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Role of Triplet States of Two Different Ligands in the Sensitized Emission of LnIII (EuIII, TbIII) in d–f Hybrid Tetranuclear Heterometal (ZnII2LnIII2, CdII2LnIII2) Complexes

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Abstract

The crystal structures of synthesized d–f hybrid tetranuclear heterometal ZnII2EuIII2 and CdII2EuIII2 complexes, involving a pair of carboxylate ligands in a rare μ422 bridging mode, reveal that the metal centers are arranged alternately in a rhombic plane within 4 Å from each other. The ZnII2EuIII2 and ZnII2TbIII2 complexes exhibit efficient sensitization compared with the CdII-analogues. Under proper excitation, the lowest ππ* triplet states of both ligands N,N′-dimethyl-N,N′-bis(2-hydroxy-3,5-dimethylbenzyl)ethylenediamine (H2L) and hexafluoroacetylacetone (Hhfac) act as donors, implying synergistic energy transfer (ET). The ratio of the sensitized efficiency for the heterocomplexes in different media at 298 K and 77 K, based on the observed transients, reveal the role of ZnII and CdII in the ET process. The ratio of the ET rate constants obtained from the time-resolved phosphorescence of the complexes at 77 K and time-dependent DFT calculations on homocomplexes support this contention.

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