J. Phys. Chem. 1083, 87, 2557-2563
2557
Optical Excitation and Emission Spectra of Eu3+ in Microcrystalline Samples of Trigonal Na,[ Eu(ODA),]*2NaCI04*6H,0 Andrew F. Kirby and F. S. Richardson' Depertment of Chemistry, Universliy of Virginia. Charlottesv/lk. Virginia 2290 1 (Received: September 20, 1982; I n F / n d Form: December 20, 1982)
Optical excitation and emission spectra are reported for microcrystalline samples of trigonal Na3[Eu(ODA)3].2NaC104-6H20 (ODA oxydiacetato (-OOCCHzOCHzCOO-))embedded in a KBr/silicone grease matrix. Detailed line assignments are made throughout the 7F0,1,2 5D0,1,z,3excitation region and throughout the 'FJ 5Do,lemission regions. Both crystal field origin lines and vibronic lines are observed in the excitation and emission spectra. Oscillator strengths are determined for the crystal field components split out of the 'F1,2,34 5Doand 'Fo,1,2 5D1multiplet-to-multiplet transitions, and these empirically deduced oscillator strengths are compared to those calculated on the basis of a theoretical intensity model for lanthanide 4f 4f transitions. The 7FJ 5D0,1emission spectra obtained in this study are compared to those reported previously for the trigonal EU(DBM)~.H~O (DBM dibenzoylmethanato ligand) system. Significant differences are found in the relative intensities observed among the 'FJ 5Do,lmultiplet-to-multiplettransitions, and these differences are discussed in terms of the intensity model.
-
-
-
-
-
-
-
Introduction In their trigonal form at room temperature, single crystals of Na3[Ln(ODA)3].2NaC104.6Hz0 have the space group R32, with three Ln(ODA),,- complexes per unit cell.1S The site symmetry at the Ln3+ions is D,,and the point-group symmetry of each Ln(ODA),3- complex is D3. The crystal structure is built up of layers containing the negatively charged LII(ODA)~&systems and C104-anions, alternating with layers containing Na+ ions and water molecules. The layers are perpendicular to the crystal c axis which coincides precisely with the trigonal (C,)s y m metry axes of the Ln(0DA):- systems. The Ln3+-Ln3+ distances are -9 A. The Ln09 coordination polyhedron in each LII(ODA),~- complex is a distorted tricapped trigonal prism with trigonal dihedral (D3)symmetry. The oxydiacetate (ODA2-)anions act as tridentate ligands, each forming two five-membered chelate rings with the Ln3+ ion. The top and bottom triangles of the Ln09 coordination polyhedron are defined by carboxylate oxygen donor atoms, while the middle (equatorial) triangle is defined by ether oxygen donor atoms. The top and bottom triangles of the Ln09 structure are not superimposable one upon the other but are twisted away from a superimposable configuration by an angle of twist T (referred to hereafter as the trigonal twist angle). Thus, the Ln09 coordination cluster has intrinsic D3 symmetry, with the equatorial ether oxygens each lying on a twofold symmetry axis. The chelate rings are nearly planar and they stretch across the diagonals of the rhombic faces of the LnOg trigonal prism. A view of the Ln(ODA)," complex along its trigonal axis, as it exists in crystalline Na3[Ln(ODA)3].2NaC104-6Hz0,is shown in Figure 1. The enantiomorphic crystals of Na,[Ln(ODA),]. 2NaC10,.6Hz0 are relatively easy to grow, and they afford a unique opportunity to study lanthanide chiroptical activity under structurally well-characterized conditions. The single-crystal properties of the Pr3+and Eu3+compounds have already received considerable attention in the literature."12 Polarized absorption, circular dichroism, optical (1)J. Albertason, Acta Chem. Scand., 22, 1563 (1968). (2)J. Albertason, Acta Chem. Scand., 24, 3527 (1970). (3)J. Albertason and I. Elding, Acta Chem. Scand. Ser. A , 31, 21 (1977). (4)A. C. Sen, S. C. Bera, and M. Chowdhury, Chem. Phys. Lett., 46, 594 (1977).
