Dihydroquinolylazotetrazole dyes in aqueous solutions: Effect of substituents and pH on spectral properties, acid-base equilibria and thermal Z‒E isomerization

Abstract

Spectral, luminescent and photochemical properties of a series of hetarylazo dyes with dihydroquinoline and tetrazole moieties, which differ by the position of methyl substituent at the tetrazole fragment, were investigated in aqueous solutions at various pH by stationary spectrophotometry, fluorimetry and time-resolved methods. The results were supported by quantum chemical calculations. From the pH-dependence of the absorption spectra, it has been shown that azonium cations and dications additionally protonated at the tetrazole fragment are formed upon protonation of the E isomers. Opposite to the azonium cations studied earlier, the long-wavelength absorption band of the azonium cations is hypsochromically shifted relative to that of the neutral form. Weak fluorescence in aqueous solutions growing in acidic media is the specific feature of the dyes. Time-resolved experiments have shown that proton transfer reactions play important role in the transformations of the dyes after excitation in aqueous solutions.