Spectroscopic and quantum-chemical investigations of chloro-bis-bipyridyl complexes of ruthenium(II) with 4-substituted pyridine ligands

Abstract

The luminescence spectra of cis-[Ru(bpy)2(L)Cl]+ (bpy is 2,2′-bipyridyl; L is pyrazine, pyridine, 4-amino-pyridine, 4-picolin, isonicotinamide, 4-cyanopyridine, or 4,4′bipyridyl) complexes are studied in alcoholic (4: 1 EtOH-MeOH) solutions at 77 K. A linear correlation is found between the energy of the lowest electronically excited metal-to-ligand charge transfer (3MLCT) state dπ(Ru) → π* (bpy) and the parameter pKa of the free 4-substituted pyridines and pyrazine used as ligands L. The [B3LYP/6-31G + LanL2DZ(Ru)] hybrid method of the density functional theory is used to optimize the geometry of complexes and calculate their electronic structure and the charge distribution on the atoms of the nearest environment of the ruthenium ion. It is shown that there exists a linear unambiguous correlation between the negative charge on the nitrogen atom (qNL) of ligands L coordinated in the complex and the parameters pKa of free ligands. The calculated energies of 3MLCT excited states almost linearly (correlation coefficient 0.958) depend on the charge qNL, which completely agrees with experimental data.