Photochemical Spin-State Switching using 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 that 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 pi-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.