Effects of europium spectral probe interchange in Ln-dyads with cyclen and phen moieties

Abstract

Herein, we have investigated spectral structure and intensity changes in a bimetallic lanthanide complex comprising La³⁺ and Eu³⁺, with the ions coordinated to silent and antenna ligands, when their positions are interchanged. Comparison of the fluorescence decay of a ligand in the presence and absence of La³⁺ has enabled internal nonradiative decay rates to be determined. The effects upon Eu³⁺ emission spectra resulting from changes in its environment at a distance of ∼10 Å, and upon changing from the solid state to solution, were also investigated. Conclusive results to these investigations were achieved from the electronic excitation spectra, emission spectra and emission decay measurements of cyc-phen, cycLn¹-phLn², cycLn-phen and phLn (Ln = La, Eu; cyc = substituted 1,4,7,10-tetrazacyclododecane; phen = 1,10-phenanthroline; ph = phen(pdtc)₃, pdtc = pyrrolidine-1-carbodithioate) recorded in the solid state, at 298 K and ∼10 K, and in solution. Ligand fluorescence was observed in all cases at room temperature, and phosphorescence was observed at 77 K, except for cyc-phen. The phosphorescence lifetimes of the La³⁺ complexes extend up to 180 ms. Our results support the concept that the lowest excited states of the complexes are localized on individual ligands, in the present case phen, rather than delocalized over the entire molecule.