rotatory dispersion, Zeeman, and magnetic circular dichroism spectra have been reported for the P P sy~tem,41~3~ and (linearly) polarized emission, absorption, circular dichroism, Zeeman, and magnetic circular dichroism spectra ~~~*~ have been reported for the Eu3+s y ~ t e m .Time-resolved emission studies have also been reported for the Eu3+ system.12 In most of these studies, measurements were carried out under variable-temperature conditions from room temperature down to 4.2 K. A t temperatures below -120 K, there is some evidence for crystal structure changes (from the known room-temperature structure), and these crystal structure changes are the subject of a recent paper by Schwartz et al.ll In the latter study, both optical measurements and electron paramagnetic resonance measurements (using Gd3+ dopant as the EPR probe) were used in characterizing the structural changes as due to a second-order crystalline phase transition in which the Ln3+site symmetry is reduced from D3 to Czand a Na+ is moved off of a threefold axis.l' It was determined that the low-temperature crystal remains uniaxial and that its space group is most likely B 1 2 1 (or P3,2l), a subgroup of the room-temperature R32 space group. Recently we have reported circularly polarized axial emission spectra and linearly pohized orthoaxial emission spectra for Eu3+ and Tb3+ in single crystals of Na3[Ln(ODA)3].2NaC104.6H,0.13J4 For Eu3+,emission spectra throughout the 7FJ(J= 1-4) 5D0transition regions were reported,13and for Tb3+emission spectra throughout the 7FJ (J = 0-6) 5D4transition regions were reported.14 These spectra were analyzed and interpreted entirely in
-
-
( 5 ) A. K. Banerjee, R. K. Mukherjee, and M. Chowdhury, J. Chem. Soc., Faraday Trans. 2, 75,337 (1979). (6)R. W.Schwartz, A. K. Banerjee, A. C. Sen, and M. Chowdhury, J. Chem. SOC., Faraday Trans. 2, 76,620 (1980). (7)A. K., Banerjee, A. C. Sen, and M. Chowdhury, Chem. Phys. Lett., 69,592 (1980). (8)A. C. Sen, M. Chowdhury, and R. W. Schwartz, J. Chem. SOC., Faraday Trans. 2,77,1293 (1981). (9)A. K. Banejee, R. W. Schwartz, and M. Chowdhury, J. Chem. SOC., Faraday Trans. 2,77,1635 (1981). (10)A. C. Sen, and M. Chowdhury, Chem.Phys. Lett., 79,165,(1981). (11)R. W.Schwartz, A. K. Banerjee, M. Chowdhury, and R. Kuroda, J. Chem. SOC., Faraday Trans. 2, 77,557 (1981). (12)D. S.Roy, K. Bhattacharyya, A. K. Gupta, and M. Chowdhury, Chem. Phys. Lett., 77,422 (1981). (13)J. P. Morley, J. D. Saxe,and F. S. Richardson, Mol. Phys., 47,379 (1982). (14)J. D.Saxe, J. P. Morley, and F. S. Richardson, Mol. Phys., 47,407 (1982).
@ 1983 American Chemical Society
2558
The Journal of Physical Chemlstty, Vol. 87, No. 14, 1983
Kirby and Richardson
resulting microcrystals were dispersed in a KBr/silicone grease matrix (mull). All excitation and emission spectra were measured with a commercial SLM Model 8000 photon-counting spectrofluorimeter equipped with a 450-W xenon arc lamp excitation source. Excitation and emission spectra were obtained over the -30-295 K temperature range by mounting the samples in the cryostat compartment of a CTI-Cryogenics closed-cyclehelium refrigerator whose temperature was controlled by a Lake Shore Cryotronics temperature controller (Model DRC-70).
Figure 1. View down the trigonal axis of the Ln(ODA)," complex as it exists in crystalline Na3[Ln(ODA),].2NaCi0,.6H,0. est-neighbor Ci0,- anions are also shown.
The SIX near-
- -
terms of origin lines associated with individual 4f 4f crystal field transitions split out of the 7FJ 6D, and 'FJ 5D4multiplet-multiplet manifolds of Eu3+ and Tb3+, respectively. The Eu3+emission spectra also exhibited a myriad of very weak features which could not be assigned to 'FJ 5 Dcrystal ~ field origin transitions. These features were not analyzed or assigned in our previous study.', In the present study, we report unpolarized excitation and emission spectra obtained on randomly oriented microcrystals of Na3[Eu(ODA)3]-2NaC104.6Hz0 embedded in a KBr/silicone grease matrix. These spectra were obtained at lower resolution but with higher (detection) sensitivity than were those reported previously,13 and special efforts were made to detect and locate the weak emissions due to vibronic transitions and 7FJ 5D1transitions. These isotropic, unpolarized emission spectra are compared to the axial and orthoaxial polarized spectra that we obtained previously on single ~rysta1s.l~Additionally, these spectra are compared to the isotropic, unpolarized emission results obtained on microcrystalline samples of the Eu(DBM),.H20 system.15 The latter system has trigonal (C3) symmetry, and so it is similar with respect to symmetry to the Eu(ODA)~,-chromophore in Na3[Eu(ODA),].2NaC1O4.6H20. However, whereas Eu3+ is seven-coordinate in EU(DBM)~.H~O, it is nine-coordinate in Eu(ODA)~~-.Furthermore, the physical and chemical properties of the DBM- and ODA2- ligands are dramatically different, and one may anticipate that these differences will be reflected in the emission spectra observed for the Eu(DBM),.H,O and Eu(ODA)?- systems. The spectroscopic results obtained in this study are analyzed by using models and procedures discussed previously.13-17
-
+-
-
Experimental Section The crystals of Na3[Eu(ODA)3].2NaC104.6H20 were grown from aqueous solution following the method of Albertason.' These crystals were then pulverized, and the (15)A. F. Kirby and F. S. Richardson, J. Phys. Chem., preceding
paper in this issue.
