Electron- and energy-transfer processes involving excited states of lanthanide complexes: evidence for inner-sphere and outer-sphere mechanisms

The interactions of the luminescent excited states of the aquo ions Eu3+ and Tb3+, their 2.2.1 cryptate complexes, and Eu3+ or Tb3+ cryptate complex - 2F- ion pairs, with M(CN)6z- complexes where M = Cr(III), Fe(II), Co(III), Ru(II), Os(II) was studied in aq. solns. by luminescence lifetime measurements. Depending on the specific lanthanide-species/CN-complex system, the lifetime of the luminescent lanthanide species was quenched (by an energy- or electron-transfer mechanism), unaffected, or enhanced. The enhancement was attributed to substitution of H2O mols. in the coordination sphere of the lanthanide ion by CN complexes that were unable to cause energy- or electron-transfer quenching. The lower values obtained for the quenching consts. of exergonic energy- and electron-transfer processes than those for diffusion attributed to poor electronic factors (nonadiabatic behavior). The results can be accounted for by making the reasonable assumption that the precursor complex has a different structure depending on the lanthanide species involved: (1) for the aquo ions, H2O mols. can be replaced by the CN complex to yield an intimate ion pair; (2) for the cryptate complexes, a CN-bridged ion pair can be formed, and (3) for the cryptate complex- -2F2/M(CN)6z- systems, only outer-sphere ion pairs can be involved in the quenching process. [on SciFinder(R)]