NUGENT, BURNETT, BAYBARZ, WERNER,TANNER, TARRANT, AND KELLER
1540
Intramolecular Energy Transfer and Sensitized Luminescence in Actinide(II1) @-DiketoneChelates1 by L. J. Nugent, J. L. Burnett, R. D. Baybarz, G. K. Werner, S. P. Tanner, J. R. Tarrant, and 0. L. Keller, Jr. Transuranium Research Laboratory, Oak Ridge National Laboratory, Oak Ridge. Tennessee (Receioed October PQ, 1 9 6 8 )
37830
Survey experiments were performed to determine whether uv-excited sharp-line sensitized luminescence (SLSL) could be detected and studied in some a-active actinide (An3+)P-diketones, namely, the hexafluoroacetylacetonate chelates CsAn(HFA)4.nHzO with the An3+ ions 241Am3+,243Am3+, 244Cm3+,249Bks, 24YCf3+,and 263Es3+.The relatively heavy HFA- ligand was chosen as the chelating agent so the work could be performed effectively at room temperature, Measurements were made on the pure crystals CsAn(HFA)4lH20 only in the Am3+and Cm3+cases, on the chelates in anhydrous ethanol solutions in all cases, and with An3+as a dopant in the CsGd(HFA)4 crystal matrix in all cases. SLSL was detected only in the case of Cm3+; it is highly efficient in all three media, resembling Eu3+ in red SLSL color and in high quantum efficiency. The results indicate that laser emission could be demonstrated in certain Cm3+-chelate solutions. The absence of SLSL over the 4000-10,200-~ experimental range in the case of Es3+suggests that the first excited state of this 5f10 electronic configuration is below 9800 em-'. Measurements of self-excited luminescence of the crystalline C S ~ ~ ~ C ~ ( H F Ashow ) ~ - that H~O it is essentially the same as the uv-excited luminescence and that the radiolytic decomposition is linear with a total decomposition time of 6 hr. This is approximately the time required for the time-integrated molar radioheat to equal the sum of the molar bond energies and crystal binding energy. Evidently the individual ligands are excited by the a activity and the energy is transferred via the ligand singlet and/or triplet state to the Cm3+,as it is with uv excitation. Estimates are made of the radiolysis lifetime for each sample studied, assuming the radiolytic decomposition is in each case linear in time and that the decomposition rate is proportional to the specific radioheat of the isotope under consideration. These results indicate that radiolytic decomposition over the time required to do the experiments is not responsible for the absence of SLSL in the Am3+,Bk3f, Cf3+,and Es3+cases.
I. Introduction Since the realization of the first laser in 1960, general interest in the study of luminescence from organic compounds has intensified. Particular emphasis has been on the lanthanide ( Ln3+) P-diketone chelates, and among these the europium and terbium complexes have attracted the most attention because they show by far the strongest uv-excited luminescence. As a result of the research on these substances, a better understanding of their unusual spectroscopic properties has begun to develop, The very strong ultraviolet absorption due to an initial electronic transition from the singlet ground state to the first excited singlet state of the organic ligand, the subsequent radiationless energy transfer to the excited states of the Lna+ 4fn electronic configuration, and the luminescence from the latter in sharp emission lines have been measured in many cases, and the results are a t least qualitatively understood in relation to electronic and vibrational structures, molecular structures, crystal structures, and solution environments. Another important result of this research was the first demonstration, in certain europium and terbium chelate solutions, of the possibility of obtaining laser emission from the liquid state, and this in turn initiated the development of a more general organic and inorganic liquid laser technology .2-5 The Journal of Physical Chemistry
I n view of the encouraging results of the lanthanide P-diketone work, we decided to survey the uv, visible, and near-ir spectra of some of the actinide (An3+) 0-diketones, comparing their previously unknown luminescence characteristics with those of the related lanthanides and determining which if any of these compounds show the very strong sharp-line sensitized luminescence (SLSL) now well known in the EuS+and Tb3+ chelates. More specifically, this work was begun for several reasons. First, it is now apparent that sufficient similarities exist between the energy levels of the 5 f n electronic configurations of the Ansf series and the energy levels of the 4f" electronic configurations of the Ln8+ series6 that efficient SLSL similar to that found in the Eu3+ and TbS+ 0-diketone chelates may be expected in some of the actinide p-diketones. Second, (1) Research sponsored by the U. S. Atomic Energy Commission under contract with the Union Carbide Corp. This paper was delivered in part by L. J. N. a t the 155th National Meeting of the American Chemical Society, San Francisco, Calif., April 1-5, 1968. (2) G . A. Crosby. Mol. Cryst., 1, 37 (1966). (3) (a) J. P.Fackler, Jr., Progr. Inorg. Chem., 7 , 361 (1967);(b) 8 . J. Lippard, ibid., 8 , 109 (1967). (4) A. Heller, P h y s . Today, 2 0 , No. 11, 35 (1967). (5) S. Bjorklund, G. Kellermeyer, C. R . Hurt, N. McAvoy, and N. Filipescu, A p p l . Phys. Letters, 10, No. 5 , 160 (1967). (6) W.T. Carnal1 and P. R. Fields, Advances in Chemistry Serios, No. 71, American Chemical Society, Washington, D. C., 1967, p 86.