(16)F. S. Richardson, J. D. Saxe, S. A. Davis, and T. R. Faulkner, Mol.
Phys., 42, 1401 (1981).
(17)F. S.Richardson and T. R. Faulkner, J. Chem. Phys., 76,1595 (1982).
Calculations 4f-Electron Wave Functions and Energy Levels. The "free-ion" wave functions and energy levels used in the present study are identical with those reported in the preceding paper.15 The parameters used to generate these wave functions and energy levels are listed in Table I of ref 15, and a major component analysis of the free-ion intermediate-couplingstate functions is presented in Table I1 of ref 15. Our crystal field calculations were carried out by assuming that the Eu3+ions reside at sites of exact trigonal symmetry. Choosing the trigonal (C,)axis dihedral (0,) as the axis of quantization, one may write the even-parity (gerade) components of the crystal field Hamiltonian as H,,= B p U p + Bh4t44) + B!J4)(U54)- U i ) + B p U p B p ( a 6 ) - UL) + B p ( Up + U!) (1) where the ) : ' u are one-electron intraconfigurational unit tensor operators, and the BL') are the even-parity crystal field coefficients. To obtain crystal field wave functions and energy levels, the H,,operator was diagonalized in a J M j intermediate-couplingbasis comprised of the 204 J M j components associated with the 22 lowest energy intermediate-coupling states derived from our free-ion calculations. The values of the Br' coefficients were chosen to optimize agreement between the calculated and observed orderings and splittings of crystal field levels within the 7FJ (J = 1-4) and 5DJ (J = 1 and 2) multiplets of Eu3+. This optimization procedure was carried out in a previous study,', and it was based on both absorption and emission results. The final values obtained for the B(') coefficients were Biz' = -70 cm-', Bh4)= -1020 cm-', B64P= *660 cm-', Bi6)= 320 cm-', Bi6)= 7285 cm-', Eli6) = 220 cm-'. (The signs of the Bi4)and Bi6' coefficients depend upon which enantiomeric form of the chiral Eu(ODA)~,-system one is considering.) All calculated results reported in the present study were obtained with these values of B('"s. The energies and major IJM,) compositions of the 7Fo,$F1, 7F2,7F3,7F4,5D0,5D1,and 5D2crystal field levels are listed in Table I. Electric and Magnetic Dipole Strengths. The electric and magnetic dipole strength calculations reported here were carried out by using the same models and procedures described in the preceding paper15 and e l s e ~ h e r e . ' ~ ~ ' ~ ~ ' ~ ~ ' ' Magnetic dipole strengths were calculated directly from the wave functions (partially) described in Table I. These same wave functions provided the 4f-electron basis set for our electric dipole strength calculations. The interconfigurational E parameters and 4f-electron radial integrals (9) required in the electric dipole strength calculations were assigned values identical with those listed in Table I of ref 15. All atoms in the ODA ligand of the Eu(ODA),% chromophores were included explicitly as perturber sites and were assigned fractional (point) charges and isotropic polarizability values. Two different sets of ligand charges (qL)and polarizabilities (cq)were used in the calculations reported here. These two sets (A and B) are given in Table 11.
