Spin fluctuations in the light-induced high-spin state of cobalt valence tautomers

We present a study of the static magnetic properties and spin dynamics in cobalt valence tautomers (VT), molecules where a low spin (LS) to high spin (HS) crossover driven by an intramolecular electron transfer can be controlled by the temperature, by the external pressure, or by light irradiation. In the investigated complex, a LS-Co(III) ion bound to a dinegative organic ligand can be reversibly converted into the HS-Co(II) bound to a mononegative one. By combining magnetization measurements with nuclear magnetic resonance (NMR) and muon spin relaxation ($\ensuremath{\mu}\mathrm{SR}$), we have investigated the static magnetic properties and the spin dynamics as a function of the temperature. Moreover, the effect of the external pressure as well as of the infrared light irradiation has been explored through magnetometry and NMR measurements to determine the spin dynamics of the HS state. The photoinduced HS state, which can have a lifetime of several hours below 30 K, is characterized by spin dynamics in the MHz range, which persist at least down to 10 K. The application of an external pressure causes a progressive increase of the LS-HS crossover, which reaches room temperature for pressures around 10 kbar.