Photomagnetic properties of the [Fe(L222(N3O2))(CN)2]•H2O complex :: a fascinating example of multi-metastability

Today, it is known that the [Fe(L-222(N3O2))(CN)(2)]center dot H2O complex exhibits an atypical thermal spin-crossover behaviour probably connected to an unusual change of the iron(II) environment from a six to a seven coordination sphere. Here, we show that the photomagnetic properties of this derivative are also atypical. When the sample is initially prepared in the LS form, by a cooling process of 3 K/min, the light irradiation gives access to a long-lived metastable HS state with a T(LIESST) value of 132 K. This photoinduced HS state is moreover characterised by a strong cooperative lattice, as reflected by the kinetic relaxations which follow a sigmoidal model and the existence of a Light-Induced Thermal Hysteresis (LITH) loop. At the opposite, when the sample is carefully placed into the LS/HS mixture state, by using a slow cooling process of less than I K/min, the light irradiation generates a metastable state with a T(LIESST) value of only 73 K and the influence of the cooperativity is weak. The relaxation curves are indeed satisfactory fitted by using a stretched exponential model and the LITH phenomenon is not present. All these data will be discussed in term of multimetastability.