Impact of tautomers on the absorption spectra of neutral and anionic alizarin and quinizarin dyes

We have used the PCM-(TD)DFT formalism to evaluate the tautomeric effects on the absorption spectra of the neutral and anionic forms of both alizarin and quinizarin dyes. Our procedure is able to predict an excitation energy with an accuracy of 0.08 eV. It is shown that for the monoanionic form of alizarin, two tautomers can coexist in solution. A resulting λmax was simulated, and was found to be in good agreement with experiment. We have subsequently quantified the extent to which excitation energy errors correlate with the spatial overlap between the HOMO and the LUMO, as measured by the quantity SHL.