J . Phys. Chem. 1984,88, 1534-1537
1534
Photophysical Properties of Europium(I I ) Cryptates N. Sabbatini,*’” M. Ciano,Ib S. Dellonte,lb A. Bonazzi,la F. Bolletta,la and V. Balzani’ Istituto Chimico “G. Ciamician” dell’liniversitci, Bologna, Italy, and Istituto di Fotochimica e Radiazioni d’Alta Energia del CNR, Bologna, Italy (Received: April 8, 1983; In Final Form: September 7 , 1983)
The photophysical properties of the complexes between EuZ+and the [2.2.1] and [2.2.2] cryptands are reported and compared to those of the EuZ+aquo ion. Both complexes show broad, relatively intense absorption bands in the near-UV region due to 4P 4f65d transitions. Some weak, narrow bands due to transitions within the 4f‘ configuration also appear in the 3 l(j32O-nrn region. Both complexes exhibit a strong blue luminescence from 4f65d. At 77 K the emission lifetime is of the order of 1 ws (see Table I), the emission quantum yield is unity, and some vibrational structure can be observed in the broad emission band. Luminescence is also maintained in aqueous solution at room temperature with T of the order of a few nanoseconds, and of the order of (see Table I), in contrast with the behavior of the E u a F ion, which does not exhibit any luminescence emission under such conditions. The results obtained are discussed in the light of the interaction between Eu2+ and water molecules and of the size and symmetry of the cryptand cage.
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Introduction It is well-known that certain diazapolyoxabicyclic ligands (“cryptands”)2 can form stable complexes (“cryptates”) with monovalent, divalent, or trivalent These ligands encapsulate the metal ion into an organic microenvironment which prevents or at least reduces interaction between metal ion and solvent molecules. Gansow et al.4-7 have recently shown that Eu3+and E d + can be encapsulated into the 4,7,13,16,21,24-hexaoxa-1,lO-diazabicyclo[8.8.8] hexacosane and 4,7,13,16,21-pentaoxa-1,lO-diazabicyclo[8.8.5]tricosane cryptands (shown schematically in Figure 1 and hereafter simply called [2.2.2] and [2.2.1], respectively2) to yield fairly stable complexes which exhibit a reversible electrochemical behavior. We have carried out an investigation on the photophysical properties of these complexes. In a preliminary communications we reported the absorption spectrum, emission spectrum, and emission lifetime of [Eu2+C2.2.2]and in this paper we report a complete picture of the photophysical properties of [Eu2+C2.2.1] and [Eu2+C2.2.2]. The photophysical properties of the corresponding Eu3+complexes will be reported elsewhere. It is well-known that EuZ+does not exhibit luminescence in aqueous solution at room temperature9-” because it is rapidly deactivated by fast radiationless transitions and by an efficient photoreaction yielding Eu3+and Hz.12-17Strong luminescence ~~
(1) (a) Istituto Chimico “G. Ciamician” dell’Universit5. (b) Istituto di Fotochimica e Radiazioni d’Alta Energia del C.N.R. (2) Lehn, J. M. Acc. Chem. Res. 1978, 1 1 , 49. (3) Burns, J. H.; Baes, C. F., Jr. Inorg. Chem. 1981, 20, 616. (4) Gansow, 0. A.; Pruett, D. J.; Triplett, K. B. J . Am. Chem. Soc. 1979,
