Coupled Potential Energy Surfaces Strongly Impact the Lowest-Energy Spin-Flip Transition in Six-Coordinate Nickel(II)...

Luminescent complexes of earth-abundant first-row transition metals are of renewed, broad interest due to their spectroscopic and photochemical properties as well as emerging applications. New strong-field polypyridine ligands have led to six-coordinate 3d³ chromium(III) complexes with intense spin-flip luminescence in solution at room temperature. The ground and emissive states both arise from the (t₂)³ electron configuration involving the d_π levels (O point group symmetry labels). Pseudoctahedral 3d⁸ nickel(II) complexes with such strong ligands are a priori also promising candidates for spin-flip luminescence. In contrast, the relevant electron configurations involve the d_σ orbitals and (e)² configurations. We have prepared the known nickel(II) complexes [Ni(terpy)₂]²⁺, [Ni(phen)₃]²⁺, and [Ni(ddpd)₂]²⁺ as well as the novel complexes [Ni(dgpy)₂]²⁺ and [Ni(tpe)₂]²⁺ forming a series with increasing ligand field strengths (terpy = 2,2':6',2″-terpyridine; phen = 1,10-phenanthroline; ddpd = N,N'-dimethyl-N,N'-dipyridine-2-ylpyridine-2,6-diamine; dgpy = 2,6-diguanidylpyridine; tpe = 1,1,1-tris(pyrid-2-yl)ethane). The lowest-energy singlet and triplet excited states of these nickel(II) complexes are analyzed based on absorption spectra using ligand field theory and CASSCF-NEVPT2 calculations for vertical transition energies and a model based on coupled potential energy surfaces, leading to calculated absorption spectra in good agreement with the experimental data. No photoluminescence signal was observed in the wavelength ranges identified through the analyses of the absorption spectra. The models provide insight into key differences between the nickel(II) complexes and their strongly luminescent chromium(III) analogues.