Light-Induced Spin State Switching in Copper(II)-Nitroxide-Based Molecular Magnet at Room Temperature.

Molecular magnets Cu(hfac)L exhibit an unusual type of photoinduced magnetostructural switching in exchange-coupled copper(II)-nitroxide clusters. Such photoswitching from strongly coupled to weakly coupled spin state (SS → WS) was recently found to be ultrafast, thus enhancing the interest in these systems and the scope of their potential applications. However, to date such SS → WS photoswitching was demonstrated only at cryogenic temperatures, being limited by the absence of suitable SS states and short relaxation times at T > 100 K. In this work we selected model compound Cu(hfac)L residing in the mixed SS/WS state at room temperature and investigated it using femtosecond optical spectroscopy. Photoinduced spin dynamics was detected, and an ultrafast SS → WS photoswitching was for the first time demonstrated at room temperature, constituting an important milestone in the development of copper(II)-nitroxide molecular magnets for practical purposes.