-101 .., A4OR . .- - . (5) Gansow, 0. A,; Kausar, A. R.; Triplett, K. B.; Weaver, M. J.; Yee, E. L. J . Am. Chem. SOC.1977, 99, 7087. (6) Yee, E. L.: Gansow, 0. A.; Weaver, M. J. J . Am. Chem. SOC.1980, 102, 2278. (7) Gansow, 0. A.; Triplett, K. B. U S . Patent 4257955, Mar 24, 1981; Chem. Abstr. 1981, 94, 194446j. (8) Sabbatini, N.; Ciano, M.; Dellonte, S.; Bonazzi, A.; Balzani, V. Chem. Phys. Lett. 1982, 90, 265. (9) Butement, F. D. S. Trans. Faraday SOC.1948, 44, 617. (10) Haas, Y.; Stein, G.; Tomkiewicz, M. J . Phys. Chem. 1970, 74,2558. (1 1) Bulgakov, R. G.; Kazakov, V. P.; Korobeinikova, V. N. Opt. Spectrosc. (Engl. Transl.) 1973, 35, 497. (12) Korolev, V. V., Bazhin, N. M.; Chentzov, S.F. Zh. Fiz. Khim. 1981, 55, 144. (13) Korolev, V. V.; Bazhin, N. M.; Chentzov, S.F. Zh. Fiz. Khim. 1981, 55, 138. (14) Brandys, M.; Stein, G. J. Phys. Chem. 1978, 82, 852. (15) Davis, D. D.; Stevenson, K. L.; King, G. K. Inorg. Chem. 1977, 16, 670. (16) Ryason, P. R. Sol. Energy 1977, 19, 445. (17) Korolev, V. V.; Bazhin, N. M.; Chentzov, S. F. Khim. Vys. Energ. 1980, 14, 542.
0022-365418412088-1534$01 SO10
emission, however, is commonly observed for Eu2+in glassy solutions at 77 Kii-I3 or in crystal^.'^-^^ This property is currently the object of much interest for theoretical reasons related to the occurrence of intra- and/or intershell transitionsis-2i,24-26,30 as well as for laser a p p l i c a t i ~ n . ~We ~ - ~have ~ found that the [Eu2+C2.2.1] and [Eu2+C2.2.2]complexes, which to our knowledge are the first molecular EuZ+compounds studied from a photophysical point of view, exhibit very strong luminescence at 77 K (ae,,, 1) and a weak emission even in aqueous solution at room temperature. Other interesting properties of these compounds are reported and discussed.
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Experimental Section Preparation of Cryptates. The [Eu2+C2.2.1] and [Eu2+C 2.2.21 complexes can be prepared either by reduction of the corresponding Eu3+ complexes or by adding a stoichiometric amount of the ligand to an Eu2+s ~ l u t i o n . The ~ , ~ first method does not offer any particular advantage from the experimental point of view and requires the preparation of pure samples of the Eu3+ complex, which is a very difficult task because of the rigorously anhydrous conditions r e q ~ i r e d .Thus, ~ the second method was preferred and the procedure adopted was as follows. An aqueous
(18) Reisfeld, R.; Jorgensen, C. K. In “Inorganic Chemistry Concepts”; Becke, M., Lappert, M. F.,Margrave, J. L., Parry, R. W., Jorgensen, C. K., Lippard, S. J., Niedeuzu, K., Yamatera, H., Eds.; Springer: West Berlin, 1977; Vol. 1. (19) Blasse, G. Struct. Bonding (Berlin) 1976, 26, 43. (20) Blasse, G. In “Handbook on the Physics and Chemistry of Rare Earths”; Gschneidner, K. A,, Jr., Eyring, L., Eds.; North-Holland: Amsterdam, 1979; Vol. 4, p 237. (21) Maestro, P.; Dougier, P. Actual. Chim. 1982, 15. (22) McClure, D. S.; Kiss, Z. J. Chem. Phys. 1963, 39, 3251. (23) Johnson, K. E.; Sandoe, J. N. J . Chem. SOC.A 1969, 1694. (24) Hoffman, M. V. J . Electrochem. SOC.1971, 118, 933. (25) Hoffman, M. V. J . Electrochem. SOC.1972, 119, 905. (26) Ryan, R. M.; Lehmann, W.; Feldman, D. W., Murphy, J. J . Electrochem. SOC.1974, 121, 1475. (27) Arakawa, T.; Takata, T.; Adachi, G. Y.; Shiokawa, J. J . Chem. SOC. Chem. Commun. 1979, 453. (28) Kobayasi, T.; Mroczkowski, S.;Owen, J. F. J. Lumin. 1980, 21, 247. (29) Hernandez, J. A,; Cory, W. K.; Rubio, J. 0. J . Chem. Phys. 1980, 72, 198.
