Optical and resonance Raman scattering study of two ‘bisazo’ pigments derived from substituted benzene‐2′‐azonaphthols

Abstract

AbstractThe UV‐visible (300–800 nm), Raman and resonance Raman (900–1700 cm−1) spectra of two bisazo scarlet RN and brown 5R pigments have been investigated in the solid state at 300 K. A detailed analysis of the resonance Raman spectra and excitation profiles of various fundamentals in the 900–1700 cm−1range is reported through the visible absorption spectrum region: the vibrational assignments confirm that both substituted benzene‐2′‐azonaphthol derivatives are stabilized in their hydroxyazo form; the excitation profiles (EPs) established for many modes in each dye indicate interferences between two overlapping electronic manifolds. Taking account of multistate interference phenomena and inhomogeneous broadening effects, the observed EPs compare well with the calculated ones under the assumption of weakly displaced harmonic oscillators; this interpretation is supported by the observation of weak overtone and combination bands in the 1700–3000 cm−1range.Finally, conclusions are drawn about the scattering mechanisms and the nature or degree of mixing of the first electronic excited states, which appear to be quite sensitive to the substituent (Cl or NO2) on the benzene rings. Such information can be helpful for the design and synthesis of new azo dyes with desirable properties.