9216
J. Phys. Chem. 1996, 100, 9216-9222
Temperature Dependence of the Eu3+ 5D0 Lifetime in Europium Tris(2,2,6,6-tetramethyl-3,5-heptanedionato) Mary T. Berry,* P. Stanley May, and Hong Xu Department of Chemistry, UniVersity of South Dakota, Vermillion, South Dakota 57069 ReceiVed: December 13, 1995; In Final Form: March 12, 1996X
The temperature dependence of the Eu3+ 5D0 relaxation rate in solid europium tris(2,2,6,6-tetramethyl-3,5heptanedionato) is presented. Eu3+ emission is strongly quenched in the region of room temperature. It is proposed that thermal quenching of 5D0 emission is via crossover from the 5D0 to a ligand-to-lanthanide charge transfer state. Activation energies for the crossover process were determined using the models of Mott and of Englman and Jortner, yielding values of 4120 ( 20 and 5700 ( 300 cm-1, respectively. The crossover process is less efficient in media in which Eu(thd)3 exists as a monomer.
Introduction The neutral lanthanide β-diketonate complex europium tris(2,2,6,6-tetramethyl-3,5-heptanedionato), Eu(thd)3, has received considerable attention in the literature, in part because of its utility as an NMR shift reagent1 and in part because of its unusual volatility.2 Each thd ligand is bidentate, with two coordinating keto oxygens. Crystallographic studies3,4 have shown that the anhydrous Eu(thd)3 exists in the crystal as the dimeric form, Eu2(thd)6, with two of the ligands serving as bridges between the metal ions. One oxygen in each of the bridging ligands is shared by both metal ions. This dimeric structure is exhibited by all of the lighter lanthanides up through and including Gd2(thd)6. The heavier lanthanides, Ho through Lu, with their smaller ionic radii, tend to crystallize with monomeric structures, the latter involving a coordination number of six rather than seven. The thd chelates of Tb and Dy crystallize in both forms.5 In hexane solution, at concentrations up to 20 mM, vapor phase osmometry studies indicate that the complex is predominantly monomeric.6 It is believed also to be monomeric in the gas phase at elevated temperatures (T > 400 K).2 Throughout this paper, the empirical formula, Ln(thd)3, is used, regardless of monomeric or dimeric structure. Gas phase studies of the temperature dependence of the fluorescence lifetime of the 5D4 level in Tb(thd)37indicate that at the temperatures under consideration (500-570 K), the relaxation rate of 5D4 is dominated by energy transfer to the ligand triplet state (T1), the minimum of which lies at about 26 000 cm-1,8 5500 cm-1 above the 5D4 level. The 5D4-T1 energy difference provides a thermal barrier to 5D4 relaxation and accounts for the observed temperature dependence of the fluorescence lifetimes. The 5D0 level of Eu3+ lies some 8000 cm-1 below the T1 state, and though the relaxation rates of gas phase Eu(thd)3 show distinct Arrhenius behavior between 450 and 500 K, the extracted activation energy, 4100 cm-1, is far too low to implicate the ligand triplet state.9 On the basis of the results presented here, we propose that the most probable path for nonradiative relaxation from the 5D0 level in crystalline Eu(thd)3 above 270 K is through a ligandto-lanthanide charge transfer (CT) state. The Arrhenius-type activation energy extracted from our crystal data closely matches that obtained from the gas phase studies discussed above, suggesting that, in the gas phase too, relaxation is through the X
Abstract published in AdVance ACS Abstracts, May 1, 1996.