(30) Joubert, M. F.; Boulon, F.; Gaume, F. Chem. Phys. Lett. 1981, 80, 367. (31) Machida, K.; Adachi, G.; Moriwaki, Y.; Shiokawa, J. Bull. Chem. SOC.Jpn. 1981, 54, 1048.
(32) Machida, K.; Adachi, G.; Shiokawa, J.; Shimada, M.; Koizumi, M.; Suito, K.; Onodera, A. Inorg. Chem. 1982, 21, 1512. (33) Weber, M. J. In “Handbook on the Physics and Chemistry of Rare Earths”; Gschneidner, K. A,, Jr., Eyring, L., Eds.; North-Holland: Amsterdam, 1979; Vol. 4, p 275.
0 1984 American Chemical Society
The Journal of Physical Chemistry, Vol. 88, No. 8. 1984 1535
Photophysical Properties of Europium(I1) Cryptates TABLE 1: Absorption and Emission Properties‘ absorption
T ,K [ E u Z + c2.2.1 ]
293
habs max, cm-l ( E ,
M” cm-’ )
39 680 (1830), 30 300 (650)
emission h&&, cm-’
22 220 22 730
77
[ EU2+C2.2.21
293
21 740
39 220 (27001, 31 450 (600)
23 810
77 EuaqZ+
293 77
T
,
ns~
b
@em
1.5 2.07. 7.0Csd 610 79OC .. 3.0 14.0, 41.0C,d 550 85OC
1.oc
200, 850d*f
0.13g
4 0 0 0 0 (22301, 31 250 (650) 22 220
’In aqueous solution, unless otherwise noted.
Values obtained by a two};or errors see text. In methanol-water 6:4 (v/v). exponential fitting procedure. e Only an order of magnitude estimation was made because the actual values were not relevant to our discusReference 13. sion. For analogous data see also ref 12 and 13.
3ol
ABSORPTION
EMISSION
I
[2211
r22
I
21
Figure 1. Schematic representation of the [2.2.1] and [2.2.2] cryptands.
solution (10 cm3) containing 1.0 X M EuC13.6Hz0 (Ventron, 99.9%) and 0.1 M NaC104.H20 (Baker) was slightly acidified to pH 4. The solution was then introduced into a three-electrode electrochemical cell and reduced at controlled potential on a mercury pool at -0.9 V vs. SCE under pH-controlled conditions. The cathodic compartment was separated by a fine-grade frit from the anodic compartment. When all Eu3+ was reduced (polarographic control), the desired amount of the cryptand (Le., 2% more than that required by the 1:1 stoichiometry) was added. This was made possible by a microsyringe in the case of the liquid [2.2.1] cryptand and by a rotatable side arm in the case of the solid t2.2.21 cryptand. Since the [2.2.1] cryptand undergoes some degradation reaction, the commercial Merck product was purified from hexane just before use, while the [2.2.2] cryptand was used as received from Merck. In both cases addition of the cryptand caused a small increase in pH, which was then adjusted to -8 for [Eu2+C2.2.1] and -7 for [Eu2+C2.2.2]. Under such conditions, practically 100% of the cryptate was formed in a few minutes, as shown by polarographic and cyclic voltammetric measurements and as expected on the basis of the stability constants of the complexe~,~ the acidity constants of the ligands,34 the hydrolysis constant of E U * + ,and ~ ~ the rate constant of formation of the two complexes.6 After preparation, the complex was transferred into a side-arm cell suitable for spectrophotometric and spectrofluorimetric measurements. Apparatus, Techniques, and Procedures. Preparative electrochemistry and dc polarograms were performed with an Amel multifunction unit Electrochemolab. For