FT-Raman, IR and UV-visible spectral investigations and ab initio computations of a nonlinear food dye amaranth

摘要

Amaranth (E123, Food Red 9, FD & C Red 2) is a sulfortated azo dye used as a color additive in foodstuffs, pharmaceuticals and cosmetics. FT-IR and FT-Raman spectra of amaranth were recorded and analyzed. Density functional theory (DFT) calculations were performed to derive the equilibrium geometry, vibrational wavenumbers, intensities and first hyperpolarizability. The results of the optimized molecular structure gave clear evidence for the intramolecular charge transfer (ICT) and intramolecular hydrogen bonding in the molecule. Azo stretching wavenumbers are lowered owing to conjugation and pi-electron delocalization. Time-dependent density functional theory (TD-DFT) calculations of the electronic spectra were performed on the optimized structure and compared with the experimental UV-visible spectrum. Vibrational spectra, natural bonding orbitals (NBO) analysis and optimized geometry indicate C-H center dot center dot center dot N hydrogen bonding in the molecule. The first hyperpolarizability of the molecule was calculated. The optical nonlinearity of the dye is due to the donation of the electron density from the hydroxyl group of the conjugated system via naphthalene (2) ring into pi*-orbital of the azo moiety. Copyright (c) 2008 John Wiley & Sons, Ltd.