Synthesis and spectroscopic properties of novel azo dyes derived from phthalimide

A set of azo dyes has been synthesised using N‐substituted phthalimides as diazo components. 3,5‐Dibromo‐N‐substituted‐phthalimidylazo derivatives were cyanated by cyanodehalogenation to give dicyano analogues. All of the synthesised intermediates and dyes have been characterised by mass spectrometry, 1H‐NMR or elemental analyses. The gamut of colour of the prepared dyes spanned much of the visible spectrum: absorption maxima of the dyes in formamide were observed in the range 436 to 609 nm. Consistent with results seen in previous studies of non‐phthalimido monoazo disperse dyes, replacement of dibromo groups with dicyano functions in the phthalimide‐based set led to large bathochromic shifts in absorption maxima, as did the introduction of acetylamino groups onto the coupler ring ortho to the azo link. However, switching an N‐propyl function on the imido ring for a benzyl or 3‐cyanopropyl substituent had little effect on absorption maxima. Good correlations between the observed absorption maxima of certain dyes and those of model analogues predicted by Pariser–Parr–Pople molecular orbital calculations were observed when steric effects were absent.