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Luminescence Probes The Eu3- and Tb3-Cryptates of Polypyridine Macrobicyclic Ligands.

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Luminescence Probes: The E u ~and
~ Tb30-Cryptates
of Polypyridine Macrobicyclic Ligands
Table 1. Absorption and emission data for Eu'" and Tb'" aquo ions and
cryptates 1 and 2 in aqueous solution.
By Beatrice Alpha, Vincenzo Balzani,*
Jean-Marie Lehn,* Siglinda Perathoner, and
Nanda Sabbatini
Complex
There is currently a growing interest in the luminescence
properties of lanthanoid complexes because of their importance as probe ion
especially for biological
applications. Particularly promising are the cryptate comp l e x e ~ 'where
~]
the luminescent lanthanoid ion is encapsulated into the ligand structure.18-'21In this way, one may
reach some important goals: (1) inertness of the complex;[8,121 (2) protection towards deactivating interactions
between the luminescent lanthanoid ion and solvent (water) ;[9-1 1I (3) efficient energy transfer from the (strongly absorbing) ligand chromophore to the luminescent metal
13] These three features are combined in the lanthanoid cryptates['z.'41of macrobicyclic ligands containing the
2,2'-bipyridine (bpy) or 1,lO-phenanthroline chromophoric
1
We have carried out a systematic investigation on the
photophysical properties of the Eu3@and Tb3@complexes
1 and 2[Iz1and we have found that these species are efficient molecular devices for the conversion of the UV light
absorbed by the ligands into visible luminescence of the
lanthanoid ions. In view of their interest, in particular as
luminescent labels in aqueous solution, we briefly report
here some results of our measurements.
Absorption and emission data for I and 2 in aqueous
solution are shown in Table 1 where data for the corresponding aquo ions['6,171
are also reported for comparison
purposes. The cryptate complexes exhibit strong ligandcentered (LC) absorption bands in the near UV region,
104-105 times more intense than the metal-centered (MC)
f-f bands characteristic of the lanthanoid ions. The wavelength of the luminescence emission originating from the
5Doand 'D4 levels of Eu3@and Tb3@,respectively,''] is almost unaffected by the coordinating environment (water or
cryptand ligands), whereas the excited-state lifetimes and
the luminescence quantum yields are strongly modified
(Table 1). From the lifetime values in H,O or D,O solution
it is possible to e v a l ~ a t e ~the
~ . ' average
~
number of coordinated water molecules (estimated error, t-0.5): 1, 2.5;
[*] Prof. Dr. V. Balzani, Dr. S . Perathoner, Dr. N. Sabbatini
Departimento di Chimica "G. Ciamician" deIl'Universita
and lstituto FRAE-CNR
1-20126 Bologna (Italy)
Prof. Dr. J.-M. Lehn, B. Alpha
lnstitut Le Bel, Universite Louis Pasteur
4, rue Blaise Pascal, F-67000 Strasbourg (France)
[**I We wish to thank V. Cacciari and G. Gubellini for technical assistance.
This work was supported by the Consiglio Nazionale delle Ricerche,
Minister0 della Pubblica Istruzione (Italy), and by ORIS-lndustrie
(France).
1266
0 VCH Verlagsgesellschaji mbH. 0.6940 Weinheim, 1987
Eu;:
2
Tb:?
Absorption [a]
a,
E,,,
[nm]
[M-'
Emission [b]
cm-']
304 (LC) 25000
393 (f-9
3
304 (LC) 29000
308 (f-9
0.3
,a,,
TL~Zrbup," >:T
[nm]
[msl [ms] [ms]
615 [el
616 [el
542 [fl
543 [fl
0.34 1.7
1.7
0.11 3.2
3.3
0.33 0.43 3.8
0.40 3.8
3.8
(D
[CI
77 Id1
0.02
0.1
0.006
0.03
0.3
0.08 [g] -
[a] Excitation wavelength; LC, ligand-centered band; f-f, metal-centered
band. [b] Experimental uncertainties: lifetimes, < 10%; quantum yields, ca.
30%. [c] H20 solution, room temperature. Id] Energy-transfer efficiency,
given as the ratio between the emission quantum yield for LC excitation and
excitation into the 'Do (for Eu3") or 'D4 (for Tb3")emitting levels. The latter
quantity has been evaluated from the ratio between the emission lifetime in
H 2 0at 300 K and the emission lifetime in D 2 0at 77 K, taken as the radiative
lifetime. [el Assigned to the 'Do+'F2 transition (most intense emission
band). [fJ Assigned to the 5D4-+7F5transition (most intense emission band).
[g] Excitation in the emitting 'D4 level [17].
