Raman and Surface Enhanced Raman Spectra of 7-hydroxyflavone and 3’,4’-dihydroxyflavone

The FT-Raman and surface-enhanced Raman (SER) spectra of two hydroxyl derivatives of flavone, namely 7-hydroxyflavone, 3’,4’-dihydroxyflavone, have been obtained. The importance of these compounds lies in the fact that they are simple precursors to the most important of the flavonoid dyes, such as quercetin. The SERS spectra were obtained on citrate reduced Ag colloids. Assignments of the experimentally obtained normal vibrational modes were aided by density functional theory (DFT) calculations using the B3LYP functional and the 6-31+G* basis set. Excellent fits were obtained for the observed spectra with little scaling. As in other flavone derivatives, the C=O stretching bands in the SERS spectra are diminished in intensity by proximity of the metal surface relatively compared to the normal Raman spectra. Additionally, the lines at lower wavenumbers, assigned to in-plane ring deformation, are strongly enhanced by the surface, indicating a perpendicular orientation of the flavonoids on the Ag surface. Finally, the influence of the 7 and 3’,4’ OH substitutions on the spectra of chrysin, apegenin, and luteolin are examined.