Photophysical properties and excitation polarization of fac/mer-ruthenium complexes with 5'-amino-2,2'-bipyridine-5-carboxylic acid derivatives.

Novel ruthenium(II) complexes, fac/mer-[Ru(MeCO-5Bpy-R)3]2+ (H-5Bpy-OH = 5'-amino-2,2'-bipyridine-5-carboxylic acid; R = -NHtBu, -NH(cHex), -N(cHex)2), have been synthesized. The fac and mer isomers have been successfully separated using HPLC techniques, and their photophysical/electrochemical properties have been investigated. In the absorption and emission spectra of fac/mer-[Ru(MeCO-5Bpy-R)3]2+ with secondary amines (R = -N(cHex)2) in acetonitrile at room temperature, the maximum wavelengths based on the MLCT are longer than those for the amide derivatives with primary amines (R = -NHtBu, -NH(cHex)). A small solvent effect on the photophysical properties between fac- and mer-[Ru(MeCO-5Bpy-NHtBu)3]2+ has been observed. The excitation polarization spectra, giving P values reflecting the relation between the absorption and the emission oscillators, for the fac- and mer-ruthenium(II) complexes (C3 and C1 symmetry, respectively) have been measured for the first time. Almost no difference in the excitation polarization spectra between the fac and mer complexes is found, and these spectra are similar to that for [Ru(bpy)3]2+ with D3 symmetry. This finding suggests that the orientations of the absorption and emission oscillators, in the case of the ruthenium(II) tris(2,2'-bipyridine) derivatives, would not be affected by the symmetries of the complexes and that the P values for any derivatives would be similar to that for [Ru(bpy)3]2+.