Ultrafast charge-transfer-induced spin transition in cobalt-tungstate molecular photomagnets

In photoinduced phase transition materials, where both charge transfer and spin transition occur, there has been a long debate on which of the two processes are leading the phase transition. Herein, we present experimental evidence supporting an optically charge-transfer-induced spin transition ( CTIST ) process, as demonstrated through femtosecond optical spectroscopy in two-dimensional cyanido-bridged cobalt-tungstate photomagnets. Optical and magnetic investigations reveal that the optical excitation of the ground low-temperature (LT) Co ^III _LS -W ^IV state drives a photoinduced phase transition towards the Co ^II _HS -W ^V state, similar to the high temperature (HT) state. Ultrafast spectroscopy further indicates that this optical excitation of the intermetallic W-to-Co charge-transfer band produces a transient photoexcited (PE) Co ^II _LS -W ^V state, which decays within 130 fs through a spin transition towards the Co ^II _HS -W ^V state. Here we show that the CTIST dynamics corresponds to the Co ^III _LS -W ^IV (LT) → Co ^II _LS -W ^V (PE) → Co ^II _HS -W ^V (HT) sequence. The present work sheds a new light on understanding optical dynamics underlying the photoinduced phase transitions.