Eu3+-Mixed

The excitation of the Tb3+ 7F6 → 5D4 transitions produces not only the emission lines of Tb3+ but also those of Eu3+, accompanied by nonexponential ...
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8850

J. Phys. Chem. B 1999, 103, 8850-8857

Luminescence and Energy Transfer Phenomena in Tb3+/Eu3+-Mixed Polyoxometallolanthanoates K15H3[Tb1.4Eu1.6(H2O)3(SbW9O33)(W5O18)3]‚25.5H2O and Na7H19[Tb4.3Eu1.7O2(OH)6(H2O)6Al2(Nb6O19)5]‚47H2O Toshihiro Yamase* and Haruo Naruke Research Laboratory of Resources Utilization, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan ReceiVed: May 10, 1999; In Final Form: August 24, 1999

The energy dissipation of Tb3+/Eu3+ cations in both heterolanthanide multinuclear polyoxometalates, K15H3[Tb1.4Eu1.6(H2O)3(SbW9O33)(W5O18)3]‚25.5H2O and Na7H19[Tb4.3Eu1.7O2(OH)6(H2O)6Al2(Nb6O19)5]‚47H2O is studied by crystal structures, emission and excitation spectra, and emission decay dynamics. The excitation of the Tb3+ 7F6 f 5D4 transitions produces not only the emission lines of Tb3+ but also those of Eu3+, accompanied by nonexponential rise and decay curves of the emission from Tb3+ and Eu3+. There is no significant exchange interaction between the lanthanide ions, as a result of the coordination of aqua and/or hydroxo ligands to the lantahanide ions. The mechanism of the Tb3+ f Eu3+ energy transfer is identified as a Fo¨rster-Dexter-type energy transfer from Tb3+ (donor) to Eu3+ (acceptor). At low temperatures 5D4(Tb) + 7F (Eu) f 7F (Tb) + 5D (Eu) governs the transfer process, and at high temperatures it is governed by 5D 0 4 0 4 (Tb) + 7F1(Eu) f 7F5(Tb) + 5D1(Eu), 5D4(Tb) + 7F1(Eu) f 7F4(Tb) + 5D0(Eu), and 5D4(Tb) + 7F2(Eu) f 7F (Tb) + 5D (Eu) interactions which involve the thermally populated 7F and 7F levels. The nearest-neighbor 5 1 1 2 energy-transfer rates by electric dipole-dipole interactions between a Tb-Eu pair at 4.2 K are estimated to be 4.5 × 104 and 4.7 × 105 s-1, and the critical radii at 4.2 K are 10.3 and 10.0 Å for K15H3[Tb1.4Eu1.6(H2O)3(SbW9O33)(W5O18)3]‚25.5H2O (with Tb-Eu separation of 5.05 Å) and Na7H19[Tb4.3Eu1.7O2(OH)6(H2O)6Al2(Nb6O19)5]‚47H2O (with 3.76 Å separation), respectively. The low symmetry (Cs for the former and C1 for the latter) of the LnO8 (Ln ) Tb and Eu) coordination polyhedra allows the nonvanishing electric dipole transition probability for the 7FJ T 5D0 (J ) 0,1) transitions which leads to a faster transter rate at high temperatures.

Introduction We have investigated the intramolecular transfer of the oxygen-to-metal charge-transfer {O f M () Nb, Mo, W) lmct} energy to the Eu3+ site in the polyoxometalloeuropate lattices for understanding the molecular insight into the transfer of the excitation energy of the host lattice to luminecence centers in the Eu3+-doped metal oxide phosphors, such as Gd2(WO4)3/ Eu, Y2WO6/Eu, and YNbO4/Eu,1 and discussed the photoluminescence properties of the polyoxometalloeuropates in terms of both the energy transfer from the O f M lmct triplet states and the nonradiative relaxation of the 5D0 state of Eu3+.2 Of particular interest for the energy transport phenomena among lanthanide centers in the oxide lattices is Eu multinuclear polyoxometalate complexes, in which the Eu3+ ions are bridged by oxo, hydroxo, and water oxygen ligands to form -O-EuO-Eu- rings, as exemplified by K15H3[Eu3(H2O)3(SbW9O33)(W5O18)3]‚25.5H2O and Na7H19{[Eu3O(OH)3(H2O)3]2Al2(Nb6O19)5}‚47H2O.3,4 Parts a and b of Figure 1 show the structural features of the central Eu3+ aggregates for K15H3[Eu3(H2O)3(SbW9O33)(W5O18)3]‚25.5H2O and Na7H19{[Eu3O(OH)3(H2O)3]2Al2(Nb6O19)5}‚47H2O, respectively. In the anion of K15H3[Eu3(H2O)3(SbW9O33)(W5O18)3]‚25.5H2O, a central trinuclear Eu3(H2O)3 core tetrahedrally arranges one B-R-type SbW9O33 and three W5O18 ligands, giving an approximate pointsymmetry of C3V. Each Eu3+ in the Eu3(H2O)3 core achieves square antiprismatically 8-fold coordination (with an approximate symmetry of Cs) by attachment to four oxygens from