S0022-3654(95)03702-6 CCC: $12.00
CT state. This CT relaxation pathway is shown to be more efficient in the solid phase than in the gas phase, presumably due to structural differences in the chelate complexes in the two phases (dimer vs monomer). In solutions in which the Eu(thd)3 complexes exist as monomers, nonradiative relaxation behavior with regard to the CT relaxation pathway appears to be more similar to the nonradiative behavior of the gas phase complexes, lending support to the explanation that the difference in efficiencies of the CT relaxation pathway arises from differences in the localized structure of the Eu(thd)3 complex. Background: Nonradiative Relaxation Mechanisms in Eu3+ Chelates Radiative lifetimes for the 5D0 state in Eu3+ compounds are typically on the order of milliseconds. Most Eu3+ chelates exhibit measured 5D0 lifetimes much shorter than their radiative lifetimes, even at low temperatures, indicating that relaxation is dominated by nonradiative mechanisms. The most important nonradiative relaxation mechanisms are (i) multiphonon emission, (ii) energy transfer to ligand-localized electronic states within the complex or to acceptor traps outside the complex, and (iii) crossover from the 4fn configuration to a CT state. In multiphonon emission, the relaxation transition from the excited state is normally to the nearest lower electronic level. The energy difference between the electronic states is converted to vibrational energy in the complex or in the matrix (solution, glass, or crystal). For large energy gaps, such as that between the 5D0 and 7F6 levels of Eu3+, the highest energy vibrational modes available usually dominate the mechanism, since multiphonon relaxation probability increases as the number of phonons required to bridge the gap decreases. For lanthanide chelates, the highest energy vibrations are usually ligandlocalized and multiphonon emission can be quite efficient, since the ligands possess vibrational modes that can bridge large energy gaps with only a few phonons. The multiphonon mechanism requires Franck-Condon (FC) overlap between the occupied vibrational levels of the excited electronic state and the resonant vibrational levels of the lower electronic state to which energy must be transferred. The potential energy surfaces for the electronic states of the 4fn configuration are very nearly parallel (due to weak coupling between the vibrational modes and the well-shielded 4f electrons), so that FC overlap between such states is small. Treatment of the multiphonon mechanism is generally within the single configurational coordinate (SCC) model,10-12 wherein © 1996 American Chemical Society
Temperature Dependence of Eu3+ 5D0 Lifetimes
J. Phys. Chem., Vol. 100, No. 22, 1996 9217
it is assumed either that only a single vibrational coordinate is operational or that an effective coordinate can represent the relevant vibrations. The temperature dependence of multiphonon emission rates within the weak coupling limit appropriate to the intraconfigurational 4fn relaxation in the lanthanides, is then given by
wmp(T) ) wmp(0)(n + 1)p
(1)
where wmp(0) is the rate of multiphonon emission at 0 K, n ) [exp(hν/kT) - 1]-1, the phonon occupancy factor, ν is the frequency of the accepting phonon mode, and p is the number of phonons required to bridge the energy gap between the initial and final states. In general, multiphonon emission rates increase with temperature, because thermal population of higher vibrational states in the excited electronic state leads to a net increase in the FC overlap. For multiphonon processes involving high-energy vibrations, however, a significant thermal enhancement to the multiphonon transition rate occurs only at relatively high temperatures. The 5D0 f 7F6 multiphonon transition rates in Eu3+ chelates are not expected to show significant temperature dependence until well above room temperature, because the high-energy vibrations involved in the transition (e.g., C-H and O-H stretching modes) are not thermally excited at lower temperatures. In Na3[Eu(oxydiacetate)3]‚2NaClO4‚6H2O, for example, in which both C-H and O-H modes are active in the 5D0 f 7F6 multiphonon transition, the 5D0 lifetime is essentially temperature independent between 77 K and room temperature.13 A strong temperature dependence in nonradiative rates at temperatures in which high-energy molecular chelate modes are not excited is, therefore, most likely due to either an electronic energy transfer process or crossover from the 5D0 state to a CT state. Electronic energy transfer processes exhibit strong temperature dependences in cases when thermal energy is required to match energy differences between the electronic transitions on the donor and acceptor species. In lanthanide chelates, transfer of electronic energy from 4fn states may occur to electronic states localized on a coordinating ligand or to states on other species in the bulk environment. In solid-state environments in which the donor concentrations are high (and the lifetime of the excited donor state is relatively long), electronic energy may migrate over long distances until the energy is finally quenched at a terminal trap. Crossover from the 5D0 state can occur when vibrational wave functions from another electronic state, such as a ligand-to-metal CT state, have FC overlap with populated vibrational states of 5D . A low-lying CT state is believed to be responsible for the 0 quenching of 5D0 luminescence in the europium cryptate [Eu⊂2.2.1]3+ 14,15 and for the quenching of the upper 5DJ levels in Eu3+:La2O2S and Eu3+:Y2O2S.16 In the latter case 5DJ f CT quenching is followed by a CT f 5D0 step, which results in Eu3+ 5D0 f 7FJ luminescence. Using an SCC description of the crossover process, the FC overlap between the two states, and, therefore, the crossover rate, will be maximized near the classical crossing point for the vibrational potentials of the two states. The energy difference between the ground vibrational state of 5D0 and the crossing point determines the height of the thermal barrier to crossover. It has been common practice to analyze the temperature dependence of crossover processes in terms of an Arrhenius-type equation (Mott17), such that the barrier height is described by an activation energy, Ea.