+
The Journal of Physical Chemlstty, Vol. 87, No. 14, 1983
Spectra of Eu3+ in Na, [Eu(ODA),] .2NaCIO,.BH2O
TABLE I: Energies and Major IJMJ) Compositions Calculated for the Crystal Field Levels Split Out of the 7 F ,(~J = 0-4) and S D( J~= 0-2) Multiplets
Operationally, the value of A may be determined by use of the relation A = (4.31 X 10-9)(Bb/B,)
~~~
energy/
cm-
level A( 'F,) E(7F1) A2(7F1)
Ea(7F,) Eb('F,) A,('F,) A,a(7F3) A1(7F3) Azb(7F3) E,('F,)
Eb(7F3) Ea('F4) Ala(7F4) Eb(7F,) A2(7F4)
E,J7F,) Alb(7F4) A,(5D,) E(5D1)
A,(5D,) A1(5D,)
Ea('D,)
Eb('D,)
0 372 388 972 1079 1141 1870 1920 1924 1934 1964 2785 2860 2923 2948 2962 3056 17547 19059 19064 21567 21579 21619
-
major components
1.00 10,O) 1.0010,T1)
-1.0011,O) k0.89 12,il) + 0.46 12,72) -0.4512,il) * 0.8812,72) * 0.1213,*1) 0.99 12,O)- 0.08 13,3) - 0.08 13,-3) 0.53 13,3) + 0.66 13,O) - 0.53 13,-3) -0.7013,3) - 0.1212,O) - 0.70 13,-3) 0.47 13,3) - 0.74 l3,O) - 0.47 13,-3) ~ 0 . 8 013,+1)+ 0.58 1 3 , ~ 2 ) T0.58 l 3 , i l ) - 0.8113,72) k0.76 14,i4) + 0.64 14,kl) -0.96 14,O) + 0.17 14,-3) - 0.17 14,3) ~ 0 . 3 14,+4) 4 + 0.47 14,+1)~ 0 . 8 1 1 4 , ~ 2 ) 0.70 14,3) + 0.70 l4,-3) i 0 . 5 5 l4,*4) - 0.60 I4,kl) i: 0.57 14,T2) -0.68 14,3) + 0.24 14,O) + 0.68 14,-3) -1.00l0,0) T1.0Oll,*l) - 1.00 11,O) 1.00 12,O) i 0 . 4 7 12,*1) - 0.8812,72) +0.88 12,+1)+ 0.47 1 2 , ~ 2 )
TABLE 11: Ligand Charge and Polarizability Parameters set
para-
atoma
meterb
A
B
O(1)
chg PO1 chg PO1 chg PO1 chg PO1 chg PO1 chg PO1
-0.20 0.47 -0.82 0.56 -0.77 0.56 -0.04 0.88 0.57 0.88 0.08 0.14
-0.20 0.65 -1.15 0.91 -0.44 0.84 -0.04 1.03 0.57 1.03 0.08 0.41
O(2) O(3) C(1)
C(2)
H
a 0 ( 1 )is the ether oxygen, O(2) is the coordinated carboxylate oxygen, O( 3 ) is the uncoordinated carboxylate oxygen, C(1) is the methylene carbon, and C(2) is the carboxylate carbon. chg = charge in units of e (the proton F m2. charte); pol = polarizability in units of
All of the transition dipole strengths reported here were calculated by assuming a spatially isotropic ensemble of emitting chromophores. Oscillator Strengths. Comparisons between calculated and observed transition intensities are made in terms of transition oscillator strengths. The oscillator strength of an a b transition observed in spontaneous emission (luminescence) can be expressed as
-
fob = ABb-'$
-
a-b
I(P) d5/p3
2559
(2)
where I ( 3 ) denotes a measured emission intensity, the integration is over the a b emission band, Bb is a normalized Boltzmann population factor for the b-th emitting level (within a given excited-state multiplet), and A is a scaling factor. In writing eq 2, we assume a quasi-thermal equilibrium to exist within each emitting multiplet, but not between multiplets. The scaling factor A depends upon nonradiative processes (rates and pathways), as well as upon excitation characteristics (i.e., anything that affecta the population of the emitting multiplet level). This factor will be different for different multiplets, but the same for different crystal field levels within a given multiplet.
1 a--b
t(p)
d Q / S I(D)D-~ d3 a-b
(3) when the a b transition can be observed in both absorption and emission. In eq 3, ~ ( 3 is ) the decadic molar extinction coefficient for the a b absorptive transition, Ba is a normalized Boltzmann population factor for the level a , and .fa+, is an integration over the a b absorption band. In the present study, the value of A for sDO emission is obtained by applying eq 3 to the 7F1 5Do transition (which is observed in both the absorption and emission spectra), and the value of A for 5D1emission is obtained by applying eq 3 to the 7F0 5D1 transition. The theoretically calculated transition oscillator strengths may be expressed as fab (s~2m,c/hez)pa,[~oab(ED) + X'Dab(MD)I (4) where Dab is the transition frequency (expressed in units of cm-'), Dab(ED)is the electric dipole strength of the transition, Dab(MD)is the magnetic dipole strength of the transition, and the x and x' are Lorentz corrections for the bulk refractivity of the medium at nab. In writing eq 4, we have included all factors related to state degeneracies and transition polarization properties in the dipole strength quantities. The oscillator strengths obtained experimentally by the use of eq 2 and calculated theoretically from eq 4 represent inherent transition oscillator strengths which are independent of initial state population factors. The dipole strength quantities appearing in eq 4 were calculated, in the present study, by using the procedures described in the preceding section (vide supra).