one W5O18, two oxygens from one SbW9O33, and two aquaoxygen ligands. The nearest Eu‚‚‚Eu distances in the trimeric [Eu3(H2O)3]9+ core singly bridged by water oxygen atoms are 5.015(5)-5.067(4) Å [average, 5.050(3) Å].3 The anion of Na7H19{[Eu3O(OH)3(H2O)3]2Al2(Nb6O19)5}‚47H2O consists of two [Eu3O(OH)3(H2O)3]4+ clusters, two Al3+ cations, and five [Nb6O19]8- (apically two and equatorially three) anions. Each Eu3+ achieves a bicapped trigonal prismatically 8-fold coordination (with a C1 symmetry) via one µ3-O, two µ3-OH- ions, one terminal water oxygen, and four oxygen atoms belonging to the equatorial [Nb6O19]8- groups. The nearest Eu‚‚‚Eu distances in the [Eu3O(OH)3(H2O)3]4+ half-core are 3.740(7)-3.777(5) Å [average, 3.756(2) Å]). Two half-cores are linked by three µ4-O atoms, each of which belongs to each of three equatorial [Nb6O19]8- ligands, with the Eu‚‚‚Eu distances of 4.609(6)4.763(6) Å [average, 4.69(2) Å].4 Such Eu multinuclear polyoxometalloeuropates exhibited a simple exponential decay of the 5D0 emission with moderate quantum yield of emission, implying that the energy migration between two Eu3+ sites due to exchange interaction is negligible.2b A variety of the Ln multinuclear polyoxometallolanthanoates (Ln ) Er, Lu, and so on) with the same structure of anions have been prepared, and the structural change by the nucleation of heterolanthanide cations was minor, as far as we compared the crystallographic structure among three {[Ln3O(OH)3(H2O)3]2Al2(Nb6O19)5}16- (Ln ) Eu, Er, and Lu) anions which showed a small change (within approximately 0.1

10.1021/jp991536d CCC: $18.00 © 1999 American Chemical Society Published on Web 09/30/1999

Luminescence and Energy Transfer Phenomena

J. Phys. Chem. B, Vol. 103, No. 42, 1999 8851 47H2O (4) is presented here in order to determine the energy transfer channels and to identify the type of the interaction between the partners involved in the transfer, together with photoluminescence properties for Tb3+ in 1 and 2. 3 is regarded as a disordered mixture of Tb/Eu ) 1:2 and 2:1, and 4 as the one of Tb/Eu ) 4:2 and 5:1. Thereby, it is reasonable to assume that the energy transfer from Tb3+ to Eu3+ for 3 and 4 occurs exclusively at the shortest distance of Tb‚‚‚Eu which is close to the shortest Eu‚‚‚Eu (5.05 Å for 3 and 3.76 Å for 4) for the coresponding pure Eu complex, although it is difficult to distinguish X-ray crystallographically between Tb and Eu. Experimental Section

Figure 1. Schematic representations and Eu‚‚‚Eu distances of the anions of K15H3[Eu3(H2O)3(SbW9O33)(W5O18)3]‚25.5H2O (a) and Na7H19{[Eu3O(OH)3(H2O)3]2Al2(Nb6O19)5}‚47H2O (b). A schematic representation of the EuO8 coordination geometry for each anion is also shown.

Å) in Ln‚‚‚Ln distances due to the lanthanide contraction.5 Nevertheless, the Er/Eu- or Tb/Eu-mixed nucleation has a noticeable impact on the time dependence of the Eu3+ emissions, and our attention has been paid to the luminescnce behavior of Tb/Eu-mixed complexes K15H3[Tb3-xEux(H2O)3(SbW9O33)(W5O18)3]‚25.5H2O and Na7H19{[Tb6-xEuxO2(OH)6(H2O)6Al2(Nb6O19)5}‚47H2O, to investigate energy transfer processes between Tb3+ and Eu3+. As will be shown below, the role of the donor is played by Tb3+ and that of the acceptor by Eu3+. Both the Tb3+ (5D4) and Eu3+ (5D0) ions in the Tb/ Eu-mixed complexes emit, where Eu3+ (5D0) excitation buildup following excitation of Tb3+ (5D4) provides strong evidence for Tb3+ to Eu3+ energy transfer. Energy transfer in solutions and solids has been extensively investigated,6 but neither has the mechanism of the Tb3+ f Eu3+ energy transfer in the polyoxometalate lattices been identified up to now nor has the dynamics of this process been studied in any detail. Tb3+ ions in polyoxometalloterbates K15H3[Tb3(H2O)3(SbW9O33)(W5O18)3]‚25.5H2O (1) and Na7H19{[Tb3O(OH)3(H2O)3]2Al2(Nb6O19)5}‚47H2O (2) show green emissions with exponential decays due to the 5D4 f 7FJ (J ) 6-0) transition. The Tb/Eu-mixed polyoxometallolanthanoates provide a favorable system for investigation of the Tb3+ f Eu3+ energy transfer in the oxide lattices, because the emission lines of donor and acceptor are well separated and can be measured without much interference by each other. For this reason, the Tb3+ f Eu3+ energy migration in K15H3[Tb1.4Eu1.6(H2O)3(SbW9O33)(W5O18)3]‚ 25.5H2O (3) and Na7H19{[Tb4.3Eu1.7O2(OH)6(H2O)6Al2(Nb6O19)5}‚

