- Email: [email protected]
Enhanced Eu3+ luminescence in a new hybrid material with an open-framework structure
ARTICLE IN PRESS
Journal of Luminescence 122–123 (2007) 492–495 www.elsevier.com/locate/jlumin
Enhanced Eu3+ luminescence in a new hybrid material with an open-framework structure F. Pelle´a,, S. Surble´b, C. Serreb, F. Millangeb, G. Fe´reyb a
LCAES–Chimie de la Matie`re Condense´e de Paris UMR 7574 CNRS ENSCP, 11 rue Pierre et Marie Curie, F 75231 Paris Cedex, France Institut Lavoisier, UMR CNRS 8637, Universite´ de Versailles St-Quentin-en-Yvelines, 45 Avenue des Etats-Unis, 78035 Versailles Cedex, France
b
Available online 24 March 2006
Abstract Organic–inorganic hybrid materials with open-framework structure are currently studied since they represent a class of materials with a great potential as luminescent materials for photonic applications. In this aim, optical properties of Eu3+ in a hybrid material Ln((C6H3)–CO2)3) (LnY, Eu, Gd) has been investigated. Its three-dimensional structure can be described as a one-dimensional inorganic sub-network related together via organic acids creating an open-framework structure. Doping with Gd3+ ion allows observing the emission of the ligand triplet state at room temperature. Eu3+ excitation spectra demonstrate an efficient energy ligand–rare-earth energy transfer process. Site-selective excitation and the temperature dependence of the 5D0 decay profile on the full concentrated sample as a function of temperature indicate energy migration along the chains. r 2006 Elsevier B.V. All rights reserved. Keywords: Rare-earth spectroscopy; Hybrid materials; Energy transfer
Since the antenna effect has been proposed to operate in lanthanide (Ln3+) ion complexes [1], it appears as an alternative to overcome the very small absorption coefficients of Ln3+ ions and to realize efficient UV light conversion devices. Then intensive research has been devoted to the design of efficient UV light converter devices and new concepts of rare-earth-doped materials have emerged [2,3]. Among them, organic–inorganic hybrid materials with open-framework structure are currently studied since they represent a class of materials with a great potential as luminescent materials for photonic applications. In this aim, a new strongly fluorescent three-dimensional Ln3+ doped trimesate MIL-78 or Ln((C6H3)–CO2)3) (LnY, Eu, Gd) has been synthesized and Eu3+ optical properties have been investigated.
Corresponding author. Tel.: +33 1 53 73 79 33; fax: +33 1 46 34 74 89
E-mail address: [email protected] (F. Pelle´). 0022-2313/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2006.01.216
1. Experimental 1.1. Synthesis and structure MIL-78 was hydrothermally synthesized at 493 K from a mixture of rare-earth nitrate M(NO3)3 5H2O (Aldrich, 98%) (MY, Ln), trimesic acid (C6H3)–(CO2H)3 (Alfa, 98%), sodium hydroxide NaOH (Prolabo, 99%) and H2O in the molar ratio 1:1–2:2:400. The infra-red spectrum of the compound clearly shows the presence of the vibrational bands characteristic of the –(O–C–O)– groups around 1550 and 1430 cm1 confirming the presence of the tricarboxylate within the solids. MIL-78 (Y, Eu) exhibits a three-dimensional structure (space group C2/m—N112) built up from eight coordinated monocapped square antiprism polyhedra and trimesate ions. However, this structure possesses a one-dimensional inorganic sub-network related together via organic acids creating an open-framework structure (Fig. 1). The steric hindrance of the organic moieties leaves no free aperture within the small channels delimited by the inorganic chains. Each yttrium or rare-earth ion is surrounded by eight
ARTICLE IN PRESS F. Pelle´ et al. / Journal of Luminescence 122–123 (2007) 492–495
493
dispersed through an HD10 Jobin–Yvon monochromator for UV excitation or the 465.8 nm Innova 300 argon laser line in the visible. The fluorescence was dispersed through through a HR1000 Jobin–Yvon monochromator. Continuous Wave selective excitation was provided by an Ar+ pumped CR590-03 laser dye, the signal detected by a RTC 56 TVP photomultiplier and amplified by a PAR 128a lock-in amplifier with data acquisition on a microcomputer. For time-resolved measurement, the Eu3+ luminescence was excited by an OPO pumped by the third harmonic of a pulsed Nd:YAG laser (501-DNS,720 BM Industrie), the luminescence was dispersed through a HR460 Jobin–Yvon monochromator and detected by a EMI 9558 QBM photomultiplier. Fluorescence transients were digitized and averaged by an oscilloscope (TDS350 Tektronix) with data acquisition on a PC. Low temperature measurements were performed by cooling the samples in a closed-cycle CTI cooling system. Fig. 1. Polyhedral representation of the MIL-78 (Y, Eu or Gd) structure along the c axis. RE polyhedra are represented in grey, C atoms and black.
