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Localization of optical excitations in crystalline Rh3+-trisdiimine chelates
Volume 166, number 5,6
CHEMICAL PHYSICS LETTERS
9 March 1990
LOCALIZATION OF OPTICAL EXCITATIONS IN CRYSTALLINE Rh’+-TRISDIIMINE CHELATES J. WESTRA and M. GLASBEEK Laboratory for Physical Chemistry, University ofAmsterdam,
Nieuwe Achtergracht 127, 1018 WSAmsterdam,
The Netherlands
Received 17 November 1989; in final form 12 December 1989
The luminescent state of [ Rh (bpy ), ] ( C104)., (bpy= 2,2’-bipyridine) has been studied by means of low-field optically detected magnetic resonance (ODMR) spectroscopy. Experiments performed on single crystals, at low temperatures (Tz 1.4 K), revealed a magnetic anisotropy characteristic of a ligand-centered %-x* excitation. Furthermore, the experimental results show that the excitation is trapped on one single ligand molecule per Rh3+ cation site only.
1. Introduction
2. Experimental
In recent years the spectroscopic characterization of the luminescent state of a number of Rh3+ (d6) chelates has attracted great interest [ l-7 1. Based on the similarity of the emission spectra of Rh3+-trisdiimine chelates and the pure ligand molecules, it has long been suggested that the emissive state in these Rh3+ complexes is of n-n* origin [ 4-7 1. Lifetime and optically detected magnetic resonance (ODMR ) results seemed indicative of a triplet IF-I[*excitation of the Rh3+ chelates [8-lo]. However, unambiguous evidence for the 3rc--3[3* nature of the photo-excited Rh3+-trisdiimine compounds has not yet been presented. It is the purpose of this paper to provide such evidence for photo-excited [ Rh (bpy ) 3] (Clod) 3 single crystals. We report on ODMR experiments performed on the system in low magnetic fields. From a study of the anisotropic behavior of the observed ODMR line splittings, in combination with the results of an X-ray analysis of the crystal structure, it will be shown that theRh (bpy ) : + chelate indeed has a 3rr-rc* phosphorescent state. Furthermore, the electronic rc-n* excitation in the investigated single crystal is shown to be localized on one particular ligand molecule per Rh (bpy ):+ entity only.
[ Rh (bpy ) 3] Cl, was synthesized following the method of Harris and McKenzie [ 111. By treating [ Rh (bpy ) 3] Cl, dissolved in an ethanol/water mixture with perchloric acid, [ Rh(bpy), ] ( C104)3 was formed. Disc-shaped single crystals of the latter compound were grown by recrystallization from water. ODMR experiments were performed using the ODMR spectrometer described previously [ 12,13 1. The crystal was mounted inside a slow-wave helix immersed in a liquid-helium bath. The crystallographic a- (or b-) axis was parallel to the (vertical) helix axis. By controlled pumping of the helium in the cryostat, the bath temperature was maintained at about 1.3 K. Optical excitation near 320 nm was by means of the light from a 100 W high-pressure mercury PEK lamp filtered by a water-cooled Schott UG11 band pass filter. The emission perpendicular to the excitation pathway was focused on the entrance slit of a Jobin-Yvon HR 1000 grating monochromator. In the ODMR experiments, the microwave power was amplified using a 20 W traveling-wavetube amplifier. The microwaves were square-waveamplitude modulated at 90 Hz. Phase-sensitive lockin detection of the emission near 456 nm was applied. Magnetic fields were obtained by means of superconducting coils, immersed in the liquid helium bath, fed by a regulated power supply.
0009-2614/90/$ 03.50 0 Elsevier Science Publishers B.V. (North-Holland )
535
Volume 166, number 5,6
CHEMICAL PHYSICS LETTERS
3. Results and discussion The zero-field ODMR spectrum of the [ Rh (bpy ) 3] ( ClO,) 3 single crystal is displayed in fig. 1. The observed resonance frequencies of this work, as well as the analogous data for [ Rh (bpy ) 3 ] Cl3 and pure 2,2’-bipyridine obtained elsewhere [ IO,1 4 1, are collected in table 1. The observed ODMR linewidths (fwhm) of the chelated bipyridine molecules is in the order of 100 MHz, whereas pure bipyridine yields ODMR signals with a typical width of 10 MHz (fwhm). The large similarity among the ODMR resonance frequencies of the [Rh(bpy),13+ chelates and those of the pure ligand molecule in the 3n-x* state, is highly suggestive of the 3~-~* nature of the emissive state of the rhodiumtrisbipyridyl cation. Unambiguous evidence regarding the triplet spin character of the luminescent state of Rh (III)-diimine complexes was obtained from ODMR experiments performed in a magnetic field. Our initial experiments concerned EPR and ODMR of the [ Rh (bpy ) 3 ] ( C104 ) 3 single crystal using an X-band EPR spectrometer.