LUMINEWENCE IN ACTINIDE(III)0-DIKETONE CHELATES many of the energy levels of the 5fn electronic configurations of the An3+ series are not well known, particularly in the second half of the series where only a few theoretical estimates are available, so measurements of the uv-excited emission spectra of these chelates could help establish some of this information. Third, it is an advantage to do emission studies with the very strongly absorbing chelates in cases such as Es3+, where the actinide ion is very weakly absorbing,’ and it is desirable to use very small samples because of limited availability or because of the radioactivity hazard. Fourth, a new and interesting part of this investigation is the observation of the effects of the interaction of the actinide ion a activity with the chelate crystal and molecular system, This has two aspects which are interrelated but not well known: one is self-excitation and energy transfer, followed in certain cases by emission from the energy levels of the first excited ligand triplet state and/or the excited energy levels of the Wn electronic configuration; the other is the radiolytic decomposition of the crystal and molecular system. Finally, it is of interest to determine whether the results of this investigation support the possibility of the application of some of the actinides in liquid laser technology. In the present work we examine the uv-excited and the a-activity self-excited optical emission characteristics of some of the hexafluoroacetylacetonate (HFA-) chelates [ C S A ~ ( H F A ) ~ . ~ Rat ~O room ] temperature. The actinides considered here are 241Am3+, 248Am3+, 244Cm3+ 249Bka+, 249Cf3+, and 263EsW. The others, Th3+, Pa3+, U3+, Np3+, and Pu3+ with possible unfilled 5fn electronic orbitals, were not investigated because only the higher oxidation states in these cases are stable in water or ethanol solution18 and Fm3+, Md3+,and No3+were not investigated because they are not yet available in the 0.1-ng ( 2 x 10” ions) minimum quantity required. The anion (HFA-) was selected as the chelating agent because it contains a minimum of H atoms and provides molecular and crystal environments of predominantly heavier atoms which are known to enhance emission by reducing thermal quenching, so the experiments could be conveniently performed a t room temperature. SLSL via intramolecular energy transfer, similar to that observed in the lanthanide chelates, is found only in the case of the Cm8+. The SLSL is observed in ethanol solution and in the crystalline state. Evidence for cation exchange, similar to that observed by Bhaumikg between Eu3+ and TbS+chelates in solution, is found between either of the ions Eu3+ or Tb3+, introduced into ethanol solution as the HFA- chelate, and Cma+ introduced as the chloride. Intermolecular triplet exciton migration, similar to that recently observed and interpreted by Kleinerman’O in certain crystalline lanthanide chelates, occurs in the crystalline chelate CsGd (HFA)4 doped with various Ln8+ and
1541
Ana+ ions. Triplet exciton migration is helpful here in transferring the excitation energy throughout the ultraviolet-opaque CsGd (HFA)4 crystals to microgram amounts of An3+ doped at 1 part/l08 parts of Gda+, This technique opened the possibility of observing optically excited emission from Es3+ ions, and because of the factor of lo3 dilution, it reduced the effects of radiolysis so the chelate crystals were not destroyed during the minimum 20-min preparation and observation time. Absence of SLSL in the actinide chelates other than Cm3+ is discussed in relation to the known electronic energy level spacings and the vibrational frequencies of the molecular environment. The energy of the first excited electronic state of Es3+is discussed in relation to the absence of SLSL in the E#+-doped CsGd( HFA) 4 crystals and recent theoretical energy level estimates.eJ1 Finally, since it became apparent that Cm8+ chelate solutions show a possibility for a liquid laser, resembling Eua+ chelates in this respect, the parameters of importance in estimating liquid laser efficiency, namely, the absorption spectrum, the emission spectrum, the quantum efficiency, and the emission decay time, are reported and compared with one of the well-known Eu3+ chelate laser solutions.
11. Experimental Section The crystalline rare earth chelates CsEu (HFA)4, CsGd (HFA)dl and CsTb (HFA) were prepared in aqueous-ethanol solutions using the technique described by Lippard12J3 for the preparation of CsY (HFA)4. The structural formula for CsTb (HFA) is presented in Figure 1. Danford, et a1.,I4have shown
0,
\ \\
HC
i”
C
Figure 1. Structure of CsTb(HFA)4 where HFA- ia the hexafluoroacetylacetonate anion. (7) B. B. Cunningham, J. R . Peterson, R. D . Baybars, and T. 0. Parsons, Inorg. Nuc2. Chem. Letters, 3 , 519 (1967). (8) B. B. Cunningham. “Comparative Chemistry of the Lanthanide and Actinide Elements,” paper presented at the XVII International Congress of Pure and Applied Chemistry, Munich, 1959, pp 64-81. (9) M. L. Bhaumik, J. Inorg. NucE. Chem., 2 7 , 243 (1965). (10) M. Kleinerman, J . Chem. Phys., in press. (11) J. G. Conway, private communication, Lawrence Radiation Laboratory, 1967. (12) 9. J. Lippard, J. Amer. Chem. SOC.,8 8 , 4300 (1966). (13) 9. J. Lippard, F. A. Cotton, and P. Legzdins, {bid., 88, 5930 (1966). (14) M. D . Danford. J. H. Burns, 0. L. Keller, Jr., J. R . Stokely, and W. H. Baldwin, Inorg. Chem., in press. Volume 7.% Number 6 M a y 1969
1542
NUGENT, BURNETT, BAYBARZ, WERNER,TANNER, TARRANT, AND KELLER
