Photophysical studies of uranyl complexes. 2. Evidence for splitting of

Photophysical studies of uranyl complexes. 2. Evidence for splitting of the luminescent excited state of the uranyl ion. Harry G. Brittain, and Dale L...
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J. Phys. Chem. 1981, 85,3073-3078

3073

Photophysical Studies of Uranyl Complexes. 2. Evidence for Splitting of the Luminescent Excited State of the Uranyl Ion Harry G. Brlttaln”7 D8partment of Chemistry, Seton W I I Universw, South Orange, New Jersey 07079

and Dale L. Perry” Earth Sciences Divlsion, Lawrence Berkeley Laboratory, Univers#y of Californis, Berkeley, Callfornla 94720 (Recelved: November 24, 1980; In Final Form: March 5, 198 I)

High-resolution luminescence spectra have been obtained for the uranyl ion in uranyl acetate hydrate [UOz(CzH30z)2~2Hz0], lithium uranyl nitrate [LiU02(N03),],rubidium uranyl nitrate [RbU0z(N03)3],disodium zinc uranyl acetate hydrate [ZnUOz(C2H30z)4~7Hz0], uranyl bis(pyridine-2,6-dicarboxylate)[Na2UOz(C7H3N04)z], bis(iminodiacetato)dioxouranium(VI) [UOz(C4H5N04)z], and uranyl sulfate hydrate [UOz(S04)z.3.5] at 77 K. The vibronic structure observed in the luminescence spectra has been analyzed in terms of both an unsplit emitting state and a slightly split excited state. The data can best be explained by invoking a split excited state, and arguments are presented which indicate that this splitting is due to a descent in symmetry experienced by the uranyl ion when it is placed in a crystal field. Introduction In spite of the fact that luminescence from the uranyl ion may be observed in the solid and solution phases (this phenomenon has been known for quite some time), the nature of the excited state responsible for the emission has remained controversial.192 The lowest excited state lies approximately at 20.5 X lo3 cm-1;3 fairly recently, Jorgensen and Reisfeld were able to deduce several spectroscopic factors relating to the nature of this excited state.4 The photoluminescent process must often compete with photochemistry, and this additional pathway out of the excited state can complicate a detailed examination of excited-state parameter^.'-^ Perry and co-workers5recently reported the structural analysis of a water-bridged dimer of uranyl nitrate, di-paquo-bis(dioxobis(nitrato)uranium(VI)) diimidazole; the luminescent spectrum of the uranyl ion at cryogenic temperatures for this compound was obtained and subsequently compared to the spectrum of the closely structurally related compound, uranyl nitrate hexahydrate6 A complicated vibronic pattern observed in the former compound led to the conclusion that the excited state is split by approximately 80 cm-l and that emission is occurring to ground-state vibrational levels from both split halves of the excited state. This luminescence behavior is quite puzzling, and it was thought necessary to determine whether the uranyl nitrate imidazole complex is merely an isolated case or whether it is a particularly clear example of a general trend. Therefore, the high-resolution luminescence spectra of the uranyl ion in as many structurally documented compounds as possible have been obtained. The vibronic patterns have been interpreted both in terms of an unsplit excited state (the more conventional analysis) and in terms of a slightly split excited state. An examination of the data leads to the conclusion that the excited state of the uranyl ion is probably split in all cases. Experimental Section Reagents and Complexes. Disodium uranyl bis(pyridine-2,6-dicarboxylate) was prepared as previously reported’ by using the sodium salt of pyridine-2,6-di‘Teacher-Scholar of the Camille and Henry Dreyfus Foundation.

