Bipyridine complexes of Ru(II) as SERS spectral probes of the mechanisms of surface -enhanced optical processes

SERS and SERRS (surface enhanced /resonance/ Raman scattering) spectra of a dicationic Ru (II) bis(2,2'-bipyridine)(4,4'-dicarboxy-2,2'-bipyridine) /Ru(bpy)2(dcbpy)/ complex in systems with aggregates of unmodified and/or chloride-modified Ag nanoparticles (NPs) were obtained and compared to those of dicationic Ru (II) tris(2,2'-bipyridine) /Ru(bpy)3 /. The splitting of several bands and the appearance of a new band at 1367 cm-1 observed solely in SERS and SERRS of Ru(bpy)2(dcbpy) in the system with unmodified Ag NP aggregates was attributed to chemisorption of the complex onto Ag NP surface via two carboxylate groups. SERS/SERRS excitation profiles obtained for the spectral bands and attributed to the Ru- dcbpy unit of the chemisorbed Ru(bpy)2(dcbpy)/ complex were found to maximize at 488 nm excitation, while those of the two Ru-bpy units peaked at 458 nm. Comparison of the profiles with the electronic absorption spectrum of free Ru(bpy)2(dcbpy) has revealed that chemisorption of the complex causes a red-shift of the Ru→dcbpy charge transfer transition band. The observed decrease of the energy of the Ru→dcbpy charge transfer is explained by an increase of the electron-withdrawing ability of the two COO- groups upon their chemisorption on AgNP surface. Concentration value of SERRS spectral..