 1032 cm-1, assigned to solvated ions, is present in all the spectra and at the same position independent of cation, as should be the case for the vibration of a free non-interacting molecular anion. From the intensity ratio of this mode and the total SO3 symmetric stretching band the amount of solvated ions for the trivalent complexes was calculated, see fig. 3. The largest concentration of free ions is found in the EU 3+ complex ( ~ 55% at room temperature) as compared to the Nd 3+ and Ce 3+ complexes ( ~ 3 0 % and ~40% respectively). We note that Eu 3+ has considerably higher charge density due to its smaller ionic radius ( 1.2/~) compared to Nd 3+ ( 1.6 ~) and Ce 3+ ( 1.85 ~,), i.e. REu3+
The larger concentration of solvated Eu 3+ ions may then be explained by their tight charge distribution which allows for strong interactions with the oxygens of the polymer chain. The relative behavior of the
A
7s[
d o 2s t
;f
•
• - ' ~ ~
w
t
N
260
I
280
300
320
Temperature
340
360
(K)
Fig. 3. Concentration of solvated ions versus temperature in M(CF3SO3)3-(PEO) (CH3 endcapped) complexes of salt concentrations M: O ~ 1 : 40, M = Eu 3+, Nd 3+ and Ce 3+.
free ion concentration in the N d 3+ and C e 3+ c o m p l e x e s , however, does not follow this scheme. We also note that the so called free-ion mode for the trivalent systems is considerably broader (6-8 cm 1) than that of the monovalent system ( ~ 4 cm-~ ). It can be explained by that the bandprofile of the trivalent complexes may include more components than used in the present analysis. The coordination number for the trivalent cations in aqueous solutions is ~ 9 as compared ~ 4 in the case of Li +. Due to configurational restrictions one can however hardly expect the polymer chain (or nearby polymer chains) to be wrapped around the trivalent ions such that the polyethers of the chain sufficiently neutralise their high ionic charge. Thus even rather well solvated cations can carry a net charge and may be coordinated not only to the ether oxygens but also to the anions. The bandprofile in the case of the trivalent systems may therefore contain modes not only due to solvated ions, ion pairs, triplets, etc. but also due to a range of anion-cation-polyether coordination possibilities. The more complicated solvation picture of the trivalent systems is further demonstrated by the behavior of the higher frequency mode in the SOy symmetric stretch profile in fig. 2 previously assigned to anion-cation pairs. For monovalent cations it has been noted that a change to a smaller cation (from Na + to Li +) results in an increased frequency shift of the ion-pair mode [ 5 ]. This is explained by the larger Coulomb attraction for the cation of higher charge density. In fig. 4a the frequency of this mode is plotted for the trivalent complexes; a larger frequency shift is indeed observed for the E u 3+ triflate pair ( 1041 c m - ~at room temperature) in consequence with its higher charge density. The Nd 3+ and C e 3+ complexes, however, show similar frequency for the ion-pair mode ( ~ 1039 cm-~ ). We also note that the frequency width of the ion-pair component increases as FEu
G. Petersen et al. / M(CF3SOs)x-polyether complexes
169
mer and for anion-cation pair formations. 1043
~ = - -
3.3. Luminescence spectra of Eu 3+ in PEOEu(CF3S03)~
1042
E
1041 r 1040
~ C
1039
E
1038 1037 Ce
I
L'
q
L,
,'
L,
~
~
I
,
,
18
'E
Ce
o
NO
•
v 16
.1I~.
@
14 Eu
1O
260
20o
300 Temperature
320
340
36o
(K)
Fig. 4. (a) Frequency shift and (b) halfwidth of the Raman mode attributed to anion-cation configurations in M(CF3SO3)3(PEO) (CH3-endcapped) complexes, M = Eu 3+, Nd 3+ and Ce 3+, respectively.
