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 resonan ce Raman scattering (SERRS) from a Lee-Meisel colloidal suspension. Th e dye molecule is present on the surface in the hydrazone form. Three semiempirical calculations were used to assign the bands. The PM3 Hami ltonian provides the best fit between experiment and theory after corr ecting 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 s hift of the principal maxima from 501 to 514 nm, suggesting an interac tion with the charged Surface sufficiently strong to effect the separa tion 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 vibrat ional modes with in plane displacements predominantly on the phenyl ri ng and bridging region. There was one strong and some weaker enhanceme nts for vibrations with predominant displacements on the naphthyl syst em. Consistent with this observation, the HOMO and LUMO indicate small er Franck-Condon overlap on certain carbon atoms of the naphthyl syste m. At 514 nm, the modes which give rise to the strongest SERR bands co incide with those assigned to the strongest resonance Raman (RR) bands at 501 nm, but there were Some differences in relative intensity cons istent with an interaction with the charged surface. The surface plasm on interaction is sufficiently strong to result in an enhancement of 1 0(5)-10(6) compared with solution resonance. The comparison of resonan ce and SERRS at 514 nm suggests that there is a significant depolariza tion and that no surface selection rules apply. Secondary maxima were observed at approximately 613 nm in the SE(R)REPs, the region of plasm on resonance. At this frequency, non-molecular-resonant bands appear i n the spectrum. In preresonance, polarization and surface selection ru les may apply. A new band appears to be preresonant due to redistribut ion of electron density in the naphthyl moiety induced by the oscillat ing field. The data are rationalized on the basis that SERRS is very s imilar to electromagnetic SERS but with resonant enhancement involved directly in the process. The molecule is postulated to lie with the pl ane of the naphthyl ring approximately vertical to the surface of the colloid particle and with the naphthyl ring close to it. Where separat ion is observed between molecular and surface plasmon resonance, the c hoice 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. F or studies of surfaces the orientation-sensitive preresonant SERS is p referred.