carboxylic acid. Bis(iminodiacetato)dioxouranium(VI) was synthesized by using the method of previous workers.*se Rubidium uranyl nitrate and lithium uranyl nitrate were prepared by evaporation of 1:l molar ratios of the appropriate nitrate salts as previously reported.1° Zinc uranyl acetate hydrate (Amend Drug and Chemical Co.), uranyl acetate hydrate (Mallinckrodt),and uranyl sulfate hydrate (ROC/RIC)l’ were obtained commercially and used without further purification. Instrumentation. Luminescence spectra were recorded on a high-resolution emission spectrometer constructed in The 365-nm the laboratory of one of the authors (H.G.B.). output of a 200-W Hg-Xe arc lamp was selected by a combination of a 0.1-m grating monochromator (Model H-10-UV-V, Instrument SA) and a UV-transmitting “black glass” filter and then focused onto the sample crystals. For low-temperature work, the crystals were contained in a Suprasil quartz tube and immersed in liquid nitrogen. The emission was collected at 90’ to the exciting beam, analyzed by a 0.5-m grating monochromator (Model 1870, Spex Industries), and detected by an EM1 9797B photomultiplier tube (5-20 response). An emission bandpass of 15 A yielded maximum resolution a t room temperature, but at liquid nitrogen temperature, a bandwidth of 5 A was necessary to achieve optimum resolution. No attempt was made to correct the emission spectra for monochromator or photomultiplier response. Excitation at wavelengths other than 365 nm did not lead t o any differences in the (1) Balzani, V.; Carassiti, V. “Photochemistry of Coordination Compounds”; Academic Press: New York, 1970. (2) Rabinowitch, E.;Belford, R. L. “Spectracopy and Photochemistry of Uranyl Compounds”;Pergomon Press: Oxford, 1964. (3) Burrow, H. D.; Kemp, T. J. Chem. SOC.Reu. 1974,3, 139. (4) Jorgensen, C. K.; Reisfeld, R. Chem. Phys. Lett. 1975, 35, 441. (5) Perry, D. L.; Ruben, H.; Templeton, D. H. Zalkin, A. Inorg. Chern. 1980,19, 1067. (6) Fittain, H. G.; Perry, D. L. J.Phys. Chem. 1980,84, 2630. This paper IS part 1 of this series. (7) Maragoni, G.; Degetto, S.; Graziani, R.; Bombieri, G.; Forsellini, E. J . Inorg. Nucl. Chem. 1974,36, 1787. (8) Krishnamurthy, M.; Morris, K. B. Inorg. Chem. 1969, 8, 2620. (9) Bombieri, G.; Forsellini, E.; Tomat, G.; Magon, L.; Graziani, R. Acta Crystallogr., Sect. B 1974, B30, 2659. (IO) Barclay, G. A.; Sabine, T. M.;Taylor, J. C. Acta Crystallogr. 1965, - -. -1.9- , -2n5 (11) The commercially obtained U02S04.3H20(ROC/RIC) yields the UOzS0,.3.5Hz0 compound when recrystallized from water. Please see Reference 18 for structural details.

0022-3654/81/2085-3073$01.25/00 1981 American Chemical Society

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The Journal of Physical Chemistry, Vol. 85, No. 21, 1981

Brittain and Perry

TABLE I: Wavelengths and Energies of the Luminescence Bands of Uranyl Acetate, U0,(C,H3O,),.2H,O, at 7 7 K waveassignment band length energy, l o 3 cm-I 1 2 system max, A A 4861 20.572 u= 0 va= 0 4879 20.496 v= 1 vb= 0

495

500

505

510

I

I

I

I

515

2.

c .-

C

c

B

C

va= 0 vb= 0

._

U = 0

u,= 0

u= 1

vb=o

wE

5071 5089

19.720 19.650

V =

0 u=l

5299 5321

18.870 18.794

C

._ VI

.-

luminescence spectra, except for intensity changes that were consistent with variations in the degree to which the excitation energy was absorbed. I

