J. PhyS. Chem. 1981, 85, 146-148
148
increased with steric hindrance in these molecules since the energy of the excited state will be reduced, for example, as the terminal tolyls in Bis-2MSB rotate to achieve a more planar conformation than in the ground state. We have previously speculated that this rotation occurs on the 5-ps timescale, leading to a risetime on this order of the relaxed excited state e m i ~ s i o n . ~However, ,~ since the lifetime of the Bis-2MSB excited state is -1.4 ns,’v8J0 the steady-state fluorescence spectra which we report here are essentially all due to relaxed molecules. The fluorescence quantum yields which we have obtained for these distyrylbenzenes (Table 111) are all very high. Only for the cases of DSB and Bis-2MSB both in heptane, and Bis-2MOSB in cyclohexane, were the observed quantum yields experimentally less than one. The
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value obtained for Bis-2MSB in cyclohexane Wem, within experiment1 error, with the value previously reported by Berlman.lo However, the fluorescent quantum yields obtained for DSB in both toluene and heptane are significantly higher than those previously reported by others.14 Thus, we do not find a strong solvent dependence on these quantum yields, in contrast to the earlier report.14
Acknowledgment. We thank Professor J. N. Pitts, Jr., of this department for the use of his spectrofluorometer and Dr. D. Lokensgard for assistance in the use of this instrument. We also thank Mr. Matthew Iwamoto and Ms. Joanne Richards for synthesizing the dyes. This research was supported by the Committee on Research of the University of California, Riverside.
Highly Efficient Energy Transfer in Cubic Lanthanide Hexachloroelpasolite Crystals Asok K. BanerJee,fFiona Stewart-Darling,$ Colin D. Fllnt,t and Robert W. Schwartz’t Department of Chemistry, Louisiana State University, Baton Rouge, Louisiana 70603, and Department of Chemistry, Birkbeck College. University of London, London, England (Received: August 29, 1960; In Final Form: October 6, 1980)
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The radiative lifetime of the 5F5 51Eluminescence of Ho3+in the cubic elpasolite Cs2NaYC1, is -5.5 X s and shows almost no concentration quenching up to the pure-phase c~~NaHoC1~. Excitation of Cs2NaHoC& via the 518 5F3,5Fz,or 3K8transitions gives strong 5F3 and 5F5 51Eemissions, but the 5F4 518is weak. This last transition is strong with resonance excitation. Low concentrations of Sm3+and Yb3+quench the 5F, 51Eemission with an efficiency of the order of 99%. Selective excitation and lifetime measurements indicate that Ho3+ Ho3+,Sm3+,and Yb3+energy transfers occur by cross relaxation.
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Introduction The application of lanthanide ions in luminescent materials for display devices, laser applications’ and upconverter phosphors2 has lead to intensive study of the emission characteristics of these ions in various crystalline environments. Almost all such investigations have been undertaken for lanthanides in noncentrosymmetric envir o n m e n t ~so~ that the f f electric dipole no-phonon transitions become allowed. Excited states of lanthanide ions in centrosymmetric environments should have much longer relaxation times. This offers interesting possibilities for optical pumping, energy storage, and transfer. A well-documented case of six-coordinate octahedral symmetry for lanthanide ions is in the elpasolite hexachlorides of formula Cs2NaLnC16(Ln = trivalent lanthanide or Y3+).4 The absorption and emission spectra as well as MCD/MCPE and Zeeman studies of a number of ions in this series have been reported in recent yearsS5 The room-temperature fluorescence lifetimes have been reported for Cs2NaNdC&and Cs2NaN&.olYo.99C&crystals; the latter possess the longest lifetime reported so far for Nd3+ in any environment.6 In this report remarkably efficient energy transfer from Ho3+ to Ho3+,Sm3+,and Yb3+ in hexachloroelpasolite crystals will be described.
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Experimental Section Crystals of appropriate compositions were grown by the Bridgeman method in evacuated quartz ampules a t +LouisianaState University.
* University of London.
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800-850 OC. All concentrations are only nominal as they represent the ratio of starting materials. The measured lifetimes are rather dependent on the sample quality; in particular, exposure to air reduces the lifetime presumably due to surface hydrolysis. Most of the luminescence spectra and lifetime measurements were carried out by the procedure as described el~ewhere.~ Some excitation experiments made use of krypton ion and argon ion pumped dye laser excitation and a Spex Raman spectrometer.