Eu,:
9.6; 2, 3.0; Tb&@',9.0. These figures show the capability of the cryptand ligand to shield the metal ion from
the solvent molecules.
In H 2 0 solution at room temperature, the quantum
yields of luminescence emission on excitation in the highintensity ligand-centered band at ca. 300 nm are 0.02 for 1
and 0.03 for 2. The reason for the lower than unity luminescence quantum yield is the competition between intraligand radiationless decay of 'LC to the ground state
and intramolecular 'LC-+(MC)'Do (for Eu3@) or
'LC-+(MC)5D4(for Tb3@)energy transfer (for details see
Ref. [18]).
In conclusion, because of their very intense absorption
bands, which collect incident light, and because of their
reasonably high ligand-to-metal energy transfer and luminescence efficiencies, the cryptates 1 and 2 are excellent
molecular devices to play the role of luminescence probes
and luminescence concentrators (antenna effect). Even in
very diluted aqueous solutions (ca.
M), they are able
to convert about 1% of the incident UV photons into emitted visible photons, a quite interesting result for several applications, in particular for labeling of biological materials.
Higher conversion efficiency may be attained if structural
modifications are found which increase both the shielding
of the enclosed ion and the yield of intramolecular energy
transfer.
Received: July 27, 1987 [Z 2376 IE]
German version: Angew. Chem. 99 (1987) 1310
[I] F. S . Richardson, Chem. Reu. 82 (1982) 541.
[2] W. D. Horrocks, Jr., D. R. Sudnick, Ace. Chem. Res. 14 (1981) 384.
[3] W. D. Horrocks, Jr., M. AIbin in S . J. Lippard (Ed.): D o g . Inorg. Chem.
31 (1984) 1.
141 J.-C. G. Biinzli, D. Wessner, Coord. Chem. Rev. 60 (1984) 191.
151 V. Balzani, N. Sabbatini, F. Scandola, Chem. Reu. 86 (1986) 319.
161 J.-M. Lehn in V. Balzani (Ed.): Supramolecular Phutochemis/ry. Reidel,
Dordrecht, The Netherlands, 1987, p. 29.
171 J.-M. Lehn, Acc. Chem. Res. 11 (1978) 49.
[S] E. L. Yee, 0. A. Gansow, M. J. Weaver, J . Am. Chem. SOC.302 (1980)
2278.
[9] N. Sabbatini, S. Dellonte, M. Ciano, A. Bonazzi, V. Balzani, Chem. Phys.
Lett. 107 (1984) 212.
[lo] G. Blasse, M. Bujs, N. Sabbatini, Chem. Phys. Lett. 124 (1986) 538.
1111 N. Sabbatini, S. Dellonte, G. Blasse, Chem. Phys. Lett. 129 (1986) 541.
[I21 B. Alpha, J.-M. Lehn, G. Mathis, Angew. Chem. 99 (1987) 269; Angew.
Chem. In?. Ed. Engl. 26 (1987) 266.
1131 M. J. Weber in K. A. Gschneidner, L. Eyring (Eds.): Handbook on the
Physics and Chemistry of Rare Earths. Vol. 4, North-Holland, Amsterdam 1979, p. 275.
0570-0833/87/1212-1266 $ 02.50/0
Angew. Chem. Int. Ed. Engl. 26 (1987) No. I2
1141 The structure of these complexes has been confirmed by the determination of the crystal structure of the La’@ cryptate of the [bpy.bpy.bpyl
macrobicyclic ligand: J. Guilhem, C. Pascard, unpublished results.
[I51 J.-C. Rodriguez-Ubis, B. Alpha, D. Plancherel, J.-M. Lehn, Helu. Chim.
Acta 67 (1984) 2264.
1161 Y. Haas, G. Stein, J . Phys. Chem. 75 (1971) 3668.
[I71 G. Stein, E. Wurzberg, J . Chem. Phys. 62 (1975) 208.
[I81 N. Sabbatini, S. Perathoner, V. Balzani, B. Alpha, J.-M. Lehn in V. Balzani (Ed.): Supramolecular Photochemistry. Reidel, Dordrecht, The Netherlands, 1987, p. 187, and unpublished results.
as multiconfigurational (MC-SCF) treatment with the latter method,[71no minimum corresponding to a “classical”
structure of 2 could be located. Like 3, a classical geometry of 2 would be expected to have nearly equal SilSi2 and
SilSi3 bond lengths. Instead, when these distances were
set to 2.3 A, optimization led automatically to extension of
the central bond (see equilibrium values in Table 1). In orTable 1. The geometry of 2 at various theoretical levels.
