A role of copper(II) ions in the enhancement of visible and near-infrared lanthanide(III) luminescence

Most of the existing optical methods for CuII detection rely on a “turn-off” approach using visible lanthanide(III) luminescence. In this work we present an innovative molecular systems where the podands bis(2-hydrazinocarbonylphenyl) ethers of ethylene glycol (L1) and diethylene glycol (L2) have been designed, synthesised and tested with an ultimate goal to create a "turn-on" lanthanide(III)-based molecular probe for the specific detection of CuII ions based on both visible (TbIII, EuIII) and near-infrared (NdIII, YbIII) emission. Quantum yields of the characteristic LnIII emission signals increases by at least two-orders of magnitude upon addition of CuII into water/acetonitrile (9/1) solutions of LnL (L=L1, L2) complexes. A detailed investigation of ligand-centred photophysical properties of water/acetonitrile (9/1) solutions of CuL, GdL and GdCuL complexes revealed that the presence of CuII ions does not significantly affect the energy positions of the singlet (32,260 cm−1) and triplet (25,640-25,970 cm−1) states, but partially or fully eliminates the singlet state quenching through an electron transfer mechanism. This effect increases the probability of intersystem crossing leading to enhanced triplet-to-singlet emission ratio and to longer triplet state lifetimes. The redox activity of hydrazine moieties and their ability to reduce CuII to CuI has been indicated by a qualitative assay with neocuproine. Finally, the probe demonstrates a good selectivity towards CuII over other transition metal ions: the addition of divalent ZnII, CdII, PdII, NiII, CoII or trivalent FeIII, GaIII, InIII ion salts into solutions of TbL either does not affect emission intensity or increases it to a maximum of 2-3 times, while, under similar experimental conditions, the presence of CuII results in a 20- to 30-times lanthanide luminescence enhancement. This new strategy results in a versatile and selective optical platform for the design of efficient “turn-on” sensors for CuII ions based on visible and near-infrared LnIII luminescence.