Spectral characterisation and computational studies of some novel phenylazoindol‐2‐one dyes

A series of novel phenylazo disperse dyes were prepared by coupling selected diazotised 3‐ or 4‐anilines with indol‐2‐one. The structures of the synthesised dyes were determined by spectroscopic and analytical methods. Solvent effects on the ultraviolet–visible absorption spectra of these novel dyes were studied. The colour of the dyes was evaluated with respect to the substituent therein. Acid and base effects on the visible absorption maxima of the dyes were also reported. The tautomerism of 3‐(phenyldiazenyl)indol‐2‐one was modelled by density functional theory at the B3LYP level. The ultraviolet–visible absorption spectra for the tautomers were reproduced with the time‐dependent density functional theory and semi‐empirical Zerner’s intermediate neglect of differential overlap/spectroscopic (ZINDO/S) method. The predominance of the keto–hydrazone tautomer was confirmed experimentally and theoretically.