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, H-1-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.