Variation of the Cooperativity and the Role of Ligand-field States in Spincrossover Compounds
The spin-crossover process involves the rearrangement of electrons within metal dorbitals from the high-spin (HS) to the low-spin (LS) configuration corresponding to the distribution that yields maximum and minimum number of unpaired electrons respectively. The relative population of the spin states is a function of various external perturbations such as temperature, magnetic fields, external pressure, and light irradiation. Therefore, these materials are of current interest in chemistry and materials science not only because of their intrinsic fundamental properties but also because of their potential applications as functional materials for the construction of sensors, as well as memory and display devices. In the present talk, the thermal and photo-induced spin switching dynamics and the variation of the cooperative effects in the spin-crossover coordination networks will be discussed. The aim is to understand the physics of cooperative effects and establish the limits of cooperativity. In addition, the role of ligand-field states in the ultrafast photophysics of the prototypical spin-crossover compound will be discussed. The involvement of ligand-field states in transition-metal photophysics is nevertheless crucial, and that they are by no means innocent is borne out by the discovery of photo-induced processes in spin-crossover compounds, which has no low-energy Metal-to-Ligand-Charge-Transfer (MLCT) states.