Photo-induced spin-transition: the role of the iron(II) environment distortion

The [FeL_n(NCS)₂] iron(II) spin-crossover complexes cover a wide range of magnetic behaviour. Owing to the large number of known structural and magnetic data, this series is perfectly adapted to the investigation of the structure–magnetic properties relationship. In this paper we propose a new structural parameter, denoted Θ, which is used to correlate the features of the spin-crossover phenomena with the distortion of the iron environment. In particular, this parameter has shed light on the role of such distortion on the limiting temperature of photo-inscription, known as T(LIESST). A strong dependence of T(LIESST) on Θ is clearly demonstrated. The stronger the distortion the higher the T(LIESST) value. This structure–property dependence represents, for instance, a powerful tool to estimate the highest potential T(LIESST) value for a series of complexes. This limit in the [FeL_n(NCS)₂] series is estimated to be around 120 K, which probably prevents their use in any industrial application.