Fig. I. Zero-field ODMR spectrum as observed for a photo-excited single crystal of [Rh(bpy),] (C104)3 at 1.4 K. Excitation wavelength is near 320 nm; detection wavelength is near 456 nm. The structure in the zero-field transitions is due to a variation of the microwave power with the microwave frequency.
9 March 1990
However, in these experiments EPR signals characteristic of an excited triplet state could not be observed. Fortunately, at much lower magnetic fields (near 500 G ) the signal-to-noise ratio of the ODMR signals is much improved and in fact a study of the anisotropy of the ODMR spectrum could be undertaken. Fig. 2 shows as a representative example an ODMR spectrum obtained in the 2-4 GHz microwave region, when the crystal is in a magnetic field of 550 G directed perpendicular to the crystallographic a(or 6-) axis. Rotation of the crystal about its a- (or b- ) axis (the magnetic field being kept perpendicular to this rotational axis) revealed the anisotropy of the ODMR spectrum. The angular dependence of the ODMR lines is given in fig. 3. The dots are representative of the experimental data (derived from the 1.18 GHz and 2.32 GHz zero-field resonances), for a series of different H-field directions in the plane perpendicular to the crystallographic a- (or b- ) axis. The Zeeman shift of the 3.49 GHz zero-field transition in the applied field of 550 G appeared relatively small, i.e. less than 1.3 GHz for all orientations. The linewidth of this transition increased from 110 MHz (fwhm at zero-field) up to 350 MHz (fwhm in H= 550 G). As a result, an accurate measurement of the splitting and the anisotropic behavior of this “high-frequency” transition was hampered and henceforth the latter will not be considered. To interpret the experimentally obtained results of fig. 3 we make use of the results of a crystallographic
AI t
Table 1 ODMR resonance frequencies (GHz) of [Rh(bpy),] (C10,)3 (this work), [Rh(bpy)j]C1, (ref. [lo]) and of the bipyridine ligand in a n-heptane matrix (ref. [ 141)
21~71 IDI - IEI IDI + IEI
536
[Rh(bpy)3l(ClO&
[Rh(bpy),lCl,
bpy
1.18 2.32 3.49
1.05 2.16 3.21
0.76 2.99 3.75
2.0
3:o
410 (~~21
Fig. 2. ODMR transitions in the microwave range of 2-4 GHz when the [Rh(bpy),] (C1O4)3 crystal is in a magnetic field of 550 Gauss in the plane perpendicular to the crystallographic u{or b-) axis.
Volume 166, number 5,6
CHEMICAL PHYSICS LETTERS
0.4 O0
90”
i8O0
Fig. 3. Anisotropy of the ODMR transitions in the 2-4 GHz rcgion, when a magnetic field of 550 G is applied in the plane perpendicular to the crystallographic a- (or b) axis. The dots represent the experimental ODMR frequencies; the drawn curves represent the angular dependences as calculated for a ‘x--x* state localized on the bipyridyl ligands indicated by an asterisk in fig. 4b.
