Sensitised lanthanide luminescence using a RuII polypyridyl functionalised dipicolinic acid chelate

Abstract

A visible light absorbing [Ru^II(tpy)₂]²⁺-type chromophore appended with a dipicolinic acid Ln^III chelator has been prepared and complexed with several differing lanthanide cations to form the corresponding heterobimetallic d–f assemblies. The subseqent solution speciation analysed by ¹H NMR spectroscopy revealed an unexpected decrease in the Ln^III chelate complex stability, in particular for the 1 : 3 complex, when compared to the parent dipicolinic acid. As a result, the desired Ln(ML)₃ complexes could not be isolated, and the 1 : 1 Ln^III–ML complexes were instead characterised and investigated using steady state absorption and emission spectroscopy. Sensitised NIR emission from the Yb^III, Nd^III and Er^III complexes was observed upon ¹MLCT excitation of the Ru^II based metalloligand in the visible region at ca. 485 nm. Investigations using transient absorption spectroscopy revealed essentially quantitative intersystem crossing to form the ³MLCT excited state, as expected, which then acts as the energy donor for the metalloligand based antennae effect, facilitating sensitisation efficiencies of 4.8, 17.0 and 37.4% respectively for the Yb^III, Er^III and Nd^III cations.