which is evident in the N d 3+ and Ce 3+ complexes and only just detectable in the E u 3+ system. For Li +containing complexes, no 1020 cm - l has been observed. For the temperature behavior of the ion solvation, see fig. 3, we notice a negative temperature dependence in accordance with the general trend observed for monovalent system [5]; for Eu 3+ and Ce 3+ the amount of solvated ions decreases considerably as temperature increases whereas for the Nd 3+ complex only a weak temperature dependence is found. The negative entropy of solution previously observed for monovalent systems has been related to the ordering imposed by the solvated cations on the polymer chains and to accompanying electrostrictive effects [5,9,10]. Similar ordering effects seem to be present in the polymer on the addition of trivalent salts. As the temperature increases the release of solvated cations from the polyethers of the chain allows for increased conformational freedom of the poly-
In the case of the E u 3+ containing complexes strong luminescence lines were observed in a frequency range 14000-17000 cm -~, see fig. 5. This corresponds to much larger frequency shifts from the laser line at ~20500 cm - t (488 nm) than those of the vibrational modes in fig. 1. The luminescent band at ~ 16200 c m - t is approximately 300 times more intense than the vibrational mode at 1032 c m - ~. Fig. 6 shows an energy level diagram of Eu 3+ (4f 6) as deduced from absorption and fluorescence spectra of LaF3:Eu 3+ [ 11 ] and the lines in fig. 5 are identified as emission from the 5Do level to the different 7Fj ground levels. Since luminescence is only observed from the 5Do level for an excitation wavelength of 488 nm, one expects efficient nonradiative transitions between the 5D~ and 5Do levels. Nonradiative transitions from 5Do state to the ground 7Fj states may also occur and are frequent in the presence of O - H bonds as has been shown for some aqueous solutions of Eu compounds [ 12 ]. The three most intense bands in fi~. 5, centred at about 14400, 16200 and 16900 c m - ~, correspond to 5 D o - 7 F J transitions to the 7F4, 7F2 and 7F l levels, respectively; the weak components at about 15400 and PEO(400)-Eu,
488
nm
excit.,
T =
7 Do - F z
5" a
300
K
7 D O- F 1
7 Do - F 4
2 5
i
14000
t
15000
7
5
t
i
t
16000
7 t
i
17000
W a v e n u r n b e r s (era 1) Fig. 5. Luminescence spectrum of Eu 3+ in a Eu (CF3SO3)3-PEO complex in the region of SD-TFj transitions, the excitation laser wavelength being 488 nm.
17 0
G. Petersen et al. / M(CF~SOsL,-pol.vether complexes 25
2 r
-
20
~
5
7
'.
5 Dj o
~O -15
o *a +a c) N
-10
17240 ¢o .0
172f30
17300
17;t20
Fig. 7. Luminescence spectra of a Eu(CF3SO3)3-PEO complex in the region of the 5D0-7F o transition of Eu 3+ for different temperatures, the excitation laser wavelength being 488 nm.
-5 4 kO
17260
Wavenumbers (cm-l/
3 2 1 0
7 Fj
Fig. 6. Energy levels of Eu 3+.
17300 cm -1 are due t o 7F 3 and 7Fo transitions. All 4Ps intraconfigurational transitions are electric dipoles (ED) forbidden in the free ion, however, become allowed as the inversion symmetry is lowered. Identification of the internal structure of the various bands shows Stark splitted components of the 7FI,2,3,4, 5 levels in low symmetry sites [11 ]. The s u p e r - f o r b i d d e n S D o - 7 F o transition achieves a weak magnetic dipole (MD) oscillator strength in low symmetry sites. Since the J = 0 states are non-degenerate the linewidth of the spectral mode corresponding to the 5Do-TFo transition gives a measure of the energy distribution of the different Eu 3+ sites. We find that the S D o - T F o mode is asymmetric in shape, see fig. 7, which indicates that the E u 3+ ions in PEO are subject to distinctly different environments and experience different chemical coordinations. In fact, Eu 3+ is commonly used to investigate the local structure and there are literature data for a wide variety of heterogeneous systems [ 13-15 ]. As the temperature increases, the total emission intensity increases, the band shifts towards higher energies, and there is a change of the bandshape, which suggest a redistribution of the E u 3+ sites as the temperature increases. This is in accordance with the vibrational data in figs. 2 and 3 from which it is concluded that the amount o f E u 3+ ions coordinated to the ether oxygens is decreasing with increasing temperature in favour of increased ion-ion pairing phenomena. To
investigate in detail the local structure of Eu 3+ from its luminescent properties we are at present applying laser excited site selective spectroscopy, a technique that is particularly well suited for structural investigations. Preliminary results show clear evidence of at least two Eu 3+ environments.