-
-
-
-
Experimental Results General Aspects. All of the excitation and emission spectra reported here were obtained on microcrystals of Nq[Eu(ODA),].2NaC1O4.6H2O dispersed in a KBr/silicone grease matrix. In all cases, the emitting samples were determined to be isotropic with respect to the spatial orientations of the emitting chromophores. No excitation dependence was observed in the qualitative features of the emission spectra at X 1466 nm (excitation). All of the corrected emission spectra reported here were obtained by using excitation centered at -396 nm. For all of the excitation spectra reported here, the emission centered at -594 nm (7F1 5Do)was monitored. Both the excitation and emission spectra were obtained at -0.5 nm (instrumental) resolution. Excitation Spectra. A survey excitation spectrum obtained at 295 K between 340 and 550 nm is shown in Figure 2. No attempt is made here to assign the 340410-nm region. Polarized single-crystal spectra would be required to make detailed line assignments in this region. The 410-550-nm excitation region is shown in greater detail in Figure 3, where the upper spectral trace was recorded at 200 K and the lower spectral trace was recorded at 295 K. Assignments for the lines and features numbered in Figure 3 are given in Tables I11 and IV. Emission Spectra. A survey emission spectrum obtained at 125 K between 520 and 770 nm is shown in Figure 4. Individual regions of this spectrum are shown with expanded wavelength and intensity scales in Figures 5-9. Assignments for the lines and features numbered in Figures 4-9 are given in Tables V and VI. At the spectral resolution achieved in the present study (-0.5 nm), there was no evidence of line splittings due to site-symmetry reduction (from D3) or crystalline phase
-
2560
The Journal of Physical Chemistry, Vol. 87, No. 14, 1983
Kirby and Richardson
17
340
-L,
15 550
WAVELENGTH (nm)
-
Flgure 2. Survey excitation spectrum obtained at 295 K, monitoring the 'F, 'Do emission centered at -594 nm.
510
WAVELENQTH
(nm)
a70
Figure 5. 125 K emission spectrum.
17
1
,
r
10
,
100 I
W
570
WAVELENGTH (nm)
610
Figure 6. 125 K emission spectrum. I
195K
P I d \ s
*
-~ ,*r
4 5
1211 5
-A
WAVELENGTH ( n m l
410
29
150
Figure 3. Excitation spectra obtained at 200 (upper trace) and 295 (lower trace) K. See Figure 2 for the relative intensities of lines 17 and 25. 10
I I 1
17
620
1
WAVELENGTH (nml
570
Figure 7. 125 K emission spectrum.
transitions down to 125 K. Such line splittings did appear in the spectra obtained at T < 120 K, but these are not presented or discussed here. The ratios of 7FJ 5D1:7FJ 5D0emission intensities increased with decreasing temperatures. Transition oscillator strengths, determined by applying eq 2 and 3 to the emission results obtained here and the absorption results reported elsewhere,leJ8 are listed in Tables VI1 and VIII. In evaluating eq 2, we confined the integrations to spectral features assigned to crystal field origin transitions (as given in Table V). Therefore, the f(expt1) numbers
-
+-
19
I
Flgure 4. Survey emlsslon spectrum obtained at 126 K. A,, nm.
= 396
Spectra of E d f in Na, [Eu(ODA),].2NaCi0,-6H20
The Journal of Physical Chemistry, Vol. 87, No. 14, 1983 2561
TABLE 111: Excitation Features Assigned to 7 F ~ ’DJCrystal Field Origin Transitions +
200 K spectra peak no.a
295 K spectra v1cm-l
assignmentsC
24 287 n.0. 23 895 n.0. n.0. 21 448 21 396 21 080 21 025
1 2 3 4 5 17 18 20 21
and
vb/cm“
A1(7F0)-* r l ( ’ D 3 )
assignmentsC
24 301 24 1 1 8 23 902 23 310 23 202 21 453 21 402 21 075 21 026
A,,E(7F1)+. rZ(’D3) A 1 ( 7 F 0 ) +Eb(’D2) A1(7Fo)-t Ea(’D,) A,,E(’F1) +. Eb(’D,)
A , R 7 F 1 ) + r1(’D3) Ea(7F2)+.ri(’D3) Eb(7F2) --i ri(’D3)
See Figure 3 for numbering of peaks. n.0. (not observed) denotes features observed at 295 K but not a 200 K. r 2denote unidentified crystal field levels.
r,
TABLE IV: Excitation Features Assigned t o 7F0+. ’D, Vibronic Transitions Peak no.a 6 7 8 9
10 11 12 13 14 15 16
19
F/cm23 055 2 3 002 22 846 22 818 22 769 22 497 21 912 21 861 21 786 21 686 21 540 21 237
origin assignments Al-Eb Ea -+E, -+Eb Ea -+E, -+
-+
--t +
Eb
Ea
-+
-t
+.
a See Figure 3 for numbering of peaks. from origin line.
AEb/cm‘l 1607 1606 1398 1370 1373 1049 464 465 338 238 90 -211 ( h o t ) Displacement
I
WAVELENQTH lnml
720
770
Figure g. 125 K emission spectrum.
