Recognition of a Bromide Ion by the Protonated Form of 2‐(1 H ‐Imidazol‐2‐ylthio)‐3‐methylnaphthalene‐1,4‐dione

The interactions of various acids, such as hydrochloric, hydrobromic, nitric, perchloric, and tetrafluoroboric acids with 2‐(1 H ‐imidazole‐2‐ylthio)‐3‐methylnaphthalene‐1,4‐dione (L) enhance the intensity of the fluorescence emission of L. Exceptionally, the interaction of hydrogen bromide with L not only enhances the emission intensity, but also leads to a sharp characteristic emission at λ =480 nm ( λ ex =350 nm), which is different from the other acids. Bromide‐ion recognition by protonated L is explained on the basis of a tautomeric equilibrium. The Stokes shifts were calculated for each case and they were dependent on the anions and, in general, were found at λ >100 nm. Fluorescence lifetimes were measured and it was shown that two independent paths operated for the emission processes in solutions of the salts of L. Various salts of L with the general composition [HL][X] (in which X=Cl − ( 1 ), Br − ( 2 ), NO 3 − ( 3 ), ClO 4 − ( 4 ), and BF 4 − ( 5 )) are structurally characterized. The coordination environment of the corresponding anion in these salts in the solid state is guided by electrostatic N + H⋅⋅⋅O interactions.