cyclic voltammograms an Amel 448 oscillographic polarograph was used. The pH of the solution was controlled with a combined microelectrode connected with a Knick KpH 34 pH meter. The absorption spectra were obtained by means of a Cary 219 spectrophotometer and the photoluminescence spectra by a Perkin-Elmer M P F 3 spectrofluorimeter. Emission quantum yields were evaluated by the method described by Parker and R e e ~ , ~ * using as a reference standard the dye POPOP (a = 0.93 in c y ~ l o h e x a n e )which , ~ ~ emits in the same spectral region as the
EuZ+compounds. The estimated error was 610%. Emission lifetimes in the microsecond region were measured by using a pulsed N, laser (Lambda Physik-Gottingen) with a fwhm of -3.5 ns and a peak power of 1 MW. The emission was monitored at a right angle by using a Hamamatsu R955 photomultiplier in combination with a Bausch and Lomb high-intensity monochromator or with narrow-band interference filters. Transient signals were acquired and reduced by using a Tektronix R 79 12 transient digitizer equipped with a 7A26 vertical amplifier and interfaced to a 2 8 0 based Cromemco microcomputer. The scattering in the lifetime values so obtained was 65%. Emission lifetimes in the nanosecond region were detected by using a modified Applied Photophysics single-photon time correlation apparatus. The samples were excited at 337 nm by a thyratron-gated flash lamp filled with deuterium. The emission was monitored at a right angle with a Philips 56TVP/03 photomultiplier cooled a t -20 O C . Narrow-band interference filters (Bakers) in combination with a 400-nm cutoff and an IR-absorbing Schott glass filter were used. Fluorescence decays were analyzed by deconvolution using a Digital PDP 11/23 microcomputer. Lifetimes were obtained by averaging five measurements, the scattering being 6 8 % . The Gaussian analysis of the absorption bands was performed by using a program developed for an Apple I1 microcomputer, on the basis of the Marquardt alg~rithm.~’All measurements were carried out in aqueous solution under argon unless otherwise noted.
(34) Lehn, J. M.; Sauvage, J. P. J . Am. Chem. SOC.1975, 97, 6700. (35) Taken equal to that of SrZC(ref 36) which has the same charge and almost equal rad-i~s.~’ (36) Sillen, L. G.; Martell, A. M. “Stability Constants of Metal-Ion Complexes”; The Chemical Society: London, 1964. (37) Sinha, S. P. Struct. Bonding (Berlin) 1976, 25, 69. (38) Parker, C. A,, Rees, W. T. Analyst (London) 1960, 85, 587.
(39) The quantum yield of POPOP at 77 K in a MeOH-H,O matrix was measured to be 0.97, using the value of 0.93 for POPOP in cyclohexane given by: Berlman, I. B. ‘Handbook of Fluorescence Spectra of Aromatic Molecules”; Academic Press: New York, 1971. (40) Bevington, P. R. ‘Data Reduction and Error Analysis for the Physical Sciences”; McGraw-Hill: New York, 1969; p 235.
h(nm)
Figure 2. Absorption and emission spectra of [Eu2+C2.2.1] (full line) and [Eu2+C2.2.2] (dashed line) in aqueous solution at room temperature. The lowest energy component of the Gaussian analysis of the absorption spectrum is also shown.