Results The luminescence spectrum of the uranyl ion remains qualitatively the same for the range of ligands covered in the present work, with a few general features that run though all spectra. One always observes sharp-line emission at 77 K, and the most intense lines are separated by anywhere from 855 to 890 cm-l. It is well known12that the totally symmetric vibration associated with the uranyl ion has a frequency of approximately 860 cm-l, so it is quite clear that the luminescence pattern represents a series of transitions from the excited state to several vibrational levels of the ground state. However, the energies of these transitions and the frequency of the totally symmetric vibration are found to be somewhat dependent on the ligands bound to the uranyl ion, as will be shown shortly. The fitting of these vibronic patterns provides valuable information regarding the spectroscopic nature of the excited state, and assignments for the patterns observed during the course of the present study are presented. Assignments are made for two alternate possibilities: (a) the excited state is not split, and all emission comes from this one state, and (b) assumption of a slightly split excited state, and emission occurs from both states. It should be noted that the assumption of a slightly split excited state was required to fit the luminescence spectrum of the octahedral uranate ion re~ent1y.l~ Uranyl Acetate Hydrate. The luminescence spectrum of U02(C2H302)2-2H20 greatly resembles the previously published spectrum for U02(N0J2.6H206in that very little vibronic structure is found to be associated with each of the emission band systems; the transition energies are essentially the same. Three vibrational band systems are noted, with the band a t highest energy (corresponding to a transition to the lowest vibrational level of the ground state) being located a t 4861 (20.572 X lo3 cm-l). The main peaks that correspond to vibrational origins are separated by an average of 853 cm-', identifying this vibration as the totally symmetric mode. This frequency is somewhat lower than that found for the uranyl nitrate hexahydrate, which was determined to be 866 cm-l. Within each band system, a second peak of lower intensity is found approximately 74 cm-' lower in energy than the main peak (the separation for uranyl nitrate hexahydrate is 82 cm-l). The wavelength maxima and transition energies obtained for all features in the uranyl acetate hydrate luminescence spectra have been collected in Table I.

a

(12) (a) Conn, G. K. T.;Wu,G. K. Trans. Faraday SOC.1938,34,1483. (b) Jones, L. H.; Penneman, R. A. J.Chern. Phys. 1963,21,542. (c) Jones,

L. H. Ibid. 1966,23, 2105. (13) Bleijenberg,K. C.; Breddels, P. A. J. Chern. Phys. 1980,72,5390.

1

485

490

,

495

1

1

500

505

Wavelength (nm)

Flgure 1. Luminescence spectra obtained at 77 K for LIU02(N03)3 (upper spectrum) and RbU0,(N03)3 (lower spectrum). Data for the B systems only are shown, bvt the A and C systems have essentially the same shape. The intensities are in arbitrary units.

TABLE 11: Wavelengths and Energies of the Luminescence Bands of Lithium Uranyl Nitrate, LiUO,(NO,),, at 77 K waveband length system max, A A 4815 4834 4876 4884

energy, l o 3 cm-'

assignment 1

2

20.768 20.687 20.509 20.475

B

5026 5046 5093 5103

19.900 19.817 19.634 19.596

C

5254 5276 5329 5340

19.033 18.953 18.765 18.727

Lithium and Rubidium Uranyl Nitrate. The luminescence spectra of the UOZ2+ion in these two salts of the U02(N03)3-ion are quite similar and differ only in the wavelength maxima and vibrational frequencies. Transitions to four vibrational levels are noted for the rubidium salt, while only three sets of transitions are found for the lithium salt. Each particular system is essentially a copy of all the others; as a result, only the B band systems for both salts are shown in Figure 1. The origin of each band system is found to lie at somewhat higher energy than the uranyl nitrate or acetate hydrate salts, with the rubidium salt showing the highest energy transitions of any salt system studied. Ground-state vibrational spacings of 867 cm-' for LiU02(N03)3and 888 cm-' for RbU02(N03)3are calculated; the value for the rubidium compound compares well with the frequency of 887.9 cm-l obtained by Dieke" during the course of the Manhattan Project studies. A summary of the LiU02(N03)3wavelength maxima and transitions is found in Table 11, while the analogous data for RbU02(N03)3are located in Table 111. Analysis of the vibronic structure associated with each band system is possible by assuming either a split or an unsplit excited state, but, as shall be seen during the rest of this report, the assumption of an unsplit excited state leads to strange and unreasonable interpretations. The assignments which result from each assumption are also shown in Tables I1 and 111,and the computed lattice modes

