For the application of the previously developed light-induced magnetically switchable nickel porphyrin complexes as contrast agents in vivo, a few improvements have yet to be achieved. 1. Switching in aqueous media, 2. higher efficiency as a contrast agent (relaxivity) by incorporation of different transition metal ions (e.g. Fe3+), 3. switching at higher wavelengths. Further work focusses on contrast agents with non-porphyrin based ligands and “smart” contrast agents that respond to pH, temperature, and other metabolic parameters. Additionally, MR-sequences have to be adjusted and developed to enable the measurement of relaxation times and the determination of the kinetics of the switching process with high spatial and temporal resolution in vitro as well as in animal studies.