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-(HO)-O-..., C-H-...pi, and pi(...)pi d hold the molecules together in the crystal lattice along with interactions of the type C-(HF)-F-... and unusual C-(FF)-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-(FF)-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.