w ) A exp(-Ea/kT)
(2)
The Mott description is, however, basically empirical. Struck and Fonger18 have pointed out that the Mott equation provides a satisfactory approximation of the classical crossing point only in situations in which there is a large relative displacement of the two potentials along the nuclear coordinate axis and only for “upward transitions,” i.e., those transitions in which the final state has a potential minimum at a higher energy than does the initial state. For “downward transitions,” the Mott activation energy tends to underestimate the classical crossing point. The potentials of ligand-to-lanthanide CT states have very large displacements along the SCC axis relative to 4fn states but may or may not involve upward transitions. A multidimensional, quantum mechanical treatment by Englman and Jortner19 has shown that in the strong coupling limit (appropriate to the 4fn f CT crossover) the crossover rate is described by an activation formula of the form
w ) cT*-1/2 exp(-Ea/kT*)
(3a)
T* ) (hν/2k) coth(hν/2kT)
(3b)
The activation energy, Ea, represents the minimum energy classical crossing point for the initial and final state multidimensional potentials, and hν represents the average energy of all vibrational modes in the molecule. The factor c is a collection of constants including the electronic coupling factor for the two states and the Stokes shift. The Englman and Jortner model assumes equal force constants and harmonic potentials for the initial and final states. The temperature dependence of any crossover rate, however, can often be more precisely and insightfully described using the general SCC overlap model for multiphonon emission.10-12 However, application of the general SCC overlap model requires some knowledge about the magnitude of the displacement of the potentials involved and about the energy of the active vibrational mode. In addition, differences in force constants for the two electronic states as well as anharmonicity in the potentials near the crossing point must also be taken into account. Experimental Section A. Sample Preparation. Powder samples of Aldrich Eu(thd)3 (Resolve-Al) were stored in a desiccator when not in use. Some powder samples were purified via vacuum sublimation immediately prior to spectroscopic measurement. Other samples were sublimed under vacuum and then recrystallized from hexane under an inert atmosphere. We noted no significant differences in the spectroscopic properties of the purified material compared to that used as received from the manufacturer. In general, spectroscopic measurements on all samples used in this study gave similar results provided that the sample had been protected from excessive exposure to moisture. Samples exposed to the atmosphere for extended periods showed some changes in the structure of the emission spectra and also began to exhibit a long-lived component in the 5D0 emission. Crystals of 4% Eu:Gd(thd)3 and 0.5%Eu:Gd(thd)3 were prepared by mixing the appropriate amounts of Eu(thd)3 and Gd(thd)3 powders (Aldrich), heating under vacuum to dry, refluxing in hexane to dissolve, and filtering the solution, which was allowed to slowly evaporate under flowing N2 or Ar atmosphere. A mass spectrum of the nominally 4% Eu:Gd(thd)3 was measured (HP 5988A) to check the relative concentrations of Eu3+ and Gd3+ to ensure that co-doping of the lanthanides was occurring as expected. The mass spectrum experiment was not performed on a single crystal but on a small
9218 J. Phys. Chem., Vol. 100, No. 22, 1996
Figure 1. Emission spectrum of Eu(thd)3 powder at room temperature and at 77 K. Excitation is into the 5D2 level at 4643 Å. Emission resolution (fwhm) is 4 Å for the room temperature spectrum and 1 Å for the 77 K spectrum.
