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) RIGHTWARDS ARROW dcbpy MLCT (metal to ligand charge transfer) transition band.
The energy decrease of the Ru(II) RIGHTWARDS ARROW dcbpy MLCT transition is explained by an increase of the electron-withdrawing ability of the two COOMINUS SIGN 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 x 10MINUS SIGN 9 M value for the chemisorbed Ru(bpy)2(dcbpy) and the 1 x 10MINUS SIGN 12 M value for the electrostatically bonded Ru(bpy)3.
The major contribution to this difference is attributed to ca. 500x higher resonance damping in SERRS of the chemisorbed complex in comparison to that in SERRS of the electrostatically bonded one.