The Journal of Physical Chemistty, Vol. 85, No. 2 1, 198 1 3075

Photophysicai Studies of Uranyl Complexes

490

TABLE 111: Wavelengths and Energies of the Luminescence Bands of RbUOJNO,),, Rubidium Uranyl Nitrate, at 77 K waveband length svstem max. a

energy,

I '

1

4719 4736 4771 4779

21.191 21.115 20.965 20.926

v=O

ua= 0

u= 1

vb

v = 3a V = 3b

4926 4944 4981 4989

20.301 20.226 20.076 20.044

u= 0 u= 1 v = 3a U = 3b

va= 1 Vb= 1 va= 0

C

5151 5171 5212 5220

19.415 19.337 19.186 19.156

v= 0 v = l u = 3a V = 3b

va= 0 Vb = 0 va= 1

D

5397 5420 5465 5474

18.529 18.450 18.298 18.268

v=O U = 1 v = 3a v=3b

ua= 0

B

B

C

505

510

I

1

I

=0

vb= 0

ua= 1 Vb=

Ub=

1

1

vb= 0

va=l Vb=1

Figure 2. Luminescence spectra of the B band systems of dlsodlum uranyl bis(pyridine-2,6dicarboxylate) (lower spectrum) and zinc uranyl acetate hydrate (upper spectrum).

TABLE IV : Wavelengths and Energies of the Luminescence Bands of Disodium Uranyl Bis(pyridine-2,6-dicarboxylate), Na,UO,(C,H,NO,),, at 77 K ~assignment band wavelength energy, system max, A. l o 3 cm" 1 2

A

500

I

assignment 2

IO3cm-'

A

495

TABLE V: Wavelengths and Energies of the Luminescence Bands of Zinc Uranyl Acetate Hydrate, ZnUO,(C,H,0,),.7H,O, at 7 7 K

v,=O

4801 4814sh 4864 4880Sh

20.827 20.773 20.560 20.492

v=O

5008 5021 sh 5076 5093sh

19.970 19.917 19.699 19.635

u=O

5232 5247sh 5306 5325sh

19.113 19.060 18.848 18.779

v r 0

va=O

vzl

vb=o

u= 5 U=6

va=l

U = U = V =

I)=

1 5 6

1

v= 5 II

=6

Vb=o

va= 1 1

Vb=

va= 0 Vb= 0 ua= 1 Vb=

1

Vb=1

are found in Table VIII. If the luminescent state is not split, then the band systems consist of the origin, the first lattice peak, and, for some reason, the third lattice peak in the progression split by 35 cm-'. In addition, the second lattice peak would be missing by using this explanation. However, if one assumes an excited state split by 81 cm-' in the lithium salt and 77 cm-l in the rubidium salt, one can fit the data easily by observing that each origin is coupled with its own lattice mode. These are 267 and 224 cm-l for the LiU02(N0J3 salt and 228 and 183 cm-l for RbU02(N03)3.One does not need to assume the absence of any portions of the progression, and this feature will be a major part of the subsequent discussion in this paper. Disodium Uranyl Bis(pyridine-2,6-dicarboxylate).The uranyl complex of pyridine-2,6-dicarboxylicacid (PDA) has a deceptively simple luminescence spectrum, as shown in Figure 2. The band systems consist of two peaks with two clear shoulders, and they are separated by 857 cm-l. The wavelength maxima and transition energies for this complex are found in Table IV. If one attempts to assign the vibronic progression of each band system assuming an unsplit excited state, then one is forced to do so by coupling a 53-cm-* vibration to the transitions. One then obtains the progression of 0, 1,5,and 6, while peaks corresponding to modes of 2,3, and 4 vibrational quanta are missing. However, a much more reasonable fit of the data is obtained by assuming an excited-state splitting of 53

waveband length svstem max. A

energy,

assignment 2

lo3cm"

