Surface-enhanced Raman scattering and surface-enhanced resonance Raman scattering excitation profiles of Ag-2,2′-bipyridine surface complexes and of [Ru(bpy)3]2+ on Ag colloidal surfaces:: Manifestations of the charge-transfer resonance contributions to the overall surface enhancement of Raman scattering

Excitation profiles of SERS (surface-enhanced Raman scattering) and/or SERRS (surface-enhanced resonance Raman scattering) spectral bands of two forms of a Ag-bpy (bpy = 2,2'-bipyridine) surface complex and of [Ru(bpy)(3)](2+) on Ag nanoparticle (hydrosol) surfaces were determined from the spectra excited in the 458-600 nm region and are reported together with the FT-SERS spectra of the Ag-bpy surface complex and FT Raman spectra of [Ru(bpy)(3)] Cl-2. Seven of the observed 11 fundamentals as well as their first overtones and combination bands are selectively enhanced in SERS of the Ag-bpy surface complex formed in the Ag colloid/HCl/bpy system. The profiles of these bands show a common maximum at similar to 540 nm. The selectively enhanced bands of the Ag-bpy surface complex have nearly the same wavenumbers as those enhanced in the SERRS and resonance Raman spectra of [Ru(bpy)(3)](2+) upon excitation close to the 453 nm maximum of its MLCT absorption band. Moreover, the intensity patterns of the bpy vibrations of the two species match both in resonance (541 nm excitation for Ag-bpy, 458 nm for [Ru(bpy)(3)](2+)) and in off-resonance (458 and 1054 nm for Ag-bpy, 1064 nm for [Ru(bpy)(3)](2+)). The distinct band shapes of the excitation profiles of the selectively enhanced vibrational modes of the Ag-bpy surface complex, as well as the observation of overtones and combination bands in the SERS spectra upon excitation into this "band", are interpreted in terms of a charge-transfer resonance contribution to the overall SERS enhancement. In view of the near-coincidence of the vibrational modes coupled to the resonant electronic transition of Ag-bpy with those coupled to the MLCT transition of [Ru(bpy)(3)](2+), the resonant electronic transition is tentatively assigned to a Ag metal to bpy (pi*) CT transition.