Color and Constitution of Azo Compounds

Attempts to explain the surprising violet color of the new azobisphosphonic acid derivatives Y 2 OPNNPOY 2 have led to the preparation and spectroscopic characterization of more than 60 azo compounds. The long‐wavelength n → π * transitions on which the color depends exhibit regularities which justify the assumption of substitutent‐specific absorption increments. The complex relation between color and constitution of azo compounds can be qualitatively explained with the aid of orbital models. Infrared and ultraviolet spectral data show that conjugation is inhibited by tetra‐coordinated phosphorus (V). the π‐system of these compounds therfore comprises only for atoms, P·N·N·P. The unexpected violet color of the phosphorus azo compounds can be ascribed to a π*/d orbital interaction. The substituent effects described have general significance, and n→π*transitions of other chromophores are similarly affected.