2. Optical properties: results and discussion
PL Intensity
2.1. Non-selective excitation
(b) (a)
400
450
500
550 600 Wavelength (nm)
650
700
Fig. 2. Emission spectra recorded with UV excitation (lexc ¼ 300 nm) at 300 K): (a) solid line Eu((C6H3)–CO2)3) and (b) dotted line Y((C6H3)–CO2)3):Gd3+ (2%).
oxygen atoms coming exclusively from the trimesate moieties. Pure rare-earth forms of MIL-78 (LnPr, Nd, Sm, Eu, Gd, Tb, Dy, Y, Er), synthesized using similar conditions, exhibit the same structure. Full details on synthesis and structure analysis have been already published [4]. 1.2. Spectroscopic measurements Optical properties of MIL-78(Y, Eu, Gd) were investigated using as excitation source a 150 W XBO xenon lamp
Under excitation at 254 nm or in the visible (465.8 nm), five transitions are observed on the luminescence spectrum of Eu3+ in (Y0.98Eu0.02)((C6H3)–CO2)3) and Eu((C6H3)– CO2)3). The emission spectrum of the full concentrated sample Eu((C6H3)–CO2)3) is represented on Fig. 2 curve (a). These transitions can be well interpreted on the basis of electronic transitions from the 5D0(Eu3+) emitting state to the 7FJ (J ¼ 0–4) multiplets. Two lines recorded at 579.1 and 579.7 nm corresponding to the 5D0-7F0 transition spectral range, reveal that two optical sites are occupied by Eu3+ in that structure. However, the intensity of the spectrum 5D0-7F0 which is observed at 579.1 nm is very weak. From laser site selective excitation (Section 2.2) this minor site can be ascribed to Eu3+ ions in a parasitic phase in very low concentration or in the lattice nearby an impurity or a distortion due to local strain which will play the role of trap for the energy migration (Section 2.4). The excitation spectrum monitoring the hypersensitive 5 D0-7F2 transition exhibits a very intense band located in the UV range (200–340 nm), the transitions from the Eu3+ ground state 7F0 to the 5L6 and 5D2 states being very weak as shown on Fig. 3. To determine the energy of the lowest ligand triplet state, the emission spectra and decay time measurements for the Gd3+-doped material were performed. Since the 6P7/2 emitting level of Gd3+ is located at high energy, it allows the identification of the lowest triplet state when situated at lower energy. The UV excitation of Y((C6H3)–CO2)3):Gd3+ (2%) gives rise to two structured emission bands located at 480 and 576 nm, respectively, at room temperature (Fig. 2 curve (b)). These emissions which cannot be ascribed to electronic transitions within the 4f7 (Gd3+) configuration are ascribed to emission from the ligand triplet states 3T1 to the ground state 1S0. The absence of Gd3+ emission means that the triplet states lye
ARTICLE IN PRESS F. Pelle´ et al. / Journal of Luminescence 122–123 (2007) 492–495
494
PL Intensity
PL Intensity
1
300K (doped) 10K 50K 125K 300K
0.1
0.01
1E-3 200
250
300
350 400 Wavelength (nm)
450
500
550
0
2000
4000 6000 Time (µs)
8000
10000
Fig. 3. Excitation spectra (T ¼ 300 K): (a) solid line: Eu((C6H3)–CO2)3) monitoring 5D0-7F2 (Eu3+) transition and (b) dotted line: Y((C6H3)– CO2)3):Gd3+ (2%), monitoring at 480 nm.
Fig. 4. 5D0 fluorescent transient as a function of temperature.
at lower energy than the lowest excited state of Gd3+ ions precluding any energy transfer from the triplet states of the ligand to Gd3+ ions [3]. These two emission bands are excited in the UV range from 320 to 260 nm (Fig. 2(b)). The Y((C6H3)–CO2)3):Gd3+ (2%) optical properties allow us a complete determination of the ligand energy level scheme. The triplet state excitation spectrum is quite similar to that of the 5D0 (Eu3+) level in the UV range. The triplet state emission is completely quenched in the 0.2 at% Eu-doped compound, the ratio of the UV band intensity to the 7 F0-5L6 and 5D2 excitation lines is 100 and 200, respectively. This result allows deducing for an efficient energy transfer from the ligand triplet state to the Eu3+ states when excited in UV.
exponential law increases with temperature (Fig. 4). This temperature dependence can be explained by assuming a temperature dependence of the migration probability.
