Ground State Nuclear Magnetic Resonance Chemical Shifts Predict Charge-Separated Excited State Lifetimes

Dichalcogenolene platinum(II) diimine complexes, (L_E,E')Pt(bpy), are characterized by charge-separated dichalcogenolene donor (L_E,E') → diimine acceptor (bpy) ligand-to-ligand charge transfer (LL'CT) excited states that lead to their interesting photophysics and potential use in solar energy conversion applications. Despite the intense interest in these complexes, the chalcogen dependence on the lifetime of the triplet LL'CT excited state remains unexplained. Three new (L_E,E')Pt(bpy) complexes with mixed chalcogen donors exhibit decay rates that are dominated by a spin-orbit mediated nonradiative pathway, the magnitude of which is proportional to the anisotropic covalency provided by the mixed-chalcogen donor ligand environment. This anisotropic covalency is dramatically revealed in the ¹³C NMR chemical shifts of the donor carbons that bear the chalcogens and is further probed by S K-edge XAS. Remarkably, the NMR chemical shift differences also correlate with the spin-orbit matrix element that connects the triplet excited state with the ground state. Consequently, triplet LL'CT excited state lifetimes are proportional to both functions, demonstrating that specific ground state NMR chemical shifts can be used to evaluate spin-orbit coupling contributions to excited state lifetimes.