Excited-state electronic structure in polypyridyl complexes containing unsymmetrical ligands

Step-scan Fourier transform infrared absorption difference time-resolved (( SFTIR)-F-2 Delta A TRS) and time-resolved resonance Raman (TR3) spectroscop ies have been applied to a series of questions related to excited-state str ucture in the metal-to-ligand charge transfer (MLCT) excited states of [Ru( bpy)(2)(4,4'-(Co2Et)(2)bpy)](2+), [Ru(bpy)(2)(4-CO2Et-4'-CH(3)bpy)](2+), [R u(bpy)(4,4'-(CO2Et)(2)bpy)(2)](2+), [Ru(4,4'-(CO2Et)(2)bpy)(3)](2+), [Ru(bp y)(2)(4,4'-(CONEt2)(2)bpy)](2+), [Ru(bpy)(2)(4-CONEt2-4'-CH(3)bpy)](2+), an d [Ru(4-CONEt2-4'-CH(3)bpy)(3)](2+) (bpy is 2,2'-bipyridine). These complex es contain bpy ligands which are either symmetrically or unsymmetrically de rivatized with electron-withdrawing ester or amide substituents. Analysis o f the vibrational data, largely based on the magnitudes of the <(nu)over ba r>(CO) shifts of the amide and ester substituents (Delta<(nu)over bar>(CO)) , reveals that the ester- or amide-derivatized ligands are the ultimate acc epters and that the excited electron is localized on one acceptor ligand on the nanosecond time scale. In the unsymmetrically substituted acceptor lig ands, the excited electron is largely polarized toward the ester- or amide- derivatized pyridine rings. In the MLCT excited states of [Ru(bpy)2(4,4'-(C o2Et)(2)bpy)](2+) and [Ru(bpy)(2)(4,4'-(CoNEt2)(2)bpy)](2+), Delta<(nu)over bar>(CO) is only 60-70% of that observed upon complete ligand reduction du e to a strong polarization interaction in the excited state between the d p i(5) Ru-III core and the excited electron.