An internal fluorescent probe based on anthracene to evaluate cation-anion interactions in imidazolium salts

A series of fluorescent imidazolium-based salts containing the cation [AnCH₂MeIm]⁺ (in which An = anthracene and Im = the imidazolium cation) with Cl^-, BF₄^-, PF ₆^-, SO₃CF₃^-, [N(CN) ₂]^-, [N(SO₂CF₃)₂] ^-, or PhBF₃^- anions have been prepared and characterized. X-ray diffraction analysis of four of the salts reveals a number of C-H⋯X-type (X = O, N, F) hydrogen bonds between the hydrogen atoms from the imidazolium ring and in some cases from the anthracene ring with the electronegative atoms of the anions. Additionally, C-H⋯π interactions can be found in all the salts analyzed by Xray diffraction, whereas π-π stacking is observed only in the salt containing the phenyltrifluoroborate anion. Fluorescence emission analysis in acetonitrile shows that the fluorescence of these salts varies significantly according to the nature of the anion, and correlates to the extent of ion pairing present in solution. Photodimerization of these salts was observed, and in one case a dimer has been isolated and characterized by X-ray crystallography.