4. Conclusion From the Raman vibrational studies we conclude that ion solvation of trivalent M(CF3SO3)3 salts in PEO is described by rich coordination possibilities. There are evidences of solvated ions and ions in various anion-cation configurations, as previously have been observed for monovalent salt-polymer complexes. The spectra however reveal new ionic configurations not reported for the monovalent complexes. We attribute the latter to combined cationanion-polyether coordinations in which the number of participant species may vary. Comparing the solubility of the studied lanthanides Eu 3+, Nd 3+ and Ce 3+, the largest concentration of solvated ions is found for the Eu 3+ which we attribute to its larger charge density. The strong luminescence spectra of the Eu 3+-complex, which fluoresce red, suggest that Eu 3+ has several coordination environments. Changing the temperature a clear redistribution of the Eu 3+ sites is noted. This is in support of the vibrational data which show decreased solubility and increased cationic-
G. Petersen et al. / M(CF3SO~)x-polyether complexes
anionic interactions as temperature increases.
References [ 1 ] R. Huq and G.C. Farrington, in: 2nd Intern. Symp. on Polymer Electrolytes, ed. B. Scrosati (Elsevier, Amsterdam, 1990) pp. 273. [2] A.S.R. Machado and L. Alcacer, in: 2nd Intern. Symp. on Polymer Electrolytes, ed. B. Scrosati (Elsevier, Amsterdam, 1990) pp. 283. [3] P.G. Bruce, J. Nowinski, F.M. Gray and C.A. Vincent, Solid State lonics 38 (1990) 231. [4] M.J. Smith and C.J.R. Silva, Solid State lonics 58 (1992) 269. [5] L.M. Torell, P. Jacobsson and G. Petersen, Polym. Adv. Techn. 4 (1993) 152. [6 ] D.H. Johnston and D.F. Shriver, Inorg. Chem. 32 ( 1993 ) 1045.
171
[7] M.G. Miles, G. Doyle, R.P. Cooney and R.S. Tobias, Spectrochim. Acta 25A (1969) 1515. [8 ] S. Schantz, J. Sandahl, L. B6rjesson, J.R. Stevens and UM. Torell, Solid State Ionics 28-30 (1988) 1047. [9] M.A. Ratner and A. Nitzan, Faraday Discuss. Chem. Soc. 88 (1989) 19. [10] R. Olender and A. Nitzan, Electrochim. Acta 37 (1992) 1505. [ 11 ] U.V. Kumar, D.R. Rao and P. Venkateswarler, J. Chem. Phys. 66 (1977) 2019. [ 12 ] G. Blasse, G.J. Dirksen, P. van der Voort, N. Sabbatini, S. Perathoner, J.-H. Lehn and B. Alpha, Chem. Phys. Lett. 146 (1988) 347. [13]A.A. Kapylanskii and R.M. Macfarlane, Spectroscopy of Solids Containing Rare Earth Ions (North-Holland, Amsterdam, 1987). [14] M.F. Hazenkamp and G. Blasse, Chem. Mater. 2 (1990) 105. [ 15] D. Levy, R. Reisfeld and D. Avnir, Chem. Phys. Lett. 109 (1984) 593.
Solid State Ionics 72 (1994) 165-171 North-Holland
IONICS Light scattering and luminescence studies of M (CF3SO 3 )xpolyether complexes containing trivalent cations G. Petersen, A. Brodin, L.M. Torell 1 Department of Physics, Chalmers Universityof Technology, S-412 96 Gothenburg, Sweden
M. Smith Department of Chemistry, Universidade do Minho, Largo do Pafo, 4 719 Braga Codex, Portugal
Raman scattering has been used to study ionic interactions in trivalent M (CF3SO 3)3-PEO complexes, M = Eu3+, Nd 3+ and Ce3÷. Both solvated ions and cation-anion pairs/multiplets were found, and, in the case of Nd 3+ and Ce3+ complexes, the spectra suggest more complicated configurations of combined cation-anion-polyethercoordinations. The findings are discussed in terms of the cationic charge density. Intense luminescence was observed in the Eu3+ complexes which converted blue laser light into red radiation. The well defined luminescence spectra support the concept of several cationic sites in these complexes.