Results The detailed analysis of the absorption and emission spectra of Cs2NaHoC16has been described elsewhere.8 Excitation into the 5F3,5Fz,or 3KElevels using an argon ion laser source produces intense red emission from the (1)M. J. Weber, “Handbook of Lasers”, Chemical Rubber Publishing Co., Cleveland, OH, 1971,p 371. (2)L. F. Johnson, H. J. Guggenheim, T. C. Rich, and F. W. Ostermayer, J. Appl. Phys., 43,1125 (1972). (3)R. Reisfeld, Struct. Bonding (Berlin), 30,65 (1976). (4)L. R. M o m and J. Fuger, Inorg. Chern., 8,1433(1969);L. R. Mom, M. Siegel, L. Stenger, and N. Edelstein, ibid., 9,1771 (1970). (5)(a) R. W. Schwartz, Mol. Phys., 30,81 (1975);(b) ibid., 31,1909 (1976);(c) R. W. Schwartz and P. N. Schatz, Phys. Reu. B, 8,3229(1973); (d) L. C. Thompson, 0. A. Serra, J. P. Riehl, F. S. Richardson, and R. W. Schwartz, Chern. Phys., 26, 393 (1977);(e) R. W. Schwartz, Inorg. Chem., 16,1694 (1977); (f) 0.A. Serra and L. C. Thompson, ibid., 15, 504 (1976); (9) R. W. Schwartz, H. G. Brittain, J. P. Riehl, W. Yeakel, and F. S. Richardson, Mol. Phys., 34,361 (1977); (h) R. W. Schwartz, T. R. Faulkner, and F. S. Richardson, ibid., 38, 1767 (1980). (6)B. C. Tofield and H. P. Weber, Phys. Reo. B, 10, 4560 (1974). (7)C. D.Flint, P. Greenough, and A. P. Mathews, J. Chern. SOC., Dalton Trans., 368 (1973). (8) J. P. Moreley, T. R. Faulkner, F. S. Richardson, and R. W. Schwartz, to be published in J. Chern. Phys.
0 1981 American Chemical Society
The Journal of Physical Chemistry, Vol. 85, No. 2, 1981 147
Energy Transfer in Cubic Eipasolite Crystals
TABLE I: Relative Intensities of the SF, '1, and 'F, + $1, Transitions with Excitation into 'F, and $F, Levels at 300 K 51,
+
SF,
excitation
Cs,NaHoCl, Cs,NaHo,,Yb
100 100 80 Cs~NaHo0.98Sm0.0~C16 Cs2NaHo0.99Tb0.01C16 63 Cs,NaYo~,Hoo~,Tbo~,C16 20
100 5.0 4.7 90 35
51,
0
--f
'F,
excitation.
100
1.7 2.2
9.5 0.8
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c q
0 TABLE 11: Lifetimes of the ,F,+ Transitions and Ho + Acceptor Energy-Transfer Efficiencies with Excitation into the 'F, Level 10-3 10-3 lifetime lifetime (300 K ) (80 K) compd 7, s Ilt 7,s Ilt Cs, NaHoC1, 4.5 5.5 Cs, NaHo, .Y,.C1, 6.1 6.6 0.98, 0.05 0.99, Cs~NaHo,,Smo0,"Cl6 0.07 Cs,NaHo,,,Yb,~o~C1, 0.035 0.99, 0.02 0.99, 0.89 0.9 0.84 Cs,NaHo,,,Tb,,,Cl, 0.5 0.035 0.99, 0.003 0.99,, Cs,NaY,~,Hoo~,Tbo~,C1,
"..