All of the reagents were of at least analytical grade without further purification. The pure Tb complexes 1 and 2 were prepared by replacing Eu(NO3)3‚6H2O as a starting material with Tb(NO3)3‚6H2O in our preparation procedures for the pure Eu complexes.3,4 Identification was done by the agreement of their IR spectra with those of the pure Eu complexes. Interlanthanide substitutions for Tb/Eu-mixed complexes 3 and 4 were easily accomplished, and the resulting Tb/Eu-mixed complexes have stoichiometries which were, conveniently, close to the original composition of the Tb(NO3)3/Eu(NO3)3 mixture. Energydispersive X-ray (EDX) spectrometry analysis was performed for the determination of the atomic ratio of Tb/Eu on a JEOL JSX-3200 instrument. The results of the EDX spectrometry analysis for 3 and 4 are shown as Supporting Information in Figures 1S and 2S, respectively. The IR spectra of 3 and 4 were consistent with those of the pure Eu complexes, too. A selected region of the IR spectra of 3 and 4 is shown in Figures 3S and 4S, respectively, as Supporting Information together with the spectra of corresponding pure Tb and Eu complexes for comparison. Diffuse reflectance and IR spectra were recorded on Hitachi 330 and JASCO FT/IR-5000 spectrophotometers at room temperature, respectively. Luminescence and excitation spectra of the sample powder pellet were obtained using a lock-in (NF L1-574) technique. The sample pellet (with a thickness of about 1 mm and a diameter of 10 mm) was prepared by pressing the sample powder under 3 × 107 Pa. The light source for the photoluminescence measurements was a Continuum 9030 YAG (355 nm, 400 mJ per pulse) laser, a 500 W xenon lamp (in a combination with a Nikon G-25 grating monochromator), or a LDL 20505 LAS dye laser (with LDC 480 dye, 459-510 nm) pumped by a Questek 2320 XeCl (308 nm, 50 mJ per pulse) laser. The 488 nm light excitation of the Tb/Eu-mixed complexes was carried out by a 50 ns pulse of 5 mJ photons from the above dye laser. The luminescence was collected at an angle of 90° to the exciting light and focused onto the entrance slit of a Spex 750M spectrometer (for high resolution) or a Nikon G-25 grating monochromator which was equipped with Hamamatsu Photonix R636 photomultiplier tube. An absolute wavenumber accuracy of (2 cm-1 for the high-resolution luminescence spectra was estimated from the dye laser alignment. Luminescence at low temperatures was measured using an Oxford Instruments CF 204 cryostat. The time profiles of the luminescence was measured on a LeCloy 9361 digital storage oscilloscope. No observable part of the original intensity of the incident light was transmitted through the sample pellet. Results Emission and Excitation Spectra. The luminescence spectra of the pure Tb complexes 1 and 2 under 355 nm light irradiation consist of transitions of 5D4 f 7FJ around 488, 545, 584, 624,

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Figure 2. High-resolution emission spectra (a) observed under the 355 nm light irradiation and excitation spectra (b) under low resolution for the Tb3+ 5D4 f 7F5 transition (at 545 nm) of K15H3[Tb3(H2O)3(SbW9O33)(W5O18)3]‚25.5H2O (1) at 4.2 and 300 K.

Figure 3. High-resolution emission spectra (a) observed under the 355 nm light irradiation and excitation spectra (b) under low resolution for the Tb3+ 5D4 f 7F5 transition (at 545 nm) of Na7H19{[Tb3O(OH)3(H2O)3]2Al2(Nb6O19)5}‚47H2O (2) at 4.2 and 300 K.

650, 668, and 682 nm for J ) 6, 5, 4, 3, 2, 1, and 0, respectively, and the excitation spectra for the strongest 5D4 f 7F5 lines of the Tb3+ emission consist of 7F6 f 5D4 lines (around 488 nm), complicated 7F6 f 5D3,2 and 5L10 lines (in the range 320-380 nm), and O f M (M ) W or Nb) lmct bands (at 77 K.2b,3 In contrast to the pure Eu complexes, none of excitation spectra of 1 and 2 exhibits hot lines (7F5 f 5D3,2 and 5L ) of the 7F state of Tb3+, since the large energy gap (about 10 5 2000 cm-1) between the 7F6 and 7F5 states for Tb3+ compared to the case (about 300 cm-1 between the 7F0 and 7F1 states) for Eu3+ makes the thermal population (at T < 300 K) of the 7F5 state prohibitive.