TABLE V: Emission Features Assigned t o 7 F +~ ’D0.1 Crystal Field Origin Transitions in the 1 2 5 K Spectra ~~
peak no.a
Figure 8. 125
v/cm-’
assignmentsb
K emission spectrum.
listed in Tables VI1 and VI11 correspond roughly to purely electronic crystal field transition oscillator strengths. However, it is likely that each of these numbers also includes some hidden (but small) contributions from vibronic transitions. Survey emission spectra obtained at 295 K for Eu(DBM)3.H20and for Na3[Eu(ODA)3]-2NaC104.4Hz0 are presented in Figure 10 for comparison purposes. Most striking are the differences in relative intensities among the various 7FJ SDo multiplet-to-multiplet transitions. Quantitative expression of these differences can be found in the oscillator strength data given in Table IX. Excepting the ‘Fo 6Do transition, the largest differences
-
a See Figures 5-9 for numbering of peaks. unidentified crystal field levels.
-
are observed in the 7F2
-
r I denote
5Doand 7F3 5D0transitions.
Calculated Results Crystal Field Wave Functions and Energy Levels. An abbreviated summary of the results obtained from our
2582
TABLE VI: Emission Features Assigned to 'FJ Vibronic Transitions Peak no.Q Y/cm-' 6 11 13 14 15 16 18 21 22 23 24 25 26 27 28 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 49 54 55 56 57 60 61
Kirby and Richardson
The Journal of Physical Chemistry, Vol. 87, No. 14, 1983
17 325 1 6 717 1 6 656 16 600 16 502 1 6 367 16 166 16 057 16 0 1 5 1 5 911 1 5 803 1 5 785 1 5 679 1 5 635 1 5 559 1 5 211 15152 15 1 0 3 15 067 15 049 1 5 004 14 945 1 4 899 1 4 870 1 4 839 1 4 791 1 4 747 14 725 1 4 689 1 4 661 1 4 514 1 4 110 13 992 13 918 13 526 13 3 2 1 13 201
+
5D,
origin assignments
AE / cm- '
A,,E('F,) + A,(5D,) A,('F,) + A,(5D,) A,('F,) A,('D,) A,,E('F,) A,,E('F,) A,(5D0) A,,E('F,) + A,(5D,) E,('F,) + A,(5D0) Et,( 'F,) + A,( 5D0) E,,A,('F,) A,('DO) Eb,E,('F,) +- A,('D,) A,,E('F,) + A,(5D,) E,('F,) + A,('D,) Eb( 'F,) A,( 'Do) A,('F,) + A,('D,) A,('F,) t A , ( 5 D , ) E,('F,) + A,('DO) A,,('F,) +- A,('D,) A,b('F,),Eb('F,) A,('D,) A,('F,) + A,('D,) AZa('F,) A,( 'Do) Azb('F3) A,(5D,) Ea('F3) A,('D,) E,('F,) A,('D,) Eb('F,) A1(53,) A2a(7F3) A,('D,) A,b('F,),Eb('F,)+ A,('D,) A,('F,) A,(5D0) Ea( 7F,) A,( 5D,) r,('F,)- A,(5D,) Eb(7F3),E,(7F,) A,('D,) A,('F,) A,(5D,) r , f 7 F , \ + A.[5D,\ ri('F:) A;(SDi) r 4 ( 7 F 4 ) A,(5D,) r 1 ( 7 F 4 ) A,(5D,) + r 2 ( 7 F 4 ) A,(5D,) r 1 ( 7 F 4 )A,(5D,) t
484 (hot) 487 548 241 339 474 86 88 237, 91 234, 341 1038 467 466 47 1 547 1041 237 241, 1 0 4 2 1042 340 340 332 1353 346 550 553, 1 3 5 4 1359 552 340 (hot) 555, 1 5 9 4 1592 239 249 323 1036 1028 238
-
+
+
+
+
+ +
+
+
+
+
+
f-
+
f-
a See Figures 6-9 for numbering of peaks. ments from origin lines.
-
TABLE VIII: Calculated and Experimental Oscillator 5D,Crystal Field Transitions Strengths for 'FJ calcd" transition
-
'F, + 'D, A, A, A, t E 'F, 5D, E t A, E-E A,+ E 'F, + 5D, A, A, A. E E; A, E, + E Ea A, E, t E +
--
+
f P ) / 10-1ox
0.31 0.21 0.10 381 0.07 380 0.49 136 0.33 1.61 51.0 21.1 19.0 43.2
Calculated according t o eq 4. Table VII.
f(MD)I 10-10x'
16.4 5.41 11.0 0.42 0.26 0.03 0.13 1580 225 109 68.2 496 268 411
106 31 75 610 1610 1620 1480 I480 I660
See footnote b of
TABLE IX: Comparison of t h e 7FJ + 5 D 0 Multiplet-to-Multiplet Transition Oscillator Strengths for Isotropic Samples of Eu(DBM),H,O (I) and Na3[Eu(ODA),]~2NaC1O,~6H,O (11) f(exptl)/lO-* transition
'F,7F, 'F, +7F, + 'F, ++
a
5D, 5D, 5D0 5D, 'Do
Data from ref 1 5 .
