Semiempirical and ab initio studies of the structure and spectroscopy of the azo dye direct blue 1: Comparison with experiment

The structure and spectroscopy of the bis-azo dye Direct Blue 1 have been studied using semiempirical, Hartree-Fock, and density functional theory (DFT) calculations, and the results have been compared with those from experiment. The calculated dye structure comprises two essentially identical near-planar halves with a twist about the central biphenyl bond; the structures produced by the semiempirical AM 1 method and the ab initio Hartree-Fock and DFT methods were found to be similar to each other. Excited-state calculations describe the transition to the lowest excited state of Direct Blue 1 as a charge transfer from the central biphenyl group onto both naphthyl rings, along with charge redistribution within the region of the hydrazone groups. Infrared and Raman spectra calculated by ab initio methods match the experimental spectra well; by contrast, IR spectra were modeled poorly by the semiempirical AM1 method. NMR resonances calculated with the DFT methods give good correlation with the experimental NMR resonances of Direct Blue 1 in solution. Overall, the match between the calculated and experimental properties is good for this large (88-atom) molecule, demonstrating that such calculations can assist greatly in analyzing the structure and spectroscopy of azo dyes, provided that a suitable level of theory is used.