Upon Tb3+ excitation (by the7F6 f 5D4 transition) at 488 nm, the Tb/Eu mixed complexes 3 and 4 exhibited both Tb3+ 5D f 7F (J ) 5-0) and Eu3+ 5D f 7F (J ) 0-4) lines in 4 J 0 J the region 540-720 nm. There was no observation of the Eu3+ 5D f 7F lines under this excitation. Figure 4 shows the 1 J excitation spectra of 3 at 4.2 and 300 K for the Tb3+ 5D4 f 7F5 and Eu3+ 5D0 f 7F2 emissions as a typical example of the Tb/Eu- mixed complexes. The excitation spectrum for the Eu3+ emission indicates the contribution of both the O f M (M ) W or Nb) lmct bands and the f-f transitions of Tb3+ in addition to the direct f-f transitions of Eu3+, while the excitation spectrum for the Tb3+ emission indicates little contribution of the Eu3+ f-f transitions. This proves that there is energy transfer from Tb3+ to Eu3+ not from Eu3+ to Tb3+ in the polyoxometallolanthanoate lattices. The 300 K excitation spectrum of the Eu3+ 5D0 f 7F2 emission shows the very weak 7F2 f 5D1lines at 554 nm, which were little observed at T e 200 K, in addition to the 7F1 f 5D0,1,2,3 lines at 589.5 and 594.5, 534.0 and 539.0, 471.0 and 474.5, and 415.0 and 418.5 nm. The weak 7F2 f 5D lines expected around 487 nm at T > 200 K may be 2 overlapped with the strong Tb3+ 7F6 f 5D4 lines at around 488 nm. The observation of the hot lines due to the population of the 7F2 state is ascribed to the relatively small gap (about 1000 cm-1) between the 7F2 and 7F0 states. Decay of Tb3+ Emission and Buildup of Eu3+ Emission. The decay patterns of the 5D4 f 7FJ luminescence for 1 and 2 were single exponentials at all temperatures. Figure 5 shows the decay characteristics of the 5D4 emission as a function of

Luminescence and Energy Transfer Phenomena

Figure 4. Excitation spectra of K15H3[Tb1.4Eu1.6(H2O)3(SbW9O33)(W5O18)3]‚25.5H2O (3) at 4.2 and 300 K under low resolution for the Tb3+ 5D4 f 7F5 (a) and Eu3+ 5D0 f 7F2 (b) emissions.

Figure 5. Temperature dependence of the lifetime of the 5D4 luminescence of Tb3+ for 1 (open circle) and 2 (filled circle). The 5D4 f 7F5 decays observed after the 488 nm light pulse exposure corresponding to the 7F6 f 5D4 transition are singly exponential at all temperatures.

temperature for 1 and 2, which is measured from the 5D4 f 7F decays observed after the 488 nm light pulse exposure 5 corresponding to the 7F6 f 5D4 transition. The intrinsic lifetimes (τD) of the 5D4 state for 1 and 2 are in the range 1.24-1.64 and 0.63-0.73 ms, respectively. The lifetime for 1 is slightly temperature-dependent [for example, τD ) 1.59 ( 0.05, 1.42 ( 0.05, and 1.30 ( 0.06 ms, at 4.2, 77, and 300 K, respectively], in contrast to the almost independent decay for 2. τD for 2 is smaller than for 1. In conjunction with the fact that the total number (n) of aqua and hydroxo ligands in the Tb3+ coordination sphere for 2 (n ) 3) is larger than that for 1, this is predicted

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Figure 6. Semilogarithmic decay curves of the Tb3+ 5D4 f 7F5 emission after pulsed excitation at 488 nm at a variety of temperatures for 3 (a) and Na7H19{[Tb4.3Eu1.7O2(OH)6(H2O)6Al2(Nb6O19)5}‚47H2O (4) (b). The dashed lines in (a) and (b) indicate the exponential decays for 1 and 2, respectively.

by the radiationless deactivation of the 5D4 state through weak vibronic coupling with the vibrational states of the aqua and hydroxo ligands’ high-frequency OH oscillators,2b although the 5D state of Tb3+ being approximately one OH phonon energy 4 (about 3000 cm-1) as high in the energetic position as the 5D0 state of Eu3+ is less efficient in the radiationless deexcitation by the OH oscillators.7 The significant temperature dependence of the 5D4 f 7FJ decay has also been observed for the other polyoxotungstoterbate K3Na4H2[Tb(W5O18)2]‚20H2O (with τD ) 3.8, 2.7, and 1.7 ms at 4.5, 77, and 300 K, respectively)8 and has been demonstrated by the temperature-dependent nonradiative transition into the Tb3+ f W6+ charge-transfer state, resulted in nonradiative relaxation of the excitation energy.9 While the pure Tb complexes 1 and 2 show simple exponential decays of Tb3+ emission, the Tb/Eu-mixed complexes 3 and 4 exhibit more complicated decay patterns. Parts a and b of Figure 6 show the semilogarithmic decay curves of the 5D4 f 7F5 lines for 3 and 4 after pulsed excitation at 488 nm at various temperatures, respectively. In Figure 6, the exponential decays at 4.2 K for 1 and 2 are added for comparison. At all temperatures, the decay curves of the 5D4 state for the Tb/Eumixed complexes are nonexponential, obviously as a result of the energy transfer from Tb3+ to Eu3+. With increasing temperature a faster decay is observed for 3 and 4. This results from a temperature dependence of the Tb-Eu transfer step, since the Tb-Tb energy migration for 1 and 2 can be regarded to be negligible as well as the Eu-Eu migration for the pure Eu complexes.2b The Tb-Eu transfer process is also reflected in the decay of the Eu3+ emission if Eu3+ is excited via the 7F6 f 5D4 transition of Tb3+. The radiative decays of the 5D0 f 7FJ (J ) 0-4)

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Figure 7. Normalized decay curves of the Eu3+ 5D0 f 7F2 emission after pulsed excitation at 488 nm at a variety of temperatures for 3 (a) and 4 (b).