2.2. Site-selective spectroscopy Selective excitation into the 5D0-7F0 lines at 579.1 and 579.7 nm was performed at low temperature (T ¼ 10 K) and a complex emission spectrum which contains the contribution of both Eu3+ types is recorded. No simplification of the spectrum can be observed. This result compared to the 5D0 decay profile recorded in the full concentrated sample as a function of the temperature suggests that energy migration is occuring. 2.3. Time-resolved spectroscopy The 5D0 fluorescence transient recorded as a function of temperature in the diluted sample (Y0.98Eu0.02)((C6H3)–CO2)3 exhibits an exponential function with time whatever the temperature. The decay time, varying from 3.51 to 3.24 ms from 10 K to room temperature reflects a very low non-radiative contribution by multiphonon relaxation. On the opposite, above 50 K, the 5D0 fluorescence transient in the full concentrated sample exhibits a non exponential behaviour with time, the discrepancy to an
2.4. Modelling the temporal transient of 5D0 state Site-selective spectroscopy allowed ascribing the nonexponential behaviour of the intensity with time to a migration process within Eu3+ ions. Taking into account of the structural data, especially the distances between rare earth ions in adjacent sites within a chain (0.392 nm) and those between adjacent chains (0.544 and 0.926 nm), threedimensional interactions seem unlikely. The decay profiles as a function of time and temperature were fit using the model of Balagurov and Vaks (BV) assuming a onedimensional migration with randomly distributed traps [5]. This model assumes that impurities are randomly distributed along the chain. The asymptotic form of the decay is expressed as " 1=3 # 16 k1 t 1=2 k1 t IðtÞ ¼ I 0 exp W r t 3 , (1) p 3p 4 where Wr is the radiative rate for the isolated centre, k1 stands for the trapping rate parameter. In Eq. (1) the term in the exponential that reflects onedimensional trapping depends on t1/3. Using Eq. (1) and Wr(T) values deduced from the diluted sample, the k1 constant was determined and its temperature dependence has been derived. Several mechanisms have to be considered to explain the temperature behaviour of the diffusion constant, i.e. one-phonon process, Orbach process, two-sites resonant Raman process and one-site resonant Raman process [6]. Fitting k1 ¼ f(T) by taking into account for all possible processes, demonstrates that the Raman processes do not contribute to the temperature dependence and the one phonon process is negligible.
ARTICLE IN PRESS F. Pelle´ et al. / Journal of Luminescence 122–123 (2007) 492–495
A plot of ln(k1) versus reciprocal temperature gives an activation energy of 168 cm1. This energy cannot be ascribed to the lowest Stark component of the 7F1 state (278 cm1). Further experiments are underway to understand the role of the electronic structure of the ligand in the UV excitation process and of the exchange interaction between Eu3+ which should operate. 3. Conclusion In this material, we have demonstrated an efficient energy transfer ligand–rare earth also called antenna effect. The ligand triplet state is located 200 cm1 higher than the 5 D0(Eu3+) emitting level. Furthermore, migration of the excitation is found to occur along the linear chains of the rare-earth polyedra with an activation energy quite
495
similar to the energy separation between the triplet state and the 5D0(Eu3+) level. Further experiments are underway to identify clearly the role of the triplet state in the migration process. References [1] S.I. Weissman, J. Chem. Phys. 10 (1942) 214. [2] G.F. de Sa´, O.L. Malta, C. de Mello Donega´, A.M. Simas, R.L. Longo, P.A. Santa-Cruz, E.F. da Silva Jr., Coord. Chem. Rev. 196 (2000) 165 and references therein. [3] D. Imbert, S. Comby, A. Chauvin, J.C. Bu¨nzli, Chem. Commun. 11 (2005) 1432 and references therein. [4] C. Serre, F. Millange, C. Thouvenot, N. Gardant, F. Pelle´, G. Fe´rey, J. Mat. Chem. 14 (2004) 1540. [5] B.Y. Balagurov, V.G. Vaks, Sov. Phys. JETP 38 (1974) 968. [6] P.A.M. Berdowski, G. Blasse, J. Lumin. 29 (1984) 243.
