Comparison of ruthenium(II) and cyclometalated iridium(III) azacrown ether phenanthroline hybrids for the detection of metal cations by electrochemiluminescence

Two azacrown ether appended iridium(III) complexes, 1 and 3, are described as probes for metal cations operating on oxidative-reduction electrochemiluminescence (ECL) with tri-n-propylamine as coreactant in acetonitrile. Complexes 1 and 3 exhibit a notable ECL sensing for Ba(2+) and Ag(+), respectively, through a 9-fold emission intensity enhancement and an unprecedented lambda(em)(max) red shift. In contrast, the structurally analogous azacrown ether appended ruthenium(II) complexes 2 and 4 only display insignificant ECL changes and no wavelength shift upon addition of the same metal ions. The key to understand this disparity is to appreciate that the azacrown ether phenanthroline ligand, responsible for metal uptake, represents the LUMO in iridium(III) complexes but is part of the HOMO for the ruthenium analogs. As a consequence, the electron transfer equilibria of the oxidative-reduction ECL process lead to a preferential formation of the metal free, excited ruthenium systems 2* and 4*. In contrast, the iridium complexes 1 and 3 remain coordinated to metal ions in the emitting state, as the oxidation involves the iridium phenylquinoline part, which is not invoked in cation binding.