Photoluminescence in Square-Planar Ni(II) Complexes: A Study of Electronic Structure and Quantum Dynamics

Although rare, photoluminescent Ni(II) complexes have wide-ranging applications, especially when they are accompanied by a spin-switch mechanism. This study explores the mechanisms behind fluorescence and its quenching in two similar Ni(II) square-planar complexes with a Schiff-base ligand attached to a phenazine fluorophore. The photorelaxation dynamics in these two complexes have been studied using the multiconfiguration time-dependent Hartree method on potential energy surfaces of 11 electronic states estimated from multiconfiguration wave function-based methods along 12 normal modes. In one complex, the triplet-state effects are marginal, allowing the singlet-state internal conversion dynamics to dominate and lead to fluorescence. In the other complex, where the methyl groups are replaced by electron-withdrawing CF₃ groups, the d-orbital splitting decreases the level of metal-ligand mixing, influencing the characters of the electronic states and the effects of the spin-orbit coupling. In the CF₃-substituted complex, the triplet states play a greater role, leading to the dominance of intersystem crossing over internal conversion in the excited-state dynamics, thereby suppressing fluorescence.