Journal of Luminescence 122–123 (2007) 492–495 www.elsevier.com/locate/jlumin
Enhanced Eu3+ luminescence in a new hybrid material with an open-framework structure F. Pelle´a,, S. Surble´b, C. Serreb, F. Millangeb, G. Fe´reyb a
LCAES–Chimie de la Matie`re Condense´e de Paris UMR 7574 CNRS ENSCP, 11 rue Pierre et Marie Curie, F 75231 Paris Cedex, France Institut Lavoisier, UMR CNRS 8637, Universite´ de Versailles St-Quentin-en-Yvelines, 45 Avenue des Etats-Unis, 78035 Versailles Cedex, France
b
Available online 24 March 2006
Abstract Organic–inorganic hybrid materials with open-framework structure are currently studied since they represent a class of materials with a great potential as luminescent materials for photonic applications. In this aim, optical properties of Eu3+ in a hybrid material Ln((C6H3)–CO2)3) (LnY, Eu, Gd) has been investigated. Its three-dimensional structure can be described as a one-dimensional inorganic sub-network related together via organic acids creating an open-framework structure. Doping with Gd3+ ion allows observing the emission of the ligand triplet state at room temperature. Eu3+ excitation spectra demonstrate an efficient energy ligand–rare-earth energy transfer process. Site-selective excitation and the temperature dependence of the 5D0 decay profile on the full concentrated sample as a function of temperature indicate energy migration along the chains. r 2006 Elsevier B.V. All rights reserved. Keywords: Rare-earth spectroscopy; Hybrid materials; Energy transfer
Since the antenna effect has been proposed to operate in lanthanide (Ln3+) ion complexes [1], it appears as an alternative to overcome the very small absorption coefficients of Ln3+ ions and to realize efficient UV light conversion devices. Then intensive research has been devoted to the design of efficient UV light converter devices and new concepts of rare-earth-doped materials have emerged [2,3]. Among them, organic–inorganic hybrid materials with open-framework structure are currently studied since they represent a class of materials with a great potential as luminescent materials for photonic applications. In this aim, a new strongly fluorescent three-dimensional Ln3+ doped trimesate MIL-78 or Ln((C6H3)–CO2)3) (LnY, Eu, Gd) has been synthesized and Eu3+ optical properties have been investigated.
Corresponding author. Tel.: +33 1 53 73 79 33; fax: +33 1 46 34 74 89
E-mail address: [email protected] (F. Pelle´). 0022-2313/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2006.01.216
1. Experimental 1.1. Synthesis and structure MIL-78 was hydrothermally synthesized at 493 K from a mixture of rare-earth nitrate M(NO3)3 5H2O (Aldrich, 98%) (MY, Ln), trimesic acid (C6H3)–(CO2H)3 (Alfa, 98%), sodium hydroxide NaOH (Prolabo, 99%) and H2O in the molar ratio 1:1–2:2:400. The infra-red spectrum of the compound clearly shows the presence of the vibrational bands characteristic of the –(O–C–O)– groups around 1550 and 1430 cm1 confirming the presence of the tricarboxylate within the solids. MIL-78 (Y, Eu) exhibits a three-dimensional structure (space group C2/m—N112) built up from eight coordinated monocapped square antiprism polyhedra and trimesate ions. However, this structure possesses a one-dimensional inorganic sub-network related together via organic acids creating an open-framework structure (Fig. 1). The steric hindrance of the organic moieties leaves no free aperture within the small channels delimited by the inorganic chains. Each yttrium or rare-earth ion is surrounded by eight
ARTICLE IN PRESS F. Pelle´ et al. / Journal of Luminescence 122–123 (2007) 492–495
493
dispersed through an HD10 Jobin–Yvon monochromator for UV excitation or the 465.8 nm Innova 300 argon laser line in the visible. The fluorescence was dispersed through through a HR1000 Jobin–Yvon monochromator. Continuous Wave selective excitation was provided by an Ar+ pumped CR590-03 laser dye, the signal detected by a RTC 56 TVP photomultiplier and amplified by a PAR 128a lock-in amplifier with data acquisition on a microcomputer. For time-resolved measurement, the Eu3+ luminescence was excited by an OPO pumped by the third harmonic of a pulsed Nd:YAG laser (501-DNS,720 BM Industrie), the luminescence was dispersed through a HR460 Jobin–Yvon monochromator and detected by a EMI 9558 QBM photomultiplier. Fluorescence transients were digitized and averaged by an oscilloscope (TDS350 Tektronix) with data acquisition on a PC. Low temperature measurements were performed by cooling the samples in a closed-cycle CTI cooling system. Fig. 1. Polyhedral representation of the MIL-78 (Y, Eu or Gd) structure along the c axis. RE polyhedra are represented in grey, C atoms and black.
