An All-Organic Photochemical Magnetic Switch with Bistable Spin States

Controlling the electronic spin state in single molecules through an external stimulus is of interest in developingdevices for information technology, such as data storage and quantum computing. We report the synthesis and operation mode oftwo all-organic molecular spin-state switches that can be photochemically switched from a diamagnetic [electron paramagneticresonance (EPR)-silent] to a paramagnetic (EPR-active) form at cryogenic temperatures due to a reversible electrocyclic reaction ofits carbon skeleton. Facile synthetic substitution of a configurationally stable 1,14-dimethyl-[5]helicene with radical stabilizinggroups at the 4,11-positions afforded two spin-state switches as 4,11-dioxo or 4,11-bis(dicyanomethylidenyl) derivatives in a closeddiamagnetic form. After irradiation with an LED light source at cryogenic temperatures, a stable paramagnetic state is readilyobtained, making this system a bistable magnetic switch that can reversibly react back to its diamagnetic form through a thermalstimulus. The switching can be monitored with UV/vis spectroscopy and EPR spectroscopy or induced by electrochemical reductionand reoxidation. Variable-temperature EPR spectroscopy of the paramagnetic species revealed an open-shell triplet ground state withan experimentally determined triplet-singlet energy gap of Delta ET-S< 0.1 kcal mol-1. The inherent chirality and the ability to separatethe enantiomers turns this helical motif into a potential chiroptical spin-state switch. The herein developed 4,11-substitution patternon the dimethyl[5]helicene introduces a platform for designing future generations of organic molecular photomagnetic switches thatmightfind applications in spintronics and related fields.