that the lanthanide compounds have the indicated elemental formula, that these crystals are isomorphous, and that they belong to the orthorhombic crystal system. Using the same method of preparation, the products CsAm(HFA)4.H2O and CsCm(HFA) 4.Hz0 were obtained, and the presence of H2O here was indicated by an X-ray powder diffraction measurement which showed these crystals to be isomorphous with monoclinic CsNd (HFA) 4 - H 2 0 rather than with CsGd(HFA)4. (These results for CsGd(HFA)4 and CsCm(HFA)4*Hz0are by the authors of ref 14, but they are as yet unpublished.) Samples of AnCls-nHzO where n is an undetermined number of H2O molecules were prepared by dissolving microgram amounts of the actinide oxide in boiling ethanol solution to which an excess of concentrated aqueous HC1 was added. This solution was taken to dryness a t room temperature by passing argon gas across the surface, thereby minimizing the formation of the actinide oxychloride. The solid product was redissolved in anhydrous ethanol and taken to dryness again to remove excess acid, and this was repeated until the final ethanol solution gave the same color reaction to pH paper as pure ethanol. (In the case of Es3+ repetition of the cycle did not yield a neutral pH color indication but one that was always slightly acid. This indicates that solutions of einsteinium trichloride are slightly more acid than are those of the other actinides.) Measurements were made on the pure crystals CsAn(HFA)d-H20 only in the Ama+ and CmS+ cases, where the radioisotopes are more plentiful, on the chelates in anhydrous ethanol solution in all cases, and with An3+ as a dopant in the CsGd(HFA)4 crystal matrix in all cases. The actinide chelate solutions were prepared by taking from 1 pg to 1 ng of the actinide to M) chloride in 5 p1 of anhydrous ethanol and introducing this solution into a quartz capillary tube containing 5 pl of 10-8 M CsTb (HFA) 4 in anhydrous ethanol. Very rapid exchange occurs between the Ln3+ chelate and the Ana+ions in solution, producing instant chelation of the latter as evidenced by the observation of SLSL from the Cm3+. For the solidstate experiments Ans+-doped CsGd( HFA) 4 crystals were prepared by taking 1 pg of the actinide chloride in 5 pl of anhydrous ethanol and introducing this solution into a quartz capillary tube containing 5 pl of anhydrous ethanol saturated at -0.6 M with CsGd(HFA)h. The combined solution was vacuum pumped to dryness leaving a residue of CsGd(HFA)d crystals doped to 1 part of An3+/103parts of Gd3+. Measurements of the solution absorption spectra were performed on a Cary 14 recording spectrophotometer. Measurements of the emission and the excitation of all samples were performed on an AmincoBowman spectrophotofluorometer. A steel enclosure with quartz entrance and exit windows was used for sample containment and for protection against radioThe Journal of Physical Chemistry
activity. To obtain the emission spectrum the excitation monochromator was set to illuminate the sample with radiation of wavelength 3400 A, in the region where the HFA- ligand absorbs strongly and the xenon lamp intensity is high, and the emission monochromator was set to scan either the region 4000-8OOOA or the region 8000-10,208 A while the emission signal vs. the emission wavelength was recorded. I n the visible region an 5-20 surface photodetector operating at room temperature provided adequate sensitivity; however, in the near-infrared region it was necessary to substitute this with a Dry Ice cooled S-1 photodetector. To obtain the excitation spectra, the emission monochromator was set on an emission line and the excitatio? monochromator was scanned from 2000 to 8000A while the emission signal vs. the excitation wavelength was recorded. Because of the difficulty in determining accurate quantum efficiencies of microcrystalline solids and because a knowledge of quantum efficiency is basic in the present work only in relation to the evaluation of the prospects for a curium chelate liquid laser, only the quantum efficiency of the curium chelate formed by exchange with terbium chelate in anhydrous ethanol solution was measured. This was determined by comparison with a chelate solution of known quantum M solution of Eu8f thenoyltriefficiency, namely, a fluoroacetonate (TTA) in dimethylformamide. (The Eu-TTA chelate was supplied by M. L. Bhaumik, who measured the quantum efficiency to be 38% a t 25'
2900
3400
x frf,
moo
Figure 2. Absorption band of CsTb(HFA)r in anhydrous ethanol at 23".
LUMINESCENCE IN ACTIKIDE(III) DIKET TONE CHELATES
1543
-
~m3*
5f 7
b
Es3+ 5 f '0 c
Yb3+ 4f'3
d
a
Figure 3. Lower lying energy levels of HFA-, Eua+, TbS+, Cm*+,E$+, and Ybs+: (a) G. H. Dieke, "Advances in Quantum Electronics," J. R. Singer, Ed., Columbia University Press, New York, N. Y., 1961, p 170; (b) theoretical energy levels calculated by Fields, Wybourne, and Carna1l;e ( 0 ) energy levels measured in absorption in 3-6 M HCl in the wavelength region 3200-6500 d;7 (d) theoretical energy levels calculated by Conway.7
in dimethylforrnamide.l6) An Aminco solid sample accessory was employed to minimize signal variations that could arise owing to differences in spatial distribution of the emission from the chelate standard compared to that of the solution to be measured. The emission decay time of the M curium chelate solution was measured by operating the spectrophotofluorometer in a flash mode, Le., by flash exciting the sample through the excitation monochromator with a 10-psec flash and observing the time dependence of the emission signal through the emission monochromator on an oscilloscope. The flash energy was provided by a 0.1-pF capacitor charged to 3 kV and fired through a 200-W xenon-mercury lamp. The latter was triggered by discharging a 10-pF capacitor energized to 68 V t,hrough an ignition coil giving a 30-kV trigger pulse through the lamp. The emission signal was detected with a 1P21 phototube impedance matched to the oscilloscope by a pulse amplifier with a factor of 10 gain and a 10-psec response.
