Electrochemiluminescence of a First‐Row d6 Transition Metal Complex

We report the electrochemiluminescence (ECL) of a 3d6 Cr(0) complex ([Cr(LMes)3]; λem=735 nm) with comparable photophysical properties to those of ECL‐active complexes of 4d6 or 5d6 precious metal ions. The electrochemical potentials of [Cr(LMes)3] are more negative than those of [Ir(ppy)3] and render the [Cr(LMes)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] (λem=520 nm), the annihilation ECL of [Cr(LMes)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. A chromium(0) complex was explored as an earth‐abundant 3d6 alternative to the precious metal complexes that dominate electrochemiluminescence (ECL) detection. A sufficiently reactive intermediate for chemi‐excitation was obtained by reduction of Ir(ppy)3, providing a multi‐colour ECL system in which the ratio of the two spectrally distinct emissions can be controlled through the applied potential.