distinguished. Upon subdividing the unit cell in two halves by means of a plane parallel to the &plane, one may show that the lower and upper half of the unit cell each contain the set of six magnetically inequivalent molecules. Note that for rotation around the crystallographic a- (or b-) axis only three magnetically inequivalent molecules can be distinguished due to orientational degeneracy. For convenience, the projection of one such half unit cell iS displayed in fig. 4b. In this figure the six magnetically inequivalent molecules are labeled (a ) to ( f ). We now discuss what Zeeman ODMR patterns can be expected for the six molecules assuming a 3x--x* luminescent state in the Rh( III)-complex molecule. Several possibilities for the n-n* excitation may be considered: (i ) complete delocalization among the three ligands per Rh(III) chelate, and (ii) localization of the R--K* excitation at one or two ligand molecules. We have found clear evidence for a oneligand localized X-X* excitation (possibility (ii) ) by computer fitting the experimental data of fig 3. In the fitting procedure use was made of a spin Hamiltonian of the form, .~Y=g~BH.S+D(S2--)+E(S~--S:))
Fig. 4. (a) Projection of the unit cell of a [Rh(bpy),] (C10,)9 single crystal (projected slightly off the c-axis). The cell constants are: a=b=30.535 A, c=20.965 A; (u=/3=90”, y= 120”. The u-axis is along the OA direction; the b-axis is along the 08 direction. (b) The upper half of the unit cell of (a). The six magnetically inequivalent molecules are labeled (a) to (f ). The ligand molecules which are electronically excited, are denoted by an asterisk. study of the crystal structure of [ Rh( bpy),]
( ClO4)3 [ 15 1. The crystal has R3c space group symmetry [ 15 1. A projection of the unit cell (projected slightly off the c-axis) is displayed in fig. 4a. There exist 18 W(bw)~13+ entities per unit cell, but due to translational symmetry in the unit cell, only six magnetically inequivalent [ Rh (bpy), ] 3+ sites can be
9 March 1990
(1)
where it is understood that the spin operators are characteristic of an S= 1 spin system and the g-factor is taken to be isotropic. The drawn lines in fig. 3 represent the best fit computer calculated results. The latter were obtained for a molecular triplet state characterized by g=2.0, IDI =2920 MHz and IEl = 590 MHz, while the molecular axes coincide with those of the bipyridine ligand molecules containing an asterisk in fig. 4b. If we now allow for the possibility that localization of the %--11* excitation could also be on the other ligand molecules in fig. 4b (not indicated by an asterisk), then much more ODMR lines than actually observed are anticipated. Furthermore, variation of the numerical values of 1D 1 and (El with respect to the best fit values, invariably yields calculated angular dependences that are not compatible with the experimental data. It is concluded that for the [Rh(bpy),](ClO,), single crystals (i) the luminescent state is a triplet state indeed, (ii) the electronic excitation is ligand localized and therefore of K--A*nature, and (iii) the excitation of the system in the luminescent state is trapped on a single specific ligand molecule only. The result that 537
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CHEMICAL PHYSICSLETTERS
the x-n* excitation is localized on a specific single ligand molecule shows that, at least in the crystal, the three bipyridine l&and molecules chelated to the same central ion are energetically inequivalent and that therefore the local symmetry of [Rh(bpy)3]3+ in the %-rc* state must be lower than Ds. Most likely the inequivalence among the ligand molecules is caused by the electrostatic perturbations exerted by nearby ClCQ counterions in the lattice.
Acknowledgement This work was supported in part by the Netherlands Foundation for Chemical Research (SON) with financial aid from the Netherlands Organization for Scientific Research (NWO).
References [ 11 A. Zilian, U. Maeder, A. von Zelewski and H.U. Giidel, J. Am. Chem. Sot. 111 (1989) 3855.
538
9 March1990
[2] D. Sandrini, M. Maestri, V. Balzani, U. Maeder and A. von Zelewski, Inorg. Chem. 27 (1988) 2640. [3] Y. Ohsawa, S. Sprouse, LA. King, M.K. DeArmond, K.W. Hanck and R.J. Watts, J. Phys. Chem. 91 (1987) 1047. [4 ] M.K. DeArmond and C.M. Charlin, Coord. Chem. Rev. 36 (1981) 325. [ 51 W. Halper and M.K. DeArrnond, J. Luminescence 5 (1972) 225. [ 61 J.E. Hillis and M.K. DeArmond, J. Luminescence 4 ( 1971) 273. [7] D.W.H. Carstens and G.A. Crosby, J. Mol. Spectry. 34 (1970) 113. [S] Y. Komada, S. Yamauchi and N. Hirota, J. Phys. Chem. 90 (1986) 6425. [9] A.P. Suisalu, A.L. Kamyshnyi, V.N. Zakharov, L.A. Aslanov and R.A. Avarmaa, Chem. Phys. Letters 134 ( 1987) 617. [lo] E. van Oort, R. Sitters, J.H. Scheijde and M. Glasbeek, J. Chem. Phys. 87 (1987) 2394. [ 111 CM. Harris and E.D. McKenzie, J. Inorg. Nucl. Chem. 25 (1963) 171. [ 121 M. Glasbeek and R. Hond, Phys. Rev. B 23 ( 1981) 4220. [ 13 1D.J. Gravesteijn and M. Glasbeek, Phys. Rev. B 19 ( 1979) 5549. [ 141 K. Vinodgopal and W.R. Lcenstra, J. Phys. Chem. 89 (1985) 3824. [ 151 C.E. Kiriakidis, J. Westra, M. Glasbeek and C.H. Stam, unpublished results.