bundle consisting of three to four needles, this being considered sufficiently few crystals that Eu would not appear as 4% of the sample if the two metals were segregated into separate crystals. IR spectra of both crystal and powder samples were run to exclude the possibility of the presence of hydrates in our samples. B. Spectroscopic Measurements. Low-resolution excitation and emission spectra were measured using a SPEX Fluoromax spectrofluorometer. High-resolution spectra and lifetime data were obtained using an optical bench system constructed in our laboratory, consisting principally of a pulsed (6 ns pulse width) N2 pumped dye laser (Laser Photonics UV-12 and DL-14), a Jobin Yvon 0.46 m monochromator (HR460), and a Thorn EMI 9658 photomultiplier tube. Photomultiplier output was monitored using a Tektronics 2440A digital oscilloscope interfaced via an IEEE port to a PC. For fluorescence lifetime measurements, a fast current-to-voltage preamplifier (Thorn EMI Model A-1) was connected between the photomultiplier tube and the oscilloscope. Temperature control (150-360 K) was achieved by mounting the sample in an optical Dewar through which cooled or warmed N2 gas was passed. The temperature was controlled by flow rate and measured using a chromel-alumel thermocouple. Measurements at 77 K were made by immersing the sample in an optical Dewar filled with liquid N2. C. Scanning Calorimetry. Resolve-Al Eu(thd)3 powder was examined using differential scanning calorimetry (Perkin-Elmer DSC-7) between 77 and 483 K to look for evidence of phase changes occurring in this temperature range. Results and Discussion A. Fluorescence Spectra of Eu(thd)3. Figure 1 shows the corrected 5D0 f 7FJ (J ) 0, 1, 2) emission spectrum of Eu(thd)3 at room temperature and at 77 K. Transitions to the J ) 3, 4, 5, and 6 levels have also been observed, but are considerably weaker than those to the lower J levels. Excitation is into the 5D2 level, which can relax nonradiatively to 5D0, either directly or indirectly via the 5D1 level. All of the peaks in Figure 1 result from 5D0 emission. On a more sensitive intensity scale, 5D emission is apparent. Emission from 5D has not been 1 2 observed. An unusually strong 5D0 f 7F0 transition is evident in the spectrum. In Eu3+ systems, this transition serves as a good diagnostic of crystal quality and purity. Since the line is unsplit by the crystal field effect, any multiplicity observed is necessarily due to a multiplicity of Eu3+ sites. A red shift in the 5D0 f 7F0 peak position is observed upon cooling.
Berry et al.
Figure 2. Laser excitation scan (laser-induced fluorescence) of the 5 D0 r 7F0 band in Eu(thd)3 at 298 and 150 K. Laser bandwidth is 0.1 Å. Emission is monitored on the 5D0 f 7F2 transition at 6116 Å with 20 Å resolution.
Figure 3. Low-resolution (10 Å) excitation scans of Eu(thd)3 at room temperature and at 77 K. Emission is monitored at 6140 Å (5D0 f 7F ) with 80 Å resolution. Asterisks indicate hot bands originating from 2 7 F1.
Figure 2 shows high-resolution, laser excitation scans of the r 7F0 region at room temperature and at 150 K. As in the emission spectrum, a single symmetrical peak is observed, indicating that only one Eu3+ species is present in the sample. Figure 3 shows excitation scans over a wider wavelength region, monitoring 5D0 f 7F2 emission so that only absorbances that populate 5D0 are observed. In the room temperature spectrum, there is an obvious decrease in the efficiency of populating 5D0 via 5D2 excitation, the 5D2 r 7F0 peak being less than twice the height of the 5D0 r 7F0 peak. Above 5D2 there are no features in the excitation spectrum corresponding to 4f f 4f absorbances, only a weak broad band excitation feature peaking at 4000 Å. At 77 K, population of 5D0 via 5D2 excitation becomes relatively more efficient and structure due to excitation of 5D3 and 5L6 is also evident. We attribute the broad band excitation to a ligand-to-metal CT state. At room temperature, the 4f6 states at or above the 5D2 level are depopulated via crossover to this CT state, and, therefore, do not relax through the 5D0 state. B. x% Eu:Gd(thd)3. The mass spectra of Eu(thd)3 and Gd(thd)3 showed dominant peaks for Ln(thd)3+, Ln(thd)3+-tertbutyl, Ln(thd)2+, Ln(thd)2H+, and, in the case of Eu(thd)3, Eu(thd)+. The latter appears for Eu(thd)3 because of the special stability of the 4f7 electronic configuration of Eu2+. The most intense peaks in each case were due to the isotopes of Ln(thd)2+. The mass spectrum of Eu(thd)3 has been previously measured,20 and similar results were reported. A comparison of the relative heights of the Eu(thd)2+ and Gd(thd)2+ peaks in the mass 5D 0
Temperature Dependence of Eu3+ 5D0 Lifetimes
J. Phys. Chem., Vol. 100, No. 22, 1996 9219 TABLE 1: Temperature Dependence of the Relaxation Rate, w(T), of the 5D0 Level in Eu(thd)3 temp/K
Figure 4. Room temperature laser excitation scan of the 5D0 r 7F0 band in doped Eu:Gd(thd)3. Emission is monitored on the 5D0 f 7F2 transition.
