Electrochemiluminescence of a First-Row d 6 Transition Metal Complex.

We report the electrochemiluminescence (ECL) of a 3d 6 Cr(0) complex ([Cr(L Mes ) 3 ]; λ em =735 nm) with comparable photophysical properties to those of ECL-active complexes of 4d 6 or 5d 6 precious metal ions. The electrochemical potentials of [Cr(L Mes ) 3 ] are more negative than those of [Ir(ppy) 3 ] and render the [Cr(L Mes ) 3 ]* excited state inaccessible through conventional co-reactant ECL with tri-n-propylamine or oxalate. ECL can be obtained, however, through the annihilation route in which potentials sufficient to oxidise and reduce the luminophore are alternately applied. When combined with [Ir(ppy) 3 ] (λ em =520 nm), the annihilation ECL of [Cr(L Mes ) 3 ] was greatly enhanced whereas that of [Ir(ppy) 3 ] was diminished. Under appropriate conditions, the relative intensities of the two spectrally distinct emissions can be controlled through the applied potentials. From this starting point for ECL with 3d 6 metal complexes, we discuss some directions for future development.