CHEMICAL PHYSICS LETTERS
9 March 1990
LOCALIZATION OF OPTICAL EXCITATIONS IN CRYSTALLINE Rh’+-TRISDIIMINE CHELATES J. WESTRA and M. GLASBEEK Laboratory for Physical Chemistry, University ofAmsterdam,
Nieuwe Achtergracht 127, 1018 WSAmsterdam,
The Netherlands
Received 17 November 1989; in final form 12 December 1989
The luminescent state of [ Rh (bpy ), ] ( C104)., (bpy= 2,2’-bipyridine) has been studied by means of low-field optically detected magnetic resonance (ODMR) spectroscopy. Experiments performed on single crystals, at low temperatures (Tz 1.4 K), revealed a magnetic anisotropy characteristic of a ligand-centered %-x* excitation. Furthermore, the experimental results show that the excitation is trapped on one single ligand molecule per Rh3+ cation site only.
1. Introduction
2. Experimental
In recent years the spectroscopic characterization of the luminescent state of a number of Rh3+ (d6) chelates has attracted great interest [ l-7 1. Based on the similarity of the emission spectra of Rh3+-trisdiimine chelates and the pure ligand molecules, it has long been suggested that the emissive state in these Rh3+ complexes is of n-n* origin [ 4-7 1. Lifetime and optically detected magnetic resonance (ODMR ) results seemed indicative of a triplet IF-I[*excitation of the Rh3+ chelates [8-lo]. However, unambiguous evidence for the 3rc--3[3* nature of the photo-excited Rh3+-trisdiimine compounds has not yet been presented. It is the purpose of this paper to provide such evidence for photo-excited [ Rh (bpy ) 3] (Clod) 3 single crystals. We report on ODMR experiments performed on the system in low magnetic fields. From a study of the anisotropic behavior of the observed ODMR line splittings, in combination with the results of an X-ray analysis of the crystal structure, it will be shown that theRh (bpy ) : + chelate indeed has a 3rr-rc* phosphorescent state. Furthermore, the electronic rc-n* excitation in the investigated single crystal is shown to be localized on one particular ligand molecule per Rh (bpy ):+ entity only.
[ Rh (bpy ) 3] Cl, was synthesized following the method of Harris and McKenzie [ 111. By treating [ Rh (bpy ) 3] Cl, dissolved in an ethanol/water mixture with perchloric acid, [ Rh(bpy), ] ( C104)3 was formed. Disc-shaped single crystals of the latter compound were grown by recrystallization from water. ODMR experiments were performed using the ODMR spectrometer described previously [ 12,13 1. The crystal was mounted inside a slow-wave helix immersed in a liquid-helium bath. The crystallographic a- (or b-) axis was parallel to the (vertical) helix axis. By controlled pumping of the helium in the cryostat, the bath temperature was maintained at about 1.3 K. Optical excitation near 320 nm was by means of the light from a 100 W high-pressure mercury PEK lamp filtered by a water-cooled Schott UG11 band pass filter. The emission perpendicular to the excitation pathway was focused on the entrance slit of a Jobin-Yvon HR 1000 grating monochromator. In the ODMR experiments, the microwave power was amplified using a 20 W traveling-wavetube amplifier. The microwaves were square-waveamplitude modulated at 90 Hz. Phase-sensitive lockin detection of the emission near 456 nm was applied. Magnetic fields were obtained by means of superconducting coils, immersed in the liquid helium bath, fed by a regulated power supply.
0009-2614/90/$ 03.50 0 Elsevier Science Publishers B.V. (North-Holland )
535
Volume 166, number 5,6
CHEMICAL PHYSICS LETTERS
3. Results and discussion The zero-field ODMR spectrum of the [ Rh (bpy ) 3] ( ClO,) 3 single crystal is displayed in fig. 1. The observed resonance frequencies of this work, as well as the analogous data for [ Rh (bpy ) 3 ] Cl3 and pure 2,2’-bipyridine obtained elsewhere [ IO,1 4 1, are collected in table 1. The observed ODMR linewidths (fwhm) of the chelated bipyridine molecules is in the order of 100 MHz, whereas pure bipyridine yields ODMR signals with a typical width of 10 MHz (fwhm). The large similarity among the ODMR resonance frequencies of the [Rh(bpy),13+ chelates and those of the pure ligand molecule in the 3n-x* state, is highly suggestive of the 3~-~* nature of the emissive state of the rhodiumtrisbipyridyl cation. Unambiguous evidence regarding the triplet spin character of the luminescent state of Rh (III)-diimine complexes was obtained from ODMR experiments performed in a magnetic field. Our initial experiments concerned EPR and ODMR of the [ Rh (bpy ) 3 ] ( C104 ) 3 single crystal using an X-band EPR spectrometer.