1536
The Journal of Physical Chemistry, Vol. 88. No. 8. 1984
Results The electrochemical behavior of the [Eu2+C2.2.1J and [Eu2+C2.2.2]cryptates was found to be in agreement with that previously reported by Gansow et aL6 Cyclic voltammograms in 0.1 M NaClO, showed that both complexes undergo reversible oxidation-reduction with a mean potential between the cathodic and anodic peaks of -425 and -225 mV vs. SCE, while the Euaq3+/Euaq2+ couple behaves irreversibly under the same conditions and has a formal potential of -625 mV vs. SCE. The absorption spectra of the two complexes are shown in Figure 2. The absorption maxima and the corresponding extinction coefficients are given in Table I, where the data concerning EU,? are also reported for comparison purposes. Both complexes exhibit a blue luminescence under all the experimental conditions examined. Figure 2 shows the emission spectrum in aqueous solution at room temperature. At 77 K the emission spectra were very similar to those shown in Figure 2, but a fine structure appeared when they were recorded under the best resolution conditions of our equipment. The emission quantum yield could not be measured in aqueous solution at 77 K because the glass was not transparent. For a methanol-water 6:4 (v/v) solution, which forms transparent glasses, the emission quantum yield was about unity for both complexes. The emission lifetimes for the same solutions were 0.79 p s for [Eu2+C2.2.1] and 0.85 p s for [Eu2+C2.2.2]. Somewhat shorter lifetimes were obtained for aqueous solutions at 77 K (Table I). In all the above cases, the emission decay was strictly first order. Both emission intensity and emission lifetime decreased with increasing temperature, but a noticeable luminescence was still observed at room temperature both in pure water and in methanol-water solutions (Table I). In aqueous solution the emission decay was still strictly first order, while in the mixed solvent the decay was clearly more complex. A reasonably good fit was obtained by using two exponential components (Table I).
Discussion Examination of space-filling models shows that encapsulation of Eu2+into the [2.2.1] and [2.2.2] cryptands yields complexes having C2, and D3has the highest possible symmetry, respectively. A detailed investigation carried out on the Eu3+complex of the [2.2.1] cryptand4' has shown that in aqueous solution three water molecules are coordinated to Eu3+ through the holes of the cryptand structure. A similar arrangement is expected for [Eu2+C2.2.1]and [Eu2+C2.2.2]with the water molecules less strongly bonded because of the reduced positive charge on the metal. It should be noted that in both cases the coordination of three water molecules leaves unchanged the C2, and D3h symmetries of the two complexes. The lowest energy configuration of the Eu2+ion is 4P.'9320 The ground state is %7,2 and the first excited state within the 4f' configuration is 6P7/2. In the free ion, such an excited state lies at 28 200 cm-I above the ground state,Is and in the Eu2+ compounds it lies at quite similar energy values regardless of the nature of the ligands, as expected for intraconfigurational f f excited states.18-20 The first excited configuration for Eu2+is 4f65d. A simple, although not r i g o r o ~ s , ~way & ~to ~ discuss the excited states deriving from the 4P5d configuration and to account for the gross features of the absorption and emission spectra is to assume that the interaction between the 4f6 electrons and the 5d electron is sufficiently weak so that the composite 4P5d system retains much of the character of the uncoupled 4f6 and 5d levels. According with this assumption, the f electrons are left in their RussellSaunders state (7FJ),the d configuration is allowed to split according to the ligand field symmetry, and the energy position of the center of gravity of the 4f65d level is determined by the nephelauxetic effect which reduces the energy separation between the 4 P and 4f65d free-ion levels.18-20 For the [Eu2+C2.2.1] complex (CZvsymmetry) complete splitting of the degenerate d levels is expected, while for the [Eu2+C2.2.2] complex (D3h +
(41) Sabbatini, N.; Dellonte, S.; Ciano, M.; Bonazzi, A.; Balzani, V., submitted for publication.