111. Results and Discussion A . Spectroscopy of the Chelates in Solution. The near-ultraviolet absorption band of CsTb (HFA) in anhydrous ethanol is presented in Figure 2; similar bands with essentially the same shape and intensity are observed for all of the other hexafluoroacetylacetonate rare earth chelates. As indicated in Figure 3, this band arises from the So+S1 transition in the singlet electronic manifold of the HFA- ligand. This allowed transition produces the very strong absorption through which relatively large amounts of uv power can be delivered into the 4f" or, as will be shown, into the 5fn electronic energy levels of very small amounts of chelated Lna+ or An3+ ions. The energy-transfer mechanism is intramolecular and is believed to proceed as shown in
Figure 4, from So+Sl+Tl--+4fn or Sf", where TI is the lowest lying electronic triplet state of the HFAligand. Efficient intramolecular energy transfer and strong SLSL similar to that known to occur in the EuS+and Tb3+ chelates are reported here for the curium hexafluoroacetylacetonate chelate. The emission and the excitation spectra of an anhydrous ethanol solution 10-3 M in CsTb (HFA) 4 and M in CmC&are shown in Figure 4a. I n addition to the strong Tbs+ SLSL a t 4880 and 5440 A, a new very strong line is observed a t 6040A and assigned to a transition from the first excited electronic state to the ground state (J = $ + J = 4 ) in the Sf7 electronic configuration of Cm3+, as indicated in the Cm3+energy level diagram of Figure 3. The excitation spectrum is essentially the same for each terbium emission line and for the curium emission line, thus demonstrating that SLSL occurs for both the Tb3+ and the Cm3+. The excitation band shape is determined by the shape of the absorption band in Figure 2, by the concentration of the CsTb (HFA) 4 in the 1-mm diameter cell, and by the spectral output of the xenon lamp. These observations and the previous work on the Ln3+ p-diketones support the conclusion that a curium chelate complex, possibly Cm(HFA)2+, or Cm(HFA)2+ and Cm(HFA)3, forms by exchange with the terbium chelate in solution, that intramolecular energy transfer occurs, and, as indicated in Figure 3, that the nonradiative ( ) and radiative ( 1 ) transitions with the 5f7 electronic configuration of Cm3+ result in efficient SLSL. 1611'
-I
(15) M. L. Bhaumik and 0.L. Telk, J . Opt. Soe. Amer., 54, 1211 (1964). (16) F. Halverson, J. 8. Brinen, and J. R. Leto, J . Chem. Phys.. 40, 2790 (1964). (17) M. L. Bhaumik and M. A. El-Sayed, i b M , 42, 787 (1965).
Volume '79, Number 6 May 196.9
NUGENT, BURNETT,BAYBARZ, WERNER,TANNER , TARRANT, AND KELLER
1544 (01
WAVELENGTH
too -
m
4ooo
-
c.
s 80E -
(h
f Emlrslon Spectrum Only)
I
,
I
I
6000 I
-
‘1066
I
Volume
-
-
SI
Room ~ p e r a t u m +EXCITATION SPECTRUM (Emiadon Wovelength* 6040h I
-
-
-2.001
I
40-6
-c b)
too
I
I
6
Room Temperature 5-20 Photodetect#
I
EMISSION SPECTRUM
L
WAVELENQTH f i b
Figure 4. SLSI. via intramolecular energy transfer in curium chelate in anhydrous ethanol solution at 23”. The actual yidth of the Cm3+ emission line at half-maximum intensity is 80 A but it is instrument-broadened here to about 100 b: (a) 1.00 X 10-* M M C S T ~ ( H F Aplus ) ~ 1.00 X 10-3 M fi44CmC13;(b) 1.00 X M 244CmC1a. C S T ~ ( H F Aplus ) ~ 1.00 X
Evidence that the 6040-A SLSL is not due to Tb3+ is shown in Figure 4b and in Figure 5. I n Figure 4b the Cm3+concentration is less by a factor of 10 than in Figure 4a while the CsTb (HFA) 4 molarity is the same, and it can be seen that the Cm3+ SLSL intensity relative to that of the Tb3+ is down by about a factor of 7. I n the plot in Figure 5 the ratio of the amplitude of the red Cm3+ line to the amplitude of the green Tb3+line is recorded as the Cma+ concentration is varied down to loReM while the CsTb (HFA) concentration is kept at 10-3 M . It is clear that a t 10-4 M Cma+ and below a linear relationship exists between these parameters. At M Cm3+ the measured point is slightly below the linear fit because the competition between the Cm3+ and Tb3+ for the HFA- becomes significant, while this is not the case at lower Cm3+ concentrations, It is worth noting that such measurements could be made the basis for quantitative determinations of Cm3+ in The Journal of Phyeical ~ h e n i s l r y
I \
I
10-3
(Crn3+)
10-1
---L
Figure 5. Logarithm of the ratio of the amplitude of the red Cm3+ emission line A(CmSt) to the amplitude of the green Tb*+emission line A(Tb3+) vs. the concentration C(Cm*+)in 1.00 X lO-aM CsTb (HFA) 4 ethanol solutions.
the presence of other cations. The power of this method for analytical chemistry is shown by the fact that a t loR6M em3+ the 5-pl sample gave an emission line at 6040k with a signal to noise ratio of 50, indicating that a sample with as little as g, or 2 X 1O’O ions, of Cm3+ could be detected. It is important to compare the SLSL of the curium chelate with that of the corresponding europium chelate because of the close similarity in the emission in these cases, so it will be clear that the Cm3+emission is not due to Eua+ impurity. The sensitized luminescence spectrum of an ethanol solution of low3M CsGd(HFA)4 with 1.0 X M CmCla and 0.59 X loR6M EuCla is presented in Figure 6. Two SLSL lines with identical excitation spectra are observed
Figure 6. SLSL via intramolecular energy transfer in a M CsGd(HFA)4 anhydrous ethanol solution with Cm3+ and Euat at 1000 and 590 partsll06 parts of Gda+, respectively.
