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

• Published in: Journal of Raman spectroscopy 2014-05, Vol.45 (5), p.338-348
• Main Authors: Šloufová, Ivana, Vlčková, Blanka, Procházka, Marek, Svoboda, Jan, Vohlídal, Jiří
• Format: Article
• Language: English
• Subjects: 2''-terpyridine) complex dication; 2':6'; Ag nanoparticles; DFT calculations; Iron(II) bis; Iron(II) bis (2,2':6',2''‐terpyridine) complex dication; resonance Raman scattering (RRS) excitation profiles; surface-enhanced resonance Raman scattering (SERRS) excitation profiles
• ISSN: 0377-0486; 1097-4555
• DOI: 10.1002/jrs.4468

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 445–780 nm is supported by density functional theory (DFT) calculations, Raman depolarization measurements, comparison of 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 plasmon 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 (with max. at 552 nm) 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, 1 × 10−11 M 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. Copyright © 2014 John Wiley & Sons, Ltd. Comparison of resonance Raman scattering (RRS)/nonresonance (or normal) Raman spectra (NRS) and surface‐enhanced resonance Raman scattering/surface‐enhanced Raman scattering excitation profiles of [Fe(terpyridine)2]2+ spectral bands in the range of 445–780 nm supported by density functional theory calculations, polarization NRS and solid state RRS spectra is reported. The electrostatic bonding of the complex dication to the chloride‐modified Ag nanoparticles does not perturb its geometric and ground state electronic structure, however, it leads to a slight change in localization of one of the three metal‐to‐ligand charge transfer electronic transition