Post-modification of 2-formylthiophene based heterocyclic azo dyes

•Three sets of novel post-modification S-containing heterocyclic azo dyes are described.•Representative crystal structures for every series are obtained and compared.•Variation of π-conjugated length and polarity shows different λmax and solvatochromism.•An α,β-unsaturated cyanoacetic ester has the largest bathochromic shift of 87 nm.

In order to further adjust the π-conjugated system, solubility and electronic spectrum of 2-amino-3-cyano-4-chloro-5-formylthiophene based blue colored heterocyclic blue azo dyes, a post-modification strategy has been used to extend their terminal aldehyde radical into either an imine, acetal or α,β-unsaturated cyanoacetic ester group. The resultant azo-azomethine dyes exhibit slight bathochromic shifts in their UV–Vis spectra with the increase of the π-conjugated system. In contrast, acetal terminated products display good solubility in organic solvents but hypsochromic shifts in their UV–Vis spectra. It is worth mentioning that the α,β-unsaturated cyanoacetic ester derivatives show significant bathochromic shifts and improvement of solubility simultaneously originating from the presence of increased π-conjugated system as well as the terminal cyano and ester groups. It is believed that the current study could provide a practical post-modification strategy for decorating and improving the dyeing performance of certain dyes with functional end groups.

Graphical abstractA post-modification strategy has been achieved for 2-amino-3-cyano-4-chloro-5-formylthiophene based heterocyclic blue azo dyes with the common terminal aldehyde group, where an α,β-unsaturated cyanoacetic ester derivative shows the largest bathochromic shift of 87 nm and improvement of solubility simultaneously.Figure optionsDownload full-size imageDownload as PowerPoint slide