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

In materials exhibiting photoinduced phase transitions, and in which both charge transfer and spin transitions occur, there has long been a debate about which process drives 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 studies revealed that the photoexcitation of the ground low-temperature (LT) Co^III_LS-W^IV state leads to a photoinduced phase transition towards the Co^II_HS-W^V state, which is 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.