Figure 5. Fluorescence lifetime of the 5D0 level in Eu(thd)3 as a function of temperature. Excitation is directly into 5D0, and emission is monitored on the 5D0 f 7F2 transition.
spectrum of our nominally 4% Eu:Gd(thd)3 sample indicated that the Eu3+ concentration in the crystals was indeed between 3 and 4%, indicating that there was little or no segregation of the metals. Figure 4 shows the room temperature excitation spectrum of the 5D0 r 7F0 region for 4% Eu:Gd(thd)3 and for 0.5% Eu:Gd(thd)3. The dominant feature is a symmetrical peak at 5787 Å as in the pure Eu(thd)3. In the 4% Eu:Gd(thd)3, a minority site with a 5D0 r 7F0 at 5809 Å is also evident. This peak appears weakly in the 0.5% Eu:Gd(thd)3 spectrum but is not as evident on the intensity scale used here. The emission spectra of the doped x% Eu:Gd(thd)3 and the pure Eu(thd)3 are essentially indistinguishable. The similarity of the emission and excitation spectra indicates that the Eu3+ ion occupies a site in the Gd(thd)3 lattice which is similar in structure to that in the pure Eu(thd)3 lattice. This is not surprising given that the two crystal structures are known to be quite similar.3 C. Temperature Dependence of 5D0 Emission. As shown in Figure 5, the lifetime of the 5D0 state in crystalline Eu(thd)3 shows an unusually strong temperature dependence in the vicinity of room temperature. From 77 to 240 K the lifetime changes very little, at which point it begins to decrease rapidly with increasing temperature. Table 1 contains lifetime data from 155 to 352 K. At 155 and 246 K, the lifetimes are 470 and 407 µs, respectively, dropping to 40 µs at 297 K and to 1.6 µs at 352 K. Thermal scans using scanning differential calorimetry show no evidence of any phase transitions occurring between 77 K and the melting point at 453 K. The temperature dependence of the 5D0 lifetime is, therefore, not due to structural changes in the solid samples.
155 186 201 216 236 246 254 258 262 267 271 273 276 279 282 286 289 293 297 302 306 309 312 315 319 322 325 328 331 335 338 341 344 348 352
w(T)/103 s-1
τ/µs
2.13 2.02 2.19 2.04 2.11 2.46 3.04 3.43 3.96 4.70 5.40 6.14 7.37 8.77 10.8 13.4 16.6 20.4 24.8 35.4 44.2 55.0 68.4 80.2 99.3 122 145 172 205 257 304 354 417 514 617
(470)a (490)a (460)a (490)a (470)a 407 329 292 253 213 185 163 136 114 92.6 74.6 60.2 49.0 40.3 28.2 22.6 18.2 14.6 12.5 10.1 8.2 6.9 5.8 4.9 3.9 3.3 2.8 2.4 1.9 1.6
wCT(T)/103 s-1
0.44 1.02 1.41 1.94 2.68 3.38 4.12 5.35 6.75 8.8 11.4 14.6 18.4 22.8 33.4 42.2 53.0 66.4 78.2 97.3 120 143 170 203 255 302 352 415 512 615
a At the lowest temperatures the curves were not perfectly single exponential. This may account for the scatter in the data here.
This is a much stronger temperature dependence than would be expected if the dominant mechanism of relaxation were 5D0 f 7FJ multiphonon emission, since several high-energy ligand modes are available. For example, the C-H oscillator at the alpha carbon of the thd ligand is in close proximity to the lanthanide with an energy of about 2900 cm-1. The temperature dependence of the 5D0 f 7FJ multiphonon transition rate, wmp(T), can be described in the weak coupling limit using eq 1. In the absence of other nonradiative mechanisms, the total relaxation rate is given by
wtotal ) wR + wmp(T) ) wR + wmp(0)(n + 1)p
(4)
where wR is the radiative rate. If the relevant vibrational energy is taken as 2900 cm-1, the temperature dependence of the 5D0 lifetime cannot be adequately described, regardless of the values chosen for wR and wmp(0). Figure 6 shows the curve generated using eq 4 when hν is constrained to 2900 cm-1 superimposed on the experimental data. The sum of wR and wmp(0) has been chosen to fit the low-temperature data. With this phonon energy, the relaxation rate is essentially independent of temperature over the range shown. The data in Table 1 were also fit to eq 4 in a completely empirical fashion, allowing all relevant parameters to be determined in the fit. The results of the empirical fit are as follows:
wR ) 2000 s-1
9220 J. Phys. Chem., Vol. 100, No. 22, 1996
Berry et al.
