Solvatochromism of new substituted 4-[(E)-(4-nitrophenyl)diazenyl]phenolate dyes

Studies on the reverse solvatochromism of dyes in pure solvents are of interest to gain a better understanding of the nature of this phenomenon. In this paper, a series of nine compounds, exhibiting phenol and 4-nitrophenyl groups linked with an azo conjugated bridge, was synthesized and characterized. These compounds were deprotonated to generate solvatochromic phenolate dyes. The azo bridge makes the phenol more acidic, in comparison with the related compounds exhibiting a CHCH or CHN bridge in their molecular structure, indicating the electron withdrawing character of the azo bridge. The solvatochromism of the dyes was investigated in 29 solvents and their intense colors in solution were attributed to π-π* electronic transitions, with an intramolecular charge transfer from the phenolate towards the 4-nitroaryl group. The probes exhibited a reversal in their solvatochromism, in a similar fashion to the behavior verified with related dyes presenting CHCH or CHN as the conjugated bridge; however, the azo dyes are much less sensitive to solvent polarity changes. Multiparametric analysis showed that the azo dyes are very sensitive to the acidity of the medium, but still to a lesser degree than the corresponding imine and stilbene dyes, due to the electronegativity of the azo group. The reverse solvatochromism of the dyes is dictated by the electron-donor phenolate and by the electron-acceptor 4-nitroaryl groups, independently of the conjugated bridge, of the additional nitro group and of the nature of substituents in the 2,6-positions of the phenolate groups. Thus, the solvatochromic reversal verified represents a committed step involving the interaction of the nitro and phenolate groups in the probe with the molecules of structurally different solvents.