U
I
--f
IO
W
0
Sm
Yb
Tb
Ho
".I
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5F5 518transition with an exponential decay corres at 300 K. This sponding to a lifetime of -4.5 X lifetime increases only slightly on cooling to 80 K. This lifetime is some 80% of the value for Cs2NaYo.99Hoo.olC& so concentration quenching is small. A weaker blue emission is assigned to the 5F3 518emission, but the 5F4 518emission is extremely broad and weak. This latter transition is generally much more intense and well resolved in CS2NaYo,~HOo.o&16and in more concentrated samples when excitation is via the 5F4transition. Each crystal field component of the free ion states has the same lifetime. The visible emission from Cs2NaSmC&is weak and has not been studied at high resolution. Absorption and MCD results will be presented elsewhere? Most of the lines are assigned as magnetic dipole allowed origins from the E" and (hot) U' componentsof the 6H5/2ground state. Typical t values for the magnetic dipole origins and electric dipole vibronic structure in the region 10 000-23 500 cm-' are 0.01-0.5 and (mol-' cm2),respectively. The red luminescence of C S ~ N ~ H O ~ , when ~ S ~the ,~C excitation is into the 5F3(Ho3+) level is reduced in intensity (Table I) and lifetime (Table 11) by 2 orders of magnitude relative to pure Cs2NaHoClG.A similar quenching occurs with excitation directly into the 5F5level. However, when the excitation is into the 5F4level, there is no quenching of the 5F4 518emission although the 5F5emission is still quenched. Clearly the energy transfer from Ho3+to Sm3+ occurs from the 5F5level, but, since there is no Sm3+level near 15000 cm-', it is necessary to assume that the quenching is by energy transfer involving the low-lying Sm3+levels (Figure 1). To test this hypothesis a similar series of experiments was carried out with Cs2NaHoo,~Ybo,olC&. With excitation into the 5F3or 5F5states, the 5F5 518emission intensity and lifetime is quenched by a factor of 200. With excitation into the 5F4level, the 5F4 518emission is not affected but the 5F5 518emission is again quenched. These results imply that the quenching is by mechanism D, Figure 1.
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(9)A. K.Banerjee and R. W. Schwartz, to be submitted for publication in Chem. Phys.
Figure 1. Portion of the experimental1 determined energy-level diagrams for Sm3+, Tb3+, Ho3+, and Yb3 . The width of the levels is a
Y
combination of crystal f i M splitting and a 250tm-' vibrational spread. The arrows show some possible cross-relaxation pathways.
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Direct evidence for this was found by monitoring the 'I7 518Ho3+emission in the infrared. This emission is weak in neat Cs2NaHoC1,, much more intense in C S ~ N ~ H O ~ . ~ Yb0,01C16,and too weak to be observed in C S ~ N ~ H O ~ . ~ ~ Smo.02Cls. Both Yb3+and Sm3+have energy levels near loo00 cm-', which corresponds to the Ho3+5F5 517separation. It was of interest to use Tb3+as an acceptor since it has no levels between 6500 and 20 000 cm-'. For Cs2NaHoo.~Tbo.olC16 there is slight quenching of the 5F4,5F5 518emissions when excited by absorption into the 5F4level and a good deal more quenching of the 5F5 518 emission with excitation into 5F3or 5F5. A dramatic quenching of the 5F5 518emission occurs in Cs2NaYo.8Hoo.lTbo.lC16 without any visible emission from the Tb being detected.
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Discussion The luminescence behavior of pure Cs2NaHoC16shows that the 5F3 5F5relaxation is fast and bypasses the 5F4 &level. It is reasonable to suppose that the mechanism in the pure material is cross relaxation, i.e. +HO~+(~I~) +
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The overall efficiency of energy transfer from one level can be calculated by using the expressionlo vt
= 1 - 7d/Tdo
where 7d0 is the radiative lifetime of the donor alone and 7d is its lifetime in the presence of the acceptor. With this expression the efficiency of the Ho3+ Sm3+transfer is >98% at 300 K and >99% at 80 K, whereas the corresponding efficiencies for Ho3+ Yb3+ are >99 and >99.5 % In Cs2NaYo.8Hoo.lTbo.lC& efficiencies are 99.4 and 99.95% at 300 and 80 K, respectively. So far as we are aware, these are the highest efficiencies found in systems of this type. In the Cs2NaLnC16crystal each Ln3+site has 12 nearest-neighbor Ln3+sites at distances of -7.5 A. If a uniform distribution of dopants is assumed, then in the Cs2Na-
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(10)R. Reisfeld and N. Lielehich-Soffr, J.Solid State Chem., 28, 391 (1979).