1. Introduction
2. Experimental
It has been shown that trivalent salts can form complexes with poly (ethylene oxide) and that reasonable ionic conductivities are o b t a i n e d [ 1-4 ]. Little is however known about the b e h a v i o r o f the solvated ions on the molecular level in these systems. In the present study we have investigated i o n - p o l y mer and i o n - i o n interactions in P E O - M (CF3SO3) 3, where M = Eu 3+, N d 3+ and Ce 3+, respectively, from the molecular vibrational dynamics using R a m a n scattering techniques. The much studied m o n o v a lent PEO-LiCF3SO3 complex has also been included for R a m a n investigations for comparisons with the trivalent systems. The Eu3+-complexes are found to be strongly fluorescent in the o r a n g e - r e d wavelength range [ 1 ], a fact which m a y have consequences for these systems as a new group o f optical materials, namely as flexible phosphors. F o r the Eu 3+ complexes we therefore also investigated the frequency range for electronic transitions to see whether the luminescence spectrum o f Eu 3+ can be used as a probe o f its local coordinations.
Low molecular weight poly (ethylene oxide) with an average molecular weight o f 400 and CH3-endc a p p e d (Polysciences) was used. By endcapping the chains, anion coordination to the chain ends could be avoided. This allows for using the low molecular weight complexes, which are easy to handle spectroscopically, as model systems for the high molecular weight complexes o f interest in technical applications [5]. The triflate salts M(CF3SO3)3, M being Li +, Eu 3+, Nd 3+, and Ce 3+ respectively, and the PEO glyme were dried separately under vacuum for several days. The p o l y m e r was then degassed on a vacuum-line by freeze-drying techniques. Samples o f concentration O: M ~ 40:1 (O: M being the ether oxygen to salt ratio) were prepared in a glovebox by dissolving the salt directly into the p o l y m e r on a hot plate ( T ~ 350 K ) . The solutions were then poured into quartz sample cells, again degassed, and thereafter sealed under vacuum. Narrow sample cells o f cylindrical shape (inner diameter ~ 2 m m ) were used to m i n i m i s e the sample volume and thereby the fluorescent background o f the R a m a n spectra, see below. The 488 nm line o f an a r g o n - i o n laser was focused into the sample cell situated inside the
To whom all correspondence should be addressed.
0167-2738/94/$ 07.00 © 1994 Elsevier Science B.V. All rights reserved.
166
G. Petersen et al. I M(CF~SO3)~-polyethercomplexes
cryostat/thermostat. A Spex double monochromator (model 1403) with 1800 lines/mm was used to resolve the spectra (resolution ~ 2 c m - ~ ) . The spectra were recorded by either a C C D (_charge _coupled _device) or a photomultiplier with typical recording times of 20 s and 5 min, respectively. Every spectrum is the result of accumulation and averaging of ~ 20 recordings.
3. Results and discussion 3.1. R a m a n spectra o f (PEO)-M(CF~SO~)x complexes
In fig. 1 the Raman spectra of Li +, Eu 3+, N d 3+ a n d Ce 3+ containing salt-polymer complexes are
presented over a wide frequency range (0-1500 cm-~). The spectra generally contain considerable background which spans the whole frequency range studied and increases with increasing frequency shift. A general observation is that the purer the salt, the smaller is the spectral background intensity. In the spectra presented in fig. 1 the background has been subtracted. The sharp modes in the spectra of the various complexes in fig. 1 (star marked in the figure) are due to the internal vibrations o f C F 3 S O 3 which is the
J!
E
i
¢~ =
I
',!