LUMINESCENCE IN ACTINIDE(III)DIKET TONE CHELATES
1545 ~
~~
Table I: Parameters for Liquid Laser Evaluation 10- M CrnCla and 10-8 M C S T ~ ( H F A )10-8 ~ M CsEu(HFA)4
Parameter Emission wavelength (AJ, A Line width at half-maximum intens (AX), Quantum efficiency (*), % (l/e) decay time (T),psec
in anhyd ethanol
in anhyd ethanol
6040 80
6140
A
7 120
40 15 (19 for 10” M) 400
K x 10-8 M Eu(BTFA)rHPyrr in acetonitrilea
6118 -40 -50 -600
From the work of E. J. Schimitschek, J. A. Trias, and R. B. Nehrich, Jr., J . Appl. Phys., 36, 867 (1965). BTFA- is benzoyltrifluoroacetonate and HPyrr+ is the pyrrolidinium ion. (I
in the red region-one at 6040A due to the Cm3+ chelate and the other a t 6140 A due to the Eu3+ chelate. From this it is clear that the red emission line in Figure 4 is due to the Cm3+ chelate and not to Eu8+ chelate which is not observably present here as an impurity. It is evident, therefore, that although the SLSL spectra of europium hexafluoroacetylacetonate and curium hexafluoroacetylacetonate chelates are not identical, they are markedly similar in emission wavelength and quantum efficiency, and so the comments above on the possibilities for a quantitative analytical determination apply as well for europium as for curium. B. Liquid Laser Considerations. The SLSL wavelength XO, the line width a t half-maximum intensity AA, the quantum efficiency 9, and the observed SLSL decay time T are listed in Table I for the Cm3+ and Tb3+ chelate mixture in ethanol, for the pure C S E U ( H F A )in ~ ethanol, and for a Eu3+ benzoyltrifluoroacetonate chelate comparison solution known to give laser emission at a relatively low threshold in acetonitrile. I n the present section these parameters are used in evaluating the possibility of the Cm3+ chelate as a liquid laser material. On the basis of the Schawlow-Townes equation for the molecular population inversion required to initiate laser e m i s s i ~ n , ’ ~the J ~ laser threshold power required for a Cm3+ chelate solution can be estimated by taking it to be directly proportional to (rrAA), where the radiation lifetime rr 2 7, and inversely and comparing it to the known proportional to ( 9,XO4), threshold for Eu (BTFA) 4HPyrr in acetonitrile (HPyrr = pyrrolidinium ion). The parameters 9 and rr to be used in such a comparison should not be taken from the first column of Table I, however, since these are for the CmS+-Tb3+ chelate mixture in ethanol and the corresponding parameters should be significantly better for pure Cm( BTFA) 4HPyrr in acetonitrile. Because of the close similarities in the emission properties of the Cm3+ and the Eu3+ chelates, the parameters and rr for Cm(BTFA)lHPyrr in acetonitrile are taken to be approximately equal to the corresponding parameters for Eu (BTFA)4HPyrr in acetonitrile. Since XO is essentially the same in both cases, only the line widths are different. Another important point in this comparison is that the Cm3+ SLSL arises from a
transition terminating on the ground state while the E u ~ SLSL + arises from a transition terminating on the 7F2energy level which is 1000 em-’ above the ground state. This requires that more than half the original population of the ground state be optically pumped to obtain population inversion in the Cm3+ case, while considerably less pumping is normally required in the Eu*+ case because here the terminal level has initially only a relatively small thermal population. I n the threshold estimate the effect of the difference in the terminal energy levels can be eliminated by taking the comparison laser threshold a t the minimum Eu (BTFA) 4HPyrr concentration required for laser M concentration and action, i.e., 850 J a t 1.5 X 10°.20 At this concentration essentially all of the Eu3+ chelate molecules must be optically pumped out of the ground state in order to achieve the minimum population inversion required for laser emission. Then with Ah(Cm3+) = 80 = 2AX(Eu3+)and all of the other parameters for the Cm3+ system the same as for the Eu3+ system, the laser threshold for Cm(BTFA) 4HPyrr is estimated to be 1700 J, a factor of 2 greater than that of E u (BTFA) 4HPyrr comparison solution and well within the 4000-5 range of conventional liquid laser test apparatus. Similarly, the minimum concentration of Cm (BTFA) cHPyrr required for laser emission in acetonitrile is estimated to be 3 X M , also a factor of 2 greater than that of the comparison solution. From this it appears that a 244Cm3+chelate liquid laser could be demonstrated. However, as discussed in section IIID, in working with 244Cmthe problem of radiolysis must be carefully considered. The preparation of the Cm3+ chelate crystals must be done in less than 1 hr to reduce radiolytic decomposition in the solid state. Once the chelate is in solution at about 10“ M, it should last for several days for experimentation, but after this a fresh preparation would have to be made. On the other hand the radiolysis problems will be essentially eliminated when the lower activity isotopes 247Cm [ t ~ , z ( m ) = 1.7 X lo7 years] and
A
(18) A. L. Schawlow, Solid State Design, 2 , 21 (1961). (19) A. Yariv, “Quantum Electronics,” John Wiley & Sons, Inc., New York, N . Y., 1967, Chapter 15. (20) E. J. Schimitschek. J. A. Trias, and R . B . Nehrich, Jr., J. Appl. P h y s . , 36, 867 (1966).
Volume 73,Number 6 M a y 1060
NUGENT, BURNETT,BAYBARZ, WERNER,TANNER, TARRANT, AND KELLER
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248Cm [ t l l z ( a ) = 5 x 106 years] become available in macroscopic quantities. C . Emission Properties of the Chelates in the CsGd(IIFA).I Matrix. The SLSL excitation and emission spectra of 244Cm3+ doped into the CsGd(HFA) 4 crystals a t 1 part of Cm3+/103parts of Gd3+ are presented in Figure 7. The wavelength of the SLSL peak of the Cm3+ chelate is shifted from 6040 A in ethanol solution to 6140 A in the CsGd(HFA)4 crystal matrix. Such wavelength shifts are common in rare earth chelate studies; they are attributed mainly to differences in the species present and t o differences in the solution and the crystal fields. The weaker lines shown in Figure 7 a t 5740, 5900, and a t 6740A are due to vibronic transitions. The ligand and crystal field splittings in each case are expected to be unresolved within each line.21 The excitation spectrum is shifted to longer wavelength and changed slightly in shape compared to the ethanol solution case because there is stronger absorption due to the higher -0.6 M CsGd (HFA) 4 concentration in the crystal compared to l O - 3 M in the solution. SLSL is not observed from Gd3+ in these experiments because the first excited electronic energy level of Gd3+ is a t 32,500 cm-’ (3100 A), in the peak of the very strong HFA- So431 absorption band and far above the lowest lying HFAtriplet state TI, nor can the relatively high-lying energy levels of the Cd3+ ion quench the actinide SLSL, so CsGd (HFA)4 is a good host matrix. Furthermore this solid-matrix technique provides the same 0.1-ng sensitivity in the cases of Cm3f or Eu3+ as the ethanol solution technique and it could be applied as effectively in quantitative analytical determinations of these ions. The CsGd(HFA) used in the present experiments was prepared from 99.999% pure Gd203 so any rare earth impurities could be present a t the most at 10 ppm. In Figure 8 the uv-excited emission spectrum from CsGd( HFA) crystals at room temperature is presented.
