Interactions of aqueous 7F and 5D1 europium(III) ions with bromide and inner-sphere coordination exciplex formation
The study of the europium(III) ion + water + bromide system shows totally outer-sphere interactions between the anion and the ground-state 7F octa-(E8) and nona-(E9) hydrated europium species. An appropriate analysis of these interactions, involving the corresponding (E8B) and (E9B) outer-sphere bromocomplexes or any other aqua-europium outer-space complexes, quantitatively rationalises the intriguing diminution of the europium-ion average hydration number, N, with dilution reported in the literature from Raman and 5D0 lifetime measurements. The [E8]/[E9] concentration ratio is calculated and is ca. 10. Values of N calculated for aqueous EuCl3 and EuBr3 solutions of various compositions indicate unambiguously a predominance of the octa-hydrated species. The projection of the solid-state coordination number of nine to aqueous solutions of middle-row lanthanide ions is unjustified. It is stressed that the symmetry-related number of components of the optical transitions is insufficient to establish the predominance of either the ennea- or octa-hydrated europium species in aqueous media, as previously claimed. Modifications of the 4ƒ and 5p radii of maximum density, accompanying the optical excitation to some (Smax– 1) state of europium(III), are shown to result in a value for the [*E8]/[*E9] ratio that is > 10. Optical absorption, steady-state and time-resolved emission measurements, stationary and transient photokinetics, as well as excited-state 5D1 energetics, show unambiguously the formation of an inner-sphere hepta-aquo-europium(III) bromoexciplex (*E7B), most probably occurring by a dissociative-interchange type of mechanism. Formation constants of *E8B and *E7B, the heat and entropy of the overall formation of *E7B, the radiative (5D1→7F) and non-radiative (5D1→5D0) probabilities of *E8 and *E7B, as well as the activation energies of their 5D1→5D0 relaxations, are determined. The 5D1→5D0 process is shown to occur by emission of one 1640 cm–1 phonon, corresponding to the H2O bending vibration, and the theoretically calculated non-radiative probabilities are in good agreement with the experimental values. Huang–Rhys numbers for the 5D1→5D0 transitions of *E8 and *E7B are evaluated and the dimensionless displacements, Δ, of the horizontal shift of the potentials are those expected from theory, if the degeneracy of the 1640 cm–1 mode is taken to be 8 and 7, respectively. The previously unclarified origin of the activation energy of the 5D1→5D0 process is identified as the point of minimum energy on the hypercurve of intersection of the two potential surfaces. The earlier claim, that the 5D1→5D0 process rate constant of the aquo-europium(III)ion is independent of isotopic substitution, is inexact. The experimental rate-constant ratio (H2O/D2O) is 2.2 and the calculated value is 2.7.