Surface‐enhanced resonance Raman spectroscopy of water‐insoluble Ni(II) porphine adsorbed on aqueous silver sol: suppression of antisymmetric scattering, antiresonance effect and higher order scattering

Abstract

With the intention of examining how adsorption on a SERS-active surface affects the electronic resonance enhancement in surface-enhanced resonance Raman scattering (SERRS) from Ni(II) porphine (NiP), we acquired high-quality SERR spectra from very low concentrations of NiP (∼10 −8 M) in aqueous Ag sol from 50 to 3500 cm −1 with excitations across both the Soret and Q absorption bands. Water-insoluble NiP was transferred to aqueous silver sol surface by using pyridine as carrying solvent, taking advantage of the fact that it both dissolves NiP and is partially miscible with water at the same time. It was found that SERS boosts the overall sensitivity of resonance Raman scattering by providing extra enhancement (the surface enhancement) to a 1g , b 1g and b 2g modes, but not to a 2g modes. As a consequence, a relative 'suppression' effect of antisymmetric scattering by a 2g modes was observed when excitation was tuned into the Q absorption band region in SERR spectra. With excitations in the valley between the Soret and Q absorption bands, an antiresonance effect was observed in the SERR spectra for the totally symmetric n 6 mode at 995 cm −1 which was attributed to the destructive interference between Franck-Condon and Herzberg-Teller mechanisms. Strong second-order and weak third-order Raman scattering were observed in the SERR spectra of NiP when excitation was tuned close to and across the Q 1 absorption band. A vibronic theory based on the extension of Albrecht's formalism was developed to account for the activation of high-order Raman scattering and for the antiresonance effect of the n 6 band. More than 100 Raman wavenumbers, including both fundamentals and overtones/combinations, both in-plane and out-of-plane modes, were observed for NiP in its SERRS spectra. The wavenumbers from the SERR spectra agree very well with those obtained from RR spectra from a homogeneous NiP solution and FTIR data from KBr (CsI) pellets, suggesting that adsorption of NiP on a SERS-active surface is physisorption in nature, and that such adsorption has a negligibly small effect to the vibrational wavenumbers. The observed Raman wavenumbers from SERR spectra are also in excellent agreement with those predicted from ab initio/DFT calculations for both in-plane and out-of-plane modes. Many out-of-plane modes and IR-active in-plane modes were observed in the SERR spectra of NiP, suggesting that the effective symmetry of the adsorbed NiP is lower than the ideal planar D 4h .

A SERRS spectroscopic investigation of nickel(ii) porphyrin complexes adsorbed at electrochemical interfaces

“…This indicates an edge-on adsorption geometry for reasons detailed above but is in direct contrast with the observations of Li et al who observed a selective enhancement of the b 2g modes for NiP. 30 Thus, while it is entirely likely that NiP is adsorbed through the CC bond, the ethyl groups attached to each b-carbon in NiOEP present a substantial steric hindrance to this adsorption geometry. Thus, from the preferential enhancement of b 1g modes, it is tentatively suggested that NiOEP is adsorbed edge-on with the C 2 00 -axis normal to the electrode surface.…”

“…The lack of significant changes in the relative intensities of the totally symmetric modes under Soret-resonance has been observed by Li et al in the SERRS spectra of nickel porphine, NiP. 30 The authors attributed the invariance of the relative intensities of the a 1g modes as indicative of a dominant Franck-Condon RR enhancement mechanism operating in conjunction with an increased scattering efficiency resultant from surface adsorption. Li et al noticed that b 1g modes were much weaker than b 2g modes in the SERRS spectra, whereas in the RR spectra, the enhancement pattern was reversed.…”

“…Lack of significant shifts in band positions upon adsorption has been reported previously for large molecules oriented edgeon to the surface. Li et al reported SERS bands of NiP to be '' almost identical '' to RR bands, 30 Hester and co-workers observed a similar effect for H 2 TPP 48 while Zhang et al report the initial observation of SERS spectra identical to the RR spectra of AgTPP and CuTPP on hydroxyl-modified Ag and Ag 2 O colloids. 41,49 The energetic stability of the porphyrinbased modes means that a face-on adsorption geometry can be discounted.…”

“…The changes in band positions detailed in Table 1 may be initially surprising for a porphyrin adsorbed in an edge-on geometry, although previous studies have shown SERRS vibrational bands for edge-on adsorbed porphyrins to be close, and in many cases identical, to RR band positions. [30][31][32] Changes in band positions upon adsorption are usually due to a face-on orientation of the porphyrin, accompanied by lowering of the vibrational energy of in-plane modes. An end-on adsorption geometry of NiOEP is supported by a small red-shift in bands associated with the ethyl substituents, i.e.…”

The observation of high order overtones and combinations in the SERRS spectra of a perylene dye spin coated onto silver island films

“…The observation of overtones and combinations of fundamental vibrations that appear above 1000 cm À1 in SERS/ SERRS experiments is very scarce. [24][25][26][27] The first experimental evidence for such overtones in SERS experiments was reported by Pettinger in the study of pyridine adsorbed on silver electrodes. 24 The next report on the observation of overtones in SERS/SERRS experiments appeared only 10 years later by Aroca and Clavijo 25 investigating the SERRS spectra of a perylene dye on a Langmuir-Blodgett layer on Ag island films.…”