The Eu3+ complexes of the tripode and tetrapode ligands 1 and 2, respectively, containing 2,2'-bipyridine coordinating units have been prepared. The UV absorption and luminescence spectra, lifetimes, and quantum yields have been measured under a variety of experimental conditions. The contributions of different paths to the decay of the luminescent excited state are evaluated, and the structures of the complexes are discussed on the hasis of spectroscopic and photophysical data.
In the last few years, there has been a growing interest in the luminescence properties of Eu3+ species because of their potential use as probes and labels for a variety of chemical and biological applications [l-51. To make use of the excellent emitting properties of the ELI'ion, one has to remedy for the lack of intense metal-centered absorption bands. Current research [6][7][8][9][10][11][12][13][14] is, therefore, oriented towards the use of ligands that exhibit intense absorption bands in the near-UV spectral region and have their lowest excited state sufficiently high in energy to be able to transfer excitation to the luminescence 'Do Eu'+ level. The 2,2'-bipyridine (bpy) chromophoric unit satisfies this requirements, since it exhibits an intense 'zn * absorption band around 280 nm and has its lowest excited state ('zz*) at ca. 23000 cm-' (both the absorption band and the lowest excited state are somewhat displaced to lower energies upon metal coordination) [ 101 [ 151. Eu3+, however, exhibits a stronger coordinating ability for H,O than for bpy so that in aqueous solution, Eu'+ complexes of the simple bpy ligand are not stable. Preorganized multidentate ligands made up by two ore more bpy units (e.g., the bpy.bpy.bpy cryptand [6] [16]) are, therefore, used to obtain kinetically inert complexes which exhibit good luminescent properties.
Continuing our studies in this field, we have prepared the Eu3+ complexes of the tripode and tetrapode ligands 1 and 2 (which contain three and four bpy units, respectively), and we have studied their luminescence behavior.