Interactions of aqueous 7F and 5D1 europium(III) ions with bromide and inner-sphere coordination exciplex formation

Abstract

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 (S_max– 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.