Trans influence and substituent effects on the HOMO-LUMO energy gap and Stokes shift in Ru mono-diimine derivatives

The ground (S₀) and excited triplet (T₁) electronic states and corresponding optical spectra of a series of cationic complexes [RuH(CO)L(PPh₃)₂]⁺ (L = 2,2′-bipyridyl) (Rubpy), 4,4′-dicarboxylic-2,2′-bipyridyl(Rudcbpy),bis-4,4′-(N-methylamide)-2,2′-bipyridyl(Rudamidebpy),bis-4,4′-(methyl)-2,2′-bipyridyl(RudMebpy),[Ru(CO)₂dcbpy(PPh₃)₂]²⁺(Ru(2CO)dcbpy),and [Ru(H)₂dcbpy(PPh₃)₂] (Ru(2H)dcbpy) have been studied by combined Density Functional/Time-Dependent Density Functional (DFT/TDDFT) techniques using different combinations of DFT exchange-correlation functionals and basis sets. PBE0/LANL2DZ provided more accurate geometries to describe S₀ whereas B3LYP/LANL2DZ predicted spectral energies that correlated better with the available experiment data. The Ru (II) complexes with different substituents emit photons ranging from 560 to 610 nm in the series RudMebpy, Rubpy, Rudamidebpy, Rudcbpy. The calculations predicted a maximum emission at about 540 nm for the complex constructed from two carbonyl π-acceptors ligands trans to the dcbpy, while an emission in the far infrared region is calculated when two H σ-donor ligands trans to the dcbpy. Our calculation results show correlations between HOMO-LUMO energy gap, Stokes shift, and T₁ distortion, which reflect the different effects of electron-withdrawing and donating groups. We proposed that these correlations can be used to predict the photophysical properties for new complexes.