IQ
IIb
f(I):f(II)
2.6 8.0 65 7.6 4.8
0 9.5 8.1 0.33 8.7
0.84 8.0 23 0.55
Data from present study.
Displace-
TABLE VII: Calculated and Experimental Oscillator Strengths for 'FJ 5D, Crystal Field Transitions +
calcda
'F, E+ A, 7F, + Ea +
'Do A, A1 'Do Ai
+
+
Ai
'F, 'Do Ea A, +
+
Ai
0.39 0.12 0.27 54 7 128 419 9.6 4.8 3.0 1.4 0.42
751 487 264 0.08 0.06 0.02 2.4 0.01 1.1 0.11 1.2 2.8 0.18 0.04 0.53 2.1
9 50 565 385 810 600 210 33 6.0 6.5
18 2.6 'F, 'Do 33 867 100 Ea Ai 7.6 320 Ai 0.18 27 E, A, 12 A2 A, 13 420 Calculated according t o eq 4. Determined by using eq 2 and 3, and the absorption results reported in ref 1 9 . A,,
-
+
+
A,
crystal field calculations is given in Table I. Calculated and observed crystal field energies and splittings within the 7F1, 7F2,and 7F3multiplets are listed in Table X. Transition Oscillator Strengths and Dipole Strengths. Electric and magnetic dipole oscillator strengths calculated according to eq 4 are given in Tables VI1 and VIII. Note
510
WAVELENGTH i n m l
170
Flgure 10. Survey emission spectra obtained at 295 K for Eu(DB&.H,O (upper trace) and for Na3[Eu(ODA)3].2NaCD,-6H20 (lower trace). The intensity scales are in arbitrary units-not necessarily the same for the two spectra.
that the calculated oscillator strengths shown in these tables have not been corrected for bulk refractivity effects of the sample medium. The electric dipole contributions to the oscillator strengths listed in Tables VI1 and VI11 were calculated by using the ligand parameter set B of Table 11. Among the crystal field components of the 7FJ(J = 1-4) 5Do transitions, only three are calculated to acquire substantial electric dipole intensity via the dynamic coupling mechanism (see eq 5-8 of ref 15 for a breakdown of the various electric dipole contributions in our intensity model). These transitions are Ea?F2), Eb(7Fz),Ea("F3) A,(5Do),and in each case the static coupling and dynamic
-
++
Spectra of Eu3+ in Na, [Eu(ODA),].2NaCI0,.6H20
The Journal of Physical Chemktry, Vol. 87, No. 14, 1983 2563
TABLE X: Calculated and Observed Energy Levels within the 7FJ (J = 0-3) Multiplets
2NaC104-6H20system is somewhat of an enigma within the context of our intensity theory. For the Eu(DBM)3.H20system, the oscillator strength of this tranobsda obsd calcd sition was overestimated by almost a factor of 2,15whereas energy E l AEI E l AEI E l AEI in the present study we underestimate its value by almost level cm-I cm-I cm-' cm-' cm-' cm-' a factor of 25. In both the Eu(DBM) and Eu(0DA) sys'F,:A. 0 0 0 tems, this transition is calculated to acquire most of its 7F1:EL A, 380 362>18 1364 (unresolved) 16 intensity from the static coupling electric dipole mechanism. Vibronic Transitions. Unique to this spectroscopic investigation of Na3[Eu(ODA)3].2NaC104-6H20 are the de7F,:A;, 1830 1816 1870 tection and identification of a large number of spectral 1920 A1 features assignable to vibronic transitions (see Tables IV 1861 E, 1876 A,, 1941gZ; 1 9 2 7 3 : ; and VI). These transitions, in excitation and emission, are 30 always observed to be at least 1-2 orders of magnitude less E, 2002 1988 1964 intense than their associated origin transitions. CharacFrom high-resolution data of ref 13. From present terizing the observed vibronic lines according to their study. "average" displacements from their assigned origins, we can list the following values of AE(average) (in cm-') for coupling contributions are calculated to be of comparable emission and excitation, respectively: ul, 89, 90; vz, 241, magnitudes, with the Ymixednstatic coupling-dynamic 238; ~ 3 335,338; , ~4,476,465;U5,551, not observed; ~ 6 1036, , coupling contributions being approximately equal to the 1049; u7, 1356, 1371; u8, 1595, 1606. Given the spectral sums of the static and dynamic coupling contributions. resolution of the data obtained in this study (-5 A), the Dcd)z That is, for these transitions we calculate differences between the AE(emission) and hE(excitation) l/Jl(ssd), in the notation of ref 15. For all the remaining values listed above cannot be considered as significant. 