The question might seem quite easy to answer but still I am unable to find any useful information. I am aware of laporte- or symmetry-forbidden transitions and for f-f also spin-forbidden transitions in lanthanides but still I can't seem to find any good solution how they can happen.
For a d-d transition there are many exceptions depending on the arguments you come up with. From inverse symmetrical polyhedra that use ligand vibrations to become distorted to s- and p-mixing with the d-orbitals.
But f-orbitals, at least for the lanthanides, are said to not contribute to chemical bonds. Then there is often the argument about spin-orbit coupling but that would probably solve the spin-forbidden transitions. I'm not sure how it would affect the actual f-f transition being forbidden. In an old paper I also read about df- and fg-mixing. That would explain it but I am not sure if this is really the case.
So the question remains can the f-f transition in lanthanide complexes be somehow influenced to become at least a tiny bit more probable and therefore cause the transition to happen?