1 ” ” 1 ” ” 1 ” - ’ 1
eo
-
eiii 6
I
cD(CITAll0N SPSTRUH
60-
j’E.Z%Z U
Q 86
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-
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-
-
07
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S-I Phototube
-
Excitation I Wavelenpth.36008
V)
s3
60-
-
I
EMISSION WAVELENGTH )(;
Figure 8. SLSL uia intramolecular energy transfer in europium chelate as an impurity in CsGd(HFA)4 a t 2 parts of Eua+/lOG parts of Gda+ in crystalline CsGd(HFA)a.
This spectrum shows two features which are worthy of note. The first is the broad phosphorescence of the HFA- ligand in the transition T1-+So in the blue-green region of the spectrum. The second is SLSL from Eu3+ estimated from flame photometry measurements to be present as an impurity a t about 2 ppm. From the phosphorescence we confirm the lowest triplet state (TI) energy band shown in Figure 3, and since phosphorescence is not observable from pure CsEu(HFA) 4, it is evident that Eu3+ ions quench triplet excitons. From the sensitized luminescence we note that the strongest Eu3+ SLSL is a t the same peak wavelength as the SLSL of Cm*+and it is half as wide, The line widths of the SLSL of these compounds are essentially unchanged in going from the solid state to solution but the emission wavelength of Cm3+ shifts slightly to the blue. These 2-ppm Eu3+ impurity lines are weak enough so they produce no measurable contribution to the Cm*+chelate SLSL presented in Figure 7, where the Cm3+concentration was 500 times larger than the concentration of the Eu3+ impurity in the CsGd(HFA)4. The absence of SLSL is not surprising for Am3+, Bkaf, and Cf3+ in the present experiments because the 5fn electronic energy level spacing, or gap, between the so-called radiative resonance level and the next lower level is relatively small in each of these cases, with the largest energy gap 4000 cm-1 for Ama+, 6180 cm-l for Bk*+,and 6410 cm-l for Cf3+.16J9,20The relatively small energy gaps enhance fast nonradiative energy transfer down the 5f“ energy level manifold in a time that is short relative to the radiative lifetime. Such rapid nonradiative cascading is well known in rare earth spectroscopy, It is believed to be the result of the time-dependent ligand field perturbation involving the interaction of moIecular vibrations with the excited 4fn or 5fn electronic states, and the faster cascading rates are associated with smaller electronic (21) J. B. (1966).
The Journal of Physical Chemistru
+.” t
Gruber and W. R. Cochran, J. Chem. Phys.,
45, 1423
LUMINESCENCE IN ACTINIDE(III)DIKET TONE CHELATES
1547
and Cf3+cases. energy level spacings and higher molecular vibrational D. Self-Excited Luminescence and Some Preliminary energies. This accounts for the fact that sensitized Radiolysis Considerations. Self-excited luminescence luminescence at room temperature is strong only for appears in solution as a weak orange-red glow as soon relatively heavy-ligand chelates of Eu3+, Tb3+, and as all of the reactants are combined in the preparation Cm3+. In these cases at least one of the electronic of the pure CsCm (HFA) 4 HzO. The water-ethanol energy level spacings is greater than 12,000 em-’, as reaction solution is stirred and partially evaporated can be seen in Figure 3, and so with the large gap and over a period of 1 hr and the glow brightens as the the lower energy vibrational modes of the heavier crystals precipitate. The power activating the glow is ligands, the nonradiative cascading rate is reduced primarily from absorption of the &MeV 244CmOL activity relative to the radiative rate and efficient luminescence (17.6-year half-life). In low resolution and with the can occur.16,22--24 sample at room temperature, the glow spectrum of the The Es3+ 5fI0 energy level structure, on the other pure compound is essentially the same as the uv-excited hand, may be intermediate between the rapid and slow emission spectrum shown in Figure 7 for Cm3+ doped cascading cases. In this case experimental work has into crystalline CsGd (HFA)4. Differences between been slower because of the availability of only a few these spectra may appear at higher resolution and micrograms of 263Es3+a t any one time, its short 20-day lower sample temperature since the crystal structure of half-life, its high 1000-W/g specific radioheat, and its C S G ~ ( H F Ais) orthorhombic ~ while CsCm(HFA) 4.Hz0 relatively low absorption coefficients. The present is monoclinic and since additional crystal and ligand status of the Es3+ 5f1° energy level work is presented in field differences are undoubtedly introduced via the Figure 3. The absorption spectrum has been measured faster radiolysis of the pure Cm3+ compound; however in the region 3200-6500 A by Cunningham, et aZ.,’ but such effects are not apparent under the present experimeasurements a t higher wavelengths have not been mental conditions. reported yet. Two independent sets of theoretical The intensity of the self-excited glow was monitored calculations have predicted the first excited electronic in two experiments. In the first experiment the state to be a t about 10,000 cm-1.6J1 However, these crystals were filtered from the solution and the glow estimates are based on parameters obtained from signal from the dry precipitate contained in a small extrapolations of corresponding parameters of the sample cell was measured in a photometer, beginning lower atomic number actinides, some of which are approximately 4 hr after compound formation. Suruncertain, so the accuracy of the present theoretical prisingly, the glow signal was not exponential in time; energy levels in the EsS+case is a t best uncertain. it decreased linearly to zero over the last 2 hr of a 6-hr In the present work the SLSL spectrum of CsGd(HFA)d crystals doped a t 1 part of Es3+/103 total radiolysis period, indicating that the overall radiolysis kinetics may be zero order, if, as seems reasonable, parts of Gd3+was searched from 4000 to 10,200 and the concentration of the organic ligand is proportional no SLSL from Es3+was detected. The sensitivity of the method in the near-infrared region was checked by to the glow intensity. In the second experiment, the crystals were kept in a slurry with the mother liquor measuring the SLSL spectrum of Yb3+ doped a t the and the glow