Directly probing spin dynamics in a molecular magnet with femtosecond time-resolution

We show that a vanadium-chromium Prussian blue analogue, which is a room-temperature molecule-based magnet, displays a fast magnetic response on a femtosecond timescale that is attributed to the super-exchange interaction between the metal ions. These dynamics are obtained from femtosecond Faraday magneto-optical (MO) measurements, performed at 50 and 300 K. Exciting at the ligand-to-metal charge-transfer (LMCT) band results in the formation of the E-2 excited state on the Cr ion via intersystem crossing (ISC) from the (LMCT)-L-4 state in less than 250 fs. Subsequent vibrational relaxation in the 2E state occurs on a 0.78 +/- 0.05 ps timescale at 50 K and 1.1 +/- 0.1 ps at 300 K. The MO measurements can detect the formation of the E-2 state on the Cr ion from the change in the super-exchange interaction taking place as a result of the corresponding spin flip associated with the formation of the E-2 state. These results open up a new avenue to study molecular magnets using a powerful method that is capable of directly probing spin dynamics on a sub-picosecond timescale in thin film environments.