Synthesis and properties of novel aziridinyl azo dyes from 2-aminothiophenes - Part 1: Synthesis and spectral properties

A series of yellow to greenish-blue aziridinyl azo dyes and their azo precursors containing a thienyl coupling moiety has been prepared from 2-aminothiophenes. The 2-aminothiophenes were readily obtained by using the Gewald reaction. It was found that cyclisation of the precursor dyes to the corresponding aziridinoazo dyes brought about bathochromic shifts in absorption maxima. Further spectral comparisons with N-phenylazo dyes derived from other terminal cyclic groups, such as four-, five-, six-, seven- and eight-membered rings, showed that the N-thienylaziridinoazo dyes are relatively bathochromic. From the viewpoint of solvatochromism, a clear contrast existed between λ_max values in different solvents; thus, a positive solvatochromism was observed in aprotic solvents, whereas a hypsochromic shift was brought about in polar protic solvents. PPP-MO calculations provided reliable predictions of absorption maxima for the various aziridinyl azo dyes and their precursor dyes.