1

A

4764 4783 4803 4824

20.990 20.907 20.820 20.729

v= 0 u= 1 u= 2 v=3

B

4966 4987 5008 5031

20.138 20.054 19.968 19.875

v=o v= 1 v= 2

5186 5210 5233 5259

19.281 19.195 19.875 19.015

v=o v= 1

C

u= 3

u= 2

v=3

cm-l and coupling of a 266- and 281-cm-' lattice vibration to each of the origins. Zinc Uranyl Acetate Hydrate. For this salt system, one again obtains a luminescence pattern of four peaks within the range of each band system, with the most intense peaks being separated by 855 cm-l. While previous luminescence studies of this compound have not been conducted, it should be pointed out that the analogous rubidium, sodium, and lead uranyl acetate spectra have totally symmetric vibrational energies of 852.1, 855.2, and 853 cin-l, re~pectively.'~An example of the band pattern obtained for this salt system may be found in Figure 2; the spectroscopic data are summarized in Table V. The data may be fit with a reasonable degree of precision if one assumes no splitting of the excited state and a coupling of a 88-cm-l lattice mode to each emission transition. In this case, there is no need to assume the existence of missing vibronic peaks, and a progression up to u = 3 is found. Alternately, if one assumes an excited-state splitting of 84 cm-', one can fit the data equally well by coupling each of the origins to lattice modes of 171 and (14) Dieke, G. H.; Duncan, A. B. F. "Spectroscopic Properties of Uranium Compounds";McGraw-Hill: New York, 1949; National Nuclear Energy Series, Div. 2, Vol. 2.

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The Journal of Physical Chemistry, Vol. 85, No. 21, 1081

Brittain and Perry

TABLE VI: Wavelengths and Energies of the Luminescence Bands of Bis(iminodiacetato)dioxouranium(VI), U02(C,H,N0,),, at 77 K band wavelength system max, A A 4828 4849 4870 4891 4913sh 4935 B

C

energy, lo' cm-' 20.713 20.623 20.534 20.446 20.534 20.264

5031 5053 5077 6100 5124sh 5148

19.877 19.790 19.697 19.607 19.517 19.426

5263 5278 5303 5328 5354sh 5380

19.037 18.947 18.856 18.767 18.6'78 18.588

assignment

1

2

v= 0 v= 1 v= 2

va= 0

3 v=4 V = 5

~ b 1 =

U=

v= 0 u= 1 v=2 3 v=4 V =

v= 5 v=O u = l v=2 u=3 v=4 u= 5

vb= 0

v,= 1 ua= 2 ~ b 2= va= 0 vb=o

1

u,= 1

energy, 103cm-'

va= 2 vb= 2

A

4919 4942 4969 4981 5012 5030 5072

20.329 20.235 20.126 20.074 19.952 19.881 19.715

B

5135 5160 5191 5203 5236 5256 5303

19.474 19.380 19.267 19.219 19.099 19.026 18.858

v=o

5371 6398 5430 5446

18.618 18.526 18.416 18.362

v=o

C

I

515

520

Flgure 3. Luminescence spectrum of the B band system of bis(imlnodiacetato)dioxouranium(VI)at 77 K.

v,=O vb=o

u,=l 1 ua=2 %= 2 vb=

assignment

1 u= 0 v=2 v=4 v=5 u= 7 v= 9 v = 12

1

510

Wavelength ( n m )

TABLE VII: Wavelengths and Energies of the Luminescence Bands of Uranyl Sulfate Hydrate, U02S0;3.5H,0, at 77 K band wavesys- length tem max, A

I

505

vb=l

2

v= 2 v= 4 u= 5

v= 7 v=9

v = 12 v= 2 u=4 v= 6

179 cm-'. It is difficult to determine the better description for this salt, since the excited-state splitting is approximately half that of the two lattice mode frequencies.

