Tautomerism of4,4′-dihydroxy-1,1′-naphthaldazine studied byexperimental and theoretical methods

Abstract

Abstract Background The title compound belongs to the class of bis-azomethine pigments. On thebasis of comparative studies on similar structures, insight into the complexexcited state dynamics of such compounds has been gained. It has been shown,for example, that only compounds that possess hydroxyl groups arefluorescent, and that the possibility for cis-trans isomerisationand/or bending motions of the central bis-azomethine fragment allows fordifferent non-radiative decay pathways. Results The compound, 4,4'-dihydroxy-1,1'-naphthaldazine (1) was synthesized andcharacterized by means of spectroscopic and quantum chemical methods. Thetautomerism of 1 was studied in details by steady state UV-Vis spectroscopyand time resolved flash photolysis. The composite shape of the absorptionbands was computationally resolved into individual subbands. Thus, the molarfraction of each component and the corresponding tautomeric constants wereestimated from the temperature dependent spectra in ethanol. Conclusions According to the spectroscopic data the prevalent tautomer is the diol form,which is in agreement with the theoretical (HF and DFT) predictions. Theexperimental data show, however, that all three tautomers coexist insolution even at room temperature. Relevant theoretical results wereobtained after taking into account the solvent effect by the so-calledsupermolecule-PCM approach. The TD-DFT B3LYP/6-31 G** calculatedexcitation energies confirm the assignment of the individual bands obtainedfrom the derivative spectroscopy.