Fig. I. Zero-field ODMR spectrum as observed for a photo-excited single crystal of [Rh(bpy),] (C104)3 at 1.4 K. Excitation wavelength is near 320 nm; detection wavelength is near 456 nm. The structure in the zero-field transitions is due to a variation of the microwave power with the microwave frequency.
9 March 1990
However, in these experiments EPR signals characteristic of an excited triplet state could not be observed. Fortunately, at much lower magnetic fields (near 500 G ) the signal-to-noise ratio of the ODMR signals is much improved and in fact a study of the anisotropy of the ODMR spectrum could be undertaken. Fig. 2 shows as a representative example an ODMR spectrum obtained in the 2-4 GHz microwave region, when the crystal is in a magnetic field of 550 G directed perpendicular to the crystallographic a(or 6-) axis. Rotation of the crystal about its a- (or b- ) axis (the magnetic field being kept perpendicular to this rotational axis) revealed the anisotropy of the ODMR spectrum. The angular dependence of the ODMR lines is given in fig. 3. The dots are representative of the experimental data (derived from the 1.18 GHz and 2.32 GHz zero-field resonances), for a series of different H-field directions in the plane perpendicular to the crystallographic a- (or b- ) axis. The Zeeman shift of the 3.49 GHz zero-field transition in the applied field of 550 G appeared relatively small, i.e. less than 1.3 GHz for all orientations. The linewidth of this transition increased from 110 MHz (fwhm at zero-field) up to 350 MHz (fwhm in H= 550 G). As a result, an accurate measurement of the splitting and the anisotropic behavior of this “high-frequency” transition was hampered and henceforth the latter will not be considered. To interpret the experimentally obtained results of fig. 3 we make use of the results of a crystallographic
AI t
Table 1 ODMR resonance frequencies (GHz) of [Rh(bpy),] (C10,)3 (this work), [Rh(bpy)j]C1, (ref. [lo]) and of the bipyridine ligand in a n-heptane matrix (ref. [ 141)
21~71 IDI - IEI IDI + IEI
536
[Rh(bpy)3l(ClO&
[Rh(bpy),lCl,
bpy
1.18 2.32 3.49
1.05 2.16 3.21
0.76 2.99 3.75
2.0
3:o
410 (~~21
Fig. 2. ODMR transitions in the microwave range of 2-4 GHz when the [Rh(bpy),] (C1O4)3 crystal is in a magnetic field of 550 Gauss in the plane perpendicular to the crystallographic u{or b-) axis.
Volume 166, number 5,6
CHEMICAL PHYSICS LETTERS
0.4 O0
90”
i8O0
Fig. 3. Anisotropy of the ODMR transitions in the 2-4 GHz rcgion, when a magnetic field of 550 G is applied in the plane perpendicular to the crystallographic a- (or b) axis. The dots represent the experimental ODMR frequencies; the drawn curves represent the angular dependences as calculated for a ‘x--x* state localized on the bipyridyl ligands indicated by an asterisk in fig. 4b.