Bis(imidoacetato)dioxouranium(VI). Each band system for the uranyl complex of iminodiacetic acid is more complicated than any of those noted before, as is shown in Figure 3. The totally symmetric vibration is found to have a frequency of 839 cm-', and this value is quite low compared to the other salts. The positions and energies of all peaks obtained for this compound have been tabulated in Table VI. In the spectrum of this particular complex, one is able to fit the data equally well by assuming an unsplit excited state and transitions coupled to lattice modes of 90 cm-l, or by assuming an excited state split by 89 cm-l and each origin coupled to modes of 180 and 181cm-'. On the basis of these computations,one cannot state whether the excited state is split or not. An examination of Figure 3, however, reveals a most unusual intensity pattern for the vibronic peaks: the second peak observed in each band system is much more intense than either of its neighbors. This immediately suggests that it is not a result of a single lattice mode coupled to a lone origin, but is in fact an origin itself; therefore, the excited state must also be split for this compound, with the only alternative for an unsplit excited state being some very different type of intensity mechanism for the vibronic peaks. As with the other systems, the vibrational parameters have been tabulated in Table VI11 for comparison to the other compounds. Uranyl Sulfate Hydrate. The most complicated luminescence obtained for the uranyl ion in any environment is found within this system. The spectrum of each band system appears to be quite complicated, as Figure 4 illustrates. An examination of the wavelength maxima and transition energies in Table VI1 enables a computation of the frequency of the totally symmetric vibration, with a

TABLE VIII: Summary of Ground-State Vibrational Frequencic !s, Lattice Frequencies, and Excited-State Splitting Energies Obtained from the 77 K Luminescence SpectrEP

compound UO,(NO, )3.6H,0 uo2(c2H307,)~~2H20

LiUO,(NO,), RbU02(N03)3 U02(C7H3N04)2

ZnU0,(C,H30,),~7H,0 U02(C4H5N04)2

UO,SO4*3.5H,O

U07,(N03)~(C3H4Nz)~'~H20

vibrational freq

lattice freqb

866 853 867 888 857 855 839 855 839

82 74 87 76 53 88 90 51 e

lattice freqC d

d = 267, vb = v a = 228, u b = v a = 266, v b = ua= 171, v b = V a = 180, v b = v a = 205, vb =

224 183 281 179 181 162 v a = 170, u b = 190

U,

excitedstate splitting"

ref

82 74 81 77 53 84 89 93 85

6 this work this work this work this work this work this work this work 6

a All frequencies are in cm-'. Lattice energies obtained by using assignments (1); an unsplit excited-state is assumed. Lattice energies obtained by using assignments ( 2 ) ;a split excited state is assumed. Cannot be calculated by assuming a split excited state, since no vibrational bands are then found. e No single lattice vibrational energy can be calculated for this compound if an unsplit excited state is assumed.

