Photochemical Spin‐State Switching of an All‐Organic Molecular System with Visible Light

Controlling the spin state of a molecule using the spatiotemporal properties of visible light is of interest for spintronic devices in information technology or (bio)medical applications. We, herein, report an all‐organic visible light‐induced photochromic system than can switch from a diamagnetic (singlet) to a paramagnetic (triplet) state. This is realized by precisely tuning orbital symmetry and internal molecular strain in a [5]helicene scaffold substituted with an indanedione π‐acceptor. Irradiation with visible light at cryogenic temperatures gives a kinetically meta‐stable paramagnetic diradical state with a solvent‐dependent ground‐state multiplicity (triplet or singlet), which can be thermally switched back to its initial diamagnetic state.