' i
1000
500
Raman
shift
(era - 1 )
Fig. 1. Raman spectra ofM(CF3SO)x-(PEO) (CH3-endcapped,
MW~400) complexes of salt concentrations M:O~I:40, M = Li +, Ce3+, Eu3+ and Nd 3+, respectively.
c o m m o n anion in all the studied complexes. The corresponding frequencies are given in table 1 and they are assigned according to the recent study by Shriver et al. [6]. It can be seen in table 1 that while the various complexes in all cases display the same set of vibrational modes for the anion significant differences in their frequencies and bandshapes arc demonstrated. Thus the triflate anions in the saltpolymer complexes are influenced by the cations and slightly differently for different cations. In the low frequency region the most intense line is the C-S stretching mode, the frequency of which changes from 313 c m - J to ~ 3 2 0 cm-~ as the valency of the cation increases from monovalent Li + to trivalent Eu 3+, Nd 3+ or Ce 3+. From literature data the 313 cm ~ mode is recognised as the vibration of a free (solvated) anion whereas the frequency increases for anions coordinated by cations in pairs or triplets [7,8]. Almost as intense as the C-S-stretch is the SOy-rocking mode at ~ 349 cm-~ observed for all the investigated salt-polymer complexes and attributed to solvated anions. In the case of the trivalent systems the mode is splitted with a smaller intensity contribution on the high frequency side. Thus the low frequency region indicates the presence of both solvated ions and ions coordinated in pairs a n d / o r multiplets in the trivalent complexes. The 500-800 c m - ~region is the range of the bending modes. The intensities of the symmetric and asymmetric SOy bending modes at about 639 and 520 c m - ~, respectively, are very weak and cannot be used for any closer analysis. The symmetric CF3 bending is strongest in intensity in this range. It is observed as a splitted mode for all the investigated complexes. The low frequency component is assigned to solvated ions [7,8] and occurs at about the same frequency ~ 755 cm-~ independent of cationic valency. The higher frequency component is stronger in intensity and is attributed to ion pairs/multiplets [ 7,8 ]. It shifts from ~ 758 c m - ~ in the Li + complex to about 763-766 cm ~ in the trivalent complexes. The band of the antisymmetric CF3 bending mode. though considerably weaker in intensity, shows in all cases splitted behavior; a low frequency component at ~ 574 c m - ~, identified as the flee ion mode, and a mode at ~ 582 cm ~ due to ion-ion coordinated configurations. The high frequency region is dominated by the S()3
G. Petersen et al. / M(CF~SO3)x-polyether complexes
167
Table 1 Comparison of (CF3SO 3 ) - bands and assignments for M (CF3SO 3 )x-PEO complexes. O: M = 40: 1.
Li
~(CS)
p(SO3)
0as(CF3)
0~(CF3)
~(SO3)
313
347
574 581
753 758
I032 1041 1051
solvated ions ion pairs multiplets
319
347 350
574 583
756 763
1021 1032 1041
ion-ion-solvent solvated ions ion pairs, multiplets
320
349 361
571 581
766
1019 1032 1039
ion-ion-solvent solvated ions ion pairs, multiplets
320
349 357
574 582
755 763
1019 1032 1039
ion-ion-solvent solvated ions ion pairs, multiplets
Eu
Nd
Ce
symmetric stretching band. This is the most intense feature of the Raman spectra and it was used for a detailed analysis. The bandshape revealed in some cases a complicated multicomponent contour. The component at ~ 1032 cm -J, attributed to free (solvated) ions, was present in all the complexes and its relative intensity was used as a measure of the concentration of solvated species. The behavior of the SO3 symmetric stretch is discussed for the various complexes below.
3.2. Ion-polymer and ion-ion interactions in PEOM(CFsSOs)x complexes In fig. 2 we show Raman spectra of the SO ~- symmetric stretch for the various complexes. Measurements were performed over a temperature range of 275-350 K and in fig. 2 the spectra obtained at the lowest and highest temperatures are demonstrated. The bandprofile was analysed in terms of a multicomponent band. In the case of the Li + complex a two-component profile with intensity peaks at 1032 c m - ~ and 1041 c m - l fitted well the spectrum at low temperatures. The two modes are assigned to vibrations of solvated triflate anions respectively anions coordinated by cations in pair configurations, (the assignment is based on a series of previous papers reviewed in ref. [5]). As temperature increases a third component at ~ 1 0 5 0 cm -~ is growing in strength. In accordance with previous observations from various monovalent (Li + and Na +) salts in
T=275
K
T=350
Li
Li
,., Eu
j
Ce
,2\
/j
,
~
=
K
Eu
'~"
.,: ,
- --
,
Ce
Nd
1020
1;60 Flaman
I
Shift
1020
-
10160
(cm "1 )
Fig. 2. Temperature evolution of the SO;- symmetric stretch band of M(CF3SO3)x-(PEO) (CH3-endcapped) complexes, M = L i ÷, Eu 3+, Nd 3+ and Ce 3÷, respectively.