Photophysical Studies of Uranyl Complexes

The Journal of Physical Chemjstry, Vol. 85, No. 21, 1981 3077

the authors believe that the assumption of a slightly split excited state leads to a much more rational interpretation of the data. With the former assumption, one must frequently postulate the existence of missing vibronic peaks in order to fit the observed peak energies to a progression. If one assumes a split excited state and the existence of two different lattice modes, on observes complete pro+ .gressions. Within this assumption, the progressions tend ul 0 c to be quite short, which indicates that the geometry of the c I x6.5 c ._ emitting state is approximately the same as the ground state. This in turn supports the previous assignment of .-c0VI .the emitting state as being predominantly f orbital in E n a t ~ r e , since ~ . ~ it is well-known that f-f transitions in w lanthanides and actinides tend to be among essentially nonbonding orbitals. The origin of this splitting is not presently clear, but several possibilities exist. If uranyl ions were located at nonequivalent sites within the crystals, one might observe different energies for the emission transitions, and each site might be coupled to a different mode. However, examination of the known crystal 510 515 520 525 530 s t r u c t ~ r e s ~ ~does ~ J ~not J ~support - ~ ~ this possibility. Wavelength ( n m ) Another possibility that immediately suggests itself is Flgure 4. Lumlnescence spectrum of the B band system of uranyl some degree of uranyl-uranyl interaction, leading to the sulfate hydrate at 77 K. existence of uranyl eximers in the crystal. Eximers have been shown to be responsible for some of the quenching value of 855 cm-I being found. This value compares faof the uranyl ion in aqueous solution.20 One may easily vorably with that of 852 cm-' reported by DiekeI4and 850 calculate uranium-uranium distances for the systems cm-' reported by Gordon.'6 The first two band systems where crystal structures have been published, and the appear to be a repetition of the same pattern, but in the following values have been obtained for the nearest urathird system, the last features are too weak to measure. nium-uranium neighbors: 3.93 A for di-p-aquo-bis[diThe vibronic pattern proves quite difficult to fit if one oxobis(nitrato)uranium(~)]diimidazole: 5.68 A for uranyl assumes an unsplit excited state, as the attempted asacetate hydrate,17 6.09A for uranyl nitrate hydrate,lg 6.25 signments of Table VI1 show. One can form an approxiA for rubidium uranyl nitrate,1° 5.84 A for bis(iminodimate fit by assuming a lattice vibration of 51 cm-'; then acetato)dioxouranium(VI),9 and 6.79A for uranyl sulfate the observed progression is missing peaks due to the hydrate.18 If the excited-state splitting were due to the coupling of 1,3,6,8,10,and 11 quanta. A much better formation of eximers, then the magnitude of the excitedfit for the patterns is obtained if one assumes an excited state splitting should increase as the uranium-uranium state split by 93 cm-l, each origin being separately coupled separation decreases. In particular, one should see a much to lattice modes of 205 and 162 cm'-', A most interesting greater splitting of the excited state for the uranyl nisituation is found in that the u = 2 vibronic peaks assotrate-imidazole complex, since the dimerization of the ciated with each transition are predicted to lie at exactly uranyl ion in this compound results in the shortest urathe same energy. In fact, one does not observe a peak at nium-uranium distance of 3.93 A. However, almost all these energies, but does find two peaks which do not match excited-state splittings are the same, with only disodium the predicted patterns and whose baricenter lies exactly uranyl bis(pyridine-2,6-dicarboxylate)having a spectrum at the predicted energy. The peaks have split apparently with appreciably different values. It can therefore be due to some sort of configuration interaction, thus leading concluded that eximer interactions are not responsible for to the observed pattern. If this is so, however, then the the splitting of the excited state. vibronic progressions may be fit with no need to invoke A most reasonable explanation for the excited state missing peaks. splitting involves the symmetry of the uranyl ion in the crystals. An examination of the existing crystal structures Discussion reveals that the symmetry of the uranyl ion is quite low, The data presented in the preceding section indicate being C2h or C a in most cases.699~10*17-19 The spectroscopy that luminescence from the uranyl ion is affected by the of the uranyl ion, however, can be interpreted in a reanature of the coordinated atoms attached to it, even in sonable manner by considering the symmetry obtained if complexes containing only the simplest of anions. While one considers only the atoms directly bound to the uranyl the nature and degeneracy of the excited state are not ion in the equatorial plane; a variety of Dnhgroups then completely understood yet, it is assumed that the lowest result for the various systems. Such an approximationhas excited states are predominantly f orbital in nature and that the luminescence of the uranyl ion is f-f in origin.2~~ been used with great effectiveness to explain the absorption spectrum of CsU02Cld.21In none of these groups can Both the data here and the earlier resultsI4show that the vibrational energies of the uranyl ion are affected by the (16)Lieblich-Sofer, N.;Reisfeld, R.; Jorgensen, C. K. Inorg. Chim. crystal environment; they also demonstrate that lattice Acta 1978,30,259. vibrational frequencies can vary widely as the crystal (17)Howatson, J.; Grev, D. M.; Morosin, B. J. h o g . Nucl. Chem. system is changed. 1975,37,1933. (18)Zalkin,A.;Ruben, H.; Templeton, D. H.; Inorg. Chem. 1978,17, While the fine structure observed in the luminescence 3701. studies can be interpreted in a fairly reasonable manner (19)Taylor, J. C.; Mueller, M. H. Acta Crystallogr. 1965, 19, 536. if one assumes a single emitting state for the uranyl ion, (20) (a) Marcantonatos, M. D. Inorg. Chim. Acta 1978,26, 41. (b) ) .

