The SMM behaviour of dinuclear Ln(III)–Pc multiple-decker complexes (Ln = Tb3+ and Dy3+) with energy barriers and slow-relaxation behaviour were explained by using X-ray crystallography and static and dynamic susceptibility measurements. In particular, interactions among the 4f electrons of several dinuclear Ln(III)–Pc type SMMs have never been discussed on the basis of the crystal structure. For dinuclear Tb(III)–Pc complexes, a dual magnetic relaxation process was observed. The relaxation processes are due to the anisotropic centres. Our results clearly show that the two Tb3+ ion sites are equivalent and are consistent with the crystal structure. On the other hand, the mononuclear Tb(III)–Pc complex exhibited only a single magnetic relaxation process. This is clear evidence that the magnetic relaxation mechanism depends heavily on the dipole–dipole (f–f) interactions between the Tb3+ ions in the dinuclear systems. Furthermore, the SMM behaviour of dinuclear Dy(III)–Pc type SMMs with smaller energy barriers compared with that of Tb(III)–Pc and slow-relaxation behaviour was explained. Dinuclear Dy(III)–Pc SMMs exhibited single-component magnetic relaxation behaviour. The results indicate that the magnetic relaxation properties of dinuclear Ln(III)–Pc multiple-decker complexes are affected by the local molecular symmetry and are extremely sensitive to tiny distortions in the coordination geometry. In other words, the spatial arrangement of the Ln3+ ions (f–f interactions) in the crystal is important. Our work shows that the SMM properties can be fine-tuned by introducing weak intermolecular magnetic interactions in a controlled SMM spatial arrangement.