A charge density study of crystalline 1-(4-fluorophenyl)-3,6,6-trimethyl-2-phenyl-1,5,6,7-tetrahydro-4H-indol-4-one (A) and 1-(4-fluorophenyl)-6-methoxy-2-phenyl-1,2,3,4-tetrahydroisoquinoline (B) has been carried out using high-resolution X-ray diffraction data collected at 113(2) K. Weak intermolecular interactions of the type C-H...O, C-H...pi, and pi...pi hold the molecules together in the crystal lattice along with interactions of the type C-H...F and unusual C-F...F-C examined via charge density analysis. The topological features are evaluated in terms of Bader's theory of atoms in molecules through the first four criteria of Koch and Popelier. The C-F...F-C contact is observed to be across the center of symmetry in B and not in A, and further, this interaction appears to possess a certain correlation with the electron density properties at the critical point which suggests that such an interaction fits into the hierarchy of weak interactions.