SERRS Spectra and Excitation Profiles of Ru(II) Polypyridine Complexes Attached to Ag Nanoparticle Aggregates: Structural, Electronic, and Resonance Damping Effects of Chemisorption

Abstract

Surface enhanced resonance Raman scattering (SERRS) spectra and SERRS excitation profiles of a dicationic Ru(II) bis(2,2′-bipyridine)(4,4′-dicarboxy-2, 2′-bipyridine) (Ru(bpy)2(dcbpy)) complex in systems with Ag nanoparticle (NP) aggregates were obtained and compared to those of a dicationic Ru(II) tris(2,2′-bipyridine) (Ru(bpy)3). SERRS spectra provided evidence for chemisorption of Ru(bpy)2(dcbpy) complex onto Ag NP surface via at least one of the carboxylate groups. SERRS excitation profiles of the spectral bands 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) indicates that chemisorption of the complex causes a red-shift of the Ru(II) → dcbpy MLCT (metal to ligand charge transfer) transition band. The energy decrease of the Ru(II) → dcbpy MLCT transition is explained by an increase of the electron-withdrawing ability of the two COO– groups upon their chemisorption on Ag NP surface. Finally, concentration values of SERRS spectral limits of detection of both complexes in system with uniform morphologies of Ag NP aggregates were determined, yielding the 1 × 10–9 M value for the chemisorbed Ru(bpy)2(dcbpy) and the 1 × 10–12 M value for the electrostatically bonded Ru(bpy)3. The major contribution to this difference is attributed to ca. 500× higher resonance damping in SERRS of the chemisorbed complex in comparison to that in SERRS of the electrostatically bonded one.