2. Optical properties: results and discussion
PL Intensity
2.1. Non-selective excitation
(b) (a)
400
450
500
550 600 Wavelength (nm)
650
700
Fig. 2. Emission spectra recorded with UV excitation (lexc ¼ 300 nm) at 300 K): (a) solid line Eu((C6H3)–CO2)3) and (b) dotted line Y((C6H3)–CO2)3):Gd3+ (2%).
oxygen atoms coming exclusively from the trimesate moieties. Pure rare-earth forms of MIL-78 (LnPr, Nd, Sm, Eu, Gd, Tb, Dy, Y, Er), synthesized using similar conditions, exhibit the same structure. Full details on synthesis and structure analysis have been already published [4]. 1.2. Spectroscopic measurements Optical properties of MIL-78(Y, Eu, Gd) were investigated using as excitation source a 150 W XBO xenon lamp
Under excitation at 254 nm or in the visible (465.8 nm), five transitions are observed on the luminescence spectrum of Eu3+ in (Y0.98Eu0.02)((C6H3)–CO2)3) and Eu((C6H3)– CO2)3). The emission spectrum of the full concentrated sample Eu((C6H3)–CO2)3) is represented on Fig. 2 curve (a). These transitions can be well interpreted on the basis of electronic transitions from the 5D0(Eu3+) emitting state to the 7FJ (J ¼ 0–4) multiplets. Two lines recorded at 579.1 and 579.7 nm corresponding to the 5D0-7F0 transition spectral range, reveal that two optical sites are occupied by Eu3+ in that structure. However, the intensity of the spectrum 5D0-7F0 which is observed at 579.1 nm is very weak. From laser site selective excitation (Section 2.2) this minor site can be ascribed to Eu3+ ions in a parasitic phase in very low concentration or in the lattice nearby an impurity or a distortion due to local strain which will play the role of trap for the energy migration (Section 2.4). The excitation spectrum monitoring the hypersensitive 5 D0-7F2 transition exhibits a very intense band located in the UV range (200–340 nm), the transitions from the Eu3+ ground state 7F0 to the 5L6 and 5D2 states being very weak as shown on Fig. 3. To determine the energy of the lowest ligand triplet state, the emission spectra and decay time measurements for the Gd3+-doped material were performed. Since the 6P7/2 emitting level of Gd3+ is located at high energy, it allows the identification of the lowest triplet state when situated at lower energy. The UV excitation of Y((C6H3)–CO2)3):Gd3+ (2%) gives rise to two structured emission bands located at 480 and 576 nm, respectively, at room temperature (Fig. 2 curve (b)). These emissions which cannot be ascribed to electronic transitions within the 4f7 (Gd3+) configuration are ascribed to emission from the ligand triplet states 3T1 to the ground state 1S0. The absence of Gd3+ emission means that the triplet states lye
ARTICLE IN PRESS F. Pelle´ et al. / Journal of Luminescence 122–123 (2007) 492–495
494
PL Intensity
PL Intensity
1
300K (doped) 10K 50K 125K 300K
0.1
0.01
1E-3 200
250
300
350 400 Wavelength (nm)
450
500
550
0
2000
4000 6000 Time (µs)
8000
10000
Fig. 3. Excitation spectra (T ¼ 300 K): (a) solid line: Eu((C6H3)–CO2)3) monitoring 5D0-7F2 (Eu3+) transition and (b) dotted line: Y((C6H3)– CO2)3):Gd3+ (2%), monitoring at 480 nm.
Fig. 4. 5D0 fluorescent transient as a function of temperature.
at lower energy than the lowest excited state of Gd3+ ions precluding any energy transfer from the triplet states of the ligand to Gd3+ ions [3]. These two emission bands are excited in the UV range from 320 to 260 nm (Fig. 2(b)). The Y((C6H3)–CO2)3):Gd3+ (2%) optical properties allow us a complete determination of the ligand energy level scheme. The triplet state excitation spectrum is quite similar to that of the 5D0 (Eu3+) level in the UV range. The triplet state emission is completely quenched in the 0.2 at% Eu-doped compound, the ratio of the UV band intensity to the 7 F0-5L6 and 5D2 excitation lines is 100 and 200, respectively. This result allows deducing for an efficient energy transfer from the ligand triplet state to the Eu3+ states when excited in UV.
exponential law increases with temperature (Fig. 4). This temperature dependence can be explained by assuming a temperature dependence of the migration probability.