polyethers the mode is assigned to vibrations ofa triflare ion in a multiplet configuration of anions-cations. By comparing the temperature dependence of the intensities of the respective modes in fig. 2 it can
168
G. Petersen et al. / M(CFsSO3)x-polyether complexes
be seen that as the Li + complex is heated the concentration of solvated ions decreases and that of the associated ions (pairs, triplets, etc.) increases as expected [ 5 ]. For the trivalent complexes a more complicated band structure is revealed, see fig. 1. The bandprofile has been analysed for the minimum number of modes to obtain a fit consistent within the experimental accuracy to the spectrum. The mode at 1032 cm-1, assigned to solvated ions, is present in all the spectra and at the same position independent of cation, as should be the case for the vibration of a free non-interacting molecular anion. From the intensity ratio of this mode and the total SO3 symmetric stretching band the amount of solvated ions for the trivalent complexes was calculated, see fig. 3. The largest concentration of free ions is found in the EU 3+ complex ( ~ 55% at room temperature) as compared to the Nd 3+ and Ce 3+ complexes ( ~ 3 0 % and ~40% respectively). We note that Eu 3+ has considerably higher charge density due to its smaller ionic radius ( 1.2/~) compared to Nd 3+ ( 1.6 ~) and Ce 3+ ( 1.85 ~,), i.e. REu3+
The larger concentration of solvated Eu 3+ ions may then be explained by their tight charge distribution which allows for strong interactions with the oxygens of the polymer chain. The relative behavior of the
A
7s[
d o 2s t
;f
•
• - ' ~ ~
w
t
N
260
I
280
300
320
Temperature
340
360
(K)
Fig. 3. Concentration of solvated ions versus temperature in M(CF3SO3)3-(PEO) (CH3 endcapped) complexes of salt concentrations M: O ~ 1 : 40, M = Eu 3+, Nd 3+ and Ce 3+.
free ion concentration in the N d 3+ and C e 3+ c o m p l e x e s , however, does not follow this scheme. We also note that the so called free-ion mode for the trivalent systems is considerably broader (6-8 cm 1) than that of the monovalent system ( ~ 4 cm-~ ). It can be explained by that the bandprofile of the trivalent complexes may include more components than used in the present analysis. The coordination number for the trivalent cations in aqueous solutions is ~ 9 as compared ~ 4 in the case of Li +. Due to configurational restrictions one can however hardly expect the polymer chain (or nearby polymer chains) to be wrapped around the trivalent ions such that the polyethers of the chain sufficiently neutralise their high ionic charge. Thus even rather well solvated cations can carry a net charge and may be coordinated not only to the ether oxygens but also to the anions. The bandprofile in the case of the trivalent systems may therefore contain modes not only due to solvated ions, ion pairs, triplets, etc. but also due to a range of anion-cation-polyether coordination possibilities. The more complicated solvation picture of the trivalent systems is further demonstrated by the behavior of the higher frequency mode in the SOy symmetric stretch profile in fig. 2 previously assigned to anion-cation pairs. For monovalent cations it has been noted that a change to a smaller cation (from Na + to Li +) results in an increased frequency shift of the ion-pair mode [ 5 ]. This is explained by the larger Coulomb attraction for the cation of higher charge density. In fig. 4a the frequency of this mode is plotted for the trivalent complexes; a larger frequency shift is indeed observed for the E u 3+ triflate pair ( 1041 c m - ~at room temperature) in consequence with its higher charge density. The Nd 3+ and C e 3+ complexes, however, show similar frequency for the ion-pair mode ( ~ 1039 cm-~ ). We also note that the frequency width of the ion-pair component increases as FEu
G. Petersen et al. / M(CF3SOs)x-polyether complexes
169
mer and for anion-cation pair formations. 1043
~ = - -
3.3. Luminescence spectra of Eu 3+ in PEOEu(CF3S03)~
1042
E
1041 r 1040
~ C
1039
E
1038 1037 Ce
I
L'
q
L,
,'
L,
~
~
I
,
,
18
'E
Ce
o
NO
•
v 16
.1I~.
