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



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.