Both imidazole and acridine contain tertiary nitrogen atoms so quaternization reactions is possible in both the cases (you can prepare salts from both acridine and imidazole). but you have not added any alkylating reagent then on what basis you are expecting quaternization?? Do you think that reaction of acridine and imidazole will produce imidazolium salt? if there is any reference regarding that then please inform . regards.

Please provide details of concentration and solvent used for the reaction as well as solvent used for 1H NMR.

Acridine and imidazole would not react under those conditions.  Something is missing from your description.  (Perhaps you are using a 9-halogenated acridine?) 

@James Demers.. Sir I also think that imidazole and acridine can't react. I think you are right, there may be haloginated acridine.