Barbituric acid-based mono/bi-heterocyclic dyes showing distinct spectral behaviors induced by solvents and pH

Abstract A series of barbituric acid-based heterocyclic dyes were prepared and fully characterized by ESI-MS, NMR and UV–Vis absorption spectroscopy. UV–Vis spectral comparisons reveal that in contrast to mono-heterocyclic dyes the absorption maxima of barbituric acid-based bi-heterocyclic dyes display significantly bathochromic shifts in wavelength due to the increasing π-conjugated system within the whole molecules. The effects of varying solvent media on the absorption spectra of this group of dyes were investigated, in which bi-heterocyclic dyes are liable to be affected by nature of solvents and present distinct solvatochromism in four kinds of organic solvents. Our results demonstrate that though the equilibrium between the hydrazone and deprotonated azo anion can be driven by pH values for this group of barbituric acid-based heterocyclic dyes, the acid-base effect on their absorption spectra behave differently. In order to further elucidate their different structural and spectral behavior, time-dependent density functional theory (TD-DFT) calculations with the implicit solvent model (SMD) were performed.