The solvatochromism and other spectroscopic properties of seven azo dyes were studied, with a particular respect to the role of the solvent basicity, and interpreted with the aid of experimental findings and semiempirical data. The electronic absorption spectra of the dyes examined in different solvents combined with theoretical calculations showed that most of the investigated compounds coexist in the hydrazone and/or azo-enamine-common anion equilibrium or in the solely anionic form depending upon the nature of the solvent employed. These interesting features open up possibilities for the use of these compounds in analytical chemistry as acid-base indicators. Furthermore, both of intermolecular and intramolecular charge transfer equilibria have been reflected by experimental absorption spectra of compounds 4 and 5. The enthalpies of formation predicted at PM6 (COSMO) and PM6/CI (COSMO) for the ground (S(0)) and excited (S(1)) states, respectively have been successfully used for the explanation of the observed bathchromic shift in non-polar solvents. The effect of time on the longer wavelength visible band of compound 7 has been thoroughly investigated.

A series of new azo dyes derived from cyclohexane- 1,3-dione have been synthesized and studied as regards the existence of prototropic tautomerism both in gas phase and in solution. The calculated equilibrium constants predicted by DFT method are in agreement with the NMR experimental results, predicting the existence of all newly studied dyes predominantly or completely in their hydrazone forms. The positions of 1H signals in NMR spectra compare qualitatively with those predicted computationally at the GIAO/DFT level of theory. Different bands observed in electronic absorption spectra have been assigned to the proper electronic transitions. The new band displayed by compound 5 in proton accepting solvents was regarded as a result of ionization. This characteristic band could be useful if the compound was used as analytical indicator. Analysis of electronic absorption spectra confirms the conclusions drawn from calculations and examination of 1H NMR as well as FT-IR spectra.