signal was measured as in the first experisame concentration as the Es3+ in the CsGd(HFA)4 crystal matrix. A t least two Yb3+ SLSL signals were ment, but beginning here approximately 2 hr after observed-one at 9720 A with a signal-to-noise ratio compound formation. In this case the signal also of 50 and the other at 9984k with a signal-to-noise decreased linearly to zero, but with a longer total ratio of 20-and in addition other weaker, more diffuse radiolysis lifetime of 12.5 hr. The OL energy liberated over the 6-hr radiolysis lifetime is 3.5 X lo6cal/mol, or, bands were observed up to 10,200k. This SLSL emission corresponds to the Yb3+transition 2F~,z-+zF7j-2 in the terminology of radiation chemistry G, ( - HFA) = illustrated in Figure 3, and multiple lines are observed 2.6 for the dry crystals; similarly, over the 12.5-hr radiolysis lifetime it is 7.4 X lo6 cal/mol, or G,( -HFA) = owing to crystal and ligand field splitting. From this evidence it is clear that a t room temperature the 1.2 for the slurry. This compares with an esticrystal matrix does not completely quench SLSL of mated total molecular bond and crystal energy of wavelength as high as 10,200 A and that the excitation CsCm(HFA)4.H20 of 6 X lo6 cal/mol, and the G and detection systems are adequate for the observation values are about as expected for the OL radiolysis of of these signals. Furthermore, it is clear from absorporganic compounds when a chain reaction is not tion measurements of Cunningham, et al.,’ and Figure involved.26 3 that some of the Es3+5f’O energy levels nearly match In the first experiment, gases, perhaps Fz or HF, the ligand triplet level TI, so the sensitized energy (22) M. L. Bhaumik, J. Chem. Phys., 40, 3711 (1964). transfer to Es3+ undoubtedly occurs. This suggests (23) J. L. Kropp and M. W. Windsor, ibid., 4 2 , 1599 (1965). that SLSL was not observed in the Es3+ case in the (24) M. L. Bhaumik and L. J. Nugent, ibid., 4 3 , 1680 (1965). region up to 10,200 A because the first excited state of (25) A. J. Swallow, “Radiation Chemistry of Organic Compounds.” Es3+is below 9800 em-’, as is true in the Am3+, Bk3+, Pergamon Press, New York, N. Y., 1960, p 36. Volume YS,Number 6 M a y 1969
NUGENT,BURNETT,BAYBARZ, WERNER,TANNER, TARRANT, AND KELLER
1548
Table 11: Some Estimated Radiolysis Lifetimes of Actinide Chelates
Chelate CsAm(HFA)*. H10 CsCm(HFA)4*HzO CSBk( HFA)n CsCf(HFA)a CSES(HFA)~ 1 part of EsS+/108 parts of Gda+ in CsGd(HFA)d 1 part of Cma+/lOaparts of Gd*+in CsGd(HFA)r
Speciflc radioheat, W/g of radioisotopt9
Isotope
Half-life
241 243 244 249 249 253
485 years 7950 years 17.6 years 314 days 360 years 20 days
1000
150 hr (6.4 days) 2.9 X 108 hr (120 days) 6 hr (measd) 47 hr (2 days) 120 hr (5 days) 0.017 hr (1 min)
253
20 days
1000
17 hrb
244
17.6 years
0.11 0.0058 2.8 0.36 0.14
The Journal of Physical Chemistru
6 X 108 hr (250 days)
2.8
a D. E. Ferguson, Nucl. Xci. Eng., 17, 435 (1963), and a currently revised private communication. periment.
formed in the radiolysis remain trapped in the dry solid and react further with the Cm3+ chelate thereby shortening the decomposition time. This is at least partially eliminated in the second experiment by the mother liquor which forces the observed gas bubbles up and away from the sample and out of the system. Moreover, in surrounding the 10-20-p chelate microcrystals with mother liquor, the a particles escaping from one crystal are dissipated by the liquid environment, while in the first experiment they proceed on through the air to attack other neighboring crystals. The escaping reactive gases and the a-energy dissipation in the solution in the second experiment both lengthen the radiolysis lifetime making it approximately twice as long as in the first experiment. It is evident that the sample particle size and container shape significantly influence the radiolysis lifetimes, with longer lifetimes associated with very small particles or dilute solutions spread out on an open surface. The present measurements indicate that a rule-of-thumb first approximation to the radiolysis lifetime of an actinide P-diketone is the time required for the time-integrated radioheat per mole to equal the sum of the molecular bond energies and the crystal binding energy per mole, or Ga( -ligand) w 2. Estimated radiolysis lifetimes of the Ana+ chelates investigated in the present work are presented in Table 11. The estimates are for the dry microcrystalline chelates and they are based on the measured 6-hr radiolysis lifetime of the dry C S ~ ~ ~ C ~ ( 4HHzO FA) microcrystals, assuming the radiolytic decomposition is linear in time and that the decomposition rate is proportional to the specific radioheat of the isotope under consideration. These estimates indicate that enough time is available before total radiolysis to do spectroscopy or crystal structure studies on any of the materials listed in Table 11, with the probable exception of pure CP3Es(HFA) where the radiolysis lifetime is estimated to be only 1 min. In order to have a,n independent check on the
Estd lifetime of chelates
* This estimate was also verified by ex-
radiolysis of the CsGd(HFA)4crystals doped to 1 part of 25aEs3+/103parts of Gd3+, a sample containing an additional 0.6 part of EuS+/lO3 parts of Gd3+ was prepared and its radioexcited and uv-excited emission spectra were measured. As before, no SLSL from Es3+ was detected. On the other hand, the characteristic Eu*+SLSL was observed and shown to be identical in both the radioexcited and uv-excited cases,z6and the 17-hr radiolysis lifetime listed in Table I1 for this sample was verified from intensity us. time measurements of the radioexcited luminescence. The fact that the radioexcited luminescence here decays linearly with time to aero, as it does for the pure CsCm(HFA)4.Hz0 crystals and as assumed for the other samples of Table 11, suggests that the number of activatable centers destroyed per unit time is a constant. These centers are apparently the individual chelated organic ligands. The ligand triplet state TI either is excited directly by the radioactivity (or by some related process) or is excited indirectly via the direct excitation of the ligand singlet state SI and the energy is transferred into the 4f" or 5f" electronic manifold as it is in the uv-excited cases.