Figure 8. Total intensitiy of the Tb3+ (open circle) and Eu3+ (filled circle) emissions normalized by I(4.2) as a function of temperature for 3 (a) and 4 (b).

emission of Eu3+, which were temperature-independent and of single exponentials with lifetimes of 1.1 ( 0.2 and 0.31 ( 0.02 ms for the pure Eu complexes, respectively,2b,3 changes drastically for the Tb/Eu-mixed complexes. Parts a and b of Figure 7 show normalized decay curves of the 5D0 f 7F2 emission of Eu3+ after pulsed excitation of the 7F6 f 5D4 transition of Tb3+ at various temperatures for 3 and 4, respectively. The emission of Eu3+ at 4.2 K shows clearly a slow rise from zero to a maximum and then nonexponential decay, which can be rationalized as a population of the 5D0 state of Eu3+ by energy transfer from the 5D4 f 7FJ emission of Tb3+, the 5D0 state decay being now governed by the time behavior of the transfer process. At long times after the excitation pulse, the decay curve becomes exponential. The radiative decay times of the Eu3+ emission derived from the tail of the decay curve below 100 K are 1.3 ( 0.1 and 0.32 ( 0.01 ms for 3 and 4, respectively. These decay times are in agreement with the 5D0 f 7FJ emission of Eu3+ for the corresponding pure Eu complexes.2b The buildup of the 5D0 state becomes faster with increasing temperature. The decay curves presented in Figure 7 give the most convincing demonstration that the Tb-Eu transfer rate increases with increasing temperature. Parts a and b of Figure 8 show the normalized relative intensities of both Tb3+ 5D4 f 7F5 and Eu3+ 5D0 f 7F2 emissions for 3 and 4, I(T)/I(4.2), as a function of temperature, which are obtained by integration of the decay curves after pulsed excitation of the 7F6 f 5D4 transition of Tb3+. The total intensity of the Tb3+ emission for 3 and 4 decreases with increasing temperature below 100 K, accompanied by the increase in the intensity of the Eu3+ emission. The temperature dependence of the experimental data above 100 K was not measured because of our inability to obtain the essential information reliably for the Tb3+ emission.

Discussion Fo1 rster-Dexter-Type Energy Transfer from Tb3+ to Eu3+. The above results seem able to be analyzed in terms of the Fo¨rster-Dexter theory for multipolar interaction between donor and acceptor.10,11 We now analyze the emission decay of the 5D4 state of Tb3+ at 4.2 K for 3 and 4 under the 7F6 f 5D line excitation. Since both the Tb-Tb transfer and the back 4 transfer from Eu3+ to Tb3+ at this temperature are negligible, the intensity I(t) of the Tb3+ emission after pulsed excitation can be described by Inokuti-Hirayama model,12,13

ln I(t) + t/τD ) -4/3π3/2nArDA3(kETt)3/s

(1)

where nA is the density of Eu3+ and kET is the nearest-neighbor transfer rate between a Tb-Eu pair at central cavity of the Tb/ Eu-mixed complex, with separation rDA. Also, s is 6, 8, or 10 depending on the multipolar nature of the Tb-Eu interaction, and τD ()1.59 ( 0.05 and 0.68 ( 0.05 ms for 3 and 4 at 4.2 K, respectively) is again the intrinsic decay time. Figure 9 shows plots of the 5D4 f 7F5 emission decays of 3 and 4 in the forms of ln I(t) + t/τD against t3/s for s ) 6 (which is used throughout this study), corresponding to an electric dipole-electric dipole interaction. The approximate straight-line behavior indicates that our assumptions above are correct and that the choice of s ) 6 is a valid one. The effect of the lanthanide contraction on the intermetalic distances between the isostructural anion species of Tb and Eu complexes is small, as implied by the small difference in mean Ln-O distances (2.42 and 2.43 Å) between K3Na4H2[TbW10O36]‚20H2O and Na9[EuW10O36]‚32H2O, respetively.14 Therefore, the rDA value for the Tb/Eu-mixed complex is estimated to be close to the nearest Eu‚‚‚Eu distance for the pure Eu complex. For 3, at nA ) 5.46 × 10-4 ions/Å3 [)1.6 × 4 /11680, where 4 and 11 680 are number (Z) of

Luminescence and Energy Transfer Phenomena

Figure 9. Tb3+ emission decay data at 4.2 K plotted according to the Inokuti-Hirayama model for the electric dipole-dipole Tb-Eu interaction for 3 (a) and 4 (b).

