Raman spectroscopic and DFT theoretical study of 4-(2-pyridylazo)resorcinol and its complexes with zinc(II) and copper(II)

The surface-enhanced Raman scattering (SERS) spectra of 4-(2-pyridylazo)resorcinol (PAR) and its Zn(II) and Cu(II) complexes were recorded using a hydroxylamine reduced silver colloid. Molecular geometry optimization, molecular electrostatic potential (MEP) distribution and vibrational frequencies calculation were performed by density functional theory (DFT) computations for the PAR molecule and its Zn(PAR)2 metal complex. The SERS spectra of the samples prepared at PAR:metal salt 3:1 molar ratio, show mainly the spectral features of Zn(PAR)2 or Cu(PAR)2 complexes, but also spectral features of PAR molecules adsorbed to the silver surface, whereas the SERS spectra of the 1:1 molar ratio samples show exclusively the spectral features of the PAR-metal complex. Differentiation between PAR complexes with Zn(II), Cu(II), Fe(III), Mn(II) and Pb(II) is shown by the SERS spectral features of each complex.