Targeting of anionic membrane species by lanthanide(iii) complexes: towards improved MRI contrast agents for apoptosisElectronic supplementary information (ESI) available: Preparation of [Ln(L)Zn2](NO3)4, preparaton of liposomes, luminescence experiments, preparation of cell cultures, and MRI imaging of cell samples. See 10.1039/c1cc13284a