Figure 6. Plot of the decay rate of the 5D0 level in Eu(thd)3 as a function of temperature. The open circles represent the experimental data, and the solid lines represent fits to the multiphonon emission model in eq 4.
wmp(0) ) 2 s-1
Figure 7. Proposed pathway for the decay of the 5D0 state through a low-lying ligand-to-metal CT state.
hν ) 317 cm-1
to make a significant contribution to the overall relaxation rate in the vicinity of room temperature. Having eliminated all other reasonable mechanistic pathways, we assert that the temperature-dependent nonradiative relaxation from 5D0 is most likely due to crossover to a low-lying ligandto-metal CT state. A proposed mechanism for the temperaturedependent quenching of 5D0 through a CT state is illustrated in Figure 7. This mechanism can account for the temperature dependence observed in the 5D0 relaxation rates since a thermal barrier exists for the 5D0 f CT step. As shown, the CT potential is expected to have a minimum at a greater value for the nuclear coordinate than does the 5D0 in at least one dimension. For example, the metall-to-ligand bond length is probably greater for Eu2+‚ ‚ ‚Lo than for Eu3+‚ ‚ ‚L-. Relaxation from the CT state itself is expected to be rapid by reason of its large potential displacement relative to the 7FJ states. Direct evidence for this CT state exists. In the excitation spectrum, a weak, broad band excitation above the 7F0 f 5D2 appears. The weakness is due to the fact that excitation of this state does not populate 5D0 with good efficiency and the detection is set to monitor 5D0 f 7F2 emission. Large crystals of sufficient optical quality to measure the absorbance spectrum have not been obtained, so direct absorbance detection of the CT state in the solid has not been achieved. However, absorbance measurements of 0.048 M Eu(thd)3 in hexane solution show a broad absorbance beginning at 15 400 cm-1, well below the energy of the 5D0 level (17 280 cm-1), and reaching a maximum at about 31 000 cm-1, where it appears as a shoulder on the ligand S1 absorbance feature. Figure 8 shows a comparison of the absorbance spectra of hexane solutions of Eu(thd)3 and Gd(thd)3. In the latter case (Gd3+: 4f7) a low-lying CT state would not be expected, because Gd3+ is stabilized by a half-filled shell electronic configuration. However, a low-lying CT state might be expected in Eu(thd)3 since the Eu2+ ion is stabilized relative to other dipositive lanthanide ions by virtue of a half-filled shell and the ligand radical is stabilized by the same resonance possibilities as exist for the anion. Treating the observed relaxation rates of the 5D0 level as a sum of the radiative rate, wR, the direct multiphonon rate, wmp(T), and the rate of relaxation via the CT state, wCT(T), we assume that the rate observed at low temperatures is due to the radiative and direct multiphonon rates. These are not expected to vary over the temperature range under consideration.