molecules in unit cell and cell volume (in Å3), respectively],3 rDA ) 5.05 Å, and from the slope ()110 s-1/2) in Figure 9 we find kET ) (4.49 ( 0.81) × 104 s-1. From this, the donoracceptor interaction parameter (R) is calculated to be (7.45 ( 0.13) × 108 ()kET5.056) Å6 s-1, and the critical radius (R0) for the Tb-Eu energy transfer by making use of R0 ) (τDR)1/6, at which energy transfer and radiative decay of the donor ()Tb3+) have the same probability, is obtained as R0 ) 10.3 ( 0.2 Å. A similar treatment for 4 yields kET ) (4.68 ( 0.90) × 105 s-1, R ) (1.31 ( 0.25) × 109 Å6 s-1 ()kET3.766), and R0 ) 10.0 ( 0.3 Å. In the crystal structure of 3, there are two nearest neighbors for Eu3+ or Tb3+ at distance 5.05 Å (Figure 1a). More distant neighbors for Tb3+ are associated with the intermolecular distance at 11.2 Å (twice).3 R0 ) 10.3 Å for 3 indicates that Tb3+ at 4.2 K enables transfer of its excitation energy to the Eu3+ ions which occupy either site of the two nearest neighbors within a sphere of radius of 10.3 Å. In 4 (as shown by the crystal structure of the pure Eu complex in Figure 1b), there are five nearest neighbors at distance 3.76 Å (twice), 4.69 Å (twice), and 6.00 Å (one). Similarly R0 ) 10.0 Å at 4.2 K for 4 indicates the transfer of the Tb3+ excitation energy to the Eu3+ ions which occupy within a central cavity of the molecule, if we consider the nearest neighboring intermolecular Tb‚‚‚Eu distance of 11.0 Å (twice).4 The next nearest Tb‚‚‚Eu distance is 4.69 Å in the central [Tb4.3Eu1.7O2(OH)6(H2O)6]8+ core for 4. This allows us to estimate the rate of the electric dipole-dipole energy transfer to next nearest neighbors to be approximately 1/4 [)(3.76/ 4.69)6] smaller than for the shortest Tb‚‚‚Eu distance of 3.76 Å. It is possible to fit the decay curves of the 5D4 f 7F5 emission at a variety of temperatures to the formula (with s ) 6) 1. Parts a and b of Figure 10 show plots of the calculated values of kET against temperature for 3 and 4, respectively. As the temperature increases, kET for 3 and 4 increases, and the increasing effect for the latter is much larger than that for the former (for example, when the temperature was altered from 4.2 to 100 K, the

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Figure 10. Temperature dependence of the nearest-neighbor Tb3+ f Eu3+ energy transfer rate (kET) for 3 (a) and 4 (b). Numbers in parentheses indicate average values of kET at selected temperatures.

calculated value of kET for 4 increased approximately 50-fold, while the one for 3 increased approximately 5-fold). Energetics for the Energy Transfer. The basic requirement for a Fo¨rster-Dexter energy transfer to occur is the spectral overlap of donor emission and acceptor absorption lines. Figure 11 shows energy situations of the emission and excitation line peaks of Tb3+ (left) and Eu3+ (right) in the range of less than 488 nm excitation energy for 3, which are estimated by the 4.2 and 300 K lines (Figures 2-4). In Figure 11, the excitation lines due to the 7F1 and 7F2 states, which are involved in the energy transfer at high temperatures, are shaded. The line widths of the Tb3+ emission lines are in the range 40-100 cm-1 at half-height. At low temperatures, T < 100 K, where the population of the 7F2 state is ignored, the population of the 7F1 manifold of Eu3+ is low and the excitation transfer occurs mainly through the 7F0 level (Figure 4b). Therefore, it becomes evident that the condition for energy transfer is fulfilled around 17 200 cm-1 (581 nm), where the 5D4 f 7F4 line emission of Tb3+ around 17 123 cm-1 overlaps with the 7F0 f 5D0 excitation of Eu3+ around 17 240 cm-1 (see a light arrow in Figure 11, symbolizing the transfer channel). At high temperatures, the transfer channels obviously increase due to the thermal population of 7F1 and 7F2 states of Eu3+; an overlap between the 5D4 f 7F5 line emission and the 7F1 f 5D1 (and 7F2 f 5D1 line only at T > 200 K) line excitation around 18 349 cm-1 with addtional overlap of the 5D4 f 7F4 line emission with the 7F1 f 5D0 line excitation around 16 960 cm-1 (see the heavy arrows as additional channels for energy transfer in Figure 11). Hence, large values of kET at high temperatures are expected after pulsed excitation of the 7F6 f 5D4 line, as shown in Figure 10a. Similar energetic situation was also observed for 4. The involvement of both the forbidden transition 7F0 f 5D0 and the magnetic dipole transition 7F1 f 5D0 in the Tb-Eu energy transfer process is associated with the low local symmetry at Tb3+ and Eu3+ site in 3 and 4. The LnO8 coordination polyhedra for 3 and 4

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Yamase and Naruke energy of ∆Eph), then one can expect kET to be increased with temperature, since it is proportional to the phonon occupation number (n) [n ) [exp(∆Eph/kT) - 1]-1].19-21 This demonstrates alternatively the temperature dependence of kET at low temperatures (in Figure 10). Time Dependence of the Eu3+ Emission. Not only the time dependence of the Tb3+ emission in the presence of energy transfer can be calculated (see eq 1) and tested against experiment (Figure 6) but also the response of the Eu3+ emission can be predicted within the framework of the Fo¨rster-Dexter mechanism. However, the latter case (Figure 7) is more complicated, because the population of the 5D0 state of Eu3+, from which the emission occurs, is now governed by the energy transfer to Eu3+ as an acceptor. If this is given by the transfer rate Wtr, one can then use rate equations to describe the behavior of the Tb3+ 5D4 and Eu3+ 5D0 states in the present system.

