Closed-to-open-shell ground state solid state photoswitching of thienyl-based acylhydrazones

Photoswitches convert a photon to molecular information through a reversible change in bonding or configuration. Applications of photoswitches are determined by the different properties in the two states and span from catalysis, to allosteric activation, to photopharmacology, up to materials and devices such as memories being able to store optical information in chemical bonds. For the latter, operation in solid state and different means to read/write/erase the information must be possible. These features, however, are challenging to achieve due to close molecular packing and intermolecular interactions. Here, we present acylhydrazones that photoswitch even in the solid state and adopt an open-shell triplet ground state electronic configuration in the photoswitched-form which is very long-lived. The optically stored information can thus be read magnetically and completely erased by applied electrochemical potential. In addition, ab initio calculations identified the closed-to-open-shell pathway and explained the singlet-triplet crossing. Photoswitching of closed/open-shell ground state electronic structure is a fundamentally new feature, which can bring new opportunities and applications. We demonstrate it on an example of the open-shell state of the photoswitch used as an initiator of radical benzylic bromination and radical debromination of an aromatic ring. However, the presented phenomenon is robust and insensitive to conditions, thus showing potential in a broad range from heterogeneous catalysis to spintronics.