Self-assembled heterodinuclear europium(III)–lanthanide(III) chelates of 2,6-bis[N,N-bis(carboxymethyl)aminomethyl]-4-benzoylphenol and their radiative 5D0→7Fj transitions of EuIII

Abstract

A photochemical study has been conducted (i) to shed light on the intramolecular T₁-relaxation pathways of 2,6-bis[N,N-bis(carboxymethyl)aminomethyl]-4-benzoylphenol (L) to initiate the radiative ⁵D₀→⁷F_j transitions of Eu^III and (ii) to elucidate the effects of different aqua-bridged non-radiative Ln^III cations on these Eu^III-specific radiative transitions, using the self-assembled heterodinuclear LEu^IIILn^IIIL chelates where the Eu^III and Ln^III cations are coordinated to each other by the aqua bridge inside the chelating cage consisting of two deprotonated phenolic hydroxy groups and four partially deprotonated 2,2′-(methylenenitrilo)bis(acetic acid) moieties. Depending on its non-radiative Ln^III cation, the encapsulated aqua-bridged Eu^IIILn^III pair may be (i) as in the case of Ln^III= Er^III, Ho^III, Nd^III or Pr^III, a double-nuclear triplet relaxation centre where the T₁-relaxation occurs mainly to the excited state of Ln^III, (ii) as in the case of Ln^III= Gd^III or Y^III, a single-nuclear triplet relaxation centre where the T₁-relaxation is energetically capable of occurring only to the excited state ⁵D_j of Eu^III, so that the resulting radiative ⁵D₀→⁷F_j transitions of Eu^III are to some extent enhanced and, finally, (iii) as in the case of Ln^III= Yb^III, a single-nuclear triplet relaxation centre where the T₁-relaxation occurs only to the excited state of Eu^III, so that the resulting excited state ⁵D₀ of Eu^III is strongly destabilized by the aqua-bridged Yb^III: however, our present experimental results do not exclude the possibility of the aqua-bridged Yb^III cation occurring also as the T₁-relaxation centre despite the large energy gap ΔE(T₁–²F_7/2)= 11 560 cm^–1.