Figure 11. Energetic comparison of the emission and excitation line peaks for Tb3+ and Eu3+ in 3. Excitation lines appearing at high temperature are shaded. The light arrow indicates energy transfer at low temperatures and the heavy arrows additional channels of energy transfer at high temperatures.

approximately have Cs and C1 symmetries, respectively, because of distortions in the bond length and angles (Figure 1) as a result of the coordination of both aqua and hydroxo ligands.3,4 Under such low symmetries, the 7FJ T 5D0 (J ) 0,1) transitions have a nonvanishing electric dipole transition probability with a resultant Fo¨rster-Dexter-type energy transfer due to the electric dipole-dipole interaction.15-17 Thereby, the decay behavior of Tb3+ emission in 3 and 4 is nonexponential, indicating that donor-donor transfer is much slower than donor-acceptor transfer, due to the disruption of the resonance energy transfer between Tb3+ ions by both aqua and hydroxo ligands.15,18 As described, the intrinsic lifetimes (τD and τA) of emissive Tb3+ and Eu3+ states for 3 are considerably larger than for 4 (with lower symmetry crystal field of Ln3+). The lower symmetry crystal field of 4 enhances the transition probabilities and broadens the excitation and emission lines for both Tb3+ and Eu3+. In addition, there seem to be spectroscopically inequivalent emission sites for 4, as suggested by the existence of at least three Eu sites for the pure Eu complex.2b Such structural features of 4 lead to a faster donor-acceptor transfer rate compared to 3 (Figure 10). The line widths of the Tb3+ emission lines increase slightly with temperature (Figures 2-4). This will increase the spectral overlap between the Tb3+ 5D4 f 7F4 emission lines and the Eu3+ 7F0 f 5D0 excitation line, which is responsible for a faster donor-acceptor transfer rate with increasing temperature below 100 K. One can remark the small energy mismatch (within 100 cm-1) between the donor-emission and acceptor-excitation spectral lines (Figure 11) which let us postulate the phonon-assisted energy transfer in the present system. Assuming a single-phonon process (with a phonon

d[5D4]/dt ) -(1/τD + Wtr)[5D4]

(2)

d[5D0]/dt ) Wtr[5D4] - [5D0]/τA

(3)

Wtr ) (1/2)4/3π3/2nAR03(τDt)-1/2

(4)

where [5D4] and [5D0] are concentrations of the radiative 5D4 and 5D0 states (with 1/τD and 1/τA as radiative decay rates) for Tb3+ and Eu3+, respectively. Integration of the differential equation for the change in population of 5D0 leads to the complicated decay function for the 5D0 state with use of a Karpov integral.22,23 However, in the case where [5D4]0 excited donors are created at t ) 0 and where no acceptors are excited, Wtr is assumed to be constant ()W′tr) and the solutions are

[5D4] ) [5D4]0 exp[-(1/τD + W′tr)t]

(5)

and

[5D0]) ([5D4]0W′tr){exp(-t/τA) - exp[-(1/τD + W′tr)t]}/ (1/τD + W′tr - 1/τA) (6) Since the Eu3+ luminescence varies as [5D0] produced as a result of the Tb3+ f Eu3+ energy transfer, [5D0] rises from zero at t ) 0, reaches a maximum, and then decays exponentially at either its own decay rate 1/τA or the decay rate of the donors (1/τD + W′tr), whichever is the smaller (Figure 7). In fact, the intrinsic decays τD for Tb3+ and τA for Eu3+ and W′tr are related, as can be seen by realizing that at the maximum in the [5D0] curve (t ) tmax) the slope is zero:

(d[5D0]/dt)t)tmax ) 0 ) -[exp(-tmax/τA)](1/τA) + {exp[-tmax(1/τD + W′tr)]}(1/τD + W′tr) Thus,

tmax ) [ln(1/τD + W ′tr) - ln(1/τA)]/(1/τD + W ′tr - 1/τA) (7) A value of W ′tr is approximately estimated to be close to the vaue of Wtr at t ≈ tmax/10 in eq 4. Substituting 1/τD ()6.3 × 102 s-1 for 3 and 1.5 × 103 s-1 for 4), 1/τA ()9.1 × 102 s-1 for 3 and 3.2 × 103 s-1 for 4), and W ′tr ()1.2 × 104 s-1 at 20 µs for 3 and 2.1 × 104 s-1 at 5 µs for 4) into eq 7 gives calculated values of tmax at 4.2 K for 3 and 4 to be 2.2 × 10-4 and 1.0 × 10-4 s, respectively. Here again an agreement of tmax between calculated and measured decay curves of Eu3+ is obtained (tmax

