N. Sabbatini

ANTENNA EFFECT IN LUMINESCENT LANTHANIDE CRYPTATES: A PHOTOPHYSICAL STUDY

Excited state emission and absorption decay measurements have been made on the cage‐type cryptate complexes [M bpy.bpy.bpy]n+, where Mn+= Na+, La3+, Eu3+, Gd3+ or Tb3+ and [bpy.bpy.bpy] is a tris‐bipyridine macrobicyclic cryptand. Excitation has been performed in the high intensity 1π‐π* cryptand band with maximum at about 300 nm. Experiments have been carried out in H2O or D2O solutions and at 300 and 77 K to evaluate the rate constants of radiative and nonradiative decay processes. For Mn+= Na+, La3+ and Gd3+ the lowest excited state of the cryptate is a 3ππ* level of the cryptand which decays in the microsecond time scale at room temperature in H2O solution and in the second‐millisecond time scale at 77 K in MeOH‐EtOH. For Mn+= Eu3+, the lowest excited state is the luminescent 5D0 Eu3+ level which in H2O solution is populated with 10% efficiency and decays to the ground state with rate constants 2.9 × 103 s_1 at room temperature and 1.2 × 103 s−′ at 77 K. The relatively low efficiency of 5D0 population upon 1ππ* excitation is attributed to the presence of a ligand‐to‐metal charge transfer level through which 1ππ* decays directly to the ground state. For Mn+= Tb3+ the lowest excited state is the luminescent 5D4 Tb3+ level. The process of 5D4 population upon 1ππ* excitation is ˜100% efficient, but at room temperature it is followed by a high‐efficiency, activated back energy transfer from the 5D4 Tb3+ level to the 3ππ* ligand level because of the relatively small energy gap between the two levels (1200 cm_1) and the intrinsically long lifetime of 5D4. At 77 K back energy transfer cannot take place and the 5D4 Tb3* level deactivates to the ground state with rate constant 5.9 × 102 s‐′ (H2O solution). The relevance of these results toward the optimization of Eu3+ and Tb3+ cryptates as luminescent probes is discussed.

Lanthanide complexes of encapsulating ligands: Luminescent devices at the molecular level

Lanthanide complexes of encapsulating ligands are studied as luminescent devices at the molecular level. The photophysical properties of the complexes which, up till now, showed the most intense luminescencepe reported. The luminescence intensity of these complexes is discussed,considering the efficiency of incident light-emitted light conversion, defined as the product of the absorption efficiency and the luminescence quantum yield. It is illustrated,how the metal luminescence intensity can be enhanced by adapting the ligands on basis of previously obtained experimental results. The possible application of these complexes in fluoroimmunoassays is examined.

Spectroscopic and photophysical properties of the Europium (III) Cryptate [Eu3+ ⊂ 2.2.1]

Abstract The spectroscope and photophysical properties of [Eu 3+ ⊂ 2.2.1] have been investigated. High-resolution emission spectra in aqueous solution are compatible with the presence of only one Eu-containing species with C 2v symmetry. The emission quantum yields indicate that the conversion of the charge-transfer levels to the 5 D 0 emitting state is relatively inefficient. Luminescence decay measurements show that encapsulation of Eu 3+ in the cryptand cage does not completely shield the metal ion from interaction with the solvent, since three water molecules are still coordinated to Eu 3+ through the cryptand holes.

The luminescence of the rare earth cryptates [Tb ⊂ 2.2.1]3+ and [Sm ⊂ 2.2.1]3+

Abstract The spectroscopic and photophysical properties of [Tb ⊂ 2.2.1] 3+ and [Sm ⊂ 2.2.1] 3+ in aqueous solution and the solid state are reported. The role played by multiphonon emission and non-radiative deactivatfon via the excited configuration state in determining the observed luminescence properties is examined.

Photophysics of Ce3+ cryptates

The species [Ce 3+ C 2.2.11 and [Ce”’ C 2.2.21 show efficient ultraviolet luminescence at room temperature. The spectra have been analysed and are discussed. The Stokes shift of the emission is smaller in the solid state than in solution. This is discussed in connection with the configuration of the species. In solid [Ce3+ C 2.2.1]C13*2Ha0 energy transfer between the cryptate species leads to a diffusion length of about 60 A. A comparison with the related Eu2+ cryptates is made. The crystal-field splitting of the excited 5d level is derived and compared for these cryptates and some crown-ether complexes.

Lanthanide luminescence in supramolecular species

Abstract Luminescence spectra and decays as well as transient absorption spectra and decays are reported for the following cage-type complexes: [Ln⊂bpy·bpy·bpy] 3+ , where Ln = Eu 3+ , Gd 3+ and Tb 3+ and [bpy·bpy·bpy] is a tris-bipyridine tris-bipyridine macrobicyclic cryptand; [Eu ⊂ L 6 ] 3+ , where L 6 is a bpy branched-macrocyclic ligand; and [Eu⊂ 1 ] 3+ , where Ln = Eu 3+ , Gd 3+ and Tb 3+ and 1 is a p -t-butylcalix[4]arenetetramide ligand. The relevance of these results towards the optimization of Eu 3+ and Tb 3+ complexes as labels for time-resolved fluoroimmunoassay is discussed.

Absorption and emission properties of a europium(II) cryptate in aqueous solution

Abstract The absorption spectrum of the complex between Eu 2+ and the [2.2.2] cryptand shows narrow, weak transitions within 4f 7 superimposed on broad, strong transitions from 4f 7 to 4f 6 5d. Emission from 4f 6 5d to 4f 7 can be observed at 77 K λ max = 420 nm, τ = 0.55 μs) and at 293 K (λ max = 460 nm, τ = 3 ns) in aqueous solution.

Luminescence of the europium(III) cryptate [Eu3+⊂2.2.1] in the solid state

Abstract The luminescence properties of solid [Eu 3+ ⊂ 2.2.1]Cl 3 ·2H 2 O are reported. Measurements were performed down to liquid-helium temperature. There are some striking differences between the present results and those reported earlier for the same cryptate in aqueous solution. These differences are discussed.

2,2'-bipyridine lariat calixcrowns: a new class of encapsulating ligands forming highly luminescent Eu3+ and Tb3+ complexes

A new class of calix[4]arene crown ethers with one or two bipyridines appended to the polyether ring (lariat calixcrowns) have been designed and synthesized; the luminescence properties of their Eu3+ and Tb3+ complexes have been studied in acetonitrile. In this solvent, long lifetimes for the metal emitting states and high metal-luminescence intensities obtained upon ligand excitation have been observed in both Eu3+ and Tb3+ complexes. The association constants in methanol have been determined for some of the complexes studied.

The solid state luminescence of the encapsulation complex of Eu3+ in p-t-butyl-calix[4]arene tetra-amide

Abstract The luminescence of solid [Eu⊂ l ] 3+ ( l = p -t- butyl-calix [4] arene tetra-amide) has been investigated at 4.2 K. The Eu 3+ -ligand charge-transfer state has been observed in the excitation spectrum. Its role in non-radiative transitions is discussed.