distinguished. Upon subdividing the unit cell in two halves by means of a plane parallel to the &plane, one may show that the lower and upper half of the unit cell each contain the set of six magnetically inequivalent molecules. Note that for rotation around the crystallographic a- (or b-) axis only three magnetically inequivalent molecules can be distinguished due to orientational degeneracy. For convenience, the projection of one such half unit cell iS displayed in fig. 4b. In this figure the six magnetically inequivalent molecules are labeled (a ) to ( f ). We now discuss what Zeeman ODMR patterns can be expected for the six molecules assuming a 3x--x* luminescent state in the Rh( III)-complex molecule. Several possibilities for the n-n* excitation may be considered: (i ) complete delocalization among the three ligands per Rh(III) chelate, and (ii) localization of the R--K* excitation at one or two ligand molecules. We have found clear evidence for a oneligand localized X-X* excitation (possibility (ii) ) by computer fitting the experimental data of fig 3. In the fitting procedure use was made of a spin Hamiltonian of the form, .~Y=g~BH.S+D(S2--)+E(S~--S:))
Fig. 4. (a) Projection of the unit cell of a [Rh(bpy),] (C10,)9 single crystal (projected slightly off the c-axis). The cell constants are: a=b=30.535 A, c=20.965 A; (u=/3=90”, y= 120”. The u-axis is along the OA direction; the b-axis is along the 08 direction. (b) The upper half of the unit cell of (a). The six magnetically inequivalent molecules are labeled (a) to (f ). The ligand molecules which are electronically excited, are denoted by an asterisk. study of the crystal structure of [ Rh( bpy),]
( ClO4)3 [ 15 1. The crystal has R3c space group symmetry [ 15 1. A projection of the unit cell (projected slightly off the c-axis) is displayed in fig. 4a. There exist 18 W(bw)~13+ entities per unit cell, but due to translational symmetry in the unit cell, only six magnetically inequivalent [ Rh (bpy), ] 3+ sites can be
9 March 1990
(1)
where it is understood that the spin operators are characteristic of an S= 1 spin system and the g-factor is taken to be isotropic. The drawn lines in fig. 3 represent the best fit computer calculated results. The latter were obtained for a molecular triplet state characterized by g=2.0, IDI =2920 MHz and IEl = 590 MHz, while the molecular axes coincide with those of the bipyridine ligand molecules containing an asterisk in fig. 4b. If we now allow for the possibility that localization of the %--11* excitation could also be on the other ligand molecules in fig. 4b (not indicated by an asterisk), then much more ODMR lines than actually observed are anticipated. Furthermore, variation of the numerical values of 1D 1 and (El with respect to the best fit values, invariably yields calculated angular dependences that are not compatible with the experimental data. It is concluded that for the [Rh(bpy),](ClO,), single crystals (i) the luminescent state is a triplet state indeed, (ii) the electronic excitation is ligand localized and therefore of K--A*nature, and (iii) the excitation of the system in the luminescent state is trapped on a single specific ligand molecule only. The result that 537
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CHEMICAL PHYSICSLETTERS
the x-n* excitation is localized on a specific single ligand molecule shows that, at least in the crystal, the three bipyridine l&and molecules chelated to the same central ion are energetically inequivalent and that therefore the local symmetry of [Rh(bpy)3]3+ in the %-rc* state must be lower than Ds. Most likely the inequivalence among the ligand molecules is caused by the electrostatic perturbations exerted by nearby ClCQ counterions in the lattice.
Acknowledgement This work was supported in part by the Netherlands Foundation for Chemical Research (SON) with financial aid from the Netherlands Organization for Scientific Research (NWO).
References [ 11 A. Zilian, U. Maeder, A. von Zelewski and H.U. Giidel, J. Am. Chem. Sot. 111 (1989) 3855.
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9 March1990
[2] D. Sandrini, M. Maestri, V. Balzani, U. Maeder and A. von Zelewski, Inorg. Chem. 27 (1988) 2640. [3] Y. Ohsawa, S. Sprouse, LA. King, M.K. DeArmond, K.W. Hanck and R.J. Watts, J. Phys. Chem. 91 (1987) 1047. [4 ] M.K. DeArmond and C.M. Charlin, Coord. Chem. Rev. 36 (1981) 325. [ 51 W. Halper and M.K. DeArrnond, J. Luminescence 5 (1972) 225. [ 61 J.E. Hillis and M.K. DeArmond, J. Luminescence 4 ( 1971) 273. [7] D.W.H. Carstens and G.A. Crosby, J. Mol. Spectry. 34 (1970) 113. [S] Y. Komada, S. Yamauchi and N. Hirota, J. Phys. Chem. 90 (1986) 6425. [9] A.P. Suisalu, A.L. Kamyshnyi, V.N. Zakharov, L.A. Aslanov and R.A. Avarmaa, Chem. Phys. Letters 134 ( 1987) 617. [lo] E. van Oort, R. Sitters, J.H. Scheijde and M. Glasbeek, J. Chem. Phys. 87 (1987) 2394. [ 111 CM. Harris and E.D. McKenzie, J. Inorg. Nucl. Chem. 25 (1963) 171. [ 121 M. Glasbeek and R. Hond, Phys. Rev. B 23 ( 1981) 4220. [ 13 1D.J. Gravesteijn and M. Glasbeek, Phys. Rev. B 19 ( 1979) 5549. [ 141 K. Vinodgopal and W.R. Lcenstra, J. Phys. Chem. 89 (1985) 3824. [ 151 C.E. Kiriakidis, J. Westra, M. Glasbeek and C.H. Stam, unpublished results.