Calorimetric measurements of the light-induced metastable high-spin state on the [Fe(PM-BiA)2(NCS)2] complex

Heat capacity measurements have been obtained on a pellet sample of [Fe(PM-BiA)2(NCS)2] formed by applying uniaxial pressure on the powder compound. They show a broad heat capacity peak near 170 K corresponding to a thermal spin crossover transition with enthalpy and entropy values around 4 kJ mol−1 and 24 J mol−1 K−1, respectively, and a hysteresis of a few kelvins. Magnetic data of the pellet sample show a gradual and incomplete thermal transition. Light irradiation (λ = 750–850 nm) at 5 K promotes the high-spin (HS) state and photomagnetic measurements give T(LIESST) = 68 ± 1 K. The heat capacity of this excited state has been measured and the relaxation from the HS to the LS state has been followed with AC calorimetry. An extended anomaly is observed centred at 66.5 K. This is the first calorimetric report of a sample on a LIESST state.