Sabbatini et al.
r
~
-
~
Figure 3. Schematic representation of the configuration coordinate diagram for Eu2' compounds. Only the lowest excited states deriving from the 4f' and 4P5d configurations are shown. For the sake of simplicity the vibrational levels have not been drawn. The figures refer to the specific energy situation of [Eu2+C2.2.1](see text).
symmetry) the d levels split into three subgroups. It should also be recalled that, while the intraconfigurational4f' excited states have the same equilibrium metal-ligand bond distance as the ground state, this is not the case for the states derived from the 4f65d configuration which are expected to exhibit shorter metal-ligand bond distancem A schematic picture of the configuration coordinate diagram for Eu2+compounds, showing the ground state and the lowest excited state of the 4f' and 4f65d configuration, is given in Figure 3. As we shall see later, such a diagram describes specifically the situation of the [Eu2+C2.2.1]complex. Absorption Spectra. According to the previous discussion, the broad absorption bands exhibited by both complexes (Figure 2) as well as by EU,? in the near-UV region can be assigned to 4P 4f65d transitions. A Gaussian analysis has shown that the absorption spectrum of [Eu2+C2.2.1] can be clearly resolved into five bands with A, at 254, 273, 293, 329, and 368 nm. This splitting is consistent with the expected C2, symmetry of the complex (see above). The lowest energy component of the Gaussian analysis (which is shown in Figure 2) has half-width 6 3600 cm-'. For [Eu2+C2.2.2]the results of the Gaussian analysis were less clear. A fairly good fit was obtained with three components (as expected for a D3hsymmetry) at 258,260, and 301 nm. The lowest energy component (Figure 2) showed a very high half-width value (6 8500 cm-'). Both the experimental absorption spectra and the Gaussian analysis show that the splitting among the d levels is smaller in [Eu2+C2.2.2],as could be expected because of the larger size of the [2.2.2] cavity ( r = 1.4 to be compared with r = 1.1 A for [2.2.1],34 and r = 1.09 A for Eu2+ (ref 37)). Closer inspection of the absorption spectra in the 3 10-320-nm region under higher resolution and higher sensitivity conditions than those used to record the spectra reported in Figure 2 showed that some weak and narrow bands are also present, as first reported by Butementg for Euaq2+and attributed to 8S 61transitions within the 4f' configuration.18 These weak narrow bands can be better observed for [Eu2+C2.2.2] than for [Eu2+C2.2.1]or Eu,?, presumably because in the first complex they lie on a relatively flat portion of the underlying 4f' 4f65d broad absorption (see Figure 1 of ref 8). Emission Properties. It is known that Eu2+ can give rise to two different types of luminescence emission, depending on the orbital nature of the lowest excited state: (i) a broad emission band associated with a 4f65d 4f7 transition and (ii) a sharp emission band due to intraconfigurational 4P transitions. 19,24-26,30 The wavelength of the former emission is strongly dependent on +
N
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The Journal of Physical Chemistry, Vol. 88, No. 8, 1984 1537
Photophysical Properties of Europium(I1) Cryptates the nature of the ligands because the energy of the lowest 4P5d level is dictated by nephelauxetic and ligand field effects, while the latter can only occur at -360 nm since the energy separation between the 6P7/2and %7/2 levels (28 200 cm-' in the free ion) is almost unaffected by the ligands.18-20When the lowest excited level of the 4f65d configuration is close to the 4f'(6P7i2)level, both types of emission can be observed.ze26~28~30*42 Another distinctive characteristic of the two types of luminescence is the "radiative" emission lifetime, which should be relatively long ( T O 1 ms) for the electric dipole forbidden 4f7(6P7/2) 4f'(*S7/2) transition 1 ~ s for ) the allowed 4P5d 4f' and much shorter (r0 transition.18-20 Both [Eu2+C2.2.11 and [Eu2+C2.2.2] exhibit a broad, short-lived luminescence emission in the visible region (Table I, Figure 2), typical of 4P5d 4f' transitions. When the emission bands of the two complexes were recorded at 77 K under the best resolution conditions allowed by our equipment, a weak fine structure consisting of numerous narrow bands separated by 100-200 cm-' could be observed. Such a fine structure is attributed to vibrational progressions, like those studied in detail by Ryan et al. for Eu2+ in C a S 0 4 and BaMg(SO& at 1.8 K.26 A very interesting result is that CPem = 1 for both complexes