Photophysical and Photochemical Properties of Europium Cryptates

Abstract

Several diazapolyoxabicyclic ligands (‘cryptands’) [1, 2] are able to encapsulate metal ions to form coordination compounds (‘cryptates’ which have been the object extensive thermodynamic, kinetic, structural, electrochemical, and analytical investigations [2–12]. By contrast, only a few studies [12–14] have been reported on the electronic absorption and emission spectra of these compounds because both the cryptands and, most cases, the encapsulated metal ions are spectroscopically ‘mute’ species. The spectroscopic behavior of rare earth complexes is a topic of great interest, both theoretical [15–17] and applicative [18–20]. We have thus begun spectroscopic studies on cryptates containing europium or the rare earth ions. The spectroscopic and photophysical properties of type complexes between En 2+ and the 2.2.1 and 2.2.2 cryptands are reported and compared to those of the Eu 2+ aquo ion. Both complexes show broad relatively intense absorption bands in the near u.v. region due to 4f 7 → 4F 65d transitions. Some weak narrow bands due to transitions within the 4f 7 configuration also appear in the 310–320 region. Both complexes exhibit a strong blue luminescence from 4f 6. At 77 K 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 τ of the order of a few nanoseconds, and ø: of the order of 10 −3, in contrast with the behavior of the Eu 2+ aq ion which does not exhibit any luminescence emission under such conditions. The results obtained are discussed in the light of the interaction between Eu 2+ and water molecules and of the size and symmetry of the cryptand cage. The spectroscopic and photophysical properties of the Eu 3+ complex of the 2.2.1 cryptand are investigated in aqueous solution. The absorption spectrum of the complex, besides the f → f transition of the Eu 3+ ion, shows two broad bands 298 and 248 nm (ϵ, 111 and 93, respectively) which are assigned as charge transfer transition from N and, respectively, O atoms of the ligands to Eu 3+. High resolution emission spectra show that in aqueous solution there is only one Eu-containing species with C 2v symmetry. The emission quantum yield is 3 × 10 −2 upon excitation at 393 nm in the 5L 6 metal centered band and 3 × 10 −3 and 1 × 10 −3 upon excitation in the charge transfer bands at 350 nm and 260 nm, showing that the conversion of the charge transfer levels to the 5D o emitting state is relatively inefficient. Luminescence decay measurements in H 2O and D 2O solutions and comparison with the data obtained for Eu 3+ aq show that encapsulation of Eu 3+ in the cryptand cage does not shield the metal ion towards interaction with solvent since three water molecules are still coordinated to Eu 3+ through the cryptand holes. The emission of [Eu + C 2.2.1] is quenched by Fe(CN) 4− 6, Ru(CN 4− 6, Mo(CN) 4− 8 with K q = 6.4 × 10 8, 1.9 × 10 8, 12 × 10 9 l mol −1 sec −1, respectively. The quenching takes place via charge-transfer interaction, as shown by the appearance of a new absorption band in the visible region for solutions containing the cryptate and the quencher. Spectrophotometric and electrochemical analysis show that 1:1 complexes are formed with stability constants of the order od 10 2 1 mol −1.