Elucidating photophysical attributes of alpha,alpha'-diimine ligands, rhodium(III) dicyano-bis (alpha,alpha'-diimine) and tris(alpha,alpha'-diimine) complexes via ab inition and density-functional calculations

Based on crystal structure data, the recently developed density functional PBE1PBE predicts ground state equilibrium geometries in good agreement with experiments. Bond length and angle α,α′-diimine ligand Mean Absolute Deviation (MAD) values of 0.0077 Å and 0.63° are obtained with the low-cost model chemistry PBE1PBE/6-21G. Theoretical trends, specifically the gs → 1ππ* absorption energies and 3ππ* → gs phosphorescence emission energies of the ligands also agree well with experiment.; Computations on [Ru(II)(1,10-phenanthroline)₃]2+ indicate that the Stuttgart ECP ECP28MWB is capable of reproducing adequately the geometries and photophysical characteristics of transition-metal complexes when paired with the DFT hybrid functional PBE1PBE and the Pople-style split-valence 6-21G basis set describing the ligands. Examination shows that the predicted photophysical properties of both [Rh(III)(s-NN)₃](PF₆) ₃ and [Rh(III)(CN)₂(s-NN)₂](PF₆) complexes agree with experimental evidence in many, but not all aspects. The experimentally observed spectroscopic trend for the gs → 1ππ* absorption energies is reproduced, namely the absorption bands of phenanthroline complexes containing progressively more methyl substituents are monotonically red-shifted relative to the parent phenanthroline in the following energy order: phen > 4-Mephen > 4,7-Me₂phen > 3,4,7,8-Me₄phen >> 5,6-Me ₂phen. Also, the trend of the experimental 3ππ* → gs phosphorescence emission energies is reproduced by the calculations.; Experimentally, the activation barriers for the onset of photochemistry in glycerol matrices are reported to be around 2500 cm−1 and 2000 cm−1 for the [Rh(III)(s-NN)₃](PF ₆)₃ and [Rh(III)(CN)₂(s-NN)₂](PF ₆) complexes, respectively. Calculations of the energy gap between the lowest 3ππ* states and the ligand-field states locate the ligand-field states ∼5000cm−1 above the 3ππ* manifolds in the [Rh(III)(s-NN)₃](PF₆) ₃ complexes, far exceeding the experimentally observed values. Analogous calculations on [Rh(III)(CN)₂(s-NN)₂](PF₆) complexes predict an energy gap closer to the experimentally observed activation barriers (∼2500 cm−1) and correctly reproduce the observed trend of increasing activation energy with increasing methyl-substitution, but the ligand field states are shown to possess substantial ligand-centered character.