Molecular Sensors Operating by a Spin‐State Change in Solution: Application to Magnetic Resonance Imaging

Recent proposals for molecular switches are reviewed that adopt a different spin state before and after the encounter with an analyte (metabolite) or physical stimulus (light). Spin‐state switching has significant potential to provide molecular MRI with a first line of probes operating by an off/on mode. Past efforts to design MRI sensors relied on strategies other than spin‐switching and thus mostly comprised probes only modulating the signal before and after encounter with the analyte (X%‐on/Y%‐on). The article thus essentially treats small‐molecule metal ion chelates, i.e. coordination compounds. Initially, a comprehensive treatment of design considerations for optimal spin‐state switches is furnished. The article then enters into a treatment of reported examples of spin‐state switches according to three response mechanisms: 1) changes in the redox state of the metal center; 2) changes in coordination geometry; and 3) modification of the surrounding ligand field, either by peripheral interaction with the analyte, or by replacement of a coordinating ligand by another.