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Abstract
Electrochemical surface-enhanced Raman scattering (SERS) of the cruciform system 1,4-bis((E)-2-(pyridin-4-yl)vinyl)naphthalene (bpyvn) was recorded on nanostructured silver surfaces at different electrode potentials by using excitation laser lines of 785 and 514.5 nm. SERS relative intensities were analyzed on the basis of the resonance Raman vibronic theory with the help of DFT calculations. The comparison between the experimental and the computed resonance Raman spectra calculated for the first five electronic states of the Ag2-bpyvn surface complex model points out that the selective enhancement of the SERS band recorded at about 1600 cm−1, under 785 nm excitation, is due to a resonant Raman process involving a photoexcited metal-to-molecule charge transfer state of the complex, while the enhancement of the 1570 cm−1 band using 514.5 nm excitation is due to an intramolecular π→π* electronic transition localized in the naphthalenyl framework, resulting in a case of surface-enhanced resonance Raman spectrum (SERRS). Thus, the enhancement of the SERS bands of bpyvn is controlled by a general chemical enhancement mechanism in which different resonance processes of the overall electronic structure of the metal-molecule system are involved.
Highlights
The analysis of the selective enhancement of particular bands recorded in surface-enhanced Raman scattering (SERS) of most systems like aromatic molecules is usually a very difficult task
This work continues a series of studies where we demonstrated the close relationship between the electronic structure of the metal-molecule complex and the SERS spectra, concerning the selective enhancements of the bands
The different Raman responses of bpyvn adsorbed on rough silver electrode when irradiated by the two different excitation laser lines, 785 nm and 514.5 nm, can be explained on the basis of the electronic structure of the hybrid metal-molecule system
Summary
The analysis of the selective enhancement of particular bands recorded in surface-enhanced Raman scattering (SERS) of most systems like aromatic molecules is usually a very difficult task. Thereafter, the theoretical electronic spectra [18,19,20] computed on the basis of the resonance Raman vibronic theory are compared with the experimental SERS in order to detect the presence of resonant Raman processes of different types This methodology has proven its powerfulness to explain the complex dependence of the spectra on the applied potential and the differentiated behaviours shown by the SERS of different molecular systems. Bpyvn is a complex system which can be used as pyridine-based linkers in DNA bis-intercalators to probe the spatial organization of DNA [22] and it is a potential candidate to be used as a molecular junction in electronic devices as happens is other related molecules [23,24] This cruciform adsorbate is a bifunctional system able to bond to two metallic electrodes of a molecular junction through the nitrogen atoms of the two opposite pyridyl rings. The electrical conductance of the junctions will be controlled by the electronic structure of the metal-molecule complex and, in particular, by the metal-molecule charge transfer states whose existence and properties will be studied in this work by means of surface-enhanced Raman scattering
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