Directly probing spin dynamics in a molecular magnet with femtosecond time-resolution† †Electronic supplementary information (ESI) available. See DOI: 10.1039/c6sc01105e Click here for additional data file.

We show that a vanadium–chromium Prussian blue analogue, which is a room-temperature moleculebased 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-tometal charge-transfer (LMCT) band results in the formation of the E excited state on the Cr ion via intersystem crossing (ISC) from the LMCT state in less than 250 fs. Subsequent vibrational relaxation in the E 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 state on the Cr ion from the change in the superexchange interaction taking place as a result of the corresponding spin flip associated with the formation of the E 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.