at 77 K in a MeOH-H20 matrix. Under such conditions the radiative lifetime coincides with the experimentally measured 4f' transitions, in the lifetime and is, as expected for 4F5d microsecond range. The emission lifetime is a little shorter in a pure water matrix a t 77 K (Table I), suggesting that water introduces radiationless deactivation and reaction channels which apparently are the predominant ones for Eu,? (CPem = 0.13 and CP, = 0.22, at 77 K).I3 The shielding effect of the cryptate cage toward water-induced radiationless deactivation processes or photochemical reactions is clearly shown by the presence of a not negligible luminescence emission for the two complexes in fluid solution at room temperature, while no emission can be observed for t h e . E u , F ion. Interestingly, the presence of at least two components in the emission decay for MeOH-H20 solutions at room temperature suggests that there are different species (presumably containing a different number of MeOH and H 2 0 molecules coordinated to E d + ) which do not fully equilibrate during their (relatively short) lifetimes. These different species are also likely to be present at 77 K. Under such conditions, however, the lifetime is almost exclusively determined by the radiative deactivation process whose rate constant is approximately the same for the species containing a different number of coordinated MeOH and H 2 0 molecules. For [Eu2+C2.2.1] the half-width of the emission band in aqueous solution at room temperature (6 3500 cm-') is quite
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-
-
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(42) Vestegen J. M. P. J.; Sommerdijk, J. L. J . Lumin. 1974, 9, 297.
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similar to the half-width of the lowest energy component of the 3600 cm-I). Gaussian resolution of the absorption spectrum (6 This indicates that for this complex the ground state and the lowest excited 4P5d state can be described by quite similar parabolic curves (Figure 3). The Stokes shift between the maxima of the corresponding absorption and emission bands is -5000 cm-I, which places the zero-zero transition between 4P(%,/z) and 4f65d at -24700 cm-' (Figure 3). For [EuZ+C2.2.2]the half-width of the emission band (-2700 cm-l) is smaller than that of the [2.2.1] complex. On the other hand, the half-width (-8500 cm-') of the lowest energy Gaussian component and the Stokes shift (- 11 500 cm-') are much larger than those found for [EuZ+C2.2.1].These results can be explained by (i) a smaller cohesion energy in the ground state, due to the larger size of the [2.2.2] cavity, (ii) a consequently larger b upon excitation, and (iii) a higher cohesion energy in the excited state compared with the ground state.
Conclusion The [Eu2+C2.2.1]and [Eu2+C2.2.2]cryptates are fairly stable Eu2+complexes which offer the opportunity to study the absorption and emission properties of the Eu2+ion in a well-defined molecular environment. Both complexes exhibit broad and relatively intense 4f' 4P5d absorption bands and, superimposed on them, some weak and narrow bands due to transitions within the 4f configuration. The minimum of the lowest 4f65d excited state lies below the lowest 4f' excited level, so that a broad d f emission band is observed which at 77 K exhibits some vibrational structure. The emission quantum yield is almost unity in a rigid matrix at 77 K, where the emission lifetime is of the order of microseconds, as expected for parity-allowed transitions. In contrast with the behavior of the Eu,? ion, both cryptates exhibit luminescence emission even in aqueous solution at room temperature, showing that encapsulation into the cryptands partly shields Eu2+ from photochemical and photophysical deactivation processes involving water. Specific spectroscopic properties, such as type and amount of ligand field splitting, half-width of absorption and emission bands, and Stokes shifts between absorption and emission maxima, are different for the two compounds since they reflect the different size and symmetry of the cage in which the Eu2+ion is contained. Further investigations are in progress in order to evaluate whether these or similar compounds could be useful as laser materials.
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Acknowledgment. We thank Professor J. M. Lehn for interesting discussions. Financial support from the Minister0 della Pubblica Istruzione and the National Research Council of Italy is gratefully acknowledged. Registry No. [EuZ+C2.2.1],73587-32-3; [Eu2+C2.2.2],73587-33-4.