IV. Concluding Remarks The present work shows that SLSL studies are feasible on Cm3+ p-diketone chelates and that these spectra resemble those of Eu3+ p-diketone chelates in emission wavelength and efficiency. An important difference between the emission characteristics of the Cma+ and Eu3+ chelates is the simplicity of the SLSL spectra of the former. The simplicity arises because the position and spacings of the 5f7 electronic energy levels of ema+ are such that only one set of transitions (J = 4 -+ J = 3) occur in the SLSL process, while, on the other hand, in the case of Eu3+, transitions occur from both (26) Very recently, Karraker reported that the or-excited and the uv-excited luminescences from lalAm3+-doped(C2Ha)aNHEu (DPPD)a. were identical for the crystalline chelate at room temperature: D. G . Karraker, J. Chem. Phys., 49, 957 (1968).
CONDUCTIVITY OF TETRAALKYLAMMONIUM HALIDES the and the 6Do states to all six states of the ’IF m ~ l t i p l e t . The ~ ~ ~complexity ~~ in the Eu3+ case, and in most of the other rare earth cases, makes it difficult to assign certain pure electronic transitions and makes it very difficult to identify vibronic transitions. The simplicity of the Cm3+ spectrum can be seen from Figure 7 where it is apparent, after examining the Cm3+ energy level diagram of Figure 3, that the 6140-A emission from Cm3+-doped CsGd (HFA) crystals corresponds to the pure electronic transition and that the 5740-, 5900-, and the 6740-A emissions must be vibronic. Thus high-resolution SLSL research on Cma+P-diketone chelates, particularly a t low temperature and using the relatively “cold” isotopes 247Cmand 248Cm,should be a fertile area for the study of the nature of vibronic and electronic transitions and their relationships to crystal and molecular structures. I n conclusion, it is understood that the possibilities of detecting SLSL from Am3+, Bk3+, Cf3+, and Es3+ chelates have not been exhausted by the present roomtemperature work. Indeed, Whan and Crosby2*have
1549
detected SLSL from each of the benzoylacetonate and dibenzoylmethide chelates of Pr3+, Nd3+, Sm3+, Eu3+, Tb3+,Dy3+,Ho3+,E$+, Tm3+,and Yb3+in EPA solution at 77°K. As expected, SLSL intensities are very low in the first two and in the last four rare earth cases, and if SLSL from the Am3+, Bk3+, Cf3+,and Es3+ actinides is detectable in similar experiments, it will also undoubtedly be very weak. On the other hand, we realize that there are good possibilities for improving this situation. Major sources of quenching are the relatively high-energy 0-H and C-H stretching vibrations of the solvent molecules and the C-H stretching vibrations of the ligands. Quenching rates dependent upon hydrogen stretching vibrations can in many cases be considerably decreased by substituting heavier atoms for H in both the solvent and the chelate ligands, and experiments along these lines are planned here for the near future. (27) L. J. Nugent, M. L. Bhaumik, 8. George, and 9. M. Lee, J. Chem. Phys., 41, 1305 (1964).
(28) R. E. Whan and G. A. Crosby, J. Mol. Spectry., 8, 316 (1962).
Electrical Conductivity of Tetraalkylammonium Halides in 1,3=Diaminopropane by Arthur M. Hartstein and Stanley Windwerl Department of Chemistry, Adeiphi Universtty, Garden City, New York
12580
(Received October 8 0 , 1968)
The electrical conductivity of a number of tetraalkylammonium halides in l13-diaminopropanewas obtained. The handling, transferring, and purification of the solvent was accomplished under vacuum conditions employing the techniques used in metal solution chemistry. The salts were purified using standard methods. The equivalent conductivity at infinite dilution and the association constants for all the salts studied were obtained by application of Shedlovsky’s method for low dielectric constant solvents.
Introduction Studies of the electrical conductivity of salts in low dielectric constant solvents have not received the strong attention given to those in the intermediate dielectric range. One reason for this is the large amount of work being expended in testing the Fuoss-Onsager theory for such systems.2 At present the above theory can only qualitatively be applied to systems in the low dielectric region. The purpose of this paper is to present conductance data for a number of symmetrical tetraalkylammonium halides in 1,3-diaminopropane, a low dielectric constant solvent,
Experimental Section Glassware. All glassware used, except the conductivity cell, was cleaned by rinsing with an HE”
cleanerla followed by boiling aqua regia and rinsing approximately ten times with distilled water. The glassware was then soaked in distilled water for 4 hr and finally dried in an oven operating at 120’. All stopcocks, standard taper joints, and ball joints were lubricated with Apiezon N grease. The conductivity cell was cleaned with fuming nitric acid. A diagram of the apparatus is shown in Figure 1. Salt Purification. All the tetraalkylammonium salts used in this study were obtained from Eastman Organic Chemicals and recrystallized by known procedures.‘ (1) T o whom requests for reprints should be addressed. (2) G. J. Janz and J. J. Tait, Can. J. Chem.. 45, 1101 (1967),and
papers cited therein. (3) Hydrofluoric acid cleaner: 2 % detergent, 33 % concentrated ”08, 5 % HF, 60 % distilled Hz0. (4) D. B. Evans, 0.Zawoyski, and R . L. Kay, J . Phys. Chem., 69, 8878 (1965). Volume 78,Number 6 May 1969
