Multiple Anti-Kasha Emissions in Transition Metal Complexes

In this manuscript we present a from-first-principlesevidence that several higher-lying excited states are responsible for the emission spectrum of [M(CO)₄(bpy)] (M = Cr, Mo, W and bpy=2,2’-bipyrimidine) complexes. These results highlight the violation of Kasha’s rule, which states that after irradiation molecules emit light with appreciable yield only from its lowest energy excited state. Furthermore, in [W(CO)₄(bpy)] and [Mo(CO)₄(bpy)], the breaking of Kasha’s rule is two-fold, because at least two different excited states besides T₁are involved in emission. To our knowledge, these are the first transition metal complexes unambiguously demonstrated to display simultaneous equilibrated and non-equilibrated anti-Kasha emissions. This work also highlights the complexity of the emissive processes of tetracarbonyl-diimine transition metal complexes, which are controlledviaa subtle interplay of electronic and geometrical effects along the excited state deactivation dynamics.