The best fit curve, using these parameters, is also shown in Figure 6. The functional form of the multiphonon model using the “best fit” parameters fits the data rather well, but the parameter values obtained are not physically reasonable. In particular, a phonon energy of 317 cm -1 would require 39 quanta to bridge the 12 200 cm-1 energy gap. Phonon numbers greater than 5 or 6 do not usually provide efficient relaxation in the weak coupling limit appropriate to the lanthanides.12 Nor is the best fit radiative rate, wR, considered reasonable. The radiative rate for Eu(thd)3 in hexane solution was measured from absorbance and emission data for 0.048 M Resolve-Al and found to be around 500 s-1. It is therefore believed that the 5D0 f 7F multiphonon transition is not responsible for the temperature J dependence of the relaxation rates. The possibility that the observed temperature dependence of the nonradiative relaxation might be due to direct phononassisted energy transfer to impurity traps was rejected on the basis that all decay curves measured were either exactly or very nearly single exponential. Direct transfer to randomly distributed acceptors would result in a noticeable deviation from singleexponential behavior. Indirect transfer to traps following rapid energy migration cannot be discounted on the basis of emission decay data, since such a process is expected to yield single-exponential decays. Rapid energy migration among Eu3+ ions is plausible in the Eu(thd)3 system given the strength and sharpness of the 5D0 T 7F transition. Accessible traps could provide for rapid relax0 ation and the temperature dependence could result from a thermal barrier to the ultimate energy transfer from Eu3+ to the trap. Kellendonk and Blasse21 have attributed the thermal quenching of 5D0 emission in EuAl3B4O12 to this type of mechanism. To examine this possibility, the lifetime of the Eu3+ 5D state in 0.5% Eu:Gd(thd) was measured. The Gd serves 0 3 as a spacial barrier to Eu3+-to-Eu3+ energy migration. At 296 K the lifetime of the 5D0 state in the dilute crystals was found to be 43 µs, consistent with the data for the pure material. The importance of energy migration in the relaxation mechanism is therefore doubted. Another possibility for electronic energy transfer is from the lanthanide to a ligand-localized electronic state. The minimum of the lowest lying ligand excited electronic state, the triplet state, lies at least 8000 cm-1 above the minimum of the 5D0 state.8 Relaxation via the triplet state is therefore not expected
Temperature Dependence of Eu3+ 5D0 Lifetimes
J. Phys. Chem., Vol. 100, No. 22, 1996 9221
Figure 10. Arrhenius plot of ln(wCT) vs 1/T for the decay of the 5D0 state in Eu(thd)3. The open circles represent the experimental data, and the solid line represents the best fit line. Figure 8. Comparison of the absorbance spectra of solutions of Eu(thd)3 and Gd(thd)3 in n-hexane. The broad band absorbance in Eu(thd)3 is attributed to a low-lying ligand-to-metal CT state. The sharp feature corresponds to an f-f transition on the Eu3+ ion.
Figure 9. Plot of ln(wCT) vs temperature for the decay of the 5D0 state in Eu(thd)3. The open circles represent the experimental data, and the solid curve represents the best fit to the modified activation model of Englman and Jortner. See text for best fit parameters.
Subtracting these low-temperature rates from the observed total rates leaves the CT rate, wCT(T), given in column 4 of Table 1. Fitting wCT(T) to the modified activation formula of Englman and Jortner (eq 3) yields
Ea ) 5700 ( 300 cm-1 hν ) 230 ( 20 cm-1 ln(c) ) 38 ( 1 Figure 9 illustrates the quality of fit. The large anticipated offsets of the 5D0 and CT potentials lend some support to the validity of the application of this model, but expected unequal force constants for the two states and likely anharmonicities found at the relatively high activation energy should make one cautious in interpreting the parameters quantitatively. An application of the Mott, Arrhenius-type activation model was also made to compare our results to those for gas phase Eu(thd)3.9 A linear fit of ln(wCT) vs 1/T (see Figure 10), gives an activation energy of 4120 ( 20 cm-1 and a pre-exponential factor, A ) 1.2 × 1013 s-1. It is interesting to note that this is the same activation energy (within the stated error) reported by Dao and Twarowski9 for gas phase Eu(thd)3. We suggest that a CT state is also responsible for the observed temperature dependence of the relaxation rate of the 5D0 level in gas phase Eu(thd)3. This would explain why the activation barrier measured for the gas phase Tb(thd)3 system matched the