@
14 Eu
1O
260
20o
300 Temperature
320
340
36o
(K)
Fig. 4. (a) Frequency shift and (b) halfwidth of the Raman mode attributed to anion-cation configurations in M(CF3SO3)3(PEO) (CH3-endcapped) complexes, M = Eu 3+, Nd 3+ and Ce 3+, respectively.
which is evident in the N d 3+ and Ce 3+ complexes and only just detectable in the E u 3+ system. For Li +containing complexes, no 1020 cm - l has been observed. For the temperature behavior of the ion solvation, see fig. 3, we notice a negative temperature dependence in accordance with the general trend observed for monovalent system [5]; for Eu 3+ and Ce 3+ the amount of solvated ions decreases considerably as temperature increases whereas for the Nd 3+ complex only a weak temperature dependence is found. The negative entropy of solution previously observed for monovalent systems has been related to the ordering imposed by the solvated cations on the polymer chains and to accompanying electrostrictive effects [5,9,10]. Similar ordering effects seem to be present in the polymer on the addition of trivalent salts. As the temperature increases the release of solvated cations from the polyethers of the chain allows for increased conformational freedom of the poly-
In the case of the E u 3+ containing complexes strong luminescence lines were observed in a frequency range 14000-17000 cm -~, see fig. 5. This corresponds to much larger frequency shifts from the laser line at ~20500 cm - t (488 nm) than those of the vibrational modes in fig. 1. The luminescent band at ~ 16200 c m - t is approximately 300 times more intense than the vibrational mode at 1032 c m - ~. Fig. 6 shows an energy level diagram of Eu 3+ (4f 6) as deduced from absorption and fluorescence spectra of LaF3:Eu 3+ [ 11 ] and the lines in fig. 5 are identified as emission from the 5Do level to the different 7Fj ground levels. Since luminescence is only observed from the 5Do level for an excitation wavelength of 488 nm, one expects efficient nonradiative transitions between the 5D~ and 5Do levels. Nonradiative transitions from 5Do state to the ground 7Fj states may also occur and are frequent in the presence of O - H bonds as has been shown for some aqueous solutions of Eu compounds [ 12 ]. The three most intense bands in fi~. 5, centred at about 14400, 16200 and 16900 c m - ~, correspond to 5 D o - 7 F J transitions to the 7F4, 7F2 and 7F l levels, respectively; the weak components at about 15400 and PEO(400)-Eu,
488
nm
excit.,
T =
7 Do - F z
5" a
300
K
7 D O- F 1
7 Do - F 4
2 5
i
14000
t
15000
7
5
t
i
t
16000
7 t
i
17000
W a v e n u r n b e r s (era 1) Fig. 5. Luminescence spectrum of Eu 3+ in a Eu (CF3SO3)3-PEO complex in the region of SD-TFj transitions, the excitation laser wavelength being 488 nm.
17 0
G. Petersen et al. / M(CF~SOsL,-pol.vether complexes 25
2 r
-
20
~
5
7
'.
5 Dj o
~O -15
o *a +a c) N
-10
17240 ¢o .0
172f30
17300
17;t20
Fig. 7. Luminescence spectra of a Eu(CF3SO3)3-PEO complex in the region of the 5D0-7F o transition of Eu 3+ for different temperatures, the excitation laser wavelength being 488 nm.
-5 4 kO
17260
Wavenumbers (cm-l/
3 2 1 0
7 Fj
Fig. 6. Energy levels of Eu 3+.
