Tautomeric Forms of 2-Hydroxynaphthalene-Based Imines: Theoretical and Experimental Comparisons

Experimental and theoretical studies on two 2-hydroxynaphthalene imine-based compounds, namely, 1-[{(2,4-dichlorophenyl)imino}methyl]naphthalen-2-ol (24Clmno) and (E)-N-(2,4-dichlorophenyl)-2-{(2-hydroxynaphthalen-1-yl)methylene}hydrazine-1-carbothioamide (24Clhhc), to establish the energetically favorable enol-imine form in their respective crystal structures are reported. The structure of 24Clmno has edge-on chlorine−π interactions and also C–H···O interactions among neighboring molecules to form chain-like arrangements. The self-assembly of 24Clhhc was devoid of chlorine−π interactions but guided by C–H···S interactions, forming double-chain-like arrangements forming a layer-like structure. Theoretically, the enol-imine form of 24Clmno is more stable by 38.52 kJ/mol than the keto-amine form, whereas in the case of 24Clhhc, the enol-imine form is more stable than the keto-amine, which was stable by 129.5 kJ/mol. Energy calculation by density functional theory on a series of compounds adopted from the literature showed that an energetically favorable tautomeric form is generally observed; the stabilities are also guided by the substituents and their positions in such molecules. The solvatochromic and solvatoemissive properties of these compounds were studied, and results were analyzed with the aid of nuclear magnetic resonance, emission lifetime measurements, and different control experiments in solution. Interactions of 24Clhhc with alkali and alkaline-earth metal ions, and under basic conditions, caused enhancement of fluorescence intensity at 509 nm. The zinc and cadmium ions do not cause fluorescence emission of the 24Clmno, but the emission of 24Clhhc changes by these two ions in a distinguishable manner. Zinc ions shifted the emission peak toward higher wavelengths, and cadmium ions to lower wavelengths.