ASSIGNMENTS AND MECHANISM OF SERRS OF THE HYDRAZONE FORM FOR THE AZO-DYE SOLVENT-YELLOW-14

Solvent Yellow 14 (CI 12055) is used to study surface enhanced resonance Raman scattering (SERRS) from a Lee-Meisel colloidal suspension. The dye molecule is present on the surface in the hydrazone form. Three semiempirical calculations were used to assign the bands. The PM3 Hamiltonian provides the best fit between experiment and theory after correcting for anharmonicity. Solution resonance excitation profiles (REP)and surface enhanced (resonance) Raman excitation profiles (SE(R)REP) were compared. Comparison of the SE(R)REPs with the REPs indicates a shift of the principal maxima from 501 to 514 nm, suggesting an interaction with the charged Surface sufficiently strong to effect the separation of the HOMO and LUMO of the adsorbed dye. These peaks correspond to the peak in the electronic spectrum of the hydrazone form at 503 nm. The principal enhancements in the resonance spectrum were for vibrational modes with in plane displacements predominantly on the phenyl ring and bridging region. There was one strong and some weaker enhancements for vibrations with predominant displacements on the naphthyl system. Consistent with this observation, the HOMO and LUMO indicate smaller Franck-Condon overlap on certain carbon atoms of the naphthyl system. At 514 nm, the modes which give rise to the strongest SERR bands coincide with those assigned to the strongest resonance Raman (RR) bands at 501 nm, but there were Some differences in relative intensity consistent with an interaction with the charged surface. The surface plasmon interaction is sufficiently strong to result in an enhancement of 10(5)-10(6) compared with solution resonance. The comparison of resonance and SERRS at 514 nm suggests that there is a significant depolarization and that no surface selection rules apply. Secondary maxima were observed at approximately 613 nm in the SE(R)REPs, the region of plasmon resonance. At this frequency, non-molecular-resonant bands appear in the spectrum. In preresonance, polarization and surface selection rules may apply. A new band appears to be preresonant due to redistribution of electron density in the naphthyl moiety induced by the oscillating field. The data are rationalized on the basis that SERRS is very similar to electromagnetic SERS but with resonant enhancement involved directly in the process. The molecule is postulated to lie with the plane of the naphthyl ring approximately vertical to the surface of the colloid particle and with the naphthyl ring close to it. Where separation is observed between molecular and surface plasmon resonance, the choice of excitation frequency will determine the information which can be obtained from SERRS. For analysis, the SERRS maximum is preferred since the signal is most intense, and it is orientation-insensitive. For studies of surfaces the orientation-sensitive preresonant SERS is preferred.