J. Phys. Chem. 1981, 85, 148-153
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Y0.8H00,1Tb0.1C16 crystal the probability is very high that each type of lanthanide, either Ho3+or Tb3+,will have at least one nearest-neighbor site occupied by the other type of lanthanide. Thus Ho T b energy transfer is possible involving no more than one Ln3+-Ln3+separation. On the other hand in CszNaHoo.ggYbo,olC16 there is only about a 12% probability that a Ho3+will have a Yb3+as a nearest neighbor. With the efficiency of Ho3+ Yb3+ energy transfer seemingly 99% (rate constant =9 X lo4 s), this implies faster and more efficient Ho3+ Ho3+ energy transfer as well. A similarly high rate constant, -2 X lo4 s, is found for Ho3+ Sm3+ energy transfer. The apparently much higher Ho3+ Tb3+efficiency in CszNaY0.8H00.1Tb0.1C16 compared with that in CszNaHoo.ggTbo.olC16may be an indication that the actual Tb3+concentration is less than nominal 1%. At present only speculation is possible concerning the detailed mechanism of these efficient energy-transfer processes. In the centrosymmetric ions the lifetimes of the various excited states are long and the relative intensities
of the relatively broad electric dipole vibronic features in the absorption and emission spectrum are enhanced. The possibilities for dipole-dipole or dipole-multipole energy transfer are thereby improved. The efficient pumping of the 5F5level by cross relaxation in the pure cs2NaHoCl6 may also be significant. The infrared results have clearly shown that pathway D is operative in the Ho3+ Yb3+energy transfer and is not operative in the Ho3+ Sm3+energy transfer. Additional experiments are in progress that promise to elucidate, experimentally, the energy-transfer pathways in these and other hexachloroelpasolite systems.
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Acknowledgment. We thank Professor R. V. Nauman for use of his emissionfexcitationequipment in preliminary experiments. This work was supported in part by a grant from the National Science Foundation, U.S.A., (DMR 78-11540) to R.W.S. and in part by the SRC and the Central Research Fund of the University of London, U.K. (to C.D.F.).
Electronegatlvlty. An Isolated Atom Property James L. Reed Depadment of Chemistry, The Atlanta University, Atlanta, Georgia 303 14 (Received: Juw 8, 1980; In Final Form: September 22, 1980)
The distinction between electronegativityas an isolated atom property and the property of an atom in a molecular environment is fundamental to the understanding and use of this quantity. A treatment similar to that performed by Klopman on the atomic Hamiltonian is applied to the molecular Hamiltonian. The treatment illustrates the strengths and weakness inherent in the practice of equalizing the isolated atom electronegativitiesof Mulliken and others. Two new approximate molecular parameters, totally derivable from atomic properties, are defined and examined.
Introduction The extrapolation of various isolated atom properties to atoms in molecules has been fundamental to both the development and practice of chemistry. Of equal importance has been the assumption of the constancy of certain atomic properties in various molecular environments. Nonetheless, the assumption that such an extrapolation can or should be made or that such constancy exists is neither required nor suggested in the molecular orbital description of molecules.’ The observation that numerous atomic properties, including electronegativity, are retained a t least in part by atoms in molecules suggests that atoms, although perturbed by the molecular environment, retain their identity. Thus it is important that an understanding of the nature and extent of these pertubations be developed. Of particular interest in this regard is the atomic property, electronegativity, which inspite of its universal acceptance is not yet fully understood. Since its introduction by Pauling,2 electronegativity has been defined in numerous ways by various author^.^-^
Nonetheless, the definition proposed by Iczkowski and Margrave5has found widespread acceptance and includes two important modifications to the original definition. First, electronegativity is an orbital property5 and thus dependent on the valence state of the atom.8 Second, electronegativity is dependent on the charge associated with a particular The neutral atom electronegativities (for appropriate valence states) computed by this method correlate well with the original scale proposed by Pauling. Nonetheless, electronegativity,as described by Iczkowski and Margrave: is a property of an isolated gaseous atom or ion. This is in contrast to the definition of Pauling which defines electronegativity as a property of an atom in a molecule.2 The success of the work of Iczkowski and Margrave, and Hinze and Jaffe, attest to the extent to which atomic properties are retained in molecular systems. The empirical expression used by Iczkowski and Margrave has been derived in a semiempirical treatment by Klopman.1° Using a similar approach, we will illustrate the
(1) R.F. Bader, Acc. Chem. Res., 8,34 (1975). (2)L.Pauling, “The Nature of the Chemical Bond”, 3rd ed, Cornel1 University Press, Ithaca, N.Y., 1960,p 88. (3)R.T. Sanderson, J. Chem. Ed., 29,539 (1952). (4)A. L. Allred and E. G. Rochow, J . Znorg. Nucl. Chem., 5 , 264 (1958).
(5)R.P. Iczkowski and J. L. Margrave, J. Am. Chem. SOC.,83,3547 (1961). (6)J. Hinze and H. H. Jaffe, J. Am. Chem. SOC.,84,540 (1962). (7)J. Hinze and H. H. Jaffe, J. Am. Chem. SOC.,85, 148 (1963). (8)A. D.Walah, Discuss. Faraday Soc., 2, 18 (1947). (9)W. Gordy, Phys., Reu., 69,604 (1946).
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0 1981 American Chemical Society
