Self-assembled heterodinuclear europium(III)–lanthanide(III) chelates of 2,6-bis[N,N-bis(carboxymethyl)aminomethyl]-4-benzoylphenol and their radiative5D0→7Fjtransitions 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→7Fj transitions of EuIII and (ii) to elucidate the effects of different aqua-bridged non-radiative LnIII cations on these EuIII-specific radiative transitions, using the self-assembled heterodinuclear LEuIIILnIIIL chelates where the EuIII and LnIII 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 LnIII cation, the encapsulated aqua-bridged EuIIILnIII pair may be (i) as in the case of LnIII= ErIII, HoIII, NdIII or PrIII, a double-nuclear triplet relaxation centre where the T1-relaxation occurs mainly to the excited state of LnIII, (ii) as in the case of LnIII= GdIII or YIII, a single-nuclear triplet relaxation centre where the T1-relaxation is energetically capable of occurring only to the excited state 5Dj of EuIII, so that the resulting radiative 5D0→7Fj transitions of EuIII are to some extent enhanced and, finally, (iii) as in the case of LnIII= YbIII, a single-nuclear triplet relaxation centre where the T1-relaxation occurs only to the excited state of EuIII, so that the resulting excited state 5D0 of EuIII is strongly destabilized by the aqua-bridged YbIII: however, our present experimental results do not exclude the possibility of the aqua-bridged YbIII cation occurring also as the T1-relaxation centre despite the large energy gap ΔE(T1–2F7/2)= 11 560 cm–1.