Novel xanthene-like dyes as pH indicators in acidic media

Abstract

In this work, a previously unknown aldazine, N'-[4-((E)-{(2Z)-[(12-{[(1E)-(dimethylamino)methylene]amino]amino}-2,3-dihydro-1H-benzo[b]xanthene-4-yl)methylene]hydrazone}methyl]-2,3-dihydro-1H-benzo[b]xanthene-12-yl]-N,N-dimethylimidoforma-mide was synthesized by reacting the corresponding aldehyde with hydrazine hydrate. The spectral characteristics of the two azomethine dyes were studied: N,N-dimethyl-N'-{4-[(E)-(phenylimino)methyl]-2,3-dihydro-1H-xanthen-9-yl}imidoformamide and N'-[4-((E)-{(2Z)-[(12-{[(1E)-(dimethylamino)methylene]amino]-2,3-dihydro-1H-benzo[b]xanthen-4-yl)methy-lene]hydrazono}methyl]-2,3-dihydro-1H-benzo[b]xanthen-12-yl]-N,N-dimethylimidoformamide. Both dyes exhibit a bathochromic shift in their absorption maxima at lower pH levels. The observed deepening of the color can be attributed to the protonation of the azomethine nitrogen atom and the change in the hybridization of the orbital that carries the unpaired electron pair of the azomethine nitrogen atom, switching from the sp2 orbital to the unhybridized p-orbital, which is capable of conjugation with the xanthene fragment. The color of N,N-dimethyl-N'-{4-[(E)-(phenylimino)methyl]-2,3-dihydro-1H-xanthen-9-yl}imidoformamide changes from yellow to red as the pH decreases, while the color of N'-[4-((E)-{(2Z)-[(12-{[(1E)-(dimethylamino)methylene]amino]-2,3-dihydro-1H-benzo[b]xanthen-4-yl)methylene]hydrazono}methyl]-2,3-dihydro-1H-benzo[b]xanthen-12-yl]-N,N-dimethylimidoformamide shifts from red to blue. The synthesized dyes have potential as pH indicators in acidic environments. These dyes demonstrate a color change not only in response to pH reduction but also in the presence of transition metal salts.