9-(2,6-Difluorophenoxycarbonyl)-10-methylacridinium trifluoromethanesulfonate and its precursor 2,6-difluorophenyl acridine-9-carboxylate: C-H...O, C-F...pi, S-O...pi and pi-pi stacking interactions.

The title compounds, C21H14F2NO2+.CF3SO3-, (I), and C20H11F2NO2, (II), form monoclinic and triclinic crystals, respectively. Adjacent cations of (I) are oriented in a ;head-to-tail' manner and are linked to one another via networks of C-H...O, C-F...pi, S-O...pi and multidirectional pi-pi interactions. Adjacent molecules of (II) are also arranged in a ;head-to-tail' manner and are linked via networks of C-H...O and multidirectional pi-pi interactions. The mean planes of the acridine moieties lie parallel in the lattices of both compounds. The benzene rings are also parallel. However, the acridine and difluorophenyl rings are mutually oriented at an angle of 17.3 (2) degrees in (I) and 5.8 (2) degrees in (II). This mutual orientation in various phenyl acridine-9-carboxylates and related compounds is strongly influenced by the nature of the substituents on the phenyl fragment.