expected position of the ligand triplet state while that for gas phase Eu(thd)3 was far too low. It is also interesting to note that the pre-exponential factor derived from the gas phase Eu(thd)3 data in ref 9 is 2.2 × 1011 s-1, a factor of 55 lower than that reported here for the crystalline material. If the gas phase Mott parameters are extrapolated back to room temperature, we obtain wCT ) 550 s-1 at 298 K. The overall rate would therefore show a considerably weaker temperature dependence than we observe since wCT would be dominated by the essentially invariant wR + wmp. This is in fact more consistent with the behavior we observe in the 1 mM hexane solution of Eu(thd)3, for which the room temperature rate of 3470 s-1 (288 µs) shows a very weak temperature dependence. One is led to speculate whether the differences we observe derive from the differences in structure, specifically the dimeric structure in the crystal as opposed to the monomeric structures thought to exist in solution and in the gas phase. The Mott activation energy should not be interpreted as the classical crossing point of the 5D0 and CT potentials since, as evidenced by the solution absorbance, this is likely a downward transition. Conclusion The thermal quenching of 5D0 luminescence of Eu3+ in Eu(thd)3 crystals and powder is through a ligand-to-lanthanide CT state. Fits of the 5D0 relaxation rates vs temperature to the models of Mott and of Englman and Jortner yield activation energies of 4120 and 5700 cm-1, respectively. The Mott activation energy is expected to underestimate the classical crossing point of the 5D0 and CT potentials, but even the Englman and Jortner activation energy should be treated as an approximation since assumptions inherent to the model may not be strictly valid. What can be asserted, however, is that the deactivation of the 5D0 state is likely through a low-lying ligandto-metal CT state, and although the minimum of the CT potential is likely lower than the 5D0 minimum, this mechanism shows a high thermal barrier as a result of the relative displacement of the two potentials along at least one nuclear coordinate. The crossover relaxation mechanism is less efficient for Eu(thd)3 in 1 mM hexane solution and in the gas phase, where the complexes exist as monomers. Presumably, the difference in crossover efficiencies between monomers and dimers can be explained in terms of changes in the CT and 5D0 potentials resulting from structural differences between the monomer and the dimer. Acknowledgment. Acknowledgment is made to the donors of the Petroleum Research Fund, administered by the American
9222 J. Phys. Chem., Vol. 100, No. 22, 1996 Chemical Society, for partial support of this project. M.T.B. acknowledges the support of the Research Corp. M.T.B. and P.S.M. thank the University of South Dakota for its support, and all of the authors thank Dr. Karen Koster for help in obtaining calorimetric data. References and Notes (1) Sievers, Robert E., Ed. Nuclear Magnetic Shift Reagents; Academic Press: New York, 1973. (2) Sicre, J. E.; Dubois, J. T.; Eisentraut, K. J.; Sievers, R. E. J. Am. Chem. Soc. 1969, 91, 3476. (3) Erasmus, C. S.; Boeyens, J. C. A. Acta Crystallogr. 1970, B26, 1843. (4) Mode, V. A.; Sisson, D. H. Inorg. Nucl. Chem. Lett. 1972, 8, 357. (5) de Villiers, J. P. R.; Boeyens, J. C. A. Acta Crystallogr. 1972, B28, 2335. (6) Desreux, J. F.; Fox, L. E.; Reilley, C. N. Anal. Chem. 1972, 44, 2217. (7) Jacobs, R. R.; Weber, M. J.; Pearson, R. K. Chem. Phys. Lett. 1975, 34, 80.
Berry et al. (8) Brinen, J. S.; Halverson, F.; Leto, J. R. J. Chem. Phys. 1965, 42, 4213. (9) Dao, P.; Twarowski, A. J. J. Chem. Phys. 1986, 85, 6823. (10) Keil, T. H. Phys. ReV. A 1965, 140, 601. (11) Struck, C. W.; Fonger, W. H. J. Chem. Phys. 1974, 60, 1988. (12) Donnelly, C. J.; Imbush, G. F. NATO ASI Ser., Ser. B 1991, 249 (AdVances in NonradiatiVe Processes in Solids), 175. (13) May, P. S.; Berry, M. T. Unpublished results. (14) Sabbatini, N.; Dellonte, S.; Ciano, M.; Bonazzi, A.; Balzani, V. Chem. Phys. Lett. 1984, 107, 212. (15) Blasse, G.; Buys, M.; Sabbatini, N. Chem Phys. Lett. 1986, 124, 538. (16) Struck C. W.; Fonger, W. H. J. Chem. Phys. 1976, 64, 1784. (17) Mott, N. F. Proc. R. Soc. London Ser. A 1938, 167, 384. (18) Struck, C. W.; Fonger, W. H. J. Lumin. 1975, 10, 1. (19) Englman, R.; Jortner, J. Mol. Phys. 1970, 18, 145. (20) McDonald, J. D.; Margrave, J. L. J. Less-Common Met. 1968, 14, 236. (21) Kellendonk, F.; Blasse, G. J. Chem. Phys. 1981, 75, 561.
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