17300 cm -1 are due t o 7F 3 and 7Fo transitions. All 4Ps intraconfigurational transitions are electric dipoles (ED) forbidden in the free ion, however, become allowed as the inversion symmetry is lowered. Identification of the internal structure of the various bands shows Stark splitted components of the 7FI,2,3,4, 5 levels in low symmetry sites [11 ]. The s u p e r - f o r b i d d e n S D o - 7 F o transition achieves a weak magnetic dipole (MD) oscillator strength in low symmetry sites. Since the J = 0 states are non-degenerate the linewidth of the spectral mode corresponding to the 5Do-TFo transition gives a measure of the energy distribution of the different Eu 3+ sites. We find that the S D o - T F o mode is asymmetric in shape, see fig. 7, which indicates that the E u 3+ ions in PEO are subject to distinctly different environments and experience different chemical coordinations. In fact, Eu 3+ is commonly used to investigate the local structure and there are literature data for a wide variety of heterogeneous systems [ 13-15 ]. As the temperature increases, the total emission intensity increases, the band shifts towards higher energies, and there is a change of the bandshape, which suggest a redistribution of the E u 3+ sites as the temperature increases. This is in accordance with the vibrational data in figs. 2 and 3 from which it is concluded that the amount o f E u 3+ ions coordinated to the ether oxygens is decreasing with increasing temperature in favour of increased ion-ion pairing phenomena. To
investigate in detail the local structure of Eu 3+ from its luminescent properties we are at present applying laser excited site selective spectroscopy, a technique that is particularly well suited for structural investigations. Preliminary results show clear evidence of at least two Eu 3+ environments.
4. Conclusion From the Raman vibrational studies we conclude that ion solvation of trivalent M(CF3SO3)3 salts in PEO is described by rich coordination possibilities. There are evidences of solvated ions and ions in various anion-cation configurations, as previously have been observed for monovalent salt-polymer complexes. The spectra however reveal new ionic configurations not reported for the monovalent complexes. We attribute the latter to combined cationanion-polyether coordinations in which the number of participant species may vary. Comparing the solubility of the studied lanthanides Eu 3+, Nd 3+ and Ce 3+, the largest concentration of solvated ions is found for the Eu 3+ which we attribute to its larger charge density. The strong luminescence spectra of the Eu 3+-complex, which fluoresce red, suggest that Eu 3+ has several coordination environments. Changing the temperature a clear redistribution of the Eu 3+ sites is noted. This is in support of the vibrational data which show decreased solubility and increased cationic-
G. Petersen et al. / M(CF3SO~)x-polyether complexes
anionic interactions as temperature increases.
References [ 1 ] R. Huq and G.C. Farrington, in: 2nd Intern. Symp. on Polymer Electrolytes, ed. B. Scrosati (Elsevier, Amsterdam, 1990) pp. 273. [2] A.S.R. Machado and L. Alcacer, in: 2nd Intern. Symp. on Polymer Electrolytes, ed. B. Scrosati (Elsevier, Amsterdam, 1990) pp. 283. [3] P.G. Bruce, J. Nowinski, F.M. Gray and C.A. Vincent, Solid State lonics 38 (1990) 231. [4] M.J. Smith and C.J.R. Silva, Solid State lonics 58 (1992) 269. [5] L.M. Torell, P. Jacobsson and G. Petersen, Polym. Adv. Techn. 4 (1993) 152. [6 ] D.H. Johnston and D.F. Shriver, Inorg. Chem. 32 ( 1993 ) 1045.
171
[7] M.G. Miles, G. Doyle, R.P. Cooney and R.S. Tobias, Spectrochim. Acta 25A (1969) 1515. [8 ] S. Schantz, J. Sandahl, L. B6rjesson, J.R. Stevens and UM. Torell, Solid State Ionics 28-30 (1988) 1047. [9] M.A. Ratner and A. Nitzan, Faraday Discuss. Chem. Soc. 88 (1989) 19. [10] R. Olender and A. Nitzan, Electrochim. Acta 37 (1992) 1505. [ 11 ] U.V. Kumar, D.R. Rao and P. Venkateswarler, J. Chem. Phys. 66 (1977) 2019. [ 12 ] G. Blasse, G.J. Dirksen, P. van der Voort, N. Sabbatini, S. Perathoner, J.-H. Lehn and B. Alpha, Chem. Phys. Lett. 146 (1988) 347. [13]A.A. Kapylanskii and R.M. Macfarlane, Spectroscopy of Solids Containing Rare Earth Ions (North-Holland, Amsterdam, 1987). [14] M.F. Hazenkamp and G. Blasse, Chem. Mater. 2 (1990) 105. [ 15] D. Levy, R. Reisfeld and D. Avnir, Chem. Phys. Lett. 109 (1984) 593.