Characterization of Ir(ppy)₃ and [Ir(ppy)₂ bpy] ⁺ by infrared, Raman spectra and surface-enhanced Raman scattering

The Raman and infrared spectra of fac-tris(2-phenylpyridinato-N,C2′) iridium(III), Ir(ppy)₃ and surface-enhanced resonance Raman spectra of bis(2-phenyl pyridinato-) (2,2′bipyridine) iridium (III), [Ir(ppy) ₂ (bpy)]⁺ cation were recorded in the wavenumber range 150-1700 cm^-1, and complete vibrational analyses of Ir(ppy) ₃ and [Ir(ppy)₂ (bpy)]⁺ were performed. Most of the vibrational wavenumbers were calculated with density-functional theory agree with experimental data. On the basis of the results of calculation and comparison of the spectra of both complexes and their analogue [Ru(bpy) ₃]²⁺, we assign the vibrational wavenumbers for metal-ligand modes; metal-ligand stretching wavenumbers are 277/307 and 261/236 cm^-1 for Ir(ppy)₃, and 311/324, 257/270, 199/245 cm ^-1 for [Ir(ppy)₂ bpy]⁺. Surface-enhanced Raman scattering spectra of [Ir(ppy)₂ bpy]²⁺ were measured at two wavelengths on the red and blue edges of the low-energy metal-to-ligand charge-transfer band. According to the enhanced Raman intensities for the vibrational modes of both ligands ppy and bpy, the unresolved charge-transfer band is deduced to consist of charge-transfer transitions from the triplet metal to both ligands ppy and bpy.