Pentasilall.l.l]propellane. Predictions Concerning
Structure, Bonding, and Strain Energy**
By Paul von Ragut Schleyer* and Rudolf Janoschek*
Because of its inverted carbons, high ring strain, the nature of the bonding between the bridgehead atoms, and,
most certainly, its availability through synthesis, [l.l.ljpropellane, 1, is an exceptionally intriguing species.[’-41
Whether or not there is a central bond in 1, or, if so, to
what extent, is being hotly debated.[’-4J Similar questions
pertain to pentasila[l.l. 11propellane (tricyclo[l. l.l.O]pentasilane, 2), the silicon analogue of 1. Small-ring silicon sysH H
H H
2
1
tems are providing instructive comparisons and contrasts
with their carbocyclic analogues. In particular, we have
predicted recently that two “bond stretch” isomers of bicyclo[l.l.O]tetrasilane, 3 and 4, should be capable of exist e n ~ e . ” . ~One
]
of these, 3, with an essentially classical
structure, has an experimental precedent,[6b1but derivatives
of the more stable 4, which can be regarded as a singlet
Method
d(SilSi2) [A]
d(SilSi3) [A]
SCF-21G* (spd) [a]
CEP-31G* [a]
MC-SCF (spd) [a]
3-21G
3-21G*
6-31G*
2.325
2.347 [b]
2.330
2.443 [c]
2.331 [d]
2.347
2.686
2.729 [b]
2.735
2.885 [c]
2.692 [d]
2.719
Ref.
[31
I101
[a] Pseudopotential method employed. [b] For comparison, the CEP-31G*
values for 4 are 2.362 and 2.915 [3b]. [c] For comparison, the 3-21G values
for 4 are 2.406 apd 2.972 A. [d] For comparison, the 3-21G* values for 4 are
2.305 and 2.731 A [5].
A
der to rule out the possibility of a second minimum with a
shorter Si-Si bridge distance, a series of two-configuration
MC-SCF (spd) calculations were carried out with various
fixed SilSi3 distances, all other parameters being allowed
to vary within the restriction of D3,, symmetry. As the results in Table 2 indicate, there was no second minimum.
Thus, particularly with regard to the length of the central
bond, the structure of 2 is similar to that of 4, but not to 1
or to 3. This conclusion is supported by the analysis of the
two-configuration wave function:
1yMc.2
= 0.97 Y~(o’) - 0.25
I~.,(u**)
Table 2. Search for a second minimum of 2, MC-SCF (spd).
~ ( s ~ I s ~[A]
z)
d(SilSi3) [A]
2.330
2.309 (opt.)
2.305 (opt.)
2.305 (opt.)
2.735
2.600 (fixed)
2.500 (fixed)
2.400 (fixed)
E,,, [kcal mol - ‘1
0.0 [a]
1.7
5.4
11.9
[a] From Table 1
3
4
diradical requiring a multiconfigurational description, are
not yet known. As we mentioned,15]similar structural possibilities exist for 2. We now describe the nature of this
species.
At all ab initio levels investigated, which include singledeterminant optimizations with the 3-21G (sp), 3-21G*
(spd), and pseudopotential-21G* (spd) basis sets, as well
This wave function indicates substantial singlet diradical
character.[’] Hence, it would be misleading to represent 2
by drawing a line between the bridgehead atoms. The 321G atom-atom overlap-weighted natural atomic orbital
bond order[’] for the central SilSi3 bond is only 0.176 vs.
0.677 for the SilSi2 bonds. These are almost identical with
the values for 4 (SilSi3=0.173, SilSi2=0.693) but contrast with those for 3 (0.600 and 0.765, respectively). The
corresponding results for bicyclo[l.l.l]pentasilane, 5 , are
0.013 for SilSi3 and 0.762 for SilSi2.
As with 1, the formally bonding o-MO involving the
central bond (HOMO) is higher-lying (but only by 0.02
[*] Prof. Dr. P. von R. Schleyer
lnstitut fur Organische Chemie der Universitat Erlangen-Niirnberg
Henkestrasse 42, D-8520 Erlangen (FRG)
Prof. Dr. R. Janoschek
lnstitut fur Theoretische Chemie der Universitat
Mozartgasse 14, A-8010 Graz (Austria)
[**I This work was supported by the Deutsche Forschungsgemeinschaft and
the Fonds der Chemischen Industrie. We thank Professor L. C. Allen
and Professor S . Nagase for preprints of their related papers (Ref. [3b]
and [lo], respectively).
Angew. Chem. In(. Ed. Engl. 26 (1987) No. 12
0 VCH Verlagsgesellschaft mbH. 0-6940 Weinheim. 1987
5
0570-0833/87/1212- 1267 S OZ.JO/O
1267
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luminescence, cryptates, eu3, tb3, polypyridine, macrobicyclic, probes, ligand
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