Comparison of SERRS and RRS excitation profiles of [Fe(tpy)2]2 (tpy = 2,2':6',2''-terpyridine) supported by DFT calculations: effect of the electrostatic bonding to chloride-modified Ag nanoparticles on its vibrational and electronic structure
Abstract
Nonresonance (or normal) Raman scattering (NRS), resonance Raman scattering (RRS), surface-enhanced Raman scattering (SERS), and surface-enhanced RRS (SERRS) spectra of [Fe(tpy)2]2+ complex dication (tpy=2,2':6',2''-terpyridine) are reported. The comparison of RRS/NRS and SERRS/SERS excitation profiles of [Fe(tpy)2]2+ spectral bands in the range of 445-780nm is supported by density functional theory (DFT) calcu lations, Raman depolarization measurements, comparisonof the solid [Fe(tpy)2](SO4)2 and solution RRS spectra, and characterization of the Ag nanoparticle (NP) hydrosol/[Fe(tpy)2]2+ SERS/SERRS active system by surface plasmo n extinction spectrum and transmission electron microscopy image of the fractal aggregates (D=1.82).
By DFT calculations, both the Raman active modes and the electronic states of the complex have been assigned to the symmetry species of the D2d point group. It has been demonstrated that upon the electrostatic bonding of the complex dication to the chloride-modified Ag NPs, the geometric and ground state electronic structure of the complex and the identity of the three different metal-to-ligand charge transfer (1MLCT) electronic transitions remain preserved.
On the other hand, the effect of ion pairing manifests itself by a slight change in localization of one of the electronic transitions (withmax. at 552nm) as well as by promotion of the Herzberg-Teller activation of E modes resulting from coupling of E and B2 excited electronic states. Finally, the very low, 1x10 11M SERRS spectral detection limit of [Fe(tpy)2]2+ at 532-nm excitation is attributed to a concerted action of the electromagnetic and molecular resonance mechanism, in conjunction to the electrostatic bonding of the complex dication to the chloride-modified Ag NP surface.