We report uniaxial stress measurements on the 2.242 eV photoinduced zero phonon line, observed on nickel- and nitrogen-containing synthetic diamonds after annealing at 1600 °C. We show that the line is an electric dipole transition, which occurs at a defect of rhombic I symmetry. The shape of the vibronic sideband and the temperature dependence of the line position and width can be understood assuming electron–phonon coupling to totally symmetric vibrational modes centred at ℏω=55 meV. Electron–lattice interaction cannot explain the large decrease of ZPL intensity with temperature, and it is suggested that some thermal activated de-excitation path is responsible for the observed behaviour.