Substitution effects and electronic properties of the azo dye (1-phenylazo-2-naphthol) species: a TD-DFT electronic spectra investigation

Abstract

DFT/B3LYP and ab initio Hartree–Fock calculations with full geometry optimization have been carried out on hydrazo and azo forms of 1-phenylazo-2-naphthol and their substituted derivatives. The predicted geometries show that a small energy difference of 1.8 kcal/mol might tune the equilibrium between both forms. Depending on the electron donating and electron accepting of the different used substituents (CF3, NH2, CH3, Cl, and NO2), the various obtained isomers show small energy differencies between meta and para substitution except for the NH2 one, indicating the coexistence of the tautomers in solution. The ortho(C12) position was found to be the less favored substitution in all cases, while the second ortho(C16) position for different substituents provides isomers competing with the most stable meta and para ones. The obtained results suggest that a judicious choice in the substituents’ use on the phenyl ring should lead to stabilization. The TD-DFT theoretical study performed on the optimized geometry allowed us to identify quite clearly the spectral position and the nature of the different electronic transitions according to their molecular orbital localization, hence, reproducing the available UV-Vis spectra. The increase in the wavelength values is in perfect agreement with red shifts and the ΔE (ELUMO – EHOMO) decreasing. Thus, from the point of view of both substitution and the used solvent, the obtained electronic spectra appear to behave quite differently.