Electrochemiluminescence of a First-Row d⁶ Transition Metal Complex

We report the electrochemiluminescence (ECL) of a 3d⁶ Cr(0) complex ([Cr(L^Mes)₃]; λ_em=735 nm) with comparable photophysical properties to those of ECL-active complexes of 4d⁶ or 5d⁶ 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 3d⁶ metal complexes, we discuss some directions for future development.