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Abstract:
Two coordination complexes EuII2EuIII4(OH)4(bza)4L2 (1) and EuIII5(H2O)(OH)4(NO3)3(bza)4L (2) have been assembled with the tetrasubstituted cyclen ligand N,N′,N′′,N′′′-tetrakis(2-hydroxy-3-methoxy-5-methylbenzyl)-1,4,7,10-tetraazacyclododecane (H4L) and the auxiliary ligand benzoic acid (Hbza). Complex 2 is a pentanuclear EuIII cluster whereas complex 1 is a huge heterovalent 2EuII/4EuIII cluster in which two divalent europium ions are trapped within the macrocyclic cavities of the designed ligands L and are further isolated from the environment by peripheral phenyl rings and the tetrahedral 4EuIII cluster, which results in the enhanced oxidative stability of Eu2+. The luminescence of complex 1 was investigated and is discussed in comparison with the luminescence of ligand H4L and complex 2. Owing to the protection of Eu2+ by the macrocyclic ligands L, the heterovalent complex exhibits a strong broad emission band with a maximum at 533 nm as well as the characteristic sharp emissions of Eu3+. The broad emission is redshifted compared with that of ligand H4L and has been attributed to the Laporte-allowed 4f65d1 → 4f7 transition of Eu2+.
