An Internal Fluorescent Probe Based on Anthracene to Evaluate Cation-Anion Interactions in Imidazolium Salts

A series of fluorescent imidazolium-based salts contg. the cation [AnCH2MeIm]+ (in which An=anthracene and Im=the imidazolium cation) with Cl-, BF4-, PF6-, SO3CF3-, [N(CN)2]-, [N(SO2CF3)2]-, or PhBF3- anions have been prepd. and characterized. X-ray diffraction anal. of four of the salts reveals a no. 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 electroneg. atoms of the anions. Addnl., C-H•••π interactions can be found in all the salts analyzed by X-ray diffraction, whereas π-π stacking is obsd. only in the salt contg. the phenyltrifluoroborate anion. Fluorescence emission anal. 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 soln. Photodimerization of these salts was obsd., and in one case a dimer has been isolated and characterized by X-ray crystallog.