Spin-Correlated Radical Pairs as Magnetic Switches for Controlling Emissive Triplet States via Triplet–Triplet Energy Transfer

Magnetic fields offer a powerful means to control molecular emission, enabling quantum sensing and spin-level control of chemical reactions. Here, we demonstrate a strategy to magnetically control red to near-infrared phosphorescence via triplet–triplet energy transfer (TTET) from donor–chiral bridge–acceptor (D–χ–A) molecules that generate spin-correlated radical pairs (SCRPs) upon photoexcitation. These SCRPs yield nonemissive triplet excited states whose formation is sensitive to magnetic fields. By transferring this energy to emissive Pt- and Pd-based π-extended porphyrins, we enable magnetic control over phosphorescence that would otherwise be unresponsive to weak magnetic fields (<1 T). This approach establishes a platform for quantifying magnetic field effects on silent triplet states while extending magnetically responsive emission into the near-infrared. Coupling SCRPs-based molecular magnetic switches to long-wavelength emissive acceptors offers a new way for probing and modulating spin-dependent processes and triplet-state populations in molecular systems.