4

(16) Gordon, B. E. Dokl. Akad. Nauk. SSSR 1960,74,913.

Marcantonatos, M. D. J. Chem. Soc., Faraday Trans. 1 1980,76,1093. (21)Denning, R.G.; Snellgrove, T. R.; Woodward, D. R. Mol. Phys. 1976,32,419.

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The Journal of Physical Chemistry, Vol. 85, No. 21, 1981 I

I

I

> r .v)

W +

..6

.-E

v)

w

I

f

Wavelength ( n m )

Figure 5. The 15 and 100 K luminescence spectra of di-paquebis[dioxobis(nitrato)uranium(IV)] dlimldarole within the most intense

emission band system.

there exist a molecular orbital of more than twofold degeneracy, and this descent of uranyl symmetry from Dmh to Dnh(or lower) could certainly result in level splitting. Since it would be reasonable to anticipate that the resulting levels would belong to different irreducible representations, it would also be reasonable to assume that the levels would couple to lattice vibrations of different symmetry (thus explaining the observations here that each electronic origin appears to be coupled to a different vibronic mode). This type of explanation has been involked to explain an apparent splitting of the excited state in the octahedral ion.13 The results obtained here indicate that the nature of luminescence from the uranyl ion is still not well understood; clearly, further studies involving high-resolution emission spectra of crystals of known structure are highly desirable. Such work is currently continuing in these laboratories. Acknowledgment. This work was supported by a grant from the Research Corporation to H.G.B. through the Cottrell Research Program (No. 8926). D.L.P. thanks the

Brittain and Perry

Miller Insitute for Basic Research, University of California, Berkeley, for a Miller Research Fellowship which sponsored the initial synthetic portion of this research. The authors also thank the U.S. Department of Energy for support of this work under Contract No. W-7405-ENG-48, and Dr. Gervain Chapuis (University of Lausanne, Switzerland) for calculation of the uranium-uranium distances in the reported crystal structures. The 15 K spectra were obtained on equipment obtained under grants from the National Science Foundation (TFI-80-23975),and from the Camille and Henry Dreyfus Foundation (through a Teacher-Scholar Award to H.G.B.). NOTEADDEDIN PROOF: Since submission of this work, the authors have acquired instrumentation that makes lower temperature (15 K) and higher resolution (1-A bandpass) studies of these uranyl systems possible. Cooling of the uranyl salt systems to 15 K resulted in a considerable simplification of the luminescent features. Four widely separated band systems were still noted (each separated by approximately 860 cm-l), with these corresponding to transitions to the lowest four vibrational levels of the ground state. Under high resolution at 15 K, each band was found to consist of a strong doublet followed by several very weak vibronic lines. Raising the sample temperature to 100 K resulted in an intensification of these weak lines, thus establishing their identity as being purely vibronic. One example of the spectra obtained for a particular band system at the two temperatures is shown in Figure 5. The lack of temperature dependence for the strong doublet bands effectively argues that each of them must be due to a purely electronic transition (transitions that do not involve the vibronic mechanism) and that the peak found at lower energy for each doublet cannot be assigned as a vibronic transition accompanying the main electronic transition. Since the spacing between the doublet components was constant for a given uranyl system, the authors believe that the excited luminescent state of the uranyl ion must be split in the crystal state. This conclusion is proposed above in the report detailing 77 K spectra, and it is further supported by the data obtained at 15 K. The magnitude of the excited-state splitting still varied with the nature of the uranyl ligands, as was discussed above.