2.2. Site-selective spectroscopy Selective excitation into the 5D0-7F0 lines at 579.1 and 579.7 nm was performed at low temperature (T ¼ 10 K) and a complex emission spectrum which contains the contribution of both Eu3+ types is recorded. No simplification of the spectrum can be observed. This result compared to the 5D0 decay profile recorded in the full concentrated sample as a function of the temperature suggests that energy migration is occuring. 2.3. Time-resolved spectroscopy The 5D0 fluorescence transient recorded as a function of temperature in the diluted sample (Y0.98Eu0.02)((C6H3)–CO2)3 exhibits an exponential function with time whatever the temperature. The decay time, varying from 3.51 to 3.24 ms from 10 K to room temperature reflects a very low non-radiative contribution by multiphonon relaxation. On the opposite, above 50 K, the 5D0 fluorescence transient in the full concentrated sample exhibits a non exponential behaviour with time, the discrepancy to an
2.4. Modelling the temporal transient of 5D0 state Site-selective spectroscopy allowed ascribing the nonexponential behaviour of the intensity with time to a migration process within Eu3+ ions. Taking into account of the structural data, especially the distances between rare earth ions in adjacent sites within a chain (0.392 nm) and those between adjacent chains (0.544 and 0.926 nm), threedimensional interactions seem unlikely. The decay profiles as a function of time and temperature were fit using the model of Balagurov and Vaks (BV) assuming a onedimensional migration with randomly distributed traps [5]. This model assumes that impurities are randomly distributed along the chain. The asymptotic form of the decay is expressed as " 1=3 # 16 k1 t 1=2 k1 t IðtÞ ¼ I 0 exp W r t 3 , (1) p 3p 4 where Wr is the radiative rate for the isolated centre, k1 stands for the trapping rate parameter. In Eq. (1) the term in the exponential that reflects onedimensional trapping depends on t1/3. Using Eq. (1) and Wr(T) values deduced from the diluted sample, the k1 constant was determined and its temperature dependence has been derived. Several mechanisms have to be considered to explain the temperature behaviour of the diffusion constant, i.e. one-phonon process, Orbach process, two-sites resonant Raman process and one-site resonant Raman process [6]. Fitting k1 ¼ f(T) by taking into account for all possible processes, demonstrates that the Raman processes do not contribute to the temperature dependence and the one phonon process is negligible.
ARTICLE IN PRESS F. Pelle´ et al. / Journal of Luminescence 122–123 (2007) 492–495
A plot of ln(k1) versus reciprocal temperature gives an activation energy of 168 cm1. This energy cannot be ascribed to the lowest Stark component of the 7F1 state (278 cm1). Further experiments are underway to understand the role of the electronic structure of the ligand in the UV excitation process and of the exchange interaction between Eu3+ which should operate. 3. Conclusion In this material, we have demonstrated an efficient energy transfer ligand–rare earth also called antenna effect. The ligand triplet state is located 200 cm1 higher than the 5 D0(Eu3+) emitting level. Furthermore, migration of the excitation is found to occur along the linear chains of the rare-earth polyedra with an activation energy quite
495
similar to the energy separation between the triplet state and the 5D0(Eu3+) level. Further experiments are underway to identify clearly the role of the triplet state in the migration process. References [1] S.I. Weissman, J. Chem. Phys. 10 (1942) 214. [2] G.F. de Sa´, O.L. Malta, C. de Mello Donega´, A.M. Simas, R.L. Longo, P.A. Santa-Cruz, E.F. da Silva Jr., Coord. Chem. Rev. 196 (2000) 165 and references therein. [3] D. Imbert, S. Comby, A. Chauvin, J.C. Bu¨nzli, Chem. Commun. 11 (2005) 1432 and references therein. [4] C. Serre, F. Millange, C. Thouvenot, N. Gardant, F. Pelle´, G. Fe´rey, J. Mat. Chem. 14 (2004) 1540. [5] B.Y. Balagurov, V.G. Vaks, Sov. Phys. JETP 38 (1974) 968. [6] P.A.M. Berdowski, G. Blasse, J. Lumin. 29 (1984) 243.