Luminescence and Energy Transfer Phenomena ) 2.0 × 10-4 and 0.7 × 10-4 s in Figure 7), supporting our view that a Fo¨rster-Dexter-type mechanism for the electric dipole-dipole interaction is responsible for the Tb3+ f Eu3+ energy transfer in the Tb/Eu-mixed polyoxometallolanthanoate system. As above-discussed, the transfer channels at high temperatures indicate an increasing contribution of the Eu3+ 5D1 state to the emission process of Eu3+. Since there was no observable 5D1 f 7FJ emission for both the present lattices 1-4 and the corresponding pure Eu complexes,2b,3 it is clear that the decay of the 5D1 manifold is governed by fast nonradiative processes via a cross relaxation with a resultant deactivation to the 5D0 state which leads to the 5D0 f 7FJ emission.6 The Eu3+ emission behavior indicated by eq 3 includes also such contribution of the 5D1 state at high temperatures in the energy transfer. Acknowledgment. One (T.Y.) of us acknowledges Grantsin-Aid for Scientific Research, no. 06241104 (for “Priority Area, New Development of Rare Earth Complexes”), no. 09354009, and no. 10304055 from the Ministry of Education, Science, Sports, and Culture for support of this work. Supporting Information Available: Figures 1S and 2S of EDX spectra for the Tb/Eu-mixed complexes 3 and 4, respectively, and Figures 3S and 4S of IR spectra for 3 and 4, respectively, together with the corresponding pure Tb and Eu complexes. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) (a) Powell, R. C.; Blasse, G. Structure and Bonding 1980, 42, 43. (b) Blasse, G. J. Chem. Phys. 1966, 45, 2350. (2) (a) Yamase, T. Chem. ReV. 1998, 98, 307. (b) Yamase, T.; Kobayashi, T.; Sugeta, M.; Naruke, H. J. Phys. Chem. A 1997, 101, 5046.

J. Phys. Chem. B, Vol. 103, No. 42, 1999 8857 (c) Yamase, T.; Sugeta, M. J. Chem. Soc., Dalton Trans. 1993, 759. (3) Yamase, T.; Naruke, H.; Sasaki, Y. J. J. Chem. Soc., Dalton Trans. 1990, 1687. (4) Ozeki, T.; Yamase, T.; Naruke, H.; Sasaki, Y. Inorg. Chem. 1994, 33, 409. (5) Naruke, H.; Yamase, T. J. Alloys Compd. 1998, 268, 100. (b) Naruke, H.; Yamase, T. J. Alloys Compd. 1997, 255, 183. (c) Naruke, H.; Yamase, T. Acta Crystallogr. 1996, C52, 2655. (6) See, for example: (a) Bu¨nzli, J.-C.-G. In Lnthanide Probes in Life, Chemical and Earth Sciences, Theory and Practice; Bu¨nzli, J.-C.-G., Choppin, G. R., Eds.; Elsevier Publishing Co.: Amsterdam, 1989; Chapter 7 and references therein. (b) Blasse, G.; Grabmaier, B. C. Luminescent Materials; Springer-Verlag: Berlin, Heidelberg, 1994; Chapter 5 and references therein. (7) (a) Horrocks, W. D., Jr.; Sudnick, D. Acc. Chem. Res. 1981, 14, 384. (b) Horrocks, W. D., Jr.; Sudnick, D. Science 1979, 206, 1194. (8) Ozeki, T.; Yamase, T. J. Alloys Compd. 1993, 192, 28. (9) (a) Blasse, G.; Dirksen, J.; Zonnevijlle, Z. Chem. Phys. Lett. 1981, 83, 449. (b) Stuck, C. W.; Fonger, W. H. J. Chem. Phys. 1976, 64, 1784. (10) Fo¨rster, T. Ann. Phys. 1948, 2, 55. (11) Dexter, D. L. J. Chem. Phys. 1953, 21, 836. (12) Inokuti, M.; Hirayama, F. J. Chem. Phys. 1965, 43, 1978. (13) Hegarty, J.; Huber, D. L.; Yen, W. M. Phys. ReV. B 1981, 23, 6271. (14) (a) Ozeki, T.; Yamase, T. Acta Crystallogr. 1994, B50, 128. (b) Sugeta, M.; Yamase, T. Bull. Chem. Soc. Jpn. 1993, 66, 444. (c) Ozeki, T.; Takahashi, M.; Yamase, T. Acta Crystallogr. 1992, C48, 1370. (15) Kahwa, I. A.; Parkes, C. C. McPherson, G. L. Phys. ReV. B 1995, 52, 51. (16) Forsberg, H. Coord. Chem. ReV. 1973, 10, 195. (17) Sinha, S. P.; Butter, E. Mol. Phys. 1966, 16, 285. (18) (a) Moret, E.; Bu¨nzli, J.-C. G.; Schenk, K. Inorg. Chim. Acta 1990, 178, 83. (b) Blasse, G. Inorg. Chim. Acta 1990, 169, 33. (c) Blasse, G.; Brixner, L. H. Inorg. Chim. Acta 1990, 169, 25. (19) Laulicht, I.; Meirman, S. J. Lumin. 1986, 34, 287. (20) Buijs, M.; Blasse, G. J. Lumin. 1986, 34, 263. (21) Holstein, T.; Lyo, S. K.; Orbach, R. In Laser Spectroscopy of Solids; Yen, W. M., Selzer, P. M., Eds.; Springer: Berlin, Germany, 1981; Chapter 2. (22) Heber, J. Phys. Status Solidi B 1971, 48, 319. (23) Kaschke, M.; Vogler, K. Chem. Phys. 1986, 102, 229.