Changes in the Excited State Relaxation Pathway of Ruthenium(II) Bipyridine Complexes with Substituted Imidazo[4,5-f]-1,10-phenanthroline Ligands upon Additional Coordination of the Second Metal Cation

A new type of mixed-ligand heterobimetallic ruthenium(II) complexes has been reported. At the first stage, monoruthenium(II) complexes with ruthenium(II) cation coordinated to the N,N fragment of imidazo[4,5-f]-1,10-phenanthroline ligand containing bridging thiophene fragment and free coordination center of pyridine have been obtained. This coordination center strongly binds to the second metal cation of the studied series (Fe³⁺, Fe²⁺, Zn²⁺, or Pb²⁺) to form heterobimetallic complex, which is confirmed by optical, luminescent, and NMR titration and quantum chemical computations. The coordination of the second cation leads to the considerable decrease of LUMO energy as compared with the initial monoruthenium complex which results in phosphorescence quenching caused by the relaxation of excited ³MLCT state typical for monoruthenium complexes. It has been shown additionally that the presence of bridging thiophene fragment increases the nucleophilicity of pyridine nitrogen atom and enables it to coordinate the second cation. In the absence of the thiophene ring, the ligand cannot coordinate doubly charged cations.