Bright Insights into Luminescent Nickel(II): Exploring Ligand Rigidity and Ultrafast Deactivation Pathways via Theory and Experiment

In this work, we elucidate the origin of the unique luminescence of a square-planar 3d nickel(II) complex, [Ni(L−CH3)], and explain its quenching in the differently substituted [Ni(L−CF3)] through a combination of high-level multiconfigurational wave function methods and transient absorption spectroscopy. The strong agreement between computed activation barriers and experimental kinetic data validates the predicted deactivation mechanisms. In addition, the new theoretical results prompted a re-evaluation of experimental UV–vis absorption spectra published earlier by some of us. The updated analysis reveals a previously overlooked, concentration-dependent oligomerization of [Ni(L−CH3)], which accounts for the complexity of the visible absorption envelope. We uncover a delicate energetic balance between competing relaxation pathways that renders non-radiative deactivating electronic states inaccessible for the −CH3 complex, yet accessible in its −CF3 analog. In addition to common metal-centered triplet and singlet states, we have identified a class of excited states that are crucial for the differing photoactivity of the two species. These states arise from antiferromagnetic coupling of two local triplet states, metal and ligand, into an overall singlet. They are (a) not described by conventional TD-DFT approaches, due to their inherently multiconfigurational character, and (b) not widely recognized within the inorganic photochemistry community.