7FJ 5D0crystal field transitions, we calculate the static Assuming that each of the "persistent" vibronic lines coupling mechanism to be the dominant contributor to corresponds to a one-phonon transition, we may speculate electric dipole intensity. The latter also holds true for the that the ul, u2, u3, u4, and v5 lines can be assigned to low7Fo 5D1crystal field transitions. However, for each of frequency Eu09 cluster modes or to chelate ring torsional the electric dipole allowed 7F1,2 5D1crystal field tranmodes, and that the u6, u7, and us lines can be assigned to 1/2D(*vd). sitions our calculated results yield higher frequency ligand-localized vibrational modes. In Electric dipole strengths calculated by using ligand pageneral, the lower frequency ~ 1 lines ~ are 5 observed to be rameter set A vs. ligand parameter set B showed signifislightly more intense than the ug-ug lines. cant variations with respect to the relative intensities Eu(0DA) us. Eu(DBM) Spectra. As the spectra in predicted for individual crystal field transitions. Overall Figure 10 and the data of Table IX reveal, the Eu(0DA) multiplet-to-multiplet dipole strengths exhibited a lesser and Eu(DBM) systems exhibit considerably different 7FJ dependence on the ligand charge and polarizability pa5D0emission properties. Most striking are the signiframeters. icantly larger 7F2 5D0and 'F3 5Dooscillator strengths observed for Eu(DBM) vs. those observed for Eu(0DA). Discussion These latter differences are satisfactorally accounted for Crystal Field Origin Transitions. The excitation asat a semiquantitative level by our intensity calculations signments given in Table I11 are in agreement with the (see Table VI1 of the present paper and Table XI of ref single-crystal polarized absorption results reported in 15). Large differences between Eu(DBM) and Eu(0DA) previous s t u d i e ~ , ~and . ~ Jthe ~ 7FJ 5Doemission assignare also found in the 7F1 5D1and 7Fz 5D1oscillator ments given in Table V are in agreement with the sinand strengths. For Eu(DBM), f(7F1 5D1) = 821 X gle-crystal polarized emission results reported previo~ly.'~ f(7F2 4-5D1) = 139 X lo4; for Eu(ODA), f(7F1 5D1)= 6.1 The 7FJ 5D1 emission assignments listed in Table V X and f(7Fz 5D1) = 16.2 X These differences augment those reported previously by Chowdhury and are also well accounted for by our intensity calculations. co-~orkers.~J~ The only multiplet-to-multiplet transition for which our The results given in Tables VI1 and VI11 demonstrate intensity calculations fail to at least qualitatively reproduce only mixed success in our attempts to calculate transition the Eu(DBM) vs. Eu(0DA) oscillator strength ratio is 7F4 oscillator strengths. The most glaring failures in these 5Do. In this case, the Eu(0DA) oscillator strength is attempts are (1)the distribution of intensity among the grossly underestimated. E, Al and Eb Al components of the 7F2 5D0tranFrom the data given above and in Table M, we note that sition, (2) the gross underestimate of 7F4 5D0intensity, the 'F3 5Doand 7F1 5D1transitions exhibit the largest and (3) the underestimate of 7F0 5D1intensity. The 7Fz intensity changes in going from Eu(DBM),.H20 to Na35D0intensity distribution problem was also encountered [Eu(ODA),].2NaClO4.6H2O. The 7F2 5D0 and 7F2 5D1 in our study of the Eu(DBM)~.H~O system,15and is most transitions exhibit significant, but lesser, changes in inlikely attributable to the neglect of ligand polarizability tensity (with changes here referring to intensity ratios). anisotropies in our dynamic coupling electric dipole inIn this case, therefore, the 7F2 5D0transition is neither tensity model (as suggested by Masonl8J9). The relatively the only nor the most hypersensitive transition, if we make intense emission (and large oscillator strength) observed use of the purely operational definition of hypersensitivity. for the 7F4 5D0 transition in the Na3[Eu(ODA),]. Acknowledgment. This work was supported by the (18)R. Kuroda,S. F. Mason, and C. Rosini, Chem. Phys. Lett., 70,ll National Science Foundation (NSF Grant CHE80-04209). (1980). Rsgistry No. Na3[Eu(ODA)3].2NaC104,43030-82-6; Eu3+, (19)R. Kuroda, 5.F. Mason, and C. Rosini, J. Chem. Soc., Faraday
~~~~$''
-
-
-
-
-
-
-
- - -
+-
-
-
-
-
-
